A 3-DIMENSIONAL PERSPECTIVE ON THE EVOLUTION OF ARCHEAN CRUST - LITHOPROBE SEISMIC-REFLECTION IMAGES IN THE SOUTHWESTERN SUPERIOR PROVINCE

In 1990-1991 the LITHOPROBE project completed 450 km of seismic reflec tion profiles across the late Archaean crust of the southwestern Super ior province. The results define a broad three-fold division of crust: upper crust in the Abitibi greenstone belt is non-reflective and is a 6-8 km veneer of volcanic and plutonic supracrustal rocks, whereas, i h the sediment-gneiss dominated Pontiac subprovince, upper crust compr ises shallow northwest-dipping turbidite sequences; mid-crust, in both the Abitibi and the Pontiac subprovinces, is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks; lower crust in b oth subprovinces has a horizontal layer parallel structure which may r epresent interleaved mafic-intermediate gneisses. The seismic signatur e of the northern Abitibi greenstone belt may be represented in an exp osed 25 km crustal section in the Kapuskasing stuctural zone. Prelimin ary tectonic models based on the seismic data are consistent with a pl ate-tectonic scenario involving oblique subduction and imbrication of sedimentary, plutonic and volcanic sequences. The northern Abitibi sup racrustal sequences either represent an allochthon, or overlie an allo chthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the Pontiac or Quetico. Seismic velocities have yet to be defined. However, crustal t hicknesses are relatively constant at 35-40 km. The thinnest crust is adjacent to the Grenville Front where Moho is very well defined.