bluuebunny/crossref_metadata_2025 huggingface dataset:
Selected elastic moduli and their temperature derivatives for olivine and garnet with different Mg/(Mg+Fe) contents: Results from GHz ultrasonic interferometry
Authors: Chen Ganglin , Spetzler Hartmut A. , Getting Ivan C. , Yoneda Akira
Published: February 2004

Selected P‐wave acoustic travel times through single crystal samples of olivine and garnet with different Mg/(Mg+Fe) ratios were measured at elevated temperatures (from 293 K to 363 K) using a newly developed GHz ultrasonic interferometer. These high precision data allow us to examine aspects of the elasticity‐composition systematics of olivine (forsterite‐fayalite) and garnet (pyrope‐almandine) solid solution series. For every 10% decrease in the Mg/(Mg+Fe) content, the P‐velocity of olivine along the b‐axis decreases about 3% and ∂lnC22/∂T decreases about 7%; for garnet ∂ln⌊C44 + (C11 + C12) / 2⌋/∂T decreases about 6%. If such velocity‐composition systematics are typical and hold at the P‐T regime of the mantle, only a 3–4% variation in the Mg/(Mg+Fe) content of olivine is required to explain the lateral seismic velocity variation in the upper portion of the mantle within one tectonic region (dlnVp∼l%), and 7–8% variation in the Mg/(Mg+Fe) content to explain the 2% lateral seismic velocity variation of the upper mantle globally.
WIND/WAVES observations of man‐made radio transmissions
Authors: Kaiser M. L. , Desch M. D. , Bougeret J.‐L. , Manning R. , Meetre C. A.
Published: February 2004

The WAVES radio and plasma wave instrument on the Wind spacecraft, launched on November 1, 1994, has detected copious shortwave radio transmissions from broadcast stations on Earth. WAVES can easily detect signals consisting of only a few kW of transmitted power, suggesting that useful ionospheric propagation research can be accomplished with very modest equipment. We also conclude that the terrestrial radio spectrum is quite “non natural” so that any extraterrestrial radio astronomer should easily deduce that Earth is populated with a civilization with technical capabilities.
Evidence for the removal of gaseous HNO3 inside the arctic polar vortex in January 1992
Authors: Höpfner M. , Blom C. E. , Blumenstock T. , Fischer H. , Gulde T.
Published: February 2004

Unexpectedly low column amounts of HNO3 (1.3×1016 cm−2) inside the Arctic polar vortex were obtained on January 9, 1992 by airborne FTIR emission measurements. After correction for dynamic variations it is concluded that about 40% of the column HNO3 had been removed from the gas phase. The observed depletion is consistent with a nearly equilibrium saturation of HNO3 over nitric acid trihydrate. It cannot be explained by the uptake of HNO3 in ternary H2SO4/H2O/HNO3 mixtures.
Bromine‐chlorine coupling in the Antarctic Ozone Hole
Authors: Danilin Michael Y. , Sze Nien‐Dak , Ko Malcolm K. W. , Rodriguez Jose M. , Prather Michael J.
Published: February 2004

The contribution from the chlorine and bromine species in the formation of the Antarctic ozone hole is evaluated. Since chlorine and bromine compounds are of different industrial origin, it is desirable, from a policy point of view, to be able to attribute chlorine‐catalyzed loss of ozone with those reactions directly involving chlorine species, and likewise for bromine‐catalyzed loss. In the stratosphere, however, most of the chemical families are highly coupled, and, for example, changes in the chlorine abundance will alter the partitioning in other families and thus the rate of ozone loss. This modeling study examines formation of the Antarctic ozone hole for a wide range of bromine concentrations (5–25 pptv) and for chlorine concentrations typical of the last two decades (1.5, 2.5 and 3.5 ppbv). We follow the photochemical evolution of a single parcel of air, typical of the inner Antarctic vortex (50 mbar, 70°S, NOy=2 ppbv, with polar stratospheric clouds(PSC)) from August 1 to November 1. For all of these ranges of chlorine and bromine loading, we would predict a substantial ozone hole (local depletion greater than 90%) within the de‐nitrified, PSC‐perturbed vortex. The contributions of the different catalytic cycles responsible for ozone loss are tabulated. The deep minimum in ozone is driven primarily by the chlorine abundance. As bromine levels decrease, the magnitude of the chlorine‐catalyzed ozone loss increases to take up the slack. This is because bromine suppresses ClO by accelerating the conversion of ClO and Cl2O2 back to HCl. For this range of conditions, the local relative efficiency of ozone destruction per bromine atom to that per chlorine atom (α‐factor) ranges from 33 to 55, decreasing with increase of bromine.
On the cross‐sectional geometry of thermal erosion channels formed by turbulent lava flows
Authors: Jarvis Richard A.
Published: February 2004

A new theoretical framework for interpreting the cross‐sectional geometry of self‐formed lava channels is introduced. The method may be applied to any lava channel, or tube, and is demonstrated by studying thermal erosion by a hot, turbulently flowing lava. The cross section of a thermal erosion channel is often characterized by undercutting of the channel walls when the upper surface of the lava flow falls beneath the original ground level. The degree of lateral undercutting is shown here to be a function of the flow parameters, which enables the shape of the channel cross section to be used to determine a lower bound upon the lava flow rate. The solution for the geometry of the lava channel is then coupled to an existing two‐dimensional model for thermal erosion by komatiite flows to determine the effects of lateral erosion and variable flow cross section upon cooling of the lava and contamination of the lava by assimilated ground material. Lateral thermal erosion is shown to lead to a thinning of the lava flow with time, even at a constant eruption rate, which reduces its erosive power and enhances cooling of the lava. Finally, a linear stability analysis of the evolution equation for the channel cross section suggests that inefficient lateral mixing in a thin lava flow can lead to a morphological instability and eventual splitting of the channel. The instability is suppressed at short wavelengths by the variation in melting rate with local wall curvature, and the fastest growing symmetric mode has a wavelength equal to half the channel width, which corresponds to the formation of midstream islands.
Revised predictions of long‐period ocean tidal effects on Earth's rotation rate
Authors: Dickman S. R. , Nam Young S.
Published: February 2004

The rotational response of Earth to long‐period tidal forces, embodied in a “zonal response function,” can be expected to vary with frequency because of variable contributions by the oceans, mantle, and core. The zonal response function has been estimated from 9 years of International Radio Interferometric Surveying (IRIS) universal time (UT1) data and compared with theoretical predictions, using a spherical harmonic tide model to compute the oceans' dynamic response, at semiannual, monthly, fortnightly, and 9‐day lunisolar tidal frequencies. Different amounts of mantle anelasticity have been considered for both the oceanic and solid earth responses; predictions have been made assuming axial core‐mantle coupling which is either complete or absent. Additionally, an extensive recalibration of the ocean model's frictional parameters was performed using constraints derived in part from Space92 polar motion data; zonal response function predictions have also been made employing this recalibrated ocean tide model. Our results indicate that any amount of core coupling can be ruled out at a fortnightly period and probably at a 9‐day period, but not at a monthly period. Our results also suggest that the mantle responds purely elastically at a 9‐day period but may behave increasingly anelastically at longer periods. A simple dispersive rule is postulated for periods ranging up to the 14‐month Chandler wobble period.
Early Cretaceous paleomagnetic results from Marie Byrd Land, West Antarctica: Implications for the Weddellia collage of crustal blocks
Authors: DiVenere Vic , Kent Dennis V. , Dalziel Ian W. D.
Published: February 2004

A new ∼117 Ma paleomagnetic pole has been defined from the study of volcanic and plutonic rocks from the eastern portion Marie Byrd Land (MBL). The new pole (185.6°E/56.8°S, A95 = 8.7°) implies that the eastern portion of MBL was an integral part of Weddellia, which included the ancestral Antarctic Peninsula, Thurston Island, and Ellsworth‐Whitmore Mountains blocks of West Antarctica. This pole is generally similar to a ∼125 Ma pole from Thurston Island. Both poles call for major clockwise rotation and poleward motion of eastern MBL and Thurston Island between the Early Cretaceous (125–117 Ma) and the mid‐Cretaceous (110–100 Ma). We propose that in the Early Cretaceous, eastern MBL and the Eastern Province of New Zealand were part of a continuous active Pacific margin of Gondwana, connecting with the Antarctic Peninsula, and distinct from western MBL, the Western Province of New Zealand, and North Victoria Land. These western terranes are thought to have accreted to Gondwana in the Devonian. Eastern MBL and the Eastern Province of New Zealand amalgamated with western MBL and the Western Province of New Zealand by the mid‐Cretaceous. Major Early Cretaceous motions of the Weddellia blocks postdate the estimated initiation of seafloor spreading in the Weddell Sea and therefore may be the result of plate reorganization during the Cretaceous Quiet Zone.
Remagnetization of the folded Belden Formation, northwest Colorado
Authors: Fruit David , Elmore R. Douglas , Halgedahl Sue
Published: February 2004

Paleomagnetic and geochemical results are used to evaluate the origin of an apparent synfolding magnetization in Belden Formation (Pennsylvanian) limestones on an asymmetric fold in Colorado. The natural remanent magnetizations contain three components: a modern viscous magnetization removed at low temperatures, a reversed Tertiary magnetization (declination (D) = 186.7°, inclination (I) = −54.6°) of thermoviscous origin removed at intermediate temperatures, and an apparently synfolding Late Cretaceous/early Tertiary characteristic magnetization (D = 338.7°E, I = 62.5°, precision parameter k = 54.9, radius of 95% probability cone ∝95 = 5.9°, 65% unfolding) removed by 580°C. Hysteresis properties suggest that the characteristic magnetization resides in single‐domain magnetite. Low burial temperatures and the presence of authigenic magnetite are consistent with a chemical origin for this magnetization. The 87Sr/86Sr values (0.7088–0.7100) show the limestones to be radiogenic, with 87Sr enriched halos around calcite‐cemented tectonic fractures. There is no control on the distribution and intensity of the magnetization by the veins or structural position (e.g., highly mineralized versus less mineralized rock). Lithology, rather than fluid alteration, controls the magnetization. Fecal pellets contain pyrite, with alteration rims identified as magnetite by scanning transmission electron microscope X ray diffraction patterns. Selective demagnetization has established that the fecal pellets carry the characteristic magnetization. It is not possible to unequivocally establish whether the characteristic magnetization is synfolding or prefolding. A synfolding magnetization caused by the syndeformational mineralizing fluids, however, can be ruled out. Structural modification of a prefolding component and a synfolding piezoremanent magnetization cannot be ruled out but are not considered likely mechanisms of remanence acquisition. Overprinting of a prefolding chemical magnetization by a postfolding viscous magnetization residing in multidomain grains is a likely explanation for the synfolding test result. The replacement of pyrite by magnetite is interpreted as prefolding in origin, perhaps triggered by an oxidizing fluid or by diagenesis of hydrocarbons.
Crustal structure of the Tuamotu Plateau, 15°S, and implications for its origin
Authors: Ito Garrett , McNutt Marcia , Gibson Richard L.
Published: February 2004

We investigate the sedimentary and volcanic structure of the Tuamotu Plateau with multichannel seismic, seismic refraction, and gravity data along a ship track crossing the plateau near 15°S. The volcanic basement of the central portion of the plateau is capped with a 1 to 2‐km‐thick sediment layer composed of two compositional sequences. The uppermost sequence, with semblance‐derivedPwave velocities of 1.6–1.9 km/s and thicknesses of 0.2–0.9 km, is composed of pelagic sediments. The underlying sequence, with velocities 2.5–3.5 km/s and thicknesses of 0.5–1.5 km, is composed of limestone and volcaniclastic sediments. Sonobuoy refraction data show the upper 1 km of the volcanic basement to have velocities 4.5–5.5 km/s. The gravity data indicate that the platform is compensated by an elastic lithosphere with effective thickness 5±5 km and that the volcanic thickness is 9–10 km thicker than normal oceanic crust with a volume of 2.0–2.6×l06km3. The inferred eruption rates of 0.1–0.13 km3/yr are comparable to those of the Hawaiian and Marquesas island chains but substantially less than those of many oceanic plateaus. Radiometric and paleontological ages for the plateau and geomagnetic dates of the surrounding seafloor indicate that the northwestern portion of the plateau formed –600 km off the axis of the paleo‐Pacific‐Farallon spreading center, on lithosphere of age ∼10–20 Ma. Linear volcanic ridges and scarps bounding deep sediment‐filled basins, however, are similar to features of oceanic plateaus which formed at or near accretionary plate boundaries. We suggest that these volcanic ridges and the gross plateau like morphology were formed by magma that was channelled along the lithospheric discontinuities left behind by a southward propagating rift segment of the nearby spreading center. We attribute the formation of the northwestern portion of the Tuamotu Plateau to the passage of two hotspots during times 50–30 Ma as they migrated beneath the Pacific plate but remained west of the Tuamotu propagator.
Evidence for diffuse extension of the Pacific Plate from Pukapuka ridges and cross‐grain gravity lineations
Authors: Sandwell David T. , Winterer Edward L. , Mammerickx Jacqueline , Duncan Robert A. , Lynch Mary Ann , Levitt Daniel A. , Johnson Catherine L.
Published: February 2004

Satellite altimeter measurements of marine gravity reveal 100 to 200‐km wavelength lineations over a wide area of the Pacific plate oriented roughly in the direction of absolute plate motion. At least three mechanisms have been proposed for their origin: small‐scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere; diffuse N‐S extension of the lithosphere resulting in lineated zones of extension (boudins); and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism. Recently, several chains of linear volcanic ridges have been found to be associated with the gravity lineations. Following ridgelike gravity signatures apparent in high‐resolution Geosat gravity measurements, we surveyed a series of volcanic ridges that extend northwest from the East Pacific Rise flank for 2600 km onto 40 Ma seafloor. Our survey data, as well as radiometric dates on samples we collected from the ridges, provide tight constraints on their origin: (1) Individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion. (2) The ridges formed along a band 50 to 70‐km‐wide in the trough of one of the more prominent gravity lineations. (3) Radiometric dates of the largest ridges show no hotspot age progression. Moreover, the directions predicted for minihotspot traces older than 24 Ma do not match observed directions of either the gravity lineations or the ridges. Based on this last observation, we reject the minihotspot model. The occurrence of the ridges in the trough of the gravity lineation is incompatible with the small‐scale convection model which would predict increased volcanism above the convective upwelling. We favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere. However, the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model. Perhaps the fault scarps are buried under more than 30 m of sediments and/or covered by the elongated ridges. Finally, we note that if ridge‐push force is much smaller than trench‐pull force, then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion.
Melting of the continental crust: Some thermal and petrological constraints on anatexis in continental collision zones and other tectonic settings
Authors: Thompson Alan Bruce , Connolly James A. D.
Published: February 2004

Useful constraints on the depth‐temperature range and degree of melting of lower crustal rocks have been deduced by combining the results from experimental petrology with plausible PTt paths calculated for continental collision zones. H2O is required to generate melts at the temperatures of crustal orogenesis. Our modeling has shown that residual free water from subsolidus dehydration reactions produces less than 0.5% granitic melt at the H2O‐saturated solidus. Breakdown of hydrous minerals results in fluid‐absent (dehydration)‐ melting. For average geotherms such melting of muscovite and biotite in metapelite will generate at most 25% granitic melt. Because of much higher solidus temperatures, fluid‐absent melting of hornblende in metabasalt (amphibolite) will not occur in collision zones without augmented heat input from the mantle. Fluid‐absent melting of epidote in metabasalt, and low Ca amphibole and biotite in metavolcanics, can produce up to 10% melt in continental collision zones. Minor melt production from dehydration‐melting of chlorite, staurolite, chloritoid, or talc will occur at pressures greater than ∼0.6 GPa. The calculated paths envelope small regions or PT space, thus identifying which mineral reactions are important for crustal anatexis and at which PT conditions future experiments should be made. In a dynamically evolving crust during uplift, thermal buffering by melting reactions is a minor component of the heat bucket. Extension of thickened continental crust will not promote dehydration‐melting without the incursion of mantle heat. Delamination of thickened eclogitic lower crust, or decompression of the asthenosphere in response to lithosphere thinning are the most promising mechanisms to induce extensive lower crustal melting including amphibolite.
Case for very low coupling stress on the Cascadia Ssubduction Fault
Authors: Wang Kelin , Mulder Taimi , Rogers Garry C. , Hyndman Roy D.
Published: February 2004

A fundamental problem in plate tectonics is the shear strength of major plate boundary faults. This translates to the question whether the generally observed small earthquake stress drops of 3–10 MPa on major faults release most of the accumulated stress or only a small fraction of it. There is strong evidence that the San Andreas fault, a major transform plate boundary, is weak (<20 MPa shear resistance). It is not yet clear whether subduction thrust faults are also weak. We present two types of evidence from the northern Cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault, comparable to that estimated for the San Andreas fault. First, the well‐defined surface heat flow and heat generation allow negligible frictional heating on the plate interface. The average shear stress on the fault must thus be very low over a time scale of a few million years. Second, focal mechanism solutions for small crustal earthquakes in the southern Vancouver Island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress. This inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low. One explanation for the weakness of the fault is the presence of near‐lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence. The conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked, accumulating strain energy toward a future great earthquake. The surface deformation responds to the small (<20 MPa) temporal changes of the stress field associated with the subduction earthquake cycle. This transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin. The weakness of the Cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards. For example, a subduction earthquake may induce large strike‐slip earthquakes in the forearc that affect a large area.
Wasp‐waisted hysteresis loops: Mineral magnetic characteristics and discrimination of components in mixed magnetic systems
Authors: Roberts Andrew P. , Cui Yulong , Verosub Kenneth L.
Published: February 2004

Rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials. Hysteresis loops that are constricted in the middle section, but are wider above and below the middle section, are commonly observed in mixed magnetic assemblages. Such “wasp‐waisted” hysteresis loops have been widely documented, particularly with respect to rare earth permanent magnets, basaltic lava flows, remagnetized Paleozoic carbonate rocks, and an increasingly wide range of other rocks. Our modelling, combined with a review of previous work, indicates that there are several conditions that give rise to, as well as magnetic properties that are characteristic of, wasp‐waisted hysteresis loops. First, at least two magnetic components with strongly contrasting coercivities must coexist. This condition can arise from either mixtures of grain sizes of a single magnetic mineral, or a combination of magnetic minerals with contrasting cocrcivities, or a combination of these two situations. Second, materials that give rise to wasp‐waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force (Bcr/Bc) because B0 is controlled by the soft (low coercivity) component, whereas Bcr is controlled by the hard (high coercivity) component. Third, values of Bcr/Bc ≥ 10 usually only occur for strongly wasp‐waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains. Fourth, a given mixture of superparamagnetic and single‐domain (SD) grains is more likely to give rise to wasp‐waisted hysteresis loops than an equivalent mixture of SD and multidomain grains. Fifth, our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component, in the absence of significant magnetic interaction between particles. Thus to contribute significantly to wasp‐waisted behavior, a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock. As a result, minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp‐waistedness in materials that also contain ferrimagnetic minerals. We outline a method for determining the magnetic components that can give rise to wasp‐waisted hysteresis loops. This method is based on high‐ and low‐temperature magnetic measurements that are used to identify the dominant remanence‐bearing mineral/s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states. The method is illustrated with several examples from archaeological, geological, and synthetic materials.
Model of earthquake recurrence: Role of elastic wave radiation, relaxation of friction, and inhomogeneity
Authors: Nielsen S. , Knopoff L. , Tarantola A.
Published: February 2004

We have constructed a dynamical model of seismicity, wherein a fault is embedded in an infinite, continuous, elastic medium. Therefore the influence of the energy dissipated in seismic wave radiation oil the sequential history of model earthquakes is fully taken into account. In this model, the drop in friction at rupture takes place gradually, thus introducing a relaxation dimension. As an example, we consider a finite homogeneous fault that is terminated by infinitely strong barriers at the ends. The features of the seismicity are dominated by the stresses that are reflected from the unbreakable barriers. There is a strong dependence of the patterns of seismicity on the ratio of the relaxation dimension to the distance between the barriers at the ends. For large values of this parameter, we find that periodicity begins after a short transient interval and a dominance of the statistical distributions by large events that break completely through the fault from end to end. For small values of this parameter, smaller‐scale seismicity is interspersed with the large events, and no periodicity is observed within the time spanned by the computations. We conclude that major unquenched heterogeneities, such as those found at barriers, which we suppose arise in nature due to the nonuniform geometry of faults, are vital ingredients for generating complex seismic histories as well as giving rise to the complex features of individual earthquake source‐time functions in models of seismicity as a dynamical process.
Crustal velocity structure of the Omineca and Intermontane Belts, southeastern Canadian Cordillera
Authors: Burianyk Michael J. A. , Kanasewich Ernest R.
Published: February 2004

We provide information on the physical properties across two important morphogeological regions of the southeastern Canadian Cordillera, the Omineca and Intermontane Belts, and tie together two previously interpreted wide‐angle seismic lines which have separately probed each zone along strike. The data analyzed here, Line 7 of Lithoprobe's 1990 Southern Cordillera Refraction Experiment in southern British Columbia is along a 410‐km profile from the Fraser River to Radium Hot Springs near the Rocky Mountain Trench. Iterative forward and inverse modeling of the seismic travel times, constrained by seismic amplitude characteristics and earlier cross‐line interpretations, defines a thin crust with relatively low velocities. The upper crust has an average thickness of 15.3 km and an average P wave velocity of less than 6.1 km s−1 with velocities at the bottom reaching 6.2–6.3 km s−1. It is further characterized by velocity gradients which decrease markedly with depth and by low‐velocity trends which correlate with the traces of crustal‐scale shear zones. A high‐velocity region at the west end of the profile is coincident with a major uplift structure. The midcrust has an average thickness of 9.2 km with velocities below 6.1 km s−1 except for the extreme west end of the profile. This defines a midcrustal low‐velocity zone (LVZ) in the Omineca Belt and partly in the Intermontane Belt. The lower crust, from 24 km to 34 km depth, has velocities averaging 6.6–6.7 km s−1. Higher velocities in the deep crust may mark the westward extent of North American cratonic rocks at the Okanagan Valley. The upper mantle velocity has an average 8.0 km s−1 increasing to 8.2 km s−1 at the maximum depth of ray penetration at 48 km. The model presented here compliments well the earlier interpretations of vertical incidence seismic reflection profiles. The velocity structure of the crust, especially the LVZ, closely correlates with the regional isotherms. While there seems to be no well‐defined velocity boundary between the Intermontane and Omineca Belts, there are distinct velocity differences between the Intermontane and neighboring Coast Plutonic Complex to the west. The low‐velocity characteristics of the Omineca and much of the Intermontane belts may be a consequence of temperature control of the physical parameters of the crust. The lithosphere in the southeastern Canadian Cordillera is clearly anomalous with respect to much of northwest North America, and in terms of crustal thickness and velocity structure, it is similar only to the Basin and Range province of the United States.
Regional recovery of the gravity field from satellite gravity gradiometer and gravity vector data using collocation
Authors: Arabelos D. , Tscherning C. C.
Published: February 2004

The method of least squares collocation has been used to investigate the regional recovery of the gravity field from satellite gravity gradiometer (SGG) data and gravity vector data derived from, e.g., satellite‐to‐satellite tracking (SST). We chose a region centered over the European Alps where the gravity anomalies showed a (large) standard deviation of 60 mGal. Mean gravity anomalies were used to generate SGG data at a satellite altitude of 200 km. As SGG data we used the second‐order derivatives in radial direction, across‐track, and the mixed radial/cross‐track derivative, all assumed to have an associated noise equal to 0.01 EU (Eötvös unit =EU, 1 EU=10−9s−2). As gravity data we used the three components of the gravity vector, assuming that the associated errors were uncorrelated noise with standard deviation equal to 1 mGal. The satellite data in the test area were used in combination to predict 0.5° mean gravity anomalies and geoid heights. Ground truth data were used to investigate the quality of the recovery. The difference between observed and computed values have a standard deviation equal to 22 mGal in the best case. The use of the gravity vector data gave no improvement when added to the SGG data. The use of topographic information with 5 arc min resolution (ETOPO5U) gave, after removal of severe errors, a 17‐mGal standard deviation of observed minus computed values. This is a much smaller improvement than expected but is due to errors in the topographic values.
Influence of frequency and fluid distribution on elastic wave velocities in partially saturated limestones
Authors: Cadoret T. , Marion D. , Zinszner B.
Published: February 2004

Elastic waves velocities were measured in the laboratory on homogeneous limestones in three different frequency ranges with varying water saturations (Sw). Measurements at frequencies of approximately 1 kHz were obtained using large (1‐m length) resonant bars. The conventional pulse transmission technique was used to measure the samples at 50 kHz and 500 kHz. In order to study the influence of the fluid distribution we used two different saturation methods: drying and depressurization. Using a computerized tomography scan to image fluid distribution, we found that during depressurization, saturation was highly homogeneous at the millimetric scale at all saturations. In contrast, during the drying, heterogeneous saturation was observed at high water saturation levels. Results from elastic wave propagation show that compressional and extensional velocities are sensitive to the saturation technique at high water saturation level. Furthermore, the dependence of velocity upon saturation technique is frequency dependent, and dispersion is greatest at high saturations. We attribute this behavior to a scale effect that varies with the size of heterogeneities of the rock and with the wavelength. In addition to global and local flow mechanisms it seems necessary to include a “path dispersion” effect to explain the amount of velocity dispersion observed. For shear waves the experimental results show little variation of the velocity with frequency and very little with fluid distribution.
Potential field power spectrum inversion for scaling geology
Authors: Maus Stefan , Dimri Vijay
Published: February 2004

We propose a method for inverting the power spectrum of gravity and magnetic data. The method is demonstrated on aeromagnetic and borewell data from the German Continental Deep Drilling Project (KTB). Density and susceptibility distributions in the Earth's crust exhibit scaling behavior with power spectra proportional to ƒ−β, where f is the wavenumber and β is the scaling exponent of the source distribution. We model the sources of the potential field by a random function with scaling properties, defined on a half‐space with its top at a specified depth beneath the observation plane. Comparing the theoretical power spectrum for this model with the power spectrum of the measured data, we obtain the best values for the depth to source and the scaling exponent as a global minimum of the misfit function. Despite the simplicity of the model, it offers a new understanding of the factors influencing the shape of the potential field power spectrum. In particular, the low wavenumber part of the power spectrum can be dominated by the scaling properties of the source distribution and not by the depth to some kind of basement. The scaling exponent of the field varies with the type of surface geology. The question of whether the scaling exponent can actually be used to identify different types of geology gives an interesting new aspect to power spectrum inversion.
History of rift propagation and magnetization intensity for the Cocos‐Nazca sspreading Center
Authors: Wilson Douglas S. , Hey Richard N.
Published: February 2004

Analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the Cocos‐Nazca spreading center. Forward models of the magnetic anomalies accounting for spreading rate variations, ridge jumps, asymmetric spreading, magnetization intensity variations, and bathymetry show excellent agreement with observed anomalies. Spreading rates can be constrained to a common finite rotation history through anomaly 4A with three changes in rates. Rate changes at about 1.5 Ma and 4.1 Ma correspond to changes in rate gradients and occur during the well‐calibrated part of the reversal timescale, so they can unquestionably be identified as true changes in plate motion. A ∼15% rate decrease at about 5.2 Ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale. The change at 4.1 Ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0–6 Ma. Rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis. Propagation has been predominantly away from the hotspot, with jumps predominantly south‐ward. Propagation rates have ranged from 30 to 120 mm/yr, commonly near 70 mm/yr. Origin of most propagation sequences is difficult to interpret, but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot. Magnetic anomaly amplitude appears to be a reliable tracer of Fe content of lavas. Several generalizations can be drawn about along‐axis variations in magnetization intensities since 8 Ma: high magnetizations are only observed at the far ends (relative to the Galapagos hotspot) of segments at least 150 km long; offset at the end of a high‐magnetization segment is at least 15 km; and there are no offsets larger than 30–45 km between high‐magnetization segments and the reconstructed position of the hotspot. We interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along‐axis flow of evolved magma. The gradients in supply result from subaxial flow of hotspot‐derived asthenosphere in a narrow conduit. This flow is only partly obstructed by an offset of 20–30 km but entirely blocked by an offset of 50 km.
Geodesy using the Global Positioning System: The effects of signal scattering on estimates of site position
Authors: Elósegui P. , Davis J. L. , Jaldehag R. T. K. , Johansson J. M. , Niell A. E. , Shapiro I. I.
Published: February 2004

Analysis of Global Positioning System (GPS) data from two sites separated by a horizontal distance of only ∼2.2 m yielded phase residuals exhibiting a systematic elevation angle dependence. One of the two GPS antennas was mounted on an ∼1‐m‐high concrete pillar, and the other was mounted on a standard wooden tripod. We performed elevation angle cutoff tests with these data and established that the estimate of the vertical coordinate of site position was sensitive to the minimum elevation angle (elevation cutoff) of the data analyzed. For example, the estimate of the vertical coordinate of site position changed by 9.7±0.8 mm when the minimum elevation angle was increased from 10° to 25°. We performed simulations based on a simple (ray tracing) multipath model with a single horizontal reflector which demonstrated that the results from the elevation angle cutoff tests and the pattern of the residuals versus elevation angle could be qualitatively reproduced if the reflector were located 0.1–0.2 m beneath the antenna phase center. We therefore hypothesized that the elevation‐angle‐dependent error was caused by scattering from the horizontal surface of the pillar, located a distance of ∼0.2 m beneath the antenna phase center. We tested this hypothesis by placing microwave absorbing material between the antenna and the pillar in a number of configurations and by analyzing the changes in apparent position of the antenna. The results indicate that (1) the horizontal surface of the pillar is indeed the main scatterer, (2) both the concrete and the metal plate embedded in the pillar are significant sources of scattering, and (3) the scattering can be reduced greatly by the use of microwave absorbing materials. These results have significant implications for the accuracy of global GPS geodetic tracking networks which use pillar‐antenna configurations identical or similar to the one used for this study at the Westford WFRD GPS site.
Kinetics of pressure solution at halite‐silica interfaces and intergranular clay films
Authors: Hickman Stephen H. , Evans Brian
Published: February 2004

Pressure solution is widely regarded as a potentially important deformation mechanism along crustal faults and during diagenesis, yet the mechanisms and kinetics of this process remain highly controversial. To better understand the fundamental factors controlling the rates of pressure solution at the grain‐to‐grain scale, we conducted experiments in which convex halite lenses were pressed against flats of fused silica in brine. Fluid pressures were maintained at 0.1 MPa; temperatures and mean contact normal stresses ranged from 8.3° to 90.2°C and 0.5 to 13.5 MPa, respectively. The geometry and growth rate of the contact spot between the two lenses and the rate at which the lenses approached one another (convergence) were monitored using reflected light interferometry and transmitted light photomicrography. Convergence occurred when halite and silica lenses were pressed together in brine (halite/silica experiments). No undercutting was observed, and dry control experiments indicated negligible dislocation creep. Convergence rates in experiments at 50.2°C ranged from 0.01 to 0.05 μm/d, depending on mean normal stress and contact spot radius. The data are consistent with intergranular pressure solution (IPS) rate‐limited by diffusion through an intergranular film with a very high diffusion coefficient (∼ 10−5–10−7 cm2/s). The data further suggest that the diffusion coefficient and/or thickness of this film increases with decreasing normal stresses, at least for normal stresses less than about 4 MPa. As no island‐channel boundary structures were observed, we propose that this film consists of a continuous layer of strongly adsorbed (i.e., structured) water that is maintained between the halite and silica lenses during deformation. Convergence rates in similar experiments conducted at a constant load of 0.11 N but at 8.3°, 50.2°, and 90.2°C were approximately constant at a given normal stress and contact spot size. The cause of this temperature insensitivity is unknown but might result from changes in interphase boundary structure or thickness with increasing temperature that are sufficient to offset the expected thermal activation. An experiment was also conducted in which a halite lens was pressed against a fused silica flat coated with an 0.8‐μm‐thick film of Na‐montmorillonite in brine. This clay film produced an approximately fivefold increase in convergence rates over those observed in a halite/sulia experiment conducted without clay at the same load and temperature. The strong sensitivity of IPS rates both to contact spot radius and to the presence of second phases along grain boundaries suggests that fine‐grained, clay‐rich fault gouges and multiphase granular aggregates should be particularly susceptible to pressure solution creep in the middle to upper crust.
Faulting processes at high fluid pressures: An example of fault valve behavior from the Wattle Gully Fault, Victoria, Australia
Authors: Cox Stephen F.
Published: February 2004

