Robert B. Hearst

Regional gravity setting of the Sudbury Structure

In the vicinity of Sudbury, Ontario, Canada, the boundary between the Southern and Superior tectonic provinces is overlain by the elliptical Sudbury Structure. On the basis of gravity modeling, genesis of the Sudbury Structure has been attributed to either a magmatic origin (having a dense hidden differentiate zone) or a meteorite impact origin (there being no dense hidden mass). The difference between the two gravity models centers on the problem of regional-residual separation. As shown by numerous previous studies, any such separation of components is nonunique. This becomes especially problematic when, as in Sudbury, a portion of the near-surface geology has a similar orientation and dimension to more deep-seated sources. In this paper, several numerical methods (upward continuation, downward continuation, wavelength filtering, trend-surface analysis) for determining the regional component of the gravity field associated with the Sudbury Structure have been applied and evaluated. Of the numerical methods used, the upward and downward continuation operators provided the most insight into the deep structural controls of the Sudbury Basin. Our preferred interpretation of the regional gravity field invokes a two-component structure. Underlying the southern half of the Sudbury Structure is a laterally continuous gravity anomaly that is probably associated with a zone of uplifted Huronian volcanics. The gravity anomaly under the northern portion of the Sudbury Structure has a more restricted spatial extent. The close association between the northern limit of the gravity anomaly and the surface outcrop of the Levack Gneiss suggests the source of this anomaly is probably a slab of dense Levack Gneiss. This interpretation favors a meteorite impact origin for the Sudbury Structure.

Magnetic interpretation along the Sudbury Structure Lithoprobe Transect

Magnetic modelling of the Sudbury Structure (SS) has been undertaken using the geometrical and lithological framework provided by high-resolution seismic reflection data. This initial constraint was required because of the complex magnetic properties exhibited by rock units of the SS. Locally remanent magnetisation (NRM) dominates over induced magnetisation, and the orientation and amplitude of the NRM vector varies both between and within individual rock units. The use of measured magnetic susceptibility and NRM values afforded an important constraint in the modelling process, and provided a valuable insight into the genesis of the various magnetic anomalies. The magnetic anomaly profile along the Lithoprobe Transect across the SS appears to arise from three main sources: a) a broad regional magnetic anomaly ascribed to a more magnetic layer of the Levack Gneiss Complex subjacent to the base of the Sudbury Igneous Complex (SIC); b) a prominent magnetic high associated with the southern contact between the Onwatin and Onaping Formations, believed to reflect a zone of hydrothermal mineralisation; and c) a magnetic high associated with the southern contact between the SIC and the Huronian mafics is related to the juxtaposition of rock units produced by northward directed thrusting and an enhanced NRM signature in the basal unit of the South Range Norite.

Near surface IP investigations: Four case studies

The use of the Induced Polarisation (IP) method of geophysical surveying for near surface site investigations is gaining acceptance within the geophysical community. In this study the IP method is evaluated as a tool for the delineation of ground water resources; contamination plume detection in a lateritic horizon; and acid mine drainage leak detection from decommissioned mine tailings. A time domain IP system was selected for this study primarily for the flexibility in the selection and setting of receiver time windows and diagnostic characteristics attributed to submitting the data to Cole-Cole analysis. Analysis of the acquired data in conjunction with available borehole and geological information illustrates the effectiveness and usefulness of the survey method for solving near surface problems. In all of the locations tested, it was found that with a properly designed IP survey it was possible to resolve the target and/or related structures.

Geophysics of the Levack Gneiss Cartier Batholith complex

high resolution seismic reflection data from the Abitibi-Grenville transect (Milkereit et al., 1992 ) combined with gravity, aeromagnetic, airborne radiotnetric, and airborne data and field mapping (Hearst et al., 1992: Fueten et al., 1992) have been used to , study the Levack Cartier Batholith Complex in the Sudbury region of Ontario. The data acquired suggest that the Archean gneisses and/or greenstone belts of the Superior Province to the immediate north and west of the Sudbury Structure are dipping towards the south under the Sudbury Basin. Geologic field mapping has suggested the presence of previously unmapped ductile shear zones, increasing foliations towards the Sudbury Structure, and possible revisions to the current “mapped” geologic boundaries. Supporting evidence for these ductile shear zones and geologic boundary revisions has been derived from the analysis of the geophysical datasets, providing a more detailed and complex structure for the Levack Gneiss Cartier Batholith Complex