extent of oceanic crust in the labrador sea1 this paper examines the mode of formation of the northern margins of the labrador sea observations of faulted and rotated crustal blocks across the northwest margin of greenland had led to the suggestion that this region is underlain by extended continental crust a detailed analysis of all geophysical data from this region presented here suggests on the contrary the possibility that it could have been formed by an excessively slow seafloor spreading process and ridge axis propagation this gave rise to rotated fault blocks at a time when spreading was mainly confined to the greenland side prior to its shift to the west supporting evidence for such a reconfiguration of the spreading axis comes from gravity and magnetic observations of this and neighbouring regions modelling of magnetic anomalies in the area based on the average rate of spreading for the entire labrador sea shows that a gradual shift of the ridge axis to the west started after the formation of anomaly 32 because the earliest seafloor spreading anomalies are highly attenuated in amplitudes other models like extended and intruded continental crust are possible if one only considers the magnetic anomalies however an extended continental crustal model poses problems in accounting for other geophysical observations based on a slow spreading model it is shown that oceanic crust extends fairly close to the foot of the slope with a narrow belt of extended continental crust along the west greenland margin while a much wider zone of perhaps attenuated continental crust extends off labrador breakup of the continental landmass occurred much closer to the west greenland coast than to labrador the applicability of a slow spreading model to other passive margins where similar geophysical observations have been reported is discussed

seismic stratigraphic record of transpression and uplift on the romanche transform margin offshore ghana abstract recently available multi channel seismic reflection data from offshore ghana have been reprocessed to probe the eastern romanche fracture zone rfz of the equatorial atlantic and the sedimentary basins of the paleotransform margin the rfz terminates landward in a submarine canyon up to 2 km deep and 50 km wide variably filled with submarine fans and projects landward into a neoproterozoic dextral shear zone by contrast the rfz to the southwest is represented by an ene submarine escarpment juxtaposing continental and oceanic crusts three major sedimentary sequences representing pre syn and post rift strata have been identified from the seismic profiles and calibrated with chronostratigraphic data from oil exploration wells the pre rift sequence consists largely of paleozoic strata ranging in age from devonian to carboniferous the overlying syn rift sequence which was deposited during the intracontinental shearing stage consists of aptian to albian silicilastic strata with distinct continental facies the seismic sections provide some of the clearest images yet of i folding associated with transform displacement and ii southwest increasing subaerial erosion and stratal discordance we attribute lateral variation in erosion to crustal thickening associated with folding and to transient thermal effect of ridge transform interaction which is predicted to produce westward increasing uplift on this margin

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

parameterization of stream channel geometry in the distributed modeling of catchment dynamics a simple and efficient procedure for incorporating the effects of stream channel geometry in the distributed modeling of catchment dynamics is developed at a station and downstream fluvial relationships are combined and the obtained laws of variability in space and time for water surface width and wetted perimeter are incorporated into a diffusion wave routing model based on the muskingum cunge method with variable parameters the parameterization obtained is applied to the approximately 840 km 2 sieve catchment central italian apennines to test the possibility of estimating channel geometry parameters from cross section surveys and to assess the impact of dynamic variations in the channel geometry on catchment dynamics the use of the estimated channel geometry in surface runoff routing produces a significant improvement in the flood hydrograph description at the catchment outlet with respect to less detailed network parameterizations in addition the results obtained from a downstream analysis of the velocity field indicate that the stream characteristics related to the locally varying cross section shape may have a strong control on flow velocities and thus they should be monitored and synthesized for a comprehensive description of the distributed catchment dynamics

evidence for transform margin evolution from the ivory coast ghana continental margin the relative motions between lithospheric plates can be reduced to three major types convergence divergence and transform motion convergence leads to an active margin and eventual collision divergence results in the construction of a rifted passive margin transform motion generates a transform or sheared margin although passive margins have been extensively studied1 2 and many models have been proposed for their origin and subsequent evolution3 5 little is known about transform margins with the exception of a few studies of their morphologies6 7 shallow structures or crustal sections8 9 here we present the main conclusions derived from a recent study of the northern gulf of guinea margins particularly off the eastern ivory coast and ghana where the continental margin is one of the best preserved examples of an extinct transform margin10 12 our observations support a four stage model for transform margin evolution technically active transform contacts first between normal continental crusts and then between thinned margins induce characteristic structures such as pull apart grabens and shear folds the next stage in which thermal exchange between oceanic and continental lithos pheres controls a complex subsidence is followed by the transition to a true intra oceanic fracture zone

crustal taper and topography at passive continental margins terra nova 23 349 361 2011 abstract the long term evolution of kilometre s high seaward facing escarpments at passive margins is linked directly to the crustal thinning gradient the development of post rift faults in the onshore parts of the margin and the associated distribution of drainage patterns landscape types and sediment dispersal patterns reflect this linkage for the seismically well imaged norwegian margin as well as for a number of passive margins worldwide we identify a scaling relationship that correlates escarpment height with the distance to the highly extended distal margin or more specifically the taper break the highest escarpments and the most asymmetric margin topography are found inboard of sharply tapering crystalline crust independent of margin age conversely escarpments are lower where the taper is gentle thus the topography of passive margins does not primarily reflect age magmatism climate or mantle convection but more probably the response to loading of the laterally variable fault controlled architecture of the crystalline crust

long term evolution of the west african transform margin estimates of denudation from benin using apatite thermochronology the benin continental margin was formed during the breakup of gondwana through oblique rifting along transform faults the evolution of topography following breakup directly affects the evolution of sedimentary basins which has major implications for hydrocarbon exploration in the region quantitative constraints on erosion across benin are limited to the cenozoic based on analysis of dissected lateritic palaeolandscapes to resolve the mesozoic erosion history we have obtained apatite fission track and single grain u th sm he data from 18 samples collected across a 600 km long transect through benin we invert these data including available geological and geomorphological constraints to obtain time temperature paths which are used to estimate magnitudes of denudation over the last 200 myr our study suggests that denudation was focused over a c 300 km long seaward sloping limb of the marginal upwarp and at the southern margin of the interior iullemmeden basin from 140 to 100 ma with lower magnitudes of denudation characterizing the continental interior and post cretaceous evolution of the margin models are consistent with modest burial c 1 km of the iullemmeden basin between 120 and 85 ma and of the continental margin between 85 and 45 ma by the eocene the first order relief of benin had developed with regional erosion rates supplementary information full details of the analytical data and modelling results including the various constraints and further data on denudation and burial magnitudes and rates are available at https doi org 10 6084 m9 figshare c 4220804

continent ocean transition at the western barents sea svalbard continental margin the change in crustal type at the western barents sea svalbard margin takes place over a narrow zone related to primary rift and shear structures reflecting the stepwise opening of the greenland sea regionally the margin is composed of two large shear zones and a central rifted margin segment local transtension and transpression at the plate boundary caused the early cenozoic tectonism in svalbard and the western barents sea and might explain the prominent marginal gravity and velocity anomalies

simplified method for the characterization of the hydrograph following a sudden partial dam break this paper presents a simplified approach to the characterization of the hydrograph following the partial collapse of concrete gravity dams the proposed approach uses a simplified representation of the reservoir geometry and is based on the numerical solution of shallow water equations to study the two dimensional evolution of the water surface within the reservoir the numerical results are made dimensionless and reorganized so as to compute the peak discharge the duration and the recession limb of the dam break hydrograph the proposed practical approach provides a quite satisfactory reproduction of the computed hydrograph for a wide set of realistic situations that have been simulated in detail

rifting assisted by shear heating and formation of the lomonosov ridge abstract we suggest that thermo mechanical feedback between shear heating and temperature dependent viscosity in continental zones of oblique extension has the potential to significantly weaken the lithosphere and facilitate subsequent rapid breakup we perform thermo mechanical experiments of strike slip and coeval but longer lasting extension using the finite element method the style of rifting in our model depends mainly on the efficiency of shear heating which in turn depends on the magnitude of lithospheric stresses and strain rates we develop and apply a numerical model to explain the formation of the lomonosov ridge microcontinent our proposed model provides a physically consistent mechanism for the detachment of the lomonosov ridge from the barents kara sea margin at 68 56 ma the narrow rift mode predicted by the numerical model is supported by the crustal structure of conjugate passive margins of the eurasia basin

crustal structure of a nonvolcanic rifted margin east of newfoundland a detailed seismic refraction study was undertaken of the rifted continental margin southeast of the grand banks of newfoundland along a preexisting deep multichannel seismic reflection profile the primary objectives were to determine the topography of moho across the margin to detect any evidence for crustal underplating and to determine the position of the continent ocean boundary data were obtained from nine ocean bottom seismometers deployed along the profile with an array of six 1000 inch3 air guns as the source two additional refraction lines were shot parallel to structure over the continental slope the resultant velocity model is well constrained and consistent with the observed gravity the crust thins abruptly beneath the continental slope to a major structural transition identified as the continent ocean boundary there is no significant magmatic underplating of the thinned continental crust seaward of the transition the seismic velocity structure is characterized by a 50 km wide zone of intermediate velocity 7 2 7 6 km s beyond this is a thin 3 5 km crust with low velocity 4 5 5 km s and high gradient over mantle with a velocity of 7 7 km s increasing to 8 0 km s the zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust suggesting little or no magmatic activity during and immediately after rifting this interpretation is consistent with results from adjacent and conjugate margins

transform marginal plateaus abstract numerous submarine plateaus form highstanding bathymetric highs at continent to ocean transitions due to their proximity to continents they have been frequently labelled marginal plateaus although this term has not been clearly defined or associated with a specific geology or geodynamic process until now these elevations have been interpreted as submerged thinned continental fragments detached from continents basaltic buildups formed by hotspots volcanic margins or oceanic plateaus many of these plateaus formed at transform margins connecting oceanic basins of contrasted ages we propose for the first time to define and review a class of marginal plateaus related to a specific tectonic setting transform marginal plateaus tmps based on a compilation of 20 tmps around the world we show that most of them have a polyphased history and have undergone at least one major volcanic phase our review highlights in particular a hitherto unrecognized close link between hotspots volcanic activity and transform margins we also propose that due to their polyphased history tmps may contain several successive basins and overlooked long lived sedimentary archives we finally highlight that because these tmps were transform plate boundaries perpendicular or oblique to surrounding rifts many of them were close to last contact points during final continental breakup and may have formed land bridges or bathymetric highs between continents therefore we discuss broader scientific issues such as the interest of tmps in recording and studying the onset and variations of oceanic currents or past biodiversity growth bio connectivity and lineage evolution

insights into the lurking structures and related intraplate earthquakes in the region of bay of bengal using gravity and full gravity gradient tensor comprehension of subsurface structures buried under thick sediments in the region of bay of bengal is vital as structural features are the key parameters that influence or are caused by the subsurface deformation and tectonic events like earthquakes here we address this issue using the integrated analysis and interpretation of gravity and full gravity gradient tensor with few seismic profiles available in the poorly known region a 2d model of the deep earth crust mantle is constructed and interpreted with gravity gradients and seismic profiles which made it possible to obtain a visual image of a deep seated fault below the basement associated with thick sediments strata gravity modelling along a ne sw profile crossing the hypocentre of the earthquake of 21 may 2014 m w 6 0 in the northern bay of bengal suggests that the location of intraplate normal dip fault earthquake in the upper mantle is at the boundary of density anomalies which is probably connected to the crustal fault we also report an enhanced structural trend of two major ridges the 85 e and the 90 e ridges hidden under the sedimentary cover from the computed full gravity gradients tensor components

a physically based model for the topographic control on shallow landsliding a model for the topographic influence on shallow landslide initiation is developed by coupling digital terrain data with near surface through flow and slope stability models the hydrologic model topog o loughlin 1986 predicts the degree of soil saturation in response to a steady state rainfall for topographic elements defined by the intersection of contours and flow tube boundaries the slope stability component uses this relative soil saturation to analyze the stability of each topographic element for the case of cohesionless soils of spatially constant thickness and saturated conductivity the steady state rainfall predicted to cause instability in each topographic element provides a measure of the relative potential for shallow landsliding the spatial distribution of critical rainfall values is compared with landslide locations mapped from aerial photographs and in the field for three study basins where high resolution digital elevation data are available tennessee valley in marin county california mettman ridge in the oregon coast range and split creek on the olympic peninsula washington model predictions in each of these areas are consistent with spatial patterns of observed landslide scars although hydrologic complexities not accounted for in the model e g spatial variability of soil properties and bedrock flow control specific sites and timing of debris flow initiation within areas of similar topographic control

reconstruction of the evolution of the niger river and implications for sediment supply to the equatorial atlantic margin of africa during the cretaceous and the cenozoic

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

channel initiation and the problem of landscape scale since the 1940s it has been proposed that landscape dissection into distinct valleys is limited by a threshold of channelization that sets a finite scale to the landscape this threshold is equal to the hillslope length that is just shorter than that necessary to support a channel head a field study supports this hypothesis by showing that an empirically defined topographic threshold associated with channel head locations also defines the border between essentially smooth undissected hillslopes and the valley bottoms to which they drain this finding contradicts assertions that landscapes are scale independent and suggests that landscape response to changes in climate or land use depends on the corresponding changes in the threshold of channelization

review of oceanic transform activity and development oceanic transforms generally have a complex morphology and structure and they do not conform to the ideal concept of simple fractures that exactly follow small circles centered on euler poles of relative plate motions it is proposed that the special properties of transforms arise because they are inherently leaky i e new plate area forms along them as transforms have finite widths the adjacent ridge tips are at different distances from the euler poles and the plate boundaries along transforms are slightly oblique to the direction of plate motions such that new plate area forms along them however along transforms the normal process of seafloor spreading is interrupted and new plate area forms by a different process transforms are underlain by crust consisting of variable proportions of a material emplaced in the transform domains and b stretched ridge generated crust the transform structure is further modified by shearing deformation related to irregularities of lateral faults and because of propagation of adjacent ridges transform leakiness is not considered to result merely from changes of plate motions though such changes will influence transform structure but it reflects the segmented structure of mid oceanic ridges the persistence of transforms and their migration in the absolute reference frame with a component perpendicular to their strikes shows that the ridge structure is decoupled from flow in deeper levels thermal anomalies along transforms especially enhanced heat losses stabilize the segmented ridge structure

post rift stratigraphic architectures along the african margin of the equatorial atlantic part i the influence of extension obliquity abstract we investigated the variability of the first order crustal structure and early post rift stratigraphy of six segments of the african equatorial atlantic margin using sub surface data seismic and wells extension obliquity of the segments varies from 0 for the west ivory coast and ghana transform segments to 30 for the togo benin oblique segment and 75 for the east ivory coast normally divergent segment the sierra leone and liberia segments underwent probably deformation during both the early jurassic rifting of the central atlantic and the early cretaceous rifting of the equatorial atlantic with contrasted divergence obliquities for segments that underwent a single rifting we show that the higher the obliquity the wider the crustal thinning domain this has a major influence on the first order geometry of all the post rift horizons including the present day slope the lower the obliquity the larger the differential subsidence across the margin and the steeper the present day slopes of post rift horizons this also has a major influence on the flexural isostatic response of the lithosphere to thermal and erosion sedimentation driven un loads during the early post rift narrow transform segments underwent higher flexural and or thermal uplifts in the proximal domain than wider divergent segments along the same margin divergent segments therefore may preserve early post rift deposits in their proximal domains whereas they are not preserved on nearby transform segments

imaging a lithospheric detachment at the continent ocean crustal transition off morocco abstract the sismar seismic survey on the moroccan atlantic margin recorded deep penetration images of the continent ocean boundary this paper focuses on the 3d observation of a landward dipping reflector overlaid by a layered unit the deep part of the reflector plunges towards the continent beneath thinned continental crust whereas its upper part finishes at the top of the basement in a transition zone where likely volcanics dipping reflectors are visible and where a magnetic anomaly named s1 marks the continent ocean boundary location as the landward dipping reflector crosses the moho it is interpreted as a lithospheric detachment with associated volcanic material originating from the asthenospheric mantle observations are used i to build a 2d model to explain the mechanisms of the lithospheric breaking off between moroccan margin and its conjugate canadian margin the landward dipping reflector allowing mantle exhumation ii to propose an ene wnw trend for the initial rifting extension in this part of atlantic during late triassic direction compatible with that of the southern grand banks transform margin

crustal structure of the lofoten vesteralen continental margin off norway abstract by compiling wide angle seismic velocity profiles along the 400 km long lofoten vesteralen continental margin off norway and integrating them with an extensive seismic reflection data set and crustal scale two dimensional gravity modelling we outline the crustal margin structure the structure is illustrated by across margin regional transects and by contour maps of depth to moho thickness of the crystalline crust and thickness of the 7 km s lower crustal body the data reveal a normal thickness oceanic crust seaward of anomaly 23 and an increase in thickness towards the continent ocean boundary associated with breakup magmatism the southern boundary of the lofoten vesteralen margin the bivrost fracture zone and its landward prolongation appears as a major across margin magmatic and structural crustal feature that governed the evolution of the margin in particular a steeply dipping and relatively narrow 10 40 km wide moho gradient zone exists within a continent ocean transition which decreases in width northward along the lofoten vesteralen margin to the south the zone continues along the voring margin however it is offset 70 80 km to the northwest along the bivrost fracture zone lineament here the moho gradient zone corresponds to a distinct 25 km wide zone of rapid landward increase in crustal thickness that defines the transition between the lofoten platform and the voring basin the continental crust on the lofoten vesteralen margin reaches a thickness of 26 km and appears to have experienced only moderate extension contrasting with the greatly extended crust in the voring basin farther south there are also distinct differences between the lofoten and vesteralen margin segments as revealed by changes in structural style and crustal thickness as well as in the extent of elongate potential field anomalies these changes may be related to transfer zones gravity modelling shows that the prominent belt of shelf edge gravity anomalies results from a shallow basement structural relief while the elongate lofoten islands belt requires increased lower crustal densities along the entire area of crustal thinning beneath the islands furthermore gravity modelling offers a robust diagnostic tool for the existence of the lower crustal body from modelling results and previous studies on and off shore mid norway we postulate that the development of a core complex in the middle to lower crust in the lofoten islands region which has been exhumed along detachments during large scale extension brought high grade lower crustal rocks possibly including accreted decompressional melts to shallower levels

temperature effects and their geological consequences at transform margins a thermal model of transform margin evolution including both shear heating and lateral conduction of heat from hot oceanic to colder lithosphere was developed to gain insight into transform margin crustal structure results indicate that over 2 km of crustal uplift may occur at the fault trace for a modelled transform fault 500 km in length with spreading half rates of 1 0 and 4 0 cm year this uplift decreases away from the fault over a distance of 60 80 km the viscosity of the lower continental crust and upper mantle adjacent to the transform margin is reduced by a factor of more than 100 in response to plate motion and asthenospheric upwelling at the spreading ridge flow of this thermally weakened material may also play a role in continental crustal thinning thermal model predictions are compared with geological observations and crustal structure across transform margins in particular we show that the geology of the southwest newfoundland transform margin eastern canada and the cape range frac

chronology of nappe assembly in the pan african dahomeyide orogen west africa evidence from 40ar39ar mineral ages abstract the pan african dahomeyide orogen represents the southwestern segments of the eastern tectonic zone along which the west african craton was incorporated into gondwana orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in ghana and togo external nappes include the atacora composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived from c 2 0 ga foreland basement the suture zone is represented by high pressure granulite facies garnet homblende gneiss we have analyzed muscovite and homblende concentrates from external and suture zone nappes in incremental heating 40 ar 39 ar experiments muscovites which define the penetrative foliation in the atacora nappes and a shear foliation in the basement parautochthon record 40 ar 39 ar plateau ages of 579 4 0 8 ma and 578 1 0 5 ma respectively in contrast c 600 km to the north muscovite in quartz schist near the base of the atacora nappes displays a spectrum which gives a minimum age of 608 1 1 2 ma whereas muscovite in the overthrust kara nappes yields a plateau age of 633 8 0 5 ma the c 30 m y difference between the atacora nappes in the northern dahomeyides and the younger dates of the southern external nappes indicate out of sequence thrust imbrication however forward imbricating thrusting is also suggested by the older age of kara nappes relative to atacora nappes in the northern segment the youngest ages are inferred to date orogen parallel nappe transport at c 575 ma hornblende from the structurally lower sections of the suture zone nappes record a 36 ar 40 ar versus 39 ar 40 ar isotope correlation age of 587 4 3 ma hornblende from a tonalitic vein in the host garnet mafic gneiss yields an isotope correlation age of 581 9 2 4 ma these ages are interpreted to date the exhumation of the suture zone nappes and thus provide a minimum age of crustal thickening in the dahomeyide orogen the age data are consistent with coeval forward and hindward propagating thrusting and nappe imbrication and constrain the amalgamation in northwest gondwana to have occurred in the latest proterozoic rather than cambrian

seismic volcanostratigraphy of the gascoyne margin western australia abstract integrated interpretation of seismic gravity and magnetic data has revealed the presence of a large breakup related volcanic complex on the rifted gascoyne margin western australia using the concept of seismic volcanostratigraphy we have identified three main volcanic seismic facies units related to the extrusive volcanism 1 landward flows 2 seaward dipping reflections sdr and 3 volcanic protrusions additionally we have identified four seismic facies units related to the tectonomagmatic evolution of the margin 4 domes 5 moho 6 top underplating and 7 sill intrusions the galah rise comprises the majority of the volcanic complex and is dominated by 100 200 km long ne striking volcanic ridges surrounded by multiple sets of deep marine emplaced sdrs an estimated excess crustal volume of the 50 000 km 2 large galah rise is 350 000 km 3 the volcanic complex is located at the outer corner of a shear rift margin setting the magmatism is sparse on the shear margin massive near the outer corner and decreases ne wards along the rifted margin segment and away from the fracture zone the gascoyne margin is comparable to the volcanic voring margin ne atlantic in terms of dimensions structural setting and distribution of the volcanic seismic facies units however the along margin variations in magmatism are more prominent on the gascoyne margin

shallow structures and evolution of the ivory coast and ghana transform margin abstract seismic profiles across the transform continental margin off the ivory coast and ghana western africa illustrate the structural style resulting from the early cretaceous phase of shear stress which leads to the final separation between the african and brazilian continental margins in this area most of the characteristic tectonic features observed along this portion of margin asymmetric grabens on the ghanean platform folds of the deep ivory coast basin the ivory coast ghana marginal ridge are believed to result from progressive transform contacts between the african and brazilian continents as their margins were created during early cretaceous time a major tectonic unconformity inferred to be of upper albian lower cenomanian age may be a direct consequence of the final separation of the continental margins the later evolution of the transform margin is chiefly explained by thermal subsidence

crustal structure of the central part of the voring basin mid norway margin from ocean bottom seismographs abstract regional ocean bottom seismograph obs data acquired in the central and northern part of the voring basin mid norway margin have been modelled by use of two dimensional 2 d ray tracing the regional dataset comprises thirty obss deployed along seven 100 170 km long profiles the deeper part of the voring basin is difficult to map using multichannel reflection data due to the presence of sills at intermediate sedimentary levels 2 5 km below sea floor but the modelling of the obs data reveals that this technique provides a reliable estimate of structures and seismic velocities from the sea floor to the upper mantle the shallow and intermediate sediments to 5 km below sea floor are characterized by a vertical increase in velocity due to increased confining pressure there is also considerable lateral variation in the velocities within sedimentary layers at all levels the obs data confirm that intrusions of sills are important at intermediate and deep sedimentary levels 2 10 km below sea floor in most parts of the area the sills seem to vary in lateral extent from about 20 km to more than 100 km and their thickness is generally inferred to be about 200 m the low velocity in the upper crystalline crust 6 2 km s confirms that the crust in the voring basin is of continental origin in most parts of the area the velocity of the lower crust is as high as 7 3 7 6 km s this high velocity layer is interpreted as a magmatic underplated body with strong lateral variations in thickness the base of the 7 3 km s layer is interpreted as the moho and the upper mantle velocity is estimated to 8 2 km s

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

structural framework and tectono stratigraphic evolution of the porcupine seabight basin offshore western ireland abstract the porcupine seabight basin is an intra cratonic rift basin which evolved through recurrent episodes of lithospheric stretching structure contour and isopach maps presented here have been constructed following the interpretation of approx 10 000 km of seismic data together with all the available well data this integrated database has allowed the tectono stratigraphic history of the basin to be traced in detail from its earliest inception the basin was initially formed during the carboniferous when up to 3 7 km of autuno carboniferous sediments were deposited on a caledonian and hercynian structured basement late permian and early mesozoic sediments at least 1 km thick are preserved in contiguous grabens and their presence is also inferred within localized sub basins late liassic crustal doming heralded the onset of the final rifting episode and in the north promoted severe erosion and a rapid discharge of clastic sediments rapid subsidence block faulting and magmatism accompanied the deposition of up to 3 5 km of middle upper jurassic sediments in asymmetrical sub basins the transition from stretching to passive subsidence across the jurassic cretaceous boundary in combination with localized inversion abrupt eustatic and environmental changes generated a basin wide unconformity the rift drift transition in the adjacent proto atlantic rift system is recorded in the porcupine seabight basin by voluminous volcanism and a late lower cretaceous unconformity cretaceous sediments are strongly transgressive and up to 6 km thick these were superseded during the early palaeogene by several regressive transgressive cycles before the arrival of an upper slope fauna in the mid middle eocene cenozoic sediments are up to 4 km thick and record the interplay between 1 the location of the basin pn the pyrenean alpine foreland superimposed on the periphery of the icelandic plume and 2 its development as a marginal atlantic shelf region the former is thought to be responsible for eo oligocene faulting subsidence and igneous activity in the north east part of the basin and the latter for unconformities and a change in the style of sedimentation principally due to oceanic circulation changes

the shallow structures of the guinea and ivory coast ghana transform margins their bearing on the equatorial atlantic mesozoic evolution abstract recent geological and geophysical data from a marine survey conducted off guinea and the ivory coast ghana region equatorial atlantic are discussed we interpret the southern guinean margin firstly as an area linked to the central jurassic atlantic and secondly as an area affected by the progressive transform opening of the equatorial atlantic in the early cretaceous off the ivory coast and ghana we have demonstrated distinct structural domains interpreted as consequences of the progressive transform contact between the west african and brazilian cratons also during the early cretaceous on both margins we observe a clear tectonic unconformity of probable late albian early cenomanian age we propose that this unconformity results from the final disruption between the two continental west african and brazilian crusts the geological structures generated along transform margins can be considered as good geological tools to constrain early phases of plate motions particularly around the equatorial atlantic

rift shear architecture and tectonic development of the ghana margin deduced from multichannel seismic reflection and potential field data abstract the ghana margin displays one of the best known transform margins studies of the margin have provided the framework for a number of conceptual models aimed at understanding transform margin development worldwide however the deep structure of the margin is poorly known as knowledge is based only on wide angle refraction measurements obtained from two separate localities on the margin consequently complexities in the rift shear margin architecture have been overlooked by current interpretations of margin development based on combined analysis of a detailed grid of 2710 km multichannel mcs lines and potential field data we provide new insights into the structural architecture and tectonic development of the ghana margin in particular we outline the deep structure of the entire margin using a series of 2d gravity modelled transects constrained by mcs and published wide angle data our study reveals more complex rift shear margin architecture than previously envisaged we demonstrate that the main transform boundary representing the continental extension of the romanche fracture zone is actually composed of two distinct margin segments i e the ene wsw trending sheared margin segment of the cote d ivoire ghana ridge and the ne sw trending rift influenced sheared margin segment of the ghana platform these segments evolved under varying stress regimes and during different time intervals west of the transform margin divergent rifting during the early cretaceous initiated the development of the deep ivorian basin essentially as a single major pull apart structure however east of the shear zone oblique rifting resulted in the development of the eastern ghana slope basin as a composite of at least two coalescing pull apart basins displaced along strike slip faults our structural interpretation of the transform boundary geometry shows that the ridge and platform margin segments were each subjected to separate thermal influences from two different migrating spreading centres tectonic uplift of the ridge began through transpression during mid albian time following a change in relative direction of plate motion from ne sw to ene wsw however the ridge uplift was amplified by thermal heating from a previously undocumented spreading centre whose progressive westward migration along the ridge followed closely after the albian transpressional phase the structural architecture of the ghana margin resulted from a combination of factors notably pre existing basement structure plate boundary geometry the relative direction of plate motion and thermal heating

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

1923 gleno dam break case study and numerical modeling on the morning of december 1 1923 the gleno dam located in the central italian alps suddenly collapsed a few days after the first complete reservoir filling nearly 4 5 1 0 6 m 3 of water was released the consequent inundation caused significant destruction along the downstream valley and a death toll of at least 356 lives this failure is the only historical case of dam break caused by structural deficiencies that has occurred in italy as a result it has deeply influenced the evolution of italian regulations regarding dam design and hydraulic risk evaluation however in spite of its relevance this event has never been characterized from a hydraulic standpoint this paper reports the main information obtained from the analysis of a vast amount of historical documents regarding the gleno dam break to set up a case study useful for validating dam break models in mountain settings moreover it presents the main results of one dimensional 1d modeling of the dam break wave propagation accomplished with a first order finite volume numerical scheme recently proposed in the literature for field applications the overall effectiveness and reliability of the model are evaluated for this case characterized by very irregular topography finally the practical relevance of several choices that the numerical reconstruction of this kind of event demands is tested doi 10 1061 asce hy 1943 7900 0000327 2011 american society of civil engineers ce database subject headings dam failures shallow water numerical models hydraulic models case studies italy author keywords dam failure 1d dam break modeling shallow water finite volume numerical model hydraulic hazard case study

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

constraints for plate reconstruction using gravity data implications for source and reservoir distribution in brazilian and west african margin basins abstract evaluation of tertiary sequences of west africa and brazil typically requires large budgets and staffs to identify drilling targets since these tertiary sedimentary systems extend for hundreds of kilometers proc petrol geol deepwater depositional syst 2001 2 marine petrol geol 1990 94 122 well beyond the limits of individual 3d and many 2d seismic surveys additional information is necessary to interpret the most favourable locations for detailed exploration potential field data are powerful but often underutilized assets in building an exploration framework for reducing costs and interpretation risks although drilling locations are normally based on 3d seismic interpretations reservoir distribution is controlled by features mappable with other methods such as potential fields other factors crucial to petroleum maturation migration and trap formation relate to deep seated controls that are well imaged and sometimes exclusively imaged on potential field data our observations from potential field data help identify reservoir distribution source pod locations source maturity possible migration pathways and potential traps this paper illustrates the power of combining both regional and basinal scale interpretations based primarily on gravity analyses with extensive knowledge of the underlying geology we review data sets and methods employed and describe results of a regional overview based on special plate tectonic reconstructions of the south atlantic examples from basin scale work offshore west africa and brazil are used to illustrate factors important to hydrocarbon exploration we conclude with a look ahead to improvements in methodology and application

cote d ivoire ghana margin seismic imaging of passive rifted crust adjacent to a transform continental margin i i summary during may 1990 and january february 1991 an extensive geophysical data set was collected over the cote d ivoire ghana continental margin located along the equatorial coast of west africa the ghana margin is a transform continental margin running subparallel to the romanche fracture zone and its associated marginal ridge the cote d ivoire ghana ridge from this data set an explosive refraction line running 150 km ene wsw between 3 55 n 3 21 w and 4 23 n 2 4 w has been modelled together with wide angle airgun profiles and seismic reflection and gravity data this study is centred on the cote d ivoire basin located just to the north of the cote d ivoire ghana ridge where bathymetric data suggest that a component of normal rifting occurred rafher than the transform motion observed along the majority of the equatorial west african margin traveltime and amplitude modelling of the ocean bottom seismometer data shows that the continental moho beneath the margin rises in an oceanward direction from 24 km below sea level to 17 km in the centre of the line where the crust thins most rapidly there exists a region of anomalously high velocity at the base of the crust reaching some 8km in thickness this higher velocity region is thought to represent an area of localized underplating related to rifting modelling of marine gravity data collected coincident meth the seismic line has been used to test the bestfitting seismic model this modelling has shown that the observed free air anomaly is dominated by the effects of crustal thickness and that a region of higher density is required at the base of the crust to fit the observed data this higher density region is consistent in size and location with the high velocities required to fit the seismic data

geophysical evidence for marine prolongation of the palghat cauvery shear system into the offshore cauvery basin eastern continental margin of india abstract the precambrian basement shear zones played a major role in developing offshore structures that are zones of weakness along the eastern continental margin of india a compilation and analysis of various geophysical datasets particularly magnetic gravity seismicity and seismic reflection data support a clear geometric connection between the shear zones and offshore structures the presence of shear zones within the crystalline basement in the continental domain and oceanic domain is demarcated primarily by prominent magnetic lineation we identified three major magnetic lineaments in the offshore cauvery basin which correspond to the plaghat cauvery shear system pcss of southern granulite terrain sgt the faults interpreted in the seismic sections are correlated to the extension of pcss and indicate the location of reactivation of the shear zones which give rise to considerable intraplate seismicity the geophysical crustal model shows deep crustal scale faults associated with the marine prolongation of pcss the shear zone extended structures in the offshore might have formed at the breakup stage at the onset of rifting and might be extending up to the continent ocean transition zone cotz the pcss partly seems to accommodate the stress created by the northward push of indian plate against the asian plate it is also possible that the shear zone might have extended to the nearby land mass that marks it as one of the principal component in gondwana reconstruction

structure and evolution of the demerara plateau offshore french guiana rifting tectonic inversion and post rift tilting at transform divergent margins intersection we present the structure and evolution of the eastern part of the demerara plateau offshore french guiana from the analysis of geophysical data collected during guyaplac cruise this area is located at the intersection of a transform segment and a divergent segment of a continental margin related to the early cretaceous opening of the equatorial atlantic the main structures are nne ssw to nnw sse trending normal faults on the eastern edge of the plateau and wnw ese to nw se trending acoustic basement ridges on its northern edge when replaced in their albian position these structures appear to be parallel to the coeval oceanic accretion axis and transform faults respectively the most striking structures are related to a post rift but syn transform tectonic inversion producing e w to wnw ese trending folds sealed by a regional unconformity this shortening can not be related to ridge push but is probably related to a plate kinematic change 105 my ago that modified the deformation in the vicinity of the transform fault late post rift evolution also includes a significant tertiary oceanward tilt of the edge of the demerara plateau the driving mechanism of this late tilt is unclear but may be related to a lithospheric flexure resulting from the loading of the abyssal plain by the orinoco and amazon deep sea fans

implications of structural inheritance in oblique rift zones for basin compartmentalization nkhata basin malawi rift ears abstract the cenozoic east african rift system ears is an exceptional example of active continental extension providing opportunities for furthering our understanding of hydrocarbon plays within rifts it is divided into structurally distinct western and eastern branches the western branch comprises deep rift basins separated by transfer zones commonly localised onto pre existing structures offering good regional scale hydrocarbon traps at a basin scale local discrete inherited structures might also play an important role on fault localisation and hydrocarbon distribution here we consider the evolution of the central basin of the malawi rift in particular the influence of pre existing structural fabrics integrating basin scale multichannel 2d and high resolution seismic datasets we constrain the border mlowe nkhata fault system mnf to the west of the basin and smaller mbamba fault mf to the east and document their evolution intra basin structures define a series of horsts which initiated as convergent transfers along the basin axis the horsts are offset along a ne sw striking transfer fault parallel to and along strike of the onshore karoo permo triassic ruhuhu graben discrete pre existing structures probably determined its location and oriented obliquely to the extension orientation it accommodated predominantly strike slip deformation with more slowly accrued dip slip to the north of this transfer fault the overall basin architecture is asymmetric thickening to the west throughout while to the south an initially symmetric graben architecture became increasingly asymmetric in sediment distribution as strain localised onto the western mnf the presence of the axial horst increasingly focussed sediment supply to the west as the transfer fault increased its displacement so this axial supply was interrupted effectively starving the south east while ponding sediments between the western horst margin and the transfer fault this asymmetric bathymetry and partitioned sedimentation continues to the present day overprinting the early basin symmetry and configuration sediments deposited earlier become increasingly dissected and fault juxtapositions changed at a small 10 100 m scale the observed influence of basin scale transfer faults on sediment dispersal and fault compartmentalization due to pre existing structures oblique to the extension orientation is relevant to analogous exploration settings

northeastern brazilian margin regional tectonic evolution based on integrated analysis of seismic reflection and potential field data and modelling abstract integration of regional seismic reflection and potential field data along the northeastern brazilian margin complemented by crustal scale gravity modelling is used to reveal and illustrate onshore offshore crustal structure correlation the character of the continent ocean boundary and the relationship of crustal structure to regional variation of potential field anomalies the study reveals distinct along margin structural and magmatic changes that are spatially related to a number of conjugate brazil west africa transfer systems governing the margin segmentation and evolution several conceptual tectonic models are invoked to explain the structural evolution of the different margin segments in a conjugate margin context furthermore the constructed transects the observed and modelled moho relief and the potential field anomalies indicate that the reconcavo tucano and jatoba rift system may reflect a polyphase deformation rifting mode associated with a complex time dependent thermal structure of the lithosphere the constructed transects and available seismic reflection profiles indicate that the northern part of the study area lacks major breakup related magmatic activity suggesting a rifted non volcanic margin affinity in contrast the southern part of the study area is characterized by abrupt crustal thinning and evidence for breakup magmatic activity suggesting that this region evolved partially with a rifted volcanic margin affinity and character

postrift stress field inversion in the potiguar basin brazil implications for petroleum systems and evolution of the equatorial margin of south america rifting and related normal stress regime in the equatorial continental margin of brazil ceased during the late cretaceous when the stress regimes in eastern south america and west africa changed to induce strike slip or reverse motion in this study we explore the postrift tectonic geomorphic magmatic and sedimentary responses to stress changes using the potiguar basin the easternmost basin in the equatorial margin of brazil as a case study we use field and topographic data 2d seismic reflection lines vertical electric soundings and geochronological and borehole data to constrain the stress evolution of the potiguar basin from the late cretaceous to the quaternary discussing the role of basin inversion on sedimentation and landforms our results indicate the presence of two strike slip stress regimes after rifting the first stress field sf1 occurred from late cretaceous to the middle miocene and consisted of a n s oriented maximum subhorizontal compression and an e w oriented extension the second stress field sf2 took place from the middle miocene to the present day and included subhorizontal e w to nw se compression combined with n s and ne sw subhorizontal extension emplacement of volcanic rocks occurred along transtensional faults with a principal peak during sf1 at 20 30 ma and a subordinate peak during sf2 at 5 10 ma in response to shortening during sf2 a 70 km long and 50 km wide dome formed where marine miocene strata were uplifted to 250 m asl this uplift induced the displacement of alluvial channels away from the dome anticlines formed by transpression along the main ne sw striking faults during both sf1 and st2 acted as traps in the petroleum system similar shifts and stress field inversions documented in other areas of the brazilian continental margin are consistent with the neogene rise of the andes and may have implications for reconstructing the tectonic history of the equatorial atlantic margin of south america

the crustal structure of the southern argentine margin summary multichannel reflection seismic profiles combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost argentine margin at the transition from a rifted to a transform margin and outline the extent of the north falkland graben based on these data we establish a regional stratigraphic model for the post rift sediments comprising six marker horizons with a new formation in the barremian lower cretaceous our observations support that a n s trending subsidiary branch of the north falkland graben continues along the continental shelf and slope to the argentine basin during the rift phase a wide shelf area was affected by the e w extension subsequently forming the north falkland graben and the subsidiary branch along which finally breakup occurred we propose the division of the margin in two segments a n s trending rifted margin and an e w trending transform margin this is further underpinned by crustal scale gravity modelling three different tectono dynamic processes shaped the study area 1 the triassic early jurassic extensional phase resulting in the formation of the north falkland graben and additional narrower rift grabens ended synchronously with the breakup of the south atlantic in the early valanginian 2 extensional phase related to the opening of the south atlantic 3 the transform margin was active in the study area from about hauterivian times and activity lasted until late cretaceous early cenozoic both the rifted margin and the transform margin are magma poor very limited structures may have a volcanic origin but are suggested to be post rift the oceanic crust was found to be unusually thin indicating a deficit in magma supply during formation these findings in combination with the proposed breakup age in the early valanginian that considerably predates the formation of the parana etendeka continental flood basalt provinces in brazil and namibia question the influence of the tristan da cunha hotspot during the initial formation of the south atlantic

numerical models of the filchner ronne ice shelf an assessment of reinterpreted ice thickness distributions recent radio echo soundings of the filchner ronne ice shelf by the german antarctic expedition 1983 1984 suggest that previous ice thickness measurements may have misinterpreted an internal radio echo reflecting horizon as the true ice shelf bottom this and the analysis of total ice thickness from surface altimetry suggests that a previously defined thin ice region comprising approximately 1 5 of the total ice shelf area may be underlain by a thick layer of possibly saline ice one possible way to verify the existence of such a layer is by measurement of its influence on the ice shelf flow regime here we evaluate this influence by conducting finite element simulations of two alternative ice thickness configurations we conclude that flow differences are sufficiently large to allow verification of the possible saline basal ice layer provided that surface strain rate measurements are conducted in certain key areas

uniform sense normal simple shear of the continental lithosphere geophysical studies suggest that the thin crust characteristic of the basin and range province extends eastward beneath the west margin of the colorado plateau and rocky mountain regions in arizona and utah zones perhaps over 100 km wide may be defined bounded on the west by the east limit of upper crustal normal faults that account for more that 10 extension and on the east by the east limit of thinning beneath the colorado plateau a discrepancy exists within these zones between the negligible extension measurable in the upper crust and the substantial extension apparent from crustal thinning assuming the discrepant zone crust was as thick as or thicker than the colorado plateau rocky mountain crust prior to extensional tectonism if various theories appealing to crustal erosion are dismissed mass balance problems evident in the discrepant zones are most easily resolved by down to the east normal simple shear of the crust moving lower and middle crustal rocks that initially were within the zones up and to the west to where they now are locally exposed in the basin and range province west of the discrepant zones in both arizona and utah east directed extensional allochthons with large displacement are exposed these geophysical and geological observations complement one another if it is accepted that the entire crust in both arizona and utah failed during extension on gently east dipping east directed low angle normal faults and shear zones over a region several hundred kilometres wide large scale uniform sense normal simple shear of the crust suggests the entire lithosphere may do the same such a hypothesis predicts major lithospheric thinning without crustal thinning will occur in plateau areas in the direction of crustal shear in the case of the arizona utah and red sea extensional systems and possibly the death valley extensional terrain a broad topographic arch typically 1500 2000 m higher than the extended terrain is present suggesting lithospheric thinning in areas predicted by the hypothesis

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

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

optimal cross sectional spacing in preissmann scheme 1d hydrodynamic models choosing a suitable set of cross sections for the representation of the natural geometry of a river is important for the efficiency of one dimensional 1d hydraulic models but only few guidelines are available for the selection of the most suitable distance between cross sections depending on the hydraulic problem at hand this issue is investigated by examining models of a 55 km reach of the river po italy and a 16 km reach of the river severn united kingdom for both of which high quality laser scanning altimetry are available the high resolution digital terrain models of the two river reaches enabled the construction of a series of hypothetical topographical ground surveys with different spacing between cross sections which could be used as input to a standard 1d model unet both historical and synthetic flood events for the two river reaches were simulated and the results were then analyzed to quantify the accuracy associated with each resolution and to assess how survey resolution impacts the performance of standard 1d models the study results agree with the available suggestions in the literature and provide useful guidelines for 1d hydrodynamic modeling

mass transport deposits and fluid venting in a transform margin setting the eastern demerara plateau french guiana abstract the eastern demerara plateau offshore french guiana was surveyed in 2003 during the guyaplac cruise multibeam bathymetry and acoustic imagery 6 channel seismic reflection and 3 5 khz echo sounding the data show the post transform cenozoic that the series located on the outer part of the plateau below c 2000 m contain at least twelve stacked mass transport deposits mtds that have recorded a history of large scale slope failure as well as two main normal fault sets that provide possible pathways for upward fluid migration through the series reaching at high as the uppermost mtds seabed data show that the area above the failures is characterized by circular to elongate slope parallel depressions interpreted as fluid seeps pockmarks some of them have been modified by along slope currents we suggest that the development of the mtds to results from the combinaiton of the presence of fluid overpressure at depth the geometry of the margin s deep structure in particular the existence of a free borderlateral border on the outermost plateau our results also emphasise the role of stratigraphic decollements within the cenozoic series

the seismic structure across an active oceanic continental transform fault zone a seismic structural section across the southern queen charlotte transform fault zone which separates the oceanic pacific plate from the continental america plate off western canada has been determined from an offshore onshore refraction experiment two explosion profiles one parallel and one perpendicular to the fault strike were recorded on three ocean bottom seismographs obs and seven land based seismographs lbs assuming lateral homogeneity along the parallel profile recorded over oceanic crust at one obs one dimensional amplitude modeling produced a velocity model with the characteristics of standard ocean crust the partially reversed perpendicular profile recorded at two obs s and one good quality lbs extended across the fault zone which in bathymetric cross section shows two linear fault escarpments separated by a flat terrace an initial velocity structure provided by time term analysis of the complete data set was modified by ray tracing until the travel time data for the three stations were satisfied by one seismic structural section this model shows three distinctive crustal blocks the oceanic terrace and continental blocks separated by two major crustally pervasive faults the outer and inner queen charlotte faults the rock units composing the terrace block have lower velocities at equivalent depths than those of the blocks on either side this may have resulted from deformation associated with oblique convergence and or the effects of a jump in the position of the active queen charlotte fault from its outer to inner positions about 1 0 5 ma ago depth to the base of the crustal section increases from 12 to 18 km below sea level across the terrace transition zone representing an eastward dip of about 20 the seismic model after conversion to a density model agrees well with the gravity anomaly along the profile the interpreted structural section is consistent with recent tectonic models that require a component of convergence along the transform fault zone

deep seismic transects across the sheared western barents sea svalbard continental margin abstract we have used recently acquired deep seismic reflection and refraction profiles together with a large amount of conventional mcs data to study the 1000 km long sheared western barents sea svalbard margin with the objective of determining its crustal structure and tectonic history the margin developed from a megashear zone which linked the norwegian greenland sea and the eurasia basin during the eocene opening the margin is composed of two large shear segments and a central rifted margin segment associated with volcanism the continent ocean boundary is confined within a narrow zone 10 20 km at the sheared margin segments but is more obscure and partly masked by volcanics at the rifted margin the crustal structure and tectonic history differ significantly between the three margin segments in the north across the hornsund fault zone the crustal thickness changes abruptly from more than 30 km thick continental crusy on the svalbard platform to 6 8 km thick oceanic crust in the greenland sea this margin segment experienced oblique continent continent and partly continent ocean shear with both transtensional and transpressional component during eocene time while post eocene extension caused crustal stretching west of svalbard the volcanic province at the central rifted margin segment probably comprises an outer part of thick oceanic crust and an inner part of thinned continental crust covered by a mixture of volcanics and sediments this formed at a releasing bend in the margin in response to eocene oligocene extension and two phases of volcanism in the south the crustal thinning occurs over a wider area landward of the senja fracture zone here the margin bounds a basinal province in which as much as 18 20 km of sediments cover a highly attenuated crystalline crust the senja fracture zone evolved due to eocene transtensional continent continent shear followed by continent ocean shear and has been passive since earliest oligocone time we conclude that the first order crustal structure along the margin and its tectonic development is mainly the result of three controlling parameters 1 1 the pre breakup structure 2 2 the geometry of the plate boundary at opening and 3 3 the direction of relative plate motion the interplay between the direction of plate motion and the geometry of the plate boundary gave rise to striking differences in the structural development of the different margin segments

numerical flow simulation at local parts of filchner ronne ice shelf antarctica the munster radio echo sounding res work in the central part of filchner ronne ice shelf has revealed new information different signatures are discernible in the airborne res data local models in the area between henry ice rise and berkner island are used to interpret these signatures in terms of thermal and dynamic parameters from our airborne measurements the shapes of the meteoric and marine ice layers were prepared as input data for dynamic model calculations a general increase in the temperature dependent flow parameter is compared with a spacially variable flow parameter that is derived from model calculations concerning freezing and melting processes velocity and strain fields were calculated by a finite difference method on a local data base between henry ice rise and berkner island

borehole evidence for a thick layer of basal ice in the central ronne ice shelf the ronne ice shelf floating between berkner island and the antarctic peninsula is the biggest ice shelf in antarctica it drains an estimated 1 2 106km2 of the antarctic ice sheet much of it resting on bedrock below sea level consequently the balance and dynamics of this ice shelf is of importance to antarctic glaciology especially with regard to the integration of this part of the cryo sphere into the global processes that control the climate of the earth extensive radio echo sounding res by robin and others revealed reflections in the central part of the ronne ice shelf at the relatively shallow depth of 100 200 m below surface1 the interpretation of these echoes which varied in strength was ambiguous and the possibility of internal reflecting horizons was thoroughly discussed but after surface elevation measurements by radar altimeter from drifting balloons appeared to fit the presence of thin ice it was decided to base a thickness map of the ronne ice shelf on these res echoes1 4 we now present direct observational evidence from boreholes that the total ice thickness is much greater than mapped and that the shallow res reflections therefore do come from internal horizons

path based methods for the determination of nondispersive drainage directions in grid based digital elevation models 1 path based methods for the determination of nondispersive drainage directions in grid based digital elevation models are presented these methods extend the descriptive capabilities of the classical d8 method by cumulating the deviations between selected and theoretical drainage directions along the drainage paths it is shown that either angular or transversal deviations can be employed accordingly two classes of methods designated d8 lad eight drainage directions least angular deviation and d8 ltd eight drainage directions least transversal deviation are developed detailed tests on four synthetic drainage systems of known geometry and on the liro catchment central italian alps indicate that the proposed methods provide significant improvement over the d8 method for the determination of drainage directions and drainage areas