Aspects of the structural evolution of the Lusitanian Basin in Portugal and the shelf and slope area offshore Portugal

The study provides a regional seismic interpretation and mapping of the Mes ozoic and Cenozoic succession of the Lusitanian Basin and the shelf and slo pe area off Portugal. The seismic study is compared with previous studies o f the Lusitanian Basin. From the Late Triassic to the Cretaceous the study area experienced four rift phases and intermittent periods of tectonic quie scence. The Triassic rifting was concentrated in the central part of the Lu sitanian Basin and in the southernmost part of the study area, both as symm etrical grabens and half-grabens. The evolution of half-grabens was particu larly prominent in the south. The Triassic fault-controlled subsidence ceas ed during the latest Late Triassic and was succeeded by regional subsidence during the early Early Jurassic (Hettangian) when deposition of evaporites took place. A second rift phase was initiated in the Early Jurassic, most likely during the Sinemurian-Pliensbachian. This resulted in minor salt mov ements along the most prominent faults. The second phase was concentrated t o the area south of the Nazare Fault Zone and resulted here in the accumula tion of a thick Sinemurian-Callovian succession, Following a major hiatus, probably as a result of the opening of the Central Atlantic, resumed deposi tion occurred during the Late Jurassic. Evidence for Late Jurassic fault-co ntrolled subsidence is widespread over the whole basin. The pattern of Late Jurassic subsidence appears to change across the Nazare Fault Zone. North of the Nazare Fault, fault-controlled subsidence occurred mainly along NNW- SSE-trending faults and to the south of this fault zone a NNE-SSW fault pat tern seems to dominate. The Oxfordian rift phase is testified in onlapping of the Oxfordian succession on salt pillows which formed in association wit h fault activity. The fourth and final rift phase was in the latest Late Ju rassic or earliest Early Cretaceous. The Jurassic extensional tectonism res ulted in triggering of salt movement and the development of salt structures along fault zones. However, only salt pillow development can be demonstrat ed. The extensional tectonics ceased during the Early Cretaceous. During mo st of the Cretaceous, regional subsidence occurred, resulting in the deposi tion of a uniform Lower and Upper Cretaceous succession. Marked inversion o f former normal faults, particularly along NE-SW-trending faults, and devel opment of salt diapirs occurred during the Middle Miocene, probably followe d by tectonic pulses during the Late Miocene to present. The inversion was most prominent in the central and southern parts of the study area. Tu betw een these two areas affected by structural inversion, fault-controlled subs idence resulted in the formation of the Cenozoic Lower Tagus Basin. Northwe st of the Nazare Fault Zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevai led. The Miocene compressional stress was mainly oriented NW-SE shifting to more N-S in the southern part. (C) 1998 Elsevier Science B.V. All rights r eserved.