tectonic evolution of the west scotia sea joint inversion of isochron and flowline data from the flanks of the extinct west scotia ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron c8 26 5 ma and extinction around chron c3a 6 65 9 ma when they are placed in a regional plate circuit the rotations predict plate motions consistent with known tectonic events at the margins of the scotia sea oligocene extension in powell basin miocene convergence in tierra del fuego and at the north scotia ridge and miocene transpression at the shackleton fracture zone the inversion results are consistent with a spreading history involving only two plates at rates similar to those between the enclosing south america and antarctica plates after chron c5c 16 7 ma but that were faster beforehand the spreading rate drop accompanies inception of the east scotia ridge back arc spreading center which may therefore have assumed the role of the west scotia ridge in accommodating eastwards motion of the trench at the eastern boundary of the scotia sea this interpretation is most easily incorporated into a model in which the basins in the central parts of the scotia sea had already formed by chron c8 contrary to some widely accepted interpretations and which has significant implications for paleoceanography and paleobiogeography

continental stretching preceding the opening of the drake passage evidence from tierra del fuego age estimates for the onset of a seaway through the drake passage range from middle eocene to early miocene complicating interpretations of the relation between ocean circulation and cenozoic global cooling here we present evidence for the presence of a latest paleocene early eocene extensional basin i e lateral rift in tierra del fuego an accurately dated postrift unconformity indicates that extensional faulting ended in the studied area ca 49 ma in concurrence with a previously reported eightfold increase in south america antarctica separation rate and the proposed onset of oceanic basins in the incipient drake passage the coincidence of these facts indicates progressive strain concentration on the zone of future crustal separation i e drake passage after abandonment of lateral rifts ca 49 ma although the described extensional depocenters did not allow the exchange of water between the pacifi c and atlantic oceans they represent a direct indication of continental lithospheric stretching preceding the recently proposed eocene opening of embryonic basins in the west scotia sea

igneous activity in the southern andes and northern antarctic peninsula a review the antarctic peninsula and the southern andean continental margin have been the sites of extensive and varied magmatic activity since at least late palaeozoic time the dominant activity has been of calc alkaline character and although andesite and tonalite are the main rock types there are well marked transverse variations in the major and trace element chemistry of the magmas erupted in both areas thus calc alkaline magmas erupted farthest from the ancestral trench are generally more silicic potassic and light ree enriched than magmas erupted closer to the trench in both regions the locus of calc alkaline activity has tended to move trenchwards since the palaeozoic although activity began to wane during the mid cenozoic when subduction of the pacific plate slowed down and ceased a northward cessation of calc alkaline magmatism along the antarctic peninsula can be linked in broad terms to the progressive northwards subduction of sections of the aluk ridge during the cenozoic extensional tectonics within and behind the main igneous belts caused the development of narrow back arc basins in southern chile during the mesozoic and in bransfield strait along the northern antarctic peninsula during recent times tholeiitic magmas with transitional calc alkaline characteristics were erupted in these basins in addition alkalic basalts and related undersaturated magma types were erupted onto the stable continental platform well behind the main calc alkaline and marginal basin magmatic belts during cenozoic and recent times probably in response to extensional tectonics as subduction slowed or ceased

oriented inclusions of magnetite in clinopyroxene source of stable remanent magnetization in gabbros of the messum complex namibia 1 crystallographically oriented and highly elongate magnetite inclusions in clinopyroxene are the dominant source of highly stable remanent magnetization in gabbros of the early cretaceous messum complex namibia rock magnetic properties determined for individual pyroxene crystals indicate a high proportion of single domain magnetite consistent with the observed sizes and shape anisotropy of the magnetite inclusions as in previous studies of similar inclusions these are inferred to have formed by exsolution two arrays of inclusions are regularly present in the messum clinopyroxenes inclined at about 74 consistent with formation at about 800 c deduced from optimization of phase boundary orientations virtual geomagnetic poles from these rocks are consistent with reference data confirming that the magnetization is of thermoremanent origin bipolar magnetizations are recorded at one site as well in individual clinopyroxene crystals suggesting that remanence acquisition upon initial cooling of the gabbro spanned a geomagnetic polarity reversal

