Dynamic Compressive Strength and Fragmentation in Felsic Crystalline Rocks

Brittle deformation in rocks depends upon loading rate; with increasing rates, typically greater than similar to 10(2) s(-1), rocks become significantly stronger and undergo increasingly severe fragmentation. Dynamic conditions required for rate-dependent brittle failure may be reached during impact events, seismogenic rupture, and landslides. Material characteristics and fragment characterization of specific geomaterials from dynamic loading are only approximately known. Here we determine the characteristic strain rate for dynamic behavior in felsic crystalline rocks, including anisotropy, and describe the resulting fragments. Regardless of the type of felsic crystalline rock or anisotropy, the characteristic strain rate is the same within uncertainties for all tested materials, with an average value of 229 +/- 81 s(-1). Despite the lack of variation of the critical strain rate with lithology, we find that the degree of fragmentation as a function of strain rate varies depending on material. Scaled or not, the fragmentation results are inconsistent with current theoretical models of fragmentation. Additionally, we demonstrate that conditions during impact cratering, where the impactor diameter is less than similar to 100 m, are analogous to the experiments carried out here and therefore that dynamic strengthening and compressive fragmentation should be considered as important processes during impact cratering. Plain Language Summary When rocks deform quickly, they can behave with properties very different to the properties that would be measured when rocks are deformed slowly. In this study, we have measured the strength of rocks deformed at different rates to find how fast they must be deformed to cause substantial changes to their properties. We chose to look at granitic rocks and gneisses as they are broadly representative of the Earth's continental crust. We found that regardless of the exact rock type, the change from slow to fast deformation occurs at the same rate. When rocks break at fast rates, they break into many small fragments. We have measured the size of those fragments from our experiments to show how the average fragment size changes as a result of the deformation rate and the rock type. In addition, we show that the changes of properties that we see in our experiments is important for the formation of impact craters and potentially earthquake rupture and landslides.