bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic reflectivity of the Sudbury Structure North Range from borehole logs
Authors: White D. J. , Milkereit B. , Wu J. J. , Salisbury M. H. , Mwenifumbo J. , Berrer E. K. , Moon W. , Lodha G.
Published: February 2004

Results from borehole geophysical logs, full waveform sonic logs, VSP and laboratory core sample measurements indicate that lithologic variations within the Sudbury Igneous Complex (SIC) and footwall rocks are the primary cause of reflections observed regionally on the multi‐channel seismic reflection profiles. The effects of macroscopic fracturing and low‐grade alteration on the seismic response are only important to a depth of ∼ 320 m. The major lithologic units of the SIC and footwall rocks in the North Range of the Sudbury structure have contrasting physical properties: Felsic norite and quartz‐gabbro have higher Vp (∼ 6300 m/s) than granophyre (∼ 6000 m/s) due to their higher pyroxene content and the presence of quartz rather than calcic plagioclase in the granophyre. Velocities are higher (Vp ∼ 6500 m/s) within the brecciated footwall rocks due to an overall increase in mafic mineral content. The contrasting velocities and densities of these units imply that the granophyre/quartz‐gabbro contact and the SIC/footwall transition can be mapped regionally using seismic reflection methods. Subunits within these units are also highly reflective, consistent with the nature of the seismic data, but are likely discontinuous laterally.
Gravity wave activity in the upper stratosphere and lower mesosphere observed with the Rayleigh lidar at Tsukuba, Japan
Authors: Murayama Y. , Tsuda T. , Wilson R. , Nakane H. , Hayashida S. A. , Sugimoto N. , Matsui I. , Sasano Y.
Published: February 2004

We delineated the climatological characteristics of the potential energy of gravity waves, Ep, using Rayleigh lidar observations made in 1990–1991 at Tsukuba, Japan (36°N, 140°E). Tendency of large and small values of Ep in winter and summer, respectively, was detected in the upper stratosphere (30–45 km), suggesting annual variation with a winter maximum. This annual cycle seemed to be consistent with lidar observations made in France and UK [Wilson et al., 1991; Mitchell et al., 1991]. The obtained Ep values in the upper stratosphere and lower mesosphere (45–60 km) were rather similar to the French data, despite the large scatter of the Tsukuba values.
Point defect diffusivites in San Carlos olivine derived from reequilibration of electrical conductivity following changes in oxygen fugacity
Authors: Wanamaker B. J.
Published: February 2004

The time dependence of electrical conductivity following step changes in oxygen fugacity has been used to deduce the diffusivities of the dominant point defects in pyroxene‐buffered San Carlos olivine. Nonlinear least squares fits of the data indicate that two relaxation times (τ) are present in the conductivity‐time records. The effective diffusivity calculated from the slower τ agrees with values reported for the chemical diffusivity of metal vacancies (VMe‖). The effective diffusivity calculated from the faster τ coincides with that measured for analogous changes in strain rate (έ) following step changes in fO2. This faster diffusivity has tentatively been associated with the chemical diffusivity of silicon vacancies (VSi‖‖). These results imply that defects on the tetrahedral and octahedral sublattices of olivine may reequilibrate independently. Furthermore, changes in electrical conductivity and strain rate resulting from changes in thermodynamic conditions are initiated by reequilibration of the faster of these defects.
Teleseismic arrivals at a mid‐ocean ridge: Effects of mantle melt and anisotropy
Authors: Kendall J‐M.
Published: February 2004

Recent observational evidence of upwelling‐mantle anisotropy at a slow spreading center has motivated the modeling of teleseismic arrivals at mid‐ocean ridges. The models consider a variety of types of anisotropy and heterogeneity where the emphasis is to ascertain whether or not travel‐times can be used to discriminate between the existence of partial melt and anisotropy. Two mechanisms for anisotropy in the upwelling asthenosphere are considered: one due to the preferential alignment of the fast axes of olivine crystals in the direction of mantle flow and the other due to the preferential alignment of cracks that feed melt towards the spreading axis. The results indicate that P‐waves are most sensitive to even modest amounts of flowinduced asthenospheric anisotropy, while S‐waves are most sensitive to the presence of mantle melt. Multiple S‐wave arrivals are predicted for many models, most notably the ones with anisotropy due to crack‐alignment where very large S‐wave separations develop. The model which best fits existing data requires a higher degree of crystal‐alignment anisotropy in the upwelling‐asthenosphere than in the lithosphere. This effect has been predicted in studies of the evolution of crystal‐alignment anisotropy in polycrystalline aggregates.
Relative paleointensity across the last geomagnetic reversal from sediments of the Atlantic, Indian and Pacific oceans
Authors: Valet Jean‐Pierre , Meynadier Laure , Bassinot Franck C. , Garnier Florence
Published: February 2004

Paleointensity records from marine sediments at three locations in the Atlantic, Indian and Pacific oceans show coherent and reproducible signals across the Brunhes‐Matuyama reversal. The low‐field susceptibility signals are predominantly anticorrelated between the Indo‐Pacific and the Atlantic sites while the normalization of the natural remanent magnetization by any rock magnetic parameter yield identical results. We deduce that in these cases climatic components do not induce first order effects in the determination of relative paleointensity. The results establish the worldwide character of the triangular pattern displayed by the decay and recovery phases of the field intensity variations across the Brunhes‐Matuyama reversal.
Backscatter measurements of stratospheric aerosols at Thule during January‐February 1992
Authors: Larsen N. , Knudsen B. , Jørgensen T. S. , di Sarra A. , Fuà D. , Di Girolamo P. , Fiocco G. , Cacciani M. , Rosen J. M. , Kjome N. T.
Published: February 2004

Within the European Arctic Stratospheric Ozone Experiment (EASOE) aerosols from the volcanic eruption of Mt. Pinatubo have been observed during 6 balloonborne backscatter soundings in January and February 1992 under different polar vortex conditions from Thule, Greenland. The vortex boundary seemed to retard stratospheric aerosol mixing into the inner parts of the vortex from lower latitudes; however, when Thule was outside the vortex large aerosol loadings were measured. The aerosols were simultaneously observed by groundbased lidar, whereby aerosol backscatter measurements in three different wavelengths made it possible to obtain information about the particle sizes. Parameters of lognormal distributions have been derived, using a least square approach between observed and calculated backscatter coefficients from Mie theory. The aerosol surface area density was increased by factors 10–50, compared to the background levels at pre‐volcanic conditions.
HCL solubility and liquid diffusion in aqueous sulfuric acid under stratospheric conditions
Authors: Luo B. P. , Clegg S. L. , Peter Th. , Müller R. , Crutzen P. J.
Published: February 2004

The solubility of HCl in cold H2SO4/H2O solutions is central to an assessment of the role of stratospheric aerosol in polar ozone destruction. The interpretation of laboratory measurements of HCl uptake by aqueous H2SO4 requires a knowledge of (1) HCl solubility and (2) the HCl liquid phase diffusion coefficient. Neither quantity is easily determined because of the high H2SO4 concentration and low temperature of the solutions, making experimental data difficult to evaluate and to apply to the real stratospheric situation. To avoid these difficulties, we have used a new thermodynamic model for an improved estimate of HCl solubility, and a viscosity‐based cubic cell model to obtain the diffusion coefficient. The calculated HCl solubility in aqueous H2SO4 below 200 K is up to three orders of magnitude greater than previously thought. This might promote chlorine activation particularly during volcanically perturbed periods with an enhanced stratospheric aerosol loading.
Activation of stratospheric chlorine by reactions in liquid sulphuric acid
Authors: Cox R. A. , MacKenzie A. R. , Müller R. H. , Peter Th. , Crutzen P. J.
Published: February 2004

Active chlorine release on H2SO4 aerosol particles via the reaction of HOCl and HCl in solution is discussed. Based on current laboratory data, the process is shown to have an appreciable rate at temperatures below about 195 K, when the aerosol particles become sufficiently dilute to allow both HCl dissolution and fast ClONO2 hydrolysis. Evolution of part‐per‐billion concentrations of Cl2 could be significant in cases in which volcanically enhanced aerosol remains liquid until the frost point (≈ 190 K) as observed on several occasions in the Arctic stratosphere.
Tropospheric rivers: A one‐year record and a possible application to ice core data
Authors: Newell Reginald E. , Zhu Yong
Published: February 2004

The morphology of tropospheric rivers has been examined for the period June 1991–May 1992 from atmospheric specific humidity and wind velocity data. Vertically integrated water vapor flux vectors are subjected to harmonic analysis and the high frequency components (periods less than 3 days) show large amplitude values in the North Atlantic, North Pacific, South Atlantic, South Indian Ocean and South Pacific plus small amplitude occurrences in the Red Sea‐Arabian Sea region in the December–February season. In June–August the latter is replaced with a large amplitude monsoon‐related region over South‐East Asia. Maximum intensities increase slightly in altitude with latitude. The correspondence between rivers, frontal boundaries and storm tracks is noted.

Sometimes rivers originating over the North Pacific cross North America and reach Greenland while at other times they pass polewards over the North Atlantic, raising the possibility that water vapor involved in ice core formation may have evaporated from surface water bodies with different temperature and salinity characteristics. Changes in oxygen isotope variations at a point may therefore reflect changes in the dynamics of tropospheric rivers in addition to local temperature changes.
Changes in the permeability, streaming potential and resistivity of a claystone from the Nankai prism under stress
Authors: Jouniaux Laurence , Lallemant Siegfried , Pozzi Jean‐Pierre
Published: February 2004

Permeability is the critical factor governing fluid flow in accretionary prisms. Accretionary wedges are highly deformed, so permeability changes in an indurated claystone sample from the Nankai accretionary prism were measured during a triaxial stress experiment by the pulse decay method. Three zones were identified from the loading test. In zone I, the sample has deformed less than 1.3% and its permeability was 5.10−20 m². In zone II, the deformation reached 1.5% and the permeability was unstable with time. In zone III, the sample deformation reached 2% and its permeability reached 3.8 10−18 m². The permeability strongly decreased (from 3.2 10−18 m² to 1.7 10−19 m²) with increasing effective pressure (from 2.3 to 10.8 MPa) after the sample was well deformed (zone III). The streaming potential was not measurable when the sample had a low permeability (zone I), but clearly occured when the permeability increased (zone III) : the streaming potential measured was 6 mV when the pore pressure pulse applied was about 1.17 MPa and the permeability 3.8 10−18 m². Our experiments suggest that shear deformation under low effective pressure increased the vertical permeability of sediments above the décollement. This increase in permeability may be detected by measuring the streaming potential. Variations of flow rate of expelled fluid in accretionary wedges may be detected by monitoring changes of electrokinetic potential, giving new insights on the state of stress related to the seismic cycles.
Temperature dependence of the C1ONO2 UV absorption spectrum
Authors: Burkholder James B. , Talukdar Ranajit K. , Ravishankara A. R.
Published: February 2004

The temperature dependence of the ClONO2 absorption spectrum has been measured between 220 and 298 K and between 195 and 430 nm using a diode array spectrometer. The absorption cross sections were determined using both: (1) absolute pressure measurements at 296 K and (2) measurements at various temperatures relative to 296 K using a dual absorption cell arrangement. The temperature dependence of the ClONO2 absorption spectrum shows very broad structure. The amplitude of the temperature dependence relative to that at 296 K is weak at short wavelengths, <2% at 215 nm and 220 K, but significant at the wavelengths important, in the stratosphere, ∼30% at 325 nm and 220 K. Our ClONO2 absorption cross section data are in good general agreement with the previous measurements of Molina and Molina [1979].
In situ measurements of nitric oxide, ozone, and aerosols in the Scandinavian Arctic stratosphere in January 1992
Authors: Aimedieu P. , Kondo Y. , Sugita T. , Koike M. , Sheldon W. R.
Published: February 2004

During the European Arctic Stratospheric Ozone Experiment (EASOE) the Arctic stratosphere was investigated with a set of instruments flown on a CNES high altitude balloon from ESRANGE, northern Sweden (67°52′N, 21°4′E) on January 22, 1992. The vertical profiles to an altitude of 30 km are presented for ozone, nitric oxide, aerosols, and the temperature. The ozone mixing ratio was less than 0.1 ppmv below 13km, and increased significantly only above the 18.5 km level. The aerosol concentration profile shows a large peak between 13 km and 21km, centered at 18.5 km, reaching as high as about 10 particles/cm³. Above 24 km the aerosols were absent. Below 22 km, the NO mixing ratio was lower than 50 pptv; however it increased rapidly from 23 km with a sharp peak at 27 km. The rapid increase in NO occurs just at the altitude where the aerosol concentration falls below the detection limit: on this day, NO was present only in the absence of aerosols.
Integrated seismic and borehole geophysical study of the Sudbury Igneous Complex
Authors: Milkereit B. , Green A. , Wu J. , White D. , Adam E.
Published: February 2004

Reflection seismic and borehole geophysical data place important constraints on the subsurface geometry of the Sudbury Structure, which is the site of the world's largest Ni‐Cu camp. Seismic reflections can be traced from outcrop within the Sudbury North Range to about 4.5 km depth beneath the center of the Sudbury Basin, where the layer thickens abruptly from 1 to 3 km. Further south the North Range norite can be followed to about 10 km depth beneath the South Range. Borehole studies show systematic variations of p‐ and s‐wave velocity, Poisson's ratio and density within the Igneous Complex. Quartz‐rich granophyre is distinguished from the norite and footwall rocks by relatively low Poisson's ratios (0.20–0.23 versus 0.23–0.25). These changes in physical rock properties define an important subdivision of the Igneous Complex, compatible with a simple model involving differentiation of melted crustal rock into dominantly felsic and mafic components. This study documents the importance of interlayering to the seismic reflection response of the crystalline crust.
Subsidence of aircraft engine exhaust in the stratosphere: Implications for calculated ozone depletions
Authors: Rodríguez J. M. , Shia R. ‐L. , Ko M. K. W. , Heisey C. W. , Weistenstein D. K. , Miake‐Lye R. C. , Kolb C. E.
Published: February 2004

The deposition altitude of nitrogen oxides and other exhaust species emitted by stratospheric aircraft is a crucial parameter in determining the impact of these emissions on stratospheric ozone. We have utilized a model for the wake of a High‐Speed Civil Transport (HSCT) to estimate the enhancements in water and reductions in ozone in these wakes as a function of time. Radiative calculations indicate differential cooling rates as large as −5K/day at the beginning of the far‐wake regime, mostly due to the enhanced water abundance. These cooling rates would imply a net sinking of the wakes of about 1.2 km after three days in the limit of no mixing. Calculated mid‐latitude column ozone reductions due to emissions from a Mach 2.4 HSCT would then change from about −1% to −0.6%. However, more realistic calculations adopting moderate mixing for the wake reduce the net sinking to less than 0.2 km, making the impact of radiative subsidence negligible.
Relationship between subduction and seismicity in the Mexican part of the Middle America Trench
Authors: Kostoglodov Vladimir , Ponce Lautaro
Published: February 2004

Two catalogs of earthquakes in the Mexican part of the Middle America trench are analyzed to elucidate principal relations between structure of the subducting Cocos plate and seismicity. A catalog of historical events that have occurred during the last two centuries with large magnitudes (Ms> 6.0) is used to obtain cumulative seismic moment (Mo) and seismic moment release rate () distributions along the Mexican subduction zone. The catalog of instrumentally observed earthquakes from 1963 to 1990 (International Seismological Center and U.S. Geological Survey) with 4.5 ≤mb< 6.0 is applied to investigate background seismicity for the region. The strength of coupling between the Cocos and North American plates would be expected to grow gradually from the southeast to the northwest according to the variation of convergence rate (V) and age (A) of the Cocos plate. This correlates in general with a steady reduction in background seismicity and a slight average increase ofand seismic energy release rate (). At the sites where the main fracture zones of the Cocos plate; Tehuantepec, O'Gorman, Orozco and Rivera, undergo the subduction the general correlation breaks down. The background seismicity increases at fracture zones whileM0anddecrease significantly. This feature is interpreted as a drop of the coupling at the areas where transform faults are being subducted. Seismic slip rates along the trench obtained fromare lower then the values of plate convergence rates but the average seismic slip is in agreement with the estimates from the V model (interaction between lithospheric plates at convergent zones through the viscous layer of subducted sediments). Variability ofand seismic slip rate in relation with tectonics should be taken into account when the seismic gap model is being used for the prediction of strong earthquakes. An examination of space‐time plots for the historical catalogs supposes a probable tendency of northwest migration of strong events with a rate of ≈10 km/yr.
Paleomagnetism of the Catoctin volcanic province: A new Vendian‐Cambrian apparent polar wander path for North America
Authors: Meert Joseph G. , Van der Voo Rob , Payne Troy W.
Published: February 2004

The Vendian/Cambrian segment of the Lauretian apparent polar wander path (APWP) has been poorly constrained and the subject of some controversy. The Catoctin volcanic province in central Virginia is well‐dated at 570±35 Ma (Rb‐Sr) and 597±18 Ma (U‐Pb) and therefore presented an excellent paleomagnetic target for resolving the Laurentian Vendian‐Cambrian APWP. A total of 206 samples from 32 sites were collected from the Catoctin basalts, feeder dikes and sills. The study revealed three ancient directions of magnetization. The youngest, C component, fails the fold test and yielded a characteristic in situ direction of D = 147°, I = +44° (k = 21, α95 = 9°). The corresponding paleopole falls along the Middle Ordovician segment of the Laurentian APWP and we consider this component to be the result of a Taconic remagnetization. The second component, the B component, is carried by hematite, exhibits dual‐polarities and passes a fold test. The tilt‐corrected B component characteristic direction is D = 92°, I = +17° (k = 16, α95 = 13°). The corresponding paleopole at 4°S, 193°E falls near a well‐established Late Cambrian (505 Ma) pole for Laurentia, and we consider this component to be a remagnetization during a Late Cambrian tectonic event in the central Appalachians. The third component isolated in the Catoctin basalts, the A component, yields a tilt‐corrected mean of D = 68°, I = +84° (k = 59, α95 = 9°). This component passes a fold and reversal test. A suite of samples was collected from two Catoctin feeder dikes and surrounding country rocks that yield a positive baked contact test. The A pole at 43°S, 128°E falls significantly away from previously proposed Vendian poles for Laurentia. A reevaluation of previous paleomagnetic studies from coeval rock units reveals similarly steep directions and leads us to propose a new APWP. This new APW track indicates that Laurentia was located near the pole during the interval 615–580 Ma and drifted rapidly (16 cm yr−1) toward its Late Cambrian equatorial position.
Neogene strike‐slip faulting in Sakhalin and the Japan Sea opening
Authors: Fournier Marc , Jolivet Laurent , Huchon Philippe , Sergeyev Konstantin F. , Oscorbin Leonid S.
Published: February 2004

We describe structural data from a 2000 km N‐S dextral strike‐slip zone extending from northern Sakhalin to the southeast corner of the Japan Sea. Satellite images, field data, and focal mechanisms of earthquakes in Sakhalin are included in the interpretation. Since Miocene time the deformation in Sakhalin has been taken up by N‐S dextral strike‐slip faults with a reverse component and associated en échelon folds. Narrow en échelon Neogene basins were formed along strike‐sup faults and were later folded in a second stage of deformation. We propose a model of basin formation along extension al faults delimitating dominos between two major strike‐slip faults, and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime, basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening. The association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in Sakhalin. We present fault set measurements undertaken in Noto Peninsula and Yatsuo Basin at the southern end of the Sakhalin‐East Japan Sea strike‐slip zone. Early and middle Miocene formations recorded the same transtensional regime as observed along the west coast of NE Honshu. During the early and middle Miocene the strike‐slip regime was transpressional to the north in Sakhalin and Hokkaido, and transtensional to the south along the west coast of NE Honshu as far as Noto Peninsula and Yatsuo basin. Dextral motion accommodated the opening of the Japan Sea as a pull‐apart basin, with the Tsushima fault to the west. The opening of the Japan Sea ceased at the end of the middle Miocene when transtension started to change to E‐W compression in the Japan arc. Subduction of the Japan Sea lithosphere under the Japan arc started 1.8 Ma ago. The evolution of the stress regime from transtensional to compressional in the southern part of the strike‐slip zone is related to the inception of the subduction of the young Philippine Sea Plate lithosphere under the Japan arc during the late Miocene. Subduction related extension is a necessary condition for the opening of the Japan Sea. Two possible mechanisms can account for dextral shear in this area: (1) counterclockwise rotation of crustal blocks due to the collision of India with Asia, (2) extrusion of the Okhotsk Sea block squeezed between the North America and Eurasia plates.
Evaluation of very long baseline interferometry atmospheric modeling improvements
Authors: MacMillan D. S. , Ma C.
Published: February 2004

We determined the improvement in baseline length precision and accuracy using new atmospheric delay mapping functions (Ifadis (Ifadis, 1986) and MTT (Herring, 1992a)) by analyzing the NASA Crustal Dynamics Project research and development (R&D) experiments and the International Radio Interferometric Surveying (IRIS) A experiments. These mapping functions reduce baseline length scatter by about 20% below that using the CfA2.2 (Davis et al., 1985) dry and Chao (Chao, 1974) wet mapping functions. With the newer mapping functions, average station vertical scatter inferred from observed length precision (given by length repeatabilities) is 11.4 mm for the 1987–1990 monthly R&D series of experiments and 5.6 mm for the 3‐week‐long ERDE series. The inferred monthly R&D station vertical scatter is reduced by 2 mm or by 7 mm in a root‐sum‐square (rss) sense. Length repeatabilities are optimum when observations below a 7–8 ° elevation cutoff are removed from the geodetic solution. Analyses of IRIS‐A data from 1984 through 1991 and the monthly R&D experiments both yielded a nonatmospheric unmodeled station vertical error of about 8 mm. In addition, analysis of the IRIS‐A experiments revealed systematic effects in the evolution of some baseline length measurements. The length rate of change has an apparent acceleration, and the length evolution has a quasi‐annual signature. We show that the origin of these effects is unlikely to be related to atmospheric modeling errors. Rates of change of the transatlantic Westford‐Wettzell and Richmond‐Wettzell baseline lengths calculated from 1988 through 1991 agree with the NUVEL‐1 plate motion model (Argus and Gordon, 1991) to within 1 mm/yr. Short‐term (less than 90 days) variations of IRIS‐A baseline length measurements contribute more than 90% of the observed scatter about a best fit line, and this short‐term scatter has large variations on an annual time scale.
Avalanche effects in phase transition modulated thermal convection: A model of Earth's mantle
Authors: Solheim L. P. , Peltier W. R.
Published: February 2004

We describe the development and application of an anelastic, multiphase model of the mantle convection process in axisymmetric spherical shell geometry. The radial structure of the anelastic reference state has been determined on the basis of elastic wave propagation data, primarily those used to construct the preliminary reference Earth model (PREM). The multiphase model is employed to examine the extent to which the pressure‐induced phase transitions in the planetary mantle may conspire to cause the flow to become radially layered. We find that the endothermic phase transition at 670 km depth profoundly influences the radial mixing process in the high Rayleigh number regime. In the Earth‐like region of parameter space the flow exhibits a low‐frequency quasi‐periodicity characterized by rather long periods of relative quiescence in which the circulation is predominantly layered followed by short periods of intense radial mixing across the endothermic horizon. These “avalanche” events are controlled by the periodic instability of the internal thermal boundary layer that develops on the endothermic horizon when the flow is layered. This hydrodynamic process appears to have important implications for the understanding of a number of characteristics of planetary evolution, especially thermal and chemical history.
Morphology of extrusive basalts and its relationship to seismic velocities in the shallow oceanic crust
Authors: Moos Daniel , Marion Dominique
Published: February 2004

Recent seismic experiments have revealed that a shallow, low velocity zone is a common feature within the uppermost few hundred meters of very young (less than 100 kyr) oceanic crust at ‘fast spreading’ ridges. The shape of this zone suggests that the low velocities are related to the constructional morphology of the extrusive (layer 2A) portion of the crust. With this in mind, we use simple analytical techniques to study the effects of characteristic morphologic elements of the extrusive basalts on their elastic properties. We find that high compliance of the contacts between pillows and pillow fragments can account for all of the observed velocity anomaly. Voids between pillows are, by comparison to their effects on porosity, considerably less important. Radial (cooling) cracks and voids within the pillows themselves will affect the velocities in direct proportion to their effect on the individual pillows. The relatively rapid increase in velocity with age in these very young extrusives can be explained by welding of pillow contacts and stiffening of the individual pillows by infilling of radial fractures, both of which may take place without a large reduction in porosity. If voids between pillows are filled initially by sediments, the cementation of these materials can also cause large increases in velocity without a large addition of mass. Because of the importance of such subtle variations in morphology, there can be no one‐to‐one relationship between seismic velocities and porosities within the extrusive layer of the oceanic crust. Therefore independent measures of crustal porosity must be found (for example, using sea floor gravity surveys).
Spatial and temporal variability in crystallization of celadonites within the Troodos ophiolite, Cyprus: Implications for low‐temperature alteration of the oceanic crust
Authors: Gallahan William E. , Duncan Robert A.
Published: February 2004

Celadonite, a low‐temperature (<30°C) hydrothermal alteration mineral, is a predominant fracture and void‐filling phase within the volcanic rocks of the Troodos ophiolite. The combined chemical and structural properties of celadonite (i.e., high K2O and Ar retentive), along with its common occurrence, provide a valuable tracer for studying the temporal and spatial variability of low‐temperature hydrothermal fluid circulation and alteration within this ancient oceanic crust. Some 54 new K/Ar age determinations of celadonites from various geographic and stratigraphic locations within the extrusive rocks of Troodos yield crystallization ages ranging from 90.9±1.0 to 49.8±0.5 Ma, with the oldest in close agreement with the estimated 91–92 Ma crystallization age of Troodos igneous rocks. The youngest age indicates that low‐temperature water/rock chemical exchange continued for at least 40 m.y. after crustal formation. This represents a 100% increase over previous estimates, based on limited numbers of K/Ar dates of celadonite, of the duration of low temperature mineral precipitation in Troodos. Correlation between celadonite ages and field relationships suggests the following: (1) There is no apparent relationship between celadonite crystallization and stratigraphic depth. (2) Celadonite crystallization occurs homogeneously on an outcrop scale, with adjacent samples from a single flow unit yielding equivalent K/Ar ages. (3) Flow units with relatively high primary permeabilities, e.g., pillows and breccias, yield celadonites with younger K/Ar ages. (4) Celadonite precipitation at distinct locations occurred rapidly, with little or no age difference between rim and core sections of single large deposits. The above correlations suggest that low‐temperature hydrothermal fluid circulation and secondary mineral precipitation are controlled by the local alteration conditions, such as intrinsic permeability, degree of fracturing, and water/rock ratios, and are independent of time and space. Integration of these local heterogeneities over the entire extrusive sequence suggests that the upper oceanic crust undergoes relatively homogeneous closure to hydrothermal fluid circulation. No evidence is found to support a progressive upward sealing of the oceanic crust.
A laboratory study of ash flows
Authors: Woods Andrew W. , Bursik Marcus I.
Published: February 2004

Hot ash flows propagate from volcanic vents during some explosive volcanic eruptions. As a flow entrains and heats ambient air and simultaneously deposits pyroclasts, part of the flow may become less dense than the air and ascend from the flow. If entrainment of air is more important than sedimentation in controlling the flowbehaviour, then a large, buoyant ash cloud may form above the flow. Conversely, if sedimentation of particles dominates, then a much smaller fraction of the particles are able to ascend in the buoyant cloud. We have carried out a series of analog laboratory experiments to simulate the motion and buoyancy generation in dilute ash flows propagating along the ground. The first series of experiments investigated buoyancy generation through entrainment, using mixtures of methanol, ethylene glycol and water. The second series examined the role of sedimentation upon the generation of buoyancy, using particle‐laden, fresh‐water currents propagating in a saline ambient fluid. The experiments showed that the rate of entrainment increases while the sedimentation rate decreases as the downward slope of the terrain increases, suggesting that massive ash‐laden clouds are more likely to rise from flows traveling on steeper slopes. Changes in slope angle also control the motion of ash flows. The experiments suggest that flow separation and hydraulic jumps resulting from sudden changes in slope angle can enhance entrainment or sedimentation and increase buoyancy. Observations of the the interactions of ash flows with topography at Redoubt Volcano in April 1990 and at Mount St. Helens on May 18, 1980, are consistent with our laboratory observations.
A unified approach to geopressuring, low‐permeability zone formation, and secondary porosity generation in sedimentary basins
Authors: Birchwood R. A. , Turcotte D. L.
Published: February 2004

Geopressuring, secondary porosity, and low‐permeability zones occur in many of the world's sedimentary basins and are often associated with major petroleum reservoirs. We present a mechanical model which simultaneously reproduces these phenomena. Assuming pressure solution to be the dominant rock deformation mechanism, a solid‐state rock skeleton viscosity is introduced which relates the deformation of the rock skeleton to the effective stress. Numerical solutions for compaction indicate that a soft sedimentary layer preferentially compacts to form a low‐permeability zone. There is a large liquid pressure gradient through the low‐permeability zone. In the more slowly compacting region beneath the low‐permeability zone, the pressure gradient remains hydrostatic but the absolute pressure can exceed the lithostatic pressure in the absence of hydraulic fracturing. This results in the formation of secondary porosity.
Mechanics of wedge‐shaped fault blocks: 2. An elastic solution for extensional wedges
Authors: Yin An
Published: February 2004

Listric, planar, low‐angle, and high‐angle normal faults are common in hanging walls of detachment faults. An elastic model has been developed to evaluate the role of basal friction, wedge geometry, pore fluid pressure within the wedge, and boundary conditions applied along the wedge rear in controlling the stress distribution in an extensional fault wedge. This model assumes a stress‐free condition on the top and fractional sliding on the base of the wedge, respectively, a linear variation of stress components as a function of depth along the wedge rear, and a uniform horizontal stress applied on the wedge toe. The model predicts that (1) for the surface slope equal to zero, a thin wedge favors development of high‐angle and planar faults, whereas a thick wedge favors development of low‐angle and listric normal faults; variation of pore fluid pressures along the basal detachment fault hardly affects the predicted fault geometry; pore fluid pressure within the wedge is critical in controlling the state of stress in the wedge: higher values of the internal pore fluid pressure promote low‐angle normal faulting and locally high‐angle reverse faulting; (2) variation of surface slope and the uniform horizontal normal stress applied at the wedge toe does not affect the predicted fault geometry appreciably, although the distribution of deviatoric stress magnitude changes for different cases; and (3) listric normal faults are predicted in all computations and the fault curvature increases as the vertical gradient of the horizontal normal stress along the wedge rear, the wedge angle (surface slope + dip angle), and the internal pore fluid pressure increase. The model provides a simple conceptual guide to deciphering the formation of complex fault geometries and cross cutting relationships as a function of mechanical parameters related to geologic processes and settings. For example, it provides an explanation for why low‐angle normal faults cut high‐angle normal faults in hanging walls of some detachment fault systems in the U.S. Cordillera.
Recent large earthquakes near Cape Mendocino and in the Gorda plate: Broadband source time functions, fault orientations, and rupture complexities
Authors: Velasco Aaron A. , Ammon Charles J. , Lay Thorne
Published: February 2004

