Application géophysique à l'interprétation structurale de la région du lac Bécancour, Thetford Mines, Québec

Abstract

The present study, based on detailed gravity anomaly (Bouguer and residual) and magnetic maps of the Thetford Mines area, seeks to explain the mechanism by which the Lac Bécancour structural unit was emplaced. The Bouguer anomaly map shows: (1) generally more positive anomalies toward the northwest; (2) local positive anomalies in the southwest and north-northwest; and (3) a small local positive anomaly in the east. Residual anomalies are characterized by a weak high of the order of 2–3 × 10−5 m/s2 (2–3 mgal) over the center of the structure. The magnetic map shows: (1) negative to neutral intensities over the center of the structure; (2) two small positive anomalies on the inner border of the structure; (3) a large magnetic mass (the ophiolite complex) in the west-northwest; and (4) small and isolated magnetic masses to the southwest and east.The hypothesis of diapir-like rise caused by an underlying serpentinite mass, as proposed by some geologists, is refuted by the absence of a positive magnetic anomaly over the structure. A second hypothesis, which calls on post-nappe granitization of the core of the structure at depth, is unlikely as gneiss is now here exposed in the area and the structure is bounded by faults. Instead, the geophysical data suggest that the Lac Bécancour structural unit is an old (Early Cambrian or Eocambrian) block that has undergone polyphase deformation during the Taconic Oxogeny rather than being a horst-like body emplaced by vertical rise. Identical overthrust slices of Hadrynian basement in the centers of imbricated blocks and nappes of lower Paleozoic rocks are commonly observed in the Caledonides of Norway and Sweden where they generally have the appearance of antiforms or synforms. [Journal Translation]