Subsurface deformation in hypervelocity cratering experiments into high-porosity tuffs

Hypervelocity impact experiments on porous tuff targets were carried out to determine the effect of porosity on deformation mechanisms in the crater's subsurface. Blocks of Weibern Tuff with about 43% porosity were impacted by 2.5mm and 12.0mm diameter steel spheres with velocities between 4.8kms(-1) and 5.6kms(-1). The postimpact subsurface damage was quantified with computer tomography as well as with meso- and microscale analyses of the bisected crater subsurface. The intensity and style of deformation in mineral clasts and the tuff matrix were mapped and their decay with subsurface depth was determined. Subsurface deformation styles include pore space compaction, clast rotation, as well as microfracture formation. Evaluation of the deformation indicates near-surface energy coupling at a calculated depth of burial of similar to 2 projectile diameters (d(p)), which is in conflict with the crater shape, which displays a deep, central penetration tube. Subsurface damage extends to similar to 2 d(p) beneath the crater floor in the experiments with 2.5mm projectiles and increases to similar to 3 d(p) for 12mm projectiles. Based on overprinting relationships and the geometrical orientation of deformation features, a sequence of subsurface deformation events was derived (1) matrix compaction, (2) intragranular crack formation in clasts, (3) deformation band formation in the compacted matrix, (4) tensile fracturing.