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.

S U M M A R YBroad-band magnetotelluric data were collected at 50 stations over a 400 km long, approximately east-west profile over the granite-greenstone terrain of Dharwar, southern India. The tensor decomposed data were interpreted using a 2-D inversion scheme. The geoelectric model is suggestive of a suture along the Chitradurga-Gadag schist belt, formed by the thrusting of the West Dharwar Craton beneath its eastern counterpart, with an easterly dip of 20-30 • . The thrust proposed here pre-dates the formation of these schists, which occurred during the Late Archean (2600 Ma). The accretionary wedge of the thrust and the depressed part of the West Dharwar Craton may have controlled the emplacement of the Chitradurga-Gadag and Shimoga-Dharwar schists. The subsequent weathering, the several episodes of tectonic activity witnessed during the Precambrian and the vertical block movements caused during the passage of the Indian Plate over the Reunion hotspot may have modified the crust, leading to the present-day geological configuration. Despite its age and several tectonothermal episodes, the signature of this thrust is adequately preserved in the Dharwar Craton. Several similarities with younger sutures, as is evident from the observed relics of the oceanic rocks present along the Chitradurga schist belt, suggest that the tectonic processes leading to this Archean event may have had a close resemblance to those witnessed in recent times. Magnetotelluric studies also image a zone of low resistivity at a depth of 40 km beneath the west Dharwar Craton. This seems to be a regional feature, extending to the north over a distance of at least 250 km beneath the Deccan volcanics. The low heat flow values and the high density associated with this feature make partial melting an unlikely explanation for the low resistivity. The grain boundary graphites and the sulphides deposited in the form of pyrites may have caused the low resistivity in the lithospheric mantle of the West Dharwar Craton, although the fluids generated and trapped in the mantle during the passage of the Indian Plate over the Reunion hotspot in the waning phase of its outburst could also be a possibility. The asthenosphere is delineated at a depth of about 100 km beneath the East Dharwar Craton.

SUMMAR YMagnetotelluric studies over the Damoh±Jabalpur±Mandla±Anjaneya pro®le in central India have delineated Vindhyan sediments which are about 5 km thick in the Damoh± Katangi region. The crust below the Vindhyan sediments shows the characteristics of lower crust, as observed from the relatively lower resistivity of about 200 V m and high seismic velocities (P-wave velocities of 6.5 km s x1 compared with 5.8±6.2 km s x1 in the surrounding region). It is conjectured that the upper crust may have been completely eroded in the uplift and erosion process and thus the Vindhyan sedimentation has occurred directly over the lower crust. An anomalous conductivity is observed at depths of 10±12 km in the Vindhyan crust. The conductance of more than 1200 S observed here may be due to either the serpentinization of the ma®c and ultrama®c rocks or the presence of grain boundary graphites. The thickness of the Deccan traps is about 100 m near Jabalpur and decreases near Mandla. On the south of Mandla, the Archaean crust is exposed. Two crustal conductors are delineated below the Deccan volcanics with a resistivity of about 30 V m. The ®rst on the immediate south of Jabalpur seems to mark the southern boundary of the Jabalpur horst block. The second conductor was delineated about 40 km southeast of Jabalpur, coincident with a positive gravity anomaly of about 30 mGal. Deep seismic sounding studies do not show any signi®cant density contrast associated with this conductive feature. It is proposed that the gravity high may be due to the upwarp of the Moho. The high electrical conductivity is attributed to the¯uids in the upper crust.

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[1] Magnetotelluric studies in the NW Himalayan region have shown anomalously high conductance of about 20,000 siemens in the crust beneath the Indus Tsangpo suture (ITS) and the adjoining Tso-Morari dome in the depth range of 1 -20 km. High heat flow and high attenuation of the seismic waves in the Himalayan crust, together with the observed high conductance indicate presence of wide spread partial melt generated from the subducted Indian crust. The Ladakh batholith appears as a resistive block to the north of ITS. A moderately conductive zone demarcates the Ladakh batholith from Karakoram batholith to the north. The similarity in the resistive structure with the results reported from the Tibetan region by Wei et al. [2001] about 1500 km to the east is rather significant, indicative of a two dimensional nature of the Himalayan collision belt, at least to the first order.