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Rift Structure: Models and Observations |
1 Department of Earth Sciences, University of Liverpool, Liverpool L69 3BX, UK
2 Badley Earth Sciences, Spilsby, Lincoln, PE23 5NB UK
3 Elf-Aquitaine Production, Rue Laribau, Pau, France
The McKenzie model of continental lithosphere extension describes the first-order lithosphere responses of crustal thinning and geothermal gradient increase following rifting, which lead to syn- and post-rift basin subsidence. At the sub-basin scale, seismic data show the fundamental importance of major basement faults in controlling the geometry and subsidence of rifted sedimentary basins. Reflection and earthquake seismology data show that these faults are generally planar and extend down 10–15 km to mid-crustal level. Below this depth deformation gives way to distributed ductile shear in the lower crust and mantle. Extensional faulting leads not only to hanging wall subsidence but also to footwall uplift. It can be successfully modelled using elastic dislocation, visco-elastic finite element and thin plate flexural-isostasy (flexural cantilever) theories. For extension on multiple faults, interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics. Mathematical forward models of rifting, incorporating upper crustal faulting, lower crust/upper mantle plastic stretching, lithosphere thermal perturbation and re-equilibration, sediment loading and flexural isostasy have been developed and applied to both syn- and post-rift stages of basin evolution. The models allow the effects on basin geometry and subsidence of fault spacing, fault polarity and extension magnitude, as well as sediment fill and footwall erosion, to be explored. Reverse post-rift modelling from present day sections may be used to constrain ß stretching estimates and to predict palaeobathymetry and topography. Both forward and reverse models have been successfully applied to many rift basins worldwide, including the Viking Graben, North Sea and the East African Rift System, both of which are documented here. The forward model is also capable of simulating the first-order geometry of metamorphic core-complexes and so-called ‘detachment faults’ documented from the Basin-and-Range Province of the western USA.
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