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Early Onset of Franciscan Subduction



Subduction zones are places where dense ocean crust descends (or subducts) beneath more buoyant plates of continental or oceanic crust. Ancient subduction zonesexposed at the Earth’s surface provide important information on past plate movements and processes nowoccurring within active subduction zones. Determining the age that subduction began in ancientsubduction zones is difficult because the earliest formed rocks are rarely preserved in the rock record. Ourstudy focuses on the Franciscan Complex of California, which is considered a type example of an ancientsubduction zone. The timing of Franciscan subduction initiation, however, has long been debated, resultingin conflicting models for the tectonic history of western North America. This study is unique because wedated the mineral zircon preserved as inclusions within garnet formed during the early stages of subduction.We also analyzed zircon outside of garnet formed during a younger event within the subduction history. Thezircon inclusions within garnet yield an age of ~176 Ma, whereas the zircon outside the garnet had a youngerage of ~160 Ma. These ages require that Franciscan subduction began by 180 Ma, significantly older thancommonly believed and constrain models for the tectonic evolution of the western North American margin.