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pronounced magnetic-anomaly low. Modelling of the magnetic data indicates that
the magnetic susceptibility of the Tibetan crust is low. A reasonable interpretation
is that the Curie-temperature isotherm (Section 3.1.3)isatadepth of
15 km
beneath the plateau and that an intracrustal granitic melt is present below about
15 km depth over much of the plateau. Such a mid-crustal layer, if widespread,
is consistent with an effective decoupling of the upper crust from the underlying
lower crust and mantle. An estimate of the average viscosity of the Tibetan
lithosphere is 10 22 Pa s, only 10-100 times greater than that of the upper mantle.
There are different reasons for the presence of partial melt beneath southern
and northern Tibet.
1. The presence of partial melt in the crust in the vicinity of the Zangbo suture results from
the effective doubling of the crust: the increase in crustal thickness causes temperatures
to rise (see Section 7.3 regarding calculation of geotherms). After a few tens of million
years temperatures would have been sufficiently high for partial melting to take place
in wet crust (i.e., temperatures exceed the wet solidus) and for granites to form. Hence,
in time, a doubling of crust causes the development of a partially molten mid-crustal
layer. Figure 10.18(c) suggests that temperatures in the partially molten mid-crust
exceed 600 C.
2. The widespread normal faulting and basaltic volcanism in northern Tibet started only
8-12 Ma ago, late in the accretionary tectonic history of the region (Fig. 10.14(b)).
The distinctive potassium-rich composition of the volcanism indicates that the source
was melted lithosphere rather than asthenosphere. A relatively sudden onset to such
volcanism, combined with extension, can be explained were part of the lower lithosphere
beneath Tibet suddenly removed. This would cause a rapid increase in temperature as
the lower lithosphere was replaced by hotter asthenosphere. Another consequence of
such a convective removal of the lower lithosphere would be an additional uplift (in
excess of 1 km) of Tibet. An additional uplift would have enhanced Tibet's role as a
major regulator on the climate of the Indian region and could account for the changes
in the monsoon that occurred during the Miocene.
The entire Himalayan mountain chain is a region of large negative Bouguer
gravity anomalies. Figure 10.19(a) shows the Bouguer gravity anomaly along
profiles perpendicular to the Himalayas at 84-86 E. Also shown in Fig. 10.19(a)
is the anomaly that has been calculated by assuming that the surface topography is
locally isostatically compensated by crustal thickening (i.e., Airy's hypothesis -
see Section 5.5.2). These calculated anomalies are different from those actually
←−
reflection sections from (a). (c) A schematic interpretation of the India/Eurasia
collision zone based on (a), (b) and structural information. MFT, main frontal thrust;
MCT, main central trust; numbers 1-4 indicate structures giving rise to similarly
labelled features in (b). Colour version Plate 28. (After Brown et al .(1996). Reprinted
with permission from Nelson, K. D. et al ., Partially molten middle crust beneath
southern Tibet: synthesis of project INDEPTH results, Science , 274 , 1684-7.
Copyright 1996 AAAS.)
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