Inversion of an extensional-ramp basin by a newly formed thrust: the Cameros basin (N. Spain)

Abstract

Abstract The Cameros basin (Iberian Chain) was developed under a very subsident extensional regime during the latest Jurassic-Early Cretaceous. Its sedimentary fill constitutes a megasequence of more than 5000 m of vertical thickness which contains six depositional sequences. Most of the sediments are continental (alluvial and lacustrine) with only minor marine intercalations. A lateral accretionary geometry at the basin scale shows a SSW-NNE migration of the depocentre and an onlap to the north of the sedimentary sequences on the Mesozoic substratum of the northern boundary of the basin. The overall Mesozoic structure of the basin, as seen from field studies and seismic profile interpretation, is a gentle WNW-ESE synclinorium 30–70 km wide and 150 km long. Extension in the pre-basin Mesozoic rocks is very small. The basin boundaries are defined by an unconformity of the basin filling rocks on the former Mesozoic substratum; it is bounded only locally by major faults at surface. The basin is interpreted as an extensional-ramp basin produced over an S-dipping ramp in a blind, flat-lying extensional fault some kilometres deep in the basement. The extensional displacement on the fault produced a synclinal basin which progressively widened with time. The depocentres of the successive depositional sequences were always located above the ramp and migrated to the north, inside the basin, as a result of the hangingwall displacement to the south. From a computer-modelled cross-section, we estimate the total displacement on the extensional fault to be about 33 km. This extension, in the hangingwall, would have taken place in the Pyrenean basin, north of the Cameros basin. Tertiary compression (Palaeogene to Lower-Middle Miocene) inverted the basin by means of a newly formed E-W north-directed thrust. Overthrusting in the Tertiary Ebro basin fill, this new fault formed in the Keuper beds, in a weakness zone with uniform dip within the extensional hangingwall (about 30 km in cross-section). During the deformation it expanded to the north and south until it branched to the Iberian sole thrust, which might coincide with the Mesozoic extensional sole fault. Although this thrust produced a slight (2–3 km) inversion of the basement-Mesozoic cover unconformity, its maximum displacement was of about 28 km. Pre-basin Mesozoic rocks overlie the thrust surface and are overlain by the basin-filling rocks. A south-directed imbricate-fan thrust system developed in the southern margin. The total shortening was about 38 km, leading to the complete inversion of the basin.