bluuebunny/crossref_metadata_2025 huggingface dataset:
Development and application of the spheroidal coordinate based T matrix solution to elastic wave scattering
Authors: Hackman Roger H. , Lim Raymond
Published: February 2004

A spheroidal coordinate based T matrix formalism is developed for the description of elastic wave scattering from a collection of obstacles embedded in a homogeneous, unbounded medium. The development of the spheroidal formalism for the scattering from a single obstacle is a refinement of the earlier work of Hackman (1984), a discussion of the extension of this approach to a many inclusion system is a generalization of Lim and Hackman (1992). Calculations are presented for the elastic wave scattering from (single) prolate spheroidal and finite cylindrical objects. The single inclusion calculations presented have bearing on a study of anechoic mechanisms in composite materials presented earlier (Lim and Hackman, 1990).
Excitation of electromagnetic waves by delta function current sheets in the ionospheric plasma
Authors: Omid Mahmoud , Hayakawa Masashi
Published: February 2004

The excitation of electromagnetic wave packets by delta function current sheets in a dispersive magnetoplasma is studied, in which the wave normal direction is vertical to the current sheet but the Earth's magnetic field is assumed to have an arbitrary direction to the wave normal direction. The temporal and spatial evolution of wave packets in the ionospheric F2 plasma layer is analyzed by an exact numerical inversion method of the Laplace transform. Properties of these excited waves are discussed in different frequency bands; (1) quasi‐free space modes, (2) whistler mode, and (3) hydromagnetic wave modes. By interpreting the waveforms at fixed points in the plasma as experimental data, the obtained results provide explicit information about the nature of the transient response, and this can be used as a diagnostic technique of the plasma.
The severe depletion of turbulent echo power in precipitation observed using the Chung‐Li VHF Doppler radar
Authors: Chu Yen‐Hsyang , Lin Chun‐Hsien
Published: February 2004

Earlier investigations of atmospheric precipitation made with a VHF radar demonstrate that below the level of melting (i.e., 0°C isotherm) the VHF echo intensities from precipitation particles are generally much weaker than those from turbulent refractivity fluctuations by about 20–30 dB. However, in this article, using Chung‐Li VHF radar data, the precipitation echoes in the height range from about 2.5 to 4 km, which is below the melting level during the season of spring over the Taiwan area, may far exceed the turbulent returns in intensity by about 15 dB, contradicting earlier observations. A comparison of the echo power profiles of the precipitation with those of refractivity fluctuations reveals that the weak backscatter from refractivity can be attributed to abnormally severe depletion of turbulent echo power in a convective cloud with fairly intense upward air motion and moderate precipitation. Furthermore, the correlation between hydrometeor terminal velocity and precipitation echo power is positive, while the air motion and turbulent echo power are anticorrelated. On the basis of these observations a plausible mechanism is proposed to explain the abnormal depletion of refractivity echo power.
A spectral technique for the detection and correction of coherent interference effects on incoherent scatter radar power profiles
Authors: Zhou Qihou , Mathews John D.
Published: February 2004

We report a scheme for the detection and removal of coherent interference from incoherent scatter radar power profile (total power versus range) results. Our approach depends on the availability of highly signal‐aliased power spectra formed at each range gate from the same data set that is employed to form the corresponding power profile point. We utilize the ratio R of the standard deviation of each power spectrum to its average to “measure” the degree of contamination by coherent interference. When interference exists, R is usually significantly larger than it is for the base statistical process. This fact is used to determine which spectra will be processed for interference removal. This approach has been verified experimentally and is now used on a “production” basis.
Three years of C band signal measurements for overwater, line‐of‐sight links in the mid‐Atlantic coast: 2. Meteorological aspects of sustained deep fades
Authors: Goldhirsh Julius , Dockery G. Daniel , Meyer James H.
Published: February 2004

The morphology of deep and long‐lived fades for line‐of‐sight, overwater propagation links in the mid‐Atlantic coast are examined. Such events, known as sustained deep fades (SDF), are analyzed employing weather maps, in situ measurements from radiosondes, an instrumented helicopter, and sensors on coastal platforms. These events occurred exclusively from November through July over the 3 years in which fade statistics were amassed; no SDF events were observed during the months August, September, or October. The SDF events biased the 3‐year fade statistics described in a companion paper. The results have demonstrated that synoptic weather conditions created by a sustained high‐pressure system over the subtropical Atlantic and occasionally by a sustained low‐pressure system whose center lies west of the coastal region result in a steady flow of warm, humid air at higher altitudes. When coupled with colder water conditions, such a flow results in a surface temperature inversion, an increase of water vapor pressure with altitude, and a positive lapse rate of the radio refractivity. This condition leads to severe subrefraction resulting in fades ranging from 20 to 60 dB for durations in excess of 2 hours and lasting as long as 2 days. Propagation modeling using a refractivity profile derived from sensors on board a helicopter during an SDF event predicted the same range of fades as were measured during that period.
Method for calculating wave attenuation coefficients in plasmas
Authors: Ghanashev I. , Shivarova A. , Schlüter H.
Published: February 2004

A new simple method for computing the attenuation (due to dielectric losses) of weakly damped electromagnetic waves in a wide class of wave‐guiding structures is suggested. The wave guiding structures considered contain time dispersive dielectric media, and restrictions are set neither on the type of modes nor on the cross‐section geometry. Only one of the dielectrics of the structure is supposed to be lossy. The wave attenuation coefficient results directly from the slope of the wave dispersion presented as a dependence of the wave number on the permittivity of this dielectric at fixed wave frequency. The method is demonstrated on structures containing cold isotropic plasmas. For this case the method was found to be applicable even for some configurations with inhomogeneous plasma filling.
Anomalous noise from whistlers and radio atmospherics due to quasi‐transverse electromagnetic mode propagation of VLF signals in the Earth‐ionosphere waveguide
Authors: Mielke T. A. , Mideke M.
Published: February 2004

Observations of unusual VLF signals in the 0–4 kHz range are presented which appear to be band‐limited segments of sferics and whistlers. These observations can be explained in terms of quasi‐transverse electromagnetic mode propagation over long distances in the Earth‐ionosphere waveguide.
First estimates of HF‐induced modifications of the D region by the HF Active Auroral Research Program facility
Authors: Milikh G. M. , Freeman M. J. , Duncan L. M.
Published: February 2004

A one‐dimensional, self‐consistent computer code was used to predict electron temperature and density changes in the lower ionosphere caused by high power, high frequency (HF) radio wave heating. The code was used to obtain first estimates of ionospheric perturbations due to the proposed HF Active Auroral Research Program (HAARP) heating instrument, which is currently in the early phases of construction. Simulations were conducted using both pulsed and continuous wave (CW) heating modes, under a variety of conditions. Results indicate that electron temperatures will be increased by up to a factor of 15, and electron densities will be changed by up to 50%relative to natural levels.
Bioelectromagnetics in Russia
Authors: Zolin V. F.
Published: February 2004

A review is given of research in Russia on electromagnetic field (EMF) mechanisms of action, biological effects, and medical diagnostic and therapeutic application. Possible applications of the sensitive multichannel radiometers at GHz and IR for dynamic medical diagnostics are also outlined. Dynamic imaging of patients is used for diagnosis of diseases such as disorders of the peripheral blood flow and accompanying diseases.
Physical interpretation of the effect of the antenna heights on multipath propagation occurrence
Authors: Blanchetière Ciarletti V. , Sylvain M.
Published: February 2004

One aim of the Propagation en Air Clair et Météorologie 2 (PACEM2) experiment was to obtain a model for clear air propagation applicable to space diversity reception. The statistical results derived from nearly 4 years of continuous recording show an asymmetry between both diversity channels, the higher antenna being significantly more affected by multipath effects than the lower one. A possible effect of the antenna heights on multipath propagation occurrence is included in the latest version of the International Radio Consultative Committee (CCIR) prediction formula, which takes the inclination angle of the path into consideration, but the difference experimentally observed between both channels is much greater than the prediction derived from this CCIR formula. The object of this paper is to give a physical interpretation of the effect of antenna height on multipath occurrence leading to a quantitative agreement with the data. The physical model we use is built on available statistics of radio meteorological data. For each particular meteorological situation the shape and size of the region of interference between multiple propagation paths are then obtained by means of ray tracing. The results of these computations lead to the determination of the relative probability of occurrence of multipath propagation as a function of path length and antenna heights. A rather good agreement with the data is obtained when these parameters are given the values of the experimental link.
An alternative method for inferring winds from spaced‐antenna radar measurements
Authors: Lataitis Richard J. , Clifford Steven F. , Holloway Christopher L.
Published: February 2004

One variation of the spaced‐antenna technique for measuring atmospheric winds using a pulse Doppler radar is based on a calculation of the complex temporal cross‐correlation function for the backscattered fields detected by a pair of spaced receiving antennas. The delay to the peak of the cross‐correlation amplitude, adjusted for the temporal evolution of the backscattered field pattern through an extended analysis, is related to the so‐called trace velocity along the antenna pair baseline, with two nonparallel baselines yielding the full horizontal wind vector. We suggest an alternative method for inferring the horizontal wind from the temporal cross correlation. We demonstrate that the slope of the cross‐correlation amplitude at zero lag, normalized by the level of the cross‐correlation amplitude at zero lag, is directly proportional to the component of the wind velocity along the antenna pair baseline. We illustrate that this measure of the horizontal wind is insensitive to the temporal evolution of the backscattered field pattern. Therefore no extended analysis is needed to estimate the horizontal wind vector. The advantage of this approach is that it is a very simple and direct method of retrieving the horizontal wind. Our theory indicates that for many cases of interest the proportionality constant relating the normalized slope to the wind component along the antenna pair baseline depends only on the transmitting and receiving antenna diameters and on the separation between the receiving antennas. This constant can be calculated, or the system can be calibrated against a rawinsonde retrieval. A disadvantage to the slope technique is that it is somewhat sensitive to the degree of vertical anisotropy of the scattering medium. We loosely define the degree of vertical anisotropy r as the ratio of the vertical to horizontal correlation length of refractive index irregularities having a spatial scale of the order of the Bragg wavelength λ/2, where λ is the radar wavelength. In the neutral atmosphere, r typically satisfies 0 < r ≤ 1. We find that for a horizontally isotropic, Kolmogorov‐type refractive index spectrum with λ/2 < L0z, where L0z is the vertical outer scale, the slope method is useful provided r is greater than the effective system beam width λ/De, where De is an effective system antenna diameter (approximately equal to the transmitter antenna diameter). We note that this same restriction ensures that the correlation scale of the backscattered field (i.e., the pattern scale) is solely a function of the antenna diameters. The slope method is therefore limited to scattering from isotropic to moderately anisotropic turbulence (i.e., λ/De < r ≤ 1) provided λ/2 < L0z and is perhaps best suited for wind profiling in the atmospheric boundary layer. A comparison of horizontal winds obtained by applying the slope technique to data from a 915‐MHz spaced‐antenna radar and winds from a rawinsonde retrieval showed good agreement.
Radio star and satellite signal scintillation by E region irregularities: A case study
Authors: Om Vats Hari , Chandra Harish , Deshpande M. R. , Vyas G. D.
Published: February 2004

Here we present the study of daytime scintillation due to E region irregularities at low latitudes from a set of simultaneous observations on June 3, 1993. The effects of these irregularities are clearly seen on (1) the 103 MHz signal of the radio star 3C 196 recorded at Rajkot and at Thaltej (near Ahmedabad, India), (2) the 244‐MHz radio beacon of Fleetsat satellite (73°E) recorded at Ahmedabad, and (3) the ionosonde being operated at Ahmedabad. These observations provide an estimate of the irregularity patch with a spatial extent of around 400 km east‐west and 80 km north‐south. As the subionospheric points of the radio source and satellite observations are quite far apart, a study on the dynamics of these irregularities is also possible which indicates a southward movement of the patch. The drift speed of ∼ 60 m/sec is calculated from the upper frequency roll off of the scintillation spectrum at 103 MHz. This is found to be consistent with earlier investigations of blanketing Es near magnetic equator.
Ionospheric radio tomography using maximum entropy 1. Theory and simulation studies
Authors: Fougere Paul F.
Published: February 2004

Ionospheric tomography is viewed as an inverse problem, the direct problem of which is linear. The total electron content (TEC) is the integral of the electron density along rays from a satellite to a set of receivers on the ground. If the continuous ionosphere is replaced by a discrete set of finite‐sized cells, in each of which the density is constant, then the integrals become simple sums of density times ray lengths in each cell. The equation of the direct problem is then a simple matrix equation. Since the electron density distribution is positive and normalizable, it is isomorphic to a probability distribution for which the maximum entropy method (MaxEnt) yields a unique solution which uses all available information and is maximally noncommittal with respect to all unavailable information. The ionospheric problem is distinct from other forms of tomography in that the rays from the satellite to the receivers are all nearly vertical; horizontal rays which would carry information on the average vertical profile are not available. Without such information the problem cannot be solved; but the nearly vertical rays do carry some such information, although very weakly. The present method is able to extract the required vertical profile for three different test cases, as follows: An arbitrary vertical profile is assumed; for convenience a linear combination of orthonormalized simple Chapman profiles is used; the coefficients of these functions are found using a nonlinear optimization procedure in conjunction with repeated MaxEnt calculations. These test results are unique: There is enough information in the simulated TECs to determine a unique reconstruction in all three test cases. No artifacts appear in the reconstructions, and the resolution is excellent.
On the influence of specular reflections in MF radar wind measurements
Authors: Sürücü Fahri , Franke Steven J. , Kudeki Erhan
Published: February 2004

