the central anomaly magnetic high implications for ocean crust construction and evolution the central anomaly magnetic high camh is a long lineated short wavelength 10 km wide at the sea surface magnetic anomaly overlying many of the world s midocean spreading centers this anomaly is particularly well developed over the northern juan de fuca ridge including the northern symmetrical and endeavour segments the ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near axis crustal plate evolution recently obtained deep tow magnetometer data from segments of the juan de fuca spreading center have caused us to revise the present models of the source of the camh and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates in this model we attribute the near axis magnetization contrast to tectonically controlled low temperature alteration of the upper vertical section of extrusive basement extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis the correlation between fissured crustal blocks and near bottom magnetic lows and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off axis tow over an outer flanking ridge

imaging of erebus volcano using body wave seismic interferometry of strombolian eruption coda 1 seismic interferometry is a recently developed theory that allows for the recovery of a medium s impulse response between two points should randomly distributed sources of white noise or equivalently a multiply scattered equipartioned wavefield be present throughout the medium we exploit the extremely scattering nature of volcanic media and seismic illumination from impulsive strombolian eruptions to extract single station body wave green s tensors at an unusually dense array of stations on erebus volcano antarctica we optimally rotate these green s tensors for each notable arrival and back project them to construct a 3 dimensional scattering map of the magma filled volcano conduit system also corroborated by an independent active source tomography experiment using the same station distribution this approach not only favors highly scattering media contrarily to most conventional methods but its passive nature i e non anthropogenic sources allows for studies of structural temporal variability and possible extension into real time monitoring of active volcanoes

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

very long period oscillations of mount erebus volcano as the signal decays vlp scalar moments up to 5 10 11 n m exceed sp moments by an order of magnitude or more suggesting distinct though genetically related sp and vlp source mechanisms we conclude that vlp signals arise from excitation of a quasi linear resonator that is intimately associated with the conduit system and is excited by gravity and inertial forces associated with gas slug ascent eruption and magma recharge vlp signal stability across hundreds of eruptions spanning 5 years the persistence of the lava lake and the rapid posteruptive lava lake recovery indicate a stable near summit magma reservoir and vlp source process index terms 4544 oceanography physical internal and inertial waves 7280 seismology volcano seismology 8419 8414 volcanology eruption mechanisms 8419 volcanology eruption monitoring 7280 keywords strombolian very long period volcano seismology

episodic dike swarms inferred from near bottom magnetic anomaly maps at the southern east pacific rise fissure eruption at 1728 0 s similar lows are observed at three other drained lava lake troughs including one which is at least 1800 years old residing 400 m away from the present day axis we attribute these lows to the presence of shallow dike swarms the degree to which other geologic features may contribute to the lows is constrained using geologic geophysical and geochemical observations and forward modeling compositional analyses of alvin samples at 1728 0 s do not support fe or ti variations as a primary source hypotheses requiring hydrothermal alteration and porosity variations are both inconsistent with geologic observations and near bottom gravity data analysis from similar areas previous mappings between paleointensity variations and the observed magnetic field over distances of several kilometers from the axis suggest that such variations do not create the field low the dominant source of the magnetization low is most likely the presence of a 100 200 m wide region of shallow dikes which are poorly magnetized relative to extrusives or a region heated above magnetic blocking or curie temperatures by intrusions during the most recent eruption though the latter interpretation cannot explain the low at the fossil trough in the first case this extrusive thinning implies a change in eruptive behavior over the last 750 1500 years given the local spreading rate for the latter case thermal models suggest the anomaly had to have been created by a dike swarms totaling at least 45 m width during the most recent eruption s corresponding to 300 years of plate spreading models indicate that the source of the low is centered slightly east of the axial trough this offset suggests that the axis has been progressively migrating westward over the past millennium consistent with other studies covering greater length and timescales westward migration provides an explanation for the preferential emplacement of recent lavas flows west of the axis evident in abe bathymetry and submersible observations index terms 3035 marine geology and geophysics midocean ridge processes 1517 geomagnetism and paleomagnetism magnetic anomaly modeling 8499 volcanology general or miscellaneous 3045 marine geology and geophysics seafloor morphology and bottom photography 3005 marine geology and geophysics geomagnetism 1550

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

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

autocorrelation of the seismic wavefield at newberry volcano reflections from the magmatic and geothermal systems we show that seismic autocorrelations provide new depth constraints on upper crustal magmatic systems autocorrelations of both ambient noise recorded on seismometers and geophones and teleseismic earthquake coda recorded on seismometers elucidate the structure of newberry volcano these autocorrelations result in the two way body wave green s function beneath a station within the caldera a reproducible coherent p wave reflection is inferred to come from the top of a magma body at 2 5 km depth and maps with the lateral extent of an anomalously low velocity body imaged tomographically on the west flank of the volcano a reflection that deepens with distance from the caldera is inferred to result from a temperature dependent change in metamorphic facies and may map the thermal structure of the edifice our results show that the autocorrelation of diffuse seismic energy reconstructs reflections from seismically sharp boundaries associated with the upper crustal magmatic structure

3 d focused inversion of near seafloor magnetic data with application to the brothers volcano hydrothermal system southern pacific ocean new zealand 1 we describe and apply a new inversion method for 3 d modeling of magnetic anomalies designed for general application but which is particularly useful for the interpretation of near seafloor magnetic anomalies the crust subsurface is modeled by a set of prismatic cells each with uniform magnetization that together reproduce the observed magnetic field this problem is linear with respect to the magnetization and the number of cells is normally greater than the amount of available data thus the solution is obtained by solving an under determined linear problem a focused solution exhibiting sharp boundaries between different magnetization domains is obtained by allowing the amplitudes of magnetization to vary between a pre determined range and by minimizing the region of the 3 d space where the source shows large variations i e large gradients a regularization functional based on a depth weighting function is also introduced in order to counter act the natural decay of the magnetic field intensity with depth the inversion method has been used to explore the characteristics of the submarine hydrothermal system of brothers volcano in the kermadec arc by inverting near bottom magnetic data acquired by autonomous underwater vehicles auvs different surface expressions of the hydrothermal vent fields show specific vertical structures in their underlying demagnetization regions that we interpret to represent hydrothermal upflow zones for example at focused vent sites the demagnetized conduits are vertical pipe like structures extending to depths of 1000 m below the seafloor whereas at diffuse vent sites the demagnetization regions are characterized by thin and inclined conduits

high resolution magnetics reveal the deep structure of a volcanic arc related basalt hosted hydrothermal site palinuro tyrrhenian sea high resolution magnetic surveys have been acquired over the partially sedimented palinuro massive sulfide deposits in the aeolian volcanic arc tyrrhenian sea surveys flown close to the seafloor using an autonomous underwater vehicle auv show that the volcanic arc related basalt hosted hydrothermal site is associated with zones of lower magnetization this observation reflects the alteration of basalt affected by hydrothermal circulation and or the progressive accumulation of a nonmagnetic deposit made of hydrothermal and volcaniclastic material and or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids to discriminate among these inferences estimate the shape of the nonmagnetic deposit and the characteristics of the underlying altered area the stockwork we use high resolution vector magnetic data acquired by the auv abyss geomar above a crater shaped depression hosting a weakly active hydrothermal site our study unveils a relatively small nonmagnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones thermal demagnetization is unlikely but the stockwork extends beyond the limits of the nonmagnetic deposit forming lobe shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones

neoproterozoic tectonic evolution of south qinling china evidence from zircon ages and geochemistry of the yaolinghe volcanic rocks abstract the south qinling terrain is located at the northern margin of the yangtze block neoproterozoic volcanic sedimentary sequences of the yaolinghe group play the most significant role in studying the tectono magmatic evolution of the south qinling based on temporal and spatial geochemical variations the yaolinghe group can be divided into three volcanic cycles basaltic lavas of cycle 1 crystallized at 847 8 ma and exhibit typical island arc basalt characteristics with enrichment of liles th u and lrees depletion of hfses nb ta zr hf ti and homogeneous initial 143nd 144nd ratios ranging from 0 51162 to 0 51172 corresponding to initial nd values of 1 6 to 3 5 these features suggest that they were derived from a depleted mantle source during a subduction period volcanic rocks of cycle 2 show a complete lithological evolution from basalt through andesite to rhyolite without a significant gap of sio2 content they formed between 731 11 ma and 768 7 ma and display trace element signatures similar to arc volcanics with positive lile and negative hfse anomalies these basalts have lower initial 87sr 86sr ratios from 0 7030 to 0 7052 and higher initial 143nd 144nd ratios from 0 51172 to 0 51194 with positive initial nd values ranging from 0 9 to 5 3 whereas intercalated felsic rocks show lower initial 143nd 144nd ratios from 0 51150 to 0 51177 with initial nd values varying from 4 5 to 1 9 suggestive of metasomatic alteration of lithospheric mantle by slab derived fluids melts cycle 3 lavas show undepleted hfses nb ta zr hf and slightly enriched liles th u and ti concentrations they were generated in a rift related setting between 680 ma and 650 ma geochronological and geochemical evidence from lavas of the yaolinghe group suggest that the panxi hannan arc system extended to northern south qinling and the magmatism in the northern margin of the yangtze block evolved from arc to rift magmatism during the neoproterozoic

a high 4 he 3 he mantle material detected under the east pacific rise 15 4 n we investigate in details helium isotope data reported in mougel et al 2014 for 14 basaltic samples collected on the east pacific rise by submersible 15 4 n where the ridge interacts with the mathematician seamounts samples locations are separated by only few hundred meters across a 15 km along axis profile the data reveal a strong geochemical variability that has never been observed at such high spatial resolution for helium isotope compositions moreover they reveal an unusually high 4he 3he mantle component also characterized by unradiogenic lead atypical in oceanic basalts he pb systematics suggests a mixture between a nonradiogenic lead and radiogenic helium pyroxenitic component recycled from the deep continental lithosphere and the ambient peridotitic mantle the he isotope difference between these two end members can be interpreted as a time evolution of two distinct mantle sources after a slight u th 3he fractionation likely due to some ancient degassing during the formation of deep continental pyroxenites

the energy partitioning and the diffusive character of the seismic coda following recent theoretical developments of the radiative transfer the ory of elastic waves we propose to use the ratio r of energies of curl and divergence part of the ground displacement to distinguish between the different possible domi nant scattering regimes in the lithosphere a consequence of the diffusion regime is the partitioning of the energy between different vibrational modes which is inde pendent of time it results in the stabilization of r this behavior is not expected if low order diffraction such as single scattering is dominant we apply our technique to seismograms recorded by a small aperture seismic array operated during june august 1997 in guerrero mexico we estimate the energy ratio r in the coda of 13 earthquakes recorded by the array we find it to be very stable in the coda window while the energy level itself changes by several orders of magnitude the value of r is 7 1 independent of the earthquake location and the magnitude the observed stabilization of r is a strong indication of the diffusive regime of the seismic coda

the rise and fall of axial highs during ridge jumps 1 we simulate jumps of ocean spreading centers with axial high topography using elastoplastic thin plate flexure models processes considered include ridge abandonment the breaking of a stressed plate on the ridge flank and renewed spreading at the site of this break we compare model results to topography at the east pacific rise between 15 25 n and 16 n where there is strong evidence of a recent ridge jump at an apparently abandoned ridge gravity data do not suggest buoyant support of topography model deflections during cooling and melt solidification stages of ridge abandonment are of small vertical amplitude because of plate strengthening resulting in the preservation of a frozen fossil high the present day high is bounded by slopes with up to a 40 grade a scenario very difficult to achieve flexurally given generally accepted constraints on lithospheric strength we model these slopes by assuming that the height at which magma is accreted increases rapidly after the ridge jumps this increase is attributed to high overburden pressure on melt that resided in an initially deep magma chamber followed by a rapid increase in temperature and melt supply to the region shortly after spreading began the high is widest at the segment center suggesting that magmatic activity began near the center of the segment propagated south and then north the mantle bouguer anomaly exhibits a bull s eye pattern centered at the widest part of the high but the depth of the axis is nearly constant along the length of the segment we reconcile these observations by assigning different cross axis widths to a low density zone within the crust

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

source depopulation potential and surface wave tomography using a crosscorrelation method in a scattering medium we use seismic prospecting data on a 40 40 regular grid of sources and receivers deployed on a 1 km 1 km area to assess the feasibility and advantages of velocity analysis of the shallow subsurface by means of surface wave tomography with green s functions estimated from crosscorrelation in a first application we measure rayleigh wave dispersion curves in a 1d equivalent medium the assumption that the medium is laterally homogeneous allows using a simple projection scheme and averaging of crosscorrelation functions over the whole network because averaging suppresses noise this method yields better signal to noise ratio than traditional active source approaches and the improvement can be estimated a priori from acquisition parameters we find that high quality dispersion curves can be obtained even when we reduce the number of active sources used as input for the correlations such source depopulation can achieve significant reduction in the cost of active source acquisition in a second application we compare rayleigh wave group velocity tomography from raw and reconstructed data we can demonstrate that the crosscorrelation approach yields group velocity maps that are similar to active source maps scattering has an importance here as it may enhance the crosscorrelation performance we quantify the scattering properties of the medium using mean free path measurements from coherent and incoherent parts of the signal we conclude that for first order velocity analysis of the shallow subsurface the use of crosscorrelation offers a cost effective alternative to methods that rely exclusively on active sources

multiple scattering from icequakes at erebus volcano antarctica implications for imaging at glaciated volcanoes we examine seismic coda from an unusually dense deployment of over 100 short period and broadband seismographs in the summit region of mount erebus volcano on a network with an aperture of approximately 5 km we investigate the energy partitioning properties of the seismic wavefield generated by thousands of small icequake sources originating on the upper volcano and use them to estimate green s functions via coda cross correlation emergent coda seismograms suggest that this locale should be particularly amenable to such methods using a small aperture subarray we find that modal energy partition between s and p wave energy between 1 and 4 hz occurs in just a few seconds after event onset and persists for tens of seconds spatially averaged correlograms display clear body and surface waves that span the full aperture of the array we test for stable bidirectional green s function recovery and note that good symmetry can be achieved at this site even with a geographically skewed distribution of sources we estimate scattering and absorption mean free path lengths and find a power law decrease in mean free path between 1 5 and 3 3 hz that suggests a quasi rayleigh or rayleigh gans scattering situation finally we demonstrate the existence of coherent backscattering weak localization for this coda wavefield the remarkable properties of scattered seismic wavefields in the vicinity of active volcanoes suggests that the abundant small icequake sources may be used for illumination where temporal monitoring of such dynamic structures is concerned

influence of magma supply and spreading rate on crustal magma bodies and emplacement of the extrusive layer insights from the east pacific rise at lat 16 n seismic reflection data from the east pacific rise at lat 16 n which is spreading at the high end of intermediate rates suggest that the depths at which axial magma chambers reside do not vary smoothly as a function of spreading rate rather magma chamber depths form two distinct populations each associated with a distinct axial morphology and with an abrupt transition occurring within the intermediate spreading rate range our data 1 show that the melt lens at high intermediate spreading ridges lies at a shallow level similar to lens depths at faster spreading ridges and 2 provide further support for the spreading rate invariance of ridges with axial highs noted in other ridge properties the axial morphology of the two ridge segments within the study area differs markedly and a large contrast in magma supply is inferred the ridge segment with greater magma supply is associated with a broader and more continuous melt lens a wider region over which the extrusive crust accumulates and a thicker extrusive layer off axis where supply to the ridge segment appears to be centered however on axis the extrusive layer is thinnest where magma supply is robust and a shallower melt lens is observed consistent with a model in which magma pressure controls the thickness of the extrusive layer accumulated above the magma lens

lapse time dependent coda wave depth sensitivity to local velocity perturbations in 3 d heterogeneous elastic media in the context of seismic monitoring recent studies made successful use of seismic coda waves to locate medium changes on the horizontal plane locating the depth of the changes however remains a challenge in this paper we use 3 d wavefield simulations to address two problems first we evaluate the contribution of surface and body wave sensitivity to a change at depth we introduce a thin layer with a perturbed velocity at different depths and measure the apparent relative velocity changes due to this layer at different times in the coda and for different degrees of heterogeneity of the model we show that the depth sensitivity can be modelled as a linear combination of body and surface wave sensitivity the lapse time dependent sensitivity ratio of body waves and surface waves can be used to build 3 d sensitivity kernels for imaging purposes second we compare the lapse time behaviour in the presence of a perturbation in horizontal and vertical slabs to address for instance the origin of the velocity changes detected after large earthquakes

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

central anomaly magnetization high constraints on the volcanic construction and architecture of seismic layer 2a at a fast spreading mid ocean ridge the epr at 9 30 50 n abstract the central anomaly magnetization high camh is a zone of high crustal magnetization centered on the axis of the east pacific rise epr and many other segments of the global mid ocean ridge mor the camh is thought to reflect the presence of recently emplaced and highly magnetic lavas forward models show that the complicated character of the near bottom camh can be successfully reproduced by the convolution of a lava deposition distribution with a lava magnetization function that describes the variation in lava magnetization intensity with age this lava magnetization function is the product of geomagnetic paleofield intensity which has increased by a factor of 2 over the last 40 kyr and low temperature alteration which decreases the remanence of lava with exposure to seawater the success of the forward modeling justifies the inverse approach deconvolution of the magnetic data for lava distribution and integration of that distribution for magnetic layer thickness this approach is tested on two near bottom magnetic profiles al2767 and al2771 collected using alvin across the epr axis at 9 31 n and 9 50 n our analysis of these data produces an estimate of the relative thickness of the magnetic lava layer which is remarkably consistent with existing multichannel estimates of layer 2a thickness from lines cdp31 and cdp27 the similarity between magnetic layer and seismic layer 2a at the 9 10 n segment of the epr crest provides independent support to the notion that seismic layer 2a in young oceanic crust represents the highly magnetic lava layer and that the velocity gradient at the base of layer 2a is related to the increasing number of higher velocity dikes with depth in the lava dike transition zone the near bottom magnetic anomaly character of the camh is a powerful indicator of the emplacement history of upper crust at mors which allows prediction of the relative thickness and architecture of the extrusive lavas independent of other constraints

crustal magnetization of brothers volcano new zealand measured by autonomous underwater vehicles geophysical expression of a submarine hydrothermal system near bottom magnetic anomaly data have been acquired using autonomous underwater vehicles at brothers volcano southern kermadec arc new zealand crustal magnetization for the study area was obtained by inverting the magnetic data and shows a strong correlation between areas of low magnetization and four hydrothermal fields one of which was unknown prior to our surveys the magnetization pattern is consistent with a model of discrete individual zones of fluid upflow focused along caldera ring faults that provide preferred pathways for the ascent of the hydrothermal fluids differences in the amplitude of the magnetization over the vent fields appear to correlate with age and temperature variations of the hydrothermal fields

segmentation and eruptive activity along the east pacific rise at 16 n in relation with the nearby mathematician hotspot the 16 n segment of the east pacific rise is the most overinflated and shallowest of this fast spreading ridge in relation with an important magma flux due to the proximity of the mathematician hot spot here we analyze the detailed morphology of the axial dome and of the axial summit trough ast the lava morphology and the geometry of fissures and faults in regard to the attributes of the magma chamber beneath and of the nearby hotspot the data used are 1 m resolution bathymetry combined with seafloor photos and videos at the dome summit the ast is highly segmented by 10 third order and fourth order discontinuities over a distance of 30 km often two contiguous and synchronous asts coexist such a configuration implies a wide 1100 m minimum zone of diking the existence of contiguous asts their mobility their general en echelon arrangement accommodating the bow shape of the axial dome toward the hotspot plus the existence of a second magma lens under the western half of the summit pla teau clearly reflect the influence of the hotspot on the organization of the spreading system the different asts exhibit contrasted widths and depths we suggest that narrow asts reflect an intense volcanic activity that produces eruptions covering the tectonic features and partially filling the asts ast widening and deepening would indicate a decrease in volcanic activity but with continued dike intrusions at the origin of abundant sets of fissures and faults that are not masked by volcanic deposits

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

triassic diorites and granitoids in the foping area constraints on the conversion from subduction to collision in the qinling orogen china abstract the western qinling is characterized by triassic granitic plutons which are keys to constrain the timing of subduction and collisional processes between the north china and south china blocks this paper presents a set of new la icpms u pb zircon geochronology major and trace element geochemistry for several representative diorite and granitoid plutons in the foping area the diorites are characterized by high mgo al2o3 y and yb contents but low sio2 sr y and la yb ratios as well as depletion of nb ta p and ti suggesting derived from a subduction related setting la icpms zircon u pb age of 216 9 1 5 ma mswd 0 69 from the diorite constrains the time of the subduction event the medium and fine grained monzogranites as well as the biotite granites exhibit relative high sio2 mgo and rb low y and yb contents and low sr y and la yb ratios as well as various depletions of nb ta p and ti these features indicate a derivation from lower continental crust but had been influenced by slab released fluids in a syn collisional setting the medium grained monzogranite and biotite granite yield la icpms zircon u pb ages of 207 3 2 1 ma mswd 1 8 and 201 6 1 2 ma mswd 1 04 respectively proposing the syn collision during ca 207 201 ma geochemical and geological evidences suggest that the granodiorites intruding in the syn collisional monzogranites were related to the post orogenic collapse after the collision between the north china and south china blocks the la icpms u pb zircon age of 189 3 2 9 ma mswd 5 6 for the granodiorite constrains the time of the post orogenic collapse our geochronological and geochemical data provide important constraints on the triassic evolution from subduction to collision then up to post collisional collapse along the qinling orogen

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

inter source body wave propagations derived from seismic interferometry summary seismicinterferometrycombinedwithsource receiverreciprocityhasrecentlybeenemployed to extract inter source wave propagations in this study we applied the technique to seismograms of earthquakes that occurred at depths of 300 500 km beneath the bonin islands the wavelets of direct p and s waves generated by the deep earthquakes were examined to compute cross correlation functions ccfs we stacked the ccfs observed at receivers around the stationary point whose location could be estimated by the ray tracing technique because the phases in such ccfs potentially emerge at similar lag times that is the stationary phases the results demonstrate the retrieval of direct p and s waves propagating between pairs of deep earthquakes in particular direct p waves are reconstructed for most event pairs the following points should be considered to effectively retrieve the propagation of body waves 1 it is important to design a geometry in which the stationary phase derived from two earthquakes can be observed at the employed stations 2 the lag time difference of the stationary phases that emerge in the employed ccfs should be as small as possible and 3 the direct waves associated with deep earthquakes have a large amplitude at stations employed for cross correlation which is accomplished by both low noise level at the stations and the radiation pattern of the sources

variations in layer 2a thickness and the origin of the central anomaly magnetic high the seismically determined off axis thickening of the extrusive layer is apparently at odds with the magnetic anomaly high t ypic ally associated with the ridge crest the positive magnenzauon contrast at the ridge crest is most likely caused by rapid alteration of the extrusive source layer which occurs over spatial scales 2 3 km comparable to that of the proposed layer 2a thickening we present magnetic rema nence data from basalts dredged on and near the east pacific rise axis at 12on which are compatible with a rapid magnetization reduction 20 k y to decay to l e together with near bottom magnetic profiles from the ultra fast spreading east pacific rise at 19 5os these data suggest that previous estimates of the time constant of alteration inferred from slow spreading ridges 0 5 m y may be more than an order of magnitude too high seismic studies of the fast spreading east pacific rise epr indicate a rapid thickening of seismic layer 2a within a few kilometers of the ridge axis vera et al 1990 toomey et ai 1990 detrick et al 1993 these seismic data have been interpreted to reflect an approximate doubling of the extrusive layer thickness from 100 200 m to 300 500 m through addition of flows whi ch may extend 1 2 km off axis christeson et al 992 vera and diebold in press such thickening of the extrusive source layer had been argued on the basis of the widths of magnetic reversal transitions e g macdonald et al 1983 however this geometry for a uniformly magnetized source layer should produce a marked magnetic anomaly minimum over the ridge axis and is therefore difficult to reconcile with the pronounced anomaly high typically associated with the ridge crest klitgord 1976 sea surface magnetic profiles from the ultra fast spreading 77 mrn yr half rate epr near 19 5os figure 1 illustrate the short wavelength axial anomaly high within the 120 km wide central anomaly which has been termed the central anomaly magnetic high camh klitgord 1976 the camh typically has a wavelength of 8 10 km in sea surface profiles at in termediate spreading ridges 15 45 mm yr the camh occupies a larger proportion of the central anomaly and at slow spreading ridges it is not readily distinguishable in sea surface profiles from the central anomaly because the wavelengths become similar near bottom magnetic profiles figure 1 typically have higher amplitude and shorter wavelength 2 4 kin axial anomalies as expected from the closer proximity to the source klitg6rd et al 1975 miller 1977 perram et al 1990 and even allow distinction of the camh from the central anomaly at some slow spreading ridges macdonald 1977 the higher temporal resolution of faster spreading ridges therefore provides the best opportunity to evaluate the processes responsible for the camh thickness variations in the extraslye layer at fast spreading fi dges have a profound effect on the axial magnetic anomaly since the presence of an axial magma chamber and the low re rnanent intensity of intrusive rocks e g smith and banerice 1986 pariso et al 1991 minimize deeper source contributions in the near ridge environment based on seismic

magnetic structure of a slow spreading ridge segment insights from near bottom magnetic measurements on board a submersible 1 near bottom magnetic measurements on board submersible nautile were carried out on the mid atlantic ridge 21 40 n segment and deep sea geomagnetic vector anomalies along 19 dive tracks were obtained by applying the processing method for shipboard three component magnetometer data a forward modeling technique using short wavelength components of the anomalies arising from local topography and vertical motion of the submersible was designed to estimate the absolute magnetization intensity of the seafloor in the vicinity of the spreading axis a considerable number of magnetization estimations are reliably confirmed by the high correlation between observed and modeled anomalies whereas less reliable estimations are obtained off axis probably because the sediment buries the basement topography the natural remanent magnetization nrm measured on basalt samples collected during these dives is compared with the magnetization from anomalies though both results give a similar range of magnetization intensity no correlation is confirmed between them possibly because the magnetization from anomalies represents laterally averaged seafloor magnetization whereas the nrm has variations at the scale of individual pillow or lava pile equivalent magnetization inverted from the sea surface magnetic anomalies shows axial magnetization increases significantly from the segment center to the segment ends however the results of eight dives conducted near the spreading axis at different locations along the segment show much less variation in magnetization intensity along the axis we ascribe the high equivalent magnetization at segment ends to preferential serpentinization of peridotite near the segment ends and the associated formation of magnetite the results of three across axis transects composed of 15 dives running in the spreading direction can be consistently interpreted as recording geomagnetic paleointensity variations during the brunhes epoch although magnetization lows are generally correspondent to periods of low paleointensity they show deeper drop than predicted from the paleointensity variation

timing of volcanism along the northern east pacific rise based on paleointensity experiments on basaltic glasses 1 samples from two adjacent and contrasting ridge segments along the east pacific rise were measured for their magnetic paleointensity in order to further explore the possibilities of dating very young volcanic samples using secular variations in the earth s magnetic field the ridge segment north of the orozco transform fault 15 22 16 20 n is the shallowest and broadest along more than 5000 km of the east pacific rise whereas the adjacent segment to the north 16 16 18 n has a typical morphology for its intermediate spreading rate both ridge segments were densely sampled during the panr01mv cruise and 36 samples of axial lava flows consisting mainly of glasses from the rims of the flows and some fragments of lobate basalts were selected from this collection for paleointensity experiments the coe version of the thellier double heating procedure in air was used twenty seven units provide internally consistent paleointensity estimates leading to precise estimates of the paleofield which range between 8 t and 57 t comparisons with reference paleointensity curves compiled from subaerial flows archeomagnetic data and sedimentary records projected to the sampling site coordinates show that the measured values can be used to constrain the volcanic history of the ridge segments over the past few thousand years a good agreement was found between apparent freshness of the glasses the geochemistry of the lavas and their magnetic paleointensity values the inflated southern segment seems characterized by recent activities as indicated by numerous flows with paleointensities clustering around today s value 39 t or around the high values typical of 2000 3000 years ago 55 t we interpret this distribution to indicate the flooding by effusive lava flows of the entire axial plateau some 2000 3000 years ago followed by a volcanic phase producing smaller volume lava flows confined to the innermost 200 m of the ridge axis the northern ridge segment is characterized by dispersed paleointensity values consistent with a series of small eruptions of diverse ages samples collected at the tips of both ridge segments across the 16 20 n axial discontinuity have the lowest paleointensities and are thus thought to be significantly older consistent with models advocating reduced magmatism near ridge axis discontinuities this study demonstrates the strong potential of paleointensity measurements as a tool to help constrain volcanic history at ridge axes

nd hf isotopic mapping of late mesozoic granitoids in the east qinling orogen central china constraint on the basements of terranes and distribution of mo mineralization abstract voluminous late mesozoic granitoids and the world s largest mo deposits occur in the east qinling this paper presents the results of nd hf isotopic mapping for the late mesozoic granitoids 155 105 ma and demonstrates their constraint on the basements and distribution of the mo deposits in the east qinling this isotopic map made by 98 21 new and 77 published whole rock nd isotopic and 29 7 new and 22 published average zircon hf isotopic data shows large variations of whole rock end t values from 22 1 to 1 5 and the correspondingly nd model ages tdm nd from 2 83 to 0 79 ga and zircon ehf t values from 26 3 to 0 1 and two stage hf model ages tdm2 hf from 2 86 to 0 96 ga three regions of variations have been identified from north to south a end t values range from 22 1 to 10 9 with tdm nd of 2 82 1 47 ga and ehf t values 26 3 to 13 5 with tdm2 hf 2 86 2 04 ga b end t values 13 9 to 1 5 with tdm nd 2 02 0 79 ga and ehf t values 16 2 to 0 1 with tdm2 hf 1 96 0 96 ga and c end t values 6 3 to 4 5 with tdm nd 1 28 1 12 ga and ehf t values 1 0 to 0 3 with tdm2 hf 1 25 1 22 ga respectively the three regions approximately correspond to the three different terranes the southern margin of the north china block ncb the north qinling belt nqb and the south qinling belt sqb respectively these demonstrate that the granitoids in the different terranes have distinct sources and their sources change from old to more juvenile from the north southern margin of the ncb to the south sqb these also reveal the distinct basements for the terranes in late mesozoic the southern margin of the ncb contains widespread neoarchaean to paleoproterozoic basement the nqb comprises archaean to neoproterozic basement and the sqb mesoproterozic to neoproterozic basement all these suggest that the three terranes underwent different tectonic evolution and the continental crust of the east qinling were mainly formed during archaean to neoproterozic different from a typical accretion orogen the old sources of the granitoids and basements of the terranes constrain the distribution scale and number of the mo mineralization and deposits mo mineralization is closely related to the small granitic bodies with old continental component sources and mo deposits are mainly hosted by the terranes with oldest basement the scale and number of the mo mineralization and deposits decreased from the southern margin of the ncb to sqb

the first second of volcanic eruptions from the erebus volcano lava lake antarctica energies pressures seismology and infrasound 1 we describe a multiparameter experiment at erebus volcano antarctica employing doppler radar video acoustic and seismic observations to estimate the detailed energy budget of large up to 40 m diameter bubble bursts from a persistent phonolite lava lake these explosions are readily studied from the crater rim at ranges of less than 500 m and present an ideal opportunity to constrain the dynamics and mechanism of magmatic bubble bursts that can drive strombolian and hawaiian eruptions we estimate the energy budget of the first second of a typical erebus explosion as a function of time and energy type we constrain gas pressures and forces using an analytic model for the expansion of a gas bubble above a conduit that incorporates conduit geometry and magma and gas parameters the model consistent with video and radar observations invokes a spherical bulging surface with a base diameter equal to that of the lava lake the model has no ad hoc free parameters and geometrical calculations predict zenith height velocity and acceleration during shell expansion during explosions the energy contained in hot overpressured gas bubbles is freed and partitioned into other energy types where by far the greatest nonthermal energy component is the kinetic and gravitational potential energy of the accelerated magma shell 10 9 j seismic source energy created by explosions is estimated from radar measurements and is consistent with source energy determined from seismic observations for the generation of the infrasonic signal a dual mechanism incorporating a terminally disrupted slug is proposed which clarifies previous models and provides good fits to observed infrasonic pressures a new and straightforward method is presented for determining gas volumes from slug explosions at volcanoes from remote infrasound recordings

laterally heterogeneous scattering explains lg blockage in the pyrenees 1 we present an algorithm to simulate the propagation of seismic energy in a model that contains both deterministic structure and statistically described small scale heterogeneities with this algorithm we study the propagation of seismic waves through the pyrenees using a series of seismograms the variations of the attenuation properties of the crust along the pyrenean axis are illustrated the western pyrenees exhibit strong attenuation of crustal phases a phenomenon known as lg blockage previous studies demonstrated that the large scale velocity structure of the western pyrenees cannot explain the strong lateral variations of attenuation along the pyrenean range in this paper we present a model that is able to explain the principal observations we propose that the lg blockage is caused by a perturbed region of the crust with scattering and anelastic properties that differ markedly from the surrounding crust numerical simulations of the elastic radiative transfer equation in a layered medium show that it is possible to model the observed lg blockage with increased scattering in the perturbed body in order to explore the relative importance of scattering and attenuation we performed an inversion of the data using a genetic algorithm our study reveals that stronger intrinsic attenuation in the perturbed body improves the fit to the data but that intrinsic attenuation alone cannot explain the observations our work puts forward the role of multiple scattering in the attenuation mechanism of crustal phases

geochemical evidence for origin of magma mixing for the triassic monzonitic granite and its enclaves at mishuling in the qinling orogen central china abstract whole rock major trace elements and sr nd isotopes mineral chemistry and zircon u pb and lu hf isotope compositions are reported for i type monzogranite and its mafic microgranular enclaves from the triassic mishuling granite in the qinling orogen central china zircon u pb dating yields an age of 212 5 ma for the mafic enclaves and 213 3 ma for the host monzogranite whole rock major trace elements sr nd isotopes and zircon hf isotopes preclude simple crystal fractionation or restite unmixing for a genetic link between the enclaves and the monzogranite mixing between mafic and felsic magmas is compatible with the geochemical data the monzogranite displays an enriched lile and lree pattern high 87sr 86sr i ratios of 0 7068 to 0 7071 and low end t values of 9 2 to 5 7 suggesting a crustal origin its high mg 47 6 to 50 7 cr and ni contents clearly indicate its derivation from mafic refractory lower crust most of the zircons in the monzogranite have negative ehf t values of 12 5 to 0 1 and two stage hf model ages of 1049 to 1683 ma and some zircons in the host monzogranite have positive ehf t values of 0 3 to 4 8 and single stage hf model ages of 665 to 845 ma this indicates that the monzogranite is most likely to be formed by partial melting of a proterozoic lower continental crust the mafic enclaves have fine grained igneous textures and contain abundant acicular apatite that indicates rapid cooling they also have a high 87sr 86sr i ratio of 0 7071 and a low end t value of 6 5 suggesting a derivation from ancient subcontinental lithosphere mantle sclm zircons in the mafic microgranular enclaves have ehf t values of 1 6 to 5 1 and single stage hf model ages of 700 to 910 ma suggesting a possible origin from partial melting of the neoproterozoic sclm that may form in association with contemporaneously supercontinental rifting in the south china block therefore the mishuling i type granite may form by anatexis of meso neoproterozoic lower crust with mixing between the mafic and felsic magmas in particular we interpret the positive ehf t values as being derived from reworking of neoproterozoic juvenile lithosphere in response to tectonic extension in the late stage of the triassic continental collision between the south china block and the north china block

time lapse monitoring of seismic velocity associated with 2011 shinmoe dake eruption using seismic interferometry an extended kalman filter approach seismic interferometry is a powerful tool to monitor the seismic velocity change associated with volcanic eruptions for the monitoring changes in seismic velocity with environmental origins such

origin of late mesozoic granitoids in the newly discovered zha shan porphyry cu district south qinling central china and implications for regional metallogeny abstract the newly discovered porphyry cu deposits in the south qinling belt sqb have not been well researched as compared with the large porphyry mo province in the southern north china block s ncb and the origin of granitoids associated with porphyry cu mineralization in the zha shan district sqb is poorly constrained here we present detailed zircon u pb geochronological whole rock elemental and sr nd isotopic data for important late mesozoic granitoid stocks associated with porphyry cu deposits in the zha shan district these data are used to constrain the age and the source of magmas that formed these granitoids and implication of regional metallogeny the new zircon la icpms u pb ages presented here indicate that the granitoids related to porphyry cu system at chigou beishagou shuangyuangou and yuanjiagou developed at 148 144 ma 144 ma 145 144 ma and 146 ma respectively these rocks are high k calc alkaline i type granitoids which are enriched in large ion lithophile elements e g th u and pb and light rare earth elements are depleted in nb ta and ti characterizing by wide variations in initial end t 3 8 to 9 5 and moderate radiogenic sr isotopes 87 sr 86 sr i 0 7046 to 0 7093 these features indicate that the magmas that formed the granitoids related to porphyry cu system in the zha shan district formed as a result of variable degrees of mixing between crustal and metasomatic lithospheric mantle the new zircon la icpms u pb ages in this study combined with previous published data suggest that regional scale late jurassic to early cretaceous granitoid stocks and associated porphyry cu and mo systems in both the s ncb and sqb formed almost contemporaneously with 147 139 ma porphyry mo deposits in the s ncb and 148 145 ma porphyry cu deposits in the sqb the cu related intrusions contained a greater contribution of lithospheric mantle component than the mo related intrusions in the east qinling orogeny

petrogenesis of early cretaceous dioritic dikes in the shanyang zhashui area south qinling central china evidence for partial melting of thickened lower continental crust abstract the dioritic dikes distributed in the shanyang zhashui area of the south qinling region play an important role in understanding the deep magmatic processes and tectonic evolution during the orogenic process the zircon u pb ages of the dioritic dikes indicate that they were emplaced at 144 ma and therefore postdate the dikes that formed in the intracontinental orogenic background after the continental collision between the north china block ncb and the south china block scb the dioritic dikes have sio2 contents of 56 86 64 93 wt k2o contents of 1 65 3 21 wt low mgo 1 50 2 66 wt y 14 4 25 5 ppm and heavy rare earth element hree contents low mg values 39 9 49 high sr contents 528 4833 ppm and high sr y ratios 32 8 189 they exhibit highly fractionated ree and flat hree patterns strong enrichment in large ion lithophile elements liles e g rb ba and u and depletion in high field strength elements hfses e g nb as well as positive sr and negative ti anomalies furthermore these dioritic dikes exhibit 87sr 86sr i ratios ranging from 0 7048 to 0 7083 end t values ranging from 3 3 to 1 4 and ehf t values ranging from 4 1 to 1 6 the geochemical patterns of the dioritic dikes indicate that they possess adakitic characteristics moreover the low mgo contents mg values ni contents th ce ratios and sr nd hf isotopic features all indicate that these dioritic dikes were generated by the partial melting of thickened mafic lower crust the high la yb and sr y ratios low y and yb contents absence of significant eu anomalies flat hree patterns and low nb ta ratios of these rocks suggest that the adakitic melts were derived from the melting of garnet bearing amphibolite the geochronologic elemental and isotopic evidence suggests that the dioritic dikes may have formed in a locally extensional environment within an overall n s compressional setting or during the transition from compressional to extensional environments in the early cretaceous this process resulted in the upwelling of the asthenospheric or lithospheric mantle causing partial melting of the mafic lower crust and forming the adakitic dioritic melts

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

near seafloor magnetics reveal tectonic rotation and deep structure at the tag trans atlantic geotraverse hydrothermal site mid atlantic ridge 26 n we take advantage of geological constraints from ocean drilling program drill holes and high resolution bathymetry to revisit the near seafloor magnetic anomaly at the trans atlantic geotraverse hydrothermal site mid atlantic ridge 26 n the dipolar anomaly associated with the site is better reduced to the pole if the magnetization vector is tilted by 34 an observation suggesting that the strongly faulted basalt surrounding the site has been rotated by 53 along an axis parallel to the mid atlantic ridge as a probable consequence of the detachment tectonics inferred in this area the faults and the deeper detachment focus and guide the hot ascending hydrothermal fluid magnetic modeling shows that the nonmagnetic stockwork zone is a significant contributor to the observed negative anomaly the rest being accounted for by a deeper source probably related to thermal demagnetization of an ascending hydrothermal pipe beneath the active part of the site

what causes low magnetization at basalt hosted hydrothermal sites insights from inactive site krasnov mar 16 38 n high resolution magnetic surveys acquired near the seafloor show that active basalt hosted hydrothermal sites are associated with zones of lower magnetization this observation may reflect the thermal demagnetization of a hot hydrothermal zone the alteration of basalt affected by hydrothermal circulation and or the presence of thick nonmagnetic hydrothermal deposits in order to discriminate among these inferences we acquired vector magnetic data 50 m above inactive hydrothermal site krasnov using the remotely operated vehicle rov victor this deep hydrothermal site located 7 km east of the mid atlantic ridge mar axis at 16 38 n is dissected by major normal faults and shows no evidence of recent hydrothermal activity it is therefore a perfect target for investigating the magnetic signature of an inactive basalt hosted hydrothermal site krasnov exhibits a strong negative magnetic anomaly which implies that the lower magnetization observed at basalt hosted hydrothermal sites is not a transient effect associated with hydrothermal activity but remains after activity ceases thermal demagnetization plays only a secondary role if any in the observed magnetic low forward models suggest that both the nonmagnetic hydrothermal deposits and an altered zone of demagnetized basalt are required to account for the observed magnetic low the permanence of this magnetic signature makes it a useful tool to explore midocean ridges and detect inactive hydrothermal sites

empirical synthesis of time asymmetrical green functions from the correlation of coda waves we demonstrate the existence of long range field correlations in the seismic coda of regional records in alaska the cross correlations between the different components of coda records at two points are measured for a set of distant earthquakes remarkably while individual correlations have a random character the correlations averaged over source and time exhibit deterministic arrivals that obey the same symmetry rules as the green tensor between the two points in addition the arrival times of these waves coincide with propagating surface waves between the two stations thus we propose to identify the averaged correlation signals with the surface wave part of the green tensor we observe the causal and anticausal parts of the green function however we find experimentally that amplitudes at positive and negative times are not equal we explain this observation by the long lasting anisotropy of the diffuse field we show that the flux of energy coming from the source can still dominate the late coda and result in nonsymmetric cross correlations when the distribution of earthquakes is not isotropic around the stations the extraction of green functions from coda waves allows new types of measurements with seismic waves along paths between stations that could not be obtained with the waves produced by earthquakes

the magnetic signature of ultramafic hosted hydrothermal sites unlike basalt hosted hydrothermal sites characterized by a lack of magnetization the magnetic signature of ultramafic hosted hydrothermal sites remains poorly known despite their wide occurrence at slow spreading ridges and their strong mineral potential the first high resolution magnetic surveys of such ultramafic hosted sites achieved by deep sea submersible on four sites of the mid atlantic ridge reveal positive magnetic anomalies and therefore a strong magnetization at the largest sites this observation reflects the presence of a wide mineralized zone beneath these sites the stockwork where several chemical processes concur to create and preserve strongly magnetized magnetite beyond pointing out the importance of subsurface chemical processes in hydrothermal activity the aging of oceanic lithosphere and the ocean chemical budget our results have immediate application for detecting and characterizing economically valuable deep sea mineral deposits

evaluation of morphological indicators of magma supply and segmentation from a seismic reflection study of the east pacific rise 15 30 17 n hydrosweep bathymetry and multichannel seismic reflection data from two contrasting segments of the east pacific rise 15 30 17 n are used to assess the relationship between crustal structure morphological indicators of magma supply and ridge segmentation from stacked and migrated seismic profiles we evaluate the width and depth of the axial magma lens and the geometry of the seismically inferred extrusive crust layer 2a in contrast to other studies correlation between axial morphology and magma lens characteristics is found with a lens that on average is twice as wide and slightly shallower 100 200 m beneath the unusually shallow and broad southern segment and that shoals and broadens beneath the shallowest part of the northern segment however large local scale variations in lens depth and width are also observed that are not related to morphology we conclude that magma supply contributes to the regional characteristics of the magma lens beneath a ridge segment with large variations within a segment due to local processes of magma eruption and delivery a negative correlation between zero age extrusive layer thickness and morphology is found with a thinner extrusive layer along the southern segment in this innermost axial region above the magma lens factors such as magma pressure not magma volume in the lens may govern the thickness of extrusives that accumulate the width of the zone over which the extrusive layer is built is positively correlated with morphology with a wide zone where extrusives approximately triple in thickness characterizing the southern segment 5 8 km and a narrower zone 2 5 km of minor thickening less than double along the northern segment and toward ridge segment ends this relationship could reflect changes in lava flow characteristics in these areas of contrasting magma supply we find a close correspondence between discontinuities in the narrow axial summit trough and changes in magma lens presence and geometry indicating a genetic link between the finest scale tectonic segmentation of the ridge and segmentation of the magma lens in several locations discrete magma lenses at different levels within the crust offset by several hundred meters are imaged beneath adjacent fourth order ridge segments

detailed analysis of near tectonic features along the east pacific rise at 16 n near the mathematician hot spot spreading processes at the axes of fast spreading ridges are mainly controlled by magmatic activity whereas tectonic activity dominates further away from the axis high resolution near bottom bathymetry data photographs videos and human observations from submersible surveys are used to develop a detailed tectonic analysis of the 16 n segment of the east pacific rise epr these data are used to evaluate how a highly magmatic segment close to a hot spot affects the nucleation and evolution of faulting patterns and impacts the evaluation of tectonic strain within 2 km of the spreading axis our study shows that 1 the growth of tectonic features differs in response to dike intrusion and tectonic extension 2 the initiation of brittle extension is strongly controlled by the location of the axial magma lens and the development of layer 2a and 3 the high magmatic budget and the off axis magma lens in the west part of the plateau do not significantly impact the initiation of brittle extension along the central portion of the 16 n segment within the axial summit region more than 2 of plate separation at 16 n on the epr is accommodated by brittle extension as is observed at other epr segments the interaction of the mathematician hot spot with this epr segment has no significant influence on the initiation of the tectonic deformation but it does reduce the development of the brittle deformation

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

fault zone imaging from correlations of aftershock waveforms we image an active fault zone environment using cross correlations of 154 15 s long 1992 landers earthquake aftershock seismograms recorded along a line array a group velocity and phase velocity dispersion analysis of the reconstructed rayleigh waves and love waves yields shear wave velocity images of the top 100 m along the 800 m long array that consists of 22 three component stations estimates of the position width and seismic velocity of a low velocity zone are in good agreement with the findings of previous fault zone trapped waves studies our preferred solution indicates the zone is offset from the surface break to the east 100 200 m wide and characterized by a 30 velocity reduction imaging in the 2 6 hz range resolves further a high velocity body of similar width to the west of the fault break symmetry and shape of zero lag correlation fields or focal spots indicate a frequency and position dependent wavefield composition at frequencies greater than 4 hz surface wave propagation dominates whereas at lower frequencies the correlation field also exhibits signatures of body waves that likely interact with the high velocity zone the polarization and late arrival times of coherent wavefronts observed above the low velocity zone indicate reflections associated with velocity contrasts in the fault zone environment our study highlights the utility of the high frequency correlation wavefield obtained from records of local and regional seismicity the approach does not depend on knowledge of earthquake source parameters which suggests the method can return images quickly during aftershock campaigns to guide network updates for optimal coverage of interesting geological features

reflection images from ambient seismic noise one application of seismic interferometry is to retrieve the impulse response green s function from crosscorrelation of ambient seismic noise various researchers show results for retrieving the surface wave part of the green s function however reflection retrieval has proven more challenging we crosscorrelate ambient seismic noise recorded along eight parallel lines in the sirte basin east of ajdabeya libya to obtain shot gathers that contain reflections we take advantage of geophone groups to suppress part of the undesired surface wave noise and apply frequency wavenumber filtering before crosscorrelation to suppress surface waves further after comparing the retrieved results with data from an active seismic exploration survey along the same lines we use the retrieved reflection data to obtain a migrated reflection image of the subsurface

a record of eruption and intrusion at a fast spreading ridge axis axial summit trough of the east pacific rise at 9 10 n 1 high resolution side scan sonar near bottom multibeam bathymetry and deep sea photo and bathymetry traverses are used to map the axial summit trough ast at the east pacific rise between 9 and 10 n we define three ridge axis morphologic types no ast narrow ast and wide ast which characterize distinct ridge crest domains spanning tens of kilometers along strike near bottom observations modeling of deformation above intruding dikes and comparisons to the geologic and geophysical structure of the ridge crest are used to develop a revised model of ast genesis and evolution this model helps constrain the record of intrusive and extrusive magmatism and styles of lava deposition along the ridge crest at time scales from hundreds to tens of thousands of years the grabens in the narrow ast domain 9 43 53 n are consistent with deformation above the most recent 10 diking events beneath the ridge crest frequent high effusion rate extrusive volcanism in this domain several eruptions every 100 years overprints near axis deformation and maintains a consistent ast width the most recent eruption at the ridge crest occurred in this area and did not significantly modify the physical characteristics of the ast the grabens in the wide ast domain 9 23 43 n originated with similar dimensions to the narrow ast spreading driven primarily by the intrusion of shallow dikes within a narrow axial zone causes the initial graben bounding faults to migrate away from the axis infrequent extrusive volcanism several eruptions every 1000 years fills a portion of the subsidence that accumulates over time but does not significantly modify the width of the ast outside of these domains lower effusion rate constructional volcanism without efficient drain back fills and erases the signature of the ast the relative frequency of intrusive versus extrusive magmatic events controls the morphology of the ridge crest and appears to remain constant over millennial time scales within the domains we have identified however over longer time scales 10 25 ka domain specific intrusive to extrusive ratios do not appear to be fixed in space resulting in a fairly consistent volcanic accretion over the length scale of the second order ridge segment between 9 n and 10 n

mesozoic intracontinental orogeny in the qinling mountains central china abstract the qinling orogenic belt has been well documented that it was formed by multiple steps of convergence and subsequent collision between the north china and south china blocks during paleozoic and late triassic times following the collision in late triassic times the whole range evolved into an intracontinental tectonic process the geological geophysical and geochronological data suggest that the intracontinental tectonic evolutionary history of the qinling orogenic belt allow deduce three stages including strike slip faulting during early jurrassic n s compressional deformation during late jurassic to early cretaceous and orogenic collapse during late cretaceous to paleogene the strike slip faulting and the infills in early jurassic along some major boundary faults show flower structures and pull apart basins related to the continued compression after late triassic collision between the south qinling belt and the south china block along the mianlue suture late jurassic to early cretaceous large scale of n s compression and overthrusting progressed outwards from inner of qinling orogen to the north china block and south china block due to the renewed southward intracontinental subduction of the north china block beneath the qinling orogenic belt and continuously northward subduction of the south china block respectively after the late jurassic early cretaceous compression and denudation the qinling orogenic belt evolved into late cretaceous to paleogene orogen collapse and depression and formed many large fault basins along the major faults

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

reconstructing green s function by correlation of the coda of the correlation c3 of ambient seismic noise analysis of long range correlation of the microseisms has been shown to provide reliable measurements of surface wave speeds that can be used for seismic imaging and monitoring in the case of an even distribution of noise sources it has been theoretically demonstrated that the correlation is the exact green s function including all types of waves this method is limited in its application by the actual source distribution in practice the azimuthal distribution of energy flux of the noise is dominated by some particular directions resulting in a clear azimuthal dependence of the quality of the reconstruction of rayleigh waves with a poor reconstruction in some azimuths to solve this problem we use noise correlations measured on the entire network we consider two stations a and b for which the rayleigh waves could not be discerned in the in the correlation of continuous records of ambient noise we computed all correlations between the station a respectively b and all the 150 other stations located at regional distances theoretically these virtual seismograms contain direct waves and coda although they are clearly contaminated by the influence of the imperfect ambient noise field and most are inadequate for direct analysis we used these correlation functions as equivalents to seismograms produced by sources acting at the 150 stations locations and recorded in a respectively b we select time windows in those virtual seismograms that correspond to coda and compute correlations between them this metacorrelation is found to exhibit the surface wave part of the green s function that was not visible in the raw correlation of ambient noise we illustrate the legitimacy of the reconstruction by comparison with raw noise correlations this procedure can be used to assess seismic velocity between stations even in presence of a directive and poorly oriented ambient noise the result shows that in spite of the small signal to noise ratios often seen in correlations of ambient noise especially at large lag time corresponding to coda their codas are better equipartitioned than was the ambient noise upon which they were based they are therefore presumably multiply scattered and contain information on both direct surface waves and also on more complex travel paths

