The Rheological Evolution Of Brittle Ductile Transition Rocks During The Earthquake Cycle

We investigate how the rheological evolution of shear zone rocks from beneath the brittle-ductile transition (BDT) is affected by coeval ductile shear and pseudotachylyte development associated with seismicity during the earthquake cycle. We focus our study on footwall rocks of the South Mountains core complex, and we use electron backscatter diffraction (EBSD) analyses to examine how strain is localized in 3 granodiorite mylonites both prior to and during pseudotachylyte development beneath the BDT. In mylonites that are host to pseudotachylytes, deformation is partitioned into quartz, where quartz exhibits lattice preferred orientation patterns and microstructures indicative of dynamic recrystallization during dislocation creep. Grain size reduction during dynamic recrystallization leads to the onset of grain boundary sliding (GBS) accommodated by fluid-assisted diffusion creep, localizing strain in quartz-rich layers prior to pseudotachylyte development. The close association of foliation-parallel zones of GBS in the mylonites, and the overwhelming presence of GBS traits in polycrystalline quartz survivor clasts indicate that GBS zones were the plastic precursors to in situ pseudotachylyte generation. During pseudotachylyte development, strain was partitioned into the melt phase, where grain size sensitive flow continued until crystallization impeded flow. Grain size piezometry shows high differential stress values in both host mylonites (~160 MPa) and pseudotachylytes (> ~200 MPa), consistent with high stresses expected for interseismic and coseismic deformation. The multiple veins of co-planar pseudotachylyte indicates a cyclicity to their development, but their lateral discontinuity suggests that the seismic events are confined to the deep crust, consistent with characteristics of deep crustal tremor. Our findings indicate that pseudotachylytes with plastic precursors may be produced during slower-slip seismic events, and can be used to identify former tremor events in the deep crust.