The oceanic crustal structure at the extinct, slow to ultraslow Labrador Sea spreading center

Abstract

Two seismic refraction lines were acquired along and across the extinct Labrador Sea spreading center during the Seismic Investigations off Greenland, Newfoundland and Labrador 2009 cruise. We derived two P wave velocity models using both forward modeling (RAYINVR) and traveltime 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 ultraslow 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 ultraslow-spreading centers like the Mohns Ridge or the South West Indian Ridge 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 ultraslow spreading, which was thinned by tectonic extension.

The velocity structure of a fossil spreading centre in the Southwest Sub‐basin, South China Sea

“…S1). The results reveal a big difference in crustal thickness: 3-4 km thick in the SCS , 5-15 km thick in the Shikoku back-arc basin (Nishizawa et al, 2011), 3.5-5 km thick in the Labrador Sea (Osler and Louden, 1992;Delescluse et al, 2015), 6 km thick in the southern Baffin Bay (Funck et al, 2007;Suckro et al, 2012) and 4-5 km thick at the Aegir Ridge Breivik et al, 2006). The crustal structure of a fossil spreading centre records many important processes from seafloor spreading regimes to spreading ridge extinction and indicates active spreading ridges.…”

“…The layer 3 velocity is mainly influenced by lithology . Moreover, the thickness of layer 3 is reported in relation to the spreading rate, melt supply and mantle serpentinization (Chen, 1992;Delescluse et al, 2015). However, the thickness of layer 3 varies from~2.5 (off-axis) to~4 km (axial) (Fig.…”

“…Hence, the SWSB ridge (NE) was characterized by a decreasing crustal thickness immediately after the cessation of spreading in the SWSB and the waning stage of spreading seems to be characterized by a reduced melt supply (Canales et al, 1998;Delescluse et al, 2015). The axial thinner crusts represent the state with little or no influence of postspreading magmatism.…”

“…Overall, they are mainly distributed in the Pacific Ocean and then the Atlantic Ocean and the Indian Ocean (Fig. S1), only the SCS Pichot et al, 2014), the Shikoku backarc basin (Nishizawa et al, 2011), the Labrador Sea (Osler and Louden, 1992;Delescluse et al, 2015), Baffin Bay (Funck et al, 2007;Suckro et al, 2012) and the Aegir Ridge Breivik et al, 2006) have been sampled by seismic refraction data. These fossil spreading ridges can be easily divided into two groups based on the post-spreading magmatism: (1) ridges with strong post-spreading magmatism showing high terrain, such as the Shikoku Basin and the Galapagos Rise ; (2) ridges with little or no postspreading magmatism maintaining their initial morphology after seafloor spreading, such as the Labrador Sea , Baffin Bay (Suckro et al, 2012) and the Aegir Ridge .…”

Seismic Evidence for Tectonically Dominated Seafloor Spreading in the Southwest Sub‐basin of the South China Sea

“…In the final velocity model of OBS973 (Figures 3b and 11), velocities beneath these M-reflectors and PmP-inverted Moho increase gradually from 6.8-7.2 km/s to 8.0 km/s rather than jump sharply, which are consistent with the almost absence of PmP phases in the OBS records over the SWSB ( Figure S1; Pichot et al, 2014;Qiu et al, 2011;Yu et al, 2017b). The velocities from 6.8-7.2 km/s to 8.0 km/s are too low for unaltered mantle and may result from serpentinization of the upper mantle (e.g., in fracture zones, Detrick et al, 1993, the COT of nonvolcanic rifted margins, Davy et al, 2016, and slow or ultraslow spreading oceanic basins; e.g., the Labrador Sea; Delescluse et al, 2015;Osler & Louden, 1992 or magma intrusion (e.g., in NW Indian Ocean; Gupta et al, 2010). In our final model (Figure 3b), velocities at depths >3 km below the top basement sit outside the envelopes for Pacific and magmatic oceanic crust newly compiled by Grevemeyer et al (2018;Figure 9a well with the velocity profiles in the models of thin oceanic crust overlying serpentinized mantle in the North Atlantic Ocean (Figure 9b; Davy et al, 2016;Funck et al, 2003;Hopper et al, 2004;Whitmarsh et al, 1996) and the exhumed mantle zone in West Iberia (Figure 9b; Dean et al, 2000;Sallarès et al, 2013) and the Central Tyrrhenian Basin (Figure 9c; Prada et al, 2014).…”