--> Detrital zircon mixture modeling demonstrates changing source terranes consistent with strike-slip motion, Ventura Basin, California

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Detrital zircon mixture modeling demonstrates changing source terranes consistent with strike-slip motion, Ventura Basin, California


Many active strike-slip fault zones lie near significant population centers (e.g. California, New Zealand, Turkey, and Indonesia). A better understanding of how strike-slip faults evolve over deep-time (i.e., >106 yr timescales) may aid future assessments of tectonic stresses, seismic hazards, and landscape evolution. Studies that reconstruct the tectonic history of a basin by characterizing its sedimentary basin fill (i.e., tectonostratigraphy) commonly use detrital zircon geochronology, but sediment recycling and non-unique source terranes can confuse interpretations of ancient sediment routing. In this study, we quantify the proportion of source terranes contributing sediment to the Ventura Basin (California) throughout Cenozoic time by employing mixture modeling of detrital zircon geochronology age distributions. The Ventura Basin has a complex tectonostratigraphic history, containing Cretaceous-Oligocene sediments deposited in a forearc basin associated with subduction of the Farallon Plate. The initiation of strike-slip tectonism in California at ~29 Ma due to juxtaposition of the North American and Pacific plates formed a chain of discontinuous pull-apart basins filled with Neogene sedimentary packages up to 6 km thick. Palinspastic reconstructions suggest that the Ventura Basin was rotated ~90° clockwise and translated 60-75 km to the northwest along the San Gabriel and Canton faults between 18 and 6 Ma. In order to reconstruct the Cenozoic position of the Ventura Basin, we utilize the basin’s stratigraphic record to infer ancient sediment routing patterns. We sampled 8 Eocene-Pliocene formations near Lake Piru, California and analyzed n=120 zircons for U-Pb ages using established LA-ICP-MS methods. Our data show a progressive northward shift in source terranes that fed the Ventura Basin, recording the basin’s northwest translation. Our total offset estimations (i.e. the distance measured from sample locations to their inferred sediment sources across the San Gabriel and Canton faults) are consistent with previously-published estimates. However, the presence of a unimodal population of ~1180 Ma zircons in an upper Oligocene sample suggests that the basin was adjacent to the San Gabriel Anorthosite at that time. Restoring this part of the Ventura Basin to be adjacent to the modern extent of the San Gabriel Anorthosite requires 35-60 km of slip since the Oligocene , up to 6 Ma earlier than previous restorations. This pre-Miocene translation has also been suggested in studies in central California, and could fundamentally change previous interpretations of the tectonic history of the California margin.