the 9th generation international geomagnetic reference field summary the international association of geomagnetism and aeronomy has recently released the 9th generation international geomagnetic reference field the latest version of a standard mathematical description of the earth s main magnetic field used widely in studies of the earth s deep interior its crust and its ionosphere and magnetosphere the coefficients were recently finalized at the xxiii general assembly of the international union of geophysics and geodesy held at sapporo in japan in 2003 july the igrf is the product of a huge collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and from observatories and surveys around the world

a new towed marine vector magnetometer methods and results from a central pacific cruise summary this paper focuses on new instrumental and methodological aspects of the acquisition processing and interpretation of marine magnetic data between two overhauser sensors towed as a longitudinal gradiometer a new fluxgate vector magnetometer was employed a second independent vector magnetometer system operated simultaneously this equipment was used during research cruise so 180 of r v sonne in the central pacific at about 120 w just south of the equator the survey area on the pacific plate is the mirror image to the currently subducting cocos plate off central america the oceanic crust was formed around 23 ma at the east pacific rise when the farallon plate broke up into the cocos and nazca plates the magnetic seafloor anomalies in the survey area strike approximately north south almost parallel to the main field resulting in very low anomaly amplitudes which had hindered detailed anomaly identification so far a new processing scheme was applied to the data which identifies the weak anomalies in the total field and those in the vertical component that as a consequence of the source body geometry have about doubled amplitude the vertical component constrains 2 d modelling much better than the total field alone processed fluxgate total field data are practically identical to the overhauser reference and even provide a reliable gradient when combined with one overhauser although towed vector magnetometers typically provide no independent estimate of yaw we illustrate that a numerical yaw bandpass filtered magnetic heading can provide reasonable estimates of the horizontal field components these component data open additional analysis tools the strike direction of magnetic lineations can be estimated from single profiles by either magnetic boundary strike ellipses in the space domain or by coherences between vertical and horizontal components in the wavenumber domain auto power spectra of the total field provide an approximate depth to the anomaly source or if in obvious contradiction to the bathymetric depth allow the detection of distortions for example by external temporal geomagnetic variations

siliceous sponge expansion and phosphogenesis in a shallow water environment in the malyi karatau range kazakhstan during the precambrian cambrian transition abstract ocean oxygenation during the precambrian cambrian transition is thought to be one of the key factors for the rapid diversification of animals known as the cambrian explosion siliceous sponges inhabiting continental slope settings were capable of shifting the respiratory oxygen demand in the water column to depth resulting in an increase in oxygen levels of shallow water to assess the global importance of the impact of ecosystem engineering by sponges we evaluated the palaeo environmental conditions that confine sponge expansion using multiple geochemical proxies including silicon and carbon stable isotopes as well as major and trace elements in cherts and siliceous sedimentary rocks that were deposited in shallow water around the precambrian cambrian transition we investigated three lithosections in the malyi karatau range in southern kazakhstan the lowermost chert samples of these sections are typically characterized by comparably low 30si values and exhibit molds of siliceous sponge spicules upsection the occurrence of cross bedding structures and granular phosphorites document the shallowing of the depositional environment where overall increasing 30si values and increasing al si indicate less settlement of sponges and higher suspended particle loads corresponding carbon isotope records of the carbonate fraction decrease from near zero to negative carbon isotope values averaging 5 9 the 13ccarb values overall coincide with y ho the redox indicator ce ce and mass fractions of carbonate fluorapatite suggesting that these ratios are predominantly controlled by the mineralization of organic matter in phosphatic sediment the studied sections document the ecological requirements for sponge settlement and proliferation such as the presence of a hard substrate and sufficiently low suspended particle concentrations and or low wave energy this work suggests that the widespread appearance of karsted surfaces around the precambrian cambrian transition that were subsequently submerged during the cambrian transgression would have created permissive habitats for sponges and large phosphate deposits the capability of sponges to oxygenate shallow seawater and extensive phosphorous burial in epicontinental basins are thus feasible mechanisms for the seawater oxygenation around the precambrian cambrian transition

a geomagnetic record over the last 3 5 million years from deep tow magnetic anomaly profiles across the central indian ridge high resolution records of the geomagnetic field intensity over the last 4 myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short lived low field intensity features associated with excursions or short polarity intervals in order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events we have collected six deep tow 500 m above the seafloor and several sea surface magnetic anomaly profiles from the central indian ridge across the brunhes matuyama and gauss chrons i e from the ridge axis to anomaly 2a after removal of topography latitude and azimuth effects we converted distances into time sequences using well dated polarity reversal anomalies as tie points we calculated the average signal to test the robustness of the short wavelength anomalies the resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the central indian ridge the east pacific rise and the pacific antarctic ridge our results suggest that in addition to polarity reversals to previously suggested geomagnetic events subchrons or excursions within the brunhes and matuyama chrons a new small scale magnetic anomaly likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas we observe a dense succession of microanomalies that are correlated excursions ontong java 1 and 2 and gilsa is found after the olduvai chron the near bottom results support the existence of three geo magnetic features between the gauss matuyama boundary and olduvai they also suggest three geomagnetic events during the c2a i n subchron within the gauss chron this study emphasizes the potential of deep tow magnetic surveys in detecting fluctuations in geo magnetic field intensity and in particular short lived excursions a poorly constrained part of the geomagnetic field temporal variation spectrum

regional variations in the carbon isotopic composition of phosphorite from the early cambrian lower tal formation mussoorie hills india abstract early cambrian lower tal phosphorites of the mussoorie syncline have been analyzed for their carbon isotopic composition of apatite bound carbonate and organic carbon two mine excavations at maldeota and durmala located on two limbs of the syncline and 14 km apart were subjected to detailed sampling along selected stratigraphic profiles field and microscopic evidence suggest a shallow marine environment at the site of phosphorite formation characterized by small scale fluctuations in the sedimentation milieu isotope data have been utilized to characterize differing redox conditions prevalent during phosphorite formation at durmala and maldeota in addition a marked negative carbon isotope excursion has been recorded at the dolomite phosphorite contact defined by the krol tal stratigraphic boundary it is thought to reflect the influx of 12 c and p rich waters from a deeper part of the sedimentary basin based on the isotopic composition of carbon in the phosphorite sulfur in the associated pyrite and petrological data it has been inferred that the durmala phosphorite reflects deposition under a suboxic diagenetic environment and the maldeota phosphorite formed within suboxic to anoxic sulfate reducing conditions

a new shipboard three component magnetometer the three components of the geomagnetic field were measured at sea by stcm shipboard three component magnetometer with a relative accuracy of 50 25 nt the stcm was controlled by a microcomputer and the three geomagnetic components were calculated by reducing the magnetic field due to the ship s remanent and induced magnetic moments the only assumption being that the magnetic field is linearly related to the magnetic moments although stcm has no particular magnetic compensation device it can be set at any place on board of any kind of ship in the kh82 5 cruise the three component measurements were conducted along latitude 42 n north of the mendocino fracture zone where there are well defined magnetic anomaly lineations trending north and south stcm measured the north component anomalies with very small amplitudes compared to the east and downward component anomalies which shows that lineations trend north and south the three measured component anomalies are almost identical to the anomalies

vector magnetic analysis within the southern ayu trough equatorial western pacific summary we present the analysis of magnetic field data of the ayu trough a divergent plate margin located in the southern philippine sea plate the survey which mapped both axial and offaxis regions of the southern ayu trough recorded the total field using a proton precession magnetometer towed behind the ship and the vector magnetic field using a shipboard threecomponent fluxgate magnetometer together with multibeam bathymetry the two magnetic field anomalies show good agreement indicating that the reduction of vector field data was successful our inspection of e w profiles shows that there is little correlation in magnetic anomalies between adjacent profiles and lack of symmetry with respect to the trough axis however a careful examination of gridded magnetic anomaly map reveals that symmetry may be found to the north of 1 n along n120e which is roughly the direction of spreading at this region as revealed by the fine scale seafloor fabric by our survey vector magnetic anomalies exhibit important differences with strike of magnetic boundaries within the inner region between tobi and mapia ridges trending predominantly n s whereas in the outer region they are almost e w this finding suggests that the outer region appears to retain the magnetization and crustal structure of the old west philippine basin while the inner region is inherited from the e w seafloor spreading in the ayu trough the short along axis segmentation and complex seafloor morphology caused by slow oblique spreading may explain the lack of symmetry in the magnetic anomalies with respect to the trough axis and a wider occurrence of 3 d magnetic source crust roughly to the south of 1 n

comprehensive analysis of marine magnetic vector anomalies we present a comprehensive description of the analysis of marine magnetic vector anomalies the marine magnetic vector field can be measured with a shipboard three component magnetometer stcm the stcm has the advantage of measuring three components but it suffers from several types of noise sources because the principles of its measurement are more complex than that of a proton precession magnetometer we have estimated the quality of stcm data constructed an optimal filter and evaluated the reliability of the filtered data then we have organized an analytical procedure composed of four steps 1 the discrimination between two and three dimensional magnetic sources 2 the determination of magnetic boundaries 3 the calculation of the boundary strikes and 4 the estimation of the boundary magnetization contrasts we show examples of these procedures using the data near the east pacific rise 28 s 31 s collected during leg 5 of the gloria expedition all the results of the above processing are merged in order to illustrate the distinct pattern of the seafloor magnetization in the surveyed area

diagenetic formation of interlayer deficient fluorophlogopite as a clay mineral in early cambrian phosphorite lesser himalaya india the trioctahedral analog of illite the occurrence of a trioctahedral analog of illite the dioctahedral interlayer deficient k mica has long been debated due to the inherent difficulties of determining structure and chemical composition of the extremely fine grained material earlier descriptions based on separated material are equivocal here we describe low temperature diagenetic formation of fluorophlogopite which is interlayer deficient and therefore analogous to illite using high resolution in situ methods transmission electron microscopy tem with preparation by focused ion beam milling combined with wavelength dispersive analysis by field emission gun electron microprobe the average composition is k0 5mg2 8v0 01fe0 005 si3 15al0 85o10 oh 0 65f1 35 including minor amounts of nh4 for charge compensation as determined by electron energy loss spectroscopy the k deficient mg mica occurs in layer packages of 10 layers and no indications for interlayering with other sheet silicate layers such as chlorite or vermiculite could be identified with tem x ray powder diffraction patterns of separated material confirm the absence of smectite components the mineral was identified in phosphorites from the lowermost cambrian tal group mussoori syncline lesser himalayas india the rocks are alternating phosphatic mudstones and phosphatic dolostones at times interbedded with phosphate poor carbonate layers which are rich in organic matter sedimentary fluorophlogopite occurs in both rock types and in two textural associations one in vesicles filled with amorphic organic matter the other as reaction rims around illite which contains up to 5 wt v2o3 in its rims textural arguments favor an early diagenetic formation of both v bearing illite and fluorophlogopite closely associated with organic matter and linked to dolomitization the high f content stabilizes phlogopite to low temperatures our findings confirm that the stability field of fluorophlogopite extends from magmatic to metamorphic and sedimentary conditions

equatorial pacific magnetic anomalies identified from vector aeromagnetic data summary it has long been challenging to identify magnetic anomalies due to seafloor spreading in the equatorial pacific here we show that project magnet vector aeromagnetic profiles from the equatorial pacific record magnetic anomalies due to seafloor spreading much more clearly than do shipboard total intensity profiles the anomalies are reliably recorded at wavelengths between 20 and 150 km in the vertical and east components which have high coherence differ in phase by 90 and resemble synthetic magnetic anomaly profiles from an analysis of a single near equatorial vector aeromagnetic profile we infer that the magnetic lineations strike 8 10 counter clockwise of north and that magnetic anomaly 7 is located 400 km further east than previously estimated the newly estimated location of anomaly 6 is consistent with a tentative estimate by wilson from a low amplitude shipboard magnetic profile because the skewness of profiles over the seafloor formed near the paleoequator changes rapidly with paleolatitude and paleostrike a skewness analysis of these data may provide useful bounds on the location of pacific plate paleomagnetic poles and indicate that this seafloor has had little if any northward motion relative to the spin axis since it formed

chronology of early cambrian biomineralization data on the first appearances of major animal groups with mineralized skeletons on the siberian platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the cambrian radiation popularly known as the cambrian explosion with a higher precision and provide a basis for the definition of cambrian stages 2 to 4 the lophotrochozoa and probably chaetognatha were first among protostomians to achieve biomineralization during the terreneuvian epoch mainly the fortunian age fast evolutionary radiation within the lophotrochozoa was followed by radiation of the sclerotized and biomineralized ecdysozoa during stage 3 the first mineralized skeletons of the deuterostomia represented by echinoderms appeared in the middle of cambrian stage 3 the fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late neoproterozoic but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians the distribution of calcium carbonate skeletal mineralogies from the upper ediacaran to lower cambrian reflects fluctuations in the global ocean chemistry and shows that the cambrian radiation occurred mainly during a time of aragonite and high magnesium calcite seas

radiolarian palaeoecology and radiolarites is the present the key to the past abstract radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint evolutionary selection pressure among silica secreting marine plankton both radiolarians and diatoms has led toward more economic usage of rapidly shrinking nutrient resources including dissolved silica of the photic zone in the late cenozoic oceans and in particular a substantial modification of oceanic cycle by the diatom explosive radiation even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history oceanic chemistry and productivity as well as patterns of circulation upwelling have changed radically during the phanerozoic radiolarites apparently represent an anachronistic facies as exemplified by their long lived and ocean wide distribution in palaeo pacific and hitherto highlighted actualistic models of localized intra oceanic wind driven upwelling loci are of largely questionable applicability in addition to plate drift hypersiliceous domains and intervals are explainable mostly by a large scale volcano hydrothermal activity during major plate boundary reconfigurations which in many ways favoured siliceous biotas acme and their skeletal remains accumulation and preservation factors tied to rapid voluminous submarine eruptions such as thermal buoyant megaplumes and basin overturns offer a viable alternative for traditional climatic circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse to icehouse climatic change the evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering volcano hydrothermal and tectonic uplift events related mostly to extensive rifting and or accelerated oceanic spreading were the endogenous driving force that created this perturbation of the exogenous system the present biogeochemical cycle is representative only for the overall silica depleted post eocene oceanic ecosystems which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal and closely linking the silica budget with phosphorus and nitrogen cycles thus an orthodox uniformitarian approach to biosiliceous sedimentation based on a silica starved vigorous ocean is of limited significance when applied to the pre neogene settings especially in the peculiar planktic habitats of epeiric seas as well as during biotic crises marked by strong geotectonic overprint the major turnovers in marine siliceous biota composition in particular after the end permian radiolarite gap may have been coupled with discernible changes in an increasing biological control on the long term oceanic silica cycling punctuated equilibrium the evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the phanerozoic contemporaneously with the general secular trend of upward scaling of nutrient related ecological processes and increased effectiveness of resource utilization

a surface towed vector magnetometer 1 we have tested the feasibility of using a commercial motion sensor as a vector magnetometer that can be towed at normal survey speeds behind a research vessel in contrast to previous studies using a shipboard mounted vector magnetometer the towed system is essentially unaffected by the magnetization of the towing vessel results from a test deployment compare favorably with an earlier vector aeromagnetic survey indicating that the towed instrument can resolve horizontal and vertical anomalies with amplitudes 30 50 nt this instrument should be particularly useful in equatorial regions where the vector anomalies are substantially greater than the corresponding total field anomalies

depth estimation from the scaling power spectrum of potential fields summary depth estimation from potential field power spectra requires a realistic assumption of the statistical properties of the source distributions density and susceptibility distributions in the earth s crust exhibit a long range dependence which is adequately described by scaling random fields with a spectral density proportional to some power of the wavenumber the theoretical power spectrum for a half space model of scaling sources explains the shape of observed power spectra of real potential field data very well minimizing the misfit between the model and the observed power spectrum yields an estimate for the depth to the top of the sources after demonstrating this approach on synthetic magnetic data we reinterpret power spectra of gravity and aeromagnetic data from utah hawaii and saskatchewan finding depth values that differ significantly from earlier interpretations all three power spectra are best explained by source distributions starting at surface level even the power spectrum from an aeromagnetic survey of a sedimentary basin with virtually non magnetic basin fill in the latter case a priori information on the intensity and the scaling exponent of the field caused by the basement had to be included to obtain an approximate estimate of the basin depth in general potential field power spectra are dominated by scaling properties of their source distributions and contain only limited depth information

microfossils from the neoproterozoic gangolihat formation kumaun lesser himalaya their stratigraphic and evolutionary significance abstract a well preserved microbial assemblage from the gangolihat dolomite kumaun lesser himalaya is described in the present paper the present assemblage contains both filamentous septate and non septate and coccoidal microfossils besides acritarchs and sponge spicules the assemblage includes 23 species belonging to 12 genera siphonophycus typicum is the most common species in the entire assemblage the assemblage shows an abundance of filaments over coccoids and acritarchs the assemblage includes siphonophycus septatum siphonophyscus typicum siphonophycus robustum siphonophycus capitaneum siphonophycus kestron and siphonophycus punctatum gunflintia minuta gunflintia grandis gunflintia barghoornii oscillatoriopsis media oscillatoriopsis obtusa oscillatoriopsis cuboides oscillatoriopsis sp nostocomorpha chlorogloeaopsis contexta obruchevella parvissima sphaerophycus parvum myxococcoides minor leiosphaeridia crassa trachyhystrichosphaera vidalii trachyhystrichosphaera sp micrhystridium pallidum cymatiosphaera minuta and sponge spicules of monaxons and hexactinellid affinity the assemblage suggests a neoproterozoic age for gangolihat dolomite

