faulting pattern along slow spreading ridge segments a consequence of along axis variation in lithospheric rheology abstract we present here results from an analogue model designed to test the relative influence of along axis variations of the crustal thickness and of the thermal structure of the lithosphere on the geometry and on the faulting pattern of the axial rift of a slow spreading ridge rheologically calculated layered models are employed using quartz sand and silicone putty as analogues of the brittle and ductile components of the lithosphere respectively two parameters have been analysed in detail 1 the thickness of the brittle layer and 2 the viscosity of the ductile layer this study shows that the thickening of the brittle layer and the increase of the viscosity of the ductile layer bring about a widening of the axial valley associated with an increase of the number of faults a decrease of the depth of the axial valley and of the vertical throw of the faults is observed with an increase of the viscosity of the ductile layer and a decrease of the thickness of the brittle layer these results are consistent with observations along mid atlantic ridge segments for segment ends the fault pattern obtained in the models is similar to that described on both sides of the axis for segments bordered by a zero offset discontinuity and on outside corners for segments bordered by a lateral offset discontinuity our results suggest that the viscosity of the ductile layer plays a more important role in the fault pattern than the thickness of the brittle layer the influence of segment length offset length and temporal variation in thermal input could explain the more or less important along axis variation in the deformation pattern observed along segments of slow spreading ridges

ridge segmentation faulting and crustal thickness in the atlantic ocean the mid atlantic ridge mar between the kane and atlantis fracture zones consists of segments 20 85 km in length1 bull s eye patterns in the mantle bouguer gravity anomaly field2 centred on several segments associated with narrow rift valleys3 have been interpreted as centres of strong mantle upwelling and thick crust here i present a map of normal faults inferred from 5 104 km2of sea beam bathymetry4 along the mar between 28 n and the atlantis fracture zone faults are mapped both on and off ridge using a criterion that distinguishes them from volcanic topography abyssal hill lineations seem to form at the rift valley walls through the growth of the normal faults towards ridge segment ends these faults are more widely spaced with larger throws than those at segment centres and often define the boundaries of the gravity bull s eyes these two different faulting styles which probably reflect changes in lithospheric strength are preserved into the rift mountains large throw faults are highly correlated with the mantle bouguer highs suggesting that amagmatic extension on the large faults contributes to the crustal thinning inferred towards the segment ends

enhancement of uranium 234 in springwaters of aguas da prata sao paulo brazil springwater samples were collected at aguas da prata an important spa in s o paulo state brazil the 238u and 234u isotopes were evaluated for platina paiol villela prata radioativa prata nova boi vitoria and prata antiga springs an inverse logarithmic correlation was found between 234u 238u activity ratio and 222rn content of the studied waters which was related to different solution mechanisms of 222rn and 234u nuclides at the rock water interface a linear correlation also exists between the 234u 238u activity ratios of the studied waters and total dissolved solids ionic strength co2 partial pressure and index of base exchange which indicates that the enhancement of 234u in solution increases with the degree of weathering in the aquifers a higher 234u 238u activity ratio was found to be directly related to a higher value of dissolution rate

234u 238u evidence for local recharge and patterns of ground water flow in the vicinity of yucca mountain nevada usa uranium concentrations and 234u 238u ratios in saturated zone and perched ground water were used to investigate hydrologic flow and downgradient dilution and dispersion in the vicinity of yucca mountain a potential high level radioactive waste disposal site the u data were obtained by thermal ionization mass spectrometry on more than 280 samples from the death valley regional flow system large variations in both u concentrations commonly 0 6 10 g l 1 and 234u 238u activity ratios commonly 1 5 6 are present on both local and regional scales however ground water with 234u 238u activity ratios from 7 up to 8 06 is restricted largely to samples from yucca mountain data from ground water in the tertiary volcanic and quaternary alluvial aquifers at and adjacent to yucca mountain plot in 3 distinct fields of reciprocal u concentration versus 234u 238u activity ratio correlated to different geographic areas ground water to the west of yucca mountain has large u concentrations and moderate 234u 238u whereas ground water to the east in the fortymile flow system has similar 234u 238u but distinctly smaller u concentrations ground water beneath the central part of yucca mountain has intermediate u concentrations but distinctive 234u 238u activity ratios of about 7 8 perched water from the lower part of the unsaturated zone at yucca mountain has similarly large values of 234u 238u these u data imply that the tertiary volcanic aquifer beneath the central part of yucca mountain is isolated from north south regional flow the similarity of 234u 238u in both saturated and unsaturated zone ground water at yucca mountain further indicates that saturated zone ground water beneath yucca mountain is dominated by local recharge rather than regional flow the distinctive 234u 238u signatures also provide a natural tracer of downgradient flow elevated 234u 238u in ground water from two water supply wells east of yucca mountain are interpreted as the result of induced flow from 40 a of ground water withdrawal elevated 234u 238u in a borehole south of yucca mountain is interpreted as evidence that natural downgradient flow is more likely to follow southerly paths in the structurally anisotropic tertiary volcanic aquifer where it becomes diluted by regional flow in the fortymile system

observations of dual slantwise circulations above a cool undercurrent in a mesoscale convective system a series of mesoscale convective systems mcss was observed in the uk during one of the intensive observation periods iop 3 in the convective storm initiation project csip a detailed case study of one of the mcss involving some novel interpretation of single doppler radar data reveals the evolution of stacked slantwise circulations associated with elevated upright convection a feature of this occasion was the existence near the surface of a strong undercurrent of cool air flowing against the direction of travel of the storm the air feeding both the upright and slantwise ascent originated not from near the surface but rather from layers between 1 and 3 km the upright convection was intense at first and was accompanied by a region of dry subsidence surrounding it together with the alternating layers of slantwise ascent and descent within the storm s precipitation area when the elevated upright convection weakened vigorous slantwise motions continued for some time but their slope decreased from the initial 1 in 4 to 1 in 9 the lowermost layer of slantwise descent corresponded to a moderately intense rear inflow jet the rear inflow jet did not penetrate the cool undercurrent and reach the surface instead beneath where it impacted the warm air capping the undercurrent the undercurrent took on the structure of a gravity wave without stagnation the raised head of the undercurrent lifted overlying air of high wet bulb potential temperature sufficiently to overcome the convective inhibition copyright c 2010 royal meteorological society

crustal structure and magmato tectonic processes in an active rift asal ghoubbet afar east africa 2 insights from the 23 year recording of seismicity since the last rifting event 1 the asal ghoubbet ag rift has sustained a major volcano tectonic rifting episode in 1978 and has been subsequently monitored with continuous geodetic and seismological surveys it is therefore an ideal place to study the transient magmato tectonic processes that operate after a rifting episode we examine the space time evolution of 2500 md 2 8 earthquakes recorded in the rift from 1979 to 2001 we focus on the relationships between this seismic activity and both the three dimensional structure of the rift and its postrifting behavior depicted from geodesy the results highlight the major role of the central magmatic system fieale shark bay on the structure seismic activity and overall behavior of the rift from 1978 to 1986 the rift opens at a fast rate yet mainly aseismically the opening is magmatically driven and accommodated since 1986 when the opening rate abruptly decreased the seismicity is concentrated in the central part of the rift and reveals pulses of activity of the central volcanic system these pulses result from that magmatic zone undergoing alternating stretching and inflating episodes thus while the plate driven induced stresses have been rebuilding in the rift since 1987 the rift opening is still essentially accommodated in the axial magmatic zone the ag rift has thus sustained a postrifting unsteady opening over more than 23 years following its stretching episode that transient opening has essentially occurred aseismically and most tectonic faults remain relaxed and locked

