surface geostrophic currents across the antarctic circumpolar current in drake passage from 1992 to 2004 the southern ocean plays an important role in the global overturning circulation as a significant proportion of deep water is converted into intermediate and deeper water masses in this region recently a secular trend has been reported in wind stress around the southern ocean and it is thought theoretically that the strength of the acc is closely related to wind stress so one consequence should be a corresponding increase in acc transport and hence changes in the rate of the global overturning there are no long term data sets of acc transport and so we must examine other data that may also respond to changing wind stress here we calculate surface currents in drake passage every seven days over 11 25 years from 1992 to 2004 we combine surface velocity anomalies calculated from satellite altimeter sea surface heights with measured surface currents since 1992 the uk has regularly occupied woce hydrographic section sr1b across the acc in drake passage from seven hydrographic sections surface currents are estimated by referencing relative geostrophic velocities from ctd sections with current measurements made by shipboard and lowered acoustic doppler current profilers combining the seven estimates of surface currents with the altimeter data reduces bias in the estimates of average currents over time through drake passage and we show that surface current anomalies estimated by satellite and in situ observations are in good agreement the strongest surface currents are found in the subantarctic and polar fronts with average speeds of 50 cm s and 35 cm s respectively and are inversely correlated so that maximum westward flow in one corresponds to minimum westward flow in the other the average cross sectional weighted surface velocity from 1992 to 2004 is 16 7 0 2 cm s a spectral analysis of the average surface current has only weakly increasing energy at higher frequencies and there is no dominant mode of variability the standard deviation of the seven day currents is 0 68 cm s and a running 12 month average has only a slightly smaller standard deviation of 0 52 0 16 cm s the southern annular mode sam measures the circumpolar average of wind stress and like the surface currents its spectrum has slightly increased energy at frequencies greater than 1 cpy a cospectral analysis of these averaging cospectra of five slightly overlapping 36 month segments improve statistical reliability suggests that there is coherence between them at 1 cpy with the currents leading changes in the southern annular mode we conclude that the sam and average drake passage surface currents are weakly correlated with no dominant co varying modes and hence predicting southern ocean transport variability from the sam is not likely to give significant results and that secular trends in surface currents are likely to be masked by weekly and interannual variability

circulation mixing and production of antarctic bottom water abstract the antarctic source of bottom water to the abyssal layer of the world ocean is examined as well as its large scale flow pattern and ultimate entrainment rate into the deep water above we make use of the available high quality station data in the southern ocean to construct bottom maps of neutral density and mean property maps including chlorofluorocarbon cfc for the abyssal layer underneath a selected neutral density surface the maximum density at the sill depth of drake passage is used to distinguish between the voluminous deep water mass that is a continuous component of the antarctic circumpolar current from the relatively denser bottom water originated along the antarctic continental margins based on water density antarctic bottom water aabw is defined here generically to include all volumes of non circumpolar water of antarctic origin over the shelf regime multiple localized sources of specific aabw types contribute to the abyssal layer of the adjacent antarctic basins characteristics of these dense bottom waters reflect closely those observed on the parent shelf water mass spreading paths of newly formed deep and bottom waters over the slope regime and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the aabw layer interior mixing and interbasin exchanges of aabw are deduced from mean characteristic curves following the southern streamline of the antarctic circumpolar current outflow and mixing of aabw from the weddell sea to the argentine basin is depicted using density and cfc distributions of two zonal hydrographic lines recirculation and mixing of deep and bottom waters within the weddell gyre are also detailed using a meridional section along the greenwich meridian the strength of all localized sources of aabw combined is estimated by two independent approaches an estimate of the total production rate of aabw is calculated based on the oceanic cfc budget for the aabw layer offshore of the 2500 m isobath the sum of all downslope inputs of well ventilated bottom water types underneath the top isopycnal must account for the measured cfc content in the bottom layer the resulting total aabw production rate is about 8 sv which is a conservative figure that neglects the loss of cfc bearing waters across the top isopycnal in recent years whereas about 9 5 sv is calculated assuming a well mixed bottom layer making use of their transient nature cfc distributions at the top of the aabw layer indicate that more direct and rapid entrainment of cfc rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing a second more ad hoc but independent oceanic mass budget of the bottom layer is also constructed a typical basin wide rate of deep upwelling of 3 10 7 m s 1 requires 10 sv 1 sv 106 m3 s 1 of newly formed aabw to sink down the slope around antarctica we have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the aabw it is expressed as a combination of wind topographic and turbulent components which in turn are functions of the isopycnal depth bottom depth and bottom layer thickness this non uniform upwelling field yields about 12 sv of aabw exported across the top isopycnal fortuitously the overall average of upward speed at the top isopycnal 3 7 10 7 m s 1 compares well with previous estimates of deep upwelling in the northern basins a series of likely sites for strong vertical entrainment of aabw are clearly identified in the modeled distribution of deep upwelling consonant with the observed cfc distributions on the top isopycnal altogether regions with relatively high upwelling rates 5 10 7 m s 1 occupy only a quarter of the total areal extent of the top isopycnal but they account for as much as 45 of the total vertical transport

southern ocean bottom water of the australian new zealand sector the bottom water characteristics of the southeast indian ocean and southwest pacific ocean reveal northward transfer of cold water across the mid ocean ridge at the discordance zone near 120 e and at the balleny fracture zone near 155 e a principal northward flow which transfers cold continental margin waters of antarctic origin to lower latitudes within the south indian basin occurs to the east of kerguelen plateau within the south indian basin near the 0 c isotherm the s o2 and si relations display significant slope changes the more variable water below 0 c is of local origin while the water above 0 c represents larger scale regional effects

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

two full depth velocity sections in the drake passage in 2006 transport estimates abstract in january february 2006 a hydrographic section under jason track 104 across drake passage dp was performed twice within 3 weeks on board of the r v polarstern two ladcps mounted on the ctd rosette provided the most comprehensive synoptic top to bottom observations of directly measured velocity available in the drake passage to date with an estimated error of less than 3 cm s 1 geostrophic velocities computed from the hydrographic data were referenced to ladcp velocities the velocity structure of the antarctic circumpolar current acc along the track and its evolution within 3 weeks reflect the frontal branches presented by barre et al 2011 and provost et al 2011 the repeated section offers two independent transport estimates computed from the ladcp velocities 145 8 8 and 137 9 10 5 sv respectively and from the adjusted geostrophic velocities 136 6 7 and 129 7 sv respectively error bars of the transport estimates are small contributions of the fronts and eddies to the total transport are modified within 3 weeks but tend to compensate each other the total transport is reduced by about 10 15 17 sv between the southward and northward journey total transport estimations are compared to previous results in particular the isos international southern ocean study data from which the canonical value of 134 sv has been estimated within the uncertainties due to uneven data distribution no trend is discernible neither in the baroclinic nor total transports from 1975 to 2006

the accretionary history of southern south america from the latest proterozoic to the late palaeozoic some palaeomagnetic constraints

southern ocean fronts controlled by wind or topography 1 the location of fronts has a direct influence on both the physical and biological processes in the southern ocean here we explore the relative importance of bottom topography and winds for the location of southern ocean fronts using 100 years of a control and climate change simulation from the high resolution coupled climate model higem topography has primary control on the number and intensity of fronts at each longitude however there is no strong relationship between the position or spacing of jets and underlying topographic gradients because of the effects of upstream and downstream topography the southern hemisphere westerlies intensify and shift south by 1 3 in the climate change simulation but there is no comparable meridional displacement of the antarctic circumpolar current s acc path or the fronts within its boundaries even over flat topography instead the current contracts meridionally and weakens north of the acc the subtropical front stf shifts south gradually even over steep topographic ridges we suggest the stf reacts more strongly to the wind shift because it is strongly surface intensified in contrast fronts within the acc are more barotropic and are therefore more sensitive to the underlying topography an assessment of different methods for identifying jets reveals that maxima of gradients in the sea surface height field are the most reliable approximating the position of fronts using sea surface temperature gradients is ineffective at high latitudes while using sea surface height contours can give misleading results when studying the temporal variability of front locations

internal dynamics of a paleoaccretionary wedge insights from combined isotope tectonochronology and sandbox modelling of the south central chilean forearc forearc accretionary wedges are cyclic systems in which material is frontally and or basally accreted material cycling involves underthrusting subduction underplating exhumation erosion transfer to the trench and underthrusting again in this study we present a novel tectonochronologic approach to constrain long term exhumation rates of basally accreted wedge complexes based on isotopic dating of structural features on petrological data and sandbox analogue simulations congruence between the structural inventory in nature and structures generated in scaled sandbox experiments allows detailed insights into wedge dynamics for the present day surface material of the paleoaccretionary wedge of south central chile valdivia area 40 s published u pb ages of detrital zircon place a maximum age of 278 ma for subduction prograde metamorphism at transitional greenschist to blueschist facies conditions 420 c 8 9 kbar was immediately followed by progressive penetrative deformation associated with basal accretion dated at 250 245 ma using rb sr internal mineral isochrons the accretion process involved duplex tectonics and antiformal stacking with formation of near horizontal mylonitic shear zones at around 241 ma continuous basal accretion at depth gave rise to an extensional tectonic regime at higher structural levels both semi ductile small scale extensional shear zones and post kinematic vein mineralizations yield rb sr ages of 235 ma tension gashes representing the latest isotopically dateable stage of structural evolution were formed at 210 ma at conditions of 230 c at 1 5 3 kbar as constrained by fluid inclusion data zircon fission track data indicate final cooling to below 200 c at 186 24 ma the results suggest continuous basal accretion for at least 50 ma with long term average exhumation rates of 0 6 0 2 mm a most probably outbalanced by similar long term average erosion rates changing plate boundary conditions at about 210 200 ma terminated the accretion process as evident from a dramatic decrease of exhumation rates at that time since then the paleoaccretionary wedge remained stably in place despite its delicate geotectonic position within the andean active margin the tectonochronologic approach complements thermochronologic and geomorphologic methods of exhumation research as it provides direct constraints on mass flux rates even for high temperature increments of p t trajectories

on the variability of antarctic circumpolar current fronts inferred from 1992 2011 altimetry abstractantarctic circumpolar current acc fronts defined as water mass boundaries have been known to respond to large scale atmospheric variabilities especially the southern hemisphere annular mode sam and el nino southern oscillation enso distinct patterns of localized variability in meridional front displacements during 1992 2011 are derived from the analysis of satellite sea surface height data major basin scale differences are found between the southeast pacific 150 90 w and the southeast indian 75 150 e sectors of the acc frontal positions in the southeast pacific show large year to year meridional fluctuations attributed mostly to enso and in part sam and no apparent seasonal cycles or long term trends in contrast summer winter frontal locations in the southeast indian extend farther to the south north of their long term mean distribution a southward drift of acc fronts is indicated over the indian sector during the past two decades this long term shift is not directly re

petrogenesis of late carboniferous a type granites and early cretaceous adakites of the songnen block ne china implications for the geodynamic evolution of the paleo asian and paleo pacific oceans abstract the petrogenesis of late carboniferous a type granites and early cretaceous adakites in ne china offers new insights into the geodynamic evolution of the paleo asian and paleo pacific oceans this study reports new la icp ms zircon u pb ages geochemical data and zircon hf isotopic compositions of syenogranites and quartz diorite porphyries exposed on the northern margin of the songnen block ne china zircon u pb age data indicate that the syenogranites and quartz diorite porphyries were emplaced at ca 319 and 120 ma respectively the syenogranites have high k2o contents of 4 15 4 79 wt and k2o na2o cao ratios of 33 102 and low mgo 0 06 0 20 wt and p2o5 0 01 0 02 wt contents their remarkably high hree contents and y nb ratios 2 07 2 40 significantly negative eu anomalies eu eu 0 15 0 28 and low sr contents 29 7 85 8 ppm indicate that the syenogranites are a type granites they have positive zircon ehf t values of 8 4 12 1 and neoproterozoic hf tdm2 ages of 795 561 ma suggesting a juvenile lower crustal source the quartz diorite porphyries have relatively high na2o and mgo and moderate k2o contents their a cnk values 0 95 1 18 indicate a transition from being metaluminous to weakly peraluminous they are enriched in liles e g rb k and ba and depleted in hfses e g nb ta ti and p their low y 11 1 14 4 ppm and yb 0 98 1 34 ppm contents and high sr 589 1131 ppm cr and ni contents and sr y ratios indicate that they were generated by partial melting of oceanic crust based on these data and regional geological investigations we propose that the syenogranites on the northern margin of the songnen block were formed in a post collision extensional setting likely due to closure of the paleo asian ocean between the xing an and songnen blocks formation of the early cretaceous quartz diorite porphyries may have been triggered by partial melting of the paleo pacific flat slab apart from rollback of the paleo pacific oceanic slab the main reason for the eastward migration of magmatism during the cretaceous was the trench ward dehydration eclogitization and sinking of the residual paleo pacific flat slab

southern ocean jets and how to find them improving and comparing common jet detection methods this study undertakes a detailed comparison of different methods used for detecting and tracking oceanic jets in the southern ocean the methods under consideration are the gradient thresholding method the probability density function pdf method and the contour method some weaknesses of the gradient thresholding method are discussed and an enhancement the whose method based on techniques from signal processing is proposed the whose method is then compared to the other three methods quantitative comparison is undertaken using synthetic sea surface height fields the whose method and the contour method are found to perform well even in the presence of a strong eddy field in contrast the standard gradient thresholding and pdf methods only perform well in high signal to noise ratio situations the whose pdf and the contour methods are then applied to data from the eddy resolving ocean general circulation model for the earth simulator while the three methods are in broad agreement on the location of the main acc jets the nature of the jet fields they produce differ in particular the whose method reveals a fine scale jet field with complex braiding behavior it is argued that this fine scale jet field may affect the calculation of eddy diffusivities finally recommendations based on this study are made the whose and gradient thresholding methods are more suitable for the study of jets as localized strong currents useful for studies of tracer fluxes the contour and pdf methods are recommended for studies linking jets to hydrographic fronts

paleomagnetism and geochronology of late paleozoic granitic rocks from the lake district of southern chile implications for accretionary tectonics k ar and sup 40 ar sup 39 ar dating shows that granite rocks in the central lake district southern chile are late paleozoic granitic rocks of similar age extend far up the coast of chile perhaps as much as 1800 km the authors have obtained paleomagnetic directions from seven sites near lago ranco these give a paleomagnetic pole at lat 57 4 degree s long 323 5 degree e with a circle of 95 confidence of 18 8 degree this is not significantly different from the mean late paleozoic reference pole for the south american craton if these results are characteristic of the late paleozoic belt as a whole it follows that post late plaeozoic accretion and terrane displacement are absent throughout much of the central andes

jets and topography jet transitions and the impact on transport in the antarctic circumpolar current the southern ocean s antarctic circumpolar current acc naturally lends itself to interpretations using a zonally averaged framework yet navigation around steep and complicated bathymetric obstacles suggests that local dynamics may be far removed from those described by zonally symmetric models in this study both observational and numerical results indicate that zonal asymmetries in the form of topography impact global flow structure and transport properties the conclusions are based on a suite of more than 1 5 million virtual drifter trajectories advected using a satellite altimetry derived surface velocity field spanning 17 years the focus is on sites of cross front transport as defined by movement across selected sea surface height contours that correspond to jets along most of the acc cross front exchange is localized in the lee of bathymetric features with more than 75 of crossing events occurring in regions corresponding to only 20 of the acc s zonal extent these observations motivate a series of numerical experiments using a two layer quasigeostrophic model with simple zonally asymmetric topography which often produces transitions in the front structure along the channel significantly regimes occur where the equilibrated number of coherent jets is a function of longitude and transport barriers are not periodic jet reorganization is carried out by eddy flux divergences acting to both accelerate and decelerate the mean flow of the jets eddy kinetic energy is amplified downstream of topography due to increased baroclinicity related to topographic steering the combination of high eddy kinetic energy and recirculation features enhances particle exchange these results stress the complications in developing consistent circumpolar definitions of the acc fronts

paleomagnetic rotation pattern of the southern chile fore arc sliver 38 s 42 s a new tool to evaluate plate locking along subduction zones the chile fore arc at 37 s 47 s represents the coseismic deformation zone of the 1960 mw 9 5 valdivia earthquake here we report on the paleomagnetism of 43 oligocene pleistocene volcanic sites from the fore arc sliver between 38 s and 42 s sites were gathered west of the 1000 km long liquine ofqui dextral fault zone lofz that represents the eastern fore arc sliver boundary nineteen reliable sites reveal that the fore arc is characterized by counterclockwise ccw rotations of variable magnitude except at 40 s 41 s where ultrafast 50 myr clockwise cw rotations occur within a 30 km wide zone adjacent to the lofz ccw rotation variability even at close sites and rapidity 10 myr suggest that the observed block rotation pattern is related to nw se seismically active sinistral faults crosscutting the whole fore arc according to previously published data cw rotations up to 170 also occur east of the lofz and have been related to ongoing lofz shear we suggest that the occurrence and width of the eastern fore arc sliver undergoing cw rotations is a function of plate coupling along the subduction zone interface zones of high coupling enhance stress normal to the lofz induce high lofz strength and yield a wide deformation zone characterized by cw rotations conversely low coupling imply a weak lofz a lack of cw rotations and a fore arc entirely dominated by ccw rotations related to sinistral fault kinematics our locking inferences are in good agreement with those recently derived by gps analysis and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5 ma

post middle oligocene origin of paleomagnetic rotations in upper permian to lower jurassic rocks from northern and southern peru abstract we report paleomagnetic data from 28 sites of upper permian to lower jurassic strata from northern and southern peru in northern peru 6 s a stable magnetic component from six permo triassic sites passes fold and reversal tests the overall mean pole agrees well with late permian to triassic poles from cratonal south america suggesting this part of peru has experienced neither significant rotation nor latitudinal transport since the permo triassic in southern peru 13 to 16 s thermal demagnetization isolates stable magnetic components in 16 of 22 upper permian to lower jurassic sites collected along the transition between the altiplano and the eastern cordillera these 16 sites are rotated 14 to 147 counterclockwise and pass an inclinations only fold test within the same structural zone three other permo triassic sites as well as 10 paleocene sites also show important counterclockwise rotations roperch and carlier j geophys res 97 1992 17233 17249 butler et al geology 23 1995 799 802 the large magnitude and exclusively counterclockwise sense of rotation suggest that the tectonic regime included an important sinistral shear component no correlation exists between rotation amount and rock age suggesting the rotations are post paleocene in age because the rotations occur along the fringe of the eastern cordillera they were likely produced during its structural formation hence from the late oligocene to present sinistral shear acting in the northern part of the bolivian orocline appears much more pronounced than that north of the abancay deflection which likely arises from differences in convergence obliquity

circumpolar structure and distribution of the antarctic circumpolar current fronts 2 variability and relationship to sea surface height 1 in part 1 of this study we showed that the antarctic circumpolar current acc consisted of multiple fronts each of which was consistently associated with a particular contour of sea surface height ssh or approximate streamline in part 2 we have used maps of ssh to examine the variability of the acc fronts between 1992 and 2007 the ssh label associated with each frontal branch is nearly constant around the circumpolar belt the front labels are also nearly constant in time the bands of enhanced ssh gradient i e fronts occur along the same streamlines throughout the 15 year period of observations both short and long period changes of the ssh frontal labels of the acc are small based on a tight relationship between dynamic height and cumulative baroclinic transport of the acc the baroclinic transport variability of the individual branches of the acc is also expected to be small the major change in the total acc baroclinic transport occurs in the drake passage the streamline associated with the northern branch of the saf saf n does not pass through drake passage and the waters carried by this branch are not observed there instead the transport of the saf n turns north in the pacific to supply the export of water to the indian ocean north and south of australia strong eddy activity in the southeast pacific acts to dissipate the hydrographic signature of the saf n there in the atlantic the saf n reappears as the same streamline is again associated with enhanced ssh gradients to the east of the brazil malvinas confluence zone while the large changes in ssh have occurred in the southern ocean between 1992 and 2007 there are strong regional differences because the acc fronts are robustly associated with particular ssh contours the changes in ssh reflect shifts in the position of the acc fronts in the circumpolar average each of the acc fronts has shifted to the south by about 60 km the changes in ssh in the southern ocean are largely due to changes in ocean circulation rather than warming and freshening by atmospheric fluxes much larger changes in ssh are observed in some locations of the southern ocean particularly where the fronts interact with large scale topography the northern branch of the pf pf n near the kerguelen plateau is an extreme example where the pf n followed a path around the northern end of kerguelen plateau between 1992 and 1997 passed through the fawn trough after 2003 and oscillated between the two paths between 1997 and 2003

pangea b and the late paleozoic ice age abstract the late paleozoic ice age lpia was the penultimate major glaciation of the phanerozoic published compilations indicate it occurred in two main phases one centered in the late carboniferous 315 ma and the other in the early permian 295 ma before waning over the rest of the early permian and into the middle permian 290 ma to 275 ma and culminating with the final demise of alpine style ice sheets in eastern australia in the late permian 260 to 255 ma recent global climate modeling has drawn attention to silicate weathering co2 consumption of an initially high greater variscan edifice residing within a static pangea a configuration as the leading cause of reduction of atmospheric co2 concentrations below glaciation thresholds here we show that the best available and least biased paleomagnetic reference poles place the collision between laurasia and gondwana that produced the greater variscan orogen in a more dynamic position within a pangea b configuration that had about 30 more continental area in the prime equatorial humid belt for weathering and which drifted northward into the tropical arid belt as it transformed to pangea a by the late permian the presence of widespread equatorial coal basins with euramerica flora in the footprint of the greater variscan orogen during the late carboniferous is more compatible with a heterogeneous horst and graben morphology characterized by uplifted crystalline massifs acting as loci of intense silicate weathering co2 consumption and supplying sediment for proximal basins as venues of organic carbon burial than a contiguous high mountain plateau as assumed in recent climate modeling of the lpia and its demise the culminating phase of the lpia occurred at about 275 ma with the transformation from pangea b to pangea a and the attendant reduction of continental area in the equatorial humid belt as well as with continued northward drift that placed what remained of the greater variscan orogen into the zechstein arid belt in the late permian by which time the geologic landscape was largely blanketed with siliciclastics the resulting warming from reduced silicate weathering and thus increasing pco2 was interrupted at 260 ma with a cooling trend that coincided with emplacement of the emeishan large igneous province on the equatorial south china craton as well as the drift of the cimmerian continental blocks through the equatorial humid belt due to opening of the neo tethys a return to ice age conditions from the increase in silicate weathering uptake of co2 was avoided by drift of the emeishan large igneous province out of the equatorial belt that in conjunction with massive outgassing from emplacement of the siberian traps in high latitudes at the end of the permian 252 ma helped steer the climate system to sustained non glacial conditions

variability of subantarctic mode water and antarctic intermediate water in the drake passage during the late twentieth and early twenty first centuries a time series of the physical and biogeochemical properties of subantarctic mode water samw and antarctic intermediate water aaiw in the drake passage between 1969 and 2005 is constructed using 24 transects of measurements across the passage both water masses have experienced substantial variability on interannual to interdecadal time scales samw is formed by winter overturning on the equatorward flank of the antarctic circumpolar current acc in and to the west of the drake passage its interannual variability is primarily driven by variations in wintertime air sea turbulent heat fluxes and net evaporation modulated by the el nino southern oscillation enso despite their spatial proximity the aaiw in the drake passage has a very different source than that of the samw because it is ventilated by the northward subduction of winter water originating in the bellingshausen sea changes in aaiw are mainly forced by variability in winter water properties resulting from fluctuations in wintertime air sea turbulent heat fluxes and spring sea ice melting both of which are linked to predominantly enso driven variations in the intensity of meridional winds to the west of the antarctic peninsula a prominent exception to the prevalent modes of samw and aaiw formation occurred in 1998 when strong wind forcing associated with constructive interference between enso and the southern annular mode sam triggered a transitory shift to an ekman dominated mode of samw ventilation and a 1 2 yr shutdown of aaiw production the interdecadal evolutions of samw and aaiw in the drake passage are distinct and driven by different processes samw warmed by 0 3 c and salinified by 0 04 during the 1970s and experienced the reverse trends between 1990 and 2005 when the coldest and freshest samw on record was observed in contrast aaiw underwent a net freshening by 0 05 between the 1970s and the twenty first century although the reversing changes in samw were chiefly forced by a 30 yr oscillation in regional air sea turbulent heat fluxes and precipitation associated with the interdecadal pacific oscillation with a sam driven intensification of the ekman supply of antarctic surface waters from the south contributing significantly too the freshening of aaiw was linked to the extreme climate change that occurred to the west of the antarctic peninsula in recent decades there a freshening of the winter water ventilating aaiw was brought about by increased precipitation and a retreat of the winter sea ice edge which were seemingly forced by an interdecadal trend in the sam and regional positive feedbacks in the air sea ice coupled climate system all in all these findings highlight the role of the major modes of southern hemisphere climate variability in driving the evolution of samw and aaiw in the drake passage region and the wider south atlantic and suggest that these modes may have contributed significantly to the hemispheric scale changes undergone by those waters in recent decades

the tectonics of the altaids crustal growth during the construction of the continental lithosphere of central asia between 750 and 130 ma ago the largest mountain belt in central asia 9 million km2 is called the altaids it was assembled between 750 and 130 ma ago around the western and southern margins of the siberian craton partl

interdecadal water mass changes in the southern ocean between 30 e and 160 e interdecadal water mass changes in the indian western pacific sectors of the southern ocean were investigated using the japanese antarctic research expeditions and historical hydrographic observations from the 1950s to 1990s freshening and cooling occurred on the neutral density surfaces of 27 0 kgm3 equatorward of sub antarctic front results for the area south of the polar front show warm and saline anomalies and oxygen decreases on the surfaces around 27 9 kgm3 which correspond to the upper circumpolar deep water these latter anomalies are most simply explained by the mixing of these shallow waters with warmer and fresher surface waters steric sea level has also increased with an average change of 1mmyr1 from the 1970s to 1990s the changes are larger north of the sub antarctic front implying a strengthening of the antarctic circumpolar current it appears that the observed changes are consistent with the results from coupled climate model results for a similar period

timeline of the south tibet himalayan belt the geochronological record of subduction collision and underthrusting from zircon and monazite u pb ages the exact timing of collision between india and eurasia is a recurring theme careful dating is critical for all tectonic events with the example of the south tibet himalaya collision system

structural anatomy of the early paleozoic laohushan ophiolite and subduction complex implications for accretionary tectonics of the proto tethyan north qilian orogenic belt northeastern tibet

accretion subduction erosion and tectonic extrusion during late paleozoic to mesozoic orogenesis in ne china abstract the high pressure heilongjiang complex records a long and complicated evolution from the late paleozoic to mesozoic in ne china which is still subject to controversy in this paper we present a new detailed study including structural geochemical and geochronological analyses ar ar dating on metamorphic minerals u pb sims dating on zircons two main periods of oceanward accretionary growth are identified between 305 ma and 195 ma and around 112 101 ma these periods of accretionary growth were associated with the emplacement of voluminous magmatism slab roll back with continental back arc extension and growth of a forearc accretionary prism between these periods of accretionary growth a major phase of regional subduction erosion took place between 195 ma and 142 ma at the eastern edge of the songliao block the main effects of this subduction erosion phase were that regional jurassic early cretaceous extrusion overlapped with mesozoic accretion and erosion in ne china which disturbed the initial orogenic architecture with a review of relevant zircon ages of this accretionary orogen we unravel the complicated geodynamic history of accretion subduction erosion and tectonic extrusion that shaped ne china from the late paleozoic to mesozoic

late paleozoic tectonic evolution of the paleo asian ocean in the northern alxa block nw china the northern alxa block occupies a key position in the southern margin of central asian orogenic belt caob and records late paleozoic subduction and closure processes of the paleo asian oce

geodynamic evolution and tectonostratigraphic terranes of northwestern argentina and northern chile in ordovician time gondwana in the area of northwestern argentina and northern chile had a west facing active margin the evolution of this margin culminated in the ocloyic orogeny at the end of ordovician time this orogeny was caused by the collision of the allochthonous arequipa antofalla terrane with this margin the early paleozoic evolution of northwestern argentina and northern chile contrasts markedly with the accretionary history of central argentina and central chile where the precordillera and chilenia terranes docked in the late ordovician and late devonian periods respectively an inspection of the available stratigraphic and geochronological data on sedimentary volcanic and plutonic units of the southern central andes of northern chile and northwestern argentina reveals a lull in magmatic and metamorphic activity lasting for 100 m y from early silurian to early late carboniferous time this is interpreted as corresponding to a tectonic scenario in which the present andean margin was a passive margin of gondwana this passive margin developed in response to the rifting off of a part of the arequipa antofalla terrane the present location of this block is unknown late carboniferous time marks the renewed onset of subduction initiating the andean plate tectonic setting still prevalent today recently proposed models explain the late ordovician orogeny by the collision of laurentia with western south america during laurentia s clockwise motion around south america and away from its position in the neoproterozoic supercontinent in its present form this hypothesis is difficult to reconcile with the paleozoic tectonostratigraphic evolution of the southern central andean region

syntectonic magnetization of the mid palaeozoic sierra grande formation further constraints on the tectonic evolution of patagonia a palaeomagnetic study was carried out in the silurian devonian clastic sedimentary rocks of the sierra grande formation exposed in northeastern patagonia 41 6 s 65 3 w thirteen sites n 88 were located on opposite limbs of a syncline anticline structure stepwise thermal demagnetization permitted the identification of a very stable magnetic component of reversed polarity carried by hematite stepwise performance of the fold test yielded negative results both in situ and after 100 bedding correction but positive after partial unfolding 19 this indicates a syntectonic origin for the isolated magnetization a pole position was computed for the partially 19 corrected remanence sg3 77 3 s 310 7 e p 7 7 m 6 6 n 13 its position is coincident with late early to late permian palaeomagnetic poles from south america suggesting that age for the previously undated folding of the sierra grande sequence and therefore for the main tectonic event that affected the northern boundary of patagonia palaeozoic palaeomagnetic poles from patagonia obtained to date agree with those from gondwana of devonian or younger age suggesting that patagonia did not undergo important displacements relative to south america since those times this and the permian age of deformation determined in this study invalidates tectonic models involving collision of a far travelled patagonia with gondwana in the mid or late palaeozoic

multiple jets of the antarctic circumpolar current south of australia abstract maps of the gradient of sea surface height ssh and sea surface temperature sst reveal that the antarctic circumpolar current acc consists of multiple jets or frontal filaments the braided and patchy nature of the gradient fields seems at odds with the traditional view derived from hydrographic sections that the acc is made up of three continuous circumpolar fronts by applying a nonlinear fitting procedure to 638 weekly maps of ssh gradient ssh it is shown that the distribution of maxima in ssh i e fronts is strongly peaked at particular values of absolute ssh i e streamlines the association between the jets and particular streamlines persists despite strong topographic and eddy mean flow interactions which cause the jets to merge diverge and fluctuate in intensity along their path the ssh values corresponding to each frontal branch are nearly constant over the sector of the southern ocean between 100 e and 180 the front positions inferred from ssh agree closely with

dynamics of sea surface temperature anomalies in the southern ocean diagnosed from a 2d mixed layer model we analyze the processes responsible for the generation and evolution of sea surface temperature anomalies observed in the southern ocean during a decade based on a 2d diagnostic mixed layer model in which geostrophic advection is prescribed from altimetry anomalous air sea heat flux is the dominant term of the heat budget over most of the domain while anomalous ekman heat fluxes account for 20 40 of the variance in the latitude band 40 60 s in the acc pathway lateral fluxes of heat associated with anomalous geostrophic currents are a major contributor dominating downstream of several topographic features reflecting the influence of eddies and frontal migrations a significant fraction of the variability of large scale sst anomalies is correlated with either enso or the sam each mode contributing roughly equally the relation between the heat budget terms and these climate modes is investigated showing in particular that anomalous ekman and air sea heat fluxes have a co operating effect with regional exceptions hence the large sst response associated with each mode it is further shown that enso or sam locked anomalous geostrophic currents generate substantial heat fluxes in all three basins with magnitude comparable with that of atmospheric forcings for enso and smaller for the sam except for limited areas enso locked forcings generate sst anomalies along the acc pathway and advection by mean flows is found to be a non negligible contribution to the heat budget exhibiting a wavenumber two zonal structure characteristic of the antarctic circumpolar wave by contrast sam related forcings are predominantly zonally uniform along the acc hence smaller zonal sst gradients and a lesser role of mean advection except in the southwest atlantic while modeled sst anomalies are significantly correlated with observations over most of the southern ocean the analysis of the data model discrepancies suggests that vertical ocean physics may play a significant role in the nonseasonal heat budget especially in some key regions for mode water formation

transport and structure within the antarctic circumpolar current to the north of south georgia the antarctic circumpolar current acc crosses rugged topography to the north of south georgia in this region the subsurface expression of the polar front pf is generally thought to occur north of the maurice ewing bank meb close to the subantarctic front saf however in january 1996 the pf was south of the bank separate from the saf in a position where it was most likely constrained by local bathymetry high rates of transport 25 sv eastward were present at the pf consistent with previous estimates for this current core high transport rates 20 sv westward were also present along the northern shelf of south georgia close to where the southern acc front saccf has previously been reported during our occupation the saccf was not encountered as it probably lay outside the survey area comparisons with recent circumpolar analyses emphasise the potential for variability in the region

emplacement of the ophiolitic melanges in the west karamay area implications for the late paleozoic tectonic evolution of west junggar northwestern china abstract the darbut and karamay ophiolitic melanges in the west karamay area at the southeastern margin of west junggar southern altaids represent the relict fragments of the paleozoic junggar ocean basin the melanges contain structurally complex tectonic blocks of ultramafic rocks gabbro basalt and chert that are dispersed within an extensively sheared serpentinite matrix this study presents new field observations structural analyses geochronological and geophysical data for the melanges and the overlying carboniferous strata three stages of deformation d1 d3 have been identified d1 is only locally preserved in the melanges and is characterized by thrust imbricated faults recording the subduction and accretion of the paleo oceanic crust in the early carboniferous d2 mainly consists of map scale upright folds f1 in the carboniferous strata and ne sw striking sub vertical right lateral shear zones in the melanges d3 is characterized mainly by ne sw striking transcurrent left lateral strike slip faults and s shaped drag folds f2 the extensive tectonic reworking during d2 and d3 deformations eradicated most of the d1 structures our structural observations combined with geophysical data suggest that the basement of the west karamay area consists of trapped oceanic crust or dismembered ophiolitic fragments and that the darbut and karamay ophiolitic melanges were extruded through right lateral shear zones our results and the published data further suggest that the emplacement of the melanges and the formation of f1 folds represent regional scale amalgamation processes between different tectonic blocks and or plates in the late carboniferous these processes resulted in the closure of the west junggar remnant ocean basin and other sub oceanic basins in the paleo asian ocean leading to the formation of the kazakhstan orocline furthermore the d3 deformation was associated with anticlockwise rotation of the west junggar region in a post collisional setting in the permian

tuffs tectonism and glacially related sea level changes carboniferous permian southern namibia abstract the carboniferous permian glacigene dwyka group in southern africa has been widely regarded as amagmatic only in the higher parts of the permian and subsequent younger strata has evidence of magmatism been recognised particularly in the form of pyroclastic fallout ash beds the source of these tuffs has been suggested to be patagonia and west antarctica forming part of an extensive volcanic arc which lay some 1500 to 2000 km to the south and west in their pre break up gondwana positions rhyolitic dacitic and basaltic andesitic tuff beds within the glaciomarine sediments of the dwyka group in namibia reveal new evidence for an early onset of proximal bimodal volcanic activity in southern africa contemporaneous tectonism is recorded by type 1 unconformities and systematic thickness changes across nw se trending extensional normal faults we suggest that this tectonomagmatic period marks initial extensional events in southern namibia and the carboniferous permian volcanic trend appears to coincide with the position of the eventual namibian continental margin significantly the stratigraphic positions of the tuff beds show a distinct association with enclosing marine transgressive depositional sequences most of them coincide with sharply defined flooding surfaces within relatively thick shaly successions reflecting their multiple transgressive nature widening the study context to the latest early permian in southern africa the main karoo basin of southern africa exhibits a similar coincidence of marine transgressions and extrusion of volcanics the relationship between tuff beds and transgressive depositional sequences is not only the result of enhanced preservational potential during transgression but of a genetic coupling between magmatism extensional tectonics and basin subsidence the latter accelerating a rise in relative sea level this interaction is significant not only for understanding potential controls on relative sea level change but also for understanding the early geodynamic evolution of the southern south atlantic continental margin

kinematic constraints on intra arc shear and strain partitioning in the southern andes between 38 s and 42 s latitude 1 on the basis of structural field work fault kinematic analysis and the analysis of digital imagery we describe the northern part of the southern andean intra arc liquine ofqui fault zone lofz as an sc like fault zone system accommodating part of the nazca south american plate convergence obliquity kinematic modeling suggests that the lofz accommodated 124 24 21 km of dextral displacement between 40 s and 42 s and 67 13 11 km between 38 s and 40 s since the pliocene associated vertical axis rotations are 31 4 clockwise and 9 1 counterclockwise along synthetic and antithetic faults respectively mean pliocene to recent shear rates along the lofz decrease northward from 32 6 mm yr to 13 3 mm yr compatible with partitioning of half of the convergence obliquity into the intra arc zone north of 40 s and complete partitioning to the south the displacement gradient along the intra arc zone results in margin parallel shortening of the fore arc

tectonic evolution of early mesozoic sedimentary basins in the north china block abstract there have been lots of debates regarding tectonic evolution of sedimentary basins in the north china block during the early mesozoic it has been widely held that the north china block was strongly affected by the collision of the north and south china blocks on the south and the terrane accretion on the north and thus early mesozoic basins of various scales in the interior and peripheries of the north china block resulted from crustal shortening based on extensive and detailed field investigations of stratigraphic sequences sedimentary processes and basin tectonics this study shows that the north china block manifested itself in general as a stable domain during the early mesozoic with rifting transcurrent faulting and magmatism occurring mainly in peripheral regions the north china block behaved as a big intracratonic basin in the triassic albeit its depositional area shifted to the west in mid late triassic time the westward migration of depocenter is assumed the consequence of uplifting of the eastern part of the north china block in response to the collision of the north and south china blocks along the tongbai dabie sulu orogenic belt and coeval transpression of the tanlu fault zone sedimentary basins had not developed until the early jurassic in the regions north of the tongbai dabie orogen and plausibly formed as the result of crustal stretching related to southeastward exhumation of the dabie high pressure metamorphic rocks the collision of the north and south china blocks led to south directed displacement of the east qinling orogen but few geological records exist indicative of mid late triassic thrusting in the southern margin of the western north china block instead some evidence is suggestive of extensional or transtensional deformations such as the existence of large normal faults controlling subsidence and sedimentation of middle late triassic basins and the presence of synsedimentary normal faults in basin sequences both the east and west qinling experienced rapid uplifting in the late triassic coeval with large volume granitic magmatism late triassic uplift and exhumation of the qinling was considered as the result of asthenospheric upwelling and voluminous granitic injections vertical motion of the northern east qinling might have triggered normal faulting along its northern edge and extensional subsidence of the southern margin of the western north china block broad denudation occurred simultaneously in the northern margin of the north china block from the late triassic to early jurassic in association with magmatism and development of a few small rift basins asthenospheric uprising is regarded as the cause for late triassic magmatism regional surface uplift and supracrustal stretching triassic subsidence and sedimentation in the interior of the north china block was punctuated by crustal uplifting possibly driven by mantle upwelling at the transition from the triassic to jurassic the whole north china block then resumed low rate subsidence from the late early jurassic and plausibly developed into a broad intracratonic basin in the middle jurassic characterized by widespread distribution of shallow water lacustrine and meandering river facies associations the sagging of the whole north china block is thought of as the consequence of thermal contraction in the aftermath of mantle upwelling backarc extension related to the paleo pacific plate subduction might have magnified subsidence of the eastern north china block resulting in relatively thick accumulation of middle jurassic strata this study restores tectonic and sedimentary history of the early mesozoic basins in different portions of the north china block and advances models to illustrate how the basins of diverse origin were generated and filled in different stages

petrogenesis and geodynamic implications of late carboniferous sanukitic dikes from the bieluagaxi area of west junggar nw china abstract sanukitic dikes mainly consisting of pyroxene diorite porphyrite diorite porphyry quartz diorite porphyrite and quartz diorite were widely distributed in the bieluagaxi area southern west junggar in this contribution we present new la icp ms zircon u pb ages major and trace elements and sr nd hf isotopes for these dioritic dikes these rocks formed in the early late carboniferous ca 319 315 ma coeval with abundant magmatic rocks in the region and exhibit high mgo cr and ni concentrations and k na ratios and also show strong enrichment in large ion lithophile elements relative to high field strength elements with pronounced negative nb ta and ti anomalies analogues to those of cenozoic sanukitoids of the setouchi volcanic belt compositionally the sanukitic dikes are divided into two groups group 1 have mgo contents greater than 6 wt 6 18 9 34 wt with mg values ranging from 58 2 to 73 7 they also have relatively low sio2 51 7 55 7 wt and sr 356 ppm in average contents and low sr y 15 1 25 4 ratios but high concentrations of cr 6 2 490 ppm and ni 12 8 163 ppm compared to the group 1 the group 2 samples have slightly lower mgo 2 12 5 58 wt mg 45 8 68 5 cr 10 1 329 ppm and ni 2 84 136 ppm but relatively higher sio2 58 6 65 7 wt and sr 304 521 ppm concentrations and sr y 19 2 41 0 ratios making them akin to adakitic rocks elsewhere these dikes also exhibit depleted sr nd hf isotopic signatures with 87sr 86sr i 0 70353 0 70480 end t 5 03 to 7 61 ehf t 10 9 to 15 3 geochemical and sr nd hf isotopic data suggest that the group 1 rocks are similar to the typical sanukitoids of the setouchi volcanic belt and were likely derived from partial melting of a depleted mantle source with subducted sediment melts while the group 2 rocks show a remarkable geochemical affinity with the northwestern karamay sanukitoids and were derived from partial melting of a depleted mantle source metasomatized by slab derived adakitic melts and fluids combined with the identified and reported subduction related magmatism i e adakites sanukitoids nb enriched basalts lagged arc volcanic rocks and i type granitoids in the western junggar region and taking into consideration the previous achievements therefore we consider a multi stage subduction accretionary orogeny model may account for the difference of subduction duration between the northern segment and southern segment of western junggar

long term trends and interannual variability of temperature in drake passage abstract temperature data collected over the last 36 years 1969 2004 in drake passage are used to examine interannual temperature variation and long term trends in the upper ocean to reduce the effect of variation from different sampling locations and temporal variability introduced by meridional shifts in the polar front pf the data were divided into two sub regions north 3800 temperature profiles and south 3400 of the pf temperature anomalies were formed by removing a temporal mean field for each profile in each sub region at 100 m depth intervals from the surface to 700 m north of the pf statistically significant warming trends of 0 02 c yr 1 were observed that were largely depth independent between 100 and 700 m a statistically significant cooling trend of 0 07 c yr 1 was observed at the surface south of the pf which was smaller 0 04 c yr 1 but still statistically significant when possible seasonal sampling biases were accounted for the observed cooling at the surface and warming at depth is largely consistent with a poleward shift of the pf due to enhancement of westerly winds in the southern ocean as recently suggested by models and observations the observed annual temperature anomalies in the upper 400 m north of the pf and in the upper 100 m south of the pf are highly correlated to variability in sea ice and also to climate indices of the antarctic oscillation and the el nino southern oscillation variability in sea ice and temperature anomalies lag el nino variability in the pacific with a phasing consistent with the observed cyclical patterns of sea ice and sea surface temperature associated with the antarctic circumpolar wave or antarctic dipole mode in the southern ocean in contrast the sea ice variability and temperature anomalies at all depths north of the pf and at 0 100 m depth south of the pf were primarily coincident with or led the antarctic oscillation index no significant correlations were found with the large scale climate variability indices in southern drake passage below 100 m depth which is occupied by upper circumpolar deep water ucdw this water mass is not formed locally is largely isolated from the surface and exhibits vertical and lateral homogeneity hence changes may be difficult to detect in the available measurements and climate variation in the source water regions of ucdw may take a long time to reach drake passage

a comparison of surface eddy kinetic energy and reynolds stresses in the gulf stream and the kuroshio current systems from merged topex poseidon and ers 1 2 altimetric data analyses of surface eddy kinetic energy eke inferred from 5 years of merged topex poseidon and ers 1 2 altimetric data are presented in the gulf stream gs and the kuroshio ks current systems focusing on anisotropy and eke seasonal interannual and long term variations as well as reynolds stresses for both gs and ks a descriptive analysis of the eke field shows that it is anisotropic along the path of the currents and assesses the close relationship between the spatial distribution of eke high levels and bottom topography a striking ocean feature is found in the gs shaped as a zonal double blade structure in the eke field centered at 37 n 71 5 w that likely corresponds to a regular and permanent lateral displacement of the mean path of the current at this geographical location maps of eke seasonal means for years 1993 1996 and 1997 provide a detailed description of the surface variability meanders pinched off eddies etc and show strong interannual variations in the ks eke monthly mean changes in the ks reveal a well marked seasonal cycle during the first 3 years 1993 1995 with eke peaking in july august this is followed by a constant increase of its level from the beginning of year 1996 till the beginning of 1997 during this increase the zonal velocity variance u 2 is up to 30 higher than its meridional counterpart suggesting a zonal acceleration of the eastward flow eke seasonal changes are also found in the ks current 30 35 n 130 140 e and also show a regular annual cycle during the first 3 years which precedes by 2 months the eke seasonal cycle in the ks extension maximum in may june the maximum in eke in both regions precedes by 2 months the maximum surface transport found by qiu et al 1991 and zlotnicki 1991 occurring in july and late september respectively no such annual cycle is found in the gs region during the first 3 years year 1996 is anomalous in both current systems with mean eke level that increases by more than 20 compared to the other years also eke monthly mean changes in the ks reveal a well marked positive tendency in the eastern part of the current of more than 300 cm2 s 2 yr 1 and a negative tendency both in its eastern part and in the ks current this feature is absent from the gs at last eddy mean flow interaction is presented both the spatial averaging of turbulent quantities and the relative contribution of the different terms of the horizontal momentum fluxes are discussed a tentative explanation is given for the anomalously large u 2 at the end of 1996 in the ks extension

evolution of the late paleozoic accretionary complex and overlying forearc magmatic arc south central chile 38 41 s constraints for the tectonic setting along the southwestern margin of gondwana stratigraphic structural metamorphic and geochronologic studies of basement rocks in the andean foothills and coast ranges of south central chile 39 41 s suggest a protracted late paleozoic to middle mesozoic deformational and metamorphic history that imposes important constraints on the tectonic development of the southwestern gondwana margin in the study area the late paleozoic paired metamorphic belt coeval magmatic arc and overlying triassic sedimentary units preserve a record of late carboniferous to early permian subduction and arc magmatism subsequent deep exhumation of the western series subduction complex and diminished uplift and erosion of the eastern series arc forearc region by the late triassic late paleozoic structural elements and metamorphic assemblages formed during early subduction and arc magmatism collectively referred to as dl are largely erased in the western series by the dominant d2 schistosity and lower greenschist grade metamorphism d1 structural features as well as original sedimentary textures are relatively well preserved in the less penetratively deformed eastern series the regional distribution of late paleozoic arc magmatism suggests that the late paleozoic convergent margin deviated from a n s trend north of this area to a nw se trend near this latitude and faced an open marine environment to the southwest a transition from f2 isoclinal folding to more open larger scale f3 folds interpreted as change in ductility during differential uplift of the western series is not apparent in the eastern series despite a lesser degree of uplift during the main exhumational d2 event delineation of unconformities and u pb dating of detrital zircons and intrusions into the eastern series allow tighter constraints to be placed on timing of uplift and denudation of the eastern series than on that in the western series a regional unconformity exposed in the lake district that separates more highly deformed eastern series lithologies from late triassic shallow marine to continental deposits suggests that substantial uplift also affected the inner forearc and magmatic arc region during the d2 event we propose that dextral oblique convergence initiated during the middle permian along this segment of the gondwana margin resulted in the transpressional uplift and juxtaposition of high pressure temperature p t western series against low p t eastern series lithologies and culminated with deposition of late triassic continental to shallow marine coarse clastic sedimentary rocks in fault bounded strike slip basins adjacent to the exhumed western series large scale dextral transpression and northward displacement of the accretionary complex during late permian to late triassic time along the chilean margin of gondwana are synchronous and kinematically compatible with widespread regional transpression extension and silicic magmatism inboard of the southern gondwana margin at this time we thank c mpodozis m gardeweg and j munoz of the servicio de geologia y mineria de chile sernageomin for their support of this work fruitful discussions with n blanco f herve h moreno c mpodozis and f munizaga have aided in our understanding of the geology of the region the hard work by the staff of sernageomin s puerto varas office is graciously appreciated we thank j d walker and w r van schmus at the university of kansas for allowing mwm use of their u pb and mass spectrometer facilities and j vargas and the staff of sernageomin s geochemistry laboratory for their assistance in this project f munizaga allowed us to cite an unpublished 40ar 39ar date we thank g ya nez for access to aeromagnetic data t kato wishes to thank w g ernst comments by i dalziel s kay and v ramos helped clarify ideas presented in this paper and are greatly appreciated this work is dedicated to our friend and colleague alberto campos c who died in a climbing accident on calbuco volcano 1996

an early cambrian plume induced subduction initiation event within the junggar ocean insights from ophiolitic melanges arc magmatism and metamorphic rocks abstract the central asian orogenic belt caob is one of the largest accretionary orogens in the world the mechanism of continental growth and tectonic evolution of the caob remain debated here we present an overview of early paleozoic ophiolitic melanges calc alkaline intrusions and metamorphic rocks in west junggar with an aim to provide constraints on the time and mechanism of subduction initiation in the junggar ocean a branch of the southern paleo asian ocean pao the early paleozoic ophiolitic melanges are composed of ultramafic mafic rocks cherts pelagic limestones basaltic breccias and tuffs the mafic rocks from these ophiolitic melanges are divided into morb like and oib like types the morb like rocks were formed in a fore arc setting but the oib like mafic rocks were formed by the intra plate magmatism related to mantle plume activities the early paleozoic intrusions are occurred as small stocks with a dominant composition of diorite trondhjemite and granodiorite these granitoids display high k calc alkaline affinities and have high and positive end t and ehf t values formed in an arc related setting the metamorphic complex is mainly composed of blueschist and amphibolite blocks with metamorphism ages ranging from 500 ma to 460 ma their protoliths are calc alkaline andesite and tholeiitic and alkaline basalts formed in an arc related and seamount setting respectively it is clearly show that the west junggar was under an intra oceanic subduction regime during the early paleozoic and the initial subduction of the southern pao might have occurred in the early cambrian based on our observations and in combination with previous work we propose the plume induced subduction initiation model for the early middle cambrian tectonic evolutionary of the junggar ocean our new model not only shed light on subduction initiation dynamics of the southern pao but also contribute to tectonic evolution of the caob

meridional displacement of the antarctic circumpolar current observed long term warming trends in the southern ocean have been interpreted as a sign of increased poleward eddy heat transport or of a poleward displacement of the entire antarctic circumpolar current acc frontal system the two decade long record from satellite altimetry is an important source of information for evaluating the mechanisms governing these trends while several recent studies have used sea surface height contours to index acc frontal displacements here altimeter data are instead used to track the latitude of mean acc transport altimetric height contours indicate a poleward trend regardless of whether they are associated with acc fronts the zonally averaged transport latitude index shows no long term trend implying that acc meridional shifts determined from sea surface height might be associated with large scale changes in sea surface height more than with localized shifts in frontal positions the transport latitude index is weakly sensitive to the southern annular mode but is uncorrelated with el nino southern oscillation

new paleomagnetic results for ordovician and silurian rocks of the tarim block northwest china and their paleogeographic implications abstract the kinematics of the tarim block is poorly understood prior to the late paleozoic and this study aims to clarify motions during ordovician and silurian times we report the paleomagnetic record from 12 sites in ordovician marine sediments and 21 sites in silurian fine grained sandstones and mudstones from the aksu and kalpin areas of northwestern tarim together with our previously reported preliminary results the ordovician sediments isolate a sporadically preserved pre cenozoic characteristic remanence with dual polarity although a positive fold test can only define the remanence as pre cenozoic the corresponding paleomagnetic pole 33 7 s 185 0 e dp dm 2 7 4 0 is removed from post silurian paleopoles from tarim and combined with evidence from magnetic fabrics indicating a quasi primary origin a characteristic remanence isolated at 17 sites in silurian sediments from the aksu and kalpin areas includes 7 sites with suspected contamination from regional permian igneous activity the 10 remaining sites all from kalpin yield a positive fold test and a paleopole at 19 1 n 172 9 e a95 5 5 significantly removed from younger paleopoles from tarim indicating a late llandovery to wenlock remanence age the new mid late ordovician paleopole places the tarim block at 32 s probably in the vicinity of the arabian margin of gondwana rapid apw between 460 and 433 ma records large differential dextral motions of tarim together with other continental blocks along the paleo asian ocean margin which brought the china continental blocks into proximity with australia by mid silurian times in low 9 n northerly paleolatitudes

paleomagnetism of the santa fe group central brazil implications for the late paleozoic apparent polar wander path for south america 1 paleomagnetic and rockmagnetic data are reported for the floresta formation santa fe group of the sanfranciscana basin central brazil this formation represents the permo carboniferous glacial record of the basin and comprises the brocoto diamictites and flow diamictites brejo do arroz red sandstones and shales with dropstones and invertebrate trails and lavado red sandstones members which crop out near the cities of santa fe de minas and canabrava minas gerais state both brejo do arroz and lavado members were sampled in the vicinities of the two localities alternating field and thermal demagnetizations of 268 samples from 76 sites revealed reversed components of magnetization in all samples in accordance with the permo carboniferous reversed superchron the magnetic carriers are magnetite and hematite with both minerals exhibiting the same magnetization component suggesting a primary origin for the remanence we use the high quality paleomagnetic pole for the santa fe group 330 9 e 65 7 s n 60 95 4 1 k 21 in a revised late carboniferous to early triassic apparent polar wander path for south america on the basis of this result it is shown that an early permian pangea a type fit is possible if better determined paleomagnetic poles become available

tectonic evolution of the chinese tianshan orogen from subduction to arc continent collision insight from polyphase deformation along the gangou section central asia the central asian orogenic belt as the largest accretionary orogen on earth is an ideal candidate to study the geodynamics of convergent plate boundaries through a prolonged period the evolution of this orogen has been explained by different tectonic models which incorporated one or a combination of the following mechanisms lateral stacking of arc systems along major shear zones arc amalgamation oroclinal bending and trench migration here we elucidate major mechanisms responsible for the tectonic evolution of the central asian orogenic belt focusing on the chinese tianshan orogen in the southern central asian orogenic belt structural observations from the 50 km long gangou section show evidence of polyphase deformation the earliest episode of orogen parallel sinistral shearing constrained to the early devonian ca 399 ma by syn deformational intrusions was possibly controlled by oblique subduction this was followed by an episode of ne sw contractional deformation dated at ca 356 ma 40ar 39ar age of syn deformational hornblende and likely associated with an episode of trench advance the third stages of deformation during the latest carboniferous and permian involved ne sw contraction orogen parallel extension and dextral transpression our new geochronological data constrain the timing of orogen parallel extension to ca 303 293 ma and confirm that dextral activation along shear zones occurred during the permian the results highlight the role of trench migration oblique tectonics and syn collisional orogen parallel extension in the build up of the central asian orogenic belt and contribute to the pre collisional reconstruction of this orogenic system

a geomagnetic polarity timescale for the carboniferous the geomagnetic polarity pattern for the carboniferous is incompletely known but with the best resolved parts in the serpukhovian and bashkirian hence data from both igneous and sedimentary units are also used in an additional polarity bias evaluation in the tournaisian to mid visean interval polarity is mainly derived from palaeopole type palaeomagnetic studies allowing identification of polarity bias chrons seven polarity bias chrons exist in the mississippian mi1nb to mi4nb with an additional 33 conventional magnetochrons and submagnetochrons mi4r to mi9r the moscovian and gzhelian polarity is best resolved in magnetostratigraphic studies from the donets basin and the southern urals dispute about the reliability of these data is ill founded since an assessment of supporting data from palaeopole type studies suggests that these datasets currently provide the best magnetic polarity data through the pennsylvanian polarity bias assessment indicates a normal polarity bias zone in the kasimovian in the pennsylvanian there are 27 conventional magnetochrons and submagnetochrons pe1n to ci1r and one normal polarity bias chron pe8nb the kiaman superchron begins in the mid bashkirian with clear data indicating brief normal polarity submagnetochrons within the superchron the magnetochron timescale is calibrated using 31 u pb zircon dates and a quantitative bayesian based age scaling procedure

strong export of antarctic bottom water east of the kerguelen plateau deep western boundary currents east of the antarctic peninsula and the kerguelen plateau are important pathways for transporting deep antarctic water masses to the global ocean an array of moored current meters used to quantify the water transport in this system reveals a flow that is stronger than any measured in a deep western boundary current at similar depths so far the primary paths for the transport of antarctic bottom water from the southern ocean into the global ocean are the deep western boundary currents east of the antarctic peninsula and the kerguelen plateau1 previous ship based observations documented distinct water properties and velocities associated with a deep western boundary current in the kerguelen region2 3 4 5 6 7 but the mean flow is as yet unconstrained here we report measurements from a coherent array of eight current meter moorings that reveal a narrow and intense equatorward flow extending throughout the water column just east of the kerguelen plateau velocities averaged over two years exceed 20 cm s 1 at depths of about 3 500 m the strongest mean deep western boundary current flow yet observed at similar depths we estimate the mean equatorward transport of water colder than 0 c at 12 3 1 2 106 m3 s 1 partially compensated by poleward flow we also estimate the net equatorward flow of water colder than 0 2 c at about 8 106 m3 s 1 substantially higher than the 1 9 106 m3 s 1 reported from the boundary current that carries dense water from the weddell sea into the atlantic ocean north of the falkland plateau8 we conclude that the kerguelen deep western boundary current is a significant pathway of the global ocean s deep overturning circulation

mean sea surface height of the antarctic circumpolar current from geosat data method and application the mean sea surface height across the antarctic circumpolar current has been reconstructed from height variability measured by the geosat altimeter without assuming prior knowledge of the geoid for this study an automated technique has been developed to estimate mean sea surface height for each satellite ground track using a meandering gaussian jet model and errors have been estimated using monte carlo simulation the results are objectively mapped to produce a picture of the mean subantarctic and polar fronts which together comprise the major components of the antarctic circumpolar current the meandering jet model explains between 40 and 70 of the height variance along the jet axes the results show that the fronts are substantially steered by topography and that the jets have an average gaussian width of about 44 km in the meridional direction and meander about 75 km to either side of their mean locations the average height difference across the subantarctic front saf is 0 7 m and across the polar front pf 0 6 m the mean widths of the fronts are correlated with the size of the baroclinic rossby radius

paleomagnetism of mesozoic rocks from the central andes of southern peru importance of rotations in the development of the bolivian orocline a paleomagnetic study has been undertaken in jurassic volcanics and cretaceous intrusions from coastal southern peru a secondary magnetization in the jurassic volcanics was identified at 21 sites from three main localities along the southern coast of peru mean virtual geomagnetic pole 59 5 n 190 1 e k 111 95 3 0 this secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the peruvian coastal batholith and or to the first andean phase of compressional deformation in late cretaceous if we assume that the remagnetization is of late cretaceous age a regional counterclockwise rotation of about 20 is recorded by the forearc of southern peru from 16 s to 18 s primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations counterclockwise rotations of 50 to 80 are documented near the localities of chala eight sites ocona two sites and arequipa four sites preliminary results based on three sites of late permian to early triassic sediments from northern bolivia also show a large counterclockwise rotation the observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization perhaps this first episode of deformation partially structured the andean chain in the central andes and induced an incipient orocline during the cenozoic a differential shortening along the chain gave the present day shape of the bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20 as indicated by the remagnetization

development of accretionary orogens preface abstract this special issue celebrates the career of cees van staal an icon of appalachian geology who retired from the geological survey of canada in 2017 the issue is an outcome of the symposium entitled development of accretionary orogens a symposium celebrating the career of cees van staal that was held in cees s honour at the 2017 geological association of canada mineralogical association of canada gac mac meeting in kingston ontario the papers in this special issue simultaneously demonstrate recent advances in the understanding of accretionary orogens cees s influence on this understanding as well new data and concepts concerning their development in keeping with the global reach of cees s research this vsi is a contribution to the international geological correlation program 648 supercontinent cycles and global geodynamics

polar front around the kerguelen islands an up to date determination and associated circulation of surface subsurface waters the circulation of iron rich shelf waters around the kerguelen islands plays a crucial role for a climatically important annually recurrent phytoplankton spring bloom over the sluggish shelf region and its downstream plume area along the antarctic circumpolar flow however there is a long standing confusion about the polar front pf in the kerguelen region due to diverse suggestions in the literature for its geographical location with an extreme difference over 10 of latitude based on abundant historical hydrographic data the in situ hydrographic and current measurements during the 2011 keops2 cruise satellite chlorophyll images and altimetry derived surface velocity fields we determine and validate an up to date location of the pf around the kerguelen islands artificial lagrangian particle trajectories computed from altimetric velocity time series are analyzed for the possible pathways and sources of different surface subsurface waters advected into the chlorophyll bloom area east off the islands studied during the keops2 cruise the pf location determined as the northernmost boundary of the winter water colder than 2 c which is also associated with a band of strong currents appears to be primarily controlled by topography the pf rounds the kerguelen islands from the south to deflect northward along the eastern escarpment up to the northeastern corner of the kerguelen plateau before making its southward retroflection it is shown that the major surface subsurface waters found within the deep basin east of the kerguelen islands originate from the shelf around the heard island rather than from the shallow shelf north of the kerguelen islands

rapid southern ocean front transitions in an eddy resolving ocean gcm the formation of persistent multiple fronts is an established feature of the antarctic circumpolar current acc front strength and location are closely linked to eddy properties and therefore have important implications for the eddy driven closure of the southern ocean meridional overturning circulation acc front structure is analyzed here by calculating regional probability density functions pdfs of potential vorticity diagnosed in an eddy resolving ocean general circulation model rapid spatial transitions in the number of fronts and in the density classes over which they occur are found front transitions are associated with the major topographic obstacles kerguelen island campbell plateau and drake passage multiple fronts are preferentially found downstream of these features these findings highlight the significant departure from zonal symmetry of the acc front structure and emphasize the importance of local dynamics on large scale southern ocean properties

the subantarctic and polar fronts in relation to deep water masses through the southwestern atlantic the two principal fronts within the antarctic circumpolar current acc the subantarctic front saf and the polar front pf are investigated with respect to their spatial patterns and relations to deep water masses from the northern drake passage to the western and southern argentine basin observations from four recent cruises in adjoining and overlapping regions are supplemented with satellite infrared imagery and trajectories from surface drifters east of drake passage the width of the polar frontal zone pfz increases rapidly when the saf turns sharply northward as a part of the falkland malvinas current while the pf remains oriented mainly toward the northeast found within this region of the pfz are mesoscale eddies the upper layers of which consist of subantarctic mode water from the pacific the contrasts in upper layer water properties that identify the saf can be traced the entire length of the falkland current to the brazil falkland confluence zone and then southward to the southern argentine basin there the saf turns eastward and later but perhaps not always merges with the pf north of ewing bank to form a single intense current core strongly influenced by bottom topography at times this eastward current loops southward through a gap in the falkland ridge into the georgia basin thereby supplying relatively warm and salty subantarctic water to the antarctic zone east of the gap the two fronts become separated and are once again distinct features in the vicinity of the islas orcadas rise at depth within drake passage and the western scotia sea the saf and pf are not lateral boundaries between distinct water masses but are instead identified by enhanced vertical displacements of property isopleths with the northward turn of the saf east of drake passage a thick layer of circumpolar deep water cdw is advected over the falkland plateau into the argentine basin there it is joined by waters entering the argentine basin via a deep spreading route through the georgia basin denser cdw deep water from the weddell sea and episodically deep water from the southeastern pacific ocean together these waters form a northward flow off the patagonian shelf that extends from the sea surface to the bottom most of which turns back toward the south with the falkland current return also flowing south along the seaward side of the falkland current return well away from the western boundary is the main core of north atlantic deep water nadw detatched masses of nadw are observed within the cdw at the western boundary as well as within the acc south of the saf in the south central argentine basin

carboniferous to early triassic magmatism and accretion in alxa nw china implications for accretionary orogenesis of the southern altaids the alxa occupies a critical position between the tianshan beishan to the west and the solonker suture to the east and is significant for understanding the accretionary orogenesis of the southern altaids to unravel the tectonic history of the alxa we undertook an integrated study of field geology geochemistry and geochronology of magmatic rocks and an accretionary complex six granites and one rhyolite from the zhusileng hangwula arc zha show peraluminous high k calc alkaline composition and one gabbro is tholeiitic they show incompatible element rich patterns and negative nb ta anomalies on the primitive mantle normalized trace element spider diagrams la icp ms zircon u pb dating reveals 348 329 and 267 250 ma magmatic events these data indicate that the zha was a japan type island arc from late paleozoic to early triassic the engger us melange comprises n morb type pillow basalt tuff chert siliceous mudstone limestone and turbidite which are tectonically juxtaposed by block in matrix structure this melange is interpreted as a carboniferous permian accretionary complex these new data combined with previous studies confirm that the paleo asian ocean subducted bidirectionally in the alxa in carboniferous early triassic a consistent divergent subduction system existed from alxa to solonker before the terminal closure of the paleo asian ocean in the mid late triassic supplementary material https doi org 10 6084 m9 figshare c 4962230