Active folding of fluvial terraces across the Siwaliks Hills, Himalayas ofcentral Nepal

We analyze geomorphic evidence of recent crustal deformation in the sub-Him alaya of central Nepal, south of the Kathmandu Basin. The Main Frontal Thru st fault (MFT), which marks the southern edge of the sub-Himalayan fold bel t, is the only active structure in that area. Active fault bend folding at the MFT is quantified from structural geology and fluvial terraces along th e Bagmati and Bakeya Rivers. Two major and two minor strath terraces are re cognized and dated to be 9.2, 2.2, and 6.2, 3.7 calibrated (cal) kyr old, r espectively. Rock uplift of up to 1.5 cm/yr is derived from river incision, accounting for sedimentation in the Gangetic plain and channel geometry ch anges. Rock uplift profiles are found to correlate with bedding dip angles, as expected in fault bend folding. It implies that thrusting along the MFT has absorbed 21 +/- 1.5 mm/yr of N-S shortening on average over the Holoce ne period. The +/- 1.5 mm/yr defines the 68% confidence interval and accoun ts for uncertainties in age, elevation measurements, initial geometry of th e deformed terraces, and seismic cycle. At the longitude of Kathmandu, loca lized thrusting along the Main Frontal Thrust fault must absorb most of the shortening across the Himalaya. By contrast, microseismicity and geodetic monitoring over the last decade suggest that interseismic strain is accumul ating beneath the High Himalaya, 50-100 km north of the active fold zone, w here the Main Himalayan Thrust (MHT) fault roots into a ductile decollement beneath southern Tibet. In the interseismic period the MHT is locked, and elastic deformation accumulates until being released by large (M-w > 8) ear thquakes. These earthquakes break the MHT up to the near surface at the fro nt of the Himalayan foothills and result in incremental activation of the M FT.