bluuebunny/crossref_metadata_2025 huggingface dataset:
Coastal meteorology
Authors: Rogers David P.
Published: February 2004

Coastal meteorology encompasses phenomena that extend from about 100 km offshore to 100 km inland. Thus understanding the meteorology of the coastal zone combines knowledge of the interaction of marine and land atmospheric boundary layers, air‐sea interaction, large‐scale atmospheric dynamics, and the circulation of the coastal ocean. While there is considerable understanding of homogeneous marine and land boundary layers, application of this knowledge is complicated by the inherent heterogeneity of the coastal zone. Most coastal environments are modified by the adjacent ocean, the coastal topography and the land‐sea thermal contrast. Complex feedbacks occur between the atmosphere, ocean and land. The thermal contrast between the land and sea creates the land‐sea breeze, coastal atmospheric fronts, coastal ocean currents and upwelling. The convergence of marine air over the coastline can result in strong convection with heavy precipitation and runoff. A lateral, orographic boundary can accelerate the wind speed while constraining the flow parallel to the coast. Variations in the strength and direction of the wind depend on the exact nature of the orographic boundary. This results in highly variable temporal and spatial air‐sea exchange processes that are dominated by scales of tens to hundreds of kilometers.
Western boundary currents
Authors: Hogg Nelson G. , Johns William E.
Published: February 2004

The past decade has seen considerable progress toward clarifying the mean circulation of the World's oceans. At the same time we have come to realize that the specification of a mean circulation is difficult as there is energy at all time scales which are quantifiable and the spectrum is usually red.

The western boundary currents (WBCs) of the oceans are the principal conduits for communication between the equatorial regions, where heat is added to the oceans and the polar regions where it is removed. Understanding how these current systems work is fundamental to understanding the earth's global climate engine. Several substantial observational programs focussed on WBCs have been undertaken in recent years especially within the North and South Atlantic. These include studies of the Brazil‐Malvinas Confluence and the South Atlantic Ventilation Experiment (SAVE) within the South Atlantic, the Western Tropical Atlantic Experiment (WESTRAX) in the tropics, and the Subtropical Atlantic Climate Study (STACS) and Synoptic Ocean Prediction experiment (SYNOP) in the subtropics. We shall concentrate in this review with a summary of the results from these programs but will also briefly cover new findings from other parts of the globe. Efforts connected with programs of the World Ocean Circulation Experiment in the Pacific and the South Atlantic (particularly the Deep Basin Experiment) are still underway and can be expected to make substantial contributions to the knowledge of WBCs in the future. The nature of this review also compells us to emphasize recent U.S. research but we will incorporate results from the international community, as well, especially in regions of the globe where the U.S. has done little. The reader may also wish to consult recent reviews by Ierley [1990] and Huang [1991], which discuss many issues relevant to WBCs in subtropical gyres.
Particle dynamics, chaos, and order in the magnetotail
Authors: Chen James
Published: February 2004

Current sheets are prevalent in space, astrophysical, and laboratory plasmas. They occur in boundary layers between regions of oppositely directed components of magnetic field, typically occupying small spatial regions inside larger systems. A current sheet, however, represents a significant magnetic energy reservoir and can play a critical role in determining the properties of the larger system in which it is embedded. Important physical processes in current sheets include magnetic reconnection and particle acceleration. The charged particles are typically collisionless in space and astrophysical plasmas while those in laboratory plasmas can exhibit a range of collisionality.
All ionospheres are not alike: Reports from other planets
Authors: Nagy Andrew F. , Cravens Thomas E. , Waite H. J.
Published: February 2004

Gauss, Lord Kelvin and Stewart Balfour suggested the existence of an electrically conducting region in the terrestrial upper atmosphere already in the last century. The first direct verification of the existence of such a region came when Marconi succeeded in sending radio signals across the Atlantic. The name ionosphere was coined by R. A. Watson‐Watt in 1926. No uniform definition of the ionosphere exists; a rather broad definition might be “the ionosphere is that region of the atmosphere (or gaseous envelope) surrounding a solar system body where significant numbers of low energy, free electrons and ions are present.” The first direct evidence for an ionosphere at a planet other than Earth came from the radio occultation measurements of electron densities by Mariner 5, as it flew by Venus on October 19, 1967. Since then spacecraft have visited all planets, except Pluto. As a minimum, radio occultation measurements of the electron densities have been obtained at all, but Mercury. However, ground‐based optical observations indicate that Mercury has a significant exosphere; thus an ionosphere, in its broad definition, must also be present. Also for Pluto, optical measurements have established the presence of an atmosphere, implying it too has an ionosphere.
Asynchronous maximum advances of mountain and continental glaciers
Authors: Gillespie Alan , Molnar Peter
Published: February 2004

The last maximum glacier advance in many mountain ranges appears to have predated the last maximum advance of the Wisconsinan continental ice sheets (∼20,000 years B.P.). Published evidence from widely spaced localities in North and South America, Europe, Asia, and Hawai'i suggests that some mountain glaciers extended farther during one or more stades early in the last glaciation, roughly 115,000–30,000 years B.P., than at its end, when low global temperature and high precipitation presumably were most favorable for the growth of glaciers. Variations in the timing of glacier advances in different mountain ranges suggest that alpine glaciers are dependent on regional climate as well as global conditions. Their study might provide sensitive tests for theories of global climate change. Glacial deposits are often used as time markers in neotectonic and geomorphic studies, and their ages sometimes are inferred from distant measurements or from proxy histories of global climate change. Undated geomorphic features such as moraines, sharply incised streams, and river terraces, which commonly have been associated with the last maximum glacier advance (∼20,000 years B.P.) or last deglaciation ( ∼13,000 years B.P.), may actually be older by a factor of 2 to 4 (or more). Conversely, deposits assigned to the maximum advances of the penultimate global glaciation ( ∼130,000 years B.P.) may actually date from early in the most recent glacial cycle and be younger by a comparable factor. Rates of faulting or folding estimated from displacements or deformation and the assumed ages of such geomorphic features might be too great (or too small) by corresponding factors.
Recent advances in land‐atmosphere interaction research
Authors: Entekhabi Dara
Published: February 2004

From the earliest attempt at numerical weather prediction up until today's efforts on improving the land surface hydrologic parameterization in General Circulation models (GCMs), it has been recognized that the realistic characterization of atmospheric phenomena requires accurate representation of surficial processes. Lewis F. Richardson who attempted numerical weather prediction using hand calculations around the First World War period includes, in his notes, a parameterization of the surface evapotranspiration and plant stomatal control of the vapor exchange between land and atmosphere. He writes: “…Let the rate of loss of water from a leaf be denoted by T, then

urn:x-wiley:87551209:media:rog1913:rog1913-math-0001
here K is the conductance of the stomatal openings and F (θleaf) is the saturated vapour density at θ.” [Richardson, 1922]. Almost three‐quarters of a century later and using high‐speed digital computers, the research community is essentially implementing Lewis F. Richardson's original scheme of stomatal resistance to vapor flux between the saturated interior of leaves at temperature 9ieaf and near‐surface air humidity wair. At first sight it may appear that not much scientific progress has been made in the interim years. Transpiration, turbulence and other processes related to land‐atmosphere exchange are complex phenomena and in fact there has been significant recent advances in the study of land‐atmosphere interaction. The land, biosphere, atmosphere and ocean systems are coupled across a wide range of space and time scales such that each discovery leads to a deeper and larger scientific question. Research inquiry in this area is now performed in both the hydrologic and the atmospheric science communities.
Middle atmospheric winds and tides over London, Canada (43°N, 81°W) during 1992–1993
Authors: Thayaparan T. , Hocking W. K. , MacDougall J.
Published: February 2004

Mean winds and tides have been measured with the new MF radar at London, Ontario. Using a 1‐year continuous set of data, the mean winds are compared with a recent empirical model. The diurnal and semidiurnal tides are discussed, and these are compared with results from other similar midlatitude ground‐based radars. There are interesting similarities and discrepancies, although we note that some of the initial comparisons use data from different years. The diurnal and semidiurnal tides are also compared with recent tidal models. Tidal characteristics are generally consistent with the models, but again some differences are also noted.
A new method for improving the interferometric resolution by compensating for the atmospherically induced phase shift
Authors: Zivanovic S. S. , Forster J. R. , Welch W. J.
Published: February 2004

Electromagnetic waves traveling through the atmosphere experience an added phase shift due to the presence of various gases. Since the water vapor distribution in the atmosphere has a significant spatial and temporal variance, the corresponding phase shift can cause a serious degradation of the maps produced by radio interferometers. This study presents the calculation of the atmospherically induced phase shift in terms of ground based measurements and introduces a new technique to compensate for it. The compensation technique is implemented on the Berkeley‐Maryland‐Illinois Array at Hat Creek, California, and various tests are performed to demonstrate the accuracy of the technique. Finally, some future work is suggested.
Electromagnetic scattering of large structures in layered earths using integral equations
Authors: Xiong Zonghou , Tripp Alan C.
Published: February 2004

An electromagnetic scattering algorithm for large conductivity structures in stratified media has been developed and is based on the method of system iteration and spatial symmetry reduction using volume electric integral equations. The method of system iteration divides a structure into many substructures and solves the resulting matrix equation using a block iterative method. The block submatrices usually need to be stored on disk in order to save computer core memory. However, this requires a large disk for large structures. If the body is discretized into equal‐size cells it is possible to use the spatial symmetry relations of the Green's functions to regenerate the scattering impedance matrix in each iteration, thus avoiding expensive disk storage. Numerical tests show that the system iteration converges much faster than the conventional point‐wise Gauss‐Seidel iterative method. The numbers of cells do not significantly affect the rate of convergency. Thus the algorithm effectively reduces the solution of the scattering problem to an order of O(N2), instead of O(N3) as with direct solvers.
Finite element analysis of bodies of revolution using the measured equation of invariance
Authors: Barkdoll Ty L. , Lee Robert
Published: February 2004

