UTILIZING LINKED THERMO-KINEMATIC MODELING AND BALANCED CROSS SECTIONS TO PREDICT AND EVALUATE THE TOPOGRAPHY, ACTIVE FAULT GEOMETRY AND RESULTING THERMAL EVOLUTION OF THE CENTRAL NEPAL HIMALAYA (Invited Presentation)

Sequentially restored, balanced geologic cross sections present a model of how structurally induced uplift varies in space and time. The spatial and temporal evolution of uplift determines the location, rate and magnitude of exhumation. Mineral cooling ages are a common tool to investigate the magnitude and rate of rock exhumation either based on tectonic or climatic drivers, but rarely uniquely differentiate between either process. 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. Changes in thermal properties and velocities alter both the slope and the age of the cooling signal, but neither affects its first-order shape. Using a kinematic model that allows for sequential displacement on structures, a thermal model to calculate the resulting thermal field and thermochronometer age, and a landscape model that depicts the subsequent shape of topography, we show how thermochronologic cooling ages and topographic analyses can be used to determine the geometry of structures, the locations of ramps and rate of faulting. We evaluate three different proposed geometries for the central Nepal Himalaya (hinterland dipping duplex, foreland dipping duplex and out-of-sequence thrust) and evaluate the impact of these geometries on both the predicted topographic evolution as well as the predicted cooling ages. The spatially and temporally varying uplift pattern is input into the TIN-based MATLAB^® landscape evolution model SIGNUM that implements the CASCADE algorithm. The landscape modeling provides quantitative metrics that can be compared to the landscape morphology of the Himalayas. A modified version of the thermo-kinematic model Pecube is used to predict thermochronometer cooling histories based on kinematics, topography, 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.