bluuebunny/crossref_metadata_2025 huggingface dataset:
An application of Parameterized Real‐Time Ionospheric Specification Model to regional ionospheric specification
Authors: Bishop G. J. , Daniell R. E. , Rao S. , Mazzella A. J. , Bullett T. W.
Published: February 2004

We are currently utilizing the Parameterized Real‐Time Ionospheric Specification Model (PRISM) as a platform for deriving real‐time regional ionospheric specification. This application involves supporting PRISM with a limited set of real‐time sensor input and obtaining from PRISM a specification of ionospheric densities over a region extending to a radius of 2000–3000 km. In the initial work on this effort we have examined both data assimilation and model validation techniques, on a regional basis. We seek improved techniques for regional ionospheric specification using primarily GPS and ionosonde data as model input and mechanisms for both after‐the‐fact and real‐time assessment of the quality of the resulting specification. We will examine, for one region, data selection and application, data quality and accuracy, and approaches for obtaining a figure of merit on the specification product. We will present some results from a short regional test of these methods and discuss issues involved in generalizing to larger studies and other regions.
Relationship between Jovian hectometric attenuation lanes and Io volcanic activity
Authors: Menietti J. D. , Gurnett D. A. , Spencer J. R. , Stansberry J. A.
Published: February 2004

Within the Galileo plasma wave instrument data a narrow (in frequency) attenuation band is seen in the hectometric (HOM) emission that varies in frequency with system III longitude. This attenuation lane is believed to be the result of near‐grazing incidence or coherent scattering of radio emission near the outer edge of the Io torus, i.e., when the ray path is nearly tangent to an L shell containing the Io flux tube [Gurnett et al., 1998]. Such a process should, therefore, be enhanced when the Io volcanic activity is increased and the Io flux tube has enhanced density. We have performed a systematic study of the existing Galileo radio emission data in an effort to determine the phenomenology and frequency of occurrence of the attenuation lanes and the association, if any, with published volcanic activity of Io. Our results indicate that the attenuation lanes are present almost all of the time but are enhanced on occasion. The best examples of attenuation lanes occur when Galileo is within ∼65 RJ of Jupiter and thus are probably more apparent because of the increased signal‐to‐noise ratio of the radio receivers. The lack of continuous monitoring of Io activity and the lack of known activity on the anti‐Earthward side of Io are problematic and make detailed correlation with radio emission very difficult at this time. Nevertheless, if the data are displayed for periods when the spacecraft is within 65 RJ (i.e., for each perijove pass), then the highest‐contrast lanes occur on most passes when the Io volcanic activity is also high for that pass. These results support our current understanding of attenuation lane formation and suggest that future efforts can be made to better understand the interaction of HOM emission with the Io flux tube.
Application of radio tomographic imaging to HF oblique incidence ray tracing
Authors: Rogers N. C. , Mitchell C. N. , Heaton J. A. T. , Cannon P. S. , Kersley L.
Published: February 2004

Radio tomography is a technique for generating images of the spatial structure of ionospheric electron density over a wide area. This paper assesses the potential use of radio tomography in HF oblique propagation and ray tracing applications. Synthetic ionograms produced by ray tracing through tomographic images and ionospheric models have been compared with experimental oblique ionograms from six paths lying close to the image plane in the United Kingdom. In particular, study has been made of the effects of various types of input information used to constrain the vertical electron density structure in the tomographic reconstructions. It was found that use of a fine height resolution (5 km) and incorporation of information from one vertical ionosonde in the reconstruction process makes significant improvements to the overall reliability of the tomographic image. As expected, E layer propagation is better defined using a climatological model than by tomography. However, in comparison with three ionospheric models, use of tomographic images can significantly reduce the RMS error in the determination of the F2 layer maximum usable frequency.
Implementation of the matrix sparse decomposition technique to the scattering of two‐dimensional homogeneous dielectric cylinders
Authors: Liu Y. W. , Song Y. M. , Mei K. K.
Published: February 2004

In this paper, a novel matrix‐thinning technique, matrix sparse decomposition (MSD) [Liu et al., 1998, 1999], has been implemented to solve the scattering of waves by two‐dimensional (2‐D) homogeneous dielectric cylinders for the first time. The MSD technique is a further development of the integral equation formulation of the measured equation of invariance (MEI) (IE‐MEI) [Rius et al., 1996a; Hirose et al., 1999a]. The MSD describes the local relationship between total currents and scattered fields rather than that between the scattered electric fields and the scattered magnetic fields in the IE‐MEI. The MSD directly thins a dense matrix from singular integral equations, such as method of moments (MOM), into two sparse matrices. The IE‐MEI method has difficulty in solving thin wire or thin plate structure problems. However, the MSD can do it without a hitch. Numerical examples for the scattering of 2‐D homogeneous dielectric circular and rectangular cylinders under both transverse magnetic and transverse electric plane wave incidences show that the MSD is a simple and effective technique to thin the MOM dense matrix.
Correct characterization of guided electromagnetic waves in chirowaveguides of arbitrary cross sections
Authors: Li Le‐Wei , Leong Mook‐Seng , Lim Seh‐Ngi , Yeo Tat‐Soon
Published: February 2004

