A Preliminary Report on New Evidence for the Late Miocene Growth of a Segment of the Chocolate Mountains Anticlinorium, Southeastern California

J. W. Ricketts, J. S. Sainsbury, K. K. Muela, and G. H. Girty
San Diego State University, San Diego, CA, [email protected], [email protected], [email protected], [email protected]

Between Indian Pass and Picacho State Recreation Area crystalline basement is overlain by lower Miocene volcanic rocks. These units are in turn unconformably overlain by undated alluvial sedimentary rocks of the informally recognized Bear Canyon conglomerate. On the north limb of the anticlinorium, three steeply dipping faults and associated splays cut this sequence of rocks, consistently offsetting them in a dextral sense, while a fourth fault at Picacho transects both limbs of the anticlinorium. Where exposed, fault surfaces contain sub-horizontal slickenlines. The three westernmost faults appear to tip out into the core of the anticlinorium. Hence, we speculate that the anticlinorium may have taken up the slip on these faults through tightening and growth. Over an ~15 km EW distance, the cumulative dextral offset on the four faults is ~3.5-4 km.

At Picacho, the Bear Canyon conglomerate lies on the northern limb of the anticlinorium. There the lower member of the Bear Canyon dips ~21^o northward, and is unconformably overlain by the ~15^o northward-dipping middle member. In contrast, the unconformably overlying upper member is not tilted.

Between Carrizo Wash and Indian Pass, the Bear Canyon conglomerate lies on the southern limb of the anticlinorium. Northwest of Carrizo Wash, it dips ~17^o southward. Just NW of Indian Pass, the Bear Canyon dips about 10-20^o southward and is interstratified with the basalts of Black Mountain. The basalts ~2 km SE of Indian Pass dip ~26^o S. Published K-Ar data suggest an age somewhere between ~9.6 and ~13.4 Ma for the basalts of Black Mountain. Hence, our data imply that the anticlinorium was growing after ~9-13 Ma ago, and that it, and the dextral strike-slip faults that we have mapped, are the likely record of the development of a segment of the Eastern California Shear Zone.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009