North Pacific Deep-Sea Ash Chronology and Age of Present Aleutian Underthrusting

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Abstract
Volcanic ash layers in North Pacific deep-sea sediments are distributed in a 1000-km wide belt bordering the volcanic arcs of the North and Northwest Pacific. Paleomagnetic and radiolarian stratigraphy combined with study of the physical properties of the ash provide tools for correlating and dating the ash layers. The belt of ash can be divided into two areas based on the chronology and composition of the ash layers; the first is in the Northwest Pacific with Japan, Kamchatka and the Kurils as its probable source, and the second is south of the active part of the Aleutian Islands (east of 175° E.), from which it was probably derived. In the northwest area the rates of sedimentation are high, in most cores exceeding 5 cm/1000 yrs. Some cores contain as many as 10 ash layers in sediments deposited in less than 300,000 years. The oldest ash encountered in this area is 1.3 m.y. old but it occurs near the outer limit of ash dispersal, so older ashes are to be expected. Ashes in cores south of the Aleutians indicate that at least 20 large andesitic eruptions have occurred since 1.8 m.y. B.P. Sediments between 1.8 and at least 2.8 m.y. contain no ashes, suggesting that 1.8 m.y. (lower Pleistocene) was the beginning of the present cycle. Previous work has shown that andesitic volcanism in island arcs is related to the depth of underthrusting of oceanic lithosphere beneath the arc. When the oceanic plate is underthrust to a depth of between 100 and 200 km the conditions are met for the formation of andesitic magma. By using measured rates and directions of sea-floor spreading it is possible to calculate that the underthrusting must have begun 10 to 14 m.y. B.P. in order to reach a depth of 100 to 200 km by 1.8 m.y. B.P. This age is in agreement with an age for the formation of the Aleutian Trench based on the end of turbidite deposition on the relict Aleutian abyssal plain. The initiation of underthrusting 10 to 14 m.y. B.P. could be explained either by a pause in sea-floor spreading prior to this, or a change in direction of the ridge axis, which caused a reorientation of the stress system of the Pacific margin and initiated a new phase of underthrusting.

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