INDEPTH geophysical and geological observations imply that a partially molten midcrustal 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.

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The Cenozoic collision between the Indian and Asian continents formed the Tibetan plateau, beginning about 70 million years ago. Since this time, at least 1,400 km of convergence has been accommodated by a combination of underthrusting of Indian and Asian lithosphere, crustal shortening, horizontal extrusion and lithospheric delamination. Rocks exposed in the Himalaya show evidence of crustal melting and are thought to have been exhumed by rapid erosion and climatically forced crustal flow. Magnetotelluric data can be used to image subsurface electrical resistivity, a parameter sensitive to the presence of interconnected fluids in the host rock matrix, even at low volume fractions. Here we present magnetotelluric data from the Tibetan-Himalayan orogen from 77 degrees E to 92 degrees E, which show that low resistivity, interpreted as a partially molten layer, is present along at least 1,000 km of the southern margin of the Tibetan plateau. The inferred low viscosity of this layer is consistent with the development of climatically forced crustal flow in Southern Tibet.

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.

S U M M A R YThe INDEPTH project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the Tibetan Plateau. In the Lhasa Block, seismic reflection surveys in 1994 detected a number of bright-spots at 15-20 km depths that indicate zones of crustal fluids (aqueous fluids or partial melt). Coincident magnetotelluric (MT) data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots. Using constrained inversion, the MT data require a minimum crustal conductance of 6000 S. This abnormally high electrical conductance can be best explained by a layered model with fluids: partial melt, aqueous fluids or a combination of partial melt and aqueous fluids. The non-uniqueness of the MT method means that a wide range of melt fraction-thickness combinations for the above models could all explain the 6000 S conductance. To distinguish between these three models, other geophysical and geological data are required. Reflection seismic data suggest that a high fluid content (>15 per cent) is present at the top of the layer. The amplitude-versus-offset data suggest that the top of this layer may be aqueous fluids rather than partial melt. Passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt. Petrological studies suggest that concentrations of aqueous fluids above 0.1 per cent at mid-crustal depth cannot be sustained. Taken together, these data show that the high conductivity in Southern Tibet is most probably the result of a relatively thin layer of aqueous fluids (100-200 m) overlying a thicker zone of partial melt (>10 km).