LATE MIOCENE AND PLIOCENE STRIKE-SLIP FAULTING IN THE EASTERN MOJAVE DESERT, CALIFORNIA

Strike-slip faults of the eastern California shear zone initiated ~12-10 Ma, so its pre-Quaternary evolution, particularly in areas with no active faults, is important for understanding regional tectonic evolution. The eastern Mojave Desert southeast of the terminus of the Garlock fault is an area of broadly spaced mountains and valleys, similar in some respects to the northern Basin and Range but differing in having only rare sedimentary basins >1 km deep and no mapped or inferred basin-margin normal faults. In the southern Ivanpah Valley the dextral Stateline fault and the sinistral Nipton fault meet at a ca. 3 km deep localized extensional basin, replete with a detachment fault associated with the termination of the Stateline fault and Pliocene basin-fill deposits. The valley formed after 8 Ma and the localized deep basin formed after 6 Ma. To the northeast the Nipton fault crosses the New York Mts., with 8-10 km of offset, toward a ca. 2 km deep basin formed by extensional faults bounding Eldorado Valley, as mapped by Ernie Anderson. These were active until at least 9 Ma, and likely step northward to connect with sinistral faults of the Lake Mead system. To the southwest the Nipton fault crosses a bedrock-floored saddle and continues through a releasing bend and associated small basin to Kelso. Farther southwest the fault bends west in a compressive orientation that probably is related to uplift of the Granite Mountains. The valleys, in places floored by bedrock, are not caused by normal faulting, leading us to hypothesize that they are synclines associated with the sinistral fault. The parallel Fenner Valley may also be a syncline with an associated sinistral fault. If so, faults are more widely spaced than Quaternary faults to the west, 20 to 25 km vs. 10 km. Post-10 Ma tectonics of the eastern Mojave Desert may reflect accommodations between the northern and southern Basin and Range provinces as well as the farfield effects of the plate boundary to the west.