Early strain localization associated with a low-angle normal fault system active across the brittle-plastic transition
Justin LaForge, University of Wyoming
Barbara John, University of Wyoming
Craig Grimes, Ohio University
Holger Stünitz, Universitetet i Tromsø
Renee Heilbronner, Basel University
The Chemehuevi detachment fault (CDF) system exposed along the Colorado River extensional corridor in the Chemehuevi Mountains (SE California) hosts exceptional exposures of a denuded fault system related to Miocene extension. Here, we characterize strain localization associated with the early history of extension along an associated small slip (1-2 km) low-angle normal fault, the Mohave Wash fault (MWF), initially active across the brittle-plastic transition. Strain localized in three principal ways across its 23-km-long down-dip exposure (temperatures of <150° to >400°C): into a brittle fault zone, localized, disseminated quartz mylonites, and syntectonic dikes hosting mylonitic fabrics. Brittle deformation in these crystalline rocks was concentrated into a 10–62-m-thick brittle fault zone that hosts localized zones of intense cataclasite series fault rocks ≤3 m thick and rare pseudotachylite. Crystal-plastic deformation localized into thin quartz mylonites hosted in the footwall that increase in thickness down-dip. A transect in the slip direction (NE), highlights variations in the mechanisms of strain localization. At initially shallow structural depths, footwall mylonites are absent; at ~9 km down dip, centimeter-scale mylonitic quartz veins and dike margins are common; at ~18 km down dip, centimeter-scale phyllonites are exposed; at ~23 km down dip direction, the footwall hosts disseminated zones of mylonitic quartz of varying intensities. Lastly, strain localized in meter-scale syntectonic intermediate–felsic dikes at ≥18 km down dip, which are rotated and attenuated into parallelism with the overriding fault zone, and host well-developed L-S tectonite fabrics. Deformation mechanisms of quartz associated with this fault system record progressively hotter dislocation-creep deformation down dip from bulging (BLG) recrystallization (deformation T of ~280–400°C), and subgrain rotation (SGR) recrystallization (deformation T of ~400–500°C) ~9–23 km down dip to SGR and grain boundary migration (GBM; deformation T of ≥500°C) ~23 km down dip. Crystallographic preferred orientations in quartz indicate NE-directed non-coaxial dislocation creep deformation, collinear the MWF slip direction, with an increase in deformation temperatures from ≤500°C to ≥500°C down-dip. Microstructures associated with mylonitic intermediate and felsic composition dikes indicate deformation by diffusion creep with grain boundary sliding in all phases, suggestive of relatively 'hot' deformation shortly after emplacement. These microstructures support previous studies that constrain the CDF system to be a gently NE-dipping structure and highlight the mechanisms active in localized deformation. Overall, the composite character of the MWF suggests localized, episodic and competing mylonitic, brittle, and magmatic processes that accommodate extension during early low-angle normal fault slip.