STRUCTURE AND CHEMISTRY OF TERTIARY VOLCANICS PICACHO STATE RECREATION AREA: IMPLICATIONS FOR THE TRANSFER OF BAJA TO THE PACIFIC PLATE

Picacho State Recreation Area lies along the lower Colorado River and contains the record of Oligocene volcanism and Miocene sedimentation. Though previous workers in this region subdivided the Tertiary volcanics into the Quechan volcanics and the overlying rhyodacite of Little Picacho Peak, our new chemical data along with detailed mapping suggests the following stratigraphic hierarchy: Quechan volcanics (33 – 27 Ma), trachyte of Marcus Wash, trachyte of Rojo Grande, bedded pyroclastic deposits, trachyte of White Wash, ignimbrite of Ferguson Wash (27 – 25 Ma), Walker andesite. With the exception of the Quechan volcanics and the ignimbrite of Ferguson Wash, all of the volcanic units listed above were included in the rhyodacite of Little Picacho Peak by previous workers. However, on a Zr/TiO₂ vs Nb/Y diagram samples from each unit cluster and do not overlap with other mapped units, a relationship that supports the above newly defined stratigraphic subdivisions. Unconformably overlying the Tertiary volcanics is the Miocene Bear Canyon conglomerate.

Tertiary volcanics are folded into ~EW trending folds, and are cut by WNW-NW trending faults of the newly defined Taylor Lake fault system (TLFS). The TLFS offsets the Tertiary volcanic section by ~1 km in a dextral sense. Locally ENE trending NW and SE dipping reverse faults bring the ignimbrite of Ferguson Wash over the younger Bear Canyon conglomerate. The above fault-fold fabric defines a strain pattern that is consistent with ~NS shortening and ~NW distributed dextral shear. Though difficult to constrain the fault-fold fabric probably developed after deposition of the Bear Canyon (~11 Ma) and prior to deposition of Colorado River gravels (≤ ~5 Ma). We speculate that the fault-fold fabric may reflect the continuation of the general strain pattern associated with the Eastern California Shear Zone. If this speculation is correct, then data from Picacho supports the distributed shear model for the transfer of Baja California to the Pacific plate as recently discussed by M.S. Wong and P.B. Gans.