DUCTILE PRECURSERS TO SEISMOGENIC RUPTURE RECORDED BY STRAIN GRADIENTS IN DEFORMED QUARTZ VEINS, HOMESTAKE PSEUDOTACHYLYTE ZONE, COLORADO

Deformed and locally recrystallized quartz veins cut by pseudotachylyte-bearing faults in the Homestake shear zone (HSZ) preserve evidence of small-magnitude crystal-plastic deformation that preceeded seismogenic slip on the faults. Strain recorded by crystallographic preferred orientation (CPO) and paleostress inferred form recrystallized grain size piezometry increase systematically toward the faults with macroscopic ductile strain concentrated within several centimeters of the rupture surface. The asymmetry of fabrics in the quartz vein are kinematically consistent with fault offsets indicating that the ductile deformation and brittle rupture probably occurred during the same deformation event. The HSZ comprises distinct high-strain mylonite zones and a >20-km-long system of pseudotachylyte-bearing faults. Deformation in one of the primary mylonite zones has been dated to ~1400 Ma by in-situ monazite U-Th-Pb chemical geochronology and the pseudotachylyte system is inferred to be broadly coeval on the basis of field relations. ⁴⁰Ar/³⁹Ar data form biotite, muscovite and hornblende indicate that ambient temperatures at ca. 1400 Ma were in the range 300-500°C. The co-occurance crystal-plastic deformaiton and brittle seismogenic rupture at the outcrop to thin section scale confirms previous results linking the brittle and plastic components of the shear zone and suggests that strain localization and consequent acceleration in strain rate during ductile deformation may have led to localized brittle rupture in the pseudotachylyte-bearing faults.