Cenozoic tectonic development of the Ancestral Cascades Arc–forearc basin system: Sedimentologic and provenance analysis of the Blue Mountain Unit

The Eocene accretion of the Siletzia oceanic plateau reconfigured the continental margin in Washington and Oregon, USA, and set the stage for development of the modern Cascades Arc and forearc system. However, new sedimentologic and geochronologic data from this study suggests a previously unrecognized record of seamount collision immediately followed the accretion of Siletzia and played an important role in the early development of the Olympic subduction complex. The Blue Mountain Unit is exposed along the periphery of the Olympic Mountains and was originally interpreted as the stratigraphic base of the Siletzia basalts. We propose a new depositional and structural model for the Blue Mountain Unit based on lithofacies mapping (six areas) and U-Pb detrital zircon geochronology (N = 5; n = 1389), whereby the unit represents sedimentary and volcanic strata thrust beneath Siletzia basalts. We grouped lithofacies into two distinct affinities that we consider associated with different tectonic plates: (1) volcanic and distal marine sedimentary strata from the subducting oceanic plate that show greenschist facies metamorphism, and (2) continentally derived trench turbidites from the upper plate. Lithofacies are structurally juxtaposed against each other in chaotic assemblages at all areas, but together, they represent the spatially defined components of an ocean plate stratigraphy mélange that comprises the oldest part of the Olympic subduction complex. Seamount collision fundamentally modifies forearc geometry and subduction zone processes, and more broadly, our study provides new insight into the structural and depositional controls on the transition of the Ancestral Cascade forearc system to an accretionary margin by the Oligocene.