Tectonic beheading of fluvial valleys in the Maestrat grabens (eastern Spain): Insights into slip rates of Pleistocene extensional faults

Interaction between faulting and landscape evolution in regions of active tectonics allows us to use subtle geomorphological markers for estimating fault slip rates. Geomorphic features of two valleys connected with the bottom of the Alcalà de Xivert graben, at the Maestrat graben system (eastern Spain), suggest that they correspond to the lowest segments of ancient valleys whose original heads were located at the axis of the neighbouring Irta range. They were beheaded owing to displacement of the Torreblanca and Irta faults during a period of active extensional faulting in Middle Pleistocene times. These faults produced a negative inversion of the relief, sinking a narrow graben (the Ametler graben) at the middle of the Irta range whose alluvial infill buried the midsegments of the beheaded valleys. This hypothesis has been tested by applying two geomorphic indices, the Stream-gradient index (SL) and the Valley width/height ratio (Vf), as well as by considering surface and subsoil information about the sedimentary infill of the Ametler graben. From this evolutionary model, after reconstructing hypothetical longitudinal profiles of the ancient rivers, and taking into account a new absolute age obtained for the alluvial infill of the Ametler graben, slip rates at the northern segment of the Torreblanca fault have been approached. The throw rate has been constrained between a minimum of 0.04–0.07mm/year for the last 1.9 to 2.6Ma, and a maximum of 0.26–0.30mm/year for the last 253.3±18.0ky. These values are comparable to those averaged on other active faults in the central-eastern Iberian Chain for the overall Late Pliocene–Pleistocene times. Nevertheless, the Torreblanca fault shows exceptionally high activity within the context of the Maestrat and Catalonian grabens, which can explain its deep imprint in landscape evolution. ► Valleys beheaded by Middle-Pleistocene normal faults are described in Spain. ► Their anomalous morphology is evidenced by morphometric analysis. ► Their mid segments were buried below the alluvial infill of a narrow graben. ► Fault throw rate up to 0.26–0.30mm/y has been inferred.