the lost inca plateau cause of flat subduction beneath peru abstract since flat subduction of the nazca plate beneath peru was first recognized in the 1970s and 1980s a satisfactory explanation has eluded researchers we present evidence that a lost oceanic plateau inca plateau has subducted beneath northern peru and propose that the combined buoyancy of inca plateau and nazca ridge in southern peru supports a 1500 km long segment of the downgoing slab and shuts off arc volcanism this conclusion is based on an analysis of the seismicity of the subducting nazca plate the structure and geochemistry of the marquesas plateau as well as tectonic reconstructions of the pacific farallon spreading center 34 to 43 ma these restore three sub parallel pacific oceanic plateaus the austral tuamotu and marquesas to two farallon plate counterparts the iquique and nazca ridges inca plateau is apparently the sixth and missing piece in an ensemble of v shaped hotspot tracks formed at on axis positions we argue the mirror image of the inca plateau the marquesas plateau is an ancient edifice overprinted by recent volcanism in disagreement with the widely accepted young

the late cretaceous to recent tectonic history of the pacific ocean basin abstract a vast ocean basin has spanned the region between the americas asia and australasia for well over 100 myr represented today by the pacific ocean its evolution includes a number of plate fragmentation and plate capture events such as the formation of the vancouver nazca and cocos plates from the break up of the farallon plate and the incorporation of the bellingshausen kula and aluk phoenix plates which have been studied individually but never been synthesised into one coherent model of ocean basin evolution previous regional tectonic models of the pacific typically restrict their scope to either the north or south pacific and global kinematic models fail to incorporate some of the complexities in the pacific plate evolution e g the independent motion of the bellingshausen and aluk plates thereby limiting their usefulness for understanding tectonic events and processes occurring in the pacific ocean perimeter we derive relative plate motions with 95 uncertainties for the pacific farallon vancouver kula pacific bellingshausen pacific and early pacific west antarctic spreading systems based on recent data including marine gravity anomalies well constrained fracture zone traces and a large compilation of magnetic anomaly identifications we find our well constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the north and south pacific in conjunction with recently published and well constrained relative plate motions for other pacific spreading systems e g aluk west antarctic pacific cocos recent pacific west antarctic spreading we explore variations in the age of the oceanic crust seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern pacific asymmetries in crustal accretion within the overall pacific basin where both flanks of the spreading system are preserved have typically deviated less than 5 from symmetry and large variations in crustal accretion along the southern east pacific rise i e pacific nazca farallon spreading appear to be unique to this spreading corridor through a relative plate motion circuit we explore the implied convergence history along the north and south america s where we find that the inclusion of small tectonic plate fragments such as the aluk plate are critical for reconciling the history of convergence with onshore geological evidence

neotectonic deformation of the central chile margin deepwater forearc basin formation in response to hot spot ridge and seamount subduction 1 underlying the midslope valparaiso terrace the valparaiso basin constitutes a unique and prominent deepwater forearc basin on the central chile margin the basin contains a 3 3 5 km thick sediment fill of late cenozoic age the seismic stratigraphic architecture of the valparaiso basin fill shows that margin wide subsidence above the underthrusting juan fernandez ridge an aseismic volcanic swell crested with seamounts on the nazca plate characterized the basin evolution contemporaneously with margin subsidence and seaward rotation of the upper slope and shelf the outer part of the forearc was deformed by thrust faulting above the leading flank of underthrusting seamounts that rotated the middle slope to a more horizontal attitude and formed seaward vergent outer forearc highs subsidence of the margin above the underthrusting juan fernandez ridge is ascribed to processes of subduction erosion enhanced by factors related to subduction of the ridge that caused dismemberment of upper plate material along the shallow aseismic part of the plate interface and transfer of material to the downgoing nazca plate accelerated midslope subsidence 55 75 km landward of the trench coincides with the seaward limit of interplate seismicity suggesting that enhanced basal erosion is related to the transition from aseismic to stick slip subduction behavior since subduction of the juan fernandez ridge began near 33 s 10 ma ago the chile trench north of 33 s is inferred to have retreated 3 km myr and the midslope valparaiso basin subsided 0 3 0 5 km myr approximating analogous neogene rates for the peruvian margin

quantitative evaluation and prediction of water inrush vulnerability from aquifers overlying coal seams in donghuantuo coal mine china the water inrush vulnerability from overlying aquifers is evaluated for a coal seam in the donghuantuo coal mine china the water inrush vulnerability is based on superposition of two zoning maps water abundance map and connectivity map the water abundance zoning map was derived from a comprehensive analysis of six types of geoscience data including 1 lithological changes of the overlying aquifer 2 geological structure 3 pumping test results 4 seepage field of water inrush event 5 geochemistry and 6 loss of drilling mud the connectivity map was constructed based on comparing the numerically calculated height of the induced fracture zone above mining areas with the thickness of formation between the coal seam and the overlying aquifer visual modflow was used to predict the mine discharges at working face 2 284 under both natural and mining conditions based on the water inrush vulnerability map and the modeling results advanced dewatering from the overlying aquifer is proposed as the main prevention and control measure against the overlying aquifer water inrush

a model for the plate tectonic evolution of the east central pacific based on seasat investigations abstract a complete seasat dataset is used to investigate the small scale bathymetry of a 15 by 15 area of the east central pacific northeast of pitcairn island which was involved in the miocene ridge jump and reorientation two fracture zones fz1 and fz2 are mapped along 450 km at an azimuth of n95 e while the signature of the austral fracture zone disappears east of 135 w several seamounts are discovered or confirmed along the oeno henderson ducie lineament and the degree of isostatic compensation of a number of islands at the eastern end of the tuamotu group is studied islands of the northern branch have practically no signatures in the geoid while the southern ones must have formed on lithosphere at least 10 ma old on the basis of these constraints we present a model of the evolution of spreading since 40 ma b p in which the deactivation of the austral fracture zone and the transfer of its offset to fz1 in the late oligocene is interpreted as due to rift propagation triggered by a northern tuamotu hotspot we propose that the oeno ducie chain is the surface expression of a southern tuamotu hotspot deviated 15 in azimuth by the presence of a young and fresh fracture zone thus our model predicts that hotspots and ridge systems can have a moderate influence on each other s kinematics and surface expressions which does not however extend to permanent trapping

the early cenozoic tectonic history of the southeast pacific abstract new marine geophysical data collected in the southeast pacific enable us to define better the early cenozoic tectonic history of the area and to interpret previously unexplained bathymetric features in terms of plate tectonic processes we have identified a major re organization in plate boundaries at anomaly 21 time when a large fragment was broken off the pacific plate and joined to the antarctic plate this interpretation is based on three pieces of evidence 1 a major ese trending feature the humboldt fracture zone marks the southern boundary of the farallon antarctic anomalies identified by weissel et al 1 2 a north south striking trough the hudson trough which represents the scar left by the pacific antarctic ridge when it rifted across old pacific crust 3 a sequence of north south striking magnetic lineations south of the humboldt fracture zone and east of the hudson trough is correlated to anomalies 27 to 22 and identified with pacific aluk spreading

stable isotope 2h 18o and 87sr 86sr and hydrochemistry monitoring for groundwater hydrodynamics analysis in a karst aquifer gran sasso central italy this paper deals with chemical and isotope analyses of 21 springs which were monitored 3 times in the course of 2001 the monitoring program was focused on the groundwater of the gran sasso carbonate karst aquifer central italy typical of the mountainous mediterranean area based on the hydrogeological setting of the study area 6 groups of springs with different groundwater circulation patterns were distinguished the hydrogeochemistry of their main components provided additional information about groundwater flowpaths confirming the proposed classification the spatial distribution of their ion concentrations validated the assumptions underlying the hydrogeological conceptual model showing diverging groundwater flowpaths from the core to the boundaries of the aquifer geochemical modelling and saturation index computation elucidated water carbonate rock interaction contribution by alluvial aquifers at the karst aquifer boundaries as well as impacts of human activities the analysis of 18o 16o and 2h h values and their spatial distribution in the aquifer substantiated the hydrogeology based classification of 6 groups of springs making it possible to trace back groundwater recharge areas based on mean isotope elevations the latter were calculated by using two rain monitoring stations 87sr 86sr analyses showed seasonal changes in many springs in winter spring the changes are due to inflow of new recharge water infiltrating into younger rocks and thus increasing 87sr 86sr values in summer autumn when there is no recharge and spring discharge declines changes are due to base flow groundwater circulating in more ancient rocks with a subsequent drop in 87sr 86sr values the results of this study stress the contribution that spatio temporal isotope monitoring can give to the definition of groundwater flowpaths and hydrodynamics in fissured and karst aquifers taking into account their hydrogeological and hydrogeochemical setting

repeat ridge jumps and microcontinent separation insights from ne arabian sea abstract microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume ridge interactions india separated from seychelles at 64 ma by asymmetric sea floor spreading initially when the spreading centre in the mascarene basin jumped towards the indian sub continent between magnetic chrons c29 and c28 the subsequent tectonics is difficult to comprehend since laxmi ridge another microcontinent formed during the later phase most of the studies considered the laxmi ridge as a sliver others considered it to be oceanic crust high resolution deep 25 km seismic data reveals that i the ridge possesses 15 km deep sea ward dipping reflector sdr packages ii normal faulted rift valleys devoid of syn rift sedimentary packages and iii axial magma chambers 5 7 km beneath the ridge top additionally from 2d forward gravity models we deduce that the ridge most possibly comprises of high density oceanic crust we conclude the laxmi ridge to be indeed composed of oceanic crust and a fossil spreading centre we thus identified the ridge jumps and their relation to the seychelles microcontinent separation previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating spreading rate at the ridge and plate ages for repeated ridge jumps the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating we find that the medium spreading rates and high magmatic heating due to the reunion plume and young plates favoured rapid and repeated ridge jumps towards the plume

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

the origin of swash cusps on beaches abstract genetically there are two types of beach cusps those formed in the surfzone by the nearshore circulation system and those formed on the beachface by the swash and backwash the latter are here called swash cusps and a simple model relating the physical dimensions of swash cusps to the properties of the incident wave field and the mean beach slope is developed as in some previous models the cusp wavelength is controlled by the longshore wavelength of edge waves but the edge waves are now required only to provide small periodic perturbations on an originally uniform beach further cusp growth occurs because of the interactions between the incident waves and the perturbed topography the maximum cusp height apex to valley is hypothesized to be proportional to the significant vertical excursion of swash motions it follows from the above assumptions that the maximum possible cusp steepness cusp height cusp longshore wavelength is proportional to the mean on offshore beach slope this result agrees with field data and the value of the ratio of maximum cusp height to beach slope differs from that obtained theoretically by guza and bowen 1981 using a complicated non linear model with rather different physics the present model implies that edge waves although necessary for initiating the initial bedform perturbation need not persist for the development of mature cusp topography

field observations on the morphodynamic evolution of a low energy rip current system abstract a field experiment was conducted at palm beach nsw australia to investigate morphodynamic relationships operating within a rip current system during a near complete sequence of beach state evolution under decreasing energy conditions observations and measurements were obtained from nearshore surveys multiple dye releases oblique photographs and the deployment of various combinations of nine ducted impellor flow meters and five pressure sensors over a seven day period in rip and feeder channels the dataset provides a unique opportunity to assess and quantify aspects of the accretionary model of wright and short mar geol 56 1984 93 118 for intermediate beaches the morphology of the main rip channel exhibited distinct sequential adjustments with a narrowing of channel width an increase in channel depth a reduction in the cross sectional area available for rip flow and an increase in morphological relief rip flow velocity increased during the observed evolution and was also modulated by the tide experiencing maximums at low tide and minimums at high tide morphologic and kinematic adjustments of the rip system were most pronounced during the transition to a transverse bar and rip state the existence of a dominant feeder in a twin feeder and rip system contributed to the migration and establishment of a rip head over the seaward slope of the opposite longshore bar net erosion in the rip channel was balanced by net deposition in the feeder channels and bar crests and qualitative observations suggest that this rip head bar both receives transported sediments from the rip channel and contributes sediments to the ongoing accretion of the beach system a direct relationship was found to exist between rip morphology and flow with rip velocity increasing as cross sectional rip channel area decreases and these results are incorporated in a conceptual modification of the model of wright and short the results of this study are restricted to a subset of commonly occurring beaches but show that accretionary beach state evolution on intermediate beaches is indeed characterised by an increase in rip current velocity the nature of which can be linked to morphologic control in addition the rip head bar is identified as a potentially key component of low energy rip system evolution

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

late cretaceous to recent plate motions in western south america revisited abstract the andean cordillera has evolved since the late cretaceous in the context of subduction of oceanic lithosphere beneath continental lithosphere making the kinematics between south america and its adjacent oceanic plates in the pacific basin valuable to analyze the development of the andean orogen the latest cretaceous cenozoic convergence history in western south america may be divided into three stages the youngest stage 1 25 0 ma is characterized by ene directed convergence of the nazca plate toward most of south america and by e w subduction of the antarctic plate beneath southern patagonia the nazca south america convergence rate in stage 1 shows a continuous decrease from the highest values in the cenozoic 15 cm yr to the present day values from gps measurements 7 cm yr stage 2 47 28 ma is characterized by ne directed subduction of farallon with the convergence rate remaining almost constant during the entire interval in those times obliquity was dextral in chile sinistral in southern peru while almost head on convergence occurred in central and northern peru during latest cretaceous to early eocene times stage 3 the farallon plate was subducted beneath peru and the phoenix plate was subducted farther south where a triple junction migrated southward along the chilean margin the subduction of the farallon plate was rather slow with variable direction imposed by the position of the triple junction whereas subduction of the phoenix plate was rapid 10 cm yr and ese directed we present a working hypothesis suggesting no major changes in the age of subducted lithosphere in the chile trench from middle eocene to late oligocene followed by subduction of progressively older oceanic lithosphere in the early neogene and progressively younger lithosphere during the late neogene and the quaternary in addition it is shown that south american motion as predicted by available hotspot models has insufficient resolution to be applied to the analysis of cenozoic andean deformation

southeast pacific tectonic evolution from early oligocene to present plate tectonic reconstructions of the nazca antarctic and pacific plates are presented from late oligocene to present these reconstructions document major plate boundary reorganizations in the southeast pacific at dirons 6c 24 ma 6 o 20 ma and 5a 12 ma and a smaller reorganization at chron 3 o 5 ma during the chron 6 o reorganization it appears that a ridge propagated into crust north of the northernmost pacific antarctic ridge between the chiloe fracture zone fz of the chile ridge and agassiz fz of the pacific nazca ridge which resulted in a northward jump of the pacific antarctic nazca pac ant naz mid ocean triple junction during the chron 5a reorganization the chile ridge propagated northward from the valdivia fz system to the challenger fz through lithosphere formed roughly 5 myr earlier at the pacific nazca ridge during this reorganization a short lived microplate the friday microplate existed at the pac ant naz triple junction the pac ant naz triple junction jumped northward 500 km as a result of this reorganization from a location along the valdivia fz to a location along the challenger fz the chron 5a reorganization also included a change in spreading direction of the chile and pacific antarctic ridges the reorganization at chron 3 o initiated the formation of the juan fernandez and easter microplates along the east pacific rise the manner of plate boundary reorganization at chron 6 o and chron 5a and possibly today at the juan fernandez microplate included a sequence of rift propagation transfer of lithosphere from one plate to another microplate formation and microplate abandonment and resulted in northward migration of the pac ant naz triple junction the associated microplate differs from previously studied microplates in that there is no failed ridge

eocene propagating rifts in the southwest pacific and their conjugate features on the nazca plate we have mapped a 1200 km long nw se trending lineament in seasat and geosat radar altimeter data crossing a remote portion of the southwest pacific basin this lineament runs obliquely both to the fracture zones fzs and magnetic lineations between the austral and agassiz fzs by examining shipboard magnetics gravity and bathymetric profiles in this region and in the conjugate regions of the nazca plate we identify this feature as the site of a series of small ridge jumps highly asymmetrical spreading and small propagating rifts during the mid to late eocene chron c21 to chron c13 between the austral and agassiz fzs there is roughly 250 to 350 km of missing pacific plate crust which must have been transferred to the nazca plate by the propagating rifts and small ridge jumps on the nazca plate the conjugate area to the gravity lineament is now being subducted beneath the chile trench north of the challenger fz this area corresponds to a region of undecipherable magnetic anomalies south of the challenger fz there is evidence for a small 50 km ridge jump offshore of the chile trench this small ridge jump by itself is not sufficient to account for the missing crust on the pacific plate and we conclude that there must have been repeated small ridge jumps at the same site one consequence of these ridge jumps is that crust on the nazca plate entering the trench is not as old as would be predicted by simply mirror imaging the anomaly pattern on the pacific plate

discrimination of water inrush source and evolution analysis of hydrochemical environment under mining in renlou coal mine anhui province china with the development of mining the hydrochemical environment has changed in the renlou coal mine anhui province china it will be of great significance to coal mine safety production by exploring the temporal and spatial evolution law of hydrochemistry of main water inrush aquifers in renlou coal mine under the effect of mining groundwater samples from three main water inrush aquifers in different periods were collected to analyze their principal components and establish the discrimination model of water inrush source the first and second principal components with eigenvalues greater than 1 were selected in which the first principal component represents sulfate dissolution and pyrite oxidation and the second principal component represents desulfurization and cation alternating adsorption the evolution law of hydrochemical environment was analyzed by combining the distribution characteristics of principal component scores with the spatial location of misjudged groundwater samples the results show that 1 quaternary aquifer is greatly affected by mining and the hydraulic connections with carboniferous limestone aquifer in the central region leads to two misjudged groundwater samples in the discriminant model 2 permian sandstone aquifer is to a certain extent affected by mining which shows a relatively closed hydrogeological environment in most areas of coal mine and an opened environment in some areas with time there are five misjudged samples in the discriminant model due to hydraulic connections with the quaternary aquifer and carboniferous aquifer in some areas 3 carboniferous limestone aquifer is controlled by typical faults and collapse columns and mining has a slight influence on it in the central area there are hydraulic connections with the quaternary aquifer and the permian sandstone aquifer due to the collapse columns which lead to four misjudged groundwater samples in the discriminant model

a trend surface technique for discrimination of surf zone morphology rip current channels a major problem facing coastal nearshore studies is the conversion of survey data to precise two and three dimensional representations of complex surf zone topography in addition to inherent problems in providing accurate survey data most signi cant topographical boundaries are often delineated in a subjective manner a trend surface technique is hereby presented which provides an objective rigorous and replicable method of identifying morphological rip system channel boundaries in the surf zone using an extensive eld dataset obtained over two eld experiments including an almost complete cycle of low energy intermediate beach state evolution this technique is shown to produce accurate planimetric dimensions for a range of topographic beach state morphologies previously absent or highly descriptive in the literature formal morphometric de nitions of geometric dimensions for rip morphologies including rip channel width relief depth and cross sectional areas are provided several new aspects of rip current morphology are discussed from results of the approach the approach described here is also intended to illustrate the potential for future trend surface applications to surf zone and other geomorphological studies copyright 2003 john wiley sons ltd

a detailed view of the south pacific geoid from satellite altimetry images of sea surface undulations in the south pacific have been constructed from geos 3 and seasat altimeter data height discrepancies at crossover points associated with long wavelength radial orbit error were suppressed by taking along track derivatives of the ascending and descending profiles these geoid slopes were then rotated and scaled to produce the north and east components of the deflection of the vertical finally the results are displayed by using the hill shading technique where gray tone images represent the innner product of the deflection vector with an assigned sun vector less apparent sea surface undulations can be enhanced by varying the sun s zenith and azimuth shorterwavelength sea surface undulations reflect seafloor topography for instance fracture zones fz s appear as elongated sharp steps in the sea surface while seamounts produce circular bumps since large areas of the south pacific are unsurveyed many previously undetected features appear on the images comparisons with bathymetric charts reveal 72 uncharted seamounts having geoid expressions greater than or equal to easter island s expression the dominant features in the images however are the large age offset fz s such as the eltanin and udintsev fz s the images reveal that the eltanin fz is connected to the louisville ridge combined they produce a continuous geoid signature across most of the south pacific this supports the hypothesis of hayes and ewing 1968 that the louisville ridge is the northwest extension of the eltanin fz

rip current type spacing and persistence narrabeen beach australia abstract long term daily rip observations on narrabeen beach australia were made together with daily measurements of wave height period direction surf zone width and beach state a total of 3513 rips were observed on 270 days over a 19 month period results show that rips accompany moderate to high waves and are associated with intermediate beach types rip spacing follows wave conditions increasing in spacing size and intensity as waves rise and conversely as they fall rips are a function of both the prevailing and antecedent wave conditions and the rate and direction of change in wave conditions rips are classified into three types erosion mega and accretion erosion rips are initiated by rising seas in the transverse bar and rip and higher beach states they accompany general beach erosion they are widely spaced narrabeen ys 300 500 m 100 200 m increasing in size and intensity with the waves until the fully dissipative ripless state is reached they are both temporally and spatially highly variable persisting in one location for only hours to a day or so except where topographically controlled they are a major mechanism for the seaward transport of sediment and under extreme conditions may extend over a kilometer seaward of the breaker zone mega rips are large scale 1 km topographically controlled erosion rips that persist when nearshore and or embayment topography prevents the development of the fully dissipative state by inducing wave refraction and persist longshore gradients in surf zone dynamics that in turn drive the rip circulation accretion rips prevail during stable or falling wave conditions usually following erosion rip formation they are associated with general beach accretion are more closely spaced narrabeen ys 170 250 m 100 m less intense and become increasingly topographically arrested by crescentic bars and rhythmic bar beach morphology of the longshore bar trough rhythmic bar and beach and transverse bar and rip beach states they are relatively stable both spatially and temporally and may persist in one location for days to weeks if wave conditions remain favorable they are fair weather rips which may disappear by infilling in the low tide terrace beach state or be destroyed by rising seas rips generally are absent from the reflective low tide terrace and fully dissipative beach states

morphodynamics of a large scale rip current system at muriwai beach new zealand abstract field measurements of rip spacing nearshore morphology water surface elevation and eulerian and lagrangian flows were made in order to investigate the morphodynamics of a large scale rip current system at muriwai beach new zealand muriwai is a high energy meso tidal beach characterised by modal breaker wave heights of 2 5 m and incident wave periods of 10 15 s the monitored rip system was characterised by a 400 m long 75 m wide longshore feeder channel and a 150 m wide rip neck channel oriented obliquely to the shore and extending over a distance of almost 400 m during the experiment the beach evolved from a longshore bar trough and rip state to a transverse bar and rip configuration mean eulerian flow velocities obtained from ducted flowmeters deployed on the margin of the feeder channel and rip neck were on the order of 1 m s 1 and instantaneous flows were commonly in excess of 2 m s 1 mean lagrangian surface flow velocities extending from the base of the feeder through to the rip head were obtained by tracking rip floaters and were on the order of 0 7 m s 1 with maximums in the rip neck region of 1 4 m s 1 a distinct tidal modulation of rip current flow existed with maximum velocities occurring at low tide and minimum velocities at high tide comparison with other rip studies suggests that although the magnitude of the morphodynamic and hydrodynamic processes occurring within large scale rip systems is extreme the behaviour of these rip systems is very similar to low energy rips with much smaller spatial scales there is evidence to suggest that distinct morphodynamic scaling relationships exist between these environments

prediction of water inrush risk areas in process of mining under the unconsolidated and confined aquifer a case study from the qidong coal mine in china the unconsolidated cenozoic formation is well developed in the qidong coal mine water inrushes from the bottom unconsolidated cenozoic aquifer which is the confined aquifer have happened several times during mining in the north mining area of the qidong coal mine to predict the water inrush risk areas in the south mining area of the qidong coal mine where some working faces have just begun to mine the engineering analogy method is used on basis of the fisher s discriminant model from the actual mining situation in the north mining area six main influence factors including the effective thickness the specific yield and the load transfer coefficient of the aquifer the effective thickness of the protective bedrock layer the fractal dimensional value of bedrock faults and the distance between the key hard stratum and the primary mineable coal seam are selected as discriminant indexes and their corresponding data in the north mining area of the qidong coal mine are served as training samples on this basis the fisher s discriminant model for water inrush is established and water inrush risk areas including the safety the medium risk and the risk areas of no 61 82 and 9 primary mineable coal seams in the south mining area of the qidong coal mine are predicted by the model the model s accuracy is 90 4 the scatter diagram of training samples shows obviously classified effect and the field verification indicates that the predicted type is consistent with the actual type results show that the discriminant model is well applied in the engineering analogy method and the water inrush risk areas predicted by the discriminant model contribute to the subsequent mining in the south mining area in the qidong coal mine

two dimensional velocity models of the nazca plate subduction zone between 19 5 s and 25 s from wide angle seismic measurements during the cinca95 project the interpretation of seismic refraction wide angle reflection data from the 1995 crustal investigations off and on shore nazca central andes cinca95 project has resulted in the derivation of nine e w two dimensional 2 d velocity cross sections for the region between the peru chile trench and the coast between 19 5 s and 25 s with three of the cross sections extending a farther 100 200 km inland these sections define the major lithospheric structures of the upper south american plate and the lower nazca plate down to uppermost mantle depths of 30 60 km beneath this part of the present day forearc region in addition to showing the nazca plate subducting at an increasing angle of 9 25 down to 30 50 km depth near the coast these cross sections show a portion of the moho dipping eastward from 43 50 km near the coast to 55 64 km up to about 240 km inland owing to a gap of almost 50 km in the data coverage of the moho near the coast it is uncertain from this data set alone whether the moho defined east of the coast should be correlated with the lower oceanic nazca plate or the upper continental south american plate however a comparison with other seismological data suggests that the moho identified here east of the coast defines the base of the continental crust of the upper south american plate immediately behind the downgoing nazca plate thus the hypothesis of delouis et al 1996 that the zone of seismic coupling between the plates correlates with the region where continental crust is in contact with the plate boundary is supported the cross sections also show between 20 s and 22 5 s a boundary extending from upper crustal levels down to 25 30 km depth subparallel to and 5 10 km above the top surface of the subducting nazca plate the discussion focuses on either that the boundary is a fossil extensional structure or preferably that the wedge below the boundary represents gabbroic material from layer 3 of the oceanic crust tectonically underplated onto the lower part of the upper plate

