SEISMIC-REFLECTION CONSTRAINTS FROM LITHOPROBE LINE-29 ON THE UPPER CRUSTAL STRUCTURE OF THE NORTHERN ABITIBI GREENSTONE-BELT

Detailed reprocessing of east-west Lithoprobe seismic reflection line 29 includes cross-dip analysis to improve the continuity of the reflec tors and median filtering to attenuate shear wave refractions. The int erpretation provides additional constraints on the tectonic models of northern Abitibi, but cannot be used to invalidate either of the two m odels recently presented for the area. However, the seismic profile de fines a large east-dipping faulted contact between the Brouillan tonal ite and the volcanic rocks exposed west of the intrusion. The moderate dip of the reflectors and their extensive lateral continuity, combine d with geological information, provide evidence for an east-dipping th rust sequence and suggest an allochthonous origin for the Brouillan to nalite. The maximum thickness of the volcanic sequence in the northern Abitibi greenstone belt is 8 km, but could be as low as 4 km if Opati ca orthogneisses are considered to underthrust northern Abitibi. The m id-crustal reflections confirm the east-west continuity of south-verge nt imbrications also observed on a north-south reflection line (28) th rough northern and central Abitibi. Breakage and displacement of some mid-crustal reflectors may define a west-to-east thrust sequence of se nse opposite to that which thrust the Brouillan pluton over the volcan ic rocks.