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Present address: Shell Minerals and Exploration, Melbourne, Australia.

Abstract

The Mesozoic and Tertiary structural and sedi-mentological evolution of the western Iberian margin can be related to the reactivation of structures within the Hercynian basement. Sedimentary basins probably formed by extensional collapse of the hanging walls of Hercynian thrust sheets. Within the Lusitanian basin, and also offshore, reactivation of north-northeast to south-south-west- and northeast to southwest-trending late Hercynian orogenic strike-slip faults in the basement strongly controlled basin geometry, facies distributions, the site of salt structures, and the location of extensional and compressional faults.

The Mesozoic of the Lusitanian basin comprises four unconformity-bounded sequences which are related to extensional events in the evolution of the North Atlantic: (1) Triassic-Callovian; (2) Middle Oxfordian-Berriasian; (3) Valanginian-Lower Aptian; (4) Upper Aptian-Turonian. Sequences 3 and 4 may be equated to synrift and postrift sequences drilled on the subsided northwestern Iberian margin, but they differ in having much lower rates of subsidence. The contrasting subsidence histories of the northern and central sectors of the western Iberian margin are consistent with a change in location and movement history ofcrustal detachments across an inferred first-order transfer fault. The fact that Mesozoic and Paleogene-Eocene igneous activity and low Cretaceous subsidence rates are confined largely to the area south of the transfer fault suggests that the Lusitanian basin developed on the upper-plate side of an asymmetrical rift. North of the transfer fault, the extensional geometry was more complex.

Two inversion episodes resulted in the reversal of Mesozoic tensional/transtensional features and were related to the Pyrenean and Betic orogenies. The earlier episode was insignificant to the south of the major transfer fault. It was followed by the deposition of as much as 1 kilometer of largely Neogene sediments and subsequent inversion during the late Miocene, which resulted in prerift and synrift sediments being uplifted by as much as 1 kilometer.

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