the early paleozoic evolution of the argentine recordillera as a laurentian rifted drifted and collided terrane a geodynamic model discussion and reply

foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

simple shear conjugate rift margins of the argentine precordillera and the ouachita embayment of laurentia the argentine precordillera was rifted from the ouachita embayment of laurentia during cambrian time the ouachita rifted margin along the texas promontory of laurentia exhibits a narrow zone of transitional crust a lack of synrift rocks and a thin passive margin succession indicating slow and limited postrift subsidence in contrast data from the western margin of the precordillera suggest more extensive synrift sediment accumulations and document a thick passive margin succession indicating more rapid and relatively greater postrift subsidence than along the ouachita margin of laurentia passive margin deposition began in latest early cambrian time around the precordillera but it did not begin before latest middle cambrian time around the ouachita embayment the contrasts in structure and stratigraphy are best explained by an asymmetric rift system in the context of simple shear low angle detachment models for continental rifting differences in subsidence rates and inferred crustal structure suggest that the rifted margin of the ouachita embayment represents an upper plate configuration whereas the conjugate margin on the western precordillera represents a lower plate configuration

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

the sandstone derived provenance record of the gualala basin northern california u s a abstract detrital zircon u pb ages and pb isotope compositions of detrital feldspars from the late cretaceous eocene gualala basin in northern california indicate that the basin received sediments from the southern sierra nevada the mojave block and the salinian block mesozoic detrital zircon age components and pb isotope compositions of detrital feldspars from the basin are similar to u pb zircon ages and pb isotope compositions from previously studied coeval conglomerate clasts the detrital zircons and feldspars however also include additional age and pb isotope components that are unobserved or poorly represented in the conglomerate based provenance record these new data suggest a more complex provenance history than that inferred from previously studied conglomerate clasts and strongly imply a connection to the mojave desert provenance evolution of the sediments in the basin indicates that significant contribution of sediment from the mojave desert began during the paleocene when it is inferred that the drainage network expanded eastward from its previous extent the presence of mojave derived late permian zircons as well as feldspars that have high 208pb 204pb ratios in the eocene age german rancho formation indicates that the basin remained in the vicinity of the mojave desert until the eocene this result demonstrates that the gualala basin had undergone very little northward motion at that time and consequently requires that no significant northward translation took place prior to 50 ma the results of this study highlight the value of additional provenance constraints combined with age distributions when identifying sediment source terranes

proterozoic early paleozoic evolution in western south america a discussion in tectonophysics 354 121 137 2002

paleozoic laurentia gondwana interaction and the origin of the appalachian andean mountain system laurentia the rift bounded precambrian nucleus of north america may have broken out from a neoproterozoic supercontinent between east and west gondwana several lines of evidence suggest that the appalachian margin of laurentia subsequently collided with the proto andean margin of the amalgamated gondwana supercontinent in different relative positions during early and mid paleozoic time in route to final docking against northwest africa to complete the assembly of pangea hence the appalachian and andean orogens may have originated as a single mountain system the overall hypothesis retains the same paleomagnetic and paleobiogeographic controls as previous global reconstructions for the paleozoic era laurentia gondwana collisions may help to explain contemporaneous unconformities in the paleozoic sedimentary cover of the laurentian gondwanan and baltic cratons

seasonal and universal time distribution of patches in the northern and southern polar caps ion density measurements from the dynamics explorer 2 and defense meteorological satellite program f8 and f9 satellites are used to examine hemispherical differences in the occurrence patterns of polar ionization patches as a function of season universal time ut and interplanetary magnetic field imf when bz 0 the greatest frequency of patch occurrence in the northern hemisphere is in the winter in the 1000 2200 ut range this time corresponds to the interval when the northern magnetic pole and hence the cusp lies the farthermost toward the dayside this fact is often used to explain the creation of patches in terms of the entrainment of dayside plasma into the cusp by high latitude convection in the southern hemisphere we see that the occurrence frequencies peak at over twice the northern hemisphere values in the same general ut region 1000 2300 ut however the southern hemisphere cusp is most dayward at approximately 0300 ut a time of minimum patch formation examination of the relationship of the terminator to the polar cap boundaries in each hemisphere leads to a simple explanation in terms of the differing offset of the magnetic poles and the ionospheric convection pattern from the geographic poles

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

neodymium isotopic composition of ordovician conodonts as a seawater proxy testing paleogeography 1 to evaluate recent ordovician paleogeographic reconstructions end values were determined in conodonts as a proxy for the isotopic variations in ancient seawater using samples from the major cratons and microplates isotopic variations reveal the existence of distinct oceanic masses and epeiric seas that constrain the position of plates and terranes relative to these water masses and hence constrain models of regional and global paleogeography the isotopic patterns show a consistent picture of the changes in the ordovician oceans the end values for water masses associated with laurentia are strongly negative 28 to 18 in the early ordovician evolving over time to higher values in the range 13 to 5 the early ordovician signature of laurentia is in marked contrast to other cratons and microplates which have a range of values from 10 to 5 samples from south china show interesting signals that reflect a greater similarity to laurentia than other peri gondwana terranes the isotopic variations are a function of both regional geology and global tectonic processes the most obvious being the taconic orogeny and onset of the closure of the iapetus ocean regional and global models of paleogeography are considered in light of these proxy signals this study also reveals that conodonts are powerful geochemical tools for obtaining information on ancient water masses the use of the nd isotopic signatures from conodonts provides a method independent of paleomagnetism and biogeography to test paleogeographic models by integrating such information a more multidisciplinary approach is possible

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

mapping geoelectric fields during magnetic storms synthetic analysis of empirical united states impedances empirical impedance tensors obtained from earthscope magnetotelluric data at sites distributed across the midwestern united states are used to examine the feasibility of mapping magnetic storm induction of geoelectric fields with these tensors in order to isolate the effects of earth conductivity structure we perform a synthetic analysis calculating geoelectric field variations induced by a geomagnetic field that is geographically uniform but varying sinusoidally with a chosen set of oscillation frequencies that are characteristic of magnetic storm variations for north south oriented geomagnetic oscillations at a period of t0 100 s induced geoelectric field vectors show substantial geographically distributed differences in amplitude approximately a factor of 100 direction up to 130 and phase over a quarter wavelength these differences are the result of three dimensional earth conductivity structure and they highlight a shortcoming of one dimensional conductivity models and other synthetic models not derived from direct geophysical measurement that are used in the evaluation of storm time geoelectric hazards for the electric power grid industry a hypothetical extremely intense magnetic storm having 500 nt amplitude at t0 100 s would induce geoelectric fields with an average amplitude across the midwestern united states of about 2 71 v km but with a representative site to site range of 0 15 v km to 16 77 v km significant improvement in the evaluation of such hazards will require detailed knowledge of the earth s interior three dimensional conductivity structure

a silurian devonian marine platform deltaic system in the san rafael block argentine precordillera cuyania terrane lithofacies and provenance fil manassero marcelo jorge consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet la plata centro de investigaciones geologicas universidad nacional de la plata facultad de ciencias naturales y museo centro de investigaciones geologicas argentina

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

40ar 39ar and nd sr isotopic characteristics of mid ordovician north american k bentonites a test of early paleozoic laurentia gondwana interactions highly altered tephra layers k bentonites of mid ordovician age are widespread throughout the midcontinent region of the united states and the appalachian taconic foreland basin the numerous ash layers their wide areal extent and their large volume provide evidence that extensive volcanic activity accompanied taconic orogenesis the mineralogical and geochemical characteristics of these widespread ordovician tephra layers can be used to constrain the nature of their source region and thereby help test recent hypotheses that gondwana collided with laurentia in mid ordovician time initial 143nd 144nd and 87sr 86sr ratios of two k bentonites that occur traced throughout the midcontinent are 0 51183 0 51187 and 0 7101 0 7120 respectively isotopic ratios for k bentonites within the taconic basin are 0 51178 0 51201 and 0 70564 0 70905 the isotopic composition of the tephra layers and the ubiquitous presence of proterozoic zircon xenocrysts within the k bentonites is consistent with an origin by crustal anatexis the cooling history of the source is constrained by 842 996 ma 40ar 39ar apparent ages of hornblende xenocrysts contained within two taconic basin k bentonites a possible south american source region of the k bentonites is the arequipa massif of peru this region contains early paleozoic arc related rocks many of which contain mid proterozoic zircon xenocrysts and early proterozoic gneisses which were affected by grenville age granulite grade metamorphism gneisses within the adirondack highlands of the grenville province have similar end 450 ma values and hornblende cooling ages to the k bentonites consistent with this region as a source caradocian k bentonites in sweden have sr isotopic ratios similar to the north american beds if the swedish and north american k bentonites were produced from the same source then baltica and laurentia were proximal in ordovician time precluding a gondwana laurentia link if caradocian k bentonites with north american characteristics are found in western south america support for a laurentia gondwana link would be established if such k bentonites cannot be found considerable doubt would be cast on this model

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

systematic deviations in source direction estimates of q bursts recorded at nagycenk hungary q bursts are globally detectable extremely low frequency elf 3 3000hz band wave packets produced by intense lightning discharges q bursts recorded in the szechenyi istvan geophysical observatory nck 16 7 e 47 6 n hungary on 1 and 2 august 2012 have been analyzed to find azimuths of their sources the location of parent lightning strokes of 320 and 205 q bursts on the 2days respectively have been identified in the records of the world wide lightning location network wwlln using the detection times at nck elf data based source azimuths were found to differ systematically from source azimuths obtained from wwlln lightning locations the difference between the corresponding azimuth values depends on the azimuth of the source this variation of the source azimuth error mirrors the symmetry of the conductance of the earth s crust inferred from magnetotelluric measurements around nck after correction for the azimuthal dependence the variation of the residual error shows a diurnal pattern with positive azimuth deviations occurring near midnight local time fullekrug and sukhorukov 1999 suggested that the anisotropic conductivity in the earth s crust below the observatory and the different daytime and nighttime conductivities in the lower ionosphere respectively may cause the identified error terms our results emphasize the substantial effect of anisotropic conductivity in the earth s crust around the recording station on the accuracy of elf direction finding the need for theoretical approach and more measurements is pointed in understanding the underlying mechanisms quantitatively and in investigating whether elf observations can be used in geophysical prospecting 2016 american geophysical union all rights reserved

basement evolution of the sierra de la ventana fold belt new evidence for cambrian continental rifting along the southern margin of gondwana u pb sensitive high resolution ion microprobe data together with geochemical and nd isotope analyses obtained in the basement complex of the sierra de la ventana fold belt indicate that the early palaeozoic passive margin history of the basin followed cambrian magmatism related to rifting in a 600 ma neoproterozoic crust the cambrian episode started with intrusion of 531 4 and 524 5 ma a and i type granites derived from a dehydrated infracrustal source e nd 530 3 1 to 5 9 and culminated with eruption of high zr peralkaline spherulitic rhyolites derived from an undepleted lithospheric mantle 509 5 ma e nd 509 0 5 to 1 0 these rift related magmatic rocks were covered by shelf sediments deposited along a once continuous passive margin encompassing the sierra de la ventana fold belt the cape fold belt the falkland malvinas microplate and the ellsworth mountains block in antarctica the cambrian rifting event defined the outline shape of the southern part of gondwana and can be regarded as the initiation of the supercontinent stage which lasted until jurassic break up the conjugate continental fragments separated from gondwana during the cambrian rifting could be the source for microcontinents with c 1000 ma basement rocks that collided with the proto andean margin during ordovician silurian times

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

mafic and ultramafic crustal fragments of the southwestern precordillera terrane and their bearing on tectonic models of the early paleozoic in western argentina new u pb zircon ages and initial nd isotope data from mafic and ultramafic rocks that have been tectonically imbricated on the southwest margin of the precordillera terrane western argentina indicate that these rocks do not constitute a single ophiolite pseudostratigraphy as previously thought rock units include late proterozoic 576 17 ma microgabbro and silurian 418 10 ma sills both of which have initial nd isotope values of 6 0 to 7 3 and identical trace element geochemistry indicating formation in extensional settings late ordovician 450 20 ma layered gabbro has a distinct initial nd isotope value of 0 6 it intrudes quartzofeldspathic gneiss and tectonized ultramafic rocks this assemblage represents the deep part of a continental margin volcanic arc that formed on the eastern margin of chilenia as the ocean basin west of the precordillera terrane began to close in the middle ordovician the new tectonic interpretations generally support existing hypotheses that the precordillera terrane rifted and drifted from southeastern laurentia in the cambrian to early ordovician and that the western margin of the precordillera terrane represents a suture between two fragments of continental crust

2 05 ga isotopic ages for transvaal mississippi valley type deposits evidence for large scale hydrothermal circulation around the bushveld igneous complex south africa abstract the known dimensions of hydrothermal fluid circulation associated with emplacement of the 2 055 ga bushveld igneous complex bic are extended with new isotopic ages for paleoproterozoic mineral deposits in wallrocks transvaal supergroup surrounding the bic a fluorite rich mississippi valley type mvt deposit in the zeerust district dated at 2 05 ga sm nd isochron age has nd isotopic compositions that are identical to coeval f rich lebowa granite late in the bic intrusive sequence suggesting transport of magmatic fluids 100 km beyond the present margins of the bic hydrothermal illite from the zn pb mvt bushy park deposit dated at 2 05 ga by k ar geochronology links widespread mvt deposits of the kaapvaal craton to large scale basinal brine circulation associated with emplacement of the bic these data in combination with gold vein phosphate mineral and paleomagnetic resetting ages indicate a regional scale bushveld age hydrothermal system circulating 700 km outward from the prese

new perspectives on graptolite distributions and their use as indicators of platform margin dynamics graptolite distributions in ordovician shelf slope and basinal facies in the great basin indicate that graptolites were scarce in open oceanic waters oceanward of the cordilleran shelf margin and that they thrived in waters above the margin this pattern is consistent with that of most zooplankton in modern oceans it follows from these observations that the depositional setting of typical graptolitic shale was the area of the sea floor under continental margin upwelling zones where graptolites flourished and within the oxygen minimum zone where their rhabdosomes were preserved with changes over time in relative sea level deep oceanic circulation and wind driven surface circulation the upwelling and oxygen minimum zones may have thickened or thinned migrated landward or oceanward and expanded laterally contracted or even disappeared the observed graptolite occurrences suggest that the primary graptolite biotope that is the habitat of diverse and abundant faunas was a relatively narrow belt of upwelling waters along and extending somewhat open oceanward from the continental margin provinces were maintained only to the extent that species could disperse along continental margins distribution of typical graptolitic strata may be used to interpret development of continental margins because such distribution incorporates a signal of sea level rise or fall oceanographic changes especially upwelling and tectonic events that led to creation and deterioration of upwelling conditions in which graptolites flourished

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

simultaneous observations of geoelectric and geomagnetic fields produced by magnetospheric ulf waves nasanational aeronautics space administration nasa nas5 02099 80nssc19k0907 u s geological survey mendenhall postdoctoral fellowshipunited states geological survey nsfnational science foundation nsf ags 1004814 ear0323311 iris under nsf cooperative agreement 478 489 ear0350030 ear0323309 ear0733069 75mt cfda 47 050 05osusage ear1261681 seismological facilities for the advancement of geoscience sage award of the national science foundation ear 1851048

nd isotopic constraints on sediment sources of the ouachita marathon fold belt nd isotopes for the overthrust deep marine ouachita marathon sedimentary assemblage of arkansas oklahoma and west texas and associated paleozoic shelf and foreland deposits resolve into three distinct populations 1 lower to middle ordovician nd 13 to 16 average t dm 2 0 ga 2 upper ordovician to pennsylvanian nd 6 to 10 average t dm 1 6 ga and 3 mississippian tuffs nd 1 to 3 average t dm 1 1 ga a rapid shift in nd from 15 passive margin shales to 7 orogenic turbidites in the ouachita assemblage at ca 450 ma implies termination of craton dominated sources and the emergence of the appalachian orogen as the primary source of sediment for sea floor lying south of north america this connection is reinforced by nd isotopes in ordovician silurian turbidites from both the ouachita assemblage and the southern appalachian sevier martinsburg taconic foredeep which are identical nd 7 to 9 the post 450 ma ouachita assemblage falls along a single nd isotopic trend that significantly is not deflected by onset of carboniferous flysch nd 7 to 10 sedimentation nor by associated regional volcanism the less negative nd 2 of mississippian ash flow tuffs that erupted from arc s to the south probably resulted from isotopic mixing of old precambrian crust with young mantle derived components within a continental margin arc there is little isotopic trace element or petrographic evidence for any significant volcaniclastic detritus in the carboniferous turbidites indicating that volcanic arc sources were minimal nd isotopes in fluvio deltaic strata of the ouachita appalachian foreland and continental interior that is arkoma illinois and black warrior basins nd 7 to 10 imply that continental margin pathways and interior basins received the same detritus as the ouachita trough by pennsylvanian time these data are consistent with a composite carboniferous ouachita submarine fan complex built down the axis of a remnant ocean basin from varied mature immature delivery systems tapping dominantly appalachian fold thrust belt sources to the east graham et al 1975 carboniferous turbidites from the marathon fold belt west texas which are isotopically similar nd 8 to 11 to ouachita turbidites may have been ultimately derived from similar sources however they probably do not represent merely distal turbidites of a ouachita fan complex it is suggested that dominantly appalachian derived detritus augmented by uplifted plutonic and fold thrust belt sources south of the marathon basin was swept up into subduction complexes on the north side of the approaching arc and recycled along the collision zone

provenance of the lower paleozoic balcarce formation tandilia system buenos aires province argentina implications for paleogeographic reconstructions of sw gondwana abstract lower paleozoic moderately sorted quartz arenites from the balcarce formation deposited in eastern argentina tandilia system comprise mainly detrital material derived from old upper crustal material the sources were magmatic sedimentary and subordinated felsic metamorphic terranes high concentrations of tourmaline and ti rich heavy minerals including zircon and nearly euhedral chromite are common trace element concentrations nb cr on rutile indicate pelitic and metabasaltic sources respectively major element analyses on chromites indicate a basic volcanic protolith of mid oceanic ridge origin which was exposed close to the depositional basin the delivery of chromite may be associated with convergent tectonics causing the consumption and obduction of oceanic crust during pre upper ordovician times the oblique orthogonal collision of the precordillera terrane with the western border of the rio de la plata craton west of the balcarce basin or source further to the east from a lower palaeozoic extensional basin are possibilities geochemical and petrographic data exclude the underlying precambrian and cambrian sedimentary rocks as dominant sources and favour the basement of the rio de la plata craton including cambrian rift related granites of south africa and the sierras australes eastern argentina as main suppliers of detritus trace element geochemistry of recycled pyroclastic material associated with the quartz arenites also suggests volcanic arc sources the provenance of the pyroclastic material may either be the puna famatina arc located in north and central argentina or a hypothetical active margin further to the south these ash layers are equivalent in age to volcanic zircons found in the devonian bokkeveld group in western south africa the deposition of a glacial diamictite of hirnantian age sierra del volcan diamictite is interpreted as a member of the balcarce formation based on the stratigraphic re location of the glacial diamictite and trace fossils the balcarce formation is considered here to be ordovician to silurian in age the balcarce formation can be correlated with similar rocks in south africa the peninsula formation and the upper table mountain group windhoek and nardouw subgroups including the hirnantian glacial deposit of the pakhuis formation

conodont palaeothermometry of contact metamorphism in middle ordovician rocks from the precordillera of western argentina the yerba loca formation middle upper ordovician exposed in the western precordillera san juan argentina is made up of clastic carbonate turbidites and basic ultrabasic rocks it is affected by regional siluro devonian very low grade metamorphism that locally reaches greenschist facies at ancaucha creek 45 conodont samples were taken from two sections that include 30 to 50 m thick sills in order to analyse the thermal alteration patterns produced by these intrusive bodies conodont colour alteration index cai is contrasted with optical petrography and x ray diffraction analyses of clay minerals the intrusions are dated as post darriwilian da2 as determined by conodont biostratigraphy of the host rock which indicates the paroistodus horridus subzone of the lenodus variabilis zone the distribution of cai values defines a thermal aureole of about 2 5 times intrusion thickness that prevailed over the later very low grade metamorphism metasomatism at ancaucha creek is recorded by cai values of 4 to 7 particularly restricted to a few layers close to the intrusions as indicated by conodont textures and rock fabric one dimensional thermal computer simulation conforms to empirical data indicating temperatures greater than 600 c for the contact zone although it points out slightly narrower thermal aureoles the clay mineral assemblage of most of the analysed samples chlorite illite smectite and i s mixed layers is complex and probably derives from several superimposed processes thus representing non equilibrium assemblages in turn ki values 0 27 0 32 indicate anchizone metamorphism in agreement with regional cai values of 4 consequently the occurrence of smectite and i s probably resulted from retrograde diagenesis processes

