Shear stress in subducting continental margin from high-pressure, moderate-temperature metamorphism in the Ordenes Complex, Galicia, NW Spain

Abstract

The Ordenes Complex, Galicia, NW Spain, preserves high-pressure, moderate-temperature metamorphism in continental margin rocks subducted during closure of the Rheic Ocean in the Variscan orogeny. The exposures extend across c90 km perpendicular to strike and include rocks that reached depths of 30 to 60 km. Estimates of P–T conditions of rocks found near the boundary between overriding and subducting plates range from 430 8C at 1.0 GPa to 520 8C at 1.65 GPa. Structural reconstructions including these data indicate an angle of subduction between 15 and 308. A mathematical solution and numerical models have been used to estimate shear heating experienced by this well-exposed paleo-subduction zone. Best fit of model to thermobarometric results occurs if shear stress in the upper reaches of the fault separating subducting and overriding slabs was c100 MPa (constant shear) or c10.0% of pressure (constant coefficient of friction) assuming a convergence rate of 6 cm year 1. At greater depths negative feedback between temperature and shear stress caused the system to approach steady state with decreasing shear stress and with little increase in temperature. The decrease in shear stress at temperatures above 400 8C occurs as the rheological properties of the rock at higher temperature and (or) pressure allow more plastic behavior. This suggests that high-temperature metamorphism is unlikely to occur in subducting crust without special circumstances. A comparison of these results with estimates of shear stress inferred from seismicity and heat flow at active convergent boundaries in the Pacific indicates that shear stress is best described as a pressure-dependent variable not as a constant shear stress.