Deformation of Brittle Clasts in a Viscous Matrix: Implications on Deformation Dynamics

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 viscous and brittle failure coexist, deformation is described as semi-brittle and both creep and stick slip are plausible. The Papoose Flat Pluton is an example of a semi-brittle system where strain has been accommodated by the viscous (quartz and mica rich matrix) and brittle (feldspar clasts) phases. In order to investigate how a failing brittle phase impacts deformation dynamics within semi-brittle environments, we use the Papoose Flat Pluton as an analog. Theoretical models suggest that the deformation of brittle clasts within a viscous matrix is controlled by the competency contrast between the viscous and brittle components as well as their relative concentrations. Mineralogy is consistent throughout the pluton suggesting that the relative abundance of the clasts is responsible for changes in strain accommodation within the brittle phase. This allows us to systematically determine whether there is a critical concentration at which the brittle phase begins to accommodate strain. Preliminary data show that as the concentration of brittle clasts decreases, the degree of fracturing within them decreases as well. This suggests that there is a clast concentration threshold at which the clasts will no longer exhibit fractures or contribute to the total strain. The presence of a critical concentration should result in deformation dynamics that diverge from pure creep. Laboratory experiments using analog materials are being conducted to further investigate concentrations of brittle clasts and their impact on deformation dynamics. The study of Papoose Flat Pluton in combination with our laboratory experiments will illuminate whether deforming semi-brittle systems can lead to deformation dynamics that show characteristics of both creep and stick-slip. Semi-brittle materials can be found not only in plutons but also in large scale faults and subduction zones. Whether such systems move via constant creep or stick-slip has a significant impact on their ability to produce destructive earthquakes.