The eigenvalue problem for the determination of propagating modes in a lossless and homogeneous chirowaveguide of arbitrary cross section is again considered in this paper. A corrected analysis of the characteristic equations and the behaviors of the wave modes is made, based on the work by Eftimiu and Pearson [1989] who incorrectly formulated the problem. As an example of the application of the generalized analysis to some specific cases, a circular cylindrical chirowaveguide is then considered in the theoretical and numerical investigations. Through the solution of the characteristic equation, a dependency of cutoff wavenumbers of the various eigenmodes upon the chirality values is found.
Using geomagnetic birefringence to locate sources of impulsive, terrestrial VHF signals detected by satellites on orbit
Authors: Jacobson Abram R. , Shao Xuan‐Min
Published: February 2004

The Earth's ionosphere is magnetized by the geomagnetic field and imposes birefringent modulation on VHF radio signals propagating through the ionosphere. Satellites viewing VHF emissions from terrestrial sources receive ordinary and extraordinary modes successively from each broadband pulse emitted by the source. The birefringent intermode frequency separation can be used to determine the value of ƒce cos β, where ƒce is the electron gyrofrequency and β is the angle between the wave vector k and the geomagnetic field B at the point where the VHF ray path intersects the ionosphere. Successive receptions of multiple signals (from the same source) cause variation in ƒce cos β, and from the resulting variation in the signal intermode frequency separation the source location on Earth can be inferred. We test the method with signals emitted by the Los Alamos Portable Pulser and received by the FORTE satellite.
Group propagation time of quasi‐monochromatic radiation in a plane layer of plasma with random irregularities
Authors: Zabotin N. A. , Bronin A. G. , Kovalenko E. S.
Published: February 2004

The influence of multiple scattering in a plasma layer with random irregularities on the propagation time of a totally reflected pulse signal has been considered. Two approaches have been implemented. The first one is based on an analytical solution of the radiative transfer equation. The second one is based on a numerical simulation in the spirit of the Monte Carlo technique. The general conclusion is that, subject to relative position of a transmitter and a receiver and the irregularity shape, one can observe both a decrease and an increase of the propagation time in comparison with the reflection from the same layer deprived of random irregularities. The numerical estimates made in this paper regard the case of radio sounding of the ionospheric plasma. It has been shown that the magnitude of the considered effect is of the order of the characteristic pulse length applied in the ionosphere vertical sounding.
Polarization observations of broadband VHF signals by the FORTE satellite
Authors: Shao Xuan‐Min , Jacobson Abram R.
Published: February 2004

Coherent very high frequency (VHF) radio observations with the pair of orthogonal log‐periodic array antennas of the FORTE satellite allow us to study thoroughly the polarization properties for a received signal. Eighty‐one broadband VHF pulses that were generated by the Los Alamos Portable Pulser (LAPP) have been analyzed. The data are analyzed by computing the Stokes parameters in the time‐frequency domain. We first examine the LAPP pulses at high time resolution so as to separate the ordinary and extraordinary ionospheric modes. The two modes have been found to be mirror images of each other in terms of polarization, as would be expected. For each mode the polarization degrades from circular toward elliptical as the nadir angle increases. Antenna pattern effects on this observation are discussed. The tilt of the detected polarization ellipse is found to be tightly associated with the azimuthal direction of the pulse source. The same set of data are then examined with much lower time resolution to intentionally mix together the two split modes, so that the ionospheric Faraday rotation can be detected. With the known geomagnetic field the total electron content (TEC) is computed, which shows good agreement with the TEC computed by dechirping the signal. A case study of an impulsive lightning emission shows that it is highly polarized, indicating that the associated breakdown processes are highly coherent and organized. Finally, we discuss the potential use of the polarization observations for locating terrestrial radio signals.
Experimental and computer simulation studies of frequency selective fading in terrestrial line‐of‐sight microwave links
Authors: Zhao Xiongwen , Zhang Yerong , Vainikainen Pertti , Yixi Xie
Published: February 2004

Rummler's three‐ray model is used for multipath propagation in modified spherical stratified air. The time delays of the three rays are obtained by using the ray tracing method. Both the time delays and the amplitudes are related to the modified refractive index gradients inside and outside the layer. Because of the random variations of the modified refractive index gradients, the computer simulation method of multipath fading is studied by using the Monte Carlo method. The experiments of frequency selective fading are performed in China on a 48.4 km terrestrial microwave link. The statistical distributions of flat fading, in‐band dispersion and the effect of diversity reception and their correlations are studied both by computer simulations and by experiment. On the basis of data measured in China and Japan, and computer simulation results, the correlation between the equal probability values of the in‐band linear amplitude dispersion and flat‐fading depth due to multipath propagation is analyzed. An excellent linear relationship is found between them.
Ductile and brittle shortening, extension‐parallel folds and maintenance of crustal thickness in the central Aegean (Cyclades, Greece)
Authors: Avigad Dov , Ziv Alon , Garfunkel Zvi
Published: February 2004

