Tectonic Evolution of the South Tibetan Detachment System, Bhutan Himalaya

Abstract

Syn-convergent low-angle normal-sense detachments (LANDs) are found in many orogens around the world. However, those tectonic processes which result in their formation are little known. The South Tibetan detachment system (STDS) is the best-studied example worldwide of a syn-convergent LAND, and formed in the Miocene due to the continental collision of India and Asia. In Bhutan, eastern Himalaya, the STDS is duplicated. Here I investigate the tectonic history of the inner STDS and particularly the outer STDS in Bhutan, to determine whether the duplicated STDS can be explained by or used to constrain models of Himalayan orogenesis. A range of geochronometric, thermochronologic, petrologic, structural, thermobarometric, thermometric, and isotopic tools are used to constrain: the onset and cessation of motion on the outer STDS; the cessation of motion on the inner STDS; the peak metamorphic conditions in the hanging wall and footwall of the outer STDS; the pressure-temperature-time paths of tectonites in the hanging wall and footwall of the outer STDS; the structural history of the hanging wall rocks of the outer STDS, and; the paleogeographic affinity of the hanging wall rocks of the outer STDS. The results of these studies are compared to thermo-mechanical models of Himalayan- type continental collision. Similarities in model predictions of the type and timing of structures, peak metamorphic conditions of hanging wall and footwall tectonites, pressure-temperature-time paths, and other regional tectonic observations lead to two main conclusions. 1. The STDS is a system of three main types of LANDs: those that formed during channel flow of low-viscosity mid-crustal rocks, those that formed by extrusion of cooled channel rocks to the surface, and those that formed by destabilization of the upper crust above a dome of mid-crustal channel rocks. 2. The STDS was duplicated by underthrusting of a crustal ramp into the Himalayan orogen since early Miocene. The underthrusting led to extrusion of a dome of weak mid-crustal above a previously-extruded channel. The crustal ramp may be local to the eastern Himalaya due to higher convergence and/or erosion rates, or due to local underthrusting of relatively strong crust behind weaker crust.