Reply on RC1

Abstract

Plate boundary shear zones generate earthquakes, which are at present unpredictable, but advances in mechanistic understanding of the earthquake cycle offer hope for future advances in earthquake forecasting. Studies of fault zone architecture have the potential to reveal the controls on fault rupture, locking, and reloading that control the temporal and spatial patterns of earthquakes. The Pofadder Shear Zone exposed in the Orange River in South Africa is an ancient, exhumed, paleoseismogenic continental transform which preserves the architecture of the earthquake source near the base of the seismogenic zone. To investigate the controls on earthquake rupture geometries in the seismogenic crust, we produced a high resolution geologic map of the mylonite zone which forms the shear zone core. The core consists of thin, pinch-and-swell layers of mylonites of variable mineralogic composition, reflecting the diversity of regional rock types which were dragged into the shear zone. Our map displays centimetric bands of a unique black ultramylonite along some mylonite layer interfaces. We present a set of criteria for identifying recrystallized pseudotachylytes (preserved earthquake frictional melts) and show that the black ultramylonite is a recrystallized pseudotachylyte, with its distribution representing a map of ancient earthquake rupture surfaces. We then compare the attributes of lithologic interfaces which hosted earthquakes with those which apparently did not, and find that their geometry differs meaningfully at wavelengths of 10 m. We argue that the pinch-and-swell structure of the mylonitic layering, enhanced by viscosity contrasts between layers of different mineralogy, is expected to generate spatially heterogeneous stress during viscous creep in the shear zone, which dictated the path that earthquake ruptures followed. The condition of rheologically layered materials causing heterogeneous stresses should be reasonably expected in any major shear zone, is enhanced by creep, and represents the pre-seismic background conditions through which earthquakes nucleate and propagate. This has implications for patterns of earthquake recurrence and explains why some potential geologic surfaces are favored for earthquake rupture over others.