bluuebunny/crossref_metadata_2025 huggingface dataset:
Volume strain within The Geysers geothermal field
Authors: Mossop Antony , Segall Paul
Published: September 2002

During the 1970s and 1980s, The Geysers geothermal region was rapidly developed as a site of geothermal power production. The likelihood that this could cause significant strain within the reservoir, with corresponding surface displacements, led to a series of deformation monitoring surveys. In 1973, 1975, 1977, and 1980, The Geysers region was surveyed using first‐order, class I, spirit leveling. In 1994, 1995, and 1996, many of the leveling control monuments were resurveyed using high‐precision Global Positioning System receivers. The two survey methods are reconciled using the GEOID96 geoid model. The displacements are inverted to determine volume strain within the reservoir. For the period 1980–1994, peak volume strains in excess of 5×10−4 are imaged. There is an excellent correlation between the observed changes in reservoir steam pressures and the imaged volume strain. If reservoir pressure changes are inducing volume strain, then the reservoir quasi‐static bulk modulus K must be <4.6×109 Pa. However, seismic velocities indicate a much suffer reservoir with K = 3.4 × 1010 Pa. This apparent discrepancy is shown to be consistent with predicted frequency dependence in K for fractured and water‐saturated rock. Inversion of surface deformation data therefore appears to be a powerful method for imaging pressure change within the body of the reservoir. Correlation between induced seismicity at The Geysers and volume strain is observed. However, earthquake distribution does not appear to have a simple relationship with volume strain rate.
Deformation kinematics in the western United States determined from Quaternary fault slip rates and recent geodetic data
Authors: Shen‐Tu Bingming , Holt William E. , Haines A. J.
Published: September 2002

We estimate the horizontal velocity gradient tensor field from Quaternary fault slip rates, and recent Global Positioning System (GPS) and very long baseline interferometry (VLBI) velocity solutions in the western United States transform plate boundary zone. The total velocity obtained from the Quaternary fault slip rate data across the entire plate boundary is within 1 mm/yr of the NUVEL‐1A predicted Pacific (PA)‐North American (NA) plate motion velocity, but directions are 5°–6° anticlockwise of directions given by NUVEL‐1A. The total velocity obtained from inversion of recent geodetic data is 2°–3° anticlockwise from the NUVEL‐1A NA‐PA velocity, but the difference between the two is not significant at the 95% confidence level. The discrepancy between the total PA‐NA motion obtained from the geological data and NUVEL‐1A indicates that a marginally significant amount of NE‐SW shortening (possibly as much as 5 mm/yr) is missing overall in the geologic data. Shortening may occur in the long‐term in the offshore and coastal areas of California where such shortening is required in the shorter‐term geodetic solution. The seismic moment released in the last 148 years is ∼59% of the total moment release rate expected from long‐term strain rate field (including both seismic and aseismic deformation) derived from the inversion of geological data with NUVEL‐1A far‐field PA‐NA motion constraints. The accumulated strain in the areas containing the southern San Andreas fault‐San Jacinto fault, the San Francisco Bay area, and the area containing the Ventura basin in the Western Transverse Ranges in the last 148 years is the equivalent to that which could be released by an Mw>7.0 earthquake in each 50 × 100 × 15 (km3) crustal volume if strain is to be released seismically in these areas.
Shallow mantle temperatures under Europe from P and S wave tomography
Authors: Goes S. , Govers R. , Vacher P.
Published: September 2002

Temperature is one of the key parameters controlling lithospheric and mantle dynamics and rheology. Using recent experimental data on elastic parameters and anelasticity, we obtain models of temperature at 50 to 200 km depth beneath Europe from the global P wave velocity model of Bijwaard et al. [1998] and the regional S wave velocity model of Marquering and Snieder [1996]. Forward modeling of seismic velocity allows us to assess the sensitivity of velocity to various parameters. In the depth range of interest, variations in temperature (when below the solidus) yield the largest effects. For a 100°C increase in temperature, a decrease of 0.5–2% in VP and 0.7–4.5% in VS is predicted, where the strongest decrease is due to the large effect of anelasticity at high temperature. The effect of composition is expected to give velocity anomalies <1% for the shallow mantle and would therefore be difficult to resolve. At depths >80 km the relative amplitudes of the European VP and VS anomalies are consistent with a thermal origin. At shallower depths, variations in crustal thickness and possibly the presence of partial melt appear to have an additional effect, mainly on S wave velocity. In regions where both P and S anomalies are well‐resolved, VP‐ and VS‐derived thermal models agree well with each other and with temperatures determined from surface heat flow observations. Furthermore, the thermal models are consistent with known tectonics. The inferred temperatures vary significantly, from around 400°C below an average mantle adiabat at 100 km depth under the Russian Platform and a 300°C increase from east to west across the Tornquist‐Teisseyre zone to temperatures around the mantle adiabat in the depth range 50–200 km under areas with present surface volcanism. In spite of the uncertainties in the calculation of temperatures due to uncertainties in the experimental elastic parameters and anelasticity and uncertainties associated with tomographic imaging, we find that the tomographic models of the shallow mantle under Europe can yield useful estimates of the thermal structure.
Mechanical prediction of fracture aperture in layered rocks
Authors: Bai Taixu , Pollard David D. , Gross Michael R.
Published: September 2002

Two types of opening‐mode fractures (joints) are commonly found in layered rocks. One is called unconfined because fracture heights are much less than the layer thickness and they behave like fractures in massive rocks. The other is called confined because the fractures terminate at the layer boundaries. We investigate the mechanical control on apertures in these systems using the theory of elasticity. An analytical solution demonstrates that the ratio of aperture to height (aspect ratio) of an unconfined fracture in a homogeneous, isotropic medium is linearly related to the average strain, the overburden stress, and the internal fluid pressure within the fracture. Numerical results based on of the finite element method (FEM) for an unconfined fracture in the central layer of a three‐layer model agree with the analytical result when the fractured layer and neighboring layers have the same elastic constants. The aspect ratio of the unconfined fracture is insensitive to the ratio of Young's modulus of the fractured layer to that of the neighboring layers and to the differences in Poisson's ratios. The FEM results for confined fractures show that their aspect ratio is linearly related to the average strain, the overburden stress, and the internal fluid pressure. However, the aspect ratio increases nonlinearly with increasing fracture spacing to layer thickness ratio because of the mechanical interaction between adjacent fractures. The interaction becomes insignificant when the spacing to layer thickness ratio is > ∼ 6.0. The aspect ratio of confined fractures depends on the ratio of Young's modulus of the fractured layer to that of the neighboring layers. This dependence is significant when the fracture spacing to layer thickness ratio is <1.3; otherwise, it is negligible. In all of these cases the aspect ratio of confined fractures is insensitive to variations in Poisson's ratios. Furthermore, the FEM results predict that fracture accommodated strain measured by the traditional scan line method may slightly overestimate the average normal strain when the spacing to layer thickness ratio is < ∼ 1.0, and may slightly underestimate this strain for greater ratios.
Rapid rotation of Fiji: Paleomagnetic evidence and tectonic implications
Authors: Taylor Graeme K. , Gascoyne Jon , Colley Howard
Published: September 2002

The timing and magnitude of rotation of Fiji in relation to the tectonic evolution of the SW Pacific region has been investigated by extensive (126 sites) paleomagnetic sampling of Late Miocene and younger units. Generally, samples yielded well‐defined characteristic remanence directions which indicate that the main Fiji platform rotated by 135±17.5° counterclockwise commencing at, or before, 10 Ma. This very large and rapid rotation ceased abruptly at 3 Ma, coincident with the onset of spreading on the major N‐S aligned spreading centers in the North Fiji Basin. Latest Miocene to Pliocene paleomagnetic results indicate that Viti Levu, the main island itself, has suffered no internal disruption by localized rotations after 5 Ma. We present a reconstruction for the origin of Fiji which differs markedly from previous models such that Fiji, Tonga, and Lau form a continuous N‐S aligned arc and that arc reversal was initiated at or near Fiji itself.
Equivalent medium representation of fractured rock
Authors: Liu Enru , Hudson John A. , Pointer Tim
Published: September 2002

A similarity may be found between various approaches for determining the effects of parallel fractures or aligned cracks on seismic wave propagation at wavelengths that are long compared with the scale length of the cracks. Fractures can be modeled using an empirical linear slip condition; however, natural fracture surfaces can also be simulated directly as planar distributions of small isolated areas of slip (cracks) (model 1) or, conversely, as planar distributions of imperfect interfacial contacts (model 2). An alternative is plane surfaces separated by thin continuous layers of viscous fluid or a soft material (model 3). We present analytic expressions for the fracture compliances for these three models and, using these analytic results, compute the effective compliances and stiffnesses of the fractured material. As a result, it is possible to relate the measured compliances or stiffnesses directly to the statistics of the microstructural details of a fracture, given appropriate a priori information on the fracture surfaces. The results for model 1 are equivalent to a volume distribution of cracks as studied by Hudson [1980, 1981] for small crack density; the results for model 2 are basically the same as those given by White [1983] for a packing of spheres; and finally, the results for model 3 are in agreement with those given by Backus [1962] for combinations of two constituent layers. These results can be extended to the case of nonaligned fractures and to allow for fluid flow between cracks and into a porous matrix rock. Finally, it is shown that the ratio of the normal to shear fracture compliance is a good indicator of the properties of the fracture infill.
A seismically constrained mass discharge rate for the initiation of the May 18, 1980 Mount St. Helens eruption
Authors: Brodsky Emily E. , Kanamori Hiroo , Sturtevant Bradford
Published: September 2002

We calculate the vertical mass discharge rate from Mount St. Helens for the first few minutes of the May 18, 1980 cataclysmic eruption using a new method based on seismic constraints. The observed seismic waves indicate that the seismic source is a series of single forces. We model these forces as thrusts due to a combination of the momentum flux of the erupted products and the pressure of the eruptive jet. The momentum discharge rate is converted to a mass discharge rate based on estimates of the velocity and jet pressure as constrained by a simple fluid dynamical model. Only two parameters are necessary for the calculation: Mach number and sound velocity. The calculated mass ejected in the first 100 s is 1.6×1011 −4.6×1011 kg. Since the total blast deposit is ∼3.2×1011 − 4.1×1011 kg, one possible interpretation is that the directed blast had a significant (>30%) vertical component.
Associations between burial diagenesis of smectite, chemical remagnetization, and magnetite authigenesis in the Vocontian trough, SE France
Authors: Katz Bodo , Elmore R. Douglas , Cogoini Monika , Engel Michael H. , Ferry Serge
Published: September 2002

