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 structure. Seismic velocities and the compositions of erupted lavas suggest that the Aleutian crust has a mafic bulk composition, in contrast to the andesitic bulk composition of continents. The silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the Aleutian intraoceanic are. Therefore, if island arcs form a significant source of continental crust, the bulk properties of are crust must be substantially modified during or after accretion to a continental margin. The pervasive deformation, intracrustal melting, and delamination of mafic to ultramafic residuum necessary to transform are crust into mature continental crust probably occur during are-continent collision or through subsequent establishment of a continental are. The volume of crust created along the are exceeds that estimated by previous workers by about a factor of two.
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