Two seismic refraction lines were acquired along and across the extinct Labrador Sea spreading center during the SIGNAL 2009 cruise. We derived two P-wave velocity models using both forward modeling (RAYINVR) and travel time tomography inversion (Tomo2D) with good ray coverage down to the mantle. Slow spreading Paleocene oceanic crust has a thickness of 5 km, while the Eocene crust created by ultra-slow spreading is as thin as 3.5 km. The upper crustal velocity is affected by fracturation due to a dominant tectonic extension during the waning stage of spreading, with a velocity drop of 0.5 to 1 km/s when compared to Paleocene upper crustal velocities (5.2-6.0 km/s). The overall crustal structure is similar to active ultra-slow spreading centers like the Mohns Ridge or the South West Indian Ridge (SWIR) with lower crustal velocities of 6.0-7.0 km/s. An oceanic core complex is imaged on a 50 km-long-segment of the ridge perpendicular line with serpentinized peridotites (7.3-7.9 km/s) found 1.5 km below the basement. The second, ridge-parallel line also shows extremely thin crust in the extinct axial valley, where 8 km/s mantle velocity is imaged just 1.5 km below the basement. This thin crust is interpreted as crust formed by ultra-slow spreading, which was thinned by tectonic extension.