Lithospheric flexure and the tectonic evolution of the Betic Cordilleras (SE Spain)

Abstract

We present flexural and gravity models along four profiles across the Betic Cordilleras (SE Spain). The flexural response of the Iberian lithosphere to the loading exerted by the thrust sheets of the internal zone of the orogen is modelled for a broken plate with an elastic rheology of the lithosphere. The conspicuously low value for the flexural rigidity ( e. e. t. = 10 km) required to produce a fit of the deflection suggests that a heating event predated the thrusting, and that decoupling takes place along crustal discontinuities. The present topographic load is insufficient to produce the observed deflection and an additional subsurface load is required, which can be explained by overthrusting of a pre-existing rifted margin. The model predicts a thickness of the overthrust units of approximately 10 km. In the western Betics, the gravity signature results from flexure of the Iberian plate combined with crustal thinning and flank uplift in the Alborán basin and the internal zone. In the eastern Betics, the flexural response is completely overprinted by post-thrusting extensional events. Rheological models of the Iberian lithosphere confirm the results of the flexural and gravity models. Lateral variations in plate rheology, primarily caused by differences in thermal structure, can explain the observed variation in tectonic configuration along strike of the Betic orogen and the eastward disappearance of the Guadalquivir foreland basin. The modelling demonstrates that at least two thermo-tectonic events must have affected the Betics during their orogenic evolution. The first event took place after the Mesozoic extension and development of a rifted margin and before the main phase of overthrusting. This phase reflects most likely the Oligocene-Early Miocene extension which is widely recognised in the Valencia trough and surrounding areas. The second phase, which produced the present configuration of the Betics, corresponds to the Tortonian-Recent extension centred in the Valencia trough and Alborán basin.