Age, sedimentology, and deformational history of the Mesozoic Franciscan accretionary complex, Angel Island, California, USA

Abstract

The Franciscan Complex represents the forearc accretionary wedge that formed during late Mesozoic−Cenozoic subduction in California, USA, and preserves detailed records of long-lived subduction and exhumation of high-pressure, low-temperature rocks. As one of the world’s largest exposed subduction complexes, it records the evolution of subduction dynamics that correspond to the tectonic development of the western USA. Lithotectonic units within the Franciscan Complex represent different stages, mechanisms, and depths of accretion in the subduction history. Previous authors have proposed subdivisions for the Franciscan Complex, but as the metasedimentary units are very similar over large spatial and temporal scales, they have been difficult to map except on the broadest scales. This restricts the potential for researchers to characterize past tectonic events and subduction/exhumation mechanisms. Angel Island in the San Francisco Bay has previously been mapped as an undivided blueschist-facies metasedimentary unit thrust over lower-grade metasediments along the Quarry Point thrust, but we find that the Angel Island units include lithologic and metamorphic diversity spanning nearly the entire range of variability for the Franciscan Complex. We present new mapping, lithologic descriptions, and detrital zircon U-Pb geochronology to propose a new lithostratigraphy for Angel Island consisting of seven structurally juxtaposed units ranging from sub-metamorphic to low blueschist. The structurally highest and structurally lowest units are sub-blueschist grade, and the center of the fault-bounded stack consists of blueschist-grade metasediments separated by a thin sliver of sodic-amphibole metabasalt. The blueschist units and structurally higher sub-blueschist−grade units have maximum depositional ages of ca. 108−110 Ma, which is consistent with similar grade units from across the Bay Area, and they match well with respect to lithology and metamorphic grade. The structurally lowest units below the Quarry Point thrust have similar provenance but younger maximum depositional ages of ca. 97 Ma. These units have been suggested by previous authors to be affiliated with either Alcatraz or Marin Headlands rocks, which have similar depositional ages but show much more diversity in detrital zircon age spectra, which makes their association debatable. These results offer the first high-resolution detrital zircon U-Pb geochronologic dataset from the Franciscan Complex that allows definition of the diversity of age and provenance within a previously defined regional unit, which should be considered in future work attempting regional correlations with regional sample distribution.