deep sea magnetic vector anomalies over the hakurei hydrothermal field and the bayonnaise knoll caldera izu ogasawara arc japan 1 we conducted deep sea magnetic measurements using autonomous underwater vehicles in the bayonnaise knoll caldera the izu ogasawara island arc which hosts the large hakurei hydrothermal field we improved the conventional correction method applied for removing the effect of vehicle magnetization thus greatly enhancing the precision of the resulting vector anomalies the magnetization distribution obtained from the vector anomaly data shows a 2 km wide belt of high magnetization trending nnw sse going through the caldera and a low magnetization zone 300 m by 500 m in area extending over the hakurei site comparison between the results obtained using the vector anomaly and the total intensity anomaly shows that the magnetic field is determined more accurately especially in areas of sparse data distribution when the vector anomaly rather than the total intensity anomaly is used we suggest a geologically motivated model that basaltic volcanism associated with the back arc rifting occurred after the formation of the caldera resulting in the formation of the high magnetization belt underneath the silicic caldera the hakurei hydrothermal field lies in the intersection of the basaltic volcanism belt and the caldera wall fault suggesting a mechanism that hot water generated by the heat of the volcanic activity has been spouting out through the caldera wall fault the deposit apparently extends beyond the low magnetization zone climbing up the caldera wall this may indicate that hot water rising from the deep through the alteration zone is transported laterally when it comes near the seafloor along fissures and fractures in the caldera wall

distributed deformation ahead of the cocos nazca rift at the galapagos triple junction the galapagos triple junction is not a simple ridge ridge ridge rrr triple junction the cocos nazca rift c n rift tip does not meet the east pacific rise epr instead two secondary rifts form the link incipient rift at 2 40 n and dietz deep volcanic ridge the southern boundary of the galapagos microplate gmp at 1 10 n recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the gmp 1 5 ma south of c n rift a band of northeast trending cracks cuts epr generated abyssal hills it is a mirror image of a band of cracks previously identified north of c n rift on the same age crust in both areas the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the epr spreading center and cut eastward into pre existing topography each crack formed a short lived triple junction until it was abandoned and a new crack and triple junction initiated nearby between 2 5 and 1 5 ma the pattern of cracking is remarkably symmetric about c n rift providing support for a crack interaction model in which crack initiation at the epr axis is controlled by stresses associated with the tip of the westward propagating c n rift the model also shows that offsets of the epr axis may explain times when cracking is not symmetric south of c n rift cracks are observed on seafloor as old as 10 5 ma suggesting that this triple junction has not been a simple rrr triple junction during that time

cambrian biostratigraphy of the tal group lesser himalaya india and early tsanglangpuan late early cambrian trilobites from the nigali dhar syncline precise biostratigraphic constraints on the age of the tal group are restricted to 1 a basal level correlative with the anabarites trisulcatus protohertzina anabarica assemblage zone of southwest china 2 a level near the boundary of the lower and upper parts of the tal group correlative with the early tsanglangpuan stage drepanuroides zone and 3 an interval low in the upper part of the tal group correlative with later in the tsanglangpuan stage palaeolenus zone these correlations are based on small shelly fossil and trilobite taxa other chronostratigraphic constraints include the marked negative 13 c isotopic excursion coincident with the transition from the krol group to the tal group this excursion is used as a proxy for the precambrian cambrian boundary in several sections worldwide and if applied to the lesser himalaya indicates that the boundary is at or just above the base of the tal group the upper parts of the tal group may be of middle or late cambrian age and might form proximal equivalents of sections in the zanskar spiti region of the tethyan himalaya both faunal content and lithological succession are comparable to southwest china furthering recent arguments for close geographic proximity between the himalaya and the yangtze block during late neoproterozoic and early cambrian time trilobites from the uppermost parts of the sankholi formation from the nigali dhar syncline are described and referred to three taxa one of which drepanopyge gopeni is a new species they are the oldest trilobites yet described from the himalaya

siliceous sponge spicules in the early cambrian chert phosphorite member of the lower tal formation krol belt lesser himalaya we present the first record of siliceous hexactinellid and demosponge spicules from lower tommotian chert and phosphorite of the lower tal formation of the mussoorie and garhwal synclines in the lesser himalaya spicules include cross shaped as well as single rayed forms and occur sporadically in a chert or phosphatic groundmass scanty records of siliceous sponge spicules in the proterozoic sedimentary rocks and their abundance and rapid proliferation during the early cambrian are interpreted as a result of major change in seawater chemistry due to enhanced availability of nutrients leading to progressive evolution of siliceous biological forms and phosphatic shelly faunas

magnetic signatures of the orogenic crust of the patagonian andes with implication for planetary exploration abstract the patagonian andes represent a good scenario of study because they have outcrops of diverse plutonic rocks representative of an orogenic crust on earth and other planets furthermore metamorphic surface rocks provide a window into deeper crustal lithologies in such remote areas satellite and aerial magnetic surveys could provide important geological information concerning exposed and not exposed rocks but they integrate the magnetic anomalies in areas of kilometres for the southernmost andes long wavelength satellite data show clear positive magnetic anomalies 100 nt for the patagonian batholith pb similar as parts of the older martian crust this integrated signal covers regions with different ages and cooling histories during magnetic reversals apart from the variability of the rocks to investigate the complex interplay of distinct magnetic signatures at short scale we have analysed local magnetic anomalies across this orogen at representative sites by decimeter scale magnetic ground surveys as expected the investigated sites have positive and negative local anomalies they are related to surface and subsurface rocks and their different formation and alternation processes including geomagnetic inversions distinct curie depths of the magnetic carriers intracrustal deformation among other factors whole rock chemistry ranging from 45 to 80 wt sio2 and from 1 to 18 wt feotot magnetic characteristics susceptibilities magnetic remanence and konigsberger ratios as well as the composition and texture of the magnetic carriers have been investigated for representative rocks rocks of an ultramafic to granodioritic intrusive suite of the western and central pb contain titanomagnetite as major magnetic carrier individual magnetic signatures of these plutonic rocks reflect their single versus multidomain status complex exolution processes with ilmenite lamella formations and the stoichiometric proportions of cr fe and ti in the oxides at the eastern margin of the pb the investigated plutons and mafic dykes have been emplaced and equilibrated at 4 6 km depth they do not contain magnetite but include variable amounts of ferrimagnetic monoclinic c4 pyrrhotite which was formed along fractures zones by a hydrothermal gold bearing mineralisation the intensity of their positive magnetic anomalies up to 220 nt is well correlated with the amount of pyrrhotite 1 4 vol in all cases high resolution ground surveys variations of the magnetic signature down to 20 nt could be used to clearly distinguish different rock types on a decimeter scale vector magnetometer moura designed for martian exploration was successfully used for these surveys and provides results comparable to a commonly used caesium scalar magnetometer this is important for near future ground surveys on mars and the moon

on reduction of long period horizontal seismic noise using local barometric pressure summary tilt from atmospheric loading has long been known to be the major source of long period horizontal seismic noise we try to quantify these effects for seismic data from the black forest observatory bfo which is known to be a very quiet station experimental transfer functions between local barometric pressure and horizontal seismic noise are estimated for two long time series by standard methods two simple analytical physical models are developed the local deformation model ldm and the acoustic gravity wave model twm subsequently these models with only two free parameters are fit using least squares to the observed seismic noise for time series of widely differing lengths the results are variable sometimes rather dramatic variance reductions are obtained and sometimes the reduction is hardly significant the method produces the best results when barometrically induced noise is high the resulting admittances for the ldm are compared to finite element calculations since the methods are simple and can result in conspicuous reductions in noise we provide one more reason for installing barometers at even the best broad band seismic stations

structural development of the jurassic magnetic quiet zone off morocco and identification of middle jurassic magnetic lineations 1 geophysical investigations carried out during two meteor cruises have revealed weak linear magnetic anomalies in those parts of the jurassic magnetic quiet zone jmqz off morocco that are not affected by cenozoic igneous activity the linear magnetic anomalies are not correlated with variations in relief or structure of oceanic crust using the reversal sequence m25 m41 the anomalies of the jmqz can be modeled with a half spreading rate of 2 2 cm yr extrapolation of the weak lineations back to the slope anomaly s1 results in a breakup age of 170 ma according to this interpretation the crust of the jmqz off morocco has formed during the time 170 155 ma bajocian to oxfordian however much lower spreading rates around 1 cm yr cannot be excluded s1 coincides with a poorly developed seaward dipping reflector sequence sdrs which is most likely its source the sdrs corroborates the earlier claim that s1 marks the ocean continent transition seaward of s1 is a 70 km wide strip of horizontal reflectors that become landward dipping to the west in the uppermost part of the basement it parallels s1 over a distance of 200 km and may indicate excessive magma supply within the first 2 m y of seafloor spreading similar landward dipping reflectors are observed also in the conjugate sohm abyssal plain off nova scotia the average magnetization of the landward dipping reflectors is much lower than that of the sdrs a lower crustal body with a seismic velocity of 7 3 km s which formed during the neogene beneath the eastern part of the essaouira rise has only a weak magnetization 0 5 a m

reducing magnetic field induced noise in broad band seismic recordings summary seismic broad band sensors are known to be sensitive to the magnetic field magnetic storms and man made disturbances of the magnetic field can produce significant noise in seismic recordings i show that variations in the magnetic field translate directly into apparent acceleration of the seismic sensor within the period range from 60 to 1200 s for all leaf spring sensors under investigation for a streckeisen sts 1v this is shown even for periods down to 1 s the sensitivity is quantified in magnitude and direction both are quite stable over many time windows and signal periods the sensitivities obtained by linear regression of the acceleration signal on magnetic field recordings during a magnetic storm can effectively be applied to reduce noise in seismic signals the sensitivity varies in magnitude from sensor to sensor but all are in the range from 0 05 to 1 2 m s 2 t 1 seismograms from sensors at black forest observatory bfo and stations of the german regional seismic network were investigated although these are mainly equipped with leaf spring sensors the problem is not limited to this type of instrument the effect is not observable on the horizontal component sts 1s at bfo while it is significant in the recordings of the vertical sts 1 the main difference between these instruments is the leaf spring suspension in the vertical component that appears to be the source of the trouble the suspension springs are made of temperature compensated elinvar alloys that inherently are ferromagnetic and may respond to the magnetic field in various ways however the lacoste romberg et 19 gravimeter at bfo which uses this material too does not respond to magnetic storms at a similar magnitude neither do the invar wire strainmeters an active shielding composed of three helmholtz coils and a feedback system is installed at station stuttgart and provides an improvement of signal to noise ratio by almost a factor of 20 at this particular station the passive permalloy shielding commonly installed with sts 1v sensors performs similarly well

birth of an intraoceanic spreading center the cocos nazca spreading center is one of the few examples of the formation of a spreading center by splitting of oceanic lithosphere it was created when the farallon plate broke up in the early miocene following the collision of the pacific farallon spreading center with the north american continent much of the ancient farallon plate corresponding to the area of opening is lost to subduction beneath central america and south america but new data from the conjugate area on the pacific plate allow the first detailed reconstruction of the break up process the opening began after chron 7 25 ma at a location of focused crustal extension caused by overlapping spreading centers that had evolved in response to a slight reorientation of a pacific farallon ridge segment beginning at chron 6b 22 7 ma eastward progressing seafloor spreading started along an axis that most likely migrated toward the region of weak lithosphere created by the galapagos hotspot by chron 6 19 5 ma plate splitting from the spreading center to the trench was complete allowing the fully detached cocos and nazca plates to move independently this kinematic change resulted in a significant ridge jump of the newly established pacific nazca spreading center a change in plate motion direction of the nazca plate by 20 clockwise and a large increase in pacific cocos plate velocity in the middle miocene

a preliminary investigation into the relationship between long period seismic noise and local fluctuations in the atmospheric pressure field summary the displacement response of an elastic half space to a plane pressure wave is examined in order to establish the conditions under which sources of this type can contribute significantly to the long period seismic noise field the study is restricted to pressure waves which propagate at velocities well below the seismic wave velocities characteristic of the half space the numerical studies indicate that pressure waves with amplitudes of 100 pbar or more can contribute significantly to the long period vertical background noise observed at the surface provided that the detectors are located on sections of alluvial fill or poorly to moderately indurated sandstones and shales whose thicknesses are greater than about a kilometre these same waves can also create significant tilt noise on long period horizontal seismographs located at or near the surface regardless of the rock type the seismic disturbances created by pressure waves decay rapidly away from the surface therefore it appears that it may be possible to eliminate the effects of atmospherically generated noise by placing the detectors at moderate depths

revised tectonic boundaries in the cocos plate off costa rica implications for the segmentation of the convergent margin and for plate tectonic models the oceanic cocos plate subducting beneath costa rica has a complex plate tectonic history resulting in segmentation new lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the east pacific rise from lithosphere formed at the cocos nazca spreading center they also define two early phase cocos nazca spreading regimes and a major propagator in addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of cocos plate over the galapagos hot spot the subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab newly identified seafloor spreading anomalies show oceanic lithosphere formed during initial breakup of the farallon plate at 22 7 ma and opening of the cocos nazca spreading center a revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the cocos nazca spreading center at 19 5 ma a major ridge jump reshaped its geometry and after 14 5 ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere a suspected cause of ridge jumps is an interaction of the cocos nazca spreading center with the galapagos hot spot

a new method for precise determination of the position and strike of magnetic boundaries using vector data of the geomagnetic anomaly field summary we present a new method which provides precise determination of the position and strike of magnetic boundaries using vector data of the geomagnetic anomaly field assuming a 2 d magnetic structure the intensity of spatial differential vectors isdv exhibits a peak at the boundary of magnetic structures the strike of the magnetic boundary is obtained using the fact that the magnetic field is zero in the direction parallel to the boundary we have also applied this method to 3 d magnetic structures our method requires only the relative changes of the three components which are more accurately measured than their actual magnitudes we have applied this method to data from the southern juan de fuca ridge obtained on the r v hakuho maru expedition in 1988 kh 88 3 the results are summarized in magnetic boundary strike diagrams mbsd these are maps that show the position strike and character of the magnetic boundaries mbsd provides valuable information for understanding the magnetic structure of oceanic crust such as magnetic lineations pseudo faults fracture zones and the central anomaly magnetic high camh therefore relatively few vector magnetic profiles provide information similar to extensive total intensity geomagnetic surveys

the 10th generation international geomagnetic reference field abstract the international association of geomagnetism and aeronomy iaga on 12 december 2004 released the 10th generation international geomagnetic reference field igrf the latest version of a standard mathematical description of the earth s main magnetic field and used widely in studies of the earth s deep interior its crust ionosphere and magnetosphere the coefficients were finalised by a task force of iaga the igrf is the product of a large collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and observatories around the world

palaeozoic oceanic crust preserved beneath the eastern mediterranean most oceanic crust is subducted back into earth s mantle within 200 million years of formation analysis of magnetic data from the eastern mediterranean reveals oceanic crust formed up to 340 million years ago as part of an ancient ocean basin

evolution of the south china sea revised ages for breakup and seafloor spreading abstract the continental breakup which gave way to the formation of the oceanic south china sea scs basin began in the latest cretaceous in the northeastern scs and propagated in southern and western direction over a long period of time possibly more than 40 m y the seafloor spreading history of the south china sea has been interpreted in different ways in the past and the debate over the correct timing of the major tectonic events continues we review the different models that have been published and present a revised interpretation of seafloor spreading anomalies based on three datasets with documented high quality which cover all of the scs but the northernmost and southernmost parts we can precisely date the onset of seafloor spreading in the central part of the scs at 32 ma after a ridge jump at 25 ma spreading also began in the southwestern sub basin and spreading ended at 20 5 ma in the entire basin followed by a phase of magmatic seamount formation mainly along the abandoned spreading ridge spreading rates vary from 56 mm yr in the early stages to 72 mm yr after the ridge jump to 80 mm yr in the southwestern sub basin we find indications for a stepwise propagation of the seafloor spreading from northeast to southwest in segments bounded by major fracture zones seafloor spreading ended abruptly probably because the subduction zone along the eastern and southern boundary of the scs of which today the manila trench remains was blocked by collision with a continental fragment possibly the northern part of palawan or a part of the dangerous grounds

on the late neoproterozoic early cambrian transition events in parts of east gondwanaland abstract the east gondwanaland evolved as a result of break up of the rodinia supercontinent the late neoproterozoic early cambrian geochemical events documented in the rocks of the east gondwanaland and siberia suggest variations in the c s and sr isotopic compositions of the contemporary seawater and systematic distribution of phosphorite and evaporite deposits the geochemical records in the peninsular india himalaya south china iran and oman regions have been discussed and used for establishing late neoproterozoic links of these widely separated sedimentary basins

organic walled microfossils from the chert phosphorite member tal formation precambrian cambrian boundary india the chert member also designated chert phosphorite member of the tal formation in the lesser himalaya contains well preserved organic walled microfossil assemblage the assemblage includes small 25 m acanthomorphic acritarchs sphaeromorphic acritarchs cyanobacterial filaments sponge spicules and thallophytic remains identified as micrhystridium lanceolatum m dissimilare m tornatum m lanatum cf lophosphaeridium sp paracymatiosphaera irregularis cymatiosphaera sp estiastra minima leiosphaeridia sp siphonophycus robustum thalloidal fragments and hexactinellid sponge spicules the present assemblage rich in small acanthomorphs appears to be best developed in upper nemakit daldynian beds based on acritarch biostratigraphy the present work suggests a nemakit daldynian to early tommotian age for the chert member which is in agreement with the formerly proposed age based mainly on small shelly fossils the assemblage is correlatable to the precambrian cambrian boundary microbiotic assemblage from china central asia and east european platform the assemblage shows low diversity of species the characteristic of inshore biota

the trace of the pacific cocos nazca triple junction in the central pacific and the formation of an overlapping spreading centre the plate tectonic mirror image to the region of the cocos and nazca plates which are currently being subducted beneath central america is preserved in the central pacific around 120 w just south of the equator cruise so 180 investigated this remote area during project central and acquired new magnetic and bathymetric data a plate tectonic model for the mirror image is presented based on the newly acquired as well as reprocessed existing data discordant magnetic anomaly patterns and bathymetric structures indicate at least two major reorganization events 19 5 and 14 7 myr which can be detected both in the cocos nazca spreading system and in the east pacific rise irregularities in the anomaly pattern and curvilinear structures on the sea floor of the survey area are interpreted in terms of a fossil overlapping spreading centre at the location where the farallon break up originated