Molasse basins of Europe: a tectonic assessment

Abstract

ABSTRACTSedimentary basins are structures that formed either by subsidence of an area relative to its surroundings, or by uplift of the surroundings. The basin is defined by its sedimentary fill, and the vertical kinematics of the fill are reflected by stratal wedging, unconformities and, or, faulting. The following basin mechanisms are distinguished: locally (a) stretch, (b) thrust and piggy-back, (c) local uplift, and regionally (d) stretch-and-cool, (e) load-and-flex and (f) cratonic uplift.Basin patterns are reviewed for the three main Phanerozoic episodes for which molasse-like features of sedimentation or tectonics are claimed. Sediment accumulation rates are used as an index of the vigour of basinal activity.Within the area of the Caledonian orogen, Devonian basinal activity was locally very rigorous, some of it being late orogenic and some post-orogenic, and mostly apparently of ‘stretch-type’. The orogenic area stood high, relative to sea level, throughout Devonian times, but outside the orogenic area, the basins were less vigorous and marine.Within the area of the Hercynian orogen, and outside it, Permian basins were generally not so active, apparently reflecting a different style of orogenesis. However, the whole area was standing high, relative to sea level. The Triassic basins, though post-orogenic, were rather more vigorous, although a marine transgression records the general lowering of the continental surface. Major evaporites accumulated in these settings.In Cenozoic times, the narrow orogenic belts formed the most active basins, and these were of load-and-flex type, reflecting the importance of thrust-sheet movement, itself perhaps a result of the presence of Triassic evaporites. Other non-orogenic basins reflect both ‘stretch’ and ‘stretch-and-cool’ mechanisms. Only the Spanish basins appear to have been standing high, relative to sea level, perhaps in response to cratonic uplift.