modern lacustrine microbialites towards a synthesis of aqueous and carbonate geochemistry and mineralogy abstract lacustrine microbialites record evidence of their depositional environments in their morphology mineralogy and geochemistry this synthesis reviews geochemical data both for microbialites and lake water for 21 modern lacustrine microbialite occurrences younger than 15 000 years although these data are limited several trends and associations are identified that provide useful criteria to aid interpretation of the environmental settings of ancient analogues these microbialites are either thrombolites or thrombolitic stromatolites they form in diverse settings including karst volcanic coastal and inland athalassic lakes surveyed lakes are mostly closed basins subject to evaporative processes lakes vary from stratified to totally mixed examples the major hydrochemical types include na cl ca so4 ca hco3 and soda lakes the salinity and alkalinity of the lakes correlates as expected with the mode of mineralization which in turn is reflected in the microstructure of the microbialites a correlation between the 13ccarbonate and the ca2 alkalinity ratio ca alk is observed generally lakes with ca alk 1 are subject to carbonate precipitation driven by either uptake of co2 that results in a broad range of positive and negative 13ccarbonate or mineralization mediated by sulphate reducing bacteria in saline to hypersaline environments that results in negative 13ccarbonate ca alk 39 are associated with hydromagnesite dolomite high mg calcite hmc and monohydrocalcite are associated with the total aqueous concentration of mg cmg occurring only in lakes with cmg 75 meq l evaporites e g gypsum are related to high lake water salinities the absolute concentration of aqueous si commonly associated with dissolution of diatom tests influences precipitation of crystalline and or amorphous silicates silica to form microbialites with si concentrations above 0 54 mmol l either as amorphous phases or as minerals such as stevensite and kerolite combined the trends observed in this survey demonstrate that modern lacustrine microbialites can preserve information symptomatic of the environments in which they formed however in a limited number of examples this is not the case as subsequent diagenesis may alter the original mineralogy to a point where the geochemical evidence conserved from the depositional environment is lost additionally some geochemical information may be the product of isolated microenvironments within living biofilms and thus not necessarily directly related to the chemistry of ambient lake water further misleading conclusions would result if the hydrochemistry at the time of study had changed from those prevailing when the microbialites were mineralized thus all the correlations developed in this work represent hypotheses to be tested rather than interpretations to be accepted this compilation demonstrates how microbialites respond to different lacustrine hydrochemical environments and is intended to provide a guide for future field studies and laboratory simulations this review also highlights the general scarcity of trace element data for lake waters and microbialites however some recent studies suggest that lacustrine microbialites may fractionate rare earth elements ree during uptake from water whereas marine microbialites appear to record the ree distribution from seawater without modification this possible difference in ree behaviour could have implications for the interpretation of palaeoenvironmental proxies

three dimensional shear wave splitting tomography in the parkfield california region 1 we developed a three dimensional 3d shear wave splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave splitting delay times along ray paths derived from a 3d shear velocity model assuming the delay times are accumulated along the ray paths the local strength of the anisotropy is indicated by a parameter of anisotropy percentage k using the shear wave splitting delay times for 575 earthquakes measured at paso and hrsn stations we imaged a detailed 3d anisotropy percentage model around the san andreas fault observatory at depth safod the anisotropy percentage model shows strong heterogeneities consistent with the strong spatial variations in both measured delay times and fast polarization directions the san andreas fault saf zone is highly anisotropic down to a depth of 4 km and then becomes less anisotropic at greater depths outside the fault zone the highly anisotropic zone extends as deep as 7 km consistent with the systematic depth dependence of the average time delays to the southwest of the saf the salinian granitic block shows relatively strong anisotropic anomalies that are presumably caused by aligned microcracks consistent with the direction of the regional maximum compressive horizontal stress to the northeast of the fault zone a strong anisotropic anomaly between depths 2 and 4 km corresponds to a serpentinite body sandwiched between franciscan rocks

extracting time domain green s function estimates from ambient seismic noise 1 it has been demonstrated experimentally and theoretically that an estimate of the green s function between two seismic stations can be obtained from the time derivative of the long time average cross correlation of ambient noise between these two stations this tdgf estimate from just the noise field includes all tensor components of the green s function and these green s function estimates can be used to infer earth structure we have computed cross correlations using 1 to 30 continuous days of ambient noise recorded by over 150 broadband seismic stations located in southern california the data processing yielded thousands of cross correlation pairs for receiver separations from 4 500 km which clearly exhibit coherent broadband propagating dispersive wavetrains across frequency band 0 1 2 hz

40ar 39ar and k ar geochronology of the dykes from the south indian granulite terrain abstract 40ar 39ar and conventional k ar data on mafic dykes and a felsic dyke together with previous data have been used to constrain the ages of the dyke magmatism in the south indian granulite terrain the age of the mainly ene wsw trending agali anaikatti swarm is estimated to be 1980 25 ma with a very limited occurrence of upper cretaceous 80 90 ma dykes in the area the nw se dharmapuri swarm is dated as at least 1800 ma possibly 1855 9 ma both the nw se and the ne sw dyke swarms of tiruvannamalai are dated at 1650 10 ma only a few dykes in north kerala are of middle proterozoic ca 1700 1650 ma age furthermore the 40ar 39ar results of dharmapuri dykes exhibit age spectrum characteristic of partial degassing they provide strong evidence of latest proterozoic overprinting by a thermal event probably related to the pan african orogeny most of the phanerozoic dykes are confined to the coastal region in kerala province the dolerites of central kerala and the majority of north kerala dolerites intruded during the 70 65 ma period whereas a large gabbro and a felsic dyke in central and north kerala respectively are of upper cretaceous age ca 85 ma and the mean 81 1 5 ma age from these dykes may represent cooling age the oldest phanerozoic upper jurassic 144 6 ma dyke intrusions have a restricted occurrence in south kerala very close to the southern tip of india the proterozoic dykes are interpreted as being lateral intrusions from under plating igneous bodies beneath the eastern shield region the geological record of the terrain does not show indications of significant pre magmatic extension and rifting but dyke magmatism may have led to extension rifting and the consequent development of the india antarctica sri lanka proterozoic high grade terrain the data are also interpreted to suggest that the south indian granulite terrain is an early proterozoic tectono metamorphic terrane with strong thermal imprints of neoproterozoic early palaeozoic age it is difficult to reconcile its extensions into proterozoic sri lanka or antarctica where tectono metamorphism is dated to be of neoproterozoic age the late phanerozoic dykes are predominantly related to the deccan trap event and are associated with mantle plume decompressional tectonics that gave rise to ocean floor spreading between the seychelles and the indian continent the few older ca 145 ma dykes in south kerala may be related to the rift tectonics that preceded the separation of australia antarctica and india

palaeomagnetism and magnetic anisotropy of proterozoic banded iron formations and iron ores of the hamersley basin western australia abstract rock magnetic properties and palaeomagnetism of weakly metamorphosed banded iron formations bifs of the palaeoproterozoic hamersley group western australia and proterozoic bif derived iron ores have been investigated the bif units sampled here are slightly younger than 2500 ma at paraburdoo mount tom price and mount newman iron ore formation was completed before 1850 ma sampling was mainly from the mount tom price and paraburdoo mining areas and for the first time a palaeomagnetic fold test on fresh unweathered and unaltered bif samples has allowed the nature of the remanence of the bifs to be defined the remanence of the bifs is carried by late diagenetic low grade metamorphic magnetite after primary haematite this remanence is pre folding and is unlikely to be greatly affected by the high anisotropy because the palaeofield inclination was demonstrably low determination of palaeofield directions from measured remanence directions is complicated by self demagnetization effects in strongly magnetic highly anisotropic bif specimens we present a method for correcting measured directions for the effects of self demagnetization and anisotropy for typical bifs the effect of magnetic anisotropy on measured remanence inclinations and inferred palaeolatitudes is minor for low palaeolatitudes but can lead to large errors in calculated palaeopoles for intermediate to moderately steep palaeolatitudes anisotropy also causes cones of confidence to be underestimated due to compression of the range of inclinations in principle deflection of post folding remanence towards the bedding plane by high magnetic anisotropy can produce an apparent syn folding signature with best agreement between directions from different fold limbs after partial unfolding thus high anisotropy cannot only bias estimated palaeofield directions and cause underestimation of errors but can also mislead interpretation of the time of remanence acquisition the anisotropy of anhysteretic remanent magnetization arm probably yields an upper limit to the anisotropy of the chemical remanent magnetization crm carried by the bifs therefore from the anisotropy of arm a maximum inclination deflection of 9 is suggested for the sampled bifs this corresponds to less than 5 change of palaeolatitude the palaeomagnetic pole position calculated for bifs at paraburdoo is 40 9 s 225 0 e dp 2 9 dm 5 8 after tilt correction but without correction for anisotropy other pole positions reported include that from flat lying bifs from wittenoom at 36 4 s 218 9 e dp 4 6 dm 9 1 from mount tom price iron ore at 37 4 s 220 3 e dp 5 7 dm 11 3 and from paraburdoo ore at 36 4 s 209 9 e dp 4 7 dm 8 8 the poles from the bifs the paraburdoo ore and the part of the tom price deposit that was sampled in this study are indistinguishable from each other and from the mount jope volcanics overprint pole the magnetization of the bifs was probably acquired during burial metamorphism of the hamersley group soon before the main folding and uplift event in the southern part of the hamersley province this tectonic event exposed magnetite rich bifs to near surface oxidizing conditions producing extensive martite goethite orebodies and also appears to have produced the syn folding overprint magnetization recorded by the mount jope volcanics of the underlying fortescue group ages of magnetization are tentatively interpreted as 2200 100 ma for the bifs 2000 100 ma for the supergene enrichment of bif to martite goethite ore recorded by the parabudoo and mount tom price orebodies and 1950 100 ma for the metamorphic martite microplaty haematite ore recorded as an overprint by the tom price orebody and as the only surviving magnetization of the mount newman orebody

syndepositional cements associated with nannofossils in the marmolada massif evidences of microbially mediated primary marine cements middle triassic dolomites italy the marmolada platform is characterized by striking globose masses evinosponges arranged in concentric bands of fibrous calcite cements ranging in size from centimeters to several decimeters the lithogenetic importance of these peculiar cements has been recognized in many middle triassic buildups of the western tethys eds microanalyses revealed that these fibrous cements contain 1 3 mol of mg and detectable amounts of sr over 1000 ppm sometimes exceeding 10 000 ppm the strontium geochemical signature in neomorphic calcite could reflect the replacement of an aragonitic carbonate precursor the boundaries between the fibrous calcite bands are marked by alignments of microcrystalline aggregates of fluorapatite and or dolomite the presence of fluorapatite may indicate a depositional microenvironment eutrophic or rich in microbial communities epifluorescence analyses showed bright bands alternated to dark ones confirming that organic matter remains occur within the studied cements and could have played a significant role in supporting the widespread syndepositional cementation high magnification sem observations on bright epifluorescent bands showed the presence of widespread more or less spherical bodies ranging in size 100 300 nm these bodies could represent the relicts of nannobacterial cells

ambient noise cross correlation observations of fundamental and higher mode rayleigh wave propagation governed by basement resonance measurement of basement seismic resonance frequencies can elucidate shallow velocity structure an important factor in earthquake hazard estimation ambient noise cross correlation which is well suited to studying shallow earth structure is commonly used to analyze fundamental mode rayleigh waves and increasingly love waves here we show via multicomponent ambient noise cross correlation that the basement resonance frequency in the canterbury region of new zealand can be straightforwardly determined based on the horizontal to vertical amplitude ratio h v ratio of the first higher mode rayleigh waves at periods of 1 3 s the first higher mode is evident on the radial radial cross correlation functions but almost absent in the vertical vertical cross correlation functions implying longitudinal motion and a high h v ratio a one dimensional regional velocity model incorporating a 1 5 km thick sedimentary layer fits both the observed h v ratio and rayleigh wave group velocity similar analysis may enable resonance characteristics of other sedimentary basins to be determined

a giant late archean lake system the meentheena member tumbiana formation fortescue group western australia abstract the 2720 ma old meentheena member of the tumbiana formation fortescue group is a 30 50 m thick succession of alternating siliciclastic and limestone beds that crop out intermittently over a distance of 680 km across the pilbara craton western australia limestones of the member consist of flat pebble intraformational conglomerate oolite grainstone calcisiltite and calcilutite siliciclastics consist of shale siltstone and sandstone and many of these are tuffaceous sedimentary structures include abundant symmetrical ripples ripple cross stratification desiccation cracks teepee structures and planar to wavy lamination the member is superficially similar in appearance wherever it crops out however the correlation of individual beds and units over distances greater than 15 km is problematic there are no regional marker beds radiometric ages from tuffaceous units young to the west by a few million years four lithofacies are defined a flat pebble conglomerate b ripple cross laminated oolite grainstone c planar to wavy laminated calcilutite calcisiltite and d shale siltstone stromatolites are found in all three of the limestone facies evaporites are rare and include halite pseudomorphs the four lithofacies are generally found in a vertical succession that characterizes a lithofacies association and reflects a deepening upward succession associations may not have all four lithofacies but do reflect the same deepening upward pattern the uppermost portions of shale siltstone facies commonly contain symmetrical ripples and desiccation cracks indicating subaerial exposure measured sections contain as many as 25 of these parasequences indicating fluctuating water levels limestone lithology and facies commonly change abruptly laterally at the meter and greater scale and vertically centimeter to meter scale the sedimentary structures differences and abrupt changes in lithofacies patterns the inability to correlate confidently between regions and the differences in age from east to west suggest a complex diachronous depositional system of fluctuating water levels in separate bodies of water the limestone is dominated by sedimentary structures indicative of shallow water within wave base however it is devoid of evidence of any tidal influence possibilities for depositional environments include a tideless sea or a lacustrine system 87sr 86sr values from limestone differ from known archean marine carbonates ree patterns are decidedly non marine as well a lacustrine system is favored for the meentheena member based on sedimentary structures the abruptness of lithologic and facies changes the facies association lateral gradation into fluvial deposits the inability to correlate many units and beds for more than a few to several kilometers geochemical data stromatolites and comparisons with younger lacustrine basins as well as marine successions there is no single line of evidence that unequivocally establishes a lacustrine origin however multiple lines of evidence are collectively consistent with a lacustrine origin the meentheena member would therefore represent the oldest known carbonate rich lacustrine system which we call the meentheena lakes system

passive monitoring of anisotropy change associated with the parkfield 2004 earthquake 1 we investigate temporal variations in the polarization of surface waves determined using ambient seismic noise cross correlations between station pairs at the time of the mw 6 0 parkfield earthquake of september 28 2004 we use data recorded by the high resolution seismic network s 3 component seismometers located along the san andreas fault our results show strong variations in azimuthal surface wave polarizations for the paths containing station varb one of the closest stations to the san andreas fault synchronous with the parkfield earthquake concerning the other station pair only smooth temporal variations of are observed two principal contributions to these changes in are identified and separated they are 1 slow and weak variations due to seasonal changes in the incident direction of seismic noise and 2 strong and rapid rotations synchronous with the parkfield earthquake for paths containing station varb strong shifts in are interpreted in terms of changes in crack induced anisotropy due to the co seismic rotation of the stress field because these changes are only observed on paths containing station varb the anisotropic layer responsible for the changes is most likely localized around varb in the shallow crust these results suggest that the polarization of surface waves may be very sensitive to changes in the orientations of distributed cracks and that implementation of our technique on a routine basis may prove useful for monitoring stress changes deep within seismogenic zones

1891 1883 ma southern bastar cuddapah mafic igneous events india a newly recognized large igneous province abstract a newly recognized remnant of a paleoproterozoic large igneous province has been identified in the southern bastar craton and nearby cuddapah basin from the adjacent dharwar craton india high precision u pb dates of 1891 1 0 9 ma baddeleyite and 1883 0 1 4 ma baddeleyite and zircon for two se trending mafic dykes from the bd2 dyke swarm southern bastar craton and 1885 4 3 1 ma baddeleyite for a mafic sill from the cuddapah basin indicate the existence of 1891 1883 ma mafic magmatism that spans an area of at least 90 000 km2 in the south indian shield this record of 1 9 ga mafic ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south eastern margin of the south indian shield is a widespread geologic phenomenon on earth similar periods of intraplate mafic ultramafic magmatism occur along the margin of the superior craton in north america 1 88 ga molson large igneous province and in southern africa along the northern margin of the kaapvaal craton 1 88 1 87 ga dolerite sills intruding the waterberg group existing paleomagnetic data for the molson and waterberg 1 88 ga large igneous provinces indicate that the superior and kalahari cratons were at similar paleolatitudes at 1 88 ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a limpopo like 2 0 ga deformation zone in the superior province the widespread occurrence of 1 88 ga intraplate and plate margin mafic magmatism and basin development in numerous archean cratons worldwide likely reflects a period of global scale mantle upwelling or enhanced mantle plume activity at this time

mapping stress and structurally controlled crustal shear velocity anisotropy in california we present shear velocity anisotropy data from crustal earthquakes in california and demonstrate that it is often possible to discriminate structural anisotropy polarization of the shear waves along the fabric of major active faults from stress induced anisotropy polarization parallel to the maximum horizontal compressive stress stress directions from seismic stations located near but not on the san andreas fault indicate that the maximum horizontal compressive stress is at a high angle to the strike of the fault in contrast seismic stations located directly on one of the major faults indicate that shear deformation has significantly altered the elastic properties of the crust inducing shear wave polarizations parallel to the fault plane

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

temporal changes in attenuation associated with the 2004 m6 0 parkfield earthquake 1 elevated seismic attenuation is often observed in fault zones due to the high degree of fracturing and fluid content however temporal changes in attenuation at the time of an earthquake are poorly constrained but can give indications of fracture damage and healing in this study spectral ratios between earthquakes within repeating clusters are calculated in an attempt to resolve temporal variations in attenuation at the time of the 2004 m6 0 parkfield earthquake a sharp increase in attenuation is observed immediately after the earthquake which then decays over the next 2 years influences of intercluster magnitude variations time window length and previously reported postseismic velocity changes are investigated the postseismic decay is fit by a logarithmic function the timescale of the decay is found to be similar to that in gps data and ambient seismic noise velocities following the 2004 m6 0 parkfield earthquake the amplitude of the attenuation change corresponds to a decrease of approximately 10 in qp at the time of the earthquake the greatest changes are recorded on the northeast of the fault trace consistent with preferential damage in the extensional quadrant behind a north westerly propagating rupture tip our analysis suggests that significant changes in seismic attenuation and hence fracture dilatancy during coseismic rupture are limited to depths of less than about 5 km

correlations between the eastern block of the north china craton and the south indian block of the indian shield an archaean to palaeoproterozoic link abstract recent tectonometamorphic data reveal that the north china craton formed by amalgamation of an eastern block eb and a western block wb along the trans north china orogen tnco and the indian shield formed by amalgamation of the south indian block sib and the north indian block nib along the central indian tectonic zone citz the eb of the north china craton and the sib of the indian shield share many similar magmatic and sedimentary features the early archaean supracrustal sequences in both the eb and sib consist of 3 6 3 4 ga fuchsite quartzites pelitic gneisses calc silicates marbles banded magnetic quartzites and amphibolites called the caozhuang group in the eb and the older metamorphic group omg in the sib the middle archaean assemblages in both the blocks can be divided into an early 3 4 3 3 ga phase represented by the chentaigou supracrustal rocks and orthogneisses in the eb and the holenarsipur supracrustal rocks and the gorur orthogneisses in the sib and a late 3 1 2 9 ga phase represented by the qianan supracrustal rocks and the yangyashan orthogneiss in the eb and the sargur group and the peninsular gneiss in sib the late archaean taishan group and its equivalents in the eb are stratigraphically similar to the dharwar supergroup in the sib moreover both the eb and sib are characterised by near contemporaneity of late archaean granitoid intrusive and metamorphic events in addition dome and basin structures are dominant in both the eb and sib and anticlockwise p t paths charaterise the metamorphic evolution of the late archaean granulites in the two blocks the palaeoproterozoic sedimentary volcanic rocks in both blocks comprise lower clastic rich middle volcanic rich and upper clastic carbonate sequences the strong resemblance of the archaean to palaeoproterozoic magmatic and sedimentary records suggests that it is possible that the eb and sib are dispersed remnants of what was once a single continent a possible fit is proposed for the reconstruction of the eb and sib where the northern margin of the eb is placed adjacent to the western margin of the sib with the tnco and wb representing the continuations respectively of the citz and nib this fit suggests that the eb sib and the wb nib represent two discrete continental blocks that joined along the tnco citz orogen during a palaeoproterozoic collisional event fragmentation of the eb and sib may have occurred at the beginning of the mesoproterozoic and by 1 4 ga the eb and sib may have separated into two different continental blocks as evidenced by striking differences in the mesoproterozoic sedimentary successions and associated bimodal volcanics in the two blocks

palaeomagnetics and igneous activity of the area adjoining the south westem margin of the cuddapah basin india summary fifty six orientated samples were collected from 13 sites on five dolerite dykes between lat 14 23 n long 77 43 e and lat 14 08 n long 77 49 e which adjoin the south western margin of the cuddapah basin in anantapur district andhra pradesh after af demagnetization two dykes five sites striking ene possess similar magnetic directions 1 d 57 1 69 k 52 95 7 and 2 d 71 i 72 k 260 95 5 again dykes 3 three sites and 4 two sites have similar strike ne and magnetic directions d 64 i 7 k 142 95 8 and d 53 i 8 k 142 95 6 and dyke 5 two sites striking nw shows d 320 i 34 k 68 95 13 remanent directions estimated from total field magnetic anomaly data agree well with these results synthesis of these data with 10 other published palaeomagnetic studies of precambrian dolerite dykes on the indian peninsula suggest that these three systems of dykes adjoining the cuddapah basin had been emplaced prior to the basin formation perhaps representing the initial thermal event responsible for the basin development and also that there have been at least three separate periods of dyke emplacement on this shield the radiometric data however are very sparse and these periods cannot be dated with confidence

