a geomagnetic induction anomaly from ims data near hudson bay and its relation to crustal electrical conductivity in central north america summary canadian geomagnetic data collected during the international magnetospheric study are used to investigate the terrestrial electrical conductivity structure of north eastern manitoba and part of the north west territories the computed transfer functions resolve a major conductor trending east west between the communities of gillam and back in manitoba regional trends in the surface geology suggest that this conductor may be linked with the north american central plains electrical conductor twodimensional modelling of the data suggests that the conductor dips to the north from a small depth beneath gillam and may extend to the lowermost cnst

natural electromagnetism in the rhine graben summary together with experiments by differential geomagnetic soundings magnetotelluric soundings have been performed in the rhine graben the results of the former have been presented and discussed before we try to show that it is not necessary to call upon the notion of induction in surrounding areas to justify the characteristics of the observed anomalous magnetic fields theoretical calculations of these fields on structures defined by the interpretation of magnetotelluric sounds or by geological and other geophysical data agree very well with experimental data we conclude that magnetotelluric experiments and interpretations are valid as the rhine graben is a well known region allowing us to verify our interpretation

geochemical interactions between constituents in acidic groundwater and alluvium in an aquifer near globe arizona abstract acidic water from a copper mining area has contaminated an alluvial aquifer and stream near globe arizona the most contaminated groundwater has a ph of 3 3 and contains about 100 mmol 1 so4 50 mmol 1 fe 11 mmol 1 al and 3 mmol 1 cu reactions between alluvium and acidic groundwater were first evaluated in laboratory column experiments a geochemical model was developed and used in the equilibrium speciation program minteqa2 to simulate breakthrough curves for different constituents from the column the geochemical model was then used to simulate the measured changes in concentration of aqueous constituents along a flow path in the aquifer the ph was predominantly controlled by reaction with carbonate minerals where carbonates had been dissolved adsorption of h by iron oxides was used to simulate ph acidic groundwater contained little or no dissolved oxygen and most aqueous fe was present as fe ii in the anoxic core of the plume fe ii was oxidized by mno2 to fe iii which then precipitated as fe oh 3 attenuation of aqueous cu co mn ni and zn was a function of ph and could be quantitatively modeled with the diffuse layer surface complexation model in minteqa2 aluminum precipitated as amorphous al oh 3 at ph after the alluvium in the column had reached equilibrium with acidic groundwater uncontaminated groundwater was eluted through the column to evaluate the effect of reactants on groundwater remediation the concentration of fe mn cu co ni and zn rapidly decreased to the detection limits within a few pore volumes all of the gypsum that had precipitated initially redissolved resulting in elevated ca and so4 concentrations for about 5 pore volumes aluminum and ph exhibited the most potential for continued adverse effects on groundwater quality as h desorbed from fe oh 3 ph remained below 4 5 for more than 20 pore volumes resulting in dissolution of alohso4 and elevated aqueous al

electromagnetic induction studies recent developments in electromagnetic induction studies of the lithosphere and the asthenosphere are reviewed attention is given to geoelectrical studies of active tectonic areas in terms of the major zones of crustal extension the basin and range province along western regions of north america and the rio grande rift studies have also been performed of tectonic activity around iceland the salton trough and cerro prieto and the subduction zones of the cascade mountains volcanic belt where magnetotelluric and geomagnetic variation studies have been done geomagnetic variations experiments have been reported in the central appalachians and submarine electromagnetic studies along the juan de fuca ridge controlled source electromagnetic and dc resistivity investigations have been carried out in nevada hawaii and in the adirondacks mountains laboratory examinations on the conductivity of representative materials over a broad range of temperature pressure and chemistry are described

a 1300 year reconstruction of paleofloods using oxbow lake sediments in temperate southwestern quebec canada the study of paleofloods provides important information on past flood frequency and intensity for regions where there is a paucity of records it therefore extends our knowledge of flood occurrence

accurate and efficient regularized inversion approach for the interpretation of isolated gravity anomalies a very fast and efficient approach for gravity data inversion based on the regularized conjugate gradient method has been developed this approach simultaneously inverts for the depth z and the amplitude coefficient a of a buried anomalous body from the gravity data measured along a profile the developed algorithm fits the observed data by a class of some geometrically simple anomalous bodies including the semi infinite vertical cylinder infinitely long horizontal cylinder and sphere models using the logarithms of the model parameters log z and log a rather than the parameters themselves in its iterative minimization scheme the presented numerical experiments have shown that the original non logarithmed minimization scheme which uses the parameters themselves z and a instead of their logarithms encountered a variety of convergence problems the aforementioned transformation of the objective functional subjected to minimization into the space of logarithms of z and a overcomes these convergence problems the reliability and the applicability of the developed algorithm have been demonstrated on several synthetic data sets with and without noise it is then successfully and carefully applied to seven real data examples with bodies buried in different complex geologic settings and at various depths inside the earth the method is shown to be highly applicable for mineral exploration and for both shallow and deep earth imaging and is of particular value in cases where the observed gravity data is due to an isolated body embedded in the subsurface

electrical conductivity structure of southeastern north america implications for lithospheric architecture and appalachian topographic rejuvenation abstract we present the first three dimensional view of the lithospheric electrical conductivity structure beneath southeastern north america by inverting earthscope long period magnetotelluric mt data we obtain an electrical conductivity image that provides new insights into both the architecture of the appalachian orogen and the cryptic post rifting geodynamic history of the southeastern united states our inverse solutions reveal several elongate electrically conductive features that we interpret as major terrane sutures within the appalachian orogen most significantly we resolve a highly electrically resistive layer that extends to mantle depths beneath the modern piedmont and coastal plain physiographic provinces as high resistivity values in mantle minerals require cold mantle temperatures the mt data indicate that the sub piedmont thermal lithosphere must extend to greater than 200 km depth this firm bound conflicts with conclusions from seismic results the boundary between the anomalously thick resistive sub piedmont lithosphere and the relatively thin moderately conductive sub appalachian lithosphere corresponds within resolution to the modern appalachian topographic escarpment this newly recognized contrast in lithospheric properties likely has important implications for appalachian topographic rejuvenation

sediment water interaction acidity and other water quality parameters in a subtropical setting pimpama river southeast queensland the geological setting and landuse activities of the pimpama river catchment are typical for many drainage systems in southeast queensland the river originates in coastal ranges of late paleozoic age crosses a floodplain developed during the late pleistocene holocene sea level fluctuations and flows into the southern part of the moreton bay the formation of sedimentary pyrite associated with the mid holocene transgression is an important feature of this coastal setting the oxidation and hydrolysis of pyrite and the consequent production of sulfuric acid are controlled by the amount and seasonality of rainfall and influenced locally by landuse activities the acid production and the leaching of dissolved metals from river alluvium and estuarine sediments impact substantially on land and aquatic habitats the water quality of the pimpama river and its tributaries reflects the lithology of the bedrock and can vary largely depending on season tidal regimes sediment lithology local topography and agricultural activities

geoelectromagnetic measurements across the southern senegal basin west africa abstract magnetotelluric and differential geomagnetic sounding surveys consisting of nine soundings were performed in 1984 along a 200 km profile across the southern senegal basin they were intended to obtain information concerning the resistivity structure of the crust and upper mantle and the distribution of the induced electric currents magnetotelluric data indicate that two dimensional resistivity models are appropriate for the region the zone above the basement is highly inhomogeneous in geoelectrical structure very conductive sediments 0 6 3 ohm m appear in the mesozoic cenozoic senegal basin these sediments lie at depths of up to 4500 m on the west end of the profile below this a modest resistivity material 10 30 ohm m extends to a maximum depth of about 3000 m the material at depth on the cast part of the traverse line is thought to be palaeozoic sediments of the bove basin the depth of the magnetotelluric basement lies between about 250 m in the east and 4800 m in the west the crust is characterised by a drop in electrical resistivity at a depth of 15 km below the east part of the profile considering the total section we observe a general trend towards lower resistivities at depths in excess of 100 km the transition from 2000 ohm m to about 2 ohm m occurs in the depth range 100 to 175 km an analysis of the geomagnetic variation field has identified a concentration of telluric current flow beneath the deep basin it appears that the additional currents flowing in the striking direction of the senegal basin are largely controlled by sedimentary rocks of high conductivity lying at depths less than 5 km model studies show that the local conductivity distribution is able to explain the currents circulating in the thick well conducting sediments

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

the electrical resistivity structure of archean to tertiary lithosphere along 3200 km of snorcle profiles northwestern canada magnetotelluric mt measurements to image the three dimensional resistivity structure of the north american continent from an archean core to a region of tertiary assembly were recorded at almost 300 sites along 3200 km of profiles on the lithoprobe slave northern cordillera lithospheric evolution snorcle transect in northwestern canada at the largest scale the mt results indicate significant lithospheric thickness variation from 260 km at the southwest margin of the slave craton to significantly 100 km at the southwestern end of the snorcle transect in the cordillera at intermediate scale the resistivity results allow broad terrane subdivisions to be made several anomalously conductive zones along the snorcle transect in rocks ranging in age from archean to tertiary are attributed to the introduction of either water or carbon into the crust and mantle during subduction processes at the local scale the mt data image two major faults crossing the study area the great slave lake shear zone and the tintina fault the resistivity images show that both the tintina fault and great slave lake shear zone form crustal scale features and that the tintina fault has a remarkably uniform resistivity signature over a 400 km strike length in the study area arguably the most controversial conclusion reached is that the mt data do not support the western extension of north american autochthonous basement suggested from interpretation of the seismic reflection data

regional induction in scotland an example of three dimensional numerical modelling using the thin sheet approximation abstract a new algorithm recently devised for the solution of three dimensional problems of electromagnetic induction by a uniform source in a non uniform thin sheet at the surface of a conducting half space is used in a numerical study of regional induction in scotland limitations imposed by grid size and the conductivity of the lower crust restrict the investigation to short periods and for comparison with the published results from a recent magneto telluric survey of scotland a period of 25 s is chosen here the variations of the field components along parallel traverses across scotland are plotted for two perpendicular polarizations of the inducing field the coastlines and other conductivity structures included in the model notably the great glen fault are clearly revealed by these plots and the importance of vertical as well as horizontal deflections of the induced currents at a conductivity discontinuity is demonstrated real induction vectors are drawn at a number of grid points whose positions correspond approximately to the sites of actual field stations agreement with the corresponding induction vectors given by the field data is quite good at the coastal stations and near the great glen fault but otherwise the calculated vectors at inland stations are much shorter than those obtained from field data apparent resistivities phases and the azimuth of the major principal axis of impedence are also calculated for each site but there is poor agreement with measured values which show a greater degree of anisotropy it seems that for many sites the numerical grid is too coarse to model the surface conductivity distribution in sufficient detail to provide realistic estimates of these parameters

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

a magnetotelluric traverse across the mauritanides orogenic belt west africa summary magnetotelluric measurements with reference magnetometers for study of the anomalous geomagnetic variation field were made in the period range 10 10000 s at 10 sites crossing the main geological zones of eastern senegal west africa the results indicate that the electromagnetic fields are controlled by the dominant structural trends of the mauritanides orogenic belt apparent resistivities reflect the major structural units identified from surface geology data in the principal directions for most of the stations exhibit anisotropic behaviour and at certain sites the magnetotelluric responses are affected by surface distortion effects the study of the geomagnetic variation field indicates the presence of a concentration of telluric current flow beneath the craton a lack of obviously conductive geological structures at the surface suggests that this flow of telluric current may be attributed to increased electrical conductivity in the crust the quantitative interpretation of the magnetotelluric results is given in terms of 1 d and 2 d resistivity models it is shown that the data require the presence of a highly anomalous crustal structure 40 rm dipping to the east from a small depth beneath the outcropping margin of the west african craton in the mauritanides the crust is resistive 3000 rm t o a depth of 30 km and overlies a conducting layer of 10 km thick in the depth range 250 300 km resistivities decrease to less than 50 rm this may coincide with the top of the asthenosphere the geoelectric model is discussed in relation to the general geological background of the region considered

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

crustal and upper mantle electrical conductivity structures in west africa geodynamic implications abstract measurements of the electrical and magnetic fields at the surface of the earth have been carried out in west africa in the period range 10 10 000 s in this area the method of magnetotelluric mt sounding finds its application in a variety of geological problems such as the deep shape of sedimentary basins the geometry of orogenic belts and craton deep structures the present article is an attempt to correlate the conductivity model with large scale tectonics mt surveys have revealed the presence of several areas where electrical resistivity is abnormally low at crustal and upper mantle depths and the existence of lateral electrical conductivity inhomogeneities within the lithosphere subsurface electrical conductivity distribution is in good relation with the geological features and its variation associated with lithological and structural changes a major discontinuity found at a depth of several hundred kilometers in the lithosphere separates two structural blocks the eastern one with higher resistivities being the west african craton the lithosphere derived from this work has a maximum thickness of 300 km under the senegalo mauritanian basin and more than 400 km beneath the craton the electrical asthenosphere is certainly not well developed under the craton a sketch of electric structures across west africa is presented and a geodynamic interpretation is proposed to explain the present structural pattern of the southern segment of the mauritanides belt

fractures as conducting dykes and corresponding two dimensional models a statistical treatment of mt data from the pannonian basin hungary based on the distortion theory of the s effect suggests that the local increase in crustal conductivity is connected with deep fractures field model measurements have recently been carried out in a shallow quasi two dimensional basin of well known tectonics using mt station distances of 1 3 km the results of the investigation and numerical modelling of near surface distortions support the idea that deep fractures faults contain the conducting formations according to 2 d model computations several conducting dykes at considerable depths can cause anomalies which are indiscernible from an anomaly due to a conducting layer the significance of these results is discussed from the point of view of geothermal energy exploitation

