Abstract

A Cretaceous turbidite sequence, a probable trench deposit, forms the central portion of a Triassic to early Tertiary deep-water sedimentary and igneous sequence exposed along the Shumagin-Kodiak shelf, southwestern Alaska. The Cretaceous turbidite sequence was deformed initially in a partially lithified state with the development of axial plane slaty cleavage. Fold axes parallel the existing continental shelf edge, trending northeast and west-northwest in the outer Shumagin and Sanak Islands, respectively. Folds are predominantly overturned seaward, axial surfaces dipping landward. Locally, units may be described as broken formations, though no mélanges were observed. The style of this early folding is consistent with, but not diagnostic of, gravity gliding. Alternatively, the rocks may have been deformed by underthrusting at the trench inner wall. At strain rates of 10−13 to 10−14 per sec (calculated assuming underthrusting), the trench sediments may have undergone “strain hardening” (caused by increasing internal grain friction and cementation during dewatering) allowing transmission of an externally applied stress.

Thick homoclinal sections dip toward the continent, suggesting tilting independent of folding. This tilting was probably associated with uplift along major high-angle faults trending parallel to the continental margin. The early folding, tilting, and uplift was followed by a second, minor, brittle deformation apparently associated with regional granodiorite and quartz diorite intrusions 60 m.y. ago.

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