inherent anisotropic stiffness of weathered geomaterial and its influence on ground deformations around deep excavations the shear modulus g0 at very small strain 0 001 or less is an important parameter for predicting ground movements of many geotechnical structures recent advances in laboratory testing enable th

late cretaceous to middle tertiary tectonic history of the northern rio grande rift new mexico apatite fission track ages for samples collected from three mountain ranges on the eastern margin of the rio grande rift are used to examine the late cretaceous to middle miocene uplift and erosional history of north central new mexico the dates indicate that uplift and erosion was in progress in the sandia mountains near albuquerque and in the taos range portion of the sangre de cristo mountains near taos at least 30 35 m y ago uplift and erosion continued in the sandia mountains at a relatively constant rate 81 m m y until 15 ma the rate of uplift and erosion in this area has approximately tripled in the past 15 m y 230 m m y igneous activity in the taos range has largely obscured the early tertiary uplift and erosional history of this portion of the sangre de cristo mountains a fission track date from one of the middle tertiary intrusions in the taos range is used to calculate the cooling rate due to uplift and erosion in this area for the past 14 m y 210 m m y the uplift and erosion rates derived from the fission track data for the past 14 15 m y are similar to those obtained from other geological evidence in contrast to the oligocene to miocene ages found in the other two areas the apatite fission track ages from the santa fe range portion of the sangre de cristo mountains near santa fe are late cretaceous to early eocene these dates record the cooling of the area due to uplift and erosion during the laramide event the preservation of these older ages indicates that the santa fe range was a low lying area during the oligocene to miocene while the surrounding areas sandia mountains and taos range underwent uplift and erosion volcanic activity occurred in the vicinity of the two areas of positive relief localized crustal extension associated with the volcanism may have contributed in part to the uplift of these areas using simple two dimensional thermal models we found that the apparent denudation rates derived from the fission track age data may represent true denudation rates given certain geologic conditions that may have existed during the uplift and erosion of the sandia mountains in the middle tertiary and the santa fe range in the late cretaceous

geochemical evidence for possible natural migration of marcellus formation brine to shallow aquifers in pennsylvania the debate surrounding the safety of shale gas development in the appalachian basin has generated increased awareness of drinking water quality in rural communities concerns include the potential for migration of stray gas metal rich formation brines and hydraulic fracturing and or flowback fluids to drinking water aquifers a critical question common to these environmental risks is the hydraulic connectivity between the shale gas formations and the overlying shallow drinking water aquifers we present geochemical evidence from northeastern pennsylvania showing that pathways unrelated to recent drilling activities exist in some locations between deep underlying formations and shallow drinking water aquifers integration of chemical data br cl na ba sr and li and isotopic ratios 87sr 86sr 2h h 18o 16o and 228ra 226ra from this and previous studies in 426 shallow groundwater samples and 83 northern appalachian brine samples suggest that mixing relationships between shallow ground water and a deep formation brine causes groundwater salinization in some locations the strong geochemical fingerprint in the salinized cl 20 mg l groundwater sampled from the alluvium catskill and lock haven aquifers suggests possible migration of marcellus brine through naturally occurring pathways the occurrences of saline water do not correlate with the location of shale gas wells and are consistent with reported data before rapid shale gas development in the region however the presence of these fluids suggests conductive pathways and specific geostructural and or hydrodynamic regimes in northeastern pennsylvania that are at increased risk for contamination of shallow drinking water resources particularly by fugitive gases because of natural hydraulic connections to deeper formations

relationships between radium and radon occurrence and hydrochemistry in fresh groundwater from fractured crystalline rocks north carolina usa article i nfo article history accepted 21 october 2008 naturally occurring radionuclides uranium radium and radon major dissolved constituents and trace elements were investigated in fresh groundwater in 117 wells in fractured crystalline rocks from the piedmont region north carolina usa chemical variations show a general transition between two water types 1 slightly acidic ph 5 0 6 0 oxic low total dissolved solids tds waters and 2 near neutral oxic to anoxic higher tds waters the uranium radium and radon levels in groundwater associated with granite rolesville granite are systematically higher than other rock types gneiss metasedimentary and metavolcanic rocks water chemistry plays a secondary role on radium and radon distributions as the 222 rn 226 ra activity ratio is correlated with redox sensitive solutes such as dissolved oxygen and mn

thermal consequences of lithosphere extension over continental margins the initial stretching phase summary we compute the thermal evolution of a lithosphere submitted to stretching during a finite duration of time in order to discuss the initial stretching phase of future continental margins the numerical method developed can handle 2 d laterally variable stretching as well as sedimentation it is shown that lateral conduction is more important than vertical conduction over most continental margins during their formation by stretching a simple way to evaluate the relative importance of lateral and vertical conduction effects at the axis of the zone of rifting just prior to oceanization is proposed a simple way to evaluate the amplitude of the thermal uplift on the edges of the zone of rifting at the end of the stretching phase is also presented for small width zones of rifting 70 100 km lateral cooling becomes so large as to prevent large scale melting and presumably prevent the transition to oceanization the effect of highsedimentation rates 100 500 m myr 1 is to increase the surface temperature of the lithosphere and consequently significantly decrease the surface heat flow

the rhinegraben extension subsidence and shoulder uplift abstract we apply a numerical model of the thermal evolution of a continental lithosphere undergoing extensional deformation to the rhinegraben using reasonable parameters the results of computations are in agreement with the present configuration of the rift and of its shoulders as well as with their evolution since the oligocene contrasting evolutions are reconstructed in the northern and southern rhinegraben whereas the northern history is accounted for by a simple model of homogeneous stretching of the lithosphere the history of the southern rhinegraben cannot be explained without introducing two main periods of heterogeneous stretching the results are locally supported by structural analyses of extensional patterns factors of extension average 1 2 in the axial portion of the rhinegraben and 1 15 for the whole rift section suggesting that the total amount of e w extension reaches 6 km

determination of the anisotropic shear stiffness of an unsaturated decomposed soil the shear modulus at very small strains 0 001 or less g0 is one of the most important parameters for predicting ground movements and dynamic responses of many engineering earth structures wit

high levels of uranium and radium in groundwaters at canada s underground research laboratory lac du bonnet manitoba canada abstract high concentrations of u and 226 ra and elevated 234 u 238 u activity ratios have been measured in groundwater samples collected from water supply wells and exploratory boreholes in the area surrounding the underground research laboratory url of atomic energy of canada limited in southeastern manitoba all groundwaters come from the lac du bonnet granite batholith or sediments overlying the batholith uranium concentrations attain almost 1 mg l in some shallow low salinity groundwaters whereas 226 ra tends to be high up to 38 bq l in deeper saline waters the u concentrations are some of the highest observed in global groundwaters yet no significant ore body or mineralization is known in the area analyses of unaltered rock samples of the lac du bonnet granite show slight u enrichment over average canadian shield granites 6 5 g g vs 4 g g and altered wall rock in fracture zones is enriched in u by up to an order of magnitude compared to adjacent bedrock low 234 u 238 u activity ratios in this altered rock indicate active and recent leaching of u by groundwater the key control on u concentration appears to be redox potential concentrations of u in rock residence time and groundwater composition are of lesser importance geochemical modelling of the shallower oxidized waters indicates that u speciation consists mainly of anionic carbonate complexes of the uranyl ion this is supported by the remarkable efficiency of an anionic filter developed to remove high levels of u from drinking water in the area in more reducing groundwaters u concentrations are similar to those determined in recent experimental work on uraninite solubility in the ph range 7 8 5 colloidal u is

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

flexural rigidity of the basin and range colorado plateau rocky mountain transition from coherence analysis of gravity and topography stochastic inversion for flexural loads and flexural rigidity of the continental elastic layer can be accomplished most effectively by using the coherence of gravity and topography however the spatial resolution of coherence analysis has been limited by use of two dimensional periodogram spectra from very large 105 km 2 windows that generally include multiple tectonic features using a two dimensional spectral estimator based on the maximum entropy method the spatial resolution of flexural properties can be enhanced by a factor of 4 or more enabling more detailed analysis at the scale of individual tectonic features this new approach is used to map the spatial variation of flexural rigidity along the basin and range transition to the colorado plateau and middle rocky mountains physiographic provinces large variations in flexural isostatic re sponse are found with rigidities ranging from as low as 8 7x102o n m elastic thickness t 4 6 km in the basin and range to as high as 4 1x1024 n m t 77 km in the middle rocky mountains these results compare favorably with independent determinations of flexural rigidity in the region areas of low flexural rigidity correlate strongly with areas of high surface heat flow as is expected from the contingence of flexural rigidity on a temperature dependent flow law also late cenozoic normal faults with large displacements are found primarily in areas of low flexural rigidity while deformation fronts of mesozoic tertiary overthrusts occur 0 to 100 km east of the low rigidity region the highest flexural rigidity is found within the archean wyoming craton where evidence suggests that deeply rooted cratonic lithosphere may play a role in deter mining the distribution of tectonism at the surface

