This paper examines the complete crustal transition across the nonvolcanic, southwest Greenland continental margin of the Labrador Sea using wide-angle and coincident vertical-incidence seismic profiles. Six ocean botton seismometers and a sonobuoy record P and S wave first and multiple arrivals from the crust and upper mantle, which are analyzed by two-dimensional dynamic ray tracing and one-dimensional reflectivity modeling. The resulting seismic velocity model requires that the preexisting 30-km thick continental crust is thinned abruptly to ~3 km across the continental slope, primarily by removal of the lower crust. Farther seaward, the crust thickens to ~6 km primarily through the addition of a high-velocity (7.0--7.6 km/s) layer in the lower crust. This lower crustal layer is 4--5 km thick, extends for a horizontal distance of ~80 km, and is interpreted as partially serpentinized upper mantle. It is overlain by a low-velocity (4.0--5.0 km/s), upper layer which is interpreted as highly fractured upper continental crust. Our model suggests that seafloor spreading did not start until chrons 27--28, 13 Ma younger than previously suggested. This interpretation is supported by two-dimensional modeling of gravity and magnetic data along the refraction line. Our results are consistent with a simple shear mechanism for the initial rifting, with the SW Greenland margin as the upper plate. However, a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin, east of Labrador. ¿ American Geophysical Union 1994