bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of confining pressure on dilatation, recrystallization, and flow of rock salt at 150°C
Authors: Peach C. J. , Spiers C. J. , Trimby P. W.
Published: February 2004

Microstructural evidence for fluid‐assisted dynamic recrystallization (FADRX) is widespread in naturally deformed rock salt. However, the principal experimental evidence for FADRX in salt has been obtained from stress relaxation experiments, and it is unclear whether the process occurs during steady state dislocation creep and what its effect might be. Here we report deformation experiments performed on natural rock salt at a constant strain rate of 3.5×10−7 s−1, 150°C, and confining pressures Pc of 3–30 MPa. Samples deformed at Pc = 3 MPa showed continuous work hardening and minor dilatation. Microstructurally, they exhibited intergranular cracking plus slip band and subgrain structures indicative of dislocation glide/creep. Electron backscatter diffraction analysis revealed a high frequency of boundaries with low‐angle misorientations in the range 5°–10°. In contrast, samples deformed at Pc ≥ 6.5 MPa showed work hardening followed by steady state flow at strains >6–7%. These samples compacted slightly, and crystal plastic deformation was accompanied by extensive FADRX, with a predominance of high‐angle boundaries (30°–50°) over low‐angle boundaries. We infer that FADRX is suppressed by dilatation at low pressures as a result of grain boundary disruption. At pressures high enough to prevent dilatation, however, FADRX acts as a “recovery” mechanism counteracting work hardening. The results offer a possible explanation for rheological variability seen in previous experiments conducted at pressures up to 30 MPa. A simple rate model for diffusion‐ and interface‐controlled FADRX indicates that FADRX should become increasingly important toward natural halokinetic conditions, although the effect on flow stresses is likely to be small.
Seismic structure across the rift valley of the Mid‐Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges
Authors: Canales J. Pablo , Collins John A. , Escartín Javier , Detrick Robert S.
Published: February 2004

The results from a 53‐km‐long, wide‐angle seismic profile across the rift valley of the Mid‐Atlantic Ridge south of the Kane transform (near 23°20′N, MARK area) provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid‐ ocean ridges. Anomalously low middle and lower‐crustal P wave velocities beneath the neovolcanic Snake Pit ridge are consistent with elevated axial temperatures and with the presence of 4±1% partial melt evenly distributed within the lower crust in preferentially oriented, elongated thin films. If the melt inclusions have larger aspect ratios, melt fractions can be up to 17±3%. This and other geological observations suggest that the study area is presently in a magmatically active period. The igneous crust is anomalously thin beneath both flanks of the median valley (≤2.3–2.5 km). Thus the mantle rocks observed along the western rift valley wall at Pink Hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting. The crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous, with igneous crustal thickness variations of ≥2.2 km over horizontal distances of ∼5 km. This heterogeneity indicates that the magma supply in the area has fluctuated during the last ∼2 m.y. Thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data.
Pressure‐volume‐temperature behavior of diaspore and corundum: An in situ X‐ray diffraction study comparing different pressure media
Authors: Grevel Klaus‐D. , Burchard Michael , Faßhauer Detlef W. , Peun Thomas
Published: February 2004

The lattice parameters of synthetic diaspore (AlO(OH)) and synthetic and natural corundum (Al2O3) have been measured at simultaneously elevated pressure and temperature up to 7 GPa and 800°C (1000°C for corundum) using a MAX 80 cubic anvil high‐pressure apparatus. Several experimental runs were carried out for both minerals, in which the samples were first compressed at room temperature and then heated; NaCl served as an internal standard for pressure calibration. In most runs the samples were mixed with Vaseline or NaCl to ensure hydrostatic pressure‐transmitting conditions. Some runs, mainly at pressures <4 GPa, were performed without using a pressure medium. A Birch‐Murnaghan equation of state (EOS) was fitted to the experimental data obtained at room temperature (assuming K′ = 4), yielding bulk moduli K (diaspore) = 134.4±1.4 GPa in contrast to 230 GPa obtained from diamond anvil cell measurements without using a pressure medium recently, and K (corundum) = 219.1±3.5 GPa, consistent with previous results based on X‐ray diffraction measurements in conjunction with multi‐anvil high‐pressure techniques. The high‐temperature Birch‐Murnaghan EOS was fitted to the high‐temperature high‐pressure diaspore data, P = 1.5KT[VT,r−7 ‐ VT,r−5][1 ‐ 0.75 (4 ‐ K′T) VT,r−2 ‐ 1], VT,r ≡ (V/VT,0)1/3, KT = K + (∂KT/∂T)P(T ‐ 298.15), VT,0 = 117.84 Å3 exp (∫a+bTdT), resulting in K = 133.6±4.0 Gpa,K′T = 4.1±1.5, (∂KT/∂T)P = −0.017±0.007 Gpa K−1, a = 2.22±0.40 10−5 K−1, and b = 2.23±0.79 10−8 K−2.
Refined kinematics of the eastern California shear zone from GPS observations, 1993–1998
Authors: Miller M. Meghan , Johnson Daniel J. , Dixon Timothy H. , Dokka Roy K.
Published: February 2004

Global Positioning System (GPS) results from networks spanning the Eastern California shear zone and adjacent Sierra Nevada block, occupied annually between 1993 and 1998, constrain plate margin kinematics. We use an elastic block model to relate GPS station velocities to long‐term fault slip rate estimates. The model accounts for elastic strain accumulation on the San Andreas fault, as well as faults of the Eastern California shear zone. South of the Garlock fault, 14 mm/yr of dextral shear is distributed across the Eastern California shear zone. Some of this slip penetrates eastward into the Basin and Range, and a collective budget of 13 mm/yr is observed to the north at the latitude of Owens Lake. Model slip rates for two important faults, the Garlock and Owens Valley faults, significantly misfit geologic estimates. By referencing station velocities to stable North America we observe northward‐increasing deformation east of our regional GPS network. At the latitude of Mojave Desert, however, some of this deformation is ascribed to elastic strain accumulation due to a locked San Andreas fault and thus does not represent additional fault‐related, permanent deformation.
Depth variation of seismic anisotropy and petrology in central European lithosphere: A tectonothermal synthesis from spinel lherzolite
Authors: Christensen N. I. , Medaris L. G. , Wang H. F. , Jelínek E.
Published: February 2004

Spinel lherzolite xenoliths from the Neogene Kozákov volcano in central Europe, yielding temperatures from 680°C to 1065°C and estimated to originate from depths of 32 to 70 km, provide an exceptionally continuous record of the depth variation in seismic and petrological properties of subcontinental lithospheric mantle. Extraction depths of the xenoliths and thermal history and rheological properties of the mantle have been evaluated from a tectonothermal model for basaltic underplating associated with Neogene rifting. The chemical depletion of sub‐Kozákov mantle decreases with depth, the Mg number in olivine decreasing from ∼91.4 to 90.5 and the Cr number in spinel, decreasing from ∼38.9 to 14.7. Texturally, the sampled mantle consists of an equigranular upper layer (32–43 km), an intermediate protogranular layer (43–67 km), and a lower equigranular layer (below 67 km). Olivine petrofabrics show strong axis concentrations, which change with depth from orthorhombic symmetry in the equigranular upper layer to axial symmetry in the lowermost layer. Calculated compressional and shear wave anisotropies, which average 8% and 6%, respectively, show significant depth trends that correlate with variations in depth of olivine fabric strengths and symmetries. Comparisons of the xenolith anisotropies with field observations of Pn anisotropy and SKS shear wave splitting in the region suggest that foliation is horizontal in the upper layer of the lithospheric mantle and vertical in the middle and lower layers. The depth variation in mantle properties and complexity in central Europe is the result of Devonian to Early Carboniferous convergence, continental accretion, and crustal thickening, followed by Late Carboniferous to Permian extension and gravitational collapse and final modification by Neogene rifting, thinning, and magmatic heating.
Superplastic deformation of ice: Experimental observations
Authors: Goldsby D. L. , Kohlstedt D. L.
Published: February 2004

Creep experiments on fine‐grained ice reveal the existence of three creep regimes: (1) a dislocation creep regime, (2) a superplastic flow regime in which grain boundary sliding is an important deformation process, and (3) a basal slip creep regime in which the strain rate is limited by basal slip. Dislocation creep in ice is likely climb‐limited, is characterized by a stress exponent of 4.0, and is independent of grain size. Superplastic flow is characterized by a stress exponent of 1.8 and depends inversely on grain size to the 1.4 power. Basal slip limited creep is characterized by a stress exponent of 2.4 and is independent of grain size. A fourth creep mechanism, diffusional flow, which usually occurs at very low stresses, is inaccessible at practical laboratory strain rates even for our finest grain sizes of ∼3 μm. A constitutive equation based on these experimental results that includes flow laws for these four creep mechanisms is described. This equation is in excellent agreement with published laboratory creep data for coarse‐grained samples at high temperatures. Superplastic flow of ice is the rate‐limiting creep mechanism over a wide range of temperatures and grain sizes at stresses ≲0.1 MPa, conditions which overlap those occurring in glaciers, ice sheets, and icy planetary interiors.
New Mesozoic apparent polar wander path for south China: Tectonic consequences
Authors: Yang Zhenyu , Besse Jean
Published: February 2004

A new paleomagnetic study in the South Sichuan basin (South China Block) has been carried out on sediments of Early Triassic to Middle Jurassic age. Detailed stepwise thermal and alternating field demagnetizations allowed us to isolate two to three components. A low‐temperature component (component A) falls close to the local present Earth field direction. A middle‐temperature component (component B) is found both in limestone and red beds of Early Triassic age, in limestones of Middle Triassic age, and finally in red beds of Jurassic age. The component B directions failed the fold test but show a significant increase of the ks/kg ratio at 60% when a progressive stepwise unfolding is applied. The directions obtained at 60% unfolding are grouped best and yield a pole position at 76.5°N, 174.1°E, A95 = 3.7° that is close to the Late Cretaceous pole of the South China Block (SCB) or the Early Tertiary pole of the Eurasian plate. Characteristic directions are finally obtained (component C) for Early Triassic and Early to Middle Jurassic sediments only, both with reversal and fold tests. They yield pole positions at 44.1°N and 217.2°E (A95 = 4.9°) and 79°N and 227°E (A95 = 4.5°), respectively. The Early Triassic result well fits other data of the same age from the SCB. The revised Late Permian to Cretaceous paleopole data provide a new Mesozoic apparent polar wander path for south China. The successive pole positions from Late Permian to Early Triassic indicate motion of the South China Block as a whole rather than local rotations linked to important strain within the block. The Early to Middle Jurassic pole is statistically indistinguishable from the compiled North China Block (NCB) poles and indicates that final suturation between the two blocks occurred by the end of the Triassic, which is coherent with the age of ultra high‐pressure metamorphism in the Dabie‐Sulu areas of eastern China. The Asian plate including the Mongol‐Central Asian Block, NCB, and SCB, however, was probably separated from Siberia by the Mongol‐Okhotsk ocean during Jurassic time.
Convection current generated prior to rupture in saturated rocks
Authors: Yoshida Shingo
Published: February 2004

Laboratory experiments were performed to study the generation mechanism of electric signals during deformation and rupture of rock, with the focus on the effects of pore water movement. We have developed a triaxial apparatus that is specially designed for this purpose, within the framework of the International Frontier Program on Earthquake Research of the Institute of Physical and Chemical Research (RIKEN). This apparatus can deform a rock specimen that is electrically isolated from the surroundings at a very slow strain rate. Using a cylindrical specimen of intact rock (Inada granite, Makabe granite, Muroto gabbro, and Fujieda sandstone) saturated with distilled water, we carried out triaxial deformation tests under a constant pore pressure at room temperature. During a deformation test we continuously measured the electric current generated in the specimen. In addition, local strains, pore pressure, and pore fluid movement from the intensifier were recorded at a sampling frequency up to 1 kHz. The volume change due to dilatancy was obtained from the average of the local strain measurements at four positions. We found that the convection current flowed before the main fracture, showing good correlation with the dilatancy rate and the water flow rate. The current density of the signal was about 1 mA/m2. This result demonstrated that the electric current is caused by an electrokinetic effect due to the water flow associated with accelerating evolution of dilatancy before the fracture.
The effect of global seismicity on the polar motion of a viscoelastic Earth
Authors: Soldati Gaia , Boschi Lapo , Piersanti Antonio , Spada Giorgio
Published: February 2004

We evaluate the polar drift of a stratified viscoelastic Earth, generated by 20 years of global seismicity (from the Centroid Moment Tensor catalog, 1977–1997). Our estimate is better than that of Soldati and Spada [1999] because it accounts for the effects, previously neglected, of earthquakes at depths larger than 80 km. In fact, we show that deep focus and intermediate‐depth focus seismic events tend to affect the Earth's inertia tensor more significantly than shallow ones; specifically, we find that the mean rate of polar drift (evaluated over a 6‐year time interval immediately following the seismic event) due to a vertical dip‐slip source at the bottom of the asthenosphere (280 km depth in our model) is 5 times larger than that generated by an analogous source, located at the boundary between lithosphere and asthenosphere (80 km depth). We conclude, nevertheless, that global seismic activity is not able to alter significantly the rotational parameters of the Earth: according to our computations, the changes that it induces in the inertia tensor result in an average polar drift that is still significantly smaller than the observed one.
A theoretical model of hotspot volcanism: Control on volcanic spacing and patterns via magma dynamics and lithospheric stresses
Authors: Hieronymus Christoph F. , Bercovici David
Published: February 2004

Many linear island chains are thought to be the result of the steady motion of the lithospheric plates over stationary hotspots. The occurrence of discrete, nearly regularly spaced volcanoes, rather than continuous ridges, are assumed to be caused by the interaction of flexural stresses (due to the volcanic loads) with magma percolation. A parameter study is performed on a simple model that incorporates these effects in addition to dike wall erosion. It is demonstrated that the strength of the erosional feedback determines whether the model generates discrete volcanoes or a continuous ridge. The intervolcanic spacing depends not only on the elastic thickness of the lithosphere but also on the magma pressure at the base of the lithosphere. The size of the eruptive region of the individual volcanoes is controlled by the elastic response of the lithosphere to magma overpressurization. If an initial off‐axis edifice is introduced, the model is able to preserve this asymmetry and produce an alternating series of volcanoes. A small initial perturbation grows over time, resulting in double lines or wider patterns depending on the width of the magma source region. Single lines of volcanoes therefore indicate very narrow magma source regions.
Crustal structure of the northern Reykjanes Ridge and Reykjanes Peninsula, southwest Iceland
Authors: Weir Nicholas R. W. , White Robert S. , Brandsdóttir Bryndís , Einarsson Páll , Shimamura Hideki , Shiobara Hajime
Published: February 2004

