coupling mechanisms in double sandbar systems part 1 patterns and physical explanation crescentic sandbars and rip channels along wave dominated sandy beaches are relevant to understand localized beach and dune erosion during storms in recent years a paradigm shift from hydrodynamic template models to self organization mechanisms occurred to explain the formation of these rhythmic features in double sandbar systems both the inner and outer bar rip channels and crescentic planshapes are now believed to be free instabilities of the nearshore system arising through self organization mechanisms alone however the occasional occurrence of one or two inner bar rip channels within one outer bar crescent suggests a forced morphologically coupled origin here we use a nonlinear morphodynamic model to show that alongshore variability in outer bar depth and the relative importance of wave breaking versus wave focussing by refraction across the outer bar is crucial to the inner bar rip channel development the coupling patterns simulated by our model are similar to those observed in the field morphological coupling requires a template in the morphology outer bar geometry which through the positive feedback between flow sediment transport and the evolving morphology that is self organization enforces the development of coupling patterns we therefore introduce a novel mechanism that blurs the distinction between self organization and template mechanisms this mechanism may also be extended to explain the dynamics of other nearshore patterns such as beach cusps the impact of this novel mechanism on the alongshore variability of inner bar rip channels is investigated in the companion paper copyright 2010 john wiley and sons ltd

sediment size effects on rip channel dynamics abstract field observations indicate that both wave conditions and sediment characteristics exert the controlling effects on the beach states and determine the development of rip channel system although the role of wave conditions on the rip channel dynamics has been extensively studied little attention has been paid to the effects of sediment characteristics in this paper we use a nonlinear morphodynamic model xbeach to investigate the impact of grain size on the formation and nonlinear evolution of rip channels under the condition of uniform sediments we find that the grain size can affect a number of aspects of rip channel dynamics with increasing grain size both longshore migration speed and growth rate decrease significantly while the response and saturation times increase on the other hand the influence of grain size on the geometrical properties of the rip channel seems to be much less pronounced as the wavelength of the channels hardly changes and the three dimensionality features only vary slightly the global analysis method is applied to investigate the mechanism underlying the variations of growth rate and migration speed for varying sediment size the calculations reveal that these variations are mainly caused by the amount of sea bed sediments being stirred up which is clearly grain size dependent quantitative relationships between predicted rip channel characteristics sediment grain size and the hydrodynamic conditions are tentatively established which is amendable to improvement by extending the analysis to other or more general situations

moho signature from wide angle reflections preliminary results of the 1993 trans hudson orogen refraction experiment abstract in 1993 a combined crustal seismic refraction and wide angle reflection experiment was conducted over the northern saskatchewan and manitoba portion of the trans hudson orogen by lithoprobe the program included three seismic lines extending over 1900 km in length the exceptionally high quality data collected during the field experiment included seismic reflections and refraction events from the moho and upper lithospheric mantle the relatively high seismic fold and signal to noise ratio of the field observations made it possible to process the wide angle portion of the data set with standard reflection signal enhancement techniques the moho is revealed as a complex interface with significant structural relief 40 52 km that correlates in first order fashion with crustal geology and structure the moho topography is interpreted as a result of variable ductile reworking and eclogitization of the lower crust

models of seismic anisotropy in the deep continental lithosphere seismological observations sks wave polarizations systematic p residual variations azimuthal dependence of pn and surface wave velocities or a dispersion of surface waves are not consistent with isotropic if laterally heterogeneous upper mantle structure therefore an anisotropy should be considered as an a priori aspect of future large scale studies of mantle structure most studies of anisotropy however have assumed horizontal or vertical axes of symmetry but such orientations cannot explain bipolar patterns of spatial variations of p residuals which we have observed at many seismological stations on the basis of the petrophysical properties of real upper mantle rocks we consider anisotropy formed either by hexagonal or by orthorhombic aggregates composed of olivine orthopyroxene and clinopyroxene rotations of the aggregates about vertical and horizontal axes allow us to find the three dimensional orientations of symmetry axes that fit combinations of both p and s seismological observations in central europe and in western north america the orientations with plunging symmetry axes velocity extremes seem to be consistent across large spatially uniform tectonic units and change abruptly at important suture zones

observations of rip spacing persistence and mobility at a long straight coastline three years of daily video observations at a long straight beach were analysed to determine the temporal trends and variability of the location spacing persistence and mobility of rips rips were identified at this site on 684 days of the total 947 days 72 occurrence when suitable images were available a median number of 7 rips were observed within the total 2 km study area on days when rips were present no tendency was identified for rips to reoccur in preferred locations alongshore following storm reset events the average alongshore distance between all rips observed was 209 m but with a high standard deviation of 98 m or approximately 47 of the mean rip spacing there was no evidence at this site that rips tended to be regularly spaced alongshore no clear relationship was identified between the number of rips and the prevailing offshore wave conditions including significant wave height peak wave period and incident wave power the majority of rips persisted for 5 or fewer days with an average of 8 days and standard deviation of 9 days rips were stationary 33 of days with migration rates of less than 5 m day observed on 47 of days when rips were present maximum migrations rates of up to 50 m day were observed and generally though not always occurred in the direction consistent with the prevailing offshore swell direction and resulting alongshore current the occurrence of inshore sea breezes may have accounted for the less frequent observations of rip migration against the opposing regional swell direction the results presented at this site complement a recent four year study at a contrasting short and embayed beach detailed in holman et al holman r a symonds g thornton e b and ransinghe r 2006 rip spacing on an embayed beach journal of geophysical research 111 c01006 doi 10 1029 2005 jc002965 17p the specific mechanism s of rip channel infilling at the long straight and littoral drift dominated coastline may have contributed to the observation of shorter rip persistence relative to the embayed site mean rip spacing was observed at both sites to be highly variable irregular and exhibited no identifiable trend associated with the offshore wave conditions this latter observation supports the notion that rips may become rapidly topographically controlled soon after a storm reset event and their location is then primarily determined by the evolving nearshore morphology rather than hydrodynamic forcing this new dataset of daily multi year observations at an open coast site cannot be reconciled with the majority of existing template and instability models for rip formation that predict a relationship between incident wave conditions and regular spacing of rips alongshore rather these observations tend to support emerging theories of rip genesis that predict the irregular and random spacing of rip channels alongshore

nearshore oblique sand bars 1 the coupling between hydrodynamics and the evolving topography in the surf zone has been theoretically examined for oblique wave incidence it is shown that positive feedback can lead to the initial growth of several types of rhythmic systems of sand bars the bars can be down current oriented or up current oriented which means that the offshore end of the bar is shifted down current or up current with respect to the shore attachment in the limit of strong current compared to wave orbital motion very oblique down current oriented b ars are obtained with a spacing of several times the surf zone width when wave orbital motions are dominant systems of up current oriented bars and crescentic down current oriented bars appear with spacings of the order of the surf zone width the latter feature consists of alternating shoals and troughs at both sides of the break line with the inner shoals being bar shaped and oblique to the coast the growth e folding time of the bars ranges from a few hours to a few days and it is favored by constant wave conditions the range of model parameters leading to growth corresponds to intermediate beach states in between the fully dissipative and the fully reflective situations preliminary comparison with field observations shows qualitative agreement

structure of a paleoproterozoic continent continent collision zone a lithoprobe seismic reflection profile across the trans hudson orogen canada abstract an 800 km reflection seismic profile across the trans hudson orogen northern saskatchewan and manitoba images crustal scale tectonic imbrication in an unprecedented picture of paleoproterozoic crustal accretion and continent continent collisional tectonism the profile is crudely symmetric about a crustal scale culmination in the western part of an accreted juvenile collage reindeer zone geologic and isotopic data suggest that this culmination is cored by microcontinental archean basement west of the culmination highly reflective juvenile crustal elements dip westward into the lower crust beneath the wathaman batholith and archean continental crust of hearne craton to the east strong reflections in the juvenile reindeer zone crust and reworked archean foreland of the thompson belt have eastward dips persisting into the middle crust and extending beneath the superior craton a continuous reflection moho well defined for 500 km in the western part of the profile shows marked relief 12 15 s including a prominent root below the crustal culmination these imaged structures give evidence of substantial crustal shortening and thickening via large scale imbrication consistent with collisional orogeny w dipping structures below the wathaman batholith and reworked hearne craton may reflect subduction polarity in this part of the orogen however geological evidence suggests that e dipping structures below superior craton are largely related to late post collisional deformation rather than to prior oceanic subduction polarity

rip spacing and persistence on an embayed beach 1 four years of daily time exposure images from an embayed beach were examined to study the spacing persistence and location preferences of rips in a natural rip channel system a total of 5271 rip channels was observed on 782 days occurrence statistics showed no evidence of the preferred location pattern associated with standing edge waves trapped in an embayed beach the histogram of rip spacing the primary diagnostic observable for most models was well modeled by a lognormal distribution mean spacing of 178 m however spacings were highly longshore variable time mean of the standard deviation longshore mean of rip spacing was 39 so they are of questionable merit as a diagnostic variable storm driven resets to the longshore uniform condition required by most models occurred only four times per year on average making rip generation models relevant to only a small fraction of the system behavior rip spacings after the 15 observed reset events were uncorrelated with bar crest distance the lifetime of the 324 individual rip channel trajectories averaged 45 6 days rips were equally mobile in both longshore directions but the coefficient of variation of rip migration rates was large even for high migration rates the mean migration rate was well correlated to a proxy for the longshore current r2 of 0 78 thus there is no significant evidence that the formation spacing and migration of rip channels on this beach can be explained by currently existing simple models moreover the alongshore uniform initial conditions assumed by these models are rare on palm beach making the models generally inapplicable

cross shore mean flow in the surf zone abstract the seaward mean returnflow or undertow in the surf zone which compensates for the shoreward mass flux above the wave trough level is found to be driven by the force imbalance between the wave momentum flux on the one hand and the set up on the other hand in qualitative agreement with the model of dyhr nielsen and sorensen 1970 a model which quantifies this imbalance is shown to yield theoretical results in good accordance with experiments when the proper boundary conditions are accounted for

high angle wave instability and emergent shoreline shapes 1 modeling of sand waves flying spits and capes 1 contrary to traditional findings the deepwater angle of wave approach strongly affects plan view coastal evolution giving rise to an antidiffusional high wave angle instability for sufficiently oblique deepwater waves with angles between wave crests and the shoreline trend larger than the value that maximizes alongshore sediment transport 45 a one contour line numerical model shows that a predominance of high angle waves can cause a shoreline to self organize into regular quasiperiodic shapes similar to those found along many natural coasts at scales ranging from kilometers to hundreds of kilometers the numerical model has been updated from a previous version to include a formulation for the widening of an overly thin barrier by the process of barrier overwash which is assumed to maintain a minimum barrier width systematic analysis shows that the wave climate determines the form of coastal response for nearly symmetric wave climates small net alongshore sediment transport cuspate coasts develop that exhibit increasing relative cross shore amplitude and pointier tips as the proportion of high angle waves is increased for asymmetrical wave climates shoreline features migrate in the downdrift direction either as subtle alongshore sand waves or as offshore extending flying spits depending on the proportion of high angle waves numerical analyses further show that the rate that the alongshore scale of model features increases through merging follows a diffusional temporal scale over several orders of magnitude a rate that is insensitive to the proportion of high angle waves the proportion of high angle waves determines the offshore versus alongshore aspect ratio of self organized shoreline undulations

the influence of sea state on formation speed of alongshore variability in surf zone sand bars abstract the formation time of alongshore morphological variability in surf zone sand bars has long been known to differ from one beach to the other and from one post storm period to another here we investigate whether the type of sea state i e distant swell waves or locally generated short period wind sea affects the formation time of the emerging alongshore topographic variability a numerical modeling approach is used to examine the emergence of alongshore variability under different shore normal wave forcing a research version of delft3d operating on the time scale of wave groups is applied to a schematised bathymetry with a single bar the model is then used to investigate several wave scenarios examining the impact of peak period frequency spread and directional spread on the formation time of alongshore variability results show that an increase in wave period has a large effect changing the formation time up to o 250 in case the wave period is changed from a representative value for the dutch coast t p 5 6 s to an australian south east coast value t p 10 12 s in contrast modifications in the directional and frequency spread of the wave field result only in a minor change in the formation time examination of hydrodynamics and potential sediment transport shows that the variations in formation time are primarily related to changes in the magnitude of the time averaged flow conditions variations in the magnitude of very low frequency f f these findings indicate that the development of alongshore topographic variability may be faster at swell dominated open coasts primarily due to the occurrence of longer period swell also at a given site the arrival of a long wave period swell after a storm can accelerate the emergence of variability

petrological constraints on seismic anisotropy the purpose of this paper is to investigate the correlations between anisotropic parameters for different orientations and mineralogies for realistic mineralogical and petrological models of the upper mantle such correlations make it possible to reduce the number of independent parameters required in seismic modelling of the upper mantle the variation with depth and with degree of random orientation is also investigated it is shown that the anisotropic combinations involved in an equivalent transversely isotropic medium azumithally averaged velocities with a vertical symmetry axis are strongly correlated but the combinations involved in azimuthal anisotropy are more weakly correlated comparison with available seismic data does not yet allow discrimination based on anisotropy between different petrological models it is shown that even if a proportion of random orientation is introduced it should be possible to achieve petrological discrimination from seismic data for some orientation configurations if the inverse problem is well posed i e takes account of the correlations between anisotropic parameters these correlations are incorporated in an inversion process to show that it is possible to explain the dataset used to derive the prem by alternative anisotropic models for the uppermost 220 km

a morphological stability analysis for a long straight barred coast a morphological stability analysis is carried out for a long straight coast with a longshore bar the situation with oblique wave incidence and a wave driven longshore current is considered the flow and sediment transport are described by a numerical modelling system the models comprise i a wave model with depth refraction shoaling and wave breaking ii a depth integrated model for wave driven currents and iii a sediment transport model for the bed load transport and the suspended load transport in combined waves and current the direction of the sediment transport is taken to be parallel to the depth integrated mean current velocity neglecting the effects of a bed slope and secondary currents an instability is found to develop around the bar crest the instability is periodic in the alongshore direction and tends to form rip channels and to steepen the offshore face of the bar between the rip channels the alongshore wave length of the most unstable perturbation is determined for different combinations of the wave conditions and the geometry of the profile

shoreline response to multi functional artificial surfing reefs a numerical and physical modelling study the results of a series of 2dh numerical and 3d scaled physical modelling tests indicate that processes governing shoreline response to submerged structures such as artificial surfing reefs are different from those associated with emergent offshore breakwaters unlike the case of emergent offshore breakwaters where shoreline accretion salient development is expected under all structural environmental conditions the principal mode of shoreline response to submerged structures can vary between erosive and accretive depending on the offshore distance to the structure the predominant wave incidence angle and structure crest level also have important implications on the magnitude of shoreline response but not on the mode of shoreline response i e erosion vs accretion based on the results obtained here a predictive empirical relationship is proposed as a preliminary engineering tool to assess shoreline response to submerged structures

linear evolution of a shoreface nourishment abstract the morphological evolution of a shoreface nourishment is investigated by interpreting the nourishment as a linear perturbation of the natural system the nourishment is projected onto the subset of linear eigenmodes with negative growth rates of the morphodynamical system the evolution of these linear modes then determines the temporal behaviour of the shoreface nourishment the method is presented and results are shown for shoreface nourishments of different length scales on a straight coast and subject to normal incidence shoreface nourishments are represented by their expansions according to the projection method on a 1 50 plane beach profile all nourishments are shown primarily to be diffusive features with long scale nourishments diffusing more slowly than shorter length scale nourishments long scale nourishments also exhibit a shoreward movement during their decay this all indicates that long length scale nourishments may be more beneficial in coastal engineering projects this study is a first step towards nonlinear projection to study shoreface nourishment behaviour

physical modeling of three dimensional intermediate beach morphodynamics experiments have been performed in a large wave tank in order to study the morphodynamics of rip current systems both accretive and erosive shore normal wave conditions were applied the beach evolving through all the states within the intermediate beach classification under the so called down state accretive and up state erosive morphological transitions results show that any prescribed change in the wave conditions drastically increases the rate at which the morphology changes the surf zone morphology tends toward a steady state when running a given wave climate for a long duration we quantitatively describe a full down state sequence characterized by the progressive evolution of an alongshore uniform bar successively into a crescentic plan shape a bar and rip channel morphology and a terrace from the analysis of a large data set of dense eulerian measurements and bathymetric surveys we depict several feedback mechanisms associated with wave driven rip current circulation wave nonlinearities and the seabed evolution at first a positive feedback mechanism drives a rapid increase in the rate of morphological change beach three dimensionality and rip intensity by the time the sandbar evolves into a bar and rip morphology a negative feedback mechanism characterized by a decaying beach change rate and an increasing beach alongshore uniformity overwhelms the former mechanism an erosive sequence characterized by both an overall offshore bar migration and an increase in beach three dimensionality is also described

on the development of large scale cuspate features on a semi reflective beach carchuna beach southern spain abstract carchuna beach located on the southeast coast of spain has a series of natural permanent horns of time dependent horizontal dimensions which do not appear to be periodically spaced and do not propagate alongshore until at present it has been assumed that the permanency of these forms is related to the existence of bed rocks nevertheless analysis of the configuration of the depth contours and the bed sediment composition does not justify this hypothesis this paper explores the formation of cuspate features by the effects of the wind wave dynamics and their development by nearshore circulation and infragravity waves including edge waves three levels of wave energy flux conditions are considered in all likelihood wave refraction of severe storms on submerged fluvial valleys is the main cause for longshore variation of breaking wave height which seems to be one of the initial conditions necessary for the cuspate to initiate moderate storms enhance the deepening of the embayments whereas mild sea states are able to maintain the features by self organization the possibility of the occurrence of edge waves due to the non linear interaction between the components of a gravity wave spectrum approaching the shore under moderate storm conditions is explored preliminary analysis of video images taken by an argus station seems to confirm this hypothesis

physical properties of crustal and upper mantle rocks with regards to lithosphere dynamics and high pressure mineralogy this paper focusses on the physical properties of rocks and property changes occurring during geodynamic processes it addresses a number of outstanding petrophysical problems in the framework of crustal and mantle deformation the major themes to be discussed will be the seismic signature of crustal and mantle shear zones the nature of crustal and mantle anisotropy and the contribution of anisotropic rocks to seismic reflectivity in particular seismic shear waves traversing anisotropic strata can reveal important information on the spatial orientation of deformed structures because the measured seismic anisotropy has a structural origin emphasis will be placed on the relations between shear wave splitting and fabric anisotropy fabric anisotropy and strain and strain and geologic tectonic processes the second part of this paper discusses the effect that metamorphic phase transitions in crustal and mantle rocks may have on rock physical properties velocity density specifically the possible role that the gabbro eclogite transition could have in crustal delamination and recycling of material into the mantle will be considered

generation and nonlinear evolution of shore oblique transverse sand bars the coupling between topography waves and currents in the surf zone may self organize to produce the formation of shore transverse or shore oblique sand bars on an otherwise alongshore uniform beach in the absence of shore parallel bars this has been shown by previous studies of linear stability analysis but is now extended to the finite amplitude regime to this end a nonlinear model coupling wave transformation and breaking a shallow water equations solver sediment transport and bed updating is developed the sediment flux consists of a stirring factor multiplied by the depth averaged current plus a downslope correction it is found that the cross shore profile of the ratio of stirring factor to water depth together with the wave incidence angle primarily determine the shape and the type of bars either transverse or oblique to the shore in the latter case they can open an acute angle against the current up current oriented or with the current down current oriented at the initial stages of development both the intensity of the instability which is responsible for the formation of the bars and the damping due to downslope transport grow at a similar rate with bar amplitude the former being somewhat stronger as bars keep on growing their finite amplitude shape either enhances downslope transport or weakens the instability mechanism so that an equilibrium between both opposing tendencies occurs leading to a final saturated amplitude the overall shape of the saturated bars in plan view is similar to that of the small amplitude ones however the final spacings may be up to a factor of 2 larger and final celerities can also be about a factor of 2 smaller or larger in the case of alongshore migrating bars the asymmetry of the longshore sections the lee being steeper than the stoss is well reproduced complex dynamics with merging and splitting of individual bars sometimes occur finally in the case of shore normal incidence the rip currents in the troughs between the bars are jet like while the onshore return flow is wider and weaker as is observed in nature

a mechanism for the generation of wave driven rhythmic patterns in the surf zone the coupling between topographic irregularities and wave driven mean water motion in the surf zone is examined this coupling occurs because the topographic perturbations produce excess gradients in the wave radiation stress that cause a steady circulation this circulation in turn creates a sediment transport pattern that can reinforce the bottom disturbance and may thereby lead to the growth of large scale bed forms to investigate this coupling mechanism the linearized stability problem with an originally plane sloping beach and normal wave incidence is solved in two different cases first the breaking line is considered to be fixed and second the perturbations in water depth that produce a displacement of the breaker line are accounted for the first case shows that the basic topography can be unstable with respect to two different modes a giant cusp pattern with shore attached transverse bars that extend across the whole surf zone and a crescentic pattern with alternate shoals and pools at both sides of the breaking line showing a mirroring effect in the second case the varying breaker line may have a strong influence on the circulation this is clear for the giant cusp topography whose growth is totally inhibited in contrast the morphology and the growth of the crescentic pattern remains almost unchanged

dynamics of a nearshore bar system in the northern adriatic a video based morphological classification abstract the aim of this paper is to define a simplified morphodynamic classification suitable for low energy beaches exposed to microtidal conditions the study site is located in the northern adriatic in italy it is an almost 2 km long rectilinear beach bordered at the northern edge by coastal structures and at the southern end by a small river inlet the mechanisms related to the evolution of the submerged part of the beach were derived from video monitoring using argus technology the morphodynamic evolution of the system was studied using an automatic procedure on images for the characterisation of nearshore bars that showed good correspondence with hand based visual interpretation to apply this automatic procedure the bar s plan crest shape was mapped using cross shore pixel luminosity transects traced on time averaged video images a careful sensitivity analysis was undertaken to determine the best spacing between transects for the correct tracing of the shape of the bar crest the error associated to a transect spacing every 25 m resulted in being comparable with the pixel resolution in the area and with the error found comparing the video interpretation with bathymetric surveys from the study of a four and a half year dataset february 2003 may 2007 the submerged beach was found to be characterised by the presence of a single bar in the area next to coastal protection structures however moving southwards of these inner and outer bars were present the morphodynamics of the outer bar and its plan shape modifications were dominated by rhythmic forms occasionally after high energy events the bar became rectilinear but during the following lower energy periods rhythmicity was re established supporting the hypothesis of self organization mechanisms the cross shore position of the bar s crests only showed limited cross shore mobility through time

crustal refraction surveys across the trans hudson orogen williston basin of south central canada eight reversed seismic refraction profiles have been recorded across the phanerozoic williston sedimentary basin that overlies the margins of the superior and wyoming archean cratons and the proterozoic trans hudson orogen these data have been modeled using two dimensional ray tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals within the southern extension of the superior craton margin thompson belt a high velocity 6 4 km s lid overlies a narrow region of high electrical conductivity and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge thompson fault of the craton margin complex zones of low velocity material and the north american central plains electrical conductivity anomaly are concluded to lie within the southern extension of the reindeer south indian lakes terrain a component of the trans hudson orogen a high velocity 7 0 km s lower crustal layer which represents a zone of complexity separating normal crust from the upper mantle extends throughout the region the crust is relatively thick with depths to moho varying smoothly between 41 and 48 km there is no evidence in the relatively low resolution seismic refraction data for abrupt changes in the level of the moho but there may be a minor 3 4 km thinning of the crust near the center of the williston basin

quantification of sand bar morphology a video technique based on wave dissipation a technique is presented to remotely measure the scales and morphology of natural sand bars based on the preferential dissipation of wind waves and swell over the crests of the bar photographic or video images are recorded and statistical uncertainties associated with incident wave height modulations removed by averaging time exposures ground truth testing of the technique was carried out as part of the superduck experiment in october 1986 the time exposures generally provided a good mapping of underlying morphology allowing detection of the bar and determination of cross shore and longshore length scales however during high waves persistent surface foam obscures the relationship of image intensity to local dissipation modeled theoretically by dissipation of a random wave field and an enhancement technique of image differencing must be done to remove the bias errors in the estimate of bar crest distance from the shoreline are generally less than 35 but this value depends on the geometry of the particular bar logistic simplicity and quantitative capabilities make this technique very attractive

on the impact of an offshore bathymetric anomaly on surf zone rip channels 1 we use a nonlinear morphodynamic model to demonstrate that the presence of a single persistent offshore bathymetric anomaly strongly affects the formation nonlinear evolution and saturation of surf zone rip channels in the case of an offshore bump or trough and waves with oblique incidence a rip channel shoreward of the anomaly is enforced by the more seaward alongshore variability in depth the degree of rip channel enforcement is controlled by the strength of the rotational nature of surf zone rip current circulations which is in turn driven by differential broken wave energy dissipation induced by wave refraction across the offshore bathymetric anomaly the alongshore location of this forced rip channel is more stable with increasing offshore anomaly amplitude decreasing offshore wave obliquity and decreasing bathymetric anomaly distance to the shore simulations show that rip channel behavior downdrift and updrift of the offshore perturbation are different in our numerical experiments downdrift rip channels have systematically larger alongshore scales smaller alongshore migration rates and more erosive megacusps than those updrift rip channels therefore self organize into patterns of different alongshore scales and migration rates as a result of an alongshore perturbation in the wave forcing enforced by wave refraction across an offshore bathymetric anomaly these simulations are qualitatively corroborated by video observations of sandbar behavior during a down state sequence at a site with a persistent offshore trough

coupling mechanisms in double sandbar systems part 2 impact on alongshore variability of inner bar rip channels double sandbar systems are common morphological features along sandy wave dominated micro to meso tidal coastlines in the companion paper we demonstrated how various alongshore inner bar rip channel patterns can develop through morphological coupling to an alongshore variable outer bar the simulated coupling patterns are however scarcely observed in the field instead inner bar rip channels more often possess remarkably smaller and more variable alongshore length scales suggesting that coupling mechanisms do not play a substantial role in the overall double sandbar dynamics here we use a numerical model to show that the relative importance of self organization and morphological coupling changes in favour of the latter with an increase in waterdepth variability along the outer bar crest furthermore we find that the typical alongshore variability in inner bar rip channel scale is indicative of a mixture of self organization and morphological coupling rather than self organization alone morphological coupling may thus be more important to understanding and predicting the evolution of inner bar rip channels than previously envisaged copyright 2010 john wiley and sons ltd

lithospheric structure of the trans hudson orogen from seismic refraction wide angle reflection studies the trans hudson orogen tho is the world s largest paleoproterozoic orogenic belt data from three refraction profiles are used to investigate its lithospheric structure in saskatchewan and manitoba r1 crosses the orogen from the hearne craton on the west to the superior craton on the east r2 and r3 are along the orogen p wave velocity structural models are generated using a ray based technique on line r1 higher crustal velocities in its eastern part coincide with rocks of the flin flon namew gneiss complex depth to moho is in the 40 45 km range and equates to that from the reflection data including a small crustal root below the sask minicontinent along lines r2 and r3 depth to moho varies from about 40 km up to 55 km at the north end of r2 and south end of r3 in general variations in crustal velocity and depth to moho do not correlate with the location and extent of geological domains they appear to reflect the complex deformation and metamorphic history of the crustal rocks mantle velo

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

morphodynamic modelling of rip channel growth the morphological evolution of a rip channel system due to normally incident waves has been investigated using two different approaches the first approach is a state of the art commercial 2dh coastal area model whereas the second approach is a linear stability analysis the results of the two different approaches have been compared and both models predict rip channel spacings and initial growth rates of similar magnitudes

self organization mechanisms for the formation of nearshore crescentic and transverse sand bars the formation and development of transverse and crescentic sand bars in the coastal marine environment has been investigated by means of a nonlinear numerical model based on the shallow water equations and on a simplified sediment transport parameterization by assuming normally approaching waves and a saturated surf zone rhythmic patterns develop from a planar slope where random perturbations of small amplitude have been superimposed two types of bedforms appear one is a crescentic bar pattern centred around the breakpoint and the other herein modelled for the first time is a transverse bar pattern the feedback mechanism related to the formation and development of the patterns can be explained by coupling the water and sediment conservation equations basically the waves stir up the sediment and keep it in suspension with a certain cross shore distribution of depth averaged concentration then a current flowing with against the gradient of sediment concentration produces erosion deposition it is shown that inside the surf zone these currents may occur due to the wave refraction and to the redistribution of wave breaking produced by the growing bedforms numerical simulations have been performed in order to understand the sensitivity of the pattern formation to the parameterization and to relate the hydro morphodynamic input conditions to which of the patterns develops it is suggested that crescentic bar growth would be favoured by high energy conditions and fine sediment while transverse bars would grow for milder waves and coarser sediment in intermediate conditions mixed patterns may occur

three dimensional near shore bar morphology a numerical morphological model has been applied to investigate the initiation and development of along shore variations in a long shore bar morphological instability two cases have been studied a case where bar generating effects are included starting with an initially plane bed q3d and a case where an otherwise stationary bar uniform in the alongshore direction is used as the initial topography 2dh in both cases the morphological evolution of the slightly perturbed initial states is simulated beyond the initially linear stage in the q3d case a long shore bar was formed with the bar crest significantly meandering down the coast the bar crest did not seem to develop any rip channel like features in the 2dh case on the other hand the bar tended to break up forming channels

three dimensional collisional structure of the trans hudson orogen canada abstract the three dimensional structure of the eastern trans hudson orogen tho part of a paleoproterozoic continent continent collision zone in central north america is revealed through a network of lithoprobe seismic reflection profiles the seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust e w profiles show strong e dipping reflections extending throughout the crust while n s profiles record events outlining antiformal and synformal structures this allows the identification of decollements that may have localized along pre existing structures e g possible basin bounding extensional faults and at major rheological boundaries e g basement cover contact upper middle crust transition the present topographic surface displays oblique crustal sections with 10 15 km of structural relief generated during post collisional intracontinental transpression of tho as a result of crustal scale folding and faulting

mountain building strike parallel motion and mantle anisotropy abstract geophysical and geological evidence indicates that tectonic movements parallel to the strike of orogenic belts and suture zones are the most important mode of displacement during continental collision and oblique subduction several hundred to a few thousand kilometers of displacement may have occurred this way resulting in a pervasive ductile deformation of the lithosphere consequently both crustal and mantle fabrics may reflect these movements and their study using both geological and geophysical methods could supply important clues to lithosphere behavior during mountain building on the basis of kinematic analysis of ductilely deformed crustal rocks in zones of movement a variety of tectonic models have been proposed which differ in 1 1 the timing of strike parallel motions early before or during continental accretion or late after accretion was completed 2 2 the geometric pattern of strike parallel displacement zones low angle or high angle and 3 3 the location of orogen parallel movement zones in outboard terranes on the subducting plate along the suture zone or within the continental hangingwall large scale displacements parallel to mountain belts also suggest that they occurred along lithospheric faults rooted in the upper mantle although few studies have been carried out from this view point thermal structural and petrological data support the conclusion that lithospheric faults involving the upper mantle may be discriminated from crustal faults rooted in some crustal decoupling zone where available data on mantle flow beneath orogenic belts are in agreement with mountain parallel displacements inferred from crustal studies this has an important bearing on the seismic anisotropy pattern which can be predicted in the lithospheric mantle below mountain belts the fast velocity direction which is parallel to the flow direction should therefore be recorded parallel to the strike of the belt

double bar beach dynamics on the high energy meso macrotidal french aquitanian coast a review the french aquitanian coast is an approximately 250 km long straight low coast exposed to high energy conditions in a mesomacrotidal setting offshore wave conditions are seasonally modulated predominantly with a wnw incidence with offshore significant wave height likely to reach 10 m during winter truc vert beach representative of most of the aquitanian coast beaches commonly exhibitstwodistinctsandbarsystems theinnerbarcangothroughallthestateswithintheintermediateclassificationandusuallyexhibitsa tansversebarandripmorphology afterafewweeksoflowerenergyconditionsduringsummer theinnerbarcommonlybecomesalow tideterracewithameanwavelengthof400mandameansoutherlymigrationrateofabout2 3m day crescenticbarshavebeenreported in the literature in nontidal to microtidal settings long term persistent crescentic patterns are however exhibited at a narrow range of wavelength mean of 700m by the outer bar at the meso macrotidal truc vert beach most of the time the outer bar is inactive and stagnates as offshore waves ofhsn3 m are required to induce a significant morphological change the crescent shape varies from a symmetric shape to a strongly asymmetric shape likely to be the result of a long period of nw wave conditions a strong and rarely observed elsewhere morphological coupling between the inner and outer bars can sometimes be observed and may be the result of the combined effectsof the initial presence of a well developed outer crescentic bar and a long period of shore normal low energy conditions a synthesis of all the data available on the area combined with observations on other environments leads to a truc vert beach state model ranging from a modal double bar configuration to an occasional triple bar configuration this work also identifies knowledge gaps to be explored by further numerical and field studies in tidal double sandbar environments 2007 elsevier b v all rights reserved

