the exponential flow law applied to necking and folding of a ductile layer summary the uniaxial exponential law has been applied to experimental results for steady state creep including that of wet and dry olivine pyroxenite and carbonates at a deviatoric stress greater than 100 mpa such stress levels likely occur in the upper mantle lithosphere and middle crust in a layered rock in layer parallel extension or shortening high deviatoric stress can occur in the stiffest layers for example a dolomite layer in fine grained marble under assumptions of isotropy and incompressibility the uniaxial law yields where j2 sijsij 2 is an invariant of the deviatoric stress tensor sij linearization about a homogeneous basic state of flow yields equations identical in form to those obtained for the familiar power law this establishes expressions for an effective viscosity and stress exponent where is an invariant of the basic state deformation rate these results allow application of existing analytical folding and necking solutions for a rock layer of power law fluid to a rock layer with an exponential flow law the effective stress exponent for the exponential flow law increases with decreasing temperature through the dependence of c on the latter and weakly with increasing deformation rate for dry olivine effective stress exponents are between 10 and 30 for temperatures between 400 and 600 c with little dependence on deformation rate finite element simulations employing full non linear forms of the flow laws show that large strain necking is nearly identical for power law and exponential flow laws the results suggest that the instability in necking and folding in ductile rock layers can be considerably stronger than inferred from results based on flow laws representing diffusion and dislocation creep the large values of the effective stress exponent ne 15 that may be attained for exponential flow laws can account for observed outcrop scale ductile necking

hydrocarbon plumbing systems of salt minibasins offshore angola revealed by three dimensional seismic analysis analysis of three dimensional seismic data from the lower congo basin offshore angola reveals numerous fluid flow features in the miocene to holocene succession and the potential for large shielded traps underneath basinward overhanging salt structures the fluid flow evidence includes present day sea floor pockmarks clustered above salt structures pliocene pleistocene stacked paleopockmarks and miocene pockmark fields other fluid flow features include high amplitude cylindrical pipe structures 60 to 300 m 197 984 ft wide and 25 to 300 m 82 984 ft high within lower and middle miocene strata thick 150 m 492 ft high reflectivity zones within the pliocene succession associated with bottom simulating reflections and subvertical low amplitude chimneys originating from the deeper section 1 km 0 6 mi beneath the sea floor the miocene pockmark fields occur at a specific horizon suggesting a regional fluid expulsion event at ca 12 ma and the miocene fluid flow regime is interpreted to be dominated by thermogenic fluids supplied via carrier beds and leaking vertically above structural highs the pliocene pleistocene fluid flow regime was dominated by short distance vertical fluid migration and expulsion related to early stage diagenetic processes involving biogenic methane and pore water the present day fluid flow regime is inferred to be dominated by thermogenic fluids primarily controlled by kilometer scale salt flank controlled migration the study emphasizes the use of seismically imaged fluid flow features in hydrocarbon systems analysis by documenting the evolution of an overburden plumbing system through time involving several fluid types and flow regimes depending on the spatiotemporal availability of thermogenic and diagenetic fluids and the tectonostratigraphic occurrence of aquifers traps and seals

geophysical characterisation of active thermogenic oil seeps in the salt province of the lower congo basin part i detailed study of one oil seeping site abstract we report the geophysical characterisation of natural oil seep sites through a combination of sea surface evidence of oil leakage from spatial imagery with a large collection of seafloor and subsurface geophysical data this paper provides a detailed characterisation of one selected active seep site and identifies possible specific feature of oil seep sites the oil seep is a complex shaped feature on the seafloor consisting of a cluster of heterometric pockmarks inside a main depression area and peripheral metre scale seafloor mounds a strong deformation related to salt tectonics controls the location of the seafloor source by fracturing the overburden the associated thermal anomaly induces a vertical modification position of the base of the gas hydrate stability zone bghsz that is used as a fluid migration route towards the crest of the diapir the combination of local depressions and seafloor amplitude anomalies linked with vertical high amplitude pipes rooted on the bghsz suggests a focused fluid flow towards the seafloor in peripheral areas the seafloor mounds are linked by shallow faults to buried high amplitude patches on sub bottom profiler sections the combination of restricted size seafloor mounds with a progressive deepening of the high amplitude from the seafloor suggests a substantial decrease of the hydrocarbon flow towards peripheral areas the proximity of actively oil supplying seafloor depressions and seafloor mounds shows that the hydrocarbon flow rapidly decreases laterally the thermogenic seep site is affected by two consistent and sub parallel reflections with negative polarity the first is interpreted as the methane related bghsz the second could correspond to the base of a thermogenic bghsz produced by a mixture of heavier gas the seafloor roughness and double bsr appear to be specific features of oil seep sites the geophysical features revealed at a localised study area will be extrapolated towards a larger province for relevance validation

physics controlled thickness of shear zones caused by viscous heating implications for crustal shear localization we evaluate the parameters that control the thickness of ductile shear zones that are generated by viscous heating we employ two dimensional thermomechanical numerical models to simulate shear zone development under compression results show that the shear zone thickness is essentially independent on the numerical resolution and the initial size of a weak inclusion that triggers shear localization a simple scaling law is derived which predicts the thickness with six physical parameters far field stress and strain rate thermal conductivity both constant and temperature dependent initial temperature activation energy and stress exponent the scaling law is confirmed by numerical simulations for a wide range of parameters for crustal deformation conditions typical temperature increase ranges between 50 c and 200 c and the predicted thickness is between 1 km and 7 km these thicknesses agree with natural crustal and lithospheric shear zones suggesting that shear heating is a process controlling crustal shear zone formation

extensional inheritance and surface processes as controlling factors of mountain belt structure surface processes and inherited structures are widely regarded as factors that strongly influence the evolution of mountain belts the first order effects of these parameters have been studied extensively throughout the last decades but their relative importance remains notoriously difficult to assess and document we use lithospheric scale plane strain thermomechanical model experiments to study the effects of surface processes and extensional inheritance on the internal structure of contractional orogens and their foreland basins extensional inheritance is modeled explicitly by forward modeling the formation of a rift basin before reversing the velocity boundary conditions to model its inversion surface processes are modeled through the combination of a simple sedimentation algorithm where all negative topography is filled up to a prescribed reference level and an elevation dependent erosion model our results show that 1 extensional inheritance facilitates the propagation of basement deformation in the retro wedge and 2 increases the width of the orogen 3 sedimentation increases the length scale of both thin skinned and thick skinned thrust sheets and 4 results in a wider orogen 5 erosion helps to localize deformation resulting in a narrower orogen and a less well developed retro wedge a comparison of the modeled behaviors to the high atlas the pyrenees and the central alps three extensively studied natural examples characterized by different degrees of inversion is presented and confirms the predicted controls of surface processes and extensional inheritance on orogenic structure

on the meaning of peak temperature profiles in inverted metamorphic sequences inverted metamorphic sequences ims are common features of main thrust systems on earth they exhibit an upwards continuous increase in peak temperature conditions and thereby constitute evidence of the close relationship between the thermal field evolution and tectonic processes heat advection and shear heating are known to allow the formation of such metamorphic signatures heat diffusion also plays an important role in temperature distribution on both sides of the thrust other advection processes such as erosion or accretion may also cause a local peak temperature inversion each one of these processes therefore affects the thermal field around the thrust however despite the crucial importance of all these processes for the interpretation of the inverted peak temperature signatures their respective influences have never been quantified and compared all together to address this issue we propose an innovative coupled approach i we use two dimensional numerical models that simulate various thrust systems allowing for a wide diversity of setups to illustrate this study we focus on intracontinental thrust systems for which all processes listed are likely to play a key role in the thermal evolution we perform a parametric study including kinematic settings i e convergence erosion and accretion thermal properties mechanical strength and heat sources ii dimensionless numbers based on parameters are used to quantify the relative contributions of each process to the thermal budget evolution hence the three thermal processes i e heat diffusion heat advection and shear heating are compared with each other via three dimensionless combinations of the peclet and brinkman numbers rdif radv and rpro respectively erosion and accretion are compared separately based on a fourth dimensionless number rea iii we analytically examine the inverted peak temperature recorded along profiles that are perpendicular to the thrust zone defined in our numerical experiments each peak temperature profile presenting an inversion can then be characterized by a function of approximation involving six meaningful parameters the location ff and width ff of the maximum peak temperature inversion the characteristic peak temperature tcte and gradient glb beneath the inversion zone and the inversion related contrasts in the peak temperature t and gradient g this coupled approach linking numerical modelling and analytical treatment allows to quantitatively interpret ims in terms of the processes involved the application of our method to intracontinental thrust systems demonstrates that shear heating and erosion support significant inversions but that the relative contributions of each process have meaningful consequences our results reveal that competition between shear heating and heat diffusion on the one hand and between erosion and accretion on the other hand have a high impact in particular the variability in the rock s mechanical strength strongly influences the features of peak temperature inversions consequently none of these processes can be ignored our results highlight the major importance of the rheology of rocks in the thermal evolution of shear zones finally our methodology is not only restricted to the analysis of numerical data but also constitutes a way of broad interest to analyse peak temperature signatures around any shear zone

lateral variation in structural style of mountain building controls of rheological and rift inheritance many mountain belts exhibit significant along strike variation in structural style with changes in the width of the orogen the geometry and kinematics of the crustal scale thrust system and the degree of partitioning between pro and retro wedge deformations although the main factors controlling first order structural style are understood the cause of these lateral variations remains to be resolved here we focus on the pyrenees characterized by significant lateral variation in structural style with a thrust system involving more and thinner thrust sheets in the eastern section than in the western part similarly the prior mesozoic rifting event was characterized by significant lateral variation in structure we integrate available geological and geophysical data with forward lithospheric scale numerical models we show that lateral variation in crustal strength attributed to inherited variscan crustal composition accentuated during mesozoic rifting explains the variation in structural style observed during pyrenean mountain building

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

patterns of carbonate authigenesis at the kouilou pockmarks on the congo deep sea fan abstract different types of seep carbonates were recovered from the kouilou pockmarks on the congo deep sea fan in approximately 3100 m water depth the carbonate aggregates are represented by pyritiferous nodules crusts and slabs tubes and filled molds the latter are interpreted to represent casts of former burrows of bivalves and holothurians the nodules consisting of high mg calcite apparently formed deeper within the sediments than the predominantly aragonitic crusts and slabs nodule formation was caused by anaerobic oxidation of methane dominantly involving archaea of the phylogenetic anme 1 group whereas aragonitic crusts resulted from the activity of archaea of the anme 2 cluster evidence for this correlation is based on the distribution of specific biomarkers in the two types of carbonate aggregates showing higher hydroxyarchaeol to archaeol ratios in the crusts as opposed to nodules formation of crusts closer to the seafloor than nodules is indicated by higher carbonate contents of crusts probably reflecting higher porosities of the host sediment during carbonate formation this finding is supported by lower 18 o values of crusts agreeing with precipitation from pore waters similar in composition to seawater the aragonitic mineralogy of the crusts is also in accord with precipitation from sulfate rich pore waters similar to seawater moreover the interpretation regarding the relative depth of formation of crusts and nodules agrees with the commonly observed pattern that anme 1 archaea tend to occur deeper in the sediment than members of the anme 2 group methane represents the predominant carbon source of all carbonates 13 c values as low as 58 9 v pdb and the encrusted archaeal biomarkers 13 c values as low as 140 v pdb oxygen isotope values of some nodular carbonates ranging from 3 9 to 5 1 v pdb are too high for precipitation in equilibrium with seawater probably reflecting the destabilization of gas hydrates which are particularly abundant at the kouilou pockmarks

shear heating induced lithospheric scale localization does it result in subduction abstract even though it is a well established fact that the earth is currently in a plate tectonics mode the question on how to break lithospheric plates and initiate subduction remains a matter of debate here we focus on shear heating as a potential mechanism to cause lithospheric shear localization and subsequent subduction initiation in oceanic plates it is shown that shear heating under some conditions i facilitates the formation of a lithospheric scale shear zone and ii is capable of stabilizing a lithospheric scale shear zone thus creating the necessary condition for subduction initiation to occur furthermore we demonstrate that not only the localization process is of importance but also the post localization stage where rapidly growing convective instabilities might prevent an incipient subduction zone from developing we develop scaling laws for both the localization and the post localization stages in the case of lithospheric localization the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere thus of the order of the competent layer thickness this shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller scale heterogeneities we compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that for earth like conditions a dry olivine rheology is more likely to result in subduction initiation a large plate age does not always increase the potential for subduction initiation as it increases the potential for convective instabilities to occur instead an optimal plate age exists for shear heating induced subduction initiation which is around 40 ma for a wet olivine rheology

channel networks a geomorphological perspective the study of channel networks has been dominated since 1966 by the random model however recent work has shown 1 that although the topological properties of small networks conform to the random model more closely than those of large ones even small networks exhibit systematic deviations from topological randomness and 2 that the topological and length properties of channel networks are controlled to a large degree by the spatial requirements of subbasins and the need for these subbasins to fit together in space by the size sinuosity and migration rate of valley bends and by the length and steepness of valley sides the factors that control the density properties of channel networks vary with the scale of the investigation and the geomorphic processes governing channel initiation although progress has been made toward a satisfactory stream junction angle model further work is needed the evolution of channel networks has been investigated by a variety of methods including the development of conceptual and simulation models the monitoring of small scale badland and experimental drainage basins and the substitution of space for time the morphology of most channel networks is largely inherited from the past or strongly influenced by inherited forms inasmuch as there is no way of ever knowing the origin or complex history of such networks the use of stochastic models in their study seems unavoidable

neogene sedimentation history of the congo fan abstract high resolution seismic reflection data were interpreted to develop a model for the neogene sedimentation in the congo fan area identification of two reflectors representing marine isotope stages 6 and 12 allows a distinction between the early and the late quaternary a single sediment source system during the late paleogene early neogene gradually changes into a two sediment source system in the late neogene since the late quaternary the congo river has been the prevailing sediment source the evolving benguela current further modified the system and led to a high biogenic productivity evidence for gas in the sediments can be found in the whole area of investigation additionally indications for a tectonic control of the congo canyon have been found in the distribution of sedimentary units and unit thicknesses

the drainage of africa since the cretaceous much of the drainage of africa is relatively youthful many of its major rivers have shown substantial changes in their courses since the break up of gondwanaland in the cretaceous in addition many of the rivers have distinctive morphological characteristics such as inland deltas cataracts and elbows of capture tectonic and climatic changes including the development of the east african rift system and the aridification of the quaternary help to explain the nature of these rivers the history of the saharan rivers the niger the nile the congo the cunene the zambezi the limpopo and the orange is reviewed

bedload entrainment travel lengths step lengths rest periods studied with passive iron magnetic and active radio tracer techniques this paper reports results from bedload transport investigations with active radio and passive iron magnetic tracers in the lainbach a step pool mountain river in bavaria southern germany the spatial distributions of the iron tracers after flood events can be best described by exponential or gamma distributions there is some indication of a tendency of size selective transport of the iron tracers but there is also a considerable amount of scatter in the correlations between weight size and travel length owing to the masking influence of other variables such as the shape of the particles and different positions in the river bed the experiments with artificial magnetic tracers showed that elongated pebbles rods had the longest mean transport distance platy ones discs remained relatively close to the starting points the particles from the pool showed the greatest transport lengths and a 100 per cent chance of being eroded the pebble transmitter system petsy consists of transmitters implanted into individual pebbles a computerized receiver a stationary antenna system with an antenna switchboard and a data logging system the particles do not move continuously but in a series of transport steps and non movement intervals a single value for a given size class is not adequate to describe the critical conditions of entrainment under natural circumstances a probability approach is much more suitable the critical unit discharges total discharge divided by active channel width along the measuring reach are dependent on river bed morphology in the steps bedload needs higher unit discharges to be entrained once entrained the transport of bedload is stochastic in nature and the single particle transport is controlled by the step lengths and the duration of rest periods the distributions of both parameters can be approximated by exponential functions applying the stochastic concept proposed by einstein the mean values of the random variables step length and duration of rest period measured with the petsy technique were used for the simulation of spatial distributions of bedload particles from point sources more field and laboratory data are needed to include varying flow and roughness conditions with tracers representing different particle characteristics

geodynamic regimes of subduction under an active margin effects of rheological weakening by fluids and melts the dynamics of subduction under an active margin are analysed by using a 2d coupled petrological thermomechanical numerical model of an oceanic continental subduction system the model includes spontaneous slab bending dehydration of the subducted crust aqueous fluid transport partial melting of both crustal and mantle rocks as well as melt extraction processes resulting in magmatic arc crust growth based on our models the following five geodynamic regimes of subduction which may potentially form at active margins are identified i stable subduction with no backarc spreading centre and without plumes in the mantle wedge ii retreating subduction with a focused backarc spreading centre and without plumes iii retreating subduction with distributed intra arc extension and trans lithospheric sedimentary plumes iv advancing subduction with underplating laterally extending sub lithospheric plumes and v stable to advancing subduction with stationary laterally limited sub lithospheric plumes transitions between these different regimes are mainly caused by the concurrence of rheological weakening effects of i aqueous fluids percolating from the subducting slab into the mantle wedge and ii melts propagating from the partially molten areas formed in the mantle wedge towards the surface the aqueous fluids mainly affect the forearc region strong fluid related weakening promotes plate decoupling and reduces subduction drag and thus results in stacking of sediments in the accretion prism in contrast reduced weakening by fluids results in strong coupling of the plates and leads to advancing collision like subduction with enhanced subduction erosion thickening of the overriding plate and large sedimentary plumes in the mantle wedge are the consequences on the other hand melts extracted from the hot regions above the slab rheologically weaken the lithosphere below the arc which thus controls overriding plate extension and shortening strong rheological weakening by melts in combination with weak plate coupling trigger retreating subduction with a pronounced backarc spreading centre also weakening of the continental lithosphere by melts extracted from trans lithospheric sedimentary plumes generate a weak channel through which these structures may be emplaced into subarc crust if there is insufficient melt related weakening plumes cannot ascend but extend horizontally and thus underplate the lithosphere

viscous heating allows thrusting to overcome crustal scale buckling numerical investigation with application to the himalayan syntaxes article i nfo the eastern and western himalayan syntaxes are crustal scale antiforms whose metamorphic evolution is coeval in the india asia collisional history finite element modelling of lithospheric shortening and the resultingcrustal scale folds havepreviously been used todebate the structuralconditions for the development of such structures however numerical models could not account for the metamorphic evolution associated with folding in continuation of previous work we present two dimensional finite element modelling of lithospheric shortening inwhich thermal effects are implemented the new models are consistent with earlier interpretations that crustal folding is the plausible mechanism that shaped the two himalayan syntaxes they further lend support to the concept that lithospheric buckling is a basic response to large scale continental shorteningandan efficientmountainbuilding process introducingviscous shearheating shows however that the buckling mode shifts to a thrusting mode within few shortening depending on the initial thermal rheological structure of the deforming lithosphere the change to the thrusting mode prohibits further fold amplification and lateral fold propagation this mayexplainwhy in continental lithosphere crustal scale folds are isolated whereas regular periodic crustal fold trains are rare focusing deformation on through limbs thrust zones accompanies the establishment of inverted metamorphic gradients these results offer new working hypotheseson howlargethrusts like thehimalayamain centralthrustnucleate results further show that the thermal structure of the lithosphere strongly controls three fundamental deformation and metamorphic modes 1 a cold lithosphere mainly deforms by thermally activated thrusting and exhibits large areas with significant tectonic overpressures twice lithostatic 2 a warm lithosphere is essentially buckled and significant tectonic overpressure builds up in the upper crust and 3 a hot lithosphere tends to thicken homogenously and mainly records lithostatic pressures below the upper crust

lithospheric pressure depth relationship in compressive regions of thickened crust we investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically our results show that in a convergent tectonic setting pressure can be considerably higher than lithostatic the pressure resulting from the weight of the overburden increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root respectively the contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle where rocks are thought to deform brittlely in these domains pressure gradients twice lithostatic can be achieved the contribution of horizontal stresses is less important in the ductile domains as diverential stresses are progressively relaxed nevertheless the evects are still noteworthy especially close to the brittle ductile transition flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust as a result of the combination of the two mechanisms the pressure gradient varies vertically through the lithosphere ranging from negative inverted gradients to gradients up to several times the lithostatic gradient the pressure values range from one to two times the lithostatic values 1rgz to 2rgz

geophysical characterisation of active thermogenic oil seeps in the salt province of the lower congo basin part ii a regional validation abstract the lower congo basin is known to discharge a substantial volume of oil towards the sea surface from more than one hundred seafloor seep sites distributed throughout the deep province of the lower congo basin a large geochemical coring survey confirmed the presence of oil on the seabed the combination with the seismic data considerably improved the identification of the origin of the oil slicks on the seabed multiple specific geophysical characteristics of thermogenic hydrocarbon seep sites were highlighted in a previous detailed analysis of seismic datasets this study aims to test the characteristics previously identified at regional scale the active discharge zone is limited to the distal province of the basin it is characterised by strong compression shortening due to the sliding of the post salt super sequence resulting in numerous salt diapirs that control the location of seafloor oil seep sites the paper describes the bathymetric reflectivity amplitude and sub bottom profiler characteristics of a group of thermogenic seep sites they correspond essentially to submarine mounds or pockmarks of complex and irregular shape surrounded by hummocky mounds active oil pockmarks are systematically associated with positive amplitude anomalies on the seabed and are linked to vertical high amplitude columns rooted in the seismic reflector and associated with the base of the gas hydrate stability zone the sub bottom profiler data shows that the hummocky mounds are connected by a network of faults to high amplitude bodies buried under a consistent sediment thickness based on the definition of specific geophysical features seafloor mounds complex shape and irregular pockmarks positive anomalies of seafloor amplitude high amplitude vertical pipes we identified a series of potential oil seep sites at basin scale the mounds are particularly recognizable using the seismic curvature attribute we have identified 2946 individual hummocks that are grouped in 50 zones with a density of 35 240 per km2 they are believed to be associated with asphalt storage on the seafloor and related to the biodegradation of heavy oils during hydrocarbon dysmigration through the sedimentary pile unusual double bsrs occur over the study area these are also a specific feature of thermogenic seep sites barely half of the potential sites identified on the seismic datasets are associated with recurrent oil slicks at the sea surface the proportion of remaining anomalies may be associated with inactive seep sites over the period of satellite based monitoring or gas dominated seep sites

from gullies to mountain belts a review of sediment budgets at various scales this paper reviews the state of the art in the concept as well as in the application of sediment budgets in sedimentary research sediments are a product of mass dispersal at the earth surface and take part in global cycles sediment budgets aim at quantifying this mass transfer based on the principle of mass conservation and are the key to determine ancient fluxes of solid matter at the earth surface this involves fundamental questions about the interplay of uplift climate and denudation in mountain belts and transfer of sediments from the continents to the oceans as well as applied issues such as soil and gully erosion reservoir siltation and coastal protection first after introducing basic concepts relevant scales and methodologies the different components of quaternary routing systems from erosion in headwaters river systems glacial and paraglacial systems lakes deltas estuaries coasts shelves epicontinental seas and deep sea fans are discussed in terms of their sediment budget most suitable are sedimentologically closed or semi closed depositional environments e g alluvial fans lakes deltas and deep sea fans in a second step the dynamics of passive active and collisional tectonic settings and sediment budgets in related sedimentary basins are explored and new concepts of sediment portioning at large geodynamic scales are introduced ancient routing systems are more or less incomplete and may be intensively fragmented or destroyed in active tectonic settings in terms of sedimentary basin types rifts intracontinental and epicontinental settings are preferred objects of sediment budgets because of their persistence and relatively simple overall sedimentary architecture however closing basins such as foreland forearc retroarc piggy back and wedge top basins may provide excellent snapshots of orogenic sediment fluxes in a third step the large long lived routing systems of the amazon the ganges brahmaputra and the rhine are reviewed for each system estimates of either sediment volumes mass or sediment fluxes of continental and marine subsystems have been compiled in order to receive a complete routing in terms of mass conservation for specific time periods since the late glacial maximum as well as the cenozoic following lessons can be taken from these case studies i depositional centers and fluxes show strong shifts in space and time and call for caution when simply looking at subsystems ii the response times of these large systems are within the milankovich time interval thus lower than predicted from diffusion models iii cyclic routing of sediments in continental basins is much more dominated by climate human control than by eustacy and iv at long time scales ultimate sinks win over intermittent storage it is concluded from this review that the quantitative understanding of global sediment cycling over historic and geologic time and its response to allogenic forcing is still in its infancy and further research is needed towards a holistic view of sediment routing systems at various temporal and spatial scales and their coupling with global biogeochemical cycles this includes i to better determine response times of large routing systems by linking quaternary with cenozoic sediment budgets and continental with marine sub systems ii to combine advanced provenance techniques with sediment budgets in order to reconstruct ancient systems iii to study sediment partitioning at the basin scale iv to reconcile continental supply dominated sequence stratigraphy with the eustatic dominated marine concept and iv to account for non actualism of ancient systems with respect to their erosion and transport mode in particular during glaciations and pronounced arid intervals glacial and eolian sediment routing may cross over hydrologic boundaries of drainage basins thus challenging the principle of mass conservation

a seismic reconnaissance survey of the northern congo fan abstract to study the time varying influence of the congo river and the benguela current on the deposition at the angola continental margin a high resolution reflection seismic survey was carried out on the northern congo fan four seismostratigraphic units have been defined for the upper 800 m 1000 ms twt of the data the units record different depositional environments ranging from pre establishment of the congo river drainage system to the influence of the benguela current an indication of a general change in the turbidite system is provided by a shift in channel distribution and a relocation of the depocentre of coarse material the ascent of salt is recorded up to the pliocene gas that has migrated out of lower cretaceous shales and that was produced from large quantities of organic matter in the younger sediments can be found on the flanks and on top of the salt domes in a few places this gas even ascends to the ocean floor along structural pathways through the topmost unit

intermediate depth earthquake generation and shear zone formation caused by grain size reduction and shear heating the underlying physics of intermediate depth earthquakes have been an enigmatic topic several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on shear zone and pseudotachylite formation our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway making it more likely to result in intermediate depth earthquakes at shallower depths furthermore grain size is reduced in and around the shear zone which agrees with field and laboratory observations where pseudotachylites are embedded in a simultaneously formed mylonite matrix the decrease in critical stress to initialize localization has important implications for large scale geodynamics as this mechanism might induce lithosphere scale shear zones and subduction initiation we suggest that the combination of grain size reduction and shear heating explains both the occurrence of intermediate depth earthquakes and the formation of large scale shear zones

spontaneous ductile crustal shear zone formation by thermal softening and related stress temperature and strain rate evolution abstract ductile crustal shear zones play an important role during the formation of orogenic wedges but the softening mechanism controlling their formation and evolution are still debated we perform two dimensional thermo mechanical numerical simulations of lithospheric shortening including an underlying asthenosphere to investigate the mechanism for ductile shear localization and to quantify the evolution of differential stress temperature and strain rate inside shear zones that form in the ductile part of the upper crust the simulations show the development of an intra continental subduction zone whereby the lower crust and mantle lithosphere subduct beneath the adjacent mantle lithosphere the upper crust forms a doubly vergent wedge in which shear zones form subsequently shear zone formation is exclusively caused by thermal softening due to shear heating and temperature dependent viscosity and is initiated by locally elevated shear stresses the shear zone thickness is physics controlled hence mesh insensitive and numerically resolved in the simulations temperature increase inside crustal shear zones is ca 100 c the tectonic overpressure inside upper crustal shear zones is up to 250 mpa and can be twice the value of the corresponding deviatoric stress stresses inside the shear zone decrease during its formation and are hence smaller inside the shear zone than outside surface processes influence the location and orientation of shear zones but not their formation and propagation by thermal softening the numerically calculated differential stress 30 260 mpa temperature 280 380 c and strain rate 10 13 s 1 inside ductile crustal shear zones agree with corresponding estimates for natural shear zones this agreement between modelled differential stress temperature and strain rate with corresponding natural estimates supports previous results which indicate that thermal softening is a controlling softening mechanism for natural shear zone formation

tectonic overpressure in weak crustal scale shear zones and implications for the exhumation of high pressure rocks 1 a two dimensional numerical simulation of lithospheric shortening shows the formation of a stable crustal scale shear zone due to viscous heating the shear zone thickness is controlled by thermomechanical coupling that is resolved numerically inside the shear zone away from the shear zone lithospheric deformation is dominated by pure shear and tectonic overpressure i e pressure larger than the lithostatic pressure is proportional to the deviatoric stress inside the shear zone deformation is dominated by simple shear and the deviatoric stress decreases due to thermal weakening of the viscosity to maintain a constant horizontal total stress across the weak shear zone i e horizontal force balance the pressure in the shear zone increases to compensate the decrease of the deviatoric stress tectonic overpressure in the weak shear zone can be significantly larger than the deviatoric stress at the same location implications for the geodynamic history of tectonic nappes including high pressure ultrahigh pressure rocks are discussed

formation of orogenic wedges and crustal shear zones by thermal softening associated topographic evolution and application to natural orogens abstract the model of an orogenic wedge has been applied to explain the tectonic evolution of many orogens worldwide orogenic wedges are characterized by 1 a first order shear zone which underthrusts the mantle lithosphere and lower crust beneath the adjacent mantle lithosphere and 2 a sequence of second order upper crustal shear zones which form tectonic nappes shear zone formation in the lithosphere is however incompletely understood we perform two dimensional thermo mechanical numerical simulations of lithospheric shortening to study shear zone formation propagation and associated wedge formation the only perturbation in the model lithosphere is a different temperature at the left 1300 c and right 1400 c half of the model bottom despite this smooth and weak perturbation simulations show self consistent and spontaneous formation of first and second order shear zones generating an orogenic wedge the shear zones are caused by thermal softening and temperature dependent rock viscosity lateral spacing of upper crustal shear zones spans between 30 and 50 km and is controlled by the depth of the boundary between upper and lower crust which acts as mechanical detachment level modelled upper crustal shear zones are active for 1 to 4 my surface processes such as sedimentation and erosion influence shear zone orientation spacing and duration but do not impact fundamental processes of shear zone formation and propagation simulations produce both singly vergent and doubly vergent wedges topographic uplift rates are controlled by the applied bulk shortening rate the modelled surface uplift and subsidence associated with crustal shear zones could explain major consecutive thrusting events and related sedimentation within flexural basins during the formation of the helvetic nappe system in western switzerland furthermore results for shear zone propagation along a mid crustal detachment and associated uplift of crustal basement could potentially explain foreland basement cored uplifts in natural orogens such as the laramide orogen taiwan or the shillong plateau

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

the interaction of viscous heating with grain size dependent rheology in the formation of localized slip zones the formation of localized shear zones is important for understanding many local and global processes in geodynamics we have developed a self consistent thermal mechanical model together with a rheology which depends on temperature strain rate and grain size distribution the grain size distribution has contributions from both dynamic recrystallization and grain growth processes and is governed locally by a nonlinear ordinary differential equation a one dimensional model with 104 points is employed to resolve all of the scales involving grain size and temperature we found that grain growth inhibits the development of shear zones and that there is a delicate interplay between viscous heating and grain growth process in determining whether narrow fault zones are developed quickly for realistic parameters of rheology and grain boundary processes for wet olivine the magnitude of the rate of grain growth is crucial to determine whether shear zones are stable or unstable at temperature t 1000 k or shear stress 100 mpa

the alps 2 controls on crustal subduction and ultra high pressure rock exhumation in alpine type orogens building on our previous results we use 2 d upper mantle scale thermomechanical numerical models to explore key controls on the evolution of alpine type orogens and the alps per se focusing on ultra high pressure u hp metamorphic rocks the models show that uhp rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit here we test the sensitivity of the models to surface erosion rate crustal heat production plate convergence divergence rates geometry of the subducting continental margin and strength of the retrocontinent surface erosion affects crustal exhumation but not early buoyant exhumation metamorphic temperatures increase with crustal radioactive heat production maximum burial depth prior to exhumation increases with plate convergence rates but exhumation rates are only weakly dependent on subduction rates onset of absolute plate divergence does not trigger exhumation in these models we conclude that contrasting peak pressures exhumation rates and volumes of u hp crust exhumed in the alps orogen primarily reflect along strike contrasts in the geometry thermal structure and or strength of the subducting microcontinent brianconnais and continental european margin crust the experiments also support the interpretation that the western alps u hp internal crystalline massifs exhumed as composite stacked plumes and that these plumes drove local crustal extension during orogen scale shortening for weak upper plate retrocrusts postexhumation retrothrusting forms a retrowedge overall these results are consistent with predictions using the exhumation number ratio of buoyancy to side traction forces in the conduit which expresses the combined parameter control of the depth volume of crustal subduction and the transition to buoyant exhumation

accelerated contribution of the paleo congo river to global seawater 87 sr 86 sr change following eocene oligocene collapse of the african surface

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

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

stress strength relationship in the lithosphere during continental collision lithospheric strength profiles generated for a shortening continental lithosphere generally predict excessively high differential stresses in the sub moho continental mantle this seems inconsistent with the relative scarcity of earthquakes at this depth this inconsistency was put forward as evidence for weak mantle rheology however this argument implicitly assumes that strength envelopes are valid in actively deforming regions we test this assumption on two end member model lithospheres having identical upper crustal rheologies but with 1 a weak lower crust and strong mantle and 2 a strong lower crust and weak mantle for this purpose we compare one dimensional 1 d with 2 d visco elastoplastic numerical models of continental shortening the 2 d models show that strongly heterogeneous deformation typically follows initially homogeneous deformation lithospheric scale buckle folds and shear zones result in strain rate variations of as much as three orders of magnitude differential stresses in the upper crust are close to yield as predicted by 1 d models stresses in deeper lithospheric regions however are significantly smaller than in 1 d models especially in actively deforming regions systematic numerical simulations as a function of temperature and deformation rate reveal that 1 d models are reliable in hot and or slowly deforming lithospheres only the relative scarcity of earthquakes at mantle levels should thus be interpreted as an intrinsic consequence of strong lithospheric deformation rather than as evidence for a weak upper mantle

shear zone and nappe formation by thermal softening related stress and temperature evolution and application to the alps we present the results of two dimensional thermo mechanical numerical simulations of the formation of kilometre scale shear zones and tectonic nappes which are generated by thermal softening during horizontal shortening this model is thermo mechanically coupled and conserves energy by converting mechanical work into heat the results show that in homogeneous material the temperature increase in shear zones caused by viscous heating can be 100 c the characteristic width of the thermally perturbed part of the deforming zone is three times larger than the width of the corresponding shear zone because of diffusive heating of the wall rock therefore temperature variations initiated by shear heating may be difficult to identify based on petrological inferences of field data during lithospheric shortening with a bulk rate of 2 5 10 15 s 1 a nappe forms in the crust at a depth of 45 km peak temperatures in the nappe reach 600 c 100 c increase due to shear heating significant shear heating occurs only locally and is not active continuously in time along the entire crustal shear zone peak values of the maximal principal stress locally reach 2 5 gpa in the nappe peak values of temperature and maximal principal stress increase locally to 770 c 300 c increase due to shear heating and 3 8 gpa respectively if the applied bulk shortening rate is 5 10 15 s 1 and if higher effective viscosities for the crust are applied in the model lithosphere high differential stresses 500 mpa and significant stress deviations from the lithostatic pressure 50 occur only locally and are transient the highest average effective viscosity of the model lithosphere is 2 1022 pa s which is in agreement with the estimates for the continental lithosphere a characteristic feature of the presented dynamic nappe model is a significant stress decrease and cooling within a hundred thousand to 2 million years these rates of decompression and cooling are discussed in the context of microstructural and metamorphic observations the results also show that differential stresses in mature shear zones can be several hundred mpa smaller than at the onset of shear zone formation which suggests that estimates based on microstructural analysis of rocks in ductile shear zones provide minimum values the presented model could explain the formation of coherent alpine nappes with inferred high pressure 1 5 2 5 gpa and ultrahigh pressure 3 gpa conditions in 40 60 km depth at the base of an orogenic wedge

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

deformation mechanism maps for feldspar rocks abstract deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates the maps display constant temperature contours in stress grain size space for strain rates ranging from 10 16 to 10 12 s 1 two fields of dominance of grain boundary diffusion controlled creep and dislocation creep are separated by a strongly grain size sensitive transition zone for wet rocks diffusion controlled creep dominates below a grain size of about 0 1 1 mm depending on temperature stress strain rate and feldspar composition plagioclase aggregates containing up to 0 3 wt water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks the strength of water bearing feldspar rocks is moderately dependent on composition and water fugacity for a grain size range of about 10 50 m commonly observed in natural ultramylonites the deformation maps predict that diffusion controlled creep is dominant at greenschist to granulite facies conditions low viscosity estimates of 10 18 10 19 pa s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures 900 c or at lower temperatures diffusion creep of fine grained rocks possibly localized in abundant high strain shear zones for similar thermodynamic conditions and grain size lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks

influence of shear heating on microstructurally defined plate boundary shear zones abstract in plate boundary scale ductile shear zones defined by microstructural weakening shear heating may lead to a temperature increase over 5 m y of up to 80 c just below the brittle ductile transition up to 120 c just below the moho and to thermal boundary zones tens of km wide on either side of the shear zone where rock strength is highest shear zones are narrow 1 km and thermal gradients within the shear zone itself are low so there is no tendency for increased localization heating results in thermal weakening but this is partly offset by grain growth related to the decrease in stress in shear zones of the order of 10 km width shear stress strain rate and hence heat generation are lower and thermal gradients are insufficient to cause additional strain localization temperature increases in the mid crust are of the order of 10 c insufficient to cause partial melting or an increase in metamorphic grade in the upper mantle shear zones may be 50 km or more wide and the temperature increase is less than 20 c in 5 m y but temperature differences between center and margin may enhance the strain rate at the center by up to 18

metamorphic record of catastrophic pressure drops in subduction zones when deeply buried in subduction zones rocks undergo mineral transformations that record the increase of pressure and temperature the fact that high pressure metamorphic parageneses are found at the earth s surface proves that rock burial is followed by exhumation here we use analysis of available data sets from high pressure metamorphic rocks worldwide to show that the peak pressure is proportional to the subsequent decompression occurring during the initial stage of retrogression we propose using a simple mechanical analysis that this linear relationship can be explained by the transition from burial related compression to extension at the onset of exhumation this major switch in orientation and magnitude of principal tectonic stresses leads to a catastrophic pressure drop prior to actual rock ascent therefore peak pressures are not necessarily as commonly believed directly dependent on the maximum burial depth but can also reflect a change of tectonic regime our results which are in agreement with natural data have significant implications for rock rheology subduction zone seismicity and the magnitudes of tectonic pressures sustained by rocks current views of subduction dynamics could be reconsidered in that perspective rocks are altered by high pressure during subduction analysis of exhumed metamorphic rocks suggests that the peak pressures recorded within minerals mark a change in tectonic regime within a subduction zone rather than burial depth

physical properties of a porcellanite layer southwest indian ridge constrained by geophysical logging abstract a distinct porcellanite layer from the southwest indian ridge intercalated in pleistocene diatom ooze was studied using nondestructive physical property measurements and sedimentological data this bed was sampled by two piston cores at a water depth of 2615 m the 3 5 cm thick porcellanite layer appears in the cores at a depth of 6 03 m core ps2089 2 and 7 73 m core ps2089 1 below the seafloor due to its characteristic physical properties the porcellanite bed can be detected with core measurements and its distribution and lateral extent mapped with echosounding the physical index properties wet bulk density and electrical resistivity increase significantly across this bed magnetic susceptibility is used to compare the lithological units of both cores and to distinguish whether resistivity anomalies are caused by a higher amount of terrigenous components or by the presence of porcellanite the porcellanite has the special characteristic to affect a positive anomaly in resistivity but not in susceptibility most marine sediments in contrast show a positive correlation of magnetic susceptibility versus electrical resistivity therefore a combination of electrical resistivity and magnetic susceptibility logs yields a definite detection of the porcellanite bed images from the x ray ct survey indicate that the porcellanite is lithified and brittle and fragmented when the piston corer penetrated the bed

the ultimate fate of a synmagmatic shear zone interplay between rupturing and ductile flow in a cooling granite pluton abstract the neoarchean cundimurra pluton yilgarn craton western australia was emplaced incrementally along the transpressional cundimurra shear zone during syndeformational cooling discrete networks of cataclasites and ultramylonites developed in the narrowest segment of the shear zone showing the same kinematics as the earlier synmagmatic structures lithological boundaries between aplite pegmatite veins and host granitic gneiss show more intense pre cataclasite fabrics than homogeneous material and these boundaries later became the preferred sites of shear rupture and cataclasite nucleation transient ductile instabilities established along lithological boundaries culminated in shear rupture at relatively high temperature 500 600 c here tensile fractures at high angles from the fault plane formed asymmetrically on one side of the fault indicating development during seismic rupture establishing the oldest documented earthquake on earth tourmaline veins were emplaced during brittle shearing but fluid pressure probably played a minor role in brittle failure as cataclasites are in places tourmaline free subsequent ductile deformation localized in the rheologically weak tourmaline rich aggregates forming ultramylonites that deformed by grain size sensitive creep the shape and width of the pluton shear zone and the regime of strain partitioning induced by melt present deformation and established during pluton emplacement played a key role in controlling the local distribution of brittle and then ductile subsolidus structures

evidence of a large upper cretaceous depocentre across the continent ocean boundary of the congo angola basin implications for palaeo drainage and potential ultra deep source rocks abstract the analysis of 2d deep seismic reflection profiles across the slope and abyssal plain of the angola oceanic basin reveals the existence of a significant and formerly unknown depocentre beneath the giant cenozoic congo deep sea fan between 7000 m and 9000 m depth deposited directly onto the aptian oceanic crust the unit which is up to 2 5 km thick and extends for more than 200 km basinwards of the continent ocean boundary is probably aged albian turonian its radial fan shaped depocentre is centred on the present day congo river outlet and contains at least 0 2 mio km 3 of sediments these observations and the results from flexural modelling indicate that 1 the location of the congo river s outlet has remained fairly stable since the late cretaceous and 2 the basal unit was indeed sourced by a palaeo congo river probably located nearby the present day one thus the atlantic sedimentary system related to the exoreism of the congo river is much older than previously thought thermal modelling indicates that the maturation history of this upper cretaceous deposits is highly influenced by the interaction between the initial high heat flow of the young oceanic crust and further increase in sediment supply due to the progradation of the overlying tertiary deep sea fan during the miocene hence despite low present day heat flow values should the unit have source rock potential its basal section may be currently generating hydrocarbons all in all the results from our models also suggest that the interplay between an initially high heat flow and the further high sediment supply in areas of major river input may be a key factor for the thermal maturation of potential source rocks deposited onto a present day cold oceanic crust

authigenic carbonates from methane seeps of the northern congo fan microbial formation mechanism abstract authigenic carbonates were collected from methane seeps at hydrate hole at 3113 m water depth and diapir field at 2417 m water depth on the northern congo deep sea fan during rv meteor cruise m56 the carbonate samples analyzed here are nodules mainly composed of aragonite and high mg calcite abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix the 13c values of the hydrate hole carbonates range from 62 5 to 46 3 pdb while the 13c values of the diapir field carbonate are somewhat higher ranging from 40 7 to 30 7 pdb indicating that methane is the predominant carbon source at both locations relative enrichment of 18o 18o values as high as 5 2 pdb are probably related to localized destabilization of gas hydrate the total content of rare earth elements ree of 5 hno3 treated solutions derived from carbonate samples varies from 1 6 ppm to 42 5 ppm the shale normalized ree patterns all display positive ce anomalies ce ce 1 3 revealing that the carbonates precipitated under anoxic conditions a sample from hydrate hole shows a concentric lamination corresponding to fluctuations in 13c values as well as trace elements contents these fluctuations are presumed to reflect changes of seepage flux

spontaneous generation of ductile shear zones by thermal softening localization criterion 1d to 3d modelling and application to the lithosphere the generation of ductile shear zones is essential for the formation of tectonic plate boundaries such as subduction or strike slip zones however the primary mechanism of ductile strain localization is still contentious we study here the spontaneous generation of ductile shear zones by thermal softening using thermo mechanical numerical simulations for linear and power law viscous flow in one dimension 1d 2d and 3d all models are velocity driven the 1d model exhibits bulk simple shear whereas the 2d and 3d models exhibit bulk pure shear the initial conditions include a small temperature perturbation in otherwise homogeneous material we use a series of 1d simulations to determine a new analytical formula which predicts the temperature evolution inside the shear zone this temperature prediction requires knowledge of only the boundary velocity flow law and thermal parameters but no a priori information about the shear zone itself such as thickness stress and strain rate the prediction is valid for 1d 2d and 3d shear zones in bulk pure and simple shear the results show that shear heating dominates over conductive cooling if the relative temperature increase is 50 c the temperature variation induced by the shear zone is nearly one order of magnitude wider than the corresponding finite strain variation so that no significant temperature variation occurs between shear zone and wall rock applying typical flow laws for lithospheric rocks shows that shear zone generation by thermal softening occurs for typical plate tectonic velocities of few cm yr 1 or strain rates between and s 1 shear stresses larger than 200 mpa can already cause strain localization the results indicate that thermal softening is a feasible mechanism for spontaneous ductile shear zone generation in the lithosphere and may be one of the primary mechanisms of lithospheric strain localization

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

geological controls on focused fluid flow associated with seafloor seeps in the lower congo basin a synthesis of backscatter imagery coupled with a large 3d seismic dataset in the lower congo basin lcb reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth with the exception of one region of anomalous backscatter positive relief mounds all inferred seep sites occur in negative relief pockmarks the extensive 3d seismic dataset in the lcb offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems and in general to reconstruct the relationship between tectonics and fluid flow in continental margins the fluid seeps in the lcb are associated with morphologically stratigraphically or tectonically controlled focused fluid flow the integration of the geophysical datasets backscatter imagery coupled to 3d seismic clearly indicates that fluid seeps are not randomly distributed but their seabed organization reflects 1 the location of the underlying structure reservoir or trap where the fluids are coming from 2 the geometry and morphology of the reservoir trap and 3 the discontinuities in the sedimentary column along which fluids have migrated in the lcb seafloor pockmarks are always associated with underlying tectonic structures fault zones salt diapirs polygonal faults or buried sedimentary bodies turbiditic channels erosional surfaces whereas they never occur above sub horizontal parallel stratified fine grained sediments even if triggering processes can not be clearly defined here we propose a model of seafloor fluid seep organization which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from this qualitative 3d model provides insight into the geohydrologic processes of continental margins

