Indications
of gravitational collapse in the Field
Up until now we have been
talking in terms of theories and experimental results, how then does this
translate into the reality? Throughout the world there are a number of
geologically significant sites that display physical structures believed
to be examples of the processes of gravitational collapse discussed in
this paper. The Himalayan Mountains are among the most famous. These mountains
run from east China to the straits of Gibraltar and are the result of a thickening
lithosphere of Mesozoic of Tertiary origin. With an average elevation of
5000m above sea level and a width of 250km, it contains the highest mountain
in the world Everest. 3,000,000km² of this region is the Tibetan Plateau
situated at about 5,400m 30°-40°N and 80°-100°E.
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Left: Tibetan ranges,
Mt Everest
Right: Basin and
Ranges, South west USA
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There are further mountain
ranges to the north, the Tein Shan and the Altai, which are also part of the
same thickening zone. Another area of interest is in Southwest in places called
the Basin and Range, the Great Plains, the Coast Ranges and the Traverse ranges.
(P.England p.276) lying above 1500m.
Previously gravitational collapse has been discussed
as a result of structural strength, gravitational force, GPE and isostacy.
These figures rely on measurements of the density, thickness of lithosphere
and depth of Moho. The crust has a constant density of 2800kgm³ and
the mantle 400-500kgm³, which translates to a root requirement of 5-7km:
1km of mountain range above sea level. Tibet’s total crustal thickness
(root included) should theoretically be about 60-75km. This can be tested
seismically by refraction studies to discover the Moho, which comes to
55-70km in Southern Tibet.
Methods of analyzing gravitational collapse:
www.es.usyd.edu.au/geology/people/staff/prey/Teaching/Geol-2001GPHS/index.html
Seismology is the study
of the normal, reflected, refracted and rarefracted primary and secondary
waves produced by the mechanical failure of fault zones. Commonly known as
earthquakes these seismic disruptions propagate through the earth’s layers
in different ways making analysis of the different layers possible and more
importantly the depth of the Moho can be deducted. These P and S waves also
give information on the kinematics of the fault responsible for the earthquake.
Basically P waves or the first motion of the earthquake propagate in such
a way that when data from two locations is triangulated the focus of the
earthquake can be determined. S waves have shadow zones on the opposite of
the earth(between 105 degrees) as they cannot travel through the liquid mantle.
P waves can but are refracted at an angle (105-143 degrees are shadow zones).
The P waves can also give information on the nature of a fault, as they
travel more strongly in one direction of extension or compression depending
on their nature. These then can be graphically represented as ‘beach balls’,
which correspond with normal, thrust or transform. The white areas are compression
and the black extension.
In the beach ball diagram of Tibet we can see
along the top and bottom boundary thrust faults. Theoretically if only tectonic
forces were acting then only trust fault should be in a region of continental
convergence as seen here with the collision of the Indian and Eurasian plates.
But in the Plateau region we also see normal faults indicating east-west
extension.(P.England p.285) These extensional fractures are some of the most
characteristic features of gravitational collapse in the field.
Features of gravitational collapse:
In the last section the issue of different types
of collapse was raised. Fixed and free boundary collapse are processes which
produce individual features that can be used to identify the sub-terrainian
movement.(Diagrams displayed in previous section)(P.Rey et al p.444).
Fixed boundary collapse
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Upper crustal creates Fans (duplexes of normal faults)
and corresponding conjugate
Fans (thrust faults), a thickening of the forelands
and little deformation of the ductile basement.
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Blind collapse or lower crustal involves the deformation
of the basement alone and can be
recognised in a widening of the plateau, thickening
of crustal forelands with no associated thrusting. No normal faults or
fan features are evident but there is crustal thinning i.e. sedimentation
occurs as the area is lower than the surrounding.
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The final scenario involves a combination of the
other two. There is both deformation of the
brittle crust and the ductile basement giving the faulting
patterns of the first and the crustal thinning of the second. This tends
to promote partial melting and low pressure, high temperature metamorphic
facies as blue shist and glauco-lauconite facies.
Free -boundary collapse
Is
the thinning of the entire thickened crust not just sections bounded by faults
and inconsistencies. There are normal faults/extensional fractures in the
brittle upper crust and ductile flow of the basement. There is usually low
angle detachment and dècollement marks the change between these two
layers and no flow of material outside of the region.(P.Rey et al p.445) The
basin and Range Province in America is believed to be a thickened unit currently
under the process of free boundary gravitational collapse. It was formed
in the Cainozoic and displays the graben and horst features common in this
type of deformation.
These are linear graben and horst features of the Basin and Range
Province in America's Southwest.
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From
the air it looks like this with the lines showing the areas of subsidence
and elevation of the blocks. A cross section of this area would show the classical
v shaped fracture zones with strike-slip movement. Over time these feature
become more pronounced. This process is deforming the Basin and Range Province
at present. This applies as well to Tibet as one of its major features is
the Guzuo graben northwest of Mt Everest .
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Tibetan plateau showing
extensional fractures as normal faults and graben features running north-south
and extending east-west.
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Other methods of
identification:
There are other methods
of identification using empirical evidence, logical assumptions and mathematical
extrapolations based on the experimental results. Extensional faulting brings
deeply buried rocks to the surface (P.England p.297) allowing the analysis
of metamorphic peaks and zircon dating of these, history and folding events
which give insight into the strengths and depth of the rock at the times of
multiple deformation events. This is helpful for the studying the gravitational
collapse’s history where a site has already been identified but what about
a suspected case? At present the best that can be used is numerical modeling
(p.rey p.78), metamorphic core samples and satellite images. Satellite images
give some indication of the extent of faulted areas, basins forming and other
tectonically anomalous macro features, which are possibly a result of gravitational
collapse.
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A pictoral representation of continent-continent collision
(a)
(b)
This diagram is a pictoral representation
of the equation below. 
minus
Top: Shows a balanced GPE with the upper lithosphere in
tension and the lower lithosphere in compression.
This diagram shows how the crust deforms
during free boundary collapse. You can see how the free boundary is
displaced and the crust is thinned.