Archaean unstable stagnant lid tectonics

English Abstract:

It has been proposed that the early Archaean Earth was in an unstable stagnant lid mode, with non-cratonic areas dominated by oceanic ‘plateau’ type crust. Numerical models of convection under a thick conductive mafic to ultramafic crustal layer imply frequent basal delamination of garnetiferous rocks; with rapid recycling of subaqueously erupted lava into the mantle. The unstable sub-crustal convection pattern would prevent development of a sub-oceanic lithospheric mantle layer (SOLM); in contrast with the modern Earth where a SOLM forms gradually as oceanic lithosphere drifts away from the ridge axis thermal anomaly. The absence of a thick, cold, stiff, dense SOLM beneath Archaean oceanic crust removes the principal driving force for subduction. Archaean continental drift as a result of mantle traction on the sub-continental lithospheric mantle root would produce broad, ‘soft’ mafic accretionary orogens. Partial subcretion and anatexis of oceanic crust beneath the continental leading edge would produce voluminous syn-tectonic TTGs. Bédard & Harris 2014 (Geology) proposed that the S.Superior craton was disaggregated by a Neoarchaean mantle overturn; followed by reassembly of detached ribbon-continents and intervening oceanic tracts when the N.Superior block drifted south. Numerical modelling of global mantle convection on an unstable stagnant lid planet implies frequent early whole-mantle overturn events so as to evacuate mantle heat buildups. Modeling results imply that overturns would cease due to the progressive decay of radiogenic heat, and would then have been replaced by a mobile lid plate tectonic mode. Global mantle overturn events can explain synchronous tectono-magmatic pulses on different cratons. Areas of high vertical flux during overturns would create unusually thick crust and would be favored sites of craton genesis or reworking. Vigorous mantle flow would drive faster continental drift and could also correspond to phases of enhanced tectonism.