on the analysis of longitudinal stress in glaciers analyse des contraintes et des deformations dans un glacier lorsque les contraintes longitudinales deviatrices sont comparables aux contraintes de cisaillement a la base on etudie aussi le cas ou la composante du poids vers l aval n est pas equilibree par la contrainte de cisaillement basale on obtient les expressions de la contrainte de cisaillement a la base en fonction de l epaisseur de la glace de la pente du gradient de deformation en surface et des proprietes de la glace

a late cretaceous and cenozoic reconstruction of the southwest pacific region tectonics controlled by subduction and slab rollback processes abstract a cenozoic tectonic reconstruction is presented for the southwest pacific region located east of australia the reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for pacific australia and lord howe rise pacific relative plate motion the reconstruction is based on a conceptual tectonic model in which the large scale structures of the region are manifestations of slab rollback and backarc extension processes the current paradigm proclaims that the southwestern pacific plate boundary was a west dipping subduction boundary only since the middle eocene the new reconstruction provides kinematic evidence that this configuration was already established in the late cretaceous and early paleogene from 82 to 52 ma subduction was primarily accomplished by east and northeast directed rollback of the pacific slab accommodating opening of the new caledonia south loyalty coral sea and pocklington backarc basins and partly accommodating spreading in the tasman sea the total amount of east directed rollback of the pacific slab that took place from 82 ma to 52 ma is estimated to be at least 1200 km a large percentage of this rollback accommodated opening of the south loyalty basin a north south trending backarc basin it is estimated from kinematic and geological constraints that the east west width of the basin was at least 750 km the south loyalty and pocklington backarc basins were subducted in the eocene to earliest miocene along the newly formed new caledonia and pocklington subduction zones this culminated in southwestward and southward obduction of ophiolites in new caledonia northland and new guinea in the latest eocene to earliest miocene it is suggested that the formation of these new subduction zones was triggered by a change in pacific australia relative motion at 50 ma two additional phases of eastward rollback of the pacific slab followed one during opening of the south fiji basin and norfolk basin in the oligocene to early miocene up to 650 km of rollback and one during opening of the lau basin in the latest miocene to present up to 400 km of rollback two new subduction zones formed in the miocene the south dipping trobriand subduction zone along which the solomon sea backarc basin subducted and the north dipping new britain san cristobal new hebrides subduction zone along which the solomon sea backarc basin subducted in the west and the north loyalty south fiji backarc basin and remnants of the south loyalty santa cruz backarc basin subducted in the east clockwise rollback of the new hebrides section resulted in formation of the north fiji basin the reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere a change in plate kinematics or slab mantle interaction

on the mechanics of surging glaciers le mecanisme des crues glaciaires est developpe a partir des ecoulements intensifs et compressifs des fortes contraintes deviatrices longitudinales et de l etat de contrainte pres de la discontinuite basale

petroleum geology of fiji abstract most petroleum exploration in fiji has been directed towards the shallow water sedimentary basins around and extending onto viti levu the bligh water and bau waters basins a large amount of commercial multi channel seismic reflection data has been acquired across these basins since 1971 and five deep wells were drilled offshore and on viti levu from 1980 to 1982 all the wells were dry although some had minor shows of gas and oil fluorescence the wells penetrated over 2500 m of sedimentary rocks largely of miocene and younger age but some oligocene or older volcanogenic rocks were also intersected the well sections comprise mainly volcaniclastics and lower to middle miocene shallow water limestones which were the primary targets were not encountered a large amount of recently released commercial seismic data from around viti levu is interpreted here to enable an assessment of petroleum potential to be made up to about 4 km and 2 km of late miocene and younger strata are interpreted in the bligh water and bau waters basins respectively but strata underlying an angular unconformity believed to be of late miocene age are not seismically resolved and thus the total sedimentary thickness is unknown the sedimentary sequences off viti levu are cut by large faults and pre pliocene rocks are affected by folding which is locally quite intense no source rocks have been identified in the deep wells nor in tests of outcrop and stratigraphic borehole material from viti levu and vanua levu but anomalous amounts of pentane in seabed sediments off northern viti levu suggest that some thermogenic hydrocarbons have been generated information on subsurface reservoirs is sparse but limestones are generally considered to have better reservoir potential than coarse volcaniclastics and are the more desirable targets because of the limited seismic resolution no traps with lower to middle miocene shallow water limestone facies can presently be identified in the basins offshore viti levu late miocene and younger sequences contain numerous structurally formed potential traps and several pliocene reef like seismic mounds also occur in the western bligh water basin and form potential traps mudstones are widespread in the offshore basins and seals are likely to be good although some relatively recent faulting may have breached some potential traps commercial and non commercial reconnaissance geophysical surveys have also been conducted across other sedimentary basins in fiji including the deep water suva and baravi basins and basins around vanua levu and on the lau ridge

clockwise and anticlockwise rotation of viti levu fiji in relation to the tectonic development of the north and the south fiji basin summary samples for palaeomagnetic study were collected from 40 sites in viti levu fiji reliable palaeomagnetic directions were obtained from 22 sites after magnetic cleaning these data cover the age between late eocene to pliocene geological study indicates that viti levu can be structurally divided into three stages stage 1 late eocene to early oligocene stage 2 early to middle miocene and stage 3 late miocene to pliocene the mean palaeomagnetic direction d i for each stage is 10d 48d in stage 1 64d 28d in stage 2 and 32d 45d in stage 3 respectively expected directions of the geomagnetic field in the past were calculated from the australian apparent polar wander path apwp and were subtracted from the obtained palaeomagnetic directions in geological sequence then the tectonic movement in and around the island of viti levu with respect to the australian plate was estimated for post eocene time the following conclusions were reached 1 a clockwise rotation of about 45d took place during oligocene 2 an anticlockwise rotation of about 75d had occurred since the end of the middle miocene time and 3 the euler poles of these rotations were situated in or near fiji a tectonic history of the fiji region since late eocene which can explain the episodes of clockwise and anticlockwise rotations of viti levu is as follows clockwise rotational movement in oligocene was closely related to the formation of the south fiji basin and anticlockwise rotational movement since the end of middle miocene was closely related to the formation of the north fiji basin the clockwise rotation of viti levu was accompanied by fan shaped spreading in the northern half of the south fiji basin in which a system ridge ridge ridge r r r triple junction of plates was involved the subsequent anticlockwise rotation of viti levu can be explained as ball bearing rotation associated by antithetical transform faults accompanied with the spreading of the north fiji basin

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

volcanism associated with extension in an oligocene miocene arc southwestern viti levu fiji the wainimala group rocks of southwestern viti levu fiji represent part of an oligocene miocene island arc some of the early volcanism in this arc took place on a substrate of eocene oligocene frontal arc crust mainly andesitic transitional calcalkaline lavas were erupted from major subaerial volcanic edifices while elsewhere tholeiitic magmas formed low lying basaltic lava fields and small felsic volcanic centres on the seafloor the intrusion of a dense bimodal basalt dacite dyke swarm into the underlying frontal arc crust indicates that significant crustal extension accompanied the eruption of the inter edifice lavas geochemical variations within the wainimala volcanic suite are thought to have been generated through fractional crystallisation processes while relatively slow ascent and mixing of magmas took place beneath the major edifices efficient shallow level fractionation and rapid ascent of magmas occurred in the areas between them the contrasting styles of volcanism may reflect the interaction of ascending magmas with pre existing heterogeneities in the frontal arc substrate

contribution of aeolian dust in japan sea sediments estimated from esr signal intensity and crystallinity of quartz 1 late quaternary hemipelagic sediments in the japan sea contain aeolian dust from east asia which potentially records past variations in the asian monsoon and the westerly jet however extracting information about aeolian dust from japan sea sediments is difficult because the sediments also contain detrital material from the japanese islands here we present a method for extracting the aeolian dust components from japan sea hemipelagic sediments using provenance analysis of different size fractions hemipelagic sediments of the japan sea can be described by a combination of two populations characterized by lognormal grain size distributions with different median grain sizes which are called the coarser 2 15 m and the finer 10 m populations we characterized these populations on the basis of provenance analyses using the electron spin resonance esr signal intensity and crystallinity of quartz the results suggest that quartz in the coarser population has been mostly supplied from northern china and siberia northeastern china by aeolian transport whereas quartz in the finer population has been dominantly supplied by rivers draining the japanese islands

clay minerals as indicators of sources of terrigenous sediments their transportation and deposition bering basin russian alaskan arctic abstract cluster analysis of clay mineral abundances in surficial sediments from 52 stations revealed the presence of four station cluster groups within the bering basin and contiguous eastern aleutian trench forearc these results can be explained using two end members of terrigenous clay mineral sources and their admixtures the first end member cluster group i consists of the highest average concentration of illite 48 and chlorite plus kaolinite 40 and lowest of expandable minerals 12 in contrast the second end member cluster group iv has the highest concentration of expandable minerals 43 and lowest concentrations of illite 30 and chlorite plus kaolinite 27 the group i assemblages come from two regions the eastern aleutian trench forearc and the northwestern margin of the bering basin off koryak mountains east siberia clays in the former region are derived primarily from the meta sedimentary and meta volcanic rocks of the chugach mountains with infiltrations from alaska peninsula and are transported from the southeastern kodiak shelf of the gulf of alaska to the trench forearc by the alaska current alaskan stream similar clays are derived from the koryak mountains group iv clays located within the komandorsky and bowers basins and unmak plateau originated from the mio pliocene andesites and basalts of the eastern kamchatka peninsula and aleutian islands respectively it is postulated that group ii clays in the southcentral aleutian basin and bowers ridge basin area and group iii clays in the northern aleutian basin are mixtures obtained primarily from terrigenous sources of group i and iv the distribution patterns and boundaries of the cluster groups reflect the dispersal pathways of clays from the primary sources and the extent of their mixing by regional and local surface currents and mesoscale gyres eddies presumably the minor infiltration of anadyr river clays into group iii is a result of clay transport by episodic turbidity currents most of the clays derived from the alaska mainland and russia proper do not reach the bering basin as these clays are trapped within the bering sea shelf

seafloor spreading in the north fiji basin southwest pacific abstract the seapso leg 3 cruise december 1985 was carried out in the north fiji basin aboard the ifremer r v jean charcot the main purpose of the cruise was to carry out a geophysical and geological study of the central part of the basin between 16 s and 22 s to this end bathymetric profiles with the help of the seabeam multichannel echosounder single channel seismic reflection profiles and magnetic and gravimetric profiles were made moreover geological samples were taken by dredging and piston coring as well as water samples with the help of a multiprobe one of the aims of the cruise was to explore the present day spreading axis of the north fiji basin we were able to show that accretion the rate of which is about 7 cm yr in the north fiji basin has been constrained in a n s direction for 3 ma very recently between 0 6 and 0 7 ma ago the accretion process changed resulting in the creation of a ridge ridge trench triple junction centered at 173 30 e and 16 40 s water samples taken in the vicinity of this triple junction show significant anomalies of methane and manganese which are probably linked to hydrothermal events in its morphological characteristics transversal as well as longitudinal the ridge of the north fiji basin shows striking similarities to the east pacific rise it presents the same differences in level the same sizes of structures and the same particular features overlapping spreading centers offsets etc another aim of the seapso cruise was to study the complex deformation area located immediately to the west of the fiji islands which we interpret as a distensive type deformation in a strike slip system

contribution of detrital materials from the yukon river to the continental shelf sediments of the bering sea based on the electron spin resonance signal intensity and crystallinity of quartz abstract bering sea sediments contain detrital materials from the yukon river these materials may contain records of past climate changes in the arctic area such as the melting of glaciers around the drainage basin of the yukon river which help to understand hydrological cycle in this area in the bering sea however the spatial extent and pattern of the detrital materials supplied from the yukon river is not yet fully understood here we developed a method to identify clay to sand sized detrital materials derived from the yukon river based on electron spin resonance esr intensity of the e 1 center and the crystallinity of quartz we then estimated the spatial pattern of quartz contributed by the yukon river on the bering sea shelf by applying the method to core top samples from the continental shelf and slope of the eastern bering sea the results showed a large contribution of sand sized quartz from the yukon river to wide areas of the continental shelf and slope whereas contributions of clay to silt sized quartz from the yukon river were small except on the northeastern shelf these spatial distribution patterns suggest that sand sized quartz was repeatedly reworked and transported by processes such as storm surges to the outer continental shelf whereas the clay to silt sized quartz on the northeastern shelf was supplied as suspended materials directly from the yukon river

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

geophysical evidence for post miocene rotation of the island of viti levu fiji and its relationship to the tectonic development of the north fiji basin geophysical studies of the island of viti levu fiji show largeamplitude magnetic anomalies of up to 1500 nt to be located above the tavua volcanic center field sampling of the volcanic rocks from the intrusive and extrusive centers of the volcano show the rock samples to have initial intensities of natural remanent magnetization of up to 380 a m radiometric dating of fifteen oriented samples show an age range of 2 9 to 7 0 m y b p for the suite of rocks collected from the mba basaltic group paleomagnetic analysis of the dated rocksshows declination vectors ranging from 325 9 for an age of 2 9 0 2 m y b p to 290 4 for an age of 7 0 0 4 m y b p as a group the paleomagnetic declination values of the samples systematically decrease with age this decrease is attributed to progressive anticlockwise rotation of viti levu during the process of formation of the north fiji basin a linear least squares analysis of the paleomagnetic data indicates that the island has rotated in an anticlockwise direction through 90 during the past 7 m y at a rate of 13 7 per million years declination values as low as 245 measured for some sites not dated suggest that anticlockwise rotation of viti levu and development of the north fiji basin could have commenced as early as 10 m y b p following the breakup of the ancient solomons new hebrides fiji lau island arc large rotations of microplates may have occurred along other island arcs and marginal basin systems elsewhere in the western pacific and such rotational histories may reflect in detail the evolutionary development of these arcs and basins

role of ridge jumps and ridge propagation in the tectonic evolution of the lau back arc basin southwest pacific gloria sidescan imagery of the central lau basin reveals a complex neotectonic framework in which the central lau spreading center is actively propagating southward at the expense of the eastern lau spreading center the latter is located about 75 km closer to the active tofua arc and the transfer of spreading motion across this distance is achieved via a relay spreading ridge segment new magnetic and bathymetric interpretations suggest that the majority of the central and southern basin was formed by the eastern lau spreading center between 5 ma and the present day the processes of arcward ridge migration ridge propagation and basinward arc migration may reflect the complex interplay of the subduction zone and regional tectonics in driving back arc spreading and may be a common feature of this tectonic environment

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

double saloon door tectonics in the north fiji basin abstract fiji platform rotated counter clockwise from at least 10 2 ma until 1 56 ma while vanuatu arc rotated clockwise from 12 10 ma until the present a revised model incorporating these opposite rotations explains the distribution of magnetic anomalies in the north fiji basin nfb the conjugate margin of southwest fiji platform is southeast vanuatu arc previous models which associate nw oriented anomalies off southwest fiji with similarly oriented anomalies northeast of vanuatu arc are therefore wrong secondly these models propose a nw oriented spreading ridge extending to 19 s 177 5 e almost 500 km southeast of the mapped extension of rifted island arc crust on vanuatu arc this creates an unacceptable overlap with fiji platform in pre rift reconstructions thirdly anomalies off sw fiji which are nw oriented in their present day position were oriented nne during initial breakup rather than aligning with nw oriented anomalies in the western nfb they are matched by nne oriented anomalies off se vanuatu arc with further rotation these conjugate anomaly sets form a fan shape in the southern nfb fourthly previous models which recognise a triple junction only from 3 ma do not explain early wnw ese separation of vanuatu arc and fiji platform required by well documented opposite rotations nfb characteristics which match the double saloon door tectonic model include opposite rotations of island arc terranes backarc seafloor spreading which is both arc parallel and arc perpendicular and rifts propagating north south northeast and northwest features which do not match the double saloon door model include the north fiji fracture zone and the west fiji spreading centre both initiated post 1 5 ma when fiji platform stopped rotating and only one terrane vanuatu arc continued to rotate the nfb developed via double saloon door tectonics from 12 10 ma until 1 5 ma whereas post 1 5 ma opening constitutes single saloon door rotation

the offshore morpho structure and tsunami sources of the viti levu seismic zone southeast viti levu fiji abstract new high resolution seabat 8160 multibeam bathymetry data reveal in detail the complex offshore morphology over part of the viti levu seismic zone vlsz in southeast viti levu the mapped seafloor is scarred by a complex network of linear submarine canyons and numerous submarine slides the submarine canyons are morphologic expressions of graben structures in the underlying basement rock submarine canyon faults are continuous with nearshore shelf faults and onshore fault controlled valleys and structural lineaments together these define continuous onshore offshore fault zones which combined form a structurally complex fault mesh of composite origin we attribute the development of the principal fault sets in the fault mesh to progressive distributed faulting in response to anticlockwise tectonic block rotations within the fiji platform during the neogene low occurrence of large earthquakes and a diffused pattern of seismicity currently observed in the vlsz may reflect complex coseismic faulting through the fault mesh the abundance of submarine slides over the vlsz which typically occur on the outer barrier reef edge as well as the submarine canyon heads and walls and mid slope imply that seismic shaking may be an important factor for triggering slope failure morphometric analysis and empirical modelling of the slides show that slides triggered in shallow water depths within 5 km of the coastline at the outer barrier reef edge and submarine canyon heads produce the largest near field tsunami amplitudes we interpret that such slides represent a significant local tsunami hazard

a possible marine mechanism for internally generated long period climate cycles abstract by reinterpreting the sea ice extent discussed in a recently proposed model of auto oscillatory climatic change saltzman et al 1981 to represent high inertia grounded and shelf sea ice forms instead of thinner marine ice forms it is possible for the model to yield a much longer period of auto oscillation perhaps as much as 100 000 years the dominant period of ice volume variations revealed by deep sea ocean cores the physical basis of the model can in principle be tested by future evidence regarding the long term variations of bulk ocean temperature and atmospheric co2 concentration

new magnetostratigraphic and 40ar39ar dating results from the suva marl fiji calibration of the early pliocene geomagnetic polarity time scale abstract the suva marl fiji provides a rare opportunity to integrate early pliocene biostratigraphic magnetostratigraphic and chronostratigraphic data we present new paleomagnetic data that extend the previously reported magnetostratigraphy p rodda i mcdougall r a cassie d a falvey r todd j a wilcoxon isotopic ages magnetostratigraphy and biostratigraphy from the early pliocene suva marl fiji geol soc am bull 96 1985 529 538 of the suva marl down section and suggest a revision of the original correlation with the geomagnetic polarity time scale gpts 40 ar 39 ar laser incremental heating analyses of mineral separates from the interbedded tuffs yield refined age constraints for previously reported k ar dates for the suva marl polarity reversals the new dates and the new magnetostratigraphic data are most consistently interpreted as supporting the revised early pliocene polarity time scale w a berggren d v kent c c swisher iii m p aubry a revised cenozoic geochronology and chronostratigraphy in w a berggren d v kent m p aubry j hardenbol eds geochronology time scales and global stratigraphic correlation soc econ paleontol mineral spec publ 54 1996 129 212 the ages of the suva marl reversals derived from the 40 ar 39 ar dates provide a revised direct calibration of the early pliocene gpts bringing it in synch with those calibrations produced by orbital tuning of milankovitch cycles astronomical time scale

recent geodynamic evolution of the north fiji basin southwest pacific a cruise of the r v jean charcot seapso iii december 1985 in the north fiji basin between 16 and 22 s obtained new structural and magnetic data concerning the central part of the basin these have been used for the reinterpretation of the national oceanic and atmospheric administration and u s naval research project aeromagnetic survey 1979 the structural surface ship magnetic and aeromagnetic data allow us to present a sketch of the evolution in four stages of the north fiji basin for the past 10 m y

a coupled marine ice stream ice shelf model theorie des variations climatiques du pleistocene a aujourd hui liees a la dynamique des couvertures marines de glace couplee aux glaces libres flottantes on neglige les effets thermodynamiques et on etudie les processus de flottaison en relation avec les rapports de densite entre la place et l eau de mer

arc dynamics and tectonic history of fiji based on stress and kinematic analysis of dikes and faults of the tavua volcano viti levu island fiji 1 stress analysis of dikes and kinematic analysis of faults within the tavua volcano of viti levu fiji indicate an apparent migration of the causative maximum principal stress axis 1 from nw toward north between 5 0 and 3 9 ma the dikes and faults are part of successive structural and magmatic events at the arc scale we present data on the chronological succession of stress indicators in the tavua volcano based on overprinting relationships and published geochronology which also equate with a counterclockwise rotation of fiji assuming an approximately constant n s regional compressive stress orientation during this time the latter interpretation is consistent with paleomagnetism studies that require substantial counterclockwise rotation of the fiji platform since fragmentation of the formerly continuous outer melanesian arc during the latest miocene to earliest pliocene the stress data suggest that the fiji platform has undergone an 50 rotation since 5 ma given that rotation apparently stopped at 3 ma then particularly fast rotation of 25 myr 1 occurred from 5 ma to 3 ma recent seismicity data indicate the platform is currently subject to n s compression the history of late miocene and pliocene events in the region can be related to the consequences arising from collision of an island arc with an oceanic plateau

sources and dispersal patterns of clay minerals in surface sediments from the continental shelf areas off alaska the three mineral suites were recognized in the gulf of alaska the glaciomarine sediments of fjords and the eastern shelf consist typically of the primary phyllosilicates micas and chlorites derived locally from argillite graywacke and metavolcanics under conditions of mild chemical weathering more glycol expandable clay minerals gecm in the north gulf substantiate clay supply to that region primarily from the yakataga and poul creek formations and to a lesser extent from the copper river the northern gulf clay mineral suite is transported to central prince william sound and northeast kodiak shelf by the alaska coastal current in the western gulf clay minerals adjacent to the alaska peninsula are derived principally from the susitna river via cook inlet whereas those of the outer shelf presumably have their provenance in kodiak island and the katmai ash submarine deposit in the southeast bering sea a progressive northwestward decrease in gecm from 65 to 11 is caused by a gradually decreased sediment export from the alaska peninsula and the aleutians with a possible minor increase in illite from the kuskokwin river the yukon river is the predominant clay source for the north bering sea as signified by an abrupt increase in that area of gecm 20 to 35 and of kaolinite chlorite ratios clay mineralogy indicates that the central chukchi sea is the major repository of the yukon clays suggesting that clay mineral stratigraphic studies have implications for interpretation of the quaternary paleogeography of beringia in the beaufort sea nearshore influence of different fluvial inputs and their dispersals by currents are sharply defined the lack of a definite pattern to the distribution of clay minerals in the outer continental shelf may be due to haphazard transport of clays by ice rafting and or reworking and redistribution of sediments resulting from ice gouging of the sea substrate movement of sediment to the west in the alaskan portion of the inner shelf of beaufort sea is evident an apparent seaward increase in illite gecm ratio in the colville delta is tentatively ascribed to regeneration of degraded illite micas by k fixation in saline waters an isolated area of sediment rich in expandable clay minerals in the central beaufort shelf presumably is relict

the tavua caldera fiji a complex shoshonitic caldera formed by concurrent faulting and downsagging the shoshonitic tavua volcano in viti levu fiji developed from a submarine to a subaerial environment and comprises lava flows dominantly of absarokite composition with debris flows and conglomerates on its flanks the tavua caldera formed in the centre of the volcano within the caldera avalanche deposits are overlain by the turtle pool formation which consists of lacustrine sediments shoshonite lavas and hydrovolcanic pyroclastic lapilli tuffs subsequently the inner caldera formed in the centre of the tavua caldera this was infilled by the morrisons pool formation which comprises lavas and pyroclastic rocks of banakite composition with minor intercalated sediment thus the magmas became progressively more evolved in addition to normal movement along regional and concentric faults subsidence which exceeds 3 km was accomplished via substantial downsagging evidence for this is the consistent centripetal dip of caldera fill material which locally exceeds 50 in parts of the turtle pool formation centripetal dips are also recognized in pre caldera rocks immediately outside the tavua caldera subsidence was episodic caused by periodic non explosive withdrawal of underlying magma to erupt on the flanks and in part within the caldera downsagging and fault movement were responsible for approximately equal amounts of subsidence and their interaction produced a caldera with the shape of a stepped funnel

multiple active spreading centres in the hot north fiji basin southwest pacific a possible model for archaean seafloor dynamics abstract the north fiji basin nfb in the southwest pacific is one of the most evolved back arc basins within the basin active spreading is taking place along several ridges some of which display the characteristics of spreading ridges observed in mature large oceans characterized by a range of spreading rates segmented accretion centres organized magnetic lineations and propagators for example however the great cumulate length of the spreading centres of the nfb relative to the basin s surface makes it an unique example among the world s oceanic basins we have reviewed a comprehensive body of geological geophysical and geochemical evidence multiple spreading centres of various trends high heat flow high geoid and slow upper mantle seismic velocities which collectively demonstrate that a hot upper mantle underlies the north fiji basin active upper mantle convection is the key process controlling the large crustal and lithospheric production in the nfb which would be thus largely independent from the evolution of even the largest structural features such as the vitiaz and new hebrides subduction zones petrological studies of the archaean greenstone belts have led to a similar picture for the entire earth during the archaean numerous short active ridges connected by unstable triple junctions would have at this time provided an efficient convective mechanism allowing for a large heat dissipation we propose that the presently active processes in the north fiji basin are modern analogues to archaean seafloor dynamics

rapid rotation of fiji paleomagnetic evidence and tectonic implications the timing and magnitude of rotation of fiji in relation to the tectonic evolution of the sw pacific region has been investigated by extensive 126 sites paleomagnetic sampling of late miocene and younger units generally samples yielded well defined characteristic remanence directions which indicate that the main fiji platform rotated by 135 17 5 counterclockwise commencing at or before 10 ma this very large and rapid rotation ceased abruptly at 3 ma coincident with the onset of spreading on the major n s aligned spreading centers in the north fiji basin latest miocene to pliocene paleomagnetic results indicate that viti levu the main island itself has suffered no internal disruption by localized rotations after 5 ma we present a reconstruction for the origin of fiji which differs markedly from previous models such that fiji tonga and lau form a continuous n s aligned arc and that arc reversal was initiated at or near fiji itself

magnitude and timing of new hebrides arc rotation paleomagnetic evidence from nendo solomon islands paleomagnetic results from nendo record a total rotation of the northwestern corner of the new hebrides arc of 52 12 clockwise reversed magnetic polarity and a nannofossil assemblage constrain the age of the nendo pole to 4 29 3 58 ma paleomagnetic data from malekula farther south in the same arc imply a rotation of only about 28 since the late miocene probably because these poles have been affected by rotation of malekula in response to collision of the arc with the d entrecasteaux zone aseismic ridge a new late miocene reconstruction of the new hebrides and vitiaz arcs based on the paleomagnetic evidence requires a rotation of 39 around the point of intersection of the two arcs the same rotation restores both the northwestern and southeastern segments of the new hebrides arc although rotation of the southeastern segment commenced earlier a series of sequential movements of segments of the arc occurring at different times but acting to keep the arc roughly collinear may be driven by a mechanism which minimizes deformation of the subducted plate propagation of spreading into the northwestern corner of the north fiji basin after 4 3 ma is implied by the paleomagnetic results and is consistent with spreading anomaly patterns in the basin when the new hebrides and vitiaz arcs are restored to their position relative to the lau ridge at 10 ma the fiji platform can be accommodated after clockwise rotation of 140 to a position in line with the new hebrides arc the large rotation for fiji is consistent with paleomagnetic evidence indicating at least 115 of rotation

oceanic mechanisms for amplification of the 23 000 year ice volume cycle situated adjacent to the largest northern hemispher ice sheets of the ice ages the mid latitude north atlantic ocean has an important role in the earth s climate history it provides a significant local source of moisture for the atmosphere and adjacent continents forms a corridor that guides moisture bearing storms northward from low latitudes and at times makes direct contact along its shorelines with continental ice masses evidence of major ice ocean air interactions involving the north atlantic during the last 250 000 years is summarized outflow of icebergs and meltwater initially driven by summer insolation over the ice sheets affects midlatitude ocean temperatures summer heat storage winter sea ice extent and global sea level these oceanic responses in turn influence the winter moisture flux back to the ice sheets as well as ablation of land ice by calving spectral data indicate that the oceanic moisture and sea level feedbacks in part controlled by glacial melt products amplify milankovitch insolation forcing of the volumetrically dominant mid latitude ice sheets at the 23 000 year precessional cycle

kinematics of active spreading in the central north fiji basin southwest pacific abstract based on a synthesis of magnetic and bathymetric data we re evaluate the kinematics of the recent opening of the central part of the north fiji basin nfb the westward motion of the pacific plate along the left lateral north fiji fracture zone nffz results in the opening of two n s trending spreading ridges located at 173 30 e and 176 e both ridges show complex features such as propagating rifts ridge jumps and overlapping spreading centres their spreading rates are similar 7 6 to 4 cm yr 1 across the western ridge 5 5 cm yr 1 across the eastern one while the nffz is purely strike slip to the east of the eastern ridge it becomes more complex to the west it changes from transpressional to transtensional then to purely transform and finally joins the western n s ridge in a rrr type triple junction our kinematic analysis shows that most of the left lateral motion along the nffz is transferred to the eastern ridge at the rtf type 176 e triple junction it suggests that the western n s ridge is probably connected to the north with another left lateral transform possibly the south pandora ridge

a crustal model for zealandia and fiji summary existing maps of satellite free air gravity and bathymetry and the crust 2 0 global crustal structure model have been used to make new bouguer gravity anomaly and crustal thickness maps of the sw pacific region to analyse the region s crustal structure the new maps clearly outline the limits of the rifted largely submerged continent of zealandia the bouguer gravity anomalies of ocean crust large igneous provinces and backarc basins vary simply and predictably with age the bathymetric fiji platform lies in a water depth of 19 km for the entire fiji platform comparable to stretched and submerged parts of zealandia the question whether the continental crustal thickness of fiji is due to its actually being a rifted part of gondwana zealandia or due to enhanced eocene pliocene magmatism remains open but testable with the latter more likely using the new accurate 3 d crustal thickness and published radiometric dating of lavas we estimate that the magmatic portions of southwest pacific cenozoic subduction related arcs three kings tonga and vanuatu have grown at net volumes which vary from 263 to 1400 km3 km 1 with net addition rates of 17 31 km3 km 1 ma 1

magmatism and associated hydrothermal activity during the evolution of the tavua caldera 40 ar 39 ar dating of the volcanic intrusive and hydrothermal events the tavua caldera in viti levu fiji occurs in the upper part of the shoshonitic tavua volcano precaldera rocks forming the edifice of the volcano are predominantly mafic absarokite in composition whereas intracaldera rocks are intermediate shoshonite and banakite in composition the caldera hosts subeconomic porphyry style copper gold mineralization and minor weakly auriferous acid sulfate epithermal alteration both associated with monzonite intrusions in the center of the caldera and adularia sericite epithermal mineralization the emperor gold deposit on the caldera margin ten tavua samples ranging from hydrothermally altered or mineralized to relatively fresh material were dated by the 40 ar 39 ar incremental heating technique these dates establish that caldera collapse occurred between 4 6 and 4 5 ma precaldera absarokites have an age greater than 4 6 ma and probably as much as 4 7 ma while intracaldera shoshonites and banakites have a maximum age of 4 51 ma so there was a gap of at least 0 1 m y between mafic and major intermediate volcanism in this area monzonite intrusion and concurrent porphyry style alteration in the center of the caldera occurred between 4 51 and 4 44 ma essentially synchronous with intracaldera volcanism the last magmatic event within the caldera a banakite vent phase pyroclastic unit occurred at 4 14 or 0 08 ma adularia deposited during epithermal mineralization at emperor was formed at 3 71 or 0 13 ma this history which is clearly indicated by 40 ar 39 ar data would not have been possible to deduce by conventional k ar dating the results are similar to those obtained in other porphyry and epithermal systems where porphyry style magmatic hydrothermal activity is deemed to be synchronous with magmatism but a measurable interval occurs between magmatism and epithermal mineralization tavua is unusual in that samples of both types of mineralization are available are amenable to dating techniques and can be tied to the history of the caldera

a glacier flow model incorporating longitudinal deviatoric stresses le modele d ecoulement de glacier developpe introduit les effets de la composante longitudinale du deviateur des contraintes dans les equations les hypotheses sous jacentes peuvent etre appliquees au developpement de modeles pour des cas de figures variees ici elles sont envisagees pour des glaciers glissant en ecoulement plan a l etat stationnaire et non stationnaire le modele se ramene a une generalisation adequate de la theorie pseudo hydrostatique en ecoulement plan si la contrainte longitudinale deviatrice est negligee devant la contrainte de cisaillement basale

diagenetic overprint on authigenic nd isotope records a case study of the bering slope abstract neodymium isotope ratios 143nd 144nd end have been used as a quasi conservative water mass tracer in the atlantic and southern oceans however boundary exchange and diagenesis can limit their use in areas where extensive interactions between seawater pore water and bottom sediments can overwrite the original water mass signature to avoid the misapplication of this proxy and to provide a new perspective on the application of nd isotopes to authigenic phases we present end records covering the past 2 4 myr for the authigenic end auth and detrital end det phases of sediments from integrated ocean drilling program site u1343 1950 m water depth on the bering slope the end det values range from 9 0 to 2 3 n 70 showing binary mixing between alaskan and aleutian sediments glacial interglacial variations are muted in the end det record because sediment delivery pathways from both source areas exist regardless of climatic conditions the end auth values range from 5 9 to 0 2 n 153 and show a strong correlation r 0 58 n 60 with end det in the deeper part of the core 160 ka where authigenic carbonate layers are frequently observed we attribute this covariation to diagenetic overprinting by pore water nd during late diagenesis the pore water nd that is incorporated into authigenic carbonates was initially released by marine silicate weathering therefore indicating that sediment lithology exerts a first order control on the end values of the pore water in deeply buried sediments in the shallower part of the core r 0 23 n 10 here end auth is typically radiogenic during interglacial periods which may be due to the diagenetic flux from the relatively stagnant n pacific or the incongruent chemical weathering of freshly ground sediments on the bering shelf the effect of dense water formation on end auth seems to be minimal when we compare the end auth records at three different depths of the bering sea our end dataset provides a unique perspective on the behavior of end auth during late diagenesis to apply end auth as a water mass tracer to long cores containing volcanogenic sediments or in productive regions we recommend testing for deep authigenesis using pore water profiles

sedimentation and volcanism in an oligocene miocene intra oceanic are and fore are southwestern viti levu fiji rocks belonging to the lower oligocene to middle miocene wainimala group in southwestern viti levu can be divided into an are assemblage representing the axial part of a volcanic are and a basinal assemblage representing the trenchward flank of the are and the inner part of the fore are basin the are assemblage consists mainly of volcaniclastic rudites and lava flows with minor shallow water limestones and is intruded by mid to late miocene gabbro and tonalite stocks the basinal assemblage to the north is floored by part of an earlier eocene to lower oligocene are the lower part of this assemblage includes volcaniclastic rudites representing the dispersal aprons of major volcanic edifices to the trenchward of the are axis and basaltic lavas and fissure fed rhyolite lava domes and breceia ridges erupted in deeper water away from these centres the rhyolites are directly related to the acidic members of a dense compositionally bimodal dyke swarm that intrudes the underlying earlier are rocks recording substantial crustal extension the basinal volcanic rocks are interbedded with and overlain by a sequence of pelagic carbonates thin bedded epiclastic turbidites and acidic tuffs the basinal succession evolved from an intra are to an inner fore are setting as volcanism within the assemblage died away in the early miocene in the late early miocene increased uplift and emergence of the are massif increased the sediment yield to the fore are basin resulting in the deposition of thicker bedded coarser grained turbidites

sedimentary records of bulk organic matter and lipid biomarkers in the bering sea a centennial perspective of sea ice variability and phytoplankton community abstract in this study organic geochemical analyses of two sediment cores bl16 and lv63 23 recovered from the western bering sea were carried out to examine the sea ice variability and its relationship to phytoplankton community evolution over the past century bulk stable organic carbon isotopic composition 13ctoc showed pronounced depletion on the northern shelf since the late 1970s indicating greater terrigenous organic matter om under warming during recent decades variation in sedimentary om in the southward core was closely associated with marine primary productivity and regional deposition processes arctic sea ice proxy ip25 throughout the two cores with different temporal profile patterns demonstrated sea ice presence with the spatio temporal variability across the study area over the past century the phytoplankton marker ip25 index pip25 a proxy for estimating semi quantitatively sea ice concentrations reflected a decreased sea ice cover with more distinct interannual fluctuations between 0 7 and 0 2 especially in core bl16 after the late 1970s coinciding with the recent warming scenario increased concentrations of phytoplankton biomarkers brassicasterol and dinosterol and their ratios as well as the pip25 record in core bl16 indicated a synchronous variability of reduced sea ice cover with the enhancement of phytoplankton productivity since the late 1970s these results suggested a coupled interaction of the sea ice condition and planktonic ecosystem in the north bering shelf our results also revealed recent since the 2000s spatial heterogeneity in sea ice coverage between the northern and southern parts of the bering sea

sea floor spreading in the lau back arc basin abstract we report the first 3 d magnetization inversion of bathymetry and magnetics data for the entire lau basin and confirm a history of southward propagating sea floor spreading during the past 4 myr magnetic lineations seafloor fabric and ridge segmentation patterns indicate that lithospheric accretion on discrete spreading centers in this back arc basin is fundamentally similar to that occurring on mid ocean ridges the brunhes chron spreading rates 65 100 mm yr are

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

excess 210pb and 137cs concentrations mass accumulation rates and sedimentary processes on the bering sea continental shelf abstract we used 210pb and 137cs concentrations in twelve cores from the continental shelf and slope of the bering sea to investigate recent mass accumulation rates in sediments and sedimentation processes no decreases in exogenic 210pb concentrations with depth were observed in sediments from the shallow shelf area suggesting that there has been considerable mixing of the sediments there however we calculated mass accumulation rates from 210pbex profiles for several cores collected from the outer shelf and the slope mass accumulation rates in the sediments from these cores ranged from 0 11 to 0 44 g cm 2 y 1 for the cores with a mixed surface layer we showed that sediments that accumulated over the last 50 78 years were mixed the 210pbex inventories in the sediment cores suggested deposition of 210pb particles was focused in the slope area 137cs was observed in the sediments collected from the northern to western shelf areas the 137cs inventories showed higher values in the shelf area which suggests that fine grained particles from the yukon river have contributed to sedimentation there

the nadi basin neogene strike slip faulting and sedimentation in a fragmented arc western viti levu fiji the nadi basin in western viti levu is one of a number of sedimentary basins controlled by e w to ne sw faults that developed on the fiji platform in the late miocene the initial basin fill belonging to the upper miocene nadi sedimentary group consists of mainly basement derived breccias and sandstones deposited at bathyal depths by sediment gravity flows on a basin floor dissected by braided e w faults the main source area lay to the south across the actively down faulting basin margin in the latest miocene a change in provenance took place as debris spread southward from a newly developed e w chain of shoshonitic volcanoes koroimavua volcanic group some 12 km north of the southern basin margin continued normal displacement along the southern margin allowed the deposition of a thick sequence of shoshonitic sandstones at bathyal depths on the downthrown side in the earliest pliocene the onset of basin inversion was marked by shallowing to neritic depths and the onlap of sandstones and shoshonitic conglomerates southward onto pre nadi sedimentary group basement as inversion proceeded the basin fill was folded and southward directed reverse displacement took place along the southern basin margin evidence from the nadi and other late miocene basins indicates that they developed as a result of broadly e w directed sinistral strike slip probably related to a shearing collision of the melanesian border plateau with the northern margin of the fiji platform during the breakup of the formerly continuous outer melanesian arc it is suggested that prior to the establishment of the fiji fracture zone at 7 6 ma much of the left lateral displacement currently taking place along that plate boundary was accommodated by strike slip deformation within the fiji platform

melanesian back arc basin and arc development constraints from the eastern coral sea abstract the eastern coral sea is a poorly explored area at the north eastern corner of the australian tectonic plate where interaction between the pacific and australian plate boundaries and accretion of the world s largest submarine plateau the ontong java plateau has resulted in a complex assemblage of back arc basins island arcs continental plateaus and volcanic products this study combines new and existing magnetic anomaly profiles seafloor fabric from swath bathymetry data ar ar dating of e morb basalts palaeontological dating of carbonate sediments and plate modelling from the eastern coral sea our results constrain commencement of the opening of the santa cruz basin and south rennell trough to c 48 ma and termination at 25 28 ma simultaneous opening of the melanesian basin solomon sea further north suggests that a single 2000 km long back arc basin with at least one triple junction existed landward of the melanesian subduction zone from eocene oligocene times the cessation of spreading corresponds with a reorganisation of the plate boundaries in the area and the proposed initial soft collision of the ontong java plateau the correlation between back arc basin cessation and a widespread plate reorganisation event suggests that back arc basins may be used as markers for both local and global plate boundary changes

glacial interglacial sediment transport to the meiji drift northwest pacific ocean evidence for timing of beringian outwashing abstract a large sediment deposit known as the meiji drift located in the northwestern pacific ocean is thought to have formed from deep water exiting the bering sea although no notable deep water forms there presently we determine the terrigenous sources since 140 ka to the drift using bulk sediment 40ar 39ar and nd isotopic analyses on the silt sized 20 63 m terrigenous fraction from ocean drilling program odp site 884 to reconstruct paleo circulation patterns there are large changes in both isotopic tracers varying on glacial interglacial cycles during glacial intervals bulk sediment 40ar 39ar ages range between 40 and 80 ma while nd isotopic values range from end 1 to 2 during interglacial intervals sediments become much younger and more radiogenic with bulk sediment ages falling to 2 15 ma and nd isotopic values ranging between end 5 and 9 these data and quantitative comparison to potential source rocks indicate that the young kamchatkan and aleutian arcs lying nw and ne of the meiji drift contribute the majority of sediment during interglacials conversely older source rocks such as those drained by the yukon river and northeast russia are the dominant origin of sediments during glacials mixing model calculations suggest that as much as 35 45 of the sediment deposited in the meiji drift during glacials is from the bering sea it remains unclear whether thermohaline type circulation or focussing of bering sea flow lead to the glacial interglacial sediment source changes observed here

slow physics of large continental ice sheets and underlying bedrock and its relation to the pleistocene ice ages very simple two component models of the earth s cryosphere lithosphere system are analyzed the models consist of a continental ice sheet coupled to a responding earth model because of relatively long response times of the bedrock at least over shorter wavelengths long period of the order of 50 000 years internal or free oscillations in the system occur over a wide range of parameters and parameterizations the oscillatory mechanism operates primarily in the ablation zone and in particular at the firn line these free oscillations could play a dynamic role in the response of the climate system to eccentricity forcing a simple analysis presented strongly suggests that for the simplest rheological models of the earth response to pleistocence ice sheets elastic response is confined to a layer considerably less than the thermal thickness of the lithosphere furthermore because of the striking lack of geological evidence for large depressions of bedrock at the leading edge of growing continental ice sheets predicted by viscoelastic models of the earth more sophisticated rheological models of the earth s uppermost layer appear necessary

a paleoclimate model of northern hemisphere ice sheets abstract a model for predicting the growth and decay of ice sheets based on the astronomical theory of climate change is presented the purpose of the study in part is to isolate the role of the ice sheet physics and earth response under varying ice load by simplifying to the extreme the role of the hydrosphere atmosphere ice sheet physics and the response of the lithosphere asthenosphere under the ice load are modeled explicitly insolation anomalies taken at a fixed latitude directly force latitudinal displacement of the snow line accumulation rate a and ablation rate a evaluated at mean sea level are specificed as external constants a a decrease linearly with ice sheet elevation rough tuning of the model to the general shape of the ice volume record of the last two major glacials determines the external constants model predictions of the ages of several events in the last major glaciation compare well with the radiological ages the six glacial terminatios i vi over the last 600 000 yr are identified and the predicted ages compare reasonably well with the 18 o record for two deep sea cores a direct comparison of model power spectra of ice volume as a function of period with spectra of the 18 o record shows apparent underprediction of power near 100 000 yr when a quantitative but heuristic method for taking into account the red noise spectrum evident in the geological records is used a much more favorable comparison is possible the model prediction lends support to the hypothesis that the nonlinearity of the ice sheet physics is responsible for the 100 000 yr periodicity in the geological record of the late pleistocene

modern shallow water graded sand layers from storm surges bering shelf a mimic of bouma sequences and turbidite systems abstract a sequence of graded sand layers interbedded with mud extends offshore over 100 km from the yukon delta shoreline across the flat shallow 100 kin such shallow water graded layers off lobate deltas may be distinguished from similar deep water turbidites by 1 the predominance of trough cross lamination perhaps resulting from wave oscillation effects in the proximal part of the system and 2 gradation to common shallow marine fossils bioturbation and storm lag layers n distal areas

the mode and mechanism of the last deglaciation oceanic evidence changes in ocean temperature carbonate productivity and ice rafted detritus in the north atlantic suggest that half of the northern hemisphere ice volume at the last glacial maximum had disappeared by 13 000 yr b p despite the still extensive limits of the ice sheets this early thinning of the ice sheets occurred during a time when summer insolation values were slowly rising but when pollen evidence south of the ice margins indicates cold dry air masses we infer that this rapid early ice disintegration 16 000 13 000 yr b p was caused by oceanic mechanisms 1 rising sea level causing increased calving along ice margins 2 the chilling of the sea surface by icebergs and meltwater reducing moisture extraction by the atmosphere and transport to the ice sheets and 3 winter freezing of the low salinity meltwater layer suppressing local moisture extraction and the regional influx of moisture bearing storms from lower latitudes in winter and hence starving the ice sheets these oceanic feedback mechanisms were strongest from 16 000 to 13 000 yr b p and weaker but still active from that date until the end of deglaciation at 6000 yr b p

quantitative mineralogy of the yukon river system changes with reach and season and determining sediment provenance the mineralogy of yukon river basin sediment has been studied by quantitative x ray diffraction bed beach bar and suspended sediments were analyzed using the rockjock computer program the bed sediments were collected from the main stem and from selected tributaries during a single trip down river from whitehorse to the yukon river delta during the summer of 2001 beach and bar sediments were collected from the confluence region of the tanana and yukon rivers during the summer of 2003 suspended sediments were collected at three stations on the yukon river and from a single station on the tanana river at various times during the summers of 2001 through 2003 with the most complete set of samples collected during the summer of 2002 changes in mineralogy of yukon river bed sediments are related to sediment dilution or concentration effects from tributary sediment and to chemical weathering during transport carbonate minerals compose about 2 wt of the bed sediments near whitehorse but increase to 14 wt with the entry of the white river tributary above dawson thereafter the proportion of carbonate minerals decreases downstream to values of about 1 to 7 wt near the mouth of the yukon river quartz and feldspar contents of bed sediments vary greatly with the introduction of pelly river and white river sediments but thereafter either increase irregularly quartz from 20 to about 50 wt or remain relatively constant feldspar at about 35 wt with distance downstream clay mineral content increases irregularly downstream from about 15 to about 30 wt the chief clay mineral is chlorite followed by illite smectite there is little to no kaolinite the total organic carbon content of the bed sediments remains relatively constant with distance for the main stem generally 1 to 2 wt with one exception but fluctuates for the tributaries 1 to 6 wt the mineralogies of the suspended sediments and sediment flow data were used to calculate the amount of mineral dissolution during transport between eagle and pilot station a distance of over 2000 km we estimate that approximately 3 wt of the quartz 15 wt of the feldspar 1 wt of the alkali and 25 wt of the plagioclase and 26 wt of the carbonates 31 wt of the calcite and 15 wt of the dolomite carried by the river dissolve in this reach the mineralogies of the suspended sediments change with the season for example during the summer of 2002 the quartz content varied by 20 wt with a minimum in mid summer the calcite content varied by a similar amount and had a maximum corresponding to the quartz minimum these modes are related to the relative amount of sediment flowing from the white river system which is relatively poor in quartz but rich in carbonate minerals suspended total clay minerals varied by as much as 25 wt with maxima in mid july and suspended feldspar varied up to 10 wt suspended sediment data from the summers of 2001 and 2003 support the 2002 trends a calculation technique was developed to determine the proportion of various sediment sources in a mixed sediment by unmixing its quantitative mineralogy results from this method indicate that at least three sediment sources can be identified quantitatively with good accuracy with this technique sediment mineralogies can be used to calculate the relative flux of sediment from different tributaries thereby identifying sediment provenance

geodynamic implications of pleistocene ultrarapid vertical axis rotations in the southern apennines italy paleomagnetic analysis from lower pleistocene sicilian mudstones in the sant arcangelo basin southern apennines italy shows no vertical axis rotations these results define the upper time constraint on the 23 counterclockwise rotations previously measured in the underlying lower pleistocene santernian emilian units of the sant arcangelo basin these rotations occurred before the jaramillo subchron in a time span of 0 5 m y paleomagnetic rotations are coeval with the major phases of thrusting along the outer front of the apennines do not extend to the adriatic foreland and were limited to the hanging wall of the active thrust sheets along the outer front of the apennines rapid counterclockwise rotations coeval with thrusting and offset on left lateral faults represent the surface manifestation of the differential slab retreat and southwest ward rollback of the trench in the calabrian arc region the unusually fast vertical axis rotations indicate that the left lateral component of deformation was particularly intense during the early pleistocene in the sant arcangelo region located along the edge of the retreating subduction zone these data suggest that the lateral breakoff of the african ionian adriatic subducting lithosphere imaged by seismic tomography and deep seismicity occurred in the sant arcangelo region during the early pleistocene

crustal and lithospheric structure of inactive volcanic arc terrains in fiji abstract the crustal and lithospheric velocity structure of the fiji area may place important constraints on the evolution of island arcs in the region here we obtain receiver functions from one permanent seismic station and three temporary networks on the fiji platform the lau ridge and other small islands to develop point estimates of crustal thickness under the networks we obtain preliminary estimates of crustal thickness and poisson s ratio from h stacking of the receiver functions we then perform an inversion for shear wave velocity structure using a joint inversion of receiver functions and rayleigh wave dispersion curves from ambient seismic noise and teleseismic events the joint inversion results show an average crustal thickness of 30 km beneath viti levu the largest fiji island suggesting an extensive period of arc crustal formation and thickening in contrast we find shallower moho depths averaging 22 km beneath the smaller fiji islands and the lau ridge the limited thickness of the high velocity lid obtained by inversion indicates that the lithosphere is thinned beneath the marginal fiji islands which may be due to the early rifting event forming the lau basin together with the crustal thinning of this area as well and to thermal lithospheric erosion processes associated with adjacent back arc spreading a relatively slow velocity middle crustal layer roughly of 11 16 km thickness exists throughout fiji suggesting that remnant arcs contain significant volumes of felsic crustal rocks necessary for continental crust formation

paleosecular variation and the average geomagnetic field at 20 latitude we assembled a new paleomagnetic directional data set from lava flows and thin dikes for four regions centered on 20 latitude hawaii mexico the south pacific and reunion we investigate geomagnetic field behavior over the past 5 myr and address whether geographical differences are recorded by our data set we include inclination data from other globally distributed sites with the 20 data to determine the best fitting time averaged field taf for a two parameter longitudinally symmetric zonal model values for our model parameters the axial quadrupole and octupole terms are 4 and 6 of the axial dipole respectively our estimate of the quadrupole term is compatible with most previous studies of deviations from a geocentric axial dipole gad field our estimated octupole term is larger than that from normal polarity continental and igneous rocks and oceanic sediments but consistent with that from reversed polarity continental and igneous rocks the variance reduction compared with a gad field is 12 and the remaining signal is attributed to paleosecular variation psv we examine psv at 20 using virtual geomagnetic pole vgp dispersion and comparisons of directional distributions with simulations from two statistical models regionally the hawaii and reunion data sets lack transitional magnetic directions and have similar inclination anomalies and vgp dispersion in the pacific hemisphere hawaii has a large inclination anomaly and the south pacific exhibits high psv the deviation of the taf from a gad contradicts earlier ideas of a pacific dipole window and the strong regional psv in the south pacific contrasts with the generally low secular variation found on short timescales the taf and psv at hawaii and reunion are distinct from values for the south pacific and mexico demonstrating the need for time averaged and paleosecular variation models that can describe nonzonal field structures investigations of zonal statistical psv models reveal that recent models are incompatible with the empirical 20 directional distributions and cannot fit the data by simply adjusting relative variance contributions to the psv the 20 latitude data set also suggests less psv and smaller persistent deviations from a geocentric axial dipole field during the brunhes

time dependent subduction dynamics driven by the instability of stagnant slabs in the transition zone abstract seismic tomographic images indicate that subducted slabs are commonly deflected within the transition zone and that accumulated slab material may be subject to localized flux into the lower mantle these observations are consistent with numerical models of mantle flow in which slab flux through phase transition at 660 km depth is impeded by the combination of rheological contrast and thermodynamic effects we use a suite of 2d numerical experiments to explore the impact of slab avalanches on plate boundary dynamics the experiments are initiated at the onset of the avalanche and two distinct classes of model geometry are considered in the first the stagnant deflected slab within the transition zone is connected to subduction at the surface of the model domain in this case we find that the location of the avalanche relative to the trench governs whether the slab dip steepens or shallows in response to the instability at 660 km depth these experiments show a relatively modest impact on plate dynamics at the surface in the second class of models the stagnant slab material is not connected to the shallow subduction and the former is interpreted as a remnant of either a relic subduction event or an accumulation of material from a distant zone of convergence in this case the impact on plate boundary dynamics is more profound for example when the avalanche is positioned under or forward of the island arc it drives a significant retreat of the trench in contrast an avalanche that initiates under the back arc leads to a thrusting of the island arc over the retro plate and a reversal in the polarity of subduction we test the influence of various boundary conditions on the evolution of these models and consider several different rheological parameterizations for the lithosphere and mantle the models are intended as general templates for the impact of slab avalanches however we discuss the potential connection of these results to various zones of convergence along the western pacific margin

southwest pacific absolute plate kinematic reconstruction reveals major cenozoic tonga kermadec slab dragging tectonic plates subducting at trenches having strikes oblique to the absolute subducting plate motion undergo trench parallel slab motion through the mantle recently defined as a form of slab dragging we investigate here long term slab dragging components of the tonga kermadec subduction system driven by absolute pacific plate motion to this end we develop a kinematic restoration of tonga kermadec trench motion placed in a mantle reference frame and compare it to tomographically imaged slabs in the mantle estimating tonga kermadec subduction initiation is challenging because another new caledonia subduction zone existed during the paleogene between the australia and pacific plates we test partitioning of plate convergence across the paleogene new caledonia and tonga kermadec subduction zones against resulting mantle structure and show that most if not all tonga kermadec subduction occurred after ca 30 ma since then tonga kermadec subduction has accommodated 1 700 to 3 500 km of subduction along the southern and northern ends of the trench respectively when placed in a mantle reference frame the predominantly westward directed subduction evolved while the tonga kermadec trench underwent 1 200 km of northward absolute motion we infer that the entire tonga kermadec slab was laterally transported through the mantle over 1 200 km such slab dragging by the pacific plate may explain observed deep slab deformation and may also have significant effects on surface tectonics both resulting from the resistance to slab dragging by the viscous mantle

isotopic ages magnetostratigraphy and biostratigraphy from the early pliocene suva marl fiji the suva marl fiji is early pliocene in age from biostratigraphical data within the 180 m sequence occur a number of tuffaceous beds from some of which biotite has been separated and used for k ar dating the measured isotopic ages when plotted against stratigraphic position within the suva marl yield a good straight line relation this implies that deposition occurred at the rate of 83 2 5 1 m m y and that the formation was deposited 5 2 to 3 2 ma calcareous nannoplankton allow identification of several zonal boundaries with the following estimated ages nn 12 nn 13 cn10b cn10c 4 82 ma or slightly younger nn 14 nn 15 cn10c cn11 3 86 ma and nn 15 nn 16 cn11 cn12 3 56 ma planktonic foraminiferal datum planes have proved difficult to locate closely apart from the first coiling direction change of pulleniatina which took place at 3 93 ma the base of zone n 21 the age of which we estimate to be 3 2 ma or slightly younger and the sphaeroidinellopsis extinction datum which is only marginally older

characteristics of currents over the continental slope of the eastern bering sea between september 1989 and 1990 twenty six current records were collected by instruments on eight moorings located in pribilof and zhemchug canyons and at a site midway between these features these records provide the first long term eulerian measurements from the slope and mid slope of the eastern bering sea results from the current records together with water property observations permit a characterization of the bering slope current moderate flow 2 to 18 cm s 1 followed the bathymetry toward the northwest and existed primarily in the upper 300 m wind and current energy increased in winter but vector mean current did not increase at all sites wind forcing accounted for only a small fraction of the current fluctuations at one mid slope location in pribilof canyon bathymetry resulted in rectification of the strong daily tidal current estimates of heat and salt fluxes indicate some significant shoreward transport however this flux did not occur preferentially in the canyons

orbital and millennial scale variations in asian dust transport path to the japan sea abstract orbital and millennial scale variations in the asian monsoon and its probable association with the dansgaard oeschger d o cycles have been demonstrated by previous studies however the origin and nature of such variations are poorly understood the japan sea is located down wind of the arid areas of the eurasian continent and thus receives significant amounts of aeolian dust consequently the hemipelagic sediments of the japan sea are expected to record a continuous aeolian dust accumulation which may provide information about the past variations in asian monsoon grain size flux and provenance of aeolian dust in the sediments were examined using two sediment cores obtained from the northern and southern parts of the japan sea in order to understand the spatial variability of asian monsoon since the silt fraction of the detrital materials in hemipelagic sediments of the japan sea is composed predominantly of aeolian dust the grain size flux and provenance of the silt fraction were examined the taklimakan desert loess plateau and siberia northeast china areas were identified as possible source areas of aeolian dust in the sediments of the japan sea based on the electron spin resonance esr signal intensity and crystallinity of quartz moreover the relative contribution of aeolian dust from each source area was estimated based on the esr signal intensity and crystallinity of quartz aeolian dust fluxes from the two source areas at the southern site were then estimated grain size and flux of eolian dust at the northern site and fluxes of aeolian dust from two source areas at the southern site show orbital scale variations in harmony with the insolation change at 30 n in june these results indicate orbital scale changes in the extent of the arid area and or frequency of dust storms in the taklimakan desert loess plateau and siberia northeast china areas and suggest that southward northward shifts of the westerly jet axis occurred together with either intensified weakened winter monsoon decreased increased winter precipitation in the siberia northeast china area or southeastward northwestward shifts of summer monsoon limit during periods of smaller larger insolation at 30 n in june in addition to these orbital scale variations millennial scale variations possibly associated with the d o cycles are observed in the grain size record of aeolian dust at the two sites based on analogy with orbital scale variations these millennial scale variations are considered to reflect millennial scale changes in the position of the westerly jet axis together with the changes either in winter monsoon intensity winter precipitation or position of summer monsoon limit our results suggest n s oscillations in the westerly jet and summer monsoon limit and changes in the intensity of the winter monsoon on orbital and millennial timescales

deep circulation of the north atlantic over the last 200 000 years geochemical evidence variations in the cadmium calcium ratio of north atlantic deep water are recorded in the fossil shells of benthic foraminifera the oceanic distribution of cadmium is similar to that of the nutrients hence the cadmium calcium ratio in shells records temporal variations in nutrient distributions data from a north atlantic sediment core show that over the past 200 000 years there has been a continuous supply of nutrient depleted waters into the deep north atlantic the intensity of this source relative to nutrient enriched southern waters diminished by about a factor of 2 during severe glaciations this evidence combined with carbon isotope data indicates that the continental carbon inventory may have been less variable than previously suggested

basal water film basal water pressure and velocity of traveling waves on glaciers la theorie de nye et weertman des ondes cinematiques est etendue pour couvrir le cas ou la presence d une abondante quantite d eau au fond ou une pression croissante d eau accroit la vitesse de glissement d un glacier sur son lit on trouve que le rapport entre la vitesse de l onde et la vitesse superficielle du glacier est independante de la quantite d eau ou de sa pression au fond

early geochemical evolution of an oceanic island arc and backarc fiji and the south fiji basin the initial 25 to 35 ma of oceanic arc volcanism in fiji the early arc stage was dominated by eruption of lree depleted low k tholeiitic basalts and andesites the oldest volcanics include the highest hfse group morb like in composition plus high mg basalts and basic andesites with flat ree patterns as low as 4x chondritic concentrations these erupted either within the volcanic arc during its first 5 ma or in an adjacent backarc during its first ma of formation subsequent volcanics of the early arc stage apparently erupted onto early backarc basin crust most are intermediate between earlier extremes in hfse contents but at least one volcano produced strongly lree depleted island arc tholeiites in contrast medium k tholeiitic and calc alkaline andesites erupted during the mature arc stage parental basalts were enriched in lree but depleted in hree relative to earlier basalts due to a change in source composition counterflowing morb source asthenosphere within the mantle wedge dominated vol

clay mineral dispersal patterns in the north bering and chukchi seas abstract characterization of the clay mineralogic constituents of the m e s d equivalent spherical diameter size fraction of contemporary marine and fluvial sediments from the north bering sea chukchi sea region and adjacent land areas has resulted in delineation of suites dominated by broadly defined illitic and expandable group components with lesser amounts of chlorite and kaolinite distribution patterns elucidated indicate net northward transport of sediments relatively enriched in the expandable group with predominant terrigen ous source from the yukon river system this material is distributed across the study area in a medial zone along a general north south trend with peripherally decreasing relative amounts of this expandable group component and concomitant relative increases in the illitic components these patterns are consistent with known physical oceanographic and regional geologic relationships in terms of sediment sources transport processes and depositional mechanisms the foregoing suggest further that consideration of the stratigraphic relationships of these clay mineral component types as a function of time in relevant marine sediments north and south of the present bering strait should be informative with respect to regional quaternary paleogeography as related to global sea level fluctuations during glacial and interglacial episodes

surface nitrate utilization in the bering sea since 180 ka bp insight from sedimentary nitrogen isotopes we present high resolution records of sedimentary nitrogen 15nbulk and carbon isotope ratios 13cbulk from piston core so201 2 85kl located in the western bering sea the records reflect changes in surface nitrate utilization and terrestrial organic matter contribution in submillennial resolution that span the last 180 kyr the 15nbulk record is characterized by a minimum during the penultimate interglacial indicating low nitrate utilization 62 80 despite the relatively high export production inferred from opal concentrations along with a significant reduction in the terrestrial organic matter fraction mterr this suggests that the consumption of the nitrate pool at our site was incomplete and even more reduced than today 84 15nbulk increases from marine isotope stage mis 5 4 and culminates during the last glacial maximum which indicates that nitrate utilization in the bering sea was raised during cold intervals mis 5 4 5 2 4 and almost complete during mis 3 and 2 93 100 this is in agreement with previous hypotheses suggesting that stronger glacial stratification reduced the nutrient supply from the subeuphotic zone thereby increasing the iron to nutrient ratio and therefore the nitrate utilization in the mixed surface layer large variations in 15nbulk were also recorded from 180 to 130 ka bp mis 6 indicating a potential link to insolation and sea level forcing and its related feedbacks millennial scale oscillations were observed in 15nbulk and 13cbulk that might be related to greenland interstadials

role of mantle flow at the north fiji basin insights from anomalous topography the north fiji basin is a complex back arc system situated between the opposite verging tonga and new hebrides subduction zones on the basis of two independent calculations we find that the back arc region is anomalously high with topography residuals reaching up to 2000 m above what should be isostatically supported our study aims to test whether a hot buoyant upwelling is required beneath the north fiji basin to support the observed high elevation the mantle thermal structure beneath the north fiji basin is inferred from the global tomographic model s20rts where seismic velocity anomalies are converted to density temperature anomalies this thermal structure is input into a three dimensional mantle convection code citcoms and the flow induced surface topography is calculated and compared with the observed anomalous topography in a series of experiments we eliminate various portions of the mantle structure to consider what type of mantle flow can support the anomalous topography the experiments suggest that the observed topography in the north fiji basin is consistent with the support of hot buoyant upper mantle upwelling and specifically rising material in the depth range from 70 to 120 km is the most critical component a relatively thin weak lithosphere is required to produce the high amplitude and short wavelength topography residuals and the presence of discrete breaks in the plate produces further localization of flow induced topography

seismotectonics and present day relative plate motions in the new hebrides north fiji basin region abstract relative motions between the pacific plate p indo australian plate ia new hebrides nh arc microplate and the north fiji basin nfb microplate are estimated using shallow seismicity 276 focal mechanism solutions bathymetry magnetism and the rm 2 plate model of minster and jordan 1978 within the nfb we define the western nfb wnfb eastern nfb enfb and southern nfb snfb microplates a 8 cm y spreading in a n72 e direction occurs along the n s trending spreading ridge of nfb wnfb enfb boundary along the enfb ia boundary located at 176 e a 3 cm y extension in a n108 e direction is proposed the wnfb p boundary which includes the hazel holme extensional zone hhez is complex and a general n25 e extension 2 cm y is inferred the fiji fracture zone ffz is composed of two segments along the eastern zone p ia boundary trending n80 e a left lateral strike slip motion occurs 9 6 cm y accommodated by minor n109 e extension pull apart while along the western zone enfb p boundary a n84 e trending left lateral strike slip motion 7 cm y is inferred the wnfb snfb boundary corresponds to a n70 e broad fracture zone along which an e w left lateral strike slip motion is proposed the snfb microplate moves rapidly eastwards 10 5 cm y at 172 e and is almost attached to the ia plate within the nfb an improved model is discussed which includes deformations within the ia and p plates and rotations with euler poles close to the nfb along the new hebrides trench the consumption rates are respectively 16 15 9 and 12 cm y at 11 12 5 15 5 and 20 s the minimum rate occurs where the d entrecasteaux ridge collides with the new hebrides arc rates of extension at the rear of the nh arc are 7 5 5 and 2 cm y at 11 12 5 and 20 s respectively back arc compression occurs between 13 30 s and 17 s the western end of the hazel holme extension zone coincides with the 13 30 s boundary separating the nh back arc compressive belt and the northern nh back arc troughs south of 21 22 s the trend of the slip vector changes along the nh trench from n70 e to n20 w this implies a n s motion of convergence along the e w trending southern part of the nh trench motion snfb ia of 1 5 cm y at 172 e and the existence of a n70 e trending left lateral fracture zone crossing the southern part of the nfb this fracture should be considered as a major plate boundary

shoshonites in southern tibet record late jurassic rifting of a tethyan intraoceanic island arc abstract detailed field mapping combined with a petrologic and geochemical investigation of the zedong terrane within the yarlung tsangpo suture zone provides insights to the evolution of now mostly subducted portions of tethys during the late jurassic the terrane is dominated by volcanic rocks of shoshonitic affinity which were erupted in a submarine oceanic island arc setting the volcanic island arc was built on a basement of oceanic crust and the shoshonites locally overlie a thin section of pillowed island arc tholeiites and red ribbon bedded radiolarian cherts geochemistry of the shoshonites suggests that their development occurred in a setting analogous to that of late miocene to early pliocene fiji and was associated with an arc rifting we speculate that this event may have been a far field response to developments associated with gondwana breakup

provenance of terrigenous detritus of the surface sediments in the bering and chukchi seas as derived from sr and nd isotopes implications for recent climate change in the arctic regions abstract strontium and nd isotope analysis of the detrital fractions extracted from the marginal sea sediments in the arctic was applied to examine regional and temporal changes in the inflow and transportation of terrigenous material in the chukchi and bering seas over the past 100 years regional distributions of the sr and nd isotopes of the surface sediments show higher sr isotope ratios and lower nd isotope ratios in the chukchi sea 87 sr 86 sr 0 7106 0 7150 e nd 10 1 to 8 3 and lower sr isotope ratios and higher nd isotope ratios in the eastern bering sea 0 7045 0 7109 8 6 to 3 0 in addition the rb and sr contents and ree patterns e g eu eu changed noticeably across the bering strait these variations mean that the sediments in the bering and chukchi seas have clearly distinct sources the terrigenous detritus in the chukchi sea is mainly derived from northeastern siberia 87 sr 86 sr 0 711 e nd 9 and bering strait inflow essentially from the yukon river and additionally from the mackenzie river basin including the canadian shield 0 732 0 734 14 the detritus in the eastern bering sea mainly consists of two components the continental material from the yukon river basin mainly underlain by mesozoic and paleozoic rocks in the alaskan mainland 0 708 0 709 9 to 8 and the aleutian arc volcanics 0 703 6 to 10 transported northward by the alaska coastal and bering shelf waters the eastern bering sediments over the past 100 years show slight variations of the sr isotope ratio and relatively wide variations of the nd isotope ratio and the variations and trends of the time series of the northeastern bering sea in the sr nd isotope diagram are probably controlled by changes in the grain size of the detritus from the yukon river basin some of the e nd time series show periodic fluctuations correlating with the annual mean surface air temperature sat for the arctic the e nd value is low in the high sat period and high in the low sat period in the warm period the yukon river was likely to supply a higher amount of the fine grain material with a relatively low e nd value such as surficial overburden probably because partial melting of continental glaciers and permafrosts in the alaskan mainland increased more detailed analysis with high time resolution is needed to clearly understand the effects the sat have on the arctic continental and marine environments

concave slab out board of the tonga subduction zone caused by opposite toroidal flows under the north fiji basin abstract an alternative scenario is proposed for the origin of a concave ne facing slab under the north fiji basin between the tonga subduction slab and vanuatu arc during rollback of the australian plate vanuatu arc rotated clockwise whereas fiji platform rotated counterclockwise from 12 10 ma until 1 5 ma ago thereafter only vanuatu arc rotated until the present during the period of opposite rotations toroidal flows entered the mantle around the northwest slab edge of vanuatu arc and from the northeast slab edge of fiji platform the latter lies close to the northern end of the tonga slab where arc parallel flows are shown by volcanic geochemistry and mantle anisotropy opposite toroidal flows with upwelling and downwelling components generate the concave form of the combined vanuatu fiji platform slab match its extent to mapped deep seismicity explain its position overlying the tonga slab provide a mechanism for high heat flow in the north fiji basin as well as enriched morb and oib basalts in the northern nfb and obviate slab collisions invoked to produce slab curvature