bluuebunny/crossref_metadata_2025 huggingface dataset:
Acceleration and heating of cold ion beams in the plasma sheet boundary layer observed with GEOTAIL
Authors: Hirahara M. , Nakamura M. , Terasawa T. , Mukai T. , Saito Y. , Yamamoto T. , Nishida A. , Machida S. , Kokubun S.
Published: February 2004

The GEOTAIL/Low Energy Particle (LEP) observations have revealed detailed features of acceleration and heating of cold ion beams in the plasma sheet boundary layer (PSBL). In the lobe region, the cold ion beams are flowing tailward nearly along magnetic field lines with small perpendicular drift toward the plasma sheet. Upon entering the plasma sheet, these cold ion beams are heated and accelerated up to several keV/q in the PSBL where a high‐speed ion flow is observed separately and simultaneously, and finally assimilated into the hot component of the plasma sheet proper. It should be noted that the acceleration of the cold ion beams in the PSBL is observed in the direction perpendicular to the magnetic field, and the perpendicular velocities of the cold ion beams during the acceleration coincide well with those of the high‐speed ion beams. This fact suggests that the perpendicular acceleration is due to an increase of the E×B drift speed in the PSBL as particles move from the lobe to the central plasma sheet. The electric field intensity for the observed E×B drift motion is estimated as 2–5 mV/m. The direction of the electric field inferred from the ion motion in the PSBL is mainly in the south‐to‐north direction rather than the dawn‐to‐dusk direction which is generally thought to be typical in the tail lobe. The direction of the acceleration is at times observed to change drastically, which suggests that the electric field direction fluctuates significantly as well.
Production of isoprene by marine phytoplankton cultures
Authors: Moore R. M. , Oram D. E. , Penkett S. A.
Published: February 2004

While release of isoprene from the terrestrial biosphere is well established and known to influence the tropospheric concentrations of a number of reactive species including ozone, very little is known of its production in the ocean and possible release to the marine boundary layer. Measurements reported here of low molecular weight trace gases produced by a series of laboratory phytoplankton have shown that isoprene is a major component. Though the results confirm that the oceans are a potential source of isoprene to the atmosphere, it is not possible to extrapolate release rates from laboratory studies to the very different conditions under which marine plants grow naturally.
The anomalous Arctic lower stratospheric polar vortex of 1992–1993
Authors: Manney G. L. , Zurek R. W. , Gelman M. E. , Miller A. J. , Nagatani R.
Published: February 2004

Potential vorticity (PV) gradients defining the lower stratospheric vortex during the 1992–1993 winter were anomalously strong and persistent compared to those during the last 16 Arctic winters. For ≈3 months PV gradients were closer to typical Antarctic values than to most Arctic values. Air motion diagnostics computed for 3‐dimensional air parcel trajectories confirm that the 1992–1993 Arctic lower stratospheric vortex was substantially more isolated than is typical. Such isolation will delay and reduce the export of the higher ozone typical of the winter lower stratospheric vortex to mid‐latitudes. This may have contributed to the record‐low total ozone amounts observed in northern mid‐latitudes in 1993.
Nonlinear soil amplification inferred from downhole strong seismic motion data
Authors: Wen Kuo‐Liang , Beresnev Igor A. , Yeh Yeong Tein
Published: February 2004

Nonlinear seismic response of soil is studied by comparison of the frequency‐dependent soil amplification functions calculated on weak and strong motion. Amplifications are obtained by dividing Fourier amplitude spectra of acceleration at the ground surface by the spectra at the depths of 11 and 47 m in a borehole. Observed weak and strong motion spectral ratios are compared with those theoretically deduced from the one of the nonlinear soil models adopted in geotechnical engineering. Significant deamplification of the strong motion having PGA (peak ground acceleration) greater than 0.16 g, relative to the weak motion, is exhibited by the experimental ratios in the frequency range that is consistent with the model prediction. Existence of the different frequency bands, in which specific features of the nonlinear soil response are recognized in the theory, is also confirmed. These results give evidence of that nonlinear response can be observed at soft sedimentary sites from the real strong motion data.
The blast wave of the Shuttle plume at ionospheric heights
Authors: Li Y. Q. , Jacobson A. R. , Carlos R. C. , Massey R. S. , Taranenko Y. N. , Wu G.
Published: February 2004

The main engine burn (MEB) of the Space Shuttle deposits ∼2×1012 joules of explosive energy and ∼3×105 kg of exhaust in almost horizontal flight at 105–110 km altitude during the period 300–550 s into the ascent. This extremely robust perturbation provides a potential active‐excitation source for a variety of geophysical processes, including (1) the effects of aurora‐like localized heating on the generation of gravity waves in the thermosphere, (2) the ducting mechanisms for long‐period infrasound in the upper atmosphere, (3) dynamo effects associated with transient charge separation, (4) interactions with ambient midlatitude current systems at E‐layer heights, and (5) effects in the Earth‐ionosphere waveguide of transient electron‐density perturbations in the D‐region. The sine qua non of such an agenda is to gain a quantitative understanding of the near‐field behavior of the MEB exhaust‐plume's quasi‐cylindrical expansion, which generates a blast wave propagating away from the explosion. We report on observed electron‐density signatures of this blast wave as manifested on lines‐of‐sight (LOSs) from a very‐long‐baseline interferometer (VLBI) illuminated by 137‐MHz beacon signals from the MARECS‐B satellite. We also compare the observations to a preliminary three‐dimensional neutral‐air acoustic model coupled to the ionospheric electron density.
The Coesite‐Stishovite Transition in a laser‐heated diamond cell
Authors: Serghiou G. , Zerr A. , Chudinovskikh L. , Boehler R.
Published: February 2004

