neotectonic reactivation of the western section of the malargue fold and thrust belt tromen volcanic plateau southern central andes abstract this study examines the neotectonic deformation and development of the tromen massif a quaternary retroarc volcanic field located in the western section of the malargue fold and thrust belt in the southern central andes the linkages between neotectonic deformation in the intra arc zone and the recent retroarc structures of the tromen volcanic plateau are not clearly understood these retroarc deformations affect the mid section of the fold and thrust belt leaving to the east a 200 km wide deformed zone that can be considered inactive over the last 12 10 ma this out of sequence deformation west of the orogenic front area has not been previously addressed in detail in this study exhaustive mapping is used to describe and discriminate structures with a neotectonic component from those fossilized by pleistocene strata two balanced cross sections are constructed showing the distribution of the youngest deformations and their relation to pre miocene structures an important means for evaluating this is the morphometric and morphological analyses that allowed identification of perturbations in the fluvial network associated with active structures in a broader perspective neotectonic activity in the fold and thrust belt is discussed and inferred to be caused by local mechanical weakening of the retroarc zone due to injection of asthenospheric material evidenced by magnetotelluric surveys thus deformation imposed by the oblique convergence between south american and nazca plates while to the south being limited to the liquine ofqui fault system that runs through the arc zone in the retroarc area is located at the site of magmatic emplacement presumably in association with a thermally weakened upper crust this control exemplifies the relationship that exists between surficial processes magmatic emplacement and upper asthenospheric dynamics in the southern central andes

central arctic surface ocean environment during the past 80 000 years stable oxygen and carbon isotope and sedimentological paleontological investigations supported by accelerator mass spectrometry 14 c datings were carried out on cores from north of 85 degrees n in the eastern central arctic ocean significant changes in accumulation rates provenance of ice rafted debris ird and planktic productivity over the past 80 000 years are documented during peak glacials i e oxygen isotope stages 4 and 2 the arctic ocean was covered by sea ice with decreased seasonal variation limiting planktic productivity and bulk sedimentation rates in early stage 3 and during termination i major deglaciations of the circum arctic regions caused lowered salinities and poor oxygenation of central arctic surface waters a meltwater spike and an associated ird peak dated to similar to 14 12 14 c ka can be traced over the southern eurasian basin of the arctic ocean this event was associated with the early and rapid deglaciation of the marine based barents sea ice sheet a separate termination ib meltwater event is most conspicuous in the central arctic and is associated with characteristic dolomitic carbonate ird this lithology suggests an origin of glacial ice from northern canada and northern greenland where lower paleozoic platform carbonates crop extensively out

fluid and melt pathways in the central chilean subduction zone near the 2010 maule earthquake 35 36 s as inferred from magnetotelluric data

low electrical resistivity associated with plunging of the nazca flat slab beneath argentina beneath much of the andes oceanic lithosphere descends eastward into the mantle at an angle of about 30 ref 1 a partially molten region is thought to form in a wedge between this descending slab and the overlying continental lithosphere as volatiles given off by the slab lower the melting temperature of mantle material2 this wedge is the ultimate source for magma erupted at the active volcanoes that characterize the andean margin but between 28 and 33 s the subducted nazca plate appears to be anomalously buoyant3 4 as it levels out at about 100 km depth and extends nearly horizontally under the continent1 5 6 above this flat slab volcanic activity in the main andean cordillera terminated about 9 million years ago as the flattening slab presumably squeezed out the mantle wedge5 6 but it is unknown where slab volatiles go once this happens and why the flat slab finally rolls over to descend steeply into the mantle 600 km further eastward here we present results from a magnetotelluric profile in central argentina from which we infer enhanced electrical conductivity along the eastern side of the plunging slab indicative of the presence of partial melt this conductivity structure may imply that partial melting occurs to at least 250 km and perhaps to more than 400 km depth or that melt is supplied from the 410 km discontinuity consistent with the transition zone water filter model of bercovici and karato7

amino acid epimerization implies rapid sedimentation rates in arctic ocean cores the palaeooceanography of the arctic ocean is less well known than any other ocean basin due to difficulties in obtaining cores and in providing a secure chronological framework for those cores that have been raised most recent investigators have suggested that low sedimentation rates 0 05 0 1 cm kyr 1 have characterized the deep basins over the past 5 myr refs 1 2 despite the glacial marine character of the sediment and proximity to major centres of shelf glaciation these calculations have been primarily based on the down core pattern in the inclination of magnetic minerals supported by uranium series 14c and micropalaeontological evidence here we analyse amino acid diagnesis in foraminifera from two gravity cores raised from the floor of the arctic ocean our results suggest that these cores span 200 kyr conflicting with the earlier estimate of 3 myr based on palaeomagnetic data the chronology of other arctic ocean cores and previous palaeoenvironmental interpretations need re evaluation

plume overriding triggers shallow subduction and orogeny in the southern central andes abstract plate tectonic theory implies that mantle plumes may be eventually overridden by lithosphere during continental drift these events have particular tectonomagmatic consequences for active margins and hence related orogenic processes since the first documentation of plume overriding and the definition of the plume modified orogeny concept only few examples have been recognized in the geologic record in this study we analyze the neogene tectonic evolution of the southern central andes between 35 and 38 s and its potential relation to the subduction of the payenia plume as a recent analogue of this process through a series of tectonic reconstructions we show that progressive payenia plume overriding correlates with neogene arc front migrations linked to slab shallowing fold belt reactivation in the main cordillera and intraplate contraction in the san rafael block additionally nazca slab tear determined from tomographic analyses and subsequent diachronous steepening of the subducted plate may also be an aftermath of plume subduction as often described in the final stages of plume modified orogeny finally we propose a modern analogue for processes previously described dating back to the mesoproterozoic which provides further insights into these complex settings

pocho volcanic rocks and the melting of depleted continental lithosphere above a shallowly dipping subduction zone in the central andes the late miocene 7 9 to 4 5 ma pocho volcanic field in argentina occurs 700 km east of the chile trench over the modern shallowly dipping andean wadati benioff zone near 32 s latitude in argentina the field is located in the sierra de cordoba which is the easternmost laramide style block faulted range in the sierras pampeanas pampean ranges the arrival of the shallowly dipping slab initiated both volcanism and the uplift of the sierra de cordoba pocho rocks 52 to 68 sio2 feo mgo 2 2 comprise an older 7 5 0 5 ma high k and a younger 5 3 0 7 ma shoshonitic series mineralogic data and fractionation models show that crystallization occurred under hydrous oxidizing conditions which were most extreme in the high k series an unusual pattern of successively lower ree at higher sio2 concentrations can be modeled by sphene apatite and amphibole removal an arc like trace element signature attributed to an arc component is strongest in the younger shoshonitic series an important depleted lower crustal mantle lithospheric source component in both series is indicated by non radiogenic sr and pb isotopic ratios at nd 0 to 2 low rb sr ratios and low u and th concentrations this depleted signature contrasts with the enriched one in potassic back arc central volcanic zone cvz lavas over the steeper subduction zone to the north and is attributed to several processes in the shallow subduction zone first deep crustal mash processes in the nearly normal thickness crust beneath pocho incorporated depleted proterozoic basement components and not complexly mixed structurally thickened crustal components as in the cvz second the association of pocho volcanism with the arrival of the slab allowed little time for modification of the mantle by subduction components third miocene shallowing of the subduction zone beneath the flat slab required thinning of both the astenosphere and the subcontinental lithosphere thus an important subcrustal component could be from blocks removed from the base of the lithosphere to the west and recycled into the asthenosphere similar magmatic sources would have existed during laramide shallow subduction in western north america

