Crustal structure northwest of the Sudbury meteorite-impact structure (LITHOPROBE reflection-seismic AGT Line 42)

Abstract

A regional seismic-reflection survey, Line 42 of the LITHOPROBE Sudbury transect traverses the Levack gneiss, northwest of the North Range, associated with the Sudbury Structure. Preliminary processing by industry contract produced a section that shows no significant reflection events at shallow depths (0–3 s), particulary in the southern portion of the line close to the North Range. The Line 42 data set was reprocessed with an objective to improve spatial resolution of the seismic image so as to resolve lingering questions related with the uplift of the Levack gneiss complex. The reprocessing emphasized spectral balance, f-k filtering, refraction statics, cross-dip correction, velocity analysis cascaded with residual statics, and offset-limited stack, which greatly improved the final seismic section. The reprocessed seismic section has revealed several reflection events in the northwestern outer portion of the Sudbury Structure. Two of these events are interpreted together with the C-Band SAR (synthetic aperture radar) image data acquired from the satellite ERS-1 of European Space Agency. A reflection event AA′ associated with the interface between the Levack gneiss and the Cartier granite dips southeast, and its surface projection falls approximately 1.5 km north of the geological boundary (Dressler, 1984a). The reflection event R2 extending through most of the line appears to represent a major fault (or a shear zone) which dips southeast and is parallel to the geologic boundary between the Levack gneiss and the Cartier granite. The seismic patterns imaged on the north side of Event R2 are considerably different from those of the southern side and may represent new geological fabrics of the rocks. The reflection event R1 in the southern portion of the line, near Pumphouse Creek, appears nearly vertical at shallow depths and may correspond to the Pumphouse Creek fault (Dressler, 1984a).