Formation and exhumation of ultra-high-pressure rocks during continental collision: Role of detachment in the subduction channel

[1] UHP rocks commonly form and exhume during the transition from oceanic subduction to continental collision. Their exhumation in subduction channels depends on the balance between down-channel shear traction and up-channel buoyancy. Thermal-mechanical upper-mantle-scale numerical models are used to investigate how variations in material properties of the subducting continental margin affect this balance. Changes in shear traction leading to crustal decoupling/detachment are investigated by varying the onset of strain weakening, thermal parameters, and convergence velocity. Variations in buoyancy force are investigated by modifying subducted material density and volume. The model results are interpreted in terms of the exhumation number E, which expresses the role of the pressure gradient, channel thickness, effective viscosity, and subduction velocity. Peak metamorphic conditions, exhumation velocity, and timing of exhumation are temporally and spatially variable and are sensitive to the evolution of E. The models reproduce natural PTt constraints and indicate that neither slab breakoff nor surface erosion is required for UHP exhumation.