Direct calculation of rupture depth for an exhumed paleoseismogenic fault from mylonitic pseudotachylyte

Abstract

Multiple generations of pseudotachylyte within granulite-facies orthogneiss (northern Norway Caledonides) are cut by ultramylonitic shear bands that contain the mineral assemblage garnet + clinopyroxene + plagioclase + quartz. Mineralogic thermobarometry yields pressure (P) = 950 ± 150 MPa at temperature (T) = 625–675 °C for equilibration conditions of the ultramylonite assemblage, requiring minimum depths of 35 ± 5 km for coseismic slip that generated pseudotachylyte. These depths are well below the base of the standard seismogenic zone (15–20 km). Ambient temperatures are 200–300 °C higher than those commonly cited for the onset of ductile creep in quartzofeldspathic rocks, but they are similar to those in modern intraplate settings of higher than average heat flow that have experienced deep crustal earthquakes. The occurrence of overprinting mylonites and pseudotachylytes requires tens of kilometers of dynamic downward rupture propagation from the brittle-frictional regime into the ductile regime. Plastic instabilities do not explain the relationship between ultramylonites and mylonitized pseudotachylytes in this setting.