PSEUDOTACHYLYTE DEVELOPMENT RESULTING FROM STRAIN LOCALIZATION IN BRITTLE DUCTILE TRANSITION QUARTZOFELSPATHIC MYLONITES, SOUTH MOUNTAINS CORE COMPLEX, ARIZONA

We study granodiorite mylonites that are host to coeval pseudotachylyte veins in the footwall of the South Mountains core complex, Arizona, to evaluate the rheology of brittle-ductile transition rocks during the earthquake cycle. We use electron backscatter diffraction to investigate processes of strain localization associated with pseudotachylyte development, collecting data from the host mylonite, coeval pseudotachylyte, and the survivor clasts preserved within the pseudotachylyte. Strain within the host mylonite is strongly partitioned into quartz, where microstructures and lattice preferred orientations are indicative of dynamic recrystallization by dislocation creep. Grain size reduction by dynamic recrystallization was first achieved by subgrain rotation recrystallization followed by bulging recrystallization, resulting in the development of finely recrystallized domains of quartz. These zones develop parallel to mylonitic foliation, and show microstructures indicative of grain boundary sliding (GBS) accommodated by diffusion creep. The transition from dislocation creep to diffusion creep helped localize strain in these substantially weaker zones of fine-grained quartz prior to pseudotachylyte development. Pseudotachylyte veins are parallel to mylonitic foliation, and survivor clasts within the pseudotachylyte are composed exclusively of fine-grained quartz. That the fine-grained quartz domains exist within the mylonites, and are present as survivor clasts within pseudotachylyte indicates that pseudotachylyte nucleated along the fine-grained, rheologically weak zones of GBS. We speculate that the strain rate acceleration associated with the onset of GBS may have triggered pseudotachylyte generation, in which case the mylonitic fabric and pseudotachylyte development are associated with the interseismic and coseismic parts of the seismic cycle, respectively. This is supported by quartz grain size piezometry that yields high differential stresses in host mylonites (~160 MPa) and higher stresses in pseudotachylyte survivor clasts (>~200 MPa). We conclude that GBS zones in the mylonites are plastic instabilities that lead to pseudotachylyte generation, and that coeval pseudotachylytes may be the manifestation of intermediate slip rate seismic events near the BDT.