tracers of uplift and subsidence in the cape verde archipelago abstract ocean islands are subject to vertical movements through their lifetime and these movements9 origins are poorly understood tracking island freeboard provides insight into the mechanisms that cause these movements the cape verde archipelago is the result of hotspot volcanism ranging from 26 ma to the present and constitutes a structural pile that has preserved a record of the palaeo positions of mean sea level these palaeo markers both sedimentary and volcanic are datable and can be used to track the island9s history of vertical movements when used in conjunction with a eustatic curve we have studied the volcanostratigraphy of cape verde and mapped the palaeo marker positions and features and we provide the sea level height information that can be estimated through them evidence exists for differential uplift histories throughout the archipelago with substantial uplift in some of the islands synchronous with volcanism other islands in contrast exhibit evidence for stability or complex uplift subsidence histories supplementary material detailed island geology information is available at http www geolsoc org uk sup18390

reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores the morpho structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass wasting processes we present here a multi disciplinary study of the successive stages of development of faial azores during the last 1 myr using high resolution digital elevation model dem and new k ar tectonic and magnetic data we reconstruct the rapidly evolving topography at successive stages in response to complex interactions between volcanic construction and mass wasting including the development of a graben we show that 1 sub aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca 0 85 ma followed by its partial destruction over half a million years 2 beginning about 360 ka a new small edifice grew on the ne of the island and was subsequently cut by normal faults responsible for initiation of the graben 3 after an apparent pause of ca 250 kyr the large central volcano cv developed on the western side of the island at ca 120 ka accumulating a thick pile of lava flows in less than 20 kyr which were partly channelized within the graben 4 the period between 120 ka and 40 ka is marked by widespread deformation at the island scale including westward propagation of faulting and associated erosion of the graben walls which produced sedimentary deposits subsequent growth of the cv at 40 ka was then constrained within the graben with lava flowing onto the sediments up to the eastern shore 5 the island evolution during the holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano tectonic depression we conclude that the whole evolution of faial island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation each pulse has been separated by considerable periods of volcanic inactivity during which the faial graben gradually developed we propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement as observed historically when immediate vertical collapse of up to 2 m was observed along the western segments of the graben at the end of the capelinhos eruptive crises 1957 58

modeling the development of marine terraces on tectonically mobile rock coasts abstract a mathematical model was used to study the effect of glacially induced fluctuations in sea level on the formation of wave cut terraces on tectonically mobile rock coasts two deep water wave sets were used to calculate breaker height and depth which along with surf zone width bottom roughness and the gradient of the submarine slope dictated the force exerted at the shoreline an erosional force threshold was employed to represent the variable strength of the rocks the model also considered tidal range the time that the water level spent at each intertidal elevation and the protective effect of debris accumulation at the cliff foot three hundred runs were made with constant sea level and with different rates of rising and falling relative sea level rates of erosion increased with the rate of rising and falling sea level but eventually decreased in some runs with very rapid changes in relative sea level fifty five longer runs were also made with a quaternary sea level model that consisted of 26 glacial cycles representing the period from 2 million to 0 9 million years ago and nine cycles of approximately twice the amplitude and wavelength in the last 0 9 million years these runs were made on a landmass experiencing slow 0 11 mm yr 1 or fast 0 74 mm yr 1 positive or negative changes in the elevation of the land it was found that on rising landmasses erosional coastal terraces are formed during interglacial stages and on subsiding landmasses during glacial stages the number of terraces increased with the rate of uplift and subsidence and with the slope of the hinterland terrace gradient increased with tidal range and decreased with rock resistance the was an inverse relationship between terrace width and the strength of the rock

