ground deformation in askja iceland its source and possible relation to flow of the mantle plume abstract the volcano askja in east central iceland erupted several times between 1920 and 1930 following a very large caldera forming eruption in 1875 the last eruption occurred in 1961 measurements of the vertical component of ground deformation in askja were commenced in 1966 and made once each year 1966 to 1972 and 1983 to 1987 distance measurements with a geodimeter were first made in 1982 the measurements have shown alternating uplift and subsidence of the central part of the askja caldera two periods of uplift and three periods of subsidence have been identified during the 21 years of observations observed vertical displacements agree fairly well with a single point source model at a depth of 1 5 3 5 km below the surface near the center of the askja caldera deviation from a single point source model can be explained by a secondary source of subsidence beneath the site of the 1961 eruption this secondary source has been diminishing in intensity and appears to have been exhausted by 1983 1987 askja has been subsiding at a rate of 6 10 cm per year during the period 1983 1987 but in 1970 1972 rapid inflation was observed possibly as much as 20 cm per year the period of no levelings 1972 to 1983 is characterized by deflation but at a slower average rate than observed in 1983 1987 this suggests continuation of the 1970 1972 inflation into 1973 or 1974

rapid forearc uplift and subsidence caused by impinging bathymetric features examples from the new hebrides and solomon arcs 1 isotopically dated corals from the central new hebrides and new georgia island group solomon islands indicate that both forearcs underwent rapid late quaternary subsidence that was abruptly replaced by hundreds of meters of uplift at rates up to 8 mm yr while total plate convergence was only a few kilometers two mechanisms that might account for these rapid reversals in vertical motion include 1 a displacement mechanism in which the forearc is displaced upward by the volume of an object passing beneath on the subducting plate as the object moves deeper and vacates the base of the forearc the forearc subsides to near its original position and 2 a crustal shortening mechanism in which the forearc thickens and uplifts because of horizontal shortening when a large object impinges on the forearc and abruptly increases interplate coupling on the shallow end of the main thrust zone rapid subsidence follows when the impinging object is broken or otherwise decoupled shallow interplate coupling becomes weak and the uplifted forearc extends and subsides the displacement mechanism surely plays a role on timescales over which plates converge tens of kilometers but it fails to explain the geographic pattern short time frame and abruptness of the change from subsidence to uplift that we observe the crustal shortening mechanism is preferred because it allows the observed abrupt uplift when an object impinges on a forearc and causes locking of a shallow segment of the interplate thrust zone

morphological evolution of the dee estuary eastern irish sea uk a tidal asymmetry approach abstract asymmetry in the tide unequal ebb and flood duration is a dominant factor in causing residual sediment transport and morphological changes in estuaries the evolution of estuarine morphology is a process of dynamic equilibrium in the short term while these features are ephemeral in the long term in this study we investigate the spatial distribution of tidal distortion and asymmetry in the dee estuary uk by 3 dimensional numerical modelling methods high resolution lidar surveys are used to underpin and explain our numerical modelling results in terms of basin hypsometry and areas of recent erosion and deposition harmonic analysis of the numerical modelling results showed that the shallower intertidal areas sand and mud banks were the most tidally asymmetric showing flood dominance the main navigation channels showed some ebb dominance but the tides here were relatively undistorted this overall flood dominance is likely to induce net sediment import to the dee which explains known historical morphological changes large scale accretion over the last two centuries and also recent morphological changes as seen from the lidar surveys which show predominantly net accretion between 2003 and 2006 hypsometrical analysis suggests the dee may be approaching equilibrium and that the flood dominance and sedimentation rate may therefore decrease in the future in an infilling estuary an increase in the area and elevation of tidal flats can eventually shift an estuary towards ebb dominance as shown by previous research and by idealised estuary modelling results presented in this study the large tidal amplitude to hydraulic depth ratio of the dee however suggests that the tidal flats would have to be very extensive indeed for this to occur

coagulation and transport of sediments in the gironde estuary the distribution of suspended particle size and concentration were measured along the gironde estuary france from the river seaward to the ocean the suspended particle size and volume concentration were measured using in situ holography and onboard optical techniques utilizing special procedures in order to avoid floc breakage sediments discharged by the rivers coagulate upon encountering the very low salinities 0 2 of the upper estuary confirmed with laboratory experiments and are then transported and deposited by currents in the remainder of the estuary this coagulation coupled with estuarine circulation produces a turbidity maximum which is offset between the surface and bottom waters the floc size maximum is oceanward of the turbidity maximum and is likewise offset along the estuary by about 30 km the estuary can be subdivided into the following zones 1 coagulation 2 hydrodynamic landward of the null point and 3 hydrodynamic seaward of the null point initial coagulation appears to be completed in coagulation zone 1 and particles are transported and settled with very little floc breakage and recoagulation in zones 2 and 3 only the floc settling velocities coupled with estuarine circulation control the concentration and size distributions of flocs in the water column and eventually control the deposition of sediments

temporal variability of flooding events of guadiana river iberian peninsula during the middle to late holocene imprints in the shallow marine sediment record abstract sedimentological geochemical and benthic foraminiferal proxies were used to interpret changes of depositional environments in a mud entrapment inside the guadiana river paleo valley northern gulf of cadiz to understand the temporal variability of fluvial flooding events and to detect patterns of latitudinal climatic variability in western iberia the period between ca 5800 and ca 1250 cal yr bp was characterized by slowly accumulating coarse grained sediments high content of biogenic sand components and high abundances of shallow water benthic foraminiferal species after ca 1250 cal yr bp the sedimentary environment was dominated by fine grained sediments with high abundances of opportunistic benthic foraminiferal species and successful colonizers and high values of geochemical ratios indicative of enhanced terrigenous supply sedimentation rates increased drastically over the last ca 500 years the genesis of these environments was mainly driven by the variable frequency of fluvial flooding events driven by the regional climate variability low terrigenous sediment input under dry conditions prevailed in the older sedimentary unit flood frequency and terrigenous sediment supply increased during the younger unit the variability of the north atlantic oscillation at centennial scales highly influenced the sediment supply during the last ca 500 years with high terrigenous supply during negative nao conditions our results corroborate an n s gradient along the atlantic iberia during middle and late holocene with more humid conditions in the northwestern and drier conditions towards the southeast the gradient weakened over the past ca 500 years with increased rainfalls and flood events during the little ice age

modelling of estuarine response to sea level rise during the holocene application to the guadiana estuary sw iberia this paper focuses on simulations of the morphological evolution of an estuary during sedimentary infilling that accompanied holocene sea level rise the simulations were conducted using the estuarine sedimentation model which uses a behaviour oriented approach supported by the chronostratigraphy of the estuary s sedimentary sequence behaviour curves were computed to represent the relationship between the estuarine channel depth below maximum high tide and the net accretion at a given location relative to the average sedimentation rate of the estuary during the holocene the model was validated by comparing the observed present day bathymetry of the guadiana river estuary southeastern portugal with the corresponding simulated bathymetries for nine control sections across the estuary the best fit between simulated and actual sediment surface elevations was obtained along the cross sections in the sheltered low energy environments of the estuary the accuracy of the sedimentary stratigraphy of the best fit model was further established using 16 radiocarbon ages obtained from five boreholes in the estuary the present approach is particularly suitable for simulating long term morphological evolution in sheltered estuarine environments where tidally driven vertical aggradation dominates at centennial to millennium timescales however the accuracy of simulated sediment surface elevations and consequently the robustness of behaviour type models based on geographical information system platforms can be enhanced by incorporating i the impacts of nearshore hydrodynamic processes and episodic flood events in highly energetic channels and ii the impacts of cross currents in meandering channel sections

volcano geodesy and magma dynamics in iceland abstract here we review the achievements of volcano geodesy in iceland during the last 15 years extensive measurements of crustal deformation have been conducted using a variety of geodetic techniques including leveling electronic distance measurements campaign and continuous global positioning system gps geodesy and interferometric analysis of synthetic aperture radar images insar results from these measurements provide a comprehensive view of the behavior of icelandic volcanoes between inflation intrusion and eruption episodes volcanoes are likely to deflate or show no sign of seismic activity subsidence rates are often in the range of a few millimeters to a few centimeters a year reducing progressively with time since the last eruption or intrusion at the volcano subsidence can be caused by cooling and contraction of magma outflow of magma it can be related to plate spreading volcano subsidence or lack of deformation is often interrupted by episodic magma flow towards near surface locations such magma recharge has been observed geodetically at hengill hekla eyjafjallajokull katla grimsvotn and krafla volcanoes with inflow inferred to last from a few months up to two decades in the last 15 years five volcanic eruptions three intrusive events and two m6 earthquakes have occurred in recent years the grimsvotn and katla volcanoes have exhibited continuous inflation of a few centimeters per year which at grimsvotn culminated in an eruption on 1 november 2004 hekla and torfajokull volcanoes have inflated at rates an order of magnitude less subsidence is occurring presently at the askja and krafla volcanoes within the period of geodetic measurement signals consistent with no deformation are typical for most of the 35 active volcanoes in iceland

inner forearc response to subduction of the panama fracture zone southern central america abstract subduction of the right lateral panama fracture zone along the convergent margin of central america creates abrupt lateral variations in convergence rate obliquity and subducting crustal thickness at its intersection with the middle america trench this intersection known as the panama co nz ca triple junction is migrating to the southeast at a rate of 55 mm yr and currently coincides with the lateral termination of the fila costena thrust belt in the inner forearc of the overriding plate mapping in the inner forearc in the area that straddles the subducting panama fracture zone reveals that cocos caribbean convergence west of the triple junction leads to the development of an inner forearc thrust belt inboard of the colliding cocos ridge while little deformation is evident inboard of nazca caribbean convergence east of the triple junction this results in the lateral termination of the fila costena thrust belt in the region of the forearc that projects over the panama fracture zone where four out of five mapped thrust faults tip out and are buried by lahars three new balanced cross sections indicate a steep gradient in shortening from the center of the thrust belt to its southeastern termination the short term history of the inner forearc recorded in the landscape and topography of the fila costena is consistent with the southeastward migration of the thrust belt and the panama triple junction throughout the past 3 ma with evidence for the growth of a new topographic divide and reorganization of stream channel networks

sediment pumping by tidal asymmetry in a partially mixed estuary 1 observations collected at two laterally adjacent locations are used to examine the processes driving sediment transport in the partially mixed york river estuary estimates of sediment flux are decomposed into advective and pumping components to evaluate the importance of tidal asymmetries in turbulent mixing at the instrumented location in the estuarine channel a strong asymmetry in internal mixing due to tidal straining is documented with higher values of eddy viscosity occurring during the less stratified flood tide as a result of this asymmetry more sediment is resuspended during the flood phase of the tide resulting in up estuary pumping of sediment despite a net down estuary advective flux at the instrumented location on the adjacent shoal where no pronounced tidal asymmetry in internal mixing was found both the pumping flux and advective flux were directed down estuary the down estuary pumping of sediment on the shoal appears to be driven by asymmetries in bed stress the impact of tidal asymmetries in bed stress at the channel location was negated because the amount of sediment available for resuspension was limited as a result the pumping flux was dominated by the overlying asymmetries in internal mixing the asymmetries in stratification appear to exert an important control on the vertical distribution of sediment by both impacting the eddy diffusivity as well as the fall velocity during the more turbulent flood tide the fall velocities are smaller suggesting the kolmogorov microscale is setting the upper bound on floc diameter

key parameters of the sediment surface morphodynamics in an estuary an assessment of model solutions abstract large scale geomorphological evolution of an estuarine system was simulated by means of a hybrid estuarine sedimentation model hesm applied to the guadiana estuary in southwest iberia the model simulates the decadal scale morphodynamics of the system under environmental forcing using a set of analytical solutions to simplified equations of tidal wave propagation in shallow waters constrained by empirical knowledge of estuarine sedimentary dynamics and topography the key controlling parameters of the model are bed friction f current velocity power of the erosion rate function n and sea level rise rate an assessment of sensitivity of the simulated sediment surface elevation sse change to these controlling parameters was performed the model predicted the spatial differentiation of accretion and erosion the latter especially marked in the mudflats within mean sea level and low tide level and accretion was mainly in a subtidal channel the average sse change mutually depended on both the friction coefficient and power of the current velocity analysis of the average annual sse change suggests that the state of intertidal and subtidal compartments of the estuarine system vary differently according to the dominant processes erosion and accretion as the guadiana estuarine system shows dominant erosional behaviour in the context of sea level rise and sediment supply reduction after the closure of the alqueva dam the most plausible sets of parameter values for the guadiana estuary are n 1 8 and f 0 8f0 or n 2 and f f0 where f0 is the empirically estimated value for these sets of parameter values the relative errors in sse change did not exceed 20 in 73 of simulation cells in the studied area such a limit of accuracy can be acceptable for an idealized modelling of coastal evolution in response to uncertain sea level rise scenarios in the context of reduced sediment supply due to flow regulation therefore the idealized but cost effective hesm model will be suitable for estimating the morphological impacts of sea level rise on estuarine systems on a decadal timescale

local earthquake tomography of central costa rica transition from seamount to ridge subduction summary the structure and seismicity of the subduction zone of central costa rica have been investigated with local earthquake tomography down to ca 50 km depth seismic traveltime data sets of three on and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations 3 d structure of p wave velocity and v p v s ratio using about 2000 highquality events the seismicity and slab geometry as well as v p and v p v s show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest a seamount province in the centre and the subducting cocos ridge in the southeast of the investigation area three prominent features can be identified in the v p and v p v s tomograms a high velocity zone with a perturbation of 4 10 per cent representing the subducting slab a low velocity zone 10 20 per cent in the forearc crust probably caused by deformation fluid release and hydration and a low velocity zone below the volcanic arc related to upwelling fluids and magma unlike previously suggested the dip of the subducting slab does not decrease to the south instead an average steepening of the plate interface from 30 to 45 is observed from north to south and a transition from a plane to a step shaped plate interface this is connected with a change in the deformation style of the overriding plate where roughly planar partly conjugated clusters of seismicity of regionally varying dip are observed it can be shown that the central costa rica deformation belt represents a deep crustal transition zone extending from the surface down to 40 km depth this transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well

intratidal variations in stratification and mixing in the hudson estuary this study represents a detailed examination of intratidal variations in boundary layer structure in a relatively narrow straight section of the hudson river estuary during high river discharge the presence of active mixing is inferred from estimates of the richardson number the maximum height above bed to which mixing occurs is comparable during flood and ebb however the regions of mixing differ in density structure during the flood active mixing is confined to a well mixed near bed layer whereas during the ebb active mixing occurs throughout regions of significant stratification tidal straining maintains stratification during the ebb while it promotes the growth of a uniform bottom mixed layer during the flood in general maximum and minimum near bed stratification occur during late ebb and flood respectively reflecting the dominant role played by tidal straining in determining intratidal variation vertical stretching was also found to be an important term in the stratification balance during the ebb

lower crustal earthquakes caused by magma movement beneath askja volcano on the north iceland rift the lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity the icelandic mid atlantic rift is characterized by high heat flow abundant magmatism generating up to 25 30 km thick crust and seismicity within the upper 8 km of the crust in a 20 seismometer survey in july august 2006 within the northern rift zone around the askja volcano we recorded 1700 upper crustal earthquakes cutting off at 7 8 km depth marking the brittle ductile boundary unexpectedly we discovered 100 small magnitude ml 1 5 earthquakes occurring in swarms mostly at 14 26 km depth within the otherwise aseismic lower crust and beneath the completely aseismic middle crust a repeat survey during july august 2007 yielded more than twice as many lower crustal events geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath askja we interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding askja they represent bursts of magma motion opening dykes over distances of a few meters facilitated by the extensional setting of the active rift zone

salt wedge propagation in a mediterranean micro tidal river mouth abstract the dynamics of a seasonally formed salt wedge propagating along the micro tidal channel of strymon river estuary northern greece and its consequences on river water quality are thoroughly studied through intensive sampling campaigns the wedge is developed at the downstream river part under the summer limited freshwater discharge conditions q 3 s the geometric features of the wedge length and thickness appeared directly related to strymon river discharge a maximum intrusion length of 4 7 km along strymon river estuary was observed under minimum river discharge of almost 6 m 3 s relations produced from in situ data illustrate that limited river flow expands the wedge horizontally reducing its vertical dimension while higher flows lead to increased wedge thickness estuarine flushing time ranges between 0 2 and 1 5 days exponentially dependent on strymon river discharge wedge velocities depicted tidal asymmetry between tidal phases with consistent inward motion even under the ebb tidal stage strong vertical stratification prevails throughout the tidal cycle proving the limited vertical mixing between the two layers although higher interfacial stresses are produced in ebb bottom topography plays an interesting role in wedge propagation as the presence of an underwater sill either prevents saline intrusion during flood or isolates the front of the wedge from its core at the ebb ecological consequences of salt wedge propagation in strymon river estuary are the frequent evidence of bottom hypoxic conditions and the increased tss levels leading to the occurrence of a turbidity maximum at the tip of the salt wedge higher bod and ammonium levels were mostly observed at the river end associated to point and non point pollution sources nitrates and silicates were found associated with freshwater fluxes while ammonia levels were related to saline intrusions the reduced phosphorus freshwater fluxes resulting from phosphorus uptake at the upstream reservoir kerkini lake and the increased bottom turbidity induced by the salt wedge seem responsible for the limited chlorophyll a levels along strymon river estuary

imaging crustal structure in south central costa rica with receiver functions an array of broadband seismometers transecting the talamanca range in southern costa rica was operated from 2005 until 2007 in combination with data from a short period network near quepos in central costa rica this data is analyzed by the receiver function method to image the crustal structure in south central costa rica two strong positive signals are seen in the migrated images interpreted as the moho at around 35 km depth and an intra crustal discontinuity 15 km depth a relatively flat crustal and moho interface underneath the north east flank of the talamanca range can be followed for a lateral distance of about 50 km parallel to the trench with only slight changes in the overall geometry closer to the coast the topography of the discontinuities shows several features most notably a deeper moho underneath the talamanca mountain range and volcanic arc under the highest part of the mountain ranges the moho reaches a depth of about 50 km which indicates that the mountain ranges are approximately isostatically compensated local deviations from the crustal thickness expected for isostatic equilibrium occur under the active volcanic arc and in south costa rica in the transition region between the active volcanic arc and the talamanca range both the moho and intracrustal discontinuity appear distorted possibly related to the southern edge of the active volcanic zone and deformation within the southern part of the central costa rica deformed belt near the volcanoes irazu and turrialba a shallow converter occurs correlating with a low velocity low density body seen in tomography and gravimetry applying a grid search for the crustal interface depth and vp vs ratio cannot constrain vp vs values well but points to generally low values 1 7 in the upper crust this is consistent with quartz rich rocks forming the mountain range

fluid accumulation along the costa rica subduction thrust and development of the seismogenic zone in 2011 we acquired an 11 55 km 3 d seismic reflection volume across the costa rica margin nw of the osa peninsula to accurately image the subduction thrust in 3 d to examine fault zone properties and to infer the hydrogeology that controls fluid accumulation along the thrust following processing to remove water column multiples noise and acquisition artifacts we constructed a 3 d seismic velocity model for kirchhoff prestack depth migration imaging images of the plate boundary thrust show high reflection amplitudes underneath the middle to lower slope that we attribute to fluid rich poorly drained portions of the subduction thrust at 5 km subseafloor beneath the upper slope the plate interface abruptly becomes weakly reflective which we interpret as a transition to a well drained subduction thrust mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude there is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a plumbing system faults commonly have fault plane reflections and are presumably fluid charged the faults and layered fabric form three compartmentalized hydrogeologic zones 1 a shallow ne dipping zone beneath the slope 2 a steeply sw dipping zone beneath the shelf slope break and 3 a ne dipping zone beneath the shelf the more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure elevates effective stress and creates greater potential for unstable coseismic slip

the role of river flow and tidal asymmetry on 1 d estuarine morphodynamics numerous research efforts have been devoted to understanding estuarine morphodynamics under tidal forcing however the impact of river discharge on estuarine morphodynamics is insufficiently examined inspired by the yangtze estuary this work explores the morphodynamic impact of river discharge in a 560 km long tidal basin based on a 1 d model delft3d the model considers total load sediment transport and employs a morphodynamic updating scheme to achieve long term morphodynamic evolution we analyze the role of stokes drift tidal asymmetry and river discharge in generating tidal residual sediment transport model results suggest that morphodynamic equilibrium is approached within millennia by vanishing spatial gradients of tidal residual sediment transport we find that the interaction between ebb directed stokes return flow river flow with tides is an important mechanism that flushes river supplied sediment seaward increasing river discharge does not induce continuously eroded or accreted equilibrium bed profiles because of the balance between riverine sediment supply and sediment flushing to the sea an intermediate threshold river discharge can be defined which leads to a deepest equilibrium bed profile as a result the shape concavity or convexity of the equilibrium bed profiles will adapt with the magnitude of river discharge overall this study reveals the significant role of river discharge in controlling estuarine morphodynamics by supplying sediment and reinforcing ebb directed residual sediment transport

mechanical interaction between active volcanoes in iceland 1 we test the possibility of mechanical interaction between eight central volcanoes in the central part of the active iceland rift zone the average distance between the volcanoes is 30 km all are thought to have shallow magma chambers and many contain collapse calderas we analyzed many finite element models with the volcanoes subject to a tensile stress of 5 mpa equal to the maximum in situ tensile strength of the crust in a direction parallel to the spreading vector n105 e the results show zones between many nearby volcanoes where the tensile stresses exceed the in situ tensile strength of the crust the results indicate that mechanical interaction between volcanoes in a pair such as simultaneous dike emplacement seismogenic faulting and deformation may be common in this part of iceland in agreement with observations

gravity and elevation changes at askja iceland ground tilt measurements demonstrate that askja is in a state of unrest and that in the period 1988 1991 a maximum 48 3 rad tilt occurred down towards the centre of the caldera this is consistent with 126 mm of deflation at the centre of the caldera with a 2 5 3 0 km depth to the source of deformation the volume of the subsidence bowl is 6 2x106 m3 when combined with high precision microgravity measurements the overall change in sub surface mass may be quantified after correction for the observed elevation change using the free air gradient of gravity measured for each station the total decrease in mass is estimated to be less than 109 kg a small residual ground inflation and net gravity increase in the southeastern part of the caldera may be caused by dyke intrusion in this region the minimum dimensions of such an intrusion or complex of intrusions are 1 m width up to 100 m deep and up to several hundred metres thick

sediment transport and dispersion in a cool temperate estuary and embayment saco river estuary maine usa abstract the saco river flows through a rock bound mid latitude estuary recognized as the primary source of sand to the saco bay barrier beach system to better constrain the conditions under which the saco river contributes sand to saco bay three moorings equipped with current meters and a ctd were deployed inside and outside the saco river jetty system from april to june 2005 these measurements characterized high discharge events and their competence for transporting sand outside the jetty system discharge values from 125 175 m 3 s 1 were identified as the threshold envelope at which the saco river transports sand in its bedload to saco bay put in an 89 year historical context with usgs stream gauge data the saco river is expected to contribute sand annually to the embayment but the amount of transport varies significantly from year to year the moored hydrographic data were augmented by geophysical langrangian drifter data and local wind observations to better ascertain nearshore dispersal patterns once sand exits the jettied river mouth these data suggest that transported sand responds to seasonably variable shelf conditions reacts to artifacts of the jetty system or exits saco bay the results and methods of this study suggest that relatively small rock bound estuaries are potentially significant sources of sand to adjacent beach systems and the inner shelf although the pathway for sand from river to shoreface to beach or inner shelf is complex and mediated by local meteorological and oceanographic conditions

fore arc motion and cocos ridge collision in central america 1 we present the first regional surface velocity field for central america showing crustal response to interaction of the cocos and caribbean plates elastic half space models for interseismic strain accumulation on the dipping subduction plate boundary fit the gps data well and show strain accumulation offshore and beneath the nicoya and osa peninsulas in costa rica but not in nicaragua since large subduction zone earthquakes occur in nicaragua we suggest that interseismic locking in nicaragua and some other parts of central america occurs but is mainly shallow 20 km depth too far offshore to be detected by our on land gps measurements our data also show significant trench parallel motion for most of the region generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates however trench parallel motion is also observed in central costa rica where plate convergence is normal to the trench and in the nicaraguan fore arc where trench parallel motion is fast up to 9 mm a 1 but mechanical coupling is low a finite element model of collision as opposed to subduction involving the aseismic cocos ridge also fits the gps surface velocity field most significantly reproducing the pattern of trench parallel motion we infer that buoyant thickened cns 2 cocos ridge crust resists normal subduction and instead acts as an indenter to the caribbean plate driving crustal shortening in southern costa rica and contributing to trench parallel fore arc motion in costa rica and perhaps nicaragua as a type of tectonic escape

volcanic eruption prediction magma chamber physics from gravity and deformation measurements one of the greatest remaining problems in modern volcanology is the process by which volcanic eruptions are triggered it is generally accepted that eruptions are preceded by magma intrusion sigurdsson and sparks 1978 the degree of interaction between previously ponded magma in a chamber and newly intruded magma determines the nature and rate of eruption and also the chemistry of erupted lavas and shallow dykes here we investigate the physics of this interaction volcano monitoring at its most effective is a synergy between basic science and risk assessment while hazard mitigation depends on reliable interpretation of eruption precursors the simple and much used mogi model relates ground deformation h to changes in magma chamber volume gravity changes g combined with ground deformation provide information on magma chamber mass changes our new models predict how the g h gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity thus by simultaneous measurement of deformation and gravity at a few key stations magma chamber processes can be identified prior to the onset of conventional eruption precursors

the dynamics of a partially mixed estuary measurements of velocity density and pressure gradient in the lower hudson river estuary were used to quantify the dominant terms in the momentum equation and to characterize their variations at tidal and spring neap timescales the vertical momentum flux assumed to be due mainly to turbulent shear stress was estimated indirectly based on the residual from the acceleration and pressure gradient terms the indirect estimates of stress compared favorably to bottom stress estimates using a quadratic drag law supporting the hypothesis that the tidal momentum equation involves a local balance between tidal acceleration pressure gradient and stress divergence estimates of eddy viscosity indicated that there was significant tidal asymmetry with flood tide values exceeding ebb values by a factor of 2 as a consequence of the asymmetry the vertical structure of the tidally averaged stress bore no resemblance to the tidally averaged shear in spite of the asymmetry of vertical mixing the tidally averaged estuarine circulation was found to depend simply on the intensity of bottom turbulence which could be parameterized by a rayleigh drag formulation based on the tidal velocity magnitude and the tidally averaged near bottom flow this seemingly paradoxical result indicates that the estuarine circulation can be modeled without detailed knowledge of the effective eddy viscosity only requiring an estimate of the bottom drag coefficient the tidal forcing conditions and the baroclinic pressure gradient a notable characteristic of this solution is an inverse dependence of the estuarine circulation on the amplitude of the tides

evolution and facies architecture of the mesotidal guadiana river delta s w spain portugal abstract the guadiana river estuary and delta is one of the most important mesotidal fluvio marine systems of the southwestern iberian peninsula the river mouth was formed as a narrow channel excavated by fluvial incision during the pleistocene lowstand and then flooded 6500 yr ago during the flandrian transgression holocene the estuary is in an advanced state of sediment infilling in its proximal part due to its narrow morphology which prevents passage of sediment through to the open coast consequently sediment is accumulating in the river mouth causing progradation as response of the interaction between coastal and fluvial processes sedimentation is controlled by the interaction of sediment load delivered by the guadiana river and the active longshore current which also supplies sand eroded from barrier islands and friable cliffs in the portuguese algarve to the west tidal range is mesotidal 2 0 m and mean significant wave height is 0 6 m with an average period of 3 6 s morphological differences exist between the two sides of the river mouth the western side has a large spit separated from the mainland by a marsh and the eastern side shows an array of old barrier islands separated by salt marsh areas a vibracore study allows us to distinguish 12 different depositional facies in the system fluvial delta tidal inlet flood tidal delta washover beach dune estuarine channel lagoon tidal creek channel margin tidal flat and salt marsh furthermore a different architectural scheme of these facies is present on each side to the east there is a longitudinal repetition of the sandy facies corresponding to the barrier islands on the west side there is a transverse growth of a single spit containing eolian dunes these differences result from asymmetric interaction between the tidal and fluvial currents at the river s mouth and the waves and tidal currents of the open coastal zone

analytical models of deformation and residual gravity changes due to a mogi source in a viscoelastic medium abstract the popular mogi model describing deformation due to a small pressurized spherical cavity embedded in a homogeneous elastic half space is shortly reviewed similarities and differences with other isotropic deformation models are discussed and the interpretation of the different volume changes accompanying source inflation is provided the model is then generalized to viscoelastic rheology the overpressure needed within the source to reproduce the large deformation sometimes observed in volcanic regions is found to be strongly dependent on rheological parameters so that reasonably low values may be obtained if the relaxation time of the medium surrounding the source is short months instead of the extremely high values inferred from the elastic theory if the inflation of the source is due to the input of new magma from remote distance the resulting residual gravity change g may be easily computed thanks to the spherical geometry of the source and the density of the newly emplaced material may be inferred from the ratio of g w between the gravity change and the uplift this ratio is independent of the overpressure and the radius of the source which are generally ill defined parameters and is slightly dependent on the relaxation time in the transient phase explicit solutions are shown for a step like overpressure history and for a constant magma supply rate a more realistic model is also proposed in which the viscoelastic properties are restricted to a small spherical volume around the source surrounded by an elastic medium even if an accurate interpretation of deformation and gravity changes in volcanic areas generally requires taking into account topography elastic layering and the triaxial geometry of the inflation source values computed according to the generalized mogi model are compared with observations made during the 1982 84 uplift episode at campi flegrei caldera italy for the sake of illustration

geodetic data shed light on ongoing caldera subsidence at askja iceland subsidence within the main caldera of askja volcano in the north of iceland has been in progress since 1983 here we present new ground and satellite based deformation data which we interpret together with new and existing micro gravity data to help understand which processes may be responsible for the unrest from 2003 to 2007 we observe a net micro gravity decrease combined with subsidence and from 2007 to 2009 we observe a net micro gravity increase while the subsidence continues we infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary mass movements at active volcanoes can be caused by several processes including water table lake level movements hydrothermal activity and magma movements we suggest that here magma movement and or a steam cap in the geothermal system of askja at depth are responsible for the observed micro gravity variations in this respect we rule out the possibility of a shallow intrusion as an explanation for the observed micro gravity increase but suggest magma may have flowed into the residing shallow magma chamber at askja despite continued subsidence in particular variable compressibility of magma residing in the magma chamber as well as compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation

sedimentological and paleoenvironmental characterisation of transgressive sediments on the guadiana shelf northern gulf of cadiz sw iberia during the transgression following the last glacial maximum four backstepping parasequences were deposited on the northern gulf of c shelf sw iberia in contrast to other areas on the shelf these transgressive deposits are at or very close to the surface to the southeast of the guadiana estuary mouth this is particularly true for transgressive parasequence tc possibly associated with the younger dryas event and the youngest parasequence td probably linked to a slow down in sea level rise at around 8 2 ka bp both of which form a sandy transgressive bulge on the upper middle shelf the older and more distal parasequence tc is characterised on seismic records by convex low angle sigmoidal clinoforms and contains high amounts of quartz and bioclasts in contrast the younger and more proximal parasequence td shows relatively steep concave prograding clinoforms and contains high amounts of quartz and other terrigenous components but comparatively low amounts of bioclasts furthermore the difference in age between both parasequences is clearly marked by a difference in the content of glauconite the results indicate that sediments found in parasequence tc are associated with deposition related to storm events with frequent reworking of components and the more proximal and younger sediments in td are the result of rapid sediment accumulation related to floods in the guadiana river basin possibly during the transition from a dry cold period to a warmer more humid period when vegetation cover was lowest and flood frequency increased additionally accumulations of terrigenous components directly associated with the guadiana river basin found on the outer shelf indicate that at least during the beginning of the transgression overflow channels active during large scale flood events may have spilled material to an area located immediately to the south of the guadiana estuary mouth r 2004 elsevier ltd and inqua all rights reserved

