Plenary Address

The Origin of Laurentia
By Paul F. Hoffman
Department of Earth and Planetary Sciences, Harvard University, and School of Earth and Ocean Sciences, University of Victoria

Recent studies demonstrate the existence, age and subduction polarity of a collisional orogen between the Hearne and Rae cratons, the Snowbird tectonic zone.  Now, the relative ages and tectonic polarities of all major Orosirian (2050-1800 Ma) collisions between the Archean cratons of proto-Laurentia are tentatively known.  The oldest geosutures bound the Rae craton, which collided first with the Slave craton in 1.97 Ga, forming the Thelon orogen, and then with the Hearne craton in 1.92 Ga.  Other Orosirian geosutures are younger than 1.90 Ga.  The Rae craton was on the upper plate for both the 1.97 and 1.92 Ga collisions.  After a stint within the lower plate for the 1.88 Ga arc-continent collision in Wopmay orogen, the Rae craton returned to the upper plate during 1.88 to 1.84 Ga subduction beneath the Great Bear magmatic arc and 1.86 to 1.83 Ga subduction beneath the Wathaman-Narsajuaq magmatic arc, the latter leading to 1.83-1.82 Ga collision with the Superior craton forming the Trans-Hudson orogen.  Subduction and mantle downflow existed beneath the Rae craton from the start of the Orosirian assembly of Laurentia.  Geodynamically, this suggests that the Rae craton was the first to be captured in the mantle downwelling region, and thereafter served as a backstop for other incoming cratons and composite cratons.  The Rae craton is the geodynamic origin around which proto-Laurentia (i.e. pre-Grenville Laurentia) was assembled.  It remains to be determined if it was also the origin of Nuna, the medial Proterozoic supercontinent, within which it occupied a central location.