quantifying regional groundwater flow between continental intercalaire and djeffara aquifers in southern tunisia using isotope methods large scale interaction between the continental intercalaire and the djeffara aquifer systems in the southeast of tunisia has been investigated with the aid of chemical and isotopic tracers two distinct groundwater types have been identified 1 the continental intercalaire groundwater characterized by elevated temperatures 50 61 4 c low 18o 8 4 to 7 87 and 2h 67 2 to 59 values and negligible radiocarbon content both testifying its great age dating from the late pleistocene period and 2 the djeffara groundwater with distinctly heavier isotopic composition 18o 8 31 to 5 80 2h 65 9 to 31 9 the djeffara groundwaters reveal a distinct changes of physico chemical and isotopic parameters near el hamma faults in the northwestern part of the djeffara basin these changes could possibly be explained by a vertical leakage from the continental intercalaire aquifer through el hamma faults the mixing proportions inferred from stable isotope mass balance prove that the contribution of the continental intercalaire to the recharge of djeffara aquifer is very significant and may reach 100 in the el hamma region and in the northern part of gabes isotope tracers strongly suggest that recent recharge to the djeffara aquifer system is very limited its current yield particularly in its central and northern parts can be maintained only thanks to large scale underground inflow from the continental intercalaire aquifer system which carries late pleistocene palaeowater consequently current exploitation of groundwater resources of the djeffara aquifer has non sustainable character

three dimensional inversion of large scale earthscope magnetotelluric data based on the integral equation method geoelectrical imaging of the yellowstone conductive mantle plume 1 interpretation of the earthscope mt magnetotelluric data requires the development of a large scale inversion method which can address two common problems of 3d mt inversion computational time and memory requirements we have developed an efficient method of 3d mt inversion based on an ie integral equation formulation of the mt forward modeling problem and a receiver footprint approach implemented as a massively parallel algorithm this method is applied to the mt data collected in the western united states as a part of the earthscope project as a result we present one of the first 3d geoelectrical images of the upper mantle beneath yellowstone revealed by this large scale 3d inversion of the earthscope mt data these images show a highly conductive body associated with the tomographically imaged mantle plume like layer of hot material rising from the upper mantle toward the yellowstone volcano the conductive body identified in these images is west dipping in a similar way to a p wave low velocity body

compartmentalization of the coso east flank geothermal field imaged by 3 d full tensor mt inversion author s lindsey nj kaven jo davatzes n newman ga abstract the authors 2016 published by oxford university press on behalf of the royal astronomical society previous magnetotelluric mt studies of the high temperature coso geothermal system in california identified a subvertical feature of lowresistivity 2 5ohmm and appreciable lateral extent g1 km in the producing zone of the east flank field however these models could not reproduce gross 3 d effects in the recorded data we perform 3 d full tensor inversion and retrieve a resistivity model that out performs previous 2 d and 3 d off diagonal models in terms of its fit to the complete 3 d mt data set as well as the degree of modelling bias inclusion of secondary zxx and zyy data components leads to a robust east dip 60 to the previously identified conductive east flank reservoir feature which correlates strongly with recently mapped surface faults downhole well temperatures 3 d seismic reflection data and local microseismicity we perform synthetic forward modelling to test the best fit dip of this conductor using the response at a nearby mt station we interpret the dipping conductor as a fractured and fluidized compartment which is structurally controlled by an unmapped blind east flank fault zone

crust and upper mantle structure beneath the pacific northwest from joint inversions of ambient noise and earthquake data 1 we perform a joint inversion of phase velocities from both earthquake and ambient noise induced rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the pacific northwest we focus particularly on the areas affected by mid miocene to present volcanic activity the joint inversion combined with the high density seismic network of the high lava plains seismic experiment and data from the earthscope transportable array provides outstanding resolution for this area in oregon we find that the pattern of low velocities in the crust and uppermost mantle varies between the high lava plains physiographic province and the adjacent northwestern basin and range these patterns may be due to the presence of the brothers fault zone which separates the clockwise rotating northwest basin and range from the relatively undeformed areas further north further to the east the owyhee plateau snake river plain srp and northeastern basin and range are characterized by high crustal velocities though the depth extent of these fast wave speeds varies by province of particular interest is the mid crustal high velocity sill previously only identified within the srp we show this anomaly extends significantly further south into utah and nevada we suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid crustal sill structures within the srp

electrical image of subduction zone beneath northeastern japan we conducted long period magnetotelluric observations in northeastern japan from 2010 to 2013 to investigate the three dimensional electrical resistivity distribution of the subduction zone incorporating prior information of the subducting slab into the inversion scheme we obtained a three dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the ou backbone range the conductive body indicates a saline fluid and or melt pathway from the subducting slab surface to the lower crust the lower crust conductor is less than 10 m and we estimate a saline fluid and or melt fraction of at least 0 7 vol other resistivity profiles in the across arc direction reveal that the conductive body segregates from the subducting slab surface at 80 100 km depth and takes an overturned form toward the backarc the head of the conducting body reaches the lower crust just beneath mt gassan one of the prominent backarc volcanoes in the system

stability of composite river banks the stability of a river bank depends on the balance of forces motive and resistive associated with the most critical mechanism of failure many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size geometry and structure of the bank the engineering properties of the bank material the hydraulics of flow in the adjacent channel and climatic conditions rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt clay bank erosion occurs by fluvial entrainment of material from the lower cohesionless bank at a much higher rate than material from the upper cohesive bank this leads to undermining that produces cantilevers of cohesive material upper bank retreat takes place predominantly by the failure of these cantilevers three mechanisms of failure have been identified shear beam and tensile failure the stability of a cantilever may be analysed using static equilibrium and beam theory and dimensionless charts for cantilever stability constructed application of the charts requires only a few simple measurements of cantilever geometry and soil properties in this analysis the effects of cracks and fissures in the soil must be taken into account these cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface following mechanical failure blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue hence the rate of bank retreat is fluvially controlled even though the mechanism of failure of the upper bank is not directly fluvial in nature this cycle of bank erosion undermining cantilever failure and fluvial scour of the toe operates over several flood events and has important implications for river engineering channel changes and the movement of sediment through fluvial systems

bank accretion and the development of vegetated depositional surfaces along modified alluvial channels this paper describes the recovery of stable bank form and development of vegetated depositional surfaces along the banks of channelized west tennessee streams most perennial streams in west tennessee were straightened and dredged since the turn of the century patterns of fluvial ecological responses to channelization have previously been described by a six stage model dendrogeomorphic tree ring techniques allowed the determination of location timing amount and rate of bank sediment deposition channel cross sections and ecological analyses made at 101 locations along 12 streams encompassing bends and straight reaches show that channel and bank processes initially react vertically to channelization through downcutting a depositional surface forms on banks once bed degradation and heightened bank mass wasting processes have eased or slowed the formation of this depositional surface marks the beginning of bank recovery from channelization dominating lateral processes characteristic of stable or natural channels return during the formation and expansion of the depositional surface suggesting a relation with thalweg deflection point bar development and meanderloop extension characteristic woody riparian vegetation begins to grow as this depositional surface develops and becomes part of the process and form of restabilizing banks the depositional surface initially forms low on the bank and tends to maintain a slope of about 24 mean accretion rates ranges from 5 9 cm yr on inside bends to 0 cm yr on most outside bends straight reaches have a mean accretion rate of 4 2 cm yr the relatively stable convex upward depositional surface expands and ultimately attaches to the flood plain the time required for the recovery process to reach equilibrium averaged about 50 years indicative pioneer speccies of woody riparian vegetation include black willow river birch silver maple and boxelder stem densities generally decrease with time after and initial flush of about 160 stems per 100 m2 together bank accretion and vegetative regrowth appear to be the most important environmental processes involved in channel bank recovery from channelization or rejuvenation

