bluuebunny/crossref_metadata_2025 huggingface dataset:
Submarine hydrogeology of the Hawaiian archipelagic apron: 2. Numerical simulations of coupled heat transport and fluid flow
Authors: Harris Robert N. , Garven Grant , Georgen Jennifer , McNutt Marcia K. , Christiansen Lizet , Von Herzen Richard P.
Published: February 2004

We perform numerical simulations of buoyancy‐driven, pore fluid flow in the Hawaiian archipelagic apron and underlying oceanic crust in order to determine the extent to which heat redistributed by such flow might cause conductive heat flow measurements to underrepresent the true mantle heat flux. We also seek an understanding of undulations observed in finely spaced heat flow measurements acquired north of Oahu and Maro Reef with wavelengths of 10 to 100 km and amplitudes of 2 to 7 mW m−2. We find that pore fluid flow can impart significant perturbations to seafloor heat flow from the value expected assuming a constant mantle flux. In the simplest scenario, moat‐wide circulation driven by bathymetric relief associated with the volcanic edifice recharges a fluid system over the moat and discharges the geothermally heated water through the volcanic edifice. The existing heat flow data are unable to confirm the existence of such a flow regime, in that it produces prominent heat flow anomalies only on the steep flanks of the volcano where heat flow probes cannot penetrate. However, this flow system does not significantly mask the mantle flux for reasonable permeabilities and flow rates. Another numerical simulation in which the upper oceanic basement acts as a aquifer for a flow loop recharged at basement outcrops on the flexural arch and discharged within a permeable volcanic edifice is capable of almost uniformly depressing conductive heat flow across the entire moat by ∼15%. Large heat flow anomalies (>20 mW m−2) are located over the recharge and discharge zones but are beyond the area sampled by our data. Presumably finely spaced heat flow measurements over the flexural arch could test for the existence of the predicted recharge zone. We demonstrate that the prominent, shorter‐wave undulations in heat flow across the Oahu and Maro Reef moats are too large to be explained solely by relief in the upper oceanic basement. More likely, shallower large‐scale turbidites or debris flows also serve as aquifers within the less permeable moat sediments. With our limited information on the structural geology of the moat, permeability structure of its major geologic units, and their variations in the third dimension, we are not able to exactly match the spatial distribution of heat flow anomalies in our data, but spectral comparisons look promising.
Teleseismic P wave delays and modes of shortening the mantle lithosphere beneath South Island, New Zealand
Authors: Stern Tim , Molnar Peter , Okaya David , Eberhart‐Phillips Donna
Published: February 2004

A high‐speed zone in the mantle directly beneath the Southern Alps of New Zealand is required by the recorded pattern of teleseismic P waves. Two parallel lines of 80 seismographs spaced at ∼2 km intervals recorded three earthquakes from the western Pacific with epicentral distances of 52°, 53° and 78°. Azimuthal bearings were all within 15 degrees of the mean trends of the seismograph lines. Differences between measured delays and those predicted from the crustal structure reach 0.8 s along one line and 1.0 s along the other, with the rays for the earliest arriving signals passing the depth of ∼120 km beneath the center of the island. Assuming these early arrivals are due to structure within the mantle shallower than 200 km, they imply that the core of the high‐speed zone lies beneath the thickest crust, which has been shortened by ∼100 km of convergence during the past 6–7 Myr. Although the shape and position of the high‐speed body cannot be fixed uniquely, a roughly symmetric body centered about a depth of 120 km, 80–100 km wide, with a depth extent of 100 km and with a maximum speed advance of ∼7% satisfies the observations. The pattern of residuals does not fit with those predicted by simple models of intracontinental subduction in which crust and mantle lithosphere are detached and one slab of mantle lithosphere underthrusts the other. Rather, the residuals favor thickening of mantle lithosphere by a more homogenous straining of it, as if mantle lithosphere beneath continental crust behaved as a continuum. An excess mass in the mantle is also required by the observed gravity anomalies, once allowance is made for the seismically determined crustal thickness. This high‐density mantle anomaly provides sufficient force (per unit length) to maintain the crustal root, which is approximately twice as thick as that necessary to support the topography.
Paleoposition of the northern margin of Armorica in Late Devonian times: Paleomagnetic and rock magnetic results from the Frankenstein Intrusive Complex (Mid‐German Crystalline Rise)
Authors: Zwing Alexander , Bachtadse Valerian
Published: February 2004

A detailed rockmagnetic and paleomagnetic study of the Frankenstein Intrusive Complex (FIC, 363±7 Ma in age), southwestern Germany, has been carried out in order to test whether the paleomagnetic declinations within the internal parts of the European Variscides follow a common trend controlled by the structural grain of the orogen or whether the distribution of declinations follows a random pattern. In addition, new paleomagnetic data will contribute to further refine the drift history of the Armorican Terrane Assemblage during late Paleozoic times. The unmetamorphosed Frankenstein Igneous Complex forms part of the Mid‐German Crystalline Rise within the Armorican Terrane Assemblage, amalgamated to Avalonia and Baltica during the Variscan orogeny, Stepwise thermal and alternating field (AF) demagnetization experiments identify four components (A, B, Cn, and Cr) of magnetization. Component A is a recent viscous overprint that parallels the present‐day geomagnetic field. The high unblocking temperatures of A in some samples can be explained by the presence of multidomain magnetite. Component B is carried by hematite and is considered to be a late Carboniferous remagnetization. Components Cr and Cn, found in 110 samples from 20 sites, are antiparallel and pass a class C reversal test. Their primary origin is further supported by a contact test. Cr and Cn have maximum unblocking temperatures of up to 580°C and occasionally above over 600°C, indicating magnetite and hematite to carry the characteristic remanent magnetization. Ore microscopy revealed that primary hematite lamellae in ilmenite carry Cr and Cn and that a secondary generation of hematite is the carrier of B. The overall site mean direction of Cr and Cn (20 sites) based on endpoints and remagnetization circle analysis of 198°/40° (declination/inclination) with a 95% confidence limit (α95) of 4.9° and a precision parameter (k) of 45.0 defines a paleopole at 15°S, 9°W. When comparing to the apparent polar wander path for stable Europe this result indicates up to 27° counterclockwise rotation of the FIC region. This is significantly less than reported previously for the Odenwald, Spessart, Black Forest, and the Vosges Massifs of the central European Variscides. Furthermore, the resulting paleolatitude indicates significant separation between Baltica and this sector of the Armorican Terrane Assemblage in the latest Devonian.
Petrogenetic insights provided by compositional transects across the Central American arc: Southeastern Guatemala and Honduras
Authors: Walker J. A. , Patino L. C. , Cameron B. I. , Carr M. J.
Published: February 2004

Two detailed cross‐arc transects of the Central American subduction zone have been completed across southeastern Guatemala and Honduras. The two transects display both contrasting and similar cross‐arc geochemical variations. Across southeastern Guatemala, ratios of fluid mobile to relatively fluid immobile incompatible elements, such as Cs/La and Ba/Nb, are typically high at the volcanic front, low behind the front, and, most importantly, unchanging once behind the front. Across Honduras, by contrast, such incompatible element ratios often systematically decline from the elevated values characterizing the volcanic front. Across both transects, many incompatible element contents and ratios of highly incompatible to moderately incompatible elements, such as the La/Yb ratio, are higher behind the front. The similarities between the two transects may be caused by (1) declining total slab inputs away from the volcanic front, (2) more contributions from subducting carbonate sediments behind the volcanic front, (3) declining degrees of melting away from the volcanic front, (4) more decompression‐induced melting away from the volcanic front, and (5) more melting of enriched domains in the mantle wedge away from the volcanic front. The distinctions between the two transects are attributed to varying proximity to the North American‐Caribbean transform boundary. Motions along this boundary have caused greater extension close behind the front in southeastern Guatemala, serving to abruptly cut off or reduce chemical transfer from the subducting Cocos plate. Behind the front in Honduras, by contrast, the chemical umbilical to the Cocos plate may be only progressively cut or, alternatively, may also be abruptly cut, but at a much greater distance from the volcanic front. In both transects, however, certain ratios of fluid mobile to relatively fluid immobile incompatible elements, such as Ba/La and Pb/Ce, remain considerably above mid‐ocean ridge basalt or oceanic island basalt values some 100 km behind the front. This common slab signature may represent “fossil” slab modification of the Central American mantle wedge.
Influence of water on plastic deformation of olivine aggregates: 2. Dislocation creep regime
Authors: Mei S. , Kohlstedt D. L.
Published: February 2004

Triaxial compressive creep experiments have been conducted over a range of hydrous conditions to investigate the effect of water fugacity on the creep behavior of olivine aggregates in the dislocation creep regime. Samples synthesized from powders of San Carlos olivine were deformed at confining pressures of 100 to 450 MPa and temperatures between 1473 and 1573 K. Water was supplied by the dehydration of talc. Water fugacities of ∼80 to ∼520 MPa were obtained by varying the confining pressure under water‐saturated conditions with the oxygen fugacity buffered at Ni/NiO. Sancles were deformed at differential stresses of ∼20 to 230 MPa. The transition from diffusion creep to dislocation creep occurs near 100 MPa for both the hydrous case and the anhydrous case. Under hydrous conditions creep experiments yield a stress exponent of n ≈ 3 and an activation energy of Q ≈ 470 kJ/mol. The creep rate of olivine is enhanced significantly with the presence of water. At a water fugacity of ∼300 MPa, samples crept ∼5–6 times faster than those deformed under anhydrous conditions at similar differential stresses and temperatures. Within the range of water fugacity investigated, the strain rate is proportional to water fugacity to the 0.69 to 1.25 power, assuming values for the activation volume of 0 to 38×10−6 m3/mol, respectively. We argue that water influences creep rate primarily through its effect on the concentrations of intrinsic point defects and hence on ionic diffusion and dislocation climb. With increasing water fugacity the charge neutrality condition changes from [FeMe•] = 2[VMe″] to [FeMe•] = [HMe′]. For the latter charge neutrality condition the concentration of silicon interstitials is proportional to
, suggesting that under hydrous conditions dislocation climb is rate limited by diffusion of Si occurring by an interstitial mechanism. Our experimentally determined constitutive equation permits extrapolation from laboratory to mantle conditions in order to assess the rheological behavior of regions of the upper mantle with different water contents, such as beneath a mid‐ocean ridge and in the mantle wedge above a subducting slab.
Air pressure and cosmogenic isotope production
Authors: Stone John O.
Published: February 2004

The cosmic ray flux increases at higher altitude as air pressure and the shielding effect of the atmosphere decrease. Altitude‐dependent scaling factors are required to compensate for this effect in calculating cosmic ray exposure ages. Scaling factors in current use assume a uniform relationship between altitude and atmospheric pressure over the Earth's surface. This masks regional differences in mean annual pressure and spatial variation in cosmogenic isotope production rates. Outside Antarctica, air pressures over land depart from the standard atmosphere by ±4.4 hPa (1σ) near sea level, corresponding to offsets of ±3–4% in isotope production rates. Greater offsets occur in regions of persistent high and low pressure such as Siberia and Iceland, where conventional scaling factors predict production rates in error by ±10%. The largest deviations occur over Antarctica where ground level pressures are 20–40 hPa lower than the standard atmosphere at all altitudes. Isotope production rates in Antarctica are therefore 25–30% higher than values calculated by scaling Northern Hemisphere production rates with conventional scaling factors. Exposure ages of old Antarctic surfaces, especially those based on cosmogenic radionuclides at levels close to saturation, may be millions of years younger than published estimates.
Dynamic elevation of the Cordillera, western United States
Authors: Lowry Anthony R. , Ribe Neil M. , Smith Robert B.
Published: February 2004

We introduce a methodology that synthesizes topography, gravity, crustal‐scale seismic refraction velocity, and surface heat flow data sets to estimate dynamic elevation, i.e., the topography deriving from buoyancy variations beneath the lithosphere. The geophysical data independently constrain the topographic effects of surface processes, crustal buoyancy, and thermal boundary layer thickness. Each of these are subtracted from raw elevation of the western U.S. Cordillera to reveal dynamic elevation that can exceed 2 km and is significant at >95% confidence. The largest (∼1000 km diameter) of the dynamic elevation anomalies resembles a numerical model of a hypothetical Yellowstone hotspot swell, but the swell model does not account for all of the significant features seen in the dynamic elevation map. Other dynamic elevation anomalies are spatially correlative with Quaternary volcanism, but partial melt can contribute no more than a few hundred meters of elevation. Hence much of the dynamic elevation likely derives from other thermodynamic anomalies. Possible alternative mechanisms include both superadiabatic upwelling and adiabatic phase boundary deflections maintained by latent heat effects. Comparison of seismicity and volcanism to effective viscosity gradients, estimated from lithospheric flexural rigidity to facilitate the numerical swell model, suggests that tectonism focuses where lithosphere with negligible mantle viscosity abuts lithosphere with significant uppermost mantle viscosity.
Constraints on cooling and degassing of pumice during Plinian volcanic eruptions based on model calculations
Authors: Hort M. , Gardner J.
Published: February 2004

During explosive volcanic eruptions, pumice clasts are transported into the atmosphere, and their thermal and degassing histories determine how much volatiles are lost syneruptively. This process is studied by combining a steady state eruption column model with a model for the cooling and degassing of pumice. In the model we investigate the impact of various parameters (e.g., mass eruption rate, Biot number, eruption temperature, and geometry) on the cooling and degassing of pumice and find that the Biot number and the eruption temperature are the most influential. During typical risetimes of pumices inside eruption columns (200–300 s), those smaller than 0.5 cm in diameter are found to lose little of their volatiles syneruptively and remain in thermal equilibrium with the plume. In the case of larger pumices the ratio of the cooling timescale (∝ rp2/k, where rp is the pumice radius and k is the thermal diffusivity) to the degassing timescale (∝ ξw2/D0′ where ξω is the vesicle wall thickness and D0′ is the species diffusivity) controls degassing. If this ratio is larger than 50, degassing of the pumice will be nearly complete with a few percent of the original volatiles left in the outer rind of the pumice. For a ratio of less than 0.1 no volatiles escape the pumice. Typical values for thermal and species diffusivity as well as vesicle wall thickness indicate that in the case of degassing Cl and H2O the transition from 0.1 to 50 will be in the pumice size range of 1–10 cm. Here volatile concentration is a strong function of position inside the pumice, suggesting that values for volatile contents measured on picked matrix glass from crushed samples may not be the best way to estimate the volatile content of matrix glass, a number which is frequently used to estimate the total volatile input of volcanic eruptions. This effect is even more pronounced when looking at hydrogen isotopic fractionation with larger pumices tending toward δD < −120‰ and small samples giving δD > −80‰.
Shear wave speeds at the base of the mantle
Authors: Castle John C. , Creager Kenneth C. , Winchester John P. , van der Hilst Rob D.
Published: February 2004

We inverted 4864 ScS‐S and 1671 Sdiff‐SKS residual travel times for shear wave speed anomalies at the base of the Earth's mantle. We applied ellipticity corrections, accounted for mantle structure outside of the basal layer using mantle tomography models, and used finite size sensitivity kernels. The basal layer thickness was set to 290 km; however, the data allow thicknesses between 200 and 500 km. The residuals were inverted using a spherical harmonic basis set of degree 30 for a model that both is smooth and has a small Euclidean norm, which limited spectral leakage of higher‐order structures into low‐order wavelengths. Hotspots dominantly overlay slow wave speed regions. Nonsightings of ultralow‐velocity zones (ULVZs) most frequently appear in fast regions, suggesting that slow regions at the base of the mantle are associated with ULVZs. However, ULVZ sightings appear in both slow and fast regions. Recently active subduction zones do not correlate with velocity anomalies; however, the locations of subduction zones active prior to 90 Ma correlate extremely well with fast anomalies, implying that slabs descend as fast as 2 cm yr−1 through the lower mantle. The correlation continues through the historical subduction record to 180 Ma, suggesting that slabs remain in the deep mantle at least 90 Myr. Fast anomalies reach +2%, while slow anomalies extend to −5%. If we assume that the anomalies are thermal and anharmonic in origin and apply a wave speed/thermal anomaly conversion, the temperature deviations would be over −500°K (cold) in fastest regions and over +1000°K (hot) in the slowest regions, which would initiate plumes much hotter than those observed at the surface. Alternative explanations for the large anomalies are widespread partial melt or compositional differences in the lowermost mantle.
Artifactual log‐periodicity in finite size data: Relevance for earthquake aftershocks
Authors: Huang Y. , Johansen A. , Lee M. W. , Saleur H. , Sornette D.
Published: February 2004

The recently proposed discrete scale invariance and its associated log‐periodicity are an elaboration of the concept of scale invariance in which the system is scale invariant only under powers of specific values of the magnification factor. We report on the discovery of a novel mechanism for such log‐periodicity relying solely on the manipulation of data. This “synthetic” scenario for log‐periodicity relies on two steps: (1) the fact that approximately logarithmic sampling in time corresponds to uniform sampling in the logarithm of time; and (2) a low‐pass‐filtering step, as occurs in constructing cumulative functions, in maximum likelihood estimations, and in de‐trending, reddens the noise and, in a finite sample, creates a maximum in the spectrum leading to a most probable frequency in the logarithm of time. We explore in detail this mechanism and present extensive numerical simulations. We use this insight to analyze the 27 best aftershock sequences studied byKisslinger and Jones[1991] to search for traces of genuine log‐periodic corrections to Omori's law, which states that the earthquake rate decays approximately as the inverse of the time since the last main shock. The observed log‐periodicity is shown to almost entirely result from the “synthetic scenario” owing to the data analysis. From a statistical point of view, resolving the issue of the possible existence of log‐periodicity in aftershocks will be very difficult as Omori's law describes a point process with a uniform sampling in the logarithm of the time. By construction, strong log‐periodic fluctuations are thus created by this logarithmic sampling.
Ground deformation associated with the March 1996 earthquake swarm at Akutan volcano, Alaska, revealed by satellite radar interferometry
Authors: Lu Zhong , Wicks Charles , Power John A. , Dzurisin Daniel
Published: February 2004

In March 1996 an intense swarm of volcano‐tectonic earthquakes (∼3000 felt by local residents, Mmax = 5.1, cumulative moment of 2.7×1018 N m) beneath Akutan Island in the Aleutian volcanic arc, Alaska, produced extensive ground cracks but no eruption of Akutan volcano. Synthetic aperture radar interferograms that span the time of the swarm reveal complex island‐wide deformation: the western part of the island including Akutan volcano moved upward, while the eastern part moved downward. The axis of the deformation approximately aligns with new ground cracks on the western part of the island and with Holocene normal faults that were reactivated during the swarm on the eastern part of the island. The axis is also roughly parallel to the direction of greatest compressional stress in the region. No ground movements greater than 2.83 cm were observed outside the volcano's summit caldera for periods of 4 years before or 2 years after the swarm. We modeled the deformation primarily as the emplacement of a shallow, east–west trending, north dipping dike plus inflation of a deep, Mogi‐type magma body beneath the volcano. The pattern of subsidence on the eastern part of the island is poorly constrained. It might have been produced by extensional tectonic strain that both reactivated preexisting faults on the eastern part of the island and facilitated magma movement beneath the western part. Alternatively, magma intrusion beneath the volcano might have been the cause of extension and subsidence in the eastern part of the island. We attribute localized subsidence in an area of active fumaroles within the Akutan caldera, by as much as 10 cm during 1992–1993 and 1996–1998, to fluid withdrawal or depressurization of the shallow hydrothermal system.
A three‐dimensional Bingham model for channeled lava flows
Authors: Tallarico Andrea , Dragoni Michele
Published: February 2004

We propose a three‐dimensional (3‐D) Bingham model for channeled lava flow. Unlike from the 3‐D Newtonian models, this model can be applied also far from the vent where the Bingham rheology cannot be neglected as a consequence of the lava cooling. We assume the lava to be an isothermal Bingham liquid flowing in a rectangular channel down a constant slope. The flow velocity is calculated by solving semianalytically the steady state Navier‐Stokes equation together with the 3‐D Bingham constitutive equation. The flow vorticity is evaluated and used to define the plug shape and position for different flows: a completely filled conduit, a partially filled conduit, and an open channel. Each component of the flow vorticity vector satisfies the Laplace equation and has been evaluated by using the relaxation method. The mass flow rate is evaluated for different values of the yield stress; it appears that the Bingham rheology causes a significant reduction in flow rate as the yield stress increases. For the highest yield stress values the plug in the center of the flow welds with the plugs in the flow corners, suggesting a possible rheological mechanism for the lava tube formation.
A three‐dimensional stochastic rock mechanics model of engineered geothermal systems in fractured crystalline rock
Authors: Jing Z. , Willis‐Richards J. , Watanabe K. , Hashida T.
Published: February 2004

A three‐dimensional (3‐D) stochastic network model for simulating a hot dry rock (HDR) or hot wet rock (HWR) engineered geothermal system formed in fractured crystalline rock is presented. The model addresses the problems of fracture network characterization from in situ field data, such as fracture orientation, size, spacing, and other mechanical properties. The model can simulate the changes that occur within the rock mass during stimulation (i.e., large volume fluid injection at pressures sufficient to allow shear slip on natural fractures). It can also be used to simulate steady state circulation of the heat exchange system thus created and includes provision for predicting tracer response curves and heat extraction history. The model has been applied to data gathered during the stimulation and circulation of a 2.2‐km‐deep HDR reservoir at Hijiori, Japan. The predicted shape of the stimulated and shear‐propped fractures closely matched the distribution of seismic source distribution of acoustic emission (AE), regardless of realization of the fracture network, suggesting that the geometry of the stimulated volume can be robustly predicted from knowledge of the fracture population and in situ stresses. However, hydraulic behavior and tracer tests during the circulation could only be satisfactorily simultaneously reproduced by a small subset of realizations. These selected realizations, obtained by matching initial circulation and tracer data, are considered to give the best prospect of satisfactory long‐term thermal modeling. The success in simultaneously modeling diverse data (hydraulic, microseismic, and tracer) lends confidence to the thermal predictions. The results indicate that a large improvement in the long‐term thermal performance of the Hijiori reservoir could result from increasing well spacing from 100 to 150 m without major degradation of the hydraulic performance.
Pore and throat size distributions measured from synchrotron X‐ray tomographic images of Fontainebleau sandstones
Authors: Lindquist W. Brent , Venkatarangan Arun , Dunsmuir John , Wong Teng‐fong
Published: February 2004

The three‐dimensional geometry and connectivity of pore space controls the hydraulic transport behavior of crustal rocks. We report on direct measurement of flow‐relevant geometrical properties of the void space in a suite of four samples of Fontainebleau sandstone ranging from 7.5 to 22% porosity. The measurements are obtained from computer analysis of three‐dimensional, synchrotron X‐ray computed microtomographic images. We present measured distributions of coordination number, channel length, throat size, and pore volume and of correlations between throat size/pore volume and nearest‐neighbor pore volume/pore volume determined for these samples. In order to deal with the ambiguity of where a nodal pore ends and a channel begins, we apportion the void space volume solely among nodal pores, with the channel throat surfaces providing the nodal pore delineations. Pore channels thus have length but no associated volume; channel length is defined by nodal pore center to nodal pore center distance. For a sample of given porosity our measurements show that the pore coordination number and throat area are exponentially distributed, whereas the channel length and nodal pore volume follow gamma and lognormal distributions, respectively. Our data indicate an overall increase in coordination number and shortening of pore channel length with increasing porosity. The average coordination number ranges from 3.4 to 3.8; the average channel length ranges from 200 to 130 μm. Average throat area increases from 1600 to 2200 μm2 with increasing porosity, while average pore volume remains essentially unchanged at around 0.0004 mm3.
Analytic crack solutions for tilt fields around hydraulic fractures
Authors: Warpinski Norman R.
Published: February 2004

The recent development of downhole tiltmeter arrays for monitoring hydraulic fractures has provided new information on fracture growth and geometry. These downhole arrays offer the significant advantages of being close to the fracture (large signal) and being unaffected by the free surface. As with surface tiltmeter data, analysis of these measurements requires the inversion of a crack or dislocation model. To supplement the dislocation models of Davis [1983], Okada [1992], and others, this work has extended several elastic crack solutions to provide tilt calculations. The solutions include constant‐pressure two‐dimensional (2‐D), penny‐shaped, and 3‐D‐elliptic cracks and a 2‐D‐variable‐pressure crack. Equations are developed for an arbitrary inclined fracture in an infinite elastic space. Effects of fracture height, fracture length, fracture dip, fracture azimuth, fracture width, and monitoring distance on the tilt distribution are given, as well as comparisons with the dislocation model. The results show that the tilt measurements are very sensitive to the fracture dimensions but also that it is difficult to separate the competing effects of the various parameters.
The Australian continental upper mantle: Structure and deformation inferred from surface waves
Authors: Debayle E. , Kennett B. L. N.
Published: February 2004

We present a new three‐dimensional model for the SV wave hetero‐geneities and azimuthal anisotropy in the upper mantle of the Australasian region. The model is constrained by the waveforms of 2194 Rayleigh waves seismograms with a dense ray coverage that ensure a lateral resolution of the order of few hundred of kilometers. The use of higher modes allows the resolution of the structure down to depths of at least 400 km. In the upper 200 km of the model, seismic velocities are lower on the eastern Phanerozoic margin of the continent compared to the Precambrian central and western cratons, in agreement with previous results for Australia. The boundary between Phanerozoic and Precambrian Australia is not clear, especially in the south, where a broad positive seismic anomaly underlays the Lachlan Fold Belt. The high‐velocity lid beneath the continent shows significant variations in thickness. Locally, it may extend down to a depth of 300 km in the mantle, but for most of Precambrian Australia the lithospheric thickness oscillates around 200 km, while it is thinner on the eastern Phanerozoic margin. We found significant SV wave azimuthal anisotropy in the upper 250 km of the mantle, with a drastic change in the organization of anisotropy between the upper 150 km of the model and the deeper part, as revealed in a preliminary inversion. In the upper 150 km of the mantle, azimuthal anisotropy appears more likely to be related to past deformation frozen in the lithosphere, and in central Australia we found clear evidence that deformation is preserved since the Alice Springs orogeny. Below 150 km, a smoother pattern of anisotropy is observed, more likely to be related to present‐day deformation due to the northward motion of the Australian plate. Our current data set allows constraint of the anisotropic directions at different depths with an unprecedented lateral resolution. The observation of significant changes of anisotropic directions with depth in the Australian continental mantle suggests that care should be taken in the interpretation of anisotropy from SKS observations.
Fluid flow at the toe of the Barbados accretionary wedge constrained by thermal, chemical, and hydrogeologic observations and models
Authors: Henry Pierre
Published: February 2004

Cores and geochemical observations from the plate boundary (decollement zone) at two sites on the Barbados wedge indicate the occurrence of episodic fluid flow along the decollement, but subsequent CORK (Circulation Obviation Retrofit Kit) experiments (pressure and temperature monitoring in cased and sealed holes) provided no evidence for temperature transients associated with fluid flow and measured pressures that indicate moderate fluid overpressuring. Comparison of observations and thermal models of steady state and transient fluid flow along the decollement suggests that the decollement has not been the main conduit for fluid flow for the last few thousand years. However, pore fluid chemistry indicates recent, and possibly still active, flow along the decollement. Comparison of results from advection‐diffusion models applied to the chloride anomaly observed at the top of the decollement at Site 948/671 indicates that fluid flow may have been continuous over the last 2000–20,000 years, but mineral vein structure and chemistry suggest that episodes of higher pore pressure and water flux also occurred in the past. In situ permeability measurements show a strong dependence of permeability on pore pressure. A decollement model taking into account this dependence suggests that the propagation of a pressure front associated with a flow pulse could occur with characteristic times compatible with those inferred from the chlorinity profile. In a wedge cross section the fluid flux associated with a pressure pulse is probably <100 m2 yr−1 and thus may not explain heat flow anomalies observed near the toe of Barbados wedge. Thermal anomalies are better explained by fluid flow within the underthrust sequence and, perhaps, in basement.
High‐velocity breakup‐related sills in the Vøring Basin, off Norway
Authors: Berndt C. , Skogly O. P. , Planke S. , Eldholm O. , Mjelde R.
Published: February 2004

Multichannel seismic reflection profiles in the Hel Graben, V0ring Basin, reveal a sill complex at approximately 5 km depth. It is associated with exceptionally high, 7.4 km s−1, seismic wide‐angle velocities. The existence of observable wide‐ angle arrivals shows that the sills act as efficient waveguides. Seismic reflection data and amplitude modeling constrain the thickness of individual sills to approximately 100 m. Sonic logs from sills of similar thickness on the nearby Utgard High show an average velocity of 7.0 km s−1. Such high velocities require an olivine‐gabbroic sill composition and emplacement under conditions which allowed growth of relatively large crystal sizes. A possible reason for such an emplacement environment is the HeI Graben's role as an intrusion center during breakup volcanism. This would provide the necessary duration of the magmatic activity as well as locally increased melt volumes and cooling times. Sill complexes of this kind decrease the accuracy of determined velocity fields and crustal geometries below the top of the sill complex, affecting depth conversion and gravity modeling. Furthermore, the results question the concept of lower crustal bodies as large‐scale, homogeneous accumulations of mafic melt.
Clinopyroxene‐liquid partitioning for vanadium and the oxygen fugacity during formation of cratonic and oceanic mantle lithosphere
Authors: Canil Dante , Fedortchouk Yana
Published: February 2004

The distribution coefficient for vanadium Dvcpx/liq between clinopyroxene and silicate liquid in the system Di‐Ab‐An has been measured at trace levels as a function of oxygen fugacity (ƒO2) at 100 kPa and 2 GPa. With increasing ƒO2 vanadium changes from compatible to incompatible in clinopyroxene at about one log ƒO2 unit below the nickel‐nickel oxide (NNO) buffer. The trend in the Di‐Ab‐An system represents a reasonable upper limit for Dvcpx/liq versus ƒO2 in natural systems. The Dvcpx/liq versus fo2 systematics from this study are combined into a melting model to evaluate the V budget during partial melting in the mantle. It is shown that fo2 has a very measurable effect on bulk Dvsource/liq along the spinel peridotite solidus, and at 7 GPa along the garnet peridotite solidus. Isobaric melting models are formulated and used to examine the distinctly different covariation of V with degree of depletion in mantle lithosphere from various tectonic settings. The spectrum of oceanic lithosphere can be explained by partial melting in the spinel peridotite stability field at an fo2 of ∼NNO‐3, identical to that recorded by midocean ridge basalts. Cratonic peridotites are not well explained by melting in the garnet peridotite stability field at any fo2 but this may be a shortcoming of both the isobaric melting model and the poor constraints for Dvgt/liq with fo2. If cratonic peridotites formed by depletion in the spinel stability field, then fo2 during melting was above ∼NNO‐2.5, requiring that mantle lithosphere in the Archean formed under more oxidizing conditions than hitherto realized. The V abundances of cratonic peridotites are collinear with subduction zone peridotites, possibly linking the mantle roots beneath Archean cratons to a more oxidized subduction zone setting.
Three‐dimensional structure of the Torngat Orogen (NE Canada) from active seismic tomography
Authors: Funck Thomas , Louden Keith E. , Wardle Richard J. , Hall Jeremy , Hobro James W. , Salisbury Matthew H. , Muzzatti Angelina M.
Published: February 2004

The crustal velocity structure and the Moho depth of the Proterozoic Torngat Orogen, NE Canada, is determined by active seismic tomography using travel times of crustal turning rays and Moho reflections. The orogen developed during oblique convergence of the Archean Superior and Nain Provinces, which trapped an interior belt of Archean crust (Core Zone) between them, with the Torngat Orogen evolving between the Core Zone and the Nain Province. Beneath the central orogen a crustal root is found with a preserved depth of >52 km and a width of ∼100 km. To the north, the root shallows to <44 km and narrows to a width of ∼45 km. The root correlates with a set of major, late orogenic shear zones that accommodated oblique convergence of the Superior and Nain Provinces. This correlation suggests that the transpressional shearing focused strain in the region of the root and contributed to the crustal thickening. Absence of postorogenic magmatic activity prevented reworking or thermal relaxation of the root. The lack of late magmatism is probably related to the depleted and refractory nature of the Archean lithosphere underlying the orogen. Upper crustal velocities are lowest in the Core Zone (∼5.7 km/s at the surface) and are compatible with laboratory measurements carried out on gneissic rock samples from that area. Higher velocities in the Nain Province (∼5.9 km/s) correlate with felsic gneiss and anorthosite rock samples. A high‐velocity region immediately to the north of the crustal root is associated with a Moho uplift (∼34 km). This is explained by extension along the Ungava transform fault, and possibly in Hudson Strait, at ∼55 Ma when rifting in the Labrador Sea was transferred into Baffin Bay.
Density structure of the lithosphere in the southwestern United States and its tectonic significance
Authors: Kaban Mikhail K. , Mooney Walter D.
Published: February 2004

We calculate a density model of the lithosphere of the southwestern United States through an integrated analysis of gravity, seismic refraction, drill hole, and geological data. Deviations from the average upper mantle density are as much as ±3%. A comparison with tomographic images of seismic velocities indicates that a substantial part (>50%) of these density variations is due to changes in composition rather than temperature. Pronounced mass deficits are found in the upper mantle under the Basin and Range Province and the northern part of the California Coast Ranges and adjacent ocean. The density structure of the northern and central/southern Sierra Nevada is remarkably different. The central/southern part is anomalous and is characterized by a relatively light crust underlain by a higher‐density upper mantle that may be associated with a cold, stalled subducted plate. High densities are also determined within the uppermost mantle beneath the central Transverse Ranges and adjoining continental slope. The average density of the crystalline crust under the Great Valley and western Sierra Nevada is estimated to be up to 200 kg m−3higher than the regional average, consistent with tectonic models for the obduction of oceanic crust and uppermost mantle in this region.
GPS‐derived strain rate field within the boundary zones of the Eurasian, African, and Arabian Plates
Authors: Kahle Hans‐Gert , Cocard Marc , Peter Yannick , Geiger Alain , Reilinger Robert , Barka Aykut , Veis George
Published: February 2004

We use the GPS velocity field (1988–1998) for eastern Mediterranean and Asia Minor to determine the crustal deformation strain rate field in an area bounded by 35°N and 43°N, and 20°E and 48°E. We calculate the normal and shear strain rate components associated with the major faults and compare these qualitatively with seismological data. Uncertainties in the calculation of the strain rates reach 50 nstrain yr−1 in sparsely observed parts of Anatolia, whereas we estimate errors <20 nstrain yr−1 in the Aegean and Marmara regions. The largest compressional strain rate components in the eastern part of the study area occur along the Greater Caucasus mountain front reaching 70 nstrain yr−1. (1 nstrain yr−1 = 0.0317 × 10−15 s−1). The North Anatolian Fault Zone is the clearest feature in the shear strain rate field. It is expressed as a pronounced dextral strike‐slip fault zone, reaching rates of up to 170 nstrain yr−1. This holds true also for the Izmit area, where the August 17, 1999, earthquake occurred. Central Anatolia is almost strain‐free, whereas extension prevails in western Anatolia. The principal axes of extension vary around the N‐S direction with strain rates of up to 85 nstrain yr−1. These extensional areas coincide with graben features and normal faulting earthquakes. The central and southwestern Aegean Sea is strain‐free with values far below 40 nstrain yr−1. The seismic cluster around the Dodekanissa islands, southeastern Aegean Sea, coincides with NW‐SE oriented extension, attaining strain rates of up to 90 nstrain yr−1. This area of extension also exhibits recent active volcanism. The entire Hellenic arc shows compressional strain rates perpendicular to the arc. The Pliny‐Strabo troughs along the eastern segment of the arc show left‐lateral shear strain rates reaching 80 nstrain yr−1. Significant extension is found in central Greece, with a NNE‐SSW oriented maximum of 120 nstrain yr−1 centered around the Gulf of Corinthos. The Kephalonia Fault Zone in NW Greece is a distinct dextral fault zone, separating Apulia from the rapidly moving Aegean microplate. Right‐lateral shear strain rates reach 150 nstrain yr−1.
Causes and consequences of flow organization during melt transport: The reaction infiltration instability in compactible media
Authors: Spiegelman Marc , Kelemen Peter B. , Aharonov Einat
Published: February 2004

Geochemical and field evidence suggest that melt transport in some regions of the mantle is localized into mesoscale “channels” that have widths of 0.1–100 m or larger. Nevertheless, the mechanisms for formation of such channels from a grain‐scale distribution of melt are poorly understood. The purpose of this paper is to investigate one possible mechanism for channel formation: the reaction infiltration instability (RII). We present numerical solutions of the full equations for reactive fluid flow in a viscously deformable, permeable medium. We show that dissolution in a compactible solid with a vertical solubility gradient can lead to significant flow localization such that >90% of the melt flux is channelized in <20% of the available area. In particular, the ability of the solid to compact enhances the instability by forming impermeable regions between channels. The combination of reaction, diffusion, and solid compaction leads to strong selection of preferred length scales with channel spacing smaller than the compaction length (δ ∼ 102 − 104 m). We explore the evolution of dissolution channels over parameter space and show that the behavior of the full nonlinear solutions is consistent with predictions from linear stability analysis. We also briefly consider the behavior of the instability in the presence of melting due to adiabatic decompression and demonstrate that significant localization can occur even in the presence of uniform melting and compaction. Using the linear analysis to extend these results for parameters expected in the Earth's mantle suggests that robust channel systems could form through the RII from a homogeneous system in ∼100,000 years with dominant channel spacing of 1–200 m.
An elastic wedge model for the development of coeval normal and thrust faulting in the Mauna Loa‐Kilauea rift system in Hawaii
Authors: Yin An , Kelty T. K.
Published: February 2004

A long‐standing enigma of the Mauna Loa‐Kilauea rift system in Hawaii is the coeval development of normal and thrust faults that are vertically partitioned. To address this question, we developed a simple elastic wedge model that explores plausible boundary conditions in terms of tractions for generating such a fault pattern. Analytical solutions that best simulate the observed faulting style and geodetically determined strain at the surface require that (1) the pore fluid pressure ratio within the wedge (λ) and along the basal decollement (λb,) must be exceedingly high (i.e., λ = λb= 0.90–0.95) and (2) a tensile stress of the order of 10–30 MPa must have existed in the very top part of the rift zone at the back side of the wedge‐shaped rift flank. The high pore fluid pressure within the rift flank may be induced by pumping of fluids during emplacement of magma, whereas the high pore fluid pressure along the basal decollement may be caused by compaction of water‐saturated sediments between the volcanic pile above and the oceanic floor below. Although the predicted tensile stress in the rift zone could be related to the presence of a relatively steep topographic slope, our results show that this is not a prerequisite. Therefore we attribute occurrence of tensile stress to either upward bending of the Hawaiian volcanic pile due to emplacement of magma, or inflation of a shallow magma chamber several kilometers beneath the surface. In any case, the results of our model indicate that magma emplacement in the shallow part of the rift zone may be a passive process, while the deep rift zone experiences forceful emplacement (i.e., active rifting via magma push).
A possible bistable evolution of soil thickness
Authors: D'Odorico Paolo
Published: February 2004

The analysis of the dynamical evolution of soil depth is of crucial importance to the understanding of landform processes in soil‐mantled landscapes (such as soil production, bedrock weathering, landscape lowering, and sediment yield). The temporal evolution of the regolith thickness is studied through a stochastic model incorporating the dynamics of both soil production and erosion. Serious and extreme erosive processes (e.g., landslides) are considered as random forcings acting discontinuously and abruptly. The dynamics of soil production is described by a deterministic function of the soil depth, and different hypotheses are considered that express such a dependence. The probability distribution of soil depth is analytically estimated as a function of the relative rates of the processes of soil erosion and bedrock weathering. It is found that for different (and likely) values of the parameters the dynamics can be either transport‐limited or weathering‐limited. Intermediate conditions correspond to bistable systems having two preferential states that are both explored by the process. The conditions controlling the occurrence of bistable systems are also discussed with respect to the steady state (linear) stability of a companion deterministic model. The existence in the deterministic model of two possible stable states separated by a range of unstable configurations is found to be a condition necessary but not sufficient to the emergence of double modes in the soil depth distribution in the stochastic process suggested by this paper.