Deep structures of the Alpine-Himalaya collision in the Zagros Mountains and Tibetan Plateau: a combined geophysical and petrological study

The central Alpine-Himalaya collision is characterised by two mountains belts, Zagros and Himalaya, and its plateaus. Although being the results of the same process, some differences have been noticed since their total amount of convergence is different and Zagros seems to be in a more recent stage of development. A thinning of the lithosphere has been proposed for both Zagros and Tibet to fit potential field data and to explain the high elevation, low seismic wave velocities in the upper mantle, low Pn velocities, higher wave attenuation and higher crustal temperatures than in the surrounding shields. The study of these belts' present-day structures provides new insights into different evolution steps of a collisional process. Combining geophysical and petrological data, we study the lithospheric structure down to 400km depth along three transects, two crossing the Zagros, and the third crossing the Himalayan front up to the Tibetan Plateau. In the Zagros the results show a lithospheric mantle thinning affecting the whole area beneath the range, more extended in the southern sector. It lengthens to the Alborz and the Central Plateau. A Phanerozoic mantle composition denotes the mantle below the range. In the Himalayan belt a more evolved composition should be expected since India-Eurasia collision occurred prior to the Arabia-Eurasia one and since the average convergence rates between the former are higher that the latter. A comparison between the Zagros and Himalaya profiles will be performed in order to discuss the LAB geometry and the compositional differences in the two collisional environments.