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

high resolution stable isotope stratigraphy of the upper cambrian and ordovician in the argentine precordillera carbon isotope excursions and correlations abstract we report the occurrence of important carbon isotope excursions in early paleozoic formations of the eastern and central argentine precordillera the steptoean positive isotope carbon excursion spice is known from north america kazakhstan south china australia and south america and the negative isotope carbon excursion snice has been described for the first time in south america we report here the record of the spice and snice in a single section in the eastern precordillera in the central precordillera a minor middle darriwilian positive carbon isotope excursion mdice and a late sandbian positive isotope carbon excursion the gice 3 c bicornis biozone are reported from two sections one pre gice positive carbon isotope excursion sandbian sa1 n gracilis biozone in the central precordillera with a 13c peak of 2 is perhaps equivalent to the positive spechts ferry 13c excursion of north america a positive 13c excursion 6 n persculptus biozone recorded at the base of the late hirnantian la chilca formation probably corresponds to hice these carbon isotope excursions resulted from more than one factor where paleoceanographic events probably played a major role a sea level fall and vigorous fluctuations in the steptoean spice b sea level rise in the sunwaptan snice c important transgression in the sandbian pre gice and gice and d sea level fall in the late hirnantian hice in the darriwilian and sandbian stages organic burial has led to a large 12c sequestration in the deep anoxic ocean with saline bottom water recorded by the graptoliferous black shales of the gualcamayo and los azules formations in central precordillera helped the building of the mdice and gice anomalies 18o values for the upper cambrian carbonates are likely near primary isotope signals that point to progressive cooling from the spice to the snice whereas for sandbian carbonates they suggest strong temperature fluctuations the 13c peak of the gice coincides with a cooler period with temperatures warming up towards the late hirnantian to rhuddanian the upper cambrian to the middle ordovician of the precordillera recorded a decrease of seawater 87sr 86sr ratios in accordance to the global picture this decrease probably reflects the influence of widespread volcanic activity from arc terranes in low latitude settings along eastern margins of laurentia and in the argentine precordillera in the early to middle ordovician five carbonate samples from san juan gualcamayo and los azules formations yielded end values that along seven already published values seem to plot along the nd isotopic evolution trend of the iapetus ocean together the record of global upper cambrian and ordovician carbon isotope excursions in the argentine precordillera is a valuable proxy in refining early paleozoic stratigraphy establishing of regional global high resolution correlations and sea level change history in south america

provenance analysis and tectonic setting of the ordovician clastic deposits in the southern puna basin nw argentina provenance studies on early to middle ordovician clastic formations of the southern puna basin in north western argentina indicate that the sedimentary detritus is generally composed of reworked crustal material tremadocquartzrich turbidites tolar chico formation mean composition q t 89 f7 l4 are followed by volcaniclastic rocks and greywackes tolillar formation mean q t 33 f42 l25 these are in turn overlain by volcaniclastic deposits mean qt24 f30 l46 of the diablo formation late arenig early llanvirn that are intercalated by lava flows all units were deformed in the ocloyic orogeny during the middle and late ordovician sandstones of the tolar chico formation are characterized by th sc ratios 1 la sc ratios 10 whereas associated fine grained wackes show slightly lower values for both ratios lree light rare earth elements enrichment of the arenites is 50 chondrite eu eu values are between 0 72 and 0 92 and flat hree heavy rare earth elements patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition the e n d t s e d values scatter around 11 to 9 the calculated nd t d m residence ages vary between 1 8 and 2 0 ga indicating contribution by a palaeoproterozoic crustal component the th sc and la sc ratios of the tolillar formation are lower than those of the tolar chico formation normalized ree rare earth elements patterns display a similar shape to paas post archaean average australian shale but with higher abundances of hrees eu eu values range between 0 44 and 1 17 where the higher values reflect the abundance of plagioclase and feldspar bearing volcanic lithoclasts average e n d t s e d values are less negative at 5 1 and nd t d m are lower at 1 6 ga this is consistent with characteristics of regional rocks of upper continental crust composition which most probably represent the sources of the studied detritus the rocks of the diablo formation have the lowest th sc and la sc ratios lower lree abundances than the average continental crust and are slightly enriched in hrees eu eu values are between 0 63 and 1 17 the nd isotopes e n d t s e d 3 to 1 t d m 1 2 ga indicate that one source component was less fractionated than both the underlying early ordovician and the overlying middle ordovician units synsedimentary vulcanites in the diablo formation show the same isotopic composition our data indicate that the sedimentary detritus is generally composed of reworked crustal material but that the diablo formation appears to contain 80 of a less fractionated component derived from a contemporaneous continental volcanic arc there are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the gondwana margin during the ordovician

strontium isotopes age and tectonic setting of cambrian salinas along the rift and transform margins of the argentine precordillera and southern laurentia abstract the argentine precordillera generally is interpreted as a fragment of continental crust and sedimentary cover that was rifted from the ouachita embayment of southern laurentia in cambrian time synrift sedimentary successions in the northern part of the precordillera and in the birmingham graben of southern laurentia include thick evaporite deposits analyses of strontium isotopes indicate that the evaporites are of late early cambrian age consistent with biostratigraphic correlations variations in strontium ratios indicate that these early cambrian salinas had a persistent base of marine water and that contributions from meteoric runoff varied through time and with location the stratigraphic and isotopic data suggest a depositional setting of restricted circulation within a system of extensional fault blocks the salinas occupied graben blocks that were connected to marine circulation and that intermittently received meteoric runoff carrying fine detritus and radiogenic strontium from horst b

global mass wasting at continental margins during ordovician high meteorite influx the break up of an asteroid led to the rapid delivery of meteorites to earth around 470 million years ago the widespread occurrence of sedimentary megabreccias deposited at the same time suggests this high influx of meteorites could have triggered mass wasting downslope movement of rock and sediment at continental margins by inducing the destabilization of sedimentary slopes

three dimensional electrical resistivity of the north central usa from earthscope long period magnetotelluric data abstract we present initial results from three dimensional inversion of long period earthscope magnetotelluric mt transportable array data from 232 sites covering the north central us the study area covers the 1 1 ga mid continent rift mcr system which cuts across a series of archean and paleoproterozoic lithospheric blocks the western arm of the mcr is clearly evident in shallow depth sections with a narrow resistive core flanked by elongate conductive basins other prominent upper crustal features mapped include the moderately conductive michigan and illinois basins and extremely high conductivities in foreland basin rocks at the southern margin of the superior craton the most prominent conductive anomalies in an otherwise relatively resistive mid lower crust are two elongate east west oriented structures which are closely aligned with previously inferred continental sutures the first underlies the southern margin of the superior craton just north of the niagara fault nf and can be associated with the 1 85 ga penokean orogeny a second further south beneath iowa and western wisconsin lies just south of the spirit lake tectonic zone sltz and can be identified with yavapai accretion at 1 75 ga both of these conductive sutures are cleanly cut by the mcr which is otherwise not clearly evident in the deeper parts of the resistivity model the break in the anomalies is narrow comparable to the surface expression of the mcr indicating that rifting impacts on the entire crustal section were highly localized both suture related anomalies are imaged as extending into and perhaps through the lithosphere as dipping diffuse zones of reduced mantle resistivity sense of dip of these structures southward for the nf anomaly northward for sltz agrees with previously inferred models for subduction and accretion suggesting that a conductive phase most likely carbon has been thrust deep into the lower crust and uppermost mantle providing a marker of the three dimensional boundary between lithospheric blocks resistivities drop below 100 m below 200 km depth in rough agreement with the seismically determined lithosphere asthenosphere boundary lab there are modest lateral variations in this deep low resistivity layer but the reliability and significance of these are not yet clear

first late ordovician paleomagnetic pole for the cuyania precordillera terrane of western argentina a microcontinent or a laurentian plateau abstract time and tectonic processes involved in docking of the argentine precordillera cuyania terrane against sw gondwana has been a matter of much debate a paleomagnetic study on the early caradoc pavon formation exposed in the san rafael block province of mendoza argentina is presented after detailed thermal and alternating field demagnetizations two geologically significant magnetic components were defined a widespread post tectonic component a is present in most sites of the pavon formation with dual polarities and is coincident with the characteristic remanence isolated from a permo triassic rhyolitic dome intruding the sediments its pole position 83 7 s 271 0 e dp 6 8 dm 9 0 n 11 sites falls on the late permian early triassic south american reference poles suggesting that this component was acquired during the choiyoi magmatic phase a second component b also shows dual polarities and a positive fold test suggesting a primary origin unblocking temperatures and rock magnetic experiments indicate that b is carried either by hematite or magnetite at different sites anisotropy of magnetic susceptibility results suggest a depositional fabric and no remanence distortion due to deformation or compaction a paleomagnetic pole computed from this remanence pv falls on 3 6 n 346 4 e dp 2 9 dm 4 6 n 22 samples it indicates a paleolatitude around 26 s for deposition of pavon sediments and constrains the paleogeographic evolution of cuyania during the ordovician which was still at subtropical latitudes by the early caradoc pv is consistent with the laurentian late ordovician reference pole if cuyania remains attached to se laurentia for the early caradoc while it shows a significant cw rotation with no paleolatitude anomaly respect to the gondwana reference pole when kept in its present position in sw south america these comparisons are interpreted in three possible alternatives for the paleogeographic and tectonic setting of cuyania in the late ordovician

salterella in the argentine precordillera an early cambrian palaeobiogeographic indicator of laurentian affinity abstract this paper documents salterella maccullochi murchison in the precordillera of western argentina in the present day southern hemisphere the record of these early cambrian small conical shaped fossils assigned to the extinct phylum agmata provides further palaeobiogeographic evidence to consider the argentine precordillera an exotic terrane to gondwana that originally rifted from laurentia for palaeogeographic reconstructions salterella can be considered as an organism equally important and biogeographically as meaningful as the olenellid trilobites due to their limited dispersal and demonstrated link to laurentian shallow water inner shelf facies it has not been found anywhere in gondwana salterella existed for only a very short interval of time but spread almost instantaneously around laurentia on this basis the early cambrian of the precordillera terrane remains the more conclusive palaeobiogeographical link to laurentia salterella provides another line of evidence supporting palaeontological sedimentologic stratigraphic palaeomagnetic and isotopic evidence for a laurentia origin of the precordillera terrane

the crustal evolution of south america from a zircon hf isotope perspective hf isotope data of 1100 detrital zircon grains from the palaeozoic south central andean gondwana margin record the complete crustal evolution of south america which was the predominant source the oldest grains with crustal residence ages of 3 8 4 0ga are consistent with complete recycling of existing continental crust around 4ga we confirm three major archaean palaeoproterozoic transamazonian and late mesoproterozoic to early neoproterozoic crust addition phases as well as six igneous phases during proterozoic to palaeozoic time involving mixing of juvenile and crustally reworked material a late mesoproterozoic to early neoproterozoic grenville age igneous belt can be postulated along the palaeo margin of south america this belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages crustal reworking likely dominated over juvenile addition during the palaeozoic era and proterozoic and archaean zircon was mainly crustally reworked from the eroding thickened ordovician famatinian arc

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

early palaeozoic brachiopods and associated shelly faunas from western gondwana their bearing on the geodynamic history of the pre andean margin

faulting and erosion in the argentine precordillera during changes in subduction regime reconciling bedrock cooling and detrital records abstract the argentine precordillera is an archetypal retroarc fold and thrust belt that records tectonics associated with changing subduction regimes the interactions between exhumation and faulting in the precordillera were investigated using apatite and zircon u th sm he and apatite fission track thermochronometry from the precordillera and adjacent geologic domains inverse modeling of thermal histories constrains eastward in sequence rock cooling associated with deformation and erosion from 18 to 2 ma across the central precordillera tracking thrusting during this time the youngest ahe ages 5 2 ma and highest erosion rates are located in the eastern and western extremities of the precordillera and indicate that recent denudation is concentrated at its structural boundaries moreover synchronous rapid pliocene cooling of the frontal cordillera eastern precordillera and sierra del valle fertil was coeval with initiation of basement involved faulting in the foreland detrital zircon u pb geochronology from the ca 16 8 1 ma bermejo foreland basin strata suggests fluvial connectivity westward beyond the frontal cordillera to the main cordillera and coast range followed by an important shift in sediment provenance at ca 10 ma at this time we suggest that a substantial decrease in permo triassic igneous sources in the frontal cordillera and concurrent increase in recycled zircons signatures of paleozoic strata are best explained by uplift and erosion of the precordillera during widening of the thrust belt bedrock thermochronology and modeling indicate a 2 6 myr lag time between faulting related cooling in the hinterland and the detrital record of deformation in the foreland basin suggesting that for tectonically active semi arid settings bedrock cooling may be more sensitive to onset of faulting we suggest that high erosion rates in the frontal cordillera and eastern precordillera are associated with increased interplate coupling during shallowing of the subducting nazca plate that may concentrate stress along weak structural boundaries of the precordillera

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

polar patches auroral zone precipitation effects 1 we look at patches observed by digital ionosondes in the northern polar cap these observations show that patches have the following properties 1 patches are relatively large typically 500 km in the sunward antisunward direction and 1000 km in the dawn dusk direction 2 when the imf bz has abnormally large positive negative swings the patches show correlation with the imf bz swings 3 for typical days the patches show no average correlation with imf by and only weak correlation with negative imf bz 4 patches are slightly weaker in summer the typical patch max to min ne variation in winter is 2 1011 el m3 and in summer it is 1 5 1011 el m3 5 there is restructuring of patches particularly the small scale structures as the patches convect across the polar cap 6 patches are formed prior to entering the polar cap and their properties stay about the same thereafter we explain the patch generation and most of these properties by low energy precipitation into the returning flow from the nightside to the dayside around the dawn convection cell

