The continent‐ocean crustal transition across the southwest Greenland margin

Abstract

This paper examines the complete crustal transition across the nonvolcanic, southwest Greenland continental margin of the Labrador Sea using wide-angle and coincident verticalincidence seismic profiles. Six ocean bottom seismometers and a sonobuoy record P and S wave first and multiple arrivals from the crust and upper mantle, which are analyzed by two-dimensional dynamic ray tracing and one-dimensional reflectivity modeling. The resulting seismic velocity model requires that the preexisting 30-kin thick continental crust is thinned abruptly to ~3 km across the continental slope, primarily by removal of the lower crust. Farther seaward, the crust thickens to -6 km primarily through the addition of a high-velocity (7.0-7.6 km/s) layer in the lower crust. This lower crustal layer is 4-5 km thick, extends for a horizontal distance of-80 km, and is interpreted as partially serpentinized upper mantle. It is overlain by a low-velocity (4.0-5.0 km/s), upper layer which is interpreted as highly fractured upper continental crust. Our model suggests fiat seafloor spreading did not start until chrons 27-28, 13 Ma younger than previously suggested. This interpretation is supported by two-dimensional modeling of gravity and magnetic data along the refraction line. Our results are consistent with a simple shear mechanism for the initial rifting, with the SW Greenland margin as the upper plate. However, a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin, east of Labrador. IntroductionA major goal of studying the structure of Ariantic-type, passive continental margins has been to discriminate between different rifting models, especially between pure [McKenzie, 1978] and simple [Wernicke, 1985; Lister et al. 1986] shear mechanisms. A simple shear model invokes a low-angle detachment fault that cuts through the continental crust and separates the resulting margins into asymmetric, upper and lower plate conjugates. The upper plate margin records more thinning in the lower crust and less syn-rift subsidence than does its conjugate, lower plate margin. This model has been supported by the existence of an S-reflector, interpreted as the detachment fault, and a smaller amount of upper crustal stretching as compared to total crustal thinning in the Bay of Biscay [Le Pichon and Barbier, 1987] and Galicia Bank [Boillot et al., 1989b, 1992]. On the other hand, the observed overall symmetry of crustal thinning across the restored rift zone for the conjugate margins of Flemish Cap [Keen and de Voogd, 1988] and Goban Spur [Peddy et al., 1989] suggests a pure shear, symmetric extension with final breakup closer to one side of the rift zone [Keen et al., 1989]. Finally, it is possible to interpret the rifting of some margins using a mixture of pure shear for the lower crust and lithosphere and simple shear for the upper crust (e.g., Galicia Bank; S ibuet [1992]). Seismic refraction methods have been used in defining crustal affinities on rifted continental margins but mainly fo...

Crustal structure of northern Baffin Bay: Seismic refraction results and tectonic implications

“…Finally, the measured S wave velocity, if this has been correctly identified, is not consistent with mafic oceanic crest but is nearer the value predicated for partially serpenfinized peridotite [Chian and Louden, 1994]. For these reasons, we suggest that this margin may be amagmatic, analogous to those of the Labrador Sea [Chian et al, 1995] and the Grand Banks [Reid, 1994], and that the 6.8-km/s layer may in fact be serpentinized mantle.…”

Southeast Baffin volcanic margin and the North American‐Greenland plate separation

“…We both compare with published surface magnetic data [e.g., Srivastava and Roest, 1999] and crustal seismic profiles [Chian and Louden, 1994; Chian et al, 1995] in order to investigate the limit between unextended buoyant continental crust and very thin (6 km) continental crust [Chalmers and Laursen, 1995], Cretaceous oceanic crust [e.g., Srivastava and Roest, 1999], or else extremely thin (< 2 km) transition crust [Chian et al, 1995].…”

Constraints on Moho depth and crustal thickness in the Liguro-Provençal basin from a 3D gravity inversion: geodynamic implications

“…Thin crust overlying a high velocity layer is found on the western margin of Iberia (3-4 km thick above a 7.3 to 7.6 km/s layer, Whitmarsh et al, 1990;Whitmarsh et al, 1993), in the Tagus abyssal Plain (2 km thick above 7.6 km/s layer increasing to 7.9 km/s towards Moho, Pinheiro et al, 1992) and its conjugate Newfoundland margin off Grand Banks (2-3 km thick above 7.2 to 7.7 km/s layer, Reid, 1994), accross Southwest Greenland margin (2.5 km thick above 7.0 to 7.6 km/s layer, Chian and Louden, 1994) and its conjugate Labrador margin (1-2 km thick above a 6.4 to 7.7 km/s layer, Chian et al, 1995) On the southern Newfoundland margin the high velocity body is limited by one or two landward dipping reflectors rising to basement surface seaward and connecting to Moho landward (Keen and de Voogd, 1988;Reid, 1994). The similarity with the T reflector of the Gulf of Lion is striking.…”