geochemical mineralogical and microbiological characteristics of sediment from a naturally reduced zone in a uranium contaminated aquifer localized zones or lenses of naturally reduced sediments have the potential to play a significant role in the fate and transport of redox sensitive metals and metalloids in aquifers to assess the mineralogy microbiology and redox processes that occur in these zones several cores from a region of naturally occurring reducing conditions in a u contaminated aquifer rifle co were examined sediment samples from a transect of cores ranging from oxic suboxic rifle aquifer sediment to naturally reduced sediment were analyzed for u and fe content oxidation state and mineralogy reduced s phases and solid phase organic c content using a suite of analytical and spectroscopic techniques on bulk sediment and size fractions solid phase u concentrations were higher in the naturally reduced zone with a high proportion of the u present as u iv the sediments were also elevated in reduced s phases and fe ii indicating it is very likely that u vi fe iii and so4 reduction has occurred or is occurring in the sediment the microbial community was assessed using lipid and dna based techniques and statistical redundancy analysis was performed to determine correlations between the microbial community and the geochemistry increased concentrations of solid phase organic c and biomass in the naturally reduced sediment suggests thatmore natural bioreduction is stimulated by a zone of increased organic c concentration associated with fine grained material and lower permeability to groundwater flow characterization of the naturally bioreduced sediment provides an understanding of the natural processes that occur in the sediment under reducing conditions and how they may impact natural attenuation of radionuclides and other redox sensitive materials results also suggest the importance of recalcitrant organic c for maintaining reducing conditions and u immobilization less

seismic anisotropy reveals a dynamic link between adjacent magmatic segments prior to dyke intrusion seismic anisotropy has increasingly been proposed as a tool in the monitoring of magmatic systems and potential forecasting of volcanic eruptions we present a detailed study of how seismic anisotropy evolves in an active magmatic rift segment before during and after a dyke intrusion in the afar depression ethiopia results show that seismic anisotropy prior to the dyke intrusion is controlled by a complex and deforming magma plumbing system beneath the adjacent dabbahu and manda hararo magmatic segments approximately eight days prior to the dyke intrusion in the dabbahu segment the pattern of anisotropy coupled with lower crustal seismicity is best explained by the inflation of a lower crustal magma reservoir in the manda hararo segment this is the only clearly observed precursory change in seismic anisotropy during the dyke intrusion the magnitude of seismic anisotropy increases twofold before rapidly returning to predyke values once the intrusion has ended combining our observations with models of magmatically induced crustal stress we propose that when the deep magma reservoir beneath the dabbahu segment becomes overpressured inflation is triggered in the magma reservoir of the neighboring manda hararo segment this provides strong evidence for a hydraulic link between the deep magma systems of the neighboring rift segments and that magma reservoirs beneath the dabbahu segment can be fed by the lateral flow of magma from an adjacent segment our results demonstrate that seismic anisotropy has the potential to be a powerful tool for monitoring deformation in the magma plumbing systems of active volcanoes

cretaceous tectonic evolution of coastal southern california insights from the catalina schist the relatively high t high p t subduction related basement in the southern california continental borderland preserves important information regarding the late mesozoic tectonic evolution of coastal southern california integrated petrologic geochemical and geochronological study of the subduction related catalina schist exposed on santa catalina island documents the formation of lawsonite blueschist through amphibolite facies rocks during progressive underplating in a progressively cooling newly initiated subduction zone petrologic and 40ar 39ar thermochronologic data demonstrate similar perhaps shared retrograde pt histories for all but the lawsonite albite facies rocks under pumpellyite actinolite facies conditions as early as 95 100 ma the 40ar 39ar data together with the abundance of pumpellyite actinolite facies vein assemblages and lack of higher p t epidote or lawsonite blueschist over printing in high grade units are consistent with the rapid emplacement of the catalina schist into the shallow to mid crust by 95 ma by combined underplating and tectonic erosional removal of overlying rocks in the accretionary complex cf platt 1986 the catalina schist and the adjacent peninsular ranges batholith potentially represent paired metamorphic belts formed along southwest north america at 120 to 90 ma combining the evidence for the evolution of the catalina schist with the previously advanced hypotheses regarding tectonic evolution of the batholith suturing of a fringing island arc to the continent may have triggered an oceanward trench stepout and the formation of a new subduction zone at 120 115 ma in which the catalina schist was accreted and metamorphosed

variations in late syn rift melt alignment inferred from shear wave splitting in crustal earthquakes beneath the ethiopian rift 1 the northern main ethiopian rift mer marks the transition from continental rifting to incipient seafloor spreading we constrain anisotropy of the upper crust in the mer and its uplifted rift flanks using shear wave splitting from 24 earthquakes located beneath 18 broadband stations along the axis of the mer the fast polarization direction is oriented between n and nne parallel to quaternary recent faults aligned cones and maximum horizontal stress delay times are highest 0 24 s where independent seismic studies show evidence of shallow partial melt we attribute anisotropy along the rift axis to aligned melt filled micro cracks and dikes at stations flanking the rift the fast polarization direction is oriented ne and delay times are smaller 0 04 0 14 s the lower amount of anisotropy is consistent with reduced melt away from the rift axis these results show melt induced anisotropy persists into the crust and magma injection localizes and accommodates strain just prior to continental break up

enrichment of trace elements in garnet amphibolites from a paleo subduction zone catalina schist southern california the abundance p t stability solubility and element partitioning behavior of minerals such as rutile garnet sphene apatite zircon zoisite and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones the influence of these minerals on the composition of subduction related magmas has been inferred and disputed from inverse modelling of the geochemistry of island arc basalt or by experiment although direct samples of the dehydration partial melting region of a mature subduction zone have not been reported from subduction complexes garnet amphibolites from melanges of circumpacific and caribbean blueschist terranes reflect high t 600 c conditions in shallower regions such rocks record geochemical processes that affected deep seated high t portions of paleo subduction zones in the catalina schist a subduction zone metamorphic terrane of southern california metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta ultramafic rocks this mafic and ultramafic complex may represent either slab derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab mantle wedge contact or both the trace element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by inaa xrf electron microprobe and sem in order of increasing alteration from a probable metabasalt protolith three common types of garnet amphibolite blocks in the catalina schist are 1 non migmatitic clinopyroxene bearing blocks which are compositionally similar to morb that has lost an albite component 2 garnet amphibolite blocks which have rinds that reflect local interaction between metabasite metaperidotite and fluid and 3 migmatites that are extremely enriched in th hfse lree and other trace elements these trace element enrichments are mineralogically controlled by rutile garnet sphene apatite zircon zoisite and allanite alkali and alkaline earth elements are much less enriched in the solid assemblage and thus appear to be decoupled from the other elements in the inferred metasomatic process es the compositions of migmatitic garnet amphibolite blocks seem to complement that of average island arc tholeiite trace element metasomatism reflects fluid solid rather than melt solid interaction the metasomatic effects indicate that h2o rich fluid perhaps with a significant component of na al silicate and alkalis carried th u sr ree and hfse fractionations of lree in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex exotic lree deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment if the paleo subduction zone represented by the mafic and ultramafic complex of the catalina schist had continued its thermal and fluid evolution a selvage of similarly enriched rocks might have been generated along the slab mantle wedge contact between 30 and 85 km depth rocks affected by subduction zone metasomatism although rarely recognized at the surface could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments

high frequency borehole seismograms recorded in the san jacinto fault zone southern california part 1 polarizations abstract two borehole seismometer arrays knw bh and pfo bh have been established in the southern california batholith region of the san jacinto fault zone by the u s geological survey the sites are within 0 4 km of anza network surface stations and have three component seismometers deployed at 300 m depth at 150 m depth and at the surface downhole horizontal seismometers can be oriented to an accuracy of about 5 using regional and near regional initial p wave particle motions shear waves recorded downhole at the knw bh indicate that the strong alignment of initial s wave particle motions previously observed at the surface knw anza site knw az is not generated in the near surface weathered layer the knw bh surface instrument which sits atop a highly weathered zone displays a significantly different 20 initial s wave polarization direction from that observed downhole and at knw az which is bolted to an outcrop although downhole initial shear wave particle motion directions are consistent with a shear wave splitting hypothesis observations of orthogonally polarized slow shear waves are generally elusive even in seismograms recorded at 300 m a cross correlation measure of the apparent relative velocities of s fast and s slow horizontally polarized s waves suggests shallow shear wave anisotropy consistent with the observed initial s wave particle motion direction of 2 3 1 7 per cent between 300 and 150 m and 7 5 3 5 per cent between 150 and 0 m

cold crust in a hot spot shear waves from locally recorded icelandic earthquakes exhibit very little anelastic attenuation even though they turn in 0 5 ma lower 15 25 km crust just above the moho lower crustal shear wave quality factors are about q 150 225 for paths that cross central iceland and about q 300 for paths in southwestern iceland values that indicate that the temperature of the lower crust is at least 250 325 c below the solidus of gabbro that is about 875 950 c based on published experimental studies of the effect of temperature on shear wave attenuation in gabbro these low temperatures are not consistent with existing models of crustal accretion which predict partial melt at lower crustal depths

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

structure and chemistry of zircon and zircon group minerals zircon zrsio4 is a common accessory mineral in nature occurring in a wide variety of sedimentary igneous and metamorphic rocks known to incorporate an assortment of minor and trace elements zircon has the ability to retain substantial chemical and isotopic information leading to its use in a wide range of geochemical investigations including studies on the evolution of earth s crust and mantle e g hanchar et al 1994 bowring 1995 vervoort et al 1996 hoskin and schaltegger this volume valley this volume as well as age dating e g gibson and ireland 1995 bowring et al 1998 solar et al 1998 bowring and schmitz this volume ireland and williams this volume parrish et al this volume the physical and chemical durability of zircon is a major factor in it being the mineral by which many of earth s oldest known rocks have been dated bowring et al 1989 maas et al 1992 buick et al 1995 bowring and williams 1999 wilde et al 2001 and is also an important factor in zircon being proposed as a candidate waste form for the geologic disposal of excess plutonium from dismantled nuclear weapons ewing and lutze 1997 ewing 1999 burakov et al 2002 burakov et al 2003 ewing et al this volume the chemical and physical properties of zircon and its ability to incorporate and retain trace elements are largely determined by its crystal structure the zircon structure is adopted by numerous minerals and synthetic compounds with the general formula at o4 in which high field strength t site cations occupy isolated tetrahedra and a site cations occupy larger eight coordinated structural sites zircon type compounds share many physical properties as well as displaying variable degrees of solid solution among end members

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

shear wave splitting and anisotropy in the charlevoix seismic zone quebec during the month of august 1984 an experiment was undertaken in the charlevoix seismic zone to test the hypothesis that shear wave splitting could be observed in a seismically active region seismograms recorded by digital 3 component seismographs located very close to the epicentres of three earthquakes showed shear wave splitting the amount of splitting was estimated to be 1 6 to 2 of the shear wave velocity from particle motion diagrams the average azimuth of the direction of polarization was 30 these observations can be explained by the presence of vertical cracks striking 30 east of north with a minimum crack density of e 0 03

seismic anisotropy observed in upper oceanic crust seismic anisotropy in the upper 150om of oceanic basement has been observed by polarization analysis of three component bore hole seismometer records the most convincing evidence for the anisotropy is shear wave splitting for explosive sources at four azimuths compressional wave particle motion deviations suggestive of anisotropy are also observed but they may be caused by lateral inhomogeneities the anisotropy was not resolved by travel time analysis the observed velocities and particle motions in the horizontal plane can be modelled to within a standard deviation by assuming a perfectly elastic homogeneous anisotropic layer 2 with hexagonal symmetry and a horizontal symmetry axis the most probable cause of the anisotropy is preferred crack orientation

seismic imaging of the geothermal field at krafla iceland using shear wave splitting abstract shear wave splitting is emerging as a useful exploration method for geothermal reservoirs as it can detect the geometry of the fracture system the intensity of cracking and possibly changes in fluid pressure within the reservoir the method is based on the analyses of polarizations and time delays of shear waves that have been distorted by the anisotropy of the medium through which the seismic waves have propagated observations of shear wave splitting within the krafla leirhnukur geothermal field iceland using a 20 station 3 component portable seismic array have provided evidence for at least two major crack systems of microfractures oriented approximately n s and e w located microearthquakes align roughly along the e w direction of the geothermal field with shallow focal depths mostly around the injection well probably related to the ongoing injection this unexpected direction is however consistent with results from a simultaneous mt magnetotelluric survey

effects of the internal shear wave window comparison with anisotropy induced splitting in the absence of anisotropy the observed polarizations of shear waves should be those radiated from the source modified only by interaction with internal interfaces and free surface topography it has been suggested that shear wave polarizations may be systematically distorted by internal interfaces which could lead to difficulties in interpreting anisotropy induced shear wave splitting since the literature now reports many investigations of shear wave splitting possible complications caused by internal interfaces would be important and need to be assessed in this paper we investigate the polarization of shear waves in full wave synthetic seismograms incident from a point source at an isotropic to isotropic interface over a range of angles of incidence particle motion due to plane and curved wave fronts has different characteristics internal shear wave windows can be defined analogous to the shear wave window at the free surface within the innermost window the polarizations at subsurface geophones as in vertical seismic profiles are recorded with little distortion but outside this window the waveforms become increasingly distorted for larger angles of incidence this is similar to the behavior of shear waves with respect to the shear wave window at the free surface except that the behavior at internal interfaces is more complicated because of additional critical angles and additional windows we conclude that the interaction of shear waves with internal interfaces is only a serious difficulty when interpretating wide offset vertical seismic profiles wide angle reflections and cross hole data

