Evolution of near-surface ramp-flat-ramp normal faults and implication during intramontane basin formation in the Eastern Betic Cordillera (the Huércal-Overa Basin, SE Spain)

The nucleation, propagation, and associated folding of ramp‐flat‐ramp normal faults were analyzed from field examples developed in a brittle/ductile multilayer sequence of the Huércal‐Overa Basin (SE Spain). Gently dipping sandy silt layers, which display a low cohesive strength (C0 = 7 kPa, μ= 34°), favor the development of extensional detachments. A tectonic origin instead of a possible gravitational origin is supported by the perpendicularity between the paleoslope direction of the fluvial‐deltaic environment inferred from imbricated pebbles, and the senses of movement deduced from fault slicken‐lines. The link between high‐angle normal faults (HANFs) —formed at different levels in the layered sequence— with horizontal fault segments comes to develop ramp‐flat‐ramp normal faults with associated roll‐over in the hanging wall. Observed extensional duplexes are formed by parallel detachments connected through synthetic Riedel faults. These Riedel faults would produce the back‐rotation of the individual blocks (horses), i.e., extensional folding of the originally subhorizontal layers. There is no correlation between the analyzed ramp‐flat‐ramp normal faults, accommodating south‐southeastward extension during Serravallian‐lower Tortonian, and either the regional Alpujarride/Nevado‐Filabride west‐directed extensional shear zone or the top‐to‐the‐north detachments within Alpujarride units, which are clearly sealed by Serravallian‐lower Tortonian sediments. Therefore, the studied normal faults are restricted to the brittle/ductile multilayer fluvio/deltaic sequence and accommodate moderate late extension instead of belonging to a large crustal extensional system connected with a regional detachment at depth. Therefore, the basin formed in a moderate crustal thickness context where small and medium‐scale extensional systems were subordinate structures. These natural examples support the development of low‐angle normal faults at very shallow crustal levels in multilayer sequences with suitable rheological conditions.Unidad de Granada, Instituto Geológico y Minero de España, EspañaDepartamento de Geodinámica, Universidad de Granada, EspañaInstituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Científicas, EspañaInstituto Andaluz de Ciencias de la Tierra, Universidad de Granada, EspañaDepartamento de Ingeniería Civil, Universidad de Granada, EspañaGéosciences Montpellier, Centre National de la Recherche Scientifique, FranciaGéosciences Montpellier, Université Montpellier, Franci