cause of dominance by sheetflood vs debris flow processes on two adjoining alluvial fans death valley california two large adjoining alluvial fans of the panamint range piedmont death valley california are composed of different facies assemblages deposited by contrasting sedimentary processes the anvil spring fan was built solely by water flow processes incised channel floods and sheetfloods whereas the neighbouring warm spring fan has been constructed principally by debris flows the boundary between these fans delineates a sharp provincial piedmont contact between sheetflood dominated fans to the south and debris flow dominated fans to the north factors such as climate catchment area fan area catchment relief aspect vegetation types and density and neotectonic setting are essentially identical for these two fans the key difference between them is that their catchments are underlain by dissimilar bedrock types which weather to yield distinctive sediment suites weathering of the granite and andesite of the anvil fan catchment produces significant volumes of medium to very coarse sand granules pebbles cobbles and boulders but minimal silt and clay in contrast the shale quartzite and dolomite that dominate bedrock in the warm spring catchment weather to yield a wide suite of sedimentary particles spanning from clay to boulders the abundance of mud and the unsorted character of the yielded sediment cause precipitation induced slope failures in the warm spring catchment to transform readily into debris flows this propensity is due to the low permeability of the colluvial sediment which causes added water to become trapped quickly and pore pressure to rise rapidly promoting transformations to debris flows in contrast the limited volume of sediment finer than medium sand yielded from the anvil fan catchment causes the colluvium to have high permeability this factor prevents the transformation of wet colluvium to a debris flow during hydrologically triggered slope failures instead maintaining sediment transport as entrained bed load or suspended load in a water flow

piggyback basin development above a thin skinned thrust belt with two detachment levels as a function of interactions between tectonic and superficial mass transfer the case of the subandean zone bolivia abstract the subandean fold and thrust belt of bolivia is characterised by two major detachment levels and large piggyback basins sand box and numerical models have been used to study sedimentation and erosion control on thrust belt evolution and to study the retroactive effects of tectonics on piggyback development in thin skinned thrust belts with two detachment levels analogue models show that surface processes play a dominant role in controlling wedge evolutions erosion promotes fault reactivation and tectonic delamination passive roof duplex while sedimentation promotes forward shifting of the frontal thrust and consequently piggyback basin development numerical models were used to understand the development of the subandean fold and thrust belt of bolivia numerical experiments show that the simultaneity of basement tilting and high sedimentation rates promotes the formation of a stable tectonic wedge outer and inner faults are alternately active during the beginning of deformation a kinematic evolution that favours the development of piggyback basins between the step by step history of the thrust belt predicts that each change in tectonic location is recorded with large unconformities in basins but these unconformities are not well preserved from progressive erosion in the final geometry

miocene gypcretes from the calama basin northern chile gypcretes of miocene age are preserved beneath a 9 53 0 36 ma ignimbrite along the eastern margin of the oligo pleistocene calama basin northern chile they are restricted to a single stratigraphic horizon developed within laterally extensive 35 km coalesced alluvial fan deposits developed along the margin of an endorheic basin two types of gypcrete are recognized type 1 comprises almost completely gypsum cemented sandstones containing alabastrine nodules and columns sub vertical and horizontal veins of fibrous gypsum and v shaped cracks infilled by clastic material and are interpreted as surface weathered gypsic crusts type 2 gypcretes are composed of massive reddened poikilitic and mesocrystalline gypsum up to 80 of the rock with isolated bedding parallel clast rich lenses 200 30 cm and sub vertical veins of fibrous gypsum the massive texture resembles that of well developed b horizons in quaternary alluvial desert soils the crystal forms suggest an origin as a subsurface gypsic crust formed by a combination of hydromorphic poikilitic and illuvial mesocrystalline processes with the fibrous gypsum veins suggestive of periodic surface exposure gypcrete horizons are up to 25 m thick and composed of both gypcrete types they represent superimposed phases of surface and subsurface gypcrete development quaternary gypcretes are developed in arid climatic regimes but are not considered to develop under hyper arid climates an arid climate is considered to have prevailed in the study area up to 9 5 ma after which a change to hyper aridity favoured gypcrete preservation

the indosinian orogeny a perspective from sedimentary archives of north vietnam the triassic stratigraphic framework for the song da and the sam nua basins north vietnam suffers important discrepancies regarding both the depositional environments and ages of the main formations they contain using sedimentological analyses and dating foraminifer biostratigraphy and u pb dating on detrital zircon we provide an improved stratigraphic framework for both basins a striking feature in the song da basin located on the southern margin of the south china block is the diachronous deposition over a basal unconformity of terrestrial and marine deposits the sedimentary succession of the song da basin points to a foreland setting during the late early to the middle triassic which contrasts with the commonly interpreted rift setting on the northern margin of the indochina block the sam nua basin recorded the activity of a proximal magmatic arc during the late permian up to the anisian this arc resulted from the subduction of a southward dipping oceanic slab that separated the south china block from the indochina block during the middle to the late triassic the song da and sam nua basins underwent erosion that led to the formation of a major unconformity resulting from the erosion of the middle triassic indosinian mountain belt built after an ongoing continental collision between the south china and the indochina blocks later during the late triassic as syn to post orogenic foreland basins in a terrestrial setting the song da and sam nua basins experienced the deposition of very coarse detrital material representing products of the mountain belt erosion

uplift history of the central and northern andes a review the elevation of the andean cordillera is a crucial boundary condition for both climatic and tectonic studies the andes affect climate because they form the only barrier to atmospheric circulation in the southern hemisphere and they intrigue geologists because they have the highest plateau on earth formed at a noncollisional plate margin the altiplano puna yet until recently few quantitative studies of their uplift history existed this study presents both 1 a review of the quantitative paleoelevation estimates that have been made for the central and colombian andes and 2 an examination of the source and magnitude of error for each estimate in the central andes paleobotanical evidence suggests that the altiplano puna had attained no more than a third of its modern elevation of 3700 m by 20 ma and no more than half its modern elevation by 10 7 ma these data imply surface uplift on the order of 2300 3400 m since the late miocene at uplift rates of 0 2 0 3 mm yr paleobotanical and geomorphological data suggest a similar uplift history for the eastern cordillera namely no more than half the modern elevation present by 10 ma no evidence exists for an exponential increase in uplift rate as has been interpreted from fissiontrack data these uplift rates mostly reflect mean surface uplift rather than rock uplift that is uplift of material points because little dissection of the western eastern cordillera has occurred south of lat 19 s and of the altiplano puna thus the central andean plateau appears to be young in the colombian andes paleobotanical data imply rapid uplift of the eastern cordillera between 2 and 5 ma at rates on the order of 0 6 3 mm yr however some of this uplift is likely rock uplift due to erosion driven isostatic rebound rather than mean surface uplift

climate tectonics and the morphology of the andes large scale topographic analyses show that hemisphere scale climate variations are a first order control on the morphology of the andes zonal atmospheric circulation in the southern hemisphere creates strong latitudinal precipitation gradients that when incorporated in a generalized index of erosion intensity predict strong gradients in erosion rates both along and across the andes cross range asymmetry width hypsometry and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial glacial and tectonic processes and show that major morphologic features correlate with climatic regimes latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential indicating that like tectonics nonuniform erosion due to large scale climate patterns is a first order control on the topographic evolution of the andes

rbsr and 40ar39ar laser probe dating of high pressure phengites from the sesia zone western alps underscoring of excess argon and new age constraints on the high pressure metamorphism abstract the combined use of the rb sr and 40ar 39ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the sesia zone western alps despite systematic 40ar 39ar plateau ages 40ar 39ar phengite ages from two different areas show integrated age spreads between 65 4 0 3 ma and 109 5 0 4 ma the youngest 40ar 39ar plateau age 65 9 0 4 ma is concordant with two rb sr phengite whole rock isochron ages from the same outcrop which display a mean age at 64 2 2 5 ma and older than a rb sr biotite whole rock isochron age at 53 0 1 0 ma this age concordance being incompatible with the concept of isotopic closure temperature is probably fortuitous and probably results from an excess argon contamination however the concordance of the rb sr phengite isotopic closure temperature with the temperature reached during the eclogite facies event suggests that the age at 64 2 2 5 ma could be a crystallization age probably contemporaneous with the eclogite facies event in the sesia zone when compared to recent age data from the piemont zone and the dora maira massif the entire data set which needs to be confirmed by extra measurements support diachronous high pressure hp events in the western alps

late cenozoic geomorphologic signal of andean forearc deformation and tilting associated with the uplift and climate changes of the southern atacama desert 26 s 28 s abstract we analyze remarkable examples of the large 10 000 km2 and local scale 100 km2 landscape forms related to late cenozoic geomorphologic evolution of the andean forearc region in the southern atacama desert we also consider the continental sedimentary deposits so called atacama gravels which are related to the degradation of the landscape during the neogene our analysis integrates 1 50 000 field cartography landsat tm images observations 1 1000 sedimentary logging data and 50 m horizontal resolution topographic data to reconstruct the late cenozoic geomorphologic evolution of this region and discuss the factors that control it i e miocene aridification of the climate and neogene central andean uplift we determine that the precordillera was already formed in the oligocene and most of the present day altitude of the precordillera was reached before that time afterward five episodes of geomorphologic evolution can be differentiated 1 the development of an oligocene deep incised drainage system cutting the uplifted precordillera up to 2000 m of vertical incision and connecting it to the ocean followed by 2 the infilling of deep incised valleys by up to 400 m of atacama gravels this infill started in the early miocene with the development of fluvial deposition and finished in the middle miocene with playa and playa lake depositions we propose that playa related deposition occurs in an endorheic context related to tectonic activity of the atacama fault system and coastal cordillera uplift however the upward sedimentologic variation in the atacama gravels evidences a progressive aridification of the climate subsequently we have identified the effects of the middle upper miocene slow tectonic deformation the neogene andean uplift is accommodated by a tilting or flexuring of the inner forearc central depression and precordillera related to some hundreds of meters of uplift in the precordillera this tilting or flexuring results in 3 the middle miocene re opening of the valley network to the pacific ocean upper miocene aridification from arid to hyperarid induces alluvial fans backfilling in the central depression 4 resulting in up to 50 m of atacama gravel deposition finally in response to an increase in the rate of tilting a new phase of vertical incision up to 800 m in the precordillera allows the development of the canyon that crosses the forearc 5

