Experimental investigation of incipient shear failure in foliated rock

Abstract

It has long been known that rock fabric plays a key role in dictating rock strength and rheology throughout Earth's crust; however the processes and conditions under which rock fabric impacts brittle failure and frictional strength are still under investigation. Here, we report on laboratory experiments designed to analyze the effect of foliation orientation on the mechanical behavior and associated microstructures of simulated fault rock sheared at constant normal stress of 50 MPa. Intact samples of Pennsylvania slate were sheared with a range of initial fabric orientations from 0 to 165° with respect to the imposed shear direction. Foliation orientations of 0–30° produce initial failure at the lowest shear stress; while samples oriented at higher angles are less favorable resulting in higher peak failure strength. In all samples, post-peak development of through-going deformation zones in the R1 Riedel orientation results in a residual strength that is lower than that observed for powdered gouge from the same material. As shear strain increases, all samples approach a residual apparent friction, defined by the ratio of shear strength to normal stress, of ∼0.4. However, the final deformation microstructure depends strongly on the orientation of the pre-existing foliation. When fabric is oriented at low angles to the shear direction (<60°) slip occurs on the pre-existing foliation. Passive rotation of spectator regions becomes apparent for samples with foliation orientation of 60–90°. At higher foliation angles, deformation typically occurs in wide, through-going zones at an R1 orientation that cross-cuts the fabric or in a P-orientation along the fabric. These samples typically exhibit greater geometric layer thinning per unit shear strain. Our results document how pre-existing foliation, in any orientation, can lower the residual shear strength of rock. In contrast, the initial yield strength and peak failure strength of foliated rock is often higher in comparison with powdered samples of the same material, due to strength anisotropy and because activating slip on pre-existing foliation requires dilation and associated work against normal stress. The relationship between rock fabric orientation and frictional strength evolution that we document can explain how incipient faulting and/or flexural slip occur in foliated rock, especially when the orientations of fractures and original fabric vary widely.