Extinct spreading center in the Labrador Sea: Crustal structure from a two‐dimensional seismic refraction velocity model

The Labrador Sea contains a rare example of an abandoned mid‐ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates. Seismic refraction data were collected along a line which transversely crosses the extinct spreading center. Two‐dimensional analyses of the refraction data, using ray‐tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center. In the extinct spreading center, a crustal thickness of approximately 4 km is determined, compared with 5.5 km for the flanks. Substantial lateral variations in P wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center. Low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration. The anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction.