Oceanic- and continental-type metamorphic terranes: Occurrence and exhumation mechanisms

Understanding the fate of subducted materials has important implications for the chemical and physical differentiation of the Earth, particularly the compositional evolution of the continental crust. Of interest here is how deeply-subducted materials return to the Earthʹs surface. We present a comprehensive global compilation of high-pressure, low-temperature metamorphic terranes for which peak metamorphic conditions have been constrained. These metamorphic terranes are classified based on tectonic setting: terranes in oceanic plate subduction zones were classified as oceanic-type and those in continent–continent collision zones were classified as continental-type. We show that oceanic-type terranes form under pressures less than ~ 2.7 GPa whereas continental-type terranes develop under greater pressure and slightly higher prograde geothermal gradients. Whereas these two terrane types probably share common descent paths (i.e. subduction), their separation in pressure–temperature space suggests that the mechanism and pathways of their exhumation likely differ. Here we present a simple buoyancy-driven model to explain the bifurcation of subducted material at depth and how exhumation regimes may change in different tectonic settings during the evolution of convergent margins. We explore two exhumation modes. In one, the hydrous nature of subducted sediments leads to a low-density, low-viscosity channel bounded by relatively rigid walls, thereby driving channel-like flow along the dipping slab surface. In the other mode, channel viscosity approaches that of the overlying mantle wedge, preventing channel flow but permitting vertical exhumation via diapirism. We show that the exhumation mode depends on slab dip and the viscosity ratio between the buoyant material and the overlying mantle (described by a dimensionless parameter, M). Due to a significant change in channel viscosity with the breakdown of hydrous minerals, we suggest that the transition in exhumation mode coincides with slab dehydration; at what depth this transition occurs depends on plate velocity and the initial thermal state of the slab. Such a model predicts channel flow to be limited to shallow depths and diapiric exhumation to greater depths, providing an internally consistent explanation for the apparent differences in peak metamorphic conditions of oceanic- and continental-type terranes if the former exhume via channel flow and the latter via diapirism. Because young, hot slabs dehydrate at shallower depths than old, cold slabs, the maximum depth to which channel flow can operate is greater in the latter. Finally, our model also predicts how the exhumation mode changes as the nature of subduction zones evolves during the closure of an ocean basin, beginning with oceanic plate subduction and culminating in continent–continent collision. In such a scenario, channel flow is favored during the subduction of dense, steeply dipping oceanic lithosphere, but a growing continental (low density) character to the subducted materials as the ocean basin closes should gradually shift the mode of exhumation to diapirism.