THE ORIGIN OF MORONGO VALLEY, A PULL-APART BASIN IN THE TRANSVERSE RANGES, SOUTHERN CALIFORNIA

Morongo Valley is a pull-apart basin situated between the San Bernardino and Little San Bernardino Mountains in the Transverse Ranges about 15 km north of Palm Springs, southern California. It is bounded by the Pinto Mountain, Morongo Valley, and San Andreas faults. The occurrence of Morongo Valley relative to these faults implies a kinematic relationship. A kinematic model that relates Morongo Valley to the Pinto Mountain, Morongo Valley, and San Andreas faults is proposed.

Preliminary findings suggest that Morongo Valley formed in response to rotation of the eastern Transverse Ranges, extension between the Pinto Mountain and Morongo Valley faults, and dextral shear on the San Andreas fault. Dextral shear on the San Andreas fault dragged the west end of the Pinto Mountain fault, which accommodates the rotation of the eastern Transverse Ranges, to the northwest. A kink formed in the Pinto Mountain fault allowing it to hinge clockwise. As the Pinto Mountain fault continued to hinge, rotation of the eastern Transverse Ranges could no longer be accommodated and the Morongo Valley fault formed to allow the rotation to continue. Consequent extension between the Pinto Mountain and Morongo Valley faults resulted in the down dropping of the Morongo Valley block. Downdropping was achieved along the Pinto Mountain, Morongo Valley, Dry Morongo Canyon, and Mission Creek Canyon faults, and also a pair of possible normal faults map between Dry Morongo Canyon and Morongo Valley inferred from the Morongo Valley 7.5 minute quadrangle. Refinement of the model is in progress.