chemical influence on α recoil damage annealing in apatite implications for u th he dating article apatite u th he ahe ages reflect the influence of several parameters such as grain size thermal history and diffusion kinetics previous work has suggested that recoil damage produced by the u th decay chains can have a significant effect on he diffusion kinetics models taking into account damage production and annealing via a parameterization similar to that for apatite fission tracks aft have been proposed comple mentary to these models in this work we demonstrate that grain chemistry influences the annealing rate of diffusivity altering damage in the same manner as for aft and thus is not constant across all apatites the key parameter of the annealing law rmr0 is examined and its theoretical impact on ahe age is tested in cer tain situations the ahe age can be significantly affected by changes in the damage annealing rate for sam ples having undergone a long stay in the ahe partial retention zone 50 80 c ahe ages can diverge by more than 15 over a reasonable range of annealing rates in addition this hypothesis is tested on a geolog ical case well samples of triassic sandstones at 67 5 1 5 c at depth in the paris basin france aft data and ahe ages have been measured on detrital apatite in addition to chemical analysis of the grains dated for aft ages for both thermochronometers present similar histogram distributions with younger ages for ahe aft 14 208 ma n 117 ahe 4 76 ma and one grain at 120 ma n 36 the apatite grains are character ized by dpar values from 1 2 m to 2 9 m and cl content ranging from 0 05 to 0 9 wt using a thermal his tory reconstruction based on aft data and geological evidence it is shown that the non correlation observed between ahe age and eu content can be simply explained by changing the rmr0 value we conclude that to fully access the ahe age meaning in terms of temperature sensitivity the parameters of the annealing law stemming from the grain chemistry need to be considered

symmetry during the syn and post rift evolution of extensional back arc basins the role of inherited orogenic structures abstract rheological heterogeneities in the lithosphere have first order control on the topographical expression of tectonic processes pre existing orogenic suture zones localise extensional deformation resulting in asymmetric basins such crustal geometries are often in contrast with the more symmetric regional lithospheric structure observed beneath extensional basins we study such a symmetries and their controlling parameters by conducting a series of 2d thermo mechanical numerical experiments of the extension of an overthickened hot lithosphere that contains a weakness zone the modelling shows that syn rift subsidence is low to moderate creating asymmetric half grabens where extension migrates in space and time grouped in an overall symmetrical appearance on a larger scale the initial lithospheric mantle asymmetry is attenuated by the lateral heat conduction and further dynamic evolution of the thermal anomaly during the post rift phase resulting in differential vertical movements of the crust including additional 2 3 km subsidence in the basin centre the modelling shows that the initial crustal and lithospheric thicknesses rate of extension and surface processes strongly control the thermo mechanical evolution of the extensional system the numerical modelling yields new insights into the mechanics of coupling between near surface kinematics and the evolution of deep lithospheric structure in the pannonian basin and the aegean two of europe s largest back arc systems

stratigraphic relations and lithologic variations in the jemez volcanic field new mexico over 100 radiometric dates and recent detailed geologic mapping allow some refinements of the stratigraphic relations of major units and generalization of temporal lithologic variations in the jemez volcanic field volcanism had begun in the area by about 16 5 ma with episodic eruptions of alkaline basalts by 13 ma alkaline volcanism had been replaced with eruptions of more voluminous olivine tholeiite high silica rhyolite derived from melts of lower crust also was erupting by about 13 ma basalt and high silica rhyolite continued to be erupted until about 7 and 6 ma respectively but effusions of dominantly andesitic differentiates of basalt that began as early as about 12 ma volumetrically overshadowed all other eruptive products between 10 and 7 ma from 7 to 3 ma the dominant erupted lithology was dacite which appears to have been generated by mixing of magmas whose compositions are approximated by earlier andesites and high silica rhyolites less than 4 3 ma volcanism was dominated by eruption of rhyolitic tuffs field relations geochemistry and dates specifically indicate the following with regards to stratigraphie relations 1 distinctions among basalt of chamisa mesa paliza canyon formation basalts and lobato basalt for other than geographic reasons are artificial basaltic volcanism was continuous in volcanic field from 13 to 7 ma 2 canovas canyon and bearhead rhyolites form a continuum of high silica rhyolite volcanism from 13 to 6 ma 3 hypabyssal and volcanic rocks of the cochiti mining district probably represent the exhumed interior of a keres group volcano s 4 temporal overlaps exist among the major stratigraphie groups which may imply some genetic relations and 5 the tewa group formation cerro rubio quartz latite may more appropriately be considered part of the tschicoma formation of the polvadera group preliminary analysis of hydrothermal alteration in the context of the volcanic stratigraphy suggests at least three distinct hydrothermal events have occurred in the volcanic field s history

the influence of burial heating on the u th he system in apatite grand canyon case study article history thermochronological data can constrain the cooling paths of rocks exhumed through the uppermost 1 2 km of earth s crust and have thus been pivotal in illuminating topographic development over timescales 0 1 ma however in some cases different methods have led to conflicting conclusions about timing of valley scale exhumation here we investigate the case of western grand canyon usa where different thermochronological datasets have been interpreted to record very different timings of canyon incision 70 ma versus 5 ma we present a method to assess key assumptions in these constraints and demonstrate that burial heating conditions of basement rocks in the mesozoic can result in incomplete annealing of radiation damage in apatite in turn this has a dramatic effect on the temperature sensitivity of the apatite u th he system and its ability to record post burial exhumation the possibility of incomplete annealing resolves the apparent conflict in time temperature paths inferred over the last 70 ma although it requires temperatures during burial that are lower than predicted by apatite fission track data a refinement of parameters that prescribe the kinetics of damage annealing and related control on 4 he diffusivity in apatite would account for this discrepancy specifically if alpha recoil damage anneals at a lower rate than fission tracks at a given temperature these effects will be important for any application of the apatite u th he system in geologic settings that experienced prolonged residence 10 ma between 50 150 c the approaches developed here provide means to assess these effects

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

6 ma age of carving westernmost grand canyon reconciling geologic data with combined aft u th he and 4he 3he thermochronologic data conflicting hypotheses about the timing of carving of the grand canyon involve either a 70 ma old or 6 ma young grand canyon this paper evaluates the controversial westernmost segment of the grand canyon where the following lines of published evidence firmly favor a young canyon 1 north derived paleocene hindu fanglomerate was deposited across the present track of the westernmost grand canyon which therefore was not present at 55 ma 2 the 19 ma separation point basalt is stranded between high relief side canyons feeding the main stem of the colorado river and was emplaced before these tributaries and the main canyon were incised 3 geomorphic constraints indicate that relief generation in tributaries and on plateaus adjacent to the westernmost grand canyon took place after 17 ma 4 the late miocene pliocene muddy creek formation constraint shows that no river carrying far traveled materials exited at the mouth of the grand canyon until after 6 ma interpretations of previously published low temperature thermochronologic data conflict with these lines of evidence but are reconciled in this paper via the integration of three methods of analyses on the same sample apatite u th he ages ahe 4he 3he thermochronometry 4he 3he and apatite fission track ages and lengths aft hefty software was used to generate time temperature t t paths that predict all new and published 4he 3he ahe and aft data to within assumed uncertainties these t t paths show cooling from 100 c to 40 60 c in the laramide 70 50 ma long term residence at 40 60 c in the mid tertiary 50 10 ma and cooling to near surface temperatures after 10 ma and thus support young incision of the westernmost grand canyon a subset of ahe data when interpreted alone i e without 4he 3he or aft data are better predicted by t t paths that cool to surface temperatures during the laramide consistent with an old grand canyon however the combined aft ahe and 4he 3he analysis of a key sample from separation canyon can only be reconciled by a young canyon additional new aft 5 samples and ahe data 3 samples in several locations along the canyon corridor also support a young canyon this inconsistency which mimics the overall controversy of the age of the grand canyon is reconciled here by optimizing cooling paths so they are most consistent with multiple thermochronometers from the same rocks to do this we adjusted model parameters and uncertainties to account for uncertainty in the rate of radiation damage annealing in these apatites during sedimentary burial and the resulting variations in he retentivity in westernmost grand canyon peak burial conditions temperature and duration during the laramide were likely insufficient to fully anneal radiation damage that accumulated during prolonged near surface residence since the proterozoic we conclude that application of multiple thermochronometers from common rocks reconciles conflicting thermochronologic interpretations and the data presented here are best explained by a young westernmost grand canyon samples spread along the river corridor also suggest the possibility of variable mid tertiary thermal histories beneath north retreating cliffs

internal triggering of volcanic eruptions tracking overpressure regimes for giant magma bodies abstract understanding silicic eruption triggers is paramount for deciphering explosive volcanism and its potential societal hazards here we use phase equilibria modeling to determine the potential role of internal triggering wherein magmas naturally evolve to a state in which eruption is inevitable in rhyolitic magma bodies whole rock compositions from five large to super sized rhyolitic deposits are modeled using rhyolite melts by running simulations with varying water contents we can track crystallization and bubble exsolution during magma solidification we use simulations with variable enthalpy and fixed pressure for the five compositions the interplay between bubble exsolution and crystallization can lead to an increase in the system volume which can lead to magma overpressurization we find that internal triggering is possible for high silica rhyolite magmas crystallizing at pressures below 300 mpa 50 wt which makes magma immobile high silica rhyolite eruptions from such depths would require external triggering but examples are scarce or entirely absent calculated crystallinities at which the critical overpressure threshold is reached compare favorably with observed crystal contents in natural samples for many systems suggesting that internal evolution plays a critical role in triggering eruptions systems in which fluid saturation happens late relative to crystallization or in which degassing is effective can delay or avoid internal triggering we argue that priming by crystallization and bubble exsolution is critical for magma eruption and external triggering serves simply as the final blow rather than being the driving force of explosive rhyolitic eruptions

crustal deformation in the south central andes backarc terranes as viewed from regional broad band seismic waveform modelling summary the convergence between the nazca and south america tectonic plates generates a seismically active backarc region near 31 s earthquake locations define the subhorizontal subducted oceanic nazca plate at depths of 90 120 km another seismic region is located within the continental upper plate with events at depths 1 80 th 45 55 km than those of the eastern sierras pampeanas vp 6 0 6 2 km s 1 vp vs 1 70 th 27 35 km in addition we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes finally we estimated average p and t axes over the charge period the entire region showed p and t axis orientations of 275 and 90 plunging 6 and 84 respectively

the moho in extensional tectonic settings insights from thermo mechanical models the lithospheric memory is key for the interplay of lithospheric stresses and rheological structure of the extending lithosphere and for its later tectonic reactivation other important factors are the temporal and spatial migration of extension and the interplay of rifting and surface processes the mode of extension and the duration of the rifting phase required to lead to continental break up are to a large extent controlled by the interaction of the extending plate with slab dynamics the finite strength of the lithosphere has an important effect on the formation of extensional basins this applies both to the geometry of the basin shape as well as to the record of vertical motions during and after rifting we demonstrate a strong connection between the bulk rheological properties of europe s lithosphere and the evolution of some of europe s main rifts and back arc systems the thermo mechanical structure of the lithosphere has a major impact on continental break up and associated basin migration processes with direct relationships between rift duration and extension velocities thermal evolution and the role of mantle plumes compressional reactivation has important consequences for post rift inversion borderland uplift and denudation as illustrated by poly phase deformation of extensional back arc basins in the black sea and the pannonian basin region

modulation of magmatic processes by co2 flushing abstract magmatic systems are the engines driving volcanic eruptions and the source of fluids responsible for the formation of porphyry type ore deposits sudden variations of pressure temperature and volume in magmatic systems can produce unrest which may culminate in a volcanic eruption and or the abrupt release of ore forming fluids such variations of the conditions within magmatic systems are commonly ascribed to the injection of new magma from depth however as magmas fractionating at depth or rising to the upper crust release co2 rich fluids the interaction between carbonic fluids and h2o rich magmas stored in the upper crust co2 flushing must also be a common process affecting the evolution of subvolcanic magma reservoirs here we investigate the effect of gas injection on the stability and chemical evolution of magmatic systems we calculate the chemical and physical evolution of magmas subjected to co2 flushing using rhyolite melts we compare the calculations with a set of melt inclusion data for mt st helens merapi etna and stromboli volcanoes we provide an approach that can be used to distinguish between melt inclusions trapped during co2 flushing magma ascent and decompression or those affected by post entrapment h2o loss our results show that co2 flushing is a widespread process in both felsic and mafic magmatic systems depending upon initial magma crystallinity and duration of co2 input flushing can either lead to volcanic eruption or fluid release we suggest that co2 flushing is a fundamental process modulating the behaviour and chemical evolution of crustal magmatic systems

mechanisms of explosive volcanism on mercury implications from its global distribution and morphology the identification of widespread pyroclastic vents and deposits on mercury has important implications for the planet s bulk volatile content and thermal evolution however the significance of pyroclastic volcanism for mercury depends on the mechanisms by which the eruptions occurred using images acquired by the mercury surface space environment geochemistry and ranging spacecraft we have identified 150 sites where endogenic pits are surrounded by a relatively bright and red diffuse edged spectral anomaly a configuration previously used to identify sites of explosive volcanism we find that these sites cluster at the margins of impact basins and along regional tectonic structural trends locally pits and deposits are usually associated with zones of weakness within impact craters and or with the surface expressions of individual thrust faults additionally we use images and stereo derived topographic data to show that pyroclastic deposits are dispersed up to 130 km from their source vent and commonly have either no relief or low circumpit relief within a wider thinner deposit these eruptions were therefore likely driven by a relatively high concentration of volatiles consistent with volatile concentration in a shallow magma chamber prior to eruption the colocation of sites of explosive volcanism with near surface faults and crater related fractures is likely a result of such structures acting as conduits for volatile and or magma release from shallow reservoirs with volatile overpressure in these reservoirs a key trigger for eruption in at least some cases our findings suggest that widespread long lived explosive volcanism on mercury has been facilitated by the interplay between impact cratering tectonic structures and magmatic fractionation

cenozoic provenance history of synorogenic conglomerates in western argentina famatina belt implications for central andean foreland development in famatina at the modern northern flat slab segment of the central andean belt synorogenicconglomeratefaciesprovideaunique opportunity to reconstruct paleogeography and tectonic setting stratigraphically constrained provenance analyses record a multi event history of the andean foreland and a complex pattern of exhumed uplands which in turn controlled the depocenter history western and eastern sources can be differentiated the western domain was composed of equigranular granitoids and lower and upper paleozoic volcano sedimentary and sedimentary units whereas the eastern domain was mainly formed by porphyrytic granitoids and minor slightly metamorphosed lower paleozoic clastic rocks the surrounding regions expose medium to high grade metamorphic rocks which are not recorded in any cenozoic conglomerate of famatina unroofing sequences can be deduced from the compositional trends conglomerate composition and paleocurrent data show input from intraba sinal sources and important participation of basement since the beginning of the neogene we infer an early miocene broken foreland during deposition of the del creston formation a significant source inversion with clasts supplied from the eastern basement domain is recorded 2000 m from the base within the middle miocene del abra formation reactivation of western granitic thrust sheets occurred by the late middle miocene and late miocene as indicated by provenance studies of the santo domingo and el durazno formations in contrast to simple asymmetric foreland models provenance analysis documents early participation of crystalline rocks suggesting early development of intrafore land basement highs within the southern central andes at a regional scale basement thrusting and associated intermontane basin development were diachronous this interpretation is supported by broad propagation of basement thrusting from the northern sierras pampeanas and famatina toward the central and eastern sierras pampeanas between early middle miocene and late miocene pliocene time early miocene reconstructions of the juan fernandez ridge position on the central andean margin suggest that during this interval shallow subduction was likely unrelated to oceanic plateau subduction under the sierras pampeanas

shallowing and steepening subduction zones continental lithospheric loss magmatism and crustal flow under the central andean altiplano puna plateau

low δ18o rhyolites from yellowstone magmatic evolution based on analyses of zircons and individual phenocrysts the yellowstone plateau volcanic field is one of the largest centers exchange time to form zoned zircons is between a few hundred and a few thousand years which reflects the residence time of lowo of rhyolitic magmatism on earth major caldera forming eruptions are followed by unusual lowo rhyolites new oxygen isotope magmas after formation and before eruption petrologic and geochemical data from rhyolites belonging to the 2 0 my eruptive history of yellowstone are presented with emphasis on the genesis of lowo magmas erupted after the huckleberry ridge tuff 2 0 ma 2500 km and lava creek tuff 0 6 ma

topographic relief driven by variations in surface rock density earth s surface topography is generated by tectonically induced variations in crustal thickness combined with erosion and to a lesser degree by vertical stresses caused by convection in the underlying mantle rock hardness and resistance to erosion are also commonly thought to influence topography because hard rocks such as granites and basalts usually form topographic highs in the landscape here we use analytical and numerical models to simulate the erosion induced isostatic rebound of rocks we find that the isostatic rebound that accompanies erosion causes denser rocks to occupy higher elevations in the landscape thereby creating topographic relief that is proportional to surface rock density differences rather than rock hardness we quantify this effect taking into account the flexural strength of the continental lithosphere we show that in a steady state erosional setting density dependent isostatic rebound can cause the densest rocks to be exhumed at double the rate of surrounding less dense rocks and has a stronger effect than typical rock hardness variations the results imply that denser rock formations should erode faster and therefore be characterized by younger thermochronological ages thermochronological data sets from the kinabalu granite in borneo and the shakhdara alichur gneiss domes in pamir confirm this counter intuitive result our findings imply that lateral variations in surface rock density have significant control on the shaping of the large scale features of earth s surface the high elevation in earth s topography of hard rocks such as granites and basalts was thought to be caused by their inherent resistance to erosion numerical modelling now demonstrates counterintuitively that erosion induced isostatic rebound of rocks which is density dependent causes granites and basalts to occupy high elevations because they are more dense than surrounding rocks

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina reply the thick skinned sierras pampeanas morphotectonic domain of western and northwestern argentina 27 s 33 s is characterized by reverse fault bounded basement blocks that delimit internally deformed neogene sedimentary basins foreland basin evolution in this part of the andes is still not very well understood for example challenging questions exist as to how thick skinned deformation develops if there are distinct spatiotemporal trends in deformation and exhumation how such deformation styles influence sedimentation patterns and whether or not broken foreland basins are related to regional plate tectonic processes such as flat slab subduction the fiambala basin of the northwestern sierras pampeanas is the largest of several intermontane basins in the transition to the southern margin of the puna plateau this basin preserves a thick continental neogene sequence that provides information on the dynamics of thick skinned deformation and resulting sedimentation the fiambala basin contains 4 km of fluvial alluvial sedimentary rocks that comprise the tamberia guanchin and punaschotter formations u pb geochronology of ashes intercalated within the fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late miocene to pliocene 8 2 0 3 ma to 3 05 0 4 ma in age sedimentology and provenance data indicate that the source of the tamberia formation was located to the west of the modern western basin bounding range the guanchin and punaschotter formations record input from local sources including the modern basin bounding range to the west and the southern puna plateau to the north suggesting reorganization of the catchment area at ca 5 5 ma the coarsening upward trends recorded by the fluvial tamberia and guanchin formations indicate enhanced tectonics and relief during sedimentation the punaschot ter conglomerates record alluvial fan sedimentation and local sources fault kinematic data document a contractional regime characterized by e w and ne sw shortening active throughout the middle late miocene and pliocene furthermore a comparison between the fiambala basin and similar sedimentary basins in the sierras pampeanas e g bermejo foreland basin and the eastern cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland basin system thin skinned for much of its early history our data suggest coeval intrabasin deformation along strike from the bermejo region northward to the eastern cordillera the coeval change at ca 6 ma from a regional to more compartmentalized thick skinned tectono sedimentary environment in the regions adjacent to the eastern cordillera the southern puna margin and other sectors within the sierras pampeanas domain may thus reflect a regional tectonic process related to flat subduction our data combined with existing sedimentological and petrological evidence imply that the passage from steep to flat subduction occurred synchronously from 30 s to 26 s

exhumation and uplift of the sierras pampeanas preliminary implications from k ar fault gouge dating and low t thermochronology in the sierra de comechingones argentina the sierras pampeanas in central and north western argentina constitute a distinct morphotectonic feature between 27 s and 33 s the last stage of uplift and deformation in this area are interpreted to be closely related to the andean flat slab subduction of the nazca plate beneath the south american plate k ar fault gouge dating and low temperature thermochronology along two transects within the sierra de comechingones reveal a minimum age for the onset of brittle deformation about 340 ma very low exhumation rates since late paleozoic time as well as a total exhumation of about 2 3 km since the late cretaceous new ar ar ages 7 54 1 91 ma of volcanic rocks from the san luis volcanic belt support the eastward propagation of the flat slab magmatic front confirming the onset of flat slab related deformation in this region at 11 2 ma although low temperature thermochronology does not clearly constrain the signal of the andean uplift it is understood that the current structural relief related to the comechingones range has been achieved after the exhumation of both fault walls circa 80 70 ma

development of the southern eastern cordillera nw argentina constrained by apatite fission track thermochronology from early cretaceous extension to middle miocene shortening 1 for the puna plateau and eastern cordillera of nw argentina the temporal and spatial pattern of deformation and surface uplift remain poorly constrained analysis of completely and partially reset apatite fission track samples collected from vertical profiles along an ese trending transect extending from the plateau interior across the southern eastern cordillera at 25 s reveals important constraints on the deformation and exhumation history of this part of the andes the data constrain the neogene andean development of the eastern cordillera as well as rift related exhumation for some of the sampled locations in the late jurassic early cretaceous an intervening eocene oligocene exhumation episode in the southern eastern cordillera was probably related to crustal shortening subsequent reburial of the area by andean foreland basin strata commenced between 30 and 25 myr magnitude and duration of sedimentation revealed by thermal modeling differ between the sample locations pointing to an eastward propagating basin system in the southern eastern cordillera andean deformation commenced at 22 5 21 myr predating both the inferred formation of significant topography by 5 7 5 myr and preservation of sediments in the adjacent cenozoic basins by 6 5 8 myr comparing the calculated structural depth of partially reset samples suggests that newly formed west dipping reverse faults along the former salta rift margin accommodated most of the neogene tectonic movement late cenozoic deformation at the southern eastern cordillera began earlier in the west and subsequently propagated eastward the lateral growth of the orogen is coupled with a foreland basin system developing in front of the range and then becomes subsequently compartmentalized by later emergent topography

reconstructing the thermal and exhumation history of the sierras pampeanas through low temperature thermochronology a case study from the sierra de velasco

larger foraminiferal assemblages from oligocene platform carbonates jamaica tethyan or caribbean abstract rocks of the white limestone group of jamaica include in the central part of the island a succession of mainly miliolid grainstones walderston limestone formation containing sparse gastropod and coral faunas of oligocene age part of the middle tertiary megabank of the northern nicaragua rise the associated larger foraminiferal genera such as the archaiasinids peneroplis and praerhapidionina together with rare lepidocyclinids had a tropical cosmopolitan distribution in the oligocene but several important tethyan forms such as the alveolinids reticulate nummulitids austrotrillina cycloclypeus and in the lower oligocene heterostegina s l have not been reported from jamaica

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

phase equilibria trigger for explosive volcanic eruptions 1 a mechanism for explosive volcanic eruptions based on multicomponent phase equilibria modelling of four explosive volcanic systems is proposed in each system either isochoric or isobaric crystallization where either crystals or crystals and exsolved fluid are chemically fractionated from melt leads inevitably to near solidus dynamical instability culminating in violent explosive eruption driving this instability is a dramatic increase in the volume fraction of fluid bubbles in the magma exceeding the limit for magma fragmentation this phenomenon is independent of magma decompression country rock may be weak allowing magma to remain in lithostatic equilibrium with its host rock or strong leading to divergence of magma and lithostatic pressure bubbles may be retained or expelled during crystallization instability is the inevitable outcome of crystallization at shallow levels in the crust for all four systems regardless of the mechanical state of host country rock we speculate that this phase equilibria mechanism driving explosive eruptions has general significance

monitoring super volcanoes geophysical and geochemical signals at yellowstone and other large caldera systems earth s largest calderas form as the ground collapses during immense volcanic eruptions when hundreds to thousands of cubic kilometres of magma are explosively withdrawn from the earth s crust over a period of days to weeks continuing long after such great eruptions the resulting calderas often exhibit pronounced unrest with frequent earthquakes alternating uplift and subsidence of the ground and considerable heat and mass flux because many active and extinct calderas show evidence for repetition of large eruptions such systems demand detailed scientific study and monitoring two calderas in north america yellowstone wyoming and long valley california are in areas of youthful tectonic complexity scientists strive to understand the signals generated when tectonic volcanic and hydrothermal hot ground water processes intersect one obstacle to accurate forecasting of large volcanic events is humanity s lack of familiarity with the signals leading up to the largest class of volcanic eruptions accordingly it may be difficult to recognize the difference between smaller and larger eruptions to prepare ourselves and society scientists must scrutinize a spectrum of volcanic signals and assess the many factors contributing to unrest and toward diverse modes of eruption

micro porous pyroclasts reflecting multi vent basaltic andesite plinian eruptions at mt tongariro new zealand abstract mt tongariro is a multi vent volcanic complex in the central north island of new zealand constructed along the axis of an active graben its five largest known explosive eruption episodes occurred 11 000 years ago during a 200 year long paroxysm during these stable plinian and collapsing sub plinian columns erupted from multiple sequential and synchronous vents along the graben axis we examined the spatially varying eruption dynamics during the 200 year paroxysm through the lens of pyroclastic textures each eruption episode produced micro vesicular basaltic andesitic juvenile lapilli with high vesicle number densities and high groundmass crystallinity textural characteristics indicate that pressurisation via second boiling drove eruptions and rhyolite like melt viscosities promoted brittle failure highly variable and mingled textures within individual juvenile lapilli evidence diverse conditions developed across the conduit during the eruption foamy magma rose rapidly through the conduit core driving explosive vigour and dragged and sheared gas poor crystallised magma from the conduit margins consistent with the northward widening graben the most stable plinian eruptions occurred at northern vents under highest extensional stresses and possibly widest most stable conduits by contrast collapsing unstable eruption columns occurred at southern vents where mainly degassed and crystalline magma erupted at lower ejection rates through narrower unstable and shifting conduits these data collectively show how spatial variations in structure and tectonics may strongly influence explosive eruption style

reply to gualda ghiorso comment on a metamodel for crustal magmatism phase equilibria of giant ignimbrites we welcome the opportunity to clarify issues raised by gualda ghiorso gg fowler spera 2008 fowler spera 2010 how cooling and crystallization and or assimilation can lead inherently to destabilization and eruption of several well known magmatic systems fig 15e fowler spera 2010 summarizes the lithostatic pressure melt fraction and initial wt h2o conditions that may lead to destabilization one contentious point raised by gg fowler spera 2010 a scale analysis aimed at providing a system thermal lifetime estimate for a particular set of initial conditions and configuration cooling of a fixed mafic magma mass assuming closed system behaviour no thermal or mass recharge a simple magma body shape a fixed heat flux and eruption of some fraction of differentiated magma formed by fractional crystallization as opposed to a scale analysis a proper thermal model would necessarily include computation of conjugate heat transfer rates across a magma body host rock boundary zone for a particular system such a thermal model should account for the threedimensional magma body geometry size shape and depth the time history of magma recharge very difficult to gauge without real time geodetic data e g lundgren et al 2003 host rock thermophysical properties notably thermal conductivity and permeability structure including anisotropy e g rosenberg spera 1990 the regional stress field an important feedback to permeability and hence heat transfer and detailed crustal stratigraphy sufficient spatial resolution of the order of 10m would be required based on conjugate heat transfer zone thickness initial conditions generally unknown would also need to be specified computation of such a model is extremely challenging requiring a great deal of focused effort we have not made this calculation neither have g g because there are so many uncertain quantities as to render any detailed model as uncertain as the simple scaling estimate embodied in equation 1 of fowler

the effect of inherited paleotopography on exhumation of the central andes of nw argentina differential exhumation in the puna plateau and eastern cordillera of nw argentina is controlled by inherited paleostructures and resulting paleotopography related to the cretaceous salta rift paleomargins the ceno zoic deformation front related to the development of the andean retro arc orogenic system is generally associated with 4 km of exhumation which is recorded by cenozoic apatite fi ssion track aft and u th sm he ages he ages in the eastern cordillera of nw argentina new aft ages from the top of the nevado de cachi document oligocene ca 28 ma cooling which combined with existing data indicates exhumation of this range between ca 28 ma and ca 14 ma however some of the highest ranges in the eastern cordillera preserve cretaceous ages indicative of limited cenozoic exhumation samples collected from an similar to 3 km elevation transect along the northern part of the sierra de quilmes paleorift fl ank laguna brava show aft ages between ca 80 and ca 50 ma and he ages between ca 45 and ca 10 ma another set of samples from an similar to 1 km elevation transect farther to the southwest la quebrada shows cretaceous aft ages between ca 116 ma and ca 76 ma and mainly cretaceous he ages in agreement with aft data analysis of existing aft and he ages from the area once occupied by the salta rift reveals a pattern characterized by cretaceous ages along paleorift highs and cenozoic ages within paleorift hanging wall basins and later foreland basin depocenters this pattern is interrupted by the sierras pampeanas at similar to 28 degrees s which record mid cenozoic ages our data are consistent with a complex inherited pattern of pre andean paleostructures likely associated with paleotopography which was beveled by the cenozoic regional foreland basin and reactivated during the late neogene ca 10 ma strongly controlling the magnitude of cenozoic uplift and exhumation and thus cooling age distribution this combined with variable lithologic erodibility resulted in an irregular distribution of themochronological ages

late middle miocene segmentation of an eocene early miocene carbonate megabank on the northern nicaragua rise tied to the tectonic activity at the north america caribbean plate boundary zone the northern nicaragua rise nnr is comprised today of the eastern honduras nicaragua and southern jamaica carbonate shelves and a series of relatively small detached carbonate banks i e pedro rosalind serranilla diriangen and bawika banks separated from each other by intervening basins and seaways the nnr basins and seaways because of their common north trending orientation have been previously interpreted as paleocene eocene and possibly oligocene rifts becoming progressively younger from the eastern to the western part of nnr results from three recent 1988 and 1992 high resolution seismic surveys within these major seaways and basins 1 walton basin 2 pedro channel and 3 seaways and serranilla basin on the western side of the nnr show that the present bank and basin configuration evolved from a once continuous megabank that covered the entire length of the nnr including the island of jamaica from eocene through early miocene times in the late middle miocene this megabank progressively broke up into a series of smaller banks basins and seaways mainly as the result of tectonic movements related to the overall strike slip displacement within the north american and caribbean plate boundary zone of the cayman trough at the same time late middlemore miocene the most eastern portion of the megabank was uplifted and today forms most of central and south jamaica the timing of the megabank segmentation has tentatively been constrained by dating several blocks of shallow water limestone dredged from parts of the megabank outcropping on the sea floor of different seaways less

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

exhumation history of the andean broken foreland revisited the andean broken foreland in west central argentina located 400 800 km from the peru chile trench is associated with flat subduction linked to collision of the oceanic juan fernandez ridge while the conditions associated with flat subduction would be expected to produce increases in rock uplift and exhumation where thrusting and dynamic forces work together a prevalence of pre cenozoic apatite fission track aft cooling ages suggests that there is no such link the lack of cenozoic cooling ages is at odds with structural reconstructions and basin studies along the foreland that show several kilometers of exhumation this paradox can be reconciled by taking into account the thermal effects of flat subduction that removed the mantle wedge and a significant source of heat flow into the crust reinterpretation of published aft exhumation data 320 ages using more realistic lower geothermal gradient values allows for substantial exhumation and explains the lack of cenozoic exhumation ages across the foreland region

phase equilibria constraints on the chemical and physical evolution of the campanian ignimbrite the campanian ignimbrite is a 4200 km trachyte phonolite pyroclastic deposit that erupted at 39 3 0 1ka within the campi flegrei west of naples italy here we test the hypothesis that campanian ignimbrite magma was derived by isobaric crystal fractionation of a parental basaltic trachyandesitic melt that reacted and came into local equilibrium with small amounts 5 10 wt of crustal rock skarns and foid syenites during crystallization comparison of observed crystal and magma compositions with results of phase equilibria assimilation fractionation simulations melts is generally very good oxygen fugacity was approximately buffered along qfm 1 where qfm is the quartz fayalite magnetite buffer during isobaric fractionation at 0 15 gpa 6 km depth the parental melt reconstructed from melt inclusion and host clinopyroxene compositions is found to be basaltic trachyandesite liquid 51 1 wt sio2 9 3 wt mgo 3 wt h2o a significant feature of phase equilibria simulations is the existence of a pseudo invariant temperature 883 8c at which the fraction of melt remaining in the system decreases abruptly from 0 5 to50 1 crystallization at the pseudo invariant point leads to abrupt changes in the composition properties density dissolved water content and physical state viscosity volume fraction fluid of melt and magma a dramatic decrease in melt viscosity from 1700 pa s to 200 pa s coupled with a change in the volume fraction of water in magma from 0 1 to 0 8 and a dramatic decrease in melt and magma density acted as a destabilizing eruption trigger thermal models suggest a timescale of 200 kyr from the beginning of fractionation until eruption leading to an apparent rate of evolved magma generation of about 10 3 km year in situ crystallization and crystal settling in density stratified regions as well as in convectively mixed less evolved subjacent magma operate rapidly enough to match this apparent volumetric rate of evolved magma production

neogene and quaternary tectonics of the eastern sierras pampeanas argentina active intraplate deformation inboard of flat slab subduction 1 the ranges of the eastern sierras pampeanas are located 600 km east of the andean cordillera in central argentina and have been interpreted to be a response to shortening related to flat slab subduction of the nazca plate uplift of the ranges has been broadly documented to occur during neogene time but many questions remain regarding the timing and style of deformation and the subsurface structural configuration in this study we address these unknowns with observations at multiple scales integrate our results into a tectonic model for the area and discuss how our structural interpretation fits with more regional tectonic models our major findings are 1 the range bounding faults thrust late proterozoic to cambrian schist and gneiss over poorly dated pliocene to pleistocene alluvial strata the timing of fault displacement and age of footwall strata suggest that deformation may have been active at least by pliocene time 2 apatite and zircon u th he thermochronometry exhibits cooling ages that range from permian to early jurassic time and suggests that rock exhumation in the area is less than 2 3 km since that time 3 deploying a local seismic array allowed for locating seismicity and calculating receiver functions these observations indicate that the moho lies at a depth of 37 km and that a midcrustal discontinuity appears to correspond to a detachment zone between 15 and 20 km depth and aligns with a plane of seismicity in our tectonic model the craton appears to act as a rigid backstop to the eastward propagation of stresses from the shallowly subducting slab deformation then propagates back to the west via westward verging faults along a midcrustal detachment

geometry and brittle deformation of the subducting nazca plate central chile and argentina summary we use data from the chile argentina geophysical experiment charge broad band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the south american subduction zone between 30 s and 36 s this region contains a zone of flat slab subduction where the subducting nazca plate flattens at a depth of 100 km and extends 300 km eastward before continuing its descent into the mantle we use a grid search multiple event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the wadati benioff zone these contours reflect slab geometries from previous studies of intermediate depth seismicity in this region with some small but important deviations our hypocentres indicate that the shallowest portion of the flat slab is a

magnetic anomaly interpretation across the southern central andes 32 34 s the role of the juan fernandez ridge in the late tertiary evolution of the margin marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of chile between 32 34 s three magnetic domains have been identified oceanic continental margin and subaerial the oceanic domain has seafloor spreading anomalies 16 37 ma to 18 39 5 ma disturbed by anomalies of the juan fernandez hot spot chain in the continental margin the most prominent fabric are e w anomalies in the upper slope corresponding to onshore e w anomalies of large intrusive bodies onshore a n s lineament of short wavelength anomalies defines the roots of a cretaceous volcanic arc a resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to 50 km landward of the trench absolute cenozoic plate motion for nazca and south american plates and dating of the juan fernandez chain provide a kinematic model of ridge continent collision the reconstruction indicates rapid southward migration of the collision point along 1400 km of the margin from 20 to 11 ma 20 cm yr 1 from 11 ma to present the collison point has migrated at a slower rate along 375 km of the margin 3 5 cm yr 1 the predicted location of the subducted portion of the juan fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship in the last 10 myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates these observations indicate that subduction of the juan fernandez chain plays a major role in arc forearc tectonics

retroarc basin reorganization and aridification during paleogene uplift of the southern central andes u s national science foundation ear 1049605 robert r shrock foundation at indiana university nsf ear award 1338583 project pdts unsj e985

chronology of neogene sedimentation stratigraphy and tectonism in the campo de talampaya region la rioja province argentina abstract chronostratigraphic paleoenvironmental and tectonic analysis of a 2320 m thick neogene continental section was conducted along the southern limit of the campo de talampaya magnetic polarity stratigraphy and fission track ages were used to develop a relatively precise geochronology the strata record a period of mixed evaporitic and terrigenous elastic sedimentation 20 6 1 ma that persisted in the distal bermejo basin during eastward migration of the precordillera fold thrust belt a critical transition of depositional environments and a change from western to eastern source areas occurred between 12 9 and 12 3 ma the saline mud flat environment of the rio manero formation derived from a western source and characterized by fine grained sandstones mudstones and evaporites yielded to a short period of playa lake deposition this was soon overwhelmed by deposition of the desencuentro formation composed of sandstones and mudstones with an eastern source area and deposited in a sand flat mud flat environment we propose that this stratigraphic transition resulted from initiation of uplift of the sierra de velasco a range of the sierras pampeanas located 65 km to the east northeast of the section basement clasts probably derived from the sierra de velasco are found in conglomeratic lenses beginning in the lower part of the desencuentro formation the modern campo de talampaya basin formed after deposition of the top of the alto de san nicolas paleomagnetic section 6 1 ma when west verging reverse faults uplifted the sierra morada and sierra de los tarjados all strata in the section were tilted eastward during these culminating uplift events

origin and evolution of silicic magmatism at yellowstone based on ion microprobe analysis of isotopically zoned zircons the origin of large volume yellowstone ignimbrites and smallervolume intra caldera lavas requires shallow remelting of enormous volumes of variably o depleted volcanic and sub volcanic rocks altered by hydrothermal activity zircons provide probes of these processes as they preserve older ages and inherited do values this study presents a high resolution oxygen isotope examination of volcanism at yellowstone using ion microprobe analysis with an average precision of 0 2o and a 10 mm spot size we report 357 analyses of cores and rims of zircons and isotope profiles of 142 single zircons in 11 units that represent majoryellowstone ignimbrites and post caldera lavas many zircons from these samples were previously dated in the same spots by sensitive high resolution ion microprobe shrimp and all zircons were analyzed for oxygen isotope ratios in bulk as a function of grain size by laser fluorination we additionally report oxygen isotope analyses of quartz crystals in three units the results of this work provide the following new observations 1 most zircons from post caldera low do lavas are zoned with higher do values and highly variable u pb ages in the cores that suggest inheritance from pre caldera rocks exposed on the surface 2 many of the higher do zircon cores in these lavas have u pb zircon crystallization ages that postdate caldera formation but pre date the eruption age by 10 20 kyr and represent inheritance of unexposed post caldera sub volcanic units that have do similar to the lava creek tuff 3 young and voluminous 0 25 0 1ma intra caldera lavas which represent the latest volcanic activity atyellowstone contain zircons with both high do and lowdo cores surrounded by an intermediate do rim this implies inheritance of a variety of rocks from high do pre caldera and low do post caldera units followed by residence in a common intermediate do melt prior to eruption 4 major ignimbrites of huckleberry ridge and to a lesser extent the lava creek and mesa falls tuffs contain zoned zircons with lower do zircon cores suggesting that melting and zircon inheritance from the lowdo hydrothermally altered carapace was an important process during formation of these large magma bodies prior to caldera collapse 5 the do zoning in the majority of zircon core rim interfaces is step like rather than smoothly inflected suggesting that processes of solution reprecipitation were more important than intracrystalline oxygen diffusion concave downward zircon crystal size distributions support dissolution of the smaller crystals and growth of rims on larger crystals this study suggests that silicic magmatism atyellowstone proceeded via rapid shallow level remelting of earlier erupted and hydrothermally alteredyellowstone source rocks and that pulses of basaltic magma provided the heat for melting each postcaldera yellowstone lava represents an independent homogenized magma batch that was generated rapidly by remelting of source rocks of various ages and do values the commonly held model of a single large volume super solidus mushy state magma chamber that is periodically reactivated and produces rhyolitic offspring is not supported by our data rather the source rocks for theyellowstone volcanism were cooled below the solidus hydrothermally altered by heated meteoric waters that caused low do values and then remelted in distinct pockets by intrusion of basic magmas each packet of new melt inherited zircons that retained older age and do values this interpretation may have significance for interpreting seismic data for crustal low velocity zones in which magma mush and solidified areas experiencing hydrothermal circulation occur side by side new basalt intrusions into this solidifying batholith are required to form the youngest volcanic rocks that erupted as independent rhyolitic magmas we also suggest that the lava creek tuff

mechanisms of unrest and eruption at persistently restless volcanoes insights from the 2015 eruption of telica volcano nicaragua many of earth s volcanoes experience well defined states of quiescence and unrest with unrest occasionally culminating in eruption some volcanoes however experience an unusually protracted i e decades long period of noneruptive unrest and are thus categorized as persistently restless volcanoes prvs the processes that drive persistently restless volcanism are poorly understood as our knowledge of prvs is currently based on a small number of case studies here we examine multidisciplinary observations of the 2015 eruptive episode at telica volcano nicaragua in the context of its long term behavior we suggest that the latter phases of the 2015 eruption were ultimately driven by destabilization of its shallow magma reservoir based on previous geodetic seismic studies of telica geirsson et al 2014 https doi org 10 1016 j jvolgeores 2013 11 009 rodgers et al 2013 https doi org 10 1016 j jvolgeores 2013 08 010 and 2015 https doi org 10 1016 j jvolgeores 2014 11 012 and on multiparameter observations at telica over a 7 year period we propose that three distinct states of unrest occur at telica over decadal timescales a stable open state involving steady conduit convection and two distinct unstable states that may lead to eruptions in the weak sealing state phreatic explosions result from steady conduit convection underlying a weak seal in the destabilized state destabilization of the top of the convecting magma in the conduit leads to rapid accumulation of high pressures leading to strong impulsive phreatomagmatic explosions our observations and interpretations suggest that continuous seismic ground based deformation gas emission and thermal monitoring and interpretation of these data within a paradigm of sustained conduit convection modulated by episodes of sealing and destabilization of shallow magma reservoirs may allow robust forecasting of eruption potential energy and duration at telica and similar prvs worldwide

neogene deformation of the flat subduction segment of the argentine chilean andes magnetostratigraphic constraints from las juntas la rioja province argentina substantial neogene deformation occurred in the part of the andes mountains and the sierras pampeanas that overlie a subhorizontal segment of the subducting nazca plate in northern argentina the history of the volcanic arc suggests that the nazca plate attained the present low angle inclination between about 17 and 10 ma with the region underlain by the flat nazca plate widening eastward thereafter we compare the history of deformation provided by magnetostratigraphy of miocene strata in foreland and intermontane basins to the history of the subducting plate at las juntas in the precordillera thin skinned thrust belt 4 780 m of neogene nonmarine strata record the initiation and termination of sedimentation in a segment of a foreland basin magnetostratigraphic data complemented by fission track dating of an interbedded tuff indicate that the strata span from 18 to 9 ma the earliest strata correlate to the end of a time of volcanic activity in the magmatic arc and the beginning of reverse faulting in the frontal cordillera subsequently the deformation front migrated eastward generating the precordillera thrust belt and finally the sierras pampeanas basement uplifts the rate of strata accumulation varied systematically as new uplifts arose with a maximum rate exceeding 1 0 mm a between 15 and 12 ma a correlation between the deformational history and the geometric history of the subducted slab suggests that thrusting migrated eastward as the angle of subduction shallowed although large scale factors affecting the plate margin beyond the flat slab segment may have partly induced the compressional deformation this correlation suggests that processes related to flat subduction are of importance in deforming the continental plate

early carboniferous sub to mid alkaline magmatism in the eastern sierras pampeanas nw argentina a record of crustal growth by the incorporation of mantle derived material in an extensional setting a recently discovered granitic intrusion at cerro la gloria in western sierra de famatina nw argentina is representative of sub to mid alkaline carboniferous magmatism in the region the main rock type consists of microcline quartz and plagioclase with amphibole magnetite ilmenite biotite epidote zircon allanite and sphene as accessory minerals we report a u pb zircon shrimp age for the pluton of 349 3 ma mswd 1 1 i e tournaisian whole rock chemical composition and nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust endt between 0 58 and 0 46 and tdm values of about 1 1 ga the pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back arc basalts coeval early carboniferous a type granites occur farther east in the sierras pampeanas probably generated during lithospheric stretching overall the early carboniferous granitic rocks show a west to east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west based on the observed patterns of geochronology geochemistry and field relationships we suggest that a type magma genesis in the eastern sierras pampeanas was linked to an andean type margin where the lithospheric mantle played a role in its generation

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

fragmentation of a foreland basin in response to out of sequence basement uplifts and structural reactivation el cajon campo del arenal basin nw argentina the style and mechanisms by which a foreland region is incorporated into an orogen depends on the tectonic style effectiveness of uplift and dynamic subsidence classical foreland basin models reflect a self similar propagation of deformation into the foreland in a thin skinned thrust belt setting governed by wedge mechanics thick skinned foreland regions which are characterized by high angle reverse fault bounded basement uplifts and intervening basins however do not fit this idealized model unlike thin skinned tectonic provinces deformation and uplift in these regions may be highly variable in time and space furthermore deformation patterns may be complicated by the presence of preexisting structures particularly those which lie at orientations that enable them to be reactivated and utilized to accommodated deformation under compression the neogene el cajon campo del are nal basin is one of a series of basins located along the eastern margin of the puna plateau within the sierras pampeanas a region that is composed of a thick skinned foreland fragmented by reverse fault bounded basement uplifts that regionally characterize an eastward younging trend this region is superimposed onto the cretaceous salta rift province which provides a series of pre existing structures that may potentially be reactivated the basin is located along the eastern margin of the puna plateau an integral component of the andean orogen which includes several filled uplifted and internally drained cenozoic intraplateau basins structural and sedimentological similarities exist between basins along the margin of the puna and those within it understanding the evolution of foreland basins such as the el cajon campo del arenal basin provides possible mechanisms for the development and incorporation of marginal basins into orogenic belts and in the case of the andean orogen the potential for these basins to be incorporated into the plateau our analysis which integrates seismic sedimentary and thermochronological data characterizes the evolution of this basin and surrounding ranges the appearance in the sedimentary section of a distinct grain age population derived from the basement erosion surface constrains the uplift and erosion of an out of sequence intrabasin high to ca 6 ma the basin fill therefore records an evolution from an undeformed foreland to one that is compartmentalized by basement uplifts and that is incorporated into the greater orogenic structure the data reveal the importance of the reactivation of preexisting structures along the basin margin in creating east dipping structures in a generally west dipping domain these opposing faults on the basin margin consequently caused the out of sequence uplift of the intrabasin range the sierra de quilmes the sierra de quilmes fragments the foreland and because its position is locked by loads to the west and east creates increased deformation within the basin basin fill uplift and incorporation into the orogen unlike basins within the plateau however the el cajon campo del arenal basin has been re excavated and integrated once more into the foreland drainage network

improved measurement of fission track annealing in apatite using c axis projection apatite fission track length data are used extensively for thermal history inversion however several studies have documented instances of poor reproducibility of length data we address this problem by using c axis projection to normalize track lengths for crystallographic angle in the extensive laboratory annealing data set acquired by barbarand et al 2003a 2003b a new simplification reduces the c axis projection model from six to four fitted parameters normalizing for track angle using c axis projection improves every aspect of length measurement reproducibility examined it accelerates convergence of mean length in single analyses increases consistency among replicate measurements by a single analyst enhances consistency of measurements of the same mounts by different analysts and improves the match between analyses conducted with and without cf irradiation c axis projection is also shown to enhance the thermal sensitivity of length data based on these results we assert that c axis projection is a good means of compensating for observer bias although it does not overcome differences caused by experimental error

refined exhumation history of the northern sierras pampeanas argentina 1 the sierra de aconquija and cumbres calchaquies in the thick skinned northern sierras pampeanas nw argentina present an ideal setting to investigate the tectonically and erosionally controlled exhumation and uplift history of mountain ranges using thermochronological methods although these ranges are located along strike of one another their spatiotemporal evolution varies significantly integrating modeled cooling histories constrained by k ar ages of muscovite and biotite apatite fission track data as well as u th he measurement of zircon and apatite reveal the structural evolution of these ranges beginning in the late stage of the paleozoic famatinian orogeny following localized rift related exhumation in the central part of the study area and slow erosion elsewhere growth of the modern topography commenced in the cenozoic during andean deformation the main activity occurred during the late miocene with varying magnitudes of rock uplift surface uplift and exhumation in the two mountain ranges the cumbres calchaquies is characterized by a total of 5 7 km of vertical rock uplift around 3 km of crestal surface uplift and a maximum exhumation of 2 4 km since that time the sierra de aconquija experienced 10 13 km of vertical rock uplift 4 5 km of peak surface uplift and 6 8 km of exhumation since around 9 ma much of this exhumation occurred along a previously poorly recognized fault miocene reactivation of cretaceous rift structures may explain along strike variations within these ranges dating of sedimentary samples from adjacent basins supports the evolutionary model developed for the mountain ranges

apatite fission track thermochronology of the sierras pampeanas central western argentina implications for the mechanism of plateau uplift in the andes over much of its length the andean orogen is characterized by a generally east vergent geometry and a progressive eastward cratonward migration of individual arc orogenic elements a departure from this model occurs in the sierras pampeanas region of central western argentina where a terrane of active basement uplifts is currently developing to the east of the main cordillera apatite fission track data from uplifted precambrian and phanerozoic basement rocks of the sierras pampeanas constrain the tectonic development of this terrane and indicate that deformation associated with exhumation may have propagated broadly westward since the late miocene two pre andean cooling events during the carboniferous permian ca 300 280 ma and the early jurassic middle jurassic ca 200 174 ma have been identified the onset of andean deformation is represented by a cooling event during the late paleocene middle eocene this cooling was followed by a period of middle miocene late miocene reheating during foreland basin style sedimentation exhumation and possible westward migration of the exhumation front in the sierras pampeanas commenced during the late miocene pliocene to the east of the dominantly east vergent precordillera fold and thrust belt the apparent convergence of deformation in these two terranes and the progressive closure of miocene pliocene intermontane basins in the sierras pampeanas may reflect the early stages of andean plateau uplift the relative timing of plateau development along strike suggests that lateral thickening of the orogen is progressing southward at least from the latitude of central bolivia ca 20 s furthermore the time space coincidence between basement uplift and flattening of the subducted slab beneath the sierras pampeanas suggests that a relationship exists between westward displacement of the terrane and the dynamics of plate interaction

cenozoic orogenic growth in the central andes evidence from sedimentary rock provenance and apatite fission track thermochronology in the fiambala basin southernmost puna plateau margin nw argentina abstract intramontane sedimentary basins along the margin of continental plateaus often preserve strata that contain fundamental information regarding the pattern of orogenic growth the sedimentary record of the clastic miocene pliocene sequence deposited in the fiambala basin at the southern margin of the puna plateau nw argentina documents the late miocene paleodrainage evolution from headwaters to the west towards headwaters in the ranges that constitute the border of the puna plateau to the north apatite fission track aft thermochronology of sedimentary and basement rocks show that the southern puna plateau was the source for the youngest middle miocene detrital population detected in late miocene rocks and that the margin of the puna plateau expressed a high relief possibly similar to or higher than at present by late miocene time cooling ages obtained from basement rocks at the southern puna margin suggest that exhumation started in the oligocene and continued until the middle miocene we interpret the basin reorganization and the creation of a high relief plateau margin to be the direct response of the source basin system to a wholesale surface uplift event that may have occurred during the late cenozoic in the puna altiplano region at this time coeval paleodrainage reorganization is observed not only in the fiambala basin but also in different basins along the southern and eastern puna margin suggesting a genetic link between the last stage of plateau formation and basin response however this event did not cause sufficient exhumation of basin bounding ranges to be recorded by aft thermochronology our new data thus document a decoupling between late cenozoic surface uplift and exhumation in the southern puna plateau high relief achieved at the puna margin by late miocene time is linked to oligocene miocene exhumation no significant erosion