Extrusion as a mechanism of gneiss dome emplacement, Himalayan orogen: kinematic and vorticity analysis of Gianbul Dome, NW India
I propose to document the mechanism by which Gianbul Dome, a gneiss dome exposed within the western Himalaya Mountain belt, formed and thereby test a model for the formation of gneiss domes during the Himalayan orogeny. A unique characteristic of Himalayan geology is the development of the broadly synchronous, subparallel, opposing slip sense structures: the Main Central Thrust (MCT) and the Southern Tibetan Detachment System (STDS). The MCT is a north-dipping thrust-slip fault zone that accommodated much of the convergence which characterizes the Indian-Asian plate boundary. The STDS is a north-dipping normal-slip fault system that accommodated normal slip parallel to the regional compression direction (Fig. 1). The formation, slip history, and kinematic relationship of these two structures is best simulated by a mid-crustal channel flow model proposed by Beaumont et al (2001, 2004). This finite-element coupled thermal mechanical channel flow model predicts that an ~30 km thick layer of ductile mid-crustal rocks, sandwiched between the MCT below and the STDS above, flowed southward from Asia towards India. Flow was driven by a horizontal gravitational pressure gradient between the high standing Tibetan Plateau and the low lands of India, decreased viscosity within the mid-crust due to the presence of a few percent partial melts, and rapid rates of denudation along the southern front of the Himalayas, (Beaumont et al., 2001, 2004).
The channel flow hypothesis predicts formation of gneiss domes by a number of mechanisms, one of which is extrusion along the channel into the footwall of the STDS (Fig. 2). Such a dome should preserve opposing shear sense across the dome and manifest an exhumation history that youngs south to north. To document deformation patterns predicted to form during dome extrusion, I propose to use microscopic and mesoscopic kinematic and vorticity analyses on samples from mid-crustal rocks exposed in the Gianbul gneiss dome, NW India (Fig. 3). To test the predicted extrusion mechanism method of dome emplacement I will also conduct a cooling history study across Gianbul Dome using 40Ar/39Ar thermochronology. Model predicted doming by other mechanisms, including ramp underthrusting and overburden extension have been tested and supported by studies conducted in the central Himalaya, Tibet region (e.g. Langille et al., 2010; Wagner et al., 2010). However, the extrusion mechanism has not been tested, nor has the application of the channel flow model in a region where convergence between India and Asia is oblique (fig 4). My proposed research will address both issues.
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