This paper is concerned with the finite element analysis of electromagnetic scattering from an axially symmetric perfectly conducting body of revolution with an arbitrary cross section in free space. The coupled azimuthal potential formulation is used to express the solution in terms of the two field components, Eф and Hф. The use of a Fourier series expansion reduces the three‐dimensional problem to a series of two dimensional problems. At the truncation boundary the measured equation of invariance (MEI) is used. The MEI method uses information about the scatterer to find an equation for the boundary nodal values in terms of the values at neighboring nodes. The MEI produces a boundary condition which allows the matrix to retain its sparse structure. This method also allows the finite element mesh to be truncated very close to the body. Numerical examples are given for spheres, finite cylinders, and a cone.
Bistatic scattering characteristics of dense randomly distributed cylinders
Authors: Clayton Craig W. , Kuga Yasuo
Published: February 2004

To study the fundamental characteristics of electromagnetic wave scattering of densely distributed random media, we conducted extensive experiments using millimeter‐wave bistatic radar and dielectric cylinders of different sizes and concentrations. Experiments were run with fractional volume densities of 10% and 30% along with the highly dense case. It is shown that for ka ≈ 6, the dense and highly dense scattering resembles the single scattering characteristics of the cylinder. For ka ≈ 3 the dense case resembles the single scattering case, but the highly dense case exhibits much different behavior. The experimental results were compared with second‐order radiative transfer theory.
Transionospheric signal recognition by joint time‐frequency representation
Authors: Qian Shie , Dunham Mark E. , Freeman Matthew J.
Published: February 2004

A challenging problem in signal processing is the detection of impulsive signals which are dispersed by the ionosphere and received by RF satellite sensors. One example of this type of signal is the so‐called transionospheric pulse pair (TIPP), thought to be associated with lightning discharges. Since these dispersed events occur sporadically and typically persist for less than 100 μs, the first objective is to detect or trigger on the signal of interest, thereby reducing the amount of data relayed back to Earth. Difficulties in detecting transionospheric impulses arise from the dispersion of the original signal, which lowers the signal‐to‐noise ratio (SNR), and the need for continuous, real‐time observation. The energy content of TIPPs has been measured from 25 MHz to more than 100 MHz, requiring sampling rates of at least 150 MHz to capture the events. Although these transionospheric signals are easily recognized in the joint time‐frequency domain, the computation involved in time‐frequency transformations prohibits application to a real‐time detector with this sampling rate. Using a joint time‐frequency representation allows us to derive a robust time convolution template to fit transionospheric signals. This template can be used to design matched chirp filter banks, which are optimum with respect to SNR, for detection of transionospheric signals. We believe that a realistic matched filter bank could have a sparse spacing while maintaining good detection ability. This type of detector is also amenable to real‐time analog hardware implementation in conventional finite impulse response (FIR) filter structures. Our work shows that transionospheric signals encountered in real data are effectively detected by matched filter banks. While a specific application is treated here, the methods described are general, and applicable to many problems in signal detection and recognition which treat nonlinearly dispersed signals.
The effect of localized ionospheric perturbations on subionospheric VLF propagation on the basis of finite element method
Authors: Baba K. , Hayakawa M.
Published: February 2004

The purpose of this paper is to propose the use of finite element method in investigating the effect of localized perturbations in the lower ionosphere on subionospheric VLF propagation. Due to the complexity of the method, we have analyzed only a two‐dimensional case where a localized perturbation lies on the great circle path between the transmitter and receiver. The formulation is first made, and a few computational results are presented in order to indicate the effectiveness of this finite element method.
Proton cyclotron echoes at the ƒQ3 resonance
Authors: Horita R. E. , Chen G. M.
Published: February 2004

We report here new observations on topside sounder ionograms from the Canadian ISIS II satellite of proton cyclotron echoes associated with the ƒQ3 resonance. Proton cyclotron echoes were first observed on swept frequency ionograms from the Canadian Alouette II satellite. The echoes occur on the ionograms at nearly constant apparent ranges which correspond to the proton cyclotron period at the satellite. The echoes were first found to be associated with the fundamental resonance of the electron cyclotron frequency, ƒH, and later reported at the 3ƒH and 4ƒH resonances. The new observations were made on ionograms when the satellite was located in regions with low dip angles less than 8°. This paper will present and discuss the features of these new proton cyclotron echoes, such as the presence often of two echo images, and possible generation mechanisms. A comparison with various ray‐tracing calculations by Muldrew (1972a, b) suggests that these proton cyclotron echoes may occur when the particular waves responsible for the ƒQ3, 3ƒH and 4ƒH resonances interact with protons energized by the satellite antenna.
Coordinated digisonde and incoherent scatter radar F region drift measurements at Sondre Stromfjord
Authors: Scali J. L. , Reinisch B. W. , Heinselman C. J. , Bullett T. W.
Published: February 2004

Comparison of drift measurements made at Sondre Stromfjord show that the apparent velocities measured by the digisonde (DGS) are in good agreement with the drift velocities observed by the collocated incoherent scatter radar (ISR). Data from December 5 to 9, 1991, show the mean DGS velocities to be within 50 m s−1 of the ISR velocities, that is, within the uncertainty levels of each instrument. The analysis highlights the dominance of the electric field in controlling the plasma motion. The measured velocities are generally height independent, as would be expected for an E field mapped along the magnetic field lines from high altitudes to ionospheric heights. In addition, the comparative analysis is used to study an ionospheric event where a large section of ionization was removed from the daytime cusp region by a strong anti‐Sunward drift when the interplanetary magnetic field (IMF) Bz component changed orientation.
Comparison of ELF/VLF generation modes in the ionosphere by the HIPAS heater array
Authors: Villaseñor J. , Wong A. Y. , Song B. , Pau J. , McCarrick M. , Sentman D.
Published: February 2004

The characteristics of three different modes of ELF and VLF excitation using the high‐power auroral simulation (HIPAS) HF heater array are compared under different ionospheric conditions. For each of these three methods, amplitude modulation (AM), phase demodulation (DM), and the double‐frequency excitation (DF), we observed signals on the order of a few picoteslas at two widely separated receiver sites for both X and O mode heater polarizations. Strong electrojet activity is essential for ELF generation at frequencies close to the Schumann range, whereas at higher frequencies, such as in the VLF range (greater than 1 kHz), signals could be observed under a wider range of ionosopheric conditions. The AM method generally produced the largest signal at both frequency ranges, while the DM mode was approximately half of this signal amplitude. The DF generated signals are comparable in strength with the AM signals at VLF frequencies and are more stable than either method as a function of time. The X mode polarization also produces a stronger signal than the O mode for these three methods of excitation by a factor of two. The polarization of the received signals follow the same variation at VLF frequencies for all three modes, indicating a common height of origin.
Theoretical study of two‐frequency coherence of MST radar returns
Authors: Chu Yen‐Hsyang , Chen Tsai‐Yuan
Published: February 2004

The theoretical expressions of two‐frequency coherence of the MST radar returns from an atmospheric thin layer with sharp boundaries, which embeds within the radar volume and cannot be resolved by conventional radar techniques, are derived in this article. It shows that the derived frequency domain interferometry (FDI) coherence is not only the function of layer thickness and the components of radar wave vector, but also related to the wavenumber power spectrum of refractivity irregularities. A quantitative examination of the derived FDI coherence indicates that the effect of the irregularity power spectrum on FDI coherence is negligible for power law spectral model, while it cannot be ignored for other spectral forms, for example, a Gaussian model. The zenith angle dependence of FDI coherence is also investigated in this article. According to the observations made with Chung‐Li VHF radar, it shows that the ratio of the observed FDI coherence at the vertical to that at 17° off‐zenith angle is between 1.3 and 2.4. This feature can be illustrated satisfactorily by using the theory developed in this article. The effect of echoing mechanism on FDI coherence is also studied, showing that the expression of FDI coherence derived from the turbulent scattering theory can be treated to be identical to that from the Fresnel reflection theory as long as the condition Δk/k ≪ 1 is met. This result implies that it seems to be impossible to identify the echoing mechanism of MST radar by using FDI technique. The problem of estimating the thickness of a thin layer having sharp boundaries by using a Gaussian function with the expression of exp (−Δk2σ2r) is also discussed. It suggests that the pertinent formula used for layer thickness estimate is L = 2.364σr. A comparison of this work with other results is also made in the text.
VLF strip holographie imaging of lightning‐associated ionospheric disturbances
Authors: Chen Jiunn‐tsair , Inan Umran S. , Bell Tim F.
Published: February 2004

A linear array of very low frequency (VLF) receivers, deployed along a line roughly perpendicular to the direction of signal propagation, enables the determination of the size and location of D region disturbances produced by lightning‐induced electron precipitation (LEP) bursts or by lightning‐induced heating in the vicinity of the great circle VLF propagation paths. The configuration essentially constitutes a strip hologram so that the width of both the amplitude and phase perturbation of the signal pattern recorded along the strip are simply related to the size and location of the disturbance for the range of altitude profiles of disturbed ionization expected for LEP events. The validity of the approach is demonstrated both analytically for single waveguide‐mode propagation and also for a realistic propagation path (Annapolis‐Stanford) by using a three‐dimensional numerical model of VLF propagation and scattering in the Earth‐ionosphere waveguide. We also discuss the criteria by which the spacing of discrete elements along the strip can be optimally choosen.
Gravity waves determined by modeling of traveling ionospheric disturbances in incoherent‐scatter radar measurements
Authors: Kirchengast Gottfried , Hocke Klemens , Schlegel Kristian
Published: February 2004

With the advent of the era of thermosphere/ionosphere weather modeling there is an increasing need to routinely acquire information on important regional‐scale phenomena in the upper atmosphere like atmospheric gravity waves (AGWs) in the thermosphere and their ionospheric signatures, the traveling ionospheric disturbances (TIDs). AGW information is especially desirable in view of its role in the momentum and energy balance of the upper atmosphere. Measuring AGWs directly is difficult, however, so they are usually traced by the TIDs, which can be routinely observed by the powerful incoherent‐scatter technique even in all fundamental ionospheric parameters. Thus an important problem, and the central concern of this study, is how to infer, as comprehensively as possible, AGW information from measured TIDs. This problem was tackled up to now by direct inversion of TID parameters into AGW parameters, but severe limitations, which we shall briefly discuss, are inherent in this approach. Contrasting with this, we introduce a novel approach to AGW retrieval: realistic modeling of AGW/TID physics, constrained by TIDs measured by the incoherent‐scatter technique. This method is found to be capable of yielding comprehensive AGW information, that is, the full polarization and dispersion information. We applied the method to four TID events, which cover different seasons and solar activities during quiet magnetic conditions, from European EISCAT radar data. One‐to‐one comparisons of modeling results with experimental data are presented and discussed. These show the feasibility and scope of the method and its potential to discriminate between TIDs from AGWs and non‐AGW sources. In view of the satisfactory results, we finally address the potential of the method for routine processing of incoherent‐scatter data with respect to AGWs, which could provide valuable AGW information for other research.
Scattering from multiple slits in a thick conducting plane
Authors: Kim Youn S. , Eom Hyo J. , Lee Jae W. , Yoshitomi Kuniaki
Published: February 2004

Electromagnetic scattering from multiple slits in a thick, perfectly conducting plane is investigated. The Fourier transform and mode matching are used to obtain the simultaneous equations. The simultaneous equations are solved to obtain a series solution which reduces to a closed form in the high‐frequency limit. The transmission coefficients are represented in fast convergent series which are efficient for numerical computations. Numerical computations are performed to illustrate their scattering behaviors in terms of frequency, slit size, and slit number.
Nonlinear inversion of the integral equation to estimate ocean wave spectra from HF radar
Authors: Hisaki Yukiharu
Published: February 2004

Since all ocean wave components contribute to the second‐order scattering of a high‐frequency radio wave by the sea surface, it is theoretically possible to estimate the ocean wave spectrum from first‐ and second‐order scattering in the Doppler spectrum measured with an HF ocean radar. To extract the wave spectral information, however, it is necessary to solve a nonlinear integral equation. This paper describes in detail how to solve the nonlinear integral equation without linearization or approximation. We show that the problem of solving the nonlinear integral equation can be converted into a nonlinear optimization problem. An algorithm to find the optimal solution is described. Examples of the algorithm applied to simulated data and measured data are shown. The wave frequency spectrum can be estimated even if the Doppler spectrum is available in only a single direction. In this case, however, the solution of the two‐dimensional wavenumber spectrum tends to converge to a spectrum that is symmetrical to the beam direction. Even if the wave spectrum is dominant in a single direction, the solution may give two peaks in the wavenumber spectrum. One of them is the true peak and the other is the mirror image of it with respect to the beam direction. This ambiguity can be avoided by using Doppler spectra measured in at least two different directions. Although there is still some room for improvement in the practical application of this method, it can be applied to estimate the wave directional spectrum up to a rather high frequency, or Bragg frequency.
Propagation of pulses at optical wavelengths through fog‐filled medium
Authors: Maitra A. , Dan Marina
Published: February 2004

The pulse propagation at optical wavelengths (0.4–10.0 μm) through the fog‐filled medium has been studied considering the effects of differential attenuation and phase dispersion on the spectrum of the propagated signal. The pulse distortion, in terms of percentage change in the width of a Gaussian pulse, has been obtained using a closed solution of Fourier integral for the time domain representation of the pulse. This technique, initially developed by Forrer [1958], approximates the propagation constants by a truncated Taylor series. It has been found that the pulse distortion can be quite severe for pulses of width 0.001 ns or smaller, depending on wavelength, fog type, pulse width, and path length. Both broadening and compression of pulses can occur according to the values of second derivatives of the propagation constants.
Series expansions for the incomplete Lipschitz‐Hankel integralYe0(a, z)
Authors: Mechaik Mehdi M. , Dvorak Steven L.
Published: February 2004

Three series expansions are derived for the incomplete Lipschitz‐Hankel integral YeO(a, z) for complex‐valued a and z. Two novel expansions are obtained by using contour integration techniques to evaluate the inverse Laplace transform representation for YeO(a, z). A third expansion is obtained by replacing the Neumann function by its Neumann series representation and integrating the resulting terms. An algorithm is outlined which chooses the most efficient expansion for given values of a and z. Comparisons of numerical results for these series expansions with those obtained by using numerical integration routines show that the expansions are very efficient and yield accurate results even for values of a and z for which numerical integration fails to converge. The integral representations for YeO(a, z) obtained in this paper are combined with previously obtained integral representations for Jeo(a, z) to derive integral representations for HeO(1) (a, z) and HeO(2) (α, z). Recurrence relations can be used to efficiently compute higher‐order incomplete Lipschitz‐Hankel integrals and to find integral representations and series expansions for these special functions and many other related functions.
On the high‐frequency asymptotic evaluation of the potentials of elemental sources on an anisotropic impedance cylinder
Authors: Pogorzelski Ronald J.
Published: February 2004

In an effort to formulate the high‐frequency coupling between antennas located on airframes composed of multilayer imperfectly conducting materials a general model was sought which would embody the electromagnetic properties of the layers and would apply over a broader range of separations of the antennas. Such a model is described here. This work is a generalization of the work of Pearson [1986, 1987a] concerning the high‐frequency asymptotic representation of the fields of elemental sources diffracted by a multilayer cylinder. In that work the source and field points were located off the cylinder surface, and they were sufficiently separated to permit the effective use of the residue series representation of the spectral integrals involved. Here the source and field points are located on the cylinder surface and are permitted to be sufficiently close as to render the residue series poorly convergent. To obtain a more effective representation in this situation, the cylinder is modeled by an anisotropic impedance cylinder, and the resulting spectral integrals are evaluated by reduction to forms amenable to multiple applications of techniques described in the literature in connection with treatment of axially uniform sources in this context.
Evidence for the eastward migration of the Alboran Sea based on regional subsidence analysis: A case for basin formation by delamination of the subcrustal lithosphere?
Authors: Docherty Craig , Banda Enric
Published: February 2004

The paradox of an extensional basin forming at a site of colliding continents is apparent in the evolution of the Neogene Alboran Sea basin of the westernmost Mediterranean. Processes that may be responsible for such basins forming at sites of continental collision include subduction and the removal of subcrustal lithosphere by either convection (extensional collapse) or delamination and have recently been invoked to explain the formation of the Alboran Sea basin. Decoupling of the thermal boundary layer can reverse the stress field from a state of compression to one of extension. The processes of extensional collapse and delamination involve removal of this layer and would have similar consequences in terms of modifying lithosphere geometry: replacement of the detached thermal boundary layer by hot asthenospheric material, which would produce a high geothermal gradient and in turn cause uplift at the surface. With identical effects, it is difficult to distinguish between the two mechanisms. Using the technique of backstripping, a subsidence history which evidences the coexistence of subsided and uplifted areas in the Alboran Sea basin through the Miocene to the Quaternary, has been reconstructed for the basin. While the deformation front of the Alboran Domain migrated westward, subsidence in the Alboran Sea basin propagated in the opposite sense, east/southeastward. This is best explained as the basin forming above a southeastward‐propagating, delaminating subcrustal lithospheric slab.
Role and effects of pore fluid pressure in thrusting: The case history of the Umbria‐Marche Apennines, central Italy
Authors: Cello Giuseppe , Deiana Giovanni
Published: February 2004

Structure, timing and rates of deformation, and the aspect ratio of the external zones of the Umbria‐Marche thrust system (UMTS) have been investigated with the aim of comparing features and processes occurring during the growth of a tectonic wedge with those inferred from mechanically based model predictions. The results of the UMTS test seem to support the idea that most of the characteristic features of a thrust system may be considered as the result of the incremental, transient increase of the pore fluid pressure (Pf) as a consequence of the amount of shortening of a crustal section. The strong dependence of Pf on strain rates and on the thickness of the rock mass involved in the deformation may also help to explain local and/or regional variations in the aspect ratio of the deformed‐state profiles of different fold and thrust belts.
The Australia‐Pacific boundary and Cenozoic plate motions in the SW Pacific: Some constraints from Geosat data
Authors: Sutherland Rupert
Published: February 2004

Finite poles of Australia‐Pacific rotation are calculated using a three‐plate (Australia‐Antarctica‐Pacific) model and published Geosat data analyses of the Indian and Pacific Oceans (GEOS‐3P solution). Features identified on maps of Geosat data from SW of New Zealand are used to determine a new best fit finite pole for the prerift (mid Eocene; ∼45 Ma) configuration of the Australia‐Pacific plate boundary and to refine Eocene‐Miocene motions on the plate boundary (∼45–20 Ma; GEOS‐NZ results).

Inverting the Australia‐Pacific best fit finite rotation to find relative displacements between east and west Antarctica since the Eocene implies dextral oblique extension in the Ross Embayment and almost pure dextral movement in the Queen Maud Range. Early‐mid Eocene convergence predicted through Antarctica conflicts with geological evidence and remains an unresolved problem.
Upper mantle reflector structure and origin beneath the Scottish Caledonides
Authors: McBride J. H. , Snyder D. B. , Tate M. P. , England R. W. , Hobbs R. W.
Published: February 2004

The mantle reflector pattern beneath the Scottish and Irish Caledonides is perhaps the most widely studied individual structure in the uppermost continental mantle. Previous investigations interpreted the reflector as a thrust related to the Caledonian orogeny (Silurian‐Early Devonian) or as a normal fault or shear zone active during post‐Caledonian (e.g., Devonian or Permo‐Triassic) extension. This study uses all available deep seismic reflection data to map mantle reflector structure along the Caledonian orogen and to constrain better its geometrical and tectonic relation to regional geology. The distribution of the mantle reflector shows that it is not spatially related to either Caledonian compressional or subsequent basin extensional structures in a simple way. A structural contour map shows two distinct discontinuous surfaces consisting of a steeply plunging antiform‐synform pair beneath the West Orkney basin (north of the Scottish mainland) and the Færoe‐Shetland basin west of Shetland, respectively, separated by an inferred zone of structural disruption. A working hypothesis for this study is that these two surfaces may have once been part of a continuous, more or less planar surface that has been offset and deformed by either (or perhaps a combination) of two processes: (1) differential extension of crust that was at least partially coupled to the uppermost mantle beneath the West Orkney and Færoe‐Shetland basins; (2) a broad, approximately orthogonal (i.e., east‐west trending) zone of simple shear corresponding to the disrupted zone that was largely confined to the upper mantle but reactivated to deform the crust. Our results also suggest that the mantle reflector structure may be inherited from Neoproterozoic events associated with rifting of the Laurentia‐Baltica supercontinent.
Tertiary or Cretaceous age for Spitsbergen's fold‐thrust belt on the Barents Shelf
Authors: Maher Harmon D. , Braathen Alvar , Bergh Steffen , Dallmann Winfried , Harland W. Brian
Published: February 2004

Several publications propose that main‐phase fold‐thrust development on Spitsbergen was Late Cretaceous and not Tertiary as previously thought. The question of timing is crucial to models for crustal response to transpressive plate motions. Involvement of Tertiary strata in fold‐thrust structures, the sedimentology of the Tertiary basin strata, and studies of paleo‐stress field evolution all indicate Paleocene to Eocene fold‐thrust development during opening of the Norwegian‐Greenland oceanic basin. A regional angular unconformity of < 1° between Paleocene and Early Cretaceous strata is consistently disconformable to the eye and precludes any significant older deformation in the immediate area. Pre‐unconformity deformation was likely strike slip in character and concentrated in the west. The proposal for Late Cretaceous fold‐thrust belt formation is inconsistent with the geology.
Structural and metamorphic evolution of the Orocopia Schist and related rocks, southern California: Evidence for late movement on the Orocopia fault
Authors: Jacobson Carl E. , Dawson M. Robert
Published: February 2004

The Pelona, Orocopia, and Rand Schists (POR schists) of southern California and southwesternmost Arizona are late Mesozoic or early Tertiary subduction complexes that underlie Precambrian to Mesozoic continental basement along the low‐angle Vincent‐Chocolate Mountains (VCM) fault system. The VCM faults are often considered to be remnants of the original subduction zone, but recent work indicates that many have undergone substantial postsubduction reactivation. In the Orocopia Mountains, for example, the Orocopia Schist exhibits an exceptionally complex structural and metamorphic history due to multiple periods of movement along the Orocopia fault. Structures in the schist include isoclinal folds with axial‐planar schistosity, open‐to‐tight folds that fold schistosity, penetrative stretching lineations, and crenulation lineations, all of which show a nearly 360° range in trend. Folds and lineations that trend approximately NE–SW occur throughout the schist and are thought to be part of an early phase of deformation related to subduction. Folds of this orientation show no consistent vergence. Folds and lineations that trend approximately NW–SE are concentrated near the Orocopia fault and are interpreted to have formed during exhumation of the schist. The NW–SE trending folds, and shear indicators in late‐stage mylonite at the top of the schist, consistently verge NE. The exhumation event culminated in emplacement of the schist against brittlely deformed upper plate. Exhumation of the Orocopia Schist was accompanied by retrograde replacement of garnet, biotite, epidote, and calcic amphibole by chlorite, calcite, and sericite. Matrix amphibole has a lower Na/Al ratio than amphibole inclusions in albite, consistent with a late‐stage decrease in pressure. As NE vergence in the Orocopia Mountains is associated with exhumation of the schist, the NE movement along other segments of the VCM fault may also be late and therefore have no bearing on the facing direction of the VCM subduction zone, contrary to past interpretations.
Structural signature of tectonic processes in the Calabrian Arc, southern Italy: Evidence from the oceanic‐derived Diamante‐Terranova unit
Authors: Cello Giuseppe , Invernizzi Chiara , Mazzoli Stefano
Published: February 2004

