bluuebunny/crossref_metadata_2025 huggingface dataset:
The dynamics of current carriers in standing Alfvén waves: Parallel electric fields in the auroral acceleration region
Authors: Wright Andrew N. , Allan W. , Ruderman Michael S. , Elphic R. C.
Published: October 2002

The acceleration of current carriers in an Alfvén wave current system is considered. The model incorporates a dipole magnetic field geometry, and we present an analytical solution of the two‐fluid equations by successive approximations. The leading solution corresponds to the familiar single‐fluid toroidal oscillations. The next order describes the nonlinear dynamics of electrons responsible for carrying a few μAm−2 field aligned current into the ionosphere. The solution shows how most of the electron acceleration in the magnetosphere occurs within 1 RE of the ionosphere, and that a parallel electric field of the order of 1 mVm−1 is responsible for energising the electrons to 1 keV. The limitations of the electron fluid approximation are considered, and a qualitative solution including electron beams and a modified E∥ is developed in accord with observations. We find that the electron acceleration can be nonlinear, (ve∥∇∥)ve∥ > ωve∥, as a result of our nonuniform equilibrium field geometry even when ve∥ is less than the Alfvén speed. Our calculation also elucidates the processes through which E∥ is generated and supported.
Stability and interaction of fast auroral solitary structures in three‐dimensional plasma
Authors: Roth I. , Muschietti L. , Carlson C. W. , Mozer F. S. , Ergun R. E.
Published: October 2002

The stability of the electrostatic, fast, solitary structures that are observed frequently on auroral field lines is analyzed as a three‐dimensional dynamic system and investigated numerically with the help of test‐particle and particle‐in‐cell simulations. The recently observed large‐amplitude, localized potential structures with a flat‐top shape and separated electric field spikes parallel to the external magnetic field are shown to be supported by a particular self‐consistent charge distribution function that differs qualitatively from the distribution that supports the previously investigated Gaussian potentials. The evolution of the solitary structures is related to changes in the trajectories of the supporting, trapped electrons. It is shown that ignoring the interaction with the ions, the planar, isolated spikes propagating into a plasma with realistic, open boundary conditions, are stable in three dimensions. The addition of a perturbation to the large‐amplitude primary structure due to a smaller, faster secondary propagating spike results in destruction of the perturbing spike by modifying electron orbits of its trapped population, while the main spike gains an average momentum without any noticeable deformation. The analysis indicates robustness of the solitary spikes on the auroral field lines over significantly long times and their ability to propagate over long distances.
Mechanics of viscous wedges: Modeling by analytical and numerical approaches
Authors: Medvedev Sergei
Published: October 2002

Although complex rheological models have been used to study the evolution of orogenic wedges, many features of simple models remain to be fully explained. Here, we analyze the plane strain evolution of model orogenic wedges under simple boundary and rheological conditions. The uniform linear viscosity wedge is driven by motion of a basal boundary at a constant velocity. Three main analysis techniques are used: analytical (algebraic analysis of scales involved), semianalytical (thin sheet approximation), and a complete numerical approach. Application of this variety of approaches provides a better understanding of the underlying physics and outlines the advantages and disadvantages of the different techniques. The evolution of wedges can be divided into three phases. Initially, wedge growth is mainly vertical and symmetrical and depends little on the viscosity. The second phase exhibits almost self‐similar growth with the appearance of surface extension, within an otherwise compressional system, and development of asymmetry. The last phase involves widening of wedge and further development of asymmetry and surface extension, the average slope of wedge decreases during this phase. The Ramberg number, the ratio of characteristic gravitational to shear stress, defines the duration of each phase. Several parameters introduced here (mean surface slope, asymmetry of the wedge, surface extension, and near‐surface strain history) allow observations from natural wedges to be linked to the bulk viscosity of the model wedges. Analysis shows that the thin sheet approximation does not correctly describe the initial stages of wedge evolution.
Internal structure of Mount Merapi, Indonesia, derived from long‐offset transient electromagnetic data
Authors: Müller M. , Hördt A. , Neubauer F. M.
Published: October 2002

A long‐offset transient electromagnetic (LOTEM) survey was carried out on Merapi volcano, Indonesia, in 1998. LOTEM data have been recorded at 41 receiver locations which cover a 10 km W–E and a 15 km S–N profile. The signals were transmitted at four locations on the south, west, and north sides of Merapi. The data were interpreted with one‐dimensional (1‐D) inversions. In addition, two particular features were investigated with 3‐D modeling. On the south flank the magnetic field data show strong 3‐D distortions consistently over the profile which can only be explained by a conductive near‐surface structure, like a fracture filled with conductive fluids. The simulation of topographic effects shows that the interpretation is not significantly affected. On both profiles the most striking feature is a conductive layer of 20 Ω m at depths of 500–1000 m below the surface and a thickness of 1–2 km. The cause of the increased conductivity may be different for the summit area, the intermediate zone, and at the flanks: Below the summit, in vicinity of the conduit, the decrease in resistivity is produced either by hydrothermal fluids, by partial melts or rocks altered by the hydrothermal system, or by a combination. In the intermediate zone between the conduit and the flanks, either alteration or hydrothermal fluids may be the source of the conductivity increase. Fluids seem to be the most likely cause for the conductive layer at the flanks and the west–east striking anomaly. From the resistivity of the conductive layer and the typical porosity of volcanic rock we estimate fluid resistivities of 0.2–1 Ω m.
Aftershock triggering by complete Coulomb stress changes
Authors: Kilb Debi , Gomberg Joan , Bodin Paul
Published: October 2002

We examine the correlation between seismicity rate change following the 1992, M7.3, Landers, California, earthquake and characteristics of the complete Coulomb failure stress (CFS) changes (ΔCFS(t)) that this earthquake generated. At close distances the time‐varying “dynamic” portion of the stress change depends on how the rupture develops temporally and spatially and arises from radiated seismic waves and from permanent coseismic fault displacement. The permanent “static” portion (ΔCFS) depends only on the final coseismic displacement. ΔCFS diminishes much more rapidly with distance than the transient, dynamic stress changes. A common interpretation of the strong correlation between ΔCFS and aftershocks is that load changes can advance or delay failure. Stress changes may also promote failure by physically altering properties of the fault or its environs. Because it is transient, ΔCFS(t) can alter the failure rate only by the latter means. We calculate both ΔCFS and the maximum positive value of ΔCFS(t) (peak ΔCFS(t)) using a reflectivity program. Input parameters are constrained by modeling Landers displacement seismograms. We quantify the correlation between maps of seismicity rate changes and maps of modeled ΔCFS and peak ΔCFS(t) and find agreement for both models. However, rupture directivity, which does not affect ΔCFS, creates larger peak ΔCFS(t) values northwest of the main shock. This asymmetry is also observed in seismicity rate changes but not in ΔCFS. This result implies that dynamic stress changes are as effective as static stress changes in triggering aftershocks and may trigger earthquakes long after the waves have passed.
Lateral variation of Lg wave propagation in southern Mexico
Authors: Ottemöller Lars , Shapiro Nikolai M. , Krishna Singh Shri , Pacheco Javier F.
Published: October 2002

In this study we investigated lateral variation of Lg wave propagation in southern Mexico from recordings of 92 crustal earthquakes along 591 travel paths. The efficiency of Lg propagation was measured in terms of Lgto Pn spectral ratio. It was found that Lgpropagation is inefficient for travel paths through the Gulf of Mexico coastal plains and the Gulf of Tehuantepec, areas with thick layers of sediments. An average Lg quality factor, QLg, as a function of frequency for southern Mexico was estimated for the efficient Lg travel paths. The relation obtained for QLg in the frequency range 1.6–8 Hz is QLg(f) = 204 f0.85. The lateral variation of QLg−1was solved as a mixed‐determined inverse tomography problem, separately for each frequency, in which a spatial smoothness constraint was imposed and a priori information was added in poorly covered regions. The spatial resolution obtained was about 200 km. It was found that the Trans‐Mexican Volcanic Belt, the Gulf of Mexico coastal plains, and the area east of 94°W are characterized by lower than average QLgvalues, i.e., higher attenuation. High QLg values were obtained for the Mixteco‐Oaxaca terranes, while for the Guerrero terrane, values similar to the average were obtained. The results show a correlation between QLg and crustal structure and provide valuable information on lateral variation of QLg, which is needed for reliable prediction of ground motion during future earthquakes.
Structural analysis of hypocentral distribution of an earthquake sequence using anisotropic wavelets: Method and application
Authors: Gaillot P. , Darrozes J. , Courjault‐Radé P. , Amorese D.
Published: October 2002

Spatial organization of earthquake sequences is investigated to localize active rupture planes and to reconstruct the geometry of the inferred rupture zone. We have developed a new approach, the Normalized Optimized Anisotropic Wavelet Coefficient (NOAWC) method, to extract from a set of hypocenters the active ruptures planes. Our approach permits the detection of organized structures within a plane regardless of its size, location, shape anisotropy, and orientation. It includes the determination of a system of three perpendicular sections, minimizing the effects of projections of the hypocenters, and intrinsically accounts for uncertainty in the location of the seismic events. The accuracy and the effectiveness of the NOAWC procedure are illustrated on both synthetic and real data. An application to the M = 5.1 Arudy (French Pyrenees) aftershock sequence shows how a combination of the possible mathematically reconstructed geometries can be combined with the available fault plane solutions and geomorphological markers to determine the active rupture planes and to propose and validate a local tectonic model. This new multitool approach lends itself to quantitative and computer‐assisted analyses of large data sets.
Low‐temperature magnetic properties of lepidocrocite
Authors: Hirt A. M. , Lanci L. , Dobson J. , Weidler P. , Gehring A. U.
Published: October 2002

The magnetic properties of synthetic lepidocrocite (γ‐FeOOH) were investigated between 5 and 300 K. The purity of the samples was verified by X‐ray diffraction and Mössbauer spectra at room temperature. The lepidocrocite is paramagnetic at room temperature with a low field susceptibility of 57.8 × 10−8 m3 kg−1. It undergoes magnetic ordering at low temperature and a peak in in‐phase susceptibility occurs around 52 K, where it is antiferromagnetic. Thermal demagnetization of low‐temperature remanence after cooling in zero field and in a 2.5‐T field display a large loss in remanence from 5 to 30 K. Hysteresis loops are open and symmetric about the origin below 50 K; the remanent magnetization probably arises from a defect magnetic moment. The field‐cooled hysteresis loop at 5 K is shifted along the field axis, which implies an exchange bias between antiferromagnetic and ferrimagnetic alignment in the lepidocrocite.
Permeability, porosity and pore geometry of chemically altered porous silica glass
Authors: Mok U. , Bernabé Y. , Evans B.
Published: October 2002

In this study, we prepared samples of a well‐controlled, internally smooth, synthetic material (i.e., porous silica glass) and submitted them to chemical alteration while measuring changes in porosity and permeability. In the early stages of alteration we observed a large drop in permeability, whereas porosity remained essentially unchanged. In the late stages the permeability decrease became gradual so that even small decreases in permeability were accompanied by substantial decreases in porosity, as has been observed previously in natural sandstones and shales. A detailed microstructure study showed that smooth pore walls were roughened during alteration. Furthermore, the pore space heterogeneity increased and the connectivity decreased. A simple conceptual model suggests that the alterations occur while fluids are flowing.
Dynamic rupture on an interface between a compliant fault zone layer and a stiffer surrounding solid
Authors: Ben‐Zion Yehuda , Huang Yueqiang
Published: October 2002

We use two‐dimensional plane strain finite difference calculations to study dynamic rupture on a material discontinuity interface between a compliant elastic layer and a stiffer elastic medium. Previous works established that rupture along a material interface governed by Coulomb friction propagates as a unidirectional narrow pulse associated with self‐sharpening and divergent behavior. These effects are generated by coupling between spatial variations of slip along a material interface and local changes of normal stress. The simulations here employ a regularized friction law with a fading memory dependence of frictional strength on normal stress rather than the instantaneous Coulomb‐type response. We find that the self‐sharpening and divergent behavior found earlier with Coulomb friction exists also with regularized friction for large enough propagation distance. The parameters of regularized friction have to be fine tuned to produce apparent stability for a given propagation distance. However, eventually, the pulse always either diverges or dies. For cases with a single material interface the pulse strength increases with increasing contrast of shear wave velocity up to a maximum at ∼30% contrast, beyond which the generalized Rayleigh wave does not exist. This is similar to earlier results with Coulomb friction. For models having layer width comparable to or somewhat larger than the imposed source size, there is strong dependence of the pulse strength and shape on the layer width and velocity. The pulse amplitude is modulated by regular oscillations with period proportional to the layer width and is amplified for a range of layer widths. The results suggest that rupture along an interface between a compliant layer and a stiffer surrounding medium, initiated by a failure of an asperity with size not larger than the layer width, can become a self‐sustaining wrinkle‐like pulse.
Seismic investigations of a vent field associated with gas hydrates, offshore Vancouver Island
Authors: Riedel M. , Spence G. D. , Chapman N. R. , Hyndman R. D.
Published: October 2002

A three‐dimensional seismic reflection survey was carried out over a gas hydrate field offshore Vancouver Island in the area of Ocean Drilling Program Leg 146, Site 889/890. The area of investigation is located on the accretionary prism of the Cascadia margin in water depths of 1200–1500 m. Several seismic blank zones were observed in the seismic data over a frequency range from 30 Hz to 3.5 kHz, where the degree of blanking increases with seismic frequency. Time slices and selected horizon slices were used to define the spatial distribution of the blank zones. The zones range from 80 m to several hundred meters in width. One blank zone, almost circular with a diameter of ∼400 m, has a distinct seafloor expression. It shows the characteristics of a mud/carbonate mound and is probably associated with free gas expulsion. Hydrate was found at four sites with a piston corer within this blank zone at depths of 3–8 m below the seafloor. We present a tectonic model that explains the blank zones to be a result of differential uplift along thrust faults. The blank zones represent conduits for fluids and gas migrating upward. Blanking of the seismic energy is believed to be mainly the result of increased hydrate formation in lenses within the faults. The blanking effect is enhanced locally because of scattering at carbonates formed at the seafloor.
Quadrupolar electromagnetic field from detonation of high explosive charges on the ground surface
Authors: Soloviev S.P. , Surkov V. V. , Sweeney J. J.
Published: October 2002

Experimental results of a study of electromagnetic fields due to detonation of high explosive charges with mass of a few kilograms are presented in this paper. Data show that in the initial stage of explosion the fields decrease as the fourth power of distance. Such a quadrupolar type of field contradicts the concept that an explosion is characterized by an effective electric dipole. A theoretical analysis is given based on expansion of an electromagnetic field in multipole moments. The theory is used to make estimates of the quadrupolar and dipole moments of the electric charge system resulting from an explosion occurring at the Earth's surface; calculations of electric fields thus produced are similar to those observed.
Remote seismic influence on large explosive eruptions
Authors: Marzocchi Warner
Published: October 2002

The physical process governing the occurrence of the greatest explosive eruptions is characterized by a very high number of degrees of freedom. In this case, as well as for any stochastic and/or complex system, the knowledge of the process can be significantly improved by identifying any kind of nonrandom pattern. For example, a correlation with other processes would imply that some degrees of freedom are more important than the others. In this view, the main goal of this paper is to test if the perturbation induced on a volcanic area by great tectonic earthquakes can modify the probability of a volcanic event. The results obtained show that the occurrence of the largest explosive eruptions of the last century is significantly correlated to the earthquakes that occurred 0–5 and 30–35 years before, at distances up to 1000 km. Such a coupling might be attributed to the coseismic and postseismic stress diffusion. This result provides new insights that may be used in volcanic risk mitigation.
Full waveform numerical simulations of seismoelectromagnetic wave conversions in fluid‐saturated stratified porous media
Authors: Garambois Stéphane , Dietrich Michel
Published: October 2002

We present a full‐waveform modeling technique of the coupled seismoelectromagnetic wave propagation in fluid‐saturated stratified porous media. Our simulation code uses the macroscopic governing equations derived by Pride [1994], which couple Biot's theory and Maxwell equations via flux/force transport equations. In this theory the coupling mechanism is explained by electrokinetic effects taking place at the pore level. The synthetic seismoelectrograms and seismomagnetrograms are computed by extending the generalized reflection and transmission matrix method and by using a discrete wave number integration of the global reflectivity obtained in the frequency wave number domain. Synthetic time sections and snapshots of the wave propagation are used to study the seismic, electromagnetic, and seismoelectromagnetic waves properties in fluid‐saturated layered porous media. Two wave phenomena are investigated: (1) the electric and magnetic fields induced by the propagation of a seismic perturbation in a homogeneous porous medium and (2) the electromagnetic waves generated at depth when seismic waves propagate through a vertically heterogeneous porous medium. Concentrating on the second effect, we show that the zone which effectively contributes to the generation of EM disturbances along a plane interface coincides with the first Fresnel zone associated with a seismic‐to‐electromagnetic wave conversion. A numerical sensitivity study shows that the EM waves generated at depth by the passage of seismic waves through an interface are particularly sensitive to contrasts in porosity, permeability, fluid salinity, and fluid viscosity. Our numerical simulations highlight the potential of artificially generated seismoelectromagnetic converted waves for the characterization of the subsurface and its fluid content.
Repeated crustal thickening and recycling during the Andean orogeny in north Chile (21°–26°S)
Authors: Haschke M. , Siebel W. , Günther A. , Scheuber E.
Published: October 2002

Understanding Neogene arc crustal thickening in the central Andes requires (1) some estimate of initial pre‐Neogene (prior to 26 Ma) crustal thicknesses and (2) mechanisms that account for the remaining deficit in crustal thickening (10–30%). Mid‐Miocene horizontal crustal shortening can explain most but not all crustal thickening in the modern central Andean arc. Systematic changes in geochemical and Sr, Nd, and selected Pb isotopic data of Late Cretaceous–Eocene (∼78–37 Ma) and older arc magmatic episodes from north Chile provide new constraints on both. First, Andean crust may have been significantly thickened by long‐term underplating of mantle‐derived basalt from Jurassic to present. Second, estimated initial (late Eocene) crustal thicknesses of ∼45 km are consistent with (1) amphibole‐ and garnet‐bearing residual mineralogies for late Eocene syntectonic/posttectonic granitoids, (2) lower crustal P wave velocities of 7.3–7.7 km s−1 compatible with underplated mafic crust, and (3) results from recent experimental petrologic work showing garnet stability in mafic mineralogies ≥12 kbar (≥40 km crustal thickness). Analogous to older Andean magmatic episodes in north Chile, newly underplated basaltic crust may account for the remaining deficit in Neogene crustal thickening. Similar evolutionary patterns in geochemistry and initial Sr and Nd isotopic characteristics of Andean (200 Ma to present) magmatic rocks suggest that the Andean orogeny in this region evolved by a combination of processes of repeated arc migration, tectonic and magmatic crustal thickening, and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust.
Temporal variations of the segmentation of slow to intermediate spreading mid‐ocean ridges 2. A three‐dimensional model in terms of lithosphere accretion and convection within the partially molten mantle beneath the ridge axis
Authors: Rabinowicz Michel , Briais Anne
Published: October 2002

