THE GEOLOGY AND STRUCTURE OF THE MIDCRUSTAL WAWA GNEISS DOMAIN - A KEY TO UNDERSTANDING TECTONIC VARIATION WITH DEPTH AND TIME IN THE LATE ARCHEAN ABITIBI-WAWA OROGEN

The amphibolite-facies central Wawa gneiss domain (CWGD) preserves str uctures that developed at the mid-crustal level of the ca. 2.7 Ga Abit ibi-Wawa orogen in the southern Superior Province, The relative ages o f these domainal structures are documented and brackets on their absol ute ages established using existing U-Pb age data. Correlation of tect onic events within the CWGD, and comparison of these events with the e volution of other structural levels of the orogen, has led to subdivis ion of orogenesis into five stages. During stage 1 (2700-2680 Ma), 2.9 and 2.7 Ga rocks were tightly folded and (or) thrusted at all crustal levels in at least one thick-skinned compression event. During stage 2 (2680-2670 Ma), folding and thrusting of Timiskaming-age sediments a t high levels of the orogen was thin-skinned and had no effect on CWGD gneisses. During stage 3 (2670-2660 Ma), while the upper crust was re latively stable, a 1 km thick package of volcanics and sediments, the Borden Lake belt, was underthrust northwards to depths of 30 km and in -folded with orthogneiss of the CWGD. During stage 4 (2660-2637 Ma), c oeval east-west extension and granulite metamorphism of the middle cru st produced gently dipping shear zones that overprinted earlier fold s tructures in the CWGD and lower structural levels of the orogen. This took place with minimal effect on the upper crust. Stage 5 (2630-2580 Ma) marks a period of east-west shortening and (or) fault reactivation in the Kapuskasing uplift and upper-crustal greenstone belts that all owed penetration of deep-crustal metamorphic fluids into the latter. I n general, analysis of the structural evolution of the CWGD indicates that deformation and metamorphism in the middle crust of the Abitibi-W awa orogen outlasted that at upper-crustal levels, resulting in the ge nerally shallower dips of planar fabrics in the deeper structural leve ls of the Kapuskasing uplift crustal cross section.