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

style and history of andean deformation puna plateau northwestern argentina topographically the puna plateau of northwestern argentina is the southern continuation of the bolivian altiplano its thickening and consecutive uplift result from the andean orogeny to better constrain the structural style and its progressive development we have studied field data topographic and satellite imagery balanced cross sections seismic reflection data kinematic analysis of fault slip data anisotropy of magnetic susceptibility ams paleomagnetic data and apatite fission track aft data across the puna plateau precambrian and paleozoic basement ranges bounded by high angle reverse faults dips 60 alternate with cenozoic intermontane basins major thrusts trend nne ssw and do not show a preferred vergence intermontane basins have various degrees of symmetry depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets there is a close correlation between the surface expression of a basin and the amount of internal deformation a line balanced cross section of the puna at 25 s has yielded a cenozoic shortening of 10 15 in a direction subperpendicular to the orogen by kinematic analysis of cenozoic fault slip data we have obtained principal directions of strain rate across the puna shortening axes are subhorizontal and trend on average wnw ese n110 stretching axes are subvertical and intermediate axes are subhorizontal and trend on average nne ssw strain ellipsoids are dominantly of plane strain type and they represent dip slip thrusting from paleomagnetic and ams data shortening axes form a radial pattern around the eastern edge of the central andes the pattern is attributed to an inhomogeneous stress field reflecting the eastward convex shape of the central andean thrust front from the history of burial and uplift andean shortening reached the northeastern part of the puna in the late eocene and the adjacent eastern cordillera in the late eocene or early oligocene this shortening was presumably due to the incaic phase of the andean orogeny in the eastern part of the orogen the onset of shortening was probably guided by preexisting paleozoic and mesozoic structures so that andean deformation propagated unevenly eastward

geomorphologic evidence for the late pliocene onset of hyperaridity in the atacama desert the atacama desert has experienced a long and protracted period of hyperaridity that has resulted in what may be the most unusual biome on earth but the duration of this aridity is poorly constrained we reconstructed aspects of the fluvial and geochemical history of this region using integrated landscape features alluvial fans hillslope soils soil chemistry river profiles in the southern portion of the present desert topographic reconstructions of a large watershed 11 000 km 2 show deep incision and sediment removal between the late miocene and the end of the pliocene and modest to negligible incision in post pliocene times these changes in incision suggest an 50 280 reduction in river discharge which should reflect corresponding changes in precipitation changes in the nature of hillslope soils in the atacama desert indicate that in the pliocene or earlier hillslopes were mantled with silicate derived soil this mantle was stripped off and locally deposited as alluvial fans late pliocene to early pleistocene that now block or otherwise cause a rearrangement of pliocene and earlier river channels finally the hillslopes have largely accreted a soil mantle of dust and salt since the apparent late pliocene stripping suggesting a decline in annual precipitation of at least 125 mm yr 1 or more mean annual precipitation map is now 1 embedded in the long post pliocene era of salt accumulation there are a variety of features suggesting overland flow on hillslopes rills striped gravel deposits piping and water spouts and large infrequent storms that infiltrated gentle alluvial fans due to the depth of salt rich horizons despite evidence for episodes that punctuate the hyperaridity the magnitude and duration of these pluvial events have been insufficient to remove the regional accumulations of sulfate chloride and nitrate the late pliocene cessation of many fluvial features is coincident with recent research on the tropical pacific which shows that the pacific was in a permanent el nino state until ca 2 2 ma at which time sea surface temperatures offshore of south america declined greatly relative to those of the western pacific in turn setting up the present el nino southern oscillation enso climate system these observations indicate that the latest period of aridity has been prolonged and largely continuous and it appears to have occurred in step with the onset of the enso climate system beginning 2 m y ago

salar de atacama basin a record of compressional tectonics in the central andes since the mid cretaceous the salar de atacama basin lies in the inner fore arc of northern chile topographically and structurally it is a first order feature of the central andes the sedimentary fill of the basin constrains the timing and extent of crustal deformation since the mid cretaceous we have studied good exposures along the western edge of the basin and have correlated them with seismic reflection sections and data from an exploration well throughout most of its history the basin developed in a foreland setting during periods of thin skinned and thick skinned thrusting growth strata provide evidence for coeval sedimentation and thrust motions during mid cretaceous paleogene and neogene times pre neogene deformation was significant in the basin and in surounding areas of the early central andes models that attempt to explain the current thickness of the central andes should consider late cretaceous and paleogene shortening as well as the more obvious neogene and quaternary shortening

high level surfaces plateau uplift and foreland development bolivian central andes newly dated tertiary strata in the bolivian central andean plateau and synthesis of the tertiary record in the adjacent subandean fold thrust belt constrain the age of deformation in both regions age of deformation within the plateau is determined by dated crosscutting relations associated with a regionally extensive high level surface known as the san juan del oro surface new 40 ar 39 ar dates on undeformed strata above the high level surface preclude significant upper crustal shortening within the eastern cordillera after 10 ma tertiary strata within the adjacent subandean region demonstrate that formation of the fold thrust belt occurred after 10 ma on the basis of these data we propose a two stage model of late cenozoic andean growth that links plateau uplift to the development of the fold thrust belt in the first stage early plateau uplift occurred in response to widespread compressional deformation of the plateau eastern cordillera and altiplano during the second stage beginning after 10 ma upper crustal deformation within the plateau terminated and the subandean fold thrust belt developed crustal scale eastward thrusting along the eastern margin of the eastern cordillera drove subandean folding and thrusting the eastern cordillera served as the bulldozer for the deforming subandean wedge

andean uplift and climate evolution in the southern atacama desert deduced from geomorphology and supergene alunite group minerals abstract supergene alunite group minerals from the late eocene el salvador porphyry cu district the el hueso epithermal gold deposit and the coya porphyry au prospect located in the precordillera of northern chile 26 to 26 30 lat s have been dated by the 40 ar 39 ar method and analyzed for stable isotopes these data support published geomorphologic and sedimentologic evidence suggesting that the precordillera in the southern atacama desert had been uplifted as early as the late eocene and thus significantly prior to the altiplano which attained its high elevation only in the late miocene the oldest supergene alunite from the damiana exotic deposit at el salvador was dated at 35 8 1 ma and yielded a d vsmow value of 74 which indicates elevations of the precordillera near el salvador of at least 3000 m in the late eocene in contrast miocene supergene alunite from el salvador el hueso and coya have less negative d signatures reaching values as high as 23 to 25 at el hueso and el salvador between about 8 2 and 14 ma late miocene to holocene supergene alunite jarosite and natroalunite ages are restricted to el hueso and coya located near 4000 m above sea level in the precordillera roughly 1000 m higher than the present elevation of el salvador the d values of samples younger than 5 ma vary between 57 and 97 the complex evolution of the d signatures suggests that meteoric waters recorded in supergene alunite group minerals were variably affected by evaporation and provides evidence for climate desiccation and onset of hyper arid conditions in the central depression of the southern atacama desert after 15 ma which agrees well with published constraints from the atacama desert at 23 24 lat s our data also suggest that wetter climatic conditions than at present prevailed in the latest miocene and early pliocene in the precordillera the new and previously published age constraints for el salvador indicate that supergene mineralization at the damiana exotic cu deposit occurred periodically over 23 ma in a locally exceptionally stable paleohydrologic and geomorphologic configuration