satellite radar interferometry 1993 1999 suggests deep accumulation of magma near the crust mantle boundary at the krafla volcanic system iceland 1 deep magma accumulation near the crust mantle boundary 21 km depth at the krafla volcanic system is suggested from insar observations a best fit model derived from four interferograms covering 1993 1999 comprises an opening dike representing plate spreading and post rifting deformation and two mogi sources a mogi source deflating at a rate of 0 3 106 m3 yr coincides with the shallow krafla magma chamber while a deeper inflating mogi source further north at 21 km depth inflates at a rate of 26 106 m3 yr the inflating source is at or near the crust mantle boundary as identified by seismic studies and is interpreted as accumulating magma

evidence of a recent magma dike intrusion at the slow spreading lucky strike segment mid atlantic ridge 1 mid ocean ridge volcanic activity is the fundamental process for creation of ocean crust yet the dynamics of magma emplacement along the slow spreading mid atlantic ridge mar are largely unknown we present acoustical seismological and biological evidence of a magmatic dike intrusion at the lucky strike segment the first detected from the deeper sections 1500 m of the mar the dike caused the largest teleseismic earthquake swarm recorded at lucky strike in 20 years of seismic monitoring and one of the largest ever recorded on the northern mar hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband 3 hz intrusion tremor suggesting a volcanic origin two submersible investigations of high temperature vents located at the summit of lucky strike seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting this magmatic episode may represent one form of volcanism along the mar where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long lived discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems index terms 3035 marine geology and geophysics midocean ridge processes 7280 seismology volcano seismology 8419 8149 tectonophysics planetary tectonics 5475 4259 oceanography general ocean acoustics 9325 information related to geographic region atlantic ocean keywords mid atlantic ridge earthquake hydroacoustic

elastic thickness control of lateral dyke intrusion at mid ocean ridges abstract magmatic accretion at slow spreading mid ocean ridges exhibits specific features although magma supply is focused at the centre of second order segments melts are episodically distributed along the rift toward segment ends by lateral dyke intrusions it has been previously suggested that an along axis downward topographic slope away from the magma source is sufficient to explain lateral dyke propagation however this cannot account for the poor correlation between dyke opening and surface elevation in the 2005 2010 series of 14 dyke intrusions of afar ethiopia using mechanical arguments constrained by both geodetic and seismological observations we propose that the large dykes that initiate near the mid segment magma source are attracted toward segment ends as a result of a thickening of the elastic brittle lithosphere in the along rift direction this attraction arises from the difference of elastic resistance between the segment centre where the lithosphere is thermally weakened by long term focusing of melts and comparatively colder hence stronger segment ends the axial topographic gradient in magmatic rifts may be more likely explained as an incidental consequence of these variations of along axis elastic brittle thickness rather than the primary cause of lateral dyke injections

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

a space time rainfall disaggregation model adapted to sahelian mesoscale convective complexes a model adapted to the disaggregation of rainfields associated to sahelian mesoscale convective complexes mccs at small time 5 min and space 1 10 km scales is presented spatial and temporal disaggregation have been decoupled in order to simplify the problem the average rain depth over the study area for each storm is first disaggregated in space by using the turning band method a geostatistical technique able to generate spatially correlated fields with a known covariance function the method has been adapted in order to simulate non gaussian rainfall fields including in particular a probability of zero rainfall the movement of the rain system is described by mapping the starting time of the rain event as the convective front moves through the domain finally at each location the temporal disaggregation of the storm rain depth is obtained by using a standard hyetogram model reproducing the typical sequence of a convective front followed by a stratiform trail in its present form the only input of the disaggregation model is the average rain depth over the domain which can be provided either by a general circulation model output or a satellite estimate to perform the various steps of the spatial and temporal disaggregation some knowledge on the climatology of the zone is needed over the sahel this knowledge is provided by the epsat niger data set which contains 170 mesoscale convective systems mcss observed over a 4 year period by a dense recording raingauge network and a weather radar in the niamey area half of these mcss were identified as mccs the analysis of the mccs data set allowed the derivation of a probability distribution and a covariance function of the point storm rain depth it also allowed the calculation of statistics for the average speed and direction of storm displacement and the inference of parameters describing the standard hyetograms a complete example of the use of the model is presented showing that it produces realistic rainfall fields perspectives for further development and validation of the model are given in the conclusion

hydro radio chemical relationships in the giant guarani aquifer brazil abstract this investigation was carried out within the parana sedimentary basin and involved the sampling of 78 pumped tubular wells for evaluating the hydrochemistry and radioactivity due to the nuclides 238 u 234 u 222 rn 226 ra and 228 ra in the brazilian part of guarani aquifer several significant correlations were found involving the geostatic pressure for instance specific flow rate temperature dissolved o 2 free co 2 ph redox potential eh conductivity na hco 3 co 3 2 si calcite cl f so 4 2 and b carbonates precipitation was evidenced by inverse correlation between co 3 2 and ca mg sr and ba whereas na exhibited an opposite trend dissolving rather than precipitating with increasing co 3 2 concentration an inverse correlation between co 3 2 and k was found possibly related to the increasing tendency of k to recombine with the thickness of the clayey layers hco 3 played an important role on na ca mg and sr dissolution the dissolved u content and 234 u 238 u activity ratio data were plotted on a two dimensional diagram that was successfully utilized on identifying an unreported zone of u accumulation though not necessarily of economic size and grade the variability in chemical and radionuclides data indicated an important influence of the underlying paleozoic sediments in the composition of waters from guarani aquifer the available data allowed estimate the groundwater residence time by two u isotopes disequilibrium methods values of 45 61 ka were initially calculated depending on the adopted porosity 15 20 but a longer residence time 640 ka was also estimated which is more compatible with the hydraulic conductivity data in guarani aquifer and groundwater flow velocity occurring at milk river aquifer alberta canada such time range agrees with previously reported 14 c ages exceeding 30 ka bp at the more central parts of the parana sedimentary basin

fracturing induced release of radiogenic 4he and 234u into groundwater during the last deglaciation an alternative source to crustal helium fluxes in periglacial aquifers external 4he sources have been invoked to explain 4he concentrations in groundwater greater than those expected from in situ u and th production in a fractured aquifer of ordovician age located in the st lawrence lowlands quebec canada 4he concentrations of up to 4 48 x 10 5 cm3 stp gh20 1 were measured such concentrations are ca 1000 times higher than would be expected from in situ production a concomitant increase in 4he concentration and 234u 238u activity ratio is shown suggesting a common release process in groundwater for 234u and 4he this process has tentatively been identified as glaciation induced rock fracturing following the laurentide ice sheet retreat the resulting increase in exposed grain surface facilitates 234u release by recoil and that of radiogenic 4he by diffusion using a model of helium diffusion from a spherical grain it is shown that rock fracturing facilitated the release of accumulated 4he at rates ranging from 4 2 x 10 10 cm3stp gh20 1 yr 1 to 1 06 x 10 8 cm3stp gh20 1 yr 1 these release rates are between 1 000 and 30 000 times higher than the local u and th steady state production rate of 3 5 1 4 x 10 13 cm3stp grock 1 yr 1 integration of 4he release rates over time yields a radiogenic 4he concentration of between 3 85 x 10 6 cm3stp gh20 1 and 7 12x 10 5 cm3stp gh20 1 in the range of concentrations measured in the st lawrence lowlands fractured aquifers results support the occurrence of a local radiogenic helium source to explain the excesses measured in groundwater without requiring a significant external he crustal flux

fingerprint of the geographic and climate evolution of the baltic white sea region in the late pleistocene holocene in groundwater stable isotopes 2h 18o abstract the isotopic studies of atmospheric precipitations snow surveys surface and groundwaters which were conducted in 2005 2017 in the baltic sea white sea region are presented groundwater formed during the last cold period under the influence of the eemian sea and the baltic ice lake and also cryogenic metamorphism was discovered it was discovered that some relatively small rivers pose as regional drains for groundwater of a significant age such reaction of groundwater on the geographic and climatic changes is a consequence of the specific geological and tectonic structure of area where the baltic shield is contact with artesian basins and fractured crystalline rocks begins to be covered by the platform sediments

neutral buoyancy and the structure of mid ocean ridge magma reservoirs the mechanical structure of the regions for shallow storage and lateral intrusion of mid ocean ridge basalt morb is explored in part by examining the density range appropriate for picritic to tholeiitic liquids and liquid crystal mixtures and its relationship to the nonlinear density depth structure of the ridge magma density spans the range 2 6 to at least 2 82 mg m 3 and incorporates the effects of fractional crystallization on the density evolution the incorporation of olivine phenocrysts and the contributions of water in melt phase density reductions the melt and melt crystal density band crosses the in situ rock density curve over the 1 to 3 km depth interval beneath the volcanic surface within that averaged interval picritic to tholeiitic melt and melt crystal mixtures are in approximate mechanical equilibrium with their surroundings and the interval is referred to as the morb horizon of neutral buoyancy hnb the detailed locations of the transition regions between the negative and neutral buoyancy and between the neutral and positive buoyancy regimes are dependent on melt phase composition as well as the presence of suspended mineral phases and their identity in the east pacific rise the hnb has a 1 1 correspondence with the inferred location of the sheeted dike complex and the seismically detected minimum compressional wave velocity region in ophiolite complexes the region corresponds to the sheeted dike complex and the uppermost isotropic gabbros above the hnb is the region of negative buoyancy seafloor to an averaged 1 km depth within which negative buoyancy forces may induce the descent of nonvesiculated melt this region corresponds to the pillow basalt sediment sequence the region of positive buoyancy extends from 3 km to the depth of melt generation and picritic melt ascends in dikes and veins of finite height 500 m to 11 km and relatively narrow width 20 mm to 100 cm the finite strengths of high temperature fluid weakened rocks mandate finite height melt parcels along the ascent pathway the magmatic source region is therefore never hydraulically connected with the surface nor with the near surface reservoir thus relatively locally integrated buoyancy has a natural role in the ascent of melt from the source region to the region of neutral buoyancy during the final stages of magma ascent hydrothermal fluid circulation enshrouds the hnb in a three dimensional envelope as suggested by the depth extent of alteration mineralogy in ophiolite complexes while hydrothermal cooling may play an important role in arresting the ascent of magma in very low spreading rate environments gravitational equilibrium represents the effectively dominant theme in intermediate and high spreading rate regions such as the east pacific rise elastic crack penetration of rheological transition zones near the hnb identify modes of crack tip morphologic changes as a function of young s modulus ratios e2 e1 above and beneath the transition and affect the style of intrusion while melt buoyancy determines the ultimate equilibrium resting place elastic crack stability considerations relate the vertical magma pressure gradients for tholeiitic pt and picritic pp magmas to the vertically varying gradient in the horizontal stress component h and suggest further subdivision into two horizons of neutral buoyancy one for tholeiitic magma hnbt and one for the picritic liquid olivine mixtures hnbp these intervals are very approximate but are estimated as 600 m to 1400 m and 1400 to 3000 m depth respectively beneath the east pacific rise axis as oceanic lithosphere is created at the rise axis and spreads laterally the horizons of negative buoyancy and neutral buoyancy ride with it and may continue to modulate the subsurface magma dynamics that influence off axis volcanism

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

strain transfer between disconnected propagating rifts in afar we showed before that both the aden and red sea plate boundaries are currently rifting and propagating along two distinct paths into afar through the opening of a series of disconnected propagating rifts here we use new geochronological tectonic and paleomagnetic data that we acquired mostly in the southeastern part of afar to examine the geometry kinematics and time space evolution of faulting related to strain transfer processes it appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected this mechanism implies the rotation about a vertical axis of small rigid blocks along rift parallel faults that are shown to slip with a left lateral component which is as important as their normal component of slip rates of 2 3 mm yr by contrast where rifts do not overlap either a classic transform fault maskali or an oblique transfer zone mak arrasou kinematically connects them the length of the aden red sea overlap has increased in the last 0 9 myr as the aden plate boundary propagated northward into afar as a consequence the first order blocks that we identify within the overlap did not all rotate during the same time span nor by the same amounts similarly the major faults that bound them did not necessarily initiate and grow as their neighboring faults did despite these variations in strain distribution and kinematics the overlap kept accommodating a constant amount of strain 7 to 15 of the extension amount imposed by plate driving forces which remained distributed on a limited number seven or eight of major faults each one having slipped at constant rates 3 and 2 mm yr for vertical and lateral rates respectively the fault propagation rates and the block rotation rates that we either measure or deduce are so fast 30 130 mm yr and 15 38 myr respectively that they imply that strain transfer processes are transient as has been shown to be the case for the processes of tearing rift propagation and strain jumps in afar

simulations of an observed elevated mesoscale convective system over southern england during csip iop 3 simulations of an elevated mesoscale convective system mcs observed over southern england during the convective storm initiation project csip provide the first detailed modelling study of a case of elevated convection occurring in the uk the study shows that many factors can influence the maintenance of elevated deep convection from large scale flow through to surface heating processes and diabatic cooling within the convective system it is also shown that interactions and feedback mechanisms between a stable layer and the storm can act to maintain deep convection the simulation successfully reproduced an elevated mcs above a low level stable undercurrent with a wave in the undercurrent linked to a rear inflow jet rij convection was fed from an elevated 840 hpa source layer with cape of about 350 j kg 1 the undercurrent in the simulation was approximately 1 km deep about half that observed unlike the observed mcs a transition from elevated to surface based convection occurred in the simulation due to the combined effects of a pre existing large scale e gradient advection and surface heating causing the system to encounter increasingly unstable low level air and a shallower stable layer that was more susceptible to penetration by downdraughts the transition to surface based convection was accompanied by the development of cold pool outflow and an increase in system velocity from about 6 to 10 m s 1 diabatic cooling from microphysical processes in the simulation enhanced the undercurrent and strengthened the rij this strengthened the wave in the undercurrent and led to more extensive convection the existence of a positive feedback process between the convection rij and stable layer is discussed uncertainty in the synoptic scale generating errors in the undercurrent is shown to be a major source of error for convective scale forecasts

long term low latitude and high elevation cosmogenic 3he production rate inferred from a 107 ka old lava flow in northern chile 22 s 3400 m a s l abstract available geological calibration sites used to estimate the rate at which cosmogenic 3he is produced at the earth s surface are mostly clustered in medium to high latitudes moreover most of them have exposure histories shorter than tens of thousands of years this lack of sites prevents a qualitative assessment of available production models used to convert cosmogenic 3he concentrations into exposure ages and or denudation rates it thus limits our ability to take into account the atmospheric geomagnetic and solar modulation conditions that might have affected the production of cosmogenic nuclides in the past for longer exposure histories and in low latitude regions we present the cosmogenic 3he production rate inferred from a new geological calibration site located in northern chile five samples were collected on the surface of the largest and best preserved lava flow of the san pedro volcano 21 934 s 68 510 w 3390 m a s l which displays pristine crease structure features 40ar 39ar dating yields a reliable plateau age of 107 12 ka for the eruption of this lava flow eight pyroxene aliquots separated from the surface samples yield a weighted average cosmogenic 3he concentration of 99 3 1 2 mat g 1 from which a local cosmogenic 3he production rate of 928 101 at g 1 yr 1 is calculated the local production rate is then scaled to a sea level high latitude slhl reference position using different combinations of geographic spatialization schemes atmosphere models and geomagnetic field reconstructions yielding slhl production rates between 103 11 and 130 14 at g 1 yr 1 consistent with the most recent estimates available from the literature finally we use the same scaling frameworks to re evaluate the mean global scale cosmogenic 3he production rate in olivine and pyroxene minerals at 120 16 at g 1 yr 1 from the compilation of previously published calibration datasets

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

fluid controlled faulting process in the asal rift djibouti from 8 yr of radar interferometry observations the deformation in the asal rift djibouti is characterized by magmatic inflation diking distributed extension fissure opening and normal faulting an 8 yr time line of surface displacement maps covering the rift constructed using radar interferometry data acquired by the canadian satellite radarsat between 1997 and 2005 reveals the aseismic behavior of faults and its relation with bursts of microseismicity the observed ground movements show the asymmetric subsidence of the inner floor of the rift with respect to the bordering shoulders accommodated by slip on three of the main active faults fault slip occurs both as steady creep and during sudden slip events accompanied by an increase in the seismicity rate around the slipping fault and the fieale volcanic center slip distribution along fault strike shows triangular sections a pattern not explained by simple elastic dislocation theory these observations suggest that the asal rift faults are in a critical failure state and respond instantly to small pressure changes in fluid filled fractures connected to the faults reducing the effective normal stress on their locked section at depth

the u isotopes modeling in aquifers from pocos de caldas plateau brazil u modeled groundwater residence times were estimated by changes in 234u 238u activity ratio ar and u content of groundwaters from pocos de caldas city situated at the pocos de caldas alkaline massif brazil the estimated ages are more realistic than others generated by the use of hydraulic conductivity data and available information about the weathering rate of rocks in the plateau the 234u 238u ar and reciprocal of the dissolved u content data also defined a ternary plot that allowed the calculation of the relative volume of three source waters for the mixed water such u isotopes modeling for mixing calculations indicated that the dominant phase 68 in the mixture is thermal

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

a study of uranium and thorium series isotopes in chalk groundwaters of lincolnshire u k a groundwater investigation is described in which uranium and thorium series radionuclides are applied to the study of the hydrogeologically well documented lincolnshire chalk aquifer system of north lincolnshire england the dominance of eh in controlling the uranium content of groundwaters is discussed and the 230th234u activity ratio used as an indicator of the speciation of uranium in highly reduced waters the 230th234u activity ratio approaches unity suggesting that the uranium in solution is behaving similarly to thorium in the 4 valence state in the oxidised environment the higher uranium content groundwaters exhibit lower 234u238u activity ratios a model of mixing young low 234u238u activity ratio fissure waters with old high 234u238u activity ratio pore waters is discussed but rejected it is considered that preferential dissolution of 234u in the lower rate weathering regimes produces the observed relationship between uranium content and activity ratio a method of in situ rn random release factor determination is suggested however the calculated values for rn are disproportionately high for a fissure flow system this along with evidence of a radium depleted aquifer matrix indicates that radium is being mobilised and transported from the pore water system to the fissure system and subsequently adsorbed by clay minerals lining the chalk fissures

why meter wide dikes at oceanic spreading centers abstract numerical models show that maximum dike width at oceanic spreading centers should scale with axial lithospheric thickness if the pre diking horizontal stress is close to the andersonian normal faulting stress and the stress is fully released in one dike intrusion dikes at slow spreading ridges could be over 5 m wide and maximum dike width should decrease with increasing plate spreading rate however data from ophiolites and tectonic windows into recently active spreading ridges show that mean dike width ranges from 0 5 m to 1 5 m and does not clearly correlate with plate spreading rate dike width is reduced if either the pre diking horizontal stress difference is lower than the faulting stress or the stress is not fully released by a dike partial stress release during a dike intrusion is the more plausible explanation and is also consistent with the fact that dikes intrude in episodes at iceland and afar partial stress release can result from limited magma supply when a crustal magma chamber acts as a closed source during dike intrusions limited magma supply sets the upper limit on the width of dikes and multiple dike intrusions in an episode may be required to fully release the axial lithospheric tectonic stress the observation of dikes that are wider than a few meters such as the recent event in afar indicates that large tectonic stress and large magma supply sometimes exist

the role of evaporating showers in the transfer of sting jet momentum to the surface the most damaging winds in a severe extratropical cyclone often occur just ahead of the evaporating ends of cloud filaments emanating from the so called cloud head these winds are associated with low level jets lljs sometimes occurring just above the boundary layer the question then arises as to how the high momentum is transferred to the surface an opportunity to address this question arose when the severe st jude s day windstorm travelled across southern england on 28 october 2013 we have carried out a mesoanalysis of a network of 1 min resolution automatic weather stations and high resolution doppler radar scans from the sensitive s band chilbolton advanced meteorological radar camra along with satellite and radar network imagery and numerical weather prediction products we show that although the damaging winds occurred in a relatively dry region of the cyclone there was evidence within the llj of abundant precipitation residues from shallow convective clouds that were evaporating in a localized region of descent we find that pockets of high momentum were transported towards the surface by the few remaining actively precipitating convective clouds within the llj and also by precipitation free convection in the boundary layer that was able to entrain evaporatively cooled air from the llj the boundary layer convection was organized in along wind rolls separated by 500 to about 3000 m the spacing varying according to the vertical extent of the convection the spacing was greatest where the strongest winds penetrated to the surface a run with a medium resolution version of the weather research and forecasting wrf model was able to reproduce the properties of the observed llj it confirmed the llj to be a sting jet which descended over the leading edge of a weaker cold conveyor belt jet

rift zone development around a plume related magma centre on the isle of skye scotland a model for stress inversions using the example of the isle of skye in scotland we investigate the influence of pressure variations in upper crustal magma reservoirs on the development of rift type normal faulting around central volcanoes the regional synmagmatic stress regime is of strike slip type in scotland during the lower tertiary during a prolonged period of overall high pressure in the skye magma reservoir gabbro intrusion stage crustal extension results from the injection of basaltic dykes parallel to the trend of the far field maximum stress during a subsequent period of pressure decrease in the reservoir granites intrusion stage normal faults trending parallel to the dykes are initiated these faults tilt the upper crustal blocks along with the former dyke swarm and associated lava pile finite element modelling shows that a decrease of magma pressure in a circular cavity may lead as in skye to a change from a regional strike slip to a local rift type normal stress regime

234u 238u 230th 232th systematics in saline groundwaters from central missouri saline groundwaters with 4 7 to 26 total dissolved solids were sampled from springs and artesian wells in mississippian and ordovician carbonates and sandstones in central missouri u th isotopic variations provide a means of evaluating processes of water rock interaction and fluid mixing and estimating the time scales of element transport recently developed mass spectrometric techniques are used to make isotopic measurements on small volume groundwater samples 0 1 4 l with high precision e g 5 for 234 u 238 u activity ratios the groundwaters have a wide range of 238 u concentrations 50 10 12 to 200 10 12 g g 234 u 234 u activity ratios 2 15 16 0 232 th concentrations 0 10 10 12 to 33 10 12 g g and 230 th concentrations 0 91 10 17 to 26 10 17 g g unfiltered and filtered 0 4 m 0 1 m aliquots of a saline sample have the same isotopic composition and concentration of u indicating that 234 u and 238 u occur almost entirely as dissolved species the concentration of 232 th is up to seven times lower in filtered vs unfiltered aliquots indicating that 232 th is predominantly associated with particulates in the groundwaters in contrast most of the 230 th is in solution previous geochemical studies indicate that 1 the saline waters originated as meteoric recharge and evolved through halite dissolution reactions with silicates and saline dilute mixing processes during a long distance flow history and 2 interaction with limestone and dolomite aquifer rocks in central missouri has been limited a consistent relationship between u ca and 234 u 238 u activity ratio is observed in the groundwaters and provides constraints on the u ca ratios and 234 u 238 u activity ratios of end member reservoirs and on the processes of isotopic exchange in this water rock system model calculations that simulate 1 saline dilute groundwater mixing and 2 limited extents of dissolution of carbonate aquifer minerals by the groundwaters can account for the variations in u ca 234 u 238 u and 18 o 16 o in the suite of water samples the model calculations demonstrate that dissolved u isotopic compositions can be a sensitive indicator of water rock interaction which in turn limits the usefulness of 234 u 238 u disequilibria for groundwater age determinations the concentration of dissolved 230 th in the groundwaters is 1 two to three orders of magnitude below 230 th 234 u equilibrium activity levels and 2 significantly in excess of concentrations estimated for the supply of th to solution via desorption and dissolution a model involving the derivation of the excess 230 th from the in situ decay of dissolved 234 u in the groundwaters indicates the operation of an adsorption mechanism on the time scale of 10 10 3 years the results reported here may have broader application to the assessment and management of hazardous chemical species in natural environments