We present three‐dimensional numerical models of convection within the partially molten mantle beneath the ridge axis. The modeling takes into account the cavity flow driven by plate spreading, the diffuse upwelling due to plate accretion, and the shearing movement generated by large‐scale mantle flow. The ridge axis is free to move in the spreading direction to adjust to the maxima of tension at the lithosphere‐mantle interface induced by the convective circulation. The melt distribution in the mantle and the crustal production at the ridge axis are estimated using the formalism of McKenzie and Bickle [1988]. During the experiments the record of the ridge axis positions and crustal production is used to compute synthetic maps of the isochrons and oceanic crustal thickness. Close to the ridge, the ascending convective flow consists of 80‐ to 100‐km‐long hot sheets oriented either roughly parallel or orthogonal to spreading. Most ridge segments fit with the top of hot upwelling sheets, while transient transform faults coincide with the top of cold downwelling flows. The crustal maps display lineations subparallel or slightily oblique to spreading, a few tens of million years long, and separated by ∼60–50 km, resulting from the lithospheric record of the excess crust produced at the junction of hot sheets. When a junction of two hot sheets migrates outside the ridge axial plane, the crustal thickness maximum splits into two maxima along axis, and the induced lineation in the crustal map splits into two branches. The merging of lineations occurs when the ridge plane traps the junction of hot sheets. When the large‐scale mantle circulation moves parallel to the ridge crest, it slowly pushs the spreading‐parallel convective sheets. The resulting lineations form V shapes pointing in the same direction as the large‐scale flow. When the large‐scale flow parallels spreading, it slowly pushes the ridge‐parallel hot sheets in the upflow direction. Thus the ridge segments attached to these hot sheets also migrate in the upflow direction. After several tens of million years the cavity flow driven by the spreading closes most of the transform segments and collects most of the ridge segments to form large continuous lines quasi‐orthogonal to spreading. Using the relationships between crustal lineations and convective flow in the models, we interpret the lineations described in the satellite‐derived gravity maps on the flanks of the Atlantic, Indian, and South Pacific plate boundaries analyzed in the companion paper by Briais and Rabinowicz [2002].
Redefinition of the Amery Ice Shelf, East Antarctica, grounding zone
Authors: Fricker Helen Amanda , Allison Ian , Craven Mike , Hyland Glenn , Ruddell Andrew , Young Neal , Coleman Richard , King Matt , Krebs Kim , Popov Sergey
Published: October 2002

New evidence is presented which shows that the Amery Ice Shelf, East Antarctica, extends ∼240 km upstream of the previously reported position. We combine a digital elevation model of the Amery Ice Shelf created from ERS‐1 satellite radar altimetry with measured ice thicknesses and a simple density model in a hydrostatic (buoyancy) calculation to map the extent of the floating ice. This reveals that the ice is floating as far south as 73.2°S. The result is confirmed by static GPS measurements collected during three consecutive field campaigns on the Amery Ice Shelf where the vertical component of the GPS shows a clear tidal signal at 72.98°S. Other evidence for the grounding zone position comes from an analysis of satellite imagery, mass flux calculations, and ice radar data. The southward extension of the grounding line substantially alters the shape and dimensions of the ocean cavity beneath the ice shelf, which has implications for modeling studies of sub‐ice shelf processes, such as basal melting and freezing, ocean circulation, and tides. The new grounding line position will also improve geophysical studies, where the computation of ocean tidal loading corrections is important for postglacial rebound estimates and correction of satellite altimetry measurements within the region.
The Bolivian Altiplano conductivity anomaly
Authors: Brasse Heinrich , Lezaeta Pamela , Rath Volker , Schwalenberg Katrin , Soyer Wolfgang , Haak Volker
Published: October 2002

A long‐period magnetotelluric study was carried out in the central Andes between latitudes 19.5°S and 21°S along two almost parallel profiles of 220 and 380 km length, respectively. The investigation area extends from the Pacific coast to the southern Altiplano Plateau in the back arc of the South American subduction zone. The main geoelectrical structure resolved is a broad and probably deep‐reaching highly conductive zone in the middle and deeper crust beneath the high plateau. Although the data show deviations from two‐dimensionality, a two‐dimensional approach is justified for large parts of the profiles. Sensitivity studies were carried out in order to constrain the depth extent. Another electrically conductive structure was resolved in the middle crust of the Chilean forearc, thought to be connected with the Precordillera fault system. The Andean Continental Research Program (ANCORP) seismic reflection profile, carried out along the same line at 21°S, revealed highly reflective zones below the Altiplano, in good correlation with the upper boundary of the Altiplano conductor. This highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp/vs ratio and, although not well resolved, with low Qp seismic quality factors. Taking into account the enhanced heat flow and a derived temperature model, the most probable explanation lies in the assumption of granitic partial melts. The good conductor below the volcanic arc which was found in regions farther south at 22°S gradually vanishes toward the north; this is consistent with the results of seismic tomography concerning Qp values and a gap of recent volcanism.
Crust and upper mantle structure beneath western China from P wave travel time tomography
Authors: Xu Yi , Liu Futian , Liu Jianhua , Chen Hui
Published: October 2002