provenance and time constraints on the formation of the first bend of the yangtze river 1 the upper yangtze river flows southward on the southeastern tibet characterizing by uniquely low and continuous relief the river makes a sharp turn at shigu heading northeast and forms the first bend of the yangtze river many previous studies assumed southward flow of the ancestral yangtze river from shigu to the south china sea however field evidence of southward flow of the paleo yangtze is lacking in this paper we report our identification based on detrital zircon u pb age distributions of a range of fluvial sands left by the paleo yangtze in tongdian madeng and nanjian basins cosmogenic10be and 26al burial dating provides burial ages for these fluvial sands from 1 7 to over 8 7 ma rerouting of the yangtze river therefore occurred within the last 1 7 ma postdating the major uplift of the central tibet we attribute the rerouting of the yangtze river to response to activation of the dali fault system and in a larger scale initiation of crustal deformation by clockwise rotation around eastern himalayan syntaxis 2 4 ma ago reorganization of the yangtze drainage pattern most likely reflects regional uplift and displacement due to lower crust flowing beneath major faults in the southeastern tibet and yunnan

evidence of quaternary tectonics along rio grande valley southern malargue fold and thrust belt mendoza argentina abstract the malargue fold and thrust belt is developed in the argentinian andes between 34 and 37 s through the tectonic inversion of upper triassic and lower jurassic depocenters of the neuquen basin with an uplift history since the cretaceous evidence of quaternary deformation has been described in the northern part of it 34 34 5 s potentially coeval to neotectonic activity along the eastern edge of san rafael block to the south compressional and extensional structures active during the quaternary were found in the dorso de los chihuidos along the agrio fold and thrust belt front 37 5 38 s contrastingly the southern segment of the malargue fold and thrust belt between these two areas with described neotectonic activity is partially covered by quaternary products of the payun matru volcanic field that may hide evidence of recent deformation in this 300 km gap of neotectonic information the landscape imprint of two individual structures aligned in the mountain front through the rio grande valley was analyzed new evidence of neotectonic deformation were recognized in particular over the western slope of the cara cura range expressed by faulting and folding of quaternary deposits and lava flows an 40ar 39ar age from a deformed lava flow at the flanks of an anticline in the foothills of the cara cura range may suggest at least an upper pleistocene compressional tectonic activity longitudinal river profile analysis revealed anomalies that show some correlation with the neotectonic structures described especially knickpoints and concavity index changes meanwhile normalized steepness index values showed a moderate response to recent deformation a proposed schematic geomorphic evolution for this segment of rio grande river is discussed to put the neotectonic activity into the context of landscape formation all together this evidence supports the idea of an active front through the rio grande valley during the quaternary coetaneous to an active broken foreland to the east in the southern central andes

10be records of sediment cores from high northern latitudes implications for environmental and climatic changes the 10be records of four sediment cores forming a transect from the norwegian sea at 70 n core 23059 via the fram strait core 23235 to the arctic ocean at 86 n cores 1533 and 1524 were measured at a high depth resolution although the material in all the cores was controlled by different sedimentological regimes the 10be records of these cores were superimposed by glacial interglacial changes in the sedimentary environment core sections with high 10be concentrations 1 109 at g are related to interglacial stages and core sections with low10be concentrations 0 5 109 at g are related to glacial stages climatic transitions e g termination ii 5 6 are marked by drastic changes in the 10be concentrations of up to one order of magnitude the average 10be concentrations for each climatic stage show an inverse relationship to their corresponding sedimentation rates indicating that the 10be records are the result of dilution with more or less terrigenous ice rafted material however there are strong changes in the 10be fluxes e g termination ii into the sediments which may also account for the observed oscillations most likely both processes affected the 10be records equally amplifying the contrast between lower glacials and higher interglacials 10be concentrations the sharp contrast of high and low 10be concentrations at climatic stage boundaries are an independent proxy for climatic and sedimentary change in the nordic seas and can be applied for stratigraphic dating 10be stratigraphy of sediment cores from the northern north atlantic and the arctic ocean

variations in the fine scale composition of a central pacific ferromanganese crust paleoceanographic implications a 47 to 60 mm thick fe mn crust from horizon guyot water depth 1800 1780 m central pacific was used to evaluate the potential of crusts as recorders of neogene paleoceanographic and paleoclimatic conditions the chemical composition was determined by microprobe for 16 elements from a polished thin section three analyses were made per millimeter and averaged to give the composition of each millimeter the age of the crust was determined by measuring the strontium isotope composition of the crust and comparing it with the tertiary seawater curve the crust represents 18 5 m y of growth of fe and mn oxyhydroxides the crust is composed of alternating botryoidal and laminated layers the botryoidal layers formed during the same time intervals that widespread neogene deep sea hiatuses were forming in bottom sediments the botryoidal layers represent growth during times of intensified deepwater flow whereas the laminated intervals represent more quiescent conditions the correspondence between the botryoidal layers and the neogene hiatuses is so strong that we were able to choose a variable growth rate model over a constant growth rate model for the crust chemical changes in the crust take two forms the first is represented by broad changes in the composition defined chiefly by fourth order polynomial fits to the chemical profiles of each element with depth in the crust the second is high frequency changes in composition the broader changes occurred primarily at about 15 11 5 7 4 6 4 5 2 and 4 6 ma which may correlate with major changes in paleoceanographic circulation and development of ice caps at the poles the periods of the high frequency changes may reflect climatic changes that resulted from orbital forcing these high frequency changes may correspond to the high order eccentricity periods of 3 47 2 04 and 1 31 m y

10be records from sediments of the arctic ocean covering the past 350 ka records of be 10 be 9 mineralogy and grain size were obtained from two cores collected by the polarstern expedition 1991 in the southern nansen basin core 2213 6 and the yermak plateau core 22

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

late quaternary coccoliths at the north pole evidence of ice free conditions and rapid sedimentation in the central arctic ocean calcareous nannofossils from approximately the past 7000 yr of the holocene and from oxygen isotope stage 5 are present at 39 analyzed sites in the central arctic ocean this indicates partly ice free conditions during at least some summers the depth of holocene sediments in the nansen basin is about 20 cm or more where influenced by turbidites on the gakkel ridge calcite preservation is excellent and holocene strata are consistently documented to be at least 4 to 20 cm thick in the amundsen basin preservation of nannofossils is sometimes poor holocene specimens are commonly only found at the surface although at one site they extend down to 26 cm the holocene assemblage is consistently recorded in the upper 5 to 20 cm of sediment on the lomonosov ridge and down to 15 cm depth in the makarov basin this implies that linear sedimentation rates for the holocene in all investigated areas are centimetres per thousand years sediments from the last glacial period oxygen isotope stages 2 4 are relatively thin suggesting significantly reduced sedimentation millimetres per thousand years during this time

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

the last deglaciation event in the eastern central arctic ocean oxygen isotope records of cores from the central arctic ocean yield evidence for a major influx of meltwater at the beginning of the last deglaciation 15 7 thousand years ago 16 650 calendar years b c the almost parallel trends of the isotope records from the arctic ocean the fram strait and the east greenland continental margin suggest contemporaneous variations of the eurasian arctic and greenland laurentide ice sheets or increased export of low saline waters from the arctic within the east greenland current during the last deglaciation on the basis of isotope and carbon data the modern surface and deep water characteristics and seasonally open ice conditions with increased surface water productivity were established in the central arctic at the end of termination lb about 7 2 thousand years ago or 6 000 calendar years b c

paleomagnetic constraints on sedimentation ratesin the eastern arctic ocean abstract deep sea sediments recovered from six sites visited during the international arctic ocean expedition of 1991 were examined to determine sedimentation rates in the eastern arctic ocean basin the dearth of age diagnostic biogenic material in these sediments precludes the application of biostratigraphic methods but ages can be deduced using paleomagnetism in conjunction with measurements of radiocarbon and carbonate concentration although no one of these techniques gives an unambiguous determination of age the interpretation most consistent with these diverse data implies that sedimentation rates in the eastern arctic are in general a few centimeters per thousand years such estimates of sedimentation rate are an order of magnitude greater than those previously determined from many sediment cores taken from the canada basin however one site examined on the morris jesup rise shows a relatively low rate of sediment accumulation less than 0 6 cm 103yr suggesting that although higher than in the canada basin sedimentation rates in the eastern arctic can be highly variable

