ACTIVE DEFORMATION ALONG THE BLUE CUT FAULT IN JOSHUA TREE PARK: RESULTS FROM AN UNDERGRADUATE FIELD MAPPING PROJECT

The Blue Cut fault bisects Joshua Tree National Park (JTNP), from the Little San Bernardino Mountains to the west through the Pinto Basin to the east. It is part of a series of active sinistral ENE-trending faults that originate along the southern San Andreas fault; these include, from S to N, the Salton Creek, Chiriaco, Blue Cut, and Pinto Mountain faults (and perhaps the Garlock fault, further north). These faults appear to have accommodated a significant amount of clockwise rotation in the Mojave since the inception of the southern San Andreas fault. Despite its location in a major national park, the Blue Cut fault has not had a focused research study on it since the 1960’s. During the spring of 2011, an advanced field mapping class from Occidental College chose the Blue Cut fault as its focus based on the observation of geomorphic structures adjacent to the fault in the center of JTNP. The most significant of these is an anticline (herein called the Blue Cut anticline) with an east-west trending axis subparallel to the inferred Blue Cut fault trace. The anticline is exposed over an area of ~ 5 km east-west and 1.5 km north-south. The rocks exposed consist of poorly lithified sandstones and fanglomerates, with the fanglomerates forming distinct ridges. Although both limbs of the fold appear to dip ~30 – 40°, we were only able to measure a few attitudes on the northern limb, as it is truncated to the north, most likely by the active Blue Cut fault (although we found no evidence for its location in the field). We can, however, use the geometry of the Blue Cut anticline to constrain the possible geometries of the Blue Cut fault, a fault that is important to understand because of its role in the evolution of the southern San Andreas fault and JTNP.