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Numerical modelling of rift dynamics: Linking observations on fault, basin and global scale

Author(s): Sascha Brune GFZ German Research Centre for Geosciences, Potsdam, Germany

Dynamic processes in rift systems operate on a variety of spatial scales: Plate tectonics and mantle convection involves global dimensions, while typical rift zones feature a width of few 100 kilometres width, where brittle faults and ductile shear zones dissect the crust. Using recently developed numerical forward modelling techniques, I link observations on fault, basin and global scale addressing two major topics concerning rift evolution and the formation of rifted continental margins: (i) thermo-mechanical rift evolution during crustal hyper-extension and (ii) the dynamic interplay between rift strength and large-scale plate velocities.

Hyper-extended domains at magma-poor continental margins are one of the most promising frontier exploration provinces. Despite the discovery of significant hydrocarbon reservoirs in frontier deep-water basins, their tectono-thermal evolution and the underlying geodynamic processes remain elusive. However, recent advances in computational geodynamics allow to reproduce crustal hyper-extension using a joint elasto-visco-plastic formulation of rheology. These models presented here suggest that rift migration is a key process during hyper-extended margin formation, which is accompanied by sequential faulting in the brittle crust and controlled by lower crustal flow. The resulting syn-rift subsidence evolution severely depends on the location and shows a complex history of thinning-related subsidence, depth-dependent stretching, lithospheric flexure and long-term thermal sag. Due to lateral migration of the rift system, the timing of rapid syn-rift motion and initiation of the post-rift sag phase differs significantly for different points along the margin. Moreover, the model implies that during rift migration large amounts of material are transferred from one side of the rift zone to the other. This concept can be applied to many hyper-extended margins worldwide, such as the Central South Atlantic, Iberia/Newfoundland, the Australia-Antarctica conjugates, the Australian North West Shelf, and the Alpine Tethys margins that are now exposed in the European Alps.

Extension velocity is a key parameter during continental rifting, controlling not only surface heat flow and the amount of partial melting but also the depth of the brittle-ductile transition and fault evolution. Investigating rift kinematics globally by applying a new geotectonic analysis technique to revised global plate reconstructions, I show that rifted margins feature an initial, slow rift phase (< 10 mm/yr, full rate) and that abrupt acceleration introduces a second, fast rift phase. The transition from slow to fast extension takes place long before crustal break-up so that significant margin area is created during each period. The two-phase behaviour and the rapid plate speed-up can be explained through numerical forward models with constant-force boundary conditions. The extension models suggest that this characteristic velocity evolution is caused by a rift-intrinsic strength-velocity feedback, which can be robustly inferred for diverse crustal and mantle rheologies. While motions of Earth’s plates are thought to be driven by slab pull, basal drag, and ridge push, this finding reveals that plate motions during continental break-up are decisively controlled by the non-linear decay of a resistive force: rift strength.

Numerical modelling of rift dynamics: Linking observations on fault, basin and global scale.
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GFZ German Research Centre for Geosciences, Potsdam
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