SHEAR STRESS AND NEGATIVE FEEDBACK IN SUBDUCTED CONTINENTAL MARGIN, IMPACT ON HIGH-PRESSURE, MODERATE-TEMPERATURE METAMORPHISM IN THE ORDENES COMPLEX, GALICIA, NW SPAIN

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 ≈ 90 km perpendicular to strike and include rocks that reached depths from 30 to 60 km. Estimates of P-T conditions of rocks found near the boundary between overriding and subducting plates range from 430 °C at 1.0 GPa to 520 °C at 1.65 GPa. Structural reconstructions including these data indicate an angle of subduction ≈ 18°.

A mathematical solution has been used to model 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 ≈ 55 MPa (constant shear) or ≈ 7.5 % of pressure (constant coefficient of friction) assuming a convergence rate of 10 cm yr^-1. At greater depths negative feedback between temperature and shear stress caused the system to approach steady state conditions with decreasing shear stress and nearly constant temperature. The decrease in shear stress at temperatures above 400 °C occurs as the rheological properties of the rock at higher temperature and (or) pressure allow more plastic behavior. A comparison of results with estimates of shear stress inferred from seismicity and heat flow at active convergent boundaries in the Pacific suggests that shear stress is best described as a pressure-dependent variable not as a constant shear stress. Furthermore, evidence of feedback causing a significant decrease in shear stress if temperature exceeds 400 °C implies that individual exposures of high-pressure, moderate-temperature rocks can be used to estimate average shear stress affecting the subducted rocks but do allow an estimate of actual shear stress at any point along the underthrust.