fluid driven nucleation and propagation of splay fractures from a permeable fault progressive development of opening mode splay or branch fractures along a permeable fault in an elastic medium subject to elevated fluid pressure from a constant influx fluid source is studied numerically using a plane strain hydraulic fracturing model that couples fracture deformation and fluid flow in situ stresses are imposed so that their resultant shear stress on the fault is lower than the frictional strength new splay fractures are initiated based on satisfying a dual criterion for both tensile strength and fracture toughness and meeting a minimum fracture spacing requirement numerical results demonstrate that spatial variations in permeability along faults can cause arrest of local slip and the created slip gradient can result in splay fracture initiation at a significant distance inward from the fault tips one splay fracture generally grows first and the kinematic coherence in displacements at the junction between it and the fault can reduce the downstream flow rate to prevent the nucleation of other splay fractures however the number of splay fractures can be increased when the branch growth extent is limited to a certain size when the fault is divided into many segments each with a linear distributed initial aperture and if the main fault is curved the generation of a number of splay fractures can act to increase the permeability of the rock mass when the splay fractures are constrained by two faults a rhomb shaped fault zone involving multiple high angle branches forms the development of the second generation splay fractures on a first generation one is promoted by fluid penetration multiple fluid driven splay fractures can be created under the condition that fluid pressure is below or slightly above the fault confining stress this implies that generation of complex splay fracture patterns requires less energy than generating a single opening mode fracture multiple fluid driven splay fracture nucleation and growth into a horsetail pattern occur when the fault parallel in situ normal stress becomes more tensile in addition the effects of fluid viscosity and tensile fracture toughness are investigated to determine their role in splay fracture initiation and growth

secular variation from mexican stalagmites their potential and problems abstract as part of a feasibility study to see whether stalagmites could provide useful records of secular variation nine oriented stalagmites were collected from the states of chiapas and san luis potosi mexico of these six have yielded measurable natural remanent magnetizations nrms throughout their length the cleaned magnetizations of one of these samples were shown unequivocally to have recorded the ambient field and there is no reason to believe that this is not true of the primary magnetizations of other samples the sequences of palaeofield directions up the samples length have varying degrees of resolution and serial correlation depending on the rate of field change as averaged by the stalagmite growth rate and the thickness of the measured specimens ages and growth rates were estimated by the u th method the main problem in stalagmite palaeomagnetic analysis is a weak nrm although this may be avoided by judicious choice of the sample samples which possess significant viscous components may be cleaned by alternating field or thermally if there is sufficient magnetic material unlike many sediments stalagmites do not appear to suffer from depositional error problems dating problems may include low initial u content yielding ages with large errors and the presence of allogenic 230 th in detritus which causes older apparent ages the dating limit is 350 ka it is usually not possible to obtain long records comparable with those of most lake sediments and there are aesthetic reasons for not spoiling caves adorned with stalagmites the method is seen to be complementary to the use of sedimentary sequences to study palaeosecular variation studies of the rock magnetism of stalagmites are presented to suggest the mineral carriers of the magnetization and the origins of the natural remanence

role of fault branches in earthquake rupture dynamics 1 we analyze earthquake ruptures propagating along a straight main fault and encountering a finite length branch fault such intersections are often observed in natural fault systems the predicted effects of the interaction with the branch that we report can be remarkable they can strongly perturb the propagation velocity on the main fault and in some cases even arrest that propagation earlier work kame et al 2003 bhat et al 2004 emphasized the role of the fault pre stress state branch geometry i e branching angle and the incoming rupture velocity at the branching junction in determining whether the rupture would follow the branch or continue on the main fault or both through simulations which did not let a rupture on the branch encounter a barrier or a fault end called infinite branch cases henceforth in this study we look at finite branch cases and study the effect also of branch length with rupture being blocked from propagation beyond the branch end it is known that sudden stoppage of a dynamic rupture front leads to the propagation of large dynamic stress perturbations in the medium these have been known to nucleate or terminate ruptures on adjacent fault segments harris et al 1991 harris and day 1993 1999 harris et al 2002 fliss et al 2005 among others we thus anticipate interaction between the rupture on the main fault and the branched one at two stages when the rupture is propagating on the branch and when it is suddenly blocked at the branch end we show that in general rupture termination on a compressional branch little affects propagation on the main fault compared to the infinite branch cases for branches on the extensional side we show in some cases that whereas an infinite branch would have allowed or stopped rupture propagation on the main fault a finite branch stops or allows propagation on the main fault such results have a dependence on branch length that we document we also illustrate branch related complexities in rupture velocity evolution which could be one of the sources of the high frequency content of strong ground motion record complexities in the slip distribution often associated with a presumed heterogeneous strength distribution along the fault can also be observed when rupture is terminated on a branch

mechanics of fluid driven fracture growth in naturally fractured reservoirs with simple network geometries 1 a numerical model has been developed for fluid driven opening mode fracture growth in a naturally fractured formation the rock formation contains discrete deformable fractures which are initially closed but conductive because of their preexisting apertures fluid flow that develops along fractures depends on fracture geometry defined by preexisting aperture distribution offsets along a fracture path and intersections of two or more fractures the model couples fluid flow elastic deformation and frictional sliding to obtain the solution which depends on the competition between fractures for permeability enhancement the fractures can be opened by fluid pressure that exceeds the normal stress acting on them and by interactions with intersecting closed fractures experiencing coulomb type frictional slip the newtonian fluid is assumed to flow through the conductive fractures according to a lubrication equation that relates the cube of an equivalent hydraulic aperture to fracture conductivity the rock material is assumed to be impermeable and elastic this paper provides the governing equations for the multiple fracture systems and the solution methods used flow distribution and fracture growth in conductive fracture sets are simulated for a range of geometric arrangements and hydraulic properties numerical results show that elastic interaction between fracture branches plays a controlling role in fluid migration although initial apertures can give rise to a preferential fluid flow direction during the early stage in the presence of offsets fracture segments subject to strong compression are difficult to open hydraulically and their resulting smaller permeability can increase overall upstream fracture pressure and opening the patterns of fluid flow become more complicated if fractures intersect each other a portion of injected fluid is lost into closed empty fractures that cut across the main hydraulic fracture and this delays the pressure increases required for fracture growth past the crosscutting fracture the nonlinear fluid loss rate depends on the geometric complexities of the fracture sets and on the fluid viscosity sometimes fracture growth can be accelerated by the fast fluid transport along an intersected relatively conductive natural fracture

climate and vegetation history of the midcontinent from 75 to 25 ka a speleothem record from crevice cave missouri usa four missouri stalagmites yield consistent overlapping records of oxygen and carbon isotopic changes and provide a climate and vegetation history with submillennial resolution from 75 to 25 thousand years ago ka the thorium 230 dated records reveal that between 75 and 55 ka the midcontinental climate oscillated on millennial time scales between cold and warm and vegetation alternated among forest savanna and prairie temperatures were highest and prairie vegetation peaked between 59 and 55 ka climate cooled and forest replaced grassland at 55 ka when global ice sheets began to build during the early part of marine oxygen isotope stage 3

u pb la icp ms dating of apatite in mafic rocks evidence for a major magmatic event at the devonian carboniferous boundary in the armorican massif france apatite is a ubiquitous accessory mineral found in most magmatic rocks and is often the only u bearing mineral available to date mafic rocks because primary zircon and or baddeleyite are not present in this paper u pb la icp ms dating of apatite was applied to seven different dike and sill samples of dolerite from the variscan belt of brittany armorican massif western france these dolerites which are characterized by a within plate tholeiite geochemical signature are organized in several dense swarms across the belt their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic metamorphic settings their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the 40ar 39ar or the u pb methods on classical minerals like mica plagioclase or zircon although the closure temperature of apatite is lower than the emplacement temperature of the magma physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less consequently the u pb dates obtained on apatite can be interpreted as the emplacement ages for these mafic intrusions our results demonstrate that in all cases the apatite grains do carry enough radiogenic pb to be dated by in situ u pb analyses and yield a 207pb corrected mean age of 363 4 5 8 ma these results reveal the existence of a major and short lived magmatic event in the variscan belt of brittany during the devonian carboniferous transition a feature further highlighted by field evidence beyond the geological implications of these results u pb la icp ms dating of apatite appears to represent an ideal tool to date small size mafic intrusions

the interaction of two closely spaced cracks a rock model study the influence of preexisting fractures on fracture development and the geometric patterns of crack growth have been investigated experimentally rectangular specimens of berea sandstone with two parallel thin cuts were compressed along their longest dimension at a confining pressure of 60 mpa a pore pressure of 20 mpa an average displacement rate of 2 10 4 cm s and room temperature the precuts are inclined 35 to the loading direction they are en echelon with each other each extending only part way from the specimen boundary when the sense of shear left or right lateral overlap left or right stepping and the amount of separation are changed significant difference of fracture interaction occurs an extensional fracture zone forms between left stepping precuts subjected to left lateral shear or right stepping precuts under right lateral shear secondary fractures tend to develop from one precut and toward the other precut the sequence of the fracture development is as follows a fractures which are about 60 to the precuts fractures which are about 30 to the precuts and fractures for large overlap pattern which are about 70 to the precuts but opposite in direction the fracture zone is confined by precuts and fractures therefore the larger the overlap the larger the fracture zone that develops for large overlap patterns a set of fractures forms nearly parallel and equally spaced within the fracture zone in cross sections fractures are normal faults with surface depression microscopic study shows that microfractures are highly concentrated in the area immediately adjacent to the macrofractures and at angles less than 30 to the macrofracture grains away from macrofractures remain unfractured for right stepping precuts subject to left lateral shear or left stepping precuts under right lateral shear a compression zone forms between the precuts the fractures propagate at precut tips and diverge away the fractures develop within the compression zone and are bounded by precuts and fractures the fractures develop within the compression zone and are bounded by precuts and fractures the fractures have reverse offsets in cross section

calcite fabrics growth mechanisms and environments of formation in speleothems from the italian alps and southwestern ireland five fabrics were identified in alpine and irish caves on the basis of morphological and microstructural characteristics and re lated to growth mechanisms and growth environment columnar and fibrous fabrics grow when speleothems are continuously wet and from fluids at near equilibrium conditions low supersaturation sicc 0 35 through the screw dislocation mechanism the highly defective microcrystalline fabrics form at the same supersaturation range as co lumnar fabric but under variable discharge and the presence of growth inhibitors dendritic fabrics which have the highest density of crystal defects develop in disequilibrium conditions high supersaturation un der periodic very low flow regime periods that result in prolonged out gassing cave calcareous tufa forms in disequilibrium conditions only the calcite crystals of fabrics formed at low supersaturation seem to precipitate near isotopic equilibrium conditions

magnetic detection of paleoflood layers in stalagmites and implications for historical land use changes abstract flooding events are major natural hazards that present significant risk to communities worldwide calculations of flood recurrence rate through time are important tools for regulating land use determining insurance rates and for the design and construction of levees and dams typically flood recurrence rates are based on limited historical data or on evidence preserved in the geologic record as overbank deposits tree ring scars or high water scour marks however these approaches are either limited in their ability to produce continuous time series of flooding events or do not consider the effects of regional land use change here we use scanning superconducting quantum interference device squid microscopy to rapidly image the magnetization associated with flood layers in a polished surface of an annually laminated stalagmite from spring valley caverns svc in southeastern minnesota a time series of magnetization peaks each of which corresponds to a flooding event yields an average flood recurrence rate of 5 events per century for the last 500 years this rate increases to 7 events per century since 1900 coincident with historical timber and agricultural land use changes in minnesota this approach produces a continuous record of well dated extreme precipitation events that can be examined within the context of land use change

advancements in the use of speleothems as climate archives abstract speleothems have become a cornerstone of the approach to better understanding earth s climatic teleconnections due to their precise absolute chronologies their continuous or semicontinuous deposition and their global terrestrial distribution we review the last decade of speleothem related research building off a similar review by mcdermott 2004 in three themes i investigation of global teleconnections using speleothem based climate reconstructions ii refinement of climate interpretations from speleothem proxies through cave monitoring and iii novel technical methods of speleothem based climate reconstructions speleothem records have enabled critical insight into the response of global hydroclimate to large climate changes this includes the relevant forcings and sequence of climatic responses involved in glacial terminations and recognition of a global monsoon response to climate changes on orbital and millennial time scales we review advancements in understanding of the processes that control speleothem 13 c values and introduce the idea of a direct atmospheric pco 2 influence we discuss progress in understanding kinetic isotope fractionation which with further advances may help quantify paleoclimate changes despite non equilibrium formation of speleothems this feeds into the potential of proxy system modeling to consider climatic hydrological and biogeochemical processes with the objective of quantitatively interpreting speleothem proxies finally we provide an overview of emerging speleothem proxies and novel approaches using existing proxies most recently technical advancements made in the measurement of fluid inclusions are now yielding reliable determinations of paleotemperatures

the paleomagnetism and u th dating of three canadian speleothems evidence for the westward drift 5 4 2 1 ka bp three speleothems from western canada have been analyzed for their paleomagnetism and u th activity ratios these cave deposits do not appear to show the kind of depositional effects on the recorded magnetic signal as is often the case with sediments in addition the signal may be referred directly to geographic coordinates so that virtual geomagnetic pole plots may be constructed this in turn has allowed assessment of any possible bias of the paleofield the u th activity ratios of two flowstones from the crowsnest pass area of british columbia indicated ages greater than the 350 ka dating limit of the dating method one of the samples was reversely magnetized besides their use in the study of the ancient field these samples have useful geomorphic applications a stalagmite from vancouver island was shown to have recorded clockwise loops of the field vector in the period 5 4 2 1 ka bp this implies a mainly westward drift of the paleofield for this period the corresponding vgp s were mostly far sided

magnetism signals in a stalagmite from southern china and reconstruction of paleorainfall during the interglacial glacial transition precipitation variations deduced from speleothem stable oxygen isotopes 18o have long been debated in monsoonal china the magnetic property in speleothems is interpreted to be related to regional precipitation and can therefore offer reliable information of regional precipitation variations here we present a speleothem from southern china to explore the magnetic signals in speleothems and to reconstruct regional precipitation saturation magnetization ms which represents the magnetic mineral concentrations is used as an indicator of effective rainfall variations the precipitation inferred from ms varies increased sharply in the period prior to 80 3 ka bp followed by a drop to precipitation anomaly afterwards indicating that regional precipitation varied greatly during the hominin occupied interval in southern china power spectrum analysis of ms values shows significant 205 90 77 and 64 yrs periodicities suggesting that the solar activity may play an important role in precipitation variations in southern china during the mis5a 4 transition

effects of mesoscopic scale fault structure on dynamic earthquake ruptures dynamic formation of geometrical complexity of earthquake faults 1 we develop a new hierarchical earthquake rupture model that takes into account mesoscopic scale fault structure shear branches nucleated on the main fault are specifically assumed as an example of mesoscopic scale fault structure we numerically investigate dynamic formation of fault geometry and its effects on dynamic earthquake rupture process based on this rupture model as long as the length of the main fault is below a certain threshold lm the growth of these branches is shown to be arrested spontaneously soon after their nucleation the spatial distribution of arrested branches is shown to form a self similar geometrical structure this suggests the existence of a simple scaling relationship between small and large events as long as the length of the main fault is below the threshold lm however once the length of the main fault exceeds lm a limited number of branches begin unstable growth and their sizes soon become comparable to that of the main fault in other words mesoscopic scale branches are spontaneously transformed into macroscopic scale ones this finding indicates the critical importance of the consideration of mesoscopic scale fault structure in understanding rupture dynamics once macroscopic scale branches are formed they change the fault geometry which will considerably affect the rupture dynamics the emergence of macroscopic branches suggests that the above mentioned simple scaling relation is never valid above a critical length lm our study thus indicates that relationship between small and large earthquakes is complicated by the spontaneous transformation of a mesoscopic scale fault structure into a macroscopic scale one

