Subsurface deformation of experimental hypervelocity impacts in quartzite and marble targets

Two impact cratering experiments on nonporous rock targets were carried out to determine the influence of target composition on the structural mechanisms of subsurface deformation. Projectiles of 2.5 mm diameter were accelerated to ~5 km s⁻¹ and impacted onto blocks of marble or quartzite. Subsurface deformation was mapped and analyzed on the microscale using thin sections of the bisected craters. Additionally, both experiments were modeled and the calculated strain zones underneath the craters were compared to experimental deformation features. Microanalysis shows that the formation of radial, tensile, and intragranular cracks is a common response of both nonporous materials to impact cratering. In the quartzite target, the subsurface damage is additionally characterized by highly localized deformation along shear bands with intense grain comminution, surrounded by damage zones. In contrast, the marble target shows closely spaced calcite twinning and cleavage activation. Crater diameter and depth as well as the damage lens underneath the crater are unexpectedly smaller in the marble target compared to the quartzite target, which is in contradiction to the marble's much weaker compressive and tensile strengths. However, numerical models result in craters that are similar in size as well as in strain accumulation at the end of transient crater formation, indicating that current models should still be viewed cautiously when compared to experimental details.