raft tectonics the effects of residual topography below a salt de collement abstract some natural examples of thin skinned extensional structures resulting from gravitational gliding on passive margins which display complex listric growth fault patterns are not just controlled by the slope direction examples from the gulf of guinea are presented it is proposed that the inherited topography of the underlying basement can control the initiation of faults oblique to the gliding direction we carried out a series of small scale physical models in order to investigate the effects of residual topography beneath a de collement on faulting in the overlying rafted sedimentary cover the experimental results sustain the hypothesis of structural inheritance and their implications are discussed in the light of natural examples from the gulf of guinea

deep structure of the east mendip hills from gravity aeromagnetic and seismic reflection data beacon hill pericline is the most easterly pericline of the mendip hills whose folds and thrusts were formed during the variscan orogeny by earth movements directed from the south the silurian volcanic rocks making up the core of the pericline are shown to be responsible for the large linear magnetic anomalies present over eastern mendip magnetic anomalies within the radstock coalfield to the north however are related to volcanic horizons in the carboniferous limestone a three dimensional model shows the presence of silurian rocks beneath the spine of the eastern mendip hills seismic reflection sections are used to control two dimensional interpretations of potential field data from beacon hill pericline although the magnetic anomalies are adequately explained by the seismic model the gravity anomalies require the presence of an additional low density body beneath the pericline9s southern flank this body is interpreted as a thrust wedge of old red sandstone emplaced in its present position by a previously unrecognized post variscan normal fault the considerable lateral extent of the structure is demonstrated by the trend of the maxima of the horizontal gradient of the gravity anomalies

gravity driven thin skinned extension above zechstein group evaporites in the western central north sea an application of computer aided section restoration techniques normal faulting and halokinesis have been important controls on the deposition and subsequent deformation of mesozoic and tertiary strata in the north sea in addition to the previously documented mechanisms of salt withdrawal dissolution and differential sedimentary loading it is recognized that gravity driven thin skinned extension above inclined salt layers has played an important part in north sea basin development commercial section restoration software has been used to facilitate depth conversion restoration and decompaction of seismic sections selected from an interpreted regional database in the western central north sea allowing validation of the interpretations and a graphical and highly quantitative description of salt assisted extension results of this work show that zechstein group evaporites were deposited in shallow sag basins during the permian triassic sedimentary pods were generated by localized deposition in synclinal basins and grabens above the evaporites bedded salt became folded while mobile salt flowed to fill anticline cores since the early jurassic regional tectonic tilting related to post rift subsidence and increasing sedimentary overburden have caused allochthonous mesozoic and tertiary strata to extend by gravity spreading above the mobile salt layer which detaches the allochthon from the underlying autochthonous late palaeozoic rocks concave up listric normal faults sole out in the salt layer propagate into the overlying cover sewuence and have been active at different geological times causing stratal thickening and folding within the allochthon antithetic and synthetic normal faults have developed producing complex upward branching fault systems in map view the listric faults form curvilinear en echelon arrays the faults linked by relay ramps fault blocks are typically 3 7 km wide 2 3 km thick and 7 10 km long salt movement during the jurassic tertiary has been driven by active extension of the cover causing salt to fill potential voids created by fault block rotation thus salt highs occur beneath sites of extension the listric faults generally dip in the same direction as the sub salt surface although there are also some major counter regional faults during extension regional dips have increased up to about 5 which is sufficient for gravity driven extension above a salt layer a total extension of about 6 has occurred the gravity driven thin skinned extension documented in the western central north sea is a phenomenon which can be recognized elsewhere in the north sea basin and can be readily compared with similar phenomena already documented in offshore angola brazil nova scotia and the gulf of mexico

clyde field growth fault secondary detachment above basement faults in north sea the clyde oil field in the southern north sea comprises a jurassic fulmar sand reservoir developed on a fault bounded terrace on the margin of the central graben palinspastic sections were constructed to honor seismic and well data and were repeatedly tested using new techniques for balancing structural sections in extensional regimes cross sectional area and bed length balance clearly demonstrate the presence of both shallow and deep fault detachments seismic remapping has substantiated this model and demonstrates the decoupling of the basement and shallow listric fault sets the new model has significance in terms of variations in reservoir quality and possible reservoir discontinuities resulting from fault seals of regional interest is the possibility of additio al growth fault plays of this type within the north sea basin

centrifuged models of gulf of mexico profiles abstract physical models dynamically scaled in a centrifuge are used to explore the importance of slope progradation in driving internal salt structures much further horizontally than they rise large increments of ductile plastic overburden added at normal gravity values simulated primary clastic sedimentary progradation of a clastic slope over a layer of buoyant fluid simulating salt during load increments at a nominal 800 g in a centrifuge this ductile topographic slope spread over the ductile decollement except for down to basin growth faults all the structures considered to be distinctive in the northern gulf of mexico are simulated in the models that are scaled such that 1 mm and 60 s in the model correspond to 1 1 5 km and 2 6 ma in the prototype ratios of the rate of advance to the rate of rise of the shelf break compared with the rate of advance and increase in relief of salt structures control how much of the deep autochthonous salt is squeezed ahead of the advancing shelf and the paths this salt takes to shallow structural levels the few three dimensional diapiric stocks which developed from many two dimensional mullions in the models spread stock canopies which advanced huge distances before some became tongue canopies by climbing over clastic sediments already aggraded in front of them the models emphasize the significance of ductile spreading of the clastic sediments now being discerned by backstripping

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

mesozoic reactivation of variscan thrusting in the bristol channel area uk new seismic reflection data from the bristol channel area provide direct evidence of a major variscan thrust in the palaeozoic basement beneath the mesozoic bristol channel syncline the thrust has an average southerly dip of about 25 and it rises into the major fault the central bristol channel fault zone cbcfz at the northern margin of the bristol channel syncline the cbcfz is attributed to reactivation in extension of the variscan thrust probably during the early cretaceous there is no evidence for major synsedimentary movement on the cbcfz and the bristol channel syncline does not represent a rollover structure section restoration suggests that the cbcfz has cut up through a previously tilted sequence of triassic and jurassic strata pervasive nnw sse cross faults are interpreted as original variscan transfer faults that have been reactivated in the mesozoic and tertiary

structural and tectonic setting of the permo triassic basins of northwest europe abstract the permo triassic basins of nw europe formed by i late orogenic spreading of the variscan mountain belt eventually leading to ocean floor formation in the mid atlantic ii continental rifting propagating south from the arctic and iii thermal subsidence following carboniferous volcanic activity in the north sea spreading values in the northernmost north sea west of shetland and west and southwest of britain are relatively high but the sediments are dominantly continental the permo triassic rifting affected lithosphere thickened during caledonian and variscan orogenesis however subsidence associated with the arctic rifting together with global sea level changes allowed marine incursions into the north sea producing the large zechstein salt basins interference of the different basin formation mechanisms led to a complex three dimensional fault pattern and multidirectional extension across britain and the north sea this fault pattern formed the basic framework for subsequent jurassic and cretaceous basin formation

a latest cretaceous hotspot and the southeasterly tilt of britain the southeasterly regional dip of england and wales is usually ascribed to miocene movements loss of up to 2 km of cover from northern england shown by apatite fission track analysis has been related to uplift and erosion induced by a mantle hotspot but its effects were far more widespread than considered hitherto outcrop patterns throughout much of england wales and eastern ireland can be related to a hotspot beneath the irish sea uplift occurred within the late maastrichtian and erosion continued into the late palaeogene after post oligocene down faulting of the irish sea the modern drainage was initiated on the eroded surface unconformities provide the best means of dating orogenic movements in some localities the time interval between the youngest rocks preserved beneath the unconformity and the oldest preserved above may be small enough to enable the dating of fold movements to be determined with some precision thus in southeastern britain the major e w structures of the hampshire basin the weald and the london basin can be dated as miocene as rocks of oligocene age are involved in the folding in the hampshire basin and pliocene sands rest in erosional pockets of the chalk of se england because fold movements tend to affect large areas of the crust at discrete intervals of time the southeasterly tilt of britain evident from the fact that a nw se transect from anglesey to london crosses outcrops of rocks getting progressively younger from the precambrian of anglesey to the tertiary of the london basin has

