EXPERIMENTAL ANALYSIS OF SEMI-BRITTLE RHEOLOGY: IMPLICATIONS FOR STRAIN TRANSIENTS

Field observations indicate that both brittle fracture and ductile flow occur simultaneously at microscopic to lithospheric scales. The interaction and co-occurrence of these behaviors may induce a wide range of deformation dynamics ranging from stick-slip to creep and may play a major role in the occurrence of strain transients in the middle crust. In order to investigate how slip dynamics are related to fracture mode and rheology, we performed physical experiments investigating semi-brittle deformation using a visco-elasto-plastic material, Carbopol, as an analog that will both fracture and flow. We varied the yield stress of Carbopol as a function of concentration to test its impact on fracture propagation and distribution. We performed experiments on a distributed simple-shear apparatus with energy conserving boundary conditions that do not impose strain-rate or force where we record force and displacement changes during deformation. We cut a preexisting shear fracture parallel to shear to localize mode I (extensional) fractures. Carbopol with a low yield stress developed S-C structures and resulted in creep-dominant behavior with no fractures. Experiments with moderate yield stresses were shown to have both creep and stick-slip with the presence of fractures and anastomosing ductile shear bands. High yield stress corresponded with many through-going fractures and larger, more frequent stick-slip events. We interpret that slip events are linked to the interaction of mode I fractures and mode II (shear) fractures. These results support that yield stress is a control on slip behavior in semi-brittle rock.