Thermal regimes and regional metamorphism in the vicinity of overthrust faults: an example of shear heating and inverted metamorphic zonation from southern California

Thermal models are developed and temperature-depth profiles computed to account for the development of sharply inverted metamorphic zonations and estimated P-T conditions beneath deep-seated thrust faults, with particular reference to the metamorphism of the Pelona Schist-Vincent Thrust system of southern California. Models dependent only upon conductive redistribution of heat from an initially hot upper plate are inadequate to account for the observed temperature distributions and the P-T time constraints can only be satisfied if a total shear heating of 3 hfu or more occurs in the region of the thrusts. These heating rates are consistent with known rates of plate movement, shear stresses along major fault zones, and shear strengths of rocks. Petrological constraints indicate the likelihood of buffering of the maximum temperature attained by shear heating due to marked reduction of shear strengths of rocks upon increase of temperature and release of water during metamorphic dehydration reactions. These models may be of general applicability to inverted metamorphic zonations, notably those beneath obducted Newfoundland ophiolites, and are consistent with our present understanding of the thermal properties of the upper parts of subduction zones.