bluuebunny/crossref_metadata_2025 huggingface dataset:
Normal faulting in a back arc basin: Geological and geodetic characteristics of the 1987 Edgecumbe Earthquake, New Zealand
Authors: Beanland S. , Blick G. H. , Darby D. J.
Published: February 2008

The ML6.3 Edgecumbe earthquake, March 2,1987, was associated with a 7‐km surface rapture of the NE striking Edgecumbe fault and 10 secondary fault raptures. Average net slip of the main fault at the ground surface was 1.7 m with an azimuth of N30°W. Regional subsidence of up to 2 m affected a large part of the Rangitaiki Plains and the Whakatane graben widened by about 0.7 m in a NW‐SE direction. Dislocation modelling suggests the fault has a dip of about 40° and extends to more than 6 km depth with 2.7 m normal slip, parameters consistent with seismological data presented in a companion paper. The slip was large for an earthquake of this magnitude in comparison with other normal faulting events, There is preliminary evidence for both regional postseismic deformation and continued fault slip. The Edgecumbe earthquake illustrates active extension of the Taupo volcanic zone, a back arc basin associated with the Hikurangi subduction zone beneath the North Island of New Zealand. The faulting was in many ways typical of worldwide historical normal faulting earthquakes and suggests that back arc spreading, where onshore, is accommodated by tectonic processes that are similar to those in other extensional environments.
Diffusion of hydrogen in olivine: Implications for water in the mantle
Authors: Mackwell Stephen J. , Kohlstedt David L.
Published: February 2008

To investigate the kinetics of diffusion of hydrogen in olivine, single crystals from San Carlos in Arizona have been annealed at temperatures between 800° and 1000°C under hydrothermal conditions at a confining pressure of 300 MPa. The hydrogen diffusivities were determined for the [100], [010], and [001] directions from concentration profiles for hydroxyl in the samples. These profiles were obtained from infrared spectra taken at 100‐μm intervals across a thin slice which was cut from the central portion of each annealed crystal. The rate of diffusion is anisotropic, with fastest transport along the [100] axis and slowest along the [010] axis. The fit of the data to an Arrhenius law for diffusion parallel to [100] yields an activation enthalpy of 130±30 kJ/mol with a preexponential term of (6±3)×10−5 m2 s−1. For diffusion parallel to [001], as there are insufficient data to calculate the activation enthalpy for diffusion, we used the same value as that for diffusion parallel to [100] and determined a preexponential term of (5±4)×10−6 m2 s−1. The diffusion rate parallel to [010] is about 1 order of magnitude slower than along [001]. The measured diffusivities are large enough that the hydrogen content of olivine grains which are millimeters in diameter will adjust to changing environmental conditions in time scales of hours at temperatures as low as 800°C. As xenoliths ascending from the mantle remain at high temperatures (i.e., >1000°C) but experience a rapid decrease in pressure, and hence hydrogen fugacity, olivine grains may dehydrate during ascent. By comparison, slow rates of carbon diffusion (Tingle et al., 1988) suggest that carbon will not be lost from olivine during ascent. Thus, low hydrogen contents within olivine and within fluid inclusions in olivine cannot be taken as support for low water contents in the mantle.
Hotspots and mantle plumes: Some phenomenology
Authors: Sleep Norman H.
Published: February 2008

The available data, mainly topography, geoid, and heat flow, describing hotspots worldwide are examined to constrain the mechanisms for swell uplift and to obtain fluxes and excess temperatures of mantle plumes. Swell upleft is caused mainly by excess temperatures that move with the lithosphere plate and to a lesser extent hot asthenosphere near the hotspot. The volume, heat, and buoyancy fluxes of hotspots are computed from the cross‐sectional areas of swells, the shapes of noses of swells, and, for on ridge hotspots, the amount of ascending material needed to supply the length of ridge axis which has abnormally high elevation and thick crust. The buoyancy fluxes range over a factor of 20 with Hawaii, 8.7 Mg s−1, the largest. The buoyancy flux for Iceland is 1.4 Mg s−1 which is similar to the flux of Cape Verde. The excess temperature of both on‐ridge and off‐ridge hotspots is around the 200°C value inferred from petrology but is not tightly constrained by geophysical considerations. This observation, the similarity of the fluxes of on‐ridge and offridge plumes, and the tendency for hotspots to cross the ridge indicate that similar plumes are likely to cause both types of hotspots. The buoyancy fluxes of 37 hotspots are estimated; the global buoyancy flux is 50 Mg s−1, which is equivalent to a globally averaged surface heat flow of 4 mWm−2 from core sources and would cool the core at a rate of 50°Cb.y.−1. Based on a thermal model and the assumption that the likelihood of subduction is independent of age, most of the heat from hotspots is unplaced in the lower lithosphere and later subducted.
Inversion for seismic slip rate history and distribution with stabilizing constraints: Application to the 1986 Andreanof Islands Earthquake
Authors: Das Shamita , Kostrov B. V.
Published: February 2008

We develop and present a method of inverting seismograms, including their absolute amplitudes, to obtain the complete seismic slip time history and distribution, using the method of linear programming. A rectangular fault is discretized into cells and the source duration into time steps. The inversion for the slip rate distribution on the fault as a function of time reduces to the solution of a system of linear equations. The influence on the solution of constraining different source parameters such as the sign of the slip rate and the seismic moment is investigated. It is found that the constraint of positiveness of slip rate and the constraint that the seismic moment determined from, the solution should be close to that obtained from the centroid moment tensor (CMT) solution are very strong constraints and are essential to obtaining a physically reasonable solution. Since the inverse problem is unstable, instead of presenting only one particular solution we examine several solutions equally well fitting the data for all practical purposes. We find that when both constraints are applied the solution becomes quite stable. The method is applied to a data set consisting of the vertical component of available digitally recorded teleseismic P waves at stations within a chosen distance window for the 1986 Andreanof Islands earthquake (Mw = 8.0). The preferred solution is the one in which the slip rate is constrained to be positive and the moment is set to the CMT value. The average rupture speed in the 90‐km segment to the east of the hypocenter was ≈ 3.2 km/s and in the 190‐km section to the west was ≈ 2 km/s. The maximum slip in the area of large moment release to the west was ≈ 5 m and the maximum slip rate, which occurs in the same place as the maximum slip, was ≈ 6.5 m/s. The slip distribution on the fault shows a corrugated pattern with the long axis in the direction of the plate convergence, rather than normal to the arc, which was the slip vector direction for the earthquake, with most of the slip occurring pn a region of the fault to the west of the rupture nucleation zone. This region continued to slip for about 55 s, the total source duration being about 80 s. The results of this analysis suggest that the long‐term motion on this portion of the Aleutian arc is indeed in the direction of plate convergence.
Frictional behavior and constitutive modeling of simulated fault gouge
Authors: Marone Chris , Raleigh C. Barry , Scholz C. H.
Published: February 2008

This paper presents an investigation of the factional properties and stability of frictional sliding for simulated fault gouge. In these experiments we sheared gouge layers (quartz sand) under saturated drained conditions and at constant normal stress (50–190 MPa) between either rough steel surfaces or Westerly granite surfaces in a triaxial apparatus. Surface roughness (60 to 320 grit) and gouge layer thickness (0–4.0 mm) were varied in the experiments with granite samples. Porosity ϕ was monitored continuously during shear. Our measurements indicate that granular gouge exhibits strain hardening and net compaction for shear strains γ less than 0.5–1.0. For γ > 0.5–1.0, sliding occurs at approximately constant shear stress and net compaction from one load/unload cycle to the next ceases. Dilatancy occurs at 1/3 to 1/2 the shear stress required for sliding and d2ϕ/dγ2 becomes negative at about the peak stress in a given loading cycle, indicating the onset of shear localization. Oblique shear bands appear in the layers at γ = 1.3–1.5. Experiments with an initial gouge layer exhibit velocity strengthening (the coefficient of friction increases with slip velocity), and initially bare granite surfaces exhibit velocity weakening. The magnitude of velocity strengthening varies inversely with normal stress and directly with gouge thickness and surface roughness. In the gouge experiments the dilatancy rate dϕ/dγ also varies with slip rate. Using a simple energy balance to relate volume change and frictional resistance, we find quantitative agreement between the measured change in dilatancy rate and friction following changes in slip rate. This indicates that velocity strengthening within granular gouge is the result of dilatancy. The slip rate dependence of dϕ/dγ increases with gouge thickness and surface roughness, in agreement with the friction data. Our data therefore suggest that slip within unconsolidated granular material, such as some natural fault gouges, is inherently stable. The results thus provide an explanation for (1) the tendency of gouge accumulation to stabilize slip in laboratory samples, and (2) the tendency for aseismic slip within shallow (< 3–5 km) unconsolidated fault gouge and within unconsolidated sediments such as shallow alluvium and accretionary prisms.
Volcanic geology of Tyrrhena Patera, Mars
Authors: Greeley Ronald , Crown David A.
Published: February 2008

Tyrrhena Patera is a large, low‐relief volcano located in the southern cratered highlands of Mars on a proposed ring fracture of the Hellas basin. The volcano is composed of four or possibly five principal units, the oldest and most areally extensive of which are suggested to be ash on the basis of their morphology and erosional characteristics. Lava flows are observed in a flank flow unit extending from the central caldera to the southwest and apparently cap the summit region. Models derived from the study of terrestrial pyroclastic flows have been applied to the deposits at Tyrrhena Patera and indicate that the distribution and lengths of the observed deposits are consistent with an origin by the emplacement of gravity‐driven ash flows generated by hydromagmatic or magmatic explosive eruptions. The hydromagmatic case requires considerable quantities of water (∼7.5×1016 kg); however, at volumetric water flow rates determined by other investigators (∼105 m3/s) and calculated from the regional slopes in the Hesperia Planum region (∼106 m3/s), the amounts necessary could be supplied relatively rapidly in geologic time. Tyrrhena Patera represents the earliest central vent volcanism identified on Mars and may reflect a transition from the flood‐style eruptions which dominated early Martian history. A hydromagmatic origin for Tyrrhena Patera is in agreement with suggested climatic changes on Mars and may explain the absence of this style of volcanism in later eras. Alternatively, a magmatic origin for Tyrrhena Patera implies an evolution in the types of magmas produced on Mars from Hesperian to Amazonian time, during which the prominent shields in the Tharsis region formed.
Test for bias in paleomagnetically determined paleolatitudes from Pacific Plate Deep Sea Drilling Project sediments
Authors: Gordon Richard G.
Published: February 2008

Paleolatitudes determined from paleomagnetic study of azimuthally unoriented sediment cores recovered by deep sea drilling on the Pacific plate have been previously used in determining reference poles for the Pacific plate apparent polar wander (APW) path. From discordant paleoinclinations and corresponding paleolatitudes determined from these data, some workers have inferred tectonic tilting or large horizontal motion of some deep‐sea drilling sites relative to other parts of the Pacific plate. Here the reliability of these paleolatitudes is evaluated by comparing them with paleolatitudes determined from a Pacific plate reference APW path determined from seamount poles, skewness and amplitudes of magnetic profiles across magnetically lineated seafloor, and paleocolatitudes from azimuthally unoriented basalt cores and from equatorial sediment facies. The reference APW path includes a new determination of the Early Cretaceous pole from several types of paleomagnetic data, which agree well with one another. Sediment paleolatitudes are found to be systematically less southerly than those predicted from the reference APW path, a discrepancy that corresponds to inclinations shallower than expected. Thus most deep‐sea drilling sediment paleomagnetic paleolatitudes are unreliable and should be omitted from the data used in determining reference APW paths. Inclination and paleolatitude discrepancies noted in prior studies may be due not to tectonic causes but to shallowly biased inclinations.
Petrologic constraints on the unroofing history of the Funeral Mountain Metamorphic Core Complex, California
Authors: Hodges K. V. , Walker J. D.
Published: February 2008

The northern Funeral Mountains, southeastern California, comprise a metamorphic core complex that lies within a region of extreme Neogene extension but did not experience a significant late Cenozoic thermal event. This circumstance, unusual among core complexes of the North American Cordillera, permits direct examination of the preextensional thermal evolution of a portion of the Cordilleran hinterland. For pelitic schists from the highest‐grade portions of the Funeral metamorphic core, thermobarometry and thermodynamic modeling of garnet zonation define a P‐T trajectory showing: (1) attainment of “peak” metamorphic conditions at 800–850 K and 800–1000 MPa (30–37 km depths); followed by (2) 400 to 600 MPa of decompression (15–22 km of exhumation) with no substantial change in temperature. Available geochronologic data indicate that peak metamorphism occurred in Early Cretaceous time and that the decompression path developed over the Early to Late Cretaceous interval. The maximum pressures indicated by the petrologic data require substantial Late Jurassic(?)‐Early Cretaceous tectonic burial even though most recognized thrust faults at this latitude that have large stratigraphie throws are assumed or demonstrated to have early Mesozoic ages. We postulate that post‐Early Cretaceous extensional faults may have excised the necessary middle Mesozoic thrust structures. While the majority of extensional structures responsible for this excisement is likely to be of Neogene age, associated with Basin and Range extension, the Cretaceous decompression path described in this paper is similar to the theoretical P‐T paths derived from numerical modeling of extensional unroofing. Given recent evidence for the development of extensional structures in compressional regimes like the Himalayan and Alpine orogens, it seems prudent to search for evidence of Mesozoic extensional structures in future studies of the hinterland of the North American Cordillera.
Single‐crystal 40Ar/39Ar dating of the Olorgesailie Formation, Southern Kenya Rift
Authors: Deino Alan , Potts