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

three dimensional geometry and kinematics of experimental piggyback thrusting the three dimensional geometry and kinematics of piggyback stacks of imbricate thrust sheets are illustrated and discussed using a single model shortened in a squeeze box strike parallel geometric elements simulated include lateral ramps eyed sheath folds splays and thrust thrust interference fine details of these structures were exposed by eroding a shortened wedge of sand using a newly developed vacuum eroding technique a kinematic analysis of the model shows a stepwise increase in imbricate thrust spacing and or a decrease in rate of nucleation of imbricate thrusts in the direction of thrust transport despite the steady forward advance of a rear wall the piggyback wedge accreted episodically recording different strain domains in longitudinal cross sections strain partitioning in single layers by bed length balancing showed an increase in layer shortening with volume loss and a corresponding decrease in imbricate thrusting and ramp folding with depth

relationship between basement linked and gravity driven fault systems in the ukcs salt basins abstract extension of basement below significant detachment layers such as salt results in the formation of structures in the cover strata above the detachment layer in two ways thick skinned cover structures evolve to balance basement stretching these are linked to the basement faults via detachment in the salt layer or are directly linked to the basement structure depending on the amount of basement fault displacement relative to salt layer thickness basement extension also results in rotation of major basement fault blocks tilting the detachment layer and precipitating gravity driven thin skinned slip of the cover involving detached extensional faulting and folding detachment layers consisting specifically of salt play two roles in facilitating the development of thin skinned systems salt constitutes the detachment layer and also lends buoyancy to the cores of anticlinal folds reducing resistance to shortening the detachment layer may become tilted away from the basement fault scarp resulting in synthetic slip systems with the same polarity as the underlying basement fault or the detachment may be tilted toward the basement scarp giving antithetic slip systems of the opposite polarity in both cases the slides may terminate downdip in detachment folds and or reverse faults both types of thin skinned system will interact with those cover structures directly balancing basement extension and all of these components must be considered during section restoration tilt associated with 3d basement fault geometry e g relay zones may result in decoupled cover faults locally trending at high angles to the trend of underlying basement faults contrasting strain rates of basement faulting and related cover slip can lead to continuation of cover structural evolution after cessation of basement extension giving for example cover faults and folds of lower cretaceous age in the central north sea though basement extension had occurred in the upper jurassic synthetic systems may include footwall derived olistostromes emplaced above basement hangingwalls these commonly occur above basement relay ramps on segmented basin margins examples from the central graben and channel basin are discussed examples of antithetic systems from the irish sea central graben and a field example from the bristol channel basin are discussed

extensive glacial advances during the last glacial maximum near the eastern himalayan syntaxis abstract climatically the eastern himalaya are mainly controlled by the indian summer monsoon ism and important region for understanding the relationship between ism and glacial change this region is therefore useful for studies of the relation between glacial changes and ism using a geomorpho stratigraphic method combined with gis we investigated and mapped the outmost lateral frontal moraine of the most extensive glaciations that occurred in the basongcuo bsc catchment near the eastern himalayan syntaxis using optically stimulated luminescence osl and 10 be surface exposure dating technique we dated five boulders and 23 sediment samples the resultant osl and 10 be ages show that the region s glaciers were at their most extensive positions between 16 and 31 ka glacial coverage was 507 5 km 2 9 9 times greater than it is today 51 5 km 2 with an equilibrium line altitude ela depression of 690 m the rapidity with which the last glacial maximum lgm glacial advance and retreat occurred suggests that temperature was the dominant control on glacial change

radial gravitational gliding on passive margins abstract gravitational gliding of uppermost sediments down a passive margin is possible if there is a basal layer of evaporite or other soft material to allow detachment in examples from the gulf of mexico and the brazilian margin gliding has produced three main structural domains an uppermost domain of downdip extension an intermediate domain of rigid gliding and a lowermost domain of downdip contraction domain boundaries are established by changes in slope in this paper we examine three kinds of gravitational gliding depending on the paths followed by material particles in ideal parallel gliding particle paths are parallel straight lines trending downslope this should occur where the margin is perfectly straight in ideal radial gliding particle paths are radii of a circle and the margin is shaped like a circular cone natural margins will not have ideal shapes but divergent gliding will tend to occur off coastal salients convergent gliding off coastal re entrants a simple kinematic model based on ductile behaviour illustrates some essential features of radial gliding changes in radius during divergent gliding produce strike parallel extension during convergent gliding they produce strike parallel contraction vertical strains also differ divergent gliding produces an uppermost domain of strong vertical thinning balanced by extensions in all horizontal directions similarly convergent gliding produces a lowermost domain of strong vertical thickening balanced by contractions in all horizontal directions these deformed states cannot be restored by simple techniques based on section balancing we have done three experiments using analogue materials sand to model the brittle behaviour of sediments silicone putty to model the ductile behaviour of basal layers of evaporite the experiments were properly scaled to account for gravitational forces experiment i reproduced convergent gliding above a basement with a conical upper surface strike parallel contraction was responsible for radial folds on the lower slopes the lowermost domain had a complex fold pattern indicating contraction in all horizontal directions experiment 2 reproduced divergent gliding above a basement with a pyramidal upper surface strike parallel extension produced radial rifts on the upper slopes the uppermost domain had a complex polygonal pattern of normal faults indicating extension in all horizontal directions experiment 3 also reproduced divergent gliding but due to non uniform sediment thickness we have examined the many seismic profiles both downdip and alongstrike available for the brazilian margin alongstrike profiles show structures which we attribute to strike parallel strains around the santos re entrant syn sedimentary growthfolds indicate strike parallel contraction increasing downslope this argues for a mechanism of convergent gliding in contrast around the campos salient normal growth faults and associated salt rollers indicate strike parallel extension decreasing downslope this argues for a mechanism of divergent gliding the pattern of oil fields in the campos area including the giant marlim field seems to be closely controlled by divergent gravitational gliding and associated processes radial gliding should occur on other passive margins as well especially where the coastline is sinuous structures around the western gulf of of mexico seem to indicate convergent gliding at the scale of the entire gulf structures at a smaller scale around the mississippi delta seem to indicate divergent gliding

the tectonic evolution of the north celtic sea and cardigan bay basins with special reference to basin inversion abstract the morphology and structural evolution of the north celtic sea basin ncsb and cardigan bay basin cbb are in large part controlled by reactivation of basement features of caledonian and possibly variscan origin from earliest triassic until maastrichtian times sedimentation was almost continuous initially in a rapidly subsiding rift system and finally in a progressive passive sag phase at the onset of the alpine orogeny both basins experienced spectacular inversion along readily identifiable axes the inversion takes the form of a compressive reactivation and reversal of the pre existing growth faults and a broad regional upwarping of the cretaceous depocentre the amount of inversion varies depending on the location within the basin but values in excess of 1000 m are possible by utilising regional seismic data the timing of inversion can be estimated as occurring between maastrichtian and eocene times which correlates with the so called laramide pulse of the alpine orogeny

salt tectonics in the norwegian danish basin a study is presented of salt tectonics in a 28 000 km2 area in the norwegian danish basin of the north sea the basin is mainly of permian to early jurassic age and contains sediments which locally reach 8 km in thickness zechstein upper permian salt occurs widely in the basin and has given rise to widespread and intense salt tectonics using seismic reflection sections with a total length of 14 000 km as well as borehole data 86 salt structures have been mapped within the study area the floor of the basin that is the base of the zechstein salt and the faults affecting this floor have also been mapped it is therefore possible to investigate the association of faults and salt structures and hence to ascertain to what extent faulting of the basin floor may have initiated salt movement various lines of investigation and reasoning have been developed to arrive at an estimate of the number of structures for which salt movement is likely to have been initiated by faulting of the basin floor it is found that only for a limited number of salt structures probably about five out of a total of 86 structures the salt movement is likely to have been initiated by faulting the very large majority of the salt structures originated independent of faulting the mechanism responsible for the initiation of salt movement in this majority has also been studied it is found that these salt structures can be divided into two groups one group comprises about 37 salt structures which are randomly distributed nearly all of them are situated where the present basin floor is relatively flat and at its deepest they are thought to be the result of classical halokinesis that is the autonomous upward movement under the influence of gravity of mobile salt at locations where the sedimentary overburden at the time of initial movement was locally less heavy a second group comprises about 43 individual structures which are aligned along nine so called trend lines it is first shown that these trend lines are independent of faults and fault trends in the basin floor the origin of these trends is then discussed by considering two alternative hypotheses for the mechanism responsible for the initiation of salt movement along these trend lines the first hypothesis assumes that salt ridges in the subsurface are initially pushed up by a prograding sedimentary overburden the second hypothesis assumes that initial ridges of mobilized salt are formed along a palaeo depth contour which migrates up the flank of the subsiding basin both hypotheses assume that the salt ridges thus formed can subsequently develop into individual salt structures which produce the observed pattern of trend lines a critical analysis of these two alternative hypotheses leads to the conclusion that the second one involving the concept of a palaeo depth contour which migrates up flank is the more acceptable the mechanism involved may be characterized as a special type of halokinesis namely as halokinesis constrained to operate along a line

geometric models of listric normal faults and rollover folds existing geometric models allow master fault shapes to be constructed given the shape and heave or displacement of a deformed marker horizon in the hanging wall these models assist in projecting normal faults to depth where the fault geometry is poorly constrained by available seismic data currently it is unclear which model best predicts the actual relation between rollover and fault geometries to address this deficiency the relative accuracies of the constant heave constant displacement constant bed length slip line and inclined shear constructions are evaluated using clay model analogs and seismic examples each of the geometric models predicts a different fault shape and depth to detachment for the same rollover shape because different hanging wall deformat on mechanisms are assumed a construction is developed for determining the hanging wall geometry from a known fault geometry this construction is based on the inclined shear model and geometric constraints imposed by material displacement paths viewed from different reference frames fixed to the hanging wall and footwall blocks in clay models the inclined shear antithetic shear greek alpha 20 degrees and slip line constructions produce the best agreement between actual and modeled fault and marker bed shapes in rollovers for subsurface examples the inclined shear model antithetic shear greek alpha 20 40 degrees and the constant displacement model predict fault trajectories that are the most similar to the positions and shapes of the master fault segments interpreted fr m offshore norway and gulf of mexico seismic lines the shear angle required in the inclined shear model can often be estimated from the orientation of minor faults in the hanging wall the shear angle may also be determined through iterative modeling in which the shear angle is systematically varied and the resultant model fault shape is compared to the shape of the shallow segment of the master fault interpreted from the seismic record

variation in fold geometry in the yuso basin northern spain implications for the deformation regime first generation structures in greywackes of the yuso group from the cantabrian mountains of northern spain show a distinct variation in geometry with depth in a regional synclinal structure curavacas and lechada synclines they are easily distinguished from other deformation events in the structurally uppermost level we find flap folds flap folds are recumbent structures with the inverted limb preserved below this level cascade folds are found these structures have a vergence opposite to that of parasitic folds the nomenclature adopted is from harrison and falcon characteristically these structures have shallowly dipping axial surfaces in agreement with the shallow dip of the axial plane regional cleavage in the lowermost structural level upright parasitic folds with a steep cleavage are present the variation in fold geometry is accompanied by a general steepening of the regional cleavage with increasing depth in the absence of overprinting relationships the f1 fold geometries are included in a single deformation event the steepening of the cleavage with depth reflects the change in orientation of the maximum shortening direction from sub vertical in the upper part of the syncline to sub horizontal in the lower part with increasing depth the deformation regime during f changed from bending to buckling the deformation regime on the regional scale however is associated with basement subsidence and passive formation of the regional synclinal structure furthermore the absence of a distinct microfabric for the different f1 folds indicates that on a small scale a similar deformation regime was present we conclude therefore that the scale at which we study a structure only reflects the deformation regime at that particular scale consequently the overall deformation regime cannot be determined from single outcrops or microstructural analysis alone

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

the gravity anomaly of the st george s channel basin southern irish sea a possible explanation in terms of salt migration gravity profiles across the st george9s channel basin show a marked discrepancy between the observed anomalies and the anomalies calculated using a model based upon an interpretation of seismic data this difference may be explained in terms of salt migration away from the centre towards the flanks of the basin evidence for such a migration path is provided by the seismic data an excess of salt on the se side of the basin has intruded into a major fault giving rise to a salt wall the gravity models suggest that the basin reaches a thickness of 10 km near its centre but that permian and carboniferous sediments are possibly thin or absent

allochthonous salt structure and stratigraphy of the north eastern gulf of mexico ii structure the kinematic evolution of allochthonous salt in the north eastern gulf of mexico proceeds in three stages 1 since its middle jurassic deposition the louann salt was loaded by sediments causing episodic basinward movement of salt ultimately leading to large concentrations of salt masses in a slope environment by the end of the lower cretaceous 2 a regime of starved sedimentation during late cretaceous and early oligocene is responsible for the stabilization of these early salt accumulations 3 with renewed rapid accumulation of sediments during the neogene and pliocene extensive allochthonous salt tongues and sheets formed by gravity spreading within the younger sediments of the slope

field examples and numerical modelling of oversteps and bends along normal faults in cross section abstract oversteps and bends along cross sections through small scale numerical modelling distinct element analysis has been carried out to simulate the patterns of deformation at oversteps and bends area increase pull apart development and localised stress concentration occur at extensional oversteps and bends stress concentration and a tendency for compaction are typical of contractional oversteps and bends such results are compatible with the structures observed in the field a sensitivity analysis of the modelling has been carried out to investigate the effects of variations in mechanical parameters on deformation although models with weaker properties are more deformable the overall geometry of deformation varies little with variations in mechanical parameters

salt flow and diapirism related to extension at crustal scale abstract the analysis of the structure and history of salt diapirs in large ancient evaporitic basins such as the southeastern north sea basin suggests that diapirism is controlled by crustal scale extension laboratory experiments on small scale models designed to simulate configurations comparable to the southeastern north sea basin show that diapirs which grow in a sedimentary cover during basement extension can develop close to basement normal faults or at a significant distance from them if the displacement on the basement faults is transfered horizontally along the salt layer for given values of salt viscosity strain rate and friction coefficient in brittle rocks the pattern of salt diapirism in a particular area depends on the ratio of cover thickness to salt thickness as well as the absolute thicknesses of cover and salt layers the experiments give criteria for identifying different types of diapirs and provide models of horizontal flow in a salt layer undergoing extension and diapir development some dynamic implications of the experiments are applied to the southeastern north sea basin

the south celtic sea bristol channel basin origin deformation and inversion history abstract the south celtic sea bristol channel basin is a relatively narrow and elongated sedimentary trough located within the northern part of the hercynian fold belt and filled with permo triassic redbeds to jurassic marine sediments its present configuration is the result of a three fold sequence of tectonic phases initiated during the late palaeozoic the origin of the basin appears to be related to localised extensional reactivation of hercynian thrust zones during the permian and early triassic followed by regional subsidence during the late triassic and jurassic uplift of the basin margins together with dextral strike slip movements in an east west sense and large scale erosion during the late jurassic to early cretaceous resulted in the present elongated configuration of the lower mesozoic basin regional subsidence from aptian time onwards resulted in widespread deposition of a relatively thin section of wealden elastics and upper cretaceous chalk unconformably overlying folded pre cretaceous strata sedimentation was interrupted during the early tertiary by a series of tectonic events that included emplacement of the lundy igneous mass during the early eocene and mild basin inversion during the oligo miocene inversion was accompanied by dextral reactivation of the nw se oriented sticklepath fault zone the total amount of uplift probably not exceeding 350 m reflects the rather stable configuration of the underlying lithospheric crust as indicated by seismic reflection and refraction data

