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.

Abstract.Twelve broadband magnetotelluric (MT) soundings were performed across the Himalaya of Central Nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics. The MT impedance tensors were obtained for frequencies between 0.001 and 500 Hz.

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S U M M A R YPrevious studies have shown that up to a few per cent porosity filled with saline fluid in the lower crust can explain many of the regions with: (1) low electrical resistivities, (2) velocities that appear to be too low for the otherwise inferred mafic composition, and (3) strong lower crustal reflectivity. Several predictions of the free porosity model are examined in this article. A compilation of approximately coincident magnetotelluric electrical resistivity and refraction seismic velocity data for the lower continental crustjs presented to test the predicted correlation. In spite of the limited geographically coincident data and the difficulties of ensuring accurate depth coincidence and of anisotropy effects, there is a general trend of decreasing velocity with decreasing resistivity. The data are scattered, but most fall between the reasonable bounds provided by pore geometry models with effective aspect ratio (for velocity) and Archie's Law pore tortuosity exponent (for resistivity) pairs of 0.03 : 2.0 and 0.1 : 1.5 respectively. As in previous compilations, shield areas tend to have both higher resistivities and higher velocities in the lower crust compared to Phanerozoic areas, although there is overlap for both parameters. A general correlation is also found between the top of low resistivity layers and the top of lower crustal reflective zones with the 400-450 "C isotherms. Possible explanations of this correlation with temperature include (1) an association with the brittleductile transition, below which pore geometries are such as to hold fluid in the required configuration, and (2) control provided by metamorphic reactions that restrict free fluid to below this depth. To constrain better the pore geometry, a compilation of the limited data on lower crustal Poisson's ratio shows most values -0.28. This is consistent with a mainly mafic composition with up to several per cent porosity. Reasonable pore geometry distributions predict a small decrease or constant Poisson's ratio with increasing porosity. While each of the three lower crustal geophysical data types have other reasonable explanations, the apparent correlations above provide support for the fluid-filled pores in the lower crust. The problems of the low permeability required to keep fluid in the lower crust, and of pore fluid consumption in retrograde metamorphic reactions during cooling are discussed briefly. Two mechanisms are suggested as means of producing a low-permeability cap in the middle to deep crust: one invokes deformation of textural equilibrium pore geometries by small deviatoric stresses, the other lower crustal shear processes. There remains some difficulty in reconciling free aqueous fluids in the lower crust with the expected retrograde metamorphism that should take up water into hydrated mineral assemblages.