International Geologiical Congress - Oslo 2008

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STT-02 Structure and formation of rift basins and passive margins from surface to depth: Observations and modelling

 

Are phase changes at the origin of the large subsidence of Barents sea basins? insights from dynamic numerical modelling

 

Sebastien Gac, University of Bergen (Norway)
Ritske S. Huismans, University of Bergen (Norway)
Nina S.C. Simon, University of Oslo (Norway)
Julia Semprich, University of Oslo (Norway)
Yuri Y. Podladchikov, University of Oslo (Norway)
 

 

Very large subsidence, with up to 20 km thick sediment layers, is observed in the eastern basins of the Barents Sea. Subsidence started in Early Palaeozoic and finished at mid-Cretaceous. The subsidence history is marked by an acceleration of subsidence rate at Permo-Triassic times. The eastern Barents Sea basin formation is not easily explained with extensional mechanisms as extensional faulting is unable to account for the observed crustal thinning. Various hypotheses have been proposed to explain this very large anomalous subsidence.
One hypothesis relates phase changes of rocks in the lower crust to anomalous vertical subsidence in the basin. It is well known that changes in pressure and temperature conditions result in modifications of the mineral composition of rocks. These phase changes are generally accompanied with density changes. For example, temperature and pressure increases cause the formation of denser minerals like clinopyroxene. Following Artyushkov (2005), we propose that the very large subsidence may result from densification in the lower crust owing to increased pressures and temperatures during shortening and buckling of the crust - mantle lithosphere system.
A finite element numerical model of lithosphere deformation has been developed in order to test the role of phase changes during lithosphere shortening and its effect on subsidence. The model consists of a crust - mantle lithosphere characterized by non-linear temperature and pressure dependent visco-elastic-plastic rheologies. Rock density depends on pressure and temperature dependent phase changes. The mechanical model is coupled with a thermal model taking into account heat advection and diffusion. Sedimentation and gravity are taken into account. Contractional boundary conditions are applied on vertical sides of the model resulting in buckling of the crust. Several models are run for different values of the main controlling parameters, crustal composition, crustal thickness, thermal structure of the lithosphere, and sedimentation. For each model, the modelled subsidence history is compared to the one observed in eastern Barents Sea basins.

 

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