Strain localization processes in a suite of sheared gabbros: examining the rheology of the lower crust

Recent studies of tremor at the base of the crust along the San Andreas Fault (SAF) indicate that there is a deep region of seismic activity (~26 km, 0.8 GPa, 600° to 700°C), which may be capable of producing strong earthquakes. To investigate the processes influencing deformation under these conditions, we examined an exposed shear zone in lower crustal gabbros from Arnoya, Norway. Field observations, thin section analysis, and thermometric calculations suggest that these rocks deformed at a temperature of 650° to 730°C and a pressure of approximately 1GPa. Over approximately one meter the gabbro progresses from relatively undeformed to highly sheared where it is adjacent to a hydrated pegmatite. With increasing proximity to the pegmatite and shear zone, there is no overall change in recrystallized plagioclase grain size mode (~100 to ~150 μm), relatively little change in the weak strength of plagioclase lattice preferred orientation, and a drastic increase in hydrous phases amphibole and zoisite at the expense of pyroxene and calcic plagioclase. The presence of amphibole and zoisite as distinct phases possessing an SPO and amphibole possessing an LPO in the sheared samples suggests that these phases were formed prior to or during deformation and that a_H2O must have been high. The stabilization of zoisite and amphibole and the reaction to less anorthitic plagioclase cannot progress without the addition of water. Diffusion curves show that it was possible to diffuse water to the undeformed parts of the gabbro on geologically realistic timescales; however, the preservation of corona structures and lack of abundant hydrous phases both suggest that abundant water was not present at distances more than one meter away from the fluid source. The fluid may have been entirely consumed in reactions in the gabbro closest to the fluid source. Based on comparisons with deformation mechanism maps, we suggest that neither a change in creep regime nor grain size reduction was the critical process for strain localization in these gabbros. Instead, the rheological behavior of the gabbro in response to the addition of fluid induced strain localization in this lower continental crust gabbro. Our results indicate that localized fluid flow in the mafic lower crust promotes syn-deformational reactions that reduce the viscosity of the rock and promote deformation localization. Lower continental crustal deformation may be accommodated in highly localized zones of enhanced fluid infiltration over small distances, thereby allowing other parts of the mafic lower crust to remain relatively undeformed. This is particularly important considering the recent observations of tremor and slow slip on deep extensions of earthquake-producing faults. Using plagioclase mean grain sizes from the most deformed samples, we obtained stress estimates of 17 to 24 MPa, which is much greater than that estimated from observations of tidally induced tremor on the SAF. This stress estimate discrepancy highlights the importance of further study to reconcile the geodetic and seismic observations with those from natural lower crust rocks.