Tectonic Control on Topographic and Exhumational Segmentation of the Himalaya

Although the Himalayan range is commonly presented as cylindrical along-strike, geological structures, topography, precipitation, and exhumation rates as recorded by low-temperature thermochronology data all vary significantly from west to east. In particular, segments of the belt that are characterized by a clear topographic step between the Lesser and Higher Himalaya, associated with a peak in precipitation and focused exhumation (e.g. central Nepal, Himachal Pradesh) alternate with segments where the topography increases more linearly to the north, precipitation peaks at lower elevations and exhumation rates appear to be lower (e.g. western Nepal, Bhutan). The potential climatic or tectonic controls on these spatially variable topographic, precipitation and exhumational patterns have been widely discussed in recent years but remain unclear.

Thermo-kinematic modelling predicts that the geometry of the main Himalayan detachment (in particular the presence or absence of a major mid-crustal ramp) strongly controls the kinematics, exhumation and topography of the orogen. Where a major crustal ramp is present, the topography shows a steep gradient that focuses exhumation and orographic precipitation whereas the topography is gentler and exhumation less focused in the absence of a ramp. We test this prediction by comparing the pattern of topography, river incision and long-term exhumation in central Nepal, where a major crustal ramp has been imaged by geophysical methods, with new results from the remote Karnali River transect in far western Nepal, where a ramp is predicted to be absent or minor. Our results therefore imply that along-strike climatic variations in the Himalaya respond to tectonics rather than driving it. The presence or absence of a mid-crustal ramp may be due to inherited structures on the underthrusting Indian Plate or, alternatively, may reflect transient behaviour of the accreting Lesser Himalayan thrust stack, which may oscillate between frontal accretion (without a ramp) or basal accretion in the presence of a ramp.