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The Chugach Terrane, a Cretaceous trench-fill deposit, southern Alaska

Summary The Chugach terrane of southern Alaska extends for approximately 2000 km along the margin of the Gulf of Alaska. A seaward flysch facies of the terrane, the Chugach flysch terrane, represents the fill of a Late Cretaceous trench and consists of structurally deformed turbidites with some mafic volcanic rocks. It is intruded by anatectic granitic plutons of early Tertiary age. The Chugach flysch terrane in most places is bounded to the north by a landward-dipping thrust fault separating it from mélange of the Chugach terrane at least as young as Late Cretaceous. To the south the flysch terrane contains oceanic volcanic rocks and is bounded by faults that separate it from Palaeogene turbidites or upper Mesozoic metavolcanic rocks. Interpretations of folds and faults in SW Alaska suggest that the Chugach terrane was deformed during NW-directed subduction. Palaeocurrents indicate primarily westward flow along the axis of the outcrop belt, and secondary southward transport. Turbidite facies associations indicate an east-to-west progression from inner-fan to middle-fan, outer-fan, fan-fringe and basin-plain deposits down the axis of the outcrop belt, and a bounding slope facies association to the north. Rock-fragment petrography of sandstone samples from the Chugach terrane indicates derivation from a magmatic arc that was increasingly dissected eastward and from an older subduction complex. The magmatic arc and adjacent shallow-marine forearc basin deposits are located to the north in southern Alaska. Palaeomagnetic data from the Chugach terrane and adjacent deposits indicate original deposition far to the south of the present latitude of Alaska. The entire magmatic arc-forearc basin-trench complex migrated northward in early Tertiary time, when it was probably oroclinally bent and accreted as a mesoplate to Alaska prior to the middle Miocene.