geophysical evidence for the east antarctic plate boundary in the weddell sea an improved gondwanaland reconstruction compatible with geological and geophysical information from the surrounding oceans and continents seems to require microplates to solve the enigmatic pre early mesozoic tectonic relation between west and east antarctica1 new multi channel seismic reflection data from the southeastern weddell sea acquired during the 1984 85 norwegian antarctic research expedition nare have outlined a linear wsw ene trending basement ridge buried below the continental slope over a distance of 700 km this structural high truncates the trend of the large sedimentary basins below the filchner and ronne ice shelves and may continue to within a few hundred kilometres of the antarctic penninsula we interpret the basement ridge as part of the east antarctic plate boundary during the break up of gondwana the morphology and structure of this boundary show greater apparent similarity to a rifted or obliquely rifted margin than to the sheared margin which is predicted by current reconstructions2 3 a linear east antarctic plate margin extending to the vicinity of the antarctic peninsula makes any post rift micro plate motion in the weddell embayment unlikely

india madagascar paleo fit based on flexural isostasy of their rifted margins the present study contributes new constraints on and definitions of the reconstructed plate margins of india and madagascar based on flexural isostasy along the western continental margin of india wcmi and the eastern continental margin of madagascar ecmm we have estimated the nature of isostasy and crustal geometry along the two margins and have examined their possible conjugate structure here we utilize elastic thickness te and moho depth data as the primary basis for the correlation of these passive margins we employ the flexure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elastic thickness gravity inversion and flexure inversion techniques are used to estimate the configuration of the moho crust mantle interface that reveals regional correlations with the elastic thickness variations these results correlate well with the continental and oceanic segments of the indian and african plates the present study has found a linear zone of anomalously low te 1 5 km along the wcmi similar to 1680 km which correlates well with the low te patterns obtained all along the ecmm we suggest that the low te zones along the wcmi and ecmm represent paleo rift inception points of lithosphere thermally and mechanically weakened by the combined effects of the marion hotspot and lithospheric extension due to rifting we have produced an india madagascar paleo fit representing the initial phase of separation based on the te estimates of the rifted conjugate margins which is confirmed by a close fit correlation of moho geometry and bathymetry of the shelf margins the matching of tectonic lineaments lithologies and geochronological belts between india and madagascar provide an additional support for the present plate reconstruction c 2014 international association for gondwana research published by elsevier b v all rights reserved

underlying fracture zone nature of astrid ridge off antarctica s queen maud land new geophysical data are used to refine the extent and setting of astrid ridge a volcanic edifice lying off the princess astrid coast section 5 e to 20 e of queen maud land in antarctica new findings presented here include a well defined fracture zone occurring as a narrow basement ridge which extends northeast from astrid ridge and an adjacent set of mesozoic magnetic anomalies m0 to m9 which are offset by 120 km from an identical previously described set these two sets of anomalies are combined with the conjugate mozambique basin anomalies to define an africa antarctica rotation pole at 10 s 29 w for anomaly m2 time the northern edge of astrid ridge and the sharp southern margin of the mozambique ridge on the african plate are probably related to the same cessation of excess volcanic activity near the end of the cretaceous normal polarity interval both features had their eastern and western limits confined by a contiguous pair of fracture zones a possible major swing in fracture zone trend prior to anomaly m9 is also suggested

new somali basin magnetic anomalies and a plate model for the early indian ocean abstract the oldest portions of the indian ocean formed via the breakup of gondwana and the subsequent fragmentation of east gondwana we present a constrained plate model for this early indian ocean development for the time period from gondwana breakup until the start of the cretaceous normal superchron the motions of the east gondwana terranes are determined using new geophysical observations in the somali basin and existing geophysical interpretations from other coeval indian ocean basins within the somali basin recent satellite gravity data clearly resolve traces of an east west trending extinct spreading ridge and north south oriented fracture zones a thorough compilation of somali basin ship track magnetic data allows us to interpret magnetic anomalies m24bn through m0r about this extinct ridge our magnetic interpretations from the somali basin are similar in age spreading rate and spreading directions to magnetic anomalies previously interpreted in the neighboring mozambique basin and riiser larsen sea the similarity between the two magnetic anomaly datasets allows us to match both basin s older magnetic anomaly picks by defining a pole of rotation for a single and cohesive east gondwana plate however following magnetic anomaly m15n we find it is no longer possible to match magnetic picks from both basins and maintain plausible plate motions in order to match the post m15n geophysical data we are forced to model the motions of madagascar india and east antarctica australia as independent plates the requirement to utilize two independent plates after anomaly m15n provides strong circumstantial evidence that suggests east gondwana breakup began around 135 ma

plate tectonic development of the south west indian ocean a revised reconstruction of east antarctica and africa a revised sequence of reconstructions for east antarctica relative to africa is generated from published seafloor spreading and fracture zone anomalies these date from the cretaceous tertiary boundary to the late jurassic and lead to a revised predrift reconstruction for east and west gondwanaland euler pole 1 67 s 35 99 e rotation angle 53 43 this reconstruction derived wholly from marine data places east antarctica adjacent to the lebombo mountains and south of and subparallel to the sabi acid volcanic suite supporting the postulate that these rocks represent mesozoic plate boundaries this refit implies that part of the coastal plain of mozambique is underlain by oceanic or very highly extended continental crust the reconstruction is supported by the alignment of 1 the cape fold belt with its antarctic equivalents and 2 the western limit of pan african reactivation of the mozambique mobile belt with the western limit of the sverdrupfjella metamorphic suite antarctica initially moved along a transform fault lying immediately east of and subparallel to the lebombo mountains the eastern face of the tugela cone the southeastern face of the falkland plateau in its reconstructed position and the eastern flank of the agulhas plateau in its reconstructed position at m10 time a triple junction formed near the tip of the falkland plateau implying that the oldest lineated magnetic anomalies in the eastern weddell sea associated with this triple junction cannot predate m10 at m2 time the mozambique ridge rifted from the astrid ridge in a ridge jump episode such an event may also explain the presence of magnetic anomalies in the southern mozambique basin gunnerus ridge conrad rise and del cano rise are all tectonically associated with the madagascar ridge

gondwana breakup messages from the north natal valley the natal valley offshore mozambique is a key area for understanding the evolution of east gondwana within the scope of the integrated multidisciplinary pamela project we present new wide angle seismic data and interpretations which considerably alter geoscience paradigms these data reveal the presence of a 30 km thick crust that we argue to be of continental nature this falsifies all the most recent palaeo reconstructions of the gondwana this 30 km thick continental crust 1 000 m below sea level implies a complex history with probable intrusions of mantle derived melts in the lower crust connected to several occurrences of magmatism which seems to evidence the crucial role of the lower continental crust in passive margin genesis

tectono thermal evolution of the southwestern alxa tectonic belt nw china constrained by apatite u pb and fission track thermochronology abstract the central asian orogenic belt caob is regarded to have undergone multiple phases of intracontinental deformation during the meso cenozoic located in a key position along the southern caob the alxa tectonic belt atb connects the northernmost tibetan plateau with the mongolian plateau in this paper we apply apatite u pb and fission track thermochronological studies on varieties of samples from the southwestern atb in order to constrain its thermal evolution precambrian bedrock samples yield late ordovician early silurian 430 450 ma and late permian 257 ma apatite u pb ages the late paleozoic magmatic sedimentary samples yield relatively consistent early permian ages from 276 to 290 ma these data reveal that the atb experienced multiple paleozoic tectono thermal events as the samples passed through the apatite u pb closure temperature 350 550 c we interpret these tectonic events to record the long lived subduction accretion processes of the paleo asian ocean during the formation of the southern caob with possible thermal influence of the permian tarim mantle plume apatite fission track aft data and thermal history modelling reveal discrete low temperature thermal events for the atb inducing cooling reheating through the aft partial annealing zone 120 60 c during the permian the samples underwent rapid cooling via exhumation or denudation from deep crustal levels to temperatures

refined spreading history at the southwest indian ridge for the last 96 ma with the aid of satellite gravity data summary the spreading history of the oceans is modelled mostly by using magnetic anomalies and the fracture zone geometry the high quality satellite derived gravity data that became available in recent years reveal the details of fracture zones which can be used as flow lines to control spreading models we have applied this approach to the southwest indian ridge swir in order to refine its spreading history this is particularly useful for the period of complex spreading between magnetic anomalies 33 and 23 where the magnetic anomalies alone cannot resolve the detailed spreading history we find four main stages in the spreading history of the swir since 96 ma including two that were not noted previously between 96 ma and anomaly 33 76 3 ma and between anomalies 23o 51 7 ma and 18o 40 1 ma o denotes old boundaries of normal magnetization period we also find that the start of the period of complex spreading was at anomaly 33 somewhat earlier than previously proposed we discuss the characteristics of the extension that the old transform faults underwent during the complex spreading phase in response to the counterclockwise rotation of spreading new transform faults appeared at that time considerably widening the transform zones

breakup and early seafloor spreading between india and antarctica summary we present a tectonic interpretation of the breakup and early seafloor spreading between india and antarctica based on improved coverage of potential field and seismic data off the east antarctic margin between the gunnerus ridge and the bruce rise we have identified a series of ene trending mesozoic magnetic anomalies from chron m9o 130 2 ma to m2o 124 1 ma in the enderby basin and m9o to m4o 126 7 ma in the princess elizabeth trough and davis sea basin indicating that india antarctica and india australia breakups were roughly contemporaneous we present evidence for an abandoned spreading centre south of the elan bank microcontinent the estimated timing of its extinction corresponds to the early surface expression of the kerguelen plume at the southern kerguelen plateau around 120 ma we observe an increase in spreading rate from west to east between chron m9 and m4 38 54 mm yr 1 along the antarctic margin and suggest the tectono magmatic segmentation of oceanic crust has been influenced by inherited crustal structure the kinematics of gondwanaland breakup and the proximity to the kerguelen hotspot a high amplitude e w oriented magnetic lineation named the mac robertson coast anomaly mca coinciding with a landwards step down in basement observed in seismic reflection data is tentatively interpreted as the boundary between continental transitional zone and oceanic crust the exposure of lower crustal rocks along the coast suggests that this margin formed in a metamorphic core complex extension mode with a high strength ratio between upper and lower crust which typically occurs above anomalously hot mantle together with the existence of the mca zone this observation suggests that a mantle temperature anomaly predated the early surface outpouring steady state magmatic production of the kerguelen lip an alternative model suggests that the northward ridge jump was limited to the elan bank region whereas seafloor spreading continued in the west enderby basin and its sri lankan conjugate margin in this case the mca magnetic anomaly could be interpreted as the southern arm of a ridge propagator that stopped around 120 ma

the agulhas plateau structure and evolution of a large igneous province summary large igneous provinces lip are of great interest due to their role in crustal generation magmatic processes and environmental impact the agulhas plateau in the southwest indian ocean off south africa has played a controversial role in this discussion due to unclear evidence for its continental or oceanic crustal affinity with new geophysical data from seismic refraction and reflection profiling we are able to present improved evidence for its crustal structure and composition the velocity depth model reveals a mean crustal thickness of 20 km with a maximum of 24 km where three major units can be identified in the crust in our seismic reflection records evidence for volcanic flows on the agulhas plateau can be observed the middle crust is thickened by magmatic intrusions the up to 10 km thick lower crustal body is characterized by high seismic velocities of 7 0 7 6 km s 1 the velocity depth distribution suggests that the plateau consists of overthickened oceanic crust similar to other oceanic lips such as the ontong java plateau or the northern kerguelen plateau the total volume of the agulhas plateau was estimated to be 4 10 6 km 3 of which about 10 per cent consists of extruded igneous material we use this information to obtain a first estimate on carbon dioxide and sulphur dioxide emission caused by degassing from this material the agulhas plateau was formed as part of a larger lip consisting of the agulhas plateau itself northeast georgia rise and maud rise the formation time of this lip can be estimated between 100 and 94 5 ma

an early cretaceous extinct spreading center in the northern natal valley abstract we have identified an extinct e w spreading center in the northern natal valley on the basis of magnetic anomalies which was active from chron m11 133 ma to 125 3 ma just before chron m2 124 ma in the early cretaceous seafloor spreading in the northern natal valley accounts for approximately 170 km of north south motion between the mozambique ridge and africa this extension resolves the predicted overlap of the continental central and southern mozambique ridge and antarctica in the chron m2 to m11 reconstructions from mesozoic finite rotation parameters for africa and antarctica in addition the magnetic data reveal that the mozambique ridge was an independent microplate from at least 133 to 125 ma the northern natal valley extinct spreading center connects to the spreading center separating the mozambique basin and the riiser larsen sea to the east it follows that the northern mozambique ridge was either formed after the emplacement of the surrounding oceanic crust or it is the product of a very robust spreading center to the west the extinct spreading center connects to the spreading center separating the southern natal valley and georgia basin via a transform fault prior to chron m11 there is still a problem with the overlap of mozambique ridge if it is assumed to be fixed with respect to either the african or antarctic plates some of the overlap can be accounted for by jurassic deformation of the mozambique ridge mozambique basin and dronning maud land it appears though that the mozambique ridge was an independent microplate from the breakup of gondwana 160 ma until it became part of the african plate 125 ma

mesozoic reactivation of the beishan southern central asian orogenic belt insights from low temperature thermochronology abstract the beishan orogenic collage boc is located in the southeast of the central asian orogenic belt caob and formed during final consumption of the palaeoasian ocean in the late palaeozoic this study applies low temperature thermochronology to constrain the meso cenozoic history of the boc apatite fission track and u th sm he data obtained for granitoid samples along a north south transect through the boc indicate three distinct phases of exhumation during 1 the late triassic early jurassic 225 180 ma 2 early cretaceous 130 95 ma and 3 late cretaceous early palaeogene 75 60 ma samples from the northern boc reveal a more profound early cretaceous signal and a weaker late triassic early jurassic signal than those in the southern boc a possible explanation for this discrepancy is the presence of crustal scale fault zones in the northern boc that are interpreted to have undergone repeated reactivation throughout the mesozoic exposing deeper exhumed sections of the boc these faults may thus have acted as a major control on exhumation in the region this pattern is consistent with results from elsewhere in the caob such as in the tianshan and the altai where regional widespread exhumation occurred since the early cretaceous while major fault zones record localised exhumation during the late cretaceous and cenozoic late cretaceous early palaeogene cooling ages were found only in the south of the boc suggesting that exhumation at that time was more localised and did not reach the northern margins of the study area the meso cenozoic cooling events described here are thought to be related to the progressive closure of the palaeo tethys ocean to the south associated collision and accretion of micro continental blocks and island arcs at the southern eurasian margin are interpreted to have induced more widespread reactivation and exhumation in central asia than previously anticipated extending to the northern margin of the tarim craton this observation hence refines the existing tectonic history models for central asia

agulhas plateau sw indian ocean new evidence for excessive volcanism a new set of seismic reflection and refraction lines has been interpreted regarding the basement and crustal structure of the southern agulhas plateau a large number of extrusion centres were identified lava flows dip away from those extrusion centres and form subparallel stratified sequences we interpret those extrusion centres as the result of excessive volcanism in course of the separation of the southern agulhas plateau from the maud rise since the sedimentary layers appear to be little affected by the volcanism that episode obviously ceased before onset of sedimentation in late cretaceous times we have not found evidence for continental fragments within overthickened predominantly oceanic crust we therefore propose that the agulhas plateau belongs to the world wide suite of large igneous provinces lip of predominantly oceanic origin

patterns of late cenozoic exhumation deduced from apatite and zircon u he ages from fiordland new zealand new apatite and zircon u th he ages from the fiordland region of new zealand s south island expand on earlier results and provide new constraints on patterns of late cenozoic exhumation and cooling across this region zircon u th he cooling ages in combination with increased density of apatite ages show that in addition to a gradual northward decrease in cooling ages that was seen during an earlier phase of this study there is also a trend toward younger cooling ages to the east distinct breaks in cooling age patterns on southwestern fiordland appear to be correlated to the location of previously mapped faults the northward decrease in ages may reflect asynchronous cooling related to migration in the locus of exhumation driven by subduction initiation or it may reflect synchronous regional exhumation that exposed different structural levels across fiordland or some combination of these effects in either case differential exhumation accommodated by major and minor faults that dissect fiordland basement rocks apparently played an important role in producing the resulting age patterns

tectonic history of the kyrgyz south tien shan atbashi inylchek suture zone the role of inherited structures during deformation propagation 1 multimethod chronology was applied on intrusives bordering the kyrgyz south tien shan suture stss to decipher the timing of 1 formation and amalgamation of the suturing units and 2 intracontinental deformation that built the bordering mountain ranges zircon u pb data indicate similarities between the tien shan and tarim precambrian crust caledonian 440 410 ma and hercynian 310 280 ma zircon u pb ages were found at the edge of the stss related to subduction and closure of the turkestan ocean and the formation of the suture itself permian triassic 280 210 ma titanite fission track and zircon u th he data record the first signs of exhumation when the stss evolved into a shear zone and the adjacent tarim basin started to subside low temperature thermochronological apatite fission track zircon and apatite u th he analyses reveal three distinct cooling phases becoming younger toward the stss center 1 jurassic cretaceous cooling ages provide evidence that a mesozoic south tien shan orogen formed as a response to the cimmerian orogeny 2 early paleogene 60 45 ma data indicate a renewed pulse of stss reactivation during the early cenozoic 3 neogene ages constrain the onset of the modern tien shan mountain building to the late oligocene 30 25 ma which intensified during the miocene 10 8 ma and pliocene 3 2 ma the cenozoic signals may reflect renewed responses to collisions at the southern eurasian border i e the kohistan dras and india eurasia collisions this progressive rejuvenation of the stss demonstrates that deformation has not migrated steadily into the forelands but was focused on pre existing basement structures

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

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

advanced insights into magmatism and volcanism of the mozambique ridge and mozambique basin in the view of new potential field data summary a new plate tectonic model is presented which describes the emplacement of the mozambique ridge off southeast africa as the result of long lasting volcanic activity 140 122 ma during the initial opening between africa and antarctica thus an oceanic origin for the mozambique ridge is proposed this model is based on a new and systematic high resolution magnetic anomaly data set acquired across the mozambique ridge and throughout the mozambique basin data from the mozambique basin allow the identification of mesozoic magnetic anomalies from m0r to m26 124 61 155 3 ma with previously unmatched accuracy small scale fracture zones are recognized by offsets of magnetic anomalies in the westernmost part of the basin additionally a bend in the major fracture zones f and e between m17r and m18n 142 84 144 04 ma and a recognized sinusoidal change in spreading direction with an amplitude of about 15 indicate that the basin experienced several small scale changes in spreading direction through time a maximum change in spreading direction to almost 0 at around m11n 135 69 ma is followed by a short lived increase in spreading half rate from 23 5 km ma 1 to about 27 5 km ma 1 in the time frame from m10r to m9n 134 30 132 83 ma we propose that this is related to the initial opening of the south atlantic ocean represented by the onset of seafloor spreading between the falkland plateau and africa in the conjugate georgia and natal basins across the mozambique ridge high amplitude magnetic anomalies at major structural boundaries suggest that the different plateaus of the ridge were formed at different times a simple 2 d gravity and magnetic model for the ridge supports the hypothesis of multiple volcanic episodes which formed the ridge though long lasting volcanic activity between about 140 and 122 ma together with new and verified rotation parameters from the mozambique basin and its conjugate the riiser larsen sea antarctica a series of plate tectonic reconstructions are presented which demonstrate when and how the different parts of the ridge evolved through time

reconstruction of the east africa and antarctica continental margins the early jurassic separation of antarctica from africa plays an important role in our understanding of the dispersal of gondwana and pangea previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent ocean boundary cob and determining the amount and distribution of extended continental crust this study focuses on the evolution of the african margin adjacent to the mozambique basin and the conjugate antarctic margin near the riiser larsen sea satellite derived gravity data have been used to trace the orientations and landward limits of fracture zones a 3 d gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust crustal thicknesses together with fracture zone terminations reveal cobs that are significantly closer to the african and antarctic coasts than previously recognized correlation of fracture zone azimuths and identified cobs suggests antarctica began drifting away from africa at approximately 171 ma in a roughly sse direction an areal balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins both margins reveal a trend of increasing extension from east to west our results suggest africa underwent extension of 60 120 km while antarctic crust was stretched by 105 180 km various models tested to determine the direction of extension during rifting suggest that antarctica moved away from africa in a wnw ese direction during the period between 184 and 171 ma prior to the onset of seafloor spreading

correlations between silicic volcanic rocks of the st mary s islands southwestern india and eastern madagascar implications for late cretaceous india madagascar reconstructions abstract the st mary9s islands southwestern india expose silicic volcanic and sub volcanic rocks rhyolites and granophyric dacites emplaced contemporaneously with the cretaceous igneous province of madagascar roughly 88 90 ma ago the st mary9s islands rocks have phenocrysts of plagioclase clinopyroxene orthopyroxene and opaque oxide moderate enrichment in the incompatible elements e g zr 580 720 ppm nb 43 53 ppm la yb n 6 9 7 2 relatively low initial 87 sr 86 sr 0 7052 0 7055 and near chondritic initial 143 nd 144 nd 0 51248 0 51249 they have mineral chemical whole rock chemical and isotopic compositions very close to those of rhyolites exposed between vatomandry ilaka and mananjary in eastern madagascar and are distinctly different from rhyolites from other sectors of the madagascan province we therefore postulate that the st mary9s and the vatomandry ilaka mananjary silicic rock outcrops were adjacent before the late cretaceous rifting that split madagascar from india if so they provide a valuable tool to check and aid traditional cretaceous india madagascar reconstructions based on palaeomagnetism matching precambrian geological features and geometric fitting of continental shelves supplementary material mineral analyses mass balance calculations and locality information are available at http www geolsoc org uk sup18332

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

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

transform continental margins part 2 a worldwide review we present a global inventory of transform continental margins based on both a worldwide comparison of continent ocean boundary identifications with oceanic fracture zones traces and a compilation of published regional studies this inventory increases the number of identified transform margins from 29 to 78 these margins represent 16 of continental margins in cumulative length and 31 of non convergent margins we include morphological data published upper crustal sections continent to ocean transition locations and published moho shape data in the new database this review confirms that continent to ocean transitions are sharper at transform margins than at divergent margins it also emphasizes the structural diversity of transform margins associated with one third of transform margins we define marginal plateaus as a new type of relief that corresponds to a flat but deep surface inside the continental slope and that may be inherited from crustal thinning prior to transform faulting transform margin initiation appears to be favoured along propagating oceans and within cold and thick lithospheres

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

cretaceous patterns of floristic change in the antarctic peninsula cretaceous radiation of angiosperms from low to high palaeolatitudes coupled with the break up of gondwana played a major role in establishing and maintaining biogeographic patterns across the southern hemisphere uncertainties in details of plate reconstructions provide conflicting hypotheses about area relationships of gondwana fragments this has led to a number of competing proposals concerning angiosperm migration across gondwana central to this debate is the role of the antarctic peninsula a region that is often envisaged as providing the main connection between east and west gondwana the initial radiation of angiosperms into the antarctic peninsula region however postdates appearances elsewhere in east gondwana e g australia strongly suggesting that the antarctic peninsula was not the main gateway at least in the early stages of gondwana radiation a steep climatic gradient in this part of the world probably acted as an effective barrier to angiosperm radiation the peak of floristic replacement coincides with the peak of cretaceous warmth turonian which in turn suggests that climatic warming acted as a forcing mechanism by pushing latitudinal belts of vegetation southwards once into the southern high latitudes angiosperms diversified and as climates cooled during the late cretaceous a number of important groups seem to have their origins here recent investigations of antarctic macro and microfloras indicate progressive floristic replacement through the cretaceous bryophytes hepatophytes bennettites and other seed plants all show a rapid decline in diversity in contrast ferns initially decline then recover while conifers remain relatively stable the ecological preferences of the replaced groups imply that angiosperms initially occupied areas of disturbance and were understorey colonizers only later replacing fern thickets and becoming important in the overstorey this pattern is consistent with those observed elsewhere through the cretaceous

the initial gondwana break up a synthesis based on new potential field data of the africa antarctica corridor abstract the initial opening of the africa antarctica corridor in the heart of gondwana is still enigmatic due to missing information on the origin of major crustal features and the exact timing of the onset of the first oceanic crust in the jurassic therefore in 2014 new ship borne magnetic data were systematically acquired in the northern mozambique basin and across beira high which we merged with all accessible magnetic data in the mozambique basin herein distinct magnetic lineations are observed which allow a refined identification of a whole set of jurassic magnetic spreading anomalies constraining the timing of the onset of oceanization beginning at m38n 2n 164 1 ma in combination with high resolution potential field data from the conjugate antarctic margin well expressed fracture zones can be traced throughout the africa antarctica corridor and allow the precise rotation of antarctica back to africa the initial fit depicts striking continuations of onshore tectonic features across the plate boundaries taking onshore aeromagnetic data of both margins into account within a tight gondwana fit the beira high can be restored along the major sinistral namama orvin shear zone of the east african antarctic orogen the beira high represents a continental block which was detached from antarctica by 157 ma at the latest simultaneously the antarctic plate cleared the area of the mcp however the crustal nature of the southern mcp remains ambiguous the northern natal valley and the mozambique ridge consist of thick oceanic crust being emplaced between m26r m18n 157 1 144 ma and m18n m6n 144 131 7 ma respectively about the half of this crust was won from the antarctic plate by a series of southwards directed ridge jumps to the northern boundary of the explora wedge a refined kinematic break up model constrained by the most extensive magnetic dataset is presented describing consistently the initial opening of the africa antarctica corridor and the somali basin

new continental margin magnetic anomalies of east antarctica over the past decade australian norwegian and russian marine surveys have collected integrated seismic gravity and magnetic data in the southern indian ocean the more than 350 000 line km of new airborne and marine magnetic observations for the east antarctic continental margin have been compiled into an improved definition of crustal magnetic anomaly patterns this compilation provides important new constraints on the breakup processes and igneous activity related to the formation of the passive margin of east antarctica the eastern sector of the map from bruce rise in the west to the d urville sea in the east is largely dominated by seafloor spreading magnetic anomalies the adelie rift block of highly stretched and extensively faulted continental crust is associated with a smooth anomaly fabric abrupt magnetic anomaly changes along the oceanic continent transition in the cooperation sea including the enderby basin anomaly extend for more than 1680 km from the kerguelen plateau towards the cosmonaut sea three sectors of the east antarctic continental margin exhibit pronounced disparities in the anomaly patterns that strongly suggest different modes of seafloor formation strong positive seafloor magnetic anomalies mark the southern margin of the kerguelen plateau the maud rise and adjacent areas in the riiser larsen sea the new compilation suggests that at least 300 km of the enderby basin and shackleton basin may be part of the cretaceous kerguelen volcanic province and possibly maps an abandoned fossil spreading center in the central enderby basin the majority of the published age models for the enderby basin and australian sector of the east antarctic margin are not in agreement with the structural grain of magnetic anomalies in the newly compiled map

cretaceous reconstructions of east antarctica africa and madagascar abstract some cretaceous reconstructions of east antarctica africa and madagascar overlap the mozambique ridge onto the margin of dronning maud land and others place madagascar west of the gunnerus ridge into the riiser larsen sea our m10ny 131 9 ma m0 120 4 ma 96 ma and 34y 83 ma marine free air gravity and topography reconstructions illustrate a solution to the overlap problem and also demonstrate that madagascar separated from antarctica east of the gunnerus ridge our reconstructions chronicle the growth of the conjugate mozambique and riiser larsen basins created at the southwest indian spreading ridge well defined spreading corridors identified by conjugate fracture zones and mesozoic magnetic anomaly identifications extend from one basin into the other we find that the eastern scarp of the mozambique ridge aligns with the astrid ridge and there is no room for madagascar west of the gunnerus ridge the overlap of the mozambique ridge is solved by taking into account the motion of an independent mozambique ridge microplate that existed from chron m11 to circa m2

high resolution inversion for south atlantic plate kinematics using joint altimeter and magnetic anomaly data we present an inversion for plate kinematics that solves for finite rotation parameters using fracture zone fz and magnetic anomaly location data jointly we define misfit functions that incorporate properties unique to each data type in particular the fz misfit function does not depend upon alignment of conjugate fz traces in the same way as magnetic lineations under the finite rotations this property is useful for fz locations in which the signals on conjugate sides of the ridge may include systematic differences or where data from one side of the ridge are sparse or missing formal error bounds estimated for the pole parameters show that the magnetic and fz data are complementary in their information content error bounds computed for the joint inversion are substantially smaller than for either the fz or magnetics data used separately indicating that simultaneous use of the data in an inversion is crucial we apply this method to seasat altimeter data and magnetic anomaly picks in the south atlantic we solve for finite poles corresponding to magnetic anomalies 5 6 8 13 21 22 25 30 33 33r and 34 these define a smooth path over the past 84 m y indicating that the plate motions have not been as erratic as found previously

seafloor spreading in the weddell sea from magnetic and gravity data abstract a re compilation of magnetic data in the weddell sea is presented and compared with the gravity field recently derived from retracked satellite altimetry the previously informally named anomaly t an east west trending linear positive magnetic and gravity anomaly lying at about 69 s forms the southern boundary of the well known weddell sea gravity herringbone north of anomaly t three major e w linear magnetic lows are shown and identified with anomalies c12r c21 29 r and c33r on the basis of these and following work by recent investigators isochrons c13 c18 c20 c21 c30 c33 and c34 are identified and extended into the western weddell sea similarly a linear magnetic low lying along the spine of the herringbone is shown and provisionally dated at 93 96 ma anomaly t is tentatively dated to be m5n in agreement with recent tectonic models although current tectonic models are generally in good agreement to the north of t to the south interpretations differ some plate tectonic models have only proposed essentially north south spreading in the region whilst others have suggested that a period of predominantly east west motion relative to present antarctic geographic coordinates occurred during the mid mesozoic spreading between east and west gondwana we identify an area immediately to the south of t which appears to be the southerly extent of n s spreading in the herringbone following recent work the extreme southerly extent of the n s directed spreading of the herringbone is provisionally dated m9r m10 in the oldest weddell sea immediately to the north and east of the antarctic shelf we see subtle features in both the magnetic and gravity data that are consistent with predominantly n s spreading in the weddell sea during the earliest opening of east and west gondwana in between however in a small region extending approximately from about 50 km south of t to about 70 s and from approximately 40 to 53 w the magnetic and gravity data appear to suggest well correlated linear marine magnetic anomalies possible isochrons perpendicular to t bounded and offset by less well defined steps and linear lows in the gravity possible fracture zones these magnetic and gravity data southwest of t suggest that the crust here may record an e w spreading episode between the two plate system of east and west gondwana prior to the initiation of the three plate spreading system of south america africa and antarctica the e w spreading record to the east of about 35 w would then appear to have been cut off at about m10 time during the establishment of n s three plate spreading along the south american antarctic ridge and then subducted under the scotia ridge

somali basin chain ridge and origin of the northern somali basin gravity and geoid low the northern somali basin located between chain ridge and the horn of africa north of 4 n is characterized by a distinct 5 m geoid low and by large negative gravity anomalies the boundaries of the basin are marked by steep gradients in both gravity and geoid basement in northern somali basin is 1 2 km deeper than on the carlsberg ridge flank to the southeast or on the sheba ridge flank to the north with a sharp discontinuity across the boundary of the basin the western somali basin to the south was created in the late jurassic and early cretaceous by the movement of madagascar away from africa reinterpretation of the magnetic anomalies in the western somali basin shows that they record both limbs of a mid ocean ridge that was active by m22 time kimmeridgian and died soon after mo aptian magnetic and gravity data allow the relict ridge crest to be traced from davie ridge near the african coast to the dhow vlcc ars fracture zone complex at 50 e davie ridge is a transform fault connecting the western somali basin spreading center with a similar age spreading center in the mozambique basin the dhow vlcc ars complex can be shown to continue north of the 4 n bathymetric high separating the northern and western somali basins and to intersect the african margin near 7 n the northern somali basin thus appears to be the third of a series of oceanic basins separated by long transform faults created during movement between east and west gondwanaland the original northern somali basin was split apart by the northward motion of india in the late cretaceous and chain ridge formed along the new boundary thermal and gravity modeling shows that the flexure resulting from differential subsidence across chain ridge combined with the difference in lithospheric thermal structure late jurassic vs early tertiary on either side of it explains well the amplitude and shape of the observed geoid step and gravity anomalies across chain ridge the geoid step up from the basin up to the african coast can be modeled as an edge effect between the juxtaposed jurassic oceanic and african continental crust thus the geoid and gravity low over the northern somali basin results from the superposition of a continental edge effect anomaly and the fracture zone edge effect anomaly

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

gondwana to asia plate tectonics paleogeography and the biological connectivity of the indian sub continent from the middle jurassic through latest eocene 166 35 ma abstract using the most up to the date information available we present a considerably revised plate tectonic and paleogeographic model for the indian ocean bordering continents from gondwana s middle jurassic break up through to india s collision with asia in the middle cenozoic the landmass framework is then used to explore the sometimes complex and occasionally counter intuitive patterns that have been observed in the fossil and extant biological records of india madagascar africa and eastern eurasia as well those of the more distal continents although the paleogeographic model confirms the traditional view that india became progressively more isolated from the major landmasses during the cretaceous and paleocene it is likely that at various times minor physiographic features principally ocean islands provided causeways and or stepping stone trails along which land animals could have migrated to from the sub continent aside from a likely link albeit broken by several marine gaps to africa for much of this time it is notable that the present day recent biota of madagascar indicates that the ancestors of five land mammal orders plus bats crossed the 400 km wide mozambique channel at different times in the cenozoic it is possible that the kerguelen plateau connected india and australia antarctica in the mid cretaceous approximately 115 90 ma later the seychelles mascarene plateau and nearby elevated sea floor areas could have allowed faunas to pass between southern india and madagascar in the late cretaceous from around 85 65 ma with an early cenozoic extension to this path forming as a result of the reunion hot spot trace islands growing on the ocean floor to the ssw of india the modelling also suggests that india s northward passage towards asia with eventual collision at 35 ma involved the ne corner of the sub continent making a glancing contact with sumatra followed by burma from 57 ma late paleocene onwards a scenario which is compatible with the fossil record indicating that india asia faunal exchanges began occurring at about this time finally we contend that a number of biologically based direct terrestrial migration routes that have been proposed for last 15 m y of the cretaceous asia to india antarctica to madagascar and or india can probably be dismissed because the marine barriers likely varying from 1000 up to 2500 km were simply too wide

is there continental crust underneath the northern natal valley and the mozambique coastal plains 1 to draw conclusions about the crustal nature and history of the natal valley and the mozambique ridge systematic potential field data were obtained during the aistek iii cruise with r v pelagia in 2009 this paper presents and interprets the results of that expedition the new magnetic data reveal a pattern of linear magnetic spreading anomalies nw se trending in the southwestern part of the mozambique ridge and e w trending on its central part the ariel graben which separates the mozambique ridge from the northern natal valley is represented by a pronounced negative anomaly in the magnetic as well as the free air gravity field the northern natal valley bears a complicated pattern of anomalies with mainly sw ne trends in the northern natal valley no indications for a continent ocean boundary between continental crust in the north and oceanic crust in the south exist either in the free air gravity or in the magnetic field the magnetic wavelengths of the mozambique coastal plains are similar to those of the northern natal valley and the mozambique ridge particularly in the gravity data the mozambique coastal plains the northern natal valley and the mozambique ridge appear as one continuous geological province we interpret the region from the mateke sabi monocline in the north to the southwestern tip of the mozambique ridge as mainly being floored by thickened oceanic crust

post break up tectonic inversion across the southwestern cape of south africa new insights from apatite and zircon fission track thermochronometry the south west african margin is regarded as an example of a passive continental margin formed by continental rifting following a phase of lithospheric extension and thinning recent attention focused on this margin has included theoretical modelling studies of rift processes plate kinematic studies of the opening geometry and timing and empirical studies focused on documenting the crustal structure and offshore sedimentary record here we examine the onshore geomorphic and tectonic response to rifting and breakup with a specific focus on the sw cape of south africa we present 75 new apatite and 8 new zircon fission track analyses from outcrop samples and onshore borehole profiles along the western margin of south africa the data are used to derive robust thermal histories that record two discrete phases of accelerated erosional cooling during the early cretaceous 150 130 ma and late cretaceous 100 80 ma respectively both periods of enhanced erosion are regional in extent involved km scale erosion and extend well inland of the current escarpment zone albeit with spatially variable intensity and style the late cretaceous episode is also expressed more locally by tectonic reactivation and inversion of major faults causing km scale differential displacement and erosion the new aft data do not exclude the possibility of modest surface uplift occurring during the cenozoic but they restrict the depth of regional cenozoic erosion on the western margin to less than c 1 km the inferred pattern and chronology of erosion onshore is consistent with the key features and sediment accumulation patterns within the offshore orange and bredasdorp basins it is suggested that the late cretaceous event was triggered by a combination of regional dynamic uplift augmented along the western margin and in the sw cape by local tectonic forces arising from dextral displacement of the falkland plateau along the falkland agulhas fracture zone

a model of plate kinematics in gondwana breakup summary an accurate model of relative plate motions in gondwana breakup is based on visual fitting of seafloor isochrons and fracture zones fz from the riiser larsen sea and mozambique basin used predictively the model precisely locates kinematic markers in the west somali basin which allows the conclusion that the spreading centres in the west somali and mozambique basins and the riiser larsen sea formed parts of the boundary between the same two plates the locations of fz and less well defined isochrons from neighbouring regions are also consistent with their formation on other lengths of this same boundary and with its relocation from the west somali basin and northern natal valley to the west enderby basin and lazarev sea during chron m10n small independently moving plates thus played no role in the breakup of this core part of gondwana in an inversion procedure the data from these areas yield more precise finite rotations that describe the history of the two plates separation breakup is most simply interpreted to have occurred in coincidence with karoo volcanism and a reconstruction based on the rotations shows the lebombo and mateke sabi monoclines and the mozambique and astrid ridges as two sets of conjugate volcanic margins madagascar s pre drift position can be used as a constraint to reassess the positions of india and sri lanka in the supercontinent

circum antarctic palaeobathymetry illustrated examples from cenozoic to recent times abstract we use a revised set of seafloor spreading isochrons and updated plate tectonic reconstructions to produce palaeo age and basement depth grids for the southern ocean the palaeo depth maps illustrate the evolution of the ocean basins widening and deepening and oceanic gateway development surrounding antarctica we provide examples of palaeo basement depth profiles at selected times between 45 ma and the present across the tasman gateway and drake passage the digital grids provide improved boundary conditions for palaeoceanographic circulation models as well as for backtracking sediments from odp drilling

application of ostracoda and charophyta from the late jurassic to early cretaceous tendaguru formation at tendaguru tanzania east africa biostratigraphy palaeobiogeography and palaeoecology abstract revisitation of the famous late jurassic early cretaceous dinosaur locality of tendaguru by a german tanzanian expedition in summer 2000 resulted in a new standard section micropalaeontologic samples from this section yielded a marine and nonmarine partially mixed ostracod fauna consisting of 40 taxa and some charophyte gyrogonites and utriculi a few marine taxa provided biostratigraphic information although comparable faunas from the west indian madagascan east african faunal province are stratigraphically older middle late jurassic fully marine and strongly endemic an middle oxfordian age is suggested for the lower part of the tendaguru formation nodosoclavatoroid utriculi charophyta suggest that the jurassic cretaceous boundary may be located lower in the section than supposed previously i e in the trigonia smeei member rather than in the upper saurian member in biogeographical terms the marine ostracod fauna is relatively endemic and has most similarities with that known from eastern tanzania only a few faunal links to madagascar northwest india and somalia exist and there are no links to south africa ostracods and charophytes also provided palaeoecological data for specific horizons in the lower part of the middle saurian member a regression is documented while the nonmarine late jurassic early cretaceous ostracods of tendaguru are not applicable to biostratigraphy so far they are important because the nonmarine setting is unique in this area of the tethys

a geotectonic history of the northern mozambique basin including the beira high a contribution for the understanding of its development abstract the interpretation of a comprehensive two dimensional 2d seismic reflection data set discloses several rifting events for a typical passive rifted margin two major phases have been identified rift i phase is in agreement with the initial rifting of gondwana postulated for the early jurassic this rift resulted in break up stage 1 governed by a north eastern motion of antarctica along the reactivated pebane shear zone indications for lava flows were identified in the seismic profiles along the offshore zambezi depression they form a link between early rifting the initial break up and early sea floor spreading the nature of these lava flows may be associated with the emplacement of thick volcanic dykes during post rift magmatism that occurred when the antarctica plate with the beira high drifted dextrally from the west to east until the mid jurassic times it is more likely that the break up in stage 1 resulted from high tension due to strain relief of rift i phase above all the extensional deformation occurred in a narrow rift mode therefore the v shape of the offshore zambezi depression suggests a possible rift failure structure this is best explained by the reeves and de wit model 2000 this model postulates that the motion of antarctica changed to southward direction at around 170 ma consequently a rift jump from the offshore zambezi depression in the northwest to the south eastern edge of the beira high occurred and rift ii phase may have started leading to the break up in stage 2 during this stage rift ii phase the extension migrated towards the east thus thinning the crust and exhuming the sub continental mantle in the continental ocean transition zone the rift ii phase shows a sequence of half graben morphologies confining the syn rift infill that is subdivided in three units syn rift i syn rift ii and rift sag all three units appear to have developed under minor extensional regimes in the crust evidenced by gently dipping low angle detachment faults with respect to the presence of the two rift phases it is deduced that break up and sea floor spreading are diachronous within rift i and rift ii segments

seasonal and spatial variations in the chemistry of a high arctic supraglacial snow cover this paper describes the physical and chemical properties of the snowpack on john evans glacier ellesmere island canadian arctic archipelago and investigates the controls on snowpack solute concentrations and atmospheric deposition the snowpack contains three layers that are traceable across the whole glacier these represent fall accumulation that has been metamorphosed to depth hoar winter accumulation mixed with snow reworked by wind from the underlying depth hoar and spring accumulation mixed with wind reworked snow the seasonal cycle in snow chemistry closely reflects changes in the composition of the atmospheric aerosol at alert with some modification of no 3 concentrations by post depositional processes mean water weighted solute concentrations in the snowpack are largely independent of accumulation while atmospheric deposition tends to increase with accumulation this suggests that for most species wet deposition is the dominant depositional process throughout the year however concentrations of ca 2 and k increase with both accumulation and elevation implying an enhanced input from dry deposition of soil dust above 800 m elevation concentrations of so 4 2 are inversely related to accumulation especially in the winter layer suggesting a significant input from non precipitating events such as dry deposition or riming during this period of very limited snowfall

seismic expression of glaciomarine deposits in the eastern riiser larsen sea antarctica abstract by studying multichannel seismic data across the continental slope and rise of the eastern riiser larsen sea and through a comparison with other east antarctic continental margins the base of the glaciomarine deposits has been traced in this area the seismic data reveal the presence of large channel levee complexes as well as multiple types of contourite accumulations downslope and alonglope processes thus interacted in forming the glaciomarine deposits the deposits are attributed to the advances of ice sheet delivering huge amounts of sediment to the shelf edge and upper slope during glacial maxima oversteepening and instability generated down slope turbidity currents forming channel levee complexes whereas the contourite accumulations were probably mostly formed during interglacials the spatial distribution of the current controlled deposits indicates that bottom currents flow along the western slope of the gunnerus ridge

mesozoic tertiary exhumation history of the altai mountains northern xinjiang china new constraints from apatite fission track data abstract this study uses apatite fission track ft analysis to constrain the exhumation history of bedrock samples collected from the altai mountains in northern xinjiang china samples were collected as transects across the main structures related to palaeozoic crustal accretion events ft results and modeling identify three stages in sample cooling history spanning the mesozoic and tertiary stage one records rapid cooling to the low temperature part of the fission track partial annealing zone circa 70 10 c stage two records a period of relative stability with little if any cooling taking place between 75 and 25 20 ma suggesting the altai region had been reduced to an area of low relief support for this can be found in the adjacent junngar basin that received little if any sediment during this interval final stage cooling took place in the miocene at an accelerated rate bringing the sampled rocks to the earth s surface this last stage linked to the far field effects of the himalayan collision most likely generated the surface uplift and relief that define the present day altai mountains

new angles on south atlantic opening existing models of relative motion between the south america and africa plates significantly misrepresent the azimuth of lower cretaceous seafloor spreading in the south atlantic ocean an improved model is derived from fits of fracture zones magnetic reversal anomalies and the edges of a rifted large igneous province at the northeast georgia rise and agulhas plateau an absolute date for this rifting event is not known but by assuming least change in spreading rates during magnetic chron c34 rifting can be estimated to have occurred at 100 ma this modelling demands a plate divergence history that involves diachronous opening of the south atlantic consistent with published estimates of the ages of breakup from sedimentary basins on the south american and african passive margins the diachronous opening lasts approximately 40 million years during which time it must be accommodated by significant intracontinental deformation a reconstruction using the intra c34 rotation also illustrates the earliest possibility for direct deep water connection between the central and south atlantic oceans one further consequence of this model is that a total reconstruction derived from it and closure of the central atlantic between north america and africa suggests that the venezuelan and gulf of mexico basins may be conjugates formed during the earliest opening of the central atlantic and not separate marginal basins

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

paleozoic accretion and cenozoic redeformation of the chinese tien shan range central asia the tien shan range in central asia contains two late paleozoic sutures the older southern suture marks the collision of a passive margin at the north of the tarim block and an active continental margin subduction under the latter was to the north the younger northern suture separates a northern carboniferous island arc from an active continental margin developed over a south dipping subduction zone the subduction direction under the island arc is unknown mesozoic elastics were deposited over the doubly sutured orogen rate and energy of sedimentation waned until deposition of oligocene conglomerates above a regional unconformity interpreted as marking the onset of deformation induced by the india asia collision molasse deposition accelerated in pliocene and quaternary time and deposition continues today as active thrusts generate relief paleozoic structures control the gross divergence of cenozoic thrusts across the orogen

early tertiary gravity field reconstructions of the southwest pacific the aim of our study is to chronicle the development of plate boundaries in the southwest pacific ocean during the early tertiary this region has been the subject of numerous and often conflicting studies that have attempted to construct the history of plate motion and plate boundary evolution as the australia and pacific plates separated from antarctica our novel approach entails reconstructing gravity fields from satellite altimeter gravity by first removing anomalies overlying seafloor younger than a selected age and then rotating the remaining anomalies through appropriate finite rotations our reconstructions reveal 1 an extensional plate boundary the iselin rift existed between west and east antarctica prior to a24 time 2 the arrival of the southeast indian ridge seir at the tasman ridge prior to a24 led to the extinction of the iselin rift as well as the conversion of the easternmost portion of the tasman plate boundary between the seir and the iselin rift into a transform fault on the pacific antarctic ridge and 3 an early a24 or younger inception of the australia pacific plate boundary our scenario for the opening of the southwest pacific ocean can explain the present day gravity anomalies and magnetic isochrons observed in the northwest ross sea we find that the east antarctic seafloor northeast of the iselin bank was generated by spreading on the tasman ridge prior to a24 time

early india australia spreading history revealed by newly detected mesozoic magnetic anomalies in the perth abyssal plain the seafloor within the perth abyssal plain pap offshorewestern australia is the only section of crust that directly records the early spreading history between india and australia during the mesozoic breakup of gondwana however this early spreading has been poorly constrained due to an absence of data including marine magnetic anomalies and data constraining the crustal nature of key tectonic features here we present new magnetic anomaly data from the pap that shows that the crust in the western part of the basin was part of the indian plate the conjugate flank to the oceanic crust immediately offshore the perth margin australia we identify a sequence of m2 and older anomalies in the west pap within crust that initially moved with the indian plate formed at intermediate half spreading rates 35 mm yr consistent with the conjugate sequence on the australian plate more speculatively we reinterpret the youngest anomalies in the east pap finding that the m0 age crust initially formed on the indian plate was transferred to the australian plate by a westward jump or propagation of the spreading ridge shortly afterm0 time samples dredged from the gulden draak and batavia knolls at the western edge of the pap reveal that these bathymetric features are continental fragments rather than igneous plateaus related to broken ridge these microcontinents rifted away from australia with greater india during initial breakup at 130ma then rifted from india following the cessation of spreading in the pap 101 103ma