geology of the argentine continental shelf and margin from aeromagnetic survey new aeromagnetics show a complex of magnetic patterns and source depths of the north and central argentine continental and adjacent oceanic crust the 40 km wide positive magnetic g anomaly reflects an end rift associated volcanic edifice at the continental margin m4 mo and cretaceous quiet zone oceanic crust are identified the colorado magnetic discontinuity forms the southern boundary of the precambrian la plata craton to the south the continental crust of the patagonia platform is composed of amalgamated late precambrian through end paleozoic weakly magnetic sedimentary prisms the sw patagonia platform crust shows a strong high frequency magnetic pattern associated with mesozoic volcanics while zones of mid crustal level intrusives and some volcanics on the outer shelf are expressed as a broad higher amplitude magnetic pattern

variations in thickness of layer 3 dominate oceanic crustal structure abstract seismic refraction studies have determined that while much of the crust underlying the world s oceans is about 7 km thick it also occurs in lesser and considerably greater thicknesses here we show that vastly different thicknesses of oceanic crust formed at ridges by seafloor spreading are generally characterized by the two gradient velocity structure recognized as typical of 6 7 km thick oceanic crust adopting a standard we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km several structures formed in midplate settings ocean islands and seamounts are also considered for comparison regardless of origin ridge or midplate we find that the percentage of the whole crust formed by layer 2 systematically decreases with increasing total crustal thickness while the average velocity of layer 2 varies widely for all crust the average velocity of layer 3 increases systematically with increases in the average whole crustal thickness and velocity crust formed in midplate settings differs in that the thickness of layer 2 velocity crust is generally two to three times that of crust formed at ridges residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle the crustal sections were restored to zero age assuming the average depth of emplacement for 7 km thick oceanic crust is 2 5 km below sea level from this restoration it appears that the depth to the top of layer 3 at the time it was formed is 4 25 1 25 km below sea level regardless of whole crustal thickness or spreading rate these results suggest that the mechanism of construction of oceanic crust is in many ways remarkably uniform despite very large changes in the total thickness of the crust produced very basic questions remain it is for instance not at all clear why layer 2 and layer 3 exist in the proportions they do while neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the layer 2 3 boundary they do not seem adequate to account for the global phenomenon

flexural ridges trenches and outer rises around coronae on venus high resolution altimetry collected by the magellan spacecraft reveals trench and outer rise topographic signatures around major coronae e g eithinoha heng o artemis and latona in addition magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere we have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim the approach was tested at northern freyja montes where the best fit elastic thickness is 18 km in agreement with previously published results the elastic thicknesses determined by modelling numerous profiles at eithinoha heng o artemis and latona are 15 40 37 and 35 km respectively at eithinoha artemis and latona where the plates appear to be yielding the maximum bending moments and elastic thicknesses are similar to those found at the middle america mariana and aleutian trenches on earth respectively estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients at heng o artemis and latona temperature gradients are less than 10 k km which correspond to conductive heat losses of less than one half the expected average planetary value we propose two scenarios for the creation of the ridge trench and outer rise topography differential thermal subsidence and lithospheric subduction the topography of heng o is well matched by the differential thermal subsidence model however at artemis and latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone in these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts rolls back and the corona diameter increases

evidence for a volcanic rifted margin and oceanic crustal structure for the falkland plateau basin marine geophysical data from the western falkland plateau show strongly lineated magnetic anomalies and a coincident linear gravity high that identify the southeastern margin of a falkland islands block as a volcanic rifted continental margin characteristics of onshore and offshore dykes suggest this margin is most probably slightly younger than 190 ma together with these data seismic reflection profiles and sonobuoy seismic refraction data from the basin province of the falkland plateau east of the islands indicate an elevated oceanic origin for the major part of the falkland plateau basin most probably within the karoo hotspot province these conclusions are compatible with but do not directly indicate the independent rotation of a falkland islands block and closely constrain models of southwest gondwana evolution

the geological evolution of the falkland islands continental shelf the falkland islands are surrounded by four major sedimentary basins the falkland plateau basin to the east the south falkland basin to the south the malvinas basin to the west and the north falkland basin to the north the four main basins appear to have formed initially as triassic through earliest cretaceous extensional rifts associated with the break up of gondwana a valanginian end to rifting was followed by thermal sag there is evidence of cenozoic uplift in at least the north falkland basin possibly coincident with andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the scotia sea there is no evidence from offshore seismic and gravity magnetic data to support interpretations that the falkland islands have rotated clockwise through up to 180 during gondwana separation with the exception of the south falkland basin all the major basins probably underwent initially more or less east west extension and had a similar orientation to adjacent south american and western southern african basins the falkland basins probably shared a similar geological history with the offshore southern african and south american basins

basal mass budget of ross and filchner ronne ice shelves antarctica derived from lagrangian analysis of icesat altimetry traditional methods of deriving temporal variability of antarctic ice shelf elevation from satellite altimetry use a fixed eulerian reference frame where the measured changes include advection of ice thickness gradients between measurement epochs we present a new method which removes advection effects by using an independent velocity field to compare elevations in a moving lagrangian reference frame applying the technique to icesat laser altimetry for the period 2003 2009 over the two largest antarctic ice shelves ross and filchner ronne we show that the lagrangian approach reduces the variability of derived elevation changes by about 50 compared to the eulerian approach and reveals clearer spatial patterns of elevation change the method simplifies the process of estimating basal mass budget from the residual of all other processes that contribute to ice shelf elevation changes we use field data and icesat measurements over ice rises and the grounded ice sheet to account for surface accumulation and changes in firn air content and remove the effect of ice flow divergence using surface velocity and ice thickness data the results show highest basal melt rates 5 m a 1 near the deep grounding lines of major ice streams but smaller melt rates 5 m a 1 near the ice shelf fronts are equally important to total meltwater production since they occur over larger areas integrating over the entire ice shelf areas we obtain basal mass budgets of 50 64 gt a 1 for ross and 124 66 gt a 1 for filchner ronne with changes in firn air content as the largest error source

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

the crustal structure of the maurice ewing bank abstract the maurice ewing bank is a bathymetric high at the eastern termination of the falkland plateau in the south atlantic ocean during the jurassic the falkland plateau was situated between the south american antarctic and african plates and therefore its crustal composition is of interest for plate kinematic reconstructions of the southern ocean on the basis of recently acquired wide angle seismic and potential field data a crustal model is established for the maurice ewing bank the adjacent eastern falkland plateau basin and the western georgia basin according to the model the falkland plateau basin is floored by thick oceanic crust an 80 km wide continent ocean transition zone is located towards the maurice ewing bank the data indicate that the maurice ewing bank is composed of continental crust of up to 29 km thickness its western part shows fast crustal velocities 7 0 km s and strong intracrustal reflections the wide angle data provide no evidence for strong tectonic or magmatic overprinting of the bank s central portion the continental crust thins eastwards to 11 km the oceanic crust of the georgia basin is of average thickness 7 km for its type here ship and airborne magnetic data confirm previously identified mesozoic spreading anomalies and the onset of oceanic crust formation at m10n 133 ma time

stratigraphy and sedimentology of the permian of the falkland islands lithostratigraphic and palaeoenvironmental links with south africa comparison of the facies ichnology and palaeocurrent patterns of the permian of the falkland islands and the dwyka ecca and beaufort part groups in south africa supports the hypothesis that the falkland islands lay east of south africa in a rotated position prior to continental break up key sections in the falklands nw lafonia and the eastern cape ecca pass would have lain about 200 km apart in permian time in the absence of accurate chonostratigraphic and biostratigraphic information from the permian of the falkland islands emphasis is placed on lithostratigraphic comparison the permian successions in both areas pass from diamictite through basin floor sediments including organic rich shale rhythmites and turbidites mud dominated delta front deposits culminating in channel deposits of a generally subaqueous delta top overlie the basin floor sediments points of close lithostratigraphic comparison are 1 the diamictites have identical appearance facies and compatible palaeocurrents 2 glacial sections are overlain by organic rich black shales 3 k bentonites occur in basin floor successions 4 presence of basin floor turbidite sandstone units with interbedded rhythmite units and a distinctive umfolozia undichna ichnofauna 5 coarsening up delta front sequences are overlain by fining up channel sandstone facies 6 there is a close petrographic similarity indicating derivation of sandstones from a contemporaneous volcanic arc the permian successions of the two areas form parts of the fill of the eastern end of the main karoo basin and were subject to the same influences of tectonic development sediment derivation eustatic variation and climatic change

late palaeozoic to mesozoic structural evolution of the falkland islands a displaced segment of the cape fold belt the falkland islands lie on a displaced crustal block presently forming part of the south american plate the islands possess two roughly orthogonal structural grains the relative chronology of which is here unequivocally established for the first time d1 structures form a southerly verging permo triassic age fold belt striking e w to wnw ese which represents a displaced segment of the gondwanian orogenic belt the strike swing from e w to wnw ese is coincident with a decrease in deformation intensity toward the west on west falkland d1 folds are superimposed by the early mesozoic d2 ne sw trending hornby mountains anticline producing localized kilometre scale type i and iii fold interference patterns the hornby mountains anticline is interpreted to be the result of dextral transpressive reactivation of a pre existing ne sw basement fault if the falkland islands are rotated 180 the structure of the d1 fold belt and tectono sedimentary features of the permian age lafonian supergroup display a remarkable correlation with the structural style and tectonic evolution of the eastern cape fold belt of south africa our data therefore support a 180 tectonic rotation of the falkland islands and reaffirm their pre gondwana break up position adjacent to the southeast coast of south africa in such a reconstruction the falkland islands constrain the easterly extension and eventual lateral termination of the cape fold belt

mesozoic seafloor spreading in the southern weddell sea a re evaluation of weddell sea magnetic anomaly data in the light of satellite gravity maps provides important constraints on plate kinematics prior to chron c34 83 ma although m series anomalies are difficult to identify the evidence favours younger ages than previously suggested based on the assumption that these anomalies formed at the south america antarctica plate boundary geosat derived free air gravity maps show a distinct anomaly anomaly t associated with the southern termination of a herring bone pattern of flow lines the associated magnetic anomaly has been identified previously as anomaly m20 on the basis of a model of very slow spreading but is interpreted here as an edge effect associated with an abrupt decline in spreading rate which occurred between m4 126 ma and m0 118 ma and which induced the fine segmentation pattern to the north although anomalies are present to the south of anomaly t they are of reduced amplitude and are difficult to correlate over distances of more than c 100 km nevertheless they appear to strike approximately w e sub parallel to the younger anomalies to the north and show a resemblance to anomalies m4 m12 modelled at a rate of 15 km ma 1 flow line modelling based on published rotations for the southwest indian ridge and southern mid atlantic ridge gives a reasonable fit to the observed trends of weddell sea gravity ridges two alternative models for the m series reconstruction of africa and antarctica are employed corresponding to the two m2 fits suggested by bergh although it is not possible to reject either of the models the tighter fit of the two gives slightly better agreement with weddell sea trends in both models a drop in rates occurs at m0 time anomalies farther south correspond with previously identified features such as the orion explora and andenes anomalies and the polarstern bank all characterized by magnetic anomaly highs attributed to excess volcanism during early break up

anatomy and formation of oblique continental collision south falkland basin 1 the south falkland basin is a partially filled cenozoic foreland basin located south of the falkland plateau it was formed by flexure of this southern edge of the south american plate when the load represented by burdwood bank collided this continental fragment belongs to the predominantly oceanic scotia plate flexure probably started in early cenozoic times and has continued to the present day the whole region is submarine and so the detailed stratigraphy and structure of the basin has been well imaged by seismic reflection profiling the clarity of this imagery has made analysis of structures within the collision zone possible the plate boundary itself is an active oblique thrust fault which has controlled the growth of a frontal fold there is evidence for older phases of thrusting and folding further south within and beneath the sediments which blanket the flexed south american plate normal faulting occurs on a variety of scales episodes of stratigraphic growth associated with the largest of these faults demonstrates that they were active during flexural bending we have modeled the development of the south falkland basin using two different approaches both of which are based upon the simplest elastic model inverse modeling of free air gravity and bathymetric profiles suggest that the elastic thickness of the loaded crust is 5 20 km a complementary approach based upon the spectral analysis of free air gravity and bathymetry shows that the elastic thickness is 15 5 km both techniques indicate that the flexed continental lithosphere is weak a conclusion supported by the presence of normal faults within the flexed plate a small increase in elastic thickness from west to east appears consistent with a change in the density and penetration of normal faulting

petrography of the paradise no 12 baker no 13 and coiltown no 14 coals of the lower portion of the sturgis formation pennsylvanian in western kentucky coal rank based on vitrinite reflectance and maceral composition were determined for 50 channel and borehole samples of the paradise no 12 baker no 13 and coiltown no 14 coals in the western kentucky coal field the three coal beds are in the lower part of the pennsylvanian sturgis formation desmoinesian coal rank trends for the three coals vary across structural provinces in a similar and predictable manner coals in the moorman syncline in the southeast are predominantly of high volatile c bituminous rank but approach high volatile b bituminous rank within the pennyrile and central fault systems that border the moorman syncline westward across the central faults rank of coals is high volatile b bituminous in the webster syncline but increases to high volatile a bituminous northward within the rough creek fault system coal rank in the henderson basin north of the rough creek faults decreases to high volatile c bituminous the paradise baker and coiltown coals have consistently high percentages of total vitrinite vitrinite and pseudovitrinite 88 for the paradise coal 87 for the baker coal 85 for the coiltown coal and concomitant uniform low percentages of inertinites less than 10 for each coal dry mineral matter free basis total reactives total vitrinite and liptinites constitute 91 of the paradise and baker coals and 90 of the coiltown coal comparison of the three coals with stratigraphically lower coals in the pennsylvanian of western kentucky indicates that rank trends and maceral composition continue to be predictable not only across the western kentucky coal field but also within a large portion of the pennsylvanian stratigraphic section

the falkland plateau in the context of gondwana breakup abstract during the jurassic the falkland plateau was part of gondwana and occupied a position between the african and antarctic plates several contrasting models exist for the breakup of gondwana that depend on assumptions about the currently unknown crustal structure of the falkland plateau here we present the results of recently acquired wide angle seismic data along the entire plateau that provide sound constraints on its role in geodynamic reconstructions in contrast to published crustal models the new data show that the falkland plateau basin consists of up to 20 km thick oceanic crust which is bounded to the east by a continental fragment the maurice ewing bank in a refined geodynamic model rifting started between the falkland islands and the maurice ewing bank at 178 ma and ceased at around 154 ma the plateau s exceptionally thick oceanic crust likely results from its position in an extensional back arc regime situated over a mantle thermal anomaly that was also responsible for the extensive onshore karoo ferrar and chon aike volcanic provinces

pb nd and sr isotope mapping of grenville age crustal provinces in rodinia new pb nd and sr isotope data are presented for geochemically similar 1 1 1 2 ga granitoids and tonalitic granitic orthogneisses from antarctica southern africa and the falkland islands and adjoining plateau areas originally within the supercontinents of rodinia and gondwana these data support contentions for the presence of a mesoproterozoic 1 2 ga destructive plate margin running from namaqua natal southern africa through the displaced microplates of the falkland plateau and falkland islands the haag nunatak crustal block west antarctica and into western dronning maud land east antarctica the bulk of these granitoids represent juvenile mesoproterozoic additions to the crust except for in parts of east antarctica i e the sverdrupfjella where older paleoproterozoic or archean crust was involved in granitoid generation our isotope data permit plate reconstructions in which southern africa east antarctica and the falkland islands and plateau were adjacent within rodinia

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

relics of a complex triple junction in the weddell sea embayment antarctica abstract interpretation of an airborne magnetic data compilation containing a key new survey together with re tracked satellite gravity data from the weddell sea embayment wse west antarctica suggests rift rift rift triple junction formation at the onset of gondwana breakup in the early middle jurassic a complex system of northwest southeast rifts was active contemporaneously with an east west trending rift this rift activity led to northward separation of the falkland plateau and formation of the weddell sea by sea floor spreading atypically the jurassic passive margin of gondwana shows evidence for coeval extension in two directions and a large volume of interpreted magmatic material this is consistent with initial doming above a mantle plume and we suggest that this resulted in the formation of a triple junction magnetic anomalies indicate a series of faults perpendicular to igneous intrusions and extrusions with outlines that range in shape from lozenges to parallel ridges they show remarkably good spatial correlation with free air gravity anomalies even in areas of sea ice we base a structural elements map and timing sequence for the events in the wse during early gondwana breakup on anomaly cross cutting relationships

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

geochemistry and isotopic evolution of the mesoproterozoic cape meredith complex west falkland whole rock major and trace element geochemical and rb sr sm nd isotopic data are presented for the mesoproterozoic 1 0 ga metamorphic and igneous rocks of the cape meredith complex west falkland the data indicate that the oldest rocks the 1 1 ga supracrustal gneisses of the big cape formation which form three petrographic and geochemical groups mafic amphibolite quartz plagioclase biotite hornblende intermediate gneiss and acid gneiss probably represent a juvenile calc alkaline basalt andesite rhyolite volcanic sequence with epsilon endt values and nd t dm ages of 3 to 6 and 1100 to 1400 ma respectively it is argued on geochemical grounds that these metavolcanics were extruded in an island arc at around 1120 ma the big cape formation was intruded by granitoids during and after a collisional orogenic event at around 1090 ma the oldest foliated g1 granodiorite was emplaced as thin sheets at approximately 1090 to 1070 ma and is characterized by end values of 1 5 to 4 t dm 1200 to 1400 ma showing its juvenile nature the 1070 ma g2 syntectonic granitoid gneisses and 1000 ma g3 post tectonic granites also exhibit juvenile characteristics e nd 0 to 5 and t dm 2200 to 1200 ma respectively the granitoids show a time composition evolution from na rich g1 granodiorite to potassic high high field strength element granites g3 the geochemical and isotopic characteristics and geological evolution of the cape meredith complex is comparable with that of the adjacent gondwana crustal blocks in natal se africa and dronning maud land east antarctica supporting models that demonstrate these areas evolved in a contiguous juvenile arc environment prior to and during a major orogenic event at 1 1 ga these events were associated with the birth of the rodinian supercontinent the three areas remained juxtaposed during rodinia break up and were subsequently incorporated into gondwana in the same relative positions

the falkland islands a key element in gondwana paleogeography stratigraphic and sedimentological fieldwork on the falkland islands confirms that they were originally part of south africa east and west falkland have contrasting geological and deformation histories and are separated by the falkland sound fault which marks a significant deformation edge to the cape fold belt the degree of structural deformation and metamorphism in west falkland is substantially less than that of east falkland adjustments to differential shortening between the two islands being reflected by structures developed along the falkland sound fault prior to fragmentation of gondwana the falkland platform and maurice ewing bank were positioned in the natal valley adjacent to the east coast of south africa this predrift reconstruction shows that the falkland islands have subsequently been rotated by 180 relative to south africa and complements recent paleomagnetic data there was a complete overlap of the maurice ewing bank and falkland platform to the exclusion of the falkland plateau basin which is a small oceanic basin formed during the jurassic fragmentation of gondwana extension in this basin was accommodated by movements along the aghulas falkland fracture zone and the gastre fault system predrift restoration of these faults shows that patagonia was positioned closer to both africa and antarctica the falkland plateau basin was an integral component of the west gondwana jurassic microplate system and provides an invaluable insight into the evolution of its western margin at some time prior to the main opening of the south atlantic the falkland platform was rotated clockwise by 120 against the agulhas falkland fracture zone such that the falkland islands were translated some 300 km to the south the driving force for this rotation is not known but could possibly have been initiated by periodic stress buildups in the falkland plateau basin as the gastre fault system within continental crust became unable to accommodate the continuous dextral shear in the agulhas falkland fracture zone the falkland islands represent an outboard extension of the eastern cape and are significant for understanding the paleogeography of both the cape and karoo supergroups east falkland is closely similar to south africa in its geology while west falkland shows significant contrasts with both the devonian and permian successions being more proximal in character but still demonstrate the eastward continuation of the cape and karoo basins

40ar 39ar ages for dykes from the falkland islands with implications for the break up of southern gondwanaland 40 ar 39 incremental heating spectra analyses were made of five microgabbro dykes from the falkland islands two of which yielded an acceptable plateau and a third a close maximum estimate all three indicate an age of dyke intrusion of about 190 ma supporting a single published k ar date this provides further support for the geological continuity of the falkland islands and the eastern karoo basin where dyke swarms of similar petrological affinities were intruded at about the same time the age constrains the previously published palaeoreconstruction of the falkland islands based on palaeomagnetic data which places the falklands immediately to the east of southern africa to be valid until at least the late early jurassic

a revised position for the rotated falkland islands microplate the early stages of transform margin formation are associated with crustal fragmentation and block rotation the restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo an example considered here is the falkland plateau this is located adjacent to the agulhas falkland fracture zone and its westernmost province is the falkland islands microcontinent the position of the plateau and the islands prior to gondwana break up remains contentious this study integrates seismic reflection and gravity data to propose a revised position of the falkland islands microcontinent constrained by 1 the presence of a mega decollement controlling the gondwanide orogen described north of the falkland islands and underneath south africa and the outeniqua basin and 2 the similar architecture of fault networks mapped north of the islands and in the northernmost outeniqua basin this revised position requires a re evaluation of the timing and rate of rotation of the falkland islands microcontinent and affects the expected crustal architecture adjacent to the islands our model yields rotation rates between 5 5 and 8 ma 1 and two potential times for rotation and predicts more unstretched crust beneath the basin east of the falkland islands than previous models

the three dimensional lithospheric structure of the falkland plateau region based on gravity modelling the lithospheric structure of the falkland plateau region has been modelled by making an initial estimate on the basis of local isostasy and then refining the geometries by inversion of gravity anomalies the model predicts a crustal thickness beneath the falkland plateau basin that is more than twice that previously inferred from seismic evidence the preferred explanation is that the seismic survey detected high velocity underplated lower crust rather than upper mantle this is compatible with the results of deep seismic experiments over the conjugate filchner block of antarctica and also appears likely on the basis of the position of the falkland plateau basin in relation to the karoo ferrar magmatic province at the time of extension continental crust is inferred to be continuous beneath the northern part of the falkland plateau as there are consistent magnetic and flexural anomalies associated with the continent ocean boundary flexural modelling of the southern margin of the plateau indicates lateral variations in strength with the strongest lithosphere beneath the falkland plateau basin this may indicate thick oceanic crust beneath the southern part of the basin or the effect of thinning relatively weak continental crust and replacing it with relatively strong underplating

the lithosphere below the rockall trough wide angle seismic evidence for extensive serpentinisation abstract the longer range data from wide angle seismic reflection experiments along an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the rockall trough in the north atlantic two subcrustal p and s wave reflections are observed the first defines a layer 3 10 km thick below the moho where p wave velocities vary from 7 5 to 7 8 km s v p v s ratios increase within this layer from 1 80 in the north to 1 83 in the south of the basin along the axial profile the second reflection from approximately 34 km depth identifies a layer 15 20 km thick with a vp velocity of approximately 8 1 km s and a v p v s ratio of 1 73 these values are typical for normal mantle peridotites both p and s wave velocities and v p v s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below the moho about 15 volume alteration of the parent mantle peridotite is required to produce the observed seismic properties this degree of alteration accounts for a systematic deficit in total tectonic subsidence when compared with that predicted from the variation in bulk crustal stretching along the axis of the basin syntectonic cooling occurred during differential lithospheric stretching as the upper to mid crust became more extended over a narrower region than the mantle lithosphere this served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle ductile transition zone in the crust migrated downwards into the mantle lithosphere the resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle

the crustal structure and sedimentation of the weddell sea embayment implications for gondwana reconstructions abstract the weddell sea embayment is central to reconstructions of the west antarctic region of gondwana some reconstructions represent this area as a pre break up component the filchner block defined by the present 2000 m isobath and coastline but take no account of crustal extension and sedimentary progradation of the continental margin since break up seismic refraction shows a wide deep sedimentary basin beneath the central part of the embayment indicating extensive rifting and stretching of continental crust during the break up of gondwana this may have increased the width of the block by about 300 km potential field data suggest extensive progradation of sediments on the weddell sea continental margin this may have moved the margin seaward between 200 and 400 km taken together these modifications in the dimensions of the pre break up filchner block represent between one half and one third the area of the present weddell sea embayment the presence of extensive rifting is indicative that hot thin crust was present during break up providing further evidence that several closely spaced mantle plumes or a single megaplume impinged on the weddell sea sector of gondwana in the early jurassic the lithospheric stretching in the embayment provided a mechanism for the rotation of the ellsworth mountains region during the early stages of gondwana break up commensurate with the paleomagnetic data

flexural unloading and uplift along the cote d ivoire ghana transform margin equatorial atlantic recent ocean drilling program sampling of the cote d ivoire ghana margin of west africa provides for the first time the opportunity to study the development of a marginal ridge that formed along a sheared passive margin adjacent to the continent ocean transition after the end of intracontinental wrenching we model its evolution using a two dimensional flexural backstripping technique the model is constrained by existing seismic refraction and reflection data on the crustal structure and stratigraphy and paleobathymetric evidence from the cores following rifting at 120 ma aptian intracontinental wrenching continued until 105 ma mid albian when south america and africa separated along this transform while the possible presence of thicker crust under the marginal ridge compared to the deep ivorian basin may explain some of the ridge s topography the entire uplift can be readily modeled as a flexural response to unloading along a shallow dipping 25 fault at the time of continental separation forward subsidence modeling of the marginal ridge suggests that an effective elastic thickness of 2 5 km at 105 ma is most appropriate to match the observed structure conduction of heat from the oceanic plate across the continent ocean transition drove temporary uplift of at least 1200 m peaking just before 89 ma when sedimentary data show deposition within the photic zone 0 50 m this study shows that significant transient thermal uplift can be found in sheared continental passive margin settings even where magmatism is insignificant

erosion and flexural uplift along transform faults summary in all geodynamic settings intracontinental continent ocean intraoceanic the main morphological feature of transform plate boundaries is a narrow and elongated valley bounded on at least one side by an uplifted shoulder several mechanisms have been proposed to explain this uplift differential thermal subsidence lateral heat transfer extension perpendicular to the transform but none can be used in all settings we propose that the erosion of a lithospheric plate along the transform boundary may be an alternative mechanism that can be active in all geodynamic settings erosional unloading produces flexural uplift along the transform border which does not exceed 35 40 per cent of the difference in height between the lithospheric plate and the transform valley our model perfectly fits the morphology of several examples in the continental lithosphere the uplifted shoulder of the dead sea transform as well as marginal ridges in continental transform margins along an oceanic transform fault the erosion of the transform border only partially explains the uplift of the highest transverse ridges but may amplify other mechanisms

the crustal structure of the continental margin east of the falkland islands abstract the 1500 km long falkland plateau is the most prominent morphological structure in the southern south atlantic ocean which crustal composition and development is mainly unknown at the westernmost boundary of the plateau the falkland islands precambrian geology provides the only insight into basement structure and age the question of whether continental basement of a similar age and origin underlies the falkland plateau further east is strongly disputed we present new high quality constraints on the crustal fabric of the plateau east of the falkland islands based on wide angle seismic and potential field data acquired in 2013 the p wave velocity model supported by amplitude and density modelling shows that the falkland plateau basin is filled with 8 km of sediments continental crust of 34 km thickness underlies the falkland islands the eastern continental margin of the falkland islands can be classified as a volcanic rifted margin the falkland plateau basin is floored by up to 20 km thick oceanic crust the exceptionally thick igneous crust and its high lower crustal velocities up to 7 4 km s indicate the influence of a regional thermal mantle anomaly during its formation which provided extra melt material the wide angle model revises published crustal models which predicted thin oceanic or thick extended continental crust below the falkland plateau basin our results provide a sound basis for future tectonic interpretations of the area

a new magnetic map of the weddell sea and the antarctic peninsula abstract as part of the antarctic digital magnetic mapping project admap workers from vniiokeangeologia russia the british antarctic survey uk and the naval research laboratory usa have brought together almost all of the available magnetic data in the area 0 120 w 60 90 s the final map covers the whole weddell sea and adjacent land areas the antarctic peninsula and the seas to the west an area comparable in size with that of the usa this paper describes the methods used during the compilation of the map and reviews briefly some of the main features shown on it distinct magnetic provinces are associated with precambrian rocks of the east antarctic craton highly extended continental crust in the weddell sea embayment the arc batholith of the antarctic peninsula and oceanic crust of the northern weddell sea which was created as a direct consequence of south america antarctica plate motion and oceanic crust generated at the pacific antarctic ridge the magnetic anomaly map thus provides an overview of the fragmentation of south western gondwana and the tectonic development of the weddell sea sector of antarctica

mesozoic rifting evolution of sw gondwana a poly phased subduction related extensional history responsible for basin formation along the argentinean atlantic margin abstract the opening of the south atlantic in the early cretaceous was the final stage of the complex rifting history of sw gondwana in this contribution we reassess the chronology of mesozoic basin formation in southern south america and africa and integrate it in the long term breakup history of sw gondwana triassic rifting is characterized by intracontinental rifting in africa karoo i phase and retro arc extension on the sw margin of gondwana in the early jurassic the impingement of the karoo plume triggered rifting in eastern africa producing the karoo ii basins and the colorado and salado basins on the argentinean shelf east africa rifting ultimately lead the breakup of eastern from western gondwana in the middle jurassic in patagonia the austral malvinas and other related basins formed in association with the synextensional emplacement of the chon aike magmatic province in the patagonian retro arc in the late jurassic the rocas verdes back arc basin opened in southern patagonia while oblique rifting in the core of the late paleozoic gondwanides orogen produced the outeniqua and rawson valdes basins the south atlantic rift initiated in the early cretaceous associated with present day e w extension rifting occurred diachronically from south to north initiating in the previously thinned rawson valdes outeniqua segment a precursor oblique rift system and a larger degree of extension in this segment could explain the lack of seaward dipping reflectors sdr south of the colorado cape fracture zones rifting and sdr emplacement occurred progressively to the north along different rift segments producing strongly asymmetric conjugate margins