CRUSTAL VELOCITY MODELS FOR THE ARCHEAN ABITIBI GREENSTONE-BELT FROM SEISMIC-REFRACTION DATA

We present velocity models for two seismic wide-angle - refraction pro files across the Archean Abitibi greenstone belt and the Pontiac Subpr ovince. The seismic profiles are 210 and 220 km long. Traveltime inver sion and amplitude forward modelling were used to obtain two-dimension al velocity structure and interface geometry. The main features of the velocity models include (1) three crustal layers; (2) variable veloci ties (5.6-6.4 km/s) in the upper crust (similar to 0-12 km), with the higher velocities generally associated with mafic metavolcanics and th e lower velocities with metasediments and granitic plutons; (3) a rela tively uniform middle crust (similar to 12 -30 km) with velocities ran ging from 6.4 to 6.6 km/s; (4) a velocity increase of 0.3 km/s across the middle crust - lower crust boundary; (5) a lower crust (similar to 30-40 km) with velocities increasing from 6.9 km/s at the top to 7.3 km/s at the base; (6) an average upper mantle velocity of 8.15 km/s; ( 7) depth to Moho of about 40 km in the north-central Abitibi belt, dec reasing southward to 37 km beneath the Pontiac Subprovince; and (8) ob served attenuation of seismic energy propagating through the Casa-Bera rdi deformation zone, suggesting a complex structure in this fault zon e. The velocity model is generally consistent with seismic reflection interpretations that suggest that the shallow supracrustal assemblages form an allochthonous veneer, overlying a mid-crustal imbricate seque nce of metaplutonic and metasedimentary rocks. The uniform-velocity st ructure below 12 km depth indicates that the tectonic zones juxtaposin g disparate crustal blocks may have limited depth extent. The 40 km th ick crust and 10 km thick high-velocity lower crustal layer exceed the thicknesses observed in other studies of Archean crust.