P/T ratio in high-pressure rocks as a function of dip and velocity of continental subduction

Abstract

High-pressure–low-temperature (HP-LT) metamorphic rocks that belong to the same orogen commonly show alignment of their peak pressure and related temperature within a pressure-temperature (PT) diagram, defining a pressure/temperature (P/T) ratio. In the Aegean region, for example, two metamorphic belts of different ages, the Eocene Cycladic blueschists and the Miocene Cretan blueschists, show contrasting P/T ratios that are characterized by the slope and the temperature at 10 kbar (ΔP/ΔT and T@10 kb). Peak PT data for the Cyclades yield a ΔP/ΔT of 0.056 and T@10 kb of 375 °C, while the Cretan blueschists give a much larger ΔP/ΔT with a lower T@10 kb. One-dimensional (1D) modeling of the thermal evolution of a subducted continental margin shows that subduction velocity controls both the slope ΔP/ΔT and T@10 kb of the P/T ratio, and the subduction dip only controls T@10 kb. On these bases, the variations of P/T ratios in the Aegean region reflect variations through time of subduction dip and velocity. Eocene subduction for Cycladic blueschists burial occurred at a rate of 1.5 cm a −1 , while subduction velocity during Cretan blueschists formation is found to be 2.75 cm a −1 . Because the convergence rate between Africa and Eurasia is constant and ~1–1.5 cm a −1 at these times, the active southward rollback of the Aegean slab during the Oligo-Miocene likely explains the larger subduction velocity for the Cretan HP-LT rocks. These results exemplify the use of this new modeling approach as a proxy to quantify dip and velocity of continental subduction from the P/T ratio of high-pressure rocks.