INTEGRATED GEOPHYSICAL INTERPRETATION OF CRUSTAL STRUCTURES IN THE NORTHERN ABITIBI BELT - CONSTRAINTS FROM SEISMIC AMPLITUDE ANALYSIS

We present a processing sequence that attempts to balance geometrical and amplitude analyses in order to recover the maximum information fro m deep seismic reflection data. The approach, which is guided by the i nterpretation of other deep geophysical data sets (magnetotellurics, r efraction), is applied to Lithoprobe seismic reflection line 28 across the central and northern Abitibi belt. We show, in particular, how am plitude analyses help to quantify the depth of penetration of seismic energy as well as the crustal reflectivity. Apparent lateral variation s of deep structures (e.g., the Moho) can be directly related to the h igh levels of noise that limit the signal penetration depth. We propos e a geological model that satisfies all deep geophysical constraints. In this model, the mid crust south of Casa-Berardi tectonic zone consi sts of imbricated volcanic-plutonic and sedimentary lithologies, which are probably comparable to the mid-crustal section of the Kapuskasing structural zone, and in this paper are referred to as ''the Abitibi p late.'' The lithologies are characterized by high reflectivity, while north of Casa-Berardi tectonic zone the mid crust is dominantly Opatic a plutonic lithologies, of lower reflectivity. In this scenario, supra crustal rocks of the Abitibi belt overlie the Opatica plutonic belt, w hereas the Abitibi plate extends beneath the Opatica plutonic belt. Th e boundary between the Opatica plutonic belt and the Abitibi plate is a northward-dipping decollement extending from mid crust in the south to lower crust in the north. The Casa-Berardi tectonic zone appears to be a crustal boundary affecting upper and middle crust down to 20 km, between northern polycyclic terranes and southern monocyclic ones. Th e uniformity of the lower crust suggests that its formation was decoup led from that of the intermediate to upper crust.