Abstract The Aleutian arc is the arcuate arrangement of mountain ranges and flanking submerged margins that forms the northern rim of the Pacific Basin from the Kamchatka Peninsula (Russia) eastward more than 3,000 km to Cook Inlet (Fig. 1). It consists of two very different segments that meet near Unimak Pass: the Aleutian Ridge segment to the west and the Alaska Peninsula- Kodiak Island segment to the east. The Aleutian Ridge segment is a massive, mostly submerged cordillera that includes both the islands and the submerged pedestal from which they protrude. The Alaska Peninsula-Kodiak Island segment is composed of the Alaska Peninsula, its adjacent islands, and their continental and insular margins. The Bering Sea margin north of the Alaska Peninsula consists mostly of a wide continental shelf, some of which is underlain by rocks correlative with those on the Alaska Peninsula. There is no pre-Eocene record in rocks of the Aleutian Ridge segment, whereas rare fragments of Paleozoic rocks and extensive outcrops of Mesozoic rocks occur on the Alaska Peninsula. Since the late Eocene, and possibly since the early Eocene, the two segments have evolved somewhat similarly. Major plutonic and volcanic episodes, however, are not synchronous. Furthermore, uplift of the Alaska Peninsula-Kodiak Island segment in late Cenozoic time was more extensive than uplift of the Aleutian Ridge segment. It is probable that tectonic regimes along the Aleutian arc varied during the Tertiary in response to such factors as the directions and rates of convergence, to bathymetry and age of the subducting

Abstract 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

Abstract Analyses of high-resolution seismic profiles have revealed the presence of a well-defined, massive submarine slide located at the north end of the Kayak Trough in the northern Gulf of Alaska. This slide is about 18 km long and 15 km wide, has a volume of about 5.9 km 3 , and has moved down a 1 ° slope. Sediment from the upper 2 m of the slide consists of low-strength, greenish-gray clayey silt. Morphologically this slide is a classic example, with a well-preserved pull-apart scarp in the headward regions, a well-developed toe, disrupted internal bedding, and hummocky surface topography. The age of the slide is unknown, but its clearly defined morphology in an area of very high sedimentation (7.5 to 15 m/1,000 yr) suggests that it is extremely young. The slide may have been generated by intense storm activity or earthquake-triggered, prolonged ground shaking. Laboratory and shipboard observations indicate that the slide sediments are very weak (peak shear strength of 0.02 kg/cm 2 ) with high water content. Other areas of the Gulf of Alaska are known to have thick accumulations of similar sediment, although a comparable slide has not been observed. The characteristics of this slide are indicative of problems that will have to be surmounted if this or similar areas in the Gulf of Alaska are to be used for pipeline emplacement or platform siting for petroleum production. High seismic risk makes the problems even more severe.