autochthonous v accreted terrane development of continental margins a revised in situ tectonic history of the antarctic peninsula the allochthonous terrane accretion model previously proposed for the geological development of the antarctic peninsula continental margin arc is reviewed in light of recent data and the geology is reinterpreted as having evolved as an in situ continental arc this is based upon the following factors 1 the presence of early palaeozoic basement and stratigraphic correlation of sequences between the autochthonous and previously proposed allochthonous terranes 2 isotopic evidence for similar deep crustal structure across the different terranes 3 ocean island basalt magmas and deep marine sedimentary rocks formed during continental margin extension within the previously proposed accretionary wedge sequence i e not formed against an active oceanic arc 4 the distribution of magnetic susceptibility measurements and aeromagnetic data locating the palaeo subduction zone along the west of the peninsula 5 a lack of clear palaeomagnetic distinction between the terranes the following alternative tectonic history is proposed 1 amalgamation and persistence of gondwana 2 subsequent silicic large igneous province magmatism and extension 3 development and history of andean subduction until its cessation in the cenozoic a number of features in the antarctic peninsula correlate with those of other circum pacific margins supporting a global evaluation of allochthonous v autochthonous margin development to aid our understanding of crustal growth mechanisms

new paleomagnetic data from the antarctic peninsula and their tectonic implications new paleomagnetic data presented here further constrain the relative motion of the antarctic peninsula relative to east antarctica during the mesozoic development of the southern ocean basins the antarctic peninsula ap is one of four crustal blocks that define west antarctica the others being the ellsworth whitmore mountains ewm thurston island eights coast ti and marie byrd land mbl a jurassic pole 155ma 124 e 64 s a95 7 1 n 10 vgps was obtained from the ap block which suggests that the ap block rotated clockwise between 175 and 155 ma due to significant early opening in the weddell sea basin a new early cretaceous paleomagnetic pole 182 e 74s a95 5 9 n 6 site mean vgps indicates that the ap block was in or near to its present day position with respect to east antarctica by 130 ma between 155 and 130 ma counterclockwise rotation of the ap ti blocks together with the southward motion of east gondwanaland probably resulted in subduction of weddell sea ocean floor beneath the southern ap block and initiated the palmer land deformational event a 130 ma pole from the ti block requires clockwise rotation of the ti and possibly the ewm blocks between 130 and 110 ma producing sinsistral strike slip motion between the ewm block and east antarctica and dextral transpressional motion between the ti ewm blocks and the ap block new ap block 110 ma and 85 ma poles from this study 199 e 74 s a95 6 9 n 13 vgps 152 e 86 s a95 7 5 n 6 vgps respectively are similar to equivalent age poles from east antarctica and suggest little or no relative motion between the antarctic peninsula and east antarctica northern and southern antarctic peninsula mid cretaceous poles are very much alike suggesting that the s shape of the antarctic peninsula is not due to oroclinal bending since 110 ma

drake passage and cenozoic climate an open and shut case drake passage opening has often been viewed as a single discrete event possibly associated with abrupt changes in global circulation and climate at or near the eocene oligocene boundary a new plate tectonic model based on recent reinterpretations of the opening history of basins in the scotia sea suggests that an effective ocean gateway may have developed even earlier during the middle eocene this is consistent with a growing body of evidence from sediment core proxy data for eocene changes in southern ocean circulation and biological productivity the period between earliest opening after 50 ma and the latest eocene was characterized by the evolution of various current pathways across the subsiding continental shelves and intervening deep basins this shallow opening may have caused important changes in southern ocean circulation contributing to eocene cooling and the growth of antarctic ice sheets

tectonic reconstructions for paleobathymetry in drake passage abstract a minimum complexity tectonic reconstruction based on published and new basin opening models depicts how the scotia sea grew by cenozoic plate divergence dismembering a jurassic sheared margin of gondwana part of the jurassic early cretaceous ocean that accreted to this margin forms the core of the central scotia plate the arc plate above a trench at the eastern end of the scotia sea which migrated east away from the antarctic and south american plates a sequence of extensional basins opened on the western edge of the central scotia plate at 50 30 ma decoupled from the south american plate to the northwest by slow motion on a long transform fault succeeding the basins seafloor spreading started around 30 ma on the west scotia ridge which propagated northwards in the 23 17 ma period and ceased to operate at 6 ma the circuits of plate motions inside and outside the scotia arc are joined via rotations that describe antarctic central scotia plate motion in powell basin until 20 ma and along the south scotia ridge thereafter the modelled relative motion at the northern edge of the scotia sea is thus constrained only by the plate circuit but nonetheless resembles that known coarsely from the geological record of tierra del fuego a paleobathymetric interpretation of nine time slices in the model shows drake passage developing as an intermediate depth oceanographic gateway at 50 30 ma with deep flow possible afterwards initially this deep flow would have been made tortuous by numerous intermediate and shallow barriers a frontal pattern resembling that in the modern scotia sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the scotia sea at 18 5 ma at shag rocks passage and after 10 ma southeast of south georgia

the curved magallanes fold and thrust belt tectonic insights from a paleomagnetic and anisotropy of magnetic susceptibility study the magallanes fold and thrust belt ftb presents a large scale curvature from n s oriented structures north of 52 s to nearly e w in tierra del fuego island we present a paleomagnetic and anisotropy of magnetic susceptibility ams study from 85 sites sampled in cretaceous to miocene marine sediments magnetic susceptibility is lower than 0 0005 si for 76 sites and mainly controlled by paramagnetic minerals ams results indicate that the sedimentary fabric is preserved in the undeformed areas of tierra del fuego and the more external thrust sheets units where an incipient lineation due to layer parallel shortening is recorded prolate ams ellipsoids indicating a significant tectonic imprint in the ams fabric are observed in the internal units of the belt ams results show a good correlation between the orientation of the magnetic lineation and the fold axes however in peninsula brunswick the ams lineations are at 20 counterclockwise to the strike of the fold axes pretectonic stable characteristic remanent magnetizations chrm were determined in seven sites a counterclockwise rotation 21 2 9 2 is documented by chrm data from four sites near the hinge of the belt in peninsula brunswick and near canal whiteside while there is no evidence of rotation near the nearly e w oriented vicuna thrust within tierra del fuego the curved shape of the cenozoic magallanes ftb is not related to vertical axis rotation and thus the magallanes ftb can be considered as a primary arc

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

the occurrence of alpine permafrost in the front range of colorado abstract permafrost distribution or ground that remains frozen for at least 2 years has been modeled using a combination of geographic information system gis techniques digital elevation model dem variables and land cover in alpine regions of the world in the front range however no such empirical models have been developed and field data are restricted in spatial extent but rock glaciers are in abundance here i present a probabilistic logistic regression model that is based on topoclimatic information elevation and aspect for rock glaciers derived from u s geological survey usgs 10 m dems classes of land cover obtained from an enhanced thematic mapper plus etm image classification were assigned weights and were then multiplied by the regression results to refine estimates the effectiveness of the model was evaluated by comparing mean probability scores with rock glacier activity categories mean annual air temperature maat from climatic stations on niwot ridge and bottom temperature of winter snow bts measurements while a monte carlo simulation was used to detect uncertainty associated with the original dem permafrost scores 50 covered about 8 9 242 km 2 of the study area 2722 km 2 with the highest scores clustered around longs and rowe peaks permafrost locations showed a strong correlation with rock glacier activity classes the 1 0 c maat isotherm and bts measurements less than 3 0 c the uncertainty analysis revealed that slight global differences exist between the original and error prone dem however local variations in aspect caused the most uncertainty these results indicate that the model accurately represents regional distribution of permafrost therefore topoclimatic information from rock glaciers and land cover when combined with an uncertainty analysis can effectively be used to map the occurrence of front range permafrost providing an imperative tool for cartographers planners and geocryologists

paleogene opening of drake passage the timing of events leading to the earliest connection between the pacific and atlantic oceans at drake passage is controversial but important because gateway opening probably had a profound effect on global circulation and climate a rigorous new analysis of marine geophysical data demonstrates a major change in the motion of the south american and antarctic plates at about 50 ma from n s to wnw ese accompanied by an eightfold increase in separation rate this would have led to crustal extension and thinning and perhaps the opening of small oceanic basins with the probable formation of a shallow b1000 m gateway during the middle eocene no change in south american antarctic motion is observed near the eocene oligocene boundary but a deep water connection developed between 34 and 30 ma when continued extension led to the initiation of seafloor spreading at the west scotia ridge these timings correlate with events seen in the oxygen isotope record from benthic foraminera and support the view that drake passage opening was the trigger for abrupt eocene oligocene climate deterioration and the growth of extensive antarctic ice sheets d 2005 elsevier b v all rights reserved

radiometric ages for basement rocks from the emperor seamounts odp leg 197 1 the hawaiian emperor seamount chain is the type example of an age progressive hot spot generated intraplate volcanic lineament however our current knowledge of the age distribution within this province is based largely on radiometric ages determined several decades ago improvements in instrumentation sample preparation methods and new material obtained by recent drilling warrant a reexamination of the age relations among the older hawaiian volcanoes we report new age determinations 40ar 39ar incremental heating method on whole rocks and feldspar separates from detroit sites 1203 and 1204 nintoku site 1205 and koko site 1206 seamounts ocean drilling program odp leg 197 and meiji seamount deep sea drilling project dsdp leg 19 site 192 plateaus in incremental heating age spectra for site 1203 lava flows give a mean age of 75 8 0 6 2 ma which is consistent with the normal magnetic polarity directions observed and biostratigraphic age assignments site 1204 lavas produced discordant spectra indicating ar loss by reheating and k mobilization six plateau ages from lava flows at site 1205 give a mean age of 55 6 0 2 ma corresponding to chron 24r drilling at site 1206 intersected a n r n magnetic polarity sequence of lava flows from which six plateau ages give a mean age of 49 1 0 2 ma corresponding to the chron 21n 22r 22n sequence plateau ages from two feldspar separates and one lava from dsdp site 192 range from 34 to 41 ma significantly younger than the cretaceous age of overlying sediments which we relate to postcrystallization k mobilization combined with new dating results from suiko seamount dsdp site 433 and volcanoes near the prominent bend in the lineament sharp and clague 2002 the overall trend is increasing volcano age from south to north along the emperor seamounts consistent with the hot spot model however there appear to be important departures from the earlier modeled simple linear age progression which we relate to changes in pacific plate motion and the rate of southward motion of the hawaiian hot spot

mafic dike swarms in the south shetland islands volcanic arc unravelling multiepisodic magmatism related to subduction and continental rifting eight groups of mafic dikes and related high level stocks cut triassic accretionary complex and mesozoic magmatic arc formations on livingston island some are affected by silicic sericitic alteration related to cretaceous hydrothermal activity and propylitic epidosite alteration analogous to that in ocean floor sheeted dikes alteration was accompanied by major and trace element metasomatism ar ar analysis of the freshest rocks indicates five intrusive events some of which are unexpectedly young groups 1 3 were intruded in the mid to late cretaceous 108 74 ma and were coeval with the calc alkaline arc between 70 and 50 ma relatively rapid and oblique plate convergence led to strike slip tectonism and a pause in magmatism at 52 ma orthogonal slow convergence resulted in extensional faulting and emplacement of calc alkaline group 2 and primitive tholeiitic dikes groups 4 6 between 51 and 45 ma extension of antarctic peninsula southern south american crust culminated in emplacement of mafic to intermediate medium grained plutons and group c porphyries between 44 and 36 ma localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby cretaceous lavas and ar ar reset ages of 40 ma in mid cretaceous hydrothermal k feldspar primitive olivine basalts group d and epithermal carbonate veins 31 29 ma were emplaced during along arc extension accompanying the opening of drake passage and powell basin excess argon occurs in two forms strongly held in melt inclusions in the primitive tholeiites and weakly held in some secondary alteration there is no radiometric evidence in the area studied for magmatism related to late cenozoic subduction nor to the pleistocene recent opening of the back arc bransfield rift

an 40ar 39ar geochronology on a mid eocene igneous event on the barton and weaver peninsulas implications for the dynamic setting of the antarctic peninsula the genesis of basaltic to andesitic lavas mafic dikes and granitoid plutons composing the subaerial cover on the barton and weaver peninsulas antarctica is related to arc formation and subduction processes precise dating of these polar rocks using conventional 40ar 39ar techniques is compromised by the high degree of alteration with loss on ignition as high as 8 in order to minimize the alteration effects we have followed a sample preparation process that includes repeated acid leaching acetone washing and hand picking followed by an overnight bake at 250 c after this procedure groundmass samples can yield accurate age plateaus consisting of 70 100 of the total 39ark released using high resolution heating schedules the different rock types studied on the barton and weaver peninsulas yielded almost coeval ages suggesting a giant igneous event in the weaver and barton peninsulas at 44 5 ma a compilation of newly published ages indicate that this event took place throughout the whole south shetland islands suggesting a dynamic incident occurred at this stage during the arc evolution history we related this igneous event to a mantle delamination mechanism during eocene times the delamination process began at 52 ma and the resultant upwelling of asthenosphere baffled the subduction of phoenix plate causing an abrupt decrease in convergence rate then multiple magmatic sources were triggered resulting in a culminating igneous activity during 50 40 ma with a peak at 45 ma along the archipelago the delamination also caused the extension regime indicated by the dike swarm plugs and sills all over the archipelago and the uplift of smith metamorphic complex and livingston island delamination process may have finished at some time during 40 30 ma leaving a weak igneous activity at that stage and thereafter the convergence rate then recovered gradually as indicated by the magnetic anomaly identifications this model is supported by seismic observation of deep velocity anomalies beneath the antarctic peninsula

geomagnetic secular variation in the cretaceous normal superchron and in the jurassic abstract it is now widely thought that geomagnetic polarity reversals occur spontaneously as a result of normal dynamo action rather than being externally triggered if this is the case then it may well be that periods of time in which the geomagnetic reversal frequency was dramatically different were characterised by different styles of secular variation two such periods were the cretaceous normal superchron cns 84 125 ma when the field was dominantly of a single polarity for 40 myr and the jurassic period 145 200 ma when reversals occurred at an average rate of as much as 4 6 myr 1 here we analyse a database of new and published palaeomagnetic directions from lavas emplaced during these periods in order to obtain first order descriptions of the palaeosecular variation psv during these times we then compare these records with one another and with that produced for the period 0 5 ma with average reversal frequency of 4 0 myr 1 our results are more equivocal than those obtained in a previous similar study mcfadden p l merrill r t mcelhinny m w lee s h 1991 reversals of the earths magnetic field and temporal variations of the dynamo families journal of geophysical research solid earth and planets 96 3923 3933 we demonstrate that this is probably a result of the previous study being affected by an artefact of their correction for within site scatter the usefulness of our jurassic record is severely limited by the restricted palaeolatitudinal span of the available data however our record for the cns is sufficient to allow us to conclude that it was likely that secular variation then was different from that in the 0 5 ma period this supports the hypothesis of a link between psv and reversal frequency and therefore endorses psv analysis as a first order tool for determining geomagnetic stability in the past

volcanic and tectonic evolution of the northern antarctic peninsula late cenozoic to recent abstract field investigation together with a number of geochemical petrographical analyses as well as absolute k ar age determinations and geophysical data allow the recognition of an evolutionary sequence of geodynamic events which have affected the northern region of antarctic peninsula and the adjacent islands a significant volcanic calc alkaline belt which developed on the northwestern margin of the antarctic peninsula during the cretaceous to middle tertiary is indicative of active subduction of the antarctic plate in that area this activity decreases during the lower miocene giving way to an expansive phase represented by the bransfield rift these extensional processes are dominant during the pliocene creating a rift system in southeastern bransfield towards larsen both the bransfield and larsen systems comprise one fan like rift system associated with the prince gustav rift and the scotia arc micro plate ejection of abundant pyroclastic material generated a large plateau of palagonite hyaloclastites of basaltic alkaline composition during the pleistocene recent the extensional activity continued as evidenced by the active volcanic fractures represented in bransfield by the deception penguin and bridgeman volcanic centres in the prince gustav rift by paulet islands and others and in larsen by the coley seal nunatak and argo volcanic centres the latter is characterized by basaltic olivine alkaline effusions these rifts and the continental blocks are affected by a series of fractures with a n60 70 w strike which could be directly associated with the hero fracture zone extending northwest of the south shetland islands trench

the role of ocean gateways in the dynamics and sensitivity to wind stress of the early antarctic circumpolar current the date of inception of the antarctic circumpolar current is debated due to uncertainty in the relative opening times of drake passage and the tasman seaway using an idealized eddy resolving numerical ocean model we investigate whether both ocean gateways have to be open to allow for a substantial circumpolar current we find that overlapping continental barriers do not impede a circumpolar transport in excess of 50sv as long as a circumpolar path can be traced around the barriers however the presence of overlapping barriers does lead to an increased sensitivity of the current s volume transport to changes in wind stress this change in sensitivity is interpreted in terms of the role of pressure drops across continental barriers and submerged bathymetry in balancing the momentum input by the surface wind stress specifically when the pressure drop across continents is the main balancing sink of momentum the zonal volume transport is sensitive to changes in wind stress changes in zonal volume transport take place via altering the depth independent part of the circumpolar transport rather than that arising from thermal wind shear in such a scenario isopycnals continue to slope steeply across the model southern ocean implying a strong connection between the deep and surface oceans this may have consequences for the meridional overturning circulation and its sensitivity to wind stress

nrm irm s demagnetization plots an aid to the interpretation of natural remanent magnetization af demagnetization of thermoremanent magnetization trm and saturation isothermal remanent magnetization irm s are used to establish diagnostic criteria for primary trm and for secondary magnetization in igneous rocks in applying the approach to the interpretation of natural remanent magnetization nrm the demagnetization of irm s documents the nature of the magnetic phases in the rock while the demagnetization of the nrm gives information about the process of magnetization the ratio of nrm irm s measures the efficiency of the magnetization process

new paleomagnetic and 40ar 39ar geochronological results for the south shetland islands west antarctica and their tectonic implications

statistical properties of paleomagnetic directions in kerguelen lava flows implications for the late oligocene paleomagnetic field 40 ar 39 ar ages are reported to aid in the calibration of the paleomagnetic results the primary contribution of this report however is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the kerguelen flood basalts in total 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 ma window from 25 to 30 ma the composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale our approach is to investigate the average normal and reversed polarity field directions over this 5 ma window we calculated a paleomagnetic pole found to be located at l 85 5n f 189 3 e a95 2 3 k 16 5 n 233 this pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the indo atlantic hot spot reference frame in the statistical analysis we tackle the specific question is the secular variation isotropic this question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles which is still a matter of debate among paleomagnetists by means of statistical tests we show that the paleomagnetic data from kerguelen agree with an isotropic model for paleosecular

evolution of cenozoic seaways in the circum antarctic region abstract a complete circum antarctic seaway did not open until both the south tasman rise cleared the oates land coast of east antarctica and drake passage opened between the southern tip of south america and the northern end of the antarctic peninsula major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the south tasman rise and antarctica as very close to the eocene oligocene boundary with an unrestricted opening deeper than 2000 m dating from 32 ma timing of the opening of drake passage is more circumstantial because the exact motions of certain micro continental fragments are not known the motion of africa with respect to south america as well as the motion of east antarctica with respect to africa are well constrained for the cenozoic these major plate motions are used with the reasonable assumption of no cenozoic motion of the antarctic peninsula with respect to east antarctica to constrain the location of the antarctic peninsula with respect to the southern tip of south america for the critical period of late eocene to late oligocene uncertainty of motion of the south georgia and south orkney microcontinents and other possible continental fragments make an exact time for opening of drake passage difficult to ascertain even so the early oligocene position of the antarctic peninsula with respect to south america requires a through going deep water seaway to have been open at drake passage prior to 28 ma even given the unconstrained motion of various high standing crustal fragments in the scotia sea with reasonable assumptions concerning motion of the crustal fragments in the western and central scotia sea it is likely that drake passage or passage through powell basin was open to deep water circulation by 31 2 ma

cretaceous and early tertiary paleomagnetic results from the antarctic peninsula paleomagnetic results are presented for early tertiary and cretaceous igneous and volcanic units from the northern antarctic peninsula when combined with previously published data they permit the construction of a polar wander path that is very similar to one derived for the east antarctic craton by rotating tertiary and cretaceous poles from india and australia into the antarctic frame of reference when considered in this context the paleomagnetic data from the orville coast which was previously interpreted as evidence in favor of oroclinal bending or relative rotation of the southern antarctic peninsula with respect to the northern part may be considered as an in situ resulte the implications of the new data are as follows 1 the antarctic peninsula down to the orville coast has been a part of the antarctic plate since 100 m y ago 2 movement of the antarctic peninsula required to avoid overlap in reconstructions of gondwanaland must have occurred before 100 m y ago

paleomagnetism and tectonics of the south shetland islands and the northern antarctic peninsula new paleomagnetic data presented here from 61 sites in paleozoic mesozoic and cenozoic igneous and sedimentary rocks from the antarctic peninsula and the south shetland islands constrain the relative motion of the antarctic peninsula since the mid cretaceous and allow the quantification of tectonic rotation between the different blocks recognized within the area paleozoic and jurassic results failed the fold test and suggest an important remagnetization in the area the similarity between these results and those obtained from cretaceous intrusives indicates a mid cretaceous age for the remagnetization the paleopoles obtained for the different blocks and for different ages suggest that there is no relative rotation among them these combined results allow us to obtain a cretaceous 90 ma and paleocene 60 ma paleopole these paleopoles document a very low apparent polar wandering of the antarctic peninsula for the last 100 ma

eocene oligocene transition in the southern ocean history of water mass circulation and biological productivity high resolution records of carbon and oxygen isotopes and benthic foraminiferal accumulation rates for the eocene oligocene section at ocean drilling program site 689 maud rise weddell sea paleodepth about 1500 m were used to infer variations in paleoproductivity in relation to changes in climate and ventilation of the deeper water column the benthic foraminiferal abundance and isotope records show short term fluctuations at periodicities of 100 and 400 ka implying orbitally driven climatic variations both records suggest that intermediate depth water chemistry and primary productivity changed in response to climate during the eocene productivity increased during cold periods and during cold to warm transitions possibly as a result of increased upwelling of nutrient rich waters in the oligocene in contrast productivity maxima occurred during intervals of low d18o values presumably warmer periods when a proto polar front moved to the south of the location of site 689 this profound transition in climate productivity patterns occurred around 37 ma coeval with rapid changes toward increasing variability of the oxygen and carbon isotope and benthic abundance records and toward larger amplitude d18o fluctuations therefore we infer that at this time temperature fluctuations increased and a proto polar front formed in conjunction with the first distinct pulsations in size of the antarctic ice sheet we speculate that this major change might have resulted from an initial opening of the drake passage at 37 ma at least for surface and intermediate water circulation

scotia sea regional tectonic evolution implications for mantle flow and palaeocirculation abstract the scotia sea and surrounding scotia arc have evolved over the past 40 ma by extension behind an east migrating subduction zone at the boundary between the south american sam and antarctic ant plates the considerable data set now available regional geology and geophysics earthquake seismology satellite altimetry global plate analyses suggest why east migrating subduction began what has been the driving force that has sustained it and what other processes have controlled the mode of back arc extension in the scotia sea a suite of six reconstructions has been developed based on this data set the reconstruction to 40 ma creates a compact cuspate continental connection between south america and the antarctic peninsula at the subducting pacific margin with fragments now dispersed around the scotia arc occupying positions within it compatible with their known geology the driving force has been subduction of south american ocean floor which began as a result of southward migration of the pole of south american antarctic plate rotation and a key modulator of back arc extension has been collision of ridge crest sections of the south american antarctic plate boundary with the east advancing trench cenozoic regional tectonic evolution has two other likely consequences which greatly increase its importance firstly this region saw the tectonic disruption of the final barrier to complete circum antarctic deep water flow that may have had a profound effect on palaeoclimate secondly it is possible that the rapid roll back of the hinge of subduction is related to shallow eastward flow in the sub lithospheric mantle both of these consequences are explored the reconstructions show that rapid roll back of the subduction hinge averaging 50 mm a over the last 40 ma with respect to the south american plate has been a feature of all of scotia sea evolution and provide a history of motion of several oceanic microplates most of which are now welded together within the scotia sea this will guide the location of seismometers and or dredge hauls to test the hypothesis of shallow mantle flow and help interpret the results the reconstructions also allow an assessment of the creation of deep water pathways that would have permitted the development of the present day antarctic circumpolar current acc an early miocene onset within the period 22 17 ma seems likely for the acc depending on the structure and palaeo elevation of davis bank and aurora bank sections of the north scotia ridge however the study shows there was a delay of one or more million years between initial provision of a deep water pathway and the major mid miocene change in global climate involving the general level of antarctic glaciation that may have been related if these changes were related then the delay suggests that other factors possibly rough elevated ocean floor but also non tectonic factors such as atmospheric co2 were important in determining palaeoclimate

new and revised set of cretaceous paleomagnetic poles from hong kong implications for the development of southeast china a paleomagnetic study of upper cretaceous 80 ma dikes from hong kong was conducted with the aim of better understanding the late cretaceous cenozoic kinematic history of southeast china twelve out of seventeen sites collected from four dikes revealed primary trms and yield a late cretaceous paleopole at 69 38n 211 28e where a95 8 98 and k 107 a synthesis of south china block cretaceous poles indicates that coastal southeast china which is bounded by the lianhuashan fault zone to the northwest and the changlenanao fault zone to the southeast has moved relative to inland central south china the discordant cretaceous paleopoles from the south china block suggest a moderate net clockwise rotation of the coastal southeast china relative to central south china since the late cretaceous this clockwise rotation is probably the result of micro blocks along the south china margin experiencing vertical axis rotations in response to the penetrative dextral shear along the se china margin during oblique subduction of the pacific plate beneath south china an alternative interpretation involves regional extension that displaced a raft block southeastwards of the margin generating apparent paleomagnetic records with no real block rotations q 2004 elsevier ltd all rights reserved

timing and climatic consequences of the opening of drake passage age estimates for the opening of drake passage range from 49 to 17 million years ago ma complicating interpretations of the relationship between ocean circulation and global cooling secular variations of neodymium isotope ratios at agulhas ridge southern ocean atlantic sector suggest an influx of shallow pacific seawater approximately 41 ma the timing of this connection and the subsequent deepening of the passage coincide with increased biological productivity and abrupt climate reversals circulation productivity linkages are proposed as a mechanism for declining atmospheric carbon dioxide these results also indicate that drake passage opened before the tasmanian gateway implying the late eocene establishment of a complete circum antarctic pathway

the coupling of bay hydrodynamics to sediment transport and its implication in micro tidal wetland sustainability abstract to investigate bay hydrodynamics and its impacts on the adjacent micro tidal wetland sustainability hourly measurements of wave tidal current and benthic suspended sediment concentration in summer winter and spring of 2015 2016 were conducted in fourleague bay louisiana usa high temporal resolution data indicate that benthic suspended sediment resuspension had a dominant periodicity of 4 8 d which was mainly caused by wind driven waves sediment flux reached 28 g m 2 s 1 during events net sediment flux direction is northwestward in summer and southeastward in winter and spring potential depth integrated sediment flux to surrounding wetland varied within 0 500 g m 1 s 1 seasonal variations of river discharge and wind direction particularly speed 3 m s 1 dominated potential sediment contribution from the bay to the surrounding wetland three sediment transport regimes were delineated bypassing season resuspension accumulation season and combined bypassing and resuspension accumulation season this study couples bay hydrodynamics to the sediment transport processes and sustainability of adjacent wetlands in a micro tidal environment it sheds light on the understanding of natural feedback mechanisms and how estuarine marsh system survive high relative sea level rising scenario in micro tidal environment which could aid in the design of future ecological engineering restoration strategies

cenozoic crustal thickening wrenching and rifting in the foothills of the southernmost andes abstract the southernmost andes form an orocline between the patagonian cordillera trending n s and the fueguian cordilleras trending e w on the foreland side is the magellan basin the area has a history of paleozoic compression triassic to early cretaceous rifting and late cretaceous to quaternary compression in response to changing plate tectonics major structures of late cretaceous and cenozoic age vary along the strike in the patagonian cordillera and foothills folds and thrusts trend nnw slightly oblique to the orogen whereas strike slip faults are parallel to the orogen and right lateral in the fueguian cordilleras and foothills folds and thrusts trend ese slightly oblique to the orogen whereas strike slip faults are parallel to the orogen and left lateral in the axial zone of the magellan basin folds and thrusts are parallel to the orogen and rifts are sub perpendicular to it to a first approximation the pattern of structures has mirror symmetry about the axis of the magellan basin in detail however wrenching appears to be more prevalent in the fueguian cordillera and foothills than it is in the patagonian cordillera and foothills minor faults of cenozoic age are common in the foothills from a kinematic analysis of fault slip data 1 shortening and stretching directions are mostly sub horizontal 2 shortening directions vary in trend from ene in the patagonian foothills to ne in the fueguian foothills and 3 stretching directions are sub parallel to traces of major thrusts in the fueguian cordillera and foothills strike slip faulting is prevalent in the patagonian foothills crustal thickening is prevalent over strike slip faulting the kinematics reflect a combination of thrusting and wrenching and they are consistent with the major structures to investigate the origin of the cenozoic structures we used analogue models on a fully lithospheric scale where an oceanic plate subducted beneath a continental corner the corner was an area of transition from frontal subduction to transcurrent motion the boundary conditions may not have been fully realistic but the experiments did account for the major elements of the structural pattern in southernmost south america including rifts that are perpendicular to the orogen and counterclockwise block rotations

observation of shrinkage cracks in ocean floor titanomagnetites abstract a characteristic of low temperature ocean floor titanomagnetite oxidation is the formation of irregular curved cracks which are caused by shrinkage of the spinel lattice the spatial distribution of the shrinkage cracks within individual titanomagnetite grains magnetically extracted from different ocean floor basalts was studied by scanning electron microscopy the cracks do not develop in grains smaller than approximately 5 microns in diameter under ocean floor conditions the critical titanomagnetite oxidation state z for the onset of cracking is between z 0 23 and z 0 37 shortly before the onset of cracking internal stress reaches a maximum which if reflected in the magnetic coercivity of the material

first order reversal curve forc diagrams for pseudo single domain magnetites at high temperature abstract the recently developed first order reversal curve forc technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations however there are still some gaps in the basic understanding of forc diagrams in particular the behavior of pseudo single domain psd grains and the contribution of magnetostatic interactions in this paper we address some of these problems we report the first forc diagrams measurements on narrowly sized and well characterized synthetic psd through multidomain md magnetite samples the forc diagrams evolve with grain size from single domain sd like to md like through the psd grain size range since each sample contains grains of essentially a single size individual psd grains evidently contain contributions from both sd like and md like magnetic moments in proportions that vary with grain size the evolving forc diagrams cannot be due to physical mixtures of sd and md grains of widely different sizes the forc diagrams were all asymmetric small psd samples have forc diagrams with a distinctive closed contour structure the distributions of the larger md grains display no peak and lie closer to the interaction field axis to assess the effect of magnetostatic interactions we measured forc diagrams between room temperature and the curie temperature on heating the forc distributions contract without changing shape until 500 c above this temperature the diagrams become more md like and in addition become more symmetric the temperature dependence of the interaction field parameter is proportional to that of the saturation magnetization in accordance with neel s interpretation of the preisach diagram the decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions the magnetic hysteresis parameters as a function of temperature were determined from the forc curves as the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors