vertical magnetic structure of ocean crust determined from near bottom magnetic field measurements consensus on the source of marine magnetic anomaly stripes has remained one of the most elusive aspects of the vine and matthews seafloor spreading model which has been of such fundamental importance to global plate tectonics the first attempt to measure the vertical magnetic structure of oceanic crust exposed at a scarp face using near bottom magnetic measurements is reported along with the theoretical basis for the reduction and analysis of these measurements magnetometers mounted to deep tow and submersible platforms were used to determine the horizontal and vertical variation of crustal magnetization of the upper 2 km of young oceanic crust exposed by a steep submarine escarpment at the blanco fracture zone in the northeast pacific ocean results show a large amplitude magnetic anomaly is consistently found at the extrusive to intrusive transition indicating that the extrusives are strongly magnetized compared to the weakly magnetized intrusive crust profiles also show the systematic presence of more than one polarity with depth indicating the possibility of dipping polarity boundaries within the extrusives forward models find that a dipping polarity boundary model correctly predicts the amplitude and shape of the overlying sea surface anomaly while a block model with vertical polarity boundaries overestimates the amplitude these in situ results are used to directly assess the contribution made by the extrusive section to the overlying magnetic anomaly signal in essence to define the magnetic source layer it is concluded that the extrusive lavas constitute the main magnetic source for magnetic anomalies in oceanic crust less than 2 m y in age

development and demise of a fringing coral reef during holocene environmental change eastern ningaloo reef western australia abstract reefs lining the western exmouth gulf located at the northern limit of the 300 km long ningaloo reef in western australia represent modern incipient coral reefs and veneers of non reef building coral algal communities on exposed pleistocene or give up holocene reef surfaces acquisition of sixteen cores alongside u series tims dates were used to confirm the nature of the pleistocene foundation and characterise holocene reef development three calcretised pleistocene units were identified as 1 the last interglacial mis 5e reef directly underlying holocene units 2 a mid pleistocene mis 7 bioclastic conglomerate unit and 3 a pleistocene alluvial fanglomerate eight holocene reef facies total thickness of 1 8 5 3 m included coral framework facies domal arborescent mixed tabulate and encrusting and detrital facies carbonate sand skeletal rubble and alluvial fan deposits holocene ages range from 7 93 to 5 8 ka bp with vertical accretion ranging from 1 46 to 9 88 mm year avg 4 11 mm year highest rates of accretion and thickest accumulation occurred in the most seaward and deepest cores composed of massive coral framestone and coralline algal crusts a six stage holocene chronology is proposed including 1 coastal inundation from 8 to 8 5 ka bp 2 initiation start up from 8 to 7 5 ka bp 3 rapid growth catch up and back step from 7 5 to 7 ka bp 4 rapid aggradational growth catch up from 7 to 6 5 ka bp 5 reef decline give up and detrital buildup from 6 5 to 5 8 ka bp and 6 detrital buildup and progradation from 5 8 ka bp to present changes in reef facies and the ultimate demise of the holocene reef probably involved a combination of increased sea level coastal flooding and erosion during the mid holocene highstand with associated increase in sedimentation turbidity and decline in water quality burial by sediment buildup during the mid holocene highstand and detrital progradation during the mid to late holocene regression and the introduction of alluvial sediment during cyclones and other severe storms to an already stressed community modern communities have thus shifted from coral dominated to bored macroalgal pavements this study shows that integration of reef development processes with response to environmental change can be used to assess future pressures on coral reef ecosystems globally

holocene reef growth dynamics on kodakara island 29 n 129 e in the northwest pacific abstract the spatial distribution of modern coral reefs in the northwest pacific nwp is restricted to approximately 30 n understanding the high latitude reef growth process and its correlation to climate change may provide important insights into future reef growth at even higher latitudes in response to global warming we conducted field surveys and obtained seven cores from the raised reefs of kodakara island 29 n 129 e in the nwp to determine the changes in and response of reef growth dynamics to millennial scale climate change through the holocene to reconstruct the timing of the reef growth 37 coral ages were determined using accelerator mass spectrometry ams radiocarbon dating the island is characterized by three holocene reef terraces terraces i to iii which were uplifted ca 2 4 ka 1 0 0 4 ka and in the modern era three growth hiatuses occurred circa 5 9 5 8 ka 4 4 4 0 ka and 3 3 3 2 ka which correlate with millennial scale climate changes hamanaka et al 2012 the reef growth began at least 8 ka relatively rapid vertical growth of 3 6 3 3 m kyr 1 occurred between 8 and 6 ka the reef started growing near land and then gradually extended seaward the reef growth around the reef slope decelerated after 6 ka and most likely coincided with the first and second hiatuses detected landward in contrast with the reef flat which resumed its growth after the third hiatus event the reef mound accretion on the slope ceased definitively terrace i was uplifted ca 2 4 ka the reef growth reactivated ca 1 3 ka and the reef grew at a rate of 9 1 m kyr 1 between 1 3 and 1 0 ka this rate is the fastest growth recorded in this study this time interval corresponds to the medieval climatic anomaly mca the cause of the delayed reef growth between 2 4 and 1 4 ka remains unclear but it may imply the existence of a late holocene hiatus due to a weaker kuroshio current kc ca 1 7 ka and to strong el nino southern oscillation enso activity between 2 and 1 5 ka in the east pacific the timing of the hiatuses agrees with the weakening of the kc and the more frequent el nino events during the positive phase of the pacific decadal oscillation pdo whereas the rapid growth coincides with la nina conditions such as the mca during the negative phase of the pdo our results indicate that the climate event at approximately 4 ka caused the largest change in the reef growth and it may have affected reefs throughout the pacific region specifically the enso and pdo phases may have impacted the reef growth and kc behavior

easter microplate evolution geochemical evidence confirms that there is current active spreading on both the east and west rift zones of the easter microplate and northwards propagation of the east rift with the active tip at 25 s the east rift is influenced by lateral flow of plume derived material from a hotspot located beneath either sala y gomez or easter island spreading rate variation does not seem to be influential in controlling the composition of erupted basalts

temporal and spatial variability in the composition of lavas exposed along the western blanco transform fault the northern scarp of the western blanco transform bt fault zone provides a tectonic window into crust generated at an intermediate rate spreading center exposing a 2000 m vertical section of lavas and dikes the lava unit was sampled by submersible during the blancovin dive program in 1995 recovering a total of 61 samples over vertical distances of 1000 m and a lateral extent of 13 km major element analyses of 40 whole rock samples exhibit typical tholeiitic fractionation trends of increasing feo na2o and tio2 and decreasing al2o3 and cao with decreasing mgo the lava suite shows a considerable range in extent of crystallization including primitive samples mg 64 and evolved feti basalts feo 12 tio2 2 on the basis of rare earth element and trace element data all of the lavas are incompatible element depleted normal mid ocean ridge basalts n morb la smn 1 the geochemical systematics suggest that the lavas were derived from a slightly heterogeneous mantle source and crystallization occurred in a magmatic regime of relatively low magma flux and or high cooling rate consistent with magmatic processes occurring along the present day southern cleft segment the bt scarp reveals the oceanic crust in two dimensional space allowing us to explore temporal and spatial relationships in the horizontal and vertical directions as a whole the data do not appear to form regular spatial trends rather primitive lavas tend to cluster shallower and toward the center of the study area while more evolved lavas are present deeper and toward the west and east considered within a model for construction of the upper crust these findings suggest that the upper lavas along the bt scarp may have been emplaced off axis either by extensive off axis flow or off axis eruption while the lower lavas represent axial flows that have subsided with time a calculation based on an isochron model for construction of the upper crust suggests that the cleft segment requires at least 50 kyr to build the lower extrusive section consistent to first order with independent estimates for the construction of intermediate spreading rate crust

structure of the lesser antilles subduction forearc and backstop from 3d seismic refraction tomography in 2007 the sismantilles ii experiment was conducted to constrain structure and seismicity in the central lesser antilles subduction zone the seismic refraction data recorded by a network of 27 obss over an area of 65 km 95 km provide new insights on the crustal structure of the forearc offshore martinique and dominica islands the tomographic inversion of first arrival travel times provides a 3d p wave velocity model down to 15 km basement velocity gradients depict that the forearc is made up of two distinct units a high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south short wavelength deformations of the outer forearc basement are observed beneath a 3 to 6 km thick sedimentary pile in relation with the subduction of the tiburon ridge and associated seafloor reliefs north offshore dominica island the outer forearc is 70 km wide it extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed its width decreases strongly to the south to terminate offshore martinique where the inner forearc acts as the backstop the inner forearc is likely the extension at depth of themesozoicmagmatic crust outcropping to the north in la desirade island and along the scarp of the karukera spur the outer forearc could be either the eastern prolongation of the inner forearc but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes or an oceanic terrane more recently accreted to the island arc

holocene turn on and evolution of the southern great barrier reef revisiting reef cores from the capricorn bunker group abstract extensive drilling of the great barrier reef gbr in the 70s and 80s illuminated the main factors controlling reef growth during the holocene however questions remain about 1 the precise nature and timing of reef turn on or initiation 2 whether consistent spatio temporal patterns occur in the bio sedimentologic response of the reef to holocene sea level rise then stability and 3 how these factors are expressed in the context of the different evolutionary states juvenile mature senile reefs combining 21 new c 14 ams and 146 existing re calibrated radiocarbon and u th ages we investigated the detailed spatial and temporal variations in sedimentary facies and coralgal assemblages in fifteen cores across four reefs wreck fairfax one tree and fitzroy from the southern gbr our newly defined facies and assemblages record distinct chronostratigraphic patterns in the cores displaying both lateral zonation across the different reefs and shallowing upwards sequences characterised by a transition from deep porites faviids to shallow acropora isopora coral types the revised reef accretion curves show a significant lag period ranging from 0 7 2 ka between flooding of the antecedent pleistocene substrate and holocene reef turn on this lag period and dominance of more environmentally tolerant early colonizers e g domal porites and faviids suggests initial conditions that were unfavourable for coral growth we contend that higher input of fine siliciclastic material from regional terrigenous sources exposure to hydrodynamic forces and colonisation in deeper waters are the main factors influencing initially reduced growth and development all four reefs record a time lag and we argue that the size and shape of the antecedent platform is most important in determining the duration between flooding and recolonisation of the holocene reef finally our study of capricorn bunker group holocene reefs suggests that the size and shape of the antecedent substrate has a greater impact on reef evolution and final evolutionary state mature vs senile than substrate depth alone

spreading evolution of the troodos ophiolite cyprus some recent developments particularly the recognition of transform tectonized and nontransform tectonized fabrics close to the southern troodos transform fault provide a framework for a new evolutionary model of the troodos ophiolite this model proposes that most of the ophiolite formed at the solea axis against a dextrally slipping southern troodos transform spreading was largely accommodated by magmatic accretion the ridge then jumped to the east a last phase of amagmatic stretching driving normal faulting above a detachment horizon in the solea graben a ridge jump boundary separates the old crust formed at the solea axis from the new crust generated at the outside corner of the new ridge with a dextrally slipping continuation of the southern troodos transform the final episode involved extension of the crust generated at the new ridge to form the larnaca graben in the eastern part of the ophiolite

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

late quaternary reef growth and sea level in the maldives indian ocean abstract based on rotary drilling and radiometric and u series dating we present the first comprehensive data on holocene reef anatomy and sea level rise as well as nature and age of underlying pleistocene limestone in the maldives holocene reefs in rasdhoo atoll central maldives are composed of four facies including 1 robust branching coral facies 2 coralline algal facies 3 domal coral facies and 4 detrital sand and rubble facies branching coral and coralline algal facies predominate the marginal reefs and domal corals and detrital facies preferentially occur in a lagoon reef in addition microbialite crusts are found in lower core sections of marginal reefs microbialites formed during the early holocene in reef cavities holocene reef thickness ranges from 14 5 m to 22 m reef growth started as early as 8 5 kyr bp marginal reefs accreted in the keep up mode with rates of 15 m kyr rate of sea level rise significantly slowed down from 7 6 kyr bp and subsequently gradually rose with rates

the quantum event of oceanic crustal accretion impacts of diking at mid ocean ridges seafloor diking eruptive events represent the irreducible quantum events of upper oceanic crustal accretion they record events by which a large portion of the oceanic crust has formed through geological history since 1993 the u s navy s real time sound surveillance system has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast pacific these diking eruptive events trigger a sequence of related rapidly evolving physical chemical and biological processes magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms some of which thrive in high temperature anaerobic environments many of the organisms identified from these systems are archaea if microorganisms can thrive in the water saturated pores and cracks within deep volcanically active portions of our planet other hydrothermally active planets may harbor similar life forms

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

discovery of lower cretaceous synmetamorphic thrust tectonics in french lesser antilles la desirade island guadeloupe implications for caribbean geodynamics located east of guadeloupe the island of la desirade exhibits the oldest rocks of the lesser antilles arc and the eastern caribbean plate an old magmatic basement is composed of 1 late jurassic ophiolitic complex with meta basaltic pillow lavas and interbedded radiolarites 2 acid igneous complex comprising meta quartz diorite and meta rhyolitic lavas flows and 3 meta diabasic microdioritic dyke swarm complex we present and discuss the discovery of synmetamorphic thrust tectonics in this island based on detailed structural analysis we evidenced two main compressive events the first event d1 is characterized by pervasive folding associated with thrust development in the northeastern part of the island a major thrust fault the grand abaque thrust has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit the second deformation event d2 is featured by a main pervasive cleavage s2 observed at the regional scale associated with upright folding f2 and by the development of two sets of conjugated steeply dipping strike slip shear zones dextral 130 n and sinistral 20 n striking respectively microstructural observations indicate that greenschist facies metamorphism is clearly contemporaneous with the development of d1 and d2 superimposed tectonic structures ar ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a d2 event these analyses yield well defined and concordant plateau ages at 106 2 1 7 ma and 107 2 1 8 ma thus shortening and thickening tectonics occurred during lower cretaceous albian in the available framework of caribbean geodynamics the synmetamorphic thrust tectonics we discovered in the french lesser antilles is related to the collision between an aptian albian oceanic plateau and the andean cordilleran east dipping subduction zone our results support subduction polarity reversal during lower cretaceous

spatial variations in wave transformation and sediment entrainment on a coral reef sand apron abstract waves are the main hydrodynamic force acting on coral reefs and are crucial in driving the evolution of reef systems previous research has mainly focused on wave breaking and transformation on the reef flat but often neglected the spatial variation of reformed wave characteristics once they have propagated into the back reef environment this study examines wave conditions on a reef flat and the adjacent back reef sand apron specifically focusing on the transformation of wave height period and spectra including changes to long period and incident wave height the ability of reformed waves in the back reef environment to entrain sediment is also investigated wave conditions were found to be distinctly different on the sand apron compared to the reef flat with the majority of wave energy dissipated during initial breaking and transformation almost all incident waves are dissipated on the reef flat under a depth threshold of 0 5 m before reaching the back reef with long period motions dominating in the back reef during these tidal stages at higher tidal stages incident waves are capable of propagating into the back reef but they are very low energy with under 1 of all waves capable of entraining sediment this suggests that higher energy events such as high frequency storms are required to significantly transport sediment and change reef geomorphology smaller scale spatial changes in wave height were observed on the sand apron that shows the influence of both cross and along reef attenuation processes a distance parameter x pd is introduced that combines the cross reef distance from the reef crest x d and the temporally specific along reef distance from the first point of wave breaking on the reef rim x p where x pd x d x p x pd is shown to accurately describe the changes in wave height and sediment entrainment if deep water significant wave height wave direction and depth over the reef flat are known the results in this study shows that wave conditions sediment entrainment and longer term trends in sediment characteristics can be predicted in back reef environments from a few simple geomorphic inputs

quaternary reef response to sea level and environmental change in the western atlantic the tropical subtropical western atlantic realm harbours abundant fringing reefs two major barrier reefs belize florida several atolls and large reef fringed carbonate platforms bahamas data from belize and south florida indicate that major barrier reefs were established during the high sea levels of the long and warm marine isotope stage 11 reefs formed during pleistocene sea level highstands whereas erosion non deposition and the formation of subaerial exposure surfaces prevailed during lowstands of sea level florida bahamas exposed reefs and platforms with siliciclastic hinterlands were characterized by deposition of clays and sands on fluvial plains belize elevated terraces and outcrops of reefs of the last interglacial of marine isotope stage 5e are evidence of higher than present sea level and temperature that sustained large reef fringed carbonate platforms and ramps in the florida bahamas region and central america belize mexico including ooid deposition in warm and shallow waters during the highstands of marine isotope stages 5c and 5a at sea levels several metres lower than today fringing reefs developed in parts of the caribbean for example in south florida outlier reefs data from the island of barbados support the contention that the postglacial sea level rise from about 120 m was rapid and punctuated by two meltwater pulses ia and ib which bracket the beginning and end of the younger dryas respectively after reflooding of shelves in the early holocene reefs developed preferentially on highs of the underlying karstified pleistocene limestone surface and on former river channel bars many reefs in the western atlantic region kept pace with holocene sea level rise whereas some were drowned reasons for reef drowning are debated and include sea level rise elevation of antecedent topography and deterioration of environmental conditions such as high nutrient input apart from sea level and antecedent topography factors such as exposure to waves and currents storms and sedimentation have controlled reef development the historical decline of acroporid corals in the caribbean realm has been attributed to cooling following the holocene thermal optimum however historical die offs of reef building corals and other reef organisms suggest that some if not all recent declines were unprecedented during the holocene rising temperatures decreasing carbonate saturation increased frequency of major storms high nutrient concentrations and sedimentation overfishing and disease certainly played a role in decline but systematic studies relating these environmental factors to holocene reef building are only in the early stages

pito and orongo fracture zones the northern and southern boundaries of the easter microplate southeast pacific abstract the easter rapanui microplate is a case example of a large dual spreading center system in a region where the fastest seafloor spreading on earth is occurring today recent theoretical models of the tectonic evolution of dual spreading center systems have explored the effects of shear and rigid rotation on the boundaries and internal structure of microplates but the models must be critically constrained by improved relative motion and structural fabric data sets during the january 1987 rapanui expedition on the n o jean charcot we conducted a sea beam magnetics gravity survey of a portion of the microplate boundaries the method that was used was to fully map selected portions of the boundaries in order to establish precise structural relationships the northern terminus of the east rift or eastern boundary of the microplate is expressed as a series of parallel nw se trending valleys including what appears to be with 5890 m depth the deepest active rift axis mapped in the pacific today pito rift the northern end of the pito rift merges with an e w to 083 narrow band of linear faults interpreted to be a transform fault between the nazca and easter rapanui plates the northern triple junction between the easter rapanui nazca and pacific plates is a rff type with the two transform faults colinear along an approximately e w direction the southwestern boundary of the easter rapanui microplate is marked by a series of en echelon offsets outlined by depressions which merge into an approximately e w zone where shear must be predominant the southern triple junction is a rrf junction with an overlapping ridge system the structural data acquired during the survey provide strong constraints for kinematic models of the microplate the structural data need to be combined with crustal age determinations in order to derive a model for the evolution of the microplate

multi stage reef development on barbados during the last interglaciation by mapping the vertical and lateral distribution of reefal facies on the west and south coast of barbados we have produced a revised model of reef development for the last interglaciation we find that reef architecture around barbados has significant complexity including evidence for wave exposure related variations in reef geometry and at least 3 stages of reef development that were controlled by variations in sea level during the main stage of development an acropora palmata dominated reef crest aggraded 22 m in response to a minimum sea level rise of 20 m during stage 2 a sea cave was cut 3 4 m above the fossil reef crest possibly indicating that reef growth was terminated before sea level reached the highstand similar sequences elsewhere in the caribbean indicate that this early reef demise may not be a local phenomenon several reefs apparently stopped growing between 2 and 4 m and only in sheltered areas did they reach the highstand at 6 m as recorded by intertidal notches this pattern of reef demise has previously been related to rapid sea level rise at the end of the interglacial but stratigraphic data are equivocal the final stage of reef development on barbados occurred when sea level began to fall this fall was rapid leaving a thin but widespread veneer of reef crest deposits over the proximal reef front and discontinuous intertidal and shallow subtidal deposits capping the distal reef front although further dating is required to differentiate these three stages our only reliable u th tims date indicates that almost 50 of the exposed reef had accreted by 129 ka giving an estimate of 15 ka for the main aggradational stage furthermore reports of relict reef crests buried beneath these exposed deposits indicate that our revised model is incomplete and that earlier stages of reef growth occurred during the last interglaciation these earlier stages imply that sea level was at an interglacial level for as long as 20 ka supporting the devils hole record of interglacial duration unfortunately these estimates could not be verified directly because most of our u th data show major stratigraphic age reversals attributed to diagenesis this pattern is also evident in all other well dated reefal units in the caribbean and leads us to conclude that only diagenetically screened precise stratigraphically consistent coral dates can be used to directly estimate the duration of the last interglaciation

rethinking the origins of the red chert at la desirade french west indies la desirade in the lesser antilles contains one of the rare fragments of jurassic oceanic crust known on caribbean islands others in the northeastern caribbean occur on puerto rico and hispaniola these fragments each include radiolarian bearing chert that has been linked to an origin in the pacific ocean of these a fragment in sierra bermeja puerto rico is clearly of pacific origin as it contains lower jurassic radiolarians that predate the opening between north and south america red ribbon chert at el aguacate dominican republic is essentially identical to widespread radiolarites found in accreted material of the pacific basin and from pacific ocean odp site 801 extensive sampling in the atlantic basin has produced no jurassic radiolarites thus based on age the older of the sierra bermeja outcrops and lithology el aguacate two of these fragments are definitely of pacific origin re evaluation of the chert pillow lava sequence on la desirade in light of recent discoveries at spreading ridges has resulted in a revised interpretation of their probable origin a wide range of features of these cherts indicate pelagic and hydrothermal sedimentation at an upper jurassic spreading ridge one that almost assuredly existed in the eastern pacific realm these features include the types of chert found on the island lack of argillaceous partings small outcrop size discontinuous chert bodies presence of limestone squeeze ups into pillow lavas and indications of hydrothermal activity including epidotization of basalt migrating outward from pillow margins with chert rinds that record pelagic and hydrothermal sedimentation at an upper jurassic spreading ridge one that almost assuredly existed in the extreme eastern pacific realm

tectonic deformation by rotating parallel faults the bookshelf mechanism abstract in various tectonic environments extension of crustal parts and shear distortion has been accommodated by the rotation of an array of parallel faults because of its kinematic resemblance to the tilting of a row of books on a shelf the tectonic process may be referred to as a bookshelf mechanism its most important manifestions are cross faulting between normal faults or parallel wrench faults the extension of deltaic slope deposits and all forms of tilted block tectonics kinematically there are two basic modes of the mechanism a the dilational mode where rotating faults tend to open up and possibly become paths for fluid migration and b the domino style where reclining faults remain closed in both cases formation and rotation of the faults may take place in one tectonic event fault displacements may therefore often remain small and difficult to detect from the mechanical point of view bookshelf operations that accommodate shearing between parallel and predominantly parallel moving rock boundaries bookshelf mechanisms proper have to be differentiated from those operations responding to an imposed crustal extension or shortening after a review of geological evidence the process is addressed from a geomechanical point of view in order to determine the geological operating conditions and controlling parameters the crucial mechanical factor in the bookshelf mechanism is the component of normal stress that acts in the direction of the imposed displacement component parallel to the driving boundary plates in contrast to the usual assumption the driving shear stress on the boundary plates not only depends on the normal stress on the plates but it also varies with the value of the normal stress that acts inside the sheared material in the direction of the plate parallel displacement component it is the relationship between the two normal stresses that also determines the type of rotating parallel faults large scale rotation of parallel normal faults in domino style tilted block tectonics is primarily associated with the extension of ductile substrata from mechanical arguments and sand box experiments it has been inferred how the process and in particular the dip direction of the parallel faults is controlled by the way substratal extension progresses by means of the direction of a substratal squeeze flow the presence of a surface slope the strength anisotropy associated with fore set beds and by the configuration of competent boundaries of the extending overburden

constraining crustal emplacement processes from the variation in seismic layer 2a thickness at the east pacific rise abstract a stochastic model for the emplacement of dikes and lava flows at a fast spreading ridge can generate an upper oceanic crustal structure similar to that observed in seismic data from the east pacific rise epr in ocean drill holes and in ophiolites in this model the location of successive dike intrusion events relative to the ridge axis is determined by a gaussian probability function and the cumulative flow lengths of the erupted lavas are chosen to build a gaussian shaped lava pile we interpret wide angle seismic reflections from the steep velocity gradient at the base of seismic layer 2a to be the extrusive sheeted dike contact seismic data from the northern and southern epr place constraints on the on axis extrusive layer thickness 230 50 m the distance over which the thickening of the extrusive layer occurs width of the accretion zone 1 3 km and its off axis thickness 300 800 m ophiolites and ocean drill holes dsdp hole 504b provide additional estimates of the thickness of the extrusive layer and constrain the thickness of the transition region from extrusives to sheeted dikes 100 200 m a simple stochastic emplacement model where the lavas are described by one mean flow length fits the thickening of the extrusive layer off axis inferred from the deepening of seismic layer 2a but the predicted transition from sheeted dikes to extrusives is too thick in order to match the dimensions and flat topped shape of the seismic layer 2a boundary as well as the thickness of the extrusive sheeted dike transition a bimodal distribution of lava flows is used short flows confined within the axial summit caldera asc build up approximately half the extrusive volume occasional voluminous flows spill out of the asc or erupt outside of the asc and pond at a considerable distance off axis to build up the remainder of the extrusive section the upper part of the final extrusive section will be dominated by the off axis flows while the lower portions will be primarily composed of short flows erupted within the asc magnetic transition widths predicted from the overlap of lavas 2 km in this model are similar to those measured in deep tow studies assuming a smoothing function which acts over one seismic wavelength the upper crustal velocity structure predicted by the bimodal lava emplacement model is consistent with the shallow seismic velocity structure measured on the epr the ages of seafloor lavas in this model are younger than the tectonic spreading model ages by 30 70 kyr in agreement with anomalously young lava ages obtained from radioisotope dating of seafloor basalts near the epr

downhole magnetic measurements of odp hole 801c implications for pacific oceanic crust and magnetic field behavior in the middle jurassic 1 downhole horizontal and vertical magnetic field measurements within the 474 m thick jurassic crustal section drilled at odp hole 801c in the western pacific show anomalies that are in phase indicating that this site formed in the southern hemisphere and moved across the paleoequator to its present location at 18 6 n the inclination computed from the horizontal and vertical anomaly logging data varies significantly downhole and can be explained by progressive rotation of the lava sequence as it is buried during accretion this is compatible with the observed dips in the formation micro scanner fms data which show up to 42 of rotation toward the fossil spreading axis we restore the magnetic inclination to prerotated values and obtain an estimated pretilt inclination of 39 9 6 6 corresponding to a paleolatitude of 22 7 5 s the magnetic logging data document six polarity units within the drilled volcanic extrusive crust the upper 132 m thick logged section of lavas contains four polarity units which apparently formed 7 myr after the formation of the underlying basement the lower 212 m logged section of the hole is reversely magnetized with an intervening normal polarity zone more than one reversal downhole would indicate a rapid reversal rate given the time it takes to form the extrusive section of oceanic crust 45 000 years at a fast spreading rate of 66 km myr previous results from a contemporaneous section in spain suggest rapid reversals throughout the late bajocian early bathonian stage 170 165 ma nevertheless the complicated formation history prevents quantification of the geomagnetic reversal rate for the 801c crustal section

origin of the sheeted dike complex at superfast spread east pacific rise revealed by deep ocean crust drilling at ocean drilling program hole 1256d 1 superfast spread 22 cm a upper oceanic crust drilled at ocean drilling program site 1256 comprises a thick sequence of extrusive lavas underlain by a thin region of sheeted dikes the sheeted dikes at this site are characterized by the intimate association of in situ hyaloclastic fragmentation and hydrothermal alteration lithostatic and magmastatic pressure estimates reveal that there is no effective level of neutral buoyancy within the extrusive layer to trap dikes in the crust this results in a situation that favors the extrusion rather than intrusion of magma fractures created by magmas rising in dikes were the loci of intense hydrothermal circulation and provided the pathways for subsequent dike intrusions magma rich conditions expected for fast spreading ridge segments give rise to a rapid increase in magmatic pressure which can result in dike intrusion even under only small deviatoric stress this allows for the emplacement of narrow dikes in the upper crust with magmas more likely to extrude and hence the high extrusive intrusive ratio for site 1256

measuring coral reef terrain roughness using structure from motion close range photogrammetry our understanding of earth surface processes is rapidly advancing as new remote sensing technologies such as lidar and close range digital photogrammetry become more accessible and affordable a very high spatial resolution digital terrain model dtm and orthophoto mosaic mm scale were produced using close range digital photogrammetry based on structure from motion sfm algorithms for a 250 m transect along a shallow coral reef flat on heron reef great barrier reef the precise terrain data were used to characterise surface roughness a critical factor affecting ecological and physical processes on the reef three roughness parameters namely the root mean square height tortuosity or rugosity and fractal dimension were derived and compared in order to asses which one better characterises reef flat roughness the typical relief across the shallow reef flat was 0 1 m with a maximum value of 0 42 m coral reef terrain roughness as characterised by the three chosen parameters generally increased towards the middle of the transect where live coral covers most of the reef flat and decreases towards the edges of the transect the fractal dimension values ranging from 2 2 to 2 59 best characterised reef roughness as evidenced by a closer agreement with the distribution of known coral benthic substrates this is the first study quantifying scale independent roughness of a coral reef at benthic and biotope patch levels cm m the readily available and cost effective methods presented are highly appropriate for data collection processing and analysis to generate very high spatial resolution dtms and orthophoto mosaics of shallow and energetic coral reefs

internal structure and accretionary history of a nearshore turbid zone coral reef paluma shoals central great barrier reef australia abstract nearshore environments directly influenced by fluvial sediments and dominated by terrigenoclastic sedimentation are generally considered marginal for coral reef growth there are however a growing number of examples from the modern and fossil record that clearly show that reef building and terrigenous sedimentation are not mutually exclusive although a number of descriptive accounts of nearshore turbid zone reefs have emerged in recent years our understanding of the accretionary architecture and history of such reefs remains extremely limited here we detail the depositional facies and accretionary history of an inshore turbid zone coral reef complex paluma shoals on the central great barrier reef shelf of australia high turbidity 40 ntu for over 30 of the time and terrigenous sedimentation both at levels close to the perceived limits for coral growth are episodically important influences at paluma shoals despite these potentially limiting conditions paluma shoals exhibits high live coral cover 50 60 but locally up to 80 with the intertidal reef flats compositionally dominated by g aspera microatolls and galaxea sp and porites sp colonies seventeen cores from 5 cross reef transects reveal that the internal reef structure comprises an unconsolidated detrital coral rubble framework 2 0 2 7 m thick dominated by fragments of acropora pulchra t frondens and m mollis these occur within a mixed terrigenoclastic carbonate sediment matrix basal facies in particular contain a high 60 proportion of terrigenoclastic sediment radiocarbon dating indicates that the reef initiated above pleistocene clays 1600 cal yr bp the present reef flat was emplaced within the last 150 100 cal yr bp within the last 60 years areas of the north shoal have become partially smothered as a result of intertidal sandflat progradation the depositional sequence at paluma shoals shows 1 that long term millennial timescale reef accretion has occurred under conditions dominated by terrigenoclastic sediment accumulation 2 that relatively rapid vertical reef growth and progradation can occur in these marginal settings and 3 that these reefs are susceptible to natural episodic mortality events associated with the remobilisation of intertidal and shallow subtidal sediments this study provides important insights into coral reef development where environmental conditions are perceived to be marginal for hermatypic coral growth

the structure and morphology of the basse terre island lesser antilles volcanic arc basse terre island is made up of a cluster of composite volcanoes that are part of the lesser antilles volcanic arc the morphology of these volcanoes and the onshore continuation of the grabens and strike slip faults that surround the island are poorly documented due to erosion and rainforest cover therefore we conducted a morphological analysis of the island using digital elevation model dem data integrated with field observations to document erosional constructional and deformational processes a dem based analysis of 1 249 lineaments and field structural measurements of 16 normal faults 3 741 veins and fractures and 46 dykes was also carried to document the structures that predominate in sub surface rocks the results indicate that the over 1 my old and elongated northern chain volcano which makes up the northern half of the island was built by high eruption rates and or a low viscosity magma injected along the n s to nnw sse striking extensional structures formed by the flexure of the lithosphere by the overall subduction regime after 1 ma the southern half of the island was shaped by an alignment of conical volcanoes likely built by a more viscous magma type that was guided by the nw se striking montserrat bouillante strike slip fault system these n to nnw and nw structural directions are however poorly expressed onshore possibly due to slow slip motion the sub surface rocks mostly contain e w striking structures which have likely guided the many flank instabilities documented in the studied area and guide hydrothermal fluids and shallow magmatic intrusions these structures are possibly part of the e w striking marie gallante offshore graben

relationship between species diversity and reef growth in the holocene at ishigaki island pacific ocean abstract coral reefs are influenced by global and local factors and living corals are currently faced with a potential loss of species diversity knowledge of the relationship between species diversity and reef growth during the holocene is important in terms of accurately reconstructing natural conditions prior to recent disturbances e g human impact pollution and over harvesting and in predicting future scenarios e g abrupt sea level rise coastal change and economic services this study seeks to characterize the holocene and present day reef at ishigaki island in the ryukyu islands focusing on spatial and temporal variations in the relationship between species diversity and reef growth the analysis is based on a drilling core obtained for the holocene reefs and quantitative species diversity data shannon and weaver s diversity index h obtained for the present day reef h was calculated for four coral communities surveyed at the ibaruma and fukido reefs the holocene sequence was dominated by the corymbose coral community e g acropora digitifera a hyacinthus goniastrea retiformis and platygyra ryukyuensis yielding an h value of 1 6 the encrusting coral community e g echinopora lamellose and pachyseris rugosa showed the highest diversity at the reef h 2 2 however this community was not one of the main reef builders during the holocene the massive coral community e g porites lutea and favites chinensis showed the lowest diversity h 0 6 it also made a minor contribution to reef building this community appeared in a shallow lagoon once sea level had stabilized the arborescent coral community e g a formosa and a nobilis was one of the main reef builders although yielding an h value of much less than 1 0 species diversity is not a prerequisite in terms of holocene reef growth thus a few species e g a digitifera a hyacinthus a formosa a nobilis g retiformis and p ryukyuensis from two main reef building communities are important for reef growth these corals that act as reefs are characterized by high growth rates having an upward reef growth rate of 2 12 m kyr 1 in the indo pacific during the holocene

dyke complex of the east pacific rise exposed in the walls of hess deep and the structure of the upper oceanic crust abstract detailed observations have been made of the extrusive and intrusive units of the uppermost crust emplaced at the axis of the east pacific rise during dives conducted with the submersible nautile on the walls of hess deep at four widely separated locations the same sequence was found an upper thin 100 200 m unit of extrusives pillow lavas underlain by a mixed zone of variable thickness 50 500 m averaging 200 m where extrusives are found and intrusives and massive subhorizontal layers which may represent sills the mixed zone itself is underlain by a sheeted dyke unit that may be up to 1200 m thick where dykes are subvertical i e untilted and parallel to the east pacific rise fabric this sequence may be a general feature of medium to fast spreading crust because it is similar to that observed in hole 504b of the costa rica rift and is consistent with images of the upper oceanic crust derived from seismic experiments on the axis of the east pacific rise dyke thickness is a good measure of the depth to the roof of the axial magma chamber the observed sections provide much needed ground truth for east pacific rise crust the thickness of the lava sequence and the overall thickness of layer 2 at most 1400 m appear to have been overestimated in previous studies of the oceanic crust

uranium series disequilibrium in a young surficial uranium deposit northeastern washington u s a a recently discovered ore grade accumulation of u in organic rich sediments of late quaternary age provides an opportunity for studying the early association of u u daughters and organic matter in a natural setting the u occurs in valley fill sediments of peat peaty clay silt and sand along the north fork of flodelle creek stevens county washington radiometric techniques delayed neutron high resolution gamma ray spectrometry thin source alpha spectrometry were employed to determine the abundance and distribution of u series nuclides the extent of secular equilibrium within the u decay series and the apparent u series ages of u incorporation sixteen lithologically distinct intervals were sampled from a 292 cm core uranium contents range from 140 to 2790 ppm and are positively correlated with organic contents measured alpha activity ratios of 234u 238u 1 31 1 38 are very similar to those reported in coexisting waters suggesting a rather constant isotopic composition of introduced u much lower th contents of 10 40 ppm are controlled by the type and abundance of silicate detritus the youth of the host sediments 15 000 a and the paucity of associated radioactivity suggested large excesses of u relative to radioactive daughters and such excesses were observed particularly in the shallowest intervals apparent ages of u emplacement determined by the alpha activity ratio of 230th daughter to 234u parent show a general increase with depth and fair agreement with estimated depositional ages this observation suggests dominantly syndepositional or early post depositional emplacement of u followed by decay generated buildup of 230th daughter with time however interval by interval comparisons of the relative abundances of other daughters particularly 226ra and 210pb indicate variability caused by processes other than closed system growth and decay probably because chemically diverse daughters that are decay generated in situ have differing mobilities and because upwelling ground water continuously adds more u and minor amounts of daughters if 230th is considered the least susceptible to these modifications the data suggest some addition of 234u in the deepest intervals and some loss of 226ra and or gain of 222rn throughout the studied core

1 ma east pacific rise oceanic crust and uppermost mantle exposed by rifting in hess deep equatorial pacific ocean abstract a series of dives with the french submersible nautile has been conducted in the region of the galapagos triple junction at the tip of the cocos nazca propagator most of the dives were devoted to the study of hess deep a rift valley produced by rifting of oceanic crust created at the axis of the very fast spreading east pacific rise the dives have enabled the structures and lithologies to be assessed and confirmed earlier dredging evidence for abundant basal crust and mantle rocks on an intra rift ridge located to the north of the axis of hess deep this shows that this situation is not specific not slow spreading ridges a nearly complete and unique log of young 1 ma crust and uppermost mantle representative of very fast accretion can be reconstructed even though the section is discontinuous and highly dismembered we propose two models that can explain the exposure on the ocean floor of the deep foundation of the oceanic crust one model stresses symmetry and vertical tectonics through isostasy diapirism of serpentinized mantle another model requires a larger amount of extension and emphasizes asymmetry with denudation of the upper crust through low angle detachment faulting in both models which differ mainly in the structure at depth amagmatic extension of east pacific rise crust by the cocos nazca propagator tip results in uplift and denudation of lower crust and upper mantle

pliocene lower pleistocene shallow water mixed siliciclastics and carbonates yanigua and los haitises formations in eastern hispaniola dominican republic the virtually unfolded sedimentary cover of the cordilleras central and oriental in the eastern dominican republic eastern hispaniola tropical north atlantic largely consists of pliocene to early pleistocene mixed siliciclastics and carbonates these deposits have been grouped into two laterally interfingering mapping units the yanigua and los haitises formations the former mainly siliciclastics comprises marl marly limestone and minor conglomerate sandstone lignite and carbonaceous clay and crops out closest to the basement the los haitises formation mainly consists of limestone and intercalating beds of marly limestone and marl lithological mapping at the 1 50 000 scale and facies analysis of twelve measured sections and of additional fourteen outcrops suggest that these deposits mainly formed on a shallow water marine platform fringing the precursor reliefs of the cordillera oriental and the southeastern end of the cordillera central only a limited proportion of sediment formed in floodplains and marshes marl and marly limestone dominated the inner platform sediments terrigenous mud decreased away from the emergent basement and carbonate sedimentation dominated the more external platform corals molluscs echinoids foraminifers bryozoans coralline algae and halimeda are the main components with varying amounts of carbonate mud the platform was generally a low energy environment with seagrass patches in the inner platform corals grew as isolated colonies or as small patch reefs dominated by porites in marly and bioclastic substrates branching corals stylophora and acropora grew in extensive carpets in more distal areas at least in the last stage of its development early pleistocene the platform was rimmed by a reef barrier similar to the holocene caribbean barrier reefs with acropora gr palmata a cervicornis porites montastrea siderastrea and diploria as the main reef builders

the development of two oceanic microplates easter and juan fernandez microplates east pacific rise in this paper we synthesize and discuss the results of recent detailed geophysical surveys and reconstructions of two active mid ocean ridge microplates on the east pacific rise we quantify their development in terms of the principal dimensions of the microplates and their boundaries and the temporal changes in these dimensions the results are discussed in the framework of the roller bearing microplate model of schouten and others which postulates that microplates are driven by shear couples imposed on them by the motions of their bounding major plates we test the specific predictions of this model for the geometry of the pseudofaults traces of the propagating tips of the microplates spreading boundaries and the histories of rotation of the microplates both microplates are currently rotating at the rates predicted by the model on the assumption of complete coupling no slip or deformation between the microplates and the major plates at instantaneous points of contact near the propagating tips of the spreading boundaries juan fernandez microplate was also rotating at the predicted rate just before 2 ma when it was driven by the nazca pacific couple rather than the nazca antarctic couple that currently applies however earlier rotations were slower than predicted implying some degree of decoupling or deformation at earlier times the microplates appear to have grown from moderate sized c 100 km probably left stepping offsets of the east pacific rise since about 5 ma the nature of these offsets is currently uncertain but they may have been propagating rifts which changed from shearing by bookshelf faulting to rigid block microplate rotation on reaching a critical size once rigid block rotation began the microplates grew by seafloor spreading accompanied by propagation of their bounding spreading centres the effects of the neighbouring easter mantle plume and pacific nazca antarctic triple junction can be discerned in the bathymetry and ridge propagation rates but do not appear to have had a major influence on the microplates development

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

dike orientations fault block rotations and the construction of slow spreading oceanic crust at 22 40 n on the mid atlantic ridge the first paleomagnetic results from oriented dike samples collected on the mid atlantic ridge shed new light on the complex interplay between magmatic accretion and mechanical extension at a slow spreading ridge segment an upper crustal section about 1 5 km thick is exposed along a west dipping normal fault zone that defines the eastern median valley wall of the southern segment of the mid atlantic ridge south of the kane fracture zone mark area two distinct groups of dikes are differentiated on the basis of orientation and paleomagnetic characteristics one group on the basis of the paleomagnetic data appears to be in its original intrusion orientation this group includes both ridge parallel vertical dikes as well as dikes in other orientations calling into question assumptions about uniform dike orientations at oceanic spreading centers the second group consists of dikes that have paleomagnetic directions that are distinct from the predicted dipole direction and we interpret them to have been tectonically rotated these also occur in many orientations the spatial relations between rotated and nonrotated dikes indicate that intrusion faulting and block rotation were contemporaneous beneath the median valley floor nonrotated dikes exposed on the eastern median valley wall indicate that there has been no net rotation of this upper crustal assemblage since magmatic construction ceased hence slip and associated uplift probably occurred in the fault zones present orientation these results provide the basis for a general model of mechanical extension and dike intrusion for this segment of the mid atlantic ridge initially a portion of crust forms beneath the median valley by synkinematic dike intrusion into laterally discontinuous fault blocks slip and associated uplift along a cataclastic normal fault zone later exposes this crustal section on the valley margin as spreading continues this valley bounding cataclastic normal fault zone is abandoned in favor of a new fault system thus passively moving the exposed crustal section away from the median valley

paleomagnetism of tilted dikes in fast spread oceanic crust exposed in the hess deep rift implications for spreading and rift propagation tilted dikes occur within the pillow lava sheeted dike and gabbroic sections of 1 million year old fast spread oceanic crust exposed on the walls of the hess deep rift during recent alvin dives oriented samples from both tilted and vertical dikes were collected from several of the outcrops these are the first oriented paleomagnetic samples ever collected directly from outcrops on the ocean floor and were obtained with the help of a specially constructed geological compass remanent magnetic inclinations measured on the dike samples are significantly different from the near horizontal inclinations expected the present attitudes of the dikes and their remanent magnetizations indicate that the dikes have been technically rotated since their intrusion submersible observations from other dive programs that examined parts of nearby escarpments suggest that the structural complexity observed in the alvin study areas may be only a local phenomenon or at least not a pervasive characteristic of oceanic crust although tectonic rotations such as those observed may have taken place as a result of the opening of the hess deep rift crosscutting vertical dikes and other geologic considerations suggest that substantial tectonic rotations occurred at or very near the east pacific rise axis probably as a result of intermittent axial collapse the observed structures suggest that tectonic processes beneath fast spreading ridges and the internal structure of crust produced at them may be significantly more complex than previously assumed

comprehensive sonar imaging of the easter microplate recent gloria and seamarc ii sidescan sonar surveys have imaged virtually the whole of the easter microplate and revealed unprecedented detail and complexity in its tectonic fabric and history there is clear evidence of rapid microplate rotation of rapidly evolving plate boundaries and of complex deformation near the northern and southern microplate boundaries

paleomagnetic constraints on deformation models for uppermost oceanic crust exposed at the hess deep rift implications for axial processes at the east pacific rise 1 studies of oceanic crust exposed in tectonic windows and in ophiolites have revealed the importance of normal faulting and attendant tilting of upper crustal rock units in the accretion process at oceanic spreading centers we present paleomagnetic remanence data from 45 fully oriented samples from dikes gabbros and a small number of basaltic lavas from fast spread crust exposed along the hess deep rift over 25 km along this escarpment dikes and dike subparallel fault zones dip consistently away from the east pacific rise epr while lava flows dip toward the ridge underlying gabbro is less deformed but contains widely spaced low angle fractures tentatively interpreted as shear zones as expected from the crustal age 1 07 1 48 ma most remanence data indicate reversed polarity magnetization and are compatible with the expected range of secular variation at the site overly steep and directionally scattered gabbro remanence and observed low angle shear structures within this unit are tentatively interpreted as the manifestation of three dimensional strain along anastomosing shear zones although some remanence directions are incompatible with any plausible deformation history and thus likely reflect orientation errors the overall data set is consistent with a model involving sequential rotations on 1 outward dipping epr parallel n s normal faults and 2 hess deep rift parallel e w normal faults average rotations for these sequential events are 22 to the east defined by the mean dike attitude and 10 to the south estimated by bathymetry respectively this model best explains the remanence data observed dikes and lava orientations presence of dike parallel fault zones and the observation of steep little deformed dikes cutting both east dipping dikes and faults the data support a structural model for spreading at the epr in which outcrop scale faulting and rotation is linked to subaxial subsidence and to consequent development of dominantly outward facing normal faults close to the spreading axis because these faults form within the neovolcanic zone they are subject to burial and are expected to have subdued to little surface expression

reworking processes and deposits in coral reefs during very high energy events example from a pleistocene coral formation 125 ka la desirade island lesser antilles abstract abundant literature has documented the ecological impact of very high energy hurricane type events and high energy storm type events on modern coral reefs and sediment transport in previous studies sedimentary processes are often deduced from geophysical image analysis and scuba diving observations few field based studies have been devoted to the impact of very high energy disturbance on the structure and ecology of holocene and pleistocene coral reefs and very few to the sedimentary processes acting around these reefs this research examines the structure of the late pleistocene end of mis5e stage reefs of la desirade island guadeloupe archipelago lesser antilles and focuses on the poorly known reworking processes and related coral breccia sedimentation surrounding these reefs fieldwork was performed to document two coral fragment rich breccia interbedded with coral patches the breccia predominantly show the same petrography but differ in their internal structure showing a massive breccia type and a stratified breccia type this work infers that these breccia are the result of gravity induced high density mass flows triggered by high to very high energy events such as storms and hurricanes damage and reworking vary according to the type of very high energy event prior to transport storms are thought to have broken the heads of branching and columnar coral colonies and broken and overturn isolated lamellar and massive colonies hurricanes particularly their eye walls may have reworked much more sediment than storms and may have also destabilised entire several metre sized columnar colonies from the flank of coral patch reefs we propose the name coral debrite for these coral breccia deposits caused by gravity induced debris flows the reworked deposits are interbedded between two coral reef levels of which the main coral species vertical succession should record the end of marine isotope stage 5e sea level rise thus reworking is believed to occur during the transitional stage between a glacial and an interglacial period

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

modes of extension at oceanic spreading centers evidence from the solea graben troodos ophiolite cyprus abstract the solea graben in the troodos ophiolite cyprus represents a preserved cretaceous age axial valley formed during a brief period of amagmatic seafloor spreading structural analysis of all crustal levels of the graben reveals two periods of deformation an extensive early phase of extensional deformation associated with seafloor spreading and a later phase of minor deformation associated with obduction and doming of the complex during the tertiary recent amagmatic extensional deformation produced networks of normal faults that dip toward the graben axis and sole into a regional detachment fault located at the sheeted dike plutonic contact and oriented subparallel to ophiolite pseudostratigraphy individual fault segments are essentially planar but link to approximate listric geometries locally bulk structural rotation of fault blocks bounding the graben occurred about a nnw trending axis parallel to the present graben trend average dips in the sheeted complex within the central portions of the graben suggest 40 50 of structural rotation although this is a minimum figure because present dike dips are not a simple function of rotation magnitude tensor analysis of fault populations associated with extensional deformation shows that 3 was oriented approximately perpendicular to the graben axis late near axis extensional deformation segmented the detachment fault along nnw trending normal faults obduction related uplift and doming of the troodos ophiolite is expressed in the solea graben by minor thrust reverse and strike slip faults principally developed during reactivation of pre existing low angle fault zones formed during seafloor deformation tensor analysis of this later phase of faulting shows that 1 was oriented approximately n s and that 2 and 3 alternated position during the deformation the style of deformation documented in the solea graben supports recent speculation for basin and range style detachment faulting at slow spreading ridge crests with low magma budgets such as parts of the mid atlantic ridge near large offset transform faults natural cross sectional exposure of the solea graben shows how extensional structures might link at depth beneath such slow spreading ridge segments and demonstrates the importance of extensive brittle deformation of the crust prior to establishment of large scale ore forming hydrothermal circulation cells

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

a global compilation of coral sea level benchmarks implications and new challenges i present a quality controlled compilation of sea level data from u th dated corals encompassing 30 studies of 13 locations around the world the compilation contains relative sea level rsl data from each location based on both conventional and open system u th ages i have applied a commonly used age quality control criterion based on the initial 234u 238u activity ratios of corals in order to select reliable ages and to reconstruct sea level histories for the last 150 000 yr this analysis reveals scatter of rsl estimates among coeval coral benchmarks both within individual locations and between locations particularly during marine isotope stage mis 5a and the glacial inception following the last interglacial the character of data scatter during these time intervals imply that uncertainties still exist regarding tectonics glacio isostacy u series dating and or coral position to elucidate robust underlying patterns with confidence limits i performed a monte carlo style statistical analysis of the compiled coral data considering appropriate age and sea level uncertainties by its nature such an analysis has the tendency to smooth obscure millennial scale and finer details that may be important in individual datasets and favour the major underlying patterns that are supported by all datasets this statistical analysis is thus functional to illustrate major trends that are statistically robust what we know trends that are suggested but still are supported by few data what we might know subject to addition of more supporting data and improved corrections and which patterns data are clear outliers unlikely to be realistic given the rest of the global data and possibly needing further adjustments prior to the last glacial maximum and with the possible exception of the 130 120 ka period available coral data generally have insufficient temporal resolution and unexplained scatter which hinders identification of a well defined pattern with usefully narrow confidence limits this analysis thus provides a framework that objectively identifies critical targets for new data collection improved corrections and integration of coral data with independent stratigraphically continuous methods of sea level reconstruction

implications for crustal accretion at fast spreading ridges from observations in jurassic oceanic crust in the western pacific 1 downhole logging data and basement stratigraphy interpretations are used to determine the spreading environment and crustal accretion history of the ocean basement cored at odp hole 801c located in the jurassic magnetic quiet zone of the western pacific high resolution microresistivity data obtained with the formation microscanner are used to measure the dip of the extrusive layers and indicate a 10 30 increase in dip down the drill hole with lava flow contacts dipping back toward the original ridge axis this structural pattern and the high proportion of massive flows relative to pillow units are consistent with prevailing crustal accretion models proposed for faster spreading ridges e g 60 km m y full rate a detailed analysis of the age data basement lithology related geochemistry and structural attitudes suggest the shallowest 100 m of the drilled section e g sequences i iii were emplaced just off the ridge sequence iii or significantly farther off axis up to 5 15 m y later sequences i and ii the remainder of the drilled section e g sequences iv viii has geochemical lithological and physical trends that are assumed to be representative of crust created at faster spreading ridges the pattern of dipping lava flow contacts from this deeper section of the drill hole suggests lava flows emanating from the ridge axis are limited to 1 2 km off axis our results suggest ocean crust drilled at hole 801c was created at faster spreading rates however caution should be used when incorporating sequences i iii into geochemical reference sections for faster spreading ridges

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

a revised morpho and chronostratigraphy of the late and middle pleistocene coral reef terraces on southern barbados west indies the barbados coral reef terraces are one of the few type localities worldwide that provide insights into interglacial sea level change during the late and middle pleistocene several sea level estimates have been established since the late 1960s and each has contributed to the barbados model of sea level change this paper presents new morpho and chonostratigraphic data from the barbados coral reef terraces developed over the last 12 years the work is based on significant advances in electron spin resonance esr dating of fossil coral air photo interpretation and a greatly improved geomorphic map of preserved fossil beach formations and reef terraces above present sea level the need for a revision of past published morpho and chronostratigraphies is best demonstrated on the southern part of the island the morpho and chronostratigraphic sequence in this region is both more complex and diverse than has been assumed so far the revised morphostratigraphy presented here includes a differentiation of coral reef terraces wave cut platforms and other erosive features such as notches and cliffs our study of these geomorphic features combined with new numeric dating results esr u th enable a revised estimate of the spatial and temporal variation in tectonic uplift rate within south barbados these new rates are an essential requirement for more precise glacio eustatic sea level reconstructions during the late and middle pleistocene from this region

a mechanism for decoupling within the oceanic lithosphere revealed in the troodos ophiolite contrasting kinematic histories recorded in the sheeted dykes and underlying plutonic rocks of the troodos ophiolite provide a new perspective on the mechanical evolution of oceanic spreading centres the kinematic framework of the decoupling zone that partitions deformation between the sheeted dykes and plutonics contrasts with low angle detachment models for slow spreading ridges based on continental rift analogues a model for the generation of multiple horizontal decoupling horizons linked by planar normal faults demonstrates new possibilities for the kinematic and rheological significance of seismic reflectors in oceanic lithosphere

large rotation of the easter microplate as evidenced by oriented paleomagnetic samples from the ocean floor one of the goals of the pito93 cruise along the perimeter of the easter microplate was to obtain oriented rock samples from the ocean floor for paleomagnetic study using a new orienting device the geocompass 14 oriented blocks were obtained during 6 dives conducted using the ifremer dsrv nautile paleomagnetic analysis shows that 13 of these 14 blocks give consistent results in thermal and af demagnetizations and reveals a mean characteristic remanent magnetization chrm direction within each block with both normal and reverse polarities after elimination of data from 5 blocks situated outside the microplate and one block which bears an anomalous direction we have computed a mean chrm direction at dm 49 0 im 39 0 k 10 2 95 19 8 n 7 although the mean inclination is in agreement with the inclination of the recent dipole magnetic field d 0 i 43 the average declination is significantly different from this direction which we interpret as due to a bulk clockwise rotation of 48 5 11 after the bivariate average of the easter microplate over the past 2 3 ma this value is in excellent agreement with previous estimates based on remote geophysical measurements and models

234u238u and 230th234u activity ratios in mineral phases of a lateritic weathered zone abstract a selective phase extraction procedure was developed for the identification of the significant phases of a typical deep soil profile sampled in the vicinity of the ranger no 1 uranium ore body alligator rivers region n t australia the significant phases were identified as amorphous iron oxide crystalline iron oxide and a clay quartz resistate the distribution of 238 u 234 u 230 th and 226 ra between the phases was measured the results indicated that the amorphous iron oxide phase is in adsorption desorption equilibrium with the ground water the crystalline iron oxide phase contains a chemical control the kinetics of which are commensurate with or less than the half life of 230 th 7 52 10 4 y the clay quartz resistate is enriched in 238 u descendants in such a way that they are not readily accessible to the ground water

geomorphology and sediment transport on a submerged back reef sand apron one tree reef great barrier reef back reef sand aprons are conspicuous and dynamic sedimentary features in coral reef systems the development of these features influences the evolution and defines the maturity of coral reefs however the hydrodynamic processes that drive changes on sand aprons are poorly understood with only a few studies directly assessing sediment entrainment and transport current and wave conditions on a back reef sand apron were measured during this study and a digital elevation model was developed through topographic and bathymetric surveying of the sand apron reef flats and lagoon the current and wave processes that may entrain and transport sediment were assessed using second order small amplitude stokes wave theory and shields equations the morphodynamic interactions between current flow and geomorphology were also examined the results showed that sediment transport occurs under modal hydrodynamic conditions with waves the main force entraining sediment rather than average currents a morphodynamic relationship between current flow and geomorphology was also observed with current flow primarily towards the lagoon in shallow areas of the sand apron and deeper channel like areas directing current off the sand apron towards the lagoon or the reef crest these results show that the short term mutual interaction of hydrodynamics and geomorphology in coral reefs can result in morphodynamic equilibrium

paleomagnetic constraints on deformation of superfast spread oceanic crust exposed at pito deep rift 1 the uppermost oceanic crust produced at the superfast spreading 142 km ma 1 full spreading rate southern east pacific rise epr during the gauss chron is exposed in a tectonic window along the northeastern wall of the pito deep rift paleomagnetic analysis of fully oriented dike 62 and gabbro 5 samples from two adjacent study areas yield bootstrapped mean remanence directions of 38 9 8 1 16 7 15 6 n 23 area a and 30 4 8 0 25 1 12 9 n 44 area b both are significantly distinct from the geocentric axial dipole expected direction at 23 s regional tectonics and outcrop scale structural data combined with bootstrapped remanence directions constrain models that involve a sequence of three rotations that result in dikes restored to subvertical orientations related to 1 inward tilting of crustal blocks during spreading area a 11 area b 22 2 clockwise vertical axis rotation of the easter microplate a 46 b 44 and 3 block tilting at pito deep rift a 21 b 10 these data support a structural model for accretion at the southern epr in which outcrop scale faulting and block rotation accommodates spreading related subaxial subsidence that is generally less than that observed in crust generated at a fast spreading rate exposed at hess deep rift these data also support previous estimates for the clockwise rotation of crust adjacent to the easter microplate dike sample natural remanent magnetization nrm has an arithmetic mean of 5 96 a m 3 76 which suggests that they significantly contribute to observed magnetic anomalies from fast to superfast spread crust

differential uplift and tilt of pleistocene reef platforms and quaternary slip rate on the morne piton normal fault guadeloupe french west indies 1 the guadeloupe islands are cut by normal faults that accommodate oblique convergence between the north american and caribbean plates such faults are responsible for part of the shallow seismicity and have produced m 5 damaging earthquakes to better assess the seismic hazard in guadeloupe we quantify the slip rate on one of the largest fault morne piton this roughly e w fault crosses the island of marie galante and uplifts a flight of reef terraces from geomorphic analysis we mapped three main terraces new u th datings show that they formed during the latest interglacials 120 and 240 kyr ago correlation with specmap isotopic records implies that the marie galante plateau emerged during the 330 ka highstand topographic profiles show that the terraces and the plateau are deformed by faulting elastic modeling of their shape constrains the geometry of the fault 70 80 dip 5 km depth and its slip rate 0 5 0 2 mm yr given its length 50 km depth and slip rate this fault might produce maximum m 6 5 earthquakes with a recurrence time of 1400 to 3300 years or more likely smaller events such as the m 5 5 16 may 1851 and 3 august 1992 shocks that might recur every 400 to 1000 years we also show that all the islands and terraces are tilted westward perpendicularly to the trench la desirade closest to the trench is uplifted by 276 m whereas subsidence 70 m is observed 10 km east of the volcanic arc this tilt probably resulted from a transient deformation episode at the subduction interface that predated the late pleistocene

a history of the easter microplate 5 25 ma to present gloria and other geophysical data are used to interpret the morphology and tectonic pattern of the easter microplate we model magnetic anomaly data and calculate spreading rates for use in euler pole calculations we calculate three plate closure and best fit poles for nazca pacific nazca easter and pacific easter plate motions and compare the results with earthquake focal mechanisms and seismicity we use the three plate closure poles to rotate digital gloria and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate s east rift first started to propagate northward at around 5 25 ma the microplate has evolved in two stages the first is from 5 25 ma to around 2 5 ma and involved continuous northward propagation of the east rift while the west rift became segmented along new transform faults as it adjusted to keep the overall nazca pacific velocities constant the transpressive northern boundary migrated with the east rift tip while the transtensional southern boundary remained more or less still at some time after 2 5 ma the east rift ceased northward propagation and the southwest rift began to open up along the transtensional pacific easter transform a convergent northern boundary formed involving southward thrusting of the nazca plate over the microplate no further nazca plate lithosphere was transferred from north of the microplate to the microplate interior and the pito deep region opened by pervasively rifting apart preexisting nazca plate lithosphere

seismic structure of 15 ma oceanic crust formed at an ultrafast spreading east pacific rise evidence for kilometer scale fracturing from dipping reflectors 1 a grid of seismic reflection data from 15 ma cocos plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading 200 mm yr full rate east pacific rise epr the data contain an unexpected quantity of bright reflectors throughout the crust generally 5 km in length significant reflection dip angles 10 30 are seen only on the isochron profiles and several reflectors have a confirmed isochron dip orientation certain upper crustal reflectors ucrs appear to form a single network with isochron dip the most prominent example apparent reflection coefficient 0 2 projects toward a 70 m disruption in the igneous seafloor and waveform analysis demonstrates that the reflector is likely the product of a low velocity zone on the order of tens of meters thick our observations suggest that the isochron dipping ucrs represent small slip kilometer scale faults fractures and none are primarily related to the seismic 2 3 boundary or to the sheeted dike gabbro transition lower crustal reflectors lcrs have no preferred orientation or regular pattern on the 30 km scale of this survey certain lcrs may be similar to the ucrs in origin while others may originate from near axis ductile flow lithologic banding off axis magmatism or some combination thereof overall reflectors in this area are abundant and complex consistent with previous off axis studies but at odds with observations of young crust 5 ma near the epr this dichotomy may be resolved by a delay in reflector formation through some combination of postaccretionary tectonics magmatism and hydrothermal alteration

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

radioelements radiogenic helium and age relationships for groundwaters from the granites at stripa sweden abstract the solution of radioelements and radiogenic 4he by groundwaters in fractured rocks is dependent upon the radioelement distribution in the rock matrix and the extent of the rock water interface the 234 u 238 u activity ratio and the dissolved u rn and he contents of such groundwaters respond to changes in the flow regime with time although 234 u 238 u activity ratios change with groundwater residence time as a consequence of 234th recoil induced solution of 234u the activity ratio is strongly influenced by the u distribution within fractures by the extent of the rock water interface and by the amount of 238u in solution a model for the quantitative evaluation of these effects is presented groundwaters from depths up to 880 m in the stripa granite have variable dissolved uranium contents and 234 u 238 u activity ratios the uranium geochemistry is primarily determined by variations in flow path rather than by groundwater age dissolved radiogenic 4he in the groundwaters increases with their depth of origin and is dependent upon the u content of the granite and upon its fracture porosity it increases with groundwater residence time but movement of 4he by diffusion and transport processes make the actual groundwater age indeterminate

structure and development of detrital reef deposits in turbid nearshore environments inhaca island mozambique coral communities are intermittently developed along the seaward margins of wide up to 500 m intertidal flats that fringe the western margins of inhaca island southern mozambique the coral communities occur in an environment that can be considered as marginal for coral growth in the sense that they are subject to high turbidity levels which results in rapid light attenuation and to the effects of periodic smothering and burial associated with a mobile siliciclastic dominated sediment substrate these constraints limit coral community development both spatially and bathymetrically in addition historical evidence suggests that the main sites of coral growth shift over decadal timescales due to periodic sediment remobilisation such rapid rates of turnover restrict the development of rigid framework deposits and in contrast the main depositional product of coral growth is an abundance of branched mainly acropora sp coral rubble rubble production is aided both by intense internal bioerosion sponge bivalve and worm traces and by periodic storm damage much of this rubble accumulates along the channel margins and provides a substrate for subsequent coral recruitment the deposits can be distinguished from those associated with clear water reef environments by 1 the lack of a rigid reef framework either comprising of in situ coral or encrusted coral rubble 2 the distinct lack of rubble encrustation syn depositional cementation and 3 the predominance of a siliciclastic rich sediment matrix the deposits indicate however a long term history of coral community occurrence in sites dominated by siliciclastic sedimentation cores recovered from a transect across the tidal flats identify a coral rubble floatstone facies extensively developed along the western margins of the island it comprises mainly branched coral rubble often heavily bioeroded but sparsely encrusted along with spheroidal rhodoliths and molluscan fragments set within a poorly sorted mixed siliciclastic carbonate sediment matrix 14c dating indicates that most of the rubble deposits date from the period 6500 4700 years bp which corresponds to a period of regional sea level highstand a detailed growth chronology cannot be established due to the lack of in situ coral but rapid rubble deposition and reef flat progradation around 350 m can be inferred for this period equivalent to an average progradation rate of around 0 2 m year 1 over the subsequent 4700 years progradation of only around 25 m has occurred this may be attributed to reworking and erosion of the seaward margin of the deposits during periods of fluctuating sea level between 4700 and 900 years bp or to a lack of seaward accommodation space

anatomy of a fringing reef around grand cayman storm rubble not coral framework abstract our fair weather perception of modern reefs has led to the implicit assumption that their development is controlled by processes that govern the siting of in place coral growth yet more ephemeral processes such as storms and hurricanes assume much greater importance over longer time scales because few reefs escape their influence to discover the importance of storms on reef development we analyze the zonation anatomy and architecture of a fringing reef complex around grand cayman we find that the surface zonation of in place corals is merely a facade and the reef core is in fact composed of meter thick layers of coral cobble rudstone capped by crusts of coralline algae the large size and abraded condition of the rudstone clasts shows that these layers are not the product of f ir weather processes but the result of destruction and deposition during hurricanes as hurricane waves cross coral mantled zones of the inner shelf they destroy live coral stands and deposit the clasts as a rubble layer covering the entire reef complex between storms this rubble foundation is stabilized by coralline algal crusts and recolonized by rapidly growing corals leading eventually to full reef regeneration before the next hurricane this cyclic pattern of destruction and regeneration consequently produces a fringing reef complex with a core composed of hurricane generated rubble not coral framework as previously assumed in addition to explaining reef anatomy hurricane control also explains the variation in reef architecture along shelf uniform reef location across shelf and reef absence along certain shelf sections as hurricane waves cross a mid shelf scarp they start to break and destroy coral growth over most of the inner shelf coral rubble generated by these waves is deposited 350 50 m from the mid shelf scarp on margins exposed to the largest waves but only 250 50 m on semi protected margins that experience smaller fetch limited waves in areas where the width of the inner shelf is 250 m hurricane waves throw rubble ashore and a fringing reef does not develop during sea level rise this influence of shelf width on rubble deposition controls the timing of reef nitiation and that in turn controls reef architecture reefs initiate first on low gradient coasts with wide shelves and gradually extend around higher gradient coasts as sea level rises and shelf width increases thus older reefs are located farther offshore front deeper lagoons and have thicker and narrower profiles than younger reefs

234u and238u concentration in brine from geopressured aquifers of the northern gulf of mexico basin abstract the 234 u and 238 u concentration in brine from six gulf coast geopressured aquifers has been determined the results reveal very low uranium concentrations from 0 003 to 0 03 g l and uranium activity ratios slightly greater than unity from 1 06 to 1 62 reducing conditions within the aquifers are responsible for the low uranium concentrations the uranium activity ratios observed are well below those calculated using theoretical considerations of alpha particle recoil effects this can be explained by interference with alpha recoil nuclides entering the liquid phase as a result of quartz overgrowths on sand grains and high temperature re equilibration that tends to minimize the effects of the alpha recoil process the fact that the uranium activity ratios of the brines are slightly greater than unity instead of the equilibrium value of 1 000 indicates that either the alpha particle recoil blocking and re equlibration effects are not complete or that another process is operative that enriches the fluid in excess 234 u by selectively removing uranium from radiation induced damage sites in the mineral sand grain matrix

experimental evidence of uraninite formation from diagenesis of uranium rich organic matter abstract this study focuses on the experimental diagenetic evolution of an uranium rich lignite from the coutras uranium deposit gironde france located in eocene sediments three samples containing 6 to 15 5 wt uranium were analysed and compared to two barren samples located outside the ore deposit in a stratigraphically equivalent unit uranium in the mineralized samples is present as uranyl carboxylic complexes dry thermal maturation was simulated in a cold seal autoclave at 150 200 250 300 and 350 c at 100 mpa for 24 h with increasing temperature uranyl carboxylic complexes decompose while uraninite forms the chemical composition of the experimentally matured barren samples when plotted in a van krevelen diagram h c vs o c parallels the natural trend of land derived type ill organic matter evolution while mineralized samples are characterized by a higher loss of hydrogen above 250 c reduction to form uraninite is directly related to the dehydrogenation of lignite the resulting uraninite has a similar unit cell edge to that of natural uraninites observed in sedimentary environments uraninite not only forms in the pore space but also replaces the lignite constituents the results demonstrate that uraninite may form during thermal evolution of uranium rich organic matter during diagenesis

reconstruction of past u migration in a sedimentary deposit coutras france implications for a radwaste repository this study is focused on ancient u mobility in unconsolidated kaolinite bearing sediments from the outcrop area of the coutras u deposit gironde france the analysis of radiation induced defects by electron paramagnetic resonance spectroscopy shows significant variations among the investigated samples and provides the paleodoses due to u at coutras the highest paleodose does not correspond to the highest u concentrations indicative of a past concentration of uranium different from the present one the paleodose gives an estimate of the equivalent u concentrations eu responsible for the defect concentration assuming an age for the kaolinites and considering extreme degrees of disequilibrium among u decay chain as uncertainties of the method the comparison between the equivalent eu and normalized present day u concentrations u norm reveals past migration of u when these two concentrations are different past occurrence of u and daughters is revealed even when this radioelement has been subsequently leached indeed surface sandy beds show a slight barely significant leaching evidence of u eu u norm in contrast several samples show u norm much greater than eu which suggests past accumulation of u younger than the sediment deposition particularly in organic rich clay layers these late enrichments may reach several hundreds ppm u isotopic constraints from the literature imply that accumulation is older than 1 ma in the organic clayey beds although the time of transfer or spatial dispersion of u cannot be specified from the methodology and the limited set of samples the quantitative analysis of radiation induced defects in clays yields intensity of past radioisotope transfers at the geological timescale it is relevant for the analysis of u mobility in sedimentary natural analogues of high level nuclear waste repositories it is also complementary to classical isotopic analysis of u and th series

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

dating open systems by the 238 u 234 u 230 th method application to quaternary reef terraces abstract the sedimentation conditions and diagenetic processes of biogenic and inorganic carbonates are such that the assumptions of a closed system evolution and the absence of initial th invoked in classical 238 u 234 u 230 th dating methods are very often not fulfilled for such material this problem is of interest for all studies requiring accurate dating of corals or aragonite rich sediments as in palaeoclimatology or neotectonics extending the approach of henderson and slowey nature 404 2000 61 66 we propose a consistent model that takes into account possible initial 230 th excess and where continuous selective redistribution gain or loss of 234 u 234 th and 230 th is controlled by recoil processes this model applied to quaternary marine terraces in barbados marie galante lesser antilles arc and pacific islands is able to explain the large scatter in isotopic compositions observed in the series of cogenetic corals or sediments we show that recoil processes generally tend to increase the 234 u 238 u and 230 th 234 u ratios with time leading to an overestimation of the apparent ages calculated using classical methods on the basis of simple assumptions identical marine initial 234 u 238 u 0 and 230 th 232 th 0 ratios we propose a method for calculating a best estimate of the age of cogenetic series our model offers an alternative way to date old reef terraces where corals are almost systematically altered and where classical dating methods do not apply it may also explain the apparent variations of the 234 u 238 u seawater initial ratio estimated from corals formed in the last 300 ka and the apparent uncoupling between sea level highstands and high latitude northern hemisphere summer insolation our set of equations can be applied to all weathering processes where the mobility of u series isotopes is controlled by recoil

uniform accretion of oceanic crust south of the garrett transform at 14 15 s on the east pacific rise using migrated common depth point reflection profiles we find the structural differences along the ultrafast spreading 150 mm yr east pacific rise south of the garrett fracture zone are second order suggesting a remarkably uniform process of crustal accretion the rise axis south of the garrett transform is underlain by a narrow 1 0 km melt lens which shows great along strike continuity the depth of the axial melt sill is approximately 1200 m beneath the seafloor which is about 400 m shallower than along the slower spreading east pacific rise at 9 30 n this observation strengthens the argument that the depth to the top of the crustal velocity inversion is spreading rate dependent melt sill width however shows little variation along the east pacific rise suggesting no dependence of magma chamber size on spreading rate the melt reservoir decreases in width toward across the 14 27 s ridge axis discontinuity by a modest 250 300 m and appears to be continuous across this feature given the small aspect ratio 1 0 km by 50 m by tens of kilometers of the axial melt lens the previously recorded jump in mgo content across the 14 27 s offset is likely the result of a mixing boundary which is sustained through an along strike impedance in convection wide angle reflections originating at the base of seismic layer 2a assumed to coincide with the extrusive layer reveal a twofold to threefold increase 200 250 to 500 600 m in thickness within 1 2 km of the rise axis the pattern of extrusive thickening imaged south of the garrett transform is similar to that observed along the slower spreading 110 120 mm yr east pacific rise at 9 n outside of the neovolcanic zone mean extrusive thickness is relatively invariant along a profile and from profile to profile this implies a degree of temporal stability of the along strike magma supply when integrated over the 10 kyr that corresponds to the width of the neovolcanic zone the inferred uniformity of off axis mean extrusive thickness is inconsistent with the conjecture that decreases in axial volume toward the 14 27 s discontinuity are caused by long term reductions in magma supply second order differences in the style of extrusive thickening may be related to structural differences within the low velocity zone underlying the rise axis and or changes within the stress field in the overlying carapace which results in the diffuse emplacement of lavas near the rise axis images of mono on cross axis profiles may be traced to within 1 0 km of the melt sill edge this observation is in agreement with rise crest models which generate the lower crustal section through the advection of material down and outward from the axial melt lens rather than through cumulate deposition at the base of a large magma chamber

a record of eruption and intrusion at a fast spreading ridge axis axial summit trough of the east pacific rise at 9 10 n 1 high resolution side scan sonar near bottom multibeam bathymetry and deep sea photo and bathymetry traverses are used to map the axial summit trough ast at the east pacific rise between 9 and 10 n we define three ridge axis morphologic types no ast narrow ast and wide ast which characterize distinct ridge crest domains spanning tens of kilometers along strike near bottom observations modeling of deformation above intruding dikes and comparisons to the geologic and geophysical structure of the ridge crest are used to develop a revised model of ast genesis and evolution this model helps constrain the record of intrusive and extrusive magmatism and styles of lava deposition along the ridge crest at time scales from hundreds to tens of thousands of years the grabens in the narrow ast domain 9 43 53 n are consistent with deformation above the most recent 10 diking events beneath the ridge crest frequent high effusion rate extrusive volcanism in this domain several eruptions every 100 years overprints near axis deformation and maintains a consistent ast width the most recent eruption at the ridge crest occurred in this area and did not significantly modify the physical characteristics of the ast the grabens in the wide ast domain 9 23 43 n originated with similar dimensions to the narrow ast spreading driven primarily by the intrusion of shallow dikes within a narrow axial zone causes the initial graben bounding faults to migrate away from the axis infrequent extrusive volcanism several eruptions every 1000 years fills a portion of the subsidence that accumulates over time but does not significantly modify the width of the ast outside of these domains lower effusion rate constructional volcanism without efficient drain back fills and erases the signature of the ast the relative frequency of intrusive versus extrusive magmatic events controls the morphology of the ridge crest and appears to remain constant over millennial time scales within the domains we have identified however over longer time scales 10 25 ka domain specific intrusive to extrusive ratios do not appear to be fixed in space resulting in a fairly consistent volcanic accretion over the length scale of the second order ridge segment between 9 n and 10 n

structural and geophysical expression of the solea graben troodos ophiolite cyprus field studies show that extensional structures in the north central part of the troodos ophiolite including steep normal faults grabens dikes and low angle detachment faults are related to e w present coordinates spreading the solea graben directly north of the mafic ultramafic rock outcrops around mount olympus consists of a 10 to 15 km wide western portion that is underlain by a low angle detachment fault between the sheeted dike complex and the plutonic section structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40 or more about subhorizontal axes the eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross cutting ophiolitic dikes and hydrothermal mineralization timing of the uplift of the central part of the troodos ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low angle normal faulting the solea graben spreading center provides a model for the structure of modern mid ocean ridges that are spreading amagmatically

uranium mobility in the sediment hosted uranium deposit of coutras france abstract recent mobility of u was investigated in the coutras u deposit gironde france located in lutetian sediments near the surface the coutras deposit is a belt of tabular ore bodies separated into two areas by the dronne river the jean vincent area close to the surface and the le fieu area located at a maximum depth of 100 m the immobility of th is supported by whole rock th contents which do not vary significantly with u content the present hydrological regime in the jean vincent area is characterized by oxidizing waters flowing toward the dronne river activity ratios ar of 230th 234u in rocks which range from 0 55 to 12 demonstrate recent u accumulation and leaching the 234u 238u ar range from 0 94 to 1 52 in the rocks and from 0 94 to 1 61 in the waters recent accumulation of u is favored along redox barriers where mo v se and as are also mobilized waters in the le fieu area are nearly stagnant they are reduced and exhibit a different chemical composition than waters in the jean vincent area with higher contents of ca mg na k cl hco3 po4 ra ba mn higher 234u 238u ar 1 33 4 63 and lower u content the le fieu rock samples do not show any isotopic evidence of recent leaching of u but both relative excesses and deficits of 234u are observed preferential mobility of 234u in the reduced le fieu area is interpreted according to the recoil model the results place new constraints on the model of cowart and osmond and should be useful in prospecting for tabular u deposits in reduced environments

uranium and organic matter in a paleodeltaic environment the coutras deposit gironde france the coutras uranium deposit located north of the aquitaine basin france was discovered by cogema in 1980 it is hosted by middle eocene arenaceous unconsolidated sediments at the surface the jean vincent area and down to 100 m depth the le fieu area the detrital components of the sediments are mainly quartz feldspars and granitic and metamorphic rock fragments the host sediments originated from the french massif central to the east orebodies are black tabular lenses occurring in sediments deposited in a delta plain environment where plant debris is abundant preservation of ligneous vegetation during burial established a reducing environment capable of concentrating uranium there is a good correlation between uranium total sulfur and organic carbon uranium is mainly fixed in organics as organo uranyl compounds uranium minerals detected include coffinite pitchblende and secondary hexavalent u minerals accumulation of uranium was favored by a permeable organic rich and slightly acid environment adsorption processes probably enhanced ore formation the lack of oxidized tongues may be the result of oxygen depleted uranium bearing solutions when they reached the organic rich zone the coutras uranium deposit is then a good example of a tabular type deposit which was deposited during early diagenesis

tectonics of the easter plate abstract we present a new model for the easter plate in which rift propagation has resulted in the formation of a rigid plate between the propagating and dying ridges we use the distribution of earthquakes eleven new focal mechanisms and existing bathymetric and magnetic data to describe the tectonics of this area both the easter nazca and easter pacific euler poles are sufficiently close to the easter plate to cause rapid changes in rates and directions of motion along the boundaries the east and west boundaries are propagating and dying ridges the southwest boundary is a slow spreading ridge and the northern boundary is a complex zone of convergent and transform motion the easter plate may reflect the tectonics of rift propagation on a large scale where rigid plate tectonics requires boundary reorientation we use simple schematic models to illustrate the general features and processes which occur at plates resulting from large scale rift propagation

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

la desirade island lesser antilles revisited lower cretaceous radiolarian cherts and arguments against an ophiolitic origin for the basal complex la desirade island in the guadeloupe archipelago is a keystone of the structural framework of the lesser antilles island arc hypothetical ophiolitic series and wide discrepancy in the radiometric datings of its igneous basement have raised considerable debate concerning the structural significance of this island and the geodynamic constraints for the caribbean plate recent detailed field work leads to a reconsideration of the problem the volcanic basement of la desirade can be subdivided into three independent units the cherts interbedded in the unit 2 basalts contain hauterivian barremian radiolarians that represent the oldest biostratigraphic assemblage found in the lesser antilles this affirms the jurassic radiometric age of the unit 1 trondhjemite outcrop observations and analyses allow us to discard the concept of an ophiolitic origin for la desirade whose basement was generated like the northern half of the lesser antilles arc by island arc processes during the mesozoic

determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge electrofacies analyses of odp iodp hole 1256d 1 the objective of this study is to construct a representative volcanostratigraphy of ocean drilling program hole 1256d the first complete penetration of intact upper oceanic crust formed at a superfast spreading rate an accurate knowledge of the volcanostratigraphy is vital to understand processes of crustal construction and submarine magmatism and to estimate chemical exchange with seawater but this is rarely achieved due to very low recovery rates in most basement holes we used two approaches to determine the rock types that form the wall rocks in the basement sections of hole 1256d 1 user guided interpretations of electrofacies acquired by imaging tools combined with other wireline tools and 2 the use of an artificial neutral network to objectively classify the responses of all available logging information great availability of formation microscanner fms images provided superior coverage of the borehole wall compared to previous attempts at core log integration this has resulted in more confident and detailed lithologic classifications such as with the distinction between pillows and different styles of breciation ten lithology types are suggested for a volcanostratigraphy model massive flows ponded lava fractured massive flows fragmented flows thin flows or thick pillows pillows breccias dikes in dike complex isolated dikes and gabbros three major lithology types in the extrusive section are massive flows both massive and fragmented massive flow 22 fragmented flows 32 and breccias 19 pillow lavas make up only 1 9 of the volcanic section and are confined to a 100 m interval below the extrusive section subvertical contacts interpreted to be dike margins are typically observed every 1 to 2 m with brecciated zones along the contacts the dikes dip steeply to the northeast indicating slight rotation away from the ridge axis we used an artificial neural network ann approach to determine a quantitative lithostratigraphy the ann is most strongly influenced by porosity and alteration degrees and the resulting stratigraphy most closely resembles the above classifications when clustered by fms texture as opposed to lithologic interpretation the ann thus provides a porosity based stratigraphy of the basement rather than the traditional lithology based stratigraphy

early establishment of seafloor hydrothermal systems during structural extension paleomagnetic evidence from the troodos ophiolite cyprus abstract paleomagnetic data from the troodos ophiolite are used to help constrain models for the relationship between extensional normal faulting and hydrothermal alteration related to production of large tonnage sulfide deposits at oceanic ridges we have sampled dikes from the troodos sheeted complex that have been subjected to variable hydrothermal alteration from greenschist alteration typical of the low water rock mass ratio interactions outside of hydrothermal upflow zones as well as from severely recrystallized rocks epidosites altered within high water rock mass ratio hydrothermal upflow zones in the root zones beneath large sulfide ore deposits these dikes are moderately to highly tilted from their initial near vertical orientations due to rotations in the hangingwalls of approximately dike parallel oceanic normal faults comparison of characteristic remanence directions from these dikes with the late cretaceous troodos reference direction therefore allows a tilt test to determine whether remanent magnetizations were acquired prior to or subsequent to tilting remanence directions for both greenschist and epidosite dikes show similar magnitudes of tilting due to rotational normal faulting and restore to the late cretaceous troodos reference direction upon restoration of dikes to near vertical positions about a nnw trending horizontal axis these data along with field observations of focused alteration along normal faults suggest that epidosite alteration occurred during the early stages of extensional tilting and prior to significant rotation this sequence of events is similar to that observed for creation of large tonnage sulfide bodies at intermediate to slow spreading centers which form soon after cessation of magmatism and during the early stages of structural extension we suggest that the dike parallel normal faults were initiated as extensional fractures during this early stage of crustal extension thus providing the necessary permeability for focused fluid flow and that later slip along these structures during rotational planar normal faulting caused reduction in permeability due to gouge formation

hurricane control on shelf edge reef architecture around grand cayman rimming the outer shelf of grand cayman is a submerged 87 km long shelf edge reef that rises to within 12 m of mean sea level it consists of an array of coral armoured buttresses aligned perpendicular to shore and separated by steep sided sediment floored canyons individual buttresses have a diverse coral dominated biota and consist of three architectural elements a shield like front wall colonized by platy corals a dome shaped crown colonized by head corals and a shoreward projecting spur covered by varying amounts of branching coral buttresses are commonly fronted by coral pinnacles that in some areas have amalgamated with buttress walls to produce pinnacle and arch structures as margin orientation changes shelf edge reef architecture shows systematic variations that are consistent with changes in fetch and height of hurricane waves along margins exposed to fully developed storm waves shelf edge reef buttresses are deep have large amplitudes and are dominated by robust head corals these characteristics are consistent with hurricane induced pruning of branching corals and the flushing of significant quantities of sand from buttress canyons by return flows along margins impacted by fetch limited storm waves reef buttresses are shallower have intermediate amplitudes and have a significantly higher proportion of branching corals these characteristics are consistent with less coral pruning and sand flushing by weaker hurricane waves along margins fully protected from storm waves the buttresses canyon architecture of the shelf edge reef breaks down producing a series of shallow undulating branching coral dominated ridges that merge laterally into an unbroken belt of coral these characteristics correspond with negligible amounts of pruning and flushing during hurricanes in addition to differences between margins local intra marginal changes in shelf edge reef architecture are consistent with changes in the angle of hurricane wave approach open sections of the shelf edge reef which face directly into storm waves are pruned of branching corals and the fragments swept back onto the shelf producing extensive spurs by contrast on more sheltered obliquely orientated sections storm waves sweep debris along and off shelf producing little or no spur development instead the debris shed seawards accumulates in front of the buttress walls and initiates the development of coral pinnacles over time repeated buttress pruning and canyon flushing during hurricanes not only controls reef architecture but may also influence accretion patterns vertical accretion is limited by the effective depth of storm wave fragmentation once this hurricane accretion threshold is reached the reef moves into a shedding phase and accretes laterally via pinnacle growth amalgamation and infilling consequently the reef steps out over its own debris in a kind of balancing act between lateral growth and slope failure a pattern widely recognized in ancient reefs

crustal faults exposed in the pito deep rift conduits for hydrothermal fluids on the southeast pacific rise 1 the escarpments that bound the pito deep rift northeastern easter microplate expose in situ upper oceanic crust that was accreted 3 ma ago at the superfast spreading 142 mm a full rate southeast pacific rise sepr samples and images of these escarpments were taken during transects utilizing the human occupied vehicle alvin and remotely operated vehicle jason ii the dive areas were mapped with a deformation intensity scale revealing that the sheeted dike complex and the base of the lavas contain approximately meter wide fault zones surrounded by fractured damage zones fault zones are spaced several hundred meters apart in places offset the base of the lavas separate areas with differently oriented dikes and are locally crosscut by younger dikes fault rocks are rich in interstitial amphibole matrix and vein chlorite prominent veins of quartz and accessory grains of sulfides oxides and sphene these phases form the fine grained matrix materials for cataclasites and cements for breccias where they completely surround angular to subangular clasts of variably altered and deformed basalt bulk rock geochemical compositions of the fault rocks are largely governed by the abundance of quartz veins when compositions are normalized to compensate for the excess silica the fault rocks exhibit evidence for additional geochemical changes via hydrothermal alteration including the loss of mobile elements and gain of some trace metals and magnesium microstructures and compositions suggest that the fault rocks developed over multiple increments of deformation and hydrothermal fluid flow in the subaxial environment of the sepr faults related to the opening of the pito deep rift can be distinguished by their orientation and fault rock microstructure some subaxial deformation increments were likely linked with violent discharge events associated with fluid pressure fluctuations and mineral sealing within the fault zones other increments were linked with the influx of relatively fresh seawater the spacing of the faults is consistent with fault localization occurring every 7000 to 14 000 years with long term slip rates of 3 mm a once spread from the ridge axis the faults were probably not active and damage zones likely played a more significant role in axial flank and off axis crustal permeability

coral reefs and sea level change abstract coral reefs provide significant evidence for former sea level positions because of their geological preservation and suitability for dating interpretation of this evidence presumes an understanding of reef geomorphology modern reef organism distributions and environmental factors influencing them fossil reef terraces formed during the last interglacial marine oxygen isotope mis substage 5e 128 116 ka are prevalent on many tropical shorelines and there has been ongoing debate as to the height reached by sea level during that highstand observations from numerous last interglacial sites suggest that sea level was at least 3 m above present sea level implying less extensive icesheets than at present an elevation of 6 m has commonly been adopted when correcting tectonically active sites for uplift recent compilations suggest elevations up to 8 9 m but incorporate few observations from reefs where the last interglacial is found below sea level oscillation of sea level during mis 5e has been interpreted from several sites with recent studies inferring rapid rise of several metres at the end of the interglacial these interpretations are at the limits to the precision with which corals can currently be dated and their palaeo water depths inferred it is not surprising that constraining last interglacial sea level changes within uncertainties of less than 1 2 m remains controversial considering sea level variations recognised between reef sites in the holocene and observed geographical variation in isostatic or flexural adjustments fossil coral reefs on uplifting margins also provide clear evidence for mis substages 5c and 5a and those on huon peninsula indicate fluctuations related to heinrich events mis 3 interpretations show considerable variability between sites with still greater uncertainties about sea level timing and elevation during previous interglacials future study of extensive sequences of fossil reefs preserved on rapidly subsiding margins could address these uncertainties submerged reefs have already yielded important information about sea level rise during the last deglaciation coring around barbados and tahiti as well as on the huon peninsula has produced a broadly consistent picture of ice melt reflecting eustatic change since the last glacial maximum these studies have shown the sensitivity of reefs to rapid sea level rise associated with meltwater pulses with some reefs drowning while others back stepped integrated ocean drilling program iodp expeditions to tahiti and recently the great barrier reef extended these records but details of timing nature and impact of deglacial meltwater pulses remain elusive studies of holocene reefs have indicated different growth strategies some kept up with sea level while others caught up when sea level decelerated holocene sea level appears to have experienced a gradual rise up to present across the caribbean providing accommodation space for reefs to accrete vertically whereas in the indo pacific sea level has been near its present level since 7 ka with many reef flats emergent following a slight fall of sea level caused by ocean siphoning microatolls on reef flats provide perhaps the clearest evidence of past sea level position but in their absence novel biological or other sea level indicators are required to better constrain palaeo water depths there is an urgent need for further research from additional key reef locations not only to decipher processes driving past sea level change and its geographical variability but also to better understand how coral reefs will respond in the context of future sea level rise