Structure of an island-arc: Wide-angle seismic studies in the eastern Aleutian Islands, Alaska

New seismic wide-angle data from the eastern Aleutian Islands show a mafic composition and a 30-km-thick island-are crust, Traveltimes of P and S refr acted arrivals and prominent crustal and mantle reflectors observed to offs ets of over 300 km were used to derive velocity models for the eastern Aleu tian Are between the islands of Atka and Unimak using a three-dimensional f inite difference modeling and tomography code. We interpret the highest cru stal P wave velocities of 7.2-7.4 km/s between about 12 and about 22 km dep th to the south and north of the main volcanic line as remainders of preexi sting oceanic crust into which are magma is intruded. Apart from the pieces of oceanic crust, the velocities suggest an overall mafic composition for the are, composed mainly of metabasalts, diorite and diabase in the upper c rust, and garnet-granulite or amphibolite-to-hornblendite in the lower crus t. Reflected arrivals from the subducting Pacific plate at depths of 45-55 km beneath the fore-are, together with P,, show a mantle wedge with P wave velocities as low as 7.4 km/s, increasing with depth to about 8.1 km/s with an average of about 7.7 km/s. A mantle composition that grades from mainly pyroxenite (probably ultramafic cumulates) near the Moho to dunite at grea ter depths best explains these observed velocities.