bluuebunny/crossref_metadata_2025 huggingface dataset:
Ionospheric heater beam scanning: A realistic model of this mobile source of ELF/VLF radiation
Authors: Barr R. , Rietveld M. T. , Stubbe P. , Kopka H.
Published: February 2008

The generation of ELF/VLF radio waves by heating the auroral ionosphere with a powerful HF transmitter, modulated at the ELF/VLF frequency, is now well documented. Recently, results were presented of the phase of the ELF/VLF signals received at a distance of 500 km from such a source as the HF beam was deflected toward and away from the receiver by up to 37° from the vertical. The phase data were modeled assuming the source to be a simple point dipole located in the ionosphere at the point of maximum HF radiation. This simple model provided good agreement with experimental phase data only when the HF beam was deflected toward the ELF/VLF receiver. The model of the ELF/VLF generation mechanism presented here includes contributions from the majority of the HF radiation pattern illuminating the ionosphere and assumes it to be a continuous source of ELF/VLF excitation. Computations based on this new model provide excellent agreement with the experimental phase data at all beam deflection angles. The use of this model to improve the power efficiency of the generation of ELF/VLF radio waves by HF heating facilities by more than a factor of 3 is discussed.
Scattering of an E‐polarized plane wave by two parallel rectangular impedance cylinders
Authors: Yoshidomi K. , Aoki K.
Published: February 2008

There have been studies to obtain scattering coefficients of an imperfect wedge by applying the impedance boundary condition. However, it is important to clarify the validity of this application, because the impedance boundary condition is known not to be valid at an edge. In this paper the scattering calculations for rectangular impedance cylinders by using the Wiener‐Hopf technique are presented. A comparison is given with the result based on the exact boundary condition. In addition, multiple scattering by two parallel rectangular impedance cylinders is studied.
Case study of polar cap scintillation modeling using DE 2 irregularity measurements at 800 km
Authors: Basu Sunanda , Basu Santimay , Weber E. J. , Coley W. R.
Published: February 2008

Although a satellite‐borne irregularity sensor obviously cannot measure scintillations, we address the question of what contribution such a sensor can make to model or predict scintillations. To pursue the problem, we have utilized the Dynamics Explorer 2 (DE 2) ionospheric electron density irregularity data obtained at approximately 800 km altitude in the winter polar cap during sunspot maximum conditions. During this period an all‐sky imaging photometer located at Thule, Greenland, within the polar cap, detected the presence of convecting ionization patches, and polar beacon satellite measurements detected several discrete, intense scintillation structures associated with these patches (E. J. Weber et al., 1984). The electron density deviation (ΔN) obtained by combining irregularity amplitudes (ΔN/N)rms processed over successive 8‐s intervals and the in situ density (N) data acquired by the DE 2 satellite also showed the presence of spatially discrete structures. These irregularity structures, both in N and ΔN, had spatial extents of ∼1000 km in the N‐S direction. The density associated with these structures, even at 800 km, showed a twofold to threefold increase in comparison to the background, and irregularity amplitudes (ΔN/N)rms as large as 20% were observed at the edges of the patches. It is shown that the observed scintillation parameters can be successfully modeled only when the satellite sensor irregularity data are combined with additional and vital information regarding maximum ionization density, irregularity anisotropy, and layer thickness in the framework of scintillation theory. We conclude that within the context of other measurements and an appropriate scintillation model, the irregularity sensor data may serve as a valuable input.
Satellite radar interferometry: Two‐dimensional phase unwrapping
Authors: Goldstein Richard M. , Zebker Howard A. , Werner Charles L.
Published: February 2008

Interferometric synthetic aperture radar observations provide a means for obtaining high‐resolution digital topographic maps from measurements of amplitude and phase of two complex radar images. The phase of the radar echoes may only be measured modulo 2π; however, the whole phase at each point in the image is needed to obtain elevations. We present here our approach to “unwrapping” the 2π ambiguities in the two‐dimensional data set. We find that noise and geometrical radar layover corrupt our measurements locally, and these local errors can propagate to form global phase errors that affect the entire image. We show that the local errors, or residues, can be readily identified and avoided in the global phase estimation. We present a rectified digital topographic map derived from our unwrapped phase values.
Simultaneous bistatic European Incoherent Scatter UHF, 145‐MHz radar and stimulated electromagnetic emission observations during HF ionospheric modification
Authors: Nordling J. A. , Hedberg Å , Wannberg G. , Leyser T. B. , Derblom H. , Opgenoorth H. J. , Kopka H. , Kohl H. , Stubbe P. , Rietveld M. T. , LaHoz C.
Published: February 2008

The ionosphere was modified with powerful, high‐frequency radio waves transmitted by the Heating facility near Tromsø, Norway. The modified ionosphere was observed using the Tromsø and Kiruna European Incoherent Scatter (EISCAT) UHF system together with a 145‐MHz portable CW radar and equipment for reception of stimulated electromagnetic emissions (SEE). Enhanced ion and plasma lines were observed. They persisted throughout heater periods of more than a minute, contrary to earlier observations at Tromsø. Spectral features in the plasma lines were repeatedly observed at frequency shifts which are not explained by current theories. The SEE amplitude variation on long time scale (seconds to tens of seconds) shows similarity with the long–time scale behavior of the amplitude of the plasma lines observed by EISCAT. No obvious correlation seems to exist between the SEE and discovered radar spectral features.
Incoherent scattering of radar waves from nonthermal F region plasma: Analytical methods of spectrum synthesis
Authors: Suvanto K.
Published: February 2008

A new method is presented of computing the theoretical spectrum of the incoherent scatter signal from nonthermal ionospheric F region plasma. An expression for the ion line‐of‐sight velocity distribution function is derived analytically and the evaluation of the Cauchy principal value integral, part of the longitudinal dielectric function, is discussed. Spectra revealing the presence of nonthermal plasma have recently been observed by the European Incoherent Scatter (EISCAT) radar during periods of rapid plasma flow, and the incorrect assumption of Maxwellian plasma at such times will cause errors in the plasma density and temperatures derived. The new procedure is tailored to satisfy the needs of non‐Maxwellian spectral analysis of data from EISCAT and other incoherent radar systems.
Enhanced backscattering phenomenon in a random continuum
Authors: Lure Y. M. , Yang C. C. , Yeh K. C.
Published: February 2008

The enhancement effect of the backscattered intensity in a random continuum under the forward scatter approximation is studied. This effect is attributed to the gain of coherence recovered from random decorrelation when the backward scattered ray passes through the same random structure as the incident ray. This effect can be completely described by the enhancement factor defined as the ratio of the average backscattered power over that in the absence of multiple scattering. In this paper, this enhancement phenomenon is investigated for two kinds of sources: a point source and a beam wave, and two kinds of power spectral functions: a Gaussian spectrum and a power law spectrum. For the case of a point source two ranges of scattering strength are considered: weak scattering and strong scattering. In the limit of weak scattering, the supereikonal approximation is applied. On the other hand, in the limit of strong scattering, the statistical saturation is assumed to be reached. For the case of a beam wave, only strong scattering is considered. Computations have been made as the beam size is varied. It is found that the enhancement effect subsides as the beam size is increased.
Radio wave propagation in presence of finite width knife edges
Authors: Wu Z. , Maclean T. S. M. , Mehler M. J. , Bagwell D. J.
Published: February 2008

An expression for the attenuation of a radio wave with respect to free space, produced by a finite width, constant height, knife edge obstacle on the ground, is derived in the form of a double integral. For the case when the ground is perfectly conducting, these integrals are evaluated in closed form. The solution for the case of cascaded, rectangular knife edges is then considered numerically. Experimental confirmation of the results for 4λ and 6λ wide, perfectly conducting knife edges is provided, together with that of two knife edges in cascade.
The natural oscillations of an infinitely long cylinder coated with lossy material
Authors: Sun W. M. , Chen K. M. , Nyquist D. P. , Rothwell E. J.
Published: February 2008

Investigations are reported of the effects of lossy coatings on the natural frequencies of a perfectly conducting cylinder with TE excitation. A general characteristic equation for extraction of the natural frequencies of a cylindrical structure is developed. It is found that the natural frequencies are substantially shifted only when the coating thickness is comparable to the radius of the cylinder. When the conductivity of the coating material is high enough, the behavior of the natural modes of a coated cylinder is essentially the same as that of a perfectly conducting cylinder.
Interpretation of layered structure in mesospheric VHF echoes induced by an inertia gravity wave
Authors: Muraoka Yoshikazu , Sugiyama Takuya , Sato Toru , Tsuda Toshitaka , Fukao Shoichiro , Kato Susumu
Published: February 2008

A remarkable layered structure of mesospheric VHF echo was observed on September 20, 1985, in the experiments with the middle and upper atmosphere (MU) radar at Shigaraki, Japan. The echoing layers were seen at the heights where the static stability in the atmosphere increases owing to the simultaneously observed inertia gravity wave motion. In this paper we show that the height profile of the observed echo power can be interpreted as due to the modulation of a mean gradient of potential refractive index by the gravity wave under the assumption of a height independent spectrum of turbulent irregularities. It is then shown that the enhancement of the observed echo agrees well with sharp gradients of electron density which are formed through the reactions between chemical species vertically transported by the wave.
First‐order Wiener‐Hermite expansion in the electromagnetic scattering by dielectric rough interfaces
Authors: Eftimiu Cornel , Pan G. W.
Published: February 2008

The electric and magnetic currents induced by an incoming electromagnetic wave on a rough surface which separates free space from a lossy dielectric medium are calculated as expansions to first‐order in Wiener‐Hermite functionals. The diffuse scattered intensity resulting from this approach is compared with the classical results following from the use of perturbation theory or the Kirchhoff approximation.
Millimeter wave scattering model for a leaf
Authors: Sarabandi K. , Ulaby F. T. , Senior T. B. A.
Published: February 2008

At millimeter wave frequencies a typical leaf is a significant fraction of a wavelength in thickness, and its nonuniform dielectric profile now affects the scattering. To provide a simple and efficient method for predicting the scattering, two types of physical optics approximations are examined. The first approximates the volume polarization current by the current which would exist in an infinite dielectric slab with the same profile, while the second (and simpler) one employs the surface current which, on the infinite slab, produces the known reflected field. It is shown that the first method is superior, and, provided the actual dielectric profile is used, it predicts the scattered field to an accuracy which is adequate for most practical purposes.
Plasma instabilities of a finite‐radius electron beam in a uniform plasma
Authors: Wong H. K. , Lin C. S.
Published: February 2008

To study electromagnetic radiation induced by electron beam injection from the space shuttle, the electromagnetic dispersion equation of a finite‐radius cold electron beam in a neutralizing background was solved numerically. The numerical solutions indicate that a keV electron beam can drive the beam and whistler modes unstable, regardless of whether the beam is homogeneous or has a finite radius. Although the beam mode is excited for frequencies up to the electron plasma frequency, only waves with frequencies below the electron cyclotron frequency can propagate outside the beam. The parallel wave numbers of the whistler waves excited by the finite‐radius and the homogenous electron beams are similar, suggesting that the finite‐radius electron beam also excites the whistler waves near the resonance cone. These results are applied to explain the whistler waves radiated from the keV electron beam injected from Spacelab 2.
A model for spaced antenna observational mode for MST radars
Authors: Liu C. H. , Röttger J. , Pan C. J. , Franke S. J.
Published: February 2008

A model, based on the principle of volume scattering and the assumption of locally frozen‐in refractive index irregularities, is proposed for mesosphere‐stratosphere‐troposphere (MST) radar observations in the spaced antenna mode. Autospectrum, cross‐correlation function, cross‐spectrum, and coherence function for the received signals are derived and related quantitatively to the atmospheric parameters. The model predicted results are compared with observational results. It will be shown that under observational and atmospheric conditions with appropriate time and spatial scales the model is consistent with experimental results. Those conditions and scales are discussed in the paper.
Aspect sensitivity of stratospheric VHF radio wave scatterers, particularly above 15‐km altitude
Authors: Hocking W. K. , Fukao S. , Tsuda T. , Yamamoto M. , Sato T. , Kato S.
Published: February 2008

The aspect sensitivity of VHF radar scatter in the stratosphere is investigated with particular emphasis being placed on the region above 15‐km altitude. It is found that the scatter is highly aspect sensitive below about 18 km in the stratosphere as is well known, but above this height the scatter becomes more isotropic again. The highest degree of aspect sensitivity occurs around 18‐km altitude, where the backscatter polar diagram has a 1/e half‐width θs of less than 1.5°, and the horizontal correlation length can exceed 30 m. At heights above 20 km, however, θs is in excess of 4° and increases with increasing height. At any one height there is a range of scatterer shapes, from scatterers with large length‐to‐depth ratios up to almost isotropic scatterers. These results are confirmed using studies of backscattered power as a function of angle, comparisons of radial velocities as a function of beam bore‐tilt angle, and studies of the spectral width compared to the expected beam‐broadened spectral width. The results complement and support the only other published study of scatterer anisotropy in the region of 15‐ to 30‐km altitude (Hocking et al., 1986).
Frequency dependence of Doppler shift for type III coherent radar echoes
Authors: Koehler J. A. , Sofko G. J. , Watermann J. , McNamara A. G. , McDiarmid D. , André D. , Prikryl P. , McKibben M. , Helgason K. H.
Published: February 2008

Coherent echoes were observed simultaneously from the same scattering region using two bistatic radars operating at 50 MHz and 60 MHz. The data were searched for the occurrence of type III echoes. If type III echoes are caused by ion‐cyclotron waves, one would expect the Doppler shift to be nearly equal to the ion‐cyclotron frequency in the scattering region for both radars, and hence to be equal. On the other hand, if type III echoes are just low velocity type I echoes, then the phase velocity should be the same, and hence the Doppler frequency shift should be scaled according to observation frequency. The results of this experiment indicate that the two radar spectra seem to scale according to frequency as would be expected for scatterers with a similar phase velocity at the two radar wavelengths.
Ultrahigh vertical resolution radar measurements in the lower stratosphere at Arecibo
Authors: Ierkic H. M. , Woodman R. F. , Perillat P.
Published: February 2008

We report on heretofore unprecedented observations of the turbulent layers in the lower stratosphere using the Arecibo 2380‐MHz radar. Spectral profiles with about 20 m height and 15 s time resolutions at altitudes in the range 16–19 km are used to parameterize relevant characteristics of the turbulence, namely vertical widths, distributions, lifetimes, and cutoff height. Our measurements validate previous deconvolved estimates and are free from contaminating factors like shear or beam broadening and partial reflections. Some theoretical predictions are verified, in particular those relating to the height of cutoff and the outer scale of the turbulence. We demonstrate the excellent capabilities of the instrument with observations of wave driven oscillatory displacements of thin turbulence structures near the tropopause. In these circumstances and because of the vertical resolution achieved, it is more direct to describe the inherent variability of spectral features following the vertical displacements of the layers rather than at a fixed height.
A simple model for the enhanced frequency spectrum of vertical velocity based on tilting of atmospheric layers by lee waves
Authors: Gage Kenneth S. , Nastrom Gregory D.
Published: February 2008

This paper is concerned with the interpretation of the enhanced frequency spectrum of vertical velocity that has been observed at many locations under strong‐wind conditions in the vicinity of rough terrain. These enhanced frequency spectra are considerably more energetic than the vertical velocity spectra observed under low‐wind conditions, and their shape resembles the shape of the oblique spectra observed by the same radars. In this paper we present the composite spectra of horizontal and vertical velocity observed over a range of wind conditions by the Platteville, Colorado, radar, located in the lee of the Colorado Rockies. The vertical velocity spectra clearly show the increase in magnitude and change in spectral slope that occurs as the troposphere becomes increasingly disturbed. We compare the observations with a simple model that relates the magnitude of the vertical velocity spectrum to the horizontal velocity spectrum and the effective tilt of atmospheric layers. Our results show that it is possible to simulate the observed vertical velocity spectrum with effective tilts in the range of 2.5–10°. Recent observations of vertical velocity spectra from the Flatland radar, located in flat terrain in central Illinois, do not exhibit this behavior, which leads us to conclude that lee waves are responsible for the enhancement in the Colorado spectra.
A diagnostic system for the study of extremely long wavelength emission produced by ionospheric modification
Authors: Baker M. R. , Collins T. W. , Lee H. S. , Ferraro A. J.
Published: February 2008

This paper describes a diagnostic system developed for the study of radio signals at frequencies below 8 kHz. Of specific interest are emissions produced by high‐frequency heating of the ionosphere which modulates natural ionospheric current systems. Dual receivers allow independent reception on orthogonal loop antennas, and microcomputer systems provide real‐time data acquisition and processing of all experimental data. Provisions are also made for automatic control of modulating frequency applied to the heating transmitters; operating frequencies may be changed on a preset schedule in tandem with receiver tuning.
Determination of D region electron densities from the ELF frequency stepping experiment
Authors: Li P. J. , Ferraro A. J.
Published: February 2008

This paper describes the use of the generation of extremely low frequency (ELF) from heating of the D region of the ionosphere at high latitudes to estimate the degree of electron density ionization of the D region. In particular, the heating facility near Fairbanks, Alaska, known as the high‐power auroral simulation (HIPAS) facility, was used in this work. The theory of electron density synthesis is described, results of actual data presented, and electron densities determined. A new future ionospheric measurement technique is discussed.
Tectonics and topography for a lithosphere containing density heterogeneities
Authors: Fleitout Luce , Froidevaux Claude
Published: February 2008

The purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities, in particular with respect to mountain building and other processes of intraplate deformation. Density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields: the product of their magnitude by their depth is shown to characterize their ability to induce deformation. This rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent. For anomalies of lateral extent that is small in comparison with their depth, the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous Newtonian lithosphere. The analytical treatment is based on Fourier transform. Continent‐continent collision thickens not only the crust but the entire lithosphere. The cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away. Thus the continental lithosphere is in a somewhat metastable mechanical state. Adiabatic, i.e. rapid, thickening (or thinning) tends to grow further once initiated. Tectonic phases of strong compression correspond to the climax of such instabilities. The response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions. They yield velocity distributions and stress fields. Instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the Alpine and Appalachian regions. In the presence of lateral variations of the mechanical properties of the lithosphere, the tectonic style is not only function of the local topography and of the nature of its compensation. Deformations in neighbouring provinces are coupled as shown by 3‐dimensional models. For example, thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness. These last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand, and of geometrical constraints related to lateral mechanical heterogeneities on the other.
Is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of North America?
Authors: Engelder Terry
Published: February 2008

On the Appalachian Plateau of New York, ENE striking joints parallel the mid‐continent contemporary stress field and do not correlate with other structures. Joints within Ohio, Indiana, Michigan, and West Virginia also parallel the mid‐continent maximum horizontal compression as indicated by hydraulic fracture measurements. In northern Indiana, where the contemporary stress field deviates significantly from the orientation of the mid‐continent stress, the attitude of the joints deviates accordingly. In addition to joints, structures on several scales from subcontinent size fracture systems to microfabrics in shales apparently parallel the mid‐continent stress field. From this I conclude that these joints are mode I cracks whose orientation corresponds to the contemporary tectonic stress field. In itself, the formation of these structures constitutes a tectonic event that has often been overlooked in favor of tectonic events that produce the more spectacular but less pervasive faults and folds.
Rock assemblages in central Asia and the evolution of the Southern Asian Margin
Authors: Leith William
Published: February 2008

By looking in detail at the rock record in Central Asia, it is possible to map rock assemblages that are diagnostic of particular plate settings. Simplest of the assemblages include the rocks of the Tadjik Depression, where Mesozoic and Paleogene sediments record the development of a south facing passive continental margin. Analysis of igneous activity in Central Asia and of the timing of deformation of these miogeoclinal rocks indicates that the Paleotethyan ocean to the south was, in the Mesozoic, subducting to the south beneath a migratory arc consisting of parts of Iran, Afghanistan, and the Pamir. Only in the late Tertiary does the Tadjik Depression develop a peripheral foreland assemblage which indicates any deformation of the miogeocline. This presumably occurred as a result of the closing of the Paleotethyan ocean and accretion of this portion of the arc to the southern margin of Asia. It appears unlikely that the Depression passive margin sequence developed in a normal rift margin setting. Analysis of assemblages in the Hercynian orogen (Tien Shan) to the north indicates that, unlike the Uralian section of the orogen, the deformation here occurred due to the closure of a marginal basin and overthrusting of its basement rocks over a south facing arc. Thus considering Depression basement composition, it appears that the passive margin assemblage is superimposed over either a late Paleozoic forearc or Paleozoic ocean floor. Paleozoic rocks exposed north of the Herat suture in Afghanistan may require the forearc basement interpretation. Noting that the geometry of subduction and the timing of Paleotethyan suturing seem to have varied along strike in the orogen, a more complicated model of tectonic accretion is required.
Formation of uppermost oceanic crust
Authors: Rosencrantz Eric
Published: February 2008

The uppermost portion of oceanic crust, extrusive basalts underlain and fed by diabasic dikes, represents the chilled roof zone or lid to mid‐oceanic spreading center magma chambers. This rigid lid forms and evolves through a steady state process of flexure, subsidence, and internal rotation, all occurring within a couple of kilometers of the spreading axis. The lid accommodates lateral spreading with dikes emplaced along a unique axis. Dikes tap underlying magma to feed overlying flows. Basalts accumulating along the axis of the lid load the lid, which responds by flexing down and away from the intrusive axis. This subsidence affects lid structure in the following ways. Emplaced dikes are tilted away from, and basalt flows are tilted toward, the intrusive axis. Lid subsidence and section rotation extend the base of the lid more rapidly at the axis than elsewhere. Consequently, dikes preferentially intrude at the existing axis to maintain existing axis shape and relative position through continued spreading. Loading and flexure of the lid, plus faster accretion at the base of the lid as a result of lid rotation, cause the lid to be uplifted distal to the axis. This creates flanking marginal highs and generates extension structures (fissuring and faulting) flanking and paralleling the loading (extrusive) axis. The crustal structure and geometry produced by this process is compatible with structure and topography observed at present‐day mid‐oceanic spreading centers and with structure observed in ophiolites, particularly the Bay of Islands Ophiolite.
Lateral stretching of the Philippine Sea Plate subducting along the Nankai‐Suruga Trough
Authors: Ukawa Motoo
Published: February 2008

Focal mechanisms are investigated for shallow earthquakes (h ≤60 km) of central Japan by using telemetered seismic data of microearthquakes. Shallow earthquakes in central and southwestern Japan can be distinguished into three groups, upper crustal earthquakes, subcrustal earthquakes, and interplate thrust earthquakes, the first and the second ones being subjects of the present study. Most of the upper crustal earthquakes are of strike slip type or reverse faulting with E‐W to SE‐NW compression, whereas the subcrustal earthquakes are characterized by strike slip or normal faulting with E‐W to NE‐SW extension. The subcrustal seismic zone, subparallel to the Nankai trough, becomes progressively deeper landward to the north and progressively shallower to the east so that it emerges into a very shallow seismic activity in the Izu Peninsula which has been interpreted as a microcontinent of the Philippine Sea plate colliding against the Eurasian plate. On the basis of hypocentral distribution, three‐dimensional seismic velocity structure, and focal mechanisms, subcrustal earthquakes are considered to occur within the subducting Philippine Sea plate that extends down to a depth of at least 60 km. On the basis of this and other focal mechanism studies, stress trajectories are drawn for the subducting Philippine Sea plate and for the overriding Eurasian plate. The overriding plate is, on the whole, under the E‐W compression which coincides with the moving direction of the Pacific plate relative to the Eurasian plate. The subducting plate, on the other hand, is under lateral extension along its strike. We interpret that the latter is a stress regime unique to an incipient subduction zone, where the subducting plate at subcrustal depths is forced to warp downward, being constrained laterally at the arc‐arc junctions, and thereby comes under lateral stretching. The lateral constraints arise either from apparent increase in flexural strength of the plate at the arc‐arc junction or from buoyancy of a microcontinent that resists subduction.
Laramide crustal thickening event in the Rocky Mountain Foreland and Great Plains
Authors: Bird Peter
Published: February 2008

The Rocky Mountain foreland and Great Plains of the western United States were formerly part of a continental platform, adjusted by erosion and deposition in Cambrian through Jurassic time to near mean sea level, and therefore to a near‐uniform crustal thickness of approximately 33 km. Today the region stands at regional elevations up to 2 km, isostatically supported by a crust exceeding 50 km in thickness. Reasonable estimates of Tertiary sedimentation and Laramide strain do not account for more than 15% of the implied thickening. However, from approximately 70‐40 m.y. B.P., this region was underlain by a horizontally‐subducting slab of Farallon plate lithosphere moving northeast; this slab could have been the cause of the thickening. Finite‐difference thermal models with specified kinematics show only a temporary cooling of the base of the North American lithosphere by this slab. The excess weight of the slab would have depressed the region; plate‐bending calculations show a quantitative agreement of predicted depression with upper Cretaceous isopachs. Since depression by this slab lasted until the Eocene at least, the latest‐Cretaceous regression was probably caused by a Laramide crustal thickening event. The Farallon slab might have caused crustal thickening in two ways. Its excess weight would have drawn in ductile lower crust from surrounding regions. However, calculations show that this effect is too slow, too local, and too reversible to explain most of the crustal thickening. Therefore it seems likely that ductile lower crust was transported from SW to NE by shear stresses which the Farallon plate exerted on the base of the North American lithosphere. A preliminary finite‐element calculation based on this hypothesis shows the correct general pattern of crustal thickening. An unexpected but encouraging result is that predicted principal compression directions are orthogonal to many Laramide basement uplifts.
Intraplate deformation by block rotation and mesostructures along the Dead Sea Transform, northern Israel
Authors: Ron H. , Eyal Y.
Published: February 2008

Combined analysis of mega‐ and meso‐structures yield the strain, inferred stress and tectonic history of the margin of the Sinai sub‐plate in northern Israel, along the Dead Sea transform. Three major stress regimes were developed and have affected the region since the middle Miocene: one is regional and controlled the left‐lateral shear along the Dead Sea transform. This one has been active since post‐early Miocene, is characterized by σ1 trending c. 345° and deformed the plate margin by horizontal shear and contemporaneous block rotation. This style of deformation is similar to simple shear. The second one was local and may be subdivided into distinct stress and strain domains. It was characterized by σ1 trending E‐W and σ3 trending N‐;S, which produced a conjugate set of strike‐slip faults. The region underwent N‐S elongation by horizontal shear and block rotation similar to the pure shear mechanism. This stress regime was active in late Miocene to early Pliocene time. The third and youngest stress regime is extentional with σ1 vertical and σ3 oriented N‐S. This one has been active since post‐middle Pliocene time producing E‐W normal faults, and minor N‐S elongation. The second and third stress regimes show local distribution and developed as a consequence of the bent geometry of the transform vis‐a‐vis the plate‐slip vector. Finite element and photoelastic models predict the development of local stress fields and elongation of the plate margin parallel and subparallel to the transform plate boundary. The strain of the plate margin in northern Israel and southern Lebanon indicate remarkable elongation parallel to the Dead Sea transform by a factor of 1.5–2.1 since the Miocene. This process of intraplate deformation and the consequent strain is unique to the parts of the plate margin which are adjacent to the bent segment of the transform.
The North China Basin: An example of a Cenozoic rifted intraplate basin
Authors: Ye Hong , Shedlock K. M. , Hellinger S. J. , Sclater J. G.
Published: February 2008

The North China basin was a stable continental region that has undergone several distinct phases of rifting and subsidence during the Mesozoic and Cenozoic eras. The last major phase, featuring block faulting, rapid subsidence and widespread calc‐alkaline basaltic volcanism, began during early Tertiary time. This intraplate rifting appears to be the result of an approximately 30% extension and thinning of the lithosphere beneath the northeastern China. By the late Tertiary period, active rifting had slowed and postrift thermal subsidence had begun. This regional extension and subsidence resulted in the present day North China basin, a large saucer‐shaped, oil producing basin with thinner crust and lower upper mantle P and S wave velocities than the surrounding regions. During the Quaternary period, however, the subsidence rate increased in the North China basin. The frequent occurrence of destructive earthquakes and observations of relatively high heat flow suggest that the Quaternary tectonic activity differs from the late Tertiary simple thermal subsidence pattern and may indicate a new phase of rifting.
Transtensive tectonics in the Strait of Sicily: Structural and volcanological evidence from the Island of Pantelleria
Authors: Cello G. , Crisci G. M. , Marabini S. , Tortorici L.
Published: February 2008

A detailed structural analysis carried out on the volcanic products of the island of Pantelleria has shown that the large‐scale setting is dominated by N‐S trending normal faults and NW‐SE right‐lateral strike‐slip faults, while mesostructural data show a more complex pattern which is characterized by four systems of discontinuity. The volcanological features show a marked difference between a north western sector characterized by products of a basaltic nature and a south central one in which evolved products connected with the presence of a magmatic chamber are present. The present‐day stress field, as derived from structural analysis, is characterized by a maximum compression oriented NNW‐SSE which allowed the activation of right‐lateral strike‐slip faults bordering the Pantelleria trough giving rise to a thinning of the underlyng crust. Inside this main dextral shear zone it is possible that the development of pull‐apart basins bounded by N‐S trending normal faults could generate further thinning of the already thinned out crust zone and give rise to the magmatic phenomena known to occur mostly along the N‐S trending extension structures. Such a model would therefore help to understand the characters of the volcanism associated with a zone of continental collision as well as its distribution, which is not rigorously in accordance with a classical rifting process.
Thermal evolution of convergent plate margins: Thermal modeling and reevaluation of isotopic AR‐ages for Blueschists in the Franciscan Complex of California
Authors: Cloos Mark
Published: February 2008

Modeling of heat flow through a wedge‐shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of Ar‐isotopic ages and metamorphic progressions across blueschist belts. If convergence rates are maintained at rates of a few centimeters per year or more, material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where Ar‐loss by diffusion and recrystallization effectively ceases. Maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider. Because of this difference in thermal history, K‐Ar and 40Ar/39Ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth. Application of this thermal modeling to the Franciscan subduction complex of northern California indicates that the coherent blueschists in the Eastern Belt (typically with 115 to 125 m.y. Ar‐isotopic ages) and the high‐grade blueschist blocks in the mud matrix melanges of the western portion of the Central Belt (typically with 140 to 150 m.y. Ar‐isotopic ages) could both have formed during a single, prolonged period of continuous convergence.
Evolution of a turbiditic foreland basin and analogy with an accretionary prism: Example of the Eocene South‐Pyrenean Basin
Authors: Labaume P. , Séguret M. , Seyve C.
Published: February 2008

The South‐Pyrenean Tertiary Basin was an east‐west trending foreland trough which developed from Early Eocene to Miocene times in a north‐south trending compressive regime. A turbiditic series of Lower and Middle Eocene age (Hecho Group) enables us to define the typical features of the tectono‐sedimentary evolution of the basin: (1) slope and shelf derived carbonate turbidites and megaturbidites bear witness to the seismic activity of the basin borders, (2) the southern basin border, evidenced by the onlap of the turbidites onto slope and shelf deposits, migrated progressively southwards in time and (3) the deformation pattern is characterized in the Meso‐Cenozoic sediments by a complex thrust‐fold system of thin‐skinned tectonics style; its specific geometry and kinematics were influenced by the pre‐deformation geometry (onlap). This thrust‐fold system is related to south‐directed basement nappes, and its front advanced progressively southwards in time. From this data, we infer an evolutionary model for this highly mobile basin: it was a flexural basin linked to the northward bending of the lithosphere in the foreland of a south‐directed crustal accretionary prism; as the nappes advanced, the basin migrated progressively southwards, in the foreland direction. As for the actual basin evolution, many analogies with accretionary prism‐trench systems of active margins exist.
Continental rifting and transform faulting along the Jurassic Transantarctic Rift, Antarctica
Authors: Schmidt Dwight L. , Rowley Peter D.
Published: February 2008

The Transantarctic rift, an extensional continental rift valley, formed between East and West Antarctica during latest Early and Middle Jurassic time and is represented today by the high Transantarctic Mountains, which contain large volumes of continental flood basalt, diabase, and gabbro. Transantarctic rifting marked the beginning of the breakup of Gondwanaland; it was contiguous and synchronous with continental rifting between East Antarctica‐India and Africa as represented by the continental basalt and diabase of Queen Maud Land and the Karroo of southern Africa. During Late Jurassic time, about 150 Ma or slightly earlier, East and West Gondwanaland separated and new oceanic crust of the earliest Indian Ocean formed between East Antarctica‐India and Africa. If, as assumed, West Antarctica and South America remained fixed through a tip‐to‐tip join between the Antarctic Peninsula and Tierra del Fuego, then this seafloor spreading required major right‐lateral transform faulting of 500 to 1000 km on the Transantarctic rift system between East and West Antarctica. The Transantarctic Mountains were elevated at about the same time in Late Jurassic; such uplifts are characteristic of active rift margins worldwide. During Cenozoic time, extensional block faulting, independent of the Jurassic rifting, further disrupted large areas of West Antarctica. During the same time, the Transantarctic Mountains were further uplifted.
Archean Foreland Basin tectonics in the Witwatersrand, South Africa
Authors: Burke Kevin , Kidd W. S. F. , Kusky T. M.
Published: February 2008

The Witwatersrand Basin of South Africa is the best‐known of Archean sedimentary basins and contains some of the largest gold reserves in the world. Sediments in the basin include a lower flysch‐type sequence and an upper molassic facies, both of which contain abundant silicic volcanic detritus. The strata are thicker and more proximal on the northwestern side of the basin which is, at least locally, bound by thrust faults. These features indicate that the Witwatersrand strata may have been deposited in a foreland basin and a regional geologic synthesis suggests that this basin developed initially on the cratonward side of an Andean‐type arc. Remarkably similar Phanerozoic basins may be found in the southern Andes. We suggest that the continental collision between the Kaapvaal and Zimbabwe Cratons at about 2.7 Ga caused further subsidence and deposition in the Witwatersrand Basin. Regional uplift during this later phase of development placed the basin on the cratonward edge of a collision‐related plateau, now represented by the Limpopo Province. Similarities are seen between this phase of Witwatersrand Basin evolution and that of active basins north of the Tibetan Plateau (e.g., the Tarim and Tsaidam Basins). The geologic evidence does not appear so compatible with earlier suggestions that the Witwatersrand strata were deposited in a rift or half‐graben.
Processes of allochthonous terrane evolution, Mindoro Island, Philippines
Authors: Sarewitz Daniel R. , Karig Daniel E.
Published: February 2008

On Mindoro, two tectonostratigraphic terranes, the North Palawan block and the Mindoro block, are separated by a steeply dipping shear zone. The North Palawan block was rifted off southern China in the Oligocene, and comprises most of the shallow southern end of the South China Sea. The eastern part of this terrane, exposed on Mindoro, includes Jurassic and Paleogene strata, probable rift basalts of Oligocene age, and Oligo‐Miocene graben fill deposits. In the late Oligocene and early Miocene, the eastern North Palawan block underwent crustal stretching, with grabens oriented at a high angle to synchronous spreading anomalies in the South China Basin. We hypothesize that grabens formed in an overall transtensional environment near a transform boundary at the eastern margin of the opening South China Basin. The Mindoro block consists of a pre‐Upper Cretaceous low‐grade metamorphic basement, overlain by Upper Cretaceous and upper Eocene strata. The Mindoro block records several episodes of intense deformation of Mesozoic and Tertiary age, and contrasts with the North Palawan block both in terms of stratigraphy and structural evolution. The Mindoro Suture, which separates the Mindoro and North Palawan blocks, is a complex structural boundary comprising slices of diverse lithologies separated by steeply dipping, anastomosing faults. Deformed tectonic slices in the suture zone include serpentinized ultramafic rock, amphibolite, and rocks derived from both of the bounding terranes. We interpret this suture as a zone of mostly transcurrent faulting that was active through much of mid‐Tertiary time. Paleogeographic considerations for early and middle Miocene time support the premise that the west side of the central Philippine archipelago was a left‐lateral transform boundary. The Mindoro and North Palawan blocks were initially juxtaposed along the transcurrent Mindoro Suture by late Miocene time. Subsequent deformation on Mindoro includes crustal shortening associated with the southern end of the Manila Trench, which projects onshore on southwest Mindoro. The onset of shortening here probably reflects a late Miocene change in Philippine‐Eurasian plate motion. The Mindoro Suture Zone is now locked, and it acts as a rigid buttress against which strata of the North Palawan block have been structurally stacked. The Mindoro Suture Zone is similar to other terrane boundaries in the northern Philippines. The history of terrane transport in this region was apparently dominated by strike‐parallel shuffling of terranes along major transcurrent boundaries within the Philippine arc. Transcurrent motion has taken place in extensional and compressional regimes, and it is ongoing today. This may be a dominant process by which complex island arc systems are disrupted, prior to accretion onto a continental margin.
Native terranes of the Central Klamath Mountains, California
Authors: Gray Gary G.
Published: February 2008

The Klamath Mountains of northern California and southern Oregon contain several good examples of terranes which have developed in situ. The term "native" is proposed for five of these terranes and all terranes whose development can be tied to an adjacent cratonal area. The Klamath terranes discussed herein include disrupted, stratigraphic, and metamorphic types. Three disrupted terranes, the Rattlesnake Creek, eastern Hayfork, and North Fork, contain fossiliferous blocks derived from both North American and exotic sources. The unique mixed faunal assemblage, stratigraphic ties to North America in the source terranes of the blocks, and paleomagnetic evidence indicate that the tectonic and sedimentary processes responsible for mixing these blocks occurred in proximity to North America, not distant from the terranes' present positions. Coeval blueschist metamorphism in a fourth, inboard terrane, the Stuart Fork, suggests that all four terranes developed during a Late Triassic to Early Jurassic subduction event. A fifth, stratigraphic terrane, the western Hayfork, was constructed upon the assembled disrupted and metamorphic terranes in the Middle Jurassic. Disrupted terranes with similar mixtures of North American and exotic faunas occur throughout the Cordillera from central California possibly as far north as British Columbia. Late Triassic deformation has been documented in several of these terranes, suggesting that (1) subduction operated along at least this portion of North America during the Late Triassic‐Early Jurassic, and (2) many additional Cordilleran terranes should also be considered native.