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

nd isotopes of atoka formation pennsylvanian turbidites displaying anomalous east flowing paleocurrents in the frontal ouachita belt of oklahoma implications for regional sediment dispersal abstract within the pennsylvanian atoka formation of oklahoma strata in the frontal ouachita belt deposited by east flowing turbidity currents yield the same end values as strata in more interior parts of the ouachita orogen deposited by west flowing turbidity currents the congruence of end values throughout the atoka formation is consistent with widespread paleozoic dispersal and recycling of detritus from the evolving appalachian orogen across the surface of the laurentian craton and along its margins sandstone in the olistostromal johns valley shale underlying the atoka formation yields a similar end value reflective of reworking appalachian derived sediment from the paleozoic oklahoma shelf succession

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

the electric field in northern england and southern scotland implications for geomagnetically induced currents summary magnetotelluric mt data in the form of mt tensors are used to estimate directly the size and spatial distribution of the electric field in northern england and southern scotland with the aim of predicting the flow of geomagnetically induced currents gic in power networks in the region mt and geomagnetic deep sounding data from a number of different field campaigns at a period of 750 s are employed the mt data are cast in the form of telluric vectors which allow a joint hypothetical event analysis hea of both geomagnetic deep sounding and mt data this analysis reveals qualitatively the pervasive effects of electric field distortion in the region two approaches are taken to understand how the spatial structure of the regional electromagnetic field is affected by local distortions and what the origin of these distortions might be the dimensionality and form of electric field distortion of the mt tensors is investigated using the weaver et al and bahr classification schemes and by examining the misfit of a galvanic distortion model as a function of rotation angle at sites where the galvanic distortion model is found to be appropriate the regional mt tensors are recovered using tensor decomposition techniques it is found that recovering the regional mt response reconciles the geometry of induced currents implied by the mt data with that of the magnetic variation anomalies lilley s central impedances are used to calculate rotationally invariant effective telluric responses in the southern uplands the magnitude of the effective telluric response is approximately 0 25 0 5 mv km 1 nt 1 but as the southern uplands fault is approached it rises steadily to 3 mv km 1 nt 1 in the midland valley the effective telluric response is approximately 0 5 mv km 1 nt 1 which rises steadily to 2 5 mv km 1 nt 1 as the southern uplands and highland boundary faults are approached to the southeast and northwest respectively therefore the increase in the magnitude of the effective telluric response correlates with the approach of a major tectonic boundary such as the southern uplands fault these results show that the induced electric field strength varies considerably throughout the central scotland region in addition the hea indicates that due to lateral changes in conductivity structure the direction of the electric field deviates significantly from the regional direction implied by the polarization azimuth of the primary geomagnetic induction therefore any attempts to model the flow of gic in the region need to account for the spatial variation of both the magnitude and azimuth of the electric field

provenance and evolution of the guarguariiz complex cordillera frontal argentina abstract the guarguardz complex basement of the cordillera frontal included in the proposed chilenia terrane consists of metasedimentary rocks deposited in clastic and carbonatic platforms turbiditic sequences point out to slope or external platform environments according to geochemical data the sedimentary protoliths derived through erosion of a mature cratonic continental basement volcanic and subvolcanic rocks with n and e morb signature were interbeded in the metasedimentary rocks during basin development a compressional stage starting with progressive deformation and metamorphism followed this extensional stage continuing deformation led to the emplacement of slices of oceanic crust conforming an accretionary prism during late devonian the guarguardz complex and equivalent units in western precordillera and also in the chilean coastal cordillera share common evolutional stages widely represented along the western gondwana margin these evidences imply that chilenia is not an allochthonous terrane to gondwana but a portion of its early paleozoic margin regional configuration indicates that the guarguardz complex and equivalent units represent the accretionary prism of the famatinian arc middle ordovician late devonian

sr c and o isotope geochemistry and stratigraphy of precambrian and lower paleozoic carbonate sequences from the western sierras pampeanas of argentina tectonic implications abstract sr c and o isotope data are presented for carbonate rocks from the sierra de pie de palo and other crystalline outcrops of the western sierras pampeanas these are used to distinguish three groups of rocks of quite different ages within the nappe pile that constitutes the sierra de pie de palo carbonates from the grenville age ophiolitic unit at the bottom of the pile probably resulted from the interaction between hot seawater and contemporaneous oceanic crust these carbonates are tentatively classified as ophicalcites carbonates from the difunta correa sedimentary sequence are part of a cover sequence to a grenvillian basement together forming the upper nappes isotope stratigraphy suggests a middle to late neoproterozoic age 580 720 ma for these carbonates this cover might be correlated with cover sequences to the archean and early to middle proterozoic cratons that are well preserved over large areas of brazil this fact together with the apparently absence of an equivalent neoproterozoic carbonate bearing cover in the appalachian margin of laurentia suggests that the western sierras pampeanas which are considered part of the exotic argentine precordillera terrane of allegedly laurentian derivation could be autochthonous or para autochthonous to gondwana other geochemical and geological arguments reinforce this hypothesis the caucete group carbonates underlie the nappe pile and are separated from it by a first order thrust the pirquitas thrust these latter carbonates are cambrian in age and isotopically similar to the carbonate platform of the precordillera we thus conclude that the pirquitas thrust is the boundary between the exotic precordillera terrane and the autochthonous or para autochthonous western sierras pampeanas

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

history of crustal growth and recycling at the pacific convergent margin of south america at latitudes 29 36 s revealed by a u pb and lu hf isotope study of detrital zircon from late paleozoic accretionary systems detrital zircon provides a powerful archive of continental growth and recycling processes we have tested this by a combined laser ablation icp ms u pb and lu hf analysis of homogeneous growth domains in detrital zircon from late paleozoic coastal accretionary systems in central chile and the collisional guarguaraz complex in w argentina because detritus from a large part of w gondwana is present here the data delineate the crustal evolution of southern south america at its paleopacific margin consistent with known data in the source regions zircon in the guarguaraz complex mainly displays an u pb age cluster at 0 93 1 46 ga similar to zircon in sediments of the adjacent allochthonous cuyania terrane by contrast zircon from the coastal accretionary systems shows a mixed provenance age clusters at 363 722 ma are typical for zircon grown during the braziliano pampean famatinian and post famatinian orogenic episodes east of cuyania an age spectrum at 1 00 1 39 ga is interpreted as a mixture of zircon from cuyania and several sources further east minor age clusters between 1 46 and 3 20 ga suggest recycling of material from cratons within w gondwana the youngest age cluster 294 346 ma in the coastal accretionary prisms reflects a so far unknown local magmatic event also represented by rhyolite and leucogranite pebbles it sets time marks for the accretion history maximum depositional ages of most accreted metasediments are middle to upper carboniferous a change of the accretion mode occurred before 308 ma when also a concomitant retrowedge basin formed initial hf isotope compositions reveal at least three juvenile crust forming periods in southern south america characterised by three major periods of juvenile magma production at 2 7 3 4 ga 1 9 2 3 ga and 0 8 1 5 ga the 176hf 177hf of mesoproterozoic zircon from the coastal accretionary systems is consistent with extensive crustal recycling and addition of some juvenile mantle derived magma while that of zircon from the guarguaraz complex has a largely juvenile crustal signature zircon with pampean famatinian and braziliano ages 660 ma originated from recycled crust of variable age which is however mainly mesoproterozoic by contrast the carboniferous magmatic event shows less variable and more radiogenic 176hf 177hf pointing to a mean early neoproterozoic crustal residence this zircon is unlikely to have crystallized from melts of metasediments of the accretionary systems but probably derived from a more juvenile crust in their backstop system

characteristics of extreme geoelectric fields and their possible causes localized peak enhancements one of the major challenges pertaining to extreme geomagnetic storms is to understand the basic processes associated with the development of dynamic magnetosphere ionosphere currents which generate large induced surface geoelectric fields previous studies point out the existence of localized peak geoelectric field enhancements during extreme storms we examined induced global geoelectric fields derived from ground based magnetometer recordings for 12 extreme geomagnetic storms between the years 1982 and 2005 for the present study two important extreme storms 29 october 2003 and 13 march 1989 are shown the primary purpose of this paper is to provide further evidence on the existence of localized peak geoelectric field enhancements and to show that the structure of the geoelectric field during these localized extremes at single sites can differ greatly from globally and regionally averaged fields although the physical processes that govern the development of these localized extremes are still not clear we discuss some possible causes

stromatolitic and trace fossils community of the cerro victoria formation arroyo del soldado group lowermost cambrian uruguay abstract the cerro victoria formation of the arroyo del soldado group uruguay comprises 400 m of mainly stromatolitic micritic and oolitic limestones in this low diversity stromatolite paleocommunity moundlike and planar laminites dominate but columnar forms ooids and pisoids occur as well stromatolites played a major role in the construction of the carbonate ramp which developed onto a passive eastern margin of the rio de la plata craton a paleobiological reconstruction of the stomatolite community which includes subtidal intertidal and supratidal environments is presented a low diversity trace fossil association dessignated as thalassinoides gyrolithes palaeophycus assemblage is preserved in syngenetic concretions the stromatolite community ichnofossil assemblage and ichnofabrics are typical for the lowermost cambrian corroborating earlier age estimations the unit represents a shallowing upward sequence deposited under predominantly subtidal normal marine high energy conditions tropical climate prevailed as shown by thick carbonate deposits evaporitic minerals halite gypsum and abundance of ooids the cambrian of western gondwana is characterized by scarcity of carbonate ramps this was mainly determined by high siliciclastic input related to erosion of the pan african rio doce orogenic belts carbonate ramps developed in blocks located at tropical paleolatitudes and characterized by low clastic input such as the nico perez terrane and argentine precordillera

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

proterozoic geologic evolution of the sw part of the amazonian craton in mato grosso state brazil abstract this paper presents new geochronologic isotopic and geochemical data bearing on the evolution of proterozoic crust in sw mato grosso state brazil which is at the southern end of the 1 6 1 8 ga rio negro juruena orogenic belt of the amazonian craton amazonia our data define three major crustal events i the alto jauru terrane occurs in the eastern part of the region and is comprised of island arc related rocks with u pb ages from 1 79 to 1 74 ga these rocks have e nd t values range from 2 8 to 2 0 with crustal residence ages t dm from 1 93 to 1 78 ga indicating a mainly juvenile signature at 1 8 ga ii the cachoeirinha suite occurs in the central part of the region and consists of calc alkaline plutons emplaced into alto jauru terrane host rocks these plutons have u pb ages from 1 56 to 1 54 ga and e nd t values ranging from 1 0 to 0 8 with crustal residence ages t dm from 1 88 to 1 75 ga we interpret this orogen as the roots of a continental margin arc built upon basement comprised of the alto jauru terrane iii the santa helena batholith occurs in the western part of the region and is a large elongate body of calc alkaline rocks ranging from granodiorite to highly evolved granite these units yield u pb ages from 1 45 ma to 1 42 ga with e nd t values ranging from 4 1 to 2 6 and crustal residence ages t dm from 1 70 to 1 50 ga the rio alegre domain occurs west of the santa helena batholith and includes juvenile 1 52 to 1 47 ga volcanic and mafic plutonic rocks regional geologic relationships suggest that these rocks are part of the crust into which the santa helena batholith was emplaced we interpret this batholith as the magmatic core of a juvenile arc accreted to the edge of the alto jauru terrane with incorporation of some older crust alto jauru terrane in the east and derivation from mainly juvenile crust rio alegre domain in the west the rio branco suite occurs to the east of exposed alto jauru terrane rocks as large hills protruding through younger aquapei group sedimentary rocks it consists of gabbro and granophyric rocks with u pb ages of 1 47 ga gabbro and 1 43 ga granophyre and inherited older nd suggesting hinterland derivation from alto jauru terrane basement during development of the santa helena batholith several undeformed 1 5 1 4 ga granitic plutons occur within the alto jauru terrane these are also regarded as inboard manifestations of subduction related magmatism associated with accretion of the rio alegre domain and formation of the santa helena batholith the siliciclastic aguapei group was deposited sometime between 1 0 and 1 4 ga it overlies jauru terrane basement rocks of the santa helena batholith and the rio alegre domain in the east it is flat lying and undeformed but in the west it is deformed and metamorphosed in the nnw trending aguapei thrust belt farther west the aguapei group is horizontal undeformed and overlies the paleoproterozoic paragua block in bolivia where it has been correlated with the sunsas group the aguapei thrust belt has k ar cooling ages of about 930 ma and is apparently a foreland fold and thrust belt formed by reactivation of an older rift basin during the 1 0 ga sunsas orogeny which occurs west of the paragua block in bolivia

detrital zircon geochronology of the northeastern tibetan plateau u pb geochronologic analyses have been conducted on 413 detrital zircon grains collected from 16 samples in the altun shan nan shan and qilian shan the samples come primarily from quartz arenites and metaturbidites of middle to late proterozoic age and from feldspathic and volcanic clast rich sandstones of early paleozoic age zircon grains in proterozoic strata resting on tarim basement yielded mainly 2 0 1 9 ga ages whereas proterozoic strata of the qaidam and qilian terranes yielded mainly ca 930 820 ma and ca 1 9 1 1 ga ages the younger grains were apparently shed from local igneous rocks whereas the grains older than 1 1 ga were shed from an undetermined continental source grains in the lower paleozoic strata are mainly ca 500 430 ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting our detrital zircon ages are consistent with a model first proposed by e r sobel and n arnaud in which early paleozoic magmatism occurred within a single northeast facing magmatic arc that was constructed across an assemblage of middle to late proterozoic accretionary complexes remnants of magmatic arcs and shallow marine strata this arc system was accreted to the tarim and sino korean cratons during silurian devonian time the resulting suture has been reactivated as tertiary thrust faults that currently define the structural and topographic margin of the tibetan plateau our data also provide two new estimates for the offset along the eastern altyn tagh fault a belt of middle proterozoic shallow marine strata is offset by 400 km whereas a belt of 490 480 ma magmatic arc rocks is offset by 370 km these values are generally similar to the 350 400 km offset reported in most previous studies

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

chemical and isotopic composition of lower to upper ordovician sedimentary rocks central andes south bolivia implications for their source abstract fifty million years of erosional history of the early paleozoic orogen in the central andes are documented in the 12 km thick ordovician siliciclastic sedimentary section from south bolivia transport directions in the north south trending basins allow a distinction between sediment sources from the proterozoic brazilian shield to the east and the early paleozoic mobile belt to the west major and trace element and isotopic nd sr pb compositions of stratigraphically well defined samples of tremadocian to arenigian ages have been studied the element patterns and isotopic composition of the ordovician sedimentary rocks end450 ma 7 5 to 11 87sr 86sr450 ma 0 71 0 72 206pb 204pb450 ma 17 2 18 8 207pb 204pb450 ma 15 60 15 73 208pb 204pb450 ma 36 9 39 1 nd depleted mantle model ages tdm 1 7 2 2 ga are typical of upper crustal felsic rocks which underwent one or more cycles of erosion and deposition the compositional range of the ordovician sedimentary rocks is similar to that of

u pb th pb and ar ar geochronology from the southern sierras pampeanas argentina implications for the palaeozoic tectonic evolution of the western gondwana margin

geoelectric hazard maps for the continental united states in support of a multiagency project for assessing induction hazards we present maps of extreme value geoelectric amplitudes over about half of the continental united states these maps are constructed using a parameterization of induction estimates of earth surface impedance obtained at discrete geographic sites from magnetotelluric survey data are convolved with latitude dependent statistical maps of extreme value geomagnetic activity obtained from decades of magnetic observatory data geoelectric amplitudes are estimated for geomagnetic waveforms having 240 s sinusoidal period and amplitudes over 10 min that exceed a once per century threshold as a result of the combination of geographic differences in geomagnetic activity and earth surface impedance once per century geoelectric amplitudes span more than 2 orders of magnitude and are an intricate function of location for north south induction once per century geoelectric amplitudes across large parts of the united states have a median value of 0 26 v km for east west geomagnetic variation the median value is 0 23 v km at some locations once per century geoelectric amplitudes exceed 3 v km

the allochthony of the argentine precordillera ten years later 1993 2003 a new paleobiogeographic test of the microcontinental model abstract recent and new faunal data from the cambrian to silurian rocks of the precordillera famatina and northwest argentina basins are used to discriminate between different paleogeographic models and especially to establish to what extent they are compatible with a previous conclusion that the precordillera is a laurentian derived microcontinent there is no paleontological evidence to support a para autochthonous gondwanan origin of the precordillera the strong differences in the cambrian trilobite faunas and lithologic successions preclude a common origin of the precordillera terrane eastern antarctica and south africa recent discoveries of brachiopods and organisms of the phylum agmata strengthened laurentian affinities during the cambrian the latest cambrian early ordovician faunas that inhabited the autochthonous northwest argentina basin including the western puna volcaniclastic successions are mostly peri gondwanan the early ordovician brachiopods ostracods and trilobites display mixed laurentian baltic and avalonian biogeographical links supporting a drifting of the precordillera across the iapetus ocean increasing gondwanan elements during the llanvirn along with varied geological evidence indicate that the first stages of collision may have begun at that time involving a major change in the plate kinematics the distribution of facies and faunas basin development and timing of deformation are interpreted as resulting from a north to south diachronous closing of the remnant basin during the last phases of convergence and oblique collision of the precordillera terrane with the gondwana margin the high level of endemism of caradoc faunas may be a consequence of the rearrangement and partial isolation of sedimentary areas during the strike slip movement of the colliding precordillera plate with respect to the gondwana margin suggested relationships between facies distribution geographic barriers and faunal migrations before and during the collision are depicted in a series of schematic reconstructions at five time slices from late cambrian to silurian

ouachita trough part of a cambrian failed rift system pre flysch cambrian mississippian strata of the ouachita mountains of arkansas and oklahoma include two main sandstone lithofacies 1 a craton derived lithofacies made up largely of mature medium to coarse grained quartzose and carbonate detritus and in some units sediment eroded from exposed basement rocks and 2 an orogen derived facies made up mainly of fine grained quartzose sedimentary and metasedimentary debris and possibly in lower units a volcaniclastic component paleocurrent and distribution patterns indicate that detritus of facies i in the benton uplift was derived from north and detritus of facies ii throughout the ouachitas was derived from south and east of the depositional basin overall sedimentological results suggest that the ouachita trough was a relatively narrow two sided basin throughout most and probably all of its existence and never formed the southern margin of the north american craton regional comparisons suggest that it was one of several basins including the southern oklahoma aulacogen reelfoot rift illinois basin and rome trough that formed as a cambrian failed rift system 150 to 250 m y after initial rifting along the appalachian margin of the north american craton

paleomagnetic confirmation of the laurentian origin of the argentine precordillera abstract several recent tectonic models have portrayed the argentine precordillera has been recently portrayed as an early paleozoic laurentian derived exotic terrane now found in southwestern south america these models have primarily been based on strong biogeographic and stratigraphic evidence however no paleomagnetic data have previously been available to independently test them a paleomagnetic study was therefore carried out on the early cambrian cerro totora formation exposed in the northern reaches of the argentine precordillera after stepwise thermal demagnetization a pre folding remanence was identified in ten sites of this formation yielding a paleomagnetic pole ct at 37 0 n 314 1 e a95 5 8 this pole is not consistent with the latest proterozoic early paleozoic apparent polar wander path for gondwana but it agrees with the early cambrian section of the laurentian path if the argentine precordillera is positioned as the conjugate margin of the ouachita embayment in southeast laurentia this result confirms that the precordillera is an allochthonous terrane derived from laurentia in cambrian times that was later accreted to gondwana probably in middle ordovician times

k bentonites in the argentine precordillera contemporaneous with rhyolite volcanism in the famatinian arc new u pb radiometric dates for k bentonite horizons within the lower cambrian to middle ordovician platform carbonates from the precordillera terrane of nw argentina provide further constraints on models for the allochthonous or parautochthonous accretion of this terrane two k bentonite layers from the talacasto section yield indistinguishable sensitive high resolution ion microprobe shrimp u pb zircon dates of 469 5 3 2 ma and 470 1 3 3 ma respectively these are within uncertainty of the u pb shrimp zircon date of 468 3 3 4 ma for a porphyritic rhyolite from the famatinian magmatic arc sierra de las planchadas near rio chaschuil geochemical and isotope data also demonstrate the similarity of the k bentonite and chaschuil rhyolite parent magmas thus it is highly probable that the famatinian arc volcanoes provided the ash for the k bentonite horizons suggesting proximity to the precordillera terrane during the deposition of the lower cambrian to middle ordovician platform carbonates this implication supports a mid ordovician collision model but could also be compatible with a parautochthonous model for docking of the precordillera terrane by movement along the pacific margin of gondwana rather than across the iapetus ocean

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