Determining impact angle from the spatial distribution of shock metamorphism: A case study of the Gosses Bluff (Tnorala) impact structure, Australia

Abstract

The majority of planetary impacts occur at oblique angles. Impact structures on Earth are commonly eroded or buried, rendering the identification of the direction and angle of impact—using methods such as asymmetries in ejecta distribution, surface topographic expression, central uplift structure, and geophysical anomalies—challenging. In this study, we investigate the potential of spatial asymmetries in shock metamorphism intensity to act as a quantitative constraint on the direction and angle of impact at the Gosses Bluff structure in Northern Territory, Australia. We measured the frequency of specific orientations of planar deformation features in quartz from nine samples around the central uplift and compared the spatial asymmetries in observed peak shock conditions with predictions from new three-dimensional numerical impact simulations of the formation of the Gosses Bluff structure. This comparison indicates formation by an impact along an approximately N→S trajectory at an angle of 52° ± 10°. The direction agrees with previous independent identification of structural asymmetry at the crater, although an attempt to constrain the impact angle has not been previously conducted. Alongside a trend of an increase in shock pressure recorded by down-range target rocks, we also observe a marked increase in shock metamorphism in the cross-range direction at Gosses Bluff. We attribute this pattern to the movement of faults in the central uplift during crater modification, displacing and dissecting the originally smooth distribution of shock metamorphism. This study provides new guidance for identifying and quantifying oblique impacts in the rock record, which is applicable to a large range of impact angles and crater sizes.