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AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Mesozoic Rifting and Tertiary Inversion of the Gulf of Cadiz

Abstract

We have studied the nature of the basement and the tectonic structure and stratigraphy of the Gulf of Cadiz from the continental margin of Morocco to the Iberia Peninsula. The study integrates wide-angle seismic data collected with ocean bottom seismometers (OBS) and 6-km-long streamer data obtained with a 4800 c.i. gun array tuned for deep penetration. The data was collected by our group in three experiments and will be completed this next summer with two new seismic cruises one with OBS and another one with streamer acquisition. The wide-angle seismic data shows that the central region of the Gulf of Cadiz is underlain by oceanic crust possibly of the Triassic-Jurassic Tethys system that extended to the east. North and south of the oceanic lithosphere there are continental margins with sedimentary series from Triassic to recent. Two other lithospheric domains form the western region of the Gulf of Cadiz. To the SW, a poorly defined continent ocean transition separates highly thinned continental crust with Triassic salt diapirs and poorly imaged underlying fault blocks, from Jurassic oceanic crust formed during early Jurassic time during the opening of the central Atlantic after break up of North America and Africa. To the NW, the wide-angle seismic data displays very high velocities in the uppermost basement (> 7 km/s) that agree with exhumed mantle under the Horseshoe and Tagus abyssal plains. Mantle exhumation possibly occurred during the opening of the North Atlantic when Iberia separated from Canada. Tertiary convergence of Iberia and Africa caused oceanic subduction and the reactivation of lithospheric boundaries and faults formed during rifting, mantle exhumation and oceanic spreading. The extensional tectonics structures and associated stratigraphy are masked across much of the Gulf of Cadiz by the development of an accretionary prism from early Miocene to end Miocene (Messinian) possibly driven by subduction of the Tethys lithosphere. The prism internal structure is characterized by rotated packages of strata in thrust sheet and out of sequence large-scale thrusting and abundant fluid-flow related structures. Pliocene-recent deformation is characterized by further reactivation of pre-exiting lithospheric structures that cut across Mesozoic and Tertiary stratigraphy and the accretionary prism. We speculate that subduction stopped and that the currently driving force of tectonic processes is continental collision between Africa and Iberia.