212 Reconstructing Earth Surface Processes in Orogenic Systems

Advances and applications of detrital and stratigraphic records for reconstructing lithospheric, climatic, and earth surface interactions over geologic time scales.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 303AB

The stratigraphic and geochronologic record for seamount collision and Eocene initiation of the Olympic subduction complex in the Blue Mountain Unit, western Washington

Paper number: 212-7
Program: Technical Sessions
Time: 3:10 PM – 3:30 PM

Authors

Abstract

The Eocene accretion of the Siletzia oceanic plateau reconfigured the continental margin in Washington and Oregon and set the stage for the development of the modern Cascades Arc, forearc basin system, and subduction complex. Key components of the forearc at this time were the Eocene—Miocene metasedimentary and volcanic rocks of the Olympic subduction complex (OSC) and age-equivalent deep-marine (peripheral rock sequence) to nonmarine (Puget Group) sedimentary strata of the forearc basin. The Eocene Blue Mountain Unit (BMU) also overlaps in age with both the strata of the subduction complex and forearc basin but is structurally positioned between Siletzia basalts (Crescent Fm.) along the Crescent Fault and against OSC strata by the Hurricane Ridge Fault. Originally, the BMU was interpreted as the stratigraphic base of the Crescent basalts (>50 Ma), but recent U-Pb detrital zircon geochronology determined it is younger (<45 Ma) and lies structurally beneath Crescent Fm. basalts.

We propose a new depositional and structural model for the BMU based on detailed lithofacies mapping and U-Pb detrital zircon geochronology (N=5; n=1389) at six locations along the northeastern Olympic Peninsula. We interpret volcanic and sedimentary strata of the BMU to represent ocean plate and seamount stratigraphy that are structurally juxtaposed against Siletzia basalts as 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. Thick successions of coherent to broken formations of volcanic strata are characteristic of seamount stratigraphy and have undergone greenschist-facies metamorphism. Additionally, new U-Pb detrital zircon geochronology (n=5; n=1389) from the trench turbidite lithofacies of the Blue Mountain Unit and a sandstone in the Needles-Gray Wolf Lithic Assemblage (OSC) show similar detrital zircon age spectra to forearc basin strata, suggesting sediment was also accumulating within the trench and was incorporated into the subduction complex by frontal accretion and/or underplating. Our study provides new insight into processes that were associated with subduction complex initiation during establishment of the Cascade Arc following Siletzia’s accretion, including a period of subsequent seamount collision.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414376

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