ROLE OF THE RAND MOUNTAINS IN ACCOMODATING EXTENSION AND TRANSROTATIONAL STRAIN NEAR JUNCTURE OF THE CENTRAL GARLOCK FAULT AND EASTERN CALIFORNIA SHEAR ZONE

The Rand Mountains of the northwestern Mojave Desert are strategically positioned to analyze strain accommodation 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). We propose a conceptual model in which displacements on two poorly exposed range-bounding faults and multiple oblique-slip cross faults within the Rand Mountains absorbed extension and transrotation in response to regional transtension affecting the Pacific-North American plate boundary since middle Miocene time. Our model draws upon geological mapping and slip vector analysis of ten north or northeast-striking cross faults whose gross displacements are determined from map offsets of distinctive south-dipping thrust (or detachment?) contacts between Rand schist, Johannesburg gneiss/marble, and Atolia granodiorite. We solved for dip-slip and strike-slip components using a spreadsheet that incorporates measured rakes, fault strike/dip, and map offsets of markers. Aggregate left-lateral displacement across these faults is 4.1 km, with a horizontal extension component of 2.2 km (~ 20%). The cross faults post-date middle Miocene movements along low-angle exhumation structures proposed by Shulaker and Grove (2015). At a larger scale, our model invokes quasi-pure shear strain in map view, with the Garlock fault and three strands of eastern California shear zone originating as conjugates with strikes of N30E and N30W, respectively, symmetrically oriented about a northerly maximum principle stress. The intervening Rand Mountains rotated clockwise along the range-bounding Cantil fault and hypothetical South Rand fault that originally had trends similar to the Garlock fault. Concomitant extension of the Rand Mountains was accomplished by clockwise rotation of cross faults that evolved from initial northwest strikes to present-day north or northeast strikes, while transitioning from right-lateral to left-lateral shear. The block rotations proposed in our model (greater than 45 degrees in places) complicate the interpretation of transport direction for older low angle structures including the Rand thrust associated detachment faults.