New, much younger ages for the Yolla Bolly terrane and a revised time line for accretion in the Franciscan subduction complex, California

摘要

The Franciscan subduction complex formed over a protracted, ～150 m.y. period, during Late Jurassic to late Cenozoic subduction of oceanic lithosphere beneath the western margin of the North American continent. Growth of the complex occurred chiefly by progressive accretion, in which voluminous sediment was eroded from the magmatic arc and continent, deposited in the trench region, and then progressively subducted and accreted soon after deposition. The Yolla Bolly terrane, a major Franciscan subunit, has stood out as a possible exception to a progressive-accretion model. Yolla Bolly clastic rocks are almost barren of fossils, but there are ~13 localities with Late Jurassic and Early Cretaceous Buchia specimens, ~3 with mid-Cretaceous ammonites or Inoceramus, plus several with mid-Cretaceous youngest detrital-zircon populations. These ages had suggested that sediments may not have been deposited into an active trench, but instead were deposited into a relatively stable Yolla Bolly basin, which was both long-lived (ca. 150 Ma through ca. 95 Ma) and far-traveled.