composition and accretion rate of fossil micrometeorites recovered in middle triassic deep sea deposits micrometeorites which are submillimeter sized extraterrestrial particles that survive atmospheric entry originate from dust producing objects such as comets and asteroids although ancient micrometeorites found in sedimentary rocks are of key interest as a historical record of meteoroid populations in the solar system they are rare and prone to severe chemical weathering here we report the recovery of well preserved micrometeorites older than 240 ma in radiolarian chert from japan the collection of micrometeorites comprised 258 cosmic spherules which are particles that totally melted during atmospheric entry and 2 coarse grained unmelted micrometeorites these micrometeorites are much older than any previous micrometeorite collection in the sedimentary record using this collection we calculated the accretion rate of iron type cosmic spherules to the earth during the anisian stage of the middle triassic the estimated accretion rate for anisian iron type spherules smaller than 125 m is 25 8 t yr 1 analysis of the accretion rate for cosmic spherules also reveals high accretion rates of small spherules 8 36 m for a 0 74 m y period in the late anisian however the possible link between an enhancement in the accretion rate of small cosmic spherules in the late anisian and variations in the flux of extraterrestrial matter to the earth requires further scrutiny

magnetic fingerprint of southern portuguese speleothems and implications for paleomagnetism and environmental magnetism environmental magnetism of speleothems is still in its early stage of development here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems and what are the factors that control its preservation and reliability to address these issues we used a multidisciplinary approach including rock magnetism petrography scanning electron microscopy stable carbon and oxygen isotope compositions and major and trace element concentrations we applied this to a set of samples from different stages of speleothem evolution present day dripwater glass plates a weathered stalactite a fresh stalagmite cave sediments and terra rossa soils these samples come from the penico and excentricas caves located in two distinct aquifers of the algarve region south portugal our results show that the main magnetic carriers of the speleothems under study are primary detrital and consist of maghemite and magnetite similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater hence they represent a regional environmental signature interestingly a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities we propose that these low intensities can be the result from i different remanence acquisition mechanisms between stalagmite and stalactite and or ii iron dissolution by fungal activity we also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock soil atmosphere while oxygen isotope composition and alkaline earth element concentrations inform about calcite water interaction processes these results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo environmental signal

origin damping and pattern of development of faults in granite observations of brittle fractures in the granite of the massif de la borne as well as in other rocks allow us to discuss the different hypotheses regarding the origin of faults it seems that all faults originate by shear along a preexisting plane which can be an important fracture joint former fault or a microflaw pore grain boundary the shear on the plane gives rise to an array of en echelon cracks that makes the rock less resistant and permits the lengthening of the fault we also found an arrangement of horsetail fractures damping the movements on the faults we then propose a pattern for the development of the faults they originate on preexisting plane and propagate by initiating en echelon cracks the movements are damped by horsetails which permit by faults connection in relays the creation of larger faults

correlation of the nappe stack in the ibero armorican arc across the bay of biscay a joint french spanish project a correlation between allochthonous units exposed in the nw iberian massif and the southern armorican massif is carried out based on lithological associations structural position age and geochemistry of protoliths and tectonometamorphic evolution the units on both sides of the bay of biscay are grouped into upper middle and lower allochthons whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the parautochthon the lower allochthon represents a fragment of the outermost edge of gondwana that underwent continental subduction shortly after the closure of a palaeozoic ocean which in turn is represented by the middle allochthon the latter consists of supra subduction ophiolites and metasedimentary sequences alternating with basic mid ocean ridge basalt morb type volcanics with inheritances suggesting the proximity of a continental domain seafloor spreading began at the cambro ordovician boundary and oceanic crust was still formed during the late devonian covering the lifetime of the rheic ocean which is possibly represented by the middle allochthon the opening of the oceanic domain was related to pulling apart the peri gondwanan continental magmatic arc which is represented by the upper allochthon

new late pennsylvanian paleomagnetic results from parana basin southern brazil is the recent giant gaussian process model valid for the kiaman superchron

the significance of silica precipitation on the formation of the permeable impermeable boundary within earth s crust the hydrological system within earth s crust is divided into the permeable zone and the underlying much less permeable zone in this study we investigated the solubility and precipitation kinetics of silica in water under the conditions of the kakkonda geothermal field japan where well wd 1a penetrates the boundary between the hydrothermal convection zone and the heat conduction zone we found that quartz solubility has a local minimum at the depth of the hydrological boundary at 3100 m depth 380 c and 24 mpa in the case of either hydrostatic conditions or fluid pressure increase above hydrostatic at deeper levels the hydrothermal experiments reveal that rapid quartz precipitation occurs via nucleation when fluids are brought from the liquid region to the supercritical region the preferential precipitation of quartz at a specific depth plays a significant role in forming and sustaining the permeable impermeable boundary in the crust

holocene enso related cyclic storms recorded by magnetic minerals in speleothems of central china extreme hydrologic events such as storms and floods have the potential to severely impact modern human society however the frequency of storms and their underlying mechanisms are limited by a paucity of suitable proxies especially in inland areas here we present a record of speleothem magnetic minerals to reconstruct paleoprecipitation including storms in the eastern asian monsoon area over the last 8 6 ky the geophysical parameter irmsoft flux represents the flux of soil derived magnetic minerals preserved in stalagmite hs4 which we correlate with rainfall amount and intensity irmsoft flux exhibits relatively higher values before 6 7 ky and after 3 4 ky and lower values in the intervening period consistent with regional hydrological changes observed in independent records abrupt enhancements in the flux of pedogenic magnetite in the stalagmite agree well with the timing of known regional paleofloods and with equatorial el nino southern oscillation enso patterns documenting the occurrence of enso related storms in the holocene spectral power analyses reveal that the storms occur on a significant 500 y cycle coincident with periodic solar activity and enso variance showing that reinforced subdued storms in central china correspond to reduced increased solar activity and amplified damped enso thus the magnetic minerals in speleothem hs4 preserve a record of the cyclic storms controlled by the coupled atmosphere oceanic circulation driven by solar activity

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

a 10 000 yr record of high resolution paleosecular variation from a flowstone of rio martino cave northwestern alps italy abstract speleothems are potentially excellent archives of the earth s magnetic field capable of recording its past variations their characteristics such as the continuity of the record the possibility to be easily dated the almost instantaneous remanence acquisition and the high time resolution make them potentially unique high quality paleosecular variation psv recorders nevertheless speleothems are commonly characterized by low magnetic intensities which often limits their resolution here we present a paleomagnetic study performed on two cores from a flowstone from the rio martino cave western alps italy u th dating indicates that the flowstone s deposition covers almost the entire holocene spanning the period ca 0 5 9 0 ka while an estimation of its mean growth rate is around 1 mm per 15 years the flowstone is composed of columnar calcite characterized by a highly magnetic detrital content from meta ophiolites in the cave s catchment this favorable geological context results in an intense magnetic signal that permits the preparation and measurement of thin 3 mm depth equivalent samples each representing around 45 yr the characteristic remanent magnetization chrm isolated after systematic stepwise alternating field demagnetization is well defined with maximum angular deviation mad generally lower than 10 paleomagnetic directional data allow the reconstruction of the psv path during the holocene for the area comparison of the new data with archeomagnetic data from italian archeological and volcanic records and using the predictions of the sha dif 14k and pfm9k 1a global geomagnetic field models shows that the rio martino flowstone represents an excellent recorder of the earth s magnetic field during the last 9 000 years our high resolution paleomagnetic record anchored by a high quality chronology provides promising data both for the detection of short term geomagnetic field variations and for complementing existing regional psv curves for the prehistoric period for which well dated data are still scarce

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

thermal modification of hematite ilmenite intergrowths in the ecstall pluton british columbia canada in this study we examine the effects of reheating on finely exsolved hematite ilmenite intergrowths from the 91 ma ecstall pluton using reflected light microscopy scanning electron microscopy sem and transmission electron microscopy tem as a result of the emplacement of the younger adjacent 52 ma quottoon pluton samples closer to a thermal boundary have experienced greater degrees of thermal alteration five main microstructural features characterize hematite ilmenite intergrowths from the ecstall i exsolution lamellae of hematite and ilmenite ii oxidation of ilmenite to form hematite rutile and other ti rich phases iii 20 50 nm magnetite precipitates in hematite iv rutile blitz texture and v exsolution of hematite in rutile based on spatial relationships textures ii through v appear to be related to reheating of the ecstall by the quottoon and samples up to 14 km from the thermal boundary intrusive contact have been affected we propose a mechanism similar to that of kontny and dietl 2001 in which reheating has driven t f o2 conditions across the hematite magnetite buffer to lower f o2 resulting in the reduction of hematite higher temperatures also enhanced oxidation in ilmenite the formation of magnetite altered the bulk magnetic properties of these samples increasing nrm intensity this study underscores the need to consider a pluton s post emplacement thermal history before making tectonic interpretations based on paleomagnetic data

deflection and propagation of fluid driven fractures at frictional bedding interfaces a numerical investigation propagation of fluid driven or hydraulic fractures deflected at bedding interfaces in layered sedimentary rocks and subsequent fluid invasion is investigated numerically using a two dimensional boundary element model the fracture is driven by an incompressible newtonian fluid injected at a constant rate the frictional stress on the interface is assumed to obey coulomb s frictional law without cohesion the bedding interface can be given a non zero minimum fluid conductivity a numerical scheme that deals with both rock deformation and fluid flow is presented and its accuracy is verified in terms of comparisons with existing results to explore the mechanisms involved in fluid and fracture invasion into the interface parametric studies are carried out for different elastic modulus contrasts in situ stresses interfacial frictional coefficients distances from the injection point to the interface and fluid viscosities the results are provided as time dependent variations of displacements fluid pressures contact stresses and fluid fronts fracture deflection and fluid invasion into the interface are found to rely essentially on local stress and deformation states at the intersection point fluid invasion and fracture growth may be delayed or inhibited when the interface is subjected to large confining stress or when fluid viscosity is relatively low for cases where the fluid driven fracture originates in the softer layer in this case a greater layer parallel tensile stress is produced and can lead to fracture propagation through the bedding contact low to medium frictional strength is found to promote fluid penetration and t shaped fracture formation by interfacial opening if the hydraulic fracture originates in a stiffer layer fluid invasion into bedding contacts can occur smoothly without the occurrence of interface closure and the fracture is thus terminated by forming a t shaped fracture at the bedding interface for fracture deflection into and growth along the interface in the absence of interface closure the long time responses resemble the solution for a fluid driven fracture growing along a frictionless interface with vanishing toughness

tectonic record magmatic history and hydrothermal alteration in the hercynian guerande leucogranite armorican massif france abstract the guerande peraluminous leucogranite was emplaced at the end of the carboniferous in the southern part of the armorican massif at the scale of the intrusion this granite displays structural heterogeneities with a weak deformation in the southwestern part whereas the northwestern part is marked by the occurrence of s c and mylonitic extensional fabrics quartz veins and pegmatite dykes orientations as well as lineations directions in the granite and its country rocks demonstrate both e w and n s stretching therefore during its emplacement in an extensional tectonic regime the syntectonic guerande granite has probably experienced some partitioning of the deformation the southwestern part is characterized by a muscovite biotite assemblage the presence of restites and migmatitic enclaves and a low abundance of quartz veins compared to pegmatite dykes in contrast the northwestern part is characterized by a muscovite tourmaline assemblage evidence of albitization and gresenization and a larger amount of quartz veins the southwestern part is thus interpreted as the feeding zone of the intrusion whereas the northwestern part corresponds to its apical zone the granite samples display continuous compositional evolutions in the range of 69 8 75 3 wt sio 2 high initial 87 sr 86 sr ratios and low e nd t values suggest that the peraluminous guerande granite a cnk 1 1 was formed by partial melting of metasedimentary formations magmatic evolution was controlled primarily by fractional crystallization of k feldspar biotite and plagioclase an20 the samples from the apical zone show evidence of secondary muscovitization they are also characterized by a high content in incompatible elements such as cs and sn as well as low nb ta and k rb ratios the apical zone of the guerande granite underwent a pervasive hydrothermal alteration during or soon after its emplacement u th pb dating on zircon and monazite revealed that the guerande granite was emplaced 309 7 1 3 ma ago and that a late magmatic activity synchronous with hydrothermal circulation occurred at ca 303 ma these new structural petrological and geochronological data presented for the guerande leucogranite highlight the interplay between the emplacement in an extensional tectonic regime magmatic differentiation and hydrothermal alteration and provide a general background for the understanding of the processes controlling some mineralization in the western european hercynian belt

high temperature 350 c thermochronology and mechanisms of pb loss in apatite abstract natural processes driven by heat flow can be understood using quantitative reconstruction of the thermal history of accessory and common minerals that were formed or modified in these processes thermochronology assumes that isotopes are lost from minerals by thermally activated volume diffusion and forms the basis of many studies of the thermal evolution of the crust however some studies challenge this assumption and suggest that the mechanisms controlling isotope transport in minerals over geological time scales are dominated by aqueous fluid flow within mineral pathways here we test these contrasting hypotheses by inverse modelling apatite uranium lead u pb dates to produce theoretical t t solutions assuming pb was lost by volume diffusion these solutions are compared with independent geological constraints and intra grain apatite u pb dates which demonstrate that volume diffusion governed the displacement of pb this confirmation combined with an inverse modeling procedure that permits reheating and cooling paths to be distinguished between 375 and 570 c provides geologists with a unique tool for investigating the high temperature thermal evolution of accessory minerals using the u pb method

palaeozoic oolitic ironstone of the french armorican massif a chemical and structural trap for orogenic base metal as sb au mineralisation during hercynian strike slip deformation in the saint aubin des ch teaux quarry armorican hercynian belt western france an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the lower ordovician gres armoricain formation the hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer these sulphide lenses are spatially associated with strike slip faults and extend laterally from them after the massive sulphidation stage fe as stage 1 subsequent fracturing allowed the deposition of base metals stage 2 and pb sb au stage 3 parageneses in veins the dominant brittle structures are vertical extension veins conjugate shear veins and strike slip faults of various orders all these structures are filled with the same paragenetic sequence deformation analysis allows the identification of structures that developed incrementally via right lateral simple shear compatible with bulk strain affecting the central armorican domain each increment corresponds to a fracture set filled with specific parageneses successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids maximum of 390 350 c during the main sulphide stage 1 the subsequent stages present a marked temperature drop 300 275 c lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying neo proterozoic basement lead isotope data relative ages of deformation and comparison with neighbouring deposits suggest that large scale fluid pulses occurred during the whole hercynian orogeny rather than pulses restricted to the late hercynian period the vicinity of the hercynian internal domain appears as a key control for deformation and fluid flow in the oolitic ironstones which acted as a chemical and structural trap for the hydrothermal fluids the epigenetic mineralisation of saint aubin des ch teaux appears to be very similar to epigenetic sulphidation described in banded iron formation hosted gold deposits

