Session 7, Lithosphere/asthenosphere interaction, oral Title: 7.2 THE BAIKAL RIFT ZONE: MAGNETOTELLURIC ARBITRATION OF GEODYNAMIC MODELS Authors: M.N.Berdichevsky (1), L.L.Vanyan (2), A.V.Kashurnikov (1) (1) Moscow State University, (2) Institute of Oceanology, Moscow, Russia E-mail: berd@geo.geol.msu.su The modern geology offers two geodynamic models of the Baikal rift zone: (1) A model with asthenospheric upwarp based on the data of gravi- metry and seismic tomography ( Yu. Zorin ). The upwarp reaches the M-dis- continuity, and its roots go to great depths. (2) A penetration model based on the seismic data ( N. Puzyrev, S. Krylov, A. Grachev ). In this model the partially melted mantle material rises through the Baikal suture and penetrates horizontally under the M- discontinuity. It would be interesting to find out which of these conceptions is in best agreement with MT data. To this end we considered MT-soundings carried out along a profile intersecting the Baikal rift. The 2D bimodal interpretation of MT-data has been performed using the operating mode of control of hypotheses. It consisted of 3 stages: (1) Construction of a starting model in accordance with the asthenospheric upwarp or penetration conception. (2) Normalization and inversion of the TE curves. The longitud- inal apparent resistivity curves were corrected by reducing their low- frequency branches to references derived from the starting model. Inver- sion of corrected TE curves was performed by the misfit minimization method with the fixed mantle conductivity distribution. At this stage we located a crustal conductive layer. It seems that the crustal conduc- tor is of a fluid nature. Moreover, some part of fluids can be of methe- oric origin (A. Popov). (3) Inversion of the TM curves with fixed crustal conductivity distribution. At this stage a mantle conductivity was optimized. Sensiti- vity of transverse apparent resistivity curves to deep conductors was provided by conductive faults. The minimum misfit attained within the asthenospheric upwarp conception was of order 50-75%. At the same time within the penetration conception we managed to minimize the misfit to 5-8%. So, we can state that the penetration conception is more realistic. Title: 7.3 Source field contamination of magnetotelluric data from northern Canada beneath the auroral zone Authors: James Cassels, Queen's University, Kingston, Canada Alan G. Jones, Geological Survey of Canada, Ottawa, Canada As part of the Lithoprobe Slave-Northern Cordillera Lithospheric Evolution (SNORCLE) transect investigations, magnetotelluric measurements have been made in northern Canada within the auroral zone. One survey was during the summer time, and the second was during the winter time (March-April). In addition, one site was operated at the Yellowknife observatory for over a year, and good data exist for the eight month period January - August 1997. Source-field contamination is a known problem that has been seen in the Yellowknife long period MT recordings. Inappropriate response estimation can lead to resulting models with the depth of the lithosphere/asthenosphere boundary altered by upwards of 50 km. The results of investigating these Yellowknife data for source-field contamination will be presented. The goal is to develop an automatic objective scheme for detecting such contamination, and reducing their effects on the response function evaluation. Session 7: Lithosphere/asthenosphere interations beneath continents and oceans and their margins Title: 7.4 The MELT Experiment: Inversion of MT Responses Authors: A.D. Chave and R.L. Evans (Woods Hole Oceanographic Institution, Woods Hole, MA 02543; alan@whoi.edu) G. Heinson and A. White (Flinders University, Adelaide, Australia) H. Toh, T. Goto, and H. Utada (University of Tokyo, Tokyo, Japan) P. Tarits (Universite de Bretagne Occidentale, Brest, France) N. Seama and K. Baba (Chiba University, Chiba, Japan) M. Unsworth and J.R. Booker (Geophysics Program, Univ. of Washington, Seattle, WA) J.H. Filloux (Scripps Institution of Oceanography, La Jolla, CA 92093) We present preliminary results from inversion of MT response and vertical-to-horizontal magnetic transfer functions for resistivity structure beneath the East Pacific Rise. A variety of inversion techniques have been applied to the responses assuming an underlying 2D electrical structure (invariant along strike) beneath each of the MELT lines. These include minimum structure techniques which seek maximally smooth models compatible with the data, and the Bayesian ABIC algorithm. Forward modeling has been employed to test specific geological models against our data. Through this process of comparative inversion and forward modeling, we are able to discern which features of our models are well constrained by the data. Chief among these is a strong, narrow conductor immediately beneath the ridge crest. We will discuss common attributes of our inversion-derived models and address issues related to the mantle structure beneath the Pacific and Nazca plates and to the presence and transport of melt beneath the ridge. Title: 7.5 The resistivity structure of the active continental margin in northern Chile Author: Friedrich Echternacht Phone: +49 (0) 331 2881254 GeoForschungsZentrum Potsdam Fax: +49 (0) 331 2881235 Telegrafenberg email: fritz@gfz-potsdam.de D-14473 Potsdam Germany A set of broadband ($10^4$ - $10^{-4}$ Hz) magnetotelluric data was collected in the forearc and arc region of Northern Chile. Decomposition of the magnetotelluric transfer functions suggests that 2D modelling of the data is possible. The forearc appears extremely resistive. The existence of a continuous conductor on top of the subducting Nazca Plate, as observed by Kurtz (1990) for the Juan de Fuca System, can probably be dismissed. The dominant structure in the model is a subvertical conductive block that reaches from 10 km to great depths. This structure is situated approximately 20 km west of the active volcanic arc. It may correspond to the Falla Oeste, a deep reaching fault system in the Chilean Precordillera, which probably provides a pathway for the rise of metamorphic fluids from the downgoing slab. In contrast to other areas in the Andes, the magmatic arc itself seems to be a poor conductor - consistent with a lack of recent volcanism in the area, and a decrease of seismic attenuation. session 7, oral. Title: 7.6 ELECTRICAL ANISOTROPY OF OLD OCEANIC LITHOSPHERE: CONSTRAINTS FROM THE PEGASUS EXPERIMENT Authors: ME EVERETT Dept of Geology and Geophysics Texas A&M University College Station TX colt45@beerfrdg.tamu.edu SC CONSTABLE Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography La Jolla CA Seismic anisotropy has been detected in oceanic crust and mantle, so it is geologically reasonable to seek anisotropy in seafloor electrical properties. Anisotropy can lead to surprising geophysical responses unanticipated by simple isotropic theory. We investigated the frequency-domain, controlled-source EM (CSEM) response of a laterally anisotropic, uniaxially conducting seafloor excited by a horizontal electric dipole oriented obliquely to electrical strike. A ``paradox of anisotropy'' is observed, in which the seafloor electric field strength is enhanced in the most conductive direction (normally, increased conductivity is associated with increased attentuation). We have applied our theory to the PEGASUS CSEM data set from the NE Pacific ocean. Relevant 8 and 24 Hz data are sparse but halfspace calculations indicate that to first order the lithosphere can be modeled as vertical conductive sheets oriented parallel to the fossil spreading direction. The electrical conductivity parallel to the sheets (0.0007 S/m) is a factor of 7 times larger than the perpendicular electrical conductivity (0.0001 S/m). Anisotropy is restricted to long range (greater than 30~km) propagation through the lower crust or uppermost mantle. Electromagnetic measurements cannot distinguish between structural and mineralogical anisotropy, but a spreading-parallel alignment of conductive trace minerals or hydrogen conduction in olivine can explain our result. Title: 7.7 The MELT Experiment: Magnetotelluric Data Authors: G. Heinson and A. White (Flinders University, Adelaide, Australia) R.L. Evans and A.D. Chave (WHOI, Woods Hole, MA 02543; alan@whoi.edu) P. Tarits (Universite de Bretagne Occidentale, Brest, France) H. Toh, T. Goto, and H. Utada (University of Tokyo, Tokyo, Japan) N. Seama and K. Baba (Chiba University, Chiba, Japan) J.H. Filloux (Scripps Institution of Oceanography, La Jolla, CA 92093) M. Unsworth and J.R. Booker (Geophysics Program, Univ. of Washington, Seattle, WA) >From June 1996 to June 1997, 47 seafloor instruments measuring the time variations of the electric and magnetic fields were deployed at 32 sites along two east-west lines in the vicinity of 17S on the East Pacific Rise. The MT and Z/B response functions display most of the characteristics of a 2D medium with a strike direction coincident with that of the ridge. For the southern line, the TM mode response