305-8 The Mechanics of Ridge Subduction: Insights from an Exhumed Accretionary Wedge Thrust Shear Zone in Southern Alaska

Session: Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology (Posters)

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Abstract:

The Chugach complex represents the Jurassic-to-Cretaceous paleo-accretionary wedge of the southern Alaskan margin. In an unusual exposure on Fox Island near Seward, greenschist-facies metasedimentary rocks of the Chugach are juxtaposed along a high-strain, thrust-sense shear zone against an underlying remnant spreading ridge known as the Resurrection ophiolite. We present new data from the Fox Island shear zone, including field and microstructural observations, Raman Spectroscopy of Carbonaceous Material (RSCM), quartz paleopiezometry, and zircon U-Th/He thermochronology, to explore the impact of spreading ridge subduction on a sediment-rich accretionary margin. 

While the Chugach metasediments exhibit variable deformation across the Kenai Peninsula, they are intensely deformed at Fox Island, displaying mylonitic fabrics distributed in anastomosing zones across a total thickness of ~1.5 km. Embedded within the shear zone are meter- to 10s-of-meters-thick metabasaltic lenses, interpreted to be remnant, detached slivers of the Resurrection Ophiolite entrained during progressive ridge subduction.  RSCM results indicate elevated peak temperatures (~380–400 °C), exceeding typical values for the Chugach metasediments elsewhere (300–350 °C) and suggesting deeper subduction. In contrast, structurally underlying metasediments record significantly lower temperatures (~270 °C) and conformably overlie the ophiolite, with a brittle fault contact between the higher-temperature, high-strain zone and the cooler ophiolite cover sequence.  These relationships suggest an early phase of deeper subduction with ductile entrainment of ridge fragments to >15 km depth, followed by mechanical jamming at shallower levels (~10-15 km) that led to the exhumation of previously subducted ridge and sedimentary material along the Fox Island shear zone. Preliminary zircon U-Th/He data indicate final exhumation of the shear zone at ~30 Ma.

We propose that the Fox Island shear zone offers a valuable case study for understanding the response of an accretionary wedge within the seismogenic zone during spreading-ridge subduction. These findings have broader implications for interpreting thermal and stress regimes in underplated wedge sediments and may serve as a paleo-analog for modern subduction-ridge systems such as those along the Chilean margin.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4899

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