imaging and observing the electrical moho abstract defining the depth to the base of the crust is of profound importance for understanding and developing theories of the secular variation of our planet including crustal formation and evolution since its discovery 100 year ago the seismic moho is almost ubiquitously used as the crust mantle boundary reference laboratory studies on crustal and mantle rocks suggest that there should be a concomitant step lie change in electrical conductivity an electric moho emoho at the crust mantle boundary we examine resolution properties of electrical and electromagnetic methods for imaging and observing an emoho for three generic models one to represent the archean one for the proterozoic and the third for the phanerozoic we show that in all three cases given the existence in most localities of a conducting lower crust compared to the upper crust and upper lithospheric mantle the problem is difficult and at the edge and often beyond confident resolution for some highly unusual localities however the lower crust is more resistive than the underlying upper mantle and in those cases it is possible to resolve an emoho but very high quality data are required examples of emoho observations around the globe are discussed focussing on the results from a site on the southwestern part of the slave craton northwestern canada

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

enhanced mantle conductivity from sulfide minerals southern sierra nevada california petrographic studies of peridotitic xenoliths entrained in late quaternary basalts from beneath the southern sierra nevada have revealed the presence of accessory sulfide minerals along grain boundaries and fractures equilibration temperatures from the xenoliths are sufficiently high that the molten sulfides coexist with the basaltic melt sulfides are extremely conductive relative to the solid matrix or the basaltic melt so a small fraction can increase the bulk conductivity of the mantle appreciably previous estimates of 2 5 partial melt from magnetotelluric measurements can be plausibly reduced to less than 1 such low melt percentages have longer residence times in the mantle and are more consistent with the volumetrically minor late quaternary basalt flows and the primitive basalt compositions

area selection for diamonds using magnetotellurics examples from southern africa abstract southern africa particularly the kaapvaal craton is one of the world s best natural laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data that exist for it the southern african magnetotelluric experiment samtex was launched to complement these databases and provide further constraints on physical parameters and conditions by obtaining information about electrical conductivity variations laterally and with depth initially it was planned to acquire magnetotelluric data on profiles spatially coincident with the kaapvaal seismic experiment however with the addition of seven more partners to the original four through the course of the experiment samtex was enlarged from two to four phases of acquisition and extended to cover much of botswana and namibia the complete samtex dataset now comprises mt data from over 730 distinct locations in an area of over one million square kilometres making samtex the largest regional scale mt experiment conducted to date preliminary images of electrical resistivity and electrical resistivity anisotropy at 100 km and 200 km constructed through approximate one dimensional methods map resistive regions spatially correlated with the kaapvaal zimbabwe and angola cratons and more conductive regions spatially associated with the neighbouring mobile belts and the rehoboth terrane known diamondiferous kimberlites occur primarily on the boundaries between the resistive or isotropic regions and conductive or anisotropic regions comparisons between the resistivity image maps and seismic velocities from models constructed through surface wave and body wave tomography show spatial correlations between high velocity regions that are resistive and low velocity regions that are conductive in particular the electrical resistivity of the sub continental lithospheric mantle of the kaapvaal craton is determined by its bulk parameters so is controlled by a bulk matrix property namely temperature and to a lesser degree by iron content and composition and is not controlled by contributions from interconnected conducting minor phases such as graphite sulphides iron oxides hydrous minerals etc this makes quantitative correlations between velocity and resistivity valid and a robust regression between the two gives an approximate relationship of v s m s 0 045 log resistivity ohm m 4 5

area selection for diamond exploration using deep probing electromagnetic surveying abstract previously proposed methods of area selection for diamond prospective regions have predominantly relied on till geochemistry airborne geophysics and or an appraisal of tectonic setting herein we suggest that a novel deep probing geophysical technique electromagnetic studies using the natural source magnetotelluric mt method can contribute to such an activity essentially diamondiferous regions must have 1 old lithosphere 2 thick lithosphere and 3 lithosphere that contains high concentrations of carbon deep probing mt studies are able to address all three of these the second and the third of these can be accomplished independently using mt but for the first the geometries produced from modelling the mt observations must be interpreted with appropriate interaction with geologists geochemists and other geophysicists examples are given from the slave and superior cratons in north america with a brief mention of an area of the rae craton and general speculations about possible diamondiferous regions

flow and electrical anisotropy in the upper mantle finite element models constraints on the effects of olivine crystal preferred orientation and microstructure long period magnetotelluric mt data shows that electrical conductivity in the upper mantle is highly anisotropic agreement between high electrical conductivity directions and seismic anisotropy fast directions suggests that anisotropic diffusion of hydrogen along oriented olivine crystals controls the anisotropy of electrical conductivity in the upper mantle since both seismic waves and hydrogen diffusion are faster along the 1 0 0 axis of olivine thus mt electrical anisotropy data like seismic anisotropy may be used to map flow patterns in the upper mantle however observed electrical anisotropies are significantly higher than seismic ones to quantify the influence of strain induced crystal preferred orientations of olivine on upper mantle bulk electrical conductivities we calculate the macroscopic electrical conductivity anisotropy of a series of naturally and experimentally deformed peridotites using an anisotropic finite element model these models which fully take into account the microstructure crystal and shape preferred orientations based on orientation maps obtained by indexation of electron back scattered diffraction ebsd patterns show macroscopic electrical anisotropy factors ranging from 3 to 16 the intensity of electrical anisotropy depends to first order on the intensity of the olivine crystal preferred orientations but the relation saturates for strong crystal preferred orientations in addition the spatial distribution of the various crystal orientations may significantly enhance influence the anisotropy in strongly textured mantle rocks the strongest anisotropy factors 10 occur in mantle rocks in which deformation by dislocation creep has produced not only crystal but also strong shape preferred orientations even if the latter is masked by recrystallization higher anisotropy factors 100 observed in a few mt experiments imply however an additional as yet unknown mechanism controlling electrical conduction at asthenospheric depths in these regions

electric lithosphere of the slave craton the archean slave craton in northwestern canada is an ideal natural laboratory for investigating lithosphere formation and evolution and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes three deep probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures the magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity 30 m at depths of 80 100 km and is interpreted to be due to dissolved hydrogen or carbon in graphite form this geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer the tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630 2620 ma

the unusual sedimentary rock record of the early triassic a case study from the southwestern united states abstract sedimentary systems as well as the earth s biota changed dramatically following the end permian mass extinction this global sedimentological transformation is reflected in the widespread occurrence of flat pebble conglomerates subtidal wrinkle structures microbialites and carbonate seafloor fans in various lower triassic sections as a case study a facies analysis of the lower triassic virgin limestone member of the moenkopi formation and the middle and upper members of the union wash formation southwestern united states was conducted and all unusual facies were documented within a palaeoenvironmental framework the occurrence of the flat pebble conglomerates and wrinkle structures is significant because these reflect a long term reduction in vertical bioturbation in the case of the subtidal microbialites and carbonate seafloor fans their formation is linked to an increase in alkalinity the presence of these facies in the uppermost lower triassic strata of the southwestern united states indicates that reduced levels of infaunal bioturbation as well as and perhaps a consequence of unusual ocean chemistry extended for 5 6 million years after the end permian mass extinction suggesting that depositional environments as well as the biota were significantly affected by the extinction event

geoelectric structure of the proterozoic wopmay orogen and adjacent terranes northwest territories canada magnetotelluric mt soundings were made along a transect in northern canada crossing the proterozoic wopmay orogen fort simpson basin and adjacent parts of the slave craton and the nahanni terrane the results are used to define the geoelectric structure and constrain the crustal and lithospheric structure and evolution across the wopmay orogen geoelectric strikes at crustal depths average n34 e and are interpreted to be related to transcurrent faulting that occurred during late distal collisions at the western margin of the orogen weak two dimensionality in the fort simpson basin is interpreted to be due to the sedimentary rocks in the basin at longer periods geoelectric strikes across the wopmay orogen rotate from n43 e at uppermost mantle penetration to n62 e at a depth of 100 km the uppermost mantle strikes are interpreted to be due to ductile shearing linked to the transcurrent faulting in the overlying crust the deeper strikes may be caused by shearing at the base of the present day lith

crustal and lithospheric scale structures of the precambrian superior grenville margin abstract the lithosphere beneath the margin of the archean superior and proterozoic grenville provinces was investigated with a northwest southeast oriented 650 km long profile of 40 magnetotelluric stations dominant geoelectric strike azimuths of n45 e and n85 e were defined for the crust and the lithospheric mantle respectively a 2 d isotropic resistivity model derived using the crustal strike images resistive laurentian margin rocks dipping southeast to the base of the crust bounded to the northwest by the grenville front and to the southeast by the central metasedimentary belt boundary zone the observation is in contrast to conductive mid to lower crust elsewhere in the region a 2 d isotropic resistivity model determined using the lithospheric mantle strike azimuth reveals an extremely resistive region in the upper 100 km of the mantle lithosphere of the northern grenville province the geometry of this body which includes a well defined base and southeast dip suggests that it is superior lithosphere a sub vertical conductor located approximately 50 km along strike from the mesozoic kirkland lake and cobalt kimberlite fields is interpreted to be due to re fertilization of an older mantle scar the resistivity model includes a horizontal conductor at 160 km depth beneath the southern superior province that is possibly the resistivity signature of the lithospheric asthenospheric boundary

