THE CHOCOLATE MOUNTAINS ANTICLINORIUM: MIOCENE GROWTH AND RE-ACTIVATION RECORDED IN VOLCANICS AND ALLUVIAL GRAVELS, INDIAN PASS TO CARRIZO WASH, SE CALIFORNIA

Between Indian Pass and Carrizo Wash, Mesozoic to early Tertiary crystalline basement is overlain by early Miocene volcanics (~23-24 Ma) and the Bear Canyon conglomerate (BCC). The basement complex, comprising the core of the Chocolate Mountains anticlinorium, includes in ascending structural order, the late Cretaceous to early Tertiary Orocopia Schist, a mafic orthogneiss unit, and the Jurassic Winterhaven Formation. The Chocolate Mountains fault separates the Orocopia Schist from the orthogneiss, while the Gatuna fault separates the orthogneiss from the structurally overlying Winterhaven Formation. Lying stratigraphically above the Winterhaven Formation is a thick section of Miocene volcanic rocks which are in turn overlain by alluvial sediments of the Bear Canyon conglomerate. Near Indian Pass, the basalts of Black Mountain (~9.6 - 13.4 Ma) are included within the upper part of the Bear Canyon conglomerate. We may have a new Ar-Ar date to report during our talk. Between Indian Pass and Carrizo Wash, the Chocolate Mountains and Gatuna faults, the section of Miocene volcanics, and the BCC are all folded about the CMA. In addition, the entire section is cut by NW trending dextral faults some of which die out into the core of the anticlinorium. We interpret this relationship to indicate that the anticlinorium may have been tightened during this period of dextral shear. Between Indian Pass and Carrizo Wash the trace of the axis of the anticlinorium forms a culmination and then between Carrizo Wash and Bear Canyon a depression. Cropping out along the east side of the structural depression is a thick section of presumably Miocene volcanic rocks. Our preliminary work suggests that these rocks may represent a dome complex along with vitrophyre and a pumaceous cap. The dome complex is in turn overlain by a thick section of welded and non-welded pyroclastic rocks that are interstratified with lahars. Our preliminary geochemical work suggests that the pyroclastic rocks are unlike the more well known pyroclastic rocks cropping out a few kilometers to the east at PSRA. In short, our continuing work suggests that the CMA was likely growing after ~9-13 Ma, and that it and the dextral strike-slip faults that we have mapped are likely the record of the propagation of the Eastern California Shear Zone southward to the latitude of Indian Pass.