late cenozoic sea level and the rise of modern rimmed atolls this paper is not subject to u s copyright the definitive version was published in palaeogeography palaeoclimatology palaeoecology 451 2016 73 83 doi 10 1016 j palaeo 2016 03 018

impact of tectonic and volcanism on the neogene evolution of isolated carbonate platforms sw indian ocean understanding the impact of tectonic activity and volcanism on long term i e millions years evolution of shallow water carbonate platforms represents a major issue for both industrial and academic perspectives the southern central mozambique channel is characterized by a 100 km long volcanic ridge hosting two guyots the hall and jaguar banks and a modern atoll bassas da india fringed by a large terrace dredge sampling geo physical acquisitions and submarines videos carried out during recent oceanographic cruises revealed that sub marine fl at top seamounts correspond to karsti fi ed and drowned shallow water carbonate platforms largely covered by volcanic material and structured by a dense network of normal faults microfacies and well constrained stratigraphic data indicate that these carbonate platforms developed in shallow water tropical envi ronments during miocene times and were characterized by biological assemblages dominated by corals larger benthic foraminifera red and green algae the drowning of these isolated carbonate platforms is revealed by the deposition of outer shelf sediments during the early pliocene and seems closely linked to 1 volcanic activity typi fi ed by the establishment of wide lava fl ow complexes and 2 to extensional tectonic deformation associat ed with high offset normal faults dividing the fl at top seamounts into distinctive structural blocks explosive vol canic activity also affected platform carbonates and was responsible for the formation of crater s and the deposition of tuff layers including carbonate fragments shallow water carbonate sedimentation resumed during late neogene time with the colonization of topographic highs inherited from tectonic deformation and volcanic accretion latest carbonate developments ultimately led to the formation of the bassas da india modern atoll the geological history of isolated carbonate platforms from the southern mozambique channel represents a new case illustrating the major impact of tectonic and volcanic activity on the long term evolution of shallow water car bonate platforms

modeling the effect of late quaternary interglacial sea levels on wave cut shore platforms abstract a mathematical wave erosional model was used to study the effect of high sea levels during the penultimate oxygen isotopic stage 7 and last interglacials substage 5e and in the late holocene stage 1 on the present morphology of wave cut shore platforms sea level was considered to have been either the same as today or 2 25 m lower during the penultimate interglacial and 2 25 4 5 or 6 75 m higher than today during the last interglacial stage the model suggested that inherited gently sloping shore platforms in resistant rocks may be essentially protected today from erosion by high storm waves the lowest platform gradients were in runs with mesotidal 3 m range and usually with low wave periods low surf attenuation rates and weak rocks modern platform gradients increased with the difference in elevation between sea levels during successive interglacial stages shore platforms were widest in runs in which sea level was the same as today in the penultimate interglacial and 4 5 m higher than today during the last interglacial constant sea level and high last interglacial sea levels with considerable overlapping between the zones of high duration values between the mean neap high and low tidal levels in stages 7 5e and 1 were conducive to the development of wide shore platforms whereas sea levels lower than today s in stage 7 tended to produce narrower platforms in general higher sea levels during the last interglacial tended to produce higher cliff platform junctions than constant sea level unless the sea was lower than today during the penultimate interglacial stage there was a lack of supratidal ledges in macrotidal 9 m model runs with high initial gradients this suggests that gently sloping inherited shore platforms are essential for the subsequent development of supratidal nonstructural ledges in high tidal environments during periods of higher sea level intertidal ledges developed in the upper portion of the modern intertidal zone under a variety of tidal and sea level conditions these ledges can develop independently of lithological or structural influences and without any change in sea level a single high sea level may also simultaneously produce two ledges at different supratidal elevations in mesotidal environments