improvements in long core measurement techniques applications in palaeomagnetism and palaeoceanography we report on new developments in the long core measurement techniques using u channels in connection with new small access cryogenic magnetometers with these new instruments u channel samples allow us to measure a wide range of magnetic parameters along entire core length with a spatial resolution and accuracy virtually equivalent to that obtained using discrete cubic samples an additional advantage is that the sediment disturbance when sampling is significantly less when using u channels we present comparisons of results from both continuous and discrete measurements and examine the effectiveness of minimal deconvolution using various techniques examples of continuous measurements of low field susceptibility nrm arm irm hrc from north atlantic deep sea cores show that it is possible to generate logs of combination of parameters related to rock magnetic characteristics of the sediments useful in palaeoceanography in particular the continuous method allows us to rapidly detect core regions with fast changing magnetic parameters useful for correlation between cores and also for understanding changes in palaeo environmental conditions the fast scan also allows us to detect regions of the core where the rock magnetic parameters are suitable or not suitable for palaeomagnetic studies related to changes in the geomagnetic field such as the relative changes in palaeointensity

geochemistry of manganese in the kalix river northern sweden dissolved and suspended mn in the kalix river northern sweden were measured weekly over a period of eighteen months during the same period four lakes in the kalix catchment were sampled at their outlets and in vertical profiles within the lakes together with a stream draining a series of mires with shallow lakes snow melting in mid may increased the dissolved mn concentration in the river tenfold compared with a concentration of 5 g l 1 during the winter discharge january to april we suggest that the increase was caused by mn rich mire water mixing with melting snow and being transported to the river large concentrations of dissolved mn built up in the hypolimnion of the lakes studied during the icecovered period break up of the ice and spring overturn in june increased the dissolved mn concentration tenfold in lake discharge and a concomitant peak in the dissolved mn concentration was observed in the river lake derived mn was the dominant source for mn in the river during this time suspended mn in the river was hosted mainly in detrital particles during flood in may in mid june non detrital suspended mn started to accumulate and reached a maximum in late july and early august the mnal ratio was 25 times higher during this period than during flood in may suggesting the precipitation of an mn oxyhydroxide phase the precipitation of the non detrital mn rich phase was correlated in time with increased temperature increased ph and increased concentration of suspended biogenic particles the precipitation of dissolved mn was biologically mediated sedimentation and mineralisation of the non detrital mn phase in river and lake sediments resulted in a steady increase of the dissolved mn concentration in the river water during autumn

magnetostratigraphy in three arctic ocean sediment cores arguments for geomagnetic excursions within oxygen isotope stage 2 3 two zones with shallow to steep negative palaeomagnetic directions have been detected within similar lithological units in three high latitude gravity cores from the arctic ocean coinciding azimuthal distributions of declinations relative to arbitrary oriented sub sampling surfaces are attributed to the systematic acquisition of magnetic components by frictional deformation of the unconsolidated sediment during the sub sampling procedure alternating field demagnetization to 50 mt was only partly successful in obtaining stable single component directions magnetic polarity zones are deduced by extrapolation of directional trends of inclinations during progressive af demagnetization high delta 18o values in sinistrally coiled n pachyderma range between 4 30 4 68 pdb mean values reflecting sediment accumulation during continental glaciation amino acid ratios of n pachyderma suggest ages less than approximately 60 000 years ago when seen in conjunction these observations suggest that the sediments were deposited within oxygen isotope stages 2 3 the reversed palaeomagnetic polarity zones are concluded to represent two short duration excursions of the geomagnetic field which occurred less than 60 000 years ago likely known candidates are the lake mungo 28 000 30 000 years ago and laschamp olby 35 000 40 000 years ago excursions all confined within oxygen isotope stage 3 24 000 59 000 years ago

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

revised magnetostratigraphies confirm low sedimentation rates in arctic ocean cores the general lack of an age diagnostic biostratigraphy in the neogene sediments of the abyssal arctic ocean has emphasized the importance of magnetostratigraphy in providing chronostratigraphic control in these sediments sedimentation rates interpreted from early magnetostratigraphic studies of cores taken from the t3 ice island in the western mendeleev plain were estimated to be on the order of 1 mm io3 yr however recent amino acid epimerization studies of a core from the same area have suggested sedimentation rates of almost 15 mm io3 yr this controversy has led us to reexamine the paleomagnetism of several of these cores our alternating field demagnetization studies indicate that many of these cores have an intense high coercivity overprint acquired after the core was opened that is adequately removed only after treatment at 20 to 70 mt we have remeasured samples from two cores after demagnetizations up to 80 mt and can confirm the position of the bnmheslmatuyama boundary originally identified in the cores in addition the jaramillo and olduvai subchrons are identified average sedimentation rates in these two cores are 2 3 mm io3 yr similar to the original estimates based on reversal stratigraphy as well as those determined from recent radiocarbon studies but incompatible with the amino acid based dates

holocene yardangs in volcanic terrains in the southern andes argentina yardangs of different sizes were developed in the payun matru volcanic field a semiarid area east of the southern andes mountains yardangs from volcanic terrains have not been described previously from earth although studies from mars interpreted linear ridges found by mariner and viking images as yardangs the payun matru volcanic field is an extensive plateau at 2000 m a s l covered by basaltic lava and ignimbrite flows strong westerly winds affect the extensive plateau micro and mesoyardangs are formed on the ignimbrite rock blanket and macroforms or megayardangs several kilometres in length are developed in the basaltic lava flows as long parallel troughs they all have a distinctive 320 azimuth which is the prevailing wind direction no yardang features are noted in the more recent lava flows younger than 1000 years indicating that their formation needed a longer time or they developed in earlier periods with stronger winds the yardang development is explained by the strong unidirectional winds the poor vegetational cover due to the aridity of the region the available quartz sand and volcanic ash particles as abrasive agents and the volcanic lithology texture and flow structure copyright 2001 john wiley sons ltd

natural radionuclides and plutonium in sediments from the western arctic ocean sedimentation rates and pathways of radionuclides abstract sediment cores collected during r v polar sea aos94 expedition from the chukchi shelf to the north pole were analyzed for several decay series natural radionuclides and pu isotopes to study sedimentation rates and pathways of radionuclides in the western arctic ocean the measured sedimentation rates vary by more than three orders of magnitude along the transect from 210pb based rates of 200 700 cm kyr 1 over the chukchi shelf and 89 cm kyr 1 at the chukchi slope to 230th based rates of 0 02 0 3 cm kyr 1 at various settings in the deep basin 230thex profiles in the central western arctic basin are characterized by a cyclic pattern and a pronounced sub surface maximum superimposed on an overall decrease with depth sediment inventories of excess 210pb and 230th in the deep basin as a whole cannot account for their in situ production and 210pb fall out the opposite is true at the slope and shallower waters we contend that as with other ocean basins boundary scavenging also exists in the arctic ocean the broad continental shelves and the slope region may have the potential of removing all or most of the particle reactive radionuclides unaccounted for in the deep basin the pu isotope data are consistent with the notion of boundary scavenging sediment inventories and concentrations of pu decrease rapidly offshore isotopic composition of pu suggests mixing of fall out pu which decreases with increasing latitudes and fuels reprocessing pu derived from the russian and atlantic sides of the arctic ocean although fuel reprocessing pu has impinged on the chukchi slope its existence over the chukchi shelf is not evident and probably overshadowed by fall out pu

paleoclimate record for northwind ridge western arctic ocean core nwr 5 from northwind ridge in the western arctic ocean contains a climatic record that extends back for over 1 million years dark brown beds with abundant planktonic foraminifers represent interglacial conditions whereas glacial and transitional conditions are represented by lithologies barren or nearly barren of planktonic foraminifers we conclude that seven and possibly all nine interglacials of the brunhes magnetic chronozone the last 780 000 years are represented in nwr 5 for about the last 800 000 years the western arctic ocean apparently had a permanent thick ice cover and was unable to support significant populations of planktonic foraminifers except during major interglacials when seasonally open and high productivity surface waters occurred at least along the basin margins our results support previous interpretations that climate variations in the arctic are in phase with glacial interglacial cycles observed in other proxy records of the brunhes

u and th series isotopes in deep baffin bay sediments tracers of detrital sources and of contrasted glacial interglacial sedimentary processes abstract uranium u and thorium th series have been used to document the sedimentary sources processes and fluxes of the last 115 ka in central baffin bay a major gateway between the arctic ocean and the north atlantic ocean which has been fed throughout the last ice age by sediments and meltwaters from the northeastern laurentide ice sheet the innuitian ice sheet and the western greenland ice sheet measurements have been made with a 1 6 ka time resolution in sedimentary core hu2008 029 016pc 70 27 n 64 39 w 2063 m water depth the lack of correlation between u concentration 234 u 238 u activity ratio 234 u 238 u ar organic carbon and redox sensitive elements suggests minimum early diagenetic u uptake if any likely due to very low organic matter sedimentary fluxes in such a subarctic environment throughout the last glaciation u and th concentrations and their mass ratio as well as 234 u 238 u ar do not show either any unequivocal linkage with detrital dolomitic carbonate and or detrital silicate fluxes data rather suggest sedimentary supplies with singular u and th series signatures linked to specific glacial margin ice streaming events and highlighting the diversity of sedimentary sources surrounding this small oceanic basin large surplus of 230 th 230 th xs 0 well above the production from dissolved u decay in the overlying water column are observed throughout the last glacial interval in response to sediment focusing and high sedimentation rates in the deepest part of the bay during this interval high frequency fluctuations in 230 th xs 0 indicate either a similar variability in sedimentation rates not captured by the age model used and or grain size or mineralogical dependent 230 th xs scavenging rates in the much lower sedimentation rate holocene core top section the 230 th xs 0 inventory falls below production in the overlying water column despite evidence for some primary productivity revival as suggested by the higher organic carbon content 230 th xs losses through lateral advection via baffin bay water masses overflowing through davis strait might account for this pattern pointing to ocean circulation drastically differing from that of the glacial interval

quaternary paleoceanography of the central arctic based on integrated ocean drilling program arctic coring expedition 302 foraminiferal assemblages the integrated ocean drilling program iodp arctic coring expedition acex hole 4c from the lomonosov ridge in the central arctic ocean recovered a continuous 18 m record of quaternary foraminifera yielding evidence for seasonally ice free interglacials during the matuyama progressive development of large glacials during the mid pleistocene transition mpt 1 2 0 9 ma and the onset of high amplitude 100 ka orbital cycles 500 ka foraminiferal preservation in sediments from the arctic is influenced by primary sea ice organic input and other environmental conditions and secondary factors syndepositional long term pore water dissolution taking these into account the acex 4c record shows distinct maxima in agglutinated foraminiferal abundance corresponding to several interglacials and deglacials between marine isotope stages mis 13 37 and although less precise dating is available for older sediments these trends appear to continue through the matuyama the mpt is characterized by nearly barren intervals during major glacials mis 12 16 and 22 24 and faunal turnover mis 12 24 abundant calcareous planktonic mainly neogloboquadrina pachyderma sin and benthic foraminifers occur mainly in interglacial intervals during the brunhes and very rarely in the matuyama a distinct faunal transition from calcareous to agglutinated foraminifers 200 300 ka in acex 4c is comparable to that found in arctic sediments from the lomonosov alpha and northwind ridges and the morris jesup rise down core disappearance of calcareous taxa is probably related to either reduced sea ice cover prior to the last few 100 ka cycles pore water dissolution or both

constraining timing and p t conditions of continental collision and late overprinting in the southern brasilia orogen se brazil u pb zircon ages and geothermobarometry of the andrelândia nappe system abstract the andrel ndia nappe system ans is an important component of the neoproterozoic southern brasilia orogen sbo in se brazil and its evolution is related to the convergence and subsequent collision between the passive margin of the sao francisco paleocontinent and paranapanema block during the brasiliano orogeny the ans is mainly composed of a pile of deformed meta sedimentary rocks that record an inverted metamorphic pattern ranging respectively from amphibolite facies at the bottom to high p granulite conditions at the top the ans is here simplified into the lower middle and upper nappes four samples from different tectonic settings within the ans were collected for zircon isotopic investigation in addition geothermobarometry modelling were performed on seven samples from the middle nappe retro eclogite slices associated to basement gneissic rocks occur along the contact between the upper and middle nappes and have zircon grains with metamorphic rims grown in high p conditions of 12 16 kbar with temperatures of 700 800 c during the continental subduction dated here by u pb shrimp in zircon at 625 6 ma a period of nappe emplacement accompanied by melting in the middle nappe is evidenced by a garnet muscovite staurolite kyanite schist with leucosome parallel to the main foliation dated by u pb shrimp in zircon at 618 5 ma re equilibrium at pressures of 4 7 kbar with temperatures of 500 700 c is ascribed to a second heat pulse under lower pressure conditions the period of melting associated to this event in the middle nappe occurred at 586 9 ma and is marked by an anatectic tourmaline garnet muscovite bearing s type granite this granite body is part of a ne sw belt of granites with ages between 605 and 563 ma and covering both the central ribeira orogen cro and the sbo the age and the alignment of this granite belt justify the interpretation that it is genetically related to the cro thus the new data provide evidence that tectonothermal events related to the collisional front of the cro extended up to 200 km overlapping the consolidated sbo at ca 580 ma

a geophysical perspective on mantle water content and melting inverting electromagnetic sounding data using laboratory based electrical conductivity profiles abstract this paper applies electromagnetic sounding methods for earth s mantle to constrain its thermal state chemical composition and water content we consider long period inductive response functions in the form of c responses from four stations distributed across the earth europe north america asia and australia covering a period range from 3 9 to 95 2 days and sensitivity to 1200 km depth we invert c responses directly for thermo chemical state using a self consistent thermodynamic method that computes phase equilibria as functions of pressure temperature and composition in the na 2 o cao feo mgo al 2 o 3 sio 2 model system computed mineral modes are combined with recent laboratory based electrical conductivity models from independent experimental research groups yoshino 2010 and karato 2011 to compute bulk conductivity structure beneath each of the four stations from which c responses are estimated to reliably allocate water between the various mineral phases we include laboratory measured water partition coefficients for major upper mantle and transition zone minerals this scheme is interfaced with a sampling based algorithm to solve the resulting non linear inverse problem this approach has two advantages 1 it anchors temperatures composition electrical conductivities and discontinuities that are in laboratory based forward models and 2 at the same time it permits the use of geophysical inverse methods to optimize conductivity profiles to match geophysical data the results show lateral variations in upper mantle temperatures beneath the four stations that appear to persist throughout the upper mantle and parts of the transition zone calculated mantle temperatures at 410 and 660 km depth lie in the range 1250 1650 c and 1500 1750 c respectively and generally agree with the experimentally determined temperatures at which the measured phase reactions olivine spinel and spinel ferropericlase perovskite occur the retrieved conductivity structures beneath the various stations tend to follow trends observed for temperature with the strongest lateral variations in the uppermost mantle for depths 300 km conductivities appear to depend less on the particular conductivity database conductivities at 410 km and at 660 km depth are found to agree overall with purely geophysically derived global and semi global one dimensional conductivity models both electrical conductivity databases point to 2 o in the upper mantle for transition zone minerals results from the laboratory database of yoshino 2010 suggest that a much higher water content up to 2 wt h 2 o is required than in the other database karato 2011 which favors a relatively dry transition zone 2 o incorporating laboratory measurements of hydrous silicate melting relations and available conductivity data allows us to consider the possibility of hydration melting and a high conductivity melt layer above the 410 km discontinuity the latter appears to be 1 regionally localized and 2 principally a feature from the yoshino 2010 database further there is evidence of lateral heterogeneity the mantle beneath southwestern north america and central china appears wetter than that beneath central europe or australia

