INTEGRATING FAULT GEOMETRY AND KINEMATICS INTO LANDSCAPE EVOLUTION MODELS IN CENTRAL NEPAL: A TALE OF TWO SECTIONS

The topographic front of the high Himalaya is markedly linear over an along-strike distance of 400 km despite marked lateral changes in the locations and traces of mapped structures and variations in lithology. To evaluate the lateral continuity of mapped structures, fault geometry and kinematics and the resulting topographic evolution we constructed three cross sections across an along-strike distance of 150 km. We evaluated these cross sections by their ability to replicate mapped surface geology and measured thermochronometric dates. We used the best fit kinematics and geometry of two of these cross sections centered along the Marsyangdi and Budhi Gandaki rivers as inputs into landscape evolution models (using CASCADE) to assess if the proposed geometry and associated kinematics can reproduce the observed topography and geomorphic metrics such as relief, normalized river steepness index (Ksn), and locations and magnitude of short term (ka) erosion rates. Topography is created by uplift over subsurface ramps, horizontal translation of this previous uplifted topography and uplift at both frontal and out-of-sequence (OOS) surface breaking faults. The experiments confirm that displacement along major Himalayan faults control the first-order topography of the orogen observed today. However, long-lived activity on the Main Boundary Thrust (MBT) at the southern edge of the Himalayas inhibited river incision to the north. Marked changes in the width of the orogen to the MBT while it was active, and the number of thrusts that repeat the Siwalk section between the MBT and Main Frontal Thrust (MFT), alters the age and magnitude of river incision between the two sections. These experiments emphasize the importance of OOS and forward propagating faults to facilitate the northward propagation of incision due to temporary inactivity of the Main Boundary Thrust. They also highlight the significance of OOS faulting in promoting the documented increase in high Himalayan topography and relief at PT2. Difference in geometry (location of the MBT) and kinematics (when and where the MBT is reactivated) between these two sections may control the location of where major river systems (such as the Trishuli, Marsyangdi and Seti Gandaki) merge and form the Narayani River, and the ability of the Narayani to breach the zone of MBT uplift.