DIR Return Create A Forum - Home
---------------------------------------------------------
FUNDAY
HTML https://funday.createaforum.com
---------------------------------------------------------
*****************************************************
DIR Return to: Updates
*****************************************************
#Post#: 178--------------------------------------------------
SURGE TECTONICS
By: Admin Date: March 22, 2017, 9:24 am
---------------------------------------------------------
Surge Tectonics
CONTENTS
Chapter 1 Why a New Hypothesis?
1.2 Former & Current Concepts of Earth Dynamics
1.2.3 Mantle Convection
1.2.4 Earth Expansion
1.2.5 Vertical Tectonics
1.2.6 Zonal Rotation
1.2.7 Continental Drift, Polar Wandering
1.2.8 Seafloor Spreading and Plate Tectonics
1.2.9 Tectonostratigraphic Terraces
1.2.10 Wedge Tectonics
1.2.11 Plate Tectonics with Fixed Continents
1.2.12 Zipper Tectonics (Spiral Tectonics)
1.2.13 Viscous Flow Model
Chapter 2 Unraveling Earth History: Tectonic Dating Sets
2.1 Data Availability
2.2 New Data Acquisition
2.2.1 Submersibles and Deep-Sea Drilling
2.2.2 Sonography
2.2.3 Accurate Bathymetry
2.2.4 Seismotomography
2.2.5 Space Geology
2.2.6 Satellite Photography
2.2.7 Satellite Radar Altimetry
2.2.8 Radar Mapping of Venus
2.2.9 Other Techniques
2.3 Data Sets Unexplained in Current Tectonic Models: Foundation
for a New Hypothesis
Chapter 3 Surge Tectonics
=3.2 Velocity Structure of the Earth's Outer Shells
3.2.1 Basic Framework
3.2.2 Continents Have Deep Roots
3.3 Contraction
3.3.2 Contraction Skepticism
3.3.3 Evidence For a Differentiated, Cooled Earth
3.4 Contraction as an Explanation of Earth Dynamics
3.5 Review of Surge and Related Concepts in Earth-Dynamics
Theory
3.6 Geotectonic Cycle of Surge Tectonics
3.7 Pascal's Law---the Core of Tectogenesis
3.8 Evidence for the Existence of Surge Channels
3.8.1 Seismic-Reflection Data
3.8.3 Seismotomographic Data
3.8.4 Surface-Geological Data
3.8.5 Other Data
3.9 Geometry of Surge Channels
3.9.1 Surge-Channel Cross Section
3.9.2 Surge-Channel Surface Expression
3.9.3 Role of the Moho
3.9.4 Formation of Multitiered Surge Channels
3.10 Demonstration of Tangential Flow in Surge Channels
3.11 Mechanism for Eastward Surge
3.12 Classification of Surge Channels
3.12.2 Ocean-Basin Surge Channels
3.12.3 Continental-Margin Surge Channels
3.12.4 Continental Surge Channels
3.13 K Structures
3.14 Criteria for the Identification of Surge Channels
Chapter 4 Examples of Surge Channels
=4.1 Ocean-Basin Surge Channels
=4.2 Surge Channels of Continental Margins
4.3 Continental Surge Channels
4.4 Surge Channels in Zones of Transtension-Transpression
Chapter 5 The Tectonic Evolution of Southeast Asia - A Regional
Application of the Surge-Tectonics Hypothesis
5.1 Surge Tectonic Framework
5.2 Surge-Tectonic History
Chapter 6 Magma Floods, Flood Basalts, and Surge Tectonics
6.2 Descriptions of Selected Continental Flood Basalt Provinces
6.3 The Use of Geochemistry in Identifying Flood Basalts
6.4 Geochemical Comparisons among Basalts Erupted in Different
Tectonic Settings
6.5 Duration of Individual Basalt Floods
6.6 Flood-Basalt Provinces and Frequency in Geologic Time
6.7 Non-Basalt Flood Volcanism in Flood-Basalt Provinces
6.8 Flood Basalts or Magma Floods?
6.9 Surge-Tectonics Origin of Magma Floods
Chapter 7 Conclusions
APPENDIX
Chapter 3
SURGE TECTONICS
3.1 Introduction
Surge tectonics is a new hypothesis quite unlike previously
proposed hypotheses, although many of its component parts are
based on ideas long known. We believe the hypothesis provides a
comprehensive and internally consistent explanation of all
tectonic phenomena without the necessity of making unsupported
assumptions or ad hoc explanations. We have found nothing that
surge tectonics cannot explain in a simpler way than other
tectonic hypotheses. Surge tectonics draws on well-known
physical laws, especially those related to Newton's laws of
motion and gravity. Fluid dynamics plays an important role in
surge tectonics. (For more information on the laws we utilize,
those mentioned in the text are defined in the Appendix; those
withing more detail are referred to two standard physics
textbooks by Sears et al. [1974] and Blatt [1983]. An excellent
state-of-the-art fluid-dynamics text is that by Tritton [1998]).
Surge tectonics is based on the concept that the lithosphere
contains a worldwide network of deformable magma chambers (surge
channels) in which partial magma melt is in motion (active surge
channels) or was in motion at some time in the past (inactive
surge channels). These surge channels play the role of the holes
in the "hole-in-the-plate" problem ("elliptical hole problem")
of civil and construction engineering, industrial engineering,
and rock mechanics (..., 1913-1991). The presence of surge
channels means that all of the compressive stress in the
lithosphere is oriented at right angles to their walls. As this
compressive stress increases during a given geotectonic cycle,
it eventually ruptures the channels that are deformed
bilaterally into kobergens (Fig. 2.15). Kobergens were named by
Meyerhoff et al. (1992b) in honor of Austrian geologist, Leopold
Kober (1921-1928). Kober observed that Alpide foldbelts have
been bilaterally deformed with the northern ranges vergent
toward Europe and the southern toward Africa (see Fig. 12 in
M-b, 1992b). Thus, bilaterally deformed foldbelts in
surge-tectonics terminology are called kobergens.
Surge tectonics involves three separate but interdependent and
interacting processes. The first process is the contraction or
cooling of the Earth. The second is the lateral flow of fluid,
or semifluid, magma through a network of interconnected magma
channels in the lithosphere. We call these surge channels for
reasons that will become apparent. The third process is the
Earth's rotation. This process involves differential lag between
the lithosphere and the strictosphere (the hard mantle beneath
the asthenosphere and lower crust), and its effects--- eastward
shifts---already discussed (Table 2.3). Because lithosphere
compression caused by cooling is the mechanism that propels the
lateral flow of fluid, or semifluid, magma through surge
channels, we discuss first the velocity structure of the Earth's
lithosphere and underlying layers, then the contraction
hypothesis (Earth cooling), and then the effects of contraction
on the Earth's outer shells.
3.2 Velocity Structure of the Earth's Outer Shells
3.2.1 BASIC FRAMEWORK
The Earth's outer shells (Fig. 3.1) consist of a "hard"
lithosphere above a "soft" asthenosphere (..., 1896-1940). The
interpreted seismic structure of these two shells is given in
Table 3.1, which is based on Press (1966) and Iyer and Hitchcock
(1989). The asthenosphere overlies another hard shell that
Bucher (1956, ...) named the strictosphere. Its seismic
characteristics (at least near the upper surface of the
strictosphere) also are given on Table 3.1. Of these shells, the
lithosphere is especially important because visible tectonic
effects provide the principal clues to the origin of
tectogenesis. It has not been too long, since the seismic
structure of the crust and upper mantle became sufficiently well
imaged to permit more than just educated guesses about it.
Before 1958, when Revelle (1958) discovered a layer of material
with a velocity of 7.3 km/s on the southern part of the East
Pacific Rise, the lithosphere was perceived as consisting of
6.6-km/s crust overlying directly the 8.1-km/s mantle. What
Revelle (1958) discovered was a lens or high-velocity crust, or
low-velocity mantle, with a P-wave velocity of 7.0-7.8 km/s
separating the "normal" crust above from the "normal" crust
below. Similar lenses were found almost everywhere beneath the
midocean ridge system (..., 1959-1965). By 1982, a similar lens
of 7.0-7.8-km/s material was found beneath most of the Earth's
rifts (Figs. 2.9, 3.2-3.8). Mooney et al. (1983) suggested that
such lenses are a characteristic of extensional tectonic belts.
During the 1970s, refraction shooting in the northern
Appalachians discovered a similar 7.0-km/s lens under the
Acadian (Devonian) foldbelt (..., 1980-1989; and Fig. 3.13). It
was not long before identical lenses beneath foldbelts were
identified in many parts of the world (Figs. 2.13, 2.18, 2.21,
3.9-3.14). Good images of these lenses were recorded on
reflection -seismic lines used for deep continental and oceanic
tectonic studies (Figs. 2.10, 2.11, 2.14, 3.15; ..., 1988).
Figures 2.10 and 2.11, from the English Channel and southwestern
Queensland respectively, are particularly good images of two of
these lenses near the base of the continental crust.
Mooney and Braile (1989), summarizing the present state of
knowledge of the structure of the Earth's crust, inserted a
7.0-7.9-km/s lower crustal layer between the 6.6-km/s sialic
crust above, and the 8.1-km/s mantle below. They showed the
layer to be absent in places. In general, it is present beneath
cratons, platforms, foldbelts, rift systems, and wrench-fault
zones. Under cratonic areas the layer is 7-15 km thick; under
tectonic belts it is thicker, ranging from 10-25 km. In many
places the 7.0-7.9-km/s velocity is gradational with mantle
velocities (>7.9 km/s). In these places, the Moho- is not a true
discontinuity but a transition zone several kilometers thick
(..., 1989). Areas were found also beneath ancient platforms and
shields where the 7.0-7.8-km/s layer is up to 30 km thick.
Examples include the Baltic Shield (3.5; ..., 1989) and parts of
the Canadian Shield (..., 1989).
The preceding suggests that the 7.0-7.8-km/s layer is
distributed rather randomly, and that its thickness in a given
area is a result of random processes. These conclusions are
almost unavoidable if one tries to explain the distribution and
thickness with any of the Earth-dynamics hypotheses published
during the last century. In surge tectonics this apparent
randomness of distribution and thickness is in part predictable.
The origin of the 7.0-7.8-km/s layer almost certainly is
closely related to the high reflectivities of the lower crust as
described and illustrated by Klemperer et al. (1986), Klemperer
(1987), Goodwin and Thompson (1988), Thompson and McCarthy
(1990), and others (see Figs. 2.10, 2.11, 2.14, 3.15). We
emphasize the fact that a lens (or lenses) or 7.0-7.8-km/s
material underlies all of the microearthquake bands that we
studied, and therefore, all of the high heat-flow bands shown on
Figure 2.26.
3.2.2 CONTINENTS HAVE DEEP ROOTS
An important aspect of upper mantle and crustal structure is
that continental cratons have deep crustal roots (...,
1963-1996). Contrary to general belief (..., 1987a), continental
roots are fixed to the strictosphere (..., 1985-1986). This
conclusion is supported by large and increasing volumes of data,
including neodymium and strontium studies of crustal rocks (...,
1979). The absence, or near-absence, of a low-velocity
asthenosphere beneath ancient cratons led Lowman (1985, 1986) to
propose an Earth-dynamics hypothesis of sea-floor spreading
between fixed continents. In this hypothesis, if sea-floor
spreading takes place, it is restricted to suboceanic regions.
Thus, the deep roots of continents are a major obstacle to any
hypothesis requiring continental movements (..., 1985-1990).
An example of deep continental roots is presented in Figure
1.1, a seismotomographic cross section of North America. The
dark shading beneath the Canadian Shield shows a root extending
to 400-450 km (..., 1987). Similar deep roots are seen beneath
part of all of the Earth's ancient cratons. In places, however,
lenses of 7.0-7.8-km/s material containing low-velocity zones
(Fig. 3.5) are present (..., 1989). Such lenses containing
low-velocity layers postdate the establishment of the deep
cratonic roots, as we show in subsequent sections.
3.3 Contraction
3.3.1 GENERAL
A discussion of the history and concept of contraction have been
presented in Chapter 1 of this book. It will not be repeated
here, but the interested reader is encouraged to review that
discussion.
3.3.2 Contraction Skepticism
Many workers today either doubt that contraction is taking place
or fail to see why the possibility should even be considered.
Bott (1971, p. 270), expressing a common opinion, wrote that
because of the success of plate tectonics in producing
foldbelts, contraction now "...is irrelevant to tectonic
problems." Two reviewers of the paper by Meyerhoff et al.
(1992b) also expressed doubts that contraction can be taking
place. However, one of them, K.B. Krauskopf (pers. comm., 1990),
conceded that "...too little is known about what goes on in the
[Earth's] interior for any definite statement to be made." He
noted that MacDonald's (1959-1965a) models could easily be as
sound today as they were in 1965 because "...not much more is
known today..." about the concentration of radioactive elements
in the Earth's interior.
3.3.3 Evidence For a Differentiated, Cooled Earth
The evidence is straightforward. The most salient facts follow.
1. The Earth includes several concentric shells, which are
explicable only if the Earth differentiated efficiently and at a
much higher temperature than today.
2. The outermost of these shells may be the oceanic crust whose
thickness ranges from about 4-7 km. This crust is characterized
by relatively constant thickness and fairly uniform seismic
properties. Both Worzel (1965) and Vogt et al. (1969) observed
that if the plate-tectonic explanation of ocean-crust generation
is correct, it is a truly remarkable process that produces such
a uniform layer in all ocean basins regardless of the spreading
rate---1.2 dm/yr or 60 cm/yr. This uniformity is explained,
however, if the oceanic crust is the outermost of the Earth's
concentric shells. There are other explanations, one of which is
discussed later.
3. Mehnert (1969), among several, noted that the further back
one looks into the geological record, the greater is the
abundance of mafic rocks. This is explained if the lithosphere
has been thickening through time by cooling, as Mehnert (1969)
suggested.
4. Miyashiro et al. (1982), reporting on studies of the Earth's
metamorphic rocks, noted that Precambrian rocks show the highest
geothermal gradients and that geothermal gradients of younger
rocks generally decrease to the present time.
5. A convincing evidence that huge segments of the lithosphere
have been and are being engulfed by tangential compression is
the existence of the previously discussed Verschluckungszonen
(swallowing or engulfment zones) of Ampferer (1906) and Ampferer
and Hammer (1911). In places along such zones, whole metamorphic
and igneous belts that are characteristic of parts of a given
foldbelt simply disappear for hundreds of kilometers along
strike (e.g., Alps: ..., 1983; K...-S... foldbelt ..., 1973; New
Zealand Alps ..., 1974: ...; southern California Transverse
Ranges: ..., 1984). Figures 2.23-2.24 and 3.16-3.17 illustrate
the characteristics of typical Verschluckungszonen. Although
Mueller (1983), Humphreys et al. (1984), and other workers
considered these features to be former subduction zones, this
interpretation is difficult to defend because all of these
zones, regardless of age, are near-vertical bodies (1) reach
only the top or middle of the asthenosphere (150 to 250 km deep)
and (2) do not deviate more than 10° to 25° from the vertical
(..., 1983-1984).
6. The antipodal positions of the continents and ocean basins
mean that Earth passed through a molten or near-molten phase
(..., 1907-1968). Such antipodal relations are unlikely to be a
matter of chance or coincidence (..., 1968).
7. Theory (..., 1970) and laboratory experiment (..., 1956)
showed that heated spheres cool by rupture along great circles.
Remnants of two such great circles (as defined by hypocenters at
the base of the asthenosphere) are active today: the
Circum-Pacific and Tethys-Mediterranean fold systems. The
importance of Bucher's (1956) experiment to contraction theory,
in which he reproduced the great circles, is little appreciated.
8. As Earth cooled, it solidified from the surface downward.
Because stress states in cooled and uncooled parts are
necessarily opposite one another, compression above and tension
below, the two parts must be separated by a surface or zone that
Davison (1887) called the level of no strain (Fig. 3.2). We, as
did Wilson (1954), equate the cooled layer with the lithosphere
(Fig. 3.1). The uncooled part below is what Bucher (1956) called
the strictosphere. Thus, as originally proposed by Scheidegger
and Wilson (1950), Davison's (1887) level of no strain must be
the asthenosphere, or a zone of no strain across which the
change in stress states is gradual (Fig. 3.1). Only in a cooling
Earth, which approximates a closed thermal system, can an
asthenosphere form.
