Rapid Late Pleistocene Frontal Fault Growth and Sutlej Drainage Reorganization in the Western Himalaya

Abstract River-network reorganization is a key process to maintain drainage connectivity in tectonically active mountain ranges. The southern Himalayan front has experienced several drainage diversion events that changed river courses in the hanging wall of the orogenic wedge and controlled growth of mega fans to the south. In this study, we dated Late Pleistocene sediments of the paleo-Sutlej River in the western Himalaya to reconstruct the last river-drainage development prior to the sediment uplift. Late Quaternary faulting along the Himalayan Frontal Thrust (HFT) and associated back-thrusting has uplifted the paleo-Sutlej sediments and preserved them as a low-relief surface on top of the Janauri frontal anticline. New luminescence (n = 11) and paired cosmogenic nuclides (10Be and 26Al) (n = 10) chronologies combined with field mapping and geomorphic analyzes constrain rapid changes of the paleo-Sutlej River. We date widespread Late Pleistocene sediment deposits from ~47 to 30 ky on the presently elevated anticline and in the infilling of a piggyback basin. Cosmogenic burial signals from fan sediments exceed ~0.5 My and suggest transient sediment storage after erosion in the source region and before deposition at the sampled location. The growing anticlinal structure was traversed by a Paleo-Sutlej channel until 26.5 ± 3.5 ky dated by an abandoned fan surface. We are able to constrain the minimum mean uplift rate to 6.5 ± 2 mm/y at the overlapping fault segment boundaries of the anticline. We also identified a second younger erosional surface on top of the anticline (14.8 ± 2.2 ky) that we relate to drainage reorganization due to tilting of the anticline surface during uplift. We propose that a combination of climatically induced sediment transport capability changes combined with tectonic uplift at the HFT is responsible for the course changes of the Sutlej River.