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

new england tidal inlets with special reference to riverine associated inlet systems abstract tidal inlets along the glaciated coast of new england exhibit a diverse morphology due to widely different physical and geological settings and sediment abundance except for the glacial sediment coasts of cape cod nantucket island and martha s vineyard most regions of new england are rocky and barrier and tidal inlet development is related to isolated glacial and riverine sediment supplies inlet connected bays and marshes encompass many different origins including drowned river valleys glacial lake discharge channels indented bedrock and glacial sediment coasts kettles and groundwater sapping channels generally inlet size is correlated to tidal range the largest inlets width 300 m occur along mesotidal coasts whereas most inlets along microtidal coasts are small width in spite of their diversity new england tidal inlets can be grouped into three broad categories based on their morphology hydrographic regime and sediment transport characteristics these classes include the wave dominated and mixed energy tidal inlets and riverine associated tidal inlets wave dominated inlets tend to occur in microtidal settings and are found primarily in southern new england they contain flood tidal deltas of variable morphology and poorly developed or absent ebb tidal deltas they have histories of migration closure and reopening during storms mixed energy tidal inlets occur mostly along the mesotidal shorelines of central and northern new england these inlets exhibit well developed ebb and flood tidal deltas they are backed by extensive marshes and tidal creeks and in some cases by a broad system of tidal flats riverine associated tidal inlets coincide with major rivers that have significant freshwater discharge especially during late winter and early spring the throat cross sections are large in comparison to other inlets in new england and are a function of their tidal prism they contain well developed subtidal ebb tidal deltas and variable developed flood tidal deltas ebb oriented bedforms grain size and mineralogical analyses of channel sediments and hydraulic data have been used to demonstrate that the estuaries are exporting sand to the nearshore a process that has likely been ongoing since deglaciation the mechanism for downstream movement of sand in the estuaries is moderate to large spring freshets and major floods when freshwater discharge supplants the saltwater tidal prism during these periods there are unidirectional seaward currents in the estuaries that may last for several days or weeks

on the inner shelf circulation in the northern gulf of cadiz southern portuguese shelf abstract a new look at the inner shelf circulation in the northern gulf of cadiz sw iberian peninsula is taken results reveal the introduction of a sharp spatial heterogeneity in the wind field by the coastal geometry around cape st vincent and nearby mountain chains direct observations taken in november december 2001 reveal the alternating nature of the coastal flow along the sw iberian peninsula featuring a sharp current inversion on time scales of less than 2 days along shore gradients of the oceanic wind stress were detected which may contribute to the coastal circulation patterns in the region over the observational period the sea level variability in the gulf of cadiz is better explained by the action of along shore winds outside the gulf than by local winds this is confirmed by a scale analysis done on the vertically integrated momentum equation it is shown that to a first approximation the expected behaviour of the inner shelf flow responds to linear dynamics in this approximation the along shore sea level slope 5 cm over 100 km is the dominant term overriding the linear bottom and surface stress terms this suggests control by large scale atmospheric pressure systems and negligible dynamical influence from local wind stress

sedimentary characterization of bed types along the guadiana estuary sw europe before the construction of the alqueva dam abstract the guadiana river estuary is one of the most important mesotidal fluvio marine systems of the south western iberian peninsula the river mouth was formed as a narrow channel that was excavated by fluvial incision during the pleistocene lowstand and was then flooded 6500 years ago the estuary is in an advanced state of sediment infilling in its proximal part due to its narrow morphology which prevents passage of sediment through to the open coast consequently sediment is accumulating in the river mouth which causes progradation the analysis of the sediments into the estuarine channel grain size organic carbon content and bedform distribution side scan sonar allows a distinction between two types of sedimentation to be made a fluvial marine sedimentation favoured by tidal action and an autochthonous sedimentation that is related to water mixing the net transport of sediments is towards the sea because of the tidal current asymmetry the autochthonous deposits and the sediments from the extreme fluvial floods that tides are unable to rework are preserved on the meandering convex margins under low tidal velocities these areas acquire a lateral tidal bar morphology with cohesive beds because the narrow geometry of the estuary inhibits the presence of longitudinal tidal bars as may be expected in tide dominated systems the higher energy zones deeper zones of the channel become bypassing channels where the flocculated material cannot be settled on the bottom this part of the channel then develops sandy beds with mesoforms as ebb oriented two dimensional and three dimensional dunes sand waves and megaripples

collision versus sliver transport in the hanging wall at the middle america subduction zone constraints from background seismicity in central costa rica earthquake focal mechanism solutions for 135 small magnitude events are inverted for best fitting partial strain rate tensors that characterize contemporary strain in five areas that span the western margin of the panama microplate in central costa rica the results indicate the predominance of subhorizontal maximum stretching subparallel to the middle america trench mat and provide constraints on the role of cocos ridge collision at the mat the trajectory of maximum stretching changes 25 45 over several tens of kilometers from the central costa rica deformed belt ccrdb where it is nearly e w to the area inboard of the cocos ridge where it is nw se this change suggests that background seismogenic deformation reflects the transition from the trailing edge of a fore arc sliver to an area of the upper plate affected by ridge collision this diffuse deformation may be localized in part on conjugate strike slip faults of the ccrdb

late miocene to recent plate tectonic history of the southern central america convergent margin new plate reconstructions constrain the tectonic evolution of the subducting cocos and nazca plates across the southern central american subduction zone from late miocene to recent because of the strong relationships between lower and upper caribbean plate dynamics along this margin these constraints have wide ranging implications for the timing and growth of upper plate deformation and volcanism in southern central america the reconstructions outline three important events in the neogene history of this margin 1 the coeval development of the panama triple junction with the initiation of oblique subduction of the nazca plate at 8 5 ma 2 the initiation of seamount and rough crust subduction beginning at 3 4 ma and 3 cocos ridge subduction from 2 to 3 ma a comparison of these events with independent geologic geomorphic volcanic and stratigraphic data sets reveals that the timing rates and origin of subducting crust directly impacted the neogene growth of upper plate deformation and volcanism in southern central america these analyses constrain the timing geometry and causes of a number of significant tectonic and volcanic processes including rapid plio quaternary arc fore arc contraction due to cocos ridge subduction the detachment of the panama microplate at 1 3 ma and the late miocene cessation of mantle wedge derived volcanism across 300 km of the subduction zone

tidal and river discharge forcing upon water and sediment circulation at a rock bound estuary guadiana estuary portugal the relative impacts of tidal neap spring and river discharge including a flood event forcing upon water and sediment circulation have been examined at the rock bound guadiana estuary near bed and vertical profiles of current salinity turbidity plus surface suspended sediment concentrations ssc at some stations only were collected at the lower and central upper estuary during tidal and fortnightly cycles in addition vertical salinity and turbidity profiles were collected around high and low water along the estuary tidal asymmetry produced faster currents on the ebb than on the flood especially at the mouth this pattern of seaward current dominance was enhanced with increasing river flow due to horizontal advection that was confined within the narrow estuarine channel the freshwater inputs and at a degree less the tidal range controlled the vertical mixing and stratification importance well mixed spring and partially stratified neap conditions alternated during periods of low river flows with significant intratidal variations induced by tidal straining especially at the partially stratified estuary highly stratified conditions developed with increasing river discharge intratidal variability in the pycnocline depth and thickness resulted from current shear during the ebb a salt wedge with tidal motion was observed at the lower estuary during the flood event depending on the intensity of turbulent mixing the residual water circulation was dominantly controlled either by tidal asymmetry or gravitational circulation the ssc was governed by cyclical local processes resuspension deposition mixing advection driven by the neap spring fluctuations in tidal current velocities more intratidal variability in stratification indicated the significance of tidal pumping at the partially and highly stratified estuary the estuary turbidity maximum etm was enhanced with increasing current velocities and displaced downstream during periods of high river discharge during the flood event the etm was expelled out of the estuary and the ssc along the estuary was controlled by the sediment load from the drainage basin under these highly variable river flow conditions our observations suggest that sand is exported to the nearshore over the long term years

seismic tomography and earthquake locations in the nicaraguan and costa rican upper mantle the central american subduction zone exhibits large variations in geochemistry downgoing plate roughness and dip and volcano locations over a short distance along the arc results from joint inversions for vp vp vs and hypocenters from the tomography under costa rica and nicaragua tucan experiment give insight into its geometry and structure in both costa rica and nicaragua the intermediate depth seismic zone is a single layer no more than 10 to 20 km thick tomographic images show that throughout nicaragua and costa rica the slowest mantle p wave velocities appear below and behind the volcanic front indicating likely zones of highest temperature extending 80 to 120 km depth a sheet of high vp vs thought to be caused by melt is imaged directly beneath the nicaraguan volcanoes whereas a weaker broader anomaly is imaged beneath the costa rican volcanoes potentially indicating a greater extent of melting beneath nicaragua within the downgoing plate anomalously low velocities occur at least 20 30 km below wadati benioff zone seismicity to depths of 140 km beneath nicaragua and to 60 km depth beneath costa rica they indicate 10 20 serpentinized upper mantle of the downgoing plate beneath nicaragua similar to that inferred from refraction seaward of the trench but continuing to subarc depths this unusually hydrated lithosphere may introduce more water into the nicaraguan mantle initiating increased amount of melting and fluid flux to the arc

morphological evolution of the guadiana estuary and intertidal zone in response to projected sea level rise and sediment supply scenarios analysis of holocene sediment accumulation in the guadiana estuary southern portugal during sea level rise since ca 13 cal ka bp was used to simulate the long term morphological evolution of the lower guadiana estuary and the associated intertidal zone for 21st century predicted sea level rises three sea level rise scenarios given by the ipcc 2007 were used in the simulations of morphology using a large scale behaviour oriented modelling approach sedimentation rate scenarios were derived both from the holocene evolution of the estuary and from a semi empirical estimation of present day sediment aggradation our results show that the net lateral expansion of the intertidal zone area would be about 3 5 of the present intertidal zone area for each 10 cm rise in sea level under constraints imposed by the lack of fluvial sediment supply the lateral expansion of the landward boundary of the intertidal zone will occur mainly in the portuguese margin of the guadiana estuary while submergence of the salt marshes will occur in the spanish margin therefore the spanish margin is highly vulnerable to both sea level rise and lack of sediment supply copyright 2011 john wiley sons ltd

gravity height correlations for unrest at calderas abstract calderas represent the sites of the world s most serious volcanic hazards although eruptions are not frequent at such structures on the scale of human lifetimes there are nevertheless often physical changes at calderas that are measurable over periods of years or decades such calderas are said to be in a state of unrest and it is by studying the nature of this unrest that we may begin to understand the dynamics of eruption precursors here we review combined gravity and elevation data from several restless calderas and present new data on their characteristic signatures during periods of inflation and deflation we find that unless the bouguer gravity anomaly at a caldera is extremely small the free air gradient used to correct gravity data for observed elevation changes must be the measured or calculated gradient and not the theoretical gradient use of which may introduce significant errors in general there are two models that fit most of the available data the first involves a mogi type point source and the second is a bouguer type infinite horizontal plane source the density of the deforming material usually a magma chamber is calculated from the gravity and ground deformation data and the best fitting model is to a first approximation the one producing the most realistic density no realistic density is obtained where there are real density changes or where the data do not fit the point source or slab model we find that a point source model fits most of the available data and that most data are for periods of caldera inflation the limited examples of deflation from large silicic calderas indicate that the amount of mass loss or magma drainage is usually much less than the mass gain during the preceding magma intrusion in contrast deflationary events at basaltic calderas formed in extensional tectonic environments are associated with more significant mass loss as magma is injected into the associated fissure swarms

chronology of the sedimentary processes during the postglacial sea level rise in two estuaries of the algarve coast southern portugal abstract four profiles of estuarine sediments obtained from boreholes drilled in the algarve southern portugal were studied in order to reconstruct the process of sediment accumulation driven by the postglacial sea level rise in addition to the sedimentological analysis the foraminifera index of marine influence fimi permitted assessment of the nature and organization of sedimentary facies in the beliche guadiana and gilao almargem estuaries the beliche guadiana cm5 and almargem g2 profiles accumulated in a sheltered environment with the former presenting an almost continuous record of the sea level rise since ca 13 000 cal yr bp the g1 and g3 profiles from the gilao almargem area represent a more discontinuous record of the last 8000 years which accumulated in the more dynamic environment of an outer estuary the integration of all radiocarbon ages of dated levels led to an estimate of sediment accumulation rates assuming a constant position of the sediment surface with respect to the tidal range and a negligible compaction of sediment the sea level rose at the rate of 7 mm yr 1 in the period from 13 000 to 7500 cal yr bp this process slowed down to ca 0 9 mm yr 1 from 7500 cal yr bp until the present the marked historical change in the rate of sediment accumulation in these estuaries also occurred with the accumulation of organic matter and is therefore important data for global biogeochemical models of carbon the main obstacle to obtain higher temporal resolution of the sedimentary processes was the intense anaerobic respiration of organic matter via sulphate reduction which did not allow any accumulation of peat and furthermore led to erasure of the palaeontological record by acid formed from the subsequent oxidation of sulphides

effects of subducting seafloor roughness on upper plate vertical tectonism osa peninsula costa rica 1 subduction of seamounts and ridges along thinly sedimented convergent margins results in deformation of the overriding forearc exposures of newly recognized late pleistocene shallow water deposits i e marenco formation record intervals of rapid subsidence and uplift across the costa rican forearc inboard of the subducting cocos ridge in general the marenco formation is a fining upward fossiliferious late pleistocene marine sand disconformably overlying beveled surfaces cut across the competent osa melange basement the 50 to 27 ka age of the marenco formation is constrained by 12 accelerator mass spectrometry and two conventional 14c dates obtained on marine macrofossils the deposition of this sequence coincident with a general fall in sea level during oxygen isotope stage 3 requires 6 mm yr 1 subsidence inboard of the northwest flank of the subducting cocos ridge presently exposures of the marenco formation are found at 75 m above sea level requiring uplift rates in excess of 6 mm yr 1 we interpret the down and up history of vertical tectonism recorded by the marenco formation as the response of the upper plate to variations in the elevation of the subducting cocos ridge on the basis of a model where the upper plate deforms through bends because of roughness on a rigid downgoing plate the rate duration and spatial distribution of vertical tectonism across the forearc are determined by the magnitude of the orthogonal component of the relative convergence vector and the bathymetry of the underthrusting plate application of this model to bathymetric data for the cocos plate offshore yields a broad agreement between predicted future rates of subsidence and uplift and rates over the last 50 kyr recorded by the marenco formation furthermore analysis suggests that the arrival of the blunt tipped leading edge of the cocos ridge 0 5 3 ma resulted in an initial period of very rapid 30 mm yr 1 uplift

deflation of the askja volcanic system constraints on the deformation source from combined inversion of satellite radar interferograms and gps measurements the askja central volcano in northern iceland has been continuously subsiding at least since 1983 gps and optical leveling tilt measurements show subsidence of at least 75 cm from 1983 to 1998 in the center of the askja caldera without any eruptive activity we have performed an interferometric analysis of synthetic aperture radar images insar of the area utilizing data from the ers satellites we observe subsidence of the askja caldera and its fissure swarm up to a distance of 25 km from the volcano we evaluate the geometry of the magma chamber at askja from a combined inversion of satellite radar interferograms and gps measurements several models were tested including a mogi point source as well as an ellipsoidal source the use of an ellipsoidal source instead of a mogi source gives an estimate of the dimensions of the magma chamber and its deflating pressure whereas these parameters are not independently resolved if a mogi source is used two source models were also considered in order to explain the additional subsidence observed along the askja fissure swarm we tested a model using two mogi sources at different depths a shallow ellipsoidal cavity with a deeper mogi source and then a shallow mogi source with a deeper elongated ellipsoid oriented along the fissure swarm results indicate that an ellipsoidal source at about 3 km depth can accommodate most of the deflation occurring in the caldera residual subsidence occurs along the askja fissure swarm suggesting the existence of a deeper source of contraction we interpret this signal in terms of subsidence of the plate boundary

precise relative earthquake location using surface waves 1 earthquake locations provide a fundamental tool for seismological investigations while dense seismic networks can provide robust locations accuracy and precision of these locations suffer outside dense networks this is particularly true in offshore areas where location analysis relies heavily on distant seismic observations we present a method for estimating precise relative seismic source epicentroid locations using surface waves several reasons including lower velocities and strength of the signal at distance make use of surface waves for event location appealing we focus on the panama fracture zone region and relocate 81 strike slip earthquakes to produce tectonically consistent epicentroid locations the resulting pattern of earthquakes more clearly delineates recently active regional structures than original body wave locations the mean shift between the us geological survey national earthquake information center epicenter and our epicentroids is about 14 km the median is about 11 km and typical origin time changes are generally less than 2 s we find that north of 6 5 n the plate boundary motion is split across two roughly north south striking structures the panama and balboa fracture zones for the last 36 years slip along these two structures roughly matches slip along the panama fracture zone to the south from 4 5 n to 6 25 n but the balboa fracture zone has roughly three times the moment than the northern panama fracture zone our analyses show that observed rayleigh wave signal to noise ratios for moderate to large shallow earthquakes are suitable for applying the procedure and that rayleigh wave observations form a self consistent set of constraints on the relative location of earthquake centroids

relationship between outer forearc subsidence and plate boundary kinematics along the northeast japan convergent margin 1 tectonic erosion along convergent plate boundaries whereby removal of upper plate material along the subduction zone interface drives kilometer scale outer forearc subsidence has been purported to explain the evolution of nearly half the world s subduction margins including part of the history of northeast japan here we evaluate the role of plate boundary dynamics in driving forearc subsidence in northeastern japan a synthesis of newly updated analyses of outer forearc subsidence the timing and kinematics of upper plate deformation and the history of plate convergence along the japan trench demonstrate that the onset of rapid fore arc tectonic subsidence is contemporaneous with upper plate extension during the opening of the sea of japan and with an acceleration in convergence rate at the trench in plio quaternary time relative uplift of the outer forearc is contemporaneous with contraction across the arc and a decrease in plate convergence rate the coincidence of these changes across the forearc arc backarc system appears to require an explanation at the scale of the entire plate boundary similar observations along other western pacific margins suggest that correlations between forearc subsidence and major changes in plate kinematics are the rule rather than the exception we suggest that a significant component of forearc subsidence at the northeast japan margin is not the consequence of basal tectonic erosion but instead reflects dynamic changes in plate boundary geometry driven by temporal variations in plate kinematics if correct this model requires a reconsideration of the mass balance and crustal recycling of continental crust at nonaccretionary margins

sand bypass and updrift beach evolution after jetty construction at an ebb tidal delta abstract the evolution of an ebb tidal delta guadiana south portugal and its updrift beach after jetty construction in 1972 1974 is analysed based on 24 ortho rectified aerial photographs 1940 2012 and 13 bathymetric maps 1969 2014 the objectives are to evaluate the re establishment of the sand bypassing process and if the disruption of the historical delta may affect the updrift beach evolution post jetty progradation of the updrift beach resulted from two large accretion events the second largest event 110 m progradation in 1985 1994 was due to beach attachment of a shoal produced by the erosion of a broad shallow area relict of the historical delta the reworking of sand from this relict area also enables the individualisation of a lateral updrift bar simultaneously with the new ebb shoal proper formation both morphological features were close to volume equilibrium in 1995 indicating that most of the sand was transported towards the downdrift side of the inlet at that time this study shows that erosion of the historical delta may enhance significantly the updrift shoreline progradation and may promote the re establishment of sand bypassing after jetty construction

the geology of the darien panama and the late miocene pliocene collision of the panama arc with northwestern south america the geology of the darien province of eastern panama is presented through a new geologic map and detailed biostratigraphic and paleobathymetric analysis of its upper cretaceous to upper miocene sediments the sequence of events inferred from the stratigraphic record includes the collision of the panama arc the southwestern margin of the caribbean plate and south american continent three tectonostratigraphic units underlie the darien region 1 precollisional upper cretaceous eocene crystalline basement rocks of the san blas complex form a series of structurally complex topographic massifs along the northeastern and southwestern margins of the darien province these rocks formed part of a 20 m y submarine volcanic arc developed in a pacific setting distant from the continental margin of northwestern south america the northerly basement rocks are quartz diorites granodiorites and basaltic andesites through dacites to rhyolites indicating the presence of a magmatic arc the southerly basement rocks are an accreted suite of diabase pillow basalt and radiolarian chert deposited at abyssal depths precollisional arc related rocks of eocene to lower miocene age consist of 4000 m of pillow basalts and volcaniclastics and biogenic calcareous and siliceous deep water sediments they consist of the eocene oligocene darien formation the oligocene porcona formation and the lower middle miocene clarita formation postcollisional deposits are mostly coarse to fine grained siliciclastic sedimentary rocks and turbiditic sandstone of upper middle to latest miocene age this 3000 m thick sedimentary sequence is deformed as part of a complexly folded and faulted synclinorium that forms the central chucunaque tuira basin of the darien the sedimentary package reveals general shallowing of the basin from bathyal to inner neritic depths during the 12 8 7 1 ma collision of the panama arc with south america the sediments are divided into the upper middle miocene tapaliza formation the lower upper miocene tuira and membrillo formations the middle upper miocene yaviza formation and the middle to upper miocene chucunaque formation the precollisional open marine units of late cretaceous middle miocene age are separated from the overlying postcollisional sequence of middle to late miocene age by a regional unconformity at 14 8 12 8 ma this unconformity marks the disappearance of radiolarians the changeover of predominantly silica deposition from the atlantic to the pacific the initiation of the uplift of the isthmus of panama and the onset of shallowing upward coarser clastic deposition this pattern is also recorded from the southern limon basin of caribbean costa rica to the atrato basin of northwestern colombia by the middle late miocene neritic depths were widespread throughout the darien region and a regional unconformity suggests completion of the central american arc collision with south america by 7 1 ma no pliocene deposits are recorded from either the darien or the panama canal basin and no sediments younger than 4 8 ma have been identified in the atrato basin of colombia suggesting rapid uplift and extensive emergence of the central american isthmus in the latest miocene northward movement of the eastern segment of the panama arc along a now quiescent panama canal zone fault during eocene oligocene time may have dislocated the pre collision arc since collision the portion west of this fault chorotega block has remained stable without rotation to the east in the darien region compression has been accommodated through formation of a panama microplate with convergent boundaries to the north north panama deformed belt and south south panama deformed belt and suturing with south america along the atrato valley deformation within the microplate has been accommodated in the darien province by several major left lateral strike slip faults that were active until the early pliocene since when the plate has behaved rigidly

ria ocean exchange driven by tides in the ria of ferrol nw spain the hydrography of the ria of ferrol situated north of cape finisterre nw spain was analyzed during a year as a function of the two main forcing factors river discharge and tide salinity temperature and tidal currents were measured at a fixed station in the middle part of the estuary during a tidal cycle 12 h in addition thermohaline variables were also measured bihourly at six stations along the main channel of the estuary the stratification and water column stability were found to vary in different sections of the estuary as a function of the estuarine richardson number and the brunt vaisala frequency well mixed conditions were found at the mouth where large tidal velocities enhance the turbulent mixing in contrast the middle and inner parts of the ria of ferrol were observed to be partially stratified during the wet season this behavior is different to the one observed in the galician rias baixas situated further south that are everywhere partially stratified despite the fact that the ratio of freshwater input to the water content is similar to that in ria of ferrol

