Geoscience Reference
In-Depth Information
The Higher Himalaya crystalline rocks (a gneiss/leucogranite tectonic package)
were then thrust over the Lesser Himalaya meta-sediments along the MCT. After
150-200 km of thrusting, the main boundary thrust (MBT) developed to the south
(as a splay thrust off the main Himalayan thrust (MHT)), and the MCT became
inactive. The Lesser Himalaya meta-sediments were then thrust over the sub-
Himalaya molasse sediments along the MHT and the MBT. The sub-Himalaya
is also underlain by the MHT, since a third stage of thrusting means that the
young sediments of the Ganga (Ganges) foreland basin are underthrust beneath
the sub-Himalaya. Estimates of the total amount of shortening in the Lesser and
sub-Himalaya are
40-70 km. Since the initiation of the MCT between 16 and
25 Ma ago, there has therefore been in total some 200-250 km of shortening,
with the average rate of shortening being 0.7-1.5 cm yr
−
1
.
The present situation has the main thrust plane between India and Eurasia being
the MFT/MHT, while the underthrust Indian continental margin does not extend
beneath the entire Tibetan Plateau. The MFT marks the southern extent of the
deformation and the thin-skinned tectonics that are taking place in the Himalayan
foothills. The main detachment between India and Eurasia is the MHT, which
extends as a shallowly dipping plane beneath the Lesser Himalaya. Beneath the
Lesser Himalaya the MHT steepens, reaching a depth of
∼
∼
25-30 km beneath the
Higher Himalaya. The earlier thrusting, folding, uplift and erosion has exposed
the MCT, the high-grade lower-crustal rocks in the overlying Higher Himalaya
and the underlying medium-grade Lesser Himalaya. The thrust relationships are
illustrated in Fig. 10.15. The main reason for uncertainty in the geology and in
understanding the formation and structure of the Himalayan region is the extreme
size and ruggedness of the terrain, which makes access and working there very
difficult.
The Himalayas are seismically active: magnitude-8 earthquakes are not
uncommon (there have been eight since 1816). Figure 10.16 shows fault-plane
solutions for some earthquakes in the Himalayas. All the fault-plane solutions
for the earthquakes immediately north of the MBT exhibit thrust faulting. The
nodal plane, which is assumed to be the thrust plane, dips at a shallow angle
of
15
◦
. The focal depth for these earthquakes is 10-20 km, which, since the
epicentres are some 100 km north of the MBT, is consistent with the earthquakes
being located at the top of the Indian plate as it is subducted beneath Eurasia
(see Figs. 10.13 and 10.19). North of this band of thrust-faulting earthquakes, the
style of deformation changes: there is normal faulting and east-west extension at
shallow depths over Tibet (Fig. 10.16). The earthquake that occurred at 78
◦
Ein
the Indian plate well to the south of the Himalayas exhibits normal faulting and
therefore is presumably indicative of the extension taking place in the top of the
Indian plate as it bends prior to subducting beneath the Himalayas.
The present-day rate of convergence in the Himalayas is estimated to be
∼
2cmyr
−
1
,which is much less than half the estimated convergence between
India and Eurasia (
∼
6cmyr
−
1
). An estimate of the rate of shortening based on
∼
