bitumen veins and eocene transpression neuquen basin argentina abstract in the northwestern part of the petroleum rich neuquen basin veins of solid hydrocarbon bitumen have been reported from over 150 localities the bitumen veins are mostly steeply dipping and show intrusive relationships with their wall rocks the largest are some 8 m wide 8 km long and probably several kilometres deep the bitumen probably formed in the earliest stages of thermal maturation of organically rich source rocks the most likely source rocks for the bitumen as well as for most of the oil and gas in the neuquen basin are black marine shales of the vaca muerta and agrio formations these were deposited in a rift setting during the late jurassic and early cretaceous according to subsidence models the source rocks should have reached thermal maturity in the latest cretaceous or paleocene and should have remained in the oil window ever since although the bitumen has not been dated directly the veins probably formed soon after maturation of the source rocks in other words in the paleocene or eocene the wall rocks of the veins are mostly the source rocks themselves other veins have been emplaced into overlying strata of early cretaceous to paleocene ages or into underlying sediments of jurassic age the bitumen veins have preferred orientations the dominant trend is 060 and subsidiary trends are either 000 to 020 or 100 as in other basins worldwide the majority of bitumen veins in the neuquen basin probably formed by tensile failure in orientations perpendicular to the least compressive stress under the combined effects of regional tectonics and fluid overpressures the orientations of the veins are compatible with the direction of oblique convergence between continental south america and the oceanic nazca plate in the eocene many major structures in the neuquen basin are also of eocene age and may have formed under right lateral transpression

physically based modeling of bedrock incision by abrasion plucking and macroabrasion 1 many important insights into the dynamic coupling among climate erosion and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision however many questions regarding incision processes and morphology of bedrock streams still remain unanswered a more mechanistically based incision model is needed as a component to study landscape evolution major bedrock incision processes include among other mechanisms abrasion by bed load plucking and macroabrasion a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts the purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above to build the model we start by developing a theory of abrasion plucking and macroabrasion mechanisms we then incorporate hydrology the evaluation of boundary shear stress capacity transport an entrainment relation for pluckable particles a routing model linking in stream sediment and hillslopes a formulation for alluvial channel coverage a channel width relation hack s law and exner equation into the model so that we can simulate the evolution of bedrock channels the model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions the results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes this supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes the model is expected to improve the current understanding of the linkage among physically meaningful input parameters the physics of incision process and morphological changes in bedrock streams

drainage basin sensitivity to tectonic and climatic forcing implications of a stochastic model for the role of entrainment and erosion thresholds long term average rates of channel erosion and sediment transport depend on the frequency magnitude characteristics of ood ows that exceed an erosion threshold using a poisson model for rainfall and runoff analytical solutions are developed for average rates of stream incision and sediment transport in the presence of such a threshold solutions are derived and numerically tested for three erosion transport formulas the howard kerby shear stress incision model the bridge dominic sediment transport model and a generic shear stress sediment transport model results imply that non linearity resulting from threshold effects can have a rst order impact on topography and patterns of dynamic response to tectonic and climate forcing this non linearity becomes signi cant when fewer than about half of ood events are capable of detaching rock or sediment predicted morphology and uplift gradient scaling is more closely consistent with observations and laboratory experiments than conventional slope linear or shear linear erosion laws these results imply that particle detachment thresholds are not details that can be conveniently ignored in long term landscape evolution models copyright 2004 john wiley sons ltd

gravel bed river evolution in earthquake prone regions subject to cycled hydrographs and repeated sediment pulses sediment often enters rivers in the form of sediment pulses associated with landslides and debris flows this is particularly so in gravel bed rivers in earthquake prone mountain regions such as southwest china under such circumstances sediment pulses can rapidly change river topography and leave the river in repeated states of gradual recovery in this paper we implement a one dimensional morphodynamic model of river response to pulsed sediment supply the model is validated using data from flume experiments so demonstrating that it can successfully reproduce the overall morphodynamics of experimental pulses the model is then used to explore the evolution of a gravel bed river subject to cycled hydrographs and repeated sediment pulses these pulses are fed into the channel in a fixed region centered at a point halfway down the calculational domain the pulsed sediment supply is in addition to a constant sediment supply at the upstream end results indicate that the river can reach a mobile bed equilibrium in which two regions exist within which bed elevation and surface grain size distribution vary periodically in time one of these is at the upstream end where a periodic discharge hydrograph and constant sediment supply are imposed and the other is in a region about halfway down the channel where periodic sediment pulses are introduced outside these two regions bed elevation and surface grain size distribution reach a mobile bed equilibrium that is invariant in time the zone of fluctuation free mobile bed equilibrium upstream of the pulse region is not affected by repeated sediment pulses under the scenarios tested but downstream of the pulse region the channel reaches different fluctuation free mobile bed equilibriums under different sediment pulse scenarios the vertical bed structure predicted by the simulations indicates that the cyclic variation associated with the hydrograph and sediment pulses can affect the substrate stratigraphy to some depth copyright 2017 john wiley sons ltd

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

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

reduction of long term bedrock incision efficiency by short term alluvial cover intermittency fluctuations of the sediment volume stored in mountain channels are driven by stochastic variations of discharge and sediment supply and can inhibit bedrock incision if sediment thickness is too large here i study how this short term stochasticity propagates into the long term reduction of bedrock incision efficiency the cover effect at geological time scales i introduce a new numerical model that resolves sediment transport and bedrock incision at daily time scales and is run for thousands of years it incorporates 1 a transport threshold and daily stochastic variations in water discharge and sediment supply 2 a freely evolving channel width and slope and 3 an explicit treatment of alluvial thickness variations and corresponding bed incision reduction for typical mountain river conditions the model predicts that alluvial cover oscillates between complete and negligible incision reduction in this intermittent regime the long term cover effect is mainly set by the fraction of time spent in full cover and the present day extent of alluvial cover is not representative of long term dynamics the long term integrated cover effect law differs strongly from proposed theoretical and experimental models and it is controlled by sediment supply stochasticity rather than the details of cover development at the hydraulic time scale model results also suggest that steady state channel configuration always depends on sediment supply rate while being never limited by transport capacity or strictly detachment limited these results point out that discharge and sediment supply stochasticity should not be considered less important than the intricate details of incision laws to model long term bedrock channel dynamics

alluvial morphodynamics of low slope bedrock reaches transporting nonuniform bed material research on bedrock rivers primarily focused on bedrock incision and to the best of our knowledge morphodynamic models have not yet considered the variability of sediment grain size and the prese

rapid incision of the colorado river in glen canyon insights from channel profiles local incision rates and modeling of lithologic controls the colorado river system in southern utah and northern arizona is continuing to adjust to the baselevel fall responsible for the carving of the grand canyon estimates of bedrock incision rates in this area vary widely hinting at the transient state of the colorado and its tributaries in conjunction with these data we use longitudinal profiles of the colorado and tributaries between marble canyon and cataract canyon to investigate the incision history of the colorado in this region we find that almost all of the tributaries in this region steepen as they enter the colorado river the consistent presence of oversteepened reaches with similar elevation drops in the lower section of these channels and their coincidence within a corridor of high local relief along the colorado suggest that the tributaries are steepening in response to an episode of increased incision rate on the mainstem this analysis makes testable predictions about spatial variations in incision rates these predictions are consistent with existing rate estimates and can be used to guide further studies we also present cosmogenic nuclide data from the henry mountains of southern utah we measured in situ10be concentrations on four gravel covered strath surfaces elevated from 1 m to 110 m above trachyte creek the surfaces yield exposure ages that range from approximately 2 5 ka to 267 ka and suggest incision rates that vary between 350 and 600 m my these incision rates are similar to other rates determined within the high relief corridor available data thus support the interpretation that tributaries of the colorado river upstream of the grand canyon are responding to a recent pulse of rapid incision on the colorado numerical modeling of detachment limited bedrock incision suggests that this incision pulse is likely related to the upstream dipping lithologic boundary at the northern edge of the kaibab upwarp copyright 2009 john wiley sons ltd

late cretaceous early tertiary sedimentation in a semi arid foreland basin neuquen basin western argentina abstract a late cretaceous transgression and a subsequent paleocene regression is recorded in the neuquen basin of western argentina in the sediments of the upper malargue group seven facies associations d through j representing different depositional subenvironments have been recognized in the younger depositional sequence facies association e green calcareous mudstones was deposited in the outer shelf facies association f bioclastic carbonates represents inner shelf deposits to littoral sediments facies association g sandy calcarenites corresponds to nearshore to coastal deposits facies association h oolitic calcarenites and facies association d evaporites composed a shallowing upward carbonate sequence facies association i cross bedded lithic sandstones and facies association j red mudstones are characterized as meandering fluvial deposits the lateral and vertical distribution of facies association changes from north to south in the basin according to several depositional controls the foreland setting of the neuquen basin at this time with different sediment input and paleoslopes on the retro arc vs the cratonic side of the basin influenced sedimentation basin embayment geometry caused differences in tidal regime which in turn affected depositional environments westerly winds generated clockwise tidal currents inside the basin which affected alongshore lateral facies association variation the semi arid climate favoured deposition of mixed carbonate siliciclastic lithologies and evaporites

abrasion set limits on himalayan gravel flux rivers sourced in the himalayan mountain range carry some of the largest sediment loads on the planet yet coarse gravel in these rivers vanishes within approximately 10 40 kilometres on entering the ganga plain the part of the north indian river plain containing the ganges river understanding the fate of gravel is important for forecasting the response of rivers to large influxes of sediment triggered by earthquakes or storms rapid increase in gravel flux and subsequent channel bed aggradation that is sediment deposition by a river following the 1999 chi chi and 2008 wenchuan earthquakes reduced channel capacity and increased flood inundation here we present an analysis of fan geometry sediment grain size and lithology in the ganga basin we find that the gravel fluxes from rivers draining the central himalayan mountains with upstream catchment areas ranging from about 350 to 50 000 square kilometres are comparable our results show that abrasion of gravel during fluvial transport can explain this observation most of the gravel sourced more than 100 kilometres upstream is converted into sand by the time it reaches the ganga plain these findings indicate that earthquake induced sediment pulses sourced from the greater himalayas such as that following the 2015 gorkha earthquake are unlikely to drive increased gravel aggradation at the mountain front instead we suggest that the sediment influx should result in an elevated sand flux leading to distinct patterns of aggradation and flood risk in the densely populated low relief ganga plain

quantitative modeling of the evolution and geometry of incised valleys abstract incised valleys are critical stratigraphic features for unraveling long term earth surface processes and depositional history and commonly exhibit stratigraphic attributes that make them desirable fluid reservoirs while there is much descriptive documentation on architecture of incised valley fills and paleoenvironmental regimes relatively little work has focused on quantitative modeling of the dynamics of incised valley evolution here we use well constrained observations of incised valleys in experiments and the field to explore controls on valley geometry and develop a simple valley model to quantify the primary dynamics of incised valley evolution we document a strong tendency for incised valleys to widen downstream independent of the details of the relative base level curve or initial surface profile due primarily to the effects of increased sediment flux this is in general agreement with measurements of experimental and natural incised valleys though the degree of widening is less in natural systems due to greater sidewall resistance and smaller water discharge relative to valley size a first order model of incised valley evolution highlights three important trends valley aggradation and widening valley incision and widening and valley incision and narrowing the model reproduces the primary geometric responses to valley formation as measured from experiments valley width and depth both increase in response to increases in 1 local relative base level fall and 2 the initial offshore water depth finally we generalize the first order controls on valley geometry via dimensional analysis to show that long term valley narrowing is not readily produced from relative base level fall alone

relationships between block quarrying bed shear stress and stream power a physical model of block quarrying of a jointed bedrock channel we examine interactions among substrate characteristics hydraulic forces and erosive processes acting on a jointed resistant substrate in a flume experiencing supercritical flow we conducted six experimental runs with no sediment in transport constant slope varying discharge varying channel width and varying geometry of concrete blocks forming the channel bed the initial downstream boundary was a vertical knickpoint lip perpendicular to flow observed changes in bed morphology occurred primarily by episodic horizontal knickpoint retreat the knickpoint lip can develop a curve that reflects interlocking of the irregular boundaries between otherwise physically disjointed blocks during rotation of the blocks sometimes a block acts like a keystone in an arch and mobilization of the key block facilitates mobilization of the linked blocks on either side resulting in a large magnitude event in contrast to previous studies where blocks were vertically plucked blocks were removed by sliding from the knickpoint building on previous work we develop a theoretical force threshold for sliding block mobilization which suggests that increased block side length parallel to flow increases block stability whereas increased side length perpendicular to flow decreases block stability stream power was also linked to observed erosion rates as a function of block length consistent with the idea that block loosening rates play a key role in controlling average erosion rates inferred amplification of bed shear stress and pressure at the knickpoint lip stream power and joint spacing correlate with the rate of block erosion the results although limited in scope and scale suggest a linear relationship between bed shear stress and stream power with the rate of block erosion block sliding especially at near vertical knickpoints can play a key or even dominant role in the morphologic dynamics of the river channel

rapid post seismic landslide evacuation boosted by dynamic river width how rivers evacuate large landslide deposits is crucial for predicting post seismic hazards a 2d hydro sedimentary model demonstrates that a narrowing river channel increases sediment transport which reduces export time by orders of magnitude

the role of sediment in controlling steady state bedrock channel slope implications of the saltation abrasion incision model abstract the saltation abrasion model is a mechanistic model for river incision into bedrock by saltating bedload which we have previously derived and used experimental data to constrain all parameter values here we develop a method for applying the saltation abrasion model at a landscape scale and use the model as a reference for evaluating the behavior of a wide range of alternative incision models in order to consider the implications of the saltation abrasion model as well as other models for predicting topographic steady state channel slope to determine the single valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock we assume all runoff can be partitioned between a low flow and a high flow discharge in which all bedload sediment transport occurs during high flow we then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for using the predictions of the saltation abrasion model as a reference we find that the threshold of motion is the most important effect that should be represented explicitly followed in order of decreasing importance by the cover effect the tools effect and the threshold of suspension effect models that lack the threshold of motion over predict incision rate for low shear stresses and under predict the steady state channel slope for low to moderate rock uplift rates and rock strengths models that lack the cover effect over predict incision rate for high sediment supply rates and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage models that lack the tools effect over predict incision rate for low sediment supply rates and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value overall the saltation abrasion model predicts that steady state channel slope is most sensitive to changes in grain size such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible but as yet poorly understood influence on the size distribution of sediments delivered to channel networks by hillslopes

emplacement of bitumen asphaltite veins in the neuquen basin argentina veins of solid bitumen asphaltite have been commercially exploited in the neuquen basin argentina for over 100 yr veins are up to 5 m wide and several kilometers in length over a region of 15 000 km2 these veins were emplaced in fractures both parallel and at high angles to bedding in close proximity to their source rocks in the vaca muerta and agrio formations late jurassic early cretaceous two or more phases of bitumen emplacement can be recognized in several localities structures bearing viscous oil are younger than structures having solid bitumen bitumen emplacement was vigorous and caused brecciation and spalling of the host rocks the bitumen was also viscous and supports rock debris ranging in size from sand grains up to meter scale slabs brecciation bedding parallel injection and wall rock impregnation suggest high fluid pressures during emplacement high fluid pressure may have been engendered by substantial hydrocarbon generation from rich source rocks in a low permeability sequence and probably caused the fractures into which the bitumen migrated the bedding parallel veins facilitated decollement during thrusting that took place during and after bitumen emplacement the timing of emplacement relative to thrusting and oil migration constrains bitumen emplacement to the eocene oligocene

travel times of nonlocal dispersion and their geoelectric approximation in nevada s fractured welded tuffs this technical note argues the importance of incorporating nonlocal dispersion effects when modeling contaminant travel times using the transport theory of di fedenco and neuman 1998 expressions for nonlocal contaminant travel times and breakthrough are developed they show that nonlocal travel times are significantly faster than their local fickian counterpart and that nonlocal breakthrough occurs significantly earlier than fickian as an illustrative example nonlocal breakthrough curves for nevada s fractured welded tuffs a common rock unit found at the u s department of energy s yucca mountain project are approximated using the electrical hydraulic conductivity correlation theory and measurements of purvance and andricevic 2000a 2000b at the 5 km compliance point the approximation shows that nonlocal contaminant breakthrough occurs 5 times earlier than what is predicted by the department of energy s fickian model

topography as a major factor in the development of arcuate thrust belts insights from sandbox experiments abstract we have used sandbox experiments to investigate and to illustrate the effects of topography upon the development of arcuate thrust belts in experiments where a sand pack shortened and thickened in front of an advancing rectilinear piston the geometry of the developing thrust wedge was highly sensitive to variations in surface topography in the absence of erosion and sedimentation the surface slope tended to become uniform as predicted by the theory of critical taper under these conditions the wedge propagated by sequential accretion of new thrust slices in contrast where erosion or sedimentation caused the topographic profile to become irregular thrusts developed out of sequence for example erosion throughout a hinterland caused underlying thrusts to remain active and inhibited the development of new thrusts in the foreland where initial topography was irregular in plan view accreting thrusts tended to be arcuate they were convex towards the foreland around an initially high area concave towards the foreland around an initially low area initial plateaux tended to behave rigidly while arcuate thrust slices accreted to them thrust motions were radial with respect to each plateau within transfer zones to each side fault blocks rotated about vertical axes and thrust motions were oblique slip at late stages of deformation the surface slope of the thrust wedge tended towards a uniform value initial mountains of conical shape representing volcanoes also escaped deformation except at depth where they detached arcuate thrust slices accreted to front and back where a developing thrust wedge was subject to local incision accreting thrust slices dipped towards surrounding areas of high topography forming vs across valleys arcuate structural patterns are to be found around the three highest plateaux on earth tibet pamirs and altiplano and around the tromen volcanic ridge in the neuquen basin of northern patagonia we infer that these areas behaved in quasi rigid fashion protected as they were by their high topography

how canyons evolve by incision into bedrock rainbow canyon death valley national park united states incising rivers may be confined by low slope erodible hillslopes or steep resistant sidewalls in the latter case the system forms a canyon we present a morphodynamic model that includes the essential elements of a canyon incising into a plateau including 1 abrasion driven channel incision 2 migration of a canyon head knickpoint 3 sediment feed from an alluvial channel upstream of the knickpoint and 4 production of sediment by sidewall collapse we calculate incision in terms of collision of clasts with the bed we calculate knickpoint migration using a moving boundary formulation that allows a slope discontinuity where the channel head meets an alluvial plateau feeder channel rather than modeling sidewall collapse events we model long term behavior using a constant sidewall slope as the channel incises our morphodynamic model specifically applies to canyon rather than river hillslope evolution we implement it for rainbow canyon ca salient results are as follows 1 sediment supply from collapsing canyon sidewalls can be substantially larger than that supplied from the feeder channel on the plateau 2 for any given quasi equilibrium canyon bedrock slope two conjugate slopes are possible for the alluvial channel upstream with the lower of the two corresponding to a substantially lower knickpoint migration rate and higher preservation potential 3 knickpoint migration occurs at a substantially faster time scale than regrading of the bedrock channel itself underlying the significance of disequilibrium processes although implemented for constant climactic conditions the model warrants extension to long term climate variation

aptian to recent compressional deformation foothills of the neuquen basin argentina abstract the neuquen basin lies in north western patagonia on the eastern foreland of the andes it formed as a rift basin in the early mesozoic and contains some 5000 m of triassic to aptian sediment mainly marine as a result of andean orogeny the western edge of the basin now forms part of the foothills we describe some of the main compressional structures drawing on surface and sub surface data we argue that regional eastward dips are due to west verging thrust displacements on the andacollo loncopue fault system during a late cretaceous peruvian phase aptian to campanian continental sediment accumulated in a retro arc foreland basin which was mainly of flexural origin nevertheless more than 1500 m of late cretaceous sediment accumulated in the footwall of the east verging agua amarga thrust the peruvian phase was responsible also for transpressional reactivation of a rift transfer zone the huincul arch the incaic phase eocene was responsible for north westerly trending folds and thrusts reactivation of cretaceous thrusts abundant andesitic lavas and high level intrusions and a swarm of north easterly trending bitumen dykes the quechua phase miocene to recent produced northerly trending folds and thrusts and a miocene foreland basin containing some 1000 m of basalt lava flows structures that formed during the incaic and quechua phases are broadly compatible with the history of convergence at the pacific margin of south america as deduced from plate reconstructions

roles apparent resistivity amplitude and phase play in an aquifer s electrical hydraulic conductivity correlation 1 this paper argues how the spectral characteristics of two borehole apparent resistivity traces further corroborate two statistically significant electrical hydraulic eh conductivity correlations previously reported in nevada s fractured welded tuffs even though the eh conductivity correlation is positive in one borehole and negative in the other as explained by low pore water electrical conductivity and the absence or presence of alteration minerals both apparent resistivity amplitude spectra are identically power law structured this is interpreted to mean that eh flow is occurring along rock fractures of a common regional fractal dimension furthermore both apparent resistivity phase spectra are strikingly linear as mandated by the condition of incompressible fluids linear phase implies a groundwater flow that is geostatistically nonstationary in the wide sense a complication normally not considered by hydrogeologists

extensional tectonics in the death valley area california transport of the panamint range structural block 80 km northwestward in the hypothesis presented here the panamint range is a structural block that was detached during late cenozoic time from underlying rocks that now form the black mountains and was transported tectonically an estimated 80 km to the northwest transport occurred along a single low angle westward dipping detachment fault or perhaps along a system of such faults the estimate of 80 km of transport distance is based on the apparent right lateral offset of late precambrian and paleozoic facies and thickness trends along the furnace creek fault zone that bounds the detached block on the northeast the death valley turtleback surfaces are considered to be gigantic mullions related to the detachment surface

predictions of steady state and transient landscape morphology using sediment flux dependent river incision models 1 recent experimental and theoretical studies support the notion that bed load in mountain rivers can both enhance incision rates through wear and inhibit incision rates by covering the bed these effects may play an important role in landscape evolution and in particular the response of river channels to tectonic or climatic perturbation we use the channel hillslope integrated landscape development child numerical model with two different bedrock incision models that include the dual role of the sediment flux to explore the transient behavior of fluvial landscapes both models predict that steady state channel slopes increase in landscapes with higher rock uplift rates however the incision models predict different transient responses to an increase in uplift rate and the behavior of each incision model depends on both the magnitude of change in uplift rate and the local drainage area in some cases the transient channel behavior is indistinguishable from that predicted for transport limited alluvial rivers in other cases knickpoints form in some or all of the drainage network as predicted by the detachment limited stream power model in all cases the response in the lower parts of the network is highly dependent on the response in the upper parts of the network as well as the hillslopes as the upper parts of the network send more sediment downstream channel incision rates may rise or fall and slopes in the lower parts of the channel may in fact decrease at times during the transient adjustment to an increase in rock uplift rate in some cases channel incision in the upper parts of the network ceases during the transient while the hillslopes adjust to the new uplift rate drainage density may also change as a function of uplift rate our results suggest that if the sediment flux strongly controls bedrock incision rates then 1 the transient fluvial response will take longer than predicted by the detachment limited stream power model 2 changes in channel slope may be much more complex than predicted by the detachment limited stream power model and 3 changes in the fluvial system will be closely tied to sediment delivery from the hillslopes importantly our results outline quantitative differences in system behavior produced by competing models and provide a framework for identifying locations in natural systems where differences in channel morphology can be used to discern between competing fluvial erosion models

structural control on volcanoes and magma paths from local to orogen scale the central andes case abstract assessing the parameters that control the location and geometry of magma paths is of paramount importance for the comprehension of volcanic plumbing systems and geo hazards we analyse the distribution of 1518 monogenic and polygenic volcanoes of miocene quaternary age of the central volcanic zone of the andes chile bolivia argentina and reconstruct the magma paths at 315 edifices by analysing the morphostructural characteristics of craters and cones then we compare these data with outcropping dykes tectonic structures and state of stress most magma paths trend n s nw se and ne sw in decreasing order of frequency the n s and nw se paths coexist in the northern and southern part of the study area whereas n s paths dominate east of the salar de atacama outcropping dykes show the same trends the regional holocene stress state is given by an e w greatest horizontal principal stress n s and nne ssw reverse faults and folds affect deposits of 4 8 3 2 and 1 3 ma bp especially in the central and southern study areas a few nw se left lateral strike slip faults are present in the interior of the volcanic arc part of which belong to the calama olacapato el toro fault the volcanic chain is also affected by several n s and nw se striking normal faults that offset pliocene and quaternary deposits the results indicate different scenarios of magma tectonic interaction given by n s normal and reverse faults and n s fold hinges that guide volcano emplacement and magma paths magma paths are also guided by strike slip and normal nw se faults especially in the northern part of the study area zones with verticalized strata with bedding striking ne sw also acted as preferential magma paths these data suggest that at convergence zones with continental crust shallow magma paths can be more sensitive to the presence and geometry of upper crustal weakness zones than to the regional state of stress

eulerian lagrangian analysis of transport conditioned on hydraulic data 4 uncertain initial plume state and non gaussian velocities in previous papers of this series we described an analytical numerical method to predict deterministically solute transport under uncertainty based on a new eulerian lagrangian theory we examined the effect that conditioning on hydraulic data has on predicted velocities and concentrations due to instantaneous point and nonpoint sources and discussed the same effect on spatial plume moments total mass flow rate across a compliance surface cumulative mass release across this surface and the corresponding travel time distribution our analysis to date assumed that the initial state of the plume the source term or initial concentration is known with certainty and that the groundwater velocity field is gaussian in reality the initial state of the plume is almost never known with certainty especially when this state is inferred by sampling a plume at some arbitrary time t0 following introduction of the solute into the subsurface as is the case at many contaminated sites likewise monte carlo simulations have shown that in gaussian log hydraulic conductivity or log transmissivity fields of the kind often indicated by in situ hydraulic test data the longitudinal velocity becomes rapidly lognormal as the variance of the heterogeneities increases in this paper we show how to handle both of these complications by means of our analytical numerical method of computation by revisiting our previous examples we explore the effects that uncertainty in plume initial state and non gaussian velocities may have on predicted plume concentrations and uncertainty due to instantaneous point and nonpoint sources with and without conditioning on measurements of log transmissivity or hydraulic head in two dimensions one of our findings is that the more common gaussian models are nonconservative in a regulatory sense when applied during early dimensionless times to plumes that are initially short in the direction of mean flow

selective sorting and abrasion of river gravel i theory most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fining is due to some combination of selective sorting by which finer grains are preferentially transported downstream and abrasion by which individual particles are reduced in size here a framework for the simultaneous treatment of both phenomena is developed the analysis is restricted to bed load transport of gravel only abrasion due to binary collisions between moving bed load and stationary bed particles is considered the formulation includes the possibility of differential basin uplift and subsidence due to tectonism and mixtures of rock types with differing coefficients of abrasion the formulation of selective sorting is based on the concept of hiding as applied to a surface based bed load transport relation abrasion is first treated in terms of a lagrangian formulation for individual grains and t

geometry and late mesozoic cenozoic evolution of the salar de atacama basin 22 30 24 30 s in the northern central andes new constraints from geophysical geochronological and field data abstract the late mesozoic cenozoic evolution of the central andes of northern chile consists of a series of events related to changing conditions along the plate margin leading to different episodes of compression and extension a long lasting basin which records these changes corresponds to the salar de atacama basin its tectonic history has been blurred by several interpretations of seismic reflection lines and multiple chronostratigraphic correlations between subsurface surface units as such many aspects of its tectonic evolution are still unclear in this study we performed an e w gravimetric profile across the barros arana syncline and the basin we analyzed interval velocities obtained from seismic reflection studies and reviewed the profiles closest to the gravity survey four u pb samples for detrital zircon analysis were obtained from late cretaceous paleogene units geophysical data show a segmentation of physical properties within the basin with denser units and faster interval velocities around the late mesozoic early cenozoic units and lighter slower units in the actual salt pan the main contrast in physical properties is observed in the evaporitic and fine grained continental deposits of the san pedro formation geophysical and geological constraints show that its thickness varies between 800 and 1200 m and that it was deformed during later events the u pb data show that the rocks within the barros arana syncline reach well into the paleogene and that they were mainly deformed during the late eocene incaic event where an east verging thrust system also involving permo triassic units was developed establishing the actual structural configuration the late oligocene early miocene units were then deposited in a post orogenic setting with minor compression a stronger phase around the late miocene reactivated previous structures and folded these deposits this continuous compressive history of the basin which is mostly a result of the incaic event is in agreement with the regional evolution of the central andes

geoelectric characterization of the hydraulic conductivity field and its spatial structure at variable scales this paper describes how to begin with extensive geoelectric measurements of an aquifer along with a few hydraulic measurements and end with an estimate of the aquifer s hydraulic conductivity field and spectrum at variable scales it is based on a preponderance of theoretical and empirical evidence indicating that electrical and hydraulic eh conductivities of aquifers are linearly correlated on a bilogarithmic scale the steps required for a geoelectric characterization are scaling up calibration and spectrum estimation scaling up estimates electrical conductivity at variable scales from the apparent resistivity measurement calibration equilibrates coincident equal scale eh conductivities using the theoretical eh conductivity correlation combined scaling up and calibration provide a variable scale expression for the hydraulic conductivity field in terms of a geoelectric measurement from this expression the hydraulic conductivity field spectrum is calculated these steps are applied to borehole data collected at the department of energy central nevada testing area and the u s geological survey cape cod site in each case a statistically significant eh conductivity correlation is observed the paper ends by conjecturing that a horizontal exploitation of the eh conductivity correlation offers for the first time an inexpensive means of characterizing the hydraulic conductivity field at a high resolution and over a large area

sedimentary processes and facies of the waterlaid anvil spring canyon alluvial fan death valley california the anvil spring canyon fan of the panamint range piedmont in central death valley was built entirely by water flow processes as revealed by an analysis of widespread 2 to 12 m high stratigraphic cuts spanning the 9 7 km radial length of this 2 5 5 0 sloping fan two facies deposited from fan sheetfloods dominate the fan from apex to toe the main one 60 95 of cuts consists of sandy granular fine to medium pebble gravel that regularly and sharply alternates with cobbly coarse to very coarse pebble gravel in planar couplets 5 25 cm thick oriented parallel to the fan surface the other facies 0 25 of cuts comprises 10 to 60 cm thick wedge planar and wedge trough beds of pebbly sand and sandy pebble gravel in backsets sloping 3 28 both facies are interpreted as resulting from rare sediment charged flash floods from the catchment and were deposited by supercritical standing waves of expanding sheetfloods on the fan standing waves were repeatedly initiated enlarged migrated and then terminated either by gradually rejoining the flood or by more violent breakage and washout the frequent autocyclic growth and destruction of standing waves during an individual sheetflood resulted in the deposition of multiple coarse and fine couplet and backset sequences 50 250 cm thick across the active depositional lobe of the fan erosional intensity during washout of the standing wave determined whether early phase backset bed deposits or washout phase sheetflood couplet deposits were selectively preserved in a given cycle two minor facies are also found in the anvil fan pebble cobble gravel lags 0 20 of cuts are present above erosional scours into the sheetflood couplet and backset deposits they consist of coarse gravel concentrated through fine fraction winnowing of the host sheetflood facies by sediment deficient water flows this reworking occurred during recessional flood stage or from non catastrophic discharge during the long intervals between major flash floods this facies is common at the surface giving rise to a braided stream appearance however it is stratigraphically limited present as thin continuous to discontinuous beds or lenses that bound 50 to 250 cm thick sheetflood sequences the other minor facies of the anvil fan consists of clast supported and imbricated thickly stratified pebbly cobbly boulder gravel present in narrow radially aligned ribbons nested within sheetflood deposits this facies is interpreted as representing deposition in the incised channel of the fan a subenvironment characterized by greater flow competence resulting from maintained depth from channel wall confinement and by more frequent water flows and winnowing events caused by its direct connection with the catchment feeder channel

extension and basin formation in the southern andes caused by increased convergence rate a mid cenozoic trigger for the andes the southern andes between 33 and 45 s latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin which itself was characterized by volcanic activity the basins are filled with thick up to 3000 m accumulations of interbedded sedimentary and volcanic strata of late oligocene early miocene age we interpret that these basins developed during a phase of moderate extension within the plate margin system triggered by an increased rate of convergence of the farallon nazca and south american plates between 28 and 26 ma this history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling although extensional basins of this age are only well described in the southern andes the convergence history and volcanic chronology are similar farther north in the central andes 18 33 s leading to the speculation that extension may have characterized the late oligocene early miocene interval regionally we hypothesize that this extension was a necessary condition to subsequent building of the modern andes mountains

hillslope derived blocks retard river incision the most common detachment limited river incision models ignore the effects of sediment on fluvial erosion yet steep reaches of mountain rivers often host clusters of large 1 m blocks we argue that this distribution of blocks is a manifestation of an autogenic negative feedback in which fast vertical river incision steepens adjacent hillslopes which deliver blocks to the channel blocks inhibit incision by shielding the bed and enhancing form drag we explore this feedback with a 1 d channel reach model in which block delivery by hillslopes depends on the river incision rate results indicate that incision dependent block delivery can explain the block distribution in boulder creek colorado the proposed negative feedback may significantly slow knickpoint retreat channel adjustment and landscape response compared to rates predicted by current theory the influence of hillslope derived blocks may complicate efforts to extract base level histories from river profiles

emplacement history and inflation evidence of a long basaltic lava flow located in southern payenia volcanic province argentina abstract the el corcovo lava flow from the huanul shield volcano in the southern mendoza province central western argentina traveled a distance of 70 km and covered a minimum area of 415 km 2 the flow emplacement was controlled both by extrinsic e g topography and intrinsic e g lava supply rate lava physicochemical characteristics factors the distal portion of the lava flow reached the colorado river valley in la pampa province where it spread and then was confined by earlier river channels cross sections through the flow surveyed at several localities show two vesicular layers surrounding a dense central section where vesicles are absent or clustered in sheet shaped and cylindrical shaped structures lavas of the el corcovo flow are alkaline basalts with low values of viscosity the morphological and structural characteristics of the flow and the presence of landforms associated with lava accumulation are the evidence of inflation this process involved the formation of a tabular sheet flow up to 4 m of thick with a large areal extent in the proximal sectors while at terminal sectors frontal lobes reached inflation values up to 10 m the numerous swelling structures present at these portions of the flow suggest the movement of lava in lava tubes we propose that this aspect and the low viscosity of the lava allowed the flow travel to a great distance on a gentle slope relief