Abstract
AbstractThe Great Valley basin of California (USA) is an archetypal forearc basin, yet the timing, structural style, and location of basin development remain controversial. Eighteen of 20 detrital zircon samples (3711 new U-Pb ages) from basal strata of the Great Valley forearc basin contain Cretaceous grains, with nine samples yielding statistically robust Cretaceous maximum depositional ages (MDAs), two with MDAs that overlap the Jurassic-Cretaceous boundary, suggesting earliest Cretaceous deposition, and nine with Jurassic MDAs consistent with latest Jurassic deposition. In addition, the pre-Mesozoic age populations of our samples are consistent with central North America sources and do not require a southern provenance. We interpret that diachronous initiation of sedimentation reflects the growth of isolated depocenters, consistent with an extensional model for the early stages of forearc basin development.
Highlights
Forearc basins occupy a critical tectonic zone above subducting plates, and their strata contain a record of subduction-related orogenesis (e.g., Dickinson, 1995; Hessler and Sharman, 2018)
Uncertainty regarding the age of the basal Great Valley Group (GVG) impedes our understanding of how the incipient forearc basin developed as the west coast of North America became a consolidated, two-plate subduction system (e.g., Ernst, 1970; Dickinson, 1995)
Dickinson and Gehrels (2009) demonstrated the utility of detrital zircon (DZ) for calculating maximum depositional ages (MDAs) in retroarc samples, noting that the youngest single grain corresponded to the true depositional age (TDA) 90% of the time
Summary
Forearc basins occupy a critical tectonic zone above subducting plates, and their strata contain a record of subduction-related orogenesis (e.g., Dickinson, 1995; Hessler and Sharman, 2018). In an arc-arc collisional model, the Smartville and Great Valley ophiolite segments of the CRO formed as backarc ophiolites atop a west-dipping subduction zone offshore western North America (Schweickert and Cowan, 1975; Ingersoll, 2000, 2019; Schweickert, 2015). These ophiolites and island arcs accreted onto the California margin during the Sierran phase of the Nevadan orogeny ca. Another sample from GVG strata mapped as Tithonian contained ~5% Cretaceous zircon (Surpless, 2014) These ages suggest Early Cretaceous rather than Late Jurassic initiation of basal GVG sedimentation. Given contradictory DZ and biostratigraphic age constraints, we seek to clarify the timing of deposition of the basal GVG
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