Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 24-5
Presentation Time: 8:30 AM-6:00 PM


PICKEREL, Molly, Dept. of Geoscience, University of Nevada - Las Vegas, Geoscience (LFG) 104B 4505 S Maryland Pkwy, Las Vegas, NV 89154-4010, ODLUM, Margo, Department of Geoscience, University of Nevada Las Vegas, 4505 S. Maryland Pkwy, Las Vegas, NV 89154-4010 and CAPALDI, Tomas, University of Nevada, Las Vegas

This study investigates a series of hematite coated fault mirrors along bedrock fault scarps in Miocene volcanic rocks near Lake Mead, Nevada. The studied faults are located at the intersection of the NE-SW trending strike slip faults of the Lake Mead fault system and N-S detachment faults of the Northern Colorado River Extensional Corridor (CREC). Fault mirrors are one type of thin (typically <1 mm thick) slip surface in exhumed fault zones and have been observed in a variety of rock types at a variety of scales (cm2 to m2). The studied fault scarps are typically m2, locally mirrored, and generally NE striking and near vertical. Two sets of slickenlines indicating both strike-slip and oblique slip are preserved on the surface, providing evidence these faults were reactivated multiple times. Initial field and SEM observations on the samples support that these discrete faults accommodated multiple episodes of slip since the Miocene. A variety of hematite particle textures, sizes, and deformed grains have been observed through SEM analysis of the fault mirror volume. Away from the slip surface, hematite is characterized by hematite plates about ~5-10 μm in width. These plates grade into broken and fragmented pieces along the lower contact with the host rock. At the slip surface, hematite is characterized by sub-rounded nanoparticles (0.1-1 μm diameter) and broken hematite plates. Along with the varying grain size and morphologies, there is an apparent gradient in size distribution with smaller particles at the surface. Sections of the fault surfaces contain a thin film coating, featureless at SEM scale, and may be evidence of hydrothermal fluids. If these fault surfaces preserve a record of seismic slip, they can inform on coseismic processes including dynamic weakening mechanisms, providing insights into seismicity along faults in the shallow crust. Future microstrucutral and microtextural characterization and hematite (U-Th)/He thermochronology will inform on fault slip behavior, timing of hematite mineralization, and slip, and fluid interactions occurring along these fault surfaces. Ultimately, this information will inform on the spatial and temporal relationships between extension, magmatism, fluids, and seismicity in the Northern CREC.