The internal structures of the Wattle Gully Fault provide insights about the mechanics and dynamics of fault systems exhibiting fault valve behavior in high fluid pressure regimes. This small, high‐angle reverse fault zone developed at temperatures near 300°C in the upper crust, late during mid‐Devonian regional crustal shortening in central Victoria, Australia. The Wattle Gully Fault forms part of a network of faults that focused upward migration of fluids generated by metamorphism and devolatilisation at deeper crustal levels. The fault has a length of around 800 m and a maximum displacement of 50 m and was oriented at 60° to 80° to the maximum principal stress during faulting. The structure was therefore severely misoriented for frictional reactivation. This factor, together with the widespread development of steeply dipping fault fill quartz veins and associated subhorizontal extension veins within the fault zone, indicates that faulting occurred at low shear stresses and in a near‐lithostatic fluid pressure regime. The internal structures of these veins, and overprinting relationships between veins and faults, indicate that vein development was intimately associated with faulting and involved numerous episodes of fault dilatation and hydrothermal sealing and slip, together with repeated hydraulic extension fracturing adjacent to slip surfaces. The geometries, distribution and internal structures of veins in the Wattle Gully Fault Zone are related to variations in shear stress, fluid pressure, and near‐field principal stress orientations during faulting. Vein opening is interpreted to have been controlled by repeated fluid pressure fluctuations associated with cyclic, deformation‐induced changes in fault permeability during fault valve behavior. Rates of recovery of shear stress and fluid pressure after rupture events are interpreted to be important factors controlling time dependence of fault shear strength and slip recurrence. Fluctuations in shear stress and transient rotations of near‐field principal Stresses, indicated by vein geometries, are interpreted to indicate at least local near‐total relief of shear stress during some rupture events. Fault valve behavior has important effects on the dynamics of fluid migration around active faults that are sites of focused fluid migration. In particular, fault valve action is expected to lead to distinctly different fluid migration patterns adjacent to faults before, and immediately after, rupture. These fluid migration patterns have important differences with those predicted by models for dilatancy‐diffusion effects and for poroelastic responses around reverse faults.
Shape of the subducted Rivera and Cocos plates in southern Mexico: Seismic and tectonic implications
Authors: Pardo Mario , Suárez Gerardo
Published: February 2004

The geometry of the subducted Rivera and Cocos plates beneath the North American plate in southern Mexico was determined based on the accurately located hypocenters of local and teleseismic earthquakes. The hypocenters of the teleseisms were relocated, and the focal depths of 21 events were constrained using a body wave inversion scheme. The suduction in southern Mexico may be approximated as a subhorizontal slab bounded at the edges by the steep subduction geometry of the Cocos plate beneath the Caribbean plate to the east and of the Rivera plate beneath North America to the west. The dip of the interplate contact geometry is constant to a depth of 30 km, and lateral changes in the dip of the subducted plate are only observed once it is decoupled from the overriding plate. On the basis of the seismicity, the focal mechanisms, and the geometry of the downgoing slab, southern Mexico may be segmented into four regions: (1) the Jalisco region to the west, where the Rivera plate subducts at a steep angle that resembles the geometry of the Cocos plate beneath the Caribbean plate in Central America; (2) the Michoacan region, where the dip angle of the Cocos plate decreases gradually toward the southeast, (3) the Guerrero‐Oaxaca region, bounded approximately by the onshore projection of the Orozco and O'Gorman fracture zones, where the subducted slab is almost subhorizontal and underplates the upper continental plate for about 250 km, and (4) the southern Oaxaca and Chiapas region, in southeastern Mexico, where the dip of the subduction gradually increases to a steeper subduction in Central America. These drastic changes in dip do not appear to take place on tear faults, suggesting that smooth contortions accommodate these changes in geometry. The inferred 80 and 100 km depth contours of the subducted slab lie beneath the southern front of the Trans‐Mexican Volcanic Belt, suggesting it is directly related to the subduction. Thus the observed nonparallelism with the Middle American Trench is apparently due to the changing geometry of the Rivera and Cocos plates beneath the North American plate in southern Mexico, and not to zones of weakness in the crust of the North American plate as some authors have suggested.