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

cocorp and the continental crust this is a speculative paper on the continental crust a major frontier of modern earth science seismic reflection profiling data from the cocorp project coupled with other kinds of geophysical and geological observations suggest some fresh perspectives on the crust for example there is evidence for great differences within the basement at a single site and from one cocorp site to another a simple layered model is surely not an adequate representation of the crust in most areas there is however the puzzling and rather common occurrence of limited zones of parallel though not always flat layered features at various depths in certain parts of the crust are the corresponding rocks metasedimentary or igneous or both there are transparent zones from which little or no coherent reflected seismic energy is detected are the corresponding rocks intrusions or highly deformed metasediments can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust there is evidence for large scale thrusting and related features that emphasizes the great importance of the wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences in some places the modern moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting the widely held concept of the moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crust

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

why do continents break up parallel to ancient orogenic belts the frequently observed parallelism between rifts and the pre existing orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents we propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropie strength in the lithosphere that guides the propagation of continental rifts subcrustal mantle mechanical anisotropy is supported by i the anisotropie strength of olivine ii an ubiquitous tectonic fabric in exposed mantle rocks and iii measurements of seismic and electrical anisotropy during major episodes of continent rifting parallel to orogenic belts ocean opening through rifting and continent break up is frequently related to the occurrence ofhotspots there is howevcr a discrepancy between hot spots acting as pin point sources of heat and the linear extent of rifts over thou sands ofkilometres moreover rifts tend to parallel pre existing orogenic fab

paleohydrologic bounds and extreme flood frequency of the upper arkansas river colorado usa article i nfo article history the upper arkansas river basin has experienced notable large floods including the event of 2 6 june 1921 that devastated the city of pueblo colorado we investigated flood and paleoflood hydrology at strategic sites to determine the frequency and geographic extent of extreme floods within the basin for a dam safety application streamgage historical and paleoflood data were utilized to develop frequency curves at sites near salida cotopaxi parkdale and pueblo soil stratigraphic descriptions radiocarbon dating and hydraulic modeling were used to estimate paleoflood nonexceedance bounds at the four sites which ranged from 400 to 2200 ybp for late holocene surfaces to late pleistocene surfaces near cotopaxi peak flow data are from lower magnitude snowmelt runoff in may and june in the upper basin and from high magnitude rainfall runoff from june to august in the lower basin flood frequency curves reflect this transition near parkdale from snowmelt to extreme rainfall runoff for similar return periods paleoflood peak discharges increase from about 480 m 3 s upstream at loma linda to about 4250 m 3 s downstream near pueblo this increase is attributed to the larger rainfall component derived from lower elevations between loma linda and pueblo return periods for design floods at pueblo dam exceeded 10 000 years based on paleoflood frequency curve extrapolations

a review of deep structure and ocean continent transition in the senegal basin west africa abstract in the mauritania senegal basin gravity and magnetic data reveal long n s lineations reflecting probable extensional structures which are possibly related to opening of the atlantic along the coast where the basin is considerably thicker a large gradient in the gravity field is closely associated with an important linear magnetic high this gravity feature which dominates the whole of the area concerned reflects a crustal thinning to the west together with the existence of elongate dense intrusive bodies suggesting that it is associated with a crustal discontinuity and a change towards a crust of oceanic type the long coastal magnetic high is presumed to be an onshore segment of the magnetic lineations identified on the mauritania senegal continental margin as the landward boundary of the magnetic quiet zone a comparison of conjugate magnetic quiet zones of west africa and eastern north america shows that the west africa coastal magnetic anomaly may be closely related to the east coast magnetic anomaly which probably marks the boundary between continental and oceanic basement such correlation implies that the cape vert peninsula senegal may rest on a basement of oceanic type this appears to be consistent with geological and geophysical interpretations which suggest seaward rapid thinning of the continental crust together with major subsidence and seaward progradation

magnetotelluric images of the crust and mantle in the southwestern taurides turkey inversion of the magnetotelluric data across the southwestern taurides reveals two subzones of crust with varying thicknesses conductive lower crust b75 v m overlain by resistive n350 v m upper crust with four resistive cores n2000 v m separated by three relatively conductive vertical zones the first and second vertical zones coincide with surface faults interpreted in anatolia suchasfethiyeburdurfaultzone thethirdoneisthemostconductiveandliesin continuitywiththestrabofaultzone in the mediterranean sea a hypocentral cross section of earthquakes along the profile shows more dense seismic activity in the second resistive core where the conductive crust is not present beneath it the depth of the crust upper mantle boundary varies between 30 and 50 km and has an undulating character the resistivity of the upper mantle reaches 500 1000 v m d 2004 elsevier b v all rights reserved

paleohydrologic techniques used to define the spatial occurrence of floods abstract defining the cause and spatial characteristics of floods may be difficult because of limited streamflow and precipitation data new paleohydrologic techniques that incorporate information from geomorphic sedimentologic and botanic studies provide important supplemental information to define homogeneous hydrologic regions these techniques also help to define the spatial structure of rainstorms and floods and improve regional flood frequency estimates the occurrence and the non occurrence of paleohydrologic evidence of floods such as flood bars alluvial fans and tree scars provide valuable hydrologic information the paleohydrologic research to define the spatial characteristics of floods improves the understanding of flood hydrometeorology this research was used to define the areal extent and contributing drainage area of flash floods in colorado also paleohydrologic evidence was used to define the spatial boundaries for the colorado foothills region in terms of the meteorologic cause of flooding and elevation in general above 2300 m peak flows are caused by snowmelt below 2300 m peak flows primarily are caused by rainfall the foothills region has an upper elevation limit of about 2300 m and a lower elevation limit of about 1500 m regional flood frequency estimates that incorporate the paleohydrologic information indicate that the big thompson river flash flood of 1976 had a recurrence interval of approximately 10 000 years this contrasts markedly with 100 to 300 years determined by using conventional hydrologic analyses flood discharge estimates based on rainfall runoff methods in the foothills of colorado result in larger values than those estimated with regional flood frequency relations which are based on long term streamflow data preliminary hydrologic and paleohydrologic research indicates that intense rainfall does not occur at higher elevations in other rocky mountain states and that the highest elevations for rainfall producing floods vary by latitude the study results have implications for floodplain management and design of hydraulic structures in the mountains of colorado and other rocky mountain states

