bluuebunny/crossref_metadata_2025 huggingface dataset:
Simulations of the September 1987 lower thermospheric tides with the National Center for Atmospheric Research Thermosphere‐Ionosphere General Circulation Model
Authors: Fesen C. G. , Roble R. G.
Published: February 2008

The National Center for Atmospheric Research thermosphere‐ionosphere general circulation model (TIGCM) was used to simulate incoherent scatter radar observations of the lower thermosphere tides during the first Lower Thermosphere Coupling Study (LTCS) campaign, September 21–26, 1987. The TIGCM utilized time‐varying histories of the model input fields obtained from the World Data Center for the LTCS period. These model inputs included solar flux, total hemispheric power, solar wind data from which the cross‐polar‐cap potential was derived, and geomagnetic Kp index. Calculations were made for the semidiurnal ion temperatures and horizontal neutral winds at locations representative of Arecibo, Millstone Hill, and Sondrestrom. The diurnal tides at Sondrestrom were also simulated. Tidal inputs to the TIGCM lower boundary were obtained from the middle atmosphere model of Forbes and Vial (1989). The TIGCM tidal structures are in fair general agreement with the observations. The amplitudes tended to be better simulated than the phases, and the mid‐ and high‐latitude locations are simulated better than the low‐latitude thermosphere. This may indicate a need to incorporate coupling of the neutral atmosphere and ionosphere with the E region dynamo in the equatorial region to obtain a better representation of low‐latitude thermospheric tides. The model simulations were used to investigate the daily variability of the tides due to the geomagnetic activity occurring during this period. In general, the ion temperatures were predicted to be affected more than the winds, and the diurnal components more than the semidiurnal. The effects are typically largest at high latitudes and higher altitudes, but discernible differences were produced at low latitudes. Additional observational studies are clearly warranted to test and refine the model predictions.
An anomalous component of neptune radio emission: Implications for the auroral zone
Authors: Desch M. D. , Farrell W. M. , Kaiser M. L.
Published: February 2008

The Voyager planetary radio astronomy experiment detected a bursty, narrow‐band radio emission originating in Neptune's magnetosphere. The time of occurrence of nearly all of the episodes of this bursty radio emission can be explained on the basis of a radio source located just above and to the east of the south magnetic offset tilted dipole (OTD) tip (Farrell et al., 1990). However, several episodes of bursty emission do not occur at the usual frequency and planetary rotation phase for emissions of this type. We show that the occurrences of these rarely seen anomalous episodes are shifted systematically in planetary longitude so as to be consistent with a source of emission to the southwest of the southern magnetic OTD pole. Owing to the proximity of these sources to the magnetic polar region we associate them with an active auroral region. Therefore, at least from the standpoint of the radio emission, the picture that emerges is of an auroral zone with two emission hot spots approximately diametrically east and west of the south magnetic pole. The possibility of a complete radio‐active auroral oval is discussed.
Solar cycle variation in the subauroral electron temperature enhancement: Comparison of AE‐C and DE 2 satellite observations
Authors: Fok M.‐C. , Kozyra J. U. , Brace L. H.
Published: February 2008

The elevation of the subauroral electron temperature is one of the phenomena showing the energy transfer from the magnetosphere and the response of the ionosphere. The present study addresses solar cycle variations in the subauroral Te peak by comparing observations of the subauroral peak by the Atmosphere Explorer C (AE‐C) satellite near solar minimum (1974 and 1977) with similar observations by the Dynamics Explorer 2 (DE 2) satellite near solar maximum (1981–1982). Electron temperature peaks with magnitudes sufficient to produce observable stable auroral red (SAR) arc emissions occurred more frequently during solar maximum than in solar minimum but the variation in the magnitudes and positions of these peaks with magnetic activity did not change significantly with solar cycle. These results will be discussed in terms of the solar cycle changes in the ionosphere and the magnetospheric energy source.
Rocket observation of equatorial plasma bubbles over Natal, Brazil, using a high‐frequency capacitance probe
Authors: Abdu M. A. , Muralikrishna P. , Batista I. S. , Sobral J. H. A.
Published: February 2008

In situ measurement of electron density height profile of the equatorial nighttime ionosphere, under a developing spread F event, was carried out using a dual mode high frequency capacitance (HFC) probe that was flown on board a SONDA‐III rocket launched from Natal on December 11, 1985. This represents the first measurement of plasma bubble characteristics using an HFC probe. A series of plasma bubbles in varying degrees of their growth phase was detected mostly during the upleg of the flight. A somewhat detailed discussion, and comparative studies, of the measurements at the two probe frequencies (6.17 MHz and 11.75 MHz) are presented in this paper. Among the important findings from the present study is an evidence of electron temperature enhancement in the plasma bubble possibly caused by electron heating from energetic particle precipitation in the South Atlantic magnetic anomaly.
On rotational discontinuities in both two‐fluid and hybrid models
Authors: Wu C. C. , Hada T.
Published: February 2008

Rotational discontinuities are studied in a two‐fluid model that includes finite ion inertia dispersion and in a hybrid model in which the full ion dynamics is retained while the electrons are treated as a massless fluid. We show that as in previous dissipative MHD studies, a rotational discontinuity is unstable in both models and evolves to a 2 → 3 intermediate shock, a slow rarefaction wave, and other waves. (However, due to the limitation in computational resources, the hybrid calculations are limited to low‐to‐medium ion beta.) In addition, we show that the so‐called Walén relation, which holds exactly for rotational discontinuities, can also be well satisfied by tran‐Alfvénic intermediate shocks. Thus intermediate shocks can be candidates for those observed structures that satisfy the Walén relation.
Experimental observations of patchy reconnections associated with the three‐dimensional tearing instability
Authors: Gekelman W. , Pfister H. , Kan J. R.
Published: February 2008

Experiments on tearing of a thin current sheet are performed in a collisionless laboratory plasma. The tearing of the finite‐dimension current sheet (Lx = 40 cm, Ly = 100 cm, Lz ≈ 1.3 cm) is fully three‐dimensional (3‐D). The 3‐D tearing is observed to lead to patchy reconnection. The wavelengths of the most unstable mode of the 3‐D tearing satisfy the scaling relationship: λy ≈ λx ≈ 30 Lz. The e‐folding time of the 3‐D tearing is about 50 percent longer than that of the 2‐D tearing: τ(3‐D)/τ(2‐D) ≈ 1.6. The sites of the patchy reconnections are centered on the X line segments which are located in the current density minima. The experimental results are consistent with the observational model proposed by Russell and Elphic (1979) for the reconnection at the dayside magnetopause, known as the elbow‐shaped flux transfer events (FTEs). The results also support the theoretical idea that elbow‐shaped FTEs can be produced by the 3‐D tearing, resulting in patchy and intermittent reconnections driven by the solar wind as proposed by Kan (1988).
On the electrodynamical state of the auroral ionosphere during northward interplanetary magnetic field: A Transpolar Arc Case Study
Authors: Marklund G. T. , Blomberg L. G. , Murphree J. S. , Elphinstone R. D. , Zanetti L. J. , Erlandson R. E. , Sandahl I. , de la Beaujardière O. , Opgenoorth H. , Rich F. J.
Published: February 2008

The ionospheric electrodynamical state has been reconstructed for a transpolar arc event during northward interplanetary magnetic field conditions. An extensive set of observations by Viking and other satellites and by ground‐based radars has been used to provide realistic model input data or to verify the modeling results. The resulting convection pattern is found to be consistent with the Viking electric field and intimately linked to the prevalent auroral distribution. It is characterized by a large evening cell, well extended across noon and split up by two separated potential minima, and a minor crescent‐shaped morning cell. The convection signatures are found to vary a lot along the transpolar arc depending on the relative role of the arc‐associated convection and the ambient convection. The transpolar arc is generally embedded in antisunward convective flow except near the connection points with the auroral oval, where sunward flow exists in localized regions.
The collision zone between the North d'Entrecasteaux Ridge and the New Hebrides Island Arc: 2. Structure from multichannel seismic data
Authors: Fisher Michael A. , Collot Jean‐Yves , Geist Eric L.
Published: February 2008

The d'Entrecasteaux zone (DEZ) collides with the central New Hebrides island arc and consists of two subparallel ridges that strike east‐west, stand 1–2 km above the surrounding oceanic plate, and subduct obliquely (15°) northward beneath the arc. Rocks dredged from the north ridge as well as reflections evident in multichannel seismic reflection data indicate that this ridge has a volcanic origin. Crystalline volcanic rocks are common along the lower flank of the ridge, but sedimentary, probably volcaniclastic, rock caps the ridge. Seismic reflection data collected over the lower arc slope reveal that mass wasting deposits locally make up most of the accretionary wedge. These deposits appear to form discrete bodies, suggesting that mass wasting occurred episodically. Large anticlines and thrust faults having large vertical separation are not readily evident where the colliding ridge intersects the arc slope; apparently, slope rocks have low strength so that mass wasting deposits formed instead of large‐relief structures. Mass wasting is thought to occur as the accretionary wedge is uplifted in response to the northward oblique subduction of the north ridge. The toe of the north ridge flank marks an abrupt transition in the lithologies that make up the footwall of the interplate decollement. Footwall lithologies change from ocean basin to volcaniclastic ridge material, and this transition probably marks a discontinuity in friction along the decollement or in rock mechanical properties because north of the transition, thrust faults deform the accretionary wedge whereas south of the transition, steep reverse faults crosscut the wedge and pierce the north flank of the ridge. This piercement means that the decollement at least locally lies within the ridge and that ridge material exotic to the New Hebrides arc may be incorporated into the accretionary wedge.
Geodesy by radio interferometry: Water vapor radiometry for estimation of the wet delay
Authors: Elgered G. , Davis J. L. , Herring T. A. , Shapiro I. I.
Published: February 2008

An important source of error in very‐long‐baseline interferometry (VLBI) estimates of baseline length is unmodeled variations of the refractivity of the neutral atmosphere along the propagation path of the radio signals. We present and discuss the method of using data from a water vapor radiometer (WVR) to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations. Data from different WVRs are compared with estimated propagation delays obtained by Kalman filtering of the VLBI data themselves. The consequences of using either WVR data or Kalman filtering to correct for atmospheric propagation delay at the Onsala VLBI site are investigated by studying the repeatability of estimated baseline lengths from Onsala to several other sites. The lengths of the baselines range from 919 to 7941 km. The repeatability obtained for baseline length estimates shows that the methods of water vapor radiometry and Kalman filtering offer comparable accuracies when applied to VLBI observations obtained in the climate of the Swedish west coast. For the most frequently measured baseline in this study, the use of WVR data yielded a 13% smaller weighted‐root‐mean‐square (WRMS) scatter of the baseline length estimates compared to the use of a Kalman filter. It is also clear that the “best” minimum elevation angle for VLBI observations depends on the accuracy of the determinations of the total propagation delay to be used, since the error in this delay increases with increasing air mass. For use of WVR data along with accurate determinations of total surface pressure, the best minimum is about 20°; for use of a model for the wet delay based on the humidity and temperature at the ground, the best minimum is about 35°.
Structural Analysis of Late Neogene Deformation in the Central Offshore Santa Maria Basin, California
Authors: Clark D. H. , Hall N. T. , Hamilton D. H. , Heck R. G.
Published: February 2008

Structural analysis of the Queenie structure, a topographically prominent, southwest vergent asymmetric fold located 35 km west of Point Sal, constrains the timing and nature of late Neogene deformation in a transect across the central offshore Santa Maria Basin. Analysis of post‐Miocene sediments mantling the fold indicates that the Queenie structure was formed in a relatively brief episode of NE‐SW directed shortening between 5 and 3 Ma, apparently in response to the onset of compression normal to the North American/Pacific plate boundary. Retrodeformation of depth‐corrected cross sections constructed from an extensive set of common depth point seismic data demonstrates that the Queenie structure overprints a Miocene extensional basin and is a fault propagation fold that formed as compression changed the normal slip on the basin margin shear zone to reverse slip. Geometric constraints and seismic data suggest that the controlling shear zone dips moderately (30° to 50°) to the northeast and is essentially nonlistric to the base of the brittle crust at a depth of about 12 km. Structural analysis of cross sections further shows that average total post‐Miocene NE‐SW shortening across the Queenie structure is about 3 % (0.39 km across the 13‐km‐wide zone of folding) and that related shortening across the 40‐km‐wide central reach of the offshore Santa Maria Basin between the Hosgri fault and the Santa Lucia Bank fault during the same period is roughly 1–2% (0.4–0.8 km). High‐resolution seismic reflection data show small folds (structural relief less than 10% of the primary Queenie structure) in post‐early/late Pliocene sediments on the southwestern flank of the Queenie structure. These folds exhibit minor coaxial deformation along the flank of the structure, suggesting that very low rates of tectonic folding (of the order of 0.005 mm shortening per year) may continue to the present. The transient nature of the Pliocene compressional event that formed the primary Queenie structure and the relatively low rates of recent seismic activity in the central offshore Santa Maria Basin suggest that postearly/late Pliocene interplate shortening at this latitude has been largely accommodated outside the basin.
Sequence stratigraphy: Eustasy or tectonic imprint?
Authors: Aubry Marie‐Pierre
Published: February 2008

Sequence stratigraphy is viewed by many geologists as the expression in the stratigraphic record of the history of sea level changes. Sequence boundaries are its most distinctive elements and are used for global correlations. Until now, correlations between unconformities and postulated lowerings of sea level have been seen primarily in terms of cause and effect relationships, the postulated fall providing an explanation for the presence of an unconformity. They have not been truly perceived in terms of stratigraphic correlations which would imply the establishment of synchrony between the cause and the effect. To decipher the presence of a global sea level signal in the stratigraphic record, the synchrony of sea level related unconformities in widely separated basins must be established, which can be achieved by comparing the degree of overlap between hiatuses in different stratigraphic sections. Outcrop sections, wells, and the deep‐sea record reveal an apparently worldwide unconformity(ies) across the lower/middle Eocene boundary, characterized by intensive reworking and/or relatively rapid shallowing in some areas (e.g., Cyrenaica, NE Libya; California; Egypt). The lower‐middle Eocene intervals in sections on the North Atlantic margins have been correlated through magnetobiostratigraphy, and the durations of the hiatuses have been precisely determined. This record is compared with that in sections in other basins where shallowing and intensive reworking occur. This study shows that there are two groups of unconformities and associated hiatuses around the lower/middle Eocene boundary. The older hiatuses, in the latest early Eocene, are generally short, even on the shelf (∼1 m.y.); the younger hiatuses, in the early middle Eocene, are somewhat longer (>2 m.y.). The stratigraphic record around the lower/middle Eocene boundary suggests that there may be two mechanisms acting simultaneously to produce unconformities. Unconformities of a given age on the shelf may result from a global fall in sea level, while their stratigraphically correlative unconformities on the slope and rise may result from tectonic instability. Unconformities related to the third‐order cycles in sea level changes in Vail's model primarily may reflect tectonoeustasy.
Tertiary ice sheet dynamics: The Snow Gun Hypothesis
Authors: Prentice M. L. , Matthews R. K.
Published: February 2008

