Magnetic evidence for slow seafloor spreading during the formation of the Newfoundland and Iberian margins

There is considerable debate concerning the nature and origin of the thin c rust within the ocean-continent transition (OCT) zones of many passive non- volcanic continental margins, located between thinned continental and true oceanic crust, This crust is usually found to be underlain by upper mantle material of 7.2-7.4 km/s velocity at shallow depths (1-2 km). It has been p roposed that such crustal material could have originated either by exhumati on of upper mantle material during rifting of continents or by slow seafloo r spreading. One of the examples of occurrence of such a crust are the conj ugate margins of Newfoundland and Iberia. Here we present an interpretation of magnetic data from these regions to show that their OCT zones are under lain by crustal material formed by slow seafloor spreading (6.7 mm/yr) soon after Iberia separated from the Grand Banks of Newfoundland in the late Ju rassic. Similarities in the magnetic anomalies and velocity distributions f rom these regions with those from the Sohm Abyssal Plain, a region lying im mediately south of the Newfoundland Basin and formed by seafloor spreading at a similar rate of spreading, give further support to such an interpretat ion. The idea that these regions were formed by unroofing of upper mantle d uring rifting of Iberia from Newfoundland may be likely but the presence of weak magnetic anomalies in these regions, which bear all the characteristi cs of seafloor spreading anomalies, makes it difficult to ignore the possib ility that these regions could be underlain by oceanic crust formed during slow seafloor spreading. The similarities in velocity structure and the pre sence of small amplitude magnetic anomalies both across this pair of conjug ate margins of the North Atlantic and that of the Labrador Sea suggest that this OCT velocity structure may be the norm rather than the exception acro ss those passive non-volcanic margins where the initial seafloor spreading was slow. Furthermore, the existence of similar velocity distributions alon g a few active spreading centers raises the possibility of formation of sim ilar crust across slow spreading ridges. (C) 2000 Elsevier Science B.V. All rights reserved.