lithospheric geometry of the wopmay orogen from a slave craton to bear province magnetotelluric transect 1 two dimensional inversions of lithospheric probing magnetotelluric mt data at a total of 20 sites acquired along an approximately east west 300 km long profile across the wopmay orogen in the northwest territories canada provide electrical resistivity models of the boundary between the archean slave craton and the adjacent proterozoic bear province an analysis of distortion effects and structural dimensionality indicates that the mt responses are primarily one dimensional or only weakly two dimensional with a depth independent geoelectric strike angle of n32 e consistent with regional structural geology the regional scale model generated from the longer period responses from all of the sites along the profile reveals significant lateral variations in the lithospheric mantle resistive cratonic roots are imaged to depths of 200 km beneath both the slave craton and the hottah terrane of the bear province these are separated by a less resistive region beneath the great bear magmatic zone which is speculatively interpreted as a consequence of a decrease in the grain size of olivine in the wopmay mantle caused by localized shearing compared to its neighboring cratonic roots focused two dimensional models from higher frequency responses at sites on specific sections of the profile reveal the resistivity structure at crustal depths beneath the region these suggest that the root of the slave craton crosses beneath the wopmay orogen and that the wopmay fault zone does not penetrate into the lower crust a comparison of these results with those obtained during the lithoprobe project farther south shows striking along strike variations in the conductivity structure associated with the wopmay orogen

wetting of mantle olivine by sulfide melt implications for re os ratios in mantle peridotite and late stage core formation this study investigates the effects of variations in the relative fugacities of oxygen and sulfur on the wetting of mantle olivine by molten sulfide experiments were performed on mixtures of san carlos olivine and synthetic fes at 1 bar and 1350 c crucibles were fabricated from san carlos olivine and the fugacities of oxygen and sulfur were controlled by mixing co2 co and so2 gases experimental conditions ranged from logfo2 7 9 to 10 3 and from logfs2 1 5 to 2 5 our experimental results demonstrate that at a given temperature and pressure the olivine sulfide melt dihedral angle is controlled by the concentration of o dissolved in an anion rich melt trace amounts of o dissolve in sulfide melt at fo2 conditions near the iron wustite oxygen buffer and the dihedral angle is 90 at fo2 conditions near the fayalite magnetite quartz oxygen buffer the concentration of dissolved o is near 9 wt and the dihedral angle is 52 allowing small amounts of sulfide melt to form an interconnected network in olivine rich rocks and to migrate via porous flow these results indicate that sulfide melt is likely to be mobile at current upper mantle fo2 and fs2 conditions in mantle peridotite the addition or removal of sulfide melt by porous flow will variably fractionate re os u pb and th pb ratios because os and pb are more chalcophile than re u and th the re os ratio of the peridotite is especially sensitive to this process the mobility of sulfide melt at oxidizing conditions implies that the addition of oxidized chondritic material during the later stages of the accretion of the earth may have facilitated the segregation of core forming material by porous flow if temperatures were in excess of the sulfide solidus

the role of the global carbonate cycle in the regulation and evolution of the earth system we review one of the most ancient of all the global biogeochemical cycles and one which reflects the profound geochemical and biological changes that have occurred as the earth system has evolved through time that of calcium carbonate caco3 in particular we highlight a mid mesozoic revolution in the nature and location of carbonate deposition in the marine environment driven by the ecological success of calcareous plankton this drove the creation of a responsive deep sea sedimentary sink of caco3 the result is that biologically driven carbonate deposition provides a significant buffering of ocean chemistry and of atmospheric co2 in the modern system however the same calcifying organisms that under pin the deep sea carbonate sink are now threatened by the continued atmospheric release of fossil fuel co2 and increasing acidity of the surface ocean we are not yet in a position to predict what the impact on caco3 production will be or how the uptake of fossil fuel co2 by the ocean will be affected this uncertainty in the future trajectory of atmospheric co2 that comes from incomplete understanding of the marine carbonate cycle is cause for concern

evidence from borehole samples for the role of accessory minerals in lower crustal conductivity the high electrical conductivity of the lower continental crust as inferred from geophysical measurements1 has defied a simple explanation both pore saturating brines2 and interconnected carbon films3 5 have been proposed as conducting pathways but their relative importance remains uncertain and may vary from place to place6 so far attempts to address this question in the laboratory have had to rely on samples of metamorphic rock collected from surface exposures7 10 although these rocks almost certainly originated in the lower crust chemical alteration during their transport to and residence at the surface is likely to have affected their conducting properties here we report conductivity data for pristine crustal rocks recovered from depths of up to 4 6 km in the ktb boreholes in southern germany our results show that the electrical conductivity of accessory phases such as fe ti oxides and sulphides can enhance or even surpass the high conductivity produced by saline fluids