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

sedimentation and preservation of the miocene atacama gravels in the pedernales chanaral area northern chile climatic or tectonic control in recent years longitudinal changes on the thin thick skinned tectonic styles of the central andes has been intensively discussed while other studies have considered the role of mass transfers on the unloading the orogen and on the stress regime along the plate interface arising from changes on the volume of sediment arriving into the peru chile trench the search for paleo climate records is therefore crucial for our understanding of the history of the central andes in this paper we focus on the atacama gravels an extensive blanket of miocene continental deposits filling a neogene paleo valley system along the southern atacama desert in northern chile an east west transect between pedernales and chanaral 26o30 s enabled us to carry out a sedimentological and tectonic study of the atacama gravels based on logging and field observations along the rio salado canyon new 39ar 40ar ages obtained on intercalated and overlying ignimbrites date the beginning of the atacama gravels sedimentation at around the oligocene miocene boundary and cessation of sedimentation in the late miocene thirteen lithofacies included within five facies associations a1 to a5 were identified depositional environments vary from proximal alluvial fan a1 a2 in the precordillera through ephemeral fluvial a3 a4 to distal playa lake a5 in the coastal cordillera no evidences of syn sedimentary deformation have been found showing that the change from sediment removal to sediment preservation cannot be explained by tectonic causes and climate change appears to be the dominant controlling factor of sediment preservation a progressive change from semi arid towards hyper arid climatic conditions during the miocene led to a reduction on the transport capacity of the fluvial system and sediment preservation along the paleovalley system formed during the oligocene

remnants of a fossil alluvial fan landscape of miocene age in the atacama desert of northern chile using cosmogenic nuclide exposure age dating we have used cosmogenic nuclides to estimate limits on the surface exposure durations and erosion rates of alluvial fans and bedrock surfaces in the atacama desert in chile the oldest landforms we studied are extensive alluvial fans referred to as atacama gravels with the exception of samples collected in antarctica the cobbles collected on these alluvial surfaces have the lowest erosion rates of any samples as determined by cosmogenic nuclides analyzed to date the oldest cobble has a model surface exposure age of 9 myr based on combined measurements of cosmogenic 10be 26al and 21ne concentrations cobbles from the alluvial fans are eroding slower than the surrounding steep mountainous bedrock surfaces maximum erosion rates for cobbles on alluvial surfaces are uniformly 0 1 m myr the survival of these gravels specifically and more generally the stability of landform features in this geographic area is made possible by the attainment of hyperarid conditions in the atacama desert resulting from global climatic cooling about 15 myr ago combined with the rain shadow effect caused by uplift of the central andes the landform features observed presently in the atacama desert are remarkably stable and despite the inevitable erosion that is detectable using cosmogenic nuclides undoubtedly bear considerable resemblance to conditions as they existed in the miocene over geologic time the atacama landscape is evolving in such a manner as to erode the higher bedrock ridges relative to the more stable but stratigraphically lower depositional surfaces through which clastic detritus now travels occasionally along the floors of incised drainage systems leaving the older permeable alluvial fan surfaces largely intact as widespread remnants of a miocene fossil landscape

origin of polygonal fractures in sand uppermost navajo and page sandstones page arizona abstract polygonal fractures in eolian sandstones occur along the upper surface of the navajo sandstone and along four additional surfaces within the page sandstone near page arizona the polygons are largely rectangular and range from 1 to 6 m across most of the fractures are less than 1 5 m deep and fracture widths range from hairlines to tens of centimeters or more fracture fill most commonly shows vertical layering and in some cases a series of layers is evident symmetrical about the midline of the fracture other fractures are filled with contorted or structureless deposits subhorizontal laminae and displaced or brecciated host strata in some cases surfaces marked by polygonal fractures can be shown to have been topographically low at the time of deposition and the fracture sandstone below these low areas contains calcite or siliceous nodules and siliceous cement interpreted as contractional in origin the polygonal fractures formed on an exposed evaporite encrusted surface in a manner similar to ice wedges irregularities in these fractures are attributed to erosion by wind or water evaporites evidenced by anhydrite inclusions pore shapes megaquartz replacement and pseudomorphs of evaporitic minerals provided the cohesion and contraction that caused the tensional stresses necessary for polygonal fracturing stresses were the result of thermal contraction such surfaces are thought to represent hiatuses in eolian sand sea development and probably represent deflation to near the water table during net deflationary conditions

paleohydrologic evolution and geochemical dynamics of cumulative supergene metal enrichment at la escondida atacama desert northern chile quantitative limonite mapping within the leached capping of the porphyry copper deposit at la escondida chile permits reconstruction of the paleohydrologic and chemical evolution of a well developed supergene ore forming system the mineralogy textures and relative abundance of supergene limonite minerals hematite goethite and jarosite are used to reconstruct the former ratio of pyrite to chalcocite and the preoxidation copper grade based on empirical limonite sulfide correlations after locke 1926 blanchard 1968 and loghry 1972 estimates of preoxidation copper grades in surface exposures and tops of drill holes at la escondida are significantly lower than actual copper grades in the underlying enrichment blanket at depth this apparent inconsistency is explained by a progressive increase in the copper grade of the sulfide enrichment blanket as it descended to its present location in response to a descending water table this systematic trend of reconstructed grades of supergene enriched copper sulfide increasing with depth offers the first quantitative proof of cumulative downward enrichment in a supergene profile as proposed by locke 1926 the consistent trends of limonite mineralogy and abundance in vertical profiles indicate that water table descent at la escondida was relatively continuous in space although not necessarily in time evidence for cumulative downward enrichment in vertical profiles through leached capping allows quantitative analysis of chemical mass balance in dynamic supergene systems slopes of linear regressions for profiles of reconstructed enriched copper grades vs depth indicate lateral fluxes into or out of a given vertical profile this method provides independent verification of conclusions from a previous study of copper mass balance at la escondida brimhall et al 1985 which showed that lateral fluxes of copper were a significant factor during supergene leaching and enrichment

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

subandean thrust and fold belt of northwestern argentina geometry and timing of the andean evolution the subandean ranges of northwestern argentina are an active thin skinned fold and thrust belt the main detachment level in silurian shales dips 2 3w and all the major east verging faults detach from it important intermediate detachment levels in the devonian shales generate lift off structures and the decoupling of the lower and upper structural levels the subandean thrust belt has a minimum shortening of about 60 km 36 at about 2240 latitude the deformation started at about 8 5 9 ma with the uplift of the el pescado range and the formation of an important back thrust at the cinco picachos range fault generation gets young to the east the pintascayo range uplift started at 7 6 ma while the baja orn range uplift began at about 6 9 ma these two ranges continued growing simultaneously until at least 4 7 ma at san antonio range fault movement began at approximately 4 4 ma and the aguarage uplift started at about 2 7 ma an important stage dominated by out of sequence movements spans from about 4 5 ma to present for both proposed models of shortening the quaternary rates of shortening between 8 and 11 mm yr coincide well with global positioning system results from the area the hydrocarbon generation and migration is contemporaneous with the deformation enhancing the possibilities of hydrocarbon entrapment in the area

petrogenesis of ca 1 95 ga meta leucogranites from the jining complex in the khondalite belt north china craton water fluxed melting of metasedimentary rocks abstract anatexis occurs in response to changing p t fluid conditions during orogenesis is thus crucial for understanding the tectonic evolution of orogenic belts here we report an integrated petrological geochemical whole rock oxygen isotopic and zircon u pb hf o isotopic study of meta leucogranites from the jining complex north china craton to unravel its petrogenesis and constrain the tectonic evolution of the paleoproterozoic khondalite belt the meta leucogranites are weakly peraluminous with homogeneous minimum melt compositions and are characterized by low rb contents and rb sr ratios high ba and sr contents and sr y ratios and moderately fractionated ree patterns without obvious eu anomalies these trace element characteristics differ from that of leucogranites derived from fluid absent melting considering their low whole rock zr saturation temperatures 746 780 c and comparable compositions with experimentally generated melts produced by water fluxed melting we infer that the meta leucogranites are low temperature granites and were generated by water fluxed melting of metasedimentary rocks at 6 8 kbar sims zircon u pb analyses suggest that the meta leucogranites were emplaced at 1 95 ga or slightly earlier and were subsequently metamorphosed and cut by a pegmatite dike at 1 92 ga the abundant inherited zircons yield nearly concordant 207 pb 206 pb ages ranging from 2 24 to 1 98 ga which are consistent with the detrital zircon age spectra of the metasedimentary rocks from the khondalite belt the hf isotopic compositions of inherited and magmatic zircons e hf t 0 2 to 7 7 closely overlap with the time evolved e hf range of detrital zircons from metasediments in the khondalite belt different types of zircons inherited magmatic and metamorphic zircons have similar 18 o values of 7 2 9 2 which is lower than that of metamorphic zircons 10 0 13 6 from surrounding metasediments thus we propose that the meta leucogranites were formed by water fluxed melting of metasedimentary rocks from the khondalite belt at 1 95 ga during the prograde upper amphibolite facies metamorphism

displacement history of the atacama fault system 25 00 s 27 00 s northern chile in the cordillera de la costa of the central andes in northern chile mesozoic arc complexes are cut by a trench parallel strike slip fault system the atacama fault system brittle faulting in the atacama fault system is superposed on steeply dipping foliations in ductile shear belts between 25 s and 27 s the western part of the fault system was active in early cretaceous time as an upper amphibolite facies down to the east dip slip ductile shear zone in the eastern part of the fault system ductile deformation is of similar early cretaceous age but occurred under lower grade metamorphic conditions at the greenschist lower amphibolite facies transition the mylonites in the eastern part of the fault system were formed by sinistral strike slip displacement the dip slip and sinistral strike slip displacements are contemporary with the development of a magmatic arc and they imply that the tectonic environment in this part of the arc was transtensional the ductile deformation was partitioned spatially into a dip slip component associated with the emplacement of magmas and a sinistral strike slip component brittle fault zones in the el salado segment of the atacama fault system define large scale sidewall ripout structures subhorizontai slickenlines ripout asymmetry and s c type fabrics in fault gouge indicate that brittle deformation involved sinistral strike slip displacements the transition from ductile to brittle sinistral strike slip displacements may have occurred due to cooling in mid cretaceous time when the magmatic arc was abandoned

global miocene tectonics and the modern world abstract an amazing congruence of seemingly unrelated diverse global events began in the middle and upper miocene and established our modern world two global orogenic belts were active mostly in the middle and upper miocene while backarc basins formed along the eastern margin of asia coincident with these events global temperatures cooled in both the ocean and atmosphere desertification occurred from central asia into and across most of northern africa and also in australia and in southern south america coincident with the expansion of the antarctic ice cap at 14 ma there was initial widespread deep sea erosion and changes in patterns of deep sea sedimentation muddy pelagic sedimentation increased six fold in the north and central atlantic and pacific oceans and global changes in circulation lead to more diatomites in the pacific and fewer in the atlantic by the end of the miocene most of the mediterranean sea had evaporated broadly coincident with these events many old large river systems were destroyed and new ones formed as much of the world s landscape changed collectively these global on shore tectonic and ocean atmospheric events provide the foundation for our modern world a mixture of new and rejuvenated orogenic belts and their far field effects distant epiorogenic uplift rain shadow deserts large alluvial aprons and distant deltas as inherited gondwanan landscapes persisted remote from plate boundaries thus at the end of the miocene much of the world s landscape except for that changed by pleistocene continental glaciation would be recognizable to us today we argue that all of these events had the same ultimate common cause an internal earth engine that drove plate motions in two broad ways first the opening and closing of seven key gateways to deep water oceanic currents radically altered global heat transfer and changed a lingering greenhouse to an icehouse world secondly these events were in part coincident with renewed heat flow in the african and pacific superplumes that energized global plate motions in the middle and upper miocene we hope this global synthesis will stimulate more research on the many global events of the miocene to understand better both our modern world and earlier global orogenies

neogene climate change and uplift in the atacama desert chile the relationship between andean uplift and extreme desiccation of the west coast of south america is important for understanding the interplay between climate and tectonics in the central andes yet it is poorly understood here we use soil morphological char acteristics salt chemistry and mass independent fractionation anomalies 17 o values in dated paleosols to reconstruct a middle miocene climatic transition from semiaridity to extreme hyperaridity in the atacama desert paleosols along the southeastern margin of the calama basin change from calcic vertisols with root traces slickensides and gleyed horizons to an extremely mature salic gypsisol with pedogenic nitrate we interpret this transition which occurred between 19 and 13 ma to represent a change in precipitation from 200 mm yr to 20 mm yr this drastic reduction in precipitation likely resulted from uplift of the central andes to elevations 2 km the uplift blocked moisture from the south american summer monsoon from entering the atacama the mid miocene gyp sisol with pedogenic nitrate is located at elevations between 2900 and 3400 m in the cal ama basin significantly higher than modern nitrate soils which occur below 2500 m modern and quaternary soils in this elevation zone contain soil carbonate and lack ped ogenic gypsum and nitrate we infer that 900 m of local surface uplift over the past 10 m y displaced these nitrate paleosols relative to modern nitrate soils and caused a return to wetter conditions in the calama basin by decreasing local air temperatures and creating an orographic barrier to pacific air masses

from alluvial fan to playa an upper jurassic ephemeral fluvial system neuquen basin argentina abstract the upper jurassic fill of the back arc neuquen basin includes a lowstand wedge known as the tordillo formation the studied deposits crop out along a n s oriented belt that runs parallel to the andean magmatic arc they are limited to the south by the east west oriented positive structure of the huincul arch formed as a result of upper jurassic tectonic inversion the tordillo deposits were formed in an arid fluvial dominated system characterised by systematic downstream changes in architectural style a gravelly and sandy bedload fluvial system is recognised in the southern upstream sector the reduced thickness and the coarse grain size suggest steep gradients excess of bedload supply and a low subsidence rate thicker and finer grained deposits prevail to the north and northeast they were formed under arid conditions in a wadi sand flat playa fluvial system this distal facies association indicates increased accommodation owing to high rates of subsidence relative to coarse siliciclastic sedimentation rates these low gradient deposits are characterised by cyclic alternations of mud dominated and sand dominated packages interpreted as high and low accommodation systems tracts the overall fining upward stacking pattern of the tordillo formation suggests a change towards higher accommodation rates this is accompanied by frequent development of soil horizons and darker primary and reworked pyroclastic deposits these attributes indicate a stronger explosive volcanic activity associated with increased precipitation and high water table emplacement towards the end of the tordillo lowstand wedge

massive middle miocene gypsic paleosols in the atacama desert and the formation of the central andean rain shadow abstract the central andes of south america are one of the highest and broadest mountain ranges on earth and have a profound influence on climate in the southern hemisphere yet the timing of andean uplift and its subsequent impact on atmospheric circulation is not well known here we use fossil soils from along an 800 km transect 19 25 s in the atacama desert of northern chile to identify the initiation of extreme aridity along the west coast of south america and infer a minimum age for the development of the central andean rain shadow paleosols record the onset of extreme aridity at the boundary between the middle and late miocene 12 10 ma lower miocene 24 20 ma soils are chemically weathered and contain soil carbonate indicative of a vegetated landscape and semi arid climate in contrast gypsum dominates massive 2 5 m thick upper middle miocene soils that formed on a hyperarid landscape devoid of vegetation the development of extreme hyperaridity is linked to the formation of the central andean rain shadow through the divergence of oxygen isotope values oxygen isotope values of lower miocene soil carbonates in the atacama desert 5 0 1 1 vpdb and central andes 7 8 1 are similar indicating no or a limited rain shadow by 15 mya however 18 o values of soil carbonate differ by 8 with values of soil carbonate in altiplano paleosols getting lower due to uplift while soil carbonate values in the atacama desert get higher due to aridity therefore by 15 mya there is clear evidence for a rain shadow from the central andes by the latest middle miocene ca 12 10 ma paleosols in the altiplano decreased to 11 8 1 0 while soils in the atacama desert increase to 1 5 4 5 this almost 14 divergence in the 18 o values of soil carbonate for soils only 250 km apart is evidence of a strong central andes rain shadow effect in place during the late middle miocene 12 10 ma

the upper jurassic lower cretaceous alluvial fan deposits of the kalaza formation central asia tectonic pulse or increased aridity the topographical evolution of tectonic systems as well as the sedimentation pattern and depositional environments in the associated basins are controlled by both tectonics and climate in the region of the tien shan central asia the jurassic lower cretaceous period was marked by complex low intensity tectonic deformation and major climate changes from humid to arid conditions jurassic to semi arid conditions cretaceous using the sediment record in the junggar tarim and fergana basins to describe the tectonic evolution of the tien shan area during the mesozoic thus requires differentiation between the tectonic and climatic influences on sedimentation the conglomerates of the upper jurassic lower cretaceous kalaza formation were commonly associated with renewed tectonic activity resulting from the docking of the lhasa block along the southern margin of asia from sedimentology and sequence stratigraphy analyses of several sections in the junggar tarim and fergana basins we reassess the main factors controlling the deposition of this formation we show that while some tectonic activity persisted throughout the jurassic cretaceous transition the switch from the sandy deposits of the upper jurassic qigu formation to the coarse deposits of the kalaza formation is largely linked to the development of an arid climate

mesozoic magmatic and tectonic events within the andean plate boundary zone 26 27 30 s north chile constraints from 40 ar 39 ar mineral ages in north chile jurassic cretaceous arc magmatism is represented by several narrow belts of plutons and two separate volcanic sequences new 40 ar 39 ar mineral ages confirm five distinct episodes of magma emplacement as plutonic complexes c 202 188 ma c 160 153 ma c 142 138 ma c 130 127 ma and c 106 103 ma conjugate dike sets were emplaced immediately before periods of plutonism and each distinct episode of magmatism prior to c 127 ma was located successively eastward inboard from the subduction boundary features interpreted to result from regional extension within the magmatic arc steeply dipping ductile mylonitic shear zones were developed in wall rocks along western contacts of late jurassic and early cretaceous plutons steeply plunging mineral stretching lineations and a variety of kinematic indicators record an east side down extension along eastern faults of the atacama fault system steeply dipping ductile mylonitic shear zones display subhorizontal mineral stretching lin

quantitative model for magma degassing and ground deformation bradyseism at campi flegrei italy implications for future eruptions campi flegrei phlegrean fields is an active volcanic center near naples italy numerous eruptions have occurred here during the quaternary and repeated episodes of slow vertical ground movement bradyseism have been documented since roman times here we present a quantitative model that relates deformation episodes to magma degassing and fracturing at the brittle ductile transition in a magmatic hydrothermal environment the model is consistent with fi eld and laboratory observations and predicts that uplift between 1982 and 1984 was associated with crystallization of 0 83 km 3 of h 2 o saturated magma at 6 km depth during crystallization 6 2 10 10 kg of h 2 o and 7 5 10 8 kg of co 2 exsolved from the magma and generated 7 10 15 j of mechanical p v energy to drive the observed uplift for comparison 10 17 j of thermal energy was released during the 18 may 1980 lateral blast at mount st helens

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

evolution of a plio pleistocene volcanogenic alluvial fan the puye formation jemez mountains new mexico the plio pleistocene puye formation north central new mexico contains 15 km 3 of volcaniclastic alluvial sediments deposited in response to rift margin volcanism associated with the later evolution of the tschicoma volcanic center northeastern jemez mountains stratigraphic and sedimentologic evidence indicates that aggradation of puye deposits was nearly continuous and contemporaneous with graben development and establishment of the rio grande as the through flowing axial drainage system in this region much of the puye sediments debouched into an adjacent subsiding depocenter resulting in near complete preservation of this wedge shaped volcanogenic fan primary pyroclastic and reworked facies in the puye formation exhibit distinctive 5 to 30 m scale cyclicity directly related to volcanic activity in the northeastern jemez highlands reworked pyroclastic and conglomeratic sequences deposited by high energy braided streams sediment charged sheet floods sediment gravity flows and in shallow lakes reflect markedly increased sediment loads during and following explosive eruptions individual sequences change markedly in character with increased distance from the source area analysis of these abrupt lateral and vertical variations reveals that puye sedimentation was controlled by the influences of semiarid climate and syndepositional tectonism although the associations that most characterize the fan resulted largely from explosive volcanism cessation of fan growth is attributed to waning sediment supply as a result of volcanic quiescence and to the onset of basinwide pedimentation associated with downcutting of the ancestral rio grande the puye fan therefore provides a detailed record of rift basin alluvial sedimentation complete with closely spaced tephrostratigraphic control and it records the growth and partial denudation of high standing northeastern jemez mountains volcanoes

