a paleoslope model of late albian to early turonian foraminifera of the western atlantic margin and north atlantic basin abstract a paleoslope model of albian to turonian benthic foraminifera reveals seven successive biofacies which are associated with increasing paleodepth the paleoslope model indicates that the biofacies occupied inner neritic to abyssal environments of deposition 10 m to 3500 m water depth biofacies 1 and 2 characterize paleodepths of less than 60 m the biofacies contain abundant species of epistomina and gavelinella and exhibit a rapid stratigraphic alternation in response to local sea level change which considerably aids in the identification of depositional sequences species of globorotalites eggerellina and neobulimina characterize biofacies 3 which principally occurs between 60 m and 130 m paleodepth biofacies 3 is the only biofacies to occur in both the new jersey coastal plain and the baltimore canyon trough biofacies 4 is composed of species of gavelinella gyroidinoides and valvulineria and is abundant in outer neritic to middle bathyal paleodepths of 130 m to 600 m it is largely replaced by biofacies 5 during times of maximum flooding during sea level rise a diverse assemblage of species of lingulogavelinella and lenticulina characterize biofacies 5 paleodepths of 600 m to greater than 3500 m contain abundant species of osangularia pleurostomella praebulimina pseudopatellinella and quadrimorphina which define biofacies 6 and 7 in addition four biofacies of much more limited distribution are recognized biofacies 1 characterizes shallow water open marine paleoenvironments of the north atlantic and is composed of spirillinids and species of epistomina turonian dysaerobic environments are indicated by biofacies dsa and dsb which are composed mostly of agglutinated species dysaerobic environments expanded rapidly from middle bathyal to outer neritic depths in the early turonian on the western atlantic margin biofacies daa represents disturbed benthic environments and is dominated by species of psammosphaera and reophax biostratigraphy and species composition of the studied biofacies are similar to previously described european and south atlantic assemblages major speciation events occur among the genera gavelinella lingulogavelinella and valvulineria well defined neritic and bathyal biofacies formed during a rapid late albian species diversification concomitant with the first major cretaceous flooding of continental margins

flexural constraints on the denudation of assymetric mountain belts the regional excavation of metamorphic rocks from mid and lower crustal levels is the consequence of erosion driven denudation of mountain belts that are in permanent isostatic equilibrium if the erosion process occurs at different rates in different parts of a topographically asymmetric mountain belt then the flexural properties of the lithosphere need to be considered in order to describe the features of the isostatic rebound such a situation is given in plateaulike mountain ranges which are likely to erode only at their steeply sloping edges whereas erosion on top stagnates this paper investigates the flexural isostatic response of a plate that is unloaded due to the continuous retreat of an erosional escarpment the isostatic response is tested for two limiting model cases 1 for a plate with a broken edge and 2 for a continuous plate with variable flexural rigidity it is shown that for both cases the rate of topography development on the exposed plate is not linear with the retreatrate of the escarpment this feature is utilized to predict the nature of the subsequent erosion of the newly formed topography and ultimately of the denudation of the plate geological limitations of the theoretical model and its relevance to plateaulike topographies in southern africa and the eastern highlands of australia are then discussed in these areas the origin of the apparently cyclic rebound forces creating stairlike topography at their continental margins and the old age of their present surface topography has previously been unexplained within the presented model these features can be explained satisfactorily two important conclusions may be drawn 1 the distance of flexural swells from great escarpments and the origin of subsequent stair like topographies may be explained by simple flexural considerations 2 the flexural rebound forces uplifting a plate exposed by a retreating escarpment are cyclic excavation times of metamorphic rocks under such mountain belts may therefore be prolonged not only by the nature of erosion of their initial topography but also by the nature of the subsequent isostatic rebound and ultimately their denudation history

isostatic response of the alula fartak and owen transforms in the eastern gulf of aden and the adjoining arabian sea summary the alula fartak and owen transforms are the active parts of major fracture zones with a distinct topographic expression in the eastern gulf of aden and the arabian sea respectively while the alula fartak transform offsets the sheba ridge by about 180 km and is associated with a broad steep sided valley with a relief of nearly 3 5 km the owen transform offsets the carlsberg ridge by nearly 300 km and is associated with a broad step like valley surrounded by deeper water depths the gravity and topography data along several profiles selected across these two transforms have been analysed using cross spectral analysis in order to investigate their isostatic compensation the observed admittance estimates have been compared with three theoretical isostatic compensation models two local compensation models airy i and ii and one regional compensation plate model comparing the longer wavelength admittance estimates suggests that the regional compensation model gives the best fit for both the alula fartak transform and the owen transform with effective elastic thickness te of 5 km and slightly less than 10 km respectively for the alula fartak transform the airy ii model might also be acceptable though with large scatter in the observed values it suggests a mean value of 9 km for the mantle layer with a 6 km thick crust for the owen transform on the other hand the two local compensation models failed the difference in te estimate between the two transforms could be ascribed to differences in thermal structure arising from their varied tectonic history a comparison with the isostatic response estimates of transform fracture zones along the slow spreading mid atlantic ridge suggests that the regional compensation model is generally applicable for transform fracture zone topography along such mid ocean ridges

continental break up along strike slip fault zones observations from the equatorial atlantic the study focuses on equatorial atlantic margins and draws from seismic well gravimetric and magnetic data combined with thermo mechanical numerical modelling our data and numerical modelling indicates that early drift along strike slip originated margins is frequently characterized by up to 10 20 spreading vector adjustments in combination with the warm thinned crust of the continental margin these adjustments control localized transpression our observations indicate that early drift margin slopes are too steep to hold sedimentary cover which results in their inability to develop a moderately steep slope undergoing cycles of gravitational instability resulting in cyclic gravity gliding these slopes either never develop such conditions or gain them at later development stages our modelling suggests that the continental margin undergoing strike slip controlled break up experiences warming due to thinning along pull apart basin systems pull apart basins eventually develop sea floor spreading ridges margins bounded by strike slip faults located among pull apart basins with these ridges first undergo cooling however spreading ridges leaving the break up trace along its strike eventually pass by these cooling margins warming them again before the final cooling proceeds as a result the structural highs surrounded by several source rock kitchens witness a sequential expulsion onset in different kitchens along the trajectory of spreading ridges

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

gravity signature and physical properties of copper deposit in the curac a valley northern bahia brazil case study this paper aims at analysing the application of the gravimetric method in the search for copper ore in the valley of curac a river in northern bahia brazil the area where this study was carried out is known as angico farm one of the claims of cara iba s a a copper producer in the northern bahia brazil there are 18 drill holes at the angico farm target drilled in order to investigate the mineralizations in depth we have obtained information such as geographic coordinates and chemical results from the company in order to test the geophysical response and the correlation with geology a 3d inverse gravimetric model was generated in order to verify the validity of the method in exploring for copper ore associated with hydrothermally altered mafic and ultramafic both mafic ultramafic rocks and copper ore present high density therefore the gravity method may not be effective for identification we have shown however that copper ore from the curac a valley presents a fairly good gravity response and 3d inverse mathematical model pointed out a well delimited copper orebody in the regions where drill holes intersect the ore and coincide with the positive gravity anomaly the ore contents were overlapped on cross sections of density extracted from the inverse model and such information helped us to check out the consistency of the gravimetric method in mapping and modelling mineralized bodies associated with mineral occurrence additionally magnetic susceptibility and gammaspectrometric data were acquired along 18 drillcores to investigate their possible correlation with orebodies

rifting process of the xihu depression east china sea basin abstract having a maximum depositional depth up to 15 km the ne trending xihu depression is evolved from a deep paleogene continental margin rifting along the present day east china sea continental shelf in this paper we use deep seismic reflection well logging and gravity data to synthesize early rifting process of the depression to study the regional tectonic subsidence history we developed a computer program that performs backstripping and simultaneous inversion for stretching factor initial lithospheric thickness and incipient age of thermal subsidence tectonic subsidence analyses of 40 industrial wells reveal nonuniform stretching preferential crustal extension along the depocenter but preferential mantle extension at the western flank uniform extension may have occurred only somewhere between the central inversion zone and the western margin in the early rifting stage subdued initial subsidence or initial uplift is observed on several wells near the western margin due possibly to flexural isostatic balance the initial mechanical lithospheric thicknesses from subsidence inversion seem to vary from about 55 km to almost 130 km suggesting significant longitudinal segmentation as well as transverse variation in the prerifting lithosphere it is likely that the southern part of the xihu depression was developed initially on a region of relatively thin lithosphere a statement conformable to geological evidences nw se trending deep seismic profiling clearly reveals the deep basement structure of the depression as well as the synrift and postrift sedimentary packages separated by a breakup unconformity crustal layer thicknesses are independently estimated from gravity modeling inversion tectonic subsidence analyses and deep seismic profiling respectively these methods all arrive at very small present day thicknesses of the crustal layer beneath the depocenter of the xihu depression

development history of the southern terminus of the central atlantic guyana suriname case study the study focuses on the offshore guyana suriname french guiana region it draws from seismic well gravimetric and magnetic data they indicate that the continental break up along the western margin of the demerara plateau took place during the callovian oxfordian associated with the central atlantic opening and accommodated by normal faults the continental break up in the se offshore guyana accommodated by strike slip faults was coeval the continental break up along the ne and eastern margins of the demerara plateau took place during the late aptian albian associated with the opening of the equatorial atlantic and accommodated by dextral strike slip and normal faults respectively different spreading vectors of the central and equatorial atlantic required development of the accommodation block during the late aptian albian paleocene in their contact region and in the region between the central atlantic and its southernmost portion represented by the offshore guyana block which were separated from each other by the opening equatorial atlantic its role was to accommodate for about 20 mismatch between the central and equatorial atlantic spreading vectors which has decreased from the late aptian albian to paleocene down to 0 differential movements between the central and equatorial atlantic oceans were also accommodated by strike slip faults of the guyana continental margin some active until the paleocene

opening the gateways for diatoms primes earth for antarctic glaciation the abrupt onset of antarctic glaciation during the eocene oligocene transition 33 7 ma oi1 is linked to declining atmospheric pco2 levels yet the mechanisms that forced pco2 decline remain elusive biogenic silicon cycling is inextricably linked to both long and short term carbon cycling through the diatoms siliceous walled autotrophs which today account for up to 40 of primary production it is hypothesised that during the late eocene a sharp rise in diatom abundance could have contributed to pco2 drawdown and global cooling by increasing the proportion of organic carbon buried in marine sediment diatom and sponge silicon isotope ratios 30si are here combined for the first time to reconstruct the late eocene early oligocene ocean silicon cycle and provide new insight into the role of diatom productivity in antarctic glaciation at odp site 1090 in the southern ocean a 0 6 rise in diatom 30si through the late eocene documents increasing diatom silicic acid utilisation with high near modern values attained by the earliest oligocene a concomitant 1 5 decline in sponge 30si at odp site 689 on the maud rise tracks an approximate doubling of intermediate depth silicic acid concentration in the high southern latitudes intermediate depth silicic acid concentration peaked at 31 5 ma coincident with the final establishment of a deepwater pathway through the tasman gateway and drake passage these results suggest that upwelling intensification related to the spin up of a circum antarctic current may have driven late eocene diatom proliferation organic carbon burial associated with higher diatom abundance and export provides a mechanism that can account for pco2 drawdown not only at but also prior to antarctic glaciation as required by a pco2 threshold mechanism for ice sheet growth

upper plate magma poor rifted margins stratigraphic architecture and structural evolution abstract although it is generally accepted that many distal magma poor rifted margins are asymmetric and can be divided into lower and upper plate margins little is known about the details of how and when this asymmetry evolves and how upper and lower plate margins can be distinguished this is due to the fact that most papers focused on the so called lower plate margins while the upper plate margins remained less well understood mainly due to the lack of public accessible drill hole data the aim of this paper is to provide a first order description of the global architecture and stratigraphic evolution of an upper plate magma poor rifted margin in order to provide such a template we focused on 2 seismic sections the ion 1000 line east indian margin and the screech 2 line newfoundland margin and describe key km scale outcrops from the fossil european margin exposed in the western central alps all of which document classical upper plate margins based on these data we show that upper plate magma poor rifted margins can be characterized by a staircase type architecture with terraces t 1 t 2 t 3 and ramps r 1 r 2 that result as a consequence of an evolution through a coupling exhumation and breakup stage we also defined key stratigraphic levels that we try to link with the evolution of the margin which enables us to link the tectonic evolution with the creation of accommodation space and formation of the staircase architecture that characterizes the upper plate margin from these observations we develop a conceptual model for the evolution of upper plate margins and discuss the applicability of this model for different strain rates rates of subsidence and sedimentation rates

the elastic properties of the lithosphere beneath scotian basin abstract to assess the possibility that the north atlantic ocean may subduct at scotian basin east of canada we investigate the present compensation state of this deep basin a fourier domain analysis of the bathymetry depth to basement and observed gravity anomalies over the oceanic area east of nova scotia indicates that the basin is not isostatically compensated moreover the analysis emphasizes that in addition to the sediments density perturbations exist beneath the basin the load produced by the sediments and these density perturbations must have been supported by the lithosphere we simulate the flexure of the lithosphere under this load by that of a thin elastic plate overlying an inviscid interior it is shown that a plate with a uniform rigidity does not adequately represent the lithosphere beneath the basin as well as the oceanic lithosphere far from the basin rather the rigidity of the lithosphere directly beneath the basin is about one to two orders of magnitude smaller than elsewhere we relate this weakening to the thermal blanketing effects of the thick sediments and the fact that the lithosphere has a temperature dependent rheology we suggest that this weak zone would have a controlling effect on the reactivation of normal faults at the hinge zone of the basin that were formed during the break up of africa and north america and were locked in the early stages after the break up the weak zone would facilitate reactivation of the faults if tensional stresses were produced by possible reorientation of the spreading direction of the north atlantic ocean in the future the reactivation of the faults would create a free boundary condition at the hinge zone allowing further bending of the lithosphere beneath the basin and juxtaposition of this lithosphere to the mantle beneath the continent this may provide a favorable situation for initiation of slow subduction due to subsequent compressional forces

total tectonic subsidence a parameter for distinguishing crust type at the u s atlantic continental margin total tectonic subsidence tts is the difference between the prerifting continental crust elevation and the present sediment unloaded basement depth in a sedimentary basin tts is calculated using observations of bathymetry and basement depth and assumptions about sediment density and the loading response of the lithosphere i use the extensional model for sedimentary basin formation to interpret the tts of a passive continental margin that model predicts that the tts on an old margin should define the major crust type boundaries and indicate the lateral distribution of extension within the extended continental crust tts analysis of the u s atlantic continental margin indicates that the boundary between extended continental and oceanic crusts lies along the east coast magnetic anomaly ecma to the north of the blake spur fracture zone bsfz and along the blake escarpment to the south there is no tts anomaly associated with the blake spur magnetic anomaly which seems to be underlain by normal oceanic crust the ecma appears to be associated with a tts minimum but i attribute that to artificially shallow estimates of depth to magnetic basement under the ecma tts estimates of extended continental crust thickness agree well with gravity models except where the problem with magnetic basement depth estimates interferes the amount of crust extension prior to seafloor spreading varies along the strike of the margin the most abrupt change is at the bsfz across the bsfz the amount of extension varies by 160 km but the volume of continental material does not vary the extension variation may be caused by lateral changes in the gross rheology of the continental lithosphere in the case of the bsfz it would be the suture of more ductile pre late paleozoic african craton with more brittle north american craton that could cause the abrupt change

