geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

role of compaction in listric shape of growth normal faults growth normal faults propagate syndepositionally at the depositional surface and their dips flatten with depth as the surrounding sediments compact as described by the equation tan thgr tan thgr0 1 phgr0 1 phgr where phgr0 and phgr are the initial and final porosities of the surrounding sediments and thgr0 and thgr are the initial and final fault dips after compaction this equation is tested on several southern louisiana growth normal faults whose shapes are known from well penetrations and whose porosity depth relationships for sand and shale are derived from well logging the initial surface fault dip thgr0 67 and the initial sand and shale porosities phgr0ss 39 3 and phgr0sh 68 respectively were determined by an inversion technique on an extensive data set from one fault once these initial conditions were determined the shapes of other faults were computed accurately based on knowledge o the sand shale stratigraphy from single well logs the compaction equation allows accurate determination of the proportion of fault flattening caused by compaction and other processes for example one fault flattens from an initial dip of 67 to a dip of 38 at 5 km depth 23 of the flattening is caused by compaction and 6 is caused by footwall folding finally in pure shale the compaction induced flattening of a growth fault is as much as 19 in the first 500 m of burial and reduces to about 1 km below this depth in pure sand the flattening is approximately 1 km throughout

mechanics of extensional wedges the extensional wedges that overlie low angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold and thrust belt but with an opposite sense of shear on the basal fault such wedges are common in zones of crustal extension such as the basin and range province of the north american cordillera the north sea and the u s gulf coast this paper examines the mechanics of these upper crustal extensional wedges using a modification of the compressional coulomb critical taper model developed for fold and thrust belts and accretionary wedges if a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline a critically tapered extensional wedge is one that is on the verge of coulomb failure everywhere a wedge whose taper is narrower may be slid stably down the same incline without internal deformation a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical after testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges we apply the theory to a pair of active extensional wedges in the brazos area offshore texas both wedges are underlain by well imaged basal detachment faults dipping at 17 o down to depths of 10 km one wedge exhibits relatively little internal deformation whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical regional values for the coefficients of basal and internal friction in excess of 0 3 together with vertical cohesion gradients in excess of 1 kpa m are consistent with the observed pore pressures and the geometry of the wedges as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults such values are comparable to values inferred from mechanical and thermal models of compressional wedges and they are consistent with laboratory rock mechanics data

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

shortening in the upper plate of the buckskin rawhide extensional detachment fault southwestern u s and implications for stress conditions during extension commonwealth post graduate research award u s geological survey national cooperative geologic mapping program under statemap g11ac20455

evidence of enso mega drought triggered collapse of prehistory aboriginal society in northwest australia the kimberley region of northwest australia contains one of the world s largest collections of rock art characterised by two distinct art forms the fine featured anthropomorphic figures of the gwion gwion or bradshaw paintings and broad stroke wandjina figures luminescence dating of mud wasp nests overlying gwion gwion paintings has confirmed an age of at least 17 000 yrs b p with the most recent dates for these paintings from around the mid holocene 5000 to 7000 yrs b p radiocarbon dating indicates that the wandjina rock art then emerged around 3800 to 4000 yrs b p following a hiatus of at least 1200 yrs here we show that a mid holocene enso forced collapse of the australian summer monsoon and ensuing mega drought spanning approximately 1500 yrs was the likely catalyst of this change in rock art the severity of the drought we believe was enhanced through positive feedbacks triggered by change in land surface condition and increased aerosol loading of the atmosphere leading to a weakening or failure of monsoon rains this confirms that pre historic aboriginal cultures experienced catastrophic upheaval due to rapid natural climate variability and that current abundant seasonal water supplies may fail again if significant change in enso occurs

partial melting of h6 ordinary chondrite kernouve constraints on the effects of reducing conditions on oxidized compositions partial melting experiments at temperatures of 950 1300 c were conducted on the h6 chondrite kernouve under reducing conditions using co co2 gas mixing and graphite buffered sealed silica tubes to examine the effect of reducing conditions during melting of starting materials that are more oxidized relative to the oxygen fugacity conditions of the experiments the experiments produced a range of mineralogical and compositional changes olivine exhibits significant reduction to compositions of fa2 5 at temperatures of 1300 c in contrast orthopyroxene exhibits only slight reduction until the highest temperatures chromite is sometimes consumed by intruding sulfides and displays increasingly magnesian compositions ranging as low as fe fe mg of 0 1 at a constant cr cr al ratio the compositional changes with increasing temperature reflect a complex set of reactions including oxidation reduction one application of these experiments address whether primitive achondrites could have formed from ordinary chondrite like precursors by partial melting under reducing conditions while changes observed in olivine and troilite compositions might support such an idea differences in oxygen isotopic composition cr cr al in chromite orthopyroxene compositions and thermodynamic evidence against reduction during melting of primitive achondrites benedix et al 2005 firmly refute such an idea

