Collaborative Research: Constraints on Initiation of Low-Angle Normal Faults Within the Seismogenic Regime

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Low-angle normal faults are widely recognized in extended continental and oceanic lithosphere, yet the mechanisms influencing their initiation and protracted slip at shallow depths through the seismogenic crust remain controversial. Compilations of normal faulting earthquake focal mechanisms show only a small fraction of all earthquakes on faults dipping less than 30°, and fault mechanics predicts that normal faults with dips less than 30° should neither form nor be reactivated unless they are anomalously weak. In contrast, field observations indicate that slip within the seismogenic crust has occurred. To address this paradox, we are conducting multidisciplinary field and laboratory structural and isotopic studies of a very well exposed low-angle normal fault in the Chemehuevi Mountains of California. Our investigation will provide macro- and microstructural characterization of natural fault rocks and documentation of the role of fluids during initiation and slip along the fault. These observations will be combined with laboratory experiments by colleagues in Europe and recent microseismicity studies, to address processes allied with low-angle normal fault initiation. The study site is the near-ideal natural laboratory to address the relationship between internal structure and mechanical behavior of low-angle normal faults, because 1) regional mapping has established key areas of fault exposure; 2) the footwall is underlain by undeformed granitoids, and it can reasonably be assumed that the protolith is dominated mechanically by a two-phase mixture of quartz and feldspar; 3) the fault system comprises multiple low-angle normal faults with variable slip (about 2 to 18 kilometers) and is well exposed (greater than 18 km down dip and 25 km along strike), facilitating analysis of spatial variations in deformation mechanisms from initiation to maturity, and 4) the thermal history of the footwall is well documented, providing an opportunity to examine rocks with initial ambient temperatures between approximately 150-400°C across the brittle-plastic transition.To illuminate the microstructural evolution and fault weakening mechanisms that lead to low-angle normal faulting, we are integrating field observations with petrographic, SEM (EBSD, image analysis characterization of microstructures, and cathodoluminescence), and stable isotope studies. low-angle normal faults exposed in the Basin and Range provide key examples of this globally significant class of faults and highlight questions about slip at gentle dips in the brittle crust, yet initiation of these fault systems has previously received little attention. In situ stable isotope analyses by ion microprobe allow us to ?see through? secondary alteration during the late slip and fluid migration history and correlate detailed microstructures with associated fluids. By combining these field and laboratory investigations with experimental studies by colleagues in Europe, this research bridges disciplinary boundaries in an effort to transform the understanding of frictional-viscous processes and their manifestation in the geologic record. Collaboration with scientists in Norway and Switzerland will contribute to scaling of laboratory experiments, providing relevant information for earthquake hazard assessment. Undergraduate students will be engaged throughout the project in field and laboratory studies, gaining experience conducting individual research and outreach. Outreach to the broader community is being achieved through the development of virtual field trips using Google Earth and associated educational materials designed for grade school through college students.