Australia: The Land Where Time Began |
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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. 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.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |