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

stable isotope composition of middle miocene carbonates of the frontal cordillera and sierras pampeanas did the paranaense seaway flood western and central argentina abstract the geographic extent interconnectedness and chronology of the miocene paranaense epeiric seaway have been the subject of considerable debate understanding the timing and location of this marine incursion is significant for documenting sea level changes paleoclimatic changes and surface uplift or subsidence stable isotope analyses of carbonate strata in the flat slab segment of the sierras pampeanas and high andean cordillera previously purported to be of marine origin provide evidence that the parana seaway did not inundate this portion of west and central argentina no unambiguously marine facies or isotopic signatures were recognized for five representative stratigraphic units the saguion formation ese of salinas grande in cordoba province anta formation in the quebrada de la yesera and lerma valley of salta province del buey and del abra formations within the famatina ranges la rioja province and chinches formation in the manantiales foreland basin in the frontal cordillera san juan province paleontologic and lithologic features in support of a marine origin are reconsidered herein instead a lacustrine origin is inferred for these formations their contemporaneity perhaps related to concurrent global climate conditions and broad tectonic setting until more substantive evidence of marine deposition is found we reject correlation of these units with the parana seaway preserved in the chaco pampean plain subsurface and discourage mapping of the seaway as extending into the central sierras pampeanas or andean foreland our findings suggest the miocene andean foreland was elevated above sea level although it remained as a largely flat lying area west of the pampas and chaco plains which were inundated by the parana seaway

present day south american climate abstract this paper documents the main features of the climate and climate variability over south america on the basis of instrumental observations gathered during the 20th century it should provide a modern reference framework for paleoclimate research in south america targeting high resolution proxies over the past few centuries several datasets suitable for present day climate research are first described highlighting their advantages as well as their limitations we then provide a basic physical understanding of the mean annual cycle of the precipitation and atmospheric circulation over the continent and the adjacent oceans in particular the diversity of precipitation temperature and wind patterns is interpreted in terms of the long meridional extent of the continent and the disruption of the large scale circulation caused by the andes cordillera the contrasting oceanic boundary conditions and the landmass distribution similarly the intensity and timing of the interannual and interdecadal climatic fluctuations exhibit considerable geographical dependence as some regions are more influenced by large scale phenomena rooted in the tropical oceans while others are more influenced by high latitude phenomena the impact of these large scale phenomena over south america is documented by a regression analysis between selected atmospheric indices and the precipitation and temperature fields we have included a discussion on the seasonality and long term stability of such impacts and complemented our general description by an updated review of the literature on climate variability over specific regions

role of continents in driving the hadley cells gcm simulations are used to investigate how forcing that originates over land surfaces influences the hadley circulation the presence of continental surfaces is found to approximately double the intensity of the winter hemisphere hadley cell and to halve the intensity of the summer cell the strengthening of the winter cell occurs because the increase in surface friction associated with land enhances the angular momentum flux into the atmosphere the development of strong monsoon circulations in the northern hemisphere summer and the convergence zones of the southern hemisphere south pacific south atlantic and south indian convergence zones shifts mass out of the subtropics lowers the zonal mean subtropical highs and weakens the summer cells the responses of the summer and winter cells are different signs and occur by different processes because heating of the land surfaces in the summer is effectively communicated through the depth of the atmosphere whereas cooling in the winter is not also the higher surface wind speeds and shears of the winter hemisphere trade winds make the winter cell more sensitive to surface friction than the summer cell these results suggest that the axisymmetric models that have provided a theoretical basis for the understanding of the hadley circulation do not capture some of the important physical processes that determine the intensity of the mean meridional circulation

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

noble gas and carbon isotopic evidence for co2 driven silicate dissolution in a recent natural co2 field secure storage of anthropogenic carbon dioxide co2 in geological reservoirs requires predicting gas water rock interactions over millennial timescales noble gases and carbon isotope measurements can be used to shed light on the nature of competing dissolution precipitation processes over different timescales from the fast dissolution of gaseous co2 in groundwater to more sluggish reactions involving dissolution and precipitation of newly formed minerals in the reservoir here we study a compilation of gas analyses including noble gases and 13c of co2 from nine different natural co2 reservoirs amongst these reservoirs the bravo dome co2 field new mexico usa shows distinct geochemical trends which are explained by degassing of noble gases from groundwater altering the composition of the gas phase this groundwater degassing is synchronous with the dissolution of co2 in groundwater progressive creation of alkalinity via co2 promoted mineral dissolution is required to explain the observed positive correlation between co2 3he and 13c of the gas phase a unique feature of bravo dome the differences between bravo dome and other natural co2 reservoirs are likely explained by the more recent filling of bravo dome reflecting co2 water rock interactions over thousands of years rather than over millions of years in older reservoirs

eocene exhumation and basin development in the puna of northwestern argentina 1 the puna is part of the larger puna altiplano plateau also known as the central andean plateau characterized by high elevation low relief and aridity located in the central andes of bolivia and argentina tertiary sedimentary rocks preserved within the puna contain a unique archive of information regarding the paleogeography depositional environments and timing of sediment source exhumation during the early stages of andean mountain building the eocene geste formation in the salar de pastos grandes area within the central puna of northwestern argentina consists of deposits that are the result of confined to unconfined flows in a sandy to gravelly braided fluvial system and alluvial fans proximal to the source terrane paleocurrent data document an overall eastward flow direction up section coarsening of the geste formation suggests that topographic relief in the source area increased through time possibly owing to enhanced tectonic activity and source terrane unroofing sandstone petrography and conglomerate clast count data document quartzose and phyllitic compositions typical of ordovician rocks preserved just west of the salar de pastos grandes area paleocene eocene detrital apatite fission track age populations p1 35 52 ma p2 52 65 ma of the geste formation and their consistent trends up section suggest moderate to rapid 0 4 mm a to 1 mm a exhumation of western sediment sources during the early to mid tertiary stages of andean mountain building sedimentation rates increase up section from 0 1 mm a to 1 mm a our data when combined with other structural stratigraphic and seismic evidence from surrounding regions suggest that the geste formation was deposited in response to crustal shortening and resulting erosion and sedimentation which started as early as cretaceous in the chilean cordillera de domeyko and in the salar de pastos grandes area by eocene time the geste formation could be interpreted either as a local wedge top accumulation on the eastward propagating central andean orogenic wedge or as a local intermontane basin the similarities between wedge top deposits preserved in bolivia and eocene deposits in northwestern argentina south of 25 s lead us to favor the wedge top scenario for the geste formation if correct this implies that the deformation front of the andean orogenic wedge incorporated both thin and thick skinned structures as it migrated possibly unsteadily from the cordillera de domeyko during the cretaceous paleocene to areas within the puna and eastern cordillera by mid late eocene time contemporaneously a regional scale foreland basin system developed over an along strike distance of at least 650 km

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

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

late paleozoic to triassic evolution of the gondwana margin evidence from chilean frontal cordilleran batholiths 28 s to 31 s upper paleozoic to triassic chilean granitoids in the andean frontal cordillera between 28 s and 31 s record crustal and mantle conditions at the gondwana margin during the final assembly and initial breakup of the pangea supercontinent this period overlaps the end of paleozoic terrane accretion and precedes andean subduction integration of new trace element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the gondwana margin in the model the carboniferous to early permian is a period of oblique convergence associated elqui complex granitoids are diverse those in the guanta and montosa units are predominantly related to subduction processes whereas those in the cochiguas and el volcan units are dominated by melting of the subduction complex and older crust progressive oblique collision of the last pre pangea terrane equis along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the elqui complex subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt which intruded and melted the crust producing the post collisional ingaguas complex the los carricitos granitoids formed in thickened crust whereas the chollay el colorado and el leon units formed in thinner crust the ingaguas complex is part of the choiyoi granite rhyolite province whose formation similar to that of other gondwana silicic provinces was probably accentuated by anomalously hot upper mantle associated with the pangea supercontinent

the long term fate of co2 in the subsurface natural analogues for co2 storage abstract co 2 is a common gas in geological systems so that planned storage of co 2 in the subsurface may do no more than mimic nature natural co 2 has a wide number of sources that can be at least partly identified by carbon stable isotope geochemistry three pairs of case studies with different reservoir characteristics and co 2 contents have been examined to assess the natural impact of adding co 2 to geological systems carbonate minerals partially dissolve when co 2 is added simply because the co 2 dissolves in water and forms an acidic solution therefore carbonate minerals in the subsurface are not capable of sequestering secondary co 2 the addition of co 2 to a pure quartz sandstone or a sandstone in which the supply of reactive aluminosilicate minerals has been exhausted by excess natural co 2 addition will have no consequences the co 2 will simply saturate the water and then build up as a separate gas phase the addition of co 2 to carbonate cemented sandstone without reactive aluminosilicate minerals will induce a degree of carbonate mineral dissolution but no solid phase sequestration of the added co 2 when co 2 is naturally added to sandstones it will induce combined aluminosilicate dissolution and carbonate cementation if the aluminosilicate minerals contain calcium or magnesium or possibly iron examination of a co 2 filled porous sandstone with abundant reactive aluminosilicate minerals that received a huge co 2 charge about 8000 to 100 000 years ago reveals minimal evidence of solid phase sequestration of the added co 2 this indicates that either dissolution of reactive aluminosilicates or precipitation of carbonate minerals is relatively slow it is very likely that the slow dissolution of reactive aluminosilicates is the rate limiting step solid phase sequestration of co 2 occurs only when reactive aluminosilicates are present in a rock and when the system has had many tens to hundreds of thousands of years to equilibrate the two critical aspects of the behaviour of co 2 when injected into the subsurface are 1 that the rock must contain reactive ca and mg aluminosilicates and 2 that reaction to produce carbonate minerals is extremely slow on a human timescale the reactive minerals include anorthite zeolite smectite and other fe and mg clay minerals such minerals are absent from clean sandstones and limestones but are present in dirty standstones lithic arenites which are mineralogically immature and some mudstones the analysis of geological analogues shows that injection of co 2 into carbonate bearing rocks that do not contain reactive minerals will induce dissolution of the carbonate whether it is a matrix cement rock fragment fault seal or part of a top sealing mudstone

paleosurfaces paleoelevation and the mechanisms for the late miocene topographic development of the altiplano plateau abstract we construct a topographic datum across the central andes between 18 s and 22 s using extensive well preserved paleosurfaces that span the entire range this datum combined with estimates of topographic change from paleoaltimetry in the altiplano and geomorphic relief generation estimates from the plateau flanks allow us to reconstruct the paleo topography of the range at 10 ma using the 10 ma topography we explore which geophysical processes can create the observed topographic change over the

an intensity ratio calibration method for the accurate determination of mg ca and sr ca of marine carbonates by icp aes an inductively coupled plasma atomic emission spectroscopy icp aes method for the accurate and precise simultaneous measurement of the mg ca and sr ca content of carbonates was established while a precision of 0 3 1 standard deviation sd is easily obtainable for both mg ca and sr ca analysis a ca matrix effect complicates achieving similar levels of accuracy with conventional calibration procedures an alternative ratio calibration procedure is proposed which overcomes the ca matrix effects and ensures the accuracy of the mg ca and sr ca analysis of marine carbonates to 0 3 almost an order of magnitude better than conventional calibration methods the longer term precision is 0 1 if the batch run average values are corrected for longer term drift the method is suitable for analysis of foraminiferal calcite and coral aragonite and can easily be adjusted for the analysis of other carbonates or microsamples

spatial and temporal evolution of injected co2 at the sleipner field north sea time lapse three dimensional 3d seismic surveys have imaged an accumulation of injected co 2 adjacent to the sleipner field in the north sea basin the changing pattern of reflectivity suggests that co 2 accumulates within a series of interbedded sandstones and mudstones beneath a thick caprock of mudstone nine reflective horizons within the reservoir have been mapped on six surveys acquired between 1999 and 2008 these horizons have roughly elliptical planforms with eccentricities ranging between two and four in the top half of the reservoir horizon areas grow linearly with time in the bottom half horizon areas initially grow linearly for about eight years and then progressively shrink the central portions of deeper reflective horizons dim with time amplitude analysis of horizons above within and below the reservoir show that this dimming is not solely caused by acoustic attenuation instead it is partly attributable to co 2 migration and or co 2 dissemination which reduce the impedance contrast between sandstone and mudstone layers growth characteristics and permeability constraints suggest that each horizon grows by lateral spreading of a gravity current this model is corroborated by the temporal pattern of horizon velocity pushdown beneath the reservoir horizon shrinkage may occur if the distal edge of a co 2 filled layer penetrates the overlying mudstone if the buoyant plume draws co 2 upward or if the effective permeability of deeper mudstone layers increases once interstitial brine has been expelled topographic control is evident at later times and produces elliptical planforms especially toward the top of the reservoir our results show that quantitative mapping and analysis of time lapse seismic surveys yield fluid dynamical insights which are testable shedding light on the general problem of co 2 sequestration

geological setting and paleomagnetism of the eocene red beds of laguna brava formation quebrada santo domingo northwestern argentina abstract the red bed succession cropping out in the quebrada santo domingo in northwestern argentina had been for long considered as upper triassic lower jurassic in age based on weak radiometric and paleontological evidence preliminary paleomagnetic data confirmed the age and opened questions about the nature of fossil footprints with avian features discovered in the section recently the stratigraphic scheme was reviewed with the identification of previously unrecognized discontinuities and a radiometric dating obtained in a tuff indicated an eocene age for the laguna brava formation and the fossil bird footprints much younger than the previously assigned we present a detailed paleomagnetic study interpreted within a regional tectonic and stratigraphic framework looking for an explanation for the misinterpretation of the preliminary paleomagnetic data the characteristic remanent magnetizations pass a tilt test and a reversal test the main magnetic carrier is interpreted to be low ti titanomagnetites and to a lesser extent hematite the characteristic remanent magnetization would be essentially detrital the obtained paleomagnetic pole pp for the laguna brava formation has the following geographic coordinates and statistical parameters n 29 lon 184 5 e lat 75 0 s a 95 5 6 and k 23 7 when this pp is compared with another one with similar age obtained in an undeformed area a declination anomaly is recognized this anomaly can be interpreted as laguna brava formation belonging to a structural block that rotated about 16 clockwise along a vertical axis after about 34 ma this block rotation is consistent with the regional tectonic framework and would have caused the fortuitous coincidence of the pp with early jurassic poles according to the interpreted magnetostratigraphic correlation the laguna brava formation would have been deposited during the late eocene with a mean sedimentation rate of about 1 4 cm per thousand years probably in relation to the onset of the andean deformation

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

diagnosis of the mjo modulation of tropical cyclogenesis using an empirical index the modulation of tropical cyclone activity by the madden julian oscillation mjo is explored using an empirical genesis potential gp index composite anomalies of the genesis index associated with the different mjo phases are consistent with the composite anomalies in tc genesis frequency that occur in the same phases indicating that the index captures the changes in the environment that are at least in part responsible for the genesis frequency changes of the four environmental variables that enter the genesis potential index the midlevel relative humidity makes the largest contribution to the mjo composite gp anomalies the second largest contribution comes from the low level absolute vorticity and only very minor contributions come from the vertical wind shear and potential intensity when basin integrated mjo composite anomalies of the gp index are regressed against basin integrated composite anomalies of tc genesis frequency the results differ quantitatively from those obtained from the analogous calculation performed on the annual climatologies in the two quantities the gp index captures the mjo modulation of tc genesis to a lesser degree than the climatological annual cycle of genesis to which it was originally tuned this may be due to weaknesses of the reanalysis or indicative of the importance of precursor disturbances not well captured in the gp index computed from weekly data to the intraseasonal tc genesis frequency fluctuations

north central pacific tropical cyclones impacts of el nino southern oscillation and the madden julian oscillation abstractboth el nino southern oscillation enso and the madden julian oscillation mjo have been documented in previous research to impact tropical cyclone tc activity around the globe this study examines the relationship of each mode individually along with a combined index on tropical cyclone activity in the north central pacific approximately twice as many tropical cyclones form in the north central pacific in el nino years compared with la nina years these differences are attributed to a variety of factors including warmer sea surface temperatures lower sea level pressures increased midlevel moisture and anomalous midlevel ascent in el nino years when the convectively enhanced phase of the mjo is located over the eastern and central tropical pacific the north central pacific tends to have more tropical cyclone activity likely because of reduced vertical wind shear lower sea level pressures and increased vertical motion the convectively enhanced phase of the mjo is also responsible for

mjo and tropical cyclogenesis in the gulf of mexico and eastern pacific case study and idealized numerical modeling the role of the madden julian oscillation mjo in modulating the frequency and location of tropical cyclogenesis over the eastern pacific and the gulf of mexico during august september 1998 is examined during the nonconvective phase of the mjo convection and low level cyclonic vorticity occurred primarily in conjunction with the intertropical convergence zone itcz during the convective phase convection low level cyclonic vorticity and convergence expanded into the northeastern pacific and the gulf of mexico this was accompanied by enhanced eddy kinetic energy and barotropic energy conversions as compared to the nonconvective phase consistent with previous research during the nonconvective phase of the mjo vertical shear was relatively weaker but tropical cyclones tended to form mainly within the itcz on the contrary during the convective phase vertical wind shear exceeded 10 m s 1 over much of this region and tropical cyclone development occurred north of the itcz near the mexican pacific coast and the gulf of mexico idealized numerical experiments are conducted using a barotropic model with time invariant basic states representative of the nonconvective and convective phases of the mjo the simulations indicate that the propagation paths as well as the amplification of the eddies differ substantially between the two phases during the nonconvective phase the waves tend to propagate westward into the eastern pacific during the convective phase stronger southerlies steer the waves into the gulf of mexico the mjo related modulation of tropical cyclogenesis in the eastern pacific and gulf of mexico thus appears to involve anomalous convergence cyclonic vorticity vertical wind shear and differing tracks of easterly waves

along strike segmentation of the andean foreland causes and consequences abstract thrust belts of tertiary age characterize the east flank of the andes and part of the adjacent foreland from venezuela to the southern tip of south america this foreland deformation comprises three basically different structural styles 1 thin skinned thrust belts detached within the sedimentary cover 2 thick skinned thrust belts with an inferred basal detachment in the basement at 10 20 km depth and 3 foreland basement thrusts which possibly affect the entire crust alternating foreland structural styles along the orogen often with sharp boundaries produce a segmentation of the andean foreland the changes in structural style are interpreted to be primarily controlled by inherited stratigraphic and structural features of the south american plate for instance thin skinned thrust belts appear to require a more or less conformable sedimentary cover at least some 3 km thick over a metamorphic crystalline basement unaffected by mesozoic extension in areas of mesozoic rifting normal faults have been reactivated during tertiary contraction resulting in the formation of thick skinned thrust belts the basement becomes involved even if potential decollement levels are present deep seated foreland basement thrusts tend to form over basement arches with a thin sedimentary cover the three basic structural styles are unequally efficient in terms of shortening thin skinned belts are typically shortened by 40 70 thick skinned belts by 20 35 and areas of foreland thrusts by less than 10 the structural segmentation of the foreland therefore implies strong along strike variations in the amount of shortening available estimates of shortening along the central andes actually suggest that shortening does not vary steadily but has several maxima and minima strongly and weakly shortened foreland segments correlate poorly with the segmentation of the subducted plate in steeper and subhorizontal segments flat slabs but closely with lithospheric structure the most strongly shortened segment coincides with thick mantle lithosphere suggesting that shortening of the edge of the south american plate has resulted in substantial underthrusting of lithospheric mantle beneath part of the andes this underthrusting of thick lithosphere may have caused steepening of a formerly flat slab in the arica bend region to the normal 25 30 dip presently observed

transport mechanisms by fluid flow in metamorphic rocks oxygen and strontium decoupling in the trois seigneurs massif a consequence of kinetic dispersion

modulation of eastern north pacific hurricanes by the madden julian oscillation abstract hurricane and tropical storm statistics verify the modulation of eastern pacific tropical systems by the madden julian oscillation mjo as hypothesized by maloney and hartmann over twice as many named tropical systems hurricanes and tropical storms accompany equatorial 850 mb westerly anomalies than accompany equatorial easterly anomalies and the systems that do exist are stronger hurricanes are over four times more numerous during westerly phases of the mjo than during easterly phases the current study constructs a composite life cycle of the mjo during may november 1979 95 using an index based on the 850 mb equatorial zonal wind equatorial kelvin waves propagating eastward from convective regions of the western pacific ocean alter dynamical conditions over the eastern pacific ocean westerly easterly equatorial 850 mb wind anomalies are accompanied by enhanced suppressed convection over the eastern pacific hurricane region convection locally amplifies the wind anomalies over the ea

chronology of motion in a complete thrust belt the precordillera 30 31 s andes mountains in the precordillera thrust belt of west central argentina faulted rotated and onlapping relations of piggyback basin units and foreland basin units yield an accurate but imprecise chronology of thrust motion addition of data from provenance history strata accumulation rates and presence of thick conglomerate units in the bermejo foreland basin results in a thrust chronology that is more precise but not entirely independent of assumptions about foreland basin dynamics seven new radiometric dates allow substantial revision of the interpreted age of first deformation in the precordillera at about 30 south units previously assigned to the paleozoic and mesozoic are of oligocene and miocene ages a redbed unit that began before 21 6 pm 0 8 ma appears to have pre dated thrusting deposits in the early bermejo foreland basin were dominated by eolian sandstones unlike the younger fluvial and alluvial units an andesitic volcanic center within the miocene foreland basin had long term effects on sub

geological carbon storage storage of the carbon dioxide that is produced by burning fossil fuels is one way to avoid the damaging consequences of climate change a range of observations suggests that geological carbon storage is much less risky than unabated carbon emissions to the atmosphere

cenozoic foreland basin system in the central andes of northwestern argentina implications for andean geodynamics and modes of deformation 1 cenozoic strata in the central andes of northwestern argentina record the development and migration of a regional foreland basin system analogous to the modern chaco parana alluvial plain paleocene lower eocene fluvial and lacustrine deposits are overlain by middle upper eocene hypermature paleosols or an erosional disconformity representing 10 15 myr this supersol disconformity zone is traceable over a 200 000 km2 area in the andean thrust belt and is overlain by 2 6 km of upward coarsening eastward thinning upper eocene through lower miocene fluvial and eolian deposits middle miocene pliocene fluvial lacustrine and alluvial fan deposits occupy local depocenters with contractional growth structures paleocurrent and petrographic data demonstrate westerly provenance of quartzolithic and feldspatholithic sediments detrital zircon ages from cenozoic sandstones cluster at 470 491 522 544 555 994 and 1024 1096 ma proterozoic mesozoic clastic and igneous rocks in the puna and cordillera oriental yield similar age clusters and served as sources of the zircons in the cenozoic deposits arc derived zircons become prominent in oligo miocene deposits and provide new chronostratigraphic constraints sediment accumulation rate increased from 20 m myr during paleocene eocene time to 200 600 m myr during the middle to late miocene the new data suggest that a flexural foreland basin formed during paleocene time and migrated at least 600 km eastward at an unsteady pace dictated by periods of abrupt eastward propagation of the orogenic strain front despite differences in deformation style between bolivia and northwestern argentina lithosphere in these two regions flexed similarly in response to eastward encroachment of a comparable orogenic load beginning during late paleocene time

the madden julian oscillation s influence on african easterly waves and downstream tropical cyclogenesis abstractthe influence of the madden julian oscillation mjo over tropical africa and atlantic is explored during the northern hemisphere summer months the mjo is assessed by using real time multivariate mjo rmm indices these indices divide the active convective signal of the mjo into 8 phases convection associated with the mjo is enhanced over tropical africa during rmm phases 8 1 and 2 convection becomes suppressed over tropical africa during the subsequent rmm phases phases 3 7 african convective signals are associated with westward propagating equatorial rossby waves the mjo modulates african easterly wave aew activity aew activity is locally enhanced during rmm phases 1 3 and suppressed during rmm phases 6 8 enhanced aew activity occurs during periods of enhanced convection over tropical africa consistent with stronger or more frequent triggering of aews as well as more growth associated with latent heat release enhanced aew activity occurs during the low level westerly wind phase of

variations in global tropical cyclone activity and the madden julian oscillation since the midtwentieth century the madden julian oscillation mjo has been documented in previous studies to significantly impact tropical cyclone activity in all ocean basins most of these studies have utilized the wheeler hendon index this index is only available since 1974 the period over which remotely sensed outgoing longwave radiation data has been available our study utilizes a long reconstructed mjo index based on surface pressures which extends back to 1905 we document consistent modulation of tropical cyclone activity by the mjo in all basins over this time period these modulations are shown to be remarkably stable over the entire analysis period we also examine the combined impacts of el nino southern oscillation and the mjo on tropical cyclone activity in each basin over multidecadal time scales

rapid reactions between co2 brine and silicate minerals during geological carbon storage modelling based on a field co2 injection experiment the dissolution of co2 into formation brines and the subsequent reactions of the co2 charged brines with reservoir minerals are two key processes likely to increase the security of geological carbon dioxide storage these processes will be dependent on the permeability structure and mineral compositions of the reservoirs but there is limited observational data on their rates in this paper we report the cation and anion concentrations and sr oxygen and carbon isotopic compositions of formation waters from four extraction wells sampled at surface over 6 months after commencement of co2 injection in a five spot pattern for enhanced oil recovery at the salt creek field wyoming sampled fluids separated from the minor oil component exhibit near monotonic increases in alkalinity and concentrations of cations but little change in cl and br concentrations and oxygen and deuterium isotope ratios the increases in alkalinity are modelled in terms of reaction with reservoir calcite and silicate minerals as the changes in fluid chemistry and sr isotopic compositions are inconsistent with simple addition of injected fluids sampled over the course of the experiment the reservoir mineral chemical and isotopic compositions are characterised by sampling core as well as surface exposures of the frontier formation elsewhere in wyoming the evolution of the fluid chemistries reflects extensive dissolution of both carbonate and silicate minerals over the course of the six months sampling implying rapid dissolution of co2 in the formation waters and reaction of co2 bearing brines with formation minerals rates of co2 diffusion into the brines and advection of co2 charged brines in the reservoir are sufficiently slow that if present calcite should react to be close to equilibrium with the fluids this allows estimation of the co2 partial pressures in the sampled fluids and comparison with the thermodynamic driving force for the relatively rapid average plagioclase dissolution rates of 10 12 mol m 2 s 1

a probability model to detect carbonate cementation in sandstones and other enigmatic wireline facies abstract zones of carbonate cementation identified within lower paaratte formation sandstones of the eastern otway basin victoria southeastern australia can be quantitatively detected by a wireline log based probability model though trained on these zones the same model appears to accurately predict carbonate cementation within late cretaceous to eocene reservoir sandstones of the latrobe group supersequence of the gippsland basin 100s of km to the east predicted carbonate cementation matches published evidence a regional petrographic study and provides a plausible interpretation for corresponding sections of formation micro imager data acquired more recently these latrobe group sandstones are thought to have once been pervasively cemented prior to development of the secondary porosity responsible for providing reservoirs to the main petroleum system of the basin however model predictions imply that discrete heavily cemented zones remain which must have acted and must still act as local obstructions to reservoir fluid migration these zones would also likely react with carbonic acid formed at plume fronts generated by dedicated co2 storage operations of the future the cemented zones that have been predicted are sparse spatially sporadic and cannot be resolved by 3d seismic reflection survey data at present day reservoir depths these predictions therefore emphasise the difficulty in mapping the distribution of carbonate cemented zones however they provide calibration data for future mapping systems two different tests of the probability log model were undertaken one benchmarks its predictions against those of a neural network trained using the original model development dataset from the otway basin predictive performance of the neural network model is significantly worse than that of the probability log model when making predictions using wireline data acquired at a well in the gippsland basin the second test demonstrates that the general probability log modelling approach is amenable to classifying other cryptic electro facies for example those representing enigmatic or ambiguous rock types not easily interpreted from common wireline log data using standard theory this test presents a case study by which the probability log modelling approach predicts the presence of both basic and evolved felsic igneous sill intrusions within the stratigraphic succession of the faroe shetland basin of nw europe

bayesian change point analysis of tropical cyclone activity the central north pacific case bayesian analysis is applied to detect change points in the time series of annual tropical cyclone counts over the central north pacific specifically a hierarchical bayesian approach involving three layers data parameter and hypothesis is formulated to demonstrate the posterior probability of the shifts throughout the time from 1966 to 2002 for the data layer a poisson process with gamma distributed intensity is presumed for the hypothesis layer a no change in the intensity hypothesis and a single change in the intensity hypothesis are considered results indicate that there is a great likelihood of a change point on tropical cyclone rates around 1982 which is consistent with earlier work based on a simple log linear regression model a bayesian approach also provides a means for predicting decadal tropical cyclone variations a higher number of tropical cyclones is predicted in the next decade when the possibility of the change point in the early 1980s is taken into account

links between tropical cyclone activity and madden julian oscillation phase in the north atlantic and northeast pacific basins abstract the leading intraseasonal mode of atmospheric and oceanic variability the madden julian oscillation mjo influences tropical and extratropical sea level pressure temperature divergent and rotational wind components moisture and deep convection as a 40 to 50 day oscillation the mjo is also known to influence tropical phenomena including tropical cyclone tc activity in various tc basins the links between the mjo and multiple measures of tc activity including genesis landfall and an integrative accumulated cyclone energy ace index were quantified for multiple tc formation basins across the western hemisphere including the north atlantic and northeast pacific ocean and subbasins for the period 1978 2006 using this relatively long 29 yr tc dataset and employing an upper tropospheric mjo diagnostic that is physically meaningful over the entire western hemisphere this study extends existing research on the relationships between the mjo and tcs the noaa climate prediction cente

paleobiology and paleoecology of an arid semiarid miocene south american ichnofauna in anastomosed fluvial deposits abstract miocene deposits of the toro negro formation in la rioja province argentina host a rich vertebrate and invertebrate ichnofauna trace fossils are recorded from the lower part of the lower member of the toro negro formation at quebrada de la troya this succession consists of sandstone intraformational breccia mudstone and few conglomerate deposited in sandy anastomosing fluvial systems developed under a semi arid climate the invertebrate ichnofauna is composed of meniscate trace fossils taenidium barretti scoyenia gracilis dwelling structures palaeophycus tubularis and grazing trails helminthopsis hieroglyphica the vertebrate ichnofauna includes avian fuscinapeda sirin incumbent footprint slender anisodactyl footprint and mammalian footprints macrauchenichnus rector cf venatoripes riojanus small heteropod footprint kidney like footprints and oval impressions tetrapod footprints and rare palaeophycus and helminthopsis are preserved on the top of exposed sandbars crevasse splay deposits preserve abundant meniscate trace fossils and dwelling tubes e g taenidium scoyenia and palaeophycus ascribed to the scoyenia ichnofacies two suites of invertebrate trace fossils can be differentiated meniscate backfill structures t barretti and other burrows p tubularis without ornamentation developed in a soft substrate and striated traces s gracilis that crosscut the former developed in a firmer substrate the toro negro depositional system displays a high preservation potential for tracks due to seasonal flooding events which rapidly buried the footprints the favorable preservational conditions and good exposure of the track bearing surface enable us to interpret the track assemblage of the toro negro formation as a sample of the original tetrapod fauna that inhabited that region at central western argentina during the miocene the identification of possible trackmakers is utilized in this work as a useful tool that contributes to our understanding of the ancient environment the paleoichnocommunity is characterized by the number of trackmakers and the relative abundance of each type of footprints a new method to measure footprint abundance is proposed the relative bioturbed area rba this measurement provides numerical characterization of the degree of bioturbation of the whole tracking surface caused by each ichnotaxa the tetrapod fauna indicated by the footprint record was compared with the body fossil record of the toro negro formation these records overlap in some ways and in other ways they are complementary the presence of large and medium to small ground sloth proterotheriid litopterns caviid rodents and birds is pointed out by both body fossils and ichnofossils body fossils include remains attributed to dasypodids glyptodontids and abrocomid rodents ichnofossils indicate the presence of macraucheniid litopterns and three types of birds shorebirds perching birds and large cursorial birds

surface uplift and erosion of the southernmost argentine precordillera article i nfo article history the timing of uplift of the precordillera is important for understanding the linkages if any between slab dynam ics shortening and topography the study region between 32 and 33 s latitude liesat the southern end of the flat slab where the subducting nazca plate is nearly horizontal south of the study region subduction occurs at normal subduction angles of around 30 while north of the study region the slab subducts at around 5 we use the geomorphology of the region to date the initiation of surface uplift and the ensuing landscape adjustment the topography of the precordillera of the argentine andes consists of both remnants of a low relief miocene landscape developed when the region was at a lower elevation and rapidly eroding fluvial systems that have been dissecting this surface since uplift this study utilizes 26 al and 10 be concentrations in stream sediment quartz to calculate erosion rates of the miocene remnant paleo landscape as well as incision rates within the ac tively incising post uplift fluvial system the remnant landscape is eroding at b10 m my while the surrounding landscape is eroding an order of magnitude faster approaching 100 m my these values show the transition from a rapidly eroding system adjusting to uplift to a system where rates of uplift approach those of erosion we used a locally determined relationship between average upstream slope and erosion rate to model erosion across the landscape dem analyses of modern river profiles are used to reconstruct paleo river profiles which suggest an average of 1 3 km of uplift uplift of the southernmost argentine precordillera is constrained to have initiated by 10 ma demonstrating that under the arid conditions typical of the southernmost precordil lera millions of years are needed for a landscape to reach equilibrium

miocene landscape evolution and geomorphologic controls on epithermal processes in the el indio pascua au ag cu belt chile and argentina the world class breccia stockwork and vein hosted epithermal au ag cu deposits of the central andean el indio pascua belt latitudes 29 20 to 30 30 s were emplaced between 6 2 and 9 4 ma immediately beneath the shoulders of an actively uplifting north south tectonic block along the axis of the cordillera principal the deposits of the pascua lama veladero el indio and tambo districts are hosted by largely felsic volcanic units of late oligocene early miocene and late paleozoic age but were associated with small hypabyssal bodies of dacitic porphyry the pascua formation there is no evidence that the epithermal centers were overlain by substantial volcanic edifices the preglacial miocene landscape previously undocumented incorporates three planar erosional landforms of regional extent 1 the ca 15 to 17 ma frontera deidad surface 2 the ca 12 5 to 14 ma azufreras torta surface and 3 the ca 6 to 10 ma los rios surface these pediplains are each vertically separated by 200 to 400 m and by analogy with the enormous erosion surfaces of the atacama desert of northern chile and southern peru are considered to have formed in a semi arid climate in direct response to uplift events although the azufreras torta surface immediately overlies most of the major hydrothermal systems and hence influenced their evolution ore formation was contemporaneous with the development of the younger los rios pediplain and was almost entirely focused around the upper extremities of stage iii valley pediments in contrast precursor high sulfidation hydrothermal alteration zones in the main mineralized districts ranging in age from 10 0 to 13 6 ma and associated with vacas heladas formation dacitic magmatism developed before incision of the los rios pediments and are barren the areal and temporal relationships between economic epithermal centers and the heads of upper miocene valley pediments provide at the least clear empirical exploration guidelines for the el indio pascua belt however it is further proposed that changes in the surficial hydrodynamic environment including the rapid lowering of the water table increased lateral ground water flow and fluid boiling and mixing all favoring ore deposition may have been directly induced by pediment incision in this and other semi arid cordilleran environments in which epithermal mineralization was not associated with significant volcanic edifices

spatiotemporal trends in erosion rates across a pronounced rainfall gradient examples from the southern central andes abstract the tectonic and climatic boundary conditions of the broken foreland and the orogen interior of the southern central andes of northwestern argentina cause strong contrasts in elevation rainfall and surface process regimes the climatic gradient in this region ranges from the wet windward eastern flanks 2 m yr rainfall to progressively drier western basins and ranges 0 1 m yr bordering the arid altiplano puna plateau in this study we analyze the impact of spatiotemporal climatic gradients on surface erosion first we present 41 new catchment mean erosion rates derived from cosmogenic nuclide inventories to document spatial erosion patterns second we re evaluate paleoclimatic records from the calchaquies basin 66 w 26 s a large intermontane basin bordered by high 4 5 km mountain ranges to demonstrate temporal variations in erosion rates associated with changing climatic boundary conditions during the late pleistocene and holocene three key observations in this region emphasize the importance of climatic parameters on the efficiency of surface processes in space and time 1 first order spatial patterns of erosion rates can be explained by a simple specific stream power ssp approach we explicitly account for discharge by routing high resolution satellite derived rainfall this is important as the steep climatic gradient results in a highly nonlinear relation between drainage area and discharge this relation indicates that erosion rates er scale with er ssp1 4 on cosmogenic nuclide time scales 2 we identify an intrinsic channel slope behavior in different climatic compartments channel slopes in dry areas 0 75 m yr with equal drainage areas thus compensating lower amounts of discharge with steeper slopes 3 erosion rates can vary by an order of magnitude between presently dry 0 05 mm yr and well defined late pleistocene humid 0 5 mm yr conditions within an intermontane basin overall we document a strong climatic impact on erosion rates and channel slopes we suggest that rainfall reaching areas with steeper channel slopes in the orogen interior during wetter climate periods results in intensified sediment mass transport which is primarily responsible for maintaining the balance between surface uplift erosion sediment routing and transient storage in the orogen

inversion of an oligocene volcano tectonic basin and uplifting of its superimposed miocene magmatic arc in the chilean central andes first seismic and gravity evidences abstract a new structural model is presented for the cenozoic part of the main range east of santiago a chain made up of thick miocene ignimbrites and andesitic lavas overlying primitive oligocene volcanoclastics deposited on a thin continental crust because the latter coya machali formation locally show tight folding close to the margins of the miocene volcanics they had previously been described as deformed prior to construction of the arc our field observations analysed along two profiles at 33 and 34 15 s however support the conclusion hinted at by ongoing geochronological and geochemical studies deformation of both formations is synchronous and related to late miocene inversion of the coya machali intra arc basin uplift of the rigid arc rocks and part of its volcanoclastic basement took place mainly along two northward shallowing boundary structures that seem to share a lower decollement the eastern fierro out of sequence thrust and a series of western back thrusts that resemble triangle zones seismic data obtained from el teniente porphyry copper mine 34 s rockbursts and two gravimetric profiles at the western end of the southern profile are compatible with a decollement located at that locality 2 3 km under the surface

long term reactivity of co2 in a low salinity reservoir seal complex abstract an understanding of the long term reactivity of different rock types to injected co2 is needed for sequestration site assessment relative to saline aquifer studies the long term reactivity of co2 in low salinity aquifers has received little attention currently in australia the surat basin is being appraised for its large scale co2 storage potential within low salinity aquifers sixteen core samples from the precipice sandstone and evergreen formation the notional target reservoir and seal complex were characterized for mineral content helium mercury injection and micro ct porosities air permeability and imaged with sem eds samples consisted of quartz rich reservoir sandstones feldspar and clay rich or calcite cemented sandstones secondary reservoir mudstones sealing complex and oolitic ironstones sealing complex derived from braided river fluvial deltaic and restricted marine shoal depositional environments respectively the reservoir sandstone samples characterized here had measured total porosity that ranged from 11 to 23 with pore throats mainly between 90 and 100 m and core air permeability from 558 to 3397 md in the precipice sandstone reservoir sample ct plugs 98 of the pore space was connected with calculated vertical permeability 145 4611 md and horizontal 4291 8200 md feldspar and clay rich sandstone and mudstone samples from the overlying evergreen formation had porosity that ranged between 0 2 and 22 9 with a wide range of pore throat sizes from 0 005 to 30 m and permeability from 0 2 to 28 1 md respectively ironstone and mudstone samples from the westgrove ironstone member evergreen formation had porosity from 0 7 to 9 7 and a low permeability of 0 04 md kinetic geochemical co2 reactivity models made predictions over two time scales 30 or 1000 years selected models also accounted for the potential presence of 10 ppm so2 gas the precipice sandstone quartz rich reservoir sandstones had consistently small amounts of reactive minerals and mineral trapping or scaling of the reservoir was not predicted over 30 years with the ph approximately 4 5 after 30 years samples from the evergreen formation included feldspar and clay rich sandstones and mudstones several contained variable amounts of carbonate cement their response to co2 was more influenced by mineral content than rock type plagioclase feldspars and fe rich chlorite were the main silicate minerals that reacted to produce siderite and ankerite mineral trapping up to 2 57 kg m3 co2 in the very unlikely event that co2 rich fluids migrated upwards as far as the westgrove ironstone member chlorite is predicted to alter to siderite this study indicates that the precipice sandstone reservoir in the study region has a low likelihood of mineral scaling which is favorable to avoid co2 injectivity issues mineral trapping as ankerite and siderite could be expected to trap co2 in the chlorite and plagioclase rich evergreen formation seal lithologies further work is suggested on validating long term predictions with observation data from natural analogue studies

links between topography erosion rheological heterogeneity and deformation in contractional settings insights from the central andes abstract orogenic structure in the central andes 15 s 34 s systematically varies with mean annual precipitation suggesting that erosional processes may be coupled to tectonic processes we explore the range of possible interactions between deformation erosional processes changing geodynamic conditions and preexisting geologic structures to assess the relative importance of each our review and synthesis indicates that the pre orogenic geologic history and changing geodynamic conditions leave a lasting imprint on the modern structure of the mountain belt in contrast changes in precipitation that result from regional scale atmospheric and oceanic circulation interact with local topography to control the climatic zoning across the central andes changes in erosion which likely correspond to changes in climate rock uplift rate and the exposure of different lithologies may in turn affect the tectonic development of the region at the scale of each morphotectonic province in this view geologic and geodynamic factors set the stage for the nature and strength of coupling between erosional processes tectonic deformation and mountain belt structure in the different parts of the central andes

fitting straight lines and planes with an application to radiometric dating conventional practice in geochronology is to fit a straight line or isochron to data consisting of two isotopic ratios by a method e g that of 1 2 or perhaps the more modern version of titterington and haliday 1979 that takes into account that fact that both ratios are measured with error in this paper we use matrix algebra to lay out a general method for fitting linear relations between any number of variables all subject to errors with known variances and covariances and the well known newton raphson method to do the optimization this leads to a good computational algorithm which may also be used e g to check whether coefficients in several linear relations are the same in many fields of science one needs to fit linear relations so our method is of wide utility its use is in no way restricted to radiometric dating

magnetic polarity stratigraphy of the miocene rio azul section precordillera thrust belt san juan province argentina the chronostratigraphy of synorogenic strata of rio azul in the bermejo foreland basin argentine andes reveals details of miocene tectonic activity and deposition chronostratigraphic interpretations are based on magnetic polarity stratigraphy and zircon fission track dates of interbedded volcanic ashes the rio azul section is located within the precordillera thrust belt uplifted and rotated by motion on subjacent faults the cenozoic strata are themselves broken by two principal zones of faulting whose displacements are great enough to affect correlation of the local magnetic stratigraphy to the magnetic polarity time scale the 3200 m thick section coarsens upward from sandstone and mudstone cuculi formation to conglomerate el corral formation the magnetostratigraphic approach could be used only in the relatively fine grained strata of the lower part of the section the radiometric ages indicate the entire section is of middle to late miocene age and we correlate the magnetic polarity patter

lowstand deltas in the frontier formation powder river basin wyoming implications for sequence stratigraphic models deposits of lowstand deltas formed on the floor of the cretaceous interior seaway of north america are found in the cenomanian lower belle fourche member of the frontier formation central wyoming sandstones located in similar distal basin locations hundreds of kilometers basinward of highstand shoreline deposits form important hydrocarbon reservoirs isolated within marine shales but interpretation of their origin has been highly controversial the distribution geometry and internal facies of these sandstones are documented by an extensive outcrop study and regional subsurface correlations to develop genetic facies models for these deposits this integrated record of lithofacies ichnofacies palynofacies paleocurrent data bedding relationships and isolith maps incorporates observations from nearly 100 measured outcrop sections and about 550 subsurface well logs four episodes of sediment progradation and subsequent transgression each left behind gradually upward coarsening deltaic sandstones that have eroded tops these deltaic sandstones have a lobate to elongate geometry basinward dipping internal clinoform bedding radiating paleocurrents a low to moderate degree of shallow marine burrowing and show variable wave influence and tidal influence on deposition delta plain paralic and nonmarine facies have been eroded from the top of deltaic successions erosion surfaces capping progradational deltaic successions are the only stratal discontinuities that can be mapped regionally and they appear to record transgressive ravinement enhanced over areas of structural uplift compared with lowstand surfaces of erosion which record the bypass of sediments basinward low accommodation during lowstands left little room for sandstones to stack vertically and successive episodes of delta progradation were offset along strike more tide influenced delta deposits formed within shoreline embayments begin page 262 defined by the topography of older wave influenced delta lobes and subtle syndepositional deformation of the basin floor standard sequence stratigraphic terminology is difficult to use in broad lowstand systems like the frontier formation because sandstones do not show simple vertical stacking patterns major stratal discontinuities can form by processes other than lowstand fluvial erosion and minor syndepositional deformation of the basin floor exerts a first order influence on depositional and sediment preservation patterns although many basin distal sandstones have been interpreted to be deposits of offshore bars shelf isolated valley fills and stranded shorelines the frontier formation examples documented here suggest that many of these deposits may be top eroded deltas formed where rivers delivered sediment to lowstand coastlines the external geometry and internal heterogeneities of hydrocarbon reservoirs found in these types of deposits reflect processes active on the low accommodation deltaic shoreline even in cases where subsequent ravinement has significantly truncated the deposits during transgression

el nino southern oscillation the madden julian oscillation and atlantic basin tropical cyclone rapid intensification 1 both el nino southern oscillation enso and the madden julian oscillation mjo have previously been documented to impact atlantic basin tropical cyclone tc activity through alterations in large scale fields such as vertical wind shear mid level moisture sea level pressure and sea surface temperature atlantic tc activity has been shown to be enhanced when la nina conditions are present in the tropical pacific while activity is reduced when el nino conditions occur atlantic tc activity is enhanced when the convectively active phase of the mjo is over africa and the western indian ocean phases 1 2 while tc activity is suppressed when the convectively active phase of the mjo is over the tropical pacific phases 6 7 these relationships are shown to extend to atlantic basin rapid intensification ri events typically defined as intensification of 30 knots or greater in 24 h with nearly three times as many ri events in la nina years when compared with el nino years in addition approximately four times more ri episodes occur when the mjo exceeds one standard deviation in phases 1 2 than when the mjo exceeds one standard deviation in phases 6 7 storms forming in phases 1 2 are twice as likely to undergo at least one ri episode during their lifetime as storms forming in phases 6 7 even stronger relationships are seen when the mjo and enso are considered in combination

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

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

lower carboniferous post orogenic granites in central eastern sierra de velasco sierras pampeanas argentina u pb monazite geochronology geochemistry and sr nd isotopes the central eastern part of the sierra de velasco sierras pampeanas nw argentina is formed by the large huaco 40 30 km and sanagasta 25 15 km granite massifs and the small la chinchilla stock 2 2 km the larger granites intrude into ordovician metagranitoids and crosscut devonian mylonitic shear zones whereas the small stock sharply intrudes into the huaco granite the two voluminous granites are biotitic muscovitic and biotitic porphyritic syeno to monzogranites they contain small and rounded tonalitic and quartz dioritic mafic microgranular enclaves the small stock is an equigranular zinnwaldite and fluorite bearing monzogranite the studied granites are silica rich sio2 70 potassium rich k2o 4 ferroan alkali calcic to slightly calk alkalic and moderately to weakly peraluminous a cnk 1 06 1 18 huaco granite 1 01 1 09 sanagasta granite 1 05 1 06 la chinchilla stock they have moderate to strong enrichments in several lil li rb cs and hfs nb ta y th u elements and low sr ba and eu contents u pb monazite age determinations indicate lower carboniferous crystallization ages 350 358 ma for the huaco granite 352 7 1 4 ma for the sanagasta granite and 344 5 1 4 ma for the la chinchilla stock the larger granites have similar nd values between 2 1 and 4 3 whereas the younger stock has higher nd of 0 6 to 1 4 roughly comparable to the values obtained for the carboniferous san blas granite 1 4 to 1 7 located in the north of the sierra the huaco and sanagasta granites have a mainly crustal source but with some participation of a more primitive possibly mantle derived component the main crustal component can be attributed to ordovician peraluminous metagranitoids the la chinchilla stock derives from a more primitive source suggesting an increase with time in the participation of the primitive component during magma genesis the studied granites were generated during a post orogenic period in a within plate setting possibly as a response to the collapse of the previous famatinian orogen extension of the crust and mantle upwelling they are part of the group of middle devonian lower carboniferous granites of the sierras pampeanas the distribution and u pb ages of these granites suggests a northward arc parallel migration of this mainly post orogenic magmatism with time

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

magnetostratigraphy of neogene andean foreland basin strata lat 33 s mendoza province argentina neogene nonmarine strata in the southernmost part of the precordillera thrust belt of the andes of argentina accumulated during the miocene and pliocene in response to eastward advance of thrust sheets located to the west to link the sequence of deformational events with the sedimentary record five stratigraphic sections in the mendoza province were dated using magnetic polarity stratigraphy calibrated with 40 ar 39 ar dates from interbedded tephras these data establish age control on tectonic activity volcanism and deposition the existence of a backarc foreland basin that developed prior to or ca 16 ma is recorded by the oldest strata the ca 15 7 12 2 ma baksi time scale marino formation deposition of this unit is linked to a major phase of thrust activity in the central part of the aconcagua fold and thrust belt in the principal cordillera initiation of deposition coincides with an eastward shift of the subduction related magmatic foci to the argentine principal cordillera the overlying ca 11 7 9 0 ma la pilona formation is thought to record the initiation of exhumation of the frontal cordillera to the northwest of the study region an extensive ash rich unit the 8 9 locally 9 5 to 8 7 ma tobas angostura formation is correlated with the latest and probably the largest extrusive episode in the principal cordillera chronologic data constrain the timing of oil generation in the cuyo basin and development of prospective closures the new data also require that land mammalian fossils found in the rio de los pozos formation are late miocene rather than pliocene as previously believed

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

a ground penetrating radar survey of a delta front reservoir analog in the wall creek member frontier formation wyoming ground penetrating radar gpr measurements in conjunction with outcrop sedimentology were carried out at murphy creek reservoir in the upper cretaceous turonian wall creek member of the frontier formation in wyoming the objectives were to apply gpr to map geometrical details of a top truncated lowstand delta front and to estimate the volumes of the prograding bar deposits of the delta lobe eleven gpr profiles totaling about 4400 m 14 435 ft were acquired using 50 mhz antennas on a coarsely spaced two dimensional grid of lines lying parallel and perpendicular to the average depositional dip ground penetrating radar reflections were detected from within the outcrop to a depth of about 10 15 m 33 49 ft four southerly dipping major surfaces identified in the gpr data are correlated with the boundaries of progradational delta front facies stacked as distal mouth bar deposits in the outcrop the major boundaries correspond to lithological changes between relatively clean sandstones that are interpreted to have been deposited during floods with high sediment supply alternating with bioturbated sandstones and mudstones deposited during interflood periods with correspondingly low sedimentation rates these two lithological units which also correspond to the two main gpr facies repeat at least three times with no change in dominant average sand grain size subsequent erosion by transgressive ravinement caused the significantly truncated lowstand delta long after the sandstones were deposited the bar assemblage volume at successive stages of growth is estimated using measurements from the outcrop and the gpr data the migrating bars have an estimated average half length of 650 m 2132 ft a lower bound on the average volume of the bar is 3 9 106 m3 1 37 108 ft3 as the volume of the bars increases the bar deposits appear to have a landward as well as a basinward component of accretion

cenozoic tectonics of the high andes of west central argentina 30 36 s latitude abstract the structure of the central andes shows three distinctive segments characterized by different geometries these geometries are superimposed on the present large scale plate tectonic setting characterized by distinct subduction segments the northern la ramada segment is a thick skinned fold and thrust belt formed by tectonic inversion of a late triassic rift the central aconcagua segment consists of a thin skinned fold and thrust belt while the southern malargue segment like the first one is a thick skinned fold and thrust belt developed by tectonic inversion of a late triassic early jurassic rift system during late cenozoic times the amount of shortening gradually decreases from north to south as indicated by the crustal roots of the central andes the different geometries along the principal cordillera controlled the abrupt changes in the shortening among segments the structure of precordillera and sierras pampeanas has also been considered in order to account for the total shortening in the la ramada segment the main shortening occurred in the precordillera in the aconcagua segment in the principal cordillera while in the malargue segment the shortening is widely distributed in a broader principal cordillera because south of the flat slab subduction segment the precordillera and sierras pampeanas are missing

the control of pre existing extensional structures on the evolution of the southern sector of the aconcagua fold and thrust belt southern andes abstract the aconcagua fold and thrust belt located in the andean mountains at 32 30 to 34 s has been described as a classic model of a thin skinned thrust belt however new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple mesozoic extensional phases overprinted by structural inversion as well as thin and thick skinned tectonics two major superimposed extensional structural styles have been identified for the mesozoic characterized by distinctly oriented stress fields a key role in the evolution of this part of the fold and thrust belt was played by a late triassic to early jurassic depocentre and by late jurassic block faulting shortening was accommodated by a combination of inversion of pre existing normal faults development of footwall short cuts and both thin and thick skinned thrusting synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults with further shortening along newly formed thin skinned thrust faults the geometry of thin skinned fault systems is controlled by the architecture of the rift basin competent footwalls forming barriers to the lateral propagation of detachments

calcite cement distribution and its effect on fluid flow in a deltaic sandstone frontier formation wyoming precipitation of extensive calcite cement during burial diagenesis can strongly modify the depositional permeability of a sandstone reservoir and affect fluid flow during production to predict subsurface flow through cemented reservoirs permeability distributions used in fluid flow models must reflect this diagenetic overprint calcite cements in sandstones commonly occur as irregularly distributed concretions which makes it difficult to predict diagenetic permeability modifications in the subsurface from typically spaced wells outcrops can provide a continuous image of heterogeneity produced by concretionary calcite cements the size and distribution of calcite concretions were mapped in outcrops of the frewens sandstone frontier formation in central wyoming large tabular calcite concretions in this deltaic sandstone generally follow basinward inclined bedding median thickness of the concretions is 0 6 m length is 4 2 m and width is 5 3 m the highest cement fraction is in the high permeability facies at the top of the sandstone body concretion centers are approximately poisson distributed within the sandstone the upward increasing cement fraction is caused by upward increasing concretion size lateral variation in the fraction of the sandstone cemented by calcite has a normal distribution with a mean of 12 s 5 spatial distribution of calcite cement in the frewens sandstone was modeled using indicator geostatistics variograms were inferred from outcrop maps of cement indicator semivariograms of cement have a range of 30 m horizontally and 2 5 m vertically dimensions that correspond approximately to the size of the largest concretions stochastic images of cement were created using indicator simulation with vertically varying cement proportion flow models indicate that concretions make flow paths more tortuous and retard flow in the coarser facies near the top of the sandstone the fastest path through the sandstone is in the lightly cemented high net to gross center of the sandstone body because the cement mainly occurs within the highest permeability facies in the sandstone body a model based on depositional facies alone would overestimate upscaled permeability of the frewens sandstone

on discrimination between carbonate and silicate inputs to himalayan rivers we review new and published analyses of river waters bedloads and their constituent minerals from the dhauli ganga and alaknanda headwaters of the ganges in garhwal and the marsyandi in nepal and their tributaries these data are used to discriminate between the inputs of major cations and sr from silicate and carbonate sources methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in sr ca mg na k 87sr 86sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component this results in factor of two overestimates of the fractions of silicate derived ca mg and sr to correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of sr ca mg na k 87sr 86sr space the compositions of the carbonate and silicate end members in the bedload are determined by sequential leaching the results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron microprobe analyses of the minerals in the bedload in the upper marsyandi catchment which drains low grade tethyan sedimentary series formations a set of mainstem samples collected over a two year period define tight correlations in sr ca mg na k 87sr 86sr space modeling of the magnitude of secondary carbonate precipitation and fractions of silicate derived ca mg and sr in sr ca mg 87sr 86sr space gives self consistent results that are compatible with both the calculations of mineral modes and published mg isotopic compositions if the ratio of chlorite to biotite weathering is high or if there is another silicate source of mg these calculations imply that between 12 and 31 percent of the sr and 44 and 72 percent of the mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate derived to carbonate derived cations modeling in sr ca na and or k space is inconsistent with the sr isotopic and mg isotopic constraints and we conclude that in this catchment dissolution of na and k are incongruent relative to sr ca mg potassium is preferentially retained in micas whereas the controls on na are unclear modeling of the catchments underlain by high himalayan crystalline and lesser himalayan series in garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature up to 60 percent of the sr in the catchment on high himalayan crystalline series and 20 to 30 percent of sr in the catchments on lesser himalayan series are estimated to be derived from silicates however it should be noted that the element budgets are not all self consistent and the use of bedrock element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties