The Role of Rift Structure on the Contractional Deformation of Magma-Poor Rifted Margins: Insights from the Reactivated Iberian Atlantic Margins

Intraplate lithospheric contractional deformation followed by subduction and continent collision affect oceanic basins and rifted margins during a complete cycle of positive tectonic inversion. Magma-poor rifted margins, in particular, display significant compositional and structural contrasts from continent to ocean that strongly influence the distribution of tectonic structures, especially during the early stages of contractional deformation that precede and/or accompany subduction initiation. The Mesozoic magma-poor Iberian Atlantic margins uniquely recorded contractional lithospheric deformation and aborted, incipient subduction in the Bay of Biscay during the Alpine Orogeny, enabling the investigation of the influence of inherited rift structures in governing the type and spatial distribution of contractional structures. Using seismic images, we map contractional tectonic structures, basement domains, extensional faults and rift basins along the North and West Iberian margins to analyse the rift parameters that conditioned deformation distribution and localisation, with particular emphasis on thrust emplacement.

Along the North Iberian margin, we identified three overlapped multi-stage Mesozoic rift systems that accommodated distinctive types of contractional structures, amplifying the inherited margin segmentation. Halokynetic-related structures developed within a diffuse rift system, whereas mild inversion of pre-existing extensional faults and the formation of reverse faults deforming the sedimentary cover occurred in a transtensional rift system. In contrast, thrusting developed distinctly within a hyperextended rift, consisting of two segments. Continentward-dipping thrusts that sole out in the pre-rift sediments and in the basement, along with inverted oceanward-dipping extensional faults, affected thinned continental crust. Continentward-dipping thrusts deformed and decoupled in the deeper transitional basement, consisting of highly thinned crust and exhumed serpentinized mantle, resulting in basement duplication and thickening. We mapped large thrusts that caused basement and sediment uplift, erosion, and landward backtilting, as well as large thrusts that produced erosion in the frontal part of the thrust sheet. The former are confined to regions of sharp top-basement deepening, suggesting significant crustal thickness variations, while the second are localised in zones of basement-structure changes that may correspond to transitions between different basement types. Distributed thrusts, however, internally deformed the upper part of ultra-thinned transitional basement and soled out at highly reflective levels that may correspond to relicts of ductile lower crust or serpentinized levels within the exhumed mantle. The West Iberian margin exhibits comparable patterns of reverse-fault formation and pre-existing faults reactivation, with extensional faults reactivated within thinned continental crust, and large, newly formed landward- and oceanward-dipping thrusts deforming the transitional basement. Our results show that rift segmentation conditions the type of reactivation along magma-poor rifted margins. Segment boundaries, together with internal variations in crustal thickness and the structure of weak transitional basement along hyperextended rift systems, localise large thrusts that may accommodate significant and potentially underestimated intraplate contractional deformation.