RELATING NEOTECTONIC INDICATORS TO SUBSURFACE STRUCTURAL GEOMETRIES IN THE NOBLE HILLS FLOWER STRUCTURE ALONG THE SOUTHERN DEATH VALLEY FAULT ZONE (CALIFORNIA)

With the goal of assessing structural interactions, this study investigates active faulting and structural geometries in the Noble Hills of southern California by combining surface structural and geomorphic observations with seismic imaging in the subsurface. The Noble Hills is a positive flower structure that develops along the Southern Death Valley Fault Zone near its intersection with the Garlock Fault. The northwest striking faults that compose the flower structure deform the Quaternary alluvial sediments, exposing some Pleistocene crystalline rocks. This neotectonic investigation assesses Late Quaternary fault activity by mapping structural and geomorphic relationships, measurement of fault-related landforms, and collection of mesoscopic structural data. Mapping was facilitated by high-resolution satellite and aerial imagery, a high-resolution digital elevation model, and field observations. Several northwest striking fault scarps were identified in the Late Quaternary units, indicating the presence of multiple active faults. To better understand the subsurface fault geometries, a seismic reflection profile was collected. A dry wash that cuts nearly perpendicularly across the flower structure served as the location for the seismic profile. The acquisition involved using a modified ‘rolling-spread’ approach with a 2 meter geophone spacing and a 10 meter shot spacing. Three line segments covered a distance of 1100 meters and provided reliable imagery down to a depth of approximately 2 km. Preliminary analysis of the seismic reflection data reveals a strong, northeast-dipping reflector. This reflector has an approximate dip of 55° and can be traced to a depth of about 2 km. This dipping reflector is closely associated with one of the surface scarps in the quaternary sediments, indicating that it is an active fault. The seismic profile also reveals that the near-surface sedimentary units possess a different strike direction than the slope of the ground surface. By combining the surficial observations with the seismic reflection profile, we are able to obtain a more comprehensive understanding of the structure and fault geometries of the Noble Hills flower structure.