Aleutian Plain sediments and lithospheric plate motions

Abstract

Airgun reflection profiles indicate that the relict Aleutian abyssal plain consists of two tongues of well-stratified sediments deposited from turbidity currents which entered the region from a northerly direction. The distribution of the turbidites appears to have been largely governed by a topographic grain imparted to the basement surface by Late Cretaceous-Paleocene plate motions associated with the generation of the Great Magnetic Bight. High resolution 3.5 kHz sub-bottom profiles and sedimentation rate data suggest that the last channelized routes of turbidity current flows to the southern portion of the area were severed during the Late Miocene, 6.9 (±0.9) m.y. ago, and since then only pelagic sediments have accumulated. The change in the depositional regime can thus be associated in time with the change in the direction of sea-floor spreading in the northeastern Pacific and to the start of a period of severe deformation in southern Alaska. The pattern of sedimentation and the time at which the Aleutian plain became isolated from its source of terrigenous sediments are both consistent with models recently proposed for plate motions in the northeastern Pacific since the close of the Mesozoic. The present sedimentary data are not sufficient, however, to determine whether large relative motions between the Pacific and North American plates may have occurred during Tertiary time.