9. Continued cooling deepens the asthenosphere and the upper
surface of the strictosphere. The stresses accumulated through
cooling are relieved episodically by rupture along the
great-circle fractures that are the Earth's cooling cracks or
the Benioff zones of current literature. Because the lithosphere
is being compressed and the strictosphere subjected to tension,
the mechanics of rupture should follow the Navier-Coulomb
maxiumum shear-stress theory (..., 1962). Accordingly, the
lithosphere Benioff zone must dip less than 45° to a tangent to
the Earth's surface (in actual fact, it dips 22° to 44°, Figs.
2.36, 3.1). In contrast, the strictosphere Benioff zone must dip
more than 45° to a tangent to the Earth's surface (50° to 75°,
Figs. 2.36, 3.1). Benioff (1949, 1954) discovered the change in
Benioff-zone dip from lithosphere to strictosphere, but
Scheidegger and Wilson (1950) recognized these dips as an
expression of the Navier-Coulomb maximum shear-stress theory
(Figs. 2.36, 3.1). The dip values of the lithosphere and
strictosphere Benioff zones confirm that the Earth is a cooling
body. (Ritsema [1957, 1960], working independently, also
discovered the abrupt dip changes in the dip of the Benioff zone
with increasing depth.)
An important fact concerning the Benioff zone is that the two
segments, one in the lithosphere and the other in the
strictosphere, do not necessarily form a single, continuous zone
as depicted in diagrams and cross sections (e.g., Figs. 2.36,
3.1). Benioff (1949, 1954) found that the two segments of the
Benioff zone, instead of joining near the base of the
asthenosphere, may be offset for distances of 100 to 200 km
(Fig. 3.18). In some places such as the Lesser Antilles arc, a
strictosphere Benioff zone may not even be present below the
lithosphere Benioff zone (e.g., Lesser Antilles and Scotia
volcanic arcs). These facts are explained in our surge-
tectonics hypothesis but not by other hypotheses. In fact, all
detailed modern studies of hypocenter distribution in Benioff
zones show the same clear division into a shallow, gently
dipping lithosphere benioff zone and a deeper, steeply dipping
strictosphere Benioff zone (Fig. 2.36; ..., 1973; ..., 1977).
Ritsema's (1957, 1960) focal-mechanism studies of shallow,
intermediate, and deep earthquakes showed that the Benioff-zone
dip in the lithosphere is only half of its dip in the
strictosphere. An even more significant discovery made by
Ritsema (1957, 1960), although he attached little importance to
it, was the revelation that earthquake foci above 0.03R
(approximately 180 km depth) show mainly compression, whereas
those below 0.03R show mainly tension. Most earthquakes above
and below 0.03R, as Scheidegger (1963) also noted, have a
strike-slip component. Thus Ritsema's findings lend support to
Scheidegger and Wilson's (1950) interpretation of the Benioff
zone as a manifestation of the Navier-Coulomb maximum
shear-stress theory.
10. Computer simulations of possible Earth thermal histories
(...,1959, 1963; ..., 1961; Reynolds et al., 1966), using a
broad spectrum of assumed initial temperatures and chemical
compositions, show that the Earth is cooling (..., 1959-1966).
The fact that Earth's Benioff zones still are active
earthquake-generating zones provides strong support for this
conclusion. Perhaps the strongest support comes from the Basalt
Volcanism Study Project (1981) report by 101 petrologists,
mineralogists, and petrographers, who wrote that the repeated
extrusion of basalt to the Earth's surface through its history
is proof of the Earth's long history of cooling.
11. Finally, the existence of Verschluckungszonen in the
lithosphere and upper mantle also constitutes evidence that the
Earth is actively cooling. Verschluckungszonen are interpreted
by us to be large masses of lithosphere and upper mantle that
were downbuckled into the upper mantle during tectogenesis as
the lithosphere readjusted its shape to fit the underlying,
cooling strictosphere (Figs. 2.24, 3.16-3.17). If this
interpretation is correct, the existence of Verschluckungszonen
may constitute proof that the Earth has been cooling to the
present day. We discuss Verschluckungszonen in a subsequent
section.
3.4 Contraction as an Explanation of Earth Dynamics
3.4.1 CONTRACTION ACTING ALONE
Despite the attraction of a cooling Earth, both Scheidegger
(1963) and Bott (1971) concluded that contraction acting alone
is inadequate to produce the crustal shortening measured in
Earth's many tectonic belts. For both geological and
seismological reasons, this conclusion appears to be
well-founded. They gave several reasons; three of which and one
of our own are crucial.
1. The total amount of shortening measured across the Earth's
foldbelts far exceeds what can be inferred on theoretical
grounds, whether one uses the contraction model of MacDonald
(1963) or of Jeffreys (1970). Even if one accepts Bucher's
(1955...; 1956...) outstanding demonstration that apparent
(measured) shortening can be and generally is four to five times
true shortening, the contraction hypothesis cannot explain all
true shortening in foldbelts. (Lyttelton's [1982] theoretical
estimate of 2,000 km of shortening adequately explained the
measured shortening, but his hypothesis requires cataclysmic
geological events that need to be sought in the field.)
2. Contraction alone is unable to explain the origins of all
types of tectonic belts---compressional foldbelts, tensional
rift zones (including midocean ridges), and strike-slip zones.
3. Ritsema (1957, 1960) and Scheidegger (1963) observed that
earthquake first- motion studies show that strike-slip motions
are most common in Benioff zones, not just in strike-slip and
rift zones. Contraction alone cannot explain the ubiquitous
strike-slip component.
4. Contraction theory requires that foldbelts are concentrated
in and adjacent to oceanic trenches. This is not observed. More
than 50% of the Earth's foldbelts lie at great distances from
the surface trace of a Benioff zone, and all Jurassic- Cenozoic
foldbelts lie within the high heat-flow bands illustrated on
Figure 2.26. This cannot be explained by any Earth-dynamics
hypothesis yet proposed. However, if contraction could lead to
tectogenesis of large parts of the lithosphere far removed from
Benioff zones, the preceding objections to the contraction
hypothesis would be irrelevant.
3.4.2 CONTRACTION ACTING AS THE TRIGGER FOR TECTOGENESIS
Figure 2.26, as we have discussed, portrays the bands of high
heat flow that crisscross the Earth's surface. We believe that
this reticulate network of high heat-flow bands is underlain by
a network of interconnected magma chambers. In our
surge-tectonics hypothesis, these magma chambers are the mantle
diapirs discussed in preceding sections and summarized in part
in Table 2.1. Figure 2.26 indicates that these mantle diapirs,
or magma chambers, are interconnected. The interconnected
channels comprise the surge channels of surge tectonics. If the
hot material in these channels is sufficiently mobile, lateral
flow through them should be possible provided a pressure
gradient is present. The compression already present in the
lithosphere would provide the force needed to initiate and
maintain flow. We emphasize that such flow would be temporally
discontinuous (.i.e., episodic) and, when it did occur, would be
extremely slow. Flow velocities are discussed later.
We have pointed out that in a cooling Earth, the lithosphere by
definition is everywhere and at all times in a state of
compression (Fig. 3.1; ..., 1887-1982). The compression is
concentrated in planes tangent to the Earth's surface, and is
equal in all directions. James C. Meyerhoff (pers. comm., 1988)
called this equiplanar tangential compression, and this
compression in the lithosphere is what Bucher (1956) referred to
(incorrectly) as all-sided compression.
The only elements in the lithosphere that disturb this
approximately equiplanar tangential stress state are the surge
channels. Flow can take place in these channels wherever a
pressure gradient develops. For example, the escape of lava from
a channel lowers the pressure at that point, and equiplanar
compression, acting at right angles to the surge-channel walls,
mobilizes the fluid elements inside the channel until pressure
equilibrium is restored. The presence above active channels of
channel-parallel fault, fracture, and fissure systems indicates
that (1) flow takes place along the full length of each tectonic
belt and (2) the channels are in communication with the Earth's
surface through the fault-fracture- fissure system.
Surge channels and their fault-fracture-fissure systems
constitute zones of weakness in the lithosphere. Because (1) the
channels are the only bodies in the lithosphere that, owing to
their potential to contain mobile fluids, they have the capacity
to upset the state of equiplanar tangential compression, and
because (2) they are constantly losing their contents to the
surface lithosphere compression ultimately destroys them. Their
deformation and ultimate destruction are the essence of
tectogenesis. Thus the cooling process in the Earth's
strictosphere effectively guarantees the presence within the
lithosphere of a powerful mechanism for tectogenesis.
3.5 Review of Surge and Related Concepts in Earth-Dynamics
Theory
Several workers proposed, on both theoretical and
geological-geophysical grounds, the presence of bodies similar
to surge channels in the lithosphere. Others developed concepts
much like those that are the basis for surge tectonics.
3.5.1 SURGE CHANNELS
Our study of this topic was by no means exhaustive, we may have
missed important references that deal with the concept of
surge-channel-like edifices in the lithosphere and uppermost
mantle. A particularly good example of a surge-channel- like
feature was proposed by Vogt (1974) for the Iceland region,
including the Kolbeindey, Reykjanes, and Faeroe-Greenland
ridges. He wrote (...), "In the model I assume there is a
pipe-like region below the spreading axis, extending
subhorizontally away from a plume such as Iceland.... This
mid-oceanic pipe extends from the base of the axial lithosphere,
about 5 or 10 km deep, down to maximum depths (30 to 50 km?)
from which basalt melts segregate and rise. Tholeiitic fluids
would be released from the entire pipe; origin depths of 23 km
for the Mid-Atlantic Ridge and 16 km for the East Pacific Rise
... Would approximate depth to the center ot the pipe. The
ultrabasic mush in this pipe is assumed to be flowing away from
the hot spot at a rate determined by pipe diameter, viscosity,
and horizontal pressure gradient...." Vogt (1974) estimated that
flow ranged laterally outward from Iceland from 500 to 600 km. A
similar study by Gorshkov and Lukashevich (1989) suggested that
channels are present beneath the full length of the midocean
ridge system. According to them, flow beneath the Mid-Atlantic
Ridge would be from hot spots located beneath Antarctica in the
south and Iceland in the north, with the two flows converging
near the equator.
Other midocean ridges where some type of axis-parallel flow
and/or rift propagation have been postulated include the Juan de
Fuca Ridge (..., 1975) and the Galapagos Rift (..., 1977). After
the discovery of systematic segmentation along the midocean
ridges beginning with the East Pacific Rise (..., 1982),
Lonsdale, MacDonald, Fos, and others commenced a series of
investigations that led to a general postulate of ridge-parallel
flow in the midocean ridges (..., 1988-1989). Macdonald et al.
(1988) proposed that mantle diapirs rise beneath the centers of
each ridge segment, thereby accounting for the greater
elevations of the central parts of such segments. From the
crest, ridge-parallel lateral flow commences. Such flow halts in
the depressed areas between adjacent segments because of mutual
impingement.
Sonographs of the midocean ridges show that the Macdonald et
al. (1988) hypothesis is not tenable. Where the diapirs rise in
the centers of the ridge segments, radial and/or annular
structures should be present. Where the flows from adjacent
segments impinge, compressional structures should be present.
Neither structural form is observed. Instead, linear fractures
and faults extend for hundreds of kilometers along strike, with
interruptions only at transform faults, we traced individual
fault traces through the fault zone from one ridge segment to
the next, which negates the Macdonald et al. (1988) hypothesis.
Surge-channel, or interconnected mantle-diapir systems have
been reported from small oceanic basins and continental areas. A
well-known example of the former is the Tyrrhenian Sea west of
Italy, where geophysical techniques and very high heat flow show
the presence of a very large diapir-like body at shallow depths
(..., 1988).
Among continental examples, the Fergana Valley in Soviet
Central Asia is the best documented Kuchay and Yeryemin (1990)
discovered a very large pipelike body, or channel, below this
large east-west-striking late Cenozoic structural depression
nestled among the western ranges of the Tian Shan. In their
summary (p. 45), Kuchay and Yeryemin concluded: "Interpretation
and interpolation of geophysical data from the [Fergana Valley]
lead to the conclusion that the base of the 'granitic' layer
undergoes partial melting, as a consequence of which there forms
a layer of lowered viscosity that has been identified
seismically as a zone of reduced velocity above the Conrad
discontinuity. It is possible to consider this zone as a
'granitic' asthenochannel, a subhorizontal layer that has a high
strain rate and in which relative lateral displacement takes
place between the contents of the asthenochannel and the
surrounding rock layers. The absence of a gravity anomaly, which
is an indication of variations in thickness and other properties
within the zone of reduced velocity, suggests that the
'granitic' layer and the 'granitic' asthenochannel have the same
density."
3.5.2 USE OF THE SURGE CONCEPT IN TECTONICS
We have found three parallel and presumably independent
derivations of the surge- tectonics concept. The most recent of
these originated with Hollister and Crawford (1986) who used
"tectonic surge" to describe rapid vertical uplift in the
structural core of a bilaterally deformed foldbelt (i.e., in the
center of what we call a kobergen). Hollister and Crawford
(1986, ...) opined that "Weakening of the crust [by] anatexis
and accompanying development of melt-lubricated shear zones..."
is essential to rapid vertical uplift. Paterson et al. (1989,
...) referred to this type of tectonics as surge tectonics.
Tobisch and Paterson (1990) used this term in two or three areas
of southeastern Australia. Their usage is close to our own.
The third and, as far as we can determine, oldest use of the
term in tectonics in recent literature was by Meyerhoff and
Meyerhoff (1977). They proposed that asthenosphere surges (1)
from beneath the Asian continent, (2) between North and South
America, and (3) between South America and Antarctica produced
the eastward- facing island arcs in the three regions. The idea
was used subsequently to explain the complexities of Caribbean
tectonics (..., 1990). Morris et al. (1990) used the term surge
tectonics (coined by Bruce D. Martin). The paper was written
during 1987-1988 and was submitted to and accepted by the
Geological Society of America in 1988. Regardless, the Paterson
et al. (1989) use of the term in print precedes by five months
that by Meyerhoff et al. (1989). The term surge tectgonics is
used in this book in the same sense that it was employed by
Taner and Meyerhoff (1990).
3.6 Geotectonic Cycle of Surge Tectonics
The asthenosphere alternately expands (during times of tectonic
quiescence) and contracts (during tectogenesis). Thus, when the
asthenosphere is expanding, the surge channels above it, which
are supplied from the asthenosphere, also are expanding; and
when tectogenesis takes place, the magma in the surge channels
is expelled. Tectogenesis is triggered by collapse of the
lithosphere into the asthenosphere along the 30° -dipping
lithosphere Benioff zones. The following is Meyerhoff et al.'s
(1992b) interpretation of the approximate sequence of events
during a geotectonic cycle (Fig. 3.19).
1. The strictosphere is always contracting, presumably at a
steady rate, because the Earth is cooling.
2. The overlying lithosphere, because it is already cool, does
not contract, but adjusts its basal circumference to the upper
surface of the shrinking strictosphere by (1) large-scale
thrusting along the lightosphere Benioff zones, and (2) normal-
type faulting along the strictosphere Benioff zones. These two
types of deformation, one compressive and the other tensile, are
complementary and together constitute an example of the
Navier-Coulomb maximum shear stress theory (..., 1962 -1979).
3. The large-scale thrusting of the lithosphere is not a
continuous process, but occurs only when the lithosphere's
underlying dynamic support fails. That support is provided
mainly by the softer asthenosphere and frictional resistance
along the Benioff fractures. When the weight of the lithosphere
overcomes the combined resistance offered by the asthenosphere
and Benioff-zone friction, lithosphere collapse ensues. Because
this process cannot be perfectly cyclic, it must be episodic;
hence tectogenesis is episodic.
4. During the anorogenic intervals between lithosphere
collapses, the asthenosphere volume increases slowly as the
strictosphere radius decreases (Fig. 3.19). The increase in
asthenosphere volume is accompanied by decompression in the
asthenosphere.
