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316 CHAPTER 10

Ocean in mid-Cretaceous times. The Bangong–Nujiang 10.4.5 Deep structure
suture separates this unit from the Karakorum granite
batholith on its northern side (Fig. 10.19).
North of the Indus–Zangbo suture, active normal Velocity models and tomographic images derived from
faulting and east–west extension are dominant. This studies of Rayleigh surface waves (Section 2.1.3) show
style of deformation has formed a series of young rift that the crust and uppermost mantle of the Indian
basins that trend approximately north–south. At most Shield are characterized by high seismic velocities
these basins record extension of a few tens of kilome- (Mitra et al., 2006). This characteristic suggests that the

ters. Most are filled with Pliocene and younger con- subcontinent is composed of relatively cool, strong
glomerates and appear to have formed since the lithosphere. At the northern edge of the Shield, com-
Miocene. Some are associated with major strike-slip paratively low velocities occur beneath the Gangetic
faults, such as the Jiali Fault, and may represent pull- plains as a result of the molasse sediments and alluvial
apart basins (Section 8.2). These observations and geo- cover in the Ganga foredeep. Low velocities also char-
chronologic data suggest that the east–west extension acterize the thick crust beneath the Himalaya and
is either younger than or outlasted the north–south Tibetan Plateau. South of the Himalaya broad-band
extension recorded by the South Tibetan Detachment teleseismic data indicate that crustal thickness ranges
System (Harrison et al., 1995). Late Cenozoic intrusive from 35 to 44 km (Mitra et al., 2005). This variability

and extrusive activity also occurs in southern and central partially reflects the flexure of the Indian plate (Fig.
Tibet (Chung et al., 2005). 10.18), as it is underthrust to the north beneath Eurasia
Between the Bangong–Nujiang suture and the (Section 10.4.3).

Qaidam Basin (Fig. 10.13) are three major mid–late Below the Himalaya, seismic reflection and shear

Cenozoic fold-thrust belts. All three are associated with wave velocity profiles (Fig. 10.20b) show a well-defi ned
the development of a foreland basin (Yin & Harrison, Moho at 45 km depth that descends as a single smooth
2000). The cumulative amount of shortening accom- surface to depths of 70–80 km beneath southern Tibet
modated by these belts is poorly constrained but may (Nelson et al., 1996; Schulte-Pelkum et al., 2005). A
reach several hundreds of kilometers. At the northern crustal décollement surface above the Moho dips north-
margin of Tibet deformation is partitioned between ward from 8 km below the Sub-Himalaya to a mid-
active folding and thrusting and several major active crustal depth of 20 km beneath the Greater Himalaya.
strike-slip faults, including the Altyn Tagh and Kunlun Above the décollement a strongly (20%) anisotropic
faults. Along the former fault, left-lateral strike-slip layer characterized by fast seismic velocities has formed
motion is transferred to as much as 270 km of north- in response to localized shearing. Slightly north of the
east–southwest shortening in the Qilian Shan. Farther Greater Himalaya the lower crust of the Indian Shield
east and southeast of the Qilian Shan, the shortening shows a high velocity region that may contain eclogite.
direction turns east–west where motion on east-striking These observations suggest that the upper and middle
strike-slip faults is transferred onto active north-striking parts of the Indian crust detach along the base of the
thrust faults in the Longmen Shan (Burchfi el, 2004). shear zone and are incorporated into the Himalaya
This latter mountain range also records Mesozoic short- while the lower crust continues its descent under south-
ening and rises more than 6 km above the rigid, virtually ern Tibet (Fig. 10.21). This conclusion is consistent with
undeformed Sichuan Basin, forming one of the steepest gravity measurements that predict an increase in Moho
fronts along the Tibetan Plateau (Clark & Royden, depth beneath the Greater Himalaya (Cattin et al.,
2000). To the south of the basin, many of the major 2001).
strike-slip faults, including the Jiali and Xianshuihe The deep structure of the Tibetan Plateau has been
faults, are curved. These faults rotate clockwise around studied using passive and active source seismic surveys,
the eastern Himalayan syntaxis relative to South China magnetotelluric measurements, and surface geologic
(Wang et al., 1998). studies as part of an interdisciplinary project called
North of the plateau active shortening also occurs INDEPTH (InterNational DEep Profi ling of Tibet
in the Tien Shan and the Altai ranges of northern China and the Himalaya). The geophysical data indicate

and Mongolia. The deformation in these regions appears that the reflection Moho beneath Tibet is rather
to be controlled mostly by pre-existing strength hetero- diffuse (Fig. 10.20b), similar to that observed on seismic
geneities in the Eurasian lithosphere. refl ection profiles across the plateaux of the Central