MAPPING THE SUBSURFACE OF JOSHUA TREE NATIONAL PARK, SOUTHERN CALIFORNIA, USING NEW GEOPHYSICAL DATA

We use gravity and aeromagnetic data to map faults, ascertain cumulative offsets, and reveal associated strike-slip basins in Joshua Tree National Park, which lies at the east end of the Big Bend of the San Andreas fault. Seismically active faults in and near the Park include those of the NW-striking dextral Eastern California Shear Zone and the E-striking sinistral Blue Cut (BCF), Chiriaco (CF) and Pinto Mountain (PMF) faults. The subsurface structure of the Park area is of interest for seismic hazard and groundwater management.

Along the PMF, two strike-slip basins are observed: the Joshua Tree basin to the west and the Dale Lake basin to the east. The Joshua Tree basin is bounded by Quaternary faults and is ~ 3 km wide, 1 km deep, and extends ~ 20 km east of the town of Yucca Valley. East of its intersection with the Mesquite Lake fault, the PMF lacks surface expression, but gravity data define the southern margin of the eastern basin, the Dale Lake basin, which is 16 km long, 6 km wide and 2 km deep.

Along the BCF, gravity, magnetic, and new electrical data define a 6 km long basin that is 1-2 km deep in Pleasant Valley. Farther west, gravity data define similar strike-slip basins in the western Pinto Valley, where the previously mapped fault does not coincide with the geophysically-defined basin margin. In the eastern Pinto Valley, the topographic expression of the Eagle Mountain strand of the BCF is marked by prominent gravity and magnetic gradients. The northern margin of the basin does not coincide with a previously inferred right-lateral strike-slip fault, but rather appears to be a normal fault at the range front. The basin is more than 2 km deep and 10 km wide.

The CF consists of left-stepping strands mapped by strong gravity and magnetic gradients. The ENE-striking Hayfield basin is 11-12 km long, ~3 km wide and 1-2 km deep with a 12 km long basin beneath Shaver Valley to the west. At the eastern end of the CF, a deeper, wider basin underlies Chuckwalla Valley.

Using the length of the basins as a proxy for cumulative left slip, the displacement increases westward along the PMF and eastward along the BCF. Estimates from this method are consistent with those derived by matching offset magnetic anomalies and best-fit bedrock geologic match-ups. Magnetic data suggest little offset on the Eastern California Shear zone in the Little San Bernardino Mountains.