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The Shumagin segment of the eastern Aleutian convergent margin extends into the Gulf of Alaska from the Alaska Peninsula. Here the shelf is narrower and the Shumagin Islands are much smaller than islands, in the adjacent Kodiak shelf area, but many of the rocks are similar. Insular exposures are comprised of late Cretaceous sedimentary rocks metamorphosed in a low-temperature environment and once buried about 10 km deep. Paleogene rocks that crop out in the Kodiak area may continue into the Shumagin area, but they have not yet been sampled beneath the Shumagin shelf. The Paleogene rocks are cut by a major unconformity that is overlain by upper Miocene rocks in the Kodiak area. A Neogene forearc basin, Shumagin basin (Bruns and von Huene, 1977; Bruns et al., in press), contains about 2.5-km-thick strata of probable late Miocene and younger age. Landward of the basin is the Border Ranges fault (Fisher and von Huene, 1984), which probably isolates the pre-Upper Cretaceous geology of the Alaska Peninsula from that of the Shumagin shelf. The Aleutian volcanic arc forms the backbone of the Alaska Peninsula. Bristol Bay and the Bering Sea shelf constitute the backarc basin.

Structurally, the Shumagin margin has all the elements associated with a major subduction zone. However, this segment differs from the central and eastern Aleutian sections (p. 10 and 20, this volume) in having no sediment ponded in the trench axis (von Huene, 1972). Seaward of the trench axis is Zodiak fan, a Paleogene deep-sea fan complex that

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