HR: 17:15h
AN: T14A-06    [Abstracts]
TI: The Influence of Mantle Petrology on Basin Subsidence During Rifting
AU: * Simon, N S
EM: nina.simon@geo.uio.no
AF: PGP, University of Oslo, PO Box 1048-Blindern, Oslo, 0316 Norway
AU: Podladchikov, Y Y
EM: y.y.podladchikov@fys.uio.no
AF: PGP, University of Oslo, PO Box 1048-Blindern, Oslo, 0316 Norway
AB: The density distribution of the lithosphere is non-linear and discontinuous due to complex mineralogy and, most importantly, phase transitions. It was shown that the garnet-spinel and spinel plagioclase transitions in the lithospheric mantle have the most profound effect on uplift/subsidence during lithospheric extension. Here we evaluate the influence of changes in mantle composition on lithospheric density and behaviour during stretching. Lithospheric mantle enters the plagioclase stability field if the crust is thin and the geotherm is relatively hot (stretching factor 2 to 3, depending on composition). This causes a large decrease in density (80-100 kg/m³). The extent of syn-rift uplift and post-rift subsidence recorded in the basin is directly proportional to the total amount of plagioclase in the column. This is controlled by the depth of the plagioclase-in reaction and the bulk Al₂O₃ in the mantle. The depth to which plagioclase is stable is mainly governed by bulk Na₂O. Since Al₂O₃ and Na₂O decrease with decreasing fertility of the mantle the effect of phase transitions on basin subsidence is most pronounced for relatively fertile mantle (and strong extension). The incompatible nature and strong mobility of sodium, however, may lead to large changes in bulk Na₂O during minor melting (Na extraction) or metasomatism (Na addition). The sensitivity of mantle mineralogy to variations in sodium can therefore cause sudden changes in density without changes in P-T regime. The combination of 2-D finite element deformation modeling and phase diagram calculations allows us to evaluate and quantify the feedback between geochemical variations and lithospheric stretching.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8169 Sedimentary basin processes
SC: Tectonophysics [T]
MN: Fall Meeting 2005