numerical modelling of lithosphere shortening application to the laramide erogenic province western usa abstract a two dimensional model of lithosphere shortening is presented which quantifies crustal thickening temperature perturbations and flexural isostatic components the model assumes that the upper brittle layer of the crust deforms by thrusting while the lower ductile lithosphere accommodates compressional deformation by a pure shear or squashing mechanism model results suggest that basement uplift foreland basin development and underlying crustal structure are controlled by the amount and configuration of the compressional deformation in the upper and lower lithosphere the perturbation of the temperature field erosion and the flexural rigidity of the lithosphere the model is applicable to regions of thick skinned thrust tectonics and is applied to the laramide orogenic province in the western usa model simulations of structural cross sections across the laramide province show basement uplifts and adjacent foreland basins that are comparable in magnitude with those suggested by geological data the model has also been used to provide insights into the tectonic evolution of the laramide province and an attempt has been made to determine the flexural rigidity of the lithosphere at the time of laramide deformation as well as quantify both the effects of the post shortening re equilibration of the temperature field and thrust uplift erosion the amount of crustal thickening and post shortening cooling of the geotherm predicted by the two dimensional model of lithosphere shortening are used in strength calculations to determine the relative strength or weakness of the lithosphere the results suggest that the lithosphere is relatively strong immediately following shortening due to the cooling of the geotherm following shortening however gradual weakening occurs as the temperature field returns back to its unperturbed state in the presence of an enhanced crustal thickness the results are compatible with the evolution of the laramide province which has experienced a change in tectonic regime to one of extension over the last 20 30 ma

tectonic subsidence and crustal flexure in the neogene chaco basin of bolivia abstract we examined the evolution of the geometry of the chaco foreland basin of bolivia rio grande parapetiarea latitude 18 50 20 s using seismic reflection gravity and well log data subsidence curves enabled us to identify three tectonic stages with a subsidence rate which ranged from 0 1 to 0 4 km myr during the late miocene we used the bouguer gravity anomaly and the power spectrum of the gravity anomalies to estimate the mean depth of the compensating interfaces in order to understand the evolution of the crustal flexure we used an elastic model with a flexural rigidity of 10 23 n m and crustal thickness of 30 31 km we observed a migration of the forebulge during the neogene of about 90 km analysis of multiple geophysical data in the chaco foreland basin of bolivia permits us to sketch out its evolution and to tentatively describe its successive geometries and topographies since 10 ma

a two phase model for compaction and damage 3 applications to shear localization and plate boundary formation a new two phase theory employing a nonequilibrium relation between interfacial surface energy pressure and viscous deformation bercovici et al this issue provides a model for damage void generation and microcracking and thus a continuum description of weakening failure and shear localization here we demonstrate applications of the theory to shear localization with simple shear flow calculations in which one phase the matrix representing for example silicate is much stronger more viscous than the other phase the fluid this calculation is motivated as a simple model of plate boundary formation in a shear zone even without shear the two phases eventually separate due to gradients in surface tension however the influence of shear on phase separation is manifest in several ways as shear velocity increases the separation rate of the phases increases demonstrating a basic feedback mechanism accumulation of the fluid phase causes focused weak zones on which shear concentrates causing more damage and void generation and thus greater accumulation of fluid beyond a critical shear velocity phase separation undergoes intense acceleration and focusing leading to a tear localization in which the porosity becomes nearly singular in space and grows rapidly like a tear or crack at an even higher value of shear velocity phase separation is inhibited such that shear localization gives way to defocusing of weak zones suggestive of uniform microcracking and failure throughout the layer our two phase damage theory thus predicts a wide variety of shear localization and failure behavior with a continuum model applications of the theory to various fields such as granular dynamics metallurgy and tectonic plate boundary formation are numerous

early carboniferous of the solway basin a tectonostratigraphic model and its bearing on hydrocarbon potential abstract the solway basin forms the western portion of the northumberland trough a carboniferous basin system trending wsw ene across northern england a study of the tectono stratigraphic variations along the margins allows certain predictions to be made regarding the hydrocarbon prospectivity of the dinantian it is proposed that earliest carboniferous extension initiated a series of half grabens separated by transfer zones that have subsequently formed fold culminations and fault belts within the basin a model for the proposed graben polarity switching in the basin system is outlined differential subsidence across active faults led to pronounced facies variations in the courcevan chadian which subsequently declined in importance until in brigantian pendleian times deposition was governed by regional subsidence the initial stages of graben formation led to the deposition of subaerial coarse clastic facies associations followed by a cyclical series of marine transgressions and regressions the model anticipates that the best development of reservior facies is in the distal nearshore equivalent of the early dinantian alluvial coarse clastics one of the hydrocarbon objectives identified is stratigraphic trapping in these sands enhanced by courceyan chadian rollover early carboniferous algal source rocks are considered as lateral equivalents to the reservoirs and are calculated as oil generating from the permo trias onwards the westphalian coals are unlikely to have generated significant gas in the basin

geodynamic evolution of the offshore indus basin pakistan the western indian plate passive continental margin a complex meso cenozoic history characterizes the offshore indus basin pakistan including rifting drifting collision transpression and recent inverted extension that is poorly understood we use reflection seismic data to document the geometry of the structures that accommodated the deformation and offshore well data to correlate the sedimentary sequences which were affected by the different tectonic episodes synthetic seismograms have been prepared for calibrating seismic data with well information for identification of key reflections representative of major lithological horizons seismic stratigraphy helped in dividing the sedimentary successions into rifted margin and transpressive margin sequences additionally the basement represented by chaotic and semitransparent reflections and moho discontinuity by moderate amplitude bright reflections are also identified on seismic records syn depositional normal faulting in extensional regime cretaceous oligocene are identified on seismic profiles that converted into strike slip faults in miocene and listric faults in recent sediments forming inverted extensional regime we infer that the basin inversion from extension to inverted extension and transpression occurred in recent times number of depositional features including shelves deltaic deposits canyons and channel levee complexes have been identified that developed in different tectonic episodes five tectonic subsidence curves using back stripping procedure have been calculated in order to constrain the geodynamic evolution of the meso cenozoic sedimentary succession of the offshore indus pakistan the analysis of tectonic subsidence curves covering a time span of 140 ma allowed us to quantify the total subsidence and to document the activities of syn rift and post rift faulting systems different stages in terms of duration and magnitude of subsidence uplift trends have been identified in the evolution of the basin two uplift events are prominent in eocene similar to 40 ma and miocene time similar to 12 ma a rapid resumption of subsidence with non conformity in early palaeocene is seen along the margin

subsidence analysis and quantitative basin modelling in the styrian basin pannonian basin system austria abstract we present the results of subsidence analysis and of quantitative basin modelling using isostatical and flexural models for basin evolution along four cross sections in the styrian basin the westernmost subbasin of the pannonian basin system subsidence analysis reveals a first ottnangian karpatian synrift phase our local isostatic models predict crustal stretching values up to 1 3 and subcrustal stretching values of 1 6 for this event stretching factors of a minor sarmatian extension phase are below 1 04 the termination of subsidence during the pannonian and a rapid quaternary uplift phase can be explained by major changes in the regional stress field a w e cross section through the northern furstenfeld subbasin provides a key for the understanding of the dynamics of basin formation it crosses a narrow karpatian rift basin the metamorphic core complex of the penninic eisenberg window and shows an eastward tilting of the easternmost part of the basin during pannonian times uplift of the penninic window can only be modelled with an extremely weak lithosphere equivalent elastic thickness eet 2 km whereas a best fit between observed and modelled tilting is obtained with an eet value of 5 km these results suggest that the lithosphere was extremely weak during the onset of basin evolution in ottnangian karpatian times probably caused by high extension rates and high heat flows associated with karpatian to early badenian magmatic activity subsequent cooling led to a pronounced increase in flexural rigidity an eet of 5 km fits well with estimations in other parts of the pannonian realm depth dependent rheology models based on palaeo heat flow estimates indicate a similar increase in lithospheric strength with time the impact of plio pleistocene volcanism on rheology appears to be relatively modest which can be explained by a deep position of the magma chamber for this event