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

isostatic rebound due to tectonic denudation a viscous flow model of a layered lithosphere a four layer model of the upper 150 km of the earth is used to calculate the viscous response of continental crust and the underlying mantle to tectonic denudation the model comprises a strong upper crustal layer a weak lower crustal layer a very strong mantle lithosphere layer and a weak mantle asthenosphere layer which is in accord with experimental constraints on strength depth profiles for continental lithosphere the strength of each layer is represented by its effective viscosity flow in the crust and mantle is driven by buoyancy forces which arise from the unloading of an allochthon along a detachment fault by a series of instantaneous displacements earthquakes or rapid creep events numerical solutions obtained by using a finite element technique predict footwall uplift moho deflection and surface topography that are consistent with observations from the basin and range province of the western united states the calculated curvature of the footwall uplift is also similar to that observed and is sensitive to the geometry of the detachment fault such bending need not be elastically controlled hence the curvatures of footwall domes do not clearly place limits on the effective elastic thickness of the extending crust the upward deflection of the moho and the surface topography are sensitive to the viscosity structure and enable us to bound the range of the various viscosities by matching observations from the basin and range province which indicate no moho deflection and low magnitude of surface topography 3 5 km we estimate the upper crustal lower crustal and mantle lithospheric viscosities in the ranges 1021 1023 pa s 1019 1021 pa s and 1021 1023 pa s respectively

evaluation of in situ anisotropy from crosshole and downhole shear wave velocity measurements downhole and cross hole shear wave velocity determinations are becoming a routine part of dynamic site evaluation studies the in situ measurements are frequently performed in boreholes using a variety of techniques a seismic cone system has been developed for determining both downhole and crosshole shear wave velocity the system consists of a source and two receivers all embodied in penetrometers at any depth three different polarized sets of shear wave signals can be generated and are recorded by both receivers located at different distances from the source it has been suggested from laboratory tests in large chambers that the measured shear wave velocities can be used to evaluate the in situ stress conditions tests have been performed at research sites in vancouver to evaluate the in situ stress dependence of the measured shear wave velocities the results of this study suggest that variations in structural anisotropy are the predominant factors controlling the directional shear wave velocities

geostatistical techniques application to dissolved radon hazard mapping an example from the western sector of the sabatini volcanic district and the tolfa mountains central italy abstract dissolved rn was determined in 192 samples collected from cold shallow volcanic and sedimentary aquifers deep thermal aquifers and from waters associated with bubbling gases in the western sector of the sabatini volcanic district and the tolfa mountains central italy shallow aquifers hosted in the quaternary volcanic complexes show values ranging from 1 0 to 352 bq l median value 55 and inter quartile distance 62 bq l while waters circulating within the permeable horizons of the sandy to clayey sediments of the tolfa flysch have values from 1 0 to 44 bq l median value 6 9 and inter quartile distance 8 1 bq l thermal waters are hosted in the mesozoic carbonate formations and move towards the surface along faults here dissolved rn values range from 0 to 37 bq l median value 3 0 and inter quartile distance 9 5 bq l waters associated with bubbling gases show dissolved rn contents ranging from 2 0 to 48 bq l median value 6 2 and inter quartile distance 23 bq l those results suggest that lithology is the main factor affecting the rn contents in shallow aquifers due to the high levels of rn progenitors u and ra in the volcanic rocks relative to sedimentary units the influence of other factors such as the presence of a fracture network seasonal flow variations type of discharge from well or spring was also investigated radon contents of thermal waters result from mixing with shallow waters from both volcanic and sedimentary rock aquifers and decrease of rn solubility with temperature while for bubbling pools the effects of strong degassing were also considered in terms of health hazard from direct ingestion of rn rich waters 20 8 of those circulating within the volcanic aquifer show values higher than the recommended value of 100 bq l while none of those circulating within the sedimentary aquifers exceed the threshold value geostatistical techniques were used for the elaboration of contour maps by using variogram models and kriging estimation aimed at defining the areas where a potential health hazard due to the direct ingestion of rn rich waters and to inhalation of air following degassing of rn from waters may be expected

redistribution of uranium and thorium series isotopes during isovolumetric weathering of granite previous studies of the distribution of u and th in parent versus weathered granites have shown both depletion and enrichment of these elements during weathering in this study the distribution of u and th decay series isotopes was determined in a weathering profile of a granitic saprolite which showed textural preservation indicating isovolumetric weathering two types of dissolution methods were used a whole rock dissolution and a sodium citrate dithionite leach to preferentially attack noncrystalline phases of weathering products using volume based activities 45 70 percent of the total 232th was gradually removed during weathering although the whole rock 228th232th activity ratios were in equilibrium there were large excesses of 228th in the leachable fraction of both parent rock 228th232th 2 06 and partially weathered saprolite 228th232th 3 6 5 due to alpha recoil and release of daughter 228th to the weathering rind of the mineral grain for the most weathered sample 81 percent of the thorium was in the teachable fraction and 228th232th 1 indicating that even the more resistant minerals were attacked the total u activities showed as much variation in the six parent rock samples as in the weathered profile and 234u238u were in equilibrium in both the whole rock and leachable fractions 230th was deficient relative to 234u and 226ra in both fractions suggesting recent addition of u and ra to the entire profile the large variation in u was not from absorption onto the intermediate weathering products because only 11 23 percent of the u was in the leachable fraction

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

natural radioelement solution by circulating groundwaters in the stripa granite natural radioelement concentrations in groundwaters from the fracture flow regime in the stripa granite were monitored during the period 1977 1986 uranium concentrations from depths of 80 m or less and in minewaters from above the hydraulic sink produced by the deepest mining level 410 m range from 10 to 90 ppb 234u238u activity ratios are characteristic of chemical leaching processes at the shallower depths whilst at about 350 m groundwaters occur which have an additional component of 234u by alpha recoil processes at greater depths u concentrations are indicative of the changed redox conditions and are subject to activity ratio enhancement by the alpha recoil process groundwater residence times may be estimated from the rate of such recoil solution of 234u in zones where chemical leaching of u is not active the 222rn contents of the minewaters are extremely high and can be related to the 222rn flux from fracture surfaces and the fracture width the fracture apertures range from 10 mm for the shallow groundwater zone 222rn contents m for water from the deepest borehole section 222rn content 100 kbq kg 1 the activity ratios 234th238u 210pb222rn and 210po222rn have been used to estimate residence times for th pb and po isotopes solution of ra is controlled by chemical rather than alpha recoil processes the 226ra content of the groundwater increases with salinity and is well correlated with groundwater ca content the 228ra226ra activity ratios in the groundwaters suggest that water rock interaction occurs predominantly with the average granitic matrix rather than with highly uraniferous fracture mineralisation the behaviour of ra th and po isotopes may be used to assess that of homologous fission product and actinide constituents of nuclear wastes in groundwaters

the flexural isostatic response of the lithosphere to extensional tectonics abstract the tectonic loading resulting from the extension of the lithosphere is examined loads resulting from crustal thinning thermal perturbations and basin infill are recognised and the resulting flexural isostatic deflection of the lithosphere is modelled in response to each of them in the presence of a listric fault during rifting regional uplift or rebound occurs as a result of both crustal thinning due to normal faulting simple shear and the elevation of the geothermal gradient while subsidence is generated by the downward acting loads created by both basin infill and thinning of the lower crust by stretching pure shear following rifting further flexural subsidence occurs in response to the cooling of the temperature field and associated sedimentary infill one of the major tectonic effects resulting dominantly from crustal thinning by normal faulting is the generation of footwall uplift the magnitude of this uplift along with basin depth and underlying crustal structure are strongly determined by the flexural rigidity of the lithosphere the amount of extension and the dip of the major basement fault along which extension is taking place depending upon the values of these parameters model calculations predict between a few tens of metres and 1 5 km of footwall uplift these values correlate closely with the magnitude of footwall uplift estimated from examples within the geological record the erosion of this rift induced topography is also proposed as a possible driving mechanism for the generation of major basin unconformities

flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension we suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound this mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes and magmatic thickening of the crust our hypothesis is based on two critical concepts first the lithosphere retains finite mechanical strength or flexural rigidity during extension second isostatic rebound uplift of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation we develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks we first investigate the isostatic consequences of finite simple slip on an initially planar dipping normal fault cutting the entire lithosphere when the lithosphere retains flexural rigidity during extension the topography resulting from this model resembles a half graben and the footwall rift flank is flexurally uplifted this simple normal faulting model explains free air gravity anomalies and topography observed at rift flanks in oceanic lithosphere such as broken ridge in the eastern indian ocean the caroline ridges sorol trough in the western equatorial pacific and the coriolis trough behind the new hebrides island arc we then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates when the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical the surface of slip can be regarded as a detachment by simplification our general model accounts for pure shear extension of the lithosphere that is uniform with depth in this case detachments have no meaning in the geologic sense however the kinematics of depth independent pure shear may nevertheless be described in terms of a surface which we term a kinematic reference surface at some depth in the lithosphere we speculate that the depth of this surface may be rheologically controlled the magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface in contrast if local isostasy is assumed when the lithosphere undergoes a given amount of depth independent pure shear the resulting topography will be the same regardless of how the kinematics of that extension are formulated the basin and rift flank topography and free air gravity anomaly over young continental rifts such as the rhine graben can be satisfied using our general extensional model with a small amount 5 km of extension along a listric shaped detachment soling into the crust mantle boundary because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension

radon 222 and its parent radionuclides in groundwater from two study areas in new jersey and maryland u s a abstract a study of groundwater chemistry and radionuclide mobility in new jersey and maryland was conducted to investigate natural processes that control the mobility of radionuclides in the water rock system groundwater was sampled from two geological units in new jersey and from six in maryland the water sampled was from aquifiers in fractured metamorphic rocks of varying composition and metamorphic grade in both areas groundwater chemistry was affected most by aquifier mineralogy and lithology concentrations of total dissolved u 226ra and 222rn were similarly affected in evey sample for which measurements were made dissolved utotal and 226ra were present in much lower concentrations than 222rn when expressed in terms of their radioactivity on the other hand the total amount of 222rn that could be produced in these rocks given their u contents is much higher than the concentrations observed in groundwater thus the emanating efficiencies of the aquifer rocks studied must be near 10 or less such low emanating efficiencies require that a fraction of the 226ra in the rock be located close to the water rock interface so that 222rn when produced can be rapidly and efficiently transferred to the aqueous phase this condition is established when a similar fraction of the u is in a readily leachable position no known u or ra solids were supersaturated in any of the samples thus adsorption processes probably play a role in limiting mobilities of utotal and 226ra concentrations of utotal and 226ra found in the water samples are comparable to those found in experimental studies of adsorption onto mineral surfaces

radium isotopes to trace uranium redox anomalies in anoxic groundwater abstract 223ra 224ra 226ra and 228ra isotopes have been measured in groundwaters from depths ranging 50 900 m in fractured crystalline bedrock forsmark sweden to understand the reason for elevated up to 150 g l aqueous uranium uaq at 400 650 m depth ra isotope data is interpreted alongside previously reported 222rn 234u and 238u data as well as phreeqc geochemical modelling and uranium mineralogy a novel 223ra 226ra gw based approach where brackets and gw subscript refer to expression of an activity ratio measured from groundwater to groundwater residence time estimation shows that elevated uaq is most common in holocene age groundwaters of marine origin although these groundwaters are geochemically reducing the 223ra 228ra corr where corr subscript refers to a correction applied to compare 223ra 228ra gw to the more commonly reported 226ra 228ra gw suggest that they interact with u rich pegmatites containing proterozoic and palaeozoic age ca u vi silicate minerals which are undersaturated in the present groundwaters local aqueous u vi can be stabilized in ca2uo2co30 complexes at pe values as low as 4 5 but is susceptible to reduction after a modest decrease in pe value alkalinity or ca concentration the 223ra 228ra corr and 224ra 228ra gw also suggest that u vi aq precipitates as uo2 x at the interface between marine and non marine groundwaters from these data local uaq is proposed to be governed by on going water rock interaction involving old u vi minerals

the growth of geological structures by repeated earthquakes 2 field examples of continental dip slip faults a strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault bounded geological structures such as basins and ranges which are built by repeated cycles of deformation along strike slip faults the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign resulting in little permanent deformation except for the fault offset for dip slip faults portions of the crust are lifted and dropped and so buoyancy forces are exerted the seismic and interseismic deformations do not balance and structures grow and become subject to erosion and deposition we consider three examples for which the structure and fault geometry are well known the white wolf reverse fault in california site of the 1952 kern county m 7 3 earthquake the lost river normal fault in idaho site of the 1983 borah peak m 7 0 earthquake and the cricket mountain normal fault in utah site of quaternary slip events basin stratigraphy and seismic reflection records are used to profile the structure and coseismic deformation measured by leveling surveys is used to estimate the fault geometry to reproduce these structures we add the deformation associated with the earthquake cycle the coseismic slip and postseismic relaxation to the flexure caused by the observed sediment load treating the crust as a thin elastic plate overlying a fluid substrate the cumulative deformation is principally dependent on the elastic plate thickness modestly sensitive to the sediment substrate density difference and insensitive to the fluid viscosity for the 4 to 8 ma structures we deduce a longterm flexural rigidity of 2 15 1019 nm this is equivalent to an elastic plate thickness of 2 4 km for a young s modulus of 2 5 1010 nm 2 this value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km but where coseismic fault slip extends to a depth of 10 15 km thus much of the seismogenic crust must weaken substantially during the life of active faults causing the fault bounded basins to narrow over time

flexural rift flank uplift at the rio grande rift new mexico like other cenozoic continental rifts the rio grande rift in colorado and new mexico exhibits prominent flanking uplifts several driving stresses and thermal mechanical processes have been proposed to explain the origin of rift flank relief which can be modeled to infer lithospheric structure although we have identified multiple uplift styles at the rio grande rift only one range the sacramento mountains is attributable to flexural upwarping of the lithosphere the process most suitable for geodynamic modeling and interpretation we demonstrate that two common assumptions in such modeling potentially introduce serious errors first presuming only one mechanism acts to uplift the flanks is inappropriate various forces influence flank topography at different depths and wavelengths and no single one is dominant second the end member boundary conditions of complete mechanical continuity or discontinuity broken plate at the range bounding normal fault are in general not applicable at rift flanks we examine alternative analytic plate flexure solutions by comparing them to finite element models of footwall flexure at a normal fault in a two dimensional elastic plate undergoing extension these simulations indicate that broken plate fits to rift flanks underestimate the plate thickness unless the uplift is large at least 1 km which promotes decoupling between the hanging wall and footwall if denudation dominates the flank unloading as may commonly be the case the best fit broken plate thickness error can be even greater our flexural analysis of the sacramento mountains suggests that the pecos river valley originated as a flexural down warp adjacent to the rift flank sensitivity tests of least squares fits to the sacramento mountains imply typical plate thickness errors of 20 although in extreme cases the combined errors may be 50 the average effective elastic lithosphere thickness is 23 km we find that elastic plastic models of rift flank flexure are unable to provide meaningful constraints on the thermal structure of continental lithosphere

wave velocity and stiffness measurements of the crag and lower london tertiaries at sizewell this paper describes an investigation into the body wave velocity and stiffness characteristics of the foundation soils at the site formerly considered for sizewell c nuclear power station results from both in situ and laboratory tests are presented in situ measurements comprised shear and compression wave velocities using cross hole and down hole techniques including the velocity of horizontally propagating horizontally polarized shear waves laboratory tests included torsional shear tests on hollow cylindrical samples triaxial compression and extension tests with local measurements of strain and resonant column tests sample quality and the effects of destructuring are evaluated the anisotropy of the wave velocity and of the stiffness characteristics is highlighted l article decrit des recherches sur la vitesse de propagation des ondes de volume et les caracteristiques de rigidite des sols d assise de l emplacement propose de la centrale nucleaire sizewell c il presente les resultats des essais

estimates of terrestrial heat flow from deep petroleum tests along the rio grande rift in central and southern new mexico twenty six new estimates of heat flow are calculated for the albuquerque belen basin and the basins of the southern rio grande rift in new mexico from these and previous data the following heat flow patterns may be suggested the regional or background heat flow in the albuquerque belen basin is 77 mw m 2 there is also a ne sw trend of higher heat flow 92 110 mw m 2 across the region in the southern rio grande rift between socorro and las cruces the regional heat flow appears to be 95 mw m 2 in this region there are also higher heat flows 117 135 mw m 2 at widely scattered sites south of las cruces near the international border between mexico and the united states preliminary and perhaps suspect data suggest the possibility of a transition to the intermediate heat flows previously estimated at sites throughout much of northeastern chihuahua mexico regional heat flow values in the albuquerque belen basin and in the basins of the southern rio grande rift are consistent with but do not constrain simple models of crustal thinning higher heat flows in these two areas are thought to be caused by hydrothermal fluids largely groundwaters but possibly magma in some areas transporting heat upward along crustal fracture zones

ra isotopes and rn in brines and ground waters of the jordan dead sea rift valley enrichment retardation and mixing abstract thirty six springs and wells from the dead sea rift valley were periodically sampled and analyzed the latter included full chemical analyses 222 rn and 226 ra by counting emanometry as well as 228 ra 226 ra and 224 ra 228 ra activity ratios spectrometry sampling stretched over almost 2 yr several hundred rn ra analyses and close to a hundred isotopic ratios were measured most waters in the rift valley have both elevated ra several to 750 dpm l and rn a few hundred to almost 60 000 dpm l content in practically all samples 222 rn activity considerably exceeds that of its parent 226 ra the ra content is the result of all dead sea rift valley waters being mixtures of fresh water with saline brines ra is efficiently extracted from surrounding rocks into the brine end member 228 ra 226 ra ratios are exceptionally low 0 07 to 0 9 mostly less than 0 2 this apparently reflects the u over th enrichment in the source rocks that contribute the ra ra enrichment both 228 ra and 226 ra is locally correlated throughout the rift valley with water salinity this correlation can be used to constrain the age of the brine freshwater mixing process in one of the hydrologic subsystems studied the fuliya block the mixing of the shallow ground water must have occurred in less than 200 to 300 yr ago probably before no more than some 30 yr high radon activities in surface waters along the dead sea rift valley result from the formation of radium enriched linings on the aquifer rock surface the mixing of the radium extracting brines with fresh water leads to continual adsorption of radium as water salinity decreases the unique combination of fast upward flow and continual mixing in the dead sea rift valley accounts for a constant replenishment of radium in the waters this causes a gradual buildup of a radium lining on the aquifer walls until eventually a steady state surface activity is established

anisotropy of small strain stiffness in ticino sand comprehensive tests performed in a large calibration chamber and consisting of the propagation of seismic body waves in dry ticino silica sand are presented by virtue of the fact that both shear and constrained compression waves have been propagated in vertical horizontal and oblique directions five independent constants of the cross anisotropic elasticity model of the test have been established the tests have also made it possible to evaluate the influence of the structural inherent and stress induced anisotropy on the velocity of the propagated seismic body waves the influence of the principal effective stresses on the velocity of these waves is also described de nombreux essais realises dans une grande chambre d etalonnage et consistant en la propagation d ondes sismiques dans des sables siliceux et sec de ticino sont presentes les ondes de cisaillement et de compression ayant w emises dans des directions verticales horizontales et obliques les 5 constantes independantes du modele eastique

trace element mobility in mine waters from granitic pegmatite u th ree deposits bancroft area ontario abstract small low grade granitic pegmatite u th ree deposits are found throughout the grenville geological province of eastern canada groundwater quality at historical mining properties in the bancroft area was investigated in order to better understand the mobility of trace elements that may pose health risks if there is renewed development of this class of mineral deposit groundwater samples were obtained from diamond drill holes flowing adits and flooded mine shafts uranium occurs almost entirely in the dissolved

buckling of a pervasively faulted lithosphere we investigated the buckling response of a faulted elastic plate under horizontal compression using the finite element technique to better understand the effect of faults on the elastic behavior of a plate we studied the effect of changes in fault spacing depth and dip on the effective young s modulus buckling stress and wavelength our model consists of a thick elastic plate whose entire upper surface is cut by evenly spaced faults we impose either an initial sinusoidal deformation with a fixed wavelength or a random deformation to the grid a fault is represented as a free surface with no resolved shear stress and is allowed to slip in a specified direction using the method of slippery nodes with the assumption of free slip on the faults our model results represent an end member case in which the buckling wavelength and buckling stress are minimized by the presence of the faults

the occurrence and behavior of radium in saline formation water of the u s gulf coast region abstract radium has been measured in deep saline formation waters produced from a variety of u s gulf coast subsurface environments including oil reservoirs gas reservoirs and water producing geopressured aquifers a strong positive correlation has been found between formation water salinity and ra activity resulting from the interaction of formation water with aquifer matrix ra isotopes enter the fluid phase after being produced by the decay of parent elements u and th which are located at sites on and within the solid matrix processes that are belived to be primarily responsible for transferring ra from matrix to formation water are chemical leaching and alpha particle recoil factors controlling the observed salinity ra relationship may be one or a combination of the following factors a ion exchange b increased solubility of matrix silica surrounding ra atoms coupled with a salinity controlled rate of reequilibration of silica between solution and quartz grains and c the equilibration of ra in solution with detrial barite within the aquifer no difference was found in the brine ra relation in water produced from oil or gas wells and water produced from wells penetrating only water bearing aquifers although the relation was more highly correlated for water bearing aquifers than hydrocarbon containing reservoirs

measurements and numerical simulations of inherent stiffness anisotropy in soft taipei clay abstractsoils are generally known not to be isotropic materials the inherent anisotropy of clays results from the deposition process which tends to induce a horizontal bedding plane in the soil layer in this paper the anisotropy of clays was studied to obtain more accurate analytical results for geotechnical problems especially for deep excavations in soft clay with nearby structures a series of k0 consolidated undrained triaxial compression ck0uc tests thus was conducted on tube samples of natural taipei silty clay with multidirectional bender elements a newly designed triaxial testing system equipped with a high precision servo motor and local strain sensors was developed a new soil suction control system was also developed to perform soil saturation for triaxial tests the suction control system was used to reduce the change in the void ratio during saturation in the triaxial tests soil samples were retrieved from a site near a well documented excavation case in taipei the anisotropy ratios

evaluation of groundwater quality based on radiological and hydrochemical data from two uraniferous regions of western iberia nisa portugal and ciudad rodrigo spain the regions of nisa portugal and ciudad rodrigo spain are characterized by a high radiation background due to the presence of uranium enriched rocks to evaluate the water quality of the shallow aquifers and the relationship between radiological and non radiological variables a set of 41 samples was collected mainly from springs and analyzed for the radioisotopes 238u 234u 226ra and 222rn as well as for some major and minor cations the results obtained for the cationic composition in both areas show some differences in terms of geological bedrock with higher mineralization and hardness in the spanish samples ca mg facies the nisa samples are included in the na k facies the indicative effective dose was calculated for several age groups on average the effective dose is higher in nisa due to the activity in 226ra and 25 of the total dataset puts 222rn above the recommended acceptable maximum despite the low scale ancient uranium exploitation works in the nisa area in comparison to those carried out on the spanish side the higher risk calculated in the portuguese region reinforces the importance of taking into account the natural background when the goal is to control the exposure of the population to ionizing radiation

the mechanics of continental extension and sedimentary basin formation a simple shear pure shear flexural cantilever model abstract deep seismic reflection data and earthquake seismology show the fundamental importance of major crustal faults during extension of continental lithosphere controlling the development of rifted sedimentary basins major basement faults are generally planar and extend down to 10 15 km depth corresponding to the base of the seismogenic layer beneath this depth deformation gives way to distributed ductile shear in the lower crust and lithospheric mantle a mathematical model is described of the geometric thermal and flexural isostatic response of the lithosphere to extension by planar faulting of the upper crust and plastic distributed deformation in the lower crust and mantle during faulting upper crustal footwall and hanging wall blocks behave as two mutually self supporting flexural cantilevers their response to isostatic forces induced by extension generates footwall uplift and hanging wall collapse for extension on multiple faults interference of footwall uplift and hanging wall collapse gives rise to the familiar block rotations of rift tectonics the flexural cantilever model is able to predict crustal structure and sedimentary basin geometry for faults of arbitrary spacing horizontal displacement and polarity the effects of fault spacing fault polarity and the amount of extension as well as erosion of footwall uplift on crustal structure and basin geometry during both syn rift and post rift stages of basin evolution are explored models of passive rifting involving lithosphere extension in response to the build up of far field tectonic forces and decompression induced partial melting of the upper asthenosphere and lower lithosphere appear to conform to the main geological observations the role of active rifting driven by locally derived deviatoric tensional forces generated by mantle plumes remains uncertain

constraints on yield strength in the oceanic lithosphere derived from observations of flexure summary the wavelength and amplitude of outer rises seaward of sub duction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing the data show the clear age dependence in the mechanical thickness of the lithosphere up to 60 100myr saturation of moment at large curvature is interpreted in terms of a depth dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation a comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long term strength below about 40 km depth with no difference between 100 and 165 myr old crust moderate axial loading forces 200 mpa can explain most variations in the moment curvature observations except in the case of the kuril trench which appears anomalous given the age of the crust regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension the observations point to a lithosphere weaker than the prediction from experimental deformation of rocks of the possible weakening mechanisms elevated pore fluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms our favoured explanation is that the activation energy q appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures if recent oceanic geotherms are reliable q in the lower lithosphere must be lower than 100kcal mol 1 the method used here is most appropriate for trench profiles with curvatures greater than 10 7m 1 for lower curvatures such as along seamount profiles small errors in the curvature estimate cause large changes in rheological parameters

