Timing And Style Of Miocene Deformation, Indian Pass And Picacho State Recreation Area, SE California, U.S.A.
Advisor Dr. Gary Girty
The Chocolate Mountains anticlinorium is a complex structure extending from the San Andreas fault on the west to south-central Arizona on the east. It is well exposed between Indian Pass and Picacho State Recreation Area, SE California, where Neogene rocks reveal a complex record of Paleogene to early Miocene extension followed by multiple episodes of Miocene fold growth culminating in the formation of a conjugate set of strike-slip faults and the Copper Basin reverse fault. New 40Ar/39Ar data presented herein suggests that the basalts of Black Mountain are 9.45 +/- 0.27 Ma. These basalts crop out prominently at Indian Pass where they are interstratified with the youngest part of the Bear Canyon conglomerate. At this locality both the basalts and bedding in the Bear Canyon conglomerate are tilted southward along the south limb of the Chocolate Mountains anticlinorium. About 2 km to the SE, the Indian Pass fault, a key dextral component of a conjugate set of strike-slip faults mapped throughout the Indian Pass – Picacho area, cuts the basalts of Black Mountain. Hence, the culminating phase of Miocene deformation in the Indian Pass area occurred after ~9.45 Ma and prior to the deposition of flat lying Quaternary alluvial sediments.
At Picacho State Recreation Area, the NW-trending Taylor Lake fault represents another key dextral component of the conjugate fault system recognized at Indian Pass. It records at least 1 km of displacement, and transects Mesozoic-Paleogene basement, a thick section of Miocene (~23 Ma) volcanic and epiclastic rocks, and the Bear Canyon conglomerate. The Taylor Lake fault subdivides the main area of study into NE and SW blocks. Structures within the NE block consist of two EW-trending anticlines, a set of EW-trending reverse faults that cut the older Sortan fault, and NW-trending dextral and NE-trending sinistral faults. In addition, NW to NS trending normal faults are prevalent at Rojo Grande and in the south-central part of the NE block. In contrast to the NE block, a set of complex EW-trending folds, the EW-trending Copper Basin reverse fault, and a set of NW-trending and NE-trending dextral and sinistral faults are the dominant structures in the SW block.
The geometry of Miocene structures identified during this study is consistent with NS shortening, EW extension, and vertical telescoping. Such shortening is not unique to the Indian Pass and Picacho region, and other authors have proposed that much of eastern California has undergone significant NS shortening during the Neogene. In addition, within the Mojave Block, such structures are superimposed on an early Miocene (22 - 18.5 Ma) extensional fabric. Though details are lacking, the record of NS shortening within the Mojave appears to have varied in both space and time, and may have accompanied dextral slip and/or developed intermittently between periods of dextral slip on major strike-slip faults within the evolving Eastern California Shear Zone. Because of the similarity of structural style and timing, structures identified during this study are interpreted to have formed in a similar manner, and thusly accommodated transient and spatially variable NS shortening in an otherwise dextral shear environment within the margin of the North American plate as the Eastern California Shear Zone evolved after ~12-10 Ma. In this context, Miocene structures identified during this study represent the embryonic development of the San Andreas system.