tracking ancient magmatism and cenozoic topographic growth within the northern andes forearc constraints from detrital u pb zircon ages a comparison of detrital zircon ages from the cenozoic forearc basins in sw ecuador nw peru and those derived from the amotapes massif offers a singular opportunity to shed some light on the evolution of the nw andean margin of south america an extensive set of pre andean u pb zircon ages older than late cretaceous was obtained from 18 detrital samples coming from the talara zorritos progreso and gulf of guayaquil tumbes forearc basins as well as from two metasedimentary rocks of the amotapes massif u pb age analysis on detrital zircon grains allows a general source to sink scenario to be determined for this section of the andes these analyses give consistent insights into the early topographic growth during the modern andean orogeny the occurrence of proto andean magmatic periods in the western margin of gondwana and the evolution of the amotapes massif within the geodynamical setting of northwestern south america zircon ages at ca 320 ma and neoproterozoic clusters for the amotapes metasediments show significant similarities along the strike of the metamorphic belt thus defining a common metasedimentary origin and can be compared with previously dated rocks from the eastern cordillera of peru these strong similarities demonstrate the presence of a wide polyphased metamorphic belt that includes the western parts of the eastern cordillera the northern section of the occidental cordillera of peru and the amotapes massif detrital sediments of the forearc are marked by a major gap in early cretaceous jurassic ages and the minor presence of ordovician famatinian orogeny and carboniferous permian 340 285 ma ages the age spectrum is not compatible with sediment input from the eastern andes and suggests that the external areas of the andean chain e g the western cordillera were already uplifted prior to or immediately after forearc initiation during the paleocene thus representing a pervasive drainage divide throughout all of the cenozoic the amotapes massif or other terrains sharing the same age affinities e g the olmos terrane occidental cordillera of peru are likely to represent the best potential sources for pre andean zircon grains in the forearc age clusters in both source and sink at ca 600 ma 570 560 ma and 520 ma most likely define magmatic periods and may represent first order constraints for the difficult correlation between proto andean magmatic and orogenic periods to the north and south of the huancabamba deflection it is unlikely that the neoproterozoic zircon grains came from the pan african braziliano belts thus suggesting the onset of significant magmatic activity in this section of the andes at around 570 560 ma and probably ca 600 ma the 520 ma cluster is compatible with the most prominent magmatic period in the southern andes related to the end of the pampean orogeny thus suggesting the existence of a regional proto andean magmatic arc during early cambrian times

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

c o sr isotopic stratigraphy of cap carbonates overlying marinoan age glacial diamictites in the paraguay belt brazil abstract the neoproterozoic paraguay belt located along the southeastern border of the amazon craton and eastern border of the rio apa block comprises a thick passive margin succession of glaciomarine turbidites carbonates and siliciclastic sedimentary rocks that were deformed during the brasiliano pan african orogeny the carbonate sequence comprises a 1300 m thick succession of platformal carbonate rocks araras formation which directly overlie marinoan glacial diamictites puga formation the late neoproterozoic age of this carbonate sequence is indicated by the presence of cloudina lucianoi and corumbela werneri found in the upper part of the corumba group southern paraguay belt carbon and sr isotope data were obtained on marine carbonate samples across three different sections of the araras formation 13 c pdb values of the carbonate from the border of the basin range between 10 5 and 2 7 including a short stratigraphic interval of cap dolomite 22 m directly overlying diamictites the isotopic profile across the basal 200 m of deep shelf laminated microcrystalline limestone and clay limestone cap carbonate that overlie glacial diamictites also show negative 13 c pdb values 5 3 and 0 6 in contrast with the lower portion of the carbonate sequence a 800 m thick unit of shallow water dolostones forming the upper part of the carbonate sequence present uniform positive 13 c pdb values ranging from 1 9 to 2 4 dolostone and arenaceous dolostone of the upper parts of the araras formation have high 13 c pdb values 4 1 to 9 6 which are followed by an abrupt decrease in 13 c values down to 1 0 the carbon isotope profile of the paraguay belt is quite different from the bambui group an other important neoproterozoic sequences in central brazil which also overlie glacial sediments negative 13 c pdb values are found only within the lowermost meters of these sequence and are followed by an important and extensive positive carbon isotope excursion values ranging from 6 9 to 16 0 on the other hand the 13 c pdb profile observed across the araras formation exhibits an approximately 200 m thick section of carbonates with low 13 c pdb values carbonates of the araras formation have 87 sr 86 sr ratios ranging from 0 70753 to 0 70803 comparable to cap carbonates that overly marinoan glacial deposits from other neoproterozoic sequences

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

isotopic studies on detrital zircons of silurian devonian siliciclastic sequences from argentinean north patagonia and sierra de la ventana regions comparative provenance the silurian devonian siliciclastic sedimentary units known as sierra grande formation and the upper part of the ventana group crop out in the eastern area of the north patagonian massif and in the ventania system toward the atlantic border of argentina both sequences show similar stratigraphical characteristics and were deposited in a shallow marine platform paleoenvironment previous contributions have provided evidence of an allochthonous patagonia terrane that amalgamate to gondwana during the permian triassic however other lines of research support a crustal continuity southward where the pampean and famatinian events extend into the northern patagonia in either case the detrital input to the eo mesopaleozoic basins generated along the passive margin tectonic setting should reflect the sedimentary sources in this contribution new age data on the sedimentary provenance of these units is provided by u pb and lu hf isotopic studies on detrital zircons using la icp ms and shrimp methodologies the main sedimentary sources of detrital zircons for both regions are of cambrian ordovician and neoproterozoic age while a secondary mode is mesoproterozoic zircons from older cratonic sources mesoarchean paleoproterozoic ages are scarcely recorded the sample from the upper section of the devonian lolen formation ventana group shows an important change in the sedimentary provenance with a main mode of mesoproterozoic detrital zircons detrital source areas considering the orogenic cycles known for southwest south america famatinian pampean brasiliano mesoproterozoic grenvillian and paleoproterozoic transamazonian are proposed

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

late leaching under deep burial conditions a case study from the miocene zhujiang carbonate reservoir south china sea abstract the zhujiang carbonates are the hydrocarbon reservoir rock of the liuhua 11 1 field they comprise three porous zones of around 20 m intercalated with tight zones parallel to bedding thin section cathodoluminescence and stable isotope data were combined with well log information in order to investigate the origin and spatial distribution of the abundant porosity the vast majority of pores in the porous zones was generated or overprinted in deep burial settings by late leaching two tight zones however lack this porosity because of meteoric cementation and increased compaction and related calcite precipitation prior to late leaching we propose that migrating corrosive fluids were diverted along bedding parallel flow barriers of the tight zones and leached the intervals in between the porous zones thereby emphasizing the depositional pattern

neoproterozoic backarc basin sensitive high resolution ion microprobe u pb and sm nd isotopic evidence from the eastern pampean ranges argentina the eastern pampean ranges comprise high grade supracrustal sequences with linear belts of mafic ultramafic bodies representing ophiolite remnants new u pb and nd isotopic data suggest that the tectonic evolution of the pampean ranges started ca 640 ma with the deposition of supracrustal sequences in a backarc basin between a neoproterozoic magmatic arc to the east and the pampia terrane to the west ophiolite remnants of this backarc basin yielded a whole rock isochron indicating the age of 647 77 ma 2 and nd initial time t of 5 2 sensitive high resolution ion microprobe u pb data for detrital metasediments show provenance patterns with two main age populations the older 1 1 0 9 ga and the younger between ca 0 7 and 0 6 ga the neoproterozoic population is relatively more abundant in sediments of the easternmost units of the eastern pampean ranges and becomes less abundant toward the west depleted mantle ages show a similar pattern with ages generally increasing from east 1 42 ga to west 1 76 ga suggesting the presence of neo proterozoic sources to the east of the ranges the provenance data do not support previous evolution models for the eastern pampean ranges according to which the supracrustal sequence represents the passive margin of the rio de la plata craton early cambrian collision and high grade metamorphism mark the final stages of evolution of the belt and were shortly followed by calc alkaline metaluminous and peraluminous granitic magmatism ca 530 514 ma the results suggest that the geological evolution of the eastern pampean ranges took place between ca 640 and 514 ma coeval with other brasiliano orogens in brazil e g the paraguay and araguaia fold belts

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

miocene forebulge development previous to broken foreland partitioning in the southern central andes west central argentina 1 in the distal region of the modern flat slab segment in the southern central andes an unusual stack of middle miocene paleosols together with regional upwarping and normal faulting indicate episodic aggradation and condensed sedimentation contemporaneous with the principal stage of foreland basin development associated with foreland flexure farther to the west these features are consistent with development of a forebulge zone during the early stages of a proposed asymmetric foreland basin system sedimentary thickness farther east and far from the cordilleran tectonic loads suggests accommodation and preservation driven by nonisostatic dynamic subsidence regional overlapping relationships and basin modeling suggest that the modern broken foreland present sierras pampeanas can be interpreted as a reactivation of a formerly partitioned broad forebulge

closure of the clymene ocean and formation of west gondwana in the cambrian evidence from the sierras australes of the southernmost rio de la plata craton argentina abstract the formation of gondwana took place across a series of brasiliano pan african suture zones that record late neoproterozoic to earliest paleozoic collisions between precambrian cratons in south america an internal suture zone marks the disappearance of the clymene ocean that separated the amazon craton from the sao francisco and rio de la plata cratons new geochronological data from the southern end of this massive collision zone in the sierras australes of central eastern argentina document paleoproterozoic crust and suggest an ediacaran age for the oldest sedimentary rocks these two observations extend the known limit of the rio de la plata craton at least 200 km sw of previous estimates new data also confirm the occurrence of late ediacaran to late cambrian magmatism in the sierras australes the age of these hypabyssal to volcanic rocks corresponds to igneous events in the pampean belt along the western margin of the rio de la plata craton although the shallow level magma emplacement in the sierra da ventana study area contrasts with the deeply exhumed rocks of the pampean orogeny type locality these new age data are compared with a broad compilation of geochronological age clymene collision belts to the north the paraguai and araguaia belts the close overlap of the timing of orogenesis indicates the age of clymene ocean closure in its northern reaches in the south the pampean belt was unconfined allowing continued tectonic activity and crustal accretion throughout the paleozoic

paleogeographic and tectonic evolution of the pampia terrane in the cambrian new paleomagnetic constraints abstract paleomagnetic magnetic fabric and rock magnetic studies were carried out in the late middle to early late cambrian campanario formation exposed in nw argentina the study also presents preliminary results from the early cambrian metasediments of the puncoviscana formation and from mojotoro intrusive a new paleomagnetic pole c2 was computed for the campanario formation 23 6 n 346 5 e a95 7 0 n 11 sites from the localities of tilcara t terma de reyes r and el perchel ep including previous data from santa victoria oeste m in northernmost argentina a positive fold test was obtained for tilcara locality while a positive regional tilt test was obtained for mean site directions for the four localities this pole can be considered as the representative pole position for the pampia terrane for the late middle to late cambrian previous paleomagnetic data for the same formation from the iruya matancillas locality show an in situ clockwise rotation of about 30 around a vertical axis when compared with the mean pampia pole the same is observed when comparing the early ordovician pole for the santa rosita formation at those localities with the recently obtained coeval pole of the la pedrera formation indicating that local tectonic rotations affected that area these results suggest that previous paleomagnetic interpretations for these late cambrian and early ordovician rocks in the region as recording displacement of the pampia terrane in cambrian times are probably incorrect additionally two virtual geomagnetic poles were computed for the puncoviscana formation tp1 34 5 n 52 3 e a95 5 0 n 22 specimens and tp2 38 3 n 33 6 e a95 5 7 n 14 specimens and one for the mojotoro intrusive mo 9 1 n 345 6 e a95 16 4 n 11 specimens these and previous early cambrian paleomagnetic data from the pampia terrane differ from gondwana coeval reference directions in inclination and or declination lacking a simple pattern

