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

The Aleutian island arc crosses from the Pacific Ocean to the North-American continent at the island of Unimak. 3-D finite-difference traveltime inversion of our onshore–offshore seismic reflection/refraction data gives a velocity model of the crust and uppermost mantle. The arc crust is on average 30 km thick, but thickens to almost 40 km under the western Alaska Peninsula. The transition from oceanic arc to continental arc is characterised by a decrease in average velocity in the upper crust from about 6.5 km/s to less than 6.0 km/s, with no systematic change in the velocity of the lower crust. Throughout our study area, in the upper 15 km of the crust the highest velocities are observed in the fore-arc 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 velocities ranging from 7.6 to 8.2 km/s, in part due to the thermal gradient from cold fore-arc to hot back-arc. Whereas the Aleutian oceanic (fore-)arc has higher seismic velocities than average continental crust throughout the crust, the Peninsula section is close to the continental average in the upper c. 20 km of the crust. We infer that repeated episodes of arc magmatism can produce a felsic-to-intermediate upper crust as is observed in the continents, but arc magmatism produces a thicker 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, which is less evident or absent in a continental arc.