The Rocky Ledge Fault, Shasta County, NE California: Development and Morphology of a Quaternary Oblique Normal Fault in Basalt

Distinct morphology, variable vertical separation, 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, Shasta County, 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 deformation along the RLF. I developed detailed maps of geologic units and structures along the ~15 km length of the RLF. Map data informs assessment of fault geometry, scarp morphology, and fault development processes. I define a step-by-step conceptual kinematic fault development model using area-balanced cross section restoration techniques that document the unique geologic structure and scarp deformation style. Offset initiates as a broad monoclinal fault propagation fold that breaks along the upper anticlinal axis as the scarp develops. The RLF has a sub-vertical (~85°) near-surface east dip that becomes less steep (40-60°) with depth. West-dipping antithetic faults with scarps that present as along-strike "furrows" or troughs, displace talus material and form uphill-facing benches and breaks in slope in the lower portion of the main RLF scarp. I developed a profile of stratigraphic separation along the RLF fault length from measurements of vertical stratigraphic separation. The RLF is divided into three en-echelon sections. The southern section is ~7 km long with ~74 m maximum vertical separation, the central section is ~3.5 km long with 40 m maximum vertical separation, and the northern section is ~3 km long with 28 m vertical separation. Sections are separated by relay ramps, which span ~175 - 550 m widths between overlapping fault tips. Future section linkage may occur with slip localizing at these overlapping tip zones. Cumulative vertical separation is considerably higher in the southern section than the central and northern sections. Along strike, vertical separation dramatically decreases over short fault lengths at the endpoints of each section, resulting in steep fault tip taper gradients (offset die-out). Global per-event average fault tip taper gradients range from 0.1 - 2.4 m/km, with 9 of 76 reported values (12%) exceeding 1 m/km (Thompson, 2017). At the RLF, 50% of average per-event displacement gradients exceed 1 m/km. Steep taper gradients suggest the RLF has significant cumulative vertical displacement relative to fault length. Frequent or large slip-per-event earthquakes are necessary to generate the observed cumulative vertical displacement at the