Vredefort Dome Origin Confirmed as Meteorite Impact Site Using Residual Strain Measured by X-ray Laue Microdiffraction

Abstract:

The Vredefort Dome in South Africa has long been attributed as a meteorite impact scar but it was not until the discovery of planar deformation features, fluid inclusions, and pseudotachylites, all indicative of shock-metamorphism, that meteorite impact 2 b.y. ago became widely accepted as its cause of formation. Prior to this study, however, the orientation of the stress field that created the Vredefort Dome could not be confirmed. In this study, synchrotron X-ray Laue microdiffraction measured at the Advanced Light Source beamline 12.3.2 was used to determine residual strain in shocked metasedimentary quartzite with accessories of stishovite from Vredefort Dome. The residual strain is measured as distortions of the trigonal lattice with X-ray Laue microdiffraction with a < 1µm beam, from which the deviatoric strain tensor is calculated using XMAS and XtalCAMP softwares. The residual strain suggests the primary direction of compression of the metasedimentary quartzite occurred perpendicular to the foliation, which agrees with the direction of proposed meteorite impact, striking the surface of the Earth, as well as the direction of stress needed for positive rhomb alignment by stress-induced Dauphiné twinning observed in previous studies. The results of this study additionally support previous studies showing evidence of shock-metamorphism, indicative of meteorite impact, and further support the concept that this method can be used as a paleo-piezometer to reveal the stress field of deformed rocks.