The Mechanics of Pseudotachylite Formation in Impact Events

Abstract

This paper presents a discussion of the basic constraints controlling the formation of pseudotachylites in the rapidly sheared rocks in the vicinity of a large meteorite impact. The prevailing opinion among many geologists is that pseudotachylites are formed by friction melting of rocks and/or shearing associated with differential shock compression of adjacent rock types. Several physical studies of friction melting have shown that, in theory, small amounts of movement (centimeters or less) are capable of producing very thin veins of melted rock. More realistic models suggest that irregularities on the sliding surface of the order of the grain size may still create primary melt veins up to a few millimeters thick. The principal mystery of pseudotachylite formation is not that friction can cause melting, but that it seems to form thick masses of it, meters to tens of meters wide. However, such thick masses ought to preclude melting by reducing the friction between sliding rock masses. I propose that one possible solution to this conundrum is that the melt produced by sliding on narrow shear zones is extruded into the adjacent country rock, thus keeping the sliding surfaces narrow, while creating thick accumulations of melt in adjacent low pressure zones that open at the end of the shear zones. For this mechanism to operate, the melted rock must be fluid enough to extrude from the shear zone during the time available during crater collapse. This places strong constraints on the viscosity and temperature of the melt. This model may be tested by future careful investigation of the geometry of pseudotachylite occurrences.