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.

.[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.

[1] We refined the three-dimensional (3-D) Vp, Vs and Vp/Vs models around the San Andreas Fault Observatory at Depth (SAFOD) site using a new double-difference (DD) seismic tomography code (tomoDDPS) that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential P, S, and S-P times. This new method is able to provide a more robust Vp/Vs model than that from the original DD tomography code (tomoDD), obtained simply by dividing Vp by Vs. For the new inversion, waveform cross-correlation times for earthquakes from 2001 to 2002 were also used, in addition to arrival times from earthquakes and explosions in the region. The Vp values extracted from the model along the SAFOD trajectory match well with the borehole log data, providing in situ confirmation of our results. Similar to previous tomographic studies, the 3-D structure around Parkfield is dominated by the velocity contrast across the San Andreas Fault (SAF). In both the Vp and Vs models, there is a clear low-velocity zone as deep as 7 km along the SAF trace, compatible with the findings from fault zone guided waves. There is a high Vp/Vs anomaly zone on the southwest side of the SAF trace that is about 1-2 km wide and extends as deep as 4 km, which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the Salinian basement rock to the southwest. The relocated earthquakes align beneath the northeast edge of this high Vp/Vs zone. We carried out a 2-D correlation analysis for an existing resistivity model and the corresponding profiles through our model, yielding a classification that distinguishes several major lithologies.