cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

allochthonous salt initiation and advance in the northern flinders and eastern willouran ranges south australia using outcrops to test subsurface based models from the northern gulf of mexico the northern flinders ranges and eastern willouran ranges south australia expose neoproterozoic salt diapirs salt sheets and associated growth strata that provide a natural laboratory for testing and refining models of allochthonous salt initiation and emplacement the diapiric callanna group 850 800 ma comprises a lithologically diverse assemblage of brecciated rocks that were originally interbedded with evaporites that are now absent using stereonet analysis to derive three dimensional information from two dimensional outcrops of stratal geometries flanking salt diapirs and beneath salt sheets we evaluate 10 examples of the transition from steep diapirs to salt sheets 3 of ramp to flat geometries and 2 of flat to ramp transitions stratal geometries adjacent to feeder diapirs range from a minibasin scale megaflap to halokinetic drape folds to high angle truncations and appear to have no relationship to subsequent allochthonous salt development in all cases the transition from steep diapirs to salt sheets is abrupt and involved piston like breakthrough of thin roof strata which permitted salt to flow laterally we suggest two models to explain the transition from steep diapirs to subhorizontal salt 1 salt top breakout where salt rise occurs inboard of the salt flank thereby preserving part of the roof strata beneath the sheet and 2 salt edge breakout where rise occurs at the edge of the diapir with no roof preservation lateral emplacement of salt sheets is dependent on the interplay between the rate of salt supply to the front of the sheet and the sediment accumulation rate when the ratio of salt supply rate to sediment accumulation rate is high to moderate thrust advance produces base salt flats and truncation ramps respectively halokinetic folds are absent because the thrust emerges at the base of the sea floor scarp and mass transport complexes are rare as a result of relatively low scarp relief if the ratio is low pinned inflation leads to drape folding of the top salt and cover into a fold ramp with occasional slumping of the sheet and its roof and further breakout on thrust or reverse faults in the shallow water depositional environments of south australia lateral emplacement of salt sheets occurred through some combination of thrust advance extrusive advance and open toed advance with no evidence for subsalt thrust imbricates shear zones or continuous rubble zones in deep water environments such as the northern gulf of mexico thrust imbricates and rubble zones which represent slumped carapace are more common the presence of slumped carapace is caused primarily by higher topographic relief related to thicker hemipelagic roofs a lack of dissolution and gravity driven transport of overburden strata to the toes of large canopies

zircon and whole rock nd pb isotopic evidence for a grenville age and a laurentian origin for the basement of the precordillera in argentina suggestions that the precordillera of argentina has an early paleozoic connection to the appalachian ouachita rifted margin of the southern u s have been based on cambro ordovician stratigraphy and faunal assemblages further support comes from comparing precambrian grenville appalachian basement with xenoliths brought up in precordil leran miocene volcanic rocks u pb zircon ages and whole rock pb and nd isotopic data from the xenoliths show that precordilleran basement along with some part of the westernmost sierras pampeanas pie de palo range to the east is essentially unique in south america and is best interpreted as being derived from laurentia a grenville age is substantiated by u pb zircon upper intersect ages of 1102 pm 6 ma for a mafic xenolith and near 1100 ma for acidic xenoliths both xenolith types contain pink doubly terminated prismatic igneous zircons with relatively high u and pb concentrations rounded colorless low u and pb zircons with overgrowths produced in a later pre

mid to late cambrian docking of the rio de la plata craton to southwestern gondwana age constraints from u pb shrimp detrital zircon ages from sierras de ambato and velasco sierras pampeanas argentina abstract the early palaeozoic stratigraphy and tectonic history of the eastern sierras pampeanas of central argentina are complicated by metamorphism and deformation resulting from the pampean 545 510 ma and famatinian 490 440 ma orogenies we report u pb sensitive high resolution ion microprobe dating of detrital zircons in two metasedimentary successions exposed at quebrada de la cebila c 28 459s 66 259w the ambato and the la cebila metamorphic complexes the ambato zircons record age peaks corresponding to pampean 530 10 ma brasiliano c 570 and c 640 ma grenville c 950 to c 1025 ma and minor neoarchaean ages similar peaks are also apparent in the la cebila sample but it additionally contains palaeoproterozoic zircons c 2 1 ga corresponding to the age of the rio de la plata craton from which they are considered to have been sourced our interpretation is that the protolith of the ambato complex was deposited prior to juxtaposition with the craton and is older than the early ordovician la cebila metamorphic complex we infer that the craton reached its current relative position in the mid to late cambrian after the main pampean tectonothermal event 530 520 ma and before deposition of the la cebila protolith and the achavil formation sierra de famatina which contain comparable detrital zircon populations

p t t evolution of an early silurian medium grade shear zone on the west side of the famatinian magmatic arc argentina implications for the assembly of the western gondwana margin abstract the geodynamic evolution of the proto andean margin of gondwana during the paleozoic was characterized by repeated subduction processes associated with the docking of several terranes including the cuyania precordillera terrane and the chilenia terrane and the development of the calc alkaline famatinian continental magmatic arc in the sierra de la huerta 30 56 31 29 s and 67 17 67 32 w at the southwestern end of the western sierras pampeanas some mafic to ultramafic igneous bodies belonging to the famatinian arc were affected by regional metamorphism of the medium pressure granulite facies 7 7 5 kbar 850 c after this regional metamorphism local mylonitization under amphibolite facies along discrete nw se to nnw sse striking ductile shear zones occurred a 40 ar 39 ar plateau age of 432 4 ma was obtained on rather homogeneous hornblende porphyroclasts from a metagabbro mylonite the textures and mineral chemistry of the mylonitized metagabbro allow its p t t evolution from magmatic crystallization to mylonitization to be constrained geochronological data obtained from mylonite provide evidence that orogenesis was active at least until the early silurian this deformational event would have been related to uplift and decompression during the later stages of the orogenesis probably associated with the accretion of the precordillera terrane to the southwestern gondwana margin

in situ cosmogenic radiocarbon production and 2 d ice flow line modeling for an antarctic blue ice area surface 14 c activity that differs by up to 25 from the activity calculated using an ablation only approximation which is used in all prior work and 3 that at high ablation margin sites solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10 as part of this effort we model twodimensional ice flow along the central flow line of taylor glacier we present two methods for parameterizing vertical strain rates and assess which method is more reliable for taylor glacier finally we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error the results presented here can inform ongoing and future 14 c and ice flow studies at ice margin

cuyania an exotic block to gondwana review of a historical success and the present problems abstract a review of the early history of the cuyania terrane and the numerous pioneering works of the past century provides the present robust framework of evidence supporting a derivation from laurentia travel towards gondwana as an isolated microcontinent and final amalgamation to the protomargin of western gondwana in middle to late ordovician times the major remaining uncertainties and inconsistencies such as the time of deformation and collision with gondwana the lack of evidence of famatinian derived zircons the effects of strike slip displacements proposed along the suture as well as the potential sutures defined by ophiolite assemblages are discussed the precise boundary along the northern and southern limits is not yet well defined the most suitable hypothesis based on present data is that cuyania originated as a conjugate margin of the ouachita embayment south of the appalachian platform during early cambrian times the subsequent travel toward the gondwana protomargin is clearly depicted by the changing faunal assemblages in the carbonate platform new geochemical and age data on k bentonites presented by several authors reinforce the strong connection between cuyania ash fall tuffs and famatina volcanics by 468 470 ma indicating cuyania and gondwana were in close proximity at that time extension related to flexural subsidence preceded by the drowning of the carbonate platform in early llanvirnian times is recorded by abrupt facies changes in the sedimentary cover during late llanvirnian and early caradocian times this episode marked the beginning of contact between cuyania and gondwana the subsequent evolution of the foreland basin indicates that deformation lasted until latest silurian early devonian times the time of collision is tracked by the cessation of arc related magmatic activity in the upper plate gondwana protomargin at about 465 ma in western sierras pampeanas and ages around 454 ma corresponding to syncollisional and postcollisional magmatism the age of the collision is also preserved in the lower plate cuyania where both angular unconformities in the sedimentary cover and the ages of peak of regional metamorphism in the basement rocks point to 460 ma as the most probable age for the beginning of the collision evidence from the upper plate is essentially identical with an age of 463 ma thermal gradients along this suture vary from 13 c km in the lower plate to 18 c km in the fore arc upper plate reaching more than 30 c km along the famatinian arc based on these data a llandelian caradocian age for the collision can be postulated on firm grounds deformation continued through most of the early paleozoic until amalgamation of the chilenia terrane by the late devonian

interpreting ancient ice in a shallow ice core from the south yamato antarctica blue ice area using flow modeling and compositional matching to deep ice cores 1 we explore methods of dating a 101 m ice core from a bare ice ablation area in the yamato mountains dronning maud land east antarctica there are two unknowns the age of the ice at the surface and the age spanned by the core the ice crystal growth rate was used to estimate the age span of the core at about 5 kyr co2 ch4 and n2o data on the core were compared with well dated records from deep cores leading to two plausible matches both within isotope stage c detailed comparison of high resolution dep records from this core and the dome fuji core support the 55 61 kyr bp fit best oxygen isotope values in the core were then used to constrain the source elevation of the snow in the core and hence the velocities in a simple flow line model we inverted the ice core surface age age span and origin site and their confidence intervals in a sensitivity study of flow model parameter space the flow line model predicts good matches to the core by reducing glacial flow rates to 70 of present day accumulation rates by 45 with 10 confidence intervals the modeled surface age for the whole meteorite field yields maximum surface ages of about 90 kyr which is consistent with known but poorly constrained tephra dating meteorite terrestrial ages and the frequency of meteorite discoveries this approach can be used quite generally to link deep ice cores to surface outcrops on blue ice fields

zircon and whole rock nd pb isotopic provenance of middle and upper ordovician siliciclastic rocks argentine precordillera graptolite bearing middle and upper ordovician siliciclastic facies of the argentine precordillera fold thrust belt record the disintegration of a longlived cambro mid ordovician carbonate platform into a series of tectonically partitioned basins a combination of stratigraphic petrographic u pb detrital zircon and nd pb whole rock isotopic data provide evidence for a variety of clastic sediment sources four upper ordovician quartzo lithic sandstones collected in the eastern and central precordillera yield complex u pb zircon age spectra dominated by 1ae05 1ae10 ga zircons secondary populations of 1ae22 1ae30 and 1ae46 ga rare 2ae2 and 1ae8 ga zircons and a minor population 2 of concordant zircons in the 600 700 ma range archaean age grains comprise 1 of all zircons analysed from these rocks in contrast a feldspathic arenite from the middle ordovician estancia san isidro formation of the central precordillera has two well defined peaks at 1ae41 and 1ae43 ga with no grains in the 600 1200 ma range and none older than 1ae70 ga the zircon age spectrum in this unit is similar to that of a middle cambrian quartz arenite from the la laja formation suggesting that local basement rocks were a regional source of ca 1ae4 ga detrital zircons in the precordillera terrane from the cambrian onwards the lack of grains younger than 600 ma in upper ordovician units reinforces petrographic data indicating that ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins nd isotopic data n 32 for middle and upper ordovician graptolitic shales from six localities define a poorly mixed signal end 450 ma 9ae 6t o 4ae5 that becomes more regionally homogenized in upper ordovician rocks 6ae 2 1 ae0 tdm 1ae51 0ae15 ga n 17 a trend reinforced by the u pb detrital zircon data it is concluded that proximal recycled orogenic sources dominated the siliciclastic sediment supply for these basins consistent with rapid unroofing of the precordillera terrane platform succession and basement starting in mid ordovician time common pb data for middle and upper ordovician shales from the western and eastern precordillera n 15 provide evidence for a minor 30 component that was likely derived from a high l u pb terrane

crustal structure of the south central andes cordillera and backarc region from regional waveform modelling summary we investigate the crustal structure in the andes cordillera and its backarc region using regional broadband waveforms from crustal earthquakes we consider seismic waveforms recorded at regional distances by the chile argentina geophysical experiment charge during 2000 2002 and utilize previous seismic moment tensor inversion results for each single station earthquake pair we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation cordillera and the accreted terranes in the backarc region our investigation indicates that synthetic seismograms for our earthquake station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters we performed a grid search around crustal thickness p wave seismic velocity vp and p t os wave seismic velocity ratio vp vs fixing mantle parameters we evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components we identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s our results indicate along strike variations in the crustal structure for the north south high cordillera with higher p wave velocity and thickness in the northern segment north of 33 s and persistently high vp vs ratio 1 85 in both segments this is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region in contrast the results for the current volcanic arc south of 33 s agree with a warmer crust consistent with partial melt related to quaternary volcanism presumably of an intermediate to mafic composition a distinctive feature in the backarc region is the marked contrast between the seismic properties of the cuyania and pampia terranes that correlates with their heterogeneous crustal composition the cuyania terrane composed of mafic ultramafic rocks exhibits high vp high vp vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern sierras pampeanas associated with low vp and low vp vs these differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper 30 km in this active compressional region in particular the seismic properties of the cuyania terrane which shows evidence for a high vp and high vp vs crust may be reflecting the complex tectonic evolution history of this terrane including accretion rifting and re accretion processes since the palaeozoic that promote a high level of crustal seismicity in the upper 30 km enhanced by the flat slab subduction in the segment between 31 s and 32 s another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the cuyania terrane between 30 s and 34 s

a laurentian grenville age oceanic arc back arc terrane in the sierra de pie de palo western sierras pampeanas argentina

early gondwanan connection for the argentine precordillera terrane abstract the precordillera of argentina is widely accepted as an exotic terrane of laurentian north american affinity newly acquired u pb ages on individual detrital zircons from lower cambrian and upper ordovician quartz sandstone beds in the argentine precordillera indicate a gondwanan provenance not associated with any known part of laurentia accordingly the precordillera terrane is likely underlain by basement rock of gondwanan affinity in addition detrital zircons from the upper ordovician sandstone bed provide no evidence for a mid ordovician position against the inboard famatina arc these results demand critical re evaluation of widely held assumptions regarding the paleogeography of the argentine precordillera

foreland basin deposits associated with cuyania accretion in la pampa province argentina abstract in the province of la pampa south central argentina adjacent to the cuyania pampia terrane boundary a late ordovician devonian marine foreland basin generated after the middle ordovician darriwilian collision of cuyania has been recognised and is herein referred to as the curaco basin this basin was formed on crust of the cuyania terrane immediately to the west of the pampia terrane its fill primarily comprises sandstones and shales of the la horqueta formation unconformably overlain by permian continental sediments carapacha formation the extent of the curaco basin is investigated using aeromagnetic data its depocentre is conspicuously delineated by a narrow nnw orientated magnetic low in addition scarce outcrops assigned to the same sequence are found on the pampian continental crust about 50 km to the east in the proximity of the valle daza estate valle daza sub basin the studied marine sequence of la pampa is equivalent to other lower to middle paleozoic units of the eastern margin of the cuyania terrane and is clearly distinguished from the overlying permian continental sedimentary sequence u pb shrimp dating of detrital zircons from valle daza indicates a maximum deposition age of ca 466 ma the minimum age is given by the intrusion of granites at ca 405 ma i e early devonian geochemical data indicate that the curaco basin was primarily sourced by an active continental margin possibly the ordovician famatinian arc

the mesoproterozoic maz terrane in the western sierras pampeanas argentina equivalent to the arequipa antofalla block of southern peru implications for west gondwana margin evolution the rocks of sierra de maz and sierra del espinal western sierras pampeanas represent pre famatinian ordovician basement isotope compositions nd and pb of metasedimentary rocks and shrimp u pb dating of detrital zircons combined with other geological evidence show that three parallel n s domains can be recognized the central maz domain contains pre grenvillian metasedimentary rocks deposited between 1 2 and 1 6 ga that underwent grenvillian granulite facies metamorphism and were intruded by mafic igneous rocks and massif type anorthosites metasedimentary rocks have high nd tdm ages 1 7 2 7 ga and very radiogenic pb 9 8 10 2 suggesting provenance from reworked early proterozoic or archean continental crust the domains to the east and west of the maz domain consist of three metasedimentary sequences with nd tdm ages between 1 2 and 1 6 ga and variably radiogenic pb 9 6 10 0 u pb shrimp dating of detrital zircons nd tdm model ages and comparison with other data suggest that these sequences are post grenvillian i e neoproterozoic and or early paleozoic the maz domain is interpreted as a suspect terrane similar to the northern arequipa antofalla craton that forms the basement of the central andes both underwent grenville age orogeny and were probably once continuous along the western margin of amazonia west gondwana

las matras block central argentina 37 s 67 w the southernmost cuyania terrane and its relationship with the famatinian orogeny abstract the las matras block in central argentina constitutes the southernmost part of the cuyania terrane which was accreted to the southwestern margin of gondwana during the early to mid ordovician famatinian orogeny the grenville aged rocks of the las matras block are represented by the tonalitic to trondhjemitic las matras pluton a new u pb conventional zircon age of 1244 42 ma confirms previous sm nd and rb sr isochron ages of this pluton mineral composition data are consistent with the tonalitic trondhjemitic character of the pluton and constrain its emplacement level to 1 9 to 2 6 kb this shallow level of emplacement and the undeformed character of the pluton are distinctive features of this southernmost basement a regional comparison indicates that the igneous metamorphic evolution of the grenville aged basement rocks of the cuyania terrane occurred over a period of more than 200 million years with ages older than 1200 ma up to those close to 1000 ma the shallowest crustal level is found in las matras suggesting a southward shallowing of the exposed level of basement the deformation and metamorphism associated with the collisional famatinian orogeny affect both the cuyania terrane and the adjacent western margin of gondwana and the gondwana margin was also the locus of the related arc magmatism but the compressive effects of the collision decrease in intensity toward the south the famatinian metamorphism and magmatism continue even further south into the patagonia region but the southern continuity of the cuyania terrane into this region remains uncertain

0 3 byr of drainage stability along the palaeozoic palaeo pacific gondwana margin a detrital zircon study the palaeo pacific margin of gondwana in the present day south central andes is marked by tectonic activity related to subduction and terrane accretion we present detrital zircon u pb data encompassing the palaeozoic era in northern chile and northwestern argentina cathodoluminescence images reveal dominantly magmatic zircon barely affected by abrasion and displaying only one growth phase the main age clusters for these zircon grains are ediacaran to palaeozoic with an additional peak at 1 3 0 9 ga and they can be correlated with grenvillian age and the brasiliano pampean and famatinian orogenies the zircon data reveal main transport from the nearby ordovician famatinian arc and related rocks the silurian sandstone units are more comparable with cambrian units with brasiliano and transamazonian ages 2 2 1 9 ga being more common because the silurian deposits were situated within or east of the extinct famatinian arc hence the arc acted as a transport barrier throughout palaeozoic time the complete suite of zircon ages does not record the accretions of exotic terranes or the palaeozoic glacial periods we conclude that the transport system along the palaeo pacific margin of gondwana remained stable for c 0 3 byr and that provenance data do not necessarily reflect the interior of a continent hence inherited geomorphological features must be taken into account when detrital mineral ages are interpreted supplementary material u pb data cl images and detailed geological maps are available at www geolsoc org uk sup18796

vestiges of an ordovician west vergent thin skinned ocloyic thrust belt in the argentine precordillera southern central andes collision of the down going laurentia derived argentine precordillera terrane with the gondwanan margin drove the ordovician ocloyic orogeny including subduction volcanism metamorphism and top to west shearing east of the precordillera in the precordillera above passive margin carbonates lower ordovician san juan limestone and older carbonates a middle to upper ordovician westward prograding synorogenic clastic wedge of black shale gualcamayo shale and coarser clastic sediment las vacas conglomerate and trapiche formation fills a peripheral foreland basin new research has identified vestiges of a west directed thin skinned ocloyic foreland thrust belt that has been fragmented by east directed andean thrusting the el corral thrust sheet with hanging wall detachment in the san juan limestone extends over a west directed footwall frontal ramp and extensive flat to low angle footwall cutoff in the gualcamayo and las vacas formations las vacas conglomerates in the footwall include olistoliths 10 m scale exclusively of san juan limestone and gualcamayo shale the beds in some olistoliths are folded the advancing el corral thrust sheet successively supplied and overrode the stratigraphically restricted olistoliths in the el corral footwall tight west vergent folds and faults within an anticlinorium in the san juan limestone and gualcamayo shale suggest a deeper unexposed thrust fault the los celestitos fault west of the anticlinorium easterly dip restored to remove andean deformation beneath an angular unconformity between las vacas and trapiche beds is consistent geometrically with the trailing limb of a west vergent fault propagation anticline in the hanging wall of the subsurface los celestitos fault the same angular unconformity truncates the el corral fault and hanging wall strata in the trapiche formation contrasting sedimentary facies from sandy turbidites westward to limestone clast megabeds and olistoliths suggest another frontal ramp from a stratigraphically deeper detachment in a break forward sequence none of these observations separately defines an ocloyic thrust belt taken together however these vestiges of thrust belt style indicate the consistent geometry of an ordovician west vergent thin skinned ocloyic thrust belt

u pb shrimp zircon dating of grenvillian metamorphism in western sierras pampeanas argentina correlation with the arequipa antofalla craton and constraints on the extent of the precordillera terrane abstract metamorphism of grenvillian age ca 1 2 ga u pb zircon dating is recognized for the first time in the western sierras pampeanas sierra de maz conditions reached granulite facies ca 780 c and ca 780 mpa comparing geochronological and petrological characteristics with other outcrops of mesoproterozoic basement particularly in the northern and central arequipa antofalla craton we suggest that these regions were part of a single continental crustal block from mesoproterozoic times and thus autochthonous or parautochthonous to gondwana

proterozoic early paleozoic evolution in western south america a discussion abstract the hypothesis of exotic terranes in peru bolivia argentina and chile generated discussions on the mode of transfer and extent of accretional events that may have occurred in the southern andes during the late proterozoic early paleozoic initially a tectogenesis based on autochthonous mobile fold belts was discussed following ideas emphasised the fragmentation of the supercontinent rodinia laurentia moving along the west gondwana border and colliding with the gondwana western margin the most important effect of this laurentia gondwana relationship was attributed to the argentine precordillera or cuyania terrane splitting off from laurentia and docking to gondwana in the early paleozoic in this study the most cited arguments for this laurentia precordillera relationship are discussed emphasising paleontological considerations it is shown that these arguments do not exclude a close original vicinity of the precordillera terrane to gondwana the precordillera terrane is suggested to be part of a hypothetical platform which developed between south america africa and antarctica safran platform and which was displaced to its actual position by transcurrent faults the collisional events in the sierras pampeanas ensued from strike slip movements and were responsible for the s and i type transpressional magmatism along the pampean and famatinian terranes the final result of this continent parallel movement of terrane slices is similar to that of a terrane split off from laurentia but the first named way of formation easier explains the general continuity of plate convergence at the western border of gondwana than the laurentia precordillera connection does

a review of the famatinian ordovician magmatism in southern south america evidence of lithosphere reworking and continental subduction in the early proto andean margin of gondwana abstract along the proto pacific margin of gondwana from venezuela to northeastern patagonia the early middle ordovician famatinian orogeny was the first orogenic event following assembly of the supercontinent previous isotope studies of the igneous and meta sedimentary rocks of southwestern gondwana yield ambiguous implications for the role of juvenile mantle addition during the early crustal growth at the supercontinental margin to interpret the geological and tectonic evolution of the orogen and the magma sources in different episodes we look at evidence from a large area of southern south america including the 700 600 km type sector of the orogen in the sierras pampeanas 27 33 s the precordillera and northeastern patagonia previous geological geochemical and geochronological results are reviewed together with new u pb shrimp crystallization ages 177hf 176hf and 18o 16o data for dated zircon and whole rock sr and nd isotope compositions four geological domains are recognized in the sierras pampeanas western central eastern and foreland famatinian domains magmatism is mostly restricted to the interval 463 4 to 486 7 ma with the most intense period of emplacement between 468 and 472 ma constituting a magmatic flare up granitoid emplacement in both northeastern patagonia and the cordon de lila puna altiplano chile was effectively synchronous with that in the sierras pampeanas defining a continuous belt combined geochemical and isotopic data whole rock sr nd hf o in zircon indicate that the source of calcic metaluminous suites is the subcontinental lithosphere both mantle and mafic lower crust with variable contamination by the early paleozoic metasedimentary country rocks the lithospheric mantle involved is assumed to underlie the outcropping 1330 1030 ma age basement of the western domain which exhibits tectonic characteristics of active continental margin in the north and oceanic arc back arc in the south the latter sector is the potential source of some minor famatinian igneous rocks with less evolved isotopic compositions although a restricted asthenospheric addition cannot be discarded in this case minor peraluminous granites are spatially associated with the metaluminous sequence but major highly peraluminous batholiths occur on the eastern flank of the central domain field relations and geochemical isotopic evidence indicate that the most obvious source of these crustal melts was the very thick post early cambrian metasedimentary sequence comprising the host country rocks episodic tectono magmatic evolution of the famatinian magmatic belt in two overlapping stages is invoked to explain different characteristics in the four recognized domains in the type sector ca 474 486 ma roll back stage this is a mainly extensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle full development of the marine ensialic basins and early emplacement of both metaluminous granites and highly peraluminous batholiths in the central and eastern famatinian domains trondhjemite plutons with an adakitic signature were emplaced in the foreland domain ca 468 472 ma slab break off stage steepening of the oceanic slab and arc migration to the southwest ended with slab break off due to subduction of continental crust during continental collision with the precordillera terrane this stage produced voluminous metaluminous magmatism at the western edge of the central domain the flare up episode k bentonites in the precordillera leucogranites in the western domain and scattered metaluminous and peraluminous plutons in all famatinian domains both slab roll back and break off stages developed during a high t regime typical of hot orogens although asthenospheric mantle was a necessary heat source for lithospheric melting its material contribution to the growth of early paleozoic crust was apparently very minor recycling of mesoproterozoic lithosphere including the subcontinental mantle coupled with crustal melting of early paleozoic metasedimentary sequences accounts for most famatinian magmatism comparable results from the central andes and east antarctica confirm that the early stages of the terra australis orogen in sw gondwana were dominated by lithospheric reworking processes

a cambrian mixed carbonate siliciclastic platform in sw gondwana evidence from the western sierras pampeanas argentina and implications for the early paleozoic paleogeography of the proto andean margin the western sierras pampeanas wsp of argentina record a protracted geological history from the mesoproterozoic assembly of the rodinia supercontinent to the early paleozoic tectonic evolution of sw gondwana two well known orogenies took place at the proto andean margin of gondwana in the cambrian and the ordovician i e the pampean 545 520 ma and famatinian 490 440 ma orogenies respectively between them an extensive continental platform was developed where mixed carbonate siliciclastic sedimentation occurred this platform was later involved in the famatinian orogeny when it underwent penetrative deformation and metamorphism the platform apparently extended from patagonia to northwestern argentina and the eastern sierras pampeanas and has probable equivalents in sw africa peru and bolivia the wsp record the outer deepest part of the platform where carbonates were deposited in addition to siliciclastic sediments detrital zircon u pb shrimp ages from clastic metasedimentary successions and sr isotope compositions of marbles from the wsp suggest depositional ages between ca 525 and 490 ma the detrital zircon age patterns further suggest that clastic sedimentation took place in two stages the first was sourced mainly from re working of the underlying neoproterozoic metasedimentary rocks and the uplifted core of the early cambrian pampean orogen without input from the paleoproterozoic rio de la plata craton sediments of the second stage resulted from the erosion of the still emerged pampean belt and the neoproterozoic brasiliano orogen in the ne with some contribution from the rio de la plata craton an important conclusion is that the wsp basement was already part of sw gondwana in the early cambrian and not part of the exotic precordillera cuyania terrane as was previously thought

basement composition and basin geometry controls on upper crustal deformation in the southern central andes 30 36 s deformation and uplift in the andes are a result of the subduction of the nazca plate below south america the deformation shows variations in structural style and shortening along and across the strike of the orogen as a result of the dynamics of the subduction system and the features of the upper plate in this work we analyse the development of thin skinned and thick skinned fold and thrust belts in the southern central andes 30 36 s the pre andean history of the area determined the formation of different basement domains with distinct lithological compositions as a result of terrane accretions during palaeozoic time the development of a widespread permo triassic magmatic province and long lasting arc activity basin development during palaeozoic and mesozoic times produced thick sedimentary successions in different parts of the study area based on estimations of strength for the different basement and sedimentary rocks calculated using geophysical estimates of rock physical properties we propose that the contrast in strength between basement and cover is the main control on structural style thin v thick skinned and across strike localization of shortening in the study area

