bluuebunny/crossref_metadata_2025 huggingface dataset:
Foreword: Ionospheric effects on communication and related systems (IES)
Authors: Goodman John M. , Klobuchar John A. , Soicher Haim
Published: February 2008

The special section contained in this issue of Radio Science is the second of two which have been developed from papers presented at the 1987 symposium on the Effect of the Ionosphere on Communication, Navigation, and Surveillance Systems (IES 1987). This technical conference was jointly sponsored by the Naval Research Laboratory (NRL), the Office of Naval Research (ONR), the Air Force Geophysics Laboratory (AFGL), and the Army Communications‐Electronics Command (USACECOM). The symposium was held in Springfield, Virginia on May 5–7, 1987 in cooperation with the Defense Nuclear Agency (DNA), the Naval Ocean Systems Center (NOSC), the Institute for Telecommunication Sciences (ITS), and the Voice of America (VOA). See the May‐June 1988 issue of Radio Science for a more complete foreword (Radio Sci., 23, 209, 1988).
Spatial distribution of mid‐latitude sporadic E scintillations in summer daytime
Authors: Ogawa Tadahiko , Suzuki Akira , Kunitake Manabu
Published: February 2008

In the summer of 1986, 150‐MHz transmissions from six U.S. Navy Navigation Satellite System satellites were received at Kokubunji, Japan, to investigate daytime scintillations due to mid‐latitude sporadic E layers. From analysis of 145 satellite passes accompanied by Es scintillations, it is found that the diurnal variation of the scintillation occurrence ratio has two maxima, one before noon (1100 JST) and the other around evening (1800 JST), consistent with the diurnal variation of the occurrence probability of ƒoEs exceeding 8 MHz. Some of the Es layers are not accompanied by scintillations, suggesting that they are insusceptible to the plasma instability giving rise to the irregularities. Horizontal, nearly north‐south scale lengths of the scintillation regions range from 20 to 600 km with a predominant peak around 50 km and have a distribution approximated by a form of L−0.8 (L, scale length) for L ≥ 40 km. These scale lengths may represent the extents of wind shear regions producing sporadic E layers. An important finding is that of the north‐south asymmetry of scintillation occurrences. A tilted irregularity layer model is hypothesized to account for this asymmetry.
ELF scattering in the Earth‐ionosphere waveguide
Authors: Pappert Richard A.
Published: February 2008

Slab model implementation of two‐dimensional ELF scattering by a cylindrically symmetric but radially varying ionospheric disturbance is described. The method allows for estimating disturbance effects on the laterally dependent part of the transverse magnetic field components. The ground‐based transmitter and receiver may be located beneath the disturbance or outside its projection on the ground. Although partial sum convergence difficulties restrict its generality, the model can be used for a variety of case studies associated with both natural and man‐made ionospheric disturbances. As an example, calculations are presented for propagation in the presence of disturbed polar cap boundary models.
A two‐component rain model for the prediction of site diversity performance
Authors: Crane Robert K. , Shieh Horng‐Chung
Published: February 2008

A two‐component rain attenuation model is presented for the prediction of single‐ and joint‐(diversity) path rain attenuation statistics and for the prediction of the relative measures of diversity performance, diversity gain, and diversity improvement factor (or advantage). The model is based entirely on the use of geophysical parameters for the prediction of the effects of rain. It contains no free parameters to adjust to fit the available attenuation observations. The model is general and will handle both balanced and unbalanced diversity system design for terrestrial and satellite communication systems. Model performance was evaluated by comparison with published single‐path and site diversity observations. The model performs well when tested against the International Radio Consultative Committee (CCIR) site diversity data base, providing predictions of the single‐path attenuation values exceeded for specified percentages of a year with a root‐mean‐square difference (RMSD) from the observed values of 49% and providing predictions of the minimum attenuation value simultaneously exceeded on two links for specified percentages of a year with a RMSD of 59%.
Experimental observation of horizontal refractivity gradients during periods of multipath propagation
Authors: Blanchetiere‐Ciarletti V. , Lavergnat J. , Sylvain M. , Weill A.
Published: February 2008

The Propagation en Air Clair et Météorologie 3 (PACEM 3) experiment was intended to provide a better understanding of the physical situations and mechanisms that cause multipath propagation on line of sight links. In this experiment we investigated both the horizontal and the vertical gradients of the refractive index. In this paper, after a description of the experiment, we present some results from the aircraft data analysis. During two nights when radio data demonstrated the existence of multipath propagation, the meteorological situations were different, but both involved important horizontal refractivity gradients. The first night showed the combined effect of the wind circulation and relief. We locally found a 15‐unit increase in the N value over 10 km. During the second night we observed the arrival and the passage of a moist air mass inducing at the transition a difference of 15–20 N units over about 5 km. A ray‐tracing computation taking into account horizontal gradients of refractivity gives evidence of the influence of these horizontal variations upon the area of interfering rays.
Wave propagation in a switched magnetoplasma medium: Transverse propagation
Authors: Goteti Venkata R. , Kalluri Dikshitulu K.
Published: February 2008

The interaction of electromagnetic radiation with a time‐switched magnetoplasma is considered. It is shown that for the case of the incident electric field normal to the static magnetic field, the original wave splits into four new extraordinary waves whose frequencies are different from the incident wave frequency. Two of these new waves are transmitted waves, and two are reflected waves. The characteristics of these waves (frequencies, power carried by them, and their damping rates) are discussed.
Effect of lossy ground on performance of planar and cylindrical arrays
Authors: Stalzer Henry J. , Fathy Ali , Hessel Alexander , Shmoys Jerry
Published: February 2008

A method for the analysis of infinite planar circular cylindrical phased arrays of waveguide fed apertures embedded in a lossy surface is presented. Use is made of the surface impedance concept and an appropriate equivalence principle to reduce considerably the number of simultaneous equations arising in the Galerkin procedure. Numerical results for the matched element gain pattern are presented for circular feed waveguides and compared with the lossless case.
Electromagnetic field of a vertical dipole over an imperfectly conducting half‐space
Authors: King Ronold W. P.
Published: February 2008

The electromagnetic field generated by a vertical electric dipole on or near the surface of the Earth is studied for continuous‐wave excitation. The analytical foundation established by Sommerfeld and developed by many others is reviewed, and simple explicit new formulas are derived for the three components of the electromagnetic field and represented graphically. Their range of validity is defined. They show that the surface waves constitute the entire far field along the air‐earth boundary.
Transient electromagnetic field of a vertical magnetic dipole above a plane conducting Earth
Authors: Kooij Bert Jan
Published: February 2008

The pulsed electromagnetic radiation from a vertical magnetic dipole above a conducting Earth is investigated theoretically. The modified Cagniard method is used to derive closed form expressions for the magnetic Hertzian vector anywhere above the conducting Earth. Numerical results are presented for the magnetic Hertzian vector for different points of excitation and observation above the Earth, as well as for different values of the Earth's material parameters, namely, permittivity and electrical conductivity.
Scattering and depolarization of waves incident upon a slab of random medium with refractive index different from that of the surrounding media
Authors: Ma Qinglin , Ishimaru Akira , Kuga Yasuo
Published: February 2008

The problem of how the difference of the refractive indices between the background medium and surrounding media affects the propagation of an electromagnetic wave is investigated in this paper. We present the general formulation for an arbitrarily polarized plane wave obliquely incident on a slab of random medium and compare the numerical multiple scattering solution of the integral‐differential vector radiative transfer equation for latex particles with that of the analytical first‐order scattering solution. The comparison shows good agreement between the two methods within the applicable range of the first‐order approximation. From the cases studied we see that when the optical distance τ > 10, the effects of the multiple‐boundary reflection on wave propagation can be neglected for the calculation of the transmitted intensity but usually must be taken into account for the calculation of the reflected intensity and the degree of polarization.
Rain attenuation measurements: Variability and data quality assessment
Authors: Crane Robert K.
Published: February 2008

Year‐to‐year variations in the cumulative distributions of rain rate or rain attenuation are evident in any of the published measurements for a single propagation path which span a period of several years of observation. These variations must be described by models for the prediction of rain attenuation statistics. Now that a large measurement data base has been assembled by the International Radio Consultative Committee (CCIR), the information needed to assess variability is available. On the basis of 317 sample cumulative distribution functions (cdf's) for the occurrence of attenuation by rain, the expected year‐to‐year variation in attenuation at a fixed probability level in the 0.001–1.0% of a year range is estimated to be 26%. The expected deviation from the predictions of any of the better attenuation models for a single year of observations is estimated to exceed 34% when a rain climate model is employed to estimate the rain rate cdf. The probability distribution for the variation in attenuation or rain rate at a fixed probability level is lognormal. The lognormal behavior of the deviations was used to compile statistics for variability, to establish hypothesis tests for identifying outliers—the observed sample cdf that deviates significantly from the expected (modeled) cdf, and to predict the attenuation level to be exceeded with a specified return period.
Reduction of nonstationarity and parameter analysis of VHF radar returns from the atmosphere
Authors: Kuo Fu‐Shong , Lue Hsiou‐Yung , Liu Shu‐Ing
Published: February 2008

An optimization process to reduce nonstationarity problems yet preserve the reliability of the radar echo signal distribution is developed combining the conventional χ2 test and numerical simulation techniques. As an application to the real data, we determine the optimized data length to be 128 elements per set for SOUSY data and 256 elements per set for Chung‐Li data. For each of such data sets, the echo power P and the modified Rice parameter γ are calculated and their statistics are examined. When their time averages are plotted as functions of height, we frequently observe the existence of maximum P along with minimum γ in Chung‐Li data. Such a coupling can be explained as an evidence of the existence of turbulence layer. The cause of the occurrence of turbulence layers in the troposphere over Chung‐Li area is briefly discussed.
Aspect sensitivity at tropospheric heights measured with vertically pointed beam of the Chung‐Li VHF radar
Authors: Chu Y. H. , Chao J. K. , Liu C. H. , Röttger J.
Published: February 2008

Aspect sensitivity at tropospheric heights measured with the vertically pointed broad antenna beam (half power beam width is 7.4°) of the new Chung‐Li VHF radar will be presented and discussed. The angular dependence of the echo power is shown to be Gaussian, and the echo power lapse rate at heights between 2 and 8 km is about 0.5 dB/deg to 1.05 dB/deg. This angular dependence levels off at approximately 4° to 10 °, depending on the heights. The observed aspect sensitivity is analyzed and related to background atmospheric parameters.
Triangle size effect in spaced antenna wind measurements
Authors: Meek C. E.
Published: February 2008

MF radar ionospheric drift measurements are usually performed by correlating signals from spaced antennas. It has long been noted that the speeds obtained tend to increase to a limiting value when the antenna spacing is increased, and noise was indicated as a likely factor. This paper takes the view that noise is the only significant factor, and, with the help of some simplifying assumptions, shows that elimination of the noise term also eliminates the speed discrepancy on experimental data from two receiving arrays, each with two available spacings.
MF radar interferometer measurements of meteor trail motions
Authors: Meek C. E. , Manson A. H.
Published: February 2008

Angle of arrival and Doppler velocity of meteor trails are measured at Saskatoon with a medium frequency radar (2.219 MHz) and compared with spaced antenna (correlation analysis) winds.
Wiener‐Hopf analysis of the asymmetric slotline
Authors: Janaswamy Ramakrishna
Published: February 2008

The asymmetric slotline is relatively new, and currently no theory exists in the literature for obtaining its guided‐wave parameters. In the present investigation we analyze the propagation of the fundamental mode on an asymmetric slotline. The problem is formulated in the spectral domain and solved rigorously using the Wiener‐Hopf technique. Numerical results are presented for the propagation constant and the characteristic impedance of the slotline.
Near‐nadir radar backscatter from a well‐developed sea
Authors: Glazman Roman E.
Published: February 2008

Recent advances in understanding dynamics and statistical geometry of wind‐generated gravity waves necessitate a re‐examination of the radar return as a function of external factors. The Kirchhoff approximation for the case of well‐developed seas is analyzed. In this peculiar case, the equilibrium range in the wave number spectrum (approximated by αk−(4−2μ) where μ > 0 can be interpreted as a fractal codimension of the surface) corresponds to a cascade pattern in the surface geometry. Its high wave number cutoff (the internal scale h), determined by the dissipation of energy due to wave breaking, is shown to be a major factor of the radar backscatter. This intrinsic scale is evaluated (h ∼ 0.4 m), and both the geometrical and the physical optics terms are related to major parameters of wind‐wave dynamics. The range of validity of the Kirchhoff approximation and the relative importance of the diffraction correction are analyzed. Finally, the radar cross section σ0 of well‐developed seas is compared with that of poorly developed seas (when μ = 0). The great qualitative difference shown in the wind speed dependence of σ0 for these two regimes is pointed out as a source of a considerable error trend recently discovered in satellite altimeter wind measurements.
Ionospheric modification ‐ An outdoor laboratory for plasma and atmospheric science
Authors: Wong A. Y. , Brandt R. G.
Published: February 2008

With the ability to induce large‐scale changes in the ionosphere by high‐power ground‐based HF transmitters, laboratory‐like experiments can be performed repetitively in space plasmas. Using satellites and rockets, as well as ground‐based magnetometers, optical and radar diagnostics, this large‐scale plasma has been investigated. The absence of walls and the long confinement time make this plasma distinctively different from conventional laboratory plasmas. Particle and energy transports can be studied in this large‐scale well‐confined plasma. Significant changes in density profiles and velocity distributions can be brought about by modest incident EM wave intensity because the wave fields are resonantly enhanced and the perturbations are cumulative in the same plasma over a long time. Free energy sources can further enhance the excitation of waves. Experimental methods are reviewed together with major experiments performed at the Platteville, Arecibo, Max Planck, and HIPAS observatories. The ionospheric plasma with a rich source of free energy is highly variable especially in the auroral zone, and fast temporal resolution techniques used in laboratory experiments have been introduced to these outdoor experiments to observe caviton collapse and other fast processes. High‐frequency and low‐frequency wave phenomena are observed to be excitable simultaneously and coupled by non‐linear processes. Laboratory experiments, computer simulation, and analytic theories are used to interpret and to guide field experiments. Future experiments include the studies of the formation of potential double layers and ion acoustic shocks from density cavities and their coupling to free energy sources in the ionosphere. Active experiments are desirable in the atmosphere whose response to a known perturbation such as the controlled heating of electrons and neutrals and the formation of negative ions can be quantified. With higher focusing of the available power, there is the exciting possibility of generating a steady state ionospheric plasma and an opening to a new era in controlled atmospheric experiments and long‐range communications.
Excitation of ELF waves in the Schumann resonance range by modulated HF heating of the polar electrojet
Authors: McCarrick M. J. , Sentman D. D. , Wong A. Y. , Wuerker R. F. , Chouinard B.
Published: February 2008

We report the successful generation and detection of ELF waves in the frequency range of 6–76 Hz by polar electrojet modification using the High‐Power Auroral Stimulation (HIPAS) HF Heater Facility near Fairbanks, Alaska. Magnetic field amplitudes of ∼1 pT and vertical electric field amplitudes of ∼0.2 mV have been observed at a receiving site 35 km from the HIPAS site. The wave amplitude does not depend strongly on the ELF frequency, however, the amplitude is closely related to the level of electrojet activity inferred from magnetometer chain data and from ionosonde measurements. The 1‐MW HF heater is modulated at low ELF frequencies using an “array dephasing” technique where the eight‐element antenna array is alternately phased for peak vertical gain and dephased for a spread pattern at the ELF rate. This method can be employed at any modulation frequency without concern for transmitter power supply resonances. Measurements of the wave amplitude are made by conventional analog lock‐in techniques and by continuously digitizing the waveform output of the wideband magnetic search coils and the electric field detector. With the digitized waveform, digital postprocessing can be used to extract the low‐level coherent signal from the Schumann resonance noise background using a software lock‐in routine.
Computer simulation of ELF injection in the Earth‐ionosphere waveguide
Authors: Carroll K. J. , Ferraro A. J.
Published: February 2008

A computer model to simulate ELF injection from an ELF ionospheric source in the 60–90 km region of the ionosphere is described. Results are presented for the geomagnetic location of the high‐power auroral simulation (HIPAS) facility. These results include the efficiency of the ELF source with altitude and a contour plot of the ELF magnetic field intensity on the ground below the source.
Evolution of pull‐apart basins and their scale independence
Authors: Aydin Atilla , Nur Amos
Published: February 2008

Pull‐apart basins or rhomb grabens and horsts along major strike‐slip fault systems in the world are generally associated with horizontal slip along faults. A simple model suggests that the width of the rhombs is controlled by the initial fault geometry, whereas the length increases with increasing fault displacement. We have tested this model by analyzing the shapes of 70 well‐defined rhomb‐like pull‐apart basins and pressure ridges, ranging from tens of meters to tens of kilometers in length, associated with several major strike‐slip faults in the western United States, Israel, Turkey, Iran, Guatemala, Venezuela, and New Zealand. In conflict with the model, we find that the length to width ratio of these basins is a constant value of approximately 3; these basins become wider as they grow longer with increasing fault offset. Two possible mechanisms responsible for the increase in width are suggested: (1) coalescence of neighboring rhomb grabens as each graben increases its length and (2) formation of fault strands parallel to the existing ones when large displacements need to be accommodated. The processes of formation and growth of new fault strands promote interaction among the new faults and between the new and preexisting faults on a larger scale. Increased displacement causes the width of the fault zone to increase resulting in wider pull‐apart basins.
Origin and age of the Border Ranges Fault of southern Alaska and its bearing on the Late Mesozoic tectonic evolution of Alaska
Authors: Pavlis Terry L.
Published: February 2008

Local geologic relationships in the western Chugach Mountains, together with regional considerations, suggest that in the Early Cretaceous a subduction zone was formed along the southern edge of the Wrangellia‐Peninsular‐Alexander composite terrane (Talkeetna superterrane of Csejtey et al., 1982) of southern Alaska. The effects of this event include (1) a shattering of older (pre‐Cretaceous) crystalline rocks along a complex fault system, (2) emplacement of a tectonic melange beneath the shattered crystalline rocks, and (3) a subgreenschist facies prograde metamorphism of the melange and retrograde metamorphism of the older crystalline rocks. This event created a regionally mappable structural contact between broken crystalline rocks and the melange; the Border Ranges fault of MacKevett and Plafker (1974). Regional stratigraphy as well as radiometric ages of rocks predating and postdating the Border Ranges fault appear to bracket the age of the Border Ranges fault and its associated deformational effects to the interval between about 135 and 120 Ma. Further regional tectonostratigraphic associations suggest that the deformation along the Border Ranges fault represents the nucleation of a north and (or) east dipping subduction zone beneath the Talkeetna superterrane and that the magmatic arc associated with this juvenile subduction zone is the Gravina‐Nutzotin belt of southeast Alaska. Mismatches in the distribution of different elements of this Early Cretaceous arc‐trench system are probably a result of Late Cretaceous or early Tertiary strike slip motion, but Early Cretaceous ridge‐trench interaction (suggested by the occurrence of Early Cretaceous near‐trench plutons) may have played a role as well. The Talkeetna superterrane is generally thought to be an exotic block that collided with the Cordillera in the middle Cretaceous (Coney et al., 1980). The age data commonly cited as evidence for a middle Cretaceous age for the collision are, however, misleading. A model is proposed here in which the initiation of convergence along the trailing edge of the Talkeetna superterrane records the time of initial impingement of two irregular margins whereas intense middle Cretaceous deformation recognized along the leading edge of the Talkeetna superterrane (Csejtey et al., 1982) records destruction of a syncollisional flysch basin during the final phase of the collision.
Cocorp seismic reflection profiling in the Ouachita Mountains of western Arkansas: Geometry and geologic interpretation
Authors: Nelson K. Douglas , Lillie Robert J. , de Voogd Beatrice , Brewer Jonathan A. , Oliver Jack E. , Kaufman Sidney , Brown Larry , Viele George W.
Published: February 2008

COCORP seismic reflection profiling in the Ouachita Mountains of western Arkansas indicates that (1) Carboniferous foreland basin deposits within the Arkoma Basin thicken dramatically toward the south, reaching an aggregate thickness in excess of 12 km (4.5 s two‐way travel time) beneath the southern edge of the Frontal Thrust Zone, (2) evidence for northward directed low‐angle thrusting within this clastic sequence is prominent, with a probable decollement surface lying at or near the contact with underlying Early Paleozoic shelf carbonates, (3) beneath the Benton Uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km (2.8 s); this structure appears to mimic the anticlinorial shape of the Benton Uplift indicated by the surface outcrop pattern, (4) beneath the Southern Ouachitas, south dipping stratified events are observed to depths in excess of 14 km (5.0 s), (5) deeper in the section a prominent, gently north dipping reflection occurs at approximately 22 km depth (7.6 s) beneath the northern Coastal Plain/Southern Ouachitas.

Extrapolation of data along strike in the Ouachita belt, and consideration of large‐scale structure observed in other collisional orogenic belts, suggest that the reflection events observed beneath the Benton Uplift represent Early Paleozoic shelf carbonates correlative with those that floor the Arkoma Basin to the north. This interpretation requires that the Early Paleozoic deep‐water sediments exposed in the core of the Ouachitas be allochthonous, and also implies significant basement uplift beneath the core zone. To the south, the prominent package of layered reflections occurring beneath the Southern Ouachitas and northern Coastal Plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and/or metasedimentary strata. At relatively ‘shallow’ levels these strata are correlative with the Carboniferous flysch cropping out in the Southern Ouachitas, and at somewhat deeper levels, they are correlative with the Early to Middle Paleozoic deep‐water sediments exposed on the Benton Uplift.
Paleogene evolution of the Kodiak Islands, Alaska: Consequences of ridge‐trench interaction in a more southerly latitude
Authors: Moore J. Casey , Byrne Tim , Plumley Peter W. , Reid Mary , Gibbons Helen , Coe Robert S.
Published: February 2008

The Kodiak Islands of Alaska are principally composed of a Late Cretaceous and Paleogene complex of accreted deep‐sea rocks. An early Paleocene magmatic event invaded the accretionary complex with mid‐ocean ridge basalt (MORB), andesite, and granodiorite. Geochemical evidence suggests both the andesite and granodiorite are mixtures of MORB and sediment. The volcanic rocks are interlayered with coarse clastic sediments and deformed with the latter; the granodioritic intrusions, with K‐Ar ages as old as 62 Ma postdate emplacement of the early Paleocene sedimentary and volcanic rocks. Interaction of the subduction complex with the Kula‐Farallon ridge most simply explains this early Paleocene magmatic episode. Paleomagnetic studies of the volcanic rocks indicate this interaction occurred at 40.3±6° north latitude, which is 25.3±9° south of the expected position of the Kodiak Islands relative to North America. The magmatic arc presently northwest of the Kodiak accretionary complex was active during the Late Cretaceous and early Paleocene, diminished after the ridge‐trench interaction, and resurged from late Eocene into Miocene time. The magmatic resurgence is coeval with the obductive offscraping of Eocene to Oligocene(?) submarine fan deposits in the Kodiak Islands. The petrology of these fan deposits indicates they are the most likely proximal equivalent for the Zodiac fan, thereby limiting its relative motion with respect to the Kodiak Islands. Possible models accounting for the northward motion of the Kodiak accretionary complex include (1) coupling the accretionary complex and possibly related arc to the northward‐moving Kula Plate and closing an ocean basin north of the arc in Paleogene time, (2) emplacing the Kodiak accretionary complex at the northward limit of the paleomagnetic data, with subsequent northward motion accomplished by closure of a small back arc basin and/or intracontinental shortening, and (3) transcurrent faulting and northward motion of the Kodiak accretionary complex without the presently adjacent arc.
Normal faulting, block tilting, and décollement in a stretched crust
Authors: Brun Jean‐Pierre , Choukroune Pierre
Published: February 2008

Extensional tectonics cover a wide range of crustal structures from narrow continental rifts to large continental areas. If we except continental and oceanic rifts, stretched portions of the crust exhibit complex faulting patterns at upper levels with high‐angle and low‐angle normal faults. Very little is known concerning the geometry of structures and physical processes at depth. It is therefore difficult to extrapolate surface observations to the crustal scale. Five general models are proposed that combine continuous and discontinuous deformation and geometrical and geological tests are proposed. Two natural examples, the West Armorican Atlantic margin and Eldorado Mountains (Basin and Range Province), are presented. They are characterized by an association of high‐angle normal faults, tilted blocks, and low‐angle normal faults. Field and geometrical arguments are given to demonstrate that low‐angle faults are controlled by décollement surfaces along preexisting interfaces. Geometrical implications of the tilted block pattern are presented and the gravity gliding hypothesis is proposed as a possible explanation of block tilting when associated with low angle normal faults. In such cases, the amount of stretching cannot be related to bulk thinning of the crust.
Anisotropy of magnetic susceptibility and magmatic structures in the Guérande Granite Massif (France)
Authors: Guillet Philippe , Bouchez Jean‐Luc , Wagner Jean‐Jacques
Published: February 2008

A multidisciplinary approach has been used in order to map out the syntectonic magmatic structures of the Guérande leucogranite in southern Brittany (Western France): on one hand, extensive field work and microscopic observations, and on the other, a detailed magnetofabric investigation, have been carried out. The former showed a magmatic flow plane trending generally to the ENE‐WSW with a moderate dip to the north and a flow direction slightly dipping toward NNE. These observations suggest a blade shaped body which is not deeply rooted. The magnetofabric as defined by the anisotropy of magnetic susceptibility is mainly related to hematite‐ilmenite grains with a mean rock susceptibility of about 3.10−6 G/Oe. There is an excellent agreement between the magnetic lineation (Kmax) and the magmatic flow direction; the agreement between the magnetic foliation (Kmax‐Kint plane) and the magmatic flow plane is not as good. The quantitative analyses of the anisotropy allows one to group the data into two domains: the one with planolinear ellipsoids corresponding to the central part of the granitic body where flow was mainly achieved on the viscous state and the other in the flattening domain related to the northern border area which exhibits incipient orthogneissification, that is, some deformation on the solid state. Such a favorable comparison with the classical structural method is encouraging for using the anisotropy of magnetic susceptibility technique more routinely for structural mapping of granitic bodies, provided that the microstructural state of the rock is already well known.
A magnetic deep structural geology interpretation of Ireland
Authors: Max M. D. , Ryan P. D. , Inamdar D. D.
Published: February 2008

A new 10 nT compilation of magnetic data for Ireland and the immediately adjacent sea area shows distinct geophysical provinces of characteristic magnetic signature separated by major magnetic linears which can be related to important geological lines. In the far northwest there is a structurally coherent Grampian tectonic‐thermal zone bounded to the southeast by the Fair Head‐Clew Bay Line. Between the Fair Head‐Clew Bay Line and the western extension of the Southern Uplands Line, which forms the northern margin to the Southern Uplands of Scotland, is a segmented zone of relatively small magnetic anomalies and geological bodies of mixed crustal affinity: Grampian massifs, Lower Palaeozoic sedimentary basins bounded by major shear faults, relics of Lower Palaeozoic oceanic and related crust, and possible older microcontinent fragments. These separate areas have quite individual structural and metamorphic histories. There is good evidence for the Grampian Massifs having undergone significant lateral movement during Lower Palaeozoic times, and it can be inferred that many of the now adjacent bodies are only docked. The original position of each block is now not known. In the far southeast of Ireland are relics of a Cadomian Craton which are now tectonically imbricated with Lower Palaeozoic rocks. This is probably the segmented margin of the stable Cadomian craton which is inferred beneath Wales and south Britain. Between the Southern Uplands Line and the northernmost extent of the relics of the southern craton is a zone of deep‐seated, low‐amplitude magnetic anomalies and major linears. These are about 30° oblique to the southern margin of this vestiges of the Iapetus ocean zone, but are nearly parallel with the northern margin.
Stress estimate for the highest mountain system in Japan
Authors: Fukao Yoshio , Yamaoka Koshun
Published: February 2008

The Hida range, the largest mountain system in Japan, is a contemporary uplifting feature (uplifting rate∼1–5 mm/yr). No large earthquakes nor active faults have been located in this mountain range but small earthquakes occur rather frequently, often as swarms, in its axial portion. Their focal mechamisms are found to be of strike slip type with the P axes in the NW‐SE to WNW‐ESE direction. This direction is perpendicular to the trend of the mountain chain and coincides with the orientation of the tectonic compression now causing intraplate earthquakes in most regions of the Japanese islands. It also coincides with the convergence direction of the Pacific plate and the Eurasian plate. The Bouguer anomaly profile across the Hida range suggests an upper crustal root beneath it. A buoyancy load associated with this upper crustal root is calculated from the Bouguer anomaly profile, which is found to be not large enough to sustain the surface topographic load isostatically. If any tectonic forces were removed from the mountain system, mountains would subside through inelastic deformation. A formula is presented to calculate, using the known topographic load and buoyancy load, the stress differences arising at the virtual instant of removal of tectonic forces. The Newtonian fluids of uniform viscosity are assumed. The calculated stress difference is horizontally tensile, about 290 bars, at the axial portion of the mountain range. This horizontal tension must be overcome tectonically to ensure the mountain uplifting and the earthquake occurrence. We suggest that the Hida range is now uplifting by horizontal compression of plate tectonic origin with a compressive stress at least of the order of 300 bars. During uplifting, the mountain system seems to maintain its upper crustal configuration as the one in a fairly close state of minimum stress difference.
Wrench faulting in northern California and its tectonic implications
Authors: Kelsey Harvey M. , Cashman Susan M.
Published: February 2008

The Plio‐Pleistocene Prairie Creek group of sediments lies unconformably on Mesozoic Franciscan Assemblage rocks in north coastal California, 120 km north of the Mendocino triple junction. The sediments are pervasively fractured and cut by three NNW trending faults, collectively called the Grogan‐Lost Man fault zone. These sediments contain a record of Quaternary deformation in an area of the California coast otherwise devoid of young sedimentary cover rocks. Detailed field mapping and structural analysis reveal a pattern of conjugate fracturing, folding, and outcrop‐scale faulting that strongly suggests the sediments have been deformed by wrench faulting. The spatial relationship of structures is typical both of other known regions with well‐documented wrench faulting and of structural patterns produced by simple shear modeling of wrench faults. The three major faults diverge slightly from expected fault trends based on the wrench fault model because the faults follow the structural grain of underlying Franciscan basement rocks. Because of the orientation of these inherited faults with respect to the Quarternary strain field, wrench faulting is accompanied by convergence. Field exposures, which show that two of the faults have high‐angle reverse separation, confirm that offset includes vertical as well as horizontal components. Net Quaternary right‐lateral displacement cannot be determined because of poor exposure and erosional stripping of the cover sediments, but it is probably less than 5 km. The Grogan‐Lost Man wrench faults are interpreted as the northernmost part of the San Andreas transform system. We propose that northward movement of the San Andreas transform boundary will entail a shift of major San Andreas type fault activity to this northernmost right‐lateral fault system. This shift will coincide with the northward migration of the Mendocino triple junction to a position close to where the Grogan‐Lost Man wrench faults intersect the continental margin.
Ages of Alpine tectonometamorphic events in the northwestern Himalaya (northern Pakistan) by 39Ar/40Ar method
Authors: Maluski Henri , Matte Philippe
Published: February 2008

The main tectonometamorphic alpine events which occurred in the western part of the western Himalayan virgation at south of the main suture, in northern Pakistan, are dated by the 39Ar/40Ar method. These events are as follows: (1) the blueschist metamorphism (80 ± 5 Ma) present in tectonic/ophiolititic melanges along the suture (this event occurred during an early subduction or obduction stage) (2) the alpine barrovian metamorphism (50–30 Ma) developed in the sedimentary cover and orthogneisses of the Indian Plate during the postcollisional northward underthrusting of this plate below Asia, and (3) the emplacement of postmetamorphic leucogranites (23 Ma) resulting from the melting of the Indian crust during the climax of the alpine metamorphism.
The neotectonics of the Tibetan Plateau
Authors: Rothery David A. , Drury Stephen A.
Published: February 2008

Part of the continuing northward convergence between India and Eurasia is probably taken up by strike slip displacement, with Tibet as a whole moving eastward away from the collision zone and undergoing east–west extension (Molnar and Tapponnier, 1975; Ni and York, 1978). On the basis of our interpretation of Landsat images we propose that strike slip faulting occurs throughout the Tibetan plateau such that its north–south cross section is being shortened, with little contemporary folding or thrusting.
Regional joint sets in the Arabian Platform as indicators of intraplate processes
Authors: Hancock P. L. , Al Kadhi A. , Sha'at N. A.
Published: February 2008

The eastern part of the Saudi Arabian craton is a platform underlain by gently tilted and horizontal Mesozoic‐Cenozoic sedimentary rocks everywhere more than 250 km distant from the Zagros deformation front. Despite its tectonic simplicity, the platform contains several laterally extensive, but subdued, structures and lineaments belonging to more than one system of fault zones and flexures. New microtectonic observations of dominant sets of extension and conjugate hybrid joints permit regionally significant extension directions to be inferred and the origin of the macrostructures to be assessed. The central Arabian arch, the E‐W crest of which coincides with an axis of Mesozoic subsidence, developed during the Late Cretaceous to Eocene when the basin inverted as a consequence of the emplacement of ophiolite‐bearing nappes on the Zagros and Oman margins. During arching, there was strike‐parallel elongation of beds. At the same time as the arch amplified, the central Arabian graben system evolved as a result of the northward displacement of an East Arabian block or sheet that became detached across an arcuate separation zone, which in the south is coincident with the crest of the arch. An important Neogene response in the Arabian platform to NE‐SW shortening in the Zagros ranges was the formation of a swarm of NE striking master joints and lineaments. These structures reflect the diffuse and peripheral expansion of the foreland beyond the Zagros deformation front.
Paleomagnetic results from the Central Sierra Nevada: Constraints on reconstructions of the western United States
Authors: Frei Leah S. , Magill James R. , Cox Allan
Published: February 2008

Paleomagnetic results from three plutons of the central Sierra Nevada limit the motion of the Sierra Nevada relative to the North American craton to 5° ± 8° of poleward motion and 7° ± 11° of clockwise rotation since about 90 Ma. The indicated rotation and translation are not significant at the 95% confidence level. A new tectonic model is presented that is consistent with all available paleomagnetic data from the Sierra Nevada, Great Valley, Klamaths, Oregon Coast Range, Cascades, and western Nevada. The paleogeographic reconstructions of this model associate the displacement of major crustal blocks with Basin and Range extension starting at about 30 Ma. This model accounts for about 15° of clockwise rotation of the Klamaths and southern Cascades, about 2° of clockwise rotation of the Sierra Nevada and Great Valley, and 26° of clockwise rotation of the Oregon Coast Range since 30 Ma. The model is consistent with 40° of clockwise rotation of the Oregon Coast Range before 30 Ma, as proposed by Magill and Cox (1980). The amount of Basin and Range extension in this model at latitude 41°N is 225 km directed E‐W and at latitude 35°N is 245 km directed WSW‐ENE.
Paleomagnetic assessment of oroflexural deformation in west‐central Nevada and significance for emplacement of allochthonous assemblages
Authors: Geissman John W. , Callian James T. , Oldow John S. , Humphries Stephan E.
Published: February 2008

Abrupt westerly deflections of north‐south trending facies boundaries, the isopleth of (87sr/86Sr)° = 0.076, and arcuate structural trends of Paleozoic, Mesozoic, and Cenozoic age in the west‐central Basin and Range Province, have been explained by (1) post‐Early Jurassic oroflexural folding and shear, (2) the existence of a pre‐Late Jurassic resistant block along the western continental margin, possibly representing late Precambrian breakup, and (3) regional flexure (“Mina deflection”) prior to intrusion of Late Cretaceous plutons which crosscut flexure‐related structural trends. Paleomagnetic data gathered from Late Cretaceous plutons and remagnetized metasedimentary rocks along and north of the northern margin of the zone of east‐west structural trends are not in support of significant oroflexural folding (and attending major clockwise rotation) since Late Cretaceous time. The data (Water Canyon pluton: D = 1°, I = 57°, α95 = 4°, k = 9, N= 132; Gunmetal pluton: D = 354, I = 60, α95 = 6, k = 21, N = 29; East Garfield Hills pluton: D = 349, I = 74, α95 = 11, k = 14, N = 12; La Panta pluton: D = 22, I = 66, α95 = 10, k = 16, N = 13; Gillis Canyon pluton: D = 354, I = 50, α95 = 4, k = 20, N = 50; and folded units of the Excelsior Mountains: D = 309, I = 79, α95 = 7, k = 12, N = 37) are dispersed from expected Cretaceous directions for the Mina deflection region, but they do not conform to a model of major systematic regional clockwise rotation. The generally small deflections of unit means from expected directions could be explained by irregular components of Late Cenozoic tilting and/or rotation. The paleomagnetic data do not deny the possibility of pre‐Late Cretaceous oroclinal bending. Nevertheless, regional structural analysis complimented by the paleomagnetic results indicate that the Mina deflection is in all likelihood a primary feature whose geometry reflects the late Precambrian fragmentation of western North America. Structures related to the accretion of allochthonous assemblages between Middle Devonian and Cretaceous time in this part of the Cordillera also reflect the primary nature of this feature.
Structural discordance between neogene detachments and frontal sevier thrusts, central Mormon Mountains, southern Nevada
Authors: Wernicke Brian , Walker J. Douglas , Beaufait Mark S.
Published: February 2008

Detailed geologic mapping in the Mormon Mountains of southern Nevada provides significant insight into processes of extensional tectonics developed within older compressional orogens. A newly discovered, WSW‐directed low‐angle normal fault, the Mormon Peak detachment, juxtaposes the highest levels of the frontal most part of the east‐vergent, Mesozoic Sevier thrust belt with autochthonous crystalline basement. Palinspastic analysis suggests that the detachment initially dipped 20–25° to the west and cut discordantly across thrust faults. Nearly complete lateral removal of the hanging wall from the area has exposed a 5 km thick longitudinal cross‐section through the thrust belt in the footwall, while highly attenuated remnants of the hanging wall (nowhere more than a few hundred meters thick) structurally veneer the range. The present arched configuration of the detachment resulted in part from progressive “domino‐style” rotation of a few degrees while it was active, but is largely due to rotation on younger, structurally lower, basement‐penetrating normal faults that initiated at high‐angle.

The geometry and kinematics of normal faulting in the Mormon Mountains suggest that pre‐existing thrust planes are not required for the initiation of low‐angle normal faults, and even where closely overlapped by extensional tectonism, need not function as a primary control of detachment geometry. Caution must thus be exercised in interpreting low‐angle normal faults of uncertain tectonic heritage such as those seen in the COCORP west‐central Utah and BIRP's MOIST deep‐reflection profiles. Although thrust fault reactivation has reasonably been shown to be the origin of a very few low‐angle normal faults, our results indicate that it may not be as fundamental a component of orogenic architecture as it is now widely perceived to be. We conclude that while in many instances thrust fault reactivation may be both a plausible and attractive hypothesis, it may never be assumed.