on the mesoscale structure of surface wind and pressure fields near tornadic and nontornadic cold fronts abstractobservations from a mesoscale network of automatic weather stations are analyzed for 15 u k cold fronts exhibiting narrow cold frontal rainbands ncfrs seven of the ncfrs produced tornadoes a time compositing approach is applied to the minute resolution data using the radar observed motion vectors of ncfr precipitation segments interpolated onto a 5 km grid the analyses resolve much of the small mesoscale structure in surface wind temperature and pressure fields postfrontal winds varied substantially between cases tornadic ncfrs exhibited a near 90 wind veer and little or no reduction in wind speed on ncfr passage these attributes were generally associated with large vertical vorticity horizontal convergence and vorticity stretching at the ncfr nontornadic ncfrs exhibited smaller wind veers and or marked decreases in wind speed across the ncfr and weaker vorticity convergence and vorticity stretching in at least four tornadic ncfrs increases in vorticity stretching preceded tor

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

strain accommodation by slow slip and dyking in a youthful continental rift east africa continental rifts begin and develop through repeated episodes of faulting and magmatism but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented in highly evolved rifts tensile stresses from far field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions1 2 3 these rifting crises are rarely observed in thick lithosphere during the initial stages of rifting here we show that most of the strain during the july august 2007 seismic crisis in the weakly extended natron rift tanzania was released aseismically deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping this event provides compelling evidence for strain accommodation by magma intrusion in addition to slip along normal faults during the initial stages of continental rifting and before significant crustal thinning

radioelements radiogenic helium and age relationships for groundwaters from the granites at stripa sweden abstract the solution of radioelements and radiogenic 4he by groundwaters in fractured rocks is dependent upon the radioelement distribution in the rock matrix and the extent of the rock water interface the 234 u 238 u activity ratio and the dissolved u rn and he contents of such groundwaters respond to changes in the flow regime with time although 234 u 238 u activity ratios change with groundwater residence time as a consequence of 234th recoil induced solution of 234u the activity ratio is strongly influenced by the u distribution within fractures by the extent of the rock water interface and by the amount of 238u in solution a model for the quantitative evaluation of these effects is presented groundwaters from depths up to 880 m in the stripa granite have variable dissolved uranium contents and 234 u 238 u activity ratios the uranium geochemistry is primarily determined by variations in flow path rather than by groundwater age dissolved radiogenic 4he in the groundwaters increases with their depth of origin and is dependent upon the u content of the granite and upon its fracture porosity it increases with groundwater residence time but movement of 4he by diffusion and transport processes make the actual groundwater age indeterminate

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

the behavior of dissolved uranium in groundwaters of the morro do ferro thorium deposit brazil groundwater samples were collected for several months in boreholes drilled at morro do ferro a thorium and rare earth deposit located on the pocos de caldas plateau minas gerais state brazil an aquifer system has developed in the weathered mantle due to in situ intense alteration the weathered zone includes a thick argillaceous laterite greater than 100 m thick the u content and 324u 238u activity ratio were measured in the groundwater samples and in spoil samples of a borehole drilled in the ore body some possible mechanisms related to the mobilization of uranium are considered such as complexation with humic substances and adsorption by clays

uranium isotopes as tracers of groundwater evolution in the complexe terminal aquifer of southern tunisia abstract the complexe terminal ct multi layer aquifer is formed by neogene paleogene sand deposits upper senonian campanian maastrichtian limestones and turonian carbonates the chemical composition and isotopes of carbon and uranium were investigated in groundwater sampled from the main hydrogeological units of the ct aquifer in southern tunisia we paid special attention to the variability of uranium contents and isotopes ratio 234u 238u to provide a better understanding of the evolution of the groundwater system uranium concentrations range from 1 5 to 19 5 ppb typical of oxic or mildly reducing conditions in groundwaters the lowest concentrations are found southeast of the study area where active recharge is supposed to take place when looking at the isotope composition it appears that all the samples including those from carbonate levels are in radioactive disequilibrium with significant 234u excess a clear cut distinction is observed between turonian and senonian carbonate aquifers on the one hand with 234u 238u activity ratios between 1 1 and 1 8 and the sandy aquifer on the other hand showing higher ratios from 1 8 to 3 2 the distribution of uranium in this complex aquifer system seems to be in agreement with the lithological variability and are ultimately a function of a number of physical and chemical factors including the uranium content of the hosting geological formation water rock interaction and mixing between waters having different isotopic signatures significant relationships also appear when comparing the uranium distribution with the major ions composition it is noticeable that uranium is better correlated with sulfate calcium and magnesium than with other major ions as chloride or bicarbonate the 14c activities and 13c values of dic cover a wide range of values from 1 1 pmc to 30 2 pmc and from 3 6 to 10 7 respectively 14c model ages estimated by the fontes and garnier model are all younger than 22 ka and indicate that the recharge of ct groundwater occurred mainly during the end of the last glacial and throughout the holocene

what controls the along strike slopes of volcanic rift zones we investigate the dynamics of viscous pressure losses associated with lateral magma transport in volcanic rift zones by performing 1 coupled elastic hydrodynamic simulations of downrift magma flow in dikes and 2 analog experiments mimicking lateral dike propagation in the presence of an along rift topographic slope it is found that near source eruptions are likely to be favored by shallow slopes while distant downrift eruptions may be encouraged by steeper slopes provided that along rift variations in the tectonic stress are negligible or uncorrelated on the timescale of multiple dike intrusions this implies the existence of a critical slope to which a volcanic rift zone would naturally evolve such behavior is produced by three dimensional 3 d elastic effects and is controlled by the ratio of the driving pressure gradient due to the along strike topographic slope to the vertical gradient in the excess magma pressure in the dike this model may be viewed as complementary to commonly cited mechanisms that appeal to magma viscosity and the dynamics of freezing of lava flows at the surface to explain the low profiles of basaltic shield volcanoes our estimated values of the critical slopes are in general agreement with observations in hawaiian rift zones but further development of fully 3 d models is required for more accurate predictions

234u and238u concentration in brine from geopressured aquifers of the northern gulf of mexico basin abstract the 234 u and 238 u concentration in brine from six gulf coast geopressured aquifers has been determined the results reveal very low uranium concentrations from 0 003 to 0 03 g l and uranium activity ratios slightly greater than unity from 1 06 to 1 62 reducing conditions within the aquifers are responsible for the low uranium concentrations the uranium activity ratios observed are well below those calculated using theoretical considerations of alpha particle recoil effects this can be explained by interference with alpha recoil nuclides entering the liquid phase as a result of quartz overgrowths on sand grains and high temperature re equilibration that tends to minimize the effects of the alpha recoil process the fact that the uranium activity ratios of the brines are slightly greater than unity instead of the equilibrium value of 1 000 indicates that either the alpha particle recoil blocking and re equlibration effects are not complete or that another process is operative that enriches the fluid in excess 234 u by selectively removing uranium from radiation induced damage sites in the mineral sand grain matrix

multi sensor observations of a wave beneath an impacting rear inflow jet in an elevated mesoscale convective system observations of a wave generated below and propagating with an elevated mesoscale convective system mcs are presented the mcs and the wave tracked directly across the 90 km extent of the observational network deployed for the convective storm initiation project csip and maintained a coherent structure and velocity during this 1 5 hour period the rear inflow jet of the mcs was observed to impact upon but not penetrate the stable undercurrent that was located beneath the mcs apparently constricting the depth of the undercurrent which did not participate in the main convective updraught the wave crest was located approximately 15 km ahead of this constriction the overall wave led to between 200 and 1000 m of ascent and approximately 1500 m of descent however undulations were superimposed on the overall wave with a wavelength of approximately 7 km and a crest totrough amplitude of approximately 1000 m these undulations were associated with double maxima in surface pressure and the elevation of the observed low level wind anomaly where low level winds were backed by more than 90 t hese winds were never at rest relative to the system and the wave was therefore a gravity wave without stagnation the hydrostatic effect expected from the observed lifting in the wave was broadly consistent with the observed surface pressure perturbation of 2 hpa no cold pool outflow was ever observed instead the observed surface cooling of approximately 4 c was shown to be consistent with reduced solar insolation the generation of the observed wave structure and its implications for the maintenance of the mcs and numerical modelling of the system are discussed copyright c

the u contents and 234u 238u activity ratios of dissolved uranium in groundwaters from some triassic sandstones in england abstract the isotopic compositions of dissolved uranium in groundwaters of age up to 36 000 a from the east midlands triassic sandstone and in saline fluids from deep geothermal boreholes in the wessex basin have been investigated groundwaters from the east midlands have u contents which lie on a normal distribution and average 0 75 g kg 1 with a standard deviation of 0 4 g kg 1 the 234 u 238 u activity ratios for the dissolved u in these groundwaters have minimum values between 3 and 4 some groundwaters have further enhancement of the 234 u 238 u activity ratio up to 7 but this is not related to groundwater age it is shown that uranium solution is primarily controlled by carbonate solution involving the cement phase of the sandstone and that the 234 u 238 u activity ratios are the results of preferential chemical etch of 234 u relative to 238 u an eh ph stability field diagram calculated for the average groundwater hco 3 contents in the aquifer shows that the uranyl carbonate complexes uo 2 co 3 2 2 and uo 2 co 3 3 4 stabilise uranium solution in the east midlands groundwaters in the wessex basin the redox conditions indicate that uranium deposition should occur and this is reflected in the very low u content of the geothermal fluids g kg 1 the residence time of these thermal waters probably exceeds 15 ma and a steady state 234 u 238 u activity ratio due to alpha recoil induced solution of 234 u should therefore have been attained the activity ratio 2 0 is very much less than the theoretical steady state ratio 10 20 for recoil processes in the interstitial fluids of the sandstone it is concluded that u in the interstitial surfaces must have been sealed against alpha recoil processes by a phase of low u content if the thermal water has evolved from groundwater similar to that in the east midlands triassic sandstone decay of excess 234 u must have occurred the time required for this decay is at least 0 25 ma if no alpha recoil solution of 234 u occurs and is 1 25 ma if an alpha recoil steady state has been attained the general implications of the study on the use of 234 u 238 u activity ratio changes for the estimation of groundwater residence times are discussed

the relationship between buoyant mantle flow melt migration and gravity bull s eyes at the mid atlantic ridge between 33 n and 35 n abstract crustal thickness and mantle bouguer anomaly mba patterns along the mid atlantic ridge mar between the oceanographer and hayes transforms 33 35 n are predicted from a three dimensional mantle flow model driven by plate spreading and mantle density variations assuming a two or three dimensional model of melt extraction our two dimensional melt extraction model focuses melt to the nearest ridge segment by assuming that melt moves only vertically and parallel to the spreading direction in contrast the three dimensional model assumes melt migrates vertically to the top of the melting region and then up the local slope of this surface the shape of this surface which is controlled by enhanced conductive cooling at ridge offsets directs melt toward the center of segments our results suggest that buoyant mantle flow driven by temperature variations and a small retained melt fraction

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

structure and evolution of a mesoscale convective system near the british isles frequent digital cloud imagery from meteosat and precipitation patterns from radar have been analysed together with surface observations and conventional and satellite soundings to develop a conceptual model of a small mesoscale convective system the system observed to the south west of england had many of the features previously found in the united states where the term mesoscale convective system or complex is applied to a cluster of thunderstorms that generates a common cirrus shield of mesoscale dimensions a mesoscale region of fairly uniform essentially stratiform rain developed beneath the cirrus shield downwind of the active convection thunder was widespread throughout the precipitation area which was also characterized by a surface mesohigh the mesoscale convective system and another similar one before it lasted for about half a day each during which time they produced large falls of rain over an extensive area the mesoscale convective systems developed in a region of locally strong baroclinicity on the eastern flank of a cut off low they happened to form over the sea and were decoupled from the surface by a cool easterly boundary layer flow their development could be traced to the onset of advection from france of warm moist air with a wet bulb potential temperature in excess of 18 c at 850 mb the arrival of this air coincided with the intensification of the se ly flow around the approaching cut off low

a comparative study of aquifer systems occurring at the parana sedimentary basin brazil u isotopes contribution the radioactivity due to 238u and 234u in three aquifer systems occurring within the parana sedimentary basin south america has been investigated uranium is much less dissolved from fractured igneous rocks than from the porous sedimentary rocks as indicated by the u mobility coefficients between 7 6 10 6 and 1 2 10 3 g cm 3 these values are also compatible with the u preference ratios relative to na k ca mg and sio2 which showed that u is never preferentially mobilized in the liquid phase during the flow occurring in cracks fissures fractures and faults of the igneous basaltic rocks experimental dissolution of diabase grains on a time scale laboratory has demonstrated that the u dissolution appeared to be a two stage process characterized by linear and second order kinetics the u dissolution rate was 8 10 16 mol m 2 s 1 that is within the range of 4 10 16 3 10 14 mol m 2 s 1 estimated for other rock types the 234u 238u activity ratio of dissolved u in solutions was higher than unity a typical result expected during the water rock interactions when preferential 234u leach from the rock surfaces takes place some u isotopes data allowed estimating 320 ka for the groundwater residence time in a sector of a transect in sao paulo state a modeling has been also realized considering all u isotopes data obtained in bauru 35 samples serra geral 16 samples and guarani 29 samples aquifers the results indicated that the bauru aquifer waters may result from the admixture of waters from guarani 1 5 and serra geral 98 5 aquifers

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

sequence of rifting in afar manda hararo rift ethiopia 2005 2009 time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling thirteen dike intrusions in the manda hararo rift afar ethiopia from september 2005 to june 2009 studied using an extensive interferometric synthetic aperture radar insar data set provide insight into the mechanics of a major active rift kinematic inversions of insar data reveal that dikes opened by 0 8 3 5 m at an average 5 km depth with volumes of 0 04 0 2 km 3 with up to 12 m opening and a volume greater than 1 km 3 for the september 2005 megadike dikes have their source in a midsegment magma reservoir which induces a local shallowing of the brittle ductile boundary presumably due to thermal weakening of the lithosphere the smaller dikes in 2006 2009 were emplaced in regions of minimum opening of the september 2005 megadike above the central magma reservoir in contrast the most voluminous dike intrusions in 2006 2009 occurred near the locus of the peak of maximum opening of the september 2005 megadike 10 km north of the magma source this may suggest that tension on the plate boundary was highest there both prior to 2005 and possibly also after 2005 evolution and distribution of normal stress on the plate boundary throughout the rifting episode may indicate that tension near to the magma reservoir is lower than toward segment ends average relief of normal stresses of tectonic origin coeval to dike intrusions is comparable with shear stress drops for earthquakes presumably because dikes in the manda hararo rift are intruded at low magma pressure and high tectonic stress time space evolution and interactions between dikes from interferometric synthetic aperture radar and static stress change modeling

effects of the crustal stress on the growth of dikes conditions of intrusion and extrusion of magma dike growth is analyzed on the basis of the fluid dynamical behavior of incompressible newtonian magma in a spatially uniform fissure of temporally variable width the length and width of the dike vary with the magma pressure at the dike entrance connected to the magma chamber and with the mean stress working normal to the dike walls the mean stress consists of the regional crustal stress and the mean elastic stress which is approximately evaluated using an analytic solution of the crack with a constant aperture the length and width of the dike are determined as a function of time by simple ordinary differential equations the dike can grow when the initial pressure of the entering magma is above the lithostatic pressure and high enough to suppress compression of the crustal stress the assumed size of the initial crack affects the growth only in the duration needed for an initial adjustment stage after this stage the width and length first grow exponentially and then the growth slows as magma is released from the chamber the growth in the later stage is classified into two types when the initial magma pressure is high enough the dike elongates unlimitedly while the width finally decreases this extrusive growth is likely to lead to eruptions the other intrusive growth in which the dike finally stops its growth with a finite length and width occurs when the crustal stress component normal to the fissure is of sufficiently strong extension buoyancy of magma is not very effective in the earlier growth stage but it shifts the marginal condition separating the extrusive and intrusive growths the extrusive and intrusive events that recently happened in izu oshima volcano and izu tobu volcano group japan are interpreted as the examples of these two growth types

hydrochemical evolution of the east midlands triassic sandstone aquifer england a detailed hydrogeochemical interpretation has been made of the red bed sherwood sandstone triassic aquifer of the east midlands of england on which a radiocarbon model has been used to establish a chronology of groundwaters along the flow line groundwater evolution is dominated by carbonate and sulphate mineral solution and precipitation and by redox reactions cation exchange reactions are negligible and the hydrogeochemistry has evolved in the absence of residual saline water three distinct zones are recognised 1 i recent groundwater mainly 100 y oxidising dominated by congruent dissolution of dolomite and by inputs from atmospheric and anthropogenic sources 2 ii groundwater age 103 104 y predominantly reducing dominated by incongruent dissolution of dolomite 3 iii groundwater age 104 y reducing conditions approach to isotopic and chemical equilibrium in the carbonate system dominance of gypsum dissolution the minor and trace element variation can be interpreted in relation to the principal geochemical reactions and recharge history na cl f and b are controlled by atmospheric inputs sr2 and mn2 trends can be explained by carbonate and sulphate mineral dissolution ba2 is derived from carbonates but controlled by barite equilibrium fe so42 no3 and u are strongly influenced by redox reactions the concentrations of transition metals and other elements remain very low only li and k are considered to represent additions by reaction of silicate minerals the extent of direct anthropogenic influence is probably limited to enhanced so4 no32 and possibly cu zn and pb in zone i

processes controlling 234u and 238u isotope fractionation and helium in the groundwater of the st lawrence lowlands quebec the potential role of natural rock fracturing abstract the goal of this study is to explain the origin of 234 u 238 u fractionation in groundwater from sedimentary aquifers of the st lawrence lowlands quebec canada and its relationship with 3 he 4 he ratios to gain insight regarding the evolution of groundwater in the region 234 u 238 u activity ratios or 234 u 238 u act were measured in 23 groundwater samples from shallow quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the becancour river watershed the lowest 234 u 238 u act 1 14 0 01 was measured in ca hco 3 type freshwater from the quaternary shallower aquifer where bulk dissolution of the carbonate allows u to migrate into water with little 234 u 238 u isotopic fractionation the 234 u 238 u act increases to 6 07 0 14 in na hco 3 cl type groundwater preferential migration of 234 u into water by recoil is the underlying process responsible for this isotopic fractionation an inverse relationship between 234 u 238 u act and 3 he 4 he ratios has been observed this relationship reflects the mixing of newly recharged water with 234 u 238 u act close to the secular equilibrium and containing atmospheric tritiogenic helium and mildly mineralized older water 14 c ages of 6 6 kyrs with 234 u 238 u act of 6 07 and large amounts of radiogenic 4 he in excess of the steady state amount produced in situ the simultaneous fractionation of 234 u 238 u act and the addition of excess 4 he could be locally controlled by stress induced rock fracturing this process increases the surface area of the aquifer matrix exposed to pore water from which produced 4 he and 234 u can be released by recoil and diffusion this process would also facilitate the release of radiogenic helium at rates greater than those supported by steady state u th production in the rock consequently sources internal to the aquifers could cause the radiogenic 4 he excesses measured in groundwater

uranium series isotopes in the groundwater of the permo triassic sandstone aquifer lower mersey basin u k abstract an investigation of u and th series isotopes in rock and groundwater samples from a permo triassic sandstone aquifer of northwest england is described the groundwaters sampled range in age and composition from fresh and modern to highly saline and ancient with at least two sources of salinity highest u and th concentrations are present in the mudrock beds within the sandstone sequences the 234u 238u and 226ra 230th rock activity ratios suggest that saline waters were once much more extensive throughout the aquifer and have been flushed by several volumes of fresh groundwater since the last glaciation uranium contents and activity ratios confirm that the isotope signatures observed in most of the sampled groundwaters are not the result of simple conservative mixing relations between high u saline waters and low u fresh groundwaters but that the isotopic content of the matrix the matrix mineralogy and fabric the redox conditions the salinity and possibly the carbonate chemistry are all important petrographic studies have demonstrated the multiphase nature of the sandstones and although the isotopic signatures are consistent with the origins of the sampled water as concluded from other hydrochemical and hydrogeological data quantitative modelling of the processes involved is limited by lack of details of u bearing phases pore architecture and the chemistry of u in high ionic strength solutions however ra concentrations are controlled by sorption processes and ra is shown to be mobilised under saline conditions the mechanisms of rn release from the aquifer matrix are reviewed and a rn release factor of 0 11 is calculated this value is believed to be inappropriate as a measure of the recoil supply of the longer lived rn precursors in the u decay series

hydrochemical trends palaeorecharge and groundwater ages in the fissured chalk aquifer of the london and berkshire basins uk the hydrochemical radiochemical stable isotope 14c and dissolved noble gas composition of groundwaters has been determined along two profiles across the confined fissured chalk aquifer of the london basin of southern england and for selected sites in the adjacent berkshire basin during downgradient flow in the london basin aquifer the groundwater chemistry is modified by water rock interactions congruent and incongruent reaction of the carbonate lithology resulting in enhanced mg ca and sr ca ratios and 13c contents with increased residence times redox and ion exchange reactions and towards the centre of the basin mixing with a residual saline connate water stored in the chalk matrix there is evidence from anomalous water chemistries for a component of vertical leakage from overlying tertiary beds into the confined aquifer as a result of historical dewatering of the aquifer dissolved noble gas contents indicate the climate was up to 4 5 c cooler than at present during recharge of the waters now found in the centres of both basins stable isotope 2h and 18o depletions correspond to this recharge temperature change for evolved waters having 13c 8 pdb a negative linear correlation is demonstrated between derived recharge temperatures and 13c values which is interpreted as mixing between relatively warm light isotopic fracture borne waters and cooler stored waters of the matrix having a 13c signature more or less equilibrated with the chalk from geochemical 14c 4he age estimates the abstracted water is interpreted as being either of wholly holocene post devensian glacial origin or an admixture of holocene and late pleistocene pre glacial cold stage interstadial recharge devensian pleniglacial stage waters of the last glacial maximum are not represented

a comparison of rift zone tectonics in iceland and hawaii observations of eroded volcanic rift zones indicate that dikes in iceland are typically several times thicker than those in hawaii geodetic and seismic observations of active rifts however suggest that dike heights in the two regions are similar provided the elastic properties of the rift zones are the same this implies that dikes are intruded with higher driving pressures magma pressure minus compressive stress perpendicular to the dike plane in iceland than hawaii a second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in iceland from this it can be infered that a lower percentage of dikes breach the surface in iceland than in hawaii thus although dikes in iceland are intruded with higher driving pressures they possess lower absolute magma pressures than in hawaii these differences can be interpreted in terms of the tectonic settings in the two regions in iceland a steady remote extension reduces the horizontal stress perpendicular to the rift zone allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available in hawaii a more continuous magma supply on the timescale over which the dike induced stresses are relaxed and perhaps a greater role for intrusions in driving long term rift extension ensure that the rift compressive stress is not relaxed significantly before the next dike is intruded thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur if the mechanism for relaxing the rift compressive stress were less efficient still then an even higher percentage of dikes would erupt and at times the rift zone trend could become an unfavorable orientation for dike intrusion such might be the case at mauna loa which lacks large rift zone faults and fissures and possesses numerous radial vents outside its two main rift zones

234u 238u activity ratios of dissolved uranium in groundwaters from a jurassic limestone aquifer in england abstract uranium contents and234u 238u activity ratios have been determined for groundwaters from the lincolnshire limestone artesian aquifer in eastern england changes in the quantitative and isotopic chemistry of the dissolved uranium are explained in terms of a mixing model involving the rapidly moving fissure water and much older water stored in the pore system of this oolitic limestone the western part of the aquifer closest to recharge is dominated by oxidising groundwaters which then enter a reducing zone towards the east where there is an abrupt decrease in eh and the chlorinity of the groundwaters begins to increase uranium contents in the oxidising zone range from 0 7 to 3 4 g kg 1 and234u 238u activity ratio of this dissolved uranium is close to unity the equilibrium value the uranium content decreases abruptly when the grounwaaters enter the reducing zone averaging 0 04 g kg 1 east of the oxidation reduction barrier simultaneously with the decrease in uranium content there is an increase in234u 238u activity ratio and this ratio increases to a maximum within 7 km of the oxidation reduction barrier this increase in activity ratio is attributed to enhanced234u solution due to234th recoil from uraniferous fissure surfaces east of the oxidising zone the activity ratio of dissolved uranium in the ancient pore waters could in principle reach high values due to234th recoil from the oolith surfaces however the activity ratio actually declines further east and this can only be explained as a consequence of mixing with pore waters in which the uranium activity ratio is closer to equilibrium 234th recoil from the oolith surfaces has probably been inhibited by sealing of the uranium bearing surfaces in the process of oolith cementation

constructing the upper crust of the mid atlantic ridge a reinterpretation based on the puna ridge kilauea volcano the volcanic morphology of a number of segments of the slow spreading mid atlantic ridge mar have been reinterpreted based on our understanding of dike emplacement dike propagation and eruption at the east rift zone of kilauea volcano hawaii and its submarine extension the puna ridge the styles of volcanic eruption at the submarine puna ridge are remarkably similar to those of the axial volcanic ridges avrs constructed on the median valley floor of the mar we use this observation to relate volcanic processes occurring at kilauea volcano to the mar we now consider that volcanic features e g seamounts and lava terraces built on the flanks of the avrs are secondary features that are fed from lava tubes or channels not primary features fed directly from an underlying dike we examine simple models of pipe flow and conclude that lava tubes can transport lava down the flanks of submarine rifts to build all of the volcanic features observed there in addition deep water lava tubes are strong enough to withstand the pressures of a few megapascals that the building of a volcanic structure 150 m high at the end of the tube would generate the volumes of individual volcanic terraces and seamounts on the puna ridge and at the mar are large 0 1 1 km3 and similar to the volumes of lava flows that are broadly distributed at the subaerial east rift zone of kilauea this striking difference in the volcanic morphology on a scale of 1 2 km producing terraces and seamounts underwater and low relief flows on land must be related to the enhanced cooling and to the greater mechanical stability of tubes in the submarine environment we suggest that at the mar a crustal magma reservoir most likely located beneath shallow flat sections of the segment provides magma to the rift axis through dikes that propagate laterally tens of kilometers the zone of dike intrusion at least in the neighborhood of the magma body is likely narrower than the width resurfaced by flows yielding a crustal structure that has a rapid vertical transition from lavas to sheeted dikes at segment ends the zone of dike intrusion is likely to be wider giving a resulting structure with a more gradual transition from lavas to dikes

seismicity during lateral dike propagation insights from new data in the recent manda hararo dabbahu rifting episode afar ethiopia seismicity released during lateral dike intrusions in the manda hararo dabbahu rift afar ethiopia provides indirect insight into the distribution and evolution of tensile stress along this magma assisted divergent plate boundary in this paper 5 dike intrusions among the 14 that form the 2005 present rifting episode are analyzed with local and regional seismic data during dike intrusions seismicity migrates over distances of 10 15 km at velocities of 0 5 3 0 km h away from a single reservoir in the center of the rift segment confirming the analogy with a slow spreading mid ocean ridge segment comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward suggesting an asymmetry of tension in the brittle elastic lithosphere seismicity during dike injections is concentrated near the propagating crack front in contrast faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions the seismic energy released during dike intrusions in the manda hararo rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion the low level of seismic energy accompanying dike intrusions despite their significant volumes is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary

broad accommodation of rift related extension recorded by dyke intrusion in saudi arabia volcanic fields at the eastern margin of the red sea rift were regarded as seismically quiet until a swarm of 30 000 earthquakes struck in 2009 geological analyses reveal the intrusion of a volcanic dyke and indicate that rift related extension is spread over a broad region far from the rift axis

use of u isotopes in exploring groundwater flow and inter aquifer leakage in the south western margin of the great artesian basin and arckaringa basin central australia abstract the distribution of uranium isotopes 238u and 234u in groundwaters of the south western margin of the great artesian basin gab australia and underlying arckaringa basin were examined using groundwater samples and a sequential extraction of aquifer sediments rock weathering the geochemical environment and recoil of daughter products control the 238u and 234u isotope distributions giving rise to large spatial variations generally the shallowest aquifer j aquifer contains groundwater with higher 238u activity concentrations and 234u 238u activity ratios close to secular equilibrium however the source input of uranium is spatially variable as intermittent recharge from ephemeral rivers passes through rocks that have already undergone extensive weathering and contain low 238u activity concentrations other locations in the j aquifer that receive little or no recharge contain higher 238u activity concentrations because uranium from localised uranium rich rocks have been leached into solution and the geochemical environment allows the uranium to be kept in solution the geochemical conditions of the deeper aquifers generally result in lower 238u activity concentrations in the groundwater accompanied by higher 234u 238u activity ratios the sequential extraction of aquifer sediments showed that recoil of 234u from the solid mineral phases into the groundwater rather than dissolution of or exchange with the groundwater accessible minerals in the aquifer caused enrichment of groundwater 234u 238u activity ratios in the boorthanna formation decay of 238u in uranium rich coatings on j aquifer sediments caused resistant phase 234u 238u activity ratio enrichment the groundwater 234u 238u activity ratio is dependent on groundwater residence time or flow rate depending on the flow path trajectory thus uranium isotope variations confirmed earlier groundwater flow interpretations based on other tracers however spatial heterogeneity and the lack of clear regional correlations made it difficult to identify recharge and inter aquifer leakage