alteration zoning and primary geochemical dispersion at the bronzewing lode gold deposit western australia the late archaean bronzewing lode gold deposit is in the yandal greenstone belt western australia it is located in a 500 m wide n s trending structural corridor consisting of an anastomosing set of brittle ductile shear zones and is chiefly hosted by tholeiitic basalts which are metamorphosed at mid to upper greenschist facies syn peak metamorphic alteration surround all ore bodies and alteration extends laterally for 80 m from individual mineralised structures individual alteration haloes partially overlap and form a 1 5 km long and 300 m wide domain the alteration sequence studied here at 140 m below the present undisturbed surface comprises distal calcite chlorite albite quartz intermediate calcite dolomite chlorite muscovite albite quartz and proximal ankerite dolomite muscovite albite quartz pyrite zones mass transfer calculations indicate that chemical changes during alteration include enrichment of ag au ba bi co2 k rb s sb te and w and depletion of na sr and y the elements al ca cr cu fe mg mn ni p ti v zn and zr are immobile the degree of chemical change increases with proximity to gold ore zones in addition abundant quartz veins indicate substantial silica mobility during the hydrothermal event although there is no large relative silica loss or gain in the host rock the broadest anomaly surrounding the bronzewing gold deposit is defined by tellurium 10 ppb which if it is a hydrothermal anomaly extends beyond the 400 600 m study area anomalous values for co2 k rb and sb also define wider zones than does anomalous gold 4 ppb although even the lithogeochemical gold anomaly extends across strike for as much as 80 m away from ore and 600 m along the n s strike of the shear zone corridor also carbonation and sericitisation indices outline large exploration targets at the bronzewing deposit sericitisation indices define anomalies that extend for tens of metres beyond visible potassic alteration whereas the anomalies defined by the carbonation indices do not extend beyond visible carbonation none of the individual alteration indices or pathfinder elements are able to define consistent gradients towards ore however the respective dimensions of individual geochemical anomalies can be used as an extensive although stepwise vector towards ore this sequence is from species with broadest dispersion first as follows te co2 ca sb 3k al rb ti au w y ti depletion ag bronzewing ore

the effect of crustal scattering on observed high frequency earthquake seismograms summary a comparison of high frequency 1 100 hz local out to 40 km earthquake seismograms at stations of the southern black forest digital seismic network is made in an attempt to isolate the effects of near receiver structure and of crustal scattering on the waveforms of the direct body waves and on the coda waves angle of incidence and azimuth of approach analyses from lower crustal earthquakes show that near surface weathering layers with low body wave velocities and with laterally varying thicknesses exist under all of the stations even those at hard rock sites of the black forest the similarity of direct p wave pulse shapes at each site for events of different sizes the time domain form of the so called fmax phenomenon is attributed to the response from this weathering zone the amplitude of the p wave coda of an event is found to be independent of the earthquake radiation pattern and to depend only on the absolute size of the event and on epicentral distance to the receiver furthermore the high frequency p wave coda is found to be generated at least in part from random crustal scattering including that within 100 m of the earthquake focus direct s waveforms are shown to be more complicated than their p wave counterparts probably due to the same near receiver and crustal scattering phenomena however in the case of s waves scattered energy can arrive before as well as at and after the direct s arrival making the identification of the direct s difficult especially for nodal arrivals

high frequency site effects at two parkfield downhole and surface stations abstract using stacked ratios of microearthquake source spectra recorded at the surface and 200 m downhole we have established the p and s wave site effect at two locations in parkfield california the frequency responses of both sites are bimodal being completely dominated by multiples and amplification below 10 hz and by attenuation above 30 hz the low frequency amplification factors show roughly three to five times more uphole motion compared to that downhole the multiples produce peaks with a maximum of nearly 10 times more uphole motion above 50 hz the downhole motion is always larger approaching a factor of 5 at the high frequency limits of our study when fit with a linear attenuation model the high frequency ratios yield compressional and shear q values between 6 and 19 the data were divided into subsets based on event location and time of year statistical comparison of these subsets shows no systematic variation with azimuth or season the results of our study accounts for the lack of high frequency surface reflections in the parkfield downhole data but not for the lack of source scaling of parkfield microearthquakes

two billion years of evolution enclosed in hydrothermal rutile recycling of the sao francisco craton crust and constraints on gold remobilisation processes abstract the quadrilatero ferrifero in the sao francisco craton brazil is known for its world class gold deposits hosted by the rio das velhas greenstone belt rdv as in other greenstone belts gold is also concentrated in late archean early paleoproterozoic supracrustal units in this case within the moeda formation this unit has been compared to the witwatersrand basin where there has been a long term debate over the nature of the gold and whether it is detrital placer hydrothermal or hybrid modified placer presently links between gold in the moeda formation and rdv are tenuous to enlighten this area of ambiguity a new approach using chemical isotopic and mineral inclusion data from detrital rutile found in the gold bearing horizons of the moeda formation is presented grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile related to as fe cu sb pb w rich mineralising fluids fe nb and v indicate that the likely source of the gold related rutiles is the archean banded iron formations within the rdv where most of the lode gold occurs average nb ta values of these rutiles are lower than average continental crust ratios which suggests fractionation driven by fluid precipitation and or competing mineral phases u pb data yield primary crystallization ages older than deposition ages 2 58 ga supporting the detrital nature of these rutiles rutiles record either resetting or new growth at 2 1 ga and at ca 500 ma during which gold was remobilised hence this study supports a modified placer model for the gold bearing horizons of the moeda formation these fluid assisted gold remobilisation events can be found in other correlated paleoproterozoic gold bearing horizon units that represent intracratonic basins related to the rifting of the kenorland supercontinent this study reinforces the suitability of rutile and mineral inclusions as a powerful tool to elucidate processes and provenance

rupture properties of clustered microearthquakes near intersecting intraplate faults of new madrid seismic zone implications for fault weakening seismic waveforms from the portable array for numerical data acquisition network were used to identify sets of 21 similar microearthquakes near ridgely tennessee where two major segments of the new madrid seismic zone nmsz intersect the events are relocated with a precision of about 30 m and are found to have occurred along four subfaults that are aligned to n50 e subparallel to the southern arkansas segment of the nmsz the separations among the subfaults are a few hundred meters while the span of hypocenters within each subfault is typically a few tens of meters these subfaults are inferred from focal mechanism solutions to be oriented at high angles to n50 e trend striking roughly north northwest and dipping moderately with reverse slip this indicates that the subfaults belong to the central segment of the nmsz and are en echelon the similar events also cluster in time the typical time span of events over one subfaults is about 3 weeks while the intercluster times are a few months or more an empirical green s function analysis of the largest event placed a lower bound of its rupture dimension at 121 32 m suggesting that this rupture overlapped with the smaller ruptures along the same subfault on the basis of variations in fault zone properties previously inferred for the ridgely junction the rupture properties of the similar events and the result of a stress calculation we infer that the similar events may be triggered by temporal weakening of the subfaults possibly associated with a temporal elevation of pore fluid pressure the nearly constant depths 5 7 km and the reverse dip of the ruptures suggest that the en echelon subfaults may be intersected by subhorizontal veins which could have been developed by episodic influx of fluid at superhydrostatic pressures

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

nucleation and growth of myrmekite during ductile shear deformation in metagranites myrmekite is extensively developed along strain gradients of continuous lower amphibolite facies shear zones in metagranites of the gran paradiso unit western alps to evaluate the role of stress strain energy and fluid phase in the formation of myrmekite we studied a sample suite consisting of weakly deformed porphyric granites wdgs foliated granites fgs representative of intermediate strains and mylonitic granites mgs in the protolith most k feldspar is microcline with different sets of perthite lamellae and fractures in the wdgs abundant quartz oligoclase myrmekite developed inside k feldspar only along preexisting perthite lamellae and fractures oriented at a high angle to the incremental shortening direction in the wdgs stress played a direct role in the nucleation of myrmekites along interfaces already characterized by high stored elastic strain because of lattice mismatch between k feldspar and albite in the fgs and mgs k feldspar was progressively dismembered along the growing network of microshear zones exploiting the fine grained recrystallized myrmekite and perthite aggregates this was accompanied by a more pervasive fluid influx into the reaction surfaces and myrmekite occurs more or less pervasively along all the differently oriented internal perthites and fractures independently of the kinematic framework of the shear zone in the mgs myrmekite forms complete rims along the outer boundary of the small k feldspar porphyroclasts which are almost completely free of internal reaction interfaces therefore we infer that the role of fluid in the nucleation of myrmekite became increasingly important as deformation progressed and outweighed that of stress mass balance calculations indicate that in al si conservative conditions myrmekite growth was associated with a volume loss of 8 5 this resulted in microporosity within myrmekite that enhanced the diffusion of chemical components to the reaction sites and hence the further development of myrmekite

