STRUCTURE OF THE HIGH-GRADE OPATICA BELT AND ADJACENT LOW-GRADE ABITIBI SUBPROVINCE, CANADA - AN ARCHEAN MOUNTAIN FRONT

Plutonic gneisses in the Archaean Opatica Belt in Canada record crustal-scale, WSW-vergent (i.e. longitudinal), high-temperature D-1 ductile shearing. Similar structures occur in adjacent greenstone belts, but only in narrow amphibolite facies zones near the base; metavolcanics in the greenschist facies portions do not record penetrative D-1 structures. The greenstone belts are interpreted as allochthonous on the gneisses; they form the uppermost D-1 thrust sheets. SSE-vergent (i.e. approximately transverse) D-2 thrusting imbricated the Opatica Belt and formed a large antiform-either a culmination or an antiformal stack-in the southern Opatica. The imbricated Opatica Belt wedged under the northern part of the Abitibi Subprovince and created a tight syncline with NNW-vergent D-2 thrusts (interpreted as backthrusts) on its northern limb. The Opatica Belt-Abitibi Subprovince contact, therefore, resembles some Phanerozic mountain fronts. Further southwards-propagating imbrication of the crust beneath the Abitibi Subprovince created a S-vergent foreland fold and thrust belt in the overlying greenstones. High-grade Opatica gneisses and adjacent low-grade Abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an Archaean mountain belt, similar in many respects to Phanerozoic examples.