on the antarctic peninsula batholith the plutonic rocks of the antarctic peninsula magmatic arc form one of the major batholiths of the circum pacific rim the antarctic peninsula batholith is a 1350 km long by 210 km wide structure which was emplaced over the period 240 to 10 ma with a cretaceous peak of activity that started at 142 ma and waned during the late cretaceous early jurassic and late jurassic early cretaceous gaps in intrusive activity probably correspond to episodes of arc compression in a northern zone of the antarctic peninsula the batholith intrudes palaeozoic mesozoic low grade meta sedimentary rocks and in a central zone it intrudes schists and ortho and paragneisses which have late proterozoic nd model ages and were deformed during triassic to early jurassic compression in a southern zone the oldest exposed rocks are permian sedimentary rocks and deformed jurassic volcanic and sedimentary rocks all these pre batholith rocks formed a belt of relatively immature crust along the gondwana margin with few exceptions jurassic plutons crop out only within the central zone many are peraluminous having s like mineralogies and relatively high 87sr 86sri they are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion by contrast early cretaceous plutons crop out along the length of the batholith few magma compositions appear to have been affected by upper crust the bulk being compositionally independent of the type of country rock they intrude they are dominated by metaluminous calcic si oversaturated 1 type granitoid rocks with relatively low 87sr 86sri intermediate silicic compositions 5 mgo we interpret these to represent partial melts of basic to intermediate igneous locally garnet bearing lower crust contemporaneous mafic magmas e g syn plutonic dykes form a more alkaline si saturated series having higher 143nd 144nd at the same87sr 86sr than the intermediate silicic series to which they are not petrogenetically related the change from limited partial fusion of upper crust in jurassic times to widespread partial fusion of lower crust in early cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time

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

reassessment of net surface mass balance in antarctica abstract recent in situ measurements of surface mass balance and improved calculation techniques are used to produce an updated assessment of net surface mass balance over antarctica a new elevation model of antarctica derived from ers 1 satellite altimetry supplemented with conventional data was used to delineate the ice flow drainage basins across antarctica the areas of these basins were calculated using the recent digital descriptions of coastlines and grounding lines the delineation of drainage basins was achieved using an automatic procedure which gave similar results to earlier hand drawn catchment basins more than 1800 published and unpublished in situ measurements of net surface mass balance from antarctica were collated and then interpolated a net surface mass balance map was derived from passive microwave satellite data being employed as a forcing field to control the interpolation of the sparse in situ observations basinwide integrals of net surface mass balance were calculated using t

contrasting styles of rifting models and examples from the eastern canadian margin we present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm lithospheric composition dry versus wet materials and strain rate the basic mechanism of continental rupture is assumed to be passive rifting and necking we use a numerical thermomechanical model of lithosphere extension based on a finite element approach when plasticity is significant i e at lower temperatures or for harder materials deformation is unstable and thinning takes place abruptly over a narrow area conversely a progressive thinning across the margin is observed when creep is dominant i e in warm or ductile conditions cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally in these circumstances rupture is likely to take place asymmetrically along one edge of the thinned area producing a narrow margin and a very wide conjugate the eastern margins of canada and their conjugates across the north atlantic provide examples which cover this range of theoretical profiles the crustal thinning patterns inferred from deep seismic data and the duration of rifting compare well with model results we discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust or the location of the ocean continent boundary in wide areas supposedly underlain by 5 km thin continental crust

the crary fan a trough mouth fan on the weddell sea continental margin antarctica abstract multichannel seismic investigations show the presence of a large trough mouth fan deposit on the continental margin in the southern weddell sea a characteristic feature of the fan is relatively few but large canyons and or channels which appear to have been stable for long periods levees associated with the channels may reach thicknesses of more than 1 km drilling during odp leg 113 recorded the presence of glaciers in east antarctica since early oligocene time and the seismic stratigraphic evidence suggests that the submarine fan is largely composed of glacigenic sediments the channel levee systems and complexes indicate several growth phases apparently controlled by glacial interglacial climate fluctuations with maximum fan growth during glacial periods the present interglacial period is characterized by strong and erosive flow of ice shelf water running down the western slope of the fan and carrying only minor amounts of suspended matter the fan is essentially sediment starved during interglacial periods the dimensions of the channel levee complexes and the persistent loci of sediment supply suggest that turbidite sedimentation resulted from melting of wet based glaciers at or near the shelf edge which has consequently prograded 70 80 km seaward

weddell sea palaeoceanography preliminary results of odp leg 113 abstract odp leg 113 drilled 22 holes at 9 sites in the weddell sea to investigate the cenozoic development of the modern cold circum antarctic water mass structure and the continental glaciation the sites sampled 4 contrasting environments open ocean pelagic sedimentation on maud rise sites 689 and 690 the east antarctic continental margin 691 693 the deep weddell basin 694 and the southeastern margin of the south orkney microcontinent 695 697 a depth transect considerable work remains to be done on the cores but preliminary results reported here are of great interest the maud rise sites together provide a near continuous section from the late cretaceous to the present day which will form a southernmost anchor for atlantic biostratigraphy biogeography and isotopic studies the sequence is calcareous through the eocene mixed calcareous and siliceous into the late miocene and almost entirely siliceous thereafter apparently continuous k t boundary sections were cored at each site the paleocene section at site 690 contains eolian sediment from east antarctica which suggests a warm semi arid continental climate at that time at the east antarctic margin lower cretaceous organic rich sediments below a 60 ma hiatus indicate restricted circulation overlying oligocene to recent mixed hemipelagic terrigenous and biosiliceous sediments suggest a pronounced middle miocene increase in intensity of east antarctic glaciation with increased ice rafting and hemipelagic deposition and canyon cutting on the margin a rapid influx of terrigenous turbidites to the weddell abyssal plain at site 694 in the earliest pliocene is considered to mark the growth of a west antarctic ice sheet subsequently stable west antarctic climatic history is also revealed at sites 695 697 nothofagus pollen and fern spores from the eocene of site 696 suggest a climate similar to present day new zealand overlying sediments indicate a cooler climate with an almost completely siliceous biota even at palaeodepths of 650 m or less from the middle miocene pliocene biosiliceous and ice rafted terrigenous deposition was extremely rapid reaching 200 m ma at times at the south orkney sites after 2 4 ma however at all leg 113 sites but 697 the deposition of biosiliceous and ice rafted sediment decreased markedly probably reflecting greater sea ice cover and grounding of the east antarctic ice sheet to the shelf edge even during interglacial periods an interesting exception to the siliceous domination of biogenic sedimentation through the neogene is the occurrence at all leg 113 sites above 3400 m of a calcareous horizon containing neogloboquadrina pachyderma and a restricted benthic foraminiferal assemblage within the quaternary

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

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

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

late gondwanide tectonic rotations within gondwanaland geologic and paleomagnetic evidence from the ellsworth whitmore mountains crustal block of west antarctica and from the falkland islands of south america indicates that tectonic displacement of major portions of the samfrau geosyncline occurred after the early mesozoic gondwanide folding but prior to seafloor spreading comparison with continental deformation in the collisional alpine himalayan belt offers a mechanism for the displacements while supporting the hypothesis that the gondwanian orogeny resulted from collision of a fore arc and magmatic arc terrane with the panthalassic margin of the gondwana craton during closure of a marginal basin the displacements had major consequences for the ensuing evolution of the south atlantic weddell sea ross sea region