complex mineral zoning patterns caused by ultra local equilibrium at reaction interfaces chemically zoned minerals are useful records of temporal variations in ambient conditions and bulk chemical composition of the fl uid from which the minerals precipitate in fl buffered systems zoning of mineral compositions is expected to refl ect directly the evolution of fl composition here we show that during rapid fl uid rock reactions ultra local equilibrium can form complex mineral zoning patterns even when the overall system is highly fl uid buffered we reacted cleaved calcite single crystals with aqueous arsenate phosphate solutions with molar ratios of as as p between 0 01 and 0 15 at 250 c and water saturated pressure we fithat complex zoning patterns and solid solution between hydroxylapatiteand arsenate bearing hydroxylapatite that pseudomorphically replaced calcite formed within hours and these zoning patterns were destroyed within days during secondary reactions we propose a two stage reaction process in the formation of the fi nal reaction product 1 on an hour time scale calcite is dissolved and replaced by compositionally heterogeneous apatite the thin reaction interface fllayer becomes extremely enriched in arsenic at an ultra local scale as the reaction removes phosphate faster than the interface fl uid can re equilibrate with the bulk fl uid 2 the heterogeneous apatite is replaced by homogeneous apatite that refl ects the bulk fl uid composition over a longer days time scale through interface coupled dissolution precipitation this paper highlights the complexity that can arise from ultra local fl uid composition variations due to rapid fl uid rock interaction in a short lived fl uid fl ow event for example during a seismic cycle subsequent interpretation of complex zoning patterns as refl ecting the evolution of bulk fl uid would be erroneous

stability of coda wave attenuation during the loma prieta california earthquake sequence the loma prieta california earthquake occurred in a densely instrumented region with a history of microearthquake recording beginning more than a decade before the october 1989 mainshock this affords an unprecedented opportunity to detect changes in seismic wave propagation in the earth s crust associated with a major earthquake in this study we use pairs of nearly identical earthquakes doublets to search for temporal changes of coda attenuation in the vicinity of the loma prieta earthquake we analyze 21 earthquake doublets recorded from 1978 to 1991 that span the preseismic coseismic and postseismic intervals and measure the change in coda q using a running window ratio of the doublet spectral amplitudes in three frequency bands from 2 to 15 hz this method provides an estimate of changes in coda q that is insensitive to other factors that influence coda amplitudes our observations place an upper bound of about 5 on preseismic coseismic and postseismic changes of coda q in the epicentral region of the loma prieta earthquake even at this low level the changes are neither spatially coherent nor correlated between adjacent frequency bands the only hint of a signal is in the preseismic data where there is a possible precursory increase in coda q of approximately 5 in the two years before the mainshock the stability of coda q throughout the loma prieta sequence is in sharp contrast to other studies that have reported much larger precursory changes in coda q for other earthquakes

site dependent coda q at mono craters california abstract the coda q of the early coda of shear waves was determined from a network of seismic stations deployed in the mono craters area during the summer of 1982 coda amplitudes determined from a moving window analysis were fit to single scattering theory with an l1 norm for center frequencies from 1 5 to 12 0 hz we find that station to station variations in coda q observed for frequencies between 6 and 12 hz are well correlated with near site geology coda q for stations on crystalline rocks is larger and more frequency dependent than for stations sited on bishop tuff adjacent to mono craters coda q from sites on bishop tuff away from mono craters is still lower and less frequency dependent these observations suggest that site effects dominate the early coda in the mono craters region and that the single scattering model is inappropriate the mechanisms generating these site effects are inferred to be 1 multiple scattering from resonant structures such as sedimentary basins volcanic edifices variable basement topography and surface topography and 2 near site attenuation

interpretation of high frequency coda at large distances stochastic modelling and method of inversion summary based on previous work a stochastic convolution model is proposed for the lg coda observed at large epicentral distances 200 km coda at these distances differ from local coda in several respects effects of various physical processes such as dispersion scattering and mode conversion on coda at large distances are discussed in detail a spectral ratio method is developed for inversion of lg coda q in which the large variance associated with inversion is greatly reduced the method is also extended to the inversion of seismic source parameters by jointly using lg and its coda we have tested the model and method exhaustively using data from two gdsn stations and found the results to be superior to other methods when those methods used only a small number of stations

dissolution and replacement creep a significant deformation mechanism in mid crustal rocks abstract zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and s c fabrics in these rocks were produced by dissolution precipitation and replacement processes even at epidote amphibolite facies metamorphic conditions the metamorphic fabric is defined by alternating layers and folia dominated by quartz feldspars and biotite epidote zoning patterns in most metamorphic plagioclase orthoclase epidote and sphene are truncated at boundaries normal to the shortening direction suggesting dissolution interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite epidote and myrmekitic intergrowths of plagioclase and quartz metamorphic plagioclase grains are also replaced by epidote we interpret these microstructures to reflect strain enhanced dissolution the cores of many grains show asymmetric overgrowths with at least two generations of beards all oriented on the ends of grains that face the extension direction we interpret these textures to reflect precipitation of components dissolved by deformation enhanced dissolution while biotite and quartz probably deformed by dislocation creep the overall deformation was accommodated by dissolution perpendicular to the shortening direction and precipitation parallel to it these chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks thus wet crust is likely to be weaker than calculated from these experimental studies

a comparison of direct s wave and coda wave site amplification determined from aftershocks of the little skull mountain earthquake abstract site amplifications of direct s waves and coda waves are studied and compared using high quality three component digital data from aftershocks of the little skull mountain neveda earthquake we use data from 12 stations installed on a variety of geological and topographic site conditions distributed widely in space with different azimuths and epicentral distances s wave site amplifications are obtained in the frequency range from about 0 5 to over 30 hz while coda wave site amplifications are obtained in a frequency range from about 1 5 to over 30 hz a thorough statistical analysis of these results was performed we find that 1 for s waves all three components at a station follow a similar frequency dependent trend the amplitudes of the two horizontal components match more closely while the vertical component shows consistently lower amplification than the horizontal components at low frequencies 2 for coda waves all three components share both similar frequency dependent trends and amplification level 3 s wave and coda wave site amplification are consistent for stations with epicentral distance greater than about 10 km which is about the average focal depth of the earthquakes we used within the epicentral distance of 10 km however some stations show discordant s wave and coda wave site amplifications possible factors are that the direct s waves are affected by particular wave propagation paths and that at short distance sv is partitioned with more energy on the horizontal components and less on the vertical components than at larger distances

lg coda q variation across eurasia and its relation to crustal evolution an extensive set of broadband and short period seismograms from regional seismic events has been used to derive a tomographic image of broad scale variations of lg coda q for most of eurasia the image indicates that lg coda q at a frequency of 1 hz varies between about 200 and 1000 lowest q values 800 are found in four regions the east european shield the siberian shield the indian shield and the southwestern and oldest portion of the altaid belt the pattern of lg coda q variation across eurasia suggests that crustal q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large scale compressional tectonic activity or upper mantle heating that occurred there we interpret the low q as resulting largely from hydrothermal fluids generated by tectonic activity or heating that now reside in cracks and permeable portions of the eurasian crust crustal q increases with time after that activity as fluids are lost to the surface or absorbed by retrograde metamorphism

antarctic and global climate history viewed from ice cores a growing network of ice cores reveals the past 800 000 years of antarctic climate and atmospheric composition the data show tight links among greenhouse gases aerosols and global climate on many timescales demonstrate connections between antarctica and distant locations and reveal the extraordinary differences between the composition of our present atmosphere and its natural range of variability as revealed in the ice core record further coring in extremely challenging locations is now being planned with the goal of finding older ice and resolving the mechanisms underlying the shift of glacial cycles from 40 000 year to 100 000 year cycles about a million years ago one of the great mysteries of climate science a discussion of past antarctic and global climate history as seen from antarctic ice cores with an outlook on future goals and drilling priorities

local site effects on weak and strong ground motion abstract this is a review of the current state of the art in characterizing effects of local geology on ground motion a new horizon is clear in this aspect of strong motion studies non linear amplification at sediment sites appears to be more pervasive than seismologists used to think several recent observations about the weak motion and the strong motion suggest that the non linear amplification at sediment sites may be very common first on average the amplification is always greater at the younger sediment sites for all frequencies up to 12 hz in the case of weak motion while the relation is reversed for frequencies higher than 5 hz in the case of strong motion secondly the application of the amplification factor determined from weak motion overestimates significantly the strong motion at sediment sites observed during the loma prieta earthquake within the epicentral distance of about 50 km thirdly the variance of peak ground acceleration around the mean curve decreases with the increasing earthquake magnitude finally the above non linear effects are expected from geotechnical studies both in the magnitude of departure from the linear prediction and in the threshold acceleration level beyond which the non linearity begins

site amplification factors in central and southern california determined from coda waves abstract site amplification factors at frequencies of 1 5 3 6 and 12 hz were determined for 158 stations of the southern california seismic network scsn from coda waves of 306 local earthquakes using a recursive stochastic inverse method our results for southern california are consistent with those of central california obtained by su et al 1992 in that the site amplification at a station is systematically related to the surface geology of the station site the site amplification is high for young quaternary sediments and decreases with increasing geologic age at all frequencies between 1 5 and 12 hz this decreasing rate is higher for younger rock sites gradually becomes lower with increasing geologic age and appears to be insignificant when the site reaches mesozoic age or older the relationship between site amplification and geologic age obtained for southern california agrees with that for central california amplification factors at 158 sites in southern california were combined with those at 132 sites in central california through a common reference site the result is a unique data base of site specific uniformly referenced amplification factors throughout central and southern california

changes in source and site effects compared to coda q 1 temporal variations using microearthquakes doublets in california we analyse spectral ratio of the coda of doublets of microearthquakes our purpose is to find evidence for temporal changes of the attenuation in the crust before a large magnitude earthquake a moving window cross spectral analysis of the coda of doublets gives a plot of the spectral ratio as a function of lapse time along the seismogram for several frequency bands sr t f plot from a certain pattern in thesr t f plot we should infer a temporal change in coda attenuation several doublets recorded in central california by the usgs network are analysed

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

questions about myrmekite in deformed rocks abstract several questions about the development of myrmekite in deformed felsic and metapelitic rocks remain unanswered although a general relationship between deformation and myrmekite appears to be well established the fact that myrmekite grows into relatively non deformed k feldspar while being simultaneously deformed and recrystallized at the rear of the growing lobes suggests that strain energy may not be a major contributor to the advance of the growth interface of the myrmekite colony furthermore because myrmekite typically nucleates on existing plagioclase rather than in k feldspar strain energy may not contribute directly to nucleation of myrmekite either at least in many instances however strain energy probably contributes indirectly by facilitating access of fluids to growth sites thereby altering the local chemical environment and so promoting development of myrmekite because myrmekite recrystallizes readily in deformed felsic rocks it is one of the main contributors to the development of folia

some garnet microstructures an illustration of the potential of orientation maps and misorientation analysis in microstructural studies the microstructures of two contrasting garnet grains are mapped using automated electron backscatter diffraction in both cases there is a very strong crystallographic preferred orientation with measurements clustered round a single dominant orientation each garnet grain is divided into domains with similar orientations limited by boundaries with misorientations of 2 or more in both samples most of misorientation angles measured across orientation domain boundaries are significantly lower than those measured between random pairs of orientation domains one sample is a deformed garnet that shows considerable distortion within the domains lines of orientation measurements within domains and across domain boundaries show small circle dispersions around rational crystallographic axes the domain boundaries are likely to be subgrain boundaries formed by dislocation creep and recovery the second sample is a porphyroblast in which the domains have no internal distortion and the orientation domain boundaries have random misorientation axes these boundaries probably formed by coalescence of originally separate garnets we suggest that misorientations across these boundaries were reduced by physical relative rotations driven by boundary energy the data illustrate the potential of orientation maps and misorientation analysis in microstructural studies of any crystalline material

a comparison of coda and s wave spectral ratios as estimates of site response in the southern san francisco bay area abstract we calculate spectral ratios for s waves and codas to evaluate the amplification of 14 sites in the southern san francisco bay area relative to a nearby bedrock site in the coyote hills our data are seismograms written by loma prieta aftershocks the epicentral distances and azimuths to the stations are effectively the same all the sites in the study are amplified with respect to the reference site at frequencies from 0 5 to 7 hz the shapes of the s wave and coda spectral ratios are similar but the coda ratios are greater than the s wave ratios by as much as a factor of 4 the difference is larger for sites on alluvial and bay mud deposits particularly at frequencies around 1 hz suggesting the presence of waves trapped in the alluvial basin in general the length of the analysis window affects the s wave spectral ratios for alluvial sites longer windows give ratios similar to coda ratios apparently because these windows include more of the phases that contribute to the coda we classify the sites according to their geological characteristics and surficial shear wave velocities for the s wave ratios the differences between the classes showed no systematic trend the softest and hardest soil classes we consider have practically identical s wave amplifications the average coda ratios for the site classes clearly increase as the soil classes include slower and softer materials after correction for differences in reference sites the coda amplifications are very similar to the relative amplifications for these site classes estimated by borcherdt and glassmoyer 1992 from the strong motion recordings of the 1989 loma prieta earthquake

site amplification of coda waves from local earthquakes in central california the goals of this study are two fold 1 to examine the site effect of coda waves believed to be the average site effect of shear waves in order to understand its spatial and frequency dependent behavior and 2 to learn what we can about the processes that generate the coda itself we use digital data from over 90 earthquakes each recorded by a subset of some 150 stations in the coast ranges of central california between san francisco and san luis obispo results from the band 1 5 to 24 hz indicate that site amplification depends strongly on surface geology and frequency at low frequencies 1 5 to 3 0 hz changes in amplification up to a factor of 20 are observed amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and franciscan basement sites at high frequencies 6 to 24 hz the pattern changes many granite sites the majority in the gabilan range exhibit increasing amplification with frequency relative to the average station this behavior differs strikingly from that observed at sediment and franciscan sites which roll off at an intermediate rate and many sediment sites adjacent to the san andreas fault zone which decay away even more rapidly with frequency these results can be explained qualitatively by appealing to variable near site impedance and attenuation however complicating phenomena such as mode excitation in valley sediments must exist at low frequencies peaked behavior at a few hard rock sites at high frequency 12 hz currently lacks explanation at a few sediment sites the low frequency excitation is so strong that the coda envelopes begin to take on different shapes this contradicts the assumption based on previous observations that the coda shape is independent of source receiver location thus some site measurements may be artificially high the working model of coda waves as body wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near site resonance excited by incident direct and coda wave energy

role of myrmekite and associated deformation fabrics in controlling development of granitic mylonites in the pofadder shear zone of southern namibia myrmekites developed within metagranites of the pofadder shear zone psz in southern namibia were investigated to understand how alkali feldspar deforms under fluid poor mid crustal conditions myrmekites showed two different morphologies 1 wormy myrmekite with a fine grained vermicular structure that occurred closest to the alkali feldspar host and 2 recrystallised myrmekite composed of regularly sized quartz grains that formed 120 boundaries with the plagioclase and occurred along the exterior of alkali feldspar and in contact with the mylonitic matrix these textures were interpreted to represent two generations of myrmekite growth recrystallised myrmekite masses were dismembered and incorporated into the fine grained matrix this suggests that myrmekite formation during psz deformation contributed to the grain size reduction of alkali feldspar and aided in the generation of the fine grained polymineralic matrix which accommodated most of the strain syn kinematic reactions such as myrmekite and muscovite production were internally balanced and t mh2o pseudosections indicated that progressive deformation could have occurred under closed conditions the biotite muscovite plagioclase alkali feldspar quartz equilibrium assemblage was stable with decreasing fluid contents until fluid saturated or slightly fluid undersaturated conditions were reached around 2 mol h2o the variability of the quartz rod spacing and the wide range of results from two feldspar thermometry and phengite barometry on associated muscovite growth indicates that peak metamorphic conditions were around 500 c 0 5 gpa and that myrmekite growth continued as the shear zone was exhumed the myrmekite forming reaction is a deformation induced interface coupled dissolution precipitation reaction with relict magmatic plagioclase supplying the needed cationic inputs these results indicate that deformation within granitic units of the psz was aided by the formation and deformation of myrmekite and could have taken place under closed relatively fluid poor conditions

rare earth and trace element mobility in mid crustal shear zones insights from the mont blanc massif western alps the behaviour of rare earth elements ree during fluid rock interaction in mid crustal shear zones has received little attention despite their potential for mass balance calculation and isotopic tracing during deformation in this study several cases of large ree mobility during alpine fluid driven shear zone development in the pre alpine granitic basement of the mont blanc massif are considered on a regional scale the undeformed granite compositions range within 5 wt sio2 70 5 75 3 wt and magmatic chemical variations are of the order of 10 20 ascribed to minor effects of crystal fractionation major and trace element mobility observed in shear zones largely exceeds these initial variations shear zones developed a range of mineral assemblages as a result of shearing at mid crustal depths at not vert similar0 5 gpa 400 c five main shear zone assemblages involve muscovite chlorite epidote actinolite and calcite respectively as major phases in most cases selective enrichments of light or heavy ree and y ta hf are observed ree mobility is unrelated to deformation style cataclastic mylonitic the intensity of strain and to the shear zone s major metamorphic mineral assemblages instead the changes in ree concentrations are ascribed to the alteration of pre existing magmatic ree bearing minerals during deformation related fluid rock interaction and to the syntectonic precipitation of metamorphic ree bearing minerals mainly monazite bastnasite aeschynite and tombarthite minor proportions 2 of these accessory phases with grain sizes mostly 20 m account for enrichments of up to 5 1 compared to the initial granite whole rock ree budget the stability of the ree phases appears to be largely dependent on the altering fluid composition ree mobility is ascribed to changes in ph and to the availability of co32 po42 and so42 ligands in the fluid such processes are likely to influence the mobility of ree y hf and ta in shear zones

successive hydration and dehydration of high p mafic granofels involving clinopyroxene kyanite symplectites mt daniel fiordland new zealand three texturally distinct symplectites occur in mafic granofels of the arthur river complex at mt daniel fiordland new zealand these include symplectic intergrowths of clinopyroxene and kyanite described here for the first time pods of mafic granofels occur within the contact aureole of the early cretaceous western fiordland orthogneiss batholith the pods have cores formed entirely of garnet and clinopyroxene and rims of pseudomorphous coarse grained symplectic intergrowths of hornblende and clinozoisite that reflect hydration at moderate to high p these hornfelsic rocks are enveloped by a hornblende clinozoisite gneissic foliation s1 narrow garnet reaction zones in which hornblende and clinozoisite are replaced by garnet clinopyroxene assemblages developed adjacent to fractures and veins that cut s1 fine grained symplectic intergrowths of 1 clinopyroxene and kyanite and 2 clinozoisite quartz kyanite and plagioclase form part of the garnet reaction zones and partially replace coarse grained s1 hornblende and clinozoisite the development of the garnet reaction zones and symplectites was promoted by dehydration most probably following cooling of the contact aureole maps of oxide weight percent and cation proportions calculated by performing matrix corrections on maps of x ray intensities are used to study the microstructure of the symplectites

nomenclature of amphiboles additions and revisions to the international mineralogical associations amphibole nomenclature the introduction of a fifth amphibole group the na ca mg fe mn li group defined by 0 50 b mg fe 2 mn 2 li 1 50 and 0 50 b ca na 1 50 apfu atoms per formula unit with members whittakerite and ottoliniite has been required by recent discoveries of b lina amphiboles this and other new discoveries such as sodicpedrizite which is herein slightly but significantly changed from the original idealized formula necessitate amendments to the ima 1997 definitions of the mg fe mn li calcic sodic calcic and sodic groups the discovery of obertiite and the finding of an incompatibility in the ima 1997 subdivision of the sodic group requires further amendments within the sodic group all these changes which have ima approval are summarized

the subglacior drilling probe concept and design in response to the oldest ice challenge initiated by the international partnerships in ice core sciences ipics new rapid access drilling technologies through glacier ice need to be developed these will provide the information needed to qualify potential sites on the antarctic ice sheet where the deepest section could include ice that is 1 ma old and still in good stratigraphic order identifying a suitable site will be a prerequisite for deploying a multi year deep ice core drilling operation to elucidate the cause and mechanisms of the mid pleistocene transition from 40 ka glacial interglacial cycles to 100 ka cycles as part of the ice lasers subglacior projects we have designed an innovative probe subglacior with the aim of perforating the ice sheet down to the bedrock in a single season and continuously measuring in situ the isotopic composition of the melted water and the methane concentration in trapped gases here we present the general concept of the probe as well as the various technological solutions that we have favored so far to reach this goal

simultaneous study of the source path and site effects on strong ground motion during the 1989 loma prieta earthquake a preliminary result on pervasive nonlinear site effects we considered the effects of source propagation path and local site conditions on strong ground motion from the loma prieta earthquake in the following steps first the local site effect was removed from the strong motion records applying the weak motion amplification factors estimated from nearby stations of usgs regional network using the coda method then we confirmed the approximate validity of the 1 r law of geometrical spreading factor for the crustal model applicable to the region and we determined the attenuation parameter q f and source spectrum s f empirically from the site effect corrected spectral amplitude both q f and s f are consistent with the results obtained earlier for major california earthquakes we then synthesized the time history of ground acceleration by applying the stochastic simulation technique the agreement between the observed and predicted records was good for duration and spectral content we found however a strong systematic discrepancy with regard to peak acceleration namely for distances less than 50 km the predicted peak acceleration consistently overestimated the observed for sediment sites and underestimated it for the franciscan formation sites since the above systematic discrepancy applies to all the available stations within about 50 km from the hypocenter it is unlikely that the effect is due to the radiation pattern near source structure or topography it appears that the strong difference in the amplification factor between the franciscan and the sediment sites observed for weak motion disappears at acceleration levels higher than about 0 1 to 0 3 g if this is due to the nonlinear response of the sediment sites it is in the range expected from geotechnical engineering studies

ordinary mesostasis and not so ordinary symplectites late stage assemblages in howardites abstract two categories of symplectites have been observed in howardites three phase composed of vermicular intergrowths of ferroan augite fayalitic olivine and silica and two phase composed of vermicular intergrowths of orthopyroxene and troilite three phase symplectites have been previously shown to represent the breakdown products of metastable pyroxene in howardites they appear to be genetically related to gabbroic eucrites in some cases and under yet to be specified conditions ferroan clinopyroxene in gabbroic eucrites may undergo only localized decomposition resulting in oriented exsolution like features breakdown phases in those cases are fayalitic olivine silica and depending on the mgo content of the system orthopyroxene as opposed to three phase symplectites two phase symplectites are most likely of diogenitic origin they probably formed via impact induced localized melting of diogenitic orthopyroxene in the presence of troilite grain boundary melting three phase symplectites in howardites occasionally contain accessory amounts of ilmenite troilite and or kamacite and are exclusively associated with medium grained feo rich pyroxene silica and plagioclase all minerals involved are late stage crystallites or mesostasis phases in general highly evolved eucritic lithologies constitute only a minor fraction of howardites however considering that three phase symplectites are generated in a low pressure i e near surface environment feo and cao rich eucritic rocks may be exposed locally on vesta s surface this in turn is highly relevant to the ongoing dawn mission

slowness power spectrum analysis of the coda composition of two microearthquake clusters in northern switzerland abstract the s wave coda composition of two microearthquake clusters in northern switzerland has been analysed using the slowness power spectrum analysis proposed by spudich and bostwick the purpose of this paper is to estimate the directions in which individual coda wavelets leave the source region for lapse times more than six times the s wave travel time the data set consists of 12 and 16 events in the magnitude range of 1 9 2 6 and 1 4 3 4 respectively the depths range from 7 to 8 km and from 6 to 7 km digital data from five vertical component stations enb rbf gef gif and tsb and two three component stations acb and che were available the hypocentral distances to the recording sites vary between 29 and 63 km for the first cluster gunsberg and 10 and 24 km for the second cluster laufelfingen respectively all events have been relocated using a master event technique with inter event arrival times estimated by a cross correlation method the slowness power spectra for the laufelfingen cluster at all stations show essentially two different patterns in agreement with the results of spudich and bostwick we found that the early coda following the direct s wave immediately was composed of wavelets leaving the source region with the same slowness vector as the direct wave we interpret this coda part as being very probably produced close to the recording site however with increasing lapse times we observe a different coda type which receives its energy from wavelets leaving the source region in a variety of directions the transition between the two coda types takes place often at 1 5 2 times and always before three times the s wave arrival time

