STRATIGRAPHIC AND STRUCTURAL CHARACTERIZATION OF THE EOCENE BLUE MOUNTAIN UNIT TO UNDERSTAND TRENCH-FOREARC INTERACTIONS DURING INITIATION OF THE OLYMPIC SUBDUCTION COMPLEX AND ANCESTRAL CASCADES ARC

The Eocene accretion of the Siletzia oceanic plateau dramatically altered the North American margin in Washington and Oregon. Following collision, renewed subduction outboard of Siletzia basalts (Crescent Fm.) set the stage for establishment of the new forearc region and Ancestral Cascade Arc. The key components of the forearc during this time were the Eocene—Miocene metasedimentary and volcanic rocks of the Olympic subduction complex (OSC) and age-equivalent deep-marine sedimentary strata of the forearc basin. The Blue Mountain Unit (BMU) is a characterized by sedimentary, volcaniclastic, and volcanic rocks and is mapped between the Crescent basalts and rocks of the OSC. Previous research suggested the BMU was interbedded with lowermost Crescent basalts (>50 Ma), but recent U-Pb detrital zircon geochronology demonstrated that it is younger (<45 Ma) and is structurally juxtaposed against Siletzia.

We propose a new depositional and structural model for the BMU based on detailed lithofacies mapping and U-Pb zircon geochronology, whereby the unit represents the oldest part of the OSC. We base this interpretation on the structural juxtaposition of two distinct lithofacies that we consider to be associated with different tectonic plates: 1) Volcanic and distal marine sedimentary strata from the subducting oceanic plate and 2) Continentally derived trench turbidites from the upper plate. Together these lithofacies represent the spatially defined components of an ocean plate stratigraphy mélange. A tuff interbedded with deep-marine strata was dated using CA-ID-TIMS and yields an eruption age of ~48 Ma, clearly indicating some ocean plate affinity rocks incorporated into the new subduction complex are latest Early Eocene. New detrital zircon geochronology (CA-LA-ICP-MS) of three samples are integrated with previous data from Eddy et al. (2017) and are compared to forearc basin age spectra to better understand sediment routing pathways to the trench. The interpretation of the BMU as the oldest part of the OSC can explain the previously debated structural and stratigraphic relationships of the BMU with adjacent units, and more broadly, provide insight into trench-forearc processes during initiation of the OSC and Ancestral Cascade Arc.