Australia: The Land Where Time Began

A biography of the Australian continent 

Ocean Arcs – Generation of Continental Crust

Decompression melting of the upper mantle at mid-ocean ridges forms thin oceanic crust, but the origin of thick, buoyant continental crust has been enigmatic. According to Gazel et al. magmas that are erupted above intra-ocean subduction zones, places where oceanic lithosphere subducts beneath other oceanic lithosphere, may form juvenile continental crust. However, it has remained unclear why the formation of andesitic continental crust at the surface would result from the subduction of oceanic crust that is dominantly basaltic. In this paper Gazel et al. reconstructed the evolution of the Central American land bridge, which formed above an intra-oceanic subduction system over the past 70 Myr, by the use of geochemical and geophysical data. It was found by the authors1 that the geochemical signatures of the erupted lavas evolved from basaltic to andesitic about 10 Ma, which coincided with the onset of subduction of more oceanic crust that had originally formed above the Galápagos mantle plume. They also found that seismic P-waves travel through the crust at velocities that are intermediate between those that have typically been observed for oceanic crust and continental crust. In order to quantitatively correlate geochemical composition with the average velocity of the P-wave of arc-crust globally they developed a continentality index. They concluded that, though the formation and evolution of continents may involve many processes, the melting of enriched oceanic crust within a subduction zone, a process that was probably more common in the Archaean, can result in the production of juvenile continental crust.

Sources & Further reading

Gazel, E., J. L. Hayes, K. Hoernle, P. Kelemen, E. Everson, W. S. Holbrook, F. Hauff, P. van den Bogaard, E. A. Vance, S. Chu, A. J. Calvert, M. J. Carr and G. M. Yogodzinski (2015). "Continental crust generated in oceanic arcs." Nature Geosci 8(4): 321-327.


Author: M. H. Monroe
Last updated: 06/04/2015
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