Progressive deformation of a Tertiary Trench Slope, Kodiak Islands, Alaska

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Abstract
The Eocene Sitkalidak Formation comprises the youngest and seaward‐most deep‐sea sequence exposed in the Kodiak Islands, which border the eastern Aleutian Trench. Fold orientation, structural style, and stereographic projections of poles to bedding define two structural units in the Sitkalidak Formation, which are geometrically distinct from overlying non‐marine and marine shelf rocks of Oligocene and Miocene age. Landward vergence, intense deformation with stratal disruption, a relatively seaward structural position and sedimentologic considerations all argue that the more deformed portion of the Sitkalidak Formation constitutes an obductively offscraped, trenchfilling fan sequence. Conversely, regular folding, stratal continuity, and relatively landward structural position suggest that the less deformed portion of the Sitkalidak Formation accumulated and was deformed as a slope basin or slope apron deposit. In the more deformed portion of the Sitkalidak Formation, zones of stratal disruption resemble melange and are demonstrably of tectonic origin as indicated by elongated, slickensided boudins with microscopic shear and solution zones. The Sitkalidak Formation was rapidly deformed, uplifted, and unconformably overlain by the Oligocene non‐marine Sitkinak Formation. Zeolite facies metamorphism of the Sitkalidak Formation followed, as a consequence of burial during Oligocene and Miocene time. Vitrinite reflectance and porosity data respectively define a peak temperature of 100–125°C and a maximum burial of 2.4–3.9 km, suggesting a paleogeothermal gradient of about 30°C/km during Miocene time.