AGE AND MODE OF EMPLACEMENT OF THE COLLECTOR ANOMALY, (CANADA) - IS IT THE NORTHWESTERN BOUNDARY OF GONDWANA

A direct study of the Collector Anomaly, a prominent positive gravity and magnetic anomaly, located between Boston (U.S.A.) and southern Spa in, 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 originate d 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 loca ted in the Ghomarids of northern Morocco. Since, in plan view, the bod y 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-Me guma boundary, in the Gulf of Maine, in the Bay of Fundy and east of N ova-Scotia, show a western extension of the Morroccan-Grand Banks sout h dipping thrust system. This system is cut by the suture which separa tes Meguma from Avalon. The geometrical relationship suggests that the seismic and mafic suture between the two zones cannot be older than t he 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 alon g its entire length. The Collector magnetic trend is not as clear in s outhern Spain as in Canada, because of Hercynian southward thrusting i nvolving the mafic and ultramafic rocks which developed in Spain durin g the pre-Variscan northward subduction. These data show that the magn etic rocks responsible for the Collector Anomaly are sometimes affecte d by tectonic transport which locally may differ in age and direction. As a consequence, the northwestern plate boundary of Gondwanaland ima ged on the deep seismic data does not everywhere correspond with the s hallow expression of the Collector Anomaly.