Abstract

Major pseudotachylyte zones constitute a spectacular component of the renowned c. 2.023 Ga Vredefort impact structure, South Africa, but it has always been difficult to explain how they were formed. In his original account, in 1916, Shand interpreted the pseudotachylyte as due to cataclasis and frictional heating but pointed out two enigmas that have remained since: there were no associated major faults, and the pseudotachylyte volumes he observed were far greater than in similar rocks located within faults elsewhere on Earth. New observations show that the Vredefort pseudotachylyte zones were indeed formed by cataclasis and frictional heating, not by faulting but owing to impact-induced seismic shaking initiated around temporarily loosened blocks in dendritic fracture systems. Progressive cataclasis of such loose blocks by intense, high-frequency oscillations of the country rock at the beginning of the cratering process led to size reduction, rounding and comminution, and frictional melting of feldspars and biotite in the comminuted parts. Most pseudotachylyte was thus not injected from anywhere but produced in situ. The process of seismic shaking is well known from impacts on the Moon and asteroids, terrestrial earthquakes and nuclear tests but has largely been overlooked in terrestrial cratering, except in the theoretical concept of acoustic fluidization.