The crustal structure of the extinct ultra-slow Labrador Sea spreading center

The SIGNAL cruise was carried out in 2009 and includes two refraction lines along and across a segment of the extinct Labrador Sea spreading center. This extinct ridge represents a rare opportunity to study the last stages of accretion of oceanic lithosphere, with an oblique ultra-slow full spreading rate of <1 cm/yr between Chron 21 (49 Ma) and the end of spreading before Chron 13 (36 Ma). Shots were recorded by 18 ocean bottom seismometers (OBS) on the 270-km-long line 4 across the spreading centre, while 10 OBSs were deployed on the 144-km-long line 5 along the axial valley. Both refraction lines follow multichannel seismic reflection profiles. For each line, P-wave velocity models have been built using RAYINVR forward modeling. The robustness of the final models has been assessed using gravity modeling and comparisons with first-arrival traveltime tomography (TOMO2D). The velocity model of line 4 across the axial valley shows three distinct crustal structures: (i) The Paleocene oceanic crust at the ends of the model is the result of slow-spreading on the NNW-SSE oriented ridge segment. The crust is composed of a 1.5-km-thick upper layer (5.0-5.8 km/s) and a 3.5-km-thick lower layer (6.1-7.0 km/s) for a total crustal thickness of 5 km. (ii) The Eocene oceanic crust including the extinct axial valley is the result of slow to ultra-slow oblique spreading at a more NW-SE oriented ridge segment after plate reorganization. The axial valley walls form a 2-km-high basement relief. The 1.5-km-thick, fractured upper crustal layer is slower (4.5-5.0 km/s) than the Paleocene upper crust. This illustrates the significant role of tectonics in the last stages of accretion, as mentioned by previous studies. The lower crustal layer is thinner (2-2.5 km) with velocities 6-7 km/s. This layer vanishes on (iii) a 50-km-long section of the profile corresponding to the region of the Cartwright-Julianehaab Fracture Zone, just after reorientation of the spreading direction and the abandonment of the fracture zone. Here, serpentinized mantle (7.2 to 8 km/s) is exhumed. The velocity model of the along-axis line 5 offers a further look at the nature of the lower crust beneath the slope of the northern axial valley wall, where line 5 crosses line 4. Here, lower crustal velocities are higher on line 5 than on line 4 (6-8 km/s vs 6-7 km/s), which might be related to anisotropic serpentinized peridotites adjacent to large normal faults imaged by the seismic reflection data. SE of the crossing area, the velocity structure of line 5 is highly variable, ranging from 5-km-thick crust (5-7 km/s), to exhumed un-serpentinized mantle (8 km/s). This variable crustal structure is typical of the mix of gabbro and serpentinized peridotite formed during ultra-slow spreading. Areas of exhumed mantle are associated with ridge discontinuities, as ridge segments attempt to reorient in a less oblique direction. In the axial valley, a 3.8-4.2 km/s layer located above the seismic basement is interpreted as a last pulse of volcanism during the waning stage of spreading.