The crestal region of the 2,200-km-long (and mostly submerged) Aleutian Ridge is commonly an irregular tableland or summit platform. Rising above this platform, which generally lies at a depth shallower than about 200 m, are the Aleutian and Komandorsky Islands. Deeper areas of the summit platform include elongate and roughly rectangular shaped basins. Three of the larger summit basins, Amukta and Amlia basins and Pratt depression, lie along the northern edge of the summit platform and are aligned geomorphically with the loci of late Cenozoic volcanic centers. These basins are underlain by thick, slightly deformed sequences of late Cenozoic beds. Amukta basin (171.7°W) and neighboring Amlia (173°W) basin have a combined area of about 4,700 sq km (1,800 sq mi), and their floors are covered by about 1,000 m of water. They are underlain by an estimated 4,000 m of gently folded late Miocene through Holocene beds. This estimate is based on interpretations of magnetic, gravimetric, and seismic-reflection data. The deposits are dislocated along normal and high-angle reverse faults (growth types), and deformation appears to be most severe adjacent to a prominent normal fault bordering the northern sides of these basins. The floor of Pratt depression (179°E), approximately 850 m deep, also may be underlain by a thick sequence of late Cenozoic beds. A major fault scarp borders this basin on the south. The summit basins are grabens or half-grabens that began to form near the end of the Miocene, but perhaps not until the Pliocene. Andesitic volcanism accompanied their formation, and, we suspect, the basins may have subsided in response to a slight distension of the ridge crest caused by the generation of deep-seated magmas. Contemporaneous with subsidence and faulting, the basins filled with erosional debris derived from the adjacent ridge crest. The crest is underlain by deformed, altered, and plutonically intruded sedimentary and igneous rocks of Eocene through early Miocene age. Volcanic ash and diatomaceous debris are probably important minor constituents of the late Cenozoic basin fills. Seismic-reflection records suggest that much of the detritus reaching the central areas of t e basins was transported there by turbidity currents. Gaps or outlets in the northern walls of Amlia and Amukta basins allowed some of this debris, especially that of Pleistocene age, to bypass the basin and excavate large submarine canyons in thick late Cenozoic beds underlying the ridge's northern insular slope. In contrast, Pratt depression, which does not have outlets, may include a thicker sequence of Pleistocene strata.
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Jan 1, 2021
Gennady A Starodvorov + 1
