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).

.[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.

A B S T R A C TWe describe an algorithm for inverting magnetotelluric (MT) data in the presence of strong bathymetry or topography. Instead of correcting distortions due to bathymetry or topography we incorporate them directly into the inversion. To achieve a high accuracy in computing MT responses we use finite-difference approximations that permit sloping discrete boundaries inside model elements. The same approach is applicable to any seafloor electromagnetic calculation and can also be used to incorporate steep topography on land. We test our approach on various topographic features and compare our results to that of a finite element approach. Finally, we present inversion examples that illustrate the effectiveness of our inversion algorithm in recovering true subsurface structures in the presence of strong bathymetry and topography. I N T R O D U C T I O NWith the recent surge in marine controlled source electromagnetic surveys worldwide, more MT data are being collected on the seafloor. Complimentary to controlled source electromagnetic data, MT data can provide valuable constraints on background electrical structures. Due to the highly conductive seawater however, the bathymetric effect on MT data can be very strong and must be accounted for in MT data inversion.Two basic approaches have been used. The first treats the bathymetric and topographic effects as a distortion. Once this distortion is removed it is assumed that the data can be inverted as if the sea floor were flat.The basic idea was introduced by Jiracek, Reddig and Kojima (1986). They assumed that a topographically distorted response can be represented by the product of a distortion tensor and the undistorted surface response. The undistorted response could be recovered by calculating the distortion coefficients and applying the inverted distortion tensor to the measured MT data. Most recently, Baba and Chave (2005) removed 3D bathymetric distortion from measured data collected on the East Pacific Rise. This approach is based on * the work of Baba and Seama (2002) and it requires a sufficiently good approximation of the sub-sea floor resistivity to be known in advance. While this approach may not be adequate for a fine-scale MT problem it is very attractive for a large scale MT study in which inverting for bathymetry resistivity becomes impractical. In a fine-scale MT study the subsea resistivity is the parameter to be determined. However, it is reasonable to assume an average sea floor resistivity if the large-scale mantle structures are the targets.The second approach is to incorporate bathymetry and topography explicitly in the inversion. Occam's inversion (DeGroot-Hedlin and Constable 1990) has been used to invert sea floor data of interest in oil exploration (Key, Constable and Weiss 2006). It uses the finite element forward modelling code of Wannamaker, Stodt and Rijo (1986) that permits topography or bathymetry to be modelled by sloping finite element edges. The non-linear conjugate gradient algorithm of Rodi and Mackie (2001), modified ...

To study the conductivity structure of crust and upper mantle as well as the thermal regime of lithosphere beneath the northern Tibetan Plateau, the project INDEPTH (III)‐MT completed super‐broadband magnetotelluric (MT) sounding along Dêqên‐Longwei Cuo (line 500) and Nagqu‐Golmud (line 600) in northern and central Tibet in 1998 and 1999. The result shows that the conductivity structure of crust and upper mantle is quite different across the Kunlun Mountains. North to the mountains the crust and upper mantle is relatively highly resistive. South to the mountains, the conductivity structure of crust and upper mantle is of obvious stratification as described following. The upper crust is dominated by discontinuous high‐resistivity bodies with intercalated low‐resistivity anomalies. And the electric structure of the upper crust in NS direction looks like complicated with discontinuous and block‐shaped features. Meanwhile the intermediate and lower crust are characterized by large‐scale high conductivity anomalies, indicated by separated high‐conductivity bodies of big sizes and differing shapes with resistivity less than 4m. Beneath the Bangong‐Nujiang and Jinshajiang River sutures, the high‐conductivity bodies in the crust tend to extend toward the upper mantle, implying existence of a low‐resistivity conduit between crust and mantle. Based on the observed electric structure of crust and mantle beneath the northern and central Tibetan Plateau, we infer that there are extensive partial melt and thermal fluids there like southern Tibet. Their origin is associated with thermal exchange between crust and mantle beneath the Bangong‐Nujiang and Jinshajiang River sutures which occurred in their own conduits. The thermal exchange conduit below the Bangong‐Nujiang River suture was formed earlier than that beneath the Jinshajiang River suture. Therefore the thermal activity of crust and mantle beneath the study area began from south and west, then spread toward north and east, leading to the current heat flow distribution in middle and northern Tibet which becomes larger from west to east and from south to north.

The features of the faults in the central and northern Tibetan plateau are discussed, based on two super-wide band magnetotelluric (MT) sounding profiles belonging to the INDEPTH (III)-MT project, which were finished between 1998 and 1999: one is from Deqing to Longweicuo (named line 500), the other is from Naqu to Golmud (line 600). This work assists research on the collision and subduction mode between the India and Asia plates. The MT results show that there is a series of deep faults, F1 to F10, in the central and northern Tibetan plateau. Of these faults, F2 is an earlier main fault which leans to the north, and F1 is a later main overriding fault. The Jiali deep fault zone, which has a very complex space structure, is composed of these two faults. F3, F4 and F5 are super-deep faults. They are high-angle faults and lean a little to the south. The main fault zone of the BangongNujiang suture is composed of these three faults. Because of later activity in the structure, several shallow faults formed in the upper crust within the Bangong-Nujiang suture. The Tanggula fault zone is composed of two main faults, F6 and F7, and a series of sub-faults. The shallow segments of the main faults are in high angles and the deep segments of main faults are in low angles. These two faults generally lean to the south and extend into the lower crust. The Jinshajiang suture is composed of the Jinshajiang fault (F8) and the Kekexili fault (F9), and there is a series of sub-faults in the upper crust between these two faults. The Jinshajiang suture is a very wide suture caused by continent-continent collision. The Middle Kunlun fault (F10), which is the main structure of the Kunlun fault zone, is a high angle, super-deep fault. It is the north boundary of the Songpan-Ganzi-Kekexili block. Based on the conductive structure of the profile, the southern part of the Middle Kunlun fault belongs to the Tibetan plateau, but it is not certain whether the northern part of the Middle Kunlun fault belongs to the Tibetan plateau. There are conductive bodies stretching from the crust into the upper mantle below the Bangong-Nujiang suture and Jinshajiang suture. This may suggest heat exchange between the crust and mantle.

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