The Exhumation of Subducted Oceanic‐Derived Eclogites: Insights From Phase Equilibrium and Thermomechanical Modeling

Abstract

AbstractThe dynamical evolution and exhumation mechanisms of oceanic‐derived eclogites are controversial conundrums of oceanic subduction zones. The previous studies indicated that density is the primary factor controlling the exhumation of oceanic rocks. To explore their density evolution, we systematically investigate the phase relations and densities of different rock types in oceanic crust, including mid ocean ridge basalt (MORB), serpentinite, and global subducting sediments (GLOSS). According to the density of eclogites, these currently exposed natural eclogites can be classified into two categories: the self‐exhumation of eclogites (ρMORB < ρMantle) and the carried exhumation of eclogites (ρMORB > ρMantle). The depth limit for an exhumation of oceanic‐derived eclogites solely driven by their own buoyancies is 100–110 km, and it increases with the lithospheric thickness of the overriding plate. The parameters of carried‐exhumation, that is, KGLOSS and KSerp, are defined in order to quantitatively evaluate the assistance ability of GLOSS and serpentinites for carrying the denser eclogites. KGLOSS is mainly controlled by pressure, whereas KSerp is dominantly affected by temperature. Using 2‐D thermomechanical models, we demonstrate that the presences of low‐density, low‐viscosity GLOSS and seafloor serpentinites are the prerequisites for the exhumation of oceanic‐derived eclogites. Our results show that oceanic‐derived eclogites should be stalled and exhumed slowly at the Moho and Conrad discontinuities (named Moho/Conrad stagnation). We propose that oceanic‐derived eclogites should undergo a two‐stage exhumation generally, that is, early fast exhumation driven by buoyancy at mantle levels, and final exposure to surface actuated by tectonic exhumation facilitated by divergence between upper plate and accretionary wedge or by rollback of lower plate.