Shear heating and the state of stress on faults

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Abstract
Evidence for shear heating associated with crustal faulting has been described for a number of fault zones in a wide variety of tectonic environments. The evidence is that a thermal anomaly associated with the fault has resulted in the metamorphism of adjacent rocks to higher grades than can be accounted for by considering other heat sources. A number of such cases are recapitulated here to conclude that if relative displacements across these faults occurred at plate tectonic rates or slower, then shear stresses of a kilobar or so must have existed on these faults in order to produce the observed shear heating. Alternatively, these faults moved at several tens of centimeters per year, much higher than observed past and present plate tectonic rates. These conditions must be maintained over time approaching the thermal time constant, ca. 107 years. Although the case for shear heating in many individual cases is arguable, the evidence in toto strongly suggests that shear stresses of a kilobar must have existed on some crustal faults, and these are minimum estimates. Transient shear heating during dynamic rupture of faults also can produce melting within fault zones. Two interesting cases of large landslides that produced melting suggested that (1) the dynamic coefficient of friction does not vanish after melting but remains at a significant level and (2) the efficiency for melting rock by frictional heating in these cases was probably less than 10%, implying that the amount of frictional heating necessary to produce melting during an earthquake may be an order of magnitude higher than previously thought. As a side issue, a new model for the origin of paired metamorphic belts is proposed.