Structural evolution of the La Espina-Pravia Thrust, El Sablón coastal sector (San Juan de La Arena, Asturias, north-west Spain): the ductile–brittle transition and superposed folding

Abstract A detailed structural study is presented for the coastal sector of El Sablón beach (San Juan de La Arena), located within the northern segment of the West Asturian-Leonese Zone (WALZ) at its contact with the Cantabrian Zone (CZ) in the Iberian Massif. The research focuses on the evolution of the La Espina–Pravia Thrust and the structural interference between superimposed Variscan deformations. The integrated analysis of field observations, stereographic projections, and geological cross-sections enables the recognition of at least two main deformational milestones (D1 and D2), whose progressive interaction controls the final geometry of the observed structures. The integrated analysis reveals the superposition of at least two main structural markers (D1 and D2). The term ‘deformation phases’ is rejected in favour of an interpretation of progressive non-coaxial deformation. Following Ramsay (1967) and Ramsay & Lisle (2000), these are regarded as instantaneous deformation increments within a kinematic continuum involving a progressive dextral rotation of the stress tensor. The D1 deformational milestone is directly related to the emplacement and development of the La Espina–Pravia Thrust. It is expressed via metre- to decametre-scale folds accompanying a major hectometre- to kilometre-scale syncline, alongside associated fracturing zones and reverse faults. In specific locations, a mechanical and geometric anisotropy of typically coarse (spaced) character is developed. The objective identification of this structural milestone is based on the stereographic projection of fold limbs and the calculation of their axes and axial planes as the most robust geometric criterion. Cartographically, a progressive dextral rotation of the D1 fold axes is recognized as they approach the thrust front. This spatial deflection is interpreted as the combined result of a non-coaxial evolution—with a layer-parallel simple shear component—of the strain field, which was locally favoured by an incipient structural paleotopography (continental slope) and by its marginal location within the syn-sedimentary basin. The D2 deformational milestone is superimposed upon the pre-existing regional folding that affects the overriding allochthonous sheet, being prominently represented by the kilometric-scale Vidrias Anticline. Geometric determinations and structural analyses demonstrate that this structure developed post-dates the main displacement of the thrust fault. The structural topography and mechanical counterweight associated with this anticline progressively introduce a marked anisotropy that thoroughly conditions the final architecture and locking of the minor structures. The structural analysis of the El Sablón Syncline, its associated minor folds, and its mechanical relationship with the La Espina–Pravia Thrust characterises the early kinematic evolution and the regional stress field. As a key point of rheological interrelation at the outcrop scale, the thrust plane is observed truncating the shales of the Formigoso Formation. This ductile pelitic horizon effectively absorbs the initial tectonic force, functioning as a plastic buffer that facilitates both the progressive rotation of the hanging-wall block and its subsequent kinematic reactivations. Finally, the geometry of the Vidrias Anticline provides the structural record of the culmination of this progressive dextral trajectory, documenting the mechanical interference between the inherited fabrics and the younger deformational milestone under a continuous, progressive stress differential. Taken together, these results provide a high-resolution view of the structural evolution of the La Espina–Pravia Thrust within this coastal sector and deliver novel, quantitative data to refine the kinematic interpretation of Variscan deformation at a regional scale.