In contrast to previous studies that concentrated on the two‐dimensional crustal strain in the central Aegean region (Cyclades), it is shown that NNE‐SSW extension via ductile and brittle stretching and low‐angle detachments was accompanied and/or alternated with horizontal shortening perpendicular to the stretching direction since at least the early Miocene. Roughly E‐W directed ductile shortening produced large‐scale overturned and upright folds having axes parallel or slightly oblique to the stretching lineation. Upright folding and arching of low‐angle normal faults occurred above the brittle‐ductile transition and brittle E‐W compression culminated with vertical axis block rotations, strike‐slip faults and minor thrusts. Since at least the early Miocene, the structure of the Cycladic massif evolved through alternating and/or coeval increments of horizontal shortening and vertical thinning associated with an approximately constant NNE‐SSW stretching. Exact magnitudes of the vertical and horizontal deformational components are difficult to assess. Nevertheless, we note that extensional tectonics that affected the Cyclades during the last 15–20 m.y. have produced no net crustal thinning. We suggest that crustal thickness was maintained by extension‐parallel folds, which represent true contractional structures, and that crust was fed into the extended region from its margins.
Constraining continental structure by integrating Os isotopic ages of lithospheric mantle with geophysical and crustal data: An example from southeastern Australia
Authors: Handler M. R. , Bennett V. C.
Published: February 2004

Rocks exposed at the Earth's surface provide an incomplete record of the varied and complex processes that have acted to form and modify the continents. The continental lithospheric mantle (CLM), sampled as xenoliths carried in explosive volcanics, is an alternative reservoir of age information that has only recently begun to be tapped. Data from southeastern Australia, a technically complex region, illustrate that the CLM can record important aspects of continental evolution that are not indicated by the exposed crustal rocks. Re‐Os isotopic data and the resultant model ages for 15 spinel peridotite xenoliths from five localities in southeastern Australia show that parts of the CLM are ∼ 1960 Myr old, some 1400 Myrs older than the oldest crust exposed immediately in the sampled region. As the five sampled localities are east of the presumed eastern edge of the Precambrian interior of Australia, the preservation of Proterozoic ages in the lithosphere indicates that the supposed Precambrian edge of the Australian continent (the Tasman Line) is a surficial feature rather than a lithospheric scale boundary. This conclusion is consistent with recent Skippy seismic tomography models for Australia. The mantle xenolith age distribution also suggests a significant age change from Proterozoic to dominantly Phanerozoic, which appears coincident with the surface expression of the boundary between two major fold belts (Lachlan and Delamerian) and with a deep lithospheric boundary previously proposed on the basis of differences in Sr and Pb isotopic systematics of southeast Australian basalts.
Cenozoic tectonics of the Cape Roberts Rift Basin and Transantarctic Mountains Front, Southwestern Ross Sea, Antarctica
Authors: Hamilton Rhea J. , Luyendyk Bruce P. , Sorlien Christopher C. , Bartek Louis R.
Published: February 2004

We conducted a multichannel seismic reflection survey offshore Cape Roberts, Antarctica, and combined our findings with the results of the Cape Roberts International Drilling Project (CRP). This allows us to interpret Cenozoic tectonics in the southwest sector of the Ross Sea including the history of uplift of the Transantarctic Mountains (TAM) and subsidence of the Victoria Land Basin (VLB). Seismic stratigraphic sequences mapped offshore Cape Roberts arc tilted eastward and thicken into the VLB where they comprise more than half the fill seen on seismic records. Normal faults a few kilometers offshore cut these sequences and define a north trending rift graben. Drilling results from the CRP show that these strata are latest Eocene (?), Oligocene, and younger in age; much younger than previously inferred. We interpret this pattern to be due to an episode of E‐W extension and related subsidence that occurred across the major basins in the western Ross Sea during the early Cenozoic. The rift graben offshore and adjacent to Cape Roberts is bounded on the west by a major north trending fault zone. At Cape Roberts this fault system may have from 6 to 9 km of vertical separation. This fault system is part of a larger zone along the coastline in southern Victoria Land that accommodated uplift of the TAM in Oligocene time. We name it here the McMurdo Sound Fault Zone. A late Oligocene angular unconformity that is seen in seismic data and sampled by CRP drilling marks the end of east tilting of the stratigraphic sequences. We interpret this as the end of the main uplift of the TAM coinciding with a change from E‐W extension to NW‐SE oblique rifting at that time. Uplift of the TAM and subsidence in the VLB may be linked with seafloor spreading on the Adare Trough to the northwest of the Ross Sea between 43 and 26 Ma. This would imply a plate boundary between East and West Antarctica crossing through the western Ross Sea in Eocene and Oligocene time.
Tectonic erosion and consequent collapse of the Pacific margin of Costa Rica: Combined implications from ODP Leg 170, seismic offshore data, and regional geology of the Nicoya Peninsula
Authors: Vannucchi Paola , Scholl David W. , Meschede Martin , McDougall‐Reid Kristin
Published: February 2004

The convergent margin off the Pacific coast of the Nicoya Peninsula of Costa Rica exhibits evidence for subduction erosion caused by the underthrusting Cocos plate. Critical evidence for efficacy of this process was recovered at the Ocean Drilling Program (ODP) drilling Site 1042 (Leg 170), positioned ∼7 km landward of the Middle America trench axis off the Nicoya Peninsula. The primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity, the so‐called base‐of‐slope sediment (BOSS) horizon or surface. The BOSS horizon, which can be traced landward from near the trench to the Nicoya coastal area and parallel to it for hundreds of kilometers, separates a low‐velocity (∼ 2.0–2.5 km s−1) sequence of slope sediment, from an underlying sequence of much higher‐velocity (>4–4.5 km s−1) rock. Site 1042 reached the acoustically defined BOSS horizon at a below sea level depth of ∼ 3900 m and yielded a carbonate‐cemented calcarenitic breccia of early‐middle Miocene age. Sedimentological, geochemical, paleontological, and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment. On the basis of coastal exposures, the BOSS horizon, as a margin‐wide geologic interface, can be temporally and lithostratigraphically correlated to a regional angular unconformity. This unconformity, known as the Mal Pais unconformity, separates Neogene and younger shelf‐to‐littoral beds from the underlying mafic units of the Mesozoic Nicoya Complex and Cretaceous and early Tertiary sedimentary sequences. At Site 1042 it is inferred that tectonism caused the vertical subsidence of the early Neogene breccia from a shallow to a deep water setting. The Mal Pais unconformity of the BOSS horizon thus connects the rock fabric of the outermost part of margin to that of coastal Nicoya and implies that in the early Neogene the Nicoya shelf extended seaward to near the present trench axis. This circumstance requires that the early Neogene trench axis was at least 50 km seaward of where it is now located. The long‐term effects of subduction erosion, similar to those described for the scientifically drilled Japan, Tonga, and Peru margins, best account for offshore and onshore evidence for a post‐Paleogene history of crustal thinning and landward trench migration of Costa Rica's Pacific margin. During the past 16–17 Myr the calculated mass removal and landward migration rates are 34–36 km³ Myr−1 km−1 of margin, and 3 km Myr−1, respectively. These values are similar to those found for other Pacific margins dominated by nonaccretionary subduction zone processes.
Stratigraphy, structure, and tectonic evolution of the Himalayan fold‐thrust belt in western Nepal
Authors: DeCelles Peter G. , Robinson Delores M. , Quade Jay , Ojha T. P. , Garzione Carmala N. , Copeland Peter , Upreti Bishal N.
Published: February 2004

Regional mapping, stratigraphic study, and40Ar/39Ar geochronology provide the basis for an incremental restoration of the Himalayan fold‐thrust belt in western Nepal. Tectonostratigraphic zonation developed in other regions of the Himalaya is applicable, with minor modifications, in western Nepal. From south to north the major structural features are (1) the Main Frontal thrust system, comprising the Main Frontal thrust and two to three thrust sheets of Neogene foreland basin deposits; (2) the Main Boundary thrust sheet, which consists of Proterozoic to early Miocene, Lesser Himalayan metasedimentary rocks; (3) the Ramgarh thrust sheet, composed of Paleoproterozoic low‐grade metasedimentary rocks; (4) the Dadeldhura thrust sheet, which consists of medium‐grade metamorphic rocks, Cambrian‐Ordovician granite and granitic mylonite, and early Paleozoic Tethyan rocks; (5) the Lesser Himalayan duplex, which is a large composite antiformal stack and hinterland dipping duplex; and (6) the Main Central thrust zone, a broad ductile shear zone. The major structures formed in a general southward progression beginning with the Main Central thrust in late early Miocene time. Eocene‐Oligocene thrusting in the Tibetan Himalaya, north of the study area, is inferred from the detrital unroofing record. On the basis of40Ar/39Ar cooling ages and provenance data from synorogenic sediments, emplacement of the Dadeldhura thrust sheet took place in early Miocene time. The Ramgarh thrust sheet was emplaced between ∼15 and ∼10 Ma. The Lesser Himalayan duplex began to grow by ∼10 Ma, simultaneously folding the north limb of the Dadeldhura synform. The Main Boundary thrust became active in latest Miocene‐Pliocene time; transport of its hanging wall rocks over an ∼8‐km‐high footwall ramp folded the south limb of the Dadeldhura synform. Thrusts in the Subhimalayan zone became active in Pliocene time. The minimum total shortening in this portion of the Himalayan fold‐thrust belt since early Miocene time (excluding the Tibetan zone) is ∼418–493 km, the variation depending on the actual amounts of shortening accommodated by the Main Central and Dadeldhura thrusts. The rate of shortening ranges between 19 and 22 mm/yr for this period of time. When previous estimates of shortening in the Tibetan Himalaya are included, the minimum total amount of shortening in the foldthrust belt amounts to 628–667 km. This estimate neglects shortening accommodated by small‐scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening.
Geometry and structural evolution of the central Andean backthrust belt, Bolivia
Authors: McQuarrie Nadine , DeCelles Peter
Published: February 2004

The central Andean backthrust belt is a large‐scale west vergent thrust system along the western side of the Eastern Cordillera in the generally east vergent Andean fold‐thrust belt of Bolivia. Although west vergent structures in the central Andes have been recognized previously, we describe the backthrust belt at a regional scale, emphasizing its implications for the kinematic development of the Andes and the subsequent influence of these kinematics on amounts of tectonic shortening. We use techniques such as line length balancing, restorability, and the viability of the progressive development of the structures to construct balanced cross sections across the backthrust belt and Altiplano. The cross sections are taken to a regional depth of detachment (basement) to examine the relationship between mapped surface structures and inferred subsurface structures. The relationship of the backthrust belt to the Altiplano suggests that the Altiplano basin is a crustal‐scale piggyback basin created as a basement megathrust propagated up and over a half‐crustal scale ramp located just west of the physiographic boundary of the Eastern Cordillera. This basement megathrust was the means by which a narrow Paleocene fold‐thrust belt located to the west of the Altiplano propagated eastward and emerged in the present Eastern Cordillera. The relationship between the basement thrusts and the physiographic boundaries of the Central Andean plateau (as defined by Isacks [1988]) suggests that extensive megathrust sheets (involving crystalline basement or quartzite) may play an important role in the formation of orogenic plateaus. The kinematic development of the Andean fold‐thrust belt indicates that the backthrust belt developed as a taper‐building mechanism after the basement megathrust overextended the system eastward. The mechanism proposed in this study for the development of the central Andean backthrust belt requires a minimum of 200 km of shortening within the Altiplano/Eastern Cordillera alone. This increases minimum shortening estimates across the fold‐thrust belt in Bolivia to as much as 300–340 km.
Miocene unroofing of the Canyon Range during extension along the Sevier Desert Detachment, west central Utah
Authors: Stockli Daniel F. , Linn Jonathan K. , Walker J. Douglas , Dumitru Trevor A.
Published: February 2004

Apatite fission track results from Neoproterozoic and Lower Cambrian quartzites collected from the Canyon Range in west central Utah reveal a significant early to middle Miocene cooling event (∼19–15 Ma). Preextensional temperatures estimated from multicompositional apatite fission track data suggest ∼4.5 to >5.6 km of unroofing during the early to middle Miocene, assuming a geothermal gradient of ∼25°C/km. The spatial distribution of these preextensional temperatures indicates ∼15°–20° of eastward tilting of the Canyon Range during rapid extensional unroofing along a moderately west dipping detachment fault (∼35°–40°). We interpret this fault to be the breakaway of the Sevier Desert Detachment fault (SDD), the existence of which has been contested. The new thermochronologic data presented in this study provide compelling evidence for the existence of the SDD and thus the general viability of low‐angle detachment faulting. The data directly date the onset of extensional faulting along the SDD starting at ∼19 Ma and constrain the fault slip rate in the SDD breakaway zone at 2.4–2.1 mm/yr between ∼19 and 15 Ma. An early Miocene apatite fission track age obtained from a Proterozoic clast from the Tertiary Oak City Formation confirms that these conglomerates were deposited in a synextensional basin in the hanging wall of the SDD. The timing of tectonic unroofing of the Canyon Range in response to faulting along the SDD appears to be synchronous with large‐magnitude extension along the Snake Range décollement and with early extension along the Cave Canyon detachment exposed in the Mineral Mountains, pointing to widespread east‐west extension in the eastern Great Basin in the early and middle Miocene.
Crustal reworking at Nanga Parbat, Pakistan: Metamorphic consequences of thermal‐mechanical coupling facilitated by erosion
Authors: Zeitler Peter K. , Koons Peter O. , Bishop Michael P. , Chamberlain C. Page , Craw David , Edwards Michael A. , Hamidullah Syed , Jan M. Qasim , Khan M. Asif , Khattak M. Umar Khan , Kidd William S. F. , Mackie Randall L. , Meltzer Anne S. , Park Stephen K. , Pecher Arnaud , Poage Michael A. , Sarker Golam , Schneider David A. , Seeber Leonardo , Shroder John F.
Published: February 2004

Within the syntaxial bends of the India‐Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.
The two main steps of the Atlas building and geodynamics of the western Mediterranean
Authors: Frizon de Lamotte Dominique , Saint Bezar Bertrand , Bracène Rabah , Mercier Eric
Published: February 2004

The Atlas system (Morocco, Algeria, and Tunisia) constitutes an important morphologic barrier fringing the Sahara platform. Its structural style changes along strike from a thick‐skinned style in Morocco to a thin‐skinned one in Algeria and Tunisia. The position relative to the Tell‐Rif system is also different in eastern Algeria and Tunisia where the two systems are adjacent and in western Algeria and Morocco where they are separated by large rigid cores (Moroccan Meseta and Algerian High Plateaux). New data, as well as a reappraisal of available data, show that the Atlas build up occurred everywhere during two main phases of late Eocene and Pleistocene‐lower Quaternary age, respectively. These phases are clearly distinct and do not represent end points of a progressive deformation. An additional Tortonian event exists in the eastern region where the Tell‐Rif is thrusting directly over the Atlas. From Oligocene to middle Miocene the development of the Tell‐Rif accretionary prism is coeval to subduction rollback of Maghrebian Tethys lithosphere and related to the opening of the western Mediterranean Sea. For kinematic and chronological reasons this process cannot account for the two specific steps of the Atlas building. They are better explained assuming that they record two jolts in the convergence of Africa with respect to Europe and correspond roughly to the initiation and the cessation of the subduction processes active in the western Mediterranean region.
Abrupt strike‐slip fault to subduction transition: The Alpine Fault‐Puysegur Trench connection, New Zealand
Authors: Lebrun Jean‐Frédéric , Lamarche Geoffroy , Collot Jean‐Yves , Delteil Jean
Published: February 2004

Swath bathymetry and other geophysical data collected over the Fiordland Margin, southwest of New Zealand are used to investigate the mechanism of transform‐subduction transition between the Alpine Fault and the Puysegur Trench, two segments of the Pacific‐Australian plate boundary. In this region the Cenozoic Southeast Tasman Basin, which obliquely underthrusts Fiordland at the Puysegur Trench, is separated from the Cretaceous Tasman Basin by the Resolution Ridge System, a major lithospheric discontinuity of the downgoing plate. Interpretation of seafloor morphology shows that the Alpine Fault extends offshore along the Fiordland coast and splits into West and East Branches. The West Branch cuts obliquely across the margin and connects sharply to the Puysegur subduction front at the northeastern tip of the Resolution Ridge System. Earthquake and seismic reflection data indicate that the West Branch is genetically controlled by downgoing plate structures associated with the Resolution Ridge System. Hence the West Branch is interpreted as the surface trace of the plate boundary segment extending between the Alpine Fault and the Puysegur Trench. We conclude that the development of the strike‐slip segment of the plate boundary and its sharp transition to the Puysegur subduction are controlled by inherited structures of the Australian plate. Furthermore, according to geophysical data presented here, a tearing of the downgoing plate can be interpreted beneath the West Branch. A review of geophysical data along the region of the Alpine Fault‐Hikurangi Trough, northeast New Zealand, shows a progressive transform‐subduction transition that is accommodated by motion partitioning between the subduction interface and strike‐slip faults. This transition is accounted for by an interplate coupling that progressively increases toward the Alpine Fault in relation with a gradual thickening of the downgoing crust. The comparison between the Fiordland and the Hikurangi strike‐slip‐subduction transitions show that presence of inherited downgoing plate crustal faults, properly oriented with respect to the plate motion, facilitates a sharp strike‐slip‐subduction transition.
Collision tectonics in the Swiss Alps: Insight from geodynamic modeling
Authors: Pfiffner O. A. , Ellis S. , Beaumont C.
Published: February 2004

This paper compares results from two‐dimensional finite element dynamic modeling with the kinematic evolution of the Swiss Alps during the collision phase. In particular, we investigate the role of inherited lateral strength heterogeneities on orogenesis. A number of first‐order characteristics are directly comparable at crustal scales. In the models the entry of continental crust into the convergent margin marks the end of near‐perfect subduction. Accretion of material of the subducting plate to the upper plate creates an orogenic wedge on the incoming (pro)side and initiates a retroshear zone (or model backthrust). The addition of material to the upper plate builds a bivergent orogen. Heterogeneities in the pro‐crust focus shear and lead to the development of “nappe structures” The combined action of pro‐shear (nappe stacking) and retroshear (backthrusting) uplifts a plug between the two shear zones. Subsequent focusing of shear along the retroshear zone results in rotation of the plug and overlying units, leading to crustal‐scale backfolds as observed in the Swiss Alps. The model experiments predict features relevant to Alpine dynamics, including (1) similar crustal thicknesses and exhumation patterns to those observed in the Swiss Alps today for erosion rates comparable to natural ones (1 mm yr−1), (2) continued accretion and subduction of upper crustal fragments allowing high‐pressure metamorphic conditions, (3) tilting and exhumation of lower crust when a midcrustal weak zone is present, and (4) “shunting” of material across the strong lower crustal wedge of the upper plate.
Critical shear stress for incipient motion of sand/gravel streambeds
Authors: Shvidchenko Audrey B. , Pender Gareth , Hoey Trevor B.
Published: February 2004

Results of an experimental study of the incipient motion of streambeds composed of sand/gravel sediment mixtures are reported and compared with the earlier findings for uniform sediments. The experiments were conducted in an 8 m long by 0.30 m wide glass‐walled tilting flume and an 18 m long by 0.80–1.10 m wide trapezoidal concrete channel. A reference transport method is used to define the beginning of bed material movement. The experiments demonstrate that the incipient motion of individual size fractions within a mixture is controlled by their relative size with respect to median size (intergranular effects), mixture standard deviation (effect of the shape of grain‐size distribution), absolute value of median size (absolute size effect), and bed slope (effect of relative depth on overall flow resistance). The shear stress at incipient motion of median‐sized grains in mixtures is found to be the same as for uniform sediment of this size. The present findings are consistent with available flume and field data. A technique for calculating the critical shear stress of different grain sizes in coarse uniform sediments and unimodal/weakly bimodal sediment mixtures is proposed.
Similitude applied to centrifugal scaling of unsaturated flow
Authors: Barry D. A. , Lisle I. G. , Li L. , Prommer H. , Parlange J.‐Y. , Sander G. C. , Griffioen J. W.
Published: February 2004

Centrifuge experiments modeling single‐phase flow in prototype porous media typically use the same porous medium and permeant. Then, well‐known scaling laws are used to transfer the results to the prototype. More general scaling laws that relax these restrictions are presented. For permeants that are immiscible with an accompanying gas phase, model‐prototype (i.e., centrifuge model experiment–target system) scaling is demonstrated. Scaling is shown to be feasible for Miller‐similar (or geometrically similar) media. Scalings are presented for a more general class, Lisle‐similar media, based on the equivalence mapping of Richards' equation onto itself. Whereas model‐prototype scaling of Miller‐similar media can be realized easily for arbitrary boundary conditions, Lisle‐similarity in a finite length medium generally, but not always, involves a mapping to a moving boundary problem. An exception occurs for redistribution in Lisle‐similar porous media, which is shown to map to spatially fixed boundary conditions. Complete model‐prototype scalings for this example are derived.
Overview of inert tracer experiments in key belgian soil types: Relation between transport and soil morphological and hydraulic properties
Authors: Vanderborght J. , Vanclooster M. , Timmerman A. , Seuntjens P. , Mallants D. , Kim D.‐J. , Jacques D. , Hubrechts L. , Gonzalez C. , Feyen J. , Diels J. , Deckers J.
Published: February 2004

To investigate relations between solute transport, soil properties, and experimental conditions, we summarize results from leaching experiments that we carried out in a range of soils, at different scales (column (0.3–1.0 m ID, 1.0 m length) and field plot scale), and using different leaching rates (0.5–30 cm d−1). The lateral mixing regime and longitudinal dispersion were derived from time series of tracer concentrations at several depths in the soil. Field‐ and column‐scale transport were similar in loam and silt loam soils. The mixing regime was related to soil morphological features, such as vertical tongues, stratification, macropores, and a water‐repellent layer. The dispersion increased in all soils more than linearly with increasing leaching rate, implying that the dispersivity is not an intrinsic soil characteristic. The change of dispersivity with leaching rate was linked to the unsaturated hydraulic conductivity using a multidomain conceptualization of the pore space.
Three‐dimensional numerical inversion of pneumatic cross‐hole tests in unsaturated fractured tuff: 1. Methodology and borehole effects
Authors: Vesselinov Velimir V. , Neuman Shlomo P. , Illman Walter A.
Published: February 2004

We describe a three‐dimensional numerical inverse model for the interpretation of cross‐hole pneumatic tests in unsaturated fractured tuffs at the Apache Leap Research Site (ALRS) near Superior, Arizona. The model combines a finite volume flow simulator, FEHM, an automatic mesh generator, X3D, a parallelized version of an automatic parameter estimator, PEST, and a geostatistical package, GSTAT. The tests are simulated by considering single‐phase airflow through a porous continuum, which represents primarily interconnected fractures at the site. The simulator solves the airflow equations in their original nonlinear form and accounts directly for the ability of all packed‐off borehole intervals to store and conduct air through the system. Computations are performed in parallel on a supercomputer using 32 processors. We analyze pneumatic cross‐hole test data, previously conducted by our group at ALRS, in two ways: (1) by considering pressure records from individual borehole monitoring intervals one at a time, while treating the rock as being spatially uniform, and (2) by considering pressure records from multiple tests and borehole monitoring intervals simultaneously, while treating the rock as being randomly heterogeneous. The first approach yields a series of equivalent air permeabilities and air‐filled porosities for the rock volume being tested, having length scales of the order of meters to tens of meters. The second approach yields a high‐resolution geostatistical estimate of how air permeability and air‐filled porosity, defined on grid blocks having a length scale of 1 m, vary spatially throughout the tested rock volume. It amounts to three‐dimensional pneumatic “tomography” or stochastic imaging of the rock, a concept originally proposed by one of us in 1987. The first paper of this two‐part series describes the field data, the model, and the effect of boreholes on pressure propagation through the rock. The second paper implements our approach on selected cross‐hole test data from ALRS.
Solute transport in heterogeneous formations of bimodal conductivity distribution: 2. Applications
Authors: Lessoff S. C. , Dagan G.
Published: February 2004

The theoretical results of Dagan and Lessoff [this issue] are applied to three types of media (Figure 1): horizontal lenses submerged in a homogeneous matrix, sparse cracks of random orientation in a matrix of contrasting permeability, and channels of high permeability at the surface of a homogeneous medium. These discrete features are modeled as sparse elliptical inclusions of arbitrary conductivity. The longitudinal macrodispersivity is determined by the methodology of part 1 as function of the parameters characterizing the medium: the conductivity ratio κ, the anisotropy ratio of the ellipsis e, the porosity ratio ϑ/ϑ0, and the volume fraction n ≪ 1 or the fracture number per unit volume. Unlike existing stochastic continuum solutions that are first order in the logconductivity variance, the model developed here applies for an arbitrary permeability variance. This is of great advantage in media with high conductivity contrasts between the matrix and the inclusions. Simple results are obtained for inclusions of low conductivity that lead to high macrodispersivity values that are underpredicted by the first‐order continuum approach. In contrast, the presence of thin and highly conductive cracks leads to a finite longitudinal macrodispersivity that depends mainly on their length and the number density. An attempt is made to compare the present approach with the numerical simulations of Desbarats [1990].
Generalized solution to multispecies transport equations coupled with a first‐order reaction network
Authors: Clement T. Prabhakar
Published: February 2004

A generalized solution methodology is presented for deriving analytical solutions to multispecies transport equations coupled with multiparent, serial, parallel, converging, diverging, and/or reversible first‐order reactions. The method is flexible for solving one‐, two‐, or three‐dimensional advection‐dispersion equations that are coupled with a set of first‐order reactions. However, a major limitation is that the method cannot be used for solving multispecies transport equations with different retardation factors. Mathematical steps are provided to illustrate how the general solution method can be derived using linear transformation principles and to show why the method would fail when the retardation factors are different. Derivations are also presented to demonstrate how the Sun et al. [1999b] method for solving sequential reactive transport problems, which was previously presented without any fundamental analysis, can be deduced from the general solution. The results of the presented solution scheme for a test problem compare well with the numerical solution computed using the RT3D code. The proposed methodology is useful for developing simple analytical models that can be used to perform screening simulations and can be used to test complex multispecies transport codes.
Quantifying the relative importance of flow regulation and grain size regulation of suspended sediment transport α and tracking changes in grain size of bed sediment β
Authors: Rubin David M. , Topping David J.
Published: February 2004

To predict changes in sediment transport, it is essential to know whether transport is regulated mainly by changes in flow or by changes in grain size of sediment on the bed. In flows where changes in suspended sediment transport are regulated purely by changes in flow (grain size of bed sediment is constant), increases in flow strength cause increases in both concentration and grain size of sediment in suspension (because stronger flows are able to suspend more sediment and coarser grains). Under this constraint of constant grain size of bed sediment concentration and median diameter of suspended sediment are positively correlated. In contrast, where transport is regulated purely by changes in grain size of sediment on the bed, concentration and median diameter of suspended sediment are negatively correlated (because increasing the median diameter of the bed sediment causes the concentration to decrease while causing the median grain size in suspension to increase). Where both flow strength and grain size on the bed are free to vary, the relation between concentration and grain size in suspension can be used to quantify the importance of grain size regulation relative to flow regulation of sediment transport, a measure defined as α. To predict sediment transport in systems that are regulated dominantly by changes in grain size on the bed, it is more useful to measure sediment input events or changes in grain size on the bed than to measure changes in flow. More commonly, grain size of bed sediment may be secondary to flow in regulating transport but may, nevertheless, be important. The relative coarseness of bed sediment (β) can be measured directly or, like α, can be calculated from measurements of concentration and grain size of suspended sediment.
Interaction between bed roughness and flow hydraulics in eroding rills
Authors: Giménez Rafael , Govers Gerard
Published: February 2004

Govers [1992] showed that the flow velocity in rills eroding loose, nonlayered materials could be predicted from knowledge of discharge only (without significant slope or soil effect). The objective of this paper is to investigate to what extent the observed slope independence of flow velocity in eroding rills can be explained by the interaction between rill bed roughness and flow hydraulics. In a laboratory study, two situations were compared: (1) rills which can freely erode a uniform soil layer and (2) rills with a fixed bed geometry. During the experiments, rill discharge and flow velocity were recorded. After each experiment, a detailed topographic survey of the rill bed was carried out using a laser scanner. From these data the main hydraulic variables (mean values of flow depth, wetted perimeter, and hydraulic radius) were estimated. The experiments confirmed the slope independence of rill flow velocities on mobile beds. When the bed is fixed, the flow velocity in rills is clearly slope‐dependent. The slope independence of flow velocity on mobile beds is due to a feedback between rill bed morphology and flow conditions. The roughness amplitude (which was assessed from the standard deviation of corrected height values) and the frequency of macroroughness elements (which was assessed by counting the number of peaks per unit length in a previously simplified longitudinal profiles) both increase and tend to counteract the effect of the increase in slope gradient on rill flow velocity. The final rill flow velocity appears to be characterized by a constant average Froude number.