Patterns of continental extension along the conjugate margins of the central and North ATLANTIC Oceans and Labrador Sea

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Abstract
The anomalously thin crust and subsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading. We estimate the amount of continental extension prior to seafloor spreading along the conjugate margins of the central and North Atlantic Oceans and Labrador Sea from the total tectonic subsidence of the margins. In the central Atlantic basin the integrated continental extension varies from 225 km for the break between the southern Appalachian segment of the North American margin and Africa to over 600 km for the break between the Blake Plateau region of North America and the Cape Verde Terrace region of Africa. The original width of the extended zone varies from 240 to 770 km. In the North Atlantic and Labrador basins the integrated extension varies from 60 km for the break between northern Labrador and Greenland to over 600 km for the break between Newfoundland and Europe. The original width of the extended zones in these basins varies from 100 km to over 1460 km. In general, continental breakup involves two to three times less extension where it follows the preexisting structural grain than where it crosses the preexisting grain. As a result, seafloor spreading tends to begin first along segments which follow the structural grain. Extension tends to be symmetrically distributed between conjugate margins where the break follows the grain and asymmetrically distributed where the break crosses the grain. Failure parallel to the structural grain exhibits greater variability from margin to margin than failure across the grain. This phenomenon appears to result from differences in the nature of preexisting weaknesses within the structural grain. We conclude that the distribution of the continental extension along passive margins is to a large extent controlled by the orientation of the break relative to the structural grain and the nature of the preexisting weaknesses within that structural grain.