The Diamante‐Terranova unit is an oceanic‐derived element within the thrust and nappe structure of the northern sector of the Calabrian Arc. Its metamorphic and structural signature reveals a long history of SE dipping subduction and collision‐related deformation of an oceanic lithospheric section underlying the narrow Diamante‐Terranova basin. The deformation history of the Diamante‐Terranova unit may be related to three main tectonic phases. The first one is characterized by a structural association which includes features that developed during a progressive deformation related to the evolution of a deep‐seated shear zone. Kinematic indicators characterizing subduction‐related structures clearly show a top‐to‐the‐northwest sense of overthrust shear. The second tectonic phase that can be recognized in the area is characterized by structures that may be related to the onset of shear deformation induced by a postcollisional northeastward motion of the overriding plate. The third tectonic phase is recorded by brittle features related to the latest tectonic events affecting the Calabrian Arc during the opening of the Tyrrhenian Sea. Shear sense indicators and available radiometric and stratigraphic information, together with plate kinematic data, allowed us to infer the time evolution of this sector of the Calabrian Arc and to assess that the Diamante‐Terranova unit cannot be considered as an “Eoalpine” element since its deformation, related to subduction‐collision processes, occurred prior to the end of the Cretaceous.
Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by40Ar/39Ar thermochronology
Authors: Krol Michael A. , Zeitler Peter K. , Poupeau Gérard , Pecher Arnaud
Published: February 2004

The40Ar/39Ar thermochronology of the Late Cretaceous Hunza plutonic complex reveals an episodic cooling and denudation history for this regional‐scale pluton. The40Ar/39Ar analyses of biotites from a vertical relief section of >3200 m reveal a pulse of rapid cooling at ∼20 Ma. In the interval of 110–27 Ma, age‐elevation distributions suggest denudation rates of the order of 0.02 ± 0.003 mm/yr. At ∼20 Ma, denudation rates increased significantly to 2.7 ± 0.7 mm/yr, then returned to much slower rates until 12 Ma. A second pulse of rapid cooling beginning at 12 Ma is revealed by inverse numerical modeling of multidiffusion domain alkali feldspars from a vertical section of 1700 m. Decreasing in elevation, these samples record the onset of rapid cooling at 12, 9, and 7 Ma, respectively. All of the alkali feldspars record a period of nearly isothermal conditions prior to the onset of rapid cooling when rates increased to 30°C/m.y. Assuming a geothermal gradient of 30°C/km, these cooling rates translate into denudation rates of 1.0 mm/yr. Apatite fission track analysis indicates denudation rates of 0.7 ± 0.1 mm/yr over the interval of 6.6 Ma to 2.4 Ma in agreement with the alkali feldspar data. These data suggest denudation of 2.9 ± 0.4 km since the Plio‐Pleistocene. Together, the alkali feldspar and apatite data indicate that a minimum of 10 km of overburden has been removed since the mid‐late Miocene. An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system).(Paper 95TC02424, Temporal variations in the cooling and denudation history of the Hunza plutonic complex, Karakoram Batholith, revealed by40Ar/39Ar thermochronology, M.A. Krol, P.K. Zeitler, G. Poupeau, and A. Pecher). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.
Geochronology and isotopic character of ultrahigh‐pressure metamorphism with implications for collision of the Sino‐Korean and Yangtze cratons, central China
Authors: Ames Leslie , Gaozhi Zhou , Baocheng Xiong
Published: February 2004

The collision zone between the Sino‐Korean and Yangtze cratons in central China is marked by subduction‐related metamorphic rocks composed of crustal protoliths. These range in metamorphic grade from ultrahigh‐pressure eclogite to low‐greenschist facies. The paragenesis of the ultrahigh‐pressure rocks (coesite and diamond‐bearing) indicates subduction to depths greater than ∼120 km as a result of continental collision. Despite extreme metamorphic conditions, zircons have not undergone Pb loss but rather illustrate two periods of growth: crystallization between 700 and 800 Ma and ultrahigh‐pressure metamorphism at 218 ± 2.5 Ma. Previous studies have predicted a time‐transgressive collision, but age data presented in this study indicate that the collision was approximately coeval along the length of the suture. U‐Pb zircon ages of these ultrahigh‐pressure metamorphic rocks are 218.4 ± 2.5 Ma and 217.1 ± 8.7 Ma for the Dabie Mountains and Shandong peninsula, respectively. The similar ages imply that the Tan‐Lu fault, which offsets the ultrahigh‐pressure metamorphic rocks, is a secondary feature not related to collision or subduction. Whole rock isotopic data show that both Rb‐Sr and U‐Pb were open systems during metamorphism but that Sm‐Nd probably remained closed. Nd initial isotopes display crustal signatures, indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth. Sm‐Nd data in conjunction with U‐Pb zircon dating indicate that the ultrahigh‐pressure rocks are not typical Yangtze craton basement (2.9 Ga) but that they originally crystallized in a rift environment between ∼700 and 800 Ma. Discrepancies between 40Ar/39Ar plateaus, Sm‐Nd isochrons, and U‐Pb zircon ages render calculation of a P‐T‐t path unrealistic.