evidence for an extensive intrusive component of the deccan large igneous province in the narmada son lineament region india from three dimensional magnetotelluric studies abstract the crustal electrical structure beneath the western segment of narmada son lineament zone in central india has been imaged using magnetotelluric mt data set of 153 stations distributed over a grid 7 8 km site spacing three dimensional modeling of this data set brought out several major crustal conductors with different geometries at different depth levels in the crustal column the conductive features correlating with gravity high anomalies and high seismic velocity zones are interpreted to be mafic ultramafic bodies derived from mantle we infer that these bodies represent the intrusive component of the large igneous province lip of the deccan volcanic episode triggered by the passage of the indian continent over the reunion hot spot during the late cretaceous the disposition and the geometry of the subsurface magmatic bodies which must have served as magma chambers for outpouring of the deccan lavas suggest that they are closely related to the plumbing geometry of the lip of the deccan volcanic episode

the arid humid transition in the sahara and the sahel during the last deglaciation at the time of the last glacial maximum the sahara and sahel regions of north africa were extremely dry new records of rainfall show that during the subsequent deglaciation the transition from arid to humid conditions in these regions occurred synchronously in two main steps comparison with other records of palaeoclimate in europe and the north atlantic ocean shows that certain common factors controlled changes in ocean and atmosphere dynamics during the deglaciation

withdrawn hydrogeochemical and isotope evidence of groundwater evolution in el guettar oasis area southwest tunisia in southern tunisia the major aquifer system is characterized by the superposition of three principal levels the upper unconfined quaternary alluvial aquifer the intermediate semi confined confined complex terminal segui beglia and zebbag formations and the continental intercalaire bou dinar and sidi aich formations which constitutes the deeper confined aquifer the hydrodynamic functioning of this aquifer system is largely influenced by tectonics gafsa and sehib faults lithologic variation and recharge conditions the tectonics has contributed to the discontinuity of the aquifer levels by creating some barriers which play the role of hydraulic sills a study of the potentiometric maps shows four principal flow directions these flow directions converge at the sill zones chott el guettar and chott djerid depressions the total mineralization evolution shows an increase from the periphery of the basin to the discharge area 1 75 tds 14 4 g l dissolution of halite gypsum and or anhydrite bearing rocks is the principal source of the salinity of the groundwater the isotopic signatures permit to classify the studied groundwaters into two different groups non evaporated groundwaters that are characterized by depleted 18o and 2h contents highlight the importance of modern recharge at higher altitude evaporated groundwaters with enriched contents reflect the significance infiltration of return flow irrigation waters and the influence of the water dam tritium data in the studied groundwaters lend support to the existence of pre 1950 and post 1960 recharge according to radiocarbon residence time estimates the deep groundwater is approximately 40 kyears old and was recharged during a period when the climate was wetter and colder based on radiocarbon content the shallow groundwater is older than 0 5 kyear and represents modern waters mixed with a limited quality of palaeo recharge moreover the insulation of certain compartments of the reservoir and inter communications between the aquifer levels seem to play a principal role in the degradation of quality of groundwater

crustal structure beneath the high lava plains of eastern oregon and surrounding regions from receiver function analysis 1 we analyze teleseismic p to s receiver functions to image crustal structure beneath the high lava plains hlp of eastern oregon and surrounding regions coverage from 206 broadband seismic stations provides the first opportunity to resolve variations in crustal composition thickness and heterogeneity on scales of a few km in depth and tens of km laterally across the hlp region we utilize both h stacking and a new gaussian weighted common conversion point stacking technique we find crust that is 40 km thick beneath the cascades idaho batholith and owyhee plateau and thinner 31 km crust beneath the hlp and northern great basin low poisson s ratios of 0 240 characterize the granitic crust beneath the idaho batholith while the owyhee plateau exhibits values of 0 270 typical of average continental crust the owyhee plateau is a thick simple crustal block with distinct edges at depth the western hlp exhibits high average values of 0 304 typical for regions of widespread basaltic volcanism combined with other geological and geophysical observations the areas of abnormally high poisson s ratios 0 320 and low velocity zones in the crust beneath north central and southern oregon are consistent with the presence of partial melt on either side of the hlp trend suggesting a central zone where crustal melts have drained to the surface perhaps enabled by the brothers fault zone thicker crust and an anomalous n s band of low poisson s ratios 0 252 skirting the steens mountain escarpment is consistent with residuum from a midcrustal magma source of the massive flood basalts supporting the view of extensive mafic underplating and intraplating of the crust from cenozoic volcanism

abrupt post glacial climate events in west asia and north africa monsoon domains regions beyond the present or past penetration of the indian and african monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14c kyr pollen and lake records from west asia western tibet and rajasthan east africa ethiopia and west africa western sahara sahel and subequatorial africa were selected on the basis of chronological control sensitivity of both site and environmental indicators to climate change the continuity of the record and interdisciplinary control of the palaeoclimatic interpretation conditions wetter than those of today prevailed during the early mid holocene period but major dry spells are recorded at all sites during the intervals 11 0 9 5 kyr bp 8 7 kyr bp and 3 4 kyr bp several records also suggest dry events of minor amplitude around 6 kyr bp potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed the solar radiation accounts for the general envelop of the post glacial monsoon fluctuations but explains neither the timing nor the amplitude of the short term changes in spite of apparent covariation between fluctuations in sea surface conditions in the north atlantic and the monsoon record no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength changes in tropical land surface conditions soil moisture negative feedback and changes in ch4 production from wetlands provide a more satisfactory hypothesis for explaining abrupt reversal events

3d geological modeling of the kasserine aquifer system central tunisia new insights into aquifer geometry and interconnections for a better assessment of groundwater resources summary the challenge of this study was to create a 3d geological and structural model of the kasserine aquifer system kas in central tunisia and its natural extension into north east algeria this was achieved using an implicit 3d method which honors prior geological data for both formation boundaries and faults a current model is presented which provides defendable predictions for the spatial distribution of geology and water resources in aquifers throughout the model domain this work has allowed validation of regional scale geology and fault networks in the kas and has facilitated the first ever estimations of groundwater resources in this region by a 3d method the model enables a preliminary assessment of the hydraulic significance of the major faults by evaluating their influence and role on groundwater flow within and between four compartments of the multi layered kas hydrogeological system thus a representative hydrogeological model of the study area is constructed the possible dual nature of faults in the kas is discussed in the context that some faults appear to be acting both as barriers to horizontal groundwater flow and simultaneously as conduits for vertical flow also discussed is the possibility that two flow directions occur within the kas at a small syncline area of near feriana in summary this work evaluates the influence of aquifer connectivity and the role of faults and geology in groundwater flow within the kas aquifer system the current kas geological model can now be used to guide groundwater managers on the best placement for drilling to test and further refine the understanding of the groundwater system including the faults connectivity as more geological data become available the current model can be easily edited and re computed to provide an updated model ready for the next stage of investigation by numerical flow modeling

regional conductivity structure of cascadia preliminary results from 3d inversion of usarray transportable array magnetotelluric data 1 in conjunction with the usarray component of earthscope long period magnetotelluric mt data are being acquired in a series of arrays across the continental us initial deployments in 2006 and 2007 acquired data 10 10 000 s at 110 sites covering the us pacific northwest distributed with the same nominal spacing as the usarray seismic transportable array 75 km the most striking and robust features revealed by initial three dimensional inversion of this dataset are extensive areas of high conductivity in the lower crust beneath all of southeastern oregon and beneath the cascade mountains contrasting with very resistive crust in siletzia and the columbia embayment significant variations in upper mantle conductivity are also revealed by the inversions with the most conductive mantle beneath the washington backarc and the most resistive corresponding to subducting oceanic mantle citation patro p k and g d egbert 2008 regional conductivity structure of cascadia preliminary results from 3d inversion of usarray transportable array magnetotelluric data geophys res lett 35 l20311 doi 10 1029 2008gl035326

vegetation sedimentary deposits and climates during the late pleistocene and holocene in eastern morocco abstract geological investigations in the oujda region eastern morocco of the period corresponding to the last hundred thousand years have made it possible to discover several geological and archaeological sites which are rich in charcoal by including information from prehistory absolute chronology and palaeoenvironmental studies the data from charcoals analyses can be placed in their context this allows us to outline the bioclimatic evolution of a poorly known region from north to south there is a decrease in climatic fluctuations as the beni snassen mountains evolved from a humid to semi arid climate supraoromediterranean to thermomediterranean the angads plain and the oujda mountains remained under thermomediterranean conditions with slight fluctuations between sub humid and semi arid southwards the alfa and artemisia steppe dominated the northern part of the hauts plateaux with some variations from an arid climate in the oujda mountains the data are abundant and the climatic fluctuations which are indicated by the flora sediments from rock shelter sites and fluvial terraces indicate that the soltanian last glacial is principally a dry period within a semi arid climate this climate is temperate to cool and interrupted by three periods of warmer and more humid conditions the rharbian holocene begins ca 12 000 b p with a slightly more humid phase which is followed first by a short arid period and later by the climatic optimum which is warm and humid 6000 4500 b p the return of the arid climatic conditions occurs at ca 4000 b p but they are interrupted by a short more humid phase 3000 to 2000 b p

three dimensional seismic velocity structure of the northwestern united states 1 we present a high resolution three dimensional p wave tomography model for the northwestern united states major features of the model include clear well resolved images of the juan de fuca gorda slab to depths of at least 500 km reduced velocities beneath newberry volcano and north central oregon increased velocities beneath the idaho batholith and a deep slab like eastward dipping feature of increased velocities beneath nevada we find no evidence for a mantle plume in this region and demonstrate that suggested disruption of the juan de fuca slab beneath oregon in the vicinity of newberry volcano is an inversion artifact we interpret the primary features of this new model within the framework of post laramide tectonomagmatism and the continuing tectonic evolution of the cascadia subduction zone a model explaining the regional tectonomagmatic history consistent with the imaged seismic velocity structure is one driven by mantle flow responding to the evolving subduction system

electrical structure of the central cascadia subduction zone the emslab lincoln line revisited abstract the emslab experiment was an ambitious onshore offshore magnetotelluric mt transect of the cascadia subduction zone when completed 1985 1988 it was the largest experiment of its kind modeling and inversion capabilities at the time were however not sufficiently sophisticated to handle a fully regularized inversion of the data including the seafloor data and bathymetric constraints with the main final model presented based on trial and error forward modeling of the responses moreover new data collected as part of the earthscope usarray program are of higher quality due to improvements in instrument technology and augment the original emslab data set presenting an opportunity to revisit the structure in this part of the subduction system we have integrated the original wide band mt data as well as several long period stations from the original emslab data set and invert these in conjunction with emslab seafloor responses and new earthscope data on land this new composite data set has been analyzed in several ways within a two dimensional geometry in which conductivity is assumed to be invariant along a strike direction roughly coincident with that of the subduction zone we have solved for fully smooth regularized models as well as solutions that allow discontinuities in conductivity along the top surface of the descending slab finally we have tested specific features in the emslab model notably a moderately shallow 30 km depth forearc conductor a feature similar to this shallow conductor is a consistent and required feature in our new inversion models but the new models highlight the connection between the slab and what is interpreted to be an accumulation of aqueous fluids in the deep crust the depth 40 km at which the conductor intersects the slab suggests that the fluids are released by the transition of hydrous basalt to eclogite at upper greenschist facies and higher metamorphic grade the nose of the mantle wedge has a conductivity consistent with a dry peridotite composition and thermal models of the system at a depth of around 80 km the mantle intersecting the slab shows a slight increase in conductivity this increase is not sufficient to require the presence of melt but a conductor indicative of melt can be inserted into the model at this depth without compromising the fit

estimation of long term diffuse groundwater discharge in the northern sahara using stable isotope profiles in soil water stable isotope profiles in soil water in the sahara have been interpreted in terms of the hydrologic processes taking place at one site where the water table is at about 10 m depth the isotope data are interpreted using the model of barnes and allison to show that the water loss by evaporation from the water table is about 2mm a 1 at another site nearby there is a net evaporative loss from the water table but the evaporation is interrupted by occasional infiltration events it is suggested that accumulation of chloride in soil water can give information about the onset of aridity in the sahara

crustal magmatism and lithospheric geothermal state of western north america and their implications for a magnetic mantle abstract the western north american lithosphere experienced extensive magmatism and large scale crustal deformation due to the interactions between the farallon and north american plates to further understand such subduction related dynamic processes we characterize crustal structure magmatism and lithospheric thermal state of western north america based on various data processing and interpretation of gravimetric magnetic and surface heat flow data a fractal exponent of 2 5 for the 3d magnetization model is used in the curie point depth inversion curie depths are mostly small to the north of the yellowstone snake river plain hotspot track including the steens mountain and mcdermitt caldera that are the incipient eruption locations of the columbia river basalts and yellowstone hotspot track to the south of the yellowstone hotspot track larger curie depths are found in the great basin the distinct curie depths across the yellowstone snake river plain hotspot track can be attributed to subduction related magmatism induced by edge flow around fractured slabs curie depths confirm that the great valley ophiolite is underlain by the sierra nevada batholith which can extend further west to the california coast range the curie depths thermal lithospheric thickness and surface heat flow together define the western edge of the north american craton near the roberts mountains thrust rmt to the east of the rmt large curie depths large thermal lithospheric thickness and low thermal gradient are found from the differences between curie point and moho depth we argue that the uppermost mantle in the oceanic region is serpentinized the low temperature gradients beneath the eastern great basin montana and wyoming permit magnetic uppermost mantle either by serpentinization metasomatism or in situ magnetization which can contribute to long wavelength and low amplitude magnetic anomalies and thereby large curie point depths

drought year changes in streambank profiles on incised streams in the sierra nevada mountains abstract after streams incise they evolve through channel widening and adjustment of bank angle and bank shape the rate of bank erosion and the influence of vegetation on the processes of adjustment on bank morphology are not well understood to evaluate the role of vegetation in streambank changes this study included monthly measurements of bank movement along incised streams during a drought data were collected throughout one year along four streams in the sierra nevada mountains streambank shape varied among five classes vertical boomerang l short cliff and constant slope each shape included similar profile segments top bank cliff mid slope and toe slope the bank shapes did not follow location for time substitution models and erosion was not related to stream velocity relationships among profile shape profile segment and vegetation apparently influenced bank erosion deposition and net change most deposition occurred on vegetated moderately steep slopes conversely most erosion occurred on steep bare banks rhizomatous plants retarded erosion whereas forbs often grew in depositional areas

crustal inheritance and a top down control on arc magmatism at mount st helens in a subduction zone the volcanic arc marks the location where magma generated via flux melting in the mantle wedge migrates through the crust and erupts while the location of deep magma broadly defines the arc position here we argue that crustal structures identified in geophysical data from the washington cascades magmatic arc are equally important in controlling magma ascent and defining the spatial distribution and compositional variability of erupted material as imaged by a three dimensional resistivity model a broad lower crustal mush zone containing 3 10 interconnected melt underlies this segment of the arc interpreted to episodically feed upper crustal magmatic systems and drive eruptions mount st helens is fed by melt channelled around a mid tertiary batholith also imaged in the resistivity model and supported by potential field data regionally volcanism and seismicity are almost exclusive of the batholith while at mount st helens along its margin the ascent of viscous felsic melt is enabled by deep seated metasedimentary rocks both the anomalous forearc location and composition of st helens magmas are products of this zone of localized extension along the batholith margin this work is a compelling example of inherited structural control on local stress state and magmatism crustal structures are as important as deep mantle melting in controlling magma ascent and the composition and distribution of erupted material according to 3d resistivity modelling geophysical data and the distribution of quaternary volcanism

relationship between senonian and deep aquifers in southern tunisia abstract steady increase in the salinity of water resources and the decline in the groundwater level are the most widespread processes that degrade water quality and endanger future water exploitation in southern tunisia hydrological geochemical and environmental isotope data were evaluated to characterize the recharge conditions of the senonian aquifer and the flow pattern of groundwater mixing the results show that groundwaters can be classified into two major water facies ca so4 to mixed and na cl the main hydrogeochemical processes that typically control the groundwater mineralization are dissolution of evaporites halite gypsum and anhydrite evaporation cation exchange and mixing process related to the up flow leakage of deep groundwaters the stable isotope signatures reveal that the main part of groundwater is significantly affected by evaporation process wide range 4 5 to 8 4 and spatial heterogeneity of 18o in groundwater indicate that the aquifer is characterized by a slow process of natural groundwater in its lateral extent the 14c activity varies between 43 and 0 33 pmc the 13c contents of the total dissolved inorganic c tdic range between 10 8 and 3 6 the calculated 13c contents of the co2 in equilibrium with the tdic varying between 15 1 to 11 4 indicate two origins of c in solution the carbonate matrix 13c 2 and the soil co2 13c from 26 and 22 for the cultivated areas mean residence times have been determined after correction of the initial activities for dead c from the rock matrix the mean residence times confirm a modern recharge of the groundwater from dahar upland and indicate the presence of palaeowaters in the northern and southern parts of the basin detailed investigations on identifying the potential recharge zones flow paths of mixing and recharge conditions will be useful to protect the groundwater resources from depletion and salinization

recharge mode and mineralization of groundwater in a semi arid region sidi bouzid plain central tunisia groundwater is the most important source of water supply in sidi bouzid plain located in central tunisia proper understanding of the geochemical evolution of groundwater is important for sustainable development of water resources in this region a hydrogeochemical survey was conducted on the mio plio quaternary aquifer system using stable isotopes radiocarbon tritium and major elements in order to evaluate the groundwater chemistry patterns and the main mineralization processes occurring in this system the chemical data indicate that dissolution of evaporate minerals and evaporation are the main processes controlling groundwater mineralization the isotopic data show that groundwater in the study area is a mixture of recent shallow waters located upstream and along wadi al fakka bed and paleowaters located towards plain limits and discharge areas low 3h and 14c contents are observed in major part of the plain indicating that recharge of the aquifer occurs mainly through direct infiltration at wadi al fakka while there is no evidence of significant recharge in major part of the plain and mountains piedmonts

flow exchange between the deep and shallow groundwaters in the sbeϊtla synclinal basin tunisia an isotopic approach stable 18o 2 h and radiogenic 3h 14c isotopes of water have been used to constrain the source origin age migration pathway and mixing processes in the sbeitla tunisia system the system is composed of an upper unconfined middle miocene aquifer with a variable thickness from 10 300 m an intermediate confined unconfined lower miocene aquifer about 100 m thick and a deeper confined lower cretaceous aquifer about 150 m thick separated by a thin clay layer a total of 53 groundwater samples from the three aquifers and spring samples were collected during february and march 2000 and isotopically analysed using conventional methods the stable isotopes composition of waters establishes that the deep groundwater depleted as compared to present corresponding local rainfall is ancient water recharged probably during the late pleistocene and the early holocene periods the relatively recent water in the superficial aquifer is composed of mixed waters resulting presumably from upward leakage from the deeper groundwater the radiogenic 3h 14c isotopes data confirm that the recent water with a tritium content between 6 5 and 19 tu represents post nuclear recharge and the ancient groundwater with low carbon 14 contents between 7 and 26 pmc infiltrated between 8 000 to 20 000 years ago when used in conjunction with the stable isotopes data the mixing process can be clearly identified especially in the sbeitla sill area groundwater of the upper aquifer exhibits isotopic signatures of both the old and recent waters by using isotopic mass balance the computed contribution of the deep groundwater in recharging the upper aquifer is up to 94

effect of bank stability on geometry of gravel rivers a bank stability analysis is incorporated into an analytical procedure for modeling the hydraulic geometry of an alluvial gravel bed channel the new analysis includes a procedure that calculates the mean bed and bank shear stress as well as assessing the bank stability the bank stability criterion accounts for the increased stability of the channel banks due to consolidation of the bank sediment cementing by fines and binding of the sediment by root masses the two key parameters in the bank stability analysis are the median grain diameter of the bank sediment d50bank and the modified friction angle of the bank sediment a sensitivity analysis of the two parameters indicate that the bank stability can exert a large influence on the channel geometry this is supported by testing the theory on published field data the bank sediment size was assumed to equal the bed sediment size and the variation of the friction angle was investigated the estimated values are seen to increase systematically

the rate of iron oxide enrichment in arid zone alluvial fan soils tunisian southern atlas measured by mineral magnetic techniques abstract rates of iron oxide enrichment of soils on a segmented alluvial fan in southern tunisia are examined mineral magnetic and chemical characteristics of soils developed on two dated relict segments and on contemporary alluvium suggest that the rate of fe oxide enrichment was more rapid in the holocene 2 6 10 8 g g 1 yr 1 over the last 5000 years than in the pleistocene 1 91 10 9 g g 1 yr 1 over the previous 45 000 years thus fe oxide concentration may be used to determine an ordinal chronology of some geomorphological surfaces in drylands but rates of enrichment cannot be assumed to be uniform over time

the effect of herbaceous plant communities and soil textures on particle erosion of alluvial streambanks soil texture and plants are understood to be important determinants of erosion however there is a paucity of research on the quantitative effect of various soils plants or the interaction of the two on streambank erosion this study measures the influence of herbaceous plant communities and sandy loam loam and clay loam soils on particle erosion rates it demonstrates that the interaction of plants and soil are important seventy one samples from the banks of 5 streams were tested using a flume multivariate regression analysis was used to analyze a dependent variable of erosion rate with independent variables of root density soil texture and plant communities autocorrelation was treated with the maximum chi square method nebraska sedge carex nebrascensis and baltic rush juncus balticus communities had the lowest erosion rates followed by mixed sedge carex lanuginosa c rostrata and others then mixed grass poa pratensis with deschampsia caespitosa communities silt had a negative effect on erosion for the nebraska sedge baltic rush and mixed grass plant communities but had no effect with the mixed sedge community due to a silt by sedge interaction an increase in percent clay correlated with an increase in erosion as percentages of clay or silt increased root volume density declined root volume density was correlated negatively with erosion clay to silt ratio correlated negatively with erosion thus knowledge of both vegetation and soil texture are necessary to predict or manage strembank erodibility

simulated climate and biomes of africa during the late quaternary comparison with pollen and lake status data new compilations of african pollen and lake data are compared with climate ccm1 ncar boulder and vegetation biome 1 2 gsg lund simulations for the last glacial maximum lgm and early to mid holocene emh the simulated lgm climate was ca 4 c colder and drier than present with maximum reduction in precipitation in semi arid regions biome simulations show lowering of montane vegetation belts and expansion of southern xerophytic associations but no change in the distribution of deserts and tropical rain forests the lakes show lgm conditions similar or drier than present throughout northern and tropical africa pollen data indicate lowering of montane vegetation belts the stability of the sahara and a reduction of rain forest the paleoenvironmental data are consistent with the simulated changes in temperature and moisture budgets although they suggest the climate model underestimates equatorial aridity emh simulations show temperatures slightly less than present and increased monsoonal precipitation in the eastern sahara and east africa biome simulations show an upward shift of montane vegetation belts fragmentation of xerophytic vegetation in southern africa and a major northward shift of the southern margin of the eastern sahara the lakes indicate conditions wetter than present across northern africa pollen data show an upward shift of the montane forests the northward shift of the southern margin of the sahara and a major extension of tropical rain forest the lake and pollen data confirm monsoon expansion in eastern africa but the climate model fails to simulate the wet conditions in western africa

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

inversion of magnetotelluric data using integral equation approach with variable sensitivity domain application to earthscope mt data abstract we have developed a multi level parallel magnetotelluric mt integral equation based inversion program which uses variable sensitivity domain the limited sensitivity of the data which decreases with increasing frequency is exploited by a receiver sensitivity domain which also varies with frequency we assess the effect of inverting principal impedances full impedance tensor and full tensor jointly with magnetovariational data tipper we first apply this method to several models and then invert the earthscope mt data we recover well the prominent features in the area including resistive structure associated with the juan de fuca slab subducting beneath the northwestern united states the conductive zone of partially melted material above the subducting slab at the cascade volcanic arc conductive features in the great basin and in the area of yellowstone associated with the hot spot and resistive areas to the east corresponding to the older and more stable cratons

deep electrical structure of the northern cascadia british columbia canada subduction zone implications for the distribution of fluids long period magnetotelluric data have been used to image the deep electrical structure of the cascadia subduction zone in british columbia canada zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab a shallow zone of high conductivity beneath vancouver island is likely due to fluids that are trapped above the subducting plate east of this structure is a conductive 0 03 s m forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized a free fluid phase is required to account for this enhanced conductivity elevated conductivities are observed in the upper mantle throughout the backarc 0 01 s m and strongly support the hypothesis of a shallow convecting asthenosphere this enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals or by a few percent partial melting 4

strontium isotope geochemistry of groundwater in the central part of the dakota great plains aquifer usa abstract the dakota aquifer of the central and eastern great plains of the united states is an important source of water for municipal supplies irrigation and industrial use although the regional flow system can be characterized generally as east to northeasterly from the rocky mountains towards the missouri river locally the flow systems are hydrologically complex this study uses sr isotopic data from groundwater and leached aquifer samples to document the complex subsystems within the dakota aquifer in nebraska and kansas the interaction of groundwater with the geologic material through which it flows has created spatial patterns in the isotopic measurements that are related to long term water rock interaction during which varying degrees of isotopic equilibrium between water and rock has been achieved and the alteration of nacl fluids by water rock interaction specifically sr isotopic data distinguish brines from kansas and western nebraska from those in eastern nebraska the former are interpreted to reflect interaction with permian rocks whereas the latter record interaction with pennsylvanian rocks the sr isotopic composition of groundwater from other parts of nebraska and kansas are a function of the dynamic interaction between groundwater and unlithified sediments e g glacial till and loess followed by interaction with oxidized and unoxidized sediments within the dakota formation this study illustrates the power of combining sr chemistry with more conventional geochemical data to obtain a more complete understanding of groundwater flow systems within regional aquifer systems where extensive monitoring networks do not exist

lithospheric dismemberment and magmatic processes of the great basin colorado plateau transition utah implied from magnetotellurics 1 to illuminate rifting processes across the transition zone between the extensional great basin and stable colorado plateau interior we collected an east west profile of 117 wideband and 30 long period magnetotelluric mt soundings along latitude 38 5 n from southeastern nevada across utah to the colorado border regularized two dimensional inversion shows a strong lower crustal conductor below the great basin and its transition zone in the 15 35 km depth range interpreted as reflecting modern basaltic underplating hybridization and hydrothermal fluid release this structure explains most of the geomagnetic variation anomaly in the region first measured in the late 1960s hence the transition zone while historically included with the colorado plateau physiographically possesses a deep thermal regime and tectonic activity like that of the great basin the deep crustal conductor is consistent with a rheological profile of a brittle upper crust over a weak lower crust in turn on a stronger upper mantle jelly sandwich model under the incipiently faulted transition zone the conductor implies a vertically nonuniform mode of extension resembling early stages of continental margin formation colorado plateau lithosphere begins sharply below the western boundary of capitol reef national park as a resistive keel in the deep crust and upper mantle with only a thin and weak moho level crustal conductor near 45 km depth several narrow steep conductors connect conductive lower crust with major surface faulting some including modern geothermal systems and in the context of other great basin mt surveying suggest connections between deep magma sourced fluids and the upper crustal meteoric regime the mt data also suggest anisotropically interconnected melt over a broad zone in the upper mantle of the eastern great basin which has supplied magma to the lower crust consistent with extensional mantle melting models and local shear wave splitting observations we support a hypothesis that the transition zone location and geometry ultimately reflect the middle proterozoic suturing between the stronger yavapai lithosphere to the east and the somewhat weaker mojave terrane to the west we conclude that strength heterogeneity is the primary control on locus of deformation across the transition zone with modulating force components

detailed three dimensional shear wave velocity structure of the northwestern united states from rayleigh wave tomography abstract since the mid miocene the northwestern united states has experienced extensive flood basalt volcanism followed by the formation of two time progressive tracks of silicic volcanism the yellowstone snake river plains ysrp and the high lava plains hlp the ysrp track progresses towards the northeast parallel to north american plate motion and has therefore often been attributed to a deep mantle plume source however the hlp track progresses to the northwest over the same time frame in a direction not consistent with any regional plate motion the causes of the mid miocene flood basalts and the tracks of the ysrp and hlp are a matter of ongoing debate we present results of rayleigh wave phase velocity inversions and inversions for 3 d shear wave velocity structure of the northwestern united states using data collected from the high lava plains seismic experiment and the earthscope usarray transportable array ta the large number of stations used in these inversions allows us to show an unprecedented level of detail in the seismic velocity structures of this tectonically complex area our velocity images indicate that low s wave velocities in the uppermost mantle do not well match the track of hlp volcanism while at the surface the newberry caldera appears to anchor the nw end of the hlp hotspot track the seismic results show that it lies in a separate north south trending low velocity band just east of the cascades that is distinct from the main hlp trace the ultra low s wave velocities beneath the ysrp track extend locally to at least 175 km depth and are by far the most prominent seismic anomalies in the region along axis the ysrp hotspot track is characterized by a discrete low velocity channel in the upper mantle that shallows narrows and intensifies to the northeast but then deepens rapidly to the north beneath yellowstone the shallowing of the low velocity anomaly to the northeast is consistent with a stationary heat source beneath a moving plate coupled with basin and range extension and heating

