EVOLUTION OF NONVOLCANIC RIFTED MARGINS - NEW RESULTS FROM THE CONJUGATE MARGINS OF THE LABRADOR SEA

The crust across each margin of the Labrador Sea, a conjugate rift mar gin pair, consists of three well-defined zones: thinned continental cr ust, 70-80-km-wide transitional crust, and oceanic crust. The transiti on zone is characterized by a low-velocity upper crust (4-5 km/s) unde rlain by a 6.4-7.7 km/s layer. We propose that the lower layer is serp entinized upper mantle; it is less likely to be gabbroic igneous lower crust. The low-velocity upper crust may be either an oceanic basaltic layer or very thin (<2 km) continental crust. Results from this first combined seismic reflection and refraction study of a conjugate margi n pair support very slow initial continental rifting (similar to 1 mm/ yr on average) resulting in an asymmetric crustal-thinning profile, wi th breakup occurring against the West Greenland shelf. This asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small-scale convection in the lower lithosphere and asthenosphe re at the sides of the rift. The thin crust in the transition zone imp lies that the amount of melt generated in the upwelling mantle is negl igible, consistent with slow spreading or rifting. The transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth, signaling an increase t o normal oceanic crustal thickness.