3d modelling of electrical anisotropy from electromagnetic array data hypothesis testing for different upper mantle conduction mechanisms abstract upper mantle electrical conductivities provide besides seismic velocities or thermal diffusivities another source of information on upper mantle structure and composition we present the results of a long period magnetotelluric dataset consisting of 64 sites covering an area of 300 km 400 km in central germany mapping electrical conductivities at the base of the lithosphere to seperate the effect of crustal and upper mantle structures we interprete the obtained dataset by 3d conductivity modelling we observe high conductivities and anisotropies at asthenospheric upper mantle depth with main bulk conductivities of 1 2 s m in west east and 4 1 0 3 s m in north south direction differing by about two orders of magnitude in comparing with results from laboratory measurements on electrical conductivites for various conduction mechanisms we are able to confine the conduction mechanisms possibly dominating at these depths with melt volume fractions of 1 only partial melt can be excluded as origin of the high upper mantle conductivities instead conductivity enhancement due to hydrogen diffusion along the olivine 1 0 0 axis is favoured as the dominant conduction mechanism at the base of the lithosphere

imaging three dimensional crustal conductivity structures reflecting continental flood basalt effects hidden beneath thick intracratonic sedimentary basin a large scale array of long period magnetic data and a deep probing magnetotelluric profile were recorded in the intracratonic parana sedimentary basin in central eastern south america which presents a thick and extensive sedimentary magmatic sequence that allows its basement to be investigated only by indirect methods integration of the results from both methods showed that the crust beneath the basin presents several quasi linear highly conductive channeled zones with limited lateral extent in coincidence with some of the main tectonic structures recognized at the surface and a moderate but pervasive lithosphere conductivity enhancement beneath its central part upward movement of co2 bearing volatiles and magmas precipitating highly conducting mineral phases along discrete subvertical fault zones that served as feeder conduits for early cretaceous voluminous continental flood basalts was a likely process responsible for the localized conductivity enhancements correlation between some of the linear conductive zones and elongated magnetic anomalies and between the maximum depth occurrence of most of these conductive anomalies and the curie depth at which crustal rocks lose their magnetism gives strong support to interconnected iron oxides especially magnetite and iron sulfides such as pyrrhotite as the main conductive sources the moderate bulk conductivity increase in the crust and upper mantle beneath the central part of the basin is unexpected for a postulated cratonic basement and is tentatively associated with impregnation of the lithosphere by conducting minerals related either to widespread tectonic events in the ordovician or late precambrian or to dispersed magmatic residues of an early cretaceous magma differentiation contaminating the entire lithosphere

platinum group element geochemistry of peridotite xenoliths from the sierra nevada and the basin and range california abstract the nature of pge re pge pt pd os ir ru behavior in subcontinental lithospheric mantle was investigated using new high precision pge re abundance measurements and previously published re os isotopic analyses of peridotite xenoliths from the sierra nevada and mojave province california ru ir ratios and ir concentrations are constant over a wide range in s content and major element fertility indices e g mg mg fe indicating that ru and ir are not only compatible during partial melting but also that their partitioning behaviors may not be controlled entirely by sulfide pt ir pd ir os ir and re ir ratios range from slightly superchondritic to distinctly subchondritic for all xenoliths except for one anomalous sample 1026v which is characterized by radiogenic 187os 188os low re os ratio and large enrichments in cu os pt pd and s relative to ir copps metasomatism assuming chondritic initial relative abundances the magnitudes of some of the depletions in pt pd os and re relative to ir and ru require incompatible behavior or substantial secondary loss in detail some samples which are otherwise characterized by fertile major element indices exhibit low s contents and subchondritic os ir and pd ir ratios indicating that depletions in pd and os relative to ir are not simple functions of the degree of melting as inferred from major elements possible mechanisms for depleting pt pd os and re relative to ir and ru include partitioning into chromian spinels and alloys partitioning between sulfide and sulfide liquids mobilization by aqueous fluids or secondary loss associated with late stage sulfide breakdown however it is not possible to explain all of the depletions in pt pd os and re by any single mechanism the preferential enrichment in os over re and ir in sample 1026v is somewhat paradoxical because this sample s radiogenic 187os 188os requires a metasomatic agent originating from a source with a high time integrated re os ratio the abundant garnet websterite xenoliths may be a suitable source because they have high re os ratios radiogenic os and abundant garnet which may sequester re over os during partial melting however their extremely low os contents require the processing of large amounts of garnet websterite to concentrate enough os into the metasomatic sulfides needed to enrich sample 1026v in os the homogeneity in 187os 188os ratio in the remaining xenoliths suggest that their os isotopic compositions were not significantly affected by pge metasomatism the singular nature of 1026v s composition emphasizes the rarity of copps metasomatism

trace fossils and the aftermath of the permo triassic mass extinction evidence from northern italy abstract the duration of early triassic anoxia in the carbonate ramp settings of the werfen formation dolomites northern italy has been constrained using ichnofabric and sedimentological data this includes fine lamination and abundant pyrite framboids commonly reworked into lags by storm events the anoxic event spanned the entire griesbachian stage and extended from deep waters into extraordinarily shallow water settings evidence for oxygen deficiency can be found in mid and inner ramp facies the extent and duration of this event may at least partially explain the magnitude of the permo triassic mass extinction normal benthic conditions returned rapidly in the dienerian stage and pervasively bioturbated distal ramp facies testify to the ventilation of the deeper water areas of the region in post griesbachian times a post mass extinction radiation is not particularly well displayed in either trace fossil or body fossil diversity compilations from the dolomites moderate low levels of trace fossil diversity are maintained throughout the post griesbachian werfen formation whilst the body fossil data shows a moderate increase from the mid smithian stage there is thus a curious lag period spanning the dienerian to mid smithian when environmental conditions were apparently normal but the marine fauna failed to show any recovery

the electrical resistivity of canada s lithosphere and correlation with other parameters contributions from lithoprobe and other programmes over the last 30 years through lithoprobe and other programmes modern high quality magnetotelluric mt measurements probing deep into the lithosphere and underlying asthenosphere have been made at over 6000 sites across canada in all provinces and territories except nova scotia some regions are well covered particularly alberta southern british columbia and western ontario whereas others remain poorly covered such as quebec and large swaths of nunavut prior publications from individual studies have added significantly to the wealth of canada s geoscience knowledge and have demonstrated that mt can contribute significantly to understanding of the tectonic processes that have shaped canada however to date no continent scale maps of lithospheric electrical parameters have been constructed from the extensive mt database herein we review the contributions made by the mt components of lithoprobe and present new continental scale maps of various electrical parameters at crustal and upper mantle

lithospheric electrical imaging of the deccan trap covered region of western india 1 we present here the results of magnetotelluric mt studies carried out along a 330 km long ne sw trending traverse from daulatabad at the northeastern end to koyna on the southwestern end across the deccan volcanic province dvp in western india the 2 d mt modeling results obtained along this traverse together with those from other mt traverses in this area are examined and integrated with the results from deep seismic sounding 3 d teleseismic tomography and heat flow studies the deep geoelectric section as deduced from mt models point out to a two layered lithospheric mantle structures with an upper very high resistive layer several thousands of ohm m and a bottom relatively conductive one a few tens of ohm m to a few hundred ohm m the lithospheric electrical structure is inferred to be linked to compositional changes associated with a depleted lithospheric upper mantle in the dharwar craton an impressive feature that the study has brought out is the presence of a well defined lithospheric upper mantle conductor in the depth range of 80 120 km limited to the middle segment of the mt traverse from sangole to partur over the dvp the electrical characterization of the region including the presence of upper mantle conductor shows close similarities with that of other similar cratonic regions like the slave and superior cratons in northwestern canada

rapid and synchronous collapse of marine and terrestrial ecosystems during the end permian biotic crisis a newly studied permian triassic p tr boundary section in jameson land east greenland contains an abundant and well preserved marine fauna as well as terrestrial palynomorphs for the first time it is possible to compare the biotic crises of the marine and terrestrial realms using the same samples from the same section the sediments record a negative excursion in 13 c carb values of 8 9 and in 13 c org values of 10 11 the presence of the conodont hindeodus parvus combined with the 13 c carb record enables correlation with the proposed global stratotype section at meishan this shows that the greenland section is the most expanded p tr section known collapse of the marine and terrestrial ecosystems took between 10 and 60 k y it took a further few hundred thousand years for the final disappearance of permian floral elements collapse of the terrestrial and marine ecosystems began at the same stratigraphic level and preceded the sharp negative excursion in the 13 c record

magnetotelluric evidence of lithospheric mantle thinning beneath the southern sierra nevada a wideband 0 01 1000 s magnetotelluric survey across the southern sierra nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10 20 km the eastern zone underlies the highest topography of the range and extends eastward this eastern conductive zone extends to depths in excess of 100 km based on model sensitivities and lies well below the moho depth of 32 38 km from seismological studies therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt such an interpretation is consistent with gravitational seismological and geologic evidence estimates of the partial melt fraction range from 2 to 5 at depths of 40 70 km which are consistent with fractions of melt inclusions observed in xenoliths from the eastern sierra nevada this partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations the other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes the western conductive zone also straddles the moho and extends into the mantle beneath western foothills of the sierra nevada and the great valley sediments to the west however xenoliths from this region indicate high velocity crustal rocks to depths in excess of 60 km and we therefore attribute the enhanced conductivity to graphitic metasediments and or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the sierran plutons

upper mantle temperature determined from combining mineral composition electrical conductivity laboratory studies and magnetotelluric field observations application to the intermontane belt northern canadian cordillera abstract using laboratory derived temperature dependences of the electrical conductivity of mantle minerals coupled with appropriate mixing laws we determine the bulk conductivity of mantle mineral assemblages for the ternary olivine orthopyroxene clinopyroxene ol opx cpx system we calculate physical property bounds hashin shtrikman bounds as a function of the fraction of different phases present these limits correspond to the extreme situations where the most conducting phase is either fully interconnected or fully disconnected the relationships we present between temperature mineral composition and bulk electrical conductivity allow constraining one of them given the other two we apply this approach to an area of the north american cordilleran intermontane belt in the yukon territory northern canada where xenolith evidence indicates bimodal upper mantle mineral assemblages harzburgite and lherzolite this locality coincides spatially with an upper mantle region of low electrical conductivity determined by long period magnetotelluric data given the mantle mineral composition and the maximum and minimum bounds on the electrical conductivity deduced by non linear model appraisal we determine the permitted extremal temperature bounds of the intermontane belt mantle rocks directly below the moho to a depth of some 80 km we show that the mantle in this region is at a minimum temperature of 820 c and a maximum temperature of 1020 c the latter is some 200 c colder than that suggested in a recent interpretation of an observed collocated low velocity zone from a teleseismic survey

