petrogenesis of volcanic rocks that host the world class ag pb navidad district north patagonian massif comparison with the jurassic chon aike volcanic province of patagonia argentina abstract we present the first study of the volcanic rocks of the canadon asfalto formation that host the navidad world class ag pb epithermal district located in the north patagonian massif patagonia argentina these volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of gondwana and represent the mafic to intermediate counterparts of the mainly silicic jurassic chon aike volcanic province lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation la icpms u pb ages of zircon crystals from the volcanic units yielded middle jurassic ages of 173 9 1 9 ma and 170 8 3 ma in the navidad district volcanic rocks of the canadon asfalto formation show arc like signatures including high k basaltic andesite to high k dacite compositions rb ba and th enrichment relative to the less mobile hfs elements nb ta enrichment in light rare earth elements lree y ti depletion and high zr contents these characteristics could be explained by assimilation of crustal rocks in the jurassic magmas which is also supported by the presence of zircon xenocrysts with permian and middle upper triassic ages 281 3 ma 246 5 218 1 and 201 3 ma and quartz xenocrysts recognized in these volcanic units furthermore sr and nd isotope compositions suggest a contribution of crustal components in these middle jurassic magmas high k basaltic andesite has initial 87sr 86sr ratios of 0 70416 0 70658 and nd t values of 5 3 and 4 high k dacite and andesite have initial 87sr 86sr compositions of 0 70584 0 70601 and nd t values of 4 1 and 3 2 the range of pb isotope values 206pb 204pb 18 28 18 37 207pb 204pb 15 61 15 62 and 208pb 204pb 38 26 38 43 of navidad volcanic rocks and ore minerals suggest mixing pb sources with contributions of mantle and crust 206pb 204pb isotopic ratios of jurassic volcanic rocks of the chon aike volcanic province and sulfides of associated epithermal deposits increase with time from the volcanic event v1 188 178 ma to volcanic events v2 172 162 ma and v3 157 153 ma reflecting variations in the radiogenic pb source as volcanism was migrating towards the proto pacific margin of gondwana

age of mineralization of the candelaria fe oxide cu au deposit and the origin of the chilean iron belt based on re os isotopes a discussion sir mathur et al 2002 present data on the systematics of re os isotopes at la candelaria and a number of other deposits in the cretaceous iron belt of chile the data provide a new and important confirmation of the idea that the la candelaria ores are related to nearby igneous rocks they have also demonstrated that additional work on this isotopic system has the potential to provide useful information on ore genesis there are however two important points that we would like to make with respect to their interpretations the first involves the origin of magnetite apatite deposits in the cretaceous iron belt and the second involves the age of mineralization at la candelaria three samples from magnetite apatite deposits cerro negro norte cerro iman and el romeral were analyzed for re and os isotopes and yielded calculated initial 187os 188os values 1 5 0 3 to 7 6 0 7 significantly higher than those for la candelaria 0 36 0 10 on the basis of these three analyses mathur et al 2002 have suggested that the magnetite apatite deposits of the chilean iron belt were derived from sedimentary rocks by nonmagmatic brines this interpretation is incompatible with their own assumptions about the sedimentary rocks that would be the potential source for os it is impossible to produce initial 187os 188os between 1 5 and 7 6 by leaching sedimentary rocks with 187os 188os of about 1 0 as assumed by mathur et al 2002 also it is unlikely that iron deposits like cerro negro norte or cerro iman with extremely low re os ratios 12 and 21 respectively could be produced from marine sedimentary rocks with re os of about 250 as assumed by mathur et al 2002 an additional problem with the mineralization model of mathur et al 2002 is that

geochronology of type santacrucian middle tertiary land mammal age patagonia argentina mammal bearing lacustrine and tuffaceous sediments from three localities of the santa cruz formation type fauna of the santacrucian land mammal age in patagonia southern argentina are calibrated by radioisotope dating with the aid of magnetostratigraphy the strata range from about 17 6 ma to perhaps 16 0 ma and are thus of late early miocene age the santacrucian land mammal age ranges from about 18 0 ma to about 15 0 ma

proxy records of holocene storm events in coastal barrier systems storm wave induced markers abstract extreme storm events in the coastal zone are one of the main forcing agents of short term coastal system behavior as such storms represent a major threat to human activities concentrated along the coasts worldwide in order to better understand the frequency of extreme events like storms climate science must rely on longer time records than the century scale records of instrumental weather data proxy records of storm wave or storm wind induced activity in coastal barrier systems deposits have been widely used worldwide in recent years to document past storm events during the last millennia this review provides a detailed state of the art compilation of the proxies available from coastal barrier systems to reconstruct holocene storm chronologies paleotempestology the present paper aims i to describe the erosional and depositional processes caused by storm wave action in barrier and back barrier systems i e beach ridges storm scarps and washover deposits ii to understand how storm records can be extracted from barrier and back barrier sedimentary bodies using stratigraphical sedimentological micro paleontological and geochemical proxies and iii to show how to obtain chronological control on past storm events recorded in the sedimentary successions the challenges that paleotempestology studies still face in the reconstruction of representative and reliable storm chronologies using these various proxies are discussed and future research prospects are outlined

rates timing and cyclicity of holocene eolian activity in north central united states evidence from varved lake sediments most of the sediment components that accumulated in elk lake northwestern minnesota during the holocene are autochthonous or biogenic delivered to the sediment water interface on a seasonal schedule preserved in distinct annual laminae varves the main allochthonous component is detrital clastic material as measured by bulk sediment concentrations of aluminum sodium potassium titanium and quartz that enters the lake mostly as eolian dust the eolian clastic influx to elk lake was considerably greater during the mid holocene 8 4 ka than it has been for the past 4000 yr when periods of increased eolian activity correspond to the time of the little ice age and the dust bowl geochemical records of eolian activity exhibit distinct cyclicities with dominant periodicities of 400 and 84 yr

pacific subduction coeval with the karoo mantle plume the early jurasssic subcordilleran belt of northwestern patagonia the early mesozoic magmatism of southwestern gondwana is reviewed in the light of new u pb shrimp zircon ages 181 2 ma 181 3 ma 185 2 ma and 182 2 ma that establish an early jurassic age for the granites of the subcordilleran plutonic belt in northwestern argentine patagonia new geochemical and isotopic data confirm that this belt represents an early subduction related magmatic arc along the proto pacific margin of gondwana thus subduction was synchronous with the initial phase of chon aike rhyolite volcanism ascribed to the thermal effects of the karoo mantle plume and heralding rifting of this part of the supercontinent overall there is clear evidence that successive episodes of calc alkaline arc magmatism from late triassic times until establishment of the andean patagonian batholith in the late jurassic involved westerly migration and clockwise rotation of the arc this indicates a changing geodynamic regime during gondwana break up and suggests differential rollback of the subducted slab with accretion of new crustal material and or asymmetrical scissor like opening of back arc basins this almost certainly entailed dextral displacement of continental domains in patagonia

