A review of cretaceous smooth-slopes extensional basins along the Iberia-Eurasia plate boundary: How pre-rift salt controls the modes of continental rifting and mantle exhumation

This article points out for the first time a striking correlation between the paleogeography of Upper Triassic deposits and the mode of crustal stretching around and inside the Northern Iberia plate during the Cretaceous transtensional event. First, we propose a review of the architecture of the basins, which opened during the mid-Cretaceous times along the Iberia-Eurasia plate boundary. Similarly to the emblematic Parentis basin, these basins exhibit a peculiar synclinal-shaped profile and are devoid of prominent block faulting. The top of the basement is characterized by gentle slopes, which dip symmetrically toward the center of the basins. As revealed by recent comparisons with geologically-constrained rifting models established from the North Pyrenean Zone, this architecture results from the thinning of a heterogeneous continental crust under greenschist facies conditions. Basement deformations are thus dominantly ductile and are characterized by large-scale boudinage, hyper-thinning and subsequent lateral extraction. Bulk deformation of the upper and middle crust leads to the formation of anastomosed shear zones and the development of mylonitic fabric. Tectonic lenses consisting of crustal material remain welded on top of the exhuming mantle. The common character shared by all the pre-rift sequences of the studied basins is the presence of a thick low-strength Upper Triassic evaporites and clays layer belonging to the Keuper group (i.e. pre-rift salt and clay unit). In the studied basins, efficient décollement along the pre-rift salt and clay unit triggers mechanical decoupling and gliding of the pre-rift cover that remains in the center of the basin. Thus, during the early rifting phase, the basement undergoes thinning while the pre-rift cover remains preserved in the basin center. In response to hyper-thinning and horizontal extraction of the continental crust, hot mantle is exhumed beneath the pre- and syn-rift cover. Subsequent thermal exchange (i) promotes ductile deformation of the basement and (ii) induces the development of HT-LP metamorphic conditions in the pre-rift sediments and at the base of the syn-rift flysch levels. This thermal event is well recorded in the axial portion of the Pyrenean realm (future North Pyrenean Zone) as well as in the pre-rift sediments of the Cameros basin (northern Spain). The architecture of the smooth-slopes type basins thus contrasts with the structure of Iberia-Newfoundland Atlantic margins which are characterized by (i) top-basement detachment faults accommodating crustal extension through rotation and translation of undeformed basement blocks, and (ii) the individualization of continental extensional allochthons tectonically emplaced over exhumed lower crust or mantle rocks. Finally, using recent paleogeographic reconstructions, we show that the distribution of the pre-rift salt and clay unit remarkably matches the distribution of the Pyrenean and peri-Pyrenean smooth-slopes type basins. This allows for the first time to propose a genetic link between the distribution of evaporite-bearing pre-rift sedimentary formations and the development of smooth-slopes basins.