Numerical simulations are employed to investigate possible biases in Medium Frequency (MF) wind estimates. To explain apparent biases inferred from Arecibo Initiative in Dynamics of the Atmosphere (AIDA) campaign measurements, Hines et al. (1993) have suggested that MF radar estimates of horizontal winds should be biased toward the phase speed of perturbing gravity waves if the MF scattering process is modulated by the passage of such waves through the scattering volume. One of the proposed scenarios involves modulation of the tilts of anisotropic irregularities by the perturbation winds associated with gravity waves. We use a two‐dimensional propagation model in which anisotropic scattering centers are represented by conducting line segments located at random positions inside the radar scattering volume. The scatterers are advected and tilted by the superposition of a mean wind and the perturbation winds associated with a gravity wave. Simulated complex time series, which would be recorded by a two‐receiver antenna array with a typical MF radar system parameters, are analyzed to obtain spaced antenna (SA) wind estimates for comparison with the specified wind speed at the center of the scattering volume. Numerical simulations are carried out for many different sets of gravity wave parameters. Each simulation is performed twice, once with essentially isotropic scatterers (line segments of length λ/2) and again with anisotropic (“specular”) scatterers of length 10 λ. Numerical results indicate that individual SA wind estimates are accurately predicted by the wave‐induced error model of Kudeki et al. (1993). This is true for both isotropic and specular scatterers as long as the gravity wave has a horizontal wavelength larger than the horizontal dimension of the radar range cell. The results also indicate that estimated winds averaged over one period of the perturbing gravity wave exhibit statistically significant biases when the scattering centers are highly anisotropic, but the bias is not necessarily toward the phase speed of the perturbing gravity wave and, more importantly, the magnitude of the bias exhibited in our simulations was always less than 15 m/s. Implications of the numerical results are discussed in light of observed discrepancies as large as 40 m/s in recent AIDA campaign comparisons between MF and incoherent scatter radar winds. In particular, it is argued that the simulation results support the wave‐induced fluctuations model put forth by Kudeki et al. (1993) as an explanation for the AIDA results.
Phase scintillation and irregularities in the northern polar ionosphere
Authors: Kersley L. , Russell C. D. , Rice D. L.
Published: February 2004

Radio transmissions from Navy Navigation Satellite System (NNSS) satellites have been used to study phase scintillation and ionospheric irregularities at northern polar latitudes. The observations were made at Ny Alesund, a station in the vicinity of the daytime cusp and under the polar cap on the nightside. The analysis concentrates on data obtained over an extended period when more than 7000 satellite passes were recorded to provide estimates of the σф phase scintillation parameter for some 300,000 data intervals. The results show a marked difference in scintillation and irregularity occurrence with season. During the summer months the main feature is a maximum around magnetic noon, apparently associated with irregularities in the cusp region. In winter, irregularities are found at all times of day though there is a noon maximum that extends in a latitudinal belt into the afternoon and evening. The premidnight sector shows the highest occurrence levels in the polar cap. Investigation of signal fading rates indicates that doppler shifting resulting from irregularities convecting rapidly towards the station in the premidnight polar cap may contribute to the occurrence patterns. Under quiet geomagnetic conditions during summer the irregularities form a belt essentially in the morning sector, following the auroral oval. There is expansion to lower latitudes with increased geomagnetic activity and increased occurrence in the afternoon. In winter, irregularities can be seen at all hours even under quiet conditions, with the afternoon and evening showing greatest occurrence at times of geomagnetic disturbance. The pattern of scintillation occurrence with respect to the station indicates that the irregularities may be in the form of field‐aligned sheets of low axial ratio. In contrast to observations of L shell confined sheets at auroral latitudes there are indications from the current work that the irregularities are stretched in the antisunwards convective flow across the polar cap.
High‐frequency analysis of Gaussian beam scattering by a two‐dimensional parabolic contour of finite width
Authors: Anastassiu Hristos T. , Pathak Prabhakar H.
Published: February 2004

A high‐frequency analysis of the reflection and diffraction of well‐focused electromagnetic Gaussian beams (GBs) by a perfectly conducting parabolic surface with an edge is presented for the two dimensional case. The fields are evaluated analytically via the physical optics (PO) approximation, and only the reflected field is expressed as a GB. The GB procedure developed here is expected to be highly efficient in applications involving large reflector antennas, since it avoids the conventional time‐consuming numerical evaluation of the PO integrals. Some numerical results are provided to indicate the accuracy of the analytical expressions obtained. The three‐dimensional case will be reported separately.
Investigating the crustal structure of a strike‐slip “step‐over” zone along the Great Glen fault
Authors: McBride J. H.
Published: February 2004

Strike‐slip step‐overs and bends are regions of anomalous deformation that may yield clues to the mechanical behavior of the Earth's crust. Experimental reprocessing of a marine deep seismic reflection profile cutting across an ∼35‐km‐wide right‐stepping step‐over developed on the Great Glen fault system north of the Inner Moray Firth basin (east of Northwest Highlands, Scotland) reveals a restricted zone of prominent reflections and diffractions beginning at 7–8 km depth, continuing as deep as the interpreted Moho discontinuity at 26–27 km. Geological interpretation of this zone suggests a concentration of possible diffractors marking sharp structural disruption underlain by a ∼12‐km thick layer of subhorizontal and moderately dipping reflectors that dominates the lower crust in the step‐over region. The known kinematic history of the Great Glen fault system, together with the observed direction of the step‐over, implies that a zone of contraction would have formed within the step‐over during early Caledonian sinistral strike slip. The seismic reflection structure can be interpreted as developing by deformation associated with contraction and possible block rotation between the two fault segments such that the diffractive zone represents steep structure in a near‐vertical zone of strike slip which passes deeper into a layer of low‐angle dipping thrusts or shear zones. The results of this study are consistent with a concept of mechanical detachment in the middle crust as documented for areas of contraction along active strike‐slip faults.
Relationship between uplift of the metamorphic core of the southernmost Andes and shortening in the Magallanes foreland fold and thrust belt, Tierra del Fuego, Chile
Authors: Klepeis Keith A.
Published: February 2004

Rocks containing upper amphibolite facies assemblages reflecting Mesozoic‐Cenozoic metamorphism form the 5000 km2 core of the southernmost Andes in Cordillera Darwin (approximately 55°S latitude). Results from mapping along a 50‐km, continuous transect across‐ the southernmost Andes on the main island of Tierra del Fuego show structural relationships between the Late Cretaceous to early Tertiary uplift of the metamorphic core in northern Cordillera Darwin and mid‐Cretaceous to Paleogene shortening in the adjacent Magallanes foreland fold and thrust belt. New data indicate that pre‐Late Jurassic basement schists exposed along the northeastern edge of Cordillera Darwin were uplifted along a discrete, brittle, continent‐vergent thrust fault. This thrust overlies a series of thin‐skinned thrusts and back thrusts that affect Mesozoic‐Tertiary rocks of the Magallanes foreland basin. The thrust that uplifts basement schists is interpreted to have formed out of sequence with respect to the thin‐skinned structures because it placed the schists over previously shortened Lower Cretaceous sedimentary strata of the Magallanes basin. The geometry of thrusts observed across the transect and their inferred sequence of deformation indicate a thick‐skinned, contractional mechanism for uplift of the pre‐Late Jurassic schists of northern Cordillera Darwin relative to the structural level of metamorphic basement in the Magallanes foreland. The relative time of out of sequence thrusting and basement uplift inferred from thrust geometries in this paper is consistent with the latest Cretaceous to early Tertiary episode of rapid exhumation and cooling of the metamorphic core of the southern Andean orogen determined in previous structural, metamorphic, and geochronological studies.
Structural and tectonic evolution of the Humber Zone, western Newfoundland 2. A regional model for Acadian thrust tectonics
Authors: Waldron John W. F. , Stockmal Glen S.
Published: February 2004

The Humber Zone of the western Newfoundland Appalachians represents a Cambrian‐Ordovician passive continental margin which was deformed in Taconian (mid‐Ordovician) and Acadian (Silurian‐Devonian) orogenic events. A deformation front is imaged in seismic reflection data offshore of western Newfoundland. Structures associated with this deformation front are exposed on Port au Port peninsula, where Silurian rocks are strongly deformed but Mississippian strata are flat lying, indicating that latest thrusting was Acadian. A gravity low in the Gulf of St. Lawrence corresponds to a sediment‐filled Acadian foreland basin. Previous models suggest that the on‐land shelf succession is autochthonous to parautochthonous. However, two Lithoprobe seismic reflection transects show sub‐horizontal reflections between 2 and 5 s two‐way travel time, which extend up to 85 km east of the thrust front. These are interpreted as autochthonous platform and basement. In this model, shallower reflectors and outcropping units include both allochthonous platform and basement, comprising the Acadian Port au Port allochthon. The Taconian Humber Arm allochthon was carried westward as a high structural slice during thrusting of this allochthon. No major structural discontinuity exists between Grenville age crystalline rocks of the Long Range massif and platform rocks interpreted as allochthonous in the northern seismic line. A monocline at the southern extremity of the Long Range probably represents an oblique or lateral hanging wall ramp above the basal detachment. Within the Long Range thrust zone at the western margin of the massif the Long Range thrust shows only a few kilometers of displacement. However, the Parsons Pond thrust, which we interpret to run offshore at Green Point, juxtaposes contrasting successions with different structural and thermal histories; it probably carries a much larger amount of the total displacement. The basal decollement of the Port au Port allochthon is therefore interpreted to pass beneath the southern part of the Long Range massif.
Structural outline of a Tertiary Basement‐cored uplift/inversion structure in western Spitsbergen, Svalbard: Kinematics and controlling factors
Authors: Braathen A. , Bergh S. G. , Maher H. D.
Published: February 2004

The Tertiary fold‐and‐thrust belt of Spitsbergen can be divided into a western basement‐involved fold‐thrust stack and a central‐eastern foreland fold‐and‐thrust belt. In western Nordenskiøld Land the first‐order structure is an ENE‐verging basement‐cored fold and fault complex involving Paleozoic to Tertiary strata. The northern part reveals an upright, monocline geometry of east tilted sedimentary cover units with associated layer parallel to low‐angle thrusts and folds. These structures consist of two populations oriented both parallel to (NNW–SSE) and oblique to (WNW–ESE) the general structural trend of the fold complex. In the central and southern parts of west Nordenskiøld Land the fold complex involves tilted basement cut by steep, transverse faults and late normal faults. The east limb of the fold complex displays repeated basement and Paleozoic strata (Orustdalen formation) in its core and Mesozoic (Triassic) strata influenced by map‐scale chevron folds and two decollement levels, all located above an eastward rotated, major detachment fault, the Kleivdalen Thrust. Establishing fold‐fault relations includes a three‐stage structural history in the fold complex as follows: (1) a phase of early NNE–SSW shortening associated with WNW–ESE folds and thrusts and (2) a dominant ENE–WSW, basement‐involved shortening leading to the first‐order, NNW–SSE‐striking fold complex, followed by (3) approximately E–W extension. The resulting structures and structural variability along strike as well as across strike appear to have been controlled by basement and Carboniferous basin structures underlying the Permian‐Cretaceous platform strata. Restored stratigraphic sections based on thrust‐repetition of basement and cover (e.g., within a type section of the Carboniferous Orustdalen formation) support such an interpretation. A tentative inversion tectonic model reproduces the position(s) of local and major thrust ramps and associated folds, as a result of inheritance from Carboniferous basin structures.
The timing of and nature of greenschist facies deformation and metamorphism in the upper Pennine Alps
Authors: Barnicoat A. C. , Rex D. C. , Guise P. G. , Cliff R. A.
Published: February 2004

The upper Pennine nappes in the western Alps are cut by structures that locally place European basement rocks over those of the Piemonte zone, which represent the sutured remains of Alpine ocean crust. In the Täschalp area of Switzerland, the Mischabel “backfold” and associated SE verging shearing carries gneisses of the Grand Saint Bernard nappe over eclogitic material of the Zermatt‐Saas zone. Greenschist‐facies assemblages are developed locally in the foot wall and hanging wall of the Mischabel structure as well as in the sheared rocks of the thrust zone. Rb‐Sr, 40Ar/39Ar, and U‐Pb studies of white micas, amphiboles and sphenes show that greenschist assemblages developed at circa 44 Ma in the Zermatt‐Saas zone of the footwall and at circa 40 Ma in the thrust zone. Rb‐Sr data from the literature document muscovite and biotite ages in strongly foliated Grand Saint Bernard gneisses of the hanging wall of circa 36 and 32 Ma, respectively. These are thought to indicate that deformation on the Mischabel structure continued for at least 5 m.y. Rapid cooling inferred for the Zermatt‐Saas ophiolites in the mid‐Eocene may document either the tectonic emplacement of the ophiolite over the underlying Monte Rosa massif or their extensional unroofing. It is not possible to determine with certainty whether the Mischabel structure is extensional or contractional on a crustal scale. The age of cooling and deformation reported here show that it is not possible that the Mischabel structure was responsible for the exhumation of the high‐pressure assemblages in the Zermatt‐Saas and Monte Rosa rocks.
Thermotectonic evolution of the Grenville Province of western Labrador
Authors: Connelly J. N. , Rivers T. , James D. T.
Published: February 2004

The Grenville Province in western Labrador comprises three lithotectonic terranes, structurally stacked from northwest to southeast, whose geometric relationships evolved over a time interval from the Early Proterozoic to the Grenvillian Orogeny (circa 1.0 Ga). Gagnon terrane lies immediately southeast of the Grenville Front and is dominated by Early Proterozoic sediments that were deposited unconformably on the Archean basement of the Superior Province. It is overlain to the southeast by the Labradorian magmatic arc of Molson Lake terrane which is itself overlain by the highest structural deck comprising Labradorian migmatites and intrusive rocks of Lac Joseph terrane. The Labradorian magmatic arc of Molson Lake terrane was founded on the margin of mid‐Proterozoic Laurentia and was fueled by northwest directed subduction of oceanic crust that existed between Molson Lake and Lac Joseph terranes. Labradorian magmatic rocks of the exotic Lac Joseph terrane were produced in a second, independent subduction zone of undetermined dip direction. No post‐Labradorian magmatic arc is known to have existed in western Labrador, implying that all three terranes were proximal (part of the same plate) by the end of the Labradorian Orogeny. Rifting of the post‐Labradorian configuration, perpendicular to the Labradorian Front led to the emplacement of the Shabogamo‐Michael Gabbro dyke swarm in a linear belt across the future northern Grenville Orogen. The present terrane geometry of western Labrador is a product of northwest directed thrusting that followed an initial period of crustal thickening during the Grenville Orogen at circa 1000–995 Ma, an event accompanied by penetrative deformation and metamorphism only in Gagnon and Molson Lake terranes.
Area balance, depth to detachment, and strain in extension
Authors: Groshong Richard H.
Published: February 2004

A new “lost‐area” cross‐section construction and validation technique is based on the area balance of a graben system formed above a detachment. Area balance requires that the area displaced below the original regional level of each bed in a graben (the lost area) be balanced by the displaced area at the boundary of the system, given by the product of the displacement times the depth to detachment. This relationship is a straight line on a plot of lost area versus depth. The slope of the line is the displacement, and the depth intercept is the depth of the detachment. In general, beds within the graben system undergo both visible and homogeneous layer‐parallel extension. The homogeneous component can be determined from the width of the graben system, the lost area, and the depth to detachment. Conversely, the detachment depth may be calculated from the total layer‐parallel extension and the lost area. The techniques are illustrated by application to models and to very well constrained natural examples, an earthquake‐induced slope failure in Alaska and kilometric‐sized horsts and grabens in the Black Warrior foreland basin of Alabama. A half graben without footwall uplift and a full graben are the same on a lost‐area diagram. A half graben with footwall uplift linked to the hanging wall extension requires that both the hanging wall and footwall areas be included to determine the detachment depth and displacement. All the models show layer‐parallel extension e in the upper levels that are less than the crustal‐scale strain β.
Late orogenic carboniferous extensions in the Variscan French Massif Central
Authors: Faure Michel
Published: February 2004