seismic stratigraphic evidence of ice sheet advances on the wilkes land margin of antarctica abstract the wilkes land continental shelf similar to other antarctic shelves is underlain by thick sequences of steeply prograded glacial diamictons on the outer shelf banks that are shallower than 400 m are separated by broad outer shelf troughs that deepen landward the prograded sequences are found preferentially in these broad outer shelf troughs we propose that these outer shelf prograding wedges were deposited by fallout from deforming till layer transport beneath ice streams at times of ice expansion onto the continental shelf such deforming till layer transport has recently been proposed to explain seismic observations beneath ice stream b of the ross embayment two prominent erosional unconformities with stratal truncations of more than 500 m indicate erosional events that overdeepened the shelf and provided the accommodation space to allow the deposition of these prograding sequences in front of advancing ice streams at times of past glacial maxima the erosional events that produced these extraordinary downcuts were caused by erosion by ice that expanded onto a shelf with water depths far too shallow for flotation these two particular erosional surfaces developed either on an initially shallow shelf or from an extraordinarily high flux of ice or both

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

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

geological evidence for a proterozoic to mesozoic link between southeastern africa and dronning maud land antarctica comparison of crustal provinces in southeastern africa and dronning maud land antarctica reveals considerable similarity in their evolution from the archaean until the mesozoic separation archaean granites and mid proterozoic supracrustal successions in these regions are so comparable that correlation is suggested a major mid to late proterozoic orogenic terrain in dronning maud land comprising the h u sverdrupfjella and heimefrontfjella subprovinces and termed the maudheim province is very similar in age lithology structural style and metamorphic history to the mozambique and natal orogenic provinces of kibaran age 1000 ma in southeastern africa deformed supracrustal sequences in all three provinces host syn tectonic granites intruded during upper amphibolite to granulite facies metamorphism isoclinal folding was accompanied by thrusting towards adjacent cratonic areas development of orogenic provinces of kibaran age in southeastern africa and antarctica reflects accretion of marginal basin volcanic arc sequences onto older continents the 500 ma pan african event was a widespread predominantly thermal overprinting of parts of the older orogenic provinces faulting and rifting of the supercontinent preceded break up and influenced the stratigraphy of phanerozoic sedimentary successions in se africa

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

a preliminary assessment of the hydrocarbon potential of the larsen basin antarctica abstract the larsen basin on the northwest margin of the weddell sea formed as a mesozoic ensialic basin during gondwana breakup deposition was either in half grabens on the extending weddell sea margin or in a restricted back arc basin at the northern end of this basin 5 6 km of sedimentary rock crop out on james ross island exposing elements of a large potential hydrocarbon system aeromagnetic and outcrop data suggest that the basin structure inferred from james ross island can be recognised at least as far south as 70 s upper jurassic anoxic marine strata deposited prior to the main phase of arc development form a rich potential source t o c up to 3 5 with both marine and terrestrial kerogens arc derived volcaniclastic sediments of barremian oligocene age form a regressive megasequence basal strata represent slope apron and rudaceous submarine fan deposits proximal to the margin fan conglomerates form lenticular bodies hundreds of metres thick and tens of kilometres across enveloped in slope apron mudstones late cretaceous fault reactivation and uplift led to dramatic shallowing of the basin with deposition of shelf facies although there are many potentially attractive reservoir targets there may be problems of pore occlusion due to the abundant labile volcanic grains however there is evidence of more quartzose sandstone towards the top of the section and inferentially toward the basin centre in the larsen basin there is moderate potential for oil generated from upper jurassic source rocks and reservoired in cretaceous and tertiary sandstones and conglomerates in large stratigraphic or structural traps caused by partial basin inversion during deposition

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

antarctic tectonics constraints from an ers 1 satellite marine gravity field a high resolution gravity field of poorly charted and ice covered ocean near west antarctica from the ross sea east to the weddell sea has been derived with the use of satellite altimetry including ers 1 geodetic phase wave form data this gravity field reveals regional tectonic fabric such as gravity lineations which are the expression of fracture zones left by early 65 to 83 million years ago pacific antarctic sea floor spreading that separated the campbell plateau and new zealand continent from west antarctica these lineations constrain plate motion history and confirm the hypothesis that antarctica behaved as two distinct plates separated from each other by an extensional bellingshausen plate boundary active in the amundsen sea before about 61 million years ago

aeromagnetic anomalies and discordant lineations beneath the niger delta implication for new fracture zones and multiple sea floor spreading directions in the meso atlantic gulf of guinea cul de sac an aeromagnetic contour map compiled over shallow water and onshore portions of the nigerian continental margin shows several elongate long wavelength anomaly closures with some alternating polarity separated by steep gradient ne lineations the lineations are interpreted as new fracture zones or extensions of previously mapped ones the ne trend in the western delta region is concordant with ee fracture zone trends of the deeper gulf of guinea aeromagnetic lineations of the se niger delta basin however discordantly trend ene their termination against the former is interpreted as evidence of early sea floor spreading in a ene wsw direction in addition to the well documented ne sw spreading of the gulf of guinea and the rest of the meso atlantic sea floor the geophysical crustal structure indicate the existence of two early cretaceous triple junctions beneath the niger delta basin the two triple junctions further support the hypothesis that the african continent was a multi plate system in the niger delta region during the early opening of the atlantic

rb sr geochronology of graham land antarctica thirty five new rb sr whole rock and mineral isochrons are reported for igneous rocks from graham land magmatism was essentially continuous but peaks of activity occurred in early jurassic early cretaceous late cretaceous and eocene times the oldest dated granite is triassic 209 3 ma volcanic rocks were erupted throughout this history and some previously supposed upper jurassic volcanic rocks must be mid cretaceous or younger a change in magma type from uniformly acid to predominantly basic or intermediate combined with westward migration of the arc may be partly responsible for previously reported transverse geochemical variations there is a marked trend of decreasing initial 87 sr 86 sr ratio of the plutonic rocks with time from c 0 707 to 0 704 although volcanic rocks tend to have higher values this trend is ascribed to the waning influence of an as yet unidentified pre magmatic crustal basement due to increasing depth of magma generation close parallels to the observed age pattern imply continuity of the andean belt through southern south america into graham land back to early mesozoic times and are not compatible with reconstructions of gondwana in which the antarctic peninsula is located w of south america

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

creation and destruction of weddell sea floor in the jurassic new paleomagnetic data invite reassessment of the breakup history of parts of the pacific margin of gondwana due to the mesozoic opening of the southern ocean basins these data indicate clockwise rotation of the antarctic peninsula relative to east antarctica between 175 and 155 ma that would have created as much as 1000 km of pre anomaly m25 ocean floor in the weddell sea basin the paleomagnetic data also suggest that between 155 and 130 ma the antarctic peninsula rotated counterclockwise relative to east antarctica rotation that along with the southward motion of east antarctica would have caused subduction of the newly created weddell sea ocean floor beneath the southern antarctic peninsula east vergent folding and thrusting of jurassic back arc basin rocks along the southeastern antarctic peninsula known as the palmer land event probably resulted from deformation along this newly created convergent margin the presence of jurassic weddell sea floor would have isolated eastern gondwana east antarctica australia and india from the antarctic peninsula and western gondwana africa and south america encouraging increased marine faunal connections between the pacific and weddell sea mozambique somali ocean basins during the 175 to 130 ma interval after 130 ma the antarctic peninsula would have been connected to east antarctica via the other west antarctic terranes thereby facilitating land migrations between eastern and western gondwana

west antarctica in gondwanaland crustal blocks reconstruction and breakup processes abstract a combined bas usarp west antarctic tectonic project has provided new constraints on the crustal structure and geological evolution of west antarctica and its relationship to the rest of gondwanaland rb sr age dating has confirmed the presence of 1200 1000 m y old precambrian gneisses within west antarctica and aeromagnetic data have defined the extent of this precambrian crustal block paleomagnetic data have constrained the position of four of the five geologically distinctive west antarctic crustal blocks within gondwanaland they can be restored to their original position prior to breakup by 15 25 counter clockwise rotation with little or no relative displacement between the blocks a major period of within plate middle jurassic magmatism associated with an important crustal melting event that is clearly related to the breakup of the supercontinent has also been identified based on new isotopic and geochemical data

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

hemispheric comparison of pmc altitudes a middle ultraviolet imager observed the vertical structure of polar mesospheric clouds pmcs in the northern and southern polar regions during the austral summer of 1997 1998 and the arctic summer of 1999 during 23 transpolar passes the imager obtained over 15 000 images at latitudes poleward of 55 the accuracy and stability of the satellite platform and the large database permit the first statistical investigation of the small scale 1 km altitude structure of pmcs on a transpolar scale during a satellite pass over either polar region pmcs clustered at different altitudes between 80 and 85 km southern clouds had a mean altitude of 83 2 1 4 km above the earth ellipsoid while northern clouds had a mean altitude of 82 6d 1 3 km so southern and northern clouds had the same altitudes to within statistical variations the mean pmc altitude was 82 7 1 3 km for both hemispheres in neither hemisphere did cloud altitudes exhibit systematic variation with latitude

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

aeromagnetic data from the southern weddell sea embayment and adjacent areas synthesis and interpretation a new regional aeromagnetic anomaly map covering the southern weddell sea and surrounding areas was produced by combining russian and british magnetic anomaly data acquired between 1973 and 1989 the map is divided into four zones on the basis of characteristic anomaly wavelength antarctic peninsula weddell sea embayment haag nunataks and the western coast of east antarctica over the southern weddell sea and the ronne and filchner ice shelves the magnetic field is relatively smooth indicating a thick sedimentary cover although there are some distinctive anomalies a broad positive anomaly is clearly visible running w e across the north of the area which is interpreted as representing the continent ocean boundary the most prominent magnetic feature is the elongate explora anomaly which runs parallel to the coast of east antarctica and continues over berkner island terminating at the dufek massif we interpret the source of this anomaly as rift related volcanic rocks probably emplaced around the same time as the dufek intrusion the rift itself is interpreted as running south between the andenes escarpment and east antarctica and terminating south of berkner island russian gravity profiles show that berkner island does not have an associated bouguer anomaly although a large positive anomaly approx 80 mgal is seen to the east of the island over the filchner ice shelf suggesting the presence of a second rift structure gravity modelling across these features shows thinned crust below the ronne and filchner ice shelves indicating that the whole area has undergone significant extension a gravity model across the flanks of berkner island is used to estimate the ratio of unstretched to stretched crustal thickness the resulting value of approximately 2 7 indicates that enough extension has taken place to induce melting supporting the idea that the source of the magnetic anomaly over berkner island is rift related volcanic rocks

crustal growth of the antarctic peninsula by accretion magmatism and extension a subduction accretion model incorporating new geophysical data is presented to explain the geology of the antarctic peninsula from late palaeozoic to cenozoic time according to the model the peninsula consists of overlapping accretionary magmatic and extensional regimes that were diachronous across the peninsula and have built the crust to its present form the crust which contains a small proportion of sialic basement was mainly formed by accretionary and magmatic processes and modified to its present shape by extension the gondwanide orogeny for the antarctic peninsula is interpreted in terms of the accretionary processes

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

campanian maastrichtian cretaceous stratigraphy of the james ross island area antarctica one of the most important outcrops of uppermost cretaceous campanian maastrichtian sedimentary rocks in the southern high latitudes occurs within the james ross island group northeastern antarctic peninsula these rocks comprise a 1500 2000 m thick sequence of predominantly shallow marine clastic sediments that were deposited within a retro arc basin they are virtually undeformed and have yielded prolific invertebrate and vertebrate faunas and a wide range of plant taxa campanian maastrichtian strata are contained within two component formations of the upper cretaceous lower tertiary marambio group the lower santa marta formation is approximately 1100m thick and has three constituent members ammonites within the santa marta formation indicate an early to late campanian age assignment dinoflagellates suggest that the lower levels of the formation may range into the santonian stage in the northern james ross island to vega island region there is a conformable transition into the overlying lopez de bertodano formation here the basal cape lamb member is unconformably overlain by the sandwich bluff member although there are some lithological and fauna1 discrepancies which are attributed to marked lateral facies changes across the basin it is believed that the cape lamb member can be correlated with the lower to middle regions of the 1200 m thick sequence of undifferentiated lopez de bertodano formation lithologies exposed on seymour island molluscan data indicate a late campanian palaeocene age range for this upper formation dinoflagellates again suggest that the lowest beds may be slightly older the correlation presented provides the basis for a formal subdivision of the campanian and maastrichtian stages in the southern high latitudes it has also revealed two major basin shallowing events in the late campanian early maastrichtian and latest maastrichtian respectively and the existence of a major fault fault zone across southeastern james ross island the improved temporal framework will aid late cretaceous palaeoclimatic and palaeobiological studies in antarctica

