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

neoproterozoic tectono thermal evolution of the gariep belt and its basement namibia and south africa abstract within the pan african gariep belt in southwestern africa a para autochthonous predominantly sedimentary sequence deposited in a rift graben and subsequently on a passive continental margin with a kibaran basement is distinguished from an allochthonous predominantly oceanic terrane that was thrust on top of the former during collision of the rio de la plata and the kalahari plates 40ar 39ar mineral cooling ages presented here set the following constraints after kibaran high grade metamorphism the consolidated basement cooled through 300 c at 1006 ma rifting between 781 and 741 ma was followed by the formation of oceanic crust the lower parts of which cooled through 500 c sometime between 630 and 600 ma after inversion from extension to compression some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge a process that is indirectly dated between 573 576 ma by hornblende plateau ages continent continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547 543 ma as indicated by further hornblende plateau ages muscovite cooling ages for the internal zone are between 526 and 529 ma those for the tectonically deeper external zone are 483 ma in the north and between 495 506 ma in the south of the belt in conjunction with published data these results suggest successive closure of first the northern adamastor ocean kaoko belt followed by the khomas sea intracontinental damara belt and finally the southern adamastor ocean gariep belt

deep structure of the western south african passive margin results of a combined approach of seismic gravity and isostatic investigations abstract the passive margin of the south atlantic shows typical features of a rifted volcanic continental margin encompassing seaward dipping reflectors continental flood basalts and high velocity density lower crust at the continent ocean transition probably emplaced during initial seafloor spreading in the early cretaceous the springbok profile offshore western south africa is a combined transect of reflection and refraction seismic data this paper addresses the analysis of the seismic velocity structure in combination with gravity modelling and isostatic modelling to unravel the crustal structure of the passive continental margin from different perspectives the velocity modelling revealed a segmentation of the margin into three distinct parts of continental transitional and oceanic crust as observed at many volcanic margins the lower crust is characterised by a zone of high velocities with up to 7 4 km s the conjunction with gravity modelling affirms the existence of this body and at the same time substantiated its high densities found to be 3100 kg m 3 both approaches identified the body to have a thickness of about 10 km yet the gravity modelling predicted the transition between the high density body towards less dense material farther west than initially anticipated from velocity modelling and confirmed this density gradient to be a prerequisite to reproduce the observed gravity signal finally isostatic modelling was applied to predict average crustal densities if the margin was isostatically balanced the results imply isostatic equilibrium over large parts of the profile smaller deviations are supposed to be compensated regionally the calculated load distribution along the profile implies that all pressures are hydrostatic beneath a depth of 45 km

deep structure of the u s atlantic continental margin offshore south carolina from coincident ocean bottom and multichannel seismic data we present the results of a combined multichannel seismic reflection mcs and wide angle ocean bottom seismic profile collected in 1988 across the carolina trough on the u s atlantic continental margin inversion of vertical incidence and wide angle travel time data has produced a velocity model of the entire crust across the continent ocean transition the margin consists of three structural elements 1 rifted continental crust comprising 1 4 km of post rift sedimentary rocks overlying a 30 34 km thick subsedimentary crust 2 transitional crust a 70 to 80 km wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10 to 24 km thick subsedimentary crust and 3 oceanic crust comprising 8 km of sedimentary rocks overlying an 8 km thick crystalline crust the boundary between rifted continental and transitional crust marked by the brunswick magnetic anomaly represents an abrupt change in physical properties with strong lateral increases in seismic velocity density and magnetic susceptibility the transitional crust contains mid crustal seaward dipping reflections observed on the mcs section and has seismic velocities of 6 5 6 9 km s in the midcrust and 7 2 7 5 km s in the lower crust modeling of potential field data shows that transitional crust also produces the prominent margin parallel gravity anomaly and the brunswick and east coast magnetic anomalies these observations support the interpretation that the transitional crust was formed by magmatism during continental breakup the prodigious thickness up to 24 km of igneous material rivals that interpreted on continental margins of the north atlantic e g hatton bank and voring plateau which formed in the vicinity of the iceland hotspot these observations when combined with other transects across the margin confirm previous suggestions that the u s atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long lived mantle plume

large igneous province on the us atlantic margin and implications for magmatism during continental breakup rifted continental margins commonly include sections of igneous rock more than twice as thick as normal oceanic crust explanations for this voluminous magmatic accretion during rifting include plume models1 3 which require a deep seated thermal or chemical anomaly in upwelling mantle and non plume models4 7 which call on broad shallow thermal anomalies and or rapid upwelling of mantle through the melting zone new seismic models from two transects across the continent ocean transition on the us atlantic margin8 10 confirm the presence of a 20 25 km thick igneous section here we argue that the similarity of the crustal structure on these and two previous transects spanning 1 000 km of the margin and the association of thick igneous crust with the east coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3 2 106 km3 the distribution of volcanic and plutonic rocks details of the seismic structure and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non plume processes created the thick igneous crust

evolution of the northeast german basin inferences from a 3d structural model and subsidence analysis abstract a 3d structural model of the northeast german basin was evaluated with special emphasis on its evolution as an intracontinental depression the study includes investigations on subsidence history and structural setting of the basin thickness evolution and calculated tectonic subsidence volumes of permian to quaternary sediments in the northeast german basin indicate that the subsidence history was related to five stages of basin evolution which differ in their subsidence mechanisms for the initial rift phase in the late carboniferous to early permian a dominant thermal event and subordinate horizontal stresses were indicated by thickness variation evolution and by structural evidence the main part of basin subsidence occurred in a nw se oriented basin in the subsequent phase of thermal relaxation with maximum subsidence from early permian rotliegend to middle triassic muschelkalk from middle triassic the thermal subsidence pattern was superposed by further tectonic events in the middle triassic regional extension led to a reconfiguration of the southern part of the basin where new nne ssw trending troughs rheinsberg and gifhorn troughs developed in the jurassic the northwestern part of the basin was uplifted while in the south the keuper subsiding areas continued to sink and nw se trending depressions related to salt margins became important differentiation continued into cretaceous times when regional compression caused uplift of the southeastern part of the basin and basin margins a final subsidence phase occurred in the cenozoic this was accompanied by intensive salt movement recent basin configuration reflects the superposition of structural elements resulting from different evolution stages the main structural characteristics of the basin are 1 a vertical tectonic zonation in a pre zechstein succession which lacks significant internal structures and a strongly deformed post zechstein succession which was decoupled due to the thick zechstein salt and 2 a marked asymmetry of the basin with a shallow northern slope and a steep bounding fault at the southern margin elbe fault system the northwestern part of the basin shows the structural properties of an intracratonic sag basin with persisting subsidence and with minor salt mobilisation in contrast initial structures in the southeastern part are strongly overprinted by younger tectonic events including middle triassic to jurassic extension late cretaceous inversion and late to post cretaceous salt movements tectonic elements that deform the whole sedimentary succession are restricted to the basin s southern eastern and northeastern margins where salt thickness decreases combined volumetric and backstripping investigations show that 2 3 of the total subsidence was induced by the sediment load and 1 3 was caused by tectonics the tectonic subsidence history varies laterally across the basin while tectonic subsidence curves in the northwestern part of the basin show fast subsidence during the permian and early triassic decreasing exponentially with time subsidence curves in the southeastern part indicate repeated tectonic activity however the tectonic subsidence volume created during permian to late triassic is significantly higher than the tectonic subsidence during younger phases of basin history

rupture dynamics and ground motion from potential earthquakes around taiyuan china abstract using the curved grid finite difference method we develop dynamic spontaneous rupture models of earthquakes on the jiaocheng fault jf near taiyuan the capital and largest city of shanxi province in north china we then model the wave propagation and strong ground motion generated by these scenario earthquakes a map of the seismic hazard distribution for a potential m 7 5 earthquake is created based on dynamic rupture and true 3d modeling the tectonic initial stress fields derived from the inversion of focal mechanisms of historical earthquakes a nonplanar fault and a rough surface are considered in the dynamic rupture simulation based on the geological structure of the taiyuan basin normal faulting with a dipping angle of 60 is implemented for the scenario earthquake simulations the largest uncertainty of a potential earthquake in the jf zone is the hypocenter four cases are used to nucleate the earthquake at different locations using these dynamic rupture sources for the jf we further simulate and analyze both the seismic wave generated by the scenario earthquake and the strong ground motion it is found that the low velocity media of the taiyuan basin redistribute the ground motion well the effects of the regional stress fields on the dynamic rupture and hazard distribution are investigated and discussed further moreover a scenario earthquake which can cause great damage to the city of taiyuan is modeled and analyzed

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

the etendeka igneous province magma types and their stratigraphic distribution with implications for the evolution of the parana etendeka flood basalt province detailed geochemical and field data for the volcanic sequence and intrusions of the etendeka igneous province are used to construct a stratigraphic framework for petrogenetic interpretation of the evolution of the etendeka parana continental flood volcanic event geochemical and petrographic characterization of over 1 000 analyzed samples allows 8 mafic and 17 silicic magma types to be recognized both silicic and mafic types can be grouped into high ti and low ti suites on the basis of elevated ti relative to other elements the mafic magmas are khumib high ti tafelberg kuidas horingbaai huab tafelkop albin and esmeralda all low ti amongst the silicic types the goboboseb springbok wereldsend grootberg and beacon low ti quartz latites and the nil desperandum high ti latite have been described previously in addition the hoas low ti nadas sechomib and hoarusib all high ti latites and the fria low ti sarusas ventura khoraseb naude and elliott all high ti quartz latites are described for the first time here there is a marked provinciality in the distribution of the high and low ti suites with the former concentrated in the northern etendeka region and the latter dominant in the southern etendeka stratigraphic distribution of magma types allows two new formations to be defined in the northern etendeka the khumib formation of basaltic flows and the skeleton coast formation dominated by silicic sheets the geochemical provinciality hampers precise correlations between northern and southern etendeka available evidence suggests that the lower part of the awahab formation in the southern etendeka is coeval with the lower part of the khumib formation and that the silicic units in the upper part of the tafelberg formation probably correlate with the skeleton coast formation the paucity of khumib dykes in relation to tafelberg dykes and their field relationships with regard to the volcanic sequence in the northern etendeka suggests that the main khumib eruptive centers lay further north consistent with southward thinning of the khumib basalts in the southern etendeka the doros complex is the eruptive center of the plume derived tafelkop basalts which probably built a shield volcano within the regional flood lava field this work indicates that the etendeka has a greater variety of both silicic and low ti mafic magma types than are known from the se parana detailed comparisons reveal that all the important silicic types in the parana have geochemical equivalents in the etendeka new correlations are santa maria fria ourinhos khoraseb guarapuava sarusas thus extending the previously recognized correlations between southern etendeka quartz latites and the caixas do sul and anita garibaldi rhyolites these correlations emphasize that very large volume silicic systems develop with pronounced lithospheric rifting in continental flood basalt provinces

a process oriented approach to modeling the gravity signature of continental margins the gravity anomaly at a continental margin can be regarded as the result of all the processes that have shaped it through time these include rifting sedimentation and magmatic underplating this article discusses a new process oriented approach to gravity interpretation which uses seismic constraints on sediment thickness and the underplate at continental margins to estimate the gravity anomaly that is associated with rifting sedimentation and magmatic underplating by comparing the calculations to the observed free air gravity edge effect anomaly we have been able to determine the elastic thickness and crustal structure and to provide an estimate of the location of the ocean continent transition since crustal structure is a proxy for the amount of heating that occurred at the time of rifting the approach has important implications for the deep water hydrocarbons industry

making ends meet in gondwana retracing the transforms of the indian ocean and reconnecting continental shear zones interpretation of the detailed patterns of ocean floor transforms revealed by satellite altimetry enables the creation of the indian ocean to be described quantitatively as four consecutive plate tectonic regimes separated at 200 136 89 and 43 ma each regime is reversed in turn by keeping transform termini coincident and colinear until conjugate points on the margins of pre existing plates regain their pre regime integrity progressive elimination of the indian ocean demonstrable as a smooth computer animation http www kartoweb itc nl gondwana leads to a refined re assembly of the continental fragments of central gondwana that is substantiated by new geological data a sequence of euler interval poles that describes the dispersal of the gondwana fragments time calibrated against available magnetic anomaly data is given the model requires a mid cretaceous position for india s southern tip about 1000 km south of madagascar prior to india s rapid northward migration

a seismic refraction investigation of the namaqualand metamorphic complex south africa the namaqualand metamorphic complex forms part of the precambrian shield of southern africa and hosts several major base metal deposits a long line seismic refraction experiment traversing the complex was conducted in order to complement geological and geophysical studies of the structure and evolution of this region seismograms were obtained at approximately 10 km intervals along the 290 km profile seismic ray tracing techniques were used to model the travel time and amplitude data it was found that the namaqualand lower crust is comprised of a substantial proportion of intermediate velocity 6 2 to 6 9 km s rocks these intermediate velocity rocks approach the surface near the midpoint of the profile moho was found to be at a depth of 42 km no correlation was found between the seismic and electrical resistivity models of the crust when the seismic velocity structure of the middle proterozoic namaqualand metamorphic complex is compared with that of adjacent geological provinces it is found to differ significantly from the archaean kaapvaal province but resemble the late proterozoic damara province these differences might be attributed to variations in the principal mechanisms of crustal growth between archaean and proterozoic times

crustal structure and evolution of the southern voring basin and voring transform margin ne atlantic abstract the dominantly passive volcanic voring and more margins ne atlantic are separated by the 200 km long voring transform margin vtm the southern voring basin and the vtm have been studied by use of four regional ocean bottom seismograph obs profiles combined by gravity modelling the models demonstrate a complex pattern of magmatism along the transform margin the distribution of magmatism seems to be related to the existence and trend of a lower crustal 8 km s body interpreted as eclogitized rocks present in the southern voring basin early tertiary breakup related magmatic leakage across the continent ocean transition cot appears to be facilitated where this layer is absent these results support earlier workers who have concluded that the jan mayen fracture zone originated from a caledonian zone of weakness we propose that partly eclogitized rocks were uplifted into the lower crust close to this zone during the caledonian orogeny and that this body acted as a barrier to magma emplacement during the late cretaceous early eocene phase of rifting breakup the eclogitized terrain also appears to have caused northeastward channeling of the late cretaceous early tertiary intrusions within the voring basin an up to 10 km thick pre cretaceous sedimentary basin in the southern voring basin may be genetically related to the ns trending late paleozoic and mesozoic rift basins in north east greenland

the moho boundary above upper mantle peridotites or lower crustal eclogites a global review and new interpretations for passive margins abstract we have performed a global study of 2d crustal scale wide angle profiles across passive margins with regard to local elevations in the moho which could possibly be interpreted as indicative of lower crustal eclogites a total of 16 candidates have been found mainly in the north atlantic and around australia these cases make up c 6 only of the total profile length studied confirming the interpretation of the moho generally representing the top of the mantle the interpreted candidates for lower crustal eclogites indicate that there may be a link between eclogite bodies and continental suture zones in the barents sea off mid norway in the newfoundland appalachians and in the yilgarn craton western australia it is also possible that there is a genetic link between the formation of caledonian eclogites and the jan mayen fracture zone which is the only major fracture zone in the north atlantic several of the inferred lower crustal eclogite bodies are located close to lines of major changes in strain such as coastlines and shelf edges indicating that lower crustal eclogite bodies may be important in guiding the evolution of basin architecture interpreting the moho beneath the bedout high on the northwest shelf of australia as the top of a body of lower crustal eclogites may imply that the northern termination of the lambert shelf represents a paleo suture zone and that the western termination of the broome platform acted as a major transfer zone the significant increase in crustal thickness implied by the eclogite model has important implications for estimates of stretching history subsidence and hydrocarbon maturation modelling

age of mesozoic igneous rocks in northwesternnamibia and their relationship to continental breakup mesozoic igneous rocks of northwestern namibia comprise continental flood basalts and quartz latites of the parana etendeka province and a suite of central sub volcanic intrusions the damaraland complexes conventional k ar ages published for these rocks range from 377 88 ma with the majority of samples yielding ages of 150 110 ma this large spread of ages is interpreted as being due to disturbance of the k ar system by low temperature alteration which is ubiquitous among the volcanic rocks rb sr and 40 ar 39 ar dating methods which are less susceptible to such disturbance yield a narrower range of ages 132 124 ma with one outlier at 149 ma new rb sr internal isochron ages of 129 8 3 8 ma 126 8 1 3 ma and 126 1 7 3 ma have been determined for an etendeka quartz latite and nepheline syenite intrusions in the messum and okorusu complexes respectively in addition 40 ar 39 ar release spectrum ages of 137 0 0 7 ma 135 0 0 7 ma and 126 9 0 6 ma have been determined for samples from the paresis comendite cape cross nepheline syenite and okorusu nepheline syenite complexes respectively a 13 ma period between 137 and 124 ma is considered to be the best estimate of the timing of mesozoic igneous activity in northwestern namibia the continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short lived 1 2 ma magmatic activity among the damaraland complexes was contemporaneous with the onset of flood basalt volcanism but continued for approximately 15 ma and terminated after the onset of sea floor spreading in this area 130 125 ma both the flood basalt volcanism and the magmatic activity of the damaraland complexes are thought to have occurred in response to rifting across the upwelling tristan mantle plume the linear distribution of intrusions in damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the pan african damara terrain to the south and proterozoic cratonic basement to the north

deep structures and breakup along volcanic rifted margins insights from integrated studies along the outer voring basin norway three dimensional multichannel seismic survey and well data allows a deeper insight into rifting mechanisms and crustal structure of the norwegian volcanic rifted margin a surprising high velocity lower crustal dome marked by a strong amplitude reflection t reflection is described below the north gjallar ridge ngr near the volcanic plateau formed during the late paleocene early eocene breakup faulting along the ngr occurs during early campanian paleocene and is controlled by the crustal dome the t reflection was previously related to either the top of mafic underplated crust or soft lower crust updomed during the continental breakup the influence and timing of the crustal dome on the basin structure shows clearly that the dome predates the continental breakup based on the structural relationship and the results of the modelling it is therefore concluded that the t reflection does not necessarily originate from an anomalous tertiary magmatic event related to the breakup underplated material but could be also partly attributed to pre breakup mafic material or and inherited high pressure granulite eclogite rocks in the continental domain q 2004 elsevier ltd all rights reserved

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

melt generation at volcanic continental margins no need for a mantle plume melt generation in a rifting environment is studied using a dynamic 2 d finite element model the lithosphere is extended to large realistic thinning factors assuming a mantle temperature of 1333 c the focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations the timing of melt production late synrift and the amounts of melt are consistent with observations at volcanic margins the dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins

mesozoic break up of sw gondwana implications for regional hydrocarbon potential of the southern south atlantic abstract this work provides new palinspastic palaeofacies reconstructions of sw gondwana incorporating rotation of a falkland malvinas microplate we discuss the implications of this for the tectonic evolution of the southern south atlantic and hence for the regional hydrocarbon potential existing gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents in most continental reconstructions the falkland malvinas plateau was assumed to be a rigid fragment of pre permian south american crust however it has been suggested on the basis of palaeomagnetic data that the falkland malvinas islands were rotated by 180 after 190 ma this rotation hypothesis has been successfully tested on the basis of devonian stratigraphy and palaeontology permian stratigraphy and sedimentology and late palaeozoic and early mesozoic structure making it unlikely that the plateau behaved as a rigid structure during breakup we have explored the consequences of accepting this hypothesis for the tectonic evolution of sw gondwana by compiling new palaeogeographic maps for the permian cretaceous of the southern atlantic area to achieve a realistic close fit we have devised a pre rift proxy for the ocean continent boundary for the south atlantic in order to produce the best fit it is necessary to subdivide south america into four plates the consequences of this are far reaching our work suggests that although sedimentary basins were initiated at different times three major tectonic phases can be recognised in regional terms these can be thought of as pre syn and post rift during the pre rift time until the late triassic the area was dominated by compressional tectonism and formed part of the gondwana foreland the falkland malvinas islands lay east of africa the falkland malvinas plateau was 33 shorter and patagonia was displaced east with respect to the rest of south america in part along the line of the gastre fault system potential source facies are dominantly post glacial black shales of late permian age deposited in lacustrine or hyposaline marine environments these rocks would also be an effective regional seal sandstones deposited in the late permian would be dominantly volcaniclastic with poor reservoir qualities triassic sandstones tend to be more mature there was significant extension from about 210 ma end triassic until the south atlantic opened at about 130 ma early cretaceous in the early syn rift phase extension was accompanied by strike slip faulting and block rotation later extension was accompanied by extrusion of large volumes of lava early opening of the south atlantic was oblique which created basins at high angle to the trend of the ocean on the argentine margin and resulted in microplate rotation in ne brazil intermittent physical barriers controlled deposition of upper jurassic cretaceous anoxic sediments during breakup some of these mudrock units are effective seals with likely regional extent during crustal reorganisation clastic sediments changed from a uniform volcaniclastic provenance to local derivation with variable reservoir quality in the late rift and early post rift phase continental extension changed from oblique to normal and basins developed parallel to the continental margins of the south atlantic this change coincides with the main rifting in the equatorial basins of brazil and the early impact of the santa helena plume it resulted in widespread development of unconformities the abandonment of the reconcavo tucano jatoba rift and the end of ne brazil plate rotation which remained attached to south america there was extensive deposition of evaporites concentrated in but not restricted to the area north of the rio grande rise walvis ridge widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements our main conclusion is that the regional hydrocarbon potential of the southern south atlantic has been constrained by the tectonic evolution

reconstruction of the southwestern african continental margin by backward modeling abstract as the evolution of the southwestern african passive continental margin and in particular the vertical movements in this area are still a matter of debate this study aims to quantify the past vertical movements by reconstructing the paleobathymetric evolution of the southwestern african margin during the post rift phase to quantify the vertical movements inferred from paleobathymetries different subsidence components that have affected the passive continental margin are calculated and interpreted with respect to the generating processes our investigation takes advantage of information on the present day sedimentary thicknesses and the crustal configuration along the sw african margin by using this detailed information and modeling backward in time we attempt to separate different subsidence components such as the load induced subsidence and the thermal subsidence for different time intervals during the post rift phase calculating the amounts of each subsidence component and subtracting them from the present day configuration yields paleobathymetries which are interpreted with respect to the evolution of the southwestern african passive margin these restored paleobathymetries for different time steps during the post rift phase indicate spatial and temporal variations of vertical movements including phases of seafloor uplift as a result our investigation allows to give minimum estimates of the vertical movements that have occurred during the post rift phase in addition to predominant subsidence about 1000 m of seafloor uplift are deduced for the northern and southern parts of the sw african margin quantifying the amounts of vertical movements cannot finally clarify the generating processes behind these movements but the timing and magnitude of seafloor uplift indicate a deep mantle mechanism

post rift stratigraphic evolution of the atlantic margin of namibia and south africa implications for the vertical movements of the margin and the uplift history of the south african plateau abstract the aim of this study is to constrain the post rift deformations of the atlantic passive margin of namibia and south africa using an extensive industrial 2d reflection seismic dataset calibrated by wells and onshore outcrops that have been revaluated in age biostratigraphy in order to discuss the evolution of the south african plateau uplift the first order evolution of the margin is tectonically driven and can be divided into three principal phases the first 131 93 5 ma comprises an overall retrogradational trend that results from a rate of accommodation created by the thermal flexure of the margin that is higher than the sediment supply the second 93 5 66 ma comprises an overall aggradational progradational trend that results from a relative increase in sediment supply due to an uplift and subsequent erosion of the margin and the inland domain the third 66 0 ma is retrogradational again in response to a decrease in sediment supply induced by a relative stability of the margin and climate conditions this study demonstrates that the present day configuration of the margin is acquired during the late cretaceous from 81 to 66 ma with a strike long wavelength and seaward tilt of the margin which could be related to the growth of the onshore main escarpment we also characterize a regional oligocene unconformity marked by a significant downward shift of the shoreline which suggests a moderate uplift of the inland domain and could be associated with a reactivation of the relic late cretaceous relief

