Rift-flank uplifts and the breakup or postrift unconformity are characteristic features of many rifted, passive, or Atlantic-type continental margins, but are not predicted as primary features of simple lithospheric stretching models. The explanations that have been proposed for their origin remain controversial, either because they call upon special circumstances or because they are difficult to test.

Plane-strain finite element models are used in this paper to explore the dynamics of lithospheric necking during rifting and rupture. The results agree with the conceptual interpretation that uplift of the rift boundaries to form flank mountains and uplift of the basin responsible for the breakup unconformity are related consequences of regional isostatic compensation of mass that is redistributed during the necking and rupture phases. Although the amplitudes of these uplifts depend on the model parameter values, the relations between and relative signs of these two effects appear to be fundamental.

The explanation we propose may have been missed in recent studies because there has been a tendency to concentrate on kinematic stretching models, which assume local isostatic equilibrium through out the rifting process. Such an approach is predicated on the assumption that the lithosphere has an insignificant strength or flexural rigidity during extension, which is not true if our explanation is correct.

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