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Shear zone evolution and the path of earthquake rupture

Abstract. Crustal 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 shear zone core mylonite zone. The core consists of ∼ 1–200 cm, 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 layers
of a unique black ultramylonite along some mylonite interfaces, locally adding to thick composite layers suggesting reactivation or bifurcation. We
present a set of criteria for identifying recrystallised pseudotachylytes (preserved earthquake frictional melts) and show that the black
ultramylonite is a recrystallised pseudotachylyte, with its distribution representing a map of ancient earthquake rupture surfaces. Pseudotachylytes
are most abundant on interfaces between the strongest wall rocks. We find that the geometry of lithologic interfaces which hosted earthquakes
differs from interfaces lacking pseudotachylyte 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 of earthquake ruptures. 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.
