Generation of fragment-rich pseudotachylite bodies during central uplift formation in the Vredefort impact structure, South Africa

Abstract

Target rocks underlying the central portions of large terrestrial impact structures are characterized by the pervasive presence of fragment-rich pseudotachylite bodies. Debates regarding the formation of these bodies include the origin of pseudotachylitic melts, i.e., friction- versus shock-induced melting, melt mobility, causes of target rock fragmentation, and timing of fragmentation and melt emplacement with respect to stages of cratering. Comprehensive structural analysis of pre-impact mineral fabrics and properties of fragment-rich pseudotachylite in the Vredefort Dome suggests that melt is allochthonous and was emplaced at an advanced stage of cratering into tensional fracture zones within the crater floor. Both concentration of bending strains imparted on target rocks during central uplift formation and thermal stresses induced by the emplacement of allochthonous melt led to fragmentation of target rock. Tensional fracture zones formed in an overall dilational strain field towards the end of cratering, likely during collapse of the central uplift, and constitute low pressure zones, into which melt was forcefully drawn. Melt may have been drained from the overlying impact melt sheet or from sites within the crater floor and transported into fragment-rich dilation zones. Our field-based analysis failed to identify the presence of bona fide shear faults that could potentially have generated in situ frictional melts and fragments. Rather, target rock fragmentation and melt generation that resulted in fragment-rich pseudotachylite bodies are processes separated in space and time during cratering.