The Variscan French Massif Central experienced two successive stages of extension from Middle Carboniferous to Early Permian. In the northern Massif Central, the first stage began in the late Visean, immediately after nappe stacking, and is well recorded by Namurian‐Westphalian synkinematic plutonism. The Middle Carboniferous leucogranites widespread in the NW Massif Central (Limousin and Sioule area) were emplaced within a crust extending along a NE–SW direction. At the same time, the hanging wall or “Guéret extensional allochton” moved toward the SE. Several examples of the synextensional plutonism are also recognized in central Limousin: Saint Mathieu dome, La Porcherie, and Cornil leucogranites. These examples illustrate the relationship between granite emplacement and crustal scale deformation characterized by NW–SE stretching and NE–SW shortening. In the central and southern Massif Central (Cévennes, Châtaigneraie, and Margeride areas), plutonism is dominantly granodioritic and exhibits the same structural features: NW–SE maximum stretching and overturning to the SE. Middle Carboniferous (Namurian‐Westphalian) extension was parallel to the Variscan belt both in the Massif Central and southern Armorican area. This extensional regime was active from the late Visean in the north, while compression dominated in the southernmost domains (Montagne Noire and Pyrenées). The second extensional stage occurred from Late Carboniferous to Early Permian. This event was responsible for the opening of intramontane coal basins, brittle deformation in the upper crust, and ductile normal faulting localized on the margin of cordierite granite‐migmatite domes. Data from the coal basins show that the half‐graben is the dominant structural style, except for basins located along submeridianal left‐lateral faults which have pull‐apart geometries. Late Carboniferous extension occurred along the NE–SW direction. The NE–SW maximum stretching direction can be found in the whole Massif Central but is more developed in the eastern part. The extensional direction is transverse to the general trend of the belt, and top‐to‐the‐NE shearing is dominant. Correlations of these two extension directions with neighboring Variscan massifs are discussed.
Structural relationships along a greenstone/shallow water shelf contact, Belingwe greenstone belt, Zimbabwe
Authors: Kusky Timothy M. , Winsky Pamela A.
Published: February 2004

The Belingwe greenstone belt, Zimbabwe, contains a thick tholeiitic basalt and komatiite sequence structurally above shallow water shelf rocks that in turn rest nonconformably over an older gneiss terrain. The nature of the contact between the basalt/komatiite sequence and underlying shallow water shelf rocks represents one of the most critical relationships upon which general models for the tectonic setting of greenstone belts and the petrotectonic environment of komatiites are based. This contact is a 20‐ to 200‐ m thick high‐strain zone characterized by a wide variety of shear zone tectonites derived from both the shelf and overlying lava succession. In the ultramafic part of the high‐strain zone, tectonites include ductile ultramafic mylonites, ultramafic ultramylonites, ultramafic phyllonites, and serpentinite schists, enclosing less‐deformed ultramafic serpentinite blocks. In the part of the shear zone derived from shallow water sedimentary rocks, shear zone tectonites include chert C‐S mylonite, phyllonite, and iron‐rich argillaceous shear bands. Both parts of the shear zone are cut by brittle fault gouge fabrics. We suggest that the tholeiitic basalt and komatiite sequence is allochthonous with respect to the shallow water shelf and refer to it as the Mberengwa allochthon. Contrary to prior suggestions, there is no evidence that the tholeiitic basalt and komatiite succession of the Belingwe greenstone belt sits conformably over the shallow water shelf rocks. Documentation of the shear zone at the base of the Mberengwa allochthon lends support to the idea that the allochthon represents a fragment of an off‐axis oceanic plateau, structurally emplaced over a shallow water shelf during a convergent plate interaction prior to 2.6 Ga.
Simulation of dense vapor migration in discretely fractured geologic media
Authors: Shikaze Steven G. , Sudicky Edward A. , Mendoza Carl A.
Published: February 2004

Density‐induced advection of organic vapors has been demonstrated to be a significant transport process in granular porous media if the permeability is sufficiently high such that density‐induced advection dominates over vapor diffusion. In the context of partially saturated fractured media, the presence of a network of open fractures may lead to rapid rates of density‐induced advection of the vapors in the fractures, even though the matrix may have low permeability. To investigate the various factors which affect vapor migration in discretely fractured porous media, a two‐dimensional finite element model has been developed whereby the porous matrix is represented by rectangular elements and the fractures are represented as one‐dimensional line elements which are superimposed onto the rectangular grid. The cross‐sectional model includes the processes of advection due to density and pressure gradients, and vapor and aqueous diffusion in both the fractures and the porous matrix. Phase partitioning between the vapor and aqueous phases and the aqueous and solid phases is assumed to be at equilibrium. Results from simulations involving a single vertical fracture indicate that density‐induced advection decreases in importance as the fracture aperture decreases. In this case, there appears to be a critical fracture aperture, above which density‐induced advection is the dominant process, and below which vapor diffusion dominates. Simulations that include a network of horizontal and vertical fractures show the importance of matrix properties such as air porosity and water content. An increase in the matrix air porosity results in a higher storage capacity for vapor‐phase contaminants and allows more diffusion to occur from the fractures to the matrix, whereas a higher matrix water content results in an increase in the degree of phase partitioning between the vapor and the water phases. Although these two processes act to retard the migration of the vapor plume, they can be problematic in the context of vapor extraction in fractured porous media. The reason for this is that these processes tend to increase the amount of contaminant mass that exists in the low‐permeability matrix. As a result, vapor extraction in fractured porous media can be very difficult.
An Eulerian scheme for the second‐order approximation of subsurface transport moments
Authors: Naff R. L.
Published: February 2004

The moments of a conservative tracer cloud migrating in a mean uniform flow field are estimated using an operator approximation scheme; results are presented for the second, third, and fourth central moments in the mean flow direction. It is assumed that the spatially variable flow field, and therefore the tracer migration problem itself, is amenable to a probabilistic description; the effects of local dispersion on cloud migration are neglected in this study. Variation in the flow field is assumed to be the result of spatial variation in the hydraulic conductivity; spatial variation in porosity is assumed negligible. The operator approximation scheme, as implemented in this study, is second‐order correct, which requires a second‐order correct approximation of the velocity field correlation structure. Because estimation of the velocity correlation structure is decidedly the most difficult aspect of second‐order analysis, an ad hoc extension of the imperfectly stratified approximation developed earlier is implemented for this purpose. The first‐order approximation resulting from the operator expansion scheme is equivalent to small perturbation Eulerian results presented earlier (Naff, 1990, 1992). The infinite‐order approximation resulting from this scheme is equivalent to the exponential operator results obtained by Van Kampen (1976).
On geomorphological dispersion in natural catchments and the geomorphological unit hydrograph
Authors: Snell John D. , Sivapalan Murugesu
Published: February 2004

Since the introduction of the geomorphological instantaneous unit hydrograph (GIUH) by Rodriguez‐Iturbe and Valdes (1979) there have been a number of different approaches to expressing the geomorphology of a catchment for determining the hydrological response of the catchment. We examine three approaches by which geomorphology can be introduced through the probabilities and lengths of the pathways available within a network: (1) using the Horton order ratios to derive analytical expressions for these pathway parameters, (2) extracting these probabilities and lengths directly from a Strahler ordered network without using the Horton order ratios, and (3) using a contributing area‐flow distance function extracted directly from the digital elevation model (DEM) of a catchment without the assumptions of Strahler stream ordering. We use these techniques to derive the GIUH and the geomorphological dispersion coefficient as defined by Rinaldo et al. (1991) for two catchments in the southwestern region of Australia. The geomorphological dispersion coefficient derived from the area‐distance function expresses the natural dispersion within the catchment in a more fundamental manner than the other methods, and the GIUHs derived from this function exhibit an underlying dispersion of an order of magnitude greater than either of the approaches based on Strahler ordering. We look at the concept of the “completeness” of a network from which we derive catchment parameters and how this concept is related to the threshold area supporting a network derived from a catchment DEM.
Hydrologic characterization of heterogeneous geologic media with an inverse method based on iterated function systems
Authors: Doughty Christine , Long Jane C. S. , Hestir Kevin , Benson Sally M.
Published: February 2004

One way to estimate the hydrologic properties of heterogeneous geologic media is to invert well test data using multiple observation wells. Pressure transients observed during a well test are compared to the corresponding values obtained by numerically simulating the test using a mathematical model. The parameters of the mathematical model are varied and the simulation repeated until a satisfactory match to the observed pressure transients is obtained, at which point the model parameters are accepted as providing a possible representation of the hydrologic property distribution. Restricting the search to parameters that represent self‐similar (fractal) hydrologic property distributions can improve the inversion process. Far fewer parameters are needed to describe a hierarchical medium, improving the efficiency and robustness of the inversion. Additionally, each parameter set produces a hydrologic property distribution with a hierarchical structure, which mimics the multiple scales of heterogeneity often seen in natural geological media. The parameters varied during the inversion create fractal sets known as attractors, using an iterated function system (IFS). An attractor is mapped to a distribution of transmissivity and storativity in the mathematical model. Thus the IFS inverse method searches for the parameters of the IFS (typically tens of parameters) rather than the values of the hydrologic property distribution directly (typically hundreds to thousands of parameters). Application of the IFS inverse method to synthetic data shows that the method works well for simple heterogeneities. Application to field data from a sand/clay sedimentary sequence and a fractured granite produces reasonable results.