geochronology of age progressive volcanism of the oregon high lava plains implications for the plume interpretation of yellowstone 1 the high lava plains province hlp is a late cenozoic bimodal volcanic field at the northern margin of the basin and range province in southeastern oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the yellowstone snake river plain ysrp trend we present 40ar 39ar ages for 19 rhyolite domes 5 rhyolite ash flow tuffs and 34 basaltic lavas from the hlp the previously identified trend of westward migration of hlp rhyolites is confirmed the rate of propagation is 33 km m y from 10 to 5 ma slowing to 13 km m y after 5 ma the duration of silicic volcanism at any locus is 2 m y three older hlp dacite domes yielded ages of 15 5 ma basalts are not age progressive we identify several episodes of increased basaltic activity at 7 5 7 8 5 3 5 9 and 2 3 ma with the younger episode likely continuing into the recent the hlp and ysrp trends emerged from the axis of middle miocene basaltic volcanism of the columbia river and steens basalts we propose a model in which 1 miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin 2 the hlp trend is the result of westward flow originating at the craton margin and 3 the ysrp trend is the trace of the motion of the north american plate over the tail of the plume

hydrochemical and isotopic study of groundwater in wadi el hechim garaa hamra basin central tunisia hydrochemical and isotopic study of miocene and mio plio quaternary m p q aquifers in wadi el hechim garaa hamra basin central tunisia was undertaken in order to investigate recharge mode and processes leading to mineralization of groundwater as well as interaction between both systems the results revealed striking differences between the two aquifer systems while the miocene aquifer contains recently recharged waters with generally low mineralization around 0 5 g l 1 stemming mainly from dissolution of carbonate minerals the m p q aquifer reveals tds values reaching 3 g l 1 controlled mainly by dissolution of evaporitic minerals isotopic data indicate that the miocene aquifer contains water recharged in past several decades bomb tritium and bomb radiocarbon detected the m p q system appears to be much slower with time scales of groundwater flow possibly reaching some thousands of years sharp discontinuity of hydrochemical and isotope characteristic of groundwater observed across the major tectonic fault separating the miocene and m p q aquifers supports the idea of very limited if any hydraulic interconnection between both studied systems this in turn calls for revision of existing conceptual models of groundwater flow in the region postulating significant groundwater fluxes crossing the fault in the direction of m p q aquifer and adjacent aquifers in the wadi al fakka plain

palaeoenvironments and palaeohydrology of a tropical closed lake lake asal djibouti since 10 000 yr b p in eastern africa republic of djibouti the hypersaline lake asal occupies an exceptional geological topographical and hydrological situation at 155 m below sea level and at 12 km from the sea it lies in the youngest and the deepest basin in a chain of closed depressions of central afar decreasing in elevation from west to east and represents the hydrogeological low of the whole system presently infiltration of marine water represents the major water input to the lake due to high evaporation conditions halite precipitates halocene water level fluctuations 315 m sedimentary conditions from fresh oligosaline waters to heavy brines and palaeohydrology are reconstructed field observations are supported by 85 14c dates on inorganic carbonates mollusc shells stromatolites and hydrothermal travertines stable isotope analyses on carbonates and sulphates mineralogy geochemistry and studies of biological remains discussion of the water balance for the major stages is based on reasonable assumptions about holocene conditions of precipitation and evaporation after an arid terminal pleistocene phase marked by coarse deposits due to exceptional floods the holocene cycle begins with complex strontium rich carbonate phases aragonite high and low mg calcites and dolomite these carbonates could reflect the first underground marine supply and waters concentrated through evaporation a high lake level ca 160 m a s l from ca 8600 to 6000 yr b p corresponds to the deposition of high mg calcite precipitated in a deep fresh water lake lake asal was draining the awash river system ethiopian plateau and escarpment through groundwater circulations the terminal basin of the system was probably the present day marine gulf of ghoubbat al kharab hydrologically separated from the open ocean by a sill at 4 m below sea level possibly due to fracture opening a dramatic regression in the lake level from 160 to 150 m occurred between ca 6000 and 5000 yr b p the inputs of continental water were consequently overwhelmed by marine infiltrations the carbonate phase exhibits clear resemblance with that of the transgressive episode from 5000 yr b p lake asal experienced minor water level fluctuations aragonite rich carbonates precipitated followed by calcium sulphates and halite stable isotopes of gypsum reflect its marine origin and various diagenetic processes the holocene evolution of asal which clearly differs from that of the neighbouring closed basins is due to both its specific geological and hydrological setting and to the regional climatic changes

groundwater evolution in the continental intercalaire aquifer of southern algeria and tunisia trace element and isotopic indicators abstract the geochemical processes taking place along an 800 km flow line in the non carbonate continental intercalaire aquifer ci aquifer in north africa are described using chemical major and trace element and isotopic indicators the aquifer is hydraulically continuous from the atlas mountains in algeria to the chotts of tunisia and the geochemical evidence corroborates this the highest discharge temperature is 73 c but silica geothermometry indicates a maximum temperature of 94 c at depth chloride concentrations increase from 200 to 800 mg l 1 and the br cl ratios confirm the dissolution of non marine evaporites or interstitial waters as the main source of salinity fluoride concentrations are low and are likely to be derived from rainfall recording oscillations in source radiocarbon ages except near outcrop are at or near detection limits and the 18o and 2h values indicate a cooler recharge regime with rainfall having lower primary evaporation than today this is shown by the fact that mean isotope ratios of ci waters are around 3 lighter than the present day weighted mean value for rain major ion ratios and most trace elements indicate that despite the complex structure and stratigraphy uniform evolution with continuous water rock interaction takes place along the flow lines which are only disturbed near the tunisian chotts by groundwater converging from additional flow lines the ageing of the water can also be followed by the smooth increase in several indicator elements such as li k and mn which are least affected by solubility controls similarly the influence of marine facies in the tunisian sector may be recognised by the changing mg ca and higher br cl as well as trace element indicators the groundwaters are oxidising up to 300 km from outcrop dissolved o2 has persisted for at least 20 ka and within this zone the concentrations of several elements forming oxy anions such as u and cr increase and no3 remains conservative beyond 300 km from outcrop the groundwaters are reducing and contain high fe concentrations the basin contains huge reserves of fresh brackish waters but these need careful development due to the limiting high salinity and scaling tendency resulting from the high temperatures and mineral super saturation caused during abstraction as well as high concentrations of some harmful elements such as cr in the oxidising section

marine deposits 35 000 25 000 years old in the chott el djerid southern tunisia the chott el djerid is a large salt covered depression in southern tunisia fig 1 the late nineteenth century idea that the basin had been invaded by the sea1 2 has been superseded by the view that any quaternary flooding of the chott must have resulted from climatic changes as the fossils found in its sediments are not exclusively marine and there is no indication of the fluctuations in sea level or of the crustal movements that would be required to fill and then empty the basin2 3 we report here evidence for a marine connection 35 000 25 000 years ago and show that it implies subsequent uplift by some 80 metres in an area long regarded as either stable2 or prone to subsidence throughout the quaternary4

subsurface mesozoic basins in the central atlas of tunisia tectonics sequence deposit distribution and hydrocarbon potential the mesozoic and cenozoic outcrops of the central and meridional atlas of tunisia have been studied by structural biostratigraphic and sedimentologic approaches we extend these studies to the subsurface employing seismic and well data addressing structures sequence stratigraphy and hydrocarbon potential the basin structures are related to strike slip fault systems associated with triassic evaporites mesozoic sedimentary sections in the areas of the gafsa sidi ali ben oun sidi aich and majoura blocks were studied by an integrated seismic sequence stratigraphic and tectonic approach calibrated to wells and surface geologic controls we identified tectonic corridors where upper triassic evaporites intruded younger mesozoic rocks beginning in the jurassic the organization of the tectonic blocks is characterized by the distribution of subsiding basin zones of graben and rim synclines and resistant areas of platforms triassic intrusions induced by the strike slip movements formed salt pillows and domes that accentuate the border elevations between the rim syncline depocenters and the platforms mesozoic sequence deposits are distributed along the flanks of the basin platform borders according to lowstand prograding downlap systems tracts overlapped by aggrading and retrograding onlap and toplap deposits the time space distribution of the systems tract deposits indicates the possibility of formation of triassic jurassic and cretaceous carbonate and sandstone prograding turbidites reefs structural and stratigraphic pinch outs and unconformities in the flanks of salt domes and basins the geometry and distribution of reservoir and seal systems tracts permit the consideration of new triassic jurassic and cretaceous combined stratigraphic and structural hydrocarbon traps around the basins and the platforms this begin page 886 is a new tectonic model of the central atlas of tunisia that shows new potential objectives and traps

resistivity structure of the olympic mountains and puget lowlands magnetotellurics has been used to study of the geo electric structure of the olympic mountains and puget lowland data were collected along an west east line extending from the pacific ocean to the cascade mountains the smoothest resistivity model consistent with the data suggests that subduction of the olympic mountains accretionary complex core rocks has been prevented by a 30 km thick sequence of volcanic rocks this may effectively couple the juan de fuca and north american plates and may explain the concentration of crustal earthquakes beneath the puget lowland

paleohydrologie isotopique de l oued el akarit sud tunisie au pleistocene superieur et a l holocene previous data from gobert and howe coque and page on the upper quaternary deposits from wadi el akarit are expanded through radiocarbon and stable isotope measurements 18c and 13c from carbonates 18o and 34s from sulphates these new results report on detailed stratigraphic profiles and indicate fluviatile and palustrine environments during the late pleistocene vegetal dams linked to spring activity caused water stagnation fine grained sediments were deposited with appreciable amounts of organic matter which locally developed into some peaty levels in the upper part of the typical cross section located on the left bank about 100 m upstream of the railway bridge pleistocene deposits show a palustrine carbonate crust this formation is attributed to the drainage induced by the major marine regression at glacial maximum during the holocene stagnation periods still occurred interrupted by flood episodes marked by bottom transport and coarse deposits in the typical cross section deposits are well drained and oxidized however downstream a stagnation episode takes place with a rich malacofauna dominated by cerastoderma cardium the uneven distribution of rainfall is documented by the alternation of fine and coarse deposits and it is accentuated by the climatic evolution toward aridity which results in the formation of a gypsum crust after 4000 yr bp later exceptional floods removed sediments from the wadi channel cutting the upper quaternary deposits for about 13 m along the central course of the wadi radiocarbon ages on carbonates are discussed according to various possible origins of total dissolved carbon atmospheric co2 organic activity in the basins and aquifer supply a simple exchange mixing model between aquifer bicarbonate and atmospheric co2 is used for calculations of initial carbonate activities chemical isotopic and hydraulic steady states are assumed in this treatment present day 14c and 13c contents of aquifer bicarbonate are thus adopted for past values the model leads to acceptable corrected ages for pleistocene carbonate samples corrected ages are younger than apparent ages by a few thousands of years and become consistent with ages given by organic matter from nearby levels however holocene samples appear overcorrected their crystallization appears to be controlled by modern organogenic co2 holocene 14c ages are probably correct 18o contents of carbonate indicate a continental origin for environmental water nodules and carbonate concretions crystallized in bottom sediments through upward leakage of confined aquifers they show 18o contents compatible with present day stable isotope compositions of artesian springs this supports the concept of a good time constancy in stable isotope contents shell carbonate values indicate a variable evaporation rate from the same initial waters 18o and 34s contents of gypsums are also compatible with aqueous sulphates from springs the data set falls within the range of permo triassic sulphate and suggests deep hydraulic connections within aquifer systems the entire formation appears to be of continental origin the occurence of cardium rich basins can only be attributed to the initial salinity of springs waters 7 9 g 1 1 sightly increased by evaporation this signifies that one can not deduce any tectonic uplift from their location with respect to sea level

assessment of the hydrogeochemical characteristics of groundwater in two aquifer systems in cumra plain central anatolia located in a semi arid climate cumra konya plain is one of the most important agricultural regions in turkey groundwater is the major source for agricultural domestic and other water related activities due to the insufficiency of surface water as groundwater is the only major source of water in this area it is important to know the effects of geological formations and anthropogenic activities on groundwater chemistry this study was carried out with the objective of identifying the hydrogeochemical characteristics and processes controlling the groundwater chemistry in cumra plain central anatolia turkey the study area is comprised of two main aquifers which are the semi confined aquifer of the neogene age and the unconfined aquifer of the quaternary age to identify the hydrogeochemical characteristics of groundwater in two aquifer systems and to understand the major factors and mechanisms controlling the groundwater chemistry graphical plots mineral saturations and multivariate statistical analysis of chemical constituents in the groundwater were used study results show that groundwater is generally neutral to slightly alkaline in nature with a ph ranging from 6 67 to 8 10 and the dominance of ions is in the order of ca2 mg2 na k for the neogene aquifer and hco3 so4 2 cl no3 mg2 ca2 na k and so4 2 hco3 cl no3 for the quaternary aquifer while the chemical composition of groundwater in the neogene aquifer is mainly controlled by water rock interaction including dissolution of carbonates and gypsum calcite precipitation and de dolomitization the main geochemical processes in the quaternary aquifer are reverse ion exchange evaporation dissolution of carbonates gypsum and soil salts calcite precipitation and silicate weathering the mechanism controlling groundwater chemistry in the neogene aquifer is actually regulated by the geogenic processes water rock interaction rather than by anthropogenic activities however the mechanism controlling groundwater chemistry in the quaternary aquifer is regulated by both geogenic processes and anthropogenic activities in addition anthropogenic nitrogen pollution in the study area is currently not serious but evaporation and leaching of soil salts due to anthropogenic activities increase the concentrations of ions in the quaternary aquifer

fluid upwelling beneath arc volcanoes above the subducting philippine sea plate evidence from regional electrical resistivity structure 1 at subduction zones aqueous fluids released from the subducting slab play an important role in island arc volcanism triggering partial melting and accretion magmatism we used the network magnetotelluric mt method to obtain the deep 100 km large scale electrical resistivity structure beneath kyushu japan with the aim of identifying regions of deep fluids and magma that feeds subduction zone volcanoes in the region network mt observations in which dipoles of about 10 30 km in length were used to measure the electric potential difference were performed over all of kyushu in the southwest japan arc where the philippine sea plate psp is subducting beneath the eurasian plate at a high angle using the network mt data we obtained the large scale electrical resistivity structure along four profiles across each of four quaternary active volcanoes at the volcanic front of the kyushu subduction zone the resistivity models have two features in common a conductor beneath each volcano whose base extends to the backarc side and a resistor along the hinge line of the subducting psp in the forearc the conductor indicates regions of fluid released from the slab or partial melting of the mantle representing a magma source for subduction zone volcanoes in this region

3 d inversion of the mt earthscope data collected over the east central united states the magnetotelluric mt data collected as a part of the earthscope project provided a unique opportunity to study the conductivity structure of the deep interior of the north american continent besides the scientific value of the recovered subsurface models the data also allowed inversion practitioners to test the robustness of their algorithms applied to regional long period data in this paper we present the results of mt inversion of a subset of the second footprint of the mt data collection covering the east central united states our inversion algorithm implements simultaneous inversion of the full mt impedance data both for the 3 d conductivity distribution and for the distortion matrix the distortion matrix provides the means to account for the effect of the near surface geoelectrical inhomogeneities on the mt data the long period data do not have the resolution for the small near surface conductivity anomalies which makes an application of the distortion matrix especially appropriate the determined conductivity model of the region agrees well with the known geologic and tectonic features of the east central united states the conductivity anomalies recovered by our inversion indicate a possible presence of the hot spot track in the area

nacr14 a 3d model for the crustal structure of the north american continent abstract based on the large number of crustal seismic experiments carried out in the last decades we create nacr14 a 3d crustal model of the north american continent at a resolution of 1 1 we present maps of thickness and average velocities of the main layers that comprise the north american crystalline crust obtained from the most recent seismic crustal models within the usgs crustal structure database however the crustal data are unevenly distributed and in some cases discrepancies exist between published models in order to construct a consistent 3d crustal model with three layers in the crystalline crust we refrained from a direct interpolation of the crustal seismic parameters in the database instead we implemented the following sequence of steps 1 definition of the geometry of the main tectonic provinces of north america 2 selection and evaluation of the reliability of seismic crustal models in the database 3 estimation of the p wave seismic velocity and thickness of the upper middle and lower crust for each tectonic province 4 estimation of the interpolated pn velocity distribution the resulting average velocity of the crystalline crust is mostly consistent with that of the seismic points the main variations of the structure of the crystalline crust of north america displayed in the model can be related to its tectonic evolution the model available in a digital form can be used in various geophysical applications such as the correction for the crustal effects in gravity and seismic tomography and models of dynamic topography in order to detect heterogeneities characterizing the underlying upper mantle

fluid and deformation regime of an advancing subduction system at marlborough new zealand newly forming subduction zones on earth can provide insights into the evolution of major fault zone geometries from shallow levels to deep in the lithosphere and into the role of fluids in element transport and in promoting rock failure by several modes the transpressional subduction regime of new zealand which is advancing laterally to the southwest below the marlborough strike slip fault system of the northern south island is an ideal setting in which to investigate these processes here we acquired a dense high quality transect of magnetotelluric soundings across the system yielding an electrical resistivity cross section to depths beyond 100 km our data imply three distinct processes connecting fluid generation along the upper mantle plate interface to rock deformation in the crust as the subduction zone develops massive fluid release just inland of the trench induces fault fracture meshes through the crust above that undoubtedly weaken it as regional shear initiates narrow strike slip faults in the shallow brittle regime of interior marlborough diffuse in width upon entering the deeper ductile domain aided by fluids and do not project as narrow deformation zones deep subduction generated fluids rise from 100 km or more and invade upper crustal seismogenic zones that have exhibited historic great earthquakes on high angle thrusts that are poorly oriented for failure under dry conditions the fluid deformation connections described in our work emphasize the need to include metamorphic and fluid transport processes in geodynamic models

genesis and evolution of a beach ridge plain reflecting relative sea level rise a case study from trab el makhadha gulf of gabes southeastern tunisia abstract the trab el makhadha beach ridge plain presents a progradational shoreline feature on the gulf of gabes tunisia diachronic analysis coupled with a morpho sedimentary approach was applied to elucidate the genesis mode of the beach ridge plain its sedimentary sources and the climatic conditions responsible for the prograding trends in the gulf of gabes shapes dimensions sedimentologic compositions and other textural parameters of the sandy ridges have been analyzed therein we found that the trab el makhdha coast is made up of two distinctive categories of eolian beach ridge complex progradational sequences implying a significant change in climatic and hydrological conditions during the deposition of each ridge in the holocene sequence about 5600 14c years old the predominance of detrital material suggests an important contribution to sediment supply from continental input the preservation of the eolian beach ridge and its adjacent marine sediment has been favored by the postglacial hydro isostatic rebound today the paucity of rainfall is reflected by the decrease in input of terrigenous material therefore the modern sequence which has been emerging since 1987 is made mainly of bioclastic sand supplied by subaqueous shoals due to a rapid rise in sea level the observed decrease of sea level around 2100 14c yr b p largely contributed to the formation of the swale the formation of the beach ridge plain shows that since the middle holocene highstand the coastal segment of trab el makhadha has been governed by equilibrium of sediment abundance

summit lake landslide and geomorphic history of summit lake basin northwestern nevada abstract the summit lake landslide northwestern nevada composed of early miocene pyroclastic debris ashdown tuff and basalt and rhyolite of the black rock range blocked the upper soldier creek snow creek drainage and impounded summit lake sometimes prior to 7840 yr b p the slide covers 8 2 km 2 and has geomorphic features characteristic of long run out landslides such as lobate form longitudinal and transverse ridges low surface gradient 7 1 and preservation of original stratigraphic position of transported blocks however estimated debris volume is the smallest reported 2 5 10 5 m 3 for a landslide of this type the outflow channel of the summit lake basin was a northward flowing stream valley entrenched by mahogany creek subsequent negative tectonic adjustment of the basin by about 35 m accompanied by concommitant progradation of a prominent alluvial fan deposited by mahogany creek argues for a probable diversion of drainage from the alvord basin southward into the lahontan basin the landslide occurred while the creek flowed southward transferring about 147 km 2 of watershed from the lahontan basin back to the alvord basin overflow northward occurred during high stands of pluvial lake parman in the basin otherwise under drier climates the summit lake basin has been closed within large depressions on the slide surface the ca 6800 yr old mazama bed and other sediments have buried a weakly developed soil disseminated humus in the soil yields an age of 7840 310 yr b p absence of older tephra such as st helens m brackets the slide age between 7840 and 19 000 yr b p projectile points found on the highest strandlines of pluvial lake parman suggest a ca 8700 yr b p age by correlation with cultural artifacts and radiocarbon ages from nearby last supper cave nevada organic matter accumulation in landslide soils suggests ages ranging from 9100 to 16 250 yr b p estimation of the age of the slide from morphologic data for the isolated summit lake population of lahontan cutthroat trout does not conflict with the radiometric ages

crust and upper mantle electrical conductivity beneath the yellowstone hotspot track combining long period magnetotelluric data from the spatially uniform earthscope usarray and higher resolution profiles we obtain a regional three dimensional electrical resistivity model in the snake river plain and yellowstone areas idaho and wyoming united states and provide new constraints on the large scale distribution of melt and fluids beneath the yellowstone hotspot track contrary to what would be expected from standard mantle plume models the electromagnetic data suggest that there is little or no melt in the lower crust and upper mantle directly beneath yellowstone caldera instead low mantle resistivities 10 m and below which we infer to result from 1 3 partial melt are found 40 80 km beneath the eastern snake river plain extending at least 200 km southwest of the caldera beneath the area of modern basaltic magmatism the reduced resistivities extend upward into the mid crust primarily around the edges of the snake river plain suggesting upward migration of melt and or fluid is concentrated in these areas the anomaly also shallows toward yellowstone where higher temperatures enhance permeability and allow melts to ascend into the crust the top of the conductive layer is at its shallowest in the upper crust directly beneath the modern yellowstone supervolcano