Asymmetric lithosphere as the cause of rifting and magmatism in the Permo-Carboniferous Oslo Graben
Christophe Pascal, Sierd A. P. L. Cloetingh, Gareth R. Davies, 2004. "Asymmetric lithosphere as the cause of rifting and magmatism in the Permo-Carboniferous Oslo Graben", Permo-Carboniferous Magmatism and Rifting in Europe, M. Wilson, E.-R. Neumann, G. R. Davies, M. J. Timmerman, M. Heeremans, B. T. Larsen
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Compared to other Permo-Carboniferous rift basins of NW Europe, the Oslo Graben has two distinct characteristics. First, it initiated inside cold and stable Precambrian lithosphere, whereas most Permo-Carboniferous basins developed in weaker Phanerozoic lithosphere, and second, it is characterized by large volumes of magmatic rocks despite relatively little extension. Seismic reflection surveys show that the crust thickens from southern Norway to southern Sweden, the most significant Moho deepening occurring from the Oslo Region eastwards. Deep seismic studies also suggest that the base of the lithosphere deepens markedly eastwards from the Oslo Region. Such a long-wavelength lithospheric geometry cannot be explained by the Permian or post-Permian evolution of the area, hence the Oslo Graben appears to have evolved at the transition between two lithospheric domains with contrasting thickness. Numerical thermo-mechanical modelling is applied to test if this transitional position can influence the dynamics of rifting. Different models with varying lithosphere thickness contrast are considered. Model results show that a crust and lithosphere thickness contrast comparable to the Oslo Region can explain rifting and focusing of magmatism in a narrow zone with minor thinning of the crust. Models also account for the major characteristics of the Oslo Graben in terms of location and strain distributions in the crust.
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Permo-Carboniferous Magmatism and Rifting in Europe
Widespread extension occurred within the Variscan orogen and its northern foreland during Late Carboniferous to Early Permian times. This was associated with magmatism and with a fundamental change, at the Westphalian-Stephanian boundary, in the regional stress field, coincident with the termination of orogenic activity and onset of dextral translation between North Africa and Europe. Rifting propagated across basement terranes with different ages and thermal histories. Most of the rift basins developed on relatively thin lithosphere; however, the highly magmatic Oslo Graben initiated within the edge of a craton. Early Stephanian regional uplift is contemporaneous with the onset of magmatism, inviting speculation that it might have been induced by a thermal anomaly within the upper mantle. The contributions to this volume suggest that the geodynamic setting in which magmatism occurred was complex, involving wrench tectonics, slab detachment, and delamination or thermal erosion of the base of the lithosphere.