Crustal thickness and images of the lithospheric discontinuities in the Gibraltar arc and surrounding areas

Abstract

The Gibraltar arc and surrounding areas are a complex tectonic region and its tectonic evolution since Miocene is still under debate. Knowledge of its lithospheric structure will help to understand the mechanisms that produced extension and westward motion of the Alboran domain, simultaneously withNW-SE compression driven by Africa-Europe plates convergence. We perform a P-wave receiver function analysis in which we analyse new data recorded at 83 permanent and temporary seismic broad-band stations located in the South of the Iberian peninsula. These data are stacked and combined with data from a previous study in northern Morocco to build maps of thickness and average νP/νS ratio for the crust, and cross-sections to image the lithospheric discontinuities beneath the Gibraltar arc, the Betic and Rif Ranges and their Iberian and Moroccan forelands. Crustal thickness values show strong lateral variations in the southern Iberia peninsula, ranging from ~19 to ~46 km. The Variscan foreland is characterized by a relatively flat Moho at ~31 km depth, and an average νP/νS ratio of ~1.72, similar to other Variscan terranes, which may indicate that part of the lower crustal orogenic root was lost. The thickest crust is found at the contact between the Alboran domain and the External Zones of the Betic Range, while crustal thinning is observed southeastern Iberia (down to 19 km) and in the Guadalquivir basin where the thinning at the Iberian paleomargin could be still preserved. In the cross-sections, we see a strong change between the eastern Betics, where the Iberian crust underthrusts and couples to the Alboran crust, and the western Betics, where the underthrusting Iberian crust becomes partially delaminated and enters into the mantle. The structures largely mirror those on the Moroccan side where a similar detachment was observed in northern Morocco. We attribute a relatively shallow strong negativepolarity discontinuity to the lithosphere-asthenosphere boundary. This means relatively thin lithosphere ranging from ~50 km thickness in southeastern Iberia and northeastern Morocco to ~90-100 km beneath the western Betics and the Rif, with abrupt changes of ~30 km under the central Betics and northern Morocco. Our observations support a geodynamic scenario where in western Betics oceanic subduction has developed into ongoing continental subduction/ delamination while in eastern Betics this process is inactive.