Shock wave-rock interaction: Planar deformation features at the Charlevoix impact structure, La Malbaie, P. Q.

Abstract

First observed in 1962 in samples taken from proposed sites of meteorite impact, POFs have since been defined as multiple sets of microscopic, optical discontinuities lying parallel to rational planes of low Miller indices. Their usefulness lies in the fact that they are unique and unequivocal evidence for meteorite impact. Due to the ubiquitous nature of the rock-forming mineral quartz, POFs have been extensively studied in both naturally and experimentally deformed quartz. Thus far, however, the occurrence of PDFs in other common rock-forming minerals has not been of great concern. Five thin sections from the Charlevoix impact structure in La Malbaie, Quebec were studied using optical microscopy, scanning electron microscopy and electron microprobe techniques. The thin sections constitute representatives of the dominant lithology within the target rocks, anorthosite and charnockitic gneisses, both of which are present in the central uplift. It has been established that, although not as frequent or well-defined as PDFs observed in quartz subjected to the same degree of shocking, POFs do occur in pyroxenes, and appear to be more predominant in the calcium-rich pyroxenes (CPX). PDFs were also observed in magnetite, and appear to be present in such common accessory phases as zircon. Taking into account pressure regimes for the coexisting phases and observed microfeatures, a pressure range of 350-450 KBar was established for the immediate vicinity of meteorite impact. This is a pressure range that roughly corresponds to depths of 1200 km into the earth and pressures up around 4000'C-conditions which would have been achieved in less than a second upon impact.