the drainage of the lake ha ha reservoir and downstream geomorphic impacts along ha ha river saguenay area quebec canada abstract on july 18 21 1996 a severe rainstorm caused widespread flooding along the north shore of the st lawrence river southern quebec canada particularly along rivers that drain the area just south of the saguenay lake st jean region at the lake ha ha reservoir inadequate available capacity to spill during the storm at the outlet dam resulted in the overtopping and erosion of a nearby earthfill saddle dyke a new outlet formed at the site of the dyke and drained the reservoir over a period of many hours decreasing its area from 8 1 to 4 7 km2 estimates of discharge range from 910 to 1380 m3 s 1 at the site of the eroded dyke to 1080 to 1260 m3 s 1 at a location 27 km downstream about 8 km above the mouth of the river the uncontrolled drainage of the lake ha ha reservoir increased flooding along the lower 35 km of ha ha river where flooding was already in progress because of the rainstorm runoff the flooding caused extensive geomorphic impacts along the river long sections of the river totalling 25 km experienced significant widening locally up to 280 m and channel incision locally up to 20 m while two reaches 6 and 4 5 km long experienced up to several metres of aggradation in general the slope of the valley was the most important variable affecting whether or not the energy of the flow was above or below the erosive threshold of the valley bottom locally a permanent channel diversion now exists where the drainage divide between the main river course and a small ravine was overtopped and extensively eroded communities infrastructure and industry located along the river were extensively damaged by the flood waters the effects of flooding along ha ha river demonstrate that rivers on the canadian shield can undergo severe geomorphic changes caused by very high magnitude flooding

nares strait a suture zone geophysical and geological implications abstract the motion of greenland relative to ellesmere island along nares strait is determined from poles of rotation which provide control for the motion independent of local geology and geophysics the plate kinematics around the north atlantic ocean the norwegian and greenland seas and the eurasian basin of the arctic ocean constrain motion along nares strait these motions are checked by examining the stability characteristics of the triple junctions these junctions are found to be stable the motion along nares strait between anomalies 34 and 13 is a combination of strike slip and compression the regional geology is found to support the plate reconstructions the local geology of the nares strait area is reviewed and found not to refute the predicted motions the geophysical and geological data are interpreted in terms of the wilson cycle the opening and closing of an ocean the nares strait area has the characteristics of a cryptic suture a join between regions of collided continental crust

the asymptotic response of three dimensional basin offsets to magnetotelluric fields at long periods the effects of current channeling a simple inexpensive numerical algorithm is used to analyze the asymptotic long period behavior of magnetotelluric mt fields in the vicinity of lateral offsets in sedimentary basins the model is based on the distortion or channeling of telluric currents in a horizontal thin sheet although a gross oversimplification of nature the model represents a class of structures which because of excessive computer costs have been relatively unstudied previously within and closely adjacent to the region of the three dimensional 3 d offset significant distortion of the mt parameters occurs skewness coefficients vary from negligible values to over 0 7 principal resistivities vary by an order of magnitude on the other hand there is not a clear correlation between the degree of distortion of the parameters usually evaluated during mt surveys and the magnitude of conventional 3 d indicators e g the skewness coefficient calculations have simulated the technique of averaging resistivity parameters from a large number of field sites in order to arrive at a regionally representative one dimensional 1 d model the results indicate that unless care is taken in adapting the nature of the averaging algorithm to the class of distortions encountered significant bias of the averaged parameters may result these results also suggest that formore this class of structures grave problems may be associated with using the principal resistivity perpendicular to geologic strike the so called transverse magnetic tm mode to infer an equivalent two dimensional 2 d model for the region a 2 d model would likely show significant modulations in the physical character of the basement which are in fact an artifact of telluric distortion caused by current channeling in the surficial heterogeneity 10 figures 4 tables less

analyses of the magnitude and frequency of a 400 year flood record in the fish river basin namibia abstract ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime mainly dominated by floods in these environments floods are a water resource and a potential hazard with important socioeconomic implications the fish river 86 600 km2 is the largest ephemeral stream in namibia and recently also the focus of new development plans including construction of the largest dam in namibia the hydrological analysis to support decisions implies large uncertainties owing to spatial limited and short hydrological records since 1962 here we investigate the current and past patterns of extreme floods combining instrumental historical and palaeoflood records palaeoflood studies were performed at two reaches with preserved sedimentary evidence at the upper sector vogelkranz upstream of hardap dam 13 catchment area and in the lower part of the river in the fish river canyon national park 70 catchment area in the hardap reach the palaeoflood record identified at least eight large floods during the last 350 years with the largest flood reaching a minimum discharge of 4800 m3 s 1 150 year return period in the fish river canyon reach the sedimentary record shows at least 12 large floods over the last 400 years the largest with an estimated minimum discharge of 8700 m3 s 1 the elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages associated with discharges of 6400 m3 s 1 and 16 140 m3 s 1 for the upper and mid lower fish river respectively in the upper reach the flood frequency analysis ffa combining systematic and palaeoflood data provided lower discharges 25 of the flood quantiles than the ones using only systematic data sets in the fish river canyon reach incorporation of the palaeoflood data into the ffa results in slightly higher values in the magnitude of the higher flood quantiles 4 7 the ffa analysis using an upper limited lognormal distribution function ln4 fitted with palaeoflood data shows a good performance with a slow behaviour approaching the upper limit our flood frequency results suggest that the hardap dam should increase the spillway capacity and safety check flood of the original design in order to satisfy the dam safety criteria however the projected reevaluation figures calculated from conventional hydrological methods results in an overestimation of the safety floods according to our estimations

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

factors which determine the hydrogeochemical behaviour of karstic springs a case study from the betic cordilleras spain the cabra alcaide karstic massif situated in the south of spain constitutes an important part of the so called natural park of the subbetic sierras this aquifer system is drained by various springs which supply a population of some 100 000 inhabitants the feed areas of these springs show very different characteristics with respect to their geological structure size of the drainage area thickness of the vadose zone elevation and degree of karstification in addition the carbonate rocks lie over a clayey substratum which contains large masses of intercalated evaporites due to these conditions the hydrochemical composition of the springs is relatively variable in this study a hydrogeochemical characterization of the aquifer in both space and time is undertaken and the factors that determine it are discussed to achieve this 19 monthly samples were taken from the 6 most significant springs of the hydrogeological system the commencement of sampling coincided with the transition from a period of several years of severe drought and a very wet two year period which amplified considerably the hydrochemical and hydrodynamic response of the springs to the recharge identification of hydrogeochemical processes was performed by studying hydrographs the temporal evolution of physico chemical parameters ionic ratios mainly mg ca and by means of simple and multivariate statistical analyses the saturation status was established using the wateqf program and the mass transfer was quantified using phreeqc with the exception of the epikarstic subsystem i e the zarza spring the majority of the results indicate that the aquifer exhibits a diffuse flow model in which piston flow phenomena are seen closely linked to the most intense precipitation along the direction of flow hydrochemical trends are seen as the water type changes from ca hco3 to ca mg hco3 at the same time enrichment in some ions derived from the dissolution of evaporitic minerals of the impermeable substratum and dedolomitization processes occur in this way almost 90 of the water samples were oversaturated in calcite the majority of cases being a consequence of the dissolution of the gypsum of the substratum only in the epikarstic springs can it be considered that the oversaturation in calcite is due to loss of co2 from the emerging groundwater it is concluded that hydrodynamic aspects together with hydrogeochemical characteristics need to be taken into account to correctly explain the hydrochemical evolution of the karstic springs moreover the use of both approaches permits a more accurate establishment of the degree of aquifer karstification which in turn needs to be known in order to assess its vulnerability to contamination and to protect recharge zones

