Abstract

Lithoprobe seismic-reflection profiles from the 2.75-2.68 Ga southern Abitibi greenstone belt reveal subhorizontal reflection zones of regional extent in the upper 15 km of crust that contrast with the steep dips of rock units and structures at the surface. The shallowest (6-12 km) regional, subhorizontal reflection zone is interpreted to be the base of the Abitibi greenstone belt supracrustal assemblage which may be an intrusive and/or tectonic contact. Shallowly dipping, subparallel reflectors in the upper crust are interpreted as low-angle faults, consistent with "out-of-sequence" stratigraphy revealed by U-Pb geochronology. Truncations of reflectors suggest that regional, steeply dipping, gold-mineralized fault zones penetrate to depths of 15 km. The profiles are consistent with the Kapuskasing crustal model of the central Superior province, wherein greenstone belts are underlain by middle-crustal felsic intrusions and gneiss and a lower-crustal zone of mafic gneiss and intrusions. The profiles support tectonic models of deformation of the Abitibi greenstone belt that invoke either folding and thrusting followed by regional steep faulting or folding and thrusting linked to the steep faults.

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