Density distribution of the India plate beneath the Tibetan plateau: Geophysical and petrological constraints on the kinetics of lower-crustal eclogitization

We combine seismological and Bouguer anomaly data with thermo-kinematic and petrological modelling to constrain the extent and kinetics of the eclogitization process in the Indian lower crust underthrusting Tibet. Based on Airy-type isostasy gravity modelling, we show that the presence of denser material (eclogites) is required beneath the Tibetan Plateau. Using the geometries of main crustal interfaces constrained by seismological experiments along three profiles perpendicular to the Himalayan arc, multilayer density-models suggest that eclogitization of the Indian lower crust is completed where the maximal depth of its descent is reached. In an integrated geophysical and petrological approach, the temperature field of the studied area is determined and realistic pressure-temperature-density grids are calculated assuming different hydration levels for the Indian lower crust. The derived density profiles are used to forward model Bouguer anomalies and to compare them to the observations. It appears that eclogitization of the Indian lower crust is delayed compared to where it is expected to occur from phase equilibria. The results show that neither dry nor fully hydrated (free water in excess) lower-crust models are satisfactory. A hydration level of ca. 1 wt.% H2O, consistent with a lower crust having experienced amphibolitic conditions, is more realistic and yields better results. On this basis, the densification delay of the Indian lower crust can be accounted for by a kinetical hindrance (overstepping) of the consumption of the plagioclase component (garnet and clinopyroxene forming reactions), which does not release water. Densification proceeds relatively rapidly (within 6 My) at higher pressure and temperature (at least 100 degrees C above equilibrium), when dehydration reactions start releasing water. These results emphasize the key role of free water in metamorphic reaction kinetics and, consequently, on geodynamical processes.