Evidence of Holocene tectonic activity affecting alluvial fill evolution in the Vega Media of the Segura River, southeastern Spain (Western Mediterranean region)

Abstract

A compressive stress pattern, associated with active faults, controlling Holocene alluvial deposits was analyzed here by adopting an integrated approach focused on direct and indirect research methods, and taking the Vega Media of the Segura River (VMSR) (SE Spain) as a case study. In particular, geological reconnaissance to determine the morphology of valley edges was carried out, and geophysical methods, boreholes, hydrochemical analysis, piezometry, seismic records, and 14C dating with the AMS technique were also used. This neotectonic activity responds to a compressive regime, maximum in a N-S orientation, which is manifested by dextral strike-slip faults (N125º-140ºE direction), with horizontal displacements of up to 5 km, and normal faults, trending N50º-70ºE with throws in the upper silt unit of up to 8 m in less than 200 m. Within the meander-belt zone of the VMSR, the age of clayey silts, at 14-15 m depth, was estimated to be between 2,754 and 7,561 cal yr BP, representing slip rates for these faults of between 0.26 and 0.53 mm/yr since the middle Holocene. Considering the whole Holocene alluvial fill in this area, the complete set of possible net slip rates comprises values from 0.12 to 0.57 mm/yr. In addition, age differences of up to 4,350 cal yr BP were found at 9 m depth in boreholes spaced less than 100 m apart. Such Holocene tectonic activity has determined the displacement of the Segura River towards the southern valley margin and the formation of sharp meander bends.