Pseudotachylite from the Vredefort Ring, South Africa, and the origins of some lunar breccias
Pseudotachylite veins in the Vredefort impact structure occur in rocks of widely varying composition, and in stratigraphic units as thin as 10 m. The compositions of the pseudotachylites, however, are the same as those of the rocks in which they occur, with some systematic variations, thus indicating in situ formation. The textures of the pseudotachylites indicate that they formed predominantly by mechanical processes, but temperatures were locally high enough to cause thermal metamorphism and possibly fusion. The host rocks of the pseudotachylites are extensively mylonitized and show selective crushing of different mineral species. Greater resistance of quartz and feldspar to crushing partly, but not entirely, explains the systematic differences in composition between host rock and pseudotachylite. Shock features in the host rocks are generally of a low order but indicate pressures far in excess of those reasonably to be expected in crustal rocks deformed by terrestrial processes. Characteristics of the pseudotachylites, breccias formed by one impact, provide simple models for lunar breccias that may have undergone more than one cycle of brecciation.