Extent of oceanic crust in the Labrador Sea

This paper examines the mode of formation of the northern margins of the La brador Sea. Observations of faulted and rotated crustal blocks across the n orthwest margin of Greenland had led to the suggestion that this region is underlain by extended continental crust. A detailed analysis of all geophys ical data from this region, presented here, suggests, on the contrary, the possibility that it could have been formed by an excessively slow seafloor spreading process, and ridge axis propagation. This gave rise to rotated fa ult blocks at a time when spreading was mainly confined to the Greenland si de prior to its shift to the west. Supporting evidence for such a reconfigu ration of the spreading axis comes from gravity and magnetic observations o f this and neighbouring regions. Modelling of magnetic anomalies in the are a, based on the average rate of spreading for the entire Labrador Sea, show s that a gradual shift of the ridge axis to the west started after the form ation of anomaly 32. Because the earliest seafloor spreading anomalies are highly attenuated in amplitudes, other models like extended and intruded co ntinental crust are possible if one only considers the magnetic anomalies. However, an extended continental crustal model poses problems in accounting for other geophysical observations. Based on a slow spreading model it is shown that oceanic crust extends fairly close to the foot of the slope, wit h a narrow belt of extended continental crust along the west Greenland marg in while a much wider zone of perhaps attenuated continental crust extends off Labrador. Breakup of the continental landmass occurred much closer to t he west Greenland coast than to Labrador. The applicability of a slow sprea ding model to other passive margins, where similar geophysical observations have been reported, is discussed. (C) 1999 Elsevier Science Ltd. All right s reserved.