genesis of tectonic inversion structures seismic evidence for the development of key structures along the purbeck isle of wight disturbance the interpretation of a densely spaced and well calibrated seismic grid sheds new light on the development and evolution of key regional and local structures in the wessex basin the results help to resolve long standing controversies concerning the tectonic significance of apparently anomalous outcrop patterns and the role of important local ancillary structures with respect to the major monoclinal folds with which they are associated although the structures are entirely consistent with the effects of contractional reactivation tectonic inversion of normal faults the subsurface data demonstrate the role that original extensional fault segmentation and associated relay ramps had on original depositional patterns subsequent inversion geometries and resultant outcrop patterns as well as illustrating regional controls on the formation of structures the new seismic based interpretations enable a reassessment of the lulworth crumple and the ballard down fault the lulworth crumple is interpreted as a parasitic fold complex generated by internal folding of the inverted incompetent syn rift fill in the immediate hanging wall to the purbeck fault a reactivated major normal fault the ballard down fault s origin is interpreted to result from the formation of a local late stage out of the syncline reverse fault which propagated southwards and upwards through a chalk succession as the chalk had already been rotated to form the northward dipping steep limb of the purbeck monocline at ballard down the structure cuts down stratigraphically the results stress the importance of understanding the nature of original extensional fault geometries and the competence of the sedimentary units incorporated in folds in gaining a full understanding of the genesis and evolution of structural styles in inverted basins

stress distribution and plate boundary force associated with collision mountain ranges abstract tectonic stresses associated with collision mountain ranges are studied by finite element modelling large horizontal deviatoric tension is produced in the strong elastic core of the upper crust by the topographical loading and the associated crustal root equally significant is the effect of the dense downbulge of cool lithospheric mantle produced by plate convergence as recently observed beneath the alpine region this dense slab with its associated downflexure produces compression this compression is 25 or less of the theoretical value computed from the density moment function because of the finite width of the slab the actual state of upper crustal stress is a superimposition of root produced tension and slab produced compression an asymmetrically located deep slab produces large compressions at one edge of a mountain belt co existing with large tensions towards the other side of the range leading to mountain building at one edge and extensional tectonics elsewhere the stresses produced by the dense slab support and accentuate the crustal root in opposition to the self inflicted root erosion when the upper crust is thrust faulted the sinking slab produces a collision slab pull force of about 1 2 10 12 n m depending on slab size which is an essential factor with ridge push in initiating and developing mountain ranges surface downflexure produced by slab downpull may give rise to wide borderland sedimentary basins such as the po basin

synchronous compressional pulses in extensional basins abstract such paradoxes as synchronous compression and tension in extensional sedimentary basins and synchronous deformation of intractonic basins and eustatic sea level falls can each be related to pulsed changes in curvature of the earth s surface australia is everywhere in a state of significant relatively uniform upper crustal horizontal compression approximately perpendicular to the continental margins and generally directed towards its centre the compression commenced in the pliocene but was absent in the late miocene during the evolution of the gippsland and bass basins under extensional stress from the late jurassic to recent seven pulses of similarly directed compression have been identified from the campanian to recent time equivalent compressional pulses can also be demonstrated in such widely separated areas as northwest and northeast australia northwest europe and central america nine earlier pulses of early triassic to albian age are similarly synchronous global events and another twenty three pulses inferred since the neoproterozoic 800 ma are probably similarly globally synchronous earth expansion with subduction can explain these observations since it causes the surface curvature to decrease resulting in flattening of the semi rigid crust flattening results in lower crustal extension which generates sedimentary basins while contemporaneous upper crustal compression inverts and folds the sediments since globally synchronous compressional pulses may reflect expansion in the ocean basins there is a correlation of such pulses with major eustatic sea level lowstands any site of expansion within the earth is transmitted to the semi rigid crust as a volume increase by the whole gravitating fluid oblate spheroid this results in ubiquitous synchronous compressional pulses in the upper crust detectable on seismic reflection data not only at basin scale but also in individual petroleum producing structures

transient regional uplift in the early tertiary of the northern north sea and the development of the iceland plume abstract forward and reverse 2d flexural modelling of syn rift and post rift stratigraphy has been used to investigate departures from mckenzie post rift subsidence for the cretaceous tertiary following late jurassic rifting in the northern north sea basin the analysis shows uplift in the palaeocene of 375 525 m relative to calibrated mckenzie subsidence curves followed by a rapid decrease in uplift in the early eocene of 160 310 m both palaeocene uplift and eocene subsidence increase northwards the timing distribution and magnitude of regional palaeocene uplift is consistent with an origin relating to the development of a mantle plume between 65 and 55 ma transient regional palaeocene uplift in the northern north sea basin was produced by long wavelength dynamic uplift generated by the flow field of a mantle plume heating of the asthenosphere or lithosphere beneath the north sea by hot plume material or magmatic underplating are not believed to have contributed to palaeocene uplift in the northern north sea basin rapid early eocene subsidence was generated by a decrease in dynamic uplift caused by a reduction in plume activity the plume responsible for north sea palaeocene uplift may have been the early iceland plume or a distinct british tertiary igneous province plume related to the same mantle convection event

late anhydrite cements mark basin inversion isotopic and formation water evidence rotliegend sandstone north sea the lower permian rotliegend sandstone in the leman field today contains formation waters which are extremely saline 165 530 238 531 mg l with cation chloride ratios indicating mixing of zechstein marine evaporite brines with meteoric water ancient rotliegend formation waters inferred from the analyses of cements were similarly a mixture of marine brine and meteoric water 18o 0 3 smow d 16 smow the strontium isotopic compositions of present day formation waters 87sr 86sr 0 7107 are also consistent with mixing between late permian marine brines 87sr 86sr 0 707 and radiogenic strontium from a detrital source 87sr 86sr 0 720 0 730 isotopic analyses of texturally late intergranular and fracture filling anhydrite suggest waters very similar to the present day formation waters 34s values of anhydrite cements 10 cdt indicate that sulphate in the rotliegend originated from the stratigraphically overlying zechstein evaporites the 87sr 86sr 150 ratios of anhydrites 0 7080 0 7113 however suggest variable mixing of strontium sourced from the overlying zechstein evaporites with more radiogenic strontium derived from diagenetic reactions in the underlying carboniferous shales the oxygen isotope compositions of anhydrite reflect precipitation temperatures between 120 and 140 c from waters with a 18o composition similar to the present day formation waters approximately 0 smow these are the maximum inferred burial temperatures for the rotliegend sandstone in the leman field and would have occurred at maximum burial depths of 3 5 4 km 11 500 13 100 ft during the late jurassic early cretaceous at the start of basin inversion these data indicate an unusual short lived catastrophic event of cross formational flow through fractures which were tectonically induced by basin inversion pore fluids may have escaped from the overlying and underlying formations into the lower pressure rotliegend sandstone regional aquifer to drain this part of the basin

inversion styles in the songliao basin northeast china and estimation of the degree of inversion abstract two inversion patterns fault reactivated and cover folded inversions are recognized in the songliao basin of northeastern china mild inversion with a reactivated fault showing normal separation in all synrift beds moderate partial inversion and total inversion are the common styles of fault reactivated inversion the magnitude of extension by rifting as well as orientation of the synrift faults in relation to the compressional direction during inversion significantly controlled the inversion structures the same amount of shortening may produce a moderate inversion style where the normal slip during the rifting was large but a total inversion in which the initial normal slip was small the cover folded inversions in the songliao basin have either no normal faults or have one or more normal faults that occurred prior to inversion and remained dormant during the inversion the geometry of cover folded inversion may be controlled by deep faults in the basement a graben like basement fault assemblage can result in an antiformal geometry while a horst like basement fault assemblage may control a synformal cover folded inversion an inversion with antiformal geometry of the sedimentary sequences is the major style in the songliao basin a new method of estimating the degree of fault inversion is discussed the degree of fault inversion is the ratio between reverse slip during inversion and normal slip during extension both measured along the fault it can be estimated in most fault reactivated inversions where the top of the basement or the floor of the synrift deposits does not fold during the inversion the horizontal inversion ratio represents the relative amount of shortening and extension of a structure or basin it can be used to analyse the degree of inversion of complex structures or basins however both fault inversion and horizontal inversion ratios are primarily influenced by the amount of extension if undergoing same amount of shortening a structure with a small amount of extension may have a much greater inversion ratio than one with a large amount of extension

fault reactivation and structural inversion backarc and intraplate compressive deformations example of the eastern sunda shelf indonesia the tectonic evolution of the eastern sunda platform in indonesia is revealed by the study of seismic profiles and comprises three general tectonic periods during the cenozoic extension with graben infill from the paleogene to the early miocene a tectonically quiet period with only broad subsidence and a middle miocene recent period of folding inversion and thrusting these structures reveal changes in the intraplate stress field which was controlled by the changes in the boundary conditions of the plates collisions on the northern and eastern sides and interplate coupling southward along the java subduction zone inversions often occurred without reactivation of pre existing normal fault planes at the upper levels of these faults the factors controlling these deformations are discussed with special interest placed on the conditions for the potential reactivation of cohesionless normal faults inversion and reactivations were mostly controlled by the orientation of the pre existing fault planes in relation with the regional stress field and by the tectonic stress regime in a strike slip regime effective principal stresses 1 and 3 horizontal with 1 2 3 the optimum conditions for reactivation in isotropic rocks are pre existing faults that strike close to the potential two vertical conjugate failure planes the same mechanical properties as for pre existing fault prevail in such conditions strike slip movements and reverse strike slip movements and associated en echelon folds were generally observed in a compressive regime 1 and 2 horizontal with 1 2 3 typical steeply dipping normal faults are difficult to reactivate by slip movements we suggest that reverse slip movements occurred in the deeper levels of faults and were accommodated by flexure above the pre existing fault planes and the inversion of grabens the strongly inverted grabens are those whose orientation is perpendicular to the regional compression nevertheless with strong regional shortening large uplifts and thrust faults were observed with reactivation in the form of reverse or reverse strike slip movements along pre existing steeply dipping normal fault planes

mechanics of basin inversion abstract the long term consequences of rifting for the thermal state of the deep crust and upper mantle reflect in part cooling induced by the reduction in heat production in the attenuated lithosphere and heating due to the burial of this heat production beneath the basin provided that the heat production is largely concentrated in the upper half of the crust these factors result in significant increases in temperature at deep crustal and upper mantle levels because the moho depth is likely to be reduced in the long term limit of an isostatically balanced basin these same factors may lead to slight cooling or slight heating of the moho depending on the nature of the basin fill for a brace goetze lithospheric rheology i e a rheology governed by a combination of frictional sliding and power law creep significant long term lithospheric weakening up to 5 per kilometre of basin fill accompanies basin formation when the lower crust is initially relatively strong and the basin fill is characterised by appreciable heat production and low thermal conductivity in contrast initially weak lower crustal rheologies may result in long term lithospheric strengthening evidence for basin inversion in the geological record may therefore imply that heat production is strongly concentrated in the upper half of the crust and under normal continental thermal regimes the lower crust is strong

structural analysis and kinematic evolution of the inverted central south celtic sea basin abstract the central south celtic sea basin shows excellent examples of tectonics and sedimentation relationships as evidenced by detailed geological interpretation of a local grid of seismic profiles these relationships together with balanced restored geological cross sections and block diagrams have been used to unravel the post variscan evolution of this part of the celtic sea basin three sedimentary mega sequences separated by two erosional surfaces have been recognized overlying a deformed palaeozoic basement in this area 1 permo triassic jurassic syn rift mega sequence sedimentation of the lowest part of this succession is strongly controlled by extensional faults whereas the rest of the sequence indicates much less fault control lateral extent of these rocks is also controlled by the disposition of the palaeozoic basement which appears as an elevation in the pembrokeshire ridge area where no sedimentation occurs 2 cretaceous post rift mega sequence 3 tertiary post rift mega sequence the cretaceous and tertiary successions are more extensive than permo triassic jurassic sediments covering the central south celtic sea basin and the pembrokeshire ridge the structural framework displayed by the syn rift and post rift sequences is very different thus the general structure beneath the jurassic cretaceous late cimmerian unconformity consists of a wsw ene trending fault system and related folds this resulted from an extensional phase which led to a sub basin compartmentalization of the area and a subsequent inversional event which caused significant reverse movement along previous extensional structures early cretaceous erosion was responsible for an important and unequal denudation the timing of the switch between extension and inversion cannot be determined due to early cretaceous erosion overlying the jurassic cretaceous unconformity cretaceous sediments are slightly deformed by broad anticlines formed during early tertiary mainly as a result of tightening of previous folds the tertiary sequence shows a subhorizontal disposition responsible for the present day smooth bathymetric relief of the south celtic sea basin

physical explanation of the formation and evolution of inversion zones and marginal troughs inversion zones are elongate structures some tens of kilometers wide and up to hundreds of kilometers long that have deformed in response to compression and produced topography inversion zones in the alpine foreland are mainly associated with mesozoic grabens and troughs and although very important in the geologic picture the conditions of their formation and evolution and their regional geologic significance are not entirely understood the internal structure of inversion zones is variable and depends on details in the pre inversion setting the inversion inducing stress field and the sedimentary fill however on a larger scale most inversion zones share certain principal observational features which sample the physical structure and the rheologic properties of the lithosphere and thereby provide an opportunity to test hypotheses of lithospheric rheology and dynamics the quantitative model presented in this paper explains how inversion zones and the associated marginal troughs are related to lithospheric zones of differential shortening and regional isostatic compensation of the induced topography

petroleum seepages and impregnations in great britain abstract this paper documents 173 occurrences of surface petroleum seepages and impregnations in great britain a brief description is given of the seven main areas where they are found the descriptions range from pore fillings of bitumen to steady flows of oil and gas the principal areas in which seepages are found are as follows the orcadian basin of north east scotland the midland valley of scotland the flanks of the pennines the welsh borderlands cornwall the dorset coast of the wessex basin and the south eastern weald stratigraphically the seepages range from caledonide basement to holocene peat the principal sources for the petroleum are devonian oil shales orcadian basin carboniferous shales and coals midland valley pennines welsh borderlands devon and cornwall and the liassic wessex and wealden basins several anomalous seepages are recorded from welsh and scottish basement there are three clearly defined styles in which the seepages occur many are found where permeable carrier beds unconformably overlie impermeable basement around the margins of basins orcadian basin midlands carboniferous basin others are fault related especially in the wessex and wealden basins the seepages in the cornish tin mines are best explained as sourced from the culm series and drawn by the convective flow of connate waters into the thermal chimneys of the granite intrusions

a vitrinite reflectance kinetic model incorporating overpressure retardation abstract the models currently used to predict maturation and hydrocarbon generation in petroleum basins use time and temperature as controlling parameters a small number of studies have indicated that overpressure can retard maturation although some of the published information from experimental investigations appears to be conflicting careful examination of the experimental conditions shows that both vitrinite reflectance and hydrocarbon generation are retarded by overpressures overpressure appears to result in the retention of volatiles within the molecular structure which prevents the molecular reorganisation necessary to produce higher reflectance values using a database containing vitrinite reflectance from both overpressured and normally pressured wells a kinetic model presro has been developed in which overpressure modifies the arrhenius a factor the model has been tested on two different basins south china sea and sable exhibiting a range of time temperature overpressure characteristics the results suggest that the maturity and timing of the onset of hydrocarbon generation differ significantly from those predicted by current models

