EXTENSIONAL STYLES AND GRAVITY-ANOMALIES AT RIFTED CONTINENTAL MARGINS - SOME NORTH-ATLANTIC EXAMPLES

Regional isostatic adjustment of the buoyancy forces created by lithos pheric stretching during rifting is used to predict the crustal struct ure and gravity anomalies across rifted continental margins. Following earlier studies, we assume that stretching and necking of the lithosp here occurs around a ''depth of necking,'' which is the level of no ve rtical motion in the absence of gravitational forces. Differences in t he depth of necking, coupled with lateral variation in flexural rigidi ty, can account for many of the variations in tectonic style observed across rifted continental margins and associated rifted basins. We inv estigate here seven transects crossing the rifted margin around the No rth Atlantic which display considerable variations in subsidence, crus tal thickness variations, and gravity signatures. These are located wh ere high-quality seismic data are available as a constraint. Two conju gate margin segments are included to test for asymmetry in depth of ne cking which might be evidence of a simple shear mode of extension. Res ults suggest that both shallow (3 to 10 km) and deep (20 to 25 km) dep ths of necking occur. The depth of necking appears to be related to th e intrinsic strength maximum within the lithosphere, rather than to th e depth of preexisting structure. Shallower depths of necking may resu lt from heating of the lithosphere during extension which decreases th e depth of maximum strength. Deeper depths of necking may occur when t he rates of extension are low and significant heating of the lithosphe re does not occur. The depth of necking on at least one margin transec t gives results very similar to a locally (Airy) compensated model, ev en though the lithosphere exhibits finite strength. Both conjugate mar gin segments display shallow depths of necking and favor a pure shear rather than a simple shear mode of extension.