a nonlinear method to estimate source parameters amplitude and travel times of teleseismic body waves abstract we present a nonlinear waveform inversion method to determine source parameters under point source approximation depth source time function centroid moment tensor and seismic moment and also travel times and amplitudes of body waves from teleseismic records we demonstrate that our source time function estimates significantly improve correlation between observed and synthetics broadband seismograms the scaling of source dimensions with seismic moment is similar for deep and shallow events in the japanese subduction area for moment magnitudes between 6 1 and 7 5 we compare event depths and moment tensors obtained with our method to those reported by different institutions for a set of japanese earthquakes this comparison reveals that our method gives hypocentral depths closer to those obtained with regional arrays than other teleseismic methods the seismic moment is properly recovered but data weighting strongly influences the moment tensor solution focal mechanisms are biased by high frequency body wave amplitude variations due to crustal structure below the stations and by errors on the instrument responses body wave differential travel times are improved compared with phase picking measurements the high quality travel times will be exploited to obtain more precise event locations and higher resolution tomographic models online material coordinates of japanese earthquakes processed in this study

latest jurassic early cretaceous events and the late cimmerian unconformity in north sea area each of several regional or local unconformities may have been identified locally as the late cimmerian unconformity a supposed major break at the base of the valhall formation or roedby formation over most of the north sea the base of the valhall formation is isochronous and conformable with underlying sediments it is detected on seismic reflection profiles because it represents a widespread facies change marking the late ryazanian transgression most of the unconformities are of eustatic origin and the effects of eustatic sea level changes were never completely masked by local tectonics thus in the modeling of individual oil fields the possibility of sedimentary breaks occurring can be predicted in part by reference to regional or eustatic events modified journal abstract

eustatic sea level changes interpreted from seismic stratigraphy a critique of the methodology with particular reference to the north sea jurassic record seismic stratigraphic methods for interpreting eustatic sea level change are based on generalized stratigraphic models and chronostratigraphic interpretations that have not been substantiated by publication of the necessary basic supportive data chronostratigraphic correlations of sea level change curves between basins are based partly on circular reasoning onlap and offlap seismic models may not indicate sea level change but flexural subsidence facies shifts or changes in sea level in the opposite direction to that proposed resolution of some of these problems may be achieved by publication of basic data now in confidential industry files the widely acclaimed vail curves should not be used as templates for basin correlation until such data have been provided and such use should be supported by appropriate conventional lithostratigraphic and biostratigraphic documentation from each new basin where such data have been made available as with the north sea jurassic serious questions of interpretation are raised

examples of structural control of hydrothermal mineralization fault zones in epicontinental sedimentary basins a review by compiling data available from progressive geoscientific research and by complementary studies it seems possible for an increasing number of mineral districts to find indications of the regional structural control of hydrothermal ore deposits the relationship between hydrothermal mineralization faulting or fault reactivation and related processes can of course best be examined at the less deformed sedimentary record of ancient epicontinental sedimentary basins there is increasing evidence that the sediment hosted submarine hydrothermal zn pb ba mineralizations such as meggen rammelsberg howards pass and mcarthur river are spatially and genetically related to deep synsedimentarily active fault systems of intracontinental basins or stretched passive continental margins for long periods of basin development the faults provided favorable conduits for episodically released basin dewatering brines which precipitated the leached metals into third order basins situated next to these faults there are many similarities to epigenetic mineralizations which formed either pre syn or postorogenetically from brines leaching the meta sediments and associated magmatites and migrating along fault zones to their near surface trap structures

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

a proposed granite batholith along the western flank of the north sea viking graben summary a pronounced positive magnetic anomaly of approximately 300 gamma occurs over the eastern edge of the east shetland platform at approximately 60 n 1 e after the removal of the regional gravity variation and the gravity effect of the known geological structure it is found that this magnetic high correlates with a negative gravity residual anomaly of approximately 30 mgal seismic data indicate that these anomalies occur in an area of relatively shallow basement on the upthrown side of the main viking graben margin fault the presence of a buried granite batholith of approximately 40 x 40km may explain the gravity magnetic and seismic observations the observed deviation of the fault defining the edge of the viking graben in the proximity of the proposed granite may be explained in terms of the tectonic influence of the buoyant granite block during the taphrogenic development of the graben

the relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls abstract we derive an analytical expression that relates the shape of a fault in cross section to the shape of the bedding horizons in its hanging wall block the expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout although this paper concentrates on normal faults the expression is equally valid and applicable to thrust faults the direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry there is no reason to believe that the simple shear occurs on vertical planes as is commonly assumed in graphical approaches to this problem and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account for a planar normal fault such compaction may result in the development of a hanging wall syncline

uplift and extension at the gulf of suez indications of induced mantle convection reconstruction of tectonic movements since the initiation of the gulf of suez rift indicates that there has been 25 27 km of extension the uplift bordering rift indicates that lithosphere heating greatly exceeds that producible by uniform lithosphere extension i propose that small scale convection induced by the rifting augments the heating introduced by extensional rifting and produces the broad uplifts flanking the rift

high resolution stratigraphy and seismic attribute mapping of a fluvial reservoir middle jurassic ness formation oseberg field continental successions of alternating fluvial reservoir sandstones and fine grained interfluvial deposits make hydrocarbon production planning and reservoir management difficult the vertical and lateral distribution of fluvial sandstones within the surrounding nonreservoir rocks in particular is a critical factor that needs to be investigated before production or injection wells can be successfully drilled in the oseberg field of the norwegian north sea about 21 7 million sm3 standard cubic meters of recoverable oil is stored within fluvial channel sandstones of the middle jurassic ness formation the ness formation reservoir is characterized by a relatively low proportion of good quality reservoir rocks approximately 30 of the total rock volume sedimentological studies indicate that reservoir sandstone bodies are developed mainly as elongate laterally restricted sandstone bodies concentrated within certain stratigraphic levels and geographic areas identifying and accurately mapping these sandstone rich intervals are important in defining possible drilling targets the distribution of reservoir rock within the upper ness formation has been the focus of detailed geological and three dimensional 3 d seismic mapping inverted seismic data integrated with geological well data give an empirical correlation between acoustic impedance and sandstone proportion in turn these relationships provide the basis for defining areas with a higher probability of encountering reservoir sandstone subsequent production drilling into the ness formation has confirmed the existence of reservoir sandstones within areas defined by the integration of high resolution stratigraphic mapping and the geophysical analysis the integration of sedimentological and 3 d seismic techniques presently offers the best potential for defining drilling targets within the ness formation in the oseberg field

displacement geometry in the volume containing a single normal fault fault displacements measured in coal mines and from seismic data are used to develop a model describing the near field displacements associated with an ideal single normal fault displacement on a fault surface ranges from a maximum at the center of the fault to zero at the edge or tip line the tip line is elliptical with the shorter axis of the ellipse parallel to the displacement direction contours of equal displacement form concentric ellipses centered on the point of maximum displacement displacement gradients vary with fault size and with mechanical properties of host rock fault radius to maximum displacement ratios range from 5 to 500 plotting of displacement contour diagrams and knowledge of displacement gradients are useful in interpreting seismic reflectio data both for quality control of interpretations and for quantitative extrapolation of limited data displacements associated with faulting decrease systematically with increasing distance along the normal to the fault surface this decrease is seen as reverse drag in both hanging wall and footwall hanging wall rollover and tilting of the reflectors cannot be used to distinguish listric from planar normal faults even where fault block rotation can be demonstrated neither listric fault geometry nor a flat detachment surface is geometrically necessary because faulting is accommodated by ductile deformation rigid fault bounded blocks cannot exist except in some special circumstances related to a free surface the displacements within the rock volume affected by a single fault are not simply related to regional extension apparent horizontal extension by faulting varies from one layer to another and a significant proportion of the extension in a basin may be due to ductile deformation

architecture and depositional sequences of tertiary fault block carbonate platforms an analysis from outcrop miocene gulf of suez and computer modelling abstract this study combines outcrop with seismic data and tectonostratigraphic modelling to characterise the stratigraphy of tertiary fault block carbonate platforms seismic sections from the middle east and southeast asia indicate that carbonate platforms commonly develop on fault blocks in the late syn rift to post rift stage of basin evolution they are characterised by retrogradational or drowning morphologies with deep water clastics evaporites sealing the platforms a new tectono stratigraphic modelling program simulates the development of carbonate platforms on domino style fault blocks and predicts a distinctive tectonic control on the evolving syn rift stratigraphy detailed field work from a three dimensionally exposed miocene syn rift platform from the gulf of suez gebel abu shaar egypt shows the existence of unconformity bound depositional sequences which are predicted by the program and are interpreted to have formed in response to fault block rotation the sequences are characterised by synchronous hangingwall subsidence and footwall uplift and erosion the seismic images the modelling and the outcrop study provide different levels of information on fault block platforms and their characteristic stratigraphy the scale of the modelling is of particular value as it provides a link between the seismic scale imaging and outcrop data

properties of fault populations in the gullfaks field northern north sea abstract there are many published examples of the size frequency distributions of natural fault populations that have been interpreted as showing scale invariant power law scaling in this paper we present the statistics of the fault throw population in the gullfaks field and show that the distribution can not be described by a single power law instead the throw population when plotted in log log space displays two distinct slopes with different exponents at scales well above the limit of seismic resolution at scales ranging from 10 to 90 m we obtained a slope of approximately 0 3 while at scales ranging from 90 to 165 m the slope is approximately 1 0 a systematic variation in fault orientation with fault size does not appear to be the cause of this break in slope and a simple explanation has yet to be found however the documentation of such a change in scaling above seismic resolution implies that similar breaks may occur equally well below the resolution limit this suggests that extrapolation of the straight line segments of population curves to sub seismic scales may give incorrect estimates of small scale faults and fractures separation of the gullfaks fault data set into subareas and sub populations based on geometric criteria revealed several sub populations of faults that do exhibit simpler power law relationships however the wide range of values found for the scaling exponent varying from 0 73 to 2 05 suggests that the choice of such subdivisions is an important step in the analysis procedure we argue here that a more thorough understanding of fault populations can be achieved by documenting this sub population variability as well as by characterizing the combined data set

tectonic evolution of the northumberland basin the effects of renewed extension upon an inverted extensional basin the northumberland sedimentary basin has a history of repeated extensional and contractional tectonic phases and a study of the deformation interference pattern and sedimentary record allows the identification of the tectonic state through time initial rifting is represented by a thick dinantian sedimentary fill but carboniferous extension is thought to have ceased by the early namurian in order to explain the development of certain facies patterns during the westphalian thermal subsidence phase of the evolution of the northumberland basin some syn depositional extensional fault movements are required at other times in the westphalian compaction induced subsidence was the dominant control on facies distribution a single phase of e w to se nw variscan compression commenced during or after late westphalian times and ceased prior to the intrusion of the whin sills and dykes subsequent permo mesozoic extension with a right lateral shear component re activated pre existing normal fault lines these later structures may be interpreted as being listric in form shallowing onto decollement surfaces within the carboniferous basin fill and overlie the buried faulted margin at the southern side of the basin

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

a dynamical model of lithosphere extension and sedimentary basin formation convection in the upper mantle can give rise to normal stresses on the base of the continental lithosphere that may cause localized elevation of its surface and extensional deviatoric stresses this possibility is investigated assuming homogeneous biaxial strain in a continental lithosphere whose strength is governed by a combination of brittle failure and power law creep in the crust and by brittle power law or dorn law behavior in the mantle for a given geotherm and for rheologies based on current laboratory data the lithosphere responds to convection induced uplift in one of three ways depending primarily on the magnitude of the driving stresses these regimes are as follows 1 when the driving stress exceeds a threshold value the resulting strain rates are high enough greater than about 3 10 16 s 1 that the lithosphere strains almost isothermally its effective rheology is strain thinning and it undergoes accelerating extension the magnitude of the dynamic uplift required to produce the threshold value of the driving stress depends on the geotherm characterized here by the moho temperature and varies from about 1 km for a moho temperature of 700 c to around 3 km for a moho temperature of 550 c an uncertainty of at least 100 c is attached to these temperatures owing to the inaccuracies in extrapolating laboratory data to geological strain rates 2 at stress levels due to less than about half of the critical uplift extensional strain is negligible 3 at intermediate stress levels moderate uplift produces extension that is bounded by strength increases due to cooling of the ductile portion of the lithosphere during extension

depth dependent stretching a different approach uplift of the crust during rifting is a major problem that has been left unresolved by uniform stretching models various depth dependent stretching models have been proposed to account for this but these require decoupling of the crust and mantle and result in space problems within the mantle a significant modification of these discontinuous nonuniform or depth dependent stretching models is proposed here that accounts for synrift uplift and doming but does not result in these problems the model involves continuous nonuniform stretching within a polygonal region bounded by symmetrical outward sloping boundaries within the mantle this geometry results in thinning of the lithosphere below crustal regions that are less stretched or unstretched crustal uplift resulting from asthenospheric upwelling below these regions this in turn results in stretching factors for the mantle that are less than those for the crust a relationship opposite to relationships derived by discontinuous nonuniform stretching models corollary predictions for mantle derived heat flow within and outside regions of stretching of these two different depth dependent models may allow them to be distinguished from each other and from regions where uniform stretching predominates

the geometry of listric growth faults in the devonian basins of sunnfjord w norway thick devonian clastic sequences accumulated in fault bounded basins in w norway at the close of the caledonian orogeny and mark the beginning of a phase of crustal extension traditionally the eastern boundary fault to the devonian basins has been regarded as a thrust but cross sections based on the use of a branch line reveal it to be an extension fault there is clear omission of structural section across the boundary and by matching up metamorphic rocks in the hanging and foot walls of the boundary fault over 40 km of extensional dip slip can be measured at the n end of the fault the boundary fault is a flat lying listric fault with rollover anticlines in the hanging wall the fault also cuts up and down section along strike to form lateral ramps parallel to the movement direction these ramps appear as wrench faults in outcrop the ramp flat geometry of the fault may have been inherited from a precursor caledonian thrust fault which reversed its sense of slip in early middle devonian times to form the basins over 25 km of onlapping sediment accumulated in the largest basin during faulting in 14 ma with an average slip rate on the fault of 3 mm yr 1

lithospheric stretching and north sea jurassic clastic sourcelands uniform lithospheric stretching cannot explain three important aspects of north sea basin evolution namely 1 the rapid growth of middle jurassic uplands from which were liberated major fluvial deltaic clastic wedges now forming the main hydrocarbon reservoirs 2 the occurrence of basin centre upwarp and basaltic volcanism and 3 widespread and significant 1 km subsidence outside the area of demonstrable crustal thinning i present here the suggestion that a combination of nonuniform stretching and lithospheric flexuring accounts for these fine tuning effects of the uniform stretching model with magnitudes close to those demanded by independent geological evidence

a deep seismic reflection profile across the northern north sea the viking graben is a major mesozoic rift basin in the northern north sea1 seismic refraction data2 gravity surveys3 and theoretical modelling4 have led to the general acceptance of an extensional tectonic origin for the viking graben involving an early mesozoic phase of extension and fault controlled subsidence followed by a more regional thermal contraction subsidence the early success of the birps deep seismic reflection profiling5 and the results from cocorp6 led to the planning and acquisition during 1983 of a deep seismic line recorded to 15 s twt two way time across the north viking graben the aims of the present study were to provide constraints on the geometry of the viking graben and its extensional faults to provide data on the geometry of crustal thinning beneath the graben and to provide an insight into the relation between the shallow extensional features of the sedimentary basin and the deep crustal thinning here we present a specific interpretation of this seismic profile further details will be given elsewhere7

