CRUSTAL GEOMETRY OF THE ABITIBI SUBPROVINCE, IN LIGHT OF 3-DIMENSIONAL SEISMIC REFLECTOR ORIENTATIONS

We provide precise estimates of reflector orientations beneath the Arc hean Abitibi Subprovince, using two distinct approaches based on Litho probe seismic reflection data. In the first, we use the dip of reflect ions observed on intersecting profiles to establish the three-dimensio nal orientation of reflective structures. In the second, the strikes a nd dips of reflectors are estimated in the crooked parts of seismic re flection profiles by calculating a measure of coherency along the trav eltime trajectories defined by a particular azimuth, dip, depth, and m edium velocity. Mid-crustal reflectors define two areas with distincti ve geometry: reflectors beneath the southern Abitibi belt are opposite ly 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 degrees towa rd the north. These north-dipping reflectors are partly disrupted by a low-reflectivity zone, which is attributed to rocks of the Opatica Su bprovince, located underneath the northern Abitibi belt. Lower-crustal reflectors have a similar, shallowly north-dipping orientation throug hout 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 du ring a north-south shortening event. Attitudes recovered in the northe rn Abitibi belt are consistent with tectonic scenarios involving under thrusting of Abitibi middle and lower crustal terranes beneath the Opa tica belt, whereas the oppositely dipping reflectors recovered in the middle crust beneath the southern Abitibi belt could be representative of a rifted volcanic are environment.