Thermal and chemical imaging of the upper mantle anomaliesapplication of to Western Mediterranean

Resumen

The closure of the Ligurian-Tethys Ocean, opened during Jurassic and consisting of highly segmented margins in between Africa and Iberia, has produced the Alboran and Algerian basins in the Western Mediterranean through subduction and slab roll-back processes during the Cenozoic. While the Cenozoic evolution of the back-arc basins in the Central and Eastern Mediterranean (i.e., Liguro-Provenca, Tyrrehenian and Aegean) are well understood, the evolution of the Alboran and Algerian basins in the Western Mediterranean is under debate, leading to the proposal of different geodynamic evolution models. Subduction processes must have left its imprint on the crust and upper mantle structure, temperature and chemical composition, which dictate the present-day physical state. Physical state inside the Earth controls the physical properties (i.e., density, seismic velocities, and thermal conductivity) which in turn control the geophysical observables at the surface (i.e., elevation, gravity anomaly, geoid height, and surface heat flow). Subducted Ligurian-Tethys lithosphere situated at present-day in the sublithospheric mantle are imaged in seismic tomography models and which forms one the main constraining basis for the geodynamic evolution of the Western Mediterranean. Quantitative interpretation of such seismic velocity anomalies in terms of temperature and/or chemical composition is necessary to understand the geodynamic processes behind, and is challenging and is at the forefronts of the modern day geophysics. Hence, the objectives of this thesis is twofold: 1) to develop a methodological framework to incorporate the sublithospheric anomalies observed in seismic tomography in the integrated geophysical-petrological modelling of the geophysical surface observables, and 2) its application to the Alboran and Algerian basins and their margins to model the present-day crust and upper mantle thermo-chemical structure yielding temperature, density (i.e., chemical composition) and seismic velocities to put constrains on their geodynamic evolution. In the first part, an already existing tool, LitMod2D_1.0, is improved into a new LitMod2D_2.0 version which allows to model the sublithospheric anomalies and to be available for the scientific community. Various synthetic models are produced to test the sensitivity of the sublithospheric mantle anomalies on the surface observables and to demonstrate the capabilities of the new LitMod2D_2.0. In the second part of the thesis, application to the Western Mediterranean, results show a thick crust (37 km and 30 km) and a relative deep LAB (130 km and 150 km) underneath the HP/LT metamorphic units of the Internal Betics and Greater Kabylies that contrast with the ~16 km thick magmatic crust of the Alboran Basin and the ~10 km thick oceanic crust of the Algerian Basin, respectively. Despite the similar LAB depth (~60 km) in both basins, the chemical composition of the lithospheric mantle beneath the Alboran Basin is slightly more fertile than beneath the Algerian Basin. At sublithospheric levels, results show that both the Alboran slab beneath the Betics and Algerian slab beneath the Kabylies, are about -400 o C colder than the ambient mantle but have different chemical composition. Alboran slab is slightly fertile compared to the typical oceanic lithospheric of the Algerian slab. Results show that the uplift related to the slab break-off is ~700–1000 m in the Betics and is ~600–1200 m in north Algeria. The Ligurian-Tethys slab beneath the SE Iberia shows an apparent dip to the SSE whereas the slab below Algeria dips to the NNW, matching the NW- and SE-tectonic transport direction of the fold and thrust belts of the Betics and Greater Kabylies-Tell-Atlas subduction-related orogens, respectively. The large-scale configuration of present-day SE Iberia and Algerian margins as well as their mantle compositions in the Alboran and Algerian geo-transects is consistent with opposite dipping subduction of two segments of the Jurassic Ligurian-Tethys domain.