Abstract

by: Fiona Darbyshire, Centre de recherche GEOTOP, Université du Québec à Montréal, Canada

Eastern Laurentia, comprising both the present-day North American and Greenland continents, records a complex tectonic history spanning at least 3 billion years and numerous supercontinent cycles. The region is an excellent natural laboratory for studies of early Earth processes, geodynamic evolution of continents and the development of modern-style plate tectonics. Over the last 10-15 years, detailed studies of crust and upper-mantle structure in this region have been facilitated via the installation of new distributed seismograph networks, such as POLARIS in Canada and GLATIS/GLISN in Greenland. Knowledge of present-day lithospheric structure is vital to understanding the history and evolution of the continents, and the geodynamic processes that gave rise to their formation.

Group-velocity tomography, using regional earthquakes recorded by the GLATIS/GLISN network, has been used to develop a pseudo-3D shear wavespeed model of the Greenland continent and western North Atlantic for the crust and the uppermost mantle (to a depth of ~100 km). Crustal thickness across Greenland ranges from ~25-50 km and significant wavespeed heterogeneity is displayed in both the upper and the lower crust.

In eastern Canada, new studies using receiver function analyses are shedding new light on crustal architecture. Here we see clear distinction between Archean, Proterozoic and Phanerozoic crust. In general, Proterozoic regions show thicker and more mafic crust than either the Archean or Phanerozoic; Archean terranes tend to be more homogeneous than younger crust, with a sharp Moho, in contrast to a more diffuse crust-mantle transition found beneath Proterozoic terranes. A similar pattern is found in northern Canada, in the Hudson Bay region. The Archean crust is relatively thin and structurally simple, in contrast to the thicker, more complex Proterozoic crust. This region records a large-scale Paleoproterozoic collision, the Trans-Hudson Orogeny, and the new crustal-structure studies suggest that this orogen has comparable characteristics to the present-day Himalayan continental collision.