The coesite‐stishovite phase boundary in the temperature range 2300–2630°C was examined in a CO2‐laser heated diamond anvil cell using argon as a pressure medium. Phase identification was provided by Raman spectroscopy of the temperature quenched samples at pressure. Our results suggest a coesite‐stishovite phase boundary curve described by P(GPa) = 8.1 + 0.001T(°C) in good agreement with Yagi and Akimoto's (1976) estimate at lower temperatures. The slope of this curve is less than one half that determined recently by Zhang et al. [1993, 1994] in a multi‐anvil press.
Constraints on the crystalline structure of the inner core: Mechanical instability of BCC iron at high pressure
Authors: Stixrude Lars , Cohen R. E.
Published: February 2004

We examine the energetics of close‐packed phases of iron (bcc, fcc, hcp) throughout the pressure regime of the earth with elaborate first principles electronic structure calculations. The calculations reproduce the relevant experimental observations including the equation of state to over 300 GPa and the pressure of the bcc to hcp phase transition. The bcc structure is found to be energetically unfavorable and mechanically unstable with respect to a tetragonal strain at high pressure (P>150 GPa). This phase is thus highly unlikely to exist in the earth's inner core as has frequently been proposed.
Interactions between the seasonal cycle and the Southern Oscillation ‐ Frequency entrainment and chaos in a coupled ocean‐atmosphere model
Authors: Chang Ping , Wang Bin , Li Tim , Ji Link
Published: February 2004

Nonlinear interactions between the seasonal cycle and interannual variations in the coupled ocean‐atmosphere system have recently been proposed as the cause of irregularity of El Niño‐Southern Oscillation (ENSO). We investigated such a hypothesis using a coupled ocean‐atmosphere model which allows coupling between total sea surface temperature (SST) and total surface winds. Numerical simulations indicate that the model is capable of capturing the essential SST variability on seasonal‐to‐interannual time scale. Furthermore, it is shown that, as the seasonal forcing amplitude is gradually increased from zero, the coupled model undergoes several transitions between periodic (frequency‐locking) and chaotic states before it finally ‘gives up’ its intrinsic ENSO mode of oscillation entirely and acquires the frequency of the seasonal forcing. Chaotic response is found as the forcing amplitude approaches the observed value and the route to ENSO chaos is identified to be the period‐doubling cascade. The study suggests that the response of a coupled system, coupled General Circulation Models of the ocean and atmosphere for example, can be very sensitive not only to changes in the internal model parameters but also to changes in the external forcing conditions.
Inflation of Long Valley Caldera from one year of continuous GPS observations
Authors: Webb Frank H. , Bursik Marcus , Dixon Timothy , Farina Frederic , Marshall Grant , Stein Ross S.
Published: February 2004

A permanent Global Positioning System receiver at Casa Diablo Hot Springs, Long Valley Caldera, California was installed in January, 1993, and has operated almost continuously since then. The data have been transmitted daily to the Jet Propulsion Laboratory for routine analysis with data from the Fiducial Laboratories for an International Natural sciences Network (FLINN) by the JPL FLINN analysis center. Results from these analyses have been used to interpret the on going deformation at Long Valley, with data excluded from periods when the antenna was covered under 2.5 meters of snow and from some periods when Anti Spoofing was enforced on the GPS signal. The remaining time series suggests that uplift of the resurgent dome of Long Valley Caldera during 1993 has been 2.5±1.1 cm/yr and horizontal motion has been 3.0±0.7 cm/yr at S53W in a no‐net‐rotation global reference frame, or 1.5±0.7 cm/yr at S14W relative to the Sierra Nevada block. These rates are consistent with uplift predicted from frequent horizontal strain measurements. Spectral analysis of the observations suggests that tidal forcing of the magma chamber is not a source of the variability in the 3 dimensional station location. These results suggest that remotely operated, continuously recording GPS receivers could prove to be a reliable tool for volcano monitoring throughout the world.
Controlling chaos by chaos in geophysical systems
Authors: Brindley J. , Kapitaniak T. , Kocarev L.
Published: February 2004

Using the Lorenz equations as an example we show that one chaotic system can be controlled by synchronizing its behavior with the chaotic behavior of another system. We particularly discuss the implications of this phenomenon in geophysical systems.
Fine tuning the radiometrically derived geomagnetic polarity time scale (GPTS) for 0–10 Ma
Authors: Baksi Ajoy K.
Published: February 2004

New 40Ar/39Ar step‐heating ages for geomagnetic field reversals at 3.2 and 4.5 Ma, along with previous data at ∼1, 2, 10 and 16 Ma, are used to derive a geomagnetic polarity time scale for 0–10 Ma. These ages for chron boundaries are fitted to the relative widths of the polarity intervals on the ocean floor, using a cubic spline technique. The interpolated ages of chrons for 0–6 Ma are in good agreement with the astronomical time scale and differ considerably from earlier attempts, based, in part, on K‐Ar dates. Significant differences between the interpolated ages and the astronomical time scale are noted for the onset of the Olduvai and Reunion (sub)chrons; there is better agreement if 40Ar/39Ar data obtained from direct dating of these chrons is utilized. This time scale is estimated to be accurate to ±1% for 0–6 Ma; for 6–10 Ma, the lack of a tie‐point limits the accuracy to about ±2%.
Micro Raman spectroscopy of olivines in L6 chondrites: Evaluation of the degree of shock
Authors: Miyamoto M. , Ohsumi K.
Published: February 2004

