TIMING OF DEFORMATION ALONG THE BORREGO SPRINGS SHEAR ZONE, IMPERIAL COUNTY, CALIFORNIA

Shear zones play a significant role in the deformation of the crust at a variety of scales. Information about structures found within the shear zone and determining an accurate age for the duration of strain is critical in characterizing the properties of shear zones and determining their tectonic significance. Exhumed shear zones provide unique exposures of materials that can be studied in detail to answer such questions. Eastern Peninsular Ranges Mylonitic Zone (EPRMZ), located between Western and Eastern Peninsular Ranges batholiths, is one such exhumed structure. The Western Peninsular Range intrusions have been dated at 126-105 Ma and the Eastern at 105-90 Ma. The timing of juxtaposition between the two batholiths, however, remain unconstrained. The focus of this research project is to date an undeformed dike that cross-cuts the mylonites to constrain the minimum age of deformation in the BSSZ. New 206Pb/238U zircon analyses indicate that shearing along the BSSZ must have terminated by 89.66 ± 1.6 Ma (2% confidence). This data is compatible with the age of a weakly foliated dike, 90.90 ± 1.6 Ma (2% confidence), which was determined in another study. These preliminary results indicate that the juxtaposition of EPRB and WPRB was likely contemporaneous with the formation of the EPRB.