Évolution géodynamique du bassin Nord-Est Algérien : un modèle de vitesse 3D révèle un flux à haute température

Abstract

The geodynamic evolution of the Mediterranean Basin is intimately linked to the relative movement of the African and Eurasian plates, as the scientific community unanimously agrees. Although it currently appears as a single entity, the Mediterranean Basin is actually made up of sub-basins of distinct ages. In its western part, the early closure of the Tethyan Ocean gave rise to several younger oceanic domains open since the Oligo-Miocene, including the Algerian Basin. Studies of the geodynamic evolution of this domain, particularly tomographic studies, have revealed the presence of low velocity anomalies showing the presence of anomalously cold material in the asthenosphere, interpreted as the trace of subduction slabs to the east and west of the basin. The existence of such slabs is currently a question mark. It should be noted that these studies were carried out without the contribution of data recorded by stations located in Algeria. It was only after CRAAG deployed a network of high-performance digital seismological stations from 2007 onwards, that the evolution of seismological studies in the Algerian basin and the western Mediterranean basin underwent a major turning point. This has made it possible to extend the scope of seismological investigations in this region through the application of new methods. In the present thesis, we propose to study the existence of possible slabs beneath the northeastern part of the Algerian basin, including data from the national seismological network, in order to contribute further to the characterization at depth of this region and to the understanding of its geodynamic evolution. To this end, we have adopted a shear-wave velocity model approach, using a 1D and then a 3D model to characterize structures at depths ranging from 50 to 400 km beneath the northeastern Algerian basin. These models are obtained from the inversion of Rayleigh wave dispersion curves with two stations, as this is the most suitable for this type of investigation. These curves are extracted from distant seismic events recorded by broadband seismological stations. 3D structure mapping enables us to identify anomalies in the study area by reading the results. The results highlight two main parallel velocity bodies in the same area and in the same direction (NE-SW). We found that between 200 and 300 km depth, a high-speed anomaly exists beneath the northeastern Algerian basin. Below 350 km, the high-speed body is no longer visible. The geometry of the high-velocity body suggests the presence of slab material beneath the Algerian northeast basin. Lower velocity zones exist between 50 and 100 km depth due to the presence of a thermal and/or velocity anomaly caused by a high-temperature flow induced in the mantle by the presence of the lithospheric slab between 200 and 300 km. The results obtained are of great interest in the search for its signature, given that the study is considered local and precise, for the first time, compared with previous studies, which were all regional in scope, and record a deficit in the precision of deep structure results.