Frontal fault growth and megafan construction control drainage development in the western Himalaya

The evolution and course of Himalayan rivers when exiting the orogen is controlled by the interplay between tectonics, climate, and associated sediment flux. We investigate these interactions by studying a Late Pleistocene deflection of the Sutlej River at the southern margin of the western Himalayan. This part of the Himalaya is also referred to as Kangra Recess. Late Quaternary faulting and folding along the Main Frontal Thrust and related back thrusts has created anticlinal structures in the south and piggyback basins in the north. Combined field observations and chronological constraints have shown that the anticline evolved as multiple fault segments, which grew through lateral propagation and led to the permanent deflection of the Sutlej River by ~ 50 km to the southeast. In this work, we present new luminescence and cosmogenic nuclide chronologies combined with previously published data to better identify the sedimentation history. Most importantly, we focus on the cause and final timing of the permanent river deflection. We show evidence for widespread aggradation and sediment deposition by the Sutlej River megafan and its tributaries starting before 47 ka and continuing until ~ 26 ka. Our ¹⁰Be and ²⁶Al results in combination with available OSL data document the last widespread throughflow of the Sutlej at ~ 30-25 ka. We argue that a combination of climate and tectonic factors, especially the variability of monsoonal strength, led to major changes in sediment supply at short time scales and therefore affected the course of the Sutlej River system.