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Australia: The Land Where Time Began |
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Archaean Crust formation in Western Australia – Secular Change Recorded
According to Yuan, controversy remains concerning the mechanisms of
formation of continental crust in the
Archaean. The competing
mechanisms for the formation of continental crust are accumulation by
horizontal accretion, as in modern
subduction zones,
or by vertical accretion above upwelling zones in the upper mantle. A
change in the characteristics of continental crust at the transition
from the Archaean to the Proterozoic suggests continental crust did not
form in subduction zones before the late Achaean, at the earliest. In
this study seismic receiver function data was used to analyse the bulk
properties of continental crust in Western Australia, which had formed
and stabilised over a period of 1 Gyr during the Archaean. Yuan found
that the bulk seismic properties of the crust clustered spatially,
similar clusters being confined within the boundaries of tectonic
terranes. Yuan demonstrated that plume and subduction processes may have
both had as role in the formation of the crust throughout the Archaean
by the use of local Archaean crustal growth models. A trend was revealed
by correlating crustal age and bulk seismic properties of the crust: the
crust thickened gradually from about 3.5 Ga to the end of the Archaean
and its composition simultaneously became more evolved. It is proposed
by Yuan that the transition between crust that had been formed
dominantly by mantle plumes to crust that was formed by subduction
processes is reflected in this trend, the transition possibly reflecting
secular cooling of the mantle.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |