paleogene igneous underplating and subsidence anomalies in the rockall faeroe shetland area abstract crustal thickening caused by igneous underplating related to continental break up in the northeast atlantic has been invoked as the cause of the apparent lack of thermal subsidence seen in many mesozoic basins in the northwest european area in this study we examine a set of commercial wells in the rockall faeroe shetland region and calculate tectonic subsidence histories the rate of post rift basement subsidence during the cretaceous cenozoic determined from backstripping calculations always implies a higher degree of extension than the total basement subsidence assuming uniform extension furthermore the size of this discrepancy increases northward along the rockall trough and westwards in the northern north sea towards scotland we interpret this depth anomaly to reflect the effect of igneous underplating rethickening the crust following mesozoic and cenozoic extension late paleocene extension is identified in subsidence reconstructions throughout the west of britain area and is greatest in the rockall trough assuming that the underplating is gabbroic in composition the subsidence depth anomalies indicate a regional pattern of 2 3 km underplated between the erris trough and porcupine basin increasing to 4 5 km in the west lewis basin and hebrides shelf area in the faeroe shetland basin underplating is 1 0 1 5 km in the east and increases to 4 5 km in the west the data support the hypothesis of a major asthenospheric thermal anomaly centered under the faeroes in the paleocene dynamic uplift of the region is often difficult to identify but is locally seen to develop rapidly around 63 ma implying rapid emplacement of the mantle anomaly the intrusion of gabbroic bodies to the base of the crust in the late paleocene may have had an impact on the thermal maturation of petroleum source rocks in the region however thermal maturity indicators show smaller peak heatflows than predicted from simple thermal models suggesting that underplating was a gradual process achieved through the accretion of several small rapidly cooled bodies

synchronous uplift of the lower crust of the ivrea zone and of southern calabria and its possible consequences for the hercynian orogeny in southern europe abstract along the border of the adriatic microplate pre alpine granulite facies rocks from the deepest crust are outcropping at only two places in the ivrea zone of the southern alps and in southern calabria in these two areas the main features of the present crustal structures i e overlapping of large continental crustal and upper mantle segments are interpreted as resulting from their hercynian geodynamic evolutions the tilted nearly complete crustal sections in both areas display very similar lithological sequences and experienced a common geological evolution as deduced from petrological and radiometric dates at the end of hercynian time 295 m y the ivrea body and the lower crustal rocks of southern calabria were contemporaneously sheared off from the upper mantle and uplifted into intermediate crustal levels where they slowly cooled during mesozoic time the tectonic uplift was accompanied by granitoid plutonism and andesitic to rhyolitic volcanism which continued after the hercynian uplift considering the presently similar crustal structures and the upper carboniferous and permian geological evolutions along the whole adriatic plate boundary the ivrea zone and southern calabria are used to resolve the pre alpine history of the boundary zone between the adriatic and the central european block the uplift of the lower crustal upper mantle flakes of the adriatic block was due to flat overthrusting of these flakes on the continental crust of central europe the material of the central european crust underthrust subducted thereby melted during the re equilibration of the geotherms which had been disturbed by the subduction process this led to an extensive calc alkaline plutonism and volcanism of crustal origin along the adria boundary in this boundary region the overlying uppermost crustal levels schiefergebirgsstockwerk were synchronously folded asturian phase in response to the overlapping of the deeper crustal levels subsequently to the orogeny the mountain chain was eroded and molasse basins developed on the overthrust adriatic crustal segment during the lower permian in this model the granulite facies flakes of the ivrea zone and of southern calabria are interpreted as pre hercynian lower crustal segments which were thrust into the middle crust during the hercynian orogeny thus giving rise to wave velocity inversions in the crust further it is proposed that similar geodynamic processes have played a role in the genesis of the conrad discontinuity which is present in many parts of the hercynian fold belt but only in the ivrea zone and in southern calabria the crustal discontinuities formed in hercynian time were uplifted to the surface as a result of alpine reactivation of the adriatic boundary zone and due to their special positions in the bends of the alpine apennine maghrebide mountain system according to the present knowledge of the carboniferous paleogeography and of the orogenic evolution on both sides of the adria sufure zone this fault zone was located within the european continent its role during the hercynian orogeny is discussed envisaging two possibilities an a subduction zone or a subfluence zone in the sense of behr and weber

debris bed friction of hard bedded glaciers 1 field measurements of debris bed friction on a smooth rock tablet at the bed of engabreen a hard bedded temperate glacier in northern norway indicated that basal ice containing 10 debris by volume exerted local shear traction of up to 500 kpa the corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0 05 and 0 08 a model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1 a small exponent n 2 is likely because stresses in ice are small and flow is transient numerical calculations of the bed normal drag force on a sphere in contact with a flat bed using n 1 show that this force can reach values several hundred times that on a sphere isolated from the bed thus drastically increasing frictional resistance various estimates of basal friction are obtained from this model for example the shear traction at the bed of a glacier sliding at 20 m a 1 with a geothermally induced melt rate of 0 006 m a 1 and an effective pressure of 300 kpa can exceed 100 kpa debris bed friction can therefore be a major component of sliding resistance contradicting the common assumption that debris bed friction is negligible

an improved method for reconstructing the stratigraphy and bathymetry of continental margins application to the cenozoic tectonic and sedimentary history of the congo margin a number of techniques such as backstripping and forward modeling have been previously used to reconstruct the tectonic sedimentary and thermal history at passive margins still these techniques are generally inaccurate in places such as the west african margin where the stratigraphic record is disrupted by continuous faulting of the sediment pile caused by salt tectonics and by erosional unconformities the ability to reconstruct the stratigraphy salt thickness and water depth along profiles of these types of margins is important for determining the factors controlling the geological history of the margin and the evolution of hydrocarbon systems to reconstruct the stratigraphy paleowater depths and salt flowage on a profile across the congo margin we combined backstripping and palinspastic reconstruction this enabled us to restore the faulting and salt flowage and simultaneously to take into account the isostatic response of the lithosphere to sediment loading and unloading this method allowed us to estimate the paleowater depth as well as the stratigraphy and the geometry of the sedimentary section on the profile through time the determination of the regional isostatic response of the lithosphere is performed with the use of a new model to estimate the flexural rigidity of continental lithosphere to demonstrate this method we perform the cenozoic reconstruction of the congo margin the results yield a coherent geological history of the margin from the eocene to the present in the eocene the margin was a carbonate ramp with a deep seated shelf break 600 m during the eocene oligocene transition intermediate water depth oceanic currents triggered submarine erosion of the eocene slope and shelf from the miocene to the present we observe a large increase in the input of terrigenous clastic sediments as a result the shelf prograded 40 km across the eocene ramp as the clastic sediments prograded across the margin into the deep basin faulting of the sediment pile and salt withdrawal caused the formation of grabens in places where salt swells had previously formed finally the reconstruction suggests that the congo margin was uplifted by at least 330 m between the late oligocene and the middle miocene

thrust structures in the eastern dauphinois zone french alps north of the pelvoux massif abstract an examination of thrust structures in the eastern part of the dauphinois zone of the external french alps referred to in the literature as the ultradauphinois zone shows that major basement thrusts climb up section to produce cover basement synclines these thrusts also climb laterally and are continuous with thrust in the cover rocks the external basement massifs are recognized as thrust sheets with variably deformed and thrust cover sequences the distinction made in the previous literature between the dauphinois and ultradauphinois zones is no longer tenable cover thrusting proceeded by both smooth slip and rough slip the latter producing a duplex of cover thrust slices restoration of this duplex indicates that a shortening of 70 km in the cover occured during its formation possible errors in this estimate include uncertainties in the original stratigraphic thickness and in the overall shape of the duplex another duplex is thought to have formed at a basement ramp created by the presence of an early basement normal fault partial footwall collapse of this basement ramp gave rise to a basement horse at the bottom of the duplex the overall relation between cover and basement thrusting is indicated using a hanging wall sequence diagram recent geophysical studies suggest that the basement thrusts developed from a mid crustal decollement which passes down dip to offset the moho model studies of thin skinned tectonics may not be appropriate to such thrust geometries

lower cretaceous lacustrine source rocks in northern gabon effect of organic facies and thermal maturity on crude oil quality abstract organic facies variation which is a key factor affecting the quality of hydrocarbon accumulations is characterized for the pre salt freshwater lacustrine source rocks in the interior basin and n komi rift areas in northern gabon by integrating geochemical and geological data the kissenda shales have 0 59 2 23 total organic carbon toc and hydrogen indices hi from 458 93 mghc goc pyrolysis of isolated kerogen and gas chromatography on rock extracts indicate that these samples are overall more waxy than those from the overlying melania shales which have 0 77 4 79 toc and hi from 896 212 in addition hopane sterane ratios and c30 4 methyl sterane indices increase systematically with depth it is proposed that the early cretaceous lacustrine system in northern gabon was initially oligotrophic and contained primarily bacteria and cyanobacteria with a small population of dinoflagellates and botryococcus as trophic level increased through time the food chain length increased eukaryotic organisms became more abundant and diversified steroids increased relative to hopanoids in the sediment and the preserved organic matter became less waxy burial history modeling shows that the onal 1 structure is charged primarily by waxy oils generated from the lower kissenda shales lower wax better quality oils are expected to the southwest where the kissenda shales are more mature and the less waxy melania shales also become effective source rocks thus variation in organic facies and thermal maturity in the cretaceous pre salt source rocks result in commercial production from the rabi field vs the subcommercial discovery in the onal 1 structure

early oligocene stratigraphic turnover on the west africa continental margin a signature of the tertiary greenhouse to icehouse transition the post rift stratigraphy on the west african margin is characterized by aggradation of a carbonate ramp during late cretaceous to eocene epochs and progradation of a terrigenous wedge from oligocene to the present such first order structure has been attributed in the past to geodynamic forcing however comparison of the stratigraphic record of the margin with eustasy 18o and 87sr 86sr curves shows a close temporal relationship with the tertiary climate cooling an increase of continental weathering and a long term lowering of sea level we suggest that the transition from low amplitude high frequency sea level changes during the greenhouse period to high amplitude high frequency sea level changes during the icehouse period may account for i the switch from an aggrading carbonate ramp to a prograding clastic wedge and ii the enhanced continental weathering and increased terrigenous influx to the margin

relating petroleum system and play development to basin evolution west african south atlantic basins abstract sedimentary basins can be classified according to their structural genesis and evolutionary history and the latter can be linked to petroleum system and play development we propose an approach in which we use the established concepts in a new way breaking basins down into their natural basin cycle division then defining the characteristics of each basin cycle including the type of petroleum systems and plays they may contain and comparing them with similar basin cycles in other basins thereby providing a means to learn through a greater population of perhaps not immediately obvious analogues furthermore we introduce the use of the trajectory plot as a new tool in such an analysis this methodology has been applied to the west african south atlantic marginal basins between cameroon and angola and we demonstrate that the similar tectonostratigraphic evolution of the individual basins along this margin has led to the development of similar types of petroleum systems and play level s consequently we can make analogue comparisons among these basins in order to evaluate and predict the presence of potential yet undiscovered hydrocarbon accumulations in less well explored parts of the margin

on the tectonics of the himalaya and the tibet plateau new data for the gravity anomalies and the uplift rate in the himalaya and the tibet plateau have imposed strict constraints on acceptable models for present tectonic processes in this region through two dimensional finite element modeling incorporating realistic rheology for the crust and upper mantle a model is found which satisfactorily predicts both the observed uplift rate and the changes of the gravity anomalies this model shows that 1 the himalaya is dynamically supported 2 the northern margin of the indian plate underthrusts the himalaya and at the same time the entire tibet thickens at a rate of 2 mm yr 3 the partition of the total shortening across the region is about 15 in the ganges basin 30 in the himalaya and 55 in tibet and 4 the extent of deformation reaches beyond the northern boundary of tibet

rheology of ice at the bed of engabreen norway a three dimensional finite element model is used to analyze field data collected as dirty basal ice flowed past an instrumented obstacle at the bed of engabreen a temperate glacier in northern norway the ice is modeled as an incompressible power law fluid with viscosity b 2 1 n 2n d where d is the second invariant of the stretching tensor and b and n are two parameters using measurements obtained in 1996 and 1997 two values of b are obtained one using the measured normal stress difference across the obstacle and the other using the measured bed parallel force over the instrument these two values are not equal probably owing to small frictional forces at the bed unaccounted for in the numerical model hence b ranges between 1 9 10 7 and 3 2 10 7 pa s 1 3 in 1996 and between 2 2 10 7 and 4 1 10 7 pa s 1 3 in 1997 these values are smaller than measured elsewhere for clean glacier or laboratory ice field measurements of water content fabric and texture of the basal ice suggest that unbound water between thin sediment layers and lamellae of clean ice may act as a lubricant and significantly weaken the ice near isotropic fabrics indicate that preferred fabric orientation does not enhance the deformation

230th and 231pa on geotraces ga03 the u s geotraces north atlantic transect and implications for modern and paleoceanographic chemical fluxes abstract the long lived uranium decay products 230th and 231pa are widely used as quantitative tracers of adsorption to sinking particles scavenging in the ocean by exploiting the principles of radioactive disequilibria because of their preservation in the pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean this includes the vertical flux of particulate matter to the seafloor the lateral flux of insoluble elements to continental margins boundary scavenging and the southward flux of water out of the deep north atlantic in a section of unprecedented vertical and zonal resolution the distributions of 230th and 231pa across the north atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the mid atlantic ridge boundary scavenging is clearly expressed in the water column along a transect between mauritania and cape verde which is used to quantify a bias in sediment fluxes calculated using 230th normalization and to demonstrate enhanced 231pa removal from the deep north atlantic by this mechanism the influence of deep ocean ventilation that leads to the southward export of 231pa is apparent the 231pa 230th ratio however predominantly reflects spatial variability in scavenging intensity complicating its applicability as a proxy for the atlantic meridional overturning circulation

