Wide-angle seismic investigation into the transition from island arc to continental crust

Fliedner, M.

Bullard Laboratories, University of Cambridge, Cambridge CB3 0EZ, UK

Klemperer, S.

Department of Geophysics, Stanford University, Stanford, CA 94305-2215, USA

We have collected seismic wide-angle data from a multi-channel reflection survey in the Aleutian Islands where the volcanic island arc meets the American continental margin. Traveltimes of refracted and reflected arrivals from the crust and uppermost mantle were used to derive P and S velocity models. The arc crust is on average 30 km thick with a maximum thickness of 40 km under the western Alaska Peninsula. The transition from island-arc to continental crust is characterized by a decrease in average velocity in the upper crust from velocities of about 6.5 km/s to less than 6.0 km/s. In the upper 15 km of the crust, the highest velocities are observed in the fore-arc just south of the volcanic line, possibly from gabbroic rocks of pre-existing oceanic crust. In the lower crust velocity changes along the arc are less pronounced, the lowest velocities of just 6.2 km/s are found close to the volcanic line. Whereas the island section has higher seismic velocities than average continental crust throughout, the Peninsula section is close to the continental average in the upper c. 20 km of the crust. This suggests that arc magmatism produces a larger mafic lower crust than the average continent retains. Some of the excess mafic material in the island-arc crust can be attributed to pre-existing oceanic crust.