SHEAR HEATING IN SUBDUCTION ZONES: IMPLICATIONS FOR THERMAL MODELS AND METAMORPHIC P-T PATHS

Popular thermal models of subduction systems are 100-500 °C colder at ~50 km depth than the pressure-temperature (P-T) conditions recorded in exhumed metamorphic rocks. This difference has been ascribed by some to a profound bias in the metamorphic record (e.g. exhumed rocks reflect subduction of anomalously warm crust, not normal subduction). Accurately determining subduction zone thermal structure, whether from real rocks or models, is crucial for predicting the depths of seismicity, fluid release and sub-arc melting conditions. In this study, we demonstrate that adding realistic shear stresses to analytical and numerical thermal models retrieves P-T conditions consistent with the rock record, suggesting that P-T conditions retrieved from real rocks are indicative of normal conditions.

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.