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

unraveling 470 m y of shortening in the central andes and documentation of type 0 superposed folding in the famatina ranges central andes of western argentina 470 m y of protracted shortening is recorded through episodic coaxial coplanar fold amplification ordovician anticlines within the east vergent thrust belt were refolded and tightened during later paleozoic and cenozoic shortening folding episodes are recorded in four major unconformities that can be measured on both flanks of one of these map scale anticlines superposed folding type 0 interpreted from mapping and bedding plots shows similar axial orientation in older and younger rocks throughout the different stratigraphic layers of the shortening 30 is pre cenozoic and allows the recording of episodic deformation during continuous plate convergence at the gondwana south american margin

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

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

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

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

a model for carbonate to terrigenous clastic sequences the development of carbonate sequences that grade toward a source region into terrigenous clastic sediment is poorly understood in both carbonate and terrigenous clastic regimes environments of rapid sedimentation have comparable rates of deposition 102 to 102 5 cm 1 000 yr slowest continuous rates are also comparable 100 5 to 101 cm 1 000 yr because of the inhibiting effect of terrigenous deposition on carbonate skeletal production the slowest rates in a transect tend to occur where mixed lithologies are deposited midway between coarse clastic and pure carbonate sediments this leads to a bimodal distribution of rapid depositional rates and invariably to the development of a basin in the intervening area to develop a model we combine concepts of depositional rate with knowledge of the middle ordovician carbonate to terrigenous clastic sequence of the southern appalachians deposition began on a shallow water ramp sloping gently eastward but everywhere within the photic zone an episode of tectonic subsidence differentiated this ramp into a shallow shelf on the northwest and a deep starved basin on the southeast separated by a linear shelf margin with organic build ups subsequent development of the sequence consisted of the filling of the deep basin first with pelagites then with turbidites during which the carbonate shelf was isolated from terrigenous deposition once the basin filled terrigenous deposition began to affect the shelf inhibition of carbonate deposition near the shelf edge led to formation of successively smaller basins within the old shelf area after the initial pulse of tectonic subsidence deposition of the sequence can be explained by sediment loading subsidence thus the ordovician sequence coupled with the concept of bimodal distribution of sedimentation rates in carbonate to terrigenous clastic facies patterns allows development of a model for basins adjacent to active tectonic lands five model stages are derived by application of calculated deposition rates and sediment loading along a transect away from the terrigenous source these stages are 1 an initial carbonate ramp stage followed by 2 a carbonate shelf starved deep basin stage 3 a carbonate shelf turbidite basin terrigenous shelf stage 4 a carbonate shelf narrowed and shallowed basin broad terrigenous shelf stage and finally 5 a stage in which the initial basin is filled a subsequent starved basin forms on the old carbonate shelf and a new carbonate shelf margin forms a few to tens of kilometres farther from the terrigenous source the model should be useful in predicting depositional facies in analogous basins

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

neoproterozoic to early palaeozoic events in the sierra de san luis implications for the famatinian geodynamics in the eastern sierras pampeanas argentina the application of the sensitive high resolution ion microprobe shrimp u pb dating technique to zircon and monazites of different rock types of the sierra de san luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains additional constraints on the famatinian metamorphic episode are provided by pb pb stepwise leaching experiments on one staurolite and two garnet separates the results indicate that the sedimentary precursors of the conlara metamorphic complex have a maximum age of c 590 ma whereas the pringles metamorphic complex metasediments appear to be sourced from the pampean orogen in the early cambrian folded xenoliths within the c 496 ma el penon pluton suggest that the host conlara metamorphic complex underwent a pampean compression from a 208 pb 232 th monazite age of 478 ma for a migmatite from the nogoli metamorphic complex the structural evolution of this basement complex appears to be entirely post pampean onset of the famatinian high grade metamorphism between c 500 ma and c 450 ma follows a period of crustal extension on the western outboard of gondwana and might not be related directly to a mid ordovician accretion of the cuyania terrane

ordovician back arc foreland and ocloyic thrust belt development on the western gondwana margin as a response to precordillera terrane accretion 1 the famatina belt southern central andes records a circa 470 m y shortening episode ocloyic orogeny affecting the peri gondwanan back arc basin in response to the accretion of the precordillera terrane collision created distinct features across and along the margin some of which persisted into the present andean structure from east to west the ocloyic deformation is recorded across the telescoped region of famatina represented by 1 an east vergent fold thrust belt evftb involving a major angular unconformity between early and middle ordovician rocks 2 a highly deformed volcano sedimentary belt and 3 an east vergent ductile shear zone affecting famatinian granites axis attitudes of ordovician refolded anticlines in the evftb show west east tectonic transport in agreement with kinematics of ocloyic high strain belts indicating shortening orthogonal to the present pacific margin eastward displacement of the ordovician volcanic arc at 27 s latitude together with a documented sinistral slip zone across the proto andean margin suggests tectonic indentation

paired isotope records of carbonate and organic matter from the middle ordovician of argentina intrabasinal variation and effects of the marine chemocline abstract we investigate the expression of the middle darriwilian isotope carbon excursion mdice across marine shelf environments in the argentine precordillera previous work identified the mdice in the las chacritas formation in argentina but did not recognize the expression of the mdice in time equivalent strata of the deeper water las aguaditas formation albanesi et al 2013 palaeogeography palaeoclimatology palaeoecology v 398 p 48 66 recent biostratigraphic investigations of these units have for the first time provided the opportunity for high resolution correlation which suggest that the mdice or at least the initiation of the mdice should be observed in both the las chacritas and las aguaditas formations here we present new paired carbon isotope data of carbonate and organic carbon from the las chacritas and las aguaditas formations we identify a 2 positive shift in the isotopic composition of marine carbonate in the las chacritas formation whereas values abruptly fall to

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

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

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

magmatology of mt pelee martinique f w i ii petrology of gabbroic and dioritic cumulates abstract the study of 24 plutonic xenoliths collected from mt pelee pyroclastic deposits shows that they can be subdivided into anorthite pargasite salite bearing gabbroic adcumulates type i olivine bearing type ii magnetite bearing and bytownite hornblende hypersthene augite bearing dioritic orthocumulates additionally olivine diopside pargasite and anorthite from a basaltic megacryst rich ash layer have been considered mineralogical data show continuous and progressive variations in the compositions of the solid phases with increase of fe fe mg ratios during the petrographic evolution of the investigated cumulate xenoliths thermobarometric data on minerals or mineral liquid pairs indicate a decrease of crystallization temperatures from about 1090 c in gabbroic assemblages to about 850 c in dioritic assemblages the gabbroic assemblage is believed to form in a water bearing environment from a derivative calc alkaline liquid at sufficient depth to stabilize al rich pargasite i e 8 9 kbar we suggest that the materials investigated may represent respectively the bottom megacrysts the walls type i and ii gabbroic adcumulates and the roof dioritic orthocumulates of stratified magma chamber s underlying mt pelee volcano

geodynamic evolution of the central appalachian orogen geochronology and compositional diversity of magmatism from ordovician through devonian

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

sediment gravity flow deposits of an ordovician deep sea fan system western precordillera argentina abstract ordovician siliciclastic sequences of portezuelo del tontal formation ptf in the western precordillera argentina are interpreted to be deep sea deposits which formed on a passive continental margin several facies and layer divisions have been identified and interpreted as high and low density turbidites debrites and contourites hemipelagites outer and inner upper middle submarine fan subenvironments have been recognized outer fan associations consist of lobe and lobe fringe sequences inner fan associations are defined by coarse grained and channelized deposits the vertical arrangement of facies associations is interpreted in terms of progradational and retrogradational processes some sharp progradational and retrogradational transitions were controlled by allocyclic factors such as sea level changes and tectonics of the source areas the submarine fan model for the ptf is an intermediate type between the pelite rich and the sand rich fans it was formed in a narrow and elongate deep sea basin controlled by the contemporaneous evolution of an oceanic rift in the direction of progradation

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

structural evolution of the argentine precordillera the rio san juan section abstract the argentine precordillera ap represents a high level fold and thrust belt which was formed during the andean tertiary crustal shortening in a w e transect studied mostly e directed imbrications are combined with folding and involve a pile of cambrian to tertiary sediments the thrust belt is detached above a main decollement within the ordovician to lower devonian strata to the east it is bounded by a back thrust zone directed westwards restored sections allow an estimate of crustal shortening and also provide insight into the structural development during pre tertiary times carbonate platform sequences of cambrian to ordovician age grade into a volcani clastic succession with transitional melanges representing deposits along a western continental slope in the western and easternmost parts of the ap cambrian to silurian deposits are affected by a first folding which to the west is combined with a slight metamorphic overprint during carboniferous to permian times intensified mobility along the eastern rim and in the western parts of the ap leads to crustal extension which partly was triggered by the formation of synsedimentary normal faults in the central ap no angular unconformities at the base of carboniferous permian deposits were found there was no field evidence found to postulate a terrane boundary within the ap area and an onset of thrust tectonics during cretaceous to early tertiary times

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

graphic correlation of middle ordovician graptolite shale southern appalachians an approach for examining the subsidence and migration of a taconic foreland basin graptolite rich shale in the valley and ridge province of alabama georgia tennessee and virginia records subsidence and migration of the sevier foreland basin which formed in response to arc continent collision during the middle ordovician blountian phase of the taconic orogeny rapid tectonically induced subsidence of what had been a shallow carbonate platform led to pelagic deposition of graptolite shale in a deep basinal setting the contact of the shale with the underlying carbonate rocks is a distinct record of initiation of subsidence graptolites were collected extensively and systematically from the shale at 50 localities in the southern appalachians correlation with zonal biostratigraphy demonstrated that the shale sequence is diachronous in localities to the southeast in alabama and georgia it correlates with the didymograptus murchisoni zone to the northwest across strike in alabama tennessee and virginia the shale becomes progressively younger and correlates sequentially with the glyptograptus teretiusculus nemagraptus gracilis and climacograptus bicornis zones this diachronism records the northwestward cratonward migration of the foreland basin which was driven by postcollisional plate convergence biostratigraphic data from 22 localities were amenable to analysis by graphic correlation at these localities graptolites were collected from measured sections and ranges of all species in the 22 sections were compiled into a csrs composite standard reference section that correlates with the biostratigraphic interval from the g teretiusculus zone to the c bicornis zone on the basis of the most reliable radiometric ages correlated into the graptolite zonation the duration of the composite standard reference section is estimated to be 5 m y it is divisible into 89 csus composite standard units each with an approximate duration of 56 000 yr recorrelation of each of the 22 measured sections with the composite standard reference section allowed the stratigraphic base of the shale in each section to be expressed in terms of composite standard units and such correlations permitted the diachronism of the basal shale contact to be measured with much greater precision than with zonal biostratigraphy in addition age differences of the basal shale contact between sections can be expressed in terms of time with the temporal information provided by the composite standard reference section and original distances between sections determined from a palinspastic reconstruction the rate of migration of the foreland basin as recorded in the diachronous basal shale contact was determined between sections in northeastern tennessee and adjacent virginia calculated rates averaged 13 mm yr and decreased northwestward from 40 to 9 mm yr this cratonward decrease in rate may represent deceleration as foreland basin migration and the arc continent collision that drove it slowed to a halt the rate of 40 mm yr may be considered as the maximum rate of plate convergence following arc continent collision precollisional plate convergence rates may have been significantly greater the rate of 40 mm yr is however well within the range of modern rates of plate convergence in addition the deceleration recorded in the sevier foreland basin is similar to the deceleration of postcollisional plate convergence at the timor trough

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

integrated reservoir characterization of enhanced oil recovery targets in mature basins an example from the tar springs formation rock hill field illinois basin united states extensively developed hydrocarbon reservoirs within the illinois basin will soon reach the end of their production history without technological intervention although many of these mature fields have been waterflooded for decades and show significant production declines substantial amounts of residual oil remain in place making them ideal candidates for enhanced oil recovery eor this research focuses mainly on a producing field in southern indiana that is being considered as a pilot study for surfactant polymer chemical eor using a methodology that includes field geology sedimentary petrology x ray diffraction fourier transform infrared spectroscopy thermogravimetric analysis evolved gas analysis and scanning electron microscopy this paper describes an integrated approach to reservoir characterization of the tar springs formation this formation is one of several key upper mississippian producing horizons that account for approximately 60 of the historical cumulative oil production within the illinois basin traditionally any upper mississippian sandstone rich unit in a producing field was considered reservoir however our analysis shows that reservoir connectivity and fluid flow parameters within a single sandstone unit are highly dependent on mineralogy the results show that the tar springs formation at the rock hill oil field is compartmentalized by five main facies each characterized by distinct physical and chemical properties in addition we document a heterogeneous distribution of different clay minerals in the reservoir that impact porosity and permeability and can contribute to injectivity problems or eor chemical adsorption this integrated reservoir characterization approach has important implications for evaluating and identifying specific intervals that are optimal eor targets in reservoirs considered close to abandonment

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

nature and regional significance of unconformities associated with the middle ordovician hagan k bentonite complex in the north american midcontinent stratal patterns of the middle ordovician hagan k bentonite complex and associated rocks show that the black river trenton unconformity in the north american midcontinent formed through the complex interplay of eustasy sediment accumulation rates siliciclastic influx bathymetry seawater chemistry and perhaps local tectonic uplift the unconformity is diachronous and is an amalgamated surface that resulted from local late turinian lowstand exposure followed by regional early chatfieldian transgressive drowning and sediment starvation the duration of the unconformity is greatest in southern wisconsin northern illinois and northern indiana where the deicke and millbrig k bentonite beds converge at the unconformity on the basis of published isotopic ages for the deicke and millbrig beds it is possible that in these regions erosion and nondeposition spanned a period of as much as 3 2 m y two broad coeval depositional settings are recognized within the north american midcontinent during early chatfieldian time 1 an inner shelf subtidal facies of fossiliferous shale spechts ferry shale member and ion shale member of the decorah formation and argillaceous lime mudstone and skeletal wackestone guttenberg and kings lake limestone members extended from the canadian shield and transcontinental arch southeastward through minnesota wisconsin iowa and missouri 2 a seaward relatively deep subtidal sediment starved middle shelf extended eastward from the mississippi valley region to the taconian foreland basins in the central and southern appalachians and southward through the pericratonic arkoma and black warrior basins in the inner shelf region the black river trenton unconformity is a composite of at least two prominent hardground omission surfaces one at the top of the castlewood and carimona limestone members and the other at the top of the guttenberg and kings lake limestone members both merging to a single surface in the middle shelf region the inner and middle shelves redeveloped later in approximately the same regions during devonian and mississippian time

provenance of the middle ordovician blount clastic wedge georgia and tennessee convergent tectonism along the appalachian continental margin in middle ordovician time resulted in cratonward prograding clastic wedges detrital modes of 52 sandstones of the blount clastic wedge in georgia and tennessee are dominated by monocrystalline quartz 68 feldspar 10 plagioclase k feldspar and pelitic rock fragments 10 with lesser amounts of polycrystalline quartz 6 chert 2 low grade metamorphic rock fragments 2 and quartzofeldspathic rock fragments 0 3 the primary source rocks were sedimentary subordinate contributions were from low grade metamorphic and plagioclase rich plutonic and or gneissic rocks the composition of sandstones from the martinsburg clastic wedge based on point counting 18 samples from the collection of mcbride and from the taconic clastic wedge based on data of hiscott is similar to the blount clastic wedge except that martinsburg and taconic sandstones have additional evidence of derivation from intermediate or mafic volcanic and deep water sedimentary source rocks which were lacking in the blount source terrane the differences in provenance among the clastic wedges may indicate along strike variations in tectonic style or variations in the distribution of rock types or differences in the level of erosion within orogenic terranes of similar origin

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

influence of pre existing plate margin structures on foredeep filling insights from the taconian blountian clastic wedge southeastern usa break up of continents gives rise to conjugate margins with irregular shapes and with thermal histories and sedimentary records that differ along strike these elements need to be considered in the integrated study of a foreland basin orogenic belt pair that formed during subsequent contractional events this paper relates the promontory and embayment configuration of southeastern laurentia north american craton to 1 along strike variations in width of the initial depositional profile of the middle to upper ordovician taconian blountian foredeep 2 northeastward toward the embayment along strike thickening and migration of siliciclastic depocenters in the foredeep that favored the resumption of carbonate ramp deposition on the promontory during the late ordovician and 3 variation in intensity of deformation of intrabasinal structures as recorded by reactivation of across strike structures located on the transition zone between the promontory and embayment geometry and composition of the blountian foreland strata indicate that the foredeep slope was narrower and steeper on the alabama promontory than on the southwestern flank of the tennessee embayment differential slope geometries were accommodated by reactivation of transverse basement faults which controlled local conglomerate deposition in deep water settings on the transition zone between the promontory and embayment in contrast conglomerate beds at the top of coarsening upward successions on the southwestern flank of the tennessee embayment record the cratonward progradation of deltaic depositional systems the results presented here may have relevance to other peripheral foreland basins formed over rifted continental margins or retro arc foreland basins formed adjacent to inverted intracratonic rifts

crustal provenance and cooling of the basement complexes of the sierra de san luis an insight into the tectonic history of the pro to andean margin of gondwana abstract the sierra de san luis constitutes the southernmost tip of the eastern sierras pampeanas its palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto andean margin of gondwana although it is largely accepted that the polyphase accretional history of the sierras pampeanas is preluded by the docking of the pampean terrane followed by the famatinian orogenic cycle that involves subduction along the margin of gondwana and the accretion of the precordillera cuyania terrane and finally ceased with the collision of the chilenia terrane a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the eastern sierras pampeanas is published in this paper those different hypothesis are presented and evaluated under the light of new isotopic data of the sierra de san luis nd systematics of the metasedimentary sequences of the sierra de san luis indicate that the studied sequences were developed on the pampean terrane an amazonian origin of the pampean terrane that was probably detached from the arequipa antofalla craton is proposed furthermore the correlation of two low grade phyllitic belts san luis formation with the widespread puncoviscana formation is not supported by sm nd data it is suggested that the sedimentary precursors of the pringles metamorphic complex and the topping phyllites were sourced on the pampean orogen and accommodated in a newly formed back arc basin during the early famatinian the cooling history of the basement complex is recorded by an extensive amount of k ar muscovite and biotite ages a high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations post famatinian to achalian crustal scale mylonite formation 359 ma and a rotational exhumation of the central basement unit are causal for the observed k ar muscovite age pattern in the range from 395 ma to 447 ma therefore the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust an even higher variability in k ar biotite cooling ages covering the range from 315 ma to 418 ma is related to the slow cooling after the famatinian orogenic cycle or reheating during the achalian orogenic cycle and consequent variable reset of the isotopic system however ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the sierra de san luis

analogous tectonic evolution of the ordovician foredeeps southern and central appalachians new stratigraphic data suggest that the diachronous evolution of the ordovician foredeeps in the southern and central appalachians was remarkably similar stratigraphic features that characterize the middle ordovician sevier basin in tennessee and the middle and late ordovician martinsbuirg basin in pennsylvania are in identical ascending order 1 disconformity on the knox group beekmantown group 2 shelf carbonates 3 slope deposits 4 submarine fan turbidites and 5 contourites and muddy turbidites we propose that diachronous attempted subduction of the north american craton beneath southeastern microplates and or volcanic arcs resulted in uplift and erosion of the western shelf followed by its rapid subsidence basinward migration of eastern and northeastern terrigenous source areas and associated submarine fan turbidites resulted from continued convergence and filled the basins finally tectonic stabilization and lowering of the source area is recorded by contourites and muddy turbidites the evolutionary model proposed for the sevier and martinsburg basins closely resembles present day tectonics of the timor foredeep and the adjoining sahul shelf north of australia similar comparisons have also been made for ordovician basins in the northern appalachians analogous tectonic mechanisms therefore appear to have operated diachronously along the eastern margin of north america from tennessee to new england and possibly to newfoundland during ordovician time

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

the tectonic significance of k ar illite fine fraction ages from the san luis formation eastern sierras pampeanas argentina the sierra de san luis forms the southern tip of the eastern sierras pampeanas in central argentina two narrow belts of low grade phyllites and quartz arenites i e the san luis formation have accommodated part of the strain related differential exhumation of the medium to high grade metamorphic domains that constitute to the basement complex of the sierra eleven phyllite samples were subjected to the k ar fine fraction dating technique results are interpreted in relation to the kubler index of the illites which indicate epimetamorphic conditions for the majority of the samples obtained ages between 330 and 290 ma cover a period of compressional tectonics in the late mississippian visean serpukhovian boundary followed by the subsidence during the formation of the paganzo basin in the provinces of la rioja and san luis these tectonic movements are coincident with the toco orogeny in northern chile and southern bolivia this suggests that the older k ar ages document the compressional stage and that younger ages record the cooling of the basement during the subsequent extensional uplift of the basement

temporal changes in nd isotopic composition of sedimentary rocks in the sevier and taconic foreland basins increasing influence of juvenile sources initial 143 nd 144 nd ratios of clastic sedimentary rocks in the ordovician taconic and sevier foreland basins increase with decreasing stratigraphic age this shift in isotopic composition reflects an increased influx of juvenile sediment to both basins for the taconic basin geologic and isotopic data are consistent with the juvenile sediment being derivedfromupliftedgrenvillecrust upthrustaspartofthetaconicallochthons sediment derivedfromthebronsonhillarc oracombinationofboth forthesevierforelandbasin there are few geologic constraints on source regions but the similar shift in isotopic ratios with time suggests a similar increase in the influence of juvenile sources such as exotic terranes sevier basin sedimentary rocks have higher nd values than those within the taconic basin thus juvenile source areas were of greater importance to sevier basin sediments