gravitational instabilities triggered by fluid overpressure and downslope incision insights from analytical and analogue modelling fluid overpressure at the base of low permeability strata reduces effective stress allowing for gravitational sliding of the overlying cover the force driving sliding is the slope parallel component of the weight of the cover whereas the resisting forces are the friction at the base of the cover and the buttressing resistance to shortening which can be critically reduced by incision at the base of the slope we developed an analytical model and undertook a series of analogue experiments to better understand the evolution of a sedimentary cover sliding above a low permeability layer subjected to fluid overpressure where a downslope buttress was present the sliding sheet length decreased with increasing pore fluid pressure in the absence of such buttress the slide s length increased exponentially with increasing pore fluid pressure another important difference dealt with geometry and kinematics buttressed slides consisted of one large slope parallel mass rigidly translated and bounded by downslope thrusts and upslope normal faults with increasing pore fluid pressure the contractional structures propagated upslope by contrast non buttressed slides showed intense strain deformation started with normal faults forming near the incision then propagating upslope throughout the slide s evolution

abrupt change in slope causes variation in the deposit thickness of concentrated particle driven density currents abstract abrupt changes in slope occur frequently in some marine environments laterally confined laboratory experiments show that when dense surge like particulate flows travel over abrupt slope reductions their deposits may have an asymmetric bell shaped thickening near the break of slope this thickening is herein called a slope break deposit and is related to the change in kinetic energy of the flow slope break deposits are particularly pronounced under relatively high velocity flows in our experiments these are flows with high particle concentrations c 5 10 by volume silicon carbide moving down slopes steeper than 3 the experiments were designed such that currents generated by a lock exchange mechanism flowed down a slope variable slope angle onto a flat surface the bodies of the flows became thicker and slower just downstream of the slope break because the flow body slows very rapidly particles are dumped forming the slope break deposit the peak in sediment thickness of the slope break deposit is downstream of the break in slope because falling particles continue to move forward in the flow during settling the slope break deposits may be several tens of percent thicker than deposits from an equivalent flow in which no change in slope occurred the parameters of the slope break deposit correlate with slope angle change

late neogene sequence stratigraphic evolution of the foz do amazonas basin brazil the margin of the foz do amazonas basin saw a shift from predominantly carbonate to siliciclastic sedimentation in the early late miocene by this time the amazon shelf had also been incised by a canyon that allowed direct influx of sediment to the basin floor thus confirming that the palaeo amazon fan had already initiated by that time 9 5 8 3 ma above this interval during a prolonged lowstand messinian third order sequences are preserved only in the incised valley fills of the canyon with no equivalent strata on the shelf third and fourth order sequences younger than messinian are preserved on the shelf after sea level rise above the shelf by the early pliocene sequences younger than 3 8 ma often show fourth order cyclicity with an average duration of 400 ka larger scale eccentricity cycles often preserved in high sedimentation rate areas of river deltas mass wasting and transportation of slope sediments to the basin began to play an important role in sediment dispersal at least as far back as the mid pliocene after rapid progradation had produced steeper slopes more prone to failure

seismic interpretation of mass transport deposits within the upper oligocene frio formation south texas gulf coast we integrated well logs and three dimensional seismic data to describe a wedge of deformed shallow frio rocks lying above a major bed parallel decollement within the upper oligocene frio formation located between the houston and norias deltas on the south texas gulf coast our analyses show that the identified deformed shallow frio rocks can be divided into proximal clay rich and low permeability sandstones characterized by discontinuous mounded and chaotic seismic events near proximal clay poor and high permeability sandstones characterized by parallel to subparallel bedded seismic events and distal sand silt and mudstones composed of a mix of proximal and near proximal rocks all of the deformed rocks are composed of acoustic impedance materials that are lower than those of the undeformed shallow frio and are underlain by low velocity overpressured shale rich rocks the mechanism that triggered the collapse of the shallow frio and subsequent development of mass transport deposits is attributed to an uprising overpressured shale rich high and the development of a shelf edge listric fault proprietary biostratigraphic data show that the collapse of the shallow frio in areas between the houston and norias deltas occurred between 27 5 and 25 3 ma approximately the same time as the hackberry collapse in the mississippi delta in the proximal area interpreted paleowater depths from biostratigraphic data based on benthic foraminifers range from 60 to 120 ft 20 40 m in a shallow neritic environment in contrast the distal area lies in paleowater depths interpreted to be between 120 and 300 ft 40 90 m in a middle neritic environment

rock physics modeling of static and dynamic reservoir properties a heuristic approach for cemented sandstone reservoirs rock physics models for fluid and stress dependency in reservoir rocks are essential for quantification and interpretation of 4d seismic signatures during reservoir depletion and injection however our ability to predict the sensitivity to pressure from first principles is poor the current state of the art requires that we calibrate the pressure dependence of velocity with core measurements a major challenge is the fact that consolidated rocks often break up during coring and hence the stress sensitivity is likely to be overpredicted in the laboratory relative to the in situ conditions furre et al 2009 for unconsolidated sands acquisition of core samples is not very feasible due to the friable nature of the sediments one physical model that has been applied to predict pressure sensitivity in unconsolidated granular media is the hertz mindlin contact theory several authors vernik and hamman 2009 among others have suggested empirical models with fitting parameters that correlate with microcr

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

mass transport processes on amazon cone western equatorial atlantic three large mass transport deposits occur on the upper to middle amazon cone these slump debris flow complexes have been mapped from 3 5 khz echograms and cover approximately 75 000 sq km or about 10 of the cone the debris flows traveled down slopes with gradients as low as 1 90 to 1 165 0 3 to 0 6 for distances up to 300 km these deposits average 10 to 50 m in thickness and the total amount of sediments displaced may exceed 3 800 cu km the upslope parts of these complexes generally consist of hummocky slump or slide deposits which are commonly bounded by scarps of varying relief downslope from this zone of removal the deposits consist of thin the occurrence of such widespread mass movements on the amazon cone indicates that mass transport is probably an important sedimentation process on deep sea fans however most studies and models of modern deep sea fans have minimized or ignored the potential importance of mass transport processes in fan development we suggest that in the future both ancient and modern deep sea fans be more closely examined for mass transport deposits so that the importance of this process to overall fan development can be more accurately assessed

late pliocene margin development and mega debris flow deposits on the antarctic continental margins evidence of the onset of the modern antarctic ice sheet abstract major global climatic changes in the cenozoic coincided with major changes in volume of polar ice sheets which are reflected in the sedimentary regime of polar continental margins we compile and review the most recent evidences of the neogene architectural change in the sedimentary units of the antarctic continental margins to provide a comprehensive interpretation of timing and circumstances of the last of these changes we review and interpret the seismic stratigraphy and the borehole data where available from 6 key sectors of east and west antarctica the continental margins of the antarctic peninsula prydz bay wilkes land weddell sea eastern and western ross sea in these margins we identify the following change in the geometry of the sedimentary deposits margin wide erosion and subsequent progradation on the continental shelf stratal downlap on the continental slope major mass wasting deposits on the continental rise the change is apparently synchronous and dated about 3 ma in the antarctic peninsula and prydz bay margins and broadly concomitant in the others sectors this suggests a common driving force which we suggest to be the late pliocene climatic change that induced the transition of the antarctic ice sheet regime from polythermal to present polar cold dry based conditions we infer that in consequence of the late pliocene global cooling meltwater decreased in quantity and focussed within the main ice troughs where large ice streams began to flow the regional erosion was followed by focussing of the sedimentary transport at the base of the large ice streams and faster progradation of sedimentary wedges at the mouth of the ice troughs this induced large failures of the continental slope while the continental rise was progressively deprived of terrigenous sediments

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

circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

interglacial collapse of crary trough mouth fan weddell sea antarctica implications for antarctic glacial history abstract in this study we investigate the stratigraphy of a trough mouth fan in the weddell sea to determine possible relationships between basin floor stratigraphy and glaciation on antarctica seismic data from the basin floor show a deep broad and elongate erosional channel filled by thick chaotic facies deposits basin floor fans and channel levee deposits from seismic stratigraphy analysis of basin floor stratigraphy we infer that this channel formed by large volume mass wasting sourced from poorly sorted glacial sediments from crary trough mouth fan and dronning maud land slopes correlation to odp leg 113 site 693 suggests that the mass wasting of the slopes occurred in the early pliocene we surmise that collapse occurred during an early pliocene interglacial and was related to major drawdown of the antarctic ice volume collapse was probably triggered by rebound and relative sea level fall a mathematically predicted geoidal effect related to antarctic ice volume reduction the record of this major ice volume reduction is not manifest in the trough mouth fan but rather is manifest in the basin floor stratigraphy as a major backfilled erosional channel which extends far beyond the trough mouth fan

