bluuebunny/crossref_metadata_2025 huggingface dataset:
The electrostatic characterization of an n‐element planar array using the singularity expansion method
Authors: Mooney J. E. , Riggs L. S. , Baginski M. E.
Published: February 2004

In this paper, the singularity expansion method (SEM) is used to describe the electrostatic charge distribution on an array of thin linear antennas placed in a uniform electric field. The SEM, which has primarily been used to analyze transient scattering problems, decomposes the electromagnetic interaction process into various quantities such as singularities and modes. Using the SEM, the step plane wave induced transient current on the array is expanded in terms of its singularities (poles) in the Laplace transform (complex frequency domain.) The continuity equation is applied to the induced current expression to obtain the transient charge. The electrostatic charge distribution on the array is found by using the final value theorem on the transient charge expression. It is well known that the SEM factorization of a single linear element reveals that a single pole exists in the fundamental resonance region (near ω L/c = π, where L is the length of the scatterer). For a two‐element array, two poles are observed in the fundamental resonance region. This trend continues such that an n‐element array has n poles in the fundamental resonance region. Associated with each pole is a unique modal current and corresponding charge distribution. For example, one of the two fundamental resonance region poles of the two‐element array produces half‐wavelength sinusoidal current distributions whose directions are the same on one scatterer but opposite on the other. The remaining fundamental resonance region pole produces half‐wavelength sinusoidal current distributions whose directions are the same on both scatterers. Corresponding to each mode is a coupling coefficient which determines how much a particular mode couples into the response. A generalization of these results for an n‐eleinent array will be given. Furthermore, the electric polarizability is derived in terms of the SEM electric charge description. The value of this research lies in the elegance and strength of the SEM to factor a problem into various quantities which depend on different variables of the problem. By using the SEM to analyze the n‐element planar array, a much deeper comprehension of the fundamental aspects of the electrostatic interaction process is achieved.
Multiple baseline measurements of ionospheric scintillation induced by high‐power HF waves
Authors: Costa E. , Basu S. , Livingston R. C. , Stubbe P.
Published: February 2004

Eight spaced antennas were used to receive 250‐MHz signals from quasi‐geostationary polar beacon satellites. The system was deployed in such a way as to intercept the ionospheric volume illuminated by the European incoherent scatter (EISCAT) high‐power HF heating facility near Tromsø, Norway (69.6°N, 19.2°E). The spaced‐receiver analysis was then used to investigate the relationship between the elongation, the characteristic velocity, and the intensity of irregularities generated by the heater. The results were also compared with those from natural irregularities. It was observed that the elongation of both classes of irregularities increased with their intensity. During daytime heating experiments the axial ratios of controlled heater‐generated irregularities were found to be a factor of 2 larger than the background natural irregularities. On the other hand, the characteristic velocity of artificial irregularities decreased with their intensity.
On the local minima in phase reconstruction algorithms
Authors: Isernia Tommaso , Soldovieri Francesco , Leone Giovanni , Pierri Rocco
Published: February 2004

Signal reconstruction from phaseless data occurs in electromagnetics both in the diagnostics of large reflector antennas and in the near‐field testing of microwave and millimeter wave antennas. The main difficulty in solving such problems arises from its inherent nonlinearity. Because of the latter, in fact, the solution procedure can converge to a function which has nothing to deal with the actual solution. In this paper we review and summarize the properties of a recently proposed approach to phase retrieval problems which allows to understand and possibly avoid the false solution problem. To this end, ill‐posedness of the problem is briefly recalled, and a convenient generalized solution is introduced as the global minimum of a proper functional whose local minima are indeed the false solutions of the problem. Then the geometrical meaning of such a functional is considered, and properties of the solution are discussed according to the formulation of the problem as a quadratic inverse one. In particular, the relevance of using nonredundant representations for the unknown of the problem and as many independent data as possible are discussed in full detail from both a theoretical and a numerical point of view. Finally, it is shown how the proposed solution procedure, descending from the adopted formulation based on a weaker nonlinearity with respect to the other ones, has indeed a better behavior as far as local minima problem is concerned.
The measurement of gravity wave momentum flux in the lower atmosphere using VHF radar
Authors: Worthington R. M. , Thomas L.
Published: February 2004

Radar measurements have often been used to estimate gravity wave momentum flux profiles in the troposphere, lower stratosphere, and mesosphere. The reliability of the measurements in the troposphere and lower stratosphere is investigated, with a comparison of three techniques employing various radar beam directions and zenith angles, including the standard “symmetric‐beam” method. The study uses the 46.5 MHz MST radar system at Aberystwyth, Wales (52.4°N, 4.1°W). For short‐period waves, errors in momentum flux profiles arising, for example, from differing data quality in the two radial beams sometimes prove difficult to eliminate in the symmetric‐beam method. For long‐period waves, including inertia‐gravity waves, problems arise from the vertical wind measurement which is implicit in any estimate of momentum flux. Furthermore, a difference is sometimes found between the echo powers of the two symmetric radial beams, and this fluctuates with the horizontal wind perturbations from long‐period waves. The possible influence in this power difference of tilted anisotropic scattering layers, which might also bias radial velocity measurements, is discussed. A momentum flux method which measures the vertical wind with a vertical beam, and the horizontal wind with a pair of radial beams, avoids some of the problems at both short and long periods. Agreement of this method with the symmetric‐beam results would allow improved confidence in momentum flux profiles.
Efficient finite element simulation of slot antennas using prismatic elements
Authors: Gong Jian , Volakis John L. , Wang Helen T. G.
Published: February 2004

