bluuebunny/crossref_metadata_2025 huggingface dataset:
Geodynamics along an increasingly curved convergent plate margin: Late Miocene‐Pleistocene Rhodes, Greece
Authors: ten Veen Johan H. , Kleinspehn Karen L.
Published: October 2002

Neogene‐Holocene outward migration of the absolute position of the convergent Hellenic plate boundary produced simultaneous increased curvature of the plate boundary, changing obliquity of plate convergence vectors and boundary‐parallel stretching of the forearc region. To study the effects of the plate boundary migration and curvature, a tectonostratigraphy is constructed from the middle Miocene–Pleistocene Apolakkia basin on Rhodes, whose easternmost location makes it a key island to assess the inner forearc's kinematic response to expansion of the overriding Aegean‐Anatolian block and thus obliquity of convergence with the African plate. The basin fill provides temporal and paleogeographic control to interpret its syndepositional and postdepositional structural assemblages. Five fault populations in the Apolakkia basin record two neotectonic deformation phases separated by a kinematic change at ∼4.5 Ma, both of which are consistent with outward expansion of the Aegean‐Anatolian block. The Apolakkia basin originated as a late Miocene fault wedge basin in response to syndepositional southwest‐northeast D1 extension with similar strain patterns in the adjacent offshore Hellenic inner forearc. The kinematic change at ∼4–5 Ma is attributed to a threshold of obliquity whereby the inner forearc started to experience sinistral‐oblique divergence. The Plio‐Pleistocene D2 transtensional phase reoriented the basin and resulted in combined syndepositional west‐northwest‐east‐southeast extension (283°) and 070° sinistral shear, orientations that are best attributed to simultaneous outward expansion of the Hellenic forearc, increasing curvature of the plate boundary and associated boundary‐parallel stretching of the forearc. Principal shear zones offshore also occur consistently at ∼070°, mimicking the D2 kinematic history of the Apolakkia basin and suggesting a consistent geodynamic regime throughout the inner eastern Hellenic forearc. Effects of sinistral‐oblique plate convergence along the subduction zone appear confined to the outer forearc and the accretionary prism of the Mediterranean Ridge. Thus the upper crust of the expanding Aegean‐Anatolian block behaved independently at its leading edge, and the Pliny “trench” constitutes a major boundary separating partitioned forearc slivers, a post‐4.5 Ma partitioning recorded reliably by the Pliocene fill of the Apolakkia basin.
Thermochronological constraints on the timing and magnitude of Miocene and Pliocene extension in the central Wassuk Range, western Nevada
Authors: Stockli Daniel F. , Surpless Benjamin E. , Dumitru Trevor A. , Farley Kenneth A.
Published: October 2002

Apatite fission track and (U‐Th)/He thermochronological data provide new constraints on the timing of faulting and exhumation of the Wassuk Range, western Nevada, where east dipping normal faults have accommodated large‐magnitude ENE‐WSW oriented extension. Extensional deformation has resulted in the exhumation of structurally coherent fault blocks that expose sections of preextensional mostly granitic upper crust in the Grey Hills and central Wassuk Range. These fault blocks display westward tilts of ∼60° and expose preextensional paleodepths of up to ∼8.5 km, based on the structural reconstruction of tilted preextensional Tertiary andesite flows that unconformably overlie Mesozoic basement rocks. Apatite fission track and (U‐Th)/He thermochronological data from the fault blocks constrain the onset of rapid footwall exhumation at ∼15 Ma. Fission track modeling results indicate rapid fault block exhumation occurred between ∼15 and 12 Ma, which is in agreement with Miocene volcanic rocks that bracket the tilting history. In addition, fission track and (U‐Th)/He data suggest reduced rates of cooling following major extension, as well as renewed cooling related to active, high‐angle faulting along the present‐day range front starting at ∼4 Ma. Thermochronological data from structurally restored fault blocks indicate a preextensional Miocene geothermal gradient of 27° ± 5°C/km. The thermochronological constraints on the timing of extensional faulting and the eruptive history in the Wassuk Range imply a model for extension where crustal heating and volcanism precede the onset of rapid large magnitude extension, and where synextensional magmatism is suppressed during the highest rates of extension.
Ignimbrite flare‐up and deformation in the southern Sierra Madre Occidental, western Mexico: Implications for the late subduction history of the Farallon plate
Authors: Ferrari Luca , López‐Martínez Margarita , Rosas‐Elguera José
Published: October 2002

