Paper No. 5
Presentation Time: 10:00 AM


AUSTIN, Lauren J., University of Oregon, Department of Geological Sciences, 1272 University of Oregon, Eugene, OR 97405 and WELDON II, Ray J., Department of Geological Sciences, University of Oregon, Eugene, OR 97403,

New LiDAR imagery of the Rocky Ledge fault east of Burney, California enables a more detailed analysis of fault kinematics and regional stress state in the transition zone between the southern Cascadia arc and the western Basin and Range. The Rocky Ledge fault is the largest of several faults constituting the western edge of the Hat Creek graben. It offsets the ~200 ka Rocky Ledge basalt with a >50 m east-facing scarp. The fault pattern is sinuous and the roughly north-trending scarp takes several right and left steps along its 9-km length, with vertical offset diminishing at the northern and southern ends. The upper quarter to third of the scarp is a thinly-mantled bedrock slope, and the lower section is dominated by loose rubble. We explore the morphology of the rubble-dominated lower scarp face to evaluate recent slip on the Rocky Ledge fault. Along-strike furrows and troughs within rubble slopes on the lower half of the scarp appear to preserve individual rupture events and provide constraints on the near-surface dip and slip per event on the fault. Furrows can be characterized as mounds of loose debris bracketed by steepened free faces above and steepened planar slopes below that appear to be tilted panels of the scarp face. Distances between the tops of the furrows and the bottoms of the troughs vary between 0-3 meters with typical values of about a meter. Furrows persist for tens of meters along the strike of the fault, are sinuous, and vary in the amount of degradation, suggesting multiple generations.

We observe that the total offset across the scarp, after accounting for tapering towards the ends and segment steps, varies systematically with orientation. We find the greatest offset occurring on northeast-trending segments and the least offset on northwest-trending segments, which suggests a component of right-lateral motion on a dominantly normal structure. Combined with older and younger offsets in the area, oblique motion on the Rocky Ledge fault could indicate that the stress state in the Hat Creek region is evolving from a mainly normal system to a regime that incorporates a shear component consistent with the northwesterly-propagating Walker Lane belt. It is also possible that oblique motion on the Rocky Ledge fault has been consistent through time and is simply difficult to detect as the fault traverses flat basalt flows.