Belly of the Beast: Detailed Mapping in the Deformation Core of a Quartz-plastic Transitional Zone Fault: Implications for Deep Fault Seismicity on Major Strike Slip Faults

Ben Melosh, McGill University
Christie Rowe, McGill University
Christopher Gerbi, University of Maine

The Pofadder Shear Zone (Namibia-South Africa) is a right lateral, strike-slip structure, exhumed from the quartz brittle-plastic transition. In respect to lithology, kinematics and scale this shear zone is an ancient analog to the modern southern San Andreas Fault. Temperatures of deformation estimated from quartz microstructures and CPO patterns in mylonites are ~350- 450°C. Detailed mapping (1:100) of the deformation core provides a first look into the guts of this transpressional shear zone at deep seismogenic depths. The shear zone is filled with many brittle faults that occur parallel to the mylonitic foliation and host off-fault tensile fracture networks suggestive of dynamic earthquake ruptures. There are no gouge or wear products on these surfaces. Some breccias are plastically healed, suggesting that a strain rate pulse, rather than exhumation to cooler temperatures is the most plausible mechanism for their formation. Unlike upper-crustal brittle structures, on which repeated seismic events may occur on a single fault, we find a distribution of earthquake markers across the width of the 30 m wide shear zone. These markers, which include dynamic fracture networks and pseudotachylyte, appear to coincide with the highest strain mylonites near competency contrasts at the shear zone boundaries. Our observations provide new insight into the distribution, evolution and interaction of strain rate sensitive mechanisms in the quartz transitional zone and contain implications for deep fault seismicity or microseismicity.