the northeastern brazil and gabon basins a double rifting system associated with multiple crustal detachment surfaces analysis of structural stratigraphic and gravimetric data from the reconcavo tucano and sergipe alagoas basins in northeastern brazil and the gabon basin in africa shows that these basins originated in a double rifting system associated with multiple crustal detachment surfaces and that the direction of dip of the detachment surfaces was reversed at the vaza barris fault system this geometry is in agreement with models and data from modern rifts but it requires the existence of a predominantly extensional stress regime in the northern part of the south atlantic during the early stages of oceanic evolution evidence for the existence of such a stress regime is given by application of kinematic constraints to early cretaceous reconstructions of the south atlantic ocean interpretation of the regional geologic setting suggests that the double rifting system and the point of final continental rupture were controlled by the preexisting structural orientation of the tectonic provinces in the precambrian basement

sedimentary basin inversion caused by igneous underplating northwest european continental shelf a considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the british isles underwent permanent exhumation during the tertiary the most dramatic indicator of this process is the present day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of scotland any explanation of this observation must take into account the fact that the entire region has very small long wavelength free air gravity anomalies this important constraint implies either that the crust has been thickened or that low density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the north atlantic tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to alpine mountain building however to remove 3 km of sedi mentary rock from a basin 100 km wide requires 15 km of shortening minor tertiary compression is observed all over the continental shelf but nowhere is it sufficient to account for the required amount of uplift and erosion in addition exhumation dramatically increases from south to north whereas the observed compression decreases markedly in the same direction at the beginning of the tertiary rifting associated with the initiation of the iceland plume generated substantial volumes of melt inversion of rare earth element concentrations of mgorich igneous rocks suggests that a minimum of 5 km of melt was produced beneath at least part of the continental shelf we infer that much of this melt remains trapped within the lithosphere presumably close to the moho which acted as a density filter such underplating will generate rapid uplift

a model for the effect of illitization on porosity and quartz cementation of sandstones abstract quartz cementation is commonly the most important porosity reducing process in deeply buried quartzose sandstones on the norwegian continental shelf the quartz cement is mostly sourced from stylolites but silica may also be derived from illitization of kaolinite illitization can proceed at silica activities several times higher than quartz saturation whereas dissolution of quartz at stylolites is very sensitive to silica supersaturations and the illitization reaction may consequently stop or retard dissolution of quartz at stylolites since the illitization reaction does not cause a net porosity loss porosity loss due to precipitation of quartz cement is reduced or totally prevented over the depth temperature range where illitization takes place illitization affects albitizatio of k feldspar because both reactions consume k feldspar oil replacement during illitization may conceivably cause greater porosity loss than if oil were not present since oil emplacement might stop or retard illitization but not quartz cementation sourced from stylolites

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

late orogenic extension in mountain belts insights from the basin and range and the late paleozoic variscan belt from two examples of orogenic domains some general mechanisms significant of late orogenic tectonic processes in mountain belts are characterized the basin and range province and the variscan belt in the french massif central have both suffered important compressional orogenic crustal thickening and the results of late orogenic processes can be observed in the field both areas are covered by deep seismic profiling providing constraints on the geometry of a crust which has been restored to a normal thickness late orogenic features from the two domains are compared at different scales and their tectonic significance for extension mechanisms is discussed at the scale of the orogenic domains the most prominent tectonic features are the metamorphic core complexes mcc which expose deformed rocks from the middle crust generally affected by high temperature low to medium pressure metamorphism partial melting and widespread granite emplacement in these mcc large scale extensional shear zones present an intense mylonitic deformation characterized by low dipping foliations and pervasive stretching lineations they show a complete evolution from early deep seated ductile deformation generally achieved under high temperature low to medium pressure metamorphism to a late shallow brittle stage characterized by cataclastic deformation the late detachment stage generally controls the development of asymmetrical extensional sedimentary basins filled by continental deposits two main geometries of mcc are defined that are characterized by differing geometry and kinematics of low angle shear zones in the first case two low angle shear zones with opposite vergence develop along the flanks of a roughly symmetrical mcc often one system is dominant over the other the second geometry characterizes asymmetrical mcc bounded by a single normal shear zone which is upwarped during uplift and doming of the core caused by tectonic denudation detailed strain analysis performed in several extensional shear zones shows that the deformation regime is heterogeneous and results from general noncoaxial flow deformation along the shear zones evolves progressively from slight homogeneous pure shear strain to intense heterogeneous noncoaxial shear strain strain distribution within the lower crust is less well constrained by field observation however analogies between cocorp and ecors deep seismic reflection profiles give important constraints on crustal structure wide zones of highly subhorizontally layered lower crust and a flat high amplitude reflection moho characterize both evolved orogenic domains suggesting that major deformations and flow occur within the lower crust during extension a kinematic model involving heterogeneous crustal deformation and regional scale flow fits relatively well with late orogenic structures observed in continental domains a weak hot upper mantle allows large scale flow of lower crust material from zones of deep ductile extension to uplifted domains of upper crustal denudation heterogeneous strain is accommodated by low angle extensional shear zones from localized zones of extension in the brittle crust to ductile lower crust combined pure and simple shear occurs along localized shear zones whereas at the scale of the whole lithosphere deformation nearly corresponds to a vertical pure shear such deformation processes which affect a thick and hot crust seem to be common in both compared domains suggesting that late orogenic extensional processes are slightly dependent of the type of contractional tectonics thus as much in the andean type west american cordilleran belt as in the collision type variscan belt late orogenic processes produced similar extensional features

elastic stress control on the pattern of tensile fracturing around a small fault network at nash point uk abstract complex trajectories of fault related opening mode fractures at nash point in south wales uk are consistent with elastic stress fields computed for the observed fault network subject to north south compression the linear elasticity theory is used to calculate local stress increments produced by slip on the fault network faults are assumed to be free of shear stress and to slip in response to some remote stress load brittle failure of rock within this stress field is determined according to a combined griffith coulomb criterion discrete fracture networks are simulated according to simple growth rules where fractures initiate according to the state of brittle failure and propagate along a surface normal to the direction of most tensile local stress results demonstrate the dominant opening mode fracture were caused by a uniform body force e g unloading and are just locally perturbed by the pre existing fault network fault heights of 2 3 times the mean fault block width and regional contraction along an azimuth of 180 10 are required to reproduce the observed fracture pattern independent kinematic analysis of nearby faults yield estimates of the azimuth of regional contraction of 170 180 some features in the observed fracture pattern are not well reproduced but these are small scale and related to small scale irregularities on the fault planes

hydrocarbon habitat of the west netherlands basin the traditional hydrocarbon charge model for the west netherlands basin was that the carboniferous coal measures have generated only gas and the jurassic posidonia shale only oil however it is now concluded that both have generated oil and gas and that an additional possible source rock for oil occurs in the lower part of the lower jurassic aalburg formation geochemical fingerprints have been established to distinguish the different groups of gas and oil the occurrence of which is consistent with their geological setting and possible migration routes the main phase of hydrocarbon generation and expulsion occurred prior to the mid cretaceous and the occurrence of biodegraded oils indicates that significant amounts of hydrocarbons were already in place at the onset of the tertiary previously it had been considered that charge ended during the late cretaceous inversion however recent modelling studies indicate that along the south west margin of the basin and in the lows between inversion highs charge continued during the tertiary at rates that should have been sufficient to at least compensate for any loss of hydrocarbons through imperfect seals the distribution of biodegraded oils in jurassic and cretaceous reservoirs suggests that biodegradation must have occurred during the earliest tertiary when meteoric fresh waters had access to the reservoirs significantly reservoirs that were at that time so deeply buried that temperatures exceeded 70 to 80 c and that were thus at levels where bacteria cannot survive do not contain biodegraded oils for the fault bounded traps in triassic reservoirs juxtaposition with triassic and jurassic sandstones and shales is the principal control of trap integrity and determines the spill point traps bounded by faults that have been reactivated during the tertiary have clearly been breached traps at upper jurassic and cretaceous levels are usually faulted dip closures and have always been found to be full to their spill points within the limits of seismic resolution

effects of diagenesis and clays on compressional velocities in rocks experimental results at ultrasonic frequencies in brine or water saturated samples cut perpendicular to bedding demonstrate that compressional velocities vary linearly with porosity and volume fraction clay in detrital silicate rocks characterized by pores with low aspect ratios p wave velocity is more sensitive to porosity than clay content obeying the empirically derived expression vp 2 4c 8 6 5 8 at 800 bars confining pressure and 400 bars pore pressure where the units of vp are kilometers per second is the volume fraction pores and c is the volume fraction clay deviations from this equation are less than 2 with the exception of the high pore aspect ratio st peter sandstone which has a compressional velocity that is 17 higher than the predicted value the equation is insensitive to the chemical compostion of the dominant clay mineral and apparently insensitive to the location of the clay grains with respect to the mineral framework e g in free pore space or in contact zones between mineral grains of lower compressibilities

real and relict direct hydrocarbon indicators in the east irish sea basin seismic amplitude anomalies associated with the presence of hydrocarbon gas now play an important role in the exploration of the east irish sea basin tilted flat spots were first observed on data acquired by jebco seismic ltd in 1986 within the productive triassic reservoir of the sherwood sandstone group located in quadrant 110 offshore north wales with hindsight direct hydrocarbon indicators dhis can now be identified on seismic data acquired in earlier phases of the exploration of the east irish sea basin although many anomalies were not always identified as such at that time this study determines the geophysical parameters by which amplitude anomalies associated with diagenetic variations could be separated from anomalies associated with the presence of gas in the reservoir the predicted model results are compared with the seismic response of proven fields discoveries and relict or palaeo dhis around the basin practical implications for effective exploration development and production methods in the sherwood sandstone are suggested anomalous seismic reflections and amplitude variations associated with the presence of hydrocarbon fluids have been observed over the last twenty years in many localities worldwide and described by various authors backus and chen 1975 white 1977 and brown 1986 the presence of dhis on seismic data acquired in the east irish sea was first reported by geoscientists working the basin in the mid and late 1970 s v colter pers comm 1991 the existence of relict or palaeo gas water contacts associated with the distribution of diagenetic platy illite in the sub surface had been determined previously by mapping borehole data for the morecambe gas field bushell 1986 the hypothesis that some of the observed seismic amplitude anomalies in this basin are associated with diagenetic boundaries in the triassic sandstones palaeo dhis or direct porosity indicators rather than the presence of present day gas was postulated the seismic reflection character of the main lithostratigraphic units is well established in the east irish sea basin ross flack 1992 seismic markers for the major formation and member boundaries of the top ormskirk calder rottington and manchester marls and also the hercynian unconformity have been recognised and used across the basin by various authors arter fagin 1993 knipe et al 1993 the permo triassic structure and stratigraphy of the east irish sea basin is well known and has been described by various authors jackson mulholland 1993 jackson et al 1987 colter 1978 the basin is linked to the contiguous onshore west lancashire basin where similar stratigraphic sequences have been described warrington et al 1980 smith et al 1974 and similar structural styles exist the formby point platform is a major structural element of the study area fig 1 the feature is associated with a positive bouguer gravity anomaly which defines the platform s from meadows n s trueblood s p hardman m cowan g eds 1997 petroleum geology of the irish sea and adjacent areas geological society special publication no 124 pp 185 194 2 53 50 n liverpool bay ribble estuary

structural controls on palaeozoic basin evolution and inversion in southwest wales the tectonic evolution of sw dyfed is presented in terms of a model of basin evolution and subsequent destruction by reactivation of the basin bounding faults in contractional mode movements along long lived palaeozoic extensional faults were reversed during variscan compression with the pre existing faults accommodating the contraction in addition to the development of a thin skinned thrust and fold system basin analysis of the pre variscan extensional system uses sedimentological and stratigraphical evidence to constrain the timing of movements of the basin bounding faults thus allowing division of the stratigraphy into depositional megasequences related to these extensional movements field examples on all scales illustrate active extension during the palaeozoic structural cross sections through sw dyfed demonstrate the interrelationship of the extensional and contractional deformations with palinspastic restorations of the cross sections illustrating the complexity of the pre variscan template

ocean circulation in the norwegian sea during the last deglaciation isotopic evidence abstract three cores from different depths in the southeastern norwegian sea have been investigated for benthic and planktonic foraminiferal stable isotope records the cores confirm that the last deglaciation occurred in two distinct steps the first step termination ia may have occurred at about 13 000 yr b p while the second step took place shortly after 10 000 yr b p the benthic oxygen isotopes reveal strong influence of deglacial intermediate waters possibly flowing in from the north atlantic with a maximum influence at about 12 000 yr b p both carbon and oxygen isotopic records show that bottom water formation in the norwegian sea was reinitiated after 12 000 yr b p during the first part of the deglaciation the deep water circulation pattern may have been opposite to that of the present the oxygen content in the norwegian sea deep water was low during the last glacial maximum and remained low during the first part of the deglaciation due to low vertical circulation rates the large variations in deep water circulation in the norwegian sea during the last deglaciation may have had important consequences for the interchange of co 2 between ocean and atmosphere

active faulting and deformation of the coalinga anticline as interpreted from three dimensional velocity structure and seismicity this work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake interpretation of hypocenters and fault plane solutions from the 1983 coalinga coast range california earthquake sequence in combination with the three dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher velocity material above the main listric blind thrust there is a conjugate fault steeply northeast dipping that provides the western limit of the aftershocks within the coalinga anticline and that corresponds in location and spatial extent with the adjacent pleasant valley syncline the character of the seismicity varies with the degree of previous deformation on each section of the anticline where the previous uplift was largest the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure diffuse seismicity characterizes the area with the least previous deformation the mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features the upward extent of the mainshock rupture ended at the approximate boundary between franciscan and great valley sequence rocks above that depth the main thrust appears to splay into a steeper segment and a near horizontal segment thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material with the three dimensional velocity model each individual hypocenter moved slightly 0 2 km in accord with the details of the surrounding velocity structure so that secondary features in the seismicity pattern are more detailed than with a local one dimensional model and station corrections the overall character of the fault plane solutions was not altered by the three dimensional model but the more accurate ray paths did result in distinct changes in particular the mainshock has a fault plane dipping 30 southwest instead of the 23 obtained with the one dimensional model

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

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

kinematic finite strain and vorticity analysis of the sisters shear zone stewart island new zealand abstract the sisters shear zone ssz on stewart island new zealand is a greenschist facies extensional shear zone active prior to and possibly during the development of the pacific antarctica spreading ridge at 76 ma we report quantitative kinematic and rotation data as well as apatite fission track aft ages from the ssz early kinematic indicators associated with the nne trending stretching lineation formed under upper greenschist facies metamorphism and show alternating top to the nnw and top to the sse senses of shear during progressive exhumation lowermost greenschist facies and brittle ductile kinematic indicators depict a more uniform top to the sse sense of shear in the topmost ssz just below the detachment plane deformed metagranites in the ssz allow the reconstruction of deformation and flow parameters the mean kinematic vorticity number w m ranges from 0 10 to 0 89 smaller numbers prevail in the deeper parts of the shear zone with a higher degree of simple shear deformation in the upper parts of the shear zone deeper and upper parts relate to present geometry high finite strain intensity correlates with low w m and high w m numbers near the detachment correlate with relatively weak strain intensity finite strain shows oblate geometries overall our data indicate vertical and possibly temporal variations in deformation of the ssz most aft ages cluster around 85 75 ma we interpret the aft ages to reflect the final stages of continental break up just before and possibly during the initiation of sea floor spreading between new zealand and antarctica

experiments on basin inversion above reactivated domino faults abstract results are presented from scaled sand box experiments which investigate the structural inversion of half graben basins generated by the contractional reactivation of extensional domino fault systems the overburden was deformed above a series of rigid basement fault blocks that were extended in a domino manner the models were constructed from either 1 sand with mica interlayers anisotropic behaviour 2 pure sand isotropic behaviour or 3 a sand clay mixture behaviour with increased competence during the extensional phase of deformation sand or alternating sand mica layers were deposited in teh half grabens associated with individual domino faults to simulate a syn extensional stratigraphic sequence together with a component of overall subsidence after termination of the extensional phase sand layers were added to represent a post rift passive subsidence stratigraphy subsequent contraction of the experimental configuration resulted in progressive reactivation of the domino fault system which was accompanied by back rotation of both the footwall and hanging wall in each of the individual fault blocks this produced null points along the profile section of the fault planes which separated net contraction above from net extension below continued shortening of the system led to further contraction being accommodated along footwall shortcut faults it is suggested that these shortcuts developed in response to 1 the accommodation of space problems generated by increased bed length in the graben acquired during extension and 2 by more efficient low angle fault trajectories in keeping with coulomb failure during horizontal to subhorizontal compression during inversion in the anisotropic model folding in the footwall partially suppressed the initiation of footwall shortcut faults footwall shortcut faulting did not occur in experiments where the dips of the original extensional faults were decreased before the inversion in all inversion experiments the contractional deformation associated with the shortening of the half graben was concentrated in the footwall to each domino basin bounding fault a control experiment in which an undeformed sand pack was deposited above an extended series of basement fault blocks revealed that the early extensional architecture formed in the other experiments had a profound influence on the resultant inversion geometries the results show clear similarities to interpreted examples of natural basin inversion structures associated with reactivated domino fault systems as well as to inferred strike slip induced flower structures

senonian basin inversion and rejuvenation of rifting in africa and arabia synthesis and implications to plate scale tectonics abstract the late paleozoic to tertiary stratigraphic record of much of the african plate reflects the effects of continental rifting and passive margin development several short lived but widespread and tectonically important compressional or wrench dominated events occurred however during the permian to recent evolution of africa we focus here on the best documented of these events which occurred during the late santonian at that time older sedimentary basins mostly ene wsw trending were folded and inverted including some basins along the tethyan margin from morocco to the syrian arc and the intraplate benue southern chad basins and the lugh mandera basin in oman ophiolites were obducted following the santonian tectonism an extensive phase of rifting occurred in central and north africa and northern arabia spanning from campanian to maastrichtian or paleocene times rejuvenation or acceleration of subsidence occurred in several basins located along the tethyan margin along the atlantic and indian ocean margins as well as within the intraplate domain rifting was sometimes accompanied by magmatic activity especially in offshore northern libya and along the indian ocean margin compressional deformations rejuvenated or developed by latest maastrichtian times along the tethyan margin and locally within the intraplate domain one of the most remarkable attributes of the santonian tectonic event which we consider representative of the major trans african stress field changes is the rapidity with which the change was reflected stratigraphically across north and central africa and its short duration the cause of the santonian compressional event is directly linked to the change in poles of rotation for the opening of the atlantic at 83 85 ma the end of the cretaceous normal magnetic quiet zone this was also synchronous with the obduction of ophiolites along the northeast margin of arabia oman the onset of separation between india and madagascar with formation of the mascarene oceanic basin and the development of the european alpine chain we interpret these phenomena as causally related aspects of a global tectonic event the santonian compressional event as well as the campanian maastrichtian rifting event and the end cretaceous compressional event illustrate the critical connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries

glacial rebound and sea level change in the british isles observations of sea levels around the coastline of the british isles for the past 10 000 15 000 years exhibit a major regional variation and provide an important data base for testing models of glacial rebound as well as models of the late devensian ice sheet a high resolution rebound model has been developed which is consistent with both the spatial and temporal patterns of sea level change and which demonstrates that the observations are the result of i the glacio isostatic crustal rebound in response to the unloading of the ice sheet over britain and to a lesser degree of the ice sheet over fennoscandia and ii the rise in sea level from the melting late pleistocene ice sheets including the response of the crust to the water loading the hydro isostatic effect the agreement between model and observations is such that there is no need to invoke vertical crustal movements for great britain and ireland of other than glacio hydro isostatic origin the rebound contributions are important throughout the region and nowhere is it sufficiently small for the sea level change to approximate the eustatic sea level rise the observational data distribution around the periphery as well as from sites near the centre of the former ice sheet is sufficient to permit constraints to be established on both earth model parameters specifying the mantle viscosity and lithospheric thickness and the extent and volume of the ice sheet at the time of the last glaciation preliminary solutions are presented which indicate an upper mantle viscosity of 3 5 1020 pas a lithospheric thickness of about 100 km or less and an ice model that was not confluent with the scandinavian ice sheet during the last glaciation and whose maximum thickness over scotland is unlikely to have exceeded about 1500 m

sandbox model studies of inversion tectonics abstract scaled plane strain sandbox models have been used to simulate basement controlled structural inversion in a sedimentary overburden two basement configurations were tested in the first one reversal of movement along vertical and normal faults that controlled a graben relief induced only a partial recovery of displacement along the pre existing overburden faults the inversion produced new reverse faults in the footwall of the pre existing overburden faults the reactivated normal faults and the new reverse faults defined wedge shaped fault blocks at the edges of the inverted structure in the second configuration the relationship between inversion and tilted fault blocks was tested when a tilting basement block does not move parallel to its bounding fault a compression is induced in the overburden which generates an inverted structure bounded by new reverse faults both experiments show that structural inversion not only reactivates pre existing faults but also generates new faults in the overburden faults related to inversion are the result of horizontal compression either associated with rotation of one of the fault blocks or with vertical uplift of basement blocks

a two layer lithospheric compressional model for the tertiary uplift of the southern united kingdom reflection seismic records show that many of the mesozoic sedimentary basins of the southern united kingdom were compressed and uplifted inverted during the late cretaceous tertiary recent quantitative studies based on apatite fission track analysis and chalk porosity data show that uplift and erosion although concentrated in the inverted basins was regional and not limited to the recognized areas of crustal shortening inversion a two layer or heterogeneous model of lithospheric compression similar to two layer models of lithospheric extension may account for this pattern of uplift upper lithospheric shortening is accommodated by reverse movement on major reactivated extensional detachments causing inversion of pre existing basins pure shear shortening of the lower lithosphere is decoupled and may be laterally displaced from that in the upper lithosphere lower lithospheric shortening and thickening displaced from that in the upper lithosphere causes initial subsidence due to the submersion of cold dense lithosphere into the surrounding asthenosphere subsequent thermal re equilibration of the lower lithosphere may have generated the uplift without attendant crustal shortening witnessed by apatite fission track and chalk porosity data

thermal history reconstruction using apatite fission track analysis and vitrinite reflectance a case study from the uk east midlands and southern north sea apatite fission track analysis afta is a method of obtaining ther mal history information in sediments in sections that have been hotter in the past afta provides estimates of maximum palaeotemperatures and the time at which a sedimentary section began cooling from maximum palaeotempera tures it is useful for studying the thermal history of sequences containing source rocks and can provide information critical to the understanding of the timing of oil generation integration of afta and vitrinite reflectance vr data allows a coherent thermal history framework to be established and a formal method of analysing palaeogeothermal gradients has been developed by combining afta and vr data this approach allows definition of the major facets of thermal history based on directly measurable parameters rather than on assumed values of palaeo heatflow and provides rigorous estimates of the amount of uplift and erosion the case study demonstrates the application of this approach to the east midlands shelf region here afta data have revealed an episode of maximum palaeotemperatures prior to cooling commencing at around 60 ma combined afta and vr data show that this cooling was due to kilometre scale early ter tiary uplift and erosion under near normal geothermal gradients our estimates of uplift and erosion are significantly greater than those of pervious workers and show that uplift and erosion was not restricted to the recognized axes of inversion but was regional in extent this has implications for the evolution of source rock maturity in this region and in other basins in around the uk where afta has indicated early tertiary heating and where the extent and amount of tertiary uplift and erosion may have also been underestimated by previous workers apatite fission track analysis afta tm is a method of obtaining thermal history information in sediments and is particularly useful for studying the thermal history of sequences containing hydrocarbon source rocks as well as providing estimates of maximum palaeotemperatures afta provides a direct estimate of the time at which a sedimentary section began cooling from its maximum palaeotemperature a key advantage over traditional geochemical methods in many situations this knowledge is critical to the understanding of the timimg of oil generation and migration in relation to trap formation information on the timing of maximum palaeotempera ture can also be of significant value in the interpretation of vitrinite reflectance vr data and combination of afta and vr data allows a consistent integrated thermal history interpretation the afta technique is applied most usefully in sedimentary sections which have been hotter in the past than they are at the present day it is therefore applicable to large regions of britain and the uk continental shelf that have been subject to deeper burial in the past or affected by other sources of heating the basis of the technique has

the ivory coast ghana transform margin a marginal ridge structure deduced from seismic data abstract the ivory coast ghana icg marginal ridge is a prominent feature of the icg transform margin and includes a fossil ridge partially buried by a thick undeformed sedimentary cover the fossil icg ridge is 130 km long and 25 km wide and towers over the adjacent rifted basin deep ivorian basin dib and the oceanic crust by 1 3 km and more than 4 km respectively it formed in three successive stages 1 1 during the rifting of the dib both vertical and horizontal motions between the dib and the south american plate varied along the plate boundary this relative motion occurred in an accommodation zone that tilted the northern slope of the icg ridge along en echelon mainly strike slip faults 2 2 after the rifting of the dib the relative motion remained constant along the transform plate boundary at this time strike slip deformation was localized into a narrow and highly deformed belt that truncated the accommodation zone 3 3 finally the transform motion occurred between the dib and an occanic plate thermal exchanges between the two adjacent plates induced thermal uplift of the icg ridge that amplified previous tectonic tilting

crustal scale geometry and kinematics of late orogenic extension in the central aegean cyclades and ewia island abstract the aegean continental domain is known to be the site of widespread back arc extension since at least the middle miocene overprinting structures related to the mesozoic cenozoic hellenic orogeny features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered hp lt metamorphism across the aegean this study summarizes the results of a structural study in the central aegean area cyclades and evvia island examining in particular the relationship between ductile and brittle deformation on a regional scale extension appears to be responsible for most of the ductile deformation within hp rock units that have experienced penetrative greenschist facies and higher grade metamorphic overprinting on each island studied progressive extensional deformation has occurred through the development of a major normal sense detachment zone down to depths of about 18 25 km the geometry of the extensional system on the scale of the central aegean is described taking into account the characteristic features of the regional scale ductile strain field at least two probably three subparallel major north dipping detachment zones are identified trending nw se in the northwestern cyclades and e w in the southeastern cyclades both with initial dips estimated at around 30 45 the present geometry of the extensional system implies that these detachment zones and related metamorphic core complexes interfere with each other two main evolutionary models are considered which can account for such a kinematic link between parallel detachment zones structural data favour the sequential development of synthetic major detachments zones in a direction opposite to the sense of dip of the detachments early oligocene lower miocene detachment type extension in the area studied is related to a late orogenic post thickening back arc setting

regional velocity depth anomalies north sea chalk a record of overpressure and neogene uplift and erosion a normal velocity depth trend for the upper cretaceous danian chalk group is determined by identifying interval velocity data that represent maximum burial in areas unaffected by overpressuring these data are derived from 845 wells throughout the north sea basin data from pelagic carbonate deposits on a stable plateau constrain the trend for shallow depths positive velocity anomalies relative to the trend are mapped along the western and eastern margins of the north sea basin and reflect regional neogene uplift and erosion of up to 1 km along the present day limit of the chalk a hiatus at the base of the quaternary increases in magnitude away from the basin center where a complete cenozoic succession is found this hiatus is consistent in size with the missing section estimated from chalk velocities when allowance is made for the quaternary reburial of the chalk negative velocity anomalies in the central and southern parts of the basin outline an area within which overpressures in the chalk exceed 10 mpa equivalent to a burial anomaly greater than 1 km relative to the normal trend the chalk pressure system is primarily dependent on overburden properties because retention of overpressure generated by the load of the upper overburden depends on the thickness and sealing quality of the lower overburden therefore the chalk is considered to represent a regional aquitard and the hydrodynamic model of long distance migration within the chalk is rejected the neogene uplift and erosion of the margins of the north sea basin and the rapid late cenozoic subsidence of its center fit into a pattern of late cenozoic vertical movements around the north atlantic

measurement of tectonic surface uplift rate in a young collisional mountain belt measurement of the rate of tectonically driven surface uplift is crucial to a complete understanding of mountain building dynamics the lack of a suitable rock record typically prevents determination of this quantity but the unusual geology of papua new guinea s finisterre mountains makes measurement of this rate possible the tectonic surface uplift rate at the finisterre range is 0 8 2 1 mmyr 1 approximately that expected to arise from crustal thickening

sedimentary basin inversion and intra plate shortening abstract sedimentary basin inversion the shortening of formerly extensional basins is accommodated mainly by compressional reactivation of extant faults and fractures across a wide range of scales as such inversion is a large scale manifestation of byerlee friction the dynamic criterion for fault reactivation governing the effective shear strength of the shallow crust basin inversion generates distinctive deformational architecture and it is implicated strongly in sedimentary basin exhumation as a principal source of horizontal stress inversion drives significant sedimentary porosity reduction and resultant fluid flow upper crustal deformation is critically dependent on fluid overpressure i e pore fluid pressures greater than would be calculated from a hydrostatic gradient and perhaps more than in any other tectonic setting overpressures are potentially large during sedimentary basin inversion this review therefore includes discussion of the role of fluid overpressure in inversion and the evidence for it collectively inversion has profound implications good and bad for the prospectivity of many petroliferous sedimentary basins thus recognizing the evidence for basin inversion quantifying its magnitude and understanding the mechanisms that accommodate inversion and other phenomena affected by it have become essential components of the basin analyst s remit