Results of a paleomagnetic, rock magnetic, geochemical, and petrographic study on Jurassic and Cretaceous carbonates in the Vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization (CRM) carried by magnetite. Where smectite has altered to other clay minerals, limestones are characterized by a prefolding, secondary, normal polarity magnetization throughout the basin. The magnetization is interpreted to be a CRM based on low burial depths which cannot cause thermoviscous resetting. Where significant smectite is still present, the CRM is absent/weakly developed, and where the clays show no evidence for burial alteration, the units are characterized by a primary magnetization. CRM intensity also varies with the amount of smectite and burial. Isothermal, anhysteretic, and natural remanent magnetization intensities increase where smectite has altered, both stratigraphically and geographically. This is interpreted to indicate magnetite authigenesis associated with clay diagenesis. Superparamagnetic magnetite is more dominant in highly altered units based on the results of low‐temperature experiments. All sections away from the Alps have 87Sr/86Sr values that are similar to coeval seawater, and stable isotopes of carbon and oxygen show no sign of alteration. Orogenic‐type fluids therefore are not a likely agent of remagnetization. Near the Alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the CRM through a reversal. A postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic‐type fluids. We conclude that burial diagenesis of smectite is the likely cause for the development of the widespread CRM in the Vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere.
Fracture morphology of tensile cracks and rupture velocity
Authors: Müller Gerhard , Dahm Torsten
Published: September 2002

Desiccation cracks in starch‐water mixtures are studied with respect to morphological features, mainly plumose structures, on their faces. Specimens have diameters of 50–100 mm and thicknesses of 2–40 mm. Structures similar to those on joints in rocks are found. Rupture velocities are measured from videos and estimated from photos. Rupture covers the range from spontaneously nucleating, dynamic cracks with velocities of 100–200 mm/s to quasi‐static cracks with velocities of 0.1 mm/s and less. Plumose lines give the rupture direction, and their relation to rupture velocity is similar to the relation between seismic rays and seismic wave velocity. A ray‐tracing method from seismology is used to calculate plumose lines for depth‐dependent rupture velocity. Moreover, an inverse method, based on finite difference travel times and conjugate gradients, is developed to invert a set of measured plumose directions into a rupture‐velocity distribution which can also depend on the horizontal coordinate on the rupture surface. The main results of this paper are as follows. (1) Plumose lines can successfully be inverted into relative rupture velocity. (2) In thin starch layers (thickness less than 0.2 times diameter), rupture velocity decreases from top to bottom by a factor of 2–5, following a decrease of tensile stress due to the increase in water concentration. (3) Horizontal variation of rupture velocity reflects horizontal variation of stress, including stress relaxation due to the propagating crack, and ranges from dynamic to quasi‐static velocities. (4) In thick starch layers (thickness about 0.5 times diameter), rupture is predominantly quasi‐static. (5) Starch cracks sometimes have a fringe zone where topographic amplitudes are higher and rupture velocities lower than on the main part of the crack; this probably also applies to joints in rocks and their fringe zones. (6) Starch‐water mixtures at rupture have a Poisson ratio close to 0.5. Cracks in starch are closest to subsidence or diagenesis joints in sedimentary rocks.
Can a narrow, melt‐rich, low‐velocity zone of mantle upwelling be hidden beneath the East Pacific Rise? Limits from waveform modeling and the MELT Experiment
Authors: Hung Shu‐Huei , Forsyth Donald W. , Toomey Douglas R.
Published: September 2002

One of the goals of the Mantle Electromagnetic and Tomography (MELT) Experiment is to determine whether a narrow zone of enhanced melt concentration consistent with focused upwelling exists beneath the East Pacific Rise. Using SKKS, sScS, and S phases from two intermediate‐depth earthquakes in the Banda Sea and the Tonga‐Kermadec region, we demonstrate that there is no positive evidence for the existence of such a zone and that travel time delays for shear waves traveling through it must be <0.5 s. To test whether diffraction and wave front healing could obscure evidence for its existence, we employ a pseudospectral method to simulate finite frequency teleseismic waves propagating through narrow, vertical low‐velocity zones. A rich set of reflected, diffracted, and guided waves is generated when S waves encounter such a low‐velocity channel, particularly at high frequencies. Limiting the frequency content to the lower‐frequency bands with good signal‐to‐noise ratios in the observed phases obscures these waveform complexities. The travel time anomaly is broadened and reduced in amplitude but remains detectable unless the low‐velocity zone is very narrow or has only modest velocity contrast. The lower limit of detectability corresponds to a 5‐km‐wide channel of partial melt extending from 10 to 60 km below the seafloor at the ridge axis with a shear velocity contrast of 0.5 km/s. Although these limits are severe, 3 to 4% melt retention might cause a large enough viscosity reduction and anomalous buoyancy to dynamically focus upwelling into a 5‐km‐wide channel that falls within the limits. Strongly focused, dynamic upwelling beneath the ridge, however, is probably not compatible with the existence of a broad region of very low shear velocities in the surrounding mantle.
The chemically zoned 1949 eruption on La Palma (Canary Islands): Petrologic evolution and magma supply dynamics of a rift zone eruption
Authors: Klügel Andreas , Hoernle Kaj A. , Schmincke Hans‐Ulrich , White James D. L.
Published: September 2002

