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Mesozoic extensional basins and their Cenozoic contractional inversion in an intraplate setting (Maestrat basin, Iberian Chain, E Spain)

Abstract

The Iberian Chain (eastern Iberian Peninsula) is a fold-and-thrust belt developed during the Cenozoic, because of the contractional inversion of the Mesozoic Iberian Rift System.

The extension in the Iberian Chain took place in two major rifting cycles (late Permian to late Triassic, and late Oxfordian to late Albian) followed by episodes of lower rifting activity (early and middle Jurassic, and late Albian to Maastrichtian).

The Maestrat basin (up to 6.5 km of Mesozoic sediments) is one of the most subsident basins during the late Oxfordian to late Albian cycle. A system of listric extensional faults, which involved the basement, bounded the basin and also divided it into minor sub-basins.

An E-W-trending, N-verging, fold-and-thrust belt developed in the northern border of the basin, as the result of the Cenozoic inversion. This belt involved the Mesozoic cover in the northern –foreland– areas, with a detachment level located within the Triassic evaporitic units: Middle Muschelkalk and Keuper. Southwards, the thrust-system also involved the Variscan basement.

A study of the region containing the transition between the thin-skinned and the thick-skinned areas is here presented, based on seismic profiles, oil-exploration wells and field data.

In the Triassic rocks, depositional thickness variations in the Middle Muschelkalk evaporitic unit are recognized, related to high angle faults active during the Triassic rifting. Salt anticlines and welds are also observed in the Middle Muschelkalk. These halokinetic structures developed during the Upper Triassic, as it is deduced after the onlap geometries of the Keuper seismic reflectors over the Middle Muschelkalk anticlines.

A progressive northward thickening of Jurassic and lower Cretaceous units can be observed, related to a S-dipping listric extensional fault located to the N.

During the Cenozoic contraction, the horizontal shortening was about 10 km. The major Mesozoic extensional fault was only inverted in its lower segment –across the basement and Lower Triassic rocks–while the rest of the Mesozoic cover was passively displaced to the North over the Middle Muschelkalk detachment level. A tectonic step of 800 m is found in the transition between the thin-skinned and the thick-skinned areas. The hanging-wall remains elevated for 40 km to the South, and to produce this step, a S-dipping, low angle ramp has been deduced in the basement thrust.