Cooling induced fracturing in impact melt dikes derived from drone photogrammetry and Python simulation: Example from the Lesutoskraal Granophyre Dike in South Africa

Large meteorite impact events produce significant amounts of crustal melt, which can be emplaced as dikes below the crater floor over protracted time periods following the cratering process. Their emplacement is theorized to be controlled by stresses associated with the presence and opening of crustal-scale fractures, hydrostatic pressures associated with the overlying melt sheet, and lithostatic stresses of the impacted crust. At least two compositionally distinct phases of impact melt are present within the impact melt dikes at the Sudbury and Vredefort Impact Structures, underpinning the debated concept of a prolonged and multi-phase emplacement process.

In this study, cooling fractures within the Lesutoskraal impact melt dike at Vredefort are investigated as a possible pathway to facilitate multi-phase emplacement. Through a combination of high-resolution (0.612 mm/pixel) drone orthophotography and numerical simulation of stress induced during cooling of impact melt shows that (1) the dominant fracture orientation within the impact melt dike is parallel to dike margins, related to a perpendicular and tensional cooling stress, and (2) the magnitude of the tensional cooling stress could reach up to −75 MPa, sufficient to overcome the lithostatic stresses at the observed depth of dike emplacement. Depending on simulation parameters such as the initial temperature of the impact melt, cooling fractures in the impact melt are shown to form within 150 days after their emplacement representing a possible mechanism for emplacement of later phases of impact melt into the centre of earlier impact melt phase.