Quantifying the Effect of Mantle Phase Transitions and Application to the Vøring Basin, Off-Shore Norway

Nina S.C. Simon, Lars H. Rupke, and Yuri Y. Podladchikov
Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway
e-mail: [email protected]

Temperature and composition of the extending lithospheric mantle strongly influence the physical properties of the stretched lithosphere and might vary significantly during the process. The physical property that is most important for subsidence and uplift in a sedimentary basin is density. The density distribution of the lithosphere is non-linear and discontinuous due to complex mineralogy and, most importantly, phase transitions. Lithospheric mantle enters the plagioclase stability field above ~50 km, which can cause a large decrease in density (80-100 kg/m³). The integrated change in density of the isostatic column due to the plagioclase-in phase transition is predominantly influenced by the aluminum and sodium contents of the mantle. The amount of subsidence caused by phase transitions therefore depends on the composition of the mantle, in addition to classical parameters such as the thickness of the crust, the initial geotherm and the amount of stretching. The phase-transition effect is most pronounced for thin crust, strong mantle thinning/upwelling and relatively fertile mantle compositions rich in aluminum and sodium, and can exceed the effect of thermal expansion. This could explain the pronounced syn-rift uplift and accelerated post-rift subsidence observed in some basins.

The Vøring basin in the Norwegian Sea is an example of a basin showing such anomalous uplift and subsidence patterns. Towards the end of the last rifting phase (~65-55 Ma) a regional uplift event occurred. The most popular explanation for this uplift is a magmatic underplating event resulting in lower effective mantle densities and thereby uplift. While this mechanism is theoretically viable it requires very high mantle stretching/thinning factors resulting in a high basement heat flow into the sedimentary section. Alternatively, mineral phase transitions can generate uplift - as described above. This 'cold' uplift scenario can operate at much lower stretching factors and results thereby in a lower basement heat flow than in the 'hot' case of extreme mantle thinning. To discriminate between the 'hot' and the 'cold' model we use 2-D kinematic reconstructions of rifting in the Vøring basin, which appear to favour the 'cold' scenario over the 'hot' one.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands