microseismicity and velocity structure in the hawkes bay region new zealand fine structure of the subducting pacific plate summary hawkes bay overlies part of the shallow zone of subduction of the pacific plate beneath the australian plate in the north island of new zealand a temporary dense network of 11 seismograph stations placed in the region recorded more than 350 micro earthquakes in two months providing a data set for velocity and structure inversion the hypocentres define a seismic band 11 12 km thick dipping 6 2 beneath hawke bay and increasing to more than 20 towards the west side of the region this seismic band is thought to delineate the crust of the subducting pacific plate s to p converted phases for deeper events are consistent with a fairly large impedance contrast across the plate interface at the top of the band p and s arrival times were inverted to give a simultaneous determination of hypocentres and velocity structure results indicated a thin layer of low 4 7 ksm 1 p wave velocity in the neighbourhood of the plate interface we interpret this layer as a thickness of subducted sediments beneath the micro earthquake station network this is consistent with seismic reflection work to the south of hawkes bay and with sediment volume calculations the stress distribution as given by focal mechanisms shows that the microseismicity represents primarily intraplate rather than interplate deformation with the main band of microseismicity indicating normal faulting throughout at least the top 10 km of the subducted pacific plate the observation of reverse faulting at depth beneath the main band is interpreted as being due to the flexure of the subducted pacific plate

imaging subduction from the trench to 300 km depth beneath the central north island new zealand with vp and vp vs summary recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central north island of new zealand here we use a subset of the best recorded earthquakes in an inversion for the 3 d vp and vp vs structure the data set includes 39 123 p observations and 18 331 s observations from 1239 earthquakes and nine explosions the subducted plate is imaged as a high vp low vp vs feature vp within the mantle of the subducted slab is almost always 8 5 km s 1 which requires the ca 120 myr slab to be unusually cold the low vp vs within the subducted plate closely parallels the lower plane of the dipping seismic zone it most likely indicates fluid resulting from dehydration of serpentine in the slab mantle and the earthquakes themselves are likely to be promoted by dehydration embrittlement we identify a region with vp 8 0 km s 1 directly above the dipping seismic zone can be interpreted as sinking entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high temperature mantle wedge material in the overlying low vp region can be interpreted as rising within a return flow within the wedge the volcanic front appears to be controlled by where this dipping low vp region meets the base of the crust the thickness of the backarc crust also shows significant variation along strike in the central taupo volcanic zone tvz the crust is ca 35 km thick while southwest of mt ruapehu the crust thickens by ca 10 km there is no significant low vp zone in the mantle wedge in this southwestern region suggesting that this thicker crust has choked off mantle return flow the seismic tomography results when combined with constraints on mantle flow from previous shear wave splitting results provide a plausible model for both the distribution of volcanism in the central north island and the exceptional magmatic productivity of the central tvz

the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

dispersal and deposition of eolian and fluvial sediments off peru and northern chile relative abundances of quartz and feldspar in fine grained terrigenous sediments have been used to recognize specific continental sediment sources along northwestern south america and to trace the dispersal of specific grain size fractions from these sources to adjacent continental margin and deep sea sedimentary environments quartz feldspar ratios are low 0 6 for all fractions of sediments derived from southern peru and northern chile but such ratios increase systematically in central and northern peru as the importance of granitic and related intrusives increases relative to more basaltic extrusives and gabbroic equivalents limited published analyses of dusts collected offshore prospero and bonatti 1969 are also consistent with a latitudinally varying continental sediment source we find that clay

anomalous high frequency wave propagation from the tonga kermadec seismic zone to new zealand summary bulletins of the international seismological centre isc show very large residuals up to 15 s early for arrivals from events in the tongakermadec subduction zone to the new zealand network of seismometers the very early arrivals are confined to events south of about 22 s and shallower than about 350 km the waveforms show two distinct phases an early emergent first phase with energy in the high frequency band 2 10 hz and a distinct second phase containing lower frequency energy arriving at about the time predicted by jb tables the residuals are attributed to propagation through the cold subducted lithosphere which has a seismic velocity 5 per cent faster on average than normal ray tracing shows that the ray paths lie very close to the slab for events south of 22 s but pass well beneath the slab for events further north corresponding to the change in residual pattern this characteristic of the ray paths is due to the curved shape of the seismic zone and in particular to the bend in the zone where the louisville ridge intersects the trench at 25 s the residuals can only be explained if the high velocity anomaly extends to a depth of 450 km in the region of the gap in deep seismicity from 32 to 36 s the very high frequency character of the first phase requires the path from the bottom of the slab to the stations to be of high q and to transmit 2 10 hz energy with little attenuation the absence of low frequency energy in the first phase is due to the narrowness of the high velocity slab which transmits only short wavelength waves the second phase which contains low frequencies is identified as a p wave travelling beneath the subducted slab in normal mantle there is no need to invoke any special structures such as low velocity waveguides or reflectors to explain any of the observations the s wave arrivals show similar effects

azimuthal anomalies of short period p wave arrivals from nahanni aftershocks northwest territories canada and effects of surface topography abstract following two large earthquakes in the northwest territories of canada in 1985 and one in 1988 four field surveys were undertaken to study the extensive aftershock activity some of the seismographs employed recorded three component digital data which allowed the directions of approach of short period p wave arrivals to be analyzed at three stations observed azimuthal deviations reached as much as 40 from theoretically expected azimuths as computed from the inferred hypocenters for the events theoretical results are presented that support the hypothesis that the observed azimuthal deviations are caused principally by local topographic slopes in the vicinity of the recording instruments similar theoretical results indicate that effects of local surface topography on arrival azimuths of sh and sv waves are nearly an order of magnitude less than for p waves so that such effects will generally be unimportant in shear wave splitting studies

late cenozoic sedimentation and metal deposition in the north pacific similar major chemical variations in cores from three sites across a wide area of the north pacific are modeled by coupling the vertical and horizontal motions of the pacific plate with a constant authigenic flux of elements into the sediments superimposed over this flux is an input of detrital material of relatively constant chemical composition the major features of sediment chemistry of cores from the north pacific apparently vary in a fairly uniform and systematic manner and these variations are consistent with cenozoic plate movement elements with a significant authigenic component e g mn co cu ni display high concentrations at depth in cores that correspond to the period when the seafloor while under the low productivity subtropical gyre of the n pacific accumulated sediment at an extremely slow rate 1 mm kyr these concentrations were diluted when the sites experienced an increase in the eolian mass accumulation rate first brought about by migration into the influence of the westerlies and then dominantly by the huge input of wind blown detritus which corresponded to the onset of northern hemisphere glaciation a comparison of 10be profiles to the profiles of metal concentrations is used to reveal the occurrence of haituses in the sediment column over the past 8 m y

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

african dust observed over canary islands source regions identification and transport pattern for some summer situations bulk filtration samples of dust were collected on fuerteventura canary islands in july 1985 in order to investigate african dust transport over this region during summer air mass trajectories and elemental and mineralogical analyses allowed us to distinguish between oceanic and continental influences three african dust transport cases occured during this period geochemical and mineralogical tracers combined with air mass trajectories indicate that two different source regions of dust sahelian and moroccan ones are involved each of these exhibits specific si al and fe al ratios that can be considered as tracers of dust origin a meteorological study based on satellite imagery meteorological maps and air mass trajectories confirms the previous identification of the source region it is also possible to describe the meteorological situation controlling the atmospheric pathway of the dusts from the source regions up to the canary islands this study suggests that some source regions especially the sahelian one of dust affecting the canary islands during summer are rather different from the moroccan ones observed in a spring situation by coude gaussen et al 1987

late cenozoic records of eolian quartz flux in the sea of japan odp leg 128 sites 798 and 799 and paleoclimate in asia abstract the quartz content at sites 798 and 799 as a proxy for eolian sediment supply gives important informations about paleoclimate in asia and the paleoenvironment of the sea of japan lower pliocene sediments are characterized by rather unique low quartz accumulation rates which we attribute to equablee warm humid climatic conditions a dramatic shift to higher eolian sediment supply represented in a distinct increase in quartz flux rates near the gauss matuyama boundary probably marks an increased aridification in asia triggered by the development of major northern hemisphere glaciation near 2 5 ma late pliocene pleistocene sediments of site 798 are characterized by short term milankovitch type changes in eolian quartz supply probably reflecting glacial interglacial arid humid climatic cycles

variations in the fine scale composition of a central pacific ferromanganese crust paleoceanographic implications a 47 to 60 mm thick fe mn crust from horizon guyot water depth 1800 1780 m central pacific was used to evaluate the potential of crusts as recorders of neogene paleoceanographic and paleoclimatic conditions the chemical composition was determined by microprobe for 16 elements from a polished thin section three analyses were made per millimeter and averaged to give the composition of each millimeter the age of the crust was determined by measuring the strontium isotope composition of the crust and comparing it with the tertiary seawater curve the crust represents 18 5 m y of growth of fe and mn oxyhydroxides the crust is composed of alternating botryoidal and laminated layers the botryoidal layers formed during the same time intervals that widespread neogene deep sea hiatuses were forming in bottom sediments the botryoidal layers represent growth during times of intensified deepwater flow whereas the laminated intervals represent more quiescent conditions the correspondence between the botryoidal layers and the neogene hiatuses is so strong that we were able to choose a variable growth rate model over a constant growth rate model for the crust chemical changes in the crust take two forms the first is represented by broad changes in the composition defined chiefly by fourth order polynomial fits to the chemical profiles of each element with depth in the crust the second is high frequency changes in composition the broader changes occurred primarily at about 15 11 5 7 4 6 4 5 2 and 4 6 ma which may correlate with major changes in paleoceanographic circulation and development of ice caps at the poles the periods of the high frequency changes may reflect climatic changes that resulted from orbital forcing these high frequency changes may correspond to the high order eccentricity periods of 3 47 2 04 and 1 31 m y

dispersion of p waves in subducted lithosphere evidence for an eclogite layer cold subducted lithosphere has relatively fast seismic velocity which leads to early arrivals for some event station paths the effect is very large for events in the tonga kermadec deep seismic zone recorded at certain new zealand stations these particular arrivals are very high frequency 3 hz or greater and sometimes resemble two distinct phases the later arrival appearing at about the time predicted by jeffreys bullen tables data from the digital station snzo in wellington confirm the travel time results of the analog stations and furthermore show frequencies above 5 hz much higher than can be seen on analog records and up to 4 dispersion in the range 1 8 hz energy in the second phase which is often absent at snzo is mainly 1 2 hz the digital data support the idea proposed earlier that the effect is caused by propagation through a thin slab which passes only short wavelength waves the essential features of the wave propagation are modeled by acoustic waves in a one dimensional high velocity slab the waveforms produced by the model are discussed in terms of the leaky modes of the system and calculated by a reflectivity method a very thin 15 km uniform slab provides the required dispersion but the waves are heavily attenuated and would never be observed at teleseismic distances a thicker slab allows the energy through but does not give enough dispersion altering the variation of velocity across the slab provides the required dispersion if a thick high velocity layer with wave speed increasing gradually with height is overlain by a thin lid of even higher velocity for the models considered the lid thickness must lie in the range 6 15 km and be continuous from a depth of about 50 km to the bottom of the earthquake zone the thick layer could arise from the thermal anomaly in the subducted lithosphere the thin lid may be the gabbroic part of the subducted crust that has transformed to eclogite

flux and grain size variation of eolian dust as a proxy tool for the paleo position of the intertropical convergence zone in the northeast pacific abstract a 328 cm long piston core kodos 02 01 02 collected from the northeast equatorial pacific at 16 12 n 125 59 w was investigated for eolian mass fluxes and grain sizes to test these proxies as a tool for the paleo position of the intertropical convergence zone itcz the eolian mass fluxes of the lower interval below 250 cm 15 5 7 6 ma are very uniform at 5 1 mg cm2 103 yr while those of the upper interval above 250 cm from 7 6 ma are over 2 times higher than the lower interval at 12 1 mg cm2 103 yr the median grain size of the eolian dusts in the lower interval increases from 8 4 to 8 0 downward while that of the upper interval varies in a narrow range from 8 8 to 8 6 the determined values compare well in magnitude to those of central pacific sediments for the upper interval and equatorial and southeast pacific sediments for the lower interval this result suggests a possibility that the study site had been under the influence of southeast trade winds at its earlier depositional period due to the northerly position of the itcz and subsequently of the northeast trade winds for a later period when the upper sediments were deposited this interpretation is consistent with a mineralogical and geochemical study published elsewhere that assigned the provenance of the study core dust to central south america for the lower interval and to asia for the upper interval this study suggests that the distinct differences in eolian mass flux and grain size observed across the itcz can be used to trace the paleo latitude of the itcz

linking ocean and continental records of paleoclimate determining accord and discord in the records climate forcing at milankovitch and sub milankovitch frequencies are evaluated by linking climate proxy data from terrestrial and ocean records at two scales of temporal resolution 1 relatively coarse resolution over the last 500 ka and 2 higher resolution over the last 200 ka we determine that climate proxy data from hole 810c rep resent regional atmospheric and oceanic processes by comparison to core v21 146 both from the northwest pacific ocean we then compare records that represent regional deposition from china and the western pacific ocean to focus on determining milankovitch type periodicities grain size and magnetic susceptibility data from hole 810c and baoji china are contrasted using a time sensitive wavelet analysis the analyses indicate a general correspondence to milankovitch frequencies although there is no specific frequency that dominates throughout the records additionally the dominant 100 000 year frequency does not occur at the same time in the records these factors suggest that differ ences in the physical mechanisms of deposition may influence the preserved apparent periodicities in these environ ments lastly climate events over the last 200 000 years in marine and terrestrial records are correlated at a high resolution in the depth domain the last interglacial glacial boundary is consistently placed at the base of a cold period c20 in the terrestrial and marine records however the cold period in the ocean data 18 o is very subdued relative to terrestrial records additionally over the last 200 ka periods of minimum insolation intensity do not correspond lin early to periods of maximum grain size and ice volume variations revealed in the loess and ocean records respectively however during the warm interstadials of the last two glaciations peaks in insolation intensity compare favourably with the times of strongest pedogenic development in the continental records and minimum ice volume in the ocean records resume le forcage du climat a des frequences milankovitch et sous milankovitch est evalue en faisant un lien entre les donnees indirectes de sources terrestres et oceaniques a deux echelles de resolution dans le temps 1 une resolu tion relativement grossiere pour les dernieres 500 ka et 2 une plus haute resolution pour les dernieres 200 ka nous determinons que les donnees indirectes du trou 810c representent des procedes atmospheriques et oceaniques regionaux par rapport a la carotte v21 146 les deux provenant du nord ouest de l ocean pacifique nous avons ensuite compare les registres qui representent une deposition regionale en provenance de la chine et de l ouest de l ocean pacifique afin de se concentrer sur la determination de frequences de type milankovitch des donnees granulometriques et de suscep tibilite magnetique provenant du trou 810c et de baoji china sont mises en contraste au moyen d une analyse tempo relle des ondelettes les analyses indiquent une bonne correspondance generale aux frequences de milankovitch bien qu aucune frequence specifique ne domine a travers les registres de plus la frequence principale de 100 000 ans ne se presente pas au meme moment dans les registres ces facteurs suggerent que des differences dans les mecanismes phy siques de deposition peuvent influencer les periodicites apparentes preservees dans ces environnements finalement dans le domaine de la profondeur il y a correlation a haute resolution entre les registres marins et terrestres des evene ments climatiques au cours des derniers 200 000 ans la derniere limite glaciaire interglaciaire est constamment placee a la base d une periode froide c20 dans les registres marins et terrestres toutefois la periode froide dans les don nees oceaniques 18 o est tres attenuee par rapport aux registres terrestres de plus au cours des dernieres 200 ka des periodes d intensite minimale d insolation ne correspondent pas de facon lineaire aux periodes de granulometrie maximale et de variations des volumes de glace qui sont revelees respectivement dans les registres oceaniques et de loess toutefois au cours des rechauffements interstadiaires des deux dernieres glaciations des sommets dans l intensite d insolation se comparent favorablement avec les temps du plus fort developpement pedogenique dans les re gistres continentaux et avec le volume minimal de glace dans les registres oceaniques traduit par la redaction 848

neogene history of the south pacific tradewinds evidence for hemispherical asymmetry of atmospheric circulation rea d k and bloomstme m k 1986 neogene history of the south pacific tradewmds evidence for hemlsphermal asymmetry of atmospherm circulation palaeogeogr palaeochmatol palaeoecol 55 55 64 eohan dust grains extracted from southeast pacific pelagic sediments provide a neogene record of southern hemisphere atmospherm processes the mass accumulation rate of dust has been low and generally constant at 1 4 mg cm 2 103 y since the late ohgocene eohan grains ze data show an increase from 8 20 3 40 t m older sediments to 7 20 6 80 t m younger materml this shift occurred about 10 5 m y ago and reflects a s gmfmant increase in the intensity of atmospheric circulation then there s no obwous response to the onset of northern hemisphere glacmtmn 2 5 m y ago m these eohan records from the south pacific compamson with existing lnformatmn from the northern hemisphere suggests that the southern hemisphere has had more intense atmospheric circulation throughout the neogene and that this asymmetry reached its greatest extent between 10 5 and 2 5 ma

velocity and anisotropy structure at the hikurangi subduction margin new zealand from receiver functions summary we examine seismic velocity and anisotropy at the interface between the australian and the subducting pacific plate in southern north island new zealand using receiver functions from teleseismic earthquakes recorded on two l shaped seismic arrays iris station snzo makes up the corner of the southern array velocity models are tested with a ray theoretical method that allows both dipping boundaries and anisotropy assuming a 2 d structure beneath each array the strongest arrival on the northern array consistently appears at 2 s the arrival is modelled by a jump in velocity at a flat boundary at 12 km which may be caused by a change in metamorphic grade this prominent arrival correlates with a weaker arrival at 1 5 s 7 km at station snzo isotropic layers dipping with the slab can explain many features in the first 20 s of the records but anisotropy is needed to explain both the transverse receiver functions and a change in the polarity of a prominent ps arrival at 3 s delay relative to p o nthe radial component this arrival is consistent with a change between an upper layer that is either isotropic or anisotropic with a horizontal symmetry axis and a supraslab metasediment layer having anisotropy with av ertical slow axis of symmetry there is tradeoff between boundary dip the plunge of the axis of symmetry layer thickness and the amount of anisotropy a dip of 20 shallowing to 15 under the tararua array thickness of 4 km and a vertical symmetry axis fits the backazimuth variation of the amplitudes and polarities of the arrivals with anisotropy of 4 6 per cent observed layer multiples are smaller than synthetic predictions to explain this discrepancy either layer boundaries are rough attenuation is strong or the layer boundaries are more gradational at the periods 1 s used in this study compared to previous studies 5 s

shallow morphology of the subducted pacific plate along the hikurangi margin new zealand abstract along the east coast of the north island new zealand the pacific plate is subducted beneath the australian plate the seismicity associated with this subduction has been determined using the results from a number of temporary and permanent microearthquake networks a synthesis of the microearthquake seismicity the depths of thrust events the depth of s to p conversions and deep reflection seismic studies leads to a coherent picture of the subducting slab both in terms of its mechanical structure and in terms of the general morphology the slab is initially subducted at a low angle of a few degrees is bent through a circular arc of few hundred kilometre radius and then becomes planar again below 120 km depth the radius of curvature varies along strike from about 280 km just north of wellington to 240 km north of hawke bay the slab shape between depths of 15 45 km is thus approximately conical it also approximates within the errors of hypocentre location a surface of constant gaussian curvature as would be geometrically expected from the simple bending of a thin spherical lithospheric shell observed seismicity is primarily concentrated within what is interpreted as the subducted crust of the pacific plate with a lower level of activity within the subducted mantle normal fault mechanisms at the top of the plate probably reflect tensional stress associated with the plate bending whereas thrust events observed at the lower bound of the observed seismicity probably reflect compression below the neutral plane

effect of subducting sea floor roughness on fore arc kinematics pacific coast costa rica fault kinematics and uplift in the costa rican fore arc of the middle america convergent margin are controlled to a large extent by roughness on the subducting cocos plate along the northwest flank of the incoming cocos ridge seafloor is characterized by short wavelength roughness related to northeast trending seamount chains onland projection of the rough subducting crust coincides with a system of active faults oriented at high angles to the margin that segment the fore arc thrust belt and separate blocks with contrasting uplift rates trunk segments of pacific slope fluvial systems typically follow these margin perpendicular faults regionally developed marine and fluvial terraces are correlated between drainages and across faults along the costa rican pacific coast terrace separations across block bounding faults reveal a pattern of fore arc uplift that coincides roughly with the distribution of incoming seamounts magnitude and distribution of quaternary uplift along the costa rican pacific coast suggests that despite a thin incoming sediment pile the inner fore arc shows an accumulation of mass a characteristic that may be due to underplating of seamounts beneath the fore arc high

late pliocene onset of glaciation ice rafting and diatom stratigraphy of north pacific dsdp cores abstract the revised diatom biostratigraphy for the north pacific allows the timing of ice rafting as recorded in deep sea drilling project cores to be determined significant ice rafting began at the time of the matuyama gauss reversal boundary about 2 48 ma other bio and lithostratigraphic data from the pacific atlantic and arctic oceans and the black sea all indicate that northern hemispherical cooling and the accumulation of continental ice began at 2 4 2 5 ma we interpret all these data to indicate northern hemisphere ice cap formation at that time if so the closing of the isthmus of panama approximately 3 2 m y ago was not the immediate cause of the plio pleistocene glaciations

iron isotopic fractionation during continental weathering abstract the effect of continental weathering on the iron isotope compositions of natural materials is investigated unweathered igneous rocks pelagic clay and dust fall within the range 56 fe 0 0 3 rivers with large suspended loads also have 56 fe values near zero dilute streams have 56 fe values that trend towards lower 56 fe 1 suggesting that dissolved riverine iron is isotopically light relative to igneous rocks bulk soil and soil leaches display systematically different 56 fe profiles indicating that isotopically distinct fe pools are generated during pedogenesis nannofossil ooze which contains fe scavenged from the ocean water column has 56 fe 0 but is consistent with seawater dissolved fe having negative 56 fe it is inferred that continental weathering under modern oxidizing earth surface conditions preferentially releases dissolved fe with negative 56 fe which is transported in rivers to the ocean a preliminary analysis of the marine fe budget suggests that riverine fe has a substantial role in determining the 56 fe of both the modern and ancient oceans but other inputs particularly that from diagenesis of marine sediments may also be important since the chemical pathways of fe processing during weathering are dependent on oxidation state and biological activity fe isotopes may prove useful for detecting changes in these parameters in the geologic past

kinematics associated with late cenozoic deformation in central costa rica western boundary of the panama microplate we present kinematic data for late cenozoic deformation in central costa rica that marks the western margin of the panama microplate i e the caribbean panama boundary this boundary extends from the north panama deformed belt west through the valle central in costa rica and then southwest along the east nicoya fracture zone to intersect the middle america trench terrace correlation and basin asymmetry indicate a major change in tectonic evolution across the boundary where three regional northeast striking faults intersect the pacific coast mesoscopic fault populations are consistent with transtension across these north east striking faults and with transpression within the east trending valle central this late tertiary and quaternary transcurrent deformation links the north panama deformed belt to the east with the middle america trench to the west earthquake focal mechanisms are consistent with mesoscopic fault data suggesting that fault populations characterize the present day stress field this deformation marks the western extent of the panama microplate a fragment of volcanic arc that separated from the caribbean plate in the late tertiary or early quaternary and is currently advancing northward due to collisions with south america to the east and the indenting cocos ridge on the cocos plate to the south

organic geochemistry of late cenozoic sediments from the subtropical south atlantic ocean abstract organic matter has been characterized in samples of pleistocene pliocene and miocene sediments from seven deep sea drilling project sites in the subtropical south atlantic ocean organic carbon concentrations average 0 3 for most samples and n alkanoic acid n alkanol and alkane biomarkers indicate extensive microbial reworking of organic matter in these organic carbon lean sediments samples from the easternmost parts of the south atlantic contain an average of 4 1 organic carbon and reflect the high productivity associated with the benguela current lipid biomarkers show less microbial reworking in these sediments eolian transport of land derived hydrocarbons is evident at most of these oceanic locations

upper mantle p wave velocities in the southeast north island new zealand abstract reversed pn velocities have been determined from regional earthquake sources for the wairarapa region in the southeast north island new zealand values were 8 64 0 13 km s in the easternmost part of the region and 8 90 0 13 km s in the central wairarapa which are consistent with subducted pacific plate lying at shallow depth under the region a value of 8 22 km s between wellington and the wairarapa region is interpreted as a possible difference in the velocity structure above the refractor between the two localities

continental and marine paleoenvironments reflected by west pacific clay sedimentation various stratigraphic and paleoenvironmental results arise from mineralogical geochemical and micromorpho logical investigations on late cretaceous and cenozoic sediments recovered by the deep sea drilling project in the west pacific stratigraphic correlations the deep pelagic red clays deposited on either side of the shatsky rise contain a variety of clay minerals which point to the importance of eolian detrital supply and allow rough stratigraphic correlations climate and paleocirculation neogene sediments of the lord howe rise area reflect closely the relationship between the evolution of the antarctic ice sheet and related aridification stages and the atmospheric and oceanic circulation pattern since 14 my regional and global tectonics the post eocene deposits around new zealand express a sequence of tectonic events and of relaxation stages associated with changes in the circulation regime along the inner wall of the japan trench clay mineral successions document the existence of an outer continental landmass until the late oligocene its collapse associated with volcanism close to the paleogene neogene boundary and an increased detrital influence from japan since the early middle miocene volcanism diagenesis and detrital supply the aptian to maastrichtian sediments in the mariana basin document a complex sequence of geochemical and geodynamical events proximal volcanism and local supply from subaerial archipelagoes early marine diagenesis during lithospheric subsidence and plate migration and finally the occurrence and evolution of terrigenous input from distant sources distributed by marine currents

quaternary fluctuations in the northern hemisphere trade winds and westerlies the mass accumulation rate and grain size of the total colian component isolated from pelagic sediment in two north pacific cores piston core kk75 02 under the prevailing westerlies and deep sea drilling project site 503 beneath the trade winds have been used to evaluate changes in the intensity of atmospheric circulation and source area aridity over the past 700 000 yr the eolian grain size a direct indicator of wind intensity fluctuates at periodicities similar to those calculated for the earth s orbital parameters of precession obliquity and eccentricity both sites display greater variability in wind intensity prior to 250 000 yr ago eolian accumulation rates an indicator of source area aridity fluctuate at periodicities similar to those of the glacial interglacial cycles lower eolian accumulation rates during colder glacial times most likely reflect increased glacial age humidity at central asian and central american source areas

a three dimensional image of shallow subduction crustal structure of the raukumara peninsula new zealand summary earthquake arrival time data from a 36 station deployment of portable seismographs on the raukumara peninsula have been used to determine the 3 d vp and vp vs structure of this region of shallow subduction a series of inversions have been performed starting with an inversion for 1 d structure then 2 d and finally 3 d this procedure ensures a smooth regional model in places of low resolution the subducted plate is imaged as a northwest dipping feature with vp consistently greater than 8 5 km s 1 in the uppermost mantle of the plate structure in the overlying plate changes significantly along strike in the northeast there is an extensive low velocity zone in the lower crust underlying the most rapidly rising part of the raukumara range it is bounded on its arcward side by an upwarp of high velocity a viable explanation for the low velocity zone is that it represents an accumulation of underplated subducted sediment while serpentinization of the uppermost mantle may be responsible for the adjacent high velocity region the low velocity zone decreases and the adjacent high velocity region is less extensive in the southwest this change is interpreted to be related to a change in the thickness of the crust of the overlying plate in the northeast the crust is thinner and subducted sediment ponds against relatively strong uppermost mantle while in the southwest the crust is thicker and the relatively weak lower crust allows sediment subduction to greater depths a narrow zone of high vp vs parallels the shallow part of the plate interface this suggests elevated fluid pressures with the distribution of earthquakes about this zone further suggesting that these pressures may be close to lithostatic the plate interface at 20 km depth beneath the raukumara peninsula may thus be a closed system for fluid flow similar to that

evaluating the accretion of meteoritic debris and interplanetary dust particles in the gpc 3 sediment core using noble gas and mineralogical tracers abstract extraterrestrial et noble gases helium and neon in 35 sediment samples from central pacific core ll 44 gpc 3 demonstrate the variable flux of interplanetary dust particles idps and major meteorite impacts over the past 70 ma 21 72 ma spinel mineralogical and chemical compositions clearly distinguish major impact events from the continuous flux of idps including the well established cretaceous tertiary k t and late eocene e o impact boundaries no spinel grains with chemical or mineralogical evidence of a distinctly et origin were found in an extensive survey of background samples i e non e o or k t boundary suggesting that either the carrier grains for et noble gas occur within the fe ti oxide mineral fraction observed in this study found to include ilmenite and ulvospinel or are too small for identification by sem the presence of ilmenite and ulvospinel suggest lunar regolith is a potential source for et noble gas rich particles noble gas analysis on both the emf extractable magnetic fraction and the bulk minus emf bulk emf show that the he and ne compositions are consistent with partially degassed noble gas signatures of zero age magnetic grains z mag and stratospheric interplanetary dust particles idps conclusive evidence for a planetary ne a noble gas signature is found only in the bulk sediments at the k t boundary although all gpc 3 k t fractions bulk emf and hf digestion plot along a mixing line between planetary ne a and solar wind sw spinels from major impact boundaries k t e o exhibit dendritic texture and elevated ni consistent with previous reports in contrast to the otherwise consistent 3he signal from idps the 3he at the known impact boundaries k t and e o actually decreases these anomalously low 3he are accompanied by significantly elevated ne and significantly lower 3he 20ne solar ratios 10 lower produced by both preferentially degassing of he relative to ne at times of increased flux of larger et material degassed degassed he enriched ne profile noble gas characteristics occur in two sample intervals that do not correspond to any known impact events 47 and 71 ma explained by an influx of larger particles sem analysis of the 47 ma sample shows spinels with dendritic textures but without distinctive markers of large meteorite impacts e g elevated ni particle size increases and degassed signatures may be caused by major bolides micrometeorites comet showers or simply a flux of larger idps potentially with a different source

crustal heterogeneity and subduction processes 3 d vp vp vs and q in the southern north island new zealand summary along the southern hikurangi subduction zone in the southern north island new zealand the subduction zone changes from volcanic with moderately weak interseismic coupling to non volcanic with strong coupling in wellington to investigate how material heterogeneity relates to subduction processes we compute 3 d seismic properties using 342 earthquakes and 2 shots recorded by a 165 station seismic array the time base for some telemetered temporary stations was not stable which required revising the inversion code to allow an additional origin time related to the telemetered stations progressive inversions from 2 d to fine 3 d were carried out for vp vp vs and qp combined interpretation of these three seismic properties highlights features of the subduction zone the subducted slab is imaged as a high velocity high q feature in the overlying crust from southeast to northwest there is low velocity in the accretionary wedge high velocity in the axial ranges and low velocity in the wanganui basin high vp vs and low qp characterize the crust east of the north island dextral fault belt high vp vs is imaged above the subducted slab to 40 km depth in the northern half of the study area low qp is also prominent above the slab particularly trenchwards of the volcanic region in marked contrast to the high vp vs the low qp above the slab extends as a continuous feature up to the low qp in the shallow crust underlying the volcanic region the low qp and high vp vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release vp vs is particularly sensitive to overpressured fluid in the compressional forearc whereas qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region the rheology of the lower crust of the overlying plate is a factor in large scale subduction coupling as evidenced by the terrane boundaries and haast schist in the section near wellington the changing rheology of the lower crust of the overlying plate in the southern north island also provides a possible explanation for the southward cessation of volcanism

three dimensional seismic imaging of the costa rica accretionary prism field program and migration examples seismic reflection techniques are the primary geophysical tool used to examine the structure of continental margins on convergent plate margins widely separated seismic reflection profiles often do not image complex structural features because out of plane reflections and diffractions obscure the seismic images yet the structural features are often critical to understanding the tectonic processes thus the resulting geologic interpretations are based on images that may not accurately represent the true subsurface structure to image the complex geologic structures of an active continental margin a three dimensional 3 d seismic survey was conducted off of the nicoya peninsula of costa rica in april 1987 over 60 000 shot points of 96 trace multichannel data were collected in a 9 22 km grid this detailed survey was located over the accretionary prism and designed to investigate the active tectonic processes and evolution of this continental margin in this paper we outline the data acquisition program including the navigation processing critical to successful imaging and the seismic processing methodology and then show several examples of images improved by 3 d surveying the resulting 3 d seismic data were migrated both as independent lines and as a 3 d volume seismic images were produced as conventional depth and time sections and as cross sectional slices in depth and time we present comparisons of the migration results that show significant improvements of the subsurface image using 3 d techniques to illustrate these improvements we present images of two complex geologic areas a mud volcano and reflections near the top of the accretionary prism and the top of the subducting oceanic basement beneath the prism comparisons between the original stacked data the data of the two dimensional migration and the results of the 3 d migration illustrate the value of 3 d techniques in studies of active margins in addition to the advantages of the 3 d imaging which include better structural delineation and improved signal to noise ratio in the final image we show that the ability to view the 3 d data volume in both section and plan view offers significant interpretational advantages

fast p wave propagation in subducted pacific lithosphere refraction from the plate p waves traveling from events in the tonga kermadec seismic zone to stations in new zealand are very fast with highly emergent dispersed waveforms ray tracing has shown the waves to travel close to the subducted pacific plate throughout their length and synthetic seismogram calculations have shown the dispersion requires a very thin 8 12 km fast layer previous work has been based on data from analogue records and one digital single component short period instrument no polarization analysis was possible and measurements of dispersion were limited by the bandwidth from january 1991 to august 1992 we deployed nine broad band three component seismometers in good sites for observing these arrivals the data are augmented by three component short period digital records from new stations of the new zealand national network in this study we analyze 1191 broad band and 2076 short period seismograms from 71 events for polarization of the initial p wave the polarization directions are found to be up to 30 off the great circle path and consistently steep 20 from vertical they are too steep to be explained by standard ray paths or refraction from a fast horizontal layer we invert the polarization directions for a tilted interface beneath the array and use arrival times to control the depth to the interface which is found to lie close to the top of the subducted plate inferred from the seismicity we conclude that these precursive emergent p waves have traveled through a fast layer close to the top of the subducted plate and refract upward to the station a second arrival with lower dominant frequency near 1 hz and normal travel time is occasionally seen on both broad band and short period stations its polarization direction is similarly steep but difficult to measure the evidence suggests that it also travels within the plate with similar ray path to the precursor

seismicity structure and tectonics of the wellington region new zealand summary the wellington region lies on the border between the pacific and australian plates the former being subducted the plate interface lies at shallow depth under the region 10 60 km so that detailed and long term monitoring of the seismicity of the zone where large thrust earthquakes are expected to occur is possible hypocentres for events recorded by a 12 station seismic network for the period 1978 1982 7231 events have been refined by the use of a threedimensional velocity model derived from a simultaneous inversion of arrival time data from 129 local events for both hypocentres and velocity the hypocentres show that the subducting pacific plate is defined by a nw dipping zone of relatively intense activity the plate interface as given by the upper envelope of this activity is offset vertically by 7 km on average along a nw se line through cook strait parallel to the direction of absolute motion of the pacific plate rather than to the direction of convergence the strike of the interface at deeper levels also changes across this line this distortion may partly reflect effects of the transition to continent continent type convergence further south within the subducting pacific plate the seismic activity defines two parallel zones separated by about 20 km the lower zone being much less active and perhaps merging with the upper zone at a depth of 80 90 km composite focal mechanisms of events in the upper zone imply predominantly down dip tension with a compression axis perpendicular to the plate interface the mechanism in the lower zone also implies down dip tension but the compression axis has rotated to be horizontal and along strike indicating a component of lateral shearing within the plate these two types of mechanism correlate well with those found previously for events at depths of 150 km or more thus it seems that the large negative buoyancy of the subducted lithosphere dominates the internal stress field throughout the length of the seismically active region the lithosphere acting as a stress guide there is no evidence for a significant number of events with mechanisms indicating thrusting along the plate interface the seismic velocity in the region corresponding to the upper seismic band is close to that typical for oceanic crust which here is probably considerably thicker than normal there is no evidence for a substantial increase in velocity down

late cenozoic changes in atmospheric circulation deduced from north pacific eolian sediments abstract isolation and analysis of the eolian component of late cenozoic pelagic sediments from the north pacific provides direct information concerning changes in atmospheric circulation a 50 increase in intensity of both the prevailing westerlies and the tradewinds coincides with increasing pole to equator temperature gradients resulting from the onset of northern hemisphere glaciation at the same time the mass flux of dust from continents to the north pacific increased by a factor of 4 5 apparently reflecting significantly increased continental aridity associated with the late cenozoic glacial ages

geochemical and mineralogical characteristics in two color core sediments from the korea deep ocean study kodos area northeast equatorial pacific abstract at sites 89 and 90 of the korea deep ocean study kodos located in the western part of the area between the clarion and clipperton fracture zones the topmost sediment column can be divided into two layers by a sharp color boundary an upper pale brown layer unit a and a lower dark brown layer unit b geochemical and mineralogical properties in units a and b are clearly different across the color boundary which can be interpreted to be a hiatus with an age gap of at least 3 ma the large gap in sediment ages results in an overall difference in sediment properties between units a and b that is older unit b sediment is higher in smectite contents more severe in fossil dissolution and lower in poc contents commonly due to prolonged diagenesis disappearance of the coarse mode 50 60 m of the size distribution in unit b is a result of prolonged dissolution of siliceous fossils however other phenomena occurring just below the color boundary the most severe dissolution of fossils the most frequent occurrence of coarse grained micronodules and smectite aggregates maximum value of metals mn cu ni and increased water contents can be explained by the active stirring of bottom sediments by intensified paleo bottom currents present during the hiatus that is turbulence as well as winnowing action caused by the paleo currents had respectively accelerated the dissolution of siliceous fossils and the concentration of coarse grains at the hiatus in addition micronodule formation was promoted by the efficient formation of oxygen depleted micro environments around in situ fauna remains which were buried efficiently by increased reworking and resedimentation of bottom sediments by the paleo current activity meanwhile increases in metal and water contents just below the color boundary are the results of the concentration of micronodules enriched in metals and probable formation of woodpile fabric by abundant spicules and spines respectively active stirring of bottom sediments by the intensified paleo bottom currents as well as the large gap in sediment age between units a and b are mostly responsible for the vertical variations of geochemical and mineralogical properties in two color core sediments

shear wave velocity variation across the taupo volcanic zone new zealand from receiver function inversion summary the taupo volcanic zone tvz new zealand is a region characterized by very high magma eruption rates and extremely high heat flow which is manifest in high temperature geothermal waters the shear wave velocity structure across the region is inferred using non linear inversion of receiver functions which were derived from teleseismic earthquake data results from the non linear inversion and from forward synthetic modelling indicate low s velocities at 6 16 km depth near the rotorua and reporoa calderas we infer these low velocity layers to represent the presence of high level bodies of partial melt associated with the volcanism receiver functions at other stations are complicated by reverberations associated with near surface sedimentary layers the receiver function data also indicate that the moho lies between 25 and 30 km deeper than the 15 2 km depth previously inferred for the crust mantle boundary beneath the tvz

observations of very high p velocities in the subducted slab new zealand and their relation with the slab geometry summary a set of high quality aftershock data recorded on a closely spaced network of seismographs at wellington new zealand has enabled the determination of apparent wavefront speeds and azimuths controlled by the subducting pacific plate the former were very high 8 7 f 0 2 km s and the latter were shifted by as much as 10 eastward of the source receiver line which made an angle of about 12 downdip with the strike of the subducted slab beneath the north island the wavefront characteristics are modelled in terms of head wave propagation along a curved refractor with a cylindrical geometry using both a simple geometrical approach as well as a 3 d ray tracing program with this structure it is not possible to explain both the high apparent velocity as well as the large azimuthal anomaly without the presence of an unusually high p wave velocity of around 9 0 km s within the upper mantle of the subducted pacific plate such high velocities although reported previously in the north island are very hard to reconcile with known upper mantle rock velocities other alternatives were sought to explain the apparent slowness in particular modifications in the slab curvature along strike according to proposed models were found to contribute to the high wavefront speeds but very high intrinsic p velocities 8 8 km s l are still required from slowness vector data it is not possible to constrain the depth at which such velocities have to exist however traveltime considerations limit them to depths between the base of the subducted oceanic crust to about 12 km deeper

clay mineralogy and chemistry as indicators of hemipelagic sediment dispersal south of the aleutian arc abstract near surface hemipelagic sediments from across the aleutian arc trench system were analyzed for clay mineral composition by x ray diffraction in addition oriented aggregates of clay sized fractions were analyzed for their chemical compositions by energy dispersive x ray analysis eds in conjunction with scanning electron microscopy the main clay minerals are illite chlorite and expandable clay minerals principally smectite smectite is diagnostic of sediment derived from the aleutian volcanic arc in contrast continental sources on the alaskan mainland produce mainly illite and chlorite backarc samples adjacent to the central arc segment contain an average of 71 smectite in the central forearc however smectite values are generally less than 30 and decrease progressively eastward only on meiji sediment tongue in the far western aleutian segment do smectite contents begin to dominate the eds data correlate well with clay mineral abundances al 2 o 3 mgo and feo all increase in abundance with increasing chlorite content and al 2 o 3 and k 2 o increase in illite rich mud silica is the only oxide which increases in abundance with increasing smectite content the compositional and chemical disparity between forearc and backarc sediments is due to dilution of arc derived mud by suspended sediment derived from mainland alaska the westward transport of mud for thousands of kilometers occurs both through isobath parallel bottom currents and by westward flowing turbidity currents these results show that interpretations of tectonic provenance cannot be based on elemental concentrations or mineralogy without also taking into account the possibility of long distance transport

milankovitch cycles in neocene deep sea sediment pelagic carbonate sediments from the world ocean basins commonly show cyclic variations in amount and or degree of preservation of biogenic calcite with periodicities of several tens to several hundreds of thousands of years the direct causes of these cycles are fluctuations in noncarbonate dilution carbonate production carbonate dissolution and or current winnowing the overall driving force however is variation in the earth s orbital characteristics milankovitch cycles through their influence on global climate and depositional processes the main inferred climatic effects of orbital perturbations are on global ice volume global temperature ocean circulation and distribution of climatic patterns eustatic sea level is directly related to ice volume as is the rate of erosion of clastic material from continental margins changes in volume of sea ice affect the volume and intensity of bottom water flow which in turn may cause changes in the intensity of sediment winnowing by bottom currents in the intensity of upwelling of nutrient rich bottom waters and in the depth of carbonate dissolution change in productivity of calcareous plankton are difficult to prove as a cause of carbonate cycles but may have contributed to the formation of carbonate cycles off northwest and southwest africa fluctuations in winnowing of fine grained components have been demonstrated as a cause of cyclic variations in the coarse fraction component of carbonate sediments in the southwest pacific dilution of carbonate by clastic material probably was a major cause of fluctuations in carbonate content of deep sea sediments off northwest and southwest africa carbonate dissolution cycles probably are the most common manifestation of fluctuations in bottom water flow dissolution cycles are common in quaternary and neogene sediments of the north atlantic caribbean and eastern equatorial pacific the main cause of the carbonate dissolution was shoaling of the carbonate compensation depth during the early neogene in response to climatically induced fluctuations in the thickness of antarctic bottom water

high frequency waves guided by the subducted plates underneath taiwan and their association with seismic intensity anomalies 1 energy from seismic events traveling up a subduction zone is frequently associated with significant large amplitude high frequency signals with sustained long coda such seismic waves guided by the subducted plate with high wave velocity and high q can cause surprisingly large seismic intensity in the fore arc area in this study we characterize the guiding behavior of the subducted philippine sea plate psp underneath taiwan and investigate their relationship with anomalous peak ground acceleration pga patterns obliquesubductionof philippine seaplatebeneathnortheast taiwan complicatestheguiding phenomena seismic waves from events deeper than 60km offshore northern taiwan reveal wave guide behavior large sustained high frequency 3 10hz signalinpandswavetrains with increasing depth a low frequency 1hz first arrival becomes more significant especially for events deeper than 100km the time separation between the low frequency onset and the later high frequency arrival slightly increases with depth while the value varies with station due to different travel distances in the shallow crust the depth dependent high frequency content confirms the association with a waveguide effect in the subducting slab rather than localized site amplification effects we attempt here to obtain a practicable quantification scheme to determine the duration of higher frequency energy which can be regarded as an indicator of the guiding effect of the philippine sea plate

late cenozoic eolian deposition in the north pacific asian drying tibetan uplift and cooling of the northern hemisphere a newly constructed record of eolian dust accumulation from the central north pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3 6 ma we associate this sudden drying with the uplift of at least the northern portion of the tibetan plateau shutting off the indian ocean moisture source to central and western china this ten fold increase in atmospheric dust loading appears to be associated with the 1 m y long shift toward heavy 18o values that occurred at 3 6 2 6 ma the dust grain size record of wind intensity begins its late cenozoic coarsening a million years before the drying event at 4 5 ma the northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric co2 in the latest miocene and the early pliocene closing of the panamanian seaway

late neogene changes of paleoclimate and paleoproductivity off northwest africa d s d p site 397 abstract the sediment record of site 397 may provide information about both the causes of changes in coastal productivity in the northeast atlantic and the history of climate in northwest africa during the late neogene this study concentrates on the drastic climatic reversal near 2 4 m y b p near 3 2 m y b p a distinct increase of accumulation rates of terrigenous matter and higher paleo trade wind speeds may suggest increased aridity of the northwest african continent this change was followed by higher paleoproductivity of coastal waters off northwest africa high paleoproductivity may be caused by both fluvial nutrient supply and or upwelling during the beginning and the end of inter glacial times river discharge was dominant whereas during phases of glacial maximum stronger fertility of increased coastal upwelling became more important

deep circulation changes in the south atlantic ocean response to initiation of northern hemisphere glaciation particle size statistics coarsest 1 percentile and mean of the nonbiogenic silt fraction of samples from deep sea drilling project sites 514 516a 517 518 and 548 were used to infer relative changes in paleospeed of deep water masses in the atlantic ocean during the late pliocene initiation of northern hemisphere glaciation grain size statistics were integrated with previous studies of geochemical and faunal assemblages to examine how climatic forcing regulates deepwater mass paleospeed and stratification on the basis of between site comparison of particle size the late pliocene was divided into four intervals of deepwater activity 3 2 to 2 9 ma 2 9 to 2 6 ma 2 6 to 2 5 ma and 2 5 to 2 3 ma with pulses of increased paleospeed occurring at 3 15 3 10 2 85 2 7 2 6 and 2 4 ma except for a brief increase in paleospeeds at 3 1 ma the first time interval is characterized by low inferred antarctic bottom water aabw paleospeed corresponding to depleted 13c and enriched 18o values in all but the shallowest core enriched 13c and depleted 18o values in the shallowest core correspond to coarse grain sizes and are inferred to represent a deep and intense antarctic intermediate water aaiw the expansion of aabw and aaiw precludes a significant north atlantic deep water nadw incursion to 30 s at this time the interval from 2 9 to 2 6 ma is dominated by climatic cooling and an influx of deep waters originating in the north atlantic the introduction of significant volumes of mediterranean outflow water mow as demonstrated by increased inferred paleospeed at site 548 and an expanding glaciation beginning at 2 9 ma resulted in increased paleospeed and an expansion of nadw in the south atlantic a pulse of increased inferred paleospeed at 2 7 ma resulted in the temporary deepening of nadw to the depth of site 518 presently 3944 m from 2 9 to 2 6 ma aabw velocity remained low but aaiw deepened and intensified the 2 6 to 2 5 ma interval represents the retreat of deep water originating in the north atlantic from the depth of site 518 to the depth of site 517 presently 2963 m increased inferred paleospeed at site 518 corresponds with depleted 13c values and a northward shift in the polar front thus suggesting the reintroduction of circumpolar water to the depth of site 518 and the establishment of a deepwater stratification similar to today s a continuing trend toward a modern stratification continued into the latest time interval 2 5 2 3 ma as inferred deepwater paleospeed at all depths increased in response to the formation of extensive northern hemisphere ice sheets

formation and evolution of gobi desert in central and eastern asia abstract although the development of gobi desert in central and eastern asia has greatly affected the regional and even the global climate its precise origin and evolution have yet to be determined the three preconditions for the formation of gobi desert are i a dry climate ii basin landforms and iii abundant sediment production in this study we present a synthesis of both new and published data on the formation and evolution of gobi desert in central and eastern asia we conclude that the combined effects of mountain building the mid latitude westerly circulation and changes in the asian monsoon accompanied by global cooling were principally responsible for the formation of modern gobi desert landscapes in central and eastern asia during the late pliocene the arid climate in central and parts of eastern asia probably developed in the early cenozoic from 50 ma related events included the collision of the indian and asian plates the closure and complete retreat of the paratethys ocean from central asia and the growth of the himalayas and the tibetan plateau in the eocene through late miocene which blocked the water vapor supply and intensified the aridification of the asian interior superimposed on the topographic changes was the process of stepwise global cooling since the early oligocene and in particular since the late miocene which controlled the formation and evolution of the gobi desert landscape global cooling weakened the asian monsoon circulation strengthened the westerly circulation and enhanced physical weathering processes in mountain areas which together promoted both the aridification of the asian interior and sediment production these processes finally resulted in the establishment of the modern gobi desert landscape in the late pliocene we estimate that the modern gobi desert landscape was formed at 2 6 ma and was the result of the stepwise evolution of asian topography and climate during the cenozoic dominated by asian tectonic deformation and uplift and the evolution of asian monsoon climate and the westerly circulation forced by global temperature change

observations on cretaceous abyssal hills in the northeast pacific abstract an abyssal hills area of 50 60 km in the northeast pacific was studied using bottom transponder navigation closely spaced survey lines and long traverse oblique photography the block faulted north south hills are bounded by scarps commonly with 40 slopes on these steep scarps sedimentation is inhibited and pillow basalts often crop out an ash layer of high acoustic reflectivity at about 7 m subbottom depth blankets the area this ash occurs in multiple beds altered to phillipsite and is highly consolidated a 24 m y age for the ash is based on ichthyolith dates from samples in the overlying sediments acoustically transparent neogene sediments above the ash are thickest in trough bottoms and are absent or thin on steep slopes these neogene sediments are composed of pale brown pelagic clays of illite quartz smectite chlorite and kaolinite dark brown pelagic clays rich in smectite and amorphous iron oxides underlie the neogene surficial sediments manganese nodules cover the bottom in varying percentages the nodules are most abundant near basement outcrops and where the subbottom ash layer is absent

miocene stable isotope record a detailed deep pacific ocean study and its paleoclimatic implications deep sea drilling project site 289 in the western equatorial pacific has yielded an extremely detailed record of the carbon and oxygen isotopic changes in the miocene deep ocean the isotopic record reflects major changes in paleoclimate and paleoceanography probably dominated by a major phase of antarctic ice cap growth the transition from a relatively unglaciated world to one similar to today occurred between 16 5 x 10 6 and 13 x 10 6 years before the present with the greatest change occurring between approximately 14 8 x 10 6 and 14 0 x 10 6 years before the present

a middle miocene hiatus in the pacific region its stratigraphic and paleoceanographic significance abstract a deep sea hiatus with a minimum duration of 1 ma beginning at 13 ma is recognised throughout the north pacific ocean based upon the re examination of four kinds of microfossil zonations which were reported from 212 dsdp sites its distributional pattern suggest that the hiatus cannot be ascribed to scouring by an oceanic bottom current but to meso scale eddies abnormally stimulated by the first onset of bi polar glaciation the beginning of this glaciation is coeval not only with such a lowering of bottom water temperature as has been regarded as a result of east antarctic ice cap development but also with a distinct fall of sea level from the highest stand during the neogene glacial bi polarity at the time is suggested by the beginning of temperature diverggence between the surface and bottom waters in the low latitude regions and also by the first development of arctic glaciation an unconformity of approximately the same age can be observed in many places in the circum pacific region and its formation might have been triggered by this sea level fall after the hiatus event a remarkable transgression seems to have commenced over the region due to spreading accompanied by crustal subsidence resulting in a sea level fall of global scale

sedimentary indicators of atmospheric activity in the northern hemisphere during the cenozoic abstract the concentrations and accumulation rates of eolian phases and the mineralogy of sediments from a north pacific core ll44 gpc3 have been used to evaluate changes in eolian sedimentation and atmospheric activity during the cenozoic the quartz content and clay mineralogy of the sediments reflect changing source areas for eolian material whereas the accumulation rates of eolian phases reflect climatic variation and wind intensity eolian sedimentation decreased from 70 to 50 m y ago at the site as it moved north out of the equatorial pacific where it was supplied with eolian debris from north america and africa low rates of eolian sediment accumulation 50 to 25 m y ago reflect the temperate humid environment of the early tertiary and the lack of vigorous atmospheric circulation at that time from 25 to 7 m y ago eolian accumulation increased as the site moved into the influence of the westerlies from 7 to 3 m y ago there was a more rapid increase in accumulation of eolian material at the site which was not accompanied by changes in mineralogy or quartz content and was therefore related to climatic change and more vigorous atmospheric circulation about 2 5 m y ago a dramatic increase in eolian sedimentation accompanied the onset of dry northern hemisphere glaciation

mineralogical and geochemical compositions of the eolian dust from the northeast equatorial pacific and their implications on paleolocation of the intertropical convergence zone 1 eolian components of a 328 cm long piston core collected from the northeast equatorial pacific at 16 12 n and 125 59 w were investigated for mineral and geochemical compositions in order to constrain the sources of dust and determine the latitudinal position of the intertropical convergence zone itcz recorded in the core the eolian components below 250 cm are characterized by smectite and phillipsite rich mineral composition depleted rare earth elements rees and high eu sm ratios indicative of volcanic rich composition these characteristics are found in equatorial and south pacific surface sediments of which eolian particles are supplied from central and south america in contrast eolian components above 250 cm are characterized by quartz and illite rich mineralogy and more shale like ree and trace element compositions which are common in surface sediments of the central pacific north of the itcz where eolian particles are sourced from the asia and north america the observed changes are attributed to the shifting of its eolian sources from the central and south america to the china and north america across the hemispheric dust barrier of the itcz this result suggests that smectite illite transition a phenomenon that smectite amount increases over illite at a depth can be used as a tracking tool for the paleolocation of the itcz in the northeast and central pacific backtrack path construction of pacific plate indicates paleolocation of the itcz north of 12 n 2 prior to late miocene

equatorial pacific sea surface temperatures faunal patterns and carbonate burial during the pliocene abstract a high resolution record of radiolarian faunal abundances from the eastern equatorial pacific is compared to records of carbonate and noncarbonate burial to examine the evolution of eastern tropical pacific climate processes during the pliocene these data provide a means to evaluate the sensitivity of the equatorial pacific to the onset of northern hemisphere glaciation around 2 8 2 5 ma to the closure of the isthmus of panama around 4 4 3 2 ma and to orogeny related weathering changes before 4 0 ma radiolarian faunal assemblages and sea surface temperature sst estimates indicate a gradual cooling from early to late pliocene but no significant changes occur near the onset of northern hemisphere glaciation records of carbonate and noncarbonate mass accumulation show a long term decrease from the miocene pliocene boundary to the upper pliocene greater carbonate burial in the early pliocene relative to the late pliocene parallels a gradual cooling from early to late pliocene and may reflect changes related to isthmus closure or widespread orogeny no significant time domain changes are seen in the eastern equatorial pacific that could be related to the onset of northern hemisphere glaciation evolutive spectral analyses of these equatorial pacific climate parameters indicate that variance in sst and seasonality commonly concentrate at frequencies not linearly related to orbital variations furthermore cross spectral comparisons with a high resolution benthic 18 o record indicate that the surface ocean and carbonate flux share little coherent variance with high latitude climate processes during the pliocene given the high degree of chronostratigraphic control in these records these results suggest that milankovitch band surface ocean processes as well as carbonate burial in the equatorial pacific are decoupled from high latitude climate processes during the pliocene

upper mantle anisotropy in the s w pacific from earthquake travel time analysis abstract the negative p residuals from readings at stations on niue rarotonga and the chatham islands of shallower events along the tonga kermadec trench and below the north island new zealand exhibit a scatter too large to be accounted for solely by observation and mislocation errors the corresponding wavepaths cover a range of propagation azimuths between n30 e and n180 e travel times from a large number of events reveal a variation of p velocity with azimuth which is consistent with large scale upper mantle anisotropy in this area the p velocity varies from a maximum of 8 4 km s 1 along n62 e to a minimum of 7 8 km s 1 along a perpendicular axis perhaps indicating a fossil spreading direction along n62 e there is also a very consistent but unexplained variation of p residuals with earthquake depths between 0 and 100 km an observation warranting further study

retrospective dry bulk density estimates from southeast indian ocean sediments comparison of water loss and chloride ion methods abstract knowledge of absolute accumulation rates permits quantitative comparison between and within sediment components from cores of differing age and lithologic character dry bulk density is an essential parameter in the calculation of absolute accumulation rates dry bulk density measurements require a wet volume term and are best made when a core is first recovered such measurements were not routinely made on eltanin cores from the southeast indian ocean in an effort to recover this information we have estimated the dry bulk density of sixty samples from five eltanin cores using a water loss method and a chloride ion method which is independent of the state of desiccation of the cores based on the consistency of the results using these two independent methods we feel confident in our estimation of dry bulk density comparison of dry bulk density and caco 3 allowed us to develop the following non linear regression equation for estimating the dry bulk density based on caco 3 a more easily and accurately measured property 3 104x10 3 caco 3 2 2 176x10 3 caco 3 0 430 this equation can be used to estimate dry bulk density to within 0 097 g cm 3 thus providing the information needed to reliably calculate absolute accumulation rates our indian ocean dry bulk density equations are different from those developed by similar techniques for the pacific and atlantic oceans indicating that different lithologies necessitate the development of regional equations

cenozoic mass accumulation rates in the equatorial pacific based on high resolution mineralogy of ocean drilling program leg 199 1 we use continuous mineralogy logs derived from multisensor track physical properties data and reflectance spectroscopy to calculate high resolution carbonate opal and terrigenous mass accumulation rates mar for the eight sites cored during ocean drilling program leg 199 in the equatorial pacific the largest change in equatorial pacific sedimentation patterns of the last 55 myr occurs at the greenhouse icehouse transition shortly after the eocene oligocene boundary calcite compensation depth deepened dramatically the equatorial zone of opal accumulation narrowed and the intertropical convergence zone shifted to the equator during the paleogene the opal equatorial accumulation bulge extended to about 12 n whereas the neogene bulge extended only to about 7 n very low eocene carbonate mar was followed by much higher oligocene and early miocene mar with the carbonate equatorial bulge extending to 4 n terrigenous mars indicate the paleoposition of the intertropical convergence zone and a pliocene pleistocene increase in asian dust flux

barium in hemipelagic sediment of the northwest pacific coupling with biogenic carbonate 1 geochemical analyses were performed on three cores of hemipelagic sediment from the northwest pacific ocean eastern edge of shikoku basin dating from the last interglacial and glacial periods the objective of this work is to delineate the relations between excess sedimentary barium ba ex content and biogenic opal carbonate and organic carbon contents calculated ba ex values show variable degrees of correlation with biogenic sediment fractions the mass accumulation rate mar of ba ex covaries with the mars of biogenic fractions and in particular with the mar of biogenic carbonate r2 0 68 and toc r2 0 50 variable relations between barium and biogenic fractions in hemipelagic sediment provide detailed insight into the behavior of sedimentary barium it appears that a significant proportion of the barium is affiliated with the carbonate fractions based on this strong correlation carbonate dissolution rates of the last glacial to interglacial periods are estimated assuming that the mars of carbonate and ba ex covary variation of the ba ca ratio in sediments reflects the extent of postdepositional carbonate dissolution the record of sedimentary ba ca ratios exhibits striking difference between sediment of the glacial and interglacial periods with a higher positive correlation during glacial intervals and a lower correlation during interglacial intervals this pattern is the result of enhanced carbonate dissolution during interglacial times sedimentary ba ca ratios may therefore be a useful tool for estimating the relative extent of carbonate dissolution ba ex curves and patterns in sedimentary ba ca ratios resemble the established pacific carbonate dissolution record i e enhanced dissolution during interglacial times and reduced dissolution during glacial times as suggested by previous studies variations in sedimentary ba ca ratios strongly support an interpretation of pronounced glacial interglacial fluctuation in carbonate dissolution in the northwest pacific ocean an especially pronounced dissolution event is evident at 75 kyr during the transition interval form interglacial to glacial conditions this suggests that ba ex may be used as a proxy for evaluating carbonate dissolution in hemipelagic settings but further study of the association between barium content and carbonate dissolution will be required to confirm this

slab low velocity layer in the eastern aleutian subduction zone summary local earthquakes in the vicinity of the alaskan peninsula s shumagin islands often produce arrivals between the main p and s arrivals not predicted by standard traveltime tables based on traveltime and polarization these anomalous arrivals appear to be from p to s conversions at the surface of the subducted pacific plate beneath the recording stations the p to s conversion occurs at the top of a low velocity layer which extends to at least 150 km depth and is 8 f 2 per cent slower than the overlying mantle the slab is 7 per cent faster than the mantle the low velocity layer contains the foci of the earthquakes in the upper plane of the double seismic zone and confines ps ray paths to lie within it these observations indicate that layered structures persist to positions well past the surface location of the volcanic front reactions forming high pressure minerals do not yield slab like velocities until beyond the point that subduction zone magma genesis occurs if the subducted oceanic crust forms the layer it is subducted essentially intact

87sr 86sr variation in north pacific sediments a record of the milankovitch cycle in the past 3 million years abstract temporal variations of sr isotopic compositions in the detrital component of north pacific sediments have been precisely examined the sr isotopic composition is controlled by the flux of eolian material from the asian continent with a high 87 sr 86 sr ratio 0 724 0 726 relative to volcanic material with a low ratio 0 703 0 705 from the izu ogasawara mariana arc and oceanic islands such as the hawaiian islands assuming that the volcanic flux is constant the temporal variation in sr isotopic ratio reflects the amount of eolian input from the continent the major characteristics of the temporal variations and the interpretation of them are summarized as follows 1 the 87 sr 86 sr records show cyclic fluctuations of 400 ky and 100 ky periodicities which are associated with the eccentricity of the earth s orbit milankovitch cycle 2 between 3 and 0 8 ma the 87 sr 86 sr ratio in the north central pacific sediment increases gradually this reflects an increased eolian input from the arid region in east asia the increase must be related to aridification of the asian continent 3 the decreased 87 sr 86 sr ratio during the past 0 8 m y implies a decreased eolian input the age of 0 8 ma may relate to the climatic event known as the middle pleistocene shift all of these phenomena reflect a fluctuation of the eolian flux corresponding to the paleoclimatic cycle of aridity in the asian continent the sr isotopic composition of pelagic sediment in the north pacific is sensitive to changes in the eolian input reflecting to the aridity of the asian continent

late cretaceous history of eolian deposition in the mid pacific mountains central north pacific ocean rea d k and janecek t r 1981 late cretaceous history of eolian deposition in the mid pacific mountains central north pacific ocean palaeogeogr palaeoclimatol palaeoecol 36 55 67 eolian dust preserved in deep sea sediment can provide a direct historical record of global atmospheric circulation data from a reasonably complete upper cretaceous section of pelagic sediments recovered at dsdp site 463 in the central north pacific provides a good record of eolian activity during the time period between about 112 hnd 66 m y ago we have isolated the eolian component from these sediments determined its mass accumulation rate and combined these data with the mineralogy of the inorganic fraction determined by others to construct a record of eolian deposition volcanic input is significant during aptian albian and maastrichtian times otherwise continentally derived minerals dominate mass accumulation rates of the continental eolian component range from over 500 mg cm 103yr during the late albian to a low of 5 mg cm2 10 yr during coniacian time for comparison the upper miocene to pleistocene rate averages about 20 mg cm2 103yr the temporal pattern of late cretaceous eolian accumulation of site 463 generally matches known changes in sea level suggesting that source availability is the dominant control of eolian sedimentation during that time

late neogene evolution of paleoclimate and paleoceanic circulation in the northern and southern hemispheres a comparison oxygen isotopes and clay minerals grain size and accumulation rates of terrigenous i e mainly eolian sediments from northeast atlantic dsdp sites 141 366 397 and 544b and from southwest pacific dsdp sites 588 590 and 591 were used to assess parallelisms and differences in the evolution of paleoclimate and paleoceanic circulation in the northern and southern hemispheres during the last 7 ma

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

temporal changes in the lead isotopic composition of red clays comparison with ferromanganese crust records abstract a record of changes in pb and sr isotopic composition of two cores dsdp 86 576a and ll44 gpc3 from the red clay region of the central north pacific has been determined for the past 60 65 million years the isotope records of the eolian silicate fraction of the red clays reflect the change in source area as the core sites migrated under different wind systems the sr isotope compositions of eolian silicate material are consistent with asian loess and north american arc volcanism that has been recognized from mineralogical studies the silicate bound eolian pb isotopic compositions similarly reflect asian loess and arc volcanism the isotope records of three ferromanganese crusts from similar locations in the central pacific are similar to the eolian component of red clays but offset to less radiogenic values this may be due to two mechanisms 1 pb that can be removed from eolian material by seawater is much less radiogenic or less likely 2 hydrothermal pb can be transported further away from venting sites through particle exchange with seawater despite hydrothermal venting acting as a net sink of oceanic pb the temporal changes in pb isotopes in the ferromanganese crusts bulk red clays and eolian silicates are similar although offset from each other suggesting that eolian deposition is an important source of pb to seawater and to ferromanganese crusts this contrasts with the atlantic and southern ocean where more intense deep water flow leads to isotopic gradients in femn crusts that do not reflect surface water conditions immediately above the crust a mechanism is proposed which accounts for pacific deepwater pb being isotopically influenced by eolian deposition

subduction and back arc activity at the hikurangi convergent margin new zealand the hikurangi margin is a region of oblique subduction with northwest dipping intermediate depth seismicity extending southwest from the kermadec system to about 42 s the current episode of subduction is at least 16 20 ma old the plate convergence rate varies along the margin from about 60 mm a at the south end of the kermadec trench to about 45 mm a at 42 s the age of the pacific lithosphere adjacent to the hikurangi trench is not known

a rock magnetic study of giant piston core ll44 gpc3 from the central north pacific and its paleoceanographic implications previous magnetostratigraphic studies of nonfossiliferous pre pliocene age pelagic clay cores from the central north pacific have proven unsuccessful because of the unstable behavior of the natural remanent magnetization the inability to obtain a reliable magnetostratigraphy has severely limited the temporal resolution that can be achieved in paleoceanographic studies of these nonfossiliferous pelagic clays we carried out a rock magnetic study of core ll44 gpc3 which spans the interval 0 70 m y ago to determine whether rock magnetic parameters can be used to construct a high resolution stratigraphic framework for paleoceanographic studies of central north pacific pelagic clays despite the instability of the natural remanence in addition we tested the ability of rock magnetic methods to detect and characterize the paleoceanographic changes that are recorded in the sediments of ll44 gpc3 stratigraphic variations in rapid and nondestructive rock magnetic parameters and related ratios reflect changes in the concentration mineralogy and grain size of the magnetic minerals within the sediments rock magnetic parameters exhibit coherent fluctuations within both the stable and unstable sections of ll44 gpc3 this result suggests the potential use of these parameters for regional correlation and relative dating of the nonfossil bearing pelagic clays of the central north pacific gyre major fluctuations in rock magnetic parameters plotted as accumulation rates correspond to intervals of paleoenvironmental change that have been previously detected by other paleoceanographic methods in addition variations in a parameter proportional to the concentration of goethite hematite correlate remarkably well with the mass accumulation rate of the total eolian component suggesting that in some situations it can be used as a proxy indicator for eolian activity

the kinematics of the plate boundary zone through new zealand a comparison of short and long term deformations summary surveys of shear strain rates derived from repeated triangulation cover about one quarter of the area of the plate boundary zone through new zealand and are well distributed within it using a method developed by haines relating shear strains to the total strain and velocity fields the dilatational and rotational strain rates and the velocities within the deformed zone are derived the plate boundary zone is shown to be two dimensional for the most part in that gradients of the velocity along the length of the zone are small or zero the shear strain rates are not uniform however they reach a maximum along a central axis and decrease gradually to zero toward the sides nor are the two independent components of strain one involving shortening or extension perpendicular and the other shear parallel to the zone equally developed everywhere there is a clear geographic separation with shortening developed over the subduction thrust and shear developed behind in a zone of wrench faulting time variations in the strain field in part of the region above the subduction interface are related to changes in the degree of coupling across the interface a velocity field is computed the relative velocity of the two plates from this data agree very well with that derived from seafloor spreading data these strain and velocity fields derived from geodetic studies give the short term kinematics of the plate boundary zone the long term kinematics of the zone are estimated from 1 uplift and subsidence data which are related to changes in crustal thickness and hence to dilatational strain and 2 variation in the declination of primary magnetization in rocks of known age related to the rotational strain the rate of rotational strain has increased in time from about 2 to 3 myr 1 10 myr ago to about 9 myr 1 today the increase can be understood as a secondary effect of a developing compressional component increasing the coupling between the plates so that more of the motion parallel to the zone is accommodated by distributed shear and less by aseismic slip the rates of strain averaged over the last 1 myr or so and at a scale length of a few tens of kilometres are closely similar in distribution and magnitude to the strain rates derived from short term kinematics while therefore at scale lengths of a few hundred metres or so faulting is very important and the deformation is discontinuous with unstrained blocks faulted with large but uncertain slip against adjacent blocks the deformation at these longer scale lengths is continuous and regular the two consequences of this study are that 1 strain from geodetic data can be directly related to the average long term deformation and therefore can be an important tool in the study of deformation in regions of current tectonic activity and 2 at scale lengths of a few tens of kilometres the deformation of a region can be described as the deformation of a continuum and we can ignore the effects of faulting

resolution of basaltic and continental aerosol components during spring and summer within the boundary layer of hawaii an important continental aerosol component up to 70 of the total dust load during the spring has been resolved from the local basaltic component using short time step 3 7 hr sampling at the hawaiian volcano ovservatory hvo a site below the inversion layer at 1 2 km altitude the median continental contribution for approximately 2 week periods from mid march to mid june 1979 ranged from 1 to 2 g m 3 whereas from mid june to early september 1979 this component was not found being less than the detection limit of its spring magnitude over the 6 month period the median concentrations for the basaltic aerosol ranged from about 0 2 to 18 g m 3 a variation associated with local rainfall this basaltic component exhibits a strong diurnal variation resulting from the local wind regime with a ratio of a mid afternoon maximum to an early morning minimum of up to 20 or more of the six lithophilic elements examined al si k ca ti and fe si shows the greatest earth air fractionation with the continental and local basaltic aerosols being depleted by more than 35 relative to an average crustal and hawaiian basaltic rock composition respectively time series analysis of the concentrations and a detailed examination of the element ratios indicate that local and continental soils are the only important determinants of the overall aerosol composition at this site for these elements compositional differences between the continental dust at hvo and that sampled concurrently at the mauna loa observatory 3 4 km altitude may be due to the presence of two distinct dust plumes over hawaii originating from different desert regions in asia

evidence and mechanisms for forearc extension at the accretionary costa rica convergent margin seismic reflection data across the upper trench slope off the nicoya peninsula costa rica reveal a wide zone of nearly trench parallel normal faults although work in the last decade has shown that normal faults are present at many convergent margins most examples e g japan peru chile and guatemala have been associated with margins experiencing subduction erosion or non accretion in contrast extension in the costa rica study area apparently is coeval with frontal accretion and underplating the normal faults across the costa rica forearc are striking in seismic section due to the well layered 2 km thick upper slope apron fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism indicating a deep seated deformation process the zone of extension is from the midslope area to within 10 km of the shelf edge a minimum width of about 20 km the estimated extension across the zone is at least 1 5 to 3 km within the apron section spacing between the faults is generally 200 500 m and nominal fault dip is 20 40 and predominantly landward activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault controlled sedimentary thickening at least some of the faults may be presently active shallow reflectors and possibly the seafloor are displaced by faulting contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out of sequence thrusting primarily below the midslope area the consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes with the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension however the three most likely causes are underplating changes in basal shear stress or a brief episode of subduction erosion

asian aridity and the zonal westerlies late pleistocene and holocene record of eolian deposition in the northwest pacific ocean rea d k and leinen m 1988 asian aridity and the zonal westerlies late pleistocene and holocene record of eolian deposition in the northwest pacific ocean palaeogeogr palaeoclimatol palaeoecol 66 1 8 a 30 000 year record of eolian deposition in the northwestern pacific ocean provides a history of the aridity of the asian source region and information on the changing latitude and intensity of the zonal westerlies the position of the maximum in eolian flux to the deep sea remained unchanged at 38 to 40 n during this entire time period but the dust flux record of aridity is greatest 6000 years ago at the holocene climatic optimum eolian grainsize data indicate that the northern margin of the westerly jet stream retreated northward during the holocene to a poleward extreme at about 6000 years ago and then moved back toward the equator the change in wind intensity across this margin was continually reduced throughout the holocene these sorts of data enhance our understanding of the behavior of the atmosphere during times of climate change and can be used to test computer generated models of past climatic conditions

anisotropic structure of the hikurangi subduction zone new zealand integrated interpretation of surface wave and body wave observations summary a model of subduction zone anisotropy for south and central north island new zealand is presented by integrating the observations of shear wave anisotropy from anomalous surface wave polarizations local s phases at a range of hypocentral depths and sks splitting analyses anomalous particle motions of the equivalent to fundamentalmode love waves have been measured for propagation paths approximately along the strike of the hikurangi subduction zone in eastern north island new zealand these observations are modelled in terms of an anisotropic crust and subducted pacific slab with a pseudo hexagonal symmetry axis plunging at 30 to the vertical and a fast axis parallel to the strike of the margin a shear wave anisotropy of 4 per cent is assumed shear wave splitting analyses of regional phases in central north island show no increase in fast slow delay time 0 21 0 03 s with increased hypocentral depth unlike previous observations in southern north island which display an increase in delay time of about 0 5 s over 250 km depth fast polarization directions are again approximately parallel to the strike of subduction observed sks splitting parameters on the other hand do not vary significantly between southern and central north island and over the mantle wedge in western north island a mean fast slow delay time of 1 5 0 3 s and a fast polarization direction of 25 7 subparallel to the strike of the subduction zone were determined for the sks observations both the surface wave polarization and local shear wave splitting observations were modelled by an anisotropic new zealand crust with a pseudo hexagonal symmetry axis plunging at 15 30 and a horizontal fast axis parallel to the strike of the subduction the variation of the splitting delay times with hypocentral depth between central and southern north island implies a horizontal symmetry axis in the subducted slab in the south but an axis plunging parallel to the plate interface in central north island in the mantle wedge above the slab in western north island and the mantle beneath the slab in central and southern north island a horizontal anisotropic symmetry axis fits the observations 4 per cent anisotropy was assumed throughout the models within the new zealand crust the plunge of the symmetry axis of the anisotropic structure is compatible with the structural trends of the regional geology and fault orientations the variation in the plunge of the anisotropic symmetry axis within the subducted pacific lithosphere from central to southern north island is interpreted to reflect the change in intraplate stress southwards towards south island where the india australian and pacific plates are locked the orientation of olivine due to trenchparallel flow would explain the observations in the mantle wedge and pacific mantle below the subducted plate

shallow subduction tectonics and fault interaction the weber new zealand earthquake sequence of 1990 1992 a series of four large earthquakes with epicenters very close to one another occurred in 1990 1992 in the weber region of the southeast coast of the north island new zealand the region is one of oblique plate convergence and subduction the plate interface being at about 20 km depth the first event february 19 1990 ml 6 1 mw 6 2 occurred within the upper part of the subducting pacific plate on a steeply northwest dipping normal fault as defined by relocated aftershocks the mechanism of this event reflects downdip tension within the subducting plate due to slab pull forces as is common in new zealand the second event may 13 1990 ml 6 2 mw 6 4 occurred on an imbricate fault in the overlying australian plate dipping shallowly northwest with components of both thrusting and dextral strike slip motion the slip vector is similar to the direction of plate convergence showing no arc normal rotation as observed for subduction thrust events in other areas of oblique convergence it is unlikely that the spatial and temporal closeness of these events is coincidence a similar pair occurred to the south in 1942 but the coupling mechanism is not clear perhaps being related to aseismic slip downdip on the subduction interface two subsequent events both of ml 5 5 occurred on august 15 1990 and march 2 1992 and extended the aftershock zone of the deeper first main shock to the northeast matching the extent of the shallower second main shock mechanisms of aftershocks of the two larger events are quite variable but as a whole are similar to the relevant mainshock the time history of the aftershocks of the first deeper event shows a period of quiescence beginning 35 days before the second shallow event the mean magnitude of those events that did occur was high this observation cannot be explained by station biases weather or changes in analysis methods and may be due to stress changes induced by precursory aseismic slip which would discourage aftershock occurrence

benham rise unveiled morphology and structure of an eocene large igneous province in the west philippine basin abstract the benham rise is an oceanic large igneous province at the western margin of the philippine sea it has an ocean island basalt geochemistry a surface area of 0 11 mkm2 and a volume of 0 13 mkm3 a crustal thickness of 15 km is estimated from gravity anomalies volcanism was most active in the early stages of its formation during the lutetian from 47 9 ma to 41 3 ma although volcanic activity extended to 26 ma multibeam bathymetry with almost 100 coverage has revealed the morphology of benham rise for the first time it consists of a main body 310 km by 330 km with the narra loro and molave spurs extending 100 200 km from its eastern side the main body is built on a shield platform from 5200 m to 3800 m sub sea the platform flanks consist of 3 15 km wide terraces with scarps as high as 100 m to 300 m the platform is surmounted by a crest exhibiting caldera morphology at an average depth of 2500 m the crest is named apolaki caldera and may be the world s largest known caldera with a diameter of 150 km features like a breached rim intra caldera benches and a resurgent dome indicate a multi phase volcanic history consisting of both quiet and explosive eruptions the bathymetry and publicly available geological and geophysical data record three main stages in the tectonic history of benham rise shield building caldera formation and post caldera late stage volcanism the platform base is interpreted to have formed over a hotspot beneath the central basin spreading center during the shield building phase between 47 9 ma to 42 5 ma the arrowhead shaped molave spur is interpreted to have formed late in this stage along a ridge that propagated away from the hotspot the caldera formation followed and is interpreted to have occurred before 41 3 41 5 ma when benham rise was still joined with urdaneta plateau the narra spur was formed by late stage hotspot related volcanism as benham rise and urdaneta plateau separated and ended with the eruptions of the vinogradov seamount at 26 ma

quaternary uplift astride the aseismic cocos ridge pacific coast costa rica the pacific coast of costa rica lies within the central american forearc and magmatic arc region that was created by northeastward subduction of the cocos plate beneath the caribbean plate at the middle america trench from the peninsula de nicoya south eastward toward the peninsula de osa and the peninsula de burica on the panamanian border the middle america trench loses its physiographic expression where it intersects the aseismic cocos ridge interaction between subduction of the buoyant aseismic cocos ridge and the overriding caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from nicoya to burica the pliocene and pleistocene stratigraphic record and holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the peninsula de osa immediately landward of the aseismic cocos ridge crustal uplift rates decrease northwest toward the peninsula de nicoya and to a lesser extent southwest toward the peninsula de burica the late quaternary stratigraphy on the peninsula de osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates crustal uplift rates calculated from these sequences systematically decrease from 6 5 to 2 1 m ka north east across the peninsula deformation of the peninsula is modeled as uplifted and down to the northeast tilted fault blocks with an angular rotation rate of 0 03 to 0 06 per thousand years although less well constrained crustal uplift rates on the peninsula de nicoya 200 km to the northwest of the peninsula de osa vary from 1 m ka for pliocene and pleistocene sediments to 2 5 m ka for holocene marine terraces in the quepos region 100 km to the northwest of the peninsula de osa calculated uplift rates derived from incision of late quaternary fluvial terraces range from 0 5 to 3 0 m ka on the peninsula de burica only 60 km to the southwest of the peninsula de osa calculated uplift rates range from 4 7 m ka for a late holocene marine terrace to 1 2 m ka for post late pliocene deep sea sediments the variations in calculated uplift rates on the peninsula de osa constrain a dynamic model for subduction of the cocos ridge and the resulting uplift of the overriding caribbean plate deflection of the caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted cocos ridge because the shape of the subducted end of the cocos ridge is unknown two scenarios are evaluated 1 a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and 2 a ridge where the subducted end was truncated by the panama fracture zone the best fit model utilizes a truncated ridge that has been subducted during the past 0 5 m y 50 km beneath the overriding caribbean plate which has an effective elastic thickness of 5 km the model predicts that the highest uplift rate should be 3 7 m ka and occur on the southwest coast of the peninsula de osa the rate of uplift slows considerably to the northeast and indicates that the peninsula de osa is tilting to the northeast which agrees with observations in that region the predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the peninsula de osa resulting in little uplift that can be attributed to cocos ridge subduction in the northwestern portions of the peninsula de nicoya

resolving eolian volcanogenic and authigenic components in pelagic sediment from the pacific ocean abstract we measured major and trace element concentrations in the operationally defined chemically extracted residual aluminosilicate component of sediment from ocean drilling program sites 1215 and 1256 in the central and eastern equatorial pacific ocean and found that this residual component contains volcanogenic and authigenic aluminosilicates in addition to inferred eolian material while the residual component younger than 20 ma from the central pacific odp site 1215 is similar compositionally to upper continental crust and suggests an increase in the delivery of asian dust material since 20 ma the residual in sediment older than 20 ma indicates significant amounts of volcanogenic and authigenic materials volcanogenic debris comprises as much as 40 of the residual between 23 40 ma which coincides with the mid tertiary ignimbrite flare up that occurred in much of western north america the residual component extracted from the 50 ma biogenic sediment reflects authigenic signatures seawater like negative cerium anomalies and elevated fe si ratios the previously interpreted increase in an andesitic detrital source in north pacific locations may instead be authigenic material presenting significant challenges for many paleoclimate proxies additionally in the eastern pacific odp site 1256 the residual component contains 70 of volcanogenic material most likely originating from central america and also includes refractory barite the ability to separately identify eolian volcanogenic and authigenic materials in the aluminosilicate component of pelagic sediment allows resolution respectively of the climatic geologic and chemical processes contributing to the paleoceanographic archive in this critical oceanic region

hydration of the crust and upper mantle of the hikurangi plateau as it subducts at the southern hikurangi margin abstract controlled source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave speeds in the crust and upper mantle near the trench similar studies also have found unusually high p wave speeds vp in the upper mantle under oceanic plateaus onshore offshore seismic profiling at the southern hikurangi margin where the 120 ma old oceanic hikurangi plateau is subducting indicates that a fast vp 8 7 0 2 km s upper mantle layer lies beneath a 25 km thick mantle layer with more regular wave speeds vp 8 0 0 2 km s under the hikurangi trough this is consistent with previous findings of upper mantle vp 8 7 9 0 km s in the margin parallel direction under the north island 100 km northwest of the deformation front at depths 8 10 km below the moho our profiles are margin perpendicular thus we show that the upper mantle lid of the subducting pacific plate is characterized by unusually high p wave speeds along all azimuths we find an area of lowered vp in the 12 1 km thick hikurangi plateau crust beneath the trough this drop in vp is 10 and a similar drop in vp is deduced to depths of 25 2 km into the upper mantle we interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low velocity zone in the faster upper mantle layer this zone formed from serpentinisation by hydration through bending induced normal faults and or due to crack porosity introduced by thermal cracking further enhanced by bending related faulting thus the regular mantle vp 8 km s is not in fact regular but rather the high speed mantle has mechanically bent fractured and altered the absolute depth of fast mantle vp under the hikurangi trough is around 50 km the onset of the lower band of seismicity of the double seismic zone and high upper mantle vp under the north island is observed at similar depths this is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough our study on the hikurangi margin is different as the subducting plate here contains a 120 ma old oceanic plateau with a 12 km thick crust but the results are similar to other subduction margins where regular oceanic crust is subducting

holocene dust records from the west african sahel and their implications for changes in climate and land surface conditions we reconstructed aeolian dust accumulation during the holocene from two radiocarbon dated lake sediment sequences from the manga grasslands in northeastern nigeria in order to investigate long term changes in the harmattan dust system over west africa and evaluate their possible causes flux values were low in the early holocene decreasing further to a minimum at around 6 2 kyr b p after which time they increased steadily until around 2 kyr b p and then more sharply after this time the long term variations in dust flux agree broadly with changes in the exposed area of the lake chad basin to the northeast of the study sites which vary inversely with the volume of paleolake megachad more proximal sources of dust including the fine fraction of local dune sand and floodplains of nearby rivers have also made a contribution to the total dust load during times of enhanced dune and fluvial activity sharp rises in dust flux over the past century may be related to human activity broad patterns of change in dust flux during the holocene agree with other reconstructions over the same period however we see no evidence for a stepped rise during the middle holocene as seen at some sites from the northeastern tropical atlantic suggesting that controls on the harmattan dust system have differed from those affecting dust deposition elsewhere across northern africa