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

the relationships between tectonics climate and exhumation in the central andes 18 36 s evidence from low temperature thermochronology abstract the central andes between 18 s and 36 s latitude strike north south for 2000 km along the chilean subduction margin cross several climate zones from hyperarid to humid and exhibit mean elevations in excess of 4000 m a s l here we investigate the relationships between tectonics climate and exhumation by inverting low temperature thermochronological data compiled from the literature 824 ages from 549 samples and new data 238 ages from 146 samples to quantify the exhumation rate history of the central andes since 80 ma our inferred exhumation rates west of the drainage divide and between 18 and 32 s did not exceed 0 25 km ma such low exhumation rates are consistent with low shortening rates and arid conditions in this region local pulses of exhumation occurred only during the eocene as response to active deformation and during the miocene probably as response to uplift of the western andean slope east of the drainage divide between 18 and 28 s the observed exhumation pattern reflects the onset and eastward propagation of deformation here exhumation occurred locally since the middle to late eocene in the eastern cordillera and the altiplano puna and subsequently affected larger parts of these regions and the north western sierra pampeanas during the oligocene in the early miocene 20 ma the interandean zone started exhuming and at 12 10 ma exhumation propagated into the subandean zone enhanced shortening rates and intensified precipitation along the eastern deformation front associated with the onset of the south american monsoon led to increased exhumation rates in the eastern interandean and the subandean zones in the plio pleistocene 0 6 km ma higher pleistocene exhumation rates are also observed in the northern sierra pampeanas 1 5 km ma that can be related to rock uplift along steep reverse faults coupled with high precipitation south of 32 s on the western side exhumation rates in the principal cordillera increased from ca 0 25 km ma in the miocene to rates locally exceeding 2 km ma in the pleistocene whereas the tectonic regime in the southern principal cordillera remained unchanged since the late miocene these higher rates are likely associated with enhanced erosion resulting from intensified pleistocene precipitation and glacial growth in this region reinforced by isostatic rock uplift and active tectonics our study shows that the onset of exhumation correlates mainly with the initiation of horizontal shortening and crustal thickening whereas the magnitude of exhumation is largely set by the amount of precipitation and glacial erosion and by the style of deformation which is controlled by inherited structures and the amount of sediments in the foreland

timing and nature of alluvial fan development along the chajnantor plateau northern chile abstract alluvial systems in the atacama desert provide a unique opportunity to elucidate the sedimentary response to climate variability particularly changes in precipitation in hyperarid environments alluvial fans along the eastern margin of the salar de atacama adjacent to the chajnantor plateau in the atacama desert of northern chile provide an archive of climate modulated sediment transfer and erosion at an extreme of earth s climate three regional alluvial fan surfaces qf1 oldest to qf3 youngest were mapped along the western flank of the chajnantor plateau the alluvial fans were examined with geomorphic and terrestrial cosmogenic 36 cl surface exposure dating methods to define the timing of alluvial fan formation and to determine the role of climatic processes on fan development in a hyperarid environment alluvial fans in the study area are comprised of hyperconcentrated flow and boulder rich debris flow deposits that reflect deposition transitioning between cohesive and noncohesive regimes alluvial fan surfaces yield exposure ages that range from 49 6 4 4 to 194 12 ka while debris flow boulders yield exposure ages ranging from 12 4 2 1 to 229 53 ka cosmogenic 36 cl exposure ages indicate that abandonment of alluvial fan surfaces qf1 qf2 and qf3 date to 175 22 6 ka mis 6 134 5 9 18 ka mis 6 and 20 07 6 26 ka mis 2 respectively a 36 cl concentration depth profile through alluvial fan qf1 suggests a simple depositional history with minimal nuclide inheritance implying relatively rapid aggradation 6 m in ca 25 kyr followed by surface abandonment ca 180 200 ka our data support a strong climatic control on alluvial fan evolution in the region and we propose that the alluvial fans along the margins of the salar de atacama form according to the humid model of fan formation

k ar and 40ar 39ar geochronology of supergene processes in the atacama desert northern chile tectonic and climatic relations new k ar and 40 ar 39 ar data obtained for 29 samples of supergene alunite jarosite and hollandite group minerals from the central depression of northern chile document almost continuous supergene oxidation from 33 to 9 ma the downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16 35 m ma 1 these calculated rates are sensitive to pre existing topography and show that the present relief observed in the central depression represents a modified miocene landscape the results presented here are in good agreement with previously published geochronological data for the atacama desert the combination of all published data including the results presented here defines a continuous skewed distribution with a maximum between 21 and 14 ma the relative paucity of ages greater than 30 ma reflects the removal of supergene profiles by erosion during and immediately after the incaic compressive deformation from 29 to 9 ma supergene oxidation profiles were generated under semi arid conditions throughout the atacama region ages between 9 and 5 ma on the northern and southern margins of the desert mark the end of supergene oxidation processes consistent with a pliocene onset of present day hyperaridity

an 40ar 39ar geochronological and electron microprobe investigation of an archean pyroxenite and its bearing on ancient atmospheric compositions a detailed 40ar 39ar step heating geochronological study of pyroxene mineral separates from the munro warden ultramafic sill in the archean abitibi greenstone belt of the canadian superior province yields a precise primary age of 2703 11 ma 1 this is in excellent agreement with previous u pb zircon dates of 2700 2712 ma for the enclosing felsic volcanics and a sm nd age of 2622 60 ma for the sill electron microprobe analyses coupled with the argon evolution patterns demonstrate that deuteric amphibole is responsible for this primary age whereas the volumetrically dominant diopside shows anomalously young ages an isotope correlation plot reveals two components of contaminating argon with differing 40ar 36ar ratios in distinctly different sites one component trapped during the deuteric formation of amphibole has an 40ar 36ar ratio that is significantly less 258 3 than the present day atmospheric value of 295 5 this may be an indication of the atmospheric ratio 2 7 ga ago the other comp

tectonic and magmatic evolution of the andes of northern argentina and chile abstract two orogenic cycles both with different evolution are developed in the western margin of the south american continent in northern argentina and chile the paleozoic hercynic cycle and the meso cenozoic andean cycle the hercynic cycle a wide marine basin extending westward of the cordillera oriental which developed in cambrian ordovician times marks the beginning of this cycle in contrast to peru and bolivia where this basin developed between two precambrian blocks the western margin of this basin in northern argentina and chile is still unknown the ordovician sedimentation and accompanied volcanism ends with the ocloyic deformation phase and its synkinematic granitic plutonism two basins were developed in the silurian devonian separated by the puna arc uplifted during this ocloyic phase the shallow water marine terrigeneous sediments which were deposited in them were deformed by a new tectono magmatic associated phase chanic phase carboniferous to lower permian marine carbonates were deposited west of the puna arc and red beds east of it later on during the permian to triassic a magmatic belt was developed along the cordillera occidental the rhyolites ignimbrites and the granitic to granodioritic related intrusives are well represented in chile although the overall geologic history of this period is known many problems concerning its origin and relations to plate tectonics are still unsolved the andean cycle during this cycle a series of magmatic arc systems related to the subduction of the pacific crust was built up along the western margin of south america huge volumes of calc alkaline lavas and related plutons were emplaced since the jurassic along belts parallel to the present coastline showing a general eastward migration trend up to the lower cretaceous an ensialic back arc basin was formed east of this magmatic arc thousands of meters of marine and continental sediments were deposited in it this basin disappeared during the middle cretaceous probably as a result of the final opening of the atlantic and the active westward movement of the south american plate since middle cretaceous times the magmatic arc has been the fundamental paleogeographic element the arc progressively migrated stepwise eastward each step marked by a tectonic phase subduction related crustal erosion could explain the lack of fore arc petrotectonic assemblages the eastward magmatic polarity which characterizes this section of the andes could also be explained by such a mechanism

mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers skeleton coast erg namibia channel shaped deposits of well sorted sand with only 2 4 fine grained material being either massive and structureless or upward fining with basal lag are found interbedded with aeolian sand in the skeleton coast erg namibia detailed analyses of the channel shaped deposits suggest that they are formed as hyperconcentrated flows within the erg grain size analysis and whole rock geochemical modeling revealed that some of the fluvial sediments contain up to 70 aeolian sand interpreted as a result of dune collapse into the fluvial system in certain cases this instantaneous supply of sand resulted in generation of sandy mass flows with laminar flow behaviour the presence of smectite as dominant clay mineral proved to be of crucial importance in formation of mass flow deposits these mass flows had no erosional capacity and drape the palaeotopography they are comparable to those generated by catastrophic collapse of dunes described in the literature this paper suggests that all these deposits should be termed intra erg mass flows as several of them carry little if any debris based on their origin intra erg mass flows can be divided into two groups

a genetic model based on evapoconcentration for sediment hosted exotic cu mineralization in arid environments the case of the el tesoro central copper deposit atacama desert chile although the formation of exotic cu deposits is controlled by multiple factors the role of the sedimentary environment has not been well defined we present a case study of the el tesoro central exotic cu deposit located in the atacama desert of northern chile this deposit consists of two mineralized bodies hosted within late cenozoic gravels deposited in an arid continental environment dominated by alluvial fans with sub surficial ponded water bodies formed at the foot of these fans or within the interfan areas both exotic cu orebodies mostly consist of chrysocolla copper wad atacamite paratacamite quartz opal and calcite the most commonly observed paragenesis comprises chrysocolla silica minerals and calcite and records a progressive increase in ph which is notably influenced by evaporation the results of stable isotope analyses 13c and 18o and hydrogeochemical simulations confirm that evapoconcentration is the main controlling factor in the exotic cu mineralization at el tesoro central this conclusion complements the traditional genetic model based on the gradual neutralization of highly oversaturated cu bearing solutions that progressively cement the gravels and underlying bedrock regardless of the depositional environment this study concludes that in exotic cu deposits formed relatively far from the source a favorable sedimentary environment and particular hydrologic and climatic conditions are essential to trap accumulate evapoconcentrate neutralize and saturate cu bearing solutions to trigger mineralization thus detailed sedimentological studies should be incorporated when devising exploration strategies in order to discover new exotic cu resources particularly if they are expected to have formed relatively far from the metal sources

coarse grained nonmarine volcaniclastic sediment terminology and depositional process modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent dilute stream flow or viscous debris flow the characteristics of these poorly sorted sediments suggest rapid deposition from high concentration dispersions but not en masse sedimentation thus appears related to high discharge flows intermediate in sediment water ratio between stream flow and debris flow the term hyperconcentrated flood flow is proposed for describing this intermediate condition hyperconcentrated flood flow deposits are distinguished from debris flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification these deposits are distinguished from normal dilute stream flow deposits by lack of cross stratification in sand facies and by very poor sorting poor imbrication and numerous clasts with long axes oriented parallel to flow direction in gravel facies the horizontal bedding that dominates sandy hyperconcentrated flood flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper flow regime and should not be confused with the more familiar thin graded laminae of fine to medium grained sand often associated with parting lineation hyperconcentrated flood flow deposits are not unique to volcanic settings they also occur in arid alluvial fan sequences debris flow and hyperconcentrated flood flow deposits however are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings in volcanic regions therefore these deposits have greater preservation potential show greater lateral variability and are more voluminous transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water recently observed at mount st helens is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow

u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the helanshan complex is located at the westernmost segment of the khondalite belt a continent continent collisional belt along which the yinshan block in the north collided with the ordos block in the south to form the western block of the north china craton the complex consists of s type granites and high grade pelitic granulite gneiss felsic paragneiss quartzite calc silicate rock and marble together called the khondalite series rocks la q icp ms u pb geochronology and la mc icp ms lu hf isotopic data of zircons combined with cathodoluminescence cl imaging enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the khondalite belt in the western block of the north china craton cl images reveal the coexistence of magmatic type detrital zircons and metamorphic zircons in most of the khondalite series rocks of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic type detrital zircon cores la q icp ms u pb analyses on magmatic type detrital zircons reveal two distinct age populations with one in the archaean 2 85 2 53 ga and the other in the palaeoproterozoic 2 2 2 0 ga suggesting that the sedimentary protoliths of the khondalite series rocks in the helanshan complex must have been deposited at some time after 2 0 ga the hf analyses show that the archaean detrital zircons possess negative ehf t values from 7 64 to 0 15 with depleted mantle model ages ranging from 3 34 to 3 10 ga this implies the existence of a paleo mesoarchaean 3 34 3 10 ga crust in the western block which underwent a recycling event in the period 2 85 2 53 ga nearly all palaeoproterozoic 2 2 2 0 ga detrital zircons possess positive ehf t values 0 86 9 38 with depleted mantle model ages of 2 45 2 15 ga suggesting a significant crustal growth event in the western block in the palaeoproterozoic metamorphic zircons yield two groups of age with one at 1 95 ga and the other at 1 87 ga of which the former is considered as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the latter is consistent with the age of 1 86 ga from two s type granites which are considered to have formed by the partial melting of pelitic granulites gneisses at the stage of the exhumation of the helanshan complex

testing the analytical protocols and calibration of volcanic glass for the reconstruction of hydrogen isotopes in paleoprecipitation in this study we investigate the foundations of the hydrated volcanic glass paleoelevation paleoenvironment proxy by testing 1 the sensitivity of volcanic glass d values to laboratory procedures 2 the reliability of young glass reconstructions of observed meteoric water values and 3 the paleoenvironmental reconstructions based on volcanic glass in comparison to those of other proxies volcanic glass d values are insensitive to variations in laboratory pretreatments displaying no signifi cant shift in d values with variations in duration of drying rinsing the temperature at which glass is dried or with the grain size that is analyzed however clay adhering to the glass particles may skew d results by 10 and hf acid pretreatment can cause signifi cant isotopic shifts parent water d values calculated from young 0 3 ma glass d values are in close accord with modern surface water d values across 3000 m of elevation in the central andes combined glass d values pedogenic carbonate 18 o values and clumped isotope paleothermometry from the 16 3 ma los cristales tuff provide a key test of resistance to resetting of d values in ancient glass hydration layers these proxies indicate hydrated volcanic glass preserves its stable isotopic information over geologic time and that the climatic patterns of the high and dry puna plateau were already in place by 16 3 ma

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

cenozoic climate change as a possible cause for the rise of the andes causal links between the rise of a large mountain range and climate have often been considered to work in one direction with significant uplift provoking climate change here we propose a mechanism by which cenozoic climate change could have caused the rise of the andes based on considerations of the force balance in the south american lithosphere we suggest that the height of and tectonics in the andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands and shear stresses in the subduction zone depend on the amount of subducted sediments we propose that the dynamics of subduction and mountain building in this region are controlled by the processes of erosion and sediment deposition and ultimately climate in central south america climate controlled sediment starvation would then cause high shear stress focusing the plate boundary stresses that support the high andes

sedimentology based reconstructions of paleoclimate changes in the central andes in response to the uplift of the andes arica region between 19 and 21 s latitude northern chile we focus on the sedimentological record of the middle miocene to modern deposits in the andes of northern chile between 19 and 21 s these sediments deposited at the western escarpment of the central depression indicate successively more moisture on the western margin of the altiplano and the western cordillera where the sources are at the pacific coast 20 ma old exposure ages and salic gypsisols reflect an existing and ongoing hyperarid climate we interpret the increased divergence of climates between the coast and the altiplano as consequence of the andean rise to elevations higher than approximately 2 500 m a s l when the topography of the altiplano was sufficiently high and areally extensive to attract atlantic moisture accordingly the inferred general increase in run off was closely coupled with the uplift of the andes if the steady rise model applies in case that the rapid rise model for andean uplift is correct the inferred changes in sediment transport would have occurred independently of uplift requiring an alternative yet unknown driver

andean uplift and climate change sedimentological data indicate that a semi arid arid climate prevailed across the central andes from 15 ma to 4 ma between 4 and 3 ma a switch to hyperaridity occurred along the western margin of south america palaeoaltitude data suggest that a substantial proto central andean mountain range was in place between 15 and 9 ma these data support the idea that the andean rain shadow existed by 15 ma and that it reinforced the pre existing climatic regime rather than changing it the change to hyperaridity in western south america is attributed to a combination of global climate cooling and enhanced upwelling of the humboldt current generated by closure of the central american seaway between 3 5 and 3 ma and not to the andean rain shadow

a geomorphological approach to determining the neogene to recent tectonic deformation in the coastal cordillera of northern chile atacama the large 10000 km2 and local scale 300 m sedimentary succession was deposited east of the afs the succession fills previously deep paleovalleys and it consists of gravel so called atacama gravels which passes laterally into fine grained playa related deposits near the afs we interpret the deposition of this succession as a result of a blocking closure of the valley flowing from the precordillera due to the activity on afs a pedimentation episode followed sediment deposition and is locally strongly re incised by the main modern day river valleys draining the precordillera incision may result from either regional uplift of the forearc and or from more localized activity on the afs furthermore recent quaternary tectonic activity on the afs has been observed which is consistent with a localized relative uplift of the crustal block west of the afs

natural controls of fluvial denudation rates in major world drainage basins we present a new compilation of estimates of modern rates of mechanical and chemical denudation for externally drained basins exceeding 5 105 km2 in area these estimates are based on sediment and solute load data selected in order to represent natural rates as far as possible chemical denudation rates have been calculated by deducting the nondenudational component of solute load mechanical denudation rates range from 1 mm kyr 1 for the st lawrence and dnepr basins to 670 mm kyr 1 for the brahmaputra basin chemical denudation rates vary from 1 mm kyr 1 kolyma niger nile and rio grande basins to 27 mm kyr 1 chiang jiang basin the kolyma basin has the lowest 4 mm kyr 1 and the brahmaputra basin the highest overall rate of denudation 688 mm kyr 1 relationships between denudation rates and a range of morphometric hydrologic and climatic variables are investigated through correlation and regression analysis morphometric variables such as mean local relief are accurately calculated for large basins for the first time by using the national geophysical data center 10 minute topographic database variables expressing basin relief characteristics and runoff are found to be most strongly associated with both mechanical and chemical denudation rates with more than 60 of the variance in total denudation being accounted for by basin relief ratio and runoff basin area runoff variability and mean temperature however are only weakly associated with rates of denudation although direct comparisons cannot be made it appears that rates of basin denudation derived from present day mass flux estimates are not overall significantly different from estimates of long term rates based on sediment volume and thermochronologic data it therefore appears that the key factors identified as controlling denudation rates here are also applicable to the geological time spans relevant to the interaction between tectonic and denudational processes

geomorphological markers of faulting and neotectonic activity along the western andean margin northern chile in the atacama desert northern chile some ephemeral channels are developed in the plio quaternary alluvial sequence that caps the neogene atacama gravels formation geomorpho logical studies and high resolution digital elevation data gps along a structural transect in the cen tral depression are used to document modern growth history of subtle folding and faulting in the fore arc region outcrop data of the most recent deposits are combined with observations of warped and faulted late quaternary pediments alluvial fans and terrace surfaces to propose unsuspected neotec tonic processes on the western flank of the domeyko cordillera neotectonic process recognition is here based largely upon the interpretation of alluvial landforms drainage organisation and evolution as the intermittent river network shows systematic patterns of course deflections successive incisions or deposition processes as it encounters the fault scarps or folds in the superficial deposits this area presents both n s trending active vertical faults in the topographically higher pampas and n s trending active folding in the lower pampas these faults seem to accommodate e w extension and compression that could be related to uplift of the western andean margin within a compressive context uplift may have taken place unevenly over the past few million years after the deposition of the superficial alluvial surfaces that cap the neogene atacama gravels copyright 2003 john wiley sons ltd

facies analysis and basin architecture of the neogene subandean synorogenic wedge southern bolivia foreland sedimentation in the subandean zone of south central bolivia spans from the upper oligocene to present it records sediment dispersal patterns in an initially distal and later proximal retroarc foreland basin and thereby contains stratigraphic information on the tectonic evolution of the adjacent andean fold thrust belt within the neogene orogenic wedge individual siliciclastic dominated depositional systems formed ahead of an eastward propagating deformation regime we defined described and interpreted eight architectural elements and 24 lithofacies from 15 outcrop locations representing the neogene foreland basin in the subandean zone and the chaco plain these are combined to interpret depositional settings the up to 7 5 km thick neogene wedge is subdivided in five stratigraphic units on the basis of facies associations and overall architecture 1 the basal oligocene miocene up to 250 m thick petaca formation consists dominantly of calcrete reworked conglomeratic pedogenic clasts and fluvial sandstone and mudstone this unit is interpreted to represent extensive pedogenesis under an arid to semiarid climate with subordinate braided fluvial processes 2 the overlying upper miocene up to 350 m thick yecua formation records numerous small scale transgressive regressive cycles of marginal marine tidal and shoreline facies of sandstone ooid limestones and varicoloured mudstone 3 the upper miocene up to 4500 m thick tariquia formation principally consists of sandstone with interbedded sandstone mudstone couplets representing frequent crevassing in anastomosing streams with an upsection increasing degree of connectedness 4 the up to 1500 m thick lower pliocene guandacay formation represents braided streams and consists principally of granule to cobble conglomerate interbedded with sandstone and sandy mudstone 5 the upper pliocene up to 2000 m thick emborozu formation consists predominantly of alluvial fandeposited cobble to boulder conglomerate interbedded with sandstone and sandy mudstone the coarsening and thickening upward pattern and eastward progradation coupled with the variable proportions of overbank facies channel size and degree of channel abandonment in the tariquia guandacay and emborozu formations reflect a distal through proximal fluvial megafan environment this long lived megafan grew by high sedimentation rates and a northeast through southeast radial paleoflow pattern on large coarse grained sediment lobes the marked overall

development of a continental forearc a cenozoic example from the central andes northern chile in order to understand the response of a continental forearc to changes in subduction zone geodynamics we constructed a high resolution chronostratigraphic cross section across the central andean forearc of northern chile 21 24 s the tectono stratigraphic development of the forearc differs from established models no relationship was found between changes in rate of relative plate convergence and amount and style of deformation forearc response to continual compression since the oligocene has been uplift and segmentation into discrete tectono stratigraphic zones from west to east these zones are the extensional coastal cordillera the extensional and or transtensional central depression and the transpressional and or compressional precordillera preandean depression each area has recorded almost continuous sedimentation from oligocene eocene time to the present day accommodation space has been generated by basin margin uplift rather than active subsidence we propose a model in which uplift of the leading edge of the south american plate is driven by subcrustal accretion of material removed at the trench by subduction erosion uplift and subduction erosion result in the trenchward gravitational collapse of the plate edge the tectono stratigraphic complexity exhibited within the central andean forearc is likely to be representative of cordilleran type margins and would be difficult to recognize in an ancient continental forearc

magmatic arc fault systems deformation partitioning and emplacement of granitic complexes in the coastal cordillera north chilean andes 25 30 s to 27 00 s three fault systems were responsible for permian to late cretaceous deformation of the overriding plate of the andean convergent margin in the coastal cordillera of northern chile 25 30 to 27 00 s displacements were linked to crustal growth expressed by the emplacement of a sequence of magmatic arcs the tigrillo fault system active from triassic to early cretaceous time was characterized by arc normal extension with increasing left oblique extension transtension from early jurassic to early cretaceous time stretching of the crust created space for triassic early jurassic and early cretaceous arc basins where epiclastic volcaniclastic and volcanic sequences accumulated in continental to shallow marine environments tabular plutonic complexes were emplaced by roof uplift floor subsidence that allowed a vertical transfer of material in the crust without significant horizontal extension the atacama fault system was initiated at c 132 ma as a mainly left strike slip fault during left oblique extension of the margin elongate tabular plutonic complexes were emplaced within the atacama fault system between c 132 and c 106 ma again by roof uplift floor subsidence mechanisms ductile brittle transitions in synplutonic mylonitic rocks of the atacama fault system provided the setting for kiruna type fe apatite and fe oxide with cu and or au ores the chivato fault system was active as an extensional fault system at the eastern side of the coastal cordillera during displacement on the tigrillo fault system and later between c 125 and c 93 ma as a partitioned left oblique extensional fault system in post early cretaceous time the chivato fault system was inverted by left oblique contraction transpression when nw trending transfer faults some probably reactivated lateral ramps in the tigrillo fault system accommodated clockwise vertical axis rotations of 35 45 contraction inverted the atacama fault system and tigrillo fault system and was responsible for west vergent thin skinned fold thrust deformation in stratified rocks throughout the margin

middle miocene climatic change in the atacama desert northern chile evidence from supergene mineralization at la escondida geochronology and paleotopographic reconstruction of the porphyry copper deposit at la escondida chile are used to calculate long term erosion rates and to deduce the timing of tertiary climatic change for a portion of the atacama desert region hypogene hydrothermal alteration and protore mineralization at la escondida took place between 33 7 1 4 and 31 0 1 4 ma based on k ar dating of hydrothermal biotite and sericite supergene weathering and copper sulfide en richment processes were active from 18 0 0 7 to 14 7 0 6 ma based on k ar dating of supergene alunite distinguished from hypogene alunite by grain size color and sulfur isotopic composition reworked lenses of volcanic ash in the vicinity of la escondida provide useful time stratigraphic markers at 8 7 0 4 6 5 0 2 and 4 2 0 2 ma within present soil profiles long term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at la escondida using mass balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid inclusion geobarometry at similar deposits the observed trend of decreasing long term average erosion rates with time is consistent with arid to semiarid conditions in the early miocene changing to hyperarid conditions during the middle miocene this climatic desiccation caused termination of significant supergene copper suffide enrichment at la escondida and elsewhere in the atacama region and preservation of surficial features including the ash horizons and the leached capping middle miocene climatic desiccation in northern chile and southern peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral humboldt current and an increase in upwelling intensity as the antarctic ice cap became established at approximately 15 to 13 ma the central andes cordillera which now provides a rain shadow protecting the atacama region from precipitation from the east must have attained at least half its present elevation prior to about 15 ma to have played a similar role in the middle miocene

sedimentology of a lacustrine fan delta system miocene horse camp formation nevada usa a 1600 m thick succession of the miocene horse camp formation member 2 exposed in east central nevada records predominantly terrigenous clastic deposition in subaerial and subaqueous fan delta environments and nearshore and offshore lacustrine environments these four depositional environments are distinguished by particular associations of individual facies 14 defined facies subaerial and subaqueous fan delta facies associations include ungraded matrix and clast supported conglomerate normally graded matrix and clast supported conglomerate ungraded and normally graded sandstone and massive to poorly laminated mudstone subaqueous fan delta deposits typically have dewatering structures distorted bedding and interbedded mudstone the subaerial fan delta environment was characterized by debris flows hyperconcentrated flows and minor sheetfloods the subaqueous fan delta environment by debris flows high and low density turbidity currents and suspension fallout the nearshore lacustrine facies association provides examples of deposits and processes rarely documented in lacustrine environments high energy oscillatory wave currents probably related to a large fetch reworked grains as large as 2 cm into horizontally stratified sand and gravel offshore directed currents produced uncommonly large typically 1 2 m thick trough cross stratified sandstone in addition stromatolitic carbonate interbedded with stratified coarse sandstone and conglomerate suggests a dynamic environment characterized by episodic terrigenous clastic deposition under high energy conditions alternating with periods of carbonate precipitation under reduced energy conditions massive and normally graded sandstone and massive to poorly laminated mudstone characterize the offshore lacustrine facies association and record deposition by turbidity currents and suspension fallout a depositional model constructed for the horse camp formation member 2 precludes the existence of all four depositional environments at any particular time rather phases characterized by deposition in subaerial fan nearshore lacustrine and offshore lacustrine environments alternated with phases of subaerial fan delta subaqueous fan delta and offshore lacustrine deposition this model suggests that high energy nearshore currents due to deep water along the lake margin reworked sediment of the fan edge thus preventing development of a subaqueous fan delta environment and promoting development of a well defined nearshore lacustrine environment low energy nearshore currents induced by shallow water along the

more than ten million years of hyper aridity recorded in the atacama gravels abstract the atacama desert s hyper aridity is closely linked to the development of world class copper and nitrate iodine ores and to regional tectonics and global paleoclimate changes in the cenozoic era the timing when the hyper aridity commenced remains controversial with proposed ages ranging from late oligocene to pleistocene in this study we provide an independent constraint on the initiation of atacama hyper aridity utilizing a 100 m deep profile within the atacama gravels and underneath porphyry copper deposit in spence northern chile the overall high concentration of sulfate up to 10 wt and a multimodal distribution of water soluble salt sulfates chlorides and nitrates indicate multiple generations of sedimentation and salt accumulation events under semi arid to hyper arid climate conditions the multiple sulfate isotope compositions 17o 18o 34s of the upper section 15 0 to 34 5 m are close to those of modern hyperarid surface sulfates while the lower section 34 5 to 65 m displays a depth dependent isotope trend that is best interpreted as marking a period of climate change from semi arid to hyper arid when these data are combined with new chronological 40ar 39ar dates obtained from a volcanic ash layer at depth of 28 0 m our results show that hyper arid condition in the atacama desert was prevailing at least prior to 9 47 ma and may go back as old as the middle miocene

lithospheric evolution of the andean fold thrust belt bolivia and the origin of the central andean plateau we combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central andean plateau between 188 and 208 s from late cretaceous to present by integrating geophysical results of upper mantle structure crustal thickness and composition with recently published structural stratigraphic and thermochronologic data we emphasize the importance of both the crust and upper mantle in the evolution of the central andean plateau four key steps in the evolution of the andean plateau are as follows 1 initiation of mountain building by 70 ma suggested by the associated foreland basin depositional history 2 eastward jump of a narrow early fold thrust belt at 40 ma through the eastward propagation of a 200 400 km long basement thrust sheet 3 continued shortening within the eastern cordillera from 40 to 15 ma which thickened the crust and mantle and established the eastern boundary of the modern central andean plateau removal of excess mantle through lithospheric delamination at the eastern cordillera altiplano boundary during the early miocene appears necessary to accommodate underthrusting of the brazilian shield replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the eastern cordillera and altiplano at 24 23 ma and further volcanism recorded by 12 7 ma crustal ignimbrites 4 after 20 ma deformation waned in the eastern cordillera and interandean zone and began to be transferred into the subandean zone long term rates of shortening in the fold thrust belt indicate that the average shortening rate has remained fairly constant 8 10 mm year through time with possible slowing 5 7 mm year in the last 15 20 myr we suggest that cenozoic deformation within the mantle lithosphere has been focused at the eastern cordillera altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination d 2005 elsevier b v all rights reserved

subaqueous sediment gravity flow deposits practical criteria for their field description and classification a new method for the description and classification of subaqueous sediment gravity flow deposits is proposed the classification scheme employs a convenient letter code and divides deposits individual beds into descriptive categories of two hierarchical levels facies and subfacies facies as the higher rank categories are distinguished chiefly on the basis of sediment type i e bed grain size texture a total of 13 facies have been distinguished g gravel gs gravel sand couplet gys gravelly sand s sand sm sand mud couplet ms mud sand couplet tm silt mud couplet mt mud silt couplet m mud mys muddy sand sym sandy mud myg muddy gravel gym gravelly mud subfacies as the lower rank categories are distinguished within the individual facies on the basis of the bed s internal structures the number of subfacies is unlimited and their labelling code includes particular facies symbols see above preceded by lower case letters denoting specific sedimentary structures and their vertical arrangement subfacies thus refer to the bed s intervals or divisions which are labelled as follows m massive unstratified and ungraded g graded unstratified and graded s plane stratified x cross stratified 1 parallel and or cross laminated q liquefied for example subfacies gsg graded to plane stratified gravel are gravel beds that have a lower graded interval and an upper plane stratified interval subfacies xg cross stratified gravel are gravel beds that are cross stratified throughout subfacies sls plane stratified to laminated sand are sand beds that have a lower plane stratified interval and an upper laminated parallel and or cross laminated interval

thrust geometry controlled by erosion and sedimentation a view from analogue models analogue sandbox models have been set up to study thrust systems formed above two decollements and affected by erosion and sedimentation the deformation was imaged by x ray scanner three erosion sedimentation conditions were tested for all the experiments a duplex develops between the two decollement levels for the experiment without erosion and without sedimentation ramp anticlines form in the upper series for the model with syntectonic sedimentation and without erosion the shortening in the upper series is first characterized by fault related folds and then by a forward propagation of the decollement that induces the development of a piggy back basin for the experiment with erosion and sedimentation a passive roof duplex is formed relationships between structural style and syntectonic superficial conditions in classical thin skinned thrust belts are similar to those found in the analogue models the ramp anticlines of the jura developed with low erosion and low sedimentation and the large sub andean thrust belt is controlled by syntectonic sedimentation whereas the passive roof duplex of pakistan is synchronized with intense erosion this study outlines the crucial effect of the superficial mass transport phenomena in the tectonic style of thrust belts

oligocene miocene age of aridity in the atacama desert revealed by exposure dating of erosion sensitive landforms the age of onset of hyperaridity in the atacama desert chile which is needed to validate geological and climatological concepts has been heretofore uncertain measurement of cosmogenic 21 ne in clasts from erosion sensitive sediment surfaces in northern chile shows that these surfaces have been barely affected by erosion since 25 ma surface exposure ages of sediment clasts give replicate values at 25 20 and 14 ma and individual values at 37 and 9 ma predominantly hyperarid conditions are required to preserve these oldest continuously exposed surfaces on earth our findings are compatible with the hypothesis that the onset of aridity in the atacama desert could be the reason for rather than the consequence of uplift of the high andes

the origin of the magnetic record in eocene miocene coarse grained sediments deposited in hyper arid arid conditions examples from the atacama desert abstract magnetic proxies for paleoclimate are tools widely used to understand climate variability yet current proxies focus on loess derived soils in humid to temperate climate zones whereas coarse grained sediments in arid hyperarid climate zones remain poorly investigated to test the potential paleoclimatic environmental significance of the magnetic record of coarse grained sediments deposited in a region with a mean annual precipitation map

a combined study of radar facies lithofacies and three dimensional architecture of an alpine alluvial fan illgraben fan switzerland alluvial fans serve as useful archives that record the history of depositional and erosional processes in mountainous regions and thus can reveal the environmental controls that influenced their development economically they play an important role as groundwater reservoirs as well as host rocks for hydrocarbons in deeply buried systems the interpretation of these archives and the evaluation of their reservoir architecture however are problematic because marked heterogeneity in the distribution of sedimentary facies makes correlation difficult this problem is compounded because the accumulated sedimentary deposits of modern unconsolidated fan systems tend to be poorly exposed and few such systems have been the focus of investigation using high resolution subsurface analytical techniques to overcome this limitation of standard outcrop analogue studies a geophysical survey of an alpine alluvial fan was performed using ground penetrating radar to devise a scaled three dimensional subsurface model radar facies were classified and calibrated to lithofacies within a fan system that provided outcropping walls and these were used to derive a three dimensional model of the sedimentary architecture and identify evolutionary fan stages the illgraben fan in the swiss alps was selected as a case study and a network of ca 60 km sections of ground penetrating radar was surveyed seven radar facies types could be distinguished which were grouped into debris flow deposits and stream flow deposits assemblages of these radar facies types show three depositional units which are separated by continuous fan wide reflectors they were interpreted as palaeo surfaces corresponding to episodes of sediment starvation that affected the entire fan an overall upward decline in the proportion of debris flow deposits from ca 50 to 15 and a corresponding increase in stream flow deposits were identified the uppermost depositional unit is bounded at its base by a significant incision surface up to 700 m wide which was subsequently filled up mostly by stream flow deposits the pronounced palaeo surfaces and depositional trends suggest that allocyclic controls governed the evolution of the illgraben fan making this fan a valuable archive from which to reconstruct past sediment fluxes and environmental change in the alps the results of the integrated outcrop geophysical approach encourage similar future studies on fans to retrieve their depositional history as well as their potential reservoir properties

magma sources and tectonic setting of central andean andesites 25 5 28 s related to crustal thickening forearc subduction erosion and delamination

rapid uplift of the altiplano revealed through 13c 18o bonds in paleosol carbonates the elevation of earth s surface is among the most difficult environmental variables to reconstruct from the geological record here we describe an approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters constrained by measurements of abundances of 13 c 18 o bonds in soil carbonates we use this approach to show that the altiplano plateau in the bolivian andes rose at an average rate of 1 03 0 12 millimeters per year between 10 3 and 6 7 million years ago this rate is consistent with the removal of dense lower crust and or lithospheric mantle as the cause of elevation gain

neogene shortening contribution to crustal thickening in the back arc of the central andes to illustrate the neogene shortening distribution in the back arc units of the central andes and to estimate the contribution of the shortening to crustal thickening two balanced crustal cross sections have been constructed across the northern and southern branches of the bolivian orocline total neogene shortening which varies from 191 to 231 km is accommodated by a crustal duplex below the cordillera oriental but is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the altiplano the best explanation for this anomalous thickening seems to be crustal underplating by material tectonically eroded from the continental margin this process probably caused the altiplano uplift the subduction of oceanic lithosphere coupled with this underplating and a brief episode of gravity spreading of the altiplano constituted the driving forces that produced neogene shortening and development of the central andes