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Up the down escalator: Formation and exhumation of (ultra) high pressure rocks

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Ultra-high-pressure (UHP) rocks, exposed in many Phanerozoic collisional orogens, are formed and exhumed during the later stages of ocean basin closure and the early stages of continental collision. The geodynamic processes and mechanisms driving their detachment from the subducting slab and subsequent exhumation are poorly understood. Thermal-mechanical, upper-mantle-scale numerical models provide insight into the detachment and exhumation of UHP material in the subduction channel. The models show that exhumation potential is affected by the competition between down-channel shear traction and up-channel buoyancy forces. The results are be interpreted in terms of the “exhumation number”, E, which expresses the relative contributions of these forces. The value of E depends on the pressure gradient (dominated by density contrasts), channel thickness, effective viscosity, and subduction velocity, and varies with both time and distance along the channel. Although buoyancy is the main driving force for exhumation, viscous strain weakening plays a critical role in the models by first reducing the downward drag and then promoting upward flow of low-viscosity UHP material. In natural systems, other weakening mechanisms may also play a role. During the later stages of exhumation, the models predict the formation of prominent low-angle extensional shear zones at high structural levels, consistent with observations from many natural examples. The models predict pressure-temperature-time paths which are consistent with natural examples.

• Beaumont, C., Jamieson, R.A., Butler J.P., Warren, C.J., 2009, Crustal structure: a key constraint on the mechanisms of ultra-high-pressure rock exhumation. Earth and Planetary Science Letters 287, 116-129. doi: 10.1016/j.epsl.2009.08.001 • Warren, C.J., Beaumont, C., Jamieson, R.A., 2008, Deep subduction and rapid exhumation: the role of crustal strength and strain weakening in continental subduction and ultra-high pressure rock exhumation. Tectonics 27, TC6002 . doi: 10.1029/2008TC002292 • Warren, C.J., Beaumont, C., Jamieson, R.A., 2008, Formation and exhumation of ultra-high-pressure rocks during continental collision: role of detachment in the subduction channel. Geochemistry, Geophysics, Geosystems, Vol. 9, Q04019 , doi: 1029/2007GC001839.

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