Lithological control on the geomorphic evolution of the Shillong Plateau in Northeast India

Abstract

The Shillong Plateau in Northeast India is a block of raised topography in the Himalayan foreland which consists of crystalline basement rocks partially covered by a Cretaceous to Miocene sedimentary succession. It is dominated by a mature, low relief landscape surrounded by high relief, fluvially dissected margins, particularly along its southern flank which is bounded by the Dauki thrust Fault. We use river profiles and geological relationships to show that the low relief plateau is a topographic expression of a re-exposed basement palaeosurface following the stripping of sedimentary cover by scarp retreat. We show that initiation of the wave of incision does not require surface rupture on the Dauki Fault or an increase in fault slip rate at the end of the Miocene, as suggested by previous studies. We propose that incision has been spatially controlled by the slope of the basement palaeosurface, likely moderated by an incision threshold. River profiles in the Shillong Plateau cannot be interpreted as simple records of surface uplift. The observed heterogeneous spatial pattern of steepness is a function of a dynamic landscape response to the erosion of layered lithology with contrasting erodibility. Such dynamics have implications for fluvial geomorphology, highlighting that near-horizontal lithological contacts can strongly influence river profiles and topography, even when no longer physically preserved. The topography of the northern Shillong Plateau is controlled by the structure of basement rocks and is reminiscent of stable cratonic interior landscapes, consistent with its surface exposure during late Cretaceous times.