thermal history of the rio muni west africa ne brazil margins during continental breakup abstract we document the thermal record of breakup of the conjugate rio muni west africa and ne brazil margins using apatite fission track analysis vitrinite reflectance data and stratigraphic observations from both margins these results permit determination of the timing of four cooling episodes and the temperature of samples at the onset of each episode all samples are interpreted to have experienced higher temperatures in the geological past due to i elevated basal heatflow palaeogeothermal gradient in rio muni 1 well decaying from 58 c km during the mid cretaceous to 21 5 c km in the late cenozoic and ii progressive exhumation from formerly greater burial depth a well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section exhumation than if a constant heatflow is assumed cooling episodes identified from the palaeotemperature data at 110 95 ma both margins and 85 70 ma rio muni only coincide with major unconformities signifying respectively the cessation of rifting breakup and compressional shortening that affected the african continent following the establishment of post rift sedimentation drift the interval between these separate unconformities is occupied by allochthonous rafts of shallow water carbonates recording gravitational collapse of a marginal platform the rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent ascension fracture zone and thermal doming due to the coeval st helena and ascension plumes the data also reveal a c 45 35 ma cooling episode attributed to deep sea erosion at the onset of eo oligocene ice growth and a c 15 10 ma episode interpreted as the record of miocene exhumation of the west african continental margin related to continent wide plume development integration of thermal history methods with traditional seismic and stratigraphy based observations yields a dynamic picture of kilometre scale fluctuations in base level through the breakup and early drift phases of development of these margins major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy but also regional plate margin generated deformation local intra basinal reorganization and the amplifying effect of negative feedbacks between these processes

north atlantic ocean circulation and abrupt climate change during the last glaciation the most recent ice age was characterized by rapid and hemispherically asynchronous climate oscillations whose origin remains unresolved variations in oceanic meridional heat transport may contribute to these repeated climate changes which were most pronounced during marine isotope stage 3 the glacial interval 25 thousand to 60 thousand years ago we examined climate and ocean circulation proxies throughout this interval at high resolution in a deep north atlantic sediment core combining the kinematic tracer protactinium thorium pa th with the deep water mass tracer epibenthic 13c these indicators suggest reduced atlantic overturning circulation during every cool northern stadial with the greatest reductions during episodic hudson strait iceberg discharges while sharp northern warming followed reinvigorated overturning these results provide direct evidence for the ocean s persistent central role in abrupt glacial climate change

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

a tutorial on the use of control methods in ice sheet modeling control methods are recommended as an efficient means to estimate undetermined physical parameters and boundary conditions and in so doing to improve the fidelity of a given ice sheet flow model to specific ice sheet velocity observations to accomplish this task the underlying dynamics of the model are inverted this permits model tuning adjustments to be represented explicitly in terms of model observation misfit

identification and numerical modelling of hydrocarbon leakage in the lower congo basin implications on the genesis of km wide seafloor mounded structures abstract we present a combined approach of interpretation of 2d seismic reflection data and numerical modelling of hydrocarbon generation and migration across the southern slope of the lower congo basin in order to investigate the factors controlling timing and distribution of hydrocarbon leakage in this area we identified three main families of past and present day leakage features 1 mid upper miocene seismic chimneys concentrated basinwards and ending up on buried pockmarks 2 plio pleistocene chimneys rather clustered to the east of the study area and ending up in seafloor pockmarks and 3 fewer scattered chimneys identified within the miocene sequences ending up in shallow enhanced reflectors flat spots stratigraphic and structural elements seem to control the distribution of these features at least two major events of leakage occurred during the middle late miocene and intermittently during the pliocene present external factors as sediment supply are associated to the miocene leakage event whilst internal structural elements probably triggered the pliocene to present day leakage a major seabed morphological feature represented by a margin paralleled belt of more than 1 km wide mounds was identified above growth faults to the east of the study area data constrained 2d hc generation and migration modelling suggests a genetic link between these structures and vertical migration leakage of thermogenic methane sourced from either currently mature oligo miocene source rocks or secondary cracking and further expulsion from over mature upper cretaceous source rocks hence the mounds are likely to represent a lineation of methane derived carbonate build ups despite the natural limitations of a 2d migration model when combined and calibrated with observations from seismic data it can be used as a valid tool to assess petroleum migration routes in sedimentary basins to the best of our knowledge this is the first integrated approach combining both seismic observations and numerical modelling carried out in the angola basin

flexural response of passive margins to deep sea erosion and slope retreat implications for relative sea level change we suggest that the magnitude of the early late oligocene eustatic fall as predicted from sequence stratigraphy has been overestimated because of flexural rebound along margins that was induced by late eocene to early oligocene erosional unloading the unloading of sediments from a continental margin during periods of deep sea erosion and slope retreat will be accompanied by some form of flexural rebound the magnitude and form of this rebound are controlled by 1 the amount of sediment removed from the margin and 2 the strength of the lithosphere at the time of sediment removal along margins with narrow continental shelves this rebound will induce a relative sea level fall resulting in a basinward shift in the shoreline and the predicted systems tracts will be similar to those expected for a eustatic fall in contrast rebound along wide continental shelves will not alter the position of the shoreline even though a relative sea level fall may be recorded across the outer shelf deep sea drilling and seismic reflection data from many margins support the interpretation of a late eocene to early oligocene 38 34 ma deep sea erosional event best developed along the continental slope and rise seismic reflection and drilling data also document a prominent canyon cutting event across the shelf during the early late oligocene 32 29 ma this event has been interpreted in terms of a large eustatic fall 150 m nevertheless there appears to be a correlation between late eocene to early oligocene deep sea erosion and the development or at least enhancement of unconformities across the shelf during the early late oligocene

predicting radar attenuation within the antarctic ice sheet abstract to better understand the ability of ice penetrating radar to diagnose the subglacial environment from bed returned power we model the englacial radar attenuation of antarctic ice first we use a one dimensional thermo mechanical model to evaluate the sensitivity of the depth averaged attenuation rates to ice temperature as a function of surface accumulation rate geothermal flux and ice thickness we find that attenuation is most sensitive to variations in geothermal flux and accumulation rate when the bed temperature is close to the pressure melting point but even if geothermal flux and accumulation rate remain fixed attenuation can easily vary with ice thickness such high sensitivities show that one should not assume a uniform attenuation rate in the radar data analysis then using ensembles of modeled ice temperatures with different boundary conditions we generate multiple attenuation predictions for the antarctic ice sheet and evaluate the resulting uncertainties the largest contributor to uncertainty in these predictions is the geothermal flux this uncertainty is localized within the deeper half of the ice sheet by combining these temperature ensembles with ice core chemistry data we show that the sea salt adds little to the attenuation but the contribution from acids accounts for 29 inland to 53 coast of the total attenuation we conclude that improving radar diagnosis of the subglacial environment using bed returned power requires both 1 better data interpretation algorithms that account for attenuation variations and 2 better constraints of geothermal flux and bulk chemistry

seismic stratigraphy and depositional signatures of the maldive carbonate system indian ocean abstract as documented by industrial seismic reflection profiles and well data the cenozoic stratigraphic evolution of the maldive carbonate system shows three distinct intervals characterized by their own specific depositional signatures late palaeocene volcanics generated by the reunion hotspot filled deeper pull apart structures and created a flat topography that promoted the onset of the maldive carbonate system in the early eocene some of the initial structural depressions remained as deep internal seaways surrounded by reefal margins until the late oligocene the eocene early oligocene stratigraphic signature is essentially aggradational and corresponds to a transgressive cycle associated with a second order sea level rise in the mid oligocene a major sea level fall exposed the neritic carbonate system and restricted sedimentation to the former deep internal basins during the next transgression a neritic carbonate system re established itself on the newly flooded platform and was finally drowned near the oligocene miocene boundary the main effect of this second order transgressive cycle was the transformation of the central troughs into one single deep basin in the neogene the palaeo maldive inner sea when a reef framework was established in the early miocene near the present location of the platform edge systematic bank margin progradation toward the central inner sea basin became enhanced this neogene progradational phase is explained as the response to a long term sea level fall associated with a second order regressive cycle finally the late pliocene pleistocene depositional signature is mainly aggradational and corresponds to the regional modification from flat topped carbonate banks to atoll physiographies the antecedent karst theory suggests that increased karstification was triggered by high frequency eustatic and climatic changes in the last 3 0 ma

acceleration of the contribution of the greenland and antarctic ice sheets to sea level rise 1 ice sheet mass balance estimates have improved substantially in recent years using a variety of techniques over different time periods and at various levels of spatial detail considerable disparity remains between these estimates due to the inherent uncertainties of each method the lack of detailed comparison between independent estimates and the effect of temporal modulations in ice sheet surface mass balance here we present a consistent record of mass balance for the greenland and antarctic ice sheets over the past two decades validated by the comparison of two independent techniques over the last 8 years one differencing perimeter loss from net accumulation and one using a dense time series of time variable gravity we find excellent agreement between the two techniques for absolute mass loss and acceleration of mass loss in 2006 the greenland and antarctic ice sheets experienced a combined mass loss of 475 158 gt yr equivalent to 1 3 0 4 mm yr sea level rise notably the acceleration in ice sheet loss over the last 18 years was 21 9 1 gt yr2 for greenland and 14 5 2 gt yr2 for antarctica for a combined total of 36 3 2 gt yr2 this acceleration is 3 times larger than for mountain glaciers and ice caps 12 6 gt yr2 if this trend continues ice sheets will be the dominant contributor to sea level rise in the 21st century

precise interpolar phasing of abrupt climate change during the last ice age a new ice core from west antarctica shows that during the last ice age abrupt northern hemisphere climate variations were followed two centuries later by a response in antarctica suggesting an oceanic propagation of the climate signal to the southern hemisphere high latitudes

glacier crevasses observations models and mass balance implications we review the findings of approximately 60 years of in situ and remote sensing studies of glacier crevasses as well as the three broad classes of numerical models now employed to simulate crevasse fracture the relatively new insight that mixed mode fracture in local stress equilibrium rather than downstream advection alone can introduce nontrivial curvature to crevasse geometry may merit the reinterpretation of some key historical observation studies in the past three decades there have been tremendous advances in the spatial resolution of satellite imagery as well as fully automated algorithms capable of tracking crevasse displacements between repeat images despite considerable advances in developing fully transient three dimensional ice flow models over the past two decades both the zero stress and linear elastic fracture mechanics crevasse models have remained fundamentally unchanged over this time in the past decade however multidimensional and transient formulations of the continuum damage mechanics approach to simulating ice fracture have emerged the combination of employing damage mechanics to represent slow upstream deterioration of ice strength and fracture mechanics to represent rapid failure at downstream termini holds promise for implementation in large scale ice sheet models finally given the broad interest in the sea level rise implications of recent and future cryospheric change we provide a synthesis of 10 mechanisms by which crevasses can influence glacier mass balance

dynamic interactions between the gulf of guinea passive margin and the congo river drainage basin 2 isostasy and uplift 1 the gulf of guinea continental margin is paralleled by a coastal relief that isolates the congo drainage basin and can play a subsequent role in interactions between margin deposits and continental denudation migration of depocenters from the ogooe and kwanza rivers to the congo fan in the oligocene low present day sediment delivery or pleistocene submarine channels instabilities are possible evidences for this interaction in order to test this hypothesis we examine successively margin isostasy regional uplift erosional unloading and flexural bulge created by sediment loads satellite free air gravity and two dimensional thermomechanical modeling including depth of necking are used to determine the structure of continental margin and strength conditions for two selected sections across south gabon and congo fan both verify similar conditions for extension geometry depth of necking 10 20 km and strength equivalent elastic thickness of 20 30 km differences in observed gravity are explained mainly by differences in sediment volumes regional uplift during miocene of at least 450 m is attributed for both continent and margin as evidenced by the elevated base level of the congo drainage basin and the uplift of shelf break and coastal plains topography corrected for regional uplift valley erosion below relics surfaces and corresponding erosion unloading has been determined as a proxy of oligocene topography it shows a typical rift shoulder topography isolating congo drainage basin from margin in such conditions small amplitude flexural uplift related to sediment loads can possibly have negative feedbacks for both oligocene and present time

meltwater production due to strain heating in storglaciaren sweden 1 storglaciaren northern sweden is temperate in most parts except for a cold surface layer in the ablation zone one of four possible sources for liquid water in temperate ice is melting due to strain heating velocity fields are calculated with an ice flow model so that calculated and observed surface velocities agree meltwater accumulation is computed by integrating strain heating along trajectories starting at the surface in the accumulation area and ending at the cold temperate transition surface in the ablation zone the distribution of moisture content due to strain heating alone is mapped in a longitudinal section of storglaciaren values reach more than 10 g of water per kilogram ice water mixture in the lowest parts of the temperate domain for this moisture content the rate factor is more than 3 times higher than for water free ice and therefore water production by strain heating is important for the modeling of temperate and polythermal glaciers

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

the long term evolution of the congo deep sea fan a basin wide view of the interaction between a giant submarine fan and a mature passive margin zaiango project abstract we have integrated the relatively unknown distal domains of the lower congo basin where the main depocenters of the congo submarine fan are located with the better constrained successions on the shelf and upper slope through the analysis of thousands of km of 2d seismic reflection profiles off shore the congo angola passive margin the basin architecture is depicted by two ca 800 km long regional cross sections through the northern congo and southern angola margin a large unit deposited basinward of the aptian salt limit is likely to be the abyssal plain equivalent of the upper cretaceous carbonate shelf that characterized the first post rift deposits in west equatorial african margins a latest turonian shelf deepening event is recorded in the abyssal plain as a long period coniacian eocene of condensed sedimentation and basin starvation the onset of the giant tertiary congo deep sea fan in early oligocene following this event reactivates the abyssal plain as the main depocenter of the basin the time space partitioning of sedimentation within the deep sea fan results from the interplay among increasing sediment supply margin uplift rise of the angola salt ridge and canyon incision throughout the neogene oligocene early miocene turbidite sedimentation occurs mainly in nw se grabens and ponded inter diapir basins on the southern margin angola seaward tilting of the margin and downslope salt withdrawal activates the up building of the angola escarpment which leads to a northward congo shift of the transfer zones during late miocene around the miocene pliocene boundary the incision of the congo submarine canyon confines the turbidite flows and drives a general basinward progradation of the submarine fan into the abyssal plain the slope deposition is dominated by fine grained hemipelagic deposits ever since results from this work contribute to better understand the signature in the ultra deep deposits of processes acting on the continental margin as well as the basin wide sediment redistribution in areas of high river input

crustal structure gravity anomalies and flexure of the lithosphere in the vicinity of the canary islands summary simple elastic plate models have been used to calculate the flexure of the lithosphere caused by volcanic loading at the canary islands and sediment loading at the moroccan continental margin by comparing the calculated flexure to observations based on seismic refraction and free air gravity anomaly data constraints have been placed on the long term 106 years elastic thickness of the lithosphere te the best fit between the calculated and observed flexure in the vicinity of the canary islands is for te 20 km this value of te also explains seismic reflection data in regions that flank the island provided that the lithosphere underlying the moroccan margin is sufficiently weak te 5 km for sediment loading to contribute little to the island flexure a backstripping study in which gravity data are used to constrain the value of te supports the suggestion that the lithosphere underlying the moroccan margin like its conjugate at the baltimore canyon trough is weak although sediment loading at the moroccan margin appears therefore to have exerted little influence on the structure of the canary islands there is evidence that island flexure may have influenced the stratigraphic development of the moroccan margin especially in the region of the slope anticline a te of 20 km is about 15 km lower than would be expected on the basis of the thermal age of the oceanic lithosphere that underlies the canary islands similar low values have been reported from oceanic islands in the pacific superswell region of french polynesia where they have been attributed to re heating of the lithosphere by one or more hotspots a general e w age progression of the volcanic rocks suggests that the canary islands were also generated by a hotspot they lack however the topographic swell and gravity geoid high which usually accompanies these features one possibility is that the low te values are the result of a pre existing weakness in the oceanic crust a te of 20 km is large enough however for a significant part of the mantle to still be involved in the support of the island loads a more likely explanation is thermal weakening by a hotspot which has been localized enough to reduce te but not to produce a swell or gravity geoid high irrespective of its cause the low te suggests that oceanic lithosphere does not necessarily progressively increase its strength with age so that even 140 ma old lithosphere is vulnerable and can in some regions be significantly weakened by later thermal and mechanical processes

radar attenuation and temperature within the greenland ice sheet 2015 american geophysical union all rights reserved the flow of ice is temperature dependent but direct measurements of englacial temperature are sparse the dielectric attenuation of radio waves through ice is also temperature dependent and radar sounding of ice sheets is sensitive to this attenuation here we estimate depth averaged radar attenuation rates within the greenland ice sheet from airborne radar sounding data and its associated radiostratigraphy using existing empirical relationships between temperature chemistry and radar attenuation we then infer the depth averaged englacial temperature the dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry where radar transects intersect boreholes radar inferred temperature is consistently higher than that measured directly we attribute this discrepancy to the poorly recognized frequency dependence of the radar attenuation rate and correct for this effect empirically resulting in a robust relationship between radar inferred and borehole measured depth averaged temperature radar inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice sheet model particularly in southern greenland this pattern suggests that past changes in surface boundary conditions temperature and accumulation rate affect the ice sheet s present temperature structure over a much larger area than previously recognized this radar inferred temperature structure provides a new constraint for thermomechanical models of the greenland ice sheet

sea level responses to rapid sediment erosion and deposition in taiwan abstract numerous studies have shown that sediment deposition can perturb sea level by several meters over millennial timescales by modifying the gravity field crustal elevation and sediment thickness relatively few studies have focused on the complementary role of erosion on sea level change despite its effects on the same quantities partly because many rapidly eroding mountains are too far from shorelines to strongly perturb sea level at the coast taiwan a mountainous island eroding rapidly within tens of km of the coast offers an opportunity to investigate the joint influences of rapid onshore erosion and rapid offshore deposition on sea level change here we develop a sediment loading history for taiwan since the previous interglacial 120 ka by compiling published erosion and deposition rate measurements and by applying a geometric marine sediment deposition and compaction model for sites without deposition rate measurements we use the resulting sediment redistribution history to drive sea level responses in a gravitationally self consistent sea level model our simulations show that the effects of rapid onshore erosion outweigh the effects of rapid offshore deposition along taiwan s east coast along the east coast of taiwan sediment redistribution induces rapid sea level fall a response that differs in sign from the coastal sea level rise induced by rapid sediment redistribution in many other river systems around the world the spatial extent of the modeled sea level fall is sensitive to the earth model particularly the effective elastic thickness of the lithosphere a sensitivity that we describe in further detail in the discussion these results suggest that sediment redistribution could have generated sea level changes of 10 m on the east coast of taiwan since 10 ka and 100 m since 120 ka this can account for some of the discrepancy between observed and modeled paleo sea level marker elevations which reduces estimates of tectonically driven rock uplift rates inferred from the elevation differences between paleo sea level markers and modeled sea level this highlights the importance of accounting for erosional unloading in interpretations of paleo sea level reconstructions and associated estimates of tectonically driven uplift rates

glacier slip and seismicity induced by surface melt many of the key processes governing fast glacier flow involve interaction between a glacier and its basal hydrological system which is hidden from direct observation passive seismic monitoring has shown promise as a tool for remotely monitoring basal processes but lack of glacier bed access prevents clear understanding of the relationships between subglacial processes and corresponding seismic emissions here we describe direct measurements of basal hydrology sliding and broadband seismicity made in a unique subglacial facility in norway during the onset of two summer melt seasons in the most pronounced of these episodes rapid delivery of surface meltwater to the bed briefly enhanced basal slip following a period of elevated high frequency seismic activity related to surface crevassing subsequent ground tilt derived from ultralong period seismic signals was associated with subglacial bedrock deformation during transient pressurization of the basal hydraulic system these signals are interpreted to represent hydraulic jacking as the supply of water to the bed exceeded the capacity of the hydraulic system enhanced slip terminated 2 5 h after it started when ice bed decoupling or increased connectivity in the basal cavity network relieved cavity overpressure the results support theoretical models for hydraulic jacking and illustrate how melt induced increases in speed can be short lived if cavity growth or ice bed decoupling allows basal water more efficient drainage

assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under pine island glacier in ice sheet models slip conditions at the base between the ice and the bed are parameterized by a friction law the most common relation has two poorly constrained parameters c and m the basal slipperiness coefficient c depends on local unobserved quantities and is routinely inferred using inverse methods while model results have shown that transient responses to external forcing are highly sensitive to the stress exponent m no consensus value has emerged with values commonly used ranging from 1 to depending on the slip processes by assimilation of pine island glacier surface velocities from 1996 to 2010 we show that observed accelerations are best reproduced with m 5 we conclude that basal motion in much of the fast flowing region is governed by plastic deformation of the underlying sediments this implies that the glacier bed in this area can not deliver resistive stresses higher than today making the drainage basin potentially more sensitive to dynamical perturbations than predicted with models using standard values m 1 or 3

dynamic interactions between the gulf of guinea passive margin and the congo river drainage basin 1 morphology and mass balance 1 a sediment budget between the congo river drainage basin and the western african margin in the gulf of guinea is proposed on the basis of published and unpublished offshore tertiary isopach maps and onshore digital elevation analysis the overall denudation of that area may be as high as 3 5 106 km3 with a maximum of 10 coming from mesozoic and cenozoic covers the southern part of the congo river basin related to uplift of the south african and kalahari shields appears as the most immature from the morphologic standpoint but provides one third of the present day sediment production it represents a maximum denudation of 150 000 km3 on the easternmost part east african rift drainage basins show a more mature relief and a maximum denudation of 270 000 km3 which can increase to 570 000 km3 if the congo drainage basin had extended up to the east branch of the rift these values are confidently established from the existence of remnant geological surfaces and can explain the volume of tertiary sediment in the gulf of guinea during upper cretaceous the most important accumulations of sediments correspond to the ogooe and kwanza fans which shows that the organization of continental drainage was different as was potential source of sediments in the northern part of the present day congo river drainage basin there was a compressional episode during santonian that may have caused significant relief and erosion along a benue chad axis

a damage mechanics assessment of the larsen b ice shelf prior to collapse toward a physically based calving law calving is a primary process of mass ablation for glaciers and ice sheets though it still eludes a general physical law here we propose a calving framework based on continuum damage mechanics coupled with the equations of viscous deformation of glacier ice we introduce a scalar damage variable that quantifies the loss of load bearing surface area due to fractures and that feeds back with ice viscosity to represent fracture induced softening the calving law is a standard failure criterion for viscous damaging materials and represents a macroscopic brittle instability quantified by a critical or threshold damage we constrain this threshold using the ice sheet system model issm by inverting for damage on the larsen b ice shelf prior to its 2002 collapse by analyzing the damage distribution in areas that subsequently calved we conclude that calving occurs after fractures have reduced the load bearing capacity of the ice by 60 10 2012 american geophysical union all rights reserved

the smoking gun for atlantic circulation changes the atlantic meridional overturning circulation amoc has long been implicated in rapid climate changes during the most recent ice age however convincing observational evidence from the deep ocean has remained limited on page 470 of this issue henry et al 1 present two independent records from a well dated high resolution sediment core from bermuda rise in the north atlantic see the figure that may be the smoking gun that paleoceanographers have been looking for

sliding of ice past an obstacle at engabreen norway at engabreen norway an instrumented panel containing a decimetric obstacle was mounted flush with the bed surface beneath 210 m of ice simultaneous measurements of normal and shear stresses ice velocity and temperature were obtained as dirty basal ice flowed past the obstacle our measurements were broadly consistent with ice thickness flow conditions and bedrock topography near the site of the experiment ice speed 0 45 m above the bed was about 130 mm d 1 much less than the surface velocity of 800 mm d 1 average normal stress on the panel was 1 0 1 6 mpa smaller than the expected ice overburden pressure normal stress was larger and temperature was lower on the stoss side than on the lee side in accord with flow dynamics and equilibrium thermodynamics annual differences in normal stresses were correlated with changes in sliding speed and ice flow direction these temporal variations may have been caused by changes in ice rheology associated with changes in sediment concentration water content or both temperature and normal stress were generally correlated except when clasts presumably collided with the panel temperature gradients in the obstacle indicated that regelation was negligible consistent with the obstacle size melt rate was about 10 of the sliding speed despite high sliding speed no significant ice bed separation was observed in the lee of the obstacle frictional forces between sediment particles in the ice and the panel estimated from hallet s 1981 model indicated that friction accounted for about 5 of the measured bed parallel force this value is uncertain as friction theories are largely untested some of these findings agree with sliding theories others do not

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

reconnaissance study of the ancient zaire congo deep sea fan zaiango project analysis of more than 19 000 km of multichannel seismic reflection data from the zaiango project covering the zaire deep sea fan allowed us to identify the oceanic crust and five seismostratigraphic units on the basin floor and abyssal plain in the absence of a direct stratigraphic tie the time frame is provided by long distance correlations a stratigraphic model for the evolution of the zaire deep sea fan is proposed a widespread major regional unconformity representing probably the eocene oligocene transition marks a drastic change of sedimentation pattern in the basin floor a pelagic aggradational sequence albian eocene overlying the oceanic crust gives way to an onlapping prograding sequence of turbidite deposits oligocene recent this change marks the onset of the ancient zaire deep sea fan triggered by the climatic change related to the greenhouse icehouse shift at the eocene oligocene transition which was responsible for a drastic increase in continental erosion and terrigenous sedimentary supply to the margin the volume of the fan is at least 0 7 mkm 3 much broader and thicker than formerly assumed a short lived episode of rapid facies progradation

continental collision tectonics evidence from the himalaya and the neighbouring regions abstract the evolution of the himalaya and the tectonics of neighbouring regions are generally believed to be due to continent to continent collision of the eurasian and the indian plates during the last decade many new data have been obtained and examined and a suite of models has been developed to explain the evolution of the himalaya and the neighbouring regions these models could broadly be classified into three categories the subdivision of the himalaya into five main structural units is now well accepted a tectogenetic diagram of the himalaya is briefly discussed ophiolites are reported to occur all along the indus tsangpo suture zone results of geochemical analysis of an ophiolite sequence from dras in kashmir himalaya are presented surface wave dispersion and attenuation investigations have revealed the crustal and upper mantle structure below the himalaya and tibet plateau the finding of low velocity layers is consistent with the concept that horizontal compression has been responsible for creating these weak zones and the inferred elevated temperatures in the vicinity of the moho discontinuity below tibet earthquake focal mechanism solutions and interpretation of landsat imagery indicate that convergence of the indian plate with eurasia continues and is accommodated partly by crustal shortening and partly by lateral mass movement at the northwest and the northeast margins of the indian plate palaeomagnetic data from the himalaya belt and central asia can be interpreted in support of molnar and tapponnier s model of continental collision

glacier motion estimation using sar offset tracking procedures two image to image patch offset techniques for estimating feature motion between satellite synthetic aperture radar sar images are discussed intensity tracking based on patch intensity cross correlation optimization and coherence tracking based on patch coherence optimization are used to estimate the movement of glacier surfaces between two sar images in both slant range and azimuth direction the accuracy and application range of the two methods are examined in the case of the surge of monacobreen in northern svalbard between 1992 and 1996 offset tracking procedures of sar images are an alternative to differential sar interferometry for the estimation of glacier motion when differential sar interferometry is limited by loss of coherence i e in the case of rapid and incoherent flow and of large acquisition time intervals between the two sar images in addition an offset tracking procedure in the azimuth direction may be combined with differential sar interferometry in the slant range direction in order to retrieve a two dimensional displacement map when sar data of only one orbit configuration are available

implications of the seismic structure for the orogenic evolution of the pyrenean range abstract the dynamic evolution of the pyrenees is discussed in the light of geophysical data recent deep seismic sounding have revealed the crustal structure of the pyrenees which is used to test the different evolutionary models proposed until now the crustal thickness of the paleozoic axial zone paz and the north pyrenean zone npz differ by more than 10 km ranging from about 30 km in the npz to 40 50 km in the paz the transition from paz to the npz identified at the surface as the north pyrenean fault npf is sharp at depth and marked by a vertical step at least in the eastern half of the range the npz is characterized by additional throws and dips of the moho in the east whereas in the west a heterogeneous middle to lower crust is encountered with high velocity anomalies the seismic results suggest that the paz and the npz belong to different plates the npf being the plate boundary these results are inconsistent with evolutionary models involving lithospheric subduction or crustal doubling and intracratonic rifting with the main tectonic lineations following nne ssw directions they rather suggest that after a period of extension two main orogenic events took place a phase involving shearing and thinning which affected mainly the present day npz and a later compressive phase which explains the building up of the chain the thickening of the crust and the enhancement of a pre existing difference in crustal thickness between the european and iberian plates

ice thickness measurements and volume estimates for glaciers in norway glacier volume and ice thickness distribution are important variables for water resource management in norway and the assessment of future glacier changes we present a detailed assessment of thickness distribution and total glacier volume for mainland norway based on data and modelling glacier outlines from a landsat derived inventory from 1999 to 2006 covering an area of 2692 81 km 2 were used as input we compiled a rich set of ice thickness observations collected over the past 30 years altogether interpolated ice thickness measurements were available for 870 km 2 32 of the current glacier area of norway with a total ice volume of 134 23 km 3 results indicate that mean ice thickness is similar for all larger ice caps and weakly correlates with their total area ice thickness data were used to calibrate a physically based distributed model for estimating the ice thickness of unmeasured glaciers the results were also used to calibrate volume area scaling relations the calibrated total volume estimates for all norwegian glaciers ranged from 257 to 300 km 3

distributed ice thickness and volume of all glaciers around the globe 1 a new physically based approach for calculating glacier ice thickness distribution and volume is presented and applied to all glaciers and ice caps worldwide combining glacier outlines of the globally complete randolph glacier inventory with terrain elevation models shuttle radar topography mission advanced spaceborne thermal emission and reflection radiometer we use a simple dynamic model to obtain spatially distributed thickness of individual glaciers by inverting their surface topography results are validated against a comprehensive set of thickness observations for 300 glaciers from most glacierized regions of the world for all mountain glaciers and ice caps outside of the antarctic and greenland ice sheets we find a total ice volume of 170 103 21 103 km3 or 0 43 0 06 m of potential sea level rise

interannual variability of glacier basal pressure from a 20 year record basal pressure has been recorded at the svartisen subglacial laboratory northern norway for 20 years and is measured by load cells installed at the ice rock interface under 200 m of glacier ice synchronous pressure variations between load cells are investigated as evidence of stress redistribution and hydrological bed connectivity a running pearson correlation is used to study the temporal variation in the response of several sensors by studying the nature of this correlation as well as the correlation between sensor pairs it is possible to investigate the evolution of the degree of synchronous response and to some extent basal connectivity at the glacier bed persistent seasonal variations associated with the melt season are observed throughout the measurement period indicating dependence on surface hydrological forcing overlying this pattern specific years with longer periods of positive and negative correlation of pressure between sensors are presented to show contrasting interannual variability in basal pressure an anticorrelated connectivity is associated with a local increase in the rate of daily subglacial discharge and is caused by load transfer or passive cavity opening stable weather appears to enhance connectivity of the sensors which is attributed to the development of a persistent drainage system and stress redistribution

geoid height age relation from seasat altimeter profiles across the mendocino fracture zone twenty eight seasat altimeter profiles crossing the mendocino fracture zone are used together with seafloor ages determined from magnetic lineations to estimate the change in oceanic geoid height with age between ages of 15 and 135 m y an unbiased estimate of the overall geoid offset along each profile is determined from a least squares fit of the along track derivative of the geoid to the geoid slope predicted from a simple two layer gravitational edge effect model uncertainties based upon the statistical properties of each profile are also determined a geoid slope age relation is constructed by normalizing the geoid offsets and uncertainties by the age offsets the results are in agreement with geoid slope age relations determined from symmetrically spreading ridges sandwell and schubert 1980 however the fracture zone estimates have smaller uncertainties and show less scatter a comparison of these results with the geoid slope age prediction of the boundary layer cooling model shows that the thermal structure begins to deviate from this model at an early age 20 40 m y a plate cooling model with a thickness of 125 km is most compatible with the geoid slope age estimates although significant deviations occur these may indicate that the lithospheric thermal structure is not entirely age dependent

neotectonic reactivation of shear zones and implications for faulting style and geometry in the continental margin of ne brazil abstract the eastern continental margin of south america comprises a series of rift basins developed during the breakup of pangea in the jurassic cretaceous we integrated high resolution aeromagnetic structural and stratigraphic data in order to evaluate the role of reactivation of ductile neoproterozoic shear zones in the deposition and deformation of post rift sedimentary deposits in one of these basins the paraiba basin in northeastern brazil this basin corresponds to the last part of the south american continent to be separated from africa during the pangea breakup sediment deposition in this basin occurred in the albian maastrichtian eocene miocene and in the late quaternary however our investigation concentrates on the miocene quaternary which we consider the neotectonic period because it encompasses the last stress field this consisted of an e w oriented compression and a n s oriented extension the basement of the basin forms a slightly seaward tilted ramp capped by a late cretaceous to quaternary sedimentary cover 100 400 m thick aeromagnetic lineaments mark the major steeply dipping ductile e w to ne striking shear zones in this basement the ductile shear zones mainly reactivated as strike slip normal and oblique slip faults resulting in a series of miocene quaternary depocenters controlled by ne e w and a few nw striking faults faulting produced subsidence and uplift that are largely responsible for the present day morphology of the valleys and tablelands in this margin we conclude that precambrian shear zone reactivation controlled geometry and orientation as well as deformation of sedimentary deposits until the neogene quaternary

the great west african tertiary coastal uplift fact or fiction a perspective from the angolan divergent margin 1 we explore exhumation in the coastal kwanza basin by combining analyses of tertiary hiatuses and apatite fission tracks planktonic biozones show five major hiatuses in the oligo miocene and plio pleistocene between gaps oligo miocene strata accumulated under marine conditions a marine setting refutes the idea of a massively raised coastal plateau in the mid tertiary marine conditions continued until 5 ma fission track data suggest three thermal events 150 ma during rifting and volcanism 100 70 ma during shortening and volcanism and 20 10 ma during exhumation tertiary uplift was spatially highly variable for the kwanza basin we infer that tertiary uplift on the west african margin is indeed a fact but that estimates of uplift timing and size are unreliable when extrapolated to adjoining areas massive uplift 2000 4000 m of the precambrian craton had little structural effect in the outer basin instead minor uplifts on the shelf drove late tertiary deformation on the slope

hydrological controls on patterns of surface internal and basal motion during three spring events haut glacier d arolla switzerland three early melt season high velocity events or spring events occurred on haut glacier d arolla switzerland during the melt seasons of 1998 and 1999 the events involve enhanced glacier velocity during periods of rapidly increasing bulk discharge in the proglacial stream and high subglacial water pressures however differences in spatial patterns of surface velocity internal ice deformation rates the spatial extent of high subglacial water pressures and in rates of subglacial sediment deformation suggest different hydrological and mechanical controls the data from two of the events suggest widespread ice bed decoupling particularly along a subglacial drainage axis creating the highest rates of basal motion and plug flow in the overlying ice the other event showed evidence of less extensive ice bed decoupling and sliding along the drainage axis with more mechanical support for ice overburden transferred to areas adjacent to decoupled areas we suggest that 1 plug flow may be a common feature on glaciers experiencing locally induced reductions in basal drag 2 under certain circumstances enhanced surface motion may be due in part to non locally forced enhanced bed deformation and 3 subglacial sediment deformation is confined to a depth of the order of centimetres to decimetres

basal conditions at engabreen norway inferred from surface measurements and inverse modelling engabreen is an outlet glacier of the svartisen ice cap located in northern norway it is a unique glacier due to the svartisen subglacial laboratory which allows direct access to the glacier bed in this study we combine both sub and supraglacial observations with ice flow modelling in order to investigate conditions at the bed of engabreen both spatially and temporally we use the full stokes model elmer ice and satellite based surface velocity maps from 2010 and 2014 to infer patterns of basal friction direct measurements of basal sliding and deformation of lower layers of the ice are used to adjust the ice viscosity and provide essential input to the setup of our model and influence the interpretation of the results we find a clear seasonal cycle in the subglacial conditions at the higher elevation region of the study area and discuss this in relation to the subglacial hydrological system our results also reveal an area with an overdeepening where basal friction is significantly lower than elsewhere on the glacier all year round we attribute this to either water pooling at the base or saturated sediments and increased strain heating at this location which softens the ice further

