propagating rift during the opening of a small oceanic basin the protector basin scotia arc antarctica abstract the opening of oceanic basins constitutes one of the key features of plate tectonics because it determines the rifting and displacement of the continental crustal blocks although the mechanisms of development of large oceans are well known the opening and evolution of small and middle size oceanic basins have not been studied in detail the protector basin located in the southern scotia sea is a good example of a small oceanic basin developed between two thinned continental blocks the pirie bank and the terror rise poorly studied up to now a new set of multibeam bathymetry multichannel seismic reflection and gravity and magnetic anomaly profiles obtained on the scan 2001 cruise led us to determine that the protector basin probably opened during the period comprised between c5dn 17 4 ma and c5acn c5abr chrons 13 8 ma forming a n s oriented spreading axis the end of spreading is slightly younger to the north the start of spreading is clearly diachronous with the most complete set of chrons up to c5dn in the southern profile c5cn in the middle section and only up to c5adn in the northern part of the basin the spreading axis propagated northwards during the basin development producing the wedge shape of the basin in addition at the ne part of the basin a reverse fault developed in the border of the pirie bank after basin opening accentuates the sharp northern end moreover the northwestern part of the pirie bank margin is an extremely stretched continental crust with n s elongated magnetic anomalies related to incipient oceanic southward propagating spreading axes the protector basin shows the oldest evidence of e w continental stretching and subsequent oceanic spreading during middle miocene related with the eastward development of the scotia arc that continues up to present the relative rotation of continental blocks during the development of small sized oceanic basins by continental block drifting favoured the opening of wedge shape basins like the protector basin and conjugate propagating rifts

western scotia sea margins improved constraints on the opening of the drake passage 1 we present a revised tectonic interpretation from 28 ma to 3 2 ma of the western sector of the scotia sea incorporating new multichannel seismic reflection profiles and magnetic anomaly identifications for the continental margin off the tierra del fuego island and available complementary data for the conjugate margin of the northwestern flank of the south scotia ridge seismic profiles show a remarkable diversity of the pair of conjugate passive margins of the western scotia sea in both their morphology and structural framework the tierra del fuego continental margin can be related to a classic rifted passive margin while the southwestern margin of the scotia sea is characterized by steep slopes mostly generated by subvertical faults that abruptly separate the continental crust of the south scotia ridge from the oceanic crust of the western scotia sea this structural difference was caused by intense strike slip tectonism mostly concentrated along the modern south scotia ridge since the early development of the western scotia sea we find evidence for a previously unrecognized magnetic anomaly 10 28 ma at the foot of the tierra del fuego continental margin the same anomaly is present at the conjugate northern flank of the south scotia ridge the timing of events leading to the earliest development of the western scotia sea which determined the opening of the drake passage is important because this gateway opening had a profound effect on global circulation and climate the thickness and the distribution of the sedimentary cover overall in the abyssal plain off the two western scotia sea margins is different this is due to the different regimes of the bottom current flows which affected the western scotia sea both in the past and in the present time

tectonic evolution of the west scotia sea joint inversion of isochron and flowline data from the flanks of the extinct west scotia ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron c8 26 5 ma and extinction around chron c3a 6 65 9 ma when they are placed in a regional plate circuit the rotations predict plate motions consistent with known tectonic events at the margins of the scotia sea oligocene extension in powell basin miocene convergence in tierra del fuego and at the north scotia ridge and miocene transpression at the shackleton fracture zone the inversion results are consistent with a spreading history involving only two plates at rates similar to those between the enclosing south america and antarctica plates after chron c5c 16 7 ma but that were faster beforehand the spreading rate drop accompanies inception of the east scotia ridge back arc spreading center which may therefore have assumed the role of the west scotia ridge in accommodating eastwards motion of the trench at the eastern boundary of the scotia sea this interpretation is most easily incorporated into a model in which the basins in the central parts of the scotia sea had already formed by chron c8 contrary to some widely accepted interpretations and which has significant implications for paleoceanography and paleobiogeography

the correction for sediment loading on the seafloor available measurements of sediment velocity and density indicate that a factor of 600 m s 1 of two way sediment reflection time will correct seafloor depth to yield basement depth in the absence of sediment loading this simple linear relation is derived from the results of six deep drill holes in abyssal sediments of the north and south atlantic oceans the relation predicts the sediment correction with an accuracy of about 50 m regardless of the lithology sampled down to a depth of 1 5 s of reflection time the cost wells on the u s continental shelf and empirical relations among velocity density porosity and depth provide estimates of the deeper behavior of abyssal sediments and suggest that the 600 m s 1 factor continues to provide a satisfactory sediment correction to a depth of 3 s

quaternary history of the antarctic circumpolar current evidence from the scotia sea within the scotia sea the axis of the antarctic circumpolar current acc is geographically confined and sediments therefore contain a record of palaeo flow speed uncomplicated by acc axis migration we outline holocene and last glacial maximum lgm current controlled sedimentation using data from 3 5 khz profiles cores and current meter moorings geophysical surveys show areas of erosion and deposition controlled by neogene basement topography deposition occurs in mounded sediment drifts or flatter areas where 500 1000 m of sediment overlies acoustic basement 3 5 khz profiles show parallel continuous sub bottom reflectors with highest sedimentation rates in the centre of the drifts and reflectors converging towards marginal zones of non deposition locally on the flanks of continental blocks e g south georgia downslope processes are dominant the absence of mudwaves on the sediment drifts may result from the unsteadiness of acc flow a core transect from the acc axis south to the boundary with the weddell gyre shows a southward decrease in biogenic content controlled by the polar front and the spring sea ice edge both these features lay farther north at lgm the cores have been dated by relative abundance of the radiolarian cycladophora davisiana and by changes in the biogenic ba content a palaeoproductivity indicator sedimentation rates range from 3 to 17 cm ka the grain size of holocene sediments shows a coarsening trend from south to north consistent with strongest bottom current flow near the acc axis though interpretation is complicated by the presence of biogenic grains year long current meter records indicate mean speeds from 7 cm s in the south to 12 cm s in the north with benthic storm frequency increasing northwards lgm sediments are predominantly terrigenous and show a clearer northward coarsening trend with well sorted silts in the northern scotia sea assuming a constant terrigenous source this implies stronger acc flow at the lgm contrasting with weaker weddell gyre flow deduced from earlier work

small basins in the scotia sea the eocene drake passage gateway after isostatic correction for their sedimentary fill the depths of two small oceanic basins in the southern scotia sea suggest that both started to open in mid to late eocene times models of the short magnetic anomaly profiles across the basins provide support for these determinations the location of the basins adjacent to the present day drake passage marine gateway and the timing of continental stretching leading up to their opening during the ypresian early eocene onset of global cooling mean that their importance is potentially far greater than their small size implies extension in the region of the two basins would have opened drake passage to shallow or intermediate depth oceanic circulation between the pacific and atlantic oceans for the first time this coincided with a reorganisation of vertical mixing patterns in the global ocean a shift in the site of carbon sequestration from coal swamps and peatlands to ocean sediments and the onset of a long decline in atmospheric carbon dioxide concentrations cenozoic global cooling may therefore have begun as a result of the shallow opening of drake passage

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

the age and origin of the central scotia sea summary opening of the drake passage gateway between the pacific and atlantic oceans has been linked in various ways to cenozoic climate changes from the oceanic floor of drake passage the largest of the remaining uncertainties in understanding this opening is in the timing and process of the opening of the central scotia sea all but one of the available constraints on the age of the central scotia sea is diagnostic of or consistent with a mesozoic age comparison of tectonic and magnetic features on the seafloor with plate kinematic models shows that it is likely to have accreted to a mid ocean ridge between the south american and antarctic plates following their separation in jurassic times subsequent regional shallowing may be related to subduction related processes that preceded backarc extension in the east scotia sea the presence of a fragment of jurassic cretaceous ocean floor in the gateway implies that deep water connections through the scotia sea basin complex may have been possible since eocene times when the continental tips of south america and the antarctic peninsula first passed each other

anomalous temperatures in central scotia sea sediments bottom water variation or pore water circulation in old ocean crust we report low temperature gradients from 60 to 12 of geothermal and extrapolated temperatures offset from modern bottom water temperatures in sediments from the central scotia sea we examine possible causes bearing in mind similar anomalous measurements 18 years previously attributed at the time to instrumental error small temperature offsets 0 1 c may reflect short term bottom temperature variation within eddies of the antarctic circumpolar current low sediment temperature gradients may be caused by horizontal advection of cold water within the upper oceanic basaltic layer documented in younger ocean floor elsewhere or by northward encroachment of colder bottom waters from the antarctic peninsula shelf or weddell sea for several years prior to measurement

the empirical age depth relation and depth anomalies in the pacific ocean basin this paper presents a new age depth relation and a comprehensive depth anomaly map for the pacific ocean age depth and sediment thickness were averaged for each 1 1 area bathymetry was corrected for isostatic effects due to sediment and water load and for the variation of sound velocity in water seafloor existing at extreme distance from hot spot tracks is found to be anomalously deep hot spots were treated as thermal anomalies and data within 800 km of them were eliminated from further analysis an age depth equation was determined by fitting a least squares line to the remaining data the resulting depth versus age distribution confirms the linear dependence of depth on age up to 80 ma all older seafloor is within 800 km of hot spots or hot spot tracks and is shallower than predicted this flattening of the age depth distribution requires that a separate function be fit to data older than 80 ma as a result data representing the deepest seafloor older than 80 ma were selected for curve fitting and the age depth distribution was approximated by an exponential function it is impossible to distinguish between the infinite half space and cooling plate models of the lithopshere using the age depth data from the pacific ocean a depth anomaly map prepared by comparing observed depth with the depth predicted by the new age depth formula shows several newly recognized features including swells over the louisville ridge in the south pacific and in the gulf of alaska seamounts

the effect of hot spots on the oceanic age depth relation hot spots located both midplate and on midocean ridges can bias the empirical age depth relation ocean floor bathymetry for seafloor greater than 80 m y b p in age has been previously used to determine if the oceanic lithosphere behaves as a cooling half space or as a cooling plate however large areas of the ocean basins do not fit a simple age depth relation anomalously shallow swells approximately 1000 km wide and 1 km high surround hot spots such as iceland the azores and the cape verde islands by sorting seafloor depth data by distance to the nearest hot spot track we show that the along isochron variation in depth in the north atlantic correlates with distance from a hot spot track due to the absence in the north atlantic of seafloor both old and far from hot spots we compare depth versus age at constant distance and do not observe flattening of the age depth relation in any distance range the age depth relation depth meters 2700 295 t m y 1 2 derived for seafloor 1200 to 1800 km from a hot spot track represents our best estimate of the age depth relation appropriate to seafloor unaffected by a hot spot track in the north atlantic the previously inferred flattening of the age depth relation appears to be caused by the inclusion of hot spot swells this implies that hot spots may be the primary means of heat transfer from the deeper mantle to the base of the lithosphere

empirical reflection travel time versus depth and velocity versus depth functions for the deep sea sediment column numerous correlations of one way reflection travel times and depths to reflecting horizons have been proposed in the initial reports of the deep sea drilling project dsdp having carefully reviewed proposed correlations we selected 233 correlated depths ranging from 27 m to 1 4 km and one way travel times ranging from 0 02 to 0 7 s from 154 dsdp sites worldwide assuming that the velocity depth function for deep sea sediments can be approximated by an exponential form we have used these data to establish statistically well constrained global models relating depth to travel time and velocity to depth there is remarkably little scatter in the data based on nonlinear least squares fits the best depth time function is z km 3 03 1n 1 0 52t s the rms error in depth is 26 m the corresponding average velocity depth function is v km s 1 59exp 0 33z we estimate that the real site to site variability of velocities at depth is less than 0 20 km s any dependence of velocity on composition is below the resolution of the data and available evidence suggests that age has no significant effect these results imply that to a depth of 1 4 km the physical state of sediments depends on in situ overburden pressure and temperature which are related to depth of burial and that chemical and mechanical equilibrium is achieved in a period of time that is geologically short our results are consistent with laboratory measurements of p wave velocities in deep sea sediments but velocity depth functions for terrigenous and calcareous sections derived from sonobuoy wide angle reflection surveys are incompatible with the time and depth data and overestimate velocities by as much as 10 and 30 respectively these systematic errors may result from the fact that the marked anisotropy of deep sea sediments is neglected in the interpretation of reflection data

