Evidence for high stress in quartz from the impact site of Vredefort, South Africa

A microstructural investigation of stishovite-bearing quartzite from the Vredefort meteorite impact site in South Africa reveals features that are attributed to shock deformation. These include abundant mechanically induced Dauphine twinning, rhombohedral deformation lamellae and associated residual stresses. Mechanical twins were identified with SEM-EBSD. Residual stresses are derived from equivalent strains that are observed on Laue diffraction images measured with microfocused synchrotron Xrays. Average lattice strains of 1400 microstrains in Vredefort quartz are much higher compared to values (500) found in quartz from granite samples subject to only regional metamorphism. While the granitic quartz shows mainly normal compressive and tensile residual stresses, in Vredefort quartz, shear stresses associated with lamellar boundaries dominate.