chlorine cycling during subduction of altered oceanic crust eclogitic rocks that have experienced devolatilization with little or no interaction with external fluid sources can be viewed as analogues of crustal material which may be transferred back into the mantle during subduction thus they can be used to evaluate the extent of the recycling of volatile elements such as chlorine we report new oxygen isotope ratios of omphacite and fluid inclusion data determined from eclogitic metagabbros of the rocciavre massif italian alps the data are compared with those obtained for the monviso cyclades and the franciscan complex high pressure rocks in all localities relics of early dehydration fluids are preserved as primary fluid inclusions in the cores of omphacite megacrysts rocciavre monviso and franciscan complex or garnet cyclades salinity estimates of the inclusion fluids range from 32 to 45 wt nacl in rocciavre 17 to 21 wt nacl in monviso and are similar to seawater in other areas omphacite and bulk rock 18o values of rocciavre 5 1 6 8 and monviso 3 0 5 3 metagabbros are markedly lower than those of cyclades 6 8 14 3 and the franciscan 6 7 13 1 metabasites the fluid salinity 18o systematics of eclogitic rocks is similar to that documented along a typical section of the altered oceanic crust and unmetamorphosed ophiolites this suggests that high pressure metamorphism and associated processes did not modify significantly the variability in chlorine concentrations and oxygen isotope ratios inherited from a stage of sea floor hydrothermal alteration under low basaltic layer and high temperature gabbroic layer conditions respectively extrapolating the estimated h2o and cl contents of eclogitic rocks to a representative section of the subducted oceanic crust indicates that a minimum of 100 to 200 ppm cl could be recycled into the mantle during subduction this yields a cl h2o ratio of 3 6 to 7 5 10 2 for the subducted oceanic crust which is similar to e morb on the basis of available cl isotopic data we infer that a large proportion 70 of the cl stored in the altered crust should be recycled to the mantle to generate an isotopic composition of the subducted crust equivalent to the source of unaltered mid ocean ridge basalt 37cl 4 7

deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

evolution of synmetamorphic veins and their wallrocks through a western alps transect no evidence for large scale fluid flow stable isotope major and trace element systematics quartz rich synfolial veins and wallrocks from different areas of a western alps cross section are examined in an attempt to constrain the scale and mechanisms of fluid flow through metamorphic terrains different portions of this cross section underwent distinct p t evolutions as reflected by the mineralogy of veins a in the greenschist external dauphinois domain veins are characterized by quartz and calcite together with an aluminosilicate pyrophyllite na k bearing phyllosilicates and a femg bearing silicate chlorite b in narrow domains of the brianconnais and piemontais zones with a non eclogitic hp lt evolution followed by rapid cooling veins are characterized by quartz calcite ca and fe mg bearing silicates lawsonite and fe mg carpholite respectively and c in the major part of the brianconnais andpiemontais zones where low grade blueschistfacies metamorphism is followed by a greenschist facies evolution veins are characterized by the replacement of earlier lawsonite and carpholite by na and k bearing micas in order to constrain the provenance of fluids involved in vein formation we determined stable isotope c 0 h and major and trace element compositions in minerals and whole rocks for a large set of veins and wallrocks these rocks are compared with some non metamorphic shales from the northern paris basin considered as protoliths of the alpine schists stable isotope compositions of calcites are regionally distinct 6r80 28 0 smow in the northern paris basin 26 0 in the brianconnais zone 22 0 in the piemontais zone and 18 0 in the dauphinois zone within each studied region calcite g o values are quite homogeneous regardless of rock type vein or wallrock this feature is explained as an isotopic homogenization with the most abundant lithology in each zone rock buffered system where the overall ratio of calcite of marine origin to detrital silicate minerals determines the final isotopic composition of the whole rock w r after equilibration during alpine metamorphism g o gd values of analysed hydrated minerals and whole rocks si80 15 to t 25 0 and 6d 60 0 smow lie within the field of alpine cover rocks and of fluids in equilibrium with these rocks interaction with an external o or d depleted fluid such as hydrothermal waters meteoric water or basement fluid can be ruled out major and trace element systematics indicate an enrichment in si ca p sr y ta and locally eu in the veins with respect to the wallrock related to massive crystallization of quartz and carbonates together with phosphate and oxide potential fluid sources chemical composition of the fluid phase and the mechanisms of fluid flow during vein formation are discussed fluid flow takes place within tectonic units at both hectometric and decimetric scale fluid may also have migrated along and across the major tectonic contacts but this process is poorly documented

data driven numerical simulations of equatorial spread f in the peruvian sector 2 autumnal equinox an ongoing effort to simulate plasma instability in the equatorial ionosphere leading to equatorial spread f esf in the american sector is described ionospheric state parameters including plasma number density and vector drift velocity profiles were measured at the jicamarca radio observatory in the period between 20 september and 3 october 2013 coherent radar backscatter from plasma irregularities was recorded simultaneously and images of the irregularities were calculated using aperture synthesis methods neutral winds were measured by the red line fabry perot interferometers at jicamarca and arequipa peru a fully 3 d numerical simulation of ionospheric irregularities initialized and forced using parameterizations derived from measurements and empirical models was used to reproduce the esf activity observed simulations were able to recover many of the features of the irregularities although some important anomalies can be noted esf events in which the first appearance of radar plumes occurred either very early or very late were not reproduced in simulation and may be indicative of nonlocal influence

interpretation of satellite elevation magnetic anomalies the magnetic field satellite magsat obtained total field data over almost the whole of the earth we have studied these data in two different ways the first way was to use spherical harmonic coefficients it is often believed that these coefficients can be separated into those originating within the core of the earth and those from the crust at about degree 13 because of the change in slope of the power spectrum using coefficients above degree 13 in two different representations we have shown that there is little latitudinal variation of the source function and that the continent to ocean source function ratio is almost unity we suggest that these unlikely observations are in reality due to the possibility that much of the signal carried in coefficients above degree 13 may originate within the core of the earth if this is true it is then possible to keep the energy external to the core of the field generated within the core at a constant even though the dipole field has varied in strength by a factor of 3 over the past 104 years we have also produced models of crustal magnetization using the 2 2 average anomalous field and have shown how it is possible to obtain a physically meaningful susceptibility model in which magnetizations are always positive this is done by using an annihilator which is a magnetization distribution within a spherical shell which produces no external field the use of an annihilator is also appropriate for many problems connected with oceanic basins since present day oceanic crust is everywhere less than 180 ma the average magnetization seen at the altitude of a satellite is either zero for crust which is produced during times of rapid field reversal or positive for crust produced during times of constant normal polarity some results from continental and oceanic areas are discussed in the light of applying an annihilator

channelized fluid flow and mass transfer along a rift related detachment fault eastern alps southeast switzerland remnants of a rift related low angle detachment system are spectacularly exposed in the continental err nappe and occur discontinuously in the underlying oceanic platta nappe in southeast switzerland rocks along the detachment fault exhibit systematic changes in mineralogy fabrics and structures that are related to deformation and hydration of granite to form cataclasite and subsequent gouge structural mineralogical and geochemical data indicate a continuous evolution of fluid flow and mass transfer along the detachment fault mass balance calculations together with stable isotope data suggest 1 an altered seawater source and local interaction with mantle rocks 2 limited fluid rock interaction and mass transfer during initial cataclasis at deeper crustal levels and temperatures of 150 to 200 c 3 increased fluid flux and mass transfer at slightly lower temperatures 100 150 c during gouge formation at shallower crustal levels and 4 strongly localized fluid rock interaction at low temperatures 65 80 c near the sea floor at a final stage of detachment faulting we propose a model in which seawater dominated fluids penetrated and locally interacted with mantle rocks before they were caught and channelized along large scale detachment faults flow direction is upward along the detachment from higher to lower temperatures suggesting that the driving force of fluid flow was thermal fluid convection associated with mantle exhumation our study suggests that this large scale detachment fault is a good analog for extensional structures along present day nonvolcanic rifted margins such structures along ocean continent transitions are important in controlling fluid pathways and mass transfer between mantle and continental crust

