Abstract

Crust formed in volcanic arcs is accreted to continents over geologic time, serving as the source material for new continental crust in Earth's present and recent past. Geochemically, arcs are found to be significantly more mafic than bulk continental crust. Reconciling this compositional difference either requires that arc crust evolves to match the composition of continents, or suggests that the processes forming arcs in the present differ from those of the geologic past. We present a new along-strike seismic model of an intact segment of the Aleutian Arc, and use thermodynamic models to show that its overall composition is mafic compared to the composition of bulk continental crust. Punctuated low velocity anomalies imaged in the lower crust beneath Atka and Tanaga are interpreted as more felsic and hotter than surrounding material, suggesting that volcanic centers within the segment are evolving at different rates.