evolution of a fold thrust belt deforming a unit with pre existing linear asperities insights from analog models abstract heterogeneity whether geometric or rheologic in crustal material undergoing compression affects the geometry of the structures produced this study documents the thrust fault geometries produced when discrete linear asperities are introduced into an analog model scaled to represent bulk upper crustal properties and compressed varying obliquities of the asperities are used relative to the imposed compression and the resultant development of thrust fault traces and branch lines in map view is tracked once the model runs are completed cross sections are created and analyzed the models show that asperities confined to the base layer promote the clustering of branch lines in the surface thrusts strong clustering in branch lines is also noted where several asperities are in close proximity or cross slight reverse sense reactivation of asperities cut through the sedimentary sequence is noted in cross section where the asperity and the subsequent thrust belt interact the model results are comparable to the situation in the dinaric alps where pre existing faults to the sw of the ne adriatic fault zone contribute to the clustering of branch lines developed in the surface fold thrust belt these results can therefore be used to evaluate the evolution of other basement involved fold thrust belts worldwide

fault reactivation control on normal fault growth an experimental study field studies frequently emphasize how fault reactivation is involved in the deformation of the upper crust however this phenomenon is generally neglected except in inversion models in analogue and numerical models performed to study fault network growth using sand silicon analogue models we show how pre existing discontinuities can control the geometry and evolution of a younger fault network the models show that the reactivation of pre existing discontinuities and their orientation control i the evolution of the main fault orientation distribution through time ii the geometry of relay fault zones iii the geometry of small scale faulting and iv the geometry and location of fault controlled basins and depocenters these results are in good agreement with natural fault networks observed in both the gulf of suez and lake tanganyika they demonstrate that heterogeneities such as pre existing faults should be included in models designed to understand the behavior and the tectonic evolution of sedimentary basins

evolution characteristics of transtensional faults and their impacts on hydrocarbon migration and accumulation a case study from the huimin depression bohai bay basin eastern china abstract the study of transtensional faults is significantly important for analysing migration and accumulation of hydrocarbons in petroliferous basins the purpose of this study is to present disparities in the temporal and spatial evolution of two boundary transtensional faults northern linshang fault and southern xiakou fault in the huimin depression by 2d structural restoration and quantitative calculations of fault activity this paper also reveals the controls of transtensional faults on the migration and accumulation of hydrocarbons by considering the stress normal to the fault plane p the shale gouge ratio sgr of the fault zone and models of migration pathways the results show that the transtensional systems within the huimin depression were generated from the reactivation of basement faults under an oblique extensional stress field the linshang fault linked two initially separated segments and then behaved as a single fault since the ek period whereas the xiakou fault retained significant three segment growth properties during the paleogene during the hydrocarbon charging period the weak activity and strong vertical sealing restricted the occurrence of vertical migration around the xiakou fault while the relatively strong movement and weak vertical sealing resulted in multilayer hydrocarbon accumulations around the linshang fault meanwhile the vertically closed fault with high sgr 0 6 acted as barriers and trapped hydrocarbons on the lateral migration pathways whereas the vertically closed fault with low sgr

the topology of evolving rift fault networks single phase vs multi phase rifts rift fault networks can be complex particularly those developed by multiple periods of non coaxial extension comprising non colinear faults with many interactions thus topology rather than simple geometry is required to characterise such networks as it provides a way to describe the arrangement of individual faults in the network topology is analysed here in terms of nodes isolated i nodes or connected y or x nodes and branches i i i c c c branches in map view the relative proportions of these parameters vary in natural single and multi phase rift fault networks and in scaled physical models at different stages of development and strain interactions in single phase rifting are limited to fault splays and along strike fault linkage i node and i i or i c branch dominated networks whereas in multi phase rifting the topology evolves towards y node and c c branch dominated networks with the degree of connectivity increasing with greater strain the changes in topology and network connectivity have significant implications for fluid flow and reservoir compartmentalisation studies furthermore topology helps to distinguish single and multiple phase extension i e tectonic histories and thus provide constraints on the geodynamic context of sedimentary basins

an experimental study of the influence of stress history on fault slip during injection of supercritical co 2 abstract the injection of super critical co2 into a depleted reservoir will alter the pore pressure of the basin which if sufficiently perturbed could result in fault slip therefore knowledge of the acceptable pressure limits is required in order to maintain fault stability a two part laboratory study was conducted on fully saturated kaolinite fault gouge to investigate this issue previously we showed that fault slip occurred once pore pressure within the gouge was sufficient to overcome the normal stress acting on the fault for kaolinite this behaviour occurred at a pressure similar to the yield stress the current study shows that following a slow reduction in the maximum principal stress as would be expected through changes in effective stress the reactivation pressure shows a stress memory consequently the pressure necessary to initiate fault slip is similar to that required at the maximum stress encountered therefore fault slip is at least partially controlled by the previous maximum stress and not the current stress state during the slow reduction in normal stress the flow characteristics of the fault remain unchanged until pore pressure exceeds shear stress and does not increase significantly until it exceeds normal stress this results in fault slip which slows the rate of flow increase as shear is an effective self sealing mechanism these observations lead to the conclusion that stress history is a vital parameter when considering fault stability

influence of rift obliquity on fault population systematics results of experimental clay models abstract we use clay models to simulate how fault population systematics vary as a function of rift obliquity rift obliquity is related to the acute angle between the rift trend and the displacement direction so that the value of is inverse to the degree of obliquity the range of azimuths in a fault population increases as rift obliquity increases i e as decreases the length of the longest faults the sum of fault lengths and the width of the deformed zone all increase as rift obliquity decreases i e as increases the majority of faults in our models are segmented and have highly tortuous traces tortuosity is maximum when 30 as segments of widely varying azimuth link during fault growth significant changes occur in the fault patterns between 30 and 45 at 30 two fault populations of equal importance develop in the center of the rift zone one approximately rift parallel and the other displacement normal between 30 and 45 the number of faults more than doubles and between 45 and 60 summed fault length more than doubles fault patterns for all models are fractal with fractal dimensions that increase with increasing and that are comparable to those found in the field fault populations are not multi fractal because the cumulative frequency distributions of fault lengths do not generally follow a power law relationship an exponential distribution best describes the data for whole faults with the characteristic length increasing with increasing segment lengths also follow an exponential distribution with characteristic length varying very little with the fault patterns in our models resemble the spatial pattern of brittle deformation observed at oblique mid ocean ridge segments and are similar in geometry to those in oblique continental rift basins at the rift margins tensional stresses are modulated and reoriented by a secondary stress field related to a change in boundary conditions resulting in the formation of two distinct sub populations of faults during oblique rifting

controls on early rift geometry new perspectives from the bilila mtakataka fault malawi we use the 110 km long bilila mtakataka fault in the amagmatic southern east african rift malawi to investigate the controls on early rift geometry at the scale of a major border fault morphological variations along the 14 8 m high scarp define six 10 40 km long segments which are either foliation parallel or oblique to both foliation and the current regional extension direction as the scarp is neither consistently parallel to foliation nor well oriented for the current regional extension direction we suggest the segmented surface expression is related to the local reactivation of well oriented weak shallow fabrics above a broadly continuous structure at depth using a geometrical model the geometry of the best fitting subsurface structure is consistent with the local strain field from recent seismicity in conclusion within this early rift pre existing weaknesses only locally control border fault geometry at subsurface

width of continental rifts and rheological layering of the lithosphere abstract a review of some well known continental rifts shows that rift features such as maximum basin depth are dependent on the amount of extension conversely the initial rift width appears to be independent of the amount of extension a comparison is made with laboratory models of rifts developed in a brittle ductile lithosphere experimental models show that normal faults which bound a graben initially meet at the brittle ductile interface a relation between initial rift width and the depth of the brittle ductile transition is thus suggested implications for natural examples are discussed

on the observability of oceanic gyres abstractthis study examines the observability of a stratified ocean in a square flat basin on a midlatitude beta plane here observability means the ability to establish in a finite interval of time the time dependent ocean state given density observations over the same interval and with no regard for errors the dynamics is linearized and hydrostatic so that the motion can be decomposed into normal modes and the observability analysis is simplified an observability gramian a symmetric matrix is determined for the flows in an inviscid interior in frictional boundary layers and in a closed basin its properties are used to establish the condition for complete observability and to identify optimal data locations for each of these flows it is found that complete observability of an oceanic interior in time dependent sverdrup balance requires that the observations originate from the westernmost location at each considered latitude the degree of observability increases westward due to westward prop

modelling the influence of pre existing brittle fabrics on the development and architecture pull apart basins abstract we use new analogue modelling experiments to analyse the development of pull apart basins in an upper crust characterised by the presence of pre existing discrete fabrics as in previous models lateral movement of rigid basal plates induced strike slip deformation of a sand pack local extension allowing the formation of a pull apart basin was produced by the step over geometry of the master faults in this area a basal silicone layer was introduced to distribute deformation and reproduced a weaker crust in the basin conditions of neutral overlapping and underlapping interacting master faults were reproduced the model upper crust modelled by a sand mixture was characterised by the presence of pre existing structures the orientation of these inherited heterogeneities was systematically varied in different experiments model results indicate that depending on their orientation with respect to the strike slip displacement pre existing structures can be reactivated both within and at the margins of the pull apart basins inside the basin reactivation occurs when the pre existing structures are orthogonal or sub orthogonal to the strike slip displacement in this case the pre existing fabrics delay the development and linkage of cross basin faults and increase the complexity of the deformation pattern giving rise to a new set of faults characterised by an atypical trend pre existing fabrics oblique to the local extension direction may be partly reactivated in the central part of the basin as segments of cross basin faults at the margins of the pull apart reactivation occurs if the fabrics spatially coincide with the lateral boundaries of the silicone layer in these conditions reactivation allows a faster development of the border faults which are less segmented than in the homogenous models this also results in a more regular final geometry of the pull apart

relay ramp forms and normal fault linkages canyonlands national park utah fieldwork within a series of mesoscale grabens in southeast utah has revealed a particularly well exposed system of interlinked extensional faults a series of down faulted grabens are developed within a 460 m thick brittle layer of upper paleozoic sandstone and shale which overlies a ductile layer with a high gypsum content all the major grabens consist of two or more overlapping elements which are composed of fault segments these segments may be hard linked fault surfaces are joined or soft linked fault surfaces are isolated but linked by ductile strain of the rock volume between them in map view relay structures are defined as zones connecting the footwalls and hanging walls of overlapping fault segments representing soft linkage of fault segments in the canyonlands grabens the transfer of displacement between soft linked fault segments is characterized by well de fined dipping relay ramps commonly rotated and extended to accommodate the ductile strain between the overlapping fault segments relay ramps develop as ephemeral structures eventually becoming breached by hard linkage of the fault segments breakdown of ramps by breaching is part of the process of fault growth by segment linkage within the canyonlands graben system four orders of segmentation are present this may be a consequence of different scales of mechanical heterogeneity but evidence from the canyonlands grabens and elsewhere points to a fundamental process of discontinuous fault growth by segment linkage

incremental growth of normal faults insights from a laser equipped analog experiment we conducted a laser equipped analog experiment aimed at quasi continuously monitoring the growth of a dense population of normal faults in homogeneous conditions to further understand the way geological faults progressively gain in slip and length as they accumulate more strain we measured with great precision the incremental slip and length changes that the analog faults sustain as they grow these measurements show that the analog faults share common features with the natural ones in particular during their growth the faults develop and maintain cumulative slip profiles that are generally triangular and asymmetric the growth takes place through two distinct phases an initial short period of rapid lateral lengthening followed by a longer phase of slip accumulation with little or no lengthening the incremental slip is found to be highly variable in both space along the faults and time resulting in variable slip rates in particular short and long term slip rates are markedly different we also find that slip measurements at local points on fault traces do not contain clear information on the slip increment repeat mode finally while the fault growth process is highly heterogeneous when considered at the scale of a few slip events it appears homogeneous and self similar at longer time scales which integrate many slip increments this is likely to be the result of a feedback between stress heterogeneities and slip development the long term scale homogeneity also implies that the long term faulting process is primarily insensitive to the short term heterogeneities that are rapidly smoothed or redistributed we propose a new conceptual scenario of fault growth that integrates the above observations and we suggest that faults grow in a bimodal way as a result of a self driven and self sustaining process

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

the role of coseismic coulomb stress changes in shaping the hard link between normal fault segments the mechanism and evolution of fault linkage is important in the growth and development of large faults here we investigate the role of coseismic stress changes in shaping the hard links between parallel normal fault segments or faults by comparing numerical models of the coulomb stress change from simulated earthquakes on two en echelon fault segments to natural observations of hard linked fault geometry we consider three simplified linking fault geometries 1 fault bend 2 breached relay ramp and 3 strike slip transform fault we consider scenarios where either one or both segments rupture and vary the distance between segment tips fault bends and breached relay ramps are favoured where segments underlap or when the strike perpendicular distance between overlapping segments is less than 20 of their total length matching all documented examples transform fault linkage geometries are preferred when overlapping segments are laterally offset at larger distances few transform faults exist in continental extensional settings and our model suggests that propagating faults or fault segments may first link through fault bends or breached ramps before reaching sufficient overlap for a transform fault to develop our results suggest that coulomb stresses arising from multi segment ruptures or repeated earthquakes are consistent with natural observations of the geometry of hard links between parallel normal fault segments

near n s paleo extension in the western deccan region india does it link strike slip tectonics with india seychelles rifting this is the first detailed report and analyses of deformation from the w part of the deccan large igneous province dlip maharashtra india this deformation related to the india seychelles rifting during late cretaceous early paleocene was studied and the paleostress tensors were deduced near n s trending shear zones lineaments and faults were already reported without significant detail an e w extension was envisaged by the previous workers to explain the india seychelles rift at 64 ma the direction of extension however does not match with their n s brittle shear zones and also those faults sub vertical ne sw nw se and few n s we report and emphasize in this work slickenside bearing fault planes brittle shear zones and extension fractures in meso scale enabled us to estimate the paleostress tensors directions and relative magnitudes the field study was complemented by remote sensing lineament analyses to map dykes and shear zones dykes emplaced along pre existing n s to ne sw nw se shears fractures this information was used to derive regional paleostress trends a nw se ne sw minimum compressive stress in the oldest kalsubai subgroup and a n s direction for the younger lonavala wai and salsette subgroups were deciphered thus a nw ne to n s extension is put forward that refutes the popular view of e w india seychelles extension paleostress analyses indicate that this is an oblique rifted margin field criteria suggest only ne sw and nw se with some n s strike slip faults brittle shear zones we refer this deformation zone as the western deccan strike slip zone wdsz the observed deformation was matched with offshore tectonics deciphered mainly from faults interpreted on seismic profiles and from magnetic seafloor spreading anomalies these geophysical findings too indicate oblique rifting in this part of the w indian passive margin we argue that the seychelles microcontinent separated from india only after much of the dlip erupted further studies of magma rich passive margins with respect to timing and architecture of deformation and emplacement of volcanics are required

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

analogue modelling of orthogonal and oblique rifting abstract the three dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled analogue sandbox models the orientation of the zone of rifting where angle between the rift axis and the extension direction was varied systematically from 90 orthogonal rifting to 75 60 and 45 oblique rifting completed models were impregnated and sectioned both vertically and horizontally orthogonal rift models are characterized by long straight rift border faults and intra rift faults perpendicular to the extension direction oblique rift models are characterized by shorter segmented rift border faults and intrarift faults in the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching whereas the intra rift faults formed at a high angle to the extension vector increased extension in the moderately oblique and oblique rift systems 60 and 45 caused the intra rift faults to rotate towards parallelism with the rift border faults no strike slip or oblique slip transfer faults were found in any experiments rift border faults and intra rift faults in the models are segmented along strike with relay ramp structures formed between overlapping fault tips along the strike of the rift variations in half graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays the experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined basement controlled strike slip or oblique slip transfer faults are absent an accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along strike switching of half graben asymmetries and offsetting of depocentres in rift systems the fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments

self consistent rolling hinge model for the evolution of large offset low angle normal faults the nature of the physical processes responsible for the formation of continental and oceanic metamorphic core complexes is widely debated the controversy focuses primarily on whether the low angle normal faults observed in these environments formed and slipped at low angles or were rotated from an original high angle orientation after large offsets we describe a self consistent numerical model for the extension of a brittle layer that can spontaneously produce normal fault structures in our formulation a fault or faults form because strength is locally reduced with increasing strain if the reduction in fault strength is 10 of the total strength of the layer then faults lock after an offset smaller than the layer thickness and new faults form larger strength reduction leads to single faults that continue to slip no matter how large the fault offset if the strength reduction occurs by the loss of cohesion then we see the unlimited offset faults for layers 11 22 km thick for reasonable values of cohesion the key result of this study is that structures very similar to those observed in both oceanic and continental core complexes are produced by rotation of the inactive part of the model fault after very large offset

the sensitivity of the greenland scotland ridge overflow to forcing changes processes that influence the volume and heat transport across the greenland scotland ridge system are investigated in a numerical model with horizontal resolution the focus is on the sensitivity of cross ridge transports and the reaction of the subpolar north atlantic ocean circulation to changes in wind stress and buoyancy forcing on seasonal to interannual timescales a general relation between changes in wind stress or cross ridge density contrasts and the overturning transport of greenland iceland norwegian seas source water is established from a series of idealized experiments the relation is used subsequently to interpret changes in an experiment over the years 1992 97 with realistic forcing on seasonal and interannual timescales there is a clear correlation between heat flux and wind stress curl variability the realistic model suggests a steady decrease in the strength of the cyclonic subpolar gyre of the north atlantic with a corresponding decrease in heat transport during the 1990s

structural geology and 4d evolution of a half graben new digital outcrop modelling techniques applied to the nukhul half graben suez rift egypt abstract lidar based digital outcrop mapping in conjunction with a new surface modelling approach specifically designed to deal with outcrop datasets is used to examine the evolution of a half graben scale normal fault array in the suez rift syn rift deposition in the nukhul half graben was controlled by the graben bounding nukhul fault the fault can be divided into four segments based on the strike of the fault the morphology of hangingwall strata and the variation in throw along strike the segments of the fault became geometrically linked within the first 2 5 m y of rifting as evidenced by the presence of early syn rift abu zenima formation strata at the segment linkage points fault perpendicular folds in the hangingwall related to along strike variations in throw associated with precursor fault segments persist for a further 1 8 m y after linkage of the segments suggesting that the fault remains kinematically segmented we suggest this occurs because of sudden changes in fault strike at the segment linkage points that inhibit earthquake rupture propagation or because displacement is geometrically inhibited at fault linkage points where the orientation of the intersection line of the segments is significantly different from the orientation of the slip vector on the fault system length throw plots and throw contour patterns for minor faults show that some faults initiated in pre rift strata whereas late east striking faults initiated in the syn rift basin fill the late initiating faults are spatially associated with the east striking baba markha fault which was active throughout the rift history but developed as a transfer fault between major block bounding fault systems around 6 7 ma after rift initiation

pre existing normal faults have limited control on the rift geometry of the northern north sea abstract many rifts develop in response to multiphase extension with numerical and physical models suggesting that reactivation of first phase normal faults and rift related variations in bulk crustal rheology control the evolution and final geometry of subsequent rifts however many natural multiphase rifts are deeply buried and thus poorly exposed in the field and poorly imaged in seismic reflection data making it difficult to test these models here we integrate recent 3d seismic reflection and borehole data across the entire east shetland basin northern north sea to constrain the long term regional development of this multiphase rift we document the following key stages of basin development i pre triassic to earliest triassic development of multiple sub basins controlled by widely distributed nnw to ne trending east and west dipping faults ii triassic activity on a single major ne trending west dipping fault located near the basins western margin and formation of a large half graben and iii jurassic development of a large e dipping n to ne trending half graben near the eastern margin of the basin which was associated with rift narrowing and strain focusing in the viking graben in contrast to previous studies which argue for two discrete periods of rifting during the permian triassic and late jurassic early cretaceous we find that rifting in the east shetland basin was protracted from pre triassic to cretaceous we find that during the jurassic most pre jurassic normal faults were buried and in some cases cross cut by newly formed faults with only a few being reactivated previously developed faults thus had only a limited control on the evolution and geometry of the later rift we instead argue that strain migration and rift narrowing was linked to the evolving thermal state of the lithosphere an interpretation supporting the predictions of lithosphere scale numerical models our study indicates that additional regional studies of natural rifts are required to test and refine the predictions of physical and numerical models more specifically our study suggests models not explicitly recognising or including thermal or rheological effects might over emphasise the role of discrete pre existing rift structures such as normal faults

extensional hard linkages eastern gulf of suez egypt the araba abu durba area on the eastern margin of the gulf of suez exhibits two superb outcrop examples of extensional hard linkages in a rift basin here three large domino style basement cored northeast dipping fault blocks are formed by a series of major northwest trending normal faults these are offset by two north northeast trending sinistral oblique slip transfer faults that terminate in horsetail normal fault splays the transfer faults do not extend across the entire rift basin detailed mapping and structural analysis show that they developed by breakage of initial low strain relay ramps along reactivated north northeast trending basement fabrics between overlapping northwest trending normal fault segments paleostrain analysis of fault slip indicators shows that both the normal and the sinistral oblique slip transfer faults were formed synchronously in response to northeast southwest extension perpendicular to the main northwest rift trend

interaction between structural inheritance and extension direction during graben and depocentre formation an experimental approach abstract analysis of structural rift architecture shows that the graben formation is commonly controlled by the contemporaneous activity of two fault trends with an angular obliquity of approximately 40 inspection of the crustal basement and geophysical data reveals that these faults are parallel to inherited oblique crustal and lithospheric discrete fabrics which are reactivated during the extension event we conducted experiments at crustal scale to determine the role of the coeval reactivation of such oblique inherited fabrics in the graben and depocentre development experimentally the oblique inherited lithospheric faults were simulated by a basal discrete velocity discontinuity vd characterised by two different angles with respect to the extension direction at the intersection of the vds our models show that besides the extension direction which induces the formation of linear or independent en echelon grabens the intersection of the two oblique vd segments controls the location of the depocentre and concentrates subsidence for different stretching direction values the depocentre geometry varies from strongly asymmetric to symmetric when the stretching direction corresponds to the bisecting line of the two vd segments applied to the upper rhine graben central segment of the west european rift our models allow interpretation of the development of the late eocene oligocene depocentres at the intersection of two main oblique inherited structures as the result of a constant nw se extension direction concerning the southern east african rift the graben geometries and the evolution of the subsidence during the mio pliocene period may be explained by the reactivation of two main pan african inherited shear zones with an e w direction of extension

the multistage tectonic evolution of the gulf of suez and northern red sea continental rift from field observations field surveys in the gulf of suez northern red sea area revealed the structural pattern and evolution of this tertiary continental rift and provided constraints on the continental rifting process the structure has been controlled at every scale by four major trends of faults n0 20 e n100 120 e n140 e 160 e and n40 60 e observations of sealed faults and folds and superposed structures and the detailed analysis of the fault surfaces are coherent and indicate that as least two different stages of deformation characterize the evolution of the rift the initiation of the rift late oligocene aquitanian corresponds to the creation of large rhomboidal basins weakly subsident and partly bounded by oblique strike slip faults this framework which resulted from the reactivation of inherited weakness zone in the basement controlled the subsequent creation of large tilted blocks upper aquitanian lower burdigalian these structures were trangressed by the upper burdigalian deposits then the blocks were sliced by a secondary horst and graben pattern which was progressively sealed by middle miocene units since the upper miocene the deformation has been focused along some major faults located in the central part of the rift the determination of paleostresses from measurements of striae confirms that the initiation of the rift was controlled by a tectonic regime characterized by an horizontal compressive stress parallel to the rift and its subsequent evolution was controlled by triaxial and radial extensional regimes with horizontal minimum stress 3 predominantly trending n70 80 e the rifting process was not induced by regional doming but by a new distribution of intraplate stresses at approximately oligocene miocene time passive rift uplift of the rift shoulders began in the upper burdigalian cross sections of the rift emphasize its asymmetry large basement blocks are tilted toward the edge of the rift where a major recently active fault and a distinct morphological unit which we call a banquette are located several homogeneous tilt direction provinces can be defined they are separated by transverse hinge zones that correspond to inherited basement trends rifting deformation is expressed in the basement by steep faults arranged in a zig zag pattern and in neogene quaternary sediments by steep faults and listric faults associated with decollements therefore the style of shallow deformation cannot be extrapolated to deeper levels structures formed during the beginning of the rift show different charateristics according to their orientation but their initial shape was generally complicated by later deformation

influence of early strike slip deformation on subsequent perpendicular shortening an experimental approach we used sandbox analogue models to study the influence of previous pure strike slip and transpressional structures on subsequent perpendicular compression we performed also a brittle viscous system in order to analyse the presence of a viscous basal level experimental results show that a previous pattern generated under pure strike slip and transpressive regime causes the reactivation of structures with favourable orientation and the nucleation of oblique thrusts and non rectilinear deformation fronts under compression perpendicular to the first fault system the presence of a viscous basal level with the imposed strain rate which does not favour the coupling between the viscous layer and cover inhibits the reactivation of previous structures comparison with a sector of northern chile margin 24 25 s located in the forearc trench parallel region supports the experimental results

2 45 ga break up of the archaean continent in northern fennoscandia rifting dynamics and the role of inherited structures within the archaean basement abstract constraints from restoration of minor rift related faults preserved from subsequent palaeoproterozoic overprint along the southern margin of the perapohja belt northern fennoscandia indicate local ne sw extension directions prevailed at the time of 2 44 ga rifting of the archaean continent this palaeostress field is attributed to pull apart setting of the perapohja belt at a left overstepping zone of major sinistral n s trending deformation zones consequently the variably overprinted nw se trending structures of the perapohja belt including the nw se central graben were originally generated as normal faults the proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step over zone between strike slip faults by contrast the faults bounding e w to ene wsw trending basement horsts are the result of reactivation of older basement structures underlying the pull apart basin based on the results we infer that the final break up of the archaean continent in northern fennoscandia utilized the 2 44 ga rift related nw se trending crustal anisotropy parallel with dyke swarms with pre existing archaean n s trending structures reactivated as second order shear zones in between the voluminous syn rift 2 5 2 4 ga and later pulses of mafic magmatism 2 32 1 98 ga are considered indicative of active type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the superior hearne and karelian kola cratons the presented model supports the continuity of the archaean craton beyond the caledonian orogen towards north west and explains its segmentation suggesting a model for the localization of the palaeoproterozoic supracrustal belts of northern fennoscandia moreover this paper provides an approach to understand the basement cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits

analogue modelling of fault reactivation tectonic inversion and oblique remobilisation of grabens abstract analogue models of sand above a silicone layer were examined to determine the effects of normal fault reactivation models were first subjected to extension which lead to the formation of two linear grabens each model was then subjected to a second phase deformation either parallel or oblique to the previous initial extension and either extensional or contractional the influence of sedimentation has been evaluated using experiments with and without sedimentation in cases of oblique secondary deformation all newly formed faults were parallel to the older grabens thus they were oblique to the direction of the principal stress directions during the second deformation phase in experiments without sedimentation all older faults were reactivated whereas in experiments with sedimentation some of the older faults were not reactivated in the case of an oblique compressional secondary deformation phase with post rift sedimentation strain partitioning occurred between reactivated older normal faults and new reverse faults 2 was vertical on reactivated normal fault planes whereas 3 was vertical on reverse fault planes in the case of oblique compressional secondary deformation phase without post rift sedimentation no strain partitioning was observed in this model 3 was vertical on every fault plane it is therefore concluded that sedimentation within grabens induces a variation of stress orientation and strain partitioning

growth and linkage of a complex oblique slip fault zone in the pearl river mouth basin northern south china sea abstract oblique slip faults are ubiquitous in sedimentary basins but have been largely neglected relative to their dip slip counterparts case studies of oblique slip faults are very limited in this study we used 3d seismic data tied to well control to investigate the geometry kinematic characteristics growth and linkage and tempo spatial evolution of a complex oblique slip growth fault zone in the pearl river mouth basin prmb of the northern continental margin south china sea scs the e w oriented 30 5 km long fault zone consists of two superimposed oblique slip fault systems the lower fault system is an integrated zig zag fault consisting of three ene wsw to e w striking right stepping sinistral en echelon fault segments which were hard linked by two nw se trending fault segments while the upper fault system is composed of nine wnw ese striking left stepping dextral en echelon component faults which had overlapping fault tips and were partly hard linked by breaching of the relay zones the lower and upper fault systems were activated in the eocene and the oligocene to pliocene which respectively correspond to the syn rift and post rift stages in the prmb the fault throws up to 288 m and activity rates up to 34 5 m ma of the upper fault system are overall much less than those of its lower counterpart reactivation of the lower syn rift fault system shows clear influence on the development of post rift en echelon fault segments a four stage evolution model of the oblique slip fault zone was presented

effect of block rotation on fault sealing an example in dongpu sag bohai bay basin china abstract pre existing faults or early formed faults may be reactivated or locked in subsequent evolution in a rift basin the reactivation or sealing of a fault affects sedimentation in the related fault block and in turn controls the accumulation of oil and gas based on fault mechanics of reactivation this paper established equations denoting the angle range of the reactivated faults or frictional sliding faults by using mohr stress circles the results were applied to explain the sealing property of pre existing faults or early formed faults in dongpu sag in the dongpu sag reactivation or sealing of the faults in fault blocks is supposed due to the rotation of the fault block which was caused by the activity of high order faults for quantitative calculation of dip range for activated faults the sum of the maximum and minimum principal stresses were determined by acoustic emission experiment based on the kaiser effect of rocks and the cohesion and inner frictional angle of rocks were measured by rock physical experiment the calculations show when the dips of early formed faults are from 87 to 45 the faults will remain activation in the subsequent deformation otherwise the faults will be locked as sealing faults the results of petroleum exploration show that the peak dips of sealing faults are from 36 to 40 and the most of sealing faults are those with dip angles less than 46 50 these results indicate that a shallower fault is of better sealing property in oil and gas reservoirs than a steeper fault in dongpu sag

optimal observations for variational data assimilation abstract an important part of ocean state estimation is the design of an observing system that allows for the efficient study of climate related questions in the ocean a solution to the design problem is presented here in terms of optimal observations that emerge as singular vectors of the modified data resolution matrix the actual computation is feasible only for scalar quantities and in the limit of large observational errors identical twin experiments performed in the framework of a 1 north atlantic primitive equation model demonstrate that such optimal observations when applied to determining the heat transport across the greenland scotland ridge perform significantly better than traditional section data on seasonal to interannual time scales optimal observations are located primarily along the continental shelf and information about heat transport wind stress and stratification is being communicated through boundary waves and advective processes on time scales of about 1 month sea surface

construction of the adjoint mit ocean general circulation model and application to atlantic heat transport sensitivity we first describe the principles and practical considerations behind the computer generation of the adjoint to the massachusetts institute of technology ocean general circulation model gcm using r giering s software tool tangent linear and adjoint model compiler tamc the tamc s recipe for fortran line by line generation of adjoint code is explained by interpreting an adjoint model strictly as the operator that gives the sensitivity of the output of a model to its input then the sensitivity of 1993 annual mean heat transport across 29on in the atlantic to the hydrography on january 1 1993 is calculated from a global solution of the gcm the kinematic sensitivity to initial temperature variations is isolated showing how the latter would influence heat transport if they did not affect the density and hence the flow over 1 year the heat transport at 29on is influenced kinematically from regions up to 20 o upstream in the western boundary current and up to 5 o upstream in the interior in contrast the dynamical influences of initial temperature and salinity perturbations spread from as far as the rim of the labrador sea to the 29on section along the western boundary the sensitivities calculated with the adjoint compare excellently to those from a perturbation calculation with the dynamical model perturbations in initial interior salinity influence meridional overturning and heat transport when they have propagated to the western boundary and can thus influence the integrated east west density difference our results support the notion that boundary monitoring of meridional mass and heat transports is feasible

successive orthogonal and oblique extension episodes in a rift zone laboratory experiments with application to the ethiopian rift small scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique type 1 and from oblique to orthogonal type 2 in both the type 1 and type 2 models faults produced during the first stage strongly control fault development during the second stage in type 1 models the oblique faults developed during the second oblique phase are confined within a first phase graben whereas in type 2 models the oblique faults produced during the first phase continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks type 1 models are compared with the structural setting of the ethiopian rift the evolution of the rift is related to a recent extensional event whose principal direction of stretching trends at around 50 to preexisting major normal faults type 1 laboratory models are fairly comparable to the northern sector of the ethiopian rift referred to here as mer they account for both the development of the en echelon oblique faults of the wonji fault belt and the sinistral shear gradient running parallel to the eastern border of the mer which formed during an oblique rifting extension the statistical analysis of the whole ethiopian rift fault pattern by reference to the experimental data allows the determination of a n100 n110 mean direction of stretching

global ocean circulation during 1992 1997 estimated from ocean observations and a general circulation model 1 a three dimensional oceanic state is estimated for the period 1992 1997 as it results from combining large scale ocean data sets with a general circulation model at the cost of increased computational load the estimation assimilation method is chosen specifically so that the resulting state estimate is consistent with the model equations having no artificial sources or sinks to bring the model into close agreement with observations its initial temperature and salinity conditions are permitted to change as are the time dependent surface fluxes of momentum heat and freshwater resulting changes of these control vectors are largely consistent with accepted uncertainties in the hydrographic climatology and meteorological analyses the assimilation procedure is able to correct for many of the traditional shortcomings of the flow field by changing the surface boundary conditions changes in the resulting flow field are predominantly on the gyre scale and affect many features that are often poorly simulated in traditional numerical simulations such as the strengths of the gulf stream and its extension the azores current and the anticyclonic circulation associated with the labrador sea tests of the results and their consistency with prior error assumptions show that the constrained model has moved considerably closer to the observations imposed as constraints but has also moved closer to independent data from the world ocean circulation experiment not used in the assimilation procedure in some regions where the comparisons remain indeterminate not enough ocean observations are available and it is difficult to ascribe the residuals to either the model or the observations although problems remain a useful first solution to the global time dependent ocean state estimation problem has been found the estimates will continue to improve through the evolution of numerical models computer power increases more data and more efficient estimation methods

critical stress difference fault orientation and slip direction in anisotropic rocks under non andersonian stress systems abstract the coulomb navier failure criterion is applied to geological faulting in the general three dimensional case of rocks containing arbitrarily oriented strength anisotropies and subject to non andersonian stress systems i e with none of the principal stresses acting in a vertical direction general expressions for the critical stress difference necessary to cause failure as a function of depth are given in terms of material parameters pore fluid pressure orientation of the stress field and orientation of the strength anisotropy the range of angles between a plane of anisotropy and the maximum principal stress direction for which slip occurs along the pre existing anisotropy rather than along a new fault is calculated as a function of depth for different stress regimes when the stress field is non andersonian and or strength anisotropies not containing the intermediate stress direction occur in the rock faulting generally is oblique slip a kinematic classification of faulting is given on the basis of the angle between the strike direction and the slip direction on the fault plane triangular diagrams analogous to those used in petrology are introduced to describe i faulting in isotropic rock subject to arbitrarily oriented stress fields and ii faulting in anisotropic rock when one principal stress direction is vertical the type of faulting as a function of stress field and anisotropy orientation can be read off directly from these diagrams

attenuation tomography an application to gas hydrate and free gas detection we estimate the quality factor q from seismic reflections by using a tomographic inversion algorithm based on the frequency shift method the algorithm is verified with a synthetic case and is applied to offshore data acquired at western svalbard to detect the presence of bottom simulating reflectors bsr and gas hydrates an array of 20 ocean bottom seismographs has been used the combined use of traveltime and attenuation tomography provides a 3d velocity q cube which can be used to map the spatial distribution of the gas hydrate concentration and free gas saturation in general high p wave velocity and quality factor indicate the presence of solid hydrates and low p wave velocity and quality factor correspond to free gas bearing sediments the q values vary between 200 and 25 with higher values 150 200 above the bsr and lower values below the bsr 25 40 these results seem to confirm that hydrates cement the grains and attenuation decreases with increasing hydrate concentration

the growth of non colinear normal fault systems what can we learn from 3d seismic reflection data abstract many rift zones exhibit a range of fault orientations rather than simple colinear faults that strike orthogonal to the inferred least principal stress the formation of non colinear fault sets has implications for assessing rift zone kinematics as well as determining palaeo stress state in extensional basins using 3d seismic reflection data we deduce the likely mechanisms responsible for the formation of a population of non colinear faults in the maloy slope area of the northern north sea three basement displacing fault populations exist on the maloy slope i large 1 km throw n s striking faults ii smaller

an adjoint sensitivity study of buoyancy and wind driven circulation on the new jersey inner shelf adjoint sensitivity analysis is used to study the new york bight circulation for three idealized situations an unforced buoyant river plume and upwelling and downwelling wind forcing a derivation of adjoint sensitivity is presented that clarifies how the method simultaneously addresses initial boundary and forcing sensitivities considerations of interpretation and appropriate definitions of sensitivity scalar indices are discussed the adjoint method identifies the oceanic conditions and forcing that are dynamically upstream to a region or feature of interest as well as the relative roles of the prior ocean state forcing and dynamical influences to illustrate the method which is quite general the authors consider coastal sea surface temperature sst variability and define the adjoint scalar index as the temporal spatial mean squared sst anomaly on a segment of the new jersey coast at the conclusion of a 3 day period in the absence of wind surface temperature advection dominates the sst anomaly with two sources of surface water identified downwelling winds amplify upstream advective influence sensitivity to temperature is separated into direct advection and the dynamic effect on density stratification and mixing for upwelling conditions this decomposition shows that coastal sst is controlled by both advection from the south and subsurface but above the 5 m depth and temperature related density stratification between 5 and 15 m to 10 km offshore by identifying the timing and location of ocean conditions crucial to subsequent prediction of specific circulation features the adjoint sensitivity method has application to quantitative evaluation of observational sampling strategies

tectonic inheritance and continental rift architecture numerical and analogue models of the east african rift system 1 the western branch of the east african rift is composed of an arcuate succession of elongate asymmetric basins which differ in terms of interaction geometry fault architecture and kinematics and patterns of uplift subsidence and erosion sedimentation the basins are located within proterozoic mobile belts at the edge of the strong tanzanian craton surface geology suggests that the geometry of these weak zones is an important parameter in controlling rift development and architecture although other processes have been proposed in this study we use lithosphere scale numerical models and crustal scale analogue experiments to shed light on the relations between preexisting structures and rift architecture results illustrate that on a regional scale rift localization within the mobile belts at the curved craton s western border results in an arcuate rift system which implies that under a constant extensional stress field part of the western branch experienced orthogonal extension and part oblique extension largest depocenters are predicted to form mostly orthogonal to the extension direction and smaller depocenters will form along the oblique parts of the rift the varying extension direction along the rift zone furthermore results in lengthwise varying rift asymmetry segmentation characteristics and border fault architecture trend length and kinematics analogue models predict that discrete upper crustal fabrics may influence the location of accommodation zones and control the architecture of extension related faults at a local scale models support that fabric reactivation is responsible for the oblique slip kinematics on faults and for the development of z shaped or arcuate normal faults typically documented in nature

crustal structures of the rhinegraben and the massif central grabens an experimental approach two of the most improtant segments of the west european rift the rhinegraben and the massif central grabens show in plan and cross section a very different crustal structure

causes of changes in the denmark strait overflow abstract the warming nordic seas potentially tend to decrease the overflow across the greenland iceland scotland ridge gisr system recent observations by macrander et al document a significant drop in the intensity of outflowing denmark strait overflow water of more than 20 over 3 yr and a simultaneous increase in the temperature of the bottom layers of more than 0 4 c a simulation of the exchange across the gisr with a regional ocean circulation model is used here to identify possible mechanisms that control changes in the denmark strait overflow and its relations to changed forcing condition on seasonal and longer time scales the authors establish links of the overflow anomalies to a decreasing capacity of the dense water reservoir caused by a change of circulation pattern north of the sill on annual and shorter time scales the wind stress curl around iceland determines the barotropic circulation around the island and thus the barotropic flow through denmark strait for the overlapping time sc

complex rift geometries resulting from inheritance of pre existing structures insights and regional implications from the barmer basin rift abstract structural studies of the barmer basin in rajasthan northwest india demonstrate the important effect that pre existing faults can have on the geometries of evolving fault systems at both the outcrop and basin scale outcrop exposures on opposing rift margins reveal two distinct non coaxial extensional events on the eastern rift margin northwest southeast extension was accommodated on southwest and west striking faults that form a complex zig zag fault network on the western rift margin northeast southwest extension was accommodated on northwest striking faults that form classical extensional geometries combining these outcrop studies with subsurface interpretations demonstrates that northwest southeast extension preceded northeast southwest extension structures active during the early previously unrecognised extensional event were variably incorporated into the evolving fault systems during the second in the study area an inherited rift oblique fault transferred extension from the rift margin to a mid rift fault rather than linking rift margin fault systems directly the resultant rift margin accommodation structure has important implications for early sediment routing and depocentre evolution as well as wider reaching implications for the evolution of the rift basin and west indian rift system the discovery of early rifting in the barmer basin supports that extension along the west indian rift system was long lived multi event and likely resulted from far field plate reorganisations

development of the north viking graben inferences from laboratory modelling abstract the north viking graben is a classic sedimentary basin in the north sea that has been thoroughly studied over the last decade despite the amount and quality of seismic and well data now available the processes of extension remain unclear and the relationship between sedimentation and deformation is still a matter of debate we present the results of small scale laboratory modelling the models are made with sand and silicone putty to simulate brittle and ductile behaviour on a crustal scale the experiments were designed to study the effects of a the degree of coupling between brittle and ductile layers b the obliquity of extension and c the reactivation of inherited crustal scale structures the results are used to discuss the symmetry of extensional processes on crustal and lithospheric scales as well as the relationship between sedimentation and deformation it is proposed that the asymmetry of faulting in the upper crust and the nearly symmetric thinning of the whole crust are not mutually exclusive a model involving nearly pure shear on the lithospheric scale is used to explain the development of an asymmetrical basin at shallow level in addition this model accounts for the nearly horizontal stratification of lower cretaceous sediments the basal cretaceous unconformity which caps many of the normal faults and the localization of deformation to the west of the graben during the lower cretaceous

anomalies of meridional overturning mechanisms in the north atlantic abstract optimal observations are used to investigate the overturning streamfunction in the north atlantic at 30 n and 900 m depth those observations are designed to impact the meridional overturning circulation moc in numerical models maximally when assimilated and therefore establish the most efficient observation network for studying changes in the moc they are also ideally suited for studying the related physical mechanisms in a general circulation model optimal observations are evaluated here in the framework of a global 1 model over a 10 yr period hydrographic observations useful to monitor the moc are primarily located along the western boundary north of 30 n and along the eastern boundary south of 30 n additional locations are in the labrador irminger and iberian seas on time scales of less than a year variations in moc are mainly wind driven and are made up through changes in ekman transport and coastal up and downwelling only a small fraction is buoyancy driven and constitutes a sl

rapid freshening of the deep north atlantic ocean over the past four decades the overflow and descent of cold dense water from the sills of the denmark strait and the faroe shetland channel into the north atlantic ocean is the principal means of ventilating the deep oceans and is therefore a key element of the global thermohaline circulation most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse gas concentrations predict a weakening thermohaline circulation in the north atlantic as the subpolar seas become fresher and warmer1 2 3 and it is assumed that this signal will be transferred to the deep ocean by the two overflows from observations it has not been possible to detect whether the ocean s overturning circulation is changing but recent evidence suggests that the transport over the sills may be slackening4 here we show through the analysis of long hydrographic records that the system of overflow and entrainment that ventilates the deep atlantic has steadily changed over the past four decades we find that these changes have already led to sustained and widespread freshening of the deep ocean

transfer zones with en echelon faulting at the northern end of the suez rift detailed field mapping of the central part of the area between the northern end of the suez rift to the nile valley cairo suez area shows that the eocene to miocene rocks are affected by e w elongated belts of left stepped en echelon normal faults these belts act as transfer zones between nw oriented normal faults synchronous with faults of the same trend in the suez rift individual faults within the en echelon fault belts are oriented e w to wnw and dip in the same direction as the linked nw faults step faulting or horsts and grabens are two possible fault arrangements in the transfer zones depending on the geometry of the linked faults the linked faults and the transfer zones are joined together in zigzag fault belts that extend northwest ward from the northern part of the suez rift into the cairo suez area and possibly further northwestward a substantial amount of the throw of the nw faults of the suez rift is transferred northwestward by these zigzag fault belts the throw generally decreases northwestward away from the rift the en echelon fault belts were probably formed by right lateral divergent wrenching on e w oriented deep seated preexisting faults this right lateral divergent wrenching is kinematically related to the dip slip movement on the linked nw oriented normal faults

adjoint sensitivity of an ocean general circulation model to bottom topography abstract bottom topography or more generally the geometry of the ocean basins is an important ingredient in numerical ocean modeling with the help of an adjoint model it is shown that scalar diagnostics or objective functions in a coarse resolution model such as the transport through drake passage the strength of the atlantic ocean meridional overturning circulation the deacon cell and the meridional heat transport across 32 s are sensitive to bottom topography as much as they are to surface boundary conditions for example adjoint topography sensitivities of the transport through drake passage are large in choke point areas such as the crozet kerguelen plateau and south of new zealand the atlantic meridional overturning circulation is sensitive to topography in the western boundary region of the north atlantic ocean and along the scotland iceland ridge many sensitivities are connected to steep topography and can be interpreted in terms of bottom form stress that is the product of bottom pre

structural and microstructural evolution of the rattlesnake mountain anticline wyoming usa new insights into the sevier and laramide orogenic stress build up in the bighorn basin the rocky mountains in western us provide among the best examples of thick skinned tectonics following a period of thin skinned tectonics related to the sevier orogeny the compressional reactivation of basement faults gave birth to the so called laramide uplifts arches the bighorn basin located in wyoming is therefore a key place to study the transition from thin to thick skinned tectonics in orogenic forelands especially in terms of microstructural and stress strain evolution our study focuses on a classic laramide structure the rattlesnake mountain anticline rma wyoming usa a basement cored anticline located in the western part of the bighorn basin stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures fault slip data and calcite twinning paleopiezometry most of the fractures are related to three main tectonic events the sevier thin skinned contraction the laramide thick skinned contraction and the basin and range extension serial balanced cross sections of rma and displacement profiles suggest that all thrust faults were coeval evidencing strain distribution in the basement during faulting the comparison of rma with another structure located in the eastern edge of the bighorn basin i e the sheep mountain anticline sma allows to propose a conceptual model for the geometric and kinematic evolution of laramide related basement cored anticlines finally the stress evolution is reconstructed at both the fold scale and the basin scale we show that the evolution of stress trends and magnitudes was quite similar in both structures rma and sma during laramide times thick skinned tectonics in spite of different stress regimes during sevier thin skinned tectonics and post laramide times stress trends and fracture patterns were different in these two structures these results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes which were likely controlled by the structural style

the effect of cover composition on extensional faulting above re activated basement faults results from analogue modelling abstract analogue models were used to investigate differences in deformation between sedimentary cover sequences of contrasting composition above reactivated basement faults during extension two types of model arrangements were used the first represented clastic sediments directly overlying faulted basement and the second represented a basal ductile layer in an overburden overlying the same faulted basement the basement configuration consisted of faults both oblique and orthogonal to the bulk extension direction particular attention was paid to the resulting map view fault patterns in experiments without a basal ductile layer in the overburden grabens only developed in areas directly above basement faults in section cover faults above oblique basement faults curved in profile and in places exhibited reverse movement at shallow depth in experiments involving a ductile layer at the base of the overburden faults developed oblique to both the bulk extension direction and the basement faults the resulting fault pattern was influenced by both bulk extension and basement fault reactivation the results indicate that cover sequences that include thick basal ductile layers above reactivated basement faults are more likely to develop faults with strikes perpendicular to the regional extension direction compared to those without faulting in an overburden lacking a basal ductile sequence is more likely to be directly influenced by the underlying reactivated basement faults

decreasing overflow from the nordic seas into the atlantic ocean through the faroe bank channel since 1950 the overflow of cold dense water from the nordic seas across the greenland scotland ridge and into the atlantic ocean is the main source for the deep water of the north atlantic ocean this flow also helps drive the inflow of warm saline surface water into the nordic seas the faroe bank channel is the deepest path across the ridge and the deep flow through this channel accounts for about one third of the total overflow previous work has demonstrated that the overflow has become warmer and less saline over time here we show using direct measurements and historical hydrographic data that the volume flux of the faroe bank channel overflow has also decreased estimating the volume flux conservatively we find a decrease by at least 20 per cent relative to 1950 if this reduction in deep flow from the nordic seas is not compensated by increased flow from other sources it implies a weakened global thermohaline circulation and reduced inflow of atlantic water to the nordic seas

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

direct control of normal fault in hydrocarbon migration and accumulation in northwestern bozhong subbasin bohai bay basin china abstract fault growth and reactivation activities have been interpreted as potential controlling factors in vertical hydrocarbon migration but the coupling and direct comparison of fault growth reactivation history and hydrocarbon distribution patterns remain poorly understood the spatial distribution of oil fields in the western subsag of the bozhong subbasin in the bohai bay basin is closely related to fault geometry and presents a unique opportunity to investigate the coupling mechanism of the evolution of normal faults and petroleum migration and accumulation the evolution history of two major normal faults faults f1 and f2 was investigated via three dimensional seismic reflection data and fault f2 was associated with the spatial distribution of oil quantitative throw analyses including throw versus distance plots throw versus depth t z plots throw contour projections and expansion indexes ei were employed fault f2 exhibited a large displacement between the footwall and hanging wall along most of the fault plane and thickened hanging walls of the upper fault surface laterally the fault comprised three segments that linked along the strike at local throw minima vertically the fault formed through the coalescence of two parts via dip linkage both the isolated and coherent fault models may have contributed to the evolution of fault f2 during different periods and can be summarized in several steps including blind fault propagation as individual segments free surface breaching lateral and dip linkage and subsequent frequent reactivation as a coherent structure a key result of this study was that petroleum migration and accumulation could be directly controlled by multiple growth and reactivation activities of faults

geometry and segmentation of an evaporite detached normal fault array 3d seismic analysis of the southern bremstein fault complex offshore mid norway abstract the halten terrace offshore mid norway is underlain by a triassic evaporitic package that is rheologically weak and led to decoupling of fault systems during middle jurassic to early cretaceous rifting we use 2d and 3d reflection seismic data constrained by wells from the southern bremstein fault complex of the halten terrace to map faults and key stratigraphic horizons and analyse throw variations along faults allowing us to constrain patterns of fault segmentation and linkage within the complex the bremstein fault complex has an overall tilted monoclinal geometry with localised fault systems at base salt level associated with overlying highly distributed systems of normal faults vertical strain partitioning across the evaporite package means that sub evaporite and supra evaporite fault populations acted as semi independent fault systems supra evaporite faults are partly gravity driven and controlled by sub evaporite faulting and consequent tilting of the evaporitic package this behaviour leads to a wide variety of possible vertical linkage patterns of faults across the evaporite package a greater variety of lateral segment linkage patterns occurs in evaporite detached normal fault systems than in normal fault systems developed in the absence of evaporite units segment boundary styles can also be modified by migration of evaporite some segment boundaries are associated with a footwall anticline and hanging wall syncline in contrast to the footwall synclines and hanging wall anticlines widely described in studies of normal fault systems

three dimensional fault geometries and interactions within experimental models of multiphase extension abstract we use experimental analog models to examine the three dimensional 3 d fault geometries and interactions that develop during two phases of noncoaxial extension in the models a homogeneous layer of wet clay undergoes two phases of extension whose directions differ by 45 the resulting fault pattern varies significantly with depth at shallow levels second phase normal faults accommodate most second phase extension at depth both second phase normal faults and reactivated first phase faults with oblique slip accommodate most second phase extension a variety of interactions occurs between first phase and second phase faults one interaction involves the upward propagation of second phase faults from tips of reactivated blind first phase faults these hybrid faults have deep segments that strike subperpendicular to the first phase extension direction and shallow segments whose strike varies with depth becoming increasingly subperpendicular to the second phase extension direction at shallow levels a second interaction involves the nucleation of second phase normal faults on the surfaces of reactivated first phase faults these splay faults propagate upward and laterally from their nucleation sites into the hanging walls of the first phase faults as they propagate they commonly encounter and link with different first phase faults the resulting composite faults have zigzag geometries in both map and cross sectional views a third interaction involves either the termination of second phase antithetic normal faults against or near first phase faults or the offset of first phase faults by second phase antithetic normal faults the 3 d fault patterns and interactions within our models closely resemble those within the taranaki basin of offshore new zealand and milne point on alaska s north slope

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

fault volcano interactions with broadly distributed stretching in rifts abstract areas undergoing pure extension are typically characterized by the formation of faults and fractures orthogonal to the maximum principal stress in these conditions the presence of a volcano and its magmatic system perturbs the local stress field leading to changes in faulting magnitude and attitude this process has been analyzed in many previous modeling works in which fault volcano interaction have been reproduced with set ups that create extension over a single discontinuity by use of a standard base plate however many natural rifts display broadly distributed extension between main border faults with no discrete discontinuities driving deformation to account for these conditions in this study we improve the previous works by reproducing and analyzing the interaction of tectonic stresses with volcanic edifices in areas characterized by broadly distributed extension in particular we run a set of analogue models which are extended by stretching a basal elastic sheet with this improved set up models are generally consistent with previous ones but better reproduce the geometry and architecture of faulting in rifts leading to improved and more rigorously results to compare with real cases we confirmed that the presence of a volcano and or an intrusive body locally modifies the fault pattern concentrating faulting and curving faults notably the rheological contrast between the intrusion and the host rock leads to faults that follow the outline of the body favoring concentric faults in contrast radial faults are developed by the volcano load the combination of intrusion host rock contrast and volcano loading creates both types of faulting which are recognized in natural examples the geometry of the resulting faults is dependent on the coexistence or not of intrusion and edifice and on their dimensions the results indicate that volcanic edifices have a stronger influence on fault attitude with respect to the presence of intrusions experimental results are compared with some natural cases from the east african rift system finally we suggest that using faulting geometries as a foundation geodetic monitoring and more precise modeling could be used to assess the rift evolution of such extensional magmatic systems

cenozoic evolution of the yilan yitong graben in ne china an example of graben formation controlled by pre existing structures abstract the ne trending yilan yitong graben yyg occurs along the yilan yitong fault zone in the northern segment of the tan lu fault zone northeastern china it shows an unusually narrow straight shape in map view with a length of ca 600 km and width of

longitudinal evolution of the suez rift structure egypt abstract a three dimensional study of the structure of the suez rift has been carried out using field and subsurface data in an attempt to determine the role of transverse faults and the longitudinal evolution of the rift as in most intracontinental rifts the structure of the gulf of suez area is governed by normal faults and tilted blocks whose crests constitute the main target of exploratory wells the fault pattern consists of two major sets of trends 1 1 longitudinal faults parallel to the rift axis and created in an extensional regime where 3 was trending ene wsw and 2 2 transverse faults with a n s to nne ssw dominant trend the transverse faults are inherited passive discontinuities while most of the longitudinal faults were created during neogene times in a purely extensional regime both sets were simultaneously active producing a zigzag pattern and rhombic shaped blocks the transverse faults can show horizontal strike slip components and act as relays between major normal faults although the suez rift appears as a simple narrow elongated trough dominated by two almost symmetrical shoulders its internal structure is asymmetrical cross profiles show that all the major blocks are tilted in the same direction however the tilt direction changes twice along the rift to the north and to the south of the rift the blocks are tilted eastward while in the central part they are tilted westward to the north the change of dip is accompan ed by a graben type twist zone without transverse faulting at least in the neogene series to the south the change of dip is accompanied by a more complex structure involving both a major transverse fault and a horst type twist zone in this latter case the transverse fault does not cut through the entire rift balanced cross sections established from subsurface data show that the tilt angle and the amount of extension increase from north to south while the width of the blocks decreases indicating a pole of opening close to the northern end of the gulf minimum values for the amount of opening range from 5 km in the north to about 20 km in the south

a near uniform fluctuation of ocean bottom pressure and sea level across the deep ocean basins of the arctic ocean and the nordic seas across the arctic ocean and the nordic seas a basin wide mode of ocean bottom pressure and sea level fluctuation is identified using satellite and in situ observations in conjunction with a global ocean circulation model and its adjoint the variation extends across the interconnected deep ocean basins of these semi enclosed arctic seas collectively called the arctic mediterranean with spatially near uniform amplitude and phase the basin wide fluctuation is barotropic and dominates the region s large scale variability from sub monthly to interannual timescales the fluctuation results from bifurcating coastally trapped waves generated by winds along the continental slopes of the arctic mediterranean and its neighboring seas including the north atlantic ocean the winds drive ekman transport across the large bathymetric gradients forcing mass divergence between the shallow coastal area and the deep ocean basins and creating ocean bottom pressure anomalies of opposite signs in the two regions the anomalies rapidly propagate away as barotropic coastally trapped waves with the coast and continental slope as respective boundaries the waves subsequently bifurcate at the shallow straits connecting the arctic mediterranean with the rest of the globe the straits transmit the shallow anomalies but not the deep variations thereby inhibiting the anomalies mutual cancelation by geographically separating the two anomalies that enter the deep arctic basins equilibrate uniformly across the domain characterized by a homogeneous depth integrated planetary potential vorticity distribution the potential vorticity s steep gradient that borders the basins shields the region from neighboring shallow variations giving rise to the observed spatially confined fluctuation compensating anomalies outside the arctic adjust similarly across the rest of the globe but are comparatively negligible in amplitude because of the global ocean s larger area relative to that of the deep arctic mediterranean the study from a technical perspective illustrates the utility of a model s adjoint in identifying causal mechanisms underlying a complex system

critical stress difference and orientation of faults in rocks with strength anisotropies the two dimensional case abstract equations are derived for the critical stress difference on thrust normal and strike slip faults with finite cohesive strength both in homogeneous isotropic rock and along pre existing strength anisotropies with different cohesion and coefficient of friction subject to the limitation that the plane of anisotropy contains the intermediate axis of stress the range of orientations for which sliding occurs along pre existing planes of weakness rather than along a new fault is given as a function of material parameters and critical stress difference for the intact rock given the principal stress directions the equations allow the direct calculation of the stress and orientation conditions for the three faulting regimes

the role of inherited crustal and lithospheric architecture during the evolution of the red sea insights from three dimensional analogue experiments abstract the red sea represents an incipient ocean basin that is in transition from continental rifting to ocean initiation in this paper we assess how pre existing mantle and crustal heterogeneities within the african and arabian lithospheres influenced rift development and propagation using three dimensional 3d brittle ductile analogue laboratory experiments with a rotational boundary condition here we investigate the effect of three different configurations i pre existing rheological heterogeneities in the crust ii in the lithospheric mantle and iii in both the experimental outcomes show that a linear weak zone in the lithospheric mantle layer exerts a first order control on the overall mechanical behaviour of extending lithosphere rift architecture and style of rifting crustal heterogeneities control second order features such as the trends of the main boundary escarpments along the rift axis our experiments reproduce the geological history of the red sea and provide key insights on its deformation history in space and time we provide for the first time 3d dynamic constraints on how pre existing weak zones in the lithosphere have controlled the temporal evolution of the red sea and determined its distinctive present day morphology the results find that certain morphological elements of the red sea can only be explained by the combined effects of crustal and mantle lithosphere heterogeneities when applied to other natural examples our findings provide significant new insights on the role of crustal and lithospheric heterogeneities during continental rifting

oblique rifting in a slow spreading ridge in oceanic rifts the spreading direction is usually nearly perpendicular to the ridge axis but at the reykjanes1 and mohns2 4 ridges this direction is highly oblique 30 to 40 to the axis these ridges possess axial valleys containing oblique raised features highs corresponding to second or third order ridge axis discontinuities5 at the reykjanes ridge searle and laughton1 observed small highs which they interpreted as volcanic domes here we present a fault map of the mohns ridge obtained from image processing of seabeam data we argue that the oblique highs in this ridge are of tectonic origin the axial valley is interpreted as the surface trace of a deformable band of oceanic lithosphere inherited from an earlier stage of perpendicular spreading we describe laboratory experiments on brittle ductile models which support this interpretation in which stretching at 30 to a deformable band reproduces the en echelon horst and graben pattern of the mohns ridge

influence of normal fault growth and linkage on the evolution of a rift basin a case from the gaoyou depression of the subei basin eastern china abstract it is widely acknowledged that the growth and linkage of a boundary fault affects the evolution of the accompanying sedimentary basin and exerts a strong influence on its sedimentary characteristics in this study we attempt to discuss the evolution of a boundary fault and rift basin by using seismic interpretation incremental fault throw and growth rate distance g d profile which are more suitable parameters than the expansion index in rift basins where footwall strata are missing because of erosion or a depositional hiatus therefore the gaoyou depression is taken as an example the gaoyou depression is the largest hydrocarbon bearing depression in the subei basin which is a representative rift basin in eastern china the depression is bounded to the south by the jiangdu wubao fault zone which includes the zhen 1 wu 1 zhen 2 and wu 2 faults using high resolution three dimensional seismic data the zhen 1 wu 1 fault can be subdivided into four segments based on its g d profile the shaobo fanchuan liuwushe and liulu segments growth rates indicate that the zhen 1 wu 1 fault and the zhen 2 and wu 2 faults developed successively from the late cretaceous to the miocene during rifting of the gaoyou depression and that development of the zhen 2 and wu 2 faults was controlled by the zhen 1 wu 1 fault the four segments of the zhen 1 wu 1 fault were linked with each other by the time the zhen 2 and wu 2 faults formed in the late paleocene an evolution model of the gaoyou depression is set up in this study integrating the fault activity with previous sedimentological studies we suggest that the evolution of the gaoyou depression was controlled by the jiangdu wubao fault zone rather than by a single fault successive slip of the zhen 1 wu 1 fault and the zhen 2 and wu 2 faults caused subsidence and migration of the depocenters this study presents a case investigation of the effects of boundary faults on rift basins in eastern china

fault growth and interactions in a multiphase rift fault network horda platform norwegian north sea abstract physical models predict that multiphase rifts that experience a change in extension direction between stretching phases will typically develop non colinear normal fault sets furthermore multiphase rifts will display a greater frequency and range of styles of fault interactions than single phase rifts although these physical models have yielded useful information on the evolution of fault networks in map view the true 3d geometry of the faults and associated interactions are poorly understood here we use an integrated 3d seismic reflection and borehole dataset to examine a range of fault interactions that occur in a natural multiphase fault network in the northern horda platform northern north sea in particular we aim to i determine the range of styles of fault interaction that occur between non colinear faults ii examine the typical geometries and throw patterns associated with each of these different styles and iii highlight the differences between single phase and multiphase rift fault networks our study focuses on a ca 350 km 2 region around the 60 km long n s striking tusse fault a normal fault system that was active in the permian triassic and again in the late jurassic to early cretaceous the tusse fault is one of a series of large 1500 m throw n s striking faults forming part of the northern horda platform fault network which includes numerous smaller 2 10 km long lower throw nd stage nw se striking faults grew interacted and linked with the n s striking tusse fault documenting a range of interaction styles including mechanical and kinematic isolation abutment retardation and reactivated relays our results demonstrate that i isolated and abutting interactions are the most common fault interaction styles in the northern horda platform ii pre existing faults can act as sites of nucleation for 2 nd stage faults or may form mechanical barriers to propagation iii the throw distribution on reactivated 1 st stage faults will be modified in a predictable manner if they are intersected or influenced by 2 nd stage faults iv sites of fault linkage and relay breaching associated with the first phase of extension can act as preferential nucleation sites for 2 nd stage faults and v the development of fault intersections is a dynamic process involving the gradual transition from one style to another

modelling of structural complexity in sedimentary basins the role of pre existing faults in thrust tectonics abstract analogue and numerical models have been used to study the role of pre existing faults in compressive regimes from a theoretical point of view reactivation is mainly controlled by fault attitude stress regime and frictional properties of fault planes in scaled down sandbox experiments precut faults are introduced in the homogeneous granular media with a nylon wire which is forced through the sand cake producing a thin planar disturbed zone systematic experiments of thrust inversion with various dips and strikes of such planar discontinuities have been modelled comparison of experimental results with theoretical diagrams indicate that disturbed zones have a friction angle which is 10 20 lower than the homogeneous sand and that the compressive regime in the sandbox has a shape factor close to 0 4 the static analysis of fault reactivation is in accordance with the experimental observations except for pre existing faults dipping at very low angle however numerical modelling using the udec code shows that low angle faults can be reactivated as a result of stress concentration in the lower part of the fault in addition sandbox experiments indicate that in thrust systems reactivation of pre existing faults is not only dependent on their attitude but also on their spacing and location relative to the thrust system

oil migration through preferential petroleum migration pathway ppmp and polycyclic faults a case study from the shijiutuo uplift bohai bay basin china abstract the shijiutuo uplift is an oil enriched uplift in the offshore bohai bay basin petroleum migration is a key factor for oil enrichment in neogene reservoirs far away from the hydrocarbon kitchen in this article an integration of geological geophysical and geochemical analyses are employed to investigate the petroleum migration and accumulation on the shijiutuo uplift hydrocarbons in the qhd32 6 and qhd33 oilfields are mainly originated from the third e 2 s 3 and first e 2 s 1 member of the shahejie formation the shallow traps have significant contributions of late stage e 2 s 1 derived oil lateral petroleum migration is a major mechanism forming large oilfields on the shijiutuo uplift the large oilfields have multiple hydrocarbon kitchens multiple source rocks and numerous preferential petroleum migration pathways ppmps once petroleum arrives at the structural highs on the shijiutuo uplift through the guantao formation n 1 g carrier beds neotectonic faults cutting through guantao n 1 g and minghuazhen n 1 m l formations could serve as effective conduits for vertical petroleum migration neotectonic faults have experienced polycyclic fault activities fluid inclusions indicate episodic hydrocarbon charging crude oils display duplex properties of biodegradation and non biodegradation which is strong evidence for multiple and episodic oil charging on the shijiutuo uplift finally episodic petroleum charging along polycyclic neotectonic faults causes the late stage e 2 s 1 derived oils to occur in the shallow reservoirs

active normal faulting beneath a salt layer an experimental study of deformation patterns in the cover sequence scaled experimental models show that the presence of a viscous layer such as salt facilitates the development of extensional forced folds above active normal faults the geometries of the extensional forced folds and their associated secondary fault patterns depend on the thickness and viscosity of the viscous layer the thickness of the cover sequence the strength and ductility of the cover sequence and the magnitude and rate of displacement on the underlying master normal fault increasing the thickness of the viscous layer and the cohesive strength and ductility of the overburden enhances the decoupling between the deep and shallow deformation alternatively increasing the viscosity of the viscous layer the thickness of the overburden and the magnitude and rate of displacement on the master normal fault reduces the decoupling between the deep and shallow deformation enhanced decoupling facilitates the formation of broad extensional forced folds and the development of detached secondary faults both near and far from the master normal faults the model predicted deformation patterns closely resemble those observed in the gulf of suez the haltenbanken area of offshore norway and the jeanne d arc basin of the grand banks offshore southeastern canada

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic

decadal variability in the outflow from the nordic seas to the deep atlantic ocean the global thermohaline circulation is the oceanic overturning mode which is manifested in the north atlantic ocean as northward flowing surface waters which sink in the nordic greenland iceland and norwegian seas and return southwards after overflowing the greenland scotland ridge as deep water this process has been termed the conveyor belt and is believed to keep europe 5 8 c warmer than it would be if the conveyor were to shut down the variability of today s conveyor belt is therefore an important component of climate regulation the nordic seas are the only northern hemisphere source of deep water and a previous study has revealed no long term variability in the outflow of deep water from the nordic seas to the atlantic ocean here i use flows derived from hydrographic data to show that this outflow has approximately doubled and then returned to previous values over the past four decades i present evidence which suggests that this variability is forced by variability in polar air temperature which in turn may be connected to the recently reported arctic warming

variability of the meridional overturning in the north atlantic from the 50 year gecco state estimation abstract the german partner of the consortium for estimating the circulation and climate of the ocean gecco provided a dynamically consistent estimate of the time varying ocean circulation over the 50 yr period 1952 2001 the gecco synthesis combines most of the data available during the entire estimation period with the ecco massachusetts institute of technology mit ocean circulation model using its adjoint this gecco estimate is analyzed here for the period 1962 2001 with respect to decadal and longer term changes of the meridional overturning circulation moc of the north atlantic a special focus is on the maximum moc values at 25 n over this period the dynamically self consistent synthesis stays within the error bars of h l bryden et al but reveals a general increase of the moc strength the variability on decadal and longer time scales is decomposed into contributions from different processes changes in the model s moc strength are strongly influenced by the southward communication of d

response of circulation and heat transport in the north atlantic to changes in thermohaline forcing in northern latitudes a model study to avoid an explicit simulation of the overflows across the greenland scotland ridge many models of the large scale ocean circulation seek to include the net effect of the inflowing dense water masses by restoring temperature and salinity near the ridge to observed conditions in this paper the authors examine the effect of different datasets for the northern restoring condition in two versions eddy resolving and non eddy resolving of the model of the north and equatorial atlantic that has been developed in recent years as a community modeling effort for woce it is shown that the use of smoothed climatological fields of temperature and salinity south of the denmark strait leads to strong deficiencies in the simulation of the deep flow field in the basin a switch to actual hydrographic data from the denmark strait ignites a rapid dynamic response throughout the north atlantic affecting the transport and vertical structure of the deep western boundary current and by virtue of the jebar efffect the transport of the horizontal gyres meridional overturning and northward heat transport too weak in the cases with climatological boundary conditions increase to more realistic levels in the subtropical north atlantic the initial response to switches in the high latitude thermohaline forcing is mediated by fast waves along the westurn boundary leading to changes in the deep western boundary current in low latitudes after about two years in the non eddy resolving cast the initial timescale depends on the horizontal grid spacing of the model in the high resolution case the first signal reaches the equator in a few months the adjustment to a new dynamic quasi equilibrium involves kelvin waves along the equator and rossby wave in the interior and is attained in less than two decades throughout the north atlantic it is suggested that these fast dynamic adjustment processes could play an important role in possible fluctuations of the thermohaline circulation or transitions between different equilibrium states of the coupled ocean atmosphere system and may have determined the timescale of the observed climatic transitions before and during the last deglaciation