DETERMINING FAULT GEOMETRY FROM MATCHING PREDICTED AND MEASURED COOLING AGES AND TOPOGRAPHIES; MELDING CROSS SECTION KINEMATICS, THERMAL MODELING AND LANDSCAPE EVOLUTION IN CENTRAL NEPAL

The magnitude and location of rock uplift and the resulting erosion sets both the age and distribution of thermochronometers as well as the height, relief and morphology of the topographic surface. One of the primary goals of tectonic geomorphology is to understand the degree to which topography of active mountain ranges reflects subsurface deformation. Our growing understanding of the geomorphic processes that govern the redistribution of mass at the surface provides new opportunities to quantitatively tie the geomorphic signal of rock uplift to the geometry and displacement history of structures producing that uplift. Sequentially restored, balanced geologic cross sections present a model of how structurally induced uplift varies in space and time as well as a predicted geometry of the active, modern fault. This spatial and temporal evolution of uplift determines the location, rate and magnitude of exhumation. To evaluate the effect of fault geometry and kinematics on cooling ages and topography we use balanced cross sections to create an evolving sub-surface structural geometry. The sub-surface geometry (location and magnitude of modern and past fault ramps) controls the locations of uplift and exhumation and thus the across-strike pattern of cooling ages for any given thermochronometer system. This kinematic model is tied to both a thermal model to calculate the resulting thermal field and thermochronometer age, and a landscape model that depicts the subsequent shape of topography. We evaluate a suite of different proposed geometries for the central Nepal Himalaya and evaluate the impact of these geometries on both the predicted topographic evolution as well as the predicted cooling ages. A modified version of the thermo-kinematic model Pecube is used to predict thermochronometer cooling histories based on kinematics, topographic, thermal parameters and shortening rates. We then match the pattern of predicted ages with the across strike pattern of measured muscovite ⁴⁰Ar/³⁹Ar, zircon (U-Th)/He, and apatite fission-track cooling ages. The spatially and temporally varying uplift pattern is input into the landscape evolution model CASCADE. The landscape modeling provides quantitative metrics that can be compared to the landscape morphology of the Himalayas.