drake passage and cenozoic climate an open and shut case drake passage opening has often been viewed as a single discrete event possibly associated with abrupt changes in global circulation and climate at or near the eocene oligocene boundary a new plate tectonic model based on recent reinterpretations of the opening history of basins in the scotia sea suggests that an effective ocean gateway may have developed even earlier during the middle eocene this is consistent with a growing body of evidence from sediment core proxy data for eocene changes in southern ocean circulation and biological productivity the period between earliest opening after 50 ma and the latest eocene was characterized by the evolution of various current pathways across the subsiding continental shelves and intervening deep basins this shallow opening may have caused important changes in southern ocean circulation contributing to eocene cooling and the growth of antarctic ice sheets

circum antarctic age modelling of quaternary marine cores under the antarctic circumpolar current ice core dust magnetic correlation sediments in the belt under the antarctic circumpolar current acc contain high quantities of siliceous microfossils mainly diatoms and very little to zero carbonate this prevents establishment of the age by the usual methods of oxygen isotope stratigraphy at most coring sites downcore variation in magnetic susceptibility ms from cores from the scotia sea closely resembles the temporal pattern of dust concentration and flux in east antarctic ice cores the validity of the temporal correlation between marine ms and epica dome c edc dust on the edc3 time scale is established through 14c dating of the acid insoluble organic matter back to 30 ka and radiolarian abundance stratigraphy of cycladophora davisiana back to 180 ka the correlations are good r 0 80 and give credence to the use of ms dust as a means of establishing chronostratigraphy in carbonate free pelagic sediments from the southern ocean this correlation has been established in 10 cores in the scotia sea the circum antarctic validity of the pattern is shown in correlations to ms in sediment cores from the indian and sw pacific sectors of the southern ocean during mis 3 57 29 ka when sedimentation rates at correlated core sites exceeded 10 cm ka 1 three ms maxima in the marine sediments and dust peaks at edc respectively correspond to the cool periods between antarctic warm events a1 a4 we have not established the precise cause of the correlation but its validity around the circum antarctic west wind belt suggests that the material was either all transported by wind or was supplemented by wind driven current transport the fact that it occurs also in sections with a low terrigenous content suggests a fine grained high susceptibility material such as magnetite as the likely source

paleogene opening of drake passage the timing of events leading to the earliest connection between the pacific and atlantic oceans at drake passage is controversial but important because gateway opening probably had a profound effect on global circulation and climate a rigorous new analysis of marine geophysical data demonstrates a major change in the motion of the south american and antarctic plates at about 50 ma from n s to wnw ese accompanied by an eightfold increase in separation rate this would have led to crustal extension and thinning and perhaps the opening of small oceanic basins with the probable formation of a shallow b1000 m gateway during the middle eocene no change in south american antarctic motion is observed near the eocene oligocene boundary but a deep water connection developed between 34 and 30 ma when continued extension led to the initiation of seafloor spreading at the west scotia ridge these timings correlate with events seen in the oxygen isotope record from benthic foraminera and support the view that drake passage opening was the trigger for abrupt eocene oligocene climate deterioration and the growth of extensive antarctic ice sheets d 2005 elsevier b v all rights reserved

shear wave velocity and seismic response of near surface sediments in charleston south carolina six major geologic units in charleston south carolina are characterized in terms of shear wave velocity v s in this article the characterization is based on in situ v s measurements at 91 sites the six units are man made fills holocene and late pleistocene deposits the wando formation the ten mile hill beds the penholoway formation and the daniel island beds and tertiary deposits median v s values for these units in the top 25 m are 145 111 189 176 285 and 399 m sec respectively for tertiary deposits in the depth intervals of 25 55 m 55 75 m and 75 100 m median v s values are 435 533 and 663 m sec respectively a seismic response parametric study is conducted assuming several soil rock models and two input rock outcrop motions with peak accelerations of 0 3 g and 0 1 g it is found that quaternary sections with v s of 190 m sec e g the wando formation and thicknesses of about 7 m to 15 m exhibit predominant peaks in the acceleration response spectra at periods of about 0 25 to 0 4 sec these predominant peaks match fundamental periods of many existing buildings in the old city district of charleston the results suggest that local site conditions contributed to building damage in the 1886 charleston earthquake

flattening of the sea floor depth age curve as a response to asthenospheric flow the flattening of sea floor depths from the age dependence predicted by considering the cooling plate as a growing thermal boundary layer is a fundamental constraint on the evolution of oceanic lithosphere1 previous explanations for the flattening have included reheating from convective instabilities that form beneath lithosphere older than 80 myr ref 2 thermal rejuvenation of lithosphere that passes over stationary hotspots3 5 or a whole mantle flow forced by two converging plates6 we suggest here that the flattening of old ocean floors can perhaps best be explained as a dynamic phenomenon reflecting flow in asthenosphere underlying the oceanic lithosphere applying the model to the pacific plate we show that a solution for flow in an asthenosphere low viscosity channel which is consumed by plate accretion and the subduction of lithosphere at trenches and replenished by near ridge upwelling and near ridge hotspots generates the observed 1 km of sea floor flattening for asthenospheric viscosities of 2 1018 pa s and an absolute pacific plate motion of 100 mm yr 1 our model can also explain the asymmetric subsidence of the south american and african plates away from the mid atlantic ridge

late quaternary glacial history of the south orkney plateau antarctica abstract piston cores from the south orkney plateau penetrated overcompacted diamictons in water depths of up to 250 m detailed textural and petrological analyses of these diamictons indicate that they are basal tills seismic records from the plateau show a widespread surface of glacial erosion and provide additional evidence of an ice cap grounded to a depth of 250 m piston cores from the slope of the plateau penetrated diatomaceous muds resting directly on poorly sorted muds with very little to no biogenic material the ice rafted debris in these glacial marine sediments is composed almost exclusively of material derived from the south orkney islands this implies deposition beneath an ice shelf as opposed to iceberg rafting in contrast diatomaceous muds contain relatively abundant exotic iceberg rafted material and reflect a glacial maritime setting similar to that of today the sharp contact separating diatomaceous surface sediments from basal tills and sub ice shelf deposits indicates that the ice cap and ice shelf retreated from the plateau rapidly radiocarbon dates for diatomaceous muds from a glacial trough on the plateau indicate that the ice cap and ice shelf retreated from the plateau prior to 6000 to 7000 years ago the homogeneity of surficial diatomaceous sediments suggests that sea ice conditions over the plateau have not changed radically since that time

an early miocene ridge crest trench collision on the south scotia ridge near 36 w abstract preliminary interpretation of marine geophysical data and geochemical analyses of dredged rocks has led to the identification of a ridge crest trench collision zone southeast of the south orkney islands the existence of a series of such collision zones along the south scotia ridge had earlier been hypothesised to explain changes in scotia sea evolution the collision zone takes the form of a double ridge with dissecting trough aligned ne sw and about 200 km long oceanic magnetic lineations in the northern weddell sea young northwestward towards the zone with the youngest identified as anomaly 6 20 ma reaching the central trough volcanics dredged from jane bank the northwestern ridge are chemically very similar to the more siliceous of the low k tholeiite series which characterises the presently active south sandwich arc jane bank is interpreted as an in situ remnant of the arc and upper fore arc of an intra oceanic arc produced by subduction of south american oceanic lithosphere like the present south sandwich islands part of the upper fore arc and all of any accretionary wedge appear to have been subducted unlike ocean floor at collision zones off chile and the antarctic peninsula the seabed in the trough and on the ridge to the southeast does not conform to the normal oceanic age depth relationship the estimated collision time 20 ma is close to the time of onset of n s extension in the central scotia sea the limited data set described here however does not establish the synchroneity of the collision over the entire 200 km of jane bank so an assessment of the extent of any causal relationship between the two events must await the analysis of a more extensive survey

heat flow in jane basin northwest weddell sea the jane basin is a marginal basin situated immediately to the east of the south orkney microcontinent in the northwest section of the weddell sea thirty five heat flow measurements made in the jane basin ranged in value from 67 5 4 3 to 92 1 3 0 mw m2 excluding values that were corrected for tilting or were on the very edge of the basin the remaining 28 values range between 75 0 8 0 and 84 6 9 1 mw m2 magnetostratigraphy on the recovered core from ocean drilling program hole 697 which was drilled in jane basin to a depth of 322 m allowed sedimentation rates to be calculated back to 4 5 ma single channel seismic reflection data from rss shackleton allowed estimations of total sediment thickness for the jane basin to be made we calculate that the measured heat flow is only 86 89 of the actual heat flow as a result of sedimentation heat generation in the sediments contributes 1 5 1 9 mw m2to the total heat flow the corrected heat flow gives an age for the jane basin of between 25 and 32 ma from age versus heat flow comparisons similar to the age determined from basement depth the scotia sea located to the north of jane basin and the south orkney microcontinent has been dated as anomaly 10 30 ma and younger our calculated age for the jane basin would indicate that it may have been created prior to the initiation of seafloor spreading in the scotia sea evidence from major plate motions indicate that antarctica began to rotate clockwise away from south america at about 65 ma such motion may have triggered subduction along the southeast side of jane bank and the opening of jane basin as a back arc basin subduction at jane bank ended at anomaly 6a time 22 ma as evidenced by the age of the identified magnetic anomalies on the antarctic plate found immediately to the east of jane bank we conclude that jane basin opened prior to the opening of the scotia sea and that the spreading center that opened jane basin may have jumped to the scotia sea and produced the seafloor spreading there

controls on back arc crustal accretion insights from the lau manus and mariana basins abstract together the lau manus and mariana basins encompass a broad range of conditions of back arc basin development marine surveys have determined the tectonic setting and reconnaissance scale geophysical and geochemical properties of the extension axes in these basins we review these data to examine crustal accretion characteristics in the back arc setting in each basin magmatism is enhanced in spreading centres near the arc volcanic front but decreases becoming deficient in axes further from the arc in the lau and manus basins the axes extend far behind the arc and develop typical mid ocean ridge mor like characteristics we propose that these variations are controlled by the subducting slab which shapes composition and flow within the mantle wedge water released from the slab lowers the mantle solidus and increases in concentration toward the volcanic arc spreading centres near the arc advect the hydrated mantle material enhancing melt production slab induced flow carries melt depleted mantle material back beneath the basin where it mixes with ambient mantle spreading centres further from the arc advect this mantle mixture and produce thinner than normal crust far from the arc spreading centres advect essentially mid ocean ridge basalt morb source mantle and their characteristics are mor like thus in addition to spreading rate effects on mantle melting that predominate at mors in back arc basins the subducting slab introduces additional systematic effects

heat flow determinations of basement age in small oceanic basins of the southern central scotia sea results are reported from seven heat flow stations in small basins of the southern part of the central scotia sea css undertaken in order to determine basement ages the basins are small which makes magnetic anomaly based ages ambiguous and preserves basin subsidence that may have been anomalous as a result of local factors the fact that these small basins formed in a back arc setting adds additional uncertainty to depth based age estimates the results confirm that basin extension commenced in the eocene and indirectly support a relatively young back arc origin for the northern css but do not affect previously published suggestions of the age of onset of the antarctic circumpolar current

contourite deposits in the central scotia sea the importance of the antarctic circumpolar current and the weddell gyre flows abstract new swath bathymetry with multichannel and high resolution seismic profiles shows a variety of contourite drift sediment wave morphologies and seismic facies in the central scotia sea the deposits are to be found at the confluence between the two most important bottom current flows in the southern ocean the eastward flowing antarctic circumpolar current acc and the northward outflow of the weddell sea deep water wsdw the contourite drifts are wedge like deposits up to 1 km thick that exhibit aggradational reflectors along axis thinning towards the margins the contourite drifts occur in areas of weaker flows along the margins of contourite channels and in areas protected by obstacles the elongate mounded drifts are best developed along the left hand margins of channelized bottom current flows due to the coriolis force a contourite fan has a main channel and two distributary channels that expand over a gentle seafloor the proximal fan exhibits sediment waves with the distal fan incised by furrows sediment wave fields are well developed in areas of intensified bottom flows without channels particularly at the confluence of the acc and the wsdw small sediment waves occur where unidirectional bottom current flows predominate sediment waves may develop under the influence of internal waves produced by the interaction of the flows and sea bottom relief the stratigraphic sequence above the oceanic crust of early to middle miocene age contains six seismic units separated by major reflectors all the units were shaped under the influence of strong bottom current flows although they exhibit distinct seismic facies changes that record the variations of the bottom current pathways over time the age of the units was calculated based on the age of the oceanic crust and sedimentation rates of deep sea deposits in the region the oldest units vi iv are of early to middle miocene age and developed under the influence of the acc they are characterized by a southward progradational pattern of the seismic units and sedimentation rates of 5 8 cm ky unit iii with an estimated middle miocene age evidences the first incursion of wsdw into the central scotia sea when plate movement caused openings in the south scotia ridge and allowed the connection with the northern weddell sea through jane basin and gaps in the ridge unit ii estimated to be of late miocene to early pliocene age extends over the area and is characterized by internal unconformities a major unconformity at the base of unit ii records an important reorganization of bottom current flows that may predate the onset of grounded ice sheets on the antarctic peninsula shelf unit i of late pliocene to quaternary age shows intensified bottom currents the unconformity at the base of unit i probably predates the onset of major northern hemisphere glaciations and the greater expansion of antarctic ice sheets during the late pliocene the extensive distribution of contourite deposits above the oceanic crust testifies to the long term production of antarctic bottom water cold deep water was swept northward from the weddell gyre interacting with the acc and possibly exerting profound influences on the global circulation system and the onset of major glaciations

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

late triassic depositional environments and molluscan associations from west central nevada abstract the sequence of lithofacies and faunal associations described from the upper norian gabbs formation in nevada formed in response to the general late triassic regression this fauna because of its position at the triassic jurassic boundary should reflect the late triassic mass extinction event however the within habitat species diversity of these five bivalve ammonite associations is comparable to that of ecologically similar mesozoic deposit and suspension feeding associations the peculiar absence of gastropods is correlated with an abundance of small heteromorph ammonites suggesting a possible causal relationship

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

mantle wedge control on back arc crustal accretion at mid ocean ridges plate separation leads to upward advection and pressure release partial melting of fertile mantle material the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away in back arc basins the subducting slab is an important control on the pattern of mantle advection and melt extraction as well as on compositional and fluid gradients modelling studies predict significant mantle wedge effects on back arc spreading processes here we show that various spreading centres in the lau back arc basin exhibit enhanced diminished or normal magma supply which correlates with distance from the arc volcanic front but not with spreading rate to explain this correlation we propose that depleted upper mantle material generated by melt extraction in the mantle wedge is overturned and re introduced beneath the back arc basin by subduction induced corner flow the spreading centres experience enhanced melt delivery near the volcanic front diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away our model explains fundamental differences in crustal accretion variables between back arc and mid ocean settings