ULTRAMAFIC PSEUDOTACHYLITE FROM THE BALMUCCIA PERIDOTITE, IVREA VERBANO ZONE, NORTHERN ITALY

This paper reports an occurrence of ultramafic pseudotachylite from a spinel Iherzolite mass in the Ivrea-Verbano zone, northern Italy. The pseudotachylite occurs as discordant dikes and veins crosscutting primary structures of the peridotite and as thin (similar to 1 cm) layers in the middle of shear zones. The pseudotachylite has an ultramafic composition similar to the host peridotite. In thin section it consists of fine-grained euhedral olivine and clinopyroxene crystals densely packed in a glassy matrix with minor amounts of elastic olivine grains and lithic mylonitic clasts. Microstructural observations on the olivines using TEM yields evidence for deformation in the wall rock and olivine clasts at a differential stress level of similar to 300 MPa. Petrological analysis of the host rock and pseudotachylite combined with an energy calculation for shear heating suggests that the pseudotachylite originated by a near total melting of the host peridotite followed by rapid crystallization on a time scale of 100 s or less, conditions consistent with an earthquake. The presence of an ultramafic pseudotachylite implies a very significant shear heating due presumably to a high shear stress on the fault plane. Faulting in an ultramafic rock associated with melting will result in a large stress drop and resultant large moment release, because resistance to sliding will almost completely be lost upon melting due to the very low viscosity of ultramafic melts.