STRUCTURAL CONSTRAINTS ON FAULTS THAT ACCOMMODATED LATE CENOZOIC EXTENSION AND TRANSROTATION OF THE RAND MOUNTAINS BLOCK, NORTHWESTERN MOJAVE DESERT, CALIFORNIA

We present a 1:3000 scale geologic map of the Rand Thrust Complex (RTC) in the eastern Rand Mountains. Geologic study was conducted in Sections 19-20, T30S, R40E of the Johannesburg 7.5 minute quadrangle to deduce the mechanisms, extent and timing of faults that displace the pre-Miocene tectonostratigraphy of the RTC. Well-exposed fault striations and distinctive marker units facilitate determination of net slip. Mapped faults that crosscut the RTC include: (1) an early south-dipping brittle-ductile shear zone with chlorite-epidote alteration that records normal and left-lateral (LL) displacements, (2) six prominent NNE-striking LL-oblique normal faults that cause significant clockwise rotation of older RTC lineations, and (3) several late ENE-dipping normal faults associated with quartz breccia and limonite striations. Calculation of net slip for the NNE-striking fault system yields a total horizontal extension and LL displacement of 11.6 km and 3.6 km, respectively. Four significant NNE-striking faults outside the study area mapped by Nourse (1989) record additional horizontal extension and LL slip of 5.7 km and 7.8 km, respectively, using assumed oblique slip vectors. Depending on the degree of domino-style block rotation that accompanied strike-slip displacements, the integrated extensional strain produced by these faults may approach 100% in the E-W direction. The Rand Mountains occupy an accommodation zone between the LL Garlock fault and three NW-striking right-lateral faults of the eastern California shear zone. Mid-Miocene to Recent displacements on these faults likely accompanied clockwise transrotation of the Rand Mountains block, facilitated by LL slip on the ENE-striking Cantil fault along the north range-front and a hypothetical fault of similar orientation located south near the termination of the right-lateral faults. Smaller scale block rotation and coincident extension occurred within the Rand Mountains along the cross faults mapped in this study.