bluuebunny/crossref_metadata_2025 huggingface dataset:
PSC and volcanic aerosol observations during EASOE by UV‐visible ground‐based spectrometry
Authors: Sarkissian Alain , Pommereau Jean‐Pierre , Goutail Florence , Kyro Esko
Published: February 2004

Twilight sky colour measurements were made using ground‐based UV‐visible SAOZ spectrometers at four stations along the Arctic circle during EASOE. The results show that volcanic aerosol from the Pinatubo eruption of June 1991 first appeared above the Arctic in September 1991. The aerosol layer thickened progressively during the autumn and had spread to all four stations, outside in the very low stratosphere, as well as inside the polar vortex, by mid‐January. For the rest of the campaign, to mid‐March, little further change was seen. The aerosol would have masked any PSCs that may have occurred at altitudes less than 22 km. However, PSCs at higher altitudes should have been detected, had their optical thickness exceeded 0.01. Only one was recorded during EASOE: on 28–29 December 1991, in contrast to their frequent occurrence in January and early February 1990.
Observation and implications of nonlinear elastic wave response in rock
Authors: Johnson P. A. , McCall K. R.
Published: February 2004

Experiments in rock show a large nonlinear elastic wave response, far greater than that of gases, liquids and most other solids. The large response is attributed to structural discontinuities in rock such as microcracks and grain boundaries. The magnitude of the harmonics created by nonlinear interactions grows linearly with propagation distance in one‐dimensional systems. In the earth, a large nonlinear response may be responsible for significant spectral alteration of a seismic wave at amplitudes and distances currently considered to be within the linear elastic regime. We argue, based on observations at ultrasonic frequencies, that the effect of nonlinear elasticity on seismic wave propagation may be large, and should be considered in modeling.
Balloon profiles of stratospheric NO2 and HNO3 for testing the heterogeneous hydrolysis of N2O5 on sulfate aerosols
Authors: Webster C. R. , May R. D. , Allen M. , Jaeglé L. , McCormick M. P.
Published: February 2004

Simultaneous in situ measurements of stratospheric NO2, HNO3, HCl, and CH4 from 34 to 24 km were made in August 1992 from Palestine, Texas, using the Balloon‐borne Laser In‐Situ Sensor (BLISS) tunable diode laser spectrometer. Although the measurements of NO2, HNO3, and NO2/HNO3 agree well with gas‐phase model calculations near 34 km where SAGE II data show little sulfate aerosol, this is not true at the lower altitudes where SAGE II shows high aerosol loadings. At 24 km the BLISS NO2 and HNO3 measurements are 70% lower, and 50% higher, respectively, than the gas phase model predictions, with a measured NO2/HNO3 ratio 5 times smaller. When the heterogeneous hydrolysis of N2O5 and ClONO2 on sulfate aerosol of surface area densities matching the SAGE II measurements is added to the model, good agreement with the BLISS measurements is found over the whole altitude range.
Kelvin waves in total column ozone
Authors: Ziemke J. R. , Stanford J. L.
Published: February 2004

Tropical Kelvin waves have been observed previously in ozone mixing ratio data from the SBUV (Solar Backscatter Ultraviolet) and LIMS (Limb Infrared Monitor of the Stratosphere) instruments on board the Nimbus‐7 satellite. The present study investigates Kelvin wave features in total column ozone, using version 6 data from the Total Ozone Mapping Spectrometer (TOMS) instrument (also on Nimbus‐7). Results show eastward‐propagating zonal waves 1–2 with periods ∼5–15 days, amplitudes ∼3–5 Dobson Units (1–2% of the time mean), and latitudinal symmetry typical of Kelvin waves. The analyses and a linear model in this study suggest that the primary source of the perturbations is slow Kelvin waves in the lower‐to‐middle stratosphere. Maximum Kelvin wave signatures occur in conjunction with westward lower‐to‐middle stratospheric equatorial zonal winds [a quasi‐biennial oscillation (QBO) wind modulation effect]. The significance of these results is that the TOMS data are shown to be useful for investigations with global coverage of a major component of tropical stratospheric dynamics, Kelvin waves. The TOMS data set with its excellent coverage and high quality should be useful in validating model studies in the relatively data sparse and dynamically difficult tropical region.
Rifting consequences of three plate separation
Authors: Sibuet Jean‐Claude , Srivastava Shiri
Published: February 2004

Submarine sedimentary basins parallel to the trends of passive continental margins are well explained by tensional processes between two lithospheric plates. This leaves open the enigma of many basins which strike across margins. We use examples from the North Atlantic to show that such basins may be formed during the initial rifting stage due to enhanced lithospheric thinning caused by the separation of three plates. We suggest that volcanics can be emplaced in these basins and, in the extreme case, where mantle temperature and consequently the degree of partial melting are particularly high, that basaltic flows can be thick enough to fill and hide the basins.
Latitudinal variation of speed and mass flux in the acceleration region of the solar wind inferred from spectral broadening measurements
Authors: Woo Richard , Goldstein Richard M.
Published: February 2004

Spectral broadening measurements conducted at S‐band (13‐cm wavelength) during solar minimum conditions in the heliocentric distance range of 3–8 Ro by Mariner 4, Pioneer 10, Mariner 10, Helios 1, Helios 2, and Viking have been combined to reveal a factor of 2.6 reduction in bandwidth from equator to pole. Since spectral broadening bandwidth depends on electron density fluctuation and solar wind speed, and latitudinal variation of the former is available from coherence bandwidth measurements, the remote sensing spectral broadening measurements provide the first determination of the latitudinal variation of solar wind speed in the acceleration region. When combined with electron density measurements deduced from white‐light coronagraphs, this result also leads to the first determination of the latitudinal variation of mass flux in the acceleration region. From equator to pole, solar wind speed increases by a factor of 2.2, while mass flux decreases by a factor of 2.3. These results are consistent with measurements of solar wind speed by multi‐station intensity scintillation measurements, as well as measurements of mass flux inferred from Lyman α observations, both of which pertain to the solar wind beyond 0.5 AU. The spectral broadening observations, therefore, strengthen earlier conclusions about the latitudinal variation of solar wind speed and mass flux, and reinforce current solar coronal models and their implications for solar wind acceleration and solar wind modeling.
A thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (time‐GCM): Equinox solar cycle minimum simulations (30–500 km)
Authors: Roble R. G. , Ridley E. C.
Published: February 2004

A new simulation model of the mesosphere, thermosphere, and ionosphere with coupled electrodynamics has been developed and used to calculate the global circulation, temperature and compositional structure between 30–500 km for equinox, solar cycle minimum, geomagnetic quiet conditions. The model incorporates all of the features of the NCAR thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) but the lower boundary has been extended downward from 97 to 30 km (10 mb) and it includes the physical and chemical processes appropriate for the mesosphere and upper stratosphere.

The first simulation used Rayleigh friction to represent gravity wave drag in the middle atmosphere and although it was able to close the mesospheric jets it severely damped the diurnal tide. Reduced Rayleigh friction allowed the tide to penetrate to thermospheric heights but did not close the jets. A gravity wave parameterization developed by Fritts and Lu (1993) allows both features to exist simultaneously with the structure of tides and mean flow dependent upon the strength of the gravity wave source, The model calculates a changing dynamic structure with the mean flow and diurnal tide dominant in the mesosphere, the in‐situ generated semi‐diurnal tide dominating the lower thermosphere and an in‐situ generated diurnal tide in the upper thermosphere. The results also show considerable interaction between dynamics and composition, especially atomic oxygen between 85 and 120 km.
Comparison of terrestrial reference frame velocities determined from SLR and VLBI
Authors: Watkins M. M. , Eanes R. J. , Ma C.
Published: February 2004

The most recent site velocity solutions from the VLBI analysis group at the Goddard Space Flight Center and the SLR analysis group at the University of Texas Center for Space Research are compared. The results indicate that the horizontal velocities agree, after removing a velocity transformation of a few millimeters per year, to the 2–3 millimeter per year level in an rms sense. The velocities of the best observed sites agree to nearly 1 mm/yr in each horizontal component. In addition, when only sites with adjusted velocities are compared, the agreement of epoch site positions is improved over previous studies to the subcentimeter level in the horizontal components. The sites in the western United States agree at the few millimeter level.
Polarization tomography for P wave velocity structure in southern California
Authors: Hu Ge , Menke William , Powell Christine
Published: February 2004

We develop a tomographic inversion method that uses teleseismic P wave polarization data to obtain velocity structure. Polarization inversion has some intrinsic advantages over travel time inversion: It is not influenced by source location and origin time errors; it is not sensitive to deep mantle velocity structure and can be used iteratively to improve the tomographic result. Polarization inversion is more sensitive to near‐station velocity structure and to velocity gradient and is complementary to travel time inversion in this sense. The method is applied to California Institute of Technology‐U.S. Geological Servey southern California array data. The result is generally consistent with previous work and also reveals that the high‐velocity feature beneath the Transverse Ranges is bounded between 40 and 200 km depths and possibly has a second small piece at about 300 km depth. The slow velocity anomaly under the Salt on Trough is limited to shallow depths, less than about 60 km.
Robust inversion of IASP91 travel time residuals for mantle P and S velocity structure, earthquake mislocations, and station corrections
Authors: Vasco D. W. , Johnson Lane R. , Pulliam R. Jay , Earle Paul S.
Published: February 2004

Using both P and S arrival time information, 41,108 events in the International Seismological Centre (ISC) catalog for the years 1964 to 1987 are relocated relative to the IASP91 velocity model. The mean absolute horizontal relocation is 7.7 km and the mean absolute depth relocation is 9.1 km, The mean absolute origin time shift is 1.2 s. The relocation procedure increased the P residual standard deviation slightly from 2.3 s to 2.5 s while decreasing the S residual standard deviation from 6.8 s to 6.1 s. When plotted as bottoming point averages, the resulting IASP91 P and S arrival time residuals show coherent patterns as a function of geographic location. An iterative lp residual norm minimization algorithm is used to estimate the set of P and S velocity variations as well as the earthquake relocation and seismographic station parameters which best explain the travel time residuals. The procedure is robust in that extremely large travel time residuals, which are common in the ISC data, do not unduly influence the velocity estimates. Both the P and S models of lateral heterogeneity contain prominent circum‐Pacific low velocities, 1% to 2% perturbations, underlying the back arc basins between 35 and 200 km depth. This ring of negative deviations continues into the depth interval 200–400 km. The continental cratons are underlain by high‐velocity anomalies with maximum amplitudes of 2%. Iceland and the Azores are underlain by low‐velocity mantle material that extends down to at least 400 km. The Benioff zones are only intermittently imaged as 1–2% high‐velocity regions in the uppermost 400 km. They are best resolved in the P velocity variations. Both the P and S velocity models contain a circum‐Pacific ring, beneath the Benioff zones, of 1–2% positive velocity deviations in the depth range 660–870 km. Coherent high‐velocity features are seen below South America, Borneo, Tonga‐Fiji, the Marianas, and the northern Kuriles. The anomalies beneath South America and Borneo extend into the 870–1070 km depth range. Below depths of 1270 km for P variations and 1070 km for S variations the amplitude of the heterogeneity has decreased significantly. It is only in the lowermost mantle, 2670 km to the core‐mantle boundary, that the level of P heterogeneity rises significantly above the estimated noise level. In this depth range a partial ring around the Pacific basin is observed, although this.
Fluid circulation in the oceanic crust: Contrast between volcanic and plutonic regimes
Authors: Hart Stanley R. , Blusztajn Jerzy , Dick Henry J. B. , Lawrence James R.
Published: February 2004

Vein carbonates from a variety of ocean crust environments have been analyzed for Sr contents and Sr and oxygen isotopic compositions. Veins from upper crustal lithologies typically have low formation temperatures (<100°C) and form within 10–15 m.y. after crust formation, from solutions containing a basaltic component of very high Ca/Sr (>30 times that of hot smoker vent fluids). Veins from the gabbro lithologies at site 735B show both high (>150°C) and very low (<10°C) formation temperatures. The high‐temperature carbonate veins have a large basaltic component (low 87Sr/86Sr) with low Ca/Sr and probably formed before unroofing of this deep crustal block. The low‐temperature veins have only a minor basaltic component, and formed after unroofing, but within 0–10 m.y. of crust formation. Hot smoker vent fluids represent the low Ca/Sr limit of fluids recorded by carbonate veins and thus cannot describe the totality of the seawater‐ocean crust Ca/Sr exchange budget.
Formation and emplacement of the Josephine ophiolite and the Nevadan orogeny in the Klamath Mountains, California‐Oregon: U/Pb zircon and 40Ar/39Ar geochronology
Authors: Harper Gregory D. , Saleeby Jason B. , Heizler Matthew
Published: February 2004

Cordilleran ophiolites typically occur as basement for accreted terranes. In the Klamath Mountains, ophiolitic terranes were progressively accreted by underthrusting beneath North America. The Josephine ophiolite is the youngest of the Klamath ophiolites and forms the basement for a thick Late Jurassic flysch sequence (Galice Formation). This ophiolite‐flysch terrane forms an east dipping thrust sheet sandwiched between older rocks of the Klamath Mountains above and a coeval plutonic‐volcanic arc complex below. The outcrop pattern of the roof (Orleans) thrust indicates a minimum displacement of 40 km, and geophysical studies suggest >110 km of displacement. The basal (Madstone Cabin) thrust is associated with an amphibolitic sole and has a minimum displacement of 12 km. A rapid sequence of events, from ophiolite generation to thrust emplacement, has been determined using 40Ar/39Ar and Pb/U geochronology. Ophiolite generation occurred at 162–164 Ma, a thin hemipelagic sequence was deposited from 162 to 157 Ma, and flysch deposition took place between 157 and 150 Ma. Tight age constraints on thrusting and low‐grade metamorphism associated with ophiolite emplacement (Nevadan orogeny) are provided by abundant calc‐alkaline dikes and plutons ranging in age from 151 to 139 Ma. Deformation and metamorphism related to the Nevadan orogeny appears to have extended from ∼155 to 135 Ma. Most of the crustal shortening took place by thrusting, constrained to have occurred from ∼155 to 150 Ma on both the roof and basal thrusts. Minimum rates of displacement are 2.4 and 3.6 mm/year for the basal and roof thrusts, respectively, but correlations with coeval thrusts yield rates of 8.4 and 22 mm/year (within the range of plate velocities). The high displacement rates and synchronous movement along the basal and roof thrusts suggest that the ophiolite may have behaved as a microplate situated between western North America and an active arc from ∼155 to 150 Ma. A steep thermal gradient was present in the Josephine‐Galice thrust sheet from ∼155 to 150 Ma, with amphibolite facies conditions developed along the basal thrust. After accretion of the ophiolite by underthrusting, the ophiolite and overlying flysch underwent low‐grade dynamothermal regional metamorphism from 150 to 135 Ma. The upper age limit is tightly constrained by a 135 Ma K‐feldspar cooling age, syntectonic plutons as young as 139 Ma, and a Lower Cretaceous angular unconformity. Very rapid exhumation is indicated by the late Valanginian to Hauterivian age (∼130 Ma) of the unconformably overlying strata, suggesting unroofing by extensional tectonics.
The use of multispectral thermal infrared image data to estimate the sulfur dioxide flux from volcanoes: A case study from Mount Etna, Sicily, July 29, 1986
Authors: Realmuto Vincent J. , Abrams Michael J. , Buongiorno M. Fabrizia , Pieri David C.
Published: February 2004

