PROTEROZOIC CRUSTAL GROWTH: INSIGHTS FROM GEOLOGICAL MAPPING AND LITHOPROBE GEOPHYSICAL PROFILES IN CANADA

Most of the Canadian Shield (Laurentia) was assembled as a result of Proterozoic accretionary and collisional tectonics around Archean nuclei, which began with the Slave-Rae plate interaction at about 2.02 Ga and ended with the final assembly of Rodinia about 1.0 Ga ago. A simplified reconstruction of the accretion and collision history suggests that tectonic assembly occurred in three phases characterized by: i) amalgamation of a “Slave-Rae-Hearne-Wyoming-Sask-Meta Incognita” continent during the interval 2.2-1.83 Ga, ii) collision the Superior and North Atlantic cratons with the above collage at about 1.83-1.82 Ga, and iii) migration of convergence to the southeastern margin of Laurentia in a Cordilleran-type framework during the interval 1.78-1.20 Ga, followed by Grenvillian (1.20 – 1.00 Ga) reactivation of part of that margin. Advances made in 3D modeling of crustal architecture by the multidisciplinary LITHOPROBE program, suggest that although a substantial amount of juvenile material was generated during the Paleo- and Mesoproterozoic, a greater proportion than previously recognized is actually represented by reactivated Archean crust that appears to be mechanically coupled with buoyant sub-continental mantle lithosphere. In general, juvenile Proterozoic crust seems to be preferentially preserved either in: i) areas of plate reentrants, ii) areas that were dominated by transpressional or strike-slip tectonic regimes, or iii) as relatively thin intra-cratonic basins or structural klippes overlying Archean crust. The above observations suggest that relatively thick, buoyant Archean sub-continental mantle lithospheres (SCML) are a unique feature of Earth’s evolution and once formed, became an essential factor in limiting further crustal growth. It also infers that crustal growth is more a question of structural preservation than “growth pulses”.