QUANTIFYING DEFORMATION IN THE BHUTAN HIMALAYA: INSIGHTS FROM A THERMAL-KINEMATIC MODEL OF THE TRASHIGANG CROSS SECTION

Reconstructions of the Trashigang cross section in the Himalayan fold-thrust belt of eastern Bhutan are used in a thermal-kinematic model to understand the first order controls on thermochronometer cooling ages and track magnitude, timing, and rates of deformation. We model the cross section with sequential ~10-km deformation steps and apply flexural loading and erosional offloading at each step to develop a high-resolution evolution of deformation, topography and foreland basin development in the fold-thrust belt over time. Variables of topography and kinematic timing of out-of-sequence thrusting along the Kakhtang Thrust affect the model’s intermediate steps in the deformational and exhumational history. We use these flexural models coupled with varying heat production values and velocities of deformation as input in thermal-kinematic model Pecube to forward model apatite fission track, zircon-helium, and white mica ⁴⁰Ar/³⁹Ar cooling ages during fold-thrust belt development. Comparing these data to one another and to published cooling ages along the section, our results reveal that cooling ages are sensitive to (1) variable shortening rates, (2) modeled topography’s ability to account for structural uplift and flexural loading, (3) kinematic timing of fault motion, and (4) cross section geometry. Models maintaining a temporally constant deformation rate do not adequately predict cooling ages that match existing AFT, ZHe, and MAr data, but better data are generated in models that use published estimates of temporally variable shortening rates in the Bhutanese Himalaya. Best fits to the suite of published cooling ages required motion along the Kakhtang Thrust before and after Upper Lesser Himalaya duplex formation, modeled topography that can dynamically account for structural uplift and flexural loading, heat production value of 2.5 μW/m3, and pulses of deformation rates ranging from 74 mm/a to 4 mm/a. Young AFT ages (2-6 Ma) observed north of the Main Central Thrust cannot be matched in even best fitting models. We attribute this misfit to (1) the geometry of the cross section used in this model, and (2) magnitude and partitioning of shortening along the KT that is both kinematically and flexurally different than our models.