WWU Assistant Professor of Geology Sean Mulcahy has had his paper, "Early Onset of Franciscan Subduction" published in the journal Tectonics.
Plain Language Summary
Subduction zones are places where dense ocean crust descends (or subducts) beneath more buoyant plates of continental or oceanic crust. Ancient subduction zones exposed at the Earth's surface provide important information on past plate movements and processes now occurring within active subduction zones. Determining the age that subduction began in ancient subduction zones is difficult because the earliest formed rocks are rarely preserved in the rock record. Our study focuses on the Franciscan complex of California, which is considered a type example of an ancient subduction zone. The timing of Franciscan subduction initiation, however, has long been debated, resulting in conflicting models for the tectonic history of western North America. This study is unique because we dated 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. The zircon inclusions within garnet yield an age of ~176 Ma, whereas the zircon outside the garnet had a younger age of ~160 Ma. These ages require that Franciscan subduction began by 180 Ma, significantly older than commonly believed and constrain models for the tectonic evolution of the western North American margin.