Unraveling Multiple Thermotectonic Events Accommodated by Crustal\u2010Scale Faults in Northern Iberia, Spain: Insights From K\u2010Ar Dating of Clay Gouges

Large\u2010scale faults in the continental crust are significant features that control the evolution of sedimentary basins and intraplate mountain chains. Deciphering their evolution is a significant task because faults slip and reactivate in a variety of geological settings. In this work, clay gouges of two major orogen\u2010scale, long\u2010lived faults in northern Iberia, the R\uedo Gr\uedo and Vall\ue8s\u2010Pened\ue8s Faults, were investigated by X\u2010ray diffraction and K\u2010Ar isotopic analysis. Illite polytype determinations of 44 subfractions (from <0.1 to 10 \u3bcm) allowed us to discriminate between authigenic/synkinematic illite crystals formed during faulting and detrital illite crystals inherited from the host rock. K\u2010Ar dating provided a detailed set of ages corresponding to key stages of the thermotectonic evolution of the Iberian Plate: (a) the Permian to Late Triassic extensional/transtensional activity associated to the emplacement of Late Variscan magmatic bodies and hydrothermal mineralizations, (b) the opening of the Central Atlantic Rift during Late Triassic\u2010Early Jurassic times, (c) the Late Jurassic\u2010Early Cretaceous rifting that led to the development of Mesozoic extensional/transtensional basins in northern Iberia, (d) the final stage of the anticlockwise rotation of the Iberian Plate with respect to Eurasia and the accommodation of the first Pyrenean compressional pulses in Campanian time, and (e) the positive inversion of Mesozoic extensional basins due to far\u2010field stresses associated with the Alpine orogeny during the Paleogene. The results highlight that thermotectonic conditions characterized by high\u2010geothermal gradients strongly favor fault movement and neoformation of clay minerals in fault gouges, regardless of the prevailing tectonic regime