Abstract

A simple model of the probable topographic and thermal consequences of subducting an oceanic spreading center at an island arc predicts three geologic effects: (1) shoaling and subaerial emergence of the crest of the arc, (2) decrease or cessation of subduction-related magmatism, and (3) regional low-grade thermal metamorphism (ΔT ≃ 100 to 300 °C) of the arc rocks. All three of these phenomena are recorded in the geology of the Aleutian Islands, and the following sequence of events is indicated: (1) diminution of magmatism on approach of the Kula Ridge in middle Eocene time (≃45 m.y. ago), marked by a conformable transition from volcanic-rich to volcanic-poor early-series rocks; (2) shoaling and emergence of the crest of the Aleutian arc in late Eocene to Oligocene time, marked by a deep- to shallow-marine transition in sedimentation and then an arc-wide unconformity above the early series and its probable “submarine equivalent,” the now-dissected Aleutian crestal platform; (3) subduction of the Kula Ridge and greenschist metamorphism of the early-series rocks at about 30 to 35 m.y. ago, inferred from K-Ar ages; (4) subsidence of the arc down the south flank of the Kula Ridge in middle Oligocene to Miocene time, as the Pacific plate was subducted; and (5) abrupt resumption of arc magmatism 15 m.y. ago. This history of events is consistent with the timing of plate motions in the North Pacific and suggests that there has been essentially continuous underthrusting at the Aleutian Trench since at least 45 to 50 m.y. ago, with subduction of 900 to 1,000 km of ocean floor since 30 to 35 m.y. ago.

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