Thin oceanic crust is formed by decompression melting of the upper mantle at mid-ocean ridges, but the origin of the thick and buoyant continental crust is enigmatic. Juvenile continental crust may form from magmas erupted above intra-oceanic subduction zones, where oceanic lithosphere subducts beneath other oceanic lithosphere. However, it is unclear why the subduction of dominantly basaltic oceanic crust would result in the formation of andesitic continental crust at the surface. Here we use geochemical and geophysical data to reconstruct the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70 Myr. We find that the geochemical signature of erupted lavas evolved from basaltic to andesitic about 10 Myr ago - coincident with the onset of subduction of more oceanic crust that originally formed above the Galápagos mantle plume. We also find that seismic P-waves travel through the crust at velocities intermediate between those typically observed for oceanic and continental crust. We develop a continentality index to quantitatively correlate geochemical composition with the average P-wave velocity of arc crust globally. We conclude that although the formation and evolution of continents may involve many processes, melting enriched oceanic crust within a subduction zone - a process probably more common in the Archaean - can produce juvenile continental crust.
Department of Geosciences, Virginia Tech, 5041 Derring Hall (0420), Blacksburg, VA, United States; Department of Geology and Geophysics, University of Wyoming, 1000 E. University Avenue, Laramie, WY, United States; GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, Kiel, Germany; Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY, United States; Department of Statistics, Virginia Tech, 250 Drillfield Drive, Blacksburg, VA, United States; Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada; Department of Earth and Planetary Sciences, Rutgers University, Wright Laboratory, 610 Taylor Road, Piscataway, NJ, United States; Department of Geological Sciences, University of South Carolina, 701 Sumter Street, Columbia, SC, United States