a self consistent model of melting magma migration and buoyancy driven circulation beneath mid ocean ridges numerical modeling and analysis are used to investigate the processes leading to the eruption of mantle derived magma at mid ocean ridges our model includes the following effects melting due to decompression magma migration by percolation and circulation of the mantle driven by both the oceanic plates and the distribution of buoyancy beneath the ridge the distribution of buoyancy is due to both the low density of the liquid and the difference in the density of the residual solids relative to unmelted mantle material the calculation of densities is based on a simple petrological model in which garnet spinel lherzolite melts to form a basaltic liquid and a harzburgite residue we find that the system spontaneously evolves to a state in which a rapid upwelling beneath the ridge axis faster than the plate velocity is confined laterally by stably stratified residual material beneath the newly formed plates this effect is exaggerated if a modest decrease in the shear viscosity of the solid upon melting is included our results provide a simple explanation for the narrowness of the zone of crustal formation at mid ocean ridges the model also predicts a transition from steady state to episodic crustal formation as the spreading velocity is reduced perhaps giving rise to along axis variations in the character of seafloor spreading the narrow rapid upwelling gives rise to substantial porosities at depths that are a large fraction of the depth to the solidus this may allow the liquid at depth to segregate into macroscopic channels which would account for the consensus from experimental petrology that the liquids parental to morb are derived from well below the base of the crust

seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

kirchhoff migration using eikonal equation traveltimes the use of ray shooting followed by interpolation of traveltimes onto a regular grid is a popular and robust method for computing diffraction curves for kirchhoff migration an alternative to this method is to compute the traveltimes by directly solving the eikonal equation on a regular grid without computing raypaths solving the eikonal equation on such a grid simplifies the problem of interpolating times onto the migration grid but this method is not well defined at points where two different branches of the traveltime field meet also computational and data storage issues that are relatively unimportant for performance in two dimensions limit the applicability of both schemes in three dimensions a new implementation of a gridded eikonal equation solver has been designed to address these problems a 2 d version of this algorithm is tested by using it to generate traveltimes to migrate the marmousi synthetic data set using the exact velocity model the results are compared with three other images an f x migration a standard for comparison a kirchhoff migration using ray tracing and a kirchhoff migration using traveltimes generated by a commonly used eikonal equation solver the f x migrated image shows the imaging objective more clearly than any of themore kirchhoff migrations and the authors advance a heuristic reason to explain this fact of the kirchhoff migrations the one using ray tracing produces the best image and the other two are of comparable quality less

off axis crustal thickness across and along the east pacific rise within the melt area wide angle seismic data along the mantle electromagnetic and tomography melt arrays show that the thickness of 0 5 to 1 5 million year old crust of the nazca plate is not resolvably different from that of the pacific plate despite an asymmetry in depth and gravity across this portion of the east pacific rise crustal thickness on similarly aged crust on the nazca plate near a magmatically robust part of the east pacific rise at 17 degrees15 s is slightly thinner 5 1 to 5 7 kilometers than at the 15 degrees55 s overlapping spreading center 5 8 to 6 3 kilometers this small north south off axis crustal thickness difference may reflect along axis temporal variations in magma supply whereas the across axis asymmetry in depth and gravity must be caused by density variations in the underlying mantle

imaging the transition between the region of mantle melt generation and the crustal magma chamber beneath the southern east pacific rise with short period love waves 1 beneath the southern east pacific rise resides a crustal magma chamber confined to the immediate vicinity of the rise axis yet the mantle melt production region is broad and asymmetric short period 4 17 s love waves propagating along the rise provide a means of probing the transition from the asymmetric melt production region to the rise centered crustal magma chamber low velocities beneath the rise act as a waveguide for love wave energy we model this complex wave propagation with a gaussian beam representation as part of our nonlinear inversion for velocity structure our results show that the asymmetry exists at the crust mantle interface or moho and increases downward the lowest velocities are located beneath the rise at the moho beneath the rise there is an asymmetric gap in the high velocity lid at the top of the mantle and there are lower asthenospheric velocities beneath the pacific plate than the nazca plate the transition from the broad upwelling region to the narrow crustal magmatic system occurs over a 15 km depth interval at the top of the mantle our observations indicate higher temperatures and greater melt production beneath the pacific plate east of the rise melting appears to abruptly shut off at shallow levels perhaps due to a component of downward mantle flow we suggest that melt percolates upward increasing in concentration until reaching a permeability barrier beneath the lithosphere it then moves up along the lithosphere to accumulate at the moho beneath the rise west of the rise higher temperatures and upward percolation of melt delay growth of the lithosphere

geochemistry of surface sediments in the southwestern east japan sea abstract we analyzed 77 surface sediment samples collected in the southwestern east japan sea from the korea strait through the ulleung basin and the korea plateau for grain size calcium carbonate organic carbon and major na mg al fe k ca and ti and trace elements p mn sr li sc v cr co ni zn cu and pb the chemical composition of the surface sediments was found to be highly variable spatially cluster analysis of surface sediment chemical compositions indicated five major geochemical sedimentary environments basin lower slope coast and upper slope inner shelf and outer shelf continental shelf sediments were rich in shell fragments and had relict and coarse grained characteristics recent fine grained sediments were only distributed in coastal slope and basin areas concentrations of al k ca ti cr and sc were highest in the coastal and upper slope areas and decreased with water depth elemental ratios using major and trace elements indicated that coastal and upper slope detrital sediments were mixtures of sediments derived from the changjiang yangtze and nakdong rivers although the concentrations of organic carbon p mn v co ni cu and pb increased with water depth their distribution patterns indicated authigenic v cu and pb and diagenetic fe p mn co and ni origins the distribution pattern with water depth suggested that the chemical composition of surface sediment was determined by sedimentologic and geochemical processes such as the supply of detrital and biogenic materials and authigenic and post depositional diagenetic processes in sediments

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

crosshole seismic tomography many tomographic interpretations of crosshole seismic traveltimes have approximated the raypaths with straight lines connecting the source and receiver this approximation is valid where the velocity does not vary greatly but in many regions of interest velocity variations of 10 20 percent or more are observed causing significant ray curvature other work has taken this nonlinear effect into account but there do not appear to be many cases of demonstrated success in its application to the crosshole seismic problem we present here an iterative inversion scheme based on two dimensional ray tracing and its successful application to field data the interpretation method iteratively ray traces and then updates the velocity model within each iteration the differences between the data and the current model traveltimes obtained by ray tracing are related to the unknown velocity perturbations through a system of linear equations a damped least squares method solves for the velocity perturbations which updat

can we image complex structures with first arrival traveltime we experienced difficulties when attempting to perform seismic imaging in complex velocity fields using prestack kirchhoff depth migration in conjunction with traveltimes computed by finite differencing the eikonal equation the problem arose not because of intrinsic limitations of kirchhoff migration but rather from the failure of finite differencing to compute traveltimes representative of the energetic part of the wavefield further analysis showed that the first arrival is most often associated with a marginally energetic event wherever subsequent arrivals exist the consequence is that energetic seismic events are imaged with a kinematically incorrect operator and turn out mispositioned at depth we therefore recommend that first arrival traveltime fields such as those computed by finite differencing the eikonal equation be used in kirchhoff migration only with great care when the velocity field hosts multiple transmitted arrivals such a situation is typically met where geological structure creat

multiarrival wavefront tracking and its applications 1 multivalued traveltimes have traditionally not been used in seismic imaging with only a handful of notable exceptions in the field of exploration geophysics for studies at local and regional scales e g local earthquake teleseismic tomography the focus has largely been on first arrivals with numerous ray and grid based schemes developed for their calculation however later arrivals often contribute to the length and shape of a recorded wave train particularly in regions of complex geology these arrivals are likely to contain additional information about seismic structure as their two point path differs from that of the first arrival in particular they are more amenable to sampling regions of lower velocity in this work the wavefront construction principle is used as the basis of a new scheme for computing multivalued traveltimes that arise from smooth variations in both velocity structure and interface geometry the idea is to represent the wavefront as a set of points in reduced phase space and use local ray tracing and interpolation to advance the wavefront in a series of time steps the scheme is robust in the presence of strong velocity heterogeneity and interface curvature with phases comprising multiple reflections refractions and triplications successfully tracked outside the field of exploration seismology wavefront construction techniques are rarely used yet they hold great potential for addressing problems in other areas of seismology this paper demonstrates the viability of the new wavefront construction scheme by applying it to a range of scenarios including multiarrival body and surface wave tomography teleseismic receiver function prediction using gaussian beams and the tracking of global phases such as pcp

upward flow of north pacific deep water in the northern south china sea as deduced from the occurrence of drift sediments 1 reflection seismic profiles show that drift sediments occur in the depth range of 1200 2800 m on the continental margin off southeast china these drift deposits were generated by upward flow near the dongsha islands of the north pacific deep water npdw which enters the south china sea via the bashi channel sill depth 2500 m this flow results in deposition of slope sediments resuspended off east and south taiwan on the slope southeast of the dongsha islands at 1200 m water depth this upward flow probably terminates and the npdw mixes with the surface water sediment ages from odp site 1144 southeast of the dongsha islands show that this process started at least 1 1 m y ago indications for mass wasting in the area of upslope water flow document the simultaneous occurrence of current and gravity controlled sedimentation processes however bottom current related drift sedimentation dominates

rare earth element and mineralogic changes in holocene soil and stream sediment a case study in the wet mountains colorado u s a size fractions of soil and stream sediment developed on plutonic metamorphic or mixed plutonic metamorphic sources in the wet mountains colorado have been analyzed for ree rare earth elements and mineralogy the clay sized fractions 0 5 m and 0 5 2 m or 2 m have ree contents that are similar to more enriched than the source eu sm ratios that are similar to the source and la lu ratios that are higher than their corresponding source the 0 5 m fractions have lower ree contents than the corresponding 0 5 2 m fraction in every stream sediment for example unweathered san isabel batholith near the sampled stream sediment contains ree 511 712 ppm and eusmratio 0 17 0 19 and lalu cn 9 8 10 3 the corresponding 0 5 and 0 5 2 m fractions of sediment formed from san isabel batholith are enriched in the ree ree 1135 and 2225 ppm respec are slightly lower in eu sm ratios 0 16 and 0 14 respec and are higher in la lu ratios 12 2 and 16 3 respec than the unweathered rocks the silts have lree light ree contents and eu sm ratios very similar to the source due to similar mineralogy but silts have hree heavy ree contents higher than the source due to high zircon for example silt sized fractions in stream sediment draining the san isabel batholith contain ree 808 ppm eusmratio 0 18 and lalu cn 7 4 in contrast the sands contain lower ree contents similar to higher eu sm ratios and lower la lu ratios than the corresponding source due to depletion in heavy minerals concentrated in the ree and containing lower eu sm ratios relative to quartz and feldspar depleted in the ree and containing higher eu sm ratios for example the sand fraction from stream sediment from the san isabel batholith has ree 206 ppm eusmratio 0 17 and lalu cn 5 6 small variations of accessory minerals concentrated in the ree like allanite or sphene may cause drastic variation in ree content in the sand fraction clays and silts derived only from the plutonic rocks have higher ree and similar to lower eu sm ratios than corresponding fractions derived only from migmatites e g ree of silt fractions from migmatites 224 ppm and from plutons 375 808 ppm eu sm ratios from migmatites 0 17 and from plutons 0 11 0 18 the ree content and the eu sm ratio of clays and silts derived from migmatites are much more similar to the average of mcps mid continent platform sediment than corresponding fractions derived from the plutons mcps has ree 192 ppm eusm 0 19 lalu 8 0 the eusm ratio of sands derived from plutons and migmatite are lower to mostly higher than the mcps the ree content of sands from plutons are not necessarily higher than the mcps due to variation in heavy minerals like allanite or sphene concentrated in the ree sediments derived from mixed sources have ree contents in fine fractions that may not necessarily reflect the ree content of the nearest source for example 0 5 2 m and silt fractions developed on the san isabel batholith with a migmatite upstream have the high ree content of the batholith ree in 0 5 2 m fraction 1100 ppm silt 573 ppm while the corresponding 0 5 m fraction has the very low ree content of the migmatites ree 110 ppm these results suggest fine grained material near the source may sample a much larger area than coarser material near the source thus analysis of fine grained sediment associated with coarse grained material near the source may give a much different impression of the source than would determination of the source using classic petrographic examination of the sand or gravels

increased contribution of terrigenous supply from taiwan to the northern south china sea since 3 ma seismic profiles provide evidence that there has been strong transport by deep water bottom currents and drift deposition on the northern slope of the south china sea earlier geochemical studies suggest that the drift sediments originated primarily from taiwan however the transport process history and origin of the deep water bottom deposition in the northern south china sea on both glacial interglacial and tectonic time scales remain unclear here we show new high resolution records of clay minerals grain size and mass accumulation rate mar of terrigenous materials from ocean drilling program odp site 1144 together with trace element concentrations in siliciclastic sediments from odp site 1146 combined with other published data we find that the primary source for sediments at odp sites 1144 1148 since 3 ma is from taiwan and not from pearl river as previously thought before 3 ma however sediment source to odp sites 1146 and 1148 was mainly from the pearl river increased contribution of terrigenous supply from taiwan to the northern south china sea since similar to 3 ma may be related to the formation of the taiwan orogen and strengthening of deep water bottom current transport in the northern south china sea variations in clay mineralogy and sedimentology at odp site 1144 located on a sediment drift shows strong glacial interglacial cyclicity this suggests that bottom current deposition is highly dependent on sea level fluctuations which control the terrigenous supply to the deep sea c 2010 elsevier b v all rights reserved

a review on the provenance discrimination of sediments in the yellow sea the yellow sea has been extensively studied for the understanding of dispersal patterns and limits of sediments from neighboring countries including china and korea although sedimentological mineralogical and geochemical approaches have been tried to solve the problems especially including the identification of sediment sources in and around the yellow sea the published results are not enough for understanding them suggestions on sediment origins budgets sediment accumulation rates mineralogical and geochemical compositions are not coincident to each other and sometimes even controversial for example conflicts on the distribution patterns of smectite and calcium carbonate the provenance of the southeastern yellow sea mud and sedimentation rates in there the ratios of v al and mn al in korea and china river sediments the origin of ba and pb in sandy sediments of the northeastern yellow sea and so on various geochemical indicators from the literature for the provenance discrimination in the yellow sea are reviewed here in depth and corresponding discussions are described separately research topics for the future study also are suggested for the proper access to the understanding of the origin and dispersal patterns of the yellow sea sediments especially focusing on the geochemical approaches d 2003 elsevier science b v all rights reserved

evidence from the asymmetry of fast spreading ridges that the axial topographic high is due to extensional stresses along fast spreading mid ocean ridges such as the east pacific rise there is an axial topographic high 5 20 km wide which stands 200 400 m above the background slope caused by thermally induced seafloor subsidence there are also smaller topographic lows flanking the axial high along most of the east pacific rise from 20 s to 15 n the existence of these lows is predicted by models of the origin of the axial high one model postulates that the axial high is created by buoyant uplift from a narrow zone ofconcentrated partial melt extending tens of kilometres down into the mantle1 2 3 4 another model requires no such buoyant zone suggesting instead that the axial high is generated by dynamic extensional stresses in the lithosphere and shallow asthenosphere5 here we show that the observed asymmetry of the flanking lows can be used to distinguish between these two proposed mechanisms although either model can be adapted to match the asymmetry on individual profiles the along axis variation in degree of symmetry favours the model of dynamic extensional stresses for the origin of the axial high and its flanking lows

astronomical forcing of the east asian monsoon mirrored by the composition of pliocene south china sea sediments abstract high precision major and minor element records of pliocene core intervals from ocean drilling program odp site 1145 located in the northern part of the south china sea were generated at a 5 cm approximately 2 kyr resolution from the geochemical data distinct proxies those indicating changes in detrital matter provenance as well as those indicating productivity variations could be derived both the terrigenous input to the northern south china sea k si and variations in productivity ba al show a response to insolation forced monsoon variability while the fluvial input k si responds to changes in the summer monsoon productivity increases as documented by ba enrichments seem to reflect variations in winter monsoon intensity a stronger winter monsoon may have increased nutrient availability via dust input and or upwelling phenomena according to our geochemical proxy records summer and winter monsoons are approximately 180 out of phase this may imply forcing of the winter monsoon through southern hemisphere summer insolation maxima the k si ratio shows a linear correlation with the la90 1 0 5 northern hemisphere summer insolation record and has therefore been used to create an astronomical timescale for the investigated sediments of odp site 1145 carbonate contents and corresponding color reflectances display 41 and 100 kyr cycles we propose that these do not reflect glacial cycles but are rather the result of an interference signal of dilution by fluvial and eolian terrigenous material and carbonate production burial flux a sudden onset of gradual changes in sediment composition occurs after 3 0 ma this is also documented by an increase in sedimentation rates and may be caused by enhanced fluvial input from the pearl river due to intensified weathering rates on the asian continent this may be linked to tectonic processes around the himalayan tibetan complex and its consequences regarding land sea heating and atmospheric circulation

grain size and geochemistry of surface sediments in northwestern continental shelf of the south china sea eighty six surface sediments collected from the northwestern continental shelf of the south china sea scs were analyzed for grain size distribution calcium carbonate organic carbon and major and trace element compositions to investigate sediment provenance and factors controlling their geochemical composition sediments from the eastern continental shelf of hainan island have higher sand and lower clay content while the samples from the nearshore hainan island have higher contents of gravel and clay calcium carbonate contents in samples show a positive correlation with water depth in northwestern shelf of scs suggesting that it is related to biological factors however the nearshore sediments have higher contents of organic carbon compared to those of the outer shelf possibly suggesting that the terrigenous organic matter usually deposited in nearshore environments such as bays and estuaries compared with the upper continental crust the samples have relatively lower contents of sio2 and al2o3 higher than those of the pearl and red river sediments the low contents of k2o and na2o in sediments from the northwestern continental shelf are consistent with intense chemical weathering in the river basin due to the seasonally hot and humid climate regime the sediments mainly consist of three components including the gravel fraction composed of calcareous debris the sand fraction composed of quartz and the silt and clay fractions mainly composed of clay minerals the content of each component depends on grain size sediment source biogenesis and hydrodynamic conditions which finally controls the chemical composition of the sediments the distributions of co al2o3 cr al2o3 and zr sc ratios for sediments in the northwestern continental shelf suggest that source rocks are mainly composed of felsic rocks rather than mafic rocks there is a distinct difference in sediment source between eastern and western shelf sediments the eastern shelf sediments are characterized by high zr sc ratios mainly derived from the pearl river while the western shelf sediments have relatively low values of zr sc indicating a main contribution possibly sourced from the red river basin terrigenous materials from hainan island usually influence the geochemistry of sediments deposited in the nearshore area

constraints on a buoyant model for the formation of the axial topographic high on the east pacific rise bathymetry and gravity data from four geophysical surveys along the 15 19 s superfast spreading section of the east pacific rise are used to study formation of the axial topographic high after removing a best fitting thermal model the averaged residual axial topography is 265 m high and 15 km wide the residual gravity shows no evidence for shallow isostatic compensation of this topography suggesting deep rooted compensation or dynamic support previous models postulate that the axial high is compensated by low densities in a zone of partial melting within the upper mantle we model the physical properties of the lithosphere and asthenosphere required to match the residual gravity and bathymetry using a nonlinear inversion assuming the high is uplifted by buoyancy forces from low density mantle deep rooted buoyancy forces cannot create a narrow axial high unless they occur in a low viscosity conduit surrounded by high viscosity mantle this high viscosity region could be created by depletion and extraction of all water during the melting process we find that the viscosity of the asthenosphere beneath the melt conduit must also be lower than the viscosity of the residual mantle surrounding the conduit our most reasonable model has a viscosity of 1020 pa s in the residual mantle surrounding a melt conduit which extends 50 km below the seafloor and a viscosity of 1016 6 pa s in the asthenosphere below the melt conduit the viscosity of the melt conduit must be about 1015 pa s or about 105 times lower than the surrounding residual mantle with a density contrast equivalent to the retention of a small fraction of melt 5 because higher melt concentrations are probably required to create even small changes in viscosity and the viscosity of the undepleted mantle is probably higher than 1016 6 pa s we conclude that it is unlikely that the axial topographic high is formed by buoyancy forces in the crust and mantle

rare earth element fractionations of the northwestern south china sea sediments and their implications for east asian monsoon reconstruction during the last 36 kyr abstract rare earth elements rees fractionations in siliciclastic marine sediments are related to the parent rock properties weathering degree grain size and heavy minerals the relations between marine sediment ree compositions and regional paleoclimate are not fully understood here the rees for detrital components of a core retrieved from the northwestern south china sea scs were measured results show the ree lree hree la yb n and light ree la sm n fractionation degrees have good responses to the regional paleoclimate over the past 36 kyr sea level played a key role in ree fractionations during 36 24 kyr bp manifesting as lower ree and lree fractionation degrees correspond to the decreased sea level the entire trends for lree hree la yb n and la sm n over the past 20 5 kyr are almost in phase with the regional east asian summer monsoon easm records because rees are mainly concentrated in heavy minerals the density difference of heavy minerals is probably the main controlling factor on ree fractionations clay minerals and feldspars released during the weathering process have a high content of light rees lrees in the warm wet periods holocene optimum and bolling allerod more precipitation promoted weathering and strengthened the estuarine hydrodynamic force which respectively resulted in high content of clay minerals and feldspars and high density heavy minerals e g monazite the process led to lree enrichment because the enriched minerals contain higher contents of the lrees in contrast the dry cold climate conditions during the periods heinrich 1 younger dryas and last glacial maximum resulted in the reversed ree fractionations as the weak hydrodynamic force can only carry the low density heavy minerals e g zircon that are enriched in hrees this model does not seem to apply to 36 24 kyr bp probably because of the re transport of ancient shelf sediments the study facilitates the understanding of ree application in paleoclimate reconstruction within slope sediments

regional teleseismic tomography of the western lachlan orogen and the newer volcanic province southeast australia summary from 1998 may to september a portable array of 40 short period digital seismograph stations was operated in western victoria southeast australia across the western end of the midpaleozoic lachlan foldbelt and the newer volcanic province consisting of four parallel almost w e oriented receiver lines the array covered an area of about 270 150 km 2 the major aim of the lf98 lachlan foldbelt survey 1998 project is to map lateral variations in p wave speeds vp in the crust and upper mantle using teleseismic arrival time tomography primarily in order to investigate whether the major surface structural zones are associated with seismic velocity signatures at depth little a priori information from seismic profiling is available we invert 4067 relative arrival time residuals for a minimum structure vp model in the upper few hundred km using non linear iteration and 3 d ray tracing the most prominent negative anomaly 3 8 per cent in vp is found at a depth of about 45 km underneath the eastern part of the newer volcanic province it correlates spatially with the highest density of pliocene and pleistocene eruption centres northwest of melbourne and is therefore interpreted as a hotspot related high temperature anomaly causing reduced mantle velocities the related coherent volume of significantly lower than average velocities extends down to depths greater than 100 km in the east and extends west underneath the newer volcanic province a strong velocity contrast with average velocities 2 per cent greater in the west is found down to about 100 km across the moyston fault zone which forms the major structural boundary between the early paleozoic delamerian orogen in the west and the lachlan orogen in the east this result suggests that the moyston fault zone should be seen as a major lithospheric boundary in the south this boundary is also expressed by a distinct discontinuity in sr isotopic ratios of xenoliths the so called mortlake discontinuity and a change in the geochemistry of plutons of similar age however if the east to west velocity contrast originally existed in this southern zone it is now overprinted by the thermally reduced mantle velocities beneath the newer volcanic province

seismic structure beneath the tonga arc and lau back arc basin determined from joint vp vp vs tomography the tonga arc and associated lau basin exhibit many geologically important processes that link subduction and mantle flow with plate separation and crustal production we create seismic tomograms of the tonga lau region by jointly inverting for vp and vp vs structure using data from the labatts ocean bottom seismograph experiment and several island deployments to better constrain dynamic processes in the mantle wedge jointly using p and s data can help distinguish between the various mechanisms responsible for seismic velocity anomalies such as temperature and the presence of melt and or volatiles because high attenuation in the wedge limits the s wave data set we focus on 2 d inversions beneath the linear obs array where resolution is best and also parameterize the solution in terms of the vp vs ratio as expected the subducting slab has fast vp and vs and a low vp vs ratio consistent with the cold downgoing plate the central lau spreading center clsc exhibits stronger anomalies in vp vs than in vp with the anomalies larger than would be predicted purely by temperature variations the clsc anomaly extends 100 km to the west of the axis suggesting a broad region of melt production driven by passive upwelling from plate separation rather than active upwelling mechanisms the anomaly is asymmetric about the axis suggesting that slab induced corner flow possibly influences mantle dynamics several hundred kilometers away from the arc there is a strong anomaly beneath the volcanic arc that gradually deepens as it trends toward the back arc likely outlining a hydrated region of melt production that feeds the volcanic front hydration possibly continues throughout the wedge to at least 400 km depth the lau ridge exhibits a thicker lithosphere relative to the rest of the basin while the fiji platform likely has a thinner lithosphere than the lau ridge from more recent extension there is also a reasonable likelihood of a small degree of partial melt in the uppermost mantle beneath the platform

shear wave splitting and implications for mantle flow beneath the melt region of the east pacific rise shear wave splitting across the fast spreading east pacific rise has been measured from records of sks and skks phases on the ocean bottom seismometers of the mantle electromagnetic and tomography melt experiment the direction of fast shear wave polarization is aligned parallel to the spreading direction delay times between fast and slow shear waves are asymmetric across the rise and off axis values on the pacific plate are twice those on the nazca plate splitting on the pacific plate may reflect anisotropy associated with spreading induced flow above a depth of about 100 km as well as a deeper contribution from warm asthenospheric return flow from the pacific superswell region

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

a full sea level cycle in 65 years barrier dynamics along caspian shores abstract the caspian sea being a closed basin shows much higher rates of sea level change up to 340 mm yr than the oceans it experienced a full sea level cycle with an amplitude of 3 m between 1929 and 1995 its coasts therefore present an attractive physical model to test predicted changes along oceanic coasts as a result of expected eustatic sea level rise and to validate sequence stratigraphical interpretations of ancient sediments along the wave dominated coasts with intermediate shore face slopes in dagestan low onshore bars were formed during the 1929 1977 sea level fall they represent minor short lived 1 2 years transgressive oscillations within the major regressional trend after the 1977 lowstand a prominent barrier lagoon system was formed which during the first phase of sea level rise 1977 1988 encroached landwards about 18 m each year by washover during storms continuous retreat between 1988 and 1995 however barrier size diminished lagoons widened and deepened apparently due to lack of coarse grained sediment supply discontinuous retreat the 1995 highstand prevented the barriers from being overstepped altogether sea level fall after 1995 again produced low bars without lagoons the caspian sea barrier coast demonstrates that for the formation of barrier lagoon complexes the combination of transgression and storms is essential barriers may show either continuous or discontinuous development at constant rate of sea level rise and constant offshore slope depending essentially on the availability of coarse grained sediments the rate of sea level rise as a factor in barrier behaviour cannot be viewed independently from barrier dimensions which in turn depend on wave base depth and maximum storm wave height relations between para sequence thickness and frequency and amplitude of sea level change in closed basins with an independent sea level regime may differ markedly from those in basins influenced by eustasy

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

andisol formation in a holocene beach ridge plain under the humid tropical climate of the atlantic coast of costa rica abstract soil formation has been studied in relation with time in a 5000 year old chronosequence on volcaniclastic beach ridges of the perhumid tropical atlantic coast of costa rica all soils are under tropical rainforest drainage conditions change by subsidence from excessively drained in the two youngest soils to imperfectly drained in the two oldest soils parent material is rather homogeneous andesitic sand with a volcanic glass component of less than 10 it has been found that under these conditions andisols form within 2000 years imperfect drainage caused mottling and accumulation of iron coatings as well as the formation of a thin o horizon in the oldest profiles sand content of the soils decreases regularly with soil age while the amount of fine material increases concurrently the increase in fine material and the accumulation of organic matter cause an increase of cec and andic properties and a decrease in bulk density and ph with soil age depth of biological influence increases with soil age but soil faunal activity is hampered in the oldest three profiles probably by imperfect drainage due to the extreme leaching conditions the sum of exchangeable cations is less than 2 cmol kg 1 in the b horizons of the older soils notwithstanding the presence of a considerable amount of weatherable primary minerals

rare earth elements in japan sea sediments and diagenetic behavior of ce ce results from odp leg 127 the relative effects of paleoceanographic and paleogeographic variations sediment lithology and diagenetic processes on the recorded rare earth element ree chemistry of japan sea sediments are evaluated by investigating ree total abundances and relative fractionations in 59 samples from ocean drilling program leg 127 ree total abundances ree in the japan sea are strongly dependent upon the paleoceanographic position of a given site with respect to terrigenous and biogenic sources ree concentrations at site 794 yamato basin overall correspond well to aluminosilicate chemical indices and are strongly diluted by sio2 within the late miocene pliocene diatomaceous sequence eueu values at site 794 reach a maximum through the diatomaceous interval as well most likely suggesting an association of eueu with the siliceous component or reflecting slight incorporation of a detrital feldspar phase ree at site 795 japan basin also is affiliated strongly with aluminosilicate phases yet is diluted only slightly by siliceous input at site 797 yamato basin ree is not as clearly associated with the aluminosilicate fraction is correlated moderately to siliceous input and may be sporadically influenced by detrital heavy minerals originating from the nearby rifted continental fragment composing the yamato rise the biogenic influence is largest at site 794 moderately developed at site 797 and of only minor importance at site 795 reflecting basinal contrasts in productivity such that the yamato basin records greater biogenic input than the japan basin while the most productive waters overlie the easternmost sequence of site 794 cece profiles at all three sites increase monotonically with depth and record progressive diagenetic lree fractionation the observed cece record does not respond to changes in oxygenation state of the overlying water and cece correlates slightly better with depth than with age the downhole increase in cece at site 794 and site 797 is a passive response to diagenetic transfer of lree except ce from sediment to interstitial water at site 795 the overall lack of correlation between cece and lanybn suggests that other processes are occurring which mask the diagenetic behavior of all lrees first order calculations of the ce budget in japan sea waters and sediment indicate that 20 of the excess ce adsorbed by settling particles is recycled within the water column and that an additional 38 is recycled at or near the seafloor data from masuzawa and koyama 1989 thus because the remaining excess ce is only 10 of the total ce there is not a large source of ce to the deeply buried sediment further suggesting that the downhole increase in cece is a passive response to diagenetic behavior of the other lrees the ree chemistry of japan sea sediment therefore predicts successive downhole addition of lrees to deeply buried interstitial waters

shipboard geophysical indications of asymmetry and melt production beneath the east pacific rise near the melt experiment near the mantle electromagnetic and tomography melt experiment seamounts form and off axis lava flows occur in a zone that extends farther to the west of the east pacific rise than to the east indicating a broad asymmetric region of melt production more seamounts slower subsidence and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west variations in axial ridge shape axial magma chamber continuity off axis volcanism and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15 s than beneath 15 degrees55 s recent volcanism occurs above mantle with the lowest seismic velocities

volcanic origin of holocene beach ridges along the caribbean coast of costa rica abstract the formation of beach ridges along the caribbean coast of costa rica ranging in age between about 100 and 5000 years appears to be related to discontinuous sand supply by volcanic eruptions in the costa rican central cordillera this is indicated by consistent and statistical significant differences in chemical composition between sediments of individual beach ridges which cannot be explained by textural differences the differences are thought to be caused by variations in magma composition of different eruptions in the volcanic hinterland sio2 contents of the ridges vary between about 52 and 59 other elements show variations corresponding to magmatic differentiation petrographically this is expressed in variations in the amount of andesitic rock fragments and pyroxene grains chemical composition of beach ridge sediments is alike in composition to the erupted products however in spite of the removal of part of the mobile elements and mixing with weathered sediments original differences between the erupted products appear to remain reflected in the resulting beach ridge sediments

influence of climate fluctuations and changes in catchment land use on late holocene and modern beach ridge sedimentation on a tropical macrotidal coast keppel bay queensland australia abstract beach ridges at keppel bay central queensland australia preserve a record of sediment accumulation from the historical period back to middle holocene times the ridges comprise fine well sorted feldspar rich quartz sand that was eroded from the fitzroy river catchment deposited in keppel bay during floods of the fitzroy river and reworked onshore into beach and foredune deposits by the prevailing currents waves and wind these floods have an average recurrence interval of at least 7 yr and are induced by the passage of cyclones onshore into the large fitzroy catchment the youngest series of beach ridges sit sub parallel to the modern beach and comprise six accretional units each unit formed by a set of ridges and delineated by prominent swales optically stimulated luminescence osl ages of beach ridges in these units indicate they were deposited in periods of rapid progradation approximately 1500 1000 450 and 230 yr bp when there was an enhanced supply of sediment to the beach from the fitzroy river via keppel bay estimates of the mass of sediment stored in the beach ridge strandplain show that it represents a significant sediment store potentially trapping the equivalent of 79 of the estimated long term 100 yr average annual bedload of the fitzroy river that is deposited in keppel bay there has been a reduction in the rate of sediment accumulation in the strandplain since around 1000 yr bp which is consistent with other coastal records in eastern australia of a relatively wetter phase of climate in the late holocene compared to the present the youngest beach ridges osl ages

trace elements as source indicators in cratonic sediments a case study from the early proterozoic libby creek group southeastern wyoming to understand how grain size and sorting effect trace element distributions in sediments we compare the compositions of closely associated quartzites pelites and glacial diamictites from the 2 ga libby creek group in se wyoming we found similar ree distributions lree enriched negative eu anomalies and that most compositional differences can be explained by variable quartz dilution trace element distributions from mineral separates in a fuchsite quartzite show that fuchsite contains most of the th u ree sc co and ni and controls the whole rock ree distributions zircon contains most of the zr and hf but 5 of the hree in the pelites and 20 in the quartzites other than zr and hf most trace elements in libby creek sediments are chiefly in micas and were probably in clays in the original sediments increased eu anomalies in upper libby creek sediments signal a sudden increase in eu depleted granitoids in sediment sources which reflects late tectonic uplift mixing models favor granite ba

rare earth elements as provenance indicators in north australian estuarine and coastal marine sediments rare earth elements ree in three grain size fractions of clay dominated estuarine and coastal sediments from the north australian coastline have been investigated for use as potential provenance indicators comparisons of data based on instrumental neutron activation analysis and inductively coupled plasma mass spectrometry revealed that hf digestions of zircon bearing samples resulted in unsatisfactory recovery of ree instead a relatively simple fast and safe partial digestion using hclo4 hno3 was used to extract ree mainly from clay minerals while excluding extraction of ree from zircons a comparison with published data for primary rock units within river catchments showed that the main ree systematics has been preserved in downstream clay rich sediments analysis of variance of la gd paas la yb paas and eu eu paas ratios in five areas showed that the 63 m fraction possessed the greatest discriminating power all five areas could be distinguished from each other at highly significant levels based on one or more of these ratios

chemical weathering of fluvial sediments during alluvial storage the macuapanim island point bar solimoes river brazil abstract modification of sediment composition by chemical weathering depends both on the overall intensity of weathering and on its duration at macuapanim island solimboes river amazon main channel brazil a number of alluvial deposits have been exposed to intense tropical weathering for differing lengths of time in the very fine sand fraction the detrital component shows a steady and marked increase in compositional maturity with increasing sediment age immature sands of arkosic to litharenitic composition show a progressive loss of lithic fragments feldspars and accessory minerals with a concomitant enrichment in quartz this process produces first cycle sands whose detrital component is of quartz arenite composition among the salts quartz is enriched over time relative to more nstable phases particularly plagioclase this enrichment is probably largely caused by the destruction of plagioclase through chemical weathering but silt size quartz may also be produced through the breakdown of sedimentary and metasedimentary lithic fragments in the sand fraction in the clay fraction an increase in the relative abundance of smectite accompanies the breakdown of volcanic lithic fragments in the sand fraction and probably reflects the degradation of volcanic glass the clay fractions of the oldest samples examined contain less smectite somewhat more kaolinite and markedly more vermiculite as compared to younger samples but an extensively leached cation poor clay fraction analogous to the quartz dominated silts and sands is never attained the compositional mat rity of the clay fraction is everywhere lower than that of associated silts and sands perhaps because of a continual influx into the clay fraction of immature cation rich clays derived from the physical breakdown of lithic fragments in coarser size fractions this effect is enhanced by the translocation of fines whereby younger less weathered fine material is juxtaposed with older more mature coarse material in the lower levels of the alluvial stratigraphy reworking of floodplains during channel migration results in the reincorporation of older material stored on the floodplains into the sediment load if sediments have been intensely weathered during storage on the floodplain then the result will be a net dilution and replacement of younger relatively immature sands transported by the river by older more mature material from the floodplain the relative proportion of the older sands in the bedload will increase downstream resulting in a progressive downstream increase in the compositional maturity of the bedload of the river

contrasts in lithospheric structure within the australian craton insights from surface wave tomography abstract contrasts in the seismic structure of the lithosphere within and between elements of the australian craton are imaged using surface wave tomography new data from the wacraton and tigger experiments are integrated with re processed data from previous temporary deployments of broad band seismometers and permanent seismic stations the much improved path coverage in critical regions allows an interpretation of structures in the west of australia and a detailed comparison between different cratonic regions improvements to the waveform inversion procedure and a new multi scale tomographic method increase the reliability of the tomographic images in the shallowest part of the model 75 km a region of lowered velocity is imaged beneath central australia and confirmed by the delayed arrival times of body waves for short paths within the cratonic lithosphere there is clearly structure at scale lengths of a few hundred kilometres resolution tests indicate that path coverage within the continent is sufficient to reveal features of this size in the upper part of our model in western australia differences are seen beneath and within the archaean cratons at depths greater than 150 km faster velocities are imaged beneath the yilgarn craton than beneath the pilbara craton in the complex north australian craton a fast wavespeed anomaly continuing to at least 250 km is observed below parts of the craton suggesting the possibility of archaean lithosphere underlying areas of dominantly proterozoic surface geology

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

variations in subsidence rates along intermediate and fast spreading mid ocean ridges summary the nature of subsidence near the ridge crest of the intermediate and fast spreading mid ocean ridges of the indian and pacific oceans is investigated using surface ship bathymetry and magnetics profiles the ridge can be divided into discrete sections apparently bounded by distinct structural features such as major fracture zones in which bathymetry plotted against crustal age forms a well defined envelope with a width roughly the amplitude of the local bathymetry the averaged bathymetry in all of the regions studied follows closely a square root of age subsidence curve which in most regions has a subsidence coefficient c1 in the range of 340 390 m myr 1 2 the best fitting subsidence curve however never reproduces the amplitude of the axial topographic high the most notable region displaying unusual behaviour is the east pacific rise between 9 s and 22 s in this region the western flank of the ridge is subsiding at 200 225 m myr 1 2 while the eastern flank is subsiding at normal rates of 350 400 m myr 1 2 other anomalous areas include the region between the easter island hot spot and the chile rise triple junction in which the ridge crest is shallow and which is subsiding at rates of about 290 m myr 1 2 and the region east of the australia antarctic discordance in which the northern flank is subsiding at 440 m myr 1 2 this area may also be subsiding asymmetrically although there is not much data from the southern flank the asymmetric subsidence in the 9 s 22 s region of the east pacific rise begins immediately at the ridge crest and the low subsidence rates on the west flank continue to at least 12 myr old crust oligocene aged crust on the western flank is subsiding at more normal rates but is 500 m shallow with respect both to the world wide average and to the conjugate crust on the eastern flank the simplest model to explain these observations is that the western flank is underlain by a hotter mantle perhaps as the result of upwelling resulting from the large scale return circulation from the trenches depending on the depth of compensation the observed asymmetry could result from a lateral temperature gradient of 0 05 0 10 c km 1 and a total lateral temperature variation of under 100 c

an alternative dynamic model of the axial topographic high at fast spreading ridges recent models that attempt to explain the formation of an axial topographic high at fast spreading ridges invoke the presence of a narrow low density low viscosity melt conduit to buoyantly support the high mantle up welling beneath mid ocean ridges must be highly focused to create the narrow melt zone required by these buoyancy models we show that an alternative model in which extensional stresses in the upper crust are relieved by diking predicts uplift at the ridge and can match the shape and magnitude of the axial high in this model the extensional stresses deepen toward the ridge axis creating dynamic moments within the crust that are balanced by the axial topography along axis variations in the height and width of the axial high may be caused by changes in the depth and width of the axial magma chamber or flexural rigidity of the lithosphere this model is similar to dynamic models of median valley formation at slow spreading ridges in that the axial topography is uncompensated and supported by extensional stresses in the lithosphere and asthenosphere in addition to being able to explain the axial topography observed at fast spreading ridges this model is consistent with either broad passive or narrow dynamic upwelling at mid ocean ridges the change from median valley to axial high is caused by differences in strength of the axial lithosphere and rate of thickening of the plate

tectonic versus chemical weathering controls on the composition of fluvial sands in tropical environments the compositions of fluvial sands collected in mountainous regions along the volcanic front in central america and along the andean front in venezuela and colombia correlate with the tectonic setting of their source terrains the sands derived from the central american volcanic arc are compositionally distinct from those derived from the andean terrains and minor variations in tectonic style can also be discriminated subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight they are strongest in sands from low order streams draining mountainous areas with modest alluvial plain development sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases with increased weathering sands from continental terrains converge toward a supermature quartz arenite end member and the imprint left by source rock composition and tectonic setting is obscured in intraoceanic terrains a similar but much less pronounced enrichment in stable phases relative to less stable phases is observed similarly overprinting the tectonic signal although detailed tectonic information is easily lost sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical weathering conditions

lithospheric structure of tasmania from a novel form of teleseismic tomography in 2001 and 2002 a temporary array of 72 seismic recorders was deployed across northern tasmania se australia with the aim of imaging the underlying crust and upper mantle using three dimensional 3 d teleseismic tomography using a recently developed adaptive stacking technique 6520 relative p wave arrival time residuals have been picked from 101 distant earthquake records spanning a 5 month period a novel iterative nonlinear tomographic procedure based on a subspace inversion scheme and the fast marching method a grid based eikonal solver is used to map the residual patterns as p wave velocity anomalies the new scheme proves to be both fast and robust making it well suited to large data sets and the reconstruction of complex anomalies the resultant tomographic images of tasmania exhibit significant lateral perturbations in p wave velocity structure 5 from a 1 d reference model a marked transition from higher velocities in the east to lower velocities in the west strongly supports the idea that eastern tasmania is underlain by dense rocks with an oceanic crustal affinity contrasting with the continentally derived siliciclastic core of western tasmania significantly the tamar fracture system does not overlie the narrow transition from relatively fast to slow velocities which suggests that it may be a near surface feature rather than a manifestation of deeper crustal scale suturing as previously thought farther west an easterly dipping zone of relatively high velocity material beneath the rocky cape group and arthur lineament may be related to remnant subduction of oceanic lithosphere associated with the mid cambrian delamerian orogeny

focusing and defocusing of teleseismic p waves by known three dimensional structure beneath pahute mesa nevada test site the focusing and defocusing of teleseismic p waves predicted by a known three dimensional structure beneath the nevada test site is calculated by dynamic raytracing and superposition of gaussian beams the 20 to 100 km scale lengths of this model having velocity fluctuations of several per cent account for a factor of 3 fluctuation in the azimuthal pattern of p amplitudes since similar sized scale lengths and intensities of velocity fluctuation are commonly resolved in three dimensional inversions of mantle structure in other regions focusing and defocusing of teleseismic p waves is likely to be a ubiquitous feature of every test site hence network averages of mb that weight azimuthal windows in which p energy is either focused or defocused will tend to either over or underestimate the yields of underground tests the results obtained with a known nevada test site structure suggest that structural inversions having similar structural resolution may be capable of reducing 25 per cent of the variance in amplitudes due to the focusing and defocusing of upper mantle structure near the source assuming that scattering processes are concentrated in the crust and upper most mantle beneath the source and receiver the integrated energy in the p coda should be more stable than direct p because it tends to homogenize the effect of focusing defocusing structure near the source for scattering into direct p near the source for direct p scattering near the receiver however even the late coda is not capable of completely removing the effects of focusing defocusing near the source

high resolution elemental records from the south china sea and their paleoproductivity implications 1 the biogenic related elements ca sr ba p cd scavenged al and ti were analyzed by inductively coupled plasma mass spectrometry icp ms and inductively coupled plasma atomic emission spectrometry icp aes for core ns93 5 from the west slope of the south china sea terrestrial input as estimated from the accumulation of ti was higher during glacials than during interglacials carbonate accumulation rates are inversely related to those of terrestrial input suggesting higher production of calcareous phytoplankton during interglacials the accumulation patterns of authigenic sr ba p and cd match that of carbonate further indicating higher calcareous phytoplankton production during interglacials scavenged al and excess sio2 which is related to biogenic opal exhibit higher accumulation rates during glacials and correspond with changes in terrestrial input this indicates that terrestrial input driven is important to siliceous phytoplankton production but not for calcareous phytoplankton production as calcareous phytoplankton is the dominant component of the biogenic sediments in the south china sea particularly during interglacials previous inference of higher productivity in the south china sea during glacials based on only the biogenic opal proxy needs to be reconsidered

influence of the kuroshio current intrusion on depositional environment in the northern south china sea evidence from surface sediment records the influence of the kuroshio current kc intrusion on the depositional environment of the northern south china sea scs is discussed based on multi proxies of surface sediments including planktonic foraminifera clay minerals and geochemical elements the results reveal that four typical planktonic foraminifera globigerinoides ruber g sacculifer neogloboquadrina dutertrei and pulleniatina obliquiloculata can be used to trace the kc flow route in the northern scs globigerinoides ruber is mainly distributed in shallower water than the south china sea branch of the kuroshio scsbk flow route while p obliquiloculata is mostly distributed in deeper water than the scsbk flow route there are high percentages of g sacculifer and n dutertrei at the scsbk flow route along the 1000 m isobath where bottom sediments consist of clayey silt and silty clay the scsbk also affects sediment distribution of various sources taiwan sourced sediments characterized by high illite and chlorite contents are transported southwestward into the deep sea environment by the scsbk and or deep water current the pearl river sediments characterized by high kaolinite and titanium ti contents are mostly transported southwestward to be distributed in the sea environment between the pearl river mouth and southeast of hainan island but hardly entered the abyssal basin the luzon island sediments characterized by high smectite content are generally deposited at the northwest of luzon island under the effect of an eddy west of luzon island these sediments were hardly transported northwestward onto the northern shelf of the scs throughout the scsbk flow route the sedimentary environment at the northwest of luzon island is relatively oxidized and higher in salinity than the northern scs which is probably connected with the kc intrusion through the luzon strait

surface wave focusing effects numerical modeling and statistical observations most techniques used in seismological practice have been performed under the assumption that surface waves propagate along great circle arcs between the epicenter and the station however lateral variations of surface wave phase velocities should result in deviation of the wave paths from great circle arcs and in corresponding anomalies of geometrical spreading so called focusing effect we performed numerical modeling of these effects using the ray approximation on the basis of recent global phase velocity maps for fundamental rayleigh mode obtained by tomographic inversion in the period range from 60 to 150 s the aim for such a modeling is to investigate where the conventional methods based on using great circles as paths fail and to obtain quantitative characteristics for the effects of rays focusing predicted and observed focusing effects are analyzed from a statistical point of view on a dataset of real seismic data records of earthquakes in a wide magnitude range 6 ms 7 and with epicenters in different seismic regions are used for the analysis over 3000 measurements synthetic and observed rayleigh wave amplitude spectra are found to be in better agreement when the focusing effect is taken into account correction of spectra for predicted focusing effect significantly improves the fit of synthetics to observations at periods larger than 75 s calculation of focusing effect based on ray theory ignores the effects of wavefront smoothing which increases with increasing period however even this approximation gives considerable improvement of synthetics at periods up to 150 s and therefore can be used in many applications such as seismic source studies magnitude measurements and q estimates we also demonstrated that the discrepancy between real data and synthetics cannot be explained by attenuation effects it means that there is still large room for improvement of the existing tomographic models which in order to correctly explain observed amplitudes must include heterogeneity of anisotropy and anelasticity up to higher degree

amplitude and phase data inversions for phase velocity anomalies in the pacific ocean basin summary rayleigh wave phase velocities at periods 30 80 s in the pacific ocean are calculated by inverting phase and amplitude anomaly data using the paraxial ray approximation and the gaussian beam method the region is divided into 5 x 5 blocks and approximately 200 source receiver pairs from 18 well studied events around the pacific ocean are used first we assume phase anomalies for the lithospheric age dependent model next conventional phase data inversions are conducted assuming great circle paths so that the phase discrepancies are reduced to less than t this procedure is essential for later inversions using amplitude data we then determine the residuals of both amplitude and phase terms by calculating ray synthetic seismograms using the born approximation for a 2 d wave equation a nonlinear iterative inversion for phase velocities is performed with both residuals frbchet derivatives for the inversion consist primarily of two wavefields 1 the wavefield at the model point from the source and 2 the green s function from the model point to the receiver these wavefields are also calculated by the paraxial ray approximation and gaussian beam methods in the inverse formulations the simple use of the conventional backus gilbert approach yields undesirable results in the non linear iterative case and an extra term is necessary to control the model perturbations in order to minimize departures from the a priori model the use of this additional term guarantees that we are able to obtain a fairly reliable phase velocity model even in the present non linear problem in most cases residual variances are significantly reduced after two or three iterations as far as the starting model is fairly correct compared with the phase data inversions this inverse scheme gives more reliable resolution and most of the inverted features in phase velocities are significantly larger than the uncertainty level while some features obtained by the great circle phase data

jasper seamount structure seafloor seismic refraction tomography the velocity structure of jasper seamount was modeled using one and three dimensional inversions of p wave travel times the results represent the first detailed seismic images of a submerged intraplate volcano two seismic refraction experiments were completed on jasper seamount incorporating ocean bottom seismometers and navigated seafloor shots the p wave travel times were first used to compute a one dimensional velocity profile which served as a starting model for a three dimensional tomographic inversion the seamount p velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths this suggests that jasper seamount is constructed predominantly of extrusive lavas with high average porosity the velocity models confirm morphological predictions jasper seamount is a shield volcano with rift zone development high seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks comparisons between p velocity models of jasper seamount and the island of hawaii reveal that these two shield volcanoes are not structurally proportional jasper seamount is far smaller than hawaii yet both volcanoes exhibit an outer extrusive layer of similar thickness this suggests that seamount size influences the intrusive extrusive proportions density equilibrium between melt and country rock may explain this behavior

geochemistry of the surface sediments of the sulu and south china seas abstract the chemical and mineralogical composition of the surface sediments of the sulu sea a deep hydrographically restricted basin in the indonesian archipelago and the neighbouring area of the south china sea have been examined the aim of the study was to characterize the geochemistry of the sediments of the isolated oxygen deficient basin and to compare them with sediments at equivalent water depths outside the basin as a prelude to a further study of the response of the composition of the sediments to oceanographic changes including possible anoxia during the last glacial maximum the concentrations of most of the chemical constituents in the surface sediments along four transects change markedly with water depth in the sulu sea ss and the south china sea scs thus caco 3 decreases from around 60 by weight in the shallowest samples 800 1000 m to around a few percent in the deepest sample at 4203 m in the scs where the ccd lies at around 4000 m in the ss however the ccd lies much deeper so that the caco 3 contents are still approximately 40 at 4500 m depth organic c also decreases with depth in both basins from values of around 1 2 in the shallowest samples to less than 1 in the deepest samples the ratios of si mg na k co cr cu ni pb rb y zr and zn to al all decrease with increasing water depth because of the change in mineralogy and sediment texture with water depth the p al is higher in shallower water in both areas because of the higher abundance of detrital phosphorite or phosphatized limestone in these sediments the sr al ratio is also higher in shallower water reflecting the supply of aragonitic shell debris from the shallow banks and shelves around the basin the ba al ratio has a maximum at 2000 m depth in the scs due to higher primary production in overlying surface waters whereas it is uniformly low in the ss manganese is also enriched at intermediate water depths in both the ss and the scs because of the presence of oxyhydroxides and this also produces a slight enrichment in mo in the same samples because of their association in this authigenic phase the concentration of i and the i c organic ratio decreases with water depth in both areas due to the higher abundance of fresh marine organic matter at the shallow water sites which scavenges proportionally more i from sea water finally the 13 c organic values tend to decrease irregularly with increasing water depth probably reflecting a higher contribution of terrestrial organic matter in deeper water a discriminant analysis of the full data set reveals that compared with the scs samples the oxygen deficient ss sediments have higher caco 3 contents lower n and s contents and lower i c organic ba al fe al k al pb al rb al ti al v al y al and zn al ratios these contrasts are brought about by the different depths of the ccd differences in the nature of the organic matter and different types of aluminosilicate detritus in the two areas although the i c organic ratios are slightly but significantly lower in the ss which may be due to the low dissolved oxygen contents of the waters of this basin the mn and mo contents and other minor elements that respond significantly to ambient bottom water oxygen levels are not significantly different in the ss and the scs moreover the organic c contents are also indistinguishable in the two areas hence the low bottom water oxygen levels in the ss do not lead to higher organic carbon contents in the surface sediments the composition of sediments in contact with waters having even lower oxygen concentrations should be evaluated in order to conduct a more definitive test of the effect of low oxygen conditions on carbon preservation comparison of glacial interglacial sediment sections from this area must take particular care to separate the effects of source variations on those potentially caused by low oxygen levels

low velocity zone atop the 410 km seismic discontinuity in the northwestern united states the seismic discontinuity at 410 km depth in the earth s mantle is generally attributed to the phase transition of mg fe 2sio 4 from the olivine to wadsleyite structure variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations for example a cold anomaly would elevate the 410 km discontinuity because of its positive clapeyron slope whereas a warm anomaly would depress the discontinuity but trade offs between seismic wave speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities and structure often appears very complicated here we simultaneously model seismic refracted waves and scattered waves from the 410 km discontinuity in the western united states to constrain structure in the region we find a low velocity zone with a shear wave velocity drop of 5 on top of the 410 km discontinuity beneath the northwestern united states extending from southwestern oregon to the northern basin and range province this low velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic receiver function observations observed above the region we interpret the low velocity zone as a compositional anomaly possibly due to a dense partial melt layer which may be linked to prior subduction of the farallon plate and back arc extension the existence of such a layer could be indicative of high water content in the earth s transition zone

a broadband ocean bottom seismometer system based on a 1 hz natural period geophone recent work with broadband sensors has demonstrated that useful data can be obtained from seismic sensors deployed on the deep seafloor for periods as long as 300 sec but ocean bottom seismometer obs systems using conventional broadband sensors have proven fragile and expensive to build and operate the intrinsic thermal brownian motion noise limit for large 1 hz geophone sensors such as the mark products l 4 is far below typical noise levels at seafloor sites in response to the need for large numbers of obs systems for experiments studying the oceanic upper mantle and crust we have developed a very low noise amplifier that can be used with short period sensors the new amplifiers push noise levels at the 100 sec period some 40 db below standard short period obs noise levels useful data can then be obtained from these short period sensors in the broad frequency range from 0 005 to 50 hz noise levels rise from 5 10 16 m sec 2 2 hz at a 20 sec period to 5 10 14 m sec 2 2 hz at a 200 sec period power requirements are less than 35 mw for three channels these amplifiers have been installed in the first 24 instruments of the new nsf sponsored ldeo obs instrument center we believe this system is an excellent compromise between cost reliability and noise level the new amplifiers may also prove useful for work on land extending the useful range of short period sensors to a much longer period

asymmetric mantle dynamics in the melt region of the east pacific rise abstract the mantle electromagnetic and tomography melt experiment found a surprising degree of asymmetry in the mantle beneath the fast spreading southern east pacific rise melt seismic team science 280 1998 1215 1218 forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 wolfe and solomon science 280 1998 1230 1232 scheirer et al science 280 1998 1221 1224 evans et al science 286 1999 752 756 pressure release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick canales et al science 280 1998 1218 1221 seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 evans et al science 286 1999 752 756 at depths of several tens of kilometers melt is more abundant beneath the pacific plate to the west of the axis than beneath the nazca plate to the east melt investigators attributed the asymmetry in melt and geophysical properties to several possible factors asymmetric flow passively driven by coupling to the faster moving pacific plate interactions between the spreading center and hotspots of the south pacific an off axis center of dynamic upwelling and or anomalous melting of an embedded compositional heterogeneity melt seismic team science 280 1998 1215 1218 forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 wolfe and solomon science 280 1998 1230 1232 evans et al science 286 1999 752 756 here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies asthenospheric flow from hotspots in the pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies

the controls on the major and trace element variation of shales siltstones and sandstones of pennsylvanian permian age from uplifted continental blocks in colorado to platform sediment in kansas usa shales siltstones and sandstones of pennsylvanian permian age from near the source in colorado to those in the platform in eastern colorado and kansas have been analyzed for major elements and a number of trace elements including the rees the near source sandstones are significantly more enriched student t test at better than the 99 confidence level in sio2 and na2o concentrations and more depleted in al2o3 fe2o3 total tio2 th hf sc cr cs rees y and ni concentrations and laco and lani ratios than the near source shales and siltstones most likely due to more plagioclase and quartz and less clay minerals in the sandstones than in the shales and siltstones there are no significant differences in k2o and sr concentrations and eueu lalu lase thsc thco and crth ratios between the near source sandstones and the near source shales and siltstones samples of the molas hermosa and cutler formations near the source that were formed in different environments in the same area contain no significant difference in eueu lalu lasc thsc thco and crth ratios so a generally silicic source and not the environment of deposition was most important in producing these elemental ratios for example crth ratios of near source shales siltstones and sandstones range from 2 5 to 17 5 and eueu range from 0 48 to 0 78 which are in the range of sources of sediments derived from mainly silicic and not basic sources near source shales and siltstones contain significantly higher student t test and more varied concentrations of most elements al2o3 fe2o3 mno tio2 ba th hf ta co sc rees nb y but significantly lower concentrations of na2o and eueu than platform shales and siltstones in kansas e g la 65 7 40 and eueu 0 55 0 07 in near source shales and siltstones and la 23 7 8 7 and eueu 0 64 0 08 in platform shales and siltstones the sio2 and cao concentrations are not significantly different in platform shales and siltstones compared to the near source shales and siltstones so dilution of other minerals by quartz and calcite is not the main reason for the lower concentration of most elements in the platform relative to the near source shales and siltstones rather the lesser concentrations of most elements in clay minerals of the platform shales and siltstones can account for the lower concentration of most elements compared to corresponding near source shales and siltstones the lower concentrations of many elements in clay minerals in the platform shales and siltstones may be a result of having been derived from recycling of clay minerals from older rocks the greater homogeneity of elemental concentrations of the platform shales and siltstones compared to those in the source is also consistent with homogeneous mixing of such recycled material also there is no significant difference in thsc laco thco lani and crth ratios of the near source sedimentary rocks in colorado to the platform shales and siltstones in kansas and the latter are also consistent with derivation from mostly silicic source rocks

contrasting the indian and east asian monsoons implications on geologic timescales the surface winds over the arabian sea and south china sea are meaningful indicators for the strength of the indian monsoon and east asian monsoon respectively paleo monsoon variability has been studied through analysis of sediment records from these two monsoon regions to facilitate interpretation of these records we focus on the impacts of internal and external forcing of the monsoon system by contrasting the annual cycle and interannual variability of two subsystems the monsoon over the indian sector 40 105 e and over the east asian sector 105 160 e differences in the annual cycle within these subsystems arise primarily from the different land ocean configurations that determines atmospheric response to the solar forcing thus factors that drive intensities of the monsoonal annual cycle share common features with the external geographic and orbital forcing that controls paleomonsoon variability we show that the differences in interannual variations between the two monsoon subsystems are primarily due to internal factors of the coupled atmosphere ocean land system such as remote impacts of el nin o la nin a and local monsoon ocean interactions the mechanisms that operate on interannual to interdecadal timescales may differ fundamentally from that on geologic orbital timescales the low level flows over the east asia and australia are essentially established by geographic forcing the amplification of the australia summer monsoon during increased solar precession is likely caused by an enhanced east asian winter monsoon rather than following an enhanced indian summer monsoon as on the interannual timescale it is also found that el nin o influences the low level flow moderately over the arabian sea but to a greater extent over the south china sea as such large changes in the pacific thermal conditions may significantly alter the intensity of the east asian monsoon but not the indian monsoon

seismic velocity anisotropy and heterogeneity beneath the mantle electromagnetic and tomography experiment melt region of the east pacific rise from analysis of p and s body waves 1 we use teleseismic p and s delay times and shear wave splitting measurements to constrain isotropic and anisotropic heterogeneity in the mantle beneath the southern east pacific rise sepr the data comprise 462 p and s delay times and 18 shear wave splitting observations recorded during the mantle electromagnetic and tomography melt experiment we estimate the mantle melt content f and temperature t variation from the isotropic velocity variation our results indicate that the maximum variation in f beneath our array is between zero and 1 2 and maximum variation in t is between zero and 100 k we favor an explanation having partial contributions from both t and f we approximate the seismic anisotropy of the upper mantle with hexagonal symmetry consistent with the assumption of two dimensionality of mantle flow our new tomographic technique uses a nonlinear inversion of p and slow s polarization delay times to simultaneously solve for coupled vp and vs heterogeneity throughout the model and for the magnitude of anisotropy within discrete domains the domain dimensions and the dip of the anisotropy are fixed for each inversion but are varied in a grid search obtaining the misfit of the models to the body wave delay data and to split times of vertically propagating s waves the data misfit and the isotropic heterogeneity are sensitive to domain dimensions and dip of anisotropy in a region centered beneath the sepr the best average dip of the hexagonal symmetry axis is horizontal or dipping shallowly 30 west given the resolution of our data a subaxial region characterized by vertically aligned symmetry axes may exist but is limited to be 80 km deep we infer that the mantle flow beneath the sepr is consistent with shallow asthenospheric return flow from the direction of the south pacific superswell

asymmetric electrical structure in the mantle beneath the east pacific rise at 17 s the magnetotelluric component of the mantle electromagnetic and tomography melt experiment measured the electrical resistivity structure of the mantle beneath the fast spreading southern east pacific rise epr the data reveal an asymmetric resistivity structure with lower resistivity to the west of the ridge the uppermost 100 kilometers of mantle immediately to the east of the ridge is consistent with a dry olivine resistivity structure indicating a mantle depleted of melt and volatiles mantle resistivities to the west of the ridge are consistent with a low melt fraction about 1 to 2 percent interconnected melt distributed over a broad region and extending to depths of about 150 kilometers the asymmetry in resistivity structure may be the result of asymmetric spreading rates and a westward migration of the ridge axis and suggests distinct styles of melt formation and delivery in the mantle beneath the two plates

the rare earth element geochemistry of surface sediments in four transects in the south china sea and its geological significance seventy five samples of surface sediment were collected from four transects in the south china sea scs for analyses of rare earth element ree geochemistry total ree abundance ree in the scs varies from 45 to 195 ppm average 128 ppm lower than that of the upper continental crust the spatial distribution of ree is strongly dependent upon the geographical location and depositional environment low ree concentrations are observed on the continental slope e g xisha and nansha islands while high ree concentrations are present in the deep basin sediments on the slope are also characterized by lower lree hree la yb n eu eu and ce ce ratios relative to deep basin sediments there is a positive correlation between al2o3 and ree and a negative correlation between cao and ree conversely the relatively insignificant fe mn oxide accumulation in the scs suggests that manganese nodules do not make a significant contribution to the bulk ree composition of sediments therefore the sediment ree geochemistry is mainly controlled by terrigenous and biogenic sources biogenic carbonate has a diluting effect on ree concentrations but a significant influence on lree hree ce ce and la yb n values is found only in sediments with especially high biogenic carbonate levels the ree geochemistry of scs surface sediments indicates that their source is mainly post archean felsic rocks in the eastern sub basin the relatively low lree hree ratios and high eu eu values of the sediments indicate that significant terrigenous materials are derived from young volcanic rocks e g rocks in the luzon arc

new archive of another significant potential sediment source in the south china sea abstract sediment provenance is an important part of the sediment source to sink transport process from land to sea however we sometimes cannot explain some contradictory problems well when assessing sediment sources in certain areas the northern south china sea scs is currently considered to be dominantly supplied with sediments from the pearl river taiwan and luzon for smectite one type of clay mineral luzon is normally thought to be a significant source when considering only the high smectite percentages in luzon derived sediments and neglecting their limited sediment discharge into the sea and longer transport distance of its sediments from this island recently clay mineral compositions of the pearl river originated sediments were reported to change from rich in smectite from 2500 years ago due to the anthropogonic activities by contrast the wide area in the east of the pearl river epr is similarly characterized by sediments with high smectite percentages and a shorter transport distance than those from luzon however a previous study showed that surface sediment samples with high smectite percentages were occasionally distributed in bays here we present a sediment archive of 16 gravity cores from neighboring bays harbors along the northern scs coast unlike surface sediments with low smectite percentages core sediments from the coastal bays harbors of the epr show high smectite percentages indicating that the epr besides the pearl river and or luzon might be as another potential sediment source for the northern scs in south china widely distributed igneous rocks volcanic materials are prodigious sources for sediments in the northern scs slope in future work weathering of local materials must be considered when differentiating various sediment sources

the geochemistry and tectonic setting of the northern section of the luzon arc the philippines and taiwan abstract the luzon arc consists of a 1200 km chain of stratovolcanoes and volcanic necks stretching from mindoro 13 n to the coastal range of taiwan 24 n this study is concerned with three of the five major segments along the arc the northern luzon babuyan and taiwan segments the late tertiary to quaternary volcanics of these segments are primarily andesitic but range in composition from basalt to rhyolite and are typical arc volcanics porphyritic plagioclase textures primarily calc alkaline with a few tholeiitic volcanic centers low tio 2 concentrations and low high field strength element hfse to large ion lithophile element lile ratios there is a large range in k 2 o and other liles from low k tholeiites to high k calc alkaline suites calayan island and mt tabungon volcanic substratum of mt cagua volcanics in the babuyan segment and most rocks from the taiwan coastal range are low k tholeiites e g nearly flat ree patterns low lree and lile concentrations and low th u ratios the dominant calc alkaline series ranges from medium k to high k rocks the high k calc alkaline rocks are mainly young and are found on batan and lutao islands when the low k tholeiitic and medium and high k calc alkaline rocks are associated in the same region the low k rocks are usually older there is a general relationship between k 2 o and other lile concentrations and k ar radiometric dates to some extent 87 sr 86 sr ratios also increase with time throughout the arc there is a correlation between latitude and 87 sr 86 sr and a positive correlation between 87 sr 86 sr and la sm cn an increase in both these geochemical parameters has been associated with an increase in the input of continental crustal material in other arc regions the lowest 87 sr 86 sr ratios found along the entire luzon arc the samples from baguio are equivalent to those found in samples from mid tertiary plutons from northern luzon and the bicol arc both groups of rocks are associated with westward subduction along the philippine trench where presumably deep oceanic sediments with little or no continental crustal component have been subducted the absence of continental crust below the arc in the northern luzon and babuyan segments has been suggested by several groups of researchers this together with the fact that metasomatized ultramafic nodules probably of mantle origin in disequilibrium with their batan host lavas but with similar 87 sr 86 sr and la sm cn ratios to the host rocks suggests that upper level assimilation afc processes does not appear to be the major influence on the geochemical signatures a collision zone between the upper crustal block of eastern china and taiwan and the manila trench has been recorded in the northern section of the arc taiwan sediments have also been shown to decrease in thickness along the south china sea basin nearly parallel to the manila trench from north to south the sediment source region is most probably taiwan and perhaps eastern china this latitudinal in variation sediment thickness may explain the crustal signature related to both geochemical and age parameters we suggest that this crustal input has taken place via subduction of sediment rich in a crustal component

seismic anisotropy in the upper mantle 2 predictions for current plate boundary flow models 1 the anisotropic seismic structure due to flow induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary the orientation distributions of olivine enstatite aggregates evolve along streamlines of each flow model based on each grains plastic response to the local stress strain field the effective elastic tensor for these textured aggregates provides predictions of p wave anisotropy and shear wave splitting throughout the model space p and s travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model comparison of predicted fast p direction for our method versus a finite strain based estimate shows that agreement is acceptable for much of the model space but notable differences occur in regions up to several tens of kilometers in size two dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle the effect of flow in the third dimension is addressed in a few calculations for one comparison of flow in the mantle wedge at a subduction zone the introduction of trench parallel flow causes significant changes in the predicted patterns of p wave anisotropy magnitude more than orientation and sks splitting