sea level rise depth dependent carbonate sedimentation and the paradox of drowned platforms a mathematical model of carbonate platform evolution is presented in which depth dependent carbonate growth rates determine platform top accumulation patterns in response to rising relative sea level this model predicts that carbonate platform evolution is controlled primarily by the water depth and sediment accumulation rate conditions at the onset of relative sea level rise the long standing paradox of a drowned platform arose from the observation that maximum growth rate potentials of healthy platforms are faster than those of relative sea level rise the model presented here demonstrates that a carbonate platform could be drowned during a constant relative sea level rise whose rate remains less than the maximum carbonate production potential this scenario does not require environmental changes such as increases in nutrient supply or siliciclastic sedimentation to have taken place a rate of relative sea level rise that is higher than the carbonate accumulation rate at the initial water depth is the only necessary condition to cause continuous negative feedbacks to the sediment accumulation rates under these conditions the top of the carbonate platform gradually deepens until it is below the active photic zone and drowns despite the strong maximum growth potential of the carbonate production factory this result effectively resolves the paradox of a drowned carbonate platform test modelling runs conducted with 2 5 m and 15 m initial sea water depths at bracketed rates of relative sea level rise have determined how fast the system catches up and maintains the keep up phase this is the measure of time necessary for the basin to respond fully to external forcing mechanisms the duration of the catch up phase of platform response termed carbonate response time scales with the initial sea water depth and the platform top aggradation rate the catch up duration can be significantly elongated with an increase in the rate of relative sea level rise the transition from the catch up to the keep up phases can also be delayed by a time interval associated with ecological re establishment after platform flooding the carbonate model here employs a logistical equation to model the colonization of carbonate producing marine organisms and captures the initial time interval for full ecological re establishment this mechanism prevents the full extent of carbonate production to be achieved at the incipient stage of relative sea level rise the increase in delay time due to the carbonate response time and self organized processes associated with biological colonization increase the chances for platform drowning due to deepening of water depth ca 10 m furthermore this implies a greater likelihood for an autogenic origin for high frequency cyclic strata than has been estimated previously

accommodation and sedimentary architecture of isolated icehouse carbonate platforms insights from forward modeling with carb3d abstract a fundamental control on the evolution of carbonate platforms is accommodation the space available for sediment deposition here we adopt a sensitivity analysis approach using the process based forward model carb3d to quantitatively investigate the sedimentary response of isolated carbonate platforms to individual accommodation drivers of eustasy subsidence and surface dissolution sediment production the mechanism invoked to fill accommodation is also investigated fourth order milankovich driven sea level cycles dominate icehouse carbonate sedimentology with 400 ky cycles modulating the effect of 100 ky cycles of comparable magnitude when both cycles are present both overwhelm higher frequency lower amplitude fifth order cycles which are rarely preserved on the platform top subsidence is a fundamental control on sedimentary stacking patterns sequence boundary frequency sbf and platform geometry with rapid subsidence increasing the preservation of platform top sequences most icehouse platforms are subject to extended periods of platform top exposure missed beats when the platform top remains emergent at a sea level maximum are common and create non milankovich sbfs furthermore during platform emergence surface lowering via surface dissolution also creates accommodation for the subsequent platform flooding event with surface dissolution rates in humid climates comparable to moderate rates of subsidence however accommodation generation by surface dissolution is self limiting in as much as accommodation generated by dissolution is rapidly refilled on flooding sediment production is a key control on unfilled accommodation which our simulations suggest is considerable on icehouse platforms increased shallow water 3 m depth sediment production enhances intra platform bathymetric relief between the margin and interior this is driven in large part by a higher sedimentation rate at the margin than in the interior platforms can be flooded to depths up to 45 m during marine transgressions and accommodation often remains unfilled there are two distinct phases of platform top sedimentation during the transgression and early highstand platform margins accumulate at a greater rate than interior sediments generating differential intra platform bathymetry subsequently accommodation in the interior fills by forced regression and progradation from the margins into the platform interior numerical forward models can augment our understanding of icehouse carbonate platform evolution by identifying the nature of the sedimentary response to individual controls they can thus contribute to improving characterization of carbonate reservoirs in icehouse platforms such as the large hydrocarbon fields in the north caspian and s e asia