The 1949 rift zone eruption along the Cumbre Vieja ridge on La Palma involved three eruptive centers, 3 km spaced apart, and was chemically and mineralogically zoned. Duraznero crater erupted tephrite for 14 days and shut down upon the opening of Llano del Banco, a fissure that issued first tephrite and, after 3 days, basanite. Hoyo Negro crater opened 4 days later and erupted basanite, tephrite, and phonotephrite, while Llano del Banco continued to issue basanite. The eruption ended with Duraznero erupting basanite with abundant crustal and mantle xenoliths. The tephrites and basanites from Duraznero and Llano del Banco show narrow compositional ranges and define a bimodal suite. Each batch ascended and evolved separately without significant intermixing, as did the Hoyo Negro basanite, which formed at lower degrees of melting. The magmas fractionated clinopyroxene +olivine±kaersutite±Ti‐magnetite at 600–800 MPa and possibly 800–1100 MPa. Abundant reversely zoned phenocrysts reflect mixing with evolved melts at mantle depths. Probably as early as 1936, Hoyo Negro basanite entered the deep rift system at 200–350 MPa. Some shallower pockets of this basanite evolved to phonotephrite through differentiation and assimilation of wall rock. A few months prior to eruption, a mixing event in the mantle may have triggered the final ascent of the magmas. Most of the erupted tephrite and basanite ascended from mantle depths within hours to days without prolonged storage in crustal reservoirs. The Cumbre Vieja rift zone differs from the rift zones of Kilauea volcano (Hawaii) in lacking a summit caldera or a summit reservoir feeding the rift system and in being smaller and less active with most of the rift magma solidifying between eruptions.
Support for differential inner core superrotation from earthquakes in Alaska recorded at South Pole station
Authors: Song Xiaodong , Li Anyi
Published: September 2002

Observational evidence for differential inner core rotation has recently been reported. The most convincing evidence has been restricted to one ray path from earthquakes in the South Sandwich Islands to stations in Alaska. Here we show that the differential travel times between PKP(DF) and PKP(BC) for a new ray path from earthquakes in southern Alaska to the station at the South Pole have increased by about 0.6 s over 37 years, providing support for a differential inner core rotation. We apply a newly developed technique to invert simultaneously for the inner core structure and the rotation rate. The rotation rate determined from the new pathway is about 0.6°/yr faster than the mantle, compatible with the estimates of the rate from earthquakes in the South Sandwich Islands to stations in Alaska. We also address some criticisms concerning the reported detection of the differential inner core rotation, including biases from event mislocations, biases from mantle heterogeneity, and uncertainty in the tilt of the inner core anisotropy axis. We show that although these potential biases would affect the determination of the rotation rate, they are unlikely to account for the observed time dependence in the BC‐DF times. We found no evidence for a correlation between BC‐DF residuals and event magnitudes as suggested previously.
Origin of elevated sediment permeability in a hydrothermal seepage zone, eastern flank of the Juan de Fuca Ridge, and implications for transport of fluid and heat
Authors: Giambalvo Emily R. , Fisher Andrew T. , Martin Jeffrey T. , Darty Lisa , Lowell Robert P.
Published: September 2002

Elevated sediment porosity and permeability may help to focus upward fluid seepage observed over a buried basement high on the eastern flank of the Juan de Fuca Ridge. Consolidation and permeability tests of fine‐grained hemipelagic and turbidite sediments from the seepage site and from nearby sites that are not experiencing seepage indicate that sediments at the seepage site (primarily hemipelagic) have 10–15% higher porosity and 10× higher permeability at a given depth and 10–100× lower hydraulic impedance for the 40‐m‐thick sediment column. The correlation of consolidation and permeability properties with sediment type rather than location indicates that the relatively high porosity and permeability do not result from fluid flow but are intrinsic properties of the hemipelagic sediment. On the basis of measured sediment properties, fluid pressure at the top of basement is <5 kPa greater than hydrostatic. A simple circulation model that incorporates this estimate and proximity to the nearest basement outcrops (4–20 km) suggests that local basement permeability is between 3×10−13 and 3×10−11 m2, within the range measured in nearby boreholes. The measured sediment properties combined with other published data indicate that a few tens of meters of fine‐grained terrigenous, hemipelagic, or calcareous marine sediment effectively seal the basement aquifer, whereas much thicker siliceous or pelagic sediment may support thermally and chemically significant fluid flow. The large contrast in hydraulic impedance among sediment columns of different types could cause regional variation in the evolution of ridge flank hydrothermal systems and in the contribution of seepage to ridge flank fluxes.
Damage zones around en echelon dike segments in porous sandstone
Authors: Weinberger Ram , Lyakhovsky Vladimir , Baer Gidon , Agnon Amotz
Published: September 2002

