International Geologiical Congress - Oslo 2008

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MPM-11 Phase transformations and geodynamics

 

Evaluation of phase transitions in the lower crust as mechanism for basin formation

 

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

 

The Barents Sea is one of the world's deepest sedimentary basins including the East Barents Sea with a depth up to 20 km. The origin of this sedimentary basin is a matter of debate due to the enormous depth, the lateral extend of the basin and very few signs of extensional faulting. Since standard crustal extension and thinning models are apparently insufficient to explain the dramatic and rapid subsidence which can be observed in parts of the stratigraphic succession, alternative models have to be developed.
We propose that phase transitions lead to a partial or complete densification of the lower crust and therefore might play an important role in the formation of deep intra-continental basins such as the East Barents Sea.
Rock densities depend significantly on composition, temperature and pressure. Higher pressures cause the formation of denser minerals like clinopyroxene while higher temperatures often trigger dehydration reactions when hydrous minerals are present, which also results in the formation of denser minerals. Therefore, the assumption of a constant average density for any part of the crust and the mantle in a model is an oversimplification.
Field observations from East Greenland and the Western gneiss region in Norway show that the lower crust consists of silicic gneisses with lenses of mafic eclogite. We therefore investigate which metapelitic compositions yield the highest densities and show that Al- and Fe-rich rocks can become very dense under certain P-T conditions due to the formation of dense garnet in these rocks. While a hydrous metabasaltic rock at 700°C and 1.5 GPa has densities of 3.2 g/cm3, an Al- and Fe-rich metapelitic rock like the Fjørtoft gneiss can be even denser.
However, when combining P-T-density diagrams with the concept of isostasy, it can be shown that even a dry basalt, which yields the highest crustal densities, will not be denser than the mantle at the seismic Moho (estimated at around 40 km in the region of the Barents Sea). Since the density is not sufficient for the crust to sink into the mantle, we propose plate buckling under a horizontal force as mechanism which could trigger the crustal phase transitions. As a result of this mechanism, the plate will deflect into a sinusoidal shape and consequently the crust-mantle boundary is shifted to greater depths. The crustal rocks are exposed to higher pressures which enable phase transitions, for example the formation of clinopyroxene. However, the rocks will also thermally equilibrate which again causes phase transitions. In order to further investigate this mechanism, we will have to determine what minimum force is required for buckling to occur and how much lithosphere can be expected to buckle under horizontal compression in the case of the East Barents Sea basin. In addition we will address the question if deformation can be faster than the heat deformation.

 

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