Fluid-present melting of the deep crust: implications for the tectonics of large convergent orogens

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The interaction of fluids with rocks in the deep continental crust plays a critical role in the redistribution of mass and energy during the buildup and collapse of mountain belts and the genesis of ore deposits. During the construction of large mountain belts, crustal thickening generates elevated temperatures and pressures in the deep crust. Widespread fluid influx into the deep hot crust beneath mountain belts promotes partial melting, which drastically weakens large regions of continental crust. These melt-weakened regions may act as large-scale shear zones that facilitate mountain buildup or may flow laterally towards orogenic fronts resulting in the formation of high-elevation plateaux. For example, geophysical imaging suggests that large regions of partially molten crust are present beneath the Himalayan mountain belt and the lateral movement of this melt-weakened material is proposed to play a first-order role in the development of the Tibetan Plateau. While widespread fluid-fluxed partial melting may have important tectonic consequences, the sources, compositions and transport pathways of these fluids are poorly constrained. This is due, in part, to the lack of access to the deep crust beneath modern mountain belts.