Abstract. We present a wide-angle seismic refraction study of an 80x40 km region of the southern Iberia Abyssal Plain, south of Galicia Bank. An intersecting grid of two E-W and four N-S wide-angle reflection/refraction profiles is used to define variations of the basement velocity blocks immediately south of Galicia Bank. This crust is underlain by a high-velocity layer (7.3-7.9 km/s) of weakly serpentinized (i.e., 0-25%) peridotite, which exists throughout the eastern part of the survey area. Basement within the OCT appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally. In the southeast sector of our survey where basement topography deepens and becomes subdued, continental fault blocks are absent; instead, basement contains an upper layer of more pervasively serpentinized (i.e., 25-45%) peridotite that is ~2 km thick. This layer is characterized by low velocity at the top of basement (4.2 km/s) that increases rapidly with depth, and it probably corresponds to a seismically unreflective layer, previously identified in reflection profiles to the south of our survey. In the western section of our survey, beneath a series of elevated basement ridges, velocities are reduced within both the upper •asement layer (3.5-6.0 km/s) and lower layer (6.4-7.5 km/s). These changes suggest that both upper and lower layers have become more highly serpentinized (with values of 60-100% in the upper layer and 25-45% in the lower layer) probably during the last stages of rifting and immediately before formation of oceanic crust. A normal or slow spreading oceanic crustal structure is not found within the survey region. Thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at ODP Site 897 and east of the J magnetic anomaly.

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...

No abstract

No abstract

No abstract