We have found that image data acquired with NASA's airborne Thermal Infrared Multispectral Scanner (TIMS) can be used to make estimates of the SO2 content of volcanic plumes. TIMS image data are most applicable to the study of partially transparent SO2 plumes, such as those released during quiescent periods or nonexplosive eruptions. The estimation procedure is based on the LOWTRAN 7 radiative transfer code, which we use to model the radiance perceived by TIMS as it views the ground through an SO2 plume. The input to the procedure includes the altitudes of the aircraft and ground, the altitude and thickness of the SO2 plume, the emissivity of the ground, and altitude profiles of the atmospheric pressure, temperature, and relative humidity. We use the TIMS data to estimate both ground temperatures beneath a plume and SO2 concentrations within a plume. Applying our procedure to TIMS data acquired over Mount Etna, Sicily, on July 29, 1986, we estimate that the SO2 flux from the volcano was approximately 6700 t d−1. The use of TIMS to study SO2 plumes represents a bridge between highly localized methods, such as correlation spectroscopy or direct sampling, and small‐scale mapping techniques involving satellite instruments such as the Total Ozone Mapping Spectrometer or Microwave Limb Sounder. We require further airborne experiments to refine our estimation procedure. This refinement is a necessary preparation for the scheduled 1998 launch of the Advanced Spaceborne Thermal Emission and Reflectance Radiometer, which will allow large‐scale multispectral thermal infrared image data to be collected over virtually any volcano on Earth at least once every 16 days.
Seismic imaging of oceanic layer 2A between 9°30′N and 10°N on the East Pacific Rise from two‐ship wide‐aperture profiles
Authors: Vera Emilio E. , Diebold John B.
Published: February 2004

We analyze two‐ship multichannel seismic wide aperture profiles (WAP) acquired on the East Pacific Rise (EPR) between 9°30′C and 10°N in May and June 1985. For offsets between 2 and 5 km, most of the WAP common depth point (CDP) gathers exhibit a strong retrograde, reflection‐like seismic arrival that corresponds to rays turned within a very strong velocity gradient zone underlying the uppermost crustal layer 2A. Stacking of this postcritical arrival for hundreds of CDP gathers along the WAP lines yields remarkably clear, and virtually continuous, images of the bottom of layer 2A for several tens of kilometers along and across the EPR. An innovative combination of this image with others of the seafloor (SF) and of the axial magma chamber (AMC) provides an unusually clear composite picture of young upper crust. We combine these images with detailed seismic velocity information, also obtained from the CDP gathers, to achieve an accurate determination of the layer 2A thickness in the area. Along the EPR axis between 9°30°N and 10°N, layer 2A shows a relatively constant thickness between 100 and 150 m (80–120 ms two‐way travel time). Across the axis, however, a line near 9°30′N indicates that layer 2A thickness rapidly increases by approximately a factor of 2 within 2–4 km of the axis and remains nearly constant afterwards. Thus the final thickness of layer 2A is attained before the development of large‐scale tectonic faulting, strongly suggesting that the thickening of layer 2A away from the axis is due to successive episodes of volcanic activity increasing the thickness of the uppermost crustal section from the top, rather than tectonic fracturing lowering the uppermost crustal seismic velocities and pushing the bottom of layer 2A downwards as the crust moves away from the axis.
An Early Carboniferous paleomagnetic pole for Gondwanaland: New results from the Mount Eclipse Sandstone in the Ngalia Basin, central Australia
Authors: Chen Zhong , Li Zheng‐Xiang , Powell Christopher M. , Balme Basil E.
Published: February 2004

