wave induced sediment transport and sandbar migration onshore sediment transport and sandbar migration are important to the morphological evolution of beaches but are not well understood here a model that accounts for fluid accelerations in waves predicts the onshore sandbar migration observed on an ocean beach in both the observations and the model the location of the maximum acceleration induced transport moves shoreward with the sandbar resulting in feedback between waves and morphology that drives the bar shoreward until conditions change a model that combines the effects of transport by waves and mean currents simulated both onshore and offshore bar migration observed over a 45 day period

the thermal history of ascending magma diapirs and the thermal and physical evolution of magmatic conduits abstract if over millions of years the positions of island arc volcanoes remain fixed relative to their source regions magma bodies must repeatedly traverse a path through the lithospheric mantle the repeated passage of such bodies thermally and chemically modifies the adjacent mantle for diapirs with radii of 3 6 km ascending at 4 10 8 to 2 10 7 m s 60 000 to 900 000 years are required to establish a magmatic conduit to crustal depths during this period wall rock temperatures increase 200 400 c below 60 km and 400 750 c between 60 and 20 km the solidification of large volumes of magma within the mantle also provides a mechanism for the introduction of kilometer scale chemical and isotopic heterogeneities into the lithospheric mantle because the crystallization sequences of magmas ascending at different stages of conduit development vary conduit thermal and physical evolution also influences magmatic differentiation these differences in crystallization sequence result from the different p t trajectories followed by individual magmatic diapirs crystal fractionation of successive diapirs with or without concurrent wall rock assimilation can therefore generate different liquid lines of descent as successive magma bodies chemically react with their wall rock and solidify at various depths the mantle also changes in composition from peridotite toward basalt depending on the stage of conduit development assimilation of wall rock could therefore yield variable magmatic differentiation trends for successive magma bodies the degree of contamination may decline as the conduit and its margin evolve physically lithospheric conduit evolution has several important ramifications for geochemical modelling first since magma ascent from source regions to crustal depths is unlikely to be isochemical radiogenic and incompatible element ratios may not always reflect characteristics of the magma source region second the earliest stage of arc magmatism e g submarine volcanism is unlikely to reflect the processes of arc magma generation rather this phase of volcanism records the interaction between ascending magma and unmodified mantle the characteristics of primary arc magmas are likely to be understood only through examining more highly developed volcanic centers supplied by mature magmatic conduits

effect of hydrodynamics and bathymetry on video estimates of nearshore sandbar position remote sensing of wave breaker patterns clearly visible as high intensity bands in time exposure video images has become a powerful tool to obtain large scale kilometers and long term years time series of nearshore sandbar position however intensity based bar crest positions xi differ from directly measured positions xb by a time varying distance x which is of o 10 m and depends on the offshore wave height h0 the water level 0 and the bathymetry itself the effect of these parameters on x was investigated from simultaneous video observations and bathymetric surveys obtained in the double barred system at egmond aan zee netherlands and from wave model predictions assuming that the roller energy represents image intensity when the wave field over a bar was predicted to be nonsaturated xi was observed and predicted to move offshore as either 0 decreased or h0 increased under saturated conditions x only responded to changes in 0 additional model investigations showed that an increase in outer bar crest depth similar to that observed during interannual bar behavior significantly reduced the x variability at the outer bar and increased the x variability at the inner bar implications of our observational and model findings for studying sandbar position from video imagery are outlined

variability in flow and temperatures within mantle subduction zones 1 a series of laboratory experiments is used to model three dimensional aspects of flow in subduction zones and the consequent temperature variations in the slab and overlying mantle wedge the effects of longitudinal rollback and slab steepening components of motions are considered along with different thicknesses of the over riding lithosphere the results show that the style of plate sinking influences the evolution of subduction zones both in terms of the speed orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab in the simplest case of longitudinal sinking without rollback motion velocities in the mantle wedge are 30 40 of the downdip plate speed return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds and there is no mass flux around the edges of a slab segment of finite width rollback subduction leads to flow both around and beneath the sinking slab with larger velocities in the wedge up to 150 of the slab speed and flow focused toward the center of the plate segment rollback subduction in which either the trench migrates or the plate steepens with time induces shallow and steep wedge return flow trajectories respectively the thermal evolution of the plate is strongly influenced by sinking style and rate highest temperatures are along the edges of the slab for longitudinal sinking but along the centerline of the slab segment for rollback motion slab surface temperatures when scaled to the mantle are higher than those given by previous two dimensional numerical models and are consistent with recent observational and experimental data on melt compositions

upper mantle compressional velocity structure beneath the west mediterranean basin p waveforms of regional crustal earthquakes have been modeled to obtain an upper mantle compressional velocity model for the west mediterranean basin data come from long period stations of the world wide standardized seismograph network and broadband stations located in the iberian peninsula synthetic waveforms have first been computed for published velocity models corresponding to different tectonic provinces and obtained with analogous techniques a model that strongly improves the fits to the data is then derived the proposed model is characterized by a 100 km thick lid overlaying a not very pronounced low velocity zone and a 3 discontinuity at 368 km where an increase of the velocity gradient also occurs these features could be explained either as a deflection of the olivine to spinel phase transition regionally detected at about 395 km resulting from the lower temperature produced by the subduction of the african plate or as being due to the presence below 300 km depth of a layer of silicate melt producing a strong reflection from its bottom and a more usual depth for the olivine spinel transition in both cases the occurrence of relatively low velocities beneath 300 km is interpreted as being caused by the presence of melt associated with the northward dipping subduction of the african plate