Three‐dimensional velocity images of the crust and upper mantle beneath western China are obtained through seismic tomography by using P wave arrival times of local/regional and teleseismic events recorded in Chinese and Kyrgyzstan seismic networks. At shallow depths, most mountain belts and tectonically elevated areas are imaged as higher velocities, relative to adjacent sediment‐filled basins and foreland depressions with lower velocities. At mid‐lower crust depths, the Kyrgyz Tian Shan is underlain by lower velocity material and a distinct low‐velocity zone in the south‐north direction is observed between the eastern and western parts of the Chinese Tian Shan. The Moho of the southern Tarim basin has a greater depth over 50 km in front of the western Kun Lun, compared to the northern and western Tarim basin. Lateral heterogeneity within the upper mantle is apparent at depths from 71 to 571 km. The uppermost mantle beneath the Kyrgyz Tian Shan, Altay, Qi Lian, and eastern Kun Lun is characterized by low‐velocity material that is probably related to the upwelling of hot mantle. The lithospheric thickness varies from 120 to 170 km beneath the Chinese Tian Shan and northern Tibetan Plateau to 200–250 km beneath the Tarim basin. Deep coupling pattern of the Chinese Tian Shan with adjacent provinces can be classified into the embedding, detaching and sinking, and underthrusting of lithospheric roots. In addition, the lithosphere of the northern edge of the Tibetan Plateau seems to be thickened, in contrast to the central part of the northern plateau. North dipping low velocities exist in the upper mantle between the Tibetan Plateau and provinces to its north, which might indicate mantle flow from the Tibetan Plateau toward surrounding areas, but it has been blocked by the thick Tarim lithosphere.
Tectonic forces controlling the regional intraplate stress field in continental Australia: Results from new finite element modeling
Authors: Reynolds Scott D. , Coblentz David D. , Hillis Richard R.
Published: October 2002

The tectonic forces controlling the present‐day regional intraplate stress field in continental Australia have been evaluated through a finite element analysis of the intraplate stresses in the Indo‐Australian plate (IAP). Constraint for the modeling is provided by an observed regional stress field based on observations in 12 stress provinces. A weighted “basis set” method has been employed to provide an efficient means to evaluate a very large number of tectonic force combinations and to make a quantitative assessment of the fit between the observed and predicted stress fields. Our modeling results indicate that the major features of the regional stress field in continental Australia can be explained in terms of a geologically plausible array of tectonic forces. While the results continue to substantiate that modeling of the Australian intraplate stress field is inherently nonunique, we are nevertheless able to draw a number of fundamental conclusions about the tectonic settings along the principal plate boundary segments including the following: (1) The Himalayan and New Guinea boundaries exert a compressional force on the IAP. (2) Fitting the stress field in the Bowen Basin requires compressional boundary forces along the Solomon and New Hebrides subduction zones directed toward the interior of the IAP. (3) East‐west compression in eastern Australia requires a small compressional force along the Tonga‐Kermadec subduction zone. (4) Fitting the stress field in southeastern Australia requires compressional forces along the New Zealand, Puysegur Trench, and Macquarie Ridge boundary segments. (5) Significant tensional slab‐pull forces exist only along the Java subduction zone.
Wetting properties of anorthite aggregates: Implications for fluid connectivity in continental lower crust
Authors: Yoshino Takashi , Mibe Kenji , Yasuda Atsushi , Fujii Toshitsugu
Published: October 2002

The connectivity of C‐H‐O fluids in anorthite aggregate was assessed by measurements of the dihedral angle, θ, at conditions of 800–1000°C and 0.8–1.2 GPa for duration of 1–21 days. The longer‐duration experiments show reduction of the effect of crystal anisotropy on pore geometry. Especially, fluid‐filled small pores (<5 μm) in anorthite aggregate quickly approach to textural equilibrium controlled by minimization of surface energies in <3 days. Anorthite aggregates with CO2 fluid show large dihedral angle over 120°. In the anorthite aggregates with aqueous fluid the dihedral angles are over 60° at pressure of 1.0 GPa and decrease across critical angle of 60° with increasing pressure. Over the range of experimental conditions, isopleths of dihedral angle show nearly zero to negative dP/dT slope with increasing temperature. The observed reduction in dihedral angle with increasing pressure and temperature may be attributed to increase in plagioclase solubility in aqueous fluid. Natural anorthite (with small amounts of albite component) shows smaller dihedral angle than pure anorthite. At higher temperature conditions the anorthite content of the run products deviates progressively from that of starting material. This suggests that preferential solubility of albite component relative to anorthite affects the equilibrium fluid distributions of plagioclase by aqueous fluid. Experimental results point to the likelihood that plagioclase‐rich lower continental crust will not allow intergranular fluid flow of C‐H‐O fluids even in the presence of high geothermal gradient.
Geochemistry of back arc basin volcanism in Bransfield Strait, Antarctica: Subducted contributions and along‐axis variations
Authors: Keller Randall A. , Fisk Martin R. , Smellie John L. , Strelin Jorge A. , Lawver Lawrence A.
Published: October 2002

Bransfield Strait is a Quaternary, ensialic back arc basin at the transition from rifting to spreading. Fresh volcanic rocks occur on numerous submarine features distributed along the rift axis, including a discontinuous neovolcanic ridge similar to the nascent spreading centers seen in some other back arc basins. Smaller edifices near the northeast end of the rift yielded basalts with the most arc‐like compositions (e.g., high large‐ion lithophile element/high field strength element and 87Sr/86Sr). The most mid‐ocean ridge basalt (MORB)‐like basalts are from a large, caldera‐topped seamount and a 30‐km‐long axial neovolcanic ridge toward the southwest end of the rift, but these two features also yielded andesite and rhyolite, respectively. The volcanic and geochemical variations are not systematic along axis and do not reflect the unidirectional propagation of rifting suggested by geophysical data. The most depleted basalts have major and trace element characteristics indistinguishable from MORB except for slightly higher Cs and Pb concentrations. Pb isotopic ratios show little variation compared to Sr and Nd isotopic ratios and do not extend to the depleted Pb isotopic ratios found in other back arc basins. Either the depleted mantle beneath Bransfield Strait has higher than normal Pb isotopic ratios or the subducted component beneath Bransfield Strait has such high Pb concentrations that it dominates the Pb isotopic composition of the Bransfield Strait mantle without significantly affecting the Sr and Nd isotopic compositions. Metalliferous sediments and fluids extracted from a subducting slab may have the necessary high concentrations of Pb.
Global variability of transition zone thickness
Authors: Gu Yu J. , Dziewonski Adam M.
Published: October 2002

We analyze recordings from more than 3000 shallow earthquakes and measure S410S – S660S differential travel times on a global scale. These differential times are obtained using a novel cross‐correlation technique that eliminates effects associated with a priori assumptions of crustal thickness and velocity variations above the upper mantle transition zone. Our measurements show absolute perturbations up to 8 s from the global average (242 km), which imply transition zone thickness variations of ±20 km on a length scale of ∼2000 km. The transition zone under major subduction zones, e.g., the western Pacific and South America, is significantly thicker than the global average; this could be associated with large‐scale depressions of the 660‐km discontinuity due to the accumulation of cold oceanic lithosphere at the base of the upper mantle. A narrow transition zone is observed under the central Pacific and northern Atlantic Oceans. We also observe a modest anticorrelation between S410S – S660S times and the delay times of shear waves (predicted using S12/WM13 [Su et al., 1994]) in the transition zone, which suggests considerable thermal influence on the large‐scale topography of the 410‐ and 660‐km discontinuities. The lack of a stronger anticorrelation suggests that (1) compositional heterogeneities may be present and (2) a significant fraction of thermal anomalies near the transition zone do not extend through the transition zone and influence both the 410‐ and the 660‐km discontinuities. A regional analysis shows that the transition zone is, on average, 6–8 km thicker under continents than under oceans; this implies a temperature difference of 50–70°C. However, we do not observe a clear correlation between thickness and the age of the crust.
Cataclastic slip band distribution in normal fault damage zones, Nubian sandstones, Suez rift
Authors: Du Bernard X. , Labaume P. , Darcel C. , Davy P. , Bour O.
Published: October 2002

In this paper, we characterize the geometry of damage zones that form around the main slip planes of normal faults. Specifically, we examine five faults of varying throws that affect the Nubian sandstones along the Suez rift. To quantify the density of cataclastic slip bands (CSBs) associated with the main slip plane, we recorded the position of all visible CSBs along a scan line perpendicular to the fault through to the damage zone. For each outcrop the scan line record is ∼30 m long. Resulting density diagrams display concentrations of CSBs and clearly indicate a widening of the damage zone with increasing throw. A correlation integral was calculated for each CSB population in order to analyze both the scaling property of the density distribution and potential correlation lengths. From centimeter to meter scale, representing 2 orders of magnitude, the correlation integral appears adequately modeled by a power law, emphasizing the fractal property for the CSB distribution. For the five faults the calculated correlation dimension is constant within its error of determination, Dc = 0.87 ± 0.05. The validity range of the fractal nature was derived from an adequate normalization of the correlation integral and from a comparison with synthetic fractal fracture networks. It appears that a finite correlation length corresponding to the damage zone width is detectable for meter‐scale throws; for larger throws the correlation length is not detectable within the sampling domain.
Influence of grain characteristics on the friction of granular shear zones
Authors: Mair Karen , Frye Kevin M. , Marone Chris
Published: October 2002

Numerical models of granular shear show lower friction and a greater tendency for stick slip than laboratory studies designed to investigate fault mechanics. Here we report on laboratory experiments designed to reproduce the conditions of numerical models and to test the role that grain characteristics play in controlling frictional behavior. Friction and microstructural data are compared for direct shear experiments on thin layers (2–3 mm) of angular quartz sand and spherical glass beads. We study the effect of grain shape, roughness, size distribution, and comminution. In a nonfracture loading regime, sliding friction for smooth spherical particles (μ ∼ 0.45) is measurably lower than for angular particles (μ ∼ 0.6). A narrow particle size distribution (PSD) of spherical beads (105–149 μm) exhibits unstable stick‐slip behavior, whereas a wide PSD of spheres (1–800 μm) and the angular gouge display stable sliding. At higher stress, where grain fracture is promoted, initially spherical particles become stable with accumulated slip, and friction increases to the level observed for angular gouge. We find that frictional strength and stability of a granular shear zone are sensitive to grain shape, PSD, and their evolution. We suggest that a low friction translation mechanism, such as grain rolling, operates in gouge composed of smooth particles. Our results show that the first‐order disparities between laboratory and numerical studies of granular shear can be explained by differences in grain characteristics and loading conditions. Since natural faults predominantly contain angular gouge, we find no evidence for a fault‐weakening mechanism associated with the presence of gouge.
Temporal variations of the segmentation of slow to intermediate spreading mid‐ocean ridges 1. Synoptic observations based on satellite altimetry data
Authors: Briais Anne , Rabinowicz Michel
Published: October 2002

The high‐resolution geoid and gravity maps derived from ERS‐1 and Geosat satellite geodetic missions reveal a set of small‐scale lineations on the flanks of slow to intermediate spreading mid‐ocean ridges. Assuming that these lineations reflect the variations in crustal structure induced by mid‐ocean ridge axial discontinuities, we use them to investigate how the discontinuities, and the segments they bound, appear, migrate, and disappear. We provide a synoptic description of the main characteristics of the crustal structure variations, as well as their evolution in time, over the flanks of the Mid‐Atlantic, Indian, and Pacific‐Antarctic Ridges. The second‐order segment length does not appear to vary with the spreading rate for the slow to intermediate spreading ridges investigated here. The amplitude of the gravity signal associated with off‐axis discontinuity traces increases with the obliquity of the ridge to spreading and decreases with spreading rate and with the proximity of a ridge section to a hot spot. The patterns of the gravity lineations appear to be very homogeneous over 500‐ to 1000‐km‐large corridors bounded by large fracture zones. Far from hot spots, corridors are characterized either by segments bounded by discontinuities migrating back and forth along the axis, implying a lifetime of 10–30 Myr for the segments, or by segments and discontinuities very stable in space and time, surviving for 40–50 Myr. Closer to hot spots, the segmentation is affected in two ways. First, segments tend to migrate along axis away from hot spots, or toward cold spots. Second, asymmetric spreading tends to keep sections of ridges closer to hot spots than normal spreading would. These observations support the hypothesis that ridge segmentation and its evolution are controlled by mantle dynamics. Our analysis provides observational constraints for further models of crustal production along ridges, which are presented in the companion paper by Rabinowicz and Briais [2002].
Climate change in India inferred from geothermal observations
Authors: Roy Sukanta , Harris Robert N. , Rao R. U. M. , Chapman David S.
Published: October 2002

Temporal variations in surface ground temperature impart a signal to the subsurface thermal regime that is captured in borehole temperature‐depth profiles. Seventy temperature‐depth profiles in India, located between 12° and 28°N, are analyzed to infer past changes in ground temperature. These profiles exhibit predominantly positive anomalous temperatures relative to the background thermal regime beginning at depths of 75–150 m and increasing toward the surface. This pattern is consistent with warming over the past century. An interpretation in terms of linear surface temperature change indicates warming of about 0.9° ± 0.1°C over the past 150 years. Relatively complete surface air temperature (SAT) records from meteorological stations near the boreholes indicate similar rates of warming over the last century. A combined analysis of borehole temperatures and SAT records yields a long‐term, preobservational mean temperature, 0.8° ± 0.1°C lower than the 1961–1990 mean SAT. When the most recent decade is included directly in the analysis, the average total warming in India from the early 1800s to the late 1990s is ∼1.2°C.
Magnetic properties of variably serpentinized abyssal peridotites
Authors: Oufi Omar , Cannat Mathilde , Horen Hélène
Published: October 2002

We have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven Deep Sea Drilling Project and Ocean Drilling Program sites. The magnetic susceptibility (K) of these samples does not increase linearly with the degree of serpentinization (S). Instead, K remains modest in partially serpentinized samples (S < 75%) and then increases rapidly. The rate of formation of magnetite during serpentinization increases as the iron content of serpentine minerals decreases: ∼6% FeO in the first stages of serpentinization and only 2–3% FeO as serpentinization proceeds beyond S ∼ 75%. Contrasting remanent behaviors observed in extensively serpentinized samples (some samples have natural remanent magnetizations (NRM) comparable to those of basalts, while other samples have low NRM values, for similarly high K values) are due to differences in the effective magnetic grain size and appear related to serpentine textures. Samples with small effective magnetic grain sizes have a well‐developed serpentine meshwork, outlined by thin vein‐like magnetite concentrations. Most serpentinized abyssal peridotites have this well‐developed serpentine mesh texture. We thus propose that high NRM (4–10 A/m on average) and K (∼0.07 SI on average) are the most likely signature for extensively serpentinized (S > 75%) peridotites in the oceanic crust. Moderately serpentinized peridotites (S < 75%) have NRM values <5 A/m and K < 0.05 SI. Low‐temperature oxidation of magnetite is found to lower the NRM and K values of seafloor samples and of cataclastically deformed drilled samples.