Results from the Reykjanes‐Iceland Seismic Experiment (RISE) show that the thickness of zero‐age crust decreases from 21 km in southwest Iceland to 11 km at 62°40′N on the Reykjanes Ridge. This implies a decrease in mantle potential temperature of ∼130°C, with increasing distance from the center of the Iceland mantle plume, along this 250 km transect of the plate boundary. The crust thins off‐axis at 63°N, from 12.7 km thick at 0 Ma to 9.8 km at 5 Ma, most likely due to a ∼40°C change in asthenospheric mantle temperature between these times. This provides evidence for the passage of a pulse of hotter asthenospheric mantle material beneath the present spreading center. A reflective body, the top of which lies at 9–11 km depth, is identified in the lower crust just west of the tip of the Reykjanes Peninsula. Synthetic seismogram modeling of the wide‐angle reflections from this body suggests that it corresponds to a zone of high‐velocity (≥7.5 km s−1), high‐magnesium rocks in the lower crust. The P to S wave velocity ratio beneath the peninsula is 1.78, implying that crustal temperatures are below the solidus. Gravity modeling shows the RISE models to be consistent with the observed gravity field. Mantle densities are lower beneath the ridge axis than beneath older crust, consistent with lithospheric cooling with age.
Stress‐induced anisotropy of partially molten media inferred from experimental deformation of a simple binary system under acoustic monitoring
Authors: Takei Yasuko
Published: February 2004

Microstructural changes of partially molten media under deviatoric stress were investigated in a newly developed apparatus by deforming a large sample (a 70‐mm cube) under a uniform pure shear stress. Borneol + melt system having a moderate dihedral angle and texturally equilibrated under hydrostatic stress was used as a partially molten rock analogue. The applied stress was small enough not to involve cataclastic‐plastic deformation of the solid grains. Shear strain rate was about 10−8 s−1, and a stress exponent indicative of diffusion creep was obtained. During the deformation, sample microstructure was observed in situ by means of ultrasonic shear waves. The development of stress‐induced anisotropy was successfully detected by shear wave splitting. The results obtained indicate that grain boundary contiguity in the direction of the least compressive stress (σ3) was reduced with respect to the equilibrium texture and also that the relative values of liquid pressure and σ3 play an essential role for development of anisotropy. The developed anisotropy persisted as long as deviatoric stress was applied, but the initial isotropic structure was recovered by releasing this stress. Several interesting phenomena were involved in the structural change; these include shear creep‐induced dilatancy, strong dependence of the timescale of structural recovery on the amount of deformation (memory effect), and relaxation creep after releasing stress. Scaling considerations using the Griffith theory shows that the structural changes observed in the present experimental system are expected to occur in the Earth as well.
Evaluation of (Mg,Fe) partitioning between silicate perovskite and magnesiowustite up to 120 GPa and 2300 K
Authors: Andrault Denis
Published: February 2004

The (Mg,Fe) partition coefficients between Al‐(Mg,Fe)SiO3 perovskite (Pv) and (Mg,Fe)O magnesiowustite (Mw) were inferred from Pv and Mw volumes measured by X‐ray diffraction (European Synchrotron Radiation Facility, Grenoble), from 22 to 120 GPa, after laser annealing up to 2300 K. The (Mg,Fe) partition coefficient is found to decrease with increasing pressure and temperature (at moderate pressures) and with the additions of Al2O3. More iron is found in perovskite than in magnesiowustite at the highest pressures and temperatures. Artifacts possibly encountered during the calculation are discussed. Perovskite was found stable up to 120 GPa and 2200 K, with iron contents (Fe/(Mg+Fe)) up to 25%. The effects of Fe and Al2O3 on the orthorhombic distortion remain reduced.
A numerical experiment of crystallization for a binary eutectic system with application to igneous textures
Authors: Toramaru Atsushi
Published: February 2004

Numerical experiments of crystallization in a binary eutectic system were carried out to understand the origin of textural variation in igneous rocks. In this study, the number density of crystals was intensively examined as a parameter of texture. The constant cooling rate experiment (no latent heat release) yields a higher number density of crystals by 1 order of magnitude than the constant heat loss experiment (full latent heat release). Differences in growth mechanism (diffusion‐limited with capillary effect versus reaction‐limited without capillary effect) result in the different dependence of cooling rate on number density of crystals. For diffusion‐limited growth with the capillary effect for spherical crystals the cooling rate dependence of number density of olivine, clinopyroxene, plagioclase, and opaque minerals seems to be consistent with the experimental data with superliquidus initial condition. For plagioclase, experimental data with subliquidus initial temperature and the observational data are not consistent with the present numerical results for both growth processes, suggesting that the plagioclase growth is governed by a different mechanism of nucleation and growth from homogeneous nucleation and both diffusion‐limited growth with capillary effect and simple reaction‐limited growth without capillary effect. Assuming the conductive cooling of a dike, it is predicted that for the diffusion‐limited growth the number density of crystals is proportional to y−3, where y is distance from the cooling boundary. This is in good agreement with some of the observational data by Gray [1978] except for plagioclase. The consistency among numerical, natural, and experimental data highly suggests that at the nucleation stage with superliquidus initial condition, crystals grow by the diffusion‐limited process, taking the spherical shape for olivine, pyroxene, plagioclase, and some oxides. The difference between volcanic and plutonic textures is caused by the difference of cooling rate by 2 orders of magnitude. The variation from intergranular, ophitic to poikilitic textures results from the difference in cooling rate dependence on number density between the plagioclase and the other minerals. The formation of porphyritic texture must satisfy the limited condition: the very small efficiency of latent heat production (nearly constant cooling rate), small values of interfacial energy of the first precipitating phase, and smaller value of interfacial energy of the second phase.
Envelope formation of long‐period Rayleigh waves in vertical component seismograms: Single isotropic scattering model
Authors: Sato Haruo , Nohechi Makiko
Published: February 2004

At long periods on vertical component seismograms, incoherent wave trains appear in addition to direct Rayleigh waves propagating around the spherical Earth. On seismograms of earthquakes with Mw>7, these incoherent wave trains continue for more than a few tens of hours. Scattering by distributed heterogeneities is the most probable origin for the excitation of incoherent wave trains. Here we propose a simple method to synthesize the mean‐square (MS) envelope of long‐period seismogram based on the radiative transfer theory for Rayleigh wave scattering. The Earth's surface can be modeled as a spherical surface on which point‐like isotropic scatterers are randomly distributed. A nonisotropically radiating source is put at the surface. Adding the energy density of surface waves singly scattered by distributed scatterers lying on isochronal curves for a given lapse time, we calculate the energy density corresponding to the MS seismogram envelope at a receiver. Spherical trigonometry is used in the calculation of travel time and geometrical spreading factor. The time trace of the resultant energy density becomes large at arrivals of direct Rayleigh waves propagating around the Earth and is concave between these repeated direct arrivals. These characteristics of the theoretical model qualitatively explain the observed MS envelopes. Applying the above model with the velocity dispersion to seismograms of the 1999 Turkey earthquake (Mw 7.4), we estimated the total scattering coefficient to be about 2×10−6 km−1 at periods between 100 and 200 s, which is 4 orders of magnitude smaller than that of shear waves at frequencies 1∼20 Hz in the lithosphere.
Indirect effect of the atmosphere through the oceans on the Earth nutation using the torque approach
Authors: de Viron Olivier , Ponte Rui M. , Dehant Véronique
Published: February 2004

The atmosphere exerts a direct torque on the Earth, perturbing its rotation. Additionally, the atmosphere exerts pressure, gravitational, and friction forces on the ocean, which induce an indirect effect of the atmosphere on the Earth. The oceanic response consists of variable currents and mass fluxes. These effects change the oceanic torque on the Earth. As the atmosphere has a large diurnal cycle, the indirect torque also presents a large diurnal component. This indirect diurnal torque is investigated using a numerical barotropic ocean model. In particular, we show that a dynamic ocean model is essential to compute the indirect effect of the atmosphere on nutation, as the oceanic response is far from that predicted by a static model. We also point out that the response of the ocean is nevertheless highly correlated with the atmospheric signal for the diurnal timescale but with a large amplification. The amount of ocean bottom friction is found to have a substantial influence on the indirect diurnal torque acting on the Earth. The effect of this indirect torque on the Earth nutation is computed and found to be much larger than the precision of current nutation observations, but results are uncertain in light of discrepancies in the angular momentum and torque balance.
Multiple scales of hydrothermal circulation in Middle Valley, northern Juan de Fuca Ridge: Physical constraints and geologic models
Authors: Stein Joshua S. , Fisher Andrew T.
Published: February 2004

A synthesis of data including seafloor heat flow, water column thermal anomalies, shallow pore pressures, pore water chemical profiles, paired borehole observations, and 3.5‐kHz records help to characterize distinct hydrothermal circulation systems within and surrounding two active vent areas in Middle Valley, a sedimented rift on northern Juan de Fuca Ridge. We estimate the total heat and fluid budgets for a 260 km2 area encompassing 17 km of ridge to be 274 MW (16 MW km−1 of ridge) and 0.157 m3 s−1, respectively. About half of the seafloor heat loss is conductive, requiring efficient lateral fluid and heat transport in basement. Lateral head gradients (fluid driving forces) in basement between the primary fluid discharge site and the surrounding formation are very low. Simple mass balance and flow calculations indicate that the majority of the recharge to the primary hydrothermal reservoir cannot occur through sediments immediately around the vent area; recharge is probably focused along valley‐bounding normal faults. Crustal‐scale lateral permeability in basement must be 10−10 to 10−12 m2 in order to allow rapid flow and maintain nearly isothermal temperatures over many kilometers. On a smaller scale, heat flow patterns within the main vent area are controlled by the depth to the primary hydrothermal reservoir, proximity to active vents, and local recharge of a secondary circulation system. Shallow underpressures and isolated zones of very low heat flow result from vigorous secondary circulation driven by pressure gradients associated with venting. Models of secondary circulation suggest that most of the overpressure observed in hydrothermal basement is lost when the fluid enters the vents and that cooler sediment pore fluids flow toward and into the vents as a result of shallow underpressuring. Secondary circulation results in significant heat flow reduction at the seafloor but adds minor sediment pore fluid to vent discharge.
Recurrence rates of large earthquakes in the South Carolina Coastal Plain based on paleoliquefaction data
Authors: Talwani Pradeep , Schaeffer William T.
Published: February 2004

We present a reanalysis of results of 15 years of paleoliquefaction investigations in the South Carolina Coastal Plain. All earlier radiocarbon age data and locations of organic material collected by various investigators were reviewed and recalibrated to obtain a uniform data set. The calibrated dates and the spatial extent of the sandblows having similar dates were used to estimate ages and magnitudes of prehistoric earthquake episodes. The results of this analysis suggest seven episodes (episodes A‐G) of prehistoric liquefaction in the past 6000 years and two possible scenarios for their occurrence. In the first scenario, three seismic sources exist within the Coastal Plain of South Carolina; at Charleston (A, B, E, and G) with magnitudesM7+, Georgetown (C and F), and Bluffton (D) with magnitudesM∼6. In the second scenario, episodes C and D are combined into one episode, episode C′. In this scenario all earthquakes occurred at Charleston and withM7+. Episodes A and B seem to be more representative of the earthquake cycle and suggest a recurrence time of 500–600 years forM7+ earthquakes at Charleston. The recurrence times and magnitudes for episodes C and D are estimated at ≥2000 years and ∼6.0, respectively. The older episodes are less frequent, a fact that may be attributable to times of low ground water table. Before ∼6000 years B.P., the ground water table was too low to permit observable liquefaction features to develop at the surface.
Toward an understanding of the multicomponent magnetization of uplifted Neogene marine sediments in New Zealand
Authors: Turner Gillian M.
Published: February 2004

New Zealand contains many sequences of technically uplifted Miocene/Pliocene marine sediments, which have been central to the development of a biostratigraphic framework for temperate southern latitudes. Chronological control is almost invariably dependent on magnetostratigraphy. Interpretation of the natural remanent magnetization (NRM) of these sediments is, however, notoriously difficult because of the weak magnetization, secondary components that resist demagnetization, and thermal instability of the clay minerals. A new paleomagnetic study of Wanganui Basin sediments has yielded unusually well defined demagnetization data from which systematic characterization, explanation, and interpretation of the multicomponent nature of the NRM have been possible. In general, the magnetization consists of three components: (1) a low (<150°C) blocking temperature (Tb) component that is interpreted to be a recent thermoviscous component, usually close to the present day field in direction; (2) an intermediate (150–250°C) Tb component, which is thought to be all that remains of the primary detrital magnetization; and (3) a high (>250°C) Tb component that is considered to be diagenetic in origin. This interpretation of the intermediate and high Tb components is supported by rock magnetic data which show that specimens carrying strong high Tb components have a distinctly different spectrum of ferrimagnetic grains, specifically a population with much higher coercivities than is seen in specimens which lack a high Tb component. It is argued that these high‐coercivity, high Tb grains are secondary in origin, leading to the conclusion that the intermediate Tb component is the primary detrital magnetization. Consistency of the paleomagnetic data within and between sites also supports this conclusion. It is suggested that failure to recognize these three components has, in some cases in the past, led to misidentification of the primary magnetization. Using this hypothesis of the multi‐component magnetization, the Turakina River data of McGuire [1989] have been reinterpreted, yielding a high‐resolution magnetostratigraphy from the upper Gilbert, Chron 3n, (circa 4 Ma) to the lower Matuyama, Chron 2r. The correlation of this magnetostratigraphy with the geomagnetic polarity timescale, is supported by biostratigraphic constraints.
A deterministic methodology for prediction of fracture distribution in basaltic multiflows
Authors: Lore Jason , Aydin Atilla , Goodson Kenneth
Published: February 2004

The fracture distribution in basalt flows is a direct result of thermal processes. Thus basalt flows present a unique opportunity to characterize a nearly perfect deterministic system with its fundamental physical parameters. Fracture distribution data collected on cliff exposures of basalt flows near the Idaho National Engineering and Environmental Laboratory (INEEL) are combined with calculations of cooling rate and temperature distribution from a finite element model to construct a predictive methodology for fracture spacing. The methodology is based on an empirical power law relationship between inverse cooling rate and fracture spacing. The methodology may be applied to unexposed basalt flows of approximately elliptical cross section whose thickness and width are constrained only by geophysical or borehole data if sufficient fracture data on nearby exposed flows are available. The methodology aids waste remediation efforts at sites involving contaminant transport through fractured basalt, such as the INEEL and the Hanford site in Washington, as well as involving transport and fluid flow through volcanic or intrusive rocks where thermal processes are responsible for fracturing.
Thorium‐uranium systematics require layered mantle convection
Authors: Turcotte D. L. , Paul D. , White W. M.
Published: February 2004

Two of the most important sources of information on the style of mantle convection come from geochemical constraints and mantle tomography. Constraints imposed by uranium‐thorium systematics are examined in this paper. Observations require that the mid‐ocean ridge basalt (MORB) source region be nearly homogeneous with a thorium‐uranium ratio κ = 2.5±0.1. Simple mass balance considerations utilizing limits on the value of κ for the continental crust (κ<6) and a bulk earth value κ = 4.0±0.2 with heat production constraints show that the whole mantle has a thorium‐uranium ratio greater than κ = 3.35±0.1. This leads to the conclusion that a substantial fraction of the mantle has a much more primitive value of κ (closer to 4.0) than the MORB source region. Our model assumes a near‐uniform, depleted upper mantle that is the source region for MORB and has κ ≈ 2.5 and a near‐primitive lower mantle with κ ≈ 4.0. Our results indicate that it is very difficult to have the upper mantle reservoir larger than 65% of the whole mantle and our preferred value is 45%. Three alternative hypotheses can be proposed: (1) A vertical stratification of κ through the mantle (since the role of whole mantle convection is to extract heat, the resulting vertical transport in the mantle would be expected to eliminate the vertical stratification of κ), (2) a mantle permeated with blobs (since the blobs would have to constitute some 65±10% of the mantle and be absent from the MORB source region, it is difficult to support this hypothesis), and (3) layered mantle convection with a compositional barrier to convection in the lower mantle. The results given here strongly favor this layered mantle hypothesis as recently set forth by Kellogg et al. [1999] and van der Hilst and Karason [1999].
Intraplate deformation in western Europe deduced from an analysis of the International Terrestrial Reference Frame 1997 (ITRF97) velocity field
Authors: Nocquet J.‐M. , Calais E. , Altamimi Z. , Sillard P. , Boucher C.
Published: February 2004