We investigate arrays of en echelon dike segments and their associated deformation in porous sandstone to infer the segmentation mechanism and the state of stress during dike emplacement. The en echelon arrays are interpreted as breakdown segments of planar parent dikes that propagated from greater depth under mixed‐mode conditions. Typically, an array consists of either continuous nonoverlapping stepped segments (offset smaller than segment thickness) or overlapping connected segments (offset larger than segment thickness). The deformation associated with the nonoverlapping stepped segment arrays consists of newly documented fan‐like patterns of deformation bands (lamellae of crushed detrital quartz grains), whereas the overlapping connected segment arrays consists of net‐like patterns of deformation bands. Thus the patterns of deformation are related to offset geometry and are likely to be diagnostic of the states of stress. We simulated the stress and deformation fields around interacting breakdown segments by applying a continuum damage mechanics model. The simulation results mainly illustrate the stress dependence of the damage distribution and the sensitivity of the damage distribution to the geometry of the segment offset and the mutuality of segment propagation. By changing the applied stress and by controlling the segment tip growth, symmetric and asymmetric distributions of damage were produced. We describe which aspects of the generated damage zones satisfactorily correlate with field observations. Damage mechanics simulations are useful tools for studying the state of stress during dike emplacement.
A new high‐resolution geomagnetic relative paleointensity record for the North American Holocene: A comparison of sedimentary and absolute intensity data
Authors: Brachfeld Stefanie A. , Banerjee Subir K.
Published: September 2002

A new high‐resolution paleointensity record for North America has been constructed using Holocene sediments from Lake Pepin, Minnesota. Lake Pepin sediments yield the same Holocene paleosecular variation curve as nearby Lake St. Croix and satisfy all of the criteria recommended for paleointensity studies. Absolute paleointensity data for North America recorded by Holocene volcanic and archeomagnetic samples provide an independent record of geomagnetic field paleointensity against which the relative intensity records from Lake Pepin and Lake St. Croix can be compared. Since the absolute field paleointensity is known a priori, the effects of the magnetic recording assemblage can be isolated. Anhysteretic remanent magnetization (ARM) is the best choice among normalization parameters for the Lake Pepin sediments as the natural remanent magnetization (NRM) normalized by ARM shows no coherence with magnetic grain size proxies and yields a record of relative paleointensity peaks and troughs whose amplitudes are very similar to those in the archeomagnetic (ARCMAG) and Lake St. Croix data sets. Features with a wavelength of 1000 years are correlative between the three paleointensity records. NRM normalized by saturation isothermal remanent magnetization (SIRM) and by susceptibility (χ) shows grain‐size dependences that cause errors in the amplitudes of paleointensity features. NRM/SIRM and NRM/χ are not coherent with their normalizers but are both strongly coherent with independent grain size proxies such as the median destructive field of the NRM and HCR. We successfully removed the grain size dependences from NRM/SIRM by applying a correction function based on the linear relationship between normalized intensity and the median destructive field of the NRM.
Subsolidus and melting experiments of K‐doped peridotite KLB‐1 to 27 GPa: Its geophysical and geochemical implications
Authors: Wang Wuyi , Takahashi Eiichi
Published: September 2002

Subsolidus and melting experiments with a K‐doped peridotite (K2O 0.64 wt %) were performed at 10–27 GPa in order to investigate the behavior of potassium in the deep mantle, and the element partitioning between Mg‐perovskite (Mg‐pv) and magnesiowustite (mw). Three new K‐rich phases were discovered, K phase I [K2FeMg4Si5O16] in coexistence with wadsleyite, pyroxene and garnet at 15 GPa, K phase II [(K,Mg)(Al,Si)O4] with CaSiO3‐rich perovskite (Capv), ringwoodite and majoritic garnet at 20 GPa, and K phase III [(K,Na,Ca)(Mg,Fe,Cr,Al,Ni,Ti,Mn)4(Si,Al)4O12] in coexistence with Mg‐pv, mw, and Ca‐pv at 25.5 GPa. These three phases are the main K‐bearing phases in the deep peridotite mantle, rather than K‐hollandite [KAlSi3O8]. With the exception of pyroxene at relatively low pressure, the solubilities of K2O in other mantle phases, such as wadsleyite, ringwoodite, and garnet are very low (<200 ppm), even under the conditions saturated with one of the K phases. At 25.5 GPa, the contents of K2O in Mg‐pv, mw, and Ca‐pv are also very low and decrease with decreasing temperature. The K‐rich phases should appear in low concentrations in the transition zone and the lower mantle, assuming a pyrolitic mantle composition (K2O, ∼300 ppm). In contrast to basaltic composition, K‐phases in peridotite are the first to melt at pressures up to 27 GPa, and thus K is incompatible in the whole experimental pressure range. The solidus temperature of K‐rich peridotite increases drastically from about 1600°C at 20 GPa to 2300°C at 25.5 GPa. Portions of the upper mantle could be partially molten even at the present mantle temperatures. Ascending lower mantle material will partially melt to a very low degree around the transition zone/lower mantle boundary, forming a K‐rich melt. This melt would strongly affect the geophysical properties of the transition zone, and may be directly related to the worldwide mantle metasomatism and K‐rich magmatism. For partitioning of Fe/Mg, Na2O, NiO, Cr2O3, and MnO between Mg‐pv and mw, no evident variation with temperature or pressure was detected. The results show that most of the reported (Mg,Fe)O and MgSiO3 inclusions in diamonds were not in equilibrium.
Constraints on magma flux from displacements data at Merapi volcano, Java, Indonesia
Authors: Beauducel F. , Cornet F.‐H. , Suhanto E. , Duquesnoy T. , Kasser M.
Published: September 2002

