NEW GEOLOGIC MAP OF THE NEWBERRY MOUNTAINS, SOUTHERN CALIFORNIA, AND IMPLICATIONS FOR MIOCENE TECTONIC HISTORY OF THE MOJAVE DESERT

The Newberry Mountains were mapped in detail and K-Ar and 40Ar/39Ar ages determined to refine the Cenozoic tectonic history of the central Mojave Desert. Lower Miocene continental strata include a basal volcanogenic assemblage (23-21.2 Ma) and medial epiclastic assemblage (21.2-20 Ma), both deposited in a broad extensional basin. An upper epiclastic assemblage (19.5-16 Ma) fills east-trending contractional basins superimposed on the extensional basin. Strata of the basal and medial assemblages are extended by east-dipping high-angle normal faults and are locally bounded to the south by a major north-dipping dextral-normal fault (Kane Springs Fault). Stratal dips decrease progressively up section from 40-80° to less than 25°, showing that deposition was contemporaneous with extension. Sequentially developed northeast-, north-, and northwest-striking normal faults imply large (~70-90°) synextensional clockwise rotation, which is corroborated by paleomagnetic data and northeast-striking Jurassic dikes. The Kane Springs Fault is locally well exposed in the southeastern Newberry Mountains and northern Rodman Mountains. However, strata younger than about 21.5 Ma apparently bury its more westerly segments, which may coincide with a linear aeromagnetic anomaly. After 21.5 Ma, local extension induced by laterally discontinuous slip of the Kane Springs Fault may account for development of an adjacent south-dipping normal fault (Sheep Spring Fault) and for emplacement of a silicic dike swarm in the eastern Newberry Mountains. The following conclusions challenge previous interpretations: (1) Regional extension generated about 20 km of dextral slip on the Kane Springs Fault, based on an offset belt of Jurassic granite. (2) Tilted fault blocks, large vertical-axis rotation, and large lateral slip of the Kane Springs Fault suggest that the Newberry Mountains are underlain by a concealed extensional detachment fault. (3) Block rotation and dextral faulting distributed across a 30-km-wide zone apparently accommodated southeastward attenuation of extension in the central Mojave Desert. (4) The cause of late early Miocene contraction in the Newberry Mountains is uncertain, but it may be related to the initial development of an intracontinental plate boundary (early San Andreas Fault) in southern California.