The Sierra Madre Occidental (SMO) of western Mexico is one of the largest silicic volcanic provinces on Earth, but the mechanism for the generation of such a large volume of ignimbrites has never been clearly defined. We present new40Ar/39Ar ages, geologic mapping, and structural data for the southern part of the SMO demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare‐up in Oligocene (31.5–28 Ma) and early Miocene (23.5–20 Ma) time, and that extensional deformation occurred mostly before the transfer of Baja California to the Pacific plate. Extensive ignimbrite successions, with40Ar/39Ar ages clustering at ∼23 and ∼21 Ma, cover most of the southern SMO, thus correlating in age with ignimbrites exposed in southern Baja California and central Mexico. Grabens with a 020° to N‐S orientation developed in the east almost concurrently with this volcanic episode. Half grabens and NNW striking listric normal fault systems formed at the end of middle Miocene as far as 150 km from the present coast. A belt of left‐lateral transpressional structures formed along the southern boundary of the SMO during the same period. We link these magmatic and tectonic events to the evolution and dynamics of the Farallon and North America plates during the Miocene. Particularly, we propose that a first detachment of the lower part of the Farallon plate in early Miocene time produced a transient thermal event and partial melting of the crust via mafic underplating. Middle Miocene extension would be related to a second detachment event, resulting from the slowing subduction that preceded the final capture of the Magdalena microplate by the Pacific plate at 12.5 Ma. Transpression at the southernmost end of the SMO occurred along the inland projection of the Magdalena‐Cocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the Magdalena plate.
Stress direction history of the western United States and Mexico since 85 Ma
Authors: Bird Peter
Published: October 2002

A data set of 369 paleostress direction indicators (sets of dikes, veins, or fault slip vectors) is collected from previous compilations and the geologic literature. Like contemporary data, these stress directions show great variability, even over short distances. Therefore statistical methods are helpful in deciding which apparent variations in space or in time are significant. First, the interpolation technique ofBird and Li[1996]is used to interpolate stress directions to a grid of evenly spaced points in each of seventeen 5‐m.y. time steps since 85 Ma. Then, attest is used to search for stress direction changes between pairs of time windows whose sense can be determined with some minimum confidence. Available data cannot resolve local stress provinces, and only the broadest changes affecting country‐sized regions are reasonably certain. During 85–50 Ma, the most compressive horizontal stress azimuth1Hwas fairly constant at ∼68° (United States) to 75° (Mexico). During 50–35 Ma, both counterclockwise stress changes (in the Pacific Northwest) and clockwise stress changes (from Nevada to New Mexico) are seen, but only locally and with about 50% confidence. A major stress azimuth change by ∼90° occurred at 33 ± 2 Ma in Mexico and at 30 ± 2 Ma in the western United States. This was probably an interchange between1and3caused by a decrease in horizontal compression and/or an increase in vertical compression. The most likely cause was the rollback of horizontally subducting Farallon slab from under the southwestern United States and northwest Mexico, which was rapid during 35–25 Ma. After this transition, a clockwise rotation of principal stress axes by 36°–48° occurred more gradually since 22 Ma, affecting the region between latitudes 28°N and 41°N. This occurred as the lengthening Pacific/North America transform boundary gradually added dextral shear on northwest striking planes to the previous stress field of SW‐NE extension.
Structure and Cenozoic kinematics of the Eastern Cordillera, southern Bolivia (21°S)
Authors: Müller Joachim P. , Kley Jonas , Jacobshagen Volker
Published: October 2002

The Eastern Cordillera is a main morphotectonic unit of the central Andes. In southern Bolivia it is predominantly composed of Ordovician sedimentary rocks as thick as 10 km. Revision of the Ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections, improved estimates of crustal shortening, and a new kinematic model for the Eastern Cordillera. New estimates of total shortening in the Eastern Cordillera are up to 95 km (41–44%), higher than previous estimates and sufficient to thicken the Andean crust to its present state between the eastern foreland and the magmatic arc. After initial Tertiary uplift at ∼58 Ma, first thrusts developed in the middle Eocene to early Oligocene (?40–?30 Ma), exhuming the central part of the Eastern Cordillera. Tectonic shortening culminated in late Oligocene and early Miocene time (25–20 Ma) when west facing thrust systems developed in the central and western part of the Eastern Cordillera. Thrusting terminated in the western part by the end of the early Miocene, and middle Miocene tectonic activity was restricted to minor strike‐slip faulting. In the central part, thrusting remained active until the late Miocene. By ∼10 Ma, shortening in the Eastern Cordillera essentially ceased. Kinematic fault analyses indicate clockwise rotation of the shortening direction, from NNE‐SSW or NE‐SW in Eocene‐early Oligocene time over E‐W to WNW‐ESE or even NW‐SE in late Oligocene to middle Miocene time. Nevertheless, WNW‐ESE directed shortening and subvertical extension predominate. Sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the Bolivian orocline. It is proposed that space problems in the oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along NE trending transfer zones.
Basement‐involved shortening and deep detachment tectonics in forelands of orogens: Insights from recent collision belts (Taiwan, Western Alps, Pyrenees)
Authors: Lacombe Olivier , Mouthereau Frédéric
Published: October 2002

New combined structural and seismotectonic analyses demonstrate basement‐involved shortening in forelands of recent collisional orogens (Taiwan, Western Alps, Pyrenees). Basement thrusts documented by seismicity (e.g., the 21 September 1999, Chi‐Chi earthquake in Taiwan) and/or structural data are triggered and localized by preexisting basement faults which constitute crustal weakness zones available for reactivation under low stress levels. Reactivation of basement faults may induce localization of folds and thrusts in the shallow thrust wedge, development of crystalline thrust sheets, out‐of‐sequence basement thrusting and late basement uplift, deformation/refolding of shallow thin‐skinned nappes, and development of accommodation structures such as transfer faults leading to a kinematic segmentation of foreland thrust belts. Reactivation of preexisting basement faults also occurs in the far foreland in response to the far‐field transmission of orogenic stresses depending on the amount of the changing‐through‐time mechanical coupling between the orogen and its foreland. Displacements related to basement shortening in forelands are accommodated at the scale of the upper crust, which requires that it is partially decoupled from the deeper lithospheric levels by a crustal detachment. This detachment presumably occurs along the midcrustal, thermally weakened brittle‐ductile transition. It may either ramp toward the surface into a shallow, upper crustal detachment beneath the fold‐thrust belt and/or extend beneath the foreland and accommodate basin inversion far away from the orogen. Occurrence and relative timing of shallow and deep detachment tectonics in forelands seem to be dependent on mechanical boundary conditions such as the presence of ductile horizons within the cover sequence or of preexisting weakness zones in the underlying basement.
Vertical stretching and crustal thickening at Nanga Parbat, Pakistan Himalaya: A model for distributed continental deformation during mountain building
Authors: Butler R. W. H. , Casey M. , Lloyd G. E. , Bond C. E. , McDade P. , Shipton Z. , Jones R.
Published: October 2002

The localization of strain in the continental crust during compressional tectonics is examined using the active structures at the Nanga Parbat massif, an exhumed tract of Indian continental crust in the Pakistan Himalaya. This large‐scale (∼40 km wavelength) structure is considered to involve the whole crust. Thrusting at the modern surface places gneisses of the Indian continental crust onto Holocene deposits. At the Raikhot transect, the thrust zone carries a relatively narrow (2 km wide) shear zone within which minor structures are asymmetric and the deformation apparently noncoaxial. However, modeling of foliation and augen preferred orientation/ellipticity suggests that the bulk deformation is a combination of relatively small simple shear strains (γ = 1) with larger stretching strains. Heterogeneous stretching within the shear zone was accommodated by localized shearing on metabasic layers so that strain is partitioned. Outside this shear zone on the transect there is penetrative deformation throughout the Nanga Parbat massif. This broadly distributed deformation shows no asymmetry or evidence of rotation. Rather this deformation is better described as near pure‐shear subvertical stretching. Augen ellipticities suggest subvertical stretches of greater than 200%. Consideration of plausible changes in crustal thickness during the amplification of the Nanga Parbat structure suggests the magnitude of vertical stretch decays with depth. Presumably these strains in the deep crust are more distributed but weaker than in the exposed middle crustal sections, assuming conservation of horizontal shortening displacement with depth. These studies suggest that penetrative vertical stretching through dominantly pure shear deformation is an effective mechanism for thickening the continental crust and that models which assume that simple shear zones penetrate the whole crust need not be of ubiquitous applicability.
Neotectonic deformation of the central Chile margin: Deepwater forearc basin formation in response to hot spot ridge and seamount subduction
Authors: Laursen Jane , Scholl David W. , von Huene Roland
Published: October 2002

Underlying the midslope Valparaiso Terrace, the Valparaiso Basin constitutes a unique and prominent deepwater forearc basin on the central Chile margin. The basin contains a 3–3.5 km thick sediment fill of late Cenozoic age. The seismic‐stratigraphic architecture of the Valparaiso Basin fill shows that margin‐wide subsidence above the underthrusting Juan Fernández Ridge, an aseismic volcanic swell crested with seamounts on the Nazca plate, characterized the basin evolution. Contemporaneously with margin subsidence and seaward rotation of the upper slope and shelf, the outer part of the forearc was deformed by thrust faulting above the leading flank of underthrusting seamounts that rotated the middle slope to a more horizontal attitude and formed seaward‐vergent outer forearc highs. Subsidence of the margin above the underthrusting Juan Fernández Ridge is ascribed to processes of subduction erosion, enhanced by factors related to subduction of the ridge, that caused dismemberment of upper plate material along the shallow aseismic part of the plate interface and transfer of material to the downgoing Nazca plate. Accelerated midslope subsidence, 55–75 km landward of the trench, coincides with the seaward limit of interplate seismicity, suggesting that enhanced basal erosion is related to the transition from aseismic to stick slip subduction behavior. Since subduction of the Juan Fernández Ridge began near 33°S ∼10 Ma ago, the Chile Trench north of 33°S is inferred to have retreated ∼3 km/Myr, and the midslope Valparaiso Basin subsided ∼0.3–0.5 km/Myr, approximating analogous Neogene rates for the Peruvian margin.
Archean granite‐greenstone tectonics at Kolar (South India): Interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion
Authors: Chardon Dominique , Peucat Jean‐Jacques , Jayananda Mudlappa , Choukroune Pierre , Fanning C. Mark
Published: October 2002

The structural study of the Kolar greenstone belt and surrounding granite‐gneiss terrains combined with U‐Pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern Dharwar craton developed during a major magmatic accretion event between 2550 and 2530 Ma. The granite‐greenstone pattern resulted from the interference of gravity‐driven sagging of the greenstones (i.e., diapirism), E‐W bulk inhomogeneous shortening combined with horizontal N‐S stretching, and syntectonic juvenile pluton emplacement. Bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive, N‐S trending vertical foliation, shallow stretching lineation, and conjugate strike‐slip shear zone pattern within and outside the greenstone belt, resulting in regional horizontal pure shear deformation. The plutons around the greenstone belt record regional contraction by developing one set of strike‐slip C‐S fabrics of the shear zone pattern. The development of the granite‐greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion, melting, and granulite metamorphism in the lower crust. The Kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain, due to mechanical homogeneity and low viscosity provided by large‐scale melting during the accretion event. This example further suggests specific boundary conditions to the craton that allowed E‐W inhomogeneous shortening to be accommodated by N‐S stretching and spreading of the crust without significant tectonic thickening. Such tectonoplutonic pattern is specific to the Archean and may develop as a consequence of mantle plume activity in intracontinental settings.
Anatomy of an escape tectonic zone: Western Irian Jaya (Indonesia)
Authors: Pubellier Manuel , Ego Frédéric
Published: October 2002

The western fold‐and‐thrust belt of New Guinea in Irian Jaya is presently a complex boundary, dominated by the Bird's Head block escape from the collision between the Australian plate and the remnants of volcanic belts carried by the Pacific plate. The escape rate given by geodetic measurements is of the order of 7 cm/yr, and movement is accommodated by a broad shear zone. We analyze this shear pattern using fault slip analyses performed in the field from 1992 to 1996, combined with focal mechanism inversions, moment tensors summations, and surface trace of structures inferred from radar and multispectral satellite images. Time control of the deformation is attained by isotope dating of the recent syntectonic intrusives. The geometry of the macro and microstructures occurred in two stages from the early Pliocene to the Present. The first stage (5 to 2 Ma) is marked by flat‐and‐ramps structures guided by N60°E lateral ramps associated with a N60°E cleavage, affecting the whole of western Irian Jaya. The second stage (2 Ma to Present) shows the collapse of the western fold‐and‐thrust belt and the escape of the Bird's Head and the Lengguru belts along N60°E transtensile faults. In the latter stage, the strike‐slip offset is distributed on the N60°E schistosity zone along which some fracture planes are reactivated as left‐lateral transtensile faults. Shallow earthquakes moment tensors have been inverted for stress and summed to get strain rates to define contrasting structural provinces. The spatial variations in both stress and strain fields from earthquakes and microtectonics show that (1) they are consistent and are assumed to be coeval, (2) they reveal that oblique convergence is partitioned, and (3) they are influenced by the existence of a free boundary. We see no significant rotation of stress axes laterally along the escape zone. Instead, stresses change according to the different orientations of basement structures and thus undergo rapid spatial variations or principal axes permutations. Eastward, in front of the Australian craton, the oblique convergence is partly distributed on reverse and strike‐slip faults. Westward, along the NW part of the craton, we infer the Seram trench is a free boundary which allows material to escape. These results show that the evolution of the escape zone depends on the importance of the geometry of the former margin of Australia (e.g., tilted blocks), which controls the style deformation relative to the regional stress, and the boundary conditions, which control the variation of the state of stress through time.
Fold and thrust belt structures and strike‐slip faulting at the SE margin of the Salar de Atacama basin, Chilean Andes
Authors: Kuhn Dirk
Published: October 2002

A tectonic reinterpretation is reported for the southeastern margin of the Salar de Atacama basin of northern Chile. Detailed structural mapping revealed the presence of an east vergent thin‐skinned fold and thrust belt affecting Oligocene‐Miocene Paciencia Group rocks and the overlying Plio‐Pleistocene volcanic rocks. Along‐strike segmentation of the main fold implies local foreland influence on footwall ramp geometry leading to local thrust sheet rotation. To the east the adjacent western slope of the Western Cordillera displays two different structural domains, probably controlled by preexisting basement structures. The southern domain comprises two N‐S oriented sigmoidal belts of linear arranged pressure ridges, indicating left‐lateral transpression. In contrast, the northern domain is characterized by east vergent fold and thrust belt structures and reactivated NW‐SE striking sinistral strike‐slip faults, governing clockwise block rotations. An indenter‐driven deformation model is proposed to explain sinistral transpression and clockwise block rotations around vertical axes. This variant of a small‐block rotation mechanism is discussed in the context of oroclinal bending of the central Andes, emphasizing the significance of ancient structures in controlling rotations.
Initiation of an active margin at the North Iberian continent‐ocean transition
Authors: Gallastegui J. , Pulgar J. A. , Gallart J.
Published: October 2002

The North Iberian or Cantabrian margin, located at the southern flank of the Bay of Biscay, underwent successive tectonic regimes of rifting, passive margin, and compression from Mesozoic to Tertiary times. A complete crustal cross section of the North Iberian Margin, from the abyssal plain to the shoreline, and a reconstruction of its undeformed Upper Cretaceous structure are presented here. It is constrained after the compilation and interpretation of different geophysical data sets recently available in the area. The reflective pattern is provided by deep seismic reflection profile ESCIN‐4, complemented by commercial profiles and well information from the North Iberian platform, and the velocity‐depth control comes from an onshore‐offshore wide‐angle seismic profile, tested also with gravity modeling. The detailed cross section of the Meso‐Tertiary basins that fill the platform and abyssal plain shows structures from the three main tectonic events: (1) normal faults and asymmetric basins from the Permian to lower Cretaceous extensional stage; (2) Upper Cretaceous sediments deposited under stable conditions during the passive margin period; and (3) inverted faults, thrusts and folds related to the Tertiary compression. The deep structure of the crust beneath the margin is poorly constrained from ESCIN‐4 profile that shows reflectivity in the lower crust only at a small area under the platform. However, the corresponding wide‐angle experiment provides a conspicuous image of the Moho that shows a continued deepening toward the continent. On the basis of the geophysical results, we propose a new model for this margin where the lower crust is detached and underthrusted to the south as a result of the partial closure of the Bay of Biscay. An interpreted Mesozoic crustal‐scale extensional detachment could be the weakness zone that allowed the lower crust to “slide” southward under the upper crust, resulting in the indentation of the Cantabrian Margin lower crust into the Iberian crust. Plate convergence stopped at an early stage, making this a unique area to study the initial stages of deformation of a passive margin.
A framework for sustainability analysis in water resources management and application to the Syr Darya Basin
Authors: Cai Ximing , McKinney Daene C. , Lasdon Leon S.
Published: October 2002

Sustainable water management in irrigation‐dominated river basins attempts to ensure a long‐term, stable, and flexible water supply to meet crop water demands, as well as growing municipal and industrial water demands, while mitigating negative environmental consequences. To achieve this delicate balance, new models are needed which can use indicators of sustainability to guide the decision‐making process. This paper presents a new long‐term modeling framework which uses quantified sustainability criteria in a long‐term optimization model of a basin, ensuring risk minimization in water supply, environmental conservation, equity in water allocation, and economic efficiency in water infrastructure development. “Current” and “future” water supply and demand are combined into a coherent system which takes account of the cumulative effects of short‐term water use decisions and deals with the tradeoffs between the benefits of current and future generations. The modeling framework is demonstrated with an application to the Syr Darya River Basin of central Asia. Model results show the effectiveness of this tool for policy analysis in the context of the river basin.
Numerical solutions of moment equations for flow in heterogeneous composite aquifers
Authors: Winter C. L. , Tartakovsky Daniel M. , Guadagnini Alberto
Published: October 2002

We analyze flow in heterogeneous media composed of multiple materials whose hydraulic properties and geometries are uncertain. Our analysis relies on the composite media theory of Winter and Tartakovsky [2000, 2002], which allows one to derive and solve moment equations even when the medium is highly heterogeneous. We use numerical solutions of Darcy flows through a representative composite medium to investigate the robustness of perturbation approximations in porous medium with total log conductivity variances as high as 20. We also investigate the relative importance of the two sources of uncertainty in composite media, material properties, and geometry. In our examples the uncertain geometry by itself captures the main features of the mean head estimated by the full composite model even when the within‐material conductivities are deterministic. However, neglecting randomness within materials leads to head variance estimates that are qualitatively and quantitatively wrong. We compare the composite media approach to approximations that replace statistically inhomogeneous conductivity fields with pseudohomogeneous random fields with deterministic trends. We demonstrate that models with a deterministic trend can be expected to give a poor estimate of the statistics of head.
Measurement and analysis of dissolution patterns in rock fractures
Authors: Dijk Peter Erik , Berkowitz Brian , Yechieli Yoseph
Published: October 2002

Nuclear magnetic resonance imaging (NMRI) is applied to noninvasively measure flow and dissolution patterns in natural, rough‐walled, water‐saturated halite fractures. Three‐dimensional images of water density and flow velocity acquired with NMRI allow quantification of the developing fracture morphology and flow patterns. The flow patterns are correlated strongly to the local apertures and the large‐scale wall roughness. The correlations of the dissolution patterns to the fracture morphology, flow patterns, and mineralogical composition of the rock matrix are a function of the overall dimensionless Damköhler number. At high Damköhler numbers the dissolution patterns are dominated by the flow structure. In addition, at high Damköhler numbers buoyancy (stratified flow) becomes important. In such cases the dissolution patterns also depend on the orientation and elevation of the fracture walls, resulting in preferential upward dissolution. At low Damköhler numbers the dissolution patterns depend mainly on the mineralogical composition of the rock matrix. These findings suggest that coupled flow and dissolution processes are much more complex and unpredictable than commonly assumed, even under simplified conditions.
Evaluating a multiclass net deposition equation in overland flow conditions
Authors: Beuselinck Laurent , Hairsine Peter B. , Sander Graham C. , Govers Gerard
Published: October 2002

The movement of sediment and related pollutants in overland flow from the point of erosion to a stream normally occurs through a series of deposition zones. Here we present the evaluation of a new multiclass model of sediment transport through these zones. This model simulates size‐selective net deposition as a result of simultaneous deposition by settling and nonselective reentrainment of previously deposited sediment [Hairsine et al., 2002; Sander et al., 2002]. Multiclass sediment deposition models provide a useful basis for simulating off‐site effects of soil erosion, but information on the grain‐size composition of deposited and exported sediment is crucial for calibrating and evaluating such models. Therefore the multiclass sediment deposition model was tested using detailed experimental data. Information on the amount of sediment deposition, on the grain‐size distribution of the deposited and exported sediment, and on the spatial pattern of the deposits was collected during sediment deposition experiments. Calibration of the multiclass model, using one part of these data, was performed by adjusting the fraction of stream power available for sediment reentrainment by minimizing errors between both simulated and observed total outflow sediment concentration and grain‐size distribution of the exported sediment. Evaluation was conducted, using the other part of the data, over a wide range of conditions. The evaluation shows that the deposition equation is capable of reasonably simulating the observed size selectivity of the deposition process. However, the model is not capable of simulating correctly the spatial pattern of the deposited sediment because of simplifications used in the hydraulic model.
Mobility of river tracer pebbles over different timescales
Authors: Ferguson Robert I. , Bloomer Daniel J. , Hoey Trevor B. , Werritty Alan
Published: October 2002

Tracer pebbles are widely used to learn about gravel transport along rivers. Movement over short times and distances is dominated by factors controlling entrainment: relative particle size and shear stress. Movement at longer scales also involves depositional factors: burial and reexposure and exchange between channels, bars, and other depositional environments. We mapped mixed‐size tracers in six reaches of a small Scottish river after 2 and 8 years to investigate differences in relative and absolute mobility and infer the importance of burial and exchange. Patterns of relative mobility according to size and shear stress, both within and between reaches, did not change significantly. Some local bunching of tracers was apparent in both surveys, with redistribution from pools into riffles and bars. The main change was that virtual velocities were ∼50% lower, and estimated gravel fluxes were also lower, in the longer term. This slowdown is attributed to vertical mixing giving decreased mobility as surface‐seeded tracers become buried, long‐term storage in bars and other less active parts of the system, and in this channel, advection of tracers downstream onto a finer bed giving higher relative size.
Karst aquifer evolution in a changing water table environment
Authors: Kaufmann Georg
Published: October 2002

A vertical cross section through a karst aquifer is modeled by means of the finite element method to study the evolution of fractures and flow in the aquifer. The karst aquifer receives a constant recharge along the top boundary by precipitation and drains toward a resurgence assumed to be the base level in a valley. Flow is allowed both in the permeable rock matrix and the fracture network, and the fractures are enlarged with time by chemical dissolution. Hence during the early evolution of the karst aquifer the conductivity increases over several orders of magnitude, and the initially high water table drops to a steady state base level niveau. As a consequence, fractures above the final water table change from phreatic to vadose flow conditions. A systematic parameter study is carried out to investigate the aquifer evolution over a wide range of parameters, such as recharge rate, initial fracture width and density, and initial calcium concentration. The numerical models cover a wide range of drainage patterns, from phreatic water table caves to deep bathyphreatic caves to vadose river caves. The models suggest that a single theoretical approach is capable of explaining most common cave passage patterns.
Comparison of travel time analysis and inverse modeling for soil water content determination with time domain reflectometry
Authors: Huisman J. A. , Weerts A. H. , Heimovaara T. J. , Bouten W.
Published: October 2002

Travel time analysis algorithms are used to extract dielectric permittivity (and thereby water content) from time domain reflectometry (TDR) waveforms. A limitation of travel time analysis is the need to define subjective analysis parameters for reproducible measurements. The aim of this paper is to compare three methods to remove subjectivity from the analysis of TDR waveforms: (1) travel time analysis with optimized analysis parameters, (2) fitting a simulated waveform based on a model with frequency‐dependent permittivity, and (3) fitting a waveform based on a model with an apparent dielectric permittivity. We performed an outflow experiment on a sandy soil sample and simultaneously collected soil water content measurements (determined from outflow) and duplicate TDR measurements. After optimizing the travel time analysis parameters by minimizing the difference between duplicate measurements, we found little difference in accuracy and reproducibility between the three methods. However, we showed that small deviations from the optimal travel time analysis parameters have a significant influence on both the accuracy and the reproducibility of travel time analysis. A comparison of the two inverse modeling approaches showed that the inclusion of frequency‐dependent permittivity did not greatly improve the fit between measured and modeled waveforms and that a frequency‐independent (apparent) permittivity adequately describes the TDR measurements made in this sandy soil. We conclude that the removal of subjectivity resulted in similar accuracy and reproducibility for all three methods. Nevertheless, we consider inverse modeling an interesting alternative to travel time analysis because it is a fully automated analysis with high accuracy and reproducibility.
Effects of pipe flow and bedrock groundwater on runoff generation in a steep headwater catchment in Ashiu, central Japan
Authors: Uchida Taro , Kosugi Ken'ichirou , Mizuyama Takahisa
Published: October 2002

It has been suggested that pipe flow and bedrock groundwater play important roles in storm runoff generation. We examined the effects of pipe flow on storm runoff generation and the roles of bedrock groundwater in rainfall‐runoff phenomena in a steep headwater catchment in central Japan. Measurements of pore water pressures, pipe flow, and streamflow showed that when the total rainfall amount was <30 mm, the runoff process could be explained by Darcy's law using the saturated hydraulic conductivity values core samples. When the total rainfall was >70 mm, the dominant runoff process shifted to pipe flow. Temperature measurements indicated four key points: (1) During base flow conditions (streamflow <0.1 mm h−1), bedrock groundwater dominated streamflow; (2) during storm flow conditions (streamflow >0.2 mm h−1) the source of pipe flow was the same as the streamflow; (3) the transient groundwater at the upper hillslope was commonly dominated by the preevent soil water; and (4) only after the large storms with wet antecedent conditions, was water emerging from the bedrock and mixing with preevent soil water in the transient saturated area at the upper hillslope. Both the hydrometric and temperature measurements indicated that once pipe flow occurs, the contributing area of streamflow extended to upper hillslope, and the transient groundwater at the upper hillslope was delivered to the stream via preferential flow paths, shortcutting the normal mixing process through the soil matrix.
Cation competition in a natural subsurface material: Prediction of transport behavior
Authors: Vulava V. M. , Kretzschmar R. , Barmettler K. , Voegelin A. , Grolimund D. , Borkovec M.
Published: October 2002

Predicting the transport of reactive chemicals in subsurface porous media requires accurate adsorption models. This study compares the ability of various cation exchange models to quantitatively capture the transport behavior of major cations (i.e., calcium and sodium) in soil laboratory columns over a wide range of pore water composition. Experimental breakthrough curves were compared with model predictions without adjustable parameters. Various one‐site and multisite cation exchange models were independently calibrated with batch adsorption data. Our results show that a one‐site cation exchange model based on the well‐known Gaines‐Thomas convention yields good predictions of cation transport behavior, although some deviations from experimental data remain. The most reliable predictions were obtained with multisite cation exchange models with a distribution of various types of exchange sites.
Sediment transport through an area of net deposition
Authors: Hairsine Peter B. , Beuselinck Laurent , Sander Graham C.
Published: October 2002

When sediment‐laden overland flow passes across an area of reducing surface slope or increased hydraulic roughness, there is often a reduction in the sediment load carried by that flow. This paper provides a new model for the description of processes and sediment fluxes through such zones. The model describes simultaneous deposition and reentrainment of a multiclass sediment mix and includes explicit description of the mass and size composition of the deposit. A range of solutions to the developed equation for time‐steady inputs is provided. These solutions are the multisediment class case when no reentrainment occurs and the single‐sediment class solution when reentrainment is significant. The more general case of multiclass reentrainment and deposition is developed here, but a solution is provided in a subsequent paper. A preliminary evaluation of the model using a laboratory data set is presented. The model is shown to predict well trends of sediment concentration leaving an area of net deposition with incoming sediment concentration and unit discharge. Advantages of this new model over current models include a physically based sorting mechanism and no reliance on an externally invoked transport capacity concept.
Optimal division of data for neural network models in water resources applications
Authors: Bowden Gavin J. , Maier Holger R. , Dandy Graeme C.
Published: October 2002

The way that available data are divided into training, testing, and validation subsets can have a significant influence on the performance of an artificial neural network (ANN). Despite numerous studies, no systematic approach has been developed for the optimal division of data for ANN models. This paper presents two methodologies for dividing data into representative subsets, namely, a genetic algorithm (GA) and a self‐organizing map (SOM). These two methods are compared with the conventional approach commonly used in the literature, which involves an arbitrary division of the data. A case study is presented in which ANN models developed using each data division technique are used to forecast salinity in the River Murray at Murray Bridge (South Australia) 14 days in advance. When tested on a validation data set from July 1992 to March 1998, the models developed using the GA and SOM data division techniques resulted in a reduction in RMS error of 24.2% and 9.9%, respectively, over the conventional data division method. It was found that a SOM could be used to diagnose why an ANN model has performed poorly, given that the poor performance is primarily related to the data themselves and not the choice of the ANN's parameters or architecture.
Evaluation of analytical solute discharge moments using numerical modeling in absolute and relative dispersion frameworks
Authors: Hassan Ahmed E. , Andricevic Roko , Cvetkovic Vladimir
Published: October 2002

Two‐dimensional numerical simulations are used to validate the analytical solutions for the solute discharge moments. In addition to the analysis of classical absolute dispersion we also consider relative dispersion whereby plume meandering (deviation from mean flow path caused by velocity variations at scales larger than plume size) is removed. The numerical simulations are used within a Monte Carlo framework to assess the accuracy and robustness of the analytical predictions of the solute discharge moments (mean and variance). Results show that the analytical predictions deviate from the numerical simulations as the log conductivity variance increases. Deviation occurs for the mean as well as the variance of the solute discharge. The absolute dispersion formulation, however, shows better agreement with the numerical simulations than does the relative dispersion for strong heterogeneity and vice versa for small variability. The relative dispersion results, however, depend on the prediction of the ensemble mean of the plume arrival time, which differs between simulations and analytical solution. Using the first‐order analytical estimate for this parameter leads to a much better agreement between the numerical and the analytical results for solute discharge moments in the relative dispersion case.
In‐stream sorption of fulvic acid in an acidic stream: A stream‐scale transport experiment
Authors: McKnight Diane M. , Hornberger George M. , Bencala Kenneth E. , Boyer Elizabeth W.
Published: October 2002

The variation of concentration and composition of dissolved organic carbon (DOC) in stream waters cannot be explained solely on the basis of soil processes in contributing subcatchments. To investigate in‐stream processes that control DOC, we injected DOC‐enriched water into a reach of the Snake River (Summit County, Colorado) that has abundant iron oxyhydroxides coating the streambed. The injected water was obtained from the Suwannee River (Georgia), which is highly enriched in fulvic acid. The fulvic acid from this water is the standard reference for aquatic fulvic acid for the International Humic Substances Society and has been well characterized. During the experimental injection, significant removal of sorbable fulvic acid occurred within the first 141 m of stream reach. We coinjected a conservative tracer (lithium chloride) and analyzed the results with the one‐dimensional transport with inflow and storage (OTIS) stream solute transport model to quantify the physical transport mechanisms. The downstream transport of fulvic acid as indicated by absorbance was then simulated using OTIS with a first‐order kinetic sorption rate constant applied to the sorbable fulvic acid. The “sorbable” fraction of injected fulvic acid was irreversibly sorbed by streambed sediments at rates (kinetic rate constants) of the order of 10−4–10−3 s−1. In the injected Suwannee River water, sorbable and nonsorbable fulvic acid had distinct chemical characteristics identified in 13C‐NMR spectra. The 13C‐NMR spectra indicate that during the experiment, the sorbable “signal” of greater aromaticity and carboxyl content decreased downstream; that is, these components were preferentially removed. This study illustrates that interactions between the water and the reactive surfaces will modify significantly the concentration and composition of DOC observed in streams with abundant chemically reactive surfaces on the streambed and in the hyporheic zone.
Conditional moment analysis of steady state unsaturated flow in bounded, randomly heterogeneous soils
Authors: Lu Zhiming , Neuman Shlomo P. , Guadagnini Alberto , Tartakovsky Daniel M.
Published: October 2002

We consider steady state unsaturated flow in bounded, randomly heterogeneous soils under the influence of random boundary and source terms. Our aim is to predict pressure heads and fluxes without resorting to Monte Carlo simulation, upscaling, or linearization of the constitutive relationship between unsaturated hydraulic conductivity and pressure head. We represent this relationship through Gardner's exponential model while treating its exponent α as a random constant and saturated hydraulic conductivity Ks as a spatially correlated random field. We linearize the steady state unsaturated flow equations by means of the Kirchhoff transformation and integrate them in probability space to obtain exact integro‐differential equations for the conditional mean and variance‐covariance of transformed pressure head and flux. After approximating these equations recursively to second order in the standard deviation σY of Y = ln Ks, we solve them by finite elements for superimposed mean uniform and divergent flows in the vertical plane, with and without conditioning on measured Y values. Comparison with Monte Carlo solutions demonstrates that whereas our nonlocal solution is nominally restricted to mildly nonuniform media with σY2 ≪ 1, it yields remarkably accurate results for strongly nonuniform media with σY2 at least as large as 2. This accords well with a previous theoretical analysis, which shows that the solution may remain asymptotic for values of σY2 as large as 2.
Cavitation during desaturation of porous media under tension
Authors: Or Dani , Tuller Markus
Published: October 2002

A potential mechanism for liquid drainage in wet porous media based on cavitation of water (the spontaneous formation of water vapor bubbles) under tension is proposed. Cavitation theory applied to an idealized pore predicts considerable differences in drainage branches of characteristic curves determined under tension and with imposed positive pressure. A potentially important aspect of cavitation is the possibility of liquid displacement (drainage) even in the absence of a continuous gaseous phase. Surface heterogeneity and impurities in soil and rock water are conducive for lowering cavitation tension threshold; on the other hand, small pore sizes and viscous resistance to flow restrain cavitation in natural porous media. The significance of cavitation in flow and transport processes is yet unknown; however, there are two important aspects to this phenomenon: (1) it may require revision of several conceptual models for liquid retention and displacement in porous media, and (2) extrapolating from evidence in plant xylem systems, cavitation may play an important role in determining the unsaturated hydraulic conductivity function.