late pleistocene and holocene depositional systems and the palaeogeography of the dogger bank north sea 3d seismic data from the dogger bank north sea allow the mapping of late pleistocene and holocene depositional systems in unprecedented detail the data demonstrate that glacial processes resulted in the development of incised tunnel valley systems during the weichselian and that these were subsequently modified by fluvial processes in a pro glacial setting subsequently the dogger bank formed an emergent plain during the holocene with a complex meandering river system associated tributary or distributary channels and lakes dominating the region prior to the sea level rising sufficiently to submerge the dogger bank around 7500 yr ago the meandering river system was replaced by a dendritic channel network of potential fluvial estuarine or intertidal origin as the holocene depositional features bear no systematic relationship to the bathymetry this study demonstrates that previously published bathymetry based models for the holocene palaeogeographic development of the north sea require modification

stable isotope composition of middle miocene carbonates of the frontal cordillera and sierras pampeanas did the paranaense seaway flood western and central argentina abstract the geographic extent interconnectedness and chronology of the miocene paranaense epeiric seaway have been the subject of considerable debate understanding the timing and location of this marine incursion is significant for documenting sea level changes paleoclimatic changes and surface uplift or subsidence stable isotope analyses of carbonate strata in the flat slab segment of the sierras pampeanas and high andean cordillera previously purported to be of marine origin provide evidence that the parana seaway did not inundate this portion of west and central argentina no unambiguously marine facies or isotopic signatures were recognized for five representative stratigraphic units the saguion formation ese of salinas grande in cordoba province anta formation in the quebrada de la yesera and lerma valley of salta province del buey and del abra formations within the famatina ranges la rioja province and chinches formation in the manantiales foreland basin in the frontal cordillera san juan province paleontologic and lithologic features in support of a marine origin are reconsidered herein instead a lacustrine origin is inferred for these formations their contemporaneity perhaps related to concurrent global climate conditions and broad tectonic setting until more substantive evidence of marine deposition is found we reject correlation of these units with the parana seaway preserved in the chaco pampean plain subsurface and discourage mapping of the seaway as extending into the central sierras pampeanas or andean foreland our findings suggest the miocene andean foreland was elevated above sea level although it remained as a largely flat lying area west of the pampas and chaco plains which were inundated by the parana seaway

radiocarbon ages from two submerged strandline features in the western gulf of maine and a sea level curve for the northeastern massachusetts coastal region abstract new radiocarbon dates provide ages for two submerged strandline features on the massachusetts inner shelf these ages provide limited control on a relative sea level rsl curve for the late wisconsinan and holocene the curve indicates a late wisconsinan high stand of rsl of 33 m about 14 000 yr ago and a very short lived relative low stand of about 43 m at about 12 000 yr ago followed by a rise to present sea level rapid changes of rsl around 12 000 yr ago may be related to changes in global glacial meltwater discharge and eustatic sea level change shown by dated corals off barbados variations in the magnitude and timing of rsl change from south to north along the coast of the western gulf of maine are due to greater crustal depression and later deglaciation to the north

popham beach maine an example of engineering activity that saved beach property without harming the beach article i nfo beach and property erosion on coasts is a widespread and chronic problem historical approaches to this issue including seawalls and sand replenishment are often inappropriate or too expensive in maine seawalls were banned in 1983 and replenishment is too costly to employ replacement of storm damaged buildings is alsonotallowed andaprecedentcase onpophambeach mainerequiredthattheownerremoveanunpermitted building from a site where an earlier structure was damaged when the most popular park in maine popham beach state park experiencedinlet associatederosionthatthreatenedparkinfrastructure abathhouse tempo

automated feature extraction and spatial organization of seafloor pockmarks belfast bay maine usa article i nfo seafloor pockmarks occur worldwide and may represent millions of m 3 of continental shelf erosion but few numerical analyses of their morphology and spatial distribution of pockmarks exist we introduce a quantitative definition of pockmark morphology and based on this definition propose a three step geomorphometric method to identify and extract pockmarks from high resolution swath bathymetry we apply this gis implemented approach to 25 km 2 of bathymetry collected in the belfast bay maine usa pockmark field our model extracted 1767 pockmarks and found a linear pockmark depth to diameter ratio for pockmarks field wide mean pockmark depth is 7 6 m and mean diameter is 84 8 m pockmark distribution is non random and nearly half of the field s pockmarks occur in chains the most prominent chains are oriented semi normal to the steepest gradient in holocene sediment thickness a descriptive model yields field wide spatial statistics indicating that pockmarks are distributed in non random clusters results enable quantitative comparison of pockmarks in fields worldwide as well as similar concave features such as impact craters dolines or salt pools published by elsevier b v

relative sea level changes from ne ireland during the last glacial termination deglacial sea level records from ne ireland between 21 and 11 cal ka bp record marine transgressions and sensitive lithospheric responses to ice loading the sawtooth sea level curve contains four intervals characterized by 1 strong net uplift subaerial channelling and a global meltwater pulse 21 19 cal ka bp 2 ice loading isostatic depression and high relative sea level 19 17 5 cal ka bp 3 ice loading renewed isostatic depression and high relative sea level for 1000 years 17 14 5 cal ka bp 4 catastrophic ice wastage rapid uplift and lowstand 14 5 13 cal ka bp geophysical models do not capture these fluctuations

a laboratory study of nmr relaxation times in unconsolidated heterogeneous sediments nuclear magnetic resonance nmr relaxation time measurements can provide critical information about the physiochemical properties of water saturated media and are used often to characterize geologic materials in unconsolidated sediments the link between measured relaxation times and pore scale properties can be complicated when diffusing water molecules couple the relaxation response of heterogeneous regions within a well connected pore space controlled laboratory experiments have allowed us to investigate what factors control the extent of diffusional coupling in unconsolidated sediments and what information is conveyed by the relaxationtime distribution under varied conditions a range of sediment samples exhibiting heterogeneity in the form of a bimodal mineralogy of quartz and hematite were mixed with varied mineral concentration and grain size nmr relaxation measurements and geometric analysis of these mixtures demonstrate the importance of two critical length scales controlling the relaxation response the diffusion length d describing the distance a water molecule diffuses during the nmr measurement and the separation length s describing the scale at which heterogeneity occurs for the condition of s d which prevails for samples with low hematite concentrations and coarser grain size coupling is weak and the bimodal relaxation time distribution independently reflects the relaxation properties of the two mineral constituents in the heterogeneous mixtures for the condition of s d which prevails at higher hematite concentrations and finer grain size the relaxationtime distribution no longer reflects the presence of a bimodal mineralogy but instead conveys a more complex averaging of the heterogeneous relaxation environments this study has shown the potential extent and influence of diffusional coupling in unconsolidated heterogeneous sediments and can serve to inform the interpretation of nmr measurements in near surface environments where unconsolidated sediments are commonly encountered

evidence for a transgressive barrier within a regressive strandplain system implications for complex coastal response to environmental change clastic depositional strandplain systems have the potential to record changes in the primary drivers of coastal evolution climate sea level and the frequency of major meteorological and oceanographic events this study seeks to use one such record from a southern brazilian strandplain to highlight the potentially complex nature of coastal sedimentological response to small changes in these drivers following a 2 to 4 m highstand at ca 5 8 ka in southern brazil falling sea level reworked shelf sediment onshore forcing coastal progradation smoothing the irregular coastline and forming the 5 km wide pinheira strandplain composed of ca 500 successive beach and dune ridges sediment cores grab samples and 11 km of ground penetrating radar profiles reveal that the strandplain sequence is composed of well sorted fine to very fine quartz sand since the mid holocene highstand the shoreline prograded at a rate of ca 1 to 2 m yr 1 through the deposition of a 4 to 6 m thick shoreface unit a 1 to 3 m thick foreshore unit containing ubiquitous ridge and runnel facies and an uppermost beach and foredune unit however the discovery of a linear 100 m wide barrier ridge with associated washover units a 3 to 4 m deep lagoon and 250 m wide tidal inlet within the strandplain sequence reveals a period of shoreline transgression at 3 3 to 2 8 ka during the otherwise regressive developmental history of the plain the protected nature of pinheira largely buffered it from changes in precipitation patterns wave energy and fluvial sediment supply during the time of its formation however multiple lines of evidence indicate that a change in the rate of relative sea level fall probably due to either steric or ice volume effects may have affected this coastline thus whereas these other potential drivers cannot be fully discounted this study provides insights into the complexity of decadal scale to millennial scale coastal response to likely variability in sea level change rates

rapid thinning of the laurentide ice sheet in coastal maine usa during late heinrich stadial 1 abstract few data are available to infer the thinning rate of the laurentide ice sheet lis through the last deglaciation despite its importance for constraining past ice sheet response to climate warming we measured 31 cosmogenic 10 be exposure ages in samples collected on coastal mountainsides in acadia national park and from the slightly inland pineo ridge moraine complex a 100 km long glaciomarine delta to constrain the timing and rate of lis thinning and subsequent retreat in coastal maine samples collected along vertical transects in acadia national park have indistinguishable exposure ages over a 300 m range of elevation suggesting that rapid century scale thinning occurred at 15 2 0 7 ka similar to the timing of abrupt thinning inferred from cosmogenic exposure ages at mt katahdin in central maine davis et al 2015 this rapid ice sheet surface lowering which likely occurred during the latter part of the cold heinrich stadial 1 event 19 14 6 ka may have been due to enhanced ice shelf melt and calving in the gulf of maine perhaps related to regional oceanic warming associated with a weakened atlantic meridional overturning circulation at this time the ice margin subsequently stabilized at the pineo ridge moraine complex until 14 5 0 7 ka near the onset of bolling interstadial warming our 10 be ages are substantially younger than marine radiocarbon constraints on lis retreat in the coastal lowlands suggesting that the deglacial marine reservoir effect in this area was 1 200 14 c years perhaps also related to the sluggish atlantic meridional overturning circulation during heinrich stadial 1

effects of latent heat release at phase boundaries on flow in the earth s mantle phase boundary topography and dynamic topography at the earth s surface abstract mantle flow models that do not consider the effects of latent heat on phase boundaries typically predict dynamic surface topography too large to be compatible with observations here these effects were implemented in a mantle flow model and resulting changes in dynamic topography and topography of phase boundaries were computed inclusion of these effects was found to reduce the rms amplitude of dynamic topography by about 50 60 m still leaving a substantial misfit from observations in most cases considered generally it also leads to a slight improvement of correlation with dynamic surface and phase boundary topography constrained by observations results thus indicate that the model implemented here is applicable to the earth but that other effects need to be considered as well in order to fully explain phase boundary and dynamic surface topographies

depth age and dynamic topography of oceanic lithosphere beneath heavily sedimented atlantic margins our understanding of the thermal structure of oceanic lithosphere is based upon observations of water loaded subsidence and plate age these observations are predominantly from areas with sparse sedimentary cover which are unperturbed by crustal thickening and by regional epeirogeny few such measurements are available from old oceanic lithosphere where sediment thickness is large in order to address this omission we have analyzed a legacy dataset of seismic reflection and wide angle profiles from the margins of the atlantic ocean our aims are twofold first we determine the isostatically corrected age depth variation of old oceanic lithosphere adjacent to continental margins and compare it with existing trends secondly we isolate residual depth anomalies and compare them with dynamic topographic predictions made from long wavelength gravity anomalies within the sedimentary pile we have used a suite of coincident seismic reflection and wide angle measurements to create a method for converting two way travel time into depth and sediment density we also correct for changes in oceanic crustal thickness on the oldest portions of the atlantic ocean the corrected age depth variation is consistent with results from the pacific ocean which suggests that the oceanic plate has a thickness of 90 95 km positive and negative deviations from the global age depth trend are common and can be as large as 1 km they mostly but not always correlate with long wavelength gravity anomalies if the admittance is 30 mgal km 1 we propose that these deviations are primarily generated by convective circulation within the upper mantle

low electrical resistivity associated with plunging of the nazca flat slab beneath argentina beneath much of the andes oceanic lithosphere descends eastward into the mantle at an angle of about 30 ref 1 a partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material2 this wedge is the ultimate source for magma erupted at the active volcanoes that characterize the andean margin but between 28 and 33 s the subducted nazca plate appears to be anomalously buoyant3 4 as it levels out at about 100 km depth and extends nearly horizontally under the continent1 5 6 above this flat slab volcanic activity in the main andean cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge5 6 but it is unknown where slab volatiles go once this happens and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward here we present results from a magnetotelluric profile in central argentina from which we infer enhanced electrical conductivity along the eastern side of the plunging slab indicative of the presence of partial melt this conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth or that melt is supplied from the 410 km discontinuity consistent with the transition zone water filter model of bercovici and karato7

amorphous silica nanofilms result in growth of misoriented microcrystalline quartz cement maintaining porosity in deeply buried sandstones deeply buried sandstones in sedimentary basins typically have low porosity due to cementation and compaction there are several known causes of anomalously high porosity in sandstones one of which is microquartz coatings on sand grains that seem to inhibit growth of quartz cement however there has been no mechanistic understanding of why or how microquartz grows or why it maintains high porosity in sandstones here we have used high resolution scanning electron microscopy electron backscattered diffraction and transmission electron microscopy to study the microquartz cemented late cretaceous heidelberg formation germany we have revealed that a nanofilm of amorphous silica 50 100 nm and a layer of chalcedony are between the detrital grain and microquartz cement the amorphous silica insulates the detrital quartz grains and prevents syntaxial growth while microquartz adopts the orientation of the underlying chalcedony with its fast growth c axis parallel to the grain surface thus preventing growth into the pore now that we know what controls microquartz growth and why it preserves porosity it can be used to help identify rank and appraise deeply buried petroleum accumulations

a model for drowned terrestrial habitats with associated archeological remains in the northwestern gulf of maine usa abstract marine geophysical techniques and geological measurement of sea level change allow detailed mapping of seafloor archeological sites and development of models in the northwestern gulf of maine the lowstand of sea level is only at 60 m depth but a prolonged stasis during the late quaternary sea level rise herein termed the slowstand occurred between 11 500 and 7500 calibrated years before present cal yr bp at depths of 15 25 m this slowdown caused substantial erosion of glacial deposits and formed terrestrial beaches wetlands and lakes these landforms attracted people whose tools were later discovered during present day scallop fishing activities our study areas at the green islands and bass harbor are within the lowstand and slowstand depth ranges but at highly exposed and sheltered sites respectively the green islands site has lost most of its former terrestrial sediment and there is no longer any context for recovered artifacts at the protected bass harbor site the remains of eroded moraines are associated with drowned spits and a former lacustrine estuarine basin cores at the bass harbor location encountered freshwater peat deposits and numerous shallow subtidal shell and plant fossils all deposited during the slowstand the two most important controls on the preservation of terrestrial landforms and associated human artifacts in the northwestern gulf of maine are the rate of sea level rise and degree of shelter of the site from waves

palaeogeography of atlantic canada 13 0 kyr abstract we combine isobase maps with a digital terrain model of atlantic canada to map coastlines from 13 14c kyr bp to the present at 13 14c kyr bp there are ridges of high relative sea level rsl values over newfoundland and the maritime provinces and a re entrant of low values in the gulf of st lawrence this pattern persists well into the holocene and reflects crustal response to the slow wasting of ice caps that persisted in newfoundland and the maritime provinces for up to five millennia after the removal of ice from the gulf of st lawrence by a migrating calving embayment the palaeogeographic reconstructions reveal an archipelago on the outer shelf from grand bank to the continent that persisted from 13 14c kyr bp until ca 8 14c kyr bp much of the magdalen shelf was exposed but the magdalen islands were never connected to the mainland prince edward island was initially separated from the mainland became connected after 11 14c kyr bp and was separated again just before 6 14c kyr bp when northumberland strait formed the reconstructions are highly sensitive to relatively small changes in isobase values especially on the shallow banks on the continental shelf

morphology and seismic stratigraphy of the inner continental shelf off nova scotia canada evidence for a 65 m lowstand between 11 650 and 11 250 c14 yr b p abstract nova scotia s position near the margin of large pleistocene ice sheets makes the scotian shelf region critical for evaluation of glaciation models and sea level change the sea floor of the inner scotian shelf was mapped using multibeam bathymetry a combination of conventional seismic and sonar techniques and sampling multibeam bathymetry provides an areal image of the sea floor combining this unique image with sub bottom seismic imaging the relationships between sea floor topography and the underlying strata were explored the inner continental shelf of nova scotia can be subdivided into five major terrain zones these are the 1 truncation zone 2 morainal zone 3 outcrop zone 4 basin zone and 5 scotian shelf end moraine complex the scotian shelf end moraine complex and basin zone are glacial depositional zones at the seaward edge of the inner shelf landward of these zones is a region of high relief bedrock with ridges and valleys largely devoid of surficial sediments extending from 80 to 120 m water depth outcrop zone this zone is interpreted as a relict bedrock surface preserved under frozen bed glacier conditions to the east of the outcrop zone in similar water depths are unmodified till ridges overlying bedrock morainal zone the truncation zone is a region of the inner shelf from 90 m to the present shoreline characterized by muted acoustic topography and planar erosional surfaces truncating bedrock and surficial sediments the truncation zone is subdivided into the valley subzone the transition subzone the platform subzone and the estuarine subzone the valley subzone is typified by sediment infilled valleys occurring in 75 to 90 m water depths these valleys contain seismic facies with a ponded style of deposition planed off at the sea floor or by erosional unconformities near the sea floor the transition subzone is marked by a relatively steep ramp and terraces the ramp at the type section extends from 75 to 65 m water depth the surface of the ramp can be either an erosional unconformity or a depositional surface formed by clinoform beds the 65 m former shoreline is interpreted as the top of the transition subzone ramp surface at one ramp locality clinoform beds form part of a progradational sequence called the sambro delta a mussel valve fragment mytilus edulis was obtained from a core in the foresets and radiocarbon dated an age of 11 650 110 yr b p was obtained adjusted for isotopic fractionation above 65 m the sea floor forms a low relief gently sloping erosional surface platform subzone estuarine deposits have been found above 50 m the new sea level curve constructed with the dated 65 m shoreline and recently published data is at variance with calculated rsl curves based on geophysical models primarily in the amplitudes and rates of rsl change reasons for the discrepancies may be late melting ice and regional variations in lithospheric strength and thickness

integrated microstratigraphic investigations of coastal archaeological soils and sediments in norway the gokstad ship burial mound and its environs including the viking harbour settlement of heimdaljordet vestfold investigations of past coastal landscape development have included soil micromorphology chemistry and microfossil recording of soils and sediments associated with marine inundation and terrestrial soil formation in marine sediments this paper reports on similarly studied site formation processes at norwegian coastal sites in vestfold norway the viking period gokstad ship burial mound and nearby contemporary harbour site of heimdaljordet at gokstad strategically focused coring revealed mound composition and an example of its buried soil and geology the latter investigation suggested that post glacial uplift led to a slowstand period of intertidal reworking of till before 700 bc emergence and development of terrestrial soils at heimdaljordet typically laminated intertidal silty clay loam sediments were sealed by beach sands into which for example a boat grave was dug post depositional processes affecting the gokstad mound were compared to those in other mounds including those recorded in experimental earthworks waterlogged conditions in the gokstad mound led to iron phosphate migration and preferential deposition of vivianite in turf layers where relict litter l layers remained visible and where wood chips from constructional activities are also very well preserved as is the long ship itself these soil insights and other paleoenvironmental studies of the buried soil and numerous turf sequences showed that the contemporary ad 900 viking landscape was totally terrestrial it had become wet sedge grassland managed for grazing the partial weathering of turves and anomalous presence within them of fresh roots apparently indicates the possibility that turves were stacked and stored ahead of mound building the 10th c robber trench had developed muddy features and rooting traces show that it was not backfilled but was slowly infilled by humic soil silting from turf mound layers this event did not affect the overall anaerobic burial conditions in the mound which can be starkly compared to those at the heimdaljordet boat grave here because of acidic subaerial weathering the wooden boat only survives as an acidic pellety humus formed of wood residues that are often ferruginised iron appears to be concentrated at iron nail locations unlike the gokstad mound no bone survives but one sample found a typical body stain of secondary iron and phosphate close by the iron encrusted sword in the grave potentially the pelvic region of the inhumation here mineralised faecal gut remains have an assumed hydroxyapatite composition and embed phytoliths and pollen spores as found in human coprolites and cess deposits studied elsewhere

cenozoic subsidence and uplift of continents from time varying dynamic topography time varying global dynamic topography has been computed for the entire cenozoic the spherical viscous flow model is driven by a distribution of density heterogeneity based on mesozoic and cenozoic subduction by using the calculated topography the uplift and subsidence histories of five regions fixed to the north american indian australian indonesian and south american continents are computed predicted vertical motions are in qualitative agreement with observed values except for india and south america for australia and north america the predicted periods of uplift and subsidence are out of phase with respect to observations but display the correct overall trend the latest phase of uplift for north america is directly related to the cessation of kula plate subduction after 48 ma dynamic topography depends on the viscosity structure of the mantle a lower mantle that is 50 times more viscous than the upper mantle yields the best agreement between modeled and predicted vertical motions the presence of a low viscosity channel beneath the lithosphere has only a small influence on our results compared to dynamic topography the time varying geoid is insignificant in controlling relative sea level

origin of quartz cements in some sandstones from the jurassic of the inner moray firth uk the extent of quartz cementation in shallow marine sandstones of the brora arenaceous formation oxfordian is closely related to the occurrence and abundance of rhaxella perforata sponge spicules three cement morphologies are identified chalcedonic quartz microquartz and mesoquartz chalcedonic quartz forms matrix supported cements which preserve moulds of rhaxella spicules chalcedonic quartz crystals have inequant development of crystal faces on average 0 1 m in diameter and are the first formed cement and reveal homogeneous dark grey tones on the sem cl bei microquartz forms 5 10 m diameter crystals which commonly grow on chalcedonic quartz substrates and show various grey tones under sem cl bei mesoquartz crystals grow in optical continuity with their host grains have 20 m a axial diameter crystals and exhibit distinctly zoned luminescence although no opaline silica is preserved the quartz cement is interpreted to have formed from an opaline precursor detrital quartz has an average 18o composition of 12 2 and mesoquartz syntaxial overgrowth has an average 18o composition of 20 0 estimates of the 18o compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types mixtures of the two give consistently higher 18o compositions than mesoquartz the higher estimate being 39 2 from oxygen isotope data the formation of quartz microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71 c in marine derived pore waters organic and inorganic maturation data constrain the upper temperature limit to less than 60 c

andean flat slab subduction through time the analysis of magmatic distribution basin formation tectonic evolution and structural styles of different segments of the andes shows that most of the andes have experienced a stage of flat subduction evidence is presented here for a wide range of regions throughout the andes including the three present flat slab segments pampean peruvian bucaramanga three incipient flat slab segments carnegie guanacos tehuantepec three older and no longer active cenozoic flat slab segments altiplano puna payenia and an inferred palaeozoic flatslab segment early permian san rafael based on the present characteristics of the pampean flat slab combined with the peruvian and bucaramanga segments a pattern of geological processes can be attributed to slab shallowing and steepening this pattern permits recognition of other older cenozoic subhorizontal subduction zones throughout the andes based on crustal thickness two different settings of slab steepening are proposed slab steepening under thick crust leads to delamination basaltic underplating lower crustal melting extension and widespread rhyolitic volcanism as seen in the caldera formation and huge ignimbritic fields of the altiplano and puna segments on the other hand when steepening affects thin crust extension and extensive within plate basaltic flows reach the surface forming large volcanic provinces such as payenia in the southern andes this last case has very limited crustal melt along the axial part of the andean roots which shows incipient delamination based on these cases a palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism the geological evolution of the andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system

reconstructing farallon plate subduction beneath north america back to the late cretaceous using an inverse mantle convection model that assimilates seismic structure and plate motions we reconstruct farallon plate subduction back to 100 million years ago models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy requiring that the farallon slab was flat lying in the late cretaceous consistent with geological reconstructions the simulation predicts that an extensive zone of shallow dipping subduction extended beyond the flat lying slab farther east and north by up to 1000 kilometers the limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten the results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized

seismic architecture stratigraphy and evolution of a subtropical marine embayment maputo bay mozambique abstract the seismic stratigraphy architecture and evolution of a large subtropical embayment maputo bay southern mozambique is described nine seismic units are documented and show repeated phases of incised valley formation and transgressive infilling trangressive infilling alternates between the classic wave dominated transgressive succession of fluvial and central estuarine bay deposits and the filling by intertidal flat and tidal bedforms developed during stages of inlet restriction and shallowing associated with barrier growth at stillstand intervals imprinted on the succession are several key facets of subtropical stratigraphy hurricane generated erosional surfaces and beachrock horizons the upper stratigraphy comprises the highstand bayhead delta a prograding feature constructed by several flood events in comparison with literature from other similar systems such as lagoons coastal lakes and estuaries a generic three stage stratigraphic model of large marine embayments is proposed that includes incision during glaciations transgressive infilling during deglaciations and shallowing during the stillstand episodes of the recent deglaciation

submerged shoreline preservation and ravinement during rapid postglacial sea level rise and subsequent slowstand submerged shorelines hold much potential for examining the interplay between the rate of sea level rise and geomorphic setting and informing the development of models of contemporary shoreline behavior this paper describes the sedimentary architecture of a submerged barrier shoreline complex off durban south africa the complex consists of several barriers that have survived the postglacial transgression and associated erosive ravinement processes the main shoreline sequence 60 m dates to 11 690 90 calibrated cal yr b p and rests on a pleistocene lagoonal deposit 35 395 592 cal yr b p the entire barrier shoreline complex is truncated by a strongly diachronous wave ravinement surface the ravinement surface seaward of the main 60 m shoreline is steep but the gradient declines across and landward of the subordinate landward shoreline complexes the steep ravinement surface is attributed to fast sea level rise possibly associated with meltwater pulse 1b while the gentle ravinement surface is associated with a subsequent slowing of the rate of sea level rise to 0 15 mm yr 1 preservation of the main barrier and back barrier deposits through overstepping is associated namely with rapid transgression and increased back barrier accommodation the smaller barriers landward of the main barrier were preserved through overstepping related to gentle antecedent gradients despite more intense ravinement during a very slow rise in sea level slowstand this process was assisted by a sediment deficit the resulting post transgressive drape is also influenced by antecedent topography created by the barriers themselves damming along the landward sides of overstepped barriers thickens the drape and creates a temporal disconnect between the migration of the shoreface and more landward elements of the littoral system in examining the rates of shoreline migration associated with the overstepped barrier system these are far greater than those calculated for the predicted rates of shoreline change on similarly steep coastal profiles future rates of shoreline migration appear to be insufficient to cause overstepping and rollover or erosional responses are more likely

infiltrated clays in fluvial jurassic sandstones of reconcavo basin northeastern brazil abstract fluvial sandstones and conglomerates of the sergi formation a jurassic unit of reconcavo basin northeastern brazil contain significant amounts of interstitial detrital clays which are interpreted as the product of early mechanical infiltration developed in an arid semi arid environment within such an environment the lowered water table allowed muddy waters of episodic runoff to infiltrate through the coarse alluvium concentrating the clays in the upper phreatic zone near the sources of influent seepage petrographic textures considered to be typical of mechanically infiltrated mi clays are herein presented using examples from the sergi formation these textures include 1 ridges and bridges 2 geopetal fabrics 3 loose aggregates 4 anisopachous coatings of tangentially accreted lamellae cutans 5 massive aggregates and 6 shrinkage patterns developed during diagenesis in addition their detrital aspect as revealed by sem observations and the presence of impurities in the interior of the clay aggregates were also used as evidence for the interpreted origin of the clays in most hydrocarbon reservoirs in the sergi formation the clays described in this paper are among the main controls on porosity and permeability distribution commonly zones with maximum clay concentration are the main internal permeability barriers within the sergi formation reservoirs

prediction of deep reservoir quality using early diagenetic process models in the jurassic norphlet formation gulf of mexico we have developed process based models for early grain coats and their impact on deep reservoir quality in the jurassic eolian norphlet formation alabama with implications for exploration and development in other conventional and tight gas continental reservoirs the norphlet a major gas reservoir to depths of 21 800 ft 6645 m and temperatures of 419f 215c displays contrasting intervals of high and low reservoir quality within compositionally similar cross bedded eolian sands study results show that grain coats formed soon after deposition are responsible for differences in deep norphlet porosity of up to 20 and permeability up to 200 md three types of grain coats were identified in norphlet dune sands each formed in a different part of a shallow groundwater system and each with distinctive impact on deep reservoir quality diagenetic chlorite coats formed where dunes subsided into shallow hypersaline groundwater preserve good deep porosity to 20 and permeability to 200 md continuous tangential illitic coats formed in the vadose zone of stabilized dunes exposed to periodic fresh water influx preserve good deep porosity to 15 associated with poor permeability 1 md due to linked formation of later high temperature diagenetic illite discontinuous grain coats formed in active dunes where grains were abraded by eolian transport are associated at depth with tight zones of pervasive quartz cement low porosity 8 and low permeability 1 md these concepts plus data from 60 wells were used to derive bay wide predictive tight and porous zone isopachs that can be used for well placement geologic models and field development

empirical prediction of porosity and permeability in sandstones current efforts to predict porosity and permeability in sandstones prior to drilling are focused on empirical and process oriented models empirical predictions are based on the correlation between porosity and permeability and a limited number of parameters obtained from calibration data sets or estimated from appropriate geologic models of these parameters the most important are detrital composition grain size sorting and temperature history or pressure history or both despite its limitations the empirical approach provides accurate predrill predictions of reservoir quality in many sandstones containing not only primary but also secondary porosity and permeability predictions of the average porosity and permeability of target sandstones are arbitrarily defined h re as accurate if they fall within 2 porosity of the mean measured porosity and within the same order of magnitude as the mean measured permeability in a sample population representing the interval of interest the effectiveness of the empirical predictive approach is illustrated by case studies from the taranaki basin new zealand and yacheng field people s republic of china these studies indicate that empirical predictions are basin specific or even play specific and require at least some understanding of fundamental processes affecting reservoir quality of a given sandstone target process oriented approaches attempting to model the effect of diagenesis on reservoir quality are hampered by inadequate quantitative understanding of the processes responsible for preserving primary porosity and generating secondary porosity and permeability until adequate quantification of the sandstone diagenesis processes is achieved empirical models have a distinct advantage over process oriented models in providing reliable predictions of reservoir quality in many sandstone intervals

preservation of a drowned gravel barrier complex a landscape evolution study from the north eastern english channel abstract landscape response to post glacial relative sea level during the quaternary is documented using an integrated dataset of multibeam bathymetry and 2d seismic reflection profiles from the hastings bank area in the north eastern english channel mapping of nine seismic stratigraphic units calibrated to lithological information from multiple vibrocores has enabled the interpretation of fluvial shoreface barrier washover fan back barrier and tidal environments of deposition the interpreted landscape evolution is as follows i fluvial incision of bedrock during sea level lowstand ii progradation of a shoreline and then development of a barrier complex as sea level rose iii recycling and breaching of the barrier iv rapid drowning of the barrier complex v landward migration of the shoreline through continued sea level rise and vi complete abandonment and submarine preservation of the barrier complex during sea level highstand the previously undocumented yet exceptionally well preserved drowned barrier complex at hastings bank records phases of barrier initiation breakdown and retreat and documents coastal response to high rates of relative sea level rise initial development of the barrier complex required a sufficient supply of sediment maintained by offshore sources to keep pace with rising sea level which permitted progradation of a shoreline and development of a barrier complex inherited topography in the north eastern english channel is an important factor in the development of the barrier complex phases of barrier breakdown occur when sediment supply is outpaced by a rapid increase in accommodation controlled by existing basement morphology and rising sea levels subsequently the barrier responds through internal reorganisation by breaching and reworking of existing sediment bodies barrier retreat is characterised by a phase of sediment surplus overstepping under rapid rates of sea level rise where increased water depths limit wave reworking followed by a phase of discontinuous retreat where the shoreline steps back through sediment deficit overstepping hastings bank presents a rare opportunity to examine the conditions and processes that control barrier response to sea level rise and to assess the preservation potential of barrier deposits as a function of the style of retreat

miocene drainage reversal of the amazon river driven by plate mantle interaction northern south america experienced significant changes in drainage patterns during the opening of the south atlantic ocean disappearance of a mega wetland in the western amazonian basins was followed by the formation of the eastward draining amazon river which has been attributed to andean uplift however south america s westward motion over cold dense subducted slabs implies that regional subsidence and uplift east of the andes may have been driven by mantle convection here we use a coupled model of mantle convection and plate kinematics to show that dynamic subsidence of up to 40 m myr 1 initially formed the amazonian mega wetland in our model the sustained westward motion of continental south america over subducted slabs resulted in rebound of the amazonian mega wetland region at rates of up to 40 m myr 1 after 30 million years ago paired with continued subsidence of the eastern amazonian sedimentary basins at 10 20 m myr 1 the resulting progressive tilt of northern south america to the east enabled the establishment of the amazon river suggesting that mantle convection can profoundly affect the evolution of continental drainage systems

the sea level fingerprint of the 8 2 ka climate event the 8 2 ka cooling event was an abrupt widespread climate instability there is general consensus that the episode was likely initiated by a catastrophic outflow of proglacial lakes agassiz and ojibway through the hudson strait with subsequent disruption of the atlantic meridional overturning circulation however the total discharge and flux during the 8 2 ka event remain uncertain we compute the sea level signature or fingerprint associated with the drainage of lakes agassiz and ojibway as well as the expected sea level signal over the same time period due to glacial isostatic adjustment gia in response to the late pleistocene deglaciation our analysis demonstrates that sites relatively close to the lakes including the west and gulf coasts of the united states have small signals due to the lake release and potentially large gia signals and thus they may not be optimal field sites for constraining the outflow volume other sites such as the east coast of south america and western africa have significantly larger signals associated with the lake release and are thus better choices in this regard

quartz flakes in lakes microdebitage evidence for submerged great lakes prehistoric late paleoindian early archaic tool making sites exploration for submerged prehistoric archaeological sites in the great lakes north america is a major challenge due to difficulties in locating scant cultural artifacts in lake bottom sediments stone tool microfragments microdebitage 10 6 per tool and more dispersed around tool making sites but have not been identified previously in an underwater context to evaluate their use as a submerged site indicator microdebitage analysis was conducted on five lake sediment cores from a shallow lagoon adjacent to a long occupied prehistoric site mcintyre site rice lake ontario we identified 155 microdebitage fragments within a distinctive muddy peat horizon 2 2 5 m depth using light microscopy and scanning electron microscopy methods the microdebitage consisted of angular to very angular quartz fragments 400 1000 m with characteristic conchoidal fractures and flake scars produced by mechanical percussion the microdebitage horizon had a distinctive bimodal particle size peak and contained a low diversity soil thecamoebian assemblage phryaginella bullinularia sp indicative of a wetland environment that formed during an early holocene shoreline transgression accelerator mass spectrometry 14 c dating of wood fragments yielded ages of 9470 8760 50 yr b p 11 070 9560 cal calibrated yr b p indicating a late paleoindian early archaic age for the deposit results demonstrate that coring and microdebitage analysis are effective tools in the search for underwater prehistoric sites and can be employed more broadly in exploration of submerged landscapes in the great lakes basins

miocene forebulge development previous to broken foreland partitioning in the southern central andes west central argentina 1 in the distal region of the modern flat slab segment in the southern central andes an unusual stack of middle miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west these features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system sedimentary thickness farther east and far from the cordilleran tectonic loads suggests accommodation and preservation driven by nonisostatic dynamic subsidence regional overlapping relationships and basin modeling suggest that the modern broken foreland present sierras pampeanas can be interpreted as a reactivation of a formerly partitioned broad forebulge

late quaternary relative sea level change in the western gulf of maine evidence for a migrating glacial forebulge new radiocarbon dated cores obtained by vibracorers in the western gulf of maine confirm that a short lived relative sea level lowstand of 55 m occurred at 11 10 5 ka these cores and younger salt marsh data also reveal that rates of transgression varied throughout the holocene probably due to local variations in glacial isostasy the isostatic component is resolved by subtracting published approximations of eustatic sea level from our well determined observations of local relative sea level a large peak in the isostatic curve coincides with the lowstand and is interpreted as a forebulge 20 25 m in amplitude forebulge migration is estimated at 7 11 km 100 yr based on the timing of lowstands across the region

tectonic and dynamic controls on the topography and subsidence of the argentine pampas the role of the flat slab abstract we analyze the pampean foreland the pampas along the modern flat slab segment of the south central andes between 31 and 33 south latitude and to the east of the argentine flat slab province using flexural and gravity studies and computations of dynamic topography bouguer anomalies and flexural analysis predict a foredeep of 250 km width and a peripheral bulge amplitude of 25 m which match the regional morphologies of the modern argentine pampean plain however these studies do not account for the subsurface miocene quaternary basin preservation represented by sedimentary thicknesses 400 m and with depocenters 400 km eastward with respect to flexural models the discrepancy suggests that two mechanisms acting at different wavelengths influence the argentine pampas the basin preservation is likely the result of a large scale geodynamic forcing models of mantle flow driven by realistic flat slab subduction geometry and density contrasts reproduce the depocenter location and the wavelength of subsidence as well as most of the remaining amplitude nonetheless more sophisticated studies e g considering lateral viscosity variations in the mantle wedge might help reduce the dynamic amplitudes and better reproduce the observed geological record

late cretaceous subsidence in wyoming quantifying the dynamic component the late cretaceous western interior basin of north america is generally considered to have been a retroarc foreland basin flexural backstripping of the stratigraphic record from 97 2 ma to 73 4 ma along a section perpendicular to the wyoming idaho salient of the sevier belt clearly demonstrates that there were components of subsidence in addition to those driven by the thrust and associated sediment loads the simulation demonstrates that the flexural foredeep is only 180 120 km wide and shows a forebulge located near the subsequent laramide rock springs uplift foredeep strata between the rock springs uplift on the east and the thrust front to the west were mostly involved in the eastward overlapping thrust belt or deeply truncated by inferred rebound after 78 5 ma the difference between the observed decompacted cumulative subsidence and the amount of subsidence that can be explained by simple flexural loading the residual subsidence increases from 800 m in eastern wyoming to 1 8 km near the thrust belt the westward thickening upper cretaceous sediment wedge filled the accommodation space generated by downward tilting to the west of the north american cratonic margin by underplating and mantle flow associated with the shallowly subducted farallon plate

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

shallow water pockmark formation in temperate estuaries a consideration of origins in the western gulf of maine with special focus on belfast bay abstract a systematic mapping program incorporating more than 5000 km of side scan sonar and seismic reflection tracklines in the western gulf of maine has identified more than 70 biogenic natural gas deposits occupying 311 km2 in nearshore muddy embayments many of these embayments also contain pockmark fields with some exhibiting geologically active characteristics including the observance of plumes of escaping fluids and sediment pockmarks hemispherically shaped depressions of various size and depths formed through fluid escape of gas and or pore water are sometimes found within or outside gas fields although many gas fields lack pockmarks altogether although the origin of the natural gas remains unclear if coastal environments at times of lower sea level were similar to the present numerous lake wetland valley fill and estuarine sources of organic rich material may have formed on the inner shelf if these deposits survived transgression and remain buried they are potential gas sources intensive mapping of the belfast bay pockmark field in 1998 produced the first nearly continuous side scan sonar mosaic of a gulf of maine pockmark field with a corresponding 3 dimensional geological model generated from seismic data statistical analysis of pockmark geometry gas deposit loci and subsurface evidence for gas enhanced reflectors suggest that gas migration from deeper lateral sources along permeable subsurface strata may be the mechanism for pockmark formation in areas lacking gas curtain seismic reflections the coarse grained transgressive ravinement unconformity between pleistocene glacial marine mud and holocene mud may act as a conduit for distributing methane to the field s margins

stratigraphic evolution of the inner continental shelf in response to late quaternary relative sea level change northwestern gulf of maine accumulations of deltaic and littoral sediments on the inner continental shelf of maine gulf of maine preserve a record of postglacial sea level changes and shoreline migrations the depositional response of coastal environments to a cycle of regression lowstand and transgression was examined with seismic reflection profiles vibracores and radiocarbon dates collected from sediments at the mouths of the kennebec and penobscot rivers sequence stratigraphic analysis of these data reveals two distinctly different successions of late quaternary deposits that represent end members in an evolutionary model for this glaciated coast seaward of the kennebec river coarse grained shorelines with foreset beds occur at depths of 20 60 m and outline the lobate margin of the kennebec river paleodelta a complex rock framed accumulation of glaciomarine and deltaic sediments capped by estuarine and marine deposits sand derived from this system today supports large barrier spits and extensive salt marshes in contrast the mouth of the penobscot river is characterized by thick deposits of glaciomarine mud overlain by marine mud of holocene age including gas charged zones that have locally evolved into fields of pockmarks the distinct lack of sand and gravel seaward of the penobscot river and its abundance seaward of the kennebec river probably reflect differences in sediment sources and the physiography of the two watersheds the contrasting stratigraphic framework of these systems demonstrates the importance of understanding local and regional differences in sediment supply sea level change bedrock structure and exposure to waves and tides in order to model river mouth deposition on glaciated coasts evolution of shelf deposits was largely controlled by relative sea level which locally fell from a highstand 60 to 70 m at 14 ka contemporary with deglaciation to a lowstand 55 m at 10 8 ka the sea level lowering was accompanied by fluvial incision of older deposits producing a regressive basal unconformity major rivers deposited abundant sediment over this surface sea level then rose at varying rates extensively reworking formerly emergent parts of the shelf and producing a shoreface ravinement surface in areas exposed to waves a tidal ravinement surface has developed in sheltered embayments where erosion is due mainly to tidal currents incised valleys in both settings preserve transgressive estuarine deposits that contain lagoonal bivalves and salt marsh foraminifera at depths of 15 30 m these deposits accumulated ca 9 2 7 3 ka locally a period of relatively slow sea level rise

how does the nazca ridge subduction influence the modern amazonian foreland basin the subduction of an aseismic ridge has important consequences on the dynamics of the overriding upper plate in the central andes the nazca ridge subduction imprint can be tracked on the eastern side of the andes the fitzcarrald arch is the long wavelength topography response of the nazca ridge flat subduction 750 km inboard of the trench this uplift is responsible for the atypical three dimensional shape of the amazonian forelland basin the fitzearrald arch uplift is no older than pliocene as constrained by the study of neogene sediments and geomorphic markers according to the kinematics of the nazca ridge subduction

modeling the evolution of the guadalquivir foreland basin southern spain 1 previous quantitative studies dealing with the origin of foreland basins have focused primarily either on the rheological basis of the lithosphere mechanical response or on the relationship between orogenic loading and sediment geometry to link the evolution of the guadalquivir foreland basin south iberia with the thermomechanical stratification of the iberian lithosphere we combine quantitative approaches to deep and shallow processes thrust loading lithospheric flexure thickness changes of the crust and the lithospheric mantle and surface mass transport a planform flexural analysis of the present day load distribution shows that basement subsidence is related not only to upper crustal thrusting but also to a deep seated additional load on the basis of the observed gravity and geoid fields we propose this additional load to be related to a lithospheric mantle thickening larger than the coeval crustal thickening further modeling of the evolution of a basin cross section reveals that the architecture of the sedimentary basin is additionally related to the lithosphere rheological response the quantitative study of the evolution of basement faulting and the forebulge uplift of sierra morena leads us to conclude that viscous stress relaxation and or plastic yielding within the lithosphere are key processes to explain the flexural evolution of the basin

topographic and hydrodynamic controls on barrier retreat and preservation an example from dogger bank north sea abstract barrier retreat can occur due to in place drowning overstepping or rollover depending on the interplay of controls such as sea level rise sediment supply coastal hydrodynamic regime and topography offshore sedimentary archives of barriers active during rapid holocene sea level rise provide important records of marine transgression which are vital analogues to support appropriate mitigation strategies for future coastal realignment under projected relative sea level rise scenarios this study analyses the sedimentary archive at dogger bank which is a formerly glaciated area in the north sea dogger bank experienced marine transgression due to early holocene rapid relative sea level rise an integrated dataset of vibrocores and high resolution seismic reflection data permits a stratigraphic framework to be established which reveals the buried coastal geomorphology of the southern dogger bank for the first time a transgressive stratigraphy was identified comprising a topographically complicated basal glacial and terrestrial succession overlain by two phases of barrier and tidal mudflat deposition prior to shallow marine sedimentation barrier phase a was a recurved barrier drowned in place and discontinuously overstepped to barrier phase b which experienced continuous overstepping by linking barrier elevations to relative sea level curves the timing of each barrier phase was established both barrier phases retreated during periods of rapid sea level rise with abundant sediment supply coastal hydrodynamics increasing wave energy and antecedent topography with spatially variable accommodation are suggested to be the main reason for differing retreat mechanisms rather than the rate of sea level rise antecedent coastal geomorphology plays a critical role in erosional and depositional patterns during transgression and therefore on the timing rate and location of marine inundation which needs to be included in models that aim to forecast hazards in coastal areas

morphology and stratigraphy of small barrier lagoon systems in maine abstract the coast of maine contains over 200 individual barrier lagoon systems most quite small with an aggregate length of nearly 100 km although they represent less than 5 of the tidally influenced coastline of maine they are widely distributed and occur in a variety of dynamic regimes and physiographic regions their morphology and backbarrier stratigraphy are different from better studied coastal plain systems and provide important clues to the holocene evolution of the maine coast in a study of geomorphic form and backbarrier stratigraphy inlet processes and holocene sea level rise have been identified as the principal controls on coarse grained barrier stratigraphy barriers in maine are found in five distinct geomorphic forms identified herein as barrier spits pocket barriers double tombolos cuspate barriers and looped barriers the few long sandy beaches in southwestern maine are mostly barrier spits the remainder of the barrier types is composed primarily of gravel or mixed sand and gravel the barriers protect a variety of backbarrier environments fresh and brackish ponds lagoons and fresh and saltwater marshes the barriers may or may not have inlets normal wave action coarse grain size and a deeply embayed coast result in barriers with steep reflective profiles several meters above mhw occasional storm events completely wash over the barriers building steep lobate gravel fans along their landward margin few if any extensive storm layers are recognized as extending into the distal backbarrier environments however during sea level rise and landward barrier retreat this abrupt storm generated transition zone inters the backbarrier sediments statistical comparisons of barrier morphology location and backbarrier environment type with backbarrier stratigraphy show that holocene backbarrier stratigraphy is best predicted by the modern backbarrier environment type this in turn is influenced most by the absence or presence and long term stability or instability of a tidal inlet geomorphic barrier form and location in coastal geomorphic compartments show little or no correlation with backbarrier stratigraphy in contrast to previous classifications of barrier lagoon systems based primarily on sandy coastal plain examples in maine the shape or origin of the backbarrier system is relatively unimportant the presence or absence of a tidal inlet is of paramount importance in shaping the holocene stratigraphy of the backbarrier region

geomorphic and stratigraphic signals of postglacial meltwater pulses on continental shelves selective development and preservation of major shoreline complexes on the continental shelf provide geomorphic evidence in support of alternating periods of holocene sea level stillstand and rapid rise during meltwater pulses on a tectonically stable far fi eld setting the southeast african shelf regionally developed well preserved shoreline complexes occur at 100 m and 60 m within an overall shelf sequence dominated by transgressive ravinement non preservation of shorelines the development of geomorphically mature submerged shorelines with equilibrium forms is attributed to extended periods of sea level stability and their preservation is the result of early cementation followed by very rapid sea level rise several centimeters per year that caused overstepping meltwater pulses 1a and 1b are recorded in the shelf stratigraphy and geomorphology setting aside local infl uences of topography and sediment supply we hypothesize that shelf stratigraphies should preferentially preserve shorelines at water depths associated with the stillstands or slowstands immediately preceding meltwater pulses a reappraisal of shelf stratigraphy and geomorphology in particular the preservation potential of former shorelines is required in the light of contemporary understanding of postglacial sea level history

