A lithosphere geotransect from the Iberia Variscan domain to the Alpine North Africa ranges crossing the Gibraltar Arc System.

Abstract

The present-day density and compositional 2D structure of the lithosphere is studied along a ~950-km-long transect crossing the Gibraltar Arc System and Atlas Mountains. The transect runs from the tectonically stable Variscan Iberian Massif (North) to the deformed alpine tectonic domains of the Gibraltar Arc System and the Atlas intracontinental orogenic belt (South). Both domains are located in the westernmost continental segment of the African-Eurasian plates, a segment which is characterized by a diffuse transpressive plate boundary. An integrated and self-consistent geophysical-petrological methodology is used to model the along transect variations of the crust structure and the thermophysical properties of the lithosphere and sublithospheric mantle. The crust is mainly constrained by active and passive-source seismic experiments and available geological data, whereas the composition of the lithospheric mantle is constrained by xenolith data and tomography models. Results highlight large lateral variations in the topography of the lithosphere-asthenosphere boundary (LAB). We distinguish different chemical lithospheric mantle domains that reproduce the main trends of the geophysical observables and the recorded P- and S-wave seismic velocities. To reproduce the low velocity regions imaged by tomography below the Iberian Massif and the Atlas Mountains and at the same time constrain the surface geophysical data, we had to define two sublithospheric mantle anomalies (+80ºC) beneath those regions. A cold body (-220ºC) of lithospheric material composition beneath the External Betics-Rif is needed to adjust the measured geoid height and bouguer anomaly. This body is interpreted as the Iberian slab observed on a W-E tomographic profile, localized just to the east of the profile and having some effect on the geoid and Bouguer anomalies. Elastic thickness of 10 km is enough to adjust the topography in most of the profile, but higher values are necessary between the Atlas and the Rif Mountains.