Crustal structure of the Flemish Cap Continental Margin (eastern Canada): an analysis of a seismic refraction profile

Abstract

S U M M A R YThe crustal structure of the NE Flemish Cap margin was determined along a 460-km-long refraction/wide-angle reflection seismic transect (FLAME Line) to define the thickness, structure and composition of the crust and uppermost mantle along the line. A P-wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by 19 ocean bottom seismometers. A coincident multichannel seismic profile was used to guide the modelling as reflections could be identified down to Moho. The model displays a sediment cover of up to 3.6-km-thick, subdivided into three layers with velocities of 1.8-1.9 km s −1 , 2.8-3.1 km s −1 and 4.7-4.8 km s −1 . For the western part of the FLAME Line over Flemish Cap, the P-wave velocity model displays an up to 32-km-thick, three-layer continental crust. The continental crust has velocities of 5.8-6.1 km s −1 , 6.3-6.45 km s −1 and 6.65-6.85 km s −1 and thicknesses of about 5 km, 7 km and 20 km in the upper, middle and lower layers, respectively. The thick continental crust thins to a two-layer, 6-km-thick crust (upper layer is 5.55-6.0 km s −1 and the layer below is 6.65-6.8 km s −1 ) over a distance of 45 km. S-wave velocities are determined in the upper layer of the thick continental crust over Flemish Cap and the transition zone by assigning Poisson's ratios in the P-wave velocity model. Comparison of calculated to observed arrival times gives a Poisson's ratio of 0.27 in the upper layer and 0.28 in the layer below, which suggests that the composition of the crust is primarily continental in both the thick crust and the thin crust of the transition zone. The thin continental crust is stretched over a width of 80 km and is underlain by a layer with velocities of 7.5-7.9 km s −1 . We interpret this layer as partially serpentinized mantle, which is consistent with observations from the Newfoundland margin to the south. The serpentinized mantle terminates 30 km seaward of the thick continental crust. At the seaward-most end of the thin continental crust, a prominent ridge feature is observed. The seismic refraction and multichannel seismic data results indicate a mixed character between serpentinized mantle with volcanic extrusions or continental crust. The reflection seismic data show a high relief basement from the ridge feature and seaward. The FLAME Line crosses magnetic anomaly 34 and extends another ∼50 km seaward well into oceanic crust. The ridge is flanked seaward by a two-layer oceanic crust. The upper layer (Layer 2) has velocities of 4.8-5.0 km s −1 for the landward-most 35 km of oceanic crust and 4.8-6.2 km s −1 for the seaward 60 km. The average thickness of Layer 2 is ∼2 km. The lower layer (Layer 3) has velocities of 6.7-7.2 km s −1 and a thickness of ∼3.5 km. The velocity model is consistent with a sharp onset of seafloor spreading seaward of the ridge feature.

Flemish Cap-Goban Spur conjugate margins: New evidence of asymmetry

“…The crust at the Flemish Cap margin thins rapidly from 32 km to 6 km over a distance of only 40 km (Gerlings et al, 2011). In comparison, the Goban Spur margin thins from 28 km to 6 km over a distance of ~80 km (Horsefi eld et al, 1994;Peddy et al, 1989).…”

“…Here also, Poisson's ratio is constrained in the subdued basement region, but values of 0.27 in the upper and 0.28 in the lower crust are obtained (Gerlings et al, 2011), indicating a crust of a continental composition. The thin continental crust is underlain by partially serpentinized mantle.…”

“…The Bullock and Minshull (2005) velocity model of the Goban Spur margin includes a wide ocean-continent transition zone with a serpentinized mantle composition. In order to determine a complete conjugate section, the Flemish Cap margin has been reexamined, including results from a 460-km-long refraction seismic profi le (Gerlings et al, 2011) situated along the original deep MCS refl ection profi le (Lithoprobe Line 85-3; Keen and de Voogd, 1988). The MCS profi le has been reprocessed and Kirchhoff time migrated (see the GSA Data Repository 1 for details).…”

Future accreted terranes: a compilation of island arcs, oceanic plateaus, submarine ridges, seamounts, and continental fragments

“…In this region, continental fragments are accreting and colliding with arcs and (2003), (6) Klingelhoefer et al (2007), (7) Funck (2003), (8) Gerlings et al (2011), (9) Fowler et al (1989), (10) Breivik et al (2012), (11) Lebedeva-Ivanova et al (2006), (12) Morewood et al (2005), (13) Vogt et al (1998), and (14) Collier et al (2009). other continental fragments. The North Palawan block is the best example of a passive margin fragment currently impinging on an island arc (the Philippine Mobile Belt).…”

“…Bulk densities are also reported from studies where the authors combined gravity and seismic data to determine crustal density. References are (1) Funck et al (2008), (2) Grobys et al (2007), (3) Cooper et al (1981), (4) Grobys et al (2009), (5) Borissova et al (2003), (6) Klingelhoefer et al (2007), (7) Funck (2003), (8) Gerlings et al (2011), (9) Fowler et al (1989), (10) Breivik et al (2012), (11) Lebedeva-Ivanova et al (2006), (12) Morewood et al (2005), (13) Vogt et al (1998), and (14) Collier et al (2009 In the geological record, large volumes of crustal accretion are carried out by the collision of composite terranes or continental fragments onto continents (Vink et al, 1984). In North America, the amalgamation of the Wrangellia and Stikinia terranes resulted in a ribbon continent (SABIYA) that was ∼ 8000 km long and ∼ 500 km wide (Johnston, 2001).…”