on the trace of the iapetus suture in ireland and britain the iapetus suture in ireland and britain is that line which separates caledonian rocks of the laurentian and avalonian continents the suture is cryptic nowhere is there an exposed fault zone containing ophiolite remnants blue schist melanges or trench deposits instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the iapetus suture zone which contains two or more tectonostratigraphic terranes several data sets are utilized to constrain this trace the distribution of faunal provinces caused by oceanic separation can be used most reliably to define early ordovician terranes fauna1 intermingling in the mid ordovician reduces provinciality and the confidence of terrane identification using faunal data the late ordovician and particularly silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes elimination of the iapetus ocean and development of the suture zone had begun by the ashgill therefore the resolution of the suture trace becomes unreliable where ordovician rocks do not crop out because silurian turbidite fans of laurentian provenance may have dispersed across a significant width of avalonian crust similarly the precision of structural identification of the suture zone by correlating the suture with a major fold facing confrontation may be weakened by late overthrusting tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by nd and pb isotopic patterns the rb sr patterns of granites record a stronger distinction between north and south of the suture despite these difficulties we depict a possible suture trace in ireland that departs significantly from the traditionally placed line by following a ne sw aligned trace through the slane fault and the navan tipperary lineament and thence through a dog leg to pass north of the dingle peninsula

electrical resistivity investigation of the arsenic affected alluvial aquifers in west bengal india usefulness in identifying the areas of low and high groundwater arsenic this study aims at finding out possible relation between lithology and spatial pattern of dissolved arsenic as in groundwater around chakdaha municipality west bengal india satellite image coupled with electrical resistivity survey and borehole drilling helps to delineate surface and sub surface lithological framework of the as affected alluvial aquifers the satellite imagery demonstrate that the high as area are presumably under active flood plain environment low lying areas that constantly receive organics due to periodic flooding thick low resistive fine grained layer was observed at the top around the high as areas which however not found in low as areas the result suggests that hydraulic properties of the surface sub surface soil sediment have an important control on the fate and transport of as in the aquifer this study demonstrates that electrical resistivity tools can be effectively used for the reconnaissance survey in characterizing the plausible lithological framework of an alluvial aquifer containing as

heat flow measurements in ireland abstract eighteen heat flow values for ireland are described they range from 52 mw m 2 to 87 mw m 2 and have a mean of 67 mw m 2 the values are discussed in the context of the tectonic and geological structure of ireland although not unusually high the irish heat flow values are higher than might be expected for a terrain which is largely palaeozoic in age moreover there is some evidence for an association of higher than average heat flow values with a recently discovered anomaly of high electrical conductivity in the crust

late quaternary sedimentary environments and landforms in the ganges delta abstract late quaternary sediments deposited since the lowest stand of sea level during the last glacial maximum in the ganges delta and its surrounding region were classified into five units according to their sedimentary facies these are the lowest unit of sandy gravels the lower unit of sand with a few gravels and the middle upper and uppermost units consisting mainly of sand and silt with occasional peat layers the contact between the lower and middle units is fairly sharp and the upper part of the lower unit is oxidized in some places this suggests a period of subaerial exposure after the deposition of the lower unit during the low stand of sea level about the time of the last glacial maximum rivers in the ganges delta and its surrounding region dissected the surfaces of the region and deposited the lowest unit on the valley floors the lower unit was deposited over the lowest unit during an early stage of the post glacial transgression between ca 12 000 yr bp and 10 000 yr bp the surface of the delta was slightly dissected and the top of the lower unit was weathered and oxidized following this period the middle and upper units of alluvial or deltaic sand and silt with occasional peat were deposited the coastline in the early holocene retreated towards the central part of the present ganges delta in the middle and late holocene the silt and clay with occasional peat layers of the uppermost unit indicate that the lowland gradually became marshy and poorly drained as the rate of transgression became slower

a review of the irish crustal structure and signatures from the caledonian and variscan orogenies this paper reviews the complex crustal and upper mantle seismic velocity structure of ireland and surrounding seas data from 11 seismic refraction profiles reveal that onshore ireland mean crustal velocities range between 6 25 and 6 5 km s 1 with crustal thickness of 28 5 32 km superimposed on a three layer crust the sedimentary layer has a thickness of approximately 6 8 km at the southern coastline but only 3 4 km in the vicinity of the shannon estuary in western ireland the lateral heterogeneity of the upper crustal layer is pervasive throughout ireland with velocities of 5 7 6 2 km s 1 and a layer thickness of 3 10 km a low velocity zone is found in the south east which is interpreted as the buried south western extension of the leinster granite the mid crustal layer 6 3 6 7 km s 1 is between 8 and 16 km thick significant changes occur in the vicinity of the shannon estuary around the location of the iapetus suture zone the lower crust is fairly uniform with velocities of 6 8 7 2 km s 1 and a thickness of approximately 8 10 km except towards the south of ireland where the moho appears as a transition zone offshore ireland a two layer crust with a thickness of 24 26 km beneath the north celtic sea basin and only 14 15 km beneath the rockall trough prevails

arsenic enrichment in groundwater of west bengal india geochemical evidence for mobilization of as under reducing conditions abstract the mechanism of as release and source s of as has been investigated in a small part of a watershed in the murshidabad district of west bengal analyses include major ion and trace element concentrations as well as o h and s isotope ratios of groundwater surface water and a thermal spring the results indicate that all water samples belong to the ca hco 3 type except for the thermal spring which is of the na hco 3 type shallow and deeper groundwaters have distinct hydrochemical features high as contents were registered only in the deeper groundwater horizon factor analysis and the distribution pattern of major and trace elements indicate that as is present in the aquifer as a scavenged phase by fe iii and to a lesser extent by mn iv phases the release of as into the groundwater occurs gradually in successive stages corresponding to the actual redox state in the aquifer the main stage of as release is related to the bacterial reduction of fe iii to fe ii i e to the simultaneous dissolution of fe oxyhydroxides low redox conditions in highly polluted areas are indicated by low so 4 concentration and high 34 s values during bacterial so 4 reduction residual so 4 in groundwater is depleted in the lighter s isotope 32 s however the cause of the gradual decrease of the redox state in the groundwater is still not well understood

impact of local recharge on arsenic concentrations in shallow aquifers inferred from the electromagnetic conductivity of soils in araihazar bangladesh 1 the high degree of spatial variability of dissolved as levels in shallow aquifers of the bengal basin has been well documented but the underlying mechanisms remain poorly understood we compare here as concentrations measured in groundwater pumped from 4700 wells 22 m 75 ft deep across a 25 km2 area of bangladesh with variations in the nature of surface soils inferred from 18 500 measurements of frequency domain electromagnetic induction a set of 14 hand auger cores recovered from the same area indicate that a combination of grain size and the conductivity of soil water dominate the electromagnetic signal the relationship between pairs of individual em conductivity and dissolved as measurements within a distance of 50 m is significant but highly scattered r2 0 12 n 614 concentrations of as tend to be lower in shallow aquifers underlying sandy soils and higher below finer grained and high conductivity soils variations in em conductivity account for nearly half the variance of the rate of increase of as concentration with depth however when the data are averaged over a distance of 50 m r2 0 50 n 145 the association is interpreted as an indication that groundwater recharge through permeable sandy soils prevents as concentrations from rising in shallow reducing groundwater

grain boundary graphite in rocks and implications for high electrical conductivity in the lower crust the origin of zones of high electrical conductivity in the lower continental crust is a long standing mystery possible explanations include the presence of brines1 2 partial melt3 serpentine4 and graphite5 when discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6 10 here we show that a fine film of graphite is present on the grain boundaries in three rocks from the laramie anorthosite complex in two of these rocks graphite was stable during igneous crystallization but in the other it was not we maintain that in all of the rocks the grain boundary graphite precipitated from a co2 rich fluid during cooling the chemical processes that produced the grain boundary graphite in these rocks are likely to operate in many lower crustal rocks we therefore contend that because the films we observe are capable of producing the high conductivity that is seen in the lower crust grain boundary graphite should be considered as a possible cause for some conductivity anomalies

bounded influence magnetotelluric response function estimation author posting blackwell 2004 this article is posted here by permission of blackwell for personal use not for redistribution the definitive version was published in geophysical journal international 157 2004 988 1006 doi 10 1111 j 1365 246x 2004 02203 x

a n s receiver function profile across the variscides and caledonides in sw ireland summary teleseismic receiver functions have been calculated from data of a temporary seismological network of broad band three component stations to investigate the lithospheric and asthenospheric structure across the late caledonian iapetus suture zone isz in southern ireland the stations were deployed during the irish seismological lithospheric experiment isle 2002 3 and straddle the killarney mallow fault zone a remnant of the variscan orogeny and the isz the inferred boundary between the laurentia and eastern avalonia plates fused together during the caledonian orogeny receiver functions from the western part of the network were projected onto the n s varnet 1996 seismic refraction profile extending from the old head of kinsale to galway bay in sw ireland laterally continuous p to s conversions from the moho at delay times of about 3 8 4 1 s are clearly observed and correspond to moho depths of about 29 32 km the moho has a transitional character to the south of the isz synthetic receiver functions calculated from a 2 d velocity model of the previous varnet experiment show moho conversions and multiple crustal phases compatible to those observed in the isle data furthermore p to s conversions from the 660 km discontinuity 66 68 s delay time are well determined at the stations in comparison the conversion from the 410 km discontinuity at about 43 45 s delay time is considerably weaker delay times of stacked receiver functions from the mantle transition zone are in agreement with the standard iasp91 earth model and thus no structural changes are observed across the isz at this depth interval