fracture types in the lower cambrian shale and their effect on shale gas accumulation upper yangtze abstract to reveal the characteristics of fractures in high quality shale reservoir and to find regions that are favorable for shale gas accumulation the characteristics of fractures and their effect on the shale gas content in the lower cambrian marine shale from 2 typical shale gas wells in the upper yangtze have been analyzed in detail by observing the shale cores scanning electron microscope photographs and rock slice pictures associated with various geochemical data five types of macrofractures high angle shearing fracture tension shearing fracture compresso shearing fracture low angle slip fracture and bedding fracture and 4 types of microfractures interlayer fracture inter particle fracture intra particle fracture and organic matter associated fracture have been identified in the lower cambrian shale the fractures in lower cambrian shale have 5 possible origins 1 generated by tectonic tension and shearing stresses 2 cracking due to loading pressure relief 3 corroded by acidic fluids during hydrocarbon generation 4 induced by overpressured fluid 5 formed due to diagenesis associated shrinkage of mineral crystals in general the shale in sichuan basin contains more nonstructural fractures including bedding fractures and microfractures whereas shale outside the basin contains more structural fractures including high angle shearing fractures tension shearing fractures and low angle slip fractures three controlling factors of fracture development be summarized as follows 1 tectonic setting 2 mineral components 3 organic matter content and maturity in conclusion structural fractures can be considered as destructive and nonstructural fractures can be considered as favorable shale gas reservoirs can be divided into 3 types type a with a shale gas content 2 m3 t destructive structural fractures 100 m type b with a shale gas content of 1 m3 t 2 m3 t destructive structural fractures 20 m or destructive structural fractures

growth and demise of cenozoic isolated carbonate platforms new insights from the mozambique channel seamounts sw indian ocean although long term evolutions of isolated shallow water carbonate platforms and demise episodes leading to guyot formation have been the subject of numerous studies during the last decades their driving processes are still the subject of active debates the mozambique channel sw indian ocean is characterized by several flat topped seamounts ranging from 11 s to 21 s in latitudes based on a comprehensive geomorphologic study and on dredged samples analysis we show that these features correspond to tropical isolated shallow water carbonate platforms coupling strontium isotopy and foraminifera biostratigraphy well constrained chronostratigraphy results indicate that shallow water carbonate production started in the mozambique channel during distinct cenozoic periods ranging from paleocene to early miocene our data also demonstrate that these carbonate platforms were subsequently characterized by different evolutions locally marked by tectonic and rejuvenated volcanism while some of them kept developed until present days forming modern carbonate systems some others were drowned during late neogene and subsided to form guyots although different factors can be discussed tectonic and volcanism appear as good potential triggers for demise episodes during late miocene early pliocene times chronology and location of this geodynamical activity tend to emphasize influence of east african rift system until southern mozambique channel

evolution of the glorieuses seamount in the sw indian ocean and surrounding deep somali basin since the cretaceous abstract little is known about the geological history of the glorieuses seamount including basic information about its age and origin related to the regional evolution of the southern tip of the somali basin this study focused on describing and reconstructing the long term stratigraphic evolution of the glorieuses seamount sw indian ocean to identify the mechanisms that have occurred through time to finally shape the emerged modern islands distinct terrace levels currently submerged along the flanks of the seamount and surrounding seamounts have already been interpreted as resulting from successive carbonate development and back stepping episodes over the last 62 myr new isotopic and biostratigraphic dating on the flanks of the seamount coupled with sequence stratigraphic interpretation of seismic profiles acquired in the adjacent basin provide new constraints for the late cretaceous and cenozoic vertical evolution of the seamount topped by carbonate platforms and sedimentation in the surrounding deep basin even if starved steep slopes prevent a straightforward source to sink continuity between the platform and the basin domains our findings propose a consistent chronostratigraphic framework for the identified seismic markers and sequences in the deep basin and discuss a long term geological model that includes the main driving factors behind deposition volcanic events subsidence vs uplift phases climate and hydro dynamism changes and their quantitative impact on the evolution of the isolated carbonate sedimentary system our results show that i the glorieuses volcanic seamount emerged from two successive late cretaceous magmatic pulses firstly during the turonian then during the maastrichtian ii at least two potential uplift phases are recognized during the tertiary paleogene and or the eocene and tortonian iii basinal sedimentation recorded an abrupt change probably related to major regional hydro dynamical changes in late eocene times in the western indian ocean iv the export of sediments from the platform towards the basin numerous gravity flow processes is strongly enhanced after the mid miocene and is probably linked to the onset of the asian monsoon winds and bipolar circulation finally the glorieuses seamount although located in the vicinity of the comoros islands appears to have a much longer history and is geologically more comparable to the nearby seychelles this long term study has enabled us to associate the glorieuses seamount with the sse nnw madagascar seychelles alignment rather than with the comoro hot spot evolution

seismic architecture and sediment distribution within the holocene barrier reef lagoon complex of mayotte comoro archipelago sw indian ocean twenty gravity cores and a large set of high resolution seismic profiles from various lagoonal settings were studied to determine the holocene sediment distribution and sequence architecture within the mayotte barrier reef lagoon complex the holocene seismic sequence comprises a type 1 sequence with lowstand transgressive and highstand systems tracts the lowstand systems tract consists of a paleosoil horizon formed during subaerial exposure the transgressive systems tract is composed of four depositional systems 1 inner transgressive layer 2 proximal and distal incised valley fills 3 mid lagoonal layer and 4 keep up or catch up fringing and barrier reef sequence the highstand systems tract comprises three depositional systems 1 of a proximal terrigenous wedge 2 mid lagoonal and distal carbonate sands or muds and 3 reefal carbonates our studies show that the nature of the holocene sequence is controlled by the rate and amplitude of sea level rise and environmental changes which are expressed by changes in clastic sediment supply and carbonate production the pre holocene topography and water dynamics steer the vertical and spatial sediment thickness distribution of the holocene additional important parameters are the proximity to a source area carbonate or terrigenous and the width of the depositional area climate dynamics are also of great importance while they determine carbonate production and terrigenous runoff sedimentation rates in the subtidal settings always lacked behind sea level rates thus a steep relief was created keeping most lagoonal parts within the deep subtidal realm in which sediment production was not efficient enough to fill up accommodation space in addition wave and or current energy might prevent the fill up of the lagoon this ultimately resulted in a typical empty bucket morphology only a high amplitude sea level fall would allow the subtidal lagoon to build up to base level unfilled accommodation space therefore must be a very common feature in the geologic record

investigating carbonate platform types multiple controls and a continuum of geometries abstract current classifications of carbonate platforms use depositional gradient as the main criterion for separating systems into two end member types ramps and flat topped platforms ftps however many examples do not conform to this simple classification to investigate why this is and to better understand probable controls on platform development we have used a series of 2d numerical forward model runs to investigate how sediment production diffusional sediment transport and other controls such as tectonic subsidence antecedent topography and relative sea level oscillation interact to determine platform geometry modeling results reaffirm that rates of down dip sediment transport relative to rates of autochthonous production are a critical factor in maintaining a ramp profile in stable cratonic settings under a constant rate of relative sea level rise type of carbonate production versus water depth curve for example euphotic versus oligophotic is not a significant control in our model cases both euphotic and oligophotic production profiles produce ftps when diffusion coefficients are low relative to production rates and ramps when diffusion coefficients are relatively high these results suggest a continuum of platform types ranging from transport dominated low gradient systems at one end of the spectrum to in situ accumulation dominated systems at the other a system may be transport dominated because high energy processes are able to break down and transport even bound sediment or because carbonate factories produce only sediment that is easily transportable under even low energy conditions time evolution is also probably important initially low gradient systems will in the absence of sufficiently high sediment transport rates tend to evolve towards high gradient flat topped steep margined platforms many observed or inferred platform geometries are therefore likely to be transient forms and this could complicate interpretation investigating how basin bathymetry and style of subsidence control platform geometry suggests that in transport dominated systems strata simply drape the underlying topography and that pre existing breaks of slope and differential fault subsidence are a stronger control on platform geometry in in situ accumulation dominated systems rotational subsidence tends to create transport dominated systems during rotation as the topographic gradient increases and transport rate increases and outpaces in situ production rate relative sea level oscillations tend to move the locus of sediment production laterally along any slope present on the platform distributing the sediment accumulation across the whole width of the platform suppressing progradation and steepening and so favoring development of low gradient systems based on all these results we suggest that a simple cutoff classification into ramp and flat topped platform types can still be useful in some circumstances but a more meaningful approach may be to describe and predict platform strata in terms of a multiple dimension platform parameter space containing a continuum of geometries controlled by sediment production sediment diffusion coefficient antecedent topography differential subsidence effects relative sea level oscillations and perhaps other as yet unappreciated controls

the magnitude of a mid holocene sea level highstand in the strait of makassar abstract knowledge on the timing and magnitude of past sea level changes is essential to understand modern and future sea level variability holocene sea level data from literature on the west coast of sulawesi central indonesia suggest that this region experienced two relative sea level highstands over the last 6000 years with magnitudes exceeding two meters however recent datasets from the indo pacific region do not support high magnitude sea level oscillations during the holocene in tectonically stable far field locations here we present a new high precision mid holocene sea level dataset from the spermonde shelf off southwest sulawesi we surveyed 21 fossil microatolls on the reef flats of two coral islands pulau panambungan and pulau barrang lompo and referred their elevations to local mean sea level and to the height of living coral radiometrically calibrated ages from emergent fossil microatolls on pulau panambungan indicate a relative sea level highstand not exceeding 0 5 m above present at ca 5600 cal yr bp the highstand is followed by a relatively rapid sea level fall towards present sea level that was reached at around 4000 cal yr bp fossil microatolls from nearby pulau barrang lompo show the same trend however with a coherent negative vertical offset of about 0 8 m compared to their equivalents on pulau panambungan the largely consistent gradients of both trends 0 14 mm yr 1 the consistent elevation of living microatolls in the spermonde and a number of instructive geomorphic features indicate a localized post formational and probably recent drop of the fossil microatolls on the densely populated island pulau barrang lompo the relative sea level trend inferred from pulau panambungan is well within the range of geophysical predictions based on anice selen ice sheet model which predict a highstand that is significantly lower than those predicted by other gia models for this area although a complete interpretation of the holocene sea level history will require additional high resolution datasets from this and surrounding territories in se asia our results suggest that there was merely a single holocene highstand in central indonesia the magnitude of which was substantially lower than hitherto assumed

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

computer modeling the internal architecture of carbonate platforms a numerical computer model is described that calculates the internal architecture of carbonate platforms in response to varying values of carbonate production subaerial and submarine erosion sediment redeposition and sea level changes the computer generated sections closely resemble large scale outcrops and interpreted seismic profiles through carbonate platforms stillstand and transgressive sequences have prograding and downlapping platform geometries with lagoons developing in transgressive systems regressive sequences have downlapping clinoforms and erosional upper surfaces glacioeustatic scale cycles have a major control on platform geometry with erosional sequence boundaries developing during low stands and platform drowning occurring during transgressive periods lowstand downlapping wedges are minor features when compared with clastic systems and major progradation and downlap of slope deposits develop with transgressions and flooding of platform tops carbonate erosion rates are varied and have an important effect on the morphology of foodback surfaces which have a major control on platform top production the computer program contributes to the analysis of carbonate systems in two ways it gives a visual picture of the quantitative effects of the many parameters controlling carbonate geometries and it aids quantitative analysis of the architectures and time scales of ancient outcrop or seismic sequences

numerical modeling of the growth and drowning of hawaiian coral reefs during the last two glacial cycles 0 250 kyr 1 drowned coral reefs on rapidly subsiding margins possess a unique archive of sea level and climate changes generally unavailable from stable or uplifting margins using available field observations and sedimentary radiometric age and numerical modeling data we propose a new model of submerged reef development around hawaii during the last two glacial cycles 250 kyr this model provides a quantitative predictive stratigraphy for the reefs that we argue if drilled will yield new information on sea level and climate changes as well as coral reef response over the last 250 kyr comparing the observational and numerical modeling data combined with sensitivity testing we present our best case scenario for the evolution of the drowned lowstand reefs now at 400 h2 and 150 m h1 we find that growth rates of 2 5 2 85 m kyr for the main shallow reef building facies a subsidence rate of 2 5 m kyr and a variable basement substrate configuration best explain the observational data modeling of the internal stratigraphic succession of the reefs shows that the number and thickness of shallow reef units as well as the frequency and duration of subaerial exposure and reef drowning events are sensitive to the frequency and amplitude of eustatic sea level variations but not the rate of subaerial erosion h2 and h1 initiated growth during stable eustatic sea level conditions during highstands circa 222 ka mis7 and circa 126 ka mis5e respectively both h2 and h1 have a long and complex growth history growing episodically for 90 kyr precessional 20 kyr and higher frequency suborbital eustatic sea level fluctuations dominate with each reef experiencing repeated but brief 5 10 kyr drowning and subaerial exposure producing a complex layer cake stratigraphy of shallow 0 30 m coral reef units separated by either subaerial exposure horizons or thin intermediate 30 60 m coralgal units final drowning of h2 and h1 occurs during the penultimate 133 134 ka and last deglaciation 12 14 ka these findings are consistent with available age data and qualitative predictions of previous studies around hawaii

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

profiles of ocean island coral reefs controlled by sea level history and carbonate accumulation rates modern and preserved coral reefs on islands exhibit a broad range of forms from actively accreting fringing and barrier reefs to terraces preserved by drowning or subaerial exposure darwin s canonical model of reef development proposes an evolutionary sequence of reef forms as a volcanic island ages and subsides from fringing reef to lagoon bounding barrier reef to atoll compiled data from modern systems show however that many islands do not follow this sequence implying that reefs are shaped by more than island subsidence alone we show that the diversity of modern reef morphology arises from the combined effects of island subsidence coral growth and glacial sea level cycles a model for the evolution of a reef elevation profile over the past 400 k y reveals that different combinations of reef accretion rate and island vertical motion produce a variety of forms that matches the observed distribution of modern reefs this match occurs only if the model is driven by pleistocene sea level oscillations few modern environments have the right combination of conditions to produce the darwinian atoll progression

observations of flexure and the rheology of oceanic lithosphere the principal evidence for the long term i e 1 myr mechanical behaviour of the oceanic lithosphere has come from studies of how it deforms in response to large loads such as volcanoes and sediments a model widely used to explain the deformation is an elastic plate in which flexural rigidity depends on the thermal age of the lithosphere at the time of loading an elastic model however is time invariant and does not take into account temporal changes that may occur in the flexural rigidity as a consequence of loading there is evidence for example that the flexural rigidity of the oceanic lithosphere also depends on load age being large at short times and small at long times thus competing effects may exist between thermal cooling which strengthens the lithosphere and some form of load induced stress relaxation which weakens it in order to investigate the relative roles of these processes we have developed a multilayered viscoelastic model which is based on the results of experimental rock mechanics that creep in the lithosphere is a thermally activated process and on a thermal structure that is given by the plate cooling model by comparing the predictions of the model with a new compilation of flexural rigidity estimates we have found that if the upper mantle viscosity is 1020 pa s the activation energy that best describes the long term mechanical behaviour of the oceanic lithosphere is 120 kj mol 1 this parameter pair explains the dependence of flexural rigidity on both plate and load age it also helps account for the subsidence and uplift history of oceanic islands and the stratigraphic patterns that develop in the flexural moats that flank them at atolls a multilayered viscoelastic model explains the rapid subsidence that follows shield building and does not require that the age of the lithosphere that supports a volcano is thermally re set to a younger value our studies suggest that while the oceanic lithosphere has viscoelastic properties the viscosity of its upper layers is so much higher than that of its lower layers that in effect it behaves as a thin elastic plate on long timescales there will therefore be a certain permanence to the observations of oceanic flexure such that they may be used with some confidence to evaluate the tectonic setting of individual features of the seafloor and in some cases their age

computer simulation of reef growth light is one of the major controls on reef growth and carbonate production the growth of present reef builders depends largely upon the amount of light available for photosynthesis as light decreases with water depth so does reef growth the computer model presented extends this principle by combining two functions one for photosynthesis and the other for the extinction of light in water the model is used to simulate the growth of alacran reef mexico two reefs of the great barrier reef and the reefs of the windward platform of st croix us virgin islands the model also gives an accurate simulation of the growth of fore reef walls in belize in agreement with the accretion hypothesis developed for this feature