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

摘要

New seismic wide-angle data from the eastern Aleutian Islands show a mafic composition and a 30-km-thick island-arc crust. Traveltimes of P and S refracted arrivals and prominent crustal and mantle reflectors observed to offsets of over 300 km were used to derive velocity models for the eastern Aleutian Arc between the islands of Atka and Unimak using a three-dimensional finite difference modeling and tomography code. We interpret the highest crustal P wave velocities of 7.2-7.4 km/s between about 12 and about 22 km depth to the south and north of the main volcanic line as remainders of preexisting oceanic crust into which arc magma is intruded. Apart from the pieces of oceanic crust, the velocities suggest an overall mafic composition for the arc, composed mainly of metabasalts, diorite and diabase in the upper crust, and garnet-granulite or amphibolite-to-hornblende in the lower crust. Reflected arrivals from the subducting Pacific plate at depths of 45-55 km beneath the fore-arc, together with P_n, show a mantle wedge with P wave velocities as low as 7.4 km/s, increasing with depth to about 8.1 k /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 greater depths best explains these observed velocities.