HIGH-FREQUENCY MAGNETOTELLURIC INVESTIGATION OF CRUSTAL STRUCTURE IN NORTH-CENTRAL ABITIBI, QUEBEC, CANADA

Thirty-one audiomagnetotelluric (AMT) soundings have been collected al ong a N-S profile across the northern Abitibi greenstone belt. The rec ording frequency ranges of 10 000-1 Hz for 25 stations and 10 000-0.00 1 Hz for the other six stations were appropriate for defining the near -surface geoelectric structures and for imaging major faults and shear zones down to approximately 10 km along the profile. Six time-domain electromagnetic (TEM) soundings were also collected near the southern end of the profile, to provide a correction for static shift. Extensiv e static distortion analysis reveals that most of the AMT data are not distorted by localized near-surface structures; however, they are aff ected by static shift. In order to remove this static shift, a spatial filter was designed and applied to the data, effectively reducing sta tic shift as verified by TEM soundings at the same sites. Extensive 2- D inversions of the filtered data were carried out. The results sugges t the presence of a thin overburden with both conductive and resistive blocks overlying a very resistive basement. Among the four geological deformation zones crossed by the profile, only two zones are imaged a s subvertical conductive crustal structures extending to the limit of resolution of AMT. These correspond to major breaks (Porcupine-Destor Fault and Casa Berardi Tectonic Zone) as evidenced by the seismic refl ection data collected over the same area.