sediment loading on the western platform of the new zealand continent implications for the strength of a continental margin abstract mechanical properties of the new zealand lithosphere are determined by forward modelling the gravity and isostatic response of a plio pleistocene sediment loaded shelf margin the giant foreset formation on the western platform of new zealand is a 2 km thick 250 km long and 100 km wide sediment package both the downward flexed pliocene miocene boundary beneath the package and a 35 50 mgal peak to trough gravity anomaly over the sediment package are best matched with a flexural rigidity in the region of 5 6 10 22 1 5 10 23 n m effective elastic thickness t e 20 28 km the giant foresets are a particularly important load to model because unlike most sediment loaded passive margins there is a time gap of about 70 80 ma between rifting and subsequent deposition of the sedimentary load furthermore the width of the giant foreset load is sufficiently small to afford a reasonable resolution of t e

effect of thermal contrasts on gravity modeling at passive margins results from the western barents sea the western barents sea passive margin is a key locality to demonstrate the effect of the thermal structure of the lithosphere on forward gravity modeling this margin developed by shear motion between the eurasian and greenland plates during the early tertiary and it is a significant border zone between young hot oceanic lithosphere and cooler continental lithosphere we construct two dimensional gravity models of 125 km thick lithosphere based on expansion of mantle rocks determined from thermal modeling the approach has a substantial impact over traditional shallow gravity models here demonstrated on a previously published model on the basis of a 140 mgal free air anomaly the old model proposes an anomalous high density oceanic crust emplaced in a leaky transform adjacent to the continent during early margin development however the lithospheric models predict a homogeneous oceanic crust while preserving regional isostasy at base lithosphere from continent to ocean two further tests agree with this conclusion a map of bouguer corrected ers 1 satellite data reveals no residual anomalies originating from the oceanic crust at the margin admittance analysis shows a strong oceanic lithosphere and the high coherence between bathymetry and free air gravity discounts a significant subsurface load the high gravity anomalies at the margin are thus an edge effect enhanced by sedimentation onto the strong oceanic lithosphere and shaped by the effect of the lithospheric thermal field other results of this work include a new continent ocean boundary map and two crustal transects across the margin

gravity anomalies crustal structure and flexure of the lithosphere at the baltimore canyon trough seismic and gravity anomaly data have been used to examine the long term mechanical properties of the lithosphere beneath the baltimore canyon trough a 15 km deep sedimentary basin in the rifted u s atlantic margin seismic data constrain the crustal structure at the time of rifting as well as the amount of sedimentation and erosion that has occurred the gravity effect of the initial crustal structure sedimentary loading and erosional unloading have been computed and compared to the observed free air gravity anomaly the best fitting model is one in which the elastic thickness of the lithosphere te changes at the east coast magnetic anomaly landward of the anomaly te is significantly smaller than previously reported values for continental or oceanic lithosphere of the same thermal age seaward of the anomaly however te follows the depth to the 450 c oceanic isotherm which also describes the response of oceanic lithosphere to seamount and oceanic island loads these results are interpreted as indicating that the shelf part of the trough is underlain by continental lithosphere which following rifting did not acquire any significant long term strength

transects of the ancient and modern continental margins of eastern canada abstract rocks of the west flank of the northern appalachian orogen miogeocline record the history of the late precambrian early paleozoic passive continental margin of eastern north america the ancient margin was destroyed by ophiolite obduction and arc collision during the ordovician taconic orogeny the present sinuous form of the miogeocline is interpreted to reflect ancient promontories and re entrants of a previous orthogonal margin bounded by rifts and transforms four major terranes are recognized east of the miogeocline in newfoundland and nova scotia from west to east these are the dunnage gander avalon and meguma the dunnage and gander terranes were linked to the miogeocline during the middle ordovician taconian orogeny the avalon terrane arrived later possibly during the mid paleozoic acadian orogeny the meguma terrane of southern nova scotia had docked with the avalon terrane by carboniferous time the dunnage terrane contains arc volcanics which lie above an ophiolitic substrate the gander terrane comprises a thick sequence of clastic sedimentary rocks underlain by basement rocks with continental affinities it has been interpreted as a continental margin perhaps once on the eastern side of the paleozoic iapetus ocean the avalon terrane consists of belts of sedimentary and volcanic rocks which are probably underlain by grenvillian basement its tectonic affinities are unclear the meguma terrane comprises a thick sequence of sediments derived from the south east it is found only in southeastern atlantic canada the boundaries between terranes are compressional in the west and steep transcurrent faults in the east the surface extent of the geological terranes is grossly correlative with deep structural zones although no direct evidence exists for linking the two because most surface structures can be traced geophysically to only a few kilometres depth a striking feature of the deep crustal structure is a lower high velocity crustal layer beneath the dunnage and gander terranes the modern margin of atlantic canada developed by rifting and by transform motion between adjacent continents stretching and thinning of the lithosphere and the consequent production of basaltic magma that in places intrudes or underplates the thinned continental crust are the most likely processes responsible for the evolution of the modern margin these processes predict the observed deep sedimentary basins along the margin the thinning of continental crust and the high seismic velocities found within the ocean continent transition zones rifting adjacent to nova scotia began in late triassic early jurassic time between the present african and north american plates these plate motions are also responsible for the major transform margin south of the grand banks separation between iberia and the eastern grand banks occurred in mid cretaceous time before the late cretaceous opening of the labrador sea while the rifted segments of the margin exhibit deep sedimentary basins and thinned continental crust the grand banks transform segment is characterized by a sharp transition zone and a relatively thin sediment cover numerous volcanic seamounts are built on the ocean crust adjacent to this transform segment mimicry of paleozoic promontories and re entrants by modern rift and transform margin segments the location of mesozoic sedimentary basins on ancestral appalachian structures and the reactivation and propagation of major precambrian and paleozoic structural boundaries during the latest phase of ocean opening attest to ancestral controls of the modern margins the rift phase of both the ancient and modern passive margins is characterized by volcanism mafic dike intrusion and by the development of basins filled with clastic sediments the drift phase of both the ancient margin and the present nova scotia margin is marked by a change in sedimentary environment such that carbonates replaced the rift phase clastic sediments two of the markers used to delineate the ancient ocean continent transition zone carbonate banks and steep gravity anomaly gradients should be used with caution as the modern analogs of these markers may lie 100 km or more of this transition zone furthermore it is naive to view the ancient transition as simple and narrow for the modern margins exhibits complex transition zones between 30 and 300 km wide in general the evolution of the ancient and modern passive margins appear to be remarkably similar predictably closing the present atlantic will mimic the evolution of the appalachian orogen

lithospheric domains and controls on kimberlite emplacement slave province canada evidence from elastic thickness and upper mantle composition 1 we have mapped the deep structure of the slave craton by combining analysis the effective elastic thickness te with data on mantle samples from numerous kimberlites three dimensional mapping of the subcontinental lithospheric mantle sclm using mantle derived xenoliths and xenocrysts in kimberlites has shown that much of the craton is underlain by a strongly layered sclm a highly depleted upper layer low in basaltic components ca al fe is separated from a relatively fertile lower layer by a sharp boundary this boundary lies at 140 150 km depth in the lac de gras area and shallows to 100 km in the northern and southern parts of the craton weak lithosphere te 56 km in the younger eastern part of the craton is separated from the older western part by a zone of steep te gradient parallel to the major locus of kimberlite intrusion which may map the deep extension of the boundary between the two domains another strong te gradient across the kilohigok basin accompanies a marked compositional change in the upper layer of the lithospheric mantle the basin probably marks a major translithospheric fault correlations between te and mantle composition suggest that te is strongly influenced by the rheology of the upper mantle

subsidence history crustal structure and thermal evolution of the valencia trough a young extensional basin in the western mediterranean the valencia trough is an extensional basin that began rifting during the late oligoceneearly miocene yet it is located within a region of compression between the european and african plates seismic reflection seismic refraction well gravity and geoid data have been used to determine the relative importance of extensional and compressional processes in controlling the structure and evolution of the trough although the trough appears symmetric on bathymetric maps seismic and gravity and geoid data indicate a fundamental asymmetry in its deep crustal and upper mantle structure two dimensional flexural backstripping techniques have been used to estimate the total amount of crustal stretching along a transect of the trough between the catalan coastal ranges and mallorca there is good agreement between the predicted depth to the mono based on a stretching model and the seismically constrained depth beneath the catalan margin but poor agreement beneath mallorca the seismic moho beneath mallorca can be explained by a model in which the crust was modified by the flexural effects of thrust fold loading geoid data suggest that the differences in the crustal structure between the two margins extend into the upper mantle the catalan margin is characterized by a thermal and mechanical boundary which marks the transition between cold unstretched lithosphere and hot thin lithosphere while mallorca is located above a broad region of lithospheric thinning the subsidence history and amount of crustal thinning in the catalan margin resemble that of typical atlantic type nonvolcanic margin however a narrow transition zone 60 km the lack of any evidence for a conjugate margin and the absence of oceanic crust suggest fundamental differences in the way that extensional processes may occur in convergent plate settings

a marine geophysical study of the wilkes land rifted continental margin antarctica summary the wilkes land margin of east antarctica conjugate to the southern australian margin is a non volcanic rifted margin that formed during the late cretaceous during 2000 01 and 2001 02 geoscience australia acquired 10 000 line km of seismic reflection and refraction magnetic anomaly and gravity anomaly data over the margin we have used the seismic data to estimate the sediment thickness along the margin the data reveal a deep 11 km rift basin that contains over 9 km of sediments seaward of the totten glacier western wilkes land the limits of oceanic and continental crust that underlies the thick post rift and the significantly thinner pre and syn rift sediments are equivocal seismic reflection data suggest a 30 100 km wide continent ocean transition zone cotz along the margin the cotz extends over 400 km seaward of the shelf break off the eastern wilkes land terre ad elie sector

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

early cretaceous motion of flemish cap with respect to north america implications on the formation of orphan basin and se flemish cap galicia bank conjugate margins bouguer gravity anomalies together with deep seismic reflection and magnetic data on both sides of the north atlantic are used to locate the hinge zones of the flemish cap and galicia bank within the iberian and north american plates regions across which there were abrupt changes in lithospheric extension the characteristic shape and alignment of these hinge zones suggest that they were conjugate features generated during chrons m25 m0 late jurassic early aptian around a distally located euler pole of rotation rifting between iberia and north america involved these two larger plates and the two smaller microplates the flemish cap and galicia bank microplates the motion of the microplates which were adjacent to eurasia was much more complex than those of the larger plates the motion between the microplates from chron m25 or older to chron m0 was complicated by the fact that they remained attached to each other for most of the time when regions to the south were rifting apart as a result continental regions landward of these segments were subjected to extension that created the orphan and flemish pass basins on the north american side and the galicia interior basin on the iberian side by comparing the hinge zones delineated off galicia bank and flemish cap using the bouguer anomalies we were able to infer that flemish cap rotated approximately 438 relative to galicia bank and iberia and moved 200 300 km se with respect to north america such motions of flemish cap and galicia bank agree remarkably well with extensional episodes deduced from industry multichannel seismic reflection data acquired in the orphan basin normal fault orientations identified in the west orphan basin trend n0208 and are approximately perpendicular to the flow lines of our proposed flemish cap north american motion during the m25 m0 period which provides an independent constraint on our proposed kinematic model therefore extensional events affected not only the galicia bank flemish cap conjugate margins but also the galicia interior and orphan basins and need to be taken into account in any assessment of the geological development of the iberian and north american continental margins during the intermargins workshop meeting modelling the extensional deformation of the lithosphere imedl held in pontresina in 2004 a benchmark exercise was proposed to model the crustal and mantle rifting processes between the western galicia bank and the se flemish cap conjugate margins where high quality seismic reflection and refraction data exist screech line 1 funck et al 2003 hopper et al 2004 and ise line 17 perez gussinye et al 2003 henning et al 2004 during the meeting it became apparent that additional boundary conditions and constraints were needed to correctly characterize the known extension across the iberian newfoundland margin the aim of this paper is to provide some of these conditions and constraints namely a knowledge of the initial positions of plates and microplates before their separation by sea floor spreading processes the positions of flemish cap and galicia bank are well constrained at chron m0 fig 1a because of the identification of correlatable sea floor magnetic anomalies in both the bay of biscay sibuet et al 2004b and in the north atlantic srivastava et al 2000 the resulting euler pole position was constrained by the geometry of the triple junction that had remained at the mouth of the bay of from karner g d manatschal g pinheiro l m eds imaging mapping and modelling continental lithosphere extension and breakup geological society london special publications 282 63 76 doi 10 1144 sp282 4 0305 8719 07 15 00 the geological society of london 2007 fig 1 a reconstruction of the north atlantic between africa af iberia ib and north america na at chron m0 with respect to eurasia eu as given by srivastava et al 2000 ib eu 43 858n 5 838w 244 768 af eu 43 618n 6 938w 240 898 na eu 69 678n 154 268e 23 178 m0 picks west of ib crosses and east of j c sibuet et al 64

crustal structure and the mechanical properties of extended continental lithosphere in the valencia trough western mediterranean seismic refraction and reflection gravity and well data have been used to constrain the structure and evolution of the valencia trough a young extensional basin in the western mediterranean seismic refraction data reveal the present day crustal structure while backstripping of seismic reflection and well data reveal the structure at the time of rifting by comparing the seismically constrained crustal structure to that inferred from backstripping we have been able to evaluate whether the moho is in the expected position given the tectonic subsidence uplift and sediment loading history of the trough the best overall fit between the predicted and seismically constrained crustal structure in the trough is for a model in which the extended continental crust and lithosphere has acquired little or no strength following rifting a low strength is in accord with the observed free air gravity anomaly as well as results from other types of rift basins such as the north sea and the continental margin off the east coast of the usa there are discrepancies between the observed and predicted moho depths beneath the flanks of the valencia trough but they are attributed to either the removal of crustal material during or following rifting iberian peninsula or to orogenic loading of previously extended continental crust mallorca the largest discrepancy up to 10 km correlates with the ebro delta which has prograded into the trough by more than 60 km since the pliocene the discrepancy cannot be explained by the same large scale processes that modified the moho beneath the trough flanks and requires that the lithosphere beneath the ebro delta region is associated with a high 60 t c

continuous sea floor spreading in red sea an alternative interpretation of magnetic anomaly pattern the magnetic anomaly pattern over the red sea can be modeled as a continuous system of sea floor spreading from the early miocene to the present by using a time varying process filter the half spreading rate is approximately 1 cm yr 0 4 in yr since initial rifting the parameters that determine the process filter and development of the transition zone are the intrusion parameter a measure of the dispersion of feeder dikes or horizontal strain about the rift axis a flow parameter a measure of the average flow width and the effusion parameter a measure of the volcanic effusion and thickness of layer 2 application of a time varying process filter appears to be a powerful tool in analyzing magnetic anomalies over passive continental margins we estimate the flow parameter to be 2 7 km 1 7 mi and the intrusion parameter to be 7 5 km 4 7 mi at early rifting these values suggest that a wide distribution of axial dikes or horizontal strain is the dominant factor in forming the magnetic anomaly pattern reduction in the width of the intrusion parameter and the effusion rate as rifting proceeded resulted in focusing of the strain thinning of layer 2 and formation of the red sea deeps the interpretation of the anomaly pattern implies that basement within the main trough of the red sea is composed of a thick accumulation of volcanic sills and flows deposited during early rifting extrusion of the lavas occurred over a wide up to 60 km or 37 mi zone of strain distributed about the rift axis the development of this thickened zone of igneous crust is the second of three successive stages of extension for the red sea basin these stages are as follows 1 an initial stage of crustal stretching or listric faulting in the oligocene accompanied by probable regional uplift 2 a period of high volcanic effusion accompanied by the emplacement of stratoid basalts the stratoid volcanics form overlapping flows and sills with intercalated sediments within the main trough the spreading center narrowed from the early miocene to the present 3 steady state sea floor spreading from pliocene to present south of a transitional zone near 22 s the process filter width derived from the magnetic anomaly pattern at earliest rifting can be used to estimate the lateral variation from igneous to continental type crust in a statistical sense our data suggest that the transition from 100 continental crust to 100 igneous or stratoid type crust should occur within an interval of approximately 32 km 20 mi our modeling suggests that phase 2 or the stratoid phase began about the time of anomaly 5c or chron c5c approximately 16 ma this age is compatible with geologic estimates of the initial rifting at the late oligocene to early miocene coleman 1974 gass 1977 the opening rate for africa arabia plate motion has remained relatively constant since early rifting although the african margin appears to be accreting faster than the arabian plate

gravity anomalies sediment loading and lithospheric flexure associated with the krishna godavari basin eastern continental margin of india abstract gravity anomalies constrained by seismic data have been used to examine the long term mechanical properties of the lithosphere beneath the krishna godavari basin a deltaic sedimentary basin on the eastern continental margin of india the gravity effect of the rift crust sedimentary loading and erosional unloading have been computed and the sum effect of these when compared with the observed gravity anomalies gives a best fitting te of 30 km at the margin the results suggest that the lithosphere which was weak during the early rifting stage has gained strength during sedimentation the onshore dipping doublet gravity anomaly at the margin reflects a weak rifted continental lithosphere abutting a strong oceanic lithosphere comparison with the results of similar investigations for the bengal basin in the north and cauvery basin in the south suggests increasing strength for the lithosphere from north to south

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

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

a two level concept of plate tectonics application to geodynamics abstract a two level plate tectonics concept is developed on the basis of data on lithosphere rheological stratification this approach differentiates between crust and subcrust plate ensembles separated by a lower crust viscoplastic asthenolayer similarly to classical plate tectonics three types of boundaries are distinguished in the lower layer which do not always coincide with crust plate boundaries especially for continents applications of this concept to geodynamics are considered and a corresponding quantitative analysis for several important processes is carried out a quantitative model of mountain formation and collision plateau origins is proposed also a geodynamic model of the evolution of passive margins taking into account a lower crust viscous flow is considered and its geological consequences are discussed a mechanism of rifting taking into consideration rheological lithospheric layering and its vertical movements caused by extension is developed both a qualitative scheme and quantitative analysis of the slow evolution of intracraton structures of shield basin type taking into account erosion and sedimentation processes are worked through also historical aspects of plate tectonics are discussed from the point of view of the proposed concept

rifting and subduction initiation history of the new caledonia trough southwest pacific constrained by process oriented gravity models summary the new caledonia trough is a bathymetric depression 200 300 km wide 2300 km long and 1 5 3 5 km deep between new caledonia and new zealand in and adjacent to the trough seismic stratigraphic units tied to wells include cretaceous rift sediments in faulted basins late cretaceous to eocene pelagic drape and 1 5 km thick oligocene to quaternary trough fill that was contemporaneous with tonga kermadec subduction a positive free air gravity anomaly of 30 mgal is spatially correlated with the axis of the trough we model the evolution of the new caledonia trough as a two stage process i trough formation in response to crustal thinning cretaceous and or eocene and ii post eocene trough fill sedimentation to best fit gravity data we find that the effective elastic thickness te of the lithosphere was low 5 10 km during phase i trough formation and high 20 40 km during phase ii sedimentation though we cannot rule out a fairly constant te of 10 km the inferred increase in te with time is consistent with thermal relaxation after cretaceous rifting but such a model is not in accord with all seismic stratigraphic interpretations if most of the new caledonia troughtopographywascreatedduringeoceneinceptionoftonga kermadecsubduction then our results place important constraints on the associated lower crustal detachment process and suggest that failure of the lithosphere did not allow elastic stresses to propagate regionally into the overriding plate we conclude that the gravity field places an important constraint on geodynamic models of tonga kermadec subduction initiation

passive margins and their subsidence three main types of subsidence dominate the vertical tectonics of rifted inter plume passive margins these are the isostatic response to 1 stretching and thinning of the crust and lithosphere syn rift stage 2 cooling and thickening of the lithosphere mainly post rift stage and 3 sediment loading both stages additionally vertical movements occur at plume hot spot margins as a result of the anomalously hot underlying upper mantle and emplacement and cooling of magma three examples demonstrate various features of subsidence at inter plume passive margins at the north biscay starved margin the subsidence history can be mainly explained by syn rift lithospheric stretching followed by post rift thermal subsidence at the us atlantic margin the much greater overall subsidence results from the additional effect of sediment loading the tucano gabon graben system bordering the south atlantic is an example of strongly asymmetrical extensional tectonics during the syn rift stage which produced deep rifted basins within the adjacent continental borderland

bathymetry depth to magnetic basement and sediment thickness estimates from aerogeophysical data over the western weddell basin we estimated the bathymetry and sediment thickness of a remote and difficult to access portion of the antarctic continental margin using aerogeophysical surveying techniques the u s argentina chile aerogeophysical survey collected magnetic and gravity data over the basins surrounding the antarctic peninsula thirty seven of these flight lines were used to estimate bathymetry and depth to magnetic basement for the western weddell basin a wavenumber technique was applied to individual magnetic anomaly profiles in an automated fashion to obtain estimates of the depth to magnetic basement the bathymetric estimates were obtained by admittance inversions of the gravity field the results were then gridded at a 40 km interval for the region 64 w 44 w 73 s and 62 s bathymetric estimates and depth to magnetic basement estimates were differenced at each grid point to obtain a regional estimate of the thickness of nonmagnetic overburden assumed to be sediment subsequent spot measurements of topography in the estimated region of the continental margin generally agree to about 52 m the estimated magnetic basement deepens from the antarctic peninsula margin eastward to a maximum of 10 12 km near 54 w we also postulate the existence of two moderately large basins flanking the eastward continuation of the jason peninsula farther east the basement steps upward with a correspondent thinning of the sedimentary layer along the east coast of the peninsula results agree well with seismic studies on james ross island and magnetotelluric studies on marambio island and the larsen nunatak as well as the british antarctic survey basement estimates from aeromagnetic data this study further demonstrates the utility of combined application of airborne and satellite geophysical techniques in the study of structure and tectonic evolution of continental margins and marine basins

temperature anomalies under the northeast atlantic rifted volcanic margins abstract subsidence analysis of odp dsdp drill sites located on oceanic crust on the southeast greenland edoras bank and voring plateau margins as well as on the iceland faeroe ridge shows that the subsidence of these areas does not follow the age relationship of normal oceanic crust by correcting for the effect of thickened oceanic crust in raising the level to which subsidence will occur and analyzing the rate of thermal subsidence it is possible to provide maximum temperature estimates for the underlying asthenosphere through time by identifying periods of anomalous depth to basement isostatic models predict crustal thicknesses of 27 km under the iceland faeoe ridge around 20 km at edoras bank and southeast greenland and 16 17 km at the voring plateau asthenospheric temperatures at the time of continental break up range from 50 c to 100 c above normal mantle which are insufficient to account for the crustal thicknesses if melting is purely a passive adiabatic process asthenospheric upwelling must thus have been more rapid than spreading following break up at edoras bank the thermal anomaly dissipated within 5 myr of rifting similar to that inferred from the eastern us margin where no plume is considered to have affected the rifting process the need to invoke thermal input from the iceland plume in generating the thickened crust at edoras bank and possibly elsewhere in the northeast atlantic is called into question however a 14 20 myr anomaly peaking at 12 myr post rift in southeast greenland suggests that although the plume did provide heat to this margin its strongest influence post dated break up

crustal velocity structure across the eastern snake river plain and the yellowstone swell teleseismic receiver functions are used to estimate the crustal structure beneath a 36 station 500 km long nw oriented linear array centered on the eastern snake river plain and crossing the yellowstone hotspot swell 250 km sw of yellowstone structure derived previously for this region from wide angle reflection data is used as an initial model and this structure explains most features observed in our receiver functions based on a combination of forward and inverse modeling our data require several modifications to the initial structure 1 moho depth is 42 km beneath most of the snake river plain shallows to 37 km to either side and thickens abruptly to 47 km beneath sw wyoming 2 a midcrustal layer interpreted previously as a 9 km thick gabbroic sill is flat topped across the entire 90 km width of the snake river plain and 3 a low velocity layer is found beneath and southeast of the snake river plain which probably is partially molten lower most crust using the seismic structure of the crust to estimate the crustal load upon the mantle and assuming local isostasy we calculate that mantle beneath the yellowstone swell is approximately uniformly as buoyant as 12 million year old ocean mantle and more buoyant than the adjacent wyoming mantle by an amount equivalent of 1 5 km of elevation the transition between these regions of greatly different mantle occurs across a major paleozoic boundary that now separates the basin and range from the rocky mountains

nanogeochemistry geochemical reactions and mass transfers in nanopores nanopores are ubiquitous in porous geologic media and may account for 90 of total mineral surface areas surface chemistry ion sorption and the related geochemical reactions within nanopores can be significantly modified by a nanometer scale space confinement as the pore size is reduced to a few nanometers the difference between surface acidity constants pk pk2 pk1 decreases giving rise to a higher surface charge density on a nanopore surface than that on an unconfined mineral water interface the change in surface acidity constants results in a shift of ion sorption edges and enhances ion sorption on nanopore surfaces also the water activity in a nanopore is greatly reduced thus increasing the tendency for inner sphere complexation and mineral precipitation all these effects combine to preferentially enrich trace elements in nanopores as observed in both field and laboratory studies the work reported here sheds new light on such fundamental geochemical issues as the irreversibility of ion sorption and desorption the bioavailability of subsurface contaminants and the enrichment of trace metals in ore deposits as well as the kinetics of mineral dissolution and or precipitation

rigidity of the atlantic oceanic lithosphere beneath new england seamounts abstract investigation of the flexural response of the oceanic plate subject to loading of new england seamounts provides an insight into the rigidity of the lithosphere fourier domain analysis of the bathymetry and gravity anomalies over new england seamounts suggests that internal density perturbations exist in the crust and or uppermost mantle the loads due to the topography of the seamounts and density perturbations must have been supported by the lithosphere we group the seamounts into three representative ages and adopt a 3 d thin elastic plate model over an invicid mantle to assess the rigidity of the lithosphere under these loads the rigidity values are about 3 7 10 23 n m for lithospheric ages ranging from 30 to 60 my the base of the elastic part of the lithosphere follows an isotherm in the range of 750 850 c suggesting a strong lithosphere that makes the north atlantic ocean difficult to initiate subduction however the thermal blanketing by the thick sediments at scotian basin reduces the rigidity by one to two orders of magnitude and may facilitate initiation of subduction of the north atlantic oceanic lithosphere at scotian margin

basin forming processes and the deep structure of the campos basin offshore brazil abstract this paper discusses a worldwide dataset of deep seismic reflection profiles that help the understanding of crustal behaviour and the genetic processes that are involved in the formation of sedimentary basins the events observed in seismic reflection data at lower crustal depths correspond to impedance contrasts associated with one or a combination of the following factors shear zones thin layers of intrusive rocks underplated material gneissic banding fluids and heterogeneities in the crust and mantle several types of sedimentary basins have been recognized with the aid of deep seismic reflection data basins formed along active master faults basins formed along inactive faults basins with no fault control basins formed by low angle detachment faults and basins formed by pervasive pure shear or an approximately pure shear where the lower crust has been locally extended by a different amount than the upper crust the mechanisms of basin forming processes recognized in deep seismic profiles are a simple shear along low angle detachment faults extending to the lower crust and maybe the upper mantle b thinning of the lower crust and mantle by ductile processes convective mechanisms or flow in the mantle and the upper crust by faulting resulting in an overall pure shear and c thinning of the lower crust associated with phase changes in the mantle or non conservation of mass two end member models of basin formation involving lithospheric stretching may be recognized in the south atlantic simple shear stretching where only the upper crust is involved in the process and mantle uplift is offset relative to the basin and pure shear stretching where uplift of the mantle vertically balances the basin fill the first mechanism has been suggested for the onshore tucano basin the data obtained for the offshore campos basin suggests regional lithospheric stretching and crustal thinning with moho uplift compensating for sediment accumulation in the basin depocentre complex relationships between sediment accumulation and crustal thinning are obtained in the western limit of the campos basin

ups and downs of the mississippi delta during the last glacial period when sea level was low meltwater discharge drove incision of the lower mississippi valley with valley filling and delta construction during holocene sea level rise isostatic modeling shows that sediment volumes removed and replaced were sufficient to induce uplift of 9 m along valley margins followed by subsidence of the same magnitude with effects dissipating only over distances of 100 150 km along the gulf of mexico coast recognition of cyclical uplift and subsidence refutes recent interpretations of delta stability and suggests that late holocene relative sea level curves from the delta region are instead a record of subsidence of the pre holocene depocenter more broadly incised valley cutting and filling is a common fluvial response to glacioeustasy and cyclical uplift and subsidence should be common to large alluvial deltaic systems elsewhere

uplift history of a transform continental margin revealed by the stratigraphic record the case of the agulhas transform margin along the southern african plateau the south and southeast coast of southern africa from 28 s to 33 s forms a high elevated transform passive margin bounded to the east by the agulhas falkland fracture zone affz we analysed the stratigraphic record of the outeniqua and durban thekwini basins located on the african side of the affz to determine the evolution of these margins from the rifting stage to present day the goal was to reconstruct the strike slip evolution of the agulhas margin and the uplift of the inland high elevation south african plateau the agulhas transform passive margin results from four successive stages 1 rifting stage from late triassic to early cretaceous 200 134 ma punctuated by three successive rifting episodes related to the gondwana breakup 2 wrench stage 134 131 ma evidenced by strike and dip slip deformations increasing toward the affz 3 active transform margin stage 131 92 ma during which the falkland malvinas plateau drifts away along the affz with an uplift of the northeastern part of the outeniqua basin progressively migrating toward the west 4 thermal subsidence stage 92 0 ma marked by a major change in the configuration of the margin onset of the shelf break passive margin morphology two main periods of uplift were documented during the thermal subsidence stage of the agulhas margin 1 a 92 ma short lived margin scale uplift followed by a second one at 76 ma located along the outeniqua basin and 2 a long lasting uplift from 40 to 15 ma limited to the durban thekwini basin this suggests that the south african plateau is an old upper cretaceous relief 90 70 ma reactivated during late eocene to early miocene times 40 15 ma

an application of the response function technique to profiles of bathymetry and gravity in the gulf of aden summary the relationship between bathymetry and gravity in the eastern and the western gulf of aden is analysed by computing response functions the form of the response functions indicates that the short wavelength topography is isostatically compensated the most appropriate mechanism of compensation is one in which the topography is not locally compensated but regionally compensated by flexure of the lithosphere though the sets of profiles from the eastern and the western gulf of aden are separated by only 500 750 km the observed response functions are very different the lithosphere in the eastern gulf of aden can be modelled by an elastic plate 15 3 km thick whereas an elastic plate 3 2km thick is probably an appropriate model for the lithosphere in the western gulf of aden a different mechanism of isostasy the pratt operates in the compensation of the long wavelength topography of the mid ocean ridge in the gulf of aden a larger negative broad bouguer gravity anomaly across the western gulf of aden than across the east indicates that upper mantle temperatures increase from east to west the long wavelength shallowing of the gulf of aden and increasing elevation of the adjacent continents from east to west are therefore probably mainly the result of thermal uplift the subdued nature of the median valley and consequently other short wave length topography in the west can be explained by hot low viscosity asthenosphere and the inferred thin lithosphere in the west also suggests high temperature mantle finally a magnetic anomaly superimposed on the axial anomaly which is probably related to the depth of the curie temperature is more common in the western gulf of aden than in the east and this is also consistent with upper mantle temperatures increasing to the west there are a number of similarities between the mid ocean ridge in the western gulf of aden and the reykjanes ridge it is suggested that the western gulf of aden is anomalous because of its proximity to the afar hot spot in the same way that the reykjanes ridge is often considered to be anomalous because of its proximity to the iceland hot spot

thermomechanical evolution of the south alpine rifted margin north italy constraints on the strength of passive continental margins abstract we have reconstructed the kinematic thermal and rheological evolution of the triassic cretaceous south alpine passive continental margin northern italy and estimated lateral strength variations associated with rifting rifting initially caused the formation of a roughly symmetrical ca 100 km wide graben located 150 200 km away from the future break up site 30 myr after the onset of rifting the rift axis shifted laterally and eventually led to oceanic crust formation the thermal evolution of the margin was first controlled by the waning of a thermal anomaly which was emplaced shortly before rifting and was cooling during the first extensional stages numerical modelling of rifting and drifting constrained by field data is used to trace thermal and rheological changes affecting the rifted margin the effects of the thermal anomaly are restricted to the first 10 20 myr of rifting subsequently isotherms are slightly affected by lithospheric thinning and show very little distortion after break up rheological modelling demonstrates that the south alpine lithosphere preserved substantial strength even shortly before break up and became stronger thereafter the modelling also shows that at break up and 100 myr later thinned domains of the margin are stronger than the less thinned ones

stress orientations in brazilian sedimentary basins from breakout analysis implications for force models in the south american plate summary regional patterns of crustal stresses in brazil were studied with a detailed breakout analysis performed in 541 wells distributed throughout the country 481 from basins along the continental margin and 60 from intracratonic basins a total of about 591 km of cumulative well length digitally sampled at 0 15 m intervals was analysed most intervals 80 per cent are confined between 400 and 4000 m only wells that deviate less than 10d from the vertical were considered for a given well 1 the mean orientation and the standard deviation of each interval with breakouts were calculated 2 all intervals with mean breakout orientations less than 10d from the orientation of the hole deviation were discarded 3 all intervals with standard deviations less than 12d were selected 4 the selected breakouts were weighted by their interval length to calculate the mean orientation and standard deviation of the whole well and 5 the final results were classified according to the world stress map criteria thus 16 wells were classified as a a total of 7 2 km of breakouts 42 as b 7 1 km 78 as c 7 4 km 205 as d 3 3 km 184 as e 0 7 km and 16 were discarded in most basins the breakout orientations from different wells were usually consistent for qualities a b and c allowing a good estimate of the regional maximum horizontal stress shmax wells classified as quality d showed a large scatter the regional shmax determined from breakouts is generally in good agreement with the available nearby focal mechanisms the main trends of shmax are 1 nw se parallel to the coast along the equatorial marginal basins 2 e w in the alagoas basin 3 nne ssw parallel to the coast in the sergipe and reconcavo basins and 4 nw se in the middle amazon basin in the equatorial and eastern continental margins north of 15ds breakouts and focal mechanism measurements indicate that the shmax orientation is remarkably parallel to the coastline following a 90d bend of the coast in northeastern brazil because the theoretically predicted intraplate stresses in northern brazil trend about wnw to ene e g meijer 1995 coblentz richardson 1996 we interpret our observations as indicating that local sources of stress at the continental margins e g flexural stresses and lateral density contrasts dominate the plate wide stresses and may have been underestimated in theoretical stress models of the south american plate the new stress data 136 a c quality out of a population of 541 wells cover a region of mid plate south america where only 23 data points had been compiled previously in the world stress map database the more detailed observed patterns of the intraplate stress field should be helpful in better constraining future models of the forces driving the south american plate

paleogene stratigraphy and sea level history of the north carolina coastal plain global coastal onlap and tectonics abstract coastal onlap of paleogene depositional sequences mapped by stage and compared to curves of global coastal onlap and eustasy facilitate differentiation of the effects of local uplift and subsidence from eustasy on the onslow and albemarle blocks about the neuse hinge in north carolina differential uplift and subsidence of these blocks has controlled the stratal geometries and patterns of relative coastal onlap on each block potential mechanisms to produce the uplift and subsidence include episodic flexural deformation resulting from sediment loading in the salisbury embayment and horizontal compressional deformation resulting from plate motion danian sea level rise produced the initial paleogene siliciclastic sequences ta1 2 and ta1 3 on both blocks after deposition of the danian sequences and prior to thanetian inundation the onslow block was elevated relative to the albemarle block the ypresian ta2 5 2 9 one is generally restricted to the downdip albemarle block has a depositional updip limit and represents a major basinward shift in local coastal onlap at a time when global coastal onlap and long term eustatic curves predict a major landward shift this suggests that both blocks were uplifted during the ypresian lutetian bartonian sequences ta3 3 ta3 5 3 6 represent the most extensive landward shift in coastal onlap during the paleogene in north carolina and show erosional updip limits on both blocks differences in the degree of overstepping on the two blocks reflects post middle eocene uplift but greater relative uplift of the albemarle block the priabonian ta4 1 and either the ta4 2 or ta4 3 has erosional updip limits on both blocks and represents a significant basinward shift in local coastal onlap this shift occurs during a time when global coastal onlap and long term eustasy indicate a landward shift as great as that during the lutetian bartonian therefore both blocks were uplifted but the distribution of the priabonian on the albemarle block indicates greater relative uplift in north carolina the rupelian ta4 4 oversteps the priabonian on both blocks but not as far as predicted by global coastal onlap and eustasy based on the offlapping relationship of the erosional updip limit on both blocks both areas were uplifted prior to rupelian deposition chattian sediments tb1 1 1 4 lower part reflect a downward shift in coastal onlap which is consistent with global coastal onlap and long term eustatic fall