the mobility of radium 226 and trace metals in pre oxidized subaqueous uranium mill tailings the exchange of 226ra and trace metals across the tailings water interface and the mechanisms governing their mobility were assessed via sub centimetre resolution profiling of dissolved constituents across the tailings water interface in cell 14 of the quirke waste management area at rio algom s quirke mine near elliot lake ontario canada shallow zones 2 m water depth fosters stagnant bottom waters and permits the development of anoxia above the benthic boundary these anoxic tailings are characterized by substantial remobilization of 226ra resulting in a relatively large flux of 226ra from the tailings to the water column the strong correlation between the porewater profiles of 226ra and ba r2 0 99 as well as solubility calculations indicate that the mobility of ra is controlled by saturation with respect to a poorly ordered and or impure barite phase ra ba so4 in the anoxic zones severe undersaturation with respect to barite is sustained by microbial so4 reduction flux calculations suggest that the increase in 226ra activity in the water cover since 1995 from 0 5 to 2 5 bq l 1 can be attributed to an increase in the spatial distribution of anoxic bottom waters caused by increased density of benthic flora the anoxic vegetated areas also exhibit minor remobilization with respect to dissolved as ni and zn the removal of trace metals in the anoxic bottom waters appears to be limited by the availability of free sulphide collectively the data demonstrate that while the water cover over the u mill tailings minimizes sulphide oxidation and metal mobility anoxic conditions which have developed in deeper areas have led to increased mobility of 226ra

changing uranium distributions during weathering of granite abstract changes in uranium distribution during chemical weathering were investigated in three granitoid types from the new england district new south wales australia over a range of samples from unweathered rock to profoundly weathered material uranium fission track micromapping was used to characterise the three principal modes of u abundance in unweathered granitoid rock low concentrations of background u in fine fractures in quartz and in deuterically altered feldspar biotite and hornblende interstitial u in concentrations between 140 and 1500 g g 1 principally in hydrated fe oxide mn oxide and clay minerals and the greatest proportion as resistate u 90 4000 g g 1 located in the accessory minerals during low temperature chemical weathering a significant net u loss occurs and the remaining u is redistributed amongst the three modes background u is depleted and the mobilised u is adsorbed by interstitial aggregates thereby increasing interstitial u concentrations resistate u is also depleted as sphene zircon monazite and acicular apatite retain their initial u concentrations but are reduced in modal abundance prismatic apatite fluorite and allanite are progressively removed the significant net loss of u during chemical weathering provides uranium for potential secondary sedimentary mineralisation and implies that weathered granitoid rock is not an appropriate element in the geological barrier for nuclear waste

petrography of the uranium bearing minerals of the liberty hill pluton south carolina phase assemblages and migration of uranium in granitoid rocks optical petrography fission track u mapping and electron microprobe analysis of granitoid rocks from the liberty hill pluton south carolina show a nonuniform distribution of u in the rocks in order of abundance u occurs in accessory minerals with u concentrations decreasing in the order uraninite coffinite uranoan thorite zircon monazite titanite allanite apatite zeolites on altered sulfaces of fe sup 2 and s bearing minerals at microcracks filled with carbonate zeolite and fe mn oxyhydroxide minerals on clay minerals of altered feldspars and in major minerals as a widely dispersed minor component thorium has been identified in the accessory minerals thorite thoroan uraninite coffinite allanite and monazite uranium and th distribution is believed to be of magmatic origin within the major u th zr phase assemblage uranoan thorite zircon plus or minus uraninite the mechanisms of u retention are similar to those found in sedimentary u deposits the u th pb isotope studies of the liberty hill pluton show that these mechanisms are effective in retaining the initial concentrations of u refs

ground water geochemistry of 228ra 226ra and 222rn abstract 228ra 226ra and 222rn activities were determined on over 150 ground water samples collected from drilled public water supply wells throughout south carolina a wide range of aquifer lithologies were sampled including the crystalline rocks of the piedmont and sedimentary deposits of the coastal plain a significant linear relationship between log 228ra and log 226ra n 182 r 0 83 was indistinguishable between piedmont and coastal plain ground water median 228 ra 226 ra activity ratios for the piedmont 1 2 and coastal plain 1 3 ground water are close to estimated average crustal 232 th 238 u activity ratios of 1 2 to 1 5 corresponding to th u weight ratios of 3 5 to 4 5 a linear correlation was also found between log 222rn and log 226ra for piedmont n 68 r 0 62 and coastal plain n 89 r 0 64 ground water however the median 222 rn 226 ra activity ratio for piedmont ground water 6100 was much higher than for coastal plain ground water 230 higher excess 222rn activities may be due to greater retention of 226ra by the chemically active piedmont aquifers compared to the more inert sand aquifers sampled in the coastal plain the relationship between log 228ra and log 226ra was used to predict total ra 228 ra 226 ra distributions in appalachian and atlantic and gulf coastal plain ground water predictions estimate that 2 4 of appalachian and 5 3 of atlantic and gulf coastal plain ground water supplies contain total ra activities in excess of the 5 pci l limit established by the u s environmental protection agency these predictions also indicate that 40 50 of these ground water wells may be overlooked using the presently suggested screening activity of 3 0 pci l of 226ra for 228ra analysis

vertebrate bearing eolian unit from the ogallala group miocene in northwestern texas the upper couch formation is part of the lower of two formations composing the ogallala group in blanco and yellowhouse canyons in northwestern texas an eolian origin for the upper couch formation is indicated by its mean grain size pedogenic carbonate nodules massive bedding and blanketlike morphology the unit conforms poorly to the usual eolian depositional models it resulted from a combination of the processes involved in loess and sand sheet formation grassland or savanna vegetation probably existed over the area and aided in sediment trapping vertebrates are unusual in eolian units but the adaptations and mode of preservation of those in the upper couch formation also support an eolian interpretation this and other widespread silty sand sheets in the ogallala indicate major fluctuations in depositional style possibly climatically controlled lateral continuity and preservation of vertebrates give silty sand sheets great potential as correlation tools

the role of disseminated calcite in the chemical weathering of granitoid rocks accessory calcite present at concentrations between 300 and 3000 mg kg 1 occurs in fresh granitoid rocks sampled from the merced watershed in yosemite national park ca usa loch vale in rocky mountain national park co usa the panola watershed ga usa and the rio icacos puerto rico calcite occurs as fillings in microfractures as disseminated grains within the silicate matrix and as replacement of calcic cores in plagioclase flow through column experiments using de ionized water saturated with 0 05 atm co2 produced effluents from the fresh granitoid rocks that were dominated by ca and bicarbonate and thermodynamically saturated with calcite during reactions up to 1 7 yr calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates principally biotite mass balance calculations indicate that most calcite had been removed during this time and accounted for 57 98 of the total ca released from these rocks experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution the lack of excess ca and alkalinity indicated that calcite had been previously removed by natural weathering the extent of ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks high ca na ratios relative to plagioclase stoichiometries indicate excess ca in the yosemite loch vale and other alpine watersheds in the sierra nevada and rocky mountains of the western united states this ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains in contrast ca na flux ratios are comparable to or less than the ca na ratios for plagioclase in the subtropical panola and tropical rio icacos watersheds in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport limited weathering these results indicate that the weathering of accessory calcite may strongly influence ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces

radium geochemistry of ground waters in paleozoic carbonate aquifers midcontinent usa the purpose of this study was to elucidate the processes controlling the distribution and behavior of the longerlived ra isotopes in continuous paleozoic carbonate aquifers of parts of missouri kansas and oklahoma activities of 228 ra and 226 ra were analyzed in fresh and saline ground waters brines and rocks the fluids have a wide salinity range 200 250 000 mg l 1 total dissolved solids the 226 ra activity ranges from 0 66 7660 dpm kg 1 and correlates with salinity and other alkaline earth element ca sr and ba concentrations the range of 228 ra 226 ra ratios in the fluids 0 06 1 48 is similar to that in the aquifer rocks 0 21 1 53 the relatively low mean fluid 228 ra 226 ra ratio 0 30 reflects the low th u ratio of the predominant carbonate aquifer rock radium occurs mostly r77 as ra 2 species in the fluids salinity dependent sorption desorption processes with log k values from 10 0 10 4 and negatively correlated with salinity involving th enriched surface coatings on aquifer flow channels can explain the rapid solid fluid transfer of ra isotopes in the system and the correlation of ra with

flexure associated with planar faulting summary thin plate flexure theory has previously assumed unconstrained end loading to model the flexure associated with a fault cutting a strong upper crust above an inviscid substratum this assumption implies that fault planes are not parallel when the density differential across the plate differs on upthrown and downthrown sides as in the development of a sediment filled half graben a downward widening extension crack would develop during planar normal faulting which conflicts with the geological experience that such cracks are closed by overburden pressure below a few hundred metres depth a new analytical solution is presented in which the fault planes are constrained to be parallel and in contact the new theory gives a substantially larger ratio of upthrow to downthrow and a significantly wider half graben and narrower uplift

effects of wetting drying and stress ratio on anisotropic stiffness of an unsaturated soil at very small strains the very small strain shear modulus of soil g0 is affected by many factors including soil properties current stress state stress history and matric suction very little research has been condu

