STRUCTURALLY CONTROLLED TOPOGRAPHY: USING INTEGRATED KINEMATIC AND LANDSCAPE EVOLUTION MODELS TO DECIPHER HOW ELEVATION IS CREATED AND MAINTAINED IN WESTERN NEPAL. (Invited Presentation)

Topography within active mountain belts is a function of the spatial pattern of rock uplift. However, the geometry of faults driving that uplift, and how they have evolved over recent geologic time is still debated. This is particularly true for compressive mountain ranges where high topography and high relief is maintained over 50-100 km. Models of surface processes highlight that topography is created and maintained via uplift along active or recently active surface-breaking faults or fault ramps, and translated topography where the horizontal advection outpaces erosion. We evaluate structural drivers of topographic uplift in western Nepal where the well-defined zone of steep slopes and high relief that marks the high Himalayas in central Nepal splits into two zones; a northern zone ~ 140 km north of the Main Frontal thrust (MFT) and a southern zone ~ 60 km north of the MFT. High topography and relief are typically associated with young apatite (U-Th)/He (AHe) cooling ages. In contrast to expectations, across the southern zone of high relief measured low-temperature cooling ages are all relatively old with both AHe and apatite fission track (AFT) ages exhibiting dates that range from 5-10 Ma. For comparison, AHe and AFT ages in the northern zone are < 1.5 Ma. To characterize the structures that control topographic uplift and exhumation, we integrate fault kinematics from a balanced cross section with both a thermokinematic model (Pecube) and a landscape evolution model (Cascade) to assess if the proposed geometry and associated kinematics can reproduce the observed cooling ages, topography and geomorphic metrics. From north to south, the northern zone of high relief is a combination of uplift over an active ramp, motion on an out-of-sequence (OOS) fault ~ 5 km south of the Main Central thrust, and translated topography that was initially uplifted during growth of the duplex at ~ 5 Ma and an OOS fault at ~3 Ma. The southern zone of high relief is a combination of active (<0.6 Ma), but low displacement, surface breaking and subsurface faults. Elevation of the translated interfluves and topography in the southern zone increases by accounting for orographic rainfall distribution that limits erosion across these regions. In regions of increased rainfall, higher erosion limits topography over recent faulting from the MFT to ~ 50 km to the north and increases relief in the High Himalaya.