characterization of underground tunnel water hydrochemical system and uses through multivariate statistical methods a case study from maddhapara granite mine dinajpur bangladesh a quality study of the drained water from maddhapara granite mine underground tunnel was undertaken to study their hydrochemical variations and suitability for various uses employing chemical analysis basic statistics correlation matrix r cluster analysis principal component factor analyses and anova as the multivariate statistical methods the results of chemical analysis of water show the modest variation in their ionic assemblage among different sampling points of the tunnel where ca hco3 type of hydrochemical facies is principally dominated the correlation matrix shows a very strong to very weak positive even negative correlation relationship suggesting the influence of different processes such as geochemical biochemical processes and multiple anthropogenic sources on controlling the hydrochemical evolution and variations of water in the mine area cluster analysis confirms that cluster 1 contains 68 75 of total samples whereas cluster 2 contains 31 25 on the whole the dominated chemical ions of first cluster groups are ca and hco3 suggesting a natural process similar to dissolution of carbonate minerals the second cluster group consisted of cl and so4 2 ions representing natural and anthropogenic hydrochemical process the results of pca fa analysis illustrate that different processes are involved in controlling the chemical composition of groundwater in the mine area the factor 1 loadings showed that ph ec tds na mg chloride and sulfate which have high loading in this factor are expected to come from carbonate dissolution to oxidation conditions one way anova describes the significance of dependent variables with respect to independent variables anova gives us the idea that ec k fetotal so 4 2 as and pb are the most important factors in controlling spatial differences in water quality in this tunnel but different results have been encountered for different independent variables which might be due to dissimilar sources of water from the qualitative analysis it is clear that water quality is not very favorable for aquatic creatures as well as for drinking purposes the water can be used for irrigation purposes without any doubt as sar and rsc analysis provides good results moreover the results of this research confirmed that the application of multivariate statistical analysis methods is apposite to inferring complex water quality data sets with its possible pollution sources at the end this research recommends 1 as water becomes more and more important water treatment plants should be built before the water being used 2 a detailed water step utilization plan should be set beforehand to guarantee tunnel water being used effectively and 3 after the water being used for agriculture elements in crops should be monitored continuously to ensure that ions and compounds that come from the tunnel water are lower than guideline values for human beings health

rifting of old oceanic lithosphere geophysical data from five regions in the pacific and indian oceans reveal that long distance 400 km spreading center jumps have occurred in the past the present day seafloor morphology is used to develop a scenario for a spreading center jump the major events are 1 thinning and weakening of the lithosphere at the future rifting site 2 rifting of the weakened lithosphere during rifting the crack is filled from above by normal faulting and wedge subsidence viscous upwelling fills the crack from below 3 spreading at the rift site results in a ridge bounded by two troughs spreading ceases at the dying spreading center resulting in a deep central graben surrounded by flexural ridges periods of slow spreading at both spreading centers produce rough topography 4 ageing and cooling that produce a general deepening of the abandoned spreading ridge and also reduce the thermal contrast across the fossil rifting site the new spreading center develops into a normal spreading rift the major topographic expressions apparent in the seafloor today are the deep trough of the abandoned spreading center and the proximal and distal troughs which formed when the emerging spreading center bisected the fossil rifting site the proximal trough nearer the new spreading ridge and the distal trough farther from the new ridge are first order topographic features 100 1000 km long and 300 km wide resembling fracture zones with which they are often confused they share with fracture zones the characteristic of bringing together fragments of lithosphere of different ages but unlike fracture zones they are generally parallel to magnetic lineations

the foundation seamounts tectonic setting of a newly discovered seamount chain in the south pacific abstract large areas of the south pacific remain unsurveyed because they are so remote a major volcanic chain the foundation seamounts lies between 33 s and 39 s and between 111 w and 125 w poorly defined on conventional bathymetric charts it becomes very apparent when combining an analysis of conventional geophysical data with the satellite altimeter data the length of the seamount chain is 1350 km and the volcanoes are clustered within a 180 km wide band trending approximately n70w which is the direction of absolute motion of the pacific plate over a fixed reference system some of these volcanoes are so shallow that they may have emerged above the sea surface at an earlier time no dates are known for the seamounts surrounding the foundation seamounts chain the tectonic setting is that of the pacific plate at the time when the strike of the spreading ridge changed from a northwesterly direction to the present northeasterly one this episode of spreading reorganization involved the existence of a short lived microplate west of the pacific antarctic ridge a captured segment of nazca plate accounts for the large discrepancies in the magnetic anomalies offsets along the agassiz fracture zone large propagators like the adventure and kurchatov troughs are described the newly discovered western segment of the chile fracture zone is the pacific plate match of the chile fracture zone on the nazca plate

community infrastructure and repository for marine magnetic identifications magnetic anomaly identifications underpin plate tectonic reconstructions and form the primary data set from which the age of the oceanic lithosphere and seafloor spreading regimes in the ocean basins can be determined although these identifications are an invaluable resource their usefulness to the wider scientific community has been limited due to the lack of a central community infrastructure to organize host and update these interpretations we have developed an open source community driven online infrastructure as a repository for quality checked magnetic anomaly identifications from all ocean basins we provide a global sample data set that comprises 96 733 individually picked magnetic anomaly identifications organized by ocean basin and publication reference and provide accompanying hellingerformat files where available our infrastructure is designed to facilitate research in plate tectonic reconstructions or research that relies on an assessment of plate reconstructions for both experts and nonexperts alike to further enhance the existing repository and strengthen its value we encourage others in the community to contribute to this effort

flow characteristics at the rip current neck under low energy conditions abstract this paper presents the flow signature of the rip current neck as determined by fourteen near bottom velocity records with a comparison made to 63 recordings in the surf zone the field experiments including wave monitoring and morphological mapping were conducted under relatively calm dissipative wave conditions tsig 3 6 s hsig 0 3 1 1 m at herzliya beach israel on the mediterranean shore the current data were analysed for shore normal and resultant components the processing of the time series in the form of instantaneous and smoothed velocities provided spectral energy orbital and current velocity specific current discharge and directional distribution data the unique characteristic of the neck environment was found to be its continuous unidirectional seaward flow due to the channeling effect this steady offshore drift constituted the fastest flow in the surf zone with a typically narrow 35 94 directional range through spectral analysis of the unidirectional return flow the morphodynamic process signature at the rip neck was defined by the relative contribution from the incident waves 78 the subharmonic edge waves 14 and the infragravity oscillations 8 the pulsatory jet like rip model with concentration of energy at low frequencies is not corroborated by this study the studied environment of the rip and bar pattern represented an inherited beach morphology at an initial accretional although not yet reflective stage

the longshore flow component in low energy rip channels the mediterranean israel abstract we examined in the field the near bed longshore v flow component in rip channels at the herzliyya beach israel on the southeastern mediterranean field experiments conducted under relatively calm wave conditions t sig 3 6 s h sig 0 3 1 1 m included current and wave monitoring and morphological mapping time series of instantaneous and smoothed shore parallel current velocities and of the orbital components were analysed including statistical and spectral processing the peak instantaneous velocities of waves and currents combined were in excess of the threshold motion of the local sand taken as 0 3 m s some velocities even exceeded the threshold for sheet flow transport 0 9 m s the feeder and the neck demonstrated the highest longshore flow components whereas the rip head typically showed the lowest longshore velocities the unsteadiness of the net longshore current was very high feeders typically showed a mean unidirectional longshore current however directional fluctuations gradually increased towards the rip head the data suggest a clear spectral structure most energy 46 67 was in the gravity range with 13 35 subharmonic and minor 13 16 in the infragravity band systematic cross shore spatial trends of the energy spectra components were linked to bar morphology and to exposure to incident waves

multivariate statistical analysis of geochemical data as indicative of the hydrogeochemical evolution of groundwater in a sedimentary rock aquifer system summary the study of groundwater hydrogeochemistry of the paleozoic basses laurentides sedimentary rock aquifer system in quebec produced a large geochemical dataset groundwater samples were collected at 153 sites over a 1500 km2 study area and analyzed for major and minor ions the large number of data can lead to difficulties in the integration interpretation and representation of the results two multivariate statistical methods hierarchical cluster analysis hca and principal components analysis pca were applied to a subgroup of the dataset to evaluate their usefulness to classify the groundwater samples and to identify geochemical processes controlling groundwater geochemistry this subgroup consisted of 144 samples and 14 parameters ca2 mg2 na k hco 3 cl so 4 2 fe2 mn2 br sr2 f ba2 hs seven geochemically distinct clusters c1 c7 resulted from the hca samples from clusters c3 c4 c6 and c7 are mostly located in preferential recharge areas the majority of these samples have ca mg hco3 recharge groundwater c3 c6 c7 and na hco3 evolved groundwater c4 samples from the other three clusters c1 c2 c5 are characteristic of an aquifer system under confined conditions the majority of these samples have na hco3 evolved groundwater c1 c5 and na cl ancient groundwater that exhibits elevated concentrations in br c2 in addition to recognizing the importance of hydrogeological conditions on groundwater geochemistry the distribution of clusters also showed the importance of the geological formations on minor and trace elements such as fe2 mn2 sr2 f and ba2 the first five components of the pca account for 78 3 of the total variance in the dataset component 1 is defined by highly positive loadings in na cl and br and is related to groundwater mixing with champlain sea water and solute diffusion from the marine clay aquitard the high positive loadings in ca2 and mg2 of component 2 suggest the importance of dissolution of carbonate rocks in this aquifer system from their characteristic loadings the first two components are defined as the salinity and hardness components respectively components 3 5 are related to more local and geological effects the integration of the hca and the pca with conventional classification of groundwater types as well as with the hydrogeological and geological contexts allowed the division of the region into four main geochemical areas providing an improved regional picture of the aquifer system dynamics and hydrogeochemical evolution of groundwater the following factors were recognized as influencing the evolution of groundwater identified in every geochemical area 1 geological characteristics including sedimentary rock type and till mineralogy 2 hydrogeological characteristics represented by the level of confinement and the hydraulic gradient and 3 the geological history including the latest glaciation and the champlain sea invasion with its integrated approach this hydrogeochemical study contributes to the characterization and understanding of complex groundwater flow systems and provides an example of the long term geochemical evolution of hydrogeological systems after a major perturbation in this case seawater invasion

wave formed structures and paleoenvironmental reconstruction clifton h e and dingier j r 1984 wave formed structures and paleoenvironmental reconstruction in b greenwood and r a davis jr editors hydrodynamics and sedimentation in wave dominated coastal environments mar geol 60 165 198 wave formed sedimentary structures can be powerful interpretive tools because they reflect not only the velocity and direction of the oscillatory currents but also the length of the horizontal component of orbital motion and the presence of velocity asymmetry within the flow several of these aspects can be related through standard wave theories to combinations of wave dimensions and water depth that have definable natural limits for a particular grain size threshold of particle movement and that of conversion from a rippled to fiat bed indicate flow velocity limits the ratio of ripple spacing to grain size provides an estimate of the length of the near bottom orbital motion the degree of velocity asymmetry is related to the asymmetry of the bedforms though it presently cannot be estimated with confidence a plot of water depth versus wave height h h diagram provides a convenient approach for showing the combination of wave parameters and water depths capable of generating any particular structure in sand of a given grain size natural limits on wave height and inferences or assumptions regarding either water depth or wave period based on geologic evidence allow refinement of the paleoenvironmental reconstruction the assumptions and the degree of approximation involved in the different techniques impose significant constraints inferences based on wave formed structures are most reliable when they are drawn in the context of other evidence such as the association of sedimentary features or progradational sequences

revision of paleogene plate motions in the pacific and implications for the hawaiian emperor bend reply understanding the relative motion between the pacific plate and its neighboring plates in the paleogene has important consequences for deciphering the relationship between absolute and relative plate motions in the pacific ocean basin the history of circum pacific subduction and the cause of the hawaiian emperor bend heb we quantitatively model the farallon vancouver pacific antarctic seafloor spreading history from 67 to 33 ma based on a comprehensive synthesis of magnetic anomaly and fracture identifications we find a well constrained increase from 75 5 mm yr to 101 5 mm yr in pacific farallon full spreading rates between 57 6 ma and 55 9 ma followed by a stepwise increase to 182 2 mm yr from 49 7 to 40 1 ma the increases in pacific farallon spreading rates are not accompanied by any statistically significant change in spreading direction the 57 6 55 9 ma surge of pacific farallon spreading reflects an eastward acceleration in farallon plate motion as it precedes west pacific subduction initiation and is not associated with any significant change in pacific antarctic spreading we interpret the increase in pacific farallon spreading rates after ca 50 ma as a consequence of further acceleration in farallon plate motion we find no indication of a major change in pacific plate absolute motion at this time our model suggests that changes in relative motion direction between the pacific and farallon and pacific and antarctic plates were insignificant around the formation time of the heb ca 47 5 ma and the bend is largely a consequence of hawaiian hotspot motion which ceased rapid motion after 47 ma

piston core record of late paleogene 31 ma to recent seafloor hydrothermal activity in the southwest pacific basin a large diameter piston core containing 8 35 m of metalliferous sediment has been recovered from a small abyssal valley in the remote southwest pacific basin 31 42 194 s 143 30 331 w 5082 m water depth providing unique insight into hydrothermal activity and eolian sedimentation there since the early oligocene a combination of fish teeth sr isotope stratigraphy and inaa geochemical data reveals an exponentially decreasing hydrothermal flux 31 ma to the present although hydrothermal sedimentation related to seafloor spreading explains this trend a complex history of late eocene early oligocene ridge jumps propagating rifts and plate tectonic reorganization of south pacific seafloor could have also played a role a possible hiatus in deposition as recorded by changes in core composition just below 2 m depth is beyond the resolution of the fish teeth sr isotope dating method employed here however the timing of this interval may be coincident with extinction of the pacific farallon ridge at 20 ma a low flux eolian component accumulating at this site shows an increase relative to the hydrothermal component above 2 m depth consistent with dust generating continental sources far to the west australia new zealand this is the first long term paleoceanographic record obtained from within the south pacific bare zone rea et al 2006 an anomalous region where pacific seafloor has largely escaped sediment accumulation since the late cretaceous

hydrochemical signatures of deep groundwater circulation in a part of the himalayan foreland basin groundwater samples were analyzed from 71 springs and wells as part of a larger study in a region of compressional tectonic regime the study site covers the peshawar basin and surroundings in the himalayan foreland of pakistan the northern portion is mountainous and the water table is discontinuous in different intermontane valleys with abundant springs with normal and anomalous temperatures and composition the southern part is divided into isolated basins with a number of drilled deep and dug shallow wells hydrochemical signatures of elevated strontium sr sio2 boron b and the geothermometric signatures all indicate a deep circulation of the emerging groundwater moreover for several of the sample sites water chemical compositions measured spring and water well temperatures and reservoir temperatures calculated for spring waters all point to origin from deep horizons within the basin remarkable proximity of all the thermal and hydrochemical anomalies to major faults suggests that the waters ascended along these faults from greater depths the area is a natural western extension of the himalayan geothermal belt described in earlier literature for the eastern and central himalayas

mapping the miocene farallon ridge jump on the pacific plate a seismic line of weakness abstract bathymetric and magnetic data are used to obtain estimates on the pacific and nazca plates of the boundaries separating lithosphere generated at the old farallon ridge from the more recent one created at the present day east pacific rise an excellent correlation is found with the sites of known teleseismically recorded intraplate seismicity suggesting that these boundaries which are lines of age discontinuity in the plate must be zones of weakness of the lithospheric plate in particular the so called region c identified by okal et al as a major site of seismic release sits on a small piece of farallon plate in the immediate vicinity of the northern extension of the fossil roggeveen rise cut across by the east pacific rise during the ridge jump

double seismic zone of the nazca plate in northern chile high resolution velocity structure petrological implications and thermomechanical modeling this paper presents an interdisciplinary study of the northern chile double seismic zone first a high resolution velocity structure of the subducting nazca plate has been obtained by the tomodd double difference tomography method the double seismic zone dsz is observed between 80 and 140 km depth and the two seismic planes is 20 km apart then the chemical and petrologic characteristics of the oceanic lithosphere associated with this dsz are deduced by using current thermal petrological seismological models and are compared to pressure temperature conditions provided by a numerical thermomechanical model our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions in the seismic upper plane located within the upper crust these reactions would affect material of basaltic morb composition and document different metamorphic reactions occurring within high p 2 4 gpa and low t 130 km lawsonite amphibole eclogite conditions the lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions the vp and vs characteristics of the region in between both planes are consistent with a partially 25 30 vol antigorite 0 10 vol brucite and 4 10 vol chlorite hydrated harzburgitic material discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions while various geophysical indicators evidence particularly cold conditions in both the descending nazca plate and the continental fore arc thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 c 50 c isotherm lower plane earthquakes are predicted to occur in the slab s flexural neutral plane where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane thus producing a sheeted layer of free fluids or a serpentinized layer therefore earthquakes may present either downdip compression and downdip tensile characteristics numerical tests indicate that the slab s thermal structure is not the only factor that controls the occurrence of inner slab compression 1 a weak ductile subduction channel and 2 a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab 3 decreasing the radius of curvature of the slab broadens the depth of inner slab compression whereas 4 decreasing upper plate convergence diminishes its intensity all these factors indicate that if dszs indeed contour inner slab compression they cannot be linked only to slab unbending but also to the transmission of high compressional stresses from the upper plate into the slab

seismic investigations of the o higgins seamount group and juan fernandez ridge aseismic ridge emplacement and lithosphere hydration the o higgins seamount group is a cluster of volcanic domes located 120 km west of the central chilean trench on the crest of the juan fernandez ridge this aseismic hot spot track is subducting under south america triggering a belt of intraslab earthquake hypocenters extending about 700 km inland the juan fernandez ridge marks the southern boundary of a shallow subduction segment subduction of oceanic basement relief has been suggested as a cause for the flat slab segments characterizing the andean trench system the juan fernandez ridge however shows only moderate crustal thickening inadequate to cause significant buoyancy in 2001 wide angle seismic data were collected along two perpendicular profiles crossing the o higgins group we present tomographic images of the volcanic edifices and adjacent outer rise trench environment which indicate a magmatic origin of the seamounts dominated by extrusive processes high resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting a pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases reduced uppermost mantle velocities are solely found underneath the juan fernandez ridge and may indicate mineral alterations enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction which prevails to the north after the southward migration of the juan fernandez ridge

study of multi aquifer groundwater interaction in a coal mining area in china using stable isotopes and major ion chemical data investigations have been undertaken to determine the interactions between the main aquifers in a coal mining area near the taihang mountains in china to determine the sources of water environmental isotopes 18o 2h 3h 34s and water chemistry ions ca2 mg2 na k hco3 so42 cl were used as tracers furthermore batch sampling and testing were conducted on mountain spring water and karst water in the aquifers of the taiyuan group fengfeng group and majiagou group on the seatearth of the coal mining area via field observations and laboratory experimental analysis the 18o and 2h values of the mountain spring water and karst water in the coal mining area showed a common distribution with the local meteoric water line el1 of karst water in the majiagou group but significantly deviated from the surface water evaporation line el2 in the coal mining area when combined with the distribution of karst water level values thus karst water in the coal mining area is mainly supplied by groundwater from mountainous areas furthermore the concentration of sulfate ions increased dramatically in the groundwater flow of karst water from mountain spring water to karst water in the fengfeng group and majiagou group of the coal mining area when equivalent concentrations of ca2 mg2 hco3 and so42 hco3 reached their peak the chemical type of groundwater gradually evolved from ca mg hco3 to ca mg so4 hco3 in addition significant positive correlation was found between the 34s and so42 values of the water samples indicating that gypsum is involved in groundwater lixiviation in contrast the relationships of ca na versus mg na na hco3 versus total dissolved solids tds and na versus cl revealed that tds and salinity accumulate from the mountain spring water and karst water in the taiyuan group of the coal mining area furthermore the chemical type of groundwater gradually evolved from ca mg hco3 to na hco3 as revealed by a piper trilinear diagram

experimental study of nearshore dynamics on a barred beach with rip channels 1 wave and current measurements are presented from a set of laboratory experiments performed on a fixed barred beach with periodically spaced rip channels using a range of incident wave conditions the data demonstrate that the presence of gaps in otherwise longshore uniform bars dominates the nearshore circulation system for the incident wave conditions considered for example nonzero cross shore flow and the presence of longshore pressure gradients both resulting from the presence of rip channels are not restricted to the immediate vicinity of the channels but instead are found to span almost the entire length of the longshore bars in addition the combination of breaker type and location is the dominant driving mechanism of the nearshore flow and both are found to be strongly influenced by the variable bathymetry and the presence of a strong rip current the depth averaged currents are calculated from the measured velocities assuming conservation of mass across the measurement grid the terms in both the cross shore and longshore momentum balances are calculated and their relative magnitudes are quantified the cross shore balance is shown to be dominated by the cross shore pressure and radiation stress gradients in general agreement with previous results however the rip current is shown to influence the wave breaking and the wave induced setup in the rip channel analysis of the longshore balance shows that the overall strength of the longshore feeder currents is governed by a balance between longshore pressure gradient forcing and an opposing radiation stress gradient in addition the longshore feeder currents show maxima in the bar trough region providing experimental evidence that longshore pressure gradients can shift longshore current maxima shoreward from the bar crest finally since the longshore radiation stress gradient in the surf zone is governed by the amount of wave dissipation on the rip current there exists a positive feedback mechanism whereby a strong rip current can weaken the radiation stress gradient opposing the feeder currents and lead to even stronger feeder currents and rips

analysis of propagators along the pacific antarctic ridge evidence for triggering by kinematic changes abstract we analyze the structure and evolution of two propagators along the pacific antarctic ridge par that we surveyed during the pacantarctic cruise of the n o l atalante a large propagator at 63 30 s 167 w shows a n50 e trending segment of the par propagating southwestward while the adjacent n45 e trending segment retreats the propagating and doomed ridges are offset by about 43 km they both curve into the offset to define an overlap zone about 25 km long the inner pseudofault is juxtaposed to a series of e w trending ridges inferred to represent the failed axes their direction and arrangement suggest an evolution as an overlapping propagator with cyclic rift failure the pseudofaults are 35 5 oblique to the propagating ridge which implies a rate of propagation of 44 8 mm yr using a 62 mm yr full spreading rate comparable to that of other propagators with similar morphology the age of the initiation of the propagation from the heirtzler fracture zone is estimated to be 5 6 ma which coincides with the age of a clockwise change in spreading direction a second smaller southwestward propagator is observed northeast of the major one at 63 15 s 165 10 w with a morphology very similar to that of the larger one it is inferred to have started near 1 ma again at the time of a clockwise change in spreading direction along the par these two propagators are likely to have evolved from extensional relay zones which developed within the heirtzler transform fault tf valley following clockwise changes in spreading direction the present day axial discontinuity is less than 40 km in offset and may not be a tf anymore the development of propagators in this area of the par appears to be triggered by kinematic changes rather than by thermal gradients along the ridge other propagators that have left similar signatures on the flanks of the par appear to have developed at similar spreading rates near 50 60 mm yr full rate as a result of kinematic changes

morphodynamics of a bar trough surf zone abstract a field study was made of the distinguishing morphodynamic processes operating in a surf zone which perennially exhibits accentuated bar trough topography the longshore bar trough and rhytmic bar and beach states as described by wright and short 1984 characteristic features of the morphology include a shallow bar with a steep shoreward face a deep trough and a steep beach face this morphology which is favored by moderate breaker heights and small tidal ranges strongly controls the coupled suite of hydrodynamic processes in contrast to fully dissipative surf zones the bar trough surf zone is not at all saturated and oscillations at incident wave frequency remain dominant from the break point to the subaerial beach the degree of incident wave groupiness does not change appreciably across the surf zone infragravity standing waves which in dissipative surf zones dominate the inshore energy remain energetically secondary and occur at higher frequencies in the bar trough surf zone analyses of the field data combined with numerical simulations of leaky mode and edge wave nodal antinodal positions over observed surf zone profiles indicate that the frequencies which prevail are favored by the resonant condition of antinodes over the bar and nodes in the trough standing waves which would have nodes over the bar are suppressed sediment resuspension in the surf zone appears to be largely attributable to the incident waves which are the main source of bed shear stress in addition the extra near bottom eddy viscosity provided by the reformed non breaking waves traversing the trough significantly affects the vertical velocity profile of the longshore current whereas the bar is highly mobile in terms of onshore offshore migration rates the beach face and inner regions of the trough are remarkably stable over time

ripex observations of a rip current system rip current kinematics and beach morphodynamics were measured for 44 days at sand city monterey bay ca using 15 instruments composed of co located velocity and pressure sensors acoustic doppler current profilers and kinematic gps surveys the morphology consisted of a low tide terrace with incised quasi periodic rip channels representative of transverse bars offshore 17 m depth significant wave height and peak period ranged 0 20 3 0 m and 5 20 s the mean wave direction was consistently near 08 resulting in rip channel morphology which evolved in response to the changing wave characteristics an inverse relationship between sediment accreting on the transverse bar and eroding in the rip channel was found the spatial distribution of sediment is reflected in the background rip current flow field the mean velocity magnitudes within the rip channel transverse bars increased offshore onshore with decreasing tidal elevations and increased with increasing sea swell energy eulerian averaged flows were predominantly shoreward on the transverse bars and seaward within the rip channel throughout the experiment resulting in a persistent cellular circulation except during low wave energy the rip current spacing to the rip channel width was less than or equal to two which suggests that the rip currents are influenced by each other and that no two dimensional bar return flow should be present the vertical velocity profile on the bar indicated that the flow was predominantly shoreward the flow field within the surf zone was depth uniform except for significant shear occurring near the surface owing to stokes drift the wave induced transport hypothesis is evaluated d 2005 elsevier b v all rights reserved

transient rip currents and nearshore circulation on a swell dominated beach 1 eulerian and lagrangian measurements of nearshore circulation on a longshore uniform beach with perpendicularly incident swell waves are described both the eulerian and lagrangian data show a variable rotational part of the current field that is distinct from but coexists with that due to infragravity waves a well defined feature of the flow field includes transient rip currents which occur in varying locations and are not topographically controlled drifter trajectories show that the rips behave like a shallow plane jet consisting of a constrained neck region that flows into a spreading head region there is continuous modulation of the flow within the rip neck estimates of the length scale across the rip neck using two particle statistics are 20 30 m well defined discrete vortex features are observed at the initiation point of one of the necks and in one of the rip heads estimated dispersion values suggest that mixing is greatly enhanced outside the surf zone by the intermittent presence of the rip heads the dispersion calculated from both inside and outside the surf zone appears to show scale dependence with a power law close to 4 3 in a similar way to oceanic turbulence at much larger scales

equilibrium slopes and cross shore velocity asymmetries in a storm dominated barred nearshore system abstract the near bed 0 10 m elevation horizontal velocities were measured across a two barred lacustrine shoreface during a storm and reveal a spatially coherent but temporally variable pattern in the low order moments of the velocity field large cross shore velocity asymmetries indicated 1 1 spatially extensive and temporally persistent offshore mean flows up to 0 18 m s 1 across the landward slope of the linear outer bar reflecting an undertow driven by a wave induced setup 2 2 an onshore directed skewness opposing the mean flow of wind wave origin and persistent under propagating bores in the inner bar system long after this asymmetry had disappeared from the outer bar extensive sediment reactivation with near zero net sediment flux over the whole of the upper shoreface indicated a sediment prism in a steady state volumetrically over the storm cycle uniform depths of sediment reactivation with zero bed elevation change over the storm cycle revealed local slopes in a steady state in the outer linear bar however time integration of the near bed velocity field at this location indicated an offshore net flux of water while sedimentary structures indicated bedform migration and thus at least bedload almost uniformly onshore when ripples and megaripples were present the sediment transport giving rise to the local equilibrium cannot be explained therefore on the basis of the mean properties of the near bed velocity vector but requires knowledge of the time varying instantaneous sediment transport vector onshore migration of the inner bar correlated well with the large and persistent onshore skewness of the velocity distribution in the inner surf zone associated with translatory surf bores throughout the storm decay period low frequency oscillations were only significant close to the shoreline but may have influenced the development of a sinuous three dimensional inner bar form as it migrated onshore

rip current review rip currents are shore normal narrow seaward flowing currents that originate within surf zone extend seaward of the breaking region rip head and can obtain relatively high velocities within the last decade there have been a significant number of laboratory and field observations within rip current systems an overview of rip current kinematics based on these observations and the scientific advances obtained from these efforts are synthesized rip current flows are partitioned into mean infragravity very low frequency vorticity and tidal contributions and it is found that each contributes significantly to the total data from the laboratory and the field suggest that the rip current strength increases with increasing wave energy and decreasing water depths the maximum mean current occurs inside the surf zone where the maximum forcing is present owing to the dissipation of waves

assessment of the impact of anthropogenic activities on the groundwater hydrology and chemistry in tarsus coastal plain mersin se turkey using fuzzy clustering multivariate statistics and gis techniques summary tarsus coastal plain tcp is an economically and ecologically important area situated in between the fertile fluvio deltaic plains of two rivers delicay and tarsus mersin se turkey where anthropogenic activities agricultural industrial and domestic are very intense twenty four water quality parameters were surveyed at 193 groundwater and 10 surface water sites during august 2008 the objective was to characterize the physico chemical properties of groundwaters in tcp assess the impact of anthropogenic activities on the groundwater hydrology and chemistry and identify the major hydrogeochemical processes occurring in the area groundwater samples were grouped into hydrochemically distinct and spatially continuous four water classes i e c1 c2 c3 and c4 using the fuzzy c means fcm clustering method where membership values were interpolated using the ordinary kriging technique principal components analysis pca was used to decipher various underlying natural and anthropogenic processes creating these distinct water classes four principal components pcs were extracted in pca which explained more than 73 of the total variance in water quality major factors responsible for the variations in chemistries of water classes are identified as 1 water rock interaction and nitrate contamination 2 salinization by seawater intrusion and evaporite dissolution 3 geogenic anthropogenic cr fe and mn and 4 anthropogenic zn pollution overexploitation of the aquifer is clearly evident especially at settlements located near the coastal zone where the water table is lowered 2 5 m below the sea level salinization is well known in the area and is attributed not only to seawater intrusion but also to dissolution of evaporitic series from the handere formation hydrochemical evidence also suggest that in the area subsurface paleo river channels and the deposits infilling the ancient lagoon area within quaternary recent alluvial deposits act as significant hydrological features where preferential groundwater flow occurs along them

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

tectonic history of the juan de fuca ridge over the last 40 million years interpretation of a new compilation of magnetic anomaly data for north of the mendocino fracture zone and younger than chron 15 39 ma indicates a transition from a pattern of stable transform faults to a pattern of persistent ridge propagation reorganizations of the propagation pattern occurred at 30 ma in response to a counterclockwise change in direction of relative motion and at 19 ma in response to a clock wise change in addition there are significant changes in the gradient of the spreading rate apparently without major changes in motion direction at 27 and 24 ma the change at 30 ma is apparently the result of the separation of the juan de fuca plate from the rest of the farallon plate along the pioneer fracture zone the pattern of absolute motion after 30 ma does not generally support menard s pivoting subduction model and suggests that juan de fuca plate motion is being significantly affected by forces transmitted across the mendocino transform fault

vertical and horizontal structure in cross shore flows an example of undertow and wave set up on a barred beach greenwood b and osborne p d 1990 vertical and horizontal structure in cross shore flows an example of undertow and wave set up on a barred beach coastal eng 14 543 580 eulerian measurements of the horizontal cross shore velocity field in the lowermost meter of the water column in association with measurements of waves and the mean elevation of the water surface across a nontidal low relief barred surf zone reveal a spatially coherent patterns of the timeaveraged first second and third moments of the velocity field which vary temporally in direct response to the incident waves b large asymmetries in the flow field with offshore directed mean flows upto 0 20 m s a and onshore directed velocity skewness the third moment normalized by the standard deviation upto 0 60 c mean flows decrease and oscillatory flows measured by the standard deviation of the velocity field increase towards the water surface d a fourth order polynomial provides the best fit to the cross shore set up reflecting the influence of variable energy dissipation due to topographic controls on wave breaking e significant linear correlations exist between the cross shore mean velocities and the maximum set up of the still water surface near to the shoreline upto 82 of the observed variability in the former can be accounted for by linear regressions f set up is highly correlated with the incident wave height 86 of the observed variability in the former can be accounted for by a linear regression g gradients in the mean water surface are significantly less than those predicted by a linear function of beach slope as originally proposed by bowen et al even using an appropriate breaking criterion 0 4 0 6 for the spilling breakers recorded cross shore circulation over the low relief barred nearshore slope is predominantly two dimensional over the outer bar in this case study the near bed mean flow is an undertow responding to the balance between the wave generated depth dependent momentum flux directed onshore the stress induced by the onshore mass transport under waves and wind and the hydraulic pressure gradient induced by set up of the mean water level

multivariate statistical analysis of water chemistry in evaluating groundwater geochemical evolution and aquifer connectivity near a large coal mine anhui china major ion chemistry of groundwater from main aquifers near the dingi coal mine anhui china was analyzed with a suite of statistical techniques in an effort to understand aquifer connectivity and groundwater geochemical evolution fifty seven groundwater samples were analyzed from four aquifers the cenozoic top middle bottom aquifers and a permian coal strata aquifer with increasing depth of the aquifers groundwater became more mineralized showing greater hardness and salinity the dissolutions of halite and sylvite carbonate sulfate and silicate minerals were the primary processes controlling groundwater chemical compositions in the cenozoic bottom and coal strata aquifers while k na and hco3 originating from silicate mineral weathering were more enriched in the coal strata aquifer principal component analysis pca identified two principal components with the first component representing hardness variation and accounting for 57 5 of total variance and the second component 23 5 of total variance controlled primarily by salinity and sulfate reduction process the cenozoic bottom aquifer had greater hardness than other aquifers whereas the coal strata aquifer was characterized by higher salinity and larger influences of sulfate reduction cluster analysis revealed that the coal strata aquifer was largely isolated from the cenozoic aquifers with possible infiltration at discrete locations indicating that the high yield thick cenozoic aquifers may serve as a potential additional source for groundwater inrush into coal mines

toward a self consistent high resolution absolute plate motion model for the pacific the hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes given geometry and ages from several trails one can reconstruct absolute plate motions apm that provide valuable information about past and present tectonism paleogeography and volcanism most apm models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles opening angles and time intervals unlike relative plate motion rpm models such apm models suffer from oversimplicity self inconsistencies inadequate fits to data and lack of rigorous uncertainty estimates in addition they work only for fixed hot spots newer methods are now available that overcome many of these limitations we present a technique that provides high resolution apm models derived from stationary or moving hot spots given prescribed paths the simplest model assumes stationary hot spots and an example of such a model is presented observations of geometry and chronology on the pacific plate appear well explained by this type of model because it is a one plate model it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the emperor chain whether there was significant relative motion within the hot spots under the pacific plate during the last 70 m y is difficult to quantify given the paucity and geological uncertainty of age determinations evidence in support of plume drift appears limited to the period before the 47 ma hawaii emperor bend and apart from the direct paleolatitude determinations may have been somewhat exaggerated

uncertainties in reconstructions of the pacific farallon vancouver and kula plates and constraints on the rigidity of the pacific and farallon and vancouver plates between 72 and 35 ma to study the rigidity of the pacific and farallon plates between 72 and 35 ma we analyzed the magnetic anomalies and fracture zones in the pacific ocean between longitudes 125 w 180 and latitudes 43 s 60 n a set of consistent rotations indicates that the pacific and farallon plates between the pioneer and agassiz fracture zones were rigid plates between 72 and 35 ma thus the location of a missing tertiary boundary within the pacific or antarctic plates is restricted to be south of latitude 43 s the farallon plate broke into two plates at the time of the change in the pacific farallon spreading direction north of the pioneer and mendocino fracture zones sometime between 59 and 49 ma the northern fragment termed the vancouver plate was subducted beneath most of north america we present rotations and uncertainties for the histories of motion of the pacific and kula plates from anomaly 32 to anomaly 25 of the pacific and vancouver plates from anomaly 21 to anomaly 13 and of the pacific and farallon plates from anomaly 32 to anomaly 13

the foundation seamount chain a first survey and sampling the foundation seamounts form a 1400 km long chain on the pacific plate from 32 s 127 w to the pacific antarctic spreading axis at 38 s 111 w previously only known from sparse single beam echosoundings and satellite altimetry we present here the first multibeam bathymetric survey and geological sampling results we confirm that the submarine topography correlates with the altimetry and that the chain is volcanic rather than tectonic or microcontinental in origin the chain can be divided up morphologically and geochemically into three section 1 west of 125 w large flat topped volcanoes composed of incompatible element depleted lavas 1 of a near ridge origin with little or no plume influence 2 between 125 and 115 w true intraplate volcanoes with incompatible element enrichment 1 9 generated over the foundation plume 3 east of 115 w e w trending volcanic ridges with compositions 2 0 0 3 suggestive of interaction between the plume and the pacific antarctic spreading axis on the spreading axis moderate incompatible element enrichments 0 8 cf 0 3 outside the foundation area also suggest plume influence it appears that the activity of the foundation plume in the last few million years has 1 significantly waned and 2 become wholly channeled towards the spreading axis the foundation plume may be in the process of dying

tectonic history and new isochron chart of the south pacific we have developed an internally consistent isochron chart and a tectonic history of the south pacific using a combination of new satellite altimeter data and shipboard magnetic and bathymetric data highly accurate vertical deflection profiles 1 2 rad derived from 22 repeat cycles of geosat altimetry reveal subtle lineations in the gravity field associated with the south pacific fracture zones these fracture zone lineations are correlated with sparse shipboard bathymetric identifications of fracture zones and thus can be used to determine paleospreading directions in uncharted areas the high density of geosat altimeter profiles reveals previously unknown details in paleospreading directions on all of the major plates magnetic anomaly identifications and magnetic lineation interpretations from published sources were combined with these fracture zone lineations to produce a tectonic fabric map the tectonic fabric was then used to derive new poles of rotation for 12 selected times in the late cretaceous and cenozoic from our reconstructions we estimated the former location of the spreading centers in order to derive a new set of isochrons interpreted unes of equal age on the ocean floor we believe that the use of new geosat altimeter data in combination with a multi plate reconstruction has led to an improvement in our understanding of south pacific tectonics

the chile ridge a tectonic framework a new chile ridge tectonic framework is developed based on satellite altimetry data shipboard geophysical data and primarily 38 500 km of magnetic data gathered on a joint u s chilean aeromagnetic survey eighteen active transforms with fossil fracture zones fzs including two complex systems the chile fz and valdivia fz systems have been mapped between the northern end of the antarctic nazca plate boundary chile ridge at 35 s and the chile margin triple junction at 47 s chile ridge spreading rates from 23 ma to present have been determined and show slowdowns in spreading rates that correspond to times of ridge trench collisions the valdivia fz system previously mapped as two fzs with an uncharted seismically active region between them is now recognized to be a multiple offset fz system composed of six fzs separated by short ridge segments 22 to 27 km long at chron 5a 12 ma the chile ridge propagated from the valdivia fz system northward into the nazca plate through crust formed 5 myr earlier at the pacific nazca ridge evidence for this propagation event includes the friday and crusoe troughs located at discontinuities in the magnetic anomaly sequence and interpreted as pseudofaults this propagation event led to the formation of the friday microplate which resulted in the transferal of crust from the nazca plate to the antarctic plate and in a 500 km northward stepwise migration of the pacific antarctic nazca triple junction rift propagation microplate formation microplate extinction and stepwise triple junction migration are found to occur during large scale plate motion changes and plate boundary changes in the southeast pacific

flow characteristics along the rip current system under low energy conditions abstract this paper aims to determine the flow signature of the rip current under low energy conditions based on current meters at herzliyya beach israel on the southeastern mediterranean shore field experiments conducted under relatively calm wave conditions t sig 3 6 s h sig 0 3 1 1 m included current and wave monitoring with morphological mapping current data were analysed for shore normal and resultant components and the time series of instantaneous and smoothed velocities provided current and orbital wave velocities statistical and spectral processing yielded energy distributions specific current discharge and directional data velocity and velocity time asymmetry increased from the feeders to the neck and subsequently slowed down towards the rip head the directional range of the flow was narrow in channeled feeders and even better concentrated at narrow necks with higher velocities the flows at the rip head were typically divergent systematic spatial change of energy spectra was observed along the rip system the subharmonic frequency band dominated the feeders and the inner surf embayments and decreased gradually toward the rip neck with a simultaneous gradual increase in incident wave frequency the infragravity band frequency was minor everywhere velocity spectra at oblique bars with sheltered areas differed from spectral changes at transverse bars with an exposed inshore the velocity at the rip neck was above the threshold for grain movement whereas at the head it was usually below the threshold textural parameters of the beach sand could not be systematically related to the current velocity i e no environmental groupiness was evident along the rip system

mean currents and sediment transport in a rip channel measurements of hydrodynamics and suspended sediment transport were carried out along a cross shore transect in a rip channel a tidally induced threshold for rip flow initiation was found this threshold depended upon the degree of wave energy dissipation and could be identified through a critical value of the ratio of significant wave height to water depth s in the rip neck with normally incident waves rip current velocities were well correlated with velocities predicted from a simple model involving the onshore mass transport in asymmetric breaking waves crossing the bar hydrodynamics and sediment transport were tidally modulated with strong offshore directed rip currents and associated offshore sediment transport due to the mean flow at low tide at high tide when the rip was inactive the tendency was for weak onshore directed mean flows and or oscillatory incident waves transporting smaller amounts of sediment landward the net result was a gradual lowering of the topographic relief along the rip channel with net deposition seaward of the rip outside the surf zone transport was mainly landward due to oscillatory incident waves

multiple episodes of volcanism in the southern austral islands flexural constraints from bathymetry seismic reflection and gravity data 1 at the southeastern end of the cook austral island chain in the vicinity of the currently active macdonald seamount multiple episodes of volcanism have left a diverse population of seamounts multichannel seismic reflection and shipboard gravity data provide observational constraints on the magnitude and wavelength of flexure and multibeam bathymetry obtained by r v ewing and f s sonne within the study area provides full spatial coverage of all volcanic loads three dimensional flexural modeling of closely spaced loads emplaced at discrete times performed both with analytical models of idealized loads and with fourier domain solutions of the observed volcanic loads partitioned into the younger and older seamounts is compared with seismic and gravity data this modeling provides a more complete view of the volcanic history than radiometric dating and geochemical analysis of sparse dredge samples alone wide angle seismic refraction data from ocean bottom hydrophones obhs and expendable sonobuoys are also consistent with the flexural modeling results volume estimates of the different volcanic episodes show that one half to two thirds of the material added to the abyssal seafloor by midplate volcanic processes is due to older volcanism that erupted on young lithosphere while the younger higher seamounts contributed the remainder