bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of the hourly variability of f0F2 at a low‐latitude station
Authors: Pasricha P. K. , Aggarwal S. , Reddy B. M.
Published: February 2008

Conventional HF communication predictions are usually based on the observed monthly median values of ionospheric parameters. The recently acquired capability of certain users to alter their terminal parameters with very short notice can be exploited only if predictions, based on real‐time observations, are available with lead times of a few hours. Such very short‐term predictions of, sav, fo F2 are possible by performing autocorrelation analysis on the deviations from mean diurnal patterns of f0F2 separately for quiettime and disturbed conditions. A Markov statistical model is used to estimate persistence in the data, and hence the extent of the forcast. The forecast is shown to be better during disturbed conditions compared to the quiettime conditions. It is shown that a forecast is likely to be valid up to 3 hours under quiet conditions, and up to 4 hours and 6 hours during daytime and nighttime, respectively, under disturbed conditions. The prediction basis for these forecasts is a representative daily diurnal cycle under quiet conditions, and a diurnal pattern averaged over three diurnal cycles under disturbed conditions. Prediction methods exist (elsewhere) for the prediction of magnetic storms 3 days in advance.
New electromagnetic resonance effects associated with cavity‐backed apertures
Authors: Ziolkowski Richard W.
Published: February 2008

A number of canonical problems that describe the coupling of electromagnetic fields through apertures into enclosed regions have been solved. The scattering data generated by these cavity‐backed aperture solutions share a common property: they are dominated by resonance features closely connected to the presence of the cavity. The behavior of these cavity‐backed aperture resonance features has been characterized for variations in frequencies, angles of incidence, look angles, aperture sizes, and interior object properties. It has been found that the scattering cross sections contain information about the interior of the cavity.
Some recent developments in iterative techniques for solving electromagnetic boundary value problems
Authors: Chan C. H. , Mittra R.
Published: February 2008

The objective of this paper is to present a brief review of recent developments in a class of iterative techniques for solving electromagnetic boundary value problems. The iterative algorithm discussed herein is based on the minimization of the norm of the boundary condition error on a body whose scattering properties are of interest. It is shown that different choices of variational functions in the minimization process lead to different iteration schemes whose convergence properties can vary considerably. To illustrate this point, three different variational functions are chosen to demonstrate their applications in solving three different types of electromagnetic boundary value problems. Some observations on the use of iterative techniques for solving scattering problems with multiple incidence angles are included in the paper.
An improved prediction method for rain attenuation in satellite communications operating at 10–20 GHz
Authors: Yamada M. , Karasawa Y. , Yasunaga M. , Arbesser‐Rastburg B.
Published: February 2008

Several prediction methods for rain attenuation presented so far are evaluated using a common long‐term data base (total 124 sets of measurements) for oblique propagation paths with frequencies of from 10 to 20 GHz, and an improved prediction method reflecting the evaluation results performed is proposed. The evaluation results indicate that CCIR methods give relatively high precision, although in this respect, there is not such a great difference from other methods. The method proposed here includes a rain area size parameter as a function of rain rate for 0.01% of the time so as to minimize the prediction error. It is verified that the method thus obtained gives the best precision, at the present time, for predicting rain attenuation on Earth‐to‐space propagation paths at 10–20 GHz.
An underground UHF‐EM transillumination experiment at the Big Nickel Mine in Sudbury, Canada
Authors: LaFleche P. T. , Jensen O. G.
Published: February 2008

The Geophysics Laboratory of McGill University investigated the use of UHF‐EM transillumintion measurements in geophysics through the construction of a prototype 445‐MHz continuous‐wave transillumination instrument designed for underground use in geophysics. Test surveys with this instrument were conducted at the Big Nickel Mine in Sudbury, Ontario, to obtain through‐rock electromagnetic absorption rates. Measurements were made at several sites between underground drifts and the surface. The results of the field surveys indicate that the UHF‐EM transillumination technique is feasible and, under strictly defined field conditions, can provide useful information on the in situ electrical properties of rocks.
Cross‐correlation analysis and interpretation of spaced‐receiver measurements
Authors: Costa Emanoel , Fougere Paul F. , Basu Santimay
Published: February 2008

Two algorithms which provide a statistical treatment to the estimation of the parameters of the cross‐correlation analysis of spaced‐receiver data are reviewed. Their results are compared using signals: (1) transmitted by a quasi‐stationary polar beacon and received at Goose Bay, Labrador (53.3°N, 60.3°W); and (2) transmitted by the orbiting Hilat satellite and received at Tromsø, Norway (69.7°N, 18.9°E). A good general agreement is displayed in this comparison. The former experiment indicates the possibility of extreme daily variations in the anisotropy of the ground diffraction pattern and in the true drift velocity of the in situ irregularities. The latter experiment displays geometrical enhancements in the intensity scintillation index S4, in the rms phase fluctuation σϕ and in the axial ratio of the ellipse which characterizes the anisotropy of the ground diffraction pattern, around the region of local L shell alignment of the ray paths. Increases of these parameters also are observed northward of Tromsø. These observations are thus consistent with a morphological model for anisotropy of high‐latitude nighttime F region irregularities proposed in the literature. Next, a possible dependence of the results of the spaced‐receiver measurements on the receiver baselines is discussed. It is argued that this mechanism could be responsible for the relatively small values of the anisotropy of the diffraction pattern obtained from the Hilat measurements at Tromsø. Finally, a procedure which combines a simple propagation model of the spaced‐receiver experiment with a nonlinear minimization algorithm is proposed to estimate the anisotropy of the in situ irregularities from that of the diffraction pattern.
Sparse‐aperture microwave radiometers for Earth remote sensing
Authors: Milman Andrew S.
Published: February 2008

Passive microwave instruments for remote sensing of the Earth's surface from space can measure many meteorological parameters in the absence of daylight and in the presence of clouds. Because of the large apertures that are required to achieve good spatial resolution (1–5 km) at microwave frequencies, these radiometers should be arrays of small antenna elements that are arranged so that the collecting area of the radiometer is much smaller than that of a conventional, solid scanning antenna with the same spatial resolution. Despite the various methods that might be used for combining the signals from the different array elements and the different ways the elements might be arranged, the sensitivity depends almost exclusively on the collecting area, integration time, RF bandwidth, and the system temperature. Interferometers and phased arrays have limitations on their bandwidths that depend on the size of the array; beam‐forming arrays are not so limited. A cross antenna is considered in some detail as an example of a sparse‐aperture radiometer. It has 5‐km resolution from an 800‐km orbit and significantly better sensitivity than the equivalent conically scanning parabolic antenna, while it has only 10% of the collecting area and no moving parts.
Ionospheric imaging using computerized tomography
Authors: Austen Jeffrey R. , Franke Steven J. , Liu C. H.
Published: February 2008

Computerized tomography (CT) techniques can be used to produce a two‐dimensional image of the electron density in the ionosphere. The CT problem requires that the measured data be the line integral through the medium of the unknown parameter; transionospheric satellite beacon total electron content data recorded simultaneously at multiple ground stations fulfill this requirement. In this paper the CT problem is formulated as it applies to ionospheric imaging and limitations of the technique are investigated. Simulations are performed assuming a 1000‐km‐altitude polar‐orbiting satellite and both five and three ground stations; the results demonstrate the feasibility of this technique.
Origin of density enhancements in the winter polar cap ionosphere
Authors: Anderson D. N. , Buchau J. , Heelis R. A.
Published: February 2008

Coherent and incoherent ground‐based radar measurements of the winter polar cap ionosphere at Thule and Sondrestrom, Greenland have established the existence of “patches” of enhanced ionization which drift across the polar cap in an antisunward, noon‐midnight direction. Associated with these patches is strong radio scintillation activity which severely disrupts ground‐to‐satellite communication systems and interferes with the operation of space surveillance radar at high latitudes. Several recent studies have shown that the source of enhanced ionization is the sunlit subcusp ionosphere rather than production by precipitating energetic particles. However, the question of what causes the patchiness has not been addressed. We study this problem by solving the time‐dependent plasma continuity equation including production by solar ultraviolet radiation, loss through charge exchange, and transport by diffusion and convection E×B drifts. Time and spatially varying, horizontal E×B drift patterns are imposed, and subsequent ionospheric responses are calculated to determine how enhanced plasma densities in the dark polar cap could result from extended transit of relevant flux tubes through regions of significant solar production. This would occur south of the cusp prior to convection as patches across the polar cap. It is found that a density enhancement in Nmax from 7×104 to 5×105 el/cm3 occurs at Thule when a time‐varying convection pattern is included in the simulation. The patch of ionization is generated when an initial convection pattern characterized by an 80‐kV cross‐tail potential and a 12° polar cap radius is abruptly changed to a 100‐kV cross‐tail potential and a 15° polar cap radius. The horizontal extent of the patch is related to the length of time the new convection pattern remains “turned on.”
Observations of ionospheric disturbances following a 5‐kt chemical explosion: 1. Persistent oscillation in the lower thermosphere after shock passage
Authors: Jacobson Abram R. , Carlos Robert C. , Blanc Elizabeth
Published: February 2008

We have observed a 6‐min‐period oscillation persisting for an hour in the ionospheric E region following a 5‐kt chemical explosion on the ground 250 km south of the measurement site. The oscillation's phase velocity in the upper atmosphere is 0.15 km/s at an azimuth 60°W. Our observations are obtained using a time‐delayed correlation technique with multistation Doppler sounding of the overhead ionosphere. Some new aspects of the technique are described.
Geomagnetic modification of the mid‐latitude ionosphere: Toward a strategy for the improved forecasting of ƒ0F2
Authors: Wrenn G. L. , Rodger A. S.
Published: February 2008

The reduction of maximum usable frequency (MUF) during a geomagnetic storm represents a serious hindrance to the efficient operation of HF transmission links. This paper outlines a new approach to the modeling and forecasting of ƒ0F2 at quiet and disturbed times. Statistical analyses of ionosonde data from the Argentine Islands (65°S) have been used to define patterns for the main phase effects of mid‐latitude ionospheric storms. Extended to a number of stations, these could be incorporated into algorithms to permit the forecasting of MUF for a few hours ahead and enhance the frequency management of shortwave radio communication. Data from a complete solar cycle, 1971–1981, are used to determine the errors in the forecasts and to demonstrate that a useful advantage can be attained by this method. The root‐mean‐square error in ƒ0F2 for 90,175 samples is 15.6%, which compares favorably with those obtained using forecasts based upon quiet time values (20.4%) or the previous day's measurements (18%). Studies of error distribution with respect to local time, season, and phase of the solar cycle highlight limitations in the technique but also indicate avenues to further improvement.
Direct comparison between in situ and radar measurements of temperature fluctuation spectra: A puzzling result
Authors: Dalaudier Francis , Crochet Michel , Sidi Claude
Published: February 2008

Investigations of temperature fluctuation spectra of atmospheric turbulence were performed simultaneously by remote sensing and in situ measurements. A balloon borne instrumented gondola measured temperature and velocity fluctuations up to an altitude of 18 km. Spectral analysis of these data led to one‐dimensional spectra. Simultaneously, the nearby stratosphere‐troposphere radar “Provence” obtained vertical profiles of reflectivity for both vertical and oblique (15°) lines of sight and for various range resolutions. According to the classical interpretation, the radar echoes result from the three‐dimensional spectrum of refractive index fluctuations, and its aspect sensitivity reflects the spectrum anisotropy at the half radar wavelength. We thus separately reconstructed the reflectivity profiles for the two lines of sight. For the oblique one, the in situ observed spectral level of temperature fluctuations (assumed isotropic) was converted into radar reflectivity. For the vertical measurements, a theoretical “universal” level was used, along with an isotropic conversion formula (a tentative account for the known anisotropy is given in the appendix). Comparison of the reconstructed reflectivity profiles with the observed one proves to be very puzzling. For oblique measurements, where the reconstruction process uses only widely accepted hypotheses, the observed profiles are poorly reproduced. By contrast, for vertical measurements, the proposed model leads to a good reconstruction of the shape of the reflectivity profiles. In both cases, the predicted level is always strongly overestimated. The discussion concludes that the discrepancy is real and calls for further studies of this problem.
Detection of near‐water ducts by the wave tilt probing method
Authors: Chrissoulidis D. P. , Kriezis E. E.
Published: February 2008

A theoretical study indicates that near‐water ducts can be detected by airborne wave tilt measurements. Sea roughness and trapping refractivity gradients are incorporated in an iterative procedure which results in wave tilt versus frequency diagrams at low altitudes above sea level. The diagram that corresponds to ducting conditions appears shifted with respect to the reference diagram, which corresponds to the standard refractivity profile. The frequency shift Δf between corresponding ducting and standard diagrams, being rather insensitive to variations of sea state and duct thickness, is strongly dependent on the duct intensity ΔM. The diagram Δf−ΔM is almost linear up to mid‐VHF. The phase quadrature technique offers itself for ΔM estimation through measurements of Δf, provided that the refractivity at sea level is accurately known.
Effects of polarization coupling loss mechanism on design of meteor scatter antennas for short‐ and long‐range communication
Authors: Weitzen Jay A.
Published: February 2008

This paper considers the effect of the polarization coupling loss phenomenon on the design of antennas for meteor scatter communication at short and long ranges. It will be demonstrated that this mechanism strongly influences the spatial arrival pattern of usable meteor trails, and antenna patterns need to be designed around this effect. Characteristics of antenna patterns which exploit the spatial arrival pattern of meteor trails at short and long ranges will be given.
Measurement of narrow pulse distortion over a short HF skywave link: Es and F2 summer results
Authors: Salous S.
Published: February 2008

A high resolution frequency modulated, continuous wave (FMCW) oblique incidence sounder was used in an experiment to measure the minimum pulse width that could be transmitted via a one‐hop F2 layer and Es propagated ionospheric paths. Measurements were made over a 234‐km link in the United Kingdom by using 3 MHz frequency sweeps at 1 MHz/s sweep rate and repeated at 10‐s intervals for 10 min. The collected data were weighted and spectrum analyzed via the fast Fourier transform (FFT) algorithm to a 8–13 μs time resolution. This permitted the resolution of the different multipath components including the magnetoionic waves. Narrow pulse distortion, and consequently the instantaneous bandwidth, was estimated by evaluating the 6‐dB width of the compressed pulses and the slope of the group time delay versus frequency of the Es and one‐hop F2 ordinary and extraordinary waves. The time history of the different modes was obtained by analyzing repetitive sweeps.The results indicated that the sporadic E layer introduced less distortion and was more stable with time than the F2 layer. Moreover, the slope of the group time delay with frequency was often found to give a more accurate estimate of the dispersion suffered by narrow pulses than the 6‐dB width determined by Fourier transforming the FMCW data.
Daytime traveling ionospheric disturbances observed at L ≈ 4.5 in western Quebec with rapid‐run ionosondes
Authors: Morgan M. G.
Published: February 2008

Daytime traveling ionospheric disturbances (TIDs) are studied with a close‐spaced triangulation network of rapid‐run ionosondes. Observations in November and March (1985–1986) found disturbances traveling only southward, with a clockwise progression during the day from southeastward to southwestward. Further observations in May and August (1987) found travel in all directions with no apparent order or progression. These observations are contrary to earlier results in May (1982), using a wider‐spaced network in northern New England together with a chain of single stations extending northward, in which source locations appeared to be bracketed between the northern stations with waves traveling northward as well as southward from those sources. These results suggest possible seasonal control but, at present, still leave as a quandary the question of what and where the sources are. With new digital equipment it is planned to obtain longer time series of observations and the vertical component of the propagation vector. With the latter and the horizontal component the vertical inclination of the phase fronts can be obtained and they in turn can be tracked backward to source regions at any given height.
Enhanced frequency spectra of winds at the mesoscale based on radar profiler observations
Authors: Nastrom G. D. , Gage K. S.
Published: February 2008

Frequency spectra of horizontal winds based on the radar profilers at Poker Flat, Alaska, and Platteville, Colorado, are studied over the range of periods from about 10 min to a day. At Platteville in the troposphere the spectrum of both zonal and meridional winds is found to obey a power law dependence on frequency at periods longer than a few hours. At shorter periods the spectra show a transition to a second power law regime which has enhanced energy relative to a simple extrapolation of the curve which fits the low‐frequency regime. The magnitude of the enhancement is about a factor of 4. The slope of the spectrum in both regions appears to be near −5/3. In the transition region, between periods of about 1/2 and 2 1/2 hours, the spectrum appears to follow a slope near −1. At Poker Flat, where useful results extend to about 20 km, a similar pattern is found in the stratosphere, above 10 km, except that the magnitude of the enhancement factor is about 1.7. This two‐regime model is found to be robust, applying when the results are sorted by season, background wind speed, altitude, and total variance under the spectrum. The results are discussed in terms of a spectrum of vertically propagating gravity waves launched by flow over rough terrain, and the implications for a spectrum due to stratified turbulence are considered.
Spaced antenna versus Doppler radars: A comparison of techniques revisited
Authors: May Peter T.
Published: February 2008

The Doppler beam‐swinging (DBS) and the spaced antenna (SA) techniques for radar wind measurement are compared on theoretical and practical grounds. It is shown that the information content is similar for both techniques and that errors introduced by horizontal variations of the vertical wind field affect both equivalently. A radar equation for a bistatic radar with close transmitting and receiving antennas is derived, and it is shown that the backscattered power is proportional to an equivalent area. The equivalent area is equal to the antennas' physical area for a monostatic radar and is twice the area of the smallest antenna in the limit of one antenna being much larger than the other. Other factors affecting the techniques' sensitivity and accuracy are discussed. VHF radars operating at about 50 MHz should give similar sensitivity using both techniques, but for UHF wind profilers the DBS method is preferable.
Mapping of the polar electrojet current down to ionospheric D region altitudes
Authors: Werner D. H. , Ferraro A. J.
Published: February 2008

The high‐power auroral stimulation (HIPAS) heating facility has been used to modulate D region ionospheric currents at high latitudes, producing extremely low frequency (ELF) radio wave emissions. The behavior of these ionospheric currents can be deduced from a comprehensive study of the ELF signals received at a local field site. This paper examines the mapping of the polar electrojet current from the E region down through the D region, where it can then be modulated by the heater beam. Results have been obtained using a simple Cowling model of the electrojet. These results indicate that for an electrojet flowing at an altitude of 110 km with a scale size in excess of 100 km, the mapping of the horizontal components of the current can be completely characterized in terms of the Pedersen and Hall conductivities. This is attributed to the fact that the mapping becomes independent of scale sizes which exceed 100 km. Several ionospheric heating experiments are proposed, including beam steering for localized ELF generation in the mapped region below electrojets.
Correlation of plate motions with continental tectonics: Laramide to basin‐range
Authors: Engebretson D. C. , Cox A. , Thompson G. A.
Published: February 2008