We successfully detected a small shift in the wavenumber position of a Raman line near 820 cm−1 of olivines in the thin sections of L6 ordinary chondrites. This shift is probably due to residual stress. The wavenumber position of the same Raman line in a sample of Yamato (Y) 7304 (L6) olivine is 821.39 cm−1 and its full width at half maximum (FWHM) is 19.0 cm−1. For a Y7305 (L6) olivine sample, the wavenumber position is 820.48 cm−1 and its FWHM is 12.0 cm−1. The wavenumber position of the Raman line in the Y7304 olivine sample shifts significantly toward a larger wavenumber compared to that of the Y7305 olivine. The FWHM of the Raman line of the Y7304 olivine is significantly larger than that of the Y7305 olivine. These results suggest that Y7304 experienced a stronger impact shock than Y7305. The result of this Raman study is consistent with those of the profile analysis of the X‐ray diffraction spots recorded using the Laue method with synchrotron radiation and of the observation of wavy extinction under an optical microscope. This technique of Raman spectroscopy may have potential for non‐destructive and quantitative measurement of residual stress in minerals.
Anomalous cosmic ray oxygen gradients throughout the heliosphere
Authors: Cummings A. C. , Mewaldt R. A. , Blake J. B. , Cummings J. R. , Fränz M. , Hovestadt D. , Klecker B. , Mason G. M. , Mazur J. E. , Stone E. C. , von Rosenvinge T. T. , Webber W. R.
Published: February 2004

We have used data from the SAMPEX, Ulysses, Voyager 1, Voyager 2, and Pioneer 10 space‐craft to determine the radial and latitudinal gradients of anomalous cosmic ray oxygen at 10 MeV/nuc during the last half of 1993. These five spacecraft cover radial distances from 1 AU (SAMPEX) to 58 AU (P10) and latitudes to 41°S (Ulysses) and 32°N (V1). We find that the radial gradient is a decreasing function of radial distance, ∼ r−n, with n = 1.7 ± 0.7. The large‐scale radial gradient between the inner and outer heliosphere is much smaller than it was during the last solar minimum period in ∼1987. The latitudinal gradient is small and positive, 1.3±0.4%/deg, as opposed to the large and negative latitudinal gradients found during 1987, but similar to the small positive latitudinal gradient measured during 1976 for anomalous cosmic ray helium. These observations confirm that effects of curvature and gradient drift in the large scale magnetic field of the Sun are important for establishing the three‐dimensional intensity distributions of these particles in the heliosphere during periods of solar minimum conditions.
Intermediate‐depth earthquakes in central Mexico: Implications for plate waves
Authors: Singh S. K. , Santoyo M. A. , Pacheco J.
Published: February 2004

Regional seismograms of intermediate‐depth earthquakes (50≤H≤80 km) that occur below the Central Mexican Plateau show a phase which, at epicentral distances of 150 to 450 km, arrives about 15 to 20 sec after the P wave and 5 to 12 sec before the S wave. This phase was previously interpreted as a seismic wave refracted from a dipping interface below the source, thus apparently providing direct evidence of the structural location of the subducted Cocos plate below México. The phase was called the plate wave. Recent intermediate‐depth events have given rise to better quality data recorded by some newly‐installed broadband seismographs. An analysis of these and previous data strongly suggests that the phase is an S‐to‐P converted phase at the free surface and, therefore, provides no information regarding the subducted plate.
Recent trends in ozone in the upper stratosphere: Implications for chlorine chemistry
Authors: Chandra Sushil , Jackman Charles H. , Fleming Eric L.
Published: February 2004

We have studied the implications of recent trends in the annual mean and the amplitude of the annual harmonic of ozone in the upper stratosphere from the 15 years of the combined data from the Nimbus‐7 SBUV and the NOAA‐11 SBUV/2 instruments. This was done in the context of the GSFC 2D model predictions of these trends which are based on plausible scenarios of anthropogenic Cly increase in the atmosphere. The comparison of the observed and model‐estimated annual mean ozone trends show some similarity in their latitude and altitude characteristics. Both the model and data show a maximum ozone decrease of −6 to −10% per decade at high latitudes in the upper stratosphere. However, there are also significant differences between the observed and computed trends which may be related to both the model uncertainty and the uncertainty in correcting for the long term instrument drift. The observations also suggest a decrease of 10–25% per decade in the annual amplitude of ozone at 2 mb between 40°–60° in both hemispheres, with a relatively larger interannual variability in the northern hemisphere. These values are in general agreement with the model predictions and thus provide additional support in favor of the chlorine induced changes in ozone in the upper stratosphere.
Source mechanism of seismic waves excited by pyroclastic flows observed at Unzen volcano, Japan
Authors: Uhira Kohichi , Yamasato Hitoshi , Takeo Minoru
Published: February 2004