allochthonous salt structure and stratigraphy of the north eastern gulf of mexico part ii structure abstract the kinematic evolution of allochthonous salt in the north eastern gulf of mexico proceeds in three stages 1 since its middle jurassic deposition the louann salt was loaded by sediments causing episodic basinward movement of salt ultimately leading to large concentrations of salt masses in a slope environment by the end of the lower cretaceous 2 a regime of starved sedimentation during late cretaceous and early oligocene is responsible for the stabilization of these early salt accumulations 3 with renewed rapid accumulation of sediments during the neogene and pliocene extensive allochthonous salt tongues and sheets formed by gravity spreading within the younger sediments of the slope autochthonous salt allochthonous salt and detached allochthonous salt are typical stages of evolution major down to the basin growth faults separate allochthonous salt sheets from their original feeder stocks extension along down to the basin master growth faults is compensated mainly by salt withdrawal and partly by basinward shortening

possible variscan thrusting beneath the somerton anticline somerset vibroseis seismic data were acquired by goal petroleum plc during 1982 in the vicinity of the somerton anticline somerset these data show the simple form of the anticline but in addition show a southward dipping discontinuous seismic event beneath gravity modelling demonstrates that this event may be correlated with an e w trending gravity gradient which can be followed for approximately 150 km the dipping seismic event may represent a variscan thrust plane which dips to the ssw in contrast to similar planes seen to the east which dip to the sse the variation in dip seen between these postulated thrust planes may explain the curvature of structural trends seen within and to the east of the mendip hills the polden hills fault runs sub parallel to the proposed thrust plane beneath the somerton anticline and may be linked to a mesozoic reactivation of the postulated thrust

synthesis of salt tectonics in the southern north sea uk abstract a structural model encompassing the southern north sea basin west of the central graben has been developed this model consists of a rift system affecting the post salt section around the basin margin and a large area of detached compressional buckle folds within the basin this pattern is initially a response to gravity sliding of the post salt section on the salt within the basin during the late triassic to late jurassic a close relationship between the location and trend of the peripheral graben system and basement structures in the pre salt is noted pre jurassic extension across the peripheral graben systems was balanced by the sum of fault heaves at the pre salt rotliegend level and shortening across salt cored buckle folds in the post salt section salt pillows and swells passively infilled the cores of these gravity induced buckle folds cretaceous and tertiary inversion involved basin tilt and renewed movement on basin bounding basement faults notably reverse movements did not propagate from basement structures up into the peripheral graben systems the post salt sedimentary section experienced gravity spreading in response to inversion related uplift resulting in syn inversion extensional faulting in the sole pit high where the mesozoic section was thickest this extension combined with a loss of fault heave in the pre salt section was balanced by amplification of salt cored buckle folds in the centre of the basin in the context of the model described here salt pillows represent passive infill of thin skinned compressional buckle folds which later amplified during thick skinned basement shortening crestal collapse of such folds occurs via normal faulting accompanied by reactive diapirism such reactive diapirs establish conduits through which salt may leak leading to pillow deflation and ultimately conduit preservation as a salt wall flanked by rim synclines in areas where the buckle folds were emergent the salt structures described here are related to cover folds and faults which in turn reflect episodes of basin extension tilting and inversion hence individual salt structures can be said to be only remotely connected with regional intraplate stresses

