Evolution of Deformation Mechanisms during Strain Localization in the Scituate Granite, Rhode Island, USA

Abstract:

The rheology of the ductile middle and lower crust affects the rate and intensity of aftershocks following major seismic events. Ductile deformation in the middle crust is commonly localized in narrow shear zones, which requires a process that causes strain weakening to operate in order to form these shear zones. To determine the mechanisms that cause strain localization in a common crustal rock, microstructures were analyzed in and around a shear zone found in the Scituate Granite. The granite, located in central RI, was emplaced in the Devonian period and was deformed during the Alleghenian orogeny. Evolution of shear zone growth was observed through optical and scanning electron microscope (SEM) microstructure analyses of three different deformed areas in the granite: incipient, centimeter-scale, and through-going shear zones.

Microstructures in the incipient shear zone (foliated granite) include undulatory extinction and recrystallized grains at the edges of porphyroclasts of albite, K – feldspar and quartz. By measuring the grain size of the porphyroclasts and undeformed biotite grains in the foliated granite, over a 50mm x 72mm petrographic slide, the amount of recrystallization in the feldspar and quartz phases found in the host rock was determined to be 35%. In the centimeter-scale shear zone, microstructures include fine-grained sometimes-mixed phases, including numerous four-grain junctions and most biotite grains within the mixed phase domains aligned parallel with the shear zone edge. In the through-going shear zone microstructures include an increase in the population of four grain junctions, aligned biotite grains and mixing of all phases. These microstructures indicate that the host granite was deformed by dislocation creep, which produced fine grains at porphyroclast boundaries. These fine grains start to deform by diffusion creep - accommodated grain boundary sliding processes, which locally weaken the rock and form shear zones. The change in microstructures indicate a transition from dislocation creep to diffusion creep as the shear zone evolves. These results indicate that the rheology of the localized plate boundaries in the continental crust is likely controlled by diffusion creep – accommodated grain boundary sliding mechanisms in mixed phase shear zones.