We observe strong negative correlation between Tertiary low‐ to mid‐latitude planktonic foraminiferal δ18O and the difference between these data and coeval benthic foraminiferal δ18O. Late Quaternary data do not show this correlation. Coupling statistical model/δ18O comparisons and evidence for Antarctic ice and ocean temperature variation, we infer that Tertiary ice volume, recorded by tropical planktonic δ18O, increased as the deep ocean warmed. Because the isotopic signatures of deepwater temperature variation and ice volume change were of opposite sign, the sum of these signals in Tertiary benthic δ18O became lost in the noise. This renders low correlation between Tertiary planktonic and benthic δ18O time series compared to late Quaternary data. We contend that Tertiary ice sheet growth was commonly driven by warming of deep water from low‐ to mid‐latitude marginal seas (snow gun hypothesis). In contrast, late Quaternary ice sheets grew as deep water, formed at high latitude, cooled. Because tectonic forcing and orbital forcing at low‐latitude primarily controlled production and temperature variations of this Warm Saline Deep Water, these influences largely dictated Tertiary ice volume fluctuations. Through the Tertiary, we infer ice volume fluctuations to be an important component of sea level history on timescales between 103 and 107 years.
Structure and early history of the Labuan Basin South Indian Ocean
Authors: Rotstein Yair , Munschy Marc , Schlich Roland , Hill Peter J.
Published: February 2008

Recent multichannel seismic reflection data from the Labuan Basin, in the Southern Indian Ocean, are used to reevaluate older, single channel data from this region. Together, they throw light on the structure and evolution of this basin, situated between the older than 100 Ma Southern Kerguelen Plateau and the younger than 43 Ma Australian‐Antarctic Basin. The Labuan Basin is a deep, extensive basement depression, more than 350,000 km2 in area, located adjacent to the eastern margin of the Southern Kerguelen Plateau. In contrast to the nearby Kerguelen Plateau, no prominent reflectors are observed within the basement of the Labuan Basin. This suggests a different mechanism of formation from that of the Kerguelen Plateau, which is believed to have been formed by thick sequences of large lava flows. The basement surface of most of the Labuan Basin is presently quite rough, as the result of a tectonic event which created prominent tilted block structures and turned it into a large northwest‐southeast trending syncline. There is a 1–1.5 km elevation difference between the Labuan Basin and the Australian‐Antarctic Basin created at the Southeast Indian Ridge, as well as a large difference in sedimentary thickness between them, indicating that the Labuan Basin is significantly older. The boundary of the Labuan Basin with the Kerguelen Plateau is generally a steep and somewhat linear feature which appears to be of tectonic origin: it seems to result from two extensive tectonic episodes, dated at 96 Ma and 75–68 Ma, associated with the prerift phase of plate breakup between Australia and Antarctica. The basement of the Labuan Basin was created 130–100 Ma ago at about the same time than the Kerguelen Plateau. An extensive tectonic episode, to the south of the Labuan Basin, initiated the formation of the boundary between the Southern Kerguelen Plateau and the Labuan Basin. This tectonic episode could be linked to the beginning of seafloor spreading between Australia and Antarctica 96 Ma ago. Sediments were regularly deposited in the basin until 75–68 Ma, at which time an important extensional tectonic event occurred. It involved two large northwest‐southeast uplifts; one, centered on the Kerguelen Plateau, affected the western part of the Labuan Basin while the other, of which only the western half is observed, affected the eastern part of the Labuan Basin. This tectonic episode seems to correspond to the prerifting episode leading to the breakup between the Kerguelen Plateau‐Labuan Basin and Broken Ridge‐Diamantina Zone at 43 Ma. After this tectonic episode, the sediments of the postuplift megasequence filled topographic lows formed by the normal faults and the tilted blocks. The Diamantina Zone, which prior to 43 Ma was continuous with the Labuan Basin, can be expected to have a similar tectonic history. In particular, the rough basement of the Diamantina Zone can be due to postformation tectonism rather than to slow spreading as previously suggested.
Distribution of geodetic shear strains across the Alpine Fault Zone: Haast, New Zealand
Authors: Pearson Chris
Published: February 2008

Geodetic shear strains are determined for a triangulation network in the southwest corner of the South Island of New Zealand, which extends from Haast Pass to the west coast across south Westland, a distance of about 50 km. The Alpine fault crosses the network. In order to examine the variation of maximum (engineering) shear strain as a function of distance across the plate boundary zone, I have reduced the observations into a series of small networks which span at least half of the actively deforming part of the Pacific‐Australian plate boundary. The azimuth of the principal axis of relative compression has a relatively constant value of 105° across the plate boundary, which is consistent with strike‐slip movement on this part of the Alpine fault. The Maximum (engineering) shear strains have a relatively constant value of about 0.5 μrad/yr over most of the study area; however, two localized high strain anomalies, both with peak strains of 1.2 to 1.4 μrad/yr, were observed. One of these appears to be restricted to a localized zone, a few kilometers in extent adjacent to the Alpine fault, while the second exists 30 km to the east of the Alpine fault near Haast Pass. The spatial extent of the second anomaly cannot be determined. I have modeled the anomaly using a dislocation model for a dipping strike‐slip fault I can produce a reasonable fit between this simple model and the observed strains if I assume that the brittle‐ductile transition is shallow (about 4 km deep) and that the slip rate on the Alpine fault is 20 mm/yr. I cannot match the observed strains if the slip rate equals the 40 mm/yr relative Pacific‐Australian plate velocity; however, suggesting that only about half the relative plate motion is accommodated on the Alpine fault. This model does not match strains of 0.5 μrad/yr which persist 10 km west of the Alpine fault or the high strain zone observed near Haast Pass which suggests that a significant amount of the extra deformation is accommodated near the Alpine fault.
A Cretaceous pole from south China, and the Mesozoic hairpin turn of the Eurasian apparent polar wander path
Authors: Enkin Randolph J. , Chen Yan , Courtillot Vincent , Besse Jean , Xing Lisheng , Zhang Zhenhai , Zhuang Zhonghai , Zhang Jingxin
Published: February 2008

To contribute to the apparent polar wander path (APWP) of the South China Block and Eurasia in general, we collected paleomagnetic samples from Mesozoic red beds around the city of Ya' an (30°N, 103°E) in the western tip of the Sichuan Basin. In this paper we present the results from 373 oriented cores taken from one section representing 3 km of sedimentary rocks. The section is dated with continental ostracods and with a magnetostratigraphic correlation between a densely sampled 272‐m sequence and the polarity time scale, giving an upper Jurassic to Upper Cretaceous or Lower Tertiary age. The remanent direction is remarkably stable throughout the section (D=2.0°, I=34.2°, k=63.1,α95=3.6°, N=26/28 sites). While this fact might suggest that the section has been remagnetized, paleomagnetic and rock magnetic tests indicate that the remanence is primary. The pole position (78.6°N, 273.4°E, dp=2.4°, dm=4.1°) corresponds to a rather low paleolatitude (λ=18.8°±2.4°) but is consistent with other Cretaceous poles from China. If one accepts the Eurasian APWP of Irving and Irving (1982), this result would imply that more than 1000 km of shortening took place between South China and Eurasia, following the acquisition of the remanence. However, there is no geological evidence for this large shortening. We propose that the remanence was acquired within the time corresponding to the tip of the hairpin turn (∼150–50 Ma) in the revised APWP of Besse and Courtillot (this issue). The local geology suggests that the syncline from which the samples were taken has been rotated by 15°±5° counterclockwise, which is reflected in a similar discrepancy between the measured paleodeclination and that predicted by the Besse and Courtillot (this issue) Eurasian APWP. After correcting for this rotation, the pole position is 70.9°N, 225.2°E (dp=2.4°, dm=6.5°). We conclude that Eurasia was fully assembled by the end of the Jurassic and that the Mesozoic Eurasian hairpin turn is a real feature.
The collision zone between the North d'Entrecasteaux Ridge and the New Hebrides Island Arc: 1. Sea Beam morphology and shallow structure
Authors: Collot Jean‐Yves , Fisher Michael A.
Published: February 2008

Sea Beam bathymetric data, closely spaced single‐channel seismic reflection sections, and geopotential field data were collected aboard the R/V J. Charcot over the collision zone between the North d'Entrecasteaux Ridge (NDR) and the New Hebrides island arc. The NDR trends east at a small angle (14°) to the plate convergence direction so that the ridge creeps northward along the trench at about 2.5 cm/yr, requiring a continual structural adjustment of the accretionary complex. In this report we study the shallow structure, tectonic erosion, and consequent mass wasting of the accretionary complex, all of which were produced by the collision of the NDR in a complex deformational environment. The accretionary complex in this collision zone can be divided into northern, central, and southern parts. The northern part of the collision zone, which lies north of the leading flank of the ridge, shows a lobate lower accretionary complex that is structured by east to southeast dipping thrust and reverse faults and N60°W trending strike‐slip faults; a middle accretionary complex that bulges seaward and appears to be rotated 45°E from the regional arc trend; and an upper accretionary complex that is shaped by slumps and a canyon network. The central part of the collision zone, which directly overlies the crest of the ridge, forms a broad shallow protrusion that is bounded on its northern and southern sides by steep scarps. Rocks forming this protrusion have been uplifted, possibly by as much as 1500–2500 m, and tilted to the north, causing northward block sliding along an extensional detachment surface. The southern part of the collision zone, which lies in the wake of the ridge, is deformed by large slumps and normal faults that trend parallel to the ridge axis. Rocks forming this southern part collapse, causing widespread mass wasting. These geophysical data indicate that normal trench convergence has apparently produced only a few trench‐parallel structures confined to the toe of the accretionary complex, whereas the along‐trench motion of the NDR has resulted in well‐developed structures that extend obliquely across the arc slope and form an asymmetric tectonic pattern. The area of the accretionary complex that is disturbed by collision appears to be about twice as wide as the width of the NDR, and the time required to heal the disturbance may be about 0.8 m.y.
Growth of natural dams by deposition from steady supersaturated shallow flow
Authors: Wooding R. A.
Published: February 2008

The frequent occurrence in nature of dams or terraces deposited from supersaturated flows prompted the hypothesis that wavelike but quasi‐steady solid profile shapes may develop after sufficient time has elapsed. Dressler's (1978) approximate equations are used to describe steady fluid flow over a curved accreting surface, including a Chezy term to represent flow resistance. The case of a transition from subcritical to supercritical flow is treated, which leads to solutions for dams but excludes terraces. It is assumed that deposition is controlled principally by the diffusive resistance of the laminar sublayer, the thickness of which is almost inversely proportional to the flow velocity, lag effects being neglected on the assumption of large length scale/depth ratios. From the equations, the length scale of both the fluid flow and the solid profile is fixed by flow depth, and is relatively unaffected by slow temporal variations of the degree of supersaturation. The depth is defined by an upstream boundary condition on flow rate. This is taken constant as the surface grows, so that for selfsimilarity the profile moves in uniform translation. The direction of growth (“growth angle”) of the profile is expected to be nonhorizontal and is not known a priori. Here it is determined by the requirements of the geometrical boundary condition needed to satisfy the quasisteady growth assumption. The profile shape can be calculated from predetermined conditions, e.g., known values of the Froude number and drag coefficient. In applications involving the inverse problem, theoretical profiles have been fitted to photographs of natural dams with apparent success, and values of the Froude number and drag coefficient have been deduced. The study of these systems may be relevant to problems of mineral deposition.
The rotation of LAGEOS
Authors: Bertotti B. , Iess L.
Published: February 2008

In view of the need of an accurate modelling of nongravitational forces on laser‐tracked satellites, it is important to understand their rotational dynamics, which determines the temperature anisotropy and the ensuing radiation recoil effects. We propose a model of the torques acting on LAGEOS due to eddy currents and gravity gradient. The electromotive forces induced in the spacecraft by its rotation in the magnetic field of the Earth dissipate angular momentum and produce a precession of the spin axis; the oblate spacecraft will precess in the gravitational field of the Earth at a rate proportional to the rotation period. Therefore the gravitational torques become more and more important with time and eventually may produce a chaotic dynamics. The predicted evolution of the spin period agrees very well with the few experimental data available and corresponds to an approximately exponential growth rate of about 3 years.
Scattering wave energy propagation in a random isotropic scattering medium: 1. Theory
Authors: Zeng Yuehua , Su Feng , Aki Keiiti
Published: February 2008

In this paper we provide a complete formulation of scattered wave energy propagation in a random isotropic scattering medium. First, we formulate the scattered wave energy equation by extending the stationary energy transport theory studied by Wu (1985) to the time dependent case. The iterative solution of this equation gives us a general expression of temporal variation of scattered energy density at arbitrary source and receiver locations as a Neumann series expansion characterized by powers of the scattering coefficient. The first term of this series leads to the first‐order scattering formula obtained by Sato (1977). For the source and receiver coincident case, our solution gives the corrected version of high‐order formulas obtained by Gao et al. (1983b). Solving the scattered wave energy equation using a Fourier transform technique, we obtain a compact integral solution for the temporal decay of scattered wave energy which includes all multiple scattering contributions and can be easily computed numerically. Examples of this solution are presented and compared with that of the single scattering, energy flux, and diffusion models. We then discuss the energy conservation for our system by starting with our fundamental scattered wave energy equation and then demonstrating that our formulas satisfy the energy conservation when the contributions from all orders of scattering are summed up. We also generalize our scattered wave energy equations to the case of nonuniformly distributed isotropic scattering and absorption coefficients. To solve these equations, feasible numerical procedures, such as a Monte Carlo simulation scheme, are suggested. Our Monte Carlo approach to solve the wave energy equation is different from previous works (Gusev and Abubakirov, 1987; Hoshiba, 1990) based on the ray theoretical approach.
Petrology of the Keweenawan Mamainse Point lavas, Ontario: Petrogenesis and continental rift evolution
Authors: Klewin Kenneth W. , Berg Jonathan H.
Published: February 2008

The Mamainse Point Formation forms a 5250‐m‐thick sequence of flood basalts, ranging from high‐MgO picrites and basalts to basaltic andesites, that formed in the Keweenawan (1100 Ma) Midcontinent Rift of North America. This section is the most continuous and complete sequence of Keweenawan lavas and records almost the entire igneous history of the rift. The section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics. A variety of petrogenetic processes including partial melting of different sources at decreasing depth, periodically replenished, tapped, and fractionated magma chambers, crustal contamination, and polybaric fractionation were responsible for the evolution of the magmas. The dominant petrogenetic process changes with stratigraphy, which suggests that the physical evolution of the rift controlled which process operated at any time. The initial fracturing of the crust presumably allowed high‐MgO liquids generated at depth to be erupted. The early stage of rifting was apparently slow and allowed the formation of long‐lived magma chambers in which complex processes such replenishment/mixing and assimilation/contamination could occur. The late stage of rifting, however, was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted. These conclusions from the chemostratigraphy agree with the physical stratigraphy, which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation.
Equilibrium point defect concentrations in MgO: Understanding the mechanisms of conduction and diffusion and the role of Fe impurities
Authors: Hirsch L. M. , Shankland T. J.
Published: February 2008

By simultaneously solving the governing equations for defect formation and the relative conservation laws, we have made a general calculation of equilibrium concentrations of point defects in MgO as functions of temperature, oxygen fugacity (ƒO2), and Fe content. The aim of this work is to better understand the mechanisms that control the transport processes of electrical conduction and diffusion and the oxidation state of Fe. The present model suggests the following regimes that are consistent with several kinds of experiments. Electrical conduction is dominated at low ƒO2by electrons and O vacancies and at high ƒO2by Fe3+on Mg sites (Fe•mg) and Mg vacancies. Defect associates involving Fe•Mgand Mg vacancies are required to explain experimental data on oxidation state of Fe as a function of ƒO2, temperature, and Fe content. At high temperatures, intrinsic formation of Mg and O vacancies are important controls on Mg diffusivity; Fe3+impurities at intermediate temperatures and impurity‐vacancy associates at low temperatures are important. Associates involving Mg and O vacancies at low temperatures and unassociated O vacancies at higher temperatures are important for O diffusivity. The concentration of Fe at which the concentrations of other defects are affected strongly depends on ƒO2and temperature. Even minute amounts of Fe (i.e., 1 ppm) may affect material properties at temperatures below 1273 K and ƒO2above 10−15MPa, whereas at temperatures above 1873 K and ƒO2below 10−14MPa, material properties may be relatively insensitive up to 1% Fe content.
Aseismic fracturing and cataclasis involving reaction softening within core material from the Cajon Pass Drill Hole
Authors: Blenkinsop Tom G. , Sibson Richard H.
Published: February 2008

Cataclastic deformation features in crystalline rocks from the Cajon Pass drill hole, located some 4 km NE of the San Andreas fault trace in southern California, appear to have developed at slow strain rates. There is no clear evidence of seismic deformation. Most of the observed structures and microstructures are inferred to have formed during pre‐Pliocene distributed deformation, before the San Andreas fault became active in this area. Extension fractures are filled by laumontite which increases in grain size from a fine amorphous habit to coarser prismatic crystals unidirectionally across fractures and does not generally display crack‐seal textures. Fragments within fractures were derived from adjacent grains without rotation or shear. Fracture opening and cementation were therefore synchronous and slow. Fluids were present at all times during fracture opening, but there was no repeated hydrofracturing after fracture formation. However pore fluid pressures probably stayed close to the least principal stress (σ3) during subsequent fracture growth. Plagioclase compositions in all fractured areas change from oligoclase to albite or anorthite as a consequence of albitization and laumontization. Particle size distributions in dilational areas, and in extension and in shear fracture fillings, show that alteration of the major phase, plagioclase, is the fundamental process of grain size reduction in a variety of rock types. Cataclastic stresses and strain rates were controlled by alteration reaction rates in a coupled process that is a form of transformation‐modified deformation. Fracturing leading to weakening was assisted by stresses due to the 60% volume increase accompanying in situ laumontization of plagioclase and by stress corrosion and subcritical crack growth facilitated by alteration. Deformation was also enhanced by replacement of plagioclase by weaker laumontite. Such dilatant cataclasis is consistent with fluid pressure levels having remained high during deformation with effective least principal compressive stress close to zero. This transformation‐modified deformation at low temperatures (90° < T < 250°C) may be a common process in feldspar‐rich rocks. The structures and microstructures described here could be used to distinguish the products of slow and fast Cataclastic deformation.
Mantle reflections beneath the Montana Great Plains on Consortium for Continental Reflection Profiling Seismic Reflection Data
Authors: Best John A.
Published: February 2008

A systematic search for reflections from the upper mantle has been carried out on 2000 km of Consortium for Continental Reflection Profiling seismic reflection data from 14 different sites across the United States. The general observation from these data is the striking contrast between the generally pervasive reflectivity of the crust and the relative lack of coherent reflectivity in the upper mantle, a contrast that cannot always be attributed to lack of signal penetration. In order to image more of the upper mantle section on the seismic data, seismic field files were increased from 16 to 28 s two‐way travel time by extended correlation processing. While the results reveal an upper mantle generally devoid of laterally continuous coherent energy, pronounced local reflections, called here the Lonesome Lake Complex, are observed at upper mantle travel times beneath the Montana Plains. Reflections at mantle travel times do not ensure mantle reflectors, as other mechanisms operating at shallower levels in the crust may be responsible for the observed reflections. However, multiple reflections and converted waves from the crust are unlikely explanations for the Lonesome Lake Complex. Although energy from outside the plane of profile cannot be ruled out, circumstantial evidence suggests that these reflections are likely due to intramantle heterogeneity. The Lonesome Lake Complex joins examples of upper mantle reflectivity observed on other deep seismic data worldwide in establishing the mantle as an productive target for future deep seismic surveys.
Numerical simulation of solute transport in rough fractures
Authors: Thompson Mollie E.
Published: February 2008

Parallel plate models are inadequate to describe the hydraulic and transport properties of natural fractures with rough surfaces. A two‐dimensional numerical simulation of flow and solute migration in a fracture with fractal surfaces is presented. The macroscopic transport properties of the fractures are derived by comparison of the numerical simulations to predictions of a one‐dimensional advection dispersion model. This makes it possible to determine effective dispersivities and solute velocities for the mathematically generated surfaces. The effects of adsorption, travel distance and channeling phenomena on the transport properties are examined. The modeling indicates that the effective velocity of the solute differs from that of the average fluid particle due to transverse and longitudinal dispersion and diffusion. These conclusions agree with field measurements of flow and transport in fractures.
Effect of Cenozoic ice sheet fluctuations in Antarctica on the stratigraphic signature of the Neogene
Authors: Bartek L. R. , Vail P. R. , Anderson J. B. , Emmet P. A. , Wu S.
Published: February 2008

Stratigraphic successions from the Gulf of Mexico‐offshore Alabama, northeast Java‐Indonesia, Ross Sea‐Antarctica, and several other continental margins have been examined. All are characterized by very similar Neogene stratal geometries. In seismic profiles and well log cross sections from these areas, a large, mid‐Oligocene (i.e., latest Early Oligocene‐earliest Late Oligocene), basinward shift in coastal onlap is followed by a major early Miocene transgression and aggradation, which is in turn followed by early middle Miocene transgressions and late middle and late Miocene progradational episodes. The succession culminates in Plio‐Pleistocene high‐frequency progradations and transgressions. The interregional character of the Neogene stratal signature and its similarity to the stratal geometry found in seismic data from the Ross Sea continental shelf (Antarctica) suggest that the Neogene stratal signature is a manifestation of glacioeustatic fluctuations. A review of the literature and an analysis of recently acquired and published data indicate that the first major ice sheet grounding event in the Ross Sea occurred during middle to late Oligoce time. The Ross Sea is the repository for ice flowing from a major portion of the continental interior. Thus the glacial record of the Ross Sea should serve as a gage of ice volume changes on the continent that were large enough to influence global eustasy. The ice advance onto the Ross Sea continental shelf during middle to late Oligocene time may have been the result of a decrease in the rate of shelf subsidence as rifting in the Ross Sea slowed or ceased. Advance of the ice sheet resulted in widespread erosion of the continental shelf and shelf overdeepening. It is hypothesized that metastable, marine‐based ice sheets have waxed and waned on the Antarctic continental shelf since at least the Oligocene and that the these waxing and waning events were responsible for the development of a global Neogene stratigraphic signature.
A model simulation of the seasonal cycle in the tropical Pacific Ocean using climatological and modeled surface forcing
Authors: Gordon C. , Corry R. A.
Published: February 2008

A general circulation model of the tropical Pacific Ocean is used to simulate the seasonal cycle in response to two sets of forcing functions. The first set consists of climatological estimates of the stress and heat fluxes, and the second set is derived from an atmospheric general circulation model. In most areas the model fluxes are within the levels of uncertainty in the climatological estimates. Although qualitatively similar, the two flux data sets produce two very different ocean model simulations of near surface conditions. In the experiment with climatological fluxes the east Pacific cold tongue is too cold whereas the west Pacific temperatures are well simulated; the North Equatorial Counter Current (NECC) is too weak and the strength of the undercurrent agrees well with observed estimates. In the experiment with model fluxes the cold tongue is simulated reasonably well but the west Pacific temperatures are too high; the NECC is too strong and the undercurrent is rather weak. These results, and how they relate to the forcing, are discussed in detail. It is concluded that the ocean model simulation is very sensitive to the differences in the surface forcing functions. The ocean model results are compared with those from other simulations of the seasonal cycle in the tropical Pacific.
Phytoplankton biomass and photosynthetic response during the winter‐spring transition in the Fram Strait
Authors: Smith Walker O. , Brightman Ross I. , Booth Beatrice C.
Published: February 2008

The biomass, taxonomy, size, and photosynthesis‐irradiance responses of phytoplankton in the high Arctic (79°N) were investigated in March–April 1987 in the Fram Strait marginal ice zone. Chlorophyll a concentrations in open water during this period were extremely low, with surface values averaging 0.022 μg L−1. Vertical mixed layers were deep, ranging from 40 to 200 m, and surface incident irradiance was low (average of 44 μumol photons m−2 s−1 at local noon). The biomass of the microplankton was dominated by three groups: autotrophic flagellates (less than 10 μm), heterotrophic flagellates (less than 10 μm), and heterotrophic dinoflagellates (from 10 to 35 μm ). Detrital carbon contributed the largest fraction (on average 95%) to the particulate carbon pool. Photosynthesis‐irradiance responses indicated that the resident phytoplankton populations were adapted to low irradiance levels, as would be expected after the long periods of darkness encountered during the Arctic winter. Estimates of primary productivity during the March–April period indicate that very little net carbon fixation was occurring (averaging 1.84 mg C m−2 d−1 with a maximum of 3.3 mg C m−2 d−1). Some small increases in surface chlorophyll concentrations (relative to those at the base of the mixed layer) were noted at a few stations, and hence the onset of spring growth may have just begun. However, in view of the biomass and productivity observed during this period, the accumulation of substantial phytoplankton standing stocks, which are often observed at the ice edge, cannot occur until much later in the growing season at this northerly location.