THE ROCKY LEDGE FAULT, NE CALIFORNIA: DEVELOPMENT AND MORPHOLOGY OF A QUATERNARY OBLIQUE NORMAL FAULT IN BASALT

Distinct morphologies, variable vertical separations, and unique structural geometries are observed along the Rocky Ledge Fault (RLF), an east-dipping oblique normal fault in the tectonically complex Hat Creek Graben, NE California. Situated in the southern Cascade arc, the RLF is located at the boundary of the Klamath Mountains, Basin and Range, and Walker Lane Seismic Belt. The primary goal of this project is to better understand Quaternary movement along the RLF using existing lidar, aerial imagery, and cosmogenic exposure age dates along fault scarps, in conjunction with field mapping. We present a step-by-step conceptual model using area-balanced restoration techniques on cross sections that documents the unique structure and deformation style of scarp development. Offset initiates as a broad monoclinal fault propagation fold that breaks along the upper anticlinal axis as the scarp develops. Analysis indicates the RLF has a sub-vertical (~80-85°) near-surface dip that becomes less steep (~55-70°) with depth. West-dipping antithetic faults accommodate some offset and present as along-strike “furrows” and troughs, displacing talus material and forming uphill-facing benches and breaks in slope in the lower portion of the scarp. A displacement-length profile was developed from topographic profiles oriented perpendicular to fault strike using maximum vertical separation (MVS) measurements. Vertical separation in three distinct tapered sections increases dramatically in the south (~7 km length, ~74 m MVS) and diminishes in the central (~3.5 km length, 40 m MVS) and northern (~3 km, 28 m MVS) sections. Fault sections, and likely individual ruptures, taper much more rapidly at the fault tips than faults in other lithologies. Material properties of the bedrock may amplify the vertical displacement (relative to fault length) as strain accumulates near the fault tips. Alternatively, observations of large displacements relative to short fault length could suggest the RLF ruptures as part of a larger, interacting system. Human safety and critical infrastructure protection underpin the importance of this project: the town of Burney is 2 km west of the RLF, a large natural gas pipeline crosses the central RLF, and several hydroelectric dams span the Pit River ~5 km to the north.