map view interference of monoclinal folds abstract outcrop observations and laboratory experiments show that many small chevron folds form by interference of monoclinal kink bands in multilayer buckling kink band interference has also been proposed for some map scale folds furthermore fault related folding provides additional mechanisms of monoclinal fold generation other than buckling and thus makes kink band interference all the more conceptually plausible as a significant large scale process in this paper we document several relatively simple examples of map scale monoclinal fold interference including three interfering monoclines of the colorado plateau a seismically imaged example in the perdido foldbelt of the gulf of mexico and a more complex example from seismic mapping in the santa barbara channel california kink band interference has normally been analyzed in cross section here we emphasize the map view phenomena and present a simple balanced three dimensional model of the interference geometry treating the monoclines as two independent kink bands which does not depend of the kink band folding mechanism this model predicts the shape of the jog produced by crossing monoclines and is used to help evaluate the role of interference in the map view geometry of our examples the documentation of these simple examples supports the concept that more complex monoclinal fold interference could be a significant phenomenon in the upper crust

numerical investigation of dip slip fault propagation effects on offshore seabed sediments abstract an enhanced understanding of the phenomenon of dip slip fault propagation through young unconsolidated offshore sediments is essential for the evaluation of the response of shallow offshore structures and subsea oil gas pipelines substantial and permanent seafloor deformation constitutes a severe geohazard for these marine structures and associated offshore elements components this paper addresses the response of seabed sand deposits subjected to underlying dip slip fault movement the resulting propagation of the faulting offset in seabed sediments is explored using 2d finite element modeling in abaqus while accounting for nonlinear soil behavior with strain softening a 2d plain strain fem seabed model is adopted to investigate the effect of different seabed soil properties and layer thicknesses on the extent and magnitude of seabed surface deformation induced by normal and reverse faults with different dip angles the results indicate that 1 the adoption of sand properties that depend on depth is necessary to realistically model the fault propagation mechanism particularly for fault propagation through thick and dense sand deposits 2 the presentation of fault propagation results in a normalized manner with respect to sand cover thickness does not eliminate the dependency on the actual layer thickness and 3 the deformation of the beds in the cover as a result of basement faulting can be modeled realistically using trishear kinematic approaches

geological structure and evolution of majorca implications for the origin of the western mediterranean abstract the island of majorca forms part of the ne sw continental balearic promontory which is surrounded by continental and oceanic extensional basins majorca has a basin and range physiography mainly resulting from late neogene extension the basins and ranges strike ne sw the structure of the ranges consists of thrust faults and associated folds involving mesozoic series paleogene series are also involved in the compressional structures located in the southeast whereas a stratigraphic hiatus embracing late cretaceous and paleogene is present in the rest of the island the age of syncompressive rocks and sediments is chattian to aquitanian in the se llevant ranges aquitanian to burdigalian in the central ranges and burdigalian to langhian in the nw tramuntana range thus compressive deformation progressed from southeast to northwest the thrust fault hangingwall displacement is to the northwest and shortening is large 44 seismic reflection sections reveal the presence of compressive structures in the substratum of the basins moreover normal faults due to wnw ese extension occurred during the serravallian and tortonian producing additional subsidence in the basins minor compression could have occurred since the onset of the pleistocene majorca was shortened when the extensional basins surrounding the island originated available models for the origin and evolution of extensional basins of the western mediterranean do not consider the structure observed in majorca

