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Supra-Salt Carapaces in the Western Betic Cordillera, Spain: Characterizing an Outcropping Example of a Key Element in Salt Tectonics

Abstract

Surface, seismic and well log data in the Alpine fold-and-thrust belt of the Betic Cordillera, in southern Spain, was integrated to reconstruct the structure and evolution of some complex units above a large-allochthonous Triassic salt nappe in the External Betics. These units are particularly well represented in the Western Betics, with a contrasting Mesozoic-to-Paleocene stratigraphy compared to the one from the surrounding Sub-Betic units. Relatively deep exploration wells, carried out in the eighties when the Betic Cordillera was actively explored, show that the Jurassic to Paleocene section located above the kilometer-thick layer of allochthonous Triassic salt, is much thinner and condensed that the sub-autochthonous section located underneath the salt nappe (representing the subsalt section). Field observations, also confirm that the suprasalt units show a different structural style and an opposite vergence (to the NNE) to the main surrounding Betic foothills thrust sheets (to the WNW). Well data, integrated with surface geological data and seismic profiles, suggest that the suprasalt units are “floating” above the allochthonous Triassic salt nappe and that they are unrooted and detached from the subsalt, Mesozoic units. Altogether, our observations suggest the occurrence in the External, Western Betics of diverse, kilometric-scale, supra-salt units with comparable characteristics to the suprasalt carapaces or rafts identified in other marine salt basins, like for example in the Gulf of Mexico. The condensed and thin stratigraphic sections of the carapaces contrast with other units that can be traced beneath the Triassic allochthonous salt nappe. We interpreted that the suprasalt carapaces in the Betics were formed during the Mesozoic and Paleocene above coalescing salt diapirs, which finally merge in the External Betic paleomargins forming large allochthonous salt canopies. Gravitational structures related to their emplacement within the Mesozoic-to-Paleocene passive margin, prior to the Alpine compression in the westernmost Mediterranean, could also help to explain the observed, stratigraphic and structural differences, with contrasting structures and vergence, between the suprasalt (canopies) and the subsalt sequences. The use of salt tectonic principles in continental folded belts, like the case of the External Betic Cordillera, is therefore an irreplaceable tool to explain the origin and evolution of orogenic systems.