Active faulting in the Betic External Zones (SE, Spain): The Alcoy Fault Zone

Evidence of active faulting is presented for a structure located in the eastern Betic External Zones (SE Spain). New fault data are provided for the Alcoy Fault Zone, whose Quaternary activity remains under debate. This study includes a detailed structural and geomorphic characterization, together with a preliminary palaeoseismological analysis. The fault is approximately 13 km long and is divided into two main sectors based on surface geometry: a SW sector striking N060E, formed by a border fault strand, and a NE sector striking N040E to N065E, composed of both border and intrabasinal fault strands. The Alcoy Fault Zone exhibits oblique kinematics, with a dominant left‑lateral component and a subordinate dip‑slip component. Quaternary activity is supported by qualitative and quantitative geomorphic analyses using three geomorphic indices (Smf, Vf, and Bs). The fault is characterized by a well‑defined mountain front with low sinuosity. Cross‑fault streams display left‑lateral deflections, knickpoints coincident with fault traces, and deeply incised valleys, while watershed morphologies are consistent with ongoing tectonic uplift. In addition, palaeoseismological analyses of deformed colluvial deposits suggest a Holocene surface‑rupture history involving two events over the last ~1160 years. These results represent a first step toward the seismogenic characterization of the Alcoy Fault Zone and provide a foundation for future seismic hazard assessments in the External Zones of the eastern Betic Cordillera. Further seismicity studies are required to determine whether the fault is basement‑involved or represents a decoupled structure controlled by the Triassic evaporitic layer separating the Variscan basement from the pre‑Neogene Betic units.