Clast-size analysis of impact-generated pseudotachylite from Vredefort Dome, South Africa

Abstract

Clast sizes in pseudotachylite from Vredefort Dome, South Africa, which is generally regarded as being of impact-origin, were studied. Clast-size analyses were conducted on the outcrop and thin section scales, and the clast-size distribution is fractal on each scale. On the outcrop scale, the clast-size distribution of the impact-generated pseudotachylite has a small D-value ( D=1.29±0.02). On the thin section scale, the D-values ( D=1.68±0.22 and 1.85±0.13) may be slightly larger than those of fault-generated pseudotachylites and natural fault gouge, and a probable lower fractal limit (≈50 μm) is on the same order as that for fault-generated pseudotachylites. On the thin section scale, proportion of clasts for the impact-generated pseudotachylites is larger than that for fault-generated pseudotachylites, and on the outcrop scale, the impact-generated pseudotachylite has a remarkably high proportion of clasts.