Wed 3rd May @ noon, Rm 312.222 

Abstract:

Field, geochronological, isotopic and geophysical studies do not support plate tectonic models for subduction and arc accretion in Archaean granite-greenstone terrains. Geophysical data suggest Neoarchaean granite-greenstone sequences of the southern Superior Province in Canada formed during rifting of an older, composite Superior I craton and not as a series of accreted arcs and micro-continents as previously interpreted; a similar model is proposed for the Yilgarn Craton. How then do we explain regional folding, ductile shearing and faulting in Archaean granite-greenstone belts?

Venus, where there is no plate tectonics, was studied to see if regional shortening and large displacements of discrete blocks can occur without plate tectonics. Structures in basalt plains on Venus imaged by Magellan radar were previously attributed to mantle plumes in a stagnant lid or transitional convection regime. Interpretations of radar and filtered bouguer gravity data, however, provide a completely new view of our ‘sister planet’ that has significant repercussions in evaluating models for Archaean tectonics on Earth:

• Bouguer gravity lows imply the presence of granitic or other felsic rocks in craton-like plana, so Venus is more similar to Earth than previously thought.
• Regional transcurrent faults produce indentation and lateral escape structures resembling the Himalayan-Tibetan orogen and intraplate tectonics of central Australia.
• Fold interference patterns, refolded shear zones and cross-cutting rifts imply superposed deformation.

Structures on Venus are interpreted to result from mantle flow directed away from upwelling, mantle plume-related rifts interacting with the deep keels of plana, along with lithospheric drips, i.e. similar to the model proposed for Archaean tectonics. Venus shows that plate tectonics is not required to produce deformation features in Archaean terrains on Earth.

Short bio:

Since 2003, Lyal Harris is a professor of structural geology at INRS-ETE, an applied research university in Québec City, Canada (he previously taught at UWA). His research integrates enhancement and interpretation of geophysical data with field studies and analogue modelling for regional structural and tectonic syntheses. He is especially interested in the link between deep crustal and upper mantle structures and mineral deposits, and non-plate tectonic models for the Archaean Earth. Reinterpreting the tectonics of Venus and developing new Archaean tectonic models with Geological Survey of Canada (GSC) colleague Jean Bédard received an award as one of the top 10 scientific discoveries in Québec for 2014 by Quebec Science and the 2015 GSC team research award.