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The Walker Andesite and the Chocolate Mountains Anticlinorium, Picacho State Recreation Area (PSRA), SE California: Evidence for Possible Neogene Reactivation of the Anticlinorium During Early Development of the Eastern California Shear Zone (ECSZ)

Melanie A. Biggs and Gary H. Girty
San Diego State University, 1887 Greenfield Drive, El Cajon, CA 92021

The Chocolate Mountains anticlinorium has been described as a Paleogene fault-bend-fold that formed during low-angle subduction of the Farallon plate, a result of Neogene extensional exhumation, and as a long-lived structure that may have been reactivated during Neogene exhumation and early development of the ECSZ. At PSRA, the Neogene Bear Canyon conglomerate unconformably overlies Miocene volcanic rocks. Along the northern limb of the anticlinorium, the volcanic section consists of, from oldest to youngest: Quechan volcanics (~23.4 Ma), trachyte of Marcus Wash, trachyte of Rojo Grande, bedded pyroclastic rocks, ignimbrite of Ferguson Wash (~23.2 Ma), trachyte of White Wash, and Walker andesite. The EW trending Walker andesite displays a well developed flow foliation that defines a number of dome culminations. We extracted a small zircon population from the Walker andesite for U/Pb laser ablation ICP-MS dating, and will report the results during our talk. The lower member of the Bear Canyon conglomerate is tilted ~21° NW, while the unconformably overlying middle member is tilted ~15° NW. The upper member is flat lying, and appears to be interstratified with the basalt(s) of Black Mountain (~13.4 - 9.6 Ma). Chemical and petrological data from clasts in the lower member suggest successive unroofing of the underlying Miocene volcanic section, while mylonitic dioritic clasts in the middle member may have been derived from the core of the anticlinorium. The main branch of the Taylor Lake fault dissects the anticlinorium displacing blocks of the ignimbrite of Ferguson Wash ~1 km in a dextral sense. In addition, a major splay off the main branch deflects a foliation defined by flattened pumice in a manner that is consistent with ~500 m of dextral slip. The above relationships suggest Neogene reactivation (between ~23.2 and ~13 to 9 Ma) of the anticlinorium during early development of the ECSZ.


AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California