Abstract

The offshore extension of the eastern Precambrian margin of the Appalachians (the Avalon Zone) across the Grand Banks of Newfoundland is shown by the extent of arcuate zones of high magnetic anomalies corresponding with volcanic ‘ridges’ separating basins containing metasedimentary rocks. A similar geophysical pattern occurs in Iberia and France, providing a good correlation with the Avalonian structures when the margins are restored to their relative positions prior to the opening of the Bay of Biscay and the North Atlantic. The linear magnetic pattern of the Avalon Zone is truncated abruptly at its southern edge indicating that the boundary between the Avalon and the Meguma Zones trends almost due east from Cape Breton across the Tail of theBank. The European landfall of this boundary, expected in southern Spain, is not obvious there. South and west of Newfoundland there is evidence for a northwest-trending fault pattern within the Avalon Zone. Northeast of Newfoundland the abrupt contrast between the gravity and magnetic character of the Avalon Zone and that of Paleozoic units of central Newfoundland indicates that the Avalon Zone boundary extends across the Canadian margin to the western end of the Charlie Fracture Zone. On the European margin, a correlative structural boundary is less obvious, but may cross Porcupine Bank and follow a sinuous path south of Ireland and thence towards Anglesey.

The western Precambrian margin of the Appalachians (the Grenvillian Humber Zone) has a prominent gravity and magnetic gradient associated with it throughout North America. This boundary extends offshore parallel to the southeastern coast of Labrador until it apparently swings eastward at 53°N. Southwestward trends across Scotland and Ireland, that may be correlative with the Grenville boundary, veer westward towards Rockall Trough indicating that there may be continuity with the eastward-veering Canadian trends, but with little concrete evidence for its continuation across the wide and dissected European margin.

Paleozoic sedimentary and volcanic rocks are well exposed in Newfoundland with slices of an ophiolite suite in the Dunnage Zone representing the remnants of the Paleozoic ocean crust that once separated the Precambrian margins. The subsurface and offshore projections of these oceanic rocks are interpreted to indicate that there was southeastward dipping subduction of that Paleozoic crust, leading to westward thrusting of the ophiolite suites now seen at Betts Cove, Hare Bay, and Bay of Islands. Following closure of the Paleozoic ocean, east-west trending belts of late Carboniferous faults extended from the Grand Banks in Iberia, and from the Scotian Shelf and Georges Bank into northwest Africa. The continuity of these belts provides latitudinal control for late Paleozoic paleogeographic reconstructions.

Reactivation of the Precambrian and Paleozoic structures on the Canadian margin is apparent from the confinement of Mesozoic sedimentary basins between structural highs that follow Precambrian trends on the Grand Banks. Vertical displacements of several kilometres between elements of the Canadian and European margins are inferred from seismic refraction data.