OCGC Seminar - Dr. Jean Bédard

   

Continental drift on subductionless stagnant lid planets,
the Archaean Earth and Venus


Dr. Jean Bédard 
Howard Street Robinson Lecturer (2014-15),
Geological Survey of Canada, Québec

Earth and Venus

Thursday, March 19th, 2015
11:30 a.m.  

233 Advanced Research Complex (ARC) 
University of Ottawa

 

Abstract

Archaean plate tectonics is inferred because of the existence of calc-alkaline magmatism and horizontal tectonics. However, Phanerozoic subduction zone lithofacies and geochemical signatures differ from those of putative Archaean arcs, despite clear evidence for bulk shortening and terrane accretion. Bédard et al. (2013 Prec Res 229) proposed that proto-cratons that develop a deep high-viscosity mantle keel would become subject to pressure from mantle currents and could drift, resolving this paradox. Immature cratons or oceanic plateaux would not have a strong mantle keel and so would be static. Drifting Archaean cratons would accrete basaltic plateaux and other debris to their leading edge. Overridden oceanic crust would be thrust (subcreted) deep enough to melt in the garnet field and generate syntectonic pulses of TTG. This differs from plate tectonics because of the absence of subduction. Similarities between Archaean and Phanerozoic magmas and tectonic styles result because modern continents also drift in response to mantle currents, not plate boundary forces as commonly assumed.  Active advances of continental masses over unusually thick or buoyant oceanic crustal segments result in flat-slab subduction, and typically enhance uplift and deformation. Compressional thickening and anatexis of the base of the thickened upper plate crust in such regions leads to localized generation of high-Sr/Y high-La/Yb TTG-like magmas similar to Archaean ones.  Venus is an analogue for a non-plate-tectonic  Archaean Earth and structures similar to those observed in the Superior Craton are interpreted from radar images. Since there is no evidence for subduction or seafloor spreading, lateral displacement of ‘craton-like’ highlands on Venus would result from mantle tractions at their base in a stagnant lid convection regime, a regime which preceded development of plate tectonics on Earth. In the southern and western Superior Craton in Canada, the formation of granite greenstone sequences in a plume-related volcanic plateau, and its subsequent deformation, can be generated through geodynamic processes similar to those on Venus without having to invoke modern-style plate tectonics. We propose that the S Superior craton was partly disaggregated and extensively reworked as a result of the arrival of a major plume swarm at ca 2750-2720 Ma, with more juvenile ‘terranes’ like the Abitibi-Wawa being new simatic crust formed as the end-result of extensive lithospheric necking and corrosion of the lithospheric mantle. Subsequently, a shift in mantle convection patterns (or the arrival of a different plume located to the N) caused the deep-keeled Hudson’s Bay terrane to drift south and re-accrete the partly-dismembered fragments in N to S sequence.