Aleutian Arc

Abstract

The Aleutian Arc is the orogenic link, some 3200 km long, between the American and Asian continents and separates off the Bering Sea Basin from the Pacific Ocean. There is clear evidence of the antiquity—Cretaceous or earlier—and of the stages of development—submarine, basic (ophiolitic) volcanism followed by subaerial, andesitic volcanism—of the arc. Deformation occurred between these stages and, in contrast to the western Pacific arcs, an outer non-volcanic arc is absent, unless represented by the Aleutian Terrace. Nor are paired metamorphic belts present here. Segment : this is a 400 km long section close to the centre of the Arc, chosen to include the Andreanof Islands (Fig. 1). The Arc as a whole varies in width from 125 km to 235 km, averaging 165 km. Zones : four zones, three entirely submarine, are recognized. On the south, the well-defined Aleutian Trench (zone 1) is over 7000 m deep. The Aleutian Terrace (zone 2) is a wide bench feature at around 4000 m beneath sea level from which the Aleutian Ridge (zone 3) rises steeply. The latter includes all the islands of the Arc. To the north of the Arc lies the Aleutian Basin (zone 4), a broad submarine plain at around 3500 m beneath sea level. History : the oldest rocks exposed are the submarine, ophiolitic, early Marine Series (late Cretaceous to early Miocene); the initiation of the volcanism must have occurred earlier, but the date is unknown. In M. Miocene times volcanism ceased, arching and faulting took place and plutonic rocks