late early triassic microbial reefs of the western united states a description and model for their deposition in the aftermath of the end permian mass extinction abstract research in the western united states to reinvestigate and characterize the previously reported stromatolites of the latest early triassic spathian virgin limestone member of the moenkopi formation has shown that these microbial build ups formed patch reef mounds detailed field analysis of these reef mounds and the surrounding beds has established that they attained 1 2 m of topographic relief above the seafloor a petrographic study of these microbial mounds illustrates that they consisted of microbial fabrics as well as voids with early marine cements and formed in the absence of in situ metazoans these reef mounds formed during the aftermath of the end permian mass extinction that spanned the early triassic due to the devastation of marine metazoans at this extinction the early triassic has often been deemed a reef gap because no metazoan reefs are found at this time whereas colonial metazoan reefs are globally absent from lower triassic strata microbial build ups similar to these described from the western united states have been found in south china armenia southern turkey and greenland a regional early permian to late early triassic unconformity exists in the study region however analysis of these microbial reefs within a global context suggests that conditions favouring microbial reef development must have existed as long as 4 8 m y after the end permian mass extinction a depositional model of these microbial mounds indicates that flooding of the shelf with anoxic and or co2 rich deep waters may have fostered their occurrence

three dimensional electrical resistivity of the north central usa from earthscope long period magnetotelluric data abstract we present initial results from three dimensional inversion of long period earthscope magnetotelluric mt transportable array data from 232 sites covering the north central us the study area covers the 1 1 ga mid continent rift mcr system which cuts across a series of archean and paleoproterozoic lithospheric blocks the western arm of the mcr is clearly evident in shallow depth sections with a narrow resistive core flanked by elongate conductive basins other prominent upper crustal features mapped include the moderately conductive michigan and illinois basins and extremely high conductivities in foreland basin rocks at the southern margin of the superior craton the most prominent conductive anomalies in an otherwise relatively resistive mid lower crust are two elongate east west oriented structures which are closely aligned with previously inferred continental sutures the first underlies the southern margin of the superior craton just north of the niagara fault nf and can be associated with the 1 85 ga penokean orogeny a second further south beneath iowa and western wisconsin lies just south of the spirit lake tectonic zone sltz and can be identified with yavapai accretion at 1 75 ga both of these conductive sutures are cleanly cut by the mcr which is otherwise not clearly evident in the deeper parts of the resistivity model the break in the anomalies is narrow comparable to the surface expression of the mcr indicating that rifting impacts on the entire crustal section were highly localized both suture related anomalies are imaged as extending into and perhaps through the lithosphere as dipping diffuse zones of reduced mantle resistivity sense of dip of these structures southward for the nf anomaly northward for sltz agrees with previously inferred models for subduction and accretion suggesting that a conductive phase most likely carbon has been thrust deep into the lower crust and uppermost mantle providing a marker of the three dimensional boundary between lithospheric blocks resistivities drop below 100 m below 200 km depth in rough agreement with the seismically determined lithosphere asthenosphere boundary lab there are modest lateral variations in this deep low resistivity layer but the reliability and significance of these are not yet clear

proliferation of early triassic wrinkle structures implications for environmental stress following the end permian mass extinction lower triassic strata in the western united states and northern italy have yielded the first reported occurrence of microbially mediated wrinkle structures in shallow subtidal siliciclastic paleoenvironments since the cambrian we hypothesize that wrinkle structures formed under reduced infaunalization conditions during the aftermath of the end permian mass extinction these wrinkle structures occur in siliciclastic sedimentary rocks in association with hummocky cross stratification and in some cases trace fossils such as asteriacites rhizocorallium planolites and gyrochorte that indicate a subtidal paleoenvironment wrinkle structures were found in the campil member in the werfen formation of northern italy and in the virgin limestone member of the moenkopi formation and the upper member of the thaynes formation exposed in the western united states the presence of wrinkle structures is likely related to a unique taphonomic window that allowed for the preservation of these delicate features on storm dominated siliciclastic shelves wrinkle structures commonly formed in a variety of subtidal paleoenvironments during the proterozoic cambrian but thereafter became restricted to intertidal supratidal and deep sea environments this restriction has been attributed to the increase in infaunalization of metazoans following the cambrian radiation the proliferation of wrinkle structures in subtidal settings during the early triassic suggests that infaunal bioturbation was reduced after the end permian mass extinction and that this reduction lasted for millions of years

can in situ measurements of mantle electrical conductivity be used to infer properties of partial melts 1 constraints on bulk conductivity from magnetotelluric measurements and petrological analyses of late quaternary peridotite xenoliths from the southern sierra nevada allow evaluation of models commonly used to relate electrical conductivity to the physical and chemical state of the upper mantle in these models two conductive melts basalt and sulfide are embedded in a resistive matrix bounds on the amount of sulfide 0 06 0 4 and the bulk conductivity 0 03 0 1 s m place constraints on the degree of interconnection between the melts because the sulfide melt is very conductive even a small fraction of well connected melt results in a bulk conductivity larger than 0 1 s m similarly completely disconnected melts result in bulk conductivities much less than 0 03 s m the only models which matched both the bulk conductivity and sulfide bounds consisted of a small fraction 1 interconnected basalt melt with a discontinuous sulfide phase such a texture is observed in laboratory experiments with much larger sulfide melt fractions but has not been reported for small melt fractions a variant of the hashin shtrikman model and a hybrid model consisting of cascaded hashin shtrikman calculations were successful in matching the magnetotelluric and petrologic constraints with a model that appropriately simulates the melt interconnectivity we suggest that electrical conductivity may be used to infer in situ melt properties in the mantle

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

temporal development of the 1999 intrusive episode in the eyjafjallajokull volcano iceland derived from insar images eyjafjallajokull volcano located in southern iceland is characterized by its quiet nature only about a handful of earthquakes associated with the volcanic system had been detected prior to the 1990s earthquake swarms did however occur in 1994 and 1999 here we investigate the spatio temporal evolution of a magmatic intrusion associated with the 1999 earthquake swarm via analysis of produced surface deformation a series of interferometric synthetic aperture radar insar images spanning various periods of the intrusion show that in 1999 surface deformation occurred mainly on the southern flanks of the volcano the deformation amounts to more than 20 cm of range change inverse modeling resolves the deformation source to be a sill intrusion at 6 3 km depth sill opening was up to 1 m and the total intruded volume amounts to 0 03 km3 the insar data display a migration of the center of deformation through time enabling us to create time dependant sill opening models furthermore we investigate the spatio temporal distribution of earthquakes and find that the distribution supports the insar derived model and additionally provides indications for a possible site of a feeder channel magmatic flow rate estimates indicate an initial intrusion rate of 4 6 m3 s declining over a few weeks

mantle heterogeneity beneath eastern california from magnetotelluric measurements 1 a 300 km long magnetotelluric profile across the great valley sierra nevada and california basin and range reveals minimum resistivity differences of 1 order of magnitude at depths from moho at 30 35 km to depths greater than 200 km horizontal wavelengths of 25 km are seen for these changes at depths shallower than 100 km and the isabella high seismic wave speed anomaly is present in the section between depths of 100 and 200 km as a minimum tenfold resistivity increase a transformation from electrical resistivity to temperature and melt fraction using a petrological model and laboratory measurements reveals that partial melt is relatively uncommon and confined primarily to depths between 30 and 100 km the remainder of the observed resistivity variation is attributed to lateral variations in temperature predicted melt fractions match those inferred from miocene quaternary xenoliths but modeled temperatures are 200 c higher than inferred from thermobarometric analyses of the xenoliths possible causes for this discrepancy include an incorrect starting material for the petrological model exotic conductive phases higher conductivity due to oxidation state and or iron content and diffusion of ions through the solid matrix i show that hydrogen diffusion or higher oxygen concentrations are plausible explanation for the higher model temperatures less than 200 ppm h 106 si in olivine would increase the conductivity of the matrix sufficiently to allow temperatures inferred from the resistivity section to match those from the xenoliths a possible source of the hydrogen would be fragments of sinking metasomatized mantle lithosphere that melted during cenozoic delamination

geoelectric response of archean lithosphere in the western superior province central canada abstract magnetotelluric mt soundings were made at more than 230 sites in the western superior province in central canada as part of the lithoprobe western superior transect investigations the superior province forms the nucleus of the north american continent its western part contains a regional pattern of east west trending 100 200 km wide granite greenstone and metasedimentary belts or subprovinces the objectives of the mt study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries and to determine if there exists a signature in the upper mantle related to the formation of the craton the exceptionally large mt database for the western superior province provides a three dimensional sampling of the resistivity structure in this paper the primary mt responses from the data set including geoelectric strike apparent resistivity and phase are used to define first order geoelectrical structures in the region apparent resistivity responses indicate the ca 3 0 ga north caribou terrane contains extremely resistive crust crust to the south accreted at ca 2 7 ga is significantly more conductive with resistivities similar to in the southeast superior province the results suggest that the supracrustal component of the north caribou terrane provides minimal enhancement of the crustal conductivity at the southern margin of the former north caribou superterrane induction arrows define a conductivity anomaly that extends in an east west direction for at least 600 km and causes a reversal of arrows in a region 100 200 km wide this anomaly is interpreted to be caused by a conductive component of the 2 7 ga metasedimentary rocks of the english river subprovince the observations demonstrate the ability of electromagnetic methods to map some neoarchean continental margins in the southwest superior province geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere this result provides support to the interpretation that the lithospheric mantle of the western superior province has remained intact since the 2 7 ga accretion of the subprovinces the mt responses from the western superior province have thus defined a number of large scale resistivity structures providing information on tectonic structure and history and confirming the value of electromagnetic imaging of archean lithosphere

a simple model for the caco3 saturation state of the ocean the strangelove the neritan and the cretan ocean 1 a simple model of the caco3 saturation state of the ocean is presented it can be solved analytically and is intended to identify the fundamental controls on ocean carbonate ion concentration it should also attract researchers unfamiliar with complex biogeochemical models despite its limitations the model calculated caco3 saturation state of today s ocean agrees well with observations in general the model reveals three distinctly different modes of operation the strangelove ocean of high supersaturation which is dominated by inorganic caco3 precipitation 2 the neritan ocean of indefinite saturation dominated by biogenic shallow water caco3 precipitation and 3 the cretan ocean of low saturation dominated by biogenic pelagic caco3 precipitation in the latter mode the deep ocean co32 is remarkably stable provided that the biogenic production of caco3 exceeds the riverine flux of ca2 and co32 this explains the overall constancy of the saturation state of the ocean documented over the last 100 ma the model is then used to address diverse questions one important result is that the recovery of the oceanic carbonate chemistry from fossil fuel neutralization in the future will be accelerated due to expected reduced biogenic calcification

a global marine sedimentary response to the end permian mass extinction examples from southern turkey and the western united states the end permian mass extinction greatly diminished marine diversity and brought about a whole scale restructuring of marine ecosystems these ecosystem changes also profoundly affected the sedimentary record data presented here attained through facies analyses of strata deposited during the immediate aftermath of the end permian mass extinction southern turkey and at the close of the early triassic southwestern united states in combination with a literature review show that sedimentary systems were profoundly affected by 1 a reduction in biotic diversity and abundance and 2 long term environmental fluctuations that resulted from the end permian crisis lower triassic strata display widespread microbialite and carbonate seafloor fan development and contain indicators of suppressed infaunal bioturbation such as flat pebble conglomerates and wrinkle structures facies considered unusual in post cambrian subtidal deposits our observations suggest that depositional systems too respond to biotic crises and that certain facies may act as barometers of ecologic and environmental change independent of fossil assemblage analyses close investigation of facies changes during other critical times in earth history may serve as an important tool in interpreting the ecology of metazoans and their environment

evidence of a large deep conductive body within the basement of the guadalquivir foreland basin betic cordillera s spain from tipper vector modelling tectonic implications abstract the betic cordillera is an alpine belt formed by the interaction of the eurasian and african plates and the westward motion of the alboran domain long period magnetotelluric observations at 26 sites in its westernmost part provide induction arrows that have been compared with 3d forward models including bathymetry and major geological bodies the results highlight the presence of a major conductive body 0 05 m unknown to date and located within the basement of the guadalquivir foreland basin aeromagnetic and field magnetic measurements further support the occurrence of magnetic anomalies related to the top of this anomalous body this major structure is interpreted as an intermediate or basic igneous rock with a high proportion of metallic mineralization its origin is discussed in the framework of the regional geological setting possibly produced in the southern iberian variscan massif by a huge concentration of volcanogenic massive sulphide vms in the prolongation of the iberian pyrite belt during devonian early carboniferous times another possibility is that the conductive anomaly is due to magmatic intrusions associated with the mesozoic fragmentation of southern iberia and the opening of the tethys

carbon isotope excursions and microfacies changes in marine permian triassic boundary sections in hungary abstract several permian triassic boundary sections occur in various structural units within hungary these sections represent different facies zones of the western palaeotethys margin the gardony core in the ne part of the transdanubian range typically represents the inner ramp while the balvany section in the bukk mountains of northern hungary represents an outer ramp setting the two sections have different patterns for their 13 c values the balvany section shows a continuous change towards more negative 13 c values starting at the first biotic decline followed by a sharp quasi symmetric negative peak at the second decline the appearance of the 13 c peak has no relationship to the lithology and occurs within a shale with low overall carbonate content indicating that the peak is not related to diagenesis or other secondary influences instead the shift and the peak reflect primary processes related to changes in environmental conditions the continuous shift in 13 c values is most probably related to a decrease in bioproductivity whereas the sharp peak can be attributed to an addition of c strongly depleted in 13 c to the ocean atmosphere system the most plausible model is a massive release of methane hydrate the quasi symmetric pattern suggests a rapid warming cooling cycle or physical unroofing of sediments through slope failure and releasing methane hydrate the gardony 1 core shows a continuous negative 13 c shift starting below the p t boundary however the detailed analyses revealed a sharp 13 c peak in the boundary interval just below the major biotic decline although its magnitude doesn t reach that observed in the balvany section based on careful textural examination and high resolution stable isotope microanalyses we suggest that the suppression of the 13 c peak that is common in the oolitic boundary sections is due to combined effects of condensed sedimentation sediment reworking and erosion as well as perhaps diagenesis