plume tectonics as a cause of mass wasting on the southeast greenland continental margin deposition of sediments on the southeast greenland shelf is limited by its narrow width low degrees of tectonic subsidence and lack of a block faulted basement topography a marked hiatus on the continental slope removing the eocene and oligocene together with a late eocene early oligocene hiatus on the continental rise can be related to mass wasting along the continental slope erosion correlates with a phase of rapid tectonic subsidence caused by relative retreat of the iceland plume from the area affecting the oceanic crust more than the continental shelf relative deepening of 1 km over 10 my steepened the continental slope and triggered erosion following the cessation of rapid subsidence a submarine fan progressively onlapped the continental slope the lack of a major mid oligocene erosion surface on the shelf despite shallow water depths indicates that the magnitude of the mid oligocene eustatic sea level fall is much less than the 200m predicted by the exxon sea level curve

on the driving processes of the atlantic meridional overturning circulation because of its relevance for the global climate the atlantic meridional overturning circulation amoc has been a major research focus for many years yet the question of which physical mechanisms ultimately drive the amoc in the sense of providing its energy supply remains a matter of controversy here we review both observational data and model results concerning the two main candidates vertical mixing processes in the ocean s interior and wind induced ekman upwelling in the southern ocean in distinction to the energy source we also discuss the role of surface heat and freshwater fluxes which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state we conclude that both wind driven upwelling and vertical mixing are likely contributing to driving the observed circulation to quantify their respective contributions future research needs to address some open questions which we outline

characteristics of subglacial drainage systems deduced from load cell measurements rock tunnels beneath engabreen northern norway permit access to the ice bedrock interface beneath a 210 m thick glacier eight load cells have been installed in the bedrock of the glacier sole along a 22 m transect with some interruptions the load cells have been logged at 15 min intervals since december 1992 here we analyse the records until 2003 load cell signals measure stresses acting normal to the bedrock and usually log the pressure of the thin water film between the basal ice and the bed occasionally there are distinct pressure events characterized by short lived hours local minima often followed by a maximum before decaying to background load cell pressure the amplitudes of these pressure events are of the order of 0 01 1 mpa and depend on the placement of the sensor and the state of the subglacial drainage system we identify winter and summer pressure regimes the winter regime is characterized by few pressure events of large pressure amplitude influencing all load cells a lag of 0 6 days is observed between surface forcing rain and pressure events during winter the summer regime typically has periods of daily pressure events of low amplitude no delay is seen between surface forcing and pressure events during the summer regime in summer the onset of a pressure event is correlated with a local maximum of the derivative of the subglacial discharge record whereas no such relation is found during the winter regime the transition from winter to summer may june is easily detectable and is strongly correlated with a rapid increase in subglacial discharge and the transition to a dominating r channel system the autumn transition is less clearly defined but has usually occurred by the beginning of november stress bridging an increase in bed pressure at the edge of low pressure channels is recorded during the summer regime water pressures at the bed are connected or unconnected to the drainage system pressure increases in the connected system beyond local normal stress values lead to an uplift of the connected system and a pressure drop in the unconnected system the occurrence of pressure events is determined by the capacity of the drainage system uplift is controlled by local normal stress values and not mean ice overburden pressure

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

episodic cenozoic tectonism and the development of the nw european passive continental margin the north atlantic margins are archetypally passive yet they have experienced post rift vertical movements of up to kilometre scale the cenozoic history of such movements along the nw european margin from ireland to mid norway is examined by integrating published analyses of uplift and subsidence with higher resolution tectono stratigraphic indicators of relative movements including results from the stratagem project three episodes of epeirogenic movement are identified in the early mid and late cenozoic distinct from at least one phase of compressive tectonism two forms of epeirogenic movement are recognised referred to as tilting coeval subsidence and uplift rotations 1 over distances of 100s of kilometres and sagging strongly differential subsidence rotations up to 4 over distances 100 km each epeirogenic episode involved relatively rapid 10 ma km scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity bounded stratigraphic units early cenozoic tilting late paleocene to early eocene c 60 50 ma caused the basinward progradation of shelf slope wedges from elongate uplifts along the inner continental margin and from offshore highs mid cenozoic sagging late eocene to early oligocene c 35 25 ma ended wedge progradation and caused the onset of contourite deposition in deep water basins late cenozoic tilting early pliocene to present 4 0 5 ma again caused the basinward progradation of shelf slope wedges from uplifts along the inner margin including broad dome like features and from offshore highs the early mid and late cenozoic epeirogenic episodes coincided with atlantic plate reorganisations but the observed km scale tectonic movements are too large to be accounted for as flexural deflections due to intra plate stress variations mantle lithosphere interactions are implied but the succession of epeirogenic episodes of differing form are difficult to reconcile with the various syn to post rift mechanisms of permanent and or transient movements proposed in the hypothetical context of a plume beneath iceland the epeirogenic movements can be explained as dynamic topographic responses to changing forms of small scale convective flow in the upper mantle tilting as coeval upwelling and downwelling above an edge driven convection cell sagging as a loss of dynamic support above a former upwelling the inferred cenozoic succession of epeirogenic tilting sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening a process that may also be the underlying cause of plate reorganisations the postulated episodes of flow reorganisation in the ne atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin as well as on other atlantic type passive margins

structural evolution and sequence formation offshore south gabon during the tertiary abstract the present investigation of the tertiary succession offshore south gabon is based on seismic surveys of 1985 1989 vintage and wells covering the southern part of gabonese waters in general the tertiary basin off south gabon evolved during two structural stages during the paleocene and early middle eocene a trough like basin subsided slowly towards the west during this period there was a cyclic development of the succession which consists of up to four sequences mostly bounded by type 2 sequence boundaries the sequential changes were induced by eustatic sea level changes this pattern changed during the neogene when the evolution was characterized by continental crustal flexuring in early middle miocene times this resulted in strong subsidence of the western part of the basin and uplift of the eastern part consequently the eastern part was exposed to strong erosion and a thick middle miocene succession was deposited west of the shelf approximately the present day shelf especially on the slope the clinoformal and lens shaped middle late miocene succession which consists of four sequences bounded by type 1 sequence boundaries has been controlled by eustatic changes in sea level in addition to the overall subsidence pattern salt tectonics has modified the external and internal architecture of the sequences on a local scale canyon formation in connection with slumping results in the establishment of well developed fan systems on the lower part of the slope and on the basin floor area

englacial water distribution in a temperate glacier from surface and borehole radar velocity analysis we have obtained common offset common midpoint cmp and borehole vertical vrp ground penetrating radar profiles close to the margin of falljokull a small steep temperate valley glacier situated in southeast iceland velocity analysis of cmp and vrp surveys provided a four layered velocity model this model was verified by comparison between the depths of englacial reflectors and water channels seen in borehole video and from the depths of boreholes drilled to the bed in the absence of sediment within the glacier ice radar velocity is inversely proportional to water content using mixture models developed by paren and looyenga the variation of water content with depth was determined from the radar velocity profile at the glacier surface the calculated water content is 0 23 0 34 velocity 0 166 m ns 1 which rises sharply to 3 0 4 1 velocity 0 149 m ns 1 at 28 m depth interpreted to be the level of the piezometric surface below the piezometric surface the water content drops slowly to 2 4 3 3 velocity 0 152 m ns 1 until 102 m depth where it falls to 0 09 0 14 velocity 0 167 m ns 1 the water content of the ice then remains low to the glacier bed at about 112 m these results suggest storage of a substantial volume of water within the glacier ice which has significant implications for glacier hydrology ice rheology and interpretations of both radar and seismic surveys