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.

No abstract

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).

Magnetotelluric and seismic reflection surveys at Parkfield, California, show that the San Andreas fault zone is characterized by a vertical zone of low electrical resistivity. This zone is ≈500 m wide and extends to a depth of ≈4000 m. The low electrical resistivity is attributed to high porosity of saline fluids present in the highly fractured fault zone. The occurrence of microearthquakes and creep in the low resistivity zone is consistent with suggestions that seismicity at Parkfield is fluid driven. Figure 1. Map of San Andreas fault in vicinity of Parkfield showing location of magnetotelluric (MT) and seismic surveys.

.[1] Magnetotelluric (MT) data were collected in northern Tibet along the Amdo to Golmud highway during the 1995 and 1999 Project INDEPTH (International Deep Profiling of Tibet and the Himalaya) surveys. Broadband and long period MT data were collected and the TE-mode, TM-mode and vertical magnetic field data were inverted to yield a minimum structure, two-dimensional resistivity model. The model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the Kunlun Shan to the Bangong-Nuijiang suture. The vertically integrated conductivity (conductance) of this crustal layer is greatest in the northern Qiangtang terrane at latitude 34°N. The electrical resistivity of the upper mantle is constrained by the MT data to be in the range of 10-30 m across the Songpan-Ganze and Qiangtang terranes. This is lower than would be expected if Asian lithosphere underthrusts northern Tibet as far as the Qiangtang terrane. The MT responses are more consistent with a model in which Asian lithosphere extends as far south as the Kunlun Shan, and the upper mantle beneath the Songpan-Ganze and Qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt.

Magnetotelluric (MT) data were used to create a three-dimensional electrical resistivity model of the Altiplano-Puna magma body (APMB) in the area surrounding Volcán Uturuncu in southern Bolivia. This volcano is at the center of a zone of surface deformation with a diameter of 150 km and persistent inflation of ~10 mm/yr. Low electrical resistivities (<3 Ωm) at a depth of 14 + 1/-3 km below sea level (16-20 km below surface) are interpreted as being due to the presence of andesite melts in the APMB, and require a mini mum melt fraction of 15%. The upper crustal resistivity structure is characterized by finite-length, dike-shaped conductors, oriented approximately east-west near sea level. A combination of dacite partial melts and aqueous fluids is required to explain the observed low-resistivity values. Geodetic data do not require any deformation in these shallow regions. The geometry of the upper surface of the APMB beneath Volcán Uturuncu is consistent with that predicted by geodynamic models that suggest that the APMB bulges upward directly beneath Volcán Uturuncu, near the measured inflation center (~3 km west of Volcán Uturuncu). Viscosity estimates from the MT-derived resistivity model gives a maximum value of 10 16 Pa•s and is consistent with models that propose diapir-like ascent of magma above the APMB. Resistivity models are compared and quantitatively correlated to seismic velocity models, showing good agreement on the spatial extent and depth of the APMB. A forward modeling study shows that the small differences in the depth to the top of the APMB between the different geophysical methods could be explained by variations in the composition of the magma body.