morb peridotite sea water interaction experimental constraints on the behaviour of trace elements 87sr 86sr and 143nd 144nd ratios the geochemical changes associated with the experimental serpentinization of peridotite are due to reaction with sea water and to the growth of new mineral phases since the rare earth elements ree and sr are primarily ensconced in clinopyroxene it is the unreactive nature of this phase in the presence of sea water that determines the ree content the 875r s6sr and 43nd 14and ratio of the experimentally produced serpentinites serpentinites after lherzolite and dunite have chondrite normalized ree abundance patterns similar to the initial peridotite indicating that the light ree lree are not selectively mobilized by peridotite sea water interaction at 300 however the serpentinites produced as a result of harzburgite sea water experiments show an increase in lree content despite the sporadic behaviour of the lree the 43nd nd ratios in experimentally produced serpentinites after lherzolite and harzburgite are identical to primary clinopyroxene in the unaltered peridotite in contrast a marked change in the strontium isotopic composition of the peridotites occurs during experimental serpentinization due to the growth of hydrous and ca rich phases which facilitates the uptake of sea water sr whereas harzburgite and dunite alter to produce serpentinites with high sr contents and 87sr 86sr 0 709 lherzolites tend to alter to serpentinites with 875r 86sr 0 709 this behaviour invalidates the use of rb sr data in understanding the origin of oceanic and ophiolitic peridotites but the relative immobility of the light ree in clinopyroxene bearing peridotites and the low ree content of sea water encourages the careful use of the ree and neodymium isotopes as a petrogenetic indicator for elucidating the origin of serpentinized abyssal peridotites serpentinized oceanic peridotites were first reported from atlantic ocean islands by darwin 1845 and later studied by tilley 1947 hess 1954 and melson et al 1972 hess 1954 believed these rocks to be of mantle origin related to basalt genesis more recently serpentinites have been recovered in dredge hauls from the large fracture zones of the atlantic indian and antarctic oceans extending from mid ocean ridges e g dick et al 1984 whereas the provenance of exposed oceanic peridotites is believed to be the upper mantle serpentinized peridotites found within fragments of ancient oceanic lithosphere ophiolites may represent a metamorphic i e mantle or igneous protolith hess 1955 gass 1968 gass smewing 1973 gass et al 1984 the ubiquitous serpentinization of peridotites on the sea floor is due to interaction with sea water at temperatures that can reach

marine magnetic anomalies as recorders of geomagnetic intensity variations in addition to providing a robust record of past geomagnetic polarity reversals marine magnetic anomalies often show shorter wavelength variations which may provide information on geomagnetic intensity variations within intervals of constant polarity to evaluate this possible geomagnetic signal we compare sea surface profiles of the central anomaly with synthetic profiles based on brunhes age 0 0 78 ma paleointensity records derived from deep sea sediments the similarity of the synthetic profiles and observed profiles from the ultra fast spreading southern east pacific rise suggests that geomagnetic intensity variations play an important role in the magnetization of the oceanic crust this interpretation is further supported by systematic variations in the pattern of the central anomaly at slower spreading ridges which are entirely consistent with a progressively smoother record of the sediment derived paleointensity if the sedimentary records as calibrated to available absolute paleointensity data accurately record variations in dipole intensity over the brunhes it follows that much of the brunhes was characterized by geomagnetic intensities lower than either the mean dipole moment for the past 10 ka or the average for the period from 0 05 to 5 0 ma furthermore the sediment paleointensity records reflect the significant increase in geomagnetic intensity from a low of 2 1022 am2 near 40 ka to a peak value 11 1022 am2 at 3 ka that has been well documented from absolute paleointensity determinations we suggest that geomagnetic intensity variations may be the most important cause of the rapid changes in the source layer magnetization near the ridge crest and the resultant central anomaly magnetic high

geophysical study of the east pacific rise 15 n 17 n an unusually robust segment bathymetric side scan sonar magnetic and gravity data from the east pacific rise epr between 15 and 17 n are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment the axial ridge between the orozco transform fault 15 30 n and the 16 20 n overlapping spreading center osc has an average elevation of 2300 m which is 300 m shallower than typical epr depths and its cross sectional area is double the average value for the northern epr the total opening rate is 86 km myr but the inflated segment appears to have spread faster to the east by more than 20 since 0 78 ma the orientation of magnetic isochrons and lineaments in the side scan sonar indicates a 3 counterclockwise rotation of the spreading direction since 1 8 ma c2 and reflects a change in the pacific cocos plate motion the side scan lineaments also show that the percentage of inward facing faults 83 and the spacing between faults 1 5 km are consistent with the spreading rate dependence shown by carbotte and macdonald 1994 however the mean fault length 4 8 km is 1 5 km shorter than expected for the spreading rate and suggests that extensive off axis volcanism has draped the faults gravity analysis shows that the inflated segment has a 12 mgal bull s eye shaped low in residual mantle bouguer anomaly we offer several possible end member models for the anomaly including a prism of 10 partial melt in the mantle and lower crust or a crustal thickness anomaly of 2 25 km kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0 8 ma and 0 3 ma and have reshaped the plate boundary geometry and inflated the segment

seismic interferometry of teleseicmic s wave coda for retrieval of body waves an application to the philippine sea slab underneath the japanese islands summary the reconstruction of surface waves from the cross correlation of random wavefields has recently been extensively inspected by theoretical and experimental approaches however the reconstruction of body waves has not been extensively studied in this study we present a method for extracting body waves that is direct p and s waves and reflected waves from the philippine sea slab we use the cross correlation of the wavefield generated by the teleseismic s waves observed by hi net tiltmeters with a passband of 0 07 0 5 hz to enhance the contribution of the s coda we reduced the source time function and the deterministic phases ss scs and ss and the surface wave in s coda the records of the processed s coda observed at every station pair are cross correlated for each teleseismic event the cross correlation functions ccfs of different earthquakes are stacked to retrieve wavefields propagating between the station pairs as a result direct p s and the reflected waves could be extracted when the stacked ccfs are aligned as a function of distance of separation between two stations the gradients of the traveltime curves for the direct p and s waves are approximately 5 7 and 3k m s 1 respectively these velocities correspond to the seismic velocities of the crust or the p wave velocity of the uppermost mantle in order to enhance the reflected waves we searched for the reflection points by assuming that the later phases in the ccfs are ss reflections and map the amplitudes onto the depth sections as a result the negative phases dipping to the north can be traced right below the region of hypocentre distribution these negative phases probably correspond to the oceanic moho within the philippine sea slab we also show that the oceanic moho can also be traced by assuming pp reflections these results indicate that the ccfs plausibly contain information regarding both p and s waves propagating between the two receivers and are capable of detecting reflected phases in addition to the direct waves

dyking at epr 16 n hypermagmatic ridge segment insights from near seafloor magnetics abstract high resolution near seafloor magnetic data have been acquired over the 16 n hypermagmatic segment of the east pacific rise using an autonomous underwater vehicle this survey proves to be ideal to test the relative efficiency of various inversion methods applied to data acquired at a more or less constant altitude above the seafloor unlike other methods a recently published bayesian inversion preserves the short wavelengths and allows for the resolution of a high resolution reduced to the pole magnetic anomaly this anomaly unveils the presence of several laterally adjacent dykes associated with individually separated axial summit troughs the observation of such anomalies and therefore of shallow dykes confirms the hypermagmatic character of the segment in a location where complex magma chambers have been imaged in seismic reflection studies variable intensity of the magnetic anomalies reflects the depth of the dyke swarms and ultimately the timing and style of eruptive events helping to constrain the spreading axis evolution

direct inversion of potential fields from an uneven track with application to the mid atlantic ridge current methods of potential field inversion require measurements to be reduced onto a level plane resulting in a loss of resolution this is especially true for draped surveys in areas of extreme topography we examine a method that bypasses this limiting step by making use of an approximately equivalent geometry from which data are directly inverted corrections are applied to an initial solution to account for errors generated by the transformation of geometries a numerical model and an example data set are used to explore this method s benefits and drawbacks in relation to the conventional approach of data reduction onto a level plane prior to inversion we show that this direct inversion method is particularly better than the conventional approach at resolving fine scale features such as the narrow zone of crustal emplacement at the mid atlantic ridge and adjacent areas of tectonic disruption and magnetic source alteration

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined