Abstract

The amphibolite-facies central Wawa gneiss domain (CWGD) preserves structures that developed at the mid-crustal level of the ca. 2.7 Ga Abitibi–Wawa orogen in the southern Superior Province. The relative ages of these domainal structures are documented and brackets on their absolute ages established using existing U–Pb age data. Correlation of tectonic events within the CWGD, and comparison of these events with the evolution of other structural levels of the orogen, has led to subdivision 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 at 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 relatively 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), coeval east–west extension and granulite metamorphism of the middle crust produced gently dipping shear zones that overprinted earlier fold structures 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 allowed penetration of deep-crustal metamorphic fluids into the latter. In general, analysis of the structural evolution of the CWGD indicates that deformation and metamorphism in the middle crust of the Abitibi–Wawa orogen outlasted that at upper-crustal levels, resulting in the generally shallower dips of planar fabrics in the deeper structural levels of the Kapuskasing uplift crustal cross section.

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