Crustal geometry of the Abitibi Subprovince, in light of three-dimensional seismic reflector orientations

Abstract

We provide precise estimates of reflector orientations beneath the Archean Abitibi Subprovince, using two distinct approaches based on Lithoprobe seismic reflection data. In the first, we use the dip of reflections observed on intersecting profiles to establish the three-dimensional orientation of reflective structures. In the second, the strikes and dips of reflectors are estimated in the crooked parts of seismic reflection profiles by calculating a measure of coherency along the traveltime trajectories defined by a particular azimuth, dip, depth, and medium velocity. Mid-crustal reflectors define two areas with distinctive geometry: reflectors beneath the southern Abitibi belt are oppositely dipping, and convergent at depth, providing a V-shape aspect to the greenstone rocks; other reflectors beneath the northern Abitibi belt are, in general, subparallel, dipping at an average of 30° toward the north. These north-dipping reflectors are partly disrupted by a low-reflectivity zone, which is attributed to rocks of the Opatica Subprovince, located underneath the northern Abitibi belt. Lower-crustal reflectors have a similar, shallowly north-dipping orientation throughout the Abitibi Subprovince. The geometry of the reflectors recovered is consistent with the different tectonic histories proposed for the southern and northern Abitibi assemblages, until common deformation during a north-south shortening event. Attitudes recovered in the northern Abitibi belt are consistent with tectonic scenarios involving underthrusting of Abitibi middle and lower crustal terranes beneath the Opatica belt, whereas the oppositely dipping reflectors recovered in the middle crust beneath the southern Abitibi belt could be representative of a rifted volcanic arc environment.