intraplate stresses and dynamical aspects of rifted basins abstract intraplate stresses play a important role during the syn and post rift evolution of sedimentary basins deviations from long term thermal patterns of post rift subsidence as well as the development of unconformities within rifted basins reflect to a large extent the temporal fluctuations of the regional stress fields in the lithosphere it appears that incorporation of intraplate stresses in dynamical models for basin development together with a quantification of the role of petrological changes in the crust lithosphere system can explain some of the observed shortcomings of thermal models of post rift basin subsidence late neogene accelerations in tectonic subsidence in the deeper parts of rifted basins in the northern atlantic and mediterranean regions could reflect a change in the level of intraplate stress over large areas of the globe non thermal contributions to the subsidence record affect estimates of crustal extension and consequently influence the depth at which hydrocarbon source rocks enter the maturity window stress induced changes in basement topography have also important consequences for the budget of fluid flow in the deeper parts of the sediment fill of the rifted basins as well as on the dynamics of diapirism

some remarks on the relation between vertical motions of the lithosphere during extension and the necking depth parameter inferred from kinematic modeling studies a well established fact of continental stretching is that during the extension phase the lithosphere retains a finite flexural strength vertical loads caused by the redistribution of material with crustal and mantle densities during lithospheric thinning can therefore be supported by the lithosphere whether these loads cause flexural uplift or subsidence depends on the strength distribution within the continental lithosphere in kinematic model studies stratigraphic and other data acquired in the vicinity of sedimentary basins and passive margins are inverted for these loads in these models vertical rebound loads are parameterized in terms of a necking depth parameter we use thermal mechanical finite element simulations to investigate whether rheological information about the continental lithosphere can be extracted from the necking depth parameter derived from kinematic models we conclude that spatial and temporal variations in necking depths near basin edges and passive margins as derived from our dynamic simulations are not consistent with the uniform and stationary necking depth that is usually assumed in kinematic models this is very important since typically the stratigraphic data used in kinematic model studies are acquired in these regions we therefore conclude that there is no one to one relation between the necking depth parameter and the strength distribution in the lithosphere

breakup between australia and antarctica a brief review in the light of new data abstract the arguments justifying the revised timing of breakup between australia and antarctica cande and mutter 1982 and the reconstruction of broken ridge and kerguelen plateau mutter and cande 1983 are reviewed and considered with respect to new subsidence data the age of breakup was revised from anomaly 22 time 55 my b p to anomaly 34 time 85 my b p the rough topography of the diamantina zone can be attributed to very slow spreading 5 mm yr beginning between the times of anomaly 34 and anomaly 19 the reconstruction of broken ridge and kerguelen plateau at anomaly 34 time shows overlap of these two features but the overlap problem is nearly resolved by anomaly 18 time 42 my b p normal seafloor spreading rates 22 mm yr commenced at anomaly 19 time 43 my b p subsidence patterns calculated from biostratigraphic data from wells drilled along australia s southern margin are interpreted as more consistent with the revised age of australia antarctic breakup subsidence curves systematically show rapid subsidence associated with the rift phase of margin development followed by much slower thermally controlled subsidence during the drift phase the timing of the rift to drift transition is believed to coincide with the age of breakup 60 to 110 my b p in addition the subsidence curves indicate a west to east propagation of breakup along the southern margin magnetic anomaly patterns and stratigraphie observations are consistent with this hypothesis

lithospheric flexure and the evolution of sedimentary basins the sediments that have accumulated in sedimentary basins during geological time represent a load on the lithosphere that should respond by flexure simple elastic and viscoelastic maxwell plate models have been used to examine quantitatively the contribution of flexure to basin formation the models have been used to predict the stratigraphy and gravity anomalies associated with basins for different thermal and loading histories the predictions of the models have been compared with observed stratigraphy and free air gravity anomalies from interior and cratonic basins the best overall fit to the observations is for an elastic plate model in which the flexural strength of the lithosphere increases with age a similar model has recently been used successfully to explain observations from continental margin basins oceanic islands and seamounts and deep sea trench outer rise systems this model explains the increase in the overall width of basins during their evolution as well as the stratigraphy of the basin edges the apparent decrease in the widths of some basins through time can be explained by the model if sediment deposition is followed by erosion of the basin and its edges the models results suggest that the flexural properties of continental and oceanic lithosphere are generally similar and that flexure is an important factor to consider in backstripping studies in which the tectonic subsidence of a basin is isolated and stratigraphic studies in which relative changes of sea level are estimated

subsidence of the paris basin we discuss the main results of a systematic quantitative analysis of the subsidence of the paris basin made on 100 wells backstripping techniques with decompaction as well as sea level and water depth corrections are used to derive tectonic subsidence curves in the absence of water and sediment loading assuming local as well as regional compensation a best fitting sea level variations curve is derived for the deepest central part on the basis of the resulting tectonic subsidence curve the adopted amplitude of the regression since upper cretaceous is 330 m the spatial and temporal distribution of tectonic subsidence is reconstructed and discussed it is shown that rigidity of the lithosphere with an adopted 50 km flexural parameter plays a significant role these results are then quantitatively interpreted in terms of the lithospheric stretching model this interpretation is reasonable provided it is made in terms of volumetric subsidence over the whole basin because of the thermal conduction and the rigidity low pass filtering effects the subsidence can be explained if the axial zone was thinned during permo triassic by a factor of about 1 3 with respect to the adjacent lithosphere it is shown that either the initial altitude was more than 500 m above sea level or the lower part of the lithosphere was lost at the beginning of the permo triassic phase of extension resulting in a rapid uplift of several hundreds of meters the similarity of the permo triassic initiating extensional phase with active continental rifting is shown

geoid anomalies over gorringe ridge north atlantic ocean abstract gorringe ridge is a strong uplifted block of oceanic crust and upper mantle lying at the eastern end of the azores gibraltar plate boundary the geoid over this structure derived from seasat altimeter data exhibits a 9 m height anomaly with a north south lateral extension smaller than 200 km an attempt is made to interpret this geoid together with the gravity anomalies and with the seismicity which has been compiled as a function of depth it is first shown that the flexure of the oceanic lithosphere due to the ridge loading does not provide a good fit of the geoid anomalies and probably should be discarded as it assumes a continuous unfractured elastic plate models involving local heterogeneities are then tested the comparison of the observed geoid anomalies with the anomalies due to the uncompensated relief indicates that the topographic high has no shallow compensation uncompensated models previously proposed to explain the gravity anomalies are tested using the geoid one model purdy and bonnin in bonnin 11 which involves an uplift of upper mantle material at depth generates too strong geoid anomalies and must be discarded another model which represents a nascent subduction zone le pichon et al 25 fits both the gravity and geoid anomalies but leads to difficulties in explaining the deep seismicity north of gorringe ridge a model in isostatic equilibrium is also able to fit both gravity and geoid anomalies this model involves a deep root of density 3 0 g cm 3 as has been previously proposed for many oceanic ridges and plateaus this model is compatible with the deep seismicity but the origin of this low density material at great depth is up to now an unresolved question more likely dynamical models taking into account the forces induced by the convection related to the slow plate convergence in this area will have to be considered

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

crustal detachment during south atlantic rifting and formation of tucano gabon basin system a series of major sedimentary basins of mesozoic to tertiary age are associated with the brazilian continental margin between 8 and 14 s fig 1 this passive continental margin formed when africa and south america split apart with the initiation of seafloor spreading in the aptian albian3 the offshore basins which border the continental margins on both sides of the atlantic show evidence of both rifting and crustal attenuation before break up and thermal subsidence after it in contrast a combined stratigraphical and gravity study of the brazilian onshore basins which are separated from the offshore basins by a northward widening strip of precambrian suggests that up to 7 km of non marine sediments were deposited during the rifting stage of the south atlantic with no apparent extension of the lower crust beneath these basins and no subsequent thermal subsidence a mechanism for the linked development of these onshore and offshore basins is proposed here along the lines suggested by wernicke4 5 in which upper crustal extension in the onshore region is connected to and balanced against deeper lithospheric extension beneath the incipient passive margin by intracrustal detachments

tertiary tectonic and sedimentological evolution of the south carpathians foredeep tectonic vs eustatic control abstract a detailed seismic sequence stratigraphy study based on a dense network of seismic profiles is integrated with structural observations from interpreted geological sections to derive a tectonic and sedimentological model for the miocene pliocene evolution of the south carpathians foredeep getic depression following paleogene and older orogenic phases the first tectonic event which affected the studied area was characterised by early miocene large scale extension to transtension which is responsible for the opening of the getic depression as a dextral pull apart basin further middle miocene contraction caused wnw ese oriented thrusts and associated piggy back basins the last tectonic episode recognised in the studied area relates to general transpressive deformations during the late miocene early pliocene interval a first nw se oriented dextral episode is followed by second n s sinistral deformations the detailed sequence stratigraphy study allows for the definition of the dominant tectonic control of the sedimentary sequences in foreland basins a eustatic control may be associated but has a clear subordinated character

paleobathymetry in the backstripping procedure correction for oxygenation effects on depth estimates abstract this paper aims to provide a straightforward and easily applicable method for estimating the depositional depth evolution of marine basins vertical movements of the basin floor can be reconstructed from the sedimentary record and more accurately constrained when information from the sedimentary history is combined with palaeodepth estimates derived from fauna to this end we propose to extend an existing method based on the percentage of planktonic foraminifera with respect to the total planktonic and benthic foraminiferal association which is expressed as the percentage planktonics p the ratio between planktonic and benthic foraminifera is related to water depth and the p generally increases with increasing distance to shore however next to water depth the oxygen level of bottom waters has a profound effect on the abundance of benthic foraminifera and as such influences the p depending on basin configuration the oxygen level at the sea floor can vary on milankovitch time scales and is reflected by the fraction of benthic foraminiferal species that indicate an effect of oxygen stress on the biotic system these species can be used as stress markers and their percentage with respect to the total benthic population is here expressed as s to assess whether the effect of sea floor oxygenation impairs depth reconstructions we studied the percentage of planktonic foraminifera p in five well dated sedimentary successions from the lower pliocene of crete corfu and milos in greece additionally we assessed whether different foraminiferal size fractions and counting methods affect the determination of the percentage of planktonic foraminifera the palaeobathymetric evolution calculated for each basin was confirmed for all successions by an independent check on depth related occurrences of benthic foraminifera after correction for bathymetry changes of the basin due to sedimentation compaction and eustatic sea level variations the vertical movement history of the basin floor was inferred we propose a standard methodology for reconstructions of palaeobathymetry of marine sedimentary successions from foraminiferal associations

structural evolution of the witch ground graben the witch ground graben is a nw se trending extensional basin in the central part of the northern north sea lying between the moray firth and the viking graben ne sw extension occurred from triassic to cretaceous times extension took place on listric normal faults along the sw margin with downthrow to the ne giving rise to the asymmetry of the basin one of the principal phases of fault controlled subsidence occurred in the lower cretaceous from the valanginian to albian compilation of biostratigraphic data from this interval shows the development of an increasing number of stratigraphic breaks across the basin from sw to ne these breaks increasing in time span also in this direction this is a result of rotation of the subsiding fault block on the listric extensional faults the rotation is recorded in dipmeter data and occurred in several distinct episodes the triassic the volgian and the valanginian to aptian in reconstructing a cross section of the basin the interpreted fault curvature is constrained by the observed rotation of bedding surfaces and the amount of extension across the listric faults is constrained by the observed basin subsidence a depth to decollement for the faults of 4 km below sea level and an extension across the basin of 40 are estimated

polyphase rift evolution of the voring margin mid norway constraints from forward tectonostratigraphic modeling the mid norwegian voring margin bounding the northern atlantic underwent several rift episodes after the caledonian orogeny permo triassic extension concentrated at the eastern inner margin followed by jurassic cretaceous extension at the center of the margin finally more westward the last rift episode led to continental breakup in the earliest eocene although the general architecture of the margin may be clear the timing and distribution of particularly the oldest rift structures are poorly constrained we use two dimensional forward tectonostratigraphic modeling for better quantitative understanding of the tectonic evolution of the margin two regional profiles crossing the margin have been analyzed modeling of the two profiles integrates the basin shape and fill as well as the underlying crustal structure of the margin the modeling allows us 1 to quantify the distribution and timing of extension of subsequent rift episodes 2 to predict the basin and crustal architecture at deep levels and 3 to test different interpretations of the seismic data the level of lithospheric necking during basin formation is constrained by basement topography and moho depth and lies within the middle crust 20 km furthermore we investigate the migration of the rift axes and the elastic lithospheric strength during the different rift episodes locations of previous extension are not reactivated during renewed rifting instead renewed rifting occurs preferentially in regions where the lithosphere has experienced less extension

plate reorganization a cause of rapid late neogene subsidence and sedimentation around the north atlantic subsidence analysis of wells in the central north sea and labrador grand banks and oft the west greenland scotian shelf and united states atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum north atlantic sites a significant departure from the overall decrease in subsidence for the pliocene occurs in many wells when the rate is found to have increased one or more orders of magnitude from oligocence miocene rates wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence although stratigraphic resolution is not detailed more basinward sites experienced up to four times larger subsidence rates in the late neogene than in the oligocene miocene with a peak in the pliocene wells at the basin edge experienced much less subsidence or showed uplift the observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in neogene subsidence major reorganizations of spreading direction and rate occurred during the pliocene along the entire atlantic spreading system possibly in conjunction with more global changes in plate motions we propose that the associated changes in intraplate stress caused the excess margin subsidence relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level

the effect of rift shoulder erosion on stratal patterns at passive margins implications for sequence stratigraphy abstract numerical modelling indicates that the erosion of uplifted rift flanks at passive margins has a profound effect on offshore stratigraphic patterns flexural uplift due to isostatic rebound in response to erosion extends far into the basin and causes uplift of the shelf as a result the contemporaneously deposited sedimentary wedge displays a characteristic offlap pattern when the rift shoulder is largely eroded onlap promoting mechanisms related to cooling of the lithosphere enable sediments to onlap onto the basin margin the initial offlapping and subsequent onlapping strata form one complete second order depositional sequence comprising a shelf margin transgressive and highstand systems tract the modelling inferences are in broad agreement with stratal patterns and basin geometries observed on the u s east coast the southeastern brazilian and southeastern australian passive margins and the transantarctic mountains ross sea shelf system the initial offlaps caused by erosion of rift shoulders have important implications for the deriviation of eustatic signals from coastal onlap patterns

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

stress induced changes in plate density vail sequences epeirogeny and short lived global sea level fluctuations the stratigraphic record contains clear evidence that broad regions have experienced rapid changes in sea level unrelated to changes in glacial ice volume e g third and higher order vail sea level variations continental foundering we propose that many of these changes can be caused by stress induced changes in plate density stress changes produce significant changes in the density of the crust and lithosphere a point missed in previous investigations and propagate across even the largest plates in less than 30 000 years lithospheric plates interacting at existing boundaries can produce stress related density changes sufficient to cause several meters change in plate elevation these may account for many of the regressions and transgressions seen in the stratigraphic record the creation of new rifts could increase plate compression enough to cause 50 m of plate subsidence plate elevation changes of up to 200 m could result from increased plate compression during continental collisions a particularly enigmatic kind of rapid nonglacial global ngg sea level change is a coupled 50 m sea level fall regression and roughly equal rise transgression occurring in less than 106 years these couplets associated with black shales and marine extinctions can be explained by the elastic snapback attending the rapid formation of a new rift isostatic disequilibria along the new rift depresses the seafloor sufficiently to cause a 50 m fall in sea level mantle flow restores isostatic equilibrium along the rift axis and erases the fall in 60 000 years unusually intense hydrothermal circulation along the new rift during the snapback promotes anoxic bottom water conditions and deposition of black shales a rapid drop in global sea levels reduces the area of ocean bottom within the photic zone causing overpopulation food exhaustion and extinctions the connections between changes in plate stress and density have many implications perhaps the most far reaching is that tectonic events on the 3 4 of the globe that is covered by oceans as well as the 1 4 that is subaerial are recorded in sedimentary strata

sedimentary basins and crustal thickening abstract we consider the development of sedimentary basins in a tectonic context dominated by horizontal shortening and vertical thickening of the crust well known examples are foreland basins others are ramp basins and buckle basins we have reproduced various styles of compressional basins in experiments properly scaled for gravity a multilayered model lithosphere with brittle and ductile layers floats on a model asthenosphere a computer driven piston provides shortening and thickening synchronous with erosion and sedimentation after a first stage of lithospheric buckling thrust faults appear mainly at inflection points slip on an isolated reverse fault is accompanied by flexure footwall flexure results in a foreland basin and becomes accentuated by sedimentation hangingwall flexure is less marked but may become accentuated by erosion motion on a fault leads to hangingwall collapse at the surface either footwall sedimentation or hangingwall erosion tends to prolong the active life of a reverse fault slip on any pair of closely spaced reverse faults of opposite vergence results in a ramp basin simultaneous slip produces a symmetric ramp basin whereas alternating slip results in a butterfly shaped basin with superposed foredeeps some well developed ramp basins become pushed down until bounding faults meet at the surface and the basin disappears from view at this stage the basin depth is equivalent to 15 km or more slip on any pair of widely spaced reverse faults of opposite vergence results in a pronounced central anticline between two distinct foredeeps in central asia and in western europe cenozoic crustal thickening is due to continental collision for central asia western china kyrgyzstan uzbekistan tajikistan we have compiled a regional structure contour map on the base of the tertiary as well as 4 regional sections foreland basins and ramp basins are numerous and associated with cenozoic thrusts large basins tarim junggar fergana tajik occur around and between mountain ranges but smaller basins issyk kul naryn occur within them in western europe the alps and pyrenees are surrounded by foreland basins ramp basins or intermediate styles in the andes and its foreland neogene thrusts and compressional basins are due to subduction of oceanic lithosphere in colombia they account for much of the cordillera oriental in nw argentina for the altiplano in west central argentina for the sierras pampeanas compressional basins are also common in other areas of older crustal thickening

flexural and gravity modelling of the merida andes and barinas apure basin western venezuela abstract the kinematic evolution of the barinas apure basin and the southern merida andes from lower miocene to the present is numerically modelled using flexural isostatic theory and geophysical and geological data two published regional transects are used to build up a reference section which is then used to constrain important parameters e g shortenings and sedimentary thicknesses for the flexural modelling to control the location of the main fault system in the flexural model earthquake information is also used the estimated flexural elastic thickness of the south american lithosphere beneath the barinas apure basin and the merida andes range is 25 km the value for the final total shortening is 60 km the flexural isostatic model shows that the andean uplift has caused the south american lithosphere subsidence and the development of the barinas apure basin in addition gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory consequently the best fit flexural model is used to build a gravity model across the merida andes and the barinas apure basin preserving the best controlled structural features from the flexural modelling e g basin wavelength and depth and slightly changing the main bodies density values and deep crustal structures the final gravity model is intended to be representative of the major features affecting the gravity field in the study area the predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the maracaibo crust over the south american shield this process would use the conrad discontinuity as a main detachment surface within an incipient nw dipping continental subduction

western versus eastern black sea tectonic evolution pre rift lithospheric controls on basin formation abstract we demonstrate the key role of pre rift rheology on the kinematics of basin formation in the western and eastern black sea basins constraints on modelling results are provided by a large data set based on more than 50 000 km of multichannel seismics offshore and onshore wells regional gravity and magnetic surveys refraction seismics and field studies the model supports the presence of important differences in the thickness and in the thermal state of the lithosphere which rifted to form the western middle barremian and the eastern middle paleocene black sea subbasins a 200 km and a 80 km thick pre rift lithosphere appear to have driven the deformation in the western and in the eastern black sea respectively differences in the geometry and in the mechanical properties of the pre rift lithosphere have a strong control on the depth of necking and thus on the basin morphology the model sheds light on palaeotectonic and palaeogeographic reconstructions duration of rifting events location of subsiding areas and erosional surfaces the western and the eastern parts of the black sea appear to be two distinct basins characterised by different evolutionary paths determined by different pre rift conditions

an active thermoelastic rift mechanism finite element solutions for stresses and displacements during rift development are obtained for a variable thickness elastic lithosphere plate subjected to thermoelastic loading by conductive heating from an axial line source in the asthenosphere the asthenosphere is modelled as a fluid foundation the prerift lithosphere is modelled as an infinite continuous plate a semi infinite plate is used to characterize the rifted lithosphere horizontal restraints are removed at the rift axis to model the transition from a continuous to separated lithosphere plate during rifting lithosphere strength envelope curves derived from olivine flow laws are used to determine the elastic lithosphere depth in the solutions conductive heating of a variable thickness continuous lithosphere plate by a 2000 w m line source at 60 km depth produces high stresses oriented orthogonal to the forming rift axis and small vertical displacements the maximum uplift obtained in the continuous lithosphere plate models subjected to thermoelastic loading was 0 12 km at a distance of 120 km from the forming rift axis this small uplift is accompanied by shallow tensile stresses of 80 mpa at a distance of 5 km from forming rift axis which decrease to 6 mpa at a distance of 210 km shallow horizontal thermoelastic tensile stresses are over 5 times greater than the horizontal stresses produced by thermal buoyancy loads near the forming rift axis the transition from a continuous to rifted lithosphere is accompanied by an increase in lithosphere uplift at the rift axis from 0 11 to 0 42 km and a reduction in shallow horizontal stress magnitude by a factor of 30 thermoelastic loading may be a source of shallow tensile stress in the lithosphere and a contributing factor for the initiation of rifting thermal loading may also produce postrifting topography changes thermoelastic loading of the lithosphere is proposed as a possible driving mechanism for rift formation which produces separation and the transition from a continuous to rifted lithosphere plate

tectonic subsidence analysis of the illinois basin tectonic subsidence curves calculated from three wells indicate that the illinois basin formed through initial fault controlled mechanical subsidence and subsequent thermal subsidence thermal subsidence involved an early stage of thermal linear 520 460 ma and subsequent thermal asymptotic subsidence 460 350 ma this thermal subsidence was in response to mantle intrusion which formed a three arm rift system a second subsidence episode of unknown origin was documented from middle mississippian through early permian time subsidence mechanisms controlled sedimentation and subsidence rate trends and also sedimentation patterns for each of the four distinct tectonic subsidence phases tectonic subsidence curves were extended across the sauk tippecanoe ordovician silurian and tippecanoe kaskaskia unconformities and fit a post rift thermal contraction model the sauk tippecanoe ordovician silurian and possibly the kaskaskia absaroka unconformities formed because rates of eustatic sea level fall exceede

the detachment era 1977 1982 and its role in revolutionizing continental tectonics after the discovery of thrust and nappe structure near the turn of the twentieth century mountain belts were viewed as a direct expression of horizontal shortening of the continental crust and continental rifting was viewed as a phenomenon distinct from it by mid century broad consensus had emerged mainly on the basis of physical reasoning that thrust and nappe structure instead reflected gravity sliding secondary to vertical motions of the crust as embodied in the influential stockwerk folding hypothesis in a noteworthy period from 1977 to 1982 informally referred to here as the detachment era not only did the last vestiges of support for the stockwerk hypothesis evaporate but large magnitude extension was discovered throughout the cordillera manifest primarily by extensional detachments and metamorphic core complexes soon afterward extensional detachments were recognized as a global phenomenon forcing first order reinterpretation of field relations in most orogens although plate tectonics is indisputably the most profound discovery in earth sciences in the twentieth century the detachment era arguably had commensurate impact on field based interpretations of continental tectonics three decades later controversy persists over the origin of metamorphic tectonites in core complexes and over the existence and mechanics of slip on shallowly dipping extensional detachments

southwestward weakening of wyoming lithosphere during the laramide orogeny the mechanism of laramide deformation in the central rocky mountains remains enigmatic it is generally agreed that the deformation resulted from low angle subduction of the farallon plate beneath the north american plate during the latest cretaceous early eocene however recent studies have suggested the importance of slab removal or slab rollback in causing this deformation here we infer wyoming lithosphere structure and surface deformation pattern by conducting 2 d flexural subsidence modeling in order to provide constraints on the mechanism of laramide deformation we assume that wyoming lithosphere behaved as an infinite elastic plate subject to tectonic loading of mountain ranges and conduct 2 d flexural subsidence modeling to major laramide basins to document lithospheric stiffness and mountain load height our results show that the stiffness of wyoming lithosphere varied slightly in each basin during the 30 myr duration of the laramide deformation and decreased from northeastern wyoming te 32 46 km to southwestern wyoming te 6 9 km our results also imply that the increase of equivalent load height of major laramide ranges accelerated during the early eocene we propose that the bending stresses induced by the topographic load of the sevier fold and thrust belt combined with crust mantle decoupling initiated by the overthickened sevier hinterland and the end loads due to the low angle subduction at the western edge of the thick wyoming craton have caused the southwestward decrease of lithospheric stiffness in wyoming moreover we attribute the accelerated load height gain during the early eocene to both dynamic and isostatic effects associated with slab rollback

modelling depositional shifts between sedimentary basins sediment pathways in paratethys basins during the messinian salinity crisis the evolution of sedimentary basins separated by uplifted topographic barriers is characterised by gradual regressive deposition until one of the basins is filled and its sedimentation bypasses to the adjacent basin defining a depositional shift one of the critical parameters controlling these depositional shifts is sea level variation its drop potentially triggering a depositional shift by cancelling the available accommodation space conversely a sea level rise can create new accommodation space resulting in a depositional shift towards a previously overfilled basin here we use a three dimensional numerical model to study the sedimentary response to sea level variations of a system of two basins in this model a single mountainous source area is feeding an intra continental basin that is separated by a submarine barrier from another basin with normal marine bathymetry the sedimentary response is modelled during a cycle of sea level drop and subsequent rebound that exposes the barrier to sub aerial erosion the examined parameters are the barrier height magnitude and duration of sea level change climate and flexural rigidity modelling demonstrates that shifting the bulk of sedimentation from the continental basin to the open marine environment requires some minimum magnitudes and durations of sea level drop moreover given the specific geometry and parameters of our model an intervening barrier causes a delay of up to 0 35 myr depending on the magnitude and duration of sea level change to the onset of an outward depositional shift when compared to a situation without a barrier these depositional shifts depend on changes in climate magnitude and duration of sea level change model results are applied to the connectivity between the black sea and the dacic basin suggesting that depositional shifts observed during the messinian salinity crisis can be explained by a sea level drop of 1000 m in the black sea

a structural transect between the central north sea dome and the south swedish dome middle jurassic quaternary uplift subsidence reversal and exhumation across the eastern north sea basin

weakening of the slab mantle wedge interface induced by metasomatic growth of talc the rheology of the slab mantle interface in subduction zones could potentially be controlled by weak hydrous minerals such as antigorite or talc formed by reactions involving silica rich aqueous fluids liberated during dehydration of the subducting slab to determine the effect of si metasomatism on the rheological properties of antigorite we performed a series of high shear strain up to 80 frictional sliding experiments on simulated antigorite 70 plus quartz 30 gouges at a pore fluid pressure of 200 mpa an effective normal stress of 200 mpa temperatures of 20 500 c and constant sliding velocities of 0 1 30 m s using a hydrothermal ring shear machine at temperatures of 300 500 c the gouges exhibited a peak friction coefficient of 0 40 0 62 followed by strain weakening toward a quasi steady state strength with 0 25 0 47 in contrast pure antigorite and pure talc at a temperature of 400 c showed steady state shearing at 0 63 and 0 21 respectively the degree of weakening of the mixtures relative to pure antigorite increased with increasing temperature and decreasing sliding velocity the increased weakening was associated with increasing amounts of talc reaction product formed in the samples the weakening was mainly due to the development of throughgoing talc bearing boundary b shears which widened until steady state sliding was attained our results demonstrate that in the lowermost part of the fore arc wedge where silica saturated fluids infiltrate from the dehydrating slab metasomatically produced talc slip surfaces or shear bands will form in the intensely sheared plate interface causing a much larger weakening effect than expected for antigorite even if the total amount of talc formed is minor

intraplate stresses and the subsidence history of the sirte basin libya abstract results of quantitative analysis of subsidence data from 20 wells covering six major basement blocks from the sirte basin libya are presented following the late cretaceous onset of extension and associated collapse of the sirte arch the tectonic history post dating this event can be separated into four distinct phases 1 1 minor subsidence during the late cretaceous 2 2 differential fault tectonics and reactivation of basement faults during latest cretaceous to paleocene 3 3 regional uplift and subsidence during the eocene 4 4 minor subsidence during the early oligocene and early miocene a late cretaceous paleocene stretching phase of finite duration can explain the overall patterns of observed tectonic subsidence during the syn and post rift evolution of the sirte basin strong short term deviations from thermally induced long term subsidence observed during the eocene can be explained by intraplate compression and subsequent stress release related to changes in atlantic flowline geometry modelling of the observed tectonic subsidence for the sirte basin yields estimates of extension in excess of estimates inferred from displacements along high angle normal faults the effect of late stage intraplate compression on tectonic subsidence may cause part of the observed discrepancy between estimates of crustal extension

subsidence history of australia s southern margin constraints on basin models the subsidence history of australia s southern margin is shown to consist of two distinct tectonic phases and has important implications for models of sedimentary basin formation the first phase is characterized by rapid tectonic subsidence about 50 m m y which lasted for about 25 30 m y during the early cretaceous the period of rapid subsidence represents the rifting phase of margin development this early stage is correlated with associated rifting phenomena faulting and extension of the brittle upper crust formation of grabens and half grabens and the rapid accumulation of volcanogenic detritus otway group the amount of extension 20 50 is estimated from a comparison of observed and modeled tectonic subsidence at several exploratory wells along the margin the model includes uniform extension and thinning of the continental lithosphere a prolonged extensional event followed by simple cooling horizontal and vertical conduction of heat and radioactive heat production by the crust the rift phase may have begun earlier but the most rapid and widespread subsidence occurred during the early cretaceous the end of rapid and widespread subsidence occurred during the early cretaceous the end of rapid subsidence coincides closely with the age of the oldest sea floor spreading magnetic anomaly about 90 ma adjacent to the continent ocean boundary cob the second stage of margin development is the post rift or drift phase a period of much slower rates of subsidence and relative tectonic quiescence subsidence during the post rift stage is modeled as cooling and contraction of extended continental lithosphere estimates of extension and thinning from subsidence modeling agree with seismic refraction measurements of the thickness of continental crust near the cob estimates of extension from the geometry of basement blocks near the cob do not agree with the analysis from subsidence modeling upper crustal extension seems to record only a small proportion of total lithospheric extension alternative or supplementary mechanisms of extension are explored with emphasis on implications from a model that includes localized shear along a lithospheric detachment system

the role of rheology in extensional basin formation modelling the rheology of the lithosphere determines its deformation under given initial and boundary conditions this paper presents a critical discussion on how rheological properties are taken into account in extensional basin modelling since strength envelopes are often used in models we review the uncertainties in temperature and rheological parameters and assumptions in type of rheology and mode of deformation involved in their construction models of extensional basins are classified into three groups kinematic kinematic with rheological constraints and dynamic rheology enters kinematic models only implicitly in the assumption of an isostatic compensation mechanism we show that there is a critical level of necking that reconciles local isostasy with the finite strength of the lithosphere which requires a flexural response kinematic models with rheological constraints make use of strength envelopes to assess the initial lateral variations of lithospheric strength and its evolution with time at the site of extension dynamic models are the only ones to explicitly introduce rheological constitutive equations usually in plane strain or plane stress they usually however require the presence of an initial perturbation thickness variations pre existing faults thermal inhomogeneities rheological inhomogeneities the mechanical boundary conditions kinematic and dynamic and the thermal boundary conditions constant temperature or constant heat flux at the lower boundary of the lithosphere may result in negative positive feedbacks leading to cessation acceleration of extension we conclude that while kinematic models with rheological constraints if possible are very successful in accounting for the observed characteristics of sedimentary basins dynamic models are necessary to gain insight into the physical processes underlying basin formation and evolution

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

cretaceous flexural history of northern oman mountain foredeep united arab emirates an important feature of the middle cretaceous arabian continental margin was the regional upwarping in the foreland of the northern oman mountains we interpret this upwarp as an early expression of a cratonward migrating peripheral swell associated with the emplacement of the semail ophiolite onto the margin subsurface stratigraphic data indicate that the swell was expressed during the cenomanian and attained its maximum relief during the turonian by the santonian further downflexure of the margin took place around an axis west of the study area burying the then inactive turonian swell beneath the deposits of the westward expanding foreland basin we suggest that establishment of the later more cratonward flexural regime took place abruptly rather than as a continuo sly migrating wave the middle cretaceous swell axis paralleled the present day axis of the oman mountains the upwarp was slightly asymmetric to the east and loosely constrained estimates of its width and height are 150 km and 244 93 mi and 800 ft respectively the dimensions suggest an anomalously low flexural rigidity 1022 n m for the middle cretaceous continental lithosphere we attribute this low apparent rigidity to the combined effects of remnant thermal and mechanical processes of an earlier rifting event and compression of the margin during the cretaceous emplacement of the semail ophiolite the small dimensions of the middle cretaceous swell suggest that other continental margins with similar geologic histories may also have narrow extinct swells buried beneath foreland deposits such swells may have significant impact on the entrapment of hydrocarbons as we propose for the sajaa margham trend

evolution of the lithosphere beneath the michigan basin using depths to formation tops from 480 wells changes in shape of the michigan basin during the phanerozoic are determined flexural rigidity of the lithosphere is then estimated for several time intervals by assuming that changes in shape were caused by deflection of an elastic plate subjected to an axisymmetric load it is found that the flexural rigidity generally increased over time from less than 10 22 n m early in the basin s history to more than 10 23 n m late in the basin s development this increase in rigidity is attributed to cooling and thickening of the elastic portion of the lithosphere as the basin was subsiding low flexural rigidity early in the basin s history indicates that the lithosphere was probably rejuvenated heated thinned and possibly uplifted prior to basin subsidence the location of the load remained approximately fixed over time the rejuvenation process is investigated using a one dimensional finite difference thermal model in which a 142 km plate is temporarily heated from below taking the depth to the 450 c isotherm to be the effective elastic thickness of the lithosphere rigidity predicted by the thermal model is consistent with the observed increase in rigidity over time furthermore phase changes produced by heating during thermal rejuvenation combined with the predicted thermal contraction following rejuvenation explain the two episodes of subsidence observed in the michigan basin we conclude that the lithosphere in this area was rejuvenated approximately 500 m y ago as the lithosphere cooled it thickened it also contracted and subsided temperatures near the surface of the crust apparently were not raised sufficiently by lithospheric rejuvenation to alter the keweenawan and older radiometric and paleomagnetic dates of basement rocks

a new multilayered model for intraplate stress induced differential subsidence of faulted lithosphere applied to rifted basins in plane horizontal stresses acting on predeformed lithosphere induce differential flexural vertical motions a high precision record of these motions can be found in the sedimentary record of rifted basins originally it was proposed that rifted basins experience flank uplift and basin center subsidence in response to a compressive change of in plane stress which agrees well with observed differential motions subsequently published models predicted that the vertical motions may be opposite because of the flexural state of the lithosphere induced by necking during extension however the total flexural and permanent geometry of the lithosphere underlying the rifted basin is the controlling parameter for the in plane stress caused vertical motions the largest part of this preexisting geometry is caused by faulting in the uppermost brittle part of the crust and ductile deformation in the underlying parts of the lithosphere we present a new multilayered model for stress induced differential subsidence taking into account the technically induced preexisting geometry of the lithosphere including faults in the upper crust as continental lithosphere may exhibit flexural decoupling due to a weak lower crustal layer the new multilayer in plane stress model discriminates the geometries of the separate competent layers at a basin wide scale the new model predicts that a compressive change of in plane force results in basin center subsidence and flank uplift confirming the original hypothesis compared to all previous models the new model requires a lower horizontal stress level change to explain observed differential vertical motions

geotectonic imaging of the north western european continent and shelf abstract gridding and merging of onshore gravity and topography information from the british national gravity and topography data bank with offshore free air gravity anomalies derived from seasat altimeter data and bathymetry from the national geophysical data center usa has allowed for the first time the production of digital images for the north western european continent and shelf 48 62 lat 348 010 long further using image processing techniques the images have been selectively filtered using artificial illumination to enhance both caledonian and variscan trends as a large amount of terrestrially acquired offshore gravity data already exists within the north sea region it has been possible to test the accuracy of the seasat derived free air gravity anomaly data hence its use in geological and geophysical interpretations by comparing the two we conclude that while the derived seasat gravity trends are essentially correct significant error may exist in the amplitude determination especially where anomaly wavelengths are short and oceanographic components in the altimeter data are significant the potential value of geotectonic images is as an aid in recognising trends and relationships between geological and or tectonic domains that are not generally apparent in the more conventional representations of these data e g profile and contour maps we offer interpretations for both the local free air bouguer gravity anomalies in terms of the known geology of the british isles and offshore regions and the regional gravity anomalies in terms of the thermal and mechanical response of the north western european lithosphere during variscan compression continental collision and thrust sheet migration and palaeozoic mesozoic and cenozoic rifting events sedimentary basin formation

sedimentary cyclicity in the marine pliocene pleistocene of the wanganui basin new zealand sequence stratigraphic motifs characteristic of the past 2 5 m y earth s climatic history since 2 5 ma has been controlled by milankovitch variations in the planetary orbit comprising alternate periods of glaciation and interglaciation with a dominant frequency of 41 000 yr concomitantly eustatic sea level has fluctuated 70 to 130 m causing rapid transgressions and regressions of the shoreline across the world s continental shelves the resulting sedimentary record is cyclothemic each cyclothem corresponding to a single climate and sea level cycle the wanganui basin new zealand contains a 2 km thick almost complete composite record since isotope stage 100 ca 2 5 ma in the form of 47 superposed cyclothems of shelf origin each cyclothem corresponds to an unconformity bound stratigraphic sequence and typically contains a transgressive systems tract sometimes a mid cycle shell bed a highstand systems tract and sometimes a regressive systems tract no advantage accrues from using transgressive regressive units rather than cyclothems and or sequences in description of the succession six basic sequence motifs represent deposition in locations between the shoreline and offshore shelf i e the hawera birdgrove turakina seafield castlecliff and rangitikei motifs a seventh the nukumaru motif which includes dominant coquina limestone represents deposition in shallow water areas of reduced terrigenous sediment on the flank of the basin the sequence motifs represented in any section change systematically in sympathy with basin scale changes in subsidence and sediment supply in contrast with the 41000 year length of individual glacioeustatic sequences these basin wide tectonic cycles have a periodicity of many hundreds of thousands to a few million years i e that of third or fourth order sequences of the exxon type this coupled with the restriction of strongly cyclothemic sediments to geological periods of known glacio eustasy permiancarboniferous pliocene pleistocene suggests that tectonic subsidence cycles rather than glacio eustasy are the driving forces behind the development of the third and fourth order unconformity bound sequences that are reported to occur throughout the stratigraphic record

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

geoid heights over the louisville ridge south pacific geoid heights over the louisville ridge seamount chain deduced from the seasat satellite altimeter data have been analyzed and interpreted two uncharted seamounts belonging to the chain have been detected the elastic thickness of the oceanic lithosphere has been determined along the chain this was done rigorously using the three dimensional geoid and bathymetry maps and was also computed approximately using spectral methods on geoid and bathymetry profiles a slightly increasing elastic thickness from the southeast to the northwest in the range 15 5 22 5 km has been estimated

subsidence analysis and tectonic evolution of the external carpathian moesian platform region during neogene times abstract the miocene pliocene subsidence of the tectonic platforms in the romanian carpathians foreland is analysed using standard 1d backstripping techniques for individual wells combined in two regional sections and six contour maps the subsidence patterns were integrated together with previous paleostress and kinematic studies in order to derive the tertiary kinematics of the buried faults in the carpathians lower plate the study revealed accelerated subsidence during the early miocene in the western part of the moesian platform getic depression in direct relationship with the opening of a wsw ene trending extensional basin the largest subsidence recorded in the front of the carpathians took place during the late miocene due to final e ward emplacement of the thrust sheets the late miocene subsidence showed anomalous high values between the intramoesian and trotus faults as a result of the orogenic collision with the east european platform northward and acceleration of the subduction process in the se carpathians corner further pliocene subsidence continued only in the latter region the depocenter being shifted southward near the junction with the south carpathians foreland

a tensile flexural model for the initiation of subduction we argue that subduction may be initiated at passive continental margins without shortening the lithosphere overcoming the lithosphere s high compressive strength requires special circumstances and these make it difficult to explain the nearly complete recycling of old sea floor instead we present a model that predicts tensile decoupling of the continental and oceanic lithosphere passive rifting and foundering of the seafloor beneath material welling up in the rift this occurs because the lithosphere in a new ocean basin establishes mechanical continuity with the continent at a depth comparable to mid ocean ridges later subsidence at the margin is therefore inhibited by flexure which implies shear stresses that promote fault slip and tensile stresses necessary to balance the component of the plate s weight directed down the margin slope we show that this tension can more than offset ridge push in our model an important additional tension arises from basal shear tractions resisting the plate s motion away from the mid ocean ridge although these tractions cannot be evaluated with confidence slip on a high angle fault decouples the oceanic and continental lithosphere when shear stresses arising from flexure and the applied tension exceed the lithosphere s shear strength under these loads a passive rift then forms allowing a mantle column to rise to the height of mid ocean ridges over 3 km above the old seafloor and flow onto the surface in a gravity current this load flexes the plate downwards which enhances the flow and lets the old oceanic lithosphere founder this model is consistent with the presence of oceanic material in continental forearcs and the youth of ophiolites when they are obducted as both might be explained by rifting a margin and underthrusting the juvenile crust formed there boninites in ophiolite complexes and tectonites at their bases show depleted and hydrated geochemistries consistent with the melting of rifted mantle lithosphere that receives volatiles from foundering oceanic crust

the effective elastic lithosphere under the cook austral and society islands abstract we have determined the elastic thicknesste of the oceanic lithosphere along two volcanic chains of the south central pacific cook austral and society islands we used a three dimensional spatial method to model the lithospheric flexure assuming a continuous elastic plate the model was constrained by geoid height data from the seasat satellite along the cook austral chain the elastic thickness increases westward from 2 4 km at mcdonald hot spot to 14 km at rarotonga at mcdonald seamount however the data are better explained by a local compensation model the observed trend shows an increase ofte with age of plate at loading time however the elastic layer under the cook austral appears systematically thinner by several kilometers than expected for normal seafloor suggesting that substantial thermal thinning has taken place in this region considering the apparent thermal age of the plate instead of crustal age improves noticeably the results along the society chainte varies from 20 km under tahiti to 13 km under maupiti which is located 500 km westward when plotting together the society and cook australte results versus age of load we notice that within the first five million years after loading te decreases significantly while tending rapidly to an equilibrium value this may be interpreted as the effect of initial stress relaxation which occurs just after loading inside the lower lithosphere and suggests that the presently measured elastic thickness under the very young tahiti load 0 8 ma is not yet the equilibrium thickness

lake bonneville constraints on lithospheric thickness and upper mantle viscosity from isostatic warping of bonneville provo and gilbert stage shorelines the deformed shorelines of the late pleistocene lake bonneville provide an excellent opportunity to analyze the response of the earth to surface loads of relatively short wavelengths 400 km and short duration 30 kyr recently revised estimates of the lake loading history and an expanded compilation of shoreline elevations for the bonneville 1550 m provo 1440 m and gilbert 1300 m shorelines have prompted us to reexamine the deflection the earth model we use is identical with that employed by nakiboglu and lambeck and consists of an elastic plate overlying a maxwell viscoelastic channel and a rigid base our calculation differs from that of nakiboglu and lambeck in two respects we used a significantly more detailed load model and we compared our results with observations from three distinct shorelines the lake elevation history spans 32 kyr in 18 piecewise linear segments the water load is computed at each time step from a digital terrain model 4 4 km cell size and a specified lake elevation we find that the use of three separate shorelines significantly reduces the acceptable region of parameter space the effective elastic lithospheric thickness is 23 2 km the mantle viscosity is 1 2 0 2 1020 pa s and the depth to a significant viscosity increase is no less than 300 km though it might be much more examination of the residual deflections differences between observed and computed shoreline elevations reveals a regional tilt down to the ne of 6 10 5 which we attribute to collapse of the peripheral bulge produced by the laurentide ice sheet

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

tectonics global changes in sea level and their relationship to stratigraphical sequences at the us atlantic continental margin of geological the principal factors that control the extent of seas through geological time are vertical movements of the lithosphere and global changes in sea level the relative height of the sea surface determines the facies and the thickness of sediments that can accumulate in a sedimentary basin backstripping studies show that the primary factors affecting the subsidence of rifted sedimentary basins are thermal contraction following heating and thinning of the lithosphere at the time of rifting and sedimentary loading factors such as compaction palaeobathymetry erosion and global sea level changes also contribute but their combined affects are small compared to those of thermal contraction and sedimentary loading simple models have been constructed which combine the effects of sedimentary loading and thermal contraction with those of compaction sub aerial erosion and global changes in sea level in the models it was assumed that the lithosphere was heated and thinned by stretching at the time of rifting sedimentary loading occurs by flexure of a lithosphere that progressively increases its flexural rigidity with age following rifting and that sediment compaction and bathymetry change across a basin but do not vary significantly with geological time furthermore different assumptions were made on the magnitude of curves of global sea level changes and the relationship between denudation rate and regional elevation the models show that tectonics in the form of thermal contraction of the lithosphere and flexure and slowly varying global changes in sea level can explain a number of the stratigraphic features of the us atlantic continental margin in this paper some of the implications of these results are examined for studies of a sea level changes through geological time and b the maturation history of continental margin basins

cyclicity in the middle eocene yamak turbidite complex of the haymana basin central anatolia turkey the haymana basin in central anatolia turkey formed on a late cretaceous to middle eocene fore arc accretionary wedge a sequential model is proposed for the 1 km thick lutetian yamak turbidite complex ytc which is the youngest paleotectonic unit of the basin the ytc represents a prograding submarine fan subdivided into three depositional sequences ds each several hundred meters thick each depositional sequence consists of a turbidite system ts with sandstone and conglomeratic sandstone beds alternating with mudstones overlain by basin plain mudstones in each turbidite system the sandstone and mudstone sequential organization allows the distinction of smaller subdivisions namely basic sequences bs and basic units bu with each basic sequence being composed of several basic units this subdivision associated with a two dimensional geometric reconstruction of the ytc leads to a better understanding of the evolution in time and space of the submarine fan system lower to middle fan depositional lobes and upper fan and slope channels are represented as a whole the ytc progressed from a sand poor to a sand rich system depositional sequences ds of the ytc may correspond to third order sea level cycles of tectonic origin accordingly fourth and fifth order cycles might be proposed for the bs and bu respectively however partly because of the limited extent of exposures the allocyclic origin of these finer subdivisions remains problematic

an integrated sedimentary systems analysis of the rio bermejo argentina megafan character in the overfilled southern chaco foreland basin abstract the chaco foreland is well known for its spectacular fluvial megafans and this basin frequently serves as a modern analog for studies that use the geological record to investigate tectonic processes in convergent orogenic systems yet little sedimentary data has been recovered from the southern chaco particularly at the latitude of northern argentina which limits both its utility as an analog and our understanding of andean megafan development in the subtropics the southern chaco is particularly amenable for a modern time slice sedimentary systems analysis as environmental gradients controlled by tectonics and climate are relatively well documented deposition in the southern chaco foreland is dominated by the rio bermejo which was studied by integrating geomorphological observations made using geographic information systems with analyses of composition texture and u pb ages of modern detrital sediments remote sensing data reveal changes in rio bermejo channel width sinuosity and bifurcation from west to east across the foreland which are compatible with aspects of geomorphic models for distributive fluvial systems the rio bermejo fully spans the foreland and connects with the south flowing rio paraguay fluvial sediments and reworked loess bury and obscure any surficial expression of a flexural forebulge therefore the rio bermejo megafan rbm stands as an important example of a megafan in an overfilled foreland basin variability in rbm morphology and landforms appears to have only subtle linkages to flexural partitioning of the chaco which is likely a consequence of sediment supply overwhelming available accommodation trend surface analysis highlighted topographic contrasts in rbm architectural elements and indicates the presence of distributary channels with variable surficial expression shallow accommodation marked by ponds is most pervasively developed at the eastern end of the rbm this pattern is consistent with model predicted locations of a back bulge depozone although three environmentally distinct hinterland areas contribute sediment to the rbm sands from the foreland cannot distinguish these zones rather provenance is controlled chiefly by parent lithologies and transport distance sands in the rbm plot within the recycled orogen provenance field mean qtfl 77 6 17 and compositional maturity increases with distance from the thrust front such that quartzose sublitharenites dominate in the distal back bulge distinct intra foreland gradients are captured in sand texture and clay mineral abundances rio bermejo sand sizes become progressively finer towards the forebulge and back bulge mean 98 m whereas mean particle sizes in the wedgetop and foredeep are 280 and 195 m respectively detrital clays suspended in the rio bermejo consist of illite smectite chlorite and kaolinite in rank order of abundance with smectite concentrated in the foredeep due to reworking of the semiarid floodplain by water and wind the results of this study provide novel sedimentary ground truth for distributive fluvial systems in overfilled retroarc forelands that can be used to improve facies models and interpretations of the geological record

gravity anomalies and flexure of the lithosphere at the middle amazon basin brazil the middle amazon basin is a large paleozoic sedimentary basin on the amazonian craton in south america it contains up to 7 km of mainly shallow water sediments a chain of bouguer gravity highs of approximately 40 to 90 mgals transects the basin roughly coincident with the axis of maximum thickness of sediment the gravity highs are flanked on either side by gravity lows of approximately 40 mgals the observed gravity anomalies can be explained by a steeply sided zone of high density in the lower crust varying in width from 100 to 200 km wide within this region the continental crust has been intruded replaced by more dense material to more than half its original thickness of 45 50 km the much wider sedimentary basin results from regional compensation of the subsurface load and the subsequent load of accumulated sediments by flexure of the lithosphere the observed geometry of the basin is consistent with an elastic lithosphere model with a mechanical thickness of 15 20 kin although this value is lower than expected for a stable cratonic region of early proterozoic age it is within the accepted range of effective elastic thicknesses for the earth rapid subsidence during the late paleozoic may be evidence of a second tectonic event or lithospheric relaxation which could lower the effective mechanical thickness of the lithosphere the high density zone in the lower crust as delineated by gravity and flexural modeling has a complex sinuous geometry which is narrow and south of the axis of maximum sediment thickness on the east and west margins and wide and offset to the north in the center of the basin the linear trough geometry of the basin itself is a result of smoothing by regional compensation of the load in the lower crust

transform margins development controls and petroleum systems an introduction this paper provides an overview of the existing knowledge of transform margins including their dynamic development kinematic development structural architecture and thermal regime together with the factors controlling these this systematic knowledge is used for describing predictive models of various petroleum system concept elements such as source rock seal rock and reservoir rock distribution expulsion timing trapping style and timing and migration patterns the paper then introduces individual contributions to this volume and their focus supplementary material location table and map of specific transform examples structural elements of the romanche transform margin and glossary of terms used in this article is available at https doi org 10 6084 m9 figshare c 3276407 gold open access this article is published under the terms of the cc by 3 0 license this volume shows dynamic linkage between new transform margin research and increasing transform margin exploration it offers a critical set of predictive tools via an understanding of the mechanisms involved in the development of play concept elements at transform margins and their controlling factors our introduction to this volume reviews the current knowledge of the transform margins focusing on their dynamic development kinematic development structural architecture thermal regimes and petroleum systems the last part of this paper introduces the contributions to this volume

eustatic control on synchronous stratigraphic development cretaceous and eocene coastal basins along an active margin abstract field studies document an apparent eustatic control on facies patterns along a tectonically active margin in the san diego embayment and northern baja california progradational retrogradational shoreline sequences characterize late cretaceous and eocene forearc stratigraphy extensive benthonic foraminifera and nannoplankton data provide control on the age and distribution of facies changes along these depositionally compact bathymetrically steep gradient margins the complete stratigraphic package is arranged into three scales and patterns of depositional sequences timing and geometry of the two largest sequences provide relative sea level curves that correlate exceptionally well with worldwide sea level curves the major depositional cycle is asymmetric hundreds of meters thick characterized by a thin basal retrogradational sequence overlain by a thick progradational sequence each cycle correlates to a coastal onlap supercycle smaller scale stratigraphic rhythms controlled by global cycles and paracycles compose depositional cycles local depostional pulses overprint these two larger order sequences of sedimentation coeval cycles and depositional rhythms in isolated coastal basins from oregon to baja california further indicate a primary eustatic control on sedimentation field based facies analysis thus supports the use of the vail curve or other coastal onlap and global sea level curves as a predictive tool in basin analysis

thermal contraction and flexure of intracratonal basins a three dimensional study of the michigan basin summary cooling of continental lithosphere by thermal conduction to the surface following a heating event imposes a load on the lithosphere and causes subsidence as the basement rocks contract accumulation of sediments in the resulting depression forms a sedimentary basin studies of the geometry of sedimentary basins with horizontal dimensions of a few hundred kilometres suggests the lithosphere responds to loads by regional flexure of a strong elastic or viscoelastic upper lithosphere two and three dimensional models for flexure of the lithosphere owing to thermal contraction are applied to the michigan basin 2 d flexural models are compatible with previously estimated rheological parameters for an elastic lithosphere an effective flexural rigidity of about 5 1029 dyne cm can explain observed gravity anomalies and structural contours across the michigan basin a viscoelastic lithosphere requires an effective flexural rigidity of about 1032 dyne cm for a viscosity of 1025 poise for a 3 d elastic or viscoelastic lithosphere effective flexural rigidity must be approximately an order of magnitude smaller 2 1028 and 1031 dyne cm respectively larger values leave an excessive remaining load at the centre of the basin which is not indicated by observed gravity anomalies the decrease in effective flexural rigidity is attributed to greater difficulty in equi dimensional bending of a 3 d plate the magnitude and spatial distribution of the computed thermal contraction load are consistent with the presumed mechanism as the thermal anomaly of the initial heating event was present mainly at great depths in the lithosphere the lack of geological evidence for an initial heating event prior to subsidence is not a fatal objection to the thermal contraction mechanism of subsidence a driving load presumably from subsurface processes including crustal stretching and emplacement of dense rocks into the crust is necessary to explain the net subsidence of the basin from the time before the heating event until the lithosphere beneath the basin cooled without this driving load the thermal expansion would produce uplift but the surface would subside only back to sea level after the lithosphere cooled theoretical gravity results indicate that the driving load is centred at a depth of approximately 15 km deviations in subsidence curves from exponentials associated with thermal contraction can be explained by changes in sediment supply spatial variations in sediment load caused by regional facies changes produce migration of the centre of maximum deposition water and basement depths are determined for sequential time intervals during basin development the predicted depositional environments are consistent with lithofacies maps of the middle devonian

the zambezi river tectonism climatic change and drainage evolution abstract the longitudinal profile of the zambezi river forms two concave upwards sections with their boundary at the victoria falls this form has been ascribed to the process of pediplantation and the victoria falls identified as one of several knick points that have traversed the river since the breakup of gondwanaland an alternative model explains the river s long profile by suggesting that the upper and middle zambezi evolved as entirely separate river systems which only joined together in comparatively recent times the alluvial sequence of the middle zambezi is described and interpreted in terms of the latter hypothesis the river capture event caused a change in grade and is marked by a deposit believed to record a cataclysmic flood capture is inferred to have resulted from overtopping of the lake that formed the end sink of the proto upper zambezi rather than from headward erosion of the proto middle zambezi this event is dated from archaeological evidence to the peak of the last interglacial at the end of the middle pleistocene subsequent drainage diversions between the middle zambezi and the kalahari are interpreted as the product of rifting of the chobe graben and aggradation of the chobe swamps river capture by overtopping implies high rainfall over central southern africa at the peak of the last interglacial which is contrary to predictions that africa s rain belts then lay north of their modern mean positions this anomaly is resolved by postulating a southern polar warming episode at that time and agrees with suggestions that the last interglacial was marked by large scale ablation of the west antarctic ice sheet