Dallas H. Abbott, Rebecca Drury and Walter D. Mooney
Continents as lithological icebergs; the importance of buoyant lithospheric roots
Earth and Planetary Science Letters(June 1997), 149(1-4):15-27
Index Terms/Descriptors: basalts; buoyancy; Circum-Pacific region; continental crust; continents; crust; igneous rocks; lithosphere; mantle; Ontong Java Plateau; Pacific Ocean; Pacific region; plate tectonics; subduction; theoretical models; thickness; volcanic rocks; West Pacific
An understanding of the formation of new continental crust provides an important guide to locating the oldest terrestrial rocks and minerals. We evaluated the crustal thicknesses of the thinnest stable continental crust and of an unsubductable oceanic plateau and used the resulting data to estimate the amount of mantle melting which produces permanent continental crust. The lithospheric mantle is sufficiently depleted to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25-27 km. These unsubductable oceanic plateaus and hotspot island chains are important sources of new continental crust. The newest continental crust (e.g., the Ontong Java plateau) has a basaltic composition, not a granitic one. The observed structure and geochemistry of continents are the result of convergent margin magmatism and metamorphism which modify the nascent basaltic crust into a lowermost basaltic layer overlain by a more silicic upper crust. The definition of a continent should imply only that the lithosphere is unsubductable over > or =0.25 Ga time periods. Therefore, the search for the oldest crustal rocks should include rocks from lower to mid-crustal levels.