weichselian and holocene growth of the northern high latitude lofoten contourite drift on the continental slope of norway abstract the weichselian and holocene sediments in the lofoten drift comprise six lithofacies brown sandy mud olive grey mud olive grey laminated mud dark brown mud massive sand and dark grey sandy mud during the weichselian the sediment input to the lofoten drift was from two main sources i e glacimarine sedimentation and ocean current winnowing during the holocene the sediment input was only from winnowing the sediment source area was the voring margin and the continental margin further south the sedimentation rate was 190 cm ka during the late weichelian and 9 cm ka during the holocene the weichselian rates are high compared with the deep sea contourite drifts comprising highly bioturbated sediments and probably explain why bioturbation is less pronounced in these slope drift sediments drifts on glaciated margins as exemplified by the lofoten contourite drift represent areas of high sedimentation rate during glacial maxima sediment input was not restricted to glacial maximum periods for sediments also accumulated during other stadial and interstadial periods

the influence of shelf crossing glaciation on continental slope sedimentation flemish pass eastern canadian continental margin abstract flemish pass is a small perched slope basin seaward of the grand banks on the eastern canadian continental margin glacial ice crossed the grand banks to reach the upper continental slope during some mid to late quaternary glaciations but terminated on the middle shelf 200 km distant at the last glacial maximum this study investigates the effect of ice extent on slope sedimentation processes sediment in flemish pass comprises muds of hemipelagic and proglacial plume origin minor thin bedded turbidite sands and thick bedded mass transport deposits mtds a regional stratigraphy based on 500 km of high resolution seismic profiles and 60 piston cores up to 11 m long was dated using 30 radiocarbon dates the presence of ash zone ii and oxygen isotope stratigraphy this provided stratigraphic control for till tongues in water depths

stratigraphic and sedimentological evidence for late wisconsinan sub glacial outburst floods to laurentian fan abstract sub glacial meltwater produces a distinctive stratigraphic and sedimentological response on the continental margin seismo stratigraphy of laurentian channel reveals thick till deposits at its seaward end that pass laterally into stratified sediment in deeper basins that may record periods of water build up beneath the ice two scales of meltwater discharge are recognised large scale that caused catastrophic erosion and transported large volumes of coarse sediment to the abyssal plain and smaller scale yielding principally muddy sediment sub glacial outburst floods from the laurentian channel ice stream delivered distinctive red sediment derived from permian carboniferous strata of the gulf of st lawrence directly to laurentian fan between ca 17 and 14 14 c ka separate from north atlantic heinrich events on levees of laurentian fan three major pulses of meltwater plume muds are separated by intervals dominated by hemipelagic sediments these meltwater intervals are recognised distally as periods of plume sedimentation on the scotian slope and ice rafting of hematite stained quartz to the north atlantic ocean in channels of laurentian fan at least one major sediment transport event is recognised that eroded the upper slope and the major fan valleys depositing a bed of gravel at least 3 m thick in the characteristically wide fan valleys and thick sand on the sohm abyssal plain the same event was probably responsible for giant flute like scours the age of the gravel bed is directly constrained only by the presence of local overlying holocene sediment much of the surface of the gravel bed was re worked by the 1929 grand banks turbidity current an erosional event on the upper slope likely correlative with the flood generated gravel bed has been dated at 16 5 14 c ka such large scale erosional flood events can be recognised back through several glacial cycles and have played an important role in the architectural evolution of laurentian fan

bed geometry used to test recognition criteria of turbidites and sandy debrites the origin of thick bedded deep water sandstones has generated much controversy in recent years two fundamentally different models have been proposed for beds with the same internal sedimentary characteristics 1 progressive particle settling from the base of a turbulent flow the turbidity current model and 2 en masse freezing of a higher concentration flow the sandy debris flow model these models predict beds with very different geometries turbidites thin gradually whereas debrites have abrupt terminations previous studies have relied upon sedimentary recognition criteria i e sedimentary features in small scale outcrop or core to interpret depositional mechanism in this study depositional mechanism is deduced from bed geometry gained from extensive correlations of individual sandstones preserved in a classic turbidite system marnoso arenacea formation italy this approach allows recognition criteria for turbidites and submarine debrites to be independently tested we find that tabular and tapered sandstones turbidites have distinctly different internal characteristics to beds with abrupt margins debrites turbidites are relatively well sorted often exhibit grading and traction structures and have relatively low matrix mud contents they may also contain massive division floating clasts and inverse grading debrites are moderate to poorly sorted ungraded structureless contain floating clasts and have elevated matrix mud contents these findings have implications for the assessment of submarine gravity flows deposits and reservoir rock characterization

the role of passive margins on the evolution of subduction transform edge propagators steps a subduction transform edge propagator step is the locus of continual lithospheric tearing which enables subduction of one part of a tectonic plate while the juxtaposed part remains at the surface a key question is the propagation direction of active steps and we suspect passive margins to play a critical role in steering steps we investigate the role of passive margins width orientation and lateral strength contrast on the step propagation direction through mechanical finite element models for straight passive margins we show that steps remain parallel to passive margins when within 15 from a trench perpendicular geometry in other cases where passive margins change strike ahead of the active step steps are captured by passive margins for abrupt strike changes radius of curvature 25 is made through a large radius of curvature lithosphere thickness steps will also propagate along the passive margin a step system evolves toward orthogonality which may explain why step faults are approximately perpendicular to trenches in nature step systems are relatively insensitive to small scale details due to large scale stresses propagating as straight features along rugged passive margins surprisingly magnitudes of lithospheric strength variation across the passive margin and subduction history which determines location and magnitude of density anomalies in the mantle are less relevant for the step propagation direction

the role of intermediate depth currents in continental shelf slope accretion canterbury drifts sw pacific ocean the late oligocene to recent canterbury drifts were deposited in water depths between c 400 and c 1500 m by northward flowing cold intermediate depth water masses sub antarctic mode water samw antarctic intermediate water aaiw and their predecessor current flows drift accumulation started at c 24 ma fed by terrigenous sediment derived from the newly rising alpine fault plate boundary in the west which has built a progradational shelf slope sediment prism up to 130 km wide at rates of eastward advance of up to 5 4 km ma 1 gentle uplift associated with the nearby plate boundary has exposed older late oligocene and miocene drifts onland bluecliffs formation ocean drilling program site 1119 located 100 km offshore at a water depth of 394 m penetrated a 428 m thickness of mid pliocene to pleistocene 0 3 9 ma drift located just seaward of the eastern south island shelf edge uniquely these large 60 000 km3 regionally extensive intermediate depth sediment drifts can be examined in outcrop in marine drill core and at the modern sea bed the drifts comprise planar bedded units up to several metres thick some sand intervals have sharp erosive bases and normally graded tops into overlying siltstone others are symmetrically graded with reverse graded bases and normally graded tops bioturbation is moderate and rarely destroys the pervasive background centimetre scale planar or wispy alternation of muddy and sandy silts displayed by formation micro scanner imagery these features are consistent with deposition from rhythmically fluctuating bottom currents texturally the drifts are poly modal quartzofeldspathic silty sands sandy silts silts and silty clays with varying admixtures of benthic and biopelagic carbonate and silica miocene samples are mostly dominated by coarse silt 45 60 m and very fine sand 70 105 m grain size modes whereas strong fine silt 11 13 m and very fine silt clay 10 km wide to smaller 1 3 km wide mounded slope drifts occurs at c 3 1 ma marking further cooling and perhaps the inception of discrete samw flows and initiation of the subantarctic front

crustal thickness and support of topography on venus united states national aeronautics and space administration nasa planetary geology and geophysics program grant

late quaternary stratigraphy and sedimentology of orphan basin implications for meltwater dispersal in the southern labrador sea abstract orphan basin is a deep water basin on the continental margin off newfoundland which throughout the late quaternary received proglacial sediment from local ice that crossed the continental shelf sediment from more distant sources was transported southward in the labrador current as proglacial plumes and in icebergs five sedimentary facies related to glacial processes are distinguished in cores recovered from orphan basin hemipelagic sediment nepheloid layer deposits layered mud beds rich in ice rafted detritus ird sand and mud turbidites and glaciogenic debris flow deposits ird rich beds correspond to periods of intensified iceberg calving and layered mud turbidites and glaciogenic debris flow deposits with glacial meltwater discharge in the late wisconsinan eight periods of meltwater discharge and iceberg calving from the newfoundland ice sheet are interpreted from the sediment facies in orphan basin these discharges coincide with the terminations of the colder periods of the d o cycles recorded in greenland ice cores the oldest minor meltwater event 27 5 28 5 cal ka corresponds to the first late wisconsinan ice advance across the grand banks and ne newfoundland shelf the following three meltwater discharges 23 23 5 23 8 24 5 and 25 27 cal ka deposited sand turbidites and glaciogenic debris flow deposits seaward of trinity trough which was occupied by an ice stream at this time and mud turbidites in the southern part of the basin derived from a mid shelf ice margin on the grand banks four periods of meltwater discharge occurred during the deglaciation and are centered at 15 18 5 19 75 and 20 75 cal ka the youngest is correlated to heinrich event 1 in the literature the 18 5 and 20 75 cal ka events have been recorded in multiple glacial settings in the north atlantic and therefore are interpreted as large scale events of meltwater discharge and iceberg calving but in orphan basin the 19 75 cal ka event is also of similar scale

flexural ridges trenches and outer rises around coronae on venus high resolution altimetry collected by the magellan spacecraft reveals trench and outer rise topographic signatures around major coronae e g eithinoha heng o artemis and latona in addition magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere we have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim the approach was tested at northern freyja montes where the best fit elastic thickness is 18 km in agreement with previously published results the elastic thicknesses determined by modelling numerous profiles at eithinoha heng o artemis and latona are 15 40 37 and 35 km respectively at eithinoha artemis and latona where the plates appear to be yielding the maximum bending moments and elastic thicknesses are similar to those found at the middle america mariana and aleutian trenches on earth respectively estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients at heng o artemis and latona temperature gradients are less than 10 k km which correspond to conductive heat losses of less than one half the expected average planetary value we propose two scenarios for the creation of the ridge trench and outer rise topography differential thermal subsidence and lithospheric subduction the topography of heng o is well matched by the differential thermal subsidence model however at artemis and latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone in these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts rolls back and the corona diameter increases

sea level change along the french mediterranean coast for the past 30 000 years abstract observational evidence for sea level change along the french mediterranean coast has been examined and compared with glacio hydro isostatic models to predict the spatial and temporal patterns of change for about the past 30 000 14c years these predictions are pertinent to discussions of changing ocean volumes during this interval the tectonic stability or otherwise of the coastal areas mantle rheology and the timing of possible human occupation of the now submerged coastal plain and caves such as cosquer cave near marseille the principal results from the analysis are i sea levels along this section of the coast have risen continually since the time of the last glacial maximum lgm and at no time during the holocene has the mean sea level been higher than that of today ii the coast has been tectonically stable between marseille and nice as well as further to the west in roussillon western corsica may have experienced a slow tectonic uplift of between 0 15 0 3 mm year for the past 3000 years but northernmost corsica appears to have been stable during this same interval iii during the lgm sea levels along the coast and immediate off shore areas stood at between 105 115 m below present level the range reflecting the importance of the isostatic contributions during oxygen isotope stage 3 sea levels do not appear to have risen locally above about 60 m iv the rebound parameters describing the mantle rheology and ice sheets required to match the limited observational evidence are consistent with the results of similar analyses carried out for other parts of europe because of its distance from the former northern ice sheets the isostatic factors are particularly sensitive to the value of the lower mantle viscosity v the model predictions for sea level change at the cosquer cave site and for its immediate environments indicate that the cave was last readily accessible before about 10 700 500 14c years about 12 500 500 cal years bp and that the cave entrance was completely flooded by 9000 200 radiocarbon years bp between about 9800 and 10 300 calibrated years bp the cave was above sea level throughout the oxygen isotope stage 3

interbedded late quaternary turbidites and contourites in flemish pass off southeast canada their recognition origin and temporal variation abstract the recognition of contourite and turbidite deposits in deep marine sequences can be challenging as deposits may show similar sedimentary characteristics in flemish pass a small 1100 m deep perched basin along the southeast canadian continental margin both types of deposits occur late quaternary sediments include thin sand layers interbedded with hemipelagic muds the distribution age sedimentary structures and petrology of these sands were used to distinguish between turbidite and contourite deposits which were then characterized by detailed grain size distribution for understanding the conditions leading to deposition of each type seven sediment facies each with a distinctive grain size distribution were recognized they are 1 transported contourites 2 winnowed beds 3 possible jokulhlaup deposits 4 sandy turbidites 5 muddy turbidites 6 hemipelagic sediment and 7 turbidites from well sorted materials sand packets comprised of several stacked turbidite sand beds some with erosive contacts were correlated from the margin to the basin floor showing clear distal fining and creating a small lobe the temporal distribution of the sand beds indicates that most turbidites and contourites were deposited at stadial to interstadial transitions this linkage may be related to variations in the oceanic circulation in flemish pass which winnowed turbidites and may have triggered turbidity currents by moving sands into canyon heads

post miocene sedimentation at the continental rise of southeast greenland the interplay between turbidity and contour currents southeast greenland continental rise depositional sequences have been studied by means of high resolution reflection seismic data major depositional features of this glaciated margin have been compared with comparable seismic architectures at the antarctic margin two seismic units with an estimated age of early to mid pliocene and mid pliocene to recent have been analysed in detail it is concluded that episodic turbidity currents interacting with more continuous contour currents have controlled the location and architecture of the sedimentary mounds found off southeast greenland in the early pliocene turbidite sediment input significantly surpassed the amount of sediments reworked and transported by southbound contour currents as a result an overbank deposit developed oriented perpendicular to the continental slope at the proximal part and parallel to the continental slope in the distal part where the effect of the turbidity processes decreased contour current action further resulted in a southward migration of the overbank deposit from the mid pliocene a relatively stronger impact of the contour current on the sedimentation pattern is observed suggesting a relative decrease of downslope sediment transport processes stronger contour current activity may have been related to generally enhanced north atlantic deep water circulation this resulted in formation of elongated contourite deposits trending parallel to the continental slope both in proximal and distal settings the morphology and seismic expression of the sedimentary mound structures at the southeast greenland margin display several features also observed at the antarctic margin it is concluded however that the sedimentary mounds in the latter area formed in a different way which is principally attributed to persisting contour current activity off antarctica during glacial stages when currents off southeast greenland had virtually ceased

low strength of earth s uppermost mantle inferred from tri axial deformation experiments on dry olivine crystals abstract we have performed tri axial compression experiments on single crystals of san carlos olivine with various orientations at temperatures relevant for the uppermost mantle between 800 and 1090 c the experiments were carried out at a confining pressure of 300 mpa in a high resolution gas medium mechanical testing apparatus at various constant strain rates from 7 10 6 s 1 to 1 10 4 s 1 they yield differential stresses ranging from 88 to 754 mpa unpolarized infrared spectroscopy analyses indicate that hydrogen concentration in the olivine lattice is very low e 1 0 2 10 6 exp 450 60 10 3 rt 1 15 4 1 2 2 where t is the absolute temperature r is the gas constant and is the differential stress in gpa subsequently small fractions of melt or incorporation of hydrogen in olivine may not be required to reconcile natural observations indicating a limited strength of earth s uppermost mantle lithosphere

geological history of the flemish pass basin offshore newfoundland in this paper we present a seismic stratigraphic interpretation of the mesozoic and tertiary geologic history of the flemish pass basin offshore newfoundland based on seismic data and data from seven wells that were integrated on the bp subsurface integrated interpretation system siis four megasequences are identified megasequence 1 ms1 is triassic to lower berriasian and includes poorly known sediments associated with triassic early jurassic rifting prior to spreading between the north african and american continents the uppermost mappable unconformity bounded unit within ms1 contains major kimmeridgian to portlandian marine source rocks deposited in two east west trending subbasins in the baccalieu and gabriel areas during the early berriasian pronounced rif ing linked to the subsequent spreading of iberia from the grand banks barremian aptian led to regional deepening and deposition of clastic sediments ms2 in two northeast southwest trending fault bounded subbasins in the baccalieu and gabriel areas in the aptian early cretaceous subbasins were inverted due to strike slip movement as extension assumed a northeast southwest orientation prior to movement of greenland europe away from labrador albian ms3 sediments fill northwest southeast trending fault bounded depocenters typically located on areas that had formed highs during the early cretaceous postrift sedimentation continued from the albian to the present with upper cretaceous pelagic limestone and later clastic sediment deposition

simulating the thermochemical magmatic and tectonic evolution of venus s mantle and lithosphere two dimensional models 1 numerical convection models of the thermochemical evolution of venus are compared to present day topography and geoid and recent resurfacing history the models include melting magmatism decaying heat producing elements core cooling realistic temperature dependent viscosity and either stagnant lid or episodic lithospheric overturn in stagnant lid convection the dominant mode of heat loss is magmatic heat pipe which requires massive magmatism and produces very thick crust inconsistent with observations partitioning of heat producing elements into the crust helps but does not help enough episodic lid overturn interspersed by periods of quiescence effectively loses venus s heat while giving lower rates of volcanism and a thinner crust calculations predict 5 8 overturn events over venus s history each lasting 150 myr initiating in one place and then spreading globally during quiescent periods convection keeps the lithosphere thin magmatism keeps the mantle temperature constant over venus s history crustal recycling occurs by entrainment in stagnant lid convection and by lid overturn in episodic mode venus like amplitudes of topography and geoid can be produced in either stagnant or episodic modes with a viscosity profile that is earth like but shifted to higher values the basalt density inversion below the olivine perovskite transition causes compositional stratification around 730 km breakdown of this layering increases episodicity but far less than episodic lid overturn the classical stagnant lid mode with interior temperature rheological temperature scale lower than tcmb is not reached because mantle temperature is controlled by magmatism while the core cools slowly from a superheated start core heat flow decreases with time possibly shutting off the dynamo particularly in episodic cases

identification of ancient sandy contourites proposed criteria for the identification of ancient sandy contourites include occurrence thin beds reworked sandy turbidites coarse lag structure bioturbated no regular vertical sequence orientation of cross lamination perpendicular to regional downslope paleocurrents grain size silt or sand little mud low or negative skewness fabric grain orientation and composition downslope and along slope provenance concentrations of heavy minerals a suggested three stage approach to recognition of sandy contourites in the field involves work on a progressively increasing scale with particular emphasis on paleocurrent data the various deep water settings in which disparate current directions may occur are summarized and those in which sandy contourites may be formed and recognizably preserved are indicated a broad definition of the term contourite is now possible because of the widespread documentation of contourites even in freshwater environments along slope flow that is persistent in time and space is a requirement

dansgaard oeschger events is there a signal off the hudson strait ice stream abstract it is generally accepted that the dramatic heinrich h events were principally sourced from the major hudson strait drainage basin of the laurentide ice sheet but it is not known whether the higher frequency dansgaard oeschger d o events have an imprint on sediments deposited during the inter h intervals in ice proximal slope sediments this paper examines this question by an analysis of data from cores on the se baffin slope because the proxies for heinrich events are so massive in scale we removed the major h events h 1 h 2 and h 4 from the records so as to better examine the structure of data in the intervening hemipelagic sediments the period of interest extends between 15 and 40 cal ka interstadial events 2 8 resolution varies with the proxy to between 210 and 700 yrs per sample in hu87033 009 from the lower slope a total of twelve variables for example 18o mass magnetic susceptibility calcite dolomite ratio number of ird 2 mm show a variety of multi millennial oscillations although not all of the same amplitude nor frequency several parameters show little correlation with the gisp2 18o record however a comparison of cie sediment color l from hu87033 009 and hu97048 007 from the upper slope with the gisp2 oxygen isotope record suggests that a substantial correlation exists between the records such changes might be associated with either detrital carbonate content or foraminiferal concentrations analysis suggests however that the first possibility is correct hence suggestive of a glaciological response of the hudson strait ice stream with d o events in the nw labrador sea the sediment records from around h 3 time 30 cal ka show excursions which might be correlative within this event but its amplitude is not unusual compared to events both before and after this reaffirms the argument that h 3 represents a different glaciological response a significant issue discussed in this paper is the inherent ambiguity of correlating records whose errors in the time domain approach that of a dominating periodicity in this case ca 1 5 kyr

submarine deposits from pumiceous pyroclastic density currents traveling over water an outstanding example from offshore montserrat iodp 340 pyroclastic density currents have been observed to both enter the sea and to travel over water for tens of kilometers here we identified a 1 2 m thick stratified pumice lapilli ash cored at site u1396 offshore montserrat integrated ocean drilling program iodp expedition 340 as being the first deposit to provide evidence that it was formed by submarine deposition from pumice rich pyroclastic density currents that traveled above the water surface the age of the submarine deposit is ca 4 ma and its magma source is similar to those for much younger soufriere hills deposits indicating that the island experienced large magnitude subaerial caldera forming explosive eruptions much earlier than recorded in land deposits the deposit s combined sedimentological characteristics are incompatible with deposition from a submarine eruption pyroclastic fall over water or a submarine seafloor hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline the stratified pumice lapilli ash unit can be subdivided into at least three depositional units with the lowermost one being clast supported the unit contains grains in five separate size modes and has a 12 phi range particles are chiefly subrounded pumice clasts lithic clasts crystal fragments and glass shards pumice clasts are very poorly segregated from other particle types and lithic clasts occur throughout the deposit fine particles are weakly density graded we interpret the unit to record multiple closely spaced 2 d hot pyroclastic density currents that flowed over the ocean releasing pyroclasts onto the water surface and settling of the various pyroclasts into the water column our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate the overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column attesting to complex sedimentation slow settling particles were deposited on the seafloor together with faster descending particles that were delivered at the water surface by subsequent pyroclastic flows the final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies this study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water

experimental and observational evidence for plume induced subduction on venus venus lacks plate tectonics but some trenches on venus resemble subduction zones laboratory experiments suggest that upwelling plumes can initiate localized subduction of a thin lithosphere such as the one on venus

the age and chronostratigraphical significance of north atlantic ash zone ii rhyolitic tephra with the geochemical characteristics of north atlantic ash zone naaz ii are described from the giant piston core md95 2006 from the northeast atlantic ocean shard size distribution indicates that the tephra represent a wind sorted primary air fall deposit but with a mode close to 200 m they are too coarse to be air fall deposits at the core site which lies ca 1000 km from the possible tephra source randomly sampled shards reveal a single geochemical population belonging to the icelandic transitional alkali magma series suggesting that they are unlikely to represent ice rafted debris derived from icelandic icebergs the tephra probably represent air fall deposits transported to the core site by sea ice within the northeast atlantic gyre the naaz ii peak coincides with the rapid climate transition cooling at the end of interstadial 15 which can be assigned an age of 53 260 2660 yr bp from direct correlation with the greenland ice core gisp2 record a comparison of the md95 2006 neogloboquadrina pachyderma sinistral relative abundance and gisp2 18o records relative to the naaz ii isochron suggests that this climatic event was synchronous across the north atlantic by direct correlation of interstadial maxima between the two records the gisp2 time scale is transferred to md95 2006 comparison of corrected and calibrated radiocarbon ages derived from monospecific foraminifers with the gisp2 ages at the same stratigraphical horizon suggest major age differences these probably result from large variations in atmospheric 14c concentration and highlight the significant uncertainties associated with radiocarbon calibration during marine isotope stage 3 copyright 2004 john wiley sons ltd

late cenozoic sea level changes and the onset of glaciation impact on continental slope progradation off eastern canada abstract late cenozoic sedimentation from four varied sites on the continental slopes off southeastern canada has been analysed using high resolution airgun multichannel seismic profiles supplemented with some single channel data biostratigraphic ties are available to exploratory wells at three of the sites uniform slow accumulation of hemipelagic sediments was locally terminated by the late miocene sea level lowering which is also reflected in changes in foraminiferan faunas on the continental shelf data are very limited for the early pliocene but suggest a return to slow hemipelagic sedimentation at the beginning of the late pliocene there was a change in sedimentation style marked by a several fold increase in accumulation rates and cutting of slope valleys this late pliocene cutting of slope valleys corresponds to the onset of late cenozoic growth of the laurentian fan and the initiation of turbidite sedimentation on the sohm abyssal plain although it corresponds to a time of sea level lowering the contrast with the late miocene lowstand indicates that there must also have been a change in sediment delivery to the coastline perhaps as a result of increased rainfall or development of valley glaciers high sedimentation rates continued into the early pleistocene but the extent of slope dissection by gullies increased gully cutting episodes alternated with sediment draping episodes throughout the southeastern canadian continental margin there was a change in sedimentation style in the middle pleistocene that resulted from extensive ice sheets crossing the continental shelf and delivering coarse sediment directly to the continental slope

surface brightness variations seen by virtis on venus express and implications for the evolution of the lada terra region venus 1 venus express is since april 11 2006 in orbit around venus virtis visible and infrared thermal imaging spectrometer has started only hours after orbit insertion to collect an immense and unique data set of special interest is the wavelength range from 1 1 5 microns which includes the atmospheric windows by averaging several hundred multi spectral images obtained during the mission we have been able to map the surface temperatures for almost the whole southern hemisphere of venus blurring in the clouds limits the spatial resolution to approximately 100 km globally the most exciting and at the same time the most complex task is to retrieve compositional information for the surface of venus from the hyperspectral data obtained by virtis we report here about indication for variations in the surface composition in the lada terra region based on brightness variations mapped from virtis measurements and present some tentative links to the evolution of this region

evidence for retrograde lithospheric subduction on venus annular moats and outer rises around large venus coronae such as artemis latona and eithinoha are similar in arcuate planform and topography to the trenches and outer rises of terrestrial subduction zones on earth trenches and outer rises are modeled as the flexural response of a thin elastic lithosphere to the bending moment of the subducted slab this lithospheric flexure model also accounts for the trenches and outer rises outboard of the major coronae on venus accordingly it is proposed that retrograde lithospheric subduction may be occurring on the margins of the large venus coronae while compensating back arc extension is occurring in the expanding coronae interiors similar processes may be taking place at other deep arcuate trenches or chasmata on venus such as those in the dali diana chasmata area of eastern aphrodite terra

state of stress within a bending spherical shell and its implications for subducting lithosphere summary the state of stress within a bending spherical shell has some special features that are caused by sphericity while most lithospheres are more like spherical shells than flat plates our ideas of the state of stress have been dominated by flat plate models as a consequence we might be missing some important aspects of the state of stress within subducting lithospheres in order to examine this problem we analyse spherical shell bending problems from basic equations we present two approaches to solve sphericalshell bending problems one by the variational approach which is suitable for globalscale problems and the other by the asymptotic equation which is valid to first order in h r where h is the thickness of the lithosphere and r is its curvature radius i e under the assumption of small curvature the form of the equation for displacement shows that wavelengths of deformation are determined by the spherical elastic evect and the gravitational buoyancy evect for which only the latter evect is included in the usual flat plate formulations in the case of the earth the buoyancy force is dominant and consequently spherical evects are suppressed to a large extent this explains why flat plate models have been successful for earth s lithospheric problems on the other hand the state of stress shows interesting spherical evects while bending fibre stress along the subduction zone is always important bending stress along the trench strike direction can also be important in particular when the subduction zone arc is small numerical results also indicate that compressive normal stress along the trench strike direction is important when a subduction zone arc is large these two stresses the bending stress and the compressive normal stress both along the trench strike direction may have important implications for intraplate earthquakes at subduction zones

an expanded view of lada terra venus new arecibo radar observations of quetzalpetlatl corona and surrounding flows 1 quetzalpetlatl a corona approximately 800 km in diameter lies in lada terra centered at 68 s 357 e it is characterized by vast radar bright lava flows stretching to the southeast for over 1500 km covering a total area of nearly 600 000 km2 magellan sar images covered most of the quetzalpetlatl flow field but 12 6 cm arecibo radar data from march 2001 gives complete coverage of the region with these new data we examine the circular polarization properties of the quetzalpetlatl flow field for comparison with other flows on venus as well as terrestrial lava our analysis suggests that quetzalpetlatl corona is primarily a source for flows with 13 cm scale roughness properties similar to terrestrial pahoehoe and which appear to have traveled down a system of channels or tubes until reaching some unresolved topographic boundary we also identify four new crater candidates

the significance of grain size breaks in turbidites and pyroclastic density current deposits turbidite beds and pyroclastic density current deposits commonly show abrupt internal changes in grain size with stepwise upward fining grain size breaks divide a turbidite bed into a lower sandy zone a middle silty zone and an upper muddy zone zone boundaries need not correspond with bouma division boundaries but the sandy zone usually consists of a b and occasionally c divisions the silty zone is composed of one or more of the b c and d divisions and the muddy zone correlates with division e pyroclastic density current deposits likewise display a tripartite zonation with prominent grain size breaks divergence of particle laden gravity currents into a lower dense body and an upper dilute turbulent wake can explain the formation of the lower grain size break fine particles are elutriated into the wake region by ambient entrainment through the current head while coarse grains are transported in the body particle size and concentration stratification is sustained by the turbulence and velocity structure of gravity currents with a velocity maximum and turbulence minimum also dividing currents into two regions two stages of deposition result the upper grain size break may be produced by buoyant lofting with fine particles convected into the water column or atmosphere as the flow wanes

geological setting and age of the flemish cap granodiorite east of the grand banks of newfoundland flemish cap a large isolated submarine knoll 600 km east of newfoundland consists of a central core of hadrynian rocks and an onlapping sequence of undisturbed to disturbed mesozoic cenozoic sediments the central core of the cap sampled with an electric rock core drill comprises pink fine to medium grained granodiorite dacite and volcanic siltstone the granodiorite samples collected in six out of eight cores are remarkably similar lithologically despite a separation of up to 65 km and probably represent a single pluton the aphanitic dacite and laminated cherty volcanic siltstone have been metamorphosed to subgreenschist prehnite facies but show no evidence of contact metamorphism u pb analyses of coarse and fine zircon fractions from a granodiorite core yielded upper intercepts of 751 and 833 ma respectively although not precise these ages probably represent the age of intrusion sometime in the 750 830 ma range and are older than those reported for similar hadrynian granitic rocks of e

contrasting atmospheric and climate dynamics of the last glacial and holocene periods our present climate is relatively stable compared to that of the last glacial maximum about 20 000 years ago palaeoclimate records obtained from ice cores1 2 and deep sea sediment cores3 for the last glacial period show abrupt temperature changes on timescales of a few hundred years which have been attributed to cycles of ice build up and release associated with large ice sheets dansgaard oeschger cycles and heinrich events 3 and their coupling to ocean circulation4 5 but little is known about the dynamics of the atmosphere during the last glaciation ice sheets influence atmospheric circulation and studies using general circulation models have suggested stormier more variable atmospheric dynamics during the last glacial maximum than today6 9 here we report the results of an analysis of temporal trends over the past 91 000 years in the oxygen isotope signatures of a high resolution ice core record from greenland1 2 this analysis provides direct evidence that atmospheric circulation during the last glaciation was more turbulent than it is today

starch columns analog model for basalt columns desiccation of starch water mixtures produces tensile crack patterns which appear to be interesting but largely unknown study objects for fracture mechanics structural geology and volcanology this paper concentrates on columnar jointing and on columns in starch starch columns have polygonal cross sections and are very similar to basalt columns they are produced by lamp drying starch specimens with dimensions of several centimeters and have diameters in the millimeter range the columns develop behind a crack front which propagates from the surface into the interior the experiments supported by x ray tomograms show that polygonal regularity of the crack pattern is not present at the surface but develops during penetration this transition is steered by a minimum fracture energy principle the analogy between basalt cooling and starch desiccation is far reaching water concentration in starch is analogous to temperature in basalt both quantities obey diffusion equations water loss is equivalent to heat loss the resulting contraction stresses have similar dependences on depth and time and in both cases the material strength is exceeded the starch experiments show that column diameters are controlled by the depth gradient of water concentration at the crack front high low gradients are connected with thin thick columns by analogy a similar relation with the temperature gradient exists for basalt columns the normalized starch gradients are about 3 orders of magnitude larger than the normalized gradients in basalt this explains why starch columns are much thinner than basalt columns the gradients are so different because the crack front speeds differ by a factor of about 10 after 3 days the speed is about 10 mm d in starch but about 100 mm d in basalt peck 1978 the speed difference in turn results from the difference of the diffusion constants the hydraulic diffusivity of starch is 2 orders of magnitude lower than the thermal diffusivity of basalt

sedimentary processes and variations in slope current activity during the last glacial interglacial episode on the hebrides slope northern rockall trough north atlantic ocean abstract shallow cores and high resolution seismic reflection data collected from two areas along the hebrides slope indicate that a variety of depositional processes have been active since the last glacial interval seismic reflection profiles from the most northerly slope area immediately south of the wyville thomson ridge reveal a slope apron and an embayed area with a relict mid upper slope channel five cores from this locality consist of distal muddy turbidites in the area of the channel with muddy contourites interbedded with glaciomarine ice rafted sediments and thin debris flow deposits most of the cores are topped by sandy contourites the preserved sequence suggests that lowered sea levels during the last glaciation 10 ka resulted in a predominance of downslope and glaciomarine sedimentation with bottom current activity greatly suppressed at the onset of deglaciation alongslope activity increased as bottom currents gained in strength with climatic fluctuation and the retreat of ice sheets from the shelf producing muddy and sandy contourites present day current activity is still much in evidence with two current meter stations on the mid and lower slope recording a northward flowing current following the bathymetric contours recorded peak velocities at 468 m and 403 m were 26 48 cm s some elements of reverse flow at the deeper site may be tidal oscillation compounded by short duration high intensity benthic storms cores from the more southerly area in the vicinity of the geikie escarpment penetrated hemipelagites erosively overlain by sandy contourites dinoflagellate and nannofossil dating of these deposits indicate that the lower hemipelagites are of late glacial age 13 ka the sandy contourites are probably of early holocene age the lack of the allerod bolling interstadial and younger dryas stadial may be indicative of a hiatus in sedimentation a possible cause of this may be the increasing slope angles up to 28 on the escarpment concentrating the effects of a thermohaline slope current current flow was also strengthened during deglaciation producing erosion or non deposition of the lower units leaving the coarser top only sandy contourites

late quaternary sedimentation in central flemish pass flemish pass is a basin in 1000 m water depth on the continental slope off the grand banks of newfoundland and has a quaternary fill principally of turbidites the late quaternary history of the pass has been investigated using mid range side scan sonargraphs high resolution seismic profiles and cores dated using c 14 the sequence of facies in the cores reveals six lithostratigraphic units deposited in the past 40 ka at 15 19 ka and 25 30 ka sedimentation was dominated by debris flow and turbidite deposits together with hemipelagic deposits of similar clay size mineralogy derived from the grand banks at other times ice rafting and hemipelagic sedimentation principally of carbonate rich sediment transported by the labrador current predominated a late quaternary regional unconformity on the slope may reflect the effects of ice sheets reaching the shelf break probably in the early wisconsinan late wisconsinan resedimentation was not related to ice marginal processes and probably resulted from

venus interpreting the spatial distribution of volcanically modified craters to understand the impact cratering record on venus we investigate two distinct resurfacing styles localized thin flows and large shield volcanoes we statistically analyze the size frequency distribution of volcanically modified craters and using monte carlo simulations their spatial distribution lava flows partially fill most craters darkening their floors in radar images we find that a model featuring localized thin flows occurring throughout geologic time predicts their observed distribution individual flows may be morphologically indistinguishable but combined they cover large provinces recent mantle plumes may drive a small amount of hot spot magmatism that produces the observed clusters of large shield volcanoes and obviously embayed craters ultimately our analysis demonstrates that two styles of volcanism are needed to explain the observed properties of impact craters and that catastrophic resurfacing is not required

sediment drifts and contourites on the continental margin off northwest britain abstract seismic reflection profiles and short cores from the continental margin off northwest britain have revealed a variety of sediment drift styles and contourite deposits preserved in the northeast rockall trough and faeroe shetland channel the sediment drifts include 1 distinctly mounded elongate drifts both single and multi crested 2 broad sheeted drift forms varying from gently domed to flat lying and 3 isolated patch drifts including moat related drifts fields of sediment waves are locally developed in association with the elongate and gently domed broad sheeted drifts the contrasting styles of the sediment drifts most probably reflect the interaction between a variable bottom current regime and the complex bathymetry of the continental margin the bulk of the mounded gently domed drifts occur in the northeast rockall trough whereas the flat lying sheet form deposits occur in the faeroe shetland channel a much narrower basin which appears to have been an area more of sediment export than drift accumulation patch drifts are present in both basins in the northeast rockall trough the along strike variation from single to multi crested elongate drifts may be a response to bottom current changes influenced by developing drift topography muddy silty muddy and sandy contourites have been recovered in sediment cores from the uppermost parts of the drift sequences on the basis of their glaciomarine origin these mid to high latitude contourites can be referred to collectively as glacigenic contourites both partial and complete contourite sequences are preserved the former consist largely of sandy mid only and top only contourites sandy contourites by their coarse grained nature and their formation under strongest bottom current flows are the most likely to be preserved in the rock record however the very large scale of sediment drifts should be borne in mind with regard to the recognition of fossil contourites in ancient successions

the influence of meltwater on the labrador current in heinrich event 1 and the younger dryas abstract the labrador current transports freshwater from the arctic regions to the northern atlantic and the freshwater flux can influence the meridional overturning circulation the response of the labrador current to meltwater processes is unclear during the deglaciation of the ice sheets around the northwest atlantic this study utilized cores from flemish pass to monitor variations in the strength of the labrador current by using the sortable silt proxy the carbonate content the amount of ice rafted detritus and xrf chemical data were measured to identify the main meltwater events during the last deglaciation this study observed a clear enhancement of the flow strength of the labrador current in heinrich event 1 and the younger dryas with the greatest flow strength during the younger dryas event the onset of the current enhancement was dated at ca 13 cal ka bp which preceded the enhanced ice rafted deposition in hudson strait by about one thousand years the current enhancement in flemish pass during the younger dryas event corresponds to the freshening of the surface water thus meltwater preceding significant iceberg supply has an important effect on the labrador current in heinrich events on the laurentide ice sheet margin

recent hotspot volcanism on venus from virtis emissivity data the questions of whether venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change nine hotspots areas analogous to hawaii with volcanism broad topographic rises and large positive gravity anomalies suggesting mantle plumes at depth have been identified as possibly active this study used variations in the thermal emissivity of the surface observed by the visible and infrared thermal imaging spectrometer on the european space agency s venus express spacecraft to identify compositional differences in lava flows at three hotspots the anomalies are interpreted as a lack of surface weathering we estimate the flows to be younger than 2 5 million years and probably much younger about 250 000 years or less indicating that venus is actively resurfacing

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

calibration of the carbon isotope composition δ13c of benthic foraminifera the carbon isotope composition 13c of seawater provides valuable insight on ocean circulation air sea exchange the biological pump and the global carbon cycle and is reflected by the 13c of foraminifera tests here more than 1700 13c observations of the benthic foraminifera genus cibicides from late holocene sediments 13ccibnat are compiled and compared with newly updated estimates of the natural preindustrial water column 13c of dissolved inorganic carbon 13cdicnat as part of the international ocean circulation and carbon cycling oc3 project using selection criteria based on the spatial distance between samples we find high correlation between 13ccibnat and 13cdicnat confirming earlier work regression analyses indicate significant carbonate ion 2 6 0 4 10 3 mol kg 1 co32 and pressure 4 9 1 7 10 5 m 1 depth effects which we use to propose a new global calibration for predicting 13cdicnat from 13ccibnat this calibration is shown to remove some systematic regional biases and decrease errors compared with the one to one relationship 13cdicnat 13ccibnat however these effects and the error reductions are relatively small which suggests that most conclusions from previous studies using a one to one relationship remain robust the remaining standard error of the regression is generally 0 25 with larger values found in the southeast atlantic and antarctic 0 4 and for species other than cibicides wuellerstorfi discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic 13c record

tectonics and volcanism of eastern aphrodite terra venus no subduction no spreading eastern aphrodite terra a deformed region with high topographic relief on venus has been interpreted as analogous to a terrestrial extensional or convergent plate boundary however analysis of geological and structural relations indicates that the tectonics of eastern aphrodite terra is dominated by blistering of the crust by magma diapirs the findings imply that within this region vertical tectonism dominates over horizontal tectonism and consequently that this region is neither a divergent nor a convergent plate boundary

labrador sea boundary currents and the fate of the irminger sea water abstract the general circulation of the labrador sea is studied with a dataset of 53 surface drifters drogued at 15 m and several hydrographic sections done in may 1997 surface drifters indicate three distinct speed regimes fast boundary currents a slower crossover from greenland to labrador and a slow eddy dominated flow in the basin interior mean eulerian velocity maps show several recirculation cells located offshore of the main currents in addition to the cyclonic circulation of the labrador sea above the northern slope of the basin the surface drifters have two preferential paths one between the 1000 m and 2000 m isobaths and the other close to the 3000 m isobath the vertical shear estimated from ctd data supports the presence of two distinct currents around the basin one current more baroclinic flows between the 1000 m and 2000 m isobaths the other one more barotropic flows above the lower continental slope the irminger sea water carried by the boundary currents is altered as it tr

lithospheric rheology and flexure at artemis chasma venus artemis chasma is an arcuate 2000 km diameter zone of convergence and lithospheric underthrusting on venus inelastic flexure modeling of the topography at artemis combined with observations of trench tectonics allow us to document local temperature gradients below 4 k km 1 and an immense compressive in plane force at the trench lithospheric rheology on venus may be stronger than would be predicted from conventional extrapolations of rock mechanics experiments in particular the brittle surface strength must reach a few tens of megapascals to satisfy the observed lack of flexurally induced surface faulting elastic plate bending models provide an adequate estimate of the bending moment at artemis but they fail to constrain or even recognize the in plane force the inelastic analysis implies an in plane force of the order of 1 1014 n m 1 a potential driving force is thermal thinning of regionally thick lithosphre in the highlands to the north the low heat flux at artemis which is a comparatively young structure is compatible with the notion that venus has experienced a prolonged period of cooling in the last several hundred million years the inference of such exceptionally low thermal gradients embraces three end member possibilities 1 the surface age is 600 ma 2 the lithospheric thermal age is greater than the surface age and or 3 the upper mantle temperature is anomalously low 1550 k

the greenland ice core chronology 2005 15 42 ka part 1 constructing the time scale the greenland ice core chronology 2005 gicc05 is extended back to 42 ka b2k before 2000 ad i e to the end of greenland stadial 11 the chronology is based on independent multi parameter counting of annual layers using comprehensive high resolution measurements available from the north greenland ice core project ngrip these are measurements of visual stratigraphy conductivity of the solid ice electrolytical melt water conductivity and the concentration of na c a 2 s o 4 n o 3 n h 4 a n uncertainty estimate of the time scale is obtained from identification of uncertain annual layers which are counted as 0 570 5 years the sum of the uncertain annual layers the so called maximum counting error of the presented chronology ranges from 4 in the warm interstadial periods to 7 in the cold stadials the annual accumulation rates of the stadials and interstadials are on average one third and half of the present day values respectively and the onset of the greenland interstadials 2 3 and 8 based on 20 year averaged d 18 o

an event stratigraphy for the last termination in the north atlantic region based on the greenland ice core record a proposal by the intimate group it is suggested that the grip greenland ice core should constitute the stratotype for the last termination based on the oxygen isotope signal in that core a new event stratigraphy spanning the time interval from ca 22 0 to 11 5 k grip yr bp ca 19 0 10 0 k 14 c yr bp is proposed for the north atlantic region this covers the period from the last glacial maximum through termination 1 of the deep ocean record to the pleistocene holocene boundary and encompasses the last glacial late glacial of the traditional northwest european stratigraphy the isotopic record for this period is divided into two stadial episodes greenland stadials 1 gs 1 and 2 gs 2 and two interstadial events greenland interstadials 1 gi 1 and 2 gi 2 in addition gi 1 and gs 2 are further subdivided into shorter episodes the event stratigraphy is equally applicable to ice core marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon dated chronostratigraphy that has been used hitherto 1998 john wiley sons ltd

holographic measurements of surface topography in laboratory models of mantle hotspots summary the evolution of surface topography produced by the rise of a buoyant droplet or diapir towards the free surface of a very viscous fluid in laboratory experiments is monitored using holographic interferometry such experiments enable us to investigate implications for surface topography of one possible unsteady model for intraplate hotspots the arrival of mantle thermals or diapirs near the base of the earth s lithosphere our model is possibly of most direct relevance to the interaction with the earth s surface and lithosphere of large spherical caps that are expected to rise at the head of new mantle plumes to previous experimental results for the axial height of topography olson nam we add further information on the height shape and width of the surface swell and on the evolution of the diapir itself when the ambient fluid has uniform density and viscosity no lithosphere surface topography is determined by the diameter density anomaly and depth of the droplet as the diapir approaches the surface a broad axisymmetric surface swell appears and initially increases in height while decreasing in width when the leading edge of the diapir is 0 2 diapir diameters below the surface the height passes through a maximum and the width through a minimum the swell then proceeds to subside and increase in width as the diapir spreads beneath the surface in separate experiments the lithosphere is modelled by a discrete surface layer of more viscous fluid whose thickness and viscosity contrast with the mantle are treated as independent parameters effects of lithosphere buoyancy relative to the mantle a property which may influence continental hotspot swells are also studied within the parameter range used the maximum swell height is independent of the lithosphere viscosity contrast but decreases with increasing lithosphere thickness and with decreasing lithosphere density surface uplift produced by the rise of two consecutive diapirs is shown to be more complex comparison of results with the characteristics of hotspot swells is attempted and should assist in determining the nature of hotspot plumes

a magmatic loading model for coronae on venus 1 models for the formation of coronae quasi circular volcanotectonic features on venus must explain four critical characteristics coronae display 1 a wide range of diameters 2 complex varied topography 3 fracture annuli and 4 sometimes extensive volcanism previous models have difficulty simultaneously satisfying all four constraints on the basis of observations and interpretations of features on venus and earth and experiments in geophysical fluid dynamics we propose that coronae form in response to magmatic loading of the crust over zones of partial melting at the tops of thermally buoyant heads of transient mantle plumes that impinge on the base of the thermal lithosphere by tying corona formation to a melt zone and not directly to an impinging upwelling our conceptual model can account for the wide range in corona diameters lateral crustal flow facilitated by magmatic heating may lead to central depressions characteristic of many coronae a thermomechanical finite element simulation with a dissipating thermal anomaly leads to crustal thinning and predicts an elevation profile consistent with one of the more populous topographic classes of coronae deformation concentrated at the transition from thicker to thinner lithosphere above the thermal anomaly yields a narrow annulus of enhanced differential stress consistent with formation of fracture annuli estimates of excess heat in an upwelling exceed the thermal energy in the melt that comprises the corona band pass filtered maps of the topography and gravity fields in the beta atla themis region are consistent with upwellings now impinging on the lithosphere beneath seven coronae previously inferred to be active

glaciogenic debris flow deposits of orphan basin offshore eastern canada sedimentological and rheological properties origin and relationship to meltwater discharge abstract glaciogenic debris flow deposits gdfs have been recognized in the last decade seaward of many shelf crossing ice streams the rheology of gdfs remains poorly understood ultra high resolution sparker seismic profiles and 25 long piston cores were used to define the architecture age and sediment properties of the gdf deposits in trinity trough mouth fan tmf offshore northeast newfoundland and hence understand their origin and emplacement the gdf deposits comprise poorly sorted gravelly mud individual gdf lenses are 5 30 m thick 2 10 km wide and up to 250 km long shear strength measurements and grain size analysis indicate that gdfs have a different more fluid rheology at their margins and tops exhibiting a surging flow behavior on the upper slope the transition from hard over consolidated till to thin proximal gdf deposits is exposed in a mid holocene landslide scar the transition between the two lithotypes appears gradual and no pre holocene failure scarps were detected a process involving the continuous release of subglacial flowing material with high pore pressure and low shear strength is invoked for the production of gdfs five stacked gdf units a e were deposited during the last glacial maximum 20 5 28 cal ka and can be correlated into a regional lithostratigraphy based on the presence of heinrich beds and 8 local meltwater events r1 8 represented by red plumite deposits south of the trinity tmf this correlation indicates that the timing of major gdf pulses corresponds to the early part of five meltwater discharge events 20 5 21 23 23 5 23 8 24 5 25 27 and 27 5 28 5 cal ka so that gdf deposits represent only a small period of time during a major glacial advance three meltwater events 19 2 20 23 23 5 and 25 27 cal ka produced hyperpycnal flows that resulted in the formation of channel systems and distal sand turbidites the presence of such erosional features in tmfs on the continental margins of the norwegian greenland sea suggests that this novel relationship between gdfs and hyperpycnal turbidites may be widespread and thus important for understanding the glaciological processes involved

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

contourites and their relevance for mass wasting along the mid norwegian margin between about 12 and 4 3 ma slope parallel bottom currents dominated the neogene sedimentation on the slope of the mid norwegian margin and large scale sediment bodies accumulated to thicknesses up to 1000 m these bedforms are morphologically classified as sheeted and elongated mounded contourite drifts an increase in sediment supply from about 4 to 3 ma due to onshore uplift and glaciation resulted in a massive influx of downslope derived material onto the continental margin only on the voring plateau was contourite sedimentation little affected by this change in sedimentary style during the last 0 5 my contouritic deposits occurred also in the storegga area as 100 150 m thick infill drifts inside the scars of two palaeo slides r and s holocene post slide deposits in the scar of the storegga slide 8 2 ka have the typical morphological features and distribution of contourite drifts with the upslope limit at the lower base of the warm northward flowing north atlantic current at 500 700 m depth and the downslope limit at depths of approximately 1200 m detailed chronostratigraphy of the post slide sediments show relatively constant high sedimentation rates in the order of 1 2 m ky during the last 8000 years this is equivalent to the estimated rates for the older infill drifts the infill drifts consist mainly of clays with silty and sandy lamina and represents marine and glaciomarine conditions the main influence of contourite drifts on slope stability relates to their physical properties forming a more sensitive and brittle deposit than the glacial deposits and their exposure to downslope sedimentation in addition the potential for build up of excess pore pressure due to rapid loading from subsequent glacial depositions their seabed smoothing effect and great lateral extent make the contourite drift deposits particularly important for slope instability in this region

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

parameters that control lithospheric scale thermal localization on terrestrial planets 1 shear heating induced localization has been suggested as a potentially important mechanism for breaking the lithosphere yet the physical parameters that control the onset of this instability remain unclear we therefore performed systematic 2 d simulations of visco elasto plastic lithospheric deformation in which we addressed the effects of various parameters and we found that a sharp transition exists between localizing and non localizing regimes in a next step we develop a semi analytical model that combines scaling laws with a 1 d lithospheric deformation model we show that the 1 d model successfully predicts the occurrence of shear localization in 2 d models if the thickness of the plastically deforming part of the lower lithosphere is employed as characteristic length scale an application of the 1 d model to the terrestrial planets shows that this type of shear localization is expected to occur most readily on earth

distribution thickness and origin of heinrich layer 3 in the labrador sea abstract the presence of heinrich layer 3 hl 3 in the northwest labrador sea has been debated in the literature calypso giant piston core md99 2233 five new standard piston cores and re interpretation of 34 cores from previous cruises confirm the presence of hl 3 in the labrador sea it is identified by high total carbonate concentration up to 45 an increase in coarse fraction content and lighter 18o values in polar species planktonic foraminifer neogloboquadrina pachyderma left coiling as low as 3 1 the age of hl 3 of 27 ka was bracketed in the various cores by about 50 14c accelerator mass spectrometer dates where it is present in ice proximal regions it consists of nepheloid flow deposits at the base and mud turbidites at the top the thickness of hl 3 varies between 4 8 m proximal to hudson strait and 0 9 m distal decreasing rapidly seaward on the upper continental slope hl 3 was too deeply buried to be sampled elsewhere hl 3 is absent in some cores probably due to slumping or erosion associated with sandy turbidity currents or debris flows

petrographic evidence from the eastern canadian margin of shelf crossing glaciations abstract it is difficult to evaluate when and where glaciers extended across the outer continental shelf off eastern canada the outer shelf and upper slope are commonly hard bottomed or have a great thickness of post glacial sediment in either case making direct sampling and dating of ice margin sediment impossible the timing and extent of glacier ice is important for understanding continental slope sedimentation and ice sheet dynamics we have therefore determined the lithologic components of the 1 4 mm granule fraction of over 100 well dated samples from 40 cores from the more distal parts of the deep water margin off nova scotia and newfoundland this fraction is demonstrably ice rafted in these cores and geographic variation in component results from input of icebergs at different calving margins the samples show that heinrich layers have a distinctive granule fraction that shows some areal variation as a result of dilution by sources other than hudson strait heinrich event 2 ca 21 ka is widespread as far southwest as the west scotian slope but by the last glacial maximum lgm numerous discrete sources of ice rafted granules are recognised implying that calving ice margins were present in several places off newfoundland and nova scotia including off banquereau a distinctive purplish lgm gravelly mud is recognised on the eastern newfoundland margin derived from a calving margin on the ne newfoundland shelf at the time of heinrich event 1 ca 14 ka there was a calving ice outlet somewhere on the grand banks and another in halibut channel retreat of ice from the gulf of st lawrence can be tracked by changing granule petrology from the lgm to 13 ka these data suggest that final ice retreat from many parts of the outer shelf was later than is shown in many recent ice margin reconstructions