aeolian sediment reconstructions from the scottish outer hebrides late holocene storminess and the role of the north atlantic oscillation northern europe can be strongly influenced by winter storms driven by the north atlantic oscillation nao with a positive nao index associated with greater storminess in northern europe however palaeoclimate reconstructions have suggested that the nao storminess relationship observed during the instrumental period is not consistent with the relationship over the last millennium especially during the little ice age lia when it has been suggested that enhanced storminess occurred during a phase of persistent negative nao to assess this relationship over a longer time period a storminess reconstruction from an nao sensitive area the outer hebrides is compared with late holocene nao reconstructions the patterns of storminess are inferred from aeolian sand deposits within two ombrotrophic peat bogs with multiple cores and two locations used to distinguish the storminess signal from intra site variability and local factors the results suggest storminess increased after 1000 cal yrs bp with higher storminess during the medieval climate anomaly mca than the lia supporting the hypothesis that the nao storminess relationship was consistent with the instrumental period however the shift from a predominantly negative to positive nao at c 2000 cal yrs bp preceded the increased storminess by 1000 years we suggest that the long term trends in storminess were caused by insolation changes while oceanic forcing may have influenced millennial variability

deglacial and postglacial vegetation changes on the eastern slopes of the central patagonian andes 47 s abstract we report pollen spore and charcoal records from lago augusta 47 05 s 72 23 w 440 m a s l a small closed basin lake located near the modern forest steppe ecotone east of the andes in central patagonia chile the record shows local ice free conditions through the last glacial termination in the rio chacabuco valley and flooding by an ice dammed lake once this proglacial lake ceased to inundate areas above 450 m a s l the valley was colonized by herbs shrubs and evergreen rainforest taxa between 15 600 and 16 000 cal yr bp indicating an open landscape under cold wet conditions millennial scale fluctuations in the hygrophilous conifer fitzroya pilgerodendron suggest precipitation variations within a cool wet climate between 11 800 and 13 400 cal yr bp followed by the establishment of dense nothofagus forests between 9800 and 11 800 cal yr bp and declines in hygrophilous and cold resistant trees herbs and shrubs this interval coincided with peak fire activity and laminated carbonate deposition nothofagus forests have persisted with little variation since 9800 cal yr bp except for a sudden decline associated with a rapid increase in rumex cf acetosella an exotic weed introduced by europeans our results and interpretations are best explained by changes in the strength position of the southern westerly winds at millennial and multi millennial timescales since the last glaciation contrary to previous interpretations we propose increased precipitation of westerly origin in the andean sector of central east patagonia between 11 800 and 16 000 cal yr bp followed by a decline between 9800 and 11 800 cal yr bp and an increase thereafter these trends are coherent with variations of the southern westerly winds identified in other terrestrial mid latitude records suggesting zonally symmetric changes in atmospheric circulation since the last glaciation

seismic ridge subduction and topography foreland deformation in the patagonian andes abstract the patagonian andes recorded several episodes of active ridge subduction in the last 80 million years an analysis of the spatial and temporal relation between the present segment of collision of the chile ridge and the digital topography of the foreland shows a correlation with the beginning of deformation and uplift in the inner sector of the patagonia fold and thrust belt several magmatic episodes related to the collision such as near trench magmatism adakite emplacement oib plateau basalts in the retroarc and the arc volcanic gap are associated with the uplift and deformation of the patagonian cordillera based on these correlations a collision of the aluk or phoenix farallon ridge during paleogene times south of 43 30 is identified changes in magmatic patterns molasses deposits deformation and uplift of the patagonian cordillera constrain the region affected by the collision similar evidence implies a third period of collision in the late cretaceous based on the occurrence of adakitic rocks arc magmatic gap and deformation along the southern patagonian andes this earliest hypothesized collision would require the existence of a new oceanic microplate between the pacific and the aluk plates during late cretaceous times present rapid isostatic rebound related to the continental ice cap retreat in the patagonian andes is restricted to the region south of chile triple junction 46 30 s the uplift rate here is more than two times more rapid than normal isostatic rebounds recorded in the northern hemisphere and requires an abnormally hot mantle with low viscosity this abnormal mantle may be a consequence of several episodes of ridge collision and development of asthenospheric windows that are inconsistent with periods of cold flat slab subduction proposed by some authors to explain the arc volcanic gaps

aeolian sediment in raised bog deposits halland sw sweden a new proxy record of holocene winter storminess variation in southern scandinavia cores of peat taken from two raised bogs in the near coastal part of halland sw sweden were examined for their content of wind transported clastic material the boarps mosse core has a length of 262 cm and covers the last 2850 years age model based on three radiocarbon dates while the hyltemossen core has a length of 347 cm and records the last 7000 years age model based on eight radiocarbon dates the peat contains aeolian quartz silt and sand from trace amount to more than 10 weight of the dry peat sample and the grain size composition of the minerogenic sediment indicates that the material is composed of both far travelled dust and local wind eroded silt and sand grains larger than 0 2mm are present in many intervals and probably record niveo aeolian transport in connection with severe snowstorms a systematic count of quartz grains between 0 2 0 35mm and larger than 0 35mm was carried out to determine variations in aeolian sand influx asi and winter wind climate with time the boarps m

how to quantify long term changes in coastal sea storminess abstract the paper discusses various aspects of local and regional storminess as well as challenges in quantifying its tendencies in the eastern section of the baltic sea the study is based on various long term data sets such as wind and sea level observation data from coastal stations and tide gauges of estonia 1899 2013 it also discusses long term wave hindcasts 1966 2013 reconstructions of shoreline changes as well as the limitations and possible shortcomings in each of the data sets mostly located on the westerly exposed and windward coast of the semi enclosed baltic sea the case study area is sensitive to wind and storm climate changes different manifestations of storminess delivered somewhat specific results when analyzed together the first principal component of storminess was connected to the intensity of regional cyclonic activity along the polar front and had relatively high stages in 1920s 1940s 1980s 1990s and probably also from 2010 the second component carried a local and more random imprint appearing independently of the high nao or general storminess phases the most notable event driven extremes in the record were related to this component based on our data we cannot confirm the existence of any general increasing long term trends for storminess wherever the increase exists it has occurred as a winter time increase accompanied by the shift of the stormy season from autumn to winter the spatially contrasting results for westerly and northerly exposed coasts reflect the corresponding variations in local wind which are connected to the changes in storm track activity above northern europe and a poleward shift of cyclones trajectories

un coupled thrust belt foreland deformation in the northern patagonian andes new insights from the esquel gastre sector 41 30 43 s the patagonian andes represent a unique natural laboratory to study surface deformation in relation to deep slab dynamics in the sector comprised between latitudes 41 30 and 43 s new apatite u th he ages indicate a markedly different unroofing pattern between the broken foreland area characterized by late cretaceous to paleogene exhumation and the adjacent andean sector to the west which is dominated by miocene pliocene exhumation these unroofing stages can be confidently ascribed to inversion tectonics involving reverse fault related uplift and concomitant erosion late cretaceous paleogene shortening and exhumation are well known to have affected also the thrust belt sector of the study area during a prolonged stage of flat slab subduction therefore the different ages of near surface unroofing documented in this study suggest coupling of the deformation between the thrust belt and its foreland during periods of flat slab subduction e g during late cretaceous paleogene times and dominant uncoupling during periods of steep slab subduction and rollback even when these are associated with high convergence rates i e 4 cm year as those documented in miocene times for the patagonian andes

tectonic development of the north patagonian andes and their related miocene foreland basin 41 30 43 s 1 the northern patagonian andes have been constructed through multiple mechanisms that range from tectonic inversion of extensional structures of early to middle jurassic age in the main andes to oligocene in the precordilleran region these have acted during two distinctive orogenic stages first in late early cretaceous and later in miocene times late oligocene extension separates these two contractional periods and is recorded by half grabens developed in the retroarc region the last contractional stage coexists with an eastward foreland expansion of the late miocene arc whose roots are presently exposed as minor granitic stocks and volcanic piles subordinately in the main andes east of the present arc as a consequence of this orogenic stage a foreland basin has developed having progressed from 18 ma in the main north patagonian andes where the mountain front was flooded by a marine transgression corresponding to the base of the nirihuau formation to 11 ma in the foreland area cannibalization of this foreland basin occurred initially in the hinterland and then progressed to the foreland zone blind structures formed a broken foreland at the frontal zone inferred from growth strata geometries during pliocene to quaternary times most of the contractional deformation was dissipated in the orogenic wedge at the time when the arc front retracted to its present position

the chronology of the last glacial maximum and deglacial events in central argentine patagonia this paper evaluates the chronology of the last glacial cycle and deglaciation in the lago pueyrredon valley of central patagonia 47 5 s argentina the valley was a major outlet of the former patagonian ice sheet and the moraines that record its fluctuations are an important proxy record of climate change in southern south america such moraines are well preserved in the lago pueyrredon valley owing in part to the semi arid environment east of the mountain front here we provide the first direct chronology for the age of the rio blanco moraine system by utilizing cosmogenic nuclide surface exposure ages boulders on the moraines give 10be exposure ages that indicate the last glacial maximum lgm maximum extent occurred by 27 25 ka subsequent advances occurred at 23 22 ka 20 18 ka and ca 18 17 ka initial deglaciation began after ca 18 17 ka and was interrupted as evidenced by the lago columna moraines up valley subsequently the outlet glaciers occupying both the lago pueyrredon basin chilean name lago cochrane and the lago buenos aires basin chilean name lago general carrera to the north rapidly retreated more than 80 km at around 16 5 15 ka the timing of the lgm maximum extent and the onset of deglaciation occurred broadly synchronously throughout patagonia deglaciation resulted in a series of interconnected glacier dammed lakes in the region that initially drained toward the atlantic ocean and later drained to the pacific ocean as a consequence of disintegrating ice in the andes

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

past changes in the north atlantic storm track driven by insolation and sea ice forcing changes in the location of northern hemisphere storm tracks may cause significant societal and economic impacts under future climate change but projections of future changes are highly uncertain and drivers of long term changes are poorly understood here we develop a late holocene storminess reconstruction from northwest spain and combine this with an equivalent record from the outer hebrides scotland to measure changes in the dominant latitudinal position of the storm track the north south index shows that storm tracks moved from a southern position to higher latitudes over the past 4000 yr likely driven by a change from meridional to zonal atmospheric circulation associated with a negative to positive north atlantic oscillation shift we suggest that gradual polar cooling caused by decreasing solar insolation in summer and amplified by sea ice feedbacks and mid latitude warming caused by increasing winter insolation drove a steepening of the winter latitudinal temperature gradient through the late holocene resulting in the observed change to a more northern winter storm track our findings provide paleoclimate support for observational and modeling studies that link changes in the latitudinal temperature gradient and sea ice extent to the strength and shape of the circumpolar vortex together this evidence now suggests that north atlantic winter storm tracks may shift southward under future warming as sea ice extent decreases and the mid to high latitude temperature gradient decreases with storms increasingly affecting southern europe

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

magmatism and tectonics in continental chiloe chile 42 42 30 s the chiloe chonos region seems to preserve the oldest depositional events in the fore arc accretionary complex of the southeast pacific margin there are isolated occurrences of low grade metamorphic rocks including slates with a devonian trilobite fauna and schists that give rb sr evidence of a ca 290 ma metamorphism pillow basalts and ultramafic rocks may represent parts of the pan thalassic ocean floor on which the palaeozoic sediments were laid down emergence of a magmatic arc is indicated by jurassic to early cretaceous volcanogenic and marine deposits during the mid cretaceous climax of plutonic activity these were intruded by monzogranites which here constitute the eastern portion of the north patagonian batholith they give rb sr isochron ages of 120 100 ma barremian albian initial 87sr 86sr ratios of 0 7040 0 7045 and ndt values of 0 5 to 1 5 indicate a simple petrogenesis with a mantle source the western part of the batholith is petrologically more primitive being composed predominantly of tonalite diorite and gabbro and initial 87sr 86sr ratios are more variable late cenozoic movement of the liquine ofqui fault zone lofz generated deep pull apart basins to the west of the uplifted batholith basement complex these were filled by thick marine sequences of volcanogenic debris indicating the wide extent of a mainly rhyolitic volcanic field during miocene times pliocene tonalite and granodiorite plutons dated by a rb sr whole rock isochron at 4 7 0 5 ma and holocene andesite basalt stratovolcanoes are located along the lofz the latter feature has thus been a major influence on the tectonic evolution of the area there is no evidence for major post palaeozoic compression or crustal shortening

storm flood impacts along the shores of micro tidal inland seas a morphological and sedimentological study of the vesterlyng beach the belt sea denmark abstract the morphological and sedimentological impacts of an extreme storm event bodil in december 2013 were investigated at a mixed sand and gravel beach vesterlyng in a micro tidal environment along the belt sea denmark the storm response was characteristic for mixed sand and gravel beaches gravelly storm berms with sandy to gravelly washover fans were formed at the upper beach where swash processes dominated during extreme water levels in combination with overtopping and overwash in addition storm scarps and gravel flats were formed the elevation of the storm berm crest was identical to a summation of the still water level 1 60 m in the belt sea during the storm flood and the wave run up 0 75 m at the shore the crest of 2 35 m is therefore a precise measure of the storm flood level this storm flood was classified as a large scale extreme event in a holocene perspective it is suggested that storm berm beach ridge heights above contemporaneous sea level can be used as a proxy of past flood events along micro tidal shores

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

pliocene extensional tectonics in the eastern central patagonian cordillera geochronological constraints and new field evidence recent field work and review of radiometric data obtained from neogene lavas and plutonic rocks exposed in the eastern central patagonian cordillera 46 48os which overlie subducted segments of the south chile ridge suggest important late miocene to pleistocene morphological changes in relation to base level variations and or tectonic events we present new field observations from a region south of lago general carrerabuenos aires between the main cordillera and the meseta del lago buenos aires demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5 4 ma and after 3 ma we also show that the 12 ma basaltic flows of the meseta del lago buenos aires 2000 m a s l have been subjected to deep incision with younger lavas dated at 1 2 ma partially filling the valleys these incisions are thought to reflect progressive eastward tilting of the entire meseta our new observations together with additional features from central patagonia strongly suggest that tectonic events led to a regional widespread morphological change after 5 ma the coincidence in time and space between the subduction of segments of the south chile ridge at 6 and 3 ma causing opening of a slab window and strong base level variations in the studied area suggests a cause and effect relationship in central patagonia compressional tectonics ended well before extensional events reported here causes of uplift and further extension are probably completely disconnected the uplift is purely tectonic in origin and occurred prior to the subduction of the south chile ridge extension should be a consequence of this subduction

the patagonian batholith s of tierra del fuego chile timing and tectonic implications reconnaissance field studies on the patagonian batholith exposed s of tierra del fuego together with 49 new k ar and rb sr mineral dates render new insights into two main aspects of the late mesozoic geology the geochronology and composition of the batholith and time relations between the emplacement of the plutons and deformation the envelope of the batholith in this area is mainly composed of late jurassic early cretaceous sedimentary and volcanic rocks formed in an island arc marginal basin system three main plutonic groups have been recognized which have yielded mineral dates interpreted as crystallization ages ranging from 141 to 34 ma the earliest is a gabbroic assemblage including gabbros diorites and hornblendites exposed as relicts within later granitoids and with dates ranging from 141 to 103 ma the canal beagle plutonic group is composed of granitoids characterized by a penetrative syn magmatic foliation and with dates of 113 to 81 ma the seno ano nuevo plutonic group is formed of granitoids with dates of 60 to 34 ma and is mainly exposed in the outer islands indicating a local oceanwards shift of plutonism with time lower cretaceous island arc volcanism was contemporaneous with the intrusion of the gabbroic assemblage and in part at least of the canal beagle plutonic group folding of the marginal basin infill which has been related to the closure of the marginal basin occurred from about 100 to 93 ma ago the gabbroic assemblage was apparently mainly pre tectonic whereas the seno ano nuevo plutonic group was clearly intruded after this deformational event a post tectonic or possibly late tectonic emplacement for the youngest plutons of the canal beagle plutonic group and a pre or syn tectonic intrusion for the oldest is inferred

tectonic evolution of the north patagonian andes 41 44 s through recognition of syntectonic strata abstract the north patagonian fold thrust belt 41 44 s is characterized by a low topography reduced crustal thickness and a broad lateral development determined by a broken foreland system in the retroarc zone this particular structural system has not been fully addressed in terms of the age and mechanisms that built this orogenic segment here new field and seismic evidence of syntectonic strata constrain the timing of the main deformational stages evaluating the prevailing crustal regime for the different mountain domains through time growth strata and progressive unconformities controlled by extensional or compressive structures were recognized in volcanic and sedimentary rocks from the cordilleran to the extra andean domain these data were used to construct a balanced cross section whose deep structure was investigated through a thermomechanical model that characterizes the upper plate rheology our results indicate two main compressive stages interrupted by an extensional relaxation period the first contractional stage in the mid cretaceous inverted jurassic lower cretaceous half graben systems reactivating the western canadon asfalto rift border 500 km away from the trench at a time of arc foreland expansion for this stage available thermochronological data reveal forearc cooling episodes and global tectonic reconstructions indicate mid ocean ridge collisions against the western edge of an upper plate with rapid trenchward displacement widespread synextensional volcanism is recognized throughout the paleogene during plate reorganization retroarc paleocene eocene flare up activity is interpreted as product of a slab rollback and fore to retroarc oligocene slab asthenospheric derived products as an expression of enhanced extension the second stage of mountain growth occurred in miocene time associated with nazca plate subduction reaching nearly the same amplitude than the first compressive stage extensional weakening of the upper plate predating the described contractional stages appears as a necessary condition for abnormal lateral propagation of deformation

reconstruction of past storminess evaluation of an indicator approach using aeolian mineral grains buried in peat deposits estonia abstract this study aims to evaluate a reconstruction method of past storminess using aeolian sand influx asi in peat deposits we use high resolution records of asi from four peat bogs along the estonian coast and compare those with local instrumentally recorded storminess data since 1920 ad and with historical climatic records around the baltic sea over the last two centuries potential effects of local changes in forest cover and anthropogenic disturbances on the asi signals are also evaluated with historical maps available for the study sites diameter of all sand grains 65 m is measured with an image analysis system to detect temporal changes in the grain size distribution our results demonstrate two major periods of high asi over the last centuries in the region during the 19th century and since the 1950s with highest values in 1980 2000 ad cross correlations between the changes in asi and the instrumentally recorded storm days during the last century are positive and significant high asi values recorded during the 19th century correspond to the positive nao indices and high cyclonic activity reconstructed in southern sweden we find that sand grains

the origin of the loncopue trough in the retroarc of the southern central andes from field geophysical and geochemical data abstract this work analyzes through field geophysical geochemical and geochronological data a particular setting of the southern andes in which an extensional system is flanking the mountain front this setting is represented by the loncopue trough whose origin is discussed for this part of the andes this work reconstructs 3 dimensionally the structure of this basin and its evolution through time since jurassic times as a series of isolated depocenters to late cretaceous eocene times with the construction of a positive relief that subsequently was relaxed through two extensional stages during the late oligocene early miocene and the pliocene quaternary respectively the last episode of extension is characterized by an initial stage with a series of caldera collapses in the latest pliocene early quaternary this passed to a stage of development of a broad basaltic plateau in pre and inter glacial times circumscribed with new radiometric data to the pleistocene quaternary products in northern and central loncopue trough have chemical relations that are intermediate between the arc front and the southern loncopue trough volcanic rocks studied in previous works thus retroarc eruptions would be part of an extended arc configuration that goes from typical arc series in the northern part of the trough to within plate series in the south low elastic thicknesses computed from gravity data in this work correlate with the area of retroarc volcanic activity magnetic data have allowed determining the curie isotherm showing two areas of relatively abnormal heat flow one along the loncopue trough itself and the other in the foreland zone this scenario is discussed through three main hypotheses an occurrence linked to a slab steepening after a shallow subduction in the area co seismic crustal stretching linked to giant earthquakes in the subduction zone and finally slab tearing associated with asthenospheric upwelling

plate reconstructions aseismic ridges and low angle subduction beneath the andes low angle subduction segments beneath peru and chile are believed to represent interaction of the subduction zone with the aseismic nazca and juan fernandez ridges on the basis of symmetric sea floor spreading models and plate hotspot reconstructions predicted continuations of the relatively buoyant ridges correspond well with the limits of the low angle subduction segments further the history of interaction of the aseismic ridges with the subduction zone as predicted by plate reconstructions is not inconsistent with available information on volcanic episodes of the andes if the contemporary correlation of volcanic gaps and low angle subduction is also applicable to the rest of the late cenozoic the occurrence of deep seismicity beneath the eastern flanks of the andes is also believed to reflect the effects of low angle subduction the deep seismic segments are adjacent to segments which are presently or were recently experiencing low angle subduction thus the deep seismicity is interpreted as representative of remnants of normal oceanic lithosphere subducted before interaction of aseismic ridges with the subduction zone other geologic phenomena are associated with low angle subduction beneath south america including landward shift in loci of tectonism surficial subsidence and laramide style deformation the aseismic ridge buoyancy hypothesis appears to be a valuable predictive tool for the interpretation of such geologic effects

storminess variation during the last 6500 years as reconstructed from an ombrotrophic peat bog in halland southwest sweden cores taken from an ombrotrophic peat bog in the coastal zone of halland southwest sweden were examined for wind transported mineral grains pollen and humidity indicators the core covers the period from 6500 cal yr bp to present ombrotrophic conditions existed from ca 4200 cal yr bp onwards bog surface wetness fluctuated strongly until ca 3700 cal yr bp with an apparent dominance of dry summer conditions from 4800 4500 cal yr bp local wet shifts occurred around 4300 2800 2400 and 1500 cal yr bp whereas the most recent 600 years of the record show increasingly dry conditions mineral grain content interpreted as aeolian sand influx asi was used as a proxy for winter storm frequency and intensity until ca 1500 cal yr bp after which increasing human impact as reconstructed by pollen analysis became a second important potential cause for increased sand drift strongly increased storminess occurred at 4800 4200 2800 2200 1500 1100 and 400 50 cal yr bp indicating a dominance of cold and stormy winters during these periods many of these storm periods apparently coincide with storm events in other sites in southwestern scandinavia suggesting that our asi record reflects a regional scale climatic signal furthermore these stormy periods correlate to well known cold phases in the north atlantic region suggesting a link to large scale fluctuations in atmospheric circulation patterns less

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

cretaceous slab segmentation in southwestern gondwana the mesozoic austral basin of patagonia in southwestern gondwana experienced a major tectonic segmentation during aptian times sometime between 121 and 118 ma aptian the northern part of the austral basin known as the aisen basin or rio mayo embayment was inverted with the sediments overlain by calc alkaline subaerial volcanic rocks of aptian to maastrichtian age in the southern segment of the austral basin known as the magallanes basin predominantly marine sediments accumulated until cenozoic times in a back arc position relative to a magmatic arc located to the west the subduction related n s trending volcanic chains of both segments were geographically displaced during aptian to late cretaceous times in the aisen segment north of 49 50 s the volcanic chain was located further east than the coeval arc in the magallanes segment a transform fault connected the trenches of both segments with the aisen segment dipping at a shallower angle than the magallanes segment

a morphotectonic analysis of central patagonian cordillera negative inversion of the andean belt over a buried spreading center 1 we use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the central cordillera of patagonia between 46 s and 48 s in a region overlying subducted segments of the chile spreading ridge this region shows a number of peculiar features not expected in a normal subduction related belt located away from a buried spreading center first it is characterized by overall short wavelength topography and highly contrasted reliefs typical of deeply incised young mountain belts but surprisingly also by an average very low elevation of 900 m second the cordillera lacks central high reliefs instead there is a central depressed domain with average low slope gradients including the deepest point of the study area 300 m below sea level in the bottom of lake general carrera buenos aires this depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips to the east the central patagonian cordillera is now separated from the flat plains of the pampean domain representing its former piedmont and several elevated basaltic plateaus mesetas by 160 n 180 n trending morphotectonic corridors and the western border scarps of the mesetas these scarps correspond to neotectonic features controlled by recent vertical faults mostly active after 3 ma remnants of a former peneplain the avellanos surface are widespread within the entire cordilleran region from geological evidence this peneplain formed between 15 and 4 ma and was connected originally with the mesetas domain and now is dissected by these corridors it is shown that a negative inversion related to extensional and strike slip faulting occurred in the frontal region of the cordillera at 3 ma synchronously with the emplacement of magmatic products timing of the extensional deformation in central patagonia is concomitant with the migration at depth of different segments of the south chile ridge which entered the trench at 6 and 3 ma respectively and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions finally migration of segments of the chile spreading ridge since 6 ma triggered regional uplift of the cordillera followed by limited extension in relation with increased thermal weakening of the crust

sand mass accumulation rate as a proxy for wind regimes in the sw barents sea during the past 3 ka aeolian sand mass accumulation rates were determined in a coastal mire complex on the north norwegian island soroya this measure of aeolian activity is used as a proxy for past storm intensity in the sw barents sea region at least until the seventeenth century when human activities began to obscure the natural signal aeolian erosion started to increase in the sixth century and consistently high values are recorded from the eleventh century onwards periods of relatively increased aeolian activity can be seen 1200 800 bc 550 150 bc ad 500 850 ad 1000 1400 and ad 1700 2000 increased aeolian activity here indicates a higher frequency of east tracking storm systems at 70 n potential causes for such conditions are increased advection of atlantic water and intensification and or northward displacement of the main winter storm tracks normally associated with moist westerlies and positive nao conditions other noteworthy climatic events are 1 a short but severe wet spell is recorded in the late tenth

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

continental response to active ridge subduction 1 apatite fission track ages from a 2000 m elevation transect from the patagonian fold and thrust belt 47 5 s allow us to quantify the denudational and orographic response of the upper plate to active ridge subduction accelerated cooling started at 17 ma predating the onset of ridge collision 14 10 ma and was followed by reheating between 10 and 6 ma thermal modeling favors reheating on the order of 60 c at 28 c ma due to east migration of a slab window after the ridge trench collision final rapid cooling since 4 ma of 18 c ma geothermal gradient of 14 c km correlates with the presence of an orographic barrier and 1 km rock uplift in this region between 17 1 and 6 3 ma increased precipitation and erosion since 4 ma caused asymmetric exhumation with 3 4 km on the leeside repeated crustal unroofing in response to active ridge subduction can explain the positive gravity anomaly south of the chile triple junction

cordilleran slab windows the geometry and geologic implications of subducted spreading ridges are topics that have bedeviled earth scientists ever since the recognition of plate tectonics as a consequence of subduction of the kula farallon and east pacific rises slab windows formed and migrated beneath the north american cordillera the probable shape and extent of these windows which represent the asthenosphere filled gaps between two separating subducting oceanic plates are depicted from the late cretaceous to the present possible effects of the existence and migration of slab windows on the cordillera at various times include cessation of arc volcanism and replacement by rift or plate edge volcanism lithospheric uplift attenuation and extension and increased intensity of compressional tectonism eocene extensional tectonism and alkaline magmatism in southern british columbia and the northwestern united states were facilitated by slab window development

stratigraphic and structural evolution of the tertiary cosmelli basin and its relationship to the chile triple junction at the taitao peninsula in southern chile 46 5 s an active mid ocean ridge is being subducted under the south american continent continental crust south of the current triple junction has experienced subduction of three ridge segments since the mid miocene a series of mid late tertiary sedimentary basins lies inboard of the triple junction the cosmelli basin lies in the region which has experienced the earlier ridge subduction events within the cosmelli basin fill abrupt facies dislocations give clear evidence of marked base level changes four sequence boundaries are delineated with an upward increase in degree of facies dislocation which is believed to reflect increasing magnitudes of base level fall the lower part of the basin fill is folded and then thrust eastward as a series of imbricate slices while the overlying greater thickness of fluvial sediments is only gently tilted westwards this geometry indicates that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited these processes controlled accommodation space and thus stratigraphic architecture during much of the basin life this complex basin history plus the presence of basaltic sheet intrusions may reflect the underlying slab history as successive ridge segments were subducted

