bluuebunny/crossref_metadata_2025 huggingface dataset:
Mass/age distribution and composition of sediments on the ocean floor and the global rate of sediment subduction
Authors: Hay William W. , Sloan James L. , Wold Christopher N.
Published: February 2008

The total mass of sediments on the ocean floor is estimated to be 262 × 1021 g. The overall mass/age distribution is approximated by an exponential decay curve: (11.02 × 1021 g)e−0.0355t Ma. The mass/age distribution is a function of the area/age distribution of ocean crust, the supply of sediment to the deep sea, and submarine erosion and redeposition. About 140 × 1021 g of the sediment on the ocean floor is pelagic sediment, consisting of about 74% CaCO3, with the remainder opaline silica and red clay. Of the sediment on the ocean floor, 122 × 1021 g is detritus, mostly terrigenous, but a small portion (about 6 × 1021 g) is volcanic. Because very little pelagic sediment is obducted, virtually all of the pelagic sediment mass and some fraction of the terrigenous sediment is being subducted at a rate estimated to be about 1 × 1021 g per million years. The composition of sediment on the ocean floor differs significantly from that of average passive margin and continental sediment, so that the loss of ocean floor sediment through subduction may drive the composition of global sediment toward enrichment in silica, alumina, and potash and toward depletion in calcium.
IRIS‐S: Extending geodetic Very Long Baseline Interferometry observations to the southern hemisphere
Authors: Carter W. E. , Robertson D. S. , Nothnagel A. , Nicolson G. D. , Schuh H. , Campbell J.
Published: February 2008

High‐accuracy geodetic very long baseline interferometry measurements between the African, Eurasian, and North American plates have been analyzed to determine the terrestrial coordinates of the Hartebeesthoek observatory to better than 10 cm, to determine the celestial coordinates of eight southern hemisphere radio sources with milliarc second (mas) accuracy, and to derive quasi‐independent polar motion, UTl, and nutation time series. Comparison of the Earth orientation time series with ongoing International Radio interferometric Surveying project values shows agreement at about the 1 mas of arc level in polar motion and nutation and 0.1 ms of time in UTl. Given the independence of the observing sessions and the unlikeliness of common systematic error sources, this level of agreement serves to bound the total errors in both measurement series.
Geochemistry of some gases in hydrothermal fluids from the southern Juan de Fuca Ridge
Authors: Evans William C. , White L. D. , Rapp John B.
Published: February 2008

Five samples of hydrothermal fluids from two vent areas on the southern Juan de Fuca Ridge were analyzed for dissolved gases. Concentrations in the end‐member hydrothermal fluid of H2 (270–527 μmol/kg), CH4 (82–118 μmol/kg), and CO2 (3920–4460 μmol/kg) are well above values in ambient seawater and are similar to concentrations reported for other ridge crest hydrothermal systems. The carbon isotopic ratios of the CH4 (δ13C = −17.8 to −20.8) and CO2 (δ13C = −3.6 to −4.7) suggest that at least some of the CH4 and CO2 in the fluids is basalt‐derived. The range of δ13C values for the basalt‐derived CO2 is −6.8 to −9.7, calculated by assuming conservation of recharge ΣCO2 during hydrothermal circulation. Apparent temperatures of equilibration between the CH4 and the basalt‐derived CO2 range from 640°C to 750°C. Small amounts of ethane (C2H6/CH4 ≅ 0.9 × 10−3−2.2 × 10−3), propane, and butane detected in the samples may also have formed in the basalt. One sample of almost pure (95.5%) hydrothermal fluid contained a significant fraction, up to 63% and 74%, respectively, of the recharge Ar and N2. This suggests that the fluid has not undergone extensive vapor‐liquid phase separation.
Gravitational effects on planetary neutron flux spectra
Authors: Feldman W. C. , Drake D. M. , O'Dell R. D. , Brinkley F. W. , Anderson R. C.
Published: February 2008

The one‐dimensional diffusion accelerated neutral‐particle transport (ONEDANT) code is augmented to explore the effects of gravity on neutron flux spectra near planetary surfaces. The lifetime of the neutron is also explicitly accounted for. The results show a qualitatively new feature in planetary neutron leakage spectra in the form of a component of returning neutrons having kinetic energies less than the gravitational binding energy (0.132 eV for Mars). The net effect is an enhancement in flux at the lowest energies that is largest at and above the outermost layer of planetary matter. This effect diminishes with increasing depth. Fluxes for kinetic energies larger than the gravitational binding potential are minimally changed by gravity. All energy spectra can be well characterized by a model consisting of the super‐position of relatively simple thermal and epithermal functions that are completely specified by four parameters; the thermal and epithermal amplitudes, α and β, respectively; the thermal temperature, Tα; and the epithermal power law exponent, p. Instrumental parameters for the initial version of the neutron mode of the Mars observer gamma ray spectrometer are used to demonstrate the ability to use measured count rates to determine thermal and epithermal amplitudes.
Secular variation of archaeomagnetic direction in the American Southwest, A.D. 750–1425
Authors: Sternberg Robert S.
Published: February 2008

Oriented archaeomagnetic samples were collected from 158 in situ features at 33 archaeological sites in the southwestern United States. Seventy‐three independently dated features were used for analysis of secular variation. A moving‐window technique with outlier rejection was developed to compute a smoothed secular variation curve. This technique incorporates weighted Fisher statistics to account for imprecision in both the age of remanence acquisition and the direction of magnetization for each feature. A mean direction and measure of dispersion is generated for each window of time. The secular variation record covers the period A.D. 750–1425, during which time the virtual geomagnetic pole (VGP) moves in a generally counterclockwise fashion from 86°N, 83°E at A.D. 750 to the lowest latitude position of 74°N, 192°E at A.D. 1075, and then to 85°N, 236°E at A.D. 1425. The median rate of VGP movement is 0.036° yr−1, similar to present‐day rates. The Southwestern record shows close correspondence to other North American archaeomagnetic, limnomagnetic, and speleomagnetic records. The archaeomagnetic record can serve as the master curve for the derivation of archaeomagnetic dates.
Near‐vertical and intermediate offset seismic reflection data from west of the Whipple Mountains, SE California
Authors: Flueh Ernst R. , Okaya David A.
Published: February 2008

During a seismic reflection survey conducted by the California Consortium for Crustal Studies in the Basin and Range Province west of the Whipple Mountains, SE California, a piggyback experiment was carried out to collect intermediate offset data (12–31 km). These data were obtained by recording the Vibroseis energy with a second, passive recording array, deployed twice at fixed positions at opposite ends of the reflection lines. The reflection midpoints fall into a 3‐km‐wide and 15‐km‐long region in Vidal Valley, roughly parallel to a segment of one of the near‐vertical reflection profiles. This data set makes three unique contributions to the geophysical study of this region. (1) From forward modeling of the observed travel times using ray‐tracing techniques, a shallow layer with velocities ranging from 6.0 to 6.5 km/s was found. This layer dips to the south from 2‐km depth near the Whipple Mountains to a depth of 5‐km in Rice Valley. These depths correspond closely to the westward projection of the Whipple detachment fault, which is exposed 1 km east of the near‐vertical profiles in the Whipple Mountains. (2) On the near‐vertical profile, the reflections from the mylonitically deformed lower plate at upper crustal and mid crustal depths are seen to cease underneath a sedimentary basin in Vidal Valley. However, the piggyback data, which undershoot this basin, show that these reflections are continuous beneath the basin. Thus near‐surface energy transmission problems were responsible for the apparent lateral termination of the reflections on the near‐vertical reflection profile. (3) The areal distribution of the midpoints allows us to construct a quasi‐three‐dimensional image on perpendicular profiles; at the cross points we determined the true strike and dip of reflecting horizons. This analysis shows that the reflections from the mylonitically deformed lower plate dip to the southwest westward of the Whipple Mountains and dip to the south southward of the Turtle Mountains. The results of this study support the interpretation of crustal reflectivity in the near‐vertical reflection profiles to be related to the mid‐Tertiary episode of extension which produced the Whipple metamorphic core complex. This association geometrically suggests a more regionally distributed mechanism for crustal thinning as compared with single detachment fault models.
Thermomechanical coupling during seismic wave propagation in a porous medium
Authors: de la Cruz V. , Spanos T. J. T.
Published: February 2008

The procedure of volume averaging is applied to the problem of seismic propagation in a fluid‐filled porous medium. The interplay of temperature variation and the mechanical motion is taken into account to first order. A complete set of equations is obtained, which carry, in addition to permeability, two mechanical parameters and one thermal parameter. When thermomechanical coupling is ignored, a subset of the equations reduces to those of Biot (1956a), allowing a correspondence between the parameters to be established.
Hot spot‐ridge crest convergence in the northeast Pacific
Authors: Karsten Jill L. , Delaney John R.
Published: February 2008

Evolution of the Juan de Fuca Ridge during the past 7 m.y. has been reconstructed taking into account both the propagating rift history and migration of the spreading center in the “absolute” (fixed hot spot) reference frame. Northwestward migration of the spreading center (at a rate of 30 km/m.y.) has resulted in progressive encroachment of the ridge axis on the Cobb Hot Spot and westward jumping of the central third of the ridge axis more recently than 0.5 Ma. Seamounts in the Cobb‐Eickelberg chain are predicted to display systematic variations in morphology and petrology, and a reduction in the age contrast between the edifice and underlying crust, as a result of the ridge axis approach. Relative seamount volumes also indicate that magmatic output of the hot spot varied during this interval, with a reduction in activity between 2.5 and 4.5 Ma, compared with relatively more robust activity before and after this period. Spatial relationships determined in this reconstruction allow hypotheses relating hot spot activity and rift propagation to be evaluated. In most cases, rift propagation has been directed away from the hot spot during the time period considered. Individual propagators show some reduction in propagation rate as separation between the propagating rift tip and hot spot increases, but cross comparison of multiple propagators does not uniformly display the same relationship. No obvious correlation exists between propagation rate and increasing proximity of the hot spot to the ridge axis or increasing hot spot output. Taken together, these observations do not offer compelling support for the concept of hot spot driven rift propagation. However, short‐term reversals in propagation direction at the Cobb Offset coincide with activity of the Heckle melting anomaly, suggesting that local propagation effects may be related to excess magma supply at the ridge axis.
Seismic reflectivity of the Whipple Mountain shear zone in southern California
Authors: Wang Chi‐Yuen , Okaya David A. , Ruppert Charles , Davis Gregory A. , Guo Tie‐Shuan , Zhong Zengqiu , Wenk Hans‐Rudolf
Published: February 2008

The Whipple Mountains shear zone of southeastern California comprises part of the Whipple Mountains metamorphic core complex of the North American Cordillera. The 3.9‐km‐thick shear zone displays excellent exposures of ductilely deformed mid‐Tertiary mylonitic gneisses, as well as kinematically related, but younger cataclasites which underlie a major low‐angle normal fault (the Whipple detachment fault). Prominent crustal reflections beginning at 3–4 s were recorded on CALCRUST seismic profiles southwest of the Whipple Mountains. In order to interpret these seismic reflections and to understand why detachment faults are imaged on some profiles but not on others, we collected oriented rock specimens from all major structural units in the Whipple Mountains and determined P wave velocities for these samples parallel to three principal fabric directions under laboratory‐induced confining pressures up to 500 MPa. Using a geologic section through the mylonitic gneisses from a previous investigation, we then constructed a detailed acoustic impedance section for the Whipple Mountain shear zone based upon laboratory results. This section was in turn used to compose two dimensional synthetic reflection seismograms for comparison with the CALCRUST records. Significant impedance contrasts are enhanced by the opposing fabric orientations between the interlayered but structurally isolated relict domains of nonmylonitized rocks and the surrounding mylonitic gneisses. The finite thickness and lateral extent of these rock units cause constructive interference in seismic modeling to create the “seismic fabric” of strong, laterally discontinuous but subparallel reflections as seen in the CALCRUST profiles southwest of the Whipple Mountains. Chloritic breccias beneath the Whipple detachment fault (WDF) have much lower seismic impedance than that of rocks occurring both structurally above and below. However, the significant thinning of the chloritic breccia zone to the southwest of the Whipple Mountains may have caused the poor image of the WDF on the CALCRUST profiles. The absence of a detachment reflection is thus not necessarily indicative of the absence of a subsurface fault zone.
Late Neogene geohistory analysis of the Humboldt Basin and its relationship to convergence of the Juan de Fuca Plate
Authors: McCrory Patricia A.
Published: February 2008

Geohistory analysis of Neogene Humboldt basin strata provides important constraints for hypotheses of the tectonic evolution of the southern Cascadia subduction margin, leading up to the arrival of the Mendocino triple junction. This analysis suggests that the tectonic evolution of the Humboldt basin area was dominated by coupling between the downgoing Juan de Fuca plate and the continental margin. This coupling is reflected in the timing of major hiatuses within the basin sedimentary sequence and margin uplift and subsidence which occur during periods of tectonic plate adjustment. Stratigraphic evidence indicates that Humboldt basin originated at the base of the continental slope in early Miocene time. Syndepositional uplift of basin strata began in the late Pliocene and was both thermal isostatic and tectonic in origin. Isostatic uplift was a function of an increasingly more buoyant slab being subducted, whereas tectonic uplift was due to imbricated thrusting of the accretionary complex and underplating of offscraped sediment during subduction. A component of margin uplift is postulated to have been caused by a change in the rate of convergence between the Juan de Fuca and North American plates. Coeval with late Pliocene uplift documented onshore was a sharp decrease in covergence rate ∼3 Ma. A reduction in rate of tectonic uplift, observed in the Eel River section, in early Pleistocene time was coeval with a marked increase in relative motion parallel to the continental margin. This localized subsidence may have been caused by syndepositional folding.
New finite element techniques for modeling deformation histories of continents with stratified temperature‐dependent rheology
Authors: Bird Peter
Published: February 2008

Previous techniques for modeling anelastic deformation of the lithosphere have included plane strain models, restricted to two‐dimensional problems, and quasi‐three‐dimensional “plane stress” or “thin plate” models, that did not accurately include the effects of the shallow frictional layer, or of kinematic detachment of crust from mantle. This paper presents techniques to remedy these deficiencies of thin plate models. An iteration strategy in which the rheology is linearized using artificial prestress and a particular effective viscosity tensor causes the calculation of horizontal velocities to converge monotonically, even with a frictional layer at the top of the lithosphere. A technique using two planar grids allows deformation and displacement to be different at the crust and mantle levels, at far less cost than that of a three‐dimensional grid. A finite element technique is developed for computing the changes in thickness of these layers caused by pure shear, simple shear, and pressure gradients. A technique based on relaxation of perturbation eigenfunctions solves the heat equation in the lithosphere during deformation. Accuracy of component numerical methods is good for simple test problems, but in realistic nonlinear problems utilizing all components, only the precision can be determined because of the lack of analytic solutions. Precision of the combined program is tested with a realistic sample problem and presented as a function of the number of iterations in each velocity solution (convergence factor 0.73 to 0.88), size of time step in the predictor/corrector time integration (convergence as Δt0.8), and number of degrees of freedom in the finite element grid (convergence as N−0.5 to −0.8 for most variables). Overall cost of a simulation varies with the fractional precision Π as Π−3.3. A new consequence of kinematic detachment, a moving wave of crustal thickness, is found; unfortunately, the form of the wave depends on the finite element size. This means that element size must be chosen to approximate the smoothing by flexural rigidity effects that were neglected because of cost.
A cohesive zone model for dynamic shear faulting based on experimentally inferred constitutive relation and strong motion source parameters
Authors: Ohnaka Mitiyasu , Yamashita Teruo
Published: February 2008

To understand constitutive behavior near the rupture front during an earthquake source shear failure along a preexisting fault in terms of physics, local breakdown processes near the propagating tip of the slipping zone under mode II crack growth condition have been investigated experimentally and theoretically. A physically reasonable constitutive relation between cohesive stress τ and slip displacementD, τ = (τi− τd)[1 + α log (1 + βD)] exp (−ηD) + τd, is put forward to describe dynamic breakdown processes during earthquake source failure in quantitative terms. In the above equation, τiis the initial shear stress on the verge of slip, τdis the dynamic friction stress, and α, β, and η are constants. This relation is based on the constitutive features during slip failure instabilities revealed in the careful laboratory experiments. These experiments show that the shear stress first increases with ongoing slip during the dynamic breakdown process, the peak stress is attained at a very (usually negligibly) small but nonzero value of the slip displacement, and then the slip‐weakening instability proceeds. The model leads to bounded slip acceleration and stresses at and near the dynamically propagating tip of the slipping zone along the fault in an elastic continuum. The dynamic behavior near the propagating tip of the slipping zone calculated from the theoretical model agrees with those observed during slip failure along the preexisting fault much larger than the cohesive zone. The model predicts that the maximum slip accelerationbe related to the maximum slip velocityand the critical displacementDcby, wherekis a numerical parameter, taking a value ranging from 4.9 to 7.2 according to a value of τi/τp(τpbeing the peak shear stress) in the present model. The model further predicts thatbe expressed in terms ofand the cut off frequencyfmaxsof the power spectral density of the slip acceleration on the fault plane asand thatin terms ofDcandfmaxsas. These theoretical relations agree well with the experimental observations and can explain interrelations between strong motion source parameters for earthquakes. The pulse width of slip acceleration on the fault plane is directly proportional to the timeTcrequired for the crack tip to break down, andfmaxsis inversely proportional toTc.
Seismic anisotropy and mantle flow
Authors: Ribe Neil M.
Published: February 2008

Seismic anisotropy in the mantle is primarily due to deformation‐induced lattice preferred orientation (LPO) of olivine crystals. The aim of this paper is to better understand how such LPO is produced by flow, and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle. The basis of the work is a two‐dimensional theory which describes how LPO evolves during an arbitrary time‐dependent deformation [Ribe, 1989]. I investigate the evolution of LPO beneath ocean ridges, above subducted slabs, and in thermal convective flows. The ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge, and that the fast axis should be more nearly horizontal. The subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs, with the fast axis aligned with the slab, regardless of the subduction angle. The evolution of LPO in typical convective flows is extremely complex, and depends strongly on such factors as the mode of heating and the surface boundary conditions. In general, there is no simple relation between the LPO and the local flow direction in any of the models considered. The attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted. However, the LPO does become aligned with the flow direction when the flow is a progressive simple shear. Such a flow occurs in limited portions of the mantle: beneath the lithosphere at a fast spreading ridge, above subducted slabs, and in boundary layers beneath rigid surfaces such as continents. Observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle.
Upper crustal structure of the accreted Chugach Terrane, Alaska
Authors: Wolf Lorraine W. , Levander Alan R.
Published: February 2008

We have developed a model for the upper crustal structure of the Chugach terrane of southcentral Alaska from U.S. Geological Survey seismic refraction data using two‐dimensional asymptotic ray tracing. The refraction profile, acquired as part of the Trans‐Alaska Crustal Transect program, extends approximately 135 km in an east‐west direction along the local strike of the Chugach Mountains. The refraction survey consists of four shots, recorded by 120 portable seismic instruments spaced at 1‐km intervals. We observe a velocity‐depth profile which has unusually high velocities at shallow depths and at least two velocity reversals. The average velocity increases from 4.0 km/s at the surface to 6.9 km/s at a depth of 9 km. Near‐surface velocities in Cretaceous Valdez Group rocks increase eastward following the metamorphic gradient observed by Hudson and Plafker (1982). Lower layers may be correlated with rocks mapped to the south of the Contact fault zone, a suture boundary between the Chugach and Prince William terranes, and units not having surface expression. The first velocity reversal occurs at a depth of 9 km. This 1‐ to 2‐km zone has velocities which vary laterally from 6.0 to 6.9 km/s and pinches out to the east in the profile. This unit may represent a zone comprising Tertiary intrusives and country rock. It is followed at a depth of 10 km by a layer with velocities of 7.2–7.23 km/s. The second velocity reversal occurs at a depth of 12 km. Velocities within this layer range from 6.6 to 6.7 km/s and may correlate with underplated oceanic crust. Higher velocities of 7.35–7.4 km/s are attributed to a thin unit at a 14‐km depth associated with ultramafic and mafic rocks. The deepest unit is modelled as a 5‐km layer having velocities of 7.2–7.25 km/s. This layer may be underplated Prince William Terrane rocks, oceanic crust, or some portion of the Yakutat Block. The refraction data alone do not resolve deeper structure.
Evolution of midplate hotspot swells: Numerical solutions
Authors: Liu Mian , Chase Clement G.
Published: February 2008

The evolution of midplate hotspot swells on an oceanic plate moving over a hot, upwelling mantle plume is amenable to numerical simulation. In our model, the plume supplies to the lithosphere a Gaussian‐shaped thermal perturbation and dynamic support dominated by thermal buoyancy. We consider the two fundamental mechanisms of transferring heat, conduction and convection, during the interaction of the lithosphere with the mantle plume. The transient heat transfer equations, with boundary conditions varying in both time and space, are solved in cylindrical coordinates using the alternating direction implicit finite difference method on a 100×100 grid. The topography, geoid anomaly, and heat flow anomaly of the Hawaiian swell and the Bermuda rise are used to constrain the models. Our results confirm the conclusion from previous work that the Hawaiian swell can not be explained by conductive heating alone, even if an extremely high thermal perturbation is allowed. On the other hand, the model of convective thinning predicts successfully the topography, geoid anomaly, and the heat flow anomaly around the Hawaiian islands, as well as the changes in the topography and anomalous heat flow along the Hawaiian volcanic chain. The model constrains the Hawaiian plume to have a half wavelength of about 500 km, a center heat flux 5–6 times the background value, and a convective current velocity 3–10 times that of the background convective current. Comparatively, the mantle plume for the Bermuda rise is much weaker. Conductive heating is probably the dominant mechanism for the evolution of the Bermuda rise.
High‐precision absolute gravity observations in the United States
Authors: Peter G. , Moose R. E. , Wessells C. W. , Faller J. E. , Niebauer T. M.
Published: February 2008

From May 1987 to June 1988 the National Geodetic Survey (NGS) made approximately 50 observations at 30 sites with one of the six absolute gravimeters built by the Joint Institute for Laboratory Astrophysics (JILA) between 1983 and 1986. Of the 10 sites where two to three observations were made, the scatter about the mean site values ranged from under ±1 μGal to ±4.0 μGal. This high degree of repeatability is the result of three factors: the improvements made to the instrument at JILA in 1986, methods of observation and quality control of the data sets, and independent monitoring of and correcting for the environmental effects on gravity. The data correction methods now employed at NGS allow the establishment of high‐precision reference gravity stations in the United States and abroad for monitoring the temporal variations of gravity and studying vertical ground motions.
Shock wave equation of state and finite strain theory
Authors: Jeanloz Raymond
Published: February 2008

The linear shock‐velocity (Us), particle‐velocity (up) relation, which is known to be exceptionally successful in describing a wide variety of Hugoniot equation of state measurements, is shown to be virtually indistinguishable from the Birch‐Murnaghan (third‐order Eulerian finite strain) equation of state. For typical values of zero‐pressure parameters, specifically for the pressure derivative of the adiabatic bulk modulus K′0S ≃ 3 to 6, the Grüneisen parameter γ0 ≃ 1.5 (±1.0), and the logarithmic volume derivative of the Grüneisen parameter q ≃ 1.0 (±1.0), the Eulerian finite strain formulation yields the appropriate compressional moduli at infinitesimal strains, and it closely reproduces the Hugoniot at large compressions when compared with the linear Us‐up relation. Thus shock wave measurements provide strong empirical support for the success of the Eulerian finite strain equation of state in describing the compression of condensed matter to high pressures.
COCORP deep reflections: Moho at 50 km (16 S) beneath the Colorado Plateau
Authors: Hauser E. C. , Lundy J.
Published: February 2008

COCORP deep reflection data on the Colorado Plateau reveal complex reflections to about 16 s two‐way travel time (50+ km), beyond which there is an abrupt decrease of reflectivity. This boundary is interpreted to represent the Moho, an inference consistent with the existing but limited refraction data sets when they are reconciled with the recent independent determination of an 8.10–8.15 km/s Pn velocity beneath the Colorado Plateau. This ∼50 km depth to Moho and 8.10–8.15 km/s Pn velocity for the Colorado Plateau closely resembles that of the High Plains just east of the Rocky Mountains. The absence of continuous strong reflections at the Moho suggests that the Moho beneath the Colorado Plateau is a transition, not a velocity step function on the scale of the reflection experiment. The reflection boundary may represent the base of complex and discontinuous reflectors of the crust and crust/mantle transition, below which the mantle may be largely peridotite with a velocity of 8.1 km/s. These results strongly reinforce the suggestion that the Moho on deep reflection data in relatively stable continental regions is represented by an abrupt decrease in reflectivity, below which the mantle is relatively seismically transparent.
Large earthquakes and crustal deformation near Taiwan
Authors: Pezzopane Silvio K. , Wesnousky Steven G.
Published: February 2008

Source parameters of 15 large (mb ≥ 6) earthquakes which occurred near Taiwan during the last 25 years are determined by synthetic seismogram analysis of teleseismic P and S waveforms. Source depths range from 13 to 88 km. Three earthquakes of intermediate depth are located within the southernmost limit of the Ryukyu arc and show thrust‐type mechanisms with T axes subparallel to a northward dipping Wadati‐Benioff zone. Shallow earthquakes (<30 km) are distributed over a broad region along and off the east coast of Taiwan and show both high‐angle reverse and strike‐slip mechanisms. The P axes of the shallow events located off the east coast of northern Taiwan are oriented near horizontal, trend south to southwest, and may reflect relative motion between the Okinawa platelet and Taiwan or complex deformation at the southwest limit of the Ryukyu Trench. In contrast, P axes of shallow earthquakes both onshore and off the east coast of central and southern Taiwan consistently trend northwest and are interpreted to reflect relative convergence between the Philippine Sea and Eurasian plates. Summation of the seismic moment tensors of eight large shallow earthquakes that occurred along and off the central and southern coast of Taiwan shows that relative plate motion between the Eurasian and Philippine Sea plates, when averaged over the last 25 years, has been accommodated by crustal contraction at a rate of 2.6–5.4 cm yr−1 along an azimuth of about 290°.
Are the iron carbonate minerals, ankerite and ferroan dolomite, like siderite, important in paleomagnetism?
Authors: Ellwood Brooks B. , Burkart Burke , Rajeshwar Krishnan , Darwin Robert L. , Neeley Richard A. , McCall Alden B. , Long Gary J. , Buhl Margaret L. , Hickcox Charles W.
Published: February 2008

Magnetic, Mössbauer effect, and thermal properties have been evaluated for specimens of the mineral ankerite and are contrasted with similar measurements reported for the mineral siderite to determine if ankerite (and ferroan dolomite), like siderite, may be important in paleomagnetism as a producer of secondary, spurious remanent moments as the result of oxidation in air. Our data indicate that ankerite and siderite both have comparable thermal stabilities and that ankerite does break down to form iron oxides at temperatures as low as 250°C during thermal demagnetization. We conclude from our data that any iron carbonate, siderite, ankerite, or ferroan dolomite, may readily oxidize in air to maghemite or hematite thus producing a spurious remanent moment (RM) that can dominate the magnetic properties of the specimen. Thermal demagnetization of ankerite or siderite at lower temperatures (below 250°C) does not appear to produce such a spurious RM. As might be expected, the quantity of secondary iron oxides produced by oxidation is limited by the iron content in the carbonate crystal lattice. Oxidized siderite produces significantly greater amounts of iron oxides than equivalent percentages of ankerite, which in turn produces more iron oxides than equal amounts of ferroan dolomite in a given sample.
The Late Permian/Early Triassic magnetic polarity time scale and plate motions of south China
Authors: Steiner M. , Ogg J. , Zhang Z. , Sun S.
Published: February 2008

Fossiliferous stratigraphic sections spanning the Permian‐Triassic boundary in south China have yielded a well‐defined magnetic polarity sequence for the Lower Triassic and a more poorly defined sequence in the Upper Permian. Four sections comprising more than 1100 continuous meters of stratigraphic section display a sequence of 11 major reversals of polarity within the Early Triassic (Griesbachian to Spathian stages) and a minimum of two periods of normal polarity in the Late Permian (Changsing and Wujiaping stages). Synthesis of all Permo‐Triassic studies indicates that the lengthy late Paleozoic interval of reversed polarity ended after about two thirds of Permian time had passed. The record of frequent changes of polarity of the geomagnetic field during the Early Triassic is the same record that has been derived from many terrestrial redbed sequences. There is notable agreement in polarity pattern between the magnetostratigraphic‐biostratigraphic record of this study in marine rocks and those of the redbed sequences, indicating that some redbeds may record geomagnetic field history with great accuracy. The similarity of the records implies that a true record of geomagnetic polarity has been obtained for the Early Triassic. The reversal frequency during the Early Triassic was one to three reversals per million years, depending on the actual duration of this time period. Three latest Permian‐Early Triassic paleopoles were derived from this study, which in combination with those of earlier studies indicate that late Paleozoic to present apparent polar wander for the South China Block involved a relatively simple northward motion and that no movement of the South China Block occurred throughout the Triassic. A combined paleopole for the latest Permian‐Triassic of south China is 223.6°E, 46.1°N (N = 4, k = 1161.2, α95 = 2.7).
Digital image processing techniques for enhancement and classification of SeaMARC II side scan sonar imagery
Authors: Reed Thomas Beckett , Hussong Donald
Published: February 2008

The recent growth in the production rate of digital side scan sonar images, coupled with the rapid expansion of systematic seafloor exploration programs, has created a need for fast and quantitative means of processing seafloor imagery. Computer‐aided analytical techniques fill this need. A number of numerical techniques used to enhance and classify imagery produced by SeaMARC II, a long‐range combination side scan sonar, and bathymetric seafloor mapping system are documented. Three categories of techniques are presented: (1) preprocessing corrections (radiometric and geometric), (2) feature extraction, and (3) image segmentation and classification. An introduction to the concept of “feature vectors” is provided, along with an explanation of the method of evaluation of a texture feature vector based upon gray‐level co‐occurrence matrices (GLCM). An alternative to the a priori texel (texture element) subdivision of images is presented in the form of region growing and texture analysis (REGATA). This routine provides a texture map of spatial resolution superior to that obtainable with arbitrarily assigned texel boundaries and minimizes the possibility of mixed texture signals due to the combination of two or more textures in an arbitrarily assigned texel. Computer classification of these textural features extracted via the GLCM technique results in transformation of images into maps of image texture. These maps may either be interpreted in terms of the theoretical relationships shown between texture signatures and wavelength or converted to geologic maps by correlation of texture signatures with ground truth data. These techniques are applied to SeaMARC II side scan sonar imagery from a variety of geologic environments, including lithified and nonlithified sedimentary formations, volcanic and sedimentary debris flows, and crystalline basaltic outcrops. Application of the above processing steps provided not only superior images for both subjective and quantitative analysis but also the critical ability to discriminate between outcrops with distinct lithologies but similar image intensity.
The inversion of gravity data into three‐dimensional polyhedral models
Authors: Richardson Randall M. , MacInnes Scott C.
Published: February 2008

A nonlinear inversion scheme is described for the inversion of gravity data into a three‐dimensional polyhedral model. As presented, the inversion scheme is quite general and could have application to a wide range of nonlinear problems. Constraints, in the form of independent geologic information, play an essential role in the analysis. The parameterization of the problem allows inclusion of exact linear constraints to limit possible model shapes and to construct multiple body models. It also significantly reduces the degrees of freedom in a problem and can help the stability of the iterative process. Plausibility constraints are used to limit the total departure of the solution from a starting model known to a specified level of confidence. The optimal trade‐off between fitting noisy data and remaining close to a plausible starting model is determined empirically for each problem. The utility of the inversion scheme is illustrated with an example of estimating depth to bedrock from gravity data in Avra Valley, Arizona. The basin is modeled using three‐dimensional polyhedra. The analysis indicates two deep portions in the elongate basin separated by a 0.8‐km‐deep saddle point. Uncertainties in the solution are estimated, and the three controlling factors are independent control of depth to bedrock, data density, and basin depth.
Effects of a temperature‐dependent rheology on large‐scale continental extension
Authors: Sonder Leslie J. , England Philip C.
Published: February 2008

The influence of a temperature‐dependent rheology on large‐scale continental extension is investigated using a thin viscous sheet model. A vertically averaged rheology is used that is consistent with laboratory experiments on power law creep of olivine and that depends exponentially on temperature. The behavior of the model depends principally on two parameters: the Peclet number, which describes the relative rates of advection and diffusion of heat, and a dimensionless activation energy, which controls the temperature dependence of the rheology. Numerical calculations of such a thin sheet subjected to an extensional boundary condition show the following results: At short times following the beginning of extension, deformation occurs with negligible change in temperature, so that only small changes in lithospheric strength occur due to attenuation of the lithosphere, and maximum rates of deformation are located close to the extensional boundary. However, after a certain time interval, thermal diffusion results in lowered temperatures in the lithosphere, strongly increasing lithospheric strength and slowing the rate of extension. This critical time depends principally on the Peclet number and is short compared with the thermal time constant of the lithosphere. The changes in strength cause the locus of high extensional strain rates to shift with time from regions of high strain to regions of low strain. Results of the calculations are compared with observations from the Aegean, where maximum extensional strains are found in the south, near Crete, but the largest present‐day extensional strain rates occur about 300 km further north. The comparisons support the hypothesis that the observed separation of the loci of maximum strain and maximum present‐day strain rates in the Aegean may be the consequence of changes in lithospheric strength arising from the temperature‐dependent mechanical properties of lithospheric materials.
Magmatism associated with lithospheric extension: Introduction
Authors: Leeman William P. , Fitton J. Godfrey
Published: February 2008

The following collection of papers is the outgrowth of a symposium held at the Fall 1987 meeting of the American Geophysical Union. This symposium focused on the general subject of magmatism associated with lithospheric extension, in all stages of development, and covering many important regions of extensional tectonism around the world. Our own interest in this subject stems from our work on the temporal development of the Cordilleran western United States. Students of this region are aware of the complexities in timing, style, and compositional features of magmatic activity in this region, but only recently has sufficient information become available on a regional scale to provide specific constraints on the physical processes themselves.
Numerical modeling of accretionary wedge mechanics: Application to the Barbados subduction problem
Authors: Borja Ronaldo I. , Dreiss Shirley J.
Published: February 2008

