PSEUDOTACHYLYTE AND SHEAR FOLDS: MID-CRUSTAL FEEDBACKS DURING THE SEISMIC CYCLE

Deformation in the middle crust is defined by a series of competing brittle and plastic deformation mechanisms which influence the behavior of continental faults. Using mid-crustal exposures of the Pofadder Shear Zone in Namibia and South Africa we propose a positive feedback cycle between the generation of pseudotachylyte and shear folding that leads to bulk strengthening at the base of the seismogenic zone. The Pofadder Shear Zone is a continental-scale, right lateral structure predominantly comprised of quartz-rich, metagranitic mylonites and ultramylonites. Equivalent depth of deformation of ~12-15 km is suggested by the presence of synkinematic amphibolite and greenschist facies assemblages in non-granitic rocks. The shear zone core is marked by a series of tight to isoclinal shear folds and foliation-parallel, plastically overprinted pseudotachylyte veins. Coseismic grain size reduction enhances crystal plastic flow and competency contrasts, which helps to drive the initiation of active shear folding during interseismic periods. The rotation of fold limbs away from the preferred shear orientation strengthens a portion of the shear zone focusing deformation in zones of high stress, which may influence seismicity patterns across the width of the shear zone. The ubiquity of folds preserved at varying angles from the shear plane suggests this process leads to bulk strengthening of the lower seismogenic zone and may influence the seismic cycle at a continental scale.