partial melt or aqueous fluid in the mid crust of southern tibet constraints from indepth magnetotelluric data summary the indepth project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the tibetan plateau in the lhasa block seismic reflection surveys in 1994 detected a number of bright spots at 15 20 km depths that indicate zones of crustal fluids aqueous fluids or partial melt coincident magnetotelluric mt data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots using constrained inversion the mt data require a minimum crustal conductance of 6000 s this abnormally high electrical conductance can be best explained by a layered model with fluids partial melt aqueous fluids or a combination of partial melt and aqueous fluids the non uniqueness of the mt method means that a wide range of melt fraction thickness combinations for the above models could all explain the 6000 s conductance to distinguish between these three models other geophysical and geological data are required reflection seismic data suggest that a high fluid content 15 per cent is present at the top of the layer the amplitude versus offset data suggest that the top of this layer may be aqueous fluids rather than partial melt passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt petrological studies suggest that concentrations of aqueous fluids above 0 1 per cent at mid crustal depth cannot be sustained taken together these data show that the high conductivity in southern tibet is most probably the result of a relatively thin layer of aqueous fluids 100 200 m overlying a thicker zone of partial melt 10 km

crustal and mantle structure beneath the southern payenia volcanic province using gravity and magnetic data the auca mahuida volcanic field lies on the southernmost payenia volcanic province one of the broadest retroarc volcanic plateaux in the southern central andes 38 s this voluminous basaltic flooding of quaternary age was originated from a deep asthenospheric source interpreted as a mantle plume product of changing slab dynamics the geometry of this source is deduced from magnetotelluric data but the limited spatial coverage of this array does not allow a detailed resolution of this anomaly in order to present a detailed geometry of the conductive anomaly and related crustal magmatic bodies we used multiple data sources we combined magnetic and bouguer anomalies curie isotherm depth t c elastic thickness t e and moho depth derived from the global earth magnetic anomaly grid emag2 and terrestrial gravity measurements all together in a holistic geophysical analysis the magnetic data depict a nearly 200 km in diameter circular anomaly that would correspond to a dense body according to the bouguer anomaly geoid data from the gravity field model eigen 6c4 have been filtered in order to isolate deeper mass influences and visualize the asthenospheric upwelling previously described from magnetotelluric data moho inversion yields a crustal attenuation at 36 to 32 km depth coinciding with t e below 20 km depth and a shallow t c 15 km depth at the site where geoid positive undulation was calculated finally surface analysis allowed defining a topographic swell compatible with the dimensions of the identified magnetic anomaly where the main rivers deviated potentially due to a recent base level change 2018 american geophysical union all rights reserved

payenia volcanic province in the southern andes an appraisal of an exceptional quaternary tectonic setting abstract the southern volcanic zone of the andes has a quaternary basaltic province along the retroarc which has a unique tectonic setting the payenia volcanic province covers an area larger than 40 000 km2 between 33 30 and 38 south latitudes with an estimated volcanic volume of about 8387 km3 erupted through more than 800 volcanic centers in the last 2 ma the mainly basaltic province developed above the san rafael block is subdivided in three segments characterized by the cerro nevado llancanelo payun matru tromen and auca mahuida volcanic fields together with hundreds of minor monogenetic basaltic centers the analysis of the different segments shows the formation of a common basalt plateau with intraplate signature from south to north between 2 0 and 1 7 ma which reached the 35 s to the north above this plateau monogenetic centers as nihuil vn 1 433 ma and cerro chato at 1 352 ma are developed followed by the large polygenetic center of cerro nevado 3980 m a s l at 1 320 ma this plateau was broken by a series of normal faults that produced volcanic cone alignments such as the nnw trending mancha jarilla lineament in the central part at about 1 0 ma extension shifted to the eastern margin of the san rafael block which concentrates tens of monogenetic centers between 0 9 and 0 7 ma extension then migrated towards the foothills in the west where many monogenetic cones were erupted through nw trending normal faults between 0 5 and 0 435 ma the collapse of the large diamante caldera at 0 445 ma coincides with that period subsequent volcanism was concentrated in 1 the payun matru volcanic field with the eruption of cerro payen between 0 272 and 0 261 ma the payun matru shield volcano with polygenetic eruptions at least since the last 0 233 ma and with the caldera formation bracketed between 0 168 0 004 ma and 0 082 0 001 ma followed by several eruptions until 7000 yrs and even historical ones and in 2 the tromen volcano where younger than 0 2 ma eruptions took place and historical eruptions were reported the understanding of these eruptions in time and space combined with geophysical data indicates the geometry of an important crustal attenuation beneath payenia associated with a hot sublithosphere the late miocene uplifted san rafael block collapsed in the early pleistocene as a consequence of the steepening of the subducted slab and the injection of hot asthenosphere produced the quaternary payenia volcanic province melts of the lower crust along the principal cordillera at these latitudes are responsible for the quaternary calderas ignimbritic flows and rhyolitic volcanism that express the crustal delamination of the andes the payun matru volcanic field concentrates this asthenospheric flow in the present

late quaternary deep stratification climate coupling in the southern ocean implications for changes in abyssal carbon storage the southern ocean plays an important role in modulating pleistocene atmospheric co2 concentrations but the underlying mechanisms are not yet fully understood here we report the laser grain size distribution and mn geochemical data of a 523 kyr long sediment record core ant30 p1 02 off prydz bay east antarctica to trace past physical changes in the deep southern ocean the core sediments are predominantly composed of clay and silt sized material three grain size end members em as well as three sensitive grain size classes sc were discerned interpreted as ice rafted debris em1 and sc1 and coarse em2 and sc2 and fine em3 sc3 materials deposited from bottom nepheloid layers respectively ratios of em2 em2 em3 and sc2 sc3 reveal changes in the local bottom current strength which is related to the deep ocean diapycnal mixing rate showing higher values during interglacial periods and lower values during glacial periods mno was enriched at each glacial termination probably caused by abrupt elevations in antarctic bottom water aabw formation rate lower aabw formation rate and reduced deep diapycnal mixing during glacial periods enhanced deep southern ocean stratification contributing to glacial atmospheric co2 drawdown the elevated aabw formation and enhanced deep diapycnal mixing during glacial terminations alleviated such deep stratification promoting deeply sequestered co2 to outgas

lithospheric and upper mantle structure of central chile and argentina summary the tectonics of central chile and argentina have been greatly affected by the shallow dips of the subducting nazca plate which controlled patterns of magmatism and deformation nearly 1000 km away from the plate boundary we calculate receiver functions from data recorded by the charge array which transected the andes and sierras pampeanas in central chile and argentina to better constrain the crustal structure of this region beneath the northern transect of the charge array where the nazca slab flattens near 100 km we find the crust is over 60 km thick beneath the andes and thin to the east the thick crust however extends 200 km to the east of the high elevations estimates of vp vs obtained from receiver functions vary along ancient terrane boundaries exhibiting higher values to the west interestingly we observe that the amplitude of the phase corresponding to the moho on receiver functions diminishes to the west complicating our images of crustal structure we proposed that the observations presented here of thickened crust within a region of low elevations diminished receiver function arrivals and reports of high shear wave speeds atop of the mantle wedge overlying the shallowly subducted nazca slab can be explained by partial eclogitization of the lower crust the moho appears simpler across the southern transect where it can be identified near 50 km depth at its deepest point beneath the andes and shallows eastwards volcanism remains active near the latitudes of our southern transect and we observe multiple crustal lowvelocity zones indicative of regions of partial melt near the centres of volcanism signals related to the nazca slab remain more elusive suggestive of a small impedance contrast between the slab and overlying mantle

sub slab mantle anisotropy beneath south central chile knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones by analysing shear wave splitting in both teleseismic and local arrivals we can determine the relative contribution from different parts of the subduction zone to the total observed sks splitting providing us with a depth constraint on anisotropy we use this methodology to determine the location orientation and strength of seismic anisotropy in the south central chile subduction zone data come from the tipteq network deployed on the forearc during 2004 2005 we obtain 110 teleseismic sks and 116 local good quality shear wave splitting measurements sks average delay times are 1 3 s and local s delay times are only 0 2 s weak shear wave splitting from local phases is consistent with a shape preferred orientation spo source in the upper crust we infer that the bulk of shear wave splitting is sourced either within or below the subducting nazca slab sks splitting measurements exhibit an average north easterly fast direction with a strong degree of variation further investigation suggests a relationship between the measurement s fast direction and the incoming ray s back azimuth finite element geodynamic modelling is used to investigate the strain rate field and predicted lpo characteristics in the subduction zone these models highlight a thick region of high strain rate and strong s wave anisotropy with plunging olivine a axes in the sub slab asthenosphere we forward model the sub slab sourced splitting with a strongly anisotropic layer of thick asthenosphere comprising an olivine a axis oriented parallel to the direction of subduction the subducting lithosphere is not thick enough to cause 1 2 s of splitting therefore our results and subsequent models show that the nazca slab is entraining the underlying asthenosphere its flow causes it to be strongly anisotropic our observation has important implications for the controlling factors on sub slab mantle flow and the movement of asthenospheric material within the earth

integrating magnetotelluric and seismic images of silicic magma systems a case study from the laguna del maule volcanic field central chile

stagnant slab wet plumes and cenozoic volcanism in east asia abstract data from seismology and mineral physics indicate the existence of low velocity zones in the deep part of the upper mantle wedge and the mantle transition zone under east asia zhao and ohtani 2009 zhao et al 2009 it suggests a link between the deep subduction and stagnation and deep dehydration of the pacific slab below this area and the cenozoic volcanism observed at the surface here we present a numerical model describing the dynamics of a hydrated layer covering a slab that is stagnating in the earth tm s mantle transition zone specifically our model includes the effects of water on density and viscosity of the mantle minerals we conclude that the presence of water is likely to induce rayleigh taylor type instabilities wet plumes that are able to transport water up to the base of the surficial lithosphere in a relatively short time scale a few million years under initial conditions representative of the subducting pacific slab under east asia melting is induced where wet plumes reach the base of the lithosphere according to our results and analysis of the known composition of cenozoic basalts from southwest japan iwamori 1991 iwamori 1992 wet plumes and their melting induced by water related to the stagnant slab provides an appealing hypothesis to explain cenozoic volcanism in east asia

arctic ocean evidence for late quaternary initiation of northern eurasian ice sheets a high resolution multiparameter stratigraphy allows the identification of late quaternary glacial and interglacial cycles in a central arctic ocean sediment core distinct sandy layers in the upper part of the otherwise fine grained sediment core from the lomonosov ridge lat 87 5 n correlate to four major glacials since ca 0 7 ma the composition of these ice rafted terrigenous sediments points to a glaciated northern siberia as the main source in contrast lithic carbonates derived from north america are also present in older sediments and indicate a northern north american glaciation since at least 2 8 ma we conclude that large scale northern siberian glaciation began much later than other northern hemisphere ice sheets

manganese and color cycles in arctic ocean sediments constrain pleistocene chronology sequential variations in manganese mn content and color of deepsea sediments retrieved from the lomonosov ridge 87 n in the central arctic ocean apparently mimic low latitude 18 o glacial interglacial cyclicity thereby providing stratigraphic information that together with biostratigraphic data permit the construction of a detailed chronological model correlation of this mn and color chronology to established apparent brunhes age estimates of geomagnetic excursions reveals a remarkable fit between these two independently derived time scales the mn and color cycles probably provide paleoenvironmental information about material fluxes in the arctic ocean over the past 1 m y we suggest that the primary source for the observed manganese variations in our sediment core is northern siberia which has extensive peat bogs and boreal forests these siberian source areas could operate in an off and on mode tuned to pleistocene glacial and interglacial periods contrasts in ventilation of arctic ocean waters during interglacial glacial cycles probably could also enhance the observed mn and color variability

amino acid epimerization in planktonic foraminifera suggests slow sedimentation rates for alpha ridge arctic ocean interpretations of the palaeoclimatic and palaeooceanographic histories of the arctic ocean1 2 have been hampered by conflicting chronologies obtained using different dating methods magnetostratigraphic and micropalaeontological data1 4 have suggested a sedimentation rate of 1 mm kyr 1 whereas the rate indicated by amino acid epimerization analyses on foraminifera5 is an order of magnitude higher to establish a firm chronology for cores from the alpha ridge arctic ocean and to help resolve the controversy between the ages assigned by different dating methods we have carried out detailed amino acid epimerization analyses on planktonic foraminifera neogloboquadrina pachyderma sinistral from a long gravity core assuming that the temperature in the arctic ocean did not exceed 3 c during the past 1 myr the alloisoleucine isoleucine alle ile ratios obtained from this core suggest a sedimentation rate of 1 mm kyr 1 supporting previous magnetostratigraphic and micropalaeontological studies

new insights on arctic quaternary climate variability from palaeo records and numerical modelling terrestrial and marine geological archives in the arctic contain information on environmental change through quaternary interglacial glacial cycles the arctic palaeoclimate and its extremes apex scientific network aims to better understand the magnitude and frequency of past arctic climate variability with focus on the extreme versus the normal conditions of the climate system one important motivation for studying the amplitude of past natural environmental changes in the arctic is to better understand the role of this region in a global perspective and provide base line conditions against which to explore potential future changes in arctic climate under scenarios of global warming in this review we identify several areas that are distinct to the present programme and highlight some recent advances presented in this special issue concerning arctic palaeo records and natural variability including spatial and temporal variability of the greenland ice sheet arctic ocean sediment stratigraphy past ice shelves and marginal marine ice sheets and the cenozoic history of arctic ocean sea ice in general and holocene oscillations in sea ice concentrations in particular the combined sea ice data suggest that the seasonal arctic sea ice cover was strongly reduced during most of the early holocene and there appear to have been periods of ice free summers in the central arctic ocean this has important consequences for our understanding of the recent trend of declining sea ice and calls for further research on causal links between arctic climate and sea ice

geometry and brittle deformation of the subducting nazca plate central chile and argentina summary we use data from the chile argentina geophysical experiment charge broad band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the south american subduction zone between 30 s and 36 s this region contains a zone of flat slab subduction where the subducting nazca plate flattens at a depth of 100 km and extends 300 km eastward before continuing its descent into the mantle we use a grid search multiple event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the wadati benioff zone these contours reflect slab geometries from previous studies of intermediate depth seismicity in this region with some small but important deviations our hypocentres indicate that the shallowest portion of the flat slab is a

retroarc volcanism in the northern san rafael block 34 35 30 s southern central andes occurrence age and tectonic setting article i nfo one of the major retroarc volcanic provinces in the southern central andes 34 and 37 s is developed in the andean foothills of the san rafael region between the orogenic front and foreland basement uplifts of late miocene age here we present the first comprehensive geochronological study of the quaternary volcanism previously dated mainly on the basis of stratigraphy the new unspiked k ar radiometric and two radiocarbon determinations encompass many volcanic centers most of them monogenetic and of basaltic composition exposed between 34 and 35 30 s the data constrains the basaltic volcanism to between 1 8 ma and the holocene the spatiotemporal distribution of the ages indicates that eruption in the retroarc was episodic with some distinct patterns the orogenic front of the san rafael block is associated with 1 8 0 7 ma volcanic eruptions while the malargue fold and thrust belt front in the andean foothills is related to younger eruptions produced at 0 1 0 01 ma both areas are associated with late cenozoic normal faults that dismembered an uplifted a late miocene peneplain as indicated by younger over older fault relationships between paleozoic rocks and tertiary strata this linkage indicates a major relationship between pleistocene holocene retroarc eruptions of the basaltic centers and extensional collapse of the foreland region that shows a migration of the last volcanic activity towards the trench

