QUANTITATIVE SUBSIDENCE ANALYSIS OF THE MESOZOIC EVOLUTION OF THE LUSITANIAN BASIN (WESTERN IBERIAN MARGIN)

Quantitative subsidence analysis of 26 wells in the Lusitanian basin p rovides new constraints on the western Iberian Mesozoic passive margin development. Backstripped tectonic subsidence curves show a three-fol d subdivision of vertical motions from Late Triassic onward. Continent al rifting was initiated during the Late Triassic and Early Jurassic. From Middle Jurassic onward a distinct different behaviour is expresse d in the subsidence curves for the North and the South Lusitanian basi n. During the Middle Jurassic the South Lusitanian basin records a str etching episode with stretching factors of about 1.08, while the North Lusitanian basin typically has a Middle Jurassic hiatus. This differe nt development is also expressed in the Late Jurassic and Early Cretac eous sedimentary sequence when both the North and South Lusitanian bas in are subjected to another stretching episode, with a more pronounced development of the southern than of the northern part of the basin. T he stretching factors for this last phase are about 1.03 for the north ern part and 1.08 for the southern part of the area. This north-south difference during the Middle Jurassic to Early Cretaceous, for which t he transition roughly coincides with the location of the Nazare fault zone, is probably a result of differences in pre-rift crustal composit ion or thickness. Late Cretaceous sediments are mostly absent in the a nalysed wells. In the southern part of the basin the absence of the Cr etaceous record is a consequence of erosion due to Cenozoic inversion of the basin. The generally low estimates for stretching factors sugge st that the analysed eastern part of the Lusitanian basin forms the di stal part of the mid-Cretaceous continental breakup. A comparison with subsidence curves of neighbouring basins of Iberia reflects general p atterns in Mesozoic basin development and confirms the generally held view that the extension leading to continental breakup migrated from s outh to north during the Middle Jurassic to Early Cretaceous in West I beria.