The crustal structure of the Flemish Cap Continental Margin (E Canada): New Evidence for Asymmetric Rifting from Goban Spur (NW Europe)

The Flemish Cap - Goban Spur conjugate margin pairs were formed by continental extension starting in Barremian (126 -128 Ma) with final breakup leading to formation of oceanic crust in Albian (~110 Ma). These conjugate margin pairs have previously been studied by deep seismic reflection profiles and used in support of a symmetric pure shear model of extension followed by an asymmetric breakup. In contrast, a more recent seismic refraction study of Goban Spur indicates that extension is more complex and includes a wide transition zone interpreted as serpentinized mantle. In order to determine a complete conjugate section, the Flemish Cap margin has been re-examined with a 460-km-long refraction seismic profile, including dense airgun shots to 21 OBS receivers along the original deep MCS reflection profile. The refraction profile extends more than 50 km seaward of magnetic anomaly 34 and hence well on to oceanic crust. P-wave and S-wave velocity modeling by forward and inverse methods were carried out to define the thickness, structure and composition of the crust and uppermost mantle along the line. A comparison of the Flemish Cap and Goban Spur velocity models gives new insight into the rifting style of the two margins. The continental crust on the Flemish Cap margin thins from a 31 km thick crust to about 7 km over a zone of 40 km. This thinning is more rapid than for the crust on the conjugate Goban Spur margin, which thins from ~27 km thick crust to 7 km over a zone of ~100 km. Furthermore, the two margins display different crustal characteristics in the transition zones. S-wave modeling indicated that the transition zone consists of thin continental crust on the Flemish Cap margin, in contrast to the serpentinized mantle on the Goban Spur margin. The Flemish Cap velocity model also shows a layer of partly serpentinized mantle below the thin continental. A landward dipping reflection, observed on the MCS profile, is coincident with the landward termination of the serpentinized layer. These new results from Flemish Cap, together with the recent results from Goban Spur show that the transition zones are of similar widths but have different characteristics. These differences indicate asymmetric extension and asymmetric breakup between the Flemish Cap - Goban Spur conjugate margin, including asymmetric transitional zones between continental and oceanic crust.