The displacement field has been monitored in the vicinity of the crater rim at Mount Merapi (Indonesia) from 1993 to 1997. During this period the volcanic activity has been quasicontinuous with dome growth, explosions, and pyroclastic flows. We measured a nine‐point network every year with the Global Positioning System static method. Interpretation of results is conducted with a three‐dimensional elastostatic boundary elements code that takes into account topography, fractures, and complex magma source geometry. The inversion technique yields an estimate of the variation with time of the boundary conditions at the magma duct interface together with the probability associated with the best model. The Young's modulus of the equivalent continuum is found to be very low (of the order of 1 GPa), a feature which suggests that a viscoelastic behavior may be more appropriate for this rock mass, given the observed seismic velocities for the domain of interest. A striking compatibility is outlined between observed deformations and the rate of occurrence of multiphase seismic events, once the main fractures of the structure have been taken into consideration. This suggests that the summit elastic (or viscoelastic) deformation field is controlled by the magma flux within the duct rather than by magma pressure variations. In addition, a nonelastic displacement has been identified at the westernmost point of the network. This was considered critical for the stability of the summit structure, a concern whose validity has been verified a posteriori by the July 1998 explosion.
Short‐time electrical effects during volcanic eruption: Experiments and field measurements
Authors: Büttner Ralf , Zimanowski Bernd , Röder Helmut
Published: September 2002

Laboratory experiments on the fragmentation and expansion of magmatic melt have been performed using remelted volcanic rock at magmatic temperatures as magma simulant. A specially designed dc amplifier in combination with high speed data recording was used to detect short‐time electrostatic field effects related to the fragmentation and expansion history of the experimental system, as documented by simultaneous force and pressure recording, as well as by high‐speed cinematography. It was found that (1) the voltage‐time ratio of electrostatic field gradients (100 to 104 V/s) reflects different physical mechanisms of fragmentation and expansion and (2) the maximum voltage measured in 1 m distance (−0.1 to −180 V) can be correlated with the intensity of the respective processes. Based on these experimental results, a field method was developed and tested at Stromboli volcano in Italy. A 0.8 m rod antenna was used to detect the dc voltage against local ground (i.e., the electrostatic field gradient), at a distance of 60 to 260 m from the respective vent. Upwind position of the detection site was chosen to prevent interference caused by contact of charged ash particles with the antenna. A standard 8 Hz geophone was used to detect the accompanying seismicity. Three types of volcanic activity occurred during the surveillance operation; two of these could be clearly related to specific electrical and seismical signals. A typical delay time was found between the electrical and the seismical signal, corresponding to the seismic velocity within the crater deposits. Using a simple first‐order electrostatic model, the field measurements were recalibrated to the laboratory scale. Comparison of field and laboratory data at first approximation revealed striking similarities, thus encouraging the further development of this technique for real‐time surveillance operation at active volcanoes.
Syntectonic melt pathways during simple shearing of a partially molten rock analogue (Norcamphor‐Benzamide)
Authors: Rosenberg Claudio L. , Handy Mark R.
Published: September 2002

Norcamphor‐benzamide aggregates were used as analogues for partially molten quartzofeldspathic rock. Drained, constant displacement rate, simple shearing of partially molten norcamphor‐benzamide aggregates produces strain localization within melt‐bearing, extensional shear fractures. These fractures interconnect to form shear surfaces whose orientation with respect to the shear zone boundary is like that of synthetic shear bands (C′ surfaces) in naturally deformed, mylonitic rocks. The shear bands channel overpressured melt from dilatant grain boundaries in the norcamphor‐benzamide aggregate to undeformed, low‐pressure areas adjacent to the deforming sample. The rapid expulsion of melt in the aggregate along shear bands hinders the attainment of a rheological critical melt percentage (20%) within the shear zone as a whole. However, this melt percentage is achieved within the melt‐bearing shear bands. Deformation within the shear bands involves a strain‐dependent switch from intergranular fracturing and dislocation creep to diffusion‐accommodated, grain‐boundary sliding, whereas the matrix adjacent to the shear bands continues to deform by dislocation creep. At a shear strain of approximately γ = 0.7 the melt‐bearing shear bands coalesce to form interconnected weak layers subparallel to the shear zone boundaries. The strength of the aggregate could not be measured but is inferred to decrease markedly once the melt‐bearing layers interconnect. When the melt completely crystallizes and deformation ceases, the former presence of melt within the shear bands is only betrayed by the alignment of benzamide grain boundaries parallel to the shear bands.
Origin of intraplate volcanoes from guyot heights and oceanic paleodepth
Authors: Caplan‐Auerbach Jacqueline , Duennebier Fred , Ito Garrett
Published: September 2002

