METAMORPHISM IN LARGE HOT OROGENS: CHANNEL FLOW MODELS AND THEIR APPLICATION TO THE HIMALAYAN-TIBETAN SYSTEM

Thermal-mechanical models for large hot orogens that include channel flow have recently been shown to be compatible with many first-order features of the Himalayan-Tibetan system. In the models, radioactive self-heating and rheological weakening of thickened orogenic crust lead to the formation of a hot, low-viscosity mid-crustal channel and a broad plateau. The channel flows toward the plateau flank in response to the topographically-induced pressure gradient, and is extruded at the edge of the plateau in response to focused surface denudation. New model results are consistent with the overall geometry and tectonic evolution of the Himalayan-Tibetan system, and with a wide array of metamorphic P-T-t data from the central part of the orogen. The model channel is bounded by coeval thrust-sense and normal-sense ductile shear zones, interpreted to represent the Main Central Thrust (MCT) zone and South Tibetan Detachment (STD) system respectively. Hot channel material corresponding to the Greater Himalayan Sequence (GHS) flows outward from the beneath the plateau to the erosion front, where it exhumed and juxtaposed with cooler, newly accreted material corresponding to the Lesser Himalayan Sequence (LHS). Inverted metamorphism associated with the model MCT zone results from the combined effects of distributed ductile shear along the MCT and extrusion of the hot channel. A variety of P-T-t path styles, including isobaric heating, isothermal decompression, and hairpin paths, are produced for points traveling through different tectonic regimes that exist simultaneously in different parts of the model system. P-T-t paths from the model GHS and LHS resemble those observed in nature. The times of peak metamorphism and cooling predicted by the model are generally compatible with observations. In particular, the M1 and M2 metamorphic 'events' observed in the GHS correspond to the model times of maximum burial and maximum heating respectively. Times of erosion of model metamorphic facies agree well with detrital mineral data from the Himalayan foreland. The results highlight the need to integrate tectonic and metamorphic data in models of collision zones, and demonstrate the importance of lateral transport of both heat and material in large hot orogens.