The Himalayas and the Tibetan Plateau are considered as the classical case of continental collision. In the meantime, some fundamental questions concerning the structure, rheology and physical processes influencing the evolution of the region's lithosphere are still pending.
The Hi-CLIMB seismology experiment deployed a large number (255) of broadband stations during three years on an 800 km profile along 85°E, across the Himalayas and the southern half of the Tibetan Plateau. The close station spacing (~4-9 km), the large amount of data (1.5 terabyte), the high-frequency receiver functions and the use of multiply converted waves result in a detailed image of lithospheric structures at all scales. These images allow to follow: (1) faults at shallow (~3-4 km) depth; (2) the Main Himalayan Thrust from its shallow part to its deep and ductile continuation; (3) shallow and localized low-velocity layers (previously referred to as "bright spots") in Tibet in correlation with grabens; and (4) underplating of the Indian lower crust beneath Lhasa block. Furthermore, our results show (5) that the Indian lower lithosphere advances northward to about the centre of the Tibetan Plateau, where it is opposed to the Eurasian lithospheric mantle; (6) that the main sutures at the surface have no pronounced signature at depth; and (7) that the upper mantle discontinuities at 410 and 670 km do not seem to be affected by the ongoing orogeny. The obtained information on geometries are then used in two applications.
Based on the improved knowledge on flexural geometry beneath the foreland basin, the rheology of the India plate is re-assessed. Thermomechanical modelling results reveal that the effective elastic thickness decreases from south to north due to decoupling, caused by flexural and thermal weakening. To explain the support of the Tibetan Plateau's topography as well as regional isostasy in the Himalayas, a strong upper mantle is required.
Combining the geometry of underplating with Bouguer anomaly data, localized densification of the Indian lower crust is shown to occur where it reaches its maximal depth. This effect is associated to eclogitization. Investigations of the thermal field and pressure--temperature--density relations assuming different hydration levels are performed using thermo-kinematic and petrological models, respectively. The results suggest that the Indian lower crust is partially hydrated, and that eclogitization is kinetically hindered compared to phase equilibria. Overstepping is explained by the absence of free water in the system, and subsists until dehydration reactions occur at higher P-T conditions.
In conclusion, constraints on geometry and internal properties, as well as evaluation of the importance of physical processes are necessary in order to better understand the build-up of the observed lithospheric structures and the evolution of their deformation.