The northward migration of the Mendocino Triple Junction is associated with complex faulting within the Gorda plate and in the convergent zone between the Gorda and North American plates. This region has experienced substantial recent large earthquake activity, and quantification of these faulting processes is essential for understanding the evolution of the triple junction. Using an empirical Green function deconvolution method for teleseismic and regional surface waves and body waves, we obtain relative source time functions for the April 25, 1992, Cape Mendocino thrust earthquake (Mw = 7.2), its two large strike‐slip aftershocks on April 26, 1992 (Mw = 6.5, 6.6), and two large strike‐slip events in the Gorda plate that occurred July 13, 1991 (Mw = 6.8), and August 17, 1991 (Mw = 7.1). The removal of propagation effects using empirical Green functions provides unusually detailed source rupture information and indicates that all the ruptures are less than 16 s in duration. Analyzing the directivity effects observed in the source time functions, we resolve rupture directions and corresponding fault orientations for the four largest events and place some constraints on the smaller aftershock. Significant differences in the rupture duration and stress drop are observed between the Cape Mendocino mainshock and its two largest aftershocks. The mainshock, which ruptured westward on a shallow dipping plane either on or paralleling the interplate contact between the Gorda and North American plates, had a smooth 9‐s‐long rupture with a 0.9–4.6 MPa static stress drop. The two largest aftershocks occurred within the uppermost mantle of the Gorda plate and have more complex ruptures with total durations of 14–15 s and static stress drops of less than 0.3 MPa, perhaps reflecting a difference between mantle and crustal earthquakes in this region. The aftershocks appear to have ruptured conjugate strike‐slip faults, accounting for differences in their damage patterns. The August 17, 1991, Gorda plate event ruptured a southwest striking fault, while the July 13, 1991, event ruptured a southeast striking fault, with both events relieving north‐south compression within the deforming Gorda plate. The existence of multiple active faults in the region constitutes a significant earthquake hazard associated with the complex stress environment of the migrating triple junction.
Thermal infrared satellite measurements of volcanic activity at Stromboli and Vulcano
Authors: Gaonac'h H. , Vandemeulebrouck J. , Stix J. , Halbwachs M.
Published: February 2004

The use of Landsat thermal infrared imagery gives a direct way of quantitatively monitoring the evolution of low‐amplitude thermal anomalies. Using nighttime Landsat thematic mapper images of Vulcano and Stromboli volcanoes on October 23, 1986, we have developed a simplified method for correcting atmospheric effects. We were then able to identify thermal anomalies around the craters. Part of the Stromboli lava flow, formed 1 year earlier, was also still warm enough to be detected. We obtained fairly accurate estimates of relative surface anomaly radiative heat fluxes: 29±5 W m−2, 21±4 W m−2, and 37±5 W m−2 for the Stromboli craters, Stromboli lava flow, and Vulcano crater, respectively. Accounting for the convective heat loss significantly increases the estimates of the relative internal heat flux of geothermal origin; we found values of 370±60 W m−2 for the Stromboli crater region, 220±50 W m−2 for the Stromboli lava flow, and 270±60 W m−2 for the Vulcano crater. Despite the low precision, these values are much higher than the 53–62 W m−2 limit (defined by Cassinis and Lechi, 1974) as the threshold where the geothermal flux can affect ground temperatures. Our study provides a unique systematic definition of thermal volcanic features found on Stromboli and Vulcano, the spatial extents of the anomalies, and their fluxes and associated errors.
Theory of partial thermoremanent magnetization in multidomain grains: 2. Effect of microcoercivity distribution and comparison with experiment
Authors: Xu Song , Dunlop David J.
Published: February 2004

We extend the thermoremanent magnetization (TRM)and pTRM theories developed in paper 1 (Dunlop and Xu, this issue) to grains in which domain walls are pinned by microcoercivities of varying magnitudes. Assuming microcoercivities to be exponentially distributed, we find that the intensity of a total TRM is linearly proportional to the inducing field H0 for small H0, to a power of roughly 1–1/n for intermediate H0, and independent of H0 for large H0, similar to the results obtained in paper 1. Here n represents the temperature dependence of microcoercivity that goes as the n th power of the saturation magnetization Ms (T). The above three field dependent regions correspond to thermally blocked, field‐blocked and reequilibrated walls, respectively. When being thermally demagnetized, a TRM induced in a high field has low unblocking temperatures, as observed. For a partial TRM acquired from T2 (<Tc)to T1, there may be no region in which walls are field blocked if the interval (T2, T1)is not large enough. This will be the case for magnetite when T2 < 565°C if n = 2 or <500°C if n = 4 for T1 = T0, independent of H0. If T1 > T0, an even higher T2 is required. In such cases, the room temperature intensity of pTRM is approximately proportional to H02 when H0 is small. The resulting thermal demagnetization curve, normalized to the intensity before heating, is independent of both H0 and the mean value of microcoercivities. Complete demagnetization will not occur at a demagnetizing temperature T2 but only at a temperature close to Tc. The theory is supported by experimental data of thermal demagnetizations of pTRMs measured for various multidomain magnetite samples.
Geometrical evolution of stressed and curved solid‐fluid phase boundaries: 1. Transformation kinetics
Authors: Heidug Wolfgang K. , Leroy Yves M.
Published: February 2004