modeling the development of wave cut shore platforms abstract a mathematical model was used to investigate the development of wave cut shore platforms with constant sea level the model considered the effects of deep water wave height spectra period and wavelength breaker height and depth breaker type the width and bottom roughness of the surf zone the gradient of the submarine slope an erosional threshold related to the strength of the rocks the number of hours each year in which the water level is at each intertidal elevation and the amount and persistence of the debris at the cliff foot intertidal erosion rates were calculated according to the force of the surf reaching the shoreline whereas submarine erosion rates were determined using a depth decay variable each of the two hundred and fifty model runs consisted of 300 000 iterations platform width increased and gradient progressively decreased through model runs and states of static equilibrium were attained by the end of one hundred and nine runs when the wave induced force at each point in the intertidal zone had became too weak to continue eroding the rock runs with mesotidal spring tidal range 3 m generally produced subhorizontal platforms with cliff platform junctions at the mean low water neap tidal level a greater variety of profiles was produced in runs with macrotidal spring tidal ranges 9 m although almost 30 of these runs produced gently sloping platforms with average gradients of only 0 3 about two thirds of the simulated profiles had average gradients ranging from about 1 to 3 simulated platform width increased with tidal range and decreased with the rate of submarine erosion rock resistance the roughness or irregularity of the platform surface the amount and persistence of the cliff foot debris and wave period because higher waves break in deeper water than lower waves there was no consistent relationship between simulated platform width and wave height

how permeable is the edge of the arctic vortex model studies of winter 1999 2000 1 the edge of the arctic vortex constitutes a strong barrier to transport however the extent of isolation of the vortex air as a function of altitude and season is relatively poorly quantified in this study by examining the transport of midlatitude air parcels across the vortex edge into the vortex we analyze the permeability of the vortex edge with the three dimensional version of clams chemical lagrangian model of the stratosphere we explore the dilution of the vortex air due to mixing in winter 1999 2000 an artificial passive tracer was initialized on 1 december 1999 inside the polar vortex with a value of 100 and with a value of zero outside the polar vortex using several different definitions of the vortex edge the resulting intrusions of midlatitude air into the vortex show the same mean features this demonstrates that the diagnosed dilution does not strongly depend on the details of the definition of the vortex edge at about the end of march 2000 the vertical structure of the vortex consisted of well isolated pure vortex layers around 500 k and 750 k with some more diluted layers in between and at the vortex bottom the influence of wave activity on the evolution of the intrusion layers is studied the divergence of the eliassen palm flux shows such a high variability during the whole period that it is not possible to assess a direct causality of certain intrusion layers and some specific patterns some characteristics of the vortex edge in particular the shape of the gradient of potential vorticity pv can influence the dilution of the vortex in cases without a distinct maximum in the pv gradient the defined vortex edge may vary substantially from day to day the comparison of some properties of the vortex wind speed pv field area of the vortex the maximum of the pv gradient of undisturbed versus diluted layers and the variation in time of the intrusions were analyzed all observed intrusions begin in conditions of weak pv gradient indicating that the value of the maximum of the pv gradient may be used as a quantitative measure of the permeability of the vortex edge

source to sink sedimentary systems and global carbon burial a river runs through it abstract source to sink sedimentary systems are important settings of carbon cycling serving as sites of carbon transfer between terrestrial and marine reservoirs and as the primary locations for organic carbon burial on earth the age and character of the carbon that is buried at the terminal ends of these systems reflects the sources and transformations of the organic carbon oc throughout their linked terrestrial and marine segments profound differences are observed between large passive and small active margin systems large passive margin systems are characterized by large floodplains and relatively broad shelves where oc has protracted exposure to oxidants rapid burial in prograding subaqueous deltaic clinoforms or bypass to submarine fans however leads to high burial efficiency of terrestrial biospheric oc in some passive margin settings the oc in small active margin systems in contrast follows relatively short pathways from headwaters to seabed this rapid transit facilitated by the important role of storm driven transport in such settings can lead to high oc burial efficiencies the study of oc sources and transformations in contemporaneous source to sink sedimentary systems informs interpretations about the systems in which oc was buried in the geologic past their stratigraphic records of environmental change and their potential to produce petroleum resources

the changing vision of marine minerals abstract non fuel marine minerals are reviewed from the perspective of resources and their value as active analogs that can advance understanding of types of ancient ore deposits that formed in marine settings the theory of plate tectonics is the largest influence in expanding our vision of marine minerals and in developing our understanding of geologic controls of mineralization in space and time prior to the advent of plate tectonics we viewed the ocean basins as passive sinks that served as containers for particulate and dissolved material eroded from land this view adequately explained marine placer deposits heavy minerals and gems aggregates sand and gravel and precipitates phosphorites and manganese nodules although numerous sites of placer mineral deposits are known on continental shelves worldwide current activity pertains to diamond mining off southwestern africa tin mining off southeastern asia and intermittent gold mining off northwestern north america which are all surpassed economically by worldwide recovery of marine sand and gravel in turn dwarfed by offshore oil and gas with the advent of plate tectonics plate boundaries in ocean basins are recognized as active sources of mineralization in the form of hydrothermal massive sulfide deposits and proximal lower temperature deposits hosted in oceanic crust mafic at ocean ridges and felsic at volcanic island arcs and of magmatic ni cu sulfide chromite and pge deposits inferred to be present in the oceanic upper mantle lower crust based on their occurrence in ophiolites some 300 sites of hydrothermal active and relict mineralization most of them minor are known at this early stage of seafloor exploration on ocean ridges in fore arc volcanoes at back arc spreading axes and in arc rifts deposits formed at spreading axes and transported off axis by spreading are present in oceanic lithosphere but are virtually unknown the tag trans atlantic geotraverse hydrothermal field in the axial valley of the mid atlantic ridge latitude 26 n is considered to exemplify a major volcanogenic massive sulfide vms deposit forming at a spreading axis the most prospective of these occurrences lie within the 200 nautical mile 370 km wide exclusive economic zone eez of the nations of the volcanic island arcs of the western pacific where metal content of massive sulfides ag au ba cu pb sb zn exceeds that at ocean ridges plate tectonics early provided a framework for mineralization on the scale of global plate boundaries and is providing guidance to gradually converge on sites of mineralization through regional scales of plate reorganization with the potential to elucidate the occurrence of individual deposits e g eocene carlin type gold deposits investigation of the spectrum of marine minerals as active analogs of types of ancient mineral deposits is contributing to this convergence consideration of questions posed by brian skinner 1997 of what we do and do not know about ancient hydrothermal mineral deposits demonstrates the ongoing advances in understanding driven by investigation of marine minerals

categorizing the morphologic variability of siliciclastic passive continental margins the basic morphology of the continental shelf slope and rise on passive continental margins has many variations in the modern ocean environment through using a satellite derived bathymetric data set of the global ocean we have developed a classification for the shape of modern siliciclastic dominated passive margins the classification subdivides 50 margins into five distinct shape categories based on each margin s pattern of sea floor slopes as a function of depth by comparing these shape categories with the underlying stratal architecture sediment input and present degree of canyon incision of the margins we find apparent correlations between these factors and basic margin shape in general gently sloped margins tend to occur in regions with high sediment input few modern canyons and unstable substrates higher gradient margins tend to have lower sediment input more modern canyons and a variety of subsurface architectures these results suggest two important conclusions 1 morphology differences among modern siliciclastic passive margins can be objectively and systematically categorized and 2 these shape differences seem to be related to aspects of the modern sedimentary environment despite the fact that the margins have evolved over geologic time

episodic swell growth inferred from variable uplift of the cape verde hotspot islands ocean islands formed over hotspots can uplift and subside as the hotspots evolve the history of the cape verde islands reveals large scale uplift owing to growth of the hotspot swell but also variable uplift of individual islands resulting from the local intrusion of magma

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

the morphology of insular shelves as a key for understanding the geological evolution of volcanic islands insights from terceira island azores shelves from volcanic ocean islands result from the competition between two main processes wave erosion that forms and enlarges them and volcanic progradation that reduces their dimension in places where erosion dominates over volcanism shelf width can be used as a proxy for the relative age of the subaerial volcanic edifices and reconstruction of their extents prior to erosion can be achieved in this study new multibeam bathymetry and high resolution seismic reflection profiles are exploited to characterize the morphology of the insular shelves adjacent to each volcanic edifice of terceira island in order to improve the understanding of its evolution subaerial morphological and geological stratigraphic data were also used to establish the connection between the onshore and offshore evolution shelf width contiguous to each main volcanic edifice is consistent with the known subaerial geological history of the island most of the older edifices have wider shelves than younger ones the shelf edge proved to be a very useful indicator in revealing the original extent of each volcanic edifice in plan view its depth was also used to reconstruct vertical movements showing that older edifices like serra do cume ribeirinha guilherme moniz and pico alto have subsided while more recent ones have not the morphology of the shelf namely the absence presence of fresh lava flow morphologies and several types of erosional depositional and tectonic features integrated with the analysis of the coastline morphology allowed us to better constrain previous geological interpretations of the island evolution

synthesizing sea surface height change including seismic waves and tsunami using a dynamic rupture scenario of anticipated nankai trough earthquakes abstract the development of offshore observation technology will provide researchers with tsunami records from within an earthquake focal area but this will create new problems because seismic waves coexist with tsunami inside a focal area the seismic waves could act as noise for the tsunami signal this study shows an efficient method to calculate sea surface height change caused by an earthquake including both seismic waves and tsunami simulation results indicate that seismic waves overlap with tsunami both affect the change in sea surface height although most previous tsunami studies have neglected the contribution of seismic waves we also numerically simulated the sea surface displacement wavefield and hypothesized results for an anticipated rupture scenario of a huge earthquake that may possibly occur in the nankai trough japan the synthesized record could be used to evaluate the performance of a real time tsunami prediction method additionally we discussed the similarity and difference between two kinds of tsunami waveforms the displacement of the sea surface and the pressure change at the sea bottom although seismic waves appeared in both waveforms the contribution of seismic waves was lower in the displacement at the sea surface than in the pressure change at the sea bottom

morphology of the faial island shelf azores the interplay between volcanic erosional depositional tectonic and mass wasting processes 1 the extents of volcanic island shelves result from surf erosion which enlarges them and volcanic progradation which reduces them however mass wasting tectonics and sediment deposition also contribute to their morphology in order to assess the relative significance of these various processes we have mapped in detail faial island s shelf in the azores archipelago based on interpretation of geophysical and geological data the nearshore substrates of the island down to 30 50 m depth are rocky and covered by volcaniclastic boulder deposits formed by surf action on now submerged lava flows below those depths sandy and gravel volcaniclastic beds dominate building clinoforms up to the shelf edge in some sectors of the coast prograding lava has narrowed the shelf but in contrast to nearby pico island we find fewer submarine emplaced lavas on the shelf in this island we interpret the distance between the coastline and the shelf edge as almost entirely a result of a straightforward competition between surf erosion and lava progradation in which erosion dominates therefore shelf width can be used as a proxy for coastline age as well as for wave energy exposure the stratigraphy of shelf deposits in boomer seismic data is examined in detail to assess the roles of different sediment sources accommodation space and wave exposure in creating these deposits we also show evidence of mass wasting at the shelf edge and discuss the possible origins of slope instability finally we discuss the contributing role of tectonics for the development of the shelf

the 40ar 39ar dating of core recovered by the hawaii scientific drilling project phase 2 hilo hawaii 1 the hawaii scientific drilling project phase 2 hsdp 2 recovered core from a 3 1 km thick section through the eastern flanks of mauna loa and mauna kea volcanoes we report results of 40ar 39ar incremental heating by broad beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate mostly from k poor tholeiites the tholeiites generally have mean radiogenic 40ar enrichments of 1 3 and some contain excess 40ar however isochron ages of glass poor samples preserve stratigraphic order in all cases a 246 m thick sequence of mauna loa tholeiitic lavas yields an isochron age of 122 86 kyr all errors 2 at its base beneath the mauna loa overlap sequence lie mauna kea s postshield and shield sequences a postshield alkalic lava yields an age of 236 16 kyr in agreement with an age of 240 14 kyr for a geochemically correlative flow in the nearby hsdp 1 core hole where more complete dating of the postshield sequence shows it to have accumulated at 0 9 0 4 m kyr from about 330 to 200 ka mauna kea s shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level then shallow submarine flows hyaloclastites pillow lavas and minor intrusions to core bottom at 3098 m most subaerial tholeiitic flows fail to form isochrons however a sample at 984 m yields an age of 370 180 kyr consistent with ages from similar levels in hsdp 1 submarine tholeiites including shallow marine vitrophyres clasts from hyaloclastites and pillow lavas were analyzed however only pillow lava cores from 2243 2614 and 2789 m yield reliable ages of 482 67 560 150 and 683 82 kyr respectively a linear fit to ages for shield samples defines a mean accumulation rate of 8 6 3 1 m kyr and extrapolates to 635 kyr at core bottom alternatively a model relating mauna kea s growth to transport across the hawaiian hot spot that predicts downward accelerating accumulation rates that reach 20 m kyr at core bottom depaolo and stolper 1996 is also consistent with all reliable ages except the deepest

crustal structure and origin of the cape verde rise the cape verde islands are located on a mid plate topographic swell and are thought to have formed above a deep mantle plume wide angle seismic data have been used to determine the crustal and uppermost mantle structure along a 440 km long transect of the archipelago modelling shows that normal oceanic crust 7 km in thickness exists between the islands and is gently flexed due to volcano loading there is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle the observed flexure and free air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m 3 the origin of the cape verde swell is poorly understood an elastic thickness of 30 km is expected for the 125 ma old oceanic lithosphere beneath the islands suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere the lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that instead the swell is supported by dynamic uplift associated with the underlying plume

dominant influence of volcanic loading on vertical motions of the hawaiian islands abstract rates of island vertical motion above intra plate hotspots record the processes that arise from interactions between lithospheric plates and mantle plumes to assess the contribution of mantle and lithospheric processes to surface motion above the hawaiian hotspot we compare simple models of lithospheric deformation to vertical motion rates measured from dated paleoshorelines and tide gauge records in the hawaiian islands our analysis shows that observed uplift and subsidence rates mainly record the flexural response of the lithosphere to volcanic loads the effective elastic plate thickness that best fits the spatial distribution of subsidence and uplift rates is 40 km consistent with previous estimates based on total vertical deflection because volcanic loading dominates the vertical motion signal hawaiian islands appear to follow a predictable trajectory of vertical motion when they reside within one flexural half wavelength of the active volcanic center islands initially subside at rapid and decreasing rates in the first 1 myr following their construction uplift relatively slowly 1 2 5 myr following their construction and eventually subside again but at slow rates within 5 myr of their construction this observed pattern of uplift and subsidence is consistent with the pattern of vertical motion predicted to result from volcanic loading at the hawaiian hotspot lithospheric migration over the long wavelength topographic swell associated with the hawaiian hotspot has a comparatively minor influence on island uplift and subsidence its contribution to island vertical motion is not readily observed in our data with the possible exception of some uplift observed in the past 500 kyr on o ahu that might correspond to non steady state behavior of the hawaiian plume

development of volcanic insular shelves insights from observations and modelling of faial island in the azores archipelago a variety of geological processes contribute to the development of volcanic island shelves although their relative importance remains unclear we have mapped the morphology of faial island s shelf in the azores from geophysical multibeam sonar chirp and boomer seismic reflection and geological data sediment sampling a wave erosional model and a eustatic record of glacially induced oscillations of sea level which modulated the elevation at which the waves operated through time was used to determine the contribution of mechanical wave erosion to shelf development wave regimes varied according to the orientation of the coast the model was run for 10 to 800 ka depending on the age of the various coastal sectors based on published radiometric data the modelling replicated the shape of the island s shelf reasonably well supporting the contention that it developed primarily through mechanical wave erosion operating in an intertidal zone that migrated landwards and seawards with changes in the level of the sea disparities between modelled and actual shelf morphologies may reflect varied geological resistance to erosion errors in the radiometric dates more recent unrecognised volcanic deposits sediment deposition and tectonic vertical movements there were some variation in rates of shelf development over different time scales but over the long term the relationship between shelf age and width was essentially linear the depth of the shelf break in some areas suggests that there have been differential vertical movements of the land around the island probably related to local faults the poor development or absence of terraces on the shelf may be attributed either to recent volcanic progradation or to sediment infilling of the space created by earlier erosion however poorly developed terraces on the southern shelf of faial are similar to those predicted by the model given appropriate adjustments for vertical movements of the land

long term growth and subsidence of ascension island constraints on the rheology of young oceanic lithosphere the dating of material from deep boreholes drilled in volcanic ocean islands allows constraints to be placed on their growth and long term subsidence rates we dated lavas from a 3 km geothermal borehole at ascension island by the laser heating 40ar 39ar technique the samples yield ages of up to 3 4 ma and volcanic growth rates of 0 4 km myr the transition from submarine to subaerial eruption occurs at 710 m below present sea level and 2 5 ma since 2 5 ma there has been 430 500 m of subsidence over and above the expected 190 260 m due to lithospheric cooling plausible elastic thicknesses and growth histories would generate a maximum elastic subsidence since 2 5 ma of 200 m we infer that the subsidence includes a component of viscous relaxation resulting from rapid loading prior to 2 5 ma and place constraints on the timescale of this relaxation and hence the viscosity of the underlying lithosphere

gps and tectonic evidence for a diffuse plate boundary at the azores triple junction abstract we use gps bathymetric structural and seismic data to define the pattern of present deformation along the northern half of the azores plateau where the nubia eurasia plate boundary terminates at the axis of the mid atlantic ridge mar new and existing campaign gps velocities from the azores islands reveal extension oblique to a series of en echelon volcanic ridges occupied by terceira s jorge pico and faial islands in a frame of reference defined by 69 continuous gps stations on the eurasia plate terceira island moves 2 1 mm yr away from eurasia consistent with the island s location within the terceira rift and plate boundary structure the volcanic ridges south of the terceira rift move toward wsw at progressively faster rates reaching a maximum of 3 5 0 5 mm yr 2 for the pico faial volcanic ridge the hypothesis that the terceira rift accommodates all nubia eurasia plate motion is rejected at high confidence level based on the motions of sites on s jorge island just west of terceira rift all of the islands move relative to the nubia plate with pico island exhibiting the slowest motion only 1 0 5 mm yr 2 detailed bathymetry from the interior of the hypothesized azores microplate reveals faults that crosscut young mar seafloor fabric these observations and the gps evidence for distributed deformation described above argue against the existence of a rigid or semi rigid azores microplate and instead suggest that nubia eurasia plate motion is accommodated by extension across a 140 km wide zone east of the mar axis most likely bounded to the north by the northern shoulder of the terceira rift the mar spreading rate along the western end of the azores deformation zone 38 5 n 39 5 n is intermediate between the eurasia north america rate measured at 39 5 n and the nubia north america rate measured at 38 5 n consistent with the joint conclusions that the nubia eurasia boundary is broad where it intersects the mar and the azores triple junction is diffuse rather than discrete