the effect of speleothem surface slope on the remanent magnetic inclination speleothems are of interest for high resolution reconstruction of the earth s magnetic field however little is known about the influence of speleothem morphologies on their natural remanent magnetization nrm record here we report on a high resolution paleomagnetic study of a dome shaped speleothem of middle holocene age from southern portugal with special attention to the anisotropy of magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm to assess the potential influence of the slope of the speleothem surface on the recorded remanent magnetization we compare magnetic directions and ams and aarm fabrics from sub horizontal to gradually sub vertical calcite growth layers collected in a transversal cross section of the speleothem a linear correlation is observed between magnetic inclinations calcite laminae slope and aarm k1 inclination the ams fabric is mostly controlled by calcite crystals with direction of the minimum axes k3 perpendicular to laminae growth magnetic inclinations recorded in inclined and vertical calcite growth layers are underestimated when compared to a global paleosecular variation psv model after extrapolating magnetic inclinations to the horizontal the corrected data better fit the psv model but are still lower than the predicted magnetic inclinations suggesting that inclination shallowing affects the entire speleothem we suggest that speleothem morphology exerts a critical role on the magnetic inclination recording which is controlled by the earth s magnetic field but also influenced by particle rolling along the sloping surfaces these observations open new avenues for reconstructing high resolution paleomagnetic secular variations records from speleothems and provide new insights into their nrm acquisition mechanisms

the paleomagnetism and u th dating of mexican stalagmite das2 a 72 cm high stalagmite from mexico das2 has been analyzed for its paleomagnetism and has provided a record of secular variation beginning about 1200 years ago to present the stalagmite did not grow uniformly and a low content of magnetic material has rendered the record less precise in some parts a topographic test involving the magnetic measurement of coeval specimens from the sides of the stalagmite has shown that the record is detectably free from depositional errors age data were provided by the u th method and concordancy checked by14c and226ra methods shallow inclinations at the base of the record are followed by a partial anticlockwise loop in the declination inclination plot there is good agreement between the das2 data and the southwest u s a archeomagnetic record and with a lacustrine record from oregon from 750 to 1400 ad comparison of the record with the present field at the site latitude shows that westward drift of the field has persisted for at least 1200 years at a drift rate of about 0 30 yr 1 a close match in declinations with the present field at the site latitude suggests that the stalagmite may have recorded the passing of the northern edge of the gulf of guinea negative anomaly about 1000 years ago though the corresponding inclination feature is not present in the record an archeomagnetic synthesis by merrill and mcelhinny of worldwide data to estimate positions of the main dipole shows that it was tilted away from mexico from 500 to 1250 ad and this may account for the shallow inclinations seen at the start of the record

rb sr and 40ar 39ar ages in blueschists from the ile de groix armorican massif france implications for closure mechanisms in isotopic systems rb sr and 40ar 39ar data mainly ranging from 345 5 to 361 5 ma and from 348 4 1 4 to 364 6 1 4 ma respectively have been obtained on phengite from the hp lt rocks from the ile de groix armorican massif france which represents one of the few occurrences of variscan blueschists petrological data show two blueschist facies units recording distinct peak p t conditions about 16 20 kbar 500 c and 12 16 kbar 450 c in the upper and lower units respectively followed by a nearly isothermal decompression in the greenschist facies rb sr and 40ar 39ar results defining two groups of ages mainly around 345 353 ma and 358 366 ma are consistent in well preserved blueschist facies rocks as well as in strongly overprinted samples from both units one sample excepted there is no evidence of inherited or excess argon and resetting of the isotopic systems oldest rb sr and 40ar 39ar ages are found in rocks with well preserved blueschist facies parageneses while younger ages are obtained on samples with a strong greenschist facies overprint this suggests that the closure of the k ar system is diachronous in samples which record the same p t evolution a closure of the k ar clock at around 450 500 c in phengite from rocks left nearly undeformed after the blueschist facies metamorphism is in agreement with petrological geochemical and structural data thus in these samples the oldest rb sr and 40ar 39ar ages around 360 370 ma represent the age of the hp metamorphism in the overprinted samples later closure of the k ar and rb sr clocks is the result of retrogression and continuing ductile deformation the small age difference between the oldest and the youngest rb sr and 40ar 39ar ages around 10 20 ma argues in favour of a relatively fast cooling

crystallization of apatite in natural magmas under high pressure hydrous conditions with particular reference to orogenic rock series two separate series of hydrous experiments involving 1 imposing apatite saturation on a series of igneous rock compositions from basanite to rhyolite and 2 crystallizing similar natural rock compositions progressively until apatite appears demonstrate a close dependence between apatite saturation and silica content of the magma and determine p2o5 levels at a given silica value and temperature at which that composition may be expected to crystallize apatite the effect of pressure on apatite solubility is not great and is most significant for silicic compositions

palaeomagnetism and 40ar 39ar dating of the tregor dolerites armorican massif france abstract the tregor doleritic dike swarm intrudes the cadomian north tregor batholith armorican massif france alternating field a f and thermal demagnetization performed on 158 samples from 18 dolerite dikes and 37 samples from 4 baked contact sites reveal a multi component magnetization and a complex magnetic mineralogy which was thoroughly investigated with thermomagnetic experiments and polished thin section examinations three magnetization components were recognized in addition to a low temperature component with a viscous origin the most frequently observed component named b d 214 i 10 was isolated at 12 sites independent upon their magnetic mineralogy and appears to be a remagnetization negative baked contact test a carboniferous age of acquisition is inferred from comparison of the b component palaeopole 29 s 318 e with an armorican apwp at some sites after demagnetization of the b component higher temperature and coercivity components were isolated their directions fall into two groups named a and c positive baked contact tests were obtained for both the c component d 174 i 50 whose magnetic carrier is magnetite yields a palaeopole 11 s 2 e which is similar to some ordovician palaeopoles from the british isles south of the lapetus suture or eocambrian palaeopoles from the tregor area the a component d 191 i 33 whose magnetic carrier is magnetite partly transformed to leucoxene yields a palaeopole 58 s 336 e and a palaeolatitude 17 9 n not previously observed for the armorican massif in palaeozoic time comparison between this palaeolatitude and those for northern spain from ordovician to carboniferous time suggests a late silurian remanence age this interpretation supports the model of apparent polar wander for gondwana between ordovician and devonian time proposed by van der voo and bachtadse and briden 40 ar 39 ar dating of mineral separates from the dikes do not allow us to place constraints on the acquisition age of the a and c components

the origin of magnetic remanence in stalagmites observations from electron microscopy and rock magnetism 1 speleothems especially stalagmites hold great potential as recorders of the earth s ancient magnetic field however our understanding of the magnetic mineral assemblages within speleothems is not well developed we present tandem rock magnetic characterization of bulk samples and electron microscopy of magnetic extracts from five north american stalagmites magnetite and goethite are present in all samples albeit in varying abundances and morphologies magnetite grains are likely detrital showing evidence of transport and weathering including plumose markings and etch pits these grains are unlikely to have precipitated abiotically in the cave environment titanomagnetite and exsolved intergrowths of fe ti oxides indicate that a significant portion of the magnetic mineral assemblage is allochthonous and was transported into the caves these ti rich grains display a range of morphologies from euhedral to well rounded indicating that earlier morphological models for speleothem magnetization do not apply in all geologic settings goethite was observed in three morphologies isolated needles needle aggregates and polycrystalline aggregates of randomly oriented nanoscale grains the magnetite and titanomagnetite as well as their partially oxidized equivalents likely hold a depositional remanent magnetization drm whereas if goethite grains were precipitated in situ on the stalagmite drip surface they are more likely to hold a chemical remanent magnetization crm because of their small size and low saturation magnetization until the remanence of goethite can be shown to be paleomagnetically meaningful we propose that paleomagnetic studies of speleothems include a 150 c thermal demagnetization step to remove any goethite remanence

the blake geomagnetic excursion recorded in a radiometrically dated speleothem one of the most important developments in geomagnetism has been the recognition of polarity excursions of the earth s magnetic field accurate timing of the excursions is a key point for under standing the geodynamo process and for magnetostratigraphic correlation one of the best known excursions is the blake geomagnetic episode which occurred during marine isotope stage mis 5 but its morphology and age remain controversial here wes how for the first time the blake excursion recorded in a stalagmite which was dated using the uranium series dis equilibrium techniques the characteristic remanent magnetisation is carried by fine grained magnetite the eventis documented by two reversed intervals b1 and b2 the age of the eventis estimated to be between 116 5 0 7 kyr bp and 112 0 1 9 kyr bp slightly younger 3 4 kyr than recent estimations from sedimentary records dated by astronomical tuning low values of relative palaeointensity during the blake episode are estimated but a relative maximum in the palaeofield in tensity coeval with the complete reversal during the b2 interval was observed duration of the blake geomagnetic excursion is 4 5 kyr two times lower than single excursions and slightly higher than the estimated diffusion time for the inner core 3 kyr

