The Tectonic Framework of Assam

A spur of ancient rocks partly covered by gently-dipping Tertiary beds extends from the Shillong Plateau and Mikir Hills north-eastwards beneath the alluvium of Upper Assam. Over this spur the Eastern Himalaya have been thrust southwards and the Naga Hills have been thrust north-westwards. The amount of movement of the overthrust masses cannot be determined but it is suggested that in each case the total displacement may be 150-300 kilometres or even more. The Shillong Plateau is separated from the Surma Valley by a faulted monocline with southerly dips. This fault, the Dauki tear-fault, is now shown to have a probable horizontal displacement of about 250 kilometres, and thus to be a major feature of the tectonic pattern of the Indian sub-continent. The horizontal movement along the Dauki tear-fault detached the Shillong Plateau from the main mass of the Indian Shield. The principal movements occurred late in the Tertiary, mostly in the Pliocene.