The height of a guyot as measured from the surrounding regional sea floor to the volcano's slope break records the water depth at the time the guyot submerged. Thus guyot heights may be used as indicators of the paleodepth of the surrounding ocean floor. We compile data on the heights of 68 intraplate guyots and atolls in the Pacific Ocean as well as 46 volcanic islands in the Pacific, Atlantic, and Indian Oceans. We find that guyot heights generally increase with the age of the lithosphere upon which they were emplaced, although there is a large amount of scatter. In nearly all cases, seamount height, and thus seafloor paleodepth, is less than expected of normal seafloor. These results suggest that most of the volcanoes in this study formed on anomalously shallow seafloor, consistent with formation at hotspots. To characterize thermal anomalies associated with these hotspot swells, we model guyot heights by calculating the isostatic uplift predicted for normal lithosphere that has been partly reheated and is underlain by anomalously hot mantle. This model is able to explain the anomalous water depth at most of the seamounts with hotspot thermal anomalies of 100°–300°C. The heights of a few volcanic chains, however, are not anomalously low, suggesting that these volcanoes are not associated with hotspots. In addition, the observed trend of Hawaiian‐Emperor guyot heights as well as the subdued morphology and gravity signature of the oldest Emperor seamounts supports our hypothesis that Cretaceous age Meiji seamount may have formed on or near a spreading center.
Deformation across the forearc of the Cascadia subduction zone at Cape Blanco, Oregon
Authors: Savage J. C. , Svarc J. L. , Prescott W. H. , Murray M. H.
Published: September 2002

Over the interval 1992–1999 the U.S. Geological Survey measured the deformation of a geodetic array extending N80°E (approximate direction of plate convergence) from Cape Blanco on the Oregon coast to the volcanic arc near Newberry Crater (55 and 350 km, respectively, from the deformation front). Within about 150 km from the deformation front, the forearc is being compressed arcward (N80°E) by coupling to the subducting Juan de Fuca plate. Dislocation modeling of the observed N80°E compression suggests that the main thrust zone (the locked portion of the Juan de Fuca‐forearc interface) is about 40 km wide in the downdip direction. The transverse (N10°W) velocity component of the forearc measured with respect to the fixed interior of North America decreases with distance from the deformation front at a rate of about 0.03 mm yr−1 km−1. That gradient appears to be a consequence of rigid rotation of the forearc block relative to fixed interior North America (Euler vector of 43.4°±0.1°N, 120.0°±0.4°W, and −1.67±0.17 ° (m.y.)−1; quoted uncertainties are standard deviations). The rotation rate is similar to the paleomagnetically measured rotation rate (−1.0±0.2 °(m.y.)−1) of the 15 Ma lava flows along the Columbia River 250 km farther north. The back arc does not appear to participate in this rotation but rather is migrating at a rate of about 3.6 mm yr−1 northward with respect to fixed North America. That migration could be partly an artifact of an imperfect tie of our reference coordinate system to the interior of North America.
Size and depth of ancient magma reservoirs under atolls and islands of French Polynesia using gravity data
Authors: Clouard Valérie , Bonneville Alain , Barsczus Hans G.
Published: September 2002

New insights into the structural and tectonic evolution of islands and atolls in French Polynesia are derived from the analysis of gravity data. Free‐air anomaly maps were constructed using gravity data from land surveys of 25 islands combined with free‐air anomaly data derived from satellite altimetry and shipborne gravimeters. Residual isostatic anomalies were calculated using a three‐dimensional (3‐D), four‐layer crustal model taking into account the bathymetry of the seafloor, the topography of the islands, and the deflection of the lithosphere under the load of the volcanoes. Twenty of the 25 islands yield a positive residual anomaly, ranging between 9 and 60 mGal. Negative residual anomalies over four islands probably correspond either to limestone deposits or to fragmented material from aerial volcanism. The occurrence of a positive anomaly provides some evidence that there is a solidified magma chamber at depth beneath each of several islands in French Polynesia. A linear relation between the amplitude of the positive gravity anomaly and island volume is also observed. Geological features which generate these positive anomalies are described by simple geometric models (3‐D ellipsoidal dense bodies) which lead to a rough estimate of the size and depth of the magma chambers. We then propose a simple linear relation between the volume of such magma chambers and the volume of islands that can be used for other extinct intraplate basaltic volcanoes. The diameter of the inferred magma chamber and its projection on the surface of the islands imply that calderas correspond mostly to collapses of the flanks of the edifices rather than to caldera vertical subsidence.
Fernandina Volcano's evolved, well‐mixed basalts: Mineralogical and petrological constraints on the nature of the Galapagos plume
Authors: Allan James F. , Simkin Tom
Published: September 2002

Fernandina Volcano, the most active of Galapagos volcanoes and the one most directly overlying the Galapagos hotspot, is one of a small number of active, plume‐related oceanic island volcanoes with a well‐documented recent eruptive record. Whole rock and glass analyses of lava and tephra from all 12 known eruptions from 1958 to the present show them to be evolved (Mg # (= Mg/Mg + Fe2+) of 0.47–0.56), plagioclase‐phyric, clinopyroxene‐ and olivine‐bearing tholeiites. Thermodynamic modeling indicates that this phase assemblage erupted at 1100–1130°C and is stable only at low pressures (<0.1–0.2 GPa), thereby representing crystallization within a shallow magma chamber underlying the well‐defined, 850‐m‐deep, 5 by 6.5 km caldera. These tholeiites are remarkably homogenous, as is shown by narrow ranges in Zr/Y (5.7–6.3), (La/Sm)N (1.4–1.7), and (La/Yb)N (3.8–4.3). Prehistoric Fernandina lavas, selected for petrographic and geographic variation over the entire subaerial volcano, are of remarkably similar composition (e.g., Zr/Y is 5.2–6.1, (La/Sm)N is 1.5–1.6, and (La/Yb)N is 3.4–4.1), with a single example of a more primitive lava having an Mg # of 0.63, with 8.5% FeO at 8% MgO. Cr‐spinel and glass melt inclusions in plagioclase, together with zoned olivine, give evidence for a range of parental melts in the recent shallow magmatic system, with Mg # as high as 0.62. Extensive, shallow magma mixing is required to explain these variations in mineral and inclusion chemistry, indicating more thermal variation within the shallow magmatic system than is implied by the relatively monotonous whole rock compositions. Apparently, Fernandina's shallow magma chamber has physically blocked passage of more primitive magmas to the surface and homogenized the erupted lavas by extensive mixing. Small variations in lava isotopic and compatible element composition, combined with the need to resupply melt to maintain thermal balance, indicate that replenishment of the magma chamber is semicontinuous, with resupply occurring of the order of a decade or less. Plagioclase‐dominated, evolved Fernandina lavas are in sharp contrast to more primitive, olivine‐dominated Hawaiian lavas; the total lack of primitive melts in the shallow Fernandina magmatic system implies that substantial fractionation and heat loss occurred during melt transportation from the mantle source to the edifice magma chamber, despite the fact that these melts were transported through relatively warm, young, and thin lithosphere adjacent to the Cocos‐Nazca ridge. The diffuse nature of Galapagos volcanism, with 20th century eruptions from eight distinct volcanoes over a 20,000 km2 area, further supports this interpretation and suggests that the Galapagos plume is relatively diffuse, weak, and significantly less thermally intense than the vigorous Hawaiian plume. It may in fact be waning.
High‐pressure elasticity of stishovite and the P42/mnm ⇌ Pnnm phase transition
Authors: Carpenter Michael A. , Hemley Russell J. , Mao Ho‐kwang
Published: September 2002

A Landau free energy expansion has been developed to describe the elastic constant variations of stishovite (SiO2) associated with the P42/mnm (rutile‐type) ⇌ Pnnm (CaCl2‐type) phase transition as a function of pressure. The transition appears to display classical second‐order character, with an equilibrium transition pressure which is renormalized by coupling of the soft optic mode with spontaneous strain. Lattice parameter data from the literature show that the symmetry breaking strain is large, while the nonsymmetry breaking strains are small. These have been used to constrain the numerical values of the strain/order parameter coupling coefficients, which in turn have been used to constrain values for the Landau coefficients. When substituted into expressions for all the individual elastic constants, taking values for experimental and calculated values of the bare elastic constants from the literature, a clear view of the pattern of elastic constant variations is produced. Variations of P and S wave velocities derived from the calculated elastic constants for a second‐order transition show a dip as the transition point is approached from high pressure and from low pressure but do not show a discontinuity. The velocity anomaly provides a signature for the presence of free silica in the lower mantle.
Alternating seismic uplift and subsidence in the late Holocene at Madang, Papua New Guinea: Evidence from raised reefs
Authors: Tudhope Alexander W. , Buddemeier Robert W. , Chilcott Colin P. , Berryman Kelvin R. , Fautin Daphne G. , Jebb Matthew , Lipps Jere H. , Pearce Robert G. , Scoffm Terence P. , Shimmield Graham B.
Published: September 2002

Well‐preserved mid‐late Holocene coral reefs are exposed in low coastal cliffs in the vicinity of the Madang lagoon on the north coast of Papua New Guinea. Results from U/Th and 14C dating of corals, surveying, and field mapping indicate several major changes in relative sea level over this period. Specifically, there is evidence for a relative sea level fall of ≥4.5 m about 3000 calendar years B.P., followed by relative sea level rises of ∼1.5 m about 2400 calendar years B.P. and ≥0.5 m about 1200 calendar years B.P. and a subsequent relative sea level fall of ≥3 m some time in the past 1000 years. Since regional eustatic sea levels are believed to have been dropping gradually over this time frame, these observed changes in relative sea level are interpreted as reflecting alternating tectonic uplift and subsidence. Furthermore, the detailed structure and age relationships of the coral deposits indicate that both uplift and subsidence occurred rapidly, most probably as coseismic events with vertical displacements of 0.5 to 4.5 m. These events may be related to rupture on NW‐SE trending reverse faults which have been mapped in the nearby Adelbert Range and possibly on NE trending cross faults which have been inferred from seismicity. This interpretation implies a much greater degree of tectonic instability and potential seismic hazard in the region than previously recognized, although the inferred coseismic vertical displacements are shown to be consistent with present‐day local seismicity. In a broader context, the study illustrates how detailed analysis of vertical changes in coral reef structure and assemblages may be used as a sensitive indicator of changing relative sea level, capable of resolving century timescale events and reversals.