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

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

small ocean basin development along the scotia antarctica plate boundary and in the northern weddell sea abstract the oceanic crust of the northern weddell sea and scotia sea contains the following principal morphostructural elements 1 the south scotia ridge and the south orkney microcontinent both with continental crust 2 the oceanic powell and jane basins and 3 jane bank which belongs to an island arc the analysis of mcs profiles and of gravimetric and magnetic data from russian italian and spanish cruises supplemented with satellite gravimetric data has enabled us to determine the relationship between these elements and to propose a model for the main stages of cenozoic evolution in the area during the early cenozoic the weddell sea oceanic crust was subducted under the southern margin of the south orkney microcontinent the subduction probably ended westwards at the south powell ridge a submarine extension of the antarctic peninsula a major transcurrent fault zone is identified in the northwestern weddell sea bounding oceanic crust of mesozoic and cenozoic ages this fault zone was probably active at least to the miocene the drifting of the south orkney microcontinent from the antarctic peninsula during the late eocene to early miocene originated the powell basin jane basin developed as a backarc related to the subduction of the weddell sea oceanic crust below jane bank the seismic stratigraphy of the depositional sequences in these two basins indicates that spreading in jane basin started simultaneously with the end of the opening in powell basin the active spreading ridge of the weddell sea collided with the trench and was subducted below jane bank at 15 20 ma drifting in jane basin and subduction below jane bank ended shortly thereafter in the middle miocene and the boundary between the antarctic scotia plates migrated north of the south orkney microcontinent along the south scotia ridge present tectonic activity in the region is minor

margin architecture reveals the transition to the modern antarctic ice sheet ca 3 ma seismic reflection data collected primarily on the continental slope of the pacific margin of the antarctic peninsula after drilling ocean drilling program odp leg 178 allow us to date a regional change in the style of margin accretion to ca 3 ma late pliocene deep erosion of continental shelf and slope which likely included the emplacement of a megadebris flow deposit was followed by prominent growth of steep relatively stable continental slope prograding wedges while the deep distal margin sedimentation rate was significantly reduced a review of the available stratigraphy from deep sea drilling project odp drilling and correlation with available seismic stratigraphic information allowed us to recognize comparable changes in terms of age and trends in continental margin evolution in several places around antarctica we argue that this late pliocene architectural change of the antarctic margin reflects a change in the texture and water content of the sediment delivered by the antarctic ice sheet following the transition from wet to dry based ice regimes this circum antarctic change coincides with the late pliocene ca 3 ma global cooling and is proposed as the marker event of the transition to the modern cold polar dry based antarctic ice sheet

autopsy on a dead spreading center the phoenix ridge drake passage antarctica new bathymetric and magnetic anomaly data from the phoenix ridge antarctica show that extinction of all three remaining segments occurred at the time of magnetic chron c2a 3 3 0 2 ma synchronous with a ridge trench collision south of the hero fracture zone this implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision spreading rates slowed abruptly at the time of chron c4 7 8 0 3 ma probably as a result of extinction of the west scotia ridge which would have led to an increase in slip rate and transpressional stress across the shackleton fracture zone spectacular high relief ridges flanking the extinct spreading center mapped for the first time using multibeam swath bathymetry are interpreted as a consequence of a reduction in spreading rate involving a temporary magma oversupply immediately prior to extinction

the norwegian greenland sea continental margins morphology and late quaternary sedimentary processes and environment the continental margins surrounding the norwegian greenland sea are to a large degree shaped by processes during the late quaternary the paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features slides trough mouth fans and channels several large late quaternary slides have been identified on the eastern norwegian greenland sea continental margin the origin of the slides may be due to high sedimentation rates leading to a build up of excess pore water pressure perhaps with additional pressure caused by gas bubbles triggering might have been prompted by earthquakes or by decomposition of gas hydrates trough mouth fans tmf are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves in the norwegian greenland sea seven tmfs have been identified varying in area from 2700 km2 to 215 000 km2 the trough mouth fans are depocentres of sediments which have accumulated in front of ice streams draining the large northwest european ice sheets the sediments deposited at the shelf break upper slope by the ice stream were remobilized and transported downslope mostly as debris flows the trough mouth fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge two large deep sea channel systems have been observed along the norwegian continental margin the lofoten basin channel and the inbis channel along the east greenland margin several channel systems have been identified the deep sea channels may have been formed by dense water originating from cooling sea ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents

the scotia antarctica plate boundary from 35 w to 45 w abstract a compilation of available multichannel seismic profiles acquired along the southern margin of the scotia sea east of the south orkney microcontinent has allowed identifying and mapping the main morphological and structural features of the central segment of the scotia antarctica plate boundary this margin is composed by several bathymetric highs of variable size and uncertain crustal nature separated by deep troughs and restricted oceanic basins some of these troughs represent pull apart basins three main segments oriented wnw ese the western sector ene wsw the central sector here named bruce deep and ne sw the eastern sector have been described these segments are separated by nnw sse trending release zones disposed in an en echelon geometry which represent mostly strike slip faults the western segment corresponds to the northern margin of the south orkney microcontinent where a subduction zone seems to be present even if its present day activity is unclear the segment further to the east corresponds to an ene oriented basin bruce deep which separates the bruce bank from the eastern promontory of the south orkney continental platform to the south of the bruce deep a wide deformation zone with n verging folds and thrusts here named jane thrust belt has been identified from seismic data the eastern segment of the plate boundary is structurally the less constrained and may be composed by a series of tectonic lineaments of different lengths from the bruce bank to the east focal mechanisms maintain a prevalent left lateral strike slip motion combined with an extensional component in this sector earthquakes are located in a 150 km wide area and on a local scale are difficult to follow unambiguously at the plate boundary lithologic analyses on dredged material recovered along a flank of one of the morphological relieves present south of the discovery bank to 35 w here collectively named irizar highs yielded a dominant granitic composition a similar composition characterizes the rocks collected in the southern flank of the south easternmost jane bank this suggests a continental crust nature for these bathymetric highs now dispersed along this sector of the scotia antarctica plate boundary we propose here a tectonic evolution for this margin dominated since the early miocene by the northward subduction of the weddell sea oceanic crust the development of a dextral en echelon transform fault system facilitated the process of fragmentation and dispersion of the crustal blocks dismembered the subduction zone and possibly inverted the direction of convergence therefore the scotia plate would subduct beneath the antarctic plate in the western sector and weddell sea would subduct beneath scotia plate in the eastern sector finally the activation of left lateral transtensional strike slip lineaments generated narrow pull apart basins in the fore arc sectors of the convergent zones

deep scientific dives in the japan and kuril trenches abstract in the summer of 1985 during the french japanese kaiko program ten dives to depths of 6000 m in the japan and kuril trenches were made in the newly launched submersible nautile the sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving seabeam mapping geomagnetic and gravimetric measurements and single channel seismic profiling the results of the dives provide new constraints on the geodynamics of these subduction zones in the japan and kuril trenches huge slump scars were observed on the landward slopes of the trenches slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material the slump scars allowed us to observe the internal structure of the margin the monoclinal structure on the northern japan trench margin deduced from the seismic profiles and dsdp drilling was confirmed several dives on kashima seamount confirmed that this volcano has recently been split into two parts by a normal fault system comparisons of lithology and paleontology on the two separated parts of the seamount were made deep sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches it can be concluded that the whole margin is venting fluids from depths of 2 3 km which is consistent with the indications of overpressure observed in drill sites on the japan trench margin the fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor

tectonics of the scotia antarctica plate boundary constrained from seismic and seismological data abstract the plate boundary between the scotia and antarctic plates runs along the broadly e w trending south scotia ridge it is a mainly transcurrent margin that juxtaposes thinned continental and transitional crust elements with restricted oceanic basins and deep troughs seismic profiles and regional scale seismological constraints are used to define the peculiarities of the crustal structures in and around the southern scotia sea and focal solutions from recent earthquakes help to understand the present day geodynamic setting the northern edge of the western south scotia ridge is marked by a sub vertical left lateral master fault locally a narrow wedge of accreted sediments is present at the base of the slope this segment represents the boundary between the scotia plate and the independent south shetland continental block along the northern margin of the south orkney microcontinent the largest fragment of the south scotia ridge an accretionary prism is present at the base of the slope which was possibly created by the eastward drift of the south orkney microcontinent and the consequent subduction of the transitional crust present to the north east of the south orkney microcontinent the physiography and structure of the plate boundary are less constrained here the tectonic regime exhibits mainly strike slip behavior with some grade of extensional component and the plate boundary is segmented by a series of nnw sse trending release zones which favored the fragmentation and dispersion of the crustal blocks seismic data have also identified along the north western edge of the south scotia ridge an elevated region the ona platform which can be considered along with the terror rise as the conjugate margin of the tierra del fuego before the drake passage opening we propose here an evolutionary sketch for the plate boundary from the late oligocene to the present encompassing the segment from the elephant island platform to the herdman bank

seafloor instabilities and sediment deformation processes the need for integrated multi disciplinary investigations abstract in this paper we present the current practice of investigations of seafloor instabilities and deformation processes based on extensive research conducted over the last years which sets the scene for future research activities in this field the mapping of the continental margins and coastal areas with ever increasing resolution systematically reveals evidence of instabilities and deformation processes both active and palaeo features in order to properly assess the hazards and risks related to these features an integrated and multi disciplinary approach is essential but challenging such an approach consists of combining field data geophysics geology sedimentology geochemistry and geotechnical data with numerical simulations constrained by results from laboratory data as such it is of paramount importance to build a common knowledge base and understanding that unify these disciplines into more complete and conceptual models constrained by all the data we review the status of this integrated approach adapted to palaeo landslides e g storegga ana vesteralen and recent deformations finneidfjord nice gulf of guinea allowing to identify gaps in our knowledge at these sites by reviewing these case studies one can conclude that each case remains highly site specific in which both the regional and local geological tectonic settings have a distinct effect of the type of instability or deformation taking place or that can take place our knowledge on the actual triggers remains poorly constrained and there is even ambiguity for historic landslides e g finneidfjord also our knowledge of the preconditioning factors is incomplete there is a general lack of geotechnical data both in situ and from laboratory and therefore modelling the dynamics e g rheology of the instabilities relies on a number of assumptions rather than facts in addition excess pore pressure and its evolution is one of the key parameters driving instabilities despite this fact in situ excess pore pressure is rarely measured or monitored much work remains to be done to relate and integrate geotechnical data with geophysics e g through inversion and rock physical models in order to obtain additional quantitative information from the sub surface but also with respect to partial saturation free gas hydrate and pore pressure behaviour or lithologies it is of critical importance to be able to identify the different processes which can lead to hazardous situations which include establishing recurrence intervals timing and frequencies through event recognition and age control and magnitudes so that proper mitigation measures can be developed in this perspective the smaller scale instabilities deserve much attention as there are many instances where such features had far reaching consequences for society e g nice finneidfjord in that perspective human interferences e g exploitation drilling blasting loading must be one of the factors that should be taken into consideration

morphologic features of the marmara sea from multi beam data abstract the bathymetrical image of the marmara sea nw of turkey was prepared using data from mainly multi beam surveys and from various other sources the marmara sea presents a unique and complicated morphological system which is mainly controlled by the active tectonic regime of the north anatolian fault zone nafz the sub bottom morphology of the marmara sea consists of a wide shelf area 55 of the total area and the marmara trough which includes the slopes ridges and deep basins the four basins separated by ridges constitute the e w trending marmara trough in an elliptical form submarine canyons and landslides constitute the secondary morphological units of the marmara sea the angle and the shape of the slopes are variable in different parts of the marmara sea the linear shaped slopes bounding the cinarcik basin eastern extremity of the marmara sea and off the ganos mountain system west of marmara sea are characteristic with their high gradient and resulted from strike slip fault the curve shaped slopes with low angles are considered to be associated with normal fault e w aligned lineaments are morphological expressions of strike slip faults and can be traced from the ganos mountain system through cinarcik basin cutting through the basins and ridges in the western marmara trough and bounding the cinarcik basin at the slope basin transitions these lineaments reflect a new rupture of nafz and must have occurred after the formation of the marmara trough

onset of submarine debris flow deposition far from original giant landslide submarine landslides can generate sediment laden flows whose scale is impressive individual flow deposits have been mapped that extend for 1 500 km offshore from northwest africa these are the longest run out sediment density flow deposits yet documented on earth this contribution analyses one of these deposits which contains ten times the mass of sediment transported annually by all of the world s rivers understanding how this type of submarine flow evolves is a significant problem because they are extremely difficult to monitor directly previous work has shown how progressive disintegration of landslide blocks can generate debris flow the deposit of which extends downslope from the original landslide we provide evidence that submarine flows can produce giant debris flow deposits that start several hundred kilometres from the original landslide encased within deposits of a more dilute flow type called turbidity current very little sediment was deposited across the intervening large expanse of sea floor where the flow was locally very erosive sediment deposition was finally triggered by a remarkably small but abrupt decrease in sea floor gradient from 0 05 degrees to 0 01 degrees this debris flow was probably generated by flow transformation from the decelerating turbidity current the alternative is that non channelized debris flow left almost no trace of its passage across one hundred kilometres of flat 0 2 degrees to 0 05 degrees sea floor our work shows that initially well mixed and highly erosive submarine flows can produce extensive debris flow deposits beyond subtle slope breaks located far out in the deep ocean

elastic piezosensitivity of porous and fractured rocks understanding the stress dependencies of seismic velocities is important for interpreting a variety of seismic data ranging from amplitude versus offset avo and velocity analysis to overpressure prediction and 4d seismic monitoring of reservoirs sometimes rather complex forms of these dependencies based on specific models of porous space geometry are used for example spherical contacts models duffy and mindlin 1957 merkel et al 2001 duffy merkel 2001 and crack contacts models gangi and carlson 1996 carcione and tinivella 2001 have been used in different studies gangi96 however usually the pore pressure velocity dependence along with the velocity dependence on the confining pressure are phenomenologically described by the following simple relationship zimmerman et al 1986 eberhart phillips et al 1989 freund 1992 jones 1992 prasad and manghnani 1997 khaksar et al 1999 carcione and tinivella 2001 kirstetter and macbeth 2001

the coulomb critical taper theory applied to gravitational instabilities the critical coulomb wedge theory has been widely applied to the structural evolution of accretionary wedges and fold and thrust belts but it also predicts the spreading of a wedge under gravity this solution may be applied to the formation of gravitational spreading and gliding along passive margins where elevated pore fluid pressure is common following the initial hypotheses of the theory we provide an alternative expression of the exact solution for a noncohesive wedge better suited to slope instabilities our formulation allows a direct calculation of the fluid pressure required for the system to deform and predicts two kinds of gravitational deformation shallow slumping and or deep gravitational spreading rooting on the basal detachment to verify the predictions of the model we performed scaled experiments with pore pressure under conditions close to the critical taper hypotheses the good agreement between our experimental results and the theory confirms the applicability of the critical taper model to the formation of gravitational structures in sedimentary wedges subjected to pore fluid pressure

continental fragmentation along the south scotia ridge transcurrent plate boundary ne antarctic peninsula abstract the study of the south scotia ridge on the basis of swath bathymetry multichannel seismic and magnetometry profiles obtained during the hesant92 93 cruise and complemented with satellite gravimetry and seismicity data illustrates the tectonics of the region the thinned continental crust fragments of the ridge are bounded by oceanic crust of the scotia sea to the north and powell basin to the south the northern boundary represents the contact between the scotia and antarctic plates this boundary is a sinistral transpressional fault with transtensional segments and moderate recent tectonic activity another fault located at the southern boundary appears inactive and does not reveal any features that would enable the kinematics to be determined both faults have associated steep scarps since they separate oceanic and continental crust types the most significant active deformation lies in the axial depression of the ridge within a band delineated by fault systems with wsw ene and sw ne strikes these faults develop pull apart basins which separate the northern and southern blocks of the ridge the northern block is being fragmented from the antarctic plate by a zone of transtensive faults and is probably a crustal element independent of the antarctic plate the axial depression which crosses the ridge slightly obliquely is characterized by deep basins locally more than 5000 m deep and associated high seismicity the fault geometry and earthquake focal mechanisms indicate an active sinistral transtensive regime for the fault system although it may locally have transpressive regimes depending on the fault plane and the stress field orientations the internal basins are characterized by an asymmetric development showing itself as depositional wedges generally thickening northward deposits onlap the southern margins and are affected by normal faults in the northern margins the seismicity around the scotia plate shows that the present stresses are compressive along the northern boundary with the south american plate 1 sw ne and subhorizontal and along the western boundary with the antarctic plate 1 wnw ese and subhorizontal for the south scotia ridge however 1 is steeply inclined and 3 is subhorizontal with a nw se trend the stress distribution in bransfield strait is similar to the ridge and the recent extension could be partially explained by the westward continuation of the active fault system of the central south scotia ridge the fragmentation of continental crustal blocks due to the tectonic activity along the transcurrent plate boundaries is a mechanism that contributes to the deformation of the northeastern end of the antarctic peninsula this area appears appropriate for the analysis of continental plate fragmentation processes

stress field evolution of the southernmost andean cordillera from paleostress analysis argentine tierra del fuego this work has been funded by projects ctm2011 30241 c02 02 ctm201460451 c2 2 p and ctm2017 89711 c22 p of the spanish r d national plan and by pict funding from anpcytfoncyt argentina

a new conceptual methodology for interpretation of mass transport processes from seismic data abstract identification and seismic mapping of mass transport deposits mtds are vital targets for marine geological studies both for a better understanding of mass wasting processes and geohazards and for economic prospects in sedimentary basins in recent decades refinements in the interpretation of these geobodies have benefited from increasingly good quality 3d seismic data however approaches to define the characteristics rheology and mechanics of such slope failure deposits still rely mainly on inferences from case dependent interpretations of these stratigraphic elements what is more features and seismic characteristics of mtds may vary significantly from one case to another implying the existence of many different environments and related physics this makes the study of submarine mass movement a challenging task for a seismic interpreter in this paper we present a new conceptual analytical method based on an objective approach for interpreting the wide range of diverse objects related to mass wasting in order to minimize seismic interpretation subjectivity we propose an ontology like methodology based on a conceptual organization of a diversity of interpretation elements arranged in a knowledge base mtds are considered as objects with representative properties each one characterized by several descriptors which are themselves impacted by multiple physical processes in a graph based conception we thus propose a method to infer the most probable interpretations for one mass movement from its deposit characteristics we applied our graph based methodology to two mtds delineated in 3d seismic data in the offshore amazon basin brazil based on the analysis of all mtd properties and their possible causes several candidate interpretations were provided the interpretations yielded by the graph are in line with the known geology and instability processes of the region thereby validating the feasibility of the approach the next development stage is a numerical definition of the knowledge base for further sharing and operability

active crustal fragmentation along the scotia antarctic plate boundary east of the south orkney microcontinent antarctica abstract the structure of the scotia antarctic plate boundary is poorly known east of the south orkney microcontinent new multichannel seismic profiles together with magnetic gravity and swath bathymetry data obtained during the scan97 cruise show a complex relief of raised blocks and elongated depressions that may reach more than 6000 m in depth these depressions develop in relation with extensional active structures and constitute an uncommon feature in the oceans where most of the trenches are formed in subduction contexts the main crustal elements of the area include the oceanic crust of the scotia plate the discovery bank composed of continental crust a tectonic domain with intermediate features between continental and oceanic crusts that includes the southern bank and the oceanic crust of the northern weddell sea representing the antarctic plate the intermediate domain was probably developed during the late cenozoic subduction of the weddell sea oceanic crust below the discovery bank the fault zone associated with the plate boundary is characterized at present by sinistral transcurrent and transtensional slips which develop a ne sw elongated deep pull apart basin with extreme crustal thinning and mantle uplift the complex bathymetry and structure of the plate boundary are consequences of the presence of continental and intermediate crusts where the deformations are concentrated between the two stable oceanic domains the location of a major part of the plate boundaries around the scotia arc is probably determined by the position of the continental and intermediate crustal fragments surrounded by oceanic crust due to the differential behavior experienced during deformation

tectonic reconstructions for paleobathymetry in drake passage abstract a minimum complexity tectonic reconstruction based on published and new basin opening models depicts how the scotia sea grew by cenozoic plate divergence dismembering a jurassic sheared margin of gondwana part of the jurassic early cretaceous ocean that accreted to this margin forms the core of the central scotia plate the arc plate above a trench at the eastern end of the scotia sea which migrated east away from the antarctic and south american plates a sequence of extensional basins opened on the western edge of the central scotia plate at 50 30 ma decoupled from the south american plate to the northwest by slow motion on a long transform fault succeeding the basins seafloor spreading started around 30 ma on the west scotia ridge which propagated northwards in the 23 17 ma period and ceased to operate at 6 ma the circuits of plate motions inside and outside the scotia arc are joined via rotations that describe antarctic central scotia plate motion in powell basin until 20 ma and along the south scotia ridge thereafter the modelled relative motion at the northern edge of the scotia sea is thus constrained only by the plate circuit but nonetheless resembles that known coarsely from the geological record of tierra del fuego a paleobathymetric interpretation of nine time slices in the model shows drake passage developing as an intermediate depth oceanographic gateway at 50 30 ma with deep flow possible afterwards initially this deep flow would have been made tortuous by numerous intermediate and shallow barriers a frontal pattern resembling that in the modern scotia sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the scotia sea at 18 5 ma at shag rocks passage and after 10 ma southeast of south georgia

deep sea volcaniclastic sedimentary systems an example from la fournaise volcano reunion island indian ocean a volcaniclastic sedimentary fan extending to water depths of 4000 m is characterized using gravity cores camera surveys high resolution sonar images seismic records and bathymetry from the submarine portion of la fournaise volcano reunion island a basaltic shield volcano in the sw indian ocean three main areas are identified from the study 1 the proximal fan extending from 500 m water depth down to 2000 m water depth 2 the outer fan extending from 2000 m water depth down to 3600 m water depth 3 the basin extending beyond 3600 m water depth within these three main areas seven distinct submarine environments are defined the proximal fan is characterized by volcanic basement outcrops sedimentary slides deep water deltas debris avalanche deposits and eroded floor in the valley outlets the outer fan is characterized by a discontinuous fine grained sedimentary cover overlying coarse grained turbidites or undifferentiated volcanic basement the basin is characterized by hemipelagic muds and fine grained turbidites interbedded with sandy and gravelly turbidite lobes the evolution of the deep sea volcaniclastic fan is strongly influenced by sector collapses such as the one which occurred 0 0042 ma ago this collapse produced a minimum of 6 km3 of debris avalanche deposit in the proximal area the feeding regime of the deep sea fan is alluvial dominated before the occurrence of any sector collapse and lava dominated after the occurrence of a sector collapse the main deep water lava fed delta is prograding among the blocks of the debris avalanche deposits as a result of turbidity flows occurring on the delta slope these turbidity flows are triggered routinely by wave action earthquakes and accumulation of new volcanic debris on top of the deltas both turbidity currents triggered on the deep water delta slope and those triggered by debris avalanche reworked volcaniclastic material as far as 100 km from the shore line

numerical modelling of the gravity induced destabilization of a slope the example of the la clapiere landslide southern france abstract a finite difference two dimensional model with hooke mohr coulomb properties and topography derived from the dem are used to reproduce the la clapiere landslide the principal factor defining the gravity driven destabilization of the model is a gradual reduction in the cohesion this reduction simulates a degradation of the material properties with time because of weathering alteration processes the inelastic deformation fracturing and faulting first occur at mountain scale and results in normal fault formation causing crest sagging later the failure process is concentrated in the lower part of the slope and leads to the formation of a localized fault subparallel to the slope surface at a depth of ca 100 m this corresponds to the initiation of the la clapiere landslide and its propagation upslope a slow crest sagging continues during the whole model evolution

occurrence and origin of submarine plunge pools at the base of the us continental slope high resolution bathymetric data from the new jersey and californian continental margins show a marked depression running along parts of the base of the continental slope detailed analysis reveals that the depressions are a series of discrete plunge pools with associated downslope topographic ramparts we have used new bathymetric data to create our own data base of over 150 examples and systematically analyse plunge pool morphology and location previous observations of plunge pools have been sparse plunge pools are up to 1100 m wide and 75 m deep with a mean diameter of 400 m and a mean depth of 21 m plunge pools only occur where there are sharp decreases in slope of more than 4 and are well developed where changes in slope exceed 15 we propose plunge pools can be created by two mechanisms firstly they may be due to reduced bed shear stress downstream of hydraulic jumps in submarine sediment laden density flows that causes the deposition of bedload and the creation of a sediment bar this bar then defines the downslope margin of a pool secondly the impact of high momentum sediment laden density flows can excavate a depression as has been observed for subaerial snow avalanches sediment deposited downslope of these impact pools is very poorly sorted and partly derived from erosion within the pool both mechanisms influence whether turbidity currents are generated from high density sediment laden density flows influence whether depositional flows are channelised and have implications for base of slope facies models

pore filling material and its effect on velocity in rocks a new method estimates the dependence of p and s velocities in rocks on elastic properties of the pore filling material by going beyond gassmann s relations and accounting for pore filling materials with nonzero shear stiffness the resulting estimates are in good agreement with experimental data and gassmann s relations for values of velocity in water saturated rocks predicted from velocity measured on dry rocks the method successfully predicts the temperature dependence of velocity 1 in frozen rocks at the ice water phase transition and 2 in rocks saturated with heavy hydrocarbon undergoing solid liquid phase transition

inverting for reservoir pressure change using time lapse time strain application to genesis field gulf of mexico there are increasing numbers of published examples from around the world in which significant 4d time shifts have been observed in the overburden above producing reservoirs

initial stages of oceanic spreading in the bransfield rift from magnetic and gravity data analysis abstract bransfield basin a 500 km long and 100 km wide extensional structure with a well marked ne sw orientation is considered a back arc basin developed since the pliocene and associated with subduction of the former phoenix plate below the south shetland islands block extension also occurs in this area as a consequence of the end of the sinistral fault zone that deforms the south scotia ridge on the basis of potential field data from marine cruises we provide new magnetic and bouguer gravity maps of the area at sea level we have characterized the central magnetic anomaly by using euler deconvolution method spectral analysis and forward modeling obtaining a thin 1 5 km and shallow 4 km b s l layer and a low total magnetization 2 6 a m the forward modeling was constrained on basis of previous seismic refraction studies our models show two situations the first presents a uniform density values along the entire crust in the basin this would be compatible with rifting in a more advanced stage or even an oceanic crust in its earliest stages while the second would support the existence of a stretched thinned and altered crust through the injection of volcanic material in the light of these models analysis of the new potential field maps presented in this work and information from previous studies we consider that the central bransfield basin is in a rifting in its latest stages or presents an incipient oceanic crust formed by recent oceanic spreading

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

morphology age and sediment dynamics of the upper headwall of the sahara slide complex northwest africa evidence for a large late holocene failure the sahara slide complex in northwest africa is a giant submarine landslide with an estimated run out length of 900 km we present newly acquired high resolution multibeam bathymetry sidescan sonar and sub bottom profiler data to investigate seafloor morphology sediment dynamics and the timing of formation of the upper headwall area of the sahara slide complex the data reveal a 35 km wide upper headwall opening towards the northwest with multiple slide scarps glide planes plateaus lobes slide blocks and slide debris the slide scarps were generated by retrogressive failure events associated with two types of mass movements translational sliding and gravitational spreading as a result of this evolution three different glide planes gp i ii and iii can be distinguished approximately 100 m 50 m and 20 m below the seafloor these glide planes are widespread and suggest failure along pronounced continuous weak layers our data suggest an age of only about 2 ka for the failure of the upper headwall area a date much younger than that derived for landslide deposits on the lower reaches of the sahara slide complex which are dated at 50 60 ka the young age of the failure contradicts the postulate of a stable slope offshore northwest africa during sea level highstands such an observation suggests that submarine landslide risk along the continental margin of northwest africa should be reassessed based on a robust dating of proximal and distal slope failures

a potential barrier to deep antarctic circumpolar flow until the late miocene the central scotia sea located between the south american and antarctic plates is an integral part of the marine conduit that permits eastward deep water flow from the pacific ocean to the atlantic ocean the geologic history of the central scotia sea is therefore critical for a full understanding of the initiation and subsequent evolution of the complete deep antarctic circumpolar current widely believed to have been a key factor in the history of antarctic glaciation here we present new evidence on the nature and age of the central scotia sea floor multibeam surveys and the first dredged samples indicate that a now submerged remnant volcanic arc may have formed a barrier to deep eastward oceanic circulation until after the mid miocene climatic optimum inception and development of a full deep antarctic circumpolar current may therefore have been important not in the drop in global temperatures at the eocene oligocene boundary as long surmised but in the subsequent late miocene global cooling and intensification of antarctic glaciation

amundsen sea sediment drifts archives of modifications in oceanographic and climatic conditions abstract drift deposits document stages of particular dynamic bottom currents and associated sedimentary transport activities the analysis of seismic reflection data from the amundsen sea southern pacific ocean reveals sediment drift formation already in eocene oligocene times this observation indicates bottom current activity and hence a cold climate for the late palaeogene in an area which today lies under the influence of antarctic bottom water aabw originating in the ross sea the generation of sediment drifts is accompanied by the occurrence of mass transport deposits leading to the identification of a phase of strong ice sheet expansion 15 4 ma which due to a change in ice regime from wet to dry based was followed by less material input during the last 4 ma

seismotectonics and relative plate motions in the scotia sea region source parameter inversion of p and sh waveforms and amplitudes for 20 shallow earthquakes using a singular value decomposition technique provides important constraints on the tectonics and relative plate motions of the scotia sea region the derived focal mechanisms show both thrust and strike slip faulting along the north scotia ridge and normal and strike slip faulting along the south scotia ridge two thrust faulting mechanisms south of tierra del fuego indicate diffuse convergence of the antarctic plate along the western margin of the scotia plate earthquakes are consistent with active subduction and back arc spreading along the south shetland trench and bransfield strait the depths of two earthquakes 35 km and 55 km and one shallow thrust faulting event suggest continued subduction along the south shetland trench but the underthrusting is largely aseismic due to the young plate age and slow subduction rate the magnitude and depth of normal faulting earthquakes along bransfield strait and the south scotia ridge are suggestive of diffuse extension rather than typical organized mid ocean spreading which is generally associated with smaller and shallower earthquakes eight strike slip and thrust faulting events provide well constrained slip vectors showing scotia plate motion relative to the surrounding larger plates using a four plate model scotia sc south sandwich ss antarctica an and south america sa and a fixed sa an euler vector we inverted these data together with published centroid moment tensor slip vectors along the south sandwich trench and published marine magnetic data from the ss back arc spreading center to derive the first quantitative plate motion model for the scotia region the sc sa euler pole is located in the south atlantic and shows large latitudinal uncertainties the sc an pole is better constrained and located in the weddell sea these results predict left lateral strike slip motion with a component of compression along the north scotia ridge left lateral strike slip with a component of extension along the south scotia ridge and east west compression in drake passage although no rate data have been published connecting the scotia plate with the global plate system euler vector magnitudes are obtained through closure the results though poorly constrained suggest relative motion rates of 0 5 cm yr along the north scotia ridge and 1 0 cm yr along the south scotia ridge and in drake passage

opening history of powell basin antarctic peninsula the opening of powell basin was part of the regional response to n55 w relative plate motion of south america away from antarctica which led to the formation of drake passage during the eocene and oligocene restoration of microplates around the basin using gridded magnetic anomalies from its margins illustrates the pre break up continuity of the pacific margin anomaly magnetic high associated with a mesozoic arc batholith newly compiled magnetic anomaly data over the powell basin show subdued linear seafloor spreading type anomalies these are used together with marginal and regional geology to constrain the opening history of the basin magnetic reversal modelling suggests that slow spreading in powell basin probably occurred between 29 7 ma and 21 8 ma following rifting of mesozoic continental crust with associated break up volcanism a simple two phase model for the rotation of the south orkney microcontinent away from the antarctic peninsula accounts for the pattern of magnetic reversals recorded in powell basin and for the structure of its margins

galicia bank sediment transport activity in response to continuous sedimentary instability dynamics a geotechnical perspective the sedimentary instability dynamics occurring over time throughout the isolated galicia bank atlantic ocean nw iberian peninsula have been studied using a sedimentological and geotechnical approach featuring a quantitative assessment of slope stability under different scenarios including earthquake activity the erosion of the scarps in the galicia bank includes the continuous development of small instabilities evacuated along gullies and channels or deposited on slopes these deposits may be subsequently mobilized evolving into new sedimentary gravity flows e g turbidity flow the studied sediments consist mostly of poorly to very poorly sorted sands and silts transported by turbidity currents and occasionally by debris flow processes the sediments in the study area identified as normally consolidated and located on gently sloping areas gradients less than 5 may become unstable if low magnitude seismic events occur pga 10 without any trigger other than oversteepening in contrast overconsolidated sediments may remain stable under static conditions and may become unstable on slope gradients 10 when earthquakes occur with the maximum peak ground acceleration pga 0 34 the sedimentological and geotechnical models presented herein are complementary approaches that can be utilized to understand the long term sedimentary instability dynamics observed within the study area such results are critical for better understanding sedimentary models of the dismantling processes of deep seamounts located far away from continental sedimentary inputs

scaling behaviour of clay rich submarine debris flows we perform a statistical study of submarine debris flows based on recent data from the storegga area off the western coast of norway due to the high quality of the data and the uniformity of materials the data from the storegga database are far less dispersed than world wide compilations available so far allowing for a fit with good statistical significance we find that both the run out and the ratio between fall height and run out are best described as a power law function of the volume for small to moderate volumes visco plastic models are able to reproduce the power law in contrast to granular frictional models however either progressive mixing of the shear layer with ambient water or hydro planing must be invoked in order to explain the extreme run out distances of larger debris flows

mass wasting processes in the western wilkes land margin possible implications for east antarctic glacial history abstract multichannel seismic data collected off western wilkes land east antarctica reveal widespread sediment mass movement events mainly in the form of mega debris flows deposited on the continental rise in places they have a seaward extent of more than 200 km and a thickness of up to 400 m debris flow units are commonly separated by thin deposits of well stratified facies interpreted as glaciomarine mixed contouritic and distal turbidite deposits possibly with some hemipelagic sediments these units due to a dominant contouritic component represent weakened layers and could then play a major role in the slope instability based on stratigraphic correlation with dsdp site 268 it is suggested that slope failures in the western wilkes land occurred mostly during the miocene inferred high sedimentation rates due to large amounts of sediment delivered from a temperate wet based ice sheet would have constituted a key factor in the sediment failures the distribution of mass wasting related features appears also strongly related to the amount of meltwater discharge the main trigger mechanism would probably have been earthquakes enhanced by isostatic rebounds following major ice sheet retreats

deriving effects of pressure depletion on elastic framework moduli from sonic logs we have developed a method to determine the effect of pore pressure depletion on elastic framework moduli by utilizing sonic logs from wells drilled at different locations through a reservoir at varying depletion stages this is done by first inverting the sonic logs for elastic framework bulk and shear moduli thus carefully removing pressure dependent fluid effects by crossplotting these elastic framework moduli against an increase in net stress which is directly related to depletion we derive the stress sensitivity of the elastic framework moduli we found that the observed stress sensitivity was consistent with time lapse seismic results and that the sensitivity derived from the sonic logs was much smaller than predicted by hydrostatic measurements on core samples this method is applicable to depletion scenarios in mature fields and can be used both for modelling and inverting time lapse seismic data for effects of pore pressure depletion on seismic data

tectonic activity evolution of the scotia antarctic plate boundary from mass transport deposit analysis the spatial distribution and temporal occurrence of mass transport deposits mtds in the sedimentary infill of basins and submerged banks near the scotia antarctic plate boundary allowed us to decode the evolution of the tectonic activity of the relevant structures in the region from the oligocene to present day the 1020 mtds identified in the available data set of multichannel seismic reflection profiles in the region are subdivided according to the geographic and chronological distributions of these features their spatial distribution reveals a preferential location along the eastern margins of the eastern basins this reflects local deformation due to the evolution of the scotia antarctic transcurrent plate boundary and the impact of oceanic spreading along the east scotia ridge esr the vertical distribution of the mtds in the sedimentary record evidences intensified regional tectonic deformation from the middle miocene to quaternary intensified deformation started at about 15 ma when the esr progressively replaces the west scotia ridge wsr as the main oceanic spreading center in the scotia sea coevally with the wsr demise at about 6 5 ma increased spreading rates of the esr and numerous mtds were formed the high frequency of mtds during the pliocene mainly along the western basins is also related to greater tectonic activity due to uplift of the shackleton fracture zone by tectonic inversion and extinction of the antarctic phoenix ridge and involved changes at late pliocene the presence of mtds in the southern scotia sea basins is a relevant indicator of the interplay between sedimentary instability and regional tectonics

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

sandbox experiments on gravitational spreading and gliding in the presence of fluid overpressures whereas in previous analogue experiments on gravitational spreading and gliding detachment occurred on a ductile layer we have used a relatively new technique of injecting compressed air into sand packs so as to simulate the effects of fluid overpressures in sedimentary strata and to trigger slope instabilities in our experiments the governing equations yield scales for dimensions stresses and fluid pressure however the more transitory phenomena of production and decrease of overpressure cannot be suitably scaled by using layers of differing permeability we are able to produce sharp detachments in models made of sand alone the experiments involve gravity spreading or gravity gliding in gravity spreading propagation of the detachment and of extensional deformation depends on the fluid pressure for medium values of fluid overpressure normal faults are closely spaced numerous and bound rotated blocks they propagate progressively toward the back of the model for the highest pressures the deformation propagates very fast and faults bound non rotated blocks which slide on an efficient basal detachment fault dips are also controlled by fluid pressure and by frictional resistance at the base to model gravitational gliding required an apparatus with a more complex system of air injection we did a series of experiments using injection windows of various lengths and compared the results with predictions from a quasi 3d analytical model of sliding in contrast with predictions for an infinite slope sliding depends on 1 the fluid overpressure on the detachment 2 the fluid overpressure in the body of the sliding sheet and 3 the shape of the detachment surface in particular we show that frictional resistance at the lower edge is a primary control on the dynamics of gliding

a new bathymetric compilation for the south orkney islands region antarctic peninsula 49 39 w to 64 59 s insights into the glacial development of the continental shelf we present a new high resolution 300 m bathymetric grid of the continental shelf surrounding the south orkney islands northeast of the antarctic peninsula the new grid derived from a compilation of marine echo sounding data improves previous regional bathymetric representations and helps to visualize the morphology of the shelf in unrivalled detail the compilation forms important baseline information for a range of scientific applications and end users including oceanographers glacial modelers biologists and geologists in particular due to the limited understanding of glacial history in this region the bathymetry provides the first detailed insights into past glacial regimes the continental shelf is dominated by seven glacially eroded troughs marking the pathways of glacial outlets that once drained a former ice cap centered on the south orkney islands during previous glacial periods grounded ice extended to the shelf edge north of the islands a large 250 km long sediment depocenter interpreted as a maximum former ice limit of one or more cenozoic glaciations suggests that ice was only grounded to the 300 350 m contour in the south hypsometric analyses support this interpretation indicating that a significant proportion of the shelf has been unaffected by glacial erosion using these observations we propose a preliminary ice cap reconstruction for maximum glaciation of the south orkney plateau suggesting an ice coverage of about 19 000 km2 the timing of maximum ice extent number of past advances and pattern of subsequent deglaciation s remain uncertain and will require further targeted marine geological and geophysical investigations to resolve

shackleton fracture zone no barrier to early circumpolar ocean circulation the opening of southern ocean gateways was critical to the formation of the antarctic circumpolar current and may have led to cenozoic global cooling and antarctic glaciation drake passage was probably the final barrier to deep circumpolar ocean currents but the timing of opening is unclear because the shackleton fracture zone could have blocked the gateway until the early miocene geophysical and geochemical evidence presented here suggests that the shackleton fracture zone is an oceanic transverse ridge formed by uplift related to compression across the fracture zone since ca 8 ma hence there was formerly i e in the miocene no barrier to deep circulation through drake passage and a deep water connection between the pacific and atlantic oceans was probably established soon after spreading began in drake passage during the early oligocene

motion of the scotia sea plates earthquake data from the scotia arc to early 2002 are reviewed in the light of satellite gravity and other data in order to derive a model for the motion of plates in the scotia sea region events with magnitude greater than or equal to5 which occurred on or near the boundaries of the scotia and sandwich plates and for which centroid moment tensor cmt solutions are available are examined the newer data fill some of the previous sampling gaps along the boundaries of the scotia and sandwich plates and provide tighter constraints on relative motions variations in the width of the brunhes anomaly on evenly spaced marine magnetic profiles over the east scotia ridge provide new estimates of scotia sandwich plate spreading rates since there are no stable fracture zones in the east scotia sea the mean azimuth of sea floor fabric mapped by sidescan is used to constrain the direction of spreading 18 newrate estimates and four azimuths from the east scotia ridge are combined with 68 selected earthquake slip vectors from the boundaries of the scotia sea in a least squares inversion for the best fitting set of euler poles and angular rotation rates describing the present day motions of the scotia and sandwich plates relative to south america and antarctica our preferred model tlp2003 gives poles that are similar to previous estimates except for scotia plate motion with respect to south america which is significantly different from earlier estimates predicted rates of motion also differ slightly our results are much more robust than earlier work we examine the implications of the model for motion and deformation along the various plate boundaries with particular reference to the north and south scotia ridges where rates are obtained by closure

linking continental slope failures and climate change testing the clathrate gun hypothesis it has been suggested that the release of clathrates rather than expansion of wetlands is the primary cause of the rapid increases observed in the ice core atmospheric methane record during the pleistocene because submarine sediment failures can involve as much as 5000 gt of sediment and have the capacity to release vast quantities of methane hydrates one of the major tests of the clathrate gun hypothesis is determining whether the periods of enhanced continental slope failure and atmospheric methane correlate to test the clathrate gun hypothesis we have collated published dates for submarine sediment failures in the north atlantic sector and correlated them with climatic change for the past 45 k y more than 70 by volume of continental slope failures during the past 45 k y was displaced in two periods between 15 and 13 ka and between 11 and 8 ka both these intervals correlate with rising sea level and peaks in the methane record during the bolling allerod and preboreal periods these data support the clathrate gun hypothesis for glacial interglacial transitions the data do not however support the clathrate gun hypothesis for glacial millennial scale climate cycles because the occurrence of sediment failures correlates with heinrich events i e lows in sea level and atmospheric methane a secondary use of this data set is the insight into the possible cause of continental slope failures glacial period slope failures occur mainly in the low latitudes and are associated with lowering sea level this finding suggests that reduced hydrostatic pressure and the associated destabilization of gas hydrates may be the primary cause the bolling allerod sediment failures were predominantly low latitude suggesting an early tropical response to deglaciation e g enhanced precipitation and sediment load to the continental shelf or warming of intermediate waters in contrast sediment failures during the preboreal period and the majority of the holocene occurred in the high latitudes suggesting either isostatic rebound related earthquake activity or reduced hydrostatic pressure caused by isostatic rebound causing destabilization of gas hydrates

the international bathymetric chart of the southern ocean ibcso version 1 0 a new bathymetric compilation covering circum antarctic waters the international bathymetric chart of the southern ocean ibcso version 1 0 is a new digital bathymetric model dbm portraying the seafloor of the circum antarctic waters south of 60 degrees s

post rift sedimentary evolution of the gebra debris valley a submarine slope failure system in the central bransfield basin antarctica abstract the combined analysis of paleomorphology stratigraphy and seismic facies allows us to present new insights into the formation and evolution of the gebra valley located on the lower continental slope of the central bransfield basin ne antarctic peninsula repeated large scale slope failure events were responsible for the cut and fill features forming the gebra valley this study revealed a mid pleistocene gebra paleovalley that was progressively and completely infilled during the last glacial cycle the infilled paleovalley was reoccupied forming the present day gebra valley both valley incisions are genetically related to large scale failures associated with high energy gravity flows or mass flows the infilling of the valley involved channelized mass flows of various dimensions and channelized and unchannelized turbidity currents alternating erosive periods during which the valley evacuated sediment from the slope toward the basin and depositional periods during which it was fully infilled allows it to be defined as the gebra debris valley taking into account the presence of faults controlling the headwall locations and the stratigraphic correlation established with glacial periods the genesis of the gebra debris valley could have been controlled by the interplay of both the tectonic history of the bransfield basin and the glacial cycles that allowed grounding events to reach the upper continental slope

earthquake induced flooding and paleoseismicity of the el asnam algeria fault related fold late quaternary activity of the el asnam thrust fault responsible for the october 10 1980 ms 7 3 earthquake and 36 km coseismic faulting is explored through diverse paleoseismological field methods we trenched through the main and secondary 1980 coseismic scarps made an analysis of earthquake induced flood deposits conducted a geomorphic study of leveling profiles and offset geological units across thrust and normal scarps and considered the geomorphology of the active fold twenty five radiocarbon ages from colluvial and alluvial deposits along with conventional fossils and artefacts dating of geological units show the timing of successive net vertical displacements on the thrust related fold and bending moment faults previous work showed that eight pre 1980 earthquakes occurred in clusters during the late holocene cumulative net vertical slip that amounts to 3 times the 1980 displacement is observed on multiple thrust fault scarps and offsets of geological units on bending moment faults maximum cumulative vertical displacements which are visible in different zones along the fault segments can reach 10 to 25 times the 1980 coseismic displacements the late quaternary uplift rate on the el asnam fault is 0 6 mm yr and taking into account slip variation along the fault this rate may range between 0 25 1 6 mm yr from which we obtain a shortening rate of 0 17 1 2 mm yr because the tell atlas mountains exhibit comparable seismogenic fault related folds with parallel and sequential structures of active faults a total average shortening rate of 2 05 mm yr during the late quaternary represents part of the convergence accommodated along the africa eurasia plate boundary

volcanic evolution of the south sandwich volcanic arc south atlantic from multibeam bathymetry abstract new multibeam bathymetry data are presented for the south sandwich intra oceanic arc which occupies the small sandwich plate in the south atlantic and is widely considered to be a simple end member in the range of intra oceanic arc types the images show for the first time the distribution of submarine volcanic tectonic and erosional depositional features along the whole length of the 540 km long volcanic arc allowing systematic investigation of along arc variations the data confirm that the volcanic arc has a simple structure composed of large volcanoes which form a well defined volcanic front but with three parallel cross cutting seamount chains extending 38 60 km from near the volcanic front into the rear arc there is no evidence for intra arc rifting or extinct volcanic lines topographic evidence for faulting is generally absent except near the northern and southern plate boundaries most of the volcanic arc appears to be built on ocean crust formed at the associated back arc spreading centre as previously proposed from magnetic data but the southern part of the arc appears to be underlain by older arc or continental crust whose west facing rifted margin facing the back arc basin is defined by the new bathymetry the new survey shows nine main volcanic edifices along the volcanic front and ca 20 main seamounts the main volcanoes form largely glaciated islands with summits 3 0 3 5 km above base levels which are 2500 3000 m deep in the north and shallower at 2000 2500 m deep in the south some of the component seamounts are interpreted to have been active since the last glacial maximum and so are approximately contemporaneous with the volcanic front volcanism seven calderas all either submarine or ice filled have been identified adventure volcano a newly discovered submarine volcanic front caldera volcano is described for the first time all but one of the calderas are situated on summits of large volcanoes in the southern part of the arc and most are associated with current or historic volcanic or hydrothermal activity shallow shelves around the islands are generally 1 10 km wide submerged banks up to 1100 m deep are interpreted as subsided erosional surfaces seamounts and emergent volcanoes experienced a range of mass wasting processes including by landsliding and smaller mass flows

sediment subduction subduction erosion and strain regime in the northern south sandwich forearc 1 the first swath bathymetry and side scan sonar imagery from the south sandwich forearc reveals detailed seafloor morphology tectonic fabric and sedimentary features in an area where plate convergence is approximately normal to the trench simultaneously collected seismic reflection gravity and magnetic data provide information about the structure and composition of the forearc crust interpretation of these data together with the marine magnetic record of seafloor spreading in the east scotia sea back arc basin since 15 ma has enabled quantitative estimation of sediment subduction and subduction erosion rates any accretionary prism present is constrained to be very small extending 6 km or less from the trench the contrast between this result and a time integrated estimate of potential accretionary prism volume suggests that 95 of the sediment which has entered the trench since 15 ma has been subducted the position of the south sandwich island arc in relation to back arc magnetic lineations implies that the arc has migrated 70 km westward relative to the sandwich plate since 15 ma taking account of published observations concerning the typical geometric evolution of arc trench systems the average rate of forearc slope retreat during this interval is inferred to be 3 1 4 7 km myr 1 gravity modeling suggests that the forearc crust averages 10 km in thickness within 110 km of the trench for 10 km thick forearc crust the rate of slope retreat implies an average rate of subduction erosion of forearc crust of 31 47 km3 km 1 myr 1 at the present convergence rate of 74 km myr 1 this erosion rate requires a 420 635 m thick layer of subducting material derived from the forearc crust a time integrated estimate of the potential volume of forearc derived trench fill sediments subducted since 15 ma implies that frontal erosion can only account for 40 of total subduction erosion therefore basal erosion must significantly exceed frontal erosion the new data also provide insights concerning forearc strain regime and forearc basin evolution in the area studied extensional faulting near the trench slope break is probably caused by gravitational instability of the steep lower forearc slope a lack of extensional strain indicators in the upper forearc and arc may be a consequence of the fact that the east scotia sea is a mature back arc basin contributing significant ridge push in the vicinity of the arc the data reveal no evidence of arc parallel extension or of large serpentinite seamounts as found in the mariana forearc and a causal link between these two observations is proposed seismic profiles across the forearc basin suggest that the balance between sediment accumulation and erosion is sensitive to changes in the elevation of the trench slope break it is hypothesized that the basin is a dynamic feature which goes through repeated cycles of growth and destruction controlled by cyclic uplift and collapse of the trench slope break

stress and fluid sensitivity in two north sea oil fields comparing rock physics models with seismic observations during 4d seismic reservoir characterization it is important to have reliable rock physics models for both static e g mineralogy porosity cement volume and dynamic e g saturation pressure temperature reservoir parameters without a good understanding of reservoir geology and associated static rock physics properties it is impossible to interpret time variant changes in pore pressure and saturation andersen et al 2009 the dry rock properties of the reservoir can be obtained from well log data combined with geological information about mineral composition and rock texture and gassmann theory to estimate the effect of pore fluid changes normally core measurements are undertaken to quantify stress sensitivity but these are often affected by induced fractures caused by the coring acquisition that will enhance the stress sensitivity of the rock holt et al 2005 duffaut and landro 2007 showed how calibrated hertz mindlin contact theory could be applied to estimate stress sensitivity

