Abstract

The distribution of shock veins, friction melts and cataclastic rocks in complex impact craters reflects the response of target lithologies to varying rates of strain. Within the Sudbury impact structure, thin (<2 mm), anastomosing veins, which can define shatter cone surfaces, permeate the target rocks in a c. 15 km wide zone around the Sudbury igneous complex (SIC). A similar relationship exists within the Vredefort impact structure, with the additional association of the high-pressure SiO₂ polymorphs coesite and stishovite. These S- (shock-dominant) type pseudotachylytes are comparable with shock veins developed in meteorites. Both are considered to form primarily by shock compression-decompression during the contact and compression stage of the collision process. Larger, thicker (up to 1 km wide) friction melt bodies constitute pseudotachylytes formed by the extreme comminution of fault walls during rebound and gravitational collapse of the transient cavity. They also appear to define the concentric fault systems of multi-ring impact basins. These E- (endogenic-) type pseudotachylytes form during the modification stage of the cratering process and post-date S-type pseudotachylytes. At Sudbury, they occur up to 80 km beyond the SIC. E-type pseudotachylytes are formed by the same mechanism as pseudotachylytes in non-impact-related fault systems: frictional melting of fault walls during seismogenic slip. Those in impact structures can be large because the extreme displacements (several kilometres) facilitated by superfaults generate massive volumes of friction melt.