the geochemical balance of the rare earth elements and neodymium isotopes in the oceans profiles of ree concentrations and neodymium isotope measurements in the western indian ocean are consistent with published data from the other major oceans ree concentrations show a strong hydrographic control the influence of intermediate water masses is particularly significant surface water concentrations vary with salinity paniculate rees are less than 5 of dissolved concentration except for ce where up to 20 of total rees are present as 0 4 m particles in contrast to dissolved profiles concentration profiles of paniculate rees including ce all increase with depth ree concentrations correlate with silica over all but the deepest waters although in detail correlations are complex the neodymium isotopic composition of indian ocean seawater avg nd 0 8 3 is intermediate between that for the atlantic and pacific oceans comparison of ree data from the major oceans shows that the hrees behave more like si than do the lrees sm nd ratios are near constant for all oceans whereas ernd ratios vary between the atlantic indian and pacific oceans the fractionation of the rees in surface seawaters can be explained by fractionation factors relative to silica from 0 12 for la to 0 05 for er use of a seven box model shows that the interoceanic variations in neodymium isotopic compositions and nd concentrations cannot be reconciled unless particle water exchange is invoked the degree of exchange required is consistent with particle residence times of 1 10 yr

rare earth element distributions among minerals in a granodiorite and their petrogenetic implications a study of the distribution of lanthanide rare earths in a granodiorite from the eastern peninsular ranges batholith southern california reveals that a large fraction of the ree in this rock resides in the accessory phases sphene and allanite the minerals plagioclase alkali feldspar biotite epidote and apatite each contribute approximately 1 or less of each ree to the whole rock with the exception of eu for which plagioclase contributes 7 sphene and allanite together contain 80 to 95 of each ree each of these phases is zoned in ree concentration with substantial decreases from core to margin textural observations argue for relatively early saturation and precipitation of sphene and allanite in the magma ree zoning trends in sphene and allanite and unexpectedly low ree concentrations in largely later crystallizing minerals such as feldspar indicate that the precipitation of sphene and allanite significantly reduced ree concentrations in residual melts these results illustrate the potential that sphene and allanite have for controlling the behavior of ree in granitic magmas available information collectively suggest that the sampled granodiorite existed as a complete melt that the ree contained in the assemblage of phases were derived by direct crystallization from the melt and that the melt behaved essentially as a closed system once crystallization of the phases now present began close correspondences between the major and trace element chemistries of the granodiorite and phenocryst poor lavas from similar tectonic settings support these conclusions the ree pattern of the granodiorite melt appears to have originated at depth and is characteristic of its source regions and derivation mechanism the high liquidus temperature of a granodiorite melt 1000 c indicates the importance of mantle derived components within the sources of batholithic magmas in the peninsular ranges

fast forward modeling of compressional arrival slowness logs in high angle and horizontal wells abstractcompressional p and shear s sonic propagation modes are routinely measured with modern borehole acoustic logging instruments slownesses inverse of velocity of p and s modes provide information on compressional and shear slownesses of rock formations surrounding the wellbore respectively when a high angle well penetrates horizontal thin layers the layers function as waveguides for the propagation of sonic waves layer trapped and borehole guided waves interfere with each other giving rise to abnormal amplitude variations and phase discontinuity in the time waveforms acquired at receivers consequently waveforms measured across the receiver array often exhibit low semblance which renders conventional waveform semblance based processing techniques inadequate for reliable estimation of formation slowness instead of calculating waveform semblance we invoke compressional first arrival times to describe the propagation properties of the p mode in high angle and horizontal ha hz wells a

distribution of ree and trace elements in size and mineral fractions of high purity quartz sands eight sample sets raw quartz sand quartz fraction 2 m clay fraction 2 20 m fraction and heavy minerals of the cretaceous quartz sand deposits of haltern and weferlingen central germany and the tertiary quartz sand of frechen western germany were studied to characterize the distribution of ree and trace elements and to get genetic information about provenance and sedimentologic history of the sediments the quartz sand samples from different localities have rather similar trace element distribution and ree patterns with enriched lree and negative eu anomalies however these relations can be modified by local changes in syn or postsedimentary geochemical conditions adsorption of ree and trace elements on humic substances under reducing conditions and postsedimentary weathering in the quartz sand of weferlingen resulted in significantly modified element abundances the concentration of trace elements is most important in the 2 m fraction and in the heavy mineral fraction where they may be enriched by a factor of up to 100 the chondrite normalized ree distribution of the 2 m fraction is quite similar to those in the north american shale nas with average eu sm ratios of 0 21 and layb cn of 6 1 low trace element abundances in the 2 and 2 20 m fractions were attributed to quartz dilution and quantified among the common minerals in the heavy mineral fractions especially zircon and the fe 1bti oxides rutile ilmenite magnetite influence the trace element distribution investigations concerning the provenance of the quartz sands illustrate that the host sediments have a rather homogeneous geochemical signature due to their sedimentological history taking into account th ta variations the quartz sands of weferlingen and haltern can be distinguished from the frechen quartz sand additionally discriminations based on element ratios within the quartz fraction th sc ba sc ba co cs sc or li sc seem to be promising

development of the east asian monsoon mineralogical and sedimentologic records in the northern south china sea since 20 ma we here reconstruct the past change of the east asian monsoon since 20 ma using samples from ocean drilling program odp site 1146 in the northern south china sea based on a multi proxy approach including a monomineralic quartz isolation procedure identification of clay minerals by x ray diffraction xrd and grain size analysis of isolated terrigenous materials terrigenous supply to odp site 1146 was dominated by changes in the strength of multiple sources and transport processes grain size data modeled by an end member modeling algorithm indicate that eolian dust from the and asian inland and fluvial input have contributed on average 20 and 80 of total terrigenous material to odp site 1146 respectively specifically about 40 53 of the total quartz feldspar and only 6 11 of the total clay is related to eolian supply at the study site detailed analysis of the sedimentary environment and clay minerals combined with previous studies shows that smectite originates mainly from luzon kaolinite from the pearl river and illite and chlorite from the pearl river taiwan and or the yangtze river the proportion and mass accumulation rate mar of the coarsest end member em1 interpreted as eolian dust ratios of illite chlorite smectite quartz feldspar and mean grain size of terrigenous materials at odp site 1146 were adopted as proxies for east asian monsoon evolution the consistent variation of these independent proxies since 20 ma shows three profound shifts in the intensity of east asian winter monsoon relative to summer monsoon as well as aridity of the asian continent occurred at similar to 15 ma similar to 8 ma and the youngest at about 3 ma in comparison the summer monsoon intensified contemporaneously with the winter monsoon at 3 ma the phased uplift of the himalaya tibetan plateau may have played a significant role in strengthening the asian monsoon at similar to 15 ma 8 ma and 3 ma c 2007 elsevier b v all rights reserved

trace element geochemistry and sedimentary provinces a study from the tasman geosyncline australia abstract the abundances of la th u and hf have been determined in flysch sequences of the tasman geosyncline australia the arc derived graywackes differ from those of rifted continental margins and marginal basins in containing low abundances of la th u and hf la 9 2 1 7 ppm th 1 4 0 6 ppm u 0 52 0 2 ppm hf 2 1 0 6 ppm reflecting similar low abundances in the andesitic source rocks graywackes of rifted continental margins and marginal basins in contrast are mature and characterised by high abundances of la th u and hf la 39 9 9 ppm th 16 0 6 ppm u 3 40 0 5 ppm hf 7 9 ppm mirroring increased abundances in the granitic and sedimentary source rocks sedimentary rocks deposited in inter arc basins show wide dispersion but have characteristics intermediate between these two types the th u ratio for magmatic arc sediments is 2 7 for most other sedimentary rocks it remains uniform at 4 5 0 3 for sediments having andesitic and dacitic rocks as provenance the la th ratios are 6 7 and 4 5 respectively this ratio remains constant at 2 6 for most quartzose sedimentary rocks

crust and upper mantle p wave velocity structure beneath valles caldera new mexico results from the jemez teleseismic tomography experiment new results are presented from the teleseismic component of the jemez tomography experiment conducted across valles caldera in northern new mexico we invert 4872 relative ital p wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km the three principle features of our model for valles caldera are 1 near surface low velocities of minus 17 percent beneath the toledo embayment and the valle grande 2 midcrustal low velocities of minus 23 percent in an ellipsoidal volume underneath the northwest quadrant of the caldera and 3 a broad zone of low velocities minus 15 percent in the lower crust or upper mantle crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast near surface low velocities are interpreted as thick deposits of bandelier tuff and postcaldera volcaniclastic rocks lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast beneath the valle grande we interpret the midcrustal low velocity zone to contain a minimum melt fraction of 10 percent while we cannot rule out the possibility that this zone is the remnant 1 2 ma bandelier magma chamber the eruption history and geochemistry of the volcanic rocks erupted in valles calderamore following the bandelier tuff make it more likely that magma results from a new pulse of intrusion indicating that melt flux into the upper crust beneath valles caldera continues the low velocity zone near the crust mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle in either case this low velocity anomaly indicates that underplating by mantle derived melts has occurred copyright 1998 american geophysical union less

rare earth element geochemistry of oceanic ferromanganese nodules and associated sediments abstract analyses have been made of ree contents of a well characterized suite of deep sea 4000 m principally todorokite bearing ferromanganese nodules and associated sediments from the pacific ocean ree in nodules and their sediments are closely related nodules with the largest positive ce anomalies are found on sediments with the smallest negative ce anomalies in contrast nodules with the highest contents of other rare earths 3 ree are found on sediments with the lowest 3 ree contents and vice versa 143 nd 144 nd ratios in the nodules 0 51244 point to an original seawater source but an identical ratio for sediments in combination with the ree patterns suggests that diagenetic reactions may transfer elements into the nodules analysis of biogenic phases shows that the direct contribution of plankton and carbonate and siliceous skeletal materials to ree contents of nodules and sediments is negligible inter element relationships and leaching tests suggest that ree contents are controlled by a p rich phase with a ree pattern similar to that for biogenous apatite and an fe rich phase with a pattern the mirror image of that for sea water it is proposed that 3 ree concentrations are controlled by the surface chemistry of these phases during diagenetic reactions which vary with sediment accumulation rate processes which favour the enrichment of transition metals in equatorial pacific nodules favour the depletion of 3 ree in nodules and enrichment of 3 ree in associated sediments in contrast ce appears to be added both to nodules and sediments directly from seawater and is not involved in diagenetic reactions

geochemistry of surface sediments off the southern and western coasts of korea abstract the continental shelf off the southern and western side of the korean peninsula is part of a modern epicontinental sea and is an effective sediment trap for clastic material derived from china and korea we have analyzed 179 surface sediment samples from this shelf for al fe na k mg ca ti p mn sr ba v cr co ni cu zn organic carbon and calcium carbonate in order to determine the geochemical characteristics of these sediments results showed a wide range of element contents which are mainly related to the textural variability of sediments exceptions to this were found in the contents of ca k sr and ba the ca and sr contents were largely determined by the amounts of carbonate present the k content on the other hand appeared to be influenced by both the contents of illite and feldspar and ba of potassium feldspar for muds i e sediments deposited under present day conditions it is possible to distinguish those in the central yellow sea from those in the keum estuary the former are characterized by high contents of fe na k mg ca and v and the latter by high contents of mn ba and co these differences become more apparent when these elements are normalized relative to al in order to minimize grain size effects the distribution of v al ratios is different from that of other elements the highest v al values occur in muds from the central yellow sea derived from the huanghe the yellow river in china whereas the highest values of mn al are found in the nearshore area adjacent to the korean peninsula based on these results it is suggested that the sources of fine sediments in the yellow and south seas can be distinguished from the geochemical characteristics of the surface sediments in the yellow sea two different sources of fine grained sediments can be seen the huanghe and the keum river the former is characterized by higher v contents and the latter by higher mn contents using the v al ratio as an end member component of the two source materials the dispersal patterns of the fine sediments 6o were also derived