an experimental investigation of gravity driven shale tectonics in progradational delta abstract many deltas exhibit gravitational deformation of their sedimentary cover in these systems the decollement layers do not always consist of rock salt but sometimes of overpressured shale unlike salt the efficiency of detachment in shale depends on the magnitude of fluid overpressures and it varies through time and space as rapid sedimentary burial progrades into deeper water as a result the gravity deformational domains are progressively translated seaward sandbox models involving high air pore pressures were used to simulate such gravity driven shale tectonics in prograding deltas models were built with sand of various permeabilities and air was injected to simulate the mechanical effects of fluid overpressure our apparatus for the injection of air allowed us to control subsurface pressures in space and time during the experiments and it was used to simulate the advance of the front of the overpressured domain during the sedimentary progradation in our models sand kept obeying a frictional behavior for medium to high pore pressures and the detachment appeared as very thin shear bands compressional belts that formed during the experiment were dominated by asymmetric basinward verging fore thrusts as is often observed in deep water shale detached foldbelts where the value of fluid pressures approached that of the lithostatic stress sand was fluidized resulting in ductile strains analogous to what occurs in highly overpressured mobile shale during progradation ancient buried thrustbelts were reactivated thereby controlling later extension during the experiments sand volcanoes analogous to mud volcanoes formed in relation with tectonic structures some of them developed near normal faults but many of them formed directly above old buried thrusts

two scale gravitational collapse in the amazon fan a coupled system of gravity tectonics and mass transport processes abstract gravity tectonics affects the entire marine sedimentary sequence of the foz do amazonas basin including the topmost amazon fan during the various stages of the margin9s evolution the sedimentary loading and bathymetric slope induced the formation of a linked extensional compressional system in which gravitational fold and thrust belts form significant structural features on the continental slope seismic analysis carried out in this study shows that sea floor relief created along these thrust belts can trigger recurrent mass transport deposits in the upper middle amazon fan suggesting a long lasting linkage between gravity tectonics and mass transport processes throughout the sedimentary evolution of the amazon fan supplementary material uninterpreted seismic sections are available at http www geolsoc org uk sup18393

seismic structure gravity anomalies and flexure of the amazon continental margin ne brazil 1 seismic and gravity data have been used to determine the structure of the sediments crust and upper mantle that underlie the amazon continental margin offshore ne brazil seismic reflection profile data reveal a major unconformity at 7 s two way travel time twtt which we interpret as marking the onset of the transcontinental amazon river and the formation of the amazon deep sea fan system in the late miocene seismic refraction data show mean sediment velocities that decrease by 1 5 km s 1 in a seaward direction we attribute this decrease to facies changes associated with sediment progradation and the development of topset foreset and bottomset beds seismic refraction data show that the sediments are underlain by oceanic crust that has a similar velocity structure compared to elsewhere in the atlantic ocean but is unusually thin 4 2 km we attribute the thin crust to either slow seafloor spreading or a limited magma supply during the initial rifting of south america and africa in the early cretaceous the seismic data have been used to construct a new sediment thickness grid that together with gravity anomaly data suggests the amazon fan loaded lithosphere with an unusually high flexural strength while a high strength lithosphere explains the overall depth of the seismic moho there are discrepancies of up to 2 km beneath the upper fan where the modeled flexed moho is shallower than the seismic moho and beneath the middle fan where it is deeper gravity and seismic modeling suggest these discrepancies are caused by lateral changes in subcrustal density such that the mantle underlying the upper fan is denser than it is beneath the middle fan we attribute these lateral density differences to proximity to the ceara rise which is believed to have formed during the late cretaceous in a mid ocean ridge setting fan loading of a relatively strong dense and hence cold lithosphere predicts stress orientations that are consistent with borehole breakout data and the location and height of the gurupe arch onshore despite its proximity to leaky transform faults the margin that underlies the amazon fan appears to be of nonvolcanic origin the main differences with other nonvolcanic margins such as west iberia and newfoundland are a greater sediment accumulation a narrower zone of transitional crust and a lack of any evidence for extreme extension and mantle serpentinization

rock physics modelling of 4d time shifts and time shift derivatives using well log data a north sea demonstration abstract rock physics models for fluid and stress dependency in reservoir rocks are essential for quantification and interpretation of 4d seismic signatures during reservoir depletion and injection for siliciclastic sandstone reservoirs the gassmann theory successfully predicts changes in seismic properties associated with fluid changes however our ability to predict the sensitivity to pressure from first principles is poor especially for cemented sandstones in this study we demonstrate how we can use a patchy cement rock physics model to quantify the combined effect of stress and fluid changes in terms of seismic time shifts and time shift derivatives during depletion or injection the time shifts are estimated directly from well log data without core calibration of stress sensitivity by assuming non uniform grain contacts where some grain contacts are cemented and others are loose we can combine the contact theory for cemented sandstones with the contact theory for loose sands in order to predict stress sensitivity in a patchy cemented sandstone reservoir time shift derivatives are also useful estimates as this parameter reveals which part of the reservoir is most stress sensitive and contributes most to the cumulative time shift we test out our new approach on well log data from troll east north sea and compare the predicted time shifts with observed 4d seismic time shifts we find that there are good agreements between predicted time shifts and observed time shifts furthermore we confirm that there are local geological trends controlling the fluid and stress sensitivity of the reservoir sands on troll east in particular we observe a lateral stiffening of the reservoir from west to east probably associated with the tectonic and burial history of the area the combined effect of a thinning gas cap and stiffening reservoir sands amplifies the eastward decrease in time shifts associated with reservoir depletion we manage to disentangle these two effects using rock physics analysis it is essential to identify and map the static rock stiffness spatial trends before interpreting time shifts and time shift derivatives in terms of dynamic i e 4d pressure and fluid changes

seismic velocities of unconsolidated sands part 1 pressure trends from 0 1 to 20 mpa knowledge of the pressure dependences of seismic velocities in unconsolidated sands is necessary for the remote prediction of effective pressures and for the projection of velocities to unsampled locations within shallow sand layers we have measured the compressional and shear wave velocities and bulk shear and pwave moduli at pressures from 0 1 to 20 mpa in a series of unconsolidated granular samples including dry and water saturated natural sands and dry synthetic sand and glass bead samples the shear wave velocities in these samples demonstrate an average pressure dependence approximately proportional to the fourth root of the effective pressure vsp 1 4 as commonly observed at lower pressures for the compressional wave velocities the exponent in the pressure dependence of individual dry samples is consistently less than the exponent for the shear wave velocity of the same sample averaging 0 23 for the dry sands and 0 20 for the glass bead samples these pressure dependences are generally consistent over the entire pressure range measured a comparison of the empirical results to theoretical predictions based on hertz mindlin effective medium models demonstrates that the theoretical models vastly overpredict the shear moduli of the dry granular frame unless the contacts are assumed to have no tangential stiffness the models also predict a lower pressure exponent for the moduli and velocitiesvp1 6 than is generally observed in the data we attribute this discrepancy in part to the inability of the models to account for decreases in the amount of slip or grain rotation occurring at grain to grain contacts with increasing pressure

late miocene onset of the amazon river and the amazon deep sea fan evidence from the foz do amazonas basin new biostratigraphic isotopic and well log data from exploration wells on the outer continental shelf and uppermost amazon deep sea fan brazil reveal that the amazon river was initiated as a transcontinental river between 11 8 and 11 3 ma ago middle to late miocene and reached its present shape and size during the late pliocene prior to the late miocene the continental shelf was a carbonate platform that received moderate siliciclastic sediment supply from the proterozoic basement in eastern amazonia average sedimentation rates on the amazon fan show three stages of development 1 11 8 6 8 ma ago low sedimentation rates 0 05 m ka prevailed on the fan because the amazon river was not yet entrenched and some sediments were partially trapped in continental basins 2 6 8 2 4 ma ago sedimentation rates 0 3 m ka increased the river entrenched and deposition fully migrated onto the amazon fan 3 2 4 ma ago to the present very high sedimentation rates 1 22 m ka with peaks of 11 m ka prevailed on the fan and the modern amazon river developed all these paleogeographic and depositional events are closely related to andean tectonism late miocene pliocene and were exacerbated by global cooling and sea level fall during the late miocene

rock physics diagnostics of depositional texture diagenetic alterations and reservoir heterogeneity in high porosity siliciclastic sediments and rocks a review of selected models and suggested work flows rock physics has evolved to become a key tool of reservoir geophysics and an integral part of quantitative seismic interpretation rock physics models adapted to site specific deposition and compaction help extrapolate rock properties away from existing wells and by so doing facilitate early exploration and appraisal many rock physics models are available each having benefits and limitations during early exploration or in frontier areas direct use of empirical site specific models may not help because such models have been created for areas with possibly different geologic settings at the same time more advanced physics based models can be too uncertain because of poor constraints on the input parameters without well or laboratory data to adjust these parameters a hybrid modeling approach has been applied to siliciclastic unconsolidated to moderately consolidated sediments specifically in sandstones a physical contact theory such as the hertz mindlin model combined with theoretical elastic bounds such as the hashin shtrikman bounds mimics the elastic signatures of porosity reduction associated with depositional sorting and diagenesis including mechanical and chemical compaction for soft shales the seismic properties are quantified as a function of pore shape and occurrence of cracklike porosity with low aspect ratios a work flow for upscaling interbedded sands and shales using backus averaging follows the hybrid modeling of individual homogenous sand and shale layers different models can be included in site specific rock physics templates and used for quantitative interpretation of lithology porosity and pore fluids from well log and seismic data

development of volcanic passive margins two dimensional laboratory models 1 continental breakup above an anomalously hot mantle may lead to the development of volcanic margins volcanic margins are characterized by 1 thick seaward dipping lava flow sequences 2 central intrusive complexes associated with dyke swarms parallel to the coast and 3 high seismic velocity bodies in the lower crust attributable to magma underplating a conceptual model for volcanic margins development has recently been proposed based on onshore studies of the greenland margins and the british tertiary igneous province it is proposed that the long lived central intrusions are genetically linked to underlying persistent zones of mantle fusion these localized melting domains or soft spots equivalent to small mantle diapirs may locally soften the extending continental lithosphere the low viscosity diapirs would 1 localize tectonic strain and 2 feed the volcanic margin with magma thus such soft spots can control the along strike magmatic and tectonic segmentation of volcanic margins recent geophysical investigations appear to show that the along strike structure of volcanic passive margins is compatible with such a segmentation process here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub moho mantle may have a mechanical effect on continental breakup four layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle the soft spots are simulated by low viscosity silicone putty emplaced within the brittle material at the scale of the entire breakup zone the soft spots display an oceanic type strength profile defining low strength zones where continental breakup is initiated the rift orientation and segmentation are strongly controlled by the distribution of the low viscosity heterogeneities rather than by the direction of regional extension the experiments are compared with the geometry and segmentation of the onshore part of the greenland volcanic margins

progressive cenozoic cooling and the demise of antarctica s last refugium the antarctic peninsula is considered to be the last region of antarctica to have been fully glaciated as a result of cenozoic climatic cooling as such it was likely the last refugium for plants and animals that had inhabited the continent since it separated from the gondwana supercontinent drill cores and seismic data acquired during two cruises shaldril i and ii in the northernmost peninsula region yield a record that when combined with existing data indicates progressive cooling and associated changes in terrestrial vegetation over the course of the past 37 million years mountain glaciation began in the latest eocene approximately 37 34 ma contemporaneous with glaciation elsewhere on the continent and a reduction in atmospheric co2 concentrations this climate cooling was accompanied by a decrease in diversity of the angiosperm dominated vegetation that inhabited the northern peninsula during the eocene a mosaic of southern beech and conifer dominated woodlands and tundra continued to occupy the region during the oligocene approximately 34 23 ma by the middle miocene approximately 16 11 6 ma localized pockets of limited tundra still existed at least until 12 8 ma the transition from temperate alpine glaciation to a dynamic polythermal ice sheet took place during the middle miocene the northernmost peninsula was overridden by an ice sheet in the early pliocene approximately 5 3 3 6 ma the long cooling history of the peninsula is consistent with the extended timescales of tectonic evolution of the antarctic margin involving the opening of ocean passageways and associated establishment of circumpolar circulation

overpressure within upper continental slope sediments from cptu data gulf of lion nw mediterranean sea data from in situ piezocone tests cptu and laboratory analyses are utilized for the interpretation of the stress history of quaternary sedimentary sequences in the upper continental slope of the gulf of lion northwestern mediterranean sea a cptu based preconsolidation pressure profile referenced to the current effective stress indicates that the deposit is underconsolidated from 12 meters below the seafloor mbsf down to at least 150 mbsf excess pore pressure below 12 mbsf is further supported by results from oedometer and dissipation tests subseafloor pockmarks and indications of free gas in seismic reflection profiles reveal four main overpressure sources si siv with overpressure ratios 0 3 at subseafloor depths coinciding with levels where the dominantly silty clayey sediment contains increased proportions of sand we relate the excess pore pressure related to free gas due to gas exsolution processes and sea level variations driven by pleistocene sea level changes

slice of intraoceanic arc insights from the first multichannel seismic reflection profile across the south sandwich island arc we present the first multichannel seismic mcs reflection line that crosses the south sandwich trench south sandwich island arc and east scotia sea backarc basin the line is used in combination with earthquake catalogue data to interpret the strain distribution across the sandwich plate and the relationship of forearc structures to processes operating at the trench the mcs data reveal a 1 2 km high fault scarp associated with a 20 km wide arcward tilted block in the mid forearc these features indicate large scale gravitational collapse and earthquake data are consistent with trench normal extension at shallow depth in this area there is however little evidence of distributed extension within the interior of the sandwich plate the mcs data show a small frontal wedge that achieves its maximum thickness only 18 km from the trench backarc magnetic data mid forearc extension and the small size of the frontal wedge are all consistent with long term and ongoing subduction erosion earthquake data suggest that this erosion is taking place in an environment of low interplate stress

geomorphology of ona basin southwestern scotia sea antarctica decoding the spatial variability of bottom current pathways abstract ona basin the westernmost oceanic basin in the southern scotia sea is affected by the opposite flows of circumpolar deep water cdw and weddell sea deep water wsdw thus it represents a key location for exploring seafloor morphologies influenced by bottom currents the present study aims to capture the spatial arrangement of recent subsurface contourite features assuming a latitudinal influence of water masses and the interactions between along and downslope processes in order to contribute to the knowledge of regional deepwater flow pathways and to the sedimentary model of small sediment starved oceanic basins to this end the investigation combines an interpretation of multibeam bathymetry and parametric echo sounder seismic data complemented with hydrological data the distribution of morpho sedimentary features in ona basin reveals two major domains the southern margin of the basin can be regarded as a mixed hybrid system containing abundant sediment drifts with channels and contourite moats and a lateral continuity interrupted by downslope morphologies in contrast the northern abyssal setting comprises relatively homogeneous large sheeted drifts with superimposed sediment waves mounded drifts and several scattered erosive features likely reflecting the more distinct influence of deepwater contourite processes our work demonstrates that tectonic features in the southern basin control the interaction between deepwater along and downslope processes as the westward flow of the wsdw is deflected channelized and intensified along its westward route in the northern region the study indicates an overall clockwise rotation of the wsdw flow with the spatial and vertical variability of cdw and wsdw affecting the distribution of bottom current features around seamounts and or structural highs the results underscore the importance of sloping interphases in the water mass vertical structure the degree of basin confinement and the influence of local bathymetric elevations in sedimentation models of small sediment starved oceanic basins

depositional processes and growth patterns of isolated oceanic basins the protector and pirie basins of the southern scotia sea antarctica abstract sedimentary processes in small isolated oceanic basins that form adjacent to continental margins but detached from continents remain poorly understood this work describes two such basins located in the southern scotia sea the protector and pirie basins we analysed multichannel seismic profiles to interpret morphostructural features and stratigraphy of these basins sedimentary stacking patterns and depocentre distribution illustrate basin development patterns basal units infill basement depressions formed by the submerged banks of thinned continental crust that abut the basin plains these lower and middle deposits of the sedimentary record are interpreted as pre and syn rift deposits the laterally extensive upper deposits are interpreted as post rift deposits these include five discrete units evident in seismic profiles a prominent regional reflection referred to as reflector c separates in these upper deposits two sets of seismic units that have recorded major shifts in the dominant sedimentary processes stacking patterns and paleo environmental conditions the most important processes controlling deposition of the older units those beneath reflector c include down slope gravity processes that infill depressions created by crustal thinning and seafloor spreading these occurred under the coeval influence of circumpolar deep water circulation the major processes influencing younger units those above reflector c include bottom water circulation of the circumpolar deep water and weddell sea deep water water masses which coursed along bathymetric contours of the seafloor the reflector c discontinuity developed concurrently with middle miocene tectonic changes which led to the opening and deepening of deep gateways in the south scotia ridge these facilitated overflow of weddell sea deep water from the weddell gyre into the scotia sea this overflow in turn forced the circumpolar deep water northwards analysis of the protector and pirie basins shows that their tectonic evolution influenced regional deep water circulation patterns in an area that makes a significant cold water contribution to the global conveyor belt system as a long term factor controlling basin evolution and sedimentary processes tectonics events in this region therefore influenced the present day climate system these results further clarify our understanding of deep isolated oceanic basins in terms their sedimentologic climatologic and oceanographic significance

deep sea pre glacial to glacial sedimentation in the weddell sea and southern scotia sea from a cross basin seismic transect abstract identification of the pre glacial transitional and full glacial components in the deep sea sedimentary record is necessary to understand the ice sheet development of antarctica and to build circum antarctic sediment thickness grids for palaeotopography bathymetry reconstructions which constrain palaeoclimate models a 3300 km long weddell sea to scotia sea multichannel seismic reflection data transect was constructed to define the first basin wide seismostratigraphy and to identify the pre glacial to glacial components seven main seismic units were mapped of these ws s1 ws s2 and ws s3 comprise the inferred cretaceous palaeocene pre glacial regime 27 ma in our age model ws s4 the eocene oligocene transitional regime 27 11 ma and ws s5 ws s6 ws s7 the miocene pleistocene full glacial climate regime 11 1 ma sparse borehole data from odp leg 113 and shaldril constrain the ages of the upper three seismic units and seafloor spreading magnetic anomalies compiled from literature constrain the basement ages in the presented age model the new horizons and stratigraphy often contradict local studies and show an increase in age from southeast to the northwest the up to 1130 m thick pre glacial seismic units form a mound in the central weddell sea basin and in conjunction with the eroded flank geometry allow the interpretation of a cretaceous proto weddell gyre bottom current the base reflector of the transitional seismic unit has a model age of 26 6 15 5 ma from southeast to northwest suggesting similar southeast to northwest initial ice sheet propagation to the outer shelf we interpret an eocene east antarctic ice sheet expansion oligocene grounding of the west antarctic ice sheet and early miocene grounding of the antarctic peninsula ice sheet the transitional regime sedimentation rates in the central and northwestern weddell sea 6 10 cm ky are higher than in the pre glacial 1 3 cm ky and full glacial regimes 4 8 cm ky the pre glacial to glacial rates are highest in the jane and powell basins 10 12 cm ky total sediment volume in the weddell sea deep sea basin is estimated at 3 3 3 9 106 km3

4d time strain and the seismic signature of geomechanical compaction at genesis genesis field which lies in the green canyon area of the gulf of mexico at a water depth of 790 m consists of a series of unconsolidated pleistocene pliocene aged turbidite sands dipping up against a salt cored ridge since first oil in 1999 the field has been under primary production and although there is a moderate natural water drive in parts of the field much of it has undergone significant compaction resulting from pressure depletion the compaction has had severe economic ramifications as recently several wells have been lost due to compaction related shear failure