Initiation of Low-Angle Normal Faults Across the Brittle-Plastic Transition - What Role do Fluids Play?
Abstract
Initiation and protracted slip of large offset, low-angle (dip <30°) normal faults (LANFs) remain controversial. Two common explanations posed for slip on such faults include high fluid pressures and/or low-friction gouge material. Here we detail the well-exposed Mohave Wash fault (MWF), a LANF active across the base of the seismogenic zone (paleodepths of 5-15 km), to test the role of fluid involvement at initiation. The regionally developed 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 directed slip, rendering the deepest MWF inactive after <2 km of slip. Throughout >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. At structurally shallow depths (5-8 km paleodepth; ambient footwall 150° 60 m thick) hosts localized zone(s) of chlorite and epidote-rich cataclasite, and rare pseudotachylite. Down dip in the structurally deepest fault 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. δ18O of quartz-epidote pairs hosted in the damage zone measured in situ by SIMS record heterogeneity within sample; low values (-1.0‰ for quartz and -5.3‰ for epidote) indicate infiltration of surface-derived fluid (brines or meteoric-sourced) during the earliest stage of fault slip to paleodepths of >12-15 km. Stable isotope temperatures determined from adjacent rims on hydrothermal minerals record a decrease in T with distance towards the fault, further indicating surface connectivity with fluids along the fault. Steep normal faults in the hanging wall likely facilitated penetration of fluids deep into the fault zone once initiated, leading to footwall refrigeration and possibly further strain localization. Although these fluids were present, their role in weakening, or elevating pore-fluid pressure, at fault initiation is not apparent from either field or lab data.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T31H..02J
- Keywords:
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- 7250 Transform faults;
- SEISMOLOGYDE: 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICSDE: 8150 Plate boundary: general;
- TECTONOPHYSICS