an investigation of upper mantle heterogeneity beneath the archaean and proterozoic crust of western canada from lithoprobe controlled source seismic experiments abstract observations of upper mantle reflectivity at numerous locations around the world have been linked to the presence of a heterogeneous distribution of rock types within a broad layer of the upper mantle this phenomenon is observed in wide angle reflection data from lithoprobe s alberta basement transect the sarex and deep probe experiments of 1995 and trans hudson orogen transect the thore experiment of 1993 sarex and deep probe image the archaean lithosphere of the hearne and wyoming provinces whereas thore images the archaean and proterozoic lithosphere of the trans hudson orogen and neighbouring areas finite difference synthetic seismograms are used to constrain the position and physical properties of the reflective layer sarex deep probe modelling uses a 2 d visco elastic finite difference routine thore modelling uses a pseudospectral algorithm in both cases the upper mantle is parameterized in terms of two media one medium is the background matrix the other is statistically distributed within the first as a series of elliptical bodies such a scheme is suitable for modelling 1 variations in lithology e g a peridotite matrix with eclogite lenses or 2 variations in rheology e g lenses of increased strain within a less strained background the synthetic seismograms show that the properties of heterogeneities in the upper mantle do not change significantly between the two lithoprobe transects beneath the trans hudson orogen in saskatchewan the layer is best modelled to lie at depths between 80 and 150 km based on observations from perpendicular profiles anisotropy of the heterogeneities is inferred beneath the precambrian domains of alberta 400 km to the west upper mantle heterogeneities are modelled to occur between depths of 90 and 140 km in both cases the heterogeneous bodies within the model have cross sectional lengths of tens of kilometers vertical thicknesses less than 1 km and velocity contrasts from the background of 0 3 to 0 4 km s based on consistency with complementary data and other results the heterogeneous layer is inferred to be part of the continental lithosphere and may have formed through lateral flow or deformation within the upper mantle

understanding coastal morphodynamic patterns from depth averaged sediment concentration this review highlights the important role of the depth averaged sediment concentration dasc to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern beach cusps surf zone transverse and crescentic bars and shoreface connected sand ridges we present a formulation and methodology based on the knowledge of the dasc which equals the sediment load divided by the water depth that has been successfully used to understand the characteristics of these features these sand bodies relevant for coastal engineering and other disciplines are located in different parts of the coastal zone and are characterized by different spatial and temporal scales but the same technique can be used to understand them since the sand bodies occur in the presence of depth averaged currents the sediment transport approximately equals a sediment load times the current moreover it is assumed that waves essentially mobilize the sediment and the current increases this mobilization and advects the sediment in such conditions knowing the spatial distribution of the dasc and the depth averaged currents induced by the forcing waves wind and pressure gradients over the patterns allows inferring the convergence divergence of sediment transport deposition erosion occurs where the current flows from areas of high to low low to high values of dasc the formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features but the physical mechanisms for their migration their finite amplitude behavior and their decay can also be explored

the spatial and temporal variability of sand bar morphology the spatial and temporal variability of nearshore sand bar morphology is quantified using a unique data set spanning 2 years the data consist of daily time exposure images of incident wave breaking on an open coast sandy beach which may be used to infer bar morphology lippmann and holman 1989 the morphology in each image is classified into an eight state morphologic scheme in which bars are uniquely defined by four independent criteria the most frequently observed morphologies are the longshore periodic rhythmic bars observed in 68 of the data linear bars occur under highest wave conditions h s 1 78 m and are unstable mean residence time 2 days shore attached rhythmic bars are the most stable mean residence time 11 days and generally form 5 16 days following peak wave events non rhythmic three dimensional bars are very transient mean residence time 3 days eighty seven percent of transitions to lower bar types defined in text occurred one state at a time supporting our selection of the ordering of states and suggesting the suitability of a sequential morphology model transitions to higher states occurred under rising wave energy and were evenly spread among the possible higher states with more substantial changes in morphology resulting from larger wave increases this suggests that up state erosional transitions based on offshore bar migration are better described by an equilibrium model where response is better correlated with incident wave energy than with preceding morphological state time exposure images were also digitized to yield quantitative estimates of bar crest location as a function of longshore distance principal component analysis was used to decompose bar position into two dimensional linear and three dimensional longshore variable components cross shore linear bar position ranges 50 m about the 2 year mean 27 m standard deviation and dominates bar variability 74 6 three dimensional bar structure accounts for 14 of the variance 12 m standard deviation changes in incident wave height precede cross shore bar migration by less than 1 day changes in longshore variability are inversely correlated to changing wave conditions with bar morphology becoming linear rapidly during storms on time scales of less than 1 day evolution to significantly three dimensional structure typically occurs over 5 7 days following peak wave events

regional setting and kinematic features of the needle falls shear zone trans hudson orogen needle falls shear zone is the southern part of a major northeast trending ductile shear system within the paleoproterozoic trans hudson orogen in saskatchewan throughout its exposed length of 400 km the shear zone separates reworked archean continental crust and infolded paleoproterozoic supracrustals of the cree lake zone to the northwest from mainly juvenile paleoproterozoic arc terrains and granitoid plutons of the reindeer zone to the southeast it also defines the northwest margin of the ca 1855 ma wathaman batholith which forms the main protolith to shear zone mylonites although not precisely dated available age constraints suggest that the shear zone formed between ca 1855 and 1800 ma toward the end of peak thermotectonism in this part of the orogen in the needle falls study area shear zone mylonites exhibit varied sequentially developed ductile to brittle fabric features including c s fabrics winged porphyroclasts especially delta type small scale compressional and extensional

crustal structure and evolution of the trans hudson orogen results from seismic reflection profiling a 400 km long deep seismic reflection transect across northeastern montana and northern north dakota reveals the crustal scale structural fabric of the early proterozoic trans hudson orogen beneath the williston basin comparison with deep seismic reflection data across the canadian portion of the same orogen 700 km to the north reveals first order similarities in crustal architecture but documents significant along strike variation in orogenic evolution both transects display a broad crustal scale antiform axial to the orogen in the north geologic data suggest that this antiform is cored by an archean microcontinent in the south west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the archean wyoming craton within the wyoming craton the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the early proterozoic trans hudson orogen as interpreted from potential field and drill core data on the basis of subsurface geochronologic data from the crystalline basement and by analogy with the glennie domain within the exposed trans hudson orogen in canada we suggest that the southern antiform is cored by an archean crustal fragment that was caught up in the terminal collision of the wyoming and superior cratons during hudsonian orogeny the eastern side of the trans hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections we interpret the easterly dip of these reflections as evidence that the superior province was thrust westward over the interludes of the orogen during terminal collision although juvenile early proterozoic terranes characterize the exposed segment of the trans hudson orogen in canada limited drill core information within the dakota segment of the orogen shows a predominance of granulitic archean age crust this difference in basement lithologies along strike within the orogen may indicate the following either juvenile crust comparable to that exposed in the northern trans hudson was never present in the south or it was removed by progressive over thickening erosion and or faulting postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north

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

spatio temporal characteristics of small scale wave current ripples on the ameland ebb tidal delta ebb tidal deltas are highly dynamic environments affected by both waves and currents that approach the coast under various angles among other bedforms of various scales these hydrodynamics create small scale bedforms ripples which increase the bed roughness and will therefore affect hydrodynamics and sediment transport in morphodynamic models sediment transport predictions depend on the roughness height but the accuracy of these predictors has not been tested for field conditions with strongly mixed wave current dominated forcing in this study small scale bedforms were observed in the field with a 3d profiling sonar at five locations on the ameland ebb tidal delta the netherlands hydrodynamic conditions ranged from wave dominated to current dominated but were mixed most of the time small scale ripples were found on all studied parts of the delta superimposed on megaripples even though a large range of hydrodynamic conditions was encountered the spatio temporal variations in small scale ripple dimensions were relatively small height n 0 015 m length 0 11 m also the ripples were always highly three dimensional these small dimensions are probably caused by the fact that the bed consists of relatively fine sediment five bedform height predictors were tested but they all overestimated the ripple heights partly because they were not created for small grain sizes furthermore the predictors all have a strong dependence on wave and current related velocities whereas the ripple heights measured here were only related to the near bed orbital velocity therefore ripple heights and lengths in wave current dominated fine grained coastal areas d50 0 25 mm may be best estimated by constant values rather than values dependent on the hydrodynamics in the case of the ameland ebb tidal delta these values were found to be n 0 015 m and 0 11 m

regularity and randomness in the formation of beach cusps abstract controversy still exists on whether rhythmic features such as beach cusps are the result of the presence of standing edge wave motions in the hydrodynamics or if they are the result of self organising processes the compatibility between the two mechanisms is here investigated through the use of a numerical model simulating the formation and development of beach cusps simulations characterised by different forcing conditions have been performed a series of random simulations each individual run simply differing from the others in the seed used in the random number generator showed the typical self organisation behaviour with the features appearing at different locations and even with slightly different spacing regular series of simulations have been run by changing the wavelength of the template superimposed by the hydrodynamic forcing and the number of cycles when the template was present results surprisingly indicate that even a minimum number of cycles with a regular forcing can deterministically induce the final shoreline configuration furthermore if the forcing template has the same wavelength as the one resulting from purely random simulations growth rates are much faster than those obtained with random conditions implications for the kind of field measurements necessary to discern which of the two mechanisms is responsible for beach cusp formation have also been considered

temporal and spatial behaviour of rip channels in a multiple barred coastal system a 15 month data set of daily time averaged video images argus has been analyzed to describe the spatial and temporal variability of the rip channels on a multiple barred coast at noordwijk aan zee the netherlands the landward boundary of the intertidal bars and a proxy of the subtidal bar crest defined as the intertidal and subtidal bar lines respectively were derived from the argus images local seaward directed deviations of the bar lines represent the cross shore and alongshore locations of the rip channels the average intertidal rip spacing was 243 m but the rips were not spaced regularly 0 47 some intertidal rips were observed to fill up during falling tide but the majority remained open the filled intertidal rip channels had more landward positions and migrated more slowly 2 4 versus 4 6 m day in the alongshore direction than the open intertidal rip channels the number and the alongshore migration rate of open intertidal rip channels increased with the preceding wave heights r 0 26 p 0 01 and alongshore component of the offshore wave power r 0 25 p 0 01 respectively the shape of the intertidal bar lines was similar to the subtidal bar line shape suggesting that the intertidal morphology is coupled to the subtidal alongshore variability the phase of two bar lines could vary from in phase 0 to out of phase 180 the phase changes gradually due to different alongshore migration rates of the intertidal and subtidal bar lines copyright 2009 john wiley sons ltd

lithospheric structure in the southern canadian cordillera from a network of seismic refraction lines lithospheric velocity structure and its relationship to regional tectonics and development of the southern canadian cordillera are derived from a synthesis of interpretations from nine in line seismic refraction wide angle reflection profiles and broadside data recorded during the lithoprobe southern cordillera refraction experiment score and other refraction experiments across southern british columbia and one profile in northwestern washington consistency of the score two dimensional models at their intersection positions is achieved through application of a simultaneous inversion of all relevant traveltime data the cross sectional and map presentations demonstrate the strong degree of three dimensional heterogeneity within the crust and upper mantle a first order characteristic is the continuous increase in crustal velocities westward from the foreland belt to the insular belt the variations do not correlate with the morphogeological belts they do correspond with large scale geological and or

a mechanism for sandbar straightening by oblique wave incidence 1 breaker bars in the surf zone of sandy beaches generally evolve between straight bars parallel to the shore and meandering crescentic bars associated with intense dangerous currents flowing seaward through rip channels understanding the behavior of such systems is fundamental as they control the entire surf zone dynamics the shape of the coastline and the exchange of floating material with the shoreface although the mechanisms behind the meandering of an originally straight bar have been studied extensively a clear physical explanation on the crescentic bar straightening was missing recent field observations have highlighted that this morphological reset can be due to wave obliquity by using a two dimensional horizontal morphological model we show that the bar straightening by oblique waves occurs because the rip current is both weakened in intensity and shifted downdrift from the channel deepest section the technique employed is useful for the study of other types of bed forms

structure of the lithospheric mantle beneath the trans hudson orogen canada abstract refraction and wide angle reflection seismic data collected in northern saskatchewan and manitoba in 1993 by lithoprobe provide new glimpses of the deep structure of the lithospheric mantle beneath the trans hudson orogen velocity models obtained through ray tracing modelling techniques revealed the presence of complicated mantle structures down to a depth of 160 km beneath the orogen like the crust the lithospheric mantle appears to show a seismically transparent upper and a highly reflective lower part suggesting that the rheological environment responsible for development of reflections in the lithospheric mantle and of that in the crust are similar this appears to be consistent with the observation that the strong and brittle upper 30 km of the mantle contain remnants of former tectonic processes in a form of velocity anisotropy and the lower more ductile zone seems highly reflective and isotropic the suspected beginning of the lithosphere asthenosphere transition zone at a depth of 160 km imaged as gradual disappearance of mantle reflections coincides well with models established using other geophysical methods

integrated teleseismic studies of the southern alberta upper mantle this paper presents results from a teleseismic experiment conducted across the hearne province in south central alberta data from an array of nine portable broad band seismographs deployed along a 500 km nw se array have been supplemented with recordings from two canadian national seismograph network stations p wave delay times from 293 earthquakes have been inverted for upper mantle velocity structure below the array the recovered model reveals high velocities beneath much of the southern hearne province to depths of 200 250 km which are interpreted as deep seated lithospheric structure contrary to recent tectonic models these results suggest that the hearne lithosphere has remained intact in particular it appears unlikely that evidence for extensive lower crustal melting derives from lithospheric delamination however the results admit the possibility that high mantle conductivity as revealed in magnetotelluric studies originates through small volumes of connected hydrous minerals or other c

morphodynamic instabilities of planar beaches sensitivity to parameter values and process formulations the initial growth of bed perturbations on planar sloping beaches under the forcing of obliquely incident breaking waves is investigated using a state of the art numerical model this allows for a systematic investigation of the sensitivity of the spatial structures of the bed perturbations and their growth and migration rates to different model formulations and parameterizations if the sediment is only transported in the direction of the net current velocity and sediment stirring is taken proportional to the wave height squared growing up current oriented crescentic bars are found with a preferred spacing of 800 m and a down current migration rate of 10 m d 1 varying the angle of wave incidence drag coefficient and bed slope results in qualitatively similar growing bed forms using an engelund and hansen transport formula very oblique down current oriented bars are obtained that grow in time no preferred wavelength however is found using the bailard transport formula results in growing up current oriented bars with a preferred spacing smaller than 300 m for wave angles smaller than 7 when using either the engelund and hansen or bailard sediment transport formulation it is essential to take the transport in the direction of the wave orbital velocity into account in order to have growing bed perturbations

timing of basaltic volcanism along the mesa butte fault in the san francisco volcanic field arizona from 40ar 39ar dates implications for longevity of cinder cone alignments using high precision stepwise heating 40ar 39ar methods we dated volcanic products of at least 12 vents cropping out along the northern segment of the mesa butte fault in the san francisco volcanic field arizona where the relationship between fault and vent alignment is unambiguous the 40ar 39ar isochron dates indicate that the mesa butte fault hosted five episodes of volcanism over a 1 million year interval from 1400 to 300 ka the first episode occurred 1 km northwest of the trace of the mesa butte fault whereas the next two episodes were centered near the intersection of the mesa butte and cedar ranch faults the final two episodes produced red mountain 750 100 ka and shadow mountain 300 100 ka respectively ages of vents representing the third mesa butte episode of volcanism 760 50 ka to 900 80 ka indicate a systematic southwestward shift in the locus of volcanism along the mesa butte fault that contrasts sharply with overall eastward younging of volcanism within the field these data demonstrate that structures in basaltic volcanic fields are capable of hosting multiple episodes of volcanism over periods of time comparable to duration periods of composite volcanoes 106 years but at significantly lower recurrence intervals these results suggest that probability models of spatiotemporal recurrence of volcanism within basaltic volcanic fields should incorporate mapped structures particularly where these structures have either hosted previous volcanic activity or are proximal to population centers or sensitive installations such as nuclear power plants or waste repositories

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

morphodynamics of intermediate beaches a video imaging and numerical modelling study four years of time exposure images of palm beach sydney australia are used to a develop a phenomenological scheme based on surf zone properties for the classification of the four intermediate beach states low tide terrace ltt transverse bar rip tbr rhythmic bar beach rbb and longshore bar trough lbt identified by wright l d short a d 1984 morphodynamic variability of surf zones and beaches marine geology 56 93 118 and b determine environmental conditions associated with the different beach states the overall agreement between beach state classifications obtained by visual classifications of images and those predicted by the phenomenological scheme exceeds 90 analysis of time series of intermediate beach states and wave data indicates that the nearshore morphology continuously cycles through the four beach states in response to variations in the incident wave energy the results also indicate that lbt morphology is produced by high energy wave conditions erosive while the progression towards ltt occurs under lower waves accretive a depth averaged morphodynamic model is employed to determine the morphodynamic processes governing the rbb to tbr and tbr to ltt transitions model results indicate that the rbb to tbr transition is governed by the erosion of the original longshore bars in the initial rbb morphology and the formation and subsequent welding of shoals in the lee of the original longshore bars model results also indicate that rbb to tbr transitions can occur under shore normal and oblique wave conditions the tbr to ltt model simulations indicate that under obliquely incident waves progressive erosion of the transverse shoals and deposition of the eroded sediment in rip channels gradually diminishes the longshore variability in morphology resulting in an ltt morphology

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

the morphological response of a nearshore double sandbar system to constant wave forcing results of 2dh morphodynamic computations are presented to quantify the temporal evolution of the crescentic patterns emerging in a double nearshore bar system in response to constant wave boundary forcing sixteen different conditions varying both offshore wave height and angle of wave incidence were applied the mean length scales of the emerging irregular crescentic patterns are linearly proportional to the local longshore velocity over the inner and outer bars for similar longshore velocities the length scales of the outer bar are larger than of the inner bar this is explained by accounting for the difference in water depth above the bar crest the variable morphological response times can be explained by including additional bathymetrical parameters the active volume of the bar defined by the breaker index plays an important role in this response time with larger active volumes the bar responds more rapidly to identical boundary conditions also bars with a smaller total volume respond more quickly this faster response is due to the steeper active volume of the bars different initial perturbations resulted in different locations of the emerging features showing that their location is sensitive to the initial bathymetry however the range in length scales and response times due to the different perturbations was significantly smaller than those obtained for the different hydrodynamic conditions based on the present findings we hypothesize that morphological length scales in the field are rarely in equilibrium with the concurrent offshore wave height and angle of incidence owing to the slow response of the sandbars under constant conditions relative to the stochastic nature of natural wave forcing

p wave anisotropy in the lower lithosphere abstract long range controlled source seismic experiments yield detailed information about the velocity depth structure of the lower lithosphere between ireland and northern britain three such profiles sample almost the same portion of the subcrustal lithosphere three zones with steep velocity gradients have been detected between 30 km and 90 km depth both the pattern and velocity of the arrivals are incompatible with isotropy preferential alignment of olivine crystals with an azimuth ca n25 e for the fast axis could explain the observations with the more highly aligned zones occurring in bands or layers separated vertically by zones in which the degree of alignment is slight or absent we suggest that a shear heating mechanism may have played a part in producing these patterns this deformation is most likely to be of mesozoic or early cenozoic age it is argued that the upper mantle is not necessarily a strain marker for the last major orogenic episode as recent findings have suggested 1 since it may undergo deformation which decouples from the brittle upper crust and hence is not transmitted to the earth s surface

receiver function studies in the trans hudson orogen saskatchewan teleseismic events were recorded on an array of three component broadband and short period seismographs in the trans hudson orogen over a period of almost 3 years receiver functions calculated from these data were used in a forward modelling study to derive the local shear wave velocity structure beneath 20 stations station ffc flin flon located within the flin flon belt was one of two stations situated on bedrock and produced the highest quality receiver functions the ffc velocity model correlates well with an interpretation of nearby reflection data and features a distinct lower crust interpreted as archean basement which cores the entire reindeer zone both radial and tangential receiver functions at ffc show strong evidence for a northeasterly dipping shallow boundary within rocks of the flin flon belt with a dip angle of 20 total crustal thickness is 37 km eighteen stations are situated on phanerozoic sedimentary cover ranging in thickness from 0 4 km to greater than 2 km receiver function

coupled sandbar patterns and obliquely incident waves 1 in double sandbar systems the alongshore variability in the inner bar often resembles that of the outer bar suggesting that the outer bar acts as a morphological template for the inner bar earlier observations have indicated that this resemblance also termed coupling may take several forms here we apply a nonlinear 2dh morphodynamic model with time invariant forcing to show that the angle of wave incidence is crucial for the alongshore variable morphodynamic evolution of the inner bar for a given crescentic outer bar in contrast to previous modeling efforts of double barred systems which mostly used highly idealized boundary conditions we force our model with realistic hydrodynamics and bathymetrical data derived from video observations at the double barred gold coast australia the results show that for small angles of wave incidence 10 over a crescentic outer bar cell circulation patterns govern the flow at the inner bar giving rise to rip channels that incise the inner bar at the locations of the landward perturbations in the outer bar horns on the other hand for obliquely incident waves 10 20 over a crescentic outer bar the circulatory nature of the flow disappears and gives way to a meandering alongshore current the offshore directed sections of this meandering alongshore current erode the inner bar downdrift of the outer bar horns leading to landward perturbations of the inner bar that are coupled to the outer bar horns an observed coupling type that had not been reproduced as yet oblique wave incidence thus proves to be crucial to the development of this type of sandbar coupling as previously hypothesized from sandbar coupling observations at the gold coast additional simulations including tidal water level variations and bar depth variations demonstrate the robustness of our findings

seismic studies of the crust under the williston basin the seismic refraction method was used in 1981 to study the crust under the northern half of the williston basin in saskatchewan a new method of spatial seismic recording based on a triangular arrangement of receivers was used for the first time to obtain three dimensional structure and velocity information the broadside seismic refraction and wide angle reflection data obtained by the technique were of particular value in defining several faulted blocks these blocks are also characterized by aeromagnetic anomalies trending in a northerly direction the crustal thickness in the southern part of the western provinces shows large variation ranging from 35 to 50 km much of the area is also notable for the presence of one or more low velocity layers and a high velocity lower crust there is good evidence for significant lateral heterogeneity and detailed deep seismic reflection and refraction studies would likely yield information on dips and strikes of beds and faults around the basin as well as defi

development of crescentic bars for a periodically perturbed initial bathymetry 1 the development of crescentic bed patterns starting from an undisturbed beach i e one only perturbed so as to simulate background noise and thereby initiate morphological development is compared with that starting from a beach where monochromatic and in some cases bichromatic bed patterns pre exist using a fully non linear model morfo55 a wide range of different lengthscales and amplitudes is investigated the findings suggest that the pre existence of crescentic bed forms can influence the subsequent morphological development of this beach significantly but that this development is related to the undisturbed beach development whether a pre existing bed form remains under certain forcing conditions depends on the position of the pre existing lengthscale along the undisturbed linear growth rate curve if the pre existing mode shows significant linear growth in the undisturbed scenario this pre existing lengthscale remains otherwise the pre existing mode will be replaced by a bed form with a lengthscale closer to the undisturbed fastest growing mode a pre existing lengthscale that is significantly longer than the undisturbed fastest growing mode will be replaced by a higher harmonic of the original lengthscale for shorter pre existing lengthscales this will be the linear fastest growing mode an increased amplitude of the pre existing bed form accelerates the development toward a new stable situation for small initial amplitudes the initial response of the system to pre existing bed forms could theoretically be described using linearized equations alone the migration rates of both the pre existing and newly arising modes correspond to the migration rate of these modes in the undisturbed scenario

