Mechanisms of extensional basin formation and vertical motions at rift flanks: Constraints from tectonic modelling and fission-track thermochronology

Abstract

We present reconstructions of vertical motions at the Saudi Arabian Red Sea margin and the Transantarctic Mountains in order to test the validity of models of rift flank uplift. The eroded rift flanks were flexurally backstacked using apatite fission-track thermochronology to determine the amount of erosion. Basin sediments were simultaneously flexurally backstripped, constrained by seismic reflection profiles. Flexural rigidities were estimated from published coherence studies of gravity and topography. When adopting these lithospheric strength values it appears that significant tectonic uplift (of the order of 3–5 km) has taken place in the present-day coastal plain areas. Forward thermomechanical modelling of rift flank uplift indicates that thermal mechanisms fail to explain the observed tectonic uplift/subsidence pattern and crustal structure; flexural uplift as a result of lithospheric necking appears to be a much more plausible mechanism. Best-fit models consistently predict mid-crustal kinematic necking levels, with depths between 10 and 30 km. Whereas uplift of the Saudi Arabian Red Sea margin can be modelled adopting a pure-shear necking model, the large uplift of the Transantarctic Mountains requires a simple shear mechanism with lithospheric stretching concentrated under the uplifted mountain range. A comparison of these model results with rheological strength profiles and dynamic models for extension of continental lithosphere suggests that the kinematics of extension are determined to a large extent by the rheological coupling of strong upper/middle crust and weak lower crust.