post eruptive gravity changes from 1990 to 1996 at krafla volcano iceland the 1975 1984 krafla rifting episode was a major lava and dyke producing event associated with the release of extensional strain accumulated over more than 200 years at the divergent plate boundary in north iceland the present work provides a unique example of gravity decreases and increases sustained over a long period following a major eruptive episode at a rift volcano after height correction persistent net gravity decreases over the source of observed mogi type deflation occur with gravity increases occurring further away from this centre of deformation gravity decreases are interpreted in terms of drainage from a shallow magma chamber the net gravity decreases require that at least 4 1010 kg of magma must have been drained during the 6 year observation period assuming a density of 2700 kg m 3 this magma would occupy 1 5 107 m3 and by analogy with results obtained for kilauea this implies a magma chamber volume change of 4 1 106 m3 this is consistent with the chamber volume change deduced from ground deformation data assuming a poisson s ratio of 0 25 and a mogi source net gravity increases are more spatially extensive and are most likely caused by migration of the steam water interface and or closure of micro fractures in lavas above the magma chamber during post eruptive cooling and contraction we present a model for the krafla magma chamber in which a cooling contracting and draining magma body causes subsidence at the surface these results contrast with observations from the askja caldera iceland where post eruptive deflation has been shown to be accompanied by negligible net gravity changes above the mogi type source in the caldera long term post eruptive deflation and magma drainage have not been observed at subduction related volcanoes this may be a function of a difference in magma viscosity

the interpretation of gravity changes and crustal deformation in active volcanic areas simple models like the well known point source of dilation mogi s source in an elastic homogeneous and isotropic half space are widely used to interpret geodetic and gravity data in active volcanic areas this approach appears at odds with the real geology of volcanic regions since the crust is not a homogeneous medium and magma chambers are not spheres in this paper we evaluate several more realistic source models that take into account the influence of self gravitation effects vertical discontinuities in the earth s density and elastic parameters and non spherical source geometries our results indicate that self gravitation effects are second order over the distance and time scales normally associated with volcano monitoring for an elastic model appropriate to long valley caldera we find only minor differences between modeling the 1982 1999 caldera unrest using a point source in elastic homogeneous half spaces or in elasto gravitational layered half spaces a simple experiment of matching deformation and gravity data from an ellipsoidal source using a spherical source shows that the standard approach of fitting a center of dilation to gravity and uplift data only excluding the horizontal displacements may yield estimates of the source parameters that are not reliable the spherical source successfully fits the uplift and gravity changes overestimating the depth and density of the intrusion but is not able to fit the radial displacements

1983 2003 decaying rate of deflation at askja caldera pressure decrease in an extensive magma plumbing system at a spreading plate boundary new deformation data from the askja volcano iceland show that the volcano s caldera has been deflating continuously for over 20 years and confirm that the rate of subsidence is slowing down the decay in subsidence rate can be fitted with a function of the form e t where is 39 years reanalysis of gps data from 1993 1998 show that these data can be fitted with a model calling for two mogi point sources one shallow and another one much deeper 16 2 km depth pressure decrease occurs in both sources the deeper source is responsible for observed horizontal contraction towards askja at distances that cannot be explained by the shallower source plate spreading of 19 mm year distributed evenly over about 100 km wide zone is also favoured by the data

observations and modelling of periodic stratification in the upper york river estuary virginia abstract two current meter moorings were deployed simultaneously for 61 days during the winter of 1989 90 in the upper york river estuary virginia at a separation along the estuary axis of one tidal excursion both moorings were equipped with four current meters measuring conductivity temperature and current velocity and a surface conductivity temperature recorder giving a detailed picture of the vertical structure of the water column the time series of the vertical density structure measured by both moorings show closely similar periods of mixed and stratified conditions along the estuary comparison with the tidal currents over the same period demonstrates lower stratification or complete mixing to be associated with the strong spring tidal currents while significant stability develops during weaker currents such a spring neap signal is caused by the modulation of tidal mixing energy which is in competition with the stratifying estuarine circulation superimposed on this tidal signal are more irregular remixing events associated with peaks in the surface wind stress a numerical model used successfully in earlier work in a shelf sea region is applied to the upper york river regime the model employs a mellor yamada level 2 turbulence closure scheme to relate the efficiency of vertical turbulent mixing to the local water column stability running the model with a depth dependent horizontal density gradient results in no significant stratification predicted for the entire deployment by incorporating depth variability of the horizontal density gradient estimated from the observations from the two moorings the model reproduces the qualitative features of the observed evolution of stratification

postglacial sea level rise and sedimentary response in the guadiana estuary portugal spain border abstract the study of sedimentological features foraminifera and molluscan fauna in samples recovered from four rotary boreholes permitted after 14 c dating the first assessment of the post glacial transgression in guadiana river estuary the holocene sequence is underlain by coarse sediments from delta fan enclosing continental swamp deposits dated 16 980 year bp the holocene sedimentary sequence was deposited either directly on this delta fan gravels or on top of fluvial sands deposited probably between 11 000 and 10 000 yr bp an accelerated phase of the estuary infilling by clayey sediments containing the salt marsh foraminifer trochammina began ca 9800 year bp when sea level was about 39 m below present between 7500 and 7000 year bp the central part of the estuary started to accommodate coarser sediments partially introduced from the continental shelf the first phase of holocene sea level rise at a rate of 0 85 m century terminated ca 6500 year bp since then lagoonal sediments in the vicinity of the estuary have been enclosed behind sand spits and predominantly sandy sedimentation was initiated within the estuary after a second phase of slower rise at the rate of 0 3 m century which lasted until ca 5000 year bp the sea approached the present level

suspended sediment variability on seasonal and tidal time scales in the winyah bay estuary south carolina usa a transient estuarine turbidity maximum etm was observed in winyah bay during field sampling in 1996 the core of this turbidity maximum migrated 8 km seaward of its traditional location at the saltefreshwater interface in response to very high river discharge events in 1996 in response to the semidiurnal tidal cycle there was a well defined variation in the suspended sediment concentration within the turbidity maximum zone between ebb and flood tides as a result of differences in tidal velocities the suspended sediment inventory in the water column during the flood tide was greater than suspended sediment inventory during the ebb tide during slack water the water column was characterized by 0 03 kg m 3 sediment concentrations whereas at times of maximum boundary shear stress near the bottom the water column was highly turbid indicating that bottom sediments were resuspended by the strong tidal currents typical of the bay throughout a tidal cycle the vertical extension of high suspended sediment concentration in the water column within the etm was well developed during the flood tide and inhibited during the ebb tide internal tidal asymmetry with well mixed conditions during the flood tide near the bottom and with stratification in the water column during the ebb tide was probably the process responsible for the enhancement of turbulence during the flood tide and inhibition of turbulence during the ebb tide with implications for the etm 2003 elsevier ltd all rights reserved

active thrusting in the inner forearc of an erosive convergent margin pacific coast costa rica 1 structural and geomorphic analyses of the fila costena thrust belt in southwest costa rica indicate active thrusting within the inner forearc the fila costena exposes three major thrust faults that imbricate the late tertiary forearc basin sequence of the terraba basin the frontal thrust of the fila costena marks the boundary between an uplifting inner forearc and a subsiding outer forearc with only local uplift astride the indenting cocos ridge on the basis of surface constraints a cross section across the thrust belt suggests that all three thrusts flatten into parallelism with a low angle decollement horizon near the contact between the basement and the cover sequence of the terraba basin this decollement lies at a depth of 4 km the minimum shortening recorded by restoration of fault related folds is 17 km or 45 observations of late tertiary marine sediments tilted and faulted late quaternary fluvial terraces and raised holocene marine terraces indicate that fila costena uplift was likely initiated in the quaternary and is ongoing given that the coastal mountains are separated from the talamanca range by a valley the decollement that delaminates the forearc basin from the underthrusting forearc must continue as a flat beneath the valley but must link with the plate boundary along a crustal scale ramp system a structural geometry that has resulted in uplift of the talamanca range the highest peaks in central america the dichotomy between uplift in the inner forearc and subsidence in the outer forearc is explained in terms of the response of an arcward thickening wedge to rough subducting crust index terms 8150 tectonophysics plate boundary general 3040 8123 tectonophysics dynamics seismotectonics 8010 structural geology fractures and faults 8015 structural geology local crustal structure 8005 structural geology folds and folding

tidal asymmetry in an estuarine pycnocline depth and thickness tidal variations in estuarine stratification are revealed by the depth and thickness of the density interface the depth of the interface may be predicted using an inviscid two layer model that combines baroclinic estuarine circulation with barotropic tidal currents helfrich 1995 here we present results from a two layer model modified to include the effects of bottom friction and interfacial mixing modeled layer thickness and speed compare favorably with prior analytic studies farmer and armi 1986 pratt 1986 we use a bulk richardson number criterion to estimate the thickness of the pycnocline from two layer model results the predicted pycnocline depth and thickness compare remarkably well with observations we also investigate the effects of changing bottom friction and barotropic currents on the pycnocline thickness

subsidence of askja caldera 2000 2009 modelling of deformation processes at an extensional plate boundary constrained by time series insar analysis abstract many calderas in the world show long term unrest in the form of elevated rates of deformation and seismicity related to pressure changes and magma movements within their magmatic plumbing systems we present new observations of the style of deformation at the askja caldera iceland since 2000 using interferometric analysis of synthetic aperture radar images insar acquired by the canadian radarsat 2 satellite when combined with previously acquired detailed geodetic observations by various techniques we obtain an overview of askja s behaviour through more than four decades the combined dataset reveals that during this non eruptive period askja continuously subsides at a rate of 2 5 3 cm yr in 2000 2009 somewhat lower than the 5 cm yr rate inferred for the 1983 1998 period this behaviour of askja is evaluated and compared to those of other restless calderas the wrapped interferograms show three main features 1 concentric fringes depicting subsidence in the centre of the main askja caldera 2 oval fringes elongated along the rift portraying subsidence and 3 subsidence in an area north of the vatnajokull glacier the average line of sight los velocity from ground to satellite was used as input for inverse modelling of a deflating pressure source beneath the caldera embedded in a homogeneous elastic half space two different source geometries were tested a point pressure source and a horizontal penny shaped crack the concentric fringes centred in the askja caldera are best fit by a point source located at 65 05 n 16 78 w at a depth of 3 2 3 8 km with a volume decrease of 0 0012 0 0017 km3 yr from 2000 to 2009 consistent with previous studies provisional 2d fem models including structural complexities in the crustal layers indicate that the tectonic setting of askja plays an important role in the continuous long term high subsidence rates observed there in order to fully understand the cause and effects of the complicated tectonic setting we encourage the use of a more realistic rheological model of the area which could lead to reinterpretation of previous model results

sand dynamics at the mouth of a rock bound tide dominated estuary abstract the lowermost kennebec river is a relatively narrow deep bedrock cut estuary situated along the highly indented central coast of maine a large tidal prism and strong tidal currents control bedload sediment transport at the estuary mouth riverine sediment is introduced to the estuary during spring freshets when freshwater discharge may increase by an order of magnitude during the same time some of the existing estuarine sediment is exported to the nearshore a barrier adjacent to the estuary is the product of an abundant riverine sand supply and numerous bedrock ridges which anchor this sand deposit a variety of sedimentologic geomorphic and hydraulic data collected over a 14 year period document a clockwise sand circulation cell that involves the exchange of bedload among the entrance channel to the estuary adjacent beaches nearshore and offshore region tidal currents transport sand from the estuary through a subsidiary channel bordered on both sides by bedrock islands to an offshore subtidal bar northeast storm waves breaking along this outer bar move sand westward and landward toward the beach leading to the development of large bar complexes the welding of a bar complex to the western end of hunnewell beach occurs every 6 9 years and adds 2 300 000 m3 of sand to the shoreline sediment transport calculations along the outer bar are consistent with this volume of sand the sediment gyre is completed as beach sands are moved eastward along the beach and dumped into the estuarine channel this movement of sand is produced by wave action and tidal currents that enter the estuary peripherally during the flooding tide during the ebb cycle strong tidal currents flowing seaward through the subsidiary channel advect water from along the beach which enhances sand transport into the estuary thirteen years of beach profile surveys demonstrate a good correspondence between bar welding events and short term 1 4 years accretionary shoreline changes however the volumetric changes of hunnewell beach cannot be accounted for by bar welding alone the apparent movement of sand between hunnewell beach and other sand is further evidenced by the 135 year record of historical shoreline changes that reveal up to 200 m of shoreline excursions over a 26 year period these data suggest that erosional depositional trends along hunnewell beach are complex and the result of the exchange of sand among the adjacent beaches nearshore zone fox and wood island tombolos kennebec river channel and offshore region the effects of major spring floods and strong bedrock influences produce patterns of sedimentation and a distribution of sand bodies at the mouth of kennebec river that differ markedly from accepted estuarine models that are based on the relative importance of wave and tidal energy this study demonstrates the need to consider structural controls when using these models

decadal morphological response of an ebb tidal delta and down drift beach to artificial breaching and inlet stabilisation abstract the morphodynamic response of a mixed energy ebb tidal delta guadiana estuary southern portugal and its down drift barrier island canela island to channel relocation and stabilisation by jetties is examined using a series of sequential bathymetric maps and vertical aerial photographs spanning five decades morphological analysis indicates that the ebb delta is in an immature state characterised by weak sediment bypassing landward migrating shoals on the swash platform have been produced by the jetty induced artificial bank breaching and by the collapse of the eastern portion of the delta the welding of these shoals has largely controlled the evolution of the coast with local accretion and erosion lasting for years and large amounts of regional accretion occurring over decades due to sand accumulation against jetties located further down drift these observations provide insights into the potential response of a coast to very large locally concentrated sand nourishment in the form of shoals the main effects of the jetties on the coast are observed at the centre of canela island with the production of an erosion hot spot associated with a temporally persistent and divergent longshore transport providing sand to the adjacent areas significant accretion is anticipated for the next decade along the entire island due to the ongoing attachment of the presently observed shoals after the depletion of this sediment source and in the context of weak sediment bypassing the most severe down drift erosion induced by the jetties is predicted to occur some decades after their construction this study demonstrates that the geomorphic response of an ebb tidal delta to jetty construction must be considered at multiple temporal and spatial scales when assessing the impacts of jetties on the down drift coast

sea level rise and anthropogenic activities recorded in the late pleistocene holocene sedimentary infill of the guadiana estuary sw iberia abstract this study reviews data on sea level rise during the last 13000 yr cal bp 13 kyr as recorded in the estuarine sediments of the guadiana river se portugal sw spain we combined new data from a 63 m long borehole drilled through the entire postglacial sedimentary sequence with information on five previously studied cores by integrating sedimentological geochemical and palaeontological proxies we were able to make a palaeoenvironmental reconstruction of the guadiana terminal palaeovalley during the last 13 kyr and propose a curve of sea level rise for the sw iberian atlantic margin our foraminifera based palaeoecological reconstruction anchored to a 14 c age model reveals rapid sea level rise from 13 kyr interrupted during the younger dryas and resuming ca 11 5 kyr the pace of marine transgression slackened ca 7 5 kyr and since then has progressed upwards at a rate of 1 2 mm yr 1 holocene anthropocene sediments from two boreholes were also analysed to assess the timing levels and sources of trace metals produced by acid mine drainage from the iberian pyrite belt study of metal aluminium ratios through the profiles allowed background metal concentrations to be estimated from lithostratigraphic units older than ca 5 kyr i e unaffected by anthropogenic activities human activities are especially evident from 4 5 kyr the beginning of the copper age with anthropogenic sources of metal fluxes prevailing over natural sources especially pb co ni and mn and to a lesser extent zn cu and ni mining activities became particularly intensive between the late bronze age and the roman period 3 1 5 kyr when the highest metal enrichment factors were recorded ef pb 2 ef cd 10 ef cr 2 ef cu 3 ef zn 1 4 this study reveals the utility of postglacial sedimentary records for reconstructing historical changes in regional water sediment quality and separating natural and anthropogenic sources of geochemical contaminants

field studies of estuarine turbidity under different freshwater flow conditions kaipara river new zealand abstract we present a first interpretation of three days of measurements made in 2013 from the tidal reaches of the kaipara river new zealand under both low and high freshwater inputs and a neap tidal cycle during the first day we occupied two stations that were approximately 6 km apart in a tidal reach that runs for 25 km from the river mouth to the upstream limit of tidal influence during the second day longitudinal surveys were conducted over a distance of 6 km centred on the upstream station the data reveal a turbidity maximum in the form of a high concentration plug of suspended mud that was advected downstream on the ebbing tide past the upper hb measurement station and which exchanged sediment with the seabed by settling at low slack water and by resuspension in the early flooding tide the data suggest that fine sediment is transported landwards and trapped in the upper part of the tidal reach under these low flow conditions on the third day of measurements we repeated the experiments of the first day but later in the year for a much higher freshwater flow this interpretation of our data set highlights the potential contribution of a range of processes to the generation of the observed suspended sediment signals including resuspension of local bed sediment advection by the tidal current settling of suspended sediment over a long timescale compared to the advection timescale advection of longitudinal gradients in suspended sediment and suppression of vertical mixing by density stratification of the water column the level of temporal and spatial detail afforded by these measurements allows a much clearer understanding of the timing and importance of vertical stratification on the transport of suspended particulate matter than is generally possible using fixed point sensors

new mass increase beneath askja volcano iceland a precursor to renewed activity askja is an active central volcano located on the ns trending en echelon rift zone marking the mid atlantic plate boundary in north iceland between 2007 and 2009 we observed a gravity increase at the centre of the caldera this contrasts with net gravity decreases recorded between 1988 and 2007 interpreted previously in terms of magma drainage the recent gravity increase is rapid but similar in terms of lateral extent to the preceding decrease this gravity increase corresponds to a sub surface mass increase of 0 68 1011kg at about 3km depth it is possible that the new gravity increases observed at askja reflect accumulation of magma beneath the caldera and thus may herald a new phase in the activity of this volcano which last erupted in 1961

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

volcanoes as elastic inclusions their effects on the propagation of dykes volcanic fissures and volcanic zones in iceland mechanically many volcanoes may be regarded as elastic inclusions either softer with a lower young s modulus or stiffer with a higher young s modulus than the host rock matrix for example many central volcanoes stratovolcanoes composite volcanoes are composed of rocks that are softer than the crustal segments that host them this is particularly clear in iceland where central volcanoes are mostly made of soft rocks such as rhyolite pyroclastics hyaloclastites and sediments whereas the host rock is primarily stiff basaltic lava flows most active central volcanoes also contain fluid magma chambers and many have collapse calderas fluid magma chambers are best modelled as cavities in three dimensions or holes in two dimensions entire calderas as holes and the ring faults themselves which commonly include soft materials such as breccias as soft inclusions many hyaloclastite basaltic breccias mountains partly buried in the basaltic lava pile also function as soft inclusions modelling volcanoes as soft inclusions or holes we present three main numerical results the first using the hole model shows the mechanical interaction between all the active central volcanoes in iceland and in particular those forming the two main clusters at the north and south end of the east volcanic zone evz the strong indication of mechanical interaction through shared dykes and faults in the northern cluster of the evz is supported by observations the second model using a soft inclusion shows that the torfajokull central volcano which contains the largest active caldera in iceland suppresses the spreading generated tensile stress in its surroundings we propose that this partly explains why the proper rift zone northeast of torfajokull has not managed to propagate through the volcano apparently torfajokull tends to slow down the rate of southwest propagation of the rift zone part of the evz the third model again using a soft inclusion indicates how the lateral propagation of a segment of the 1783 laki fissure became arrested in the slopes of the hyaloclastite mountain laki

lateral and vertical distributions of living benthic foraminifera off the douro river western iberian margin impact of the organic matter quality abstract in this study we assess the impact of the quantity and quality of the organic matter on the distribution of live benthic foraminifera on a cross margin transect off the douro river western iberian margin surface sediments from five stations ranging from 50 to 2000 m water depth were collected in march 2011 at the end of the period of maximal river runoff in order to better evaluate the importance of terrestrial input living stained benthic foraminiferal assemblages 150 m and environmental parameters grain size total organic carbon toc total nitrogen tn c n ratio 13 c toc pigments and amino acids were investigated organic matter amounts follow the sediment grain size pattern with higher concentration in fine sediments and seem to control faunal density except at the shallowest station where toc is low and faunal density high at this coastal site a massive sandy deposit is identified and associated with very low faunal diversity labile organic matter is richest at the shelf stations where benthic foraminifera standing stocks are high this is particularly evident on the douro mud patch where both density and diversity are high foraminiferal faunas living at the shelf stations are dominated by species e g nonion scaphum ammonia beccari i bulimina aculeata and eggerella scabra characteristic of the rich trophic conditions with organic matter of mixed estuarine and marine origin very low total standing stocks on the upper slope are associated with coarse sediments probably resulting from intense bottom currents at this depth resulting in poor trophic conditions deeper environments are characterized by species e g hoeglundina elegans uvigerina mediterranea and reophax scorpiurus indicative of fairly abundant but low quality organic matter finally the faunal differences between the shelf stations may reflect different successional stages an early stage at 50 m water depth where unstable conditions are observed with faunas dominated by opportunistic species and a more mature fauna in the mud patch where more stable conditions prevail

seasonal variation of sediment deposition in the hudson river estuary abstract during and following the spring freshet rapid rates of sediment deposition were observed in the hudson river estuary side scan sonar surveys and sediment coring studies revealed a large amount of spatial variability in sedimentation within the estuary and a distinct seasonal progression during the freshet sediment was deposited in the seaward reaches of the estuary in the two month period after the freshet this sediment was eroded and sedimentation occurred 10 30 km further landward at the estuarine turbidity maximum etm zone the thickness of the new freshet deposit was as great as 40 cm in the etm indicating intense trapping and rapid deposition the location of this depocenter corresponded to a frontal zone where water column observations and modeling studies have indicated enhanced sediment trapping the mass of the new deposits was estimated at 300 000 metric tons during both 1998 and 1999 with similar spatial distributions in 1998 the estimated fluvial input was 560 000 metric tons significantly greater than the 120 000 metric tons estimated for 1999 thus the mass of the observed deposits appear to be more influenced by the redistribution of sediment within the estuary due to seasonal variations in erosion deposition conditions rather than by the direct input of sediment from the watershed

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

tidally driven residual circulation in shallow estuaries with lateral depth variation tidally driven residual circulation in shallow estuaries with lateral depth variation has been studied analytically using a two dimensional depth averaged model the solution is presented for a v shaped channel exchange flow is found to be correlated with the topography the magnitude of this exchange flow depends mainly on four parameters the ratio between the minimum depth on the shoal and the maximum depth in the channel the ratio between the tidal amplitude at the mouth and the mean depth the ratio between the length of the estuary and the tidal wave length and the ratio between the tidal timescale and the decay timescale due to friction generally a net landward flow occurs over the shoals and is balanced by a return flow in the channel the along channel residual velocity changes nearly linearly across the estuary the residual velocity decreases monotonically toward the head of the estuary the transverse residual velocity is convergent in the channel and divergent on the shoals the residual velocity is highly dependent on the length of the estuary when the length of the estuary is much smaller than a quarter of the wave length the residual velocity is relatively small when the length of the estuary is about a quarter of the wave length the magnitude of the residual velocity reaches its maximum when the length of the estuary is much larger than a quarter of the wave length the residual circulation approaches the pattern found in an infinite length estuary on the basis of the analysis of the solution we conclude that exchange flow generated by tides in a shallow estuary is the result of competition among several processes an inward flux is caused by local nonlinearity both in the bottom friction and from propagation of the tidal wave of finite amplitude this inward flux is larger on the shoals and smaller in the channel owing to larger nonlinearity in the shallow water the residual inward flow creates a setup residual surface elevation of water at the head of the estuary that produces a pressure gradient this residual pressure gradient is approximately uniform across the estuary and drives an outward flow that is larger in the channel than on the shoals in all of the parameter space the competition results in a net inflow on the shoals and a net outflow in the channel

sensitivity of bottom stress and bottom roughness estimates to density stratification eckernforde bay southern baltic sea thermohaline density stratification may significantly alter the classic near bottom logarithmic velocity profile in many weak to moderately energetic partially mixed estuaries results from eckernforde bay suggest fits to log profiles which neglect thermohaline stratification may lead to overestimates of bottom stress and roughness of the order of 130 and 600 respectively measurements of velocity obtained at four heights within 1 m of the seabed are input to theoretical models for velocity shear derived via dimensional arguments for the overlap layer previous investigators applying dimensional arguments to thermohaline stratification in estuaries have assumed buoyancy flux to be independent of height within the overlap layer this may be a poor assumption since there is no significant source or sink of thermohaline buoyancy at the sediment water interface in this paper dimensional arguments which do not assume constant buoyancy flux are used to reduce estimates of the drag coefficient and bottom roughness to below the unreasonably high values predicted by simple log profiles formulations assuming very weak and relatively strong stratification are applied and estimates of buoyancy frequency derived from fits to velocity profiles are compared with independent estimates of stratification estimates of bottom stress and roughness derived from velocity profiles are also found to be sensitive to fluid acceleration uncertainties in instrument settling and limitations in current meter accuracy but these latter effects appear secondary in eckernforde bay to the impacts of thermohaline stratification

interaction between central volcanoes and regional tectonics along divergent plate boundaries askja iceland activity within magmatic divergent plate boundaries mdpb focuses along both regional fissure swarms and central volcanoes an ideal place to investigate their mutual relationship is the askja central volcano in iceland askja consists of three nested calderas namely kollur askja and oskjuvatn located within a hyaloclastite massif along the nne ssw trending icelandic mdpb we performed an extensive field based structural analysis supported by a remote sensing study of tectonic and volcanic features of askja s calderas and of the eastern flank of the hyaloclastite massif in the massif volcano tectonic structures trend n 10 e to n 40 e but they vary around the askja caldera being both parallel to the caldera rim and cross cutting on the western side structural trends around the oskjuvatn caldera are typically rim parallel volcanic vents and dikes are preferentially distributed along the caldera ring faults however they follow the nne ssw regional structures when located outside the calderas our results highlight that the askja volcano displays a balanced amount of regional fissure swarm related and local shallow magma chamber related tectonic structures along with a mutual interaction among these this is different from krafla volcano to the north of askja dominated by regional structures and grimsvotn to the south dominated by local structures therefore askja represents an intermediate tectono magmatic setting for volcanoes located in a slow divergent plate boundary this is also likely in accordance with a northward increase in the spreading rate along the icelandic mdpb

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

integration of micro gravity and geodetic data to constrain shallow system mass changes at krafla volcano n iceland new and previously published micro gravity data are combined with insar data precise levelling and gps measurements to produce a model for the processes operating at krafla volcano 20 years after its most recent eruption the data have been divided into two periods from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at krafla is decaying exponentially the net micro gravity change at the centre of the caldera is shown using the measured free air gradient to be 85 gal for the first and 100 gal for the second period after consideration of the effects of water extraction by the geothermal power station within the caldera the net gravity decreases are 73 17 gal for the first and 65 17 gal for the second period these decreases are interpreted in terms of magma drainage following a mogi point source model we calculate the mass decrease to be 2 1010 kg year reflecting a drainage rate of 0 23 m3 s similar to the 0 13 m3 s drainage rate previously found at askja volcano n iceland based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems we suggest a pressure link between askja and krafla at deeper levels at the lower crust or the crust mantle boundary after the krafla fires co rifting pressure decrease of a deep source at krafla stimulated the subsequent inflow of magma eventually affecting conditions along the plate boundary in n iceland as far away as askja we anticipate that the pressure of the deeper reservoir at krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber possibly initiating a new rifting episode we have demonstrated that by examining micro gravity and geodetic data our knowledge of active volcanic systems can be significantly improved

multiple volcano deformation sources in a post rifting period 1989 2005 behaviour of krafla iceland constrained by levelling tilt and gps observations abstract the krafla rifting episode which occurred in north iceland in 1975 1984 was followed by inflation of a shallow magma chamber until 1989 at that time gradual subsidence began above the magma chamber and has continued to the present at a declining rate pressure decrease in a shallow magma chamber is not the only source of deformation at krafla as other deformation processes are driven by exploitation of two geothermal fields together with plate spreading in addition deep seated magma accumulation appears to take place with its centre 10 km north of the krafla caldera the relative strength of these sources has varied with time new results from a levelling survey and gps measurements in 2005 allow an updated view on the deformation field deformation rates spanning 2000 2005 are the lowest recorded in the 30 year history of geodetic studies at the volcano the inferred rate of 2000 2005 subsidence related to processes in the shallow magma chamber is less than 0 3 cm yr whereas it was 5 cm yr in 1989 1992 currently the highest rate of subsidence takes place in the leirbotnar area within the krafla caldera and appears to be a result of geothermal exploitation

constraints on inner forearc deformation from balanced cross sections fila costena thrust belt costa rica 1 the fila costena thrust belt in the forearc of costa rica is accommodating a significant portion of the convergence of the cocos plate and panama microplate geologic mapping of the thrust belt depicts a duplex with three horses that incorporate eocene limestones and oligocene to early miocene clastics inboard of the subducting cocos ridge axis by constructing a cross section at this location along a ne sw trending transect perpendicular to the thrust belt we constrain a shortening rate of approximately 40 mm a and propose that as much as 50 of the total plate convergence rate is taken up in the inner forearc the eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the cocos ridge axis owing to decrease in slip on faults and a lateral ramp in the basal decollement the thrust belt terminates near the panama border at the onland projection of the subducting panama fracture zone these observations suggest that shortening is propagating to the east with the migration of the panama triple junction and the onset of shallow subduction of the thickened edge of the cocos plate the absence of similar features in the nicaraguan forearc where the subducting crust is older subducts more steeply and lacks incoming ridges and seamounts indicates that deformation of the forearc basin in costa rica reflects greater coupling between the converging plates inboard of the cocos ridge

standardization in gravity reduction gravity reduction standards are needed to improve anomaly quality for interpretation and to facilitate the joining together of different data sets to the extent possible data reduction should be quantitative objective and comprehensive leaving ambiguity only to the interpretation process that involves qualitative subjective and geological decisions the term bouguer anomaly describes a field intended to be free of all nongeologic effects not modified by a partial geologic interpretation measured vertical gradients of gravity demonstrate considerable variation but do not suggest as often reported that the normal free air gradient is in error or needs to be locally adjusted such gradients are strongly influenced by terrain and to a lesser extent by the same geologic sources which produce bouguer anomalies a substantial body of existing literature facilitates the comprehensive treatment of terrain effects which may be rigorously implemented with current computer technology although variations in topographic rock density are a major source of bouguer anomalies a constant density appropriate to the area under investigation is normally adopted as a data reduction standard leaving a treatment of the density variations to the interpretation a field example from british columbia illustrates both the variations in vertical gravity gradients which can be encountered and the conclusion that the classical approach to data reduction is practically always suitable to account for the observed effects standard data reduction procedures do not and should not include reduction to datum the interpreter must be aware however that otherwise smooth regional bouguer anomalies caused by regional sources do contain high frequency components in areas of rugged topography

morphodynamics stratigraphy and sediment transport patterns of the kennebec river estuary maine usa abstract sediment transport and circulation patterns within the lower kennebec river estuary maine 20 km have been investigated over a two year and nine month period using fathometer profiles and side scan sonograms in conjunction with flow measurements fresh water discharge data and grain size data the geologic history of the estuary is inferred from high resolution seismic profiles and bridge borings subbottom data corroborate a five stage evolutionary history that has been determined for other areas of the west central maine coast scattered deposits of glacial till diamict and stratified drift overly a precambrian to paleozoic metasedimentary bedrock basement the glacio marine blue clay of the presumpscot formation unconformably overlies the diamict and drift and drapes the basement where till is absent the clay surface is an erosional unconformity formed during the last sea level lowstand during subsequent sea level rise a relatively coarse grained estuarine fill was deposited within a flood dominated relatively large paleo kennebec river estuary as the rates of sea level rise slowed the system shifted to an ebb dominated estuary in which the estuarine fill underwent reworking and downstream net transport bathymetric data show a hierarchical arrangement of bedforms ranging in size and morphology from well developed ebb oriented transverse bars to superimposed simple straight crested megaripples the transverse bars were stable over the study period the reworking and migration of the smaller forms are closely linked to seasonal variations in the relative contributions between tidal flow and fresh water discharge during the spring large magnitude discharge events augment ebb tidal flows the ebb reinforced flows dominate the system and result in a net downstream transport of medium to coarse grained sand estuarine stratification plays an important role in sediment transport during non spring months from mid summer to fall salinity gradients enhance flood tidal flows and result in minor quantities of upstream transport in addition bedrock bathymetric highs and abrupt changes in channel geometry may influence sediment transport within the estuary

nature of crustal terranes and the moho in northern costa rica from receiver function analysis 1 the central american subduction zone in northern costa rica shows along strike variations in both the incoming and overriding plates by analyzing the subducting oceanic moho m1 and the upper plate moho m2 with receiver functions we investigate the variability in the hydration state of the subducting cocos plate and the nature of crustal terranes within the overriding caribbean plate we calculate high quality p and pp wave receiver functions using broadband data of the global seismology network geoscope project and the crseize pocosol and corisubmod experiments in addition we estimate the depth h and vertically averaged vp vs to moho and present a sensitivity study to explore the effects of a dipping interface on receiver functions and the h and estimates our results are consistent with a drier oceanic mantle subducting beneath the southernmost part of the nicoya peninsula as compared to a serpentinized oceanic mantle subducting beneath the northern part in the caribbean plate we describe the nature of the mesquito nicoya and chorotega terranes by integrating new and published vp vs estimates both the nicoya and chorotega terranes display high vp vs 1 80 1 92 consistent with their oceanic character in contrast the oceanic mesquito terrane mostly displays lower vp vs 1 62 1 80 more compatible with continental crust which may indicate that subduction zone magmatism is modifying the crust to display continental character our estimates show that the deepest m2 42 km is observed in the volcanic arc region whereas the shallowest m2 27 33 km is observed in parts of the fore arc and back arc regions