The Central Asia region is dominated by one of the largest areas of distributed deformation on Earth, which spans eastern Turkey, northern Middle East, central and south- eastern Asia, covering the central and eastern sectors of the Alpine-Himalayan mountain belt. It is composed by the Zagros orogen in the western sector and the Himalaya-Tibetan orogen in the eastern sector, which are the results of the subduction of the Tethys oceanic lithosphere towards the NNE and the subsequent collisions between Arabia and India plates with the Eurasia plate during the Cenozoic. The strong and resistant Archean-to-Proterozoic shields of Arabia and India plates collided with the complex mosaic structure of the Eurasian ancient margin, which was formed by different Gondwana-derived continental blocks accreted by Late-Mesozoic time. The collisions resulted in tectonic escapes toward lateral regions (in Anatolia and south-eastern Asia), oblique convergence in the Zagros fold-and-thrust belt, the formation of the Makran accretionary wedge, convergence in the Hindukush, shortening in the Himalaya, Karakorum and Tibetan Plateau, and the development of two syntaxis at the edge of the Indian sub-continent. In addition, the Zagros and Himalaya-Tibetan orogens are excellent examples of diffused deformation, with wide deforming areas in the continent interiors, and the development of other mountain belts further north with respect to the Arabia-Eurasia and India-Eurasia suture zones, such as Caucasus, Alborz, Kopet Dagh, Pamir and Tian Shan mountains.

The lithosphere structure plays an important role in controlling the surface deformation and its propagation to the continental interiors. The compositional and strength heterogeneities within the lithosphere directly affect to the tectonic behaviour of the region and, hence, to the evolution of the orogenic systems. This thesis focalizes on the characterization of the present-day lithospheric structure of the Zagros and the Himalayan- Tibetan orogens and the role of the lithospheric structure and rheology in the accommodation of the deformation related to the Arabia and India convergence against Eurasia.

By combining geophysical and petrological information, the crust and upper mantle of the Zagros and the Himalaya-Tibetan orogens have been characterized from the thermal, compositional and seismological point of view. Four 2-D lithospheric profiles (two crossing the Zagros orogen and other two crossing the Himalaya-Tibetan orogen) have been modelled down to 400 km depth, in which the resulting crust and upper mantle structure is constrained by available data on elevation, Bouguer anomaly, geoid height, surface heat flow and seismic data including tomography models. In the Zagros orogen, the results on the crustal thickness show minimum values beneath the Arabia platform and Central Iran (42-43 km), and maximum values beneath the Sanandaj Sirjan Zone (55-63 km), in agreement with seismic data. Major discrepancies in Moho depth from those derived from seismic data are locally found in the Sanandaj Sirjan Zone (central Zagros) and Alborz Mountains where more moderate crustal thicknesses are modelled. Results on the lithosphere thickness indicate that the Arabian lithosphere is ~220 km thick along both profiles, whereas the Eurasian lithosphere is up to ~90 km thinner, especially below the Central Iran and Alborz Mountains. The lithosphere-asthenosphere boundary (LAB) shows different geometries between the two transects. In the northern profile (northern Zagros), the LAB rises sharply below the Sanandaj Sirjan Zone in a narrow region of ~90 km, whereas in the southern profile (central Zagros), rising occurs in wider region, from the Zagros Fold-and-Thrust Belt to the Sanandaj Sirjan Zone. The best fit of seismic velocities (Vp, Vs) and densities requires lateral changes in the lithospheric mantle composition. Our results are compatible with Proterozoic peridotitic mantle compositions beneath the Arabian Platform, the Mesopotamian Foreland Basin and the accreted terrains of Eurasia plate, and with a more depleted Phanerozoic harzburgitic-type mantle composition below the Zagros Fold-And-Thrust Belt and Imbricated Zone.

In the Himalaya-Tibetan orogen, the results show a Moho depth of ~40 km beneath the western Himalayan foreland basin, progressively deepening north-eastwards to ~90 km below the Kunlun Shan. Tarim Basin and Tian Shan show a nearly flat crust-mantle boundary at 50-65 km depth. The lithosphere-asthenosphere boundary lies at 260-290 km depth below the western Himalaya and Tibetan Plateau, Tian Shan and Altai Range, and it shallows to ~230 km depth below the southern Tarim Basin and to ~170 km below the Junggar region. The north-eastern Tibetan Plateau is underlined by a thinner lithosphere (LAB depth at ~120 km) with respect to its southern sector, confirming the results of previous 2D-geophysical integrated models carried out in this region. The modelled lithospheric mantle composition is generally compatible with a lherzolitic mantle-type, slightly changing to a more undepleted composition in the deep lithosphere beneath the Tarim Basin due to metasomatism. However, the mantle beneath Tian Shan, Junggar region and Altai Range is characterized by a FeO- MgO-rich composition, likely related to subduction slab-derived fluids, and the north-eastern Tibetan Plateau is highly depleted in MgO and enriched in FeO, Al2O3 and CaO, as retrieved by xenolith samples. Our results of the geophysical-petrological study finally suggest that the Himalaya-Tibetan orogen is supported by a thick buoyant lithospheric mantle in the western profile and by a lithospheric mantle thinning in the north-eastern sector of the Tibetan Plateau along the eastern profile.

The combination of the present-day lithospheric structure of the Zagros and the Himalaya-Tibetan orogens with plate kinematics, geodetic observations and stress data allowed investigating the neotectonic deformation related to the collision of the Arabia and India plates against Eurasia. A geodynamic modelling technique based on the thin-sheet approximation has been used for this purpose. The crustal and lithospheric mantle thickness has been inferred from previous studies based on the combination of geoid and elevation data and thermal analysis. The surface velocity field, stress directions, tectonic regime and strain distribution are calculated after imposing velocity conditions at the model boundaries and rheological parameters at the crust and lithospheric mantle.

The results allow obtaining a first order approximation of the velocity field and of the stress directions in the whole Central Asia, reproducing the counter-clockwise rotation of Arabia and Iran, the westward escape of Anatolia, and the eastward extrusion of the northern Tibetan Plateau by only imposing the convergence of Arabia and India plates respect to the fix Eurasia. The simulation of the observed extensional tectonics within the Tibetan Plateau requires, instead, a weaker lithosphere, which can be provided by i) a change in the rheological parameters or ii) reducing the lithosphere thickness in the NE-Tibet. Furthermore the temperature increase generated by the lithospheric thinning in the NE-Tibet would permit to reconcile the model with the high heat flow values and the low mantle seismic velocities observed in this area.

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