Blueschist‐facies metamorphism, shear heating, and P‐T‐t paths in subduction shear zones

Abstract

The low temperatures recorded by blueschist‐facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15–50 km. In this paper, steady state and transient pressure‐temperature‐time (P‐T‐t) paths followed by subducted material are calculated using analytical expressions presented by Molnar and England (1990), supplemented by two‐dimensional numerical calculations. In the absence of shear heating, calculated steady state P‐T‐t paths are very cold, intersecting 1 GPa (10 kbar) at T < 200°C for a wide range of subduction parameters. For the case of constant shear stress (τ) in the subduction shear zone and typical thermal parameters, steady state P‐T‐t paths intersect the blueschist facies for τ = 10–60 MPa and 0–100 MPa for convergence rates of 10 and 3 cm yr−1, respectively. Blueschist‐facies metamorphism will not occur in rapid subduction zones if shear stresses exceed 60 MPa. For the case of shear stress increasing linearly with depth, steady state P‐T‐t paths intersect the blueschist facies when τ = 1–9% P and 0–14% P for convergence rates of 10 and 3 cm yr−1, respectively. Subduction zone shear stresses may be considerably lower, and possibly zero, if blueschist terrains represent material metamorphosed during the early stages of subduction (first 5–10 m.y.) while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation. Subduction zone shear stresses of several tens of MPa (at 35 km depth) or a few percent of lithostatic pressure are consistent with a steady state model of Franciscan metamorphism, a progressive underplating model for the formation of the Pelona Schist inverted metamorphic gradient, and surface heat flow measurements in the NE Japan subduction zone.