LOCKWOOD VALLEY, CALIFORNIA—A RESULT OF PLIOCENE AND QUATERNARY BASIN CONTRACTION BETWEEN THREE MOUNTAIN UPLIFTS

New geologic mapping in the Mt. Pinos-Frazier Mountain region in the “big bend” region of the San Andreas fault has resulted in several new tectonic, stratigraphic, and geomorphologic interpretations. One reinterpretation involves the NE extent of the Big Pine fault (BPF) and the late Neogene contractional history of Lockwood Valley, which is bounded on 3 sides by converging thrust or reverse-fault systems, with pre-Cenozoic crystalline rocks of the Mt. Pinos, Frazier Mountain, and Lockwood Peak uplifts in the hanging walls. The valley is underlain by strongly folded nonmarine sedimentary rocks of Pliocene and Miocene age, unconformably overlain by synorogenic boulder conglomerates of Quaternary and (or) Pliocene age. The NW structural boundary of the valley is the NE-striking Lockwood Valley fault (LVF), a SE-directed reverse fault with a component of sinistral movement that formerly was mapped as the NE extent of the BPF. The LVF is on strike with the BPF to the SW, but is not continuous with it. The LVF places the early Miocene Plush Ranch Formation and subjacent crystalline rocks of the Mt. Pinos massif over the middle to late Miocene Caliente Formation. A separate SW-directed thrust 1-2 km SE of the LVF in Lockwood Valley probably roots into the LVF and places the Caliente above the Pliocene Quatal Formation. The eastern structural boundary of the valley is the SW-directed north Frazier Mountain thrust system, comprising at least 3 low-angle thrusts that place Early Proterozoic gneiss above rocks of the Caliente Formation and, slightly farther west, place the Caliente above the Quatal Formation. The south end of the valley is bounded by the north-directed Lockwood Peak thrust, which places Cretaceous granite above Quatal. None of the faults that bound Lockwood Valley is known to be active, although movement is thought to have extended into Quaternary time. The overall geometry suggests that Lockwood Valley is a contractional basin with a minimum amount of shortening along a NW line across the widest (6-km wide) part of the basin of about 4 km. Lockwood Valley is only one example in the region of a late Neogene contractional basin that resulted from accentuated transpression along the “big bend” region of the San Andreas fault system; Ridge Basin is another example.