anisotropy of compressional wave velocities complex electrical resistivity and magnetic susceptibility of mylonites from the deeper crust and their relation to the rock fabric abstract laboratory measurements of compressional wave velocities v p complex electrical resistivity and magnetic susceptibility have been carried out on a mylonitic clinopyroxene amphibolite mca and an epidote muscovite quartz ultramylonite mqu from the ivrea zone both mylonites exhibit significant anisotropies that are related to the rock fabric at low pressure of about 50 mpa the anisotropy of v p for the mca and mqu is 17 and 34 respectively it is due to oriented microcracks intragranular transgranular grain boundary cracks and cleavage cracks in addition to preferred orientation of anisotropic rock forming minerals at high pressure 600 mpa where most of the cracks are closed a residual anisotropy of about 10 and 12 respectively is observed that is mainly controlled by preferred orientations of mineral constituents the anisotropies of the complex electrical resistivity measured with an electrolyte tap water at 0 1 mpa are 40 and 36 for the mca and mqu whereas at 80 mpa these values increase to 44 for both samples in contrast measured with an electrolyte of 0 1 m nacl solution the resistivity is lower and gives anisotropies of 71 mca and 63 mqu at 0 1 mpa the anisotropies of the magnetic susceptibility ams are about 27 mca and 14 mqu for both mylonites the axes of the ams ellipsoid coincide with the macroscopic fabric elements where the main axis of magnetic susceptibility k max lies approximately parallel to the lineation an intercorrelation of the different physical properties in terms of rock composition and anisotropic rock fabric is difficult nevertheless the laboratory determination of anisotropic in situ physical properties provide more objective data for the modelling of in situ crustal conditions

crustal and upper mantle electrical conductivity structure in southern scotland summary magnetotelluric measurements were made during 1978 79 at nine sites on a linear traverse across south scotland extending from kinloch rannoch in the highlands to borthwickbrae in the southern uplands both one and two dimensional modelling of the apparent resistivities and phases indicate the existence of a low resisitivity 75 am zone at 20 90 km depth beneath the southern uplands rising closer to the surface beneath the midland valley to the north a zone of slightly higher resistivity 500s2m but still relatively conducting compared to the surrounding structure is shown to exist beneath the highlands the real part of the ratio of vertical to horizontal magnetic fields is shown to be suitably modelled by a combination of induction and conduction

electrical conductivity models for the continental crust based on laboratory measurements on high grade metamorphic rocks summary deep electrical conductivity soundings are increasingly being used as an additional source of information regarding the nature of the lower continental crust however a lack of relevant laboratory based conductivity measurements makes interpretation of such soundings difficult laboratory measurements have been made on saturated and unsaturated samples of possible lower crustal rock types subjected to confining pressures up to 0 4 gpa temperatures up to 300 c and with variable pore fluid pressure up to the confining pressure extrapolation of these results suggests that the surprisingly high conductivities deduced for depths of approximately 20 km in certain stable continental areas may result from a combination of basic rock type and high pore fluid pressures for whereas the conductivities measured in acid rock types can be explained in terms of conduction through the pore fluid alone the conductivities measured in basic rock types imply enhanced conduction presumably through the matrix or along grain boundaries the lower conductivities deduced from field experiments for the upper crust may be due to more acid rock types and or lower pore fluid pressures perhaps due to hydration reactions in areas where the high conductivity layer is coincident with a low velocity layer an explanation in terms of changing pore fluid pressure i e low to high with increasing depth seems more likely

electrical resistivity structure of the upper mantle beneath northeastern china implications for rheology and the mechanism of craton destruction abstract the north china craton ncc and central asian orogen belt caob in northeastern china experienced a range of tectonic events during the phanerozoic dominated by lithospheric thinning of the eastern ncc in the late mesozoic and cenozoic in order to better understand the tectonic evolution of the ncc and the caob new broadband and long period magnetotelluric data were collected along a north west to south east trending profile that extended from the caob across the yanshan belt the tanlu fault zone to the liaodong peninsula a two dimensional 2 d resistivity model was derived from inversion of the transverse electric mode transverse magnetic mode and vertical magnetic field data in the crust of the caob the resistivity model shows a northwest dipping low resistivity zone beneath the solonker suture that is identified as the suture zone formed by the collision between the siberian and north china cratons the upper mantle of the caob is characterized by moderate resistivity values 300 1000 m that are best explained by the presence of hydrogen dissolved in olivine the water concentration of the caob mantle is comparable to values reported for the asthenosphere and cratons that have been significantly hydrated the ncc upper mantle is generally lower in resistivity than the caob upper mantle and a zone of lower resistivity is observed in the upper mantle at the southeast end of the profile beneath the ncc superimposed on this southeast decrease in upper mantle resistivity three low resistivity zones were imaged 1 below the xilamulun fault 2 close to the north south gravity lineament and 3 between the northern yanshan belt and tanlu fault zone the low resistivities can be explained as regions of partial melts or fluids perhaps caused by asthenospheric upwelling together with seismic imaging results and geochemical data the resistivity model shows that the modification of the lithosphere associated with craton destruction has occurred in a spatially non uniform manner in the region of the ncc investigated in this study

now a bone then calcite abstract critical transformation and degradation of skeletal tissues most frequently affect their organic components while the mineral component of bones and fossils comprises one of the most stable mineral groups on earth the apatites the case we present here relates to fossil bones from palaeolake shores whose shapes are preserved within the sediment itself but which disintegrate into a chalky disaggregated substance at the slightest touch or breath the rarity of bone apatite transformation into calcite is due to the extreme insolubility and stability of bone mineral which is highly tolerant of ionic substitutions and inclusions without changing its basic mineral nature and composition observation of this chalky substance under high magnifications reveals a delicate structure of microcrystals these crystals enclose microtunnels and grooves identical in shape and size to the characteristic bone bioerosion caused by aquatic microorganisms this microtunnelling preservation allows us to confirm that disaggregate calcite crystals replaced areas where once there had been bone furthermore the bone chalky material transformation had to occur when microorganisms were still in the interior of these tunnels otherwise the delicate empty microtunnels observed today would have collapsed two calcareous palaeo lakeshore sites show identical phenomenon detailed taphonomic environmental and chemical studies indicate that the transformation process was initiated by microorganisms arising from a combination of their metabolism and the alkaline environment they inhabited that caused a local and sudden increase in local acidity this chalky material is referred to as biogenic in contrast to environmental calcite deposits geodes and calcite infilling bone fissures that formed in the lake conditions the calcium carbonate phase of both biological and environmental origin in these two palaeolakes is aragonite which forms under restricted situations of aridity and warming and may provide a precise indication of environmental conditions when these sites formed similar situations may occur today in arid areas

water exsolution from quartz implications for the generation of retrograde metamorphic fluids available data from the literature suggest that the solubility of h2o in nominally anhydrous minerals such as quartz should increase with increasing metamorphic grade if this is the case water should precipitate from the host mineral during cooling and uplift of regionally metamorphosed terranes calculations indicate that this mechanism could result in the release of significant quantities of h2o which would then be available to participate in retrograde hydration reactions and the formation of fluid inclusions

textural changes of graphitic carbon by tectonic and hydrothermal processes in an active plate boundary fault zone alpine fault new zealand graphitization in fault zones is associated both with fault weakening and orogenic gold mineralization we examine processes of graphitic carbon emplacement and deformation in the active alpine fault zone new zealand by analysing samples obtained from deep fault drilling project dfdp boreholes optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain raman spectroscopy allowed interpretation of the degree of graphite crystallinity which reflects both thermal and mechanical processes in the amphibolite facies alpine schist highly crystalline graphite indicating peak metamorphic temperatures up to 640 c occurs mainly on grain boundaries within quartzo feldspathic domains the subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions 500 600 c resulting in clustered in protomylonites and foliation aligned graphite in mylonites in cataclasites derived from the mylonitized schists graphite is most abundant 50 as opposed to 10 elsewhere and has two different habits inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon tectonic hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences the increasing abundance of graphite towards the fault zone core may be a significant source of strain localization allowing fault weakening

syngenetic thin graphite rich horizons in lower crustal rocks from the serre san bruno calabria italy and implications for the nature of high conducting deep crustal layers abstract in order to better understand the nature of deep crustal high electrical conductivity we studied the electrical properties of a tilted section of a former lower continental crust exposed in the calabrian arc of the alpine apennine mountain system geoelectric field measurements and impedance measurements on rock samples showed that these high grade metamorphic rocks are generally highly resistive as expected for crystalline electrolytically conducting rocks of low porosity this holds for graphite free metabasites as well as for metapelites which generally contain accessory up to 3 biogenic graphite in the form of isolated grains clearly as an exception a group of thin stratiform black horizons with thicknesses of 1 15 cm within the metapelitic series was detected by means of self potential sp measurements rock samples from these horizons exhibit high quasi metallic bulk conductivities of up to 50 s m 0 02 m in agreeement with up to 20 syngenetic graphite forming a network of interconnected streaks or crack fillings the high amount of carbon most probably originates from organic matter of c org rich black shales relative enrichment of the low mobility graphitic matter compared to the carbon content of the assumed protoliths may have been due to pressure solution and partial melting during prograde metamorphism without major contribution of a fluid phase resulting in isolated graphite flakes although enriched graphite in this form has little effect on electrical conductivity for the calabrian black horizons microscopic analyses make conceivable that in a further decisive step isolated graphite grains were mechanically smeared to continuous pathways during uplift by shearing producing hereby the observed graphitic network which is needed to generate high conductivity as c org rich black shales are common members of sedimentary sequences throughout the earth s history good conductors of this type may be expected in the continental crust at any depth depending on tectonic and metamorphic history with the exception of magmatic protoliths regarding the extremely high conductivity of the meta black shale samples containing syngenetic sheared graphite a total thickness of a few meters of such rocks is sufficient to explain magnetotelluric high conductivity anomalies in the deep crust

on the detectability of a highly conductive layer in the upper mantle beneath the black forest crystalline using magnetotelluric methods summary simultaneous geomagnetic and telluric variations 600 s 4 hr were observed at two stations on the rhine graben sediments and on the neighbouring black forest crystalline in order to extend the pulsation observations 10 600 s of former magnetotelluric mt surveys to longer periods these observations completely describe the anomalous magnetic and telluric fields in terms of a set of transfer functions using the black forest station as normal in the period range from 30 s to 4 hr the horizontal and vertical magnetic transfer functions of the rhine graben anomaly are characterized by continuously decreasing amplitudes the local schmucker induction arrows point uniformly 45 south east for all long period geomagnetic transfer functions and the nw se striking direction in the upper mantle derived from the induction arrows is also consistent with the preferred telluric direction in the black forest in both areas large offsets of apparent resistivity and phase curves for the e and b polarization directions are observed except that the phase curves converge above 400s in the long period range the interpretation of the period dependences of the observed magnetic and telluric transfer functions from 600 s to 4 hr by a 2 d conductivity model requires a highly conductive layer in the upper mantle at a depth of 120 km beneath the black forest this confirms bahr s 1992 model for a highly conductive layer in the upper mantle that has also been found south of the alps braitenberg et al 1994 and which is interrupted by low conductive alpine roots however no information about such a layer beneath the rhine graben can be derived 2 d model studies have shown that an improved depth resolution of highly conductive layers in the upper mantle is possible using telluric phases but the transfer functions for the vertical magnetic fields are not sensitive enough for the determination of the depth of an upper mantle conductive layer

fluid generation and pathways beneath an active compressional orogen the new zealand southern alps inferred from magnetotelluric data 1 forty one wideband magnetotelluric mt soundings were collected in a 150 km long transect across the southern alps of the central south island of new zealand an active compressional orogen decomposed mt impedance tensors vertical magnetic field relations and reconnaissance soundings at two locations off line imply an approximately two dimensional geometry here with average regional geoelectric strike of n40 e similar to surface geologic trends two independent two dimensional inversion algorithms were applied to the mt data and both imply a concave upward u shaped middle to lower crustal conductive zone beneath the west central portion of the island the average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along strike conductive strands narrower than the transverse magnetic cross strike mode mt data can resolve the deep crustal conductor under the southern alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust fluid interconnection and electrical conduction are promoted by shear deformation the conductor rises northwestward toward the trace of the alpine fault but attains a near vertical configuration at a depth of 10 km and reaches close to the surface 5 10 km inland of the fault trace itself the transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle ductile transition in uplifting schist and approach the surface through induced hydrofractures the high grade schist becomes resistive after depletion of fluids and continues to extrude toward the alpine fault shallow extensions of the deep high conductivity are coincident with modern hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance to the southeast high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent southern alps orogen which also exhibits evidence for expulsion of high temperature fluids the higher conductivity inferred along strike possible anisotropy could reflect more efficient fluid interconnection in this higher strain direction as well as possible contributions by sheared fluid deposited graphite conductivity of the uppermost mantle of the south island is low consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by p wave delay studies

the lithospheric structure of the solonker suture zone and adjacent areas crustal anisotropy revealed by a high resolution magnetotelluric study

metamorphic fluids and anomalous porosities in the lower crust abstract because of the plastic behavior of rocks at high temperature and confining pressure the porosity of lower crustal rocks is a variable dependent on the amount of free fluid present on the basis of hydrologic theory it is predicted that the permeability k of such rocks can be expressed by a relationship of the form k n where and n are material specific constants the exponent n is likely to vary between 3 and 6 whereas which is ultimately related to grain size and shape probably varies over several orders of magnitude consequently substantial contrasts between the k relationships of lower crustal rocks are to be expected the case in which thin crustal layers aquitards are characterized by relatively low values of or n with respect to the crust in general is of particular interest such layers would acquire high porosities in the presence of miniscule steady state crustal fluid fluxes and are a possible explanation for anomalous seismic brightspots and high electrical conductivities in the lower crust modelling of collision belt metamorphism suggests that even long after the cessation of active metamorphism and isostatic reequilibration relict metamorphic fluid fluxes could generate aquitard porosities between 0 1 and 0 4 adequate to explain such geophysical anomalies

optimization of signal to noise ratio in dc soundings geoelectric dc sounding methods are usually limited to electrode spacings smaller than 1 km which restrict the depth of investigation to only several hundreds of meters greater depths of investigation require both a larger electrode spacing and motor generator driven transmitters in order to increase the depth of investigation into the middle crust the distance between transmitter and receiver dipole must be increased up to 100 km as well instead of a schlumberger electrode configuration we use a dipole dipole electrode arrangement which avoids cable connections of several tens of kilometers it takes less logistics in the field and requires less precautions to control the cable circuits however the electric field of a grounded dipole decays by the power of 3 even if a strong current source with a power of 30 kw is employed at distances beyond 10 km the amplitude of the transmitted dc signal becomes considerably smaller than the naturally inductive field and cultural noise here we present a technique which resolves the transmitted dc signal at sites up to 60 km apart using an electric dipole source the suggested technique removes the inductive part in the time series and therefore reduces the noise level it requires applying the magnetotelluric technique before and or after the geoelectrical sounding both the magnetic and electric variational fields are recorded during geoelectrical sounding if the magnetotelluric transfer function is obtained at a site then the natural inductive electric field can be predicted which in turn is used to remove the inductive electrical part from the receiver dipole record

electrical properties of granite with implications for the lower crust the electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure structurally bound water oxygen fugacity and other parameters the effect of sulfur fugacity may be important but is experimentally unconfirmed in addition to changing the magnitude of electrical properties the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties with increasing temperature changes in water content retain large but lessened effects on electrical properties near room temperature a monolayer of water will decrease the electrical resistivity by an order of magnitude several weight percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5 at elevated temperatures just below granitic melting a few weight percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2 above the melting temperature 650 to 1100 c depending upon water pressure a few weight percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy remarkably the few weight percent water must be present as free water experiments with hydrated hornblende schist with structural water indicate an electrical resistivity very similar to that for dry granite the implications of these results together with the findings of deep magnetic sounding and magnetotelluric surveys suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle

scale integrated architecture of a world class gold mineral system the archaean eastern yilgarn craton western australia abstract world class mineral systems such as those found in the archaean eastern yilgarn craton are the product of enormous energy and mass flux systems driven by lithospheric scale processes these processes can create big footprints or signatures in the lithosphere which can be observed at a range of scales and via a variety of methods including geophysics isotopes tectonostratigraphy and geochemistry we use these datasets to describe both the architecture structure of world class gold systems of the yilgarn craton and the signatures of their formation by applying an understanding of the most critical elements of the process and their signatures new areas especially undercover may be targeted more predictably than before knowledge of the major architectural elements of the yilgarn craton has increased greatly over the last decade through the collection of a wide range of geophysical geochemical and isotopic data these data range in scale from 1 lithospheric scale which provide information on the entire craton and their underlying volumes down to depths in excess of 350 km through 2 regional scale which provide information at the province scale and to depths of 30 40 km to 3 mine and camp scale which provide information on the top few kilometres of the crust the architecture structure in the upper mantle and lower crust can be inferred from broadband seismic tomography and analysis of long wavelength gravity data geophysical data such as magnetotellurics and deep crustal reflection seismic also provide evidence for the signatures of the flow of fluids through this architecture the pathways and they illustrate that the scale of the system is many orders of magnitude larger than the immediate deposit itself of particular importance is the role of deep crustal penetrating shear zones or faults which link the mantle with domes in the upper crust together these data provide first order insights into the physical and chemical properties of the yilgarn craton these insights provide geodynamic clues and constraints as to why the yilgarn is so well endowed in metals such as gold groves et al 1989 the intersection of favourable architectural features provides connections between the mantle and the upper crust in the kalgoorlie region possibly explaining why this area is so well endowed in comparison to others

a review of geomagnetic variation measurements in the eastern united states implications for continental tectonics geomagnetic variation data for all available stations in eastern north america are presented here in the form of induction arrow maps at periods of 6 20 and 64 min the region is subdivided on th

geoelectromagnetic measurements across the moine thrust and the great glen in northern scotland summary results are presented for a magnetotelluric and magnetovariational study in northern scotland for nine of the 20 sites at which observations were made the results of the analysis are presented fully together with one dimensional 1 d monte carlo inversions for representative sites from the lewisian foreland the north west highlands the great glen region and the grampian highlands for the remaining sites the results are summarized the 1 d models of all sites situated in a traverse extending from the lewisian foreland to the neighbourhood of the highland boundary fault have been used as the basis for preliminary 2 d modelling maps of magnetic field induction vectors and transfer functions for an inducing field perpendicular to the average geological strike are presented for periods of 23 819s the 2 d conductivity models were constrained by both the magnetotelluric and magnetovariational responses the period range of observation in this study was too limited to resolve structural differences in the upper crust but adequate to reveal a moderately good conductor at upper mantle lower crustal depths in the caledonian metamorphic zone in contrast the whole of the crust in the lewisian foreland is highly resistive and there is no evidence of an intermediate conducting layer between the crust and mantle in this region

exhumed lower crust in nw ireland and a model for crustal conductivity granulite facies gneisses within the ox mountains inlier nw ireland are interpreted as an exhumed segment of sub dalradian lower crust formed at 605 37 ma and later wedged upwards by crustal imbrication along the n dipping highland boundary fault clew bay terrane boundary a lower crustal provenance for the gneisses is suggested by evidence for eclogite decompression and for the later replacement of sillimanite by kyanite in response to slow cooling at depth granulite facies rocks are believed to be present throughout much of the lower continental crust granulites are electrically resistant in contrast to the lower crust which is usually electrically conducting this paradox may be resolved by inferring that the granulite facies rocks form dry kilometre scale lenses mega augen wrapped around by anastomosing retrograded brine soaked electrically conducting shear zones retrogressive hydration is believed a to seal the margins of the dry mega augen with expanded reaction products and b to concentrate the residual brine amphibolite facies rocks associated with the lower crustal shear zones above about 350 c would equilibrate with strong unsaturated brine and become electrically conducting in the upper crust similar rocks would concentrate the brine to the point of saturation whereupon halite would precipitate all water be consumed and the rock would become an insulator

deep crustal geoelectric structure beneath the northumberland basin in terms of lateral variations in conductivity structure the southern southern uplands and northumberland basin are characterized by a region of attenuated vertical magnetic fields with small spatial gradients reflecting the presence of a substantial conducting zone five magnetotelluric data sets from the region have been analysed to provide accurate and unbiased estimates of the impedance tensor the response data are used to investigate the deep geoelectric crustal structure of the region three appropriate sets of response data have been subjected to two construction algorithms for 1 d inversion the geoelectric profiles recovered identify a deep crustal conducting zone underlying the northumberland basin the zone modelled as a layered structure dips steeply from mid crustal depths underneath the northumberland basin to lower crustal depths to the nw the structure thus correlates in location and geometry with a deep crustal reflecting wedge detected offshore by a deep seismic reflection profile

geomagnetic variation studies in the southern appalachians preliminary results magnetic field variations have been recorded at 12 stations located on a traverse across the blue ridge and the piedmont provinces of the southern appalachian system in georgia the records were fourier analyzed and the transfer functions relating the anomalous vertical magnetic field variations to the horizontal field were obtained for different periods ranging between 16 and 4000 s the transfer functions for the southeasternmost stations seem to be affected by the coast effect and possibly by the coastal plain sediment in the northwestern part of the survey the direction of the transfer functions was away from the coast this suggests the presence of a strong conductor in the lower crust or the upper mantle beneath the blue ridge and extending farther toward the northwest

a deep crustal electromagnetic sounding in the georgia piedmont electromagnetic soundings in the georgia piedmont using a horizontal loop source indicate an upper layer with conductivity variations from less than 2 10 5 to 2 5 10 4 s m extending to about 15 km underlain by a zone with conductivity of more than 10 3 s m at one site the conductance of any intermediate layer is limited to 2 s to a depth of 15 km the conductivities of the upper layer agree with those expected from the surface geology and the increase in conductivity occurs at a depth which correlates generally with reflections detected by the consortium for continental reflection profiling s seismic profile of the area a possible source of the high conductivity is water saturated sediments trapped when the region was overthrust in the appalachian orogeny

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

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

electrical resistivity structure of the eastern moroccan atlas system and its tectonic implications the electrical resistivity structure of the crust and upper mantle of the atlas mountain system was studied using magnetotelluric and geomagnetic deep soundings field experiments were done in eastern morocco along a traverse from the anti atlas to the rides rifaines in two campaigns in 1983 and 1988

a preliminary geoelectrical model of the karelian megablock of the baltic shield abstract three groups of electromagnetic data have been considered in order to construct a preliminary geoelectrical depth model of the old basement of the baltic shield i audiomagnetotelluric data in the period range 1 3700 1 8 s ii magnetotelluric data in the period range 25 1000 s and iii global magnetovariation sounding data in the period range 6 h these data sets fit together on a common apparent resistivity curve indicating the existence of a unified geoelectrical model this model does not support the existence of conducting crustal or asthenospheric layers

magnetometer array studies and deep schlumberger soundings in the damara orogenic belt south west africa summary a zone of concentrated induced electric currents crossing parts of zimbabwe botswana and south west africa was discovered during a magnetovariational study conducted in 1972 in 1977 a second study was made with 27 recording magnetometers distributed across the width of south west africa between latitudes 19 and 22 s several geomagnetic disturbances were recorded with high recording efficiencies three of these time sequences were digitized for analysis magnetograms and fourier transform amplitude and phase maps in the period range 22 128 min were used to delineate the westward continuation of the conductive structure revealed by the earlier investigation the conductive zone runs approximately east west from the botswana border 21oe to 17 e longitude from here to the atlantic coast it trends in a ne sw direction anomalous fields normalized to the horizontal field at a station recording the normal field were used to obtain maximum depth estimates of around 45 km for the induced currents several deep schlumberger soundings were done over the anomalous zone and the results showed that the conductive structure is in places only 3 km from the surface and that it has a resistivity of less than 20 qm the resistivity of the upper crust outside the structure ranges from 5000 to more than 20000slm some 14 postkaroo alkaline igneous complexes occur along the course of the resistivity anomaly these intrusive complexes represent the youngest igneous activity in the damara orogenic belt and were most probably emplaced along a line of weakness in the lithosphere the resistivity anomaly would seem to delineate this line of weakness

geophysical lineaments and deep continental structure lineaments may be detected remotely by imaging electromagnetic radiation both inside and outside the visible spectrum and by examining the natural force fields particularly the magnetic and gravitational most electromagnetic imagery shows shallow structure which is readily correlated with surface geology gravimetry and magnetometry may show much deeper structure which is unrelated to surface structure or incompatible with its most obvious downward extrapolation seismology then offers scope for the high resolution study of buried lineaments revealed in this way from such examinations it is clear than many lineaments do not traverse the entire vertical extent of the lithosphere internal horizontal detachment may cause once through going structures to become segmented a number of examples will be given including local scottish examples the great glen fault and the cruachan lineament

imaging the magmatic system beneath the krafla geothermal field iceland a new 3 d electrical resistivity model from inversion of magnetotelluric data

imaging of deep fluids in archaean crust despite the importance of aqueous fluids in tectonic metamorphic and magmatic activity in the earth s crust few techniques are available for directly observing their distribution at depth even in stable continental crust such observations as have been made electromagnetic deep sounding which by inference1 2 images saline fluids and super deep drilling3 suggest that fluids are present in varying amounts throughout the crust in direct conflict4 with the normal petrological inference of a very dry deep crust here we report the results of an unusually high resolution electromagnetic sounding method to image the deep electrical conductivity structure below a 30 km profile in ontario which crosses archaean lower crustal material of high metamorphic grade exposed by a proterozoic thrust fault the ivanhoe lake cataclastic zone the electrical resistivity image obtained shows embedded in very resistive crystalline crust two nearly continuous sub horizontal zones of enhanced electrical conductivity at depths of 2 3 and 5 6 km below this the commonly observed2 5 7 mid crustal rise in conductivity is seen at depths of 15 20 km the image does not correlate well with the structure inferred from surface geology suggesting that the transport and distribution of fluids was not strongly controlled by lithological boundaries or the thrust fault itself

the southern cape conductive belt south africa its composition origin and tectonic significance abstract magnetometer array studies have led to the discovery and mapping of the southern cape conductive belt sccb crossing the southern tip of africa from west to southeast coasts the sccb lies just south of the namaqua natal belt of cratonic rocks remobilized about 1000 m y b p it is shown that it coincides with a zone of weakness which has been exploited by three major geosynclinal accumulations over some 600 m y relationships between the sccb and the basement geochronology geology and tectonics are considered in detail these relationships support the view that the conductive belt was formed by an accumulation of marine sediments and oceanic lithosphere at the top of a proterozoic subduction which stopped about 1000 to 800 m y b p associated with this subduction we propose a proterozoic range of andean mountains whose roots are now exposed in the namaqua natal belt later subduction further south near the present south coast is proposed to account for the intrusion between the south coast and the sccb of the cape granites in the time interval 600 500 m y b p there is some evidence for a third yet more distant subduction episode off permian gondwanaland after outlining this tectonic history the paper turns to a closer examination of the hypothesis that the southern cape conductive belt consists of partly serpentinized basalt accumulated at the top of a proterozoic subduction a large static magnetic anomaly which correlates with the sccb over most of its length is well fitted by a model which strongly supports this hypothesis bouguer gravity anomalies along western and central profiles likewise support the hypothesis a discussion follows of the process of formation of the proposed block of serpentinized marine rocks beginning with serpentinization of the crust near oceanic ridges by reaction of warm porous newly extruded basalt with seawater convecting through it the serpentinized basalt is stable at crustal temperatures and pressures and so is transported in the seafloor until it reaches a subduction where it accumulates because of its low density examples of such accumulations are cited finally it is shown that any iron in the olivine and pyroxene in the original basalt precipitates when these silicates are hydrated to serpentine as magnetite which is the dominant mineral conferring high electrical conductivity and high magnetic susceptibility on serpentinites in particular the beattie static magnetic anomaly requires in our model calculation a very high susceptibility readily attainable in basalt at 15 20 serpentinization the authors know of no other rock able to provide this high susceptibility a similar percentage of serpentinization provides the density required to model the gravity anomalies

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

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

electromagnetic geophysics and global tectonics lateral variations of electrical conductivity in the lithosphere and below it can be informative concerning both contemporary and ancient global tectonics the methods of investigation include classical four electrode resistivity depth probing which is of special value in continental shields magnetotellurics long used on land and recently in the pacific to show that in electrical terms the lithosphere is thicker near an oceanic ridge than had been expected and magnetometer arrays which are useful primarily in mapping highly conductive structures examples of the use of all three methods will be reviewed with emphasis on the last in conjunction with other geophysical geological and geochemical data magnetometer arrays have been useful in mapping partial melt under the western united states in discovering an extension of the southern end of the african rift system to the atlantic coast in detecting a major fracture zone in central north america which may be a proterozoic interplate collision feature and in discovering a conductive belt under southernmost africa which may result from accumulation of serpentinized marine crustal rocks at the top of a subduction under proterozoic gondwanaland inherent difficulties of quantitative modeling limit the utility of magnetometer arrays largely to discovery and mapping of two and three dimensional conductive structures this remains valuable however particularly when other physical properties can be used to limit the range of possible interpretations of the conductive structures recent array studies now in progress in western canada have defined two new conductivity anomalies one of these may represent a local partial melt concentration under the rocky mountains trench the other a precambrian rift valley in the lower crust previously suggested by others on the basis of seismological gravity and static magnetic data both are being investigated by more concentrated arrays

magnetotelluric interpretation of crustal and mantle structure in the grenville province summary accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust a magnetotelluric mt station bat in the grenville province of the precambrian shield in eastern canada has been in operation since 1975 for time dependency studies of electrical resistivity changes related to earthquakes the mt response of the station displays low skew with small to moderate anisotropy one dimensional inversion of the apparent resistivity and phase reveals two well defined conductors in the crust one at 10 km and the second at the base of the crust the latter has a resistivity less than 50 m these results are substantiated by three additional mt stations located up to 40 km distant data from other new mt stations and from stations previously published in the literature are compared with two dimensional computer model results and with the three dimensional analogue scale model results of dosso et al while additional data for periods less than 100 s would be desirable the results from a number of the mt stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the grenville the inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 m this may coincide with a seismic low velocity zone observed in the mantle under the canadian shield

episodic rifting and volcanism at krafla in north iceland growth of large ground fissures along the plate boundary the rifting of the plate boundary in north iceland forms new ground fissures and reactivates old ones continuous growth of fissures is observed in the crust above a magma chamber beneath the krafla caldera as it responds to changes in subterranean magma pressure rapid episodes of fissure formation and reactivation are observed in conjunction with magma intrusions at depth along the rift zone that constitutes the plate boundary between the north american and eurasian plates the krafla caldera contains several fissures that open during periods of uplift lasting 100 to 300 days and that close incompletely in subsidence episodes lasting several hours to 3 weeks the average rate of opening differs from one period to another ranging from 0 1 to 0 5 mm d fissure closing and average closing rates also vary from one subsidence episode to another ranging from minimal values of 2 1 mm and 2 3 mm d to maximal values of 49 mm and 18 mm d respectively during the two intrusion episodes of september 1977 and march 1980 the magma migrated toward the south with an average velocity 0 6 and 0 5 m s respectively in the september 1977 episode some fissures in the rift zone widened with a rate greater than 190 mm d whereas during the march episode that probably resulted in a thinner intrusion the maximum widening rate was only 50 mm d the data suggest that the magnitude of a subsidence episode is determined at least partly by the available excess magma pressure in the chamber to explain the extensive growth of open fissures and normal faulting in the rift zone it is necessary to assume that in addition to the intrusion of the main central dike at depth many small dikes are intruded at shallow depths less than 1 km