a comparison of satellite and observatory estimates of geomagnetic secular variation a comparison is made between secular variation values computed at the positions of 168 magnetic observatories from two spherical harmonic models based almost entirely on the 7 month spread of magsat data and observed secular variation values derived from the observatory annual mean values the two models differ from each other rms difference in z 27 8 nt yr and from the observed values rms differences in z of 22 1 and 30 6 nt yr it appears that a time span of more than 7 months is needed to produce accurate models of the secular variation from satellite data

oxygen and carbon isotope evidence for seawater hydrothermal alteration of the macquarie island ophiolite abstract rocks of the miocene macquarie island ophiolite south of new zealand have oxygen and carbon isotopic compositions comparable to those of seafloor rocks basalt glass and weathered basalts have 18o values at 5 8 6 0 and 7 9 9 5 respectively similar to drilled seafloor rocks including samples from the leg 29 dsdp holes near macquarie island compared to the basalt glass the greenschist to amphibolite facies metaintrusives are depleted in18o 18o 3 2 5 9 similar to dredged seafloor samples whereas the metabasalts are enriched 18o 7 1 9 7 although the gabbros are only slightly altered in thin section they have exchanged oxygen with a hydrothermal fluid to a depth of at least 4 5 km there is an approximate balance between18o depletion and enrichment in the exposed ophiolite section the carbon isotopic composition of calcite in the weathered basalts 13c 1 0 2 0 is similar to those of drilled basalts but the metamorphosed rocks have low 13c values 14 6 to 0 9 these data are compatible with two seawater circulation regimes in the upper regime basalts were weathered by cold seawater in a circulation system with high water rock ratios 1 0 based on calcite compositions weathering temperatures were less than 20 c and the carbon was derived from a predominantly inorganic marine source as previously suggested for the samail ophiolite it is postulated that the lower hydrothermal regime consisted of two coupled parts at the deeper levels seawater circulating at low water rock ratios 0 2 0 3 and high temperatures 300 600 c gave rise to18o depleted gabbro and sheeted dikes via open system exchange reactions during reaction the seawater underwent a shift in oxygen isotopic composition 18o 1 0 5 0 and subsequently caused18o enrichment of the overlying metabasalts in the shallower part of the hydrothermal regime the metabasalts were altered at relatively high water rock ratios 1 0 10 0 and temperatures in the range 200 300 c the relatively low water rock ratios in the hydrothermal regime are supported by the low 13c values of calcite interpreted as evidence of juvenile carbon in contrast to the inorganic marine carbon found in the weathered basalts

fluid infiltration and volume change during mid crustal mylonitization of proterozoic granite king island tasmania the development of 10 30 m wide mylonite zones at mid crustal depths in late proterozoic granitoids on king island tasmania was associated with pervasive infiltration of low 18o fluids 5 to 7 on the scale of the shear zones syndeformational fluid rock interaction produced substantial differences in mineral composition and bulk rock chemistry among several adjacent shear zones which are hosted by the same granite in a shear zone at cape wickham with a normal slip component changes in whole rock chemistry between granite and mylonites indicate a gain of ca and losses of k and na during deformation which was nearly isovolumetric notable losses of k rb and si occurred during partial retrograde alteration of mylonites near the western margin of this shear zone the alteration suggests a component of up temperature fluid flow in contrast 3 km to the south east in a strike slip shear zone at disappointment bay complete albitization of plagioclase was associated with na gain and ca loss deformation also involved losses of mg and fe up to 60 volume gain occurred during the formation of closely spaced mesoscopic to microscopic quartz veins during mylonitization the substantial silica gain in this as well as in two mylonite zones further south east is interpreted to have been associated with upward flow of aqueous fluids along these shear zones on the basis of a gradient reaction model minimum time integrated fluid fluxes of 106 m3 m2 are estimated for the disappointment bay west shear zone

high resolution magnetic field modeling application to magsat and orsted data abstract launched on 23rd february 1999 the orsted satellite opened the decade of geopotential field research this is the first satellite to measure the three components of the earth s magnetic field since magsat 1979 1980 orsted orbital parameters are very similar to those of magsat allowing a first order comparison of the 1979 and 2000 magnetic fields using the available vector and scalar data over the first 14 months of the orsted mission and applying classical selection criteria local time external magnetic activity we compute a 29 degree order main field model and a 13 degree order secular variation model for the period 1999 2000 the applied method and the accuracy of the derived model are discussed we compare the resulting main field model to a similar one derived from magsat data results of this comparison are presented such as i morphology and energy spectrum of the secular variation and ii morphology of the crustal magnetic field at magsat and orsted epochs

resetting of oxybarometers and oxygen isotope ratios in granulite facies orthogneisses during cooling and shearing adirondack mountains new york the petrography petrology and oxygenisotope geochemistry of granulite facies granitic and syenitic orthogneisses of the diana and stark complexes adirondack mountains new york show that the extent and nature of resetting of isotopic and mineralogic systems is highly variable there is a strong correlation between retrogression and shearing and the rocks may be divided texturally into 1 unsheared lithologies that preserve little retrogressed pyroxene or hornblendebearing peak metamorphic mineralogies and 2 sheared rocks that underwent retrogression marked by the growth of late biotite in centimetre to metre wide shear zones after the peak of metamorphism oxygen fugacities in the unsheared lithologies were estimated for reintegrated mineral compositions from magnetiteilmenite mt ilm and ferrosilite magnetic quartz fs mt qtz equilibria mt ilm yields logfo2mt ilm values of 15 9 to 17 6 0 6 to 1 3 log units below the fayalite magnetite quartz buffer fmq and temperatures of 670 745 c that agree with those from other geothermometry and phase equilibria studies these data suggest that aside from oxyexsolution of ilmenite from magnetite the fe ti system underwent only minor resetting during cooling and the fe ti oxides yield good estimates of peak metamorphic temperatures and fo2 in unsheared ilmenite magnetite orthopyroxene quartz assemblages values of logfo2mt ilm are lower than logfo2fs mt qtz by an average of 0 6 when the orthopyroxene activity model of sack and ghiorso is used minor resetting of the fe ti oxides analytical errors and errors in the placement of end member reactions probably account for this relatively small difference in fo2 values whole rock 18o values of unsheared diana and stark lithologies range from 4 0 to 10 3 reflecting pre regional metamorphic oxygen isotope ratios peak metamorphic minerals preserve high temperature oxygen isotope fractionations and in many samples the effective diffusion of oxygen in minerals ceased at higher temperatures than predicted from wet experimental diffusion data these data suggest that the rocks did not contain an aqueous fluid phase during cooling the combination of petrologic isotopic and textural data also permits a detailed study of shearing and retrogression ilmenites in the sheared lithologies underwent greater degrees of hematite loss than in the unsheared rocks resulting in logfo2mt ilm values as low as 24 1 3 1 log units below fmq and mt ilm temperatures that are up to 175 c below regional estimates sheared rocks also have higher 18o values up to 13 3 during shearing 18o values of biotite k feldspar and magnetite reset readily while the degree of isotopic resetting of quartz correlates with the intensity for recrystallization

permeability generation and resetting of tracers during metamorphic fluid flow implications for advection dispersion models advection dispersion fluid flow models implicitly assume that the infiltrating fluid flows through an already fluid saturated medium however whether rocks contain a fluid depends on their reaction history and whether any initial fluid escapes the behaviour of different rocks may be illustrated using hypothetical marble compositions marbles with diverse chemistries e g calcite dolomite quartz are relatively reactive and will generally produce a fluid during heating by contrast marbles with more restricted chemistries e g calcite quartz or calcite only may not if the rock is not fluid bearing when fluid infiltration commences mineralogical reactions may produce a reaction enhanced permeability in calcite dolomite quartz or calcite quartz but not in calcite only marbles the permeability production controls the pattern of mineralogical isotopic and geochemical resetting during fluid flow tracers retarded behind the mineralogical fronts will probably be reset as predicted by the advection dispersion models however tracers that are expected to be reset ahead of the mineralogical fronts cannot progress beyond the permeability generating reaction in the case of very unreactive lithologies e g pure calcite marbles cherts and quartzites the first reaction to affect the rocks may be a metasomatic one ahead of which there is little pervasive resetting of any tracer centimetre scale layering may lead to the formation of self perpetuating fluid channels in rocks that are not fluid saturated due to the juxtaposition of reactants such layered rocks may show patterns of mineralogical resetting that are not predicted by advection dispersion models patterns of mineralogical and isotopic resetting in marbles from a number of terrains for example chillagoe marulan south reynolds range australia adirondack mountains old woman mountains notch peak usa and stephen cross quarry canada vary as predicted by these models

historic and paleomagnetic secular variation and the earth s core dynamo process this paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth s magnetic field secular variation and its implications for the core dynamo process secular variation is defined in this paper as the general spatio temporal variation of the field secular change will refer to the first time derivative of the spatial field strict geomagnetic definition of secular variation we will emphasize here the long term 10 year or longer behavior of the historic secular variation hsv for this is the variation that is comparable with the paleomagnetic secular variation psv two important related topics the 1969 1970 geomagnetic jerk in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources are treated more fully in other reviews

observatory crustal magnetic biases during magsat and orsted satellite missions 1 computing main field models derived from magsat and orsted satellite magnetic measurements the crustal influence on satellite data can be treated as random noise satellite data can thus be used in conjunction with magnetic observatory measurements to isolate the non core field at the observatory locations crustal biases for the horizontal northward x eastward y and vertical downward z components are computed for all magnetic observatories where data are available for magsat 1979 1980 and orsted 1999 2000 epochs to study their correlation over twenty years for a set of observatories installed after 1979 new crustal biases are computed

oxygen isotope geothermometers for metamorphic rocks the chicago mineral carbonate oxygen isotope fractionation curves have been combined with mineral water fractionation data for jadeite zoisite and rutile and new data for grossular water to provide a set of self consistent mineral pair calibrations the a coefficients in the equation 1000 in a 106t 2 of the new mineral pair fractionations are jadeite zoisite grossular rutile quartz 1 69 2 00 3 03 5 02 jadeite 0 31 1 34 3 33 zoisite 1 03 3 02 grossular 1 99 the isotopic fractionation properties of natural pyralspite garnet ca fe mg mn 3al2si3o12 can be approximated by those of the grossular end member appropriate substitutions also yield coefficients for the solid solution minerals sodic pyroxene and epidote e g aquartz sodic pyroxene 2 75 1 06xjd aquartz epidote 2 00 0 75xps where xjd and xps are the mole fractions of the jadeite and pistacite components respectively the new data set is particularly suitable for the geothermometry of metamorphic rocks 18o data from minerals of the high pressure metamorphic rocks of the sesia zone of italy and cyclades complex of greece yield well constrained mean temperatures of 572 and 478d c respectively type iii blueschist metabasalts of the franciscan formation of california give mean quartz garnet temperatures of 354d c

asbestos fibre identification vs evaluation of asbestos hazard in ophiolitic rock melanges a case study from the ligurian alps italy in recent years the high incidence of harmful health effects through inhalation of airborne asbestos from amphibole bearing rock melanges has been thoroughly documented here we present a field based multi scale geological approach aimed at illustrating the occurrence of amphibole fibrous mineralisation in an ophiolitic suite from the ligurian alps italy and discussing the implication on in situ determination of the asbestos hazard the rock melange is composed of plurimetre sized blocks of different lithotypes metagabbro serpentinite chloritoschist juxtaposed by the meaning of tectonic structures the geological structural survey revealed that the fibrous mineralisation is localised in specific structural sites of the rock volume including veins and schistosity both micro chemical and crystal structure analyses on selected fibrous samples revealed that actinolite fibres grow in veins within the metagabbro and in chloritoschists while fibrous tremolite occurs in serpentinite schistosity the morphological features of these amphibole fibres have been analysed in tem images and used for classifying them as asbestiform or non asbestiform the results show that the asbestos hazard determination is not unequivocally identified when different procedures for asbestos fibre identification and classification are applied this may have impact on normatives and regulations in defining environmental hazards due to asbestos occurrence

the mechanism of veining and retrograde alteration of alpine eclogites the introduction of externally derived fluids into rocks of the zermatt saas zone of the swiss alps gave rise to the simultaneous formation of shear and hydraulic fractures these fractures are now filled with albite rich assemblages and surrounded by alteration halos up to c 2 m wide the alteration assemblages are zoned and an examination of reactions in p t ah2o space implies that the parageneses developed by the hydration of fluid absent eclogites a mechanical analysis of the veins after sibson 1981 shows that pfluid pload must have been at least 0 96 fluid migration into the country rocks must have been driven by excess hydraulic head either derived from the vertical extent of the veins or due to their connection to a deeper external reservoir possibly tapped along thrust surface s diffusive and capillary transport were insignificant the fluids may have been derived from underlying metasediments that were dehydrating during the quasi isothermal uplift of this part of the alps or they may have originated during the prograde mesoalpine metamorphism documented in the area

impact of meridional winds on equatorial spread f revisited 1 the impact of meridional winds on the onset and evolution of equatorial spread f esf is studied using the nrl ionosphere model sami3 esf the prevailing belief is that meridional winds have a stabilizing effect on the onset of esf in this letter it is shown that this is not necessarily the case and that meridional winds can be destabilizing we find that a wind profile with a positive gradient as a function of latitude v m 0 is a stabilizing influence on the generalized rayleigh taylor instability a wind profile with a negative gradient v m 0 is a destabilizing influence here a northward wind is positive and increases in the northward direction it is suggested that meridional wind profiles may account for in part the longitudinal and day to day variability of esf

geochemical and stable isotope resetting in shear zones from taschalp constraints on fluid flow during exhumation in the western alps fluid flow at greenschist facies conditions during exhumation of the western alps occurred in several penecontemporaneous systems including shear zones at lithological contacts deformed contacts between serpentinite bodies and metabasalts albite veins within metabasalts and calcite quartz veins within calcareous schists fluid flow in shear zones that juxtapose metasediments and ophiolitic rocks within the piemonte unit reset o and h isotope ratios 18o values are buffered by the wall rocks however calculated fluid 2h values are similar within all the shear zones suggesting that they formed an interconnected network the similarity of 2h values of the sheared rocks and those of unsheared calcareous schists suggests that the fluids were derived from or had equilibrated with the schists that envelop the ophiolite rocks time integrated fluid fluxes at the sheared contacts estimated from changes in si in metabasalts were up to 105 m3 m 2 with the fluid flowing up temperature driven either by topography or seismic pumping individual shear zones were active for c 2 3 myr implying average fluid fluxes of up to 10 9 m3 m 2 s 1 rocks in shear zones within the ophiolite away from contacts with the metasediments show much less marked isotopic and geochemical changes implying that fluid volumes decreased into the ophiolite unit consistent with the source of fluids being the metasediments fluids were generated by dehydration reactions that were intersected during exhumation and while many rocks show the affects of fluid rock interaction large scale fluid flow between major units was not common

secular variation crustal contributions and tectonic activity in california 1976 1984 five to ten years of data from an array of 34 total field magnetometers are used to define the temporal and spatial characteristics of secular variation throughout central and southern california for this period well determined rates of secular variation are obtained at each site these rates are temporally linear but spatially variable ranging from 45 nt a near san francisco to 54 nt a near the mexican border least squares analysis of all data indicates secular variation decreases in a general southeasterly direction according to f k1 k2 k where f is in nanoteslas per year and are the geographic latitude and longitude and k1 k2 and k are 1 66 0 13 nt a deg 0 13 0 10 nt a deg and 123 2 0 2 nt a respectively deviations of as much as 1 nt a occur on scales of a few tens of kilometers these apparent small scale secular variation anomalies result in part from differences in local induction and remanent magnetization and may be reduced by determination of a site transfer function a planar surface fit to the corrected data has the form f k1 k2 k where k1 k2 and k are now 1 50 0 08 nt a deg 0 23 0 06 nt a deg and 129 2 0 1 nt a respectively residual field variations obtained after correction of all data for secular variation are most apparent on the san andreas fault in southern california between palm springs and the salton sea and marginally so along the recent coyote ml 5 9 of august 6 1979 and morgan hill ml 5 8 of april 24 1984 aftershock zones these residuals could be explained by stress localization in these regions and particularly in the case of the southern san andreas anomaly may indicate the location of a future damaging earthquake incomplete correction for complex site effects may be an alternative explanation in the first large scale test of global secular variation models we find that the models for this region do not predict either the amplitudes or the mean isogram directions of these data to better than several tens of nanoteslas per year and several tens of degrees respectively this may result from a failure to correct for site response effects at some observatories before using the data in global spherical harmonic expansions local magnetization response can therefore bias estimates of secular variation and yield apparent impulsive behavior when external fields are perturbed

data driven numerical simulations of equatorial spread f in the peruvian sector 3 solstice we present results from a continuing effort to simulate equatorial spread f esf using observations from the jicamarca radio observatory near lima peru jicamarca measures vertical and zonal plasma drifts along with plasma number density profiles overhead the number density profiles are used to initialize a three dimensional regional model of the ionosphere capable of simulating plasma density irregularities produced during esf conditions the vertical drifts measurements are used to drive the numerical simulation continuously neutral winds are derived from the new horizontal wind model 14 hwm 14 model and the zonal winds are scaled so as to make the zonal plasma flows at the start of the simulation agree with the isr profile measurements coherent scatter radar imagery from jicamarca is used to validate the simulation results campaign data were collected in april and december 2014 and a few events representative of low and high esf activity were selected for analysis the numerical simulations are able to reproduce the level of activity observed along with the gross features of the esf irregularities and radar plumes data from a network of hf beacons are being incorporated into the forecast analysis in order to elucidate radar plumes which sometimes appear even when the simulation fails to predict them

stable isotope geochemistry of alpine ophiolites a window to ocean floor hydrothermal alteration and constraints on fluid rock interaction during high pressure metamorphism the subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle however despite its potential importance fluid rock interaction during high pressure metamorphism is relatively poorly understood the stable isotope and major element geochemistry of pennine ophiolite rocks from italy and switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites suggesting that they interacted with little pervasive fluid during high pressure metamorphism cover sediments also have oxygen isotope ratios within the expected range of their protoliths in the rocks that escaped late greenschist facies retrogression different styles of sub ocean floor alteration may be identified using oxygen isotopes petrology and major or trace element geochemistry within the basalts zones that have undergone high and low temperature sub ocean floor alteration as well as relatively unaltered rocks can be distinguished serpentinites have 18o and 2h values that suggest that they were formed by hydration on or below the ocean floor the development of high pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub ocean floor alteration and which contained hydrated fine grained reactive assemblages given that the transformation of blueschist facies metabasic rocks to eclogite facies assemblages involves the breakdown of hydrous minerals e g lawsonite zoisite and glaucophane and will thus liberate considerable volumes of fluids metamorphic fluid flow must have been strongly channelled high pressure quartz calcite omphacite glaucophane titanoclinohumite veins that cut the ophiolite rocks represent one possible channel however stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids fluids were more likely channelled along faults and shear zones that were active during high pressure metamorphism such strong fluid channelling may cause fluids to migrate toward the accretionary wedge especially along the slab mantle interface which is probably a major shear zone this may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting additionally because fluids probably interact with relatively small volumes of rock in the channels they cannot scavenge elements from the subducting slab efficiently

the flow of surface derived fluids through alice springs age middle crustal ductile shear zones reynolds range central australia the south east reynolds range central australia is cut by steep north west trending alice springs age c 334 ma shear zones that are up to hundreds of metres wide and several kilometres long with reverse senses of movement amphibolite facies 550 600 c 500 600 mpa shear zones cut metapelites while greenschist facies shear zones 420 535 c 400 650 mpa cut metagranites the sheared rocks commonly underwent metasomatism implying that the shear zones were the pathways of significant fluid flow altered granites within greenschist facies shear zones have gained si and k but lost ca and na relative to their unsheared counterparts suggesting that the fluid flowed down temperature and hence probably upward through the shear zones time integrated fluid fluxes calculated from silica addition are up to 2 1 1010 mol m 2 c 4 2 105 m3 m 2 similar time integrated fluid fluxes are also estimated from changes in k and na the sheared granitic rocks locally have 18o values as low as 0 which is much lower than the 18o values of the adjacent unsheared granites 7 to 9 implying that the fluid which flowed through these shear zones was derived from the surface for the estimated time integrated fluid fluxes the fluids would be able to retain their isotopic signature for many tens to hundreds of kilometres the flow of surface derived fluids into the ductile middle crust with subsequent expulsion upwards through the shear zones may have been driven by seismic activity accompanying the alice springs deformation

horizontal contraction or horizontal extension heterogeneous late eocene and early oligocene general shearing during blueschist and greenschist facies metamorphism at the pennine austroalpine boundary zone in the western alps mylonitic structures related to two orogenic events are described from the upper and lower contacts of the combin zone and the immediately overlying upper austroalpine dent blanche nappe mont mary klippe and the directly underlying lower austroalpine etirol levaz slice the first event late eocene in age commenced during blueschist facies p t conditions but pre dated the peak of subsequent greenschist facies overprint the second event early oligocene in age took place during retrograde greenschist facies conditions most sense of shear indicators associated with the retrograde mylonites indicate top se shearing but subordinate top nw displacing shear sense indicators have also been mapped mylonitic top se shearing appears to be restricted to the combin zone and its upper and lower contacts within the dent blanche nappe and mont mary klippe and at the base of the etirol levaz slice structures were observed which developed during blueschist greenschist facies conditions and are in conjunction with the p t t history of these rocks inferred to be older associated kinematic data indicate a top nw shear sense comparable blueschist greenschist facies shear sense indicators have not been observed in the combin zone nonetheless the foliation in the combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions this indicates that the combin zone and the immediately over and underlying austroalpine units shared a common tectono metamorphic evolution since the late eocene finite strain data reveal oblate strain fabrics which are thought to result from a true flattening strain geometry flow path modelling reveals a general non coaxial deformation regime and corroborates significant departures from a simple shear deformation in the study area mylonitic top se shearing in the combin zone is attributed to early oligocene backfolding and backthrusting of the mischabel phase temperature time curves suggest slight reheating in the monte rosa nappe underneath and cooling in the dent blanche nappe above the combin zone hence confirming a thrust interpretation for this event the top nw displacing structures are thought to result from late eocene emplacement of the dent blanche nappe and the combin zone onto the middle pennine barrhorn series along the combin fault as related structures initiated during mildly blueschist facies conditions in the dent blanche nappe and the underlying combin zone and both were emplaced together onto the greenschist facial barrhorn series it is concluded that the structures developed as the nappes moved upward relative to the earth s surface thus the combin fault is regarded as a thrust the geometry of this structure indicates that the combin fault is an out of sequence thrust that locally cut down section hence top nw out of sequence thrusting caused local thinning of the metamorphic structural section in association with horizontal shortening out of sequence thrusts cutting down section and back thrusts offer the possibility of explaining the pronounced break in the grade of metamorphism across the combin fault i e the contact between the eclogite facial zermatt saas zone and the overlying lower grade combin zone by contractional deformation

interactions between serpentinite devolatilization metasomatism and strike slip strain localization during deep crustal shearing in the eastern alps the greiner shear zone in the tauern window eastern alps changes from a zone of distributed dominantly sinistral shear in supracrustal rocks to a series of narrow gully forming dextral splays where it enters basement gneisses within these splays granodiorite is transformed into quartz poor biotite and or chlorite schists reflecting hydration removal of si ca and na and concentration of fe mg and al stable isotope analyses show a prominent increase in d and a decrease in 18o from granodiorite into the shear zones these changes indicate significant channelized flow of an externally derived low 18o high d fluid through the shear zones the shear zone schists are chemically similar to blackwall zones developed around serpentinite bodies elsewhere in the greiner zone and the stable isotope data support alteration via serpentinite derived fluid monazite in schist from one shear zone yields spot dates of 29 20 ma indicating that the fluid influx and switch from sinistral to dextral shear occurred at or shortly after the thermal peak of the alpine orogeny c 30 ma we suggest that alpine metamorphism of serpentinites released large amounts of high d low 18o si undersaturated fe mg saturated fluids that became channelized along prior zones of weakness in the granodiorite infiltration of this fluid facilitated growth of chlorite and biotite which in turn localized later dextral strain in the narrow splays via cleavage parallel slip this dextral strain event can be linked to other structures that accommodated tectonic escape of major crustal blocks during dextral transpression in the eastern alps this study shows that serpentinite devolatilization can play an important role in modifying both the chemistry and rheology of surrounding rocks during orogenesis

assessing the geochemical and tectonic impacts of fluid rock interaction in mid crustal shear zones a case study from the intracontinental alice springs orogen central australia the reynolds anmatjira ranges central australia form part of a high grade basement terrane dissected by intensely metasomatized transpressional shear zones active during the ordovician carboniferous alice springs orogeny unlike typical retrograde structures associated with discrete fluid flow the mid crustal setting and intracontinental nature of these shear systems present significant problems for the source and ingress mechanism of the fluid involved in their rehydration to address these issues we describe two detailed traverses across deformed and metasomatized basement rocks in this region and interrogate their record of fluid rock interaction from various perspectives both traverses combine structural and petrological observations with zr in rutile and ti in quartz thermobarometry oxygen and hydrogen stable isotope analysis and major trace and rare earth element mobility trends each technique is critically evaluated for its utility in this study and its more widespread applicability to alternative field areas providing a strategic framework for the general investigation of fluid affected shear zones ultimately the integrated data sets specify pressure temperature conditions of 530 c and 4 5 kbar implying average apparent thermal gradients of 29 36 c km 1 and depths of 14 18 km other characteristic features to emerge include strongly variable element mobilities and pronounced isotopic depletion fronts consistent with the alteration effects of an externally derived non equilibrium fluid this is confirmed by calculated fluid compositions indicative of contributions from a fluid of meteoric origin with estimated 18o and d values as low as 2 3 and 59 8 respectively we propose that these surficial fluid signatures are imposed on the mid crust by the prograde burial and dehydration of hydrothermally altered fault panels produced during pre orogenic basin formation progressive fluid release with continued subsidence then leads to the accumulation of increasing fluid volumes in the vicinity of the brittle ductile transition promoting extensive hydration metasomatism and reaction softening at the locus of stress transmission from plate boundary sources the sustained injection of externally derived fluids into refractory crustal material may thus stimulate a critical reduction in the long term strength of the lithosphere providing strong impetus for the initiation and advancement of intracontinental orogenesis

the gabbro eclogite transformation an oxygen isotope and petrographic study of west alpine ophiolites both magmatic and eclogitic parageneses are preserved in the gabbros of western alpine ophiolites samples with relic magmatic mineralogies display partial transformation to eclogitic assemblages along cracks and grain boundaries gabbros with eclogitic mineralogies contain zoned pseudomorphs after olivine comprising talc rich cores with kyanite mg chloritoid and omphacite in outer cores and garnet rims the compositional zonation of these olivine pseudomorphs closely parallels that shown by olivines in hydrothermally altered ocean floor gabbros the eclogitic gabbros are hydrous containing paragonite zoisite and other water bearing minerals and it has been suggested that water was introduced during high pressure metamorphism however the similarity of olivine alteration patterns to those of ocean floor gabbros suggests that hydration and local metasomatism leading to the stability of aluminous minerals in olivine sites occurred during hydrothermal alteration prior to subduction oxygen isotope systematics are consistent with this proposal alpine gabbros with magmatic relics have a mean 18o value of 5 7 0 7 similar to that of unaltered oceanic crust whereas eclogitic gabbros have a mean 18o value of 4 8 0 9 this statistically significant difference is consistent with the eclogitic samples having undergone high temperature ocean floor alteration the preservation of magmatic and hydrothermal 18o values in ocean floor gabbros that have been metamorphosed at 2 2 5 gpa 60 75 km implies that the deeper levels of ocean crust have not experienced pervasive fluid flow during subduction or subsequent exhumation magmatic assemblages were preserved despite an overstep of eclogitization reactions by at least 0 6 1 1 gpa implying that equilibrium was not attained in undeformed parts of the system because of slow diffusion in water deficient rock volumes

small scale features in the earth s magnetic field observed by magsat a spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of magsat data global maps representing the contribution due to terms of the expansion above n 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar even in polar regions an expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high order field structure at a constant altitude than do component maps derived independently the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 008 2 50 payment must accompany order

new evidence of a meridional current system in the equatorial ionosphere an interesting variation in the geomagnetic d component has been found by analysis of the magsat data characteristics of this variation are that it appears everyday on the low latitude duskside and is antisymmetric about the dip equator negative in the northern hemisphere that the magnitude from 5 to 25 nt depends upon the geographic longitude and altitude and that no corresponding change can be seen on the ground magnetic data the change suggests a meridional current system in the equatorial ionosphere produced in association with the electrojet current along the dip equator this effect should be taken into account in the study of geomagnetic anomaly of internal origin with the magsat data

active hydrothermal systems during the recent uplift of nanga parbat pakistan himalaya during the last 10 m y the nanga parbat haramosh massif in the northwestern himalaya has been intruded by granitic magmas has undergone high grade metamorphism and anatexis and has been rapidly uplifted and denuded as part of an ongoing project to understand the relationship between tectonism and petrologic processes we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m y we suggest that the rapid uplift of the massif created a dual hydrothermal system consisting of a near surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic metamorphic volatiles meteoric fluids derived from glaciers near the summit of nanga parbat were driven deep into the massif along the transpressional faults causing 18 o and d depletions in the gneisses and marked oxygen isotopic disequilibriuui between mineral pairs from the fault zones the discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif at deeper levels within the massif infiltration of low 18 o magmatic fluids caused 18 o depletions in the gneisses within the migmatite zone these low 18 o fluids were derived from the young 4 ma relatively low 18 o granites 8 that are found within the core of the massif geochronological evidence in the form of fission track and 40 39 ar cooling ages and u pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined fluid infiltration in the migmatite zone rocks located along the tato traverse was coeval with metamorphism granite emplacement and rapid denudation in the interval 0 8 3 3 ma finally we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the nanga parbat haramosh massif

sequence and style of major post nappe structures simplon pennine alps abstract in the upper pennine nappe complex of the simplon pennine alps switzerland and italy at least three phases of major post nappe folding in places associated with thrusting can be distinguished these are superimposed on an earlier formed partly chaotic complex of tectonic units including the bernhard and monte rosa continental flakes and the zermatt saas and antrona ophiolite complexes the earliest post nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas later two successive phases of back folding led to the present overall structure both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition this has been demonstrated by systematically mapping the major axial traces over the whole region successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the bernhard and monte rosa nappes

fluid flow and metasomatic fault weakening in the moresby seamount detachment woodlark basin offshore papua new guinea low angle normal faults play a prominent role in discussions about fault strength as they require significant weakening to remain active at low angles the submerged moresby seamount detachment msd is arguably the best exposed active low angle detachment worldwide we analyzed dredged msd protoliths cataclasites and mylonites to investigate deformation mechanisms and fault weakening processes deformation is accompanied by important syntectonic fluid induced mass transfer controlling the rheological behavior of the msd while the mafic protolith behaves brittlely at the onset of deformation the metasomatic mineralogical and chemical changes cause a transition to plastic flow as the rock is progressively exhumed immobile elements provide a reference frame for total material gains and losses si ca and k are syntectonically enriched while fe ti mg and al are depleted mass increase is about 10 in the cataclasites and about 48 in the mylonites main mechanism is syntectonic veining causing enrichment in calcite and quartz thus making the mylonites capable to flow plastically minimum time integrated fluid flux is calculated as 3 105 m3 m 2 indicating that the msd is an important fluid conduit the fluids have a deep crustal source a bottom water temperature and turbidity anomaly suggests that the hydrothermal system is still active syntectonic veining in fault rocks and recent seismic activity both suggest that the msd is intermittently brittle implying a brittle plastic transition at unusually high temperature and low differential stress we conclude that fault zone metasomatism is crucial in forming weak detachments at passive margins and may be a prerequisite for successful crustal breakup

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

the near earth magnetic field at 1980 determined from magsat data data from the magsat spacecraft for november 1979 through april 1980 and from 91 magnetic observatories for 1978 through 1982 are used to derive a spherical harmonic model of the earth s main magnetic field and its secular variation at epoch 1980 0 the model is called gsfc 12 83 constant coefficients are determined through degree and order 13 and secular variation coefficients through degree and order 10 the first degree external terms and corresponding induced internal terms are given as a function of dst preliminary modeling using separate data sets at dawn and dusk local time showed that the dusk data contain a substantial field contribution from the equatorial electrojet current the final data set was therefore selected first from dawn data and then augmented by dusk data to achieve a good geographic data distribution for each of three time periods 1 november december 1979 2 january february 1980 and 3 march april 1980 a correction for the effects of the equatorial electrojet was applied to the dusk data utilized the solution included calculation of fixed biases or anomalies for the observatory data although similar in many respects gsfc 12 83 differs from international geomagnetic reference field 1980 by 3 6 nt in the g10 term and shows a slightly negative b in the northern polar region as well as other differences in secular variation pattern

the geometry and timing of orogenic extension an example from the western italian alps contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements low p upon high p contacts are commonly interpreted as extensional i e material points on either side of the contact moved apart relative to the palaeo horizontal but dating of deformation and metamorphism is essential in testing such models in the western alps the piemonte ophiolite consists of eclogites t 550 600 c and p 18 20 kbar structurally beneath greenschist facies rocks t 400 c and p 9 kbar mapping shows that the latter form a kilometre wide shear zone the gressoney shear zone gsz dominated by top se movement related to crustal extension rb sr data from micas within different gsz fabrics which dynamically recrystallized below their blocking temperature are interpreted as deformation ages ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping they show that the gsz had an extensional deformation history over a period of c 9 myr between c 45 36 ma this overlaps in time with the eclogite facies metamorphism the gsz operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation the discrete contact zone between eclogite and greenschist facies rocks is the last active part of the gsz and truncates greenschist facies folds in the footwall these final movements were therefore not a major component of eclogite exhumation pressure estimates associated with old and young fabrics within the gsz are comparable indicating that during extensional deformation there was no significant unroofing of the hangingwall since there are no known extensional structures younger than 36 ma at higher levels in this part of the alps exhumation since the final juxtaposition of the two units at 36 ma seems to have been dominated by erosion key words deformation age eclogite exhumation rb sr dating tectonic

interaction of chemical and physical processes during deformation at fluid present conditions a case study from an anorthosite leucogabbro deformed at amphibolite facies conditions we present microstructural and chemical analyses of chemically zoned and recrystallized plagioclase grains in variably strained samples of a naturally deformed anorthosite leucogabbro southern west greenland the recorded microstructures formed in the presence of fluids at mid crustal conditions 620 640 c 7 4 8 6 kbar recrystallized plagioclase grains average grain size 342 m with a random crystallographic orientation are volumetrically dominant in high strain areas they are characterized by asymmetric chemical zoning an80 cores and an64 rims that are directly associated with areas exhibiting high amphibole content and phase mixing analyses of zoning indicate anisotropic behaviour of bytownite plagioclase with a preferred replacement in the left langle 0 10 right rangle direction and along the 001 plane in areas of high finite strain recrystallization of plagioclase dominantly occurred by bulging recrystallization and is intimately linked to the chemical zoning the lack of cpo as well as the developed asymmetric zoning can be explained by the activity of grain boundary sliding accommodated by dissolution and precipitation creep dpc in low strain domains grain size is on average larger and the rim distribution is not related to the inferred stress axes indicating chemically induced grain replacement instead of stress related dpc we suggest that during deformation in high strain areas pre existing phase mixture and stress induced dpc caused grain rotations that allowed a deformation enhanced heterogeneous fluid influx this resulted in local plagioclase replacement through interface coupled dissolution and precipitation and chemically induced grain boundary migration accompanied by bulging recrystallization along with neocrystallization of other phases this study illustrates a strong interaction and feedback between physical and chemical processes where the amount of stress and fluids dictates the dominant active process the interaction is a cause of deformation and external fluid infiltration with a result of strain localization and chemical re equilibration at amphibolite facies conditions

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

was the 1970 geomagnetic jerk of internal or external origin annual mean data from worldwide magnetic observatories show that there was a jerk1 step change in the second time derivative in the geomagnetic field which took place over an interval of 2 yr around 1970 if such a short lived phenomenon originated in the earth s core and were still detectable after passing through the mantle to the surface this would imply a much lower conductivity for the lower mantle than had been believed previously here we show by an objective test that most of the jerk has an internal origin

focused fluid flow during subduction oxygen isotope data from high pressure ophiolites of the western alps abstract during subduction hydrothermally altered oceanic crust undergoes extensive dehydration the water released migrates upwards and may escape along the slab mantle interface or continue rising to hydrate the mantle overlying the subducting slab oxygen isotope ratios from eclogitic ophiolites of the western alps correlate closely with lithology samples with little altered basalt as a protolith have 18 o values of 5 1 6 0 material representing low temperature alteration zone rocks have 18 o values of 5 9 6 6 and rocks orginating in high temperature alteration zones have 18 o values of 4 6 5 3 these isotopic values closely approach those of the ocean crust and unmetamorphosed ophiolites these data suggest that the alpine rocks have not experienced significant fluid flow during metamorphism implying that the escape of fluid from subducted ocean crust is highly focused probably along structurally controlled pathways to the top of the slab the fluid may then continue largely along the slab mantle interface although a portion of it rises further to hydrate the overlying mantle

kyanite eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks adula nappe central alps in the southern adula nappe central alps two stages of regional metamorphism have affected mafic and pelitic rocks earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite hornblende eclogites was followed by a tertiary overprint which varied from greenschist to high grade amphibolite facies despite a common metamorphic history contrasting equilibration conditions are often recorded by high pressure mafic eclogite and adjacent predominantly lower pressure pelite assemblages this pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading the rates are controlled by a mechanism in which dehydrating metapelites provide the h2o required for simultaneous overprinting of enclosed mafic eclogites by hydration

high pressure metamorphism of the zermatt saas ophiolite zone switzerland within the zermatt saas ophiolite zone of the western alps a record of polystadial high pressure metamorphism is well preserved early blueschist assemblages retained as inclusions in garnets were succeeded by eclogitic assemblages which in some rocks contained lawsonite kyanite talc and chloritoid these eclogitic assemblages formed by prograde reaction from the early blueschists subsequently reaction of these eclogites with a mixed h 2 o co 2 vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite garnet paragenses by assemblages containing glaucophane paragonite and ankerite high pressure eo alpine metamorphism took place under conditions of 550 600 c and 17 5 20 kbar values of a h 2 o between 0 55 and 1 are high enough to accommodate equilibration with a water rich vapour phase under the highest grade conditions the presence of such a fluid phase is locally indicated by the presence of quartz rich veins containing omphacite and kyanite this vapour phase was absorbed during retrogression later rehydration is limited to areas close to albite veins tectonic contacts and bodies of metasediment the metamorphic conditions determined for the zermatt saas zone are compatible with those suggested for over and under lying units these conditions and the p t path inferred by comparing the reaction histories with a petrogenetic grid for the system na 2 o cao mgo a1 2 o 3 sio 2 h 2 o suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the alpine orogeny

the timing of and nature of greenschist facies deformation and metamorphism in the upper pennine alps the upper pennine nappes in the western alps are cut by structures that locally place european basement rocks over those of the piemonte zone which represent the sutured remains of alpine ocean crust in the taschalp area of switzerland the mischabel backfold and associated se verging shearing carries gneisses of the grand saint bernard nappe over eclogitic material of the zermatt saas zone greenschist facies assemblages are developed locally in the foot wall and hanging wall of the mischabel structure as well as in the sheared rocks of the thrust zone rb sr 40ar 39ar and u pb studies of white micas amphiboles and sphenes show that greenschist assemblages developed at circa 44 ma in the zermatt saas zone of the footwall and at circa 40 ma in the thrust zone rb sr data from the literature document muscovite and biotite ages in strongly foliated grand saint bernard gneisses of the hanging wall of circa 36 and 32 ma respectively these are thought to indicate that deformation on the mischabel structure continued for at least 5 m y rapid cooling inferred for the zermatt saas ophiolites in the mid eocene may document either the tectonic emplacement of the ophiolite over the underlying monte rosa massif or their extensional unroofing it is not possible to determine with certainty whether the mischabel structure is extensional or contractional on a crustal scale the age of cooling and deformation reported here show that it is not possible that the mischabel structure was responsible for the exhumation of the high pressure assemblages in the zermatt saas and monte rosa rocks

the use of magsat data to determine secular variation the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 004 2 50 payment must accompany order a combined spatial and secular variation model of the geomagnetic field labeled m061581 is derived from a selection of magsat data secular variation sv data computed from linear fits to midnight hourly values from 19 magnetic observatories were also included in the analysis but were seen to have little effect on the model the sv patterns from this new model are compared with those from the 1980 igrf and with those for 1970 computed by the dgrf and with the 1960 patterns computed using the gsfc 12 66 model most of the features of the m061581 are identical in location and level with those of the 1980 igrf together they confirm that the reversals in sign of field change seen over asia and north america between 1965 and 1975 are reverting to the pre 1965 states the m061581 model gives 32 nt yr for the dipole decay rate larger than the 70 increase already reported since 1965 this abnormally high value is interpreted as being a defect of the model because it appears to result from a much larger 100 nt yr drop in field over the polar regions not indicated by the 1980 igrf this north polar decrease is shown to be of external origin as the result of a combination of the seasonal effect of the north polar ionospheric counterclockwise afternoon sqp cell increasing in intensity from the beginning november 1979 to the end june 1980 of the magsat data collection period coupled with an enhancement of its effect as the orbit lowered from the 350 550 km initial altitudes to near 200 km just prior to burnup this experiment indicates that secular variation can be obtained from satellite data for intervals of less than a full year if corrections can be made for seasonal effects and that annual snapshots of the field by a satellite would allow easy and accurate models of secular change without the use of any surface data

contrasting serpentinization processes in the eastern central alps stable isotope compositions have been determined for serpentinites from between davos arosa platta nappe switzerland and the valmalenco italy d and 18o values 120 to 60 and 6 10 respectively in the arosa platta nappe indicate that serpentinization took place on the continent at relatively low temperatures in the presence of limited amounts of metamorphic fluids that contained a component of meteoric water one sample of chrysotile has a 18o value of 13 providing evidence of high w r ratios and low formation temperature of lizardite chrysotile in this area in contrast relatively high d values 42 to 34 and low 18o values 4 4 7 4 for serpentine in the eastern part of the valmalenco suggest a serpentinization process that took place at moderate temperatures in fluids that were dominated by ocean water the antigorite in the valmalenco is the first reported example of continental antigorite with an ocean water signature an amphibole sample from a metasomatically overprinted contact zone to metasediments d 36 indicates that the metasomatic event also took place in the presence of ocean water lower d values 93 to 60 of serpentines in the western part of the valmalenco suggest a different alteration history possibly influenced by fluids associated with contact metamorphism low water rock ratios during regional metamorphism and metasomatism have to be assumed for both regions

effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread f we use radar observations from the jicamarca observatory from 1968 to 1992 to study the effects of the f region vertical plasma drift velocity on the generation and evolution of equatorial spread f the dependence of these irregularities on season solar cycle and magnetic activity can be explained as resulting from the corresponding effects on the evening and nighttime vertical drifts in the early night sector the bottomside of the f layer is almost always unstable the evolution of the unstable layer is controlled by the history of the vertical drift velocity when the drift velocities are large enough the necessary seeding mechanisms for the generation of strong spread f always appear to be present the threshold drift velocity for the generation of strong early night irregularities increases linearly with solar flux the geomagnetic control on the generation of spread f is season solar cycle and longitude dependent these effects can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance dynamo electric fields the occurrence of early night spread f decreases significantly during equinox solar maximum magnetically disturbed conditions due to disturbance dynamo electric fields which decrease the upward drift velocities near sunset the generation of late night spread f requires the reversal of the vertical velocity from downward to upward for periods longer than about half an hour these irregularities occur most often at 0400 local time when the prompt penetration and disturbance dynamo vertical drifts have largest amplitudes the occurrence of late night spread f is highest near solar minimum and decreases with increasing solar activity probably due to the large increase of the nighttime downward drifts with increasing solar flux

spherical cap harmonic analysis of geomagnetic secular variation over canada 1960 1983 the new technique of spherical cap harmonic anaylsis has enabled a model of secular variation to be constructed that applies to an area of over 30 million square kilometers and a time interval of over 20 years this technique is a regional analog of the ordinary method of spherical harmonic analysis and ensures that the curl and divergence of the secular variation vector are zero by making each harmonic coefficient a polynomial in time the model provides both spatial and temporal continuity the secular variation data from 1960 to 1983 comprise 1227 first central differences of annual mean values from 24 observatories in canada united states greenland and iceland and 939 first central differences from 138 canadian and american repeat stations divided by their respective time intervals the secular variation model includes terms up to spatial index 4 and temporal degree 2 although such a model allows for 75 coefficients only 39 were found to be statistically significant the temporal behavior of the model at observatories and repeat stations is compared on time plots to that of the secular variation data itself the scatter about the regression model or standard error of estimate was 11 8 nt yr the model is required both to update aeromagnetic survey data for producing canadian geomagnetic main field charts at 1985 0 and to provide annual change information for the extrapolation of those charts up to 1990 the average secular variations derived from the definitive dgrf and provisional pgrf international geomagnetic reference fields agree well with the model but the average secular variation for 1980 1985 from the predictive international geomagnetic reference field igrf does not

identification of the scales of differential element mobility in a ductile fault zone two fundamentally different scales of element mobility have been identified in the upper amphibolite facies hunts brook fault zone of south central connecticut usa field relations mineral chemistry and 80 bulk rock analyses provide overwhelming evidence that the blastomylonitic schists and gneisses of the fault zone were derived from the enclosing granodioritic rope ferry orthogneiss two sample mass balance calculations between the rope ferry gneiss and fault rocks strongly suggest that the rope ferry gneiss was segregated into biotite rich schists and quartz plagioclase rich gneisses through the centimetre scale transfer of si al ca na ba and zr from schists to gneisses three and four sample mass balance calculations provide convincing evidence that al ca na and ba were lost and si and k gained by the entire fault zone both scales of metasomatism have been identified at two localities separated by 20 km implying that the metasomatic processes were pervasive throughout the fault zone from the mass balance calculations an apparent disparity in k mobility arises k appears to be gained by the entire fault zone but is immobile during the centimetre scale segregation this is because mass balance calculations can only detect differential element mobility differential element mobility requires that the mechanism of metasomatism produces chemical potential gradients therefore the identification of differentially mobile elements provides insight into the mechanisms of metasomatism the immobility of k during segregation of schists and gneisses implies that the segregation was not achieved through the extraction of partial melt however the element mobility is reconcilable with deformation driven metamorphic differentiation the gains of si and k and losses of ca na al and ba for the entire fault zone are also inconsistent with the intrusion or extraction of partial melt but may reflect the infiltration of a metasomatizing aqueous fluid

timing and nature of fluid flow and alteration during mesoproterozoic shear zone formation olary domain south australia the development of shear zones at mid crustal levels in the proterozoic willyama supergroup was synchronous with widespread fluid flow resulting in albitization and calcsilicate alteration monazite dating of shear zone fabrics reveal that they formed at 1582 22 ma at the end of the olarian d3 deformational event and immediately prior to the emplacement of regional s type granites two stages of fluid flow are identified in the area first an albitizing event which involved the addition of na and loss of si k and fe and a second phase of calcsilicate alteration with additions of ca fe mg and si and removal of na fluid fluxes calculated for albitization and calcsilicate alteration were 5 56 109 to 1 02 1010 mol m 2 and 2 57 108 5 20 109 mol m 2 respectively these fluxes are consistent with estimates for fluid flow through mid crustal shear zones in other terranes the fluids associated with shearing and alteration are calculated to have 18o and d values ranging between 8 and 11 and 33 and 42 respectively and nd values between 2 24 and 8 11 our results indicate that fluids were derived from metamorphic dehydration of the willyama supergroup metasediments fluid generation occurred during prograde metamorphism of deeper crustal rocks at or near peak pressure conditions shear zones acted as conduits for major crustal fluid flow to shallow levels where peak metamorphic conditions had been attained earlier leading to the apparent retrograde fluid flow event thus the peak metamorphism conditions at upper and lower crustal levels were achieved at differing times prior to regional granite formation during the same orogenic cycle leading to the formation of retrograde mineral assemblages during shearing

deep fluid circulation in alpine shear zones pyrenees france field and oxygen isotope studies a combined field stable isotope and whole rock chemical study was made on late cretaceous to tertiary metasomatic shear zones cutting hercynian gneisses in the aston massif pyrenees france mylonitisation occurred during the early stages of alpine compression under retrograde conditions at 400 450 c and about 10 km depth whole rock 18o values of 11 to 12 in the gneisses was lowered to 5 to 9 in the shear zones with the quartz muscovite 18o 16o fractionations of about 2 to 4 essentially unchanged these 18o 16o systematics together with d muscovite 40 to 50 indicate that large volumes of formation waters or d rich meteoric waters passed through the shear zones during deformation the same fluids also redistributed major elements as shown by the correlation of 18o shift with muscovitisation and albitisation reactions in granitic wall rocks however even though 18o was universally lowered within the shear zones the 18o 16o ratios were not homogenised nor do they correlate in detail with the presence or absence of muscovitisation suggesting that fluid flow was probably fracture controlled and episodic field mapping shows that along the length of a particular shear zone muscovitisation of granite gneiss dies out 150m above the contact with underlying sillimanite gneiss thus muscovitisation and albitisation of granite gneiss in shear zones and their wall rocks probably occurred during re equilibration of acidic chloride rich aqueous fluids that had previously moved upward within the shear zones through underlying sillimanite gneiss extremely high material balance fluid rock ratios 103 are required to explain the extent of muscovitisation along this shear zone implying integrated fluid mass fluxes of about 108 kg m2 this is probably close to the maximum value for other shear zones in the network similar volumes of a more chemically evolved fluid must have passed through the unmuscovitised mylonites showing that the absence of alteration cannot necessarily be used to infer low values of fluid flux for reasonable pressure gradients and time scales of fluid movement effective permeabilities of 10 15 to 10 17 m2 are required such values can be accounted for by short lived widely spaced cracks produced during seismic activity a model is presented in which formation waters were seismically pumped down an underlying shallow southward dipping decollement and then upward through the steeply dipping shear zone network

the time varying geomagnetic field the scope of this review is limited to papers on the large scale geomagnetic field of deep internal origin and its secular variation published by u s authors and their colleagues during the 1983 1986 quadrennial the many fine papers on this broad topic by authors outside the u s are regretfully excluded fortunately most of them are cited by the u s authors listed in the reference section of this review labreque and mayhew this issue review papers treating the smaller scale fields usually attributed to visco inductively and remanently magnetized crustal geologic structures review articles dealing with external fields are surveyed by m schulz in the magnetosphere section of this u s national report booker and chave this issue review papers concerning the use of more rapidly fluctuating externally induced internal fields to probe the electrical properties of the crust and upper mantle lund and olson this issue review papers concerning paleo secular variation and core geodynamo theory

an o isotope profile through the hp lt corsican ophiolite france and its implications for fluid flow during subduction abstract subduction zones are important sites for fluid generation as the downgoing slab undergoes progressive high pressure metamorphism these fluids are thought to play an important role in the generation of arc magmas but their affect on the subducted slab itself is less clear in this study the stable isotope geochemistry of the corsican ophiolite which is variably affected by subduction related metamorphism was investigated to assess the impact of subduction zone fluid flow on the downgoing slab a total of 170 stable isotope analyses oxygen and carbon showed the following features 1 overall the ophiolite preserves the same oxygen isotope profile with depth as unmetamorphosed ophiolites 2 the variation in oxygen isotope values for different areas does not correlate with metamorphic grade 3 oxygen and carbon isotope values for calcite in the ophiolite are not in equilibrium with the ophiolite but are similar to those of the overlying calc schists schistes lustres and 4 hydrogen isotopes indicate that serpentinitisation of ultramafics occurred during sub seafloor hydrothermal alteration and probably also during tectonic emplacement of the ophiolite these features indicate that large scale fluid flow did not affect the corsican ophiolite during subduction zone metamorphism calcite in the ophiolite was probably precipitated from co2 bearing fluids derived from decompression dehydration reactions affecting the overlying schistes lustres during exhumation of the ophiolite bearing schistes lustres nappes given that ocean crust must undergo at least some dehydration during prograde subduction zone metamorphism the evolved fluids must be either highly channelled or exit the slab relatively quickly so as to not interact with it the results of this study indicate that the important fluid pathways on corsica have yet to be identified but are probably the shear zones and fault systems that bound different slices of the high pressure nappe stack

evidence for extension in the western alpine orogen the contact between the oceanic piemonte and overlying continental sesia units abstract in the alps and other orogens major tectonic contacts must have been established during convergence and overthrusting what is less clear is how strongly these contacts were modified as the orogen evolved during alpine collision continental crust of the overriding plate the sesia unit is inferred to have been thrust northwest over oceanic material of the piemonte unit both units preserve in parts eclogite facies metamorphism however all structures along this contact in our study area indicate that final movement was southeast directed with no clear evidence of the kinematics of earlier movement these structures include se verging folds se directed shear bands and larger normal faults downthrowing to the southeast it is inferred that there was a se directed shear regime below the contact which although locally buckling and shortening earlier lithological contacts was dominantly extensional with respect to pre existing tectonic layering the lower limit of this shear regime has not been identified although se directed shear is common throughout the upper greenschist facies part of the piemonte unit we show that this shear passes beneath the sesia zone and does not re emerge it is therefore a shear which was net extensional relative both to the modern surface and to the palaeosurface it may have contributed to the unroofing of eclogite facies rocks in the lower part of the piemonte unit although additional timing data are required to clarify this nearby se directed shears buckle and imbricate earlier layering within the piemonte unit these are normally identified as backthrusts yet they do not appear to breach the base of the sesia unit and might merge with the extensional shear even though these structures shorten layering they too could have been extensional relative to the earth s surface the significance of alpine backthrusts should be reappraised in this context this study indicates that hinterland directed extension could have been an important phenomenon during alpine evolution

sm nd isotopic evidence on the age of eclogitization in the zermatt saas ophiolite rocks within the zermatt saas ophiolite of the western alps have undergone eclogite facies metamorphism during subduction prior to the alpine collision the metamorphic history of these rocks is well defined with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite subsequent greenschist alteration occurs adjacent to faults veins and metasediments away from such sources of water retrogression is very limited sm nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 18 ma the large uncertainty is due to the presence of very small amounts of nd rich epidote present as inclusions within garnet as the closure temperature of garnet to sm nd is thought to be 600dc resetting due to post high pressure diffusion is thought to be insignificant given the fine grained protolith to the sample analysed and its extensive deformation under eclogite facies conditions incomplete homogenization of pre metamorphic isotopic variations is also considered unlikely to be responsible for the young age a tertiary age of eclogitization means that models of early alpine evolution based on the cessation of high pressure metamorphism in the cretaceous need to be revised

band limited global scalar magnetic anomaly map of the earth derived from magsat data the magnetometer satellite magsat data are separated into two distinct sets the dawn and the dusk sets the two sets are analyzed independently because of the dawn dusk asymmetry of the external magnetic field components of the data two scalar magnetic anomaly maps the dawn and the dusk maps are derived using the spherical harmonic expressions of the respective data set the significant correlation of the two maps in the harmonic range of 18 41 indicates the repeatability of the magsat data in these harmonics the shortest repeatable wavelength is about 920 km a band limited global scalar magnetic anomaly map is derived at about 410 km altitude by arithmetic averaging of the dawn and the dusk maps the map is then downward continued to about 10 km altitude the downward continued magnetic anomalies are relatively more localized the main features of the anomalies are as follows 1 continents have relatively stronger magnetic anomalies than oceans especially the younger oceans 2 many circum pacific subduction zones have magnetic signatures and 3 there is no overall correlation between anomalies and major shields

data driven numerical simulations of equatorial spread f in the peruvian sector ionospheric state parameters including plasma number density and vector drift velocity profiles were measured at the jicamarca radio observatory during a campaign running from 12 to 18 april 2013 neutral winds were measured by the red line fabry perot interferometer at jicamarca coherent radar backscatter from plasma irregularities associated with equatorial spread f esf was also recorded a numerical simulation of ionospheric irregularities initialized and forced using parameterizations derived from a combination of measurements and empirical models was used to try to reproduce the esf activity that occurred on three different nights simulations were able to recover the salient features of the irregularities that formed in each case and produce bottom type bottomside and topside esf realistic simulations require the accurate representation of the vertical currents that generate irregularities initially at the base of the f region as well as the zonal currents necessary for irregularities to expand in altitude and penetrate to the topside the campaign data numerical simulations and protocols used to associate them are presented and evaluated