crack induced seismic anisotropy in the oceanic crust across the east pacific rise 9 30 n the seismic anisotropy of the shallow oceanic crust across the east pacific rise 9 30pn is studied with p wave refraction data collected during a controlled source three dimensional tomography experiment the anisotropy is indicated by a cos 2a pattern of travel time residuals where a is the receiver to shot azimuth the travel time data are best fit by a model containing 4 anisotropy from 500 m to 1 km depth below the sea floor 2 from 1 to 2 km and 0 below 2 km depth the upper 500 m of crust is not independently constrained by the data since ray paths at these depths are oriented near vertically the fast direction of p wave propagation is aligned along a trend oriented 6 10 counterclockwise from the rise axis the results are consistent with the presence of vertically aligned cracks that form on or very near the rise axis are oriented nearly parallel to the rise trend penetrate to less than 2 km depth and provide 6 10 of the total pore space in the shallow crust in comparison with similar studies across slow and intermediatespreading ridges there is no apparent spreading rate dependence of the magnitude of anisotropy fl 2001 published by elsevier science b v

evolution of a lateral dike intrusion revealed by relatively relocated dike induced earthquakes the 2014 15 barðarbunga holuhraun rifting event iceland abstract understanding dikes is vital as they serve both as bodies that build the crust and as conduits that feed eruptions and must be monitored to evaluate volcanic hazard during the 2014 15 bar arbunga rifting event iceland intense seismicity accompanied the intrusion of a 50 km lateral dike which culminated in a 6 month long eruption we here present relocations of earthquakes induced by the lateral dike intrusion using cross correlated sub sample relative travel times the 100 m spatial resolution achieved reveals the complexity of the dike propagation pathway and dynamics jerky segmented and allows us to address the precise relationship between the dike and seismicity with direct implications for hazard monitoring the spatio temporal characteristics of the induced seismicity can be directly linked in the first instance to propagation of the tip and opening of the dike and following this after dike opening indicate a relationship with magma pressure changes i e dike inflation deflation followed by a general post opening decay seismicity occurs only at the base of the dike where dike imposed stresses combined with the background tectonic stress from regional extension over 200 yr since last rifting are sufficient to induce failure of pre existing weaknesses in the crust while the greatest opening is at shallower depths emplacement oblique to the spreading ridge resulted in left lateral shear motion along the distal dike section studied here and a prevalence of left lateral shear failure fault plane strikes are predominately independent of the orientation of lineations delineated by the hypocenters indicating that they are controlled by the underlying host rock fabric this high resolution study provides unprecedented opportunity for comparison with both geodetic and field frozen dike observations and development and consolidation of analytical and analogue models with implications for rifting processes and real time monitoring of magma intrusion

local shear wave observations in joao câmara northeast brazil 1 we present shear wave splitting data from a 10 km aperture eight station digital three component seismograph network which operated from december 1992 to august 1994 near joao c mara 5 33 s 35 51 w to record shear waves from continuing microearthquake activity persisting 6 years after a main shock in november 1986 previous aftershock recording had shown remarkably impulsive simple seismograms and hypocentral errors of typically 200 m in a structure of near uniform wave speed but only one station was three component the pattern of polarization directions shown is consistent with the trend of precambrian foliation observed in the field it is therefore concluded that these shear wave observations are controlled by the seismic anisotropy associated with the precambrian foliation and hence bear the signature of the paleo stress field i e the precambrian deformation regime this conclusion has important implications for the interpretation of shear wave splitting observations made in crustal crystalline rock because other authors have asserted that shear wave splitting normally results from the effect of stress aligned fluid filled cracks extensive dilatancy anisotropy or eda eda is by contrast a signature of the present day stress field in this region there is firm evidence that the present day maximum compressive stress is e w this evidence comes from earthquake focal mechanisms and borehole breakout data and is summarized in the world stress map we therefore conclude that our present data are in maximum conflict with an interpretation in terms of eda our interpretation is supported by the observation that delay times are consistent with those expected from modeling of elastic waves in the crust

tectonics of the thingvellir fissure swarm sw iceland abstract the thingvellir fissure swarm dissects 9000 year old pahoehoe lava and contains about 100 fractures of average orientation n29 3e the average length of fractures is 620 m the minimum being 57 m and the maximum 7 7 km the maximum width and throw on a single fracture are 68 m and 40 m respectively most fractures are vertical at the surface and must be the result of an absolute tensile stress the geometry and arrangement of the fractures indicate that they have grown by coalescence of initially offset small fractures it is concluded that most fractures attain depths of the order of several hundred meters or less but that the largest faults attain depths of many kilometers comparison with the vogar fissure swarm on the reykjanes peninsula suggests that the thingvellir swarm may have the greater rate of dilation the total maximum postglacial dilation of the vogar swarm is only 15 m whereas the corresponding figure for the thingvellir swarm is about 100 m

the oxygen isotopic composition of uranium minerals a review abstract uranium ore is an essential material in the preparation of nuclear fuel for civilian as well as military uses uranium is first extracted from uranium bearing minerals using a variety of reagents and is precipitated from solutions as yellow cake prior to isotope enrichment processes the disintegration of the former eastern bloc in the 1990s and frequent unrest in the middle east have underscored the need for better characterizing source uranium ores for forensic and attribution purposes the world s major deposits of u occur in several distinctly different geological environments fourteen principal types of u deposits and rocks with elevated uranium contents are recognized with more than 40 subtypes combining our own analysis and literature data we have amassed over 250 oxygen isotope data from 13 major u producing countries which vary widely from 32 to 11 however interpreting the oxygen isotopic composition of uraninite in terms of the composition of the fluid from which it precipitated or interacted with requires knowledge of the fractionation factor and temperature of interactions which are not always available since each deposit type occurs within a unique geologic setting and is generally formed from chemically distinct fluids the chemical compositions of the uranium ores are also distinct uranium deposits that form in igneous rocks have higher trace element compositions relative to sandstone hosted deposits our data shows that one of the most useful techniques for distinguishing between uranium ore is to use a combination of 18 o values and rare earth elements e g la yb ratios these methods may allow investigators to trace uranium ore back to the source

cyclic variations of uranium concentrations and oxygen isotopes in tufa from the middle miocene barstow formation mojave desert california uranium concentrations of 42 to 169 ppm in fibrous calcite from spring deposited tufa in the middle miocene barstow formation california are among the highest reported for calcite fission track maps of multiple bands of uniformly dull luminescent fibrous calcite show that the concentration of u increases in the outward growth direction of the calcite of each individual band homogeneous dull luminescence in the fibrous calcite indicates no change in redox conditions of the fluid from which the calcite was precipitated it is proposed that the cyclic pattern of increase in u concentration reflects a cyclical change in the u ca ratio in the fluid episodic mixing between ca rich spring water with a low u ca ratio and ca poor saline alkaline lake water with a high u ca ratio could produce the episodes of formation of fibrous calcite with increasing u concentrations the spring water supplies the ca for calcite precipitation and the u concentrations increase with the decreasing fraction of spring water and increasing fraction of lake water these cycles reflect a variable recharge of groundwater into the lake by springs the large variation in u concentrations suggests mixing of 0 to 45 spring water with the lake while the narrow range in 18 o of 6 37 to 6 87 limits the variation of the proportions of spring and lake water to 5

ductile precambrian fabric control of seismic anisotropy in the acu dam area northeastern brazil 1 in the joao c mara area ne brazil seismic anisotropy has been interpreted as the result of either precambrian ductile fabric or stress aligned fluid filled grain boundary caused by high pore fluid pressures the first interpretation is based on the fact that the direction of the fastest s wave splitting agrees with the north south trending precambrian fabric the second interpretation related to the extensive dilatancy anisotropy eda is based on the stress field orientation and on the assumption that the pore fluid pressure in the region is high enough to modify microcrack distribution and cause s wave polarization to become parallel to the north south oriented minimum horizontal stress shmin here we address this question by investigating this ambiguity we have analyzed the seismicity in the acu dam area 150 km west of joao c mara over a 3 year period in the acu area the northeast southwest trending ductile precambrian fabric is not parallel or orthogonal to the east west trending maximum horizontal stress shmax we combined the interpretation of fastest s wave polarizations their time delays and fabric orientation measured in the field with side looking airborne radar imagery taken before the lake impoundment thus we discriminate between 1 stress aligned fluid filled cracks 2 high pore pressure fluids modifying the microcrack distribution and therefore causing 90 flips and 3 fabric or paleostrain as causes for the observed s wave anisotropy our main result indicates that the direction of the fastest s wave polarization in the acu dam area agrees with the northeast southwest trending fabric n40 e eda or 90 flips control of seismic anisotropy if occurring in acu have minor effects

a melange of subduction temperatures evidence from zr in rutile thermometry for strengthening of the subduction interface abstract the catalina schist contains a spectacular km scale amphibolite facies melange zone thought to be part of a cretaceous convergent margin plate interface in this setting blocks ranging from centimeters up to 100 m in diameter are surrounded by finer grained matrix that is derived from the blocks blocks throughout the melange represent a diversity of protoliths derived from basalts cherts and other sediments and hydrated mantle but all contain assemblages consistent with upper amphibolite facies conditions suggesting a relatively restricted range of depths and temperatures over which material within the melange was metamorphosed this apparent uniformity of metamorphic grade contrasts with other melanges such as the franciscan complex where coexisting rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface this mixing has been ascribed to flow of material within relatively low viscosity matrix the zr content of rutile in samples from across the amphibolite facies melange of the catalina schist was measured to determine peak metamorphic temperatures identify whether these temperatures were different among blocks and whether the spatial distribution of temperatures throughout the melange was systematic or random resolvably different zr contents between 290 and 720 10 40 ppm are found among the blocks corresponding to different peak metamorphic temperatures of 650 to 730 2 16 c at an assumed pressure of 1 gpa these results are broadly consistent with previous thermobarometric estimates no systematic distribution of temperatures was found however like other melange zones material flow within the catalina schist melange was likely chaotic but appears to have occurred on a more restricted scale compared to some other localities progressive metamorphism of melange matrix is expected to produce rheologically stiffer matrix minerals such as amphiboles and pyroxenes at the expense of weaker matrix minerals sheet silicates affecting the overall rheological behavior of the melange and dictating the scale of flow the catalina schist amphibolite facies melange matrix appears to provide a snapshot of hotter stiffer portions of a subduction interface perhaps more representative of rheological behavior at depths approaching the subarc than is found in some other exhumed melange zones

the 1912 iceland earthquake rupture growth and development of a nascent transform system we have mapped in detail surface ruptures of the 1912 magnitude 7 0 strike slip earthquake in south iceland this earthquake ruptured fresh basalt flows that had covered the pre existing fault the observed style of surface fracturing closely matches both theoretical predictions of the first stages of shear fracture development and microscopic scale observations from laboratory experiments the shear offset distributed across the zone of surface fractures produced by this earthquake is right lateral and is in the range of 1 to 3 m total mapped rupture length is 9 km but total rupture length is probably at least 20 km this interplate earthquake had an exceptionally high ratio of slip to fault length and by inference stress drop the north south trending rupture of the 1912 earthquake is part of the bookshelf faulting in the east west trending south iceland seismic zone we ascribe the bookshelf faulting in the south iceland seismic zone to a combination of the early development stage of the transform and regional strength anisotropy of the crust

clay alteration associated with proterozoic unconformity type uranium deposits in the east alligator rivers uranium field northern territory australia clay minerals associated with the unconformity type uranium deposits of the east alligator rivers uranium field have been used to determine the spatial distribution of hydrothermal alteration and the conditions of uranium deposition within the altered basement underlying the kombolgie unconformity alteration develops a similar pattern at regional and smaller scales with a close relationship between the degree of alteration and the amount of structural discontinuities observed on both sides of the unconformity e g faults fractures breccias these discontinuities were mechanically active during the hydrothermal activity the hydrothermal clay minerals partly to totally replace diagenetic dickite and illite in the kombolgie sandstones and metamorphic phengite and chlorite in the basement rocks the time space alteration sequence established in this study can be summarized as follows 1 early crystallization of illite illite smectite i s mixed layer minerals lree sr bearing aluminum phosphate sulfates aps in sandstones above the unconformity illite i s mixed layer minerals sudoite aps on both sides of the unconformity and in fault gouges and illite i s mixed layer minerals chlorite apatite deeper in the basement and 2 late precipitation of chlorite in the open fracture network mainly centered below the unconformity the main stage of uranium deposition was associated with chloritization in the basement rocks smaller particle size lath shaped or flaky morphology common interstratification with expandable layers lower crystallinity and mg rich chemical compositions can be used to distinguish the hydrothermal clays from the other diagenetic and metamorphic illite and or chlorite hydrothermal illite can be distinguished from diagenetic and metamorphic illite phengite by its poor crystallinity along the c axis interstratification with minor amounts of smectite layers and a 1m polytype in which trans vacant octahedral sites predominate 1 mt chemically the hydrothermal illite differs from diagenetic illite by a lower interlayer charge and a higher mg content the hydrothermal chlorite is distinguished from the metamorphic chlorite by its smaller particle size spherulitic habit poor crystallinity along the c axis and common association with corrensite and or chlorite corrensite mixed layers most of the hydrothermal chlorite is mg rich and has a structural formula close to that of clinochlore xfe 0 20 near the uranium deposits the spherules of hydrothermal chlorite are chemically zoned from xfe 0 10 to xfe 0 40 which is interpreted as the result of a change in the oxidation state of the hydrothermal solution during the chlorite crystallization the space time alteration sequence established for clay minerals and the observed transition of aps to apatite are interpreted as the result of an increasing degree of interaction of the basement rocks with infiltrating acidic and oxidizing sub basinal brines in turn these fluids tend to be neutralized and reduced by their ongoing interaction with ferrous iron bearing phyllosilicates fe chlorite and biotite and iron sulfides of metamorphic origin the ore deposits are surrounded by zones in which the degree of fluid rock interaction was very high and chloritization is most abundant the octahedral mg content of the associated hydrothermal illite from the sandstone can be used as a guide to indicate the possible presence of uranium orebodies in the basement rocks tens to hundreds of meters below the crystal structures of the hydrothermal illite and chlorite suggest that these minerals crystallized at temperatures approximatively 30 to 50 c lower than those expected at the peak diagenesis of the kombolgie sandstones near the unconformity

estimation of source parameters and q from acceleration recorded in the vatnafjoll earthquake in south iceland the earthquake in vatnafjoll 25 may 1987 m 5 8 was the largest earthquake in south iceland since 1912 m 7 and it was one of the first earthquakes recorded on a local network of strong motion acceleration recorders in this study the strong motion records are used to obtain an estimate of the source parameters and the attenuation of high frequency waves the source parameters are obtained by fitting the data by a dislocation source model combined with an exponential model for the attenuation the root mean squared acceleration is also estimated and compared to predicted values using the source parameters the source parameters estimated here m 8 2 1024 dyn cm r 4 6 km 38 bars u 36 cm td 3 sec and estimated moment magnitude mw 5 9 agree well with values obtained previously from teleseismic waves and strain meter data estimates of path averaged crustal shear wave quality factors give results in the range q 128 to 425 and k 0 05 sec for the spectral decay parameter confirming recent measurements of higher crustal q for this region than in earlier studies

the role of density in the accumulation of basaltic melts at mid ocean ridges it is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust at the east pacific rise this neutral buoyancy level lies only 100 400 m below the sea floor significantly shallower than the depths 1 2 km of the magma bodies imaged in multichannel reflection data suggesting that other factors must control the collection of melt in these reservoirs the apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis not the buoyancy of melt is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid ocean ridges

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

an investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 n mid atlantic ridge summary a detailed seismic refraction survey using three 3 component ocean bottom seismographs over young 1 1 3 4 ma old mid atlantic ridge at 45 n recorded compressional wave velocity anisotropy in the upper crust of about 5 per cent and associated particle motion deviations of up to 8 the magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near vertical water filled cracks in layer 2 the directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions in the range 30 60 with respect to the ridge axis we explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis the principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis the apparently negligible seismic influence exerted by ridge parallel sets of shallow cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge normal compression we find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust lateral variability across the 25 km square survey is limited in magnitude typically 0 05 s variations in delay times and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography

strain release and strain build up in the south iceland seismic zone abstract the south iceland seismic zone is a 70 km long and 10 20 km broad area of land in southern iceland where historically the most destructive earthquakes in iceland have occurred the zone is part of the e w bridge between the mid atlantic ridge and the neovolcanic zone which is an eastward offset of the ridge through iceland the basis for the present study is a historical and an instrumental earthquake catalogue the nature of strain release in earthquakes and as it is revealed in earthquake cracks and observations of aseismic strain release and source propagation the strain build up is modelled as an interplay between the general e w drift across the mid atlantic ridge and stresses caused by the horizontal outflow of upwelling mantle material a complex model of strain release is outlined including seismic as well as aseismic components while seismic strain release plays the dominant role in the eastern and central part of the zone to a depth even below 15 km aseismic slip takes up most of the motion in the western part of the zone and in its continuation in the reykjanes peninsula towards the reykjanes rid e

uraniferous paleoplacers of the mesoarchean mahagiri quartzite singhbhum craton india depositional controls nature and source of 3 0 ga detrital uraninites abstract the quartz pebble conglomerate qpc hosted detrital uranium mineralization is unique in character in terms of their restricted distribution before 2 2 ga atmosphere during pre great oxidation event pre goe such qpc paleoplacer deposits over the world are good targets for moderate to high tonnage and low grade uranium deposits and more importantly for their gold content the mahagiri quartzite dated c 3 02 ga for their youngest detrital zircon population is developed unconformably over the mesoarchean singhbhum granite 3 44 ga to 3 1 ga the mahagiri quartzite includes a conglomerate pebbly sandstone dominated subaerial alluvial fan to coastal braided plain sequence in the lower parts and shallow marine mature quartz arenite in the upper parts the alluvial fan braided plain deposits in the lower parts host a number of pyritiferous and uraniferous conglomerate and pebbly sandstone beds the uraninite grains are rounded to subrounded in outline suggesting mechanical transport and detrital origin together with detrital pyrite and uraninite constitute the example of 3 0 ga paleoplacer closely comparable to the witwatersrand au u deposits epma and sem eds studies suggest that the uraninite grains are rich in th 4 wt s and ree y chemical formula calculations from epma analyses suggest uraninite grains belong to two populations with different oxidation states as revealed from y ree and cation u4 u6 apfu ratios the u contents of the detrital uraninite grains from mahagiri are significantly lower than that of the ideal stoichiometric composition of uo2 this is mainly due to higher amount of heterovalent cationic substitution by th ree y pb and ca in mahagiri qpc uraninite structures and partial alteration and metamictization of uraninites alteration due to metamictization resulted in elevated concentration of si al p and ca in more altered and metamict uraninite grains the ree pattern is typically flat with comparable lree hree concentration the high th content flat ree pattern suggests that the uraninitere presents high temperature phases 350 c and are magmatic in origin the mahagiri detrital uraninite grains suggest existence of highly felsic and k rich richer than ttg granodiorite granite monzogranite suites ggm of rocks older than 3 1 ga in the singhbhum craton

azimuthal anisotropy and phase velocity beneath iceland implication for plume ridge interaction abstract we have determined the pattern of azimuthal anisotropy beneath iceland from shear wave splitting and rayleigh wave tomography using seismic data recorded during the icemelt and hotspot experiments the fast directions of shear wave splitting are roughly n s in western iceland and nnw sse in eastern iceland in western iceland azimuthal variations in rayleigh wave phase velocity show that fast directions are close to the plate spreading direction at short periods of 25 40 s and roughly parallel to the mid atlantic ridge at longer periods of 50 67 s beneath the rift zones in central iceland we find a ridge parallel fast direction at periods of 25 40 s and significantly weaker azimuthal anisotropy at periods of 50 67 s the 2 d variation of isotropic phase velocity at periods of 33 67 s indicates that the lowest velocities are beneath the rift zones in central iceland rather than above the plume conduit in southeast iceland while ridge parallel alignment of melt films might contribute to anisotropy above 50 km depth beneath the rift zones the overall observations are consistent with a model in which plume influenced hot buoyant mantle rises beneath the mid atlantic ridge at depths greater than 50 km and is preferentially channeled along the ridge axis at the base of the lithosphere beneath iceland the shear wave splitting results are attributed primarily to a n s mantle flow at depths greater than 100 km

crustal shear wave anisotropy in southern hawaii spatial and temporal analysis split shear wave arrivals are analyzed in seismograms from local earthquakes in southern hawaii recorded at five temporary arrays and one permanent network station we identify split shear wave arrivals by their orthogonally polarized pulses linear particle motions and similar waveforms and estimate the delay time for the slow shear wave arrival s 2 using a waveform cross correlation method consistent leading shear wave polarizations were measured at the majority of our stations comparison of observed and predicted shear wave polarizations confirms that the former are due to anisotropy rather than earthquake source mechanism agreement between fast shear wave s 1 polarizations and independently estimated directions of the maximum horizontal compressive stress h for the ainapo and punaluu gulch arrays leads us to conclude that the predominant source of the observed anisotropy for these two areas is stress aligned cracks consistent with the extensive dilatancy anisotropy eda hypothesis two distinct s 1 polarization directions were observed over distances less than 1 km for the bird park and south flank arrays s 1 polarizations parallel to the ne striking kaoiki pali fault system for the bird park array combined with a nonhorizontal maximum principal stress 1 for the south flank region suggest stress induced cracks aligned by nearby faulting as a source for the observed anisotropy large station to station variations in s 1 polarization and the relationship between delay time and event depth for arrays in the kaoiki and south flank regions provide evidence for anisotropy that is predominantly shallow rather than pervasive average delay times for the five arrays vary from about 100 to 230 ms with standard deviations of the order of 30 ms estimated anisotropic velocity variations and crack densities exceeding 10 indicate that the upper crust of southern hawaii is highly fractured a search for possible temporal changes in delay time associated with the 1983 kaoiki main shock m l 6 6 at a station near the epicenter finds no evidence for change

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland

the 1991 eruption of hekla iceland the eruption that started in the hekla volcano in south iceland on 17 january 1991 and came to an end on 11 march produced mainly andesitic lava this lava covers 23 km2 and has an estimated volume of 0 15 km3 this is the third eruption in only 20 years whereas the average repose period since 1104 is 55 years earthquakes as well as a strain pulse recorded by borehole strainmeters occurred less than half an hour before the start of the eruption the initial plinian phase was very short lived producing a total of only 0 02 km3 of tephra the eruption cloud attained 11 5 km in height in only 10 min but it became detached from the volcano a few hours later several fissures were active during the first day of the eruption including a part of the summit fissure by the second day however the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed the average effusion rate during the first two days of the eruption was about 800 m3 s 1 after this peak the effusion rate declined rapidly to 10 20 m3 s 1 then more slowly to 1 m3 s 1 and remained at 1 12 m3 s 1 until the end of the eruption site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption a notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption but it levelled off on the 6th day and then remained essentially constant the volume and initial silica content of the lava and tephra the explosivity and effusion rate during the earliest stage of the eruption as well as the magnitude attained by the associated earthquakes support earlier suggestions that these parameters are positively related to the length of the preceeding repose period the chemical difference between the eruptive material of hekla itself and the lavas erupted in its vicinity can be explained in terms of a density stratified magma reservoir located at the bottom of the crust we propose that the shape of this reservoir its location at the west margin of a propagating rift and its association with a crustal weakness all contribute to the high eruption frequency of hekla

seismic structure and crustal magmatism at the mid atlantic ridge 35 n we have determined the three dimensional p wave velocity structure and azimuthal anisotropy within the shallow crust of the inner valley at the mid atlantic ridge mar near 35 n the best fitting one dimensional model is 0 8 km s lower in velocity at a given depth than for fast spreading ridges while the isotropic three dimensional velocity structure shows a strong lateral heterogeneity up to 1 4 km s peak to peak variation at 1 km depth in both the along and cross axis directions an anomalous region with velocities as much as 0 8 km s lower than average and 3 5 km in lateral extent is located beneath a near axis seamount we interpret this feature as a region of near solidus temperatures with possibly a small degree of partial melt from the variation of travel time residuals with azimuth we determine a best fitting p wave anisotropy model that has 4 anisotropy from 500 m to 1 km depth 2 anisotropy in the depth range 1 1 5 km and no anisotropy at greater depth this finding is consistent with vertical cracks aligned parallel to the axial valley we find no evidence for the shallow high velocity ridge parallel feature commonly observed at the axis of fast spreading ridges and attributed to high velocity dikes beneath a thin extrusive layer the absence of this anomaly at the mar implies that the axis of crustal accretion is not at a stable location with respect to the inner valley walls on timescales greater than about 25 000 years an unstable axis of accretion will result in a thicker transition from extrusive rocks to dikes which may account for the generally lower velocities beneath the mar compared with the east pacific rise and a lateral variation in upper crustal lithology

crustal shear wave splitting from local earthquakes in the hengill triple junction southwest iceland the hengill region in sw iceland is an unstable ridge ridge transform triple junction between an active and a waning segment of the mid atlantic spreading center and a transform that is transgressing southward the triple junction contains active and extinct spreading segments and a widespread geothermal area we evaluated shear wave birefringence for locally recorded upper crustal earthquakes using an array of 30 three component digital seismographs fast polarization directions are mostly ne to nne subparallel to the spreading axis and probably caused by fissures and microcracks related to spreading however there is significant variability in throughout the array the lag from fast to slow s is not proportional to earthquake depth ray length being scattered at all depths the average wave speed difference between qs1 and qs2 in the upper 2 5 km of the crust is 2 5 our results suggest considerable heterogeneity or strong s scattering

observations of shear wave splitting on the southeast flank of mauna loa volcano hawaii we use a combination of particle motion covariance matrix decomposition particle motion plots and cross correlation to estimate the polarization and split times of shear wave arrivals from local earthquakes in the kaoiki and hilea regions of southern hawaii consistent leading shear wave polarizations and identifiable fast and slow shear wave arrivals were frequently observed average delay times for the four arrays vary from 100 to 230 ms the leading shear wave polarizations at stations in the kaoiki region agree with the direction of the local maximum horizontal compressive stress h determined from focal mechanism studies and field observations of ground rupture for the bird park bp array in the kaoiki region the relationship between delay time and event depth shows evidence for both dominant shallow and weaker pervasive anisotropy the correlation between delays and event depth is much weaker for the ainapo an array than for bp however a smaller slope is apparent conservative estimates of the crack densities for the bp and an arrays are e 12 and e 6 respectively in the hilea region the leading shear wave polarization for the punaluu gulch pg array differs by approximately 30 from the direction of h determined from focal mechanism studies while the waihaka gulch wg array shows no predominant polarization direction no correlation between delay time and event depth was observed for either of the two hilea arrays

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

azimuthal variation of the p phase in icelandic receiver functions summary a curious observation has been made on radial receiver functions calculated from teleseisms recorded by 29 broad band seismometers distributed over iceland the arrival time of the direct p phase of the radial receiver functions depends critically upon the azimuth of the teleseismic source for a seismic station in west iceland the direct p phase of the radial receiver function arrives consistently later for easterly source azimuths than for westerly source azimuths the reverse applies for stations in east iceland in the original seismograms the delayed p phase of the receiver function appears up to 450 ms later on the radial than on the vertical component the seismometer locations in east and west iceland are separated by the neovolcanic zone a constructive plate boundary the delayed p phases occur for seismic rays travelling across this zone however it is not obvious how wave propagation across the plate boundary zone could cause the observed delays the tentative explanation proposed here involves the regional dip of the icelandic lava sequences towards the neovolcanic zone a dipping interface at shallow depth results in a p s converted phase arriving shortly after the p phase these phases cannot be separated in the radial receiver functions given the bandwidth of the observed signals however a calculation of receiver functions from estimates of the p sv and sh wavefields clearly reveals a p s converted phase at about 500 ms for easterly source azimuths in west iceland and for westerly source azimuths in east iceland the amplitudes of the direct p phase and the p s phase converted at a dipping interface would be expected to vary strongly with azimuth in accordance with the observed behaviour

a four year study of shear wave splitting in iceland 1 background and preliminary analysis this is part 1 of a report of a four year study of seismic shear wave splitting in iceland designed to seek temporal variations before earthquakes shearwave splitting is observed routinely in iceland whenever shear waves arrive within the shear wave window of seismic stations and whenever adequate data are available temporal and spatial variations in shear wave splitting are observed before both earthquakes and volcanic eruptions shear wave splitting is caused principally by the stress aligned fluid saturated microcracks and pore throats in almost all in situ rocks fluid saturated microcracks are the most compliant elements of the rock mass and changes in splitting can be directly interpreted and modelled as the effects of changing stress on the microcrack geometry in the rock mass often at considerable distances from the immediate earthquake source zone such changes were found and are reported in paper 2 this paper presents the background preliminary observations and analysis of shear wave splitting in iceland

secondary fe mn oxides in minerals heavily damaged by α recoil possible implications for palaeomagnetism sub micron fe mn oxides in columbite tantalite minerals are bound to metamict domains in the host these nano oxides are secondary minerals as the metamict zones formed through accumulation of damages from recoil each of which in a small volume destroys the crystal lattice of the u and th bearing columbite tantalite host transmission electron microscope investigations demonstrate that the oxides fall in the compositional range of magnetite jacobsite iwakiite fe mn 3o4 and of hematite type minerals along the fe2o3 mn2o3 join i e minerals that are known to be major carriers of magnetic remanence measured magnetic properties of isolated columbite tantalite crystals demonstrate in some samples magnetic remanence which is bound to sub micron pseudosingle domain crystals rather than to the host thus the magnetic remanence is not related to the formation of columbite tantalite but to the crystallization of the nano oxides which occur in metamict domains of the host and therefore are secondary depending on the content and distribution of u and th the nano oxides may form diachronously within an individual and among several host crystals magnetic secondary nano oxides are not restricted to columbite tantalite minerals but may occur in other minerals where high contents of fe and mn are associated with high contents of u and th rocks that show the same spatial distribution for u and fe as for instance some red sandstones may display magnetic properties that are controlled by recoil induced growth of secondary fe oxides

seismic anisotropy of the shallow crust at the juan de fuca ridge microearthquake data recorded on four ocean bottom seismometers are used to study shear wave splitting on the endeavour segment of the juan de fuca ridge the covariance matrix decomposition method is used to determine the sensor orientation from explosive shot data and to estimate the anisotropy parameters for 238 earthquake records at three of the four sites the results show a remarkably consistent fast direction parallel to the ridge axis the time delays between the fast and the slow waves range from 40 to 200 ms with an average of 90 ms they are not clearly related to earthquake range focal depth or source receiver azimuth the splitting of the shear waves is interpreted as an effect of structural anisotropy due to the presence of ridge parallel cracks in the shallow crust if we assume that anisotropy is concentrated in the upper 1 2 km the splitting times require a high crack density of 0 1

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

shear wave splitting and crustal anisotropy at the mid atlantic ridge 35 n 1 shear wave splitting observed in microearthquake data at the axis of the mid atlantic ridge near 35 n has a fast polarization direction that is parallel to the trend of the axial valley the time delays between fast and slow s wave arrivals range from 35 to 180 ms with an average of 90 ms and show no relationship with ray path length source to receiver azimuth or receiver location the anisotropy is attributed to a shallow distribution of vertical fluid filled cracks aligned parallel to the trend of the axial valley joint modeling of the shear wave anisotropy and coincident p wave anisotropy results using recent theoretical models for the elasticity of a porous medium with aligned cracks suggests that the crack distribution that causes the observed p wave anisotropy can account for at most 10 ms of the shear wave delay most of the shear wave delay thus likely accrues within the shallowmost 500 m seismic layer 2a and the percent s wave anisotropy within this highly fissured layer is 8 30 isolated fluid filled cracks at 500 m to 3 km depth that are too thin or too shallow to be detected by the p wave experiment may also contribute to the shear wave delays the joint analysis of p and s wave anisotropy is an important approach for constraining the crack distributions in the upper oceanic crust and is especially suited for seismically active hydrothermal systems at slow and intermediate spreading mid ocean ridges

fate of trace elements during alteration of uraninite in a hydrothermal vein type u deposit from marshall pass colorado usa alteration of uraninite from a hydrothermal vein type u deposit in marshall pass colorado has been examined by electron microprobe analysis in order to investigate the release and migration of trace elements w as mo zr pb ba ce y ca ti p th fe si al during alteration under both reducing and oxidizing conditions the release of trace elements from uraninite is used to establish constraints on the release of fission product elements from the uo2 in spent nuclear fuels uraninite occurs with two different textures 1 colloform uraninite and 2 fine grained uraninite the colloform uraninite contains 1 04 1 75 wt of wo3 0 16 1 70 wt of as2o3 0 06 0 88 wt of moo3 whereas the fine grained uraninite retains 2 25 4 93 wt of wo3 up to 5 76 wt of moo3 and 0 26 0 60 wt of as2o3 the near constant concentration of incompatible w in the colloform uraninite suggests w incorporation into the uraninite structure or homogeneous distribution of w rich nano domains incorporation of w and mo into the uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of cs during the corrosion of spent nuclear fuel as there is a strong affinity of cs with w and mo zoning in the colloform texture is attributed to variation in the amount of impurities in uraninite for unaltered zones the calculated amount of oxygen ranges from 2 08 to 2 32 apfu atom per formula unit and defines the stoichiometry as uo2 x and u4o9 whereas for the altered zones of the colloform texture the oxygen content is 2 37 2 48 apfu which is probably due to the inclusion of secondary uranyl phases mainly schoepite the supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of uraninite four stages of colloform uraninite alteration are proposed i formation of an oxidized layer at the rim ii corrosion of the oxidized layer iii precipitation of u6 phases with well defined cleavage and iv fracture of the uraninite surface along the cleavage planes of the u6 phases

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

seismic attenuation in the crust at the mid atlantic plate boundary in south west iceland summary very little seismic attenuation occurs in the mid to lower crust of south west iceland the lowest path averaged quality factor for a wave turning in the mid to lower crust 12 20 km is qp 110 for p waves and q5 250 for s waves with most of the data having higher values typically qp 200 300 and qr 400 600 attenuation estimates based on a 1 d layered inversion give correspondingly high values qp 800 and q 800 2000 these q values are inconsistent with thermal models that predict a broad 100 km wide region of above solidus temperatures centred on the volcanic zones the observed attenuation implies an upper limit for mid to lower crustal temperature in the 700 775 c range assuming a gabbroic lithology much higher attenuation qp 60 qr 100 occurs in the uppermost 4 km of crust this is most likely apparent attenuation caused by strong near surface seismic heterogeneity resulting from fissures faults and extreme changes in porosity up to 20 30 per cent the quality factor of the near surface layer varies regionally and is lower in volcanic zones than in either the reykjanes peninsula or the south iceland lowland

biogeochemical aspects of uranium mineralization mining milling and remediation abstract natural uranium u occurs as a mixture of three radioactive isotopes 238 u 235 u and 234 u only 235 u is fissionable and makes up about 0 7 of natural u while 238 u is overwhelmingly the most abundant at greater than 99 of the total mass of u prior to the 1940s u was predominantly used as a coloring agent and u bearing ores were mined mainly for their radium ra and or vanadium v content the bulk of the u was discarded with the tailings finch et al 1972 once nuclear fission was discovered the economic importance of u increased greatly the mining and milling of u bearing ores is the first step in the nuclear fuel cycle and the contact of residual waste with natural water is a potential source of contamination of u and associated elements to the environment uranium is mined by three basic methods surface open pit underground and solution mining in situ leaching or in situ recovery depending on the deposit grade size location geology and economic considerations abdelouas 2006 solid wastes at u mill tailings umt sites can include both standard tailings i e leached ore rock residues and solids generated on site by waste treatment processes the latter can include sludge or mud from neutralization of acidic mine mill effluents containing fe and a range of coprecipitated constituents or barium sulfate precipitates that selectively remove ra e g carvalho et al 2007 in this chapter we review the hydrometallurgical processes by which u is extracted from ore the biogeochemical processes that can affect the fate and transport of u and associated elements in the environment and possible remediation strategies for site closure and aquifer restoration this paper represents the fourth in a series of review papers from the u s geological survey usgs on geochemical aspects of umt management that span more than three decades the first paper landa 1980 in this series is a primer on the nature of tailings and radionuclide mobilization from them the second paper landa 1999 includes coverage of research carried out under the u s department of energy s uranium mill tailings remedial action program umtra the third paper landa 2004 reflects the increased focus of researchers on biotic effects in umt environs this paper expands the focus to u mining milling and remedial actions and includes extensive coverage of the increasingly important alkaline in situ recovery and groundwater restoration

seismic anisotropy in the crust at the mid atlantic plate boundary in south west iceland summary seismograms of microearthquakes recorded on stations of the sil network in south western iceland exhibit strong shear wave splitting and are consistent with being caused by aligned parallel cracks in the uppermost crust splitting times of 0 1 0 3 s are observed with the larger values 0 2 0 3 s occurring beneath stations in the highly heated western volcanic zone and the smaller values 0 1 s in the younger less fractured south iceland seismic zone five of the stations have a fast shear wave polarization azimuth of n30 e n40 e7 parallel to the axis of the western volcanic zone i e the mid atlantic ridge in iceland and in the direction of maximum horizontal compression defined by fault dyke and fissure strikes and microearthquake focal mechanisms station gyg in the north western part of the network has the significantly different azimuth of n70 e this is parallel to one of the local strike slip fault trends and is probably due to a heated rock fabric caused by those faults

shear wave splitting across the iceland hot spot results from the icemelt experiment 1 we report on observations of upper mantle anisotropy from the splitting of teleseismic shear waves sks skks and pks recorded by the icemelt broadband seismometer network in iceland in a ridge centered hot spot locale mantle anisotropy may be generated by flow induced lattice preferred orientation of olivine grains or the anisotropic distribution of magma splitting measurements of teleseismic shear waves may thus provide diagnostic information on upper mantle flow and or the distribution of retained melt associated with the iceland mantle plume in eastern iceland fast polarization directions lie between n10 w and n45 w and average n24 w delay times between the fast and slow shear waves are generally 0 7 1 35 s in western iceland in contrast the fast polarization directions while less well constrained yield an average value of n23 e and delay times are smaller 0 2 0 95 s we propose that splitting in eastern iceland is caused by a 100 to 200 km thick anisotropic layer in the upper mantle the observed fast directions in eastern iceland however do not correspond either to the plate spreading direction or to a pattern of radial mantle flow from the center of the iceland hot spot we suggest that the relatively uniform direction and magnitude of splitting in eastern iceland situated on the eurasian plate may therefore reflect the large scale flow field of the north atlantic upper mantle we hypothesize that the different pattern of anisotropy beneath western iceland part of the north american plate is due to the different absolute motions of the two plates by this view splitting in eastern and western iceland is the consequence of shear by north american and eurasian plate motion relative to the background mantle flow from absolute plate motion models in which the eurasian plate is approximately stationary and the north american plate is moving approximately westward the splitting observations in both eastern and western iceland can be satisfied by a background upper mantle flow in the direction n34 w and a velocity of 3 cm yr in a hot spot reference frame this inference can be used to test mantle flow models in particular it is inconsistent with kinematic flow models which predict southward flow or models where flow is dominated by subduction related sources of mantle buoyancy which predict westward flow our observations are more compatible with the flow field predicted from global seismic tomography models which in particular include the influence of the large scale lower mantle upwelling beneath southern africa while the hypothesized association between our observations and this upwelling is presently speculative it makes a very specific and testable prediction about the flow field and hence anisotropy beneath the rest of the atlantic basin index terms 7218 seismology lithosphere and upper mantle 8120 tectonophysics dynamics of lithosphere and mantle general 8155 tectonophysics plate motions general keywords iceland anisotropy hot spot plate motion shear wave splitting mantle flow

metasomatism and partial melting in a subduction complex catalina schist southern california partial melting accompanied high pressure amphibolitization of eclogitic blocks and metasomatism of blocks and their peridotite matrix in a unit of the catalina schist a subduction zone metamorphic terrane migmatitic blocks contain leucocratic zones veins and pods with albitic plagioclase quartz muscovite mineralogically similar dikes and veins cut the ultra mafic matrix a few veins can be traced back into migmatitic blocks uniform phase proportions and igneous textures indicate that the dikes and veins crystallized from silicate melts field relations mineral and bulk compositions and comparisons with experimental data suggest that the migmatites and dikes are partial melts of variably metasomatized amphibolites element partitioning phase equilibrium and fluid inclusion data are compatible with partial melting of the amphibolites at p 8 11 kbar and t 640 750 c in the presence of a low salinity aqueous fluid although the catalina metamorphism occurred at much shallower levels than those inferred for subduction related magmatism the process envisioned is one of multistage slab derived metasomatic contamination of the mantle in the hanging wall of a subduction zone

geometallurgy of australian uranium deposits abstract australian uranium ores are often composed of complex mineral assemblages differences in ore compositions and textures are seen between deposits as well as within a single deposit which can host a range of ore types such a wide variety of uranium ores make it impossible for a single extraction or treatment process to be developed that will accommodate all of the ores from a mineralogical perspective key issues confronting the australian uranium mining industry include the prevalence of low grade ores a lack of detailed chemical and mineralogical information uranium speciation texture grainsize for the various ore deposit types and the presence of refractory uranium bearing minerals and highly acid consuming gangue minerals this paper reviews some of the main controls on uranium geometallurgy by linking concepts relating to ore genesis and the resulting ore mineralogy with the processing behaviour of specific australian uranium ore types emphasis is placed on the value of detailed ore mineralogical analysis and the insight this provides into the factors of importance when considering uranium extraction