U-PB ZIRCON CRYSTALLIZATION AGES FOR PLUTONIC ROCKS WITHIN THE SIERRA EL MAYOR AND SIERRA CUCAPA, NORTHWESTERN BAJA CALIFORNIA, MEXICO

The Sierra El Mayor and Sierra Cucapa are key basement exposures within the Salton Trough of northeastern Baja California. They are situated immediately west of the dextral Cerro Prieto fault and the Mexicali seismic zone, a transform fault-spreading center pair of the Pacific-North America plate margin that accommodates most of the modern plate motion at this latitude. To the west, both ranges are separated from the Peninsular Ranges by the actively extending, sub-sea level Laguna Salada basin. In an effort to recognize potential sources of transient heating that could affect interpretations of thermochronologic data bearing upon late Miocene-Pliocene extension in this area, we have performed spatially comprehensive measurements of U-Pb zircon emplacement ages in both ranges. Results obtained to date from 15 samples indicate that volumetrically significant intrusion spanned 110-75 Ma, with the bulk of the plutons emplaced between 85-75 Ma. Hence, the preponderance of latest Cretaceous-early Tertiary (~70-40 Ma) biotite and K-feldspar 40Ar/39Ar total ages we have measured in the Sierra Cucapa and Sierra El Mayor appear to be due to post-batholithic slow-cooling rather than transient pluton-related heating. The crystalline rocks of the Sierra El Mayor and Sierra Cucapa were exhumed along W-dipping detachment faults in the late Miocene-Pliocene. Our new results help to constrain the magnitude of displacement along this system. Specifically, because so many of the intrusive ages significantly post-date the 96±3 Ma La Posta tonalite-trondhjemite-granodiorite (TTG) suite that underlies much of the eastern Peninsular Ranges batholith, it appears highly unlikely that the footwall rocks could have originated from positions as far west as the La Posta TTG dominated Sierra Juarez range of the easternmost Peninsular Ranges. Hence, thermal-kinematic modeling suggesting 5-7 km of late Neogene tectonic denudation and 10-12 km of horizontal extension to unroof the samples from ~15 to 2 Ma (e.g., Axen et al., 2000) are bolstered by these new data.