TECTONICS, VOL. 8, NO. 5, PAGES 1059-1077, OCTOBER 1989 PATFERNS OF CONTINENTAL EXTENSION ALONG THE CONJUGATE MARGINS OF THE CENTRAL AND NORTH ATLANTIC OCEANS AND LABRADOR SEA John A. Dunbar 1 Department of Geological Sciences and the Institute for Geophysics, The University of Texas at Austin Dale S. Sawyer Department of Geology andGeophysics, Rice University, Houston, Texas Abstract. The anomalously thincrust andsubsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading. We estimate theamount of continental extension priorto seafloor spreading along the conjugate margins of the central and North Atlantic Oceans and Labrador Seafrom the total tectonic subsidence of the margins.In the central Atlanticbasin 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 breakbetween the Blake Plateau region of NorthAmerica andtheCape VerdeTerrace region of Africa. The original widthof theextended zone varies from240 to 770 kni. In the North Atlantic andLabrador 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. Theoriginal width of the extended zones in these basins varies from 100 km to over 1460 km. In general, continental breakup involves twoto three times less extension where it follows thepreexisting structural grain than where it crosses thepreexisting grain. As a result, seafloor spreading tends to begin firstalong segments which follow the structural grain. Extension tends to be symmetrically distributed between conjugate margins where thebreak follows thegrain and asymmetrically distributed where thebreak crosses thegrain. Failure parallel to the 1Now at Shell Development Company, Houston, Texas. Copyright1989 by theAmerican Geophysical Union. Paper number 89TC00955. 0278-7407/89/89TC-00955510.00 structural grain exhibits greater variability frommargin to margin than failureacross thegrain. Thisphenomenon appears to result fromdifferences in the nature of preexisting weaknesses withinthestructural grain. We conclude thatthe distribution of the continental extension alongpassive margins is to a large extent controlled by theorientation of thebreak relative to thestructural grain andthenature of thepreexisting weaknesses within thatstructural grain. INTRODUCTION Conjugate passive margins represent thebroken edges of oncecontinuous continental plates. The thin crust and subsidence history of such margins suggest thatcontinental breakup does notoccur in a perfectly rigid-brittle manner but instead involves several tens or even hundreds of kilometers of extension within continental lithosphere before seafloor spreading begins [McKenzie,1978; Royden andKeen,1980; Le Pichon andSibuet, 1981]. The amount of prespreading extension taken up withinpassive margins varies substantially bothfrom marginto margin[Le Pichon andS ibuet, 1981] and alongthe strikeof individual margins [Sawyer,1985]. Previous studies[Vink, 1982; Pindell, 1985; Dunbar and Sawyer,1987] havedealtwith the implications of continental extension for platetectonic reconstructions. In this paper we address themechanical causes of differential extension along passive continental margins. To investigate the distribution of continental extension along passive margins we haveundertaken a study of the subsidence alongthe conjugate margins of thecentral and North Atlantic Oceans and Labrador Sea. The amount of subsidence whichoccurs on passive margins canbe directly related to the amount of continental extension during breakup [Le Pichon and S ibuet, 1981]. Therefore subsidence analysis canbe used to map the variation in extension along passive margins. Sawyer [1985] used this approach to estimate the