Exhumation of Ultrahigh-Pressure Rocks: Thermal Boundary Conditions and Cooling History

Abstract

We investigate the exhumation of ultrahigh pressure (UHP) and high-pressure (HP) rocks in the framework of a dynamic simulation that considers heat advection, heat conduction, heat production, and consequent time-dependent changes in the geothermal gradient. In the absence of lateral heating, rocks exhuming from great depth cool or decompress isothermally and the main cooling period follows the main period of exhumation. Even for a constant exhumation rate, UHP rocks undergo a two-stage cooling history at the end of which the pressure-temperature (P-T) paths of all rocks approach a steady state or “final” geotherm at crustal levels; the shape of the steady-state or final geotherm is mainly a function of exhumation rate. Reconstruction of pressure-temperature-time (P-T-t) paths permits a qualitative distinction between “fast” and “slow” UHP exhumation: fast exhumation is characterized by extremely rapid crustal cooling following small temperature increases or isothermal decompression, whereas slow exhumation is characterized by steady cooling following more modest heating. Rocks exhuming from different depths (e.g., crustal and mantle levels) follow substantially different PT paths (e.g., heating and cooling during decompression), even if all rocks in an orogen are exhumed by the same orogen-scale process. P-T paths of UHP rocks of the Qinling-Dabie-Hong’an area of central China are consonant with our modeling in that the rocks exhumed from the greatest depth show nearly isothermal decompression, whereas more modestly buried rocks underwent heating during exhumation. The shapes of the P-T-t paths suggest an exhumation rate closer to 5 mm/a rather than 1 mm/a, and the apparent two-stage cooling history often interpreted for the Dabie area could have been produced by a single-stage, constant exhumation rate.