Quantitative subsidence analysis of the Mesozoic evolution of the Lusitanian basin (western Iberian margin)

Abstract

Quantitative subsidence analysis of 26 wells in the Lusitanian basin provides new constraints on the western Iberian Mesozoic passive margin development. Backstripped tectonic subsidence curves show a three-fold subdivision of vertical motions from Late Triassic onward. Continental rifting was initiated during the Late Triassic and Early Jurassic. From Middle Jurassic onward a distinct different behaviour is expressed in the subsidence curves for the North and the South Lusitanian basin. During the Middle Jurassic the South Lusitanian basin records a stretching episode with stretching factors of about 1.08, while the North Lusitanian basin typically has a Middle Jurassic hiatus. This different development is also expressed in the Late Jurassic and Early Cretaceous sedimentary sequence when both the North and South Lusitanian basin are subjected to another stretching episode, with a more pronounced development of the southern than of the northern part of the basin. The stretching factors for this last phase are about 1.03 for the northern part and 1.08 for the southern part of the area. This north-south difference during the Middle Jurassic to Early Cretaceous, for which the transition roughly coincides with the location of the Nazaré fault zone, is probably a result of differences in pre-rift crustal composition or thickness. Late Cretaceous sediments are mostly absent in the analysed wells. In the southern part of the basin the absence of the Cretaceous record is a consequence of erosion due to Cenozoic inversion of the basin. The generally low estimates for stretching factors suggest that the analysed eastern part of the Lusitanian basin forms the distal part of the mid-Cretaceous continental breakup. A comparison with subsidence curves of neighbouring basins of Iberia reflects general patterns in Mesozoic basin development and confirms the generally held view that the extension leading to continental breakup migrated from south to north during the Middle Jurassic to Early Cretaceous in West Iberia.