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

lower cretaceous to eocene sedimentary transverse ridge at the romanche fracture zone and the opening of the equatorial atlantic abstract a transform parallel transverse ridge runs for several hundred kilometers along the northern side of the romanche fracture zone rfz being most prominent opposite to the eastern mid atlantic ridge transform intersection rti seismic reflection profiles and rock sampling indicate that the western part of the transverse ridge is made of uplifted slivers of oceanic lithosphere in contrast the eastern part east of the rti consists of a thick 4 km sedimentary succession affected by faulting and thrusting we call it the romanche sedimentary sequence rss if this sedimentary sequence were underlain by oceanic crust its predicted age should be not more than 60 ma however dolomitized maiolica type pelagic limestones containing calpionellids of early cretaceous age 140 ma were sampled from the rss in addition to paleocene eocene quartzs and stones siltstones claystones diatomites and limestones we present a description of the rock samples and a reconstruction of their environment of deposition and of their age the rss may represent material deposited during initial continent continent transform motion in a narrow deep e w elongated basin that communicated with the central atlantic the rss was later subjected to transpression and uplift due to transform related tectonics gabbros but not basalts were sampled from the igneous basement underlying the rss it is not clear whether the rss overlies oceanic crust or some sort of pre oceanic northern red sea type crust the presence of lower cretaceous pelagic deposits near the romanche eastern rti implies an age for the initial stage of the opening of the equatorial atlantic older than the generally assumed aptian albian

rocky mountain foreland uplifts products of a rotating stress field or strain partitioning stress tensor analysis of minor faults near the structural range fronts of diversely oriented laramide foreland uplifts in the central rocky mountains wyoming indicates that paleo 1 was horizontal and oriented nearly perpendicular to uplift trends trend directions for 1 vary from east northeast in the north trending laramie range to northeast in the northwest trending bighorn mountains to north at east trending casper mountain and in the east trending central owl creek mountains although these compression directions are compatible with models invoking temporally changing stress directions during the laramide orogeny the apparent similarity in timing of uplift in these ranges suggests that the computed stress tensors may not reflect regional stress conditions recent stress determinations in transpressional regimes suggest that strain partitioning may be an important factor in considering local stress patterns in the vicinity of mountain belt structures oblique to regional trends in light of these studies caution should be used in interpreting regional paleostress from structures measured close to the tectonic fronts of laramide uplifts

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

geometry and kinematic evolution of inversion structures positive inversion structures form by the compressional reactivation of preexisting extensional structures experimental models and observations of natural structures are used to develop quantitative models for the geometry and kinematic evolution of inversion structures in this paper i analyze two main formation mechanisms of inversion structures 1 fault propagation folding on planar faults and 2 fault bend folding on listric faults inversion structures formed by fault propagation folding occur in the southern north sea the central montana platform and the kangean basin during extension a broad fault propagation or drape fold develops above the master fault with the fault subsequently breaking through the fold synextensional growth units deposited in the hanging wall typically thicken into the basin compressional reactivation results in slip reversal on the master and secondary faults their rotation to shallower dips and the development of a compressional fault propagation fold inversion structures formed by fault bend folding on listric faults occur in the taranaki basin and possibly in the southern north sea rollover folding in the hanging wall occurs during extension possibly accompanied by a small component of fault propagation folding in the vicinity of the fault tip deformation is primarily localized along a system of antithetic faults synextensional growth sediments typically thicken into the fault but also show thinning in the immediate vicinity of the fault during compression the extensional fold is first unfolded and then folded into a compressional fault bend fold the characteristic variations in bed geometry and thickness provide predictive models for interpreting the subsurface geometries of these two classes of inversion structures in areas with poor seismic data these models are particularly useful in exploring for structural traps in the complex and relatively unexplored synextensional growth units

large scale quaternary detachments in ventura basin southern california the ventura basin is an east north east trending trough in the california transverse ranges which records major quaternary detachment faulting at three levels the earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin plain turbidite sandstone because sedimentation continued during thrusting the age rate and direction of thrusting can be worked out faulting began 1 3 m y ago and ceased 0 65 m y ago with a maximum slip rate of 2 8 mm yr to the southeast the fault set moved up a 45 ramp and ended as a blind thrust the ramp had topographic expression on the seafloor diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest following the end of deposition 0 2 m y ago the competent basin plain turbidites underwent flexural slip folding over an incompetent miocene sequence dominated by shale underlying competent paleogene strata were not folded the south flank of the ventura avenue anticline tilted at 3 4 rad yr the anticlinal crest rose at a rate of 15 16 mm yr decelerating to 4 3 5 2 mm yr and the anticline and an adjacent syncline shortened at a rate of 20 mm yr the high rate of folding in the ventura avenue oil field resulted in overpressured sandstone reservoirs and oil water interfaces which have not had time to reach gravity equilibrium the red mountain san cayetano and santa susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the ventura basin at rates as high as 23 mm yr total convergence across the eastern san cayetano fault near fillmore is 11 600 2000 m in the last million years

lithostratigraphic geophysical and morpho tectonic studies of the south cameroon shelf abstract a geological and structural sketch of the southern cameroon shelf was drawn on the basis of a high resolution seismic survey dredgings and kullenberg core sampling of the outcropping rocks at sea bed palynological and mineralogical data x ray diffractometry supported by occasional foraminiferal evidence revealed four main lithostratigraphic sets resting on a precambrian basement upper cretaceous albian to senonian paleocene lower eocene miocene pliocene and pleistocene three successive deformational types characterise the structure of this margin 1 n s trending fractures parallel to the onshore precambrian structural directions formed during the rifting period and crosscut by n60 e trending cross faults induced by the transform movement of the south atlantic 2 compressive structures governed by local stresses during the eocene transcurrent reactivation along the n60 e transform faults excluding a salt controlled deformation 3 a set of approximately n s faults affecting the unfolded sequence of the miocene pliocene or the quaternary and indicative of distensive conditions the geodynamic evolution is placed in the general framework of the central and south atlantic rifting and passive margin history interestingly the age of the compressive deformation varies from late albian to mid eocene depending on the regional plate motion events which are generated along the narrow fracture zones extending as far as or even into the continental domain

crustal extension in the northern apennines the transition from compression to extension in the alpi apuane core complex an interpretation of northern apennine geology is presented which relates the temporal and spatial occurrence of both compressional and extensional deformation features in terms of the changing dynamic evolution within an accretionary wedge after a model proposed by platt 1986 followed by the initiation and development of continental rifting during cretaceous to eocene time an accretionary wedge formed as remnant tethyan oceanic crust subducted beneath the rotating corsica sardinia microplate microplate collision during the oligocene was characterized by the rapid imbrication of buoyant continental crust of the italian continental margin the record of which is preserved within the duplex structure geometry of the alpi apuane region the overthickened wedge geometry returned to a more stable configuration by developing extensional features during the miocene both listric normal faults at upper crustal levels and shear zones indicating evidence of distributed ductile extensional strain at mid crustal levels are recorded it is proposed that large scale regional extension with associated volcanism beginning in the messinian was represented by the intrusion of asthenospheric material from the subducted plate into the already attenuated accretionary complex further rifting perhaps aided by subduction and back arc processes in the southern apennines led to the formation of the tyrrhenian sea as an oceanic basin both the apennines and north american core complexes record evidence of crustal thickening followed by crustal thinning and finally of continental rifting this suggests that the similar histories of these regions with vastly different plate tectonic settings may both be explained by processes linked to the changing internal dynamics of accretionary wedges

late oligocene recent stress evolution in rift basins of northern and central thailand implications for escape tectonics abstract the tertiary rift basins of thailand are known through sub surface seismic and borehole data collected for hydrocarbon and coal exploration outcrops in open cast coal mines and a few natural outcrops these data provide mixed perspectives on how the basins evolved some providing data on ages and structural geometries others palaeostress data in general the region evolved under approximately e w extension although the extension direction probably changed periodically within basins extension was episodically interrupted by inversion events associated with sub horizontal nw se to ne sw 1 direction there is no evidence to suggest that the string of rift basins running from n thailand and laos into the gulf of thailand evolved in a similar way on the contrary the evolution of each basin seems different although certain regional trends along the rift system are apparent as follows 1 oligocene lower miocene extension is widespread in the region 2 in central and northern thailand middle miocene extension is also important and extension persisted into the upper miocene pliocene 3 in the extreme south of the area w natuna basin penyu basin malay basin extension ceased in the earliest miocene 4 in the northern gulf of thailand extension ceased in the middle miocene 5 thermal subsidence is greatest up to 4 km in the south malay pattani w natuna and penyu basins and least in northern thailand a few tens to hundreds of metres 6 inversion in the southern gulf of thailand was intense and occurred during the lower and middle miocene in the northern gulf of thailand inversion is very mild and occurred during the lower and middle miocene onshore inversion is patchy during the miocene but is strongest in the northwestern rift basins particularly the li basin the most widespread inversion event that affects the north occurred during the plio pleistocene it might be associated with the change from left to right lateral motion on the red river fault the relationship between strike slip faults and rift basins in terms of timing of extensional and inversion events and palaeostress orientation and evolution is more complex than can be explained by simple escape tectonic models

the interplay between fluids folds and thrusts during the deformation of a sedimentary succession abstract in this paper field evidence is presented to support the idea that fluid pressures in excess of lithostatic pressure were operating during the initiation and reactivation of large thrusts and folds in the wenlock shales of llangollen north wales it is also argued that as major thrusts are reactivated and as major folds amplify into finite structures they in turn exert an influence on the subsequent migration and concentration of fluids within the deforming sedimentary succession

an alternative model for the growth of faults conventional growth models suggest that faults become larger due to systematic increases in both maximum displacement and length we propose an alternative growth model where fault lengths are near constant from an early stage and growth is achieved mainly by increase in cumulative displacement the model reconciles the scaling properties of faults and earthquakes and predicts a progressive increase in fault displacement to length ratios as a fault system matures this growth scheme is directly applicable to reactivated fault systems in which fault lengths were inherited from underlying structure and established rapidly the model may also apply to some non reactivated fault systems near constant fault lengths during subsequent growth are attributed to retardation of lateral propagation by interaction between fault tips the model is validated using kinematic constraints from growth strata which are displaced by a system of reactivated normal faults in the timor sea nw australia

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

petroleum migration alteration and remigration within troll field norwegian north sea troll field represents the largest petroleum discovery within the entire north sea area in oil equivalents with 74 of the accumulated petroleum present as dry gas and 26 as a heavy biodegraded oil leg the field is divided into several provinces based on the distribution of gas and oil and the gas and oil have been suggested to be cogenetic the migration and filling model presented in this paper suggests that the oil and gas represent two different migration phases and that gas migration and filling predate oil emplacement two different oil populations have been characterized and mapped in troll field applying conventional geochemical techniques we suggest that the two oil populations migrated into the structure through two different migration systems oil and gas were subsequently biodegraded within the reservoir the two oil populations have been found in neighboring oil and gas discoveries and an oil oil correlation with these discoveries has been used to determine the location of field filling points and regional migration routes when oil biodegradation terminated fresh oil continued to migrate into the reservoir and mixed with the residue of the biodegraded oil the field was tilted downward to the west in the neogene and oil and gas remigrated within the field with a possible spillage of gas tilting resulted in a dominantly upward movement of the oil phase whereas gas migrated laterally residual oils in the water zone copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received july 18 1995 revised manuscript received march 14 1996 final acceptance august 16 1996 2saga petroleum a s kjorboveien 16 p o box 490 n 1301 sandvika norway 3newcastle research group drummond building university of newcastle upon tyne newcastle upon tyne ne1 7ru united kingdom we would like to thank the partners in the troll license who allowed us to publish these results per erling johansen and tone molvik are acknowledged for the analytical work and snorre olaussen nigel mills and terje hellem are thanked for helpful discussions and for assistance with the back tilting of the irap reservoir field model jill sonrier is acknowledged for typing we acknowledge k m wolgemuth john f brint and two anonymous reviewers for helpful comments that improved the paper significantly

recognition of tectonic events in undeformed regions contrasting results from the midland platform and east midlands shelf central england apatite fission track and vitrinite reflectance data from central england demonstrate how these techniques can reveal otherwise unrecognized tectonic and or palaeothermal episodes in apparently tectonically stable areas the results document the transition from an inverted basinal region in the north east midlands shelf to a tectonically stable platform in the south midland platform afta data from the region reveal two discrete cooling episodes in the early and late tertiary maximum palaeotemperatures from afta and vr data in outcrop samples define a consistent increase from 50 c in lower cretaceous and upper jurassic units in the se to around 80 90 c in triassic and older units in the nw these early tertiary palaeotemperatures reflect a combination of deeper burial and elevated basal heat flow results from the rufford 1 well define an early tertiary palaeogeothermal gradient of 40 5 c km 1 32 50 c km 1 at 95 confidence limits corresponding to deeper burial by 1450 m of additional section 1 1 2 2 km at 95 confidence limits subsequently removed by tertiary erosion in contrast geological considerations suggest a maximum overburden of 800 900 m above the base of the lower jurassic in the vicinity of rugby where palaeotemperatures at outcrop are similar to those near the rufford 1 location the discrepancy between stratigraphic and palaeo thermal reconstruction of former burial depths often noted in earlier studies remains unresolved the late tertiary episode is much less well constrained but results from rufford 1 may require between 910 and 1650 m of eroded section thus much of the total amount of removed overburden may have been removed during the late tertiary results from the apley barn borehole oxfordshire reveal a late tertiary palaeothermal episode characterized by a highly non linear palaeotemperature profile which probably reflects local heating due to passage of hot fluids despite stratigraphic evidence for some early tertiary erosion results from this borehole show no evidence of early tertiary effects major early and late tertiary exhumation was limited to regions underlain by older palaeozoic basins while regions overlying palaeozoic basement were more stable experiencing significantly less exhumation we suggest this reflects the preferential reactivation of the weaker basinal regions as a result of compressional events at plate margins our results emphasize the importance of incorporating results from both inverted and non inverted areas in understanding the causal mechanisms of uplift and inversion and highlight the importance of testing apparent stability using palaeo thermal methods

compressional velocity and porosity in sand clay mixtures laboratory measurements of porosity and compressional velocity were conducted on unconsolidated brine saturated clean ottawa sand pure kaolinite and their mixtures at various confining pressures a peak in p velocity versus clay content in unconsolidated sand clay mixtures at 40 percent clay by weight was found the peak in velocity is 20 30 percent higher than for either pure clay or clean sand a minimum in porosity versus clay content at 20 40 percent clay by weight is also observed such behavior is explained using a micro geometrical model for mixtures of sand and clay in which two classes of sediments are considered 1 sands and shaley sands in which clay is dispersed in the pore space of load bearing sand and thus reduces porosity and increases the elastic moduli of the pore filling material and 2 shales and sandy shales in which sand grains are dispersed in a clay matrix for these sediments the model reproduces the extrema in velocity and porosity and accounts for much of the scatter in