upwarp of anomalous asthenosphere beneath the rio grande rift continental rifts are possible analogues of mid ocean ridges although major plate tectonic features are less clearly observed1 current thermal models of mid ocean ridges2 4 consist of solid lithospheric plates overlying the hotter less viscous asthenosphere with plate thickness increasing away from the ridge axis the lithospheric lower boundary lies at or near the melting point isotherm so that at greater depths higher temperatures account for lower viscosity lower seismic velocities and possibly partial melting upwarp of this boundary at the ridge axis concentrates heat there thus lowering densities by expansion and raising the sea floor to the level of thermal isostatic equilibrium at slow spreading ridges a major central graben forms owing to the mechanics of magma injection into the crust5 topography heat flow gravity and seismic studies support these models on the continents a low velocity channel has been observed although it is poorly developed beneath ancient cratons6 9 plate tectonic models have been applied to continental basins and margins10 12 but further similarities to the oceanic models remain elusive topographic uplift is often ascribed to airy type isostatic compensation caused by crustal thickening rather than thermal compensation in the asthenosphere here we discuss the rio grande rift in southwestern united states teleseismic p wave residuals show that regional uplift is explained by asthenosphere uplift rather than crustal thickening

hydrogeochemistry groundwater ages and sources of salts in a granitic batholith on the canadian shield southeastern manitoba abstract the hydrogeochemistry of the lac du bonnet granitic batholith has been determined for the region of the whiteshell research area wra in southeastern manitoba canada this work forms part of the geosciences studies performed for the canadian nuclear fuel waste management program over the period 1980 1995 by atomic energy of canada limited aecl knowledge of the variation of groundwater chemistry and its causes is useful in assessing the performance and safety of a nuclear fuel waste vault located at depths of up to 1000 m in a crystalline rock formation of the canadian shield groundwaters and matrix pore fluids have been obtained by standard sampling methods from shallow piezometers in clay rich overburden from packer isolated borehole zones intersecting fractures or fault zones in the bedrock and from boreholes in unfractured rock in aecl s underground research laboratory url eighty six individual fracture groundwaters have been sampled and analysed from permeable zones in 53 boreholes drilled to depths of up to 1000 m in the lac du bonnet batholith in addition 28 groundwaters from piezometers in a large wetland area near the url have been sampled and analysed to determine the influence of clay rich overburden on the bedrock hydrogeochemistry analyses have been made for major and minor ions ph eh trace metals and stable and radioactive isotopes to characterise these groundwaters and relate them to their hydrogeologic regimes shallow groundwaters in the fractured bedrock are generally dilute tds 500 m na ca cl so4 waters of increasing salinity up to 50 g l with depth are found and in some fractures the waters have evolved to a ca na cl composition isotopic data indicate that these waters are warm climate and pre glacial in origin with residence times of over 1 ma pore fluids observed to drain from the unfractured rock matrix in the url facility are almost pure ca cl in composition 90 g l salinity and have a 2h 18o composition displaced well to the left of the global meteoric water line about which all other wra groundwaters lie this information indicates that these pore fluids have undergone prolonged water rock interaction and have residence times of 101 103 ma most of the deeper fracture groundwaters and pore fluids have low br cl ratios and moderate to high 34s values of dissolved so4 which indicates that their salinity could be derived from a marine source such as the basinal sedimentary brines and evaporites to the west of the batholith these fluids may have entered the batholith during early paleozoic times when sedimentary rocks were deposited over the granite and were driven by a hydraulic gradient resulting from higher ground in western canada the hydrogeochemical data and interpretations show that below 500 m in the wra fracture hosted groundwaters are very saline reducing and old and are therefore indicative of stagnant conditions over the period of concern for nuclear waste disposal 1 ma the intact rock matrix at these depths is extremely impermeable as indicated by the presence of pore fluids with unusual geochemical and isotopic characteristics the pore fluids may represent basinal brines that have evolved geochemically and isotopically to their current composition over periods as long as 103 ma

in situ radionuclide transport and preferential groundwater flows at ineel idaho decay series disequilibrium studies uranium and thorium decay series disequilibria in groundwater occur as a result of water rock interactions and they provide site specific natural analog information for assessment of in situ long term migration of radionuclides in the far field of a nuclear waste disposal site in this study a mass balance model was used to relate the decay series radionuclide distributions among solution sorbed and solid phases in an aquifer system to processes of water transport sorption desorption dissolution precipitation radioactive ingrowth decay and recoil isotopes of u 238u 234u th 232th 230th 228th 234th ra 226ra 228ra 224ra and rn 222rn were measured in 23 groundwater samples collected from a basaltic aquifer at the idaho national engineering and environmental laboratory ineel idaho the results show that groundwater activities of th and ra isotopes are 2 4 orders lower than those of their u progenitors which average 1 35 0 40 dpm 238u l with 234u 238u ratios of 1 6 3 0 222rn activities range from 20 to 500 dpm l modeling of the observed disequilibria places the following constraints on the time scale of radionuclide migration and water rock interaction at ineel 1 time for sorption is minutes for ra and th time for desorption is days for ra and years for th and time for precipitation is days for th years for ra and centuries for u 2 retardation factors due to sorption average 106 for 232th 104 for 226ra and 103 for 238u 3 dissolution rates of rocks are 70 to 800 mg l y 4 ages of groundwater range from 10 to 100 years contours of groundwater age as well as spatial patterns of radionuclide disequilibria delineate two north south preferential flow pathways and two stagnated locales relatively high rates of dissolution and precipitation and recoil of 222rn occur near the groundwater recharging sites as well as in the major flow pathways decay of the sorbed parent radionuclides e g 226ra and 228ra on micro fracture surfaces constitutes an important source of their daughter 222rn and 228th activities in groundwater

tectonic and climatic significance of a late eocene low relief high level geomorphic surface colorado new paleobotanical data suggest that in the late eocene the erosion surface which capped the front range colorado was 2 2 3 3 km in elevation which is similar to the 2 5 km present elevation of surface remnants this estimated elevation casts doubt on the conventional belief that the low relief geomorphic surface was formed by lateral planation of streams to a base level not much higher than sea level and that the present deeply incised canyons must represent neogene uplift of colorado description of the surface calculations of sediment volume and isostatic balance and fluvial landsculpting models demonstrate that while the high elevation of the erosion surface was due to tectonic forces its smoothness was mostly a result of climatic factors a sediment balance calculated for the front range suggests that from 2 to 4 km of material were eroded by the late eocene consistent with fission track ages this amount of erosion would remove a significant portion of the 7 km of laramide upper crustal thickening isostatic modeling implies that the 2 2 3 3 km elevation was most likely created by lower crustal thickening during the laramide a numerical model of fluvial erosion and deposition suggests a way that a late eocene surface could have formed at this high elevation without incision a humid climate with a preponderance of small storm events will diffusively smooth topography and is a possible mechanism for formation of low relief high level surfaces paleoclimate models suggest a lack of large storm events in the late eocene because of cool sea surface temperatures in the equatorial region return to a drier but stormier climate post eocene could have caused the incision of the surface by young canyons by this interpretation regional erosion surfaces may represent regional climatic rather than tectonic conditions

modeling the formation of a half graben using realistic upper crustal rheology the evolution of a half graben in a strong upper crust with a byerlee strength envelope underlain by an inviscid substratum has been modeled using elasto viscoplastic finite element analysis the nonlithostatic stresses regions of failure and flexure profiles have been computed as the layer is extended by increments the weaker upper part of the layer fails on stretching and more extensive regions of tensional failure extend progressively downward beneath the downbending half graben and upward from the bottom of the layer beneath the footwall uplift compressional failure occurs locally due to bending or unbending plastic yielding progressively reduces the widths of the half graben and footwall uplift as extension proceeds as the outer hinge line of the half graben migrates toward the fault the plastic deformation reduces the effective elastic thickness of the layer by up to 67 helping to explain the anomalously low observed values associated with normal faulting throughgoing failure in tension eventually occurs beneath the footwall uplift near the fault when subsidence slows down and ceases the strength of the upper crust thus limits the amount of subsidence that can occur for an 8 km thick layer and sediment infill 400 kg m3 less dense than the basement the half graben width decreases from over 80 km at the onset of extension to 42 km when the subsidence has reached its limit of 2 3 km the equivalent elastic layer thickness is then reduced to 3 3 km thicker layers have correspondingly greater widths and limiting depths less dense sediments result in a narrower half graben and smaller limiting depth

simple approach to predicting ground displacements caused by tunnelling in undrained anisotropic elastic soil an attempt to develop a kinematically based approach to predicting ground settlements due to tunnelling is represented by using the more reliable data of the commonly observed settlement troughs rather than imposing the boundary conditions on the tunnel surface which have a high level of uncertainty the anisotropic elastic model used in this work is highly simplified and the resulting analytical solution is of an approximate nature nevertheless it seems to provide good agreement with both the shape and the magnitude of the observed settlement trough based on this solution a modification is proposed for the widely used empirical gaussian curve by correlating the trough width parameter k with the ratio between the two anisotropic shear moduli ghh gvh

transversely isotropic stiffness parameters and their measurement in colorado shale drained stress path triaxial tests and confined torsion tests were conducted on colorado shale core samples to investigate transversely isotropic stiffness parameters at small strain deformation nonlinear plastic behaviour occurred along the primary loading path even at strains less than 0 01 nonlinear hysteric behaviour was only observed during the loading unloading path however the shale material displayed transverse isotropy in deformation and very small or nondetectable poisson s ratios in vertical and horizontal directions this special response alludes to the postulate that the colorado shale could be approximated by a transversely isotropic elasticity model at small strain levels with negligible yielding key words anisotropy elasticity stiffness triaxial test torsion test shale

cenozoic sedimentation and paleotectonics of north central new mexico implications for initiation and evolution of the rio grande rift the rio grande rift is one of the major late cenozoic continental rifts of the world sharing most geophysical geochemical and geological characteristics with other rifts cenozoic evolution of the rift was synchronous with lithospheric plate interactions along and under the western north american margin paleocene eocene laramide primarily amagmatic compression related to flat slab subduction oligocene intermediate to silicic volcanism related to collapse of the slab miocene to present rifting related to complex plate interactions overprinted on the previous history cenozoic paleogeographic and paleotectonic characteristics are consistent with a passive mantle mode of rifting north central new mexico provides a unique opportunity to constrain models for rift initiation and evolution it is one of the few locations within any rift where excellently exposed pre rift and syn rift basin fill has been studied thoroughly enough to allow detailed paleogeographic reconstruction for almost the entire cenozoic cenozoic paleogeography for the study area is summarized as follows 1 eocene 58 37 ma a single amagmatic sedimentary basin el rito galisteo trended northwest southeast with laramide basement uplifts on three sides 2 early to late oligocene 37 28 ma intermediate magmatism with volcaniclastic aprons derived from the san juan and ortiz cerrillos volcanic fields and residual laramide uplifts 3 late oligocene early miocene 28 21 ma initiation of bimodal volcanism with widely dispersed volcaniclastic aprons derived from the predominantly silicic san juan and latir volcanic fields 4 early to middle miocene 21 15 ma continued volcaniclastic dispersal from silicic volcanic centers concurrent with initiation of block faulting to form half grabens internal drainage and erosion of phanerozoic strata and precambrian basement 5 middle to late miocene 15 8 ma continued deepening of half grabens widespread exposure of precambrian terranes formation of complex depositional environments in basin centers and continued bimodal volcanism 6 late miocene to present 8 0 ma concentration of extension in central grabens linked by accommodation zones major bimodal volcanism for example taos plateau and jemez mountains regional uplift and integration of rio grande drainage dispersal patterns petrofacies analysis k ar ages and chemical analyses of volcanic clasts provide details concerning three primary volcanic centers 1 san juan mountains 27 29 m y high k andesite and rhyodacite 2 latir volcanic field 25 28 m y high k andesite and rhyolite and 3 the previously unrecognized servilleta plaza center 22 23 m y latite high k andesite and rhyodacite which may have been a southern extension of the latir field these three volcanic centers provided detritus to the following units respectively 1 esquibel mbr of los pinos fm upper abiquiu fm middle picuris fm bishops lodge mbr of tesuque fm and volcaniclastic units in the northern albuquerque basin zia and abiquiu 2 cordito mbr of los pinos fm and uppermost abiquiu fm and 3 chama el rito mbr of tesuque fm and upper picuris fm use of petrofacies 1 esquibel 2 cordito and 3 plaza simplifies the chaos of stratigraphic nomenclature and promotes regional correlations of poorly exposed units

crustal structure gravity anomalies and heat flow in the southern rio grande rift and their relationship to extensional tectonics abstract the rio grande rift is a neogene structure which has undergone a two phase history of extension and volcanism significant amounts of new data are available in the southern rio grande rift region which is the area of this study the first phase of extension 15 30 ma was of the order of 30 50 and was directed ne sw the second phase 3 10 ma involved extension of about 10 which was directed e w crustal structure in the region of the rift can be mapped using recent seismic and gravity data and the observed thinning associated with the rift is consistent with extension of about 10 however heat flow data in the southern rift cannot be explained simply in terms of extension both the heat flow and uplift history of the area require crustal thickening possibly by intrusion in the lower crust a process not directly related to extension

lithospheric rheology and flexure at artemis chasma venus artemis chasma is an arcuate 2000 km diameter zone of convergence and lithospheric underthrusting on venus inelastic flexure modeling of the topography at artemis combined with observations of trench tectonics allow us to document local temperature gradients below 4 k km 1 and an immense compressive in plane force at the trench lithospheric rheology on venus may be stronger than would be predicted from conventional extrapolations of rock mechanics experiments in particular the brittle surface strength must reach a few tens of megapascals to satisfy the observed lack of flexurally induced surface faulting elastic plate bending models provide an adequate estimate of the bending moment at artemis but they fail to constrain or even recognize the in plane force the inelastic analysis implies an in plane force of the order of 1 1014 n m 1 a potential driving force is thermal thinning of regionally thick lithosphre in the highlands to the north the low heat flux at artemis which is a comparatively young structure is compatible with the notion that venus has experienced a prolonged period of cooling in the last several hundred million years the inference of such exceptionally low thermal gradients embraces three end member possibilities 1 the surface age is 600 ma 2 the lithospheric thermal age is greater than the surface age and or 3 the upper mantle temperature is anomalously low 1550 k

radium content and the 226ra228ra activity ratio in groundwater from bedrock the relative abundance of 226ra and 228ra were determined in the groundwater from 125 drilled wells containing from 0 1 to 51 3 pci l of 226ra the determination of 228ra was carried out with a liquid scintillation counter by measuring only the weakly energetic particles emitted from 228ra thus the interference from the daughter nuclides of 226ra was avoided without specific separation of 228ac the direct measurement of 228ra made the method decisively simpler and faster in terms of the chemistry involved the concentration of 228ra was found to be independent of the amount of 226ra present in the samples the concentrations of 228ra were nearly the same over the whole range of 226ra concentrations and the average sol226ra228ra ratio sharply increased as the 226ra content of water increased the 226ra228ra ratio in the drilled wells varied from 0 3 to 26 abnormally high 226ra228ra ratios were found in areas with known uranium deposits as well as in several drilled wells at other locations the abnormally high 226ra228ra ratios present in groundwater suggest that the radioactivity anomaly is caused by uranium deposits and not by common rocks in samples with a low radioactivity level the average 226ra228ra ratio was slightly below unity corresponding to the typical u th ratio of granite the most common kind of rock in the study area the samples from the rapakivi area proved to be exceptional in that they had a low 226ra228ra ratio independent of the concentration of 226ra

state of disequilibrium between 238u 234u 226ra and 222rn in groundwater from bedrock abstract approximately one thousand drilled wells were investigated for their natural radioactivity the determinations of 238u 234u 226ra and 222rn from 310 samples showed a high state of radioactive disequilibrium between the members of the uranium series present in water the 238 u 226 ra activity ratio usually fell in the range 1 20 and the 238 u 222 rn activity ratio in the range 1 20 10 4 the highest activity ratios being from samples with an elevated uranium content the 234 u 238 u activity ratio varied between 0 76 and 4 67 the most frequent values showing a 60 excess of 234u in the samples most of the 234 u 238 u activity ratios near unity were found in samples with a high uranium content several drilled wells with anomalously high uranium contents were found in southern finland the average 226ra and 222rn contents of these wells were not exceptionally high which suggests high mobility of uranium in groundwater from the areas involved

using radon 222 and radium 226 isotopes to deduce the functioning of a coastal aquifer adjacent to a hypersaline lake in nw iran abstract this study aims to assess the hydrogeochemistry of coastal groundwater the occurrence of 222rn and 226ra and their isotopic response to salinity and associated chemical compositions of groundwater in the coastal urmia aquifer ua at the western side of urmia lake ul the results of the pca show that 87 3 of groundwater chemistry changes are controlled by six principal components the interaction between groundwater and coastal igneous and metamorphic rocks in eastern areas next to the ul results in complex hydrogeochemical conditions than western areas based on correlation of u and salinity some coastal samples display conservative and the others non conservative behaviors differed from most previous studies 226ra and 222rn concentrations in coastal groundwater samples of ua do not show a good correlation with salinity given 10 of groundwater 222rn is originated from host rocks the radon concentrations recorded in the coastal groundwater samples are relatively in range that can effectively be supplied by the local rocks 5 49 bq l results of different chemical and isotopic parameters in this area indicate that there is no direct connection between fresh groundwater and ul saltwater this is because that the hard and thick salty layer in the lakebed acts as an impermeable barrier to prevent the underground hydraulic connection results show that removing the salty layer of ul as an option to progress in rehabilitation program of this lake may result in more hydraulic connection between the lake and groundwater resources in some areas

