A QUATERNARY ASYMMETRIC ANTICLINAL RIDGE IN THE NORTHERN SAN BERNARDINO FOOTHILLS, CALIFORNIA

The northern piedmont of the San Bernardino Mountains contains structual elements characteristic of the Mojave block plate boundary zone between the North American and Pacific plates and the complex convergent Transverse Ranges represented by the North Frontal thrust system. The northwest trending Helendale fault conforms to the dextral transcurrent plate boundary kinematics, but westerly anticlinal flexures and north-verging thrust faults have deformed Pleistocene alluvial fans in the piedmont on both sides of the fault. Some of these anticlines create active geomorphic ridges cored by erosionally resistant Pleistocene and older lithologies. One such ridge is present east of the Helendale fault and south of Cougar Buttes in Lucerne Valley. Here the Pleistocene alluvial deposits contain resistant petrocalcic conglomerates that cap the anticlinal ridge. Quaternary alluvium that paves the present depositional surface around the anticline has largely been removed from the ridge by erosion. The anticline is strongly asymmetric with a steeply dipping southern limb and gently inclined northern limb. At the present level of exposure, the fold limbs are relatively straight and the hinge region is thus angular. In some transects, the anticline contains a south dipping thrust fault with minimal offset. Along strike, however, the fault is not always evident. GPR profiles across the anticline's hingeline show that the shape of the anticline is consistent in the subsurface at least to the depth of radar penetration of about 10 m. The orientation and geometry of this anticline are commensurate with north-south contraction. If the fold-fault pair are co-genetic, it is not a simple accommodation of fault-tip propagation. Furthermore, systematic orthogonal joint sets occur throughout the petrocalcic conglomerate. One set trends parallel with the fold axis. In the hinge region of the fold, carbonate mineralization formed veins along this joint set and within the voids created by separation of bedding surfaces. The joint sets, if coeval with each other, imply uniaxial extension during or after folding. Where this anticline is cored by granite along strike, the fold's geomorphic expression is minimized because of the extent of weathering within the crystalline rocks.