evidence of active ground deformation on the mid ocean ridge axial seamount juan de fuca ridge april june 1988 since september 1987 a precision bottom pressure recorder bpr has been deployed within the summit caldera of axial seamount on the central juan de fuca ridge the instrument is capable of measuring pressure to 1 mbar resolution and recording these measurements at 64 samples per hour for up to 15 months after removal of oscillatory signals due to tidal and oceanographic effects and linear trends caused by sensor drift any significant change in the pressure record should indicate a change of depth associated with vertical ground movement in subaerial volcanic systems such ground movements commonly indicate active inflation or deflation of underlying magma bodies and are frequently coincident with rift eruptions axial seamount was selected for this pilot study for three primary reasons 1 its tectonic setting 2 the presence of a well formed summit caldera and 3 observational evidence of geologically recent volcanic activity results from the first 9 months of the bpr deployment revealed a significant change in pressure which is interpreted to represent a 15 cm subsidence of the caldera floor during two 2 to 3 week periods in april june 1988 also during these periods an anomalous decline in temperature at the site was recorded that is correlated with an apparent increase in current velocity at the axial seamount hydrothermal emissions study ashes vent field suggesting vigorous advection of cold water into the caldera concurrent oceanographic data from geosat and from current meter arrays do not indicate any large scale oceanographic phenomena capable of generating these simultaneous events one mechanism to explain simultaneous ground subsidence and temperature decline at the caldera center and increased bottom current at the caldera margin is the generation of a buoyant parcel of heated water in response to the intrusion or the eruption of magma associated with volcanic deflation such a parcel must have been sufficiently buoyant and proximal to induce horizontal entrainment currents capable of producing the bottom water conditions that were observed at both sites similar volcanic events also may have generated large midwater plumes that have been described previously along the southern juan de fuca ridge

the juan de fuca ridge hot spot propagating rift system new tectonic geochemical and magnetic data underway geophysics deep tow camera work and preliminary geochemical and magnetic studies from a 1980 cruise to the juan de fuca ridge allow the following interpretations 1 the central third between 45 5 n and 46 5 n of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the brunhes matuyama magnetic reversal boundary 2 the apparent terminus of surficial igneous activity at the northern tip of the active cobb propagator has been identified by dredging and camera work the tip appears curved or offset slightly to the west does not correspond to the northernmost tip of the central magnetic anomaly elvers et al 1974 and spreading has been initiated beneath sediments farther north and 3 iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward then a pronounced reversal to more normal concentrations in the vicinity of the cobb offset

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

numerical modelling of deep dipole dipole mhd sounding abstract distortion effects of surface geoelectrical inhomogeneities on deep mhd soundings must be taken into account in the interpretation of the measurements computer programs which calculate these effects for the steady state part of the mhd pulses using the thin sheet approximation and which present the results graphically are used to study the distribution of apparent resistivity for various kinds of models in which the mhd generator is located off the coast of the kola peninsula

seismic and geoelectric evidence for collisional and extensional events in the fennoscandian shield implications for precambrian crustal evolution extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles normal incidence reflection profiles several hundred magnetotelluric soundings and several magnetovariational array observations results are compiled into four maps 1 moho depth 2 thickness of the lower crustal high velocity layer vp 7 km s 3 depth to the upper interface of the high velocity layer and 4 distribution of upper and middle crustal electrical conductors the moho depth is approximately 40 km in the archaean domain and varies between 40 and 65 km in the svecofennian domain most of the thickness variation 0 24 km is concentrated in the lower crustal high velocity layer electrically the crust is formed of several rather homogeneous either resistive or conductive blocks that are bounded by upper and middle crustal conductive zones the cratonized archaean crust experienced an extensional phase during which a thin mafic lower crustal high velocity layer was produced in association with mafic dykes 2 2 2 1 ga and well conducting volcanogenic and metasedimentary belts 2 5 2 0 ga the moho of the archaean domain was consequently reformed in the early proterozoic major thickening of the crust 50 65 km occurred when a svecofennian island arc and the archaean craton collided the svecofennian orogeny 1 76 1 9 ga resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc the internal terrane boundaries are seen as good reflectors and or inclined or vertical conductors that extend at least to the middle crust the thick high velocity 7 km s lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of late svecofennian subduction the collisional boundary is also preserved as a discontinuous conductive zone between the archaean and svecofennian domains the thickened crust was subsequently thinned to 42 km along e w striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths coeval mafic dykes and gabbro anorthosite bodies 1 6 1 5 ga the crust of the central part of the shield has since remained virtually intact

a new methodology to estimate magnetotelluric mt tensor relationships estimation of local transfer functions by combining interstation transfer functions elicit summary anewmethodologytoestimatemagnetotelluric mt tensorrelationships calledestimationof local transfer functions by combining interstation transfer functions elicit is proposed wherebythemttensorrelationshipsofthelocalsitearederivedusingonlyinterstationtransfer functions the mt impedance tensor and the geomagnetic transfer function at the local site are characterised by combining interstation tensor relationships between electric and magnetic fields at the local site with the horizontal magnetic fields acquired at a neighbouring site the main property of the proposed method is that the employed interstation transfer functions are independently constrained without the need to acquire the electric and the magnetic fields at the local site simultaneously to recover the local mt tensor relationships due to this property the elicit method offers new possibilities for mt data acquisition and processing providing significant improvements when the magnetic time series at the local site are affected by local noise or are truncated error analysis shows that even when magnetic fields are truncated the quality of the results obtained following the elicit method are similar to those we would obtainifthemagneticfieldshadnotbeentruncated anotherimportantpropertyisthatdifferent neighbouring sites can be used to recover the tensor relationships at the local site averaging of results obtained using different neighbouring sites can be performed to improve the statistics for our example data when the elicit method is used to improve the statistics errors of the estimates for periods between 1000 and 20000 s periodicities are clearly reduced all interstation transfer functions are calculated doing remote reference and the bootstrap method is used to compute the errors when necessary long period magnetotelluric data acquired in the pyrenees and in the atlas mountains in morocco and magnetic data provided by

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

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

high resolution body wave tomography beneath the svekalapko array ii anomalous upper mantle structure beneath the central baltic shield high resolution body wave tomography beneath the svekalapko array ii anomalous upper mantle structure beneath the central baltic shield

the magnetic structure of axial seamount juan de fuca ridge axial seamount is a large volcanically active seamount located in the eastern pacific on the central juan de fuca jdf ridge at 46 n 130 w sea surface magnetic anomaly data show that axial lies completely within crust formed during the brunhes normal polarity epoch the edifice however does not produce a simple positive magnetic anomaly related to the excess mass above mean seafloor several pronounced negative magnetic anomalies are associated with the seamount including both a circular shaped low located west of the summit and an elongate low trending nw on the southeast flank of the volcano these two anomalies form part of a nw se magnetic trough which extends across the seamount edifice oblique to the general jdf ridge trend of n20 e a third negative anomaly is observed on the northern flank of the seamount associated with north helium basin a deep elongate basin which indents the northeast flank of the seamount both inversion of sea surface magnetics data and forward modelling of topography constrained by magnetization values of dredged samples show that bathymetric effects can explain the helium basin anomaly but topography alone cannot produce the low magnetization values which extend across the seamount summit magnetization of crustal rocks does not vary spatially over the seamount so that negative anomalies at the summit can be explained by variations in the effective thickness of the magnetic source layer a simple forward model of uniform magnetization shows that the anomaly source layer would have to thin by 50 to less than 700 m thick in order to generate the observed magnetic response a thinned source layer is consistent with the presence of either a substantial magma chamber at depth beneath the summit of axial or a zone of substantial crustal alteration the shape of the anomaly implies the trend of either of these sources must be oriented obliquely to the general ridge trend but parallel to the direction of the absolute plate motion

in situ electrical resistivity and bulk porosity of the oceanic crust costa rica rift in situ electrical resistivity was measured in dsdp hole 504b to a depth of 1 013 m into oceanic basement apparent resistivities are about 10 m in about 600 m of pillow lavas sharply increasing to nearly 1 000 m in the underlying dykes bulk porosities calculated from resistivities show a distinct layering apparently corresponding to geophysical layers 2a 2b and 2c

