WHAT ROLE DO FLUIDS PLAY AT INITIATION OF LOW-ANGLE NORMAL FAULTING? (Invited Presentation)

Initiation and protracted slip on large offset, low-angle (dip <30°) normal faults (LANFs) remain controversial. Explanations for the early slip history include elevated fluid pressure and/or reaction weakening. We detail the well-exposed Mohave Wash fault (MWF), a LANF active across the base of the seismogenic zone (initially ~5-15 km), to test these mechanisms.

The regional Chemehuevi detachment system (SE CA) formed at ≤20° dip in heterogeneous crystalline rocks, and is characterized by three stacked LANFs; the Chemehuevi detachment preferentially localized ≥ 18 km of NE slip, rendering the deepest MWF inactive after <2 km of slip. Across >16 km of down dip exposure (T <150° to >400°C), MWF strain is localized into a brittle fault zone, thin, disseminated quartz mylonites, and mylonitic syntectonic Miocene dikes, downdip. At structurally shallow levels (5-8 km paleodepth; ambient footwall 150° <T< 250°C), the MWF cuts isotropic granitic rocks; the brittle damage zone (10 to > 60 m thick) hosts localized zone(s) of chlorite and epidote-rich cataclasite, and rare pseudotachylite. In the structurally deepest exposures (12-15 km; T ≥ 400°C), the MWF juxtaposes granitoids against gneissic basement, cut by mylonitic dikes and meter-scale quartz shear bands. Dislocation creep with subgrain rotation recrystallization dominated deformation at this paleodepth.

SIMS δ¹⁸O of qtz-ep pairs hosted in damage zone cataclasites record significant within sample heterogeneity (<5 mm² scale), resulting from influx of rock dominated, unevenly δ¹⁸O-shifted fluids along disconnected micro-cracks, over variable temperature with time. Low δ¹⁸O values (-1.0‰ for qtz and -5.3‰ for ep) indicate infiltration of surface-derived fluid (brines or meteoric-sourced) during earliest fault slip to paleodepths of >12-15 km. Stable isotope temperatures from hydrothermal mineral rims record a decrease with distance towards the fault, indicating heterogeneous surface connectivity with fluids along the fault. Steep hanging wall normal faults likely facilitated penetration of fluids deep into the fault zone once initiated, leading to footwall refrigeration and possibly, strain localization. Yet despite fluids being present, their role in weakening or elevating pore-fluid pressure, at fault initiation, remains unclear.