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 reflectors and median filtering to attenuate shear wave refractions. The interpretation provides additional constraints on the tectonic models of northern Abitibi, but cannot be used to invalidate either of the two models recently presented for the area. However, the seismic profile defines a large east-dipping faulted contact between the Brouillan tonalite and the volcanic rocks exposed west of the intrusion. The moderate dip of the reflectors and their extensive lateral continuity, combined with geological information, provide evidence for an east-dipping thrust sequence and suggest an allochthonous origin for the Brouillan tonalite. 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 Opatica orthogneisses are considered to underthrust northern Abitibi. The mid-crustal reflections confirm the east-west continuity of south-vergent imbrications also observed on a north-south reflection line (28) through northern and central Abitibi. Breakage and displacement of some mid-crustal reflectors may define a west-to-east thrust sequence of sense opposite to that which thrust the Brouillan pluton over the volcanic rocks.