the geometry of the iapetus suture zone in central ireland deduced from a magnetotelluric study abstract the crustal geometry of the iapetus suture zone isz in central ireland is inferred from an electrical resistivity model derived from broadband and long period magnetotelluric measurements along a 200 km long nnw sse profile the model highlights an undulating high conductivity layer of thickness 10 15 km and resistivity 2 5 m at middle to lower crustal depths interpreted as sulphide bearing graphitic sediments deposited between the converging continents of laurentia and avalonia during the closure of the iapetus ocean the sediments underwent sinistral transpressional deformation during convergence and movements along the faults transported the sediments to depths of 5 10 km forming the u shaped conductor beneath the centre of the profile correlative with the inferred location of the iapetus suture zone two high resistivity blocks identified above the u shaped conductor are interpreted as caledonian granite bodies and likely relate to late caledonian igneous activity

shear wave splitting observations across southwest ireland 1 shear wave splitting analyses have been carried out on teleseismic data from the southwest of ireland acquired during the irish seismological lithospheric experiment isle the data were gathered over a ten month period by a temporary network of 23 broadband and short period stations the results are compared with data from two permanent broadband seismic stations which have recorded sks and skks phases for up to 10 years the purpose of the experiment was to investigate possible anisotropy within the crust and mantle related to caledonian deformation here we report splitting results which show an average delay time of 1 2 s and a variation of fast polarisation direction with back azimuth that surprisingly suggests a much deeper origin for anisotropy than was anticipated

oceans island arcs and olistostromes the use of fossils in distinguishing sutures terranes and environments around the iapetus ocean the shallow water benthos of cratonic areas distinguish different continents and the closure of the iapetus ocean during the ordovician and silurian can be monitored by the faunas which crossed the ocean at different geological times brachiopod and trilobite distributions carry different palaeobiological evidence than for example contemporary conodont provinces which probably reflected a combination of temperature differences and water depths the celtic 9province9 is reviewed and it is concluded that each of the island faunas should be treated separately since many have links with adjacent continents rather than together as a province in newfoundland faunas help to locate the site of the main iapetus closure at the reach fault although a substantial island arc in the notre dame bay area accreted on to america in early caradoc times in the late ordovician and early silurian of newfoundland fossils assist in the dating of quite local fault controlled marginal basins some of which include large olistostromes undisturbed assemblages of brachiopods in life position give indications of the depth of water in some basins

spatial vertical and temporal variation of arsenic in shallow aquifers of the bengal basin controlling geochemical processes a detailed understanding of the geochemical processes that regulate the spatial temporal and vertical variation of dissolved arsenic as in shallow aquifers 50m is a prerequisite for sustainable drinking water management in the bengal basin the present study conducted at chakdaha block of the nadia district west bengal india combined a high resolution hydrogeochemical monitoring study over 20months from two sets of piezometers 2 5 to the sediment geochemistry at areas with high average 146 g l n 5 and relatively low average 53 3 g l n 10 dissolved as concentrations in groundwater the determination of the isotopic composition of 2h and 18o in groundwater of the two sites indicated the recharge of evaporative surface water to the aquifer the concentrations of major aqueous solutes ca2 mg2 na k hco3 and cl and electrical conductivity were considerably higher in wells at the high as site compared to the low as site additionally at the high as site the major ions fe so4 2 electrical conductivity 2h and 18o showed markedly greater enrichment in the shallowest part 24m of the aquifer compared to the deeper part reflecting vertical layering of groundwater composition within the aquifer the oxidation of pyrites has been attributed to the high rate of mineral dissolution resulting in such greater enrichments in this part of the aquifer in addition the anthropogenic input with recharge water possibly increased the concentrations of cl in this part of the aquifer the vertical layering of groundwater was absent in the aquifer at the low as site the absence of such layering and relatively low major ion concentrations and electrical conductivity could be linked to the enhanced aquifer flushing and decreased water sediment interactions influenced by local scale groundwater abstraction the seasonal variations of as concentrations in groundwater were observed only in the shallowest part of the aquifers 30m furthermore the as concentrations in groundwater at the uppermost part of the shallow aquifers 21m increased continuously over the monitoring period at both sites this study supports the view that the reductive dissolution of fe oxyhydroxides coupled with competitive po4 3 sorption reactions in the aquifer sediment enriches as in groundwater of the bengal basin however the additional fe released by the weathering of silicate minerals especially biotite or the precipitation of fe as secondary mineral phases such as siderite vivianite and acid volatile sulfides may result in the decoupling of as and fe enrichment in groundwater the redox zonation within the aquifer possibly regulates the vertical distribution of as in the groundwater

a north south seismic profile across the caledonian suture zone in ireland abstract coole i caledonian onshore offshore lithospheric experiment line i is a 284 km seismic refraction profile extending from bundoran n to ardmore s in ireland fifty nine recording stations were deployed along this profile and it was served by a total of six shotpoints the profile crosses the predominant ne sw caledonian strike at a steep angle using both one dimensional and two dimensional seismic inversion techniques a travel time and relative amplitude study of the observed p wave data was undertaken gravity data along the profile was modelled to test the plausability of the derived velocity depth section the upper crustal structure along the profile is highly variable but some features stand out a large low velocity body towards the south of the traverse correlates well with a bouguer anomaly low and is interpreted as the southwest extension of the buried leinster batholith the mid crust is approximately 10 km thick and is bound at its top and bottom by two laterally continuous reflecting horizons both reflectors shallow northwards by about 5 km over a horizontal distance of some 60 km this northwards shallowing is a feature also displayed by a number of isolines in the upper crust and it marks a broad region of crustal change along the traverse in the zone of proposed caledonian suturing crustal thickness beneath coole i is approximately 30 km and the moho is a transition zone about 2 km thick which shallows slightly towards the coasts

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

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

a geological and tectonic cross section of the caledonides of western ireland geological and geophysical criteria allow division of the caledonides of western ireland into four principal zones zone 1 in the north contains grenvillian basement with late proterozoic supra crustals zone 2 contains the westport complex the deer park complex and the south mayo trough connemara and the delany dome formation occupy zone 3 zone 4 contains the south connemara group and mid to late ordovician and silurian sediments and volcanic rocks the various tectonic models proposed for these zones are discussed and a minimum tectonic model is presented which involves early ordovician collision of the northern part of zone 2 with the laurentian margin zone 1 followed by subduction flip northern part of zone 4 the site of early collision between zones 1 and 2 subsequently underwent orogenic collapse and strike slip dismemberment leading to the sinistral transpressive emplacement of zone 3 assemblages in zone 4 were accreted to the now south facing arc northwards stress transfer consequent on suturing deformed the thinned crust and overlying silurian sequences in zones 2 and 3

the impact of aquifer flushing on groundwater arsenic across a 35 km transect perpendicular to the upper brahmaputra river in assam india well testing in the floodplain of the brahmaputra river in golaghat and jorhat districts of assam india shows that groundwater arsenic as concentrations increase with distance from the river to establish the origin of this pattern an additional 900 wells 60 m deep were tested for as and 9 sites were drilled along a 35 km transect perpendicular to the river the field data show no relation between groundwater as concentrations ranging from 1 to 660 g l along the transect and a as concentrations of 1 5 mg kg in cuttings of aquifer sand recovered while drilling or b the degree of reduction of iron oxides in these cuttings the drilling indicates however a marked increase in the thickness of a clay layer capping the aquifer starting from 1 5 m near the river to over 60 m at the most distant site towards the base of the naga foothills organic radiocarbon ages of 18 46 kyr obtained from all but one of 13 clay samples indicate pre holocene deposition of the underlying sands across the entire transect radiocarbon ages of dissolved inorganic carbon of 0 2 4 7 and 17 8 kyr were measured in groundwater from 3 monitoring wells installed to 30 60 m depth at distances of 10 20 and 40 km from the river respectively a conceptual groundwater flow model consistent with monitored heads and groundwater ages suggests that thick clay layers capping the aquifer further from the river inhibited flushing of the aquifer and as a result preserved higher as levels in groundwater

a magnetic deep structural geology interpretation of ireland a new 10 nt compilation of magnetic data for ireland and the immediately adjacent sea area shows distinct geophysical provinces of characteristic magnetic signature separated by major magnetic linears which can be related to important geological lines in the far northwest there is a structurally coherent grampian tectonic thermal zone bounded to the southeast by the fair head clew bay line between the fair head clew bay line and the western extension of the southern uplands line which forms the northern margin to the southern uplands of scotland is a segmented zone of relatively small magnetic anomalies and geological bodies of mixed crustal affinity grampian massifs lower palaeozoic sedimentary basins bounded by major shear faults relics of lower palaeozoic oceanic and related crust and possible older microcontinent fragments these separate areas have quite individual structural and metamorphic histories there is good evidence for the grampian massifs having undergone significant lateral movement during lower palaeozoic times and it can be inferred that many of the now adjacent bodies are only docked the original position of each block is now not known in the far southeast of ireland are relics of a cadomian craton which are now tectonically imbricated with lower palaeozoic rocks this is probably the segmented margin of the stable cadomian craton which is inferred beneath wales and south britain between the southern uplands line and the northernmost extent of the relics of the southern craton is a zone of deep seated low amplitude magnetic anomalies and major linears these are about 30 oblique to the southern margin of this vestiges of the iapetus ocean zone but are nearly parallel with the northern margin

implications for the lithospheric geometry of the iapetus suture beneath ireland based on electrical resistivity models from deep probing magnetotellurics

magnetotelluric studies in and adjacent to the northumberland basin northern england abstract during the past decade broadband magnetotelluric mt soundings with d c resistivity soundings at some sites have been undertaken in three separate field studies in and around the northumberland basin a region of great interest to earth scientists on account of the proposed location there of the iapetus suture as a result of an increase in cultural noise during this period the data from the last two studies have been processed using a new robust constrained impedance tensor estimation program the resulting apparent resistivity and phase data from these studies together with those from the first broadband study and some earlier mt responses from the region have now all been modelled using an interpretative modelling procedure new information has been provided by the mt models on basement depths and by integrating these new estimates with those from gravity modelling and seismic studies both on land and offshore a detailed basement topography map has been compiled for the region the deep eletrical resistivity structure has been modelled along a nw se traverse from the weardale granite of the alston block across the northumberland basin to the southern uplands of scotland underlying the more conductive sedimentary rocks the basement rock is found to have resistivities which range from about 100 m in the northumberland basin to more than 1000 m in the alston block and probably of the same order in the southern uplands a mid crustal conductor exists along the whole traverse which is well resolved and has a southward dip beneath the weardale granite under the northumberland basin the conductor is less well resolved and thus an apparent northward dip can only be regarded as tentative comparison of the pseudo 2d and full 2d models resulting from this study and from earlier mt and magnetovariational mv studies in southern scotland with new mt and joint mt and mv inversions of livelybrooks et al phys earth planet inter 81 67 84 1993 for a subset of the same response estimates reveals both comparable and disparate features the features common to the models presented in this paper and at least one of the inversions of livelybrooks et al include differences in the resistivity and thickness of the upper crust in each of the alston block the northumberland basin and the southern uplands and a conducting zone at mid crustal depths which vary along the traverse from about 10 km near the southern edge of the northumberland basin to more than 30 km under the alston block and about 30 km in the southern part of the southern uplands the main difference between the pseudo 2d and the 2d forward models and the 2d inversions of livelybrooks et al is in the resistivity of the alston block the former being more than 1000 m whereas the latter is about 400 500 m a value which is unexpectedly low for a crust of known granitic composition

joint inversion of long period magnetotelluric data and surface wave dispersion curves for anisotropic structure application to data from central germany geophysical datasets sensitive to different physical parameters can be used to improve resolution of earth s internal structure herein we jointly invert long period magnetotelluric mt data and surface wave dispersion curves our approach is based on a joint inversion using a genetic algorithm for a one dimensional 1 d isotropic structure which we extend to 1 d anisotropic media we apply our new anisotropic joint inversion to datasets from central germany demonstrating the capacity of our joint inversion algorithm to establish a 1 d anisotropic model that fits mt and seismic datasets simultaneously and providing new information regarding the deep structure in central germany the lithosphere asthenosphere boundary is found at approx 84 km depth and two main anisotropic layers with coincident most conductive seismic fast axis direction are resolved at lower crustal and asthenospheric depths we also quantify the amount of seismic and electrical anisotropy in the asthenosphere showing an emerging agreement between the two anisotropic coefficients

crustal trace of a hot convective sheet the iceland plume has played an influential role in the evolution of the north atlantic ocean and margins over the past 60 m y it is believed that this plume formed at the conjunction of a tetrad of hot subvertical convective sheets the impingement of these hot sheets at the base of the lithospheric lid caused decompressional melting generating substantial quantities of high temperature magma that were injected into the cold overlying lid over the next 10 m y these sheets partly coalesced to form a crudely axisymmetric plume head here we analyze the lithospheric fingerprint of one of these hot convective sheets by forward and inverse modeling of densely sampled wide angle seismic data in conjunction with gravity observations we determined the three dimensional shape of magmatic underplating trapped within the lithosphere the injection of this melt into the lithosphere generated substantial permanent and minor transient uplift of earth s surface predicted and measured amounts of consequent denudation and sedimentation agree within error temporal variations in the patterns of deposition and oceanic circulation adjacent to the convective sheet show its evolution through time and space our results suggest that this linear sheet has probably been directly and indirectly responsible for cyclical events over 60 m y these events have 0 5 1 and 4 6 m y periodicities the existence of which may help to elucidate the dynamic behavior of convective sheets during and after impingement thus in particular circumstances surficial geological processes yield an indirect record of mantle convection and melt generation processes

varnet 96 influence of the variscan and caledonian orogenies on crustal structure in sw ireland summary varnet an international multidisciplinary project was designed to examine the variscan front in the southwest of ireland as part of the seismic experiment a 200 km long wide angle seismic pro le line a was recorded from the old head of kinsale in the south of ireland to galway bay on the west coast along line a 170 seismic stations were deployed at approximately 1 km intervals results from ray trace modelling indicate a multilayered crust the upper crust extending to a depth of about 14 km is laterally variable south of the shannon estuary sedimentary basins alternate with uplifted basement 5 8 6 0 km s 1 this correlates well with the surface geology the larger sedimentary basin in the south reveals a shallowly south dipping structural feature that coincides with the killarney mallow fault zone north of the shannon estuary surprisingly high velocities 6 4 km s 1 at shallow depths were observed between the upper and middle crust there is a transition layer about 2 4 km thick the middle crust thickens towards the shannon estuary these structural changes coincide with the proposed location of the iapetus suture zone the middle crust is underlain by a lower crust with a variable velocity structure and the total crustal thickness varies from 29 32 km detailed traveltime investigations have also revealed a re ector in the upper mantle at depths between 39 and 44 km the seismic model shows that there are no deep crustal variations beneath the killarney mallow fault zone indicating a thin skinned mechanism of deformation

monsoonal influence on variation of hydrochemistry and isotopic signatures implications for associated arsenic release in groundwater summary the present study examines the groundwater and surface water geochemistry of two different geomorphic domains within the chakdaha block west bengal in an attempt to decipher potential influences of groundwater abstraction on the hydrochemical evolution of the aquifer the effect of different water inputs monsoon rain irrigation and downward percolation from surface water impoundments to the groundwater system and concomitant as release a low land flood plain and a natural levee have been selected for this purpose although the stable isotopic signatures of oxygen 18o and hydrogen 2h are largely controlled by local precipitation the isotopic composition falls sub parallel to the global meteoric water line gmwl the cl br molar ratio indicates vertical recharge into the wells within the flood plain area especially during the post monsoon season while influences of both evaporation and vertical mixing are visible within the natural levee wells increase in mean doc concentrations from 1 33 to 6 29 mg l from pre to post monsoon season indicates possible inflow of organic carbon to the aquifer during the monsoonal recharge concomitant increase in ast fe ii and hco3 highlights a possible initial episode of reductive dissolution of as rich fe oxyhydroxides the subsequent sharp increase in the mean as iii proportions by 223 particularly in the flood plain samples during the post monsoon season which is accompanied by a slight increase in mean ast 7 may refer to anaerobic microbial degradation of doc coupled with the reduction of as v to as iii without triggering additional as release from the aquifer sediments

sensitivity of texture of high resolution images of forest to biophysical and acquisition parameters abstract this article presents a quantitative analysis of the sensitivity of textural information of high resolution remote sensing images of a forest plantation les landes france to a number of biophysical parameters crown diameter distance between trees and rows tree positioning leaf area index lai and tree height influence of acquisition parameters spatial resolution spectral domain and viewing and illumination configurations is also investigated the work is realized with the discrete anisotropic radiative transfer model dart simulated images with which we quantify texture with variograms results point out the complex dependency of variogram characteristics range sill amplitude of oscillations on biophysical and acquisition parameters neglect of spatial variations of the reflectance of canopy elements as in most geometric optical models can lead to important errors this stresses the interest of accurate radiative transfer models such as dart although tree crown diameter is the most influential biophysic parameter its influence may be totally masked by acquisition parameters finally theoretical results were tested against high resolution airborne data 1 67 m resolution although encouraging results were obtained this work both confirms the difficulty of extracting reliable texture information from real remote sensing data and stresses the usefulness of radiative transfer models for studying the texture of high resolution satellite images

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

use of the resistivity method in geological mapping case histories from raniganj coalfield india the effectiveness of the electrical resistivity method has been studied using various configurations with different spacings over the southern boundary fault in the northwestern part and across a dolerite dyke named salma dyke in the central part of the raniganj coalfield india it has been observed that the delineation of the fault and the dyke was made possible under differential tropical weathering conditions geoelectric sections across the fault and the dyke have been prepared on the basis of schlumberger sounding results in profiling wenner two electrode half schlumberger and part of al chalabi s configurations were applied over the southern boundary fault across the dyke alpha beta and gamma wenner schlumberger half schlumberger and two electrode configurations were employed azi muthal two electrode sounding was also studied over the fault the interpretation of the results of apparent resistivity profiles across the southern boundary fault suggests that the wenner and two electrode configurations possess certain diagnostic features which help in mapping a single lithological contact provided sufficient resistivity contrast exists although schlumberger configuration seems to be quite suitable other configurations may also be usefully employed over the dyke

a geoelectric section across ireland from magnetotelluric soundings abstract a magnetotelluric survey across ireland has been completed the 40 sites were positioned on a line normal to the predominant caledonian geological trend i e ne sw of the country the average site separation was 5 km and frequencies from 640 to 0 01 hz were sampled bostick niblett and occam inversions of the data suggest that the carboniferous limestone of the main part of the profile is underlain by ordovician volcanic rocks in a block and trough structure with a wavelength of 15 km this supports previous gravity and magnetic interpretations at greater depths which vary significantly along the traverse an anomalous mid lower crustal conductor is apparent this conductor is probably a complex manifestation of the caledonian tectonic zone known as the iapetus suture in this respect it is significant that comparable conducting zones of similar geometry have previously been found in the suture region in southern scotland and northern england

