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INDEPTH geophysical and geological observations imply that a partially molten mid-crustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff indian mantle lid. The results suggest that during Neogene time the underthrusting indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet., The Himalaya and adjacent Tibetan Plateau play a central role in the study of collisional orogens. It is generally assumed that the tectonic processes at work in this active continent-continent [...]

The crust north of the Himalaya is generally electrically conductive below depths of 10 to 20 km. This conductive zone approaches the surface beneath the Kangmar dome dipping north) and extends beneath the Zangbo suture. A profile crossing the northern Yadong-Gulu rift shows that the high conductivity region extends outside the rift, and its top within the rift coincides with a bright spot horizon imaged on the INDEPTH CMP (common midpoint) profiles. The high conductivity of the middle crust is atypical of stable continental regions and suggests that there is a regionally interconnected fluid phase in the crust of the region., The INDEPTH magnetotelluric (MT) investigation undertaken during April to July of 1995 was designed to study the electrical structure of the lithosphere beneath southern Tibet. Two earlier MT studies were [...]

Magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north-to-south width of the Tibetan plateau. The integrated conductivity (conductance) of the Tibetan crust ranges from 3000 to greater than 20,000 siemens. In contrast, stable continental regions typically exhibit conductances from 20 to 1000 siemens, averaging 100 siemens. Such pervasively high conductance suggests that partial melt and/or aqueous fluids are widespread within the Tibetan crust. In southern Tibet, the high-conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust. In northern Tibet, the conductive layer is at 30 to 40 kilometers and is due to partial melting. Zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow., The Tibetan plateau is the largest area of thickened and elevated continental crust on Earth and a direct consequence of the ongoing India-Asia collision (1). Knowledge of the structure and [...]