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

the transport of u and th series nuclides in sandy confined aquifers a comprehensive evaluation of the transport of u th ra and rn nuclides of the 238 u and 232 th decay series in an unconfined sandy aquifer long island ny was conducted groundwater data are compared with results of a theoretical transport model of weathering of aquifer grains and interaction with surface coatings to establish relationships between the concentrations of the radionuclide activities in the water and flow line distance the data provide estimates for geochemical parameters including weathering rates and chemical reactivities in both the vadose zone and the aquifer a theoretical treatment of the transport is presented that considers the reaction between the water and a reactive surface layer it is found that a model with chemical exchange between the surface layer and the water for all species is not valid and that the effects of saturation and irreversible precipitation of th is required the water table shows a relatively wide range in u activities the only element in the u th series for which vadose zone input is significant in the aquifer high weathering of u and recoil inputs of 234 u to the water occur in the upper 3 m of the vadose zone while lower weathering and removal of u from the water occur below the deeper aquifer has variable 238 u activities that can be accounted for by input from the vadose zone and is not a result of non conservative behavior the isotopic composition of u is shown to be directly related to the recoil rate relative to the weathering rate the wide range of 238 u in the aquifer waters is a reflection of diverse vadose zone inputs showing that dispersive mixing is not a dominant effect the higher values of 234 u in the aquifer reflect the recoil weathering input ratios from within the aquifer where the weathering rate is lower than the vadose zone both high u activities and high 234 u values cannot be obtained in the vadose zone or within reasonable flow distances in the aquifer radium isotopes are found to be in exchange equilibrium with the surface layer 224 ra 228 ra and 226 ra have comparable activities throughout the aquifer in the vadose zone the dominant input of ra to groundwater is weathering and recoil as found elsewhere the 222 rn in the water is a large fraction 5 of the rn produced in the aquifer rock this cannot be due to ra precipitation onto surface coatings in the aquifer as supported by present weathering with th in exchange equilibrium with the surface layer it is found that th is saturated in the waters under oxidizing conditions so that the weathering input is irreversibly precipitating onto surfaces however it is shown that under somewhat reducing conditions th activities are much higher and the th u ratio in the solution is approximately that of the rock we propose that under oxidizing conditions the source of rn is a surface coating enriched in 232 th and 230 th this th was precipitated in an earlier phase during rapid dissolution of readily weathered phases that contain 10 of the u th inventory of the rock with the associated u carried away in solution therefore the previously precipitated 230 th and 232 th produce daughter nuclides in the surface coating which are the dominant contributors of ra and rn to the ground water in particular rn is provided by very efficient losses by diffusion or recoil from the surface coating this then does not require recent large recoil losses from the parent rock or the presence of nanopores in the rock the first data of both long lived 232 th and short lived 234 th and 228 th in ground water is reported the th isotope activities indicate that desorption kinetics are slow and provide the first estimate based on field data of the th desorption rate from an aquifer surface the mean residence time of th in the surface coating is 3000 y while in the water it is 1 h ra is in partition equilibrium with the aquifer surface layer however the strong fixation of th on surface coatings is very susceptible to changes in oxidation state as is shown by a comparison of two adjacent aquifers this makes it difficult to define with certainty the retentive characteristics in natural systems in general it is shown that the distributions of naturally occurring nuclides can be used to calculate values for transport parameters that are applicable to the transport of anthropogenic nuclides

implications of carbonate like geochemical signatures in a sandstone aquifer radium and strontium isotopes in the cambrian jordan aquifer minnesota usa water rock interaction in sandstone aquifers can potentially mobilize u and th series radionuclides resulting in elevated levels of radioactive constituents such as radium ra in groundwater previous studies have delineated the sources of ra in groundwater by assuming that 228ra 226ra of groundwater parallels the 232th 238u activity ratio of aquifer solids we present an integrated geochemical and isotopic study of groundwater and aquifer solids for major and trace elements radium 224ra 226ra 228ra strontium isotopes 87sr 86sr and tritium in the cambrian jordan quartz sandstone aquifer of southeastern minnesota usa the jordan aquifer part of the cambrian ordovician aquifer system of the upper midwestern usa is characterized by a wide range of ra activities in groundwater uniform ca mg hco3 major element composition and patterns in 87sr 86sr indicate that interaction with carbonate minerals controls water chemistry at least one contributing sr source is radiogenic yielding 87sr 86sr ratios up to 0 71347 radium 226 activities exhibit especially large variations 1 418 mbq l 1 and the highest radium activities occupy a 100 km long north south region consistent with the carbonate like elemental and sr isotope signatures 226ra activity median 85 mbq l 1 is higher than 228ra median 47 mbq l 1 yielding low 228ra 226ra ratios median 0 5 that would be expected from a lithology preferentially containing u relative to th such as carbonate rocks collectively results indicate the primary source of radium to the aquifer is alpha recoil from aquifer solids rather than external inputs of high ra waters indicated by 1 similar 228ra 226ra ratios in aquifer solids and groundwater including low carbonate like values in the high 226ra samples 2 some association between whole rock 226ra levels and groundwater 226ra activities and 3 groundwater 224ra 228ra averaging near unity furthermore hydrogeologic setting e g presence or absence of overlying carbonate units is inconsistent with systematic inputs of high ra waters from overlying carbonate aquifers the most proximate source of carbonate material in the sandstone aquifer seems to be the calcite and dolomite cement that yields different geochemical and isotopic fingerprints than silicate minerals would impart whole rock 226ra 238u 1 implies that any u enrichment in the carbonate cement occurred at least 350 000 years ago high groundwater ra activities relative to solids are attributable to anoxic conditions that play a role in maintaining inefficient radium removal for a fresh groundwater system indicated by the estimated dimensionless whole rock ra distribution coefficient of 150 920 median 560 coprecipitation of ra into barite is a potential but unquantified ra removal mechanism overall radium in the jordan aquifer provides an example of a naturally occurring trace element of health significance that may be derived not mainly from the quartz sandstone matrix but from carbonate material such as calcite and dolomite cement especially when combined with complementary tracers such as 87sr 86sr this study demonstrates the usefulness of the 228ra 226ra ratio as an indicator of the key mineral phases silicate vs carbonate involved in supplying radium to groundwater

lithospheric necking a dynamic model for rift morphology abstract rifting is examined in terms of the growth of a necking instability in a lithosphere consisting of a strong plastic or viscous surface layer of uniform strength overlying a weaker viscous substrate in which strength is either uniform or decreases exponentially with depth as the lithosphere extends deformation localizes about a small imposed initial perturbation in the strong layer thickness for a narrow perturbation the resulting surface topography consists of a central depression and uplifted flanks the layer thins beneath the central depression the width of the rift zone is related to the dominant wavelength of the necking instability which in turn is controlled by the layer thickness and the mechanical properties of the lithosphere for an initial thickness perturbation with a width less than the dominant wavelength deformation concentrates into a zone comparable to the dominant wavelength if the initial perturbation is wider than the dominant wavelength then the width of the zone of deformation is controlled by the width of the initial perturbation deformation concentrates in the region of enhanced thinning and develops periodically at the dominant wavelength a surface layer with limiting plastic stress exponent n behavior produces a rift like structure with a width typical of continental rifts for a strong layer thickness consistent with various estimates of the maximum depth of brittle deformation in the continental lithosphere the width of the rift is essentially independent of the layer substrate strength ratio for a power law viscous surface layer n 3 the dominant wavelength varies with layer substrate strength ratio to the one third power and is always larger than for a plastic surface layer of the same thickness the great widths of rift zones on venus may be explained by unstable extension of a strong viscous surface layer

the origin of large local uplift in extensional regions large localized uplift is commonly observed in continental regions undergoing extension these observations can be modelled by planar high angle normal faulting of an elastic upper crust overlying an inviscid lower crust isostasy provides the necessary driving force the model quantifies the role of flexural rigidity density variations in the crust and erosion and deposition of sediment

rift flank uplift in tibet evidence for a viscous lower crust analysis of high resolution digital topography across 21 plio pleistocene grabens in central and southern tibet shows that high topography occurs along the graben margins profiles across these marginal highlands fit curves describing the flexure of an elastic plate under vertical loading we interpret the features as flexural rift flank uplifts resulting from regional isostatic compensation of low density grabens the short wavelengths of the uplifts 30 40 km imply a low flexural rigidity for the plateau d 2 6 1020 n m similar to values inferred for the basin and range of western north america in addition the grabens themselves are extremely narrow compared to typical continental rifts taken together these observations suggest the existence of a low viscosity lower crust and suggest that isostatic compensation is taking place within this ductile lower crust simple channel flow calculations suggest that crustal viscosities below 1022 pa s are necessary to accommodate this compensation over reasonable timescales