5. Decompression is accompanied by rising temperature,
increased magma generation, and lowered viscosity in the
asthenosphere, which gradually weakens during the time intervals
between collapses.
6. Flow in the asthenosphere is predominantly eastward as a
consequence of the Earth's rotation (Newton's Third Law of
Motion; ..., 1977). Magma flow in the surge channels above the
asthenosphere also tends to be eastward, although local barriers
may divert flow in other directions for short distances.
Coriolis force also must exert an important influence on
asthenosphere and surge-channel flow, which by its nature is
Poiseuille flow. Therefore, the flow at the channel walls is
laminar and is accompanied by viscous, or backward drag. The
viscous drag produces the swaths of faults, fractures, and
fissures (streamlines) that are visible at the surface above all
active tectonic belts. These bands or swaths are examples of
Stoke's Law (one expression of Newton's Second Law of Motion).
7. During lithosphere collapse into the asthenosphere, the
continentward (hanging wall) sides of the lithosphere Benioff
zones override (obduct) the ocean floor (..., 1906-1911). The
entire lithosphere buckles, fractures, and founders. Enormous
compressive stresses are created in the lithosphere.
8. Both the lithosphere and the strictosphere fracture along
great circles at the depth of the strictosphere's upper surface,
as predicted by theory (..., 1959-1976) and demonstrated in the
laboratory (..., 1956). Only two partial great circle fracture
zones survive on the Earth today. These include the fairly
extensive, highly active Circum-Pacific great circle and the
almost defunct Tethys- Mediterranean great circle.
9. When the lithosphere collapses into the asthenosphere, the
asthenosphere- derived magma in the surge channels begins to
surge intensely. Wherever the volume of the magma in the
channels exceeds their volumetric capacity, and then compression
in the lithosphere exceeds the strength of the lithosphere that
directly overlies the surge channels, the surge-channel roofs
rupture along the cracks that comprise the
fault-fracture-fissure system generated in the channel by
Poiseuille flow before the rupture. Rupture is bivergent,
whether it forms continental rifts, foldbelts, strike-slip
zones, or midocean rifts. The foldbelts develop into kobergens,
some of them alpinotype and some them germanotype. The tectonic
style of a tectonic belt depends mainly on the thickness and
strength of the lithosphere overlying it (Fig. 3.19).
10. Tectogenesis generally affects an entire tectonic belt and,
in fact, may be worldwide; the worldwide early to late Eocene
tectogenesis is an example (M-b, 1992b) This indicates that the
lithosphere collapse that generates tectogenesis transmits
stresses everywhere in a give belt at the same time. Thus
Pascal's Law is at the core of tectogenesis. Sudden rupture and
deformation of surge channels may therefore be likened to what
happens when someone stamps a foot on a full tube of toothpaste.
The speed or rapidity of tectogenesis, then, is related to the
number of fractures participating in the event, as well as to
the thickness of lithosphere involved, the size of the surge
channel or surge-channel system, the volume and types of magma
involved, and related factors.
11. Once tectogenesis is completed, another geotectonic cycle
or subcycle sets in, commonly within the same tectogenic belt.
3.7 Pascal's Law---the Core of Tectogenesis
Pascal's Law (or theorem) states that pressure applied to a
confined liquid at any point is transmitted undiminished through
the fluid in all directions and acts upon every part of the
confining vessel at right angles to its interior surfaces and
equally upon equal areas. This law applies in part to all
fluids, but wholly to Newtonian fluids; it is the principle
behind every hydraulic machine, notably the hydraulic press. A
most important condition of Pascal's Law is that the pressure
(force per unit area) acts equally upon equal areas. This
condition lies at the very core of tectogenesis.
The Earth, according to our surge-tectonics hypothesis, is a
very large hydraulic press. Such a press consists of three
essential parts: a closed vessel, the liquid in the vessel, and
a ram or piston. The collapse of the lithosphere into the
asthenosphere is the activating ram or piston of tectogenesis.
The asthenosphere and its overlying lithospheric surge
channels---which are everywhere connected with the asthenosphere
by vertical conduits---are the vessels that enclose the fluid.
The fluid is magma generated in the asthenosphere. The magma
fills the lithosphere channels. When the piston (lithosphere
collapse) suddenly compresses the channels and the underlying
asthenosphere, the pressure is transmitted rapidly and
essentially simultaneously through the worldwide interconnected
surge-channel network; the surge channels burst and the
tectogenesis is in full swing. The compression everywhere of the
asthenosphere compensates for the fact that the basaltic magma
of the surge channels is non-Newtonian.
A possible objection to this simple picture of tectogenesis is
that the sudden application of pressure against the surge
channels would consolidate the magma in the channels, and
thereby prevent the bursting of the channel roof. This would be
true if the channels had no communication with the surface at
the onset of tectogenesis, but this is not the case. As
Meyerhoff et al. (1992b) noted, the channels are connected to
the surface by swaths of belt-parallel faults, fractures, and
fissures.
A second possible ofjection is that the magma in surge channels
is non-Newtonian; i.e., it is too viscous to transmit the added
stress to all of the interconnected parts of the surge-channel
system. This objection would be valid for a tectonic model in
which the added stress is applied only at a single point in the
system. In a contracting Earth, however, compression in the
lithosphere is omnipresent. Hence, the added stress is applied
everywhere along the interconnected lithosphere channels so that
the viscosity argument is invalid; the added stress is being
applied at an infinite number of points in the system. As shown
in Figure 3.19, the thickness of the lithosphere overlying each
channel is extremely important, because the thickness determines
the resulting tectonic style of the channel during
tectogenesis---rift valley, germanotype foldbelt, alpinotype
foldbelt, midocean rift, and so forth.
Although Pascal's Law applies to all tectonic settings, it is
especially important in midocean ridge systems. The law states
that pressure applied to a confined liquid acts equally on equal
areas of the walls of the confining vessel. The surge channels
beneath midocean ridges can be thousands of kilometers wide.
Hence, they are much larger than their continental-margin and
continental counterparts. Morevoer, the lithosphere above
midocean ridges is only 10 to 15 km thick, less than half of the
thickness found in a continent-margin/continental setting. This
means that the total force acting on the walls of a
midocean-ridge surge channel is vastly greater than in any other
setting. Thus, during tectogenesis, midocean ridges presumably
rupture throughout their lengths and across widths far greater
than those of continental surge channels, thereby producing
veritable magma floods on the ocean floors. If one keeps in mind
the fact that the most massive Phanerozoic continental flood
volcanism took place from Late Permian through Middle Jurassic
time (...), such magma flooding in the oceans during the same
time interval would account for the fact that the oldest basalts
thus far penetrated by deep sea dirlling beneath the abyssal
plains are Middle Jurassic. On the midocean ridges themselves,
however, basalts older than Middle Jurassic are common
(Meyerhoff et al., 1992a).
3.8 Evidence for the Existence of Surge Channels
3.8.1 SEISMIC-REFLECTION DATA
As noted above, reflection-seismic techniques (...) have shown
that the continental crust of the upper lithosphere is divisible
in a very general way into an upper moderately reflective zone
and a lower highly reflective zone (...). Closer scrutiny of the
newly-acquired data soon revealed the presence in the lower
crust of numerous cross-cutting and dipping events. When many of
these cross-cutting events were preceived to be parts of
lens-like bodies, various names sprang up: .... Strictly
nongenetic names include lenses, lenticles, lozenges, and pods
(...). Finlayson et al. (1989) found that the lenses have P-wave
velocities of 7.0-7.8 km/s, commonly with a low-velocity zone in
their middle. Thus we equate the lenses with the pods of
"anomalous lower crust" and "anomalous upper mantle" that we
discussed in a preceding section. Klemperer (1987) noted that
many of the lenses are belts of high heat flow. Hyndman and
Klemperer (1989) observed that the lenses generally have very
high electrical conductivity.
Meyerhoff et al. (1992b) discovered that there are two types of
undeformed reflective lenses, and that many of these lenses have
been severely tectonized. The first type of lens is transparent
in the middle (Fig. 3.29); the second type is reflective
throughout (Fig. 2.11). Tectonized lenses also may have
transparent interiors, or parts of interiors; many, however, are
reflective throughout (Fig. 3.21). Where transparent zones are
present (Fig. 3.20), bands of high heat flow, bands of
microearthquakes, belts of high conductivity, and bands of
faults, fractures, and fissures are present. Where a transparent
layer is not present, high heat flow and conductivity, however,
are commonly still present. Meyerhoff et al. (1992b) also found
that lenses with transparent interiors are younger than those
without transparent interiors; moreover, there is a complete
spectrum of lenses from those with wholly transparent interiors
to those without.
The best explanations of thes observations are that (1) the
lenses with transparent interiors are active surge channels with
a low-velocity zone sandwiched between two levels of 7.0 to 7.8
km/s material; (2) the lenses with reflective interiors are
former surge channels now cooled and consisting wholly of 7.0 to
7.8 km/s material; and (3) the tectonized lenses are either
active or former surge channels since converted into kobergens
by tectogenesis.
3.8.2 SEISMIC-REFRACTION DATA
After Revelle (1958) discovered the presence of a body of
7.0-7.8 km/s material on midocean ridges (the East Pacific
Rise), a similar body was discovered on the Mid- Atlantic Ridge,
and the general lens shape was reported for the first time
(...). Subsequently Talwani et al. (1965) combined gravity and
seismic data, and detailed the lens shape of the surge channel
across the entire Mid-Atlantic Ridge (Fig. 2.27). Fuchs and
Landismann (1966) found a similar but much smaller lens beneath
the Upper Rhine graben with a P-wave velocity of 7.6 km/s. Now
such a 7.0-7.8-km/s lens is known to underlie every well-sutdied
tectonic belt, regardless of tectonic origin. Figure 3.6 shows a
7.0-7.8-km/s lens under the northern Appalachians (...).
Meyerhoff et al. (1992a, 1992b) summarized the worldwide
evidence for the presence of such lenses under every type of
tectonic belt. These same authors showed that the lenses under
older tectonic belts contain no low-velocity zone, but that
lenses in younger tectonic belts contain low-velocity zones.
3.8.3 SEISMOTOMOGRAPHIC DATA
Seismotomographic data, wherever detialed studies have been
made, indicate that the lenses seen in seismic-refraction and
seismic-reflection studies form an interconnected, reticulate
network in the lithosphere. Although only one highly detailed
seismotomographic study has been made on a continental
scale---this in China, it leaves no room for doubt that the
7.0-7.8-km/s lenses with transparent interiors and the
seismotomographically detected low-velocity channels in the
lithosphere are one and the same (...). Figure 2.31 shows the
active surge channels at a depth of about 50 km in southwestern
China. Figure 3.9 is a seismotomographic cross section to a
depth of 300 km across the Yunnan surge channel shown in Figure
2.31. Figure 3.14 is a more detailed cross section (above a
depth of 65 km) of the central part of Figure 3.9. Velocity data
from refraction surveys have been added. The 7.6-7.9 km/s layer
below 50 km is the Yunnan surge channel shown in Figure 2.31;
the low-velocity layer between 22 and 44 km (5.4-6.0 km/s) is a
part of the Yunnan surge-channel complex (...). Using
seismotomographic techniques, it will be possible to map active
surge channels over the world with comparative ease. The reader
should note that Figure 3.22 shows a seismotomographic image of
the active kobergen of the Hengduqn Shan-Shaluli Shan that
overlies the Yunnan surge channel, a further demonstration of
the validity of our tectonic interpretation.
3.8.4 SURFACE-GEOLOGICAL DATA
Direct evidence for the existence of surge channels comes from
tectonic belts themselves, and from one type of magma flood
province. The latter include rift igneous rocks that crop out
nearly continuously for their full lengths. Examples include the
rhyodactic Sierra Madre Occidental-Sierra Madre del Sur
extrusive and intrusive belt of Mexico and Guatemala, some 2,400
km long; the 1,600-km-long Sierra Nevada-Baja California
batholith belt; the 4,000-km+ batholith and andesite belt of the
Andes south of the equator; the 4,000-km-long Okhotsk-Chukotka
silicic volcanic belt; the 5,800-km-long Wrangellia linear
basaltic province extending from eastern Alaska to Oregon, which
erupted in less than 5 Ma; and many other similar continental
magma belts. The ocean basins are equally replete with them,
ranging from the 60,000-km-long midocean ridge system through
the 5,800-km-long Hawaiian- Emperor island and seamount chain to
many similar belts of shorter lengths. Geochemical studies also
show that most of these belts are interconnected. Another linear
flood-basalt belt, which has been studied only relatively
recently, is the subsurface Mid-Continent province that extends
2,400 km from Kansas through the Great Lakes to Ohio (Figs.
3.23, 3.24).
3.8.5 OTHER DATA
Other data mentioned in the preceding sections corroborate the
interconnection of active surge channels. One of these is the
coincidence of the 7.0-7.8-km/s lenses of the active surge
channels (Figs. 2.9, 2.31, 3.6, 3.9, 3.14, 3.20) with the belts
of high heat flow (Fig. 2.26) and with belts of microseismicity.
Both the presence of high heat flow and microseismicity indicate
that magma is moving within active surge channels.
However, an even more dramatic example is the June 28, 1992,
Landers, California, earthquake-related activity shown on Figure
3.25. This figure shows that the 7.5- magnitude earthquake was
strong enough to affect areas up to 1,250 km from the epicenter
(...) and provides an exampole of Pascal's Law in action. Given
the importance of Pascal's Law in surge-channel systems, the
fact should be noted that the viscosity of the magma in the
surge channels affected by the Landers event is sufficiently low
that, when the stress was applied at a single hypocentral point
(Landers), the effects could still be transmitted for 1,250 km!
3.9 Geometry of Surge Channels
3.9.1 SURGE-CHANNEL CROSS SECTION
In cross sections, surge channels have a variety of shapes, and
are of many sizes and depths within the lithosphere (M-a). Two
models proposed for sill-and-laccolith complexes may be ideal
representation of surge-channel complexes because, despite the
differences in scale, the same physical principles apply. Corry
(1988) published the "Christmas Tree" model shown in Figure 2.8;
Bridgwater et al. (1974) published the more complex model shown
in Figure 3.26. Either of these could be cross sections of surge
channels. Both are multitiered with one or more magma chambers
above the main chamber. Both are formed on the basis of Newton's
Law of Gravity or, more specifically, the Peach-Kohler climb
force (...).
Seismotomographic images are available from hundreds of surge
channels in different parts of the world. They are complex
structures as Figures 3.9 and 3.27 demonstrate. Figure 3.27 is a
tracing of a seismotomographic image of the multitiered Yunnan
surge channel (Fig. 2.31 ...). It is also quite large for a
continental channel.
Figure 3.23 shows a partly deformed ("kobergenized") inactive
Proterozoic surge channel that underlies the Midcontinent Rift
of central North America (...). A surge-tectonic structural
interpretation is shown on Figure 3.24. The figures show that
the channel before deformation consisted of a large lower
chamber at the Moho-, and a smaller, higher chamber at about 20
km. Kobergenic structure developed during tectogenesis (Fig.
3.24). The P-wave velocity in the channel is 7.0 to 7.2 km/s,
and the top of the channel extends to within 10 km of the
surface. The Midcontinent Rift was a major flood-basalt province
of 1,100 Ma.
3.9.2 SURGE-CHANNEL SURFACE EXPRESSION
Study of Figures 2.8, 2.9, 2.11, 2.31, 3.6, 3.9, 3.13, 3.14,
3.20, 3.23 and 3.24 might lead one to believe that surge
channels are everywhere fairly simple structures expressed at
the surface by a single belt of earthquake foci, high heat flow,
bands of faults-fractures-fissures (streamlines), and related
phenomena which, during tectogenesis, deform into a single
kobergen. Although this simple picture is true of many
kobergens, it is not true of all. Study of Figures 3.26 and 3.27
suggests that, during tectogenesis of the surge-channel
complexes shown on these figures, two or more parallel kobergens
may exist at the surface. Such a complex surface expression is
in fact quite common. Well-documented examples are found in the
Western Cordillera of North America, the Mediterranean-Tethys
orogenic belt (including the Qinghai-Tibet Plateau), and the
Andes, inter alia. Within the Western Cordillera, the
Qinghai-Tibet Plateau, and the Andes, we have found four or more
parallel kobergens side by side at the surface as documented and
illustrated by Meyerhoff et al. (1992b).