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

the geometry of planar domino style normal faults above a dipping basal detachment abstract geometrical analysis of planar domino style normal faults rooted into a dipping basal detachment fault allows derivation of equations which relate 1 horizontal extension within the upper plate 2 dip of the detachment 3 final fault dips 4 rotation that faults and beds undergo and 5 net slip on domino style faults past geometrical models have focused on extremely idealized and non unique geometries in which domino style faults are parallel and similar cut off points are all at the same elevation after faulting this corresponds to evenly spaced domino style faults above a horizontal detachment considering non parallel faults and dipping detachments introduces unique geometries which allows calculation for example of permissible depth ranges to the detachment horizontal extension varies significantly 1 with dip of the detachment and 2 for synthetic and antithetic cases in which the domino style faults dip in the same or opposite direction respectively as the detachment when other factors e g rotation and fault dips are held constant horizontal extension greatly increases for moderate detachment dips and moderately decreases for antithetic detachment dips as compared to the horizontal detachment case this is important because the synthetic case has been widely reported

mesozoic regional tectonics and south viking graben formation evidence for localized thin skinned detachments during rift development and inversion abstract a regional thin skinned model of south viking graben development is presented in which zechstein marginal evaporites and interpreted salt have acted as a key de collement horizon along which mechanical decoupling of post salt triassic and younger strata from pre salt basement has occurred the western margin of the south viking graben defines an asymmetrical late jurassic half graben controlled by episodic extension along a nne ssw trending eastward dipping fault system an initial minor phase of rifting in the triassic is suggested by the slight westward thickening and associated divergence of interpreted triassic sequences at depth the scarcity absence of upper triassic and lower jurassic sequences is believed to result from a subsequent phase of uplift which affected the region in the mid cimmerian attributed to the development of a thermal dome at the future site of a jurassic rift triple junction to the southwest the onset of large scale extension from the late callovian onwards marked the major phase of half graben formation and created a regional hanging wall tilt which locally caused gravity gliding of post salt section to the wnw with resultant low angle extensional faulting upslope and salt cored buckling downslope the late jurassic half graben was subsequently tightened by a phase of thin skinned inversion in the latest jurassic early cretaceous associated with back steepening of the western margin fault system with resulting reactivation of the extensional detachment system the effects of this inversion have been noted in many parts of the northern north sea within the south viking graben shortening estimates vary between 14 and 18 and are expressed on seismic data by localized hanging wall folds and the back steepening of faults along the western margin fault complex of the graben together with out of graben thrusting up the hanging wall dip slope variation in both the extension and inversion mechanisms appears to be controlled by interpreted nw se transfer systems which compartmentalize the graben into three subgraben within which differential zechstein basin development occurred

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

late jurassic half graben control on the siting and structure of hydrocarbon accumulations uk norwegian central graben abstract the late jurassic structure of the central graben is discussed both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures the regional kinematics of the north sea rift suggest that during the late jurassic the central graben opened approximately orthogonally to the main nnw sse fault trend within the basin some evidence that triassic extensional faults were reactivated at this time is seen towards the basin margins and a significant triassic extensional history is inferred the late jurassic basin comprises the axial c 100 km of the central graben s l the basin axis is offset progressively to the se by both transfer zones and discrete transfer faults footwall uplift of the basin margins during extension provided the source for the upper jurassic reservoir sands encountered by exploration drilling these sands were probably preferentially fed into the basin at transfer zones in the margins salt structures are ubiquitous within the central graben and their siting is everywhere controlled by the underlying fault block topography salt structures form preferentially towards the crests of half graben or buttressed against their boundary faults inversion structures in the norwegian sector reactivated with a reverse sense of displacement late jurassic extensional faults a model is proposed which shows the two main inversion structures feda graben norway tail end graben denmark to be detached on antithetic reverse faults riding in the salt these faults terminate at tip folds within major salt walls the geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late jurassic half graben

growth of faults by accumulation of seismic slip the accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults observation shows that as a fault accumulates displacement it grows in size i e its surface area and its length increase here we address the question for an increase in the amount of displacement on a fault by how much would the length of the fault change it is argued by cowie and scholz 1992a that the displacement on a fault is linearly related to the length of the fault this simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes two different approaches are presented the first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows the available energy is provided by changes in strain energy when the fault slips the second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture the total displacement on a fault is the sum of the slips contributed by many earthquakes the usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data the models presented here predict that 1 the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1 of its previous length and 2 the work of faulting may account for 10 of the total energy available during an earthquake the energy lost from the system is accounted for by work done against friction and seismic radiation consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory