paleogene synorogenic sedimentation in the altiplano plateau and implications for initial mountain building in the central andes sedimentologic data and palynological ages from paleogene clastic deposits of the northern and central altiplano plateau suggest foreland basin development in the central andes by mid paleocene time the nonmarine potoco formation 3000 6500 m thick constitutes the majority of cenozoic basin fill the potoco overlies the santa lucia formation 50 300 m thick previously dated as mid paleocene by mammal fossils and magnetostratigraphy new geochronologic data for the potoco formation include late eocene to oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels these ages and published 40 ar 39 ar and k ar ages from overlying upper oligocene lower miocene volcaniclastic rocks indicate 1 nondeposition or greatly reduced deposition average sediment accumulation rates 10 m m y from mid paleocene to middle eocene time top santa lucia to lowermost potoco formation followed by 2 rapid deposition average sediment accumulation rates up to 500 m m y throughout late eocene and oligocene time majority of potoco formation lithofacies and paleocurrent data confirm both depositional phases a 20 100 mthick interval of superimposed paleosols in the basal potoco supports reduced sediment accumulation and low rates of subsidence during mid paleocene to middle eocene time the overlying main body of the potoco contains facies assemblages and stratigraphic architecture dominantly nonero

melting and mingling of phonolitic pumice deposits with intruding dykes an example from the otago peninsula new zealand abstract a mound of bedded phonolitic pumice lapilli ash was deposited on the marine shelf modern otago peninsula new zealand during a shallow subaqueous eruption in the miocene basaltic dykes subsequently intruded the unconsolidated pumice deposit and formed an unusual peperite dyke sediment interaction began with weak steam driven mixing accompanied by fluidisation at the contact zone that produced a metres broad zone in which bulbous tongues up to 3 m of the dykes extended into and locally engulfed host sediment irregular sinuous contacts of dykes with the pumice are marked by black glass that formed by partial melting shearing and remoulding of the glassy pumice such melting and remoulding of host sediment is not a typical peperite feature temperature evolution volume of water vapour load pressure and time as well as initial contact temperature control the welding and remoulding of pumice and glass shards a slow cooling rate of the dykes at the dyke contacts is inferred from remoulding textures and is attributed to continuous feeding of the dyke seawater may have promoted softening and remoulding of the pumice because na and k if partially taken into the glass structure can decrease the melting point of glass dramatically and reduce the viscosity of the shards remoulding may have occurred by continuous conversion of seawater in the peperite to steam which is inferred to have enhanced transfer of na and k from seawater to pumice glass

erosional control on the geometry and kinematics of thrust belt development in the central andes long term erosion rates in the central andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt a drastic along strike erosional gradient exists in the modern central andes from a high erosion region directly north of the 17 5 s bend in the andes to a low erosion region south of the bend this gradient has existed since 10 15 ma based on fission track analyses of middle miocene to holocene denudation and qualitative evaluations of the preservation potential of middle late tertiary volcanic edifices and synorogenic sediment global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17 5 s bend whereas present day shortening in the thrust belt north of the bend is distributed over much of the width of the belt south of the bend it is concentrated near the eastern frontal margin structural data suggest a similar kinematic situation during late miocene to holocene shortening an out of sequence chronology of thrusting in the narrow 200 km wide thrust belt to the north versus a forward breaking sequence of thrusting in the relatively wide 350 km wide thrust belt to the south the long term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust wedge conditions induced by rapid erosion rates since 10 15 ma such conditions inhibit thrust front advance and favor distributed deformation within the thrust belt interior in the thrust belt south of the bend a progressive eastward migration of thrusting is interpreted to be the result of long term critical thrust wedge conditions promoted by extremely low rates of denudation since middle miocene time

bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the altiplano and eastern cordillera 1 the rapid rise of the central andean plateau between 10 and 6 8 ma implies that mantle lithosphere including eclogitized lower crust was removed from beneath the region in that time interval we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum rayleigh taylor instability we place bounds on the average viscosity coefficient for central andean lithosphere when compared with laboratory measurements of flow laws for olivine and eclogite the allowed range of viscosity coefficients yields bounds on the temperature of 500 800 c at the moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than 50 mpa this range of temperature is comparable with if a slightly lower than we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity the mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part

evidence for lithospheric detachment in the central andes from local earthquake tomography abstract data from three temporary seismic networks were merged for tomographic inversion although the deployments did not coincide in time spatial overlap was achieved by re occupying existing sites travel times and t operators of about 1600 earthquakes were inverted for 3d models of p p s and p wave attenuation q p 1 all three attributes provide a consistent image of the entire subduction zone on a lithospheric scale the tomographic images reveal low velocities and high attenuation in the crust and mantle underlying the western cordillera and most of the puna plateau indicative of weak rheology and mostly asthenospheric mantle in contrast forearc and eastern foreland are characterized by high q p values corresponding to cold temperatures in accordance with thermal models in the backarc between 23 s and 24 s a high velocity high q p structure beneath the eastern cordillera and eastern puna is interpreted as detaching continental lithosphere that has been thickened in the orogenic process south of this structure the mantle is characterized by low velocities high p s ratios and low q p values here it is believed that lithosphere originally underlying andean crust has already been removed this is supported by new estimates of crustal thickness and volcanic activity

evolutionary cycles during the andean orogeny repeated slab breakoff and flat subduction tectonomagmatic similarities between the modern chilean flat slab region and pre neogene magmatic episodes suggest that they represent analogues to flat subduction evolutionary patterns in each magmatic suite include i increasing la yb ratios and sr and nd isotopic enrichment through time ii eastward migration of magmatism after periods of transpressional transtensional intra arc deformation and iii subsequent termination and virtual absence of main arc activity for 5 10 myr these patterns may reflect slab shallowing followed by flat subduction and thickening of the overlying crust if repeated they require interchanging episodes of slab steepening increasing convergence rates force slab kinking and eventual failure of the oversteepened slab followed by rebound of the slab tip owing to lack of further slab pull flat subduction and termination of subduction related magmatism rapid subduction leads to shallow overriding of the detached slab fragment eclogitization of the gradually steepening slab tip at depth and subsequent slab pull permits asthenospheric corner flow and subduction related magmatism

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

denudation rates and a topography driven rainfall threshold in northern chile multiple cosmogenic nuclide data and sediment yield budgets abstract the quantification of geomorphic process rates on the outcrop and the orogen scale is important to describe accurately the interaction between the relative effects of erosion tectonics and climate on landscape evolution we report single and paired cosmogenic nuclide 10 be 26 al and 21 ne derived erosion rates and exposure ages on hillslope interfluves from the tectonically active western central andes that show a distinct spatial variation a positive correlation of erosion rates with elevation and present day rainfall rates is observed erosion rates at lower altitudes the hyperarid coastal cordillera and the western escarpment with the northern part of the atacama desert are extremely low and of the order of 10 100 cm my nominal exposure ages 1 6 my in contrast erosion rates at higher altitudes the semiarid western cordillera range up to 4600 cm my nominal exposure ages 0 02 0 1 my this latter average long term bedrock erosion rate record suggested to be coupled to an orographically controlled pattern of rainfall is also reflected in the pattern of denudation rates derived from a short term decadal record of limited sediment yield data specifically denudation rates calculated from sediment flux data are of a similar order of magnitude as erosion rates deduced from long lived cosmogenic nuclides from bedrock hillslope interfluves of the western cordillera nevertheless the production and the supply of sediment from the western andean slope are very limited analysis of multiple cosmogenic nuclides allows simultaneous determination of erosion rates and exposure ages but also reveals complex exposure histories of non bedrock samples such as boulders or amalgamated clast samples notably this study shows that saturation of nuclides usually assumed in studies where only a single nuclide is analyzed is rather the exception than the rule as revealed by erosion island plots constant erosion that started much later than the formation age of the rocks or episodic erosion by spalling can partially explain non steady state concentrations and more complicated exposure scenarios furthermore the use of multiple nuclides with different half lives allowed us to infer that no significant variations in long term erosion rates have occurred and that at the western escarpment erosion rates have been low and constant for most of the late neogene nevertheless the time intervals necessary to reach steady state concentrations for cosmogenic nuclides can be quite different from those needed for landscapes to reach steady state

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

stratigraphy and chronology of upper cretaceous lower paleogene strata in bolivia and northwest argentina integration of sequence stratigraphy magnetostratigraphy ar ar dating and paleontology considerably advances knowledge of the late cretaceous early paleogene chronostratigraphy and tectonic evolution of bolivia and adjacent areas the partly restricted marine el molino formation spans the maastrichtian and danian 73 60 0 ma deposition of the alluvial to lacustrine santa lucia formation occurred between 60 0 and 58 2 ma the widespread erosional unconformity at the base of the cayara formation is 58 2 ma this unconformity separates the upper puca and corocoro supersequences in bolivia and is thus coeval with the zuni tejas sequence boundary of north america the thick overlying potoco and camargo formations represent a late paleocene oligocene foreland fill the onset of shortening along the pacific margin at 89 ma initially produced rifting in the distal foreland santonian campanian eastward onlapping deposits indicate subsequent waning of tectonic activity along the margin significant tectonism and magmatism resumed along the margin at 73 ma and produced an abrupt increase in subsidence rate and other related phenomena in the basin subsidence was maximum between 71 and 66 ma due to the early maastrichtian global sea level high marine waters ingressed from the northwest into this underfilled basin subsidence decreased during the late maastrichtian and was low during the danian it increased again in the latest danian for which a slight transgression is recorded and peaked in the early selandian tectonism between 59 5 and 58 2 ma produced a variety of deformational and sedimentary effects in the basin and correlates with the end of emplacement of the coastal batholith the subsequent 58 2 ma major unconformity marks the onset of continental foreland basin development which extended into andean bolivia during the late paleocene oligocene interval this basin underwent internal deformation as early as eocene time in the altiplano and cordillera oriental these early structures previously assigned to the late oligocene early miocene orogeny probably accommodated observed tectonic rotations in the eocene oligocene

columnar jointing in vapor phase altered non welded cerro galan ignimbrite paycuqui argentina columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits however the non welded cerro galan ignimbrite at paycuqui argentina contains well developed columnar joints that are instead due to high temperature vapor phase alteration of the deposit where devitrification and vapor phase crystallization have increased the density and cohesion of the upper half of the section thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures above 630 c but the lack of welding textures indicates temperatures below the glass transition temperature in order to remain below the glass transition at 630 c the minimum cooling rate prior to deposition was 3 0 10 3 8 5 10 2 c min depending on the experimental data used for comparison alternatively if the deposit was emplaced above the glass transition temperature conductive cooling alone was insufficient to prevent welding crack patterns average 4 5 sides to each polygon and column diameters average 75 cm are consistent with relatively rapid cooling where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer the presence of regularly spaced complex radiating joint patterns is consistent with fumarolic gas rise where volatiles originated in the valley confined drainage system below joint spacing is a proxy for cooling rates and is controlled by depositional thickness valley width we suggest that the formation of joints in high temperature non welded deposits is aided by the presence of underlying external water where vapor transfer causes crystallization in pore spaces densifies the deposit and helps prevent welding

mechanisms of dike propagation in layered rocks and in massive porous sedimentary rocks the mechanisms of dike propagation are analyzed through detailed examination of small structures associated with the dike intrusion exposures in the ramon area israel offer an opportunity to evaluate theoretical models of tensile fracture propagation and to better understand the factors governing the dike geometry dike related structures are studied in two jurassic units the ardon formation a well stratified sequence of shales and dolomites and the inmar formation which is composed of porous massive sandstones with a few shale lenses it is shown that for the different host rocks the propagation mechanisms and structures differ significantly dikes in the ardon formation are dominated by 1 20 m wide segments confined to distinct layers the mechanisms of segment containment are determined by calculating the stress intensity factor for an extension crack approaching a mechanical interface it is apparent that dike segmentation and containment is controlled mainly by in situ stress and shear moduli differences between adjacent layers and partly by bedding plane slippage dikes in the inmar formation are dominated by smaller segments and by 1 10 cm wide and 10 100 cm long fingers with intermittent smooth patches and slickensides opposite dike walls display exact mirror images of these features dikes propagate in alternating stages of viscous flow and brittle deformation the viscous stages include fluidization of the sandstone by the dike related fluids and increase of the pore pressure in the rock a viscous instability develops between the highly viscous fluidized sandstone and the less viscous dike related fluids forming a zone of viscous fingering in front of the dike stages of brittle fracturing occur when the pore pressure is released and the flux of fluids is insufficient to fluidize the rock

blocky versus fluidal peperite textures developed in volcanic conduits vents and crater lakes of phreatomagmatic volcanoes in mio pliocene volcanic fields of western hungary abstract volcanic fields in the pannonian basin western hungary comprise several mio pliocene volcaniclastic successions that are penetrated by numerous mafic intrusions peperite formed where intrusive and extrusive basaltic magma mingled with tuff lapilli tuff and non volcanic siliciclastic sediments within vent zones peperite is more common in the pannonian basin than generally realised and may be also important in other settings where sediment sequences accumulate during active volcanism hajagos hegy an erosional remnant of a maar volcano was subsequently occupied by a lava lake that interacted with unconsolidated sediments in the maar basin and formed both blocky and globular peperite similar peperite developed in kissomlyo a small tuff ring remnant where dykes invaded lake sediments that formed within a tuff ring lava foot peperite from both hajagos hegy and kissomlyo were formed when small lava flows travelled over wet sediments in craters of phreatomagmatic volcanoes at sag hegy a large phreatomagmatic volcanic complex peperite formed along the margin of a coherent intrusion all peperite in this study could be described as globular or blocky peperite globular and blocky types in the studied fields occur together regardless of the host sediment

relationships among climate erosion topography and delamination in the andes a numerical modeling investigation cordilleran orogenic systems such as the andes are controlled by shortening rates climatically controlled erosion rates and in some cases eclogite production and delamination all of these processes are coupled however making it diffi cult to uniquely determine the relative importance of each process and the feedbacks among them in this paper we develop a massbalanced numerical model that couples an actively shortening orogen and crustal root with eclogite production delamination and climatically controlled erosion the model provides a fi rst order quantifi cation of the sources shortening and sinks erosion and eclogite production and delamination of crustal volume during the cenozoic in the andes as a function of latitude and time given reasonable estimates for the rates of eclogite production and the threshold size of the eclogitic root required for delamination the model suggests that in the central andes between 5 s and 32 s the orogen has grown to a suffi cient height to produce and maintain eclogite which in turn has promoted delamination in the lower crust and mantle in this region climatically controlled erosion rates infl uence the size of the orogen through two separate mechanisms by exporting mass via surface processes and by controlling the lithostatic pressure in the lower crust which modulates the rates of eclogite production and or delamination to the north and south of the central andes relatively low shortening rates and high precipitation and erosion rates have slowed eclogite production such that delamination likely has not occurred during the cenozoic

origin of enigmatic structures field and geochemical investigation of columnar joints in sandstones island of bute scotland abstract columnar joints resembling those in igneous rocks are present in devonian and early carboniferous sandstones at two localities on the island of bute in southwestern scotland new data concerning field relations and geochemical changes constrain processes responsible for generation of columnar fracture patterns in sedimentary rocks the columns are mostly normal to bedding and occur at several discrete stratigraphic levels the host rocks are cut by early carboniferous volcanic necks or plugs which acted as heat sources but development of columnar jointing was strongly controlled by small sills and dikes of a recessive purple fine grained rock geochemistry of these rocks points to an igneous origin where contacts are exposed columnar joints appear above subconcordant intrusions and disappear upward the lowest parts of the columnar jointed sandstones are si depleted and enriched in ca mg fe al rare earth elements rees and some base metals ree patterns from columnar jointed sandsto

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

lava sediment interaction in desert settings are all peperite like textures the result of magma water interaction abstract this study reports on lava sediment interaction in dry depositional settings focusing on the early cretaceous volcano sedimentary sequence in the huab basin nw namibia as a detailed example here an active aeolian sand sea erg system was progressively engulfed by lavas of the etendeka flood basalt province 133 ma bp this volcanic flooding resulted in 1 the unprecedented preservation of large parts of the active dune system 2 the development of a variety of sediment interlayers and 3 a record of excellent examples of the dynamic interaction between lava and aeolian sand the lava sediment interaction happens on a variety of scales from simple sediment interbeds within the lavas at a large scale down to complex breccia horizons and bulbous lava sediment contacts at small scales peperite like textures are found where hot lava has dynamically interacted with unconsolidated aeolian sands such interaction could involve lava cascading down the front of dune faces or active aa style flow fronts bulldozing into and oversteepening dune faces causing rapid influx of sand into the active flow front resulting breccia horizons have been found up to 5 m thick locally the aeolian sand is baked to a quartzite by the hot lava in contrast on other surfaces the lava leaves striations in the sand as it flows across it indicating local flow directions and less pronounced interaction in places wind ripples and topset beds are preserved on the stoss side of the dune indicating the passive nature of emplacement for some of the lava flows peperites are often thought to form due to the presence of water within the unconsolidated sediments that the juvenile magma comes into contact with however examples from arid environments demonstrate that peperite forming processes are diverse and may not always require water therefore it is suggested that the term peperite should be used to describe deposits consisting of sediments mixed with juvenile magmatic components where it is clear that there has been a dynamic interaction between the two this leaves the presence or lack of water as an issue for interpreting the processes by which the peperite formed

quantification of mechanical compaction and cementation during contact metamorphism of sandstone abstract both mechanical compaction and mineral reactions reduce the porosity and permeability of sedimentary rocks these processes occur during burial and diagenesis as well as during contact and regional metamorphism although the mechanisms governing porosity reduction have been studied extensively in diagenetic sedimentary systems relatively limited work has been carried out on metamorphic rocks here we present a petrographic method to isolate and quantify the effects of mineral cementation and mechanical compaction in sandstone altered during contact metamorphism we sampled rocks from the inmar formation in makhtesh ramon israel where indurated sandstone is present near contacts with dykes and analyzed them using high resolution scanning electron microscopy and cathodoluminescence using image segmentation to differentiate between quartz cement and detrital grains we observed both grain rearrangement and brittle fracturing near intrusive contacts at the highest level of porosity reduction mechanical compaction reduced porosity by 18 31 while cementation caused a reduction of 11 we also found that the maximal level of mechanical compaction as well as its spatial extent scale with the size of the intrusion by contrast the maximal level of cementation was unaffected by the size of the dyke our findings demonstrate that to model porosity and permeability evolution in contact aureoles both mechanical and chemical processes must be taken into account

sims microanalysis of the strelley pool formation cherts and the implications for the secular temporal oxygen isotope trend of cherts abstract the significance of oxygen isotope ratios in archean chert has long been debated cherts from the c 3 4 ga strelley pool formation spf pilbara craton western australia host some of the oldest stromatolite and microfossil evidence for life but the genesis and timing of silica cements has been unclear field relations petrography and a combination of laser fluorination and in situ sims measurements of 18o in quartz show that bedded cherts of the spf were originally precipitated as carbonates and were later widely replaced by quartz three localities were studied and analyzed for 18o qz in chert 1 camel creek foliated metamorphosed bedded cherts and meter scale black chert veins 2 unconformity ridge and abdp8 drill core stromatolitic and bedded chert overlying basal detrital quartz sandstone and 3 the trendall locality bedded stromatolitic chert replacing original dolomite low temperature hydrothermal quartz and mm to decimeter scale chert quartz veins laser fluorination mm scale values of 18o qz range from 14 2 to 18 2 vsmow at camel creek 9 3 to 18 9 at unconformity ridge and 13 7 to 25 7 at trendall values of 18o qz in cm to decimeter scale hydrothermal chert veins cutting bedded carbonates at trendall range from ca 15 to 16 whereas bedded cherts are 17 to 26 these laser data include the highest 18o values reported for cherts in paleoarchean sediments and are up to 4 higher than the upper limit of 22 reported in other studies in apparent contrast to the long standing secular temporal trend which shows such high 18o only in younger chert however analysis by laser fluorination at the 1 mm scale cannot resolve microtextures seen petrographically in contrast in situ sims analyses can resolve petrographic microtextures and show 18o qz at 10 m scale have an even greater range of 7 31 in bedded cherts at the trendall locality up to 9 above the secular temporal trend textures observed optically at the trendall locality were classified as microquartz mesoquartz chalcedony megaquartz veins and cavity megaquartz sem cl imaging shows two generations of meso and megaquartz bright cl with well developed growth zoning and dark cl with massive or mottled texture microquartz is the earliest textural generation of quartz and has a maximum 18o qz of 22 by sims dark cl mesoquartz has similar 18o to microquartz and is interpreted to also be early bright zoned cl mesoquartz which formed post archean has even higher 18o up to 29 vein megaquartz crosscuts most quartz generations and has a restricted range of 18o mostly from 16 to 19 chalcedony pseudomorphs rhombic cavities and fractures lines the edges of veins and has similar 18o to veins 16 19 late cavity megaquartz is bright and zoned by cl grows into late open cavities and has the highest 18o qz values reported from the pilbara up to 31 3 thus the highest 18o quartz cements at the trendall locality are the youngest and may be related to weathering early silicification and the formation of microquartz chalcedony and low 18o mesoquartz occurred during low temperature hydrothermal activity in the archean none of the spf quartz examined is interpreted to have formed as a direct precipitate from paleoarchean seawater thus values of 18o qz do not record either water chemistry or temperature of archean oceans in situ sims analysis shows that high 18o qz values above 22 are only found in late forming cavity megaquartz and bright zoned cl mesoquartz at the trendall locality the spf results from our sample suite demonstrate the ability to resolve complex history using detailed petrography and sims analysis similar studies may show equal complexity of 18o qz data for other localities that are interpreted to show secular temporal trends for chert the apparent increase of 18o qz through time may reflect differences in diagenesis and or an inherent and previously unrecognized sampling bias that compares fundamentally different populations of quartz such as archean hydrothermal chert from volcanic greenstone belts with unrelated phanerozoic biogenic quartz

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

rise of the andes the surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening leading to their gradual rise recent studies of the andes indicate that their elevation remained relatively stable for long periods tens of millions of years separated by rapid 1 to 4 million years changes of 1 5 kilometers or more periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable dense lower lithosphere after long term thickening of the crust and lithospheric mantle

preparation and deformation of synthetic aggregates of quartz synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder impure silica gel and high purity silicic acid by hydrothermal isostatic pressing at 300 mpa the resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests the flow stress at 1200 1300 k was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity it is questionable that the natural quartz origin specimens were fully equilibrated in respect to the activity of the water but equilibration has probably been achieved in the amorphous silica origin specimens which crystallized under the experimental conditions in the deformation of the latter materials the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kj mol 1 for both however the impure gel origin material gave a stress exponent of about 2 3 whereas the high purity silicic acid origin material gave a stress exponent of about 4 in spite of the grain sizes being about 90 m and 20 m respectively it is concluded that while the impure specimens have higher intragranular strengths there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high purity specimens

effect of volcanic dykes on coastal groundwater flow and saltwater intrusion a field scale multiphysics approach and parameter evaluation volcanic dykes are common discrete heterogeneities in aquifers however there is a lack of field examples of and methodologies for comprehensive in situ characterization of their properties with respect to groundwater flow and solute transport we have applied an integrated multi physics approach to quantify the effect of dolerite dykes on saltwater intrusion in a coastal sandstone aquifer the approach involved ground geophysical imaging passive magnetics and electrical resistivity tomography well hydraulic testing and tidal propagation analysis which provided constraints on the geometry of the dyke network the subsurface saltwater distribution and the sandstone hydrodynamic properties and connectivity a three dimensional variable density groundwater model coupled with a resistivity model was further calibrated using groundwater and geophysical observations a good agreement of model simulations with tide induced head fluctuations geophysically derived pore water salinities and measured apparent resistivities was obtained when dykes hydraulic conductivity storativity and effective porosity are respectively about three one and one orders of magnitude lower than the host aquifer the presence of the dykes results in barrier like alterations of groundwater flow and saltwater intrusion preferential flowpaths occur parallel to observed dyke orientations freshwater inflows from upland recharge areas concentrate on the land facing side of the dykes and saltwater penetration is higher on their sea facing side this has major implications for managing groundwater resources in dyke intruded aquifers including in coastal and island regions but also provides wider insights on preferential pathways of groundwater flow and transport in highly heterogeneous aquifer systems this article is protected by copyright all rights reserved

crustal thickness variations in the central andes we estimated the crustal thickness along an east west transect across the andes at lat 20 s and along a north south transect along the eastern edge of the altiplano from data recorded on two arrays of portable broadband seismic stations banjo and seda waveforms of deep regional events in the downgoing nazca slab and teleseismic earthquakes were processed to isolate the p to s converted phases from the moho in order to compute the crustal thickness we found crustal thickness variations of nearly 40 km across the andes maximum crustal thicknesses of 70 74 km under the western cordillera and the eastern cordillera thin to 32 38 km 200 km east of the andes in the chaco plain the central altiplano at 20 s has crustal thicknesses of 60 to 65 km the crust also appears to thicken from north 16 s 55 60 km to south 20 s 70 74 km along the eastern cordillera the subandean zone crust has intermediate thicknesses of 43 to 47 km crustal thickness predictions for the andes based on airy type isostatic behavior show remarkable overall correlation with observed crustal thickness in the regions of high elevation in contrast at the boundary between the eastern cordillera and the subandean zone and in the chaco plain the crust is thinner than predicted suggesting that the crust in these regions is supported in part by the flexural rigidity of a strong lithosphere with additional constraints we conclude that the observation of airy type isostasy is consistent with thickening associated with compressional shortening of a weak lithosphere squeezed between the stronger lithosphere of the subducting nazca plate and the cratonic lithosphere of the brazilian craton

deformation of poorly consolidated sediment during shallow emplacement of a basalt sill coso range california a 150 m long wedge shaped unit of folded and faulted marly siltstone crops out between undeformed sedimentary rocks on the north flank of the coso range california the several meter thick blunt end of this wedge abuts the north margin of a basaltic sill of comparable thickness chaotically deformed siltstone crops out locally at the margin of this sill and at one locality breccia pipes about one meter in diameter crosscut the sill the sill extends about 1 km south up the paleoslope where it merges through continuous outcrop with a lava flow that in turn extends 1 4 km to a vent area marked by more than 100 m of agglutinate and scoria apparently lava extruded at this vent flowed onto unconsolidated sediments burrowed into them and fed a sill at about 40 m depth within the sedimentary sequence the sill initially propagated by wedging between sedimentary beds but eventually began to push some beds ahead of itself forming a remarkable train of folds in the process the sediments apparently were wet at the time of sill emplacement because hydrothermal alteration is common near the contact between the two rock types and because the breccia pipes that crosscut the sill apparently resulted from phreatic explosions of pore water heated at the base of the cooling sill comparison of deformation of the host material at the coso locality with that reportedly caused by emplacement of sills elsewhere indicates that the character of deformation differs greatly among the various localities the specific response of host material depends upon such parameters as initial properties of magma and host material rate of sill growth and attendant rate of strain of host material and depth of sill emplacement some properties may change considerably during an intrusive deformational episode thus complicating accurate reconstruction of such an event

dike intrusion into unconsolidated sandstone and the development of quartzite contact zones abstract shallow near surface magma emplacement into a porous unlithified sandstone occurred by forceful dike dilation and was accompanied by a localized hydrothermal event petrographic fabric authigenic mineralogy and petrophysical data grade smoothly through the quartzite contact zone outward into the friable inmar sandstone of makhtesh ramon israel quartz grain deformation fracturing and rehealing of grains and pressure solution constitute overwhelming evidence of a compressive environment adjacent to dike margins albeit with temperatures and pressures insufficient to fully recrystallize detrital grains dikes were accommodated primarily by the repacking of the sandstone with both brittle and elastic grain interaction post emplacement heating together with elastic compressive stress dissipation lead to fracture healing and pressure solution the occasional columnar jointing must be related to thermal contraction of the quartzite during later cooling and resulting mode i cracks the alteration of both contact zone and dike material occurred via a transitory and confined hydrothermal seepage up dike margins elsewhere around the world highly altered intrusions bordered by indurated sometimes uniformly jointed sediments may also be the result of forceful magma emplacement and hydrothermal alteration at near surface levels

effects of basic intrusions on ree mobility of sandstones and their geological significance a case study from the qianjiadian sandstone hosted uranium deposit in the songliao basin abstract ree geochemistry of sandstones and their carbonate cements from the upper cretaceous yaojia formation in the qianjiadian uranium deposit southern songliao basin are illustrated in order to reveal effects of basic intrusions on ree mobility of sandstones and their geological significance compared with the sandstones far away from the diabase dikes the sandstones near the diabase dikes are obviously depleted in rees lree content hree content and ree content all increase with an increase of the distance away from the diabase dikes carbonate cements in the sandstones comprise of dolomite siderite ankerite and calcite which are successively formed from the early to late stage in the sandstones near the diabase dikes the dolomite and siderite are partly dissolved and the ankerite is accumulated lree content and hree content are both gradually decreasing in turn from the dolomite to siderite to ankerite and to calcite dolomite content is positively correlated with hree content siderite content is positively correlated with lree content or hree content and ankerite content is inversely correlated with lree content dissolution of the dolomite and siderite and formation of the ankerite were aroused by intrusion of the diabase dikes resulting in the sandstones near the diabase dikes being depleted in rees the study suggests that high grade uranium mineralization may be attributed to the diabase dikes ree depletion of the sandstones can be severe as a new prospecting indicator and sandstones affected by igneous intrusions should be avoided during sampling for sandstone provenance by rees

initial plate geometry shortening variations and evolution of the bolivian orocline comparisons of newly published cross sections across the bolivian andes with existing cross sections through argentina emphasize significant along strike changes in crustal shortening a sharp decrease in the magnitude of crustal shortening from 530 km to 150 km north to south occurs at 23 s a 20 40 m y difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening extending the western margin of south america to account for 530 km of shortening in the bolivian andes and 150 km of shortening in argentina produces a central andean salient that is perpendicular to the nazca plate shortening direction from 60 to 26 ma during this same time interval the chilean coast south of 23 s was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike slip offset and backarc extension deformation within the andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of 23 s inhibited mountain building whereas north of 23 s normal convergence to a central andean salient facilitated contractional deformation the magnitude of deformation north of 23 s is a consequence of both plate convergence direction providing a longer period of contractional deformation from ca 70 ma to the present and a thick phanerozoic sedimentary package that permitted large magnitudes of thin skinned deformation significant along strike changes in the shape of the south american margin allowing for convergence to change from compression to extension along the strike of the orogen may help explain the dramatic differences in timing amount and style of deformation in the andes

