Structural variations in basal decollement and internal deformation of the Lesser Himalayan Duplex trigger landscape morphology in NW Himalayan interiors

Abstract. The Kishtwar Window (KW) of the NW Himalaya exposes the northwestern termination of the orogen-parallel anticlinal stack of thrust nappes, termed as the Lesser Himalayan Duplex and its evolution portrays rapid exhumation at least over the last 2–3 Myr. However, speculations remain if it still actively deforming. Here we combine morphometric analyses with structural and field evidences to describe the spatial pattern of internal deformation of the duplex. We suggest that the variations in the geometry of the basal décollement, the Main Himalayan Thrust (MHT) and internal faulting within the duplex define the observed neotectonic deformation. We recognize two significant steep stream segments/ knickzones, one in center of the window, and a second one along its western margin, which we relate to fault-ramps emerging from the MHT. The larger of the knickzones, in the core of the window, show an increase in the angle of foliations towards downstream. Highly-fractured and folded rocks at the base of the steep stream segment, suggest internal deformation of the duplex, possibly linked to surface-breaking thrust fault-ramp at the core of the duplex. The second steepened knickzone coincides with the western margin of the window and is identified by a narrow channel through a comparatively weaker bedrock gorge. Summarizing our findings, we favor a structural and active tectonic control on the growth of the duplex even over geomorphic timescales. Corroborating with previous studies, we suggest that the differential uplift and growth of the duplex is linked to several flat-ramp structures along the MHT.