a simple parameterization of strain localization in the ductile regime due to grain size reduction a case study for olivine we propose a simple parameterization of the transition between dislocation creep and grain size sensitive creep under conditions characteristic of the lithospheric mantle and derived from the results of laboratory experiments on olivine rich rocks through numerical modeling and linear stability analysis we determine the conditions under which this transition takes place and potentially leads to strain localization we pay particular attention to the effect of cooling rate and strain rate which are likely to be dominant parameters in actively deforming tectonic areas we conclude that at constant temperature strain localization can only take place if the rheology of the material is nonlinearly related to grain size that strain localization is facilitated by syndeformation cooling that there is only a narrow region in the strain rate versus cooling rate parameter space where localization is likely to take place and that grain growth inhibits strain localization at fast cooling rates but may lead to grain growth localization at low cooling rates we draw attention to the potential consequences of our analysis of strain localization for the style of plate motions at the earth s surface

conspicuous seismic reflections in upper cretaceous sediments as evidence for black shales off south africa abstract the upper cretaceous is commonly associated with greenhouse climate oceanic anoxic events oae and the ongoing break up of gondwana which resulted in strong variations of the ocean s currents flow paths and climate change little is known about these changing conditions in particular south of south africa a high resolution seismic reflection data set from the submarine transkei basin off south africa revealed various depositional stages for this region for the past 90 ma in these seismic sections a recurrently appearing high amplitude horizon within weak to homogeneous upper cretaceous reflections was observed due to the inaccessibility of any drill hole data from the transkei basin and adjacent regions the origin of this reflector is difficult to specify it is roughly dated at 80 85 ma which falls within the last big oae in upper cretaceous oae 3 a seismostratigraphic analysis led to an exclusion of several features as the origin for this reflector such as gas hydrates opal bearing horizons or biogenic oil other possible features such as lithologic variations intrusions or black shales are discussed in more detail according to its appearance and reflection characteristics it could possibly be the first report of black shales in this region

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

fluid flow features in hydrocarbon plumbing systems what do they tell us about the basin evolution abstract fluid flow features such as sandstone intrusions mud volcano systems and pockmarks represent well known geological structures generated due to flow of fluids in the subsurface however the integration of fluid flow features in quantitative and qualitative basin analysis is still in its infancy this study presents a review and a qualitative integration of fluid flow features in basin analysis of the danish north sea area and of salt mini basins offshore angola based on results from six case studies focusing on the detection mapping and evaluation of features related to focused fluid flow in these areas in the study the diversity of seismically imaged fluid flow features in hydrocarbon plumbing systems is reviewed and their utility is demonstrated this is done by focusing on the aspects of fluid flow related features that can be integrated into basin analysis studies and how these may help us increase the understanding of basin evolution and to achieve the major goals of basin analysis such as understanding the depositional and tectonic context of the basin and the presence and extent of hydrocarbons particularly concerning the determination of fluid flow timing triggers sources and routes fluid flow features represent a major but underused tool in basin analysis

nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks a subduction shear zone can be modeled as a long narrow channel with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential which combines the effect of both pressure and buoyancy if the channel walls are effectively rigid very slight narrowing or broadening of the channel convergence angles 2 gpa in the channel at only 40 km depth the model is consistent with a horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment 650 m the practical limit for overpressures attainable in subduction zones is determined by the strength and permeability of the channel walls at 40 km depth the channel is effectively confined on both sides by cold lithospheric mantle which should be strong enough to support a significant tectonic overpressure episodic failure of the upper plate to produce great earthquakes at 30 40 km focal depth could vent overpressured fluid from the channel allowing a cyclical buildup and release of both rock and fluid pressure topography on the subducting plate e g seamounts and thinned continental crust may lead to an anvil like jamming of the channel and local high overpressures tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks a decrease in the convergence rate or cessation of subduction with a consequent general warming within the channel and associated viscosity decrease promotes exhumation by buoyant reverse flow the most rapid reverse flow occurs in the region of previously greatest overpressure since the exhumation distance is shorter than for a simple lithostatic pressure distribution and any increase in temperature is coupled with a strong increase in the rate of exhumation preservation of high pressure assemblages at the surface in fossil subduction zones is promoted for such a model

evidence for congo river freshwater load in late quaternary sediments of odp site 1077 5 s 10 e abstract late quaternary fluctuations in the intensity of congo river freshwater load were reconstructed using three different proxies marine and freshwater diatoms and the 18o record of globigerinoides ruber preserved in the sediments of ocean drilling program odp site 1077 located at the northern rim of the congo river fan 5 10 s 10 26 e an abrupt change in the diatom assemblage is evident at termination ii a two to four fold increase in a the relative abundance of a marine planktonic diatom tolerant of low salinity conditions cyclotella litoralis and b in the concentration of freshwater diatoms the microfossil data suggest a change in the environmental conditions surrounding site 1077 from predominantly marine to mixed marine brackish fresh the 18o record of the planktic foraminifera g ruber pink revealed negative deviations from the global oxygen isotope signal since termination ii which occurred during warm stage 1 and substages 3 2 5 1 5 3 and 5 5 comparison of the isotopic signal of odp site 1077 with the record from a pelagic location core geob1041 at 3 48 s 7 05 w confirms these results the construction of an artificial 18o curve using alkenone derived sea surface temperature sst data from a nearby core geob1008 at 6 s 10 e allowed us to estimate salinity and temperature effects on the odp site 1077 isotopic signal although increased ssts may account for lighter 18o values during warmer periods they do not explain the extremely light values documented in the sediments of site 1077 we used the oxygen isotope difference 18o between our site and geob1041 as a proxy for freshwater input a general trend in the 18o was observed with more negative values since termination ii in addition conspicuous 18o negative pulses coincided with periods of northern hemisphere summer insolation maxima over the african continent suggesting an increase in the freshwater discharge from the congo river due to enhanced precipitation on the hinterland here we propose that the abrupt change in environmental conditions at site 1077 since termination ii is a consequence of a major reorganization in the depositional environment of the congo river delta this reorganization involved sustained equatorward displacement of the angola benguela front causing a northward deflection of the congo river plume thus moving plume waters further north than normal and over site 1077

deep structure of lithospheric fault zones 1 we calculate the cumulative width w of ductile shear zones accommodating plate motion in continental lithosphere based on the assumptions that 1 the flow stress is controlled by the yield strength of intact rock at any given depth 2 the yield strength profile through the crust can be constrained from observations in exhumed shear zones and 3 strain localization is primarily caused by grainsize reduction leading to a switch to grainsize sensitive creep we use a mid crustal stress temperature profile measured in the whipple mountains california and calculate stress profiles at depth from published flow laws for feldspar and olivine we conclude thatwfor a plate boundary shear zone accommodating 50 mm yr displacement comparable to the san andreas transform may reach 180 km in the quartz rich mid crust depending on water fugacity and thermal gradient it narrows to a few meters in feldspathic lower crust and in the uppermost mantle and then widens rapidly with depth in the lower lithosphere we explore the effects of differences in crustal thickness and composition thermal gradient and water activity

temperatures heat flux and frictional stress near major thrust faults we examine the two dimensional advective and conductive transport of heat in a region of thrust faulting both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor s 1 b z vsin below what they would be with the same heat sources but in the absence of advection in this expression z is the depth to the fault v is the slip rate is the component of the dip of the fault in the direction of underthrusting is thermal diffusivity and b is a dimensionless factor that is essentially equal to one for most forms of heating initial changes in temperatures near the fault are given by t z t t z 0 12vsin t z 0 z these simple formulae are successful because of the neglible influence of lateral conduction of heat at least for slip rates of a few mm yr or more two time constants govern the transition from the initial change in temperature to steady state t1 u v where u is the distance along the fault in the direction of underthrusting from the surface to the depth in question and t2 z 2 2 where z is the depth to the fault when elapsed times exceed the sum of these two time constants temperatures differ from their steady state values by only about 10 percent the numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted putain an application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 mpa and are greater than 30 mpa calculations of temperatures appropriate for the himalaya suggest that shear stresses of 100 mpa on the main central thrust probably are required to account for the tertiary granites of the region if melting took place after slip began on the thrust similarly the cut off in seismicity at a depth of about 15 km in the himalaya if due to temperatures exceeding 350 to 450 c implies a deviatoric stress close to 100 mpa

evolution of the cretaceous astrid thrust belt in the ultradeep water lower congo basin gabon the lower congo basin contains the greatest salt based fold and thrust belt off africa s atlantic margin our study area in the anton marin and astrid marin exploration blocks is in the northern part of the basin gravity driven tectonic shortening began soon after the aptian salt deposition forming gentle west trending salt cored anticlines which together with salt diapirs created a template for later thrusting in the late cretaceous a thrust front propagated landward into the study area and thrusts formed above salt anticlines and diapirs formation of a hanging wall wedge of growth strata was recorded when each thrust fault ruptured the seabed thrusting began after widespread salt thinning as autochthonous salt was expelled into older passive diapirs thinning stiffened the detachment so that thrusts verge strongly seaward structural restorations dip corrected isochron maps and fault activity graphs all show that the landward edge of the thrust belt propagated landward three main pulses of shortening episodically reactivated thrust faults as the thrust front broke landward as thrusting culminated precursor passive diapirs were squeezed and extruded small allochthonous sheets translation culminated in major erosional scouring from which we infer epeirogenic slope steepening in the late cretaceous as shortening spread updip into the previously extensional domain during the late cretaceous to paleogene older extensional faults were inverted and new extensional faults formed orthogonally parallel to the regional paleoslope the structural pattern created in the late cretaceous when the paleoslope dipped southward remains recognizable in the little deformed neogene strata although the present continental slope dips westward

a test of laboratory based rheological parameters of olivine from an analysis of late cenozoic convective removal of mantle lithosphere beneath the sierra nevada california usa summary we use the apparent change in mantle structure beneath the sierra nevada since ca 10 ma which suggests convective removal of eclogite rich mantle lithosphere and scaling laws developed for rayleigh taylor instability to place constraints on the average viscosity coefficient of the mantle lithosphere by treating the lithosphere as a non newtonian fluid obeying power law creep with an exponent of n 3 5 we may compare the inferred values of viscosity coefficient with those obtained from laboratory experiments on olivine and eclogite the values that we obtain overlap those predicted by laboratory based flow laws for the range of geotherms implied by heat flux measurements within the sierra nevada and by metamorphic geothermometry and geobarometry of xenoliths in volcanic rock erupted in the sierra nevada at ca 10 ma thus this comparison offers support for laboratory derived flow laws and specifically for the high stress limit suggested by evans goetze 1979 conversely this agreement shows that the high strength of cold mantle minerals does not prohibit its removal by convective instability

calciclastic submarine fans an integrated overview abstract calciclastic submarine fans are rare in the stratigraphic record and no bona fide present day analogue has been described to date possibly because of that and although calciclastic submarine fans have long intrigued deep water carbonate sedimentologists they have largely been overlooked by the academic and industrial communities to fill this gap we have compiled and critically reviewed the existing sedimentological literature on calciclastic submarine fans thus offering an updated view of this type of carbonate slope sedimentary system calciclastic submarine fans range in length from just a few to more than 100 km three different types can be distinguished 1 coarse grained small sized most calciclastic submarine fans were formed on low angle slopes and were sourced from distally steepened carbonate ramps subjected to high energy currents under these conditions shallow water loose grainy sediments were transferred to the ramp slope and eventually funnelled into the submarine fan by sediment gravity flows these conditions seem to have been more easily met on leeward margins in which the formation of reefs was hampered by cool waters nutrient enrichment or oligophoty another circumstance that contributes to the transfer of shallow water sediments to the distal ramp slope is a low sea level forcing the carbonate factory closer to the slope break and destabilizing sediments by increased pore water pressure however the most important factor controlling the development of calciclastic submarine fans was the existence of an efficient funnelling mechanism forcing sediment gravity flows to merge downslope and build up a point sourced sedimentary accumulation in most cases this occurred through a major slope depression associated with tectonic structures an inherited topography or large scale mass failures

revisiting scaling laws in river basins new considerations across hillslope and fluvial regimes 1 increasing availability of high resolution 1 m topography data and enhanced computational processing power present new opportunities to study landscape organization at a detail not possible before here we propose the use of directed distance from the divide as the scale parameter instead of horton s stream order or upstream contributing area for performing detailed probabilistic analysis of landscapes over a broad range of scales this scale parameter offers several advantages for applications in hydrology geomorphology and ecology in that it can be directly related to length scale dependent processes it can be applied seamlessly across the hillslope and fluvial regimes and it is a continuous parameter allowing accurate statistical characterization higher order statistical moments across scales application of this scaling formalism to three basins in california demonstrates the emergence of three distinct geomorphic regimes of divergent highly convergent and moderately convergent fluvial pathways with notable differences in their scaling relationships and in the variability or spatial heterogeneity of topographic attributes in each regime we show that topographic attributes such as slopes and curvatures conditional on directed distance from the divide exhibit less variability than those same attributes conditional on upstream contributing area thus affording a sharper identification of regime transitions and increased accuracy in the scaling analysis

constraining uplift and denudation of west african continental margin by inversion of stacking velocity data an inverse model has been developed to determine the magnitude of denudation at seabed and subsurface unconformities by using root mean square rms stacking velocity data derived from processing a set of seismic reflection profiles this approach provides superior spatial coverage in comparison to other methods such as vitrinite reflectance apatite fission track and sonic velocity modeling which are restricted to borehole locations the model assumes exponential porosity decay with depth and a standard velocity porosity relationship in order to compute a synthetic rms velocity profile denudation values at two levels in the stratigraphy are then adjusted until the fit between the model and the data is optimized successful modeling is dependent upon independent estimates of the initial porosity of sediment since significant trade off occurs between initial porosity and denudation application to the west african shelf shows that 0 5 1 km of denudation occurred along the entire margin probably during late neogene times the amount of denudation decreases oceanward and was probably triggered by regional tilting associated with initiation and or regeneration of continent wide mantle convective upwelling which is thought to have affected much of subequatorial africa a subsurface oligocene unconformity represents as much as 2 5 km of denudation and was probably produced by initiation of an oceanic current

overcoming pitfalls of vitrinite reflectance measurements in the assessment of thermal maturity the case history of the lower congo basin abstract vitrinite reflectance has been always considered the most reliable thermal maturity indicator in sedimentary basins nevertheless the reliability of vitrinite reflectance measurements can be affected by many external factors that change from basin to basin e g organic facies temperature and pressure conditions leading to wide uncertainties in thermal modelling calibration and thus in the evaluation of timing and amount of hydrocarbons generation we demonstrate that in case of vitrinite suppression and or retardation in siliciclastic successions the classical sweeney and burnham kinetics do not match the actual thermal evolution of the basin in these cases thermal evolution can be successfully modelled using a complementary approach based on smectite illitization kinetics or more appropriate kinetics e g presro tm and t p ro to evaluate the effect of pressure on organic matter maturation where pressure data are available this approach was applied in the lower congo basin where 32 cuttings from a 5 km deep well drilled in the upper oligocene miocene malembo formation have been analysed to derive organic and inorganic constraints to model the basin thermal evolution vitrinite and bitumen reflectance measurements show values between 0 3 and 1 0 along the well whilst the illite content of mixed layer i s ranges between 35 and 88 passing from r0 to r1 to r3 stacking order with increasing depth present day temperature distribution through the well indicates a geothermal gradient of about 40 c km and measured pressures show an overpressured interval between 3 and 4 km depth retardation and suppression phenomena affected vitrinite reflectance in this interval but did not influence smectite illitization the comparison between thermal maturity scenarios constrained by organic and inorganic thermal indicators shows differences in the calculation of the depth of the onset of hydrocarbon generation that is overestimated of about 500 m in the lower miocene interval when only suppressed retarded vitrinite reflectance values are used to calibrate models