Lithosphere Structure and Mantle Characterization of the Alpine-Himalayan Belt: Atlas, Zagros and Tibet

Abstract:

By combining geophysical and petrological information, we investigate the crust and upper mantle of three orogens of the Alpine-Himalayan Belt (Atlas, Zagros and Tibet), characterizing the lithosphere from the thermal, compositional and seismological viewpoint. The modeling is based on an integrated geophysical-petrological methodology combining elevation, gravity, geoid, surface heat flow, seismic and geochemical data.
The results show prominent lithospheric mantle thickening beneath the Moroccan margin followed by thinning beneath the Atlas Mountains. Different convergence accommodation between the crust and lithospheric mantle suggests a decoupled crustal‐mantle mechanical response. In the northern Zagros the lithosphere-asthenosphere boundary rises sharply below the Sanandaj Sirjan Zone in a narrow region, whereas in the central Zagros the thinning is smoother and affects a wider region. The transition from the Arabian to the Eurasian lithospheric domain is located beneath the Zagros range, and it is marked by a change in the mantle velocity anomaly and in the lithospheric mantle composition. In the western Himalaya-Tibetan orogen, the lithosphere thickening is gradual reaching the maximum below the northern edge of the Plateau. The Indian lithospheric mantle underlies the whole Tibetan Plateau up to the boundary with the Tarim Basin. In the eastern sector, the thickening generates sharp steps beneath the Himalaya Range, and it thins abruptly beneath the Qiangtang and the Songpan Ganzi terrains. The Indian underthrusting is restricted to the southern Plateau. Different Eurasian domains have been also identified beneath the Tarim Basin, the Altaids region and NE Plateau by means of different lithospheric mantle compositions. The lithospheric models crossing Zagros and Tibetan Plateau show that the present-day lithosphere mantle structure of the Arabia-Eurasia and India-Eurasia collision zones are laterally-varying along the strike of both orogens, not just in terms of crust and lithospheric mantle thickness, but also in mantle density, temperature and composition. These models show that the three studied ranges (Atlas, Zagros and Himalaya-Tibet) are characterized by the presence of crustal roots, but not necessarily underlain by lithospheric mantle roots.