Alpine overprinting of a Variscan orogen: Insights from U-Pb dating of fracture-filling carbonates in the Cantabrian Mountains, NW Iberia

Tectonic overprinting in polyorogenic zones hampers the reconstruction of the fracture and fluid history in basement rocks involved in successive tectonic events. In this regard, geochronological methods such as the U-Pb dating of fracture-filling carbonates enables the identification fractures formed during different orogenic cycles. In this study, we present 23 reliable U-Pb ages of fracture-filling carbonates crosscutting Carboniferous limestones within the Cabuérniga Fault System in the Cantabrian Mountains, incorporated into the Alpine Pyrenean-Cantabrian Orogen. Structural analyses of fractures of eight well-exposed fault zones reveal orientations comparable to E-W, NE-SW and NW-SE Variscan structures. The obtained U-Pb ages ranging from 280.8 ± 2.7 Ma (Early Permian) to 4.71 ± 2.59 Ma (Early Pliocene) are grouped into three main clusters: 1) Early Permian-Late Triassic (280.8 ± 2.7 to 237.0 ± 5.3 Ma), recording post-orogenic extensional collapse of the Variscan Orogen and the coeval onset of the North Iberian Rifting cycle (Kungurian to Carnian); 2) Early-earliest Late Cretaceous (144.5 ± 5.2 to 107.5 ± 5.9 Ma), corresponding to extension during the opening of the Bay of Biscay and coeval with Zn-Pb mineralization in faults and diapirs from the Cantabrian Zone and the overlying Basque-Cantabrian Basin (Berriasian to Albian); and 3) Early Oligocene-Middle Miocene (30.26 ± 0.76 to 13.99 ± 0.78 Ma), constraining the Alpine Orogeny compression during the Iberia-Eurasia collision (Rupelian to Langhian). The youngest Pliocene age of 4.71 ± 2.59 Ma (Zanclean) records later extensional collapse of the Pyrenean-Cantabrian Orogen. The widespread occurrence of Mesozoic F-Ba and Pb-Zn mineralization across the Iberia reveals the critical role of Alpine Cycle extensional tectonics in driving large-scale fluid systems that played a crucial role in the Alpine tectonic overprinting of the Variscan basement. This tectonic event established hydraulic conductivity between the Paleozoic basement and the overlying Mesozoic cover, governing the transport and trapping of mineral resources. • We present the first U-Pb ages of fracture-filling carbonates in NW Iberia; • Most of carbonates precipitated in Alpine fractures inheriting Variscan trends; • The ages record three main tectonic events from Early Permian to Late Miocene; • The age clusters also record major events of the tectonic history of north Iberia; • The results accounts for the Alpine overprint of the western Variscan Orogen