Age and mode of emplacement of the collector anomalY, (Canada): Is it the northwestern boundary of Gondwana?

Abstract

A direct study of the Collector Anomaly, a prominent positive gravity and magnetic anomaly, located between Boston (U.S.A.) and southern Spain, suggests that this Paleozoic feature, which follows or corresponds with a suture, may be modelled magnetically, in the Grand Banks area, by a northward dipping dyke-like body. This body could have originated when Gondwanaland and Avalon collided. The magnetic and seismic data available in the eastern part of the Grand Banks do not permit us to understand the geometric relationship where this body intersects with two post-Givetian south deepening thrusts, the roots of which are located in the Ghomarids of northern Morocco. Since, in plan view, the body causing the Collector Anomaly cuts Meguma sediments—which are Emsian in their upper part—this mafic body could be younger than Emsian. But recent geochronological data have been interpreted to indicate a 400–415 Ma metamorphic age as the collision age between Avalon and Meguma. The study of the previous seismic lines cutting across the Avalon-Meguma boundary, in the Gulf of Maine, in the Bay of Fundy and east of Nova-Scotia, show a western extension of the Morroccan-Grand Banks south dipping thrust system. This system is cut by the suture which separates Meguma from Avalon. The geometrical relationship suggests that the seismic and mafic suture between the two zones cannot be older than the Frasnian, which therefore gives a maximum age for the last docking of Meguma. This is the first study where the magnetic modelling of the Collector Anomaly is associated with the seismic reflection data along its entire length. The Collector magnetic trend is not as clear in southern Spain as in Canada, because of Hercynian southward thrusting involving the mafic and ultramafic rocks which developed in Spain during the pre-Variscan northward subduction. These data show that the magnetic rocks responsible for the Collector Anomaly are sometimes affected by tectonic transport which locally may differ in age and direction. As a consequence, the northwestern plate boundary of Gondwanaland imaged on the deep seismic data does not everywhere correspond with the shallow expression of the Collector Anomaly.