CRUSTAL EXTRUSION BETWEEN CRATONS: NEW INTERPRETATIONS OF THE PALEOPROTEROZOIC KINEMATIC AND METAMORPHIC HISTORY OF THE SOUTHEASTERN CHURCHILL PROVENCE

The southeastern Churchill Province of the Canadian Shield preserves a complex history of terrane accretion, basin closure, extension, and collision between the Archean Superior craton, the North Atlantic craton, and a continental fragment known as the Core Zone. At least two orogenic pulses are recognized from that system. The Torngat Orogen formed primarily between ca. 1.87-1.86 Ga as a result of orthogonal collision between the Core Zone and the North Atlantic Craton and contains granulite facies metamorphic assemblages representing an accretionary prism (the Tasiuyak gneiss) and a crustal root of an arc terrane (the Lac Lomier Complex). The New Quebec Orogen formed between ca. 1.83 Ga and 1.77 in a SW-directed dextral transpressional regime when the Core Zone collided with the Superior craton and its marginal sequences (the Labrador Trough) creating a complex accretionary collage. The Superior – Core Zone collision was preceded by voluminous 1.88 Ga mafic magmatism in a continental margin and possibly extensional back-arc setting. The transpressional collision preserved a number of Archean basement exposures as thrust slices in the northern New Quebec Orogen, possibly as a consequence of basement structures inherited from previous rifting of the paleo-Superior margin.

The Core Zone between these cratons is an elongated ribbon continent which contains two Archean crustal blocks and a Paleoproterozoic block, apparently unrelated to each other in age and tectonic history, suggesting large amounts of lateral displacement. These blocks are up to 100 km wide and hundreds of km long and display a general decrease in metamorphic grade and degree of exhumation towards the south and are separated by a network of anastomosing, ductile shear zones, some of which may represent sutures. These subvertical NNW-SSE trending shear zones accommodated bulk dextral top-to-the-south (in present day coordinates) displacement through a combination of dextral (mostly) and sinistral shear zones and are connected by moderately S-dipping extensional ductile shear zones. The combination of dextral shear and south-directed extension suggest that the oblique convergence between the NAC and Superior cratons produced bulk asymmetrical extension and crustal extrusion of the Core Zone towards the south.