PSEUDOTACHYLYTES GENERATED DURING SEISMIC FAULTING AND ECLOGITIZATIONOF THE DEEP CRUST

Pseudotachylytes are typically interpreted to have formed by frictiona l melting during coseismic faulting within the upper to middle crust. Pseudotachylytes in the Bergen arcs of western Norway contain microlit es including omphacite, garnet, plagioclase, and quartz. This eclogite facies assemblage is stable at temperatures of about 800 degrees C an d pressures of 18 to 19 kilobars, corresponding to depths of 60 kilome ters or more. The pseudotachylytes are exposed in Grenvillian granulit es that locally underwent fluid-induced eclogitization and correspondi ng volume reduction of approximately 10 percent during the Caledonian continental collision. The pseudotachylytes may have formed as a resul t of the rapid relaxation of stresses caused by the eclogitization pro cess.