A number of pyroclastic flows have occurred at Unzen volcano (Unzendake) in Kyushu, Japan, since the end of May 1991. Seismic waves, which are excited by the pyroclastic flows, contain low‐frequency components in their initial phases. They were recorded by two long‐period seismographs in the seismic network deployed around the volcano. This paper is an attempt to clarify the source mechanism of the seismic waves triggered by the pyroclastic flows. Visual data show that the pyroclastic flow, which excites these seismic waves, occurs in three successive steps: first, fall of a lava dome front (dome collapse), then its collision with the mountain slope and its fragmentation into small blocks, and, finally, descent of the blocks along the slope. We can build a simple mechanical model which gives theoretical time histories of vertical and horizontal components of a force acting on the spot of dome collapse on the mountain slope. The model of a mechanism generating these seismic waves was constructed through analysis of the low‐frequency components with the aid of inversion methods which can estimate source time function of the force acting on the source. The estimated time histories are close to those predicted by the simple mechanical model. We conclude that the three steps mentioned above are essential in exciting the low‐frequency seismic signals. This idea is supported by the fact that the volume of fallen material estimated from the visual data is proportional to the maximum vertical amplitude of seismic waves triggered by the pyroclastic flow. This relation holds for the two pyroclastic flows of June 22,1991, and May 28,1992, which were studied in this paper. These findings seismically confirm a visual observation that the pyroclastic flow is a result of dome collapse.
Global seismic event detection using a matched filter on long‐period seismograms
Authors: Shearer Peter M.
Published: February 2004

An image derived by stacking long‐period seismograms is used as an empirical matched filter to detect and locate earthquakes. Records from 564 events (mb ≥ 6) recorded at very long periods (T ≥ 60 s) by the 15 to 20 stations of the International Deployment of Accelerometers (IDA) network are stacked using a method that emphasizes the surface wave arrivals. The first 3 hours of this time versus range image are used to construct a matched filter for continuous application to the IDA data. The output of this filter contains spatial and temporal peaks that define the location and origin time of probable seismic events. Implementation of this technique to 11 years of IDA data from 1981 to 1991 identifies 4061 events. These include 65% of cataloged events of mb ≥ 5.5. Earthquakes which appear anomalously strong relative to their surface wave magnitudes are mainly located on oceanic transform faults and probably represent unusually slow ruptures. The method successfully detects two eruptions of the El Chichón volcano in southern Mexico, from the anomalous low‐frequency energy radiated during the eruptions. In addition, 32 earthquakes are detected which are not in the standard global catalogs. These events appear to be about Ms = 5 and are mainly located in the southern oceans, where there are gaps in the coverage of the high‐frequency networks. Although these earthquakes are probably “slow” since they occur mostly on oceanic transform faults, they are not “silent” as they also can be observed in higher‐frequency data.
Regional scale tectonic evolution and the seismic velocity structure of the lithosphere and upper mantle: The Mediterranean region
Authors: de Jonge M. R. , Wortel M. J. R. , Spakman W.
Published: February 2004

We present a numerical modeling approach, designed for the verification of tectonic reconstructions and for a quantitative analysis of the seismic velocity structure of the lower lithosphere and upper mantle. In this forward modeling procedure we use published reconstructions of the tectonic evolution of a region to predict the thermal structure, from which we calculate physical quantities like P wave velocity, basin subsidence and basement heat flow. We show an application of this approach to two reconstructions of the late Mesozoic and Cenozoic evolution of the Mediterranean region. The resulting synthetic velocity models are tested against seismological models of the mantle structure, obtained independently by delay time tomography. Subsidence and heat flow predictions are compared with actual observations to further constrain the geological assumptions underlying the forward models. The mantle models predicted from reconstructions of the evolution of the Mediterranean region correlate with major features in the tomographic images. From our model results we conclude that the present distribution of seismic velocities in the Mediterranean region is largely determined by the tectonic evolution during at least the last 40 m.y. This means we can verify processes represented in different tectonic scenarios with the seismologically determined velocity structure. Seen from a different perspective, the model results contribute to understanding the causes of the velocity structure imaged by delay time tomography by showing how the tectonic evolution results in this structure.
Combined vertical‐incidence and wide‐angle seismic study of a gas hydrate zone, Blake Ridge
Authors: Katzman Rafael , Holbrook W. Steven , Paull Charles K.
Published: February 2004

Wide‐angle ocean bottom seismic data and single‐channel seismic reflection data were collected in June 1992 over an area where gas hydrates are thought to be extensive on Blake Ridge, offshore South Carolina. Wide‐angle reflections were observed on four Woods Hole Oceanographic Institution ocean bottom hydrophones at offsets up to 15 km. Results from traveltime inversion show that the bottom simulating reflector (BSR) that marks the base of the hydrate stability field lies 400–500 m below the seafloor and is overlain by a 200‐ to 300‐m‐thick layer of average velocity 1.9 km s−1. There is no evidence for significant lateral velocity variation associated with lateral changes in BSR character. Ray‐tracing calculations show that the observation of the BSR out to large offsets (7 km) constrains the maximum vertical velocity gradient to about 0.5 s−1. Amplitude‐versus‐offset (AVO) analysis was performed for ranges up to about 6 km and incidence angles of more than 70°. The high amplitude of the BSR in the normal incidence data can be explained by two models, both of which include a strong negative impedance contrast: the “hydrate wedge” model and the free gas layer model. Synthetic seismogram calculation for the wedge model, using the reflectivity method, shows a strong increase in the BSR amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge. This amplitude increase is clearly not observed in the data, which shows only a relatively modest increase in amplitude with offset. This observation agrees better with the calculated AVO for the gas layer model. The lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and/or that the base of the gas layer is a gradational boundary.
The Krafla spreading segment, Iceland: 2. The accretionary stress cycle and nonshear earthquake focal mechanisms
Authors: Arnott S. K. , Foulger G. R.
Published: February 2004

The continuous geothermal seismicity of the Krafla volcanic system, NE Iceland, was monitored in 1985, immediately following a major spreading episode (Foulger et al., 1989; Arnott, 1990; Arnott and Foulger, this issue). Focal mechanisms of 153 well‐located, shallow earthquakes were determined using P wave polarity data. Errors resulting from poorly modeled crustal structure were reduced by calculating hypocentral locations using a three‐dimensional crustal model derived from a simultaneous inversion and by deriving takeoff angles and azimuths using three‐dimensional ray tracing. The final data are thus of exceptionally high quality. Most of the events display radiation patterns consistent with a double‐couple source model. The source orientations exhibit no systematic pattern beneath the Bjarnarflag well field, a little coherence in the Krafla caldera, and a general tendency for the greatest principal stress to be oriented normal to the rift zone in a zone of recent dike injection. Normal, thrust, and strike‐slip shear events are mixed together, and five events have radiation patterns inconsistent with a double‐couple source model and are attributed to nonshear source processes. Variable non‐double‐couple events were observed, with implosive and explosive/volume‐conserving mechanisms and no clear systematic pattern of type or orientation for the system as a whole, though patterns were discernable in some subsets of events. The nonshear suite, like the shear suite, was therefore in general heterogeneous. The results indicate that a systematic deviatoric stress field was absent in the Krafla spreading segment in 1985, in contrast with the Reykjanes segment in 1977 and the Hengill segment in 1981 (Klein et al., 1977; Foulger, 1988b). The observations are consistent with a model where a strong, systematic deviatoric stress field characterizes the accretionary plate boundary during the interrifting and prerifting phases of the rifting cycle. This systematic stress is partially or wholly released during episodic rifting and spreading episodes, such as that experienced by the Krafla system 1975–1984, and is absent in the immediate postrifting phase of the rifting cycle. Nonshear geothermal earthquakes have been reported from all of the three Icelandic spreading segments studied in detail to date.
Shock wave profile study of tuff from the Nevada Test Site
Authors: Erskine Dave , Nellis W. J. , Weir S. T.
Published: February 2004

Shock wave equation of state and release behavior of tuff from the Nevada Test Site were measured in the range 8 to 107 GPa by recording shock wave profiles produced by impact of projectiles accelerated by a two‐stage light‐gas gun. Shock wave profiles were measured with a fast optical interferometer. Hugoniot, sound speed, and effective Gruneisen parameter data were determined from the profiles. The specimen geometry allows direct observation of the shape of the leading edge of the shock, in which complex compressive dynamics caused by pore crushing or phase transitions can manifest themselves. The tuff specimens were tested in both a water‐saturated and semidry state. Hydration of the rock increases both the shock and sound speeds for a given compression. The Us‐Up relation for our water‐saturated tuff is slightly nonlinear and can be approximated as Us = 2.00 + 1.42 Up for 0 < Up < 6 km/s, and Us = 2.29 + 1.29Up for 0 < Up < 3 km/s. For semidry tuff at pressures less than 18 GPa, Us = 1.70 + 1.28Up.
Duration of eruption at the Giant Crater Lava Field, Medicine Lake volcano, California, based on paleomagnetic secular variation
Authors: Champion Duane E. , Donnelly‐Nolan Julie M.
Published: February 2004

Nearly 500 cores were collected from the postglacial Giant Crater lava field on the south flank of Medicine Lake volcano. The basaltic lavas form a continuous set of lava flows which display strong chemical zonation from initially erupted calc‐alkaline basaltic andesite to final primitive basalt of tholeiitic affinity. Six chemical‐stratigraphic groups have been recognized and mapped. The eruptive sequence was sampled at numerous sites both to determine the characteristic paleomagnetic direction of each chemical group and to estimate the duration of the eruption inferred from secular variation of the geomagnetic field. Well‐grouped mean directions of magnetization were obtained for 41 sites in the Giant Crater lava field. Mean directions of magnetization determined for the lava field are nearly identical. The likelihood of any extended time interval for the eruption of the different lava types is extremely small, and the data suggest an eruptive event of less than 30 years duration, analogous to historic Hawaiian eruptions. However, the average of groups 1–4, which cannot be distinguished paleomagnetically from each other, is slightly different statistically from that of the average of groups 5 and 6, which have similar directions. A time gap of 10±5 years is inferred between eruption of group 4 and 5 lavas based on analysis of the probability of the observed angular difference of 1.27°±0.84° between their mean directions and by comparison of this angular difference to calculated field directions with similar declination and inclination determined from spherical harmonic models of the geomagnetic field for the time period 1945–1990. About 200 oriented cores were also collected from predecessor and successor basaltic lava flows on the upper flanks of the volcano. Together with remanent directions from lavas of the Snake River Plain the data define a clockwise loop of secular variation.
Paleomagnetic evidence for Cenozoic block rotations in the Tadjik depression (Central Asia)
Authors: Thomas J.‐C. , Chauvin A. , Gapais D. , Bazhenov M. L. , Perroud H. , Cobbold P. R. , Burtman V. S.
Published: February 2004

This paper presents results of a paleomagnetic study of Oligo‐Miocene red beds of the Tadjik depression in Central Asia. We sampled about 530 cores at 69 sites and six localities across the depression and along the western border of the Pamirs. Samples were thermally demagnetized and high‐temperature components appear to predate folding of upper tertiary age. Throughout the depression, paleomagnetic inclinations are consistent with those observed on the stable Turan platform, at the western margin of the depression. However, they are shallower by about 30° than the inclination predicted from the reference apparent polar wander path. This appears to indicate a 23° difference in latitude, which is incompatible with paleogeographic reconstructions for the Tertiary. A sound interpretation of this anomaly would require a better‐constrained Tertiary paleomagnetic reference for Asia. Inside the Tadjik depression, paleomagnetic declinations are all significantly rotated, counterclockwise with respect to those measured on the Turan platform. The eastern part of the depression is a domain of large rotation (52°±13° to 46°±15°), whereas smaller amounts of rotation have occurred in the western part (27°±14° to 14°±15°). The similarity between Tertiary and Cretaceous data available for the area shows that rotations have occurred since the Miocene. Little or no paleomagnetic rotations are observed in the ranges bordering the northern and western parts of the depression. Paleomagnetic and structural data suggest that block rotations in the Tadjik depression are associated with indentation of the Pamirs into stable Asia. At a larger scale, observed rotations are compatible with a model of regional sinistral wrenching, along a strip running from the Gulf of Oman to Lake Baikal.
The latest Cretaceous‐Early Paleogene rapid growth of accretionary complex and exhumation of high pressure series metamorphic rocks in northwestern Pacific margin
Authors: Kimura Gaku
Published: February 2004

Episodic growth of accretionary complexes along the northwestern Pacific margin is well documented from detailed biostratigraphy and radiometric dating of metamorphic rocks. The latest Cretaceous to early Paleogene rapid growth of the accretionary complexes observed in Hokkaido and Sakhalin Islands was caused by the supply of a large amount of terrigenous sediments from an intracontinent collision zone: the Mongolia‐Okhotsk zone in the northern Asian continent. Rapid growth of the accretionary prism resulted in episodic tectonic events in the forearc and magmatic arc. Rapid underplating caused uplift and exhumation of previously accreted complexes along with high pressure series metamorphic rocks, now exposed as the Kamuikotan complex in Hokkaido and the Susunai complex in Sakhalin. The forearc basin shallowed as a result of the uplift. The uplift collapsed the old accretionary complex and supplied olistoliths onto the forearc region. Magmatic activity along the continental margin was also active at the same time. Subduction of a large amount of terrigenous sediments into the mantle may have contributed to active magmatism at that time. A large supply of water trapped in the subducted sediments entered the mantle and increased partial melting of the mantle wedge.
Experimental determination of the thermal conductivity of molten CaMgSi2O6 and the transport of heat through magmas
Authors: Snyder Don , Gier Elizabeth , Carmichael Ian
Published: February 2004

We have designed and built a new apparatus for measuring the thermal conductivity of molten silicates, an adaptation of the transient hot‐wire technique to high temperature. The method is the most suitable for high temperatures, as it suppresses convection and minimizes radiative heat transfer. Because of the simple geometry, the radiative transport that does occur can be estimated and subtracted from the total response, yielding true phonic conductivities. Measurements of molten CaMgSi2O6 at approximately 1673, 1773, and 1873 K yield thermal conductivities of 0.31, 0.17 and 0.04 W m−1 K−1 respectively. These values are considerably lower than previous estimates for molten silicates and indicate that the current estimates for the cooling times of magma bodies are probably too short.
Southern Basin and Range Province crust‐mantle boundary: Evidence from gabbroic xenoliths, Wikieup, Arizona
Authors: McGuire Anne V.
Published: February 2004

Gabbro xenoliths in Tertiary alkali olivine basalts near Wikieup, Arizona provide samples from the crust‐mantle boundary beneath the Basin and Range to Colorado Plateau transition zone. The xenoliths consist of two pyroxenes+plagioclase+spinel (gabbronorite) and olivine+clinopyroxene+plagioclase+spinel (olivine gabbro). Pyroxenes and spinel are aluminous; plagioclase is Ca‐rich (An59–71), and olivine is Mg‐rich (Fo69–79). Major element compositions suggest that the gabbros are cumulates formed by fractional crystallization from an alkalic parent magma. Thermobarometry indicates high equilibration temperatures of 900–1050°C. Abundant composite xenoliths of gabbro intruded into spinel and plagioclase peridotite constrain the depth of origin to the uppermost mantle or lowermost crust (about 30 km depth, 0.8 GPa pressure) and suggest that these rocks represent fragments from a crust‐mantle transition zone formed by basaltic underplating during extension‐related magmatism. This transition zone consists of peridotite intruded by veins of gabbro. The amount of peridotite decreases and gabbro increases upward until, in the lower crust, there is only gabbro. Elevated lower crust and upper mantle temperatures estimated from the xenoliths are in accord with the elevated geothermal gradient determined by surface heat flow (>80 mW m−2). Estimated compressional seismic velocities (Vp), corrected for temperature and pressure, are about 6.4 to 7.2 Ion s−1 for these gabbros. Geophysical data may be compatible with the presence of a thin, <2‐km thick, crust‐mantle transition zone beneath Wikieup. The Wikieup gabbro xenoliths are samples from this thin transition between crust and mantle. There is no evidence that these gabbros are representative of the lower crust as a whole.
The equation of state, amorphization, and high‐pressure phase diagram of muscovite
Authors: Faust Jessica , Knittle Elise
Published: February 2004

The equation of state of (KAl3Si3O10(OH)2) muscovite has been measured to 20 GPa using X ray diffraction through the diamond anvil cell. The isothermal bulk modulus (K0T) of muscovite is 61.4 (±4.0) GPa with a pressure derivative of 6.9 (±1.4), in good agreement with Brillouin spectroscopic and shock wave measurements. Above 18 GPa at room temperature, muscovite loses its long‐range crystalline order and amorphizes near 27 GPa The phase diagram of muscovite between 2 and 22 GPa at approximately 800°C has been investigated using the laser‐heated diamond anvil cell. Between 1.8 and 3.7 GPa, muscovite transforms into an assemblage containing KAlSi3O8‐sanidine, Al2O3‐corundum, H2O, and cymrite‐structured KAlSi3O8·H2O, consistent with previous piston‐cylinder studies. The K‐cymrite phase appears to persist metastably to greater than 10 GPa. Between 3.7 and 4.0 GPa, muscovite breaks down to wadeite‐structured K2Si4O9, Al2SiO5‐kyanite, Al2O3‐corundum, and H2O, and between 10.9 and 12.0 GPa, muscovite transforms to an assemblage containing hollandite‐structured KAlSi3O8, Al2O3‐corundum, and H2O: this assemblage remains stable to at least 21 GPa. In accord with previous work on granitic and K‐feldspar‐phlogopite systems our results indicate that muscovite may transform into the hydrous phase K‐cymrite under deep crustal PT conditions.
Mapping oceanic ridge segments in Oman ophiolite
Authors: Nicolas A. , Boudier F.
Published: February 2004

This paper presents the results of detailed mapping of high‐temperature flow structures in the mantle and crust of two massifs of the Oman ophiolite. In these massifs, the dominant structures, including large‐scale folds, shear zones, and fractures, were generated at elevated temperatures and are ascribed to the ridge or ridge environment activity; this means that the structural maps presented can be viewed as those of partly dissected ridge segments. It has been possible in the two massifs to locate the paleoaxis of the oceanic ridge which created this crust. This location, which is constrained by several independent tests, is a prerequisite to reconstruct the structure and to investigate the dynamics of a fast spreading ridge. In the Nakhl‐Rustaq massif, high temperature tectonic activity at the ridge rotated the Moho toward a vertical attitude and folded the layered gabbros on the scale of several hundred meters. This tectonism is attributed to a propagating ridge deforming a slightly older lithosphère. The propagating ridge segment extends in the field from a diapir area to a domain located along strike some 20 km away, where the sheeted dike complex roots directly in the mantle, without layered gabbros in between. The diapir area represents the mantle feeder for the ridge segment, and the rooted dikes represent the propagating tip. Other results include the detailed mapping of two mantle diapirs and of the diverging mantle flow issued from them. Magma chambers are centered over diapirs and are tent‐shaped, in accord with our previous models.
Air contamination of basaltic magmas: Implications for high 3He/4He mantle Ar isotopic composition
Authors: Burnard P. G. , Stuart F. M. , Turner G. , Oskarsson N.
Published: February 2004

Combined mass spectrometric and manometric techniques permit simultaneous analysis of noble gases and major volatiles from the same sample extraction. He, Ar, and CO2 data of inclusion‐and vesicle‐trapped fluids from two high 3He/4He islands (Réunion and Iceland) presented here show that air contamination is probably present in all trapped fluids and that the 36Ar content of high 3He/4He mantle volatiles is low. The least contaminated fluid compositions indicates a 36Ar abundance in these fluids (CO2/36Ar = 2.7 × 108) indistinguishable from published mid‐ocean ridge basalt (MORB) values. Although the high 40Ar/36Ar ratio of the uncontaminated fluids (≈2000) may be due to incorporation of radiogenic Ar (derived from MORB volatiles or the overlying crust), it is likely that high 40Ar/36Ar ratios are characteristic of the high 3He/4He mantle source region. Degassing of primordial 36Ar from the source region must have occurred while retaining a primordial He component. Helium isotope ratios, 40Ar/36Ar, and CO2/36Ar of olivines entrained in Icelandic picrite glasses support petrographic evidence for their origin as lower crustal material entrained by the picrite magma. It is probable that olivine “phenocrysts” from other oceanic islands (e.g., Réunion) are similarly unrelated to their host lavas and that cumulate or xenolith phases may represent more suitable “bottles” of mantle volatiles.
The 1989 Macquarie Ridge earthquake: Seismic moment estimation from long‐period free oscillations
Authors: Kedar S. , Watada S. , Tanimoto T.
Published: February 2004