Paleomagnetism of fourteen diamond‐drill cores from the uppermost Devonian‐Lower Carboniferous Mount Eclipse Sandstone in the Ngalia Basin, central Australia, has been studied. The samples were oriented using the borehole directions, bedding planes identified in the drill cores, and regional bedding attitudes. Three magnetic remanent components have been revealed: (1) a drilling‐induced, low‐ to moderate‐temperature remanence (C1) from all the drill cores, which is oriented parallel to the drill cores and is most likely a combination of isothermal remanent magnetization and piezoremanent magnetization; (2) a moderate‐ to high‐temperature, syndeformational overprint (C2: D = 226.7°, I = 82.3°, α95 = 6.2°, k = 68.9) in 65 samples from nine drill cores, which was probably acquired during the mid‐ to Late Carboniferous Mount Eclipse movement; and (3) a moderate‐ to high‐temperature, likely primary remanence (C3: D = 057.1°, I = −36.9°, α95 = 9.6°, K = 29.6) from 64 samples in nine drill cores, which exhibits a positive fold test. C2 and C3 are found only in reddish siltstones and sandstones, whereas C1 is best developed in grey reduced sandstones and siltstones. A palynological examination gives an early to mid‐Viséan (Early Carboniferous) age of deposition for samples carrying the interpreted primary component. C2 gives a mid‐ to Late Carboniferous pole at 32.1°S, 119.5°E with A95 = 11.9°, which is in agreement with results of a previous study on surface outcrops. C3, when combined with results from one drill core in a previous study, gives a Viséan pole at 37.6°S, 52.6°E with A95 = 8.7°. This pole suggests a south polar position for central Africa during the Early Carboniferous and therefore indicates the existence of a large ocean between Gondwanaland and Laurussia at that time. The paleo‐south pole shifted from central Africa in the Early Carboniferous (∼340±5 Ma) to East Antarctica in the mid‐ to Late Carboniferous (∼310±10 Ma) with a minimal continental drifting rate of 15–20 cm/yr.
Relative array analysis of the southern California lithosphere
Authors: Powell Christine A. , Mitchell Bruce T.
Published: February 2004

Lateral velocity structure in the lithosphere beneath southern California is investigated using relative array analysis. In this form of analysis, P wave polarization values measured by a subset of receivers (subarray) are compared to values measured by the entire set of receivers (array). The effects of lateral velocity structure beneath the subarray can be distinguished from the effects of average velocity structure beneath the entire array. Lateral velocity gradients can be mapped as the subarray window is moved across the network. The method is insensitive to effects of near‐source and lower mantle heterogeneity, does not lose resolution near the periphery of the array, and utilizes a locally derived ray geometry. Lateral velocity variations beneath the California Institute of Technology‐United States Geological Survey network detected by relative array analysis can be compared directly to velocity anomalies found by inversion of travel time residuals. Two regions near the edge of the tomographic image developed by Humphreys and Clayton are investigated; both regions are experiencing active tectonics. Relative array analysis suggests that relatively high velocity may be present beneath the westernmost Transverse Ranges. This feature extends to a depth of roughly 80 km and may be related to the high velocity upper mantle anomaly imaged beneath the central and western Transverse Ranges. Relatively low velocity is associated with the crust and uppermost mantle south of the Salton Sea. This feature is probably related to hot, ascending mantle but is located east of the surface expression of the spreading axis in the Imperial Valley.
Constraints on the tectonic development of the eastern Gulf of Mexico provided by magnetic anomaly data
Authors: Hall Stuart A. , Najmuddin Ilyas J.
Published: February 2004

Boundaries between oceanic and transitional crust in the eastern Gulf of Mexico have been delineated using detailed, accurately navigated, high‐quality aeromagnetic data. These data indicate that considerably more oceanic crust may be present than previously suggested by other geophysical data, especially seismic reflection data. The N‐S extent of oceanic crust substantially increases westward from approximately 280 km near 87°W to roughly 390 km near 90°W, indicating the proximity (<10°) of the pole of rotation. Variations in the amount of oceanic crust together with discontinuities in the magnetic anomaly patterns, interpreted as the traces of fossil transform faults or fracture zones, have been used to obtain a rough estimate of the location of the pole of rotation (24°N, 81.5°W) for the seafloor spreading phase of opening. The magnetic data have been used to develop a model for the opening of the Gulf and to identify some geometrical relationships between its conjugate continental margins. Based upon the estimated pole position, the eastern Gulf is interpreted to have opened in an approximately NNE‐SSW direction with the counterclockwise rotation of Yucatan away from North America. The seafloor spreading phase of movement accounts for about 25° of the counterclockwise motion of Yucatan. Rotating the Yucatan block clockwise by this amount to restore it to its predrift position places the Campeche Bank adjacent to the Mississippi Trough‐Louisiana shelf. Further clockwise rotation of Yucatan by an additional 30–35° about this pole restores extended crust to its original thickness and is consistent with rotations of Yucatan predicted from paleomagnetic data from Chiapas.
Geometrical evolution of stressed and curved solid‐fluid phase boundaries: 2. Stability of cylindrical pores
Authors: Leroy Yves M. , Heidug Wolfgang K.
Published: February 2004

The present study deals with the morphological alterations resulting from the pressure solution of fluid‐filled tubular pores embedded in a stressed solid matrix. Building upon previous work concerning transformation kinetics and the kinetically unstable nature of cylindrical solid‐fluid phase boundaries subjected to nonhydrostatic stress, we investigate the response of such a pore to circumferential and longitudinal perturbations in its shape by means of a linear stability analysis. We obtain criteria for the onset of instability, determine the wavenumber of the disturbance with fastest growth, and demonstrate that with increasing pore pressure the dominant instability changes from a circumferential to a longitudinal mode. This last feature could distinguish interface migration that is limited by the dissolution/precipitation kinetics from that controlled by diffusion through the pore fluid. We finally obtain estimates of the number and spacing of the inclusions formed when a cylindrical pore breaks up following unstable circumferential or longitudinal growth.
On the electrical nature of the axial melt zone at 13° N on the East Pacific Rise
Authors: Evans Rob L. , Sinha M. C. , Constable S. C. , Unsworth M. J.
Published: February 2004