thermal subsidence and sedimentary processes in the south china sea basin abstract we coupled analysis of regional reflection seismic data with core data from international ocean discovery program iodp expedition 349 and found differences in both depositional features and calculated subsidence between the east and southwest subbasins we calculated the original decompacted thicknesses of sediment for different time slices the depositional features of the basin show that the sedimentation is controlled by both climate and tectonics decoupling of sedimentation rates and fluxes is often observed between the continental margin and deep basin an abrupt increase in sedimentation rates since the pliocene suggests that glacial interglacial climate variability impacted erosion rates the sedimentation rate peaked during the pliocene in the southwest subbasin but peaked during the quaternary in the east subbasin showing strong influence of surrounding tectonic events such as the buildup of taiwan orogen on erosion rates we examine thermal subsidence of the two subbasins by fitting the isostatically balanced basement depths from the seismic profiles with the half space cooling model the estimated thermal diffusivity of the lithosphere in the southwest subbasin is much higher than in the east subbasin these contrasting observations suggest that lithospheric rock compositions may contribute to different tectonic subsidence curves between the east and the southwest subbasins

wavelet and multitaper coherence methods for assessing the elastic thickness of the irish atlantic margin summary there have been some inconsistencies in estimates of the effective elastic thickness of the continental lithosphere te based upon admittance or coherence relationships between gravity and topography this paper compares multitaper and wavelet methods to analyse the coherence between bouguer gravity and bathymetric data over the irish atlantic margin the analyses show that similar lateral te variations can be recovered from the data but demonstrate that the size of the data window can give rise to a significant downward bias in te estimates a seismically constrained 3 d gravity inversion over the rockall basin shows the presence of surface and subsurface loads whose ratio is loosely correlated with load ratio variations generated from the wavelet coherence method the te and load ratio f variations can be plausibly related to major geological structures on the margin if the load ratio variations can be interpreted geologically it implies that spectral based methods to estimate effective elastic thickness must incorporate subsurface loads within the underlying theoretical model on the irish atlantic margin te is generally low 6 18 km and is associated with a ne sw caledonian trend the weakest lithosphere is in the southern rockall basin porcupine bank and porcupine basin and the strongest lithosphere is along the rockall hatton region the low te values are consistent with results from other passive margins the reasons for such low te values on the irish atlantic margin remain unclear but may be the consequence of te being frozen into the lithosphere when loads were emplaced during continental breakup and temperature gradients were high the process of sedimentation and the presence of fluids may be contributory factors there is an indication of a geological and rheological divide between the rockall hatton region and the rockall basin possibly associated with the caledonian orogenic front

tectonic development of the mid norway continental margin abstract the area reviewed covers the mid norway continental margin between latitudes 62 n and 68 n main structural elements as defined at the base cretaceous level are the trondelag platform underlying the inner shelf the more and voring basins located beneath the outer shelf and slope and the more platform and the outer voring plateau forming a base of slope trend of highs sediments contained in the mid norway basin range in age from late palaeozoic to cenozoic the basement was consolidated during the caledonian orogenic cycle devonian and early carboniferous wrench movements along the axis of the arctic north atlantic caledonides are thought to have preceded the namurian onset of crustal extension rifting processes were intermittently active for some 270 my until crustal separation between greenland and fennoscandia was achieved during the early eocene during the evolution of the norwegian greenland sea rift system a stepwise concentration of tectonic activities to its axial zone the area of subsequent continental separation is observed during the late palaeozoic to mid jurassic a broad zone was affected by tensional faulting during the late jurassic and cretaceous the trondelag platform was little affected by faulting whilst major rift systems in the more and voring basins subsided rapidly and their shoulders became concomitantly upwarped during the latest cretaceous and early palaeogene terminal rifting phase only the western more and voring basins were affected by intrusive and extrusive igneous activity following the early eocene crustal separation and the onset of sea floor spreading in the norwegian greenland sea the voring segment of the mid norway marginal basin subsided less rapidly than the more segment during the early and mid tertiary minor compressional deformations affected the voring basin and to a lesser degree the more basin tensional forces dominated the late palaeozoic to early cenozoic evolution of the mid norway basin and effected strain mainly in the area where the crust was weakened by the previous lateral displacements the lithosphere thinned progressively and the effects of the passively upwelling hot asthenospheric material became more pronounced massive dyke invasion of the thinned crust preceded its rupture

geoid and gravity anomaly data of conjugate regions of bay of bengal and enderby basin new constraints on breakup and early spreading history between india and antarctica 1 timing of breakup of the indian continent from eastern gondwanaland and evolution of the lithosphere in the bay of bengal still remain as ambiguous issues geoid and free air gravity data of bay of bengal and enderby basin are integrated with shipborne geophysical data to investigate the early evolution of the eastern indian ocean geoid and gravity data of the bay of bengal reveal five n36 w fracture zones fzs and five isolated ne sw structural rises between the eastern continental margin of india ecmi and the 85 e ridge 86 e fz the fzs meet the 86 e fz at an angle of 39 the rises are associated with low gravity and geoid anomalies and are oriented nearly orthogonal to the fzs trend the geoid and gravity data of the western enderby basin reveal a major kerguelen fz and five n4 e fzs the fzs discretely converge to the kerguelen fz at an angle of 37 we interpret the fzs identified in bay of bengal and western enderby basin as conjugate fzs that trace the early cretaceous rifting of south ecmi from enderby land structural rises between the fzs of bay of bengal may either represent fossil ridge segments possibly have extinct during the early evolution of the bay of bengal lithosphere or may have formed later by the volcanic activity accreted the 85 e ridge two different gravity signatures short wavelength high amplitude negative gravity anomaly and relatively broader low amplitude negative gravity anomaly are observed on south and north segments of the ecmi respectively the location of continent ocean boundary cob is at relatively far distance 100 200 km from the coastline on north ecmi than that 50 100 km on the south segment on the basis of geoid gravity and seismic character and orientation of conjugate fzs in bay of bengal and western enderby basin we believe that transform motion occurred between south ecmi and enderby land at the time of breakup which might have facilitated the rifting process in the north between combined north ecmi elan bank and macrobertson land and in the south between southwest sri lanka and gunnerus ridge region of east antarctica approximately during the period between the anomalies m1 and m0 and soon after detachment of the elan bank from north ecmi the rifting process possibly had reorganized in order to establish the process along the entire eastern margins of india and sri lanka

passive margin evolution initiation of subduction and the wilson cycle we have constructed finite element models at various stages of passive margin evolution in which we have incorporated the system of forces acting on the margin depth dependent rheological properties and lateral variations across the margin we have studied the interrelations between age dependent forces geometry and rheology to decipher their net effect on the state of stress at passive margins lithospheric flexure induced by sediment loading dominates the state of stress at passive margins this study has shown that if after a short evolution of the margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favourable for transformation into an active margin although much geological evidence is available in support of the key role small ocean basins play in orogeny and ophiolite emplacement evolutionary frameworks of the wilson cycle usually are cast in terms of opening and closing of wide ocean basins we propose a more limited role for large oceans in the wilson cycle concept