boundary fault linkage and its effect on upper jurassic to lower cretaceous sedimentation in the gudian half graben songliao basin northeastern china abstract the songliao basin northeastern china is one of the few ultra large petroleum basins in the world fault superposition linkage and stratigraphic architecture have produced a complicated basin evolutionary history in this study we report the boundary fault linkage and associated upper jurassic and lower cretaceous stratigraphic record using 3d seismic reflection data and well data our results show that the geometry of the boundary gudian fault was originally controlled by six shorter fault segments which grew and were linked during a syn rift stage then frictionally weakened and inverted in a post rift stage this boundary fault has variably controlled coeval syn rift sedimentation fault segments in the middle section controlled the development of the limited but thickest huoshiling and the first member of shahezi formations in the early syn rift stage ca 150 133 9 ma later that control gradually weakened and vanished on succeeding the second member of shahezi and yingcheng formations in the late syn rift stage ca 133 9 105 ma the processes and the diverse syn rift sedimentation response to gudian fault growth and linkage have further implications for graben evolution and can guide hydrocarbon exploration in the songliao basin this study will help elucidate the different controls of boundary fault linkage on associated sedimentation as well as rift basins development in eastern china and similar areas throughout the world

release fault a variety of cross fault in linked extensional fault systems in the sergipe alagoas basin ne brazil abstract two types of cross faults are herein recognized transfer faults and the newly termed release faults transfer faults form where cross faults connect distinct normal faults and horizontal displacements predominate over vertical ones in contrast release faults form where cross faults associated with individual normal faults die out within the hangingwall before connecting to other normal faults and have predominantly vertical displacements release faults are geometrically required to accommodate variable displacements along the strike of a normal fault thus they form to release the bending stresses in the hangingwall and do not cut normal fault planes nor detachment surfaces at depth release faults have maximum throws adjacent to normal faults and may be nearly perpendicular or obliquely oriented to the strike of the latter such geometry appears not to depend upon pre existing weaknesses but such variable orientation to normal faults is an inherent property of release faults release faults commonly appear as simple normal faults in seismic sections without implying extension along the strike of rift and basins three dimensional strain deformation occurs in the hangingwall only between the terminations of an individual normal fault but regionally release faulting is associated with plane strain deformation in linked extensional fault systems

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

high resolution fault displacement mapping from three dimensional seismic data evidence for dip linkage during fault growth abstract detailed mapping using high resolution three dimensional seismic data has revealed a number of sub horizontal anomalies in the distribution of vertical displacement throw on the planes of growth faults in the gulf of mexico recognition of these anomalies is highly sensitive to the interval at which the fault displacements are sampled because they represent local decreases in throw which are confined to small discrete parts of the fault planes the distribution of the anomalies is inconsistent with model displacement fields of quasi elliptical concentric contours and is therefore incompatible with models of fault growth by uniform slip distribution and radial tip line propagation an alternative model is proposed whereby the evolution of a fault plane is established by the propagation and linkage of precursor fault segments in the dip direction dip linkage overlap and linkage of fault tips in the dip direction results in relay structures that are sub parallel to fault strike and therefore displacement minima that are sub horizontal on normal and thrust faults however since they are orthogonal to the main slip direction on the fault these structures have a low preservation potential and are therefore unlikely to be well resolved on cross sectional seismic profiles

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

influence of pre existing pervasive fabrics on fault patterns during orthogonal and oblique rifting an experimental approach abstract pervasive mechanical anisotropy e g metamorphic foliation in the pre rift basement rocks often appears to control the orientation of new faults in natural rift systems and thus tend to influence formation of fault bounded sedimentary basins however the extent to which this influence works in nature has remained debatable and contrasting ideas exist regarding the issue of structural inheritance in natural rifts like the east african rift system earlier experimental works have investigated different kinematic aspects of faulting in simulated orthogonal and oblique rift systems including the role of a crustal scale pre existing zone of weakness on continent scale rifting these experiments have tested the influence of discrete pre existing weak zones e g pre existing faults or an ancient orogenic belt on localization of subsequent rifting but have not considered the control of pervasive fabric anisotropy e g metamorphic foliation etc in pre rift basement rocks on the fault patterns during rifting here we present a series of simple roughly scaled analog model experiments where pervasive strength anisotropy in the basement was created by brushing plaster of paris on a viscous pitch substratum this way a transverse anisotropy of tensile strength was generated in the plaster of paris layer due to thickness variation across the brush marks the basement was overlain by a dry sand pack simulating weak sedimentary rocks overlying a foliated crystalline basement orthogonal and oblique extension of the models at various angles and with differently oriented strength anisotropy exhibited the control of the anisotropy on the orientations of new faults in the sand layer in general anisotropy could significantly control the fault pattern only when it was at a high angle 45 with respect to the maximum instantaneous horizontal stretching direction e h1 otherwise they largely disregarded the anisotropy and formed orthogonal to e h1 faults never formed orthogonal to the displacement vector d in oblique rifting unless the fault controlling anisotropy was perpendicular to d faults strongly following the anisotropy were much longer than those not following that trend faults oblique to the anisotropy were short segmented and tortuous in nature and mainly grew by segment linkage the experimental results support many field observations made in the east african rift system and are likely to be applicable to other rift systems like gondwana basins of india where field data on such structural inheritance are meager

influence of crustal heterogeneity on normal fault dimensions and evolution southern south africa extensional system abstract the tectonic configuration of southern south africa is dominated by a continental scale mesozoic extensional system superimposed upon a significant and well constrained palaeozoic lithospheric scale heterogeneity through integrating onshore structural analysis with offshore subsurface studies it is possible to evaluate the applicability of established normal fault growth models in a heterogeneous crustal setting at a basin scale for the first time the 480 km long mesozoic extensional system comprises a number of fault arrays that vary in length from 78 to 230 km coupled with displacements of up to 16 km the fault arrays are amongst the longest and largest displacement of high angle normal faults dips of 45 60 documented in continental lithosphere although comprised of co linear segments rather than the en echelon segment configuration typical of many other extensional systems this atypical geometry is considered to be a consequence of the high degree of structural inheritance between the underlying paleozoic cape fold belt and the subsequent extensional system furthermore it is proposed that the overall displacement length dimensions of the extensional faults were inherited from the underlying compressional faults the establishment of a seismic stratigraphic framework for the pletmos and gamtoos offshore basins reveals that the faults established their long lengths 160 and 90 km respectively within 6 myr of rift initiation prior to accruing their substantial displacement 16 km furthermore there is no evidence for the development of intra basin faults the southern south africa extensional system presents an end member case of structural inheritance where extensional structures are parallel to and reactivate underlying compressional structures in this study structural inheritance is considered to have a significant effect on mechanisms of fault growth the pre existing structures not only result in the rapid establishment of fault lengths of up to 160 km but additionally to strain being localised onto the pre existing fabric to such an extent that no intra basin faults evolve and strain is accommodated entirely on the bounding faults

normal fault development during two phases of non coaxial extension an experimental study we use scaled experimental analog models to study the effect of a pre existing fault fabric on fault development during extension in our models a homogeneous layer of wet clay undergoes two noncoaxial phases of extension whose directions differ by up to 45 the normal faults that develop during the first phase create a pronounced fault fabric that influences normal fault development during the second phase in all models the pre existing faults are reactivated during the second phase of extension their sense of slip depends on the angle between the first and second phase extension directions specifically the component of dip slip relative to strike slip decreases as the angle between the first and second phase extension directions increases new normal faults also form during the second phase of extension in all models the number of new fault segments increases as the angle between the first and second phase extension directions increases the orientations of the new normal fault segments are both orthogonal and oblique to the second phase extension direction indicating that both the secondphase extension direction and the pre existing fault fabric control the orientation of new fault segments some of the new normal faults cut and offset the pre existing faults whereas others terminate against them producing complex fault patterns and interactions the modeling results explain fault interactions observed in the jeanne d arc rift basin of offshore newfoundland canada and the reactivation of abyssal hill normal faults at outer highs near subduction zones

morphometric analysis of the submarine arc volcano monowai tofua kermadec arc to decipher tectono magmatic interactions morphometric analysis of multibeam bathymetry and backscatter data is applied to monowai a submarine volcano of the active tofua kermadec arc to map and document the structure and evolution of the volcanic centre low rates of erosion and sedimentation and pervasive tectonic and magmatic processes allow quantification through detailed structural analysis and measurement of deformation the slope aspect curvature rugosity and hydrology flow tools of arcgis provide a robust structural interpretation and the development of a model of monowai evolution a nested caldera structure with a volume of 31 km3 and a stratovolcano of 18 km3 dominate the magmatic constructs the outer caldera is elongate along 125 and the inner caldera along 135 numerous parasitic cones and fissure ridges are also observed oriented at 039 and 041 respectively northeast trending faults with a regional average strike of 031 are widespread within this part of the backarc forming a nascent rift graben to the west of the monowai caldera complex the distribution of throw varies spatially reaching a maximum total along rift of 320 m and across rift of 120 m with greater throw values measured in the west elongation directions of the two nested calderas are near perpendicular to the trends of faults and fissure ridges the inner caldera is more orthogonal to the magmatic constructs fissure ridges and aligned vent cones and the outer caldera is approximately orthogonal to the regional fault fabric suggesting a strong interaction between magmatic and tectonic processes and the directions of the horizontal principal stress we present a detailed morphometric analysis of these relationships and the data are used to interpret the spatial and temporal evolution of the tectono magmatic system at monowai and classify the type of rifting as transtensional similar analysis is possible elsewhere in the kermadec backarc and within other regions of submarine volcanism

early stage evolution of growth faults 3d seismic insights from the levant basin eastern mediterranean the geometry and kinematic evolution of small growth faults were analysed from a high resolution 3d seismic dataset located at the margins of the levant basin in the eastern mediterranean the 3d geometry segmentation history and throw distribution of one particular fault was reconstructed to evaluate and illustrate the changes in dimension and displacement distribution that occurred during the transition from purely blind propagation to propagation at the free surface the fault is considered to have grown by blind radial propagation of three main segments that hard linked prior to surface interaction the fault subsequently reached the seabed and continued to accrue displacement as a syn sedimentary fault most of the fault surface area formed during the blind propagation phase but most of the displacement was added during the syn sedimentary phase of the growth history with little increase in surface area the interaction of the fault with the free surface led to a change in the position of the point of maximum displacement as well as modifying the vertical throw distribution the amount of displacement added after this transition from blind fault to growth fault is discussed with respect to existing fault growth models

strain migration during multiphase extension observations from the northern north sea many rifts develop through multiphase extension it can be difficult however to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried using 2 d and 3 d seismic reflection and borehole data from the northern north sea we examine the style magnitude and timing of reactivation of a preexisting permian triassic rift phase 1 fault array during a subsequent period of middle jurassic to early cretaceous rift phase 2 extension we show that rift phase 2 led to the formation of new n s striking faults close to the north viking graben but did not initially reactivate preexisting rift phase 1 structures on the horda platform we suggest that at the beginning of rift phase 2 strain was focused in a zone of thermally weakened lithosphere associated with the middle jurassic north sea thermal dome rather than reactivating extant faults diachronous reactivation of the permian triassic fault network eventually occurred with those faults located closer to the middle jurassic to early cretaceous rift axis reactivating earlier than those toward the eastern margin this diachroneity may have been related to flexural down bending as strain became focused within the north viking graben and or the shifting of the locus of rifting from the north sea to the proto north atlantic our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field

origin of contrasting devonian supradetachment basin types in the scandinavian caledonides supradetachment basins that formed during the devonian extensional collapse of the scandinavian caledonides have been explained as hanging wall basins formed along the listric breakaway zone of a major detachment fault system this model involves significant rotation of bedding as the detachment flattens to an approximately horizontal orientation and explains the large stratigraphic thicknesses of east dipping layers overlying in tectonic contact the top to the west nordfjord sogn detachment zone nsdz however it fails to explain one of the basins the hasteinen basin where east dipping devonian strata rest unconformably on a metamorphic substrate that forms part of the upper plate of the detachment system based on detailed field mapping we present a model where the hasteinen forms as a ramp basin that develops on the upper plate above a major west facing ramp in the nsdz we test both of these models by forward modeling using 2dmove two dimensional kinematic modeling software the results show how two different types of supradetachment basins can coexist in a subhorizontal detachment zone in an area that underwent many tens of kilometers of lateral crustal extension

scale matters the influence of structural inheritance on fracture patterns abstract fracture systems are often geometrically invariant across a range of scales but the impact of structural inheritance on this relationship is poorly understood this paper shows how fracture orientations in sedimentary rocks vary at different scales when influenced by pre rift basement structures we use high resolution unmanned aerial vehicle uav orthophotos to map folds and fractures in the basement and cover rocks of the gippsland basin southeast australia outcrop scale observations are compared with 1 km long faults previously interpreted from potential field data we use length coloured rose diagrams of fracture traces to compare trends in fracture orientations early cretaceous syn rift normal faults exhibit the same ene wsw trend at basin 1 km and outcrop meters scales pervasive outcrop scale subvertical nnw sse striking joints record a subsequent regional shortening event but at the basin scale this is only expressed as reverse reactivated ene wsw striking faults thus fabrics and or faults in the underlying basement exert significant control on the orientation of basin scale fractures in the cover but appear to have limited influence on outcrop scale fracture orientations our observations show that fracture systems influenced by structural inheritance are not scale invariant and that a proper understanding of structural architecture can only be achieved by analysing data that span multiple scales

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

deep seated faults and hydrocarbon leakage in the snohvit gas field hammerfest basin southwestern barents sea abstract high quality 3d seismic data are used to analyze the history of fault growth and hydrocarbon leakage in the snohvit field southwestern barents sea the aim of this work is to evaluate tectonic fracturing as a mechanism driving hydrocarbon leakage in the study area an integrated approach was used which include seismic interpretation fault modeling displacement analysis and multiple seismic attribute analysis the six major faults in the study area are dip slip normal faults which are characterized by complex lateral and vertical segmentation these faults are affected by three main episodes of fault reactivation in the late jurassic early cretaceous and paleocene fault reactivation in the study area was mainly through dip linkage the throw distance plots of these representative faults also revealed along strike linkage and multi skewed c type profiles the faults evolved through polycyclic activity involving both blind propagation and syn sedimentary activity with their maximum displacements recorded at the reservoir zone the expansion and growth indices provided evidence for the interaction of the faults with sedimentation throughout their growth history soft reflections or hydrocarbon related high amplitude anomalies in the study area have negative amplitude reverse polarity and are generally unconformable with structural reflectors the interpreted fluid accumulations are spatially located at the upper tips of the major faults and gas chimneys four episodes of fluid migration are inferred and are linked to the three phases of fault reactivation and neogene glaciations hydrocarbon leakage in the snohvit gas field is driven by tectonic fracturing uplift and erosion the interpreted deep seated faults are the main conduits for shallow hydrocarbon accumulations observed on seismic profiles

oblique rifting and sequential faulting in the jurassic development of the northern north sea abstract jurassic structural development of the northern north sea has characteristics of oblique rifting this is interpreted to result from an obliquity between jurassic nw se to wnw ese directed extension and a preexisting n s trending zone of deformation related to permo triassic rifting and thinning of the crust the zone of maximum jurassic extension i e the viking and sogn grabens defines a narrow 25 40 km depression which trends diagonally nne ssw across a broader 130 150 km permo triassic basin the jurassic rift zone consists of a system of ene chelon rift segments with changing symmetry along strike bounded by faults oblique to the overall graben envelope sequential jurassic fault development as well as the interference between n s and ne sw striking faults are explained within the context of oblique rifting the early late bajocian oxfordian rift stage was dominated by reactivation of n s striking major faults whereas a number of coeval but smaller faults have a ne sw orientation ne sw striking faults show increasing importance during the jurassic stretching phase and major faults defining this orientation accommodated most of the kimmeridgian volgian extension faults striking nesw are observed to cross cut earlier n s trending faults they interfere with n s trending faults to define faultblocks with rhombohedral shape and they are associated with partial collapse of crestal areas of n s oriented fault blocks to form structural terraces

fabric anisotropy controls faulting in the continental crust the influence of pre existing structural anisotropy on faulting in the continents is best tested in recently exhumed crust e g nanga parbat massif nw himalayas where earlier brittle structures have been annealed the kinematics of young faults formed in a single continuing tectonic regime nnw compression are distinctly different depending upon the orientation of the early ductile foliations around them faulting is subparallel and statistically simple where foliation is moderately dipping but highly complex where foliation is steeply dipping thus structural anisotropy does control faulting in the continental crust a result with important implications for seismogenesis fluid flow and basin evolution

3d seismic analysis of the structure and evolution of a salt influenced normal fault zone a test of competing fault growth models abstract in this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3d seismic reflection dataset from the suez rift egypt our analysis indicates that the october fault zone is composed of two fault systems that are locally decoupled across a salt bearing interval of late miocene messinian age the sub salt system offsets pre rift crystalline basement and was active during the late oligocene early middle miocene it is composed of four planar nw se striking segments that are hard linked by n s striking segments and up to 2 km of displacement occurs at top basement suggesting that this fault system nucleated at or more likely below this structural level the supra salt system was active during the pliocene holocene and is composed of four nw se striking listric fault segments which are soft linked by unbreached relay zones segments in the supra salt fault system nucleated within pliocene strata and have maximum throws of up to 482 m locally the segments of the supra salt fault system breach the messinian salt to hard link downwards with the underlying sub salt fault system thus forming the upper part of a fault zone composed of i a single amalgamated fault system below the salt and ii a fault system composed of multiple soft linked segments above the salt analysis of throw distance t x and throw depth t z plots for the supra salt fault system isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history the fault tips were then effectively pinned and the faults accumulated displacement via predominantly downward propagation we interpret that the october fault zone had the following evolutionary trend i growth of the sub salt fault system during the oligocene to early middle miocene ii cessation of activity on the sub salt fault system during the middle miocene to early pliocene iii stretching of the sub and supra salt intervals during pliocene regional extension which resulted in mild reactivation of the sub salt fault system and nucleation of the segmented supra salt fault system which at this time was geometrically decoupled from the sub salt fault system and iv pliocene to holocene growth of the supra salt fault system by downwards vertical tip line propagation which resulted in downward breaching of the salt and dip linkage with the sub salt fault system the structure of the october fault zone and the rapid establishment of supra salt fault lengths are compatible with the predictions of the coherent fault model although we note that individual segments in the supra salt array grew in accordance with the isolated fault model our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre scale basin bounding faults furthermore we highlight the role that fault reactivation and dip linkage in mechanically layered sequences can play in controlling the three dimensional geometry of normal faults

postcollisional extension of the caledonide orogen in scandinavia structural expressions and tectonic significance the scandinavian caledonides are much more affected by postcontractional devonian extension than previously thought the extensional deformation is not restricted to the internal parts of the orogen but can be traced for more than 200 km from the west coast of norway toward the foreland it involved significant reactivation of the basal thrust zone as a low angle extensional detachment whic consequently contains abundant top to the north west sense of shear indicators the extensional deformation involved substantial backsliding of the orogenic wedge eduction of the baltic shield together with stretching of the basement by west dipping extensional shear zones after 30 km of backsliding the extensional detachment splayed to penetrate the overlying orogenic edifice in the coastal region in southern norway which led to inactivation of the main portion of the basal thrust zone as an extensional detachment

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

displacement loss on growth faults due to sediment compaction compaction models of normal growth faults combined with displacement data from a natural growth fault cape egmont fault new zealand are used to quantify the loss of displacement arising from compaction in circumstances where growth faults are blanketed and buried by compacting sediment in both footwall and hangingwall comparison of imposed tectonic displacements with the preserved displacements indicates that although the thicknesses of sand shale sequences may decrease by up to c 55 due to compaction associated losses in displacement on growth faults are typically 70 and post faulting burial is large i e km scale

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

unconformities related to the jurassic cretaceous synrift post rift transition of the northern north sea in the jurassic cretaceous north sea basin the synrift sequence is separated from the post rift sequence by the base cretaceous or late cimmerian unconformity the unconformity covers almost the entire basin has a distinct character in seismic reflection data and wireline logs and hence is easily identified and correlated making it the most important marker horizon in the area the unconformity displays great local complexity in many localities and great variability on a regional scale from one locality to another thus the unconformity is classified as a nonconformity a disconformity and an angular unconformity we suggest that these variations basically reflect different structural position within the basin so that the short wavelength variation is dominated by local structural development e g the rotational history of a fault block whereas the long wavelength variation reflects basin scale tectonic thermal and isostatic processes the merging of this unconformity with younger erosional surfaces its complex configuration and polychronous character makes the general term base cretaceous unconformity inadequate thus the term northern north sea unconformity complex is used here

origin and significance of intra basement seismic reflections offshore western norway we use 3d seismic data to image a series of enigmatic sw dipping reflection packets within pre mesozoic crystalline basement offshore western norway based on their low angle dip and complex reflection wave train our preferred interpretation is that the reflection packets are the seismic expression of mylonitic zones generated by nappe emplacement during the caledonian orogeny late jurassic faults truncate and offset these reflection packets by several hundred metres suggesting that these faults did not exploit pre existing basement weaknesses our observations suggest that older basement fabrics may not always play a significant role in determining the geometry of later fault systems supplementary materials details of the time depth relationships from wells that have been used to depth convert interpretations s1 a supplementary figure showing an uninterpreted version of the seismic profile presented in fig 2 s2 and a throw v distance plot for fault f1 at the structural levels of the basement reflection and intra basement reflection 1 are available at www geolsoc org uk sup18683

analogue modelling of multiphase rift systems abstract analogue models of multiphase rift systems show that structures produced by the first phase of extension exertsignificant control on the geometries of the later phase extensional faults two experimental series series 1 and series 2 used different modelling media subjected to two phases of extension series 1 models used a simple homogeneous sandpack whereas series 2 models used a sand polymer multilayer in each series kinematic scenarios included orthogonal extension followed by oblique extension or vice versa extension directions for oblique rift kinematics varied from 75 to 30 to the axis of rifting in all models where oblique rifting superseded earlier orthogonal rift structures the second deformation phase produced segmented fault systems and reorientation of fault tips into high angles with respect to the extension vector higher angles of obliquity resulted in increased fault segmentation orthogonal extension superposed on earlier oblique rifting leads to coalescence of segmented faults although remnants of the inherited initial rift structure remained even for low angles of obliquity the sand polymer multilayer models produced more complex fault patterns than those in the simple homogeneous sandpack showing the possible complexities that can be generated during polyphase rifting persistent first phase rift faults caused boundary fault segmentation and salients and embayments along the boundary faults during the second deformation phase the natural occurrence of similar structural styles particularly in map view may indicate that previously unrecognised multiphase rifting has occurred in these natural examples

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

calcium isotopic evidence for rapid recrystallization of bulk marine carbonates and implications for geochemical proxies abstract strontium and calcium isotopic data for bulk carbonate solids and pore fluids from odp sites 1170 and 1171 are presented the data suggest that bulk carbonate sediments actively exchange with coexisting pore fluids over tens of millions of year time scales recrystallization rates constrained by sr isotopes and sr elemental data are 3 per ma at 1170a and 7 per ma at 1171a the pore fluid chemistries at both sites are affected by advection which occurs in the downwards direction at 1170 25 m ma and upwards at 1171a 250 m ma both the direction and the rate of advection are reflected by the width of the diffusive boundary layer for sr at both 1170a 300 m and 1171a 50 m compared to odp site 807a 150 m where no chemically detectable advection is occurring recrystallization is supported not only by interpretations of pore fluid data but also by the alteration of the bulk solid this is especially true at 1171a where advection drives significant alteration of sr ca mg ca and 87sr 86sr numerical simulations of pore fluid geochemical and isotopic evolution over tens of millions of years conducted with a depositional time dependent reactive transport model suggest that recrystallization rates in the upper tens of meters of the sedimentary section at both sites are more rapid than suggested by the sr geochemical data when the sr constrained rates are applied to the pore fluid ca isotope data the model does not predict pore fluid 44ca within analytical uncertainty the simulations indicate rates that are initially 20 to 40 per ma in young

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

inversion and propagation of the late paleozoic porjianghaizi fault north ordos basin china controls on sedimentation and gas accumulations abstract in contrast to typical intracratonic basins the ordos basin comprises marginal deformation belts and complex fault systems this study uses 3d seismic data tied to sonic and stratigraphic data from exploration wells to document the geographic extend of the porjianghaizi fault and its control on sedimentation and gas accumulation in the northern ordos basin our results show that the geometry of large porjianghaizi fault is controlled by five small fault segments the growth history of the porjianghaizi fault composes four stages 1 an initiation stage late carboniferous to early permian 2 a reactivation stage during the late triassic 3 an inversion stage during the middle jurassic and 4 an interaction and linkage stage during the late jurassic detailed studies show that the porjianghaizi fault had significant control on sedimentation and gas accumulation during the initiation stage late carboniferous to early permian five fault segments and associated relay ramps controlled the sedimentation in the study area the relay ramps formed transport pathways in which sediment extended from north basin margin to the basin center in contrast sedimentation along the fault was more localized based on shale gouge ratios sgr and formation water salinity the fault sealing capacity of the porjianghaizi fault is characterised by horizontal segmentation which means the fault composes laterally sealed areas and laterally connected areas along the fault strike as a corollary we propose that gas resources were mainly generated from the south where there are thick source rocks migrating vertically through interbedded open fractures first and then laterally through the junction zone of fault relay ramps it eventually accumulated in structural traps found in the northern areas understanding the evolution of the porjianghaizi fault is important for predicting the distribution of sedimentary facies in upper carboniferous and lower permian units as well as understanding the migration and distribution of gas resources in the north ordos basin these results can further help in hydrocarbon exploration in the north ordos basin

the influence of structural inheritance and multiphase extension on rift development the northern north sea the northern north sea rift evolved through multiple rift phases within a highly heterogeneous crystalline basement the geometry and evolution of syn rift depocentres during this multiphase evolution and the mechanisms and extent to which they were influenced by pre existing structural heterogeneities remain elusive particularly at the regional scale using an extensive database of borehole constrained 2d seismic reflection data we examine how the physiography of the northern north sea rift evolved throughout late permian early triassic rp1 and late jurassic early cretaceous rp2 rift phases and assess the influence of basement structures related to the caledonian orogeny and subsequent devonian extension during rp1 the location of major depocentres the stord and east shetland basins was controlled by favorably oriented devonian shear zones rp2 shows a diminished influence from structural heterogeneities activity localises along the viking sogn graben system and the east shetland basin with negligible activity in the stord basin and horda platform the utsira high and the devonian lomre shear zone form the eastern barrier to rift activity during rp2 towards the end of rp2 rift activity migrated northwards as extension related to opening of the proto north atlantic becomes the dominant regional stress as rift activity in the northern north sea decreases through documenting the evolving syn rift depocentres of the northern north sea rift we show how structural heterogeneities and prior rift phases influence regional rift physiography and kinematics controlling the segmentation of depocentres as well as the locations styles and magnitude of fault activity and reactivation during subsequent events

land offshore tectonic links in western norway and the northern north sea aeromagnetic and marine gravity data sets for the northern north sea and western norway have been interpreted in terms of land to offshore tectonic links the data allow identification of linking structures both in crystalline basement and overlying sedimentary rocks and consequently facilitate a more comprehensive interpretation of regional structural features and trends with the help of bathymetric data in the inshore region major fault structures can be traced between western norway and the offshore region west of the oygarden fault zone in this way spatial and chronological links have been established between tectonic events recognized on land and the development of the north sea basin an interpretation of basement geology is proposed for the region between western norway and the uer terrace this is complemented with a depth to top crystalline basement map covering the north sea basin between the uer terrace and the east shetland basin two regional nw se lineaments were recognized offshore western norway and given the names marflo lineament and troll lineament the marflo lineament is interpreted to be a caledonian or older zone of weakness that accommodated development of a 20 30 km offset in the jurassic viking graben axis the lineament can be traced into the north atlantic where it parallels oceanic fracture zones and transform faults the troll lineament relates to a deep geological discontinuity crossing the uer and lomre terraces a grouping of earthquake epicentres along part of the lineament suggests that the discontinuity is tectonically active

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

slip distributions on intersecting normal faults abstract slip distributions on normal faults often are asymmetric and display multiple local maxima numerically computed slip distributions from elastic models indicate that such irregular slip distributions can be caused by mechanical interaction between intersecting faults that produce local perturbations of the stress field resolved on the faults a three dimensional boundary element model based on the displacement discontinuity method has been used to analyze the mechanical interaction of faults that form y or t shaped intersections slip distributions are asymmetric with a steeper slip gradient toward the line of intersection multiple slip maxima occur depending on the angle between the faults but generally they are not located along the intersections nor at the fault centers examples of intersecting normal faults taken from oil reservoir seismic surveys and sandbox experiments have been used to confirm the application of the theoretical results despite considerable simplifications in terms of geometry and boundary conditions the computed slip distributions capture the characteristics of the observed slip distributions

intertidal foraminifera and stable isotope geochemistry from world s largest mangrove sundarbans assessing a multiproxy approach for studying changes in sea level abstract recent changes in sea level have appeared as a major threat to the existence of coastal habitats like mangroves and to the biodiversity characterizing such habitats in this study benthic foraminifer analysis along with carbon isotopes 13c and ratio between organic carbon and nitrogen c n were analyzed from five intertidal stations in sundarbans the world s largest mangrove ecosystem to test the efficiency of these signatures toward tracking relative rise in sea level the stability of these signatures with respect to gap of twenty months across varying elevations of the targeted stations has been tested benthic foraminifer abundance ranged from 0 to 118 individuals per 10 g and the assemblage was found to be mostly dominated by agglutinated species the 13c values 26 6 to 23 8 reflected that the origin of carbon is majorly from vascular land plants in some inputs from estuarine phytoplankton that are known to characterize tidal water of sundarbans the values of c n 0 48 1 43 represented a microbially degraded total organic carbon toc pool and thus were not a suitable proxy out of three signatures 13c showed a strong co relation with elevation and thus could be used as a reliable proxy to track relative sea level rise in mangrove environments

spatio temporal evolution of strain accumulation derived from multi scale observations of late jurassic rifting in the northern north sea a critical test of models for lithospheric extension abstract we integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern north sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire viking graben and its flanking platform areas these data show how syn rift accommodation within the basin varied in space and time with sub kilometer scale spatial resolution and a temporal resolution of 2 3 myr regional interpretations of 2d seismic reflection refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust these image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio temporal evolution of strain accumulation throughout the lithosphere during the 40 myr 170 130 ma period of late jurassic extension across this basin for the first 25 30 myr strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined as strain rates across the whole basin were consistently very low 16 s l thermally induced strength loss cannot explain this phenomenon the strain localisation is manifest in the near surface by a systematic migration of fault activity the pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control especially when estimates of flexural rigidity for this area are considered the best explanation for what is observed in this time period is a coupling between near surface strain localisation driven by brittle or plastic failure and the evolving thermal structure of the lithosphere we demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate dependence of frictional strength observed in laboratory studies during the final 10 myr of basin formation strain accumulation was focused within the axis and strain rates declined rapidly replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline

mechanisms of inheritance of rift faulting in the western branch of the east african rift tanzania the western branch of the east african rift system is commonly cited as a result of phanerozoic reactivation of the paleoproterozoic ubendian belt in western tanzania geological evidence is provided to show that prominent mechanical anisotropies successively appeared during proterozoic evolution of the precambrian basement and that their different reactivation behavior contributed to the phanerozoic rift pattern the ubende belt 1950 1850 ma is a nw oriented amphibolite facies ductile lateral shear belt in which older 2100 2025 ma and complex granulite facies terranes are included along trend retrograde multiphase sinistral strike slip mylonites developed along the nw oriented ductile shear belt they reflect persistent proterozoic wrench fault reactivation of the latter shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the proterozoic wrench fault reactivation neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone which appears as an elongate and nw trending positive flower structure locally enhanced by late proterozoic contraction phanerozoic rifting is demonstrated by others to occur in three distinct episodes during which the complex rift segment formed upon the multiphase proterozoic wrench fault zone the evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered the proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern which became selectively reactivated by sinistral transpressional ductile brittle mylonites proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults according to the position of these mylonites in the center or in the external parts of their nw oriented neoproterozoic transpression they reactivate as complex and multiphase rift faults or as normal and recent faults respectively the paleoproterozoic nw oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy its reactivation in phanerozoic stress fields is likely dextral oblique transtension considered as a leading mechanism of the pluriphase and nw oriented deep rift basins

impact of pre existing fabrics and multi phase oblique extension on cenozoic fault patterns wichianburi sub basin of the phetchabun rift thailand abstract cenozoic fault patterns associated with rift development in the wichianburi sub basin show a wide range of controls at different scales including influence by the margin of a permian basin and nw se to ne sw trending triassic folds and faults during the late oligocene early miocene phase 1 normal faults nnw sse to nne ssw trends developed in response to e w extension faults predominantly trend nnw sse to n s during this phase during the middle miocene pliocene the extension direction rotated to a more nw se orientation the old fault orientations were reactivated and new faults also developed with predominantly nne ssw to ne sw orientations many new faults nucleated within the syn rift section and were not directly influenced by pre existing fabrics in the pre rift section four types of phase 2 fault can be identified type 1 newly formed faults that strike sub orthogonal to the phase 2 extension direction type 2 newly formed faults that strike between the phase 1 fault orientations and type 1 faults type 3 newly formed faults that strike at angles greater than 90 with respect to the phase 2 extension direction type 4 reactivated phase 1 faults local stress rotations and fault linkage rotations explain many of the type 2 and 3 fault orientations the stress rotation in the middle miocene is observed in other basins in central thailand notably the suphan buri and phitsanulok basins where a range of fault patterns are produced by the effects of inherited fabrics plus multi phase rifting

high resolution record of tectonic and sedimentary processes in growth strata growth strata are used to determine the kinematics of synsedimentary structures such as faults classical methods of analysis such as thickness versus throw plot consider that the available space created by fault slip in the hanging wall of faults is instantaneously filled up by sediments this has lead many previous works to identify a cyclic activity for growth faults here we perform a careful analysis of the variation of strata thicknesses on each side of a very well documented normal growth fault in the niger delta we show that these thickness variations are induced by the alternation of sedimentary processes during continuous fault slip suspended load processes induce either uniform or slightly variable thickness of a large majority of mudstone layers bedload processes result in a preferential thickening of sand layers in the hanging wall these high quality data thus provide strong grounds for doubting the polycyclic growth diagnosed for some faults at the scale of sedimentary cycles and supports the notion that fault displacement rates can be very well behaved our study emphasizes the important conclusion that stable fault growth and related displacement rates can appear to be punctuated when viewed at the scale of sedimentary cycles it follows that care should be taken when attempting to derive displacement rates on temporal scales equivalent to those of alternating sedimentological cycles

temporal and spatial development of a gravity driven normal fault array middle upper jurassic south viking graben northern north sea abstract three dimensional seismic and well data from the south viking graben northern north sea basin is used to investigate the temporal and spatial development of a gravity driven normal fault array above an evaporite rich detachment two moderate throw 500 900 m middle to upper jurassic normal faults the gudrun and brynhild faults are developed within the study area both faults die out laterally and tip out upwards at different structural levels within the syn rift succession both faults terminate downwards into late permian evaporites zechstein group and do not offset pre evaporite basement units this thin skinned fault array developed in response to westwards tilting of the hangingwall of the south viking graben during late jurassic rifting and consequent westward gliding and extensional break up of units above the mechanically weak evaporite horizon isochron mapping and well based correlation of middle to upper jurassic syn rift units allow constraints to be placed on the temporal evolution of the fault array several stages of structural development are observed which document i a period of relatively minor early i e pre rift halokinesis ii variable spatial activity on individual faults within the array and iii the progressive upslope migration of active faulting within the array as a whole the progressive upslope migration of fault activity is interpreted to reflect progressive unbuttressing and extensional faulting of upslope post evaporite units the overall structural style and kinematic evolution identified here shares many characteristics with both rift raft tectonics documented in other rifts developed above an evaporitic sub stratum and raft tectonics described from passive margin basins containing thick mobile salt or shale intervals this style of fault array evolution differs from that observed in rifts lacking mobile layers at depth and highlights the importance of these units in the structural development of rifts

interaction of permo triassic and jurassic extensional fault blocks during the development of the northern north sea the main structural elements resulting from late permian early triassic extension have been resolved across the total width of the northern north sea the northern segment of the basin north of 61 n bounded to the west by the hutton fault and to the east by the norwegian mainland was occupied by four westerly tilted half grabens 25 35 km in width prior to jurassic extension the central segment of the basin 60 20 61 n exhibits a full graben beneath the jurassic horda platform flanked by half grabens tilted towards the basin margins the change in permo triassic fault block configuration defining structural domains is largely constrained by a basement discontinuity which runs diagonally ne sw across the basin maximum jurassic extension is represented by several centres of subsidence aligned nne ssw across a n s oriented and broader permo triassic basin jurassic extension was concentrated within the viking graben proper south of 60 n which in this area is represented by single westerly tilted en echelon half graben segments 20 25 km wide the area affected by jurassic extension becomes progressively wider to the north again governed by the basement discontinuity eventually encompassing the total width of the basin c 200 km north of 61 n the central segment of the northern north sea displays the greatest contrast in permo triassic and jurassic structural styles and basin geometry jurassic stretching and faulting was negligible to the east horda platform precisely the area which had been the site of maximum permo triassic stretching and fault activity it has generally been assumed that jurassic fault configuration along the eastern margin of the central segment of the viking graben to a large extent reflects the reactivation of major permo triassic faults this study demonstrates the interference of permo triassic and jurassic basin geometries in this area due to major faults of the two generations dipping in opposite directions as a consequence the e tilted oseberg fault block of jurassic origin and c 20km in width is underlain by a wider c 35 km and w tilted permo triassic fault block

planar detaching faults in the southern horn graben danish north sea the danish horn graben is located in the south eastern north sea and suffered fault controlled differential subsidence during the triassic within the horn graben several faults cut the triassic succession but detach at or close to the top pre zechstein surface an e w striking trend of planar detaching faults is mapped using 2 d seismic sections and the displacement on the faults is analysed with respect to timing and spatial distribution of the displacement the spatial distribution of the displacement is analysed using the fault analysis projection system badley earth sciences whereas the timing is analysed by reconstructing the seismic sections and thus obtaining the spatial distribution of displacement at different times the occurrence of the detaching planar faults appears to be controlled by the difference in basal friction across the pinch out line of the mobile zechstein sediments a dipping detachment surface is necessary to create a driving mechanism but is of minor importance in initiating the faulting the timing of the faulting is controlled by a general change in tectonic pattern enabling a n s extension of the sediments covering the top pre zechstein surface

exhumation of ultrahigh pressure rocks beneath the hornelen segment of the nordfjord sogn detachment zone western norway the nordfjord sogn detachment zone of western norway represents an archetype for crustal scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure uhp terranes in this paper we investigate the role of normal sense shear zones with respect to uhp exhumation using structural geology thermobarometry and geochronology of the hornelen segment of the nordfjord sogn detachment zone the hornelen segment of the zone is a 2 6 km thick top w shear zone primarily developed within amphibolite grade allochthonous rocks that juxtaposes the uhp rocks of the western gneiss complex in its footwall with lower grade allochthons and carboniferous devonian basins in its hanging wall new thermobarometry and sm nd garnet geochronology show that these top w fabrics were initiated at lower crustal depths of 30 40 km between 410 ma and 400 ma structural geology and quartz petrofabrics indicate that top w shear was initially relatively evenly distributed across the shear zone and then overprinted by discrete ductile brittle detachment faults at slower strain rates during progressive deformation and exhumation these results require a three stage model for uhp exhumation in which normal sense shear zones exhumed uhp rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile brittle structures

quantitative analysis of triassic extension in the northern viking graben it has been recognized for over a decade that large displacement pre jurassic faults are present in the northern viking graben part of the north sea rift these faults define a series of major fault blocks below the more obvious jurassic rift basin we attempt here to quantify the amount of extension associated with this older rift event which is probably of early triassic age quantitative modelling of the triassic rift and the succeeding period of thermal subsidence has been undertaken using a combination of flexural backstripping and flexural cantilever forward modelling these techniques suggest that triassic extension across the horda platform norwegian sector reached c 40 s 1 4 the consequences of this extension were deposition of a thick 3 km triassic upper jurassic syn rift and post rift sequence prior to renewed but minor extension in the late jurassic earliest cretaceous the thickness of the viking group reservoirs in the troll area appears to have been almost entirely controlled by sediment loading during post triassic thermal subsidence jurassic extension on the horda platform was in the uk sector of the viking graben triassic structures are less obvious than those below the horda platform because of jurassic overprinting they are however still present average triassic extension across the east shetland basin was c 15 comparable with the magnitude of jurassic extension in the same area we believe that the tern eider and cormorant fault blocks with proven shallow basement comprised a large eroded horst during the early triassic uplifted in the footwalls of major faults flanking the magnus and statfjord half graben during the triassic the magnus half graben was contiguous with the unst basin now situated in the western footwall of the younger jurassic basin the presence of the unst basin suggests that triassic extension occurred across the area that is now the northern shetland platform continuing into the west shetland area although the more obvious structures in the viking graben are jurassic in age the earlier triassic event was equally as important in controlling the structural and stratigraphic history of the basin

patterns of basement structure and reactivation along the ne atlantic margin a lineament pattern on the ne atlantic margin is discussed illustrated by gravity and magnetic images in the norwegian sea and reviewed in the context of onshore field evidence while most possible fault trends exist three major sets predominate a ne sw left stepping lineament set defines the gross geometry of the margin while interposing northerly trends impose a rhomboidal geometry at a variety of scales the margin is segmented by nw se transfer zones sometimes involving significant offsets the principal trends are primarily a function of mesozoic cenozoic plate wide extensional stress fields certain proterozoic and caledonian lineaments were however opportunistically reactivated according to the extension direction caledonian ne sw orogen oblique shears typified by the more trondelag fault zone were reactivated via jurassic strike slip or oblique slip and were further exploited during cretaceous early cenozoic extensional episodes leading to continental break up jurassic e w extension may also have reactivated n s faults existing in the basement or generated in duplex systems between the ne sw shears precambrian and caledonian basement lineaments striking at a low angle to the extension direction probably predisposed the formation of major transfer zones

re orientation of the extension direction and pure extensional faulting at oblique rift margins comparison between the main ethiopian rift and laboratory experiments in this study we draw on a unique combination of well resolved fault slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending main ethiopian rift east africa these data show that both boundary and internal faults oblique and orthogonal to the plate divergence pd respectively exhibit almost pure dip slip motion and indicate significant local deflection in orientation of the extension direction at rift margins scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep seated pre existing weakness oblique to the direction of pd that is able to cause a local rotation in the orientation of the extension direction at rift margins minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems as supported by existing and new palaeomagnetic data from the rift

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

3d structure and evolution of an extensional fault network of the eastern dampier sub basin north west shelf of australia abstract insights of spatial and temporal development of fault network in 3d is crucial for understanding the process evolution of complex fault network and for evaluating the regional and local stresses control on structure development we demonstrate a fault network on the eastern dampier sub basin north west shelf of australia which consists of 1 a ene trending fault array that has a through going segment at depth and a series of left stepping fault splays at upper levels and 2 a network of ene and nne trending intersecting faults decoupled from the basement structures this research shows that the segmented ene trending fault array developed through three extensional phases in the late paleozoic in the early jurassic and in the late middle jurassic fault analysis shows that the summed displacement of the segmented en echelon faults behaves as a single fault and that the basement fault controlled the fault array in the upper section through vertical linkages a typical coherent fault system the nne and ene trending intersecting faults formed simultaneously in the late middle jurassic as such they might have controlled by 3d strain field released from the rosemary and mermaid fault systems bounding the fault network this implies that fault geometry derived from 3d seismic interpretation need to be treated with caution as the alignment of fault sets may not directly relate to regional far field stress but in some cases significantly modified by local stresses induced by reactivated larger faults this study provides an analogue for the interpretation of other rift systems where structures were controlled by competing forces of regional and local stresses and where reactivated and newly formed structures coexist in polyphase of extensions

how do the properties of a pre existing normal fault population influence fault development during a subsequent phase of extension abstract we use scaled experimental analog models to investigate how the properties of a population of pre existing normal faults influence fault development during a subsequent phase of extension in the models a homogeneous layer of wet clay undergoes two phases of extension whose extension directions differ by 45 to vary the properties of the first phase fault population we vary the magnitude of the first phase extension as the magnitude of the first phase extension increases the number average and maximum length and average and maximum displacement of the first phase normal faults increase for a poorly developed first phase fault population new normal faults which strike perpendicular to the second phase extension direction form during the second phase for a well developed first phase fault population many first phase normal faults are reactivated as oblique slip faults during the second phase new normal faults also form these second phase normal faults are shorter and have displacement maxima adjacent to the reactivated first phase faults they are less likely to cut the pre existing first phase faults than second phase normal faults that form in models with a less developed first phase fault population in all models pre existing faults serve as nucleation sites for new faults in models with a well developed first phase fault population the pre existing faults also act as obstacles to the propagation of the second phase normal faults fault geometries in the models vary considerably parallel zig zag or intersecting fault geometries develop depending on whether the first phase fault population is poorly moderately or well developed respectively

structural analysis of the smeaheia fault block a potential co2 storage site northern horda platform north sea abstract smeaheia a prominent fault block located on the horda platform northern north sea is identified as a potential subsurface co2 storage site we utilise the gn1101 3d seismic survey to generate a high resolution subsurface geomodel to inform the structural style and evolution of the fault block to investigate geological controls on proposed co2 storage and provide a geometric framework as a basis for future analyses two basement involved first order north south trending fault systems the vette fault zone vfz and the oygarden fault complex ofc bound the 15 km wide fault block the vfz bifurcates down section where it is hard linked with two separate basement structures a phenomenon we term as dual rooted apart from activity during the permo triassic rift phase 1 and the late jurassic early cretaceous rift phase 2 we present evidence that rifting in this part of the north sea continued into the late cretaceous with minor reactivation in the palaeocene eocene two segments of the vfz interacted and linked at a relay ramp during rift phase 2 second order thin skinned faults show basement affinity and developed during rift phase 2 in two distinct pulses a population of polygonal faults intersects the overburden and developed during the eocene to middle miocene we have revised the areal extent of two structural closures that define the smeaheia fault block alpha vfz footwall and beta ofc hanging wall which consist of upper jurassic viking group target formations simplified cross fault juxtaposition analysis of the vfz and second order intra block faults are presented and inform pressure communication pathways between the smeaheia and tusse fault block as well as reservoir integrity and compartmentalisation the geomodel further identifies important geological controls on co2 storage in the fault block including a thinning caprock above the alpha structure and identification of hard linkage between deep tectonic faults and shallow polygonal faults