orographic barriers high resolution trmm rainfall and relief variations along the eastern andes 1 quantifying the degree to which tectonic and erosive processes shape landscapes is key to understanding the evolution of tectonically active mountain belts here we explore the interplay of these two processes along the humid eastern flank of the south american andes we use high resolution tropical rainfall measurement mission trmm and srtm data to characterize elevation relief and hillslope angle of peak rainfall at orographic barriers over a distance of more than 3500 km along the eastern flanks we find that peak rainfall 3 5 m yr occurs at a mean elevation of 1 3 0 17 km a mean relief of 0 95 0 08 km and at moderate mean hillslope angles of 18 3 1 7 we suggest that topographic relief is the best first order rainfall predictor and we demonstrate how relief changes along strike south of the andean orocline alter rainfall distribution changes in climatic sedimentary and tectonic processes prevent the formation of high relief amounts at the mountain front and these areas are not characterized by pronounced rainfall peak

re examining pluton emplacement processes abstract previous pluton emplacement studies have attempted to explain how space is made during pluton emplacement in fact the only means of making space during emplacement of mantle derived magmas in the crust are 1 lowering the moho or 2 outward displacement of the earth s surface other pluton emplacement mechanisms are material transfer processes mtps that do not increase the volume of the crust one frequently cited mtp for emplacement of concentrically zoned and ballooning plutons is ductile flow of wall rocks around the plutons the expected structures and total strains near such plutons are dependent on the three dimensional pluton shape the width of the deforming aureole whether the pluton is a piercing or non piercing diapir and in the latter case the number of body radii the pluton travels we have re examined a few such plutons and calculated the amount of material transfer caused by ductile flow in no case is more than 40 and in most cases only 15 35 of the required material transfer accommodated by wall rock flow the contact aureoles around these plutons are too narrow and or strains too low indicating that the plutons are predominantly discordant bodies with narrow 0 1 0 4 pluton radii concordant aureoles a comparison of strains in natural contact aureoles and those made in mechanical models of diapirs indicates that natural strains are at least an order of magnitude less than model strains we suggest that the similarities between models of diapiric ascent of spherical bodies and concentrically zoned plutons are superficial because the models are too simplistic instead we argue that the ascent and emplacement of magmas require multiple near field and far field mtps these mtps will show gradients with depth distance from the magma and with time rates of near field mtps must be rapid whereas far field rates are probably nearer to long term average rates of orogenic processes

reappraising columnar joints in different rock types and settings abstract columnar joints comprise a network of opening mode fractures that divide material into prismatic columns they are known in several materials and rock types including basalt mud sandstone and starch in all types the resultant prismatic columns have a similar geometry facilitating a comparison of columnar joints in different materials and processes such as cooling and drying in this study we provide a unique example of columnar joints in low grade meta chalks that are the product of spontaneous combustion metamorphism of organic rich chalks in the mottled zone complex israel although the geometry of the prismatic columns observed in these rocks resembles those seen in basalts our approximate calculations based on thermal expansion and temperature difference show that contraction due to cooling is two orders of magnitude less than the measured contraction moreover the cooling effect should be much more significant in high grade ca 550 900 c rather than in low grade ca 200 c rocks but only the latter contain prismatic columns the prismatic columns are restricted to a discrete stratigraphic level between 1 and 10 meters above a chert and phosphorite unit we suggest that burning of organic matter in the protolith results in mass and volume losses and is the principal process that leads to contraction changes in texture and pore structure recrystallization accompany the contraction resulting in the rearrangement of the protolith into columnar meta chalk this intriguing example of columnar joints in meta chalk indicates that the geometric similarity of prismatic columns should not serve as the sole argument for their formation during cooling

the nature of orogenic crust in the central andes 1 the central andes 16 22 s are part of an active continental margin mountain belt and the result of shortening of the weak western edge of south america between the strong lithospheres of the subducting nazca plate and the underthrusting brazilian shield we have combined receiver function and surface wave dispersion results from the banjo seda project with other geophysical studies to characterize the nature of the continental crust and mantle lithospheric structure the major results are as follows 1 the crust supporting the high elevations is thick and has a felsic to intermediate bulk composition 2 the relatively strong brazilian lithosphere is underthrusting as far west 65 5 w as the high elevations of the western part of the eastern cordillera ec but does not underthrust the entire altiplano 3 the subcrustal lithosphere is delaminating piecemeal under the altiplano ec boundary but is not completely removed beneath the central altiplano the altiplano crust is characterized by a brittle upper crust decoupled from a very weak lower crust that is dominated by ductile deformation leading to lower crustal flow and flat topography in contrast in the high relief inland sloping regions of the ec and sub andean zone the upper crust is still strongly coupled across the basal thrust of the fold thrust belt to the underthrusting brazilian shield lithosphere subcrustal shortening between the altiplano and brazilian lithosphere appears to be accommodated by delamination near the altiplano ec boundary our study suggests that orogenic reworking may be an important part of the felsification of continental crust

source localization in a time varying ocean waveguide one of the most stringent impairments in matched field processing is the impact of missing or erroneous environmental information on the final source location estimate this problem is known in the literature as model mismatch and is strongly frequency dependent another unavoidable factor that contributes to model mismatch is the natural time and spatial variability of the ocean waveguide as a consequence most of the experimental results obtained to date focus on short source receiver ranges usually 5 km stationary sources reduced time windows and frequencies generally below 600 hz this paper shows that mfp source localization can be made robust to time space environmental mismatch if the parameters responsible for the mismatch are clearly identified properly modeled and time adaptively estimated by a focalization procedure prior to mfp source localization the data acquired during the advent 99 sea trial at 2 5 and 10 km source receiver ranges and in two frequency bands below and above 600 hz provided an excellent opportunity to test the proposed techniques the results indicate that an adequate parametrization of the waveguide is effective up to 10 km range in both frequency bands achieving a precise localization during the whole recording of the 5 km track and most of the 10 km track it is shown that the increasing mfp dependence on erroneous environmental information in the higher frequency and at longer ranges can only be accounted for by including a time dependent modeling of the water column sound speed profile

steady state erosion of critical coulomb wedges with applications to taiwan and the himalaya 1 orogenic structure appears to be partially controlled by the addition to and removal of material from the mountain belt by tectonic accretion and geomorphic erosion respectively we developed a coupled erosion deformation model for orogenic wedges that are in erosional steady state and deform at their coulomb failure limit erosional steady state is reached when all material introduced into the wedge is removed by erosion that is limited by the rate at which rivers erode through bedrock we found that the ultimate form of a wedge is controlled by the wedge mechanical properties sole out depth of the basal decollement erosional exponents basin geometry and the ratio of the added material flux to the erosional constant as this latter ratio is increased wedge width and surface slopes increase we applied these models to the taiwan and himalayan orogenic wedges and found that despite a higher flux of material entering the former the inferred ratio was larger for the latter calculated values for the erodibility of each wedge showed at least an order of magnitude lower value for the himalaya relative to taiwan these values are consistent with the lower precipitation regime in the himalaya relative to taiwan and the exposure of crystalline rocks within the himalayan orogenic wedge independently determined rock erodibility estimates are consistent with the accretionary wedge sediments and metasediments and the crystalline and high grade metamorphic rocks exposed within taiwan and the himalaya respectively therefore differences in rock type and climate apparently lead to key differences in the erosion and hence orogenic structure of these two mountain belts

influence of andean uplift on climate and paleoaltimetry estimates article i nfo article history recent elevation reconstructions of the andean plateau suggest a rapid 2 5 1 0 km rise of the central andes between 10 and 6 ma this rapid rise has been attributed to a catastrophic removal of a dense lithospheric mantle root beneath the andes however these findings are based on the assumption that climate did not change during deposition of paleoaltimetry proxies here we evaluate south american climate change due to andean uplift and its influence on interpretations of plateau elevation from climate sensitive paleoaltimetry data a series of experiments are presented using the regcm3 regional general circulation model rcm to characterize changes in andean precipitation amount surface temperature and wind direction vapor source as a function of changing plateau elevation results indicate that south american and andean climate changed significantly in response to plateau growth more specifically rising of the plateau results in up to a 900 mm increase in rainy season december january february precipitation over the plateau plateau uplift also results in a decrease in non adiabatic surface temperature of up to 6 5 c in addition to adiabatic cooling directly related to elevation change through the lapse rate finally the prevailing wind direction and the vapor source for precipitation switches from the south pacific ocean to the equatorial atlantic as plateau elevation increases above of its present day elevation taken together these changes in paleoclimate would have substantially depleted the oxygen isotopic concentration of paleoprecipitation through the cenozoic unless this climatic effect is taken into consideration paleoaltimetry reconstructions based on stable isotope methods may overestimate the rapid rise of the andes by up to several kilometers we conclude that some or all of the apparent rapid rise of the andean plateau from paleoaltimetry data could be an artifact of large changes in paleoclimate

alterations of cretaceous siltstones and sandstones near basalt contacts nugssuaq greenland abstract the influence of the intrusion of basaltic dykes and sills was investigated on sandstones and siltstones of the atane formation turonian coniacian from the sarqaq area nugssuaq peninsula central west greenland in the unaltered rock sequence the siltstones are dominated by kaolinite quartz and feldspar no cementation was observed the sandstones which are prevalent in this formation are arkoses and lithic arkoses with quartz feldspar ratios of about 1 with variable contents of rock fragments and with minor amounts of matrix the matrix consists mainly of kaolinite less frequently of illite muscovite and smectite or interstratified illite smectite cement minerals include calcite fe calcite both mainly in concretions and subordinate fe oxide hydrates the porosity of the sandstones and siltstones indicates a former maximum depth of burial of 1000 m in the sarqaq region adjacent to thin dykes and sills 1 2 m the above described rocks are altered as follows in sandstones albite fibrous quartzine smectite goethite were all formed at the expense of kaolinitic matrix and aragonite cement in siltstones interstratified illite smectite illite smectite and low cristobalite replaced kaolinite these minerals especially smectite exclude temperatures above 200 c they were probably formed during a cooling period adjacent to thicker basalt intrusions only sills over 5 m thick and no dykes are known from this area the following alterations are observed in sandstones two textural types of albite were formed in addition to 1 fibrous albite mentioned above 2 undulose lath shaped albite 3 coarse twinned non undulose replacement albite moreover brownish luminescing undulose quartz muscovite chlorite epidote nontronite and anatase occur in siltstones the mineral association muscovite chlorite pyrophyllite albite occurs suggesting temperatures in the range of 250 500 c additionally a 22 26 a mixed layer mineral chlorite muscovite formed at the contacts of thick as well as thin basalt intrusions the detrital grains directly adjoining the basalt are partly fractured potassium feldspar grains show also a chemical disintegration the temperatures calculated by application of a cooling model are higher than those indicated by the mineral alterations observed convective heat transfer by pore water is suggested as an explanation