the early paleozoic evolution of the argentine precordillera as a laurentian rifted drifted and collided terrane a geodynamic model paleontologic and stratigraphic evidence points to the early paleozoic precordilleran terrane of western argentina as being the conjugate rift pair of the appalachians stratigraphic similarities of the cambrian and early arenig carbonate series and very strong affinities among trilobite conodont and brachiopod faunas show their close relationship the most probable provenance areas are the appalachian ouachita rifted margin and the newfoundland appalachians although the former fits better with geometric and drifting paths suggested by faunal affinities increasing celtic and baltic brachiopod genera and divergent stratigraphy since the arenig indicate the drifting of the precordilleran terrane collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed the collision of the precordillera with western gondwana occurred during the mid llanvirn to llandeilo a magmatic arc related to eastward subduction present coordinates was active in the famatina range east of the precordillera this region of celtic affinity shows faunal exchange with the precordilleran terrane since the late arenig and may represent accreted intra iapetus volcanic island arc complexes the rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the precordilleran terrane from periequatorial to peripolar latitudes between the cambrian and latest ordovician the deep ocean to the west of the precordillera started to close by the late ordovician with the eastward drift of the chilenian terrane absence of volcanic or pyroclastic arc derived rocks in the precordillera indicate west dipping subduction as chilenia approached the continental margin a new forebulge was established on the former collided precordilleran terrane developing an erosional unconformity in central precordillera talacasto tambolar arch a gondwanic signature was fully developed by the middle silurian when the malvinokaffric clarkeia fauna flourished before then the late ordovician glacial record and associated hirnantia fauna were the first clear tie to gondwana during the silurian the marginal basin behaved as a foreland with lithosphere rheology and eustasy governing the sequence stratigraphy wrench faulting along its eastern boundary displaced the precordillera toward the south continued shortening during closure with the chilenian terrane in the mid devonian produced thrust loading of the basin and generated a thick graywacke succession final accretion of chilenia late devonian generated a regional angular unconformity between the lower and upper paleozoic new eastward subduction was initiated west of the accreted chilenian terrane during the late permian triassic as indicated by the choiyoi volcanic complex which presently outcrops in the frontal cordillera

isotope sr c and u pb shrimp zircon geochronology of marble bearing sedimentary series in the eastern sierras pampeanas argentina constraining the sw gondwana margin in ediacaran to early cambrian times abstract the sierras de cordoba metasedimentary series consists of marbles and metasiliciclastic rocks of ediacaran to early cambrian age ca 630 and 540 ma that underwent high grade metamorphism during the collisional pampean orogeny in the early cambrian the ages of the marbles were determined from the sr isotope composition blind dating of screened samples of almost pure calcite marble and were further constrained with c and o isotope data and u pb shrimp detrital zircon ages of an interbedded paragneiss two groups of samples are recognised with sr isotope composition ca 0 7075 and 0 7085 that are considered stratigraphically significant the first is inferred early ediacaran the second late ediacaran to early cambrian the sierras de cordoba metasedimentary series is correlated for the first time with marble bearing metasedimentary series in several sierras to the west and north of sierras de cordoba e g the difunta correa sedimentary sequence and the ancajan series implying that all were probably parts of an originally extensive sedimentary cover these series bear evidence of sedimentary sources in the mesoproterozoic and paleoproterozoic basement of the western sierras pampeanas part of the large mara continental block and farther west laurentia in terms of the age of limestones marbles and detrital zircon patterns the sierras de cordoba metasedimentary series differs strongly from the older section of late ediacaran to early cambrian puncoviscana formation of northwestern argentina which mostly outcrops in northern sierra chica and sierra norte with sedimentary input from western gondwana sources the sierras de cordoba metasedimentary series and the puncoviscana formation were probably juxtaposed during the pampean orogeny along a complex suture zone that was further folded and or imbricated at mid crustal depths the peak of metamorphism was attained at 527 2 ma according to the evidence found here most of the sierras pampeanas to the west of the sierras de cordoba were part of the lower colliding plate during the final amalgamation of sw gondwana

the proto andean margin of gondwana an introduction a basic background is presented for the discussion of the early palaeozoic geology of western argentina covered by this book this includes the definition and terminology of orogenic cycles on this part of the gondwana margin represented by the eastern sierras pampeanas the pampean orogeny early cambrian relates to an intense but short lived period of terrane collision predating the rifting of the precordillera terrane from laurentia the famatinian cycle is predominantly represented by intense subductionrelated magmatism of early middle ordovician age developed on the continental margin of gondwana during the rifting and drifting of the precordillera terrane the grenvillian basement of the latter is further exemplified by a new rb sr whole rock isochron age of 1021 12 ma for orthogneisses from the western sierras pampeanas a mid ordovician granite in this area dated at 481 6 ma by u pb ion microprobe data may be related to rifting while the precordillera terrane was still attached to laurentia a divergence of opinion is pointed out between some authors in this book who favour mid ordovician collision of the precordillera with gondwana and others who place it much latter in silurian or devonian times

record of cosmogenic in situ produced 14c in vostok and taylor dome ice samples implications for strong role of wind ventilation processes we present results of studies of in situ cosmogenic 14 c in several ice samples from the vostok and taylor dome cores spanning the time intervals of 20 and 11 k y b p respectively the results are in variance with our findings for the greenland ice sheet project 2 gisp 2 samples where near quantitative 14 c retention was observed the partitioning of 14 c in the co and co 2 phases is however quite similar in the gisp and two antarctic ice samples noting that most of the in situ 14 c is produced in the ice during its accumulation to thicknesses of up to 10 m we interpret the observed 14 c deficiencies in antarctic ice samples as due to grain metamorphism recrystallization and sublimation or evaporation caused by wind ventilation simplified models for wind ventilation exhibit an interesting feature of the firnification processes they differently affect the concentrations of in situ 14 c and of the cosmogenic nuclides 10 be and 36 cl scavenged from the atmosphere in the accumulating firn by studying the concentrations of in situ cosmogenic 14 c and the atmospheric cosmogenic nuclides in the same ice samples one can hope to obtain fairly realistic models of the chemical impacts of firnification processes specifically relationships between precipitation and accumulation and the main processes contributing to modifications in the nuclide concentrations

the iglesia basin in the southern central andes a record of backarc extension before wedge top deposition in a foreland basin abstract the iglesia basin hosts a key sedimentary archive of the evolution of the southern central andes this basin is classically interpreted as recording the transition from an outer to an inner wedge top piggyback basin depozone in the early bermejo foreland basin system in this study we reevaluate the tectonic evolution of the iglesia basin based on the interpretation of 2d seismic reflection data structural geology zircon u pb geochronology and provenance analysis we report an extensional event that initiated the basin opening our results indicate normal faulting development and synextensional deposition from the latest oligocene earliest miocene to middle miocene since the middle miocene extensional activity experienced a progressive decrease in changing to shortening and basin inversion between the middle late miocene the latter stage coincided with the full development of pampean flat slab subduction indicating maximum plate coupling at that time our findings challenge the idea of uninterrupted paleogene to neogene flexural subsidence related to the andean orogenic load in the backarc area therefore the results of this study have important consequences for the early evolution of the bermejo foreland basin and ultimately for the growth of the southern central andes

early evolution of the proto andean margin of south america from a detailed study of a 500 km transect in the sierras pampeanas central west argentina two pre silurian tectono magmatic episodes are recognized and defined each culminating in micro continental collisions against the proto andean margin of gondwana the pampean orogeny started in early cambrian time with short lived subduction indicated by ca 535 ma calc alkaline granitoids following pampean terrane collision burial to granulite facies conditions ca 9 kbar generated widespread migmatites and ca 520 ma highly peraluminous granites in the eastern sierras pampeanas after brief quiescence a second major episode the famatinian orogeny started with subduction ca 490 ma forming a wide continental arc and ensialic backarc basin this heralded the approach of laurentia to gondwana during which the precordillera terrane separated from the southern appalachian region finally colliding with gondwana in silurian devonian time

structural evolution of a composite middle to lower crustal section the sierra de pie de palo northwest argentina 1 the sierra de pie de palo of northwest argentina preserves middle to lower crustal metamorphic rocks that were penetratively deformed during ordovician accretion of the precordillera terrane to the gondwana margin new structural petrologic and geochronologic data from a 40 km structural transect reveals that the sierra de pie de palo preserves a middle to lower crustal ductile thrust complex consisting of individual structural units and not an intact ophiolite and cover sequence top to the west thrusting occurred intermittently on discrete ductile shear zones from 515 to 417 ma and generally propagated toward the foreland with progressive deformation ordovician crustal shortening and peak metamorphic temperatures in the central portion of the sierra de pie de palo were synchronous with retro arc shortening and magmatic flare up within the famatina arc accretion of the precordillera terrane resulted in the end of arc flare up and the onset of synconvergent extension by 439 ma continued synextensional to postextensional convergence was accommodated along progressively lower grade shear zones following terrane accretion and the establishment of a new plate margin west of the precordillera terrane the results support models of cordilleran orogens that link voluminous arc magmatism to periods of regional shortening the deformation metamorphic and magmatic history within the sierra de pie de palo is consistent with models placing the region adjacent to the famatina margin in the middle cambrian and not as basement to the precordillera terrane

ordovician collision of the argentine precordillera with gondwana independent of laurentian taconic orogeny abstract the argentine precordillera a rifted fragment of laurentian crust and sedimentary cover collided with gondwana in middle ordovician time the time of collision ocloyic orogeny is similar to that of the taconic orogeny of eastern laurentia three hypotheses have been proposed to explain ordovician docking of the precordillera with western gondwana a the precordillera microcontinent was rifted from laurentia in cambrian time and following solitary drift collided with gondwana independent of the laurentian taconic orogeny b a continent continent collision of laurentia with gondwana producing a continuous taconic ocloyic orogenic belt was followed by rifting that left the precordillera attached to gondwana and c the precordillera at the tip of a distal plateau on greatly stretched laurentian crust collided with gondwana and subsequently separated from laurentia contrasts in several aspects of taconic and ocloyic orogenic history provide for discrimination between the microcontinent and continent continent collision hypotheses stratigraphic gradients and lithologic assemblages within the synorogenic clastic wedges are incompatible with a single continuous orogenic belt which in palinspastic location places the thin fine grained southern fringe of the taconic clastic wedge adjacent to the thickest and coarsest part of the ocloyic clastic wedge separate temporal and spatial distribution patterns of volcanic ash bentonite beds in laurentia and the precordillera indicate originally separate dispersal systems late ordovician hirnantian gondwanan glacial deposits in the precordillera indicate substantial latitudinal separation from laurentia post collision faults with large vertical separation in the precordillera have no coeval counterparts on the laurentian foreland these contrasts indicate originally separate not initially continuous and subsequently dismembered orogenic belts favoring the microcontinent hypothesis and eliminating the continent continent collision hypothesis initial taconic tectonic loading near the southern corner of the alabama promontory of laurentia and the lack of post taconic extension there are inconsistent with the tectonic history required by the plateau hypothesis but are consistent with the tectonic history required by the microcontinent hypothesis paleobiogeography distribution of bentonite beds and the hirnantian glacial deposits all indicate wide separation iapetus ocean between the precordillera and southern laurentia at the time of the ocloyic and taconic orogenies further favoring the microcontinent hypothesis

coda q in different tectonic areas influence of processing parameters abstract published results of coda q show a large variation in values these variations are often claimed to be related to different tectonics whereas they might just be related to using different assumptions in the processing leading to different input parameters for the analysis in this study the effect of using different processing parameters is investigated and significant differences particularly at low frequencies are observed we find a new set of optimal parameters which we recommend using in future studies using a short lapse time of 30 s and optimal parameters data from both similar and very different tectonic regions are used to calculate coda q using the same program and the same parameters the regions considered are eastern anatolia the azores jan mayen northwestern and central argentina the shanxi rift system in china and southwestern norway we obtain the following relations eastern anatolia q 88 f 0 66 azores q 86 f 0 70 jan mayen q 90 f 0 72 northwestern and central argentina q 89 f 0 94 shanxi rift system q 99 f 0 89 and southwestern norway q 124 f 0 91 the results show that coda q is very similar for regions of similar tectonics and significantly different for regions with varying tectonics using alternative more common parameters gives different q but the regional differences remain so which parameters to use to get correct coda q values is still uncertain however coda q can clearly distinguish different tectonic areas provided identical processing parameters are used even if they are not optimal

corrigendum to early gondwanan connection for the argentine precordillera terrane earth planet sci lett 205 2003 349 359

did the taconic appalachians continue into southern south america the appalachian mountains now terminating abruptly at the gulf of mexico coastal plain may have formerly continued into southern south america rocks forming the basement of the argentine andes can be interpreted as remnants of an early paleozoic orogen the famatinian belt not unlike the taconic appalachians both orogens are bordered to the west present coordinates by lower paleozoic carbonate platforms bearing the olenellid trilobite fauna that is characteristic of laurentia paleomagnetic and geologic data indicate that they could have formed as one continuous mountain chain possibly extending into antarctica during ordovician closure of an ocean basin southern iapetus between laurentia and gondwana the taconic and famatinian segments of the chain may have been truncated during late ordovician separation of laurentia and gondwana along the preexisting late neoproterozoic to cambrian rift system that initiated formation of the ouachita embayment and the southern margin of north america

early paleozoic orogenic belt of the andes in southwestern south america result of laurentia gondwana collision the late precambrian to early paleozoic age rock units of the pampean ranges the puna and the north patagonian massif of southwestern south america constitute the famatinian orogenic belt they are interpreted as an ordovician collisional belt between the occidentalia terrane and the gondwana craton they include mafic and ultramafic belts of neoproterozoic to early paleozoic age an intense tectonothermal event resulted from the collision syntectonic granitoids represent crustal melting in that collision syntectonic to late tectonic foreland basins developed the recently proposed juxtaposition of laurentia and east antarctica australia in the neoproterozoic raises the possibility that laurentia and western south america were close together in the early paleozoic and therefore that the famatinian belt resulted from laurentia gondwana collision occidentalia which is bordered by a cambrian carbonate platform similar to that of eastern north america may be a sliver detached from laurentia during late ordovician time

geophysical characterization of the upper crust in the transitional zone between the pampean flat slab and the normal subduction segment to the south 32 34 s andes of the frontal cordillera to the sierras pampeanas fil sanchez marcos ariel universidad nacional de san juan facultad de ciencias exactas fisicas y naturales instituto geofisico sismologico volponi argentina consejo nacional de investigaciones cientificas y tecnicas argentina

dating antarctic blue ice areas using a novel ice flow model 1 we present a new type of flow model suitable for antarctic blue ice areas with application to dating ice for paleoclimate purposes the volume conserving model uses field data for surface velocities mass balance and ice thickness along a flow line with parameterized variation of ice rheology with depth to produce particle trajectories and isochrones the model is tested on the contrasting allan hills near western ice field and the scharffenbergbotnen blue ice fields in antarctica by comparing predicted ages with ages inferred from meteorites and 14c data during the glacial periods ice surface velocities at the allan hills must have been 25 less and accumulation rates 50 less than present day values in order to match meteorite ages in contrast scharffenbergbotnen velocities have probably been fairly constant over time due to the atypical valley where it resides

peraluminous grenvillian ttg in the sierra de pie de palo western sierras pampeanas argentina petrology geochronology geochemistry and petrogenetic implications abstract combined petrological geochemical isotopic and geochronological data shed light on the origin and evolution of a peraluminous garnet bearing two mica granitoid el tigre granitoid etg cropping out in southwestern sierra de pie de palo 31 31 30 s 68 15 12 w and to constrain the age and petrogenetic conditions of this intrusive event etg experienced amphibolite to greenschist facies metamorphism after igneous crystallization followed by strong deformation restricted to narrow mylonite zones etg shear zone and partial dynamic recrystallization under lower t conditions a dextral shear sense is compatible with kinematic observations registered along the nne striking regional las pirquitas overthrust active at 473 10 ma k ar on 1 1 normative corundum low cao values between 1 72 and 2 41 plotting mostly in the granite trondhjemite fields of the ab an or diagram the trace element contents show a relatively low abundance of rb hfs elements such as y nb ta ga and zr and high concentrations of ba sr and lree the chondrite normalized ree pattern has a high slope with la yb n 9 48 55 32 and a negative or absent europium anomaly relationships between trace elements suggest the classical setting of granitoids produced in a convergent plate setting a u pb shrimp crystallization age on zoned igneous zircon of 1105 5 4 1 ma suggests that the etg could be part of the magmatic complex forming the grenvillian basement of the western sierras pampeanas 87 sr 86 sr 1105 values of 0 70543 and nd of 4 2 indicate a rather immature source for its origin with similar initial isotopic ratios to those found in orthogneisses from elsewhere in the western sierras pampeanas on the other hand sm nd model ages t dm for the etg range from 1 20 to 1 39 ga geochemical and isotopic signatures of the etg could be explained by low pressure partial melting from a basaltic source under high geothermal gradient conditions this thermal anomaly could be associated with the 1 1 ga global period of enhanced mantle plume activity developing widespread global magmatism

dynamics and mass balance of taylor glacier antarctica 3 state of mass balance 1 taylor glacier flows from the east antarctic ice sheet and terminates in the mcmurdo dry valleys where it has left a geomorphologic record of past incursions here we use new data on the flow thickness and surface balance of taylor glacier to calculate ice fluxes and assess the current state of mass balance overall the glacier is close to a state of zero net balance and has largely adjusted to reduced snowfall on the taylor dome source region in the mid holocene one region of the upper ablation zone appears to be losing mass evidence from ice surface morphology and stable isotope profiles suggests that this mass loss is a long term phenomenon hence probably a lingering response to the earlier drying of taylor dome our data give a general indication of the size of ablation or accumulation changes needed for ice to advance far into taylor valley as occurred 70 to 130 ka ago

age constraints on the tectonic evolution and provenance of the pie de palo complex cuyania composite terrane and the famatinian orogeny in the sierra de pie de palo san juan argentina abstract new u pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the pie de palo complex represent a mesoproterozoic fragment of suprasubduction zone oceanic crust a gabbroic pegmatite interpreted to have formed during arc rifting or subsequent back arc spreading yielded a u pb age of 1204 5 3 4 7 ma highly tectonized ultramafic mafic cumulates occurring at the structural base of the pie de palo complex and previously interpreted to represent remnants of a primitive arc phase prior to rifting and back arc spreading could not be dated but should be older than 1204 ma if these inferences are correct tabular sill like bodies of leucogabbro diorite and calc alkaline tonalite granodiorite sills yielded ages of 1174 43 and 1169 8 7 ma respectively they may represent a younger more evolved arc phase established after arc rifting or a younger tectonically unrelated mesoproterozoic arc shrimp analysis of metamorphic zircon rims with low th u ratios in vvl 110 gave a 206pb 238u age of 455 10 ma similar to lower intercept dates determined by discordia lines combined these data indicate that the bulk of the amphibolite facies metamorphism present in the pie de palo complex was generated during the famatinian orogeny analysis of six single detrital zircon grains in a metasedimentary quartzofeldspathic garnet mica schist tectonically interleaved with the igneous rocks of the pie de palo complex and tentatively correlated with the difunta correa metasedimentary sequence of other workers yielded three age populations 1150 1160 ma 1050 1080 ma and 665 ma indicating that these sedimentary rocks were deposited during the late neoproterozoic or early paleozoic in addition they confirm structural evidence that intercalation of rocks of the pie de palo complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications these ages are also consistent with a laurentian provenance and earlier interpretations that these rocks once represented a sedimentary cover to the pie de palo complex the zircon population of 1050 1080 ma could be derived from grenville age felsic plutons identified elsewhere in the pie de palo complex by other workers however no evidence has been found in our samples for a grenville age orogenic event invoked previously to explain accretion of the oceanic pie de palo complex to laurentia prior to the late neoproterozoic early cambrian rifting and drift of cuyania

the neoproterozoic early paleozoic metamorphic and magmatic evolution of the eastern sierras pampeanas an overview the eastern sierras pampeanas were structured by three main events the ediacaran to early cambrian 580 510 ma pampean the late cambrian ordovician 500 440 ma famatinian and the devonian carboniferous 400 350 ma achalian orogenies geochronological and sm nd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event ediacaran dividing into two mesoproterozoic major crustal blocks source of the grenvillian age peaks in the metaclastic rocks this event would be coeval with the development of arc magmatism along the eastern margin of the eastern block closure of this eastern margin led to a cambrian active margin sierra norte arc along the western margin of the eastern block in which magmatism reworked the same crustal block consumption of a ridge segment input of oib signature mafic magmas which controlled granulite facies metamorphism led to a final collision pampean orogeny with the western mesoprotrozoic block sm nd results for the metamorphic basement suggest that the tdm age interval of 1 8 1 7 ga which is associated with the less radiogenic values of end 540 6 to 8 can be considered as the mean average crustal composition for the eastern sierras pampeanas increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the sierras de cordoba is associated with a younger tdm age and a more positive end 540 value pampean pre 540 ma granitoids form two clusters one with tdm ages between 2 0 and 1 75 ga and another between 1 6 and 1 5 ga pampean post 540 ma granitoids exhibit more homogenous tdm ages ranging from 2 0 to 1 75 ga ordovician re activation of active margin along the western part of the block that collided in the cambrian led to arc magmatism famatinian orogeny and related ensialic back arc basin in which high grade metamorphism is related to mid crustal felsic plutonism and mafic magmatism with significant contamination of continental crust tdm values for the ordovician famatinian granitoids define a main interval of 1 8 1 6 except for the ordovician ttg suites of the sierras de cordoba which show younger tdm ages ranging from 1 3 to 1 0 ga in devonian times achalian orogeny a new subduction regime installed west of the eastern sierras pampeanas devonian magmatism in the sierras exhibit process of mixing assimilation of depleted mantle signature melts and continental crust achalian magmatism exhibits more radiogenic end 540 values that range between 0 5 and 4 and tdm ages younger than 1 3 ga in pre devonian times crustal reworking is dominant whereas processes during devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism

crustal seismicity in the andean precordillera of argentina using seismic broadband data abstract in this study we analyze 100 crustal precordilleran earthquakes recorded in 2008 and 2009 by 52 broadband seismic stations from the siembra and esp two temporary experiments deployed in the pampean flat slab region between the andean cordillera and the sierras pampeanas in the argentine andean backarc region in order to determine more accurate hypocenters focal mechanisms and regional stress orientations we relocated 100 earthquakes using the jhd technique and a local velocity model the focal depths of our relocated events vary between 6 and 50 km we estimated local magnitudes between 0 4 ml 5 3 and moment magnitudes between 1 3 mw 5 3 focal mechanisms were determined from new hypocenter relocations and first motion p wave polarities our solutions exhibit a majority of the earthquakes with reverse faulting mechanism regional stress tensor from the inversion of p and t axis orientations shows a maximum stress axis 1 almost horizontal with a strike of 85 and a minimum stress axis 3 almost vertical we correlate this small to moderate magnitude seismicity with the presence of large basement structures beneath the iglesia calingasta basin in the west and the eastern precordillera in the east the nucleation of deep earthquakes beneath the iglesia basin could be related to the presence of a major ramp accommodating the crustal shortening between the frontal cordillera and the precordillera the crustal seismicity beneath the precordillera seems to correlate with west dipping structures rooting deep into the cuyania basement suggesting a thick skinned basement deformation system beneath the precordillera and its shallow thin skinned fold and thrust belt

instruments and methods a novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon we present techniques for obtaining large 100 l stp samples of ancient air for analysis of 14 c of methane 14 ch4 and other trace constituents paleoatmospheric 14 ch4 measurements should constrain the fossil fraction of past methane budgets as well as provide a definitive test of methane clathrate involvement in large and rapid methane concentration ch4 increases that accompanied rapid warming events during the last deglaciation air dating to the younger dryas preboreal and oldest dryas bolling abrupt climatic transitions was obtained by melt extraction from old glacial ice outcropping at an ablation margin in west greenland the outcropping ice and occluded air were dated using a combination of d 15 no f n 2 d 18 oo f o 2 d 18 oice and ch4 measurements the ch4 blank of the melt extractions was 4 ppb measurements of d 18 o and d 15 n indicated no significant gas isotopic fractionation from handling measured ar n2 cfc 11 and cfc 12 in the samples indicated no significant contamination from ambient air ar n2 kr ar and xe ar ratios in the samples were used to quantify effects of gas dissolution during the melt extractions and correct the sample ch4 corrected ch4 is elevated over expected values by up to 132 ppb for most samples suggesting some in situ ch4 production in ice at this site

neoproterozoic a type magmatism in the western sierras pampeanas argentina evidence for rodinia break up along a proto iapetus rift a type orthogneisses of mid neoproterozoic age 774 6 ma u pb shrimp zircon age are reported for the first time from the grenvillian basement of the western sierras pampeanas in argentina these anorogenic meta igneous rocks represent the latest event of rodinia break up so far recognized in grenvillian basement exposures across andean south america moreover they compare well with a type granitoids and volcanic rocks along the appalachian margin of laurentia blue ridge thus adding to former evidence that the western sierras pampeanas grenvillian basement was left on the conjugate rifted margin of eastern laurentia during rodinia break up and the consequent opening of the iapetus ocean

shear wave velocities in the pampean flat slab region from rayleigh wave tomography implications for slab and upper mantle hydration 1 the pampean flat slab region located in central argentina and chile between 29 and 34 s is considered a modern analog for laramide flat slab subduction within western north america regionally flat slab subduction is characterized by the nazca slab descending to 100 km depth flattening out for 300 km laterally before resuming a more normal angle of subduction flat slab subduction correlates spatially with the track of the juan fernandez ridge and is associated with the inboard migration of deformation and the cessation of volcanism within the region to better understand flat slab subduction we combine ambient noise tomography and earthquake generated surface wave measurements to calculate a regional 3d shear velocity model for the region shear wave velocity variations largely relate to changes in lithology within the crust with basins and bedrock exposures clearly defined as low and high velocity regions respectively we argue that subduction related hydration plays a significant role in controlling shear wave velocities within the upper mantle in the southern part of the study area where normal angle subduction is occurring the slab is visible as a high velocity body with a low velocity mantle wedge above it extending eastward from the active arc where flat slab subduction is occurring slab velocities increase to the east while velocities in the overlying lithosphere decrease consistent with the slab dewatering and gradually hydrating the overlying mantle the hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere helping to drive flat slab subduction

early paleozoic accretionary orogenies in nw argentina growth of west gondwana abstract two early paleozoic orogenic cycles in the gondwana margin of nw argentina were driven by subduction and interrupted by collision of continental ribbons of laurentian affinity subduction started at the passive margin of gondwana possibly in response to the end of amalgamation of the supercontinent the passive margin was underlain by a hyper extended continent ocean transition and low angle subduction gave rise to a wide and hot fore arc region during the pampean orogenic cycle the arrival of the first continent ribbon ended the pampean cycle steepened the subduction further heating the fore arc and restarting arc magmatism after a magmatic lull 250 300 km trenchwards initiating the famatinian orogenic cycle this cycle started with a 30 ma long period of extension and marine sedimentation followed by the arrival of the second continent ribbon and inversion the back arc initiating a 20 ma long shortening event culminating with the shutting down of the arc this event gave rise to a 300 km wide low topography hot orogeny thus the many peculiarities of the two paleozoic orogens of nw argentina result from a subduction history that efficiently transferred heat to a 300 400 km wide belt of turbidite dominated sediments that was first part of the pampean fore arc and then part of the famatinian retroarc developed at the extended continental margin of gondwana the two orogenies with continued high heat flux lasted 110 ma giving rise to two calc alkaline arcs separated by a 300 km belt of high t low p migmatites and peraluminous granites formed by anatexis of sediments deposited on the passive margin as well as those deposited in the wide pampean fore arc these turbidite dominated sequences were metamorphosed and melted to form a continental crystalline basement and accreted to the cratonic margin adding 500 km of crystalline rocks to the margin of gondwana at the same time that a similar process was happening in eastern australia along the same continental margin interestingly this wide accretionary orogen has now become the region where the andean system developed a wide orogeny above flat slab subduction

ordovician metamorphic event in the carbonate platform of the argentine precordillera implications for the geotectonic evolution of the proto andean margin of gondwana an ordovician metamorphic event is recorded by means of conodont paleothermometry in the slope sedimentary sequences of the western precordillera allochthonous conodonts from reworked deposits of the eastern carbonate platform with cai 4 5 5 and autochthonous conodonts from the olistostrome matrix with cai 3 allow for constraining the metamorphic age within the paltodus deltifer and lenodus variabilis zones i e ca 480 465 ma the conodont data support an ordovician instead of an alternative devonian collision for the cuyania terrane with the proto andean margin of gondwana which is consistent with the earlier proposed but still contentious microcontinent model in the latter scenario the collision of the cuyania terrane caused unroofing and cannibalism of the leading edge of the carbonate succession and basement rocks the erosion of these rocks supplied detritus and metamorphosed conodonts to the deeper basinal settings of the western precordillera as the deformation front migrated into the peripheral foreland of the eastern and central precordillera nonmetamorphosed conodonts were eventually incorporated into the west prograding synorogenic clastic wedge the occurrence of allochthonous and autochthonous conodonts in the slope facies of the western precordillera provides a time constraint for the early accretion of the cuyania terrane and its transition to an evolving foreland mountain system in the ordovician period