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

composition and textures of chromite and platinum group minerals in chromitites of the western ophiolitic belt from pampean ranges of cordoba argentina abstract chromitite bodies hosted in the neoproterozoic western ophiolitic belt of pampean ranges of cordoba argentina were studied at los congos and los guanacos ultramafic bodies with regard to the composition and textures of the chromite and platinum group minerals primary chromite composition is only preserved in some massive chromitites from the los guanacos ultramafic body and is similar to al rich ophiolitic chromitites suggesting that they crystallized from melts with back arc basin basalts babb affinity in the suprasubduction mantle subsequently these chromitites underwent a prograde metamorphism chromites from chromitites and associated metamorphosed ultramafic rocks show complex replacement and exsolution textures mineral chemistry and texture indicate that the chromite composition records two main metamorphic trends a first trend defined by chromite from massive chromitite in which there is an enrichment in fe3 and fe2 cr remain relatively constant and slightly depleted in al mg a second trend is defined by chromite from disseminated chromitite and metamorphosed dunite and harzburgite in which a fe rich phase is replacing the al rich chromite this alteration trend is characterized by enrichment in the total iron content fe3 fe2 and a strong depletion in al and mg the chemical composition of all analyzed spinels from los guanacos and los congos as plotted on the ternary fe3 cr al diagram correlates well with the cr spinels from the upper amphibolite to granulite facies metamorphism platinum group minerals pgm identified include native osmium laurite erlichmanite irarsite platinum and a number of inadequately identified phases such as an oxide or hydroxide of ru pt and ir ru pt telluride ir ru as se and ir ru ti compounds native osmium was the only pgm which remained unaltered other pgm underwent mineralogical reworking during metamorphism although it is difficult to establish the extent of platinum group element mobilization based on mineralogical observation our results suggest that the ru os ir pgm in the los guanacos and los congos chromitites were modified in situ producing re distribution of these pge on a small scale the presence of rare pt and pge as se minerals was possibly related to remobilization of pt as and se by fluids during the alteration processes

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

syn deformational anatexis along the santa rosa river section argentina feedback relations between deformation metamorphism and melt extraction abstract mass transference from lower to upper crustal levels requires an efficient network of pathways that act as channels for draining melts from the source and transferring magmas through the crust we report on a mid crustal section in the santa rosa river from the eastern pampean ranges that underwent partial melting under active deformation the study focuses on the feedback relations between structural features and the position and shape of leucosomes that record the remnant magma network in both metasedimentary and meta igneous rocks a complete range of migmatites encompassing from patch to stromatic metatexite to schollen schlieren and nebulitic diatexite formed during a single high grade metamorphic event that postdated a previous sub solid evolution the diversity in migmatites structures suggests that the layered sequence behaved heterogeneously under a non coaxial deformational regime of progressive deformation the relationships between migmatitic structures and the progressive production of melts resulted in feedback relations that persisted as a time marker during the pampean event u pb dating of leucogranitic zircons allows constraining the time of melt generation and segregation to the lower cambrian 534 520 ma migmatites in the lower crust of the pampean orogen acted as channels of melt migration towards upper crustal levels

influence of joint anisotropy on the fracturing behavior of a sedimentary rock abstract fracture toughness ft and tensile strength ts indicate a rock s susceptibility to fracturing past studies showed that mechanical properties and ft of homogeneous rocks can be correlated well in the present study this capability has been extended to the jointed sedimentary rocks sandstone with a wide combination of analogue joints were tested in the laboratory and the control of joint geometrical properties on the ft ts and development of fracture process zone fpz were investigated a ft prediction model was developed based on the ts of the jointed specimens multifractal scaling law mfsl was utilized to extend the laboratory results to the field scale further the interaction of propagating crack with the already existing joints were investigated and post peak behavior were studied results show that ft and ts decrease with decreasing joint spacing but they are more sensitive at higher joint spacing it is also possible to construct a linear prediction model between the ft and ts of rock fpz of jointed rock is less sensitive to joint orientation and more related to joint spacing a negative correlation between the fpz size and joint spacing is established in this study post peak behavior of the jointed specimens indicate that crack joint interaction increases the frictional resistance and dissipated fracture energy of the specimens further comparison of mixed mode fracture criteria with the experimental results show that maximum tangential stress mts criterion can successfully predict the mixed mode fracture behavior of jointed sandstone

time constraints on the tectonic evolution of the eastern sierras pampeanas central argentina the application of the shrimp u pb dating technique to zircon and monazite of different rock types of the sierras de cordoba provides an important insight into the metamorphic history of the basement domains additional constraints on the pampean metamorphic episode were gained by pb pb stepwise leaching pbsl experiments on two titanite and garnet separates results indicate that the metamorphic history recorded by crd free gneisses m2 started in the latest neoproterozoic earliest cambrian 553 and 543 ma followed by the m4 metamorphism at 530 ma that is documented in the diatexites zircon ages of 492 ma in the san carlos massif correlate partly with rather low th u ratios 0 1 suggesting their growth by metamorphic fluids this age is even younger than the pbsl titanite ages of 506 ma it is suggested that the fluid alteration relates to the beginning of the famatinien metamorphic cycle in the neighbouring sierra de san luis and has not affected the titanite ages the ptt evolution can be correlated with the plate tectonic processes responsible for the formation of the pampean orogene i e the accretion of the pampean basement to the rio de la plata craton m2 and the later collision of the western pampean basement with the pampean basement

the pacific gondwana margin in the late neoproterozoic early paleozoic detrital zircon u pb ages from metasediments in northwest argentina reveal their maximum age provenance and tectonic setting abstract u pb detrital zircon ages are reported from puncoviscana formation late neoproterozoic early cambrian and meson group late cambrian greywackes of northwest argentina to constrain provenance and depositional environment the new data are combined with previously published detrital zircon ages to show that puncoviscana formation age patterns contain two broad groups late mesoproterozoic early neoproterozoic 1150 850 ma and late neoproterozoic early cambrian 650 520 ma with their relative proportions varying inversely with youngest component age the 1150 850 ma age components are dominant in greywackes with oldest late neoproterozoic components 600 ma the former diminish considerably when late neoproteozoic components become dominant and younger to 520 ma a northernmost greywacke sample from purmamarca jujuy is distinctive whilst its zircon age pattern partly resembles other puncoviscana formation samples it contains no cambrian late neoproterozoic ages the youngest ages being early neoproterozoic this may reflect an early neoproterozoic passive margin depocentre for the formation or an older early neoproterozoic succession within it which may predate the brasiliano orogeny in brazil the youngest age components c 520 ma in a greywacke from rancagua cachi salta province dominate an almost unimodal pattern suggestive of contemporary volcanic sources at a late early cambrian depocentre detrital zircon age patterns of the meson group lizoite formation have major cambrian latest neoproterozoic components resembling those of the puncoviscana formation but its mesoproterozoic component is diminished and there are no significant age components of this age small youngest components at c 500 ma suggest a maximum late cambrian stratigraphic age the puncoviscana formation detrital zircon patterns suggest a provenance in a continental hinterland having a stabilised extensive late mesoproterozoic orogen with minor paleoproterozoic and archean precursors and a more variable late neoproterozoic orogen containing an evolving sequence of less extensive subcomponents a direct relationship with the brazilian shield is suggested with sediment supplies originating within active margin orogens of the interior and collisional orogens at the suture between african and south american cratons but ultimate deposition in passive margin environments of western gondwanaland

the ordovician sierras pampeanas puna basin connection basement thinning and basin formation in the proto andean back arc article i nfo the ordovician sierras pampeanas located in a continental back arc position at the proto andean margin of southwest gondwana experienced substantial mantle heat transfer during the ordovician famatina orogeny converting neoproterozoic and early cambrian metasediments to migmatites and granites the high grade metamorphic basement underwent intense extensional shearing during the early and middle ordovician contemporaneously up to 7000 m marine sediments were deposited in extensional back arc basins covering the pre ordovician basement extensional ordovician tectonics were more effective in mid and lower crustal migmatites than in higher levels of the crust at a depth of about 13 km the separating boundary between low strain solid upper and high strain lower migmatitic crust evolved to an intra crustal detachment the detachment zone varies in thickness but does not exceed about 500 m the formation of anatectic melt at the metamorphic peak and the resulting drop in shear strength initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies p t conditions p t d t data in combination with field evidence suggest significant ca 52 crustal thinning below the detachment corresponding to a thinning factor of 2 1 ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25 and 44 constituting total crustal thinning factors of 1 7 2 0 while the migmatites experienced retrograde decompression during the ordovician rocks along and above the detachment show isobaric cooling this suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface upper crustal extension and thinning is compensated by newly deposited sediments maintaining constant pressure at detachment level thinning of the migmatitic lower crust is compensated by elevation of the crust mantle boundary the degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension basin formation and sediment thickness in the back arc basin the initiation of crustal extension in the back arc however crucially depends on the presence of anatectic melt in the middle and lower crust consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension

goce satellite derived gravity and gravity gradient corrected for topographic effect in the south central andes region summary global gravity field models derived from satellite measurements integrated with terrestrial observations provide a model of the earth s gravity field with high spatial resolution and accuracy the earth gravity model egm08 a spherical harmonic expansion of the geopotential up to degree and order 2159 has been used to calculate two functionals of the geopotential the gravity anomaly and the vertical gravity gradient applied to the south central andes area the satellite only field of the highest resolution has been developed with the observations of satellite goce up to degree and order 250 the topographic effect a fundamental quantity for the downward continuation and validation of satellite gravity gradiometry data was calculated from a digital elevation model which was converted into a set of tesseroids this data is used to calculate the anomalous potential and vertical gravity gradient in the southern central andes region the geological structures are very complex but not well resolved the processing and interpreting of the gravity anomaly and vertical gradients allow the comparison with geological maps and known tectonic structures using this as a basis a few features can be clearly depicted as the contact between pacific oceanic crust and the andean fold and thrust belt the seamount chains over the oceanic nazca plate and the famatinian and pampean ranges moreover the contact between the rio de la plata craton and the pampia terrain is of great interest since it represents a boundary that has not been clearly defined until now another great lineament the valle fertil desaguadero mega lineament an expression of the contact between cuyania and pampia terranes can also be clearly depicted the authors attempt to demonstrate that the new gravity fields can be used for identifying geological features and therefore serve as useful innovative tools in geophysical exploration

detrital zircon provenance from the neuquen basin south central andes cretaceous geodynamic evolution and sedimentary response in a retroarc foreland basin the surface response in terms of drainage pattern changes to the cretaceous geodynamic reorganization of the andean subduction zone between 36 s and 41 s is reconstructed through the geochronology based provenance study of alluvial detrital zircons the age spectra obtained by 500 spot u pb ages record an eastward provenance of detritus coming from the foreland during the early cretaceous backarc extensional stage followed by westward sourced clastics coming from the cordillera during the cenomanian this drainage pattern reversal fits the regional unconformity in the sedimentary record that is linked to the geodynamic reorganization of the continental margin from an extensional to a compressional regime forcing the neuquen basin to evolve from a retroarc to a foreland stage after this inversion the clastic systems progressively returned to be mainly fed by the foreland due to the uplift of the peripheral bulge as a consequence of the late cretaceous thrust front migration this tectonic evolution of the neuquen basin and the related response of the drainage pattern are thought to be the surface expression of the dip decrease of the benioff subduction zone

geodynamic evolution of the eastern sierras pampeanas central argentina based on geochemical sm nd pb pb and shrimp data whole rock geochemical analyses using major and trace elements in combination with the sm nd and pb pb isotope systems together with shrimp age dating on metasedimentary rocks from the sierras de chepes the sierras de cordoba the sierra norte and the san luis formation in the sierra de san luis have been carried out to unravel the provenance and the geodynamic history of the eastern sierras pampeanas central argentina the geochemical and the sm nd data point to a slightly stronger mafic and less fractionated material in the provenance area of the sierras de cordoba when compared to the other units the tdm model ages from the sierras de chepes 1 82 ga and the sierra norte 1 79 ga are significantly older than the data from the sierras de cordoba 1 67 ga the pb data are homogeneous for the different units only the 208pb 204pb ratios of some samples from the sierras de cordoba are higher a late pampean detrital zircon peak around 520 ma from the sierras de chepes is in accordance with the new data from the san luis formation this is similar to the literature data from the famatina belt located to the northwest of the sierras de chepes and also fits the detrital zircon peaks in the meson group these maximum depositional ages were also reported from some locations in the puncoviscana formation but are absent in the sierras de cordoba an improved model for the development of the eastern sierras pampeanas in the area between the sierras de cordoba and the puncoviscana formation is provided this gives new insights into the late pampean development of the sierra de san luis and the complex development of the eastern sierras pampeanas this new model explains the younger detrital ages in the puncoviscana formation compared with the older ages of the sierras de cordoba another model of the sierra de san luis explains the younger depositional ages of the pringles metamorphic complex and the san luis formation when compared to the nogoli metamorphic complex and the conlara metamorphic complex additionally the rather fast change of the high grade metamorphic conditions in the pringles metamorphic complex and the low grade metamorphic conditions in the san luis formation is explained by extension the ascent of ultra mafic material and later folding and erosion

tectono stratigraphic evolution of the andean orogen between 31 and 37 s chile and western argentina fil charrier reynaldo universidad de chile facultad de ciencias fisicas y matematicas departamento de geologia chile universidad andres bello chile

western parana suture shear zone and the limits of rio apa rio tebicuary and rio de la plata cratons from gravity data abstract we present a new gravity map between 45 70 w and 5 40 s integrating open source terrestrial gravity data of argentina with the south american gravity model 2004 sagm04 a 5 min arc resolution gravity model the bouguer anomaly map reveals a 2000 km long linear gravity feature from 15 s to 30 s at longitude 55 w with a steep horizontal gradient separating two gravity domains the eastern domain is the parana basin with ne sw trending bouguer anomalies of 80 mgal in average the western domain comprises the chaco parana chaco tarija and pantanal basins with circular positive anomalies of up to 20 mgal in amplitude previous seismic studies mapped a thinner crust of less than 35 km in the western domain and the present gravity models indicate a 10 20 kg m 3 denser crust on the other hand the eastern domain has a thicker crust of more than 40 km seismic tomography models also show p and s wave velocity reduction in the western domain whereas high velocity characterises the parana basin these geophysical data indicate that the gravity gradient marks a transition between two distinct lithospheres the gravity gradient is associated with a tectonic feature referred to as the western parana suture shear zone granites of 530 570 ma ages located parallel or over the gravity gradient suggest a neoproterozoic to early cambrian age suture shear zone thus approximately synchronous and parallel to the pampean belt sediment corrected residual gravity map and its vertical derivative allow us to define the limits of the rio apa rio de la plata and rio tebicuary cratons their eastern and western limits are the western parana suture and the pampean belt respectively this study unravels precambrian tectonic elements concealed by the phanerozoic sedimentary basins adding new constraints for the amalgamation history of sw gondwana

lithospheric evolution of the pre and early andean convergent margin chile abstract the proto andean and early andean evolution of the southwestern gondwana margin comprises three stages that differ in their magmatic evolution and deformational style the gondwana cycle 330 280 ma the pre andean stage 280 210 ma and the early andean cycle 210 100 ma these stages have been traditionally interpreted as the upper crustal response to changes in the tectonic setting which include cordilleran style continental arc gondwana cycle orogenic collapse and possibly slab break off that led to continental rifting and extensive crustal melting pre andean stage and subsequent subduction re initiation in oceanic arc style context early andean cycle the petrological and geochemical characteristics of carboniferous to jurassic igneous rocks from this region however do not support the described model elemental and sr nd pb isotopic data of 86 samples along with a compilation of 1230 samples from the literature suggest that subduction was the most likely process by which the magmatic record was generated sub alkaline affinities lile enrichment over hfse nb ta troughs porphyritic textures and hornblende and biotite bearing lithologies are present in all studied units whereas isotopes suggest that magma sources are a mixture of depleted mantle and variable contribution from the continental crust even though the aforementioned features are common to all igneous rocks some changes point to a decline in the contribution of crustal lithospheric sources to the magmatism with time thus sio2 lan ybn and 87sr 86srinitial exhibit a systematic decrease from 285 to 150 ma whereas the endinitial parameter increases in the same period these changes were accompanied by the shift from dominant compressional carboniferous early permian to transtentional middle permian jurassic stresses in the upper crust suggesting that the margin went from advancing to retreating due to pangea reorganization and break up following a potential flat slab event slab roll back may have induced extension in the upper crust and lithospheric loss as a consequence of delamination or thermal erosion

a provenance study of the paleozoic ventania system argentina transient complex sources from western and eastern gondwana abstract the u pb and lu hf isotopic analyses of the different sedimentary sequences of the ventania system an old paleozoic orogenic belt exposed in the southern region of the rio de la plata craton in the province of buenos aires argentina provide new evidence for the understanding of the tectonic evolution of the western sector of the gondwanides mountain belt these ranges formed as result of the late paleozoic collision of the patagonia terrane against the continental margin of gondwana the provenance analysis together with the sedimentary paleocurrents confirm a dominant source from the tandilia system a paleoproterozoic mountain belt formed during the amalgamation of the rio de la plata craton at about 1800 2200 ma and incorporated to western gondwana during the brasiliano orogeny at 550 530 ma the local dominant source at the base of the early paleozoic changed to more distant supplies toward the top of the sequences when an increasing participation of detritus from first cambrian 560 520 ma zircons from the pampean orogen and later on ordovician 480 460 ma zircons from the famatinian orogen is recorded the detrital zircon patterns and the maximum age of the units shed light on some previous discrepancies in the early paleozoic stratigraphy the balcarce formation an early paleozoic sedimentary cover of the tandilia metamorphic and igneous basement shows striking differences when compared with the new data from the ventania system the two data sets reveal different sources for the two regions the late paleozoic foreland basin deposits mark an abrupt change of 180 in the paleocurrent directions in the petrographic composition of the sediments and in the provenance of detrital zircons these data indicate a southern provenance with the first evidence of carboniferous and permian magmatic zircons the oldest archean zircons together with the finding of clasts with archeocyathids support the provenance from patagonia which was derived from eastern gondwana the u pb ages of the ash fall tuffs in the tunas formation confirm the early permian age of the eurydesma fauna in the ventania system the u pb data together with the lu hf isotopic data enhance the comprehension of the tectonic evolution of the ventania system as part of the larger gondwanides belt that amalgamated to western gondwana during late permian times with some independent pieces derived from eastern gondwana

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

a major mid crustal decollement of the paleozoic convergent margin of western gondwana the guacha corral shear zone argentina abstract the guacha corral shear zone gcsz is one of the most significant and widest thrusts in the western gondwana margin of argentina it is 10 16 km wide and thrusts 525 515 ma forearc migmatites of the pampean orogeny eastwards onto greenschist facies rocks metamorphosed at 460 450 ma during the famatinian orogeny it is divided into two sections at the highest structural levels the shear zone is 7 5 km wide and records high t amphibolite facies deformation at lower structural levels this is overprinted by a 2 5 km wide low t greenschist facies shear zone the two are separated by a transitional zone where overprinting relationships are preserved motion in the two sections had the same kinematics syn anatectic structures in migmatites indicate that thrusting started during pampean forearc melting and continued into subsolidus during hanging wall exhumation and cooling greenschist facies reactivation at the end of the famatinian orogeny thrust the pampean migmatites over famatinian age metamorphic rocks at this stage the shear zone became part of a system of contractional shear zones defining the eastern boundary of a 300 km wide belt of famatinian foreland shortening thus the uniquely wide gcsz is a major tectonic boundary developed in two steps first as an amphibolite facies mid crustal detachment controlling the widening of the compressional pampean forearc then as the easternmost foreland thrust system of the famatinian orogeny