RIEDEL SHEARS ALONG THE OWENS VALLEY FAULT AT CRATER MOUNTAIN, EASTERN CALIFORNIA

Riedel shearing occurs during the initial stage of fault creation and results in conjugate fault sets oriented obliquely to the eventual through-going fault. Based on clay-cake modeling, synthetic faults (R-shears) form about 15 degrees to the main shear direction and antithetic faults (R’-shears) form about 75 degrees to that direction. As shearing progresses, new synthetic faults (P-shears) develop about 10 degrees to the final fault. Eventually, linkage of the Riedel shears creates a through-going fault containing a braided pattern.

The Owens Valley fault in Owens Valley, California is an active right-lateral, down-to-the-east oblique slip fault with 6:1 horizontal to vertical components of movement. The northern 20 km of the fault, which extends 20 km from the Poverty Hills to 7 km north of the town of Big Pine, is designated the Big Pine segment. A well-defined fault scarp 1 to over 20 m vertical height marks the trace of this segment. Along most of the segment’s length, the scarp is developed in young alluvial sediments. However, near Crater Mountain, the fault scarp cuts Pleistocene basalt flows for a distance of 7 km.

LIDAR imagery of the area where the fault trace is in basalt reveals several examples of R-shears. Near the southeast side of Crater Mountain three en-echelon R-shears, each of which is 200 to 300 m long, are particularly prominent. These three faults intersect the through-going fault at angles of 15 to 25 degrees. Developed between the R-shears are 10 to 15 m high up-warps of basalt interpreted to have formed by horizontal shortening related to R-shear displacement. The R-shears and associated up-warps are strongly reminiscent to Riedel shear patterns created from clay cake experiments.

A few lineaments that appear to be R’-shears are also evident in the Crater Mountain basalts. These faults strike approximately 75 degrees west of the through-going fault and appear to displace older R-shear scarps left-laterally. Displacement along the R’-shears is only a few meters.

North and south of Crater Mountain, where the Owens Valley fault cuts alluvial deposits, Riedel shears are not evident. Apparently the cohesive basalt flows of Crater Mountain are conducive to Riedel shearing, whereas the weakly- to non-cohesive young alluvial sediments are not.