A hybrid finite element‐boundary integral (FE‐BI) simulation technique to treat narrow slot antennas etched on a planar platform is discussed. Specifically, the prismatic elements are used to reduce the redundant sampling rates and ease the mesh generation process. Numerical results for an antenna slot and frequency selective surfaces are presented in order to demonstrate the validity and capability of the technique.
A localized space‐frequency algorithm for computerized ionospheric tomography
Authors: Na Helen R. , Biswas Chaitali
Published: February 2004

Ionospheric tomography is a rapidly developing technique for imaging ionization distributions. Owing to the system configuration, the resolving capability of these systems is still limited. A number of reconstruction algorithms have been developed which strive to form high resolution images by enhancing the available data with information provided by model ionospheres. This paper presents a new Fourier domain algorithm for ionospheric tomography which uses unique characteristics of ionospheric spectra to enhance the reconstructions. Unlike most model based algorithms, this solution is not limited to a linear combination of model ionospheres and data. This algorithm also uses a localized approach which enables reconstructions of varying resolution in different regions of the ionosphere.
Diffraction of a plane wave by a wedge with thin material coatings
Authors: Lyalinov M. A.
Published: February 2004

The problem of diffraction of a normally incident plane wave by a wedge with thin material coatings is studied. The thin coatings are simulated by the generalized impedance boundary conditions of the third order. At the points of junction of the coatings, that is, on the edge of the wedge, additional constraints are formulated. These constraints, or contact conditions, ensure uniqueness of the solution in the diffraction problem. The solution is determined in an explicit form by means of the Maliuzhinets' technique. The uniform and local asymptotics of the far field are deduced.
Effect of atmospheric turbulence on the spot size of a radio‐acoustic sounding system: A calculation revisited
Authors: Lataitis Richard J. , Clifford Steven F.
Published: February 2004

A theory developed almost 20 years ago suggests that atmospheric turbulence does not significantly alter the average spot size of the radar return from a radio acoustic sounding system (RASS). This theory predicts that the primary effect of turbulence is a reduction of the on‐axis intensity and the formation of extended tails in the spot profile, with little or no broadening of the central core of the spot. For a narrow‐beam radar, the width of the central core was found to be proportional to the diameter of the radar transmitting antenna. We demonstrate that this conclusion is based on an erroneous numerical evaluation of a fourfold integral that rigorously describes the spot profile. We use an alternate formulation to show that the spot profile can be expressed as a convolution of the radar antenna illumination function and a function whose width depends on the strength of turbulence. This convolution is governed by the ratio W/P0, where W is the horizontal dimension of the radar beam and p0 is the characteristic transverse scale of the turbulence‐induced acoustic wavefront distortions, both evaluated at the range R of interest. In weak turbulence (i.e., when W ≪ p0) the spot profile approximates the antenna illumination function. In strong turbulence (i.e., when p0 ≪ W) the spot profile is significantly broadened and has a diameter proportional to λR/p0, where λ is the radar wavelength. The spot broadening is consistent with recent theoretical and experimental work. The approximate spot profile as a function of turbulence strength for a variety of circularly symmetric antenna illumination functions is examined.
Extended observations of fading on a terrestrial microwave link
Authors: Webster Alan R.
Published: February 2004

Results are presented describing the cumulative fading distribution determined over 3 consecutive years during the fading season (June–August) on a 51‐km terrestrial microwave link. The fading is more severe at low signal levels than expected from the International Radio Consultative Committee predictions based on accumulated data from many such links over the years. An attempt is made to isolate the cause of the deep fading in terms of the atmospheric processes involved.
An asymptotic three‐dimensional technique to study radio wave propagation in the presence of a localized perturbation of environment
Authors: Soloviev O. V. , Agapov V. V.
Published: February 2004

This paper presents an analytical‐numerical approach to model problems that are ubiquitous in the theory of radio wave propagation. The method is particularly efficient for considering three‐dimensional diffraction by a localized perturbation of the environment. Here we are concerned with VLF and LF (less than 100 kHz) radio wave propagation in the presence of an irregularity of finite lateral extent (ionospheric disturbance or Earth's surface inhomogeneity). The surface impedance concept is used to model the Earth‐ionosphere waveguide. Our approach is based on preliminary asymptotic integration of the rigorous two‐dimensional integral equation in order to facilitate its further numerical handling. An original computational algorithm is proposed to invert an approximate equation. By reducing CPU time, the developed procedure enables study of both small and comparatively large irregularities. In spite of accepted asymptotic compromises, most of the three‐dimensional effects that are intrinsic to similar problems are obtained during numerical simulations.
Effects of 450‐kg surface explosions on HF radio reflection from the E layer
Authors: Fitzgerald T. Joseph , Carlos Robert C.
Published: February 2004

We describe ionospheric disturbances caused by the passage of acoustic shock waves at the E layer generated by the surface detonation of ordinance with effective yields of 450 kg of high explosive during an exercise conducted by the U.S. Air Force at a bombing range near the Nevada Test Site. We deployed a network of HF bistatic ionospheric sounders consisting of two transmitter and two receiver stations at the Nevada Test Site on April 4, 1991. The frequencies of the transmissions were chosen so that the HF radio waves were totally reflected in the E layer of the ionosphere at an altitude of approximately 100 km. The transmissions were highly stable continuous wave (CW) tones at two frequencies separated by 100 kHz so that two altitudes separated by approximately 0.5 km could be probed. The network sampled four geographic locations in the ionosphere in a line covering a horizontal range of 60 km. At two time periods during the day (1930 and 2400 UT), we detected a series of disturbances in the sounder data that continued for 10 min and consisted of Doppler‐shifted peaks that chirped rapidly from positive to negative Doppler. We describe the effects produced by the surface explosions and interpret the disturbance in terms of diffraction induced by electron density changes accompanying the passage of the acoustic waves from the explosions through the reflection altitudes of the HF transmissions.
Long alternating codes: 1. Search by playing dominoes
Authors: Nygrén Tuomo , Markkanen Markku
Published: February 2004

A new extremely effective method of finding strong alternating codes for incoherent scatter radars has been invented. The power of this method is described by the fact that the longest code known at the moment contains 4,194,304 bits, whereas the older methods could find codes only up to the length of 32 bits. This result means that strong alternating codes are now available for all needs of any present or future incoherent scatter hardware which one can imagine. The search method can be explained in two ways. The first one is an astonishing game of dominoes, which is not useful in the practical search but allows the search method to be easily visualized. The second presentation is a mathematical description of the method in a form suitable for computer search. The method is explained in a series of two papers. In this first paper the presentation of alternating codes in terms of Walsh indices is reviewed and the game of dominoes is described. The second paper gives the underlying mathematical formalism as well as several long codes in terms of their Walsh sequences.
A line‐of‐sight propagation model for calculating atmospheric, sea, terrain multipath, and clutter at microwave frequencies
Authors: Ott Randolph H.
Published: February 2004

The two‐dimensional parabolic wave equation (PE) is solved asymptotically in order to derive a propagation/scattering model for propagation in a stratified atmosphere, together with scattering and diffraction from the sea/terrain surface, within line of sight. The asymptotic solution to the PE begins by expanding the unknown field in a Luneberg‐Kline expansion. The first‐order solution to the Luneberg‐Kline expansion yields the eikonal equation for the ray trajectory, and the second‐order solution yields the transport equation for the energy density of the rays along the trajectory. The solution to the eikonal and transport equations is used to derive an asymptotic result for the plane wave spectrum solution for the PE, referred to as the wave‐theoretic model. The asymptotic solution for the field shows the relationship between a properly directed tube of rays in the atmosphere satisfying the eikonal and conservation of energy equations and the same tube of rays striking an edge and being diffracted, as in the geometrical theory of diffraction (GTD). The solution for the eikonal yields the ray trajectory in the stratified atmosphere and therefore the grazing angle on the sea/terrain profile necessary for calculating multipath and clutter. Also, a new derivation for the backscatter from a deterministic sea/terrain profile is given which agrees with Collin's [1992] new full wave theory and at grazing reduces to the perturbation result with the correct polarization dependence. Incorporating the various electromagnetic propagation/scattering mechanisms in one model yields a prediction tool that can be applied to a large class of propagation/scattering problems such as the analysis of radar performance in a littoral scenario, or the performance of a line‐of‐sight communication link operating over irregular terrain.
Zero‐order time domain scattering of electromagnetic plane waves by two quarter spaces
Authors: Sampaio Edson E. S. , Popov Mikhail M.
Published: February 2004

The zero‐order term of the time domain scattered electric field of an electromagnetic plane wave normally incident upon the surface of two quarter spaces is determined. The general solution is a development from a previous exact and complete solution in the frequency domain. The zero‐order term of the scattered electric field has been computed in the upper medium (z < 0). The incident wave in the frequency domain assumes the same function for three cases: (1) The conductivity vanishes everywhere; (2) only the conductivity of the upper medium is zero; and (3) the three media are conductors. Case 1 helps to understand cases 2 and 3. Case 2 is applicable to geophysical exploration. For cases 1 and 2 a causal time function decaying exponentially with time at every point above the fault (z < 0) describes the waveform of the incident plane wave. The zero‐order term of the scattered field has been computed above the fault. At x = 0 it reduces to a closed expression for case 1 and to a single integral for the other two cases. In the three cases it contains an integral of a Hankel function for x ≠ 0. The computation of the high‐frequency part of the inverse Fourier transform for x ≠ 0 employs asymptotic expressions for the Hankel function using analytical techniques of the geometrical theory of diffraction for cases 1 and 2. For case 3 the inverse Fourier transform may have two possible contributions: either from the residue at a single pole or from the integral along a branch cut in the ω plane. The wave front of the scattered field is well defined in shape, phase, and amplitude. Its amplitude is discontinuous at x = 0, and varies smoothly but presents a sharp jump for ∣ x ∣<<∣ z ∣. For ∣ x ∣ = O(z), there is a numerical noise that oscillates at 100 MHz.
Wideband model of man‐made HF noise and interference
Authors: Lemmon John J.
Published: February 2004

A mathematical model of the waveform generated by man‐made high frequency (HF) noise and interference is presented and discussed. The model is based on wideband (800 kHz) recordings of the noise and interference at various frequencies in the HF band. Representative examples of first‐ and higher‐order statistics of the measured waveforms are described, including probability distributions of the envelope and phase of the noise and interference in the time and frequency domains, power spectra, autocorrelation functions, and level crossing distributions. The statistics of waveforms generated by the model closely resemble the statistics of the measured data.
A comparison of winds measured by meteor radar systems and an MF radar at Buckland Park
Authors: Valentic T. A. , Avery J. P. , Avery S. K. , Vincent R. A.
Published: February 2004

This paper describes a comparison of winds measured by the meteor echo detection and collection (MEDAC) system and a collocated spaced antenna MF radar at the Buckland Park field station near Adelaide, Australia (35°S, 138°E). A previous paper has described a detailed comparison between the MEDAC and Adelaide meteor radar system which was also operated during the campaign. In this paper, which uses only hand‐classified underdense meteor echoes, the meteor horizontal zonal wind velocities were compared to the those produced by the MF spaced antenna system. Care was taken when interpreting the comparison results in light of the varying meteor data rates. A point‐to‐point comparison showed that the excursions seen in the meteor data tended to be larger than those in the MF data. Excellent agreement was found between the two systems and relevant model results across the observed heights for both the mean winds and tides.
System design, signal‐processing procedures, and preliminary results for the Canadian (London, Ontario) VHF atmospheric radar
Authors: Hocking W. K.
Published: February 2004

Hardware, software, and design features of a new VHF atmospheric radar situated in Canada are described, with particular emphasis being placed on the flexibility which has been implemented at quite low cost. Called CLOVAR (Canadian (London, Ontario) VHF atmospheric radar), the instrument has now been operational since November 1993. It is located at 43°04.44′N, 81°20.20′W, operates at a frequency of 40.68 MHz, and is owned and operated by the nearby University of Western Ontario in London, Ontario, Canada. There are some unique features about this system, including its low‐cost design, flexible beam‐steering, and on‐line software analysis procedures. In this paper we elaborate on these new developments and especially demonstrate the new signal processing algorithms currently in use. These new algorithms include procedures for rejection of signals due to aircraft, removal of instrumental drift, and full on‐line spectral fitting of Gaussian functions. Typical data from the system are presented, including experimental data acquired with multibeam experiments, monthly mean vertical velocities, and some interesting results obtained during a solar eclipse. The radar can also function as an efficient meteor radar for determination of high‐level winds, and this capability will also be briefly described. A special program of comparisons with colocated radiosonde flights is also discussed.
A new approach to MF radar interferometry for estimating mean winds and momentum flux
Authors: Thorsen Denise , Franke Steven J. , Kudeki Erhan
Published: February 2004

New techniques are applied to measured Doppler velocity and angle of arrival to estimate horizontal wind vectors, variances, and momentum fluxes, from MF radar data. The approach used to estimate mean winds was first introduced as “time domain interferometry” (TDI) by Vandepeer and Reid [1995]. In the present paper, the algorithm is refined and used with data from the Urbana MF radar, which employs a single vertical antenna beam, to obtain a monthly mean wind climatology which is compared with the results from conventional spaced antenna full correlation analysis. The comparison validates the scattering model used in the development of the TDI technique and highlights instrumental and processing biases that differ between the two techniques. An extension of the TDI method that can provide estimates of the Reynolds stress tensor associated with propagating gravity waves is also proposed, and some preliminary results are presented.
Measurement of the propagation speed of plasma inhomogeneities in the solar corona using an uplink/downlink cross‐correlation method
Authors: Wohlmuth Rainer , Plettemeier Dirk , Edenhofer Peter , Bird Michael K. , Pätzold Martin , Asmar Sami W.
Published: February 2004

Radio sounding investigations were performed over a period of about 30 days during the 1991 solar occultation of the Ulysses spacecraft (August and September 1991). Dual‐frequency Doppler and ranging measurements were obtained using the NASA Deep Space Network. For this experiment the radio ray path from Ulysses to Earth moved essentially parallel to the solar equator, sounding the circumsolar plasma on the east (ingress phase) and west (egress phase) solar limbs at solar offset distances from about 4 to 40 RS (solar radii). Dual‐frequency, two‐way Doppler data were used to determine the propagation speed of coronal plasma inhomogeneities by cross‐correlation analysis between the uplink and downlink ray paths. The motion of Earth and spacecraft with respect to Sun results in a spatial separation (typically 20,000 km) between the uplink and downlink ray path during the round‐trip travel time of the signal. The specific configuration of the Ulysses radio system (dual‐frequency S/X band downlinks coherently locked to a S band uplink) provides the means for separating the uplink and downlink plasma contributions. The propagation speed of plasma inhomogeneities intersecting both ray paths can be determined by computing the cross‐correlation of the Doppler data from both ray paths as a function of time lag. The time lag of maximum cross‐correlation was used to calculate the propagation speed of plasma inhomogeneities. By evaluating the Ulysses data using this uplink and downlink cross‐correlation method, significant cross‐correlation peaks were obtained in the range of 7 to 34 RS. A high symmetry of the propagation speeds for both sides of the solar limb was found. An increase in propagation speed with increasing solar distance can be seen, starting from about 50 km/s at a distance of 7 RS up to 390 km/s at a distance of 29.7 RS (east) and from about 120 km/s at 9.8 RS up to 415 km/s at 33.8 RS (west). A significant enhancement (550–660 km/s) was observed at a distance of about 18 RS on both sides of the solar limb due to a solar event still being investigated.
Derivation and error analysis of echo phase parameters for the dynasonde
Authors: Tsai L.‐C. , Berkey F. T. , Stiles G. S.
Published: February 2004

Ionospherically reflected echoes are received with a four‐element multiplexed interferometer array and two phase‐matched receivers in implementations of the National Oceanic and Atmospheric Administration dynasonde. From these data, six phase parameters (Φ0, Φx, Φy, Φt, Φp, and Φƒ) are obtained and used to derive echo location, Doppler velocity, wave polarization, and virtual range. Since 2 π aliasing is an inherent feature of interferometric spaced antenna phase measurements, the phase parameters cannot be derived directly from the measured phase values using the method of least squares. In this work, we introduce a general procedure for the derivation of these parameters that (1) employs a “zero‐freedom” technique to derive initial estimates of the phase parameters, (2) derives shifted values of the measured phases from the six estimates, and (3) uses the method of least squares in conjunction with the shifted phases to improve the phase parameter estimates. This procedure minimizes the phase ambiguity inherent in interferometric phase measurements and derives phase parameters that approach the ideal least squares result. Furthermore, the ionospheric echo “quality” is quantified by two error parameters, defined asŝ, the least squares of the measured phase errors, and EP, the RMS phase error. It is also shown that the value of ŝ relative to the square of the standard deviation of measured phase is equal to the number of degrees of freedom in the phase parameters. Applying these techniques to data acquired with the Utah State University dynasonde, we derive a standard deviation of <2° in the measured echo phase.
Demodulation of complex baseband radar signals for the analysis of multiple narrow spectral lines
Authors: Ragaini Enrico , Woodman Ronald F.
Published: February 2004

This paper proposes a technique for processing signals in which the relevant spectral feature consists of one or more narrow lines which have a nonzero Doppler shift with respect to the reference frequency. The proposed technique consists of applying a second demodulation process to the demodulated signal obtained as the receiver output. The purpose of such a demodulation is to remove the frequency shift between the observed spectral line and the center of the observed spectral band. This way, the selected spectral line is brought to baseband, allowing the study of its envelope as a slowly varying process. One efficient way to perform the second demodulation step is by a digital version of a quadrature demodulator. This idea is translated into an efficient algorithm, employing real multiplications and accumulators. An advantage of such an algorithm is that in order to observe two spectral lines in symmetrical positions with respect to the center frequency, common multiplication and accumulators can be used. A possible simplification of the algorithm is discussed in which sinusoids used for demodulation are replaced by square waves, so that multiplication reduces to a synchronous sign reversal. Possible distorsions due to this simplified algorithm are mentioned. Possible implementations of double demodulation in a digital signal processing system are discussed in the final part of the paper.
An analytic solution for verification of computer models for low‐frequency magnetic induction
Authors: Dawson Trevor W. , Stuchly Maria A.
Published: February 2004

This paper considers the analytical solution to the problem of low‐frequency induction by an applied axial uniform magnetic field, in an equatorially stratified sphere having the conductivity distribution σ (ϕ) = σ0e−λcos(pϕ), where p ∈ {1, 2} is a periodicity factor, λ > 0 is a conductivity contrast parameter, σ0 is a conductivity scale factor, and ϕ is the equatorial angle. The resulting induced electric and current density fields are fully three‐dimensional. While of interest in its own right, the model should prove useful in the validation of low‐frequency electromagnetic computer modeling codes. The development includes a full discussion of the underlying Green's function, which can also be used for modeling induction by alternative excitation fields.
Paleogene seafloor spreading in the southeast Tasman Sea
Authors: Wood R. , Lamarche G. , Herzer R. , Delteil J. , Davy B.
Published: February 2004

Magnetic anomalies 11–18 have been identified in the southeast Tasman Sea, the area of ocean crust southwest of Fiordland, New Zealand, west of the Puysegur Trench (the present plate boundary) and southeast of the older (80–58 Ma) ocean crust of the Tasman Sea basin. Structures associated with two changes in spreading direction are preserved in the area. Spreading between the Australian and Pacific plates began in the Eocene, about 40 Ma in this area, orthogonal to Cretaceous‐Paleocene spreading between them in the Tasman Sea. The boundary between the two ages of ocean crust is abrupt and associated with what we have interpreted as marginal uplift blocks and rift basins of the Resolution Ridge system. Rifting appears to have propagated northeast along a fracture zone. The northern blocks of the Resolution Ridge system may be isolated fragments of continental crust of the Campbell Plateau. Between anomalies 18 and 11 (40–30 Ma) the spreading rate was about 1.5 cm/yr. Swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11, about 30 Ma, and led to a change from tension to transcurrent motion along the plate boundary. The timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data.
Present‐day stress field changes along the Baikal rift and tectonic implications
Authors: Petit Carole , Déverchère Jacques , Houdry Frédérique , Sankov Vladimir A. , Melnikova Valentina I. , Delvaux Damien
Published: February 2004

Intraplate extension, in a frame of a global compressional stress field, seems linked to local lithospheric perturbations (lithospheric thinning or thickening) able to modify the resulting state of stress [Zoback, 1992]. The Baikal Rift Zone (BRZ), Siberia, is located north of the India‐Asia collision zone and exhibits no direct communication with any oceanic domain. It can thus be fully considered as an area of continental extension, dominated by the “global compressional intraplate stress field” resulting from plate driving forces. In order to address the problem of its dynamics and kinematics and their links with the India‐Asia collision, a comprehensive stress tensor analysis is presented, based on 319 focal mechanisms of earthquakes located along the whole Baikal rift. The stress field is varying at different scales of observation: when looking at central Asia (several thousands kilometers), the maximum horizontal stress SHmax directions remain rather constant (with a fan‐shape geometry) when the tectonic regime goes from compressional (Himalayas) to extensional (Baikal). When observing the Baikal rift (about 1000 km long), clear variations of the stress regime are observed, from an extensional regime in the central part of the rift to wrench ones in its northern and southern ends. Finally, at the scale of 100 km, systematic SHmax reorientations occur close to major rift faults. We thus infer that the interaction between collisional processes and inherited structures may have a strong influence on rift dynamics. We then use computed stress tensors to predict slip vectors on major rift faults. Deformation patterns show two distinct parts of the rift: the South Baikal Rift (SBR) is characterized by a constant trending (around N100°E) slip vector, meanwhile the North Baikal Rift (NBR) exhibits a complex block rotation behavior involving at least three crustal blocks. We propose to interpret these surficial structures and motions as the result of an interaction between the regional compression coming from the India‐Asia collision and the geometry of the hardly deformable Siberian platform. This particular setting can explain most of the surficial deformation patterns, which suggest a large‐scale cracking of the lithosphere in the Baikal region. Other possible sources of stress could also be considered, like deep mantellic upwelling, or trench suction linked to the Pacific subduction.
Timing of Cretaceous extension and Miocene compression in northeast South Island, New Zealand: Constraints from Rb‐Sr and fission‐track dating of an igneous pluton
Authors: Baker Joel , Seward Diane
Published: February 2004

The Tapuaenuku Plutonic Complex is a large alkaline intrusion that was emplaced into Upper Jurassic‐Lower Cretaceous Torlesse greywacke/argillite near the axis of the Inland Kaikoura Range (northern South Island, New Zealand). Concordant Rb‐Sr, K‐Ar, and titanite/zircon fission‐track cooling ages of circa 105 to 93 Ma record a phase of igneous activity that was related to a period of mid‐Cretaceous extension in South Island, New Zealand. The updoming and uplift which accompanied the extension and igneous activity are also reflected by the uplift of the marine Split Rock Formation above sea level where it was eroded. On this was deposited a terrestrial sequence followed by shallow marine deposits which may represent the onset of a long period of transgressive marine sedimentation, the result of slow thermal subsidence after magmatism and rifting, from Late Cretaceous to Oligocene. This burial of the pluton is identified not only from the sedimentary relationships of the local stratigraphy but also by the apatite fission‐track ages which record later exhumation. During this time, New Zealand remained passively part of the Pacific Plate. In the early Miocene at circa 22 Ma the intrusion cooled through the partial annealing zone of apatite (approximately 120°–60° C). Exhumation was the result of early Miocene thrusting and shortening in Marlborough and inception of the present day transpressive plate boundary between the Indo‐Australian and Pacific Plates through New Zealand. Further evidence for this period of uplift and accompanying denudation is seen through the large quantities of coarse‐grained clastic sediments which were deposited in Marlborough at this time. The uplift rate required to raise the top of pluton to its present altitude (2885 m), based on a constant rate from 22 Ma until the present day, is approximately 0.2 mm yr−1. However, estimates of present‐day uplift rates of 10 mm yr−1 from summit heights and raised marine benches suggest that two periods of late Cenozoic uplift must have occurred with a long period of quiescence between, a conclusion supported by a major mid‐Miocene unconformity below Quaternary sediments in Marlborough.
Kinematic evolution of the lower plate during intracontinental subduction: An example from the Scandinavian Caledonides
Authors: Hurich C. A.
Published: February 2004

Geophysical data combined with pressure‐temperature‐time data form the basis for a reconstruction of the kinematic evolution of the Scandinavian Caledonides collisional orogenic belt. The reconstruction suggests that the early stages of collision between Baltica and Laurentia involved intracontinental subduction of a minimum of 150 km of Baltic continental crust to a depth of at least 100 km. The subduction juxtaposed strong, Laurentian upper mantle rocks in the upper plate against weak, Baltic crustal rocks in the lower plate. Continued plate convergence, coupled with the buoyancy of the Baltic rocks, resulted in thrust imbrication of a substantial portion of the lower plate and extrusion of the crustal rocks from the subduction zone. Thrusting also resulted in incorporation of fragments of the Laurentian upper mantle into the Baltic crust. The reconstruction also suggests that direct interaction between the Laurentian and Baltic crustal rocks, and the time of greatest crustal thickness in the orogen, occurred after extrusion of the Baltic crust from the subduction zone. One implication of this suggestion is that the eclogite/coesite assemblage presently exposed at the surface in the hinterland of the Caledonides probably formed early in the collision sequence during subduction rather than during the time of greatest crustal thickness. The reconstruction also suggests that exposure of the high‐pressure mineral assemblage was a two‐phase process, involving extrusion from the subduction zone by thrusting during convergence and unroofing by a combination of extension on low‐angle normal faults and erosion. The kinematic model derived for the Scandinavian Caledonides provides a possible scenario for the evolution of other collisional orogenic belts, containing very high‐pressure mineral assemblages formed in situ in continental rocks and associated ultamafic rocks of mantle origin.
40Ar/39Ar and Nd‐Sr isotopic characteristics of mid‐Ordovician North American K‐Bentonites: A test of early Paleozoic Laurentia‐Gondwana interactions
Authors: Samson Scott D.
Published: February 2004

Highly altered tephra layers (K‐bentonites) of mid‐Ordovician age are widespread throughout the midcontinent region of the United States and the Appalachian Taconic foreland basin. The numerous ash layers, their wide areal extent, and their large volume provide evidence that extensive volcanic activity accompanied Taconic orogenesis. The mineralogical and geochemical characteristics of these widespread Ordovician tephra layers can be used to constrain the nature of their source region, and thereby help test recent hypotheses that Gondwana collided with Laurentia in mid‐Ordovician time. Initial 143Nd/144Nd and 87Sr/86Sr ratios of two K‐bentonites that occur traced throughout the midcontinent are 0.51183–0.51187 and 0.7101–0.7120, respectively. Isotopic ratios for K‐bentonites within the Taconic basin are 0.51178–0.51201 and 0.70564–0.70905. The isotopic composition of the tephra layers and the ubiquitous presence of Proterozoic zircon xenocrysts within the K‐bentonites is consistent with an origin by crustal anatexis. The cooling history of the source is constrained by 842–996 Ma 40Ar/39Ar apparent ages of hornblende xenocrysts contained within two Taconic basin K‐bentonites. A possible South American source region of the K‐bentonites is the Arequipa massif of Peru. This region contains early Paleozoic arc‐related rocks, many of which contain mid‐Proterozoic zircon xenocrysts, and Early Proterozoic gneisses which were affected by Grenville‐age granulite grade metamorphism. Gneisses within the Adirondack Highlands of the Grenville Province have similar εNd(450 Ma) values and hornblende cooling ages to the K‐bentonites, consistent with this region as a source. Caradocian K‐bentonites in Sweden have Sr isotopic ratios similar to the North American beds. If the Swedish and North American K‐bentonites were produced from the same source, then Baltica and Laurentia were proximal in Ordovician time precluding a Gondwana‐Laurentia link. If Caradocian K‐bentonites with North American characteristics are found in western South America, support for a Laurentia‐Gondwana link would be established. If such K‐bentonites cannot be found, considerable doubt would be cast on this model.
Displacements, strains, and rotations within the Afar depression (Djibouti) from restoration in map view
Authors: Rouby D. , Souriot Th. , Brun J. P. , Cobbold P. R.
Published: February 2004

Within the Afar depression (Djibouti) the stratiform basaltic lava flows (emplaced between 4 Ma and 1 Ma) cover the two thirds of the depression. Because of the desert climate, they are almost non eroded. The topography can then be used as a reference surface to measure the vertical offset of younger faults. Using a detailed map the southeastern part of the depression showing the fault pattern and the vertical offset on each fault (obtained from stereoparts of SPOT images), we have restored the topography in map view. We have computed the fields of finite displacement, rotation and strain. The field of horizontal displacements shows vectors trending to the NE on average. This direction is compatible with the displacement to the NE of the Arabian plate with reference to the African plate. In detail however, displacement vector trend from NNE‐SSW to NE‐SW. Major axes of strain ellipse trend NE‐SW suggesting that major faults (Nl20) are dominantely extensional. Strain intensities and magnitudes of block rotations are increasing from southwest to northeast of the restored area. Magnitudes of block rotations are very small compared with those obtained from paleomagnetic measurements. Therefore we computed a second restoration that covers a smaller area where the true vertical offset of the major faults taking into account the sedimentary fill of basins were avalaible. This second model gives rotation magnitudes in better agreement with paleomagnetic data but still underestimates. We discuss these results with the available kinematic models of the southeastern part of the depression.
Thrust‐related very low grade metamorphism in the marginal part of an orogenic wedge, Scandinavian Caledonides
Authors: Warr Laurence N. , Greiling Reinhard O. , Zachrisson Ebbe
Published: February 2004

The pattern and characteristics of thrust‐related very low grade metamorphism in the marginal part of an orogenic wedge have been investigated by combining a clay mineral crystallinity survey with detailed structural mapping of the thin‐skinned foreland thrust belt along the external part of the Scandinavian Caledonides. This external part is composed of late Neoproterozoic to Ordovician sedimentary sequences of the autochthonous cover and Lower Allochthon, which are overlain by the higher Caledonian nappes (Middle and Upper Allochthonous units). Stages of the Scandian phase of thrust wedge development are described which are related to the very low grade metamorphic history. The initial stage involved the emplacement of cooled nappes belonging to the Middle and Upper Allochthons, with very low grade peak metamorphic conditions attained within the underlying Lower Allochthon (cover) sediments as they were progressively buried and deformed beneath the thrust wedge. During this initial emplacement the isotherms are considered to have been undisturbed and dipping parallel to the wedge surface. The following stages of wedge development consisted of extensive post‐metamorphic imbrication of the underthrusted cover sediments, with a transition from basal accretion and uplift at the rear, to accretion and forward propagation at the wedge's toe. During accretion into the wedge, the externally dipping isograd surfaces were extensively displaced from deeper levels toward higher tectonic horizons. The last stage of wedge development considered here was characterized by late out‐of‐sequence thrusting with enhanced (epizonal) metamorphic grades developed in the vicinity of the fault zones, which either resulted from further displacement of the isograds toward higher levels, or from localized heating via intense fluid activity. Overall, the pattern of metamorphic grade, fabric relationships, and physical calculations of heat transfer based on the geometry of the thrust wedge, suggest that neither inverted temperature gradients nor shear heating were likely causes of the metamorphism in this flat‐lying part of the orogenic wedge. The description of inverted very low grade metamorphic isograds in other marginal parts of the Scandinavian Caledonides, which have been previously attributed to either the rapid emplacement of hot thrust nappes, or the effects of dissipative shear heating, are discussed in terms of variations in both the critical wedge geometry and its controlling boundary conditions.
Low‐angle crustal ramp and basin geometry in the Gulf of Lion passive margin: Oligocene‐Aquitanian Vistrenque graben, SE France
Authors: Benedicto A. , Labaume P. , Séguret M. , Séranne M.
Published: February 2004

With more than 4000 m of Oligocene‐Aquitanian sediments, the Vistrenque graben (SE France) is the deepest synrift depocenter of the Gulf of Lion passive margin, NW Mediterranean. Detailed analysis of industrial seismic reflection profiles and borehole data show that the steep Nîmes fault, which bounds the graben to the NW, forms at depth a low‐angle (25°) crustal ramp. Along‐strike changes of hangingwall geometry allow us to infer along‐strike changes of fault shape: A rollover structure and divergent Oligocene‐Aquitanian basin fill are associated with a listric geometry of the fault in the southern part of the graben, while a pseudo‐rollover and compensation graben result from a two‐segments planar geometry of the fault in the northern part. Preexisting structures inherited from Mesozoic extension and Late Cretaceous‐Eocene Pyrenean thickening controlled the location of the Nîmes fault and the transfer zones which divide the graben into different compartments. Since both hangingwall and fault profile are well constrained, restoration techniques can be used to estimate the prerift topography. The Vistrenque graben was formed close to sealevel, but at the front of a > 1 km‐high elevated area resulting from the Pyrenean orogeny. In the studied transect, the Nîmes fault formed the landward (NW) boundary of the basement faulted domain of the margin. Extensional deformation was restricted to this domain during most of the rifting interval. Small amounts of extension were transmitted landward to Mesozoic cover décollement rooted in the Nîmes fault, only during short episodes, probably resulting from gravitational instability during margin collapse. The Nîmes low‐angle crustal ramp, as well as the other crustal ramps of the margin of similar orientation, are probably newly formed extensional structures rather than reactivated Pyrenean thrusts. Their activation at a low‐angle may have been allowed by crustal weakening resulting from the previous Pyrenean thickening. Upper crustal extension corresponding to the graben formation was transmitted basinward through an intracrustal detachment, or/and distributed in the lower crust across the margin. In contrast to the more stretched areas of the margin which do not display thick synrift series due to their initial high surface elevation, the Vistrenque basin fill records the whole rifting episode because of its location at the front of the Pyrenean orogen.