MECHANISMS OF EXTENSIONAL BASIN FORMATION AND VERTICAL MOTIONS AT RIFT FLANKS - CONSTRAINTS FROM TECTONIC MODELING AND FISSION-TRACK THERMOCHRONOLOGY

We present reconstructions of vertical motions at the Saudi Arabian Re d Sea margin and the Transantarctic Mountains in order to test the val idity of models of rift flank uplift. The eroded rift flanks were flex urally backstacked using apatite fission-track thermochronology to det ermine the amount of erosion. Basin sediments were simultaneously flex urally backstripped, constrained by seismic reflection profiles. Flexu ral rigidities were estimated from published coherence studies of grav ity and topography. When adopting these lithospheric strength values i t appears that significant tectonic uplift (of the order of 3-5 km) ha s taken place in the present-day coastal plain areas. Forward thermome chanical modelling of rift flank uplift indicates that thermal mechani sms fail to explain the observed tectonic uplift/subsidence pattern an d crustal structure; flexural uplift as a result of lithospheric necki ng appears to be a much more plausible mechanism. Best-fit models cons istently predict mid-crustal kinematic necking levels, with depths bet ween 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 rang e. A comparison of these model results with rheological strength profi les and dynamic models for extension of continental lithosphere sugges ts that the kinematics of extension are determined to a large extent b y the rheological coupling of strong upper/middle crust and weak lower crust.