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

an upper mantle upwelling underneath ireland revealed from non linear tomography summary we present results of a non linear teleseismic p wave traveltime tomography in ireland relative traveltime residuals are calculated from the data set of the irish seismological lithospheric experiment isle 2002 3 and permanent stations onshore ireland at each of the 28 station sites a local crustal 1 d p wave velocity vp model is determined because the 3 d crustal vp structure of ireland is well known these 1 d models are used for a traveltime correction for crustal effects the corrected residuals indicate two large scale traveltime anomalies of low amplitude a negative traveltime anomaly in the western part of ireland and a positive anomaly in the eastern part of the country the subsequent traveltime tomography is calculated using an iterative non linear inversion using a variable parametrization and 3 d ray tracing our inversion result contains a low p wave velocity vp zone underneath central ireland at 31 120 km depth the amplitudes of the low velocity zone reach up to 2 5 per cent vp at 31 60 km depth but decrease to 0 5 to 1 per cent vp at 120 150 km depth reconstruction tests confirm that large anomalies in our model are well resolved although the resolution decreases towards the edges of the model we infer that the low velocity zone underneath central ireland is related to lithospheric thinning towards the north which was determined by a previous s receiver function study this anomaly is interpreted as presumably ancient or even recent mantle upwelling related to the spreading of the iceland plume head

a crustal model for a northeast southwest profile through ireland abstract the irish caledonian suture seismic project icssp consisting of a 250 km seismic refraction line was carried out in conjunction with and as an extension of the caledonian suture seismic project cssp of the university of durham 51 seismic recording sites at 5 km intervals were deployed on the northwest flank of the suture zone in ireland recordings were made from three shot points at the southwest of the line and from 31 shot points of the durham programme in the north irish sea a crustal model is presented which is based exclusively on a travel time interpretation the p g velocities recorded in eastern ireland rise quite rapidly with depth to 6 3 6 5 km s but in the western part of the profile lower velocities between 6 0 and 6 3 km s were recorded prominent energy in later arrivals can be correlated by two reflected phases from a mid crustal boundary and the moho a low velocity zone is present at the northeastern end of the line and a mid crustal layer with an average velocity of about 6 4 km s is separated from the lower crust mean velocity 6 8 km s by a well established mid crustal boundary zone of 1 2 km thickness at about 20 km depth the moho around 32 km at its deepest shallows towards the sea at both ends of the line it is a sharp discontinuity under western ireland but a transition zone of 3 4 km thickness in the east an interpretation is given which takes into account 1 a the geologic and tectonic history of ireland and 2 b data derived from the gravity and magnetic fields

two dimensional interpretation of three dimensional magnetotelluric data an example of limitations and resolution summary interpretation of magnetotelluric mt data for three dimensional 3 d regional conductivity structures remains uncommon and two dimensional 2 d models are often considered an adequate approach in this paper we examine 2 d interpretation of 3 d data by considering the synthetic responses of a 3 d structure chosen specifically to highlight the advantages and limitations of 2 d interpretation 2 d models were obtained from inversion of the synthetic 3 d data set with different conditions noise and distortion applied to the data we demonstrate the importance of understanding galvanic distortion of the data and show how 2 d inversion is improved when the regional data are corrected prior to modelling when the 3 d conductive structure is located below the profile the models obtained suggest that the effects of finite strike are not significant if the structure has a strike extent greater than about one half of a skin depth in this case the use of only tm mode data determined better the horizontal extent of the 3 d anomaly when the profiles are located away from the 3 d conductive structure the use of only tm mode data can imagine phantom conductive structures below the profile in this case the use of both polarizations produced a better determination of the subsurface structures it is important to note that the main structures are identified in all the cases considered here although in some cases the large data misfit would cause scepticism about features of the models

geoelectric resistivity sounding of riverside alluvial aquifer in an agricultural area at buyeo geum river watershed korea an application to groundwater contamination study twenty profiles of vertical electric soundings ves were obtained in a riverside alluvium at the buyeo area south korea to examine the variations of subsurface geology and associated groundwater chemistry the combination of the ves data with the borehole data provided useful information on subsurface hydrogeologic conditions the vestige of an ancient river channel e g oxbow lake was identified on the resistivity profiles by the lateral continuation of a near surface perched aquifer parallel to the river such a perched aquifer is typically developed in the area with a clay rich silty surface alluvium which prohibits the infiltration of oxygen therefore groundwater below the oxbow lake shows a very low nitrate concentration and eh values under the strong anoxic condition the distribution of water resistivity is correlated with that of measured total dissolved solids concentration in groundwater while the earth resistivity of the aquifer shows a significant spatial variation it is interpreted that the earth resistivity of the aquifer is mainly controlled by the soil type rather than by the water chemistry in the study area

causes of high crustal conductivity beneath the iapetus suture zone in great britain summary two magnetic variation anomalies in southern scotland and northern england have been linked to the position of the iapetus suture previous magnetotelluric measurements along a 140 km profile that crosses both anomalies have been supplemented by new high quality broad band observations groom bailey decomposition of the impedance and hypothetical event analysis of the magnetic variation data agree in defining different strike directions for the two halves of the mt profile n50 e for the northern part over the southern uplands and n90 e for the southern part over the northumberland trough te and tm mode apparent resistivity and phase data at representative frequencies have been inverted for the two sub profiles separately using the smooth inverse of smith booker 1990 the uppermost layer has a low resistivity is variable in thickness and correlates well with the known position and thickness of carboniferous sedimentary rocks the second layer has a very high resistivity thousands of m and reaches the surface where the lower palaeozoic metamorphic rocks of the southern uplands crop out a relatively rapid transition to low resistivities occurs at depths of between 8 and 16 km the conducting layer appears to be quasi continuous but where the profile crosses the anomalies identified by magnetic variation measurements the conductance increases and the upper surface is shallower the spatial coverage of the magnetic variation data has enabled us to extrapolate conductive features away from the line of section and project the electrical image onto the nec vertical incidence seismic reflection profile in the north sea there is excellent agreement between a number of features in the acoustical and electrical images the shallowing of the low resistivity layer to form a narrow wedge like feature corresponds to the offshore position of the stublick fault while to the north of the fault the top of the layer coincides with a south dipping reflector thought to be a thrust however the zones of high conductance and high lower crustal reflectivity do not in general correlate the good conductor beneath he northumberland trough spans two zones which were differentiated on ihe nec profile in terms of their reflectivity the shape of the conductor s upper surface in the vicinity of the stublick fault agrees well with the model of chadwick holliday 1991 who proposed that the iapetus suture was a whole crustal shear with a gently dipping central ramp the coincidence between the present day low resistivity layer and a surface of weakness that was active 300 400 myr ago is much more readily explained in terms of mineralogy the presence of graphite than the presence of fluids there is also strong support for interpreting the northern conductor in the same way its upper surface is relatively flat and occupies the position predicted as the horizontal detachment surface over which wedges of the southern uplands rocks were thrust its northeastward extension is sampled by a group of carboniferous vents from which xenolith suites of crust and upper mantle origin have been obtained the middle to upper crust is believed to be represented by quartzo feldspathic gneisses that contain abundant graphite

distribution of groundwater arsenic in xinjiang p r china although potential contamination of groundwater as is expected to occur in xinjiang p r china few data are available for the regional distribution of groundwater as in this study the spatial distribution of groundwater as was investigated in the chepaizi cpz n and shihezi shz n areas of northern xinjiang the balikun yiwu basin by e in eastern xinjiang and the tarim trm s and yanqi yq s basins in southern xinjiang arsenic concentrations greater than 10 g l were found in 12 of analyzed groundwaters all groundwater samples collected in cpz n had as concentrations greater than 10 g l 25 185 g l 30 in shz n 10 g l was found in the eastern and southern trm s and yq s distribution of groundwater as showed a tremendous spatial variability which greatly varied over a short distance horizontally arsenic concentration generally increased with increasing sampling depth the spatial distribution of groundwater as would be regulated by as source and hydrogeochemical processes higher ph and or lower orp values were generally observed in high as groundwater 10 g l in comparison with low as groundwater 10 g l arsenic mobility in shz n and cpz n may result from as desorption under relatively high ph conditions and more tentatively from reductive dissolution of fe iii oxides in by e and trm s however detailed mechanisms of as mobilization in these regions need further investigation

mobility of arsenic in the sub surface environment an integrated hydrogeochemical study and sorption model of the sandy aquifer materials groundwater and aquifer materials have been characterized geochemically at a field site located in the chakdaha municipality of west bengal india sorption experiments were also carried out on a sandy aquifer material to understand the mobility of arsenic as in the sub surface environments the result shows that the areas associated with high groundwater as mean 1 8 m is typically associated with low eh mean 129 mv and high fe mean 0 11 mm where fe2 fe oh 3 couple is controlling groundwater redox potential analysis of the aquifer material total concentrations showed the dominance of as range 8 9 22 mg kg 1 fe range 3 0 9 7 as fe2o3 and mn range 0 05 0 18 as mno in the silt clay rich sediments whereas fine medium sand rich sediment contains considerably lower amount of as 8 1 mg kg 1 fe range 1 6 3 9 as fe2o3 and mn range 0 02 0 08 as mno the acid extractable as do not correlate with ascorbate extractable fe oxyhydroxide however fe oxyhydroxide is generally high in the sediments from low groundwater as areas chemical speciation computations indicated fe ii ca ii mg ii and mn ii to be at equilibrium with respect to calcite dolomite and rhodochrosite or slightly over saturated with respect to siderite these carbonate minerals may therefore participate to the as immobilization the measured total organic carbon 1 and groundwater temperature 26 32 c coupled with sorption studies strongly favors microbially mediated fe iii oxyhydroxide reduction as the dominant mechanism for the release of as in the groundwater oscillations of as mn and fe concentrations with depth reflected pco2 oscillations consecutive to microbial respiration intensity

natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water the example of west bengal and its worldwide implications in order to investigate the mechanism of as release to anoxic ground water in alluvial aquifers the authors sampled ground waters from 3 piezometer nests 79 shallow 80 m wells in an area 750 m by 450 m just north of barasat near kolkata calcutta in southern west bengal high concentrations of as 200 1180 mug l 1 are accompanied by high concentrations of fe 3 13 7 mgl 1 and po4 1 6 5 mg l 1 ground water that is rich in mn 1 5 3 mg l 1 contains 50 mug l 1 of as the composition of shallow ground water varies at the 100 m scale laterally and the metre scale vertically with vertical gradients in as concentration reaching 200 mug l 1 m 1 the as is supplied by reductive dissolution of feooh and release of the sorbed as to solution the process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit in well waters thermotolerant coliforms a proxy for faecal contamination are not present in high numbers 10 cfu 100 ml in 85 of wells showing that faecally derived organic matter does not enter the aquifer does not drive reduction of feooh and so does not release as to ground water arsenic concentrations are high much greater than50 mug l 1 where reduction of feooh is complete and its entire load of sorbed as is released to solution at which point the aquifer sediments become grey in colour as feooh vanishes where reduction is incomplete the sediments are brown in colour and resorption of as to residual feooh keeps as concentrations below 10 mug l 1 in the presence of dissolved fe sorbed as released by reduction of mn oxides does not increase as in ground water because the as resorbs to feooh high concentrations of as are common in alluvial aquifers of the bengal basin arise because himalayan erosion supplies immature sediments with low surface loadings of feooh on mineral grains to a depositional environment that is rich in organic mater so that complete reduction of feooh is common c 2004 published by elsevier ltd

spatial variation in arsenic and fluoride concentrations of shallow groundwater from the town of shahai in the hetao basin inner mongolia abstract twenty nine wells were selected for groundwater sampling in the town of shahai in the hetao basin inner mongolia four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2 5 20 m results show that groundwater as exhibits a large spatial variation ranging between 0 96 and 720 g l with 71 of samples exceeding the who drinking water guideline value 10 g l fluoride concentrations range between 0 30 and 2 57 mg l there is no significant correlation between as and f concentrations greater as concentrations were found with increasing well depth however f concentrations do not show a consistent trend with depth groundwater with relatively low eh has high as concentrations indicating that the reducing environment is the major factor controlling as mobilization low as concentrations 2 into the aquifers and lead to reducing conditions that favor as release sediment composition is an additional factor in determining dissolved as concentrations in aquifers composed of yellowish brown fine sands at depths around 10 m groundwater generally has low as concentrations which is attributed to the high as adsorption capacity of the yellow brown fe oxyhydroxide coatings fluoride concentration is positively correlated with ph and negatively correlated with ca 2 concentration all groundwater samples are over saturated with respect to calcite and under saturated with respect to fluorite dissolution and precipitation of ca minerals such as fluorite and calcite and f adsorption desorption are likely controlling the concentration of f in groundwater

mobility of arsenic in west bengal aquifers conducting low and high groundwater arsenic part i comparative hydrochemical and hydrogeological characteristics abstract the present study demonstrates the importance of hydrogeochemical characteristics groundwater flow and recharge of an aquifer in the release of as to groundwater the study area 20 km 2 is located in chakdaha block nadia district west bengal which hosts groundwaters of variable as content the spatial distribution pattern of as is patchy with areas containing groundwater that is high in as 200 g l 1 found in close vicinity to low as 1 groundwaters within 100 m the concentration of groundwater as is found to decrease with depth in addition the data shows that there is no conspicuous relationship between high groundwater as concentration and high groundwater abstraction although the central cone of depression has enlarged over 2 a and is extending towards the se of the study area the river hooghly which forms the nw boundary of the study site shows dual behaviour effluent and influent during pre and post monsoon periods respectively complicating the site hydrogeology the observed groundwater flow lines tend to be deflected away from the high as portion of the aquifer indicating that groundwater movement is very sluggish in the as rich area this leads to a high residence time for this groundwater package prolonging sediment water interaction and hence facilitating groundwater as release

hydrogeochemical characteristics of surface water sw and groundwater gw of the chinnaeru river basin northern part of nalgonda district andhra pradesh india groundwater and surface water samples from 47 locations 28 groundwater 10 tanks and 9 stream channel were collected during the pre monsoon may june and post monsoon season november from chinnaeru river basin chinnaeru river basin is situated 30 km east of hyderabad city and its area covers 250 km2 and falls in the survey of india toposheet no 56 k 15 the extensive agricultural industrial and urbanization activities resulted in the contamination of the aquifer to study the contamination of groundwater water samples were collected from an area and analyzed for major cations and anions various widely accepted methods such as salinity sodium absorption ratio kelly s ratio residual sodium carbonate soluble sodium percentage permeability index and water quality index are used to classify groundwater and surface water tank and stream for drinking as well as irrigation purposes besides this piper trilinear diagram wilcox diagram doneen s classification and gibb s plot were studied for geochemical controls and hydrogeochemistry of groundwater and surface water samples were studied

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

three dimensional joint inversion for magnetotelluric resistivity and static shift distributions in complex media accurate interpretation of magnetotelluric mt data in the presence of static shift arising from near surface inhomogeneities is an unresolved problem in three dimensional 3 d inversion while it is well known in 1 d and 2 d studies that static shift can lead to erroneous interpretation how static shift can influence the result of 3 d inversion is not fully understood and is relevant to improved subsurface analysis using the synthetic data generated from 3 d models with randomly distributed heterogeneous overburden and elongate homogeneous overburden that are consistent with geological observations this paper examines the effects of near surface inhomogeneity on the accuracy of 3 d inversion models it is found that small scale and shallow depth structures are severely distorted while the large scale structure is marginally distorted in 3 d inversion not accounting for static shift thus the erroneous near surface structure does degrade the reconstruction of smaller scale structure at any depth however 3 d joint inversion for resistivity and static shift significantly reduces the artifacts caused by static shifts and improves the overall resolution irrespective of whether a zero sum or gaussian distribution of static shifts is assumed the 3 d joint inversion approach works equally well for situations where the shallow bodies are of small size or long enough to allow some induction such that the effects of near surface inhomogeneity are manifested as a frequency dependent shift rather than a constant shift

effective petroleum source rocks of the world stratigraphic distribution and controlling depositional factors six stratigraphic intervals representing one third of phanerozoic time contain petroleum source rocks that have provided more than 90 of the world s discovered original reserves of oil and gas in barrels of oil equivalent the six intervals are 1 silurian generated 9 of the world s reserves 2 upper devonian tournaisian 8 of reserves 3 pennsylvanian lower permian 8 of reserves 4 upper jurassic 25 of reserves 5 middle cretaceous 29 of reserves and 6 oligocene miocene 12 5 of reserves this uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval maps that show facies structural forms and petroleum source rocks were prepared for this study analysis of the maps indicates that several primary factors controlled the areal distribution of source rocks their geochemical type and their effectiveness i e the amounts of discovered original conventionally recoverable reserves of oil and gas generated by these rocks these factors are geologic age paleolatitude of the depositional areas structural forms in which the deposition of source rocks occurred and the evolution of biota the maturation time of these source rocks demonstrates that the majority of discovered oil and gas is very young almost 70 of the world s original reserves of oil and gas has been generated since the coniacian and nearly 50 of the world s petrole m has been generated and trapped since the oligocene the term petroleum as used in this study includes oil gas condensate and gas but does not include bitumen and other solid and semisolid oil derivatives

electrical conductors in archean mantle result of plume interaction 1 from long period magnetotelluric data across the gawler craton we obtain a three dimensional resistivity image of the lithosphere and provide constraints on tectonothermal events dating back to the proterozoic contrary to common observations of archean cratons displaying high electrical resistivity in the mantle lithosphere the magnetotelluric data show low resistivity of around 10 m at 80km depth underneath the 1595ma gawler range volcanics a silicic large igneous province the resistivity distribution appears to be a signature of plume modified orogenesis with low degree partial melting at the base of the lithosphere in a back arc setting the enhanced conductivity is explained through higher hydrogen and iron content in the crystal lattice and also along the grain boundaries of the mantle constituting minerals older arc related magmatism to the southwest across 1620 1610ma st peter suite does not display enhanced conductivity and suggests a depleted mantle lithosphere the data show that yellowstone type mantle plume analogues are preserved through time and still display an elevated electrical signature in the lithosphere today

anisotropic rayleigh wave tomography of ireland s crust implications for crustal accretion and evolution within the caledonian orogen 1 the irish landmass now at the western extremity of the eurasian plate was formed in the caledonian orogeny during the palaeozoic assembly of pangea the associated closure of the iapetus ocean is recorded in the ne sw structural trends that dominate the tectonic set up of ireland today the deep crustal dynamics of the orogeny and the effect on the crust of the subsequent extension and magmatism in the north atlantic are debated fabrics within deep crustal rocks preserve a record of deformation during and after the continental collisions here we measured rayleigh wave phase velocities using seismograms recorded by permanent and temporary intermediate band stations in ireland and inverted the data for phase velocity maps including azimuthal anisotropy the observed isotropic phase velocity heterogeneity reflects moderate crustal thickness and seismic velocity variations across ireland anisotropy of rayleigh waves at 10 20 s periods shows a ne sw fast propagation direction and is largest up to 2 at a 15 s period at which rayleigh waves sample primarily the middle and lower crust the ne sw trend of the deep crustal anisotropic fabric is parallel to tectonic trends in particular the iapetus suture zone which indicates that suture parallel flow in the middle and lower crust accommodated the continental collision the apparent preservation of the caledonian age fabric also shows that the deep crust of the eurasian margin in ireland was neither stretched by the nw se extension associated with the opening of the north atlantic nor modified significantly by the cenozoic magmatism in the region

the erosional and uplift history of ne atlantic passive margins constraints on a passing plume new apatite fission track analyses from nw britain indicate that a maximum of 2 5 km of erosion has occurred there during the cenozoic similar to values for se greenland and the east greenland coast north of scoresby sund the erosion may have been facilitated by magmatic underplating during break up however at kangerdlugssuaq east greenland 4 6 km of erosion is measured since 45 ma lower mid eocene marine sedimentary rocks overlying the lavas on the blosseville coast indicate that magmatic underplating on the central greenland coast substantially post dated flood volcanism and break up behaviour not predicted by simple plume rift models subsidence reconstructions of the hebrides shelf and the east and west greenland coasts show that rapid dynamic uplift was effectively synchronous at 63 ma and preceded volcanism by c 400 m is compatible with a mantle temperature anomaly of c 100 c these data suggest very rapid lateral flow of the impacting iceland plume head the predicted crossing of the plume by the east greenland coast in the mid late eocene would account for post rift magmatic underplating and dynamic support on the greenland but not the european side of the north atlantic basin

