DECIPHERING SHORELINES FROM SCARPS: FAULT TRACE MAPPING AND PALEOSEISMIC INVESTIGATION OF THE BONHAM RANCH FAULT ZONE, NORTH OF RENO, NEVADA

The 56-km-long Bonham Ranch fault zone is an east-dipping normal fault zone that extends along the western side of Smoke Creek Desert, Nevada and accommodates extensional deformation along the boundary between the Walker Lane and Basin and Range tectonic provinces. The fault zone consists of several subparallel range bounding, piedmont, and intra-basin faults that cut across pluvial lake and alluvial fan deposits. The origin of some scarps in previous studies has been difficult to reconcile due to the presence of lacustrine shorelines leading to uncertainties in the total amount of displacement accommodated across the faults. Thus, to determine whether the scarps were formed by tectonic displacement or lacustrine processes, we conducted a morphological study and compared the landforms to ancient lake high stands and desiccation events.

We present preliminary morphologic analyses and a refined fault trace map of the Bonham Ranch fault zone. To generate the map, we combined observations from satellite imagery, site specific Quaternary geologic mapping utilizing Structure from Motion DEM’s generated from low-altitude drone photography, as well as reconnaissance field mapping and description of stream channel outcrops.

Our preliminary results indicate that one scarp follows contour lines matching ancient shorelines associated with the Younger Dryas high stand at an elevation of ~1200-1220 m along the western margin of the basin. The sinuosity of the scarp, relatively flat morphology of the upper and lower surfaces, long incision of streams across the upper surface, and lack of vertical deformation within stream cut outcrops, indicate a lacustrine shoreline origin. Two scarps located along the edge of the Smoke Creek playa strike N-S, and are characterized by linear sections, and steep scarps. Several stream exposures across these scarps show vertically offset lacustrine stratigraphy including a white tephra, indicating a tectonic origin. Faulting is also inferred to be the origin of another scarp that cuts across the Younger Dryas shoreline. A scarp profile along this trace indicates that the scarp is up to ~3 m high. Additional morphologic, stratigraphic, and tephra correlation studies are planned and intended to develop constraints on earthquake timing, recurrence, and slip rate for the Bonham Ranch fault zone.