Geoscience Reference
In-Depth Information
the seismic moment for large earthquakes over a century is 1.8 cm yr
−
1
,while
GPS measurements indicate that 1.8 cm yr
−
1
of contraction was occurring in
Nepal between 1991 and 1995. The GPS data further imply that, south of the
Higher Himalaya, the MHT was locked, while to the north a mid-crustal fault
wasmoving at 0.2 cm yr
−
1
. The remaining Indo-Asian convergence is taking
place over a very large area north and east of the Himalayas, which is consistent
with the extensive tectonic, seismic and local volcanic activity in these regions.
The Tibetan plateau has anomalously high elevation and the presence of active
normal faults indicates that it is extending. This may be visualized as similar
to a blof of viscous fluid spreading out and thinning. Convergence across the
Tian Shan is due to the clockwise rotation of the apparently rigid Tarim Basin.
Figure 10.17(b) shows results from a plasticine model of Southeast Asia and
the deformation that resulted when a rigid block (India) was pushed northwards
into it. The large-scale internal deformation and eastward squeezing of regions
appropriate for Tibet and China show up clearly.
There have been several major seismic experiments to determine the details
of crustal and uppermost-mantle structure across the Himalayas and into Tibet.
These have been international experiments conducted by American, Chinese and
French institutions. Detailed seismic-reflection data from the Higher Himalaya
and southern Tibet are shown in Fig. 10.18 and the crustal and lithospheric
structure are shown in Fig. 10.13. The main feature of the crustal structure across
the Himalayas is the major increase in depth of the Moho from 35-40 km beneath
India, to
70 km beneath the Himalayas, a further increase to about 70-80 km
beneath the southern Lhasa block, a decrease to 60-70 km beneath the northern
part of the Lhasa block and a further decrease to less than 60 km beneath the
Qaidam Basin. The reflections from the MHT beneath the Higher Himalaya
show up clearly, as do the reflections from the Moho in the subducting Indian
plate. There is a low-velocity zone in the crust to the north of the Zangbo suture.
The Tibetan Plateau is a huge region at an elevation of
∼
4.5 km, which suggests
that the underlying crust and the compensation mechanism should be uniform
for the entire plateau. However, this does not seem to be the case. Across the
plateau, the crust has an average P-wave velocity of 6.1-6.3 km s
−
1
,lower than
is normal for continental crust (6.45
>
0.21 km s
−
1
), and a low-velocity zone
may be present at mid-crustal levels. However, the crustal thickness decreases
by 10-20 km on going from south to north; Poisson's ratio for the crust is much
higher than normal in the north; there are zones of low S-wave velocity in the
lower crust; seismic velocities for the upper mantle are low; and the upper mantle
in the north is anisotropic and does not transmit S-waves well. Overall these
results indicate that there are extensive regions of partial melting in the Tibetan
crust and mantle and regions of lateral flow in the Tibetan uppermost mantle.
The presence of widespread melt within the Tibetan crust is also supported by
the facts that seismicity is generally shallower than 10 km, that there is a mid-
crustal low-resistivity zone and that the Tibetan plateau is associated with a
±
