The Getxo crustal-scale cross-section: Testing tectonic models in the Bay of Biscay-Pyrenean rift system

The Bay of Biscay-Pyrenean rift system is a world-class example of an inverted hyperextended rift. However, tectonic questions such as the linkage of the extensional system, the magnitude of crustal extension, the interpretation of the syn-rift sequences and the amount of shortening during tectonic inversion remain controversial. This paper presents a sequentially restored crustal-scale transect across a key sector of the central Basque-Cantabrian Basin, which allows the prevailing tectonic models to be evaluated. Two separated phases of rifting during Permo-Triassic and late Jurassic-Early Cretaceous times led to breakup. In the second phase, a southerly-dipping extensional detachment accommodated at least 28 km of horizontal extension while separating the Iberian and European plates, and exhuming mantle rocks in the footwall. The locus of volcanism migrated to the north, peaking during the hyperextension and post-rift stages. Mantle sources were influenced by deep OIB-type components most likely triggered by the ascending asthenospheric mantle. Therefore, gravitational forces linked to asthenospheric doming are interpreted as the primary driving mechanism during the peak strain rate. Evacuation of Upper Triassic salt during the syn-rift and post-rift stages determined the development of a salt-floored ramp-syncline basin next to the lower detachment ramp, and the sinking of minibasins in other areas. About 41 km of Cenozoic shortening (21%) were resolved by underthrusting of the crust beneath the shallow lithospheric mantle, leading to the formation of thrusts with opposite vergences at the distal basin boundaries, and by the reactivation of the extensional detachment as a thrust fault. The buttress of the overburden over the salt in the hyperextended domain created outstanding examples of squeezed diapirs, chevron folds and thrust splays. Our results have implications for a better understanding of the connection between the Atlantic Ocean and the Western Tethys along the Bay of Biscay-Pyrenean realm, the processes of crust denudation prior to breakup, and the incorporation of rifted domains into collisional orogens.