Crustal and Upper Mantle Structure of the Central Aleutian Arc

Abstract

Seven reflection profiles over the central Aleutian trench and terrace are presented. Undeformed trench turbidites terminate abruptly against the inner wall which is acoustically opaque for the next 30 km. The southern half of the Aleutian terrace is an uplifted anticlinal arch, Hawley ridge. Acoustically visible sediments thicken to at least 2 km in the northern part of the terrace. A sonobuoy refraction profile near the terrace axis indicates 5 to 8 km of sediment over an oceaniclike crust (6.4 km per sec). Incorporation of this data with previous near-bottom geophysical and dredge work suggests that thrusted and complexly deformed sediments outcrop between the trench and terrace. Two alternative two-dimensional gravity models extending 200 km south and 350 km north of the trench are presented which take into account both the crustal and upper mantle structure. The fundamental ambiguity between the two models is the dip of the Benioff zone beneath the Aleutian terrace (between 5° and 20°). Low-density (2.2 to 2.4 g per cc) deformed sediments appear to make up the first 30 to 70 km north of the trench axis. The total thickness of sediments near the terrace axis appears closer to 7 to 8 km according to the gravity data. The density contrast between the descending lithosphere and surrounding aesthenosphere appears to be near +0.05 g per cc for an 80-km-thick plate, but a low-density zone in the crust and mantle beneath the volcanic chain is also necessary to fit the observed gravity. Assuming that the Pacific crust inverts into ecologite at a depth of 30 km produced reasonable results, but this estimate is difficult to confirm with present data. The effects of modifying some of these boundaries and density contrasts are calculated and discussed.