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 prim ary structures of the peridotite and as thin (similar to 1 cm) layers in the middle of shear zones. The pseudotachylite has an ultramafic co mposition similar to the host peridotite. In thin section it consists of fine-grained euhedral olivine and clinopyroxene crystals densely pa cked in a glassy matrix with minor amounts of elastic olivine grains a nd lithic mylonitic clasts. Microstructural observations on the olivin es using TEM yields evidence for deformation in the wall rock and oliv ine clasts at a differential stress level of similar to 300 MPa. Petro logical analysis of the host rock and pseudotachylite combined with an energy calculation for shear heating suggests that the pseudotachylit e originated by a near total melting of the host peridotite followed b y rapid crystallization on a time scale of 100 s or less, conditions c onsistent with an earthquake. The presence of an ultramafic pseudotach ylite implies a very significant shear heating due presumably to a hig h shear stress on the fault plane. Faulting in an ultramafic rock asso ciated with melting will result in a large stress drop and resultant l arge moment release, because resistance to sliding will almost complet ely be lost upon melting due to the very low viscosity of ultramafic m elts.