Extensional styles and gravity anomalies at rifted continental margins: Some North Atlantic examples

Regional isostatic adjustment of the buoyancy forces created by lithospheric stretching during rifting is used to predict the crustal structure and gravity anomalies across rifted continental margins. Following earlier studies, we assume that stretching and necking of the lithosphere occurs around a “depth of necking,” which is the level of no vertical motion in the absence of gravitational forces. Differences in the depth of necking, coupled with lateral variation in flexural rigidity, can account for many of the variations in tectonic style observed across rifted continental margins and associated rifted basins. We investigate here seven transects crossing the rifted margin around the North Atlantic which display considerable variations in subsidence, crustal thickness variations, and gravity signatures. These are located where high‐quality seismic data are available as a constraint. Two conjugate margin segments are included to test for asymmetry in depth of necking which might be evidence of a simple shear mode of extension. Results suggest that both shallow (3 to 10 km) and deep (20 to 25 km) depths of necking occur. The depth of necking appears to be related to the intrinsic strength maximum within the lithosphere, rather than to the depth of preexisting structure. Shallower depths of necking may result from heating of the lithosphere during extension which decreases the depth of maximum strength. Deeper depths of necking may occur when the rates of extension are low and significant heating of the lithosphere does not occur. The depth of necking on at least one margin transect gives results very similar to a locally (Airy) compensated model, even though the lithosphere exhibits finite strength. Both conjugate margin segments display shallow depths of necking and favor a pure shear rather than a simple shear mode of extension.