SHEAR HEATING IN SUBDUCTION ZONES: IMPLICATIONS FOR THERMAL MODELS AND METAMORPHIC P-T PATHS
We derive realistic effective coefficients of friction (μ) from heat flow measurements of modern fore-arcs that indicate a range of μ from 0.025 to 0.1. We include these μ values in an analytical model of subduction zone interface temperatures for an average plate that is 50 Myr old, and subducting at 6 cm/yr with a central Chilean geometry. This model retrieves temperatures at 50 km depth that are ~200-700 °C warmer than models without shear heating. At high temperatures, however, thermal softening will reduce shear heating. Adding thermal weakening to our analytical model produces concave-upward P-T paths that are 100-500 °C warmer than models with no shear heating. The P-T conditions and concave-upward shape of the shear-heating + thermal softening models closely matches the distribution of P-T conditions derived from a compilation of exhumed rocks. Recent numerical models of modern subduction zones that include shear heating also retrieve P-T conditions consistent with the metamorphic rock dataset. Thus, exhumed metamorphic rocks record the conditions of normal, not anomalous, oceanic subduction. As a result, numerous geochemical, petrologic, and geophysical interpretations that have been founded on models that lack shear heating must be re-evaluated. More specifically, P-T paths for paleosubduction zones constructed based on thermal models without shear heating often feature a component of isobaric heating that is rare in models that include shear-heating+thermal weakening. These paths must be reconsidered.