the role of subsidence in shelf widening around ocean island volcanoes insights from observed morphology and modeling abstract on reefless volcanic islands insular shelves are thought to have formed essentially by the combined effects of wave erosion and glacio eustatic sea level oscillations subsidence however has also been recognized as having an important role in the development of these morphologies yet few studies have quantified the relative contribution of subsidence to shelf generation and development particularly to shelf width a better understanding of this contribution however is key to understand the long term evolution of coasts at volcanic islands particularly given that subsidence may exacerbate the effects of marine erosion in this study we assess quantitatively the role of subsidence in shelf development by comparing real cross shore shelf profiles with modeled profiles using varied rates of subsidence to achieve this we used shelf bathymetric profiles from faial and sao jorge islands in the azores which we compared with predictions of a numerical model of coastal erosion that has been calibrated previously against other field data the first set of model runs was made to calibrate the model by determining the values that produced shelves with break depths widths and profile shapes that were similar to those observed the second set of runs which served to evaluate the contribution of subsidence to shelf widening revealed that subsidence may have been responsible for increasing shelf widths by almost 2 5 times relative to shelves formed only by the combined effects of wave erosion and glacio eustatic oscillations modeling shelf formation on the same islands but with increased subsidence rates up to 2 2 mm yr resulted in shelf profiles up to 19 km wide a value 3 to 6 times greater than observed on these islands our study therefore reinforces the idea that subsidence is a key contributor to the generation of broad insular shelves given its role in enhancing coastal retreat our shelf evolution modeling also suggests that notwithstanding the crucial role subsidence plays in increasing the width of shelves on islands subjected to energetic wave regimes as it happens in the azores and hawaii modification by erosion is important enough to result in shelf morphologies that are not constructional in essence but rather dominantly erosive surfaces this study therefore contributes to a better understanding of how insular shelves and submarine terraces form and develop by proving important new quantitative insights of the role of subsidence on the generation of these morphologies

geological evolution of the volcanic island la gomera canary islands from analysis of its geomorphology abstract erosion and landslide processes played an important role in shaping the geomorphology of the canary islands but for the heavily eroded la gomera island those processes have been unclear we conducted an integrated study of its offshore extension onshore geology and links between them in order to determine the geomorphological evolution of the island relative to the rest of the archipelago the characteristics of the island s barrancos and its interfluves have led us to differentiate four types of morphological areas whose fluvial networks are in different stages of evolution those barrancos offshore are interrupted by la gomera s shelf and its erosion has been much greater than for the rest of the canary islands such erosion is both a function of time and varied swell action la gomera s insular slope is carved by a system of submarine canyons and channels that have morphology indicative of turbidity current erosion many of the barrancos onshore and the canyons offshore have steps or knickpoints some are the result of varied resistance to erosion and others are explained by other mechanisms a ridge whose crest is covered by cone like shape structures is interpreted as of volcanic origin with the particularity that it lacks onshore continuation the opposite of what has been observed elsewhere in the canary islands also in contrast with the other islands of the archipelago la gomera s margin does not seem to have resulted from destructive mass wasting and the only offshore sign of catastrophic events are the presence of two embayments along la gomera s northern shelf edge and a lobe structure on the insular apron at 3000 m depth these features may be the remains of the catastrophic failures that took place on the island 9 4 8 7 ma ago alternatively they could be due to recent catastrophic failures on the upper slope due to sediment steeping since the conclusion of the main volcanic activity 4 0 ma ago la gomera has been intensively eroded by gradual fluvial denudation and secondary failures la gomera s insular margin reveals much regarding its evolution but more comprehensive studies including seismic and coring are needed to understand its whole geological history

geomorphologic equifinality a comparison between shore platforms in hoga kusten and faro sweden and the vale of glamorgan south wales uk this paper presents a case study comparing shore platforms of the same lithology and different rock structures limestone with those of different lithologies but similar rock structures limestone dolerite in wales and sweden results of this study illustrate that different erosion mechanisms i e wave action and ice scour produce similar erosion products and meso scale cm m landforms i e stepped shore platforms in structurally similar lithologies in contrast lithologically similar rocks i e limestones generate different meso scale landforms and shore platform morphologies thus structural controls appear to be more critical than lithological controls in generating meso scale erosion products such as rock blocks in this comparative study the similarities in the meso scale geomorphology appear to be caused by uniform rock structure i e rock hardness joint density spacing of bedding horizontal jointing and the slope and direction of bedding horizontal jointing in relation to the sea blocks produced by the jointing have been eroded by high energy waves in the case of wales and drifting sea ice in the case of one of the swedish sites meanwhile different rock structures in a similar lithology i e limestone resulted in very different landforms under the same environmental conditions as the previous example as such this research also provides observational evidence supporting the concept of equifinality

unraveling the tilting history of the submerged reefs surrounding oahu and the maui nui complex hawaii there is considerable uncertainty concerning the history of lithospheric flexure of oahu and the maui nui complex associated with the hawaiian hot spot previous reconstructions based on models and sparse observational data place the zone of maximum uplift forebulge either under oahu or between oahu and molokai to address this issue we have compiled and analyzed all existing and new high resolution multibeam bathymetry dive observations and chronologic data from submerged terraces in the region we have identified 89 separate submarine terraces and mapped their distribution across the volcanic flanks of oahu and the maui nui complex these data are used to systematically measure the direction and amplitude of terrace tilting within the maui nui complex and to provide new constraints on lithospheric flexure associated with volcanic loading at the hot spot the terraces are divided into six separate geographic regions oahu molokai penguin bank lanai maui and east maui defined by significant discontinuities in either terrace morphology or depth following the sequential development of new substrate as the new volcanoes formed throughout the development of the mnc 18 new and 51 published 87sr 86sr isotopic ages show that the terraces in these regions were also initiated at different times and the tilting data show that they also responded to lithospheric flexure independently the general pattern of terrace tilting records the plate s response to lithospheric flexure with each region continually tilting toward the zone of maximum subsidence since their formation our individual tilt calculations are based on the most complete and highest resolution bathymetric data available and they show that plate response to volcanic loading is a gradual process finally we demonstrate that the shallowest terraces we identified in the oahu and molokai regions tilt toward the north away from the current zone of maximum subsidence indicating that these regions have recently passed the zone of maximum uplift we compare our results with two separate published numerical models and differentiate between these resolving a long standing uncertainty in the position of the forebulge

on the role of discontinuities in mediating shore platform erosion in this paper we quantitatively demonstrate that dominant erosion processes on different shore platforms can be explained in part by geological properties we selected sites with a considerable history of rock coast geomorphology study to evaluate whether geological data can improve our understanding of spatial variations in erosion between sites schmidt hammer tests and discontinuity data were collected on blue lias limestone shore platforms in wales uk and sandstone platforms in victoria australia significant differences in joint spacing persistence of discontinuities and block size were found between the australian and welsh study sites these data were related to evidence of meso scale erosion to examine how geological contingency influences the nature and scale of erosion processes and thresholds this study quantitatively demonstrates that discontinuities play a fundamental role in altering rock resistance the findings of this research have important implications for the design of rock coast erosion studies concerned with measuring rates of change and determining the relative contributions of different processes to platform development

comment on reconstructing the architectural evolution of volcanic islands from combined k ar morphologic tectonic and magnetic data the faial island example azores by hildenbrand et al 2012 j volcanol geotherm res 241 242 2012 39 48 abstract hildenbrand et al 2012 have presented a model for the geological evolution of faial island in their interpretation the oldest volcanic system extended much further than the present day eastern part of the subaerial island with a ridge like morphology into the faial s jorge channel furthermore the western side of the system was suggested to have been later destroyed by mass wasting based on now extensive multibeam bathymetry collected surrounding faial island we find no evidence supporting such interpretations the geometry of the shelf edge suggests that a near circular edifice occupied the eastern part of the present day subaerial island in addition the submarine morphology does not show massive mass wasting deposits around the base of the edifice rather bathymetric gradients and morphology suggest that the submarine slopes were formed from volcanic emplacements or from material displaced in only small scale mass movements or sedimentary flows and have not failed catastrophically evidence that a ridge like feature extended into the faial s jorge channel is also lacking these observations highlight the potentially valuable information that sonar data can provide to evaluating ocean island evolution from the more limited extend of outcrops on land