origins of amplitude variations in seismic doublets source or attenuation process summary accurate estimates of spectral ratios of two doublets i e similar seismic events occurring along the san andreas fault in the hollister region have been computed using a cross spectral method both events of each doublet occurred within a 24 hr interval although both waveforms of a doublet are remarkably similar computed spectral ratios exhibit strong but steady variations with frequency in the first arrivals the attenuation process can explain neither the sign nor the order of magnitude of these variations on the contrary they are closely related to the source mechanism we show that the slope computed from the linear and coherent part of the spectral ratio is an estimation of the variation in the apparent rupture duration between the two events one doublet exhibits an azimuthal distribution of the spectral ratio variations that is interpreted as the result of a change in the position of the hypocentres relative to the rupture area for this doublet location of hypocentres varies only from 10 to 30 per cent of the source length this points out that small changes in rupture kinematics can lead to significant variations in the spectral ratio of earthquake doublets i e in the amplitude of each event for the other doublet the variations are related to a change in source length we also have computed spectral ratios all along the seismograms using a moving window technique when an azimuthal distribution of the spectral ratio is observed in the first arrivals we notice a decrease of the spectral ratio all along the seismograms the doublet with strong but non azimuthal variations of the spectral ratio in the first arrivals does not indicate such a decrease of the spectral ratio along the seismogram this implies that the variations of the coda decay are related not only to temporal or spatial changes of attenuation but also to source parameters these variations have the same order of magnitude as those provoked by a temporal change in coda q of 10 per cent they can occult time changes of spectral ratio for instance in case the attenuation in the crust would vary before a strong earthquake

complexities in high frequency seismic waveforms due to three dimensional structure in the new madrid seismic zone effects of three dimensional structure on 2 to 10 hz seismic signals in the new madrid seismic zone nmsz are investigated using waveforms from similar microearthquakes that occurred near ridgely tennessee slowness power spectral sps analysis is used to study the plane wave composition of p s and coda waves the observed sps at four stations suggest that the early s coda is composed mainly of wavelets leaving the source with slownesses of the direct s wave at greater lapse times coda is increasingly affected by random scattering sps observed on the vertical components and or near the s wave nodes are more comlplex due to practical limitations of the algorithm p coda and early s coda at a fifth station located southwest of the junction show the most complex sps analysis of that complexity together with waveform synthetics and travel time analysis strongly suggests that scattering multipathing due to source zone velocity heterogeneities occurs near the path probably due to dilatancy occurring southwest of the ridgely junction or due to internal fault zone complexities coda q when averaged over three components show no station variations we infer that 1 coda at 1 hz is mainly caused by scattering in the sedimentary layer where q is frequency independent 2 at higher frequencies the coda contains an increasing amount of scattered body waves and 3 the source radiation pattern and path variable scattering multipathing effects in early coda do not affect coda q

redistribution of high pressure fluids during retrograde metamorphism of eclogite facies rocks voltri massif italian western alps abstract high pressure clinopyroxenes in metasomatic mafic rocks rodingites and metagabbros containing titanoclinohumite and eclogites from the voltri massif ligurian western alps retain primary two phase aqueous inclusions h 2 o nacl kcl in the metasomatized rocks the inclusions align with the c axes and the cleavage planes of diopside the eclogites display mylonitic textures primary fluid inclusions are hosted in early porphyroclasts of omphacite omphacite i and are absent in later fine grained omphacite omphacite ii that forms the mylonitic foliation omphacite i has been frequently replaced by retrograde symplectites clinopyroxene plagioclase amphibole in such cases the fluid inclusions are either absent or occur in much lower number the textural observations suggest that formation of the symplectites has been controlled by availability of the fluid inclusions inside omphacite i the symplectites have been in turn overgrown by glaucophanic and winchitic amphiboles indicating that breakdown of the omphacite i to symplectites still occurred at relatively high pressure post entrapment changes have caused a large range of densities in the analyzed inclusions although the textural evidence constrains the entrapment of fluid inclusions to high pressure conditions the highest density isochores in the eclogites indicate pressure temperature conditions 8 kbar and 550 c lower than those attained during the high pressure stage such p t estimates approach those of symplectite formation after omphacite i thus the fluid inclusions preserved in omphacite were formed by low pressure re equilibration of a fluid presumably trapped at eclogite facies conditions the inclusions kinetically triggered the retrograde breakdown of the eclogitic clinopyroxenes the compositional overlap between the analyzed inclusions and those related to the latest greenschist evolutionary stage suggests further redistribution of the high pressure fluids and close system behaviour of these rocks during their overall exhumation path

microstructures and late stage magmatic processes in layered mafic intrusions symplectites from the sept iles intrusion quebec canada replacive symplectites vermicular intergrowths of two or more minerals are an important feature of layered igneous intrusions recording evidence of late stage reactions between interstitial liquid and crystals they are common throughout the layered series of the 564 ma sept iles layered intrusion in quebec canada and fall into three types oxy symplectites type i symplectites and type ii symplectites oxy symplectites are comprised of magnetite and orthopyroxene nucleate on olivine primocrysts and form via the reaction olivine o2 orthopyroxene magnetite type i symplectites of which there are 3 distinct categories are comprised of anorthitic plagioclase with pyroxene amphibole or olivine vermicules grow from primocryst oxide grains and replace primary plagioclase and type ii symplectites of which there are 2 distinct categories are comprised of anorthitic plagioclase with orthopyroxene amphibole vermicules grow from primocryst olivine

seismic trapped modes in the oroville and san andreas fault zones three component borehole seismic profiling of the recently active oroville california normal fault and microearthquake event recording with a near fault three component borehole seismometer on the san andreas fault at parkfield california have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals these wave trains are interpreted as fault zone trapped seismic modes parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers as shallow as 4 kilometers and extend 12 kilometers along the fault on either side of the recording station selected oroville and parkfield wave forms are modeled as the fundamental and first higher trapped sh modes of a narrow low velocity layer at the fault modeling results suggest that the oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second whereas at parkfield the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1 1 to 1 8 kilometers per second these low velocity layers are probably the rupture planes on which earthquakes occur

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

intragranular dynamic recrystallization in naturally deformed calcite marble diffusion accommodated grain boundary sliding as a result of subgrain rotation recrystallization abstract intragranular microshear zones within a greenschist facies calcite marble were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains intragranular recrystallized grains within large twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study lattice distortions low angle boundaries and some high angle boundaries 15 in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb accommodated dislocation creep involving subgrain rotation recrystallization changes in the ratio of host and twin domain as the deformation zone is entered show that twin boundary migration also occurred recrystallized grains have similar sizes 10 60 m to subgrains suggesting that they formed by subgrain rotation however within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15 and misorientation axes are randomly oriented moreover recrystallized grain orientations average around the porphyroclast orientation we suggest that the recrystallized grains once formed are able to deform partly by diffusion accommodated grain boundary sliding which is consistent with predictions made from lab flow laws

temporal change in attenuation intensity before and after the eastern yamanashi earthquake of 1983 in central japan a crutal earthquake of magnitude 6 0 took place on august 8 1983 in the middle of the kanto tokai microearthquake observation network of the national research center for disaster prevention in central japan by analyzing vertical seismograms of microearthquakes occurring in a small volume in the aftershock region we measured temporal change in attenuation intensity for a period from 22 months before to 9 months after the main shock attenuation change was estimated by using two different kinds of analyses in the first analysis station magnitude calculated from the maximum amplitude at each station was compared with the average of station magnitudes deviations of station magnitude from the average magnitude at five stations in the vicinity of the aftershock area are found to have increased after the main shock compared with that before in the second analysis s coda duration from the origin time was measured and was compared with the average magnitude the increase of s coda duration for the same average magnitude is found after the main shock at two stations close to the main shock epicenter the simplest interpretation of the results is that the attenuation intensity in the upper crust in the close vicinity of the focal region decreased after the main shock for frequencies f from 2 to 20 hz qs 1 1 4 10 2f 1 since other precursors appeared in the preparation stage it is possible to interpret the change in qs 1 as a precursor the attenuation intensity would have been anomalously large before the main shock

studies of the seismic coda using an earthquake cluster as a deeply buried seismograph array loosely speaking the principle of green s function reciprocity means that the source and receiver positions in a seismic experiment can be exchanged without affecting the observed seismograms consequently the seismograms observed at a single observation location o and caused by a cluster of microearthquakes at locations ei are identical to the time series that would be measured by an array of stress meters emplaced at positions ei recording waves generated by a source acting at o by applying array analysis techniques like slant stacking and frequency wave number analysis to these seismograms we can determine the directions and velocities of the component waves as they travel in the earthquake focal region rather than at the surface we have developed a computationally rapid plane wave decomposition which we have applied to single station recordings of aftershocks of the 1984 morgan hill california earthquake the analysis is applied to data from three seismic stations having considerably different site geologies one is a relatively hard rock station situated on franciscan metamorphics one is within the calaveras fault zone and one is on semiconsolidated sand and gravels we define the early coda to be the part of the coda initiating immediately after the direct s wave and ending at twice the s wave lapse time the character of the s wave and early coda varies from being impulsive at the first station to highly reverberative at the last we examine waves in sequential time windows starting at the s wave and continuing through the early part of the coda at all seismic stations the early coda is dominated by a persistent signal that must be caused by multiple scattering probably within 2 km of each seismic station despite clear station to station differences in the character of the early coda coda q values measured in the late coda greater than twice the s lapse time agree well among stations implying that the mechanisms causing the varying behavior of the early coda do not control the coda decay rate at the stations we have considered coda q values measured on horizontal components of motion agree within a factor of 2 with those measured on vertical components we have not been able to determine the composition of the late coda because of a low signal to noise ratio our analysis technique however is quite appropriate for such a task

tomographic investigation of the wear along the san jacinto fault southern california the seismic properties of the wear along the san jacinto fault are investigated using local seismic records for clustered fault zone earthquakes spatial reciprocity allows us to use the seismicity along fault plane to mimic the situation where receiver arrays are placed at depth on the fault plane a seismic record at the event location is constructed by stacking the original station records waveform cross correlations is then used to calculate the traveltimes of shear waves traveling between the pair of events we analyze 157 small earthquakes occurring between 1998 and 2004 on the san jacinto and elsinore fault zones low shear velocities in the san jacinto fault zone related to wear and gouge are persistently observed up to the depth of brittle ductile transition zone 16 km on the other hand low velocities in the elsinore fault zone are only visible in the northwestern portion at shallow depth the wear structures in the san jacinto fault zone display lower shear velocities by 6 8 than those of the wall rock 2005 elsevier b v all rights reserved

time space variation of seismic s wave coda attenuation qc 1 and magnitude distribution b values for the petatlan earthquake average s wave coda q qc at 6 hz and to about 10 sec after the s wave arrival shows 1 temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 petatlan mexico earthquake 2 spatial variation through the aftershock area positively correlated with b high qc b values at the two activity centers intermediate values in between and landward and low values toward the middle america trench which remain low for two weeks after the main shock and 3 five foreshock average qc 135 1 6 and eleven aftershock average qc 175 1 3 for the eastern asperity the b values and possibly qc values seem to correlate with the level of aftershock activity

seismological studies at parkfield iii microearthquake clusters in the study of fault zone dynamics abstract more than half of the microearthquakes that occur near parkfield california when located with high resolution methods are seen to define some 80 small clusters of 2 to 12 similar events each cluster occupies a patch typically 100 to 200 m in length within the fault zone cluster members have nearly identical waveforms correlation coefficient of 0 9 or greater to frequencies of 50 to 100 hz as recorded by the borehole installed seismographic network the clusters are distributed throughout the fault zone around the presumed nucleation region in the locked section to the se and in the creeping part of the nw of the previous m 6 hypocenter they are also found in the salinian block several kilometers sw of the fault zone the total area occupied by all of the clusters constitutes only a small fraction 1 to 2 of the fault zone approaching failure at parkfield such clusters provide insight into the dynamics of the failure process through their spatial temporal characteristics and their mechanisms they also serve as highly repetitive sources distributed throughout the fault zone suitable for monitoring the nucleation zone for possible precursory changes in physical properties that affect wave propagation in this article we demonstrate these applications with a detailed analysis of selected clusters

attenuation of body waves in the central new madrid seismic zone abstract data from the portable array for numerical data acquisition panda deployment in the central part of the new madrid seismic zone have been used to study the attenuation of body waves the spectral decay technique is used to analyze acceleration amplitude spectra of both p and s waves in a frequency range of 5 to 25 hz in order to obtain the high frequency attenuation parameter a multiple weighted least squares method is applied to obtain the site attenuation factor 0 at each station and to get d dr which were interpreted in terms of a two layer q structure consisting of a sedimentary layer overlying the upper crust the 0 values for s waves are stable and show a positive correlation with sediment thickness the 0 values for p waves in the sediments show significant scatter which may be due to greater sensitivity of p wave attenuation to water content of the soils than that of s waves which gave more stable 0 values consistency checks on the assumptions made in obtaining the p and s values are made by studying the source spectrum of several events and the differential sp s from the spectral ratio of the converted sp to s phases the three sets of values are found to be internally consistent but our results are limited to deep sediment sites since the complete vertical distribution of q with sediment depth is not known

viscosity and density of a two phase drilling fluid many ice core drilling projects expect to reach depths greater than 1000m at such depths it is necessary to fill the borehole with a fluid to compensate the ice pressure and avoid the resulting significant hole closure knowledge of the ice chips and drilling fluid circulation parameters e g flow rate pressure drop velocity is essential for understanding the behaviour of the fluid around the drill and will support drill design through a better parameterization in models a characterization of the dynamic viscosity and of the density of the ice chips and drilling fluid mixture is required to calculate these circulation parameters the goal of this study is to propose a method to calculate to a first approximation both these physical properties the relationships presented here have been established after experimentation building on prior assumptions taken from the literature

alkali diffusion in plagioclase feldspar tracer diffusion kinetics at 1 atm pressure under dry conditions using 6li 41k and 87rb in four plagioclase feldspars have been determined diffusion profiles were measured using an ion microprobe sims the arrhenius parameters and 2 uncertainties for li k and rb respectively in albite are pre exponential factor log do 3 8 1 0 3 4 1 4 and 5 8 3 8 in m2 s and activation energies 146 14 296 30 and 283 85 kj mol the diffusion rate of k in albite at 700 c and 100 mpa water pressure equals that at 1 atm dry suggesting that diffusion of k is not dependent on water fugacity between 0 004 and 100 mpa the k diffusion rate depends on plagioclase anorthite content and is approximately 20 times slower in labradorite than albite li diffusion rates in albite and anorthite are identical a linear relation between log d for the alkalis measured and their ionic radius squared permits an estimate of cs diffusion kinetics diffusion rates are isotropic or nearly so in albite

symplectite formation in the presence of a reactive fluid insights from hydrothermal experiments this study describes the microstructural and chemical development of symplectites obtained in fluid mediated mineral replacement experiments during the experiments polymineralic feldspar rich samples were exposed to aqueous na sio2 solution at 600 c and 2 kbar confining pressures for durations of 12 hr to 20 days the resulting reaction rims display high mineralogical and structural complexity and contain two varieties of symplectites represented by nanometre scale intergrowths of gehlenite zeolite and grossular zeolite grains the experimental fluid was enriched in 18o isotope in order to trace oxygen redistribution during the reaction the elevated 18o concentration in the reaction products and the heterogeneity in its distribution suggest that symplectite formation was controlled by dissolution precipitation mechanisms rather than volume diffusion processes microstructural and chemical observations suggest that symplectite formation occurred in multiple stages in response to spatially heterogeneous and temporarily evolving fluid composition at the reaction interfaces hence our results shed light on the fundamental processes involved in symplectite formation improving our ability to interpret symplectite microstructures this article is protected by copyright all rights reserved

deciphering high pressure metamorphism in collisional context using microprobe mapping methods application to the stak eclogitic massif northwest himalaya the stak massif northern pakistan is a newly recognized occurrence of eclogite formed by the subduction of the northern margin of the indian continent in the northwest himalaya although this unit was extensively retrogressed during the himalayan collision records of the high pressure hp event as well as a continuous pressure temperature p t path were assessed from a single thin section using a new multiequilibrium method this method uses microprobe x ray compositional maps of garnet and omphacitic pyroxene followed by calculations of 200 000 p t estimates using appropriate thermobarometers the stak eclogite underwent prograde metamorphism increasing from 650 c and 2 4 gpa to the peak conditions of 750 c and 2 5 gpa then retrogressed to 700 650 c and 1 6 0 9 gpa under amphibolite facies conditions the estimated peak metamorphic conditions and p t path are similar to those of the kaghan and tso morari high to ultrahigh pressure hp uhp massifs we propose that these three massifs define a large hp to uhp province in the northwest himalaya comparable to the dabie sulu province in china and the western gneiss region in norway

reduced seismic velocities in the source zone of new madrid earthquakes a three dimensional inversion of p wave travel time residuals from local earthquakes reveals a remarkable pattern of low seismic velocities in crustal rocks immediately adjacent to the active portions of the new madrid fault system seismic velocities are lowest in regions of greatest concentration of earthquake activity near two intersections of linear trends in seismicity the maximum reduction in compressional wave velocity is at least 7 per cent in the upper 5 km of the crust and at least 4 per cent in the depth range of 5 to 14 km the reductions are consistent with a velocity decrease which would be expected if crustal rocks in the source zone contain fluid filled cracks in which pore pressure is a substantial fraction of external pressure the presence or absence of such fluids may explain why some portions of the faults in and surrounding the upper mississippi embayment are active while others are not

anisotropic scattering and travel time delay analysis in kilauea volcano hawaii earthquake coda waves we study the s wave coda of microearthquake multiplets earthquakes showing similar seismograms recorded on the kilauea volcano and we analyze the direction of emission for wavelets composing the coda our data set consists of 71 small magnitude well relocated clustered events that occurred in an area of about 1 km2 in the southern flank of kilauea recorded at hypocentral distances of 10 to 20 km for each station we apply a cross spectral moving window technique to each pair of multiplet records to estimate the travel time delays due to source location offsets these delays are computed for consecutive windows starting at the p wave arrival time and ending at 4 times the s wave travel time they are next used to estimate the slowness vector for each window along the seismograms our analysis shows 1 that scattered waves are emitted anisotropically along some preferred directions as late as 4 times the s wave travel time and 2 that for some stations the azimuthal plane rotates during the seismogram with a clear travel time delay signature our analysis shows that travel time delay variation in the coda can be purely due to geometrical propagation effects this conclusion is of special interest when trying to use travel time delays to infer temporal changes in the crust our results do not support a coda diffracted isotropically in the crust in the magnitude range and at the hypocentral distances we investigated

the initiation of strain localisation in plagioclase rich rocks insights from detailed microstructural analyses abstract in order to shed light on the cause for onset of strain localisation in plagioclase rich rocks we have performed detailed microstructural analyses on a sheared anorthosite leucogabbro using optical microscopy electron backscatter diffraction ebsd and chemical analyses the analysed sample is from an archaean unit sw greenland deformed at lower to mid crustal conditions t 675 700 c and moderate pressure the initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation recrystallised plagioclase grains average size 80 m are dominantly found in i clusters ii lenses and iii continuous bands subparallel to shear zone boundaries recrystallised grains in clusters and lenses display inherited crystallographic orientations their bulk crystallographic preferred orientation cpo is random however crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems recrystallised grains in continuous bands show a cpo with a single dominant active slip system 001 aligned with the structural xyz framework for these parent and daughter grains misorientation axes are random and the dominant slip system is different grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast we infer that the cause for strain localisation is recrystallisation and development of a cpo in continuous recrystallised bands microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation accommodated grain boundary sliding disgbs in continuous bands this inferred shift in dominant deformation mechanism would lower the strength of the shear zone

ch4 and δ18o of o2 records from antarctic and greenland ice a clue for stratigraphic disturbance in the bottom part of the greenland ice core project and the greenland ice sheet project 2 ice cores the suggestion of climatic instability during the last interglacial period eem based on the bottom 10 of the greenland ice core project grip isotopic profile has been questioned because the bottom record from the neighboring greenland ice sheet project 2 gisp2 core 28 km away is strikingly different over the same interval and because records of the 18o of atmospheric o2 from both cores showed unexpected rapid fluctuations here we present detailed methane records from the vostok antarctica grip and gisp2 cores over the relevant intervals the grip and gisp2 data show rapid and large changes in methane concentration which are correlative with variations of the 18o of the ice while the vostok record shows no such variations this discrepancy reinforces the suggestion that the bottom sections of the greenland records are disturbed by combining the methane data with measurements of 18o of o2 in the same samples we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the grip core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation

a combined tof sims and emp sem study of a three phase symplectite in the los angeles basaltic shergottite a rare three phase symplectite consisting of ca rich pyroxene fe rich olivine and a silica phase is frequently found rimming pyroxene in the martian meteorite los angeles this assemblage is usually interpreted as the breakdown product of metastable pyroxferroite a very rare pyroxenoid mineral itself however its origin is not entirely understood mainly because the extremely small average size of the constituent phases represents a challenge for precise high resolution analysis in addition to electron microbeam methods the present study uses time of flight secondary ion mass spectrometry tof sims to overcome the limits of spatial resolution and to comprehensively study this mineral assemblage the prevailing breakdown hypothesis is supported by the following results 1 the three symplectite phases are very homogenous in composition from 100 m down to the micrometer scale 2 the silica phase could be shown to be almost pure sio2 3 the symplectite bulk composition is consistent with pyroxferroite sub micrometer sized ti oxide grains are found within the symplectite but not within the ca rich pyroxene and probably represent a minor breakdown phase in addition to the three main phases

textural evolution of plagioclase feldspar across a shear zone implications for deformation mechanism and rock strength caledonian amphibolite facies shear zones developed in granulitefacies anorthosites and anorthositic gabbros of the bergen arcs western norway allow a detailed study of the relationships between fluid infiltration mineral reactions the evolution of microstructure and deformation mechanisms a sequence of rocks from the relatively pristine granulites into a shear zone has been studied by optical microscopy electron microprobe microanalysis empa scanning electron microscopy sem electron backscatter diffraction ebsd and transmission electron microscopy tem focusing on the progressive development of microstructure in the plagioclase feldspars leading up to their deformation in the shear zone at the outcrop scale fluid infiltration into the granulites is marked by a distinct colour change in the plagioclase from lilac brown to white this is associated with the breakdown of the intermediate composition plagioclase an50 in the granulite to a complex intergrowth of na rich and ca rich domains ebsd analysis shows that this intergrowth retains the crystallographic orientation of the parent feldspar but that the ca rich domains contain low angle grain boundaries within the shear zone this complex intergrowth coarsens by grain boundary migration annihilating these grain boundaries but retaining the na rich and ca rich zoning pattern analysis of nearestneighbour misorientations of feldspar grains in the shear zone demonstrates that local crystallographic preferred orientation cpo is inherited from the parent granulite grain orientations random pair misorientation angle distributions show that there is no cpo in the shear zone as a whole nor is there significant shape preferred orientation spo in single grains these observations are interpreted in terms of fluid induced weakening and deformation by dissolution precipitation pressure solution creep

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

stability of coda q in the region of parkfield california view from the u s geological survey parkfield dense seismograph array many investigators have proposed that changes in the rate at which the coda decays may be an intermediate term precursor to moderate to large earthquakes parkfield california on the san andreas fault is a promising location for studying premonitory changes in coda q qc because a large earthquake is likely to occur there we have investigated qc using recordings from the u s geological survey parkfield dense seismograph array which is a digital array with 14 triaxial sensors and an aperture of about 1 km for each earthquake we can measure qc from up to 42 recordings their average is more stable than the measurement from a single station using clustered seismicity we have developed criteria for selecting events and reducing scatter in the measurement the qc value determined from a seismogram depends on the position and length of the analysis window thus qc should always be measured from the same length window starting at the same lapse time regardless of the source location in addition the band limited signal to noise ratio at the end of the analysis window is important qc determined in two frequency bands 4 8 hz and 8 16 hz from a tight cluster of 26 events which occurred between december 1989 and january 1994 has not changed despite m 4 7 and m 4 6 events in october 1992 and november 1993 qc measured from local events 60 km in three frequency bands shows larger scatter but has also not changed during this period for monitoring qc observations should include array averaged measurements from a single lapse time because qc measurements made using an analysis window that starts at a constant multiple of the s wave lapse time depend on epicentral distance a procedure combining the evaluation of the time and distance dependences of qc also gives stable observations