3.9.3 ROLE OF THE MOHOROVIC DISCONTINUITY
The principal forces acting on the lithosphere are compression,
rotation, and gravity. We have discussed the first two briefly
and now endeavor to describe gravity's role in lithosphere
development.
Gravity controls the depths of all magma chambers which, because
the Earth cools at the asthenosphere level, have to be in the
upper asthenosphere and lithosphere. The mantle above the
asthenosphere is believed to be peridotite from which basalt has
already been extracted (Ringwood, 1979). It has a P-wave
velocity just below the Moho- greater than 7.9-8.0 km/s. The
oceaenic crust above the Moho- has been assumed until very
recently to have a P-wave velocity of 6.8 km/s and less; the
continental crust above the discontinuity has been assumed to
have a P-wave velocity of about 6.5 km/s and less.
The liquid generated by the removal of basalt from the mantle
cannot be ordinary basalt, but may be akin to tholeiitic picrite
(Green et al, 1979). In any case, it is less dense than the
peridotite above the asthenosphere and more dense than the
basalt found in deep-sea drillholes. Therefore, while in the
asthenosphere, it is gravitationally unstable. Consequently
Newton's Law of Gravity works to bring the magma upward to a
level where it is gravitationally stable. The mechanism for this
is the Peach-Kohler climb force (Weertman and Weertman, 1964;
Weertman, 1971). This force compels the magma to rise through
available conduits to its level of neutral buoyancy, i.e., the
level where the lithosphere density and the magma density are
the same (...). At this level the magma can only move
horizontally.
As we have stated repeatedly, the P-wave velocity of the walls
of active surge channels is in the 7.0-7.8-km/s range; that of
inactive channels is 7.0-7.8 km/s throughout. In the crustal
models generally accepted until the late 1980s, there was no
layer having a velocity in the 7.0-7.8-km/s range. In recent
years, however, a lower crustal layer has been reported in large
areas of North America and elsewhere with velocities in the
7.0-7.8-km/s range (...). Mooney and Meissner (1992) in fact
state that the layer is omnipresent, is ca. 3.5 km thick, and is
a transition zone between the mantle and the crust.
Thus, when the postulated tholeiitic picrite magma reachs the
Moho- (i.e., the zone between 8.0-km/s mantle below and 6.6-km/s
above), it has reached its level of neutral buoyancy and spreads
laterally. Under the proper conditions---abundant magma supply
and favorable crustal structure---a surge channel can form. We
suggest the possibility that the entire 7.0-7.8-km/s layer may
have formed in this way. In support of this suggestion, we note
that the main channel of every surge channel studied, from the
Archean to the Cenozoic, is located precisely at the surface of
the Moho-. This indicates that the discontinuity is very
ancient, perhaps as old as the Earth itself. This fact and the
great difference in P-wave velicities above and below the Moho-
surface suggest in turn that the discontinuity originated during
the initial cooling of the Earth. Hence, Mooney and Meissner's
(1992) "transition zone" was the level of neutral buoyancy at
the time the 7.0-7.8-km/s material was emplaced.
The suboceanic Moho- is some 10-15 km below sea level; its
subcontinental counterpart is deeper, at about 40 km below sea
level. Hence the neutral buoyancy levels of the two regimes are
separated vertically by 25 to 30 km. This fact probably is
closely related to the fact that surge channels underlie all
continental shelf-slope breaks. Thus these breaks are transition
zones between continental and oceanic regimes. MacDonald (1963,
1964) long ago stated that this transition zone must be the site
of vertical faulting. Writing of the differences in the vertical
distribution of radioactive heat sources under continents and
ocean basins, MacDonald (1963, p.589) stated that these
differences ensure "...that the boundaries between them
[continents and ocean basine] become discontinuities in
subsurface vertical motion. The boundary regions are thus narrow
regions where faults are formed and volcanic activity is
concentrated." In summary, the ocean- continent boundary is a
weakness zone due to differing crustal thicknesses and
compositions on either side of that boundary, and the
accompanying changes in temperature regimes, specific gravities,
and velocities. This weakness zone is therefore a focus for
faulting and ascending magma.
It is a fact that volcanism does take place along continental
mragins, not just in the "active" margins, but in the "passive"
ones as well, as for example along the eastern seaboard of North
America (...) and the Atlantic margin of Africa (...). In fact,
a recent study of North America's eastern seaboard by Hollbrook
and Kelemen (1993) shows the presence beneath the outer shelf of
a linear pod of volcanic material at the Moho-. The pod has a
P-wave velocity of 7.2-7.5 km/s and is at least 3,300 km long.
It should be added that the above explanation does not mean that
all surge channels form at ocean-continent boundaries, rather,
surge channels are found in every tectonic setting, one of which
is the ocean-continent boundary.
The formation of the Christmas-tree-like structures (Figs. 2.8,
3.26) at the Moho- is simply an extension of the larger scale
process of magma transfer from the asthenosphere to the
discontinuity. Once surge channels are established at the
discontinuity, the same processes take over that brought the
magma to the discontinuity in the first place, specifically,
magma differentiation in the channels and the Peach-Kohler climb
force (...). After lighter magmas have formed by differentiation
and related processes, they rise to their own neutral buoyancy
levels, forming channels above the main surge channel (Figs.
3.23, 3.27).
#Post#: 179--------------------------------------------------
Re: SURGE TECTONICS
By: Admin Date: March 22, 2017, 5:17 pm
---------------------------------------------------------
SURGE TECTONICS
Chapter 6 Magma Floods, Flood Basalts, and Surge Tectonics
6.1 Introduction
... The old term, "plateau basalt", had functioned with
comparative efficiency and illustration, but Tyrrell's "flood
basalts" gave an immediate and striking image of basalts poured
out in broad areal effusion. "Plateau basalt" has continued in
the literature to a considerable degree, but "flood basalts" has
become by far the preferred term in mafic volcanology.
The origin of flood basalts has sparked controversy since they
were first identified in the last century [the 1800s]. The
purpose of this chapter is to re- examine the critical data,
including descriptions of many flood-basalt provinces, to
introduce the new term "magma floods" for flood basalts--a term
that we consider more appropriate and encompassing--and to
propose an explanation of our own in terms of surge tectonics.
6.1.1 SIGNIFICANCE OF FLOOD BASALTS
Some 63% of the ocean basins are covered flood basalts. At least
5% of the continents are likewise covered with flood basalts.
Thus 68%---a minimum figure--- of the Earth's surface is covered
with these basaltic rocks. Flood basalts, then, are not the
oddities that many suppose them to be. In spite of this, they
receive little attention among the scientific community. We
examined nearly twenty geologic textbooks and reference works
published since 1969, and found only two with more than three
paragraphs on flood basalts. ... Such treatment---or lack of
treatment---seems unusual, out of place, if one considers that
flood basalts are the most important rock exposed at the Earth's
surface (..., 1986...).
Engel et al. (1965) long ago demonstrated that deep ocean-floor
tholeiitic basalts are the oceanic equivalent of the continental
flood basalts. The Basalt Volcanism Study Project (1981)
differentiated between the continental flood basalts and
"ocean-floor basalts," while recognizing that the principal
differences were the abundance of minor and rare-earth elements.
Press and Siever (1974...) recognized the fact that the
ocean-floor basalts and continental flood basalts are nearly the
same, and that their differences are explained readily by
contamination in the continental crustal setting. Yoder (1988),
one of the world's authorities on basaltic magmas, stated
essentially the same thing.
In fact, as increasing numbers of basalts are analyzed, the
difference between the oceanic and continental floods blurs even
further. For example, ... (1991) found groups of samples from
the Siberian Traps that are essentially indistinguishable from
midocean ridge basalts. Fitton et al. (1991) found numerous
Great Basin basalts that are chemically indistinguishable from
midocean ridge basalts, and Sawlan (1991) observed a complete
chemical continuum from midocean ridge basalts to the flood
basalts in the Baja California, Gulf of California, and Mexican
basin- and-range province.
These extremely close--in places identical--genetic
relationships are well established. In a subsequent section of
this chapter, we shall present geochemical data to support this
statement.
6.1.2 CLASSIFICATION
Continental flood-basalt provinces are geometrically of two
types. The first is broadly ovate, or even round, with the
maximum diameter ranging from about 500 km (Columbia River
Basalt) to more than 2,500 km (Siberian Traps). The second is
distinctly linear, with a width of 100 to 200 km and lengths up
to and even exceeding 3,000 km.
Oceanic flood-basalt provinces at first appearance are
difficult to classify. However, as more ... data ... become
available, it is possible to distinguish the same two types of
geometries there as well. Ovate to semi-ovate shapes
characterize many oceanic submarine plateaus. The maximum
diameters of these plateaus, excluding the Kerguelen Plateau,
are in the order of 1,200 to 1,600 km.
Linear ridges are of two types. The larger is the
midocean-ridge system with widths between 1,200 and 3,600 km;
the smaller is exemplified by the various linear island and
seamount chains with widths of 100-200 km and lengths of
thousands of kilometers.
Ovate flood-basalt provinces include [over 13 places]....
Linear flood-basalt provinces include [over 14 places]....
Tectonism and metamorphism can severely disrupt any
flood-basalt province after its formation. For example, ... the
Antrim Plateau Volcanics of northern Australia ... parts ...
have been removed by erosion. ... Similarly, only very
scattered, strongly flooded, and metamorphosed remains of the
Willouran Mafic rocks are preserved in ... South Australia, but
their distribution shows that [it] is a linear flood-basalt
province.
6.1.3 THE PETROGRAPHIC CHARACTER OF FLOOD-BASALT PROVINCES
To judge from the geological literature, many earth scientists
assume that flood- basalt provinces are composed mainly of
basalt and little else. This characterization is justified for
some provinces but it is incorrect for many more. For example,
the Columbia River flood-basalt province consists nearly 100% of
tholeiitic basalt with small volumes of basaltic andesite and
minuscule amounts of dacite and rhyolite (..., 1979-1988). In
contrast the Snake River flood-basalt province on the
southeastern side of the Columbia River province consists more
than 50% of rhyolite and siliceous (rhyolitic) ignimbrites (...,
1989). A second example is the Lebombo monocline region of the
Karroo flood-basalt province in southern Africa. Here are thick
sequences of rhyolite (and perhaps ignimbrite) which, for most
of the length of the monocline--at least 600 km--comprise 30 to
55% of the volcanic section (..., 1983). Yet another example is
the Keweenawan (Midcontinent) flood-basalt province where every
region has large volumes of rhyolite associated with the basalt.
Our first point is that many flood-basalt provinces are bimodal,
and the volume of associated silicic extrusive (or intrusive)
rocks can be substantial.
A second common assumption is that tholeiitic basalt and
related tholeiitic rocks constitute the principal mafic rock
types. Here again, the field evidence proves the assumption is
incorrect. It is true that the Columbia River flood-basalt
province consists 99% of tholeiitic mafic rocks. Yet the huge,
3,000-km-long Arabian flood-basalt province consists mainly of
alkalic basalt. In fact, Camp and Roobol (1981) and Camp et al.
(1991) refer to this example as the "Arabian continental alkalic
basalt province." Thus our second point is that many types of
basalts may be present in flood-basalt provinces. Tholeiitic
basalt is just one of those types.
6.2 Descriptions of Selected Continental Flood Basalt Provinces
We present here some brief geological descriptions of
representative ovate and linear continental flood-basalt
provinces in order of decreasing age. Many additional
continental provinces could have been added to this list, but we
believe that those selected adequately illustrate the points we
wish to make. Undoubtedly, some earth scientists will not agree
that all of our examples are, in fact, flood- basalt provinces.
Therefore, we include data on areal extent, volume, thickness,
composition, and age which led us to conclude that we were
dealing with flood- basalt provinces. Data concerning the ages,
areal extent and volume of these provinces and others are
summarized on Table 6.1.
6.3 The Use of Geochemistry in Identifying Flood Basalts
6.3.1 INTRODUCTION
Geochemical/petrochemical studies of igneous rocks for many
decades were restricted to (1) studies of the bulk chemistry
(major compounds only) of each rock type, and (2) deviations
from the "norm" determined for each rock type. High- pressure
... and high-temperature studies were conducted in the search
for the chemical phases and eutectics of rock melts. Such
studies were invaluable in determining the origins of various
rock types, and led to many classical papers, especially the
Yoder and Tilley (1962) and Yoder (1976) treatises on the origin
and generation of basalt.
With the advent of plate tectonics, petrochemistry was used
increasingly as a supplement to traditional methods of
identifying tectonic environments. The assumption was made that
each tectonic environment had its own petrochemical "signature."
When major-element studies failed to bear out this assumption,
however, increasing attention was given to minor (trace) and
rare earth elements. Regrettably, nearly all large-scale
petrochemical research concentrated on the basalts (e.g., the
NASA-sponsored Basaltic Volcanism Study Project published in
1981), and other rock types have failed to receive anything like
the attention that the basalts received. As an inevitable
consequence, many conclusions were made on the basis of basalt
geochemistries alone. Our points are: (1) that a great deal of
research---many decades, in fact---will be necessary before
sound conclusions regarding the chemical "signature" of tectonic
environments will or can be soundly based; and (1) even though
the more silicic magma types are in very large part aggregates
of crustal compounds and processes, they too have important
scientific "messages" to impart. It is too early to reach final
conclusions based only on basalt data.
The results of minor and rare-earth element studies, however,
have been helpful, for they document in part the history of each
sample with the use of spidergrams (Fig. 6.16). They also
discriminate easily between midocean-ridge basalts and other
basalts, although this already was possible from major element
data alone. However, as we document below, the ability of
spidergrams to discriminate among most tectonic settings is
doubtful without much additional information, partly from
isotope data and, in the long run, with the aid of actual field
data.
An important step that must be taken now is to standardize the
order in which the trace and rare-earth elements appear on a
spidergram (Fig. 6.16). Second, there is no consistency about
which elements are included or excluded (Fig. 6.16), and this
problem also must be resolved. Too often elements important to
an interpretation are omitted on spidergrams. Finally, there is
no consistency about which material is used for "normalizing"
element plots. Currently some are chondrite-normalized; some are
normalized against an idealized midocean-ridge basalt
composition; and many are normalized against the composition of
an hypothetical primordial mantle, a practice which, as Thompson
et al. (1983) have noted, introduced unnecessary subjectivity
into interpretations.
6.3.2 BASALT MAGMAS
... [Skipping 3 paragraphs]
It is important to be aware that the concentration of
incompatible trace elements* [those most likely to be
transported by melts and other fluids passing through the mantle
and therefore most likely to preserve evidence of mantle
enrichment and depletion processes in their relative abundances]
changes greatly in this basaltic liquid, depending on their
relative partition coefficients, initial concentrations, and
dilution rates. In the midocean-ridge basalts, the volume of
incompatible minor elements is very small, a fact that suggests
that the parental material has already undergone some partial
melting and loss of liquid, but still retains parts of all major
melt phases (..., 1988).
Several processes involved in the emplacement of magmas in the
crust complicate the above picture. The composition of surface
samples from rocks that were molten and under high pressure is
not necessarily that of the parental liquid at depth. This is
true because (1), as the liquid rises, internal reaction
relations take place that successively eliminate olivine and
orthopyroxene (..., 1967-1988). Hence the composition of the
basalt may be altered considerably during its rise from ca. 130
km; (2) of heat loss; (3) the change in pressure further changes
the liquid composition; and (4) the rise of the melt produces a
change in the stable phases within the liquid.
The reasons for the differences between continental flood
basalts and midocean-ridge basalts are related in part to the
above factors, but differences in the thickness of the
lithosphere clearly must exert an important influence as well
(..., 1988). The penetration of an old, thicker, continental
massif by basalt melt is clearly more difficult than that of the
much thinner oceanic lithosphere, although the rising magma
rises in the same way under both lithospheres, following the
Peach-Kohler climb force (Newton's Law of Gravity; ...,
1964-1989) and stops when the level of neutral buoyancy has been
reached (..., 1989). The longer---or slower---the rise beneath
the continental crust, the greater the fractionation, as
reflected in the more iron-rich character of the continental
lavas (..., 1981- 1988). Deep-seated magma segregation beneath
the continents provides for more alkalic parental magmas, a
greater range of enrichments, and a greater variation that
depends on repose time, interactions with the continental crust,
and the rates of ascent. The bimodal character of so many
continental flood basalts implies the presence for periods of
time of multiple magma chambers.
6.3.3 STUDIES OF MINOR AND RARE EARTH ELEMENTS
When studies of major elements and compounds revealed
difficulties in discerning chemical signatures peculiar to each
tectonic environment, research began to focus on studies of
minor (trace) elements, rare-earth elements, and chemical
isotopes. Although a high degree of success has been claimed for
such studies, the facts tell quite a different story. Indeed, it
is a poor reflection on the state of current geoscientific
resaerch that the eagerness of some researchers to satisfy
preconceived hypotheses and models has led some into publishing
material that is scientifically sound [unsound?]. Minor (trace)
element, rare-earth element and chemical isotopes studies are
summarized for the following environments.
Midocean-Ridge Basalts (Ocean-Floor Volcanism) ...
Ocean-Island Basalts (Oceanic Intraplate Volcanism) ...
Continental Flood Basalts (Continental Intraplate Volcanism)
...
Volcanic Arc Basalts ("Subduction" Basalts) ...
Island Arc Basalts ...
Continental Margin Volcanic Arcs ...
6.4 Geochemical Comparisons among Basalts Erupted in Different
Tectonic Settings
... 6.4.7 CONCLUSION
Our examination of the literature on basalt rocks has led us to
conclude that geochemistry is useful in distinguishing between
midocean-ridge basalts and other basalts. This is true of bulk
geochemistry, major-element geochemistry, and minor (trace)
element and rare-earth element tectonic settings other than that
of the midocean ridge. Exceptions to this statement do exist,
but only in areas where the investigator has exceptional
knowledge of the field relations among the various igneous units
that he/she is investigating. Geochemical techniques are useful,
however, in deciphering the chemical histories of the various
igneous units, subject once again to the proviso that field
relations among the various units being studied are well
understood.
6.5 Duration of Individual Basalt Floods
6.5.1 INTRODUCTION
The length of time during which a particular basalt flooding
episode lasts differs greatly among the various flood-basalt
provinces. Some, such as the Siberian flood-basalt province,
have been active more than 200 Ma. Others---the Wrangellian
province, for example---probably completed their flood activity
in 5 Ma or less. Even in flood-basalt provinces of long
duration, the largest volume of basalts may have been extruded
in one, or perhaps two or three relatively short bursts. A close
relationship seems to exist between times of tectogenesis and
times of major basalt flooding.
6.5.2 FLOOD-BASALT PROVINCES OF LONG DURATION
Radiometric and/or paleontologic constraints are available for
only a few flood- basalt provinces. Therefore, we mention only
places where good dating is available. The radiometric data are
summarized on Table 6.1.
[2.5-283 Ma are indicated.]
6.5.4 CONCLUSION
We have discussed several flood-basalt provinces which were
active during periods that ranged from more than 210 Ma (long
duration) to less than 12 Ma (short duration). We have found no
evidence to suggest that there are any time controls or any
rules of thumb that guide the length of time during which a
flood-basalt province may remain active. Nor is there a
relationship between type of flood- basalt province may remain
active. Nor is there a relationship between type of flood-basalt
province and the duration of its extrusion. For example, the
Columbia River province is ovate while the Wrangellian province
is linear; yet the two endured for approximately the same
lengths of time. Reports that the Deccan and Siberian
flood-basalt provinces were in fact of very short duration are
based on a lack of information. In fact, information adequate to
determine the "lifespans" of most flood-basalt provinces,
including Siberian province, is not yet available.
6.6 Flood-Basalt Provinces and Frequency in Geologic Time
As we observed near the beginning of this chapter, the commonly
used textbooks of physical geology, structural geology, and
geotectonics rarely list more than 10 to 20 flood-basalt
provinces. However, the magnificent review of basalts by the
participants in the Basalt Volcanism Study Project (1981)
mentions or figures not less than 56 flood-basalt provinces and
45 additional provinces of dike swarms which the project
participants thought might have fed flood-basalt provinces that
have since been removed by erosion.
The participants in the Basalt Volcanism Study Project (1981)
concurred on a large number of phenomena that characterize
flood-type volcanism. However, they showed considerable
confusion, ambivalence, and lack of agreement on which, and what
type of, provinces should or should not be described as
flood-basalt volcanism. This confusion and ambivalence manifest
themselves with respect to the differences between flood-basalt
provinces and continental rift-related provinces. Additionally,
they used interchangeably the terms "flood basalt," "plateau
basalt," "continental rift volcanism," and "hot-spot volcanism."
We summarize here briefly their overall remarks on the ages of
flood-basalt activity. They wrote that (1) most flood provinces
are less than 200 Ma; (10 no major flood-basalt activity took
place in the interval 1,100-200 Ma (yet ... they list eight
provinces within this time span, two of which are huge---the
Siberian Flood-Basalt Province and the Emeishan Flood-Basalt
Province); (3) reasonably well-preserved remnants of flood
provinces are known from the time interval 2,150- 1,100 Ma, and
(4) a few poorly preserved remnants are present in the
geological record to 3,760 Ma (..., 1981, ...). Yet, on page 41,
the same authors state that flood-basalt provinces older than
1,200 Ma are unknown.
The participants ... have for the first time, to the best of
our knowledge, provided solid evidence that flood-basalt
volcanism is a phenomenon that has persisted since the beginning
of--or since very early in---the Earth's history. However, we
have not seen any convincing evidence to support the claim by
Rampino and Stothers (1988), and a similar claim by White and
McKenzie (1989), that flood- basalt volcanism is periodic, with
large outpourings every 32 to 30 Ma. We suspect, but cannot
prove, that flood volcanism is triggered by tectogenic
(orogenic) pulses that are episodic. In our opinion, the
available evidence all but demonstrates an endogenic origin.
Which of the various possible endogenic causes is the correct
one must await the careful sampling and dating of thousands of
more carefully located igneous-rock samples in every major
flood-basalt province.
Yoder (1988, ...) wrote that "Great basaltic 'floods' have
appeared on the continents throughout geologic time (Table 1),"
but showed on his Table 1 none older than 1,200+/- 50 Ma. He
also ... made it clear that he regards midocean-ridge and other
oceanic basalts as flood basalts, as have a number of earlier
workers (..., 1974). We concur absolutely with their
interpretation. We also concur with the participants of the
Basalt Volcanism Study Project (1981) that evidence of the
existence of flood provinces extends back in time to at least
3,760 Ma, and very likely to the Earth's earliest (but nowhere
preserved) history. Interestingly, most of what Press and Siever
(1974), Yoder (1988), and we concur in what was anticipated by
the pioneer work of Engel et al. (1965).
6.7 Non-Basalt Flood Volcanism in Flood-Basalt Provinces
The bimodal nature of many flood-basalt provinces has been known
and stressed for many years (..., 1981). Time seems not to be a
major factor (the idea being that, the longer an underlying
magma chamber is present, the more the magma will interact with
the continental crust above it). The most important factor may
be the crustal stress state.
Estimates of the volume of non-basaltic rocks in a given
flood-basalt province are difficult to find. Accordingly in
Table 6.2 we have left many blank spaces and the percentages
that we have supplied are poorly documented except in local
areas.
... [Skipping 6 paragraphs]
We believe that the evidence from these examples demonstrates
convincingly that there is a complete gradation from all-basalt
and basaltic andesite flood provinces to bimodal provinces
containing mainly rhyolite and ignimbrite. Hence, there are
basalt floods and rhyolite floods.
... [Skipping most of 1 paragraph] The volumetric predominance
of these ash-flow tuffs has led to recognition of the [Sierra
Madre Occidental] as the world's largest rhyolite-dominated
volcanic province" (Fig. 6.28).
... [Skipping one paragraph]
Thus, from 38 Ma until 17 Ma, a truly bimodal column of
extrusive rocks accumulated in northern Mexico and adjoining
parts of the United States, with rhyolite at one end, basaltic
andesite at the other, and very little rock of intermediate
compositions. ... [Skipping remainder of paragraph]
We believe that these basalts of the "southern cordilleran
basaltic andesite" suite are flood basalts. And if they are
flood basalts, then we have demonstrated that the same mechanism
that leads to continental and oceanic basalt outpourings also
produces the "orogenic andesite suite".
The Okhotsk-Chukotka Volcanic Belt, a linear belt of Cretaceous
volcanics, is similar to the Sierra Madre Occidental. It extends
3,000 km from the mouth of Uda Bay (northwestern Sea of Okhotsk)
to the Bering Sea almost at St. Lawrence Island. It seems to
have every type of volcanic from andesitic through rhyolite.
Basalts are scarce. Soviet geologists either ignore it or say
that it is the remnant of a volcanic arc.
6.8 Flood Basalts or Magma Floods?
Although we advocate the continued use of the term "flood
basalt," it is clear that another term is needed to describe
floods of andesite, dacite, and rhyolite. For future studies, we
suggest the all-encompassing term magma floods. In this way, we
can include all of the various lava types, dikes, necks, and
sills. It is a term that even embraces situations such as the
Ferrar Dolerite of Antarctica and the network of sills and dikes
of the Amazon basin.
6.9 Surge-Tectonics Origin of Magma Floods
In the preceding pages we have referred to the presence of
several flood-basalt provinces around the world, and have shown
that some flood provinces include large volumes of silicic
rocks, usually rhyolite and/or dacite. We have also shown by the
northern Mexican example that flood basalts can interfinger with
the andesite orogenic suite. In addition, we have presented
evidence that spidergrams are not more effective at identifying
the tectonic setting than bulk chemistry. The available evidence
has led us to the conclusion that the same mechanism causes
volcanism in the midocean ridges, linear island and seamount
chains, oceanic plateaus, island arcs, and continental
interiors. We next attempt an explanation of our conclusion.
Many attempts have been made to explain flood volcanism in the
framework of the plate-tectonics hypothesis. The two principal
explanations involve (1) hot spots, or mantle plumes and (2) an
extraterrestrial cause (e.g., an asteroid impact).
Extraterrestrial causes have been proposed by Alt et al.
(1988), who applied this hypothesis to the Columbia River
flood-basalt province. A major problem with this concept is that
it does not explain linear flood-basalt provinces such as the
Keweenawan (Mid-Continent) rift and Wrangellia. Furthermore,
Mitchell and Widdowson (1991) pointed out that impact and shock
phenomena should be present in the area surrounding the Columbia
River province if it resulted from extraterrestrial action, but
they are entirley absent.
Mantle diapirism or asthenosphere upwelling constitutes the
hot-spot or mantle- plume hypothesis (..., 1971) used widely in
tectonic models today. Recent literature on mantle plumes
include works by ... (1988-1991). Hot spots are often portrayed
as diapiric bodies that are essentially cylindrical, mushrooming
plumes. While this might account for isolated volcanoes, it does
not account for the massive ovate and linear flood basalt
provinces found in many parts of the world.
Mantle upwelling also has been invoked by many writers to
explain the presence of long, linear continental rifts (...,
1983), which are, for the most part, similar to one another. ...
[Skipping remainder of paragraph listing widths and lengths of
numerous linear rifts etc]
As we noted in Chapters 3 and 4, Mooney et al. (1983) observed
that all active rifts studied by them have an anomalous lower
crust with P-wave velocities in the 7.0 to 7.7 km/s range (Fig.
6.36). [Others] obtained the identical result.... Fuchs (1974)
believed that this pod of anomalous lower crustal material
houses the mechanism that causes rifting. It is interesting to
note that all midocean ridges have a pod of 7.0-7.7 km/s as well
(..., 1959-1965). (Furthermore, each island arc and foldbelt
also has a pod of 7.0-7.7 km/s material that pinches out from
the center of the arc or foldbelt (..., 1987-1989 ... for the
Japan arc ... [and] for the Appalachians.)
Figure 3.6 is a cross section across the Baykal rift, from
Krylov et al. (1979) and Sychev (1985). Years of refraction work
have shown the Lake Baykal is underlain at about 32 km by a pod
that is connected to the deeper asthenosphere. The shallow pod
contains a low-velocity zone that presumably is a partial melt.
The pod extends the full length of the rift. It is, in short, a
channel containing partly molten magma and an excellent example
of one of our surge channels. Were it to burst, we believe that
it would produce another linear flood-basalt province.
According to our surge tectonic hypothesis, magma in surge
channels moves both vertically and horizontally. When two surge
channels come in contact, their magmas join together. If they
are oriented at an appreciable angle to one another, we believe
that the result is a "collision". These5 "collisions" are
responsible for the eruption of round or ovate flood-basalt
provinces worldwide.
CHAPTER 7
CONCLUSIONS
We have proposed a new hypothesis of global tectonics in this
book, one that is different and will be considered unorthodox by
many scientists and non-scientists alike. However, we believe
that current tectonic hypotheses cannot adequately explain the
increasing volume of data being collected by both old and new
technologies. We believe that the hypothesis of surge tectonics
does explain these data sets, in a way that is simple and more
accurate.
The major points of the surge-tectonics hypothesis can be
summarized as follows:
1. All linear to curvilinear mesoscopic and megascopic
structures and landforms observed on Earth (and similar features
seen on Mars, Venus, and the moons of Jupiter, Saturn and
Uranus), and all magmatic phenomena are generated, directly or
indirectly, by surge channels. The surge channel is the common
denominator of geology, geophysics, and geochemistry.
2. Surge channels formed and continue to form an interconnected
worldwide network in the lithosphere. They contain fluid to
semifluid magma, or mush, differentiated from the Earth's
asthenosphere by the cooling of the Earth. All newly
differentiated magma in the asthenosphere must rise into the
lithosphere. The newly formed magma has a lower density and
therefore, is gravitationally unstable in the asthenosphere. It
rises in response to the Peach-Kohler climb force to its level
of neutral buoyancy (that is, to form a surge channel).
3. Lateral movements in the Earth's upper layers are a response
to the Earth's rotation. Differential lag between the more rigid
lithosphere above and the (more) fluid asthenosphere below
causes the fluid, or mushy, materials to move relatively
eastward.
4. Surge channels are alternately filled and emptied. A
complete cycle of filling and emptying is a geotectonic cycle.
The geotectonic cycle takes place along this sequence of events:
a. Contraction of the strictosphere is always underway, because
the Earth is cooling;
b. The overlying lithosphere, which is already cool, does not
contract, but adjusts its basal circumference to the upper
surface of the shrinking strictosphere by large-scale thrusting
along lithosphere Benioff zones and normal-type faulting along
the strictosphere Benioff zones.
c. Thrusting of the lithosphere is not a continuous process,
but occurs when the lithosphere's underlying dynamic support
fails. When the weight of the lithosphere overcomes combined
resistance of the asthenosphere and Benioff zone friction,
lithosphere collapse begins in a episodic fashion. Hence,
tectogenesis is episodic.
d. During anorogenic intervals between lithosphere collapses,
the asthenosphere volume increases slowly as the strictosphere
radius decreases and decompression of the asthenosphere begins.
e. Decompression is accompanied by rising temperature,
increased magma generation, and lowered viscosity in the
asthenosphere, which gradually weakens during the time intervals
between collapses.
f. During lithosphere collapse into the asthenosphere, the
continentward (hanging wall) sides of the lithosphere Benioff
zones override (obduct) the ocean floor. The entire lithosphere
buckles, fractures, and founders. Enormous compressive stresses
are created in the lithosphere.
g. When the lithosphere collapses into the asthenosphere, the
asthenosphere- derived magma in the surge channels begins to
surge intensely. Where volume of magma in the channels exceeds
volumetric capacity, and when compression in the lithosphere
exceeds the strength of the lithosphere that directly overlies
the surge channels, the surge-channel roofs rupture along the
cracks that comprise the fault-fracture-fissure system generated
before the rupture. Rupture is bivergent and forms continental
rifts, foldbelts, strike-slip zones, and midocean rifts. We call
such bilaterally deformed belts kobergens.
h. Once tectogenesis is completed, another geotectonic cycle or
subcycle sets in, commonly within the same belt.
5. Movement in the surge channel during the taphrogenic phase
of the geotectonic cycle is parallel with the channel. It is
also very slow, not exceeding a few centimeters per year. Flow
at the surge-channel walls is laminar as evidenced by the
channel-parallel faults, fractures, and fissures observed at the
Earth's surface (Stoke's Law). Such flow also produced the more
or less regular segmentation observed in tectonic belts.
6. Tectogenesis has many styles. Each reflects the rigidity and
thickness of the overlying lithosphere. In opcean basins where
the lithosphere is thinnest, massive basalt flooding occurs. At
ocean-continent transitions, eugeosynclines with alpinotype
tectogenesis form. In continental interiors where the
lithosphere is thicker, either germanotype foldbelts or
continental rifts are created.
7. During the geotectonic cycle, and within the eugeosynclinal
regime, the central core (crest of the surge channel) evolves
from a rift basin to a tightly compressed alpinotyhpe foldbelt.
Thus a rift basin up to several hundred kilometers wide narrows
through time until it is a zone no more than a few kilometers
wide that is occupied by a streamline (strike-slip) fault zone
(e.g. the San Andreas fault). Then as compression takes over and
dominates the full width of the surge-channel crest, the
streamline fault zone is distorted, until it and the adjacent
rocks are severely metamorphosed. If the underlying, and now
severely deformed surge channel still contains any void space,
the overlying rocks may collapse into it, and through this
process of Verschluckung (engulgment) become a
Verschluckungzone.
8. The Earth above the strictosphere resembles a giant
hydraulic press that behaves according to Pascal's Law. A
hydraulic press consists of a containment vessel, fluid in that
vessel, and a switch or trigger mechanism. In the case of the
Earth, the containment vessel is the interconnected
surge-channel system; the fluid is the magma in the channels;
and the trigger mechanism is worldwide lithosphere collapse into
the asthenosphere when that body becomes too weak to sustain the
lithosphere dynamically. Thus tectogenesis may be regarded as
surge-channel response to Pascal's Law.
9. Surge channels, active or inactive, underlie nearly every
major feature of the Earth's surface, including all rifts,
foldbelts, metamorphic belts, and strike-slip zones. These belts
are roughly bisymmetrical, have linear surface swaths of faults,
fractures, and fissures, and belt-parallel stretching
lineations. Aligned plutons, ophiolites, melange belts, volcanic
centers, kimberlite dikes, diatremes, ring structures and
mineral belts are characteristic. Zoned metamorphic belts are
also characteristic. In some areas, linear river valleys, flood
basalts, and/or vortex structures may be present. A lens of
7.8-7.0 km/s material always underlies the belt.
10. Active surge channels are most easily recognized by the
presence of high heat flow (Fig. 2.26), microseismicity, lines
of thermal springs, small negative Bouguer gravity anomalies,
and a 7.8-7.0 km/s lens of material that is transparent in the
center or throughout.
11. Inactive surge channels possess a linear positive magnetic
anomaly, a linear Bouguer positive gravity anomaly, and a
linear, lens-shaped pod of 7.8-7.0 km/s material that is
reflective throughout.
12. A surge-tectonics approach to geodynamics provides a new
means for determining the origin of the Earth's features and
their evolution through time, for analyzing regions prone to
earthquakes and volcanism, and for predicting the location and
formation of mineral deposits throughout the globe.
#Post#: 181--------------------------------------------------
Re: SURGE TECTONICS
By: Admin Date: March 23, 2017, 10:51 am
---------------------------------------------------------
CHAPTER 1
WHY A NEW HYPOTHESIS?
1.1 Introduction
Before 1962, the year in which H.H. Hess revived and revised
Arthur Holmes's (1931) concept of seafloor spreading (which also
was proposed by Ampferer [1941]), the geology and geophysical
departments of the world taught several geodynamics hypotheses.
These hypotheses stimulated lively discussions and resulted in
the publication of a highly diversified spectrum of ideas. After
Hess's version of seafloor spreading was published, diversity in
geodynamics thinking began to wane, and outside of Asia and
Eastern Europe, had all but vanished by the end of 1963. ... it
is the belief of these authors that as intensive geotectonic
research has vastly increased the database for Earth-dynamic
studies, plate tectonics has not adequately and completely
explained the geology of many regions of the world.
The purpose of this book is to present a comprehensive and
internally consistent hypothesis of global tectonics, an
hypothesis that we call surge tectonics. [Skipping most of 2
paragraphs] ...
... a huge body of evidence has accumulated to show that this
lithosphere mosaic is in a state of equiplanar tangential stress
(..., 1979). That is, compressive stress is ubiquitous in the
lithosphere; moreover it is tangential and directed
approximately equally in all directions of the compass, in
accord with Newton's Third Law of Motion. This fact alone means
that, for one part of the mosaicwork to move laterally (and
tangentially), all parts must shift in order to accommodate the
movement of the one part (..., 1966). One of several convincing
proofs of this involves the classical hole-in-the-plate-problem
or architecture and architectural engineering (..., 1913-1991).
Within any body (or plate) subjected to equiplanar tangential
stress (e.g., compression), stresses in all directions are
approximately equal and opposite, in accord with Newton's Third
Law, unless there is a flaw (hole) in the body. Wherever a flaw,
or 'hole', is present, the compressive stresses must of fault
zone in California, where the axis of maximum compressive stress
is everywhere at right angles to the fault trend (..., 1987;
...).
We also know from geological field mapping that objects within
the lithosphere mosaic are moved substantial distances, both
vertically and laterally. However, the argument that large
lithosphere plates, each 50 to 200 km thick, each extending for
thousands of kilometers in all directions, and each weighing
incalculable tons, can be moved freely and systematically about
the Earth's surface defies all physical laws and common sense.
Strictly lateral tangential movements are out of the question to
explain the observed lateral and vertical motions that have been
mapped in the field. To accommodate these visible, measurable,
large lateral movements, rock bodies within the lithosphere
mosaic must be able to move. To do this requires (1) upward
(vertical) motion of rock bodies to positions of least
resistance, followed by (2) lateral outward motions of the newly
freed bodies on the upper lithosphere surface where the stresses
required for lateral movements are far less than those required
within the lithosphere. To accomplish the observed
motions---which are not confined to relatively narrow mobile
belts but occur everywhere within the lithospheric plates---a
geodynamic explanation other than conventional plate tectonics
and any other existing geodynamic hypothesis is required.
Surge tectonics is a new hypothesis which proposes that the
Earth acts like a hydraulic press. The containment vessel for
this press is an interconnected network of magma chambers and
channels in the lithosphere; the fluid in the chambers is magma
from the asthenosphere; and the trigger mechanism, or press, is
episodic collapse of the lithosphere into the asthenosphere
along points of weakness. Three interdependent and interacting
processes are involved: (1) lateral flow of fluid, or semifluid
magma through the interconnected channels; (2) cooling of the
Earth causing contraction, which contributes to tectogenesis;
and (3) the Earth's rotation. Surge tectonics draws on
well-known physical laws, especially those related to the laws
of motion, gravity, and fluid dynamics. [Skipping last
paragraph]
1.2 Former and Current Concepts of Earth Dynamics
1.2.1 GENERAL
1.2.2 CONTRACTION
[Skipping 5 paragraphs]
Even though MacDonald (1963) answered many of the growing
objections to the contraction hypothesis, the hypothesis fell
from grace. ... A third reason was the observation that the
amount of measured foreshortening in foldbelts is far greater
than the amount that contraction can account for ... [which] in
our judgment, is valid. If contraction does take place, another
mechanism must produce the foldbelts. Regardless, many
geophysicists (..., 1981-1982) still regard contraction as an
ongoing process within the Earth.
A contracting Earth is an extremely attractive model for
tectonic processes, because---in theory at least---it can
provide directly for tangential compression at the Earth's
surface. However, contraction as the sole cause of tectogenesis
is highly unlikely for many reasons, most of which were
discussed by Scheidegger (1963) and Bott (1971). Not the least
of these is the fact that in neither the contraction envisioned
by Jeffreys (1970) nor that described by MacDonald (1963) can
all of the true shortening in mobile belts be accounted for.
However, if weak zones---surge channels---containing magma are
present in the lithosphere, contraction can play a role much
different than that usually attributed to it.
1.2.3 MANTLE CONVECTION
[Skipping almost all of the section]
... However, now that data are available---especially
seismotomographic data---that suggest that convection cells are
not present in the upper mantle, it may soon be unnecessary to
discuss the pros and cons of convection on such a theoretical
level.
1.2.4 EARTH EXPANSION
... Finally, MacDonald (1963) has shown that, whereas expansion
probably was important during the first three eons of Earth
history, it was rather minor and almost certainly is not taking
place today.
CHAPTER 2
...
2.3 Data Sets Unexplained in Current Tectonic Models: Foundation
for a New Hypothesis
2.3.1 LINEAR STRUCTURES
Sonographs of the midocean ridges reveal the presence everywhere
of long, linear, ridge-parallel faults, fractures, and fissures.
The ridge-parallel sets of faults, fractures, and fissures are
not restricted to the crestal regions of the ridges, but extend
down the ridge flanks to levels where the sediments of the
adjacent abyssal-plain basins lap onto the ridges (..., 1979).
Because several of the midocean ridges extend into adjacent
continents (..., 1960-1992b), we extended our study of the
ridge-parallel faults, fractures, and fissure systems to embrace
all tectonic belts within the continental regions.
So that there will be no misunderstanding, it is necessary to
define here our use of the therm tectonic belt. In general, a
tectonic belt is any structural megafeature developed at the
Earth's surface above what we call a surge channel. Thus a
tectonic belt includes the full spectrum of linear tectonic
features known on Earth. In continental regions, these include
continental rifts, strike-slip fault zones, germanotype and
alpinotype foldbelts, and continental volcanic arcs. They also
include such linear cross-strike features as the Colorado
Mineral Belt and the Lower Yangzi Valley plutonic-volcanic belt.
In oceanic regions, tectonic belts include midocean ridges,
"aseismic ridges," linear island and seamount chains, and
oceanic island arcs.
Tectonic belt-parallel systems of faults, fractures, and
fissures were found in every tectonic belt examined, whether a
continental rift, a strike-slip fault zone, or a foldbelt.
Examples include the Western Cordillera of the United States
(Fig. 2.1; ..., 1978) and, at a smaller scale within the same
tectonic province, the California Coast Ranges-San Andreas fault
zone (Fig. 2.2; ..., 1976). Other examples are the East African
Rift system (Fig. 2.3; ..., 1976-1987), the Rhine Graben (Fig.
2.4; ..., 1979), the Front Range of New Mexico, Colorado, and
Wyoming (Fig. 2.5; ..., 1986), and the Reelfoot Graben beneath
the Mississippi Embayment (Figs. 2.6, 2.7; ..., 1978-1982).
Together, these systems involve a huge body of data that are not
well explained in plate tectonics, and with rare exceptions,
have not been addressed. The fact that faults, fractures, and
fissures parallel the strike of each tectonic belt indicates, as
a simple consequence of Stoke's Law (see Appendix), that each of
these belts has been, or is underlain by a mobile body that
moves parallel with the tectonic belt. Thus the primary motions
producing these systems of faults, fractures, and fissures are
not at right angles to the tectonic belts (..., 1986).
Linear evaporite trends and many types of linear basins
originate in half-gravens, grabens, and compression-produced
topographic (synclinal) lows, and generally are explained as a
consequence of tension or compression. However, all linear
basins and all oval basins (e.g., Paris basin, Williston basin,
Illinois basin, Moscow basin, Sichuan basin), both on
cratons/platforms and in less stable regions such as rifts and
foldbelts, are underlain by lenses of 7.0-7.8 km/s material.
Linear valleys and mountain systems commonly can be explained
as inherited from the strike of underlying older structures.
Mountains that are transverse to regional structure, however,
pose bigger problems (e.g., the California Transverse Ranges,
the Uinta Mountains of the Rocky Mountains, the Wichita and
Arbuckle Mountain of the United States Great Plains). Similarly,
long, straight river courses across regional strike do not
always have an obvious explanation. Examples include the lower
courses of the Mississippi River (Mississippi Embayment), the
St. Lawrence River, and the Yangzi River. These linear to
curvilinear valleys are underlain by lenses of 7.0-7.8 km/s
material at the Moho-.
2.3.2 LITHOSPHERE DIAPIRS AND LITHOSPHERE MAGMA CHAMBERS
Ever since the publication of Wegmann's (1930, 1935) pioneer
papers on the topic, mantle diapirism has been invoked
increasingly as a mechanism for generating or promoting
tectogenesis. Van Bemmelen (1933) and Glangeaud (1957, 1959),
for example, favored mantle diapirism and subsequent lateral
sliding and/or compression for creating the structures of the
Mediterranean Sea region. Mantle-diapirism hypotheses have found
favor at different times with many geologists (..., 1968) for
explaining the structural evolution of the Mediterranean belt,
and indeed still do (..., 1988).
The evidence adduced for extensive lithosphere diapirism is now
formidable (..., 1980-1992). Shallow magma chambers are
ubiquitous beneath active tectonic belts, whether they be rift
zones, streamline (strike-slip) fault zones, or foldbelts. Some
rift-valley examples of shallow magma chambers or diapirs
include the East African Rift system (..., 1992), the Red Sea
Graven (..., 1988), the Rhine Graben (..., 1984), the Baykal
Rift (..., 1979-1985), the Rio Grande Rift (..., 1982), Iceland
(..., 1982), the Hetao-Yinchuan Graben (..., 1989), the Fen Wei
(Wei He) Graben (..., 1989), and many, many more. Streamline
(strike-slip) fault zone examples include the San Andreas Fault
zone (..., 1980), the Dead Sea Fault zone (..., 1989), the
Alpine Fault (..., 1991), the Queen Charlotte Fault zone (...,
1988), and many more. One problem with finding examples of magma
chambers at shallow depths along streamline (strike-slip) fault
zones is that these zones have not been studied in the same way
as rifts and foldbelts. Hence the discovery of shallow melt, or
potential melt, zones beneath streamline fault zones has been
largely serendipitous. Examples of shallow diapirs beneath
foldbelts are also abundant. A few examples include the
California Coast Ranges (..., 1983-1985), the Alps (..., 1983),
the Dinaric Alps (..., 1974), the Himalaya and Qinghai-Xizang
(Tibet) Plateau (..., 1991), the Yunnan Himalaya (..., 1989),
the Japan Arc (..., 1987), and the Pyrenees (..., 1989).
Almost since the beginning of the plate tectonics era,
geophysicists such as Lliboutry (1971), Bonini et al. (1973),
and many others have pointed out the important role that
diapirism must play in any scheme of Earth dynamics. Despite
this, mantle diapirism and related upwelling processes received
little consideration as intrinsic parts of plate tectonics until
Dewey (1988a) recognized their possible importance throughout
the Alpide-Mediterranean and parts of the Circum-Pacific
tectonic belts. Dewey's (1988a) explanations, however, do not
account for coexisting states of compression and tension, as the
field data from many areas required (e.g., Alboran Sea; ...,
1989). In contrast, surge tectonics requires the simultaneous
formation of side-by-side compressional and tensil regimes
during tectogenesis. Table 2.1, based on random sampling of some
recent literature, shows how widespread the idea of mantle
diapirism and upwelling has become. More than 50% of the
examples listed are alpinotype or germanotype foldbelts; the
remainder are tensile belts. Our point is that, whereas mantle
diapirism may have a place in the tensile regimes of plate
tectonics, it cannot be accommodated in the compressional
regimes.
2.3.3 MAGMA CHAMBER-RELATED PHENOMENA
Lithosphere magma chambers and related asthenosphere upwellings
form zones of reduced seismic velocity, the low-velocity zones
of the literature. Commonly a large magma chamber forms close to
the mantle-crust boundary, followed by the formation at still
higher levels in the middle to upper crust of smaller magma
chambers whose sizes decrease upward, thereby forming a
"Christmas Tree" structure as described by Corry (1988) for sill
complexes in the upper crust (see Fig. 2.8). The large magma
chamber close to the mantle-crust boundary is pod-shaped (see
Fig. 2.9), and is referred to in the literature by various
names--lenses, lenticles, lozenges, pillows, rift pillows, pods,
shear pods, anastomosing networks of shear zones, and so
forth---terms that show the lack of knowledge of their
origin(s). These lenses, for many years, have been termed layers
of "anomalous upper mantle" or, conversely, "anomalous lower
crust." Although they have been observed most commonly at the
mantle-crust boundary, such lenses do occur in some tectonic
belts in the middle to upper crust (..., 1989-1990; Figs. 2.10,
2.11).
The lens at the mantle-crust boundary typically has a P-wave
velocity in the 7.0-7.8 km/s range (..., 1959-1983). Many of
them contain a low-velocity zone (5.4-6.6 km/s) near their
centers (..., 1970-1979). Beneath continents and many parts of
the ocean basins, these lenses are typically between 100 and 500
km wide, most commonly in the 150-250-km range (..., 1980-1983).
Where not present at the mantle-crust boundary, they pinch out
laterally into a thin but nearly omnipresent zone with a
velocity range of 6.9 to 7.9 km/s (..., 1987-1989).
An identical but much larger lens occupies the crust-mantle
boundary zone beneath midocean ridges (..., 1959-1965), where
they were first discovered by Revelle (1958). Here beneath the
midocean ridges, the lenses are typically 1,000 to 3,000 km
across and they occupy the full 65,000-km length of the midocean
ridge system
Because these lenses pinch out laterally from the centers of
the midocean ridges, they were at first perceived as an obstacle
to the newly formulated hypothesis of sea-floor spreading (...,
1961), but were soon provided with an explanation conforming to
plate tectonic models. _____The problem, as it was perceived,
was that, if the "anomalous mantle" lens formed at the midocean
ridge crests (as it had to do, in sea-floor spreading), then
some process had to remove the 7.0-7.8-km/s material as the
oceanic crust moved away from the midocean ridge crest toward
its laterally coeval and subparallel subduction zones. Two
speculative solutions to the problem were suggested and, to the
best of our knowledge, were accepted without benefit of
additional research.
The first solution was proposed by Drake and Nafe (1968, ...):
"Velocity-depth data indicate that velocities in the range
7.2-7.7 km/s are almost completely absent in the deep ocean
basins away from ridges or prominent seamounts and under the
low-lying continental shields, but are present in all other
regions to some degree. The material in this velocity range must
be derived from the mantle but is of lower density than normal.
If, as is suggested by the data, it is of a transient nature,
its appearance and disappearance may be related to the changes
in elevation associated with tectonic activity." Elsewhere in
the same paper, Drake and Nafe (1968, ...) wrote that oceanic
crustal layer 3 (the lowest ocean crustal layer which overlies
the Moho-) "...would receive permanent additions of rock with
the properties of gabbro, and a 7.2- to 7.7-km/sec layer would
first develop and then vanish. In this view, the principal
contribution of the 7.2- to 7.7-km/sec layer is to increase
total thickness and, through isostatic adjustment, to increase
surface elevation during the orogenic process, and then to
disappear with an accompanying reduction in thickness and
elevation."
Referring to the 7.2- to 7.7-km/sec layer as low-velocity
mantle, Vogt et al. (1969) wrote that, "The occurrence of
low-velocity mantle under [the midocean ridge] crest could well
be a steady-state phenomenon. That is, it may be constantly
created under the axis and converted to normal mantle under the
flanks" (..., 1969, ...). In further explanation, Vogt et al.
(1969, ...) wrote that the low-velocity mantle "...under the
ridge axis is most likely an ultrabasic crystal slush through
which basaltic fluids must rise to feed the growing layers 2 and
3.... This slush then probably solidifies and becomes 'normal'
mantle as it withdraws from the axis." This second explanation
is no more satisfactory than that proposed by Drake and Nafe
(1968).
The problem has not been researched further, to the best of our
knowledge, and remains unsolved. The problem is crucial, because
these lenses are found under every type of tectonic belt. Under
the continents, for example, long linear lenses of 7.2-7.8-km/s
material underlie all rifts (..., 1983), all streamline
(strike-slip) fault zones (..., 1989), and all foldbelts (...,
1968-1989). Under the ocean basins, identical lenses underlie
the midocean ridges (..., 1959-1965), linear island and seamount
chains (..., 1968), and other aseismic oceanic ridges (...,
1975). The lenses are a common denominator for all tectonic
belts and, therefore, cannot be transient features, as
maintained by Drake and Nafe (1968). Nor can the material that
forms them become "normal" mantle as it withdraws from the axis
of each tectonic belt, as suggested by Vogt et al. (1969). In
plate tectonics, the midocean ridges are the only tectonic belts
from which rock materials (i.e., the new crust formed at the
axes of midocean ridges) can withdraw. If Vogt et al. (1969) are
right, then a second explanation must be developed to explain
the presence of identical lenses in other types of tectonic
belts.
In many foldbelts, the "anomalous" lenses have been deformed
together with the shallower rocks (e.g., Figs. 2.13-2.15). Where
a foldbelt has been found to be deformed bilaterally (i.e., the
belt is bivergent), one side of the belt is said to have a zone
of "backthrusts" (..., 1984-1986), although few of the
"backthrusts" exhibit the criteria of backthrusts (..., 1951).
In this work, we demonstrate that all foldbelts are bilateral
(i.e., bivergent), an observation made long ago by Kober (1925),
Vening Meinesz (1934), and many others. We call these bivergent
foldbelts kobergens, a concept that we define and explain in
detail in the following chapter. Examination of our many figures
illustrating bivergent foldbelts reveals at once that such
features combine the effects of compression and tension (Fig.
2.15). Along the two flanks of the foldbelts are folds, thrusts,
and nappes whose vergence on one flank is the opposite of the on
the other flank. Between the two flanks is a zone of tension.
Thus compression and tension act together, side by side, in
belts hundreds, even thousands, of kilometers long.
Consequently, the seemingly contradictory evidence for stress
regimes noted by workers in foldbelts (..., 1988a-1989) is not
at all contradictory but is an inevitable consequence of
tectogenesis. The literature on bivergent foldbelts dates at
least to Suess (1885) and has increased steadily to the present
(..., 1989).
#Post#: 184--------------------------------------------------
SURGE TECTONICS HIGHLIGHTS
By: Admin Date: March 29, 2017, 9:31 pm
---------------------------------------------------------
SURGE TECTONICS
3.1 Introduction
_Surge tectonics is based on the concept that the lithosphere
contains a worldwide network of deformable magma chambers
(surge channels) in which partial magma melt is in motion
(active surge channels) or was in motion at some time in the
past (inactive surge channels).
_The presence of surge channels means that all of the
compressive stress in the lithosphere is oriented at right
angles to their walls. As this compressive stress increases
during a given geotectonic cycle, it eventually ruptures the
channels that are deformed bilaterally into kobergens (Fig.
2.15).
_Thus, bilaterally deformed foldbelts in surge-tectonics
terminology are called kobergens.
_Surge tectonics involves
1. contraction or cooling of the Earth
2. lateral flow of fluid, or semifluid, magma through a network
of interconnected magma channels in the lithosphere
3. Earth's rotation, which involves differential lag between the
lithosphere and the strictosphere and its effects, i.e.
eastward shifts (Table 2.3)
=the strictosphere is the hard mantle beneath the asthenosphere
and lower crust
._lithosphere compression caused by cooling propels the lateral
flow of magma through surge channels
ST_3.2.2 CONTINENTS HAVE DEEP ROOTS
_Contrary to general belief continental roots are fixed to the
strictosphere [as shown] by large and increasing volumes of
data, including neodymium and strontium studies of crustal
rocks (..., 1979).
_the deep roots of continents are a major obstacle to any
hypothesis requiring continental movements (..., 1985-1990).
_deep roots are seen beneath part of all of the Earth's ancient
cratons.
_In places, however, lenses of 7.0-7.8-km/s material containing
low-velocity zones (Fig. 3.5) are present (..., 1989).
_Such lenses containing low-velocity layers postdate the
establishment of the deep cratonic roots, as we show in
subsequent sections.
_3.3.2 Contraction Skepticism
_3.3.3 Evidence For a Differentiated, Cooled Earth
_1. The Earth includes several concentric shells, which are
explicable only if the Earth differentiated efficiently and at
a much higher temperature than today.
_2. The outermost of these shells may be the oceanic crust whose
thickness ranges from about 4-7 km.
<<contradicts sed strata & oceanization
_This crust is characterized by relatively constant thickness
and fairly uniform seismic properties.
_This uniformity is explained if the oceanic crust is the
outermost of the Earth's concentric shells.
_5. A convincing evidence that huge segments of the lithosphere
have been and are being engulfed by tangential compression is
the existence of Verschluckungszonen (engulfment zones)
_In places along such zones, whole metamorphic and igneous belts
that are characteristic of parts of a given foldbelt simply
disappear for hundreds of kilometers along strike
_Although [some] considered these features to be former
subduction zones, this interpretation is difficult to defend
because all of these zones, regardless of age, are
near-vertical bodies (1) reach only the top or middle of the
asthenosphere (150 to 250 km deep) and (2) do not deviate more
than 10° to 25° from the vertical (..., 1983-1984).
_6. The antipodal positions of the continents and ocean basins
(unlikely a matter of chance) mean that Earth passed through a
molten phase
_7. Theory (..., 1970) and laboratory experiment (..., 1956)
showed that heated spheres cool by rupture along great circles.
Remnants of two such great circles (as defined by hypocenters
at the base of the asthenosphere) are active today: the
Circum-Pacific and Tethys-Mediterranean fold systems. The
importance of Bucher's (1956) experiment to contraction theory,
in which he reproduced the great circles, is little
appreciated.
3.8 Evidence for the Existence of Surge Channels
3.8.1 SEISMIC-REFLECTION DATA
_As noted above, reflection-seismic techniques (...) have shown
that the continental crust of the upper lithosphere is
divisible in a very general way into an upper moderately
reflective zone and a lower highly reflective zone (...).
Closer scrutiny of the newly-acquired data soon revealed the
presence in the lower crust of numerous cross-cutting and
dipping events.
_When many of these cross-cutting events were preceived to be
parts of lens-like bodies, various names sprang up: ....
Strictly nongenetic names include lenses, lenticles, lozenges,
and pods (...). Finlayson et al. (1989) found that the lenses
have P-wave velocities of 7.0-7.8 km/s, commonly with a
low-velocity zone in their middle.
_Thus we equate the lenses with the pods of "anomalous lower
crust" and "anomalous upper mantle" that we discussed in a
preceding section. Klemperer (1987) noted that many of the
lenses are belts of high heat flow. Hyndman and Klemperer (1989)
observed that the lenses generally have very high electrical
conductivity.
_Meyerhoff et al. (1992b) discovered that there are two types of
undeformed reflective lenses, and that many of these lenses
have been severely tectonized. The first type of lens is
transparent in the middle (Fig. 3.29); the second type is
reflective throughout (Fig. 2.11). Tectonized lenses also may
have transparent interiors, or parts of interiors; many,
however, are reflective throughout (Fig. 3.21). Where
transparent zones are present (Fig. 3.20), bands of high heat
flow, bands of microearthquakes, belts of high conductivity,
and bands of faults, fractures, and fissures are present. Where
a transparent layer is not present, high heat flow and
conductivity, however, are commonly still present. Meyerhoff et
al. (1992b) also found that lenses with transparent interiors
are younger than those without transparent interiors; moreover,
there is a complete spectrum of lenses from those with wholly
transparent interiors to those without.
_The best explanations of thes observations are that (1) the
lenses with transparent interiors are active surge channels
with a low-velocity zone sandwiched between two levels of 7.0
to 7.8 km/s material; (2) the lenses with reflective interiors
are former surge channels now cooled and consisting wholly of
7.0 to 7.8 km/s material; and (3) the tectonized lenses are
either active or former surge channels since converted into
kobergens by tectogenesis.
_3.8.3 SEISMOTOMOGRAPHIC DATA
_Seismotomographic data, wherever detialed studies have been
made, indicate that the lenses seen in seismic-refraction and
seismic-reflection studies form an interconnected, reticulate
network in the lithosphere. Although only one highly detailed
seismotomographic study has been made on a continental
scale---this in China---it leaves no room for doubt that the
7.0-7.8-km/s lenses with transparent interiors and the
seismotomographically detected low-velocity channels in the
lithosphere are one and the same....
_Using seismotomographic techniques, it will be possible to map
active surge channels over the world with comparative ease.
_3.8.4 SURFACE-GEOLOGICAL DATA
_Direct evidence for the existence of surge channels comes from
tectonic belts themselves, and from one type of magma flood
province. The latter include rift igneous rocks that crop out
nearly continuously for their full lengths. Examples include
the rhyodactic Sierra Madre Occidental-Sierra Madre del Sur
extrusive and intrusive belt of Mexico and Guatemala, some
2,400 km long; the 1,600-km-long Sierra Nevada-Baja California
batholith belt; the 4,000-km+ batholith and andesite belt of
the Andes south of the equator; the 4,000-km-long
Okhotsk-Chukotka silicic volcanic belt; the 5,800-km-long
Wrangellia linear basaltic province extending from eastern
Alaska to Oregon, which erupted in less than 5 Ma; and many
other similar continental magma belts. The ocean basins are
equally replete with them, ranging from the 60,000-km-long
midocean ridge system through the 5,800-km-long Hawaiian-
Emperor island and seamount chain to many similar belts of
shorter lengths. Geochemical studies also show that most of
these belts are interconnected. Another linear flood-basalt
belt, which has been studied only relatively recently, is the
subsurface Mid-Continent province that extends 2,400 km from
Kansas through the Great Lakes to Ohio (Figs. 3.23, 3.24).
_3.8.5 OTHER DATA
_Other data mentioned in the preceding sections corroborate the
interconnection of active surge channels. One of these is the
coincidence of the 7.0-7.8-km/s lenses of the active surge
channels (Figs. 2.9, 2.31, 3.6, 3.9, 3.14, 3.20) with the belts
of high heat flow (Fig. 2.26) and with belts of microseismicity.
Both the presence of high heat flow and microseismicity
indicate that magma is moving within active surge channels.
_However, an even more dramatic example is the June 28, 1992,
Landers, California, earthquake-related activity shown on
Figure 3.25. This figure shows that the 7.5- magnitude
earthquake was strong enough to affect areas up to 1,250 km from
the epicenter (...) and provides an exampole of Pascal's Law in
action. Given the importance of Pascal's Law in surge-channel
systems, the fact should be noted that the viscosity of the
magma in the surge channels affected by the Landers event is
sufficiently low that, when the stress was applied at a single
hypocentral point (Landers), the effects could still be
transmitted for 1,250 km!
_3.9 Geometry of Surge Channels
_3.9.1 SURGE-CHANNEL CROSS SECTION
_Corry (1988) published the "Christmas Tree" model shown in
Figure 2.8; Bridgwater et al. (1974) published the more complex
model shown in Figure 3.26. Either of these could be cross
sections of surge channels. Both are multitiered with one or
more magma chambers above the main chamber.
_3.9.2 SURGE-CHANNEL SURFACE EXPRESSION
_Study of Figures 2.8, 2.9, 2.11, 2.31, 3.6, 3.9, 3.13, 3.14,
3.20, 3.23 and 3.24 might lead one to believe that surge
channels are everywhere fairly simple structures expressed at
the surface by a single belt of earthquake foci, high heat
flow, bands of faults-fractures-fissures (streamlines), and
related phenomena which, during tectogenesis, deform into a
single kobergen. Although this simple picture is true of many
kobergens, it is not true of all. Study of Figures 3.26 and
3.27 suggests that, during tectogenesis of the surge-channel
complexes shown on these figures, two or more parallel
kobergens may exist at the surface. Such a complex surface
expression is in fact quite common. Well-documented examples are
found in the Western Cordillera of North America, the
Mediterranean-Tethys orogenic belt (including the Qinghai-Tibet
Plateau), and the Andes, inter alia. Within the Western
Cordillera, the Qinghai-Tibet Plateau, and the Andes, we have
found four or more parallel kobergens side by side at the
surface as documented and illustrated by Meyerhoff et al.