the ardeche palaeomargin of the south east basin of france mesozoic evolution of a part of the tethyan continental margin geologie profonde de la france programme abstract this paper incorporates the results of seismic and gravity investigations borehole data and studies of sedimentology biostratigraphy sequence stratigraphy and structural geology carried out in the framework of the deep geology of france programme the main purpose of the paper is to define the structure and to reconstruct the evolution of the ardeche palaeomargin of the south east basin of france from the stephanian to the end of the mesozoic the acquisition and processing of three seismic reflection profiles the drilling of two scientific boreholes ba1 and mm1 and subsequent structural modelling with the use of cross section balancing techniques applied to both the depth migrated seismic and geological sections provide new geometrical constraints which lead to the following hypothesis of the geodynamic evolution of the margin 1 1 late palaeozoic extension with the development of high angle basement faults asymmetrical permian half grabens pre extension sedimentary cover 2 2 during the middle triassic a pre rift stage with the creation of small fault blocks and in the late triassic the formation of an e w flexure 3 3 end lias early jurassic to middle jurassic formation of a major detachment in the carboniferous coal measures leading to complete decoupling of the mesozoic sedimentary cover from the basement this episode of extension coeval with the opening of tethys can be related to a phase of episodic rifting between the late sinemurian and the late bathonian i e during 30 ma 4 4 late jurassic to early cretaceous thermal subsidence with regional downwarping of the basement towards the east 5 5 after the late cretaceous the period of maximum burial pyrenean and alpine deformation in the south and east causing progressive uplift of the ardeche palaeomargin as much as two kilometres of mesozoic deposits subsequently being eroded from the section during the tertiary

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

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

dips of normal faults in british coal measures and other sedimentary sequences fault dip measurements n 3291 from 122 faults in british coalfields are presented two classes of normal fault are recognized a gently dipping 55 post coalification tectonic faults formed at depths of about 2 5 km which comprise 90 of the faults analysed dip data from the steeply dipping faults are variable standard deviation 9 both on single faults and between faults and 70 is a better approximation of fault dip than the commonly accepted value of 60 data on tectonic normal faults with comparable depths of formation using the modified griffith fracture criterion the dips of faults at varying depths up to 4 km are calculated for both unsaturated dry and saturated rocks the dips of normal faults are expected to vary from 90 at the surface to 60 at 3 4 km and this variation is consistent with the limited field data available the coalfield data are consistent with fault formation at standard hydrostatic pore fluid pressures at depths of about 2 5 km below depths of a few kilometres normal faults are expected to flatten out gradually to a dip of 45 but such flattening is not necessarily an indication of an underlying detachment

geometric and experimental models of extensional fault bend folds the geometric models ranged from simple models of listric normal faults with continuous hanging wall deposition to complex models of normal faults with ramps and flats and footwall deposition and or erosion the inclined shear angle i e the angle between the horizontal and the shear direction was 70 in all models withjack and peterson 1993 describe the modeling technique in the experimental models a 4 cm thick layer of either wet clay or dry sand covered a flat metal base consisting of overlapping plates or precut blocks movement of the plates or blocks produced a listric normal fault in the overlying clay or sand layer we added growth layers after every centimeter of displacement withjack and callaway 2000 describe the properties of the wet clay and dry sand

half graben structures balanced models of extensional fault bend folds one common structural style in extensional regimes is a half graben bounded on one side by a master normal fault and on the other side by a domain of beds dipping toward the master fault this geometry is modeled as being caused by a bend in the master fault the hanging wall beds dipping toward the master fault are bounded by axial surfaces formed as the hanging wall moves past the bend resulting in an extensional fault bend fold the footwall beds remain undeformed and unrotated the major assumptions used in the derivation are that the geometry is area balanced and that the axial surfaces in the hanging wall have dips equal and opposite to the dip of the master fault above the bend important consequences of the model include a relationship between master fault dip above and below a bend and the amount of hanging wall dip the horizontal width of the dipping beds in the hanging wall is twice the heave on the master fault above the bend the asymmetry of the axial surfaces in the hanging wall requires that the rotated beds be strained hanging wall beds dipping toward the master fault are predicted for a fault bend wherein the dip decreases downward hanging wall beds dipping away from the master fault are predicted for a bend wherein the dip increases downward the model fits measurements taken from a published ramp and flat clay model by e cloos and is used to develop a balanced and restorable cross section of the schell creek fault a basin and range structure in nevada

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

origin of coast perpendicular extensional faults western gulf of mexico the relationship between an early stage ridge and a late stage fault an area near corpus christi texas comprises two large coast parallel listric normal growth faults detached on mobilized shale like with most large gulf of mexico listric normal i e growth faults fault planes dip basinward accommodate large amounts of extension and link along strike with other growth faults less characteristically the hanging wall of the growth fault system comprises an elongate ridge whose strike is coast perpendicular and hence orthogonal to the growth faults a high angle coast perpendicular normal fault orthogonal to the growth faults nucleates on the roof of the ridge a model for the concomitant development of coast parallel and coast perpendicular ridges and faults is proposed to explain the development of the orthogonal fault first the early development of coast parallel and coast perpendicular ridges could have been related to convergent thin skinned shortening differential loading or a combination of the two second when large seaward translation occurred ridges having different orientations evolved differently those oriented parallel to the direction of maximum extension coast perpendicular were subjected to little or no extension conditions were thus set for a later nucleation of the orthogonal fault above the coast perpendicular ridge during a much later stage of extension in contrast ridges striking perpendicular to the direction of maximum extension coast parallel underwent collapse and acted as nucleation points for coast parallel growth faulting

cross section restoration and balancing as aid to seismic interpretation in extensional terranes structural cross sections in extensional terranes can be improved significantly by restoration and balancing principles originally developed for fold and thrust belts in rift basins and passive continental margins use of restoration techniques in analysis of growth faults elongate salt swells and other extensional structures results in geologically valid seismic interpretations sections must be in depth as opposed to two way time and oriented in the direction of material transport and decompaction must accompany restoration hanging wall deformation is characterized by different processes depending on the mechanical properties of the rocks being extended and can be approximated by various geometric and kinematic models antithetic faulting is analogous to antithetic shear combined antithetic and synthetic faulting is approximated by vertical shear domino style extension corresponds to rigid body rotation and flexural slip flow is modeled by bed length balance methods selection of the appropriate algorithm is crucial for proper restoration and balancing applications of these techniques illustrated here using seismic data from the gulf coast and north sea include 1 evaluating and adjusting seismic interpretations 2 projecting listric fault trajectories to depth 3 delineating more accurately the geometry and extent of hydrocarbon reservoirs 4 identifying new exploration leads 5 determining the deformation history of an area and 6 constraining the timing and direction of hydrocarbon migration and entrapment

development and growth of basement involved structural wedges in the northwestern qaidam basin china structural wedges contain two connected fault segments a fore thrust and back thrust that bound a triangular or wedge shaped fault block coeval displacements on both faults drive the wedge into surrounding rock causing distinctive patterns of uplift and folding we identified a series of thick skinned structural wedges in northwestern qaidam basin china where syntectonic strata record deformation timing and kinematics the qaidam basin is located at the northern margin of the tibetan plateau and contains more than 100 large anticlinal structures many of which exhibit characteristics of structural wedges we interpreted and modeled these structures using seismic reflection data well logs surface geological exposures remote sensing images and digital elevation data these wedge structures are distinguished by having elevated and deformed strata in the footwalls of the back thrusts and synclinal folds that extend upward from the wedge tips with the constraints of fold shape growth strata structural relief and shallow fault geometry we developed a series of forward models that describe the geometry and kinematic evolution of these wedge structures the analysis suggests that some of the wedge structures reactivate preexisting normal faults our results provide a better understanding of how to identify thick skinned wedge systems and model their geometry and kinematic evolution which has implications for studying the tectonic history of the basin understanding the geometry and kinematics of the wedge structures is also crucial for petroleum exploration because both hanging wall and footwall traps constitute an important component of current exploration targets

origin of basin scale syn extensional synclines on the southern margin of the northern carnarvon basin western australia fault geometry exerts a dominant structural control on the deformation of hanging wall sequences during extension and contraction numerical kinematic and sandbox modelling studies have demonstrated that characteristic anticline syncline pairs are produced during the extension of ramp flat faults these features are commonly recognized in smaller fault scale structures but remain underappreciated in larger basin scale settings the lewis trough situated within the northern carnarvon basin western australia is a basin scale largely unfaulted syncline with an associated anticline along its western flank rather than the fault related graben typical of the region we present kinematic models demonstrating that a se dipping ramp flat fault geometry can produce relative highs and lows in jurassic strata as well as honouring the asymmetrical onlap pattern within the lewis trough this study indicates that the lewis trough formed during the early jurassic a period typically associated with high rates of extension and not during the late triassic fitzroy compression event this study also highlights the importance of the locker shale in partitioning deformation of the permian and mesozoic fault systems and as a diffuse zone that variably partitions displacement between stacked permian and mesozoic fault systems

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

composite sedimentary record of falling stages of pleistocene glacio eustatic cycles in a shelf setting crotone basin south italy abstract a thick pleistocene shelf and nearshore cyclical succession was deposited in the s mauro sub basin of the crotone basin southern italy the regressive units of the cycles are mostly represented by coastal siliciclastic and bioclastic prograding wedges showing a clinoform geometry these are separated by blanket like deposits of high lateral persistence recording major transgressive episodes the aim of this paper is 1 to describe facies patterns and depositional setting of two prograding wedges particularly focussing on their polycyclic internal architecture 2 to analyze these units within a sequence stratigraphic framework and 3 to speculate on the possible origin of the small scale cyclicity the two wedges analyzed in this paper consist of a number of shingles individual shingles consist of two physically connected units 1 a relatively thin package of sigmoid clinoforms grading into 2 a volumetrically dominant package of oblique tangential clinoforms with toplap terminations the shingles are bounded by seaward dipping surfaces with sigmoid clinoform geometry which are ravinement surfaces updip passing into conformable flooding surfaces downdip the wedges are thus organized into high frequency small scale sequences each comprising transgressive highstand and falling stage systems tracts as a whole individual prograding wedges are interpreted as forced regressive units as the shoreline was subject to an overall shift basinwards and downwards along a low angle trajectory in spite of the repeated minor relative sea level rises tectonic subsidence and particularly the syndepositional growth of gentle synclines are thought to have been the key factors allowing the preservation of these forced regressive units progradation of the wedges took place in a high energy wave climate characterized by high frequency of storms and very efficient alongshore redistribution of sediments recurrent storm driven offshore currents led to intense reworking of sediments on the topset platform and gravity spreading on the foreset slope of the prograding wedges well oxygenated conditions over the shelf due to intensified storm activity during glacial periods may have enhanced the rate of production of skeletal foramol type carbonates it can reasonably be assumed that progradation took place from a line source and that the sand bodies are to be regarded as coastal prograding bodies in spite of active syndepositional tectonics the cycles can be correlated to pleistocene high amplitude sea level oscillations the older of the two wedges can be correlated through bio magnetostratigraphy to the major climatic transition which occurred from the marine oxygen isotope stage 25 to 24 22 rio et al 1996 the younger probably developed during the sea level fall that ended with substage 18 2 as suggested by sequence and bio stratigraphic data the prograding wedges are thus interpreted to record long lived sea level falls of fourth order cycles due to the particular depositional setting we are inclined to exclude authigenic mechanisms in the origin of small scale cyclicity although the concomitance and interaction of different controlling factors may be taken into account it is tempting to ascribe this cyclicity to minor eustatic changes punctuating long lived erratic falling stages possibly accompanied by climate driven fluctuations of sediment supply shelf perched and shelf edge prograding units consisting of foramol type carbonates are apparently a common falling stage to lowstand depositional feature in the mediterranean area during the late pliocene and pleistocene

structural restoration of seismic and geologic sections in extensional regimes seismic sections from extensional basins can be structurally transformed to produce restored seismic sections showing subsurface geology of the past restoration is achieved using a simple and effective algorithm which is suited for use in an interactive seismic interpretation environment seismic data are resampled along paleovertical trajectories which are the deformed equivalents of what were originally closely spaced vertical lines paleoverticals are generated within the present day section as follows 1 origin points are equally spaced along a reference horizon that corresponds to an original depositional surface 2 paleovertical segments are vertical within fault blocks 3 paleoverticals are offset across faults by the reference horizon slip and 4 restor d depth is equated to distance down a paleovertical these rules can be modified to conform with such styles of extensional deformation as vertical simple shear inclined shear or rigid rotation and to account for compaction structures with complex fault patterns readily are restored several examples from the gulf of mexico and offshore west africa show that seismic restoration is useful for 1 unravelling structural and stratigraphic history 2 revealing tectonically masked features and 3 checking interpretations across faults for seismic correlation and structural balance geologic sections can be restored in similar fashion

a forward modeling strategy for balancing cross sections 1 a strategy to balance cross sections of complex structures is documented and illustrated by the interpretation of a compressional structure in the deep water gulf of mexico the strategy is applicable to structures formed in sedimentary rocks under low temperatures in both compressional and extensional environments and involves the comparison of the observed structure with simple balanced forward models forward models generated using fault related fold theory help in understanding the processes and kinematics involved in the deformation further forward models are completely constrained and easy to balance whereas it is difficult to balance data therefore forward models are useful in evaluating ideas without completely solving the structure models are constructed assuming parallel behavior preservation of layer thickness no net distortion where layers are horizontal and conservation of bed length unmetamorphosed sedimentary rocks are generally observed to deform obeying the assumptions of parallel behavior in field map well and seismic data

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