is comparatively featureless across strike. The TE mode shows several interesting features: 1) a sharp, narrow (10-20 km wide) decrease in the phase and apparent resistivity at the ridge crest from 100-8000 s which suggests a narrow and shallow (10-20 km peak depth) axial high conductivity zone, 2) an increase in the phase and apparent resistivity at short (<1000 s) and long (>30000 s) periods away from the ridge crest, suggestive of a shallower asthenosphere off axis, and 3) a long period (>10000 s) decrease in the phase and apparent resistivity at the ridge axis extending to the west, suggestive of higher conductivity at asthenospheric depths in this direction. Title: 7.8 Times Rheological Modeling of the Southern Alps, New Zealand Based on Magnetotelluric Determination of a Fluidized Region of Low Viscosity Authors: George R. Jiracek, Allen D. Porter, Victor M. Gonzalez San Diego State University, San Diego, CA, USA Philip E. Wannamaker, John A. Stodt University of Utah, Salt Lake City, UT, USA Roger J. Phillips Washington University, St. Louis, MO, USA T. Grant Caldwell, J. Donald McKnight Institute of Geological and Nuclear Sciences, Wellington, NZ Two-dimensional (2-D) inversion of 54 magnetotelluric (MT) soundings gathered in 1995-1998 has revealed a spectacular, isolated, high conductivity feature (~ 10 ohm-m) under the Southern Alps, New Zealand. The 40 km-wide dipping zone has a maximum depth to its top of ~20 km and shallows to less than 10 km where closest (5 km) to the Alpine fault. It is surrounded by very resistive rock exceeding 1000 ohm-m. The conductive feature may lie conformably within a dipping 20-30 km thick sequence of metamorphic rocks exhumed adjacent to the Alpine fault by ongoing continent-continent collision. A likely cause of the conductive zone is interconnected aqueous fluid within the ductile portion of the crust. There is evidence that the top of such a zone defines an isotherm which would be advectively deflected upward near the Alpine fault. A fluidized region has profound ramifications on the deformation history of the Southern Alps since it is rheologically weak. To estimate this effect, 2-D geodynamic models using the known horizontal compressive strain rate of 13 mm/yr were investigated assuming that a region of low viscosity is coincident with the conductive region. This allows a comparison between observed and calculated deformation of the lithosphere (including uplift and delamination) as a function of variables including anomalous viscosity and temperature estimated from MT observations. Title: 7.9 Electrical Conductivity Structure Beneath the Bay of Bengal Authors: E. John Joseph, Ocean Research Institute, University of Tokyo, Tokyo, 164 JAPAN. Email : john@ori.u-tokyo.ac.jp H. Toh, Ocean Research Institute, University of Tokyo, Tokyo, 164 JAPAN. H.Utada Earthquake Research Institute, University Of Tokyo Tokyo, JAPAN. R.V.Iyengar, Indian Institute of Geomagnetism, Mumbai, India. H.Fujimoto, Ocean Research Institute, University of Tokyo, Tokyo, 164 JAPAN. B.P.Singh, Indian Institute of Geomagnetism, Mumbai, India. J.Segawa, School of Marine Science and Technology, Tokai University, Japan Seafloor magnetometer array experiments were conducted in the Bay of Bengal to delineate the subsurface electrical conductivity structure in the close vicinity of the 85 degree East Ridge, Ninety East Ridge (NER), and also to study the upper mantle conductivity structure beneath the Bay of Bengal. The experiments were conducted in three phases. Array 1991 consisted of five seafloor stations across the 85 degree East Ridge along the 14 degree North latitude with a land reference station at Selam (SLM). Array 1992 also consisted of five seafloor stations across the 85 degree East Ridge along 12 degree North latitude. Here we used the data from Annamalainagar (ANN) Magnetic Observatory as land reference. Array 1995 consisted of four seafloor stations across 9 degree North latitude with a land reference station at Tirunelveli (TIR). OBM-S4 magnetometers were used for seafloor measurements. Geomagnetic Depth Sounding (GDS) method was used to investigate the subsurface lateral conductivity contrasts. Vertical Gradient Sounding (VGS) and Horizontal Spatial Gradient (HSG) methods were used to study the depth-resistivity structure beneath the Bay of Bengal. The seafloor 3-component magnetic field variations observed show that the electromagnetic induction process in the Bay of Bengal may be 3-dimensional. We made an attempt to solve this 3-D problem numerically and followed two approaches, namely (1) thin-sheet modelling and (2) 3-D forward modelling. Thin-sheet and 3-D forward model calculations jointly show that the observed induction arrows could be explained in terms of near surface features such as deep sea fans of the Bay of Bengal, the 85 degree East Ridge, and the sea water column above the seafloor stations. VGS and HSG responses provided depth-resistivity profile as a resistive oceanic crust and upper mantle followed by a very low resistive zone at a depth of about 250-450 km. This depth-range fairly agrees with the seismic low velocity zone beneath the northeastern Indian Ocean, derived from seismic tomography. 3-D forward model responses fairly agree with the observed responses. Thus we propose an electrical conductivity structure for oceanic crust and upper mantle of the Bay of Bengal. Title: 7.10 ELECTRICAL STRUCTURE FROM THE GANGETIC PLAIN TO THE HIMALAYAS Authors: C. Lemonnier (1), G. Marquis (1), F. Perrier (2), M. R. Pandey (3), R. P. Tandukar (3), B. Kafle (3), S. Sapkota (3), M. Chitrakar (3) and the Nepal MT 96 Team (4) (1) Imagerie Tectonique, EOST, CNRS UMR 7516, 5 rue Descartes, 67084 Strasbourg, France (2) Laboratoire de G\'eophysique, CEA Bruy\`eres-Le-Ch{\^a}tel, France (3) Department of Mines and Geology, Lainchur, Kathmandu, Nepal (4) M. Bano, U. Gautam, D. Tiwari, Resham, B. Tissot, P. Henry, P. Le Fort We present here a preliminary analysis of magnetotelluric (MT) data acquired across Central Nepal to study the conductivity structure of the crust and upper mantle of the active Himalayan region. Our data show that Central Nepal's present-day geoelectrical structure is of growing complexity from south to north: under the Gangetic plain, the data are close to one-dimensional behaviour and their inversion reveals a depth to the Indian basement of about 5 km. The data from the surroundings of MBT and MCT show that the latter is associated to a strong conductor. This is especially true in the north. We can link this result to hydrothermal activity or to the presence of graphite within the Nawakot formation. In central Nepal, the high-grade rocks from the Kathmandu klipp are very resistive and extend to depths of about 7 km. We will also present the results of 2D and 3D modelling currently in progress. Title: 7.11 The electrical resistivity structure of the Valu Fa Ridge, Lau Basin, from controlled source electromagnetic sounding. Authors: L.M. MacGregor, M.C.Sinha, Department of Earth Sciences, University of Cambridge. Dr. Lucy MacGregor Bullard Laboratories, Dept. of Earth Sciences University of Cambridge Madingley Road Cambridge CB3 0EZ Tel : +44 1223 337183 Fax : +44 1223 360779 mcgregor@esc.cam.ac.uk S.C Constable, IGPP, Scripps Institution of Oceanography, California. In December 1995 we performed a controlled source EM sounding experiment over a section of the intermediate spreading Valu Fa Ridge (VFR), as part of a combined EM and seismic study of the crustal structure. Over eighty hours of high quality electric field data were recorded by an array of sea-bottom receivers deployedon and around the ridge. On axis, layer 2 resistivities are typically less than 10 Ohmm, suggesting high porosities and pervasive seawater penetration. Surprisingly, data collected 10km off axis, on 300,000 year old crust, show an unambiguous low resistivity zone at 1-2km below the seafloor. There is no evidence for a coincident seismic anomaly, and therefore the low resistivities are unlikely to be caused by the presence of melt, or an increase in porosity. An decrease in pore fluid resistivity could produce the type of anomaly observed, without affecting the seismic velocity. There is evidence in hydrothermal emissions from the VFR for possible phase separation in the deep hydrothermal system, which would generate highly conductive briney fluids. Such fluids could flow away from the axis following a permeability boundary at the top of layer 3, producing the off axis anomaly detected. Title: 7.12p Conductivity structure of the lithosphere and asthenosphere in the Pacific Ocean near 25N, 138W Authors: K. Baba and N. Seama (baba@earth.s.chiba-u.ac.jp, seama@earth.s.chiba-u.ac.jp, Graduate School of Science and Technology, Chiba University, Chiba, Japan) A. D. Chave (achave@whoi.edu, Woods Hole Oceanographic Institution, Woods Hole, U.S.A.) J. H. Filloux (jfilloux@ucsd.edu, Scripps Institution of Oceanography, La Jolla, U.S.A.) We analyzed five MT data sets from the Northeast Pacific near 25N, 138W, and estimated the conductivity structure of the lithosphere and asthenosphere. The sites are located about 100 km north of the Molokai Fracture Zone which has a strike of N80E. The lithospheric age of the area is about 43 Ma. The segment to the south of the fracture zone is about 10 Ma older than that to the north. The regional strike from a tensor decomposition is approximately parallel to the fracture zone. The result of 2D (invariant along the fracture zone) forward modeling and inversion which take account of the fracture zone and bathymetric feature shows that two factors are required to explain the MT responses: 1) the lithosphere to the south of the fracture zone is thicker than that to the north. 2) the asthenosphere under the thinner lithosphere is slightly more conductive by a factor of 1.3 than that under the thicker lithosphere. All of these are probably due to the difference in age. Title: 7.13p Baltic Electromagnetic Array Study (BEAR) Authors: BEAR Working Group Mail P.O. Box 96, FI-02151 Espoo, Finland Tel +358-(0)205 502 530 Fax +358-(0)205 5012 Email toivo.korja@gsf.fi (tkorja@adpser4.gsf.fi) The Baltic Electromagnetic Array Research project, BEAR, is an integral part of EUROPROBE's multidisciplinary SVEKALAPKO project. The SVEKALAPKO project consists of ten sub-projects including e.g. geoelectromagnetic, seismic tomography, geothermal, and xenolith sub-projects and is targeted to investigate geophysical, geological and geochemical properties of the lithosphere-asthenosphere system beneath the Baltic Shield. The BEAR project realizes an ultra deep electromagnetic sounding using a shield-wide array of 50 magnetotelluric and 20 magnetometer sites covering an area of 1000 km by 1400 km. Spatial sampling distance is ca. 150 km. Instruments will be installed in early June after which 1.5 month long time series will be collected simultaneously at each BEAR array site. The BEAR Working Group include research teams from Apatity, Moscow, St. Petersburg and Troitsk in Russia, Lviv and Kiev in Ukraine, Goettingen, Potsdam, Berlin and Frankfurt in Germany, Uppsala in Swedewn, Edinburgh in UK and Oulu and Helsinki in Finland. Title: 7.14p Thin sheet electric imaging of the Carpathian lithosphere Authors: V. Cerv(1), M. Menvielle(2,3), J. Pek(1), O. Praus(1) (1) Geophysical Institute, Acad.Sci. Czech Rep. Bocni II/1401, CZ-14131 Prague 4, CZECH REPUBLIC (2) Centre d'Etudes des Environnements Terrestres et Planetaires, CNRS/UVSQ, 4, Avenue de Neptune, F-94107 SAINT MAUR DES FOSSES CEDEX, FRANCE (3) Universite Paris Sud, F-91405 ORSAY CEDEX, FRANCE Electromagnetic investigations carried out in the area of West Carpathians and their transition to surrounding pre-Alpine units evidenced the existence of significant contrasts of conductivity in the deeper crust. Their electromagnetic signature is superimposed with that of both contrasts of conductivity in the superficial crustal layers and effects of variations in the depth of the conductive asthenosphere. Studying the electric structure of the earth's crust thus requires the elimination of the asthenosphere and surface electromagnetic signatures. We used both 3-D and thin sheet forward modelling to estimate these two signatures. We used as data the induction arrows, as well as available magnetotelluric data from the region involved. Data corrected for surface and asthenosphere effects were used to map the conductivity of the deeper crust. We used an electromagnetic imaging program based upon the thin sheet approximation. The thin sheet is digitised in homogeneous square cells, and we use a Markov chain to determine the a posteriori law of the conductance in each cell given the data and an a priori knowledge on the conductance. The obtained map of conductivity is presented and discussed. Title: 7.15p ON THE RESOLUTION BOUNDS OF BI-MODAL INVERSION AND THE VALIDITY OF 2D APPROACH ON THE LINCOLN LINE (EMSLAB EXPERIMENT) Authors: N.G. Golubev, E.Yu. Sokolova and Iv.M. Varentsov (Geoelectromagnetic Research Institute RAS, Troitsk, Russia; e-mail: igemi1@pop.transit.ru) We are verifying in this study the deep 2D conductivity model, constructed to fit both modes of MT soundings at the Lincoln Line (Varentsov et al., 1996) and updating these results with new algorithms. The focus is set on the vertical resolution of crustal and asthenospheric conducting objects at the continent and on the lateral tracing of the asthenosphere structure at the continent margin. Primarily, we check how different elements of this model could be resolved when simulated data with different noise level are inverted. Then possible 3D distortions of the inverted electric and magnetic data by local and mid-scale structures are examined and the validity of 2D approach is discussed. Finally, we summarize a number of approaches how to extract at this profile less distorted MT responses from geomagnetic observations and how to use them within inversion studies. Title: 7.16p Constraints on crustal fluid penetration at spreading ridges from intergrated seismic and electromagnetic observations. Authors: A. Greer(1), S. Constable(2), L. MacGregor(1), M. Sinha(1). (1) Department of Earth Sciences, University of Cambridge (2) Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography Studies of active spreading ridges using seismic and controlled source EM techniques provide separate but complementary constraints on the extent to which penetration by fluids alters the physical properties of the crust. At three contrasting spreading ridges, the two techniques have been used at the same locations providing coincident datasets. For the case of either aqueous or magmatic fluids filling crustal pore spaces, relationships can be found for given pore geometries, that allow pore fractional volume to be mapped into variations in seismic P-wave velocity. Similarly, for given pore geometries, there are also straight forward relationships which map the porosity onto variations in resistivity. Since these two geophysical properties - seismic velocity and electrical resistivty - respond differently to variations in pore geometry, it is possible by combining the two datasets to overcome the trade off that otherwise exists between pore geometry and pore fractional volume. This approach provides a powerful new tool for investigating the penetration of hydrothermal fluids and magma into the crust, in regions of active high temperature mineralisation. Title: 7.17p Determination of the upper mantle electrical conductivity Authors: C. Lemonnier (1), F. Perrier (2), P. Tarits (3), M. Alexandrescu (4) and G. Petiau (4) (1) Imagerie Tectonique, EOST, CNRS UMR 7516, 5 rue Descartes, 67084 Strasbourg, France (2) Laboratoire de Geophysique, CEA, Bruyeres-Le-Chatel, France (3) UBO - IUEM ,UMR Domaines Oceaniques 29280 Plouzane, France, (4) IPGP, Chambon-La-Foret, France Ninety four days of continuous and simultaneous recordings of magnetic and electric field variations at three sites in France, Chambon-La-Foret and Garchy in the Parisian Basin and Sur-Fretes in the French Alps, have been analysed. We have calculated the magnetotelluric impedance by a robust method (Chave et al. 1987) for frequencies between 10-5 to 10-3 Hz. It reveals a real static distortion at all sites. It is spectacular at Sur-Fretes. In a first approximation it is independent of frequency. We have decomposed the MT tensor using the method proposed by Counil et al. (1985) to characterize the effects of this distortion. The 1D inversion of the corrected impedances are presented and discussed in regard of results obtained in other continental areas 7.18p AN ATTEMPT TO CORRELATE THE MAGNETOTELLURIC AND TELLURIC DATA ALONG THE VARSAND-LUGOJ-CARANSEBES PROFILE. Authors: Nistor H., Stanica D., Furnica Cornelia, Ivanov Ana, Asimopolos L. Geological Institute of Romania 78344 1, Caransebes Str, Bucharest 32 e-mail: anaiv@igr.sfos.ro Varsand-Lugoj-Caransebes profile, with NNW-SSE orientation, crosses the Eastern limit of Pannonian Depression as well as a short portion of western part of Apuseni Mountains. The most important geological feature of this direction is the Major Tethyan Suture representing an old oceanic crust setting in contact the Inner Dacides and the Median Dacides - two blocks with continental crust. The available geophysical material consists of telluric information on a quasiregular network and magnetotelluric data carried out in 22 soundings. The goal of this work is gathering the different parametric distributions - both telluric and magnetotelluric - into an image of a geological structure taking into account their correlation. Title: 7.19p THE USE OF MTS DATA IN MODELS OF CONTINENTAL RIFTINFG (ON THE EXAMPLE OF THE BAIKAL RIFT ZONE (BRZ)) Authors: Popov A.M. and Kiselev A.I. Institute of the Earth's Crust, Siberian Division of the Russian Academy of Sciences, Irkutsk, Russia Fax: (3952) 46 29 00 E-mail: popov@earth.crust.irk.ru According to MT soundings, the asthenospheric top beneath BRZ is at a depth of 100-110 km. At the same time, the model of active rifting, proposed by Zorin et al. proceeding from inversion of seismological, gravity, and geothermal data advocates the presence of an asthenospheric upwarp reaching about 40 km deep. Discrepancy between this model and the MT soundings can be due to data distortion and, on the other hand, to the fact that ultrabasic rocks show no elasticity and density jump marking the phase transition boundary. Recently performed teleseismic tomography scanning of BRZ and its surroundings yielded velocity variations and attenuation of seismic waves in the mantle that fit the conditions where no partially molten material is present immediately beneath the crustal base, and the 900 C isotherm is at a depth of about 50 km. These results are perfectly consistent with the MTS data according to which temperature at Moho does not exceed 800 . Electromagnetic studies, along with teleseismic tomography results allow a more reasonable explanation for Baikal rifting in terms of a model implying convection of molten material from beneath the thick lithosphere of the Siberian platform at its active margin into the adjacent rifted area involved in ongoing activity. Such a process can be induced by plate collisions. In the case of Baikal rifting it may have been the India-Eurasia collision. Title: 7.20p MAGNETOTELLURIC TRAVERSES ACROSS THE NORTHERN MARGIN OF THE EASTERN GHATS, EASTERN INDIA Authors: By K.K. Roy, A.K. Singh and S. Srivastava Dr. AJAY KISHORE SINGH Department of Geology and Geophysics Indian Institute of Technology KHARAGPUR, PIN 721302, INDIA Phone: 091 03222 55221 4842(ext) Email: ajay@gg.iitkgp.ernet.in Fax : 091 3222 55303 Abstract Magnetotelluric survey was conducted across the well known and well identified margins of the Archaean low grade granite/granitoid and iron ore group of rocks of Singhbhum and the Proterozoic high grade granulitic terrain of the Eastern Ghats. MT data was collected along two profile across the Sukinda collision zone near Kamakhyanagar and Bhuban. Data processing is done using a robust processing software ProcMT (Metronix, Germany). Signals upto 4096 seconds period could be retrieved. 2D models are generated using RRI algorithm of Smith and Booker (1991). Rotation invariant parameters viz., Rho_av, Phi_av, Rho_det, Phi_det, (after Berdichevskii and Dmitriev (1976) and Rho_cen, Phi_cen of Lilly(1993) were used for generating th geoelectrical model along with TE, TM and TE+TM mode data. Static shift free parameters viz., (I) all the phases, (ii) All the parameters originated out of the magnetic transfer function viz., Hx/Hz, Hy/Hz, tipper, tipper skew, tipper ellipticity, (iii) Groom and Bailey's twist and shear are useful parameter for qualitative interpretation of MT data. Pseudosections of these static shift parameters show a series of faults in the collision zone and these die down within 12 to 15 km from the surface. Most of these pseudosections show the presence of brittle upper crust, the ductile lower crust and upper mantle. Surface plots based on apparent resistivity and phases of Caniard(1967), Eggers(1982), Bahr(1991), Yee and Paulson(1987), distinctly demarcates the collision zone. MT models and pseudosections show the contact of the Archaean and Proterozoic formations. MT can be used successfully for mapping the lateral inhomogeneities. Oral presentation preferred. Title: 7.21p Electrical resistivity structure of mid-ocean ridges: global variability and its implications. Authors: M.C.Sinha(1), S.C.Constable(2), L.M.MacGregor(1) & A. Greer(1). (1) Department of Earth Sciences, University of Cambridge (2) Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography Controlled-source electromagnetic sounding (CSEM) experiments have now been carried out in 3 locations on the global ridge system, as well as on mature oceanic crust. Comparisons of the results at varying spreading rates and for differing tectonic settings reveal some important and expected similarities, but also some significant differences. The most consistent feature in young crust at all sites is the dramatic increase in resistivity in the upper few hundred metres of the crust, believed to correspond to rapidly decreasing porosity at the base of oceanic layer 2A. At greater depths, in some locations, resistivity decreases again with depth, due to the presence of a crustal magma reservoir - although in one location (the Lau Basin), a deeper low resistivity zone may, instead, be due to anomalously conductive aqueous fluid ponded within the crust off axis. The observed electrical and other geophysical properties of crustal melt bodies beneath ridges provide strong evidence for non-steady-state accretion of new crust, particularly at slower spreading rates. 7.22p THE PLACEMENT OF TRANS-EUROPEAN SUTURE ZONE BY RELIABLE ARGUMENTS, ON THE ROMANIAN TERRITORY. M.Stanica, D.Stanica and C.Furnica Geological Institute of Romania,Caransebes str.1, RO-78 344 Bucharest, email:stanica@igr.sfos.ro Initially, a few 2D forward modellings are presented in order to emphasize the placement of the Trans-European Suture in the rooted zone of the Eastern Carpathians's Flysh Nappes System. Then, a conclusive review of the distribution of the main geoelectrical properties on this area is accomplished, pointing out the accuracy of the image reflecting the two types of contrasting basement, without neglecting the complex aspect related to the Carpathian Arc Bend. By correlating the two maps, at the lower crust and basement levels, elaborated by electromagnetic data, new information concerning the particularities of this distinguished tectonic element are brought. In addition to it, the two extreme top limits on the map of the lower crust are revealed - the highest one (20 km) in the middle of the Pannonian Basin, and the deepest one (50 km) in the Vrancea area Title: 7.23p SOME RESULTS OF MAGNETOTELLURIC INVESTIGATIONS IN THE POLISH CARPATHIANS Authors: M.Stefaniuk*, W.Klitynski* *Univeristy of Mining and Metallurgy, al.Mickiewicza 30, 30-059 Cracow, Poland e-mail: stefan@geolog.geol.agh.edu.pl Magnetotelluric investigations in the Polish Carpathians have been conducted for over twenty years by the Geophysical Exploration Company, Warsaw, Institute of Geophysics of Polish Academy of Sciences, Warsaw, and Institute of Geophysics of University of Mining and Metallurgy, Cracow. This paper presents results of interpretation of MT data from the eastern part of the Polish Carpathians. The objective was to identify the topography of the Carpathian basement. Interpretation techniques included 1D inversion, 1D and 2D forward modelling, and transformation of apparent resistivity into apparent velocity. Results of MT data interpretation and geological identification of geoelectric horizons were verified by parametric soundings made near several deep boreholes penetrating the high-resistivity basement in the outer part of the flysch orogen. Interpretation was made along several profiles transverse to the orogen axis and crossing regions with different style of geological structure. The main problem in interpretation was a choice of a geoelectric model, and geological identification of high-resistivity and low-resistivity horizons. Parametric sounding interpretation shows that the high-resistivity horizon is connected with the roof of Mesozoic, Palaeozoic or Precambrian basement, while the low-resistivity complex in the outer part of the region is built of shales. Title: 7.24p 2D Model Study of the MELT Seafloor MT data by ABIC minimization Authors: Hiroaki Toh*, Tadanori Goto**, Kiyoshi Baba*** and Hisashi Utada**** * Ocean Research Institute, University of Tokyo ** Department of Environmental Earth Sciences, Aichi University of Education *** Department of Earth Sciences, Chiba University **** Earthquake Research Institute, University of Tokyo >From May 1996 through July 1997, one-year deployment of about 50 seafloor MT instruments was conducted along two east-west lines around the southern East Pacific Rise (EPR) at the latitude of 17S. The objective of this Mantle ELectromagnetic and Tomography (MELT) experiment is to give observational constraints to the geometry, distribution and connectedness of the partial melt bodies beneath the super spreading centre by a joint seismo-EM sea experiment (Forsyth and Chave, 1995). The body wave tomography and surface wave dispersion analyses conducted so far show: (1) The melt body is quite broad and anisotropic, and thus implies the passive mantle flow. (2) It is asymmetric as well, which means a significant difference in the upper mantle structure beneath the Pacific plate and the Nazca plate. To test these seismic results, a seafloor version of ABIC inversion (Uchida, 1993) is applied to the MELT data. As shown by previous 2D inversions (Heinson et al., 1998) and/or synthetic 2D model studies of this specific area (Unsworth et al., 1993), TM mode inversions are relatively insensitive to the structure below while TE mode inversions reveal signatures of conductors beneath the south EPR. This comparative 2D model study will extract the most robust feature of the electrical conductivity structure beneath the MELT area.