electrical resistivity structure of the senegal basin as determined from magnetotelluric and differential geomagnetic soundings summary the methods of magnetotelluric mt and differential geomagnetic soundings dgs have been applied to study the electromagnetic response of the structure of the senegal sedimentary basin the measurements at 10 sites were carried out along a profile running perpendicularly across a zone of north south flexures and faults over the period range 10 1000 s variation of periods up to 10000 s were obtained at two sites the anomalous geomagnetic variation field across the major fault zone is characterized by an increase in the magnetic eastward component two dimensional modelling of the apparent resistivities and phases reveals a highly anomalous upper crustal structure involving a low resistivity zone in the central part of the basin less than 30 am to satisfy the additional long period data at two sites a deep conductor is also required at a depth below 300 km with a resistivity less than 10 am the crustal discontinuity located 200 km off the coast is possibly related to the opening of the atlantic ocean the difficulty in resolving the question of the nature of the crust in this region results in part from the limitations of the methods used at present for this determination

cryptic signatures of neoproterozoic accretionary events in northeast brazil imaged by magnetotellurics implications for the assembly of west gondwana abstract the borborema province in northeast brazil is a complex orogenic system severely affected by deformational metamorphic and magmatic processes mostly during the gondwana convergence in late neoproterozoic early phanerozoic brasiliano pan african orogeny new magnetotelluric mt data collected along the northwestern part of the province and eastern part of the contiguous parnaiba basin are combined with previous mt data to assess the regional deep electrical resistivity structure dimensionality analysis shows that a 3d electrical structure predominates in the subsurface and thus 3d inversion was carried out the final geoelectric model allows delineating the geometry and variation in physical properties of different lithospheric blocks bounded by major electrical discontinuities these lithospheric blocks constitute a coalesced mosaic made up of four main terrane compartments a resistive cratonic keel detected along the western part of the study area currently hidden beneath the parnaiba basin parnaiba block two complex domains in the center characterized by several resistive and conductive zones throughout the crust and upper mantle ceara central and rio grande do norte domains and a conductive block in the east with the geoelectric response being controlled by possible remains of late neoproterozoic subduction activity to the south central sub province the interfaces between these blocks are interpreted as suture zones correlated to their neoproterozoic collage one curved conductor concealed by the sediments of the parnaiba basin and bordering the eastern margin of the basin another huge conductor corresponding to the location of the oros jaguaribe subdomain on the surface and a third interface coinciding with the position of the patos shear zone the presence of these proposed sutures could be a conspicuous evidence of a neoproterozoic accretion system in northeast brazil and would support tectonic evolution models for the west gondwana assembly involving oceanic closure and collision of continental blocks during the brasiliano orogeny

reconstruction palaeoflood hydrology using slackwater flow depth method in the yanhe river valley middle yellow river basin china abstract water depth above the flood deposits should be taken into account in calculations of the palaeoflood peak stages which can provide more accurately estimate of palaeoflood stage here we present a new method slackwater flow depth to assess palaeoflood peak stage and to reduce the underestimation of palaeoflood stage palaeoflood slackwater deposits swds were identified by palaeohydrological criteria in cliff riverbank on the yanhe river middle yellow river basin palaeoflood events recorded in four layers of swd were dated by optical stimulated luminescence to 9 5 8 5 ka the estimation of palaeoflood maximum stage was 778 3 m using the slackwater flow depth method and the palaeoflood peak discharge is 15 000 m 3 s using the step backwater method palaeoflood results greatly extend the current flood data series in the yanhe river basin the regional flood history including gauged flood historical and palaeoflood data was compiled and evaluated for the major tributaries of the middle yellow river the relationship between palaeoflood peak discharges and drainage areas in this region fit well with the global maximum curves the results of site specific and regional palaeoflood evaluations demonstrate that the approach estimates the true palaeoflood peak stage and discharges and improves the flood frequency analysis of extreme and rare floods for a particular basin meanwhile the advantages and uncertainties of this method need ongoing discussion in palaeoflood investigations

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

trace metals in acid sediments and waters pimpama catchment southeast queensland australia the pimpama river floodplain has developed over the last several thousand years as a result of sea level fluctuations that shaped the lower catchment and enabled the formation of sedimentary pyrite the subsequent production of sulfuric acid due to the oxidation of this pyrite enhances the breakdown of metal bearing sediments and can lead to leaching of major and trace metals into the waters of the region the seasonal pattern of rainfall and current land use activities are important aspects that intensify the natural production of acid and influence the release and distribution of metals to identify the source and migration of metals in the pimpama catchment and to understand the impact of pyrite oxidation on the distribution of metals in sediments and waters several components of the drainage system were analyzed bedrock sediments from river bed and bank and water the elements analyzed in this study v cr co ni cu zn and pb are all present in the bedrock material which explains their occurrence in the unconsolidated sediments of the floodplain these metals concentrate in the upper section of the sedimentary sequence and their presence is related to clay minerals such as smectite organic matter and iron phases however zn mo and co occur in higher amounts than the local background and within standard shale this comparison suggests that the diagenetic processes alone cannot explain the higher concentrations and it is concluded that these metals also have an anthropogenic source the formation of sulfuric acid creates conditions for higher mobility of some metals such as cr co ni cu zn but does not affect less mobile ones such as mo and pb over the longterm the production of acid influences the breakdown of mineral phases and enhances the process of weathering over the short term every rain event leaches acid from sediments and mobilizes metals resulting in a substantial reduction in the quality of river water

modeling of water rock interactions in an aquitard of sandy clay in the coastal area near beihai china unconsolidated sand gravel and clay deposits near beihai and in the leizhou peninsula in southern china form an unconfined aquifer aquitard and a confined aquifer water and soil samples were collected from the two aquifers in the coastal beihai area for the determination of chemical compositions minerals and soluble ions hydrogeochemical modeling of three flow paths through the aquitard are carried out using phreeqc to determine water rock interactions along the flow paths the results indicate that the dissolution of anorthite fluorite halite rhodochrosite and co2 and precipitation of potash feldspar and kaolinite may be occurring when groundwater leaks through the aquitard from the unconfined aquifer to the confined aquifer cation exchanges between na and ca can also happen along the flow paths

