Crustal structure transition from oceanic arc to continental arc, eastern Aleutian Islands and Alaska Peninsula

The Aleutian island are crosses from the Pacific Ocean to the North-America n continent at the island of Unimak. 3-D finite-difference traveltime inver sion of our onshore-offshore seismic reflection/refraction data gives a vel ocity model of the crust and uppermost mantle. The are crust is on average 30 km thick, but thickens to almost 40 km under the western Alaska Peninsul a. The transition from oceanic are to continental are is characterised by a decrease in average velocity in the upper crust from about 6.5 km/s to les s than 6.0 km/s, with no systematic change in the velocity of the lower cru st. Throughout our study area, in the upper 15 km of the crust the highest velocities are observed in the forearc just south of the volcanic line. In the lower crust, the lowest velocities of just 6.2 km/s are found close to the volcanic line. The uppermost mantle is quite heterogeneous with velocit ies ranging from 7.6 to 8.2 km/s, in part due to the thermal gradient from cold fore-arc to hot back-are. Whereas the Aleutian oceanic (fore-)arc has higher seismic velocities than average continental crust throughout the cru st, the Peninsula section is close to the continental average in the upper c. 20 km of the crust. We infer that repeated episodes of are magmatism can produce a felsic-to-intermediate upper crust as is observed in the contine nts, but are magmatism produces a thicker mafic lower crust than the averag e continent retains. Some of the excess mafic material in the island-arc cr ust can be attributed to pre-existing oceanic crust, which is less evident or absent in a continental are. (C) 2000 Elsevier Science B.V. All rights r eserved.