Deformation of brittle clasts within a viscous matrix: Field and experimental observations

Abstract

Rock deformation is often described by either viscous or brittle failure. These end-member cases lead to deformation dynamics of creep and stick-slip, respectively. When a viscous and brittle phase coexist, deformation can be described as semi-brittle and deformation dynamics may diverge from the end-member cases. For example, the deformation of semi-brittle rock is a proposed explanation for slow slip events. Energy transfer between the seismic and aseismic zone is a potential consequence of these events, conceivably intensifying or alleviating stresses that are responsible for earthquakes. Natural examples of semi-brittle deformation often include a fractured brittle phase, but its influence on deformation dynamics is not well established. We conduct field and laboratory experiments to investigate the impact of a failing brittle phase on semi-brittle deformation. Observations made at the Papoose Flat pluton use fractured feldspar clasts embedded in a quartz/mica rich matrix to show that there is a weak relationship between clast concentration and the degree of brittle failure. Fracturing is observed at clast concentrations of 2-12% implying that stress transfer between the viscous matrix and brittle clasts can impose differential forces that lead to fracturing. To investigate the deformation dynamics of a semi-brittle systems where the brittle phase can fracture, we conduct physical experiments using mixtures of a viscous gel (Carbopol) and brittle beads (HydroOrbs) within a ring shear apparatus. Carbopol only experiments represent purely viscous systems and HydroOrb only experiments represent brittle/granular systems. Semi-brittle experiments use a mixture of Carbopol and HydroOrbs with 25% and 64% HydroOrbs by volume. The presence of a viscous phase heavily impacts the deformation dynamic by dampening stick-slip signals originating from frictional sliding and fracturing of the brittle phase. Experimental findings support field evidence that the viscous phase can impose differential forces capable of fracturing the brittle phase.