THE ORIGIN OF THE NACIMIENTO BLOCK: A GEO-/THERMOCHRONOLOGIC ANALYSIS

Through the analysis of both U-Pb and (U-Th)/He ages from detrital zircon grains we can constrain thermal histories by establishing ages which correspond to crystallization and exhumation events, respectively. This combined geo-/thermochronologic approach is used here to constrain the history of the polyphase Nacimiento fault. This fault traverses central and southern California and has accumulated at least 150 km of slip since Late Cretaceous time. While there is some agreement that major displacements along the structure occurred between 75 and 56 Ma, with Neogene to Holocene strike-slip remobilization linked to the San Andreas fault system there is no agreement on the nature of Late Cretaceous to early Tertiary slip along the fault. U-Pb zircon ages from Franciscan subduction assemblages west of the Nacimiento fault suggest that deposition of these rocks occurred between ca. 95 and 80 Ma. (U-Th)/He ages from the same grains indicate that these grains have been partially reset during Franciscan subduction, with resetting taking place largely between ca. 75 and 55 Ma. These results suggest that zircon grains experienced burial and exhumation during fault activity. However, some zircon grains record a more complex history with (U-Th)/He ages well after fault activity, as young as 9 Ma. These young ages may be related to passage of the Mendocino triple junction beneath central California. Young ages tend to be concentrated further north while zircon grains with Late Cretaceous resetting ages are located in the southern portion of the sampling area. Most ages from this preliminary dataset suggest: 1) that the Nacimiento fault placed Cretaceous batholithic rocks of the Salinian block above the Cretaceous forearc basin and Franciscan subduction complex and 2) that the Franciscan complex west of the Nacimiento fault was deposited, subducted, and exhumed within an ~ 10 to 20 Myr window.