bluuebunny/crossref_metadata_2025 huggingface dataset:
Décollement folding as a mechanism for thrust‐ramp spacing
Authors: Goff Douglas F. , Wiltschko David V. , Fletcher Raymond C.
Published: February 2004

The hypothesis that early formed décollement folds localize thrust ramps, explaining the regular spacing of major thrust ramps observed in many mountain belts, is examined. The model for folding instability consists of a rigid basement, a weak décollement zone, and a stiff thrust sheet. The model incorporates the effects of topographic stresses, and syntectonic erosion and redeposition and uses a linearized power law rheology for the thrust sheet. This approximation is valid for small angles of fold limb dip (<15°) and is appropriate for analysis of the wave‐length selection of folds at the onset of deformation. The spacing of major thrust ramps and the stratigraphic thicknesses of the décollement zone and thrust sheet from the Wyoming portion of the Sevier fold‐and‐thrust belt are used to test the model. The ratio of the fold dominant wavelength to thrust sheet thickness, Ld/h2, increases with the ratio, R, of the viscosity of the stiff layer to that of the décollement zone and decreases with the power law stress exponent, n2. The fold amplification factor increases with both n2 and R. As the topographic decay ratio, D, increases, Ld/h2 increases at fixed n2 and R. Acceptable models for the Absaroka and Darby thrust sheets have n2 ≥ 3. Only when R < 1000 and significant erosion and redeposition of sediments accompany the folding of the thrust sheet (D ≥ 103) does the required R fall below a reasonable limit. Thus a model in which décollement folding is accompanied by syntectonic erosion and redeposition precedes thrusting is consistent with the available observational constraints.
The 1992 Cape Mendocino earthquake: Broadband determination of source parameters
Authors: Hagerty Michael T. , Schwartz Susan Y.
Published: February 2004

The April 25, 1992, Cape Mendocino earthquake (MS 7.1) has renewed speculation about the nature of subduction along the Cascadia subduction zone and the associated seismic hazard. This event may represent the first large (M > 6) thrust event along the entire Cascadia subduction zone in historic times (last 200 years). We analyze long‐period surface waves and broadband body waves in order to estimate the mainshock source parameters. We also examine broadband body waves from the nearby 1991 Honeydew earthquake (M 6) in order to assess the contributions of both rupture complexity and unmodeled source and receiver structure in the Cape Mendocino waveforms. From both body and surface wave inversions, as well as forward modeling of body waves, we estimate a best double couple mechanism for the Cape Mendocino earthquake (strike = 330 ± 10°, dip = 12 ± 2°, rake = 75 ± 15°, seismic moment = 1.93 × 1019 N m, and Mw = 6.8). This mechanism contains a significant component of slip in the estimated direction of Gorda‐North America plate convergence. Although this earthquake had suitable geometry for relieving strain accumulated by Gorda‐North America plate convergence, we cannot resolve whether it occurred on the interplate megathrust or on a fault within the overriding accretionary prism. We find evidence for southwest (offshore) rupture toward an azimuth of 240°. In addition, we find evidence for early aftershocks in both the teleseismically recorded body waves and in locally recorded strong motions. We model one aftershock delayed ∼13 s from the mainshock with a mechanism that is different from the mainshock mechanism but is consistent with the north‐south trending, horizontal compression found offshore within the Gorda plate. We postulate that this aftershock and two additional large, strike‐slip aftershocks that ruptured the Gorda plate within 24 hours of the mainshock were caused by the transfer of stress accumulated across the Cascadia subduction zone and accretionary prism far offshore, to the Gorda plate, where it reduced the normal stress across NW‐SE oriented faults, triggering failure. The complexity of fault interactions near the Mendocino triple junction needs to be understood before potential seismic hazards of the southern Cascadia subduction zone can be quantified.
Modeling two‐dimensional structure at the core‐mantle boundary
Authors: Helmberger D. V. , Garnero E. J. , Ding X.
Published: February 2004

Recent studies of SKS waveform modeling emphasize the strong variation of seismic properties at the core‐mantle boundary (CMB) and the need for two‐dimensional and three‐dimensional waveform modeling capabilities. In particular, the bifurcation of SKS into SP dKS and SKP dS near 110° shows strong regional variations. The first of these phases has a P wave diffraction along the bottom of the mantle near the source, while the latter phase occurs at the receiver end. Generalized ray theory proves effective in generating theoretical seismograms in this type of problem because each of these diffractions is associated with a particular transmission coefficient: Tsp which transmits shear waves into primary waves when crossing the CMB and Tsp which transmits the primary waves back into shear waves at the receiver end. Each region can then be isolated and have its separate fine structure, sharp or gradational. Two classes of boundaries are explored: the CMB as a simple, sharp interface and the CMB with a very low velocity transition layer (10% slower than reference models). The two diffractions produced by these structures have diagnostic arrival times and wave shapes and when combined with the geometric SKS produce distinct waveform characteristics not easily generated by other means. Since the ray paths associated with these three phases are virtually identical in the mantle and only differ along a short sample of CMB and in the one‐dimensional fluid core, we can isolate the small localized CMB region sampled. Thus the waveform character of the extended SKS in the range of 105° to 120° becomes an excellent CMB probe which we demonstrate on a small sample of observations from the Fiji‐Tonga region as recorded in North America.
Changes in plate motions and the shape of Pacific fracture zones
Authors: McCarthy Mary C. , Kruse Sarah E. , Brudzinski Michael R. , Ranieri M. Elaine
Published: February 2004

Geosat passes, the new 2‐min global gravity grid [Smith and Sandwell, 1995], and shipboard bathymetry across central Pacific fracture zones were used to identify features common to fracture zone segments that formed during times of changes in plate motions. These features are not predicted by current “locked fault” fracture zone models. During a change in spreading direction that induces tension across the transform fault, large‐offset (greater than ∼500 km) transforms develop multiple parallel faults, spaced 50 to 100 km apart. The gravity signature of small‐offset transform faults under tension includes a broader and more symmetric trough than observed on segments that formed during periods of steady spreading. Parts of fracture zones that form subsequent to a spreading reorientation that causes compression across the transform fault generally exhibit a single fault scarp that fits the locked fault model. Seafloor formed during a period of change usually marks a transition between structural styles, for example, between multiple fracture zone strands and a narrower single‐fault fracture zone. Widening of the transform fault zone under tension and narrowing under compression are consistent with the assumption that during a change in spreading direction the new spreading ridges propagate to, but not across, the old transform fault.
Resonance and elastic nonlinear phenomena in rock
Authors: Johnson Paul A. , Zinszner Bernard , Rasolofosaon Patrick N. J.
Published: February 2004

In a great variety of laboratory experiments over large intervals in stress, strain, and frequency, rocks display pronounced nonlinear elastic behavior. Here we describe nonlinear response in rock from resonance experiments. Two important features of nonlinear resonant behavior are a shift in resonant frequency away from the linear resonant frequency as the amplitude of the disturbance is increased and the harmonics in the time signal that accompany this shift. We have conducted Young's mode resonance experiments using bars of a variety of rock types (limestone, sandstone, marble, chalk) and of varying diameters and lengths. Typically, samples with resonant frequencies of approximately 0.5–1.5 kHz display resonant frequency shifts of 10% or more, over strain intervals of 10−7 to 10−6 and under a variety of saturation conditions and ambient pressure conditions. Correspondingly rich harmonic spectra measured from the time signal progressively develop with increasing drive level. In our experiments to date, the resonant peak is observed to always shift downward (if indeed the peak shifts), indicating a net softening of the modulus with drive level. This observation is in agreement with our pulse mode and static test observations, and those of other researchers. Resonant peak shift is not always observed, even at large drive levels; however, harmonics are always observed even in the absence of peak shift when detected strain levels exceed 10−7 or so. This is an unexpected result. Important implications for the classical perturbation model approach to resonance results from this work. Observations imply that stress‐strain hysteresis and discrete memory may play an important role in dynamic measurements and should be included in modeling. This work also illustrates that measurement of linear modulus and Q must be undertaken with great caution when using resonance.
Geoelectric signals in China and the earthquake generation process
Authors: Chu Jean J. , Gui Xietai , Dai Jingan , Marone Chris , Spiegelman Marc W. , Seeber Leonardo , Armbruster John G.
Published: February 2004

We present original apparent resistivity records for stations located in the Chinese provinces of Hebei and Sichuan, along with seismicity patterns, fault maps, and limited hydrologic data. These records are part of a 30‐year database of geoelectric signals that have been recorded in China by a nationwide network of more than 100 stations set up by the State Seismological Bureau of the People's Republic of China. The database includes the time period during which the M 7.8 1976 Tangshan and M 7.2 1976 Songpan earthquakes occurred. Resistivity data recorded in and around the epicentral region of the Tangshan event show coherent changes, both temporal and in areal distribution, years and months before and after the earthquakes. A regional change in water table occurred more than 3 years prior to the Tangshan earthquake. The resistivity and water table signals appear to correlate in the 2‐ to 3‐year period prior to the earthquake. Rainfall records from the region around Tangshan, extending to the cities of Beijing and Tianjin, do not appear to account for the resistivity changes. Regional seismicity and active fault patterns in north China indicate that deformation occurs mainly along the borders of large tectonic blocks. The source region for the geoelectric signals appears to lie within a large shear zone, at the edge of a tectonic block undergoing rotation. The Tangshan epicentral area is thus only part of a larger zone of deformation. We suggest that dilational strain associated with the opening of fractures may lower resistivity and water table. For shallow crustal depths, such as those monitored by the Chinese geoelectric network, dilation and resistivity changes are expected to begin at stress levels well below the peak values of the seismic cycle. Our study suggests that in some cases the areal extent of geoelectric signals may be significantly larger than that of the eventual earthquake source dimension. The Chinese geoelectric database offers an opportunity to improve our understanding of possible geoelectric precursors to earthquakes.
Seismic refraction investigations on the crustal structure of the western Jameson Land Basin, East Greenland
Authors: Fechner Notker , Jokat Wilfried
Published: February 2004

In 1990, a combined land‐sea experiment in the Hall Bredning, Scoresby Sund (East Greenland) revealed new information about the crustal structure of the Jameson Land Basin. The sediments and the upper and lower crust have been imaged by a 710‐km‐long network of seismic refraction lines. Our data indicate that the crust‐mantle boundary is at a depth of 24 km close to the western coast of Jameson Land and deepens to 31 km toward the western termination of Hall Bredning. The sediments thin from 8‐km thickness in the east to almost 0 km close to Milne Land. Our cross sections indicate that Jameson Land is an asymmetrical, half‐graben shaped sedimentary basin with a N‐S striking axis.
Evolution of a mafic volcanic field in the central Great Basin, south central Nevada
Authors: Yogodzinski G. M. , Naumann T. R. , Smith E. I. , Bradshaw T. K. , Walker J. D.
Published: February 2004

Evolution of a mafic volcanic field is investigated through a study of Pliocene age rocks in the Reveille Range in south central Nevada. Pliocene activity began with the eruption of relatively abundant hawaiite (episode 1, 5–6 Ma), which was followed by trachytic volcanism (4.3 Ma) and by a second episode of lower‐volume hawaiite and basanite (episode 2, 3.0–4.7 Ma). Incompatible elements indicate an asthenospheric source. Isotopically, episode 2 basalts cluster around 87Sr/86Sr = 0.7035 and εNd = +4.2, but episode 1 samples vary to high 87Si/86Sr (up to 0.7060) over a narrow range of εNd (+0.8 to +4.5). Trachytic rocks (MgO ∼ 0.5%) are isotopically akin to the episode 1 basalts. Geochemical variation requires the addition of a crustal component (high 87Sr/86Sr, Sr/Nd, Pb/La, low εNd) to the episode 1 hawaiites and trachytic samples, probably by assimilation of carbonate‐rich sedimentary wall rock. The volcanic field developed in at least two eruptive cycles of approximately equal duration. Basanites (deeper and lower percentage melts) appear only in the younger episode. Eruptive episodes were apparently linked to separate melting events in the mantle. Through time, basalts were produced in diminishing volumes by lower percentage melting, magma generation and storage was at greater depths, and magma ascent was at higher velocities. Spatially, the melting anomalies were large in the Pliocene but progressively diminished in size so that by Pleistocene time, volcanism was restricted to a small area near the northern end of the initial outbreak.
Geological indexes of hydrothermal venting
Authors: Baker Edward T.
Published: February 2004

Hydrothermal venting occurs on mid‐ocean ridge axes with a diverse array of morphological, structural, and petrological characteristics. Consequently, it is not clear if certain geological environments are more conducive than others to hydrothermal activity. This paper uses complementary data sets recently collected on multiple‐tectonic‐segment scales along intermediate to superfast spreading ridge sections to systematically examine this issue. The spatial density of venting is gauged by plume incidence, the fraction of linear ridge crest overlain by hydrothermal plumes. Five geological indexes are compared with plume incidence: ridge‐axis elevation, ridge‐axis cross‐sectional area, the percentage of ridge axis underlain by an axial magma chamber (AMC) reflector, MgO wt% in basaltic glass, and spreading rate. A mean value for plume incidence and each index, where available, is calculated for each of 14 second‐ to fourth‐order tectonic segments from the Juan de Fuca Ridge (44°30′ to 48°45′N) and the northern (9° to 11°50′N) and southern (13°50′ to 18°40′S) East Pacific Rise. Plumes are most common on segments with a cross‐sectional area >3.5 km2, a net elevation >0.35 km, AMC coverage >60%, and MgO >7 wt%. Spreading rate is not deterministic on the segment scale. The data suggest two classes of ridge segments. Segments with relatively low geological index values have a uniformly low plume incidence indicative of a presently feeble magmatic budget. Those with high index values have a plume incidence ranging from low to complete coverage, apparently a consequence of a magma supply rate sufficient to produce frequent dike intrusions and highly variable hydrothermal activity. The best individual predictors of hydrothermal activity are cross‐sectional area and net elevation. Applying these indexes to the East Pacific Rise between 18°N and 33°S suggests that the unexplored segments with the highest probability of activity are those at 15°30′–16°N, 6°–9°N, 5°–6°S, 27°–28°S, and 30°30′–33°S.
Using two‐point correlation functions to characterize microgeometry and estimate permeabilities of sandstones and porous glass
Authors: Blair Stephen C. , Berge Patricia A. , Berryman James G.
Published: February 2004

We have developed an image processing method for characterizing the microstructure of rock and other porous materials and to provide a quantitative means for understanding the dependence of physical properties on the pore structure. Our method is based upon the statistical properties of the microgeometry as observed in scanning electron micrograph (SEM) images of cross sections of porous materials. This method uses a simple statistical measure of microstructure called the spatial correlation function. We formulate a two‐point spatial correlation function and show how it can be used to estimate porosity, specific surface area, and other microstructural features such as pore and grain sizes. The porosity and specific surface area are of special interest as they can be used in a Kozeny‐Carman relation to predict permeability of porous materials. We explore the Kozeny‐Carman relation and show how it incorporates a characteristic microstructural length parameter similar to that used in other analyses of permeability. We analyze SEM images of several different porous glasses and natural sandstones using the two‐point correlation function, discuss the importance of image resolution, and show for the sandstones studied here how the appropriate choices of image resolution can be made so the measured parameters are consistent with those used in a simple flow model for computation of permeability. Estimates of permeabilities for several different porous glasses and natural sandstones are presented. Comparison of these estimates to laboratory measurements shows good qualitative agreement and quantitative agreement within about a factor of 2 for most samples and 3 for all samples.
Mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge: East Pacific Rise, 17° to 19°S (Naudur cruise, 1993) phase separation processes controlled by volcanic and tectonic activity
Authors: Charlou Jean Luc , Fouquet Yves , Donval Jean Pierre , Auzende Jean Marie , Jean‐Baptiste Philippe , Stievenard Michel , Michel Stéphane
Published: February 2004

During the Naudur cruise in December 1993, 23 dives using the French submersible Nautile were conducted on the axis of the East Pacific Rise between 17° and 19°S where the spreading rate is among the fastest in the ocean (14 to 16 cm/yr). Twenty hydrothermal fluids located at the topographic high of each segment in the axial domain were collected between 2573 and 2669 m depth on three segments centered, respectively, at 17°25′S, 18°15′S, and 18°26′S. The fluids exhibit a very wide range of temperature, chemical, and gas compositions. On the 17°25′S and 18°26′S segments, fluids have quite uniform compositions, low chlorinities, are gas‐enriched and are low in dissolved metals relative to fluids from the 18°15′S segment which show high chlorinities, are less gas‐enriched and show high‐metal concentrations. Chloride and metal depletion associated with gas enrichment is consistent with phase separation. Whereas CH4 endmembers show large variations between sites, the 13CH4 data are very similar, with 13C values in a narrow range −22.0 to −23.9‰ versus pee‐dee belemnite (PDB). δ13CO2 measured in fluids within the 18°15′S and 18°26′S segments are, respectively, −7.9 and −5.8‰ versus PDB, similar to 13C of CO2 trapped in mid oceanic ridge basalts, suggesting a magmatic origin. The variability in fluid composition is linked to the variability of the accretion process observed on the three segments. The uniform venting of low‐chlorinity fluids in the 17°25′S and 18°26′S segments is connected with volcanic activity which causes boiling with preferential venting of vapor‐enriched fluids. High‐salinity fluids are emitted on the 18°15′S segment where the ridge is tectonics‐dominated and subseafloor circulation controlled by faults. Phase‐separated effluents induced by volcanic and tectonic activity are delivered to the deep ocean in this area, as previously observed on the Juan de Fuca Ridge or in the North Fiji Basin Ridge.
Segmentation and morphotectonic variations of the Central Indian Ridge (21°10′S–22°25′S)
Authors: Sauter Daniel , Nafziger Jean‐Michel , Whitechurch Hubert , Munschy Marc
Published: February 2004

Bathymetric, magnetic, and petrographic data have been used to analyze the segmentation and the morphotectonic variations on the Central Indian Ridge (CIR) between 21°10′S and 22°25′S. Volcanoes, abyssal hills, and steep slopes, corresponding to fault scarps and to volcanic constructional slopes, have been obtained by image processing methods applied to Sea Beam data. In the survey area, segmentation of the CIR is similar to that of a slow spreading ridge. The plate boundary morphology is defined by a linear zone composed of an ensemble of median ridges and elongate depressions. Hourglass‐shaped valleys are bounded by second and third‐order discontinuities located at local along‐axis depth maxima (400–700 m depth variations) and marked by oblique extensional basins and en échelon jogs, respectively. Large‐throw faults are prevalent toward segment ends and suggest greater amagmatic extension in these places. Along the axial valley, the petrographic characteristics of the dredged basalts are correlated with the morphotectonic segmentation and suggest different magma reservoirs. The morphology and structure of the rift valley reflect the three‐dimensional structure of the axial lithosphere and the geometry of mantle upwelling beneath the spreading center. Abyssal hills are bounded by both inward and outward facing faults in ∼1.5 Ma crust, indicating that the formation of abyssal hills is not solely resulting from a combination of volcanic and tectonic processes within the central part of the ridge but that tectonic processes may also occur as far as 20–40 km from the axis by initiating or reactivating some faults at the outward facing slope of the abyssal hills. Reconstructions of the plate boundary geometry through time, using abyssal hills as isochrons, provide evidence to show that the nontransform discontinuity, located at 21°47′S, with an offset as small as 2–3 km, is associated with V‐shaped off‐axis traces in the form of alignments of disrupted abyssal hill lineations. These reconstructions suggest that a small ridge jump is responsible for the initiation of this discontinuity and that this latter migrates away from areas of higher melt delivery. Three‐dimensional inversion of magnetic data shows that this discontinuity has been initiated between 0.780 and 0.984 Ma and suggests postaccretionary crustal block rotations within the discordant zones.
Generation of low‐frequency electric fields by explosion crater formation
Authors: Adushkin V. V. , Soloviev S. P.
Published: February 2004

A low‐frequency electric field is observed during crater formation by detonation of high explosive charges. The disturbance of the atmosphere's electric potential gradient at ground level was measured in the range of frequencies between 0 and 200 Hz. Our results indicate that explosions are capable of creating large electrical charge separations in the atmosphere. A simple model of field generation is proposed. Furthermore, a correlation between the magnitude of the explosion and the electric charge of the dust cloud is established. Using these results, estimates of the intensity of the electric field generated during meteorite impact crater formation is proposed.
Viscosity structure beneath northeast Iceland
Authors: Pollitz Fred F. , Sacks I. Selwyn
Published: February 2004

The dynamics of crustal rifting in Iceland depend strongly on the lower crustal rheology, which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume. While magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt, observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle. Since viscosity is also sensitive to the degree of partial melt present, viscosity estimates for these regions could shed light on the factors responsible for these observations. In this study we utilize horizontal and vertical displacement vectors determined in GPS campaigns in northeast Iceland since 1986. These are modeled in terms of steady state tectonic loading plus postseismic/postdiking relaxation following the 1975–1984 Krafla rifting episode, as first proposed by Foulger and others. With the elastic part of the model fixed by external constraints, these data have a high sensitivity to the viscosity structure beneath Iceland. Lower crust and upper mantle viscosities of about 3 × 1019 Pa s and 3 × 1018 Pa s, respectively, yield the closest agreement with the data. Our lower crustal viscosity estimate is consistent with the low attenuation and low (subsolidus) temperature for the lower crust inferred in recent studies. Inversions for fissure opening during the Krafla rifting episode yield about 7 m of opening centered on the Krafla rift, as is observed. Allowing for contemporaneous deep rifting on vertical faults along the Askja segment partially accounts for the observed increase in separation across the rift during 1987–1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the Askja rift during 1987–1990. Recent deep normal faulting beneath the Askja rift and further south might explain all of these remaining features.
Stress analysis in the intraplate area of Gazli, Uzbekistan, from different sets of earthquake focal mechanisms
Authors: Bossu Rémy , Grasso Jean‐Robert
Published: February 2004

Occurrence of intraplate earthquakes have been related either to reactivation of preexisting zones of weakness in a uniformly oriented regional stress field or to local stress changes. More recently, buildup of pore pressure anomalies has been proposed to explain both the low recurrence rate and some misorientation of rupture planes in the prevailing regional stress field. In this study, we analyze local stress field orientation in the area of the three Ms = 7.0 Gazli (western Uzbekistan) earthquakes by using different sets of earthquakes focal mechanisms. We show that stress tensor inversions from both seven 4.2 ≤ Ms ≤ 7.0, forty‐five 0.1 ≤ ML ≤ 3.2 and twenty‐five 1.0 ≤ ML ≤ 3.2 earthquake focal mechanisms give constant maximum horizontal stress direction in agreement with the one deduced from the India‐Asia collision. The best model for the local stress tensor exhibits a significant (25°) discrepancy from vertical for one of the principal stresses. We note, moreover, that the common assumption that one principal stress axis is oriented vertically is not strictly supported by stress states derived from earthquake focal mechanisms available in the world stress database. By evaluating each hypothesis of stress inversion methods from earthquake focal mechanisms, it is shown that, especially in intraplate settings, the first motion focal solution should be the most appropriate one for a formal inversion and that the magnitude of the considered event may not be the only criterion to assess the quality of a stress inversion. This is particularly timely because more and more focal mechanisms are moment tensor solutions instead of first motion solutions. Concerning the Gazli sequence, the slip conditions (pore pressure and frictional coefficient) are evaluated for each of the three mainshocks in terms of an independently determined local stress field. Because variations in orientations between local and regional stress field are not detected, and because the three mainshock fault planes seem to be relatively well oriented for a frictional activation in the local stress field, it is concluded that this seismic sequence is localized on preexisting weakness zones and is not due to local stress orientation anomalies.
Laboratory and theoretical models of fluid recharge in superheated geothermal systems
Authors: Woods Andrew W. , Fitzgerald Shaun D.
Published: February 2004

The phase changes that occur as water invades a superheated geothermal reservoir were studied experimentally. Cold liquid ether was injected into a hot, isothermal bed of sand, and the rate of vaporization and thermal evolution of the sand bed were measured. When the ether was injected from below, the ascending vaporization front remained nearly planar. In accord with theoretical models, an isothermal layer of saturated liquid developed behind the front, and the mass fraction vaporizing was proportional to the reservoir superheat. In a second experiment, liquid was injected from below into a thermally stratified reservoir in which an initially superheated layer was overlain by a supercooled layer. As vapor produced at the ascending front rose into the supercooled region, it condensed and produced a two‐phase saturated layer above the superheated zone, even though injection was from below. In a third experiment, liquid was injected from above into an isothermal superheated bed of sand. As the vaporizing liquid front migrated downward, hot vapor rose through the descending liquid. This vapor maintained the upper part of the reservoir close to the saturation temperature even though cold liquid was continuously supplied from above. In addition, fingers of liquid descended ahead of the main front into the superheated zone. Liquid transported in these fingers accumulated at the base of the reservoir in a layer of saturated liquid. Much of the subsequent vaporization occurred at the ascending front as the thickness of this liquid layer increased. Only the intermediate part of the reservoir remained superheated. Our experiments demonstrate that the geometry of recharge and the initial thermal structure of the reservoir have an important control on the phase changes in active geothermal systems. In many cases, recharge of liquid in a superheated region of a reservoir can lead to vapor production and transport and this may cause initially supercooled regions to become saturated. Also, if the liquid front becomes unstable, a two‐phase zone may develop from an initially superheated region.
Magnetotelluric evidence of lithospheric mantle thinning beneath the southern Sierra Nevada
Authors: Park Stephen K. , Hirasuna Brian , Jiracek George R. , Kinn Christopher
Published: February 2004

A wideband (0.01–1000 s) magnetotelluric survey across the southern Sierra Nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10–20 km. The eastern zone underlies the highest topography of the range and extends eastward. This eastern conductive zone extends to depths in excess of 100 km, based on model sensitivities, and lies well below the Moho depth of 32–38 km from seismological studies. Therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt. Such an interpretation is consistent with gravitational, seismological, and geologic evidence. Estimates of the partial melt fraction range from 2 to 5% at depths of 40–70 km, which are consistent with fractions of melt inclusions observed in xenoliths from the eastern Sierra Nevada. This partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations. The other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes. The western conductive zone also straddles the Moho and extends into the mantle beneath western foothills of the Sierra Nevada and the Great Valley sediments to the west. However, xenoliths from this region indicate high‐velocity crustal rocks to depths in excess of 60 km, and we therefore attribute the enhanced conductivity to graphitic metasediments and/or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the Sierran plutons.
A three‐dimensional magnetotelluric investigation of the California Basin and Range
Authors: Mackie Randall L. , Madden Theodore R. , Park Stephen K.
Published: February 2004

Band‐limited magnetotelluric measurements were collected at several sites across a wide area in the California Basin and Range and northern Mojave Desert from early 1988 until March 1989. Due to the large spacing between sites and the limited frequency range of the data, there are limits to what the data can resolve. However, a three‐dimensional (3‐D) magnetotelluric modeling study and sensitivity analysis of a subset of the data support the following conclusions. (1) Electrical currents from the Pacific Ocean are channelled across the western Mojave Desert and into the southern Basin and Range. (2) The 3‐D geometry of the basins significantly influences the fit to the data mode aligned along the strike of the basins (transverse electric mode). (3) The lower crust in the California Basin and Range is more conductive than the lower crust under the Mojave Desert and Pacific Ocean. The resistivity values we modeled for the lower crust in the California Basin and Range, though inconsistent with mineral semiconduction, can be explained by electrical conduction through small amounts of saline fluids.
The 1954 Rainbow Mountain‐Fairview Peak‐Dixie Valley earthquakes: A triggered normal faulting sequence
Authors: Hodgkinson Kathleen M. , Stein Ross S. , King Geoffrey C. P.
Published: February 2004

In 1954, four earthquakes of M > 6.0 occurred within a 30 km radius in a period of six months. The Rainbow Mountain‐Fairview Peak‐Dixie Valley earthquakes are among the largest to have been recorded geodetically in the Basin and Range province. The Fairview Peak earthquake (M = 7.2, December 12, 1954) followed two events in the Rainbow Mountains (M = 6.2, July 6, and M = 6.5, August 24, 1954) by 6 months. Four minutes later the Dixie Valley fault ruptured (M = 6.7, December 12, 1954). The changes in static stresses caused by the events are calculated using the Coulomb‐Navier failure criterion and assuming uniform slip on rectangular dislocations embedded in an elastic half‐space. Coulomb stress changes are resolved on optimally oriented faults and on each of the faults that ruptured in the chain of events. These calculations show that each earthquake in the Rainbow Mountain‐Fairview Peak‐Dixie Valley sequence was preceded by a static stress change that encouraged failure. The magnitude of the stress increases transferred from one earthquake to another ranged from 0.01 MPa (0.1 bar) to over 0.1 MPa (1 bar). Stresses were reduced by up to 0.1 MPa over most of the Rainbow Mountain‐Fairview Peak area as a result of the earthquake sequence.
The lower crust of the Bay of Islands ophiolite, Canada: Petrology, mineralogy, and the importance of syntexis in magmatic differentiation in ophiolites and at ocean ridges
Authors: Bédard Jean H. , Hébert Réjean
Published: February 2004

The lower crust of the North Arm Mountain massif (Bay of Islands ophiolite) is composed of interfolded, porphyroclastic‐textured peridotite, pyroxenite, and gabbro, belonging to several differentiation series. Low‐Ti pyroxenes, high An/Fo, abundant orthopyroxene, and high spinel Cr/Al suggest arc affinities. Discordant contacts and inclusion relationships show that many peridotites are synkinematic sills. Some host rocks were altered to sulphide‐bearing, greenschist‐grade assemblages prior to intrusion. Most feldspathic peridotites and melagabbros are hybrids formed by disaggregation of host gabbros into primitive intrusions. Many pyroxenites and chromitites are by‐products of incongruent dissolution reactions. Much of the pyroxene component of peridotites and olivine pyroxenites is xenolithic. Synmagmatic shear augmented the efficiency and extent of hydridization and assimilation reactions. Syntexis (the melting and dissolution of xenocrysts) was as important as fractional crystallization in generating petrological diversity and controlling melt evolution. Synmagmatic extension at spreading ridges could produce similar results, and so account for many enigmatic features of mid‐ocean ridge basalts (xenocrysts of anorthite and aluminous spinel, excess chlorine in basalts, and the pyroxene paradox). If syntexis is an important ridge‐axis process, then assuming that fractional crystallization alone controls melt evolution may generate incorrect calculated parental melt compositions, and so lead to erroneous conclusions about mantle sources and processes.
Tomographic inversion for P wave velocity structure beneath the northeastern Japan arc using local and teleseismic data
Authors: Sato Tamao , Kosuga Masahiro , Tanaka Kazuo
Published: February 2004

We have determined three‐dimensional P wave velocity structure up to a depth of 296 km beneath northern Tohoku, Honshu, Japan, by inverting arrival times of local and teleseismic events simultaneously. Velocities in the upper crustal layers correlate well with the surface geology and Bouguer gravity anomalies. Low‐velocity zones are ubiquitous in the crust and the uppermost mantle beneath the Quarternary volcanic areas. A well‐known feature of a pronounced low‐velocity mantle wedge underlain by the dipping high‐velocity (high‐V) Pacific plate is generally recovered over the study area. Strong heterogeneity within the high‐velocity Pacific plate is suggested by the presence of a pronounced low‐velocity anomaly confined to the depths of 130–200 km. The decreased seismicity of intermediate‐focus events may be correlated with the low‐velocity zone. A two‐dimensional inversion scheme was invoked to determine stably the average mantle structure perpendicular to the island arc. In the depths of 50–150 km, the inclined high‐V zone has a thickness of 80–100 km with its top coinciding with the upper plane of the double‐planed deep seismic zone. Below the depth of 200 km, the high‐V zone is shifted toward the bottom of the 80‐km‐thick reference slab. In the intermediate‐depth layer (150–200 km), the high‐V zone is not clearly visible within the 80‐km‐thick reference slab.
Mixing in vigorous, time‐dependent three‐dimensional convection and application to Earth's mantle
Authors: Schmalzl J. , Houseman G. A. , Hansen U.
Published: February 2004

An understanding of the mechanism of mixing in highly viscous convecting fluids is of crucial importance in explaining the observed geochemically heterogeneous nature of Earth's mantle. Using constant viscosity numerical experiments, we describe the mixing mechanism of time‐dependent Rayleigh‐Bénard convection with an infinite Prandtl number in a three‐dimensional (3‐D) rectangular container. Mixing is observed by following the positions of passive tracers advected by the flow. The major mixing mechanisms may be described in terms of the within‐cell mixing and the cross‐cell mixing. The flow structure in which tracers move on toroidal surfaces, that was previously observed in steady state 3‐D convection systems is perturbed by boundary layer instabilities in the time‐dependent experiments. This flow structure allows a very efficient exchange of mass between the boundary layers and the core of the convection cell even in the absence of time dependence. We compare this result with calculations carried out in two spatial dimensions. In similar two‐dimensional (2‐D) experiments, exchange of mass between boundary layers and core of the convection cell is entirely effected by the boundary layer instabilities. Mixing between neighboring cells appears much slower in three dimensions than in similar 2‐D experiments, perhaps because the 3‐D cell structure is more stable relative to the boundary layer instabilities. The inferred mixing rates are observed to be relatively insensitive to initial tracer location, but the timescale for mixing, tm, decreases with increasing Rayleigh number (tm goes approximately as Ra(−3/2)). The timescale of mixing is an important constraint on the large scale structure of Earth, because large‐scale geochemical heterogeneities persist to the present day, implying that the mantle is not well mixed.
Carbonaceous material in metamorphosed carbonate rocks from the Waits River Formation, NE Vermont, and its effect on electrical conductivity
Authors: Léger Albert , Mathez Edmond A. , Duba Al , Pineau Françoise , Ginsberg Susan
Published: February 2004

The relations among electrical conductivity and graphite content, metamorphic grade, and fluid:rock interaction are investigated for a suite of regionally metamorphosed graphitic carbonate rocks from the Waits River Formation, NE Vermont. Graphitization was complete by the lowest grade of metamorphism attained in the area (∼450°C and 450 MPa). In low‐grade rocks, graphite occurs as inclusions within calcite and along grain boundaries, and in medium‐grade rocks, it concentrates near porphyroblasts. In neither suite does graphite form interconnected networks. Average reduced carbon abundances are 4400 and 2800 ppm for the low‐ and medium‐grade rocks, respectively. High‐grade rocks are almost void of graphite, except for one specimen. The δ13C values were determined for graphite and carbonates and δ18O values were determined for the carbonates and silicate residues. There is no change in isotopic values between low‐ and medium‐grade rocks. In contrast, high‐grade rocks show marked decreases in δ13C and δ18O values of ∼4‰ compared to low‐ and medium‐grade rocks. Both isotopic analyses and graphite depletion in high‐grade rocks were caused by influx of large quantities of magmatic water during peak metamorphism. Electrical conductivity was measured in the laboratory on representative specimens. Rocks with high carbon content (>7000 ppm) from both the low‐ and high‐grade zones display almost an order of magnitude higher electrical conductivity than expected from their fluid content alone. Graphite does not form an interconnected network in these rocks, yet it combines with the saline fluids to significantly increase electrical conductivities.
Evolution and interaction of migrating cross‐arc volcanism and backarc rifting: An example from the Southern Havre Trough (35°20′–37°S)
Authors: Wright I. C. , Parson L. M. , Gamble J. A.
Published: February 2004

The active southern Havre Trough (35°20′ – 37°S) backarc basin is interpreted to form from the evolution and interaction of migrating cross‐arc magmatism, and the progressive development of longitudinal rift grabens. The proposed migration of the proto‐Kermadec arc front from the Colville arc at ∼5 Ma to the active Kermadec arc margin is recorded by the construction of arc edifices trailed across the intervening, and contemporaneously rifting, backarc complex. Migration trails are identified for at least four arc volcano sources. The Rumble V arc migration trail is the most prominent, forming a continuous, high‐standing, magmatic arc ridge. These postulated arc trails segment the backarc region and initially limit rift development to the intervening proto‐arc crust blocks. Early rifting between the arc volcanoes forms fully developed rift grabens which, with progressive basin widening, propagate longitudinally across the trails of migrating constructional arc magmatism. A model of the balance between rates of constructional arc magma production MR and destructive back‐arc (rifting) extension VE is proposed. When VE is high, MR is insufficient to keep abreast of destructional rifting, resulting in small, isolated arc massifs quickly dismembered by rifting. Conversely, when VE is low, MR is sufficiently greater than rifting to produce a continuous, high‐standing cross‐arc ridge which segments longitudinal rift development. Migrating arc magmatism will be best observed, and preserved, in rifting backarc basins when MR is >600 km3 m.y.−1 100 km−1 of plate boundary and VE is <25 mm a−1.
Earthquake size‐frequency relations from an analytical stochastic rupture model
Authors: Heimpel Moritz
Published: February 2004

An analytical stochastic model of earthquake rupture is presented. Although the general formulation makes no a priori self‐similarity assumption, scale independent earthquake size‐frequency (ESF) statistics emerge from the interaction between fault strength heterogeneity and rupture mechanics. The model also gives theoretical derivations for scale dependent ESF statistics, including gradual frequency saturation for small earthquakes, and a continuum of possible transitions in the statistics of large earthquakes. A model fault is characterized as a surface composed of a large number of asperities, defined as small, discrete contact surfaces of finite strength. An earthquake rupture grows via successive asperity‐breaking subevents. The probability of rupture to a certain size earthquake or greater is the total product of incremental rupture growth probabilities. These are governed by a new asperity failure probability distribution, derived as a mixture of Weibull distributions. By including fracture mechanics, the resulting mixture distribution, characterized by the scale and shape parameters Λ and Γ, is written in terms of the rupture size. The important difference between the mixture distribution and the Weibull distribution is that the mixture results in power law ESF statistics. Self‐similarity occurs when material heterogeneity and rupture mechanics are mutually “tuned” to give Λ = Γ = 1. Frequency saturation for small earthquakes is shown to be a simple consequence of a general aseismic nucleation process. The nucleation magnitude mν defines the roll‐off point in ESF statistics that occurs when the area of seismic rupture equals the area of precursory, aseismic rupture growth. By fitting the model to the Parkfield earthquake data set, mν = 0.9 is obtained. The change in ESF scaling for the transition from moderate to large earthquakes depends on the changes in stress at the growing rupture front. The Northern California Earthquake Data Center seismicity data set is fitted for a transition at magnitude m ≃ 5 with Λ′ = 0.4 and Γ′ = 0.75. This result indicates that the change in the ESF relation associated with the transition from moderate to large earthquakes in California and Nevada is caused by surface rupture accompanied by partial width and slip saturation.
Measurements of length of day using the Global Positioning System
Authors: Ray J. R.
Published: February 2004

Length‐of‐day (LOD) estimates from the seven Global Positioning System (GPS) analysis centers of the International GPS Service for Geodynamics have been compared to values derived from very long baseline interferometry (VLBI) for a recent 16‐month period. All GPS time series show significant LOD biases which vary widely among the centers. Within individual series the LOD errors show time‐dependent correlations which are sometimes large and periodic. Clear correlations between ostensibly independent analyses are also evident. In the best case the GPS LOD errors, after bias removal, approach Gaussian with an intrinsic scatter estimated to be as small as ∼21 µ/d and a correlation time constant of perhaps 0.75 day. Integration of such data to determine variations in UT1 will have approximately random walk errors which grow as the square root of the integration time. For the current best GPS performance, UT1 errors exceed those of daily 1‐hour VLBI observations after integration for ∼3 days. Assuming the stability of LOD biases can be reliably controlled, GPS‐derived UT1 can be useful for near real time applications where otherwise extrapolations for several days from the most current VLBI data can be inaccurate by up to ∼1 ms.