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

multicycle dynamics of nonplanar strike slip faults 1 we perform two dimensional dynamic models of strike slip faults with a change in strike a bend over multiple earthquake cycles to examine the long term effects of nonplanar fault geometry a viscoelastic model a proxy for off fault deformation and tectonic loading is introduced for the interseismic process to avoid pathological stress buildup around the bend a finite element method with an elastodynamic model is used to simulate dynamic earthquake ruptures we find that stresses near the bend differ strongly from the regional stress field and that the fault develops a relatively steady state in which the stress level and the event pattern on the fault are stable reduced normal stress on the dilatational side and increased normal stress on the compressive side of the bend during dynamic ruptures result in the bend serving as an initiation and or a termination point s for rupture typical events on such a fault consist of two classes unilateral events that rupture only the favorable segment and bilateral events that rupture the favorable segment and part of or the entire unfavorable segment in the latter class of events a time delay in rupture around the bend results from a high yield stress on the compressive side of the bend other effects of the bent fault geometry include higher displacement on the inward wall than on the outward wall higher slip on the more favorable segment than on the less favorable segment and a large slip velocity on the compressive side of the bend

simple and efficient separation of magnetic minerals from speleothems and other carbonates abstract trace concentrations of iron oxide minerals in carbonate sediments can preserve fine details about earth processes from high resolution recordings of the earth s ancient magnetic field to microscopic remnants of extraterrestrial impacts this paper presents a novel flask extraction method which uses a neodymium magnet and an orbital shaker for simple and efficient separation of magnetic minerals from carbonate sediments a mineral assemblage of magnetic standards titanomagnetite magnetite goethite and hematite combined with other mineral standards kaolinite quartz and nanoscale tio2 was subjected to the extraction procedure and compared to a natural speleothem sample exposure of the magnetic standards to a mildly acidic acetate buffer ph 4 did not cause physical or chemical alteration the strongly magnetic minerals were reproducibly extracted with greater than 90 efficiency by mass from mixtures of the mineral standards xrd and low temperature magnetic characterization demonstrated phase purity of the extracts quantitative comparison with two commonly used literature methods showed that the flask extraction method was more reproducible and efficient the addition of surfactant na po3 6 did not significantly improve extraction efficiency sequential dissolution and flask extraction of a simulated speleothem containing magnetic particles resulted in consistent extraction efficiencies for samples containing large 1 m strongly magnetic grains but a reduction in efficiency was observed for smaller 1 m grain sizes no method successfully extracted the weakly magnetic goethite and hematite however unprecedented representative characterization of these minerals was possible through quantitative analysis of the remainder after the collection of the magnetic extract this approach may facilitate detailed characterization of a wide range of carbonates such as pelagic limestone dolostone unlithified carbonate ooze speleothems and freshwater and pedogenic carbonates such mineral extractions can lead to new insights into paleoenvironmental processes as well as an improved understanding of the recording of the earth s magnetic field

contact analyses and permeability measurements of polished fracture surfaces in sambagawa metapelitic schists and ryoke mylonites from the median tectonic line japan 1 fractures play a vital role in fluid flow through rocks and highly fractured zones can be the most permeable part of a fault zone however the evaluation of the hydraulic properties of natural fractured rocks at great depths is not easy because of the complexity of fracture geometry and the scaling problem from laboratory to nature as a first step toward the estimation of flow rate through fractured zones at depth this paper examines the issue of permeability estimation in the case of flow between polished surfaces of known topography or roughness via an integrated study of permeability measurements at effective pressures up to 180 mpa surface roughness measurements and contact analyses using the hertzian theory of surface asperities of fractures as a function of stress apertures between two polished surfaces in specimens of sambagawa pelitic schist and ryoke mylonite were estimated from permeability measurements assuming the cubic law the aperture of contacting surfaces has also been calculated from contact analyses of representative asperities despite some imperfections in the agreement between these two approaches the overall features of progressive reduction in aperture upon an increase in effective pressure were successfully predicted deviations between the two approaches are probably due to complex deformation of multiple contacting asperities or limitations of our contact analysis method

routine low damage apatite u pb dating using laser ablation multicollector icpms 1 apatite is a common u bearing accessory mineral with a u pb closure temperature of 500 c making u pb dating of apatite a potentially valuable thermochronometer however its low u concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods we overcome previous limitations by using a nu plasma multicollector icpms with an attached short pulse excimer laser and by identifying two new matrix matched reference apatites to correct for elemental fractionation gem quality 485 myr old apatite from madagascar which we independently characterized by id tims analysis and 523 5 ma apatite from the mcclure mountain syenite source of the40ar 39ar reference mmhb common pb is corrected using measured 204pb isobarically corrected for hg interference and a five step iterative process using stacey and kramers common pb model we accurately reproduce ages of numerous independently characterized apatites regularly achieving precision of 2 2 by pooling as few as five 30 m spot analyses data quality in apatite with low u concentrations low 206pb 204pb values 30 and young ages 75 ma is compromised by the goal of avoiding significant grain damage such limitations can be overcome by using spot sizes 65 m or greater but at the expense of substantial grain damage for single detrital apatite grains with ages of 500 ma precision of 4 2 was achieved by pooling 2 to 3 spots per grain the accuracy of our detrital results is supported by a good age match with similar closure temperature 40ar 39ar detrital hornblende ages from the same sediment

a speleothem record of younger dryas cooling klamath mountains oregon usa awell dated d 18 o record in a stalagmite from a cave in the klamath mountains oregon with a sampling interval of 50 yr indicates that the climate of this region cooled essentially synchronously with younger dryas climate change elsewhere in the northern hemisphere the d 18 o record also indicates significant century scale temperature variability during the early holocene the d 13 c record suggests increasing biomass over the cave through the last deglaciation with century scale variability but with little detectable response of vegetation to younger dryas cooling d 2005 university of washington all rights reserved

stress state and fluid pressure level along the whipple detachment fault california brittle structures in the upper 200 m of the footwall of the whipple detachment fault are used to characterize the state of stress under which the detachment slipped they are younger than or coeval with chloride brecciation and those that record shear typically are either moderately northeast dipping normal structures or steep northwest striking southwest down structures that we interpret as conjugate shears from this we infer that the maximum principal stress was oriented 55 80 from the detachment while they were active the angle between the conjugate faults averages about 30 indicating failure in the transtensional regime during conjugate faulting the effective least principal stress was nearly equal to the tensile strength the whipple fault probably had a normal coefficient of static friction and it slipped under hydrostatic to only moderately elevated fluid pressure rapid mineralization in the surroundings of major fault zones allows transtensional failure there and permits differential stress to be higher than if cohesionless friction limits strength this causes resolved shear stress sufficient for slip on faults severely misoriented relative to the maximum principal stress

observations under electron microscopy of magnetic minerals extracted from speleothems abstract magnetic minerals have been extracted from a selection of british speleothems and studied by electron microscopy and energy dispersive x ray analysis edxa observations of the surface morphology size and elemental composition of these grains has assisted in determining their origin thereby direct evidence as to the carriers of the natural remanent magnetization nrm of speleothems is provided three groups of grain morphology were identified 1 detrital grains ranging in size from 0 01 m to 10 m composed of magnetite hematite and titanomagnetite 2 unabraded grains ranging in size from 0 01 m to 0 1 m composed of magnetite these grains were further subdivided into those having an affinity with bacterial magnetites and those having an affinity with inorganic authigenic magnetites 3 needle like grains less than 2 m in length probably composed of goethite preliminary laboratory experiments suggest that authigenic magnetite and goethite may precipitate under conditions likely to prevail during speleothem growth the presence of detrital magnetic grains in all samples studied is strong evidence for the presence of a detrital remanent magnetization drm in all speleothems the presence of authigenic magnetite in some samples is the first direct evidence for the presence of a chemical remanent magnetization crm in some speleothems

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

synchronous millennial scale climatic changes in the great basin and the north atlantic during the last interglacial stalagmites from goshute cave located in the great basin of the western united states preserve 20 000 yr of millennial scale oxygen isotopic variability during marine isotope stages 5c and 5b similar in timing and structure to dansgaard oeschger d o events 23 21 from the greenland ice sheet project 2 record that d o interstades 23 21 were of longer duration than many of the later d o events coupled with the asymmetric shape of the d o oxygen isotope curve and the direct u th dating of the goshute cave stalagmites allows for an improved understanding of the synchroneity of climatic changes between the western continental united states and the north atlantic eastern pacifi c atmosphere interactions are a likely mechanism for transmission of millennial scale climate variability into the great basin

multidecadally resolved polarity oscillations during a geomagnetic excursion polarity reversals of the geomagnetic field have occurred through billions of years of earth history and were first revealed in the early 20th century almost a century later details of transitional field behavior during geomagnetic reversals and excursions remain poorly known here we present a multidecadally resolved geomagnetic excursion record from a radioisotopically dated chinese stalagmite at 107 91 thousand years before present with age precision of several decades the duration of geomagnetic directional oscillations ranged from several centuries at 106 103 thousand years before present to millennia at 98 92 thousand years before present with one abrupt reversal transition occurring in one to two centuries when the field was weakest these features indicate prolonged geodynamo instability repeated asymmetrical interhemispheric polarity drifts associated with weak dipole fields likely originated in earth s deep interior if such rapid polarity changes occurred in future they could severely affect satellites and human society

trace element indiscrimination diagrams abstract we tested the accuracy of trace element discrimination diagrams for basalts using new datasets from two petrological databases petdb and georoc both binary and ternary diagrams using zr ti v y th hf nb ta sm and sc do a poor job of discriminating between basalts generated in various tectonic environments continental flood basalt mid ocean ridge basalt ocean island basalt oceanic plateau basalt back arc basin basalt and various types of arc basalt the overlaps between the different types of basalt are too large for the confident application of such diagrams when used in the absence of geological and petrological constraints none of the diagrams we tested can clearly discriminate between back arc basin basalt and mid ocean ridge basalt between continental flood basalt and oceanic plateau basalt and between different types of arc basalt intra oceanic island and continental arcs only ocean island basalt and some mid ocean ridge basalt are generally distinguishable in the diagrams and even in this case mantle normalized trace element patterns offer a better solution for discriminating between the two types of basalt

modeling speleothem δ13c variability in a central sierra nevada cave using 14c and 87sr 86sr abstract carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate progressing downward from the soil zone overlying the cave these processes include soil respiration fluid rock interaction in the host limestone degassing of co2 and precipitation of calcite upflow from the speleothem drip site and calcite precipitation at the drip site here we develop a new approach to independently constrain the roles of water rock interaction and soil processes in controlling stalagmite 13c this approach uses the dead carbon proportion dcp estimated from coupled 14c and 230th u measurements in conjunction with sr isotope analyses on stalagmite calcite from a central sierra nevada foothills cave in california a region characterized by a highly seasonal mediterranean type climate to determine the roles of water rock interaction and soil processes in determining stalagmite 13c increases in stalagmite dcp between 16 5 and 8 8 ka are coincident with decreased 13c indicating a varying yet substantial contribution from the soil organic matter som reservoir likely due to significantly increased average age of som in the soil veneer above the cave during wet climatic intervals we use geochemical and isotope mixing models to estimate the host carbonate contribution throughout the 13c time series and determine the degree of degassing and calcite precipitation that occurred prior to precipitation of stalagmite calcite the degree of degassing and prior calcite precipitation we calculate varies systematically with other climate indicators with less degassing and prior calcite precipitation occurring during wetter climatic intervals and more during drier intervals modeled 13c values and degassing calculations suggest that some degree of prior calcite precipitation is necessary at all time intervals to explain measured stalagmite 13c values even during relatively wet intervals these results illustrate the importance of constraining degassing and prior calcite precipitation in the interpretation of speleothem 13c records particularly those from caves that formed in seasonal semi arid to arid environments

opening of a fault and resulting slip due to injection of fluid for the extraction of geothermal heat the behavior of a fault with a fluid filled region is studied for the extraction of heat from hot dry rock masses a fault is viewed as a prefractured plane in the earth s crust and is modeled by an interface across which two elastic bodies contact with each other where frictional forces against the relative movement between two surfaces of the interface are taken into account once fluid is injected between the two surfaces the fault will be opened up over a region where the pressure acting on the fault plane before the injection is locally smaller than that on the neighboring regions the size of the fluid filled region is compared with that of a crack opened by a hydraulic pressure in a homogeneous medium to show that the fluid filled region could be used as a reservoir for the extraction of geothermal heat the resulting slip caused by the reduction of frictional forces due to the injection of fluid is analyzed by the use of two models i e an interface with frictional resistance only and an interface with both frictional resistance and fracture toughness it is shown that the two models do not exhibit any significant difference in the size of the slipped region when the fluid filled region is of dimensions of practical interest

paleoclimate variability during the blake geomagnetic excursion mis 5d deduced from a speleothem record abstract to evaluate possible connections between climate and the earth s magnetic field we examine paleoclimate proxies in a stalagmite pa 8 recording the blake excursion 112 116 4 ka from cobre cave n spain trace element 13 c 18 o 234 u fluorescent lamination growth rate and paleomagnetic records were synchronized using a floating lamina counted chronology constrained by u th dates providing a high resolution multi proxy paleoclimate record for mis 5d the alpine cave setting and the combination of proxies contributed to improve the confidence of the paleoclimatic interpretation periods of relatively warm and humid climate likely favored forest development and resulted in high speleothem growth rates arguably annual fluorescent laminae low 13 c and mg and increased sr and ba colder periods limited soil activity and drip water availability leading to reduced speleothem growth poor development of fluorescent lamination enhanced water rock interaction leading to increased mg 13 c and 234 u and episodic flooding in the coldest and driest period recorded evaporation caused simultaneous 18 o and 13 c enrichments and perturbed the trace element patterns the blake took place in a relatively warm interestadial at the inception of the last glacial period but during a global cooling trend recorded in pa 8 by an overall decrease of 18 o and growth rate and increasing mg that trend culminated in the cessation of growth between 112 and 101 ka likely due to the onset of local glaciation correlated with greenland stadial 25 that trend is consistent with a link between low geomagnetic intensity and climate cooling but it does not prove it shorter term changes in relative paleointensity rpi relate to climate changes recorded in pa 8 particularly a prominent rpi low from 114 5 to 113 ka coincident with a significant cooling indicated by all proxy records suggesting a link between geomagnetic intensity and climate at millennial time scales although the reliability of such inference is limited by the inferior resolution of the paleomagnetic data and a possible contamination of the rpi data by unaccounted changes in magnetite concentration the agreement of those data with the marine record of cosmogenic be suggests that the rpi record of pa 8 may reflect truly geomagnetic intensity variations

displacement transfer between intersecting regional strike slip and extensional fault systems interaction and displacement transfer between active intersecting strike slip or transform and extensional fault systems are examined outcrop data from a well preserved strike slip fault and rift intersection in new zealand are compared to a global data set of 13 such intersections in both continental and oceanic crust displacement transfer between strike slip and normal faults is typically accomplished by gradual changes of fault orientations and slip vectors close to the intersection zone for two and three plate configurations these changes result in sub parallelism of the slip vectors of the component faults with their line of intersection the dimensions of the area over which fault strike and slip vectors change are principally controlled by the extent to which displacements on the dominant of the two intersecting fault systems are confined to a single slip surface or distributed across a zone where slip is spatially distributed the region in which the two displacement fields are superimposed produces transtension and associated oblique slip this distributed off fault deformation facilitates the development of a quasi stable configuration of the fault intersection region maintaining both the regional geometry and kinematics of the intersection zone which in many cases would not be possible for rigid block translations the dimensions of the transition zone are larger for continental crust than for oceanic crust because oceanic crust is thinner fault geometries in oceanic crust are simpler two plate configurations and the slip vectors of the component intersecting fault systems are sub parallel

hydraulic stimulation of natural fractures as revealed by induced microearthquakes carthage cotton valley gas field east texas we produced a high resolution microseismic image of a hydraulic fracture stimulation in the carthage cotton valley gas field of east texas we improved the precision of microseismic event locations four fold over initial locations by manually repicking the traveltimes in a spatial sequence allowing us to visually correlate waveforms of adjacent sources the new locations show vertical containment within individual targeted sands suggesting little or no hydraulic communication between the discrete perforation intervals simultaneously treated within an 80 m section treatment i e fracture zone lengths inferred from event locations are about 200 m greater at the shallow perforation intervals than at the deeper intervals the highest quality locations indicate fracture zone widths as narrow as 6 m similarity of adjacent source waveforms along with systematic changes of phase amplitude ratios and polarities indicate fairly uniform source mechanisms fracture plane orientation and sense of slip over

cadomian tectonics in northern brittany a contribution of 3 d crustal scale modelling abstract both available and newly acquired geological and geophysical data processed in the framework of the armor project have been used to construct a 3 d geometric and geological model of the cadomian domain of northern brittany the major results of the data processing are presented and discussed relative to a composite profile transecting the studied domain and constructed from the surface geology reflection seismics magnetotelluric soundings and gravimetric data they are then extrapolated at regional scale through a series of cross sections compiled from the surface geology and gravimetric data and calibrated onto the composite profile the 3 d crustal scale models were computed according to two procedures the first using a strim matra datavision modeler defines various interfaces through correlation between adjacent cross sections and is well adapted to the 3 d representation of geological structures the second based on voronoi diagrams leads to a volumetric model of the geological units and is a powerful tool for calculating 3 d geophysical responses of the model when the physical properties of the rock units are known the tectonic implications of the computed 3 d crustal scale geometry of the cadomian domain are then discussed in terms of volcanic arc thrusting with special emphasis being placed on the obliquity of the shortening direction by reference to the thrust fault trend finally the data are used to demonstrate the reactivation of some cadomian faults during the deposition of ordovician sandstones and following the emplacement of the paleozoic dolerite dyke swarm of northern brittany

structural aspects of fluid rock interactions in detachment zones metamorphic core complexes and associated detachment faults of the north american cordillera represent gently dipping normal displacement shear zones detachment zones along which hot deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate structures that formed during the ductile to brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones mylonitization which occurred along deeper segments of detachment zones below the brittle ductile transition was locally accompanied by tensile failure indicative of low deviatoric stress and high fluid pressure this condition persisted into the earliest phases of brittle deformation after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress brittle deformation of upper plate rocks occurred largely under conditions of low fluid pressure structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems 1 an upper plate system driven by convection and dominated by meteoric and connate fluids at near hydrostatic pressures and 2 a system within deeper levels of the shear zone where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping the late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear zone rocks to be overprinted by mineralization related to the upper plate fluid system

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

long term changes in precipitation recorded by magnetic minerals in speleothems speleothems are important paleoclimate archives researchers typically compile measurements of stable isotopic ratios dated using high precision u th radiometric techniques to reconstruct regional and global climate magnetic material incorporated within speleothems can provide an independent means of connecting large scale climatic changes with their impact on more localized processes in soils overlying cave systems under certain environmental conditions pedogenic processes can produce magnetite nanoparticles enhancement of pedogenic magnetite in soil profiles depends strongly on local precipitation pedogenic magnetite can be subsequently transferred via drip waters into underlying cave systems and incorporated into speleothems as they grow here we employ high resolution magnetic methods to analyze a well dated stalagmite from buckeye creek cave west virginia usa and find that changes in magnetite concentration follow both changes in stable isotopes measured in the same stalagmite and global climate proxies we interpret the changes in magnetite concentration as reflecting variations in local pedogenic processes controlled by changes in regional precipitation this record demonstrates how magnetic measurements on speleothems can constrain interpretations of speleothem climate proxies

apatite monazite and xenotime in metamorphic rocks this chapter focuses on phosphates that are significant in metamorphic rocks a quick survey of phosphate mineral descriptions at a commercial mineral web site revealed over 500 phosphate mineral names remarkably only three are common in metamorphic rocks apatite monazite and xenotime and this chapter is restricted to discussion of these minerals apatite monazite and to a lesser extent xenotime have enjoyed intensive study during the previous half century following world war ii considerable study was made of minerals that contain fissionable materials and the sometimes large concentrations of u and th in monazite culminated in a number of seminal papers on the occurrence of that mineral e g overstreet 1967 during the 1970s and early 1980s the study of apatite and monazite turned towards their role as a sink for rees and other trace elements in rocks e g watson 1980 watson and capobianco 1981 throughout this period monazite and to some extent xenotime enjoyed attention as a geochronometer e g parrish 1990 and through today this attention has continued to grow accessory minerals such as monazite xenotime and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons first it has recently been recognized that trace elements in metamorphic rocks and especially trace element zoning in garnet and other major phases contain considerable detailed information about the reaction history a rock has experienced coupled with the fact that diffusion of many trace elements in metamorphic minerals is in general considerably slower than diffusion of major elements in those same minerals the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements attention has naturally turned to their paragenesis it has also recently been discovered that

apatite in igneous systems apatite is a minor but ubiquitous mineral in most igneous rocks although the modal proportion of apatite in common rocks is generally low it can reach high concentrations in enclaves cumulates and other rocks of low abundance i e rocks that constitute a small volume of the crust and mantle e g nelsonites the presence of apatite in most rocks is due not only to its low solubility in naturally occurring melts and aqueous solutions but also to the limited ability of common rock forming minerals to accept the amount of phosphorus that occurs in most rocks into their structure in this paper we will discuss some aspects of the occurrence texture composition physical chemistry and petrogenetic significance of apatite in felsic rocks i e andesite to rhyolite and their plutonic analogs in mafic i e basalts and related rocks and plutonic analogs and ultramafic rocks of the earth s crust and mantle fluorapatite is by far the most common member of apatite family found in igneous rocks however most natural fluorapatite contains some chlorine and hydroxyl as well and these constituents can attain high concentrations in some cases the other halogens bromine and iodine also occur in apatite but their concentrations are much lower than chlorine and fluorine many cations commonly substitute for calcium and phosphorus in apatite however they rarely reach concentrations that warrant the definition of a separate mineral species apatite can be described by the general formula a5 xo4 3z following sommerauer and katz lehnert 1985 the a site accommodates large cations e g ca2 sr2 pb2 ba2 mg2 mn2 fe2 ree3 eu2 cd2 na pan and fleet ch 2 in this volume and comprises two sites that exhibit vii fold