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

hydrologic and geologic factors controlling surface and groundwater chemistry in indian wells owens valley area southeastern california usa the indian wells owens valley area is located in the semi arid basin and range province which is characterized by alternating mountains and alluvial basins surface water resources are limited in this arid region and water demand is mainly met by groundwater pumpage in a classic basin and range groundwater system water flows from recharge areas in the mountains to discharge areas in adjacent valleys discharge areas are generally occupied by playas where large amounts of salt deposition occur due to evaporating groundwater hydrochemical data from a total of 1368 spring surface and well water samples collected over an 80 year period were used to evaluate water quality and to determine processes that control water chemistry q mode hierarchical cluster analysis hca was employed for partitioning the water samples into hydrochemical facies also known as water groups or water types five major water groups resulted from the hca analysis the samples from the area were classified as recharge area waters ca na hco3 water and na ca hco3 water transition zone waters na hco3 cl water and discharge area waters na cl water and more concentrated na cl water spatial plots of the major statistical groups show that the samples that belong to the same group are located in close proximity to one another suggesting the same processes and or flowpaths inverse geochemical models of the statistical groups were developed using phreeqc to elucidate the chemical reactions controlling water chemistry the inverse geochemical modeling demonstrated that relatively few phases are required to derive water chemistry in the area in a broad sense the reactions responsible for the hydrochemical evolution in the area fall into four categories 1 silicate weathering reactions 2 dissolution of salts 3 precipitation of calcite amorphous silica and clay minerals and 4 ion exchange

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

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

use of the magnetotelluric method for a better understanding of the west african shield magnetotelluric soundings have been carried out in the republic of upper volta and in the niger republic to gain a better understanding of the structure of the west african shield the sounding stations are situated from east to west on a sedimentary basin on the central african mobile belt pan african orogeny at 550 100 m y and on the west african craton eburnean orogeny at 1850 250 m y the results show that the craton is characterized by a zone of high resistivity in the crust and uppermost mantle the absence of a conductive zone at the interface between the crust and the uppermost mantle is consistent with the hypothesis of hyndman and hyndman 1968 on the dehydration of the crust of the stable shields the first conductive layer of the craton is situated at a depth of 130 km with a temperature of about 860 c in the mobile belt and the basin the presence of a low resistivity layer at a depth of 30 40 km has been established if water is present in the lower crust it can explain the origin of this conductive zone a plausible explanation is that this layer may have been formed by the slow infusion of water from the mantle during the last thermal reactivation regional differences in electrical conductivity structure between the central african mobile belt and west african craton appear to extend deeper than 200 km modeled conductivity variations with depth in the upper mantle obtained from magnetotelluric results are compared to other continental models thermal model seismic structure in order to define a lower lithospheric boundary for west africa at the present stage the problem is not yet resolved mobile crustal plate or anchorage in the african plate

geochemical models of the impact of acidic groundwater and evaporative sulfate salts on boulder creek at iron mountain california abstract during dry season baseflow conditions approximately 20 of the flow in boulder creek is comprised of acidic metals bearing groundwater significant amounts of efflorescent salts accumulate around intermittent seeps and surface streams as a result of evaporation of acid rock drainage those salts include the fe sulfates rhomboclase h 3 o fe 3 so 4 2 3h 2 o ferricopiapite fe 3 5 so 4 6 o oh 20h 2 o and bilinite fe 2 fe 2 3 so 4 4 22h 2 o al sulfates alunogen al 2 so 4 3 17h 2 o and kalinite kal so 4 2 11h 2 o and ca and mg sulfates gypsum caso 4 2h 2 o and hexahydrite mgso 4 6h 2 o the dissolution of evaporative sulfate salt accumulations during the first major storm of the wet season at iron mountain produces a characteristic hydrogeochemical response so called rinse out in surface waters that is subdued in later storms geochemical modeling shows that the solutes from relatively minor amounts of dissolved sulfate salts will maintain the ph of surface streams near 3 0 during a rainstorm on a weight basis fe sulfate salts are capable of producing more acidity than al or mg sulfate salts the primary mechanism for the production of acidity from salts involves the hydrolysis of the dissolved dissolved metals especially fe 3 in addition to the lowering of ph values and providing dissolved fe and al to surface streams the soluble salts appear to be a significant source of dissolved cu zn and other metals during the first significant storm of the season

regional flood frequency analysis using extreme order statistics of the annual peak record a regional flood frequency model is developed for estimating recurrence intervals of extreme floods the regionalization procedure developed in this paper differs notably from the u s geological survey index flood method dalrymple 1960 in that a large quantile rather than the mean annual flood is used as the index flood based on a result from extreme value theory exceedances of the specified quantile are modeled by a generalized pareto distribution the generalized pareto distribution has two parameters a scale parameter and a shape parameter it is assumed that the shape parameter does not vary from basin to basin implying that annual peak distributions for all basins have the same upper tail thickness the scale parameter on the other hand not only varies from basin to basin but may also depend on covariate information such as drainage area or indicator functions for basin geology likelihood based inference procedures are developed for the regional flood frequency model the model is applied to extreme floods of the central appalachian region of the united states

magnetotelluric imaging of the sw japan forearc a lost paleoland revealed abstract we investigated the crustal structure of the kii peninsula in the southwestern japan forearc using the magnetotelluric mt method we found two upper crustal anomalies 1 a sub horizontal conductive layer dipping northwest at depths of 5 10 km beneath the cretaceous accreted terrane 2 a resistive block at depths of 2 15 km beneath the tertiary accreted terrane the former is interpreted as a flysh unit the latter is consistent with the high velocity structure detected by refraction experiments and may be a lost continental fragment of the kuroshio paleoland such a continental fragment has been predicted by sedimentological studies but there had been no geophysical supporting evidence the mid crustal seismicity clusters near the bottom of the resistive body this suggests that the lost paleoland is beneath the current forearc we also found a conductive layer in the deep crust consistent with seismic reflectors at 15 25 km depth this implies trapped free water reflecting a geotherm of 400 c

some factors affecting tds and ph values in groundwater of the beihai coastal area in southern guangxi china groundwater of low total dissolved solids tds of 10 mg l and low ph ranging commonly from 4 0 to 7 0 occurs in the unconsolidated unconfined and confined aquifers in the coastal plain near beihai guangxi china minerals in the unconsolidated sediment of quaternary are predominated by quartz more than 50 with small amount of clay minerals such as kaolinite illite and chlorite sio2 accounts for most more than 70 of the chemical compositions of the sediments the sediments contain low concentrations of soluble constituents below 300 g g and are predominated by so4 abundant recharge from low tds precipitation and a long time s dissolution of the unconsolidated sediments containing small amount of soluble compositions lead to low tds of the groundwater in the area

