FORMATION OF OCEANIC-CRUST AT SLOW-SPREADING RATES - NEW CONSTRAINTS FROM AN EXTINCT SPREADING CENTER IN THE LABRADOR SEA

We present a joint analysis of seismic reflection and refraction profi les across an extinct spreading center in the Labrador Sea. The refrac tion data are used to constrain the crustal and subcrustal velocity st ructure, and the deep multichannel reflection data are used to determi ne the nature of the tectonic fabric within the crust, The resulting c rustal model shows that major reductions in crustal thickness and velo city across the rift are associated with a broad zone of rotated fault blocks. Enhanced faulting persists over a width of 100 km, creating a deep rift valley and adjacent elevated ridges. The location of crusta l-scale faults beneath the sides of the rift valley bound a region of reduced mantle velocity, consistent,vith its formation by 5%-10% serpe ntinization of mantle peridotite. The variation in crustal thickness a cross the rift indicates a 30%-40% reduction in the crustal thickness for full spreading rates <20 km/m.y. However, our results suggest that most of the observed crustal thinning is produced by postaccretionary tectonic extension of the crust, during an extended interval when the rate of melt supply did not keep pace with the rate of plate separati on, Such a discontinuous process may not be properly represented by th eoretical models of crustal formation that assume steady-state viscous flow.