internal tide coherence and decay over a wide shelf sea 1 a quasi synoptic hydrography and velocity section is used to determine the structure and the decay rate of the internal tide it across the broad continental shelf of the celtic sea in these observations the it is coherent over more than 170 km about five wavelengths with an estimated shoreward energy decay scale of 42 km the inferred it wavelength averaged dissipation rate near the shelf edge is estimated as 2 08 10 7 wkg 1 in close agreement with tidally and vertically averaged measurements from the region these results provide the firstin situ evidence of it coherence over many wavelengths in a shelf sea

paleomagnetism of late cretaceous and eocene limestones and chalks from haiti tectonic interpretations we have conducted a paleomagnetic study of limestones and chalks from haiti which suggests that the island of hispaniola is a composite of at least two tectonically independent blocks sampling sites are distributed among three widely separated localities namely les cayes nine sites 18 3 n latitude 73 6 w longitude and beloc five sites 18 4 n latitude 72 6 w longitude which are located on the southern peninsula of haiti and a northern locality ennery six sites 19 5 n latitude 72 5 w longitude stable magnetization components of dual polarity and moderately high blocking temperature were revealed through progressive thermal demagnetization of samples from all of the beloc and ennery sites while seven of the nine les cayes sites possessed unstable magnetizations mean paleolatitudes calculated from beloc and ennery data show 8 of latitudinal separation that is of at least eocene age pole positions beloc 43 2 n latitude 185 8 e longitude 95 25 1 and ennery 66 1 n latitude 31 3 e longitude 95 19 7 show both colatitudinal and angular discordance with the north american apparent polar wander path based on these disparities we demonstrate that the implied tectonic displacements tend to support some aspects of previously published caribbean plate tectonic models

diurnal variability in currents and temperature on the continental shelf between central and southern california 1 long time series are used to describe the diurnal variability in the meteorological and astronomical tidal forcing and the temperature and current response on the continental shelf of california between port san luis and port hueneme spectral characteristics are used to attribute the response to each of the forcing mechanisms the diurnal temperature field is decomposed into two dominant vertical modes a surface enhanced mode is coherent with the meteorological forcing suggesting that it is a direct response to local heating and wind forcing the amplitude of the second mode is maximum at middepth spectra of this mode exhibit multiple peaks at frequencies corresponding to the astronomical tidal forcing the phase difference between the middepth mode and the tidally dominated sea level is not consistent across the region we conclude that this mode is a localized baroclinic response to the astronomical tidal forcing the diurnal velocity field is dominated by a clockwise component at the surface this component is coherent across the study region and it is also coherent with the diurnal wind stress suggesting that it is a direct response to the diurnal heating and wind forcing coherence with the surface enhanced temperature mode further confirms this a weaker barotropic counterclockwise component is also present in the diurnal velocity field spectra of this component exhibit multiple peaks at periods corresponding to the astronomical tidal forcing the phase difference between this component and the surface elevation varies little between the observational sites we conclude that this velocity component represents a barotropic response to astronomical forcing that is coherent over the study area

instability of baroclinic tidal flow in a stratified fjord abstract the taylor goldstein equation is used to investigate the stability of a baroclinic tidal flow observed in a stratified fjord the flow is analyzed at hourly intervals when turbulent dissipation measurements were made the critical gradient richardson number is often close to the miles howard limit of 0 25 but sometimes it is substantially less although during 8 of the 24 periods examined the flow is marginally stable it is either very stable or very unstable in others for the unstable flow the e folding period of the fastest growing disturbances is 83 455 s about 46 of the buoyancy period at the levels where the fastest growing disturbances have their maximum amplitude these disturbances to the flows have wavelengths about 20 72 of the water depth and have mostly a second mode structure simultaneous measurements of the flow and turbulence allow for testing of the hypothesis that the growth rates of the most unstable disturbances are related to the turbulent dissipation rates dissipati

opening of the grenada back arc basin and evolution of the caribbean plate during the mesozoic and early paleogene abstract geological and geophysical data indicate that the grenada basin was presumably created during the paleocene by sundering of a proto eastern caribbean arc into a remnant arc to the west aves swell and an active arc to the east lesser antilles ridge grenada basin spreading is thought to have been penecontemporaneous with the creation of the yucatan basin located at the opposite side of the caribbean sea i suggest that a continuous mesozoic caribbean arc m c a including the greater antilles the aves lesser antilles system and the aruba blanquilla chain netherland venezuelan antilles was initiated in the pacific probably about 130 120 ma ago its arrival in front of and its subsequent motion inside the central atlantic tethyan seaway caused the opening of both yucatan and grenada basins which occurred at the two initial points of contact with the north and south american cratons in contrast to the style of many other island arcs this back arc spreading event occurred only once in the long history of the m c a the lesser antilles appear to be the oldest currently active intra oceanic island arc

the vertical structure of turbulent dissipation in shelf seas abstract the free fall fly profiler has been used to determine the variation in energy dissipation in the water column over a tidal cycle at mixed and stratified sites in the irish sea it was found that exhibits a strong m4 variation with a pronounced phase lag that increases with height above the bed in mixed conditions this m4 signal which extends throughout the water column is reasonably well reproduced by turbulent closure models of the vertical exchange in the summer stratified situation the m4 signal in is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed the lowest levels of dissipation 10 5 w m 3 measured in the pycnocline are significantly above the system noise level and much higher than predicted by a model using the mellor yamada level 2 closure scheme my2 0 however when allowance is made for the diffusion of tke the model my2 2 simulates the depth time distribution of dissipation in the stratified case satisfactorily if

storms modify baroclinic energy fluxes in a seasonally stratified shelf sea inertial tidal interaction observations made near the celtic sea shelf edge are used to investigate the interaction between wind generated near inertial oscillations and the semidiurnal internal tide linear baroclinic energy fluxes within the near inertial f and semidiurnal m2 wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone over the 2 week deployment period the semidiurnal tide drove 28 48 w m 1 of energy directly on shelf little spring neap variability could be detected horizontal near inertial energy fluxes were an order of magnitude weaker but nonlinear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semidiurnal internal tide led to a 25 43 increase in positive on shelf energy flux the phase relationship between f and m2 determines whether this nonlinear interaction enhances or dampens the linear tidal component of the flux and introduces a 2 day counter clockwise beating to the energy transport two very clear contrasting regimes of a tidally and b inertially driven shear and energy flux are captured in the observations

strong inertial currents and marginal internal wave stability in the central north sea solar insolation stabilizes the water column and suppresses vertical exchange observations from the central north sea clearly show that increased heating in summer is accompanied by enhanced de stabilizing vertical current differences shear surprisingly to such extent that the equilibrium state is marginally stable under calm weather conditions the shear is predominantly generated by near inertial motions while the internal wave spectrum primarily results from nonlinear interaction between the dominating tidal and near inertial motions in terms of the associated enhanced vertical mixing across the largest vertical temperature gradients shelf seas are not different from the abyssal ocean despite the proximity to energy sources near boundaries in the former by the lack of sufficiently strong wind and tidal mixing this internal mixing is considered to be responsible for the diapycnal transport of nutrients leading to the observed increase in near surface values and triggering the late summer phytoplankton bloom

topographically induced drag and mixing at a small bank on the continental shelf recent turbulence measurements over a small bank on the continental shelf off oregon reveal a previously undetected site for intense mixing of the coastal ocean the flow is hydraulically controlled and turbulence diffusivities over the bank are more than 100 times greater than estimates made on the shelf away from topography the total drag exerted by the bank on the flow field is a combination of bottom friction plus form drag analogous to mountain drag and is comparable to the coriolis force this drag is sufficient to decelerate the flow over the bank in a matter of hours

the physical oceanography of jones bank a mixing hotspot in the celtic sea abstract new measurements are presented of the currents and hydrography at jones bank in the celtic sea these measurements collected during the summer of 2008 identify a highly energetic internal wavefield generated by the local interaction of seasonally stratified flow with topography observations close to the bank crest reveal internal waves of up to 40 m amplitude approaching 50 of the local water depth these waves occur during spring periods and are predominantly associated with off bank tidal flow we provide evidence that these waves are the result of hydraulic control of the tidal currents which result in supercritical flow in the bottom mixed layer bml on the upper slopes of the bank the waning tide produces a transition from super to subcritical flow identifiable as a regular hydraulic jump on the bank slope microstructure measurements identify a turbulent bore associated with such jumps that increases bml turbulence by several orders of magnitude and produces a regular burst of enhanced mixing at the base of the ever present thermocline additional mixing in the surface mixed layer is provided during gale force conditions during the first of the two spring tides observed occurring at the start of our three week measurement period the average thermocline mixing rate during stormy spring tide conditions is 1 9 1 0 95 10 3 m 2 s 1 driven by both surface mixing and hydraulic jumps during the later calm spring period the average thermocline mixing rate is 3 3 1 7 10 4 m 2 s 1 dominated by jump associated events by comparison the intervening neap tide period produces no hydraulic jumps and is characterised by relatively calm weather a strong shear layer is however maintained during this quiet period sufficient to enhance thermocline turbulence by a factor 1000 above background levels a short lived peak in thermocline turbulence during the neap period produces diapycnal mixing as high as 1 6 10 2 m 2 s 1 however a more representative background thermocline mixing rate is 2 8 0 6 10 5 m 2 s 1

a regionalized upper mantle rum seismic model seismic velocity heterogeneity in the earth s mantle is strongly concentrated near its top the shallow heterogeneity of the mantle correlates strongly with surface tectonics we use these observations as constraints of a tomographic experiment aimed at building a regionalized upper mantle rum reference model we use a select set of teleseismic travel times to minimize the mapping of mislocation into structure the data selection emphasizes the robustness of individual picks the form of the rum model is a set of velocity profiles as functions of depth through the upper mantle for each of the different tectonic provinces of earth together the profiles constitute a three dimensional model which incorporates considerable structural detail but is described by only 90 parameters and has only about 22 degrees of freedom this is achieved by irregularly sampling a detailed regionalization of the globe by detailed mapping of subducted lithosphere in the mantle as defined by seismicity and by combining these structures in an irregular grid in which bookkeeping is efficiently handled the resulting rum model includes subducting slabs as sharp fast features in the upper mantle old continents are fast young oceans are slow models have been derived for both compressional and shear velocity the rum model is designed to represent as much of upper mantle heterogeneity as seen by body wave travel times as possible with a simple model it can be useful as a reference model for individual tectonic regions travel times are efficiently generated for the rum model mislocations of explosions of known location are significantly reduced when corrections for the rum model are applied to travel time residuals for a spherically symmetrical earth model

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin

systematic bias in baroclinic energy estimates in shelf seas abstracta simple model of an internal wave advected by an oscillating barotropic flow suggests flaws in standard approaches to estimating properties of the internal tide when the m2 barotropic tidal current amplitude is of similar size to the phase speed of the m2 baroclinic tide spectral and harmonic analysis techniques lead to erroneous estimates of the amplitude phase and energy in the m2 internal tide in general harmonic fits and bandpass or low pass filters that attempt to isolate the lowest m2 harmonic significantly underestimate the strength of m2 baroclinic energy fluxes in shelf seas baroclinic energy flux estimates may show artificial spatial variability giving the illusion of sources and sinks of energy where none are actually present analysis of previously published estimates of baroclinic energy fluxes in the celtic sea suggests this mechanism may lead to values being 25 60 too low

investigation of the generation and propagation of low frequency internal waves a case study for the east coast of india sar synthetic aperture radar images in the northwest bay of bengal indicate the existence of internal waves and their occurrence and intensity is topography dependent as indicated by in situ data and satellite information to complement and comprehend the observations a three dimensional princeton ocean model is applied to study the generation and propagation of internal waves the model domain is configured with a variable curvilinear grid and the input fields comprise bathymetry initial temperature and salinity wind stress air sea heat flux and tidal information the numerical investigation indicated a predominant activity of internal waves in the north and the rationale is three fold the first one could be the stable stratification due to fresh water discharge from head bay major river system secondly the significant magnitude range of the tides and finally the bathymetry in the coastal waters off paradip is about 12 shallower compared to that of visakhapatnam and further south the cumulative effect of these causes the predominance of internal waves in the north the core of the energy is essentially in the low frequency range and the model is able to simulate semi diurnal and diurnal components reasonably well up to 6 h frequency 0 162 cph

diurnal period continental shelf waves along vancouver island a comparison of observations with theoretical models abstract current and sea level data from the coastal oceanic dynamics experiment conducted of vancouver island from may 1979 to september 1980 reveal shelf waves of diurnal period whose motions dominate the flow over the continental shelf in this paper current meter data from mooring lines off brooks peninsula estevan point and carmanah point are compared with theoretical solutions for a combination of free shelf waves and barotropic kelvin waves over uniform alongshore topography use of brink s scheme for baroclinic shelf waves gives close agreement between the modeled and observed k1 period horizontal currents over the continental margin off central vancouver island and accurately predicts vertical velocities and sea levels the shelf waves am shown to be entirely dominated by the first baroclinic mode wavelength derived from the model closely approximate wavelenghts computed from the alongshore change in phase of the observed diurnal current ellipses the baroclinic model successfully predicts the

mixing in seasonally stratified shelf seas a shifting paradigm although continental shelf seas make up a relatively small fraction ca 7 of the world ocean s surface they are thought to contribute significantly 20 50 of the total to the open ocean carbon dioxide storage through processes collectively known as the shelf sea pump the global significance of these processes is determined by the vertical mixing which drives the net co 2 drawdown which can occur only in stratified water in this paper we focus on identifying the processes that are responsible for mixing across the thermocline in seasonally stratified shelf seas we present evidence that shear instability and internal wave breaking are largely responsible for thermocline mixing a clear development from the first order paradigm for the water column structure in continental shelf seas the levels of dissipation observed are quantitatively consistent with the observed dissipation rates of the internal tide and near inertial oscillations it is perhaps because these processes make such a small contribution to the total energy dissipated in shelf seas that they are not well represented in current state of the art numerical models of continental shelf seas the results thus present a clear challenge to oceanographic models

internal tide dissipation mixing and vertical nitrate flux at the shelf edge of ne new zealand an internal tide on the ne shelf of new zealand was observed with a combination of moored temperature loggers and current meters and vertical profiling with a microstructure probe internal tide energy flux across the shelf edge was calculated to be 400 w m with considerable variability driven by the passage of a storm through the region energy associated with the internal tide was significantly greater than the energy of the barotropic tide or of inertial shear dissipation of the internal tide calculated from the energy loss between two of the moorings was estimated to be 15 10 mw m 2 the associated vertical eddy diffusivity was 4 3 x 10 4 m 2 s 1 the microstructure observations indicated internal tide driven vertical diffusivities at the nitracline of 7 x 10 4 m 2 s 1 the observations of vertical eddy diffusivities are combined with measurements of the vertical nitrate gradient to suggest that mixing driven by the internal tide is the dominant mechanism for driving diapycnal nutrient supply during summer the calculated flux of about 12 mmol n m 2 d 1 into the photic zone is suggested to drive significant new subsurface production throughout the summer amounting to a possible contribution to annual new production on the shelf of 100 g c m 2

tectonic evolution and mantle structure of the caribbean we investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic p wave velocity structure under the caribbean region in the upper mantle positive seismic anomalies are imaged under the lesser antilles and puerto rico these anomalies are interpreted as remnants of atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the lesser antilles island arc during the past 45 myr the imaged lesser antilles slab consists of a northern and southern anomaly separated by a low velocity anomaly across most of the upper mantle which we interpret as the subducted north america south america plate boundary the southern edge of the imaged lesser antilles slab agrees with vertical tearing of south america lithosphere the northern lesser antilles slab is continuous with the puerto rico slab along the northeastern plate boundary this results in an amphitheater shaped slab and it is interpreted as westward subducting north america lithosphere that remained attached to the surface along the northeastern boundary of the caribbean plate at the muertos trough however material is imaged until a depth of only 100 km suggesting a small amount of subduction the location and length of the imaged south caribbean slab agrees with proposed subduction of caribbean lithosphere under the northern south america plate an anomaly related to proposed oligocene subduction at the nicaragua rise is absent in the tomographic model beneath panama a subduction window exists across the upper mantle which is related to the cessation of subduction of the nazca plate under panama since 9 5 ma and possibly the preceding subduction of the extinct cocos nazca spreading center in the lower mantle two large anomaly patterns are imaged the westernmost anomaly agrees with the subduction of farallon lithosphere the second lower mantle anomaly is found east of the farallon anomaly and is interpreted as a remnant of the late mesozoic subduction of north and south america oceanic lithosphere at the greater antilles aves ridge and leeward antilles the imaged mantle structure does not allow us to discriminate between an intra americas origin and a pacific origin of the caribbean plate key points tectonic reconstructions for the caribbean validated by seismic tomography cenozoic lesser antilles and mesozoic greater antilles subduction confirmed tomography gives insight into geodynamics of lesser antilles

tearing and breaking off of subducted slabs as the result of collision of the panama arc indenter with northwestern south america we present two regional lithospheric cross sections that illustrate eastward and southeastward dipping subducted slabs to depths of 315 km beneath the surface of colombia in northwestern south america these cross sectional interpretations are based on relocated earthquake hypocentral solutions models supported on gravity and magnetic regional data and coda q q c tomography the method of tomographic imaging based on spatial inversion of the coda wave has advantages of providing information on the lateral variations of the anelastic properties and thermal structure of the lithospheric system mapping of earthquake defined benioff zones combined with tomographic imaging reveals the presence of an 240 km long east west striking slab tear named here the caldas tear the proposed caldas tear separates a zone of shallow 20 30 dipping southeastward subduction in the area of colombia adjacent to panama and the caribbean sea which is not associated with subduction related volcanism from an area of steeper 30 40 dipping slab adjacent to the eastern pacific ocean that is associated with an active north south chain of active arc volcanoes we propose that the caldas slab tear separating these two distinct subducted slabs originally formed as the southern boundary of the panama indenter an extinct island arc that began subducting beneath northwestern south america about 12 ma the area south of the panama indenter is miocene oceanic crust of the nazca plate which subducts eastward beneath northwestern south america at normal angles and melts to form a north south trending active volcanic arc in addition to the formation of the caldas tear we propose that impedance of the thicker crustal area of the panama arc indenter over the past 12 ma may have led to down dip break off of previously subducted oceanic crust that is marked by an extremely concentrated and active earthquake swarm of intermediate depth earthquakes beneath east central colombia

oceanic heat delivery via kangerdlugssuaq fjord to the south east greenland ice sheet acceleration of the greenland ice sheet gris tidewater outlet glaciers has increased the ice sheet s contribution to global sea level rise over the last two decades coincident increases in atmospheric temperatures around greenland explain some of the increased ice loss but warm atlantic origin water aw is increasingly recognized as contributing to the accelerating ice mass loss particularly via the outlet glaciers of south east se greenland however there remains a lack of understanding of the variability in heat content of the water masses found to the east of greenland and how this heat is communicated to the outlet glaciers of the gris here a new analysis is presented of ocean gris interaction in which the oceanic heat flux toward the ice sheet in kangerdlugssuaq fjord 0 26 tw is an order of magnitude greater than that reported for the other major outlet glacier of se greenland helheim heat delivered by aw to the calving front of kangerdlugssuaq is equivalent to 10 m d 1 melt i e 30 60 of the ice flow speed and thus is highly significant during the observational campaign in september 2010 warm polar surface water psww melted a substantial volume of ice within the fjord equivalent to 25 of the volume melted by aw alone satellite derived sea surface temperatures show large interannual variability in psww over the 20 year period 1991 2011 anomalously warm psww was observed within the fjord prior to the well documented major ice front retreats of may 2004 and november 2010

near inertial waves on the new england shelf the role of evolving stratification turbulent dissipation and bottom drag abstract energetic variable near inertial internal waves were observed on the springtime new england shelf as part of the coastal mixing and optics cmo project surface warming and freshwater advection tripled the average stratification during a 3 week observational period in april may 1997 the wave field was dominated by near inertial internal waves generated by passing storms wave evolution was controlled by a balance among wind stress bottom drag and turbulent dissipation as the stratification evolved the vertical structure of these near inertial waves switched from mode 1 to mode 2 with associated changes in the magnitude and location of wave shear the growth of mode 2 waves was attributable to a combination of changing wind stress forcing and a nonlinear coupling between the first and second vertical modes through quadratic bottom stress to explore both forcing mechanisms an open ocean mixed layer model is adapted to the continental shelf in this model surface wind stress and bottom stre

identification and tectonic implications of a tear in the south american plate at the southern end of the lesser antilles 1 in the southeast corner of the caribbean westward subduction of atlantic oceanic south america beneath the lesser antilles transitions to east west transform motion between continental south america and the caribbean plate this geometry requires negatively buoyant subducting oceanic south american lithosphere to progressively detach from positively buoyant continental south american lithosphere the most widely accepted model is slab break off with oblique arc continent collision and northwest dipping continental subduction precipitating narrow rifting in the subducting slab in contrast the subduction transform edge propagator step model conceptualizes progressive detachment along a vertical dip slip tear through the lithosphere with stress focused at the edge of the propagating transform boundary we present four types of seismic data to resolve the ongoing lithospheric detachment local seismicity receiver functions wide angle seismic velocity inversion and a regional balanced cross section constrained by petroleum industry data these four data sets image a near vertical tear extending through the entire lithosphere revealing a key mechanism for the structural evolution of venezuela

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

the cenozoic tectonostratigraphic evolution of the barracuda ridge and tiburon rise at the western end of the north america south america plate boundary zone the barracuda ridge and the tiburon rise two major oceanic basement ridges lie at the western end of the diffuse north america south america plate boundary zone where they enter the subduction zone beneath the lesser antilles island arc there is a large degree of uncertainty in the motion between the north american and south american plates predicted by kinematic models of plate motion for the region of these two ridges during the cenozoic and quaternary from the analysis of new multibeam and seismic reflection profiles acquired in 2007 together with older geophysical and geological data we provide new information on the geological history of this area including the timing of the formation of the barracuda ridge and tiburon rise in their present day configurations the timing of the deformation in this region is now much better constrained through the correlation of several key seismic horizons with existing dsdp and odp holes the seafloor topography inherited from the process of formation of the crust at the mid oceanic ridge was buried by distal turbidites by the end of the paleogene beginning in the middle late miocene and then the pleistocene the tiburon rise and barracuda ridge respectively were uplifted and acquired their present day forms and elevation which is much more recent than previously believed in the quaternary the uplift was accompanied by the deposition of very large mass transport deposits the causes of uplift and deformation of the ridges have been convergence between the north american and south american plates and the flexure of these plates as they enter the lesser antilles subduction zone the zone of uplift and deformation migrated northward during the neogene and quaternary

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

on the characteristics of internal tides and coastal upwelling behaviour in marguerite bay west antarctic peninsula abstract internal waves and coastal upwelling have important roles in both physical oceanography and marine ecosystems via processes such as mixing of water masses and transfer of heat and nutrients to biologically active layers in this paper we use quasi weekly hydrographic profiles and moored records of temperature salinity and water velocity to investigate the nature of internal tides and coastal upwelling behaviour in northern marguerite bay at the western antarctic peninsula within ryder bay a near coastal site in northern marguerite bay atmospherically forced oscillations of the water column with periods around 2 7 days are observed associated with wind induced coastal upwelling and downwelling sea ice cover is seen to play a role in the seasonal suppression of these oscillations significant internal tides also are observed at this site a range of processes are seen to be important in controlling internal tide variability including changes in local stratification and sea ice conditions both diurnal and semidiurnal internal tidal species are observed despite the study region being poleward of the critical latitude for diurnal internal tides this suggests that at least the diurnal internal tides are generated close to the study location and we investigate likely sources

subduction history of the tethyan region derived from seismic tomography and tectonic reconstructions 1 in the mantle underneath the tethyan suture zone large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present day signature of subducted tethyan lithosphere we investigate the mesozoic cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions three different subduction scenarios for the tethyan oceanic lithosphere representative for the available tectonic reconstructions are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late mesozoic next these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images in this analysis we include among others the possible effects of ridge subduction and slab detachment after the cenozoic continental collisions absolute plate motion and slab thickening in the mantle our preferred subduction model comprises the opening of large back arc oceanic basins within the eurasian margin the model points to slab thickening by a factor of 3 in the mantle in which case the estimated volumes allow for active oceanic spreading 1 2 5 cm yr in the tethyan lithosphere during convergence our results further indicate the occurrence of early oligocene slab detachment underneath the northern zagros suture zone followed by both westward and eastward propagation of the slab tear and diachronous eocene to miocene slab detachment below the eastern to western himalayas free sinking rates of the detached material of 2 cm yr in the lower mantle provide the best fit between the tomographic mantle structure and our tethyan subduction model

observations of internal bores and waves of elevation on the new england inner continental shelf during summer 2001 1 during july august 2001 oceanographic variability on the new england inner continental shelf was investigated with an emphasis on temporal scales shorter than tidal periods mooring and ship survey data showed that subtidal variation of inner shelf stratification was in response to regional ekman upwelling and downwelling wind driven dynamics high frequency variability in the vertical structure of the water column at an offshore mooring site was linked to the baroclinic internal tide and the onshore propagation of nonlinear solitary waves of depression temperature salinity and velocity data measured at an inshore mooring detected a bottom bore that formed on the flood phase of the tide during the ebb tide a second bottom discontinuity and series of nonlinear internal waves of elevation iwoe formed when the water column became for a time under hydraulic control a surface manifestation of these internal wave crests was also observed in aircraft remote sensing imagery the coupling of iwoe formation to the offshore solitary waves packets was investigated through internal wave breaking criterion derived in earlier laboratory studies results suggested that the offshore solitons shoaled on the sloping shelf and transformed from waves of depression to waves of elevation the coupling of inshore bore formation to the offshore solitary waves and the possible impact of these periodic features on mixing on the inner shelf region are discussed

biological controls on resuspension and the relationship between particle size and the kolmogorov length scale in a shallow coastal sea abstract measurements of suspended particle matter spm and turbulence have been obtained over five tidal surveys during spring and summer 2010 at station l4 50 25 n 04 22 w depth 50 m in the western english channel the relationship between turbulence intensity and bed stress is explored with an in line holographic imaging system evaluating the extent to which material is resuspended image analysis allows for the identification of spm above a size threshold of 200 m capturing particle variability across tidal cycles and the two seasons dissipation of turbulent kinetic energy which exceeds 10 5 w kg 1 yields maximum values of bed stress of between 0 17 and 0 20 n m 2 frequently resulting in the resuspension of material from the bed resuspension is shown to promote aggregation of spm into flocs where the size of such particles is theoretically determined by the kolmogorov microscale lk during the spring surveys flocs of a size larger than lk were observed though this was not repeated during summer it is proposed that the presence of gelatinous biological material in spring allows flocculated particles to exceed lk this suggests that under specific circumstances the limiting factor on the growth of flocculated spm is not only turbulence as previously thought but the presence or absence of certain types of biological particle

impact of internal waves on sound propagation off bhimilipatnam east coast of india internal waves iws are identified off bhimilipatnam east coast of india from the time series ctd hourly interval and thermistor chain data 2 minute interval collected during 23 25 feb 2007 the measurements were carried out at 94 m water depth on the continental shelf edge these data sets are used to describe the characteristics of iws and their impact on acoustic fields garrett and munk gm model has been used to predict the characteristics of low frequency lf iws with space and time active iws are seen in the layer 54 m 94 m with a velocity of 0 548 km hr 1 and the wavelengths of the order of 0 03 km to 21 8 km the model could capture the iw features in the thermocline region accurately than at the bottom this could be due to the limitation of the model which considers linearity high frequency iws observed at the bottom could be due to the advection of tidal currents over the shallow irregular bottom in the presence of stratification the study emphasizes linear iws rather than transient non linear waves induced by tidal interaction with topography acoustic simulation results for low frequency iw field reveal that the intensity loss anomaly of eigenrays was found to be 2 86 db to 15 59 db in the water column and maximum 38 48 db was observed at the bottom due to the bottom interaction our results are well compared with those reported earlier from simulation and acoustic field experiments in the northern indian ocean

mixing in a coastal environment 1 a view from dye dispersion 1 dye release experiments were performed together with microstructure profiling to compare the two methods of estimating diapycnal diffusivity during summer and fall stratification on the continental shelf south of new england the experiments were done in 1996 and 1997 as part of the coastal mixing and optics experiment during the 100 hours or so of the experiments the area of the dye patches grew from less than 1 km2 to more than 50 km2 sundermeyer and ledwell 2001 diapycnal diffusivities inferred from dye dispersion range from 10 6 to 10 5 m2 s at buoyancy frequencies from 9 to 28 cycles hour diffusivities estimated from the dye and those estimated from dissipation rates in the companion paper by oakey and greenan 2004 agree closely in most cases estimates of diffusivities from towed conductivity microstructure measurements made during the cruises by duda and rehmann 2002 and rehmann and duda 2000 are fairly consistent with the dye diffusivities the dye diffusivities would be predicted well by an empirical formula involving shear and stratification statistics developed by mackinnon and gregg 2003 from profiling microstructure measurements obtained at the same site in august 1996 all of the measurements support the general conclusion that the diffusivity averaged over several days is seldom greater than 10 5 m2 s in the stratified waters at the site and usually not much greater than 10 6 m2 s severe storms such as a hurricane that passed over the cmo site in 1996 can dramatically increase the mixing at the site however

an investigation of internal mixing in a seasonally stratified shelf sea the shelf sea seasonal thermocline is a critical interface within the marine environment separating the euphotic zone from nutrient rich deep water fluxes across the thermocline therefore represent a key biogeochemical pathway in this paper we quantify the rate of mixing across the seasonal thermocline for a location in the celtic sea and investigate the processes responsible for driving thermocline fluxes profiles of the rate of dissipation of turbulent kinetic energy e show enhanced dissipation within the thermocline region similar to 6 x 10 5 w m 3 the diffusivity implied by these measurements is similar to 0 5 cm 2 s 1 similar to previous shelf sea studies and is sufficient to explain the observed warming of the deep water suggesting that vertical mixing is the dominant control on water column structure two potential sources of mixing energy are identified the internal tide and near inertial waves the mechanism of energy transfer from the candidate mixing mechanisms to turbulence is not clear thermocline dissipation rates were found to have no richardson number dependence but scaled positively with n 2 and s 2 in agreement with a previous turbulence parameterization application of this model to our data does a good job of capturing the mean characteristics of the observed heating flux across the thermocline although none of the short term fluctuations in mixing were reproduced

observations of the internal tide and associated mixing across the malin shelf 1 an energetic internal tide was observed at two contrasting shelf continental sites to the west of scotland the first site was 5 km shoreward of the continental shelf break and the second was 45 km directly to the east of the first site each site was instrumented with a seabed mounted acoustic doppler current profiler and thermistor chain for a 2 week period in july 1996 when thermal stratification was well developed in addition to the measurements of the evolution of the water column structure and flow field two series of measurements of the rate of dissipation of turbulent kinetic energy were made at each of the sites the barotropic tide was found to contain an unusually strong diurnal component thought to be due to a coastally trapped wave as a result there is a significant diurnal component to the baroclinic energy spectra significant peaks are also present in the baroclinic energy spectra at interaction frequencies between the main semidiurnal m2 and diurnal k1 tidal constituents at both sites observations of profiles of the rate of dissipation of turbulent kinetic energy show a significant proportion of the measured rate of dissipation took place within the thermocline close to the shelf edge detailed observations of a hydraulic jump and nonlinear internal waves are analyzed to show significantly stronger mixing at this site characterized by diffusion coefficients of 1 6 and 6 1 10 4 m2 s 1 than at the site farther onshore where the diffusion coefficient is estimated to be 0 7 10 4 m2 s 1

influence of stratification upon diurnal tidal currents in shelf edge regions abstract a three dimensional stratified tidal model of the hebrides shelf area off the west coast of scotland is used to examine the spatial distribution of the m2 o1 and k1 tides in the region the model has a finer horizontal finite difference grid than previous models of the region with a biharmonic form of horizontal diffusion and the total variation diminishing scheme for density advection by this means horizontal diffusion is kept to a minimum and internal tidal effects can be included preliminary calculations assuming a homogeneous sea region show that the model can accurately reproduce the m2 o1 and k1 tidal elevations in the region an enhancement of the o1 and k1 tidal elevations and currents at the shelf edge to the northwest of the hebrides is found in the model in agreement with theory and observations however the enhancement of the diurnal tides in particular tidal currents along the shelf edge is larger than that found in the observations with some regions of local enhancement

impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break the vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the malin shelf has been determined using a combination of the free falling light yo yo profiler and acoustic doppler current profilers two distinct internal wave regimes were encountered period i in which large amplitude high frequency nonlinear internal waves niws occurred around neap tides and period ii in which the internal wave spectral continuum was not dominated by any particular frequency band around spring tides empirical orthogonal function analysis shows that for the low frequency waves 76 of the variance was described by mode 1 rising to 95 for the high frequency waves during period i the dissipation and vertical mixing were characterized by the niws and 70 of the dissipation occurred in the bottom boundary layer during period ii the depth integrated dissipation was more evenly distributed throughout the tidal cycle whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification during both periods i and ii 30 of the total measured dissipation occurred within the thermocline when averaged over 12 4 hours the remainder occurred within the bottom boundary layer bbl tidal average values for depth integrated dissipation and vertical eddy diffusivity for period i ii were 1 1 10 2 w m 2 4 0 10 2 w m 2 and 5 cm2 s 1 12 cm2 s 1 respectively decay rates and internal damping are discussed and vertical heat fluxes are estimated observed dissipation rates are compared with a simple model for bbl dissipation

internal tide generation at a shelf break by an oblique barotropic tide observations and analytical modeling an analytical model has been developed to describe the generation of a linear interfacial wave over a steep ocean margin by a barotropic tide propagating toward the shelf break with a variable angle of incidence the stratification is reduced to a two layer system and the model uses step like shelf geometry in both shallow and deep regions an open boundary is assumed this implies particularly that there is no reflection at the coast the model is forced with the amplitude and direction of an incident barotropic poincare wave propagating toward the shelf break with this forcing the model gives the amplitude direction of propagation and wavenumber of both barotropic and baroclinic poincare waves transmitted onto the shelf and reflected into the deep region in the vicinity of the shelf break after comparison with baines 1973 model our model is compared with internal tide observations made in july 1996 on the malin shelf within the u k land ocean interaction study shelf edge study program the observations are from moored thermistor chains and acoustic doppler current profilers sited at two locations one near the shelf break and the other 47 5 km onto the shelf observations at this latter mooring are not described by the generation model because of nonlinear changes and damping as the internal wave propagates through shallow water baroclinic displacements of the thermocline of 8 m amplitude were observed close to the shelf break and total baroclinic energy was estimated at 2 4 j m3 during neap tide and 3 j m3 during spring tide both amplitude and energy of the shoreward propagating internal tide waves are compared with the model using a barotropic forcing taken from bibliographical data good agreement with observations is found during neap tide when the density structure is close to being two layer for an incident barotropic tide of 1 m amplitude and propagation direction on a bearing of 23 t t means from true north in a clockwise sense

quantifying the net slab pull force as a driving mechanism for plate tectonics 1 it has remained unclear how much of the negative buoyancy force of the slab fb is used to pull the trailing plate at the surface into the mantle here i present three dimensional laboratory experiments to quantify the net slab pull force fnsp with respect to fb during subduction results show that fnsp increases with increasing slab length and dip up to 8 12 of fb making fnsp up to twice as large as the ridge push force the remainder of fb is primarily used to drive rollback induced mantle flow 70 to bend the subducting plate at the trench 15 30 and to overcome shear resistance between slab and mantle 0 8

the steeply subducting edge of the cocos ridge evidence from receiver functions beneath the northern talamanca range south central costa rica the deep structure of the south central costa rican subduction zone has not been studied in great detail so far because large parts of the area are virtually inaccessible we present a receiver function study along a transect of broadband seismometers through the northern flank of the cordillera de talamanca south costa rica below moho depths the receiver functions image a dipping positive conversion signal this is interpreted as the subducting cocos plate slab compatible with the conversions in the individual receiver functions in finite difference modeling a dipping signal such as the one imaged can only be reproduced by a steeply 80 dipping structure present at least until a depth of about 70 100 km below this depth the length of the slab cannot be determined because of possible scattering effects the proposed position of the slab agrees with previous results from local seismicity local earthquake tomography and active seismic studies while extending the slab location to greater depths and steeper dip angle along the trench no marked change is observed in the receiver functions suggesting that the steeply dipping slab continues until the northern flank of the cordillera de talamanca in the transition region between the incoming seamount segment and cocos ridge considering the time predicted for the establishment of shallow angle underthrusting after the onset of ridge collision the southern costa rican subduction zone may at present be undergoing a reconfiguration of subduction style where the transition to shallow underthrusting may be underway but still incomplete

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

thermocline mixing in summer stratified continental shelf seas 1 a key process in the shelf sea pumping of co2 to the open ocean is vertical mixing in the summer stratified zones of continental shelf seas here we present measurements of profiles of the rate of dissipation of turbulent kinetic energy from which vertical mixing is inferred for several locations in the summer stratified zones of the nw european shelf seas at both sites we find significant thermocline mixing with 20 of the observed turbulent dissipation taking place within the thermocline we then present evidence to demonstrate that the thermocline mixing is a result of shear instability within the thermocline this is contrary to the conventional view that vertical mixing in shelf seas is largely driven by boundary shear stresses

slab detachment control on mafic volcanic pulse and mantle heterogeneity in central mexico seismic tomography studies and plate reconstructions suggest that the farallon slab broke off shortly before subduction ended off southern baja california however the progress of detachment in time and space and its consequences on the volcanism of central mexico have not so far been considered here i use the neogene geologic record of central mexico to propose a lateral propagation of slab detachment beneath the trans mexican volcanic belt during the late miocene i suggest that the trace of the detachment is expressed by a short 2 3 m y eastward migrating pulse of mafic volcanism that took place from ca 11 5 to ca 6 ma to the north of the pliocene quaternary volcanic arc as hot subslab material flowing into the slab gap produced a transitory thermal anomaly in the mantle wedge slab detachment of the deeper and denser part of the plate was initiated in the southern gulf of california area by the incoming of progressively younger oceanic lithosphere at the paleotrench that produced an increasing coupling between the magdalena microplate and the overriding north american plate the tear in the slab propagated eastward from the gulf of california to the gulf of mexico paralleling the southern mexico trench system the decrease in the rivera north america convergence rate between ca 9 and 7 ma appears to be related to the loss of slab pull after the detachment sparse oceanic island type basalts emplaced since the end of the miocene in the trans mexican volcanic belt are located above a trench parallel slab window between the inferred detachment trace and the leading edge of the present slab which has been detected seismically in this context the occurrence of these unusual intraplate magmas is easily explained by the infiltration of enriched asthenosphere into the subarc mantle