collapse of archean orogens and the generation of late to postkinematic granitoids intrusion of late stage granitoids into archean granite greenstone terranes may be spatially and temporally related to late orogenic upper crustal extensional collapse and formation of metamorphic core complexes the late to postkinematic granitoid magmas are derived through partial melting of upper mantle and lower crustal sources and are locally associated with late stage extensional structures thick roots of depleted lithospheric mantle of archean age tectosphere beneath archean cratons makes melting of the upper mantle difficult it is proposed that the archean tectosphere is made of imbricated slabs of buoyant archean oceanic crust and lithosphere together with trapped wedges of fertile mantle this would explain several seemingly contradictory phenomena decompression from upper crustal extension generates significant amounts of basaltic melts from the trapped wedges of fertile mantle these basaltic magmas intrude and partially melt the lower crust becoming more silicic in composition they migrate upward to solidify in the middle and upper crust forming the late to postkinematic granitoid suite the tectosphere in this model is less dense and colder than surrounding asthenosphere and is consistent with eclogitic xenoliths from subducted oceanic lithosphere found entrained in kimberlites the direction and magnitude of shear wave anisotropy in the upper mantle is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere

inhomogeneous structure of the senegal lithosphere from deep magnetotelluric soundings data from 23 magnetotelluric sites were used to determine electrical conductivities within the crust and upper mantle in senegal west africa along a profile about 600 km long data have been obtained in a variety of tectonic zones the west african craton stable since 1850 250 m y the west african mobile belt with ages in the range of 350 650 m y and the senegal sedimentary basin an earth model is derived from two dimensional modeling the model shows several heterogeneities extending deep in the lithosphere and perhaps more deeply in the asthenosphere to a depth of about 460 km at the stations in the western part of the basin the average resistivity is anomalously low 20 30 ohm m down to about 10 km the basin anomaly in the upper crust is assumed to mark an initial rift zone it may reveal a pattern of crustal weakness in this area beneath this zone the resisitivity is in the range of 1000 3000 ohm m down to at least 300 km in the eastern basin the resistivity is decreasing in the depth range of 20 30 km and the top of the ultimate mantle conductor is at a depth of 300 km at the sites in the marginal part of the west african craton conductive layers can be recognized in the depth range of 30 40 and 80 100 km the resistivity drops to 10 ohm m at a depth of about 460 km the model in this region is compared with available geological and geophysical information and an attempt is made to explain some of the observed discontinuities by plate tectonics it is suggested that the major discontinuity which separates the basin and the mobile belt and extends to about 460 km depth could be interpreted as marking a precambrian subduction slab dipping eastward in the west african craton a low resistivity has been determined for a depth range of 130 150 km the upper 130 km were found to have the rather high resistivities of 1000 3000 ohm m a sharp rise in conductivity occurs at a depth of about 460 km beneath the craton in contrast a surprising feature in the kedougou region in the craton is that the major portion of the lower crust is conductive the existence of a birrimian geosynclinal pair in eastern senegal consisting of a eugeosynclinal marine andesitic volcanic trough particularly volcanics with ophiolite affinity and a miogeosynclinal sedimentary sequence could characterize a modern plate tectonic phase dominated by widespread ocean opening and continental collision the conductive material in the lower crust might be explained by the buried ocean crust

laboratory measurements of electrical conductivity of hydrous and dry silicic melts under pressure an interpretation of the electrical signature of molten rocks within the earth s interior in terms of nature and temperature conditions of magma requires additional laboratory data on the electrical conductivity of silicate melts this paper describes an experimental setup and presents measurements of the electrical impedance of dry and hydrous 1 3 wt h2o metaluminous obsidians determined in an internally heated pressure vessel in the range of 50 400 mpa and 350 1325 c it is shown that the temperature and pressure dependences of electrical conductivity of both hydrous and dry obsidian can be fitted by an arrhenius law applying in the melt and the glass regions this suggests that a similar transport mechanism operates in both melt and glass the determined activation energies are 70 kj mol for the dry obsidian and 65 and 61 kj mol for the 1 and 3 wt h2o melts respectively the activation volume is 20 cm3 mol combination of tracer diffusion and electrical conductivity reveals that sodium is the dominant charge carrier in the hydrous and dry obsidians the temperature and pressure effects on conductivity are therefore interpreted in terms of activation energy and activation volume for na mobility in dry and hydrous rhyolites an increase in conductivity associated with addition of water was observed and is shown to reflect the effect of water incorporation in melts on the mobility of sodium as a prospective it is anticipated that both the mobility and content of sodium could control the electrical conductivity of most terrestrial silicate melts magma under differentiation becomes more conductive due to sodium and water enrichment associated with fractional crystallization therefore its electrical signature must reveal its nature and maturity

deep structure of fennoscandia from fundamental and higher mode dispersion of rayleigh waves abstract surface wave dispersion for the baltic shield area has been investigated through analysis of phase velocities up to periods of about 250 s for the fundamental rayleigh mode and about 80 s for the first two higher modes this has enabled investigation of the lithosphere asthenosphere system and deeper regions down to about 400 km the shear wave velocitydepth relationship obtained from the inversion of dispersion data for the paths cop kev cop krk can be considered as representative of an average model for the baltic shield moreover the inversion of dispersion data supplemented by previous results from the scandinavian area suggest the presence of significant lateral heterogeneity down to depths of about 350 km moving from the older precambrian shield cop kev cop krk to the peripheral area nur cop models with small if any differences in average shear velocity are consistent with the data in the depth range 350 450 km for the regions studied

deep dc soundings in southwestern finland using the fenno skan hvdc link as a source abstract the fenno skan high voltage direct current hvdc power line links finland and sweden across the bothnian sea a 180 km long cable has been installed at the sea bottom a return current up to 1280 a flows through water and sea bottom rocks this source gives a unique possibility for large scale dc sounding first test measurements were done by the department of geophysics university of oulu in may 1991 in june 1992 a bilateral experiment was carried out by finnish and russian geophysicists on the base of 42 sites measured during two field seasons apparent resistivity values were calculated for the interpretation of the measured data a thin sheet approximation was used the model consisted of two thin sheets the uppermost sheet had a conductance s due to the sea water and sediments according to a priori information while the second one had a depth integrated resistivity r due to the earth s upper crust a good conductor was located beneath comparisons of the experimental and theoretical apparent resistivities show that the r value for the earth s upper crust in southwestern finland is about 3 10 8 m 2 which is in good agreement with the value calculated from the magnetotelluric mt data in the proterozoic central finland granitoid complex however it is more than a factor of 10 lower than the value calculated similarly from the mt data in the kuhmo region in the archaean part of the fennoscandian shield in eastern finland and the reported value based on the magnetohydrodynamic mhd data in the kola peninsula the results do not reveal any distinct crustal conductors within the research area this experimental indicates that deep geoelectrical resistivity studies can be carried out successfully at rather large distances at least 50 km in the surroundings of a high voltage dc power link

deep schlumberger sounding and the crustal resistivity structure of central australia summary three 200 km schluniberger resistivity soundings have been conducted over the central australian shield using telephone lines to obtain the large electrode spacings these represent the first crustal scale controlled source electrical study to be carried out in this continent a computer controlled data acquisition system was used which allowed precise measurements to be made with only modest emission currents 0 1 0 5 a the three soundings centred on the towns of renner springs wauchope and aileron showed the southern part of the study area the arunta block to be an order of magnitude more resistive than the more northerly section the tennant creek block this difference correlates with the higher heat flow of the tennant creek block a lowering of apparent resistivity at large electrode spacings for one sounding wauchope is taken to indicate the presence of a low resistivity layer in the middle crust at a depth less than 20 km however the effect of the highly conductive overburden characteristic of inland australia combined with the large transverse resistance of the crust prevented the other two soundings from detecting such a layer without support from these two soundings it is impossible to be sure that the lowered resistivity at wauchope is not caused merely by lateral variations in near surface resistivity the data also show that crustal resistivities are much lower than the expected values for dry rock whether or not a low resistivity layer is included in the model this implies a widespread occurrence of free water in the crust with greater amounts occurring at depth if the low resistivity zone exists

a new interpretation of kinematic stormflow generation the paper reviews a number of possible fast and slow hydrological flow mechanisms to account for rapid runoff generation within a catchment a new interpretation of the kinematic wave process is proposed which develops some of these concepts to explain rapid subsurface flow from a watershed evidence for the process is provided by the results from a laboratory soil core experiment and an investigation of the hydrology of a dartmoor hillslope a tension response was monitored in the soil core in which pressure waves were propagated downwards and expelled water from the base the transmission of the wave down the core was considerably faster than the movement of a chloride tracer the concept of this kinematic wave process and associated water flux was then extended to the dartmoor watershed raindrops reaching the wet soil surface caused pressure waves to travel laterally downslope during large rainstorms the hillslope became hydrologically highly connected and the pressure waves forced existing water from seepage faces into the saturated area adjacent to the stream contributing substantially to the stream discharge a kinematic contributing area was defined as determined by both rainfall runoff ratios and geostatistical analyses of hillslope soil moisture contents which extended over at least 65 of the catchment area this kinematic wave theory is consistent with results of translatory flow and macropore flow models and stable isotope field studies of old new water copyright 2002 john wiley sons ltd

crustal structure of the india asia collision zone southern tibet from indepth mt investigations abstract project indepth was initiated in 1992 to develop a better understanding of the deep structure and mechanics of the himalaya tibet region and magnetotelluric surveying was added during indepth phase ii in 1995 broadband and long period data were acquired in 1995 along a n s transect crossing the india asia collision zone the yarlung zangbo suture at 90 e longitude however these data failed to penetrate to the deep crustal and mantle depths due to low solar activity and the presence of the thick conductive crust deep crustal and upper mantle information is key to test the model of subcretion to southern tibet by the stiff indian mantle lid in order to try to obtain such information and to test along strike variability of the yarlung zangbo suture zone a magnetotelluric experiment was performed in 2001 roughly along longitude 92 e containing an ultra long period component mt data were acquired at 28 locations along the new profile with long period data at 15 of these and ultra long period data at five of these 15 initial comparisons of the models generated from the 1996 and 2001 datasets indicate little variation in electrical conductivity structure along strike

introduction to electrical properties of the earth s mantle the mantle is the largest of the concentric regions that constitute the earth s interior lying beneath an oceanic or continental crust which may be 6 to 75 or more km thick the mantle region extends to the boundary with the earth s liquid core the most detailed characterization of the mantle is provided by the velocity depth profiles obtained from the seismic waves dziewonski and anderson 1981 a generally abrupt rise in velocity the mohorovicic discontinuity bounds the crust and the upper mantle between about 100 and 220 km deep a low velocity zone is encountered beyond which the velocity increases continuously and in steps near 400 670 and perhaps 770 km the name lower mantle is assigned to the region from about 1000 km to the start of the core at 2890 km deep

numerical models on the influence of partial melt on elastic anelastic and electrical properties of rocks part ii electrical conductivity abstract partial melting influences both the elastic and electrical properties of mantle rocks significantly but in a different manner a set of theoretical models of the electrical conductivity of partially molten rocks is developed for different geometries of the melt distribution the models correspond to a similar set on elasticity and anelasticity presented in a preceding paper part i both model sets allow combined interpretations of seismic and geoelectric observations from anomalous hot mantle regions by a systematic variation of the melt geometry the following melt geometries are assumed in the conductivity models melt films tubes isolated ellipsoidal inclusions films and ellipsoidal inclusions with a variable degree of interconnection distributed geometries and superpositions of spheres with films and with tubes unexplored conduction mechanisms near the melting temperature film thicknesses of molecular sizes or effects of volatiles are not considered resistor network theory is utilized to account for a variable degree of interconnection and distributed geometries thereby a new possible interpretation of archie s law is found in terms of varying interconnection furthermore it is found that a reduced degree of interconnection may have an important influence on the conductivity at melt fractions characteristic for a weak to moderate decrease in seismic velocity the model results are compiled together with the elastic models of part i in a set of diagrams which may be applied systematically to observed electrical and seismic data to find constraints on the state of melting

low frequency electrical impedance of partially molten gabbro the effect of melt geometry on electrical properties abstract electrical impedance of partially molten gabbro was measured as a function of the frequency of the applied electric field the impedance of the partially molten gabbro was mainly controlled by the volume and geometry of partial melt which were observed with an optical microscope study of the quenched sample the experimental results are interpreted by a theory that formulates the electrical property of rocks containing partial melt taking ionic diffusion in the melt into account the theory evaluates the contributions from two types of melt geometry distributed in the rock matrix namely 1 a nearly isolated melt pockets and 2 b connected melt along grain boundaries according to the theory connected melt mainly contributes to the increase of net conductivity while melt pockets increase displacement current and cause the frequency dependence of impedance the characteristic ionic diffusivity in melt pockets was determined from the peak frequency of the imaginary part of impedance the magnitude of diffusion coefficient is estimated to be of the order of 10 7 to 10 6 cm 2 sec this suggests that sodium iron magnesium and or calcium ions mainly carry the electric charge in partial melt the observed frequency dependence of impedance is considered to reflect ionic polarization in melt pockets due to the migration of these alkali ions toward solid melt interfaces the present experimental and theoretical work suggests that electrical properties of the earth s upper mantle may involve an intrinsic frequency dependence associated with ionic diffusion in the frequency range of geomagnetic observation if a partial melt zone exists the analysis of the earth s conductivity structure may be inadequate if such intrinsic frequency dependence of mantle media is neglected

conductivity and fluids in the oceanic upper mantle abstract three deep electromagnetic soundings in the north pacific ocean were re examined our inversion of existing data showed the presence of conductors below the three sites in very good agreement with previous results we showed that h 2 o rich fluids as well as partial melting account for the conductivities observed this led us to determine temperatures ranging firstly from 1050 to 1150 c when only h 2 o rich fluids were present and secondly from 1300 to 1350 c for partial melting only we suggest that the upper mantle below the two oldest sites 30 and 72 ma is characterized by about 1 of partial melting in the presence of about 1 of h 2 o rich fluids below the third site that nearest to the ridge zone 1 ma the degree of partial melting might reach 10 the presence of 1 of h 2 o rich fluids would reduce the effect of the asthenospheric outflows on the lithospheric magmatism this agrees with the principle of thermodynamic decoupling of the asthenosphere and lithosphere the weakening effect of fluids particularly water on upper mantle rheology is also discussed we suggest that the asthenosphere and lithosphere might be decoupled mechanically which if true would affect small scale convection below the oceanic lithosphere

structure of the crust in the vicinity of the banggong nujiang suture in central tibet from indepth magnetotelluric data 1 magnetotelluric data from a 150 km long profile crossing the banggong nujiang suture bns central tibet acquired as part of the international deep profiling of tibet and the himalaya indepth project have been examined for crustal and upper mantle structure strike and dimensionality analyses demonstrate that regional scale electrical structures are two dimensional and oriented approximately parallel to surface geological strike as seen elsewhere in tibet the double thickness crust is generally characterized by resistive upper crust hundreds to thousands of ohm meters overlying conductive middle and lower crust tens to hundreds of ohm meters but in detail there are lateral variations at all levels regionally a northward transition from thick 45 km to thin 15 km resistive upper crust coincides with 1 the surface trace of the bns 2 a prominent strand of the karakorum jiali fault system 3 northward decrease in upper mantle seismic velocities and increase in attenuation and 4 pronounced northward onset of seismic polarization anisotropy the latter two seismological features have been taken to mark the northern limit of indian mantle lithosphere thrust beneath southern tibet on the basis of our electrical model we speculate that 1 the resistive upper crustal root beneath the neogene lunpola and duba basins was produced by crustal shortening localized along the northern edge of the lhasa terrane 2 the low midcrustal resistivity beneath the bns reflects enhanced neogene melting and or metamorphic dewatering of relatively fertile subduction zone complex rocks 3 observed steep upper crustal low resistivity anomalies are produced by hydrothermal fluids within active faults localized within and adjacent to the bns and 4 these strike slip and extensional fault arrays are surface manifestations of lithosphere penetrating shear localized along the northern edge of the underthrust indian plate

source field problems in northern parts of the baltic shield abstract theoretical calculations using a stationary ionospheric electrojet model are presented for a resistivity model of interest in studies of the baltic shield the earth model includes the undistorted east european platform resistivity versus depth model and crustal and asthenospheric conducting layers the results show that close to the source region for periods 15 min the magnetic field depends only weakly on the earth s conductivity accordingly the resolution of apparent resistivity curves obtained by the horizontal spatial gradient hsg technique becomes poor at longer periods close to the source region a comparison with experimental hsg apparent resistivity from kiruna and kevo in northern scandinavia supports the existence of high conductivity in the lower crust the sharp decrease of apparent resistivity at periods 1 h may within the limits of experimental error be a result of the non uniform source rather than an indication of a good conductor in the upper mantle

imaging the laguna del maule volcanic field central chile using magnetotellurics evidence for crustal melt regions laterally offset from surface vents and lava flows abstract magnetotelluric mt data were collected at the laguna del maule volcanic field ldmvf located in central chile 36 s 70 5 w which has been experiencing unprecedented upward ground deformation since 2007 these data were used to create the first detailed three dimensional electrical resistivity model of the ldmvf and surrounding area the resulting model was spatially complex with several major conductive features imaged at different depths and locations around laguna del maule ldm a near surface conductor c1 0 5 m approximately 100 m beneath the lake is interpreted as a conductive smectite clay cap related to a shallow hydrothermal reservoir at 4 km depth a strong conductor c3 0 3 m is located beneath the western edge of ldm the proximity of c3 to the recent pleistocene to holocene vents in the northwest ldmvf and nearby hot springs suggests that c3 is a hydrous 5 wt h2o rhyolitic partial melt with melt fraction 35 and a free water hydrothermal component c3 dips towards and is connected to a deeper conductor c4 1 m c4 is located to the north of ldm at 8 km depth below surface and is interpreted as a long lived rhyolitic to andesitic magma reservoir with melt fractions less than 35 it is hypothesized that the deeper magma reservoir c4 is providing melt and hydrothermal fluids to the shallower magma reservoir c3 a large conductor directly beneath the ldmvf is not imaged with mt suggesting that any mush volume beneath ldm must be anhydrous 10 km as it moves from the deep magma reservoir c4 to create small ephemeral volumes of eruptible melt c3 it is hypothesized that there may be a north south contrast in physical processes affecting the growth of melt rich zones since major conductors are imaged in the northern ldmvf while no major conductors are detected beneath the southern vents the analysis and interpretation of features directly beneath the lake is complicated by the surface conductor c1 which attenuates low frequency signals the attenuation from c1 does not affect c3 or c4 at 1 km depth directly beneath ldm a weak conductor c2 50 silicic reservoir with 5 wt h2o at 2 to 5 km depth beneath the inflation center is not supported by the mt data however a smaller 10 km3 eruptible volume could go undetected even with relatively high melt fraction 50 the location of large melt regions to the north has important implications for long term volcanic hazards at ldmvf as well as other volcanoes as it raises the possibility that the vent distribution is not always indicative of the location of deeper source regions of melt

crustal and upper mantle structure of northern tibet imaged with magnetotelluric data 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

flat slab thermal structure and evolution beneath central mexico recent seismic and magnetotelluric experiments aimed at better characterizing the shape and state of the subducting slab and continental crust beneath central mexico exposed significant differences with conclusions of previous studies a new slab geometry is revealed in which the subducting cocos slab is perfectly flat between 120 to 290 km from the trench after which it plunges into the asthenosphere at a dip angle of 65 in sharp contrast with the previously proposed 20 dip angle seismic tomography studies show negative p wave velocity anomalies 2 to 4 in the mantle wedge beneath the mexican volcanic belt and positive anomalies 2 to 3 for the subducted cocos slab magnetotelluric experiments exposed a very low resistivity area 1 10 m located within the continental crust just below the mexican volcanic arc finally several spots of non volcanic tremors nvts have been recorded inside the continental crust above the flat slab segment while all these experiments provide a better picture of the subduction system beneath central mexico several key processes need further investigation in this study we take advantage of these new observations to better constrain the thermal structure beneath central mexico two different thermal models are computed for a mantle potential temperature t p of 1 350 and 1 450 c respectively the new thermal structures are then converted into p wave velocity anomalies and compared with the observed v p anomalies we found that a t p of 1 450 c produced larger v p anomalies that do not fit the observations however using a t p of only 1 350 c our predicted v p anomalies are positive 2 to 3 for the cold slab and negative 2 to 4 in the mantle wedge these v p estimates are consistent with the observed seismic tomography from p wave arrivals and therefore we conclude that a t p of 1 350 c is a better estimate for the mantle potential temperature beneath central mexico the new thermal model in conjunction with phase diagrams for sediments hydrated basalt and lithospheric mantle have been used to estimate the amount and location of fluids released from the subducting cocos slab several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat slab we found a good correlation between the pattern of these dehydration pulses and the location of nvts suggesting that slab dehydration is responsible for triggering the tremors we suggest that nvt bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening a process that has been going on since 15 ma ago when the cocos slab entered into a flat slab regime such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15 ma allowing the slab to easily roll back the continuous low resistivity region recorded beneath the volcanic front in central mexico might represent the evidence of slab dehydration and crust weakening over time

depth of crustal conducting layer and asthenosphere in the pannonian basin determined by magnetotellurics abstract on the basis of numerical modelling and statistical investigations of the horizontal electrical inhomogeneities narrow fractures change of conductance of the surface sediments min magnetotelluric mt sounding curves were found to be less distorted and therefore they can be used for the determination of deep conducting formations in the crust and upper mantle carefully selected mt curves indicate the crustal conductive layer at depths of 7 13 km and the conductive asthenosphere between 50 and 80 km the relatively shallow position of the asthenosphere corresponds to the high heat flow q 80 mw m 1 in the pannonian basin according to the empirical relation derived by adam 1978

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

properties of the lithosphere asthenosphere system in europe with a view toward earth conductivity over the past 20 years the study ofp ands wave velocities in the upper mantle of the mediterranean area and continental europe has been the subject of intensive research work we present a summary of results based on the inversion of available surface wave dispersion data andp wave trave time observations for areas characterized by different tectonic settings and very large lateral variations a discussion is made about structural models based on seismological geothermal and electrical conductivity data

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

electromagnetic images of crustal structures in southern and central canadian cordillera data from more than 400 magnetotelluric soundings made since the early 1980 s in the canadian cordillera over a 300 000 km2 area between 49 and 53 5 n are used to image qualitatively regional three dimensional crustal variation in electrical conductivity by means of phase maps phase frequency sections and maps of resistivity at depth two hundred of the soundings were acquired as part of lithoprobe southern canadian cordillera transect activities and their locations were coordinated with the seismic reflection and refraction experiments the lower crust has a generally pervasive low resistivity 1 100 m throughout the cordillera west of the foreland belt within this canadian cordilleran regional conductor the magnetotelluric data reveal both two dimensional structures with highest conductivities along the coast belt and omineca belt and three dimensional variation along geological strike this conductor mapped over a volume in excess of 106 km3 is most probably caused by fluids saline wat

magnetotelluric soundings over a precambrian contact in australia summary the willyama complex near broken hill represents a major exposure of the precambrian terrain in australia however the eastern limits are obscured by recent cover in the murray darling basin the nature and extent of the contact with the younger basement to the east is unknown magnetotelluric data have now been obtained as an aid to regional mapping profiles are based on observations at 14 sites in a traverse east of broken hill apparent resistivities presented as pseudosections reflect the major structural units identified from surface geology continuity between sites is indicated from similarity in apparent resistivities and lateral changes east of the block are generally gradational characteristic divergences in orthogonal resistivity components are consistent with 2 d structure the foliated precambrian complex appears to form a semi vertical contact at the margin of the murray darling basin thus generating the required anisotropy at depths from 15 35 km seismic evidence indicates an equivalent structural contact possibly extending into the mantle

electrical resistivities of crustal fluids results of calculations on the electrical resistivities of crustal fluids as a function of temperature pressure and fluid composition indicate that crustal fluids can range in resistivity from approximately 1 m the dominant contribution to decreasing the resistivities of crustal fluids comes from increased concentrations of chloride and bicarbonate salts and increased temperature and density of the fluids calculations indicate that the sharpest decrease in fluid resistivity occurs at shallow depths 10 km and temperatures 300 c can not be readily explained as a product of variations in resistivities of crustal fluids these results indicate that a transition in mineralogy or pore shape geometry is more likely responsible for the observed decrease in electrical resistivity of the middle crust than a major change in fluid resistivity

conductivity discontinuities in the upper mantle beneath a stable craton we present evidence for approximate collocation of seismic and electrical transitions in the upper mantle more than two years of very long period magnetotelluric mt data were recorded at a lakebottom observatory in the central canadian shield after processing to contend with non stationary source effects and removal of galvanic distortion the underlying structure is 1d for periods of one hour to four days the response was extended to periods of 100 days by appending geomagnetic depth sounding data to the mt curves minimum structure linearised inversion nonlinear extremal inversion and a new genetic algorithm for nonlinear hypothesis testing reveal discrete jumps in conductivity at depths near the major upper mantle seismic discontinuities the jumps occur over limited depth ranges

geological constraints on the origin of the mantle root beneath the canadian shield cratonic north america is composed of a cluster of archaean microcontinents centred on the canadian shield and juvenile proterozoic crust that lies mainly buried beneath the sedimentary cover of the western and southern interior platforms the shield is underlain by an anomalous low temperature mantle root that is absent beneath the platform as there appears to be no systematic difference in crustal thickness or density between the shield and the platform the long lived arching of the shield implies an intrinsic buoyancy imparted by the mantle root that more than offsets its colder temperature isotopic and seismic anisotropy data indicate an archaean age for the mantle root close to the time of formation of the overlying crust the preferential development of the mantle root beneath archaean crust is consistent with an origin by imbrication of partly subducted slabs of highly depleted oceanic lithosphere assuming that buoyant subduction was more common in the archaean formation of the mantle root was not dependent on collisional orogenesis as has been suggested but the archaean cratonic mantle was sufficiently buoyant and refractory to survive later tectonic thickening the mantle root persists beneath archaean crust that was transected by mafic dyke swarms and subjected to short lived episodes of post orogenic crustal melting but the root is reduced at mantle plume initiation sites the partitioning of archaean and proterozic crust between the shield and the platform respectively causes the shield to misrepresent precambrian crust as a whole studies of the shield falsely conclude that a high percentage of precambrian crust formed in the archaean and that the proterozoic was characterized by epicontinental volcanism and sedimentation and crustal reworking furthermore the isotopic ratios of detritus eroded from the craton may tend to overestimate the mean age of continental crust

depth extent of cratons as inferred from tomographic studies we address the question of the depth extent of mantle high velocity zones under ancient cratons by using seismic velocity maps from recent mantle tomographic studies we divided old continents into two age provinces 800 1700 ma middle proterozoic and older than 1700 ma archean and early proterozoic the areas included in these age provinces are cross correlated with the tomography at different depths to quantify the global occurrence of high velocity anomaly hva extensions beneath cratons statistically significant hvas underlie the oldest cratons to a depth of 250 km there is no significant consistent correlation between tomography and younger cratons the older cratons have correlated hvas extending to depths varying from 100 to 450 km there is no significant difference in the average velocity anomaly beneath the early proterozoic part and the archean part of the canadian shield whereas the middle proterozoic part does not contain any hva the mantle under early precambrian crust is distinctly different from mantle elsewhere we propose two different hypotheses to explain these results 1 cratons of early proterozoic age or older contain ancient and deep physically attached roots or 2 these cratons contain a small permanent physical root that induces cold downwellings beneath it a root thickness of 200 km can be explained by an feo poor olivine rich mineralogy whereas a requirement of high strength probably demands in addition a volatile poor composition different root depths for cratons of similar age may be related to their location in the various supercontinents in which they were involved and to their subsequent deformational history or they may represent different snapshots in time of transient roots such as thermal boundary layers

magnetotelluric and seismological determination of the lithosphere asthenosphere transition in central europe abstract we assembled all available magnetotelluric mt sounding curves that cover characteristic provinces of europe the depths of layers with increased electrical conductivities estimated from mt sounding curves by the respective authors were compared with a model of the lithosphere thickness derived from seismic p residuals regions of a relatively thin and thick lithosphere are characterized by positive and negative p residuals respectively we have found a good correspondence with both sets of results in the central part of the pannonian basin the depth of the lithosphere asthenosphere transition is estimated at 60 80 km and 100 120 km at the flanks the results for the bohemian massif show deeper levels of this transition between 100 and 200 km these depths seem to be typical of the lithosphere base beneath the variscan domain of central europe

magnetotelluric array data analysis from north west fennoscandia abstract new magnetotelluric mt data in north west fennoscandia were acquired within the framework of the project magnetotellurics in the scandes masca the project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable precambrian cratonic interior to passive continental margin beneath the caledonian orogen and the scandinavian mountains in western fennoscandia an array of 59 synchronous long period and 220 broad band mt sites was occupied in the summers of 2011 to 2013 we estimated mt transfer functions in the period range from 0 003 to 10 5 s the q function multi site multi frequency analysis and the phase tensor were used to estimate strike and dimensionality of mt data dimensionality and strike analyses indicate generally 2 d behaviour of the data with 3 d effects at some sites and period bands in this paper we present 2 d inversion of the data 3 d inversion models are shown in the parallel paper we choose to invert the determinant of the impedance tensor to mitigate 3 d effects in the data on our 2 d models seven crustal scale and four lithospheric scale 2 d models are presented the resistive regions are images of the archaean and proterozoic basement in the east and thin caledonian nappes in the west the middle and lower crust of the svecofennian province is conductive the southern end of the kittila greenstone belt is seen in the models as a strong upper to middle crustal conductor in the caledonides the highly conductive alum shales are observed along the caledonian thrust front the thickest lithosphere is in the palaeoproterozioc svecofennian domain not in the archaean the thickness of the lithosphere is around 200 km in the north and 300 km in the south west

the conductivity of the crust and mantle beneath the kapuskasing uplift electrical anisotropy in the upper mantle summary kapuskasing uplift has been interpreted as an oblique cross section of up to 25km of crust and thus provides the opportunity to examine the properties of exposed mid to lower crustal material a magnetotelluric mt survey mapped a remarkably uniform upper crust and provided no evidence for upper crustal conductive zones that could be related to the often observed increase in mid and lower crustal conductivity the only shallow conductive anomaly is related to the ivanhoe lake cataclastic zone but its electrical signature does not appear to extend more than one kilometer in depth a regional apparent resistivity curve was determined from mt data and confirmed by subsequent controlled source electromagnetic surveys regional curves are essential for proper structural interpretation but are often difficult to determine because of electric field distortions the data show a decrease in resistivity at depths below 15 km which is typical of continental crust in many areas of the world there is a clear difference in orthogonal apparent resistivity and phase curves at periods greater than 10s which is most pronounced in a n65 e and n25 w coordinate frame two different models are proposed to explain the data the first consists of a conducting 2 d slab of approximately 50 km width in the lower crust and striking n65 e the strike direction is consistent with a number of regional structural trends but there is no other direct supporting evidence for it the second model invokes either micro or macro electrical anisotropy in the upper mantle there is considerable evidence for seismic anisotropy in the region the electrical anisotropy could be the result of preferential conduction along the c axis of olivine crystals hydrogen dissolved in the olivine lattice with preferential conduction along the a axis or possible alignment of dykes joints and faults in the upper mantle if the anisotropy is related to strain induced orientations of crystals or fabric it will provide evidence for the motions of the mantle associated with plate tectonics

electrical conductivity from australian magnetometer arrays using spatial gradient data quiet daily magnetic variations recorded by magnetometer arrays in australia are analysed to obtain electromagnetic response parameters for two parts of the australian continent remote from electrical conductivity anomalies the parameters are based on measurements of vertical field and horizontal field spatial gradient and three different methods are followed in their computation the response parameters are checked for consistency with a compilation of globally determined earth apparent resistivities and are then interpreted for one dimensional conductivity structure in the two different parts of the continent there is evidence that the rise in electrical conductivity from 10 1 s m 1 to 100 s m 1 which occurs at a depth of order 500 km beneath central australia may occur at a substantially shallower depth of order 230 km beneath southeast australia

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

the 410 km depth discontinuity a sharpness estimate from near critical reflections the abrupt increase in seismic velocity and density about 410 km below the surface of the earth occurs at the depth where a phase change in olivine is expected akaogi et al 1989 katsura and ito 1989 ringwood 1969 we have assembled recordings from 22 short period seismometers in washington and british columbia of 35 nuclear explosions in nevada these data reveal relatively noise free reflections from the 410 discontinuity most of the transition is completed within a depth interval of less than 10 km under the western united states this result adds to the accumulating seismic evidence of a fairly sharp 410 transition

reservoir characterization and basement estimates in the papuan fold belt papua new guinea png from reanalysis of the png mt data set abstract the png mt data set covering a section of the papuan fold belt pfb in papua new guinea png which is known for its extensive hydrocarbon resources re analyzed using advanced dimensionality technique and inversion algorithms although the phase tensor analysis retrieved a dominant 2d conductivity direction matching the regional tectonic trend a mild 3d behavior with some local strong 3d effect is apparent the 3d and 2d inversions carried out yielded resistivity features related to the broad stratigraphic column of the area but with variations in the depth of occurrence and thicknesses of resistivity structures between the inversion models the preferred 3d model shows an about 1 km thick resistive miocene darai limestone cover thickening towards northeast of the profile and supports a thick skinned thrust model the 3d resistivity section shows an additional differentiation within the underlying mesozoic column a highly conductive 1 m and moderately conductive 10 40 m layer sandwiched between the darai formation and crystalline basement the upper highly conductive layer can be related to the cretaceous ieru shale formation which acts as the regional cap rock in the pfb and the below moderately conductive layer represents the stack of mesozoic strata that possibly comprise of the neocomian toro sandstone reservoir rocks and underlying jurassic source rock formations nevertheless the resolution tests do not fully support the demarcation between the ieru formation and underlying mesozoics containing the reservoir and source formations and demand additional constraints for confirmation

electrical conductivity of olivine a dunite and the mantle laboratory studies of the electrical conductivity of rocks and minerals are vital to the interpretation of electromagnetic soundings of the earth s mantle to date the most reliable data have been collected from single crystals we have extended these studies with electrical conductivity measurements on a dunite from north carolina in the temperature range of 600 1200 c and under controlled oxygen fugacity observations of conductivity as a function of oxygen fugacity and temperature demonstrate that conduction in the dunite is indistinguishable from conduction in single olivine crystals thus the common practice of exaggerating the single crystal conductivities to account for conduction by grain boundary phases in the mantle is unnecessary because the dunite conductivity is consistent with that published for single crystals under similar conditions we have made a combined analysis of these data conductivity as a function of temperature between 600 and 1450 c displays three conduction mechanisms whose activation energies may be recovered by nonlinear least squares fitting yielding activation energies of 0 21 2 56 10 19 j 0 13 1 60 ev below 720 c 2 56 0 02 10 19 j 1 60 0 01 ev between 720 c and 1500 c and 11 46 0 90 10 19 j 7 16 0 56 ev above 1500 c the behavior of conductivity as a function of oxygen fugacity is well explained by a model in which an o2 independent population of charge carriers is supplemented at high oxygen fugacities with a population that is proportional to o20 3 this parametrization produces a clear correlation of the o2 dependent term with iron content which is otherwise obscured by variations in conductivity among olivines

the electrical conductivity of the mantle beneath europe derived from c responses from 3 to 720 hr summary the c response which connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic fluctuations forms the basis for estimating the conductivity depth profile of the earth this paper describes new estimates of the c response obtained from observatory hourly mean values the z y method is applied which means that the vertical component is used locally whereas the horizontal components are used globally by expansion into terms of spherical harmonics a special evort is made to obtain unbiased estimates of c when applied to 90 months of hourly mean values from about 90 observatories the method yields consistent results for european observatories in the entire period range from 3 to 720 hr and for two diverent source mechanisms s and d st a good description of the source structure for individual time segments is essential this was achieved by a separate spherical harmonic analysis of the data for each month for d st or each day for s and by estimating a large number 120 of expansion coeycients the results are interpreted by means of 1 d conductivity models which show that the upper mantle has remarkably little structure with a monotonic decrease of resistivity from 100 v m near z 200 km to 0 7 v m below z 1000 km

low frequency electrical response of polycrystalline olivine compacts grain boundary transport the complex electrical impedance of sintered polycrystalline compacts of san carlos olivine grain size 75 180 m has been determined over the frequency range 104 to 10 4 hz at temperatures between 800 and 1400 c at the fo2 of the wustite magnetite buffer complex plane plots im z vs re z show separate effects of grain interior and grain boundary conductivity grain interior transport controls the response above 102 hz whereas grain boundary transport dominates between 102 and 10 2 hz overall response of the compacts is modeled with an equivalent circuit in which parallel rc circuit elements representing grain interior and grain responses act in series bulk dc conductivity is lower than either grain interior or grain boundary conductivity alone inversion of magnetotelluric profiles that employ lower bulk conductivities of the grain interior grain boundary system yield upper mantle temperature estimates 30 to 50 c higher than those based on single crystal values

structure coupled 3 d imaging of magnetotelluric and wide angle seismic reflection refraction data with interfaces magnetotelluric mt and wide angle seismic reflection refraction surveys play a fundamental role in understanding the crustal rheology and lithospheric structure of the earth in recent years the

electromagnetic evidence for lateral inhomogeneities within the earth s upper mantle abstract the composition of the upper mantle is of great significance to our understanding of plate tectonics and global evolution information about the physical properties of the earth at upper mantle depths including lateral variations in electrical conductivity can be deduced from measurements of the electric and magnetic fields at the earth s surface electromagnetic methods appear to give poorer resolution than do some other methods for example seismics but as they are sensitive to quite different properties of a medium they provide a different and complementary class of information the basic theory of electromagnetic sounding methods is briefly reviewed below and evidence regarding lateral conductivity inhomogeneities in the earth s upper mantle is examined while lateral electrical conductivity inhomogeneities appear to be the rule rather than the exception the interpretation of electromagnetic data still presents difficulties and the results from many regions are not as yet unambiguous where the data are of sufficient resolution a rapid increase in electrical conductivity can usually be identified within the upper mantle the depth to this highly conductive zone is different in different tectonic environments but is broadly consistent between analogous but widely separated tectonic environments a comparatively shallow conducting region is found beneath the ocean lithosphere the depth of this region is dependent on lithospheric age many of the more shallow conducting regions in both continental and oceanic environments are associated with high heat flow values and seismic low velocity zones these highly conducting regions may be zones of partial melt

upper mantle heterogeneity in the northern part of eurasia abstract a two dimensional velocity model of the upper mantle has been compiled from a long range seismic profile crossing the west siberian young plate and the old siberian platform it revealed considerable horizontal and vertical heterogeneity of the mantle a sharp seismic boundary at a depth of 400 km outlines the high velocity gradient transition zone its base lying at a depth of 650 km several layers with different velocities velocity gradients and wave attenuation are distinguished in the upper mantle they likewise differ in their inner structure for instance the uppermost 50 70 km of the mantle are divided into blocks with velocities from 7 9 8 1 to 8 4 8 6 km s 1 comparison of the travel time curves for the siberian long range profile with those compiled from seismological data for europe distinguished large scale upper mantle inhomogeneities of the eurasian continent and allowed for the correlation of tectonic features and geophysical fields the velocity heterogeneity of the uppermost 50 100 km of the mantle correlates with the platform age and heat flow i e the young plates of western europe and western siberia have slightly lower velocities and higher heat flows than the ancient east european and siberian platforms at greater depths 150 250 km the upper mantle velocities increase from the ocean to the inner parts of the continent the structure of the transition zone differs significantly beneath western europe and the other parts of eurasia the sharp boundary at a depth of 400 km traced throughout the whole continent as the boundary reflecting intensive waves transforms beneath western europe into a gradient zone this transition zone feature correlates with positions of the north atlantic west europe geoid and heat flow anomalies

electrical lithosphere beneath the kaapvaal craton southern africa 1 a regional scale magnetotelluric mt experiment across the southern african kaapvaal craton and surrounding terranes called the southern african magnetotelluric experiment samtex has revealed complex structure in the lithospheric mantle large variations in maximum resistivity at depths to 200 250 km relate directly to age and tectonic provenance of surface structures within the central portions of the kaapvaal craton are regions of resistive lithosphere about 230 km thick in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography but thinner than inferred from seismic body wave tomography the mt data are unable to discriminate between a completely dry or slightly damp a few hundred parts per million of water structure within the transitional region at the base of the lithosphere however the structure of the uppermost 150 km of lithosphere is consistent with enhanced but still low conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for kaapvaal xenoliths the electrical lithosphere around the kimberley and premier diamond mines is thinner than the maximum craton thickness found between kimberley and johannesburg pretoria the mantle beneath the bushveld complex is highly conducting at depths around 60 km possible explanations for these high conductivities include graphite or sulphide and or iron metals associated with the bushveld magmatic event we suggest that one of these conductive phases most likely melt related sulphides could electrically connect iron rich garnets in a garnet rich eclogitic composition associated with a relict subduction slab

very long period magnetotellurics at tucson observatory implications for mantle conductivity in a companion paper egbert et al this issue we describe the estimation of very long period 0 16 t 91 days magnetotelluric mt impedances from 11 years of data collected at the tucson geomagnetic observatory here we discuss the implications of these data for mantle conductivity using minimum norm flattest and smoothest inversions we find simple one dimensional models of electrical conductivity in the depth range 0 1500 km we use forward modeling a linearized resolution analysis and constrained one dimensional inverions to delineate the range of models which are consistent with the estimated impedances although the mt data have limited resolution large scale vertical averages of mantle conductivity are well constrained we reach the following conclusions concerning mantle conductivity beneath tucson 1 the upper 200 km has a conductance of order 104 siemens s this anomalously high conductance may be concentrated in an aesthenospheric high conductivity layer but the geometry of the conductive zone is not constrained 2 typical conductivities in the transition zone 400 700 km are 0 1 0 3 s m 1 a step increase to reach this value at or near the 400 km olivine spinel phase transition is consistent with but not required by the data an upper mantle which is resistive throughout 0 05 s m 1 or less is not allowed by the data 3 resolvable large scale averages of conductivity increase from 0 2 s m 1 to 1 0 s m 1 between 600 and 900 km depth a range of models including those with step increases and step decreases at the 670 km seismic discontinuity are consistent with the data 4 between 900 1500 km conductivity increases slowly average conductivities in this region are of the order of 1 s m 1 to within a factor of 2 or 3 while limited zones of highly resistive mantle are consistent with the data a lower mantle which is resistive throughout is not models in which conductivity is always above 5 s m 1 below 1000 km can also be ruled out in conjunction with improved laboratory estimates of electrical conductivities of mantle minerals at high temperatures and pressures these constraints can provide important clues to the composition and physical state of the mantle

lithospheric dismemberment and magmatic processes of the great basin colorado plateau transition utah implied from magnetotellurics 1 to illuminate rifting processes across the transition zone between the extensional great basin and stable colorado plateau interior we collected an east west profile of 117 wideband and 30 long period magnetotelluric mt soundings along latitude 38 5 n from southeastern nevada across utah to the colorado border regularized two dimensional inversion shows a strong lower crustal conductor below the great basin and its transition zone in the 15 35 km depth range interpreted as reflecting modern basaltic underplating hybridization and hydrothermal fluid release this structure explains most of the geomagnetic variation anomaly in the region first measured in the late 1960s hence the transition zone while historically included with the colorado plateau physiographically possesses a deep thermal regime and tectonic activity like that of the great basin the deep crustal conductor is consistent with a rheological profile of a brittle upper crust over a weak lower crust in turn on a stronger upper mantle jelly sandwich model under the incipiently faulted transition zone the conductor implies a vertically nonuniform mode of extension resembling early stages of continental margin formation colorado plateau lithosphere begins sharply below the western boundary of capitol reef national park as a resistive keel in the deep crust and upper mantle with only a thin and weak moho level crustal conductor near 45 km depth several narrow steep conductors connect conductive lower crust with major surface faulting some including modern geothermal systems and in the context of other great basin mt surveying suggest connections between deep magma sourced fluids and the upper crustal meteoric regime the mt data also suggest anisotropically interconnected melt over a broad zone in the upper mantle of the eastern great basin which has supplied magma to the lower crust consistent with extensional mantle melting models and local shear wave splitting observations we support a hypothesis that the transition zone location and geometry ultimately reflect the middle proterozoic suturing between the stronger yavapai lithosphere to the east and the somewhat weaker mojave terrane to the west we conclude that strength heterogeneity is the primary control on locus of deformation across the transition zone with modulating force components

magnetotelluric investigations of the lithosphere beneath the central rae craton mainland nunavut canada new magnetotelluric soundings at 64 locations throughout the central rae craton on mainland nunavut constrain 2 d resistivity models of the crust and lithospheric mantle beneath three regional transects responses determined from colocated broadband and long period magnetotelluric recording instruments enabled resistivity imaging to depths of 300 km strike analysis and distortion decomposition on all data reveal a regional trend of 45 53 but locally the geoelectric strike angle varies laterally and with depth the 2 d models reveal a resistive upper crust to depths of 15 35 km that is underlain by a conductive layer that appears to be discontinuous at or near major mapped geological boundaries surface projections of the conductive layer coincide with areas of high grade archean metasedimentary rocks tectonic burial of these rocks and thickening of the crust occurred during the paleoproterozoic arrowsmith 2 3 ga and trans hudson orogenies 1 85 ga overall the uppermost mantle of the rae craton shows resistivity values that range from 3000 m in the northeast beneath baffin island and the melville peninsula to 10 000 m beneath the central rae craton to 50 000 m in the south near the hearne domain near vertical zones of reduced resistivity are identified within the uppermost mantle lithosphere that may be related to areas affected by mantle melt or metasomatism associated with emplacement of hudsonian granites a regional decrease in resistivities to values of 500 m at depths of 180 220 km increasing to 300 km near the southern margin of the rae craton is interpreted as the lithosphere asthenosphere boundary

the electrical structure of the slave craton the slave craton in northwestern canada a relatively small archean craton 600400 km is ideal as a natural laboratory for investigating the formation and evolution of mesoarchean and neoarchean sub continental lithospheric mantle sclm excellent outcrop and the discovery of economic diamondiferous kimberlite pipes in the centre of the craton during the early 1990s have led to an unparalleled amount of geoscientific information becoming available over the last 5 years deep probing electromagnetic surveys were conducted on the slave using the natural source magnetotelluric mt technique as part of a variety of programs to study the craton and determine its regional scale electrical structure two of the four types of surveys involved novel mt data acquisition one through frozen lakes along ice roads during winter and the second using ocean bottom mt instrumentation deployed from float planes the primary initial objective of the mt surveys was to determine the geometry of the topography of the lithosphere asthenosphere boundary lab across the slave craton however the mt responses revealed completely serendipitously a remarkable anomaly in electrical conductivity in the sclm of the central slave craton this central slave mantle conductor csmc anomaly is modelled as a localized region of low resistivity 10 15 v m beginning at depths of f80 120 km and striking ne sw where precisely located it is spatially coincident with the eocene aged kimberlite field in the central part of the craton the so called corridor of hope and also with a geochemically defined ultra depleted harzburgitic layer interpreted as oceanic or arc related lithosphere emplaced during early tectonism the csmc lies wholly within the ne sw

electrical conductivity distribution of the lithosphere in the central fennoscandian shield abstract geoelectric information on the crustal structures of the central fennoscandian shield consists of magnetovariational mv and magnetotelluric mt data mv data are obtained from magnetometer arrays devised to map lateral variations of subsurface resistivity mt data are obtained from soundings yielding knowledge on vertical resistivity distribution in the form of cross sections the electromagnetic data base for fennoscandia includes data from about 350 magnetometer sites and about 600 magnetotelluric soundings the results show that the crustal resistivity of the archaean and palaeoproterozoic crust varies greatly both laterally and vertically the model resistivities range from several tens of thousands of m to below 1 m which are extreme values amongst those normally observed in crustal rocks several large elongated and inclined crustal conductors which mark fossil tectonic boundaries have been discovered in the shield these include 1 the lake ladoga outokumpu kainuu schist belt and oulu conductors that form a discontinuous conductive boundary between the domains of archaean and palaeoproterozoic crust 2 the southern finland conductor caused partly by nearly vertical several kilometres deep graphite bearing metasedimentary layers that were tilted and deeply buried in a svecofennian collision 3 the kokkola bothnian skelleftea storavan and oulu conductors representing a collisional belt in a northwards accreted palaeoproterozoic terrain and 4 the lapland granulite belt conductors imaging a sequence of sheared thrust sheets that developed as the granulites were upthrusted southwestwards onto the archaean craton zones of enhanced electrical conductivity delimit more resistive and homogeneous crustal blocks including the archaean kuhmo block in eastern finland and the svecofennian central finland granitoid complex cfgc highly resistive upper crust 30 000 m in the cfgc coincides with the granitoid intrusions groundwater which fills the fractures and pores of the granitoids is non conductive 100 m and is thus not able to enhance the bulk conductivity therefore the resistivity of the upper crust approaches that of dry rocks at depths of 10 to 14 km a decrease in resistivity by one order of magnitude coincides with a seismic low velocity vp zone that appears related to a porous fractured zone filled with saline water a lower crustal conductor beneath the cfgc at depths of 30 to 40 km has a thickness of 10 to 15 km and a resistivity of 20 to 100 m it is most probably caused by graphite that can have been precipitated from co2 rich fluids during the peak metamorphism of the svecofennian orogeny and the subsequent cooling period free fluids trapped in the lower crust appear less plausible magnetotelluric soundings in the resistive cfgc in the central fennoscandian shield have not provided indications of a well developed electrical asthenosphere within the uppermost 300 km

seismic and electrical anisotropies in the lithosphere across the grenville front canada the grenville front is a prominent tectonic feature of the canadian shield which has been the focus of a lithoprobe transect a strong electrical anisotropy has been detected in a frequency range of 0 1 0 01 hz 50 to 150 km the most conductive direction being oriented n80e in order to compare electrical and seismic anisotropies ten seismometers have been deployed across the grenville front to record teleseismic events s wave splitting measurements show an average delay of 1 54 0 21 s between the fast and the slow components the fast axis direction is approximately e w at all stations showing a strong coherence between the stations along the complete transect this azimuth correlates well with that of the electrical anisotropy the electrical anisotropy originates in the subcrustal lithosphere between 50 and 150 km the similarity of the magnetotelluric and seismic anisotropy directions across the grenville front is an argument in favor of an origin of the seismic anisotropy in the subcrustal lithosphere as advocated by silver and kaneshima 1993

obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle teleseismic shear wave splitting and magnetotelluric experiments across the grenville front between the archean craton and the proterozoic grenville province in the regions of the pontiac subprovince and northwestern grenville province canada show a consistent obliquity between the polarization direction of the fast split shear wave and the most electrically conductive direction mt in the upper mantle transcurrent shear zones at all well recorded stations is nearly n103 e and mt is approximately n80 e the obliquity may be considered a potential kinematic indicator because the seismic and electrical anisotropies are thought to be controlled by lattice preferred and shape preferred orientations of mantle minerals mainly olivine respectively the dextral movement sense of the transcurrent shear zones in the mantle inferred from the observed obliquity is consistent with that inferred from surface geology of the crustal shear zones this consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region

olivine flotation in mantle melt abstract molten komatiite and peridotite have been compressed in an octahedral multi anvil device up to 10 gpa densities of the melts were measured at pressure intervals in the range 7 to 10 gpa by observing sinking and floating san carlos olivines and synthetic forsterite marker spheres the multi anvil results for komatiite when combined with piston cylinder measurements done at 4 to 6 gpa and a calculated reference density at 105 pa yield a birch murnaghan isothermal bulk modulus of k1900c 26 gpa and pressure derivativek 4 25 the pressure of neutral buoyancy for olivine in komatiite is confirmed to be near 8 gpa as predicted in earlier work olivine flotation in the experimental komatiite commences at a pressure close to where the liquidus phase changes from olivine to denser garnet leading to the possibility of density driven crystal sorting during fractionation molten peridotite klb 1 shows an isothermal compression 2000 c of 0 065 g cm 3 gpa 1 in the interval 105 pa to 8 2 gpa the olivine liquid peridotite density crossover is predicted to lie between 9 and 11 gpa indicating that olivine flotation can operate at depths of 300 500 km in a molten peridotitic mantle

inversion of ocean bottom magnetotelluric data revisited three ocean bottom magnetotelluric data sets from sites on the pacific plate are reinterpreted the initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site that work also suggested that the resistivity increased below the conductor this analysis which includes new methods for constructing one dimensional conductivity models shows that the postulated increase in resistivity is not demanded by the data it also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data the previously reported trends with lithospheric age still exist but they are not as strong as initially believed finally it is shown rigorously that the different age sites are distinct in that no one dimensional model can account for all three data sets

europe from the bottom up a statistical examination of the central and northern european lithosphere asthenosphere boundary from comparing seismological and electromagnetic observations abstract the lithosphere asthenosphere boundary lab is a fundamental boundary in the plate tectonic paradigm it is the most pervasive on the planet yet comparatively it is one we know little about defined initially on the basis of the mechanical response of the earth to loading its usage has become ubiquitous across the geosciences but the natural differences in its definition due to differences in thermal physical and chemical parameters cause confusion to advance this debate comparisons are made both qualitatively and quantitatively between the delineation of the lab for europe based on seismological and electromagnetic observations we examine statistically using robust methods the labs derived from independent datasets and methods essentially all definitions of the lab as an impedance contrast from receiver functions slabrf a seismic anisotropy change slaba and an increase in conductivity from magnetotellurics elab are consistent with a deeper lab beneath precambrian europe and a shallower lab beneath phanerozoic europe with some exceptionally deep regions in phanerozoic europe such as the alps all three labs increase in depth significantly and rapidly at a location consistent with the surface expression of the trans european suture zone tesz which separates precambrian europe to the north and east from phanerozoic europe in the centre two of the definitions slabrf and elab are consistent for phanerozoic europe with mean values of 90 100 km compared to an average slabrf of 135 km a different two slaba and slabrf are consistent for precambrian europe with mean values of 170 180 km compared to an elab mean of 250 km the deeper elab depths for precambrian europe are more consistent with body wave and surface wave tomography models than are the slaba and slabrf ones suggesting that the lower lithosphere beneath precambrian europe between 170 to 250 km is responding to shearing on the base of the lithosphere from plate driving forces taken together the definitions provide strong constraints on the nature of the lab for example the electrical elab beneath the tesz is anomalously thick to possibly the transition zone whereas slaba and slabrf seismological definitions would place it at far shallower depths given the sensitivity of electrical conductivity to partial melt and water content the thick elab beneath the tesz excludes interpretation of the seismic lab in that location in terms of a thermal structure that would induce partial melting or of hydration

northeastern pacific mantle conductivity profile from long period magnetotelluric sounding using hawaii to california submarine cable data we present results of a long period magnetotelluric mt investigation of the electrical structure beneath the eastern north pacific the electric field data consist of 2 years of continuously recorded voltages across an unpowered 4000 km long submarine telephone cable haw 1 extending from point arena california to oahu hawaii the electric field measurements are coherent to some degree with magnetic field measurements from honolulu observatory at periods of 0 1 to 45 days this coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length significantly diminishing their effect robust controlled leverage mt response estimates and their jacknife confidence limits are computed for the haw 1 to honolulu data an equivalent scalar mt response obtained from honolulu magnetic variations data is used to correct the haw 1 mt response for static shift and to extend the mt response estimate to periods of 100 days the composite response function satisfies necessary and sufficient conditions for consistency with a one dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km inversion of the mt response reveals a conductive zone 0 05 0 1 s m between 150 and 400 km depth and a positive gradient below 500 km these observations are consistent with previous mt studies in the north pacific this upper mantle conductivity is too high to be explained by solid state conduction in dry olivine using reasonable mantle geotherms calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that h dissolved in olivine possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy provide sufficient conductivity enhancement to explain the inversion results the high conductivity may also be explained by the presence of gravitationally stable partial melt comparison of the haw 1 results with long period mt studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes in particular the upper mantle beneath the pacific ocean is considerably more conductive than that beneath the canadian shield and similar in conductivity to that beneath the basin and range

on the electrical crust mantle structure in fennoscandia no moho and the asthenosphere revealed summary an interpretation of the geomagnetic inductive response function c 0 observed at kiruna in northern sweden is herein undertaken the bounds of acceptable solutions are initially discovered by a monte carlo random search procedure and the best fitting solutions are examined by the application of linear theory to the problem the data are shown to have a higher degree of internal consistency than that described by the estimated variances of each datum a further monte carlo inversion of the variance reduced data set gives solutions with well defined model parameters the two major features of the models are 1 a small or non existent electrical conductivity variation across the seismic moho boundary and 2 the unequivocal existence of an electrical asthenosphere under the fenno scandian shield beginning at a depth of between 155 185 km and of 60km minimum thickness both of these observations have seismic counterparts finally possible mantle temperature profiles are deduced which depend on the assumptions and laboratory data employed

mid paleocene dropstones in the whangai formation new zealand evidence of mid paleocene cold climate abstract sedimentological micropalaeontological and organic geochemical rock eval pyrolysis criteria suggest that calving glaciers probably from antarctica likely reached the sea and floated by new zealand during the middle paleocene dropping angular coarse detritus into marine mud the dropstone event coincides with marked shifts in organic geochemistry of the associated marine shale as well as changes in foraminiferal productivity the implied cool climate event may have only been a short term climatic oscillation perhaps of only thousands of years however it was significant enough to have affected the biota and organic matter in the water column the dropstones may represent a deep water example of a correlative conformity referred to in sequence stratigraphy literature although the dropstones are easily recognizable in the sediments the interpretation relies on associated palaeontological and organic geochemical data sea level curves presented elsewhere in the literature show mid paleocene sea level falls which approximately coincide with the time when the dropstones in eastern new zealand were deposited

the lithosphere asthenosphere system in fennoscandia abstract we present a summary of the available information on rayleigh wave dispersion data for the fennoscandian region the observations have been combined to produce regional dispersion relations which have then been subjected to the hedgehog inversion procedure the results are presented on a map outlining the thickness of the lid and the shear velocities in both the lid and the asthenosphere channel lid thickness up to around 135 km is found in the bothnia north central finland area with if any weak shear velocity contrast to the underlying layer the surrounding areas are characterized by lid thickness up to around 75 km a stronger low velocity zone to lid contrast may be found in the caledonian and baltic sea area 0 25 0 45 km s taking into account moho depth data and the aforementioned results a map of the lithosphere asthenosphere system was derived

lithospheric structure evolution and diamond prospectivity of the rehoboth terrane and western kaapvaal craton southern africa constraints from broadband magnetotellurics abstract a 1400 km long 2 d magnetotelluric mt profile consisting of 69 sites at 20 km intervals across the western part of the archaean kaapvaal craton the proterozoic rehoboth terrane and the late proterozoic early phanerozoic ghanzi chobe damara belt reveals significant lateral heterogeneity in the electrical resistivity structure of the southern african lithosphere the lithospheric structures of the rehoboth terrane and ghanzi chobe damara belt have not been imaged previously by geophysical methods temperature is the primary control on the resistivity of mantle minerals and the mt derived lithospheric thicknesses therefore provide a very reasonable proxy for the thermal thickness of the lithosphere i e the thickness defined by the intersection of a conductive geotherm with the mantle adiabat allowing approximate present day geotherms to be calculated the work indicates the following present day average lithospheric thicknesses to a precision of about 20 km for each of the terranes traversed inferred geotherms in brackets eastern kimberley block of the kaapvaal craton 220 km 41 mw m 2 western kimberley block 190 km 44 mw m 2 rehoboth terrane 180 km 45 mw m 2 and ghanzi chobe damara belt 160 km 48 mw m 2 a clear relationship between the electrical resistivity structure of the lithosphere and the tectonic stabilisation age of the terrane is evident good agreement between the inferred present day lithospheric geotherms and surface heat flow measurements indicate the latter are strongly controlled by variations in lithospheric thickness a significant difference in lithospheric thickness is observed between the eastern and western kimberley blocks and is consistent with previous seismic tomography images of the kaapvaal craton the present day lithospheric thickness and reduced depth extent into the diamond stability field accounts for the absence of diamondiferous kimberlites in the gibeon and gordonia kimberlite fields in the rehoboth terrane previously published mantle xenolith p t arrays from the gibeon gordonia and kimberley fields however suggest that the rehoboth terrane had equilibrated to a cooler conductive palaeo geotherm 40 42 mw m 2 very similar to that of eastern kimberley block of the kaapvaal craton at some time prior to the mesozoic eruption of the kimberlites the timing and nature of both the thermal equilibration of the rehoboth terrane and the subsequent lithospheric heating thinning event required to account for its present day lithospheric structure are not well constrained a model consisting of the penetration of heat transporting magmas into the lithosphere with associated chemical refertilisation at an early stage of mesozoic thermalism appears to be the most plausible model at present to account for both the present day lithospheric structure of the rehoboth terrane and an earlier cooler palaeo geotherm some problems however remain unresolved in terms of the isostatic response of the model based on a compilation of xenocryst cr ca in pyrope barometry observations the extent of depleted mantle in the rehoboth terrane is found to be significantly reduced with respect to the eastern kimberley block 117 km versus 138 167 km it appears most likely that the depletion depth in both terranes at least in the vicinity of kimberlite eruption is explained by refertilisation of the lower lithospheric mantle

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

tectonics of the northern canadian cordillera imaged using modern magnetotelluric analysis abstract magnetotelluric data from lithoprobe snorcle corridor 2 in western canada crossing the rocky mountain trench tintina fault between 56 and 60 n have been analyzed using modern mt methods to provide new insights into the tectonics of the north american cordillera and ancestral north america resistivity images of the crust were obtained using two independent sets of unconstrained 2 d inversions and constrained 2 d inversions incorporating seismic constraints structure to the west of the tintina fault includes uniform moderate mid to lower crustal resistivity in a seismically defined westward tapering wedge of ancestral north american rocks to the east between the tintina fault and the liard line a transfer zone in asymmetric neoproterozoic rifting the wedge includes a major synformal conductor that parallels seismic reflections the large lateral changes in resistivity can be explained by transition between different blocks of asymmetric rifting the synformal conductor is attributed to graphitic rocks from the aida formation of the mesoproterozoic muskwa assemblage that were re distributed in a sag basin on the lower plate margin in the blackwater block spatially complex upper crustal resistivity in the cordillera can be interpreted in terms of lithological variations and alteration processes an east dipping conductor in stikinia is attributed to east dipping jurassic and relatively high conductivity beneath the eskay rift may be explained by crustal fertilization a conductor in the cache creek terrane is attributed to listwanite alteration of ultramafic bodies on the king salmon fault and high conductivity in the slide mountain terrane within the sylvester allochthon to localized carbonaceous alteration a series of conductors above north american basement is attributed to deformation and metamorphic processes associated with mesozoic thrust faulting to the west of the tintina fault the source material for these conductors is likely from the stikinia terrane and to the east it is likely from the muskwa assemblage