CRUSTAL STRUCTURE OF THE LABRADOR-SEA CONJUGATE MARGIN AND IMPLICATIONS FOR THE FORMATION OF NONVOLCANIC CONTINENTAL MARGINS

Wide-angle seismic studies have determined the detailed velocity struc ture along a 350-km-long profile across the Labrador margin. Combinati on of this model with a previously published cross section for the sou thwestern Greenland margin constitutes the first combined conjugate ma rgin study based on seismic velocity structure. The results indicate t hree distinct zones across the Labrador margin, similar to the structu re of the conjugate Greenland margin. Zone 1 represents 27 to 30-km-th ick continental crust thinning gradually seaward over similar to 100 k m distance. Farther seaward, zone 2 is 70-80 km wide, characterized by a distinct lower crust, 4-5 km thick, in which velocity increases wit h depth from 6.4 to 7.7 km/s. Interpretation for this lower crustal bl ock favors an origin by serpentinized peridotite rather than by magmat ic underplating. Zone 3 represents two-layered, normal oceanic crust. The cross sections from both margins are reconstructed to an early dri ft stage at Chron 27. This demonstrates that the serpentinites in zone 2 are symmetrically distributed between previous identifications of C hrons 31 and 33 on both margins. Zone 1 shows a marked asymmetry, with a gradual thinning of continental crust off Labrador contrasted with a rapid thinning off Greenland. The abundant serpentinization of upper mantle peridotite in zone 2 and the asymmetric shape of zone 1 are bo th probably related to a very slow rate of continental rifting which p roduced little if any melt.