Although tectonic deformation in western Europe is essentially concentrated in the Appenines and Alpine (Alps, Pyrenees) mountain ranges, several large historical and instrumentally recorded earthquakes (M>6) are known in the supposedly “stable” part of Europe. Because of its accuracy and internal consistency at a global scale, the International Terrestrial Reference Frame 1997 (ITRF97) velocity field allows testing of intraplate rigidity in western Europe at a millimeter per year level. Using the full statistical information available on the ITRF97 velocities, we identify a subset of sites located in central Europe that satisfies a rigid cap rotation with residual velocities <1 mm/yr and therefore provide a stable Europe reference frame (SERF). In this reference frame we find residual velocities at European ITRF sites that are consistent with known active tectonic features. We identify a northward motion at sites located in Italy, with internal deformation of the Adriatic block rather than rigid plate motion and a westward motion of the westernmost part of Europe of the order of 1–2 mm/yr relative to central Europe. The relative motion of the Adriatic block and western Europe agrees with the current extension known in the Tyrrhenian sea and the Apennines. In central Europe, we find active deformation <1 mm/yr in the eastern Alps and western Carpathians. In the Alpine range our results indicate E‐W extension across the western Alps and N‐S compression across the central and eastern Alps, in agreement with the strain regime deduced from seismotectonic observations. In Belgium and the Netherlands we find residual velocities of 1–1.5 mm/yr to the northwest at all the sites, most likely accommodated along the Upper and Lower Rhine Graben structures. An important outcome of this study is the identification of internal deformation of the order of 1–2 mm/yr in an area usually interpreted as “stable” Europe. This result should be further checked as new, denser, and more accurate space geodetic data sets with longer observation time spans, become available for Europe.
Comparison of the microfracture localization in granite between fracturation and slip of a preexisting macroscopic healed joint by acoustic emission measurements
Authors: Jouinaux Laurence , Masuda Koji , Lei Xinglin , Nishizawa Osamu , Kusunose Kinichiro , Liu Liqiang , Ma Wentao
Published: February 2004

Experiments of fracturation and slip of a preexisting macroscopic healed joint have been performed under triaxial deformation on granite from Mayet de Montagne (France). This granite shows high grain‐scale inhomogeneity. Acoustic emissions have been recorded and hypocenters have been determined during the entire experiments. For both rupture experiment and slip experiment, precursory localization of microfractures in the final rupture plane has been observed in the early stage of deformation, well before the dilatancy. It is likely that not only initial closure of favorably oriented cracks but also breaking of partially cemented grains or slipping between grains may occur in the pseudoelastic phase and are already localized on the final rupture plane where the shear stress seems to be concentrate. This behavior is observed in both cases where stress heterogeneity and rupture nucleation are controlled by (1) medium‐scale heterogeneity at the grain scale (HS sample) or (2) macroscopic heterogeneity in the form of a preexisting healed joint (JS sample). The sample with the healed joint exhibited ∼1.6 times more acoustic emission events than the intact sample. The presence of the healed joint significantly weakened the sample.
Prehistoric fault offsets of the Hilina Fault System, south flank of Kilauea Volcano, Hawaii
Authors: Cannon Eric C. , Bürgmann Roland
Published: February 2004

Historical accounts of earthquakes on the Island of Hawaii date only to 1823 but lava flows as old as 1500–3000 years B.P. contain fault offsets from prehistoric earthquakes. The M7.2 1975 Kalapana earthquake produced over 25 km of fault rupture along the Hilina fault system. We compare fault offsets in prehistoric lava flows with Kalapana earthquake fault offsets in neighboring 1969–1974 Mauna Ulu lava flows to estimate the frequency of prehistoric major earthquakes on the Hilina fault system. Horizontal and vertical fault offset rates across the Hilina fault system are 4.0 to 12.0 and −2.0 to −20.0 mm/yr, respectively, based on lava flows <750 years B.P. age. Assuming prehistoric earthquakes produced similar fault offsets compared to the Kalapana earthquake, three to five events are recorded in 400–750 years B.P. old lava flows yielding recurrence intervals of 260–80 years. Vertical fault offsets in prehistoric lava flows suggest that hanging wall rotation of the Hilina fault system contributes to fault offset. The assumption that past faulting resulted from prehistoric earthquakes with similar magnitude and fault offset to the Kalapana earthquake is an oversimplification. Large south flank earthquakes most likely do not have uniform recurrence intervals.
Limited syntectonic fluid flow in carbonate‐hosted thrust faults of the Front Ranges, Canadian Rockies, inferred from stable isotope data and structures
Authors: Kirschner David L. , Kennedy Lori A.
Published: February 2004

Samples collected from five carbonate‐hosted thrust faults in the Front Ranges (Canadian Rockies) were isotopically and structurally analyzed in order to document syntectonic fluid‐rock interaction. The most deformed rocks are adjacent to the thrust contacts, corresponding to a general increase in (micro)fractures, twins, foliation development, crystallographic preferred orientation fabric, and veins toward the thrusts. Subhorizontal veins that accommodated subvertical extension are present locally in the thrusts and help to define the foliated fabric of the fault rocks. The δ18O and δ13C values of fault rocks meter(s) away from the thrusts are typical of Paleozoic carbonates, while many rocks adjacent to the thrusts have lower values. There are not strong correlations between these values and the type and/or density of structures in the thrusts. Far from the thrusts, most syntectonic veins have isotopic values similar to their host rocks. Only within decimeters to a meter of the thrusts, do some veins have lower values. There is little isotopic evidence for significant fluid flow across the faults between hanging wall and footwall rocks. The structural data are consistent with the fault rocks having deforming by a variety of deformation mechanisms that probably resulted in both crystal plastic and brittle behavior. Transient high pore fluid pressure in the faults must have occurred locally and episodically in order to account for the formation of the subhorizontal veins. The isotopic data are consistent with a limited amount of fluid contained in very narrow zones adjacent to the thrusts. If true, this fluid must have been maintained in a semi‐closed system and not continuously replenished by large quantities of fluids from the thrusts' footwalls or from the hinterland. The long‐term rheology of the fault rocks in the Front Ranges was not necessarily dominated by pressure‐dependent frictional sliding, and the role of high pore fluid pressure in thrust movement might have been less important than is widely believed.
Crustal structure of the outer Vøring Plateau, offshore Norway, from ocean bottom seismic and gravity data
Authors: Mjelde Rolf , Digranes Per , Van Schaack Mark , Shimamura Hideki , Shiobara Hajime , Kodaira Shuichi , Naess Ole , Sørenes Nils , Vågnes Erling
Published: February 2004

Four ocean bottom seismograph (OBS) profiles acquired across the continent‐ocean transition zone of the outer Vøring Plateau have been modeled by inversion and forward modeling of the OBS vertical component traces (P waves). The thickness of the sedimentary layers deposited since the early Eocene continental breakup varies from up to 2.2 km above the basaltic inner flows (IF) to a minimum of 0.2 km about 20 km seaward of the Vøring Escarpment (VE), with a velocity of 1.6–2.55 km/s. The thickness of the uppermost layer of basalt varies from 0.5 to 4.5 km, and its velocity ranges from 3.4 to 5 km/s. The modeling of the two dip profiles suggests that strongly intruded preopening sedimentary rocks extend ∼40 km seaward of the VE. Farther seaward, another layer of extrusives/intrusives is found beneath the uppermost layer of basalt. The lateral variations in velocity in the main crustal layer along the dip profiles indicate that the continent/ocean transition extends over a 30–50 km wide zone in between (strike) profile 3 (oceanic; 6.6–7.35 km/s) and (strike) profile 4 (continental; 6.1–7.0 km/s). An up to 7 km thick lower crustal high‐velocity body (7.1–7.4 km/s) is interpreted in terms of intrusions in the lower crust beneath and landward of the continent/ocean transition. The total thickness of the normal oceanic crust is estimated to be 7–9 km, the anomalously thick oceanic crust reaches a maximum thickness of about 25 km, and the crustal thickness is estimated to be 17–22 km beneath the IF. Gravity modeling of the profiles confirmed all main aspects of the velocity models and provided constraints on the crustal structures toward the ends of the profiles where the ray coverage was low.
Heat and fluid flow in contact metamorphic aureoles with layered and transient permeability, with application to the Notch Peak aureole, Utah
Authors: Cui Xiaojun , Nabelek Peter I. , Liu Mian
Published: February 2004

We have investigated the control of layered and transient permeability structures on fluid flow and thermal evolution in contact metamorphic aureoles using two‐dimensional numerical modeling with petrological and geochemical constraints from the Notch Peak aureole, Utah. The model includes interbedded aquitard and aquifer lithologies on the scale of formations and transient enhancement of permeability due to devolatilization reactions. The results show that a layered permeability structure causes focusing of fluids into aquifer layers, resulting in flow pattern that is strongly time‐dependent and significantly different from the predicted simple convection cells in aureoles with homogeneous permeability. The model predicts temporal development of three distinctive flow regimes: (1) early radial down‐temperature flow due to release of magmatic fluid, (2) small flat convection cells near the pluton‐aquifer contacts during peak metamorphism, (3) late unidirectional down‐temperature flow in the upper aureole and up‐temperature flow in the middle to lower aureole. When the bulk permeability of host rocks is >10−16 m2, heat advection by flowing fluids is significant. At lower bulk permeability, heat conduction dominates, and the effects of detailed permeability structure on the thermal field become negligible. Metamorphic reactions enhance permeability within the inner aureole, localizing fluid convection and increasing fluid flux. The observed spatial distribution of mineral assemblages and oxygen isotopic shifts in the upper part of the Notch Peak aureole is consistent with the predicted time‐integrated fluid flux, which over duration of metamorphism is dominated by the down‐temperature flow of a large amount of water from the pluton into the wall rocks.