geologic sections across the onshore senegal mauritania basin derived from geoelectric studies on the basis of magnetotelluric two dimensional models along three traverses the regional electrical structure within the senegal mauritania basin sediments has been studied correlation of the computed models with the wells drilled in the region shows that the main gross lithologic units were detected the models can be generalized into three resistivity layers i post turonian ii pre senonian and iii basement in the southern part of the basin there are relatively conductive formations below the mesozoic that are interpreted as sediments of the paleozoic bove basin sequence by combining the information that has been provided by the geoelectric cross sections and the lithologic and electric log data a schematic model for the generalized sedimentary structure of senegal has been generated the interpretation that emerges shows that the basin is a westward sloping open homocline in which the structure is controlled by north south trending basement faulting that portrays a staircase structural

geomagnetic variation anomalies in northern england processing and presentation of data from a non simultaneous array summary the block and basin structure of northern england has been investigated by an array of 35 magnetometer stations over a grid with a spacing of 1okm recordings at these sites were made simultaneously with those at a reference site using the common reference data transfer function analysis enables the three anomalous field components to be estimated at each site for the period range 10 104s using the transfer functions obtained maps of the anomalous field components may be generated across the array and used for interpretation of electrical conductivity structure maps thus obtained from the non simultaneous array data in northern england are used to illustrate the advantages of the method and provide evidence of an east west current flow concentrated along the southern margin of the northumberland trough the limitations of the procedure although slight are considered the errors which arise in the method are deduced and techniques for their removal are suggested

drainage and flooding in karst terranes flooding in karst terranes is a commonly occurring geo hazard it causes damage to property businesses and roadways it can lead to the formation of cover collapse sinkholes and groundwater contamination generally three types of flooding or their combinations are related to karst recharge related sinkhole flooding flow related flooding and discharge related flooding understanding of the type of flooding is essential for solving the flooding problem areas prone to karst flooding should be recognized and restrictions and laws on land use should be implemented runoff and erosion control plans should address the unique characteristics of karst features digging out clogged sinkholes creating retention basins or installing class v injection wells are possible solutions to improve drainage of storm water solutions to flooding problems in karst areas should also be coordinated with the water quality control to prevent groundwater contamination

a magnetovariational study of a geothermal anomaly summary during 1980 september and october an array of 31 threecomponent magnetometers was operated over a known geothermal anomaly in the rocky mountains foothills of alberta the purpose of the array was to investigate the possibility of the existence of a geomagnetic induction anomaly coincident with the geothermal anomaly examples of magnetograms and fourier transform anomaly maps are presented and show that although there is some indication of an anomaly this is to a great extent masked by the presence of large external source field gradients a detailed transfer function analysis of the data is presented both single station transfer functions and a solution of the induction matrix are calculated the latter involves the representation of the normal fields by least squares fitted planes and a justification of the validity of this method is given contour plots of the response of the anomalous vertical field to horizontal hypothetical events and pseudosections of iza hni for traverses across the array are shown these indicate the existence beneath the geothermal anomaly of a body of anomalously high electrical conductivity by treating the observed values of the in phase part of iza ha i along a traverse across the anomaly as due to vertical dipoles it is shown that the top of the conductive body probably lies at 10 20km depth the pseudosections suggest that the body extends at least to lower crustal depths the correlation of the conductivity and geothermal anomalies supports the inference that both are due to the presence of partial melt in the crust

electrical conductivity profiles and implications for the absence or presence of partial melting beneath central and southeast australia abstract a previous paper presented measurements by australian magnetometer arrays of earth response to the magnetic daily variation source field and interpreted these data by simple representative models of one dimensional structure found by forward model fitting this paper now supplements the earlier interpretation by seeking to clarify the ranges of acceptable models which fit the data that is to demonstrate the extent of the non uniqueness of the interpretation searches for models which fit the data have been carried out on both a systematic and a random basis with similar results the major conclusion of the earlier paper is confirmed that there is a substantial difference in conductivity structure between central and southeast australia beneath central australia the structure is consistent with a traditional continental geotherm and published laboratory measurements on the temperature dependence of the electrical conductivity of recognized upper mantle crystalline olivine materials beneath southeast australia a higher conductivity by an order of magnitude in the depth range 200 300 km is most directly interpreted in terms of a small degree perhaps 5 of basalt melt such a partial melt zone under southeast australia is consistent with previous natural electromagnetic measurements for the area has earlier and independently been indicated by a variety of seismic studies and correlates with proposed thermal models which involve crustal intrusion from some sub lithospheric magma source

magnetotelluric soundings and the geological structure and tectonics of the senegalo mauritanian basin in northern senegal west africa the results of the magnetotelluric soundings provide new insights on the geological structure and tectonics of the onshore senegalo mauritanian basin in northern senegal west africa electrical resistivities can be correlated with the major geological units in the upper 1000 m resistivity variations indicate a transition from freshwater saturated sediments near the surface to saline water saturated sediments above the basement the resistivity of the sediments is dominated by their porosity and the resistivity of the contained fluids the resistivity of the cretaceous sediments is unexpectedly low especially in the western part of the basin the top of the metamorphic granitic basement cannot be located on the west portion of the profile because of the feeble resistivity contrast between the impervious compacted aptian jurassic limestones and the metamorphic crystalline rocks the magnetotelluric estimates of the depth of the basement range from 600 m in the east to 4000 m in the west based on the magnetotelluric interpretation and other independent geophysical and geological information a cross section through the senegalo mauritanian basin along the magnetotelluric traverse is presented a zone of high electrical conductivity is observed in the crust at depths of 4 6 km on the eastern margin of the deep basin this conductive structure may be the extension of one further south that was revealed by an earlier investigation the entire conductive zone which runs north south between longitudes 15 and 16 w may be a major fracture zone related to the opening of the atlantic ocean however its origin remains to be established

constraints on the crustal structure of the internal variscan belt in sw europe a magnetotelluric transect along the eastern part of central iberian zone iberian massif 1 the iberian massif is the best exposed segment of the european variscan belt it includes relatively well preserved terranes that were accreted by transpression along time and resulted in a number of geotectonic units that formed part of the late paleozoic assembly of the pangaea supercontinent in sw iberia these units are the central iberian zone ciz ossa morena zone omz and the south portuguese zone spz a 210 km long ne sw magnetotelluric profile was carried out through the ciz from the omz ciz boundary toward the north reaching the tagus cenozoic basin data dimensionality analysis resulted in a suitable 2 d electrical resistivity structure allowing a 2 d inversion of the data set complementary available geophysical data deep seismic gravity and aeromagnetic and a comparison with a detailed geological cross section led us to constrain the interpretation of the 2 d electrical resistivity structure of the ciz crust the results show for the upper crust the existence of diverse conductive resistive bodies that correlate well with known geological features sedimentary basins faults granitic plutons mineralized systems a mild but steady conductive band is located along the middle and lower crust that is interpreted as a mafic granulite basement the upper section of this band connects with several elongated shallow conductors providing further evidence for the existence in the central iberian zone of a complex decollement system where the major faults are rooted such a crustal architecture is viewed as the northward continuation of the variscan large scale structures previously recognized in the southern sectors omz and spz

geoelectrical structure of the northern part of the senegal basin from joint interpretation of magnetotelluric and geomagnetic data magnetotelluric mt and differential geomagnetic sounding dgs surveys were made in the senegal basin west africa to investigate the conductivity structure of the crust and upper mantle magnetic and electric variations were recorded at 11 sites along a east west profile 220 km in length the period range of the measurements was 10 10 000 s for the western stations the mt response displays low skew with moderate anisotropy the dgs data indicate the presence of an anomalous geomagnetic variation field in the deep basin and on the eastern margin of our study area a telluric current concentration in the thick conductive sediments of the deep basin might cause this anomaly both mt and dgs data are then compared with two dimensional model results to explain the observed features along the profile results reveal a strong anomalous crustal structure in the central part of the profile involving a conductive zone 2 ohm m at depths in the range 4 23 km within the deep basin the results of the two dimensional modeling are not conclusive acceptable fits between observed and calculated data have been obtained with alternate models a continuous conductive zone 0 7 2 ohm m may exist beneath the uppermost layers at a depth of approximatively 1 km but the observed data can be explained equally well by poorly conducting material intruded into the highly conductive sedimentary cover this ambiguity is inherent in the two dimensional models themselves both models show that the sediments increase westward to reach a thickness greater than 4000 m when approaching the coast it is suggested that the flow of current observed in this study is largely controlled by strong lateral variation in the resistivity of the basin and or by variation of the sedimentary thickness the models also indicate the existence of two decreased resistivity layers in the eastern part of the profile one at the base of the crust and the second at 80 km with a resistivity of 20 ohm m no such layers were detected at these depths below the deep basin at some depth greater than 150 km there is a general trend toward lower resistivities from 2000 to about 2 ohm m across the entire profile this may indicate the presence of a small degree of partial melt

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

bayesian flood frequency analysis with paleohydrologic bound data 1 it is valuable to construct likelihood functions that rigorously incorporate measurement errors and annual peak discharge historical and paleohydrologic bound information in bayesian flood frequency analyses estimates of primary posterior modes for common three parameter frequency distributions are constructed using simulated annealing and the simplex method parameter and flood frequency probability intervals are calculated directly by systematic parameter space integration bayesian flood frequency analyses with annual peak discharge historical and paleohydrologic bound data for the santa ynez river california and the big lost river idaho demonstrate that paleohydrologic bounds reduce quantile biases by placing large observed peak discharges in their proper long term contexts and substantially narrow peak discharge confidence intervals when estimating floods with low exceedance probabilities

comparative study of the modern flood slackwater deposits in the upper reaches of hanjiang and weihe river valleys china abstract modern flood slackwater deposits swd were investigated and sampled in the upper reaches of hanjiang river and the weihe river valleys china magnetic susceptibility and grain size distribution were analyzed in the laboratory the results show that the magnetic susceptibility values of the modern flood swd vary between 20 10 8 m 3 kg 1 and 60 10 8 m 3 kg 1 very close to that of the malan loess over the weihe river basin these sediments were newly deposited and uninfluenced by weathering and pedogenesis the grain size distribution of the modern flood swd is dominated by silt with sand and clay indicating that these sediments were sourced from the suspended sediment load of floodwater however the grain size distribution of the modern flood swd has more sand sized sediments in the upper reaches of hanjiang river valley than that in the weihe river valley the modern flood swd are defined as sandy silt and silty sand in the upper reaches of hanjiang river valley whereas the modern flood swd are defined as silt sandy silt and clayey silt in the weihe river valley the modern flood swd have different sediment sources and transporting forces the modern flood swd are sourced mainly from the qinling and the bashan bedrock mountains and the related residual deposits slopewash and mud rock flow deposits in the upper reaches of hanjiang river valley during rainstorms torrential tributary rivers can bring much coarse deposits into the mainstream of the hanjiang river however the sources of the modern flood swd are the eolian loess and the related surface soil in the weihe river valley comparative study shows different sedimentary features between the modern flood swd in the upper reaches of hanjiang river valley and that in the weihe river valley these results give insight into the different sediment sources and transporting force of the modern flood swd which provide a basic reference to mitigate flood disasters and to soil and water conservation

paleoflood hydrology on the lower green river upper colorado river basin usa an example of a naturalist approach to flood risk analysis abstract through a comprehensive paleoflood hydrological investigation we document natural evidence for at least 27 high magnitude paleofloods at six sites on the lower green river utah hydraulic analysis using the sedimentation and river hydraulic 2d model srh 2d shows that the responsible peak paleoflood discharges ranged between 500 and 7500 m3 s at least 14 of these paleoflood discharge peaks exceed a level twice that of the maximum systematic record of gauged flows 1929 m3 s geochronological analyses employing optically stimulated luminescence osl and radiocarbon dating techniques demonstrate that these 14 largest paleoflood peaks occurred during the past 700 years integration of the paleoflood data into flood frequency analyses ffa reveals considerably higher values for the upper tails of the flood distribution than does a ffa based solely on the systematic gauged record indicating that extreme floods are larger and more frequent than implied by the relatively short gauged record through examination of three approaches to extreme flood estimation conventional ffa probable maximum flood estimation pmf and paleoflood hydrology pfh we show the significance of the natural evidence for advancing scientific understanding of extreme floods that naturally occur in the colorado river system we argue that this kind of scientific understanding is absolutely essential for achieving a credible evaluation of extreme flood risk in a watershed of immense importance to economic prosperity of the southwestern u s

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

evidence for middle triassic to miocene dual subduction zones beneath the shan thai terrane western thailand from magnetotelluric data abstract kanchanaburi province western thailand is in the middle of shan thai terrane paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the shan thai terrane during the late triassic there was a westward subduction in the east where the lampang chiang rai block subducted beneath the shan thai terrane in addition in the early tertiary the western burma terrane subducted underneath the shan thai in the west resulting in an eastward subduction a pioneer survey of this region was therefore conducted using magnetotelluric mt techniques with the aim of finding deep structures associated with these ancient subduction zones 39 mt stations were deployed to cover most of the province resistivity structures are obtained from the 3 d inversion the near surface resistivity structures correspond well with the surface geology the mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3 porosity in which the remnant dehydrated fluid was accumulated during the subduction in addition the westward and eastward subduction zones generated mafic ultramafic intrusions which appear as two conductive zones on the east and west respectively these two conductors and the conductive lower crust support the hypotheses of the middle triassic to miocene dual subduction zones