We have analyzed the long‐period (∼1000 s) part of the Earth's free oscillation spectrum of the 1989 Macquarie Ridge earthquake. This event stands out as the largest in the last decade and as the first great earthquake to have been recorded digitally on the new generation of very broadband seismic sensors around the globe. Its spectrum was modeled for five spectral peaks, 0S4, 0S6, 0S9, 1S8, and 5S3, taking into account the effects of lateral heterogeneity, rotation and ellipticity which cause coupling within the spectral peaks (self coupling) and between different multiplets (multiplet coupling). The effects of self coupling were calculated following the formulation of Woodhouse and Girnius (1982). Multiplet coupling only weakly affected the spectral peaks that were used. A large discrepancy in phase and amplitude between the calculated spectrum and the data was observed, which suggests that the seismic moment was ∼50% larger than that inferred from surface wave analysis (Satake and Kanamori, 1990). An error analysis was performed in order to detect inaccuracies that might have resulted in this discrepancy; centroid time, source finiteness correction, and directivity effects are likely to cause a significant bias both in the moment and phase determination. We saw no direct evidence that a precursory event could have been responsible for the additional energy release, as was suggested by Ihmle et al. (1993).
Melting experiments on anhydrous peridotite KLB‐1 from 5.0 to 22.5 GPa
Authors: Zhang Jianzhong , Herzberg Claude
Published: February 2004

Melting experiments have been performed on anhydrous mantle peridotite KLB‐1 at 5 to 22.5 GPa using the multianvil press, with special attention paid to precision and accuracy of temperature and pressure measurement, oxygen fugacity, equilibrium, temperature gradient, and the effects of temperature gradient on the phase diagrams. The new phase diagram reveals complexities in the liquidus and solidus phase relations that were not reported by Takahashi (1986). At no pressure do the liquidus and solidus converge to a common temperature or to a narrow range of temperatures, a result that refutes the conjecture that mantle peridotite formed on the solidus as a partial melt (Herzberg and O'Kara, 1985) or a residual liquid. However, mantle peridotite could have formed as a cotectic liquid because KLB‐1 exhibits nearly cotectic liquidus crystallization behavior in garnet plus magnesiowüstite [L + Gt + Mw] from 18 to 22 GPa. The implication is that mantle peridotite may be the product of a large‐scale differentiation event and that MgO/SiO2for mantle peridotite may have been phase equilibrium controlled by cotectic crystallization in a magma ocean. Fractionation of magnesiowüstite or majorite garnet is inferred at 18 to 22 GPa for a bulk Earth MgO/SiO2that is higher or lower than mantle peridotite, respectively. Alternatively, if the Earth formed from materials that were the same in MgO/SiO2as mantle peridotite, then the liquidus phase equilibria reported here may be irrelevant or incidental in understanding Earth structure.
Effect of the brittle‐ductile transition on the topography of compressive mountain belts on Earth and Venus
Authors: Williams Charles A. , Connors Chris , Dahlen F. A. , Price Evelyn J. , Suppe John
Published: February 2004

The Coulomb critical taper model has been very successful in explaining the large‐scale topography of a number of terrestrial accretionary wedges; however, this model is limited to cases of purely brittle‐frictional deformation. In this paper we extend the range of applicability of the critical taper model by explicitly including the effects of temperature‐dependent ductile deformation. The new model includes temperature‐dependent power law flow, an assumed velocity field, and linear thermal gradients in the atmosphere and within the crust. Flexural isostasy is also incorporated so that the decollement geometry is computed as a response to the applied load of the wedge material. We assume that ductile deformation within the decollement zone is controlled primarily by diffusion flow, whereas ductile deformation within the wedge itself is controlled by dislocation creep. The topographic profiles predicted by the model are very similar to those of a number of fold‐and‐thrust belts on both Earth and Venus. A typical wedge profile includes three distinctive topographic regions: a narrow taper toe, where both the wedge and the decollement zone deform in a brittle‐frictional manner; a region of relatively steep slope, where the wedge base deforms ductilely and the decollement zone is still frictional; and a flat plateau region, where both the wedge base and the decollement zone are deforming by ductile flow. We have applied the model to two fold‐and‐thrust belts on Venus (Maxwell Montes and Uorsar Rupes) and to the Andes on Earth, and we find good agreement between observed and predicted topography using reasonable parameter values. The model accounts for the observed positive correlation between relief and elevation of Venusian fold‐and‐thrust belts on the basis of different thermal environments at different elevations. It is also able to explain the first‐order differences between terrestrial and Venusian fold‐and‐thrust belts; fundamentally, this difference is due to a combination of the lower temperatures and the presence of water on Earth.
Strain accumulation along the Laguna Salada fault, Baja California, Mexico
Authors: Savage J. C. , Lisowski M. , King N. E. , Gross W. K.
Published: February 2004

Strain accumulation observed over the 1978–1991 interval in a 30×100 km aperture trilateration network spanning the Laguna Salada fault is described by the principal strain rates 0.101±0.012 μstrain/yr N80°E±2° and −0.021±0.012 μstrain/yr N10°W±2°, extension reckoned positive. These strain accumulation rates have been corrected to remove coseismic effects of the nearby 1979 Imperial Valley (M = 6.5), 1980 Victoria (Baja California) (M = 6.4), 1987 Superstition Hills (M = 6.5), and 1987 Elmore Ranch (M = 5.9) earthquakes. The observed strain rates indicate extension at a rate of about 0.08 μstrain/yr perpendicular to the trend (N35°W) of the Salton trough as well as a right‐lateral tensor shear strain rate 0.05 μstrain/yr across it. The extension perpendicular to the trough is observed neither farther north near the Salton Sea nor farther south across the Gulf of California. However, Holocene slip on the Laguna Salada fault, about equal parts right‐lateral and normal slip, is consistent with the observed strain accumulation. A simple dislocation model intended to explain the observed strain accumulation as a product of slip at depth on the Laguna Salada fault would require that the fault be listric.