a simple model for interannual sandbar behavior time and length scales of beach variability have been quantified using 16 years of beach surveys sampled at the army corps of engineers field research facility located on the u s atlantic coast between 50 and 90 of the bathymetric variability at this site was explained by alongshore uniform response over the approximately 1 km alongshore span of the surveys although the incident wave height variance was dominated by frequencies at or higher than 1 cycle yr more than 80 of the bathymetric variance at all cross shore locations was explained by frequencies 1 cycle yr interannual cycles consisting of sandbar formation migration and decay contributed to the low frequency variability the observed behavior can be explained by a simple heuristic model the model assumes that bars migrate toward a wave height dependent equilibrium position this position was shown to coincide with the wave breakpoint additionally the rate of bar response is taken to be variable and was empirically determined to be proportional to the wave height cubed the net effect of a variable response rate is to shift the expected long term mean sandbar position offshore toward the equilibrium position associated with the largest waves the model explained up to 80 of the observed bar position time series variance and up to 70 of the variance of bar crest velocity time series which were extracted from three different sandbars characteristic bar response times related to the inverse of the response rate were found to be long relative to the characteristic timescale of the forcing 1 year in our case as a result transient response i e bar position far from equilibrium tended to persist for many cycles of the forcing transient bar behavior appears in the observations when bars formed near the shoreline or when outer bars decayed and inner bars faced a changed wave climate while the present model is able to explain the evolution of these transients it does not contain a mechanism for their introduction

observations of sand bar evolution on a natural beach waves currents and the location of the seafloor were measured on a barred beach for about 2 months at nine locations along a cross shore transect extending 255 m from 1 to 4 m water depth the seafloor location was measured nearly continuously even in the surf zone during storms with sonar altimeters mounted on fixed frames the crest of a sand bar initially located about 60 m from the shoreline moved 130 m offshore primarily when the offshore significant wave height exceeded about 2 m with 1 5 m of erosion near the initial location and 1 m of accretion at the final location an energetics type sediment transport model driven by locally measured near bottom currents predicts the observed offshore bar migration but not the slow onshore migration observed during low energy wave conditions the predicted offshore bar migration is driven primarily by cross shore gradients in predicted suspended sediment transport associated with quasi steady near bottom offshore flows these strong 50 cm s currents intensified near the bar crest by wave breaking are predicted to cause erosion on the shoreward slope of the bar and deposition on the seaward side the feedback amoung morphology waves circulation and sediment transport thus forces offshore bar migration during storms

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

morphologic properties derived from a simple cross shore sediment transport model this paper builds on the now classical discussions by bowen 1980 and bailard 1981 on the applicability and implications of bagnold s 1963 sediment transport model to nearshore profile modeling we focus on the morphologic implications of both the strengths and weaknesses of bagnold s model isolating the transport terms that are well predicted i e mean flow terms from those that are not well predicted i e transport due to correlations between flow and sediment load we factor bagnold s model into a dimensional transport magnitude and a nondimensional term the nondimensional term describes the relative importance of transport due to undertow gravity and correlations between flow and sediment load the transport magnitude largely determines the response time of nearshore profiles for typical nearshore environments this response time was estimated to vary as a function of incident rms wave height hrms from 500 years hrms 0 5 m to 2 years hrms 3 m the relative importance of competing transport mechanisms is shown to depend strongly on the relative wave height defined as the ratio of the rms wave height to the local depth simplified nearshore transport parameterizations that are a function of this variable were derived and were interrogated for the existence and form of equilibrium profiles several differences from previously computed equilibrium profiles were noted first because the relative wave height saturates in natural surf zones equilibrium profiles converge to a relatively flat profile near the shoreline second under some situations a seaward sloping equilibrium profile may not exist third the long response times combined with unknown stability of an equilibrium profile make it difficult to assess the physical connection between theoretical equilibrium profiles and profiles observed in nature

medium term bar behavior and associated sediment transport at hasaki japan 1 the medium term movement of a longshore bar and the associated cross shore sediment transport were investigated using beach profile data obtained nearly daily for 8 years from the seaward foot of a dune to a water depth of about 5 m at hazaki oceanographical research station hors bar crests were found to move seaward repeatedly with a period of a year the duration time of the seaward bar migration is shorter than those on the other coasts in the united states the netherlands and new zealand although bars migrated in one direction seaward the cross shore sediment transport rate associated with the bar migrations fluctuated seaward and shoreward seaward sediment transport occurred on and around bar crests whereas shoreward sediment transport occurred in trough regions the cross shore sediment transport was active when the offshore wave energy was large from winter to spring and in autumn however the direction of the sediment transport in a region where bars were most developed was different during the two periods it was seaward from winter to spring and shoreward in autumn a numerical model for beach profile change confirmed that the development and decay of a bar was caused by the spatial and temporal variations of the cross shore sediment transport rate

breakup of young oceanic lithosphere in the upper part of a subduction zone implications for the emplacement of ophiolites it is investigated by means of numerical modelling whether the interaction between young oceanic lithosphere and a subduction zone can lead to the breakup of a young and thin oceanic plate first a thermal model of the upper part of a subduction zone the region between the trench and the volcanic arc is presented temperatures are modelled for situations in which the age of the subducting oceanic lithosphere gradually decreases culminating in the arrival of a spreading ridge at the trench temperatures are used to infer the strength of material within the subducting plate from a pressure and temperature dependent rheology next it is investigated by means of finite element modelling whether the forces acting upon young subducting oceanic lithosphere recently created at a spreading center in the vicinity of a subduction zone can lead to its breakup our modelling results show that very young oceanic lithosphere may indeed break up during the early phase of its subduction whether breakup occurs depends upon the spreading velocity and the length of the ridge segment that interacts with the trench if breakup occurs some young oceanic lithosphere situated in the upper part of the subduction zone and in the region between the ridge and the trench will be detached from the other parts of the subducting plate the detached sheet of thin oceanic lithosphere will not be subducted to mantle depths instead subduction of the trailing plate at the opposite side of the ridge can lead to the incorporation of this sheet into the forearc region our modelling provides a mechanism to incorporate thin oceanic lithosphere into an arc trench region and eventually upon closure of the ocean basin into an orogenic belt it is shown that a large subset of ophiolites those with a harzburgite dominated mantle section has properties in good agreement with an emplacement history in which the breakup of young oceanic lithosphere in the upper part of a subduction zone is the first phase

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

on melting of the subducted oceanic crust effects of subduction induced mantle flow abstract based on petrological and geochemical arguments it is possible that arc magma is derived from subducted oceanic crust in this paper regional thermal models have been constructed to study the feasibility of melting cold subducted oceanic crusts at shallow depth i e at depths of about 100 km by a dynamic mantle calculated results suggest that plate subduction will generate an induced flow in the wedge above the subducting slab this current continuously feeds hot mantle material into the corner and onto the slab surface a high temperature thermal environment can be maintained in the vicinity of the wedge corner immediately beneath the over riding plate our regional models further demonstrate quantitatively that production of local melting is possible just about 30 km down dip from the asthenosphere wedge corner additional geological processes such as reasonable amounts of shear heating and minor dehydration which will lower the local melting temperature will further increase the probability of melting a cold subducted oceanic crust at shallow depth

ultra high pressure uhp terrains lessons from thermal modeling the thermal history of subducted exhumed crustal fragments is explored to depths of 125 km using a forward modeling approach the finite element code uses an adaptive gridding technique that allows deformation and differential movement of subgrids to simulate tectonic mass flow the geometry and velocity of plate convergence and the large scale strain partitioning for example subduction channel mobile tectonic fragments are described explicitly as functions of time the evolution of metamorphic conditions during descent and subsequent exhumation of a terrane within the subduction channel is computed for different kinematic scenarios these include a range of velocities relative to a stable continental block for the subducting slab the exhuming uhp fragment and a subduction channel of given width though essentially generic in emphasis our models are designed to help understand real uhp occurrences most especially their exhumation history boundary conditions and the geometric set up were chosen according to three case studies of known uhp terranes eastern china western norway western alps simulations indicate that the temperature reached at a given depth during the subduction stage is crucially dependent on the convergence rate width of the subduction channel location of the fragment within the subduction channel and on rates of shear heating and convection of the subcontinental mantle the p t t path experienced by a tectonic fragment during its exhumation from pmax is determined chiefly by its size thickness and exhumation velocity the rate of ongoing subduction is less relevant given the rapid exhumation required to preserve uhp assemblages the depth at which tmax is reached by an exhuming uhp fragment and the dp dt slope of its exhumation path may critically affect the extent of irreversible phase transitions during decompression comparing data for the three uhp case studies with corresponding simulation results indicates general agreement of the p t paths predicted t t evolution paths are also compatible with well constrained mineral chronometry from the study areas this agreement between field data and simulation results leads to a number of observables that may be useful in relating uhp occurrences to their evolution for example no metamorphic field gradients are expected for fragments of less than 5 km thickness and the spread in mineral ages reflecting the high pressure stage is predicted to be less than 1 ma therefore observed variations in metamorphic conditions for the stage near tmax and p tmax may reflect real differences of evolution in very large fragments only and the same is valid for a substantial spread in metamorphic age data pertaining to stages near pmax or tmax tectonic extrusion of fragments along a subduction channel is not the only mechanism proposed to explain uhp occurrences however the broad agreement between several test cases and the present simulation results lend support to the viability of the kinematic scenario adopted where field evidence indicates a similar emplacement mechanism and allows the initial size of the uhp fragment to be inferred the melonpit model provides a powerful integrative tool to produce a quantitative picture of the dynamics of uhp terranes

significance of slab derived partial melts and aqueous fluids for the genesis of tholeiitic and calc alkaline island arc basalts evidence from mt arayat philippines abstract mt arayat an inactive stratovolcano is located behind the volcanic front of the bataan segment of the taiwan luzon arc in the central valley of luzon the philippines volcanism in this arc segment results from subduction of south china sea basin crust along the manila trench eruption products of mt arayat comprise pyroclastics and porphyritic basalt and andesite lavas of high k calc alkaline affinity compared to typical oceanic island arc volcanics the basalts display slightly elevated abundances of ta and nb relative to zr and hf and of ta nb zr hf and light rare earth elements lree s relative to y and heavy rare earths hree s their ree distribution suggests mixing between high hree low lree hree and low hree high lree hree components the latter suggesting the presence of residual garnet in the source region of mt arayat basalts enrichment of lree s relative to hree s is typical of calc alkaline and high k calc alkaline island arc basalts but is usually missing or small in island arc tholeiites lree enrichment is often attributed to a metasomatizing aqueous fluid generated during dehydration of the subducted slab for this model to be applicable extremely low abundances in the fluid of agents like f oh and or co2 3 which form hard complexes with the ree s are a prerequisite due to the temperature dependence of the eu 3 eu 2 redox potential divalent eu predominates in aqueous fluids of low complex forming capacity in the subduction zone environment whereas the other ree s with the possible exception of yb are still trivalent a lree enriched aqueous fluid generated by slab dehydration should therfore display a positive eu anomaly however lree enriched island arc basalts generally do not show positive eu anomalies implying that this lree enrichment is unlikely to result from ree transfer by an aqueous fluid instead a model is favoured which assumes contributions from both slab derived aqueous fluids and partial melts in the source region for island arc tholeiites the mantle wedge is only affected by a lile enriched aqueous fluid whereas the source region for calc alkaline and high k calc alkaline basalts is additionally metasomatized by small volumes of melt generated by small degrees of partial melting of the subducted slab with garnet as a residual phase these lree enriched partial melts may react with the depleted peridotite of the mantle wedge to produce domains of refertilized garnet peridotite subsequent partial melting produces calc alkaline melts whose trace element compositions display signatures of both depleted peridotite and more fertile garnet peridotite

modeling cross shore sandbar behavior on the timescale of weeks we compare predictions of a coupled wave averaged cross shore waves currents bathymetric evolution model to observations of onshore and offshore nearshore sandbar migration the observations span a 10 and 44 day period with onshore offshore bar migration at duck north carolina and at hasaki kashima coast japan respectively a 3 5 month period of onshore bar migration at duck and a 22 day period of offshore bar migration at egmond netherlands with best fit parameter values the modeled temporal evolution of the cross shore bed profiles agrees well with the observations model skill defined as 1 minus the ratio of prediction to no change error variances ranges from 0 50 at egmond to 0 88 for the prolonged onshore bar migration at duck localized in time and space reductions in model skill coincide with alongshore variations in the observed morphology consistent with earlier observations simulated offshore bar migration takes place during storms when large waves break on the bar and is due to the feedback between waves undertow suspended sediment transport and the sandbar simulated onshore bar migration is predicted for energetic weakly to nonbreaking conditions and is due to the feedback between near bed wave skewness bedload transport and the sandbar with negligible to small effects of bound infragravity waves and near bed streaming under small waves and conditions when breaking and nonbreaking conditions alternate with the tide the sandbar is predicted to remain stationary the intersite differences in the optimum parameter values are at least partly induced by insensitivity to parameter variations parameter interdependence and errors in the offshore wave forcing

evolution of the downgoing lithosphere and the mechanisms of deep focus earthquakes summary the thermal evolution of the lithospheric slab at subduction zones and its geophysical effects are numerically calculated an alternating direction implicit finite difference scheme is used to compute the thermal models taking into account all heating sources and phase boundaries these models with the appropriate spreading rates and dip angles are compared with different island arc systems temperatures inside the slab are strongly controlled by the conductivity and by the time elapsed since the initiation of descent the depth to which temperature anomalies persist is generally about 700 km or less the thermal results are used to construct the seismic velocities and ray paths density anomalies and the resulting stress distribution comparing the theoretical stress distribution and the focal mechanism studies of intermediate and deep focus earthquakes indicates the importance of both the mantle s rheology and the temperature dependence of the slab s elastic properties the intermediate and deep focus earthquakes are located along the coolest region of the slab the theoretical results explain the source mechanisms and the orientation of principal stresses under major island arcs

the thermal regime of a three dimensional subducting lithospheric slab and its electromagnetic response a numerical model abstract a three dimensional subducting lithospheric slab is simulated by a numerical technique and its temperature regime investigated a single slab model or models with multiple slab sections can be considered an example is shown of two slab sections that dip at different angles the near surface heat flow above the slab is considered and compared with a two dimensional model the temperature changes of particular points within the slab are monitored as it subducts and compared with the two dimensional case considerable differences exist between the two and three dimensional cases depending on the location of the point considered an electromagnetic model is derived from the thermal model and the perturbations of time varying electromagnetic fields caused by the electrical resistivity variations associated with the temperature variations are investigated

numerical predictability experiments of cross shore sandbar migration 1 surf zone sandbars common features along the world s sandy coastlines continuously change their position in response to time variable offshore wave conditions process based predictions of cross shore sandbar migration relevant to the understanding of autonomous and artificially altered evolution of beaches are intrinsically imprecise because of uncertainty in the model equations and potentially the sensitive dependence on the initial bathymetry however the magnitude of the resulting predictability limit and its dominant source are unknown here we show that cross shore sandbar migration on the time scale of years is deterministically forced rather than deterministically chaotic and that the unpredictability of sandbar migration results primarily from model inadequacy during major wave events because the unpredictability of sandbar migration is related to the stochastic nature of the forcing the predictability limit is not a fixed value but depends on the timing of the wave event we anticipate that detailed experiments to understand nearshore evolution from the underlying first principles will eventually pay off by extending the range of plausible simulated behavior and consequently will increase our ability to understand and predict how coasts may respond to changing climate

effective magnetic susceptibility of the oceanic upper mantle derived from magsat data passive ocean continent boundaries have little magnetic signatures at magsat altitude 400 km whereas subducting oceanic lithosphere has well defined magnetic anomalies over some of the subduction zones based on this observation and a thermal evolution model of the oceanic lithosphere subducted along the kuril kamchatka trench we determined an effective magnetic susceptibility of 0 0085 si for the upper 35 km of the oceanic upper mantle

a behavioral template beach profile model for predicting seasonal to interannual shoreline evolution 1 this contribution presents a simple behavioral template model for beach profile evolution which is calibrated and tested against a 6 year time series of shoreline position derived from a coastal imaging system at the gold coast australia from a coastal management perspective the position of the shoreline is arguably one of the most important morphodynamic parameters to be monitored and predicted and therefore forms the focus of this paper the template model attempts to encapsulate some of the current understanding of coastal dynamics deduced from both scientific investigations and observations the model framework is based on a simple relaxation model whereby the profile approaches a template exponentially with time under conditions of constant forcing the template evolves dynamically in response to incident wave forcing such that when a threshold value of the dimensionless fall velocity hs wtp where hs offshore significant wave height tp peak wave period and w sediment fall velocity is exceeded the beach face erodes and flattens and the shoreline retreats landward forming an erosional scarp and a breakpoint bar conversely when the falls below threshold progradation and steepening of the shoreface ensues accompanied by the evolution of a swash bar and breakpoint step the gold coast data are used to calibrate the threshold value of 4 0 which is close to the time series mean 3 8 testing the calibrated model on unseen data shows that it is able to reproduce the dominant seasonal interannual shoreline succession observed at this site and up to 77 of the detrended shoreline variability

plio pleistocene arc magmatism controlled by two overlapping subducted plates central japan abstract the plio pleistocene volcanoes of the ryohaku hida mountains of central japan have been dated using the k ar method and newly determined ages of 65 lava flows and two pyroclastic flow deposits are presented on the basis of temporal and spatial distribution and the nature of the lavas the volcanism of the area has been split into four stages 1 an early eruptive stage 5 0 2 7 ma of the e w trending kuzuryu volcanic chain of stratovolcanoes 2 the ueno basalts and genetically related rocks which formed monogenetic volcanoes and large scale rhyolite pyroclastic flows during the 2 7 1 6 3 a late eruptive stage 1 5 0 8 ma of the kuzuryu volcanic chain and 4 n s trending andesite dacitic volcanism of the hakusan 0 6 0 ma and the norikura volcanic chains 0 9 0 ma this volcanic evolution was controlled by the relationship between the two overlapping slabs of the philippine sea and the pacific plates which are subducting beneath the central japan arc the kuzuryu volcanic chain with its e w trend is caused by magma generation on the northward subducted slab of the philippine sea plate and the n s trending volcanic chains are caused by magma generated on the westward subducted slab of the pacific plate the change of motional direction of the philippine sea plate from northwards to west northwestwards 4 million years ago caused the disappearance of the subducted slab of the philippine sea plate which had obstructed the ascent of magmas derived from the underlying subducted slab of the pacific plate during the change in direction of movement of the philippine sea plate there was a temporary change in the stress field from compression to extension during which the second stage of volcanism consisting of monogenetic basaltic and rhyolitic volcanoes occurred

new geophysical insight into the origin of the denali volcanic gap summary volcanic gaps are segments of subduction zones that lack the volcanic activity usually found at these convergent margins they are regions where the necessary conditions to produce melt may appear favourable but where volcanoes are surprisingly absent from the surface in this study we present a new model that can explain the occurrence of such volcanic gaps it is based on seismic imaging and geodynamic modelling of the denali volcanic gap a 400 km wide region at the eastern end of the alaska aleutian subduction zone here the thick crust of the pacific plate and yakutat terrane subduct at shallow angle beneath north america a high resolution seismic profile clearly images the subducting crust undergoing progressive dehydration between 50 and 120 km depth and a negative subhorizontal velocity contrast at 60 km depth in the overlying mantle wedge we interpret this 60 km discontinuity as marking the top of a layer of partial melt that pools at the base of the overriding plate in steadystate subduction models melt accumulates at the apex of a vaulted mantle wedge the pinch zone from where it may break through the overlying lithosphere to the surface beneath the denali volcanic gap the pinch zone is absent or greatly reduced because shallow subduction of the yakutat terrane progressively cools the system and causes the slab to advance and replace the hot core of the mantle wedge this regime can be seen as the opposite of subduction rollback it prevents the formation of a pinch zone reduces the length of the melting column and causes melt to pool at the base of the overriding plate thus inhibiting magma generation and extraction

regional scale tectonic evolution and the seismic velocity structure of the lithosphere and upper mantle the mediterranean region we present a numerical modeling approach designed for the verification of tectonic reconstructions and for a quantitative analysis of the seismic velocity structure of the lower lithosphere and upper mantle in this forward modeling procedure we use published reconstructions of the tectonic evolution of a region to predict the thermal structure from which we calculate physical quantities like p wave velocity basin subsidence and basement heat flow we show an application of this approach to two reconstructions of the late mesozoic and cenozoic evolution of the mediterranean region the resulting synthetic velocity models are tested against seismological models of the mantle structure obtained independently by delay time tomography subsidence and heat flow predictions are compared with actual observations to further constrain the geological assumptions underlying the forward models the mantle models predicted from reconstructions of the evolution of the mediterranean region correlate with major features in the tomographic images from our model results we conclude that the present distribution of seismic velocities in the mediterranean region is largely determined by the tectonic evolution during at least the last 40 m y this means we can verify processes represented in different tectonic scenarios with the seismologically determined velocity structure seen from a different perspective the model results contribute to understanding the causes of the velocity structure imaged by delay time tomography by showing how the tectonic evolution results in this structure

on the dip angle of subducting plates abstract an approximate analytic model to study the thermal structure of a subducting plate with finite length has been developed the model is able to take into consideration the effects of a moving front edge and adiabatic compression of slab material based on this model torque balance upon a descending slab is examined and the resultant implications to the evolution of slab dip angles are investigated thermal structure within a descending slab of finite length is found to be similar to that predicted by a steady state assumption except near the slab tip the effect of the leading edge is felt only within a distance equivalent to the slab thickness away from this region slab temperature is largely dependent on the subduction velocity the distance from the surface and the initial temperature profile of the plate on the basis of this analytic thermal model gravitational torques acting on subducting slabs can be evaluated for a complete range of dip angles a comparison with field observations suggests that short slabs i e slabs with penetration depth 400 km are likely under torque equilibrium at present for slabs that have penetrated the olivine spinel phase boundary at about 400 km depth the increase of gravitational torque due to the upward deformation of the phase boundary forces the slabs to move away from equilibrium as a result long slabs are speculated to be transient at present rotating towards steeper dip angles this transient behavior is likely to cause the mantle wedge to be pulled downward and may explain the observed deeper bathymetry within many back arc basins relative to normal sea floors of the same age

bar generating cross shore flow mechanisms on a beach random waves normally incident on a dissipative beach induce a variety of cross shore flows such as asymmetric oscillatory flow wave grouping induced long wave flow breaking induced turbulent flow and momentum decay induced undertow these flows are identified analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two dimensional case of bar generation on a beach

three dimensional numerical modeling of earthquake migration along a northwestern pacific subduction slab three dimensional numerical modeling has been performed to account for possible earthquake migration phenomena along a northwestern pacific subduction slab on the assumption that the lithosphere is a linear visco elastic body but the asthenosphere and the mesosphere respond to the applied stress as non linear newtonian materials obeying the power law of creep on the basis of a three dimensional thermal structure of the northwestern pacific region the preliminarily computed stress regimes show that 1 to induce a stick slip type trench event prefailures at a depth of about 40 km in the front of the locked zone between two lithospheric plates are required 2 the compositional density contrast due to phase change at typical depths is prerequisite of stress concentration to cause a dislocation type deep focus earthquake and 3 the co seismic change in gravitational potential is a controlling factor for earthquake migration downward along the subduction slab

on cross shore migration and equilibrium states of nearshore sandbars 1 the location of submerged sandbars in the nearshore zone changes over time in response to variability in wave conditions here cross shore sandbar migration is studied with an empirical model consisting of a differential equation relating cross shore sandbar migration to wave forcing model parameters are fitted to data sets containing multiple years of daily observed positions of five sandbars at three field sites from the fitted model parameters we determine the cross shore location of zero migration equilibrium location the stability of this equilibrium and the rate response time at which a sandbar migrates toward or away from the equilibrium location we find that for breaking waves a sandbar moves toward a stable wave height dependent equilibrium location for non to slightly breaking waves however a sandbar moves away from the equilibrium location implying that the beach profile evolves toward a state without submerged sandbars the response times can differ up to a factor three for different sandbars at a particular field site and a factor ten between sandbars at different sites for breaking waves response times are found to decrease with increasing wave height from months to days while for non to slightly breaking waves response times exceed several months in general response times remain larger than the timescale of variability in wave climate several days implying that sandbars spend most of their lifetime out of equilibrium with the wave forcing the model is able to hindcast the relevant features of cross shore sandbar migration on timescales of days to several years and outperforms a baseline prediction of no change for all sandbars when the equilibrium location is reached by both the hindcasted and actual sandbar the hindcast error depends on the accuracy of the predicted equilibrium location only and no longer on the history of accumulated errors as a result sandbars that occasionally reach their equilibrium location can accurately be predicted from the wave forcings over their entire lifespan however it is difficult to predict the exact rate of the yearly to interannual trends that are observed at two field sites because the sandbars at these sites never reach the offshore located equilibrium states associated with high waves

quantification of nearshore morphology based on video imaging abstract the argus network is a series of video cameras with aerial views of beaches around the world intensity contrasts in time exposure images reveal areas of preferential breaking which are closely tied to underlying bed morphology this relationship was further investigated including the effect of tidal elevation and wave height on the presence of wave breaking and its cross shore position over sand bars computerized methods of objectively extracting shoreline and sand bar locations were developed allowing the vast quantity of data generated by argus to be more effectively examined once features were identified in the images daily alongshore mean values were taken to create time series of shoreline and sand bar location which were analyzed for annual cycles and cross correlated with wave data to investigate environmental forcing and response these data extraction techniques were applied to images from four of the argus camera sites a relationship between wave height and shoreline location was found in which increased wave heights resulted in more landward shoreline positions given the short lag times over which this correlation was significant and that the strong annual signal in wave height was not replicated in the shoreline time series it is likely that this relationship is a result of set up during periods of large waves wave height was also found to have an effect on sand bar location whereby an increase in wave height resulted in offshore bar migration this correlation was significant over much longer time lags than the relationship between wave height and shoreline location and a strong annual signal was found in the location of almost all observed bars indicating that the sand bars are migrating with changes in wave height in the case of the site with multiple sand bars the offshore bars responded more significantly to changes in wave height whereas the innermost bar seemed to be shielded from incident wave energy by breaking over the other bars a relationship was also found between a site s mean wave height and inner sand bar location sites with the highest wave heights tended to have sand bars farther from shore than those with relatively low wave heights

buoyancy rather than rheology controls the thickness of the overriding mechanical lithosphere at subduction zones the thickness of earth s mechanical lithosphere is poorly defined to investigate whether rheology controls the thickness of the overriding plate s mechanical lithosphere in subduction zones the thermal structure was modelled numerically assuming a temperature dependent mantle viscosity it was found that the overriding lithosphere was ablated such that very high temperatures reached close to the surface near the apex of the wedge corner leading to unrealistically high heat flow since temperature dependent rheology clearly does not control the thickness of the mechanical lithosphere we suggest that it is instead controlled by buoyancy the source of buoyancy we assume is compositional e g buoyant crust two end member models with crustal thickness of 10 and 70 km respectively were then undertaken these had lower heat flow this work supports the assumption of some earlier workers e g plank and langmuir 1988 who equated the mechanical lithosphere with the crust of the overriding plate

application of fluid mechanic principles to the study of geodynamic processes at trench arc back arc systems geological processes at trench arc back arc systems are some of the most complex tectonic processes in need of study a large amount of data based on geological and geophysical observations has been accumulated to synthesize these data mathematical models have proven to be very useful because geological processes are directly related to the dynamical behavior of the solid earth many of them can be investigated in terms of fluid flow models some applications of fluid flow principles in studying tectonic processes at convergent plate boundaries are discussed in this paper when mantle processes are modeled as convective systems they are found to have direct implications for the determination of slab dip angles additionally they can also account for the high heat flows in the back arc basins provide a mechanism for back arc opening and resolve the question whether subducted oceanic crust can reach melting at shallow depth for island arc magma generation besides mantle processes flow models can also be used to study surface processes a simple one parameter plane flow theory is used to model the evolution of trench geometries this model is able to fit simulataneously the trench curvature and the differential paleomagnetic rotation between volcanic islands for the mariana despite the simplicity of many of these models their ability to synthesize geological and geophysical data at convergent plate boundaries is quite remarkable

remote response of the east asian winter monsoon to tropical forcing related to el nino southern oscillation 1 the mechanism of the east asian winter monsoon variability in response to el nino southern oscillation enso related tropical forcing is investigated using japanese long term reanalysis project data additionally aided by the japan meteorological agency climate data assimilation system there are at least two different responses zonally symmetric and asymmetric of the asian jet over south asia to the enso related tropical convective forcing during the northern hemisphere winter the zonally symmetric response induced by zonally extended anomalous convection from the philippine sea through southern india and sri lanka is pronounced at the mature phase of enso the zonally asymmetric response is intimately associated with anomalous convection localized in the vicinity of the philippine and south china seas accompanied by an anomalous walker circulation cell between the maritime continent and tropical indian ocean when this asymmetric response is prominent enso related anomalous convection can give rise to a change in the east asian winter monsoon system through stationary rossby wave propagation along the south asian waveguide the north atlantic oscillation nao related extratropical forcing is also a crucial factor and contributes not only to the downstream development of subpolar teleconnections across northern eurasia but also to the reinforcement of the zonally asymmetric pattern of the asian jet over south asia resulting in a significant effect on the east asian winter monsoon circulation a combination of the enso and nao related forcing plays a vital role in triggering the occurrence of extraordinary anomalous monsoon circulations such as extremely heavy snowfall in the 2005 2006 winter in japan

fluctuation of the seasonal snowpack in a mountainous area of the heavy snow district in the warm temperate zone of japan the seasonal snowpack and meteorological factors associated with the accumulation and ablation of the snowpack were monitored for 11 years in a mountainous area in the warm temperate zone of japan no notable rise was observed in mean wintertime air temperature but an increase was seen in the difference between the maximum and minimum air temperatures precipitation exhibited annual variability but no notable reduction over the measurement period the length of the continuous snow cover period increased slightly over the 11 years but no trend in variability was observed the maximum snow depth and maximum water equivalent of snow varied greatly from year to year depending on the amount of snowfall in a heavy snow year about 1600 mm of water which is almost the mean annual precipitation for the whole of japan was found to be temporarily stored in the snowpack

medium term variations of bar properties and their linkages with environmental factors at hasaki japan beach profile data obtained every weekday for 15 years along a 427 meter long pier were analysed by complex empirical orthogonal function analysis the seaward bar migration was represented by the first mode and the migration frequency and the temporal variation of the bar amplitude were estimated with the complex time coefficient of the first mode although the seaward bar migration frequency which is the reciprocal of the duration time of the bar migration cycle was relatively constant before 1993 and ranged between 1 3 to 0 8 cycles year 0 7 to 1 25 years cycle it gradually decreased after 1994 and reached 0 1 cycle year 10 years cycle at the end of 1996 subsequently it gradually recovered to the frequency before 1993 the bar amplitude had a similar long period fluctuation it was large before 1993 but small from 1995 to 1997 and then returned to the pre 1993 value the bar migration frequency was weakly correlated with the bar amplitude and the offshore wave energy flux

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

on the origin of high alumina arc basalt and the mechanics of melt extraction most models of high alumina arc basalt petrogenesis rely heavily on the supposition that the abundances of certain trace elements in particular the relatively unfractionated rare earth element ree patterns and the unusually high concentrations of k rb sr and ba are incompatible with a garnet bearing subducted oceanic crustal quartz eclogite source rock we have carefully examined this apparently unequivocal evidence in light of recent progress on the physics of melt extraction and the heat transfer and mechanics of magma ascent the weakest element of all trace element models involving a quartz eclogite source is the assumption that the element concentrations are fixed at the source and only later modified in the near surface environment we expand on such models by monitoring the concentrations of r ee and major and trace elements during initial melting ascent and extraction of magma this is done by combining calculated cooling curves for ascending magmatic bodies with high pressure phase equilibria the amount that each phase contributes to the melt is monitored along with the composition of the melt and residual solids with quartz eclogite initial melting initiates gravitational instability of the entire source material melt plus solids before melt extraction can occur during ascent of this mush melting increases until the solids can be repacked to free the melt this extraction takes place some 15 20 km above the slab after about 50 per cent melting at which point the melt has a pattern of ree and other trace element concentrations almost identical to those observed in high alumina arc basalts assuming an initial composition equivalent to altered oceanic crust plus 5 per cent pelagic sediment sr abundances are the only ones which are not well matched by this process the major element concentrations of the extracted melt also closely match those of high alumina arc basalt a similar but less detailed evaluation of both fertile and depleted peridotite source rocks yields good agreement for the ree and other trace element concentrations assuming a lree enriched source rock strongly enriched in k rb sr and ba ni cr and co abundances are satisfied only through substantial low pressure fractionation of mafic phases in particular olivine though not rigorously tested such a process may be compatible with the observed major element concentrations of high alumina basalt however the experimentally verified fact that high alumina basalts could never have been in equilibrium with either an olivine beari ng magma or source rock eliminates this possibility altogether thus the simultaneous consideration of the mechanics of ascent and melt extraction along with phase equilibria clearly shows that partial melting of quartz eclogite best satisfies the chemical constraints of major trace and ree characteristics of high alumina arc basalts

short term changes in the morphodynamic states of beaches and surf zones an empirical predictive model a time series of 6 5 years of daily observations of beach states and wave characteristics from southeastern australia was analyzed with the aim of determining the degree to which time varying beach state can be explained and predicted the six commonly occurring beach states dissipative longshore bar trough rhythmic bar and beach transverse bar and rip low tide terrace and reflective discussed earlier were related by means of discrete discriminant analyses to the parameter hb wst where hb is breaker height ws is the fall velocity of the beach sand and t is peak breaker period using a total of 1545 cases two values were used in the analyses the immediate value occurring on the day the particular state was observed and a weighted mean value expressing recently antecedent conditions the immediate value made a negligible contribution to explaining day to day beach state observations however the antecedent conditions showed a strong relationship and provide a successful means of prediction by examining cases where the time derivatives of both state and were near zero it was possible to define the equilibrium conditions associated with each state directions of change erosional or accretionary are predicted in terms of departures of from the equilibrium value appropriate to the beach state prevailing at the time change begins

a possible mechanism for the thermal asymmetry of the ligurian basin we propose a thermo mechanical model and a new interpretation of heat flux data for the ligurian basin that may be a key to understanding the evolution of the nw mediterranean the model incorporates the removal of a portion of mantle lithosphere to explain the heat flux and subsidence anomaly of the eastern corsican margin of the basin this process is envisaged as a result of eastward asthenosphere flow induced by the apennines subduction system after a heating phase time dependent conductive cooling and re thickening of the lithosphere result in re equilibrium of the thermal gradient to its initial value such a rifting mode can account for the asymmetric heat flux and subsidence pattern observed across the basin and the present day lithospheric thickness