Using the Tertiary Volcanic and Gravelly Sedimentologic Record to Test the Timing and Evolution of the Chocolate Mountains Anticlinorium, SE California

San Diego State University Foundation, San Diego CA

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). The origin of the Chocolate Mountains anticlinorium (CMA) and its significance to the Tertiary structural and tectonic history of the southwestern United States remains enigmatic. Discontinuously exposed throughout southeastern California, these Mesozoic to Tertiary age rocks may have been derived from a subduction zone or forearc basin setting located initially ~300 - ~400 kilometers to the west within the ancient Pacific Ocean, but they now form dome-like structures commonly referred to as the CMA. The goal is to determine whether or not the CMA formed initially during Paleogene low-angle subduction of the ancient Farallon plate, which lay between the Pacific and North American plates, or somewhat later during middle Tertiary extensional exhumation of the southwestern United States. In addition, the investigators will evaluate the potential role that early strike-slip faults of the Eastern California Shear Zone, a precursor to the modern day San Andreas fault, may have played in the development of the anticlinorium. To address these issues, a series of geochronologically constrained time slices that portray the evolution of the rocks and sediments involved in the formation of the CMA will be developed. The expected outcome of this research is an enhanced model for the development of the Chocolate Mountains and their significance to the formation and evolution of the southwestern U.S. Key to the development of the CMA model time slices is the Neogene Bear Canyon conglomerate, which can be subdivided into three unconformably bound units. Significantly, preliminary work indicates that the lower and middle members of the Bear Canyon lie on both limbs of the anticlinorium and record its incremental growth with the last increment recorded by the tilting of the interstratified ~9-13 million year old basalts of Black Mountain. The lead investigator and a large number of students will commence their work by tracing out the extent of the unconformities bounding the lower, middle, and upper members of the Bear Canyon conglomerate over ~300 km2. Their work will include detailed characterization of bedding types, clast and grain-size distributions, and sedimentary structures and paleocurrent directions. In addition, laser-ablation U-Pb zircon and selected Ar-Ar geochronological ages will be determined for all key Mesozoic and Tertiary volcanic units underlying the Bear Canyon, as well as for clasts included in the Neogene conglomerates. Additional analyses will be made on Eocene(?)/Oligocene(?) sedimentary rocks lying between Mesozoic basement and Tertiary volcanic rocks. By sequentially removing fold growth and fault slip that has accumulated over the last ~60 million years, the PI and students will be able to assess the incremental growth of the anticlinorium and the potential role that subduction, extensional exhumation, and pre-San Andreas strike-slip faulting may have played in its development and the evolving landscape of southeastern California during the Paleogene (~65.5 to ~33.9 million years ago) and Neogene (~23 to ~2.6 million years ago).