observations of nearshore crescentic sandbars 1 the temporal and spatial variability of crescentic sandbars is analyzed with hourly long term months video observations collected at four barred sites and are qualitatively compared to the temporal and spatial variability predicted by hypotheses underpinning existing approaches and models for crescentic bar formation edge wave template model linear stability analysis and nonlinear models the observations coming from the single barred beaches at duck north carolina usa and miyazaki kyushu japan and from the double barred beaches at the northern gold coast queensland australia and noordwijk netherlands show that crescentic sandbar wavelength and amplitude variations over space and time are very common for instance at any moment in time the wavelength of the smallest and longest crescentic bar can differ by a factor of 2 temporal changes in wavelength and amplitude result from merging and splitting of individual crescents causing the final configuration of a crescentic sandbar system to be very different from the initial configuration the gold coast data indicate that these intrinsically nonlinear interactions are an attempt of the crescentic bar system to self organize into a more uniform pattern as splitting is usually confined to the longest crescentic bar observed whereas merging usually combines the smallest crescentic bars into a longer bar the observed spatial and temporal crescentic bar behavior contrasts qualitatively with behavior predicted from the edge wave template model and implies that the predictive skill of linear stability models is limited nonlinear models are potentially better suited for a comparison against these field observations several suggestions to improve these models and hence to facilitate a data model comparison are made

sandbar straightening under wind sea and swell forcing abstract sandbar straightening on natural beaches is known to occur following storm events however on a typical south west western australia beach sandbar straightening is observed under low amplitude wind sea forcing in summer and remains alongshore variable following high wave events associated with winter storms during summer diurnal variations in wave height and direction are associated with a strong sea breeze cycle at the peak of the sea breeze the local wind sea approaches the beach obliquely driving strong alongshore currents the occurrence of the seasonal obliquely incident wind sea provides the opportunity for testing the role of the wave angle on sandbar straightening the location of the bar crest is observed using time exposure video images and the alongshore variability is quantified using the standard deviation of the sandbar position the role of wave incidence angle on alongshore variability is highlighted as the sandbar straightens in the presence of oblique wave forcing associated with summer sea breezes and remains alongshore variable with normally incident wave forcing the transition from alongshore variable to alongshore uniform also depends on the antecedent bathymetry in particular the distance between the shore and the bar crest when the sandbar is further offshore alongshore variability persists in the presence of strong sea breezes

on the structure and mechanical behaviour of the extending lithosphere in the baikal rift from gravity modelling abstract the crustal structure and lithospheric flexure of the baikal rift zone siberia are examined by means of gravity modelling we model the bouguer anomaly ba along five 1200 km long gravity profiles we first evidence that continuous elastic plate flexure due to surface loading cannot explain the observed ba then we introduce plate discontinuities coupled with a realistic brittle elasto ductile plate rheology which allow us to model of external tectonic forces acting on the plates and determination of the moho geometry we show that the clearest expression of extensional processes occurs in the central part of the rift which exhibits the highest crustal thinning it evolves southwards to a rapidly increasing compression resulting in an overthickening of the southern plate s crust and in the long wavelength flexure of the siberian plate north of the central rift crustal thinning which is always less than 7 km gives way to a more diffuse zone of deformation inside the sayan baikal folded belt based on plate flexure models we propose that the rift shoulders surrounding the central and north baikal basins are not supported by upward bending plate but have a deep crustal root caused by a downward flexure the other parts of the rift depict two adjacent plates with antithetic flexures we also infer that the axial mantle material upwarping is not related to a large scale asthenospheric upwelling since the lithosphere rheological interfaces are not significantly disturbed our results favour the role of horizontal forces and motions resulting from the india asia collision combined with the effect of inherited tectonic structures and especially the paleozoic suture bounding the siberian craton for explaining the crustal structure and plate flexure modeled

quasi three dimensional modelling of the morphology of longshore bars a morphological quasi three dimensional q3d area model for barred coasts has been developed the model combines a two dimensional depth integrated model for wave driven currents with a model for undertow circulation currents the combined model makes a simultaneous simulation of the bar forming processes associated with the undertow and the horizontal wave driven circulation currents which may cause instabilities of the bar and the formation of rip channels situations with normal and oblique wave incidence are considered compared to the depth integrated approach the q3d model produces less pronounced alongshore irregularities for obliquely incident waves for normal incident waves the q3d model produces a crescentic bar while the depth integrated model predicts almost straight sections of the bar interrupted by rip channels the sensitivity to variation of wave angle and beach slope is further investigated

rhythmic morphology in a microtidal low energy beach abstract observations of rythmic features along the inner side of the trabucador barrier beach are coupled to two numerical models to unravel the mechanisms of its formation the trabucador is a long 6 km narrow 125 m barrier and microtidal beach at the sw side of the ebro delta catalonia its inner side is a low energy beach with a sandy shallow terrace featuring an intricate alongshore rhythmic morphology sixteen aerial orthophotos from 1946 to 2014 have been analyzed and complemented with field observations from 1986 to present this morphology is dynamic but it is usually characterized by a long finger transverse bars lftb and b large scale shoreline undulations lssu the lftb are thin and elongated with a length of the order of their spacing they are intertidal and typically attach to the shoreline by a megacusp commonly opening an anti clockwise angle of 10 40 with the shore normal there can be many up to 90 with both the mean and the most frequent alongshore spacing in the range 15 25 m spectral analysis always shows peaks in this range and sometimes additional peaks in the range 30 65 m that correspond to the spacing between the largest bars with smaller bars in between the lssu typically have wavelengths in the range 150 250 m their apexes sometimes coincide with the shore attachment of the largest bars but not always numerical modelling shows that both features could emerge out of feedbacks between hydrodynamics and morphology during the sw wind events involving a deflection of the longshore current by the bars combined with the refractive wave focusing and b gradients in total alongshore sediment transport rate triggering the high angle wave instability

sandbars in motion nearshore surf zones are turbulent places yet despite the apparently chaotic nature of the sea surface in these areas sandbars often show complex patterns with an intricate structure in their perspective stive and reniers discuss recent efforts to understand how these patterns form they highlight the report by hoefel and elgar who consider a new transport mechanism based on flow acceleration within the waves this mechanism should help to improve the predictability of sandbar structures

evidence for azimuthal anisotropy in southwest iberia from deep seismic sounding data abstract data from the iberian lithospheric heterogeneity and anisotropy iliha deep seismic sounding experiment have been used to study the seismic structure of the lower lithosphere beneath the ossa morena zone southwest iberia the interpretation of the mantle phases observed in the record sections evidences a layered lower lithosphere with alternating high and low velocity zones two to three main reflectors are observed in the uppermost mantle between 30 and 90 km depth a simple three layered crust with no significant lateral inhomogeneities over the area explored means that the mantle phases are unlikely to be affected by variations in crustal structure one reversed line and two intersecting unreversed lines define an area beneath southwest iberia where the same lithospheric reflectors segments are sampled along different azimuths the independent analysis of the different lines locates the reflectors at the same depths but significant variations appear between the velocities of the corresponding layers derived from each profile this fact is interpreted as evidence for azimuthal anisotropy in the lower lithosphere the anisotropy orientation deduced from our data is not consistent with results derived from other techniques such as sks splitting this fact already reported for other continental areas must be interpreted as evidence of the existence of changes with depth in the anisotropic properties of the mantle the different hypotheses suggested to explain the origin of the continental mantle anisotropy are discussed and related to the result of our study

suture zone geometry along an irregular paleoproterozoic margin the superior boundary zone manitoba canada lithoprobe acquired a 190 km long deep seismic reflection profile across the superior boundary zone at the eastern margin of the paleoproterozoic trans hudson orogen the profile is located 250 km south of a major promontory along the ancient margin of the superior craton and complements earlier profiles closer to the promontory the new data image the remnants of a craton verging thrust belt younger than 1864 ma within the superior boundary zone extending to a maximum depth of 15 km above a shallowly west dipping basal decollement across the surface suture zone a dip reversal to east dipping reflections occurs within the adjacent reindeer zone where a crustal scale imbricate stack is imaged reindeer zone lower crust and upper mantle extend eastward beneath the superior craton margin for 40 100 km preservation of a foreland thrust belt within the superior boundary zone is explained as a result of reduced collisional convergence and subsequent exhumation within a reentrant flanking the thompson promontory