new paleomagnetic data from thurston island implications for the tectonics of west antarctica and weddell sea opening palcomagnetic data from three west antarctic crustal blocks antarctic peninsula ap thurston island eights coast ti and the ellsworth whitmore mountains ewm indicate that there has been motion between the individual blocks and motion relative to east antarctica during the mesozoic a triassic palcomagnetic pole from the ti block 116oe 61os a95 19 4o n 3 vgps appears to indicate that the block has rotated 90 o relative to east antarctica between 230 ma and 110 ma our previously reported middle jurassic palcomagnetic pole from the ewm block indicates that a 90 o rotation relative to east antarctica occurred sometime between the cambrian and 175 ma we believe that the 90 o counterclockwise ewm rotation occurred between 220 ma and 175 ma related to the development of post gondwanide orogeny shear zones the motion of the ap ti and ewm blocks appears to be linked during the mid to late mesozoic to three major events in the evolution of the southem ocean basins opening in the mozambique somali weddell sea basins may have produced major counterclockwise rotation of the ti block with respect to east antarctica between the jurassic and early cretaceous based on new late jurassic 145oe 64 5os a95 7o n 5 vgps poles we believe that the ti rotation as well as deformation in the southem ap block was caused by collision and shearing of the ewm block against the other two as the ewm block moved southward with east antarctica an early cretaceous palcomagnetic pole 232oe 49os a95 7 9o n 5 vgps from the ti block requires that between the early and mid cretaceous there was clockwise rotation with respect to east antarctica of the ap ti ewm blocks an entity we call weddellia a change in the opening history of the weddell sea basin caused by initiation of spreading in the south atlantic ocean basin at 130 ma probably started weddellia s clockwise rotation two new 110 and 90 ma poles from the ti block 210oe 73 os a95 7 6o n 7 vgps and 161oe 81os a95 3 9o n 18 vgps respectively are similartoequivalent agepoles from the apblock and east antarctica and indicate that weddellia was at ornearits present day position with respect to east antarctica by 110 ma this corresponds to a time of major plate reorganizaion in the south atlantic and southeast indian oceans based on both the new ti palcomagnetic data and previously reported data from marie byrd land mbl dextral shearing would be expected to have occurred between mbl and weddellia since the mid cretaceous pine island bay the area between the ti and mbl blocks marks a fundamental and complex tectonic boundary in west antarctica that we propose has largely been a zone of transcurrent sheafing

palaeomagnetic evidence for a displaced terrain in western antartica an outstanding problem in antarctic tectonics is the relationship between the east antarctic craton and the late palaeozoic cenozoic orogenic belts that comprise western antarctica1 antarctic peninsula ellsworth land eights coast and marie byrd land see fig 1a related problems are the place of western antarctica in reconstructions of gondwanaland and the evolution of the pacific margin of gondwanaland we present here palaeomagnetic data derived from folded cambrian argillites that crop out in the ellsworth mountains which were collected during the 1979 80 austral field season although the early palaeozoic palaeomagnetic field of gondwanaland is characterized by rapid apparent polar wander at the present level of coverage the data are consistent with rotation of the ellsworth mountain block with respect to the east antarctic craton as postulated by schopf2 this result supports a microplate nature of western antarctica