Decoding the rock record of near-surface fragmentation processes: Experimental constraints on the strain rate dependence of tensile rock fragmentation

Abstract:

Dynamic fragmentation of rocks occurs at a range of strain rates in surface processes including landslides, earthquakes, volcanic eruptions, and impact cratering. Fragment populations and fracture arrays formed during these processes contain important information about their origins. However, the majority of current rock fragmentation data is derived from compression experiments where fragment populations are easily generated and measured. In contrast, many processes that drive rock fragmentation in nature involve rapid expansion and associated transient tensile stress states. Most common tension experiments such as the direct tension and Brazilian disk test are purposely-designed to measure tensile strength, but do not produce fragment distributions consistent with those found in nature. We investigate the strain rate dependence of tensile fragmentation under minimal confining pressures using a modified sample configuration for a Split Hopkinson Pressure Bar (SHPB). The sample configuration consists of a rock disk bonded to two outer disks composed of more compliant material. When the sample configuration is loaded axially, radial strain in the outer disks produces transversely isotropic tension in the inner disk. We compare the resulting damage fabrics produced in these experiments to field analogs using petrography, computed tomography, and scanning electron microscopy. Our experiments produce a bimodal fragment population: one population consisting of large polygonal fragments and the other comprising smaller fragments associated with individual fracture process zones. Energy based fragmentation models can only be used to predict the fragment distribution of the large polygonal fragments. The frequency and spatial distribution of smaller fragments is controlled by factors such as local mineralogy and microstructures, which are not accounted for in energy based fragmentation models. The results of this study show that fragmentation under isotropic tensile loading conditions is independent of strain rate at strain rates less than ~10³ s^-1. These modified SHPB experiments offer new insights into the formation of fault damage zones, fragmentation in landslides and volcanism, and deformation produced during impact cratering.