(1992b).
3.9.3 ROLE OF THE MOHOROVIC DISCONTINUITY
The principal forces acting on the lithosphere are compression,
rotation, and gravity.
Thus, when the postulated tholeiitic picrite magma reachs the
Moho- (i.e., the zone between 8.0-km/s mantle below and
6.6-km/s above), it has reached its level of neutral buoyancy
and spreads laterally. Under the proper conditions---abundant
magma supply and favorable crustal structure---a surge channel
can form. We suggest the possibility that the entire
7.0-7.8-km/s layer may have formed in this way. In support of
this suggestion, we note that the main channel of every surge
channel studied, from the Archean to the Cenozoic, is located
precisely at the surface of the Moho-. This indicates that the
discontinuity is very ancient, perhaps as old as the Earth
itself. This fact and the great difference in P-wave
==velicities above and below the Moho- surface suggest in turn
that the discontinuity originated during the initial cooling of
the Earth. Hence, Mooney and Meissner's (1992) "transition
zone" was the level of neutral buoyancy at the time the
7.0-7.8-km/s material was emplaced.
?>The formation of the Christmas-tree-like structures (Figs.
2.8, 3.26) at the Moho- is simply an extension of the larger
scale process of magma transfer from the asthenosphere to the
discontinuity. Once surge channels are established at the
discontinuity, the same processes take over that brought the
magma to the discontinuity in the first place, specifically,
magma differentiation in the channels and the Peach-Kohler
climb force (...). After lighter magmas have formed by
differentiation and related processes, they rise to their own
neutral buoyancy levels, forming channels above the main surge
channel (Figs. 3.23, 3.27).
SURGE TECTONICS
Chapter 6 Magma Floods, Flood Basalts, and Surge Tectonics
_6.1.1 SIGNIFICANCE OF FLOOD BASALTS
_Some 63% of the ocean basins are covered with flood basalts. At
least 5% of the continents are likewise covered with flood
basalts. Thus 68%---a minimum figure--- of the Earth's surface
is covered with these basaltic rocks. Flood basalts, then, are
not the oddities that many suppose them to be. In spite of this,
they receive little attention among the scientific community.
_ Engel et al. (1965) long ago demonstrated that deep
ocean-floor tholeiitic basalts are the oceanic equivalent of
the continental flood basalts. The Basalt Volcanism Study
Project (1981) differentiated between the continental flood
basalts and "ocean-floor basalts," while recognizing that the
principal differences were the abundance of minor and
rare-earth elements. Press and Siever (1974...) recognized the
fact that the ocean-floor basalts and continental flood basalts
are nearly the same, and that their differences are explained
readily by contamination in the continental crustal setting.
_6.1.2 CLASSIFICATION
_Continental flood-basalt provinces are geometrically of two
types. The first is broadly ovate, or even round, with the
maximum diameter ranging from about 500 km (Columbia River
Basalt) to more than 2,500 km (Siberian Traps). The second is
distinctly linear, with a width of 100 to 200 km and lengths up
to and even exceeding 3,000 km.
_ Tectonism and metamorphism can severely disrupt any
flood-basalt province after its formation. For example, ... the
Antrim Plateau Volcanics of northern Australia ... parts ...
have been removed by erosion. ... Similarly, only very
scattered, strongly flooded, and metamorphosed remains of the
Willouran Mafic rocks are preserved in ... South Australia, but
their distribution shows that [it] is a linear flood-basalt
province.
_6.6 Flood-Basalt Provinces and Frequency in Geologic Time
As we observed near the beginning of this chapter, the commonly
used textbooks of physical geology, structural geology, and
geotectonics rarely list more than 10 to 20 flood-basalt
provinces. However, the magnificent review of basalts by the
participants in the Basalt Volcanism Study Project (1981)
mentions or figures not less than 56 flood-basalt provinces and
45 additional provinces of dike swarms which the project
participants thought might have fed flood-basalt provinces that
have since been removed by erosion.
_ Yoder (1988, ...) wrote that "Great basaltic 'floods' have
appeared on the continents throughout geologic time (Table 1),"
but showed on his Table 1 none older than 1,200+/- 50 Ma. He
also ... made it clear that he regards midocean-ridge and other
oceanic basalts as flood basalts, as have a number of earlier
workers (..., 1974). We concur absolutely with their
interpretation. We also concur with the participants of the
Basalt Volcanism Study Project (1981) that evidence of the
existence of flood provinces extends back in time to at least
3,760 Ma, and very likely to the Earth's earliest (but nowhere
preserved) history.
_6.7 Non-Basalt Flood Volcanism in Flood-Basalt Provinces
The bimodal nature of many flood-basalt provinces has been known
and stressed for many years (..., 1981). Time seems not to be a
major factor (the idea being that, the longer an underlying
magma chamber is present, the more the magma will interact with
the continental crust above it). The most important factor may
be the crustal stress state.
_ We believe that the evidence from these examples demonstrates
convincingly that there is a complete gradation from all-basalt
and basaltic andesite flood provinces to bimodal provinces
containing mainly rhyolite and ignimbrite. Hence, there are
basalt floods and rhyolite floods.
_ ... The volumetric predominance of these ash-flow tuffs has
led to recognition of the [Sierra Madre Occidental] as the
world's largest rhyolite-dominated volcanic province" (Fig.
6.28).
_ Thus, from 38 Ma until 17 Ma, a truly bimodal column of
extrusive rocks accumulated in northern Mexico and adjoining
parts of the United States, with rhyolite at one end, basaltic
andesite at the other, and very little rock of intermediate
compositions. ... [Skipping remainder of paragraph]
_ We believe that these basalts of the "southern cordilleran
basaltic andesite" suite are flood basalts. And if they are
flood basalts, then we have demonstrated that the same
mechanism that leads to continental and oceanic basalt
outpourings also produces the "orogenic andesite suite".
_ The Okhotsk-Chukotka Volcanic Belt, a linear belt of
Cretaceous volcanics, is similar to the Sierra Madre
Occidental. It extends 3,000 km from the mouth of Uda Bay
(northwestern Sea of Okhotsk) to the Bering Sea almost at St.
Lawrence Island. It seems to have every type of volcanic from
andesitic through rhyolite. Basalts are scarce. Soviet
geologists either ignore it or say that it is the remnant of a
volcanic arc.
_6.9 Surge-Tectonics Origin of Magma Floods
In the preceding pages we have referred to the presence of
several flood-basalt provinces around the world, and have shown
that some flood provinces include large volumes of silicic
rocks, usually rhyolite and/or dacite. We have also shown by the
northern Mexican example that flood basalts can interfinger
with the andesite orogenic suite.
_The available evidence has led us to the conclusion that the
same mechanism causes volcanism in the midocean ridges, linear
island and seamount chains, oceanic plateaus, island arcs, and
continental interiors. We next attempt an explanation of our
conclusion.
_ Many attempts have been made to explain flood volcanism in the
framework of the plate-tectonics hypothesis. The two principal
explanations involve (1) hot spots, or mantle plumes and (2) an
extraterrestrial cause (e.g., an asteroid impact).
_ Extraterrestrial causes have been proposed by Alt et al.
(1988), who applied this hypothesis to the Columbia River
flood-basalt province. A major problem with this concept is
that it does not explain linear flood-basalt provinces such as
the Keweenawan (Mid-Continent) rift and Wrangellia.
Furthermore, Mitchell and Widdowson (1991) pointed out that
impact and shock phenomena should be present in the area
surrounding the Columbia River province if it resulted from
extraterrestrial action, but they are entirley absent.
_ As we noted in Chapters 3 and 4, Mooney et al. (1983) observed
that all active rifts studied by them have an anomalous lower
crust with P-wave velocities in the 7.0 to 7.7 km/s range (Fig.
6.36). [Others] obtained the identical result.... Fuchs (1974)
believed that this pod of anomalous lower crustal material
houses the mechanism that causes rifting. It is interesting to
note that all midocean ridges have a pod of 7.0-7.7 km/s as
well (..., 1959-1965). (Furthermore, each island arc and
foldbelt also has a pod of 7.0-7.7 km/s material that pinches
out from the center of the arc or foldbelt (..., 1987-1989 ...
for the Japan arc ... [and] for the Appalachians.)
_ Figure 3.6 is a cross section across the Baykal rift, from
Krylov et al. (1979) and Sychev (1985). Years of refraction
work have shown [that] Lake Baykal is underlain at about 32 km
by a pod that is connected to the deeper asthenosphere. The
shallow pod contains a low-velocity zone that presumably is a
partial melt. The pod extends the full length of the rift. It
is, in short, a channel containing partly molten magma and an
excellent example of one of our surge channels. Were it to
burst, we believe that it would produce another linear
flood-basalt province.
_ According to our surge tectonic hypothesis, magma in surge
channels moves both vertically and horizontally. When two surge
channels come in contact, their magmas join together. If they
are oriented at an appreciable angle to one another, we believe
that the result is a "collision". These5 "collisions" are
responsible for the eruption of round or ovate flood-basalt
provinces worldwide.
CHAPTER 7
CONCLUSIONS
We have proposed a new hypothesis of global tectonics in this
book, one that is different and will be considered unorthodox
by many scientists and non-scientists alike. However, we
believe that current tectonic hypotheses cannot adequately
explain the increasing volume of data being collected by both
old and new technologies. We believe that the hypothesis of
surge tectonics does explain these data sets, in a way that is
simple and more accurate.
The major points of the surge-tectonics hypothesis can be
summarized as follows:
1. All linear to curvilinear mesoscopic and megascopic
structures and landforms observed on Earth (and similar
features seen on Mars, Venus, and the moons of Jupiter, Saturn
and Uranus), and all magmatic phenomena are generated, directly
or indirectly, by surge channels. The surge channel is the
common denominator of geology, geophysics, and geochemistry.
2. Surge channels formed and continue to form an interconnected
worldwide network in the lithosphere. They contain fluid to
semifluid magma, or mush, differentiated from the Earth's
asthenosphere by the cooling of the Earth. All newly
differentiated magma in the asthenosphere must rise into the
lithosphere. The newly formed magma has a lower density and
therefore, is gravitationally unstable in the asthenosphere. It
rises in response to the Peach-Kohler climb force to its level
of neutral buoyancy (that is, to form a surge channel).
<<So no vertical channels are needed
3. Lateral movements in the Earth's upper layers are a response
to the Earth's rotation. Differential lag between the more
rigid lithosphere above and the (more) fluid asthenosphere
below causes the fluid, or mushy, materials to move relatively
eastward.
4. Surge channels are alternately filled and emptied. A
complete cycle of filling and emptying is a geotectonic cycle.
<<I rather think they don't empty; they solidify
The geotectonic cycle takes place along this sequence of events:
a. Contraction of the strictosphere is always underway, because
the Earth is cooling;
<<...with minor exceptions due to major impacts
b. The overlying lithosphere, which is already cool, does not
contract, but adjusts its basal circumference to the upper
surface of the shrinking strictosphere by large-scale thrusting
along lithosphere Benioff zones and normal-type faulting along
the strictosphere Benioff zones.
<<Benioff zones were caused by recent impacts, so little
shrinkage has occurred since then, though major local and
sometimes minor global effects have likely occurred
c. Thrusting of the lithosphere is not a continuous process,
but occurs when the lithosphere's underlying dynamic support
fails. When the weight of the lithosphere overcomes combined
resistance of the asthenosphere and Benioff zone friction,
lithosphere collapse begins in a episodic fashion. Hence,
tectogenesis is episodic.
<<Such collapse is likely frequent and minor, due to daily
electrified tides
d. During anorogenic intervals between lithosphere collapses,
the asthenosphere volume increases slowly as the strictosphere
radius decreases and decompression of the asthenosphere begins.
e. Decompression is accompanied by rising temperature,
increased magma generation, and lowered viscosity in the
asthenosphere, which gradually weakens during the time
intervals between collapses.
f. During lithosphere collapse into the asthenosphere, the
continentward (hanging wall) sides of the lithosphere Benioff
zones override (obduct) the ocean floor. The entire lithosphere
buckles, fractures, and founders. Enormous compressive stresses
are created in the lithosphere.
<<Again, the stresses should be minor, since they're frequent
g. When the lithosphere collapses into the asthenosphere, the
asthenosphere- derived magma in the surge channels begins to
surge intensely. Where volume of magma in the channels exceeds
volumetric capacity, and when compression in the lithosphere
exceeds the strength of the lithosphere that directly overlies
the surge channels, the surge-channel roofs rupture along the
cracks that comprise the fault-fracture-fissure system
generated before the rupture. Rupture is bivergent and forms
continental rifts, foldbelts, strike-slip zones, and midocean
rifts. We call such bilaterally deformed belts kobergens.
<<This all occurred during the relatively recent major impact
event
h. Once tectogenesis is completed, another geotectonic cycle or
subcycle sets in, commonly within the same belt.
<<Surge channels likely only form during major impact events
5. Movement in the surge channel during the taphrogenic phase
of the geotectonic cycle is parallel with the channel. It is
also very slow, not exceeding a few centimeters per year. Flow
at the surge-channel walls is laminar as evidenced by the
channel-parallel faults, fractures, and fissures observed at the
Earth's surface (Stoke's Law). Such flow also produced the more
or less regular segmentation observed in tectonic belts.
6. Tectogenesis has many styles. Each reflects the rigidity and
thickness of the overlying lithosphere. In opcean basins where
the lithosphere is thinnest, massive basalt flooding occurs. At
ocean-continent transitions, eugeosynclines with alpinotype
tectogenesis form. In continental interiors where the
lithosphere is thicker, either germanotype foldbelts or
continental rifts are created.
7. During the geotectonic cycle, and within the eugeosynclinal
regime, the central core (crest of the surge channel) evolves
from a rift basin to a tightly compressed slpinotype foldbelt.
Thus a rift basin up to several hundred kilometers wide narrows
through time until it is a zone no more than a few kilometers
wide that is occupied by a streamline (strike-slip) fault zone
(e.g. the San Andreas fault). Then as compression takes over
and dominates the full width of the surge-channel crest, the
streamline fault zone is distorted, surge channel still contains
any void spaces, the overlying rocks may collapse into it, and
through this process of Verschluckung (engulgment) become a
Verschluckungzone.
8. The Earth above the strictosphere resembles a giant
hydraulic press that behaves according to Pascal's Law. A
hydraulic press consists of a containment vessel, fluid in that
vessel, and a switch or trigger mechanism. In the case of the
Earth, the containment vessel is the interconnected
surge-channel system; the fluid is the magma in the channels;
and the trigger mechanism is worldwide lithosphere collapse
into the asthenosphere when that body becomes too weak to
sustain the lithosphere dynamically. Thus tectogenesis may be
regarded as surge-channel response to Pascal's Law.
9. Surge channels, active or inactive, underlie nearly every
major feature of the Earth's surface, including all rifts,
foldbelts, metamorphic belts, and strike-slip zones. These
belts are roughly bisymmetrical, have linear surface swaths of
faults, fractures, and fissures, and belt-parallel stretching
lineations. Aligned plutons, ophiolites, melange belts,
volcanic centers, kimberlite dikes, diatremes, ring structures
and mineral belts are characteristic. Zoned metamorphic belts
are also characteristic. In some areas, linear river valleys,
flood basalts, and/or vortex structures may be present. A lens
of 7.8-7.0 km/s material always underlies the belt.
10. Active surge channels are most easily recognized by the
presence of high heat flow (Fig. 2.26), microseismicity, lines
of thermal springs, small negative Bouguer gravity anomalies,
and a 7.8-7.0 km/s lens of material that is transparent in the
center or throughout.
11. Inactive surge channels possess a linear positive magnetic
anomaly, a linear Bouguer positive gravity anomaly, and a
linear, lens-shaped pod of 7.8-7.0 km/s material that is
reflective throughout.
12. A surge-tectonics approach to geodynamics provides a new
means for determining the origin of the Earth's features and
their evolution through time, for analyzing regions prone to
earthquakes and volcanism, and for predicting the location and
formation of mineral deposits throughout the globe.
*****************************************************