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

radix a minimal resources rapid access drilling system determining the expected age at a potential ice core drilling site on a polar ice sheet generally depends on a combination of information from remote sensing methods estimates of current accumulation and modelling this poses irreducible uncertainties in retrieving an undisturbed ice core of the desired age although recently perfected radar techniques will improve the picture of the ice sheet below future drilling sites rapid prospective drillings could further increase the success of deep drilling projects here we design and explore a drilling system for a minimum size rapid access hole the advantages of a small hole are the low demand for drilling fluid low overall weight of the equipment fast installing and de installing and low costs we show that in theory drilling of a 20mm hole to a depth of 3000m is possible in 4 days first concepts have been realized and verified in the field both the drill cuttings and the hole itself can be used to characterize the properties of the ice sheet and its potential to provide a trustworthy palaeo record a candidate drilling site could be explored in 2 weeks which would enable the characterization of several sites in one summer season

corona networks as three dimensional records of transport scale and pathways during metamorphism coronal textures in metamorphic rocks are physical evidence that diffusion was the rate limiting process in the local reactions that produced these layered structures mass balance however requires that chemical communication must also take place at length scales significantly greater than those of the corona evidence for the anatomy and scale of pathways along which chemical communication occurred is rarely recorded in metamorphic rocks but orthoamphibole cordierite gneiss from the thor odin dome british columbia canada preserves multi layered coronal reaction textures around garnet and al2sio5 connected by anastomosing networks of cordierite and biotite that represent relict pathways along which mass transport occurred during reaction texture development calculations of component fluxes between reaction textures and surrounding mineral assemblages indicate that relatively long distance centimeter scale diffusional exchange was required for development of 150 500 m thick corona results of electron backscattered diffraction analysis and high resolution x ray computed tomography give new information about the scale and pathways of chemical communication during high grade metamorphism and show that length scales of material transport were two orders of magnitude greater than those of the reaction textures

fault zone waves observed at the southern joshua tree earthquake rupture zone waveform and spectral characteristics of several aftershocks of the m 6 1 22 april 1992 joshua tree earthquake recorded at stations just north of the indio hills in the coachella valley can be interpreted in terms of waves propagating within narrow low velocity high attenuation vertical zones evidence for our interpretation consists of 1 emergent p arrivals prior to and opposite in polarity to the impulsive direct phase these arrivals can be modeled as headwaves indicative of a transfault velocity contrast 2 spectral peaks in the s wave train that can be interpreted as internally reflected low velocity fault zone wave energy and 3 spatial selectivity of event station pairs at which these data are observed suggesting a long narrow geologic structure the observed waveforms are modeled using the analytical solution of ben zion and aki 1990 for a plane parallel layered fault zone structure synthetic waveform fits to the observed data indicate the presence of ns trending vertical fault zone layers characterized by a thickness of 50 to 100 m a velocity decrease of 10 to 15 relative to the surrounding rock and a p wave quality factor in the range 25 to 50

coda q before the 1983 hawaii ms 6 6 earthquake abstract coda q values were derived for more than 300 microearthquakes that occurred in a 6 yr period before the 16 november 1983 kaoiki hawaii earthquake m s 6 6 the sources were located within a 14 16 km rectangular region surrounding the main shock epicenter and most of them occurred at depths between 5 and 10 km digital recordings from three stations at epicentral distances ranging from 0 to 18 km were used coda q was calculated from the amplitude decay rate of the s wave coda in nine frequency bands from 4 5 to 27 hz the average q of the nw part of the studied area is about 15 per cent higher than that of the se part these two subregions also showed differences in seismicity b value and microearthquake source mechanisms in the high q volume the b value was 1 0 and the rate of earthquakes per unit volume was about 50 per cent of the rate in the low q volume where b 1 3 one interpretation of these observations is that more extensive faulting in the se kaoiki fault zone leads to lower q higher b value and a higher seismicity rate during the 5 to 6 yr before the mainshock the 1 yr average q values were stable no significant q change could be identified as a precursor to the main shock

provenance of rifted continental crust at the nexus of east gondwana breakup the bruce rise a prominent bathymetric feature offshore the bunger hills east antarctica has basement of unknown crustal affinity and age in east gondwana the bruce rise is reconstructed near the naturaliste plateau offshore sw australia and microcontinents now submerged in the eastern indian ocean new zircon u pb hf data from two c 1150 ma granite samples dredged from the eastern escarpment of the bruce rise demonstrate that these rocks are dominated by xenocrystic cargo mesoproterozoic xenocrystic cores show textural evidence of melt mediated coupled dissolution precipitation to form rim domains with apparent ages that skew towards c 1150 ma the zircon u pb hf signatures from the xenocrysts in the bruce rise granites and from c 1230 to 1180 ma felsic intermediate granites and orthogneisses from the conjugate naturaliste plateau basement suggest late mesoproterozoic magmatism occurred at a transition in the regional tectonic architecture between a reworked archean cratonic margin and proterozoic juvenile crust on the basis of plate reconstructions exhumation and thinning of the basement to the bruce rise naturaliste plateau occurred predominantly during rifting of india prior to c 120 ma minor further thinning likely occurred leading up to the onset of seafloor spreading between australia naturaliste plateau and antarctica bruce rise from c 90 to 84 ma

rates and mechanisms of isotopic exchange background the proliferation of stable isotope laboratories in recent years has led to a marked increase in the routine use of stable isotope analyses in geochemical studies this trend is likely to continue because of the development of micro analytical techniques e g laser ablation ion microprobe femtomole carrier gas methods etc for stable isotope analysis and the potential of these techniques to reveal the detailed sequence of thermal and fluid histories preserved in the rock record see mckibben et al 1998 equilibrium isotope fractionation factors and rates of isotopic exchange form the cornerstones for the interpretation of stable isotope data from natural systems indeed determination of the distribution of the stable isotopes of c o h s in gases fluids and minerals has become a standard and extremely powerful approach in elucidating the temperatures material fluxes rock and fluid origins and time scales associated with ancient and active fluid rock interaction processes in the earth s crust see valley et al 1986 although the thermodynamic principles underlying isotopic fractionation behavior were developed more than fifty years ago by urey 1947 the calibration of fractionation factors has proven to be difficult during the last four decades a great deal of progress has been made in determining the equilibrium fractionations of 18o 16o d h 13c 12c and 34s 32s among fluid and mineral components at high temperatures 400 c both from experimental exchange studies theoretical calculations using vibrational spectra semi empirical calculations using bond type considerations and empirical calibrations based on natural assemblages see references in o neil 1986 kyser 1987 criss 1991 1999 chacko et al this volume of these the experimental method is the most direct because it involves the least number of a priori assumptions despite the utility of the equilibrium approach in quantifying the isotopic behavior in select water rock

development of the british antarctic survey rapid access isotope drill the british antarctic survey rapid access isotope drill is an innovative new class of electromechanical ice drill which has recently been used to drill the deepest dry hole drilled by an electromechanical auger drill the record breaking depth of 461 58 m was drilled in just over 104 hours at little dome c the drill collects ice chippings for water stable isotope analysis rather than an ice core by not collecting a core the winch can be geared for speed rather than core breaking and is lightweight furthermore emptying of the chippings is performed by simply reversing the drill motor on the surface reducing the overall drilling time significantly the borehole is then available for instrumentation we describe the drill in its current state including modifications carried out since it was last deployed test seasons and the lessons learned from each are outlined finally future developments for this class of drill are discussed

imaging laterally varying regional heterogeneities from seismic coda using a source array analysis abstract seismic coda is composed of scattered waves originated from various kinds of heterogeneities whose locations are not confined in the great circle ray path direction between the source and receiver we devise a technique to image laterally varying regional heterogeneities using single station seismograms for clustered events we analyze regional seismic records of the borovoye seismic station for the balapan nuclear explosions in kazakhstan scattered waves that arrive coherently over the source arrays are extracted by directional beamformings the locations of scatterers responsible for the scattered waves are mapped from the beamforming direction travel time and slowness equivalently phase velocity the illuminated locations of heterogeneities are highly correlated with the structural variations in surface topography crustal thickness and sedimentary thickness the influence of the structural variations on seismic waves is quantified in terms of scattering intensity and quality factor the scattering properties are observed to vary with phase due to the difference in frequency content and phase velocity the proposed technique appears to be useful for a study of active tectonic regions with limited monitoring stations

the replacement of plagioclase feldspars by albite observations from hydrothermal experiments oligoclase and labradorite crystals have been experimentally replaced by albite in an aqueous sodium silicate solution at 600 c and 2 kbars the replacement is pseudomorphic and is characterised by a sharp chemical interface which progresses through the feldspar while preserving the crystallographic orientation reaction rims of albite up to 50 m thick can be readily achieved within 14 days re equilibration of plagioclase in an 18o enriched sodium and silica bearing solution results in oxygen isotope redistribution within the feldspar framework structure the observed characteristics of the reaction products are similar to naturally albitised plagioclase and are indicative of an interface coupled dissolution reprecipitation mechanism chemical analyses demonstrate that the albitisation is accompanied by the mobilisation of major minor and trace elements also including elements such as al and ti which are commonly regarded as immobile during hydrothermal alteration the results contribute to developing our understanding of the close association between large scale albitisation and secondary ore mineralisation which is common in nature

the clinopyroxene plagioclase symplectite in retrograde eclogites a potential geothermobarometer the retrograde p t t path of intermediate and high temperature eclogites t 500 50 c is characterized by the transformation of omphacite into a symplectitic intergrowth of clinopyroxene and plagioclase lamellae often this symplectite shows a two stage microstructural evolution i e a coarse lamellar width symplectite s1 followed by a thinner lamellar width symplectite s2 each symplectite may also evolve independently into a final globular microstructure sg