arctic sea ice melt in 2008 and the role of solar heating there has been a marked decline in the summer extent of arctic sea ice over the past few decades data from autonomous ice mass balance buoys can enhance our understanding of this decline these buoys monitor changes in snow deposition and ablation ice growth and ice surface and bottom melt results from the summer of 2008 showed considerable large scale spatial variability in the amount of surface and bottom melt small amounts of melting were observed north of greenland while melting in the southern beaufort sea was quite large comparison of net solar heat input to the ice and heat required for surface ablation showed only modest correlation however there was a strong correlation between solar heat input to the ocean and bottom melting as the ice concentration in the beaufort sea region decreased there was an increase in solar heat to the ocean and an increase in bottom melting

structural features of the southwest african continental margin according to results of lithosphere scale 3d gravity and thermal modelling abstract to understand the structure of the southwest african continental margin a lithosphere scale 3d structural model has been developed covering the marginal cretaceous cenozoic orange luderitz walvis and namibe basins the easternmost walvis ridge offshore onshore the model includes two late proterozoic owambo etosha and nama basins this 3d model integrates published thickness maps sediment isopach maps shallow seismic and well data as well as published deep seismic information and has been additionally constrained by 3d gravity and thermal modelling using 3d gravity modelling the first order configuration of the crystalline crust has been resolved with respect to the location of the continent ocean boundary the distribution of a high density lower crustal layer indicates a continuous body extending below the cretaceous cenozoic depocentres and aligned parallel to the coast line in addition high density zones within the continental crystalline crust had to be included in the model to fit observed and calculated gravity the obtained moho topography correlates with the major tectonic units of this continental margin the results of the 3d thermal modelling indicate that there is a clear relationship between the location of thickened sediments and areas with increased temperatures within the upper 10 km of the 3d model this indicates that the low thermal conductivity of the sediments increases heat storage within the areas covered by thick sediments within the deeper crust the main feature of the temperature distribution is the transition across the continental margin from the relatively cold oceanic part to the warm continental one this regional pattern is controlled by the thickness of the crystalline continental crust which is characterized by an increased radiogenic heat production at a depth of 80 90 km the temperature becomes higher beneath the oceanic domain than beneath the continent reflecting the configuration of the lower thermal boundary which is represented by an isothermal lithosphere asthenosphere boundary

palaeogeographic consequences of conservational models in the south atlantic ocean conservational models like simple shear pure shear or polyphase models that exclude exchanges between the lower continental crust and upper mantle are usually proposed to explain the lithospheric stretching and consequent crustal thinning of passive continental margins these models need large amounts of horizontal movement and have therefore important implications for plate kinematic reconstructions and intraplate deformation in this paper we propose to show these implications in the central segment of the south atlantic ocean in the angola brazilian system these models imply about 240 km of horizontal movement this movement can be com pensated by two end member mechanisms 1 an intraplate deformation located in africa and 2 an intraplate deformation located in south america we detail for each solution the strong geological and geodynamical implications and discuss the consequences for the genesis of passive continental margins

three dimensional subsidence analysis and gravity modelling of the continental margin offshore namibia summary seismic re ection pro les and gravity anomaly data have been used to determine the structure and evolution of the namibian continental margin in comparison to other margins the gravity anomaly at the namibian margin shows a number of distinctive features it lacks an oishore gravity low and despite the presence of up to 9 km of sediments the gravity high is displaced landwards of the maximum sediment thickness in an attempt to explain these features the early cretaceous recent stratigraphic record is analysed using a combined 3 d backstripping and gravity modelling technique that enables constraints to be placed on the long term mechanical properties of the lithosphere a neutral depth of necking and a relatively high exural rigidity or equivalent elastic thickness te o f 25 km can explain part of the anomaly at least in the south of the margin however large residual anomalies exist that can be reduced by the presence of pre rift low density sediments within the zone of maximum stretching south of 22 0 s high density volcanic material at the eastern pinch out of the lower rift sequence and a lateral sediment density variation across the shelf slope and rise in addition the presence of a magmatic body at the base of the crust provided that the exural strength of the margin is high signi cantly improves the t between observed and calculated gravity anomalies recovered stretching factors enable predictions of the geometry of the moho to be made palaeobathymetry is estimated along the entire margin through time by comparing the subsidence determined from backstripping to that predicted by simple models of rifting these estimates are within the errors of observed palaeowater depths where well control exists the requirement of high te rather than te that increases with time suggests that stretched continental crust extends oishore for some distance and the continent ocean boundary occurs in relatively deep water near magnetic anomaly m4 reconstruction of the gravity eld of the south atlantic at 100 ma illustrates that in terms of their gravity signature the south atlantic margins are asymmetric

south atlantic volcanic margins the early cretaceous south atlantic continental break up and initial sea floor spreading were accompanied by large scale transient volcanism emplacing the parana etendeka continental flood basalts and voluminous extrusive constructions on the conjugate margins south of the torres arch abutment plateau on the north namibia margin we interpret four main tectono magmatic crustal units 1 oceanic crust 2 thickened oceanic crust covered by huge seaward dipping wedges 3 a c 150 km wide break up related rift zone partly covered by the dipping wedges and 4 thicker continental crust partly deformed by palaeozoic extension east of the early cretaceous rift similar settings also characterize other south atlantic margin segments we infer an up to 300 km wide and 2400 km long rift zone representing lithospheric extension leading to breakup and formation of the south atlantic volcanic margins comparison with other volcanic margins demonstrates in spite of local and regional differences gross similarities in tectono magmatic style crustal units and dimensions

backward modelling of the subsidence evolution of the colorado basin offshore argentina and its relation to the evolution of the conjugate orange basin offshore sw africa abstract in this study we focus on reconstructing the post rift subsidence evolution of the colorado basin offshore argentina we make use of detailed structural information about its present day configuration of the sedimentary infill and the crystalline crust this information is used as input in a backward modelling approach which relies on the assumption of local isostasy to reconstruct the amount of subsidence as induced by the sedimentary load through different time stages we also attempt a quantification of the thermal effects on the subsidence as induced by the rifting here included by following the uniform stretching model of lithosphere thinning and exponentially cooling through time based on the available information about the present day geological state of the system our modelling results indicate a rather continuous post rift subsidence for the colorado basin and give no significant evidence of any noticeable uplift phase in a second stage we compare the post rift evolution of the colorado basin with the subsidence evolution as constrained for its conjugate sw african passive margin the orange basin despite these two basins formed almost coevally and therefore in a similar large scale geodynamic context their post rift subsidence histories differ based on this result we discuss causative tectonic processes likely to provide an explanation to the observed differences we therefore conclude that it is most probable that additional tectonic components other than the ridge push from the spreading of the south atlantic ocean are required to explain the observed differences in the subsidence of the two basins along the conjugate passive margins such additional tectonic components might be related to a dynamic mantle component in the form of either plume activity africa or a subducting slab and the presence of an ongoing compressional stress system as revealed for different areas in south america

were aspects of pan african deformation linked to iapetus opening the convergence recorded in some pan african deformational belts sensu lato in south america africa madagascar southern india sri lanka and antarctica is temporally correlated with opening of the iapetus ocean we propose a model in which continent continent collision and closure of the adamastor ocean between the amazon west african rio de la plata cratons and the sao francisco congo kalahari cratons in the late neoproterozoic are linked to rifting and orthogonal spreading between laurentia and the south american cratons by the early cambrian the cratons in south america and africa were assembled as west gondwana closure of the mozambique ocean which appears to have extended across antarctica between lutzow holm bay and the shackleton range resulted in continued convergence between the congo kalahari queen maud land block and east gondwana in the cambrian coeval deformation in the transantarctic mountains may be related to the obliquity of the antarctic margin relative to iapetus spreading directions initiation of voluminous arc magmatism along the paleo pacific margin of gondwana in the early cambrian is broadly synchronous with the cessation of intra gondwana pan african deformation possibly reflecting a change in plate motions at the time of final gondwana assembly the new subduction regime along the gondwana margin in the early cambrian may be linked to the closure of the iapetus ocean basin