a seafloor and sea surface gravity survey of axial volcano seafloor and sea surface gravity measurements are used to model the internal density structure of axial volcano seafloor measurements made at 53 sites within and adjacent to the axial volcano summit caldera provide constraints on the fine scale density structure shipboard gravity measurements made along 540 km of track line above axial volcano and adjacent portions of the juan de fuca ridge provide constraints on the density over a broader region and on the isostatic compensation the seafloor gravity anomalies give an average density of 2 7 g cm 3 for the uppermost portion of axial volcano the sea surface gravity anomalies yield a local compensation parameter of 23 significantly less than expected for a volcanic edifice built on zero age lithosphere three dimensional ideal body models of the seafloor gravity measurements suggest that low density material with a density contrast of at least 0 15 g cm 3 may be located underneath the summit caldera the data are consistent with low density material at shallow depths near the southern portion of the caldera dipping downward to the north the correlation of shallow low density material and surface expressions of recent volcanic activity fresh lavas and high temperature hydrothermal venting suggests a zone of highly porous crust seminorm minimization modeling of the surface gravity measurements also suggest a low density region under the central portion of axial volcano the presence of low density material beneath axial caldera suggests a partially molten magma chamber at depth

conductivity anomalies in the baltic shield in finland in 1981 84 six arrays of 30 or 31 magnetometers were operated on the baitic shield in central and southern finland to deduce information about the electrical conductivity within the earth s crust the magnetometer sites cover the area with 20 60 km spacing allowing the identification of large scale electrical structures within the crust each array recorded the three orthogonal magnetic field components on analogue film for about two months simultaneously at all the stations altogether 17 magnetic variation events of 2 6hr length have been digitized and analysed this paper includes results of the last array in south western finland a comparison of two methods of induction vector estimation and combination of results from all the arrays four zones of anomalously high electrical conductivity have been observed they are all interpreted as lying at depths greater than 5 km the oulu anomaly is wider than the others and rather short about 100 km the southern finland anomaly is probably a 500 km long narrow zone this and the ostrobothnian anomaly surround three sides of the large central finland granite area the outokumpu anomaly is probably deeper lying than the others blocks separated by these anomalies are presented the blocks have different average resistivities

magnetotelluric array in the central finnish lapland i extreme data characteristics abstract we present magnetotelluric mt data from the northern fennoscandian shield which show extreme behavior due to the complex geoelectric setting left behind by the tectonic reworking of the archaean bedrock during the palaeoproterozoic the core of the mt data set from the central finnish lapland were acquired in 2014 in the context of the masca project extreme data are defined by the condition where the determinant of the real and or imaginary part of the impedance tensor becomes negative this is related to but is a more anomalous condition than out of quadrant off diagonal impedance phases physically these are associated to reversal of e g the electric field as compared to its direction for one dimensional resistivity structure so far only few examples of extreme data have been explicitly reported in geophysical literature data previously classified as erroneous might belong to the class of extreme data instead and data quality analysis should take this into account a resistivity contrast of at least ca 1 1000 is required to observe extreme data as indicated by synthetic modelling studies extreme data are challenging from a modelling point of view and thus also drive the need for new developments in forward and inverse codes for mt in this study behavior of the phase tensor parameterization in case of extreme data is clarified the quadrants of the principal value phases a piece of information inherently missing from the phase tensor can be recovered if the parameters are followed as a function of period the behavior of the determinant average of the impedance tensor is investigated since out of quadrant determinant phases are sometimes observed in conjunction with extreme data a special condition is demonstrated which links out of quadrant determinant phases and extreme data to negative determinant of the galvanic distortion tensor

high resolution studies of the summit of axial volcano sea beam bathymetry sea marc i side scan 30 khz towed camera and submersible data from axial volcano juan de fuca ridge jdfr have been integrated to produce detailed maps of the volcano s summit region in order to elucidate its volcanic and tectonic evolution the proximity of axial volcano the youngest edifice of the cobb eickelberg seamount chain to the jdfr superposes a hotspot derived radial stress pattern onto the linear stress field of a mid ocean ridge spreading center the morphology of the summit of axial volcano reflects the differing eruptive styles produced from this superposition long rift zones begin at the edges of the caldera and extend as much as 75 km to the north and south axial volcano has a distinct summit plateau similar to those found on many other submarine volcanos the process by which summit plateaus are constructed is illustrated by the lava flows originating from the south rift zone which have partially filled the southern half of the caldera and have smoothed the flanks the summit plateau contains penetrative ne sw fracturing that is approximately parallel to the jdfr the unusual rectangular caldera is 3 8 km in size and its trend is about 160 three possible mechanisms are discussed for the caldera s formation 1 canary island type trapdoor formation 2 overlapping spreading center interaction and 3 magma source migration although none of these models is fully compatible with the present data base the overlapping spreading center model is preferred postcaldera volcanism probably began with the formation of the central caldera eruptive complex which was characterized by relatively low volume high viscosity flows the most recent volcanism has apparently taken place along the northern and southern rift zones which have erupted more fluid lavas resulting in areally extensive flows covering up to 60 of the caldera floor extensive low temperature venting and areas of high temperature venting near the northern and southern rift zones and caldera walls suggest a shallow magmatic source beneath the summit of axial volcano

crustal conductivity anomalies in central sweden and southwestern finland summary lateral conductivity variations in the earth s crust in central sweden and southwestern finland were mapped using magnetic field variations recorded simultaneously by 29 gough reitzel type magnetometers in summer 1985 three events of 3 or 4 h were digitized and analysed to obtain single station induction arrows and interstation horizontal field transfer functions the 1985 magnetometer array covered part of the bear baltic electromagnetic array research array operated in 1998 on the fennoscandian shield results from eight bear sites are used to control and to supplement the 1985 array results the conductivity of the crust was found to increase in the western part of the array in western central sweden crustal conductors were found both in the western part of the array and in the baltic sea area in the eastern part of the array 3 d modelling based on the induction arrows revealed a major conductivity anomaly in the middle and lower crust under the archipelago of southwest finland

the composition of sedimentary organic matter in relation to the dynamic features of a mangrove fringed coast in french guiana the sedimentary organic matter content of a series of 2 m deep cores was examined in relation to the evolution of mangrove forest on the basis of geochemical analyses and optical observations avicennia dominated forest deposits developing along the highly dynamic coastline of french guiana were collected in five stations based on stage of forest evolution the sedimentary organic matter in the upper sediment of the youngest mangrove swamp is mainly derived from algal mats with low carbon nitrogen ratios c n ratio from 6 to 8 and typical greyish amorphous organic flakes as observed in optical studies indeed rare young avicennia trees are present and effectively geochemical parameters do not give evidence of a litter made up of higher plant debris these rare debris being probably exported by the tides a slight increase with depth in the first decimetres of both total organic carbon toc content and c n ratio results from the development of the radial cable root system of the pioneer avicennia germinans early diagenetic conditions of this young forest are rather controlled by dominant suboxic processes as suggested by high eh values range 200 400 mv and local anoxic processes occurrence of pyrite in micro environments this is mainly due to the oxygen available by roots and crab bioturbation the organic content of the senescent mangrove sediment is mainly derived from higher plant debris in the uppermost 30 cm as indicated by relatively high c n ratios and the predominance of ligno cellulosic debris the strong decrease in hydrogen index values results from the degradation of the higher plant debris losing hydrogen bounds through decay processes moderately acidic ph values low ehs and the presence of pyrite framboids point towards the reducing decay processes in surficial layers of the senescent mangrove mediated by sulphate reducing bacteria whatever the stage of evolution of the forest the geochemical characteristics of the sediment below 30 cm are those of the shoreface one with opaque refractory debris derived from the amazon river detritus discharge the sediment collected from dead mangrove forests subsequently recolonized by pioneer mangroves contains organic markers which predate recolonization recording previous phases of erosion and accretion

marine controlled source electromagnetic sounding 2 the pegasus experiment the marine controlled source electromagnetic sounding method developed over the past 15 years at scripps institution of oceanography employs a towed seafloor electric dipole transmitter of moment 4 104 am and multiple free vehicle seafloor electric field recorders a survey of 40 ma normal oceanic lithosphere in the northeast pacific using frequencies of 0 25 to 24 hz and synchronous stacking of 0 25 to 12 hour duration detected signals at transmitter receiver ranges between 5 and 95 km one dimensional electrical conductivity structure is recovered from the data using the occam process of nonlinear regularized inversion repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km well into the upper mantle with bounds placed on conductivity to depths of 60 km structure shallower than about 1 km is comparable to that obtained by a similar experiment on the east pacific rise and by borehole logging with a sharp increase in resistivity at depths of 600 800 m although strictly our experiment is sensitive only to integrated square root of conductivity or total attenuation in the surface layers the lower crust and upper mantle has a resistivity between 2 and 7 104 m and a transverse resistance of at least 109 m2 suggesting at most 0 3 volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle although resolution is weak below 30 km our data are compatible with a dry olivine model of mantle conductivity temperature

a seafloor long baseline tiltmeter long term monitoring of seismicity and deformation has provided constraints on the eruptive behavior and internal structure and dynamics of subaerial volcanoes but until recently such monitoring of submarine volcanoes has not been feasible little is known about the formation of oceanic crust or seamounts and we have therefore developed a stand alone long baseline tiltmeter to record deformation on active seafloor volcanoes the instrument is a differential pressure two fluid sensor adapted for use on the seafloor combined with an autonomous data logger and acoustic navigation release system the tiltmeter can be installed without use of remotely operated vehicles or manned submersibles and to first order is insensitive to noise driven by temperature or pressure gradients we recorded 65 days of continuous data from one of these tiltmeters on axial seamount on the juan de fuca ridge during a multidisciplinary experiment that included ocean bottom seismographs magnetotelluric instruments and short baseline tiltmeters after instrument equilibration the 100 m iono tiltmeter provided a record with long term drift rates of 0 5 5 rad day and higher frequency variations of the order of 5 10 rad comparison with records of subaerial volcanic tilt shows that this instrument can discriminate volcanic deflation events though none occurred during our deployment a conclusion supported by nearby short baseline tilt and bottom pressure recordings the short and long baseline data constrain volcanic inflation of axial seamount to be below 0 5 1 rad day 1 during mid 1994 analysis of the long baseline tilt data in conjunction with electric field temperature and short baseline tiltmeter data shows that high frequency signals are largely driven by ocean currents improved coupling between the tiltmeter and seafloor should reduce this noise improve stability and drift and further enhance our ability to record tilt related to active submarine volcanism

short and long baseline tiltmeter measurements on axial seamount juan de fuca ridge abstract long term observations of seismic activity and ground deformation at mid ocean ridges and submarine volcanoes are required for an understanding of the spatial and temporal characteristics of magma transport and intrusion to make precise records of tilt on the seafloor we have installed short baseline tiltmeters in six ocean bottom seismometers tilt obs and developed a long baseline 100 500 m two fluid tiltmeter lbt in the tilt obs the seismometer platform is levelled to better than 1 after deployment the tiltmeter consists of a pair of electrolytic bubble sensors mounted on a secondary levelling stage on the seismometer platform the levelling stage uses two motor driven micrometers on a triangular mounting plate to bring the sensors to null the sensitivity of these tiltmeters is 0 05 rad at a dynamic range of 0 2 mrad a long baseline instrument was developed to achieve a better spatial average of deformation most approaches used on land to measure stable long baseline tilt cannot be applied to a submarine instrument but tiltmeters in which the pressure of a fluid in tubes is measured are amenable to installation on the seafloor the development resulted in a device that is essentially a center pressure instrument folded back on itself with fluids of different densities in the two tubes during july to september 1994 these instruments were deployed on axial seamount on the juan de fuca ridge off washington state for a test of their relative performance on volcanic terrain yielding 9 weeks of continuous data seismic tilt and temperature from five tilt obs and one long baseline instrument drift on all instruments was of the order of 1 rad day with higher frequency variations of order 5 10 rad initial drift on the tilt obs is shown to be associated with platform settling rather than with the sensor or its mounting high frequency noise is coherent across instruments and tidal in character and we conclude that tidal currents moving the sensors are responsible

comment on the electrical conductivity of the oceanic upper mantle by g heinson and s constable in a recent paper heinson constable 1992 hereafter hc have attempted to construct an oceanic upper mantle electrical conductivity model by combining laboratory measurements on dry olivine as a function of temperature with a geotherm inferred from a variety of geophysical and petrological data hc argue that the 1 d magnetotelluric mt response computed from their model structure is not compatible with three sets of sea floor electromagnetic em data collected in the late 1970 s and proceed to examine those data in more detail they infer that some of the em data are not consistent with a 1 d structure on the basis of statistical tests and further conclude that the presence of an upper mantle high conductivity zone is not required by the data finally hc attempt to reconcile these disparate observations by suggesting that a high resistive upper mantle which is required by their model combined with the presence of resistive coastlines will result in significant distortion of the em response at points in the interior of the ocean they construct sample mt responses

on induction effects at eiscat and image magnetometer stations summary when ground based magnetometer data are used for ionospheric magnetospheric studies it is important to know the external ionospheric magnetospheric and internal telluric contributions to the observed total variation to study them we calculate geomagnetic induction vectors and apply the siebert kertz separation method using data from the eiscat and image magnetometer stations in fennoscandia induction vectors in the period range 20 2560s show the expected and dominating ocean effect near the arctic ocean at masi a small scale inland anomaly is detected in addition to the ocean effect at hankasalmi a man made anomaly due to the nearby stare radar is present the source effect on induction vectors at high latitudes is briefly investigated by separating the magnetic variations of several bay events at image stations into external and internal parts we study the importance of the large scale induction in the recordings depending very much on the particular event the internal horizontal part is 10 30 per cent of the total field in the auroral electrojet region and its relative contribution increases in more distant regions the internal contribution of the vertical field at inland stations is 10 40 per cent having an opposite sign to the total field or 10 30 per cent having the same sign as the total field depending on the location with respect to the ionospheric currents and to the ocean results are also discussed using a thin sheet model

the ramesses experiment v crustal accretion at axial volcanic ridge segments a gravity study at 57 45 n on the slow spreading reykjanes ridge summary presented in this paper are the results of a two stage analysis of gravity data acquired during a multidisciplinary geophysical survey of a magmatically active axial volcanic ridge avr segment located at 57 45 n on the reykjanes ridge part of the slow spreading mid atlantic ridge south of iceland modelling of the free air anomaly in 2 d shows that across axis the observed anomaly results largely from density and layer thickness variation in the mid lower crust although seismic control on crustal thickness along axis is limited modelling also suggests that both crustal density and thickness also vary towards avr tips using the 2 d modelling results as crustal reference the residual mantle bouguer anomaly rmba is calculated to assess whether magma related density anomalies are present in the mantle and to investigate the structure of and relationship between adjacent avrs along axis rmba lows are associated with both an along ridge trend encompassing a number of adjacent avrs and with individual more topographically robust avrs modelling of the rmba low associated with the 57 45 n avr further suggests that along axis density variation is confined to the central region of this avr and that the anomaly can largely be accounted for by density variation within layer 3 and a degree of crustal thinning towards avr tips the nature of the along axis variation in crustal density further suggests that it may result from repeated phases of magma supply to the crustal system from the mantle within the resolution of the rmba modelling does not confirm or preclude the presence of a subcrustal density anomaly associated with retention of a small percentage of melt in the mantle however a melt free model for at least the top 40 km of the mantle is preferred as this is consistent with the results of modelling a coincident magnetotelluric data set the ridge trend characteristics of the rmba also suggest that magma delivery may take place along this trend and in an episodic fashion that controls initiation of an avr tectonomagmatic cycle avrs may therefore be created along a spreading orthogonal direction by tapping of a recent magma injection into the ridge trending crustal system with flow of this magma laterally along avr parallel faults and fissures accommodating spreading and crustal accretion

organic carbon sources and transformations in mangrove sediments a rock eval pyrolysis approach abstract a rock eval pyrolysis study was carried out on sedimentary cores and leaf and woody tissue of vascular plant species from the mangroves of french guiana these forests develop on moving mudbanks and have a lifetime limited to few decades before being eroded our main purpose was to complete the understanding of carbon cycling in this specific environment using a method that allows monitoring the depth evolution of sources and transformation of organic matter om within a tight depth span leaf and woody tissue showed clearly distinct rock eval parameters however these parameters strongly varied with om decomposition rapidly limiting their efficiency as mangrove plant tracers the decrease in values of tps2 pyrolysis temperature corresponding to the maximum release of hydrocarbons with depth clearly showed a line between a pedogenetic layer enriched in autochthonous om and marine sediments enriched in allochthonous om this allochthonous om originated from the particulate discharge of the amazon river which is partly deflected northwestwards by the current of the guianas within the soil layer decay processes induced a strong dehydrogenation and slight oxidation of om the two processes occurring simultaneously the deeper layers were characterized by a very strong increase in oico2 quantity of pyrolysed co2 relative to total organic carbon beneath both the young and the senescent forests this trend resulted rather from an increasing content of authigenic carbonate than from increasing concentration of oxidized allochthonous debris carbonate can form as a result of the more anoxic conditions prevailing in buried layers vs the upper suboxic ones beneath the senescent mangrove a greater increase in oico2 probably reflected greater carbonate precipitation resulting from the decomposition of higher amounts of om

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

dynamical consequences of compositional and thermal density stratification beneath spreading centers extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers this compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow the relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity for a low enough mantle viscosity buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone for a mantle viscosity of 1022 pa s upwelling due to plate spreading is dominant and crustal thickness is predicted to be a strong function of spreading rate however for a mantle viscosity of 1019 pa s buoyancy dominates mantle upwelling and the calculated crustal thickness like the oceanic crust is nearly independent of spreading rate thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling for a mantle viscosity of 1020 pa s and a low 1 cm yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single well defined period for even slightly lower mantle viscosity the frequency increases and crustal thickness oscillations become more complex with two or more frequencies present while numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure

magnetotelluric and audiomagnetotelluric measurements in finland abstract joint finnish hungarian mt magnetotelluric and amt audiomagnetotelluric measurements were carried out in finland in the framework of the international elas project the conditions for mt measurements are favorable at these latitudes five mt and 150 amt stations gave information on the electrical conductivity distribution in the area amt results guided the choice of mt sites with minimal near surface distortion effects and helped the interpretation of the mt soundings the mt measurements indicate the presence of large conductivity anomalies and can be best interpreted as lateral induction effects of near surface dyke structures this result is confirmed by a certain correspondence between the directions of the maximum impedances and of the tectonic zones of the area any information about the upper mantle would require the use of sq harmonics because of the crustal conductivity anomalies detected by the mt measurements

aspects of the geoelectric models of the baltic shield abstract the geoelectric structure of the baltic shield in finland has been studied since 1980 four types of geoelectric model have been considered so far numerical models for the galvanic distortions of the upper 10 km horizontal distribution of crustal inductive anomalies vertical cross sections above relevant crustal anomalies and vertical conductivity profiles on a regional scale the models are based on extensive on going measurements being conducted with magnetovariational magnetotelluric and frequency sounding methods geoelectric studies in finland are a part of both the international elas electrical conductivity of the asthenosphere and the egt european geotraverse projects in finland the most relevant crustal conductivity anomalies south of 67 n are a conductive band surrounding the central finland granite area in the west and south and the oulu mikkeli and outokumpu anomalies n s e w and n s respectively the anomalies are located in the middle crust typically from 6 12 km downwards several regional conductivity boundaries and the existence of a conductive layer in the lower crust at least in parts of central finland have also been noted a vertical cross section across the oulu anomaly shows a wide crustal conductor which seems to dip towards the west the anomaly has exceptionally good conductivity 0 5 5 m and cannot be attributed to the known sandstone formation or the schist zone on the surface along the sveka dss deep seismic sounding profile some increases in conductivity of the upper crust correlate with the kainuu and the kuhmo schist zones the upper crustal parts of the raahe ladoga zone has also a higher conductivity than the surroundings some local conductivity anisotropies seem to coincide with deep faults located by dss the area east of kuhmo seems to be more or less free of crustal conductivity anomalies allowing electric soundings down to the upper mantle a deep conductivity model based on the measurements in this area differs from earlier models for northern scandinavia where the presence of a good conductor at asthenospheric depths has been reported variations in the electrical conductivity of the earth s crust and upper mantle thus yield information about the structural composition and the tectonic development of the earth the results now presented are only preliminary and their detailed geological tectonic importance is not yet fully understood later the models will be completed with the results from work done in both the eastern and the western parts of the shield

climatic and geomagnetic influences on the lac du bouchet palaeomagnetic sv record through the last 110 000 years abstract palaeomagnetic secular variation records stacked from five cores 20 m long extend back through the last glacial cycle to 100 000 years ago the time scale has been constructed from conventional and accelerator radiocarbon ages back to 45 ka bp and beyond this by comprehensive pollen studies which identify forest and steppe cycles that have been related to marine oxygen isotopic stages 4 and 5 spectral analyses of the intensity and directional records using both fourier and maximum entropy methods reveal strong low frequency components which fall in the range of the milankovitch obliquity and precession periodicities 40 20 ka it is concluded that although part of this signal may have fed indirectly into the natural remanence via the influence of climatic change on sediment facies part has been impressed directly by perturbation of the geomagnetic dynamo process at the core boundary by variations in the earth s obliquity and precession these long periods envelop a well defined pattern registered also by the independent mackereth core stack back to 50 ka bp of irregular secular oscillations of the geodynamo with periods of a few thousands of years

a preliminary palaeomagnetic study of the holocene and late wurmian sediments of lac du bouchet haute loire france summary lac du bouchet is a small maar type lake located at 45 n 4 e in central france the bottom sediments have been sampled down to a depth of 6 m with mackereth type corers a chronology has been established from pollen analytical and diatom zonations and by radiocarbon age determinations throughout the period covered which runs from 22 000 years ago through to the present time declination and inclination variations consist of oscillations of up to about 30 amplitude no abnormally large amplitude excursions in direction are recorded the main contribution of this study is the extension of the european secular variation sv records to the interval from 13000 bp years before present to 22000 bp the sediments deposited through this interval ancient dryas intc wurm originated from the erosion of the internal walls of the volcano spectrum analyses of the declination and inclination curves from 13000 to 22000 bp reveal peaks at periods of 3000 4000 1800 1150 and 700 yr cross spectrum analysis shows strong coherence of the two signals at the longest of these periods the phase relationship of all these peaks indicates clockwise rotation of the palaeomagnetic vector possibly suggesting westward drifting sources although the palaeomagnetic sv record through the holocene 0 10300 bp is compressed into 1 7m of highly organic sediments the principal features exhibited by the type curves established for uk can be identified and the resulting magnetic ages are consistent with the palynology the sense of rotation of the magnetic vector is biased equally in both senses the late glacial 10300 bp to 13000 bp is characterized by oscillating climatic conditions recent dryas allerod and bolling which resulted in poorly resolved and probably discontinuous palaeomagnetic sv records which show a strong bias to anti clockwise rotation all ages are given in radiocarbon years

recent climatic and anthropogenic imprints on lacustrine systems in the pyrenean mountains inferred from minerogenic and organic clastic supply vicdessos valley pyrenees france high resolution seismic profiling has been combined with geochemical analyses of both watershed samples and five lacustrine cores retrieved from two natural lacustrine basins of glacial origin lake majeur and lake sigriou 1630 m a s l and 1995 m a s l respectively eastern french pyrenees identifying specific minerogenic and organic markers of autochthonous and allochthonous supply data allow documenting past climatic and anthropogenic pressures over the past century the lacustrine sediment of lake majeur has been essentially composed of algae drastically contrasting with the natural sedimentary infill of the basin mainly resulting from soil erosion from the mid late holocene since ad 1907 the lake majeur has been used for hydroelectricity production human induced lake level regulations affecting up to 37 of the lacustrine surface have increased by fourfold the accumulation rate of the lake and favoured water enrichment rubidium abundance within the lacustrine sediments of the two lakes reflects the mid late holocene palaeohydrology after dam construction in ad 1907 greater quantities of rubidium found in lake majeur sedimentary infills indicate drier climatic periods such as from ad 1975 to ad 1982 during which water reservoirs were particularly in demand inversely before the dam was built rubidium fluctuations were correlated with wetter conditions and hydrological events were recorded as sandy layers deposited by canyon reactivation synchronous with european climatic deterioration phases we notably document that the mediaeval climate anomaly was interrupted by some humid periods dated c ad 940 ad 1080 ad 1100 and ad 1250 we also date the onset of the little ice age c ad 1360 and identify that this period was wetter after c ad 1500

deep array electromagnetic sounding on the baltic shield external excitation model and implications for upper mantle conductivity studies abstract the bear array of simultaneous electromagnetic em observations probes the deep crustal and upper mantle conductivity structure of the baltic shield searching for the lithosphere asthenosphere boundary beneath the adequate interpretation of the results of this unique high latitude natural field em sounding requires proper understanding of the actual external excitation conditions because conventionally used plane wave model assumptions may be substantially violated in the vicinity of inhomogeneous polar sources the paper presents an overview of the morphology and statistics of source distortions in the bear em field transfer functions tf and the ways of their suppression the stability of the final tf estimates obtained with the exclusion of intensive non stationary auroral effects is further justified the external excitation model effective for the whole bear observation period is inferred from the array distribution of the inter station geomagnetic transfer functions the model is supported by the results of polar ionosphere magnetosphere current system studies based on the simultaneous ground and satellite geomagnetic observations and sets bounds for the plane wave approach in the bear data interpretation to avoid unfounded inferences on the upper mantle electrical properties the signatures of the lithosphere asthenospere boundary under fennoscandia derived from the bear data are summarized and its resolution within the traditional plane wave interpretational paradigm is analysed assuming the presented external source pattern and estimated tf uncertainties caused by the source inhomogeneity

relationships between organo mineral supply and early diagenesis in the lacustrine environment a study of surficial sediments from the lac du bouchet haute loire france the relationships between organo mineral composition and early diagenetic processes in surface 50 cm sediments from the lac du bouchet haute loire france are presented parallel pore water studies reveal downcore increases in dissolved organic carbon inorganic carbon and ch4 illustrating the dominant methanogenetic degradation but also highlighting regions of enhanced degradation due to sulphate reduction confirmed by solid phase sulphur and pyrite distributions three sedimentary facies are observed ia 0 25 cm where the majority of compaction and degradation of the organic matter by diagenesis occurs in the absence of any significant change in the type of organic material or sedimentation rate ib 25 40 cm characterised by an increased grain size and diatom abundance ii below 40 cm whose base dates at 2500 350 bp i e within the sub atlantic shows high organic carbon contents and large grain size autochthonous amorphous organic matter is the dominant petrographical organic component whereas pedogenetic allochthonous amorphous organic matter is slightly enriched in facies ii and ia this is coincident with the disappearance of the spores and pollen and suggests a greater continental runoff from the surrounding basin at these times proportions of kaolinite chlorite and illite content do not vary greatly indicating that the detrital input has not changed significantly over the last 2500 years downcore decreases in the sial curve proxy for biogenic silica parallel those of total organic carbon suggesting the dissolution of solid phase silica over time the anthropogenic deforestation and consequent greater nutrient runoff into the oligotrophic lake encouraged an increased production of diatoms around 30 cm the increased sedimentation rate at the beginning of the sub atlantic facies ii as a result of greater continental runoff due to installation of present climatic conditions enhanced the preservation of the organic material by reducing the dia genetic effects and by increasing the proportion of diagenetically inert and relatively resistant organic matter these findings underline the critical role that bulk sedimentation rate and the nature of organic matter play in the final determination of sediment composition both in terms of input and in post depositional degradation

non detrital si concentrations as an estimate of diatom concentrations in lake sediments and suspended material abstract this paper evaluates a method based on normalization with al to estimate the concentration of diatoms in suspended and deposited sediments provided that al exists primarily in the detrital phase one can calculate the fraction of non detrital si i e biogenic si suspended matter and sediment in two lakes in northern sweden have been analyzed results from the normalization method are compared with microscope count data the results from the two methods differ somewhat partly dissolved diatom frustules may explain part of this discrepancy these remnants can hardly be recognized as diatoms therefore they are not counted during the microscopic analysis although they contribute to the concentration of biogenic si another factor is the differences in si content among different types of diatoms we show that the calculated concentrations of non detrital si are good estimates of the diatom concentrations by using normalization as a tool reasonable estimates of the stocks of biogenic si can be obtained rapidly using these estimates as a guide sites where considerable changes in diatom productivity and or dissolution rates have occurred can be identified

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

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

high resolution magnetotelluric studies of the archaean proterozoic border zone in the fennoscandian shield finland summary the archaean proterozoic collisional zone is a complex mixture of the archaean complexes e g iisalmi complex ic proterozoic supracrustal belts e g kainuu belt kb and savo belt sb and oceanic arc lithologies in the central fennoscandian shield the zone was formed in the savo orogeny when the keitele microcontinent collided with the archaean karelian craton in the palaeoproterozoic time the crustal architecture of this palaeosuture is studied using new broad band magnetotelluric data from 104 sites 2 d conductivity models across the border zone between the palaeoproterozoic svecofennian domain and the archaean karelian province are constrained using the recent partly collocated reflection seismic data from the finnish reflection experiment fire dimensionality analyses in particular the q function analysis show that magnetotelluric data represent reasonably well regional 2 d structure at periods 100 s which is the longest period used in this study strike determinations gave a stable strike of n15w for the inversions the data are projected into three parallel profiles with an azimuth of n75e the determinant inversion is selected as the most suitable method for the data set especially the phase data are useable only from the determinant since one of the polarizations have the out of quadrant phase at several sites the interpreted final geological more appropriate models where smoother thick conductive areas are replaced by thinner layers are constructed from the results of the unconstrained smooth inversions with the help of forward modelling synthetic and prior model inversions and reflection seismic models the two major sets of crustal conductors are identified they have an opposite dip and together they form a bowl shaped conductor in the west the eastward dipping sb conductors are located at the bottom of the formation underlain by the keitele microcontinent the sb conductors extend to the east possibly cutting the westward dipping conductors of the kb the conductive kb is flanked above and below by the resistive archaean ic rautavaara and eastern finland complexes and represents the remnants of a basin that was squeezed between two archaean blocks in the collision the crustal conductors revealed by the inversion extend to the surface where they can be associated with the near surface conductors mapped by airborne electromagnetic surveys geological mapping has shown that the near surface conductors are caused by graphite and or sulphide bearing metasedimentary rocks thus the deep conductors related with the proterozoic kb and sb were formed on the surface and transported into the upper and middle crustal levels in the savo orogeny the crust in the karelian craton is highly resistive having only minor resistivity variations in our research area also the archaean lower crust is resistive on the contrary to the conductive lower crust of the proterozoic sb and the keitele microcontinent

geoelectric studies and conductivity structures of the eastern and northern parts of the baltic shield abstract large resistive regions ideally having a horizontal size of about 200 km are needed to study reliably the conductivity distribution of the lower crust of the baltic scandinavian shield by deep electromagnetic em methods the overburden of the shield consists of quaternary sediments and is thin with a low conductance this makes the shield favourable for em studies the numerous conductive zones e g schist belts in the uppermost crust however require careful two dimensional modelling in order to obtain reliable electrical cross sections of the earth s crust electromagnetic methods magnetometer array studies and magnetotelluric profiling have proven to be a useful tool in studies of the lithospheric geophysics of the shield the crust of the finnish and soviet baltic shield can be divided into geoelectrically different blocks in central finland the blocks are surrounded by narrow highly conductive band like formations they are located in the middle crust and their upper surface often occurs in the 6 10 km region the resisitivities are typically 5 10 m the greatest anomalous crustal regions have a conductance of more than 20 000 s the lower crust is very conductive in most parts of the shield two dimensional models of long magnetotelluric profiles indicate the great variety and complexity of the geoelectric structure of the crust

magnetotellurics along the fennoscandian long range profile summary estimates of magnetotelluric transfer functions along the fennoscandian long range fennolora profile are presented and discussed in relation to the major lithotectonic subdivisions of the swedish part of the baltic shield none of the transfer functions are found to be consistent with a one dimensional earth structure thereby making a quantitative interpretation difficult however the geological units are found to display characteristic differences in the obtained transfer functions and in some areas a quantitative interpretation can be justified the most interesting structure indicated is a low resistive 4 am crustal structure of lateral extension more than 150 km centred around the skellefte ore district a minimum thickness for this structure of 15 km is inferred

sedimentation rate and early diagenesis of particulate organic nitrogen and carbon in aydat lake puy de dome france abstract a core of low compacted sediment from a eutrophic lake aydat lake was extracted in order to study the early diagenesis of particulate organic carbon and nitrogen we use a simple model where the advection rate was estimated with the classical 210 pb method we show that the 210 pb data are not sufficient by themselves to provide a correct estimation of this parameter but a model which includes the sediment compaction leads to a better understanding of the sedimentation process compaction is also taken into account in the diagenetic model which allows us to make an estimation of the carbon and nitrogen mineralization rates

coupled rock eval pyrolysis and spectrophotometry for lacustrine sedimentary dynamics application for west central african rainforests kamalete and nguene lakes gabon in recent years nguene lake and kamalete lake gabon west central africa have been studied repeatedly providing comprehensive reconstructions of environmental changes over the last millennia both lakes are in different geomorphological and environmental settings they are therefore excellent sites to test new methodological approaches indeed the sedimentary cores provide various facies and the previous studies provide references for calibrating the results of new methods in this methodological issue the present study aims to evaluate the potential of spectrophotometric and rock eval coupled analysis to describe the holocene lake and marsh deposits from tropical moist forests this assessment is carried out on samples taken from two well documented reference cores the spectrophotometric analysis provides reproducible colour measurements which inform about the nature of the main colour bearing constituents coupled with rock eval pyrolysis this technique can be used to describe lithological changes and identify the probable source of sedimentary organic matter in the studied cases this approach identified the facies dominated by detrital terrigenous inputs iron bearing signature and high oi values and those associated with a more abundant primary production chlorophyll signature low oi and high hi providing a distinction between palustrine and lacustrine dynamics however although the facies are comparable sedimentary dynamics and sediment sources may vary depending on geomorphological and climatic contexts

extension of the lac du bouchet palaeomagnetic record over the last 120 000 years a high resolution profile of the palaeomagnetic variations recorded in the upper 20 m of the lac du bouchet sedimentary sequence has been reconstructed from several parallel mackereth and livingstone type cores inter core correlations were established by comparing individual core profiles of magnetic susceptibility individual records of af cleaned directions were then stretched to a common depth scale and merged the mean record was then filtered and smoothed in order to reduce the background noise accelerator radiocarbon determinations and pollen analyses show that the sequence covers the last interglacial glacial cycle back to ca 120 kyr b p and allow to perform a depth to time transformation there is a noted absence of widely divergent directions back to 90 kyr b p it is estimated that the recorded signal a pdrm signal represents the geomagnetic input signal attenuated by about 19 it follows that the excursions reported for the laschamp and olby lavas if they existed at all must have been of rather short duration two durable excursions appear in narrow diameter sections at the base of three independent cores b c and d while a reverse to normal transition is possibly documented in the lowermost meter core d this part of the sequence is dated from the recognition of major climatic events related to the 18 o isotopic stages defined in the oceanic record between 116 and 95 kyr this introduces the possibility of new excursions at ca 104 and 95 kyr i e immediately after the recovery of normal polarity documenting the end of the blake event

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

tectonic styles of early proterozoic ore deposition in the fennoscandian shield the early proterozoic metallogenic events of the fennoscandian shield are connected with two major stages of crustal evolution the karelian continental development 2 5 2 1 ga and the svecofennian orogeny 2 0 1 75 ga the karelian continental development was dominated by deposition of terrestrial sediments and tholeiitic volcanism on archaean crust a tensional regime probably associated with a mantle plume and partial melting of the upper mantle culminated in the generation of an ene wsw trending rift zone which was intruded by tholeiitic magma 2 44 ga ago some 15 layered igneous bodies hosting cr pge ni cu and fe v ti deposits are known in the kemi suhanko and koillismaa belts these mafic ultramafic bodies originally formed either several lopoliths or one single layered igneous complex with a minimum length of 260 km nw se and nnw sse trending dolerite dykes intruded the rifted and attenuated archaean crust 2 25 2 0 ga ago preceding ocean opening the coeval ne sw trending dolerite dykes in the northern part of the shield are interpreted as manifestation of an aulacogen the failed arm of a triple armed rift system oceanic crust is preserved in 1 96 ga old ophiolite complexes the cu co zn ni and cr contents of the outokumpu type deposits derive from hydrothermal leaching of oceanic crust and were discharged together with chemogenic cherts and carbonates from ascending hydrothermal fluids on the seafloor above fissure vents in the northern part of the shield rifting and probably ocean opening resulted in the formation of the layered tholeiitic sedimentary complex of the petsenga ni cu district intruded by mineralized mafic ultramafic magma the age of this event is poorly constrained the basin was subsequently closed and continental collision of the kola suture was accompanied by southward obduction of the lapland granulite belt on the archaean crust during the svecofennian orogeny the divergent continental margin was converted into an active margin early subduction of oceanic crust produced oceanic island arcs intruded by 1 93 1 9 ga old tonalites and trondhjemites subsequent subduction of oceanic crust generated 1 9 1 87 ga old continental island arcs associated with massive sulphide cu zn pb ag au deposits the island arcs were accreted to the archaean crust in a concave belt containing the ore districts of kiruna skellefte vihanti pyhasalmi aijala orijarvi and bergslagen bimodal volcanic suites with felsic calc alkaline and mafic tholeiitic volcanites are attributed to rifting of the overriding plate above subduction zones ni cu deposits in synorogenic 1 9 1 87 ga old mafic ultramafic intrusions occur in linear fault controlled belts at the margin of the archaean crust the generation of ni bearing tholeiitic to komatiitic magma is explained by partial melting of the mantle wedge above the early subduction zone magma differentiation and magma upwelling along transcurrent faults on the continental margin sulphur was probably assimilated from the supracrustal wall rocks 1 89 1 87 ga old early svecofennian granitoids are associated with porphyry type cu mo au deposits widespread late svecofennian s type 1 84 1 75 ga granitic magmatism generated minor mo u w sn and ree deposits emphasis is laid on the importance of geotectonic models as a basis for further exploration in the fennoscandian shield

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

the electrical conductivity of the oceanic upper mantle summary previous inversions of sea floor magnetotelluric mt sounding data have predicted upper mantle electrical conductivities which are more than an order of magnitude higher than laboratory measurements of the conductivity of olivine would suggest and controlled source field electromagnetic csem soundings require a lithospheric mantle conductivity of 3 x lop5 sm which is so low that the electromagnetic em coast effect would produce more anisotropy in mt soundings than is observed we address these issues by constructing an olivine mantle model for conductivity and examining the inversion of mt data from three sea floor sites and show that the incompatibilities can be largely resolved if the effects of oceans and coastlines are considered our mantle model is based on recent measurements of olivine conductivity the conductivity of tholeiite melt a thermal model for the upper mantle based on lithospheric cooling and the temperature of a a 3 olivine transition the pyrolite model of mantle petrology and conductivities derived from csem sounding we propose archie s law with exponent 2 and interconnected tubes as realistic lower and upper bounds for the effect of partial melt on rock conductivity and archie s law with exponent 1 5 as the preferred estimate the 1 d forward response of this model is not compatible with observed sea floor mt data three data sets presented by oldenburg 1981 pass a test for one dimensionality based on the size of the residuals when fit with parker s d algorithm but two of the three soundings fail a test for independence of residuals we also find that the presence of the upper mantle high conductivity zone previously inferred from these data is highly dependent on data misfit and not required when the misfit criterion is relaxed a reasonable amount re inversions of the mt data produce models which are incompatible with our petrological model of mantle conductivity however by adding an ocean with various coastlines of simple geometry to our petrological model and solving the forward 3 d problem using thin sheet analysis we predict mt responses which are distorted in a manner that is remarkably similar to the observed data

induction arrows from offshore floating magnetometers using land reference data summary induction arrows are a traditional output of magnetovariational experiments and represent transfer functions used quantitatively in the inversion of geomagnetic depth sounding data to give earth electrical conductivity structure in this paper a technique is tested in which total eld variations are combined with horizontal eld data recorded simultaneously at remote stations in order to derive induction arrows the method is rst demonstrated using total eld data recorded on land by an aeromagnetic base station and then applied to data obtained from magnetometers oated oishore on the sea surface the oating magnetometers were deployed in two con gurations one tethered to the sea oor the second free oating

decomposition analysis of the bear magnetotelluric data implications for the upper mantle conductivity in the fennoscandian shield decomposition analysis of the bear magnetotelluric data implications for the upper mantle conductivity in the fennoscandian shield

review paper instrumentation for marine magnetotelluric and controlled source electromagnetic sounding we review and describe the electromagnetic transmitters and receivers used to carry out magnetotelluric and controlled source soundings in the marine environment academic studies using marine electromagnetic methods started in the 1970s but during the last decade these methods have been used extensively by the offshore hydrocarbon exploration industry the principal sensors magnetometers and nonpolarizing electrodes are similar to those used on land but magnetotelluric field strengths are not only much smaller on the deep sea floor but also fall off more rapidly with increasing frequency as a result magnetotelluric signals approach the noise floor of electric field and induction coil sensors 0 1 nv m and 0 1 pt at around 1 hz in typical continental shelf environments fluxgate magnetometers have higher noise than induction coils at periods shorter than 500 s but can still be used to collect seafloor magnetotelluric data down to 40 100 s controlled source transmitters using electric dipoles can be towed continuously through the seawater or on the sea bed achieving output currents of 1000 a or more limited by the conductivity of seawater and the power that can be transmitted down the cables used to tow the devices behind a ship the maximum source receiver separation achieved in controlled source soundings depends on both the transmitter dipole moment and on the receiver noise floor and is typically around 10 km in continental shelf exploration environments the position of both receivers and transmitters needs to be navigated using either long baseline or short baseline acoustic ranging while sea floor receivers need additional measurements of orientations from compasses and tiltmeters all equipment has to be packaged to accommodate the high pressure up to 40 mpa and corrosive properties of seawater usually receiver instruments are self contained battery powered and have highly accurate clocks for timekeeping even when towed on the sea floor or in the water column behind a transmitter

inventory of sedimentary organic matter in modern wetland marais vernier normandy france as source indicative tools to study holocene alluvial deposits lower seine valley france palynofacies analysis was developed for petroleum exploration but it is now commonly used to differentiate the major constituents of the sedimentary organic matter som in postglacial deposits our work aimed at studying the som distribution in a present wetland of the seine estuary vernier marsh in order to provide comparative elements to interpret the composition of som in the lower seine valley alluvial infilling in the vernier marsh the distribution of major constituents is heterogeneous thus for each depositional environment studied soils drainage ditches channels ponds the composition of som has been characterized by using two ternary diagrams amorphous om preserved phytoclasts transformed phytoclasts and opaque amorphous gelified particles op ap gp this approach allowed us to define four optical indices that can be used to estimate the relative abundances of principal organic fractions in mixed deposits based on the origin of som aquatic terrestrial its source areas vegetation soils river and controls of early diagenetic changes aerobic anaerobic conditions advanced oxidation amorphous om contents increase from terrestrial environments to fluvial deposits and evaluate the abundance of planktonic contribution preserved transformed phytoclasts ratio decreases from sites of production to those of deposition indicating the degree of degradation of terrestrial materials op contents allow one to define fluvial high values and palustrine low values environments gp ap ratio allows one to discern soil low values and plant high values supplied areas finally these optical compositional indices are measured in holocene deposits and used to complete palaeoenvironmental reconstructions

crustal evolution of fennoscandia palaeomagnetic constraints abstract palaeomagnetic poles from fennoscandia ranging in age from archaean to tertiary are compiled and graded using a modified briden duff classification scale an new filtering technique is applied to select only the most reliable poles for analysis the filtering takes into account the following information 1 1 source block of rock unit 2 2 age of rock 3 3 age of magnetization component 4 4 scatter of palaeomagnetic directions 5 5 information from multicomponent analysis of natural remanent magnetization nrm 6 6 whether the pole considered belongs to a cluster or subcluster of poles 7 7 magnetic polarity and 8 8 the author s original assignment of results data are still insufficient for the drawing of separate apparent polar wander paths apwp for different blocks or cratons of fennoscandia treating fennoscandia as a single plate a new apwp from archaean to permian is constructed from the five previously drawn apwp loops or hairpins only one the jatulian loop 2200 2000 ma disappears in filtering the loops during 1925 1700 ma and during 1100 800 ma ago are linked to svecofennian and sveconorwegian orogenies respectively palaeomagnetic data support the concept that these orogenies took place episodically three distinct orogenic pulses early middle and late can be distinguished in the cluster plots of palaeopoles the drift history of fennoscandia from archaean to permian is presented during most of geological history fennoscandia has occupied low to moderate latitudes and undergone considerable latitudinal shifts and rotations the svecofennian and sveconorwegian orogenies have different kinematic characteristics during the svecofennian orogeny fennoscandia drifted slowly while rotating a large amount in an anticlockwise sense during the sveconorwegian orogeny it drifted rapidly and rotated first clockwise and then anticlockwise the most striking feature in the drift velocity curves is however the pronounced maxima in the latitudinal drift and rotation rates 9 cm yr and 0 8 ma respectively during the late subjotnian jotnian anorogenic magmatism and rifting phase 1450 1250 ma ago possibly reflecting the passage of fennoscandia across a thermal upwelling hotspot at equatorial latitudes the use of palaeomagnetism in delineating and dating movements between blocks is demonstrated with three examples from the polar profile area the northernmost section of the european geotraverse

crustal conductors along the sveka profile in the fennoscandian baltic shield finland summary electrical properties of the crust in the central fennoscandian baltic shield are examined along the 435 km magnetotelluric profile sveka a 2 d geoelectric model shows large variations in the crustal resistivity with the model resistivities ranging from 0 1 to 35 000 m the main geoelectrical features of the crust and upper mantle are 1 a resistive archaean lower crust conductance 30 s 2 a conductive palaeoproterozoic lower crust beneath the central finland granitoid complex 170 900s free h2o rich fluids trapped into the lower crust seem less plausible causes for enhanced conductivity as long retention times are required and no possible sources seem to be available to produce fluids into the dry lower crust most likely causes for permanent enhancement of lower crustal conductivity include the conductive fill of ductile shear zones conductive graphite bearing lamellae conduction in mafic rocks perhaps via conduction in magnetite or conductive solid remnants of an ancient shear zone between two crustal blocks thin graphitic grain boundary films precipitated from co2 rich fluids also seem less plausible 3 a very resistive 10 000 35 000 m upper crust of the palaeoproterozoic central finland granitoid complex and the archaean domain 4 narrow and elongated conductors 0 1 100 m beneath the palaeoproterozoic kainuu ksb and tampere tsb schist belts are caused by graphite bearing metasedimentary rocks buried deep into the crust in the collisions of crustal terranes during the formation of the palaeoproterozoic svecofennian crust the 500 km long tsb conductor represents an internal svecofennian terrane boundary whereas the ksb conductor is a part of a discontinuous archaean proterozoic boundary conductor 5 resistive upper mantle there are no indications of an upper mantle conducting layer within the uppermost 300 km in the central part of the fennoscandian shield

episodic melt transport at mid ocean ridges inferred from magnetotelluric sounding oceanic crust is generated at mid ocean ridges by decompression melting of upwelling mantle at depths of between 50 and 120 km geodynamic and geochemical models of upwelling melt extraction and melt emplacement into crustal magma reservoirs present a variety of possible migration geometries most of which assume steady state or near steady state processes here we present results from marine magnetotelluric mt measurements carried out as part of the ramesses experiment on the slow spreading reykjanes ridge which support a model of melt extraction and migration that is episodic rather than steady state

magnetotelluric observations across the juan de fuca subduction system in the emslab project a magnetotelluric mt transect has been obtained near latitude 45 n from the active juan de fuca spreading center across the subduction zone and cascades volcanic arc and into the back arc deschutes basin region this paper presents the mt data set and describes its major characteristics as they pertain to the resistivity of the subduction system in addition we discuss the measurement and processing procedures employed as well as important concerns in data interpretation broadband audiomagnetotelluric amt mt soundings approx 0 01 500 s period were collected on land with considerable redundancy in site location and from which 39 sites were selected which constrain upper crustal heterogeneity but sense also into the upper mantle fifteen long period mt recordings about 50 10 000 s on land confirm the broadband responses in their common period range and extend the depths of exploration to hundreds of kilometers on the juan de fuca plate offshore 33 out of 39 sea floor instruments at 19 locations gave good results of these locations five magnetotelluric soundings plus two additional geomagnetic variation sites covering the period range 200 105 s approximately constitute the ocean bottom segment of our profile the feature of the land observations which probably relates most closely to the subduction process is a peak in the impedance phase of the transverse magnetic mode around 30 50 s period this phase anomaly with a corresponding inflection in the apparent resistivity is continuous eastward from the seacoast and ends abruptly at the high cascades it signifies an electrically conductive layer in otherwise resistive lower crust or upper mantle with the layer conductance decreasing eastward from the coast to a minimum under the coast range but increasing suddenly to the east of the central willamette basin the higher conductance to the east is corroborated by the vertical magnetic field transfer function whose real component shows negative values in the period range 100 1000 s over the same distance the transverse electric mode apparent resistivity and phase on the land display a variety of three dimensional effects which make their interpretation difficult conversely both modes of the ocean floor soundings exhibit a smooth progression laterally from the coastal area to the spreading ridge indicating that the measurements here are reflecting primarily the large scale tectonic structures of interest and are little disturbed by small near surface inhomogeneities the impedance data near the ridge are strongly suggestive of a low resistivity asthenosphere beneath resistive juan de fuca plate lithosphere approaching the coastline to the east both impedance and vertical magnetic field responses appear increasingly affected by a thick wedge of deposited and accreted sediments and by the thinning of the seawater

seafloor magnetotelluric sounding above axial seamount axial seamount is a large active ridge axis volcano located on the central segment of the juan de fuca ridge in the northeast pacific ocean magnetotelluric mt data have been collected at three sites approximately 4 km apart around the eastern rim of the volcano during a 65 day deployment mt responses in the bandwidth of 10 105 s are almost isotropic with a weakly defined principal direction of strike parallel to the main topographic trends of axial seamount and are relatively flat over the whole bandwidth apparent resistivities are of the order of 7 20 m and phases are as low as 30 at the short periods diagonal terms of the mt tensor are an order of magnitude smaller than the off diagonal terms suggesting that three dimensional effects on the data are minimal two dimensional inversions suggest that seafloor bathymetry and the distant coastlines have a surprisingly small effect on the mt response and one dimensional inversions fit the mt data to within the errors with no serial correlation in the residuals a low crustal resistivity is the most robust part of the model probably due to seawater in fractures and possibly due to a magma chamber an electrical asthenosphere although less well constrained exists over a depth range 30 60 km and the resistivity of this region is compatible with about 8 fraction of melt

3 d analysis and interpretation of magnetotelluric data from the aluto langano geothermal field ethiopia the main ethiopian rift valley encompasses a number of volcanoes which are known to be actively deforming with reoccurring periods of uplift and setting one of the regions where temporal changes take place is the aluto volcanic complex it hosts a productive geothermal field and the only currently operating geothermal power plant of ethiopia we carried out magnetotelluric mt measurements in early 2012 in order to identify the source of unrest broad band mt data 0 001 1000 s have been acquired at 46 sites covering the expanse of the aluto volcanic complex with an average site spacing of 1 km based on this mt data it is possible to map the bulk electrical resistivity of the subsurface down to depths of several kilometres resistivity is a crucial geophysical parameter in geothermal exploration as hydrothermal and magmatic reservoirs are typically related to low resistive zones which can be easily sensed by mt thus by mapping the electrical conductivity one can identify and analyse geothermal systems with respect to their temperature extent and potential for production of energy 3 d inversions of the observed mt data from aluto reveal the typical electrical conductivity distribution of a high enthalpy geothermal system which is mainly governed by the hydrothermal alteration mineralogy the recovered 3 d conductivity models provide no evidence for an active deep magmatic system under aluto forward modelling of the tippers rather suggest that occurrence of melt is predominantly at lower crustal depths along an off axis fault zone a few tens of kilometres west of the central rift axis the absence of an active magmatic system implies that the deforming source is most likely situated within the shallow hydrothermal system of the aluto langano geothermal field

lacustrine organic fluxes and palaeoclimatic variations during the last 15 ka lac du bouchet massif central france to assess the influence of climatic changes on organic lacustrine sedimentation two cores recovered from the centre of the lac du bouchet were studied by petrographical palynofacies and geochemical methods only core ldb h was used for estimation of the organic fluxes the variation of these fluxes with climo stratigraphic periods showed low organic fluxes during the lateglacial an increase at the beginning of the holocene a minimum at the end of the atlantic period resulting from the climatic cooling and a maximum at the end of the sub boreal related to the installation of the present climatic conditions

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

electromagnetic studies in the fennoscandian shield electrical conductivity of precambrian crust abstract electromagnetic em investigations of the 1980s in the fennoscandian baltic shield produced an unique and unified em data set studies include regional investigations by the magnetovariational mv method with large lateral sampling distance investigations of anomalous conductivity structures by magnetotelluric mt soundings and other em and electrical methods audio mt soundings d c dipole dipole and vlf resistivity profilings with shorter sampling distance and studies of the near surface conductivity by airborne em surveys the variety of methods provide an ability to map efficiently crustal conductivity structures from a regional scale of hundreds of kilometres down to local details of some metres in the anomalous structures the precambrian of the fennoscandian shield is characterized by roughly nw se directed elongated belts of conductors which separate more resistive crustal blocks the latter serve as transparent windows through which to probe deep electrical structure and belts of conductors as tectonic markers of ancient orogenic zones including 1 the kittila vetrenny poyas conductor 2 the lapland granulite belt and inari pechenga imandra varzuga conductors 3 the archaean proterozoic boundary conductor and 4 the southern finland conductor the conductive belts orogenic conductors indicate places where crustal masses collided and were finally sealed together enhanced conductivity in the orogenic conductors is caused primarily by an electronic conducting mechanism in graphite and sulphide bearing metasedimentary rocks estimations of the lower crustal conductivity indicate a laterally heterogeneous lower crust in the fennoscandian shield archaean lower crust seems to be in general more resistive than the early proterozoic lower crust of the karelian and svecofennian domains the lower crust in the southwestern part of the svecofennian domain and in the sveconorwegian domain seems to be more resistive than in the central part of the svecofennian domain