Localized strain and partial melts in shear zones may control seismicity

A study by the Scientists from Wadia Institute of Himalayan Geology, Dehradun an autonomous institute under the Department of Science and Technology (DST), Govt. of India suggests that, unlike other parts of the western Himalaya, in Kumaun, partial melting of the crust is caused by activation of a tabular to sheet like, planar or curvi-planar zone composed of rocks that are more strained than rocks adjacent to the zone (major shear zones), instead of continuous zone of mid-crustal partial melts.

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The study also suggests that brittle deformation of these shear zones/ thrust planes may still control exhumation and seismicity in this region of the Himalaya.

The research, published in the Scientific journal ‘Lithos’, shows inverted metamorphism (condition in which higher-grade metamorphic rocks lie on top of lower-grade rocks), leading to partial melting of the Himalayan crust in two short spans of 27 to 32 and 22 to 26 million years.

The study agrees with recent studies which suggest that one uniform mechanism cannot explain the evolution of the Himalayan metamorphic core.

Activation of major thrusts and different mechanisms may operate at different cross-sections of this orogeny with channel flow (long-lasting flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding crustal slabs) being dominant in the hinterland, and at ductile regime.

On the other hand, the critical taper, or wedge-like extrusion and forward propagation of thrust sheets, is more pronounced on the foreland and in the brittle regime.

It has also been shown that the effect of channel flow is completely absent in various sections of the Himalaya. Therefore, different transects may have different mechanisms of exhumations (exposure of land surface that was formerly buried), thrust activity, and hence, different seismic and erosional characteristics.