The mechanisms for the development of accretionary prisms in subduction complexes is numerically simulated using finite element plasticity analysis and the Barbados problem as an example. The accreted and subducting sediments are modeled as elastoplastic continua separated by rigid‐plastic contact elements to simulate the thrusting mechanism. Results of the analyses demonstrate the importance of both lateral compression and topographic slope in the development and propagation of accretionary prisms in the Barbados subduction complex, affirming the analytical theory on the mechanics of thin‐skinned fold‐and‐thrust belts presented by Chapple [1978]. Numerical analyses also reveal that the shape of the accretionary prism requires a critical balance in material characteristics such as the frictional strength of the basal layer, the modulus of the décollement, and the flow laws governing the yielding of the elastoplastic accretionary prism itself. Finally, it is shown that, unless the high normal stress at the base of the overriding plate is counteracted by a high fluid pressure localized in the décollement region, the frictional component of the basal traction will make the horizontal propagation of accretionary prisms difficult.
Transport of fluid and electric current through a single fracture
Authors: Brown Stephen R.
Published: February 2008

One expects the hydraulic and electrical conductivities of rock to be related, since there is an analogy between the differential equations describing each process. Fractures in rock are commonly described by the parallel plate model, where the fracture surfaces are smooth and parallel with a separation or aperture of d. For this model the hydraulic conductivity is proportional to d3, whereas the electrical conductivity is proportional to d. Deviations from the parallel plate model are expected, since real fracture surfaces are rough and in partial contact. Computer simulations of fluid flow and conduction of electricity were performed on simulated fractures composed of rough surfaces generated with a fractal algorithm. The finite difference method was used to calculate the volume flow rate and electric current. This solution was used in the parallel plate model to get an effective hydraulic aperture dh and electric aperture de. The electric and hydraulic apertures are nearly the same when the surfaces are widely separated. However, de is always smaller than dh, with the difference increasing as the fracture closes. Additionally, the local directions of fluid flow and electric current are not the same. Thus contrary to the common assumption, the actual path length of a fluid particle as measured by the tortuosity is different for each process. The results of these simulations are consistent with the “equivalent channel model,” which shows that by introducing the tortuosity of the fluid flow or electric current paths, one can relate the microscopic physics of the transport properties to the macroscopic behaviors described by Darcy's and Ohm's laws.
The influence of groundwater flow on thermal regimes in mountainous terrain: A model study
Authors: Forster Craig , Smith Leslie
Published: February 2008

Groundwater flow in high‐relief mountainous terrain is modeled to examine how geology, surface topography, climate, and regional heat flow influence the pattern and magnitude of an advective thermal disturbance. A numerical procedure is used to estimate the position of the water table within constraints provided by the available infiltration rate and the permeability of the mountain massif. Results show that where the water table is located at depth within the mountain massif, the rate of groundwater recharge, rather than permeability, is the appropriate factor to characterize the potential for an advective disturbance. Thermal disturbances should be assessed within the framework of the two‐dimensional character of the flow system because interpretations based on one‐dimensional models are prone to significant error. Modeling of site‐specific systems can exploit the existence of a permeability “window,” where temperatures in discharge areas and in springs discharging from fracture zones reach peak values. Temperatures of thermal springs reflect the complex interaction between flow within the mountain massif and flow through permeable fracture zones. Consequently, simple calculations relating the geothermal gradient, depth of maximum groundwater circulation, and discharge temperature improperly represent the physics of the process.
Seismicity and active compressional tectonics in Santa Monica Bay, southern California
Authors: Hauksson Egill , Saldivar Geoffrey V.
Published: February 2008

During 1973–1986 the seismicity in Santa Monica Bay consisted of an average of four events per year of ML ≥ 2.5, in addition to the 1979 Malibu (ML = 5.0) mainshock‐aftershock sequence. The 1979 sequence ruptured a north dipping thrust fault at 10–15 km depth beneath the bay. A diffuse distribution of seismicity extends across the bay from the terminus of the Palos Verdes fault to Point Dume. Two thirds of the single‐event focal mechanisms of 36 events that are located within the bay show mostly thrust or reverse faulting on west to west‐northwest striking planes. A third of the focal mechanisms exhibit strike‐slip faulting on north to north‐northwest striking planes. A stress inversion of the focal mechanism data shows that the maximum principal stress is oriented N13°E in a horizontal plane and the minimum principal stress is oriented N102°E and plunging 47°. Hence the tectonics in Santa Monica Bay are dominated by compression, with thrust, reverse, and strike‐slip faulting. The coexistence of thrust and strike‐slip faults suggests that the bay is not a part of the Peninsular Ranges terrane but forms a transition zone that accommodates the change from strike‐slip in the Peninsular Ranges to the south to reverse faulting in the Transverse Ranges to the north. Mapped anticlines and the previously unrecognized thrust faults beneath the bay form two fold and thrust belts that present additional seismic hazards to Los Angeles and the southern California coastal zone through seismic shaking and possible generation of small tsunamis.