seismic characteristics and identification of negative flower structures positive flower structures and positive structural inversion negative and positive flower structures and positive inverted structures imply specific modes of formation and their distinctive characteristics make them important criteria for the identification of certain structural styles a negative flower structure from the andaman sea consists of a shallow synform bounded by upward spreading strands of a wrench fault that have mostly normal separations paralleling monoclines and oblique en echelon normal faults flank the divergent wrench fault a positive flower structure from the ardmore basin oklahoma consists of a shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations en echelon folds are present on either side of this convergent wrench fault flower structures may resemble extensional or contractional fault blocks on individual seismic profiles flower structures are identified by their unique internal fault and fold architecture by their association with a straight and throughgoing zone of deformation and by their lack of a consistent sense of separation or regional upthrown side positive structural inversion at the rambutan oil field south sumatra basin has formed a shallow anticlinorium and has partly uplifted the underlying graben deeper fault segments bounding the graben have retained their normal fault profiles but at shallow levels some of these faults have reverse separations similar changes of separation with depth may be produced by strike slip but sets of faults with listric profiles and nearby blocklike trend patterns identify the inversion as a dip slip event where correlation of reflections is uncertain or deep control is unobtainable positive inverted structures may also resemble contrational fault blocks detached normal fault assemblages and certain negative inverted structures

on the evolution of rifted continental margins comparison of models and observations for the nova scotian margin summary a model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed model predictions are then compared with geological and geophysical observations from a transect of the 185 ma old rifted margin off nova scotia through the scotian basin three kinematic models of the rifting process are discussed these are 1 the uniform extension model in which the amount of extension is uniform with depth but varies with position across the margin 2 the uniform extension and melt segregation model which has similar properties but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust and 3 the depth dependent extension model in which the first order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth each of which undergoes differing amounts of extension these rift models predict the form of crustal and lithospheric thinning subsidence and temperature change once the amount of extension has been determined this is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness equal to that currently measured in the adjacent continental region before rifting occurred rifting is also considered to occur instantaneously a time stepping thermo mechanical model is used to predict the cooling of rifted margin additional thermal contraction subsidence and its amplification by water and sediment loading thermal aspects are calculated using a finite difference model of time dependent conductive heat transport whereas regional isostatic response to loading is calculated by a finite element model the models are coupled because the temperature distribution is used to define a rheological lithosphere an elastic region with thickness that varies in time and space as the model evolves for flexural calculations in the mechanical model secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment the model predictions of 1 sedimentary basin stratigraphy 2 moho position 3 free air gravity anomaly 4 age depth relations for deep exploratory wells and 5 subsidence and temperature histories agree well with observations from the scotian basin additional effects due to sedimentary and crustal radiogenic heat production lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered it is concluded that a model in which rifting occurred by depth dependent extension and which includes a finite thickness for the rheological lithosphere radiogenic heat production in the sediments and crust and a brittle listric faulted crustal layer affords an accurate description of the first order processes that occur during rifting and evolution of this margin

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

development of rift basins illustrated by the structural evolution of the oseberg feature block 30 6 offshore norway the mesozoic tilted fault block in norwegian north sea block 30 6 containing the giant oseberg oil and gas field is used to illustrate basin development during the jurassic and cretaceous there is seismic evidence for an early rifting phase in the northern viking graben on which the jurassic cretaceous rifting phase early bathonian to berriasian is overprinted the oseberg structure is representative of other fault blocks bordering the northern viking graben axis planar normal faults and listric faults which were restricted to the rifting phase dominate both fault types exhibit syn fault sedimentation listric faulting is recognized on seismic sections by the differential tilting reverse drag antithetic faulting and a wedge shaped syn rift reflector package the initiation of rifting is dated by baselap onto pre rift reflectors and cessation is dated by an unconformity beneath the generally onlapping post rift sediments steeply dipping planar normal faults often active along previously established fault zones played a significant part in the post rift development of the basin the last movement on planar normal faults in the oseberg fault scarp becomes younger away from the rift axis and this produced the complex fault scarp visible on many seismic lines the topography defined by the post rift unconformity is a composite surface which formed in response to subsidence processes it is not the result of orogenic events such as the kimmerian 9orogeny9 footwall uplift associated with planar normal faults caused local unconformities in complex fault scarps and on fault block crests

an extensional model of graben subsidence the first stage of basin evolution abstract the formation of grabens in continental crust is generally associated with tensile faulting crustal thinning anomalously warm mantle below the rift zone and an updoming of the outer flanks of the rift zone these features are also common to the initial stages of sedimentary basin formation and continental rupture creating new sea floor this similarity suggests that similar tectonic processes but of varying duration may be responsible for all three phenomena in this study subsidence within continental grabens which have been formed by a sudden local stretching event is examined within the zone of extension two competing effects act to determine the sense uplift or subsidence and magnitude of initial epeirogenic movements these are the thinning of the low density crust and the upwelling of hot mantle material on continental crust depending on which effect is dominant either rapid uplift or rapid subsidence may occur initially subsequently thermally controlled subsidence occurs as the anomalously warm mantle below the rift cools due to lateral heat flow subsidence occurs more rapidly in grabens than in basins and may be accompanied by a transient uplift of the graben flanks

effects of finite rifting times on the development of sedimentary basins abstract most thermo mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post rift development of the basin this assumption greatly simplifies the mathematical treatment but is not in accord with what is found in nature where 10 to 50 m y rifting events commonly accompany the formation of sedimentary basins and continental margins the effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer this technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced the effect of a finite duration extension event is that heat is lost during rifting increasing the syn rift subsidence at the expense of the post rift lateral heat flow which was not included in previous studies of the effect of finite rifting times has a significant effect on the subsidence history distribution of sediments and thermal history in particular the post rift subsidence is decreased by more than 25 for a 20 m y rifting event and by more than 10 15 for a rifting event as short as 10 m y this will significantly decrease the subsidence rates in the post rift stage and implies that inferences concerning the structure development and thermal history of the basin derived from using curves to interpret backstripped subsidence can be greatly in error variations in syn rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors such as the flexural response of the lithosphere to sediment loading to affect the final width of the basin the total amount of sediments that accumulate and the basin stratigraphy

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

basin analysis predictions of known hydrocarbon occurrences the north sea viking graben as a test case abstract basin analysis can assimilate structural burial and thermal histories to predict hydrocarbon prone zones in speculative areas the validity of the modelling approach was field tested in the north sea viking graben an area for which good data are available a recent birps deep seismic reflection profile across tectonic strike of the graben was interpreted and depth converted with good well control the burial and structural histories were determined by backstripping the chronostratigraphic layers using a one dimensional fluid flow compaction model on pseudo wells after removal of each sequence the section was manually reconstructed the thermal history was examined using thermal indicator tomography extensional models and calculations of depth to detachment lack of resolution required that three different paleo heat flow trends be tested to encompass the range in variability a gradual warming trend a constant heat flow through time and a low heat flow until the cretaceous tertiary boundary where a heat pulse occurred basement subsidence extension depth to detachment calculations and thermal indicator inversions all suggest that the uk side of the graben has developed differently to the norwegian section the generation modelling predicts the position of the oil window in the cross section the areas where this window enveloped the draupne formation kimmeridge clay source rock broadly correlate with the positions of the gullfaks statfjord and thistle oilfields of the uk sector however in the norwegian sector the draupne formation was generally overmature except in the area around the troll oilfield predicted middle to late cretaceous generation in the rift axis may have caused updip oil migration towards the horda platform the correct prediction of oil prone zones validates the use of quantitative basin analysis as a tool for evaluating frontier areas where data are limited

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

heterogeneous stretching simple shear and basin development abstract the models of basin development which involve either homogeneous stretching of the whole lithosphere or displacement on a lithospheric scale shear zone are but two end members of a range of possible extensional models the homogeneous extension model thins the lower lithosphere beneath the thinned upper crust and superimposes a thermal subsidence basin on the earlier fault bounded basin the shear zone model offsets the zone of lower lithospheric stretching and thermal subsidence it is more likely that the zones of upper and lower lithospheric stretching will be heterogeneous and patchy but will often overlap in plan view this will produce localised uplift and subsequent thermal subsidence within the faulted basin and may explain many of the anomalies between the various stretching estimates made using different structural stratigraphic and geophysical techniques the model which combines heterogeneous lithospheric stretching and associated simple shear may explain 1 variations in dip of the major detachment zones in the large basins 2 variations in types of strain on or beneath the detachment zones 3 regional uplift of part of a basin to erode the earlier fault blocks and 4 the development of volcanism in basins with only low values of upper crustal extension the zone of stretched lower crust and lithospheric mantle may lie beneath the centre of the zone of upper crustal stretching or to one side it may be linked to the upper crustal zone by faults which dip consistently in one direction or by extensional systems which change their dominant dip direction with depth pulling out the mid crust as one or more wedges possible examples are discussed from the basin and range province and northwest european continental shelf

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

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

numerical simulation of rifting in the northern viking graben the mutual effect of modelling parameters numerous basin modelling studies have been performed on the viking graben in the northern north sea during the past decades in order to understand the driving mechanisms for basin evolution and palaeo temperature estimations in such modelling it is important to include lithospheric flexure the values derived for the lithospheric strength by these studies vary considerably i e up to a factor of 30 in this study which is based on new interpretation of a regional transect we show that both the estimated value of the effective elastic thickness and the derived b profile are dependent on the assumed value of the depth of necking the use of models that implicitly set the level of necking at a depth of 0 km generally leads to an underestimation of the lithospheric strength and an overestimation of the thinning factors in the northern viking graben a necking depth at intermediate crustal levels gives results comparable to the observations extension by faulting is modelled to be a significant factor in conclusion rifting in the northern viking graben can be explained with various models of effective elastic thicknesses eet varying from 1 km for a zero necking depth to the depth of the 450 jc isotherm for an intermediate level of necking it is also shown that the development of the basin during the post rift phase cannot be explained by pure shear simple shear extension two mechanisms are proposed here to explain the post rift subsidence pattern namely intra plate stress and phase boundary migration the two extreme models for eet mentioned above 1 km for a zero necking depth to the depth of the 450 jc isotherm for an intermediate level of necking give very different responses to compressional stress the latter gives basically no response for realistic intra plate stress d 2004 elsevier b v all rights reserved

chemical stabilization of low mg calcite an example of brachiopods abstract trace element study of brachiopod shells from the upper ordovician ellis bay formation anticosti island quebec canada shows that they underwent only a slight 20 diagenetic equilibration with meteoric waters despite 450 million years of postdepositional history this in contrast to large trace element repartitioning observed in their enclosing rocks attests to the relative stability of low mg calcite even within the realm of meteoric waters paleozoic brachiopods apparently exerted a biological control on the distribution of na and mg in their low mg calcitic shells preferentially incorporating the former and discriminating against the latter trace constituent

tectonic subsidence flexure and global changes of sea level tectonic models for the evolution of passive continental margins predict that following rifting sediments should progressively onlap basement at the edge of a margin as the lithosphere cools and increases its flexural rigidity with age the pattern of modelled onlap is strikingly similar to that used by vail and colleagues to estimate sea level rise through geological time this similarity suggests that major portions of stratigraphical sequences at margins may have a tectonic rather than eustatic control the patterns of onlap used by vail and colleagues may be widespread however because several widely separated passive margins rifted at similar times but they are unlikely to be worldwide

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

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

extension and rifting the zeit region gulf of suez abstract a field analysis of faults and fractures in the ras gharib ras gemsa region of the gulf of suez shows that the main late cenozoic extension occurred perpendicular to the rift axis three main types of dip slip normal faults successively developed as the tilt of blocks bounded by antithetic normal faults increased determinations of the amount of extension from structural data are compatible with estimates made using subsidence data through a simplified model of lithospheric stretching the uplift of rift shoulders is related in chronology and volume to the subsidence of the rift the geometry of fault patterns and directions of extension suggests that the late cenozoic total movement corresponds to a counterclockwise rotation of 4 5 of sinai relative to africa with a pole close to cairo

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