A Three-dimensional Structural, Thermal, and Dynamical Study of the Arcuate Himalaya

My research investigates the three-dimensional (3-D) evolution of the Himalayan megathrust and its role in controlling mid-lower crustal deformation and plateau growth during intercontinental collision. Integrating surface geologic data, I construct a 3-D structural model of major shear zones for central-western Himalaya that reveals significant along-strike variations in strain accumulated in the mid-lower crust of the orogenic wedge. Comparing the model with geology, coupling along the megathrust, channel steepness, micro-seismicity, thermochronologic data, and topography, I reconcile the coeval development of orogen-parallel extensional features (supra-extensional basin and metamorphic core complex on the orogenic plateau), orogen-normal shortening features (thrust faults and duplexes formed at depth), and along-strike tectonic segmentation in central-western Himalaya (expressed as an embayment in plateau landscape, coupling pattern of the megathrust, and concentrated microseismicity zone). The proposed conceptual model invokes the effects of oblique convergence in an arcuate orogen and rheologic variability in both dip and strike directions of the megathrust. To test its mechanical feasibility, I develop a 3-D visco-plastic creeping mechanical model that simulates the formation of the first-order regional features and investigated the effects of mechanical properties of different parts of an orogenic wedge. The results show that a model with a proper combination of mechanical properties can produce a complex strain distribution pattern required by the formation of the first-order above-mentioned geological features. Different parts of the model are comparable with other representative orogenic areas including the Alps, the Cascade, and the Zagros. Because my conceptual model posits that the along-strike variability in rheology is primarily controlled by the 3-D megathrust geometry, especially the mid-lower crustal ramps, in central-western Nepal, I investigate the 3-D geometry of the megathrust by conducting low-temperature thermochronology analysis coupled with inversions of 3-D thermokinematic modeling. The results confirm the along-strike variation in the megathrust geometry in central-western Nepal and suggest that the models that can produce observation-fit cooling ages require crustal accretion on a mid-lower crustal ramp. By synthesizing other geological data, I further propose an evolution model of the plateau landscape and regional drainage systems, highlighting the determining role of crustal accretion at depth.