This contribution is concerned with the fundamental thermodynamic aspects of solid‐fluid phase transformations in stressed rocks, specifically in the context of pressure solution. We concentrate in particular on the formulation of a kinetic law governing the migration of stressed and curved solid‐fluid phase boundaries, an objective that is achieved by using the methods of the thermodynamics of irreversible processes. We then apply our result to the study of the geometrical evolution of a fluid‐filled cylindrical pore embedded in an isotropic, linear elastic solid that is subject to a hydrostatic remote stress, assuming that the interface kinetics controls the phase boundary migration and allowing for the effects of capillarity. On the basis of this investigation, we obtain an analytical expression for the pore's growth and show that phase equilibrium along the cylindrical solid‐fluid phase boundary is possible only when the pore pressure exceeds a critical value. The phase equilibrium is found to be kinetically unstable: when subjected to a small perturbation of its radius, the pore will either grow or shrink. The nature of this instability is further explored in the companion paper.
Silurian paleogeography of Armorica: New paleomagnetic data from central Bohemia
Authors: Tait J. A. , Bachtadse V. , Soffel H.
Published: February 2004

Due to the lack of reliable paleomagnetic data from Armorica for mid‐Ordovician to Late Devonian times, the drift history for this tectonic element is poorly understood. In order to help rectify this problem, a paleomagnetic study of Upper Silurian volcanic, intrusive and sedimentary rocks from the Barrandian Basin of the Central Bohemian Massif (Czech Republic) has been carried out. This basin comprises a sequence of unmetamorphosed lower Paleozoic rocks, the folding of which has an uppermost age limit of Early Carboniferous. Stepwise thermal demagnetization of the samples collected (182 samples from 21 sites), and principal component analysis of both great circle and stable endpoint data reveal a total of three directions of magnetization. The first (labelled A) is in the direction of the present‐day local Earth's magnetic field (D/I = 360°/67°) and is thus interpreted as being of recent origin. The other two directions, termed B and C, comprise the intermediate and high blocking temperature directions respectively. The overall in situ mean direction obtained for B, identified between 150° and 320°C, is 195°/−8° (D/I), k = 18.5, α95 = 13.2° (eight sites). It fails the fold test and yields an in situ paleo‐south pole position of 42°S, 354°E. This coincides with the Late Carboniferous segment of the apparent polar wander (APW) path for stable Europe and is considered to be a secondary overprint of this age. The third magnetization (termed C) is identified between 320° and 600°C, gives an overall mean direction of 208°/−40°, k = 96.8, α95 = 5.7° (eight sites), after bedding correction. It passes fold and reversal tests on both local and regional levels and is interpreted as representative of the Late Silurian paleofield direction in the Bohemian Massif; it yields a paleo‐pole position of 55°S, 325°E. Assuming a normal polarity, the inclination value obtained translates into paleolatitudes of 23°S for this part of Armorica and implies a position adjacent to the southern margin of Baltica. This therefore indicates closure of the intervening ocean by Late Silurian times. However, adopting this polarity also implies large‐scale (up to 140°) anticlockwise rotations of the Bohemian Massif prior to deformation in the Early Carboniferous.
Detection of atmospheric pressure loading using very long baseline interferometry measurements
Authors: vanDam T. M. , Herring T. A.
Published: February 2004

Loading of the Earth by the temporal redistribution of global atmospheric mass is likely to displace the positions of geodetic monuments by tens of millimeters both vertically and horizontally. Estimates of these displacements are determined by convolving National Meteorological Center (NMC) global values of atmospheric surface pressure with Farrell's elastic Green's functions. An analysis of the distances between radio telescopes determined by very long baseline interferometry (VLBI) between 1984 and 1992 reveals that in many of the cases studied there is a significant contribution to baseline length change due to atmospheric pressure loading. Our analysis covers intersite distances of between 1000 and 10,000 km and is restricted to those baselines measured more than 100 times. Accounting for the load effects (after first removing a best fit slope) reduces the weighted root‐mean‐square (WRMS) scatter of the baseline length residuals on 11 of the 22 baselines investigated. The slight degradation observed in the WRMS scatter on the remaining baselines is largely consistent with the expected statistical fluctuations when a small correction is applied to a data set having a much larger random noise. The results from all baselines are consistent with ∼60% of the computed pressure contribution being present in the VLBI length determinations. Site dependent coefficients determined by fitting local pressure to the theoretical radial displacement are found to reproduce the deformation caused by the regional pressure to within 25% for most inland sites. The coefficients are less reliable at near coastal and island stations.