subduction dynamics and the origin of andean orogeny and the bolivian orocline the building of the andes results from the subduction of the oceanic nazca plate underneath the south american continent however how and why the andes and their curvature the bolivian orocline formed in the cenozoic era 65 5 million years myr ago to present despite subduction continuing since the mesozoic era 251 0 65 5 myr ago is still unknown three dimensional numerical subduction models demonstrate that variations in slab thickness arising from the nazca plate s age at the trench produce a cordilleran morphology consistent with that observed the age dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate causing it to thicken thus subducting older nazca plate below the central andes can explain the locally thickened crust and higher elevations here we demonstrate that resultant thickening of the south american plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the bolivian orocline additionally the varying forcing along the margin allows stress belts to form in the upper plate interior explaining the widening of the central andes and the different tectonic styles found on their margins the eastern and western cordilleras the rise of the central andes and orocline formation are directly related to the local increase of nazca plate age and an age distribution along the margin similar to that found today the onset of these conditions only occurred in the eocene epoch this may explain the enigmatic delay of the andean orogeny that is the formation of the modern andes

pre requisites processes and prediction of chlorite grain coatings in petroleum reservoirs a review of subsurface examples abstract deeply buried reservoirs containing chlorite coated quartz sand grains commonly have higher than expected porosity and permeability although prediction of such positive anomalies still remains elusive a total of 54 published examples based on information and data from 62 scientific papers was collated quantification of some of the most common parameters including depositional environment age and latitude of sand deposition effect on reservoir quality and chemical composition of chlorite is presented the dataset indicates that chlorite coats are found in sandstones deposited in a range of depositional environments but most commonly occur in delta related environments 44 with fluvial environments the second most common 19 age relationships indicate that there is an overall exploration bias in published examples with chlorite coats becoming increasingly common through time the latitude at the time of deposition of sands with chlorite coats is wide 60 n 60 s and indicates that temperate and tropical climates are important for the generation of this clay mineral chlorite can have a variable effect on reservoir quality but is typically positive iron rich chlorites occur overwhelmingly in coastal environments while mixed iron and magnesium rich chlorites are principally found in marine and terrestrial sandstones analysis of these factors suggests that hinterland geology basinal soil development and geochemical weathering and proximity to river systems are essential to the formation of chlorite precursor phases these characteristics have been combined to define situations where chlorite coats were more likely or more unlikely to form these parameters will provide insights into the formation of chlorite and to further refine predictive models for the presence or absence of chlorite coated sandstones

rise of the andes the surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening leading to their gradual rise recent studies of the andes indicate that their elevation remained relatively stable for long periods tens of millions of years separated by rapid 1 to 4 million years changes of 1 5 kilometers or more periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable dense lower lithosphere after long term thickening of the crust and lithospheric mantle

depths and focal mechanisms of crustal earthquakes in the central andes determined from teleseismic waveform analysis and insar 1 we investigate the depths of crustal earthquakes mw 5 5 earthquakes from the global cmt catalog and previous work spanning 1944 2007 for 38 well recorded events we use teleseismic p and sh waveforms to model the strike dip rake focal depth and source time function we use insar observations of surface deformation from 9 earthquakes to compare inferred fault parameters with the waveform inversions and global catalogs to assess their accuracies while the depths from the insar and waveform analyses generally agree within error horizontal and depth errors in global catalogs are 10 to 50 km as found elsewhere as noted in previous work the majority of crustal earthquakes occur in the eastern cordillera and foreland regions of the central andes although a few normal and strike slip earthquakes occur beneath the altiplano plateau and in the forearc in southern peru and northernmost chile we propose a new interpretation of one of the basement thrusts shira mountain peru as a pop up block on the basis of our new earthquake depths we confirm that earthquakes in the flat slab areas of peru and argentina are within the sometimes aseismic lower crust lower crustal earthquakes are globally found in all types of tectonic settings only when the thermal lithosphere is more than 80 km thick and the amount of recent shortening extension is 30

on the predictability limit of convection models of the earth s mantle reconstructing convective flow in the earth s mantle is a crucial issue for a diversity of disciplines from seismology to sedimentology the common and fundamental limitation of these reconstructions based on geodynamic modeling is the unknown initial conditions because of the chaotic nature of convection in the earth s mantle errors in initial conditions grow exponentially with time and limit forecasting and hindcasting abilities in this work we estimate for the first time the limit of predictability of earth s mantle convection following the twin experiment method we compute the lyapunov time i e e folding time for state of the art 3 d spherical convection models varying rheology and rayleigh number our most earth like and optimistic solution gives a lyapunov time of 136 13 myr rough estimates of the uncertainties in best guessed initial conditions are around 5 leading to a limit of predictability for mantle convection of 95 myr our results suggest that error growth could produce unrealistic convective structures over time scales shorter than that of pangea dispersal

exploiting radiation damage control on apatite u th he dates in cratonic regions abstract apatites from four pairs of samples of precambrian basement from the western canadian shield were analyzed by u th he thermochronometry to test for the influence of radiation damage on apatite u th he dates in this cratonic region recent studies have demonstrated that the accumulation of radiation damage increases the apatite he retentivity so that apatites with a span of effective u concentrations eu that experienced the same thermal history may be characterized by a range of closure temperatures in this investigation each sample pair consisted of a mafic dike cross cutting felsic gneisses from a single outcrop or nearby outcrops that contained apatites with a span of eu the apatites yielded u th he dates from 846 to 123 ma and were positively correlated with eu within each sample pair these results can be explained using a model that tracks the evolution of he mobility in response to the accumulation of radiation damage when coupled with regional geological constraints the data appear to require partial to complete he loss due to burial and reheating in phanerozoic time new apatite fission track dates and length data were obtained for five of these samples the apatite fission track dates are proterozoic regardless of apatite eu thermal history simulations indicate that the apatite fission track data are compatible with the u th he results although the thermal histories are not identical in detail and the fission track results alone do not require phanerozoic heating together the data are consistent with burial of this region by 1 km of phanerozoic strata that were subsequently denuded thus pointing toward significant phanerozoic deposition in the north american cratonic interior hundreds of kilometers east of where previously documented the results suggest that exploiting radiation damage control on apatite u th he dates through investigation of surface sample apatites with a span of closure temperatures can impose tighter restrictions on thermal histories in low relief cratonic landscapes than was previously possible

integrated high precision analyses of holocene relative sea level changes lessons from the coast of maine a suite of salt marsh peat samples from four sites along the coast of maine wells phippsburg gouldsboro and machiasport has been analyzed using high precision techniques to determine local relative sea level trends and to evaluate proposed along coast warping a spatially variable set of relative sea level records in maine would have important implications for geophysical models that predict the response of the lithosphere during deglaciation and postglacial isostatic relaxation the amplification of m sub 2 tidal range in the gulf of maine and the bay of fundy during the holocene is modeled and applied to the mean high water data yielding best estimate envelopes of mean tide level change for each location average long term thousands of years mean tide level rise did not exceed approx 2 mm yr at any time during the late holocene at wells phippsburg and machiasport between 4 5 and 3 ka calibrated cal the apparent rate of rise at gouldsboro was higher than at any other site studied this along coast variation in the rate of mean tide level rise may reflect time of deglaciation neotectonics or differential isostatic adjustments a slight acceleration of mean tide level rise has occurred during the past millennium in gouldsboro and machiasport if 12 m downwarping inmore easternmost maine occurred as suggested in other publications it must have happened prior to 5 7 ka cal 84 refs 13 figs 8 tabs less

archaeology of the continental shelf marine resources submerged landscapes and underwater archaeology we provide a critical review of the evidence for the long term use of marine resources and coastal environments in human evolution and later development we emphasise the importance of the coastal archaeological record in understanding patterns of human settlement and dispersal and draw attention to the large potential biases introduced by the destructive or obscuring effects of pleistocene sea level change we note that lowered sea levels have been the norm for most of the pleistocene and that periods of high sea level have been too short lived to provide other than a fragmentary coastal record and one that is beset with ambiguities and uncertainties we examine the geological processes of coastal uplift and underwater preservation that may help to mitigate these biases coastlines elevated by isostatic and tectonic processes or with very steep offshore drop offs at plate boundaries are important in providing a potential window into coastal landscapes and archaeology formed during periods of lowered sea level however we conclude that the opportunities afforded by these geological factors are too limited to obviate the need for underwater exploration we review the evidence now available from submerged landscapes across the africa eurasia interface from the atlantic coastlines of northwest europe to the southern red sea we show that geomorphological conditions for the preservation of archaeological and palaeoenvironmental data are commonly present that much material has already been discovered and that new techniques technologies and projects are providing the momentum for a rapidly expanding field of investigation the results do not simply add to what we already know from sites on land but are likely to produce qualitatively different evidence for coastal adaptations and landscapes that have no analogue on present day coastlines we note the strong probability that many coastal landscapes exposed at lowered sea level provided relatively fertile and productive refugia for plants land mammals and humans at a time when increased aridity would have reduced or deterred hinterland occupation we conclude that underwater investigation is essential if hypotheses of early human adaptation and dispersal are to be fully tested

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

ground water silicification and leaching in sands example of the fontainebleau sand oligocene in the paris basin in the fontainebleau sand oligocene superposed lenses of quartzite are restricted to outcrops and structurally low zones of the formation the quartzite lenses result from a silicification process controlled by water table levels during the recent geomorphologic evolution of the landscape the quartzite lenses pinch out rapidly under the limestone capped plateaus and are related to the development of a bleached sand profile above the water table the sand is bleached and the former quartzite lenses are corroded along their margins moreover flint pebbles are dissolved and primary smectite and illite are weathered to kaolinite dissolved silica can thus reach supersaturated levels allowing quartz precipitation quartz grains are overgrown at the base of the profile in zones of ground water outflow related to spring lines the adjustment of the water table during the landscape incision led to formation of quartzite lenses at deeper levels this silicification process has no climatic significance but is dependent on the stability of the landscape and on its progressive dissection the rate of quartz cementation indicated by geomorphologic considerations is consistent with hydraulic and geochemical calculations it appears to be very rapid approximately 30 000 yr for a quartzite lens

drowned coastal deposits with associated archaeological remains from a sea level slowstand northwestern gulf of maine usa drowned terrestrial wetland environments such as lakes marshes and beaches were thought to be rare in formerly glaciated regions like the gulf of maine united states in the northwestern gulf of maine postglacial relative sea level changes include a slowstand between 11 5 ka and 7 5 ka when the ocean rose 5 m this period of sea level stability permitted erosion of glacial materials and concomitant construction of spits and sheltered wetland habitats attractive to human occupation between 17 m and 22 m below modern sea level this work underscores the importance of a well constructed sea level chronology to predict the location of drowned terrestrial environments and associated cultural resources

influence of peruvian flat subduction dynamics on the evolution of western amazonia convection in the earth s mantle is mainly driven by cold dense subducting slabs but relatively little is known about how 3d variations in slab morphology and buoyancy affect mantle flow or how the surface above deforms in response i e dynamic topography we investigate this problem by studying the dynamics of an active region of flat slab subduction located in peru in south america here the slab geometry is well known based on the regional seismicity and we have observations from the local geological record to validate our models of particular interest is the widespread subsidence and deposition of the solimoes formation across western amazonia that coincided with the development of the peruvian flat slab during the mid late miocene this formation covers an extensive area from the foredeep to the purus arch located view the mathml source 2000km away from the trench close to the andes the preservation of several kilometers of sedimentary thicknesses can be easily accounted for by flexure based on an estimate of the andean loading we predict 2 8 to 3 6 km of accommodation space that spans 100 km the spatial and temporal history of the solimoes formation however particularly the thick distal foreland accumulations up to 1 2 km deep can only be matched with the addition of a longer wavelength dynamic source of topography following the transition from normal to flat subduction we predict over 1 km of dynamic subsidence view the mathml source 1500km wide that propagates over 1000 km away from the trench tracking the subduction leading edge this is followed by a pulse of dynamic uplift over the flat segment behind it we therefore propose that a combination of uplift flexure and dynamic topography during slab flattening in peru is responsible for the sedimentation history and landscape evolution of western amazonia that eventually led to the configuration of the amazon drainage basin we know today

nuclear magnetic resonance characterization of porosity preserving microcrystalline quartz coatings in fontainebleau sandstones the almost pure quartz cemented fontainebleau formation paris basin france sandstones are known to preserve their porosity because of microcrystalline quartz coatings here we use nuclear magnetic resonance nmr techniques petrography scanning electron microscopy porosity and permeability measurements hysteresis and mercury injection capillary pressure curves to identify and analyze their porosity structure nuclear magnetic resonance experiments include transverse relaxation time t2 distributions and t2 filtered t2 t2 exchange t2f trex a technique that provides estimates on the diffusion coupling by comparing the evolution of families of pores in t2 distributions at different exchange times samples were divided according to their texture composition and abundance of microcrystalline quartz crystals comprising group 1 samples with very low amounts of coatings and group 2 samples with entire grains coated by microquartz both groups show three or four peaks in nmr 2 mhz t2 distribution at approximately 1 peak a 10 1 peak b and 10 2 s peak c group 2 samples present a slight shift to shorter t2 times in comparison with other samples the longest t2 peak a is because of intergranular macropores whereas the shortest peak c is because of the microporosity associated with the microcrystalline quartz coating at the surface of the pores peak b is also because of microporosity associated with microcrystalline quartz but with a different surface volume ratio being likely related to flat shaped pores within the microcrystalline coating the t2f trex indicates the proton exchange is higher between macropores and the pore surface micropores peak c than between macropores and the internal flat shaped micropores no exchange between the two sets of micropores can be observed our results show the potential of nmr techniques in characterizing the microporosity in fontainebleau sandstones which is key for the mechanism of porosity preservation in these rocks

the structure of the nazca ridge and sala y gomez seamount chain from the dispersion of rayleigh waves summary we report single station measurements of group velocity for fundamental mode rayleigh waves that propagated along two aseismic ridges in the eastern pacific ocean basin the nazca ridge and the sala y gomez chain we find that the nazca ridge dispersion curves when regionalized to isolate the ridge contribution indicate group velocities of about 3 5 km s for short periods these values are slower than predicted for the geometry of the propagation paths interpreting this velocity heterogeneity is complicated by the recognition that the nazca ridge is about 1000 km longer than previously thought but models of vyv for the ridge derived by inverting the dispersion curves reveal abnormally thick 18 f 3 km oceanic crust along its longitudinal axis this value compares favourably with previously published results over the tuamotu plateau thus strengthening the hypothesis that the nazca ridge and tuamotu plateau are mirror images formed by a hotspot coincident with the farallon pacific spreading centre before 20 ma in contrast the measurements along the sala y gomez chain are consistent with typical oceanic crust and mantle when these values are inverted they reveal no thick crust 6 km along the sala y gomez chain we conclude from this result that the chain was likely formed as a leaky fracture zone not as a simple hotspot track

denudation of the continental shelf between britain and france at the glacial interglacial timescale the erosional morphology preserved at the sea bed in the eastern english channel dominantly records denudation of the continental shelf by fluvial processes over multiple glacial interglacial sea level cycles rather than by catastrophic flooding through the straits of dover during the mid quaternary here through the integration of multibeam bathymetry and shallow sub bottom 2d seismic reflection profiles calibrated with vibrocore records the first stratigraphic model of erosion and deposition on the eastern english channel continental shelf is presented published optical stimulated luminescence osl and 14c ages were used to chronometrically constrain the stratigraphy and allow correlation of the continental shelf record with major climatic sea level periods five major erosion surfaces overlain by discrete sediment packages have been identified the continental shelf in the eastern english channel preserves a record of processes operating from marine isotope stage mis 6 to mis 1 planar and channelised erosion surfaces were formed by fluvial incision during lowstands or relative sea level fall the depth and lateral extent of incision was partly conditioned by underlying geology rock type and tectonic structure climatic conditions and changes in water and sediment discharge coupled to ice sheet dynamics and the drainage configuration of major rivers in northwest europe evidence for major erosion during or prior to mis 6 is preserved fluvial sediments of mis 2 age were identified within the northern palaeovalley providing insights into the scale of erosion by normal fluvial regimes seismic and sedimentary facies indicate that deposition predominantly occurred during transgression when accommodation was created in palaeovalleys to allow discrete sediment bodies to form sediment reworking over multiple sea level cycles saalian eemian early weichselian by fluvial coastal and marine processes created a multi lateral multi storey succession of palaeovalley fills that are preserved as a strath terrace the data presented here reveal a composite erosional and depositional record that has undergone a high degree of reworking over multiple sea level cycles leading to the preferential preservation of sediments associated with the most recent glacial interglacial period

post paleozoic alluvial gravel transport as evidence of continental tilting in the u s cordillera the western united states contains three thin but remarkably widespread alluvial conglomeratic units that record episodes of large scale tilting across the u s cordilleran orogen in post paleozoic time these units are 1 the shinarump conglomerate of late triassic age exposed in northern arizona and adjacent parts of utah nevada and new mexico 2 lower cretaceous gravel deposits that overlie the morrison formation throughout the rocky mountain region and 3 the gravel rich parts of the miocene pliocene age ogallala group in western nebraska and adjacent southeastern wyoming paleoslopes of the rivers depositing these units were in the range of 10 24 to 10 23 based on paleohydraulic calculations however depositional thickness trends of these units are not sufficient to have generated such steep paleoslopes thus long wavelength tilting of the earth s surface must have occurred for these gravels to be transported although these units were deposited adjacent to large tectonic features including an evolving and migrating continental arc and for the ogallala group the northward propagating rio grande rift the tilting occurred over wavelengths too broad to be directly generated by these features these widespread gravel units attest to the interplay between the creation of subduction related isostatic and dynamic topography and continental sedimentation hence paleotopography as determined from calculated transport gradients of sedimentary deposits provides a

shallow stratigraphic control on pockmark distribution in north temperate estuaries abstract pockmark fields occur throughout northern north american temperate estuaries despite the absence of extensive thermogenic hydrocarbon deposits typically associated with pockmarks in such settings the origins of the gas and triggering mechanism s responsible for pockmark formation are not obvious nor is it known why pockmarks proliferate in this region but do not occur south of the glacial terminus in eastern north america this paper tests two hypotheses addressing these knowledge gaps 1 the region s unique sea level history provided a terrestrial deposit that sourced the gas responsible for pockmark formation and 2 the region s physiography controls pockmarks distribution this study integrates over 2500 km of high resolution swath bathymetry chirp seismic reflection profiles and vibracore data acquired in three estuarine pockmark fields in the gulf of maine and bay of fundy vibracores sampled a hydric paleosol lacking the organic rich upper horizons indicating that an organic rich terrestrial deposit was eroded prior to pockmark formation this observation suggests that the gas which is presumably responsible for the formation of the pockmarks originated in holocene estuarine sediments loss on ignition 3 5 10 not terrestrial deposits that were subsequently drowned and buried by mud the 7470 pockmarks identified in this study are non randomly clustered pockmark size and distribution relate to holocene sediment thickness r2 0 60 basin morphology and glacial deposits the irregular underlying topography that dictates holocene sediment thickness may ultimately play a more important role in temperate estuarine pockmark distribution than drowned terrestrial deposits these results give insight into the conditions necessary for pockmark formation in nearshore coastal environments

holocene sea level database for the atlantic coast of the united states we have constructed a database of holocene relative sea level rsl observations for the atlantic coast of the united states the database contains 492 index points which locate the position of rsl in time and space and 344 limiting dates which constrain the minimum or maximum limit of former sea level the majority of the index points in the database are from 6 ka bp to present with only 7 older than 6 ka bp spatially index points are distributed between maine and south carolina but there is an absence of data from georgia and the atlantic coast of florida the database is sub divided into 16 regions based on the distance from the former laurentide ice sheet and are classified depending upon their susceptibility to compaction the index points and limiting data demonstrate that rsl did not exceed present 0 m during the holocene except potentially in regions 1 and 2 eastern maine and southern maine rates of rsl change were highest during the early holocene and have decreased over time due to the diminishing response of the earth s mantle to glacial isostatic adjustment and reduction of ice equivalent meltwater input along the atlantic coast of the united states the linear rate of rsl rise prior to 4 ka bp ranged from 0 5 to 4 5 mm a 1 compared to 0 6 1 8 mm a 1 from 4 ka bp to ad 1900 the database suggests minimal 0 3 mm yr changes in these rates of rsl rise during the late holocene deglaciation of the laurentide ice sheet caused the spatial variability captured by the database the maximum rate of late holocene and ongoing rsl rise occurred in mid atlantic regions new jersey and inner delaware because of collapse of the peripheral forebulge

pleistocene cold climate groundwater silicification jbel ghassoul region missour basin morocco surficial silicifications have been long considered to be indicative of warm and dry climates here we describe various forms of supergene silicification in a miocene lacustrine sequence in the missour basin near jbel ghassoul morocco in a landscape with accentuated relief the silicification is almost exclusively limited to a 10 40 m wide zone from the edges of scarp and mesa landforms this distribution is interpreted to record the locations where groundwaters which produced the silicification discharged from a higher level palaeolandscape the main component of the silica was imported late and significantly post dates the deposition of the sediments this implies that significant volumes of silica bearing solutions flowed through these formations in response to a hydraulic gradient generated by relief silicification thus occurred only after uplift and incision of the sedimentary fill of the missour basin the zones of silicification of the jbel ghassoul sequence can be linked geomorphologically to remnants of high level pediments that have been dated in the literature to the early to middle pleistocene and interpreted to have been formed during cold climates low temperatures in outcrops near the discharge zones during cold periods are considered to be a key factor in silica precipitation from groundwaters

influence of geometry and eclogitization on oceanic plateau subduction abstract subduction is driven by the negative buoyancy of the slab which depends on both the temperature and composition of the lithospheric plate in the case of subduction of an oceanic plateau larger thickness of the lower density crust and harzburgite layers can locally decrease the negative buoyancy available to drive subduction however the ability of a plateau to have an impact on subduction depends on the total buoyancy of the slab and therefore also depends on size of the plateau and the basalt to eclogite transition which substantially increases the density of the crust using 2 d numerical finite element models of subduction we investigate the role of eclogitization and oceanic plateau size thickness and length in the process of oceanic plateau subduction model results show that eclogitization of the crust substantially increases the chances that a plateau will be subducted because the extra buoyancy of larger plateaus is lost through the phase transition for kinematically driven models all plateaus are subducted regardless of the thickness or width extrapolation of these results to 3 d geometry shows that sufficient slab buoyancy is available to sustain subduction for narrow plateaus with slab widths 6 7 times the plateau width depending on plateau thickness and length in dynamically driven models subduction of oceanic plateaus of various sizes can substantially slow subduction and large plateaus 200 km long and 25 km thick can lead to break off of the slab and underplating of the plateau

drainage reversal of the amazon river due to the coupling of surface and lithospheric processes abstract the uplift of central and northern andes occurred concomitantly with an important reorganization of the drainage pattern in the northern south america in early miocene the fluvial systems that flowed from the amazonian craton towards the sub andean basins and northward to the caribbean were replaced by a megawetland the pebas system covering more than 10 6 km 2 in western amazonia by late miocene the pebas system progressively disappeared and gave place to the transcontinental amazon river connecting the andes and the equatorial atlantic margin a previous work suggested that the reversal of the amazon river and the disappearance of the pebas system were driven by dynamic topography promoted by mantle convection based on a three dimensional numerical model that couple surface processes flexural isostasy and crustal thickening due to orogeny here i propose that the response of the surface processes to the uplift of the central and northern andes along with the flexural isostasy of the lithosphere can explain the drainage reversal of the amazon river during miocene time with no need to invoke dynamic topography induced by mantle convection moreover according to this new numerical model the timing of the drainage reversal in the northern south america is mainly controlled by the efficiency of the sediment transport within the drainage basins and the rate of the andean uplift also the present numerical experiments were able to predict the increase in sedimentation rate in the amazon fan after the drainage reversal of the amazon river as observed in late miocene pliocene sedimentary record however the proposed model fails to fully reproduce the spacial and temporal evolution of the pebas system as observed in geological data further investigation is therefore needed in order to understand the dynamic interaction between surface and tectonic processes and their implications on the development of a megawetland that preceded the reversal of amazon river drainage nevertheless this work can give new insights into the landscape and stratigraphic evolution of northern south america during andean orogeny providing quantitative constraints for the paleogeography reconstructions of amazonia

holocene relative sea level changes and glacial isostatic adjustment of the u s atlantic coast the first quality controlled holocene sea level database for the u s atlantic coast has been constructed from 686 sea level indicators the database documents a decreasing rate of relative sea level rsl rise through time with no evidence of sea level being above present in the middle to late holocene the highest rates of rsl rise are found in the mid atlantic region we employ the database to constrain an ensemble of glacial isostatic adjustment models using two ice ice 5g ice 6g global ice sheet reconstructions and two mantle viscosity models vm5a vm5b vm radial variation of viscosity in the sublithospheric mantle variations to assess whether the spherically symmetric viscoelastic models are able to survive intercomparison with a more refined database of postglacial rsl history we identify significant misfits between observations and predictions using ice 5g with the vm5a viscosity profile ice 6g provides some improvement for the northern atlantic region but misfits remain elsewhere decreasing the upper mantle and transition zone viscosity by a factor of 2 to 0 25 10 21 pa s vm5b removes significant discrepancies between observations and predictions along the mid atlantic coastline although misfits remain in the southern atlantic region these may be an indication of the importance of laterally heterogeneous viscosity in the upper mantle

continental and oceanic crustal structure of the pampean flat slab region western argentina using receiver function analysis new high resolution results summary the pampean flat slab of central chile and argentina 30 32 s has strongly influenced cenozoic tectonics in western argentina which contains both the thick skinned basement cored uplifts of the sierras pampeanas and the thin skinned andean precordillera fold and thrust belt in this region of south america the nazca plate is subducting nearly horizontally beneath the south american plate at 100 km depth to gain a better understanding of the deeper structure of this region including the transition from flat to normal subduction to the south three iris passcal arrays of broad band seismic stations have been deployed in central argentina using the dense siembra array combined with the broader charge and esp arrays the flat slab is imaged for the first time in 3 d detail using receiver function rf analysis a distinct pair of rf arrivals consisting of a negative pulse that marks the top of the oceanic crust followed by a positive pulse which indicates the base of the oceanic crust can be used to map the slab s structure depths to moho and oceanic crustal thicknesses estimated from rf results provide new more detailed regional maps an improved depth to continental moho map shows depths of more than 70 km in the main cordillera and 50 km in the western sierras pampeanas that shallow to 35 km in the eastern sierras pampeanas depth to moho contours roughly follow terrane boundaries offshore the hotspot seamount chain of the juan fernandez ridge jfr is thought to create overthickened oceanic crust providing a mechanism for flat slab subduction by comparing synthetic rfs based on various structures to the observed rf signal we determine that the thickness of the oceanic crust at the top of the slab averages at least 13 19 km supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat the overthickened region is broader than the area directly aligned with the path of the jfr however and indicates along with the slab earthquake locations that the flat slab area is wider than the jfr volcanic chain observed in the offshore bathymetry further rfs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench parallel faults one explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted alternatively it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region

dynamic topography and anomalously negative residual depth of the argentine basin a substantial portion of earth s topography is known to be caused by the viscous coupling of mantle flow to the lithosphere but the relative contributions of shallow asthenospheric flow versus deeper flow remains controversial the argentine basin located offshore of the atlantic margin of southern south america is one of the most anomalously deep ocean regions as it is significantly deeper than its age would suggest previously the anomalous depth has been attributed to asthenospheric flow and the coupling of the south american plate s westward motion to the shallow mantle using a combination of geophysical observations and geodynamic modeling we propose that subducted slab driven dynamic topography has primarily driven the long wavelength anomalous residual basin depth since the opening of the south atlantic using an inverse mantle convection model with plate motions since the early cretaceous we suggest that the median of present day dynamic topography of the basin is 400 m when the residual basement depth is low pass filtered the depth anomaly is 730 m suggesting that more than half of the residual basement depth can be attributed to deep seated mantle dynamics we conclude that coupled plate tectonic mantle convection models tied to seismic tomography bathymetry and sediment thickness data can help to elucidate the driving forces behind earth s topography one of the most fundamental characteristics of this planet

subsidence in intracontinental basins due to dynamic topography the origin of anomalous tectonic subsidence ats of large intracontinental basins long after their most recent phase of extension and last thermal perturbation is the subject of a long standing debate we show that deep earth processes may contribute to the subsidence of these tectonically stable basins by analysing the tertiary mantle convection driven topography history of a global set of more than 220 intracontinental basins integrated into a plate kinematic framework most basins are affected by increasing negative dynamic topography over the last 70 myr due to the motion of many continents away from large mantle upwellings and towards downwellings previously located along the perimeter of the supercontinent pangaea during continental dispersal increasing negative dynamic topography causes dynamic subsidence of the basins creating additional accommodation space we utilise a parameter from a global crustal analysis of intraplate basins termed anomalous tectonic subsidence to quantify sediment accumulation not related to crustal stretching we propose that dynamic subsidence due to plate motions relative to the underlying mantle as well as variations in the large scale convection patterns can significantly contribute to the creation and destruction of accommodation space in intraplate basins

dynamic uplift during slab flattening subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents including past events of large scale marine flooding and tilting the mechanism is dynamic deflection the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface here we show that subduction does not always lead to downward deflections of the earth s surface subduction of young lithosphere at shallow angles flat subduction leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite accounting for neutrally buoyant flat segments along with large variations in slab morphology in the south american subduction zone explains along strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the cenozoic our results show that the transition from normal subduction to slab flattening generates dynamic uplift preventing back arc marine flooding