Lithoprobe leads to new perspectives on continental evolution

Lithoprobe, Canada's national earth science research project, was established in 1984 to develop a comprehensive understanding of the evolution of the northern North American continent. With rocks representing 4 b.y. of Earth history, the Canadian landmass and offshore margins provide an exceptional opportunity to gain new perspectives on continental evolution. Lithoprobe's 10 study areas span the country and geological time. A pan-Lithoprobe synthesis will bring the project to a formal conclusion in 2003. Each transect involves an integrated, collaborative, multidisciplinary scientific program. Two transects are highlighted here. The first, across southern British Columbia, illustrates elements of evolution of the Canadian Cordillera and the Cascadia subduction zone. A key result is that crustal rocks of accreted terranes are detached from their subducting lithosphere and attached as thin flakes to the craton. Accretion at Cascadia is characterized by both underplating and duplexing of old oceanic crust below the backstop and near-surface thrusting to form an accretionary wedge. The second, a lithospheric section across the southeastern Superior province of Quebec, provides direct evidence for plate tectonics in the Late Archean. Complementary studies indicate that the northdipping collisional subduction zone(s?) imaged by reflection data stepped southward with time. Postcollisional modification of the lower crust occurred across the southern part of the region.