bluuebunny/crossref_metadata_2025 huggingface dataset:
Geochronologic constraints on syntaxial development in the Nanga Parbat region, Pakistan
Authors: Winslow David M. , Zeitler Peter K. , Chamberlain C. Page , Williams Ian S.
Published: February 2004

40Ar/39Ar data (hornblende, biotite, muscovite, and K‐feldspar) and U/Pb data (zircons) were obtained from the Nanga Parbat‐Haramosh Massif (NPHM), NW Pakistan, along three transects in the southern regions of the NPHM. We have based our interpretations on our new data as well as geochronologic dates from previous studies in the northern regions of the massif. Geochronologic data show that the NPHM has experienced exceptionally high denudation and cooling rates over the past 10 m.y. U/Pb ages determined through sensitive high‐resolution ion microprobe (SHRIMP) “depth‐profiling” experiments on metamorphic zircons and conventional U/Pb monazite dates suggest that the timing of metamorphism varied across the massif. In addition, we have documented that the massif has experienced postmetamorphic, differential cooling both along and across strike. Thermochronologic data on currently exposed surface rocks suggest that cooling occurred more recently and at greater rates in the south‐central regions of the massif (representing deeper crustal levels) than along the margins and northern regions of the massif. Within the Tato region, cooling following peak metamorphic temperatures of 600°–700 °C was as high as 140 °C/m.y. following partial melting of pelitic units. Biotites from this area record plateau ages of 0.9 ± 0.1 Ma. Along the Astor and Indus gorges, cooling was less rapid (approximately 70°–80°C/m.y.) following peak metamorphism as indicated by U/Pb monazite ages of 6–8 Ma and 40Ar/39Ar muscovite cooling ages of 2.2–3.4 Ma. Cooling over the last 3 m.y. occurred at rates of 100°–140 °C/m.y. The overall cooling age pattern within the massif is interpreted syntaxial growth through the development of north plunging antiforms prior to 3 Ma, followed by reverse faulting along east dipping fault zones. Along the Raikot River transect the biotite cooling age pattern is consistent with the folding of isotherms during folding of the foliation surfaces. The age pattern was disrupted at 1 Ma due to faulting along the Raikot and Tato faults. An electronic supplement of Tables A1, A2, and A3 may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GUEST and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC00032, Geochronologic constraints on syntaxial development in the Nanga Parbat region, Pakistan, David M. Winslow, Peter K. Zeitler, C. Page Chamberlain, and Ian S. Williams). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, DC 20009; $$15.00. Payment must accompany order.
Thermal models of core‐complex evolution in Arizona and New Guinea: Implications for ancient cooling paths and present‐day heat flow
Authors: Ketcham Richard A.
Published: February 2004

Two‐dimensional conductive thermal models of an evolving metamorphic core complex provide insights into synextensional and postextensional thermal history. The models assume a rotating‐hinge geometry with a low‐angle (20°–30°) detachment fault that flattens out at a depth of 12–15 km. Simulations reveal details of the temperature‐time paths experienced by footwall rocks during extension that have significant implications for interpretation of thermochronological data. The evolving isotherms are roughly horizontal near the surface breakout of the detachment fault and at depth are rotated toward the detachment ramp angle as the ramp‐flat interface is approached. This leads to highly curved apparent geothermal gradients along the detachment fault surface, with local gradients near the surface being up to 3 times those at depth. Cooling accelerates as rocks approach the surface, and rates vary by up to an order of magnitude with position within the unroofing footwall block and the temperature interval in question. Isotherms advance upward along the detachment fault surface as extension progresses. As a result, calculations of extension rate made by plotting 40Ar/39Ar or fission‐track dates against distance along the detachment slip direction can underestimate the true rate by up to 40%. Estimates of detachment dip can be similarly affected by the nonlinearity of the along‐fault synextensional thermal gradient. Simulation of cooling paths in the D'Entrecasteaux Islands core complex in Papua New Guinea reveals a remarkable congruence between model predictions and the cooling path determined from P‐T‐t data. The models also serve to examine possible mechanisms behind the heat flow low of 20–40% that parallels the band of core complexes in Arizona and southern California. The present‐day heat‐flow patterns and low‐relief Moho are most closely matched by a balanced geometry in which unroofing of the core complex is compensated by pure shear extension off center from the complex, with the core itself being a megaboudin of little or no internal shear. This geometry may also be analogous to a scenario in which lower crustal flow compensates for tectonic denudation. However, models based on detailed heat‐production data from the Santa Catalina Mountains suggest that core‐complex formation may not be responsible for the low heat flow in that area. Furthermore, the net amount of extension in area‐balanced crustal cross sections based on Arizona core complexes does not appear to be sufficient to explain the high heat flow that characterizes the overall southern Basin and Range, indicating that additional sources of heat over that introduced by regional extension may be required to explain the regional thermal anomaly.
Late Cenozoic structure and tectonics of the northern Mojave Desert
Authors: Schermer E. R. , Luyendyk B. P. , Cisowski S.
Published: February 2004

In the Fort Irwin region of the northern Mojave desert, late Cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age. Kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral‐thrust slip on northwest striking, moderately dipping segments at the east ends of the blocks. Crustal blocks ∼7–10 km wide by ∼50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block. Faulting initiated after ∼11 Ma, and Quaternary deposits are faulted and folded. We document a minimum of 13 km cumulative sinistral offset in a north‐south transect from south of the Bicycle Lake fault to north of the Drinkwater Lake fault. Paleomagnetic results from 50 sites reveal two direction groups in early and middle Miocene rocks. The north‐to‐northwest declinations of the first group are close to the middle Miocene reference pole. However, rock magnetic studies suggest that both primary and remagnetized directions are present in this group. The northeast declinations of the second group are interpreted as primary and 63.5° ± 7.6° clockwise from the reference pole and suggest net post middle Miocene clockwise rotation of several of the east trending blocks in the northeast Mojave domain. The Jurassic Independence Dike Swarm in Fort Irwin may be rotated 25–80° clockwise relative to the swarm north of the Garlock fault, thus supporting the inference of clockwise rotation. Using a simple‐shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks, we predict ∼23° clockwise rotation using the observed fault slip, or one‐third that inferred from the paleomagnetic results. The discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks, where most of our paleomagnetic sites are located. However, at least 25°–40° of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible “rigid‐body” rotation of the region evidenced by clockwise bending of northwest striking domain‐bounding faults. Our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the Eastern California Shear Zone over the last 10 m.y. occurred within the northeast Mojave Domain. The remainder must be accommodated in adjacent structural domains, e.g., east of the Avawatz Mountains and west of the Goldstone Lake fault. Supporting Appendices 1 and 2 are available on diskette or via Anonymous FTP from kosmos.agu.org, directory APEND (Username ‐‐ anonymous, Password = guest). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W, Washington, DC 20009 or by phone at 800‐966‐2481; $15.00. Payment must accompany order.
Archaean granulites of the Limpopo Belt, Zimbabwe: One slow exhumation or two rapid events?
Authors: Kamber B. S. , Biino G. G. , Wijbrans J. R. , Davies G. R. , Villa I. M.
Published: February 2004

The peak of high‐temperature, low‐pressure anticlockwise granulite facies metamorphism in the northernmost part of the Northern Marginal Zone of the Limpopo Belt has been dated at 2591 ± 4 Ma using conventional U‐Pb zircon geochronology on a syntectonic charnockite intrusion. The thermal peak of 800°C at 0.5 GPa reflects a strong perturbation of the geotherm and was reached during compressional deformation and coincides with vast granite intrusion into the overlying craton. The coincidence of the temperature (T) maximum and compression is thought to be a response to lithospheric mantle thinning that resulted in granite emplacement and a consequent change in upper crustal rheology. After initial exhumation and cooling, these high‐temperature, low‐pressure rocks resided at a comparatively shallow depth (≈10 km) for 620 Ma. Field, microstructural, and geochronological investigation of a low‐temperature overprint establishes that reworking and final exhumation occurred at 1.97 Ga along discrete greenschist facies thrusts. The 40Ar/39Ar laser ablation of hornblende and biotite and Pb‐Pb step leaching of sphene are shown to be useful geochronometers in dating the low‐grade assemblages. Our results indicate that early Proterozoic deformation lasted 10 to 20 Ma. However, a number of ages intermediate between the events at 2.6 and 2.0 Ga have been reported for the Northern Marginal Zone. These include Pb‐Pb garnet, Ar‐Ar step heating amphibole, and U‐Pb zircon ages. Such ages suggest that these minerals were partially rejuvenated below their commonly believed closure temperature. Reinterpreting these intermediate ages as partial overprint or inheritance was possible in the Northern Marginal Zone due both to the large time gap between the two tectonometamorphic events and the use of a multimethod multichronometer approach. These observations call for great caution when extrapolating mineral ages into cooling histories, especially so when the resulting cooling is slower than thermally and/or mechanically possible and suggests that the geochronometry in other regions that underwent multiple tectono‐metamorphic events may need to be reassessed.
Structural divergence and transpression in the Teslin tectonic zone, southern Yukon Territory
Authors: Stevens Robert A. , Erdmer Philippe
Published: February 2004

The structural and tectonic evolution of the Teslin tectonic zone, a complex belt of ductilely deformed rocks forming the southern extension of the Yukon‐Tanana terrane in the northern Canadian Cordillera, is elucidated with new field data and by the reinterpretation of existing data. The zone includes greenschist to amphibolite facies metasedimentary and metaplutonic rocks of the Nisutlin and Anvil assemblages characterized by S and L‐S tectonite fabrics. Primary contact relationships and ages show that most rocks in the zone were contiguous by Mississippian time. Mapping identified a number of strain domains which preserve S fabrics dipping northeasterly or southwesterly, variable mineral lineation orientations, and a variety of shear directions including easterly and westerly vergent thrust shear, down‐to‐the‐east and down‐to‐the‐west normal shear, and dextral strike‐slip shear. Regional constraints and microstructures suggest that latest ductile deformation was Late Triassic to Early Jurassic in age. Structural characteristics are explained most effectively by convergent‐dominated transpression in combination with tectonic wedging and associated back thrusting above a low‐angle west dipping detachment. Oblique eastward and upward movement of the rocks over the detachment, which may have coincided with the top of thinned North American continental crust, produced easterly vergent shear, and localized or widespread tectonic wedging and back thrusting produced westerly vergent shear. The transpressive deformation postdates formation of the Nisutlin assemblage by Mississippian time and subduction of the Slide Mountain Ocean beneath the Nisutlin assemblage in Permian to Early Triassic time. The western margin of the Teslin tectonic zone was truncated by Cretaceous strike‐slip faults and translated northward. Rocks of the peraluminous orthogneiss assemblage of the Yukon‐Tanana terrane in central Yukon acted as the tectonic wedge and were overthrust by rocks of the Nisutlin and Anvil assemblages in Early Jurassic time before unroofing by local extension in Cretaceous time.
Rapid emplacement of the Oman ophiolite: Thermal and geochronologic constraints
Authors: Hacker B. R. , Mosenfelder J. L. , Gnos E.
Published: February 2004

In understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting, and the rate and duration of that thrusting. Hornblende 40Ar/39Ar ages demonstrate that the igneous oceanic crust in Oman crystallized and cooled to ∼825 K in 1–2 m.y. Hornblende ages from metamorphic rocks and from cross‐cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below ∼825 K in 1–2 m.y. Motion along the sole thrust accounted for 200 km of displacement at a rate of 100–200 mm/yr. On the basis of age relationships and thermal considerations, we favor a two‐stage model for the initial stages of Samail ophiolite emplacement: intraoceanic thrusting over < 2‐m.y.‐old lithosphere at 150 km/m.y. parallel to a spreading ridge for 1–2 m.y., followed by equally rapid and brief thrusting over cold and old lithosphere. Preservation of the Samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure, and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting.
Dating deformation using Rb‐Sr in white mica: Greenschist facies deformation ages from the Entrelor shear zone, Italian Alps
Authors: Freeman S. R. , Inger S. , Butler R. W. H. , Cliff R. A.
Published: February 2004

Ages of deformation have been obtained by Rb‐Sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation. Since white micas commonly define deformation fabrics in medium‐grade metamorphic rocks, these ages can be directly related to structural geometries with regional context. This direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust. It does require that the dated minerals attained isotopic equilibrium with the dominant Sr reservoir at temperatures lower than the closure temperature. This resetting was apparently achieved during dynamic recrystallization of white micas in greenschist‐facies metasediments and metagranitoid units in the western Alps. The results suggest that the Sr isotopic composition of the new mica is buffered by the coexisting high‐Sr phases (calcite, feldspar or epidote) via the grain boundary network. High‐strain rocks from the Entrelor shear zone system of the western Alps have yielded indistinguishable white mica Rb‐Sr ages along 30 km of individual and kinematically linked shear zones. The age of the back‐thrusting event is constrained at 34±1Ma, the age yielded by the younger generation of synkinematically crystallized white micas. This event was short‐lived, involving at least 20 km of shortening in ∼1 m.y. or less. An earlier, variably overprinted component, dating from 38 to 37 Ma, has been identified in the mica fabric, but its kinematic significance is uncertain. This method of dating strain fabrics offers a powerful tool for tectonic studies, since isotopic resetting can be directly linked to structural geometries, microstructural textures, and PT conditions. It allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust.
Svartfjella, Eidembukta, and Daudmannsodden lineament: Tertiary orogen‐parallel motion in the crystalline hinterland of Spitsbergen's fold‐thrust belt
Authors: Maher Harmon D. , Bergh Steffen , Braathen Alvar , Ohta Yoshihide
Published: February 2004

Within metamorphic basement rocks of the hinterland of Spitsbergen's Tertiary fold‐thrust belt, a 35‐km‐long zone of notably deformed Carboniferous strata and Cretaceous intrusives forms a major orogen‐parallel lineament from Svartfjella to Eidembukta to Daudmannsodden (SEDL). Orientations and geometries of map‐scale fault duplexes and associated fault plane‐striae populations, of folds and associated cleavage, and of tension gashes all indicate orogen‐parallel motion occurred along the SEDL. Structural analyses indicates three phases: 1) ENE‐directed overthrusting, 2) sinistral motion with a backthrust component, and 3) dextral strike‐slip motion. This history indicates a change from orogen‐perpendicular to orogen‐parallel movements. Orogen‐parallel movement was likely coeval with orogen‐perpendicular fold‐thrust transport to the ENE in the foreland. A model where dextral transpressive motion between Greenland and Svalbard was decoupled explains the hinterland‐foreland difference. Basement fabric aligned with Carboniferous carbonates on the steep foreland face of an antiformal stack provided a through‐going weak surface, a prerequisite for decoupling. Sinistral orogen parallel motion is explicable by short‐lived episode of sinistral plate motion or by local wedge extrusion during dextral transpression. The evolution of decoupling patterns has significant implications for deducing plate motions from local kinematic and paleostress studies.
Flank uplift and topography at the central Baikal Rift (SE Siberia): A test of kinematic models for continental extension
Authors: van der Beek Peter
Published: February 2004

Uplifted flanks at intracontinental rifts are supported by flexural isostasy, as shown by the pattern of isostatic residual gravity anomalies associated with them. Models for flexural rift flank uplift differ in their kinematic description of extension, with respect to the asymmetry of rifting and the importance of brittle versus elastic upper crustal deformation. In this paper, I test different kinematic models of continental extension by comparing their predictions of rift flank topography and crustal structure with observations from the Baikal rift (SE Siberia). The rift is characterized by prominent flank topography on both sides of lake Baikal. The flanks reach similar elevations but differ in their structure: the tilt of the footwall flank is away from the basin, whereas the basinward part of the hanging wall flank tilts toward the basin center. Fission track data indicate that very little erosion affected the flanks since rifting started; geomorphological and sedimentological observations suggest that prerift relief was minor. Thus, the present‐day topography reflects rift‐related tectonic uplift. Pure‐shear “necking” and pure‐shear/simple‐shear “detachment” models of extension predict the topographic and Bouguer gravity anomaly patterns observed along a profile across the central Baikal rift equally well. They do not permit to discriminate between different scenarios that have been proposed for the central Baikal rift; that is, half‐graben versus full graben development; rifting at a continuous rate since the Oligocene versus a large increase in extension rate since the Pliocene. The models predict that the kinematics of rifting in Baikal are controlled by a midcrustal (20 km) depth of necking and/or a mid to lower crustal (20–30 km) detachment level; best‐fit elastic thicknesses are in the range 30–50 km. These predictions are in agreement with results from coherence studies of Bouguer gravity and topography, as well as with the rheology of the lithosphere underneath Baikal as inferred from heat flow, seismic refraction and seismological observations. In contrast, a “flexural cantilever” model with low (< 10 km) elastic thickness predicts topographic patterns which are very different from those observed, for a wide range of rifting scenarios. Significant (> 3 km) erosion of the footwall flank is required to fit the topography if a flexural cantilever model is applied; this is incompatible with the fission track data. Thus, the kinematics of extension at deep and narrow intra‐continental rifts such as Baikal appear to be controlled by a strong elastic lithosphere and require significant brittle deformation of the upper crust, as suggested by dynamic models for continental rifting.
Growth of the South Pyrenean orogenic wedge
Authors: Meigs Andrew J. , Burbank Douglas W.
Published: February 2004

A six‐step reconstruction of the South Pyrenean foreland fold‐and‐thrust belt in Spain delineates the topographic slope, basal décollement angle, internal deformation, and thrust‐front advance from the Early Eocene until the end of contractional deformation in the Late Oligocene. Style of thrust‐front advance, dip of the basal décollement, slope of the upper surface, and internal deformation are decoupled and not simply related. Internal deformation increased, decreased, and maintained surface slope angle at different stages. From the onset to the cessation of deformation, the basal décollement angle decreased overall suggesting translation of the thrust belt onto stronger crust with time. Taper angle of the Pyrenean thrust wedge was fundamentally controlled by the flexural rigidity of the lower plate, the relative rate of creation of structural relief in the rear versus the front of the wedge, the extent of deposition of eroded material within the deforming wedge, and the taper of the pretectonic stratigraphic wedge.
Alternating contractional and extensional events in the Alpujarride nappes of the Alboran Domain (Betics, Gibraltar Arc)
Authors: Balanyá Juan C. , García‐Dueñas Víctor , Azañón José M. , Sánchez‐Gómez Mario
Published: February 2004

In the western Alpine system, Neogene extensional tectonics triggered the development of marine basins on the concave side of tight orogenic arcs, as happened within the Alboran Crustal Domain, the hinterland of the Gibraltar Arc. A detailed analysis of the structural and metamorphic records of one of the main Alboran Domain complexes, however, plainly reveals a complex tectonic evolution prior to the development of the Miocene arc/back arc system, which includes a major intraorogenic extensional event. This large‐scale subvertical shortening, that can be assessed from the PT paths of representative tectonic units, was subsequent to the continental crust subduction inferred from high pressure‐low temperature mineral asssemblages. The crustal section was thinned in nearly isothermal conditions, its thickness being reducted to at least 1/3 of the initial value. Yet still before the Miocene, a second contractional event led to the overthrusting of high‐grade metamorphic rocks over other low‐grade rocks, accompanied by subordinate overturning of the metamorphic zones. Since migration of the Gibraltar Arc is roughly balanced by crustal spreading in the back arc, available data concerning Miocene extension suggest that the Alboran Domain can be restored to its appropriate position several hundred kilometers to the east. Thus a collision belt that underwent significant intraorogenic extension must have existed in what is now the western South‐Balearic basin.
Effects of First‐Order Approximations on Head and Specific Discharge Covariances in High‐Contrast Log Conductivity
Authors: Van Lent Thomas , Kitanidis Peter K.
Published: February 2004

The hydraulic head and the specific discharge fluctuations depend nonlinearly on the hydraulic conductivity. However, the methods most commonly used in the stochastic analysis of groundwater flow are based upon the linearization of these relations. In this paper we apply a numerical spectral approach to investigate the range of validity of the small perturbation approximation for head and specific discharge moments in two‐dimensional finite domains. We find that the small perturbation approximation tends to underestimate the variance of large‐scale head and specific discharge fluctuations and error increases with increasing log‐conductivity variance and increasing domain size. The head fluctuations do not appear ergodic even when the small perturbation approximation predicts they will be ergodic. The specific discharge fluctuations, on the other hand, do appear ergodic. The small perturbation approximation performs well in estimating specific discharge variance in the longitudinal direction but significantly underestimates transverse specific discharge variance.
Preferential Flow Paths in a Water Repellent Clay Soil with Grass Cover
Authors: Dekker Louis W. , Ritsema Coen J.
Published: February 2004

Grass‐covered heavy basin clay soils in the Netherlands appeared to be water repellent. Water repellency in the top layers of these soils occurred mainly as a coating on the aggregates. The variation in soil moisture content over short distances was studied by sampling the soil 10 times during the period August 31, 1993, to December 22, 1994. Each time, 35 samples (100 cm3) were taken in close order over a distance of 195 cm at depths of 0–5, 10–15, 20–25, and 30–35 cm. Differences between minimum and maximum soil moisture contents were high in all layers sampled, occasionally as much as 28 vol %. When the clay soil is dry, a major proportion of the water from precipitation or sprinkler irrigation may flow rapidly through shrinkage cracks to the subsoil, bypassing the matrix of the clay peds. However, preferential flow is not limited to macropore flow: irregular wetting patterns are also formed through the small pores of the matrix. The relationship between dry bulk density and volumetric water content was found to be positive when the clay soil was relatively dry and negative when it was relatively wet.
Probability density functions of turbulent velocity and temperature in the atmospheric surface layer
Authors: Chu Chia R. , Parlange Marc B. , Katul Gabriel G. , Albertson John D.
Published: February 2004

The probability density functions (pdf's) for the longitudinal and vertical velocities, temperature, their derivatives, and momentum and sensible heat fluxes were measured in the atmospheric surface layer for a wide range of atmospheric stability conditions. The measured pdf's for both the velocity and the temperature fluctuations are near‐Gaussian and consistent with corresponding laboratory measurements for near‐neutral and stable stability conditions. Hence the first‐ and second‐order moments are sufficient to predict the heat and momentum flux pdf's. The lower‐order moments can be estimated from mean meteorological conditions using surface layer similarity theory. For unstable conditions the pdf for temperature is non‐Gaussian and is strongly skewed due to local convective thermal plumes. For near‐neutral and stable conditions the pdf's for the velocity and temperature longitudinal gradients have long exponential tails, in agreement with findings in laboratory experiments and numerical simulations.
Relationship Between Monthly Atmospheric Circulation Patterns and Precipitation: Fuzzy Logic and Regression Approaches
Authors: Özelkan Ertunga C. , Ni Fenbiao , Duckstein Lucien
Published: February 2004

In order to link the monthly areal precipitation to large‐scale circulation patterns, a fuzzy indexing technique is used in conjunction with a fuzzy rule‐based technique and also a standard linear regression. After clustering the lag‐correlation centers, fuzziness is introduced, and several representative indices of the monthly areal precipitation in Arizona are calculated and interpreted. The relation between the indices and the precipitation is analyzed to develop the fuzzy model and then a multivariate linear regression model. To measure the forecasting capability of the models, the data are divided into a calibration period (1947–79) and a validation period (1980–1988). A comparison of the results shows that the fuzzy rule‐based model performs better than the regression model and has potential for monthly precipitation forecasting. Moreover, an adaptive fuzzy rule‐based framework is described so that the model can be used under climate change.
Inferring shallow groundwater flow in saprolite and fractured rock using environmental tracers
Authors: Cook P. G. , Solomon D. K. , Sanford W. E. , Busenberg E. , Plummer L. N. , Poreda R. J.
Published: February 2004

The Ridge and Valley Province of eastern Tennessee is characterized by (1) substantial topographic relief, (2) folded and highly fractured rocks of various lithologies that have low primary permeability and porosity, and (3) a shallow residuum of medium permeability and high total porosity. Conceptual models of shallow groundwater flow and solute transport in this system have been developed but are difficult to evaluate using physical characterization or short‐term tracer methods due to extreme spatial variability in hydraulic properties. In this paper we describe how chlorofluorocarbon 12, 3H, and 3He were used to infer groundwater flow and solute transport in saprolite and fractured rock near Oak Ridge, Tennessee. In the shallow residuum, fracture spacings are <0.05 m, suggesting that concentrations of these tracers in fractures and in the matrix have time to diffusionally equilibrate. The relatively smooth nature of tracer concentrations with depth in the residuum is consistent with this model and quantitatively suggests recharge fluxes of 0.2 to 0.4 m yr−1. In contrast, groundwater flow within the unweathered rock appears to be controlled by fractures with spacings of the order of 2 to 5 m, and diffusional equilibration of fractures and matrix has not occurred. For this reason, vertical fluid fluxes in the unweathered rock cannot be estimated from the tracer data.
Capacity Reliability of Water Distribution Networks and Optimum Rehabilitation Decision Making
Authors: Schneiter Calvin R. , Haimes Yacov Y. , Li Duan , Lambert James H.
Published: February 2004

Deterioration in pipes contributes to uncertainty about the carrying capacity in water distribution networks. Capacity reliability, defined as the probability that the carrying capacity meets the flow demand, is investigated in this paper to aid in maintenance and rehabilitation decision making for water distribution networks. This paper describes the use of capacity reliability for networks with more than one demand node through finding the probability of a feasible flow given probability distributions of flow capacities in the pipes and fixed nodal demands. The solution procedure generates a set of inequalities that represents a necessary and sufficient condition for feasible flow. A solution procedure is proposed for evaluating the probability that all the inequalities are satisfied by (1) eliminating redundant inequalities and (2) determining bounds for the probability of feasible flow. A decision‐making framework is developed that applies both the capacity reliability measure and the solution approach for maintenance and rehabilitation decision making.
Planning attitudes, lay philosophies, and water allocation: A preliminary analysis and research agenda
Authors: Syme Geoffrey J. , Nancarrow Blair E.
Published: February 2004

Despite the important societal consequences of water policy, community attitudes toward planning, ethics, and equity for allocation of water have received little research attention. This preliminary research was conducted to assess the range and structure of planning attitudes and equity and ethical considerations which might be relevant to the general public's evaluation of water allocation systems. The relationship of these to priorities for water allocation were also examined. The results showed a complex structure for planning attitudes. There were also generalized but clearly defined community approaches to water allocation. A number of significant relationships between planning attitudes and philosophies of allocation were shown. Planning attitudes also related to priorities for water allocation. In practical terms the research provides some preliminary, ethically based evaluative criteria which could be applied to allocation decision‐making systems. Theoretical research possibilities are also outlined.
A New Quantitative Test of Geomorphic Models, Applied to a Model of Braided Streams
Authors: Murray A. Brad , Paola Chris
Published: February 2004

Recent simple cellular models of self‐organized geomorphic patterns embody a new understanding of complex, spatially extended systems. Such models can be difficult to test quantitatively because the statistics traditionally used can be insensitive even to visually obvious variations in a complex pattern. Here we develop a new approach to evaluating such models. We begin by applying to spatial patterns the state‐space reconstruction techniques developed for dynamical systems, producing plots that summarize the patterns in a way that preserves more information than do the statistics usually used in geomorphology. Methods exist for characterizing some aspects of such plots. Here we develop a complementary method for quantitatively comparing state‐space plots in a way that more directly evaluates the similarity between the typical features of spatial patterns. An application of this method to the patterns produced by a cellular braided‐stream model and real braided streams indicates that this approach provides a relatively sensitive way of comparing model‐generated and real spatial patterns.
Influence of subcritical fracture growth on the connectivity of fracture networks
Authors: Renshaw Carl E.
Published: February 2004

A fracture simulation model which incorporates the physics of fracture growth is used to investigate how the mechanics of fracture formation affect the flow characteristics of fractured systems. Fractures are assumed to grow subcritically with the growth rate given by a power law function of the energy available for fracture growth. Flow characteristics are quantified in terms of the percent of networks percolating and the average effective conductivity as a function of the fracture density. For all flaw densities considered and for values of the growth rate exponent α ≤ 1, the flow characteristics primarily depend on the fracture spatial density and are similar to the flow characteristics of networks generated stochastically by assuming the fractures are randomly located. For α ≫ 1, the mechanical interaction of the flaws and fractures imparts an organized structure to the network resulting in isolated fractures, or zones of fractures, which form extensive, connected pathways at significantly lower fracture densities. Experimentally measured values of α for subcritical fracture growth are typically greater than one, suggesting that the flow characteristics of randomly located fractures may not be representative of natural fracture networks thought to have grown subcritically. An analysis of published fracture trace maps suggests that many natural fracture networks have fracture spatial densities near the percolation threshold. It is suggested that this may be due to the existence of a self‐limiting mechanism in fracture network formation.
The Temporally Integrated Monitoring of Ecosystems (TIME) Project Design: 2. Detection of Regional Acidification Trends
Authors: Stoddard John L. , Urquhart N. Scott , Newell Avis D. , Kugler Danny
Published: February 2004

The Temporally Integrated Monitoring of Ecosystems (TIME) project utilizes a hybrid sampling approach to achieve its goal of assessing whether emissions controls, mandated by the Clean Air Act Amendments of 1990, have had their intended effect on lakes and streams. A randomly placed triangular grid is used to select, with known probability, lakes and streams for sampling on either an annual basis (in the case of acid‐sensitive sites) or on a 4‐year rotation. Data from these sites will be used to detect trends in regional characteristics of the target populations. The ability of TIME to detect trends in acid‐neutralizing capacity (ANC) and SO42− is dependent on the amount of variability (year‐to‐year, site‐to‐site, within‐season, and site‐by‐year interaction) exhibited by the sites. In particular, high levels of year‐to‐year variability lead to very low power to detect trends. One method to minimize year‐to‐year variability is to group together sites with similar characteristics. By performing trends tests on seven separate subpopulations the TIME design for northeastern U.S. lakes is predicted to be able to detect trends of the expected magnitudes in ANC (0.5 μeq L−1 yr−1) and SO42− (1.2 μeq L−1 yr−1 with power at or above the 0.90 level and α=0.10.
A Two‐State Integral‐Balance Model for Soil Moisture and Groundwater Dynamics in Complex Terrain
Authors: Duffy Christopher J.
Published: February 2004

A dynamical model is devised for a hydrologic system where unsaturated and saturated storage serve as the principal control on rainfall‐runoff and where complex topography, drainage area, and variable depth of moisture penetration describe the flow geometry. The model is formed by direct integration of the local conservation equation with respect to the partial volumes occupied by unsaturated and saturated moisture storage, respectively. This yields an “integral‐balance” model in just two state variables. The relationship of the dynamical model to field data in complex terrain is found through a joint probability density for terrain features. This serves as a “volume” weighting function to construct conditional averages for the state variables and fluxes over a specified range of terrain features. The scale of averaging could range from hillslopes to river basins. Two examples of the joint probability of terrain features (altitude and aspect) are demonstrated for Valley, Ridge, and Appalachian Plateau digital elevation models. The strategy of a dynamical model formed by conditional averages of state variables with respect to terrain features is proposed as a way of simplifying the dynamics while preserving the natural spatial and temporal scales contributing to runoff response. The parametric form of the storage‐flux or constitutive relationships for the proposed model is determined from numerical experiments in a simple hillslope flow geometry. The results show that a competitive relation exists between unsaturated and saturated storage except for the lowest precipitation rates. Saturation overland flow is proposed to be a storage‐feedback relation. Solutions to the integral‐balance model are presented in terms of the phase portrait, which represents all possible solution trajectories in state‐space. The timing and magnitude of peaks in the runoff hydrograph from pulse‐type input events demonstrate quick flow from near‐stream saturated storage, saturation overland flow including rejected rainfall (storage‐feedback), and late‐time infiltration from upslope subsurface flow.
Hydrology of Spring‐Dominated Streams in the Oregon Cascades
Authors: Manga Michael
Published: February 2004

Spring‐dominated streams in the Oregon Cascades are often characterized by nearly constant discharge and by peak flows that occur in late summer or fall, several months after the annual snowmelt. A model is presented that can account for the temporal variations of discharge and the delay between snowmelt and the period of peak streamflow. Springs are assumed to be fed by an unconfined aquifer that is recharged by the annual snowmelt. Model results depend primarily on the effective permeability and the dimensions of the aquifer. Four spring‐fed streams in the Deschutes River basin in the Oregon Cascades are studied. The effective permeability of the young (<2 Ma) volcanic rocks that comprise the aquifers is inferred to be O(10−11) m2.
Evaluating the Reliability of the Stream Tracer Approach to Characterize Stream‐Subsurface Water Exchange
Authors: Harvey Judson W. , Wagner Brian J. , Bencala Kenneth E.
Published: February 2004

Stream water was locally recharged into shallow groundwater flow paths that returned to the stream (hyporheic exchange) in St. Kevin Gulch, a Rocky Mountain stream in Colorado contaminated by acid mine drainage. Two approaches were used to characterize hyporheic exchange: sub‐reach‐scale measurement of hydraulic heads and hydraulic conductivity to compute streambed fluxes (hydrometric approach) and reachscale modeling of in‐stream solute tracer injections to determine characteristic length and timescales of exchange with storage zones (stream tracer approach). Subsurface data were the standard of comparison used to evaluate the reliability of the stream tracer approach to characterize hyporheic exchange. The reach‐averaged hyporheic exchange flux (1.5 mL s−1 m−1), determined by hydrometric methods, was largest when stream base flow was low (10 L s−1); hyporheic exchange persisted when base flow was 10‐fold higher, decreasing by approximately 30%. Reliability of the stream tracer approach to detect hyporheic exchange was assessed using first‐order uncertainty analysis that considered model parameter sensitivity. The stream tracer approach did not reliably characterize hyporheic exchange at high base flow: the model was apparently more sensitive to exchange with surface water storage zones than with the hyporheic zone. At low base flow the stream tracer approach reliably characterized exchange between the stream and gravel streambed (timescale of hours) but was relatively insensitive to slower exchange with deeper alluvium (timescale of tens of hours) that was detected by subsurface measurements. The stream tracer approach was therefore not equally sensitive to all timescales of hyporheic exchange. We conclude that while the stream tracer approach is an efficient means to characterize surface‐subsurface exchange, future studies will need to more routinely consider decreasing sensitivities of tracer methods at higher base flow and a potential bias toward characterizing only a fast component of hyporheic exchange. Stream tracer models with multiple rate constants to consider both fast exchange with streambed gravel and slower exchange with deeper alluvium appear to be warranted.
Subsidence of Agricultural Lands in the Sacramento‐San Joaquin Delta, California: Role of Aqueous and Gaseous Carbon Fluxes
Authors: Deverel Steven J. , Rojstaczer Stuart
Published: February 2004

To examine the causes of land subsidence on marshes drained for agriculture, carbon fluxes and changes in land‐surface elevation were determined on three islands in the Sacramento‐San Joaquin Delta, California. Over the time period of March 1990 to May 1992, gaseous CO2fluxes were determined approximately monthly using closed chambers, and dissolved carbon fluxes were determined from the dissolved carbon loads of drainage ditches adjacent to each field site. Surface elevation changes were measured continuously by measuring the distance between the land surface and an elevated structure anchored beneath the organic soil layer. Gaseous CO2fluxes accounted for most of the permanent subsidence measured over the monitoring period. Gaseous CO2fluxes are strongly affected by soil temperature. Net subsidence rates for the three islands, which have different depths of organic soils and water‐management practices, range from 0.46 to 1.06 cm/yr. Estimates of dissolved organic carbon fluxes for all three islands were small relative to gaseous CO2losses and represent <1% of the measured subsidence.
Tracer transport in a stochastic continuum model of fractured media
Authors: Tsang Y. W. , Tsang C. F. , Hale F. V. , Dverstorp B.
Published: February 2004

A stochastic continuum model of a fractured medium conditioned on a specific set of field data is developed. Both the more conductive fractures and the less permeable matrix are generated within the framework of a single‐continuum stochastic model based on nonparametric indicator geostatistics. In the stochastic model the fracture zones are distinguished from the matrix by imposing a long‐range correlation structure for a small fraction (the highest approximately 11%) of the hydraulic conductivity in the preferred planes of fracture zones. Results of flow arid transport simulation in three dimensions (3D) are used to illustrate the large spatial variability of point measurements, but for spatially integrated quantities the variability is reduced and results become less sensitive to correlation structure. Therefore it is suggested that spatially integrated quantities may be a more appropriate choice for predicting flow and transport in a strongly heterogeneous medium in that they are more commensurate with the level of our ignorance of the site. The issue of spatial variability giving rise to uncertainty in the site characterization of a heterogeneous medium and the prediction of transport results is also addressed. Simulations are carried out for 3D transport from point tracer sources at hundreds of locations in the medium. The breakthrough curve from each point source release is characterized by two parameters: the mean transport velocity and the dispersion coefficient. The results are presented as a statistical distribution of the transport parameters, thus quantifying the uncertainty in predicting flow and transport based on a limited amount of site characterization data.
Estimating Unsaturated Soil Hydraulic Properties from Tension Disc Infiltrometer Data by Numerical Inversion
Authors: Šimůnek J. , van Genuchten M. T.
Published: February 2004

Tension disc infiltrometers are becoming increasingly popular devices for in situ measurement of the unsaturated hydraulic properties of soil. Tension infiltration data are generally used to evaluate the parameters Ks and α in Gardner's exponential model of the unsaturated hydraulic conductivity. Either two measurements using different disc diameters or measurements with a single disc but using multiple pressure heads are then used. In this paper we describe a parameter estimation procedure which combines the Levenberg‐Marquardt nonlinear parameter optimization method involving weighted least squares, with a quasi‐three‐dimensional numerical model which solves the variably saturated flow equation. By numerical inversion of Richards' equation the unknown parameters in van Genuchten's model of the unsaturated soil‐hydraulic properties are estimated from observed cumulative infiltration data during transient water flow. Additional measurements of the pressure head or water content, as well as a penalty function for constraining the unknown parameters to remain in some feasible region (Bayesian estimation), can be optionally included into the parameter estimation procedure. The problem of optimal sampling design, that is, selecting the best points in space and time for making measurements, is addressed by studying the sensitivity of the objective function to changes in the optimized hydraulic parameters. We calculate objective functions based on available cumulative infiltration, pressure head, and water content measurements and also on several combinations of these data. The behavior of the objective function in three‐dimensional parameter space is evaluated by means of a series of two‐dimensional response surfaces. The utility of the parameter estimation procedure is demonstrated using numerically generated data. The sensitivity of the procedure to different initial estimates of the model parameters is also discussed.