Geology Reference
In-Depth Information
Cambrian to Eocene marine strata,
mainly carbonates and shales, origin-
ally laid down on the Indian continental
shelf. The lower boundary of this belt
is a north-dipping normal fault - the
South Tibetan detachment
(STD).
vectors across the orogen to be calcu-
lated (Figure 11.4); these range generally
between 5 and 15 mm/year, confirm
the lateral extrusion model, and also
indicate a gradual diminution of flow
velocity northwards - there is thus no
clearly defined northern margin to the
deformation resulting from the colli-
sion as there is in the south. GPS and
InSAR
-derived data on slip rates along
strike-slip faults are typically in the
range 5-15 mm/yr; interestingly, the slip
rates on the very large strike-slip faults
such as the Altyn Tagh and Karakorum,
formerly believed to have accommo-
dated the bulk of the N-S compression,
are little different from the others.
This pattern of distributed defor-
mation, achieved mainly by faulting,
applies only to the upper (seismogenic)
crust. Beneath this, the middle crust,
being warmer and more ductile (
see
below), will have deformed in a more
continuous manner, employing shear
zones rather than discrete faults.
the juxtaposition of the high-grade
gneisses of the Greater Himalayan
complex (GHC) and the Tethyan shelf
sediments (TSS) above them, sepa-
rated by the South Tibetan detachment
(Figure 11.3B). The 'orthodox' ramp-
flat thrust model, as indicated above,
suggests that successive forward-
propagating thrusts would cause the
GHC and its TSS cover to be arched
up and subjected to erosion. Gravita-
tional spreading then caused the TSS
to slide down to the north, exposing
the higher-grade rocks of the GHC.
The other explanation relies on
the
channel-flow mechanism
(
see
Figure 10.4). In this model, the GHC
is regarded as a hot, partially molten
piece of Indian crust which has flowed
under gravitational pressure, from its
original position beneath the Asian
plate, upwards to the surface between
the TSS and the Lesser Himalayan
schists beneath. It is possible that
both mechanisms may play a part.
The 5 km-high Tibetan plateau,
underlain by 70-90 km thick crust, is
believed from seismic evidence to be
supported by relatively cool and strong
lower crust and lithospheric mantle
of the Indian plate (Figure 11.3D).
This broad, semi-rigid slab has under-
thrust the Asian plate for a distance
of 200-300 km up to the BNS suture
and is the main driver for the defor-
mation of the orogen. The movement
has absorbed 36-40 mm/yr of relative
motion between India and Asia since
the initial collision (i.e. ~2000 km of
relative motion) of which an estimated
800-1000 km is taken up by shortening
of Indian upper crust in the Himalayas
and a further shortening of possibly
~600 km (accomplished mainly by
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The northern margin of the central
Himalayan sector is the south-dipping
Indus-Tsangpo-Yarlung suture
zone
(ITS), which contains an ophiolite
complex of Cretaceous to early Ter-
tiary age. In the central sector of the
orogen, the suture is cut by the south-
dipping
Renbu-Zedong thrust
(RZT).
The Asian plate
The geology north of the suture is less
well known than that of the main Hima-
layan belt. Immediately north of the
suture is an elongate granite batholith
(the
Gangdese batholith
), which rep-
resents part of a volcanic arc resulting
from the subduction of Indian oceanic
lithosphere in the early Cenozoic. The
Asian plate has been less obviously
affected by the Himalayan compres-
sional deformation, which is partly
concentrated along the suture zones
between the microplates (e.g. the South
Tibet, North Tibet, and Tarim blocks)
that had previously accreted to Central
Asia and partly taken up by movements
along a network of faults. Numerous
north-south oriented graben systems
indicate significant E-W extension and
this, coupled with movements on a
con-
jugate
set of strike-slip faults, has been
interpreted to indicate that much of the
north-south convergence between India
and Asia has been accommodated by
the sideways extrusion of Asian crust.
Repeated precise GPS measurements
have enabled accurate movement
Orogenic history
The three northern units of the central
Himalayan belt all represent packages
of material scraped off the top of the
continental Indian plate as it descended
beneath the Asian plate after the initial
collision and emplaced as thrust sheets.
This process is illustrated schematically
in Figure 11.3C, D. It is assumed that the
thrust sheets developed by propagating
forwards with a ramp-flat geometry as
described in Chapter 5 (
see
Figure 5.7).
As each sheet ramped up, the orogen
would shorten and thicken, although
erosion would continuously remove
material from the roof of the structure.
Two alternative explanations
have been put forward to explain
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