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

A detailed study of the coastal sector of El Sablón beach (San Juan de La Arena) is presented, located within the northern section of the West Asturian-Leonese Zone (WALZ) at its contact with the Cantabrian Zone (ZC). The evolution of the La Espina-Pravia Thrust is described, where the superposition of at least two main deformational milestones (D1 and D2) is recognized, whose interaction controls the final geometry of the structures. The use of the term 'deformation phases' is discarded in favour of a progressive non-coaxial deformation interpretation, given that the geometric relationships suggest a kinematic continuum with a progressive dextral rotation of the stress field of up to 078°. Following Ramsay (1967) and Ramsay & Lisle (2000), these are considered instantaneous increments of deformation. The D1 deformational milestone culminates with the development of the La Espina-Pravia Thrust. It is expressed by means of metric-to-decametric folds accompanying a major (hectometric-to-kilometric) syncline, fracturing zones, and associated reverse faults. In specific locations, it generates a visible geometric and mechanical anisotropy of a normally coarse (spaced) character. The D2 deformational milestone manifests over the previous structures, being prominently represented by the Vidrias Anticline, whose structural relief introduces, from a certain moment and in a progressive manner, a geometric and mechanical anisotropy that abounds in the final configuration of the structures. A progressive dextral rotation of the fold axes of the allochthon is recognized as they approach the thrust front, interpreted as the combined result of a non-coaxial evolution (with a general shear component) of the strain field, favoured by an 'active' structural relief and by its marginal location within the ancient sedimentary basin. Its identification is based on the projection of the fold limbs and on the determination of their axes and axial planes as the most robust criterion. Furthermore, the structural analysis of the El Sablón Syncline, its associated minor folds, and their relationship with the La Espina-Pravia Thrust, allow for the characterization of the initial kinematic evolution and the stress field of the region. As a point of mechanical interaction in the outcrop, the thrusting of the Cándana Group materials is observed, superposing onto the Formigoso Shales, which 'feel' this initial push and facilitate the rotation of the thrust sheet and the subsequent reactivations. Finally, it is the geometry of the Vidrias Anticline that provides the most visible record of the culmination of said dextral trajectory, evidencing the mechanical interference between the inherited structures and the new deformational milestone under a progressive stress differential. Overall, the results contribute a detailed view of the structural evolution of the La Espina-Pravia Thrust in this sector and provide new data for the regional-scale kinematic interpretation of the assemblage.