nannobacteria as a by product of enzyme driven tissue decay spheroidal features 50 to 200 nm in size and found in sedimentary rocks have been described as nannobacteria the idea that they are minute fossilized life forms and especially the discovery of such features in martian meteorite alh84001 sparked a lively debate with regard to identification of ancient microbial life because biologists consider 200 300 nm to be the lower viable size limit for microorganisms an alternative explanation is needed for features that have been described as nannobacteria by geologists we report here on tissue decay experiments that produced abundant proteinaceous spheroids in the size range of nannobacteria described as nannoballs in the remainder of this paper experimental conditions were comparable to those found in earth9s surface sediments and diagenetic mineralization of these spheroids may be a common process for preservation of nannoballs that are observed in the rock record

composition structure and evolution of a lacustrine carbonate margin dominated by microbialites case study from the green river formation eocene wyoming usa abstract during mid eocene times the northwestern margin of the gosiute lake wyoming has been characterised by the extensive development of a microbialite dominated carbonate sequence corresponding to the laney member of the green river formation thus providing a potential analogue for petroleum carbonate reservoirs developed in similar marginal lacustrine settings a detailed sedimentological study carried out on the little mesa plateau near labarge sw wyoming allows the reconstruction of 1 lateral and vertical facies variations of the carbonate sequence across the margin 2 development patterns of complex microbial bioconstructions and 3 the evolution of the margin through time the carbonate sequence up to 18 m thick starts with the deposition of sandy limestones which fill channels in the underlying siliciclastic unit of the cathedral bluff formation the increasing microbial contribution in the upper part of the sequence results in the development of large insect microbial build ups which represent up to 80 of the total volume of the carbonate deposits and which aggregated to form reef like geometries up to several tens of metres in diameter including bioherms spur and groove alignments and reef flat structures the initiation and development of the lacustrine carbonate platform was controlled both by the general lake expansion associated to the eocene climatic optimum and by local drainage diversions responsible for a new water inflow in the basin from the northern volcanic provinces short term lake level changes driven by high frequency climate fluctuations which are superimposed to this general transgressive trend may explain the discontinuous and polygenic growth of the individual insect microbial build ups the basin infilling by prograding alluvial volcaniclastic deposits originated from the northwestern volcanic province caused the rapid turn off of the marginal lacustrine carbonate factory during mid eocene time

paleomagnetism of a lateritic paleoweathering horizon and overlying paleoproterozoic red beds from south africa implications for the kaapvaal apparent polar wander path and a confirmation of atmospheric oxygen enrichment the olifantshoek group in southern africa contains paleoproterozoic red beds that are exceptionally well preserved lying unconformably atop a regionally extensive lateritic paleoweathering profile we studied the basal unit of this succession known as the gamagara or mapedi formation and the lateritized substrate so called drakenstein or wolhaarkop paleosol on which it developed two ancient magnetic components are observed one int usually with a distributed unblocking spectrum between 300 and 600 c but occasionally persisting to 675 c is directed shallowly southward or northward a mesoscale fold test at south sishen mine indicates that this component was acquired during deformation similarity of the direction to previous results suggests that it was acquired at 1240 ma during early namaqua orogenesis combining our int results with existing data from the namaqua eastern zone nez we calculate the nez pole at 44 9 n 021 5 e k 23 2 a 95 12 8 q 5 the most stable component from our data set hig always persisting as a nonzero endpoint to demagnetization at 665 680 c is observed in 32 samples from south sishen and beeshoek mines directed moderately east downward sishen or west upward beeshoek this component predates the mesoscale fold at sishen more importantly a conglomerate test at beeshoek indicates that hig was acquired prior to paleoproterozoic deposition of the gamagara mapedi formation the concordance between directions from the paleoweathering zone and immediately overlying red beds indicates that hig is a primary magnetic remanence for the basal gamagara mapedi bgm formation the dual polarity bgm paleomagnetic pole 02 2 n 081 9 e dp 7 2 dm 11 5 q 6 lies neatly between previous kaapvaal poles with ages of 2220 ongeluk lavas and 2060 ma bushveld complex our data thus support recent correlations of the gamagara mapedi formation with pre bushveld sediments of the pretoria group a pre bushveld age for bgm is also consistent with its substantial distance from a new albeit less reliable paleomagnetic pole from the 1930 ma hartley lavas higher within the olifantshoek succession 12 5 n 332 8 e k 18 6 a95 16 0 q 3 our conglomerate test at beeshoek confirms previous allegations that the intense hematitization observed in the drakenstein wolhaarkop paleosol occurred during paleoproterozoic weathering under a highly oxygenated atmosphere

replacement of aragonite by quasi rhombohedral dolomite in a late pleistocene tufa mound great salt lake utah u s a abstract fabric preserving quasi rhombohedral dolomite forms as cement and replacement of aragonite in a tufa mound 2 m thick on the western shoreline of the great salt lake the rounded crystals range in size from 5 to 20 m in diameter crystal faces are not pitted and have numerous sharp edged steps and kinks the quasi rhombohedral forms are the result of crystal growth phenomena and not of post crystallization dissolution microprobe analyses n 92 of the dolomite yield an average 46 mole mgco3 in agreement with the 44 to 45 mole mgco3 content determined from x ray diffraction 30 8 2 position of the 104 peak the great salt lake dolomite is poorly ordered with relative ordering of 28 5 and a reflection ratio of 0 9503 compared to a well ordered standard dolomite the great salt lake dolomite has the poorest ordering of any natural dolomite recorded to date more than half of the electron microprobe analyses show elevated sr abundance indicating that dolomite formed in a rock buffered diagenetic system where much of the bicarbonate and ca were supplied by dissolution of aragonite and mg from the lake water preservation of the tufa fabric by dolomite was accomplished by the closely related dissolution of aragonite and precipitation of dolomite in a fluid undersaturated with respect to aragonite and supersaturated with respect to ca rich dolomite formation of spheroids might reflect a high growth rate from a high alkalinity fluid that was undersaturated with respect to other carbonate minerals maintenance of this growth rate might have been aided by the ca enrichment and cation disordering of the precipitating dolomite phase and the large surface area of a sphere compared to that of an inscribed rhomb

seismic waveform attributes before and after the loma prieta earthquake scattering change near the earthquake and temporal recovery during 1987 1995 several clusters of nearly identical seismic events multiplets occurred near the loma prieta source region these multiplets allow us to investigate and demonstrate spatial and temporal changes in seismic wave character associated with the 1989 loma prieta main shock for seismogram pairs we use a moving window technique to compute coherencies depending on lapse time and frequency post loma prieta events have reduced coherencies with pre loma prieta events in a spatially limited region close to the loma prieta hypocenter while other paths remain nearly unaffected these changes gradually recover within a time interval of 5 years after the loma prieta earthquake a possible explanation for the time dependence is coseismically opened cracks which cause scattering increase for wavefields after the loma prieta event postseismic relaxation processes such as crack healing fluid diffusion or after deformations lead to progressive closure of these cracks with time after the main shock thus the scattering properties of the local crust approach the pre main shock state

palaeomagnetic and rock magnetic study of a deeply exposed continental section in the charnockite belt of southern india implications to crustal magnetisation and palaeoproterozoic continental nucleii abstract this study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south india metamorphosed to granulite facies close to the archaean proterozoic boundary 2600 ma at former depths of 20 35 km the metamorphism is characterised by extensive development of charnockite produced by flushing of co2 rich fluids through older crust rock magnetic studies thermomagnetic high and low temperature susceptibility hysteresis and irm show that very low ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types it has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes magnetic susceptibility shows little increase with temperature falling to zero at 550 c implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust induced magnetisations are able to explain 50 of observed magnetic anomalies of deep origin intensities of magnetisation and qn values are high especially in charnockite where metasomatism has resulted in new magnetite formation and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation the magnitude of anisotropy of magnetic susceptibility ams is also increased by charnockite formation as are degrees of lineation and foliation however directions of ams in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism no appreciable phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by a1 d i 280 8 a2 d i 319 11 and a3 d i 1 10 this swathe is interpreted as an uplift related cooling acquisition following charnockite formation at 2600 ma and a remanence age of 2600 2400 ma is implied by the absence of most of the swathe in early palaeoproterozoic and younger dyke swarms cutting the region a second group of component directions b mean d i 238 87 is similar to magnetisations found in dykes linked to an age of 2370 ma and interpreted as somewhat younger than the a swathe we argue that the entire magnetic record was acquired before final basement uplift prior to 2000 1800 ma collective late archaean early proterozoic palaeopoles from india define a low to high latitude movement of the shield at 2600 2300 ma which is replicated in contemporaneous results from the other ancient gondwana nucleii of southern africa and australia the cratonic nucleii of these three shields have the oldest laterally extensive supracrustal covers and comprise the core of the protocontinent of ur the 2900 2200 ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities but not configurations were similar within the palaeozoic supercontinent of gondwana

low latitude glaciation in the palaeoproterozoic era one of the most fundamental enigmas of the earth s palaeoclimate concerns the temporal and spatial distributions of precambrian glaciations through four billion years of precambrian history unequivocally glacial deposits have been found only in the palaeoproterozoic and neoproterozoic record nonetheless some of these deposits are closely associated with tropical rather than just polar palaeolatitudinal indicators such as carbonate rocks red beds and evaporites these observations are quantitatively supported by palaeomagnetic results indicating a 5 latitude for neoproterozoic glaciogenic rocks in australia similarly reliable palaeolatitudes for the older palaeoproterozoic glaciogenic rocks have not yet been obtained as such deposits commonly suffer from poor preservation and secondary magnetic overprinting the archaean palaeoproterozoic transvaal supergroup on the kaapvaal craton in south africa is however exceptionally well preserved and is thus amenable to the palaeomagnetic determination of depositional palaeolatitudes within this supergroup the 2 2 billion year old ongeluk lavas are a regionally extensive largely undeformed and unmetamorphosed extrusive volcanic succession which conformably overlies glaciogenic deposits the makganyene diamictite here we report a palaeomagnetic estimate of 11 5 depositional latitude for the lavas and hence for the underlying contemporaneous glacial rocks the palaeoclimate enigma is thus deepened a largely ice free precambrian world was apparently punctuated by two long ice ages both yielding glacial deposits well within tropical latitudes

palaeoproterozoic assembly of the napier complex southern india and western australia implications for the evolution of the cuddapah basin abstract a detailed examination of lithostratigraphic tectonothermal geochronologic and palaeomagnetic database of the dharwar bastar cratons of south india the yilgarn craton of western australia and the east antarctic shield has shown broad similarity two palaeomagnetic key poles of 2400 ma age from the dharwar and yilgarn cratons suggest near neighbor positions matching patterns of craton outlines orientation of mafic dyke swarms and palaeo north direction for 2400 ma have established a continental assembly of siwa an acronym for south india si and western australia wa at 2400 ma in this assembly the yilgarn craton fits against the bastar craton and the singhbhum craton the available geological data from the napier complex of the eastern antarctica and the dharwar craton of south india were used to prepare the barcode style reconstruction of archaean tectonostratigraphic events of both the cratons the method of matching continental outlines the internal features of two continental blocks and the palaeomagnetic data demonstrate that the napier complex was also a part of siwa during the period 2200 1900 ma and was located at the position of the cuddapah basin of india adjacent to the dharwar craton and the yilgarn craton the separation of the napier complex from the south indian block led to the development of the cuddapah basin at 1950 ma

