Spatially Concentrated Erosion Focuses Deformation Within the Himalayan Orogenic Wedge: Sutlej Valley, NW Himalaya, India

Long-term erosion processes in the NW-Himalaya have not only shaped the distribution of topography and relief, but may also exert a regional control on the kinematic history of the Himalayan orogenic wedge. The topographic front of the orogenic wedge forms the southern margin of the High Himalaya and may be related to subsurface structures such as a crustal ramp or a blind thrust. Drastic along- and across-strike erosional gradients characterize the modern Himalaya and range from high-erosion regions along the southern High Himalayan front where monsoonal precipitation is able to penetrate far into the range, to low-erosion sectors across the moderately elevated Lesser Himalaya to the south and the high-elevation, arid sectors to the north. Published paleo-elevation estimates from the Thakkhola Graben (Nepal) suggest that by ~11 Ma the southern Tibetan Plateau and probably the High Himalaya had been uplifted to elevations comparable to the recent conditions. Thus, the presently observed pronounced erosional gradients have likely existed across the orogen since then. However, the cause of high rock-uplift and exhumation rates along distinct segments of the southern front of the High Himalaya are still a matter of debate. New apatite fission track (AFT) and ⁴⁰Ar/³⁹Ar data sampled along an orogen-perpendicular transect following the Sutlej Valley, approximately perpendicular to the Himalayan orogen, constrain the distribution patterns of rapid cooling related to rock uplift and exhumation. Combined with published thermochronologic data, this comprehensive AFT dataset from south of the High Himalaya mountain front to the interior of the Tethyan Himalaya allows us to derive a regional uplift and exhumation scenario. Our new ⁴⁰Ar/³⁹Ar ages ranging between 17 and 4 Ma reveal diachronous exhumation of two crystalline nappes (Higher and Lesser Himalayan crystalline) during Miocene-Pliocene time. In contrast, the AFT data ranging from 1.3 to 4.6 Ma indicate synchronous, fast rock uplift and exhumation of both units. AFT ages increase with increasing elevation and distance from the Sutlej. Importantly, the AFT ages are particularly young where monsoonal precipitation is concentrated and the deeply incised Sutlej River crosses the southern front of the High Himalaya, thus indicating focused exhumation. The gradual change in AFT ages across and along strike of the major tectonic boundaries (STDS, MCT) in the High Himalaya shows that apparently none of them has been reactivated in the Sutlej area during Pliocene - Quaternary time. Contrary to previous interpretations we suggest that differential uplift is accommodated by widely distributed and pervasive brittle deformation rather than large-scale structures. We therefore conclude that the locus of maximum exhumation coincides with a reorganization of the orogenic wedge in order to compensate the erosional loss of material in the sector with focused precipitation.