partial melting below the magmatic arc in the central andes deduced from geoelectromagnetic field experiments and laboratory data abstract magnetotelluric and geomagnetic deep soudings in northern chile revealed a pronounced high conductivity zone hcz below the western cordillera which constitutes the present magmatic arc with active volcanism of the south american continental margin conductivities in the range of 1 s m are observed the anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km are interpreted in terms of partial melting other geophysical observations such as a zone of low seismic velocities lvz at similar depths high heat flow values 100 mw m2 and a pronounced negative anomaly in the residual gravity field are also considered impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities at sub solidus temperatures electrical behavior is described by defect electrons with an activation energy of 1 34 ev and a conductivity of 2 5 ms m at 900 c model calculations using a modified brick layer model mbl were compared with experimental observations a good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 s m at 1250 c assuming an activation energy of 1 ev the same mbl model is used to calculate melt proportions beneath the western cordillera between 14 and 27 vol of interconnected melt are necessary to explain the observed hcz the stability of the melt rich crust is explained by a dynamic melting crystallisation behavior during crustal anatexis and by magma filled dikes

a geodynamic model linking cretaceous orogeny arc migration foreland dynamic subsidence and marine ingression in southern south america abstract this study synthesizes the tectonomagmatic evolution of the andes between 35 30 s to 48 s with the aim to spotlight early contractional phases on andean orogenic building and to analyze their potential driving processes we examine early tectonic stages of the different fold thrust belts that compose this andean segment additionally we analyzed the spatio temporal magmatic arc evolution as a proxy of dynamic changes in andean subduction during critical tectonic stages of orogenic construction this revision proposes a hypothesis related the existence of a continuous large scale flat subduction setting in cretaceous times with a similar size to the present largest flat slab setting on earth this potential process would have initiated diachronically in the late early cretaceous and achieved full development in late cretaceous to earliest paleocene times constructing a series of fold thrust belts on the retro arc zone from 35 30 s to 48 s moreover we assess major paleogeographic changes that took place during flat slab full development in maastrichtian danian times at this moment an enigmatic atlantic derived marine flooding covered the patagonian foreland reaching as far as the andean foothills based on flexural and dynamic topography analyses we suggest that focused dynamic subsidence at the edge of the flat slab may explain sudden marine ingression previously linked to continental tilting and orogenic loading during a high sea level global stage finally flat subduction destabilization could have triggered massive outpouring of synextensional intraplate volcanic rocks in southern south america and the arc retraction in late paleogene to early neogene times

quaternary history of sea ice and paleoclimate in the amerasia basin arctic ocean as recorded in the cyclical strata of northwind ridge the 19 middle early pleistocene to holocene bipartite lithostratigraphic cycles observed in high resolution piston cores from northwind ridge in the amerasia basin of the arctic ocean provide a detailed record of alternating glacial and interglacial climatic and oceanographic conditions and of correlative changes in the character and thickness of the sea ice cover in the amerasia basin glacial conditions in each cycle are represented by gray pelagic muds that are suboxic laminated and essentially lacking in microfossils macrofossils trace fossils and generally in glacial erratics interglacial conditions are represented by ochre pelagic muds that are oxic and bioturbated and contain rare to abundant microfossils and abundant glacial erratics the synglacial laminated gray muds were deposited when the central amerasia basin was covered by a floating sheet of sea ice of sufficient thickness and continuity to reduce downwelling solar irradiance and oxygen to levels that precluded photosynthesis maintenance of a biota and strong oxidation of the pelagic sediment except during the early part of 3 of the 19 synglacial episodes when it was periodically breached by erratic bearing glacial icebergs the floating arctic ocean sea ice sheet was sufficiently thick to block the circulation of icebergs over northwind ridge and presumably other areas of the central arctic ocean interglacial conditions were initiated by abrupt thinning and breakup of the floating sea ice sheet at the close of glacial time which permitted surges of glacial erratic laden icebergs to reach northwind ridge and the central arctic ocean where they circulated freely and deposited numerous and relatively thick erratic clast rich beds breakup of the successive synglacial sea ice sheets initiated deposition of the interglacial ochre mud units under conditions that allowed sunlight and increased amounts of oxygen to enter the water column resulting in photosynthesis and biologic productivity and strong oxidization of the pelagic sediment the lithostratigraphy of northwind ridge suggests that during at least late pleistocene time glacial conditions in the arctic ocean were initiated abruptly and continued unabated until terminated also abruptly by onset of the succeeding interglacial warming variations in abundance of glacial erratics within the interglacial units of the late pleistocene indicate that during at least most interglacial episodes northern north america was glaciated but with generally diminishing severity until onset of the succeeding continental glaciation magnetostratigraphy suggests that the glacial interglacial cycles on northwind ridge had an average periodicity of approximately 93 5 k y during the brunhes normal and approximately 105 k y during the latter part of the matuyama reverse polarity zone these average periodicities are close to the 100 k y temperature cycles found in north atlantic deep water sediments of the brunhes normal polarity chron which have been ascribed to forcing by a milankovitch eccentricity cycle they are also close however to the average interval 101 k y between the aperiodic glacial terminations in the 500 k y pleistocene continental climate record from devil s hole nevada which have been ascribed to nonlinear feedbacks within the earth s atmosphere ice sheet ocean system

paleomagnetism of quaternary sediments from lomonosov ridge and yermak plateau implications for age models in the arctic ocean abstract inclination patterns of natural remanent magnetization nrm in quaternary sediment cores from the arctic ocean have been widely used for stratigraphic correlation and the construction of age models however shallow and negative nrm inclinations in sediments deposited during the brunhes chron in the arctic ocean appear to have a partly diagenetic origin rock magnetic and mineralogical studies demonstrate the presence of titanomagnetite and titanomaghemite thermal demagnetization of the nrm indicates that shallow and negative inclination components are largely unblocked below 300 c consistent with a titanomaghemite remanence carrier following earlier studies on the mendeleev alpha ridge shallow and negative nrm inclination intervals in cores from the lomonosov ridge and yermak plateau are attributed to partial self reversed chemical remanent magnetization crm carried by titanomaghemite formed during seafloor oxidation of host detrital titanomagnetite grains distortion of paleomagnetic records due to seafloor maghemitization appears to be especially important in the perennially ice covered western mendeleev alpha ridge and central arctic ocean lomonosov ridge and to a lesser extent near the ice edge yermak plateau on the yermak plateau magnetic grain size parameters mimic the global benthic oxygen isotope record back to at least marine isotope stage 6 implying that magnetic grain size is sensitive to glacial interglacial changes in bottom current velocity and or detrital provenance

three dimensional electrical conductivity in the mantle beneath the payun matru volcanic field in the andean backarc of argentina near 36 5 s evidence for decapitation of a mantle plume by resurgent upper mantle shear during slab steepening

electrical conductivity of the pampean shallow subduction region of argentina near 33 s evidence for a slab window 1 we present a three dimensional 3 d interpretation of 117 long period 20 4096 s magnetotelluric mt sites between 31 s and 35 s in western argentina they cover the most horizontal part of the pampean shallow angle subduction of the nazca plate and extend south into the more steeply dipping region sixty two 3 d inversions using various smoothing parameters and data misfit goals were done with a nonlinear conjugate gradient nlcg algorithm a dominant feature of the mantle structure east of the horizontal slab is a conductive plume rising from near the top of the mantle transition zone at 410 km to the probable base of the lithosphere at 100 km depth the subducted slab is known to descend to 190 km just west of the plume but the wadati benioff zone cannot be traced deeper if the slab is extrapolated downdip it slices through the plume at 250 km depth removal of portions of the plume or blocking vertical current flow at 250 km depth significantly changes the predicted responses this argues that the plume is not an artifact and that it is continuous the simplest explanation is that there is a wedge shaped slab window that has torn laterally and opens down to the east with its apex at the plume location stress within the slab and seismic tomography support this shape its northern edge likely explains why there is no deep seismicity south of 29 s

imprint of late pleistocene continental processes visible in ice rafted grains from the central arctic ocean abstract ice rafted quartz sand grains of sediment samples representing marine isotope stages mis 3 and 4 from the lomonosov ridge arctic ocean are here analysed with a scanning electron microscope sem to characterise processes in the sediment source area the microtextural characteristics of the grains were observed and their microtexture frequencies calculated specific sets of microtextures were identified and classified group i iii using principal component analysis pca and cluster analysis the statistically analysed microtextural data were used to identify signals of continental processes that operated during the late pleistocene mis 4 to mis 3 in the eurasian arctic the microtextures of group i generally formed as a result of prolonged fluvial alluvial or aeolian transport the glacial microtextures of groups ii and iii indicate subglacial conditions and transport based on the data two principal components seem to characterise late pleistocene continental environmental conditions glacial and non glacial there is a signal of non glacial continental processes in the ice rafted grains at around 62 64 and 67 ka ago a signal of subglacial processes was observed in the grains at around 26 34 42 and 45 ka and concurrent with non glacial at around 62 and 64 ka ago we suggest that these signals in mis 4 sediments are related to deglaciation with an increased supply of glacial sediment to the arctic ocean whereas during mis 3 they were probably related more to iceberg calving from the oscillating barents kara ice sheet

radiocarbon chronology of depositional regimes in the western arctic ocean abstract the foraminiferal abundance and percentage of coarse ice rafted detritus ird define glacial deglacial and interglacial depositional regimes in ams radiocarbon dated box cores from the western arctic ocean sediment deposition rates are generally less than 0 5 cm ka for glacial regimes greater than this for deglacial regimes and greater than 1 2 cm ka for interglacial regimes these differences in deposition rates might account for the much lower average sedimentation rates for the last 780 ka of 1 3 mm ka in cores from the central arctic ocean if glacial regimes dominated this interval foraminiferal abundances are less than 500 g during glacial maxima and mostly higher than 2000 g during deglacial and interglacial regimes slightly higher coarse ird percentages occur in deglacial intervals 5 10 up to 30 compared with interglacial intervals mostly 5 which characterize the last 10 12 ka in the western arctic ocean glacial regimes occurred from about 40 to 11 ka except for a brief interglacial or deglacial interval around 24 28 ka in the central arctic ocean the coarsest deglacial events occurred prior to 45 ka the late wisconsin deglaciation sediments approximately 9 16 ka are difficult to detect in the central arctic ocean sediments because they are only slightly coarser than the holocene and in some box cores less coarse than the holocene a previously unrecognized coarse ird event occurred near the core tops 0 3 cm between 1500 and 3500 radiocarbon years ago in the western arctic ocean only sediment older than 40 ka is coarser than this recent ird event which might correspond to a neoglaciation recognized in the north atlantic and elsewhere

the temporal evolution of back arc magmas from the auca mahuida shield volcano payenia volcanic province argentina in order to better constrain the temporal volcanic activity of the back arc context in payenia volcanic province pvp argentina we present new k ar dating petrographic data major and trace elements from 23 samples collected on the auca mahuida shield volcano our new data coupled with published data show that this volcano was built from about 1 8 to 1 0 ma during five volcanic phases and that auca mahuida magmas were extracted from at least two slightly different oib type mantle sources with a low partial melting rate the first one containing more garnet was located deeper in the mantle while the second contains more spinel and was thus shallower the high mgo basalts or primitive basalts and the low mgo basalts or evolved basalts produced from the deeper and shallower lherzolite mantle sources respectively are found within each volcanic phase suggesting that both magmatic reservoirs were sampled during the 1 myr lifetime of the auca mahuida volcano however a slight increase of the proportion of low mgo basalts as well as of magmas sampled from the shallowest source can be observed through time similar overall petrological characteristics found in the pleistocene holocene basaltic rocks from los volcanes and auca mahuida volcano suggest that they originated from the same magmatic source consequently it can be proposed that the thermal asthenospheric anomaly is probably still present beneath the pvp finally our data further support the hypothesis that the injection of hot asthenosphere with an oib mantle source signature which was triggered by the steepening of the nazca subducting plate induced the production of a large volume of lavas within the pvp since 2 ma

mantle subducting slab structure in the region of the 2010 m8 8 maule earthquake 30 40 s chile we present a new tomographic model of the mantle in the area of the 2010 m8 8 maule earthquake and surrounding regions increased ray coverage provided by the aftershock data allows us to image the detailed subducting slab structure in the mantle from the region of flat slab subduction north of the maule rupture to the area of overlapping rupture between the 1960 m9 5 and the 2010 m8 8 events to the south we have combined teleseismic primary and depth phase arrivals with available local arrivals to better constrain the teleseismic earthquake locations in the region which we use to conduct nested regional global tomography the new model reveals the detailed structure of the flat slab and its transition to a more moderately dipping slab in the maule region south of the maule region a steeply dipping relic slab is imaged from 200 to 1000 km depth that is distinct from the moderately dipping slab above it and from the more northerly slab at similar depths we interpret the images as revealing both horizontal and vertical tearing of the slab at 38 s to explain the imaged pattern of slab anomalies in the southern portion of the model in contrast the transition from a horizontal to moderately subducting slab in the northern portion of the model is imaged as a continuous slab bend we speculate that the tearing was most likely facilitated by a fracture zone in the downgoing plate or alternatively by a continental scale terrane boundary in the overriding plate

the late jurassic panjeh submarine volcano in the northern sanandaj sirjan zone northwest iran mantle plume or active margin abstract the tectonic setting in which jurassic igneous rocks of the sanandaj sirjan zone sasz of iran formed is controversial sasz igneous rocks are mainly intrusive granodiorite to gabbroic bodies which intrude early to middle jurassic metamorphic basement jurassic volcanic rocks are rare here we report the age and petrology of volcanic rocks from the panjeh basaltic andesitic rocks complex in the northern sasz southwest of ghorveh city the panjeh magmatic complex consists of pillowed and massive basalts andesites and microdioritic dykes and is associated with intrusive gabbros the overall sequence and relations with surrounding sediments indicate that this is an unusually well preserved submarine volcanic complex igneous rocks belong to a metaluminous sub alkaline medium k to high k calc alkaline mafic suite characterized by moderate al2o3 13 7 17 6 wt and variable fe2o3 6 0 12 6 wt and mgo 0 9 11 1 wt contents zircon u pb ages 145 149 ma define a late jurassic tithonian age for magma crystallization and emplacement whole rock compositions are enriched in th u and light rare earth elements lrees and are slightly depleted in nb ta and ti the initial ratios of 87sr 86sr 0 7039 0 7076 and end t values 1 8 to 4 3 lie along the mantle array in the field of ocean island basalts and subcontinental metasomatized mantle immobile trace element ti v zr y nb yb th and co behavior suggests that the mantle source was enriched by fluids released from a subducting slab i e deep crustal recycling with some contribution from continental crust for andesitic rocks based the chemical composition of panjeh mafic and intermediate rocks in combination with data for other gabbroic to dioritic bodies in the ghorveh area we offer two interpretations for these and other jurassic igneous rocks of the sasz as reflecting melts from a subduction modified oib type source above a neo tethys subduction zone or b plume or rift tectonics involving upwelling metasomatized mantle mostly reflecting the 550 ma cadomian crust forming event