azimuthal anisotropy of rayleigh waves in the crust in southern tohoku area japan to reveal seismic anisotropy within the crust in the southern tohoku area japan we analyze the azimuthal anisotropy of rayleigh waves in the frequency range of 40 65 mhz rayleigh wave anisotropy is estimated by array analysis of 102 teleseismic events that occurred between 2007 and 2014 the frequency variation of the estimated azimuthal anisotropy indicates depth variation of the seismic anisotropy within the crust 1 north south fast velocity direction around the upper crust and 2 trench normal fast velocity direction in the deeper crust we also conduct a time lapse analysis of rayleigh wave anisotropy to demonstrate the stability of the estimated anisotropy in time and to estimate possible changes in seismic anisotropy associated with the 2011 tohoku oki earthquake we demonstrate that the azimuthal location of teleseismic events influences the accuracy of time lapse analysis moreover a decrease of azimuthal anisotropic ratio in time lapse results might be related to changes in the velocity structure associated with release of crustal stress during the tohoku oki earthquake

a 2 5 ga porphyry cu mo au deposit at malanjkhand central india implications for late archean continental assembly abstract eight re os ages from six molybdenite samples representative of the cu mo au mineralization at the giant malanjkhand deposit in madhya pradesh were obtained using id ntims these data provide a clear late archean early paleoproterozoic age for this deposit and by implication for its calc alkaline granitoid host among other diverse models the origin of malanjkhand has been debated as orogenic lode style associated with development of a quartz reef in a shear zone or as porphyry style associated with a pink granitoid intrusion that is conspicuously present in the vicinity of the brittle ductile deformed quartz reef previous models included middle proterozoic ages for the cu mo au mineralization based on poorly constrained rb sr and k ar data direct dating of molybdenite shows that stringer mineralization in the quartz reef and disseminated and vein mineralization in the granite were contemporaneous at 2490 8 ma 2489 5 1 4 ma based on regression without uncertainty in the 187re decay constant mswd 0 5 n 5 additional molybdenite from the quartz reef dated at 2475 and 2450 ma represents at least two pulses of reworking that we suggest configured the elongate and arcuate quartz reef into its present position we suggest that the quartz reef is a high silica stockwork cap embedded in its primary potassic alteration halo and subsequently tilted so that it lies convex to the east with deeper levels exposed to the north petrographic evidence coupled with high re concentrations for molybdenites 400 650 ppm support this scenario and suggest that malanjkhand is a subduction related stockwork porphyry style cu mo au deposit involving mantle malanjkhand geology shares little affinity with three surrounding and tectonically juxtaposed terranes we suggest that the malanjkhand cu mo au deposit and the surrounding batholith containing multiple cu mo au prospects may be the vestiges of a microplate captured along the north margin of the nearly amalgamated southern indian craton at 2 5 ga as the southern craton began to converge with the northern indian craton or other protocontinents now embedded or consumed within the central indian tectonic zone citz this scenario could imply an early paleoproterozoic age for the earliest development of the citz at least in the region of the central indian shear suture cis whose marked ene trend imprints the northwestern part of the malanjkhand batholith and the cu mo au deposit based on a compilation of 2 5 ga events in both the southern and northern indian cratons we suggest that some units in the citz linked an extensive and continuous late archean early paleoproterozoic orogenic belt that may extend to the moyar bavali bhavani attur palghat and cauvery shear zones in southern india and include inboard units of the eastern ghats and the aravalli delhi orogenic belts robust geochronology that can be tied to the early history within these orogenic belts is scarce and extensive reworking during grenvillian and pan african time creates a pervasive overprint at some localities this late archean early paleoproterozoic belt preserves sutures within a coalesced late archean continent that included east antarctica napier and vestfold hills complexes at 2 5 ga

paleomagnetism and u pb geochronology of the lac de gras diabase dyke swarm slave province canada implications for relative drift of slave and superior provinces in the paleoproterozoicgeological survey of canada contribution 20080350 lac de gras diabase dykes trend north to nne across the central slave province of the canadian shield u pb baddeleyite ages of 2023 2 and 2027 4 ma are interpreted as dyke emplacement ages th

kinematic inversion of the 2004 m 6 0 parkfield earthquake including an approximation to site effects the 2004 m 6 0 parkfield earthquake yielded one of the largest amounts of near source strong ground motion seismic data ever we invert strong motion seismograms to obtain a model for the space time distribution of coseismic slip on the fault to reduce noise in the inversion we take into account local amplifications that affect each station by using records of the 1983 m 6 5 coalinga earthquake site amplification correlates well with large peak ground velocities registered during the 2004 parkfield mainshock the inversion for a kinematic rupture model yields a nonunique solution we therefore analyze various rupture models that explain the data equally well our preferred rupture model identifies a primary zone of high slip surrounding the hypocenter where the maximum slip is 57 cm a secondary slip area over which contours are not well resolved is located northwest of the hypo center the rupture speed is highly heterogeneous we infer an average rupture ve locity of 2 8 km sec close to the hypocenter and of 3 3 km sec in the secondary region of large slip to the northwest of the hypocenter by correlation of our rupture model with both microseismicity and velocity structure we identify six patches on the fault plane that behave in seismically distinct ways online material kinematic rupture model parameters

petrography and characterization of microbial carbonates and associated facies from modern great salt lake and uinta basin s eocene green river formation in utah usa

formation of aragonite cement by nannobacteria in the great salt lake utah brine shrimp egg cases in growth cavities in modern stromatolites in the great salt lake utah are replaced by aragonite and cemented together by aragonite cement the fabric of the cement changes dramatically as the distance from the egg case increases the cement within 50 to 70 m of the egg case exhibits a random fabric of 10 to 20 m equant crystals the surface of the cement is covered by bead like bumps 0 1 m in diameter interpreted as nannobacteria overlying the random beaded fabric with a relatively abrupt transition are epitaxial prismatic aragonite crystals that have smooth crystal surfaces lacking bead like bodies the smooth surfaced prismatic aragonite crystals are interpreted to be normal abiotic precipitates whereas the beaded microspar is interpreted to result from biotic processes where the nannobacteria serve as catalysts for creation of the cement a population explosion of bacteria occurs as the organic material of egg case rots which alters the microchemical environment and induces a rapid precipitation of aragonite enclosing tens of thousands of nannobacteria as the organic material is destroyed reproduction of bacteria slows and epitaxial prismatic aragonite crystals nucleate and grow abiotically on the structureless biotic layer

regional variation in paleomagnetic polarity of the matachewan dyke swarm related to the kapuskasing structural zone ontario the 2 45 ga matachewan dykes from the abitibi subprovince of the canadian shield yield a mean paleomagnetic pole of 42 n 58 e 95 3 n sites 36 which is a composite of new and previously published data domains of paleomagnetic polarity are defined an area of dykes predominantly of reversed magnetization in the abitibi subprovince contrasts with an area of exclusively normal dykes to the north the polarity domains are separated by faults related to the 1 95 ga uplift and exposure of the lower crust in the kapuskasing structural zone and therefore reflect hudsonian age tectonics in the archean superior province

palaeomagnetism of precambrian dyke swarms in the north china shield the 1 8 ga lip event and crustal consolidation in late palaeoproterozoic times abstract the north china shield ncs is cut by a laterally extensive dyke swarm emplaced at 1 78 1 76 ga when an extensional regime succeeded regional metamorphism and completion of cratonisation by 1 85 ga palaeomagnetic study of these dykes and adjoining metamorphic country rocks identifies a dominant shallow axis comprising a contiguous population with ne to n declinations and rare opposite polarity dykes with ne shallow magnetic declination a1 d i 36 1 recognised from previous study and emplaced in granulite terranes in the north are displaced by more northerly declinations a2 d i 8 2 in lower grade metamorphic terranes to the south contact tests indicate a primary cooling related origin to these magnetisations although tests are in part ambiguous because magnetisations in the granulite basement are comparable petrologic and rock magnetic considerations imply that magnetisation of the dykes occurred during uplift from depths as deep as 20 km following the peak of metamorphism at 1 85 ga a temporal migration a2 a1 is implied by the higher crustal level and earlier acquisition of the former and the deeper source and later acquisition of the latter a third population of dyke magnetisations a3 d i 18 43 is distributed towards steeper inclinations and close to the mesozoic recent palaeofield these are either partial or complete overprints of a1 a2 magnetisations with greater degrees of alteration indicated by demagnetisation and thermomagenetic spectra or are much younger dykes of mesozoic tertiary age a minority fourth later precambrian but presently undated dual polarity population has a magnetisation 11 dykes d i 108 7 with contact tests indicating a primary cooling related origin the 1 78 1 76 ga time of emplacement of the dominant dyke swarms in this study is widely represented by contemporaneous igneous rocks in other major shields linked to major large igneous province lip related events the new definition of a 1 83 1 76 ga apw swathe from the north china shield permits a comparison with other shields and yields a constraint to continental configurations during the late palaeoproterozoic a quasi integral reconstruction of palaeopangaea is tested here and supported by conformity of predominantly of uplift related palaeopoles from the 1 90 1 70 ga tectono thermal belts and from sw ne trending apw implied by the distribution of poles from the 1 80 ga igneous suites including the lip events this trend incorporates the a2 a1 migration and the granulite terrane cooling polar swathe from north china the reconstruction indicates that continental crust consolidated in palaeoproterozoic times by accretion of 2 3 1 7 ga orogenic belts around a hemispheric and crescent shape core already established by late archaean times the north china shield is interpreted to have bordered the western cratonic margin of the indian shield in a proximity supported by correlation of geological features and suggested by a number of previous workers the central orogenic zone of the north china shield characterised by tectono thermal activity prior to 1 85 ga was then contiguous with a comparable zone running through the centre of the indian shield and continuing into the capricorn belt of western australia the 1 78 1 76 ga dykes in north china continue into dyke swarms in the south india shield and may have been sourced in a plume related lip focussed near the continental margin in the xiong er aulacogen

microbial biscuits of vaterite in lake issyk kul republic of kyrgyzstan abstract microbial biscuits of vaterite a polymorph of caco3 were found in lake issyk kul republic of kyrgyzstan four sections were defined from the inner part to the outer part of these microbial biscuits 1 a nucleus made up of coated discrete bacterial forms 2 a massive coating attributed to the calcification of the mucilaginous sheath of the bacterial community 3 a thick lamina composed of large fibers of vaterite and 4 a discontinuous external coating incorporating foreign elements such as diatoms the origin of these microbial biscuits is attributed to bacterial activity the vaterite formation seems to be related to the hexagonal structure of the mucilaginous sheath and to the inhibitor role of the phosphorous present in the water during calcite precipitation

a 2082 ma radiating dyke swarm in the eastern dharwar craton southern india and its implications to cuddapah basin formation abstract using consistent paleomagnetic data together with precise pb pb baddeleyite ages on a series of mafic dykes occurring over an area of at least 70 000 km2 a 2081 8 1 1 ma weighted mean of 4 dykes dyke swarm was identified intruding the archean basement rocks in the eastern dharwar craton skirting the cuddapah basin on its north northwest and western flanks the geometry of these dykes collectively due to their progressive variation in trend from n134 w to n28 e defines a fan angle of about 162 and forms a spectacular radiating swarm converging toward a focal point beneath the cuddapah basin anisotropy of magnetic susceptibility ams investigations on these dykes based on the orientation of principal eigenvectors indicate magma flow was vertically upward in them suggesting the magma source of these dykes to be proximal to the sampling sites these features together with reported geophysical evidence for high density material below the cuddapah basin suggest that this mafic volcanic province probably formed due to the impact of an asthenospheric mantle upwelling perhaps triggered by a plume head or other causal mechanisms like global warming of mantle or small scale instability like edge driven convection this may have resulted in the domal uplift of the continental lithosphere large scale crustal extension and thinning followed by thermal relaxation and subsidence that may have been responsible for the formation of the intra cratonic cuddapah basin shortly after 2082 ma dyke swarms of this age fort frances dykes 2076 5 4 ma or of slightly younger lac esprit dykes 2069 1 ma and older cauchon lake dykes 2091 1 1 8 2 1 ma ages are fairly wide spread in the superior province however a reconstruction of the paleopositions of dharwar cuddapah dykes 38 n 180 e a95 4 and superior at 2080 ma using paleomagnetic data fort frances dykes p 43 n lp 184 e does not suggest a close proximity for these provinces at that time their disparate locations could therefore suggest these were two distinct nodes of wide spread magmatism between 2080 ma and 2065 ma

paleomagnetism of the 1 1 ga umkondo large igneous province in southern africa 1 the umkondo dolerites are present over a wide area in the kalahari craton southern africa thirty nine sampling sites in botswana and south africa yielded tightly grouped paleomagnetic directions due south with shallow inclinations and three sites of opposite polarity the dolerites have u pb single crystal baddeleyite or zircon crystallization ages of 1112 0 5 to 1108 0 9 ma these results can be combined with published data from 39 additional umkondo sites and 33 sites in the grunehogna province of antarctica after restoring east antarctica to its position next to southern africa grouping the sites geographically yields 10 site mean poles with mean at 64 0 n 38 8 e a95 3 7 this umkondo pole can be correlated with keweenawan poles from laurentia because both sets of poles are precisely of the same age as well as predominantly of one polarity the relative orientation of the two cratons within the rodinia supercontinent is fixed this implies that the namaqua natal maud belt which rims the southern part of the kalahari craton faced away from laurentia the umkondo pole combined with published poles suggest that the kalahari craton remained distinctly south of the laurentian craton between 1 1 and 1 0 ga making it highly unlikely that the two cratons collided

proterozoic palaeomagnetism of the mafic dyke swarms in the high grade region of southern india abstract palaeomagnetic investigations have been carried out on four major dyke swarms 38 sites in the high grade metamorphic region of the south indian shield a total of 37 sites 243 samples in four dyke swarms have yielded precambrian palaeomagnetic vectors after progressive ac and thermal demagnetisation 1 the agali anaikatti swarm bordering the nilgiri massif to the south 2 the tiruvannamalai swarm to the interior of the pondicherry coast 3 the dharmapuri swarm towards the west of tiruvannamalai in the interior of tamil nadu and 4 a few dykes from a swarm in north kerala on the west coast ages of the palaeomagnetic vectors are estimated from available k ar isotope dates on samples from a number of sites from all swarms and rb sr isotope results from the older agali anaikatti dykes the vectors and their corresponding ages are as follows d 13 9 i 27 94 95 26 pole 75 7 n 153 2 e for the 2000 ma agali anaikatti swarm n 2 sites d 359 8 i 9 2 51 95 10 pole 79 7 n 101 1 w for the 1800 1750 ma dharmapuri swarm n 5 sites d 217 2 i 50 103 95 24 pole 51 n 130 e for the 1700 1650 ma northern kerala dykes n 2 sites and d 106 9 i 77 6 42 95 9 4 pole 18 8 s 125 2 w for the 1650 ma tiruvannamalai swarm n 7 sites these results in conjunction with other available good quality data have been used to compile an apparent polar wander curve of india for the proterozoic eon the palaeolatitudes derived from the data show that the indian indian landmass was situated at mid latitudes of the northern hemisphere during 2000 ma at a low latitude near the equator 10 n during 1800 1750 ma close to the south pole region 60 s by 1650 ma again near the equator 1000 ma near the north pole 80 n region at 850 800 ma and moved to 10 30 n by 750 ma

passive seismic imaging with directive ambient noise application to surface waves and the san andreas fault in parkfield ca summary this study deals with surface waves extracted from microseismic noise in the 0 1 0 2 hz frequency band with passive seismic correlation techniques for directive noise we explore the concept of passive seismic noise tomography performed on three component sensors from a dense seismic network from the nine component correlation tensor a rotation algorithm is introduced that forces each station pair to re align in the noise direction a necessary condition to extract unbiased traveltime from passive seismic processing after rotation is performed the new correlation tensor exhibits a surface wave tensor from which rayleigh and love waves can be separately extracted for tomography inversion methodological aspects are presented and illustrated with group speed maps for rayleigh and love waves and ellipticity measurements made on the san andreas fault in the parkfield area california usa

s wave splitting in the aftershock region of the 1995 hyogo ken nanbu earthquake the 1995 hyogo ken nanbu earthquake kobe earthquake m7 2 occurred on january 17 1995 a temporary aftershock observation was carried out by the geophysical research group organized by universities for prediction seismology in 1995 from october 1995 to january 1996 a dense seismic network was deployed in and around the aftershock region and some seismic stations were set directly on the active fault traces we analyze the s wave splitting in and around the aftershock region of the hyogo ken nanbu earthquake we detect a spatial variation of s wave splitting which can be related to crack orientations in an area where a large earthquake has occurred at stations more than 1 km away from the hyogo ken nanbu earthquake fault zone the leading shear wave polarization directions lspd are found to be parallel to the axis of the regional maximum horizontal compressional stress e w suggesting that cracks caused by regional tectonic stress exist in the area however at stations within 500 m of the earthquake fault zones the lspds are parallel to the fault strikes n45 50 e this suggests that new fractures were produced parallel to the faults by shear faulting of the hyogo ken nanbu earthquake and resulted in s wave splitting along the earthquake fault zone nevertheless shear fault origin anisotropy was not evident at stations on the active faults which ruptured in 1596 this implies that healing processes have already closed the fractures produced by shear faulting in 1596 therefore we can expect that s wave splitting can be a useful tool for monitoring healing processes of active faults

epitaxial growth of nanophase magnetite in martian meteorite allan hills 84001 implications for biogenic mineralization crystallographic relationships between magnetite sulfides and carbonate rosettes in fracture zones of the allan hills alh 84001 martian meteorite have been studied using analytical electron microscopy we have focused on those magnetite grains whose growth mechanisms can be rigorously established from their crystallographic properties individual magnetite nanocrystals on the surfaces of carbonates are epitaxially intergrown with one another in stacks of single domain crystals other magnetite nanocrystals are epitaxially intergrown with the surfaces of the carbonate substrates the observed magnetite carbonate hkl miller indices orientation relationships are 1 1 3 m 1 1 0 c and 1 1 1 m 0 0 3 c with lattice mismatches of approximately 13 and approximately 11 respectively epitaxy is a common mode of vapor phase growth of refractory oxides like magnetite as is the spiral growth about axial screw dislocations previously observed in other magnetite nanocrystals in alh 84001 epitaxy rules out intracellular precipitation of these magnetites by martian organisms provides further evidence of the high temperature 120 degrees c inorganic origins of magnetite in alh 84001 and indicates that the carbonates also have been exposed to elevated temperatures

microbialites in a modern lagoonal environment nature and distribution tikehau atoll french polynesia abstract microbialites including stromatolitic structures gelatinous masses and mats develop in tikehau lagoon tuamotu french polynesia on the flanks and slopes of numerous pinnacles and motu islets between 0 and 25 m deep on coral colonies algal tufts or on lagoonal sediments individual stromatolitic structures consist of hemispherical domes and are produced by monospecific populations of filamentous cyanobacteria belonging to three distinctive phormidium species symploca and possibly to one new species of schizothrix gelatinous masses hanging from the ceiling of caverns are produced by phormidium laysanense microbial mats are well developed at all depths on pinnacle slopes and are produced by either monospecific or mixed cyanobacterial populations the study of micro and ultrastructure of stromatolites reveals that they result both from trapping and binding of sediments and from carbonate precipitation in addition to phototrophic biomineralization pervasive carbonate precipitation occurred in association with decaying organic matter the basic constructional elements of the precipitated carbonate consist of 0 1 0 2 m rounded bodies that form very fine anhedral to subhedral micrite their surfaces comprise extremely small carbonate granules 20 50 nm gradual changes ageing in the ultrastructure of carbonate precipitates include the precipitation of successive euhedral calcite layers around these grains widespread development of microbialites in this pristine lagoonal environment results from rapid blooms of microbial benthic communities possibly induced by temporary and local nutrient pulses nutrients seem to undergo a restricted circulation between the sediment and pinnacle reef frameworks pinnacles and reef islets are repeatedly affected by catastrophic storm events which cause redistribution of sediments and nutrients

miocene phosphorites from the murray ridge northwestern arabian sea abstract phosphorites from the murray ridge nw arabian sea comprise nodules bioclasts and bone fragments the nodules are made up of a homogeneous light colored phosphate nucleus consisting of rivulariacean filamentous cyanobacteria and a thin dark grey colored phosphate cortex showing abundant microbial filaments and microborings the bioclasts comprise of 14 14 5 ma old planktonic foraminifers accepted as the time of deposition spherical to ovoid shaped apatite microparticles resembling fossil bacteria are distinct components in the bioclasts bone fragments exhibit apatite fillings the nodules and bone fragments consist entirely of carbonate fluorapatite cfa with low al k and th concentrations suggesting absence of continental detritus shale normalized ree patterns of the samples support a seawater derived composition the highly uniform initial nd values of 4 8 to 5 1 are interpreted as the seawater value at the onset of phosphatization 14 ma ago in contrast 87sr 86sr ratios show a large range of 0 709055 to 0 709124 corresponding to unusually young stratigraphic ages of 1 to 3 ma the data are interpreted as evidence for post depositional sr exchange of the recrystallizing phosphorites with fluids isotopically not much different from modern seawater it is concluded that the phosphorites formed under oxic shallow water conditions where microbial populations assimilated phosphorus primarily from seawater and mediated precipitation of cfa during early diagenesis at the sediment water interface on different substrates

amorphous calcium carbonate associated with biofilms in hot spring deposits abstract calcium carbonate nanoparticles are intimately associated with crystalline calcite and aragonite in the eryuan gongxiaoshe and zhuyuan hot springs water temperature 75 c which are located in yunnan province china the nanoparticles

a palaeomagnetic investigation of the mashonaland dolerites north east zimbabwe summary earlier palaeomagnetic work on the mashonaland dolerite sills and dykes 1830 230 ma showed that they record a geomagnetic field reversal 28 new sites have been sampled in sills 26 and dykes 2 in north east zimbabwe detailed alternating field demagnetization afd is more effective in removing secondary components of remanence than thermal demagnetization td and enables an assumed primary remanence to be isolated at 16 sill sites combined with earlier results there are 11 sill sites with a positive inclination p d 287 i 53 k 40 and 18 with a negative inclination n d 114 i 47 k 35 the data lead to a positive class b reversal test field tests suggest that the n remanence is the younger and that the period of mashonaland igneous activity spanned a reversal of the earth s magnetic field from p to n the areal distribution of polarity is not random sills with only p remanence are found in the more elevated central region of the mashonaland igneous province whilst predominantly n bearing sills occupy the lower surrounding terrain

paleomagnetic study of neoarchean paleoproterozoic dykes in the kaapvaal craton 431 oriented samples were collected from 27 dolerite dykes at 17 sites belonging to 2 95 2 65 and 1 90 ga swarms that trend se e and ne respectively from the bushveld igneous complex into the eastern kaapvaal craton ages determined by olsson et al 2010 olsson in soderlund et al this volume samples were analyzed for paleomagnetism and also anisotropy of magnetic susceptibility ams for the 2 95 ga se trending dykes high temperature coercivity p component has unblocking temperatures up to 590 degrees c and coercivity 40 90 mt and demonstrate ssw declination and intermediate positive inclination based on positive contact and conglomerate tests we argue for a primary origin of this component the paleopole bad calculated from component does not correspond to any of the previously obtained archean paleoproterozoic paleopoles for the kaapvaal craton and represents a new key pole for 2 95 ga the high coercivity h component for the 2 65 ga old e trending dykes has a ssw declination and steep positive inclination paleomagnetic pole ryk recalculated from this component is close to the paleopoles obtained by wingate 1998 and strik et al 2007 for 2 78 ga ventersdorp volcanics the third group ne trending dykes of the 1 90 ga black hill swarm demonstrate an m component with dual polarity high coercivity component with sse declination and negative intermediate inclination the paleopole bhd calculated from this component is close to the 1 87 ga pole of the kaapvaal craton obtained by hanson et al 2004 overprint directions include a very well developed thermo chemical overprint dec 329 degrees inc 36 degrees which is believed to be associated with a similar to 0 18 ga regional karoo thermal event c 2010 elsevier b v all rights reserved less

geochemistry and paleomagnetism of late cretaceous mafic dikes in kerala southwest coast of india in relation to large igneous provinces and mantle plumes in the indian ocean region new geochemical and paleomagnetic results are presented on two late cretaceous dikes of the 85 90 ma leucogabbroic and doleritic dikes and the 65 70 ma dolerites in kerala india the dikes are rich in incompatible elements have fractionated patterns with light rare earth element enrichment and are akin geochemically to cretaceous basalts on the east coast of madagascar the magmas were formed at garnet lherzolite depths above the marion plume constituting part of a large igneous province in madagascar in contrast the 65 70 ma dolerites are moderately depleted in incompatible elements with almost flat rare earth element patterns and resemble the upper formations of the deccan traps and the tholeiitic dikes of the seychelles these dolerites were formed by melting of spinel lherzolite over the reunion plume paleomagnetic data from the dikes and the other coeval igneous units from south india provide the 90 ma pole latitude 24 longitude 293 a 95 5 9 n 18 sites for india the 65 70 ma dolerites possess both normal and reverse polarities and the mean pole latitude 36 longitude 283 a 95 5 7 n 10 sites compares well with the deccan superpole paleolatitude estimates indicate 5 southward migration for the marion plume and a northward migration for the reunion plume in conformity with global mantle circulation models however distinguishing migration of the reunion plume from the effects of true polar wander is difficult furthermore the geodynamic reconstructions extending the shear zones of southern madagascar into south india are not tenable

chemostratigraphy of the paleoproterozoic duitschland formation south africa implications for coupled climate change and carbon cycling the paleoproterozoic duitschland formation lies stratigraphically beneath the timeball hill formation which contains the only unequivocal glacial unit of this era in the transvaal basin south africa lithologic evidence in paleoproterozoic successions of north america however indicates the existence of three discrete and potentially global ice ages within this 300 my interval carbonates of the duitschland formation are significantly enriched in 13c up to 10 1 permil in the upper part of the succession above a notable sequence boundary in contrast the lower part of this unit contains carbonates with consistently negative 13c values trace and major element compositions of these carbonates as well as carbon isotopic compositions of coexisting organic matter support a primary origin for the markedly positive carbon isotope anomaly the stratigraphic constraints indicate that 13c enriched carbonates were deposited prior to paleoproterozoic glaciation in southern africa similar to carbonates stratigraphically beneath neoproterozoic glacial diamictites worldwide also mirroring the neoproterozoic record are strongly negative 13c values in cap carbonates atop glacial diamictites in paleoproterozoic strata of wyoming and ontario the litho and chemostratigraphic constraints indicate that the interval of negative carbon isotope values in well preserved carbonates of the lower duitschland formation may reflect a second paleoproterozoic ice age in the transvaal succession this interpretation is further supported by recently discovered bullet shaped clasts with striations in diamictite from the basal part of the succession thus the emerging temporal pattern of carbon isotope variations and glaciation in the paleoproterozoic has a close analogue to neoproterozoic events suggesting a coupling of climatic and biogeochemical changes at both ends of the eon

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

preliminary report on the paleomagnetism of 1 88ga dykes from the bastar and dharwar cratons peninsular india abstract we report the results of a preliminary paleomagnetic study on well dated 1 9 ga dykes from the bastar craton india this suite of nw se trending dykes was linked to similarly aged magmatic activity in the dharwar craton and cuddapah basin in india as part of a large igneous province this igneous activity may have extended across many cratons in the columbia supercontinent including the north china craton laurentia baltica australia siberia and the kaapvaal and zimbabwe cratons southern africa the bastar dykes along with the cuddapah traps and dyke yield a dual polarity magnetization with d 126 i 15 2 k 27 95 11 9 and a corresponding paleomagnetic pole at 31 n 330 e dp 6 3 dm 12 2 the relatively robust paleomagnetic and geochronologic database at 1 85 1 90 ga allow us to test one of the configurations of the supercontinent columbia there are some critical differences between our paleomagnetically based reconstruction and the archetypal and geologically based columbia configuration of zhao et al 2004 most notably siberia india and australia cannot be linked to laurentia in the archetypal columbia fit proposed links between western australia and the kaapvaal and zimbabwe cratons are possible as is the fit between baltica and laurentia the robust paleomagnetic data reported in this paper require that either the columbia supercontinent did not exist at 1 9 ga or requires major modification given that our data provide only a snapshot on the paleoproterozoic we conclude that the columbia supercontinent remains a viable possibility although relationships between individual elements should be re evaluated as more data become available

postcrystallization disturbance in the neodymium and lead isotope systems of metabasalts from the ramagiri schist belt southern india archean metabasaltic rocks from three separate blocks that constitute the north south trending ramagiri schist belt of the eastern dharwar craton have large variations in nd2750 even samples of the same rock type from one outcrop that are less than 50 cm apart from each other show a range of up to 5 5 nd2750 units the trends of the fields defined by the three suites in smnd vs nd2750 space intersect at smnd 0 1 to 0 2 and nd2750 8 to 12 suggesting that these rocks have lost lrees and have interacted with and exchanged at least nd with a common extraneous phase the isotopic composition of this phase suggest derivation of its neodymium from sources which had a long term history of lree enrichment this phase may have derived its isotopic composition from older continental crust one of the unaltered dikes cutting the three blocks in the belt and the surrounding gneisses gives a 147sm144nd 143nd144nd age of 2454 100 ma a minimum for the time of block assembly and alteration of rees including nd while a set of samples from the central block lie along an isochron with a pb pb age of 2746 64 ma the samples from the eastern and western blocks show large uncorrelated variations in 207pb204pb with 206pb204pb residues after acid leaching of sample powders have lead isotope ratios that are distinct for samples from each of the block and are compatible with their derivation from archean mantle the lead isotope composition of the acid leaches suggest disturbance of the metabasalts by an isotopically extraneous phase that had variable and high 207pb204pb and 208pb204pb ratios for a given 206ph204pb relative to expected archean mantle values both lead and neodymium isotope disturbances are best explained as a result of alteration of these rocks by an extraneous phase which had derived its isotopic composition from ca 3 5 ga continental crust

paleomagnetism of precambrian dike swarms in the harohalli area south of bangalore india seventy two oriented core samples from twelve sites in three dike swarms in the harohalli area two of which have been dated radiometrically ikramuddin and stueber 1976 have been studied two old metadolerite dikes were completely unstable and gave no consistent results after detailed af and thermal demagnetization and principal component analysis three sites in e w quartz dolerite dikes in the group dated at 2370 230 ma gave consistent vectors n 3 d 79 6 i 82 3 k 191 95 9 0 pole 9 5 s 242 4 c one n s dike from this group is roughly reversed to the above the fifth dike an olivine dolerite gave a completely distinct vector two of five alkali syenite dikes dated at 814 34 ma gave a consistent vector n 2 d 26 9 i 83 0 k 1910 95 5 7 pole 24 75 n 264 1 e the three e w dolerite dike vectors correlate well with results from three other published dike studies the mean of thirteen site vectors presumed to date from 2 4 ga yielding a pole at lat 15 2 n long 55 6 e 95 10 2

palaeoproterozoic indian shield in the global continental assembly evidence from the palaeomagnetism of mafic dyke swarms abstract palaeomagnetic study of palaeoproterozoic mafic dykes in the basement along the margins of the cuddapah basin the largest precambrian sedimentary basin in south india is presented in detail for a general discussion of palaeoproterozoic igneous activity in india the results are compared with all earlier published data on mafic dykes in india and are integrated with recently published high precision u pb baddeleyite ages to provide a comprehensive account of palaeoproterozoic igneous activity in india the analysis consolidates palaeomagnetic poles for six age divisions between 2 45 and 1 85 ga with robust statistical criteria our best estimates of overall mean poles from 241 dykes are situated at 1 17 7 n 106 0 e a95 9 0 7n 24 at c 2 45 ga 2 7 1 n 57 2 e a95 4 5 n 69 at c 2 37 ga 3 41 6 s 5 5 e a95 5 1 n 34 at c 2 22 ga 4 4 7 n 343 0 e a95 4 4 n 31 at 2 18 ga 5 49 2 n 332 9 e a95 4 8 n 24 at 1 99 1 89 ga and 6 73 7 n 282 6 e a95 2 9 n 39 at 1 86 ga the data permit us to construct an apparent polar wander path for the indian shield for an 600 ma interval of the palaeoproterozoic eon 2 45 1 85 ga testing and evaluation of continental reconstructions for this interval which are mostly based on geological correlations reveal many inconsistencies between 2 45 and 2 37 ga the indian shield was situated at higher latitudes similar to the yilgarn craton of australia it was subsequently located near the equator at 2 22 2 18 1 99 and 1 86 ga thus an india australia connection is supported during these times but a proposed australia kaapvaal link in zimvaalbara and a dharwar india slave connection in sclavia or a superior zimbabwe india connection in superia are inconsistent with indian data in addition the close palaeomagnetic comparison between the palaeoproterozoic dykes of dharwar bastar bundelkhand cratons in india indicates an age of 2 45 ga for orogenic activity along the central indian tectonic zone hence matching this zone with the 2 0 1 8 ga trans north china orogenic belt or positioning north china adjacent to india or juxtaposition of the indian shield along the western margin of laurentia in the columbia reconstructions is not supported the indian data appear to be in accord with the essential features of the refined protopangaea model and the original ur configuration finally the results are interpreted in terms of four dyke emplacement events in the age range 2 45 2 18 ga linked to short lived 5 10 ma lips developed over mantle plumes the dykes of 1 99 1 89 ga age probably relate to continued long lived igneous activity while the 1 86 ga dykes are relatively fewer in number and may represent waning stage of a large igneous event related to a major mantle plume

paleomagnetism of ca 2 3 ga mafic dyke swarms in the northeastern southern granulite terrain india constraints on the position and extent of dharwar craton in the paleoproterozoic abstract the pole positions reported for the mafic dykes occurring in the northeastern southern granulite terrain and easterly trending dykes of dharwar craton are alike while their ages differ by ca 700 ma confounding tectonic reconstruction to resolve the discordance and to constrain the position of the indian continent in the proterozoic paleomagnetic and geochronological studies have been carried out on the mafic dyke swam in the northeastern southern granulite terrain two distinct pole positions are obtained one from the group of tiruvannamalai dykes as 27 7 s 231 5 e dp dm 12 14 with corresponding mean declination dm and inclination im as 125 and 73 8 95 22 7 6 respectively and second from the east coast dykes as 2 32 s 188 2 e dp dm 5 8 with corresponding mean dm and im as 88 9 and 33 8 95 48 7 1 respectively the remanent efficiency of magnetization values between 0 01 and 0 08 suggest that the dykes have been cooled through curie temperature of magnetite which is the carrier of magnetization the baked contact test confirms that the characteristic remanent magnetization is the primary magnetization the sm nd mineral whole rock isochron from one of the tiruvannamalai dykes yielded an age of 2318 60 ma mswd 1 9 this age is the time of intrusion of the tiruvannamalai dykes and is similar within errors to the age of dykes in the dharwar craton thus the contiguity of dharwar craton with the northern block of southern granulite terrain as early as 2 3 ga ago is established the pole position thus obtained corresponds to a high latitudinal position of dharwar craton during early paleoproterozoic

palaeomagnetic and geochemical studies on dolerite dykes from tamil nadu india abstract dolerite dykes of the south arcot north arcot and salem districts of tamil nadu can be broadly divided into four major groups depending on their remanent magnetic directions they are a d 86 i 35 k 400 95 5 pole position at 189 e 1 s b d 33 i 48 k 21 95 18 pole position at 220 e 38 n c d 133 i 75 k 134 95 6 pole position at 54 e 31 n and d d 294 i 52 k 18 95 23 pole position at 49 w 13 n vgp s of these dykes are in good agreement with the earlier precambrian data available for peninsular india and their pole positions indicate that all these dyke events could be older than 1830 200 ma petrochemical studies suggest that the dykes are quartz normative tholeiites and must have formed at shallow depths their differentiation trends reveal that they were probably derived from two different magmatic sources these dykes are transitional in character between an oceanic and a continental setting which suggests that they intruded into thinned crust during an extensional event

carbonate and organic matter sedimentation and isotopic signatures in lake chungara chilean altiplano during the last 12 3 kyr abstract sediments in lakes in the andean volcanic setting are often made up of diatomaceous ooze together with volcaniclastics and small amounts of carbonates despite their scarcity carbonates along with organic matter provide significant paleoenvironmental information about lake systems this study focuses on the carbonates in lake chungara their morphologies distribution and origin deduced from the isotopic markers these markers reflected changes in the water and the biomass between the onset of the holocene and around 9 6 cal kyr bp these changes are marked by general increases in toc tn and tn 15nair and by fluctuating values of toc 13cvpdb in its sediments and are probably related to major shifts in the lake surface volume associated with rises in lake level an increase in salinity around 10 cal kyr bp is thought to be linked to a short dry period giving rise to the onset of carbonate production the mid holocene arid period between 7 3 and 3 5 cal ka bp with a maximum of aridity around 6 0 cal kyr bp was deduced from 18ovpdb values in the endogenic carbonates these results match the reconstructions in lake titicaca based on benthic diatoms and paleoshore levels offshore sediments mainly consist of a diatomaceous ooze laminated in the lower half unit 1 and banded massive with tephra layers in the upper half of the sequence unit 2 toc 13cvpdb and the c n ratio confirm that phytoplankton was the main source of organic matter in these sediments shallower sediments units 3 to 5 developed in platform and littoral settings providing evidence of subaqueous macrophytes and to a lesser extent land plants carbonate content ranges between 0 1 and 6 wt in offshore settings 30 to 40 m water depth and reaches the maximum values in the lower part of unit 2 carbonate minerals low magnesium calcite and minor amounts of high magnesium calcite and aragonite are scarce and are arranged in mm thick layers commonly forming cm thick levels or bioclasts carbonate layers are made up of euhedral to subhedral spindle shaped calcite crystals and to a lesser extent aragonite needles all in the m range aragonite spheroids coexist in littoral sediments with other carbonate shapes and charophyte remains where carbonate reaches locally up to 20 wt co2 photosynthetic depletions related to seasonal phytoplankton blooms were responsible for the high frequency deposition of mm thick carbonate layers the average values for 13cvpdb in lake water plankton and sediments of lake chungara as organic matter or as carbonate are around 15 higher than commonly reported values in other lakes this 13c enrichment is attributed to carbon assimilation from a dic affected by methanogenesis in which hco3 is the dominant species the 13cvpdb and 18ovpdb variations and their covariation in endogenic carbonates suggest that lake water volume and lake level increased along the holocene

shear wave anisotropy in the crust around the san andreas fault near parkfield spatial and temporal analysis summary we systematically analysed shear wave splitting sws for seismic data observed at a temporary array and two permanent networks around the san andreas fault saf observatory at depth the purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the saf in this segment and its temporal behaviour in relation to the occurrence of the 2004 parkfield m 6 0 earthquake the dense coverage of the networks the accurate locations of earthquakes and the high resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the saf zone the results show that the primary fast polarization directions pds in the region including the saf zone and the northeast side of the fault are nw se nearly parallel or subparallel to the saf strike some measurements on the southwest side of the fault are oriented to the nne ssw direction approximately parallel to the direction of local maximum horizontal compressive stress there are also a few areas in which the observed fast pds do not fit into this general pattern the strong spatial variations in both the measured fast pds and time delays reveal the extreme complexity of shear wave anisotropy in the area the top 2 3 km of the crust appears to contribute the most to the observed time delays however substantial anisotropy could extend to as deep as 7 8 km in the region the average time delay in the region is about 0 06 s we also analysed temporal patterns of sws parameters in a nearly 4 yr period around the 2004 parkfield main shock based on similar events the results show that there are no appreciable precursory coseismic or post seismic temporal changes of sws in a region near the rupture of an m 6 0 earthquake about 15 km away from its epicentre

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

rbsr ages of proterozoic kimberlites of india evidence for contemporaneous emplacement abstract rb sr analyses of leached phlogopite macrocrysts from kimberlite pipes 1 2 5 and 7 in south india give concordant ages of 1091 20 1092 15 1093 20 and 1091 10 ma respectively indicating contemporaneous emplacement of these pipes the present results are in contradiction with earlier measurements groundmass mineral assemblages with low rb sr ratios give a tight set of initial sr ratios at 0 7029 2 0 7019 2 0 7029 2 and 0 7030 4 suggesting derivation of the primary kimberlite magma from a relatively uniform and depleted mantle source a similar age and initial sr isotopic ratio of one sample of the majhgawan lamproite from central india imply that the proterozoic kimberlite lamproite activities in india though widely separated in space were nearly contemporaneous

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

mn and fe rich black travertine shrubs bacterially and nanobacterially induced precipitates abstract black shrubs i e arborescent forms are present within hot water travertine deposits from morocco the shrubs are commonly a few centimeters high and laterally grouped along individual laminae other than in mineral composition the black shrubs closely resemble the calcite shrubs from other hot water travertine deposits e g bagni di tivoli italy and mammoth hot springs yellowstone national park they are composed of a variety of minerals that are mn rich and also have a significant fe oxide component similar mn and fe rich shrub like forms have been described from desert varnish deep sea mn nodules and marine carbonates frutexites sem analyses show that bacterial bodies are densely packed within the mn rich black travertine shrubs whereas no bacterial bodies are evident within the enclosing aragonite and calcite laminae the bacterial bodies range in shape and size from rods commonly 0 8 m long by 0 1 m in diameter to spherical forms commonly 0 3 m in diameter that form chains up to 1 m long to nanobacterial spheres 150 to 200 nm in diameter that make up densely packed chains commonly 15 m in length it is well documented within the microbiological literature that not only do bacteria have the ability to induce the precipitation of mn and fe they can concentrate mn in mineral precipitates from waters with very low to essentially undetectable concentrations of mn the black mn rich shrubs within the moroccan travertines other hot water travertines and related features such as desert varnish deep sea mn nodules and frutexites are postulated to also be the result of bacterially induced precipitation

paleomagnetism u pb geochronology and geochemistry of marathon dykes superior province and comparison with the fort frances swarm we report the first detailed study of the paleomagnetism u pb geochronology and major element geochemistry of paleoproterozoic north trending marathon dykes north of lake superior the paleomagnetic and geochemical results demonstrate that marathon dykes can be divided into two subsets one of normal magnetic polarity the other of reverse polarity normal and reverse marathon paleomagnetic poles at 43 n 196 e dm 9 dp 7 number of dykes n 16 and 51 n 175 e dm 9 dp 6 n 12 respectively are statistically distinct and may indicate different ages of normal and reverse dyke emplacement a u pb baddeleyite age of ma has been obtained at a normally magnetized marathon paleomagnetic site the reversely magnetized marathon dykes are undated but have a paleopole rather close to that of the reversely magnetized ma fort frances dykes and major element geochemical signatures as portrayed on jensen plots that are identical to those of the fort frances swarm therefore reverse marathon an

coseismic and postseismic elastic wave velocity variations caused by the 2008 iwate miyagi nairiku earthquake japan 1 we analyze coseismic and postseismic velocity variations caused by the june 13 2008 iwate miyagi nairiku earthquake mw 6 9 using passive image interferometry pii seismic noise is correlated in order to reconstruct the green s function between two sensors shear wave velocity changes are determined by relating the coda parts of the daily green s functions to a long term reference green s function our study extends from january 2008 to august 2010 and includes the correlations for 190 station pairs in three different frequency ranges from 0 125 to 1 0 hz we show that combining the 9 different component cross correlation functions stabilizes the velocity change estimation and increases analysis resolution the observed velocity change curves can be fitted by model time series consisting of a coseismic velocity drop followed by logarithmic postseismic recovery and seasonal velocity variations the coseismic velocity drops are stronger at higher frequencies and are concentrated in the southern part of the fault zone a tomography algorithm was developed to reproject the observed velocity variations of the different sensor pairs onto the single sensors the depth distribution of coseismic changes was modeled for the three stations with the largest velocity drops at two stations the coseismic velocity changes are located in the upper several hundred meters the third station shows indications for deeper changes in the order of kilometers postseismic recovery takes significantly longer than the analyzed two year period seasonal velocity variations with periods of one year are observed at all analyzed frequencies for most station pairs

paleomagnetism and u pb geochronology of easterly trending dykes in the dharwar craton india feldspar clouding radiating dyke swarms and the position of india at 2 37 ga abstract a u pb baddeleyite age of 2367 1 ma from a diabase dyke together with previously published age data suggests that a major early proterozoic dyke swarm cuts across the structural grain of the archean dharwar craton in india paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 with convergence to the west it was originally emplaced at high latitudes and together with the widgiemooltha dykes of the yilgarn block of australia may have been a segment of a larger radiating swarm related to a long lived plume event that was active for about 50 my from 2418 to 2367 ma a regional change in the intensity of brown feldspar clouding in the dykes suggests that the dharwar craton was tilted northwards in harmony with previous observations on the structure and metamorphism of the archean rocks towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker more sooty appearance the black clouding whose precise origins remain unknown is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at 800 ma and with a shear zone and change in structural trend related to pan african deformation at 550 ma paleomagnetic studies suggest that i the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and ii as a more general observation diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black

nature and environmental significance of microbialites in quaternary reefs the tahiti paradox drill cores from the bamer reef edge of tahiti exhibit 85 93 m thick coralgal sequences recording at least 13 500 years of continuous reef growth in optimal environmental conditions the cored reef sequences form an overall shallowingupward succession with assemblages of branching or massive colonies of porires at the base overlain by a robust branching community acropom gr dmmi robirstn heavily encrusted by coralline algae sessile vermetid gastropods and arborescent foraminifers which grew at depths less than 6 m microbialites generally form the last stage of encrustation of coral colonies or more commonly of related encrusting organisms thus appearing as a major structural component of the reef sequence where they may locally form 809 of the rock by volume they developed in an open cavity system of the reef framework with freely circulating normal marine water microbialites include laminated crusts and clotted micritic masses commonly associated in compound crusts probably feflecting differences in the composition of the involved biological communities and in biomineralization processes that controlled the accretion of the crusts the isotopic composition of the microbialites 2 05 to 3 92 i they underwent also indivitlual processes of biominrlali2ation besicles the overall decrease in lizht and enqy conditions rttlecting progressive burial by conil growth the widespread development of microbialites within the reef framework may be related to increased alkalinity and niitrient availability in interstitial waters due to temstrial groundwwr seepage and puiodic runoffs the tlevelopment of micrubialites in the cryptic niches of the reef framework ceased about 6000 years

northeast trending early proterozoic dykes of southern superior province multiple episodes of emplacement recognized from integrated paleomagnetism and u pb geochronology integrated paleomagnetic and u pb geochronologic studies have been conducted to establish the paleomagnetic directions and ages of early proterozoic tholeiitic dykes of northeast trend in the southern superior province previously referred to collectively as preissac dykes it is demonstrated that they are readily separated on the basis of paleomagnetism into subsets referred to as the biscotasing and senneterre swarms in addition a pair of unnamed dykes may be associated with the north and northwest trending matachewan swarm farther west biscotasing dykes have a down west magnetization of single polarity with a corresponding paleopole at 27 8 n 136 7 w dm 12 3 and dp 9 4 senneterre dykes carry an up north or occasionally down south direction with corresponding paleopole at 15 3 s 75 7 w dm 7 0 dp 4 4 the senneterre direction is indistinguishable from the primary n1 remanence direction that dominates the magnetization of nipissing sills of the southern province paleomagnetic fiel

anisotropy in the shallow crust observed around the san andreas fault before and after the 2004 m 6 0 parkfield earthquake local seismic arrays were deployed at two locations along the san an dreas fault saf near parkfield california before and after the 2004 m 6 0 parkfield earthquake using local earthquakes we determine the anisotropic field within 1 2 km of the main trace of the saf at the two array locations separated by 12 km the initial array near the safod site was deployed for six weeks in october and november 2003 and the second array located near the town of parkfield was de ployed for 3 months following the 28 september 2004 m 6 0 parkfield earthquake we find the fast shear wave polarization direction nearly fault parallel n40w for stations on the main fault trace and within 100 m to the southwest of the saf at both array locations these fault parallel measurements span the 100 to 150 m wide zone of pervasive cracking and damage interpreted from fault zone trapped waves associated with the main fault core li et al 2004 2006 outside of this zone the fast orientations are scattered with some preference for orientations near n10e roughly parallel to the regional maximum horizontal compressive stress direction rh in addition fast directions are preferentially oriented parallel to a northern branch of the saf recorded on stations in the 2004 parkfield deployment the measured anisotropy is likely due to a combination of stress aligned micro cracks away from the fault and shear fabric within the highly evolved fault core the majority of our measurements are taken outside of the main fault core and we es timate the density of microcracks from the measured delay times apparent crack densities are approximately 3 with large scatter the data suggest weak depth dependence to the measured delay times for source depths between 2 and 7 km below 7 km source depth the delay times do not correlate with depth suggesting higher confining pressure is forcing the microcracks to close no coseismic variation in the anisotropic parameters is observed suggesting little to no influence on measured splitting due to the 2004 m 6 0 parkfield earthquake however the premainshock and postmainshock data presented here are from arrays separated by 12 km limiting our sensitivity to small temporal changes in anisotropy

the parkfield california earthquake prediction experiment five moderate magnitude 6 earthquakes with similar features have occurred on the parkfield section of the san andreas fault in central california since 1857 the next moderate parkfield earthquake is expected to occur before 1993 the parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process observations and reports of seismicity and aseismic slip associated with the last moderate parkfield earthquake in 1966 constitute much of the basis of the design of the experiment

the intensity of the geomagnetic field from 2 4 ga old indian dykes precambrian paleomagnetic records from dyke swarms provide a unique source of information regarding the archean geomagnetic field and more specifically the average field strength produced by the early dynamo we sampled 16 paleomagnetic sites from the dharwar giant dyke swarm in southern india which was emplaced between 2 365 and 2 368 ga despite taking great care in selecting locations exempt of any geological disturbance only two of these sites provided primary directions with very steep inclinations and therefore were emplaced in close to a magnetic pole paleointensity experiments were conducted on a subset of samples from the dyke margins the characteristic magnetization is carried by single domain magnetite grains with a very narrow range of unblocking temperatures inferred from the sharp decrease by at least 75 of their remanence above 520 c the paleointensity results indicate an average low field of 9 2 7 t consistent with reported values from canadian dyke swarms for the same period these results combined with the thellier thellier determinations obtained so far for the precambrian suggest that a low field period prevailed from circa 2 3 to 1 8 ga while the preceding and following time intervals are characterized by significantly stronger paleointensities although this suite of episodes is not fully incompatible with previous models for the long term evolution of the geodynamo it is tempting to make the link with the recent suggestion of an early dynamo sustained within a conductive magma layer at the base of the mantle from 3 5 to 2 5 ga which progressively declined until convection became sufficiently efficient to reactivate a strong dynamo process within the earth s liquid core

organotemplate silica deposition in neoproterozoic deep marine environments evidence from the penganga group adilabad india silica in bedded cherts interstratified with manganese carbonates in a deep water carbonate ramp succession of the neoproterozoic penganga group india displays film like spherical to rod shaped and tubular branching microstructures the microstructures resemble mineralized extracellular polysaccharides biofilms and bacterial morphologies the microstructures suggest silicification by nucleation of silica on organic templates or indirectly as sorbed species accumulating on organic templates given that similar microstructures have also been documented in archean cherts it is proposed that organotemplates might have been an important sink for the deposition of silica in precambrian deep water marine environments

paleomagnetism and geochronological studies on a 450 km long 2216 ma dyke from the dharwar craton southern india abstract paleomagnetic and geochronological studies were carried out on a 450 km long from 17 sites n s striking paleoproterozoic dyke swarm exposed along a natural crustal cross section of about 10 km increasing from north to south in the dharwar craton to study the characteristics of paleomagnetism and geochronology in vertical dimension u pb pb pb dating on baddeleyite gives a crystallisation age of 2216 0 0 9 ma for long dyke akld paleomagnetic data from this well dated 2216 ma dyke swarm in dharwar craton are of excellent quality high coercivity and high blocking temperature components are carried by single domain magnetite and show dual polarity remanence directions combined normal and reverse polarity remanences on akld and other n s dykes define the most reliable paleomagnetic pole for 2216 ma at latitude 36 s and longitude 312 e a95 7 though paleomagnetic data is unavailable on other n s dykes below the cuddapah basin high precision geochronology suggest that they are of similar age within errors though there is a variation in the crustal depth of dharwar craton from north to south consistent pb pb u pb baddeleyite geochronology and paleomagnetic studies along the akld established its continuity and preservation along its entire strike length the virtual geomagnetic poles of these sites confirm a stable remanence and are almost identical to the previously reported paleomagnetic pole and also supported by positive reversal test positive paleomagnetic reversal test on these dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions updated apparent polar wander path of dharwar craton indicates relatively low drift rate during 2 21 2 08 ga interval magnetogranulometry and sem studies show that remanent magnetization in this dyke was carried by single domain magnetite residing within silicate minerals

the paleomagnetic significance of new u pb age data from the molson dyke swarm cauchon lake area manitoba u pb geochronology paleomagnetism and petrography indicate that the molson dyke swarm along the western margin of the superior craton is a composite of at least two ages of intrusion the more extensive younger dyke set the molson swarm sensu stricto generally has a 030 trend is mainly pyroxenitic to noritic with subordinate diabase and has been related to rifting in a back arc environment during closure of the manikewan ocean at about 1920 1800 ma which culminated in the trans hudson orogen a u pb zircon age from one of these dykes located at cauchon lake indicates emplacement at 1877 7 150 4 similar to two previous u pb age determinations on molson dykes another dyke from cauchon lake yields a baddeleyite zircon u pb date of 2091 2 ma and appears to be part of an older mainly diabasic suite of east northeast trending dykes that may represent a continental rifting episode that preceded the opening of the manikewan ocean the new u pb age data require a revision to the interpretation of