bluuebunny/crossref_metadata_2025 huggingface dataset:
Dike orientations, fault‐block rotations, and the construction of slow spreading oceanic crust at 22°40′N on the Mid‐Atlantic Ridge
Authors: Lawrence Róisín M. , Karson Jeffrey A. , Hurst Stephen D.
Published: February 2004

The first paleomagnetic results from oriented dike samples collected on the Mid‐Atlantic Ridge shed new light on the complex interplay between magmatic accretion and mechanical extension at a slow spreading ridge segment. An upper crustal section about 1.5 km thick is exposed along a west‐dipping normal fault zone that defines the eastern median valley wall of the southern segment of the Mid‐Atlantic Ridge south of the Kane fracture zone (MARK area). Two distinct groups of dikes are differentiated on the basis of orientation and paleomagnetic characteristics. One group, on the basis of the paleomagnetic data, appears to be in its original intrusion orientation. This group includes both ridge‐parallel, vertical dikes as well as dikes in other orientations, calling into question assumptions about uniform dike orientations at oceanic spreading centers. The second group consists of dikes that have paleomagnetic directions that are distinct from the predicted dipole direction, and we interpret them to have been tectonically rotated. These also occur in many orientations. The spatial relations between rotated and nonrotated dikes indicate that intrusion, faulting, and block rotation were contemporaneous beneath the median valley floor. Nonrotated dikes exposed on the eastern median valley wall indicate that there has been no net rotation of this upper crustal assemblage since magmatic construction ceased. Hence slip and associated uplift probably occurred in the fault zones' present orientation. These results provide the basis for a general model of mechanical extension and dike intrusion for this segment of the Mid‐Atlantic Ridge. Initially, a portion of crust forms beneath the median valley by synkinematic dike intrusion into laterally discontinuous fault blocks. Slip and associated uplift along a cataclastic normal fault zone later exposes this crustal section on the valley margin. As spreading continues, this valley‐bounding cataclastic normal fault zone is abandoned in favor of a new fault system thus passively moving the exposed crustal section away from the median valley.
Fluid‐rock interactions in the lower oceanic crust: Thermodynamic models of hydrothermal alteration
Authors: McCollom Thomas M. , Shock Everett L.
Published: February 2004

Rocks from the lower oceanic crust show both petrologic and isotopic evidence for extensive alteration by circulating hydrothermal fluids derived from seawater. Deeply circulating fluids may have a substantial, but as yet largely undetermined, impact on the thermal regime of spreading centers, magmatic processes at mid‐ocean ridges, and elemental fluxes at ridge hydrothermal systems. In the present study, the consequences of water‐rock interactions during the penetration of seawater into hot lower oceanic crust were investigated using thermodynamic reaction path models. Alteration assemblages predicted for reaction of an evolving seawater with olivine gabbro, gabbronorite, and troctolite during heating from 300° to 900°C closely resemble alteration assemblages observed in rock samples from the lower oceanic crust. These assemblages include sodic plagioclase + actinolite ± diopsidic clinopyroxene ± epidote ± chlorite ± prehnite ± analcime ± quartz at lower temperatures (300°–500°C) and plagioclase + hornblende ± diopside ± chlorite ± trace magnetite at intermediate temperatures (500–700°C). The relative abundances of these alteration products show much greater variation with temperature than with water/rock (W/R) ratio, indicating that variations in the amounts of hydrated minerals such as amphiboles may not be an accurate indication of the W/R ratio experienced by rocks in the lower crust since the same amount of fluid may produce widely differing amounts of amphibole at slightly different temperatures. Depending on the temperature of interaction, some rocks in the lower oceanic crust may have experienced much higher W/R ratios than has previously been proposed based on the amount of amphibole present. At higher temperatures (750°–900°C),. the models predict that very little alteration occurs before the fluid equilibrates with the rock, producing only trace amounts of amphibole as an alteration phase, with the remainder of the rock retaining its igneous mineral composition essentially unchanged. This result suggests that extensive penetration of rocks by aqueous fluids may occur in the lower ocean crust at temperatures >∼700°C and leave very little conspicuous petrologic evidence for their presence. Such cryptic alteration is consistent with isotopic evidence for hydrothermal alteration of oxygen and Sr isotopes by seawater in lower ocean crust rock samples which otherwise retain their igneous compositions and textures. Taken together, the model results allow for much more extensive circulation of fluids in the lower oceanic crust and at much higher temperatures than has been proposed by previous studies based on petrologic interpretations.
Dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in Antarctica and Greenland
Authors: De La Chapelle S. , Castelnau O. , Lipenkov V. , Duval P.
Published: February 2004

The preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip; but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is, at high temperature, at the origin of recrystallization textures. The purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures. The study was based on two deep ice cores from Greenland and Antarctica, the GReenland Ice core Project (GRIP) and Vostok ice cores. The structure along the GRIP core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom. Only grain growth and rotation recrystallization appear to occur in the Vostok ice core. The transition between these recrystallization regimes was studied, estimating, for interglacial ice, the evolution with depth of the dislocation density. This calculation has shown the efficiency of grain boundary migration for the absorption of dislocations. At Vostok, the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate. It is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at Vostok. The observed textures were compared with those predicted by the self‐consistent approach. Recrystallization textures are interpreted by assuming that the less stressed grains, i.e., the best oriented for basal slip, are favored by the size advantage of subgrains. The recrystallization textures are compared with those of other materials.
Magmatic fabric acquisition mechanisms in a syenite: Results of a combined anisotropy of magnetic susceptibility and image analysis study
Authors: Launeau P. , Cruden A. R.
Published: February 2004

The structural significance of the anisotropy of magnetic susceptibility (AMS) and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the Archean Lebel syenite intrusion, Ontario, Canada. Bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples. Intensities, shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding AMS ellipsoids. Differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre‐full crystallization textures, and structural overprinting in samples deformed after crystallization. Subfabrics of clinopyroxene and magnetite in samples showing pre‐full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes. The resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite, which is similar to the overall petrofabric of the samples. Crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma. Analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains, analogous to a logjam in a river. The final characteristics of the AMS and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma, rather than transient late features.
Paleomagnetic evidence of localized vertical axis rotation during Neogene extension, Sierra San Fermín, northeastern Baja California, Mexico
Authors: Lewis Claudia J. , Stock Joann M.
Published: February 2004

Paleomagnetic data from Sierra San Fermín in the Gulf of California Extensional Province indicate that localized clockwise rotations about vertical axes occurred during Pliocene through Recent extension and dextral shear. Relative declination discordances in upper Miocene and Pliocene ash flow tuffs indicate a net clockwise rotation of 30°±16°. Clockwise rotation between 12.5 and 6 Ma is statistically insignificant (11°±17°). Structural observations and geochronological data suggest that rotations in this area began post‐6 Ma, comprising uniform‐sense block rotations (oblique divergence) associated with extension and dextral slip in the northwest striking boundary between the Pacific and North American plates. Northeast striking sinistral‐slip faults and north striking normal faults accommodate distributed dextral shear in this area, allowing fault blocks to rotate in a clockwise sense. A model for oblique divergence predicts ∼21 km of shear in the direction of relative plate motion and ∼20% (∼7 km) ENE directed extension, perpendicular to the Main Gulf Escarpment. A broad region of northeastern Baja California may have undergone similar distributed shear. Two possible dynamic models may explain this shear. In one model, rotation accumulates above a deep, subhorizontal, basal shear zone. Rotating blocks may extend downward to a detachment beneath the extensional province, either a low‐angle eastward continuation of the San Pedro Mártir fault or to a basal shear surface on top of a subducted remnant of the Farallon plate. Alternatively, distributed dextral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults. In either case, shear may have transferred northward onto faults west of the San Andreas fault, contributing to late Miocene to Recent clockwise rotation of the Western Transverse Ranges. This shear is not accounted for in the 300 km of dextral slip computed from cross‐gulf geologic tie points.
T waves from the great 1994 Bolivian deep earthquake in relation to channeling of S wave energy up the slab
Authors: Okal Emile A. , Talandier Jacques
Published: February 2004

We report the observation of T phases recorded at a number of Pacific island sites, following the great Bolivian deep earthquake of June 9, 1994. By studying their arrival times at the various stations, we unravel the process of conversion from seismic to acoustic energy at the South American coastline. We find that the maxima of amplitude of the phase are incompatible with propagation along the great circle from source to receiver but rather require the generation of the acoustic wave at a common point at the Arica Bight for all stations in the South Pacific, and even farther south for stations in Hawaii and the Bonin Islands. The timing of these conversions corresponds to the arrival of regional S waves at the conversion points. The high‐frequency nature of the T phase requires the channeling of the S wave from the seismic source to conversion point through low‐attenuation material, which in turns adds supportive evidence for the mechanical continuity of the downgoing slab under that section of South America, despite the observed gap in seismicity between 300 and 600 km.
Bayesian seismic waveform inversion: Parameter estimation and uncertainty analysis
Authors: Gouveia Wences P. , Scales John A.
Published: February 2004

The goal of geophysical inversion is to make quantitative inferences about the Earth from remote observations. Because the observations are finite in number and subject to uncertainty, these inferences are inherently probabilistic. A key step is to define what it means for an Earth model to fit the data. This requires estimation of the uncertainties in the data, both those due to random noise and those due to theoretical errors. But the set of models that fit the data usually contains unrealistic models; i.e., models that violate our a priori prejudices, other data, or theoretical considerations. One strategy for eliminating such unreasonable models is to define an a priori probability density on the space of models, then use Bayes theorem to combine this probability with the data misfit function into a final a posteriori probability density reflecting both data fit and model reasonableness. We show here a case study of the application of the Bayesian strategy to inversion of surface seismic field data. Assuming that all uncertainties can be described by multidimensional Gaussian probability densities, we incorporate into the calculation information about ambient noise, discretization errors, theoretical errors, and a priori information about the set of layered Earth models derived from in situ petrophysical measurements. The result is a probability density on the space of models that takes into account all of this information. Inferences on model parameters can be derived by integration of this function. We begin by estimating the parameters of the Gaussian probability densities assumed to describe the data and model uncertainties. These are combined via Bayes theorem. The a posteriori probability is then optimized via a nonlinear conjugate gradient procedure to find the maximum a posteriori model. Uncertainty analysis is performed by making a Gaussian approximation of the a posteriori distribution about this peak model. We present the results of this analysis in three different forms: the maximum a posteriori model bracketed by one standard deviation error bars, pseudo‐random simulations of the a posteriori probability (showing the range of typical subsurface models), and marginals of this probability at selected depths in the subsurface. The models we compute are consistent both with the surface seismic data and the borehole measurements, even though the latter are well below the resolution of the former. We also contrast the Bayesian maximum a posteriori model with the Occam model, which is the smoothest model that fits the surface seismic data alone.
Lead isotopic evidence for synextensional lithospheric ductile flow in the Colorado River extensional corridor, western United States
Authors: Feuerbach Daniel L. , Reagan Mark K. , Faulds James E. , Walker J. Douglas
Published: February 2004

Temporal changes in the Pb isotopic compositions of Miocene lavas erupted in the northern Colorado River extensional corridor suggest that lithospheric mantle and middle to deep crust migrated from beneath the Colorado Plateau into the corridor during extension. Basaltic to rhyolitic lavas erupted in the extensional corridor prior to 12.2 Ma have Pb isotopic values that are similar to those of Tertiary to Quaternary lavas erupted through Proterozoic Mojave crust, which comprises surface exposures of basement in the corridor and much of the extended territory to the west. In contrast, most post‐12.2 Ma lavas from the same region have Pb isotopic compositions similar to those of lavas erupted through Arizona crust, which forms the basement of the Colorado Plateau. The changes in isotopic compositions of the basaltic lavas, and perhaps a portion of the changes in isotopic compositions of silicic lavas, are attributed to a change in the composition of the mantle source. However, the 206Pb/204Pb ratios for lavas erupted before and after 12.2 Ma in the corridor decrease with decreasing MgO concentrations, suggesting that the Pb isotopic compositions of crustal assimilants changed at about the same time as the composition of the mantle. In the area of the Black Mountains accommodation zone, the surface boundary between the Arizona and Mojave crustal provinces lies a minimum of 60–80 km to the east of the westernmost lava with an Arizona Pb isotopic signature. This distance cannot be accounted for by displacements along nearby major faults, suggesting that middle to deep Arizona crust flowed a significant distance to the west during extension.
Velocity variations in the crust and upper mantle beneath the Tien Shan inferred from Rayleigh wave dispersion: Implications for tectonic and dynamic processes
Authors: Mahdi Hanan , Pavlis Gary L.
Published: February 2004

We used data from nuclear explosions and earthquakes recorded by the Kyrgyzstan telemetered broadband seismic network to examine crustal structure in the Tien Shan region of central Asia. Seventy‐nine measurements of fundamental and first higher mode Rayleigh waves were analyzed using the single‐station multiple filtering technique to determine group velocity dispersions. We found that there was a distinct difference in Rayleigh wave group velocity values between different regions, but the data could be combined in three groups: (1) paths from the west and south across the Turan platform and western Tarim Basin, (2) paths across the eastern Tarim Basin, and (3) nuclear explosions from the Lop Nor test site. Group velocity values were inverted to derive one‐dimensional shear wave velocity models for the Tien Shan region. Results reveal crustal thickness varying from 50 to 60 km with thickness increasing toward the eastern Tien Shan region in agreement with published crustal thickness maps for the region. Array analysis indicates that paths along the eastern Tien Shan and Tarim‐Tien Shan are affected by lateral refractions and multipathing, but we argue that these have a small effect on the measured dispersion curves. Our results indicate that upper mantle velocities in the eastern Tarim and Tien Shan are significantly lower than those beneath the western Tien Shan and Turan platform. Our results suggest that the boundary between these two regimes lies somewhere west of the depocenter of the Tarim Basin and east of Kashgar, China.
A procedure for estimation of source and propagation amplitude corrections for regional seismic discriminants
Authors: Taylor Steven R. , Hartse Hans E.
Published: February 2004

We outline a procedure for the estimation of frequency‐dependent source and propagation amplitude corrections for regional seismic discriminants (source path amplitude correction (SPAC)). For a given station and phase a number of well‐recorded earthquakes are inverted for source and path corrections. The method assumes a simple Brune [1970] earthquake‐source model and a simple propagation model consisting of a frequency‐independent geometrical spreading and frequency‐dependent power law Q. The inverted low‐frequency levels are then regressed against mb to derive a set of corrections that are a function of mb and distance. Once a set of corrections is derived, effects of source scaling and distance as a function of frequency are applied to amplitudes from new events prior to forming discrimination ratios. The resulting discriminants are normally distributed and amenable to multivariate feature selection, classification, and outlier techniques. To date, most discrimination studies have removed distance corrections once a particular amplitude ratio is formed (distance corrected ratio (DCR)). DCR generally works well for phase ratios taken in a particular frequency band. However, when different frequency bands are combined (for phase spectral ratios or cross spectral ratios), significant source‐scaling effects (e.g., corner‐frequency scaling) can remain, causing the discriminants to vary as a function of event size and to be nonnormally distributed. It is then often necessary to construct nonphysical transformations in an attempt to make the discriminants multivariate normal. The SPAC technique can be used to construct discriminants that are multivariate normal by using simple physical seismic source and propagation models. Moreover, phase amplitude residuals as a function of frequency can be spatially averaged and used as additional path‐specific corrections to correct for additional propagation effects such as phase blockages.
Crustal accretion at the Reykjanes Ridge, 61°–62°N
Authors: Smallwood John R. , White Robert S.
Published: February 2004

We report results of a seismic, gravity, and magnetic survey of the Reykjanes Ridge spreading center at 61°–62°N, about 600 km from the center of the Iceland mantle plume. Anomalously shallow water on the ridge crest enabled us to record seismic refractions on a 2.4 km hydrophone streamer. The velocity within layer 2A is 2.4±0.3 km s−1, and its mean thickness is 400±100 m. The velocity at the base of layer 2A is 3.3±0.3 km s−1 on the ridge axis, increasing with crustal age to ∼4.0 km s−1 at 1.5 Ma and ∼4.5 km s−1 at 5 Ma. Assuming that seismic layer 2A on the ridge axis is also the extrusive layer, i.e., the magnetic source layer, we have successfully modeled the variations in amplitude of the magnetic field. The best magnetic model includes enhanced magnetization within layer 2A at the sites of recent volcanic activity as independently recognized in side‐scan sonar data. We also present a full crustal seismic model, based on wide‐angle seismic recordings on digital ocean bottom hydrophones and disposable sonobuoys. The seismic model is complemented by gravity modeling, which further suggests that the ridge crest is in isostatic equilibrium. The zero age crust is 10.0 km thick, while crust of age 5 Ma is 7.8 km thick. These crustal thicknesses are greater than those of normal oceanic crust, which we attribute to the presence of anomalously hot asthenospheric mantle beneath the spreading center. We suggest that the variation in thickness between 0 Ma and 5 Ma crust is caused by temporal variation in the plume‐fed asthenospheric temperature beneath the Reykjanes Ridge.
Single magnetic domains in magnetotactic bacteria
Authors: Frankel Richard B. , Zhang Jin‐Ping , Bazylinski Dennis A.
Published: February 2004

Magnetotactic bacteria construct an internal, permanent magnetic dipole based on single magnetic domain particles of magnetite or greigite. The organisms exert a high degree of control over the size and morphology of the particles. This may be relevant in distinguishing biogenic from nonbiogenic iron mineral particles.
Saw‐toothed variations of relative paleointensity and cumulative viscous remanence: Testing the records and the model
Authors: Meynadier Laure , Valet Jean‐Pierre , Guyodo Yohan , Richter Carl
Published: February 2004

A recent model [Kok and Tauxe, 1996a] that attempts to explain asymmetrical sawtooth patterns of relative paleointensity from sediments [Valet and Meynadier, 1993] as the consequence of cumulative long‐term viscosity effects has been tested theoretically and experimentally. A mathematical description of the model demonstrates that only a sharp distribution of relaxation times would be able to match the paleomagnetic record, which implies that all the magnetic grains would be identical. This has direct consequences for the distribution of the blocking temperatures (Tb), which would not exceed 260°C, and for the coercivities of the natural remanent magnetization (NRM). This Tb spectrum derived from the model is inconsistent with the unblocking temperatures of the NRM and with the Tb spectrum of the experimental thermoremanent magnetization (TRM) of the sediment that recorded the sawtooth. New results including thermal demagnetization and Thellier‐Thellier experiments across Brunhes‐Matuyama confirm the overall stability of the original records. The stability of the older part of the record (2 to 4 Ma) has also been strengthed. The sawtooth is unchanged between 20 mT and 65 mT, and thermal demagnetization in a neutral atmosphere at 300°C and 450°C duplicates the results obtained by alternation field (a.f.) demagnetization. Thus viscous processes, at least within the framework of a “cumulative viscous model” and for the existing data sets, are not responsible for saw‐toothed paleointensity records.
Lithospheric structure of the Qiangtang Terrane, northern Tibetan Plateau, from complete regional waveform modeling: Evidence for partial melt
Authors: Rodgers Arthur J. , Schwartz Susan Y.
Published: February 2004

We report models of P and S wave velocity and attenuation for the the crust and uppermost mantle of the Qiangtang Terrane, northern Tibetan Plateau, inferred by fitting reflectivity synthetic seismograms to observed complete regional waveforms. The data are three‐component broadband seismograms recorded by the 1991–1992 IRIS‐PASSCAL (Incorporated Research Institutions for Seismology‐Program for Array Seismic Studies of the Continental Lithosphere) Tibetan Plateau Experiment and Global Seismic Network stations in the region. The Qiangtang Terrane has thick crust (65±5 km) with P and S wave velocities of 6.1–6.3 and 3.34–3.43 km/s, respectively, yielding an anomalously high crustal Poisson's ratio of 0.29±0.02. Seismic velocities of the upper mantle of the Qiangtang Terrane are normal for P waves and slow for S waves (8.10 and 4.35–4.41 km/s, respectively) with a high mantle Poisson's ratio of 0.29±0.01. Attenuation in the crust and upper mantle is high (QP = 100–200 and QS = 44–89). Modeling of the broadband P waveforms suggests that a decrease in mantle velocity occurs at about 160 km depth in the mantle; however, this is not unambiguously supported by the data and modeling. The crust and uppermost mantle of the Qiangtang Terrane probably contains partial melt based on the high Poisson's ratio, low shear wave velocities, and low Q. The absence of high‐frequency Sn and the presence of volcanism of mantle lithospheric origin support the presence of partial melt. Crustal and uppermost mantle structure in the Qiangtang Terrane is different from that for the Lhasa Terrane (immediately to the south) based on results of our previous studies. The average crustal P and S wave velocities are 4% faster and 2% slower, respectively, in the Qiangtang Terrane relative to the Lhasa Terrane. This yields a significant difference in the crustal Poisson's ratio with values of 0.29 for the Qiangtang Terrane and 0.25 for the Lhasa Terrane. Differences in the uppermost mantle P and S wave velocities and Poisson's ratios of these two adjacent terranes cannot be explained by temperature differences alone. Using the mantle temperature estimates of McNamara et al.. [1997] we suggest that partial melt of an ultramafic composition beneath the Qiangtang Terrane fits the velocity and Poisson's ratio estimates.
Experimental observation of fluid flow channels in a single fracture
Authors: Brown Stephen , Caprihan Arvind , Hardy Robert
Published: February 2004

A method for obtaining precise replicas of real fracture surfaces using transparent epoxy resins was developed, allowing detailed study of fluid flow paths within a fracture plane. A natural rock fracture was collected from the field and prepared for study. Silicon rubber molds of the fracture surfaces were used to make a transparent epoxy replica of the original fracture. Clear and dyed water were injected into the fracture pore space allowing examination of the flow field. Digitized optical images were used to observe wetting, saturated flow, and drying of the specimen. Nuclear magnetic resonance imaging was used for quantitative measurements of flow velocity. Both video imaging and nuclear magnetic resonance imaging techniques show distinct and strong channeling of the flow at the submillimeter to several‐centimeter scale. Each phenomenon, including wetting, drying, dye transport, and velocity channeling, has its own distinct geometric structure and scale. We find that fluid velocities measured simultaneously at various locations in the fracture plane during steady state flow range over several orders of magnitude, with the maximum velocity a factor of 5 higher than the mean velocity. This suggests that flow channeling in fractured rock can cause the breakthrough velocity of contaminants to far exceed the mean flow.
Numerical models of faulting at oblique spreading centers
Authors: Tuckwell G. W. , Bull J. M. , Sanderson D. J.
Published: February 2004

Obliquely spreading mid‐ocean ridges, such as the Reykjanes Ridge, display two distinct fault sets distinguishable by orientation and position: on‐axis faults are oriented oblique to both the trend of the axis and the normal to the relative plate separation vector, while faults on the flanks strike approximately parallel to the ridge axis. Numerical modeling techniques are used here to simulate the development of faulting on the Reykjanes Ridge. Stresses acting in a cross section through the lithosphere at a slow spreading ridge are investigated using the fast Lagrangian analysis of continua (FLAC) explicit difference modeling software. The predicted stresses from the cross‐sectional models are imposed as a condition in boundary element models of fracture propagation and linkage. On‐axis fault simulations run under conditions similar to the Reykjanes Ridge successfully reproduce the mapped distribution of faults and predict the observed orientation of the axial volcanic ridges. Simulations of fractures away from the axis show the development of axis‐parallel faults by the interaction and linkage of fractures which have been rafted off‐axis, also in accord with observations. Stresses modeled in cross section favor downdip displacement on faults dipping toward the ridge axis.
Estimation of current plate motions in Papua New Guinea from Global Positioning System observations
Authors: Tregoning Paul , Lambeck Kurt , Stolz Art , Morgan Peter , McClusky Simon C. , van der Beek Peter , McQueen Herbert , Jackson Russell J. , Little Rodney P. , Laing Alex , Murphy Brian
Published: February 2004

Plate tectonic motions have been estimated in Papua New Guinea from a 20 station network of Global Positioning System sites that has been observed over five campaigns from 1990 to 1996. The present velocities of the sites are consistent with geological models in which the South Bismarck, Woodlark, and Solomon Sea Plates form the principal tectonic elements between the Pacific and Australian Plates in this region. Active spreading is observed on the Woodlark Basin Spreading Centre but at a rate that is about half the rate determined from magnetic reversals. The other major motions observed are subduction on the New Britain Trench, seafloor spreading across the Bismarck Sea Seismic Lineation, convergence across the Ramu‐Markham Fault and left‐lateral strike slip across the Papuan Peninsula. These motions are consistent with a 8.2° Myr−1clockwise rotation of the South Bismarck Plate about a pole in the Huon Gulf and a rotation of the Woodlark Plate away from the Australian Plate. Second order deformation may also be occurring; in particular, Manus Island and northern New Ireland may be moving northward relative to the Pacific Plate at ∼5–8 mm yr−1(significant at the 95% but not at the 99% confidence level) which may suggest the existence of a North Bismarck Plate.
High‐temperature creep of partially molten plagioclase aggregates
Authors: Dimanov A. , Dresen G. , Wirth R.
Published: February 2004

We have investigated the high‐temperature creep of synthetic labradorite (An60Ab40) between 1323 K and 1523 K at atmospheric pressure and low stresses (2–65 MPa). Average grain size varies from 12 to 16 μm. Samples contained up to 12 vol % melt (An60Ab40 glass). Fourier transform infrared measurements indicate that trace amounts of water (∼0.1–0.15 wt % H2O) were incorporated during crystallization. This water was only partly released at high temperature and atmospheric pressure. At low stresses and temperatures, deformation of labradorite was controlled by grain boundary diffusion creep yielding a stress exponent of n∼1 and an activation energy of 365±25 kJ/mol (regime I). With increasing stress and temperature, mechanical data and microstructural observations indicate a transition to power law creep with n∼3 (regime II). This transition occurs in both partially molten and melt‐free aggregates. In regime I the strain rate of samples containing as much as 12 vol % melt is between 3 and 5 times higher than in fully crystalline specimens. The activation energy is only slightly affected by the melt content between 0 and 10±2 vol %. Transmission electron microscopy shows that the amorphous phase resides in connected triple junctions or in unconnected pockets. Grain boundaries are not wetted except surrounding large melt pockets. The melt distribution of undeformed and deformed samples is similar. The strength and activation energy of synthetic labradorite are comparable to those for pure anorthite with trace amounts of water. Both plagioclase compositions are slightly weaker than synthetic diopside.
Polar magnetopause crossings of May 29, 1996: Implications for magnetic field modeling
Authors: Urquhart Andrew L. , Reiff Patricia H. , Toffoletto Frank R. , Hill Thomas W. , Konkel Todd R. , Russell Christopher T. , Le Guan , Savin Sergey P. , Romanov Stanislav A.
Published: February 2004

During 1996, the magnetometer and particle instruments on NASA's Polar spacecraft detected several apparent magnetopause crossings. In constast, with a geocentric apogee of only 9 RE over the northern pole, this spacecraft was not expected to leave the magnetosphere. We have investigated the modifications that are required in existing magnetospheric magnetic field models in order to account for these observations. In order to match the observed particle distributions (reported elsewhere,) this model includes northward‐IMF reconnection. For the event of May 29, 1996, unusual magnetic and particle observations began at an altitude of 5 RE while the spacecraft was sunward of the expected northern cusp position. Using a cylindrically symmetric magnetopause, we are able to reproduce the measured field reasonably well but with problems in the timing of the boundary crossings. The crossing times are much improved by making an adjustment in the spacecraft position which could arise either from motion of the boundary or an indentation in the magnetopause at the cusp. The results from this model are compared against results from the analytic open‐magnetosphere Toffoletto and Hill [1993] model and the empirical Tsyganenko [1996] model; significant improvement over both prior models is shown. This improvement is shown not only with respect to data from Polar but also with respect to data from the Interball‐Tail spacecraft. The model indicates that the Polar spacecraft encountered regions of “overdraped lobe” field lines that connect to the southern polar cap but exit the magnetosphere through the northern cusp.
A morphological study of black vortex streets
Authors: Kimball J. , Hallinan T. J.
Published: February 2004

Black aurora has been defined as the lack of auroral emissions in well‐defined regions within diffuse aurora [Royrvik, 1976; Davis, 1978]. A particularly intriguing class of black aurora is the occurrence of black vortex streets (black curls) exhibiting clockwise vorticity [Davis, 1978; Trondsen and Cogger, 1997.] An archive of low‐light level all‐sky and narrow field television camera data was searched, yielding 20 examples of black curls. Black curls are concluded to result from the Kelvin‐Helmholtz instability occurring in regions of positive space charge. Calculated peak electric fields ranged from 80 to 300 mV/m. White curls seen in aurora that was intermediate between diffuse and discrete were sometimes found in association with black curls and even paired with black curls to form newly discovered Karman vortex streets. A spectroscopic measurement indicated mixed proton and electron precipitation. Electric field configurations are proposed for the black and white curl systems and the Karman vortex streets.
Introduction to special section: The Freja Mission
Authors: Lundin Rickard , Haerendel Gerhard , Grahn Sven
Published: February 2004

Freja was a joint Swedish and German scientific satellite project to study the interaction between hot magnetospheric plasma and the topside atmosphere/ionosphere. Freja was launched on October 6, 1992, and it operated successfully during 4 years until October 1996 when the command system ceased to work. Freja enabled high temporal/spatial resolution measurements of auroral plasma characteristics. With a high telemetry rate (520 kbit/s) and ∼15 Mbyte distributed onboard memories Freja could resolve mesoscale and microscale phenomena in the 100 m range for particles and the 1–10 m range for electric and magnetic fields. Novel plasma instruments enabled Freja to increase the spatial/temporal resolution orders of magnitudes above that achieved by its predecessors. The main scientific objective of Freja was to study the interaction between the hot magnetospheric plasma with the topside atmosphere/ionosphere. This interaction leads to a strong energization of magnetospheric and ionospheric plasma and an associated erosion, and loss, of matter from the Terrestrial exosphere. Freja orbited with an altitude of ∼600–1750 km, thus covering the lower part of the auroral acceleration region. This altitude range hosts processes that heat and energize the ionospheric plasma above the auroral zone, leading to the escape of ionospheric plasma and the formation of large density cavities.
Iron freeze‐in temperatures measured by SOHO/CELIAS/CTOF
Authors: Aellig M. R. , Grünwaldt H. , Bochsler P. , Wurz P. , Hefti S. , Kallenbach R. , Ipavich F. M. , Axford W. I. , Balsiger H. , Bürgi A. , Coplan M. A. , Galvin A. B. , Geiss J. , Gliem F. , Gloeckler G. , Hilchenbach M. , Hovestadt D. , Hsieh K. C. , Klecker B. , Lee M. A. , Livi S. , Managadze G. G. , Marsch E. , Möbius E. , Neugebauer M. , Reiche K.‐U. , Scholer M. , Verigin M. I. , Wilken B.
Published: February 2004

The CELIAS particle experiment on SOHO contains the Charge Time Of Flight (CTOF) mass spectrometer which measures the ionic and elemental composition of minor ions in the solar wind. In this paper we present iron freeze‐in temperatures derived with a time resolution of 5 min. They indicate that some of the filamentary structures of the inner corona observed in Hα survive in the interplanetary medium as far as 1 AU.
Hydromagnetic equilibrium and instabilities in the convectively driven near‐Earth plasma sheet
Authors: Wu C.‐C. , Pritchett P. L. , Coroniti F. V.
Published: February 2004

Recent particle simulations have suggested that the convectively driven near‐Earth plasma sheet can develop a structure in which a thin current sheet is embedded within the much thicker plasma sheet and that finite‐ky modes with the character of kink and interchange modes can be excited in this system. Here the ideal magnetohydrodynamic (MHD) equations are used to investigate the equilibrium and linear stability properties of such a system. It is shown that the embedded current sheet configuration satisfies the conditions of pressure balance and represents an approximate two‐dimensional (x,z) MHD equilibrium state. The stability analysis, in which the boundary conditions are imposed in terms of MHD characteristic waves, indicates that two types of pressure‐driven modes are unstable. One mode is associated with the presence of a tailward gradient in the equatorial magnetic field profile. As the wavelength of this mode is made shorter, the mode becomes localized on the field lines crossing the region of increasing field and has the character of an interchange/ballooning mode. A second mode is associated with the existence of a magnetic island and is localized within the island; it has a structure similar to that of the classical kink mode for a plasma column. While the MHD growth rates continue to increase at short wavelengths, at longer wavelengths determined by the ion gyroradius (kyρi ≲ 1, where ρi is computed in the local equatorial field) they are comparable to those for the dominant nonlinear modes observed in the kinetic simulations.
Solar wind plasma correlations between IMP 8, INTERBALL‐1, and WIND
Authors: Paularena K. I. , Zastenker G. N. , Lazarus A. J. , Dalin P. A.
Published: February 2004

Solar wind plasma flux correlations between data from three spacecraft (IMP 8, WIND, and INTERBALL‐1) were analyzed for approximately 4 months during late 1995 and mid 1996 (near solar minimum) in order to investigate the local homogeneity of the solar wind. The data were split into 6‐hour segments, resulting in a total of 397 segments where data from at least one pair of spacecraft could be correlated. The results show that the average flux correlation was 0.7 over distances ranging from 0 to 220 RE in the radial direction and up to 80 RE perpendicular to the Earth‐Sun line. 43% of the segments studied had correlation coefficients of at least 0.8, while only 19% of the segments had correlation coefficients less than 0.5. The additional lags, after performing radial advection shifts at the plasma bulk speed, cluster near zero (71% of the best correlations occur with lags under 10 min), implying that the advection shift is a good approximation of the propagation time for the structures being correlated. There appeared to be no dependence of the correlation on spacecraft separation in either XGSE or YGSE. The best organizers of the flux correlation appear to be the value of the flux and the standard deviations of the flux and the density.
Solar wind plasma correlations between L1 and Earth
Authors: Richardson J. D. , Dashevskiy F. , Paularena K. I.
Published: February 2004

Solar wind plasma data from ISEE 3 at the L1 point and IMP 8 in Earth orbit are compared to determine how well an Ll monitor predicts plasma conditions at Earth. These data cover the time period August 1978 to February 1980, approaching solar maximum. Data are divided into 6‐hour blocks, time shifted to compensate for the radial separation of the spacecraft, and interpolated to provide identically sampled time series; then linear correlation coefficients are calculated as a function of lag. The average correlations of solar wind speed, density, and flux are all about 0.6. The most important factors determining the degree of correlation are the radial separation of the spacecraft and the standard deviation of the density. For the largest standard deviations of density, the correlation coefficients are 0.85. The lag between observation of solar wind features at the two spacecraft varies with both the radial and azimuthal separation of the spacecraft, indicating that both radial propagation of the solar wind and the rotation of the Sun are important effects for determining solar wind arrival time. Plasma structures seem to be less well correlated than magnetic field structures.
Empirical modeling of the solar corona using genetic algorithms
Authors: Gibson S. E. , Charbonneau P.
Published: February 2004

Many remote sensing applications encountered in astronomy and space science involve the solution of nonlinear inverse problems. These are often difficult to solve because of nonlinearities, ill‐behaved integration kernels, and amplification of data noise associated with the inversion of the integral operator. In some cases these difficulties are severe enough to warrant repeated evaluations of the forward problem as an alternate approach to formal inversion. Because a forward approach is intrinsically repetitive and time consuming, an efficient and flexible forward technique is required for this avenue to be practical. We show how a forward technique based on a genetic algorithm allows us to fit magnetostatic models of the solar minimum corona to observations in white light to a degree that would otherwise have been computationally prohibitive. In addition, and perhaps equally important, the method also allows the determination of global error estimates on the model parameters defining the best fit solution.
Toward a closer integration of magnetospheric research: Magnetospheric currents inferred from ground‐based magnetic data
Authors: Akasofu S.‐I. , Kamide Y.
Published: February 2004

A new approach is needed to advance magnetospheric physics in the future to achieve a much closer integration than in the past among satellite‐based researchers, ground‐based researchers, and theorists/modelers. Specifically, we must find efficient ways to combine two‐dimensional ground‐based data and single points satellite‐based data to infer three‐dimensional aspects of magnetospheric disturbances. For this particular integration purpose, we propose a new project. It is designed to determine the currents on the magnetospheric equatorial plane from the ionospheric current distribution which has become available by inverting ground‐based magnetic data from an extensive, systematic network of observations, combined with ground‐based radar measurements of ionospheric parameters, and satellite observations of auroras, electric fields, and currents. The inversion method is based on the KRM/AMIE algorithms. In the first part of the paper, we extensively review the reliability and accuracy of the KRM and AMIE algorithms and conclude that the ionospheric quantities thus obtained are accurate enough for the next step. In the second part, the ionospheric current distribution thus obtained is projected onto the equatorial plane. This process requires a close cooperation with modelers in determining an accurate configuration of the magnetospheric field lines. If we succeed in this projection, we should be able to study the changing distribution of the currents in a vast region of the magnetospheric equatorial plane for extended periods with a time resolution of about 5 min. This process requires a model of the magnetosphere for the different phases of the magnetospheric substorm. Satellite‐based observations are needed to calibrate the projection results. Agreements and disagreements thus obtained will be crucial for theoretical studies of magnetospheric plasma convection and dynamics, particularly in studying substorms. Nothing is easy in these procedures. However, unless we can overcome the associated difficulties, we may not be able to make distinct progresses. We believe that the proposed project is one way to draw the three groups closer together in advancing magnetospheric physics in the future. It is important to note that the proposed project has become possible because ground‐based space physics has made a major advance during the last decade.