The first controlled source electromagnetic experiment directly on a ridge, with the potential to identify the presence of an axial melt body beneath a fast‐spreading center, was conducted at 13°N on the East Pacific Rise (EPR) in 1989. Transmission for 36 hours was achieved by a deep towed horizontal electric dipole source, of moment 6000 Am, operating at frequencies between 1/4 and 8 Hz. Signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100,000‐year‐old crust. Data above ambient noise levels were obtained at ranges of up to 10 km. The results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low (∼1 Ω), indicating a heavily fractured, high‐porosity surficial layer. Below this topmost layer, the upper 2 km of crust is found to be moderately resistive (∼100 Ωm). We find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust. If a partial melt body is present, which is continuous along strike and which comprises a connected, and therefore conductive, melt texture, it must be of very limited volumetric extent. This picture is consistent with recently proposed models of a thin sill‐like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent. The larger region below the sill, characterized by low seismic velocities, must contain at best a very small melt fraction distributed in isolated pockets, providing further evidence that the EPR at 13°N is currently in a state of relative magmatic quiescence.
Array measurements and modeling of sources of shallow volcanic tremor at Kilauea Volcano, Hawaii
Authors: Goldstein Peter , Chouet Bernard
Published: February 2004

We show that resonances in near‐surface layers (path effects), or a combination of near‐surface resonances and source resonances (source effects), could be responsible for the sharp spectral peaks found in the records of gas‐piston activity and volcanic tremor recorded near Puu Oo crater on Kilauea Volcano, Hawaii. Two types of sources are found to be compatible with observations and can not be distinguished using our data. In one model, impulsive, explosion point sources are embedded in the Puu Oo structure. In the other model, impulsive, point sources of pressure excite the resonances of a fluid‐filled crack embedded in that structure. Both models require shallow sources (z < 100 m) and display a strong dependence of the radiated spectrum on source depth over a depth range comparable to the overall thickness of the surficial layers. Based on these results, it may be possible to track changes in source depth through temporal changes in observed spectra, a potentially useful tool for monitoring volcanic activity at this site. We estimate the depth and spatial extent of volcanic tremor and gas‐piston activity using two dense arrays with respective apertures of 800 and 120 m located near Puu Oo. Measurements of slowness (ray parameter) and azimuth as a function of time clearly indicate that the sources of volcanic tremor and gas‐piston activity are located beneath or in close proximity to the Puu Oo crater at depths shallower than approximately 1 km. Based on slowness and particle motion analyses we find that the records of volcanic tremor and gas‐piston events at Puu Oo consist of a complex combination of body and surface waves.
On modeling the thermal evolution of the oceanic upper mantle: An assessment of the cooling plate model
Authors: Carlson Richard L. , Johnson H. Paul
Published: February 2004

The plate cooling model for the thermal evolution of the oceanic upper mantle has been widely accepted to explain observed variations of depth to oceanic basement and conductive heat flow with the age of the seafloor. Several estimates of “best fitting” plate model parameters, derived from depth, heat flow, and age data, have been proposed, but the viability of the plate model itself has not been rigorously evaluated. We have used published mean depths and depths to basement at Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) drilling sites to test the plate cooling model based on two criteria: First, viable plate models must have coefficients that are consistent with the slope of the corresponding root t line because the half‐space (or root t) subsidence of young seafloor is implicit in the plate model (i.e., the slope of the root t line can be calculated directly from the coefficients of the plate subsidence model). Second, any viable physical model must fit the data with an acceptable degree of systematic misfit; large systematic misfits indicate that the model cannot explain the observations. Fits of half‐space (root t) models to depth versus age data for young (< ∼80 Ma) seafloor indicate basal temperatures in the range 1300 to 1370°C. Based on the age at which the depths deviate from the root t line, the minimum plate thickness that is compatible with the best fitting half‐space models for young seafloor is 120 km. In contrast, all best fitting plate models yield systematically higher temperatures (1450 to 1470°C) and thinner plates (102–118 km). The plate model can explain the depth to basement at DSDP/ODP drill sites with a satisfactory degree of systematic misfit, but we find that there is no plate cooling model that can explain the variation of the mean depths (derived from the DBDB5 database) with age over the entire range of ages (0 to 165 Ma). Models that “best fit” the entire data set have unacceptably large systematic misfits over the entire range of seafloor age, whereas models that minimize the systematic misfit for young seafloor (0 to 81 Ma) fail for older seafloor. The plate model clearly fails to explain the observations. The cooling history of old seafloor is not simply an extension of the cooling history of young seafloor according to a simple plate cooling model. Previous studies have suggested, as an alternative to the plate model, that observed variations of basement depth with age are best explained by the combined effects of a “normal” half‐space cooling process and the dynamic and/or thermal effects of hot spots or mantle plumes. Our results are entirely consistent with the half‐space cooling model, and we find that the best reference model for “normal” subsidence is d(t) = (2600±20)m + (345±3)m (m.y.)−½ t½. We also find that the heat flow predicted by this half‐space model is consistent with the most reliable average heat flow values from the Pacific.
Temporal and spatial variability of hydrothermal manganese and iron at Cleft segment, Juan de Fuca Ridge
Authors: Massoth Gary J. , Baker Edward T. , Lupton John E. , Feely Richard A. , Butterfield David A. , Von Damm Karen L. , Roe Kevin K. , Lebon Geoffrey T.
Published: February 2004

A unique data set for hydrothermal Mn and Fe was collected at Cleft segment on the Juan de Fuca Ridge between 1983 and 1991. The data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting. Manganese/heat and iron/heat ratios for plumes from the north end of the Cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for Mn of 0.36±0.17 mol s−1 and for Fe of 0.61±0.34 mol s−1. Over 6 years of plume measurements at North Cleft segment, observed episodic hydrothermal discharge accounted for −15% of the total vented Mn and −35% of vented Fe. The chronic fluxes for Mn and Fe at a second venting center located at the south end of the Cleft segment were estimated to be approximately equal to the fluxes at North Cleft segment. Chronic plumes at North Cleft segment are mixtures of focused and diffuse discharge that contribute heat, Mn, and Fe in variable proportions. Similar examination of South Cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in Mn and Fe but contributing substantially to the overall heat. Temporal and spatial variations in the concentrations of Mn and Fe and in Mn/heat and Fe/heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes. Manganese/heat and iron/heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes. The accurate assessment of segment‐scale hydrothermal fluxes of Mn and Fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge.
The magma body at Kilauea Iki lava lake: Potential insights into mid‐ocean ridge magma chambers
Authors: Barth G. A. , Kleinrock M. C. , Helz R. T.
Published: February 2004

Although geological and geophysical measurements at active ridges have provided indirect samples and remote images and ophiolites have provided fossil evidence, the lack of direct sampling of in situ physical and chemical properties severly hinders our understanding of subsurface magmatic systems at mid‐ocean ridges. In contrast with the ridge setting, substantial information has been gained from direct sampling of Kilauea Iki lava lake. We reanalyze time series physical and petrological data from both in situ sampling and remote seismic and electromagnetic imaging of this lava lake in order to gain insights into the evolution of an enclosed basaltic magma body with many characteristics similar to a mid‐ocean ridge magma chamber. Although the analogy is imperfect, our analysis of observations from Kilauea Iki suggests that at a mid‐ocean ridge (1) a fine structure exists at the top of the magma body in the form of horizontal segregation veins (i.e., sills), (2) sustained convection within the magma chamber is highly unlikely, (3) a “dry‐out” zone above the magma body and below the deepest penetration of hydrothermal circulation may exist, (4) with high magma supply (e.g., the East Pacific Rise), where quasi steady state melt zones are present, the magma chamber must be completely replenished very frequently (at least every 300–600 years), and (5) the onset of rigid behavior of the basaltic partial melt does not happen at a unique “critical” crystal fraction. These results also show that significant differences may occur in interpretation of the rheology of a magmatic system depending upon the type of petrologic, seismic, or drilling data analyzed.
Compositions, growth mechanisms, and temporal relations of hydrothermal sulfide‐sulfate‐silica chimneys at the northern Cleft segment, Juan de Fuca Ridge
Authors: Koski Randolph A. , Jonasson Ian R. , Kadko David C. , Smith Virginia K. , Wong Florence L.
Published: February 2004

Three active hydrothermal vents forming sulfide mounds and chimneys (Monolith, Fountain, and Pipe Organ) and more widely distributed inactive chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern Cleft segment, Juan de Fuca Ridge. The formation of zoned tubular Curich chimneys (type I) on the Monolith sulfide mound is related to focused flow of high‐temperature (to 328°C) fluid. Bulbous chimneys (type II or “beehives”) at the Monolith and Fountain vents are products of diffuse high‐temperature (to 315°C) discharge. A broader zone of vigorous mixing between the hydrothermal fluid and seawater results in quench crystallization of anhydrite‐rich shells. Columnar Zn‐sulfide‐rich chimneys with narrow channelways (type III) are constructed where focused and relatively low‐temperature (261°C) fluid vents directly from the basalt substrate. The bulk chemistry (low Cu; high Pb, Ag, and SiO2 contents), mineralogy (pyrite‐marcasite‐wurtzite‐amorphous silica‐anglesite), colloform and filamentous textures, and oxygen isotope characteristics of inactive (type IV) chimneys indicate a low‐temperature (<250°C) origin involving diffuse and sluggish flow patterns and conductive cooling. Seafloor observations and 210Pb data indicate that (1) type IV chimneys are products of an earlier period of hydrothermal activity that ended no more than 60 years ago but prior to the sheet flow eruption, (2) the high‐temperature Monolith and Fountain vents are manifestations of the same heating event (shallow emplacement of magma) that led to the sheet flow eruption and recent megaplumes, and (3) the Pipe Organ Vent is in a very youthful stage of development and chimney deposition postdates the sheet flow eruption.
Wear processes during frictional sliding of rock: A theoretical and experimental study
Authors: Wang Weibin , Scholz Christopher H.
Published: February 2004

An experimental study of frictional wear of rock, conducted with a rotary apparatus, shows that wear loss, the loss of material from the interface of two bodies during frictional sliding, is a function of normal stress and the initial roughness of the sliding surface. Under a given normal stress and initial roughness, the wear loss‐sliding displacement relationship indicates that wear rate is initially high and then gradually decreases to a constant value. The wear process results in an evolution of the surface topography as shown from profilometer measurements of the surface at different stages of wear. The distribution of asperity heights shows that the top of the sliding surface is progressively truncated with accumulated slip. Based on a model of two rough surfaces in elastic contact, a numerical model for wear can describe wear in two different stages: a transient stage and a steady state stage. In the transient stage, the wear mechanism is interpreted as shearing off of interlocking asperities. The total contribution by this mechanism is proportional to the overlapping volume of the asperities, V*, which is related to the initial roughness as well as the normal stress. In the steady state stage, wear is almost linear with displacement and the wear rate is proportional to the real area of contact between the two rough surfaces. Introducing a parameter h, the interlocking distance between two contacting asperities, provides a better understanding of the wear process with different surface roughnesses. Below a critical value of interlocking distance hc, the contacting asperities deform elastically and slide over each other. When h>hc, they tend to shear off. The model predicts the effects of normal stress and initial roughness, and its predictions are in good agreement with the experimental results.
Tectonics of rift propagation into a continental margin: Western Woodlark Basin, Papua New Guinea
Authors: Benes Vladimir , Scott Steven D. , Binns Ray A.
Published: February 2004

Oblique convergence along the irregular boundary between the Pacific and Indo‐Australian plates in the SW Pacific has resulted in a transpressional regime. The drag of the overriding Pacific plate has produced a breakup of the Solomon microplate from the Indo‐Australian plate and has induced its rotation within the mega‐shear zone between the two major plates. The microplate is being separated from the Indo‐Australian plate by passive rifting and seafloor spreading in the Woodlark Basin which is propagating westward along the continental margin of Papua New Guinea. The present‐day pole of the microplate rotation with respect to Indo‐Australian plate is estimated to be near the Owen Stanley Fault Zone which represents the suture after Paleocene‐Eocene arc‐continent collision. The average rate of the seafloor spreading propagation is estimated to be 150 mm yr−1 over the last 3.4 Ma. The transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension. Variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault. The style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening. Three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks.
Quantitative reflectance spectroscopy of buddingtonite from the Cuprite mining district, Nevada
Authors: Felzer Benjamin , Hauff Phoebe , Goetz Alexander F. H.
Published: February 2004

Buddingtonite, an ammonium‐bearing feldspar diagnostic of volcanic‐hosted alteration, can be identified and, in some cases, quantitatively measured using short‐wave infrared (SWIR) reflectance spectroscopy. In this study over 200 samples from Cuprite, Nevada, were evaluated by X ray diffraction, chemical analysis, scanning electron microscopy, and SWIR reflectance spectroscopy with the objective of developing a quantitative remote‐sensing technique for rapid determination of the amount of ammonium or buddingtonite present, and its distribution across the site. Based upon the Hapke theory of radiative transfer from particulate surfaces, spectra from quantitative, physical mixtures were compared with computed mixture spectra. We hypothesized that the concentration of ammonium in each sample is related to the size and shape of the ammonium absorption bands and tested this hypothesis for samples of relatively pure buddingtonite. We found that the band depth of the 2.12‐μm NH4 feature is linearly related to the NH4 concentration for the Cuprite buddingtonite, and that the relationship is approximately exponential for a larger range of NH4 concentrations. Associated minerals such as smectite and jarosite suppress the depth of the 2.12‐μm NH4 absorption band. Quantitative reflectance spectroscopy is possible when the effects of these associated minerals are also considered.
Magnetite dissolution in deep sediments and its hydrologic implication: A detailed study of sediments from site 808, leg 131
Authors: Lu Ran , Banerjee Subir K.
Published: February 2004

Our previous study of deep sea sediments from site 808, leg 131 of the Ocean Drilling Program has demonstrated that the two anomalously low‐intensity zones of natural remanent magnetization (NRM), 675 to 925 and 1080 to 1243 m below seafloor, were caused by unusually low magnetite content in the sediment sections and that this was not related to the variations in sediment sources, calcite dilution, or magnetite destruction occurring in the top sediment layer during early diagenesis. For an explanation, we now suggest that these low INRM intensity zones were produced by catagenesis at great depth by organic matter decomposition in sediments, which in turn causes magnetite dissolution and hence a lower magnetite content. A similar process also applies to manganese ions; as a consequence, iron and manganese concentration, grain size and content of magnetite, and NRM intensity all decrease, whereas sulfur content increases in these sediments. Our interpretation is also supported by other studies of organic geochemistry and sedimentology of these same sediments. We suggest, therefore, that detailed rock magnetic and geochemical tests should be carried out before geomagnetic field variations are studied using ocean sediments from such great depth. Additionally, we suspect that catagenesis may have been retarded and magnetic degradation was prevented near a décollement due to cold water percolation.