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 vertical‐incidence 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‐km 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 that 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.