petrogenesis of extension related alkaline volcanism in karaburhan sivrihisar eskisehir nw anatolia turkey abstract alkaline lavas were erupted as phonolites and trachytes around karaburhan sivrihisar eskisehir nw anatolia within the izmir ankara erzincan suture zone these volcanic rocks were emplaced as domes close and parallel to the ophiolite thrust line according to 40ar 39ar geochronological analyses of sanidine crystals from the phonolites the age of the alkaline volcanics is 25 ma late oligocene early miocene the flow textured phonolites are porphyritic and consist mainly of sanidine clinopyroxene and feldspathoid crystals the clinopyroxenes show compositional zoning with aegirine na0 82 0 96fe 30 68 0 83 rims and aegirine augite cores containing calcium magnesium and fe 2 some aegirine augites are replaced with sodium calcium and magnesium rich amphibole hastingsite feldspathoid hauyne crystals enriched with elemental na and ca have been almost completely altered to zeolite and carbonate minerals the fine grained trachytes with a trachytic texture consist of feldspar oligoclase and sanidine phenocrystals and clinopyroxene microphenocrystals within a groundmass made up largely of alkali feldspar microlites although there are some differences in their element patterns the phonolites and trachytes exhibit enrichment in liles sr k rb ba th and lrees la ce pr nd and negative anomalies in nb and ta these geochemical characteristics indicate a lithospheric mantle enriched by fluids extracted from the subduction component in addition the high 87sr 86sr 0 706358 0 708052 and low 143nd 144nd 0 512546 0 512646 isotope concentrations of the alkaline lavas reflect a mantle source that has undergone metasomatism by subduction derived fluids petrogenetic modeling indicates that the alkaline lavas generated from the subduction modified lithospheric mantle have undergone assimilation fractional crystallization and crustal contamination acquiring high pb ba rb and sr contents and pb isotopic compositions during their ascent through the thickened crust in an extensional setting

persistent non solar forcing of holocene storm dynamics in coastal sedimentary archives considerable climatic variability on decadal to millennial timescales has been documented for the past 11 500 years of interglacial climate1 2 3 this variability has been particularly pronounced at a frequency of about 1 500 years with repeated cold intervals in the north atlantic1 3 however there is growing evidence that these oscillations originate from a cluster of different spectral signatures4 ranging from a 2 500 year cycle throughout the period to a 1 000 year cycle during the earliest millennia here we present a reappraisal of high energy estuarine and coastal sedimentary records from the southern coast of the english channel and report evidence for five distinct periods during the holocene when storminess was enhanced during the past 6 500 years we find that high storm activity occurred periodically with a frequency of about 1 500 years closely related to cold and windy periods diagnosed earlier1 2 3 we show that millennial scale storm extremes in northern europe are phase locked with the period of internal ocean variability in the north atlantic of about 1 500 years4 however no consistent correlation emerges between spectral maxima in records of storminess and solar irradiation we conclude that solar activity changes are unlikely to be a primary forcing mechanism of millennial scale variability in storminess

spatial gaps in arc volcanism the effect of collision or subduction of oceanic plateaus abstract one of the major processes in the formation and deformation of continental lithosphere is the process of arc volcanism the plate tectonic theory predicts that a continuous chain of arc volcanoes lies parallel to any continuous subduction zone however the map pattern of active volcanoes shows at least 24 areas where there are major spatial gaps in the volcanic chains 200 km a significant proportion 30 of oceanic crust is subducted at these gaps all but three of these gaps coincide with the collision or subduction of a large aseismic plateau or ridge the idea that the collision of such features may have a major tectonic impact on the arc lithosphere including cessation of volcanism is not new however it is not clear how the collision or subduction of an oceanic plateau perturbs the system to the extent of inhibiting arc volcanism three main factors necessary for arc volcanism are 1 source materials for the volcanics either volatiles or melt from the subducting slab and or melt from the overlying asthenospheric wedge 2 a heat source either for the dehydration or the melting of the slab or the melting within the asthenosphere and 3 a favorable state of stress in the overlying lithosphere the absence of any one of these features may cause a volcanic gap to form there are several ways in which the collision or subduction of an oceanic plateau may affect arc volcanism the clearest and most common cases considered are those where the feature completely resists subduction causing local plate boundaries to reorganize this includes the formation of new plate bounding transform faults or a flip in subduction polarity in these cases subduction has slowed down or stopped and the lack of source material has created a volcanic gap there are a few cases most notably in peru chile and the nankai trough where the dip of subduction is so shallow that effectively no asthenospheric wedge exists to produce source material for volcanism the shallow dip of the slab may be a buoyant effect of the plateau imbedded in the oceanic lithosphere the cases which are the most enigmatic are those where subduction is continuous the oceanic plateau is subducted along with the slab and the dip of the slab is clearly steep enough to allow arc volcanism yet a volcanic gap exists in these areas the subducted plateau may have a fundamental effect on the physical process of arc volcanism itself the presence of a large topographic feature on the subducting plate may affect the stress state in the are by increasing the amount of decoupling between the two plates alternatively the subduction of the plateau may change the chemical processes at depth if either the water rich top of the plateau with accompanying sediments are scraped off during subduction or if the ridge is compositionally different

patagonian broken foreland and related synorogenic rifting the origin of the chubut group basin abstract the central patagonia is characterized by prominent continental deposits that belong to the cretaceous chubut group basin whose tectonic setting remains controversial it has been interpreted as a foreland basin an extensional basin and even as a sag stage of a rift event related deposits outcrop as part of the patagonian broken foreland east of the patagonian andes particulary the san bernardo fold and thrust belt between 42 and 48 s as the main sector of the broken foreland constitutes a nnw trending intraplate belt located 450 km away from the trench coincidentally at these latitudes 1200 km east of the trench intraplate deformation has caused tectonic inversion of mesozoic basins presently located at the atlantic offshore area additionally at the san jorge gulf area this orogenic system interferes with a conspicuous e w extensional system transversally disposed to the andean strike mainly active between the late early cretaceous and late paleocene we carried out an integrated analysis from structural 2 d and 3 d seismic data at the san bernardo ftb and the adjacent rio mayo basin studying sections of the chubut group basin to unravel the different evolutionary stages thus surface and subsurface evidence of syn compressional deposition indicate the development of a broken foreland basin related to the formation of the san bernardo ftb suggesting an initial growth during the late early cretaceous and probably up to the late paleocene our results imply that the cretaceous compression acted in concert with transversal foreland extension hence we propose a syncontractional rift reactivation of a potential lithospheric anisotropy triggered by regional contraction finally the origin of compression during this time is discussed in relation to multiple processes that go from high convergence between plates and trenchward motion of south america collision of mid ocean ridges to potential dip changes in the subducted slab as suggested by arc expansion

significance of 40ar 39ar encapsulation ages of metapelites from late palaeozoic metamorphic complexes of aysen chile the ages obtained by the 40 ar 39 ar encapsulation technique retention and total gas ages on 2 m fractions of five metapelites from the eastern andean metamorphic complex and two from the chonos metamorphic complex allow discussion of the latest recorded metamorphic event in each zone the kubler index ki of illite muscovite principal component of the metapelites varies between 0 15 and 0 45 2 indicating regional variation from diagenetic to epizonal metamorphic grade the 40 ar 39 ar encapsulation analyses reveal 39 ar loss varying between 21 and 25 which shows a limited positive correlation with ki values the obtained retention and total gas metapelite ages reflect distinct metamorphic conditions retention ages most probably indicate burial or regional metamorphic events without plutonic influence in the southern eastern andean metamorphic complex total gas ages reflect contact metamorphic ages for metapelites close to intrusions in the northern and southern eastern andean metamorphic complex and in the chonos metamorphic complex the thermal overprinting of metapelites occurred in early cretaceous times at 130 ma and 145 ma and is related to the contact metamorphism of an emplacement pulse of the north patagonian batholith total gas metapelite ages obtained from the western belt of the chonos metamorphic complex suggest a thermal event related to a distinct pulse of the north patagonian batholith

comparison of laser grain size analysis with pipette and sieve analysis a solution for the underestimation of the clay fraction classically the grain size of soil and sediment samples is determined by the sieve method for the coarse fractions and by the pipette method based on the stokes sedimentation rates for the fine fractions results from the two methods are compared with results from laser diffraction size analysis which is based on the forward scattering of monochromatic coherent light from a point of view of laboratory efficiency the laser sizing technique is far superior accuracy and reproducibility are shown by measurements on certified materials it appears that laser grain size measurements of certified materials correspond very well with the certificated measurements tests were also done on a set of randomly selected sediments of fluvial aeolian and lacustrine origin except for the 2 m clay fraction there is a coarsening of the mean diameter of one to two size classes 0 25 caused by the non sphericity of the particles the platy form of the clay particles induces considerable differences eight size classes between pipette and laser measurements the 2 m grain size defined by the pipette method corresponds with a grain size of 8 m defined by the laser particle sizer for the studied sediments using a higher grain size level for the clay fraction when laser analysis is applied enables workers in the geological and environmental field to compare classical pipette analysis with a laser sizing technique

palaeomagnetism applied to magnetic anomaly interpretation a new twist to the search for mineralisation in northern chile the chilean iron belt is an important location for fe and cu fe au deposits and includes the recently developed candelaria deposit which is located some 20 km south of the city of copiapo in northern chile this mine is now a major cu producer in chile but its discovery in the late 1980s was relatively fortuitous the exploration programme included a ground magnetic survey from which lessons can be learnt in the search for further such deposits palaeomagnetic studies throughout the northern part of the chilean iron belt indicate major crustal rotation of the region probably related to oblique convergence and transpression at the andean margin the application of palaeomagnetic techniques to a magnetite apatite deposit mina fresia indicates that magnetite rich ores in this area are capable of maintaining a significant remanence component which will contribute to their magnetic anomaly as with the volcanics intrusives and sediments of the region this remanence is clearly rotated clockwise using 2 5d and 3d magnetic modelling it is demonstrated that the magnetic anomaly associated with the candelaria deposit is also dominated by a remanence component which is significantly rotated but of reversed polarity recognition of clockwise rotated remanence dominated anomalies should provide a new key to the search for deposits in this part of chile and elsewhere an example of an unexplored anomaly showing this key feature of a clockwise rotated remanence component dominating over the induced component is presented modelling of this anomaly indicates a pair of sources which are either ten times as big or ten times as magnetic as the candelaria deposit it is suggested that the low cost of palaeomagnetic study of an exploration target should be a prerequisite to magnetic anomaly interpretation of targets in tectonic areas where vertical axis crustal rotations may be a significant element of the overall deformation pattern

age of mineralization of the candelaria fe oxide cu au deposit and the origin of the chilean iron belt based on re os isotopes re os isotopes are used to determine the age of iron oxide cu au mineralization at candelaria chile and to explore possible genetic links with the batholithic intrusions in the area re os ages calculated from molybdenite are 114 2 0 6 ma and 115 2 0 6 ma and they are interpreted to represent the age of mineralization these ages are consistent with previously reported ages for biotite alteration that range from 114 to 116 ma an isochron calculated by re os ratios from hydrothermal magnetite and sulfides constrains an initial 187 os 188 os ratio of 0 36 0 10 the initial 187 os 188 os ratio for sulfide from bronce mine a small satellite of the candelaria orebody is 0 33 0 01 these values are broadly similar to the calculated initial 187 os 188 os ratio for magmatic magnetite in nearby batholithic rocks that range from 0 20 to 0 41 the relatively radiogenic initial 187 os 188 os ratio represents a mixture of mantle and crustal components in the ores and batholitic rocks the similarity in initial 187 os 188 os ratio of the ore and magmatic oxides suggest that the granitic plutons could be the source of metals in the candelaria district these data are consistent with the existence of a significant magmatic fluid component in the hydrothermal system as suggested by previously published work in order to establish a regional perspective we analyzed ore minerals from other chilean deposits of the iron oxide cu u au rare earth elements ree class magnetite from the manto verde iron oxide cu au deposit has os and re concentrations of 11 to 17 parts per trillion ppt and 4 to 6 parts per billion ppb respectively the initial 187 os 188 os ratio is approximately 0 20 and is similar to that of ore minerals from candelaria and of the early cretaceous batholithic intrusions these data indicate a similar metal source for the mineralization at manto verde and candelaria magnetite from three magnetite apatite deposits of the chilean iron belt have re concentrations between 0 8 and 3 ppb and os concentrations between 11 and 76 ppt calculated initial 187 os 188 os ratios of these magnetites range from 1 2 to 8 4 and are distinctly different from those of the iron oxide cu au deposits the cause of the comparatively high radiogenic signatures in the magnetite apatite ores is probably related to fluid interactions with i e leaching the surrounding sedimentary rocks during their genesis in contrast to the iron oxide cu au systems the os in magnetite apatite ores could be derived from sedimentary rocks this evidence suggests that predominantly basin derived nonmagmatic brines formed these magnetite apatite deposits

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

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

regional mid pleistocene glaciation in central patagonia abstract southern south america contains a glacial geomorphological record that spans the past million years and has the potential to provide palaeoclimate information for several glacial periods in earth s history in central patagonia two major outlet glaciers of the former patagonian ice sheet carved deep basins 50 km wide and extending over 100 km into the andean plain east of the mountain front a succession of nested glacial moraines offers the possibility of determining when the ice lobes advanced and whether such advances occurred synchronously the existing chronology which was obtained using different methods in each valley indicates the penultimate moraines differ in age by a full glacial cycle here we test this hypothesis further using a uniform methodology that combines cosmogenic nuclide ages from moraine boulders moraine cobbles and outwash cobbles 10 be concentrations in eighteen outwash cobbles from the moreno outwash terrace in the lago buenos aires valley yield surface exposure ages of 169 269 ka we find 10 be inheritance is low and therefore use the oldest surface cobbles to date the deposit at 260 270 ka which is indistinguishable from the age obtained in the neighbouring lago pueyrredon valley this suggests a regionally significant glaciation during marine isotope stage 8 and broad interhemispheric synchrony of glacial maxima during the mid to late pleistocene finally we find the dated outwash terrace is 70 100 ka older than the associated moraines on the basis of geomorphological observations we suggest this difference can be explained by exhumation of moraine boulders

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

jurassic extensional tectono magmatism and associated mineralization of the el faldeo polymetallic district chilean patagonia geochemical and isotopic evidence of crustal contribution the el faldeo is the southernmost andean mineral district and consists of paleozoic polymetamorphic basement unconformably overlain by a 200 m thick homoclinal sequence of sedimentary and felsic volcanic beds of the ibanez formation in addition two epizonal intrusive units and related subvolcanic porphyries sills and dikes which intrude both the metamorphic basement and the ibanez formation are recognized the quebrada colorada granodiorite qcg and the cerro esmeralda tonalite cet the polymetallic mineralization was formed in four stages and includes pyrite arsenopyrite gold silver sphalerite galena and chalcopyrite hosted in felsic tuffs of the ibanez formation hydrothermal breccias and epizonal intrusions field observations and geochronological data u pb ar ar and fission track on cet rocks suggest 1 the development of subsiding pull apart basins and coeval formation of spaces for magmatic emplacement and mineralization in an extensional tectonic regime in mid jurassic time 2 a tectonic exhumation of the rocks of the district probably triggered by the chile ridge trench interactions that started in late miocene time sr nd isotopic data from cet samples and the pb isotope characteristics of galenas from the second and third mineralization stages are indicative of crustal contribution to the origin of the magmas and galena mineralization rocks of the paleozoic metamorphic basement are prime candidates for the crustal source components of the analyzed galenas

high precision u pb zircon dating of the chalten plutonic complex cerro fitz roy patagonia and its relationship to arc migration in the southernmost andes 1 we report new high precision u pb ages and geochemical data from the chalten plutonic complex to better understand the link between magmatism and tectonics in southern patagonia this small intrusion located in the back arc region east of the patagonian batholith provides important insights on the role of arc migration and subduction erosion the chalten plutonic complex consists of a suite of calc alkaline gabbroic to granitic rocks which were emplaced over 530 kyr between 16 90 0 05 ma and 16 37 0 02 ma a synthesis of age and geochemical data from other intrusions in patagonia reveals a striking similarities between the chalten plutonic complex and the neogene intrusions of the batholith and differences to other back arc intrusions such as torres del paine b a distinct e w trend of calc alkaline magmatic activity between 20 and 17 ma we propose that this trend reflects the eastward migration of the magmatic arc and the consistent age pattern between the subduction segments north and south of the chile triple junction suggests a causal relation with a period of fast subduction of the farallon nazca plate during the early miocene previously proposed flat slab models are not consistent with the present location and morphology of the southern patagonian batholith we advocate alternatively that migration of the magmatic arc is caused by subduction erosion due to the increasing subduction velocities during the early miocene

middle pleistocene glaciation in patagonia dated by cosmogenic nuclide measurements on outwash gravels the well preserved glacial record in argentine patagonia offers a 1 ma archive of terrestrial climate extremes in southern south america these glacial deposits remain largely undated beyond the range of radiocarbon dating at ca 40 ka dating old glacial deposits several 105 a by cosmogenic surface exposure methods is problematic because of the uncertainty in moraine degradation and boulder erosion rates here we show that cobbles on outwash terraces can reliably date old glacial deposits in the lago pueyrredon valley 47 5 s argentina favorable environmental conditions e g aridity and strong winds have enabled continuous surface exposure of cobbles and preservation of outwash terraces the data demonstrate that nuclide inheritance is negligible and we therefore use the oldest surface cobbles to date the deposit 10be concentrations in outwash cobbles reveal a major glacial advance at ca 260 ka concurrent with marine isotope stage 8 mis 8 and dust peaks in antarctic ice cores a 10be concentration depth profile in the outwash terrace supports the age and suggests a low terrace erosion rate of ca 0 5 mm ka 1 we compare these data to exposure ages obtained from associated moraines and find that surface boulders underestimate the age of the glaciation by 100 ka thus the oldest boulders in this area do not date closely moraine deposition the 10be concentration in moraine cobbles help to constrain moraine degradation rates these data together with constraints from measured 26al 10be ratios suggest that all moraine boulders were likely exhumed after original deposition we determine the local last glacial maximum lgm occurred at 27 25 ka consistent with the maximum lgm in other parts of patagonia

mid to late holocene aeolian activity recorded in a coastal dunefield and lacustrine sediments on andoya northern norway in an effort to reconstruct past aeolian activity a foredune stratigraphy and a continuous lake sediment record from the largest dunefield on andoya northern norway have been investigated the dunefield extends landwards in a north eastward direction and consists of several parabolic dunes foredunes and blowouts the sediment record 169 cm from the nearby lake latjonna and the foredune stratigraphy 10 m covers the last 6200 and 3700 cal yr bp respectively both sites possess sediments deposited after the tapes transgression maximum 6800 cal yr bp which reached a level of 7 8 m a s l at the study site the lake sediment record consists of several units dominated by sand grains interspersed by more organic rich beds the core has been examined by x ray fluorescence xrf magnetic susceptibility ms and loss on ignition loi mineral grains were detected by wet sieving of the ignition residue ir and the influx of sand grains to latjonna was calculated based on the weight of sand grains

sedimentary record of historical extreme storm surges on the gulf of gdansk coast baltic sea abstract the baltic sea is commonly viewed as a region with a low frequency of coastal hazards such as tsunamis or extreme storm surges however historical sources indicate that in the past several catastrophic storm surges resulted in coastal floods and related casualties their sedimentological records and reconstructions of paleostorminess were so far studied mainly in the westernmost denmark and easternmost estonia parts of the baltic sea the present study focus on southern baltic coast gulf of gdansk where storm surge sedimentary deposits left within coastal peatlands were investigated and related to historical record in order to compare recent risk estimations and predictions of storm surge levels to geological data to achieve this aim the collected sediment cores were subjected to sedimentological grain size and diatom analyses supported with 137cs 210pb and 14c datings two types of sandy event deposits were identified and interpreted to be storm deposits formed under an inundation regime and overwash regime respectively the deposits formed under the inundation regime were characterised by an erosional lower boundary massive structure and presence of rip up clasts of underlying sediments whereas the sediments formed under the overwash regime were low angle planar cross stratified medium sands with intercalations of massive fine sand the recognized sandy storm deposits were dated mainly to the little ice age period xvith xixth centuries they were most likely left during the historical storms of 1825 1872 and 1914 ce it extends the sedimentary record of known historical events as mainly storms from 1497 ce and 1872 ce were reported previously from baltic sea notably none of the historical storm events during the past 100 year period have formed a distinguishable sedimentary record these findings clearly show that risk models and storm return period predictions which are based on instrumental measurements from the last century should also include geological evidence the strongest storm surges from the past century may not be adequate reference of a worst case scenario

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region