Shear heating not a cause of inverted metamorphism

An archetypal example of inverted metamorphism purportedly resulting from shear heating is found in the Pelona Schist of southern California (United States). Recent studies demonstrate that the Pelona Schist was subducted and accreted at the onset of Laramide flat subduction under thermal and kinematic conditions not considered in earlier numerical models. To test the shear heating hypothesis under these conditions, we constructed a thermokinematic model of flat subduction initiation involving continuous accretion of the schist. A neighborhood algorithm inversion demonstrates that available metamorphic and thermochronologic constraints in the Sierra Pelona mountains are satisfied only if accretion rates were 0.2-3.6 km/m.y and shear heating was minimal (shear stress 0-19 MPa). Minimal shear heating is also consistent with an inversion of models constrained by thermochronology of the East Fork (of the San Gabriel River) exposure of the schist. Shear heating inhibits the formation of modeled inverted gradients during accretion and should not be considered an important factor in their generation.