arsenic levels in groundwater from quaternary alluvium in the ganga plain and the bengal basin indian subcontinent insights into influence of stratigraphy abstract late quaternary stratigraphy and sedimentation in the ganga alluvial plain and the bengal basin have influenced arsenic contamination of groundwater arsenic contaminated aquifers are pervasive within lowland organic rich clayey deltaic sediments in the bengal basin and locally within similar facies in narrow entrenched river valleys within the ganga alluvial plain these were mainly deposited during early mid holocene sea level rise arsenic was transported from disseminated sources as adsorbed on dispersed phases of hydrated iron oxide these were preferentially entrapped as sediment coatings on organic rich fine grained deltaic and floodplain sediments arsenic was released later to groundwater mainly by reductive dissolution of hydrated iron oxide and corresponding oxidation of sediment organic matter strong reducing nature of groundwater in the bengal basin and parts of affected middle ganga floodplains is indicated by high concentration of dissolved iron maximum 9 35 mg l groundwater being virtually stagnant under these settings released arsenic accumulates and contaminates groundwater the upland terraces in the bengal basin and in the central ganga alluvial plain made up of the pleistocene sediments are free of arsenic contamination in groundwater these sediments are weakly oxidised in nature and associated groundwater is mildly reducing in general with low concentration of iron

chemodynamics of an arsenic hotspot in a west bengal aquifer a field and reactive transport modeling study abstract extremely high as concentrations in drinking water of the ganges delta west bengal and bangladesh has emerged as an issue of great concern in the past decade because of its serious impact on the health of millions of people the distribution pattern of as concentrations in the ganges delta region is patchy and there are numerous as hotspots the present study is perhaps the first attempt in west bengal to characterize such a hotspot by geophysical and geochemical methods and to model the transport of the enrichment plume using a 1d reactive transport model phreeqc the study site is located along the hooghly river 60 km north of kolkota city near the city of chakdaha total as concentrations in the groundwater range from 0 5 to more than 6 mol l 1 the who recommended maximum drinking water concentration is 0 13 mol l 1 i e 10 g l 1 results show groundwater is in chemical equilibrium with siderite and calcite a mineral phase previously shown to be an efficient trap for as iii groundwater redox potential is controlled by the fe oh 3 am fe2 couple the as iii versus as v distribution 42 as iii and 58 as v on average is not at equilibrium with measured eh values no evidence of sulfide solid phases such as as rich pyrite or arsenopyrite was found although amorphous fe dissolution is confirmed to play an important role in the release of as selective dissolution extractions indicate that adsorption of as on carbonates and micas may also be an important component of as cycling in the sediment modelling results demonstrate the role of po 4 3 hco 3 and fe ii in mobilizing the as plume thereby increasing the threat to the 75 000 inhabitants of chakdaha

electromagnetic imaging of variscan crustal structures in sw iberia the role of interconnected graphite abstract the western part of the iberian peninsula iberian massif is the best exposed fragment of the variscan orogen in europe its southern half was generated by an oblique collision between three continental terranes belonging to the margins of laurassia avalonia the south portuguese zone spz and gondwana the ossa morena zone omz and the central iberian zone ciz the boundaries between them are considered to be suture zones a 200 km long magnetotelluric profile across the three variscan terranes was done in a nne direction approximately perpendicular to the main tectonic features the results of two dimensional inversion of the mt dataset reveal high conductivity zones coinciding with the transitions spz omz and omz ciz these conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime a high conductivity layer extending along the whole omz was found at a depth of 15 25 km the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile the high conductivity was interpreted as caused by the precambrian serie negra graphite rich rocks carbon and oxygen x ray mapping with electron microprobe on polished sections of serie negra samples from omz revealed the presence of interconnected graphite which supports the hypothesis that graphite is determinant for the high conductivity two graphite types which help to record the geological evolution were identified graphite accumulations in the schistosity surfaces produced by folding and metamorphism and metallic films of graphite developed along late faults the conductive layer shows blobs of higher conductivity suggesting macro anisotropy additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity given that the detachment level is located within the serie negra several high resistivity features were found in the upper crust related to devonian and carboniferous successions and probably to some unexposed plutons in the spz and the palaeozoic series of omz plus some granitic intrusions in the ciz a high resistivity zone extending to the whole crust is correlated with extensive late variscan granite intrusions

gravity gradients and upper crustal tectonic fabrics ireland gravity gradients derived from an updated bouguer anomaly map of ireland reveal large scale lineaments many of which can be related to geological structures and tectonic development first horizontal derivatives of the anomaly calculated at various azimuths accentuate linear trends in different directions while the second derivative helps resolve near surface structure the dominant ne sw caledonian trending lineaments are interpreted as due to fault boundaries and folds in pre carboniferous basement and caledonian fault reactivation during the variscan orogeny subsidiary nne ssw trending lineaments are present as well as weaker nw se trending lineaments the largest negative anomalies are produced by late caledonian granites and widespread buried granitic rock in the middle to upper crust is proposed both exposed and inferred granites appear to be spatially related to intersections of ne sw with nne ssw and nw se trending lineaments the orientation of the dominant gravity trend changes from ne sw towards ene wsw going westwards across ireland and forms part of a more regional change in the caledonian gravity fabric recognised throughout the northeast atlantic region variscan e w gravity trends in the south of ireland overlay the caledonian trends relationships between gravity lineaments and anomalies support a thin skinned model for the variscan cover

terrane boundaries in ireland inferred from the irish magnetotelluric profile and other geophysical data this paper presents explanations for some important geophysical features associated with terrane boundaries in the iapetus suture zone in ireland the acquisition of data along the irish magnetotelluric profile imp provided the basis for a re assessment of magnetic gravity and seismic refraction results and further resolved sub surface structures associated with terrane development during the caledonian orogeny the magnetotelluric data located two upper crustal conductors associated with the fair head clew bay and virginia magnetic lineaments one of the conductors probably extends along strike to scotland and its top surface may be associated with the t reflector seen on the north east coast nec deep seismic reflection profile the other conductor has a southeastern edge directly beneath a major throw in seismic basement and defines the northern boundary of the midland valley terrane in ireland the conductors and magnetic lineaments are probably due to black shales in sedimentary basins which have been thrust under the avalonian and laurentian continents during the closure of the iapetus ocean the magnetotelluric data also suggest that the southern uplands magnetic lineament represents a crustal break but the navan tipperary magnetic lineament is probably the consequence of a shallow volcanic block sedimentary trough structure

characterisation of organic matter in a shallow reducing arsenic rich aquifer west bengal abstract elevated arsenic in groundwaters extensively exploited for irrigation and drinking water in west bengal and bangladesh is causing serious impacts on human health a key mechanism for the genesis of arsenic as in these waters is microbially mediated reductive transformation of arsenic bearing fe iii hydrated oxides the role of organic c in this process whether from in situ organic matter om i e om from within the sediment or from other sources is widely recognised despite this there is a paucity of data about the characteristics of om in these as rich aquifers extraction and analysis of the polar and apolar fractions from seven different sediments from a known groundwater as hotspot in west bengal revealed om characteristic of the original terrestrial depositional environment however this was overprinted by abundant hydrocarbons with thermally mature e g petroleum distributions these hydrocarbons included abundant high molecular weight n alkanes and unresolved complex mixtures ucms as well as thermally mature distributions of hopanes and steranes additionally at certain depths surface sands 8 and 13 m deeper sands 25 m the om appeared to be biodegraded with the preferential removal of petroleum sourced n alkanes suggesting that indigenous microbes within the aquifer can utilise this carbon source the presence of this previously unreported source of bioavailable organic carbon is of importance as it has the potential to promote microbial activity and subsequent as release in the aquifers

the electric field in northern england and southern scotland implications for geomagnetically induced currents summary magnetotelluric mt data in the form of mt tensors are used to estimate directly the size and spatial distribution of the electric field in northern england and southern scotland with the aim of predicting the flow of geomagnetically induced currents gic in power networks in the region mt and geomagnetic deep sounding data from a number of different field campaigns at a period of 750 s are employed the mt data are cast in the form of telluric vectors which allow a joint hypothetical event analysis hea of both geomagnetic deep sounding and mt data this analysis reveals qualitatively the pervasive effects of electric field distortion in the region two approaches are taken to understand how the spatial structure of the regional electromagnetic field is affected by local distortions and what the origin of these distortions might be the dimensionality and form of electric field distortion of the mt tensors is investigated using the weaver et al and bahr classification schemes and by examining the misfit of a galvanic distortion model as a function of rotation angle at sites where the galvanic distortion model is found to be appropriate the regional mt tensors are recovered using tensor decomposition techniques it is found that recovering the regional mt response reconciles the geometry of induced currents implied by the mt data with that of the magnetic variation anomalies lilley s central impedances are used to calculate rotationally invariant effective telluric responses in the southern uplands the magnitude of the effective telluric response is approximately 0 25 0 5 mv km 1 nt 1 but as the southern uplands fault is approached it rises steadily to 3 mv km 1 nt 1 in the midland valley the effective telluric response is approximately 0 5 mv km 1 nt 1 which rises steadily to 2 5 mv km 1 nt 1 as the southern uplands and highland boundary faults are approached to the southeast and northwest respectively therefore the increase in the magnitude of the effective telluric response correlates with the approach of a major tectonic boundary such as the southern uplands fault these results show that the induced electric field strength varies considerably throughout the central scotland region in addition the hea indicates that due to lateral changes in conductivity structure the direction of the electric field deviates significantly from the regional direction implied by the polarization azimuth of the primary geomagnetic induction therefore any attempts to model the flow of gic in the region need to account for the spatial variation of both the magnitude and azimuth of the electric field

on the scalability of hydrogeochemical factors controlling arsenic mobility in three major inland basins of p r china abstract high as groundwater has been widely found in inland basins of p r china which has posed a serious health risk to the local residents although these inland basins experience the same arid to semiarid climate and are filled with quaternary sediments as concentrations show big variations three inlands basins have been investigated to characterize hydraulic conductivities and geochemistry of the groundwater and sediments and evaluate their controls on dissolved as concentrations dissolved as concentrations ranged between

structure of the central scandinavian caledonides and the underlying precambrian basement new constraints from magnetotellurics summary we investigate the structure of the accretionary wedge of the caledonian orogen the underlying autochthonous parautochthonous carbonaceous alum shales and the precambrian basement we have conducted 60 broad band magnetotelluric soundings along a 180 km long profile in j amtland sweden across the eastern section of the central scandinavian caledonides dimensionality analysis and regional strike estimates indicate that the conductivity structure can be approximated by a 2 d model having a n40 e strike direction consistent with the dominant geological strike the determinant average of the impedance tensor together with the tipper transfer function from the best 34 sites were inverted by the rebocc 2 d inversion code an electrically highly conducting layer beneath the caledonides images alum shales the autochthonous cambrian carbon bearing black shales on top of the precambrian basement based on the comparison of electrical conductivity and seismic reflectivity models we suggest that the caledonian accretionary wedge thickens in a step wise manner from ca 1 to 5 6 km towards the west in the east the wedge is composed of the lower allochthon in the west the wedge reaches the thickness of 15 km and is composed of the lower allochthon at the bottom the middle upper allochthons at the top and resistive allochthonous basement slices the upper crust of the autochthonous precambrian basement is homogeneous and resistive from surface

near surface lithology and spatial variation of arsenic in the shallow groundwater southeastern bangladesh this study investigated the relationship between near surface lithology and the spatial variability of as concentrations using sediment grain size analysis and electromagnetic induction survey in the southeast bangladesh it has been observed that the aquifers overlain by finer sediments have higher concentrations of as in groundwater whereas as concentrations are remarkably low in aquifers having permeable sandy materials or thinner silt clay layer at the surface the near surface lithology acts as a controlling factor for spatial distributions of groundwater as within the very shallow depths 15 m shallow alluvial aquifers can provide low as drinking water in many areas of the country when tube wells are properly installed after investigation of the overlying near surface sediment attributes and hydraulic properties

arsenic in groundwater testing pollution mechanisms for sedimentary aquifers in bangladesh in the deltaic plain of the ganges meghna brahmaputra rivers arsenic concentrations in groundwater commonly exceed regulatory limits 50 m gl 21 because feooh is microbially reduced and releases its sorbed load of arsenic to groundwater neither pyrite oxidation nor competitive exchange with fertilizer phosphate contribute to arsenic pollution the most intense reduction and so severest pollution is driven by microbial degradation of buried deposits of peat concentrations of ammonium up to 23 mg l 21 come from microbial fermentation of buried peat and organic waste in latrines concentrations of phosphorus of up to 5 mg l 21 come from the release of sorbed phosphorus when feooh is reductively dissolved and from degradation of peat and organic waste from latrines calcium and barium in groundwater come from dissolution of detrital and possibly pedogenic carbonate while magnesium is supplied by both carbonate dissolution and weathering of mica the 87 sr 86 sr values of dissolved strontium define a two component mixing trend between monsoonal rainfall 0 711 6 0 001 and detrital carbonate 0 735

the seismic reflection moho beneath the united kingdom and adjacent areas abstract onshore and offshore deep seismic reflection data have been integrated to produce two way travel time and depth maps of the seismic reflection moho beneath the uk moho travel times are mostly in the range 9 5 11 5 s showing only weak correlation with specific upper crustal structural features but decreasing generally northwestward towards the atlantic margin moho depths show a much stronger correlation with upper crustal structure than do travel times there is pronounced crustal thinning moho depths 32 km are found generally beneath basement massifs particularly onshore short wavelength topographic features on the moho are quite rare and are generally restricted to ancient zones of continental collision and or subduction some of these ancient collision and or subduction zones are spatially associated with a highly reflective moho this is particularly evident where low angle shear zones in the lower crust or uppermost mantle converge with or intersect the moho it is suggested that development of a moho detachment surface can tectonically smooth the moho and thereby markedly increase its reflectivity a tendency in certain areas for the moho to remain at roughly constant two way travel time though not constant depth beneath both sedimentary basins and intervening basement massifs is interpreted as indicating that structural features on the scale of several tens of kilometres are in regional isostatic equilibrium conversely travel time perturbations beneath smaller features 10 km across indicate that these are at least partially uncompensated the uk crust is therefore weak but not sufficiently weak to attain local airy isostasy basement massifs west of scotland show systematically smaller moho two way travel times than similar structures farther south within the constraints of regional isostasy this can be interpreted in two main ways greater rigidity and strength of lewisian rocks within the west scottish massifs manifest as higher seismic velocity relative to density could be a cause of the observed travel time differences alternatively low density material beneath the crust could buoy up the western scottish massifs if this buoyancy were the consequence of igneous underplating then any underplated material would have to lie beneath the seismic reflection moho

a high resolution magnetotelluric survey of the iapetus suture zone in southwest scotland summary magnetic variation mv surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the southern uplands of scotland from the north sea to galloway a new magnetotelluric mt survey across the conductor in galloway was designed to improve the resolution of structure in the depth range 1 15 km and at the same time assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition processing analysis and modelling a 40 km long profile was measured doubled for part of its length at a separation of 1 km with a typical site spacing of 2 km or less it was supplemented by shorter parallel and orthogonal lines giving a total of 40 sites galvanic distortion models involving regional 2 d structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites confirming the picture from induction arrow maps that many sites were influenced by 3 d structures on scales of a few kilometres and upwards other indicators of the electrical strike regional and local mv measurements the spatial structure of the groom bailey regional impedance phase and of the rotated off diagonal phase gave values more in agreement with the geological strike of n52 e which was adopted as the most appropriate coordinate system into which to resolve the data the mt impedances were accordingly rotated into directions n38 w tm and n52 e te and inverted using the 2 d code of rodi mackie the investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters and to different data subsets the te mode data were insensitive to structural detail and the best fit achieved was unacceptable suggesting the influence of structure outside the plane of the section the tm mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies forward modelling showed that the model generated from the tm mode also fitted the mv data adequately the inversion resolved the conductive zone into distinct blocks with edges matching faults mapped in the surface geology the leadhills fardingmullach and orlock bridge faults and the moffat valley lineament in the resistivity image the faults are vertical structures disrupting other features to a depth of 15 km which agrees with the view that although they originated as thrusts they were reactivated by strike slip motion during oblique closure of the iapetus ocean within some of the blocks are highly conductive regions no more than 4 km wide the most likely explanation is graphite or other metallic mineralization localized in shear zones

electromagnetic images of the trans hudson orogen the north american central plains anomaly revealed magnetotelluric studies of the trans hudson orogen over the last two decades prompted by the discovery of a significant conductivity anomaly beneath the north american central plains nacp from over 300 sites yield an extensive database for interrogation and enable three dimensional information to be obtained about the geometry of the orogen from southern north dakota to northern saskatchewan the nacp anomaly is remarkable in its continuity along strike testimony to along strike similarity of orogenic processes where bedrock is exposed the anomaly can be associated with sulphides that were metamorphosed during subduction and compression and penetratively emplaced deep within the crust of the internides of the orogen to the boundary of the hearne margin a new result from this compilation is the discovery of an anomaly within the upper mantle beginning at depths of 80 100 km this lithospheric mantle conductor has electrical properties similar to those for the central slave craton mantle conductor

proto iceland plume caused thinning of irish lithosphere abstract the lithosphere asthenosphere boundary lab separates the rigid lithospheric plates from the hot convective mantle in the region of the north atlantic this boundary is widely influenced by the spreading head of the proto iceland plume p kumar et al the lithosphere asthenosphere boundary in the north west atlantic region earth planet sci lett 236 2005 249 257 that also caused the evolution of the british tertiary volcanic province btvp with the giant s causeway basalts as famous landmark using teleseismic shear wave receiver functions we map the lab underneath ireland and find a distinctive lithospheric thinning of about 30 km towards the northern part of the island as this region is part of the btvp we explain the lithospheric thinning in agreement with geochemical results by thermal erosion originating from the head of the proto iceland plume

hydrological control of as concentrations in bangladesh groundwater 1 the elevated arsenic as content of groundwater from wells across bangladesh and several other south asian countries is estimated to slowly poison at least 100 million people the heterogeneous distribution of dissolved arsenic in the subsurface complicates understanding of its release from the sediment matrix into the groundwater as well as the design of mitigation strategies using the tritium helium 3h 3he groundwater dating technique we show that there is a linear correlation between groundwater age at depths 20 m and dissolved as concentration with an average slope of 19 g l 1 yr 1 monitoring wells only we propose that either the kinetics of as mobilization or the removal of as by groundwater flushing is the mechanism underlying this relationship in either case the spatial variability of as concentrations in the top 20 m of the shallow aquifers can to a large extent be attributed to groundwater age controlled by the hydrogeological heterogeneity in the local groundwater flow system

lateral variations of conductivity structure across southern scotland and northern england abstract high frequency 100 0 01 hz magnetotelluric mt soundings have been carried out along a 140 km profile across southern scotland and northern england where previously longer period soundings had been undertaken examples of the results of the processing of data from 15 sites along the profile and of the application of a number of 1d inversion procedures are presented the latter include the joint inversion of d c resistivity and mt data at rookhope where borehole stratigraphy is also available although the data were severely contaminated by near field sources at some sites and the station spacing was too large in parts of the traverse for good resolution of model parameters by incorporation of results from the earlier studies it is possible to obtain a preliminary 2d model of the electrical resistivity structure of the upper crust in this region significant differences have been found in the three main geological zones crossed by the survey the alston block the northumberland basin and the southern uplands of scotland in the discussion special attention has been paid to four sites in the neighbourhood of the southern uplands fault for which the data have been subjected to tensor decomposition this has revealed two dominant regional azimuths one of which dominates at the shorter periods and corresponds to the strike of the southern uplands fault and the other at longer periods which corresponds to the strike of the well known southern uplands magnetovariational mv anomaly moreover in this region the electrical resistivity models indicate the presence of a highly conducting zone beginning at a depth of about 4 km at station cap which is located on the axis of the mv anomaly and extending to a depth of at least 25 km the depth to this conducting zone increases both to the north and the south of the mv anomaly axis resulting in a structure which has many features in common with those along parallel traverses in se scotland and in ireland in discussing the implications of the irish model another researcher proposed either metamorphosed sediments and or serpentinised island are crust as being possible sources of the high conductivity and other geophysical and geological data the probability that the arguments in that study are equally applicable to southern scotland is currently under consideration

contributions of floodplain stratigraphy and evolution to the spatial patterns of groundwater arsenic in araihazar bangladesh extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fluviodeltaic environments here we examine patterns of arsenic heterogeneity in the shallow 1 10 3 m in the 25 km 2 area indicate that the concentration of arsenic in shallow groundwater largely varies with the grain size thickness and distribution of fine grained 63 m sediments that overlie buried aquifer sands the overall pattern shows that lower arsenic concentrations are typically found where aquifer sands outcrop at or near the surface whereas higher arsenic levels typically underlie or are adjacent to thicker fine grained deposits furthermore chronostratigraphic reconstructions of aquifer sediments indicate that sediment distribution and consequently the patterning of dissolved arsenic is readily explained in the context of local river history and flood plain development within the past 1000 yr an important implication is that complex patterns of groundwater arsenic in afflicted fluviodeltaic settings can be better understood through reconstructions of local aquifer history this finding is especially relevant because the village and tube well locations are closely linked with surface landforms such as former levees and bars an additional and worrisome finding is that the artificial filling of villages to protect from flooding can mimic the natural fine grained stratigraphy commonly associated with high concentrations of arsenic

the significance of large sediment supply active tectonism and eustasy on margin sequence development late quaternary stratigraphy and evolution of the ganges brahmaputra delta abstract borehole data from the ganges brahmaputra delta system unveil an intriguing late quaternary history controlled by immense sediment discharge tectonics and eustasy sea level first intersected a major portion of the lowstand surface 10 000 11 000 cal yr bp marking the onset of sediment trapping and delta growth despite rapid sea level rise 1 cm yr sediment load was sufficient to maintain relative shoreline stability during this time by 7000 cal yr bp accommodation was quickly infilled with slowed sea level rise leading to upstream channel migration and widespread dispersal of sands this forced coastal progradation along the western delta front where the ganges was situated and much of the river s fine grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf concurrently brahmaputra sediments were sequestered to an inland tectonic basin thus starving the adjacent shoreline and leading to transgression along the eastern delta by 5000 cal yr bp the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river mouth estuary present delta configuration appears to have developed by 3000 cal yr bp evolution of the tectonically active ganges brahmaputra g b system reveals important similarities and differences with other deltas overall facies succession follows that of basic models progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain however the timing thickness and controls of these deposits differ immense sediment discharge from the ganges and brahmaputra rivers allowed initial delta growth 2000 3000 yr earlier than the global average despite debouching onto a canyon incised high energy margin subsequently a thick 50 m transgressive systems tract was formed during the early holocene phase of delta aggradation contrasting with extensive shoreline retreat documented along most margins highstand progradation of coastal and shelf sequences ensued by the middle holocene but subsiding inland basins also favored accumulation of a thick to 40 m highstand sequence in the lower floodplains unlike many other delta systems subsidence in the subaerial g b delta is not dominated by compaction but rather by plate driven tectonic processes that generate rates up to 4 mm year overall the huge sediment load tectonic subsidence major seismic events and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta shelf and deep sea bengal fan throughout the late quaternary

decoupling of as and fe release to bangladesh groundwater under reducing conditions part i evidence from sediment profiles this study reexamines the notion that extensive as mobilization in anoxic groundwater of bangladesh is intimately linked to the dissolution of fe oxyhydroxides on the basis of analyses performed on a suite of freshly collected samples of aquifer material detailed sediment profiles extending to 40 to 70 m depth below the surface were obtained at six sites where local groundwater as concentrations were known to span a wide range the sediment properties that were measured include 1 the proportion of fe ii in the fe fraction leached in hot 1 2 n hcl 2 diffuse spectral reflectance and 3 magnetic susceptibility in parallel with local concentrations of dissolved as ranging from 5t o 600g l fe ii fe ratios in shallow gray holocene sands tended to gradually increase with depth from values of 0 3 to 0 5 to up to 0 9 in deeper orange aquifers of presumed pleistocene age that were separated from shallow sands by a clay layer and contained 5 g l dissolved as leachable fe ii fe ratios averaged 0 2 there was no consistent relation between sediment fe ii fe and dissolved fe concentrations in groundwater in nearby wells the reflectance measurements indicate a systematic linear relation r 2 of 0 66 n 151 between the first derivative transform of the reflectance at 520 nm and fe ii fe the magnetic susceptibility of the shallow aquifer sands ranged from 200 to 3600 x 10 9 m 3 kg si and was linearly related r 2 of 0 75 n 29 to the concentrations of minerals that could be magnetically separated 0 03 to 0 79 dry weight no systematic depth trends in magnetic susceptibility were observed within the shallow sands although the susceptibility of deeper low as aquifers was low up to 200 10 9 m 3 kg si

deep electrical conductivity structures of the appalachian orogen in the southeastern u s long period magnetotelluric data across the southeastern appalachians image deep crustal and upper mantle relics of ancient orogenic events inversions of the responses show 1 beneath the appalachian mountains there is a sub horizontal conductor at 15 20 km depth which dips to the southeast at the surface trace of the brevard fault 2 at the location of the central piedmont suture there is a crustal conductor which dips towards the southeast interpreted as a structure related to the acadian suture 3 upper mantle conductors were found at 80 km depth northwest of the blue ridge and at 140 km depth southeast of the eastern piedmont between these there is a northwest dipping resistive gap possibly representing the remnant structure of the alleghanian collision

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

mobility of arsenic in west bengal aquifers conducting low and high groundwater arsenic part ii comparative geochemical profile and leaching study aquifer sediments from areas of low and high as groundwater were characterized mineralogically and geochemically at a field site in the nadia district of west bengal india leaching experiments and selective extraction of the sediments were also carried out to understand the release mechanism of as in the sub surface the correlation between measured elements major minor and trace from low and high as groundwater areas are only significant for as fe and mn the borehole lithology and percentage of silt and clay fraction demonstrates the dominance of finer sediments in the high as aquifer multivariate analysis of the geochemical parameters showed the presence of four different mineral phases heavy mineral fraction phyllosilicates biotite fe oxyhydroxides carbonates and sulphides in the sediments selective extraction of sediment reveals that amorphous fe oxyhydroxide acts as a potential sink for as in the sub surface the result is consistent with microbially mediated redox reactions which are controlled in part by the presence of natural organic matter within the aquifer sediments the occurrences of as bearing redox traps primarily formed of fe and mn oxides hydroxides are also important factors that control the release of as into groundwater at the study site

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

inversion of electromagnetic induction data for the iapetus suture zone in the uk abstract the iapetus suture zone isz corresponding to the join between the european crust of northern england and the north american derived crust of scotland has been examined by numerous seismic and induction experiments over a 20 year period the university of edinburgh electromagnetic em induction group is in the process of building a database comprising magnetotelluric and magnetic variation stations from this region in this paper we present 2d inversions of selected induction sites along roughly north south traverses across the isz in northern england and southern scotland the first inversion involves both the transverse electric te and transverse magnetic tm modes for 10 audio magnetotelluric approximately 100 0 1 hz sites care has been taken to remove the effects of static shifts this inversion resolves a conductive feature beginning under the southern northumberland trough at depths of 10 km dipping to the south below the alston block in the second inversion 12 longer period magnetic variation sites were added to the original data set improving the resolution of this feature which correlates well with a dipping bright reflector detected by the north east coast nec seismic reflection experiment in addition the southern uplands anomaly is imaged no north dipping mid to deep crustal conductor is observed which would correlate to north dipping palaeozoic subduction under laurentia

varnet 96 three dimensional upper crustal velocity structure of sw ireland during the varnet 96 seismic experiment three seismic refraction wide angle reflection profiles were acquired in order to examine the crustal structure in the southwest of ireland a total of 170 seismic stations were used on 300 recording sites the shotpoint geometry was designed to allow for inline and offline fan shot recordings on the three profiles using a total of 34 shots results from 3 d ray trace and inversion modelling illustrate the pervasive lateral heterogeneity of the crust south of the shannon estuary about 5 km of interpreted palaeozoic sediment at the south coast was associated with the sedimentary infill of the munster and south munster basins this sedimentary layer which thins to approximately 2 km in the northern munster basin is significantly thinner than previously estimated from geological field studies high velocity zones beneath dingle bay and the kenmare river region may be associated with the deep traces of the killarney mallow fault zone and the cork kenmare line a zone of high velocity upper crust 6 4 6 6 km s 1 beneath the south munster basin is found in the area between the kenmare killarney and the leinster granite gravity lows the depth to the moho varies from approximately 28 29 km at the south coast to approximately 32 33 km in the dingle shannon basin the interpretation of the 2 d and 3 d velocity models suggests that variscan deformation is confined to sedimentary and upper crustal structures in the southwest of ireland