Structure and composition of the Aleutian island arc and implications for continental crustal growth

We present results of a seismic reflection and refraction investigation of the Aleutian island are, designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust. The Aleutian are has the requisite thickness (30 km) to build continental crust, but it differs strongly from continental crust in its composition and reflectivity structu re. Seismic velocities and the compositions of erupted lavas suggest that t he Aleutian crust has a mafic bulk composition, in contrast to the andesiti c bulk composition of continents. The silicic upper crust and reflective lo wer crust that are characteristic of continental crust are conspicuously la cking in the Aleutian intraoceanic are. Therefore, if island arcs form a si gnificant source of continental crust, the bulk properties of are crust mus t be substantially modified during or after accretion to a continental marg in. The pervasive deformation, intracrustal melting, and delamination of ma fic to ultramafic residuum necessary to transform are crust into mature con tinental crust probably occur during are-continent collision or through sub sequent establishment of a continental are. The volume of crust created alo ng the are exceeds that estimated by previous workers by about a factor of two.