OCGC Seminar - Illuminating the Architecture and Evolution of the North American Continent

   

ILLUMINATING THE ARCHITECTURE AND EVOLUTION OF THE NORTH AMERICAN CONTINENT USING MILLIONS OF BROADBAND SEISMOGRAMS 

 

Dr. Andrew Schaeffer
University of Ottawa 

gravity map

 

Thursday, November 26th, 2015
11:30 a.m.  

233 Advanced Research Complex (ARC)  
University of Ottawa 

 

Abstract

The North American continent has had a long, eventful tectonic history. The assembly of the stable cratonic core has undergone numerous collisions and accretion at its boundaries, major rifting episodes within it, as well as the loss of ancient lithosphere beneath parts of it. Seismic tomography offers rich evidence on the structure and evolution of the cratonic lithosphere. Furthermore, with the continued deployment of the USArray during the last decade, the North American continent has now been densely sampled with broadband seismic data, spanning from the west to east coasts, and now the north. We utilize a sophisticated, efficient waveform inversion method applied to a massive dataset of more than 1.5 million vertical component broadband seismograms to constrain the upper mantle beneath North America.

Using this new model, the internal structure of the Craton is resolved in detail, with clear delineation of the ancient cratonic lithosphere from the recently and actively deforming continental margins. The northern boundaries of the cratonic lithosphere closely follow the coastlines, with North America’s and Greenland’s lithospheric roots clearly separated. Strong lateral velocity gradients at depth observed in western Canada indicate the transition from cratonic lithosphere to Cordillera closely follows the surface trace of the Deformation Front. Elevated velocities between the Great Bear Arc and Beaufort Sea provide convincing evidence for the recently proposed ‘MacKenzie Craton’, which remains unexposed at the surface due to overlying sediments.  On the eastern margin of the continent, where multiple episodes of continental rifting are superimposed, the craton boundary coincides with the western extent of the Appalachian orogenic front, with significantly lower lateral velocity gradients than in the west.

Within the cratonic interior, the lithosphere surrounding the 1 Ga failed Mid-Continental Rift shows a reduction in wave speeds compared to the surrounding high-velocities indicative to the craton. When combined with geochemical and petrological analysis of nearby diamondiferous kimberlites and peridotites, the evidence indicates a thermo-chemical alteration of the sub-continental lithospheric mantle resulted in the loss of compositional buoyancy, and subsequent lithospheric thinning throughout the region. Finally, we examine the spatial extent of the lithospheric mantle root and variations in the depth of the mechanical boundary of the lithosphere across the continent, and compare them with respect to the spatial location of past kimberlite eruptions.