A MODEL FOR LATE CENOZOIC EXTENSION AND TRANSROTATION OF THE RAND MOUNTAINS BLOCK, NORTHWESTERN MOJAVE DESERT, CALIFORNIA

The ENE-trending Rand Mountains block (RMB) occupies a key location between the left-lateral Garlock fault and the termination of three right-lateral faults of the eastern California shear zone (Lockhart, Harper Lake-Gravel Hills and Blackwater faults). The geometry resembles two other structural domains of the Mojave Desert (Eastern Transverse Ranges and Fort Irwin area) that record mid-Miocene to Recent clockwise transrotation in response to distributed dextral shear across the Pacific-North American transform plate boundary. We present a kinematic model in which the RMB rotated ~45 degrees between two ENE-trending boundary faults with inferred sinistral displacements: the Cantil fault along the northern range front and a hypothetical “South Rand fault” to the south of the RMB. Essential to this model is a system of north- and northeast-striking cross faults that accommodated internal deformation of the RMB. Cross fault displacements and slip vectors are constrained by offsets of distinct marker units in the Rand Thrust complex and observed rakes of fault striations. Overprinting relations coupled with sequential reconstruction of rotating sub-blocks in map view suggest a complex movement history. The cross faults initially formed as northwest-striking right-lateral shear fractures, then absorbed normal slip as clockwise rotation of the RMB reoriented them into northerly strikes favorable for extension. Several normal faults with strikes closer to N25-30E display oblique-slip striations that record later components of left-lateral displacement. We analyzed net slip on 10 cross faults to calculate cumulative extension and left-lateral displacement of 18±3 km and ~11 km, respectively, depending on rakes assumed for 3 faults of the western Rand Mountains that are now inaccessible. Our model offers a mechanism to accommodate Late Cenozoic strain within the northwestern Mojave Desert where dextral faults of the eastern California shear zone abut the Garlock fault.