Some of the major tectonic and magmatic events of the last 150 Ma in the Western Cordillera can be correlated with a new model for the displacement histories between western North America and adjacent oceanic plates. Sierra Nevada plutonism ended and Laramide compression began during increasingly rapid convergence of the Farallon plate with North America and during a moderate increase in the westward motion of North America in the hotspot reference frame. The end of the Laramide and beginning of widespread arc magmatism and extension correlates with slowing of both of these motions. The spectacular slowing of Faralion‐North America convergence is attributed to the decreasing age of the Farallon plate entering the trench and thus to a change from negative to positive buoyancy. The transition from widespread arc‐related magmatism and rapid extension to basaltic volcanism and moderate extension in the Basin and Range province finds no ready explanation in the plate motions of the Pacific basin. A change to oblique spreading in the Basin and Range province accompanied growth in length of the San Andreas fault as the Mendocino triple junction progressed northward.
Thermal evolution of the north‐central Gulf Coast
Authors: Nunn Jeffrey A. , Scardina Allan D. , Pilger Rex H.
Published: February 2008

The subsidence history of the North Louisiana Salt Basin, determined from well data, indicates that the region underwent extension during rifting and has since passively subsided due to conductive cooling of the lithosphere. Timing of the rifting event is consistent with opening of the Gulf of Mexico during Late Triassic to Early Jurassic time. Crustal extension by a factor of 1.5 to 2 was computed from “tectonic” subsidence curves. However, data from the early subsidence history are insufficient to distinguish between uniform and nonuniform extension of the lithosphere. The magnitude of extension is in good agreement with total sediment and crustal thicknesses from seismic refraction data in the adjacent Central Mississippi Salt Basin. The temperature distribution within the sediments is calculated using a simple heat conduction model. Temperature and subsidence effects of thermal insulation by overlying sediments are included. The computed temperature distribution is in good agreement with bottom hole temperatures measured in deep wells. Temperature histories predicted for selected stratigraphic horizons within the North Louisiana Salt Basin suggest that thermal conditions have been favorable for hydrocarbon generation in the older stata. Results from a two‐dimensional heat conduction model suggest that a probable cause for the early formation of the adjacent uplifts is lateral heat conduction from the basin. Rapid extension of the lithosphere underneath areas with horizontal dimensions of 50–100 km produces extremely rapid early subsidence due to lateral heat conduction. The moderate subsidence rate observed in the North Louisiana Salt Basin during the Jurassic and Early Cretaceous suggests slow extension over a long period of time.
Middle Cambrian paleomagnetism of the Avalon Terrane in Cape Breton Island, Nova Scotia
Authors: Johnson Rex J. E. , Van der Voo Rob
Published: February 2008

Interbedded volcanics and sediments of the Bourinot Group (Mid‐Cambrian) in central Cape Breton Island were studied as part of a paleomagnetic investigation to quantify the geologically plausible displacements of the Avalon terrane with respect to the North American craton. Both volcanics and sediments reveal prefolding magnetizations with steep to intermediate inclinations. Dual‐polarity magnetizations in different volcanic flows are probably primary and they yield a mean direction of D=293°, I=−66° (N=9, k=41.8, α95=8.1°). The sediments reveal, upon detailed thermal demagnetization, complex multivectorial remanences. However, a few samples show characteristic components and a large number of samples (38) have intersecting great‐circle remagnetization trajectories; when combined, this yields a mean direction of D=267°, I=−65° (k=16, mean radius of error ellipse is 8.5°). The mean directions for the Bourinot rocks correspond to a paleolatitude of 49°S±11°, which is different from the range of possible values for the predicted Mid‐Cambrian paleolatitudes for Nova Scotia (29°S to 18°N), inferred from the North American apparent polar wander path. This discrepancy between predicted paleolatitudes and the observed paleolatitude implies that Nova Scotia was not part of North America during the Mid‐Cambrian, but rather, they were separated by an ocean with a minimum width of 1000 km (20°±10°). This interpretation of the relative positions of Nova Scotia and North America based on paleomagnetism is consistent with the geological record in the northern Appalachians and supports previous interpretations of the dissimilar Early Paleozoic faunas in Avalon and North America. It is paleomagnetically quite plausible that the Avalon terrane formed part of a larger continent, comprising Africa (Gondwana) and Armorica during the Cambrian.
Prefolding and premegakinking magnetizations from the Devonian Comerong Volcanics, New South Wales, Australia, and their bearing on the Gondwana Pole Path
Authors: Schmidt P. W. , Embleton B. J. J. , Cudahy T. J. , Powell C. McA.
Published: February 2008

Palaeomagnetic results from the late Middle to early Late Devonian Comerong Volcanics in the Budawang Synclinorium, Lachlan Fold Belt (LFB), New South Wales, Australia, satisfy a fold test and support the hypothesis that foliated parts of the LFB have been folded by megakinks during the terminal mid‐Carboniferous orogenic phase. The preferred pole position is at 76.9°S, 330.7°E (A95= 7.2°), allowing calibration of the Australian pole path at around 370 Ma ago. An alterative pole position at 71.3°S 283.1°E (same A95) takes into account a possible rotation of the megakinked terrane by 15° clockwise with respect to the Australian craton. Although there are still problems unravelling previously published results from Silurian rocks in the LFB, both because of unreliability and uncertainty as to where the LFB crustal units lay in relation to the Australian craton, there is no geological reason to suspect any large relative movement between the LFB and the craton since the Middle Devonian, and thus the Comerong Volcanics pole position is representative of the Late Devonian for Australia. The new pole requires reassessment of many other poles that have previously been accepted as representing the Siluro‐Devonian for Australia, and Gondwanaland. We present reasons for believing that the magnetization, or remagnetization of many rock units (e.g., Mereenie Sandstone and Mulga Downs Group, Australia, and the Gneigura Supergroup, Africa) was later than sedimentation, probably during widespread Early Carboniferous deformation. Another rock unit (Msissi Norite, Africa) could be of Early Carboniferous, rather than Late Devonian, age.
Introduction [to Special Section: Magnetotectonics]
Authors: McClelland E. , Courtillot V. , Tapponnier P.
Published: February 2008

The application of methods frompalaeomagnetism and rock magnetism totectonic problems already has a longhistory of success. However, in recentyears there has been a growing awarenessof the remarkable potential of trulyintegrated studies performed in closecooperation by geophysicists and structuralgeologists. The scale of tectonic problemsencountered goes from planetary to metric,and now to microscopic. Most early studiesof deformed rocks used the classicalunfolding method of Graham [1949], in whichall rotations are assumed to be rigid bodyand around horizontal axes. Subsequently,more complicated cases of rigid rotationshave been envisioned [e.g., MacDonald,1980], and now problems involving multiplephases of folding and internal deformationare being tackled. The recognition of theimportance of remagnetizations hasperformed an important role in this evolution,with the advent of more sensitivemagnetometers and of more sophisticated means ofanalyzing demagnetization diagrams. Suchstudies are made even more powerful whencombined with age determinations, chemicaland petrographical analyses.
Effects of tectonic deformation on the remanent magnetization of rocks
Authors: Lowrie W. , Hirt A. M. , Kligfield R.
Published: February 2008

Paleomagnetic investigations have usually been impossible in deformed rocks because of the effects of distortion on the remanent magnetization vector. Deformation produces a magnetic anisotropy in rocks which can deflect the remanence away from the minimum susceptibility axis. However, the assumption that the intersection points of great circles joining these two directions represent an improved estimate of the undisturbed remanence is invalid because the amount of the deflection of each remanence is not really known. Anisotropy of magnetic susceptibility (AMS) provides a quantitative basis for strain determination. The principal axes of the magnetic anisotropy ellipsoid have the same directions as those of the strain ellipsoid. A linear relationship between the normalized principal susceptibility differences and the logarithmic strains has been found in several rock types. If the correlation method used is valid, AMS observations can be used to establish the magnitudes and directions of the principal strains. Strain modifies bedding planes and realigns the grains that carry magnetic remanence. Compensation for deformation requires application of a modified bedding tilt correction and correction of the remanent vector for distortional strain. The effects of strain on bedding can be corrected using passive marker theory. Attempts to compensate for distortional strain by treating the remanent vector as a passive marker have proved inconclusive. During deformation linear and planar elements deflect away from the axis of maximum shortening. The passive marker correction shifts the remanent vectors toward the local shortening axis for the site and reduces the directional scatter. Tests of the passive marker method in strongly deformed rocks resulted in a remanence distribution dominated by the shortening axis directions. The passive marker model becomes inapplicable when the deformation results in recrystallization of the rock matrix.
A magnetotectonic study of the Hercynian Montagne Noire (France)
Authors: Courtillot V. , Chambon P. , Brun J. P. , Rochette P. , Matte P.
Published: February 2008

A paleomagnetic study of eleven sites of Devonian limestones sampled along the flanks and hinge of a polyphase Hercynian recumbent fold in the Montagne Noire reveals well defined directions of characteristic remanent magnetization (ChRM). The directions have good within‐site precision but a large between‐site scatter, which fails to be reduced by a classical tilt correction. This is readily understood if widespread remagnetization occurred between the two major Hercynian phases of folding (A and B), which are known to have shaped the Montagne Noire. This remagnetization appears to be an instance of a continent‐wide chemical event that wiped out much of the magnetic memory of Devonian and lower Carboniferous rocks in Laurasia, Baltica and northern Gondwana, possibly due to groundwater circulation following the major Hercynian tectonic phases. Comparison of observed and predicted Hercynian magnetic directions allows one to determine, through a fairly simple geometric construction, the tectonic elements of the later phase B. It is found that different rotations affected the flanks and hinge zone of the folds generated by phase A, thus demonstrating how a pre‐existing structure can control the development of deformation in a later tectonic phase. The effects of the non cylindrical phase B can next be removed to reveal the broad tectonic features of phase A deformations. The reversed (Kiaman) polarity of the ChRM allows a determination of the age of phase B, which is found to be about 300–310 Ma and of the age of the remagnetization itself at about 310–320 Ma. The search for the age of remagnetization included paleomagnetic measurements at two lower Permian sites from the nearby Lodève basin. These provide new data for the Permian pole of Europe and point out the limitations of some earlier studies, due to insufficient magnetic cleaning. Finally, the principal shortening directions of the two phases can be determined from the magnetic data and are found to have rotated counterclockwise by 30° between phase A and phase B. This feature, already observed in the collision of India against Eurasia, may be related to the progressive suturing of former subduction zones during the Hercynian collision of Gondwana against Laurasia. This study demonstrates the potential of remagnetizations for the solution of rather complex tectonic problems. We propose the term “magnetotectonics” for this rapidly expanding field of studies, in which the methods of paleomagnetism and structural analysis are closely associated.
Paleomagnetism and the Adriatic promontory: A reappraisal
Authors: Lowrie W.
Published: February 2008

The geodynamic evolution of the Western Mediterranean area was influenced strongly by the Adriatic promontory of the African plate, which can be outlined at present by current seismicity and by facies and ophiolite distributions. Paleomagnetic results from different parts of the promontory contribute useful information about its past history. The evaluation of these data is made difficult by the lack of a well defined apparent polar wander path for Africa. This has resulted in occasional overinterpretation of some paleomagnetic studies. Results from Umbria and the Southern Alps cover a long enough period of time to define part of an APW path and support the idea that the promontory moved as an adjunct to the African plate during the late Mesozoic and early Cenozoic. A conservative evaluation of Cretaceous paleomagnetic data, the most abundant for the promontory as a whole, shows no strong evidence for previously proposed counterclockwise rotations of Sicily or the Southern Alps relative to the African plate. Paleomagnetic data, however, suggest about 17 degrees counterclockwise rotation of autochthonous Gargano and Istria, and about 25 degrees counterclockwise rotation of allochthonous Umbria. Tectonic rotation of the middle part of the promontory appears to have occurred without significant rotation of its northern and southern extremities.
Tectonic implications of canyon directions over the Northeast Atlantic Continental Margin
Authors: Lallemand Serge , Sibuet Jean‐Claude
Published: February 2008

The basis of this study is a new bathymetric map of the northeast Atlantic compiled from previously published maps made from conventional echosounder data, plus all Sea Beam data acquired on board the R/V JEAN CHARCOT since 1977. As most of the Sea Beam data have been obtained on the continental margin from Porcupine Seabight to the south of the Iberian Peninsula, a precise picture of the continental slope is given. A statistical analysis of the canyons, based on 750 measurements, reveals that many of the canyons present sharp changes in their direction, indicating a structural control mainly linked to the late Hercynian trends, especially around the Iberian Peninsula. Nevertheless, the paths of canyons may merely reflect recent gravity processes, as in the Porcupine Seabight. Canyons locally follow the directions of listric and associated transecting faults (Permian to Triassic and upper Jurassic to lower Cretaceous), as on the Celtic margin, and every type of tectonic lineament—for example, the North Pyrenean Paleogene thrust front which fringes the Gouf of Cap Breton. A comparison of diagrams for the northern and southern Bay of Biscay margin (especially trends predating the opening) is compatible with a 25° rotation of Iberia with respect to Europe.
Tectonically controlled origin of three unusual rock suites in the Woodlark Basin
Authors: Abbott Dallas , Fisk Martin
Published: February 2008

We propose alternative mechanisms for the origin of three unusual rock suites, high‐Mg andesites, NaTi basalts, and arclike rocks, that have been dredged from the Woodlark basin, southwest Pacific Ocean. We show that the high‐Mg andesites and NaTi basalts are associated with an unusually cool ridge environment. The cooling is due to increased hydrothermal circulation, stimulated by an unusually high crustal permeability. The high permeability is mainly due to cracking of the Woodlark basin lithosphere as it passes over the flexural bulge in front of the subduction zone, although local processes, such as faulting in fracture zones, may also make some contribution. This increased convective cooling affects magma dynamics and chemistry at the ridge crest. The high‐Mg andesites occur where the hydrothermal circulation lowers the temperatures in the upper oceanic crust, causing the magma chamber to sit in the mantle rather than in the crust and promoting the interaction of basalt magma with harzburgite. The NaTi basalts are also the indirect result of increased crustal cooling, which causes anomalously low degrees of partial melting of their depleted mantle source. The arclike rocks are caused by interaction with volatiles from lithosphere that was emplaced beneath the edge of the basin less than 6 m.y. ago by a now inactive subduction zone to the north of the currently active one. Simple thermal models indicate that this formerly subducting plate is still cold enough to retain volatiles and to remain seismically active. As the old plate loses volatiles, they rise into the mantle convection cell which feeds the Woodlark basin ridge crest. Because ridge subduction increases the likelihood of ophiolite obduction, these observations and explanations of Woodlark basin tectonics are potentially important for ophiolites.
Kinematics of ductile shearing from outcrop to crustal scale in the Monte Rosa Nappe, Western Alps
Authors: Lacassin Robin
Published: February 2008

Significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the Mesozoic cover quartzites and marbles of the Monte Rosa nappe, in the inner part of the Western Alps. We summarize here the results of a detailed microstructural study at several scales of observation in the Northern part of the nappe and discuss the kinematic significance and compatibility of such results. Most of the observed deformations can be accounted for by progressive WNW‐vergent shearing during and after the late Eocene, within a ductile shear zone of crustal scale. The displacement of the hanging wall may have been in excess of several tens of kilometers. However, some ESE‐vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the Monte Rosa nappe.
Basement tectonics of the Southern Pampean Ranges, Argentina
Authors: Salda Luis Dalla
Published: February 2008

Upper Precambrian to lower Paleozoic crystalline basement of the Southern Pampean Ranges is composed of (1) a narrow western zone, having low to medium grade metamorphic rocks and relatively few granites and migmatites and (2) a wide central eastern Hercynotype low‐pressure zone with abundant granitoids. The internal structures fall into three distinct domains characterized by the following orientations: Dl, nearly E‐W; D2, NNW‐SSE; and D3, NNE‐SSW.

The tectonic evolution is interpreted as having begun in an upper Precambrian orogenic cycle commencing with lithospheric rifting, followed by compression forming D1 structures. Next was a mobile belt cycle composed of two main stages. The first one, which generated the main tectonic features (D2) of the igneous—metamorphic complex, coincided with a syntectonic to late tectonic granitic intrusive event, with the principal metamorphic episode, and with Paleozoic Andean tectonic trends related to the activity of the Gondwanaland Pacific margin. The last one, was a fracturing and an open weak folding (D3) event with local low‐grade metamorphism; it occurred during the Silurian.
Did rifting on Australia’s Southern Margin result from tectonic uplift?
Authors: Houseman Gregory A. , Hegarty Kerry A.
Published: February 2008

Subsidence analyses from Australia’s southern margin are used to investigate the idea that rifting on the margin was caused by an extensional lithospheric stress field induced by a hot thermal plume impinging on the base of the lithosphere from beneath. The continental lithosphere is modelled as a thin sheet with depth‐dependent rheology; the model does not explicitly include elastic, flexural, or thermal stresses. The excess buoyancy of a thermal plume pushes the lithosphere upward by as much as 1 km, thereby increasing its potential energy. If the horizontal components of lithospheric stress are fixed at some distant plate boundary (the lithosphere is a stress guide), the lithospheric stress field above the plume will produce horizontal extension that may ultimately be followed by continental separation and seafloor spreading. The uplift is modelled by a time‐dependent function which increases smoothly from zero to its peak value and then decreases back to zero. Using a plausible mantle rheology, the amount of extension inferred at well sites on the margin (typically 20% to 30% extension) is explained by uplift of the lithosphere to approximately 900 m, with a subsequent decay of the uplift over a period of approximately 60 m.y. The rise time of the uplift function is poorly constrained. In the uplift model, extension occurs in the subaerial environment during the late Jurassic ‐ early Cretaceous. The initial stage of rapid subsidence observed at most well sites on the margin is due in large part to decrease of the vertical stress anomaly caused by the decay of the thermal plume (or by the lithosphere moving away from above it). The rapid subsidence is followed by very gradual continued subsidence attributed to cooling of the extended lithosphere, as in previous studies.
Ordovician docking of the Carolina Slate Belt: Paleomagnetic data
Authors: Vick H. K. , Channell J. E. T. , Opdyke N. D.
Published: February 2008

Paleomagnetic results from eight sites in argillaceous metasediments from Union County, North Carolina, in the central Carolina slate belt yield a consistent high blocking temperature magnetization component. The precision of the mean declination and inclination of all site means is maximized at 50% tilt correction, and the improvement in grouping relative to 0% and 100% tilt correction is significant at the 95% confidence level. This tends to indicate that the high blocking temperature component was acquired during folding. The metamorphism and deformation in this area are rather well constrained, by recent 40Ar/39Ar data, to the Late Ordovician (−450 Ma). We therefore interpret the computed pole position (29.6°N, 122.1°E, α95 = 5.11) as Late Ordovician in age and note that it lies close to some of the Late Ordovician poles for cratonic North America. The paleolatitude (22.2°S) of the sampling site is certainly unlike those derived from Armorica and Gondwana for this time interval and is rather similar to that proposed for the Iapetus coast of Laurentia during the Late Ordovician. Indeed, the magnetization (and deformation) may be contemporaneous with the docking of the Carolina slate belt onto the Laurentian continent.