Lithosphere rheology during intraplate basin extension and inversion - Inferences from automated modeling of four basins in western Europe

Forward reconstructions of the (mainly) Mesozoic and younger rheological ev olution have been made for four basins (Broad Fourteens Basin, Sole Pit Bas in, Brittany Basin and the Iberian Basin) in a very consistent way by backs tripping and automatic forward modeling of subsidence data, including poten tially important effects of heat production, sediment infill and sedimentar y blanketing. For default compositional and thermal parameters, the modelin g results show strengthening in all basins, and in particular during invers ion, with strength increases up to about 2 TN m(-1) (20%) relative to their initial values. Given predominantly relatively constant intraplate stresse s in continental lithosphere, this is in disagreement with repeated localiz ation of basin deformation. In a thorough sensitivity analysis we explore the possibilities that permis sible variations in tectonic history, compositional, rheological and therma l parameters can, in a particular combination, result in slight weakening o f the basin, in agreement with reactivation during inversion. However, such a combination reflects an extreme scenario, which is not founded by geolog ical evidence and, statistically, is very unlikely to apply for all basins. A far more likely explanation for relative and permanent weakening of the b asins is the presence of pre-existing weak zones, deviating from standard r heological assumptions. At (upper) crustal levels, weakening can be attribu ted to pre-existing faults marked by a reduced friction angle. This weakeni ng has a pronounced influence on lithospheric strength provided that the re duction in friction angle of pre-existing faults can be extrapolated to the upper mantle. Alternatively, weakening of the upper mantle can be attribut ed to (1) ductile localization mechanisms, as reflected by the occurrence o f upper mantle shear zones, or (2) the occurrence of theologically weak mat erial, as indicated by upper mantle reflectors. (C) 2000 Elsevier Science B .V. All rights reserved.