SNAP, CRACKLE, POP: DILATIONAL FAULT BRECCIAS RECORD SEISMIC SLIP BELOW THE BRITTLE-PLASTIC TRANSITION

Implosion fault breccias have been proposed to mark sites of earthquake rupture, however, quantitatively distinguishing fast from slow slip dilational breccias has not yet been achieved. Recent advances in dynamic fracture studies provide some diagnostic indicators of fast slip breccia formation, these include: 1) high fracture density, 2) large angles between fractures and the slip surface, and 3) a sub-parallel curved or undulating geometry between adjacent fractures. We present new quantitative geologic evidence of dilational fault breccias with attributes highly similar to fast-slip fracture networks created in experiments. Preserved in a mid-crustal exposure of the Pofadder Shear Zone, located in Namibia and South Africa, breccia fracture networks display densities ranging from 0.5 – 2.4 (fractures/cm), form angles with the principle slip surface ranging between 90 ± 14° and 50 ± 23°, and have a sub-parallel undulating geometry with amplitudes and wavelengths of 0.9 ± 0.4 cm and 10.9 ±1.8 cm, respectively. Crackle breccia textures give way to more chaotic breccias but dominant large clast sizes create low D-values of 1.47 ± 0.37, suggesting a short-lived shear strain history. This is further supported by the lack of gouge or cataclasite along slip surfaces, showing there was insufficient grinding to build up a layer of wear material. As the breccias are only found in dilational jogs and extensional stepovers, we conclude that extensional failure is the major control on breccia formation. Equally oriented quartz plastic stretching lineations and brittle slicken lines indicate a consistent slip direction during changes between frictional slip and plastic creep. Lastly, a plastic deformation overprint on some breccias requires formation below the brittle-plastic transition. We favor a model of breccia formation by down fault propagating strain rate pulse and estimate earthquake magnitudes of 0.4 – 1.9 M_w.