Crustal structure of an extinct rift axis in the Labrador Sea: preliminary results from a seismic refraction survey

A preliminary crustal structure for the extinct rift axis in the Labrador Sea is presented based on a one-dimensional analysis of ocean bottom seismometer (OBS) refraction profiles. Within the extinct rift, low P wave velocities are observed for crustal (3.6–6.3 km/s) and upper mantle (7.3–8.1 km/s) refractors. The velocity-depth structures for OBSs sampling crust within the extinct rift suggest a marked thinning and variability in crustal thickness from 4.7 km to 2.6 km in comparison to typical thicknesses of 6.4 km observed in the adjacent flanks. The crustal structures at the extinct rift resemble those observed at the active and inactive transforms of oceanic fracture zones. Following arguments for crustal thinning at fracture zones, we speculate that the first order thinning of the lower crust is due to a reduction in the supply of partial melt to the accretionary axis. At the extinct rift, the reduced supply is attributed to changes in the thermal field and spreading rate as rifting ceased. This is consistent with thermal models which predict that crustal thickness decreases appreciably for spreading rates less than 20 mm/yr when plate spreading predominates over buoyancy in driving mantle upwelling.