investigating the history of east asian monsoon and climate during the last glacial interglacial period 0 140 000 years mineralogy and geochemistry of odp sites 1143 and 1144 south china sea abstract monsoon climate is an important component of the global climatic system a comprehensive understanding of its variability over glacial interglacial time scales as well as of its effects on the continent and in the ocean is required to decipher links between climate continental weathering and productivity a detailed multiproxy study including bulk and clay mineralogy grain size analysis phosphorus geochemistry sedex extraction organic matter characterization and nitrogen stable isotopes was carried out on samples from odp sites 1143 and 1144 leg 184 south china sea covering the past 140 000 years we tentatively reconstruct the complex sedimentation and climatic history of the region during the last glacial interglacial cycle when sea level variations linked to the growth and melting of ice caps interact with monsoon variability during interglacial periods of high sea level summer monsoon was strong and humid and warm climate characterized the adjacent continent and islands clay minerals bear signals of chemical weathering during these intervals high calcite and reactive phosphorus mass accumulation rates mars indicate high productivity especially in the southern region of the basin during glacial intervals strong winter monsoon provided enhanced detrital input from the continent as indicated by high detrital mar glacial low sea level resulted in erosion of sediments from the exposed sunda shelf to the south and clay mineral variations indicate that warm and humid conditions still prevailed in the southern tropical areas enhanced supply of nutrients from the continent both by river and eolian input maintained high primary productivity reduced circulation during these periods possibly induced active remobilization of nutrients such as phosphorus from the sediments intense and short cold periods recorded during glacial and interglacial stages correlate with loess records in china and marine climatic records in the north atlantic confirming a teleconnection between low and high latitude climate variability

major and trace element variations of the sediments at odp site 1144 south china sea during the last 230 ka and their paleoclimate implications abstract we present here the major and trace element data of the sediments at odp site 1144 of the last 230 ka with time resolution about 1 5 kyr to investigate their relations to the climate changes estimated from tio2 and cao contents over 70 in weight of the sediments are terrestrial materials and caco3 abundance ranges between 7 and 19 the variation patterns of some of the major elements such as al fe k mg and mn and some of the trace elements such as the alkali elements such as rb and cs the alkali earth elements such as ba and most transition metals such as sc v co cr zn well match that of the oxygen isotopes of the planktonic foraminifer after normalized with ti with higher values during interglacial periods and lower values during glacial periods whilst the na abundance exhibits inverse variation patterns with lower values during interglacial periods and higher values during glacial periods however most refractory elements such as y zr nb hf ta rees th and u do not show such patterns these agree well with the behaviors of these elements during chemical weathering therefore the changes of chemical weathering intensity in south china the source area of these sediments may account for such variation patterns during interglacials wet and warm climate might occur at south china which resulted in stronger chemical weathering and dry climate might occur during glacials which resulted in weaker chemical weathering such paleoclimate interpretation agrees with those from other paleoclimate proxies such as pollen records indicating that change of the east asian monsoon especially change of the winter monsoon is the key for such paleoclimate variation patterns

detrital fine grained sediment contribution from taiwan to the northern south china sea and its relation to regional ocean circulation article i nfo results of clay mineralogy in 140 samples collected in major rivers and lakes in southwestern taiwan and on the seafloor off taiwan in the northeastern south china sea scs combined with clay mineral records of the pearl river drainage basin rivers in luzon and the south china shelf and slope are used to semi quantitatively evaluate the detrital fine grained sediment contribution of taiwan to the northern scs the clay mineral assemblage of the taiwan sourced sediments consists dominantly of illite average 56 and chlorite 41 with very scarce kaolinite and smectite their respective distribution from the rivers and lakes to the seafloor off taiwan does not show obvious basin wide differences linear correlations of illite chemistry index with illite crystallinity and of illite crystallinity with kaolinite present two end members of provenances the pearl river and taiwan for the south china shelf and slope assuming that kaolinite in the northern scs is provided completely from the pearl river the contribution of taiwan in clay minerals is evaluated as 29 to the south china shelf and 23 to the south china slope respectively accordingly the contribution of the pearl river to the south china shelf and slope is 52 and 31 respectively the luzon arc accounts for the rest of clay mineral components for the northern scs mainly by providing smectite the bashi strait crossed branches of the southward deep north pacific deep water and the northward surface kuroshio current in the western pacific may transport taiwan sourced suspended sediments westwards to the northern scs

geochemical and nd isotopic variations in sediments of the south china sea a response to cenozoic tectonism in se asia secular variations in geochemistry and nd isotopic data have been documented in sediment samples at odp site 1148 in the south china sea major and trace elements show significant changes at ca 29 5 ma and 26 23 ma whereas ond values show a single change at ca 26 23 ma increases in al ti al k rb sr and la lu ratios and a decrease in the th la ratio of the sediments beginning at 29 5 ma are consistent with more intense chemical weathering in the source region the abrupt change in nd isotopes and geochemistry at ca 26 23 ma coincides with a major discontinuity in the sedimentology and physical properties of the sediments implying a drastic change in sedimentary provenance and environment at the drill site comparison of the nd isotopes of sediments from major rivers flowing into the south china sea suggests that pre 27 ma sediments were dominantly derived from a southwestern provenance indochina sunda shelf and possibly northwestern borneo whereas post 23 ma sediments were derived from a northern provenance south china this change in provenance from southwest to north was largely caused by ridge jumping during seafloor spreading of the south china sea associated with a southwestward expansion of the ocean basin crust and a global rise in sea level thus the geochemical and nd isotopic changes in the sediments at odp site 1148 are interpreted as a response to a major plate reorganization in se asia at ca 25 ma b 2003 elsevier science b v all rights reserved

clay mineral distribution in surface sediments of the northeastern south china sea and surrounding fluvial drainage basins source and transport clay mineralogy of surface samples throughout the northeastern south china sea scs and surrounding fluvial drainage basins has been investigated to reveal source and transport of detrital fine grained sediments three characteristic end members in clay mineral compositions were observed from three major provenances respectively major kaolinite and moderate illite and chlorite from the pearl river predominant illite and chlorite from taiwan and principal smectite from the luzon arc system the transport of four clay mineral species in the northeastern scs is influenced by various mechanisms smectite derived from the luzon arc system is transported mainly by surface current with significant influence of the kuroshio intrusion illite and chlorite from taiwan are mainly carried by deep water current while kaolinite from the pearl river is affected obviously by the westward guangdong coastal current and longshore current a westerly tongue shaped distribution of high contents of illite and chlorite mainly from taiwan on the eastern shelf may result from the westward guangdong coastal current during the winter the results suggest that clay mineral assemblages in the northeastern scs are mainly controlled by provenance supply and current transport and their downcore records do not bear contemporaneous paleoclimatic features the implication of clay minerals for proxies of the late quaternary east asian monsoon evolution is realized through both the provenance supply and current transport processes c 2010 elsevier b v all rights reserved

caribbean tectonics and relative plate motions during the last century three different ways of interpreting the tectonic evolution of the gulf of mexico and the caribbean have been proposed taking into account the bailey willis school of a permanent pre jurassic deep sea basin the edward suess school of a subsided continental terrain and the alfred wegener school of continental separation the present investigation is concerned with an outline of an interpretation which follows that of pindell and dewey 1982 an attempt is made to point out ways in which the advanced hypotheses can be tested the fit of africa north america and south america is considered along with aspects of relative motion between north and south america since the early jurasic attention is given to a framework for reconstructing caribbean plate evolution the evolution of the caribbean the plate boundary zones of the northern and southern caribbean and the active deformation of the caribbean plate

coupled seismic slip on adjacent oceanic transform faults 1 in a four and a half hour period more than 60 events in an earthquake swarm on the western boundary of the easter microplate were detected by an array of ocean bottom seismometers the larger events of the swarm were strike slip earthquakes located on two transform faults separated by about 25 km slip on the faults was closely coupled with activity alternating back and forth randomly between the two transforms coupled seismic activity is usually attributed to triggering by static stress changes or dynamic stresses in propagating shear waves generated by another earthquake but these earthquakes are too small for either mechanism to be plausible we suggest that the swarm may have been the seismic manifestation of a larger primarily aseismic slip event or slow earthquake involving both transforms perhaps triggered by dike injection on the easter pacific spreading center

holocene evolution of the coast of holland abstract the holocene evolution of the coast of holland was controlled by the complex interaction of such diverse parameters as wave and tidal climate the rate of sea level rise and the morphology of the pre transgressional surface the latter in combination with the rate of sea level rise mainly determines the location of sediments sources and sinks while the hydrodynamic parameters mainly determine the rate and direction of sediment transport in the early holocene the low sea level in the shallow southern north sea strongly affected wave climate and tidal regime however the barrier and back barrier sedimentary record since 5000 14c yrs b p gives us no reason to assume major changes in these parameters since then so we conclude that breaks in barrier development are due to variations in rate of sea level rise inl combination with the morphology of the pre transgressional surface the transformation from an open tide dominated to a closed wave dominated coast which occurred in the subboreal period and the concomitant change in barrier movement from transgressive to regressive around 5000 yrs b p are the main events in the development of the coast of holland up to the middle ages the morphology of the pre transgressional surface gives a shoreline in the late atlantic which consists of two protruding headlands separated by a large tidal basin the southern headland is the alluvial plain of the rhine and the meuse the northern headland constitutes the moraines of the texel high the tidal basin in between is connected to the north sea by a large number of inlets the rate of sea level rise at that time 1 m century outran the supply of sediment to the tidal basin after 6000 yrs b p the rate of sea level rise decreased gradually whereas the rate of sediment supply remained constant this led to a gradual decrease in the tidal prisms of the inlets as the tidal basin was filled in with sediment shortly before 5000 yrs b p the first channels silted up and closed the last tidal channel disappeared around 3300 yrs b p leaving two inlets along the coast which were both connected to the river rhine the closure of the first inlets occurred at the same time as the barrier began prograding because 1 the rate of sea level rise diminished considerably 2 not all the sand supplied by longshore and cross shore transport disappeared into the tidal basin but instead could be used for barrier progradation and 3 ebb tidal deltas of the closed inlets provided major sand sources the prograding barrier sequence enclosed between the two headlands roughly forms a closed system using the relationships between tidal prism cross sectional area of inlets and volume of the ebb tidal delta and the results of modelling of longshore transport along the subboreal coast under present day wave conditions a simple sand budget for the coast of holland is given which shows that a large part of the sand now stored in the barrier sequence was obtained by cross shore transport from the north sea

surface wave tomography of the upper mantle beneath the reykjanes ridge with implications for ridge hot spot interaction 1 we modeled fundamental mode love and rayleigh waves to study the seismic properties of the upper mantle beneath the reykjanes ridge these waves were generated by regional earthquakes occurring in the north atlantic to the south of iceland and were recorded by stations located on iceland over 12 000 measurements of the phase group arrival time and amplitude of narrow pass filtered waveforms over the period range of 9 5 100 s were used to solve for mantle shear wave velocity structure and anisotropy in a vertical plane oriented normal to the ridge axis the velocity structure contains a broad and deep low velocity zone in the upper mantle beneath the ridge a joint analysis of the seismic structure with gravity data reveals that the low velocities are consistent with elevated temperatures 75 our study shows that plume material spreads broadly outward beneath the reykjanes ridge from iceland and is not confined to a narrow lithospheric channel at distances 200 km from the ridge shear wave anisotropy indicates a predominant horizontal alignment of the fast axes of anisotropic crystals mainly above 50 km depth which can be interpreted as past horizontal ridge perpendicular flow within 200 km of the ridge the anisotropy indicates a general vertical alignment of the fast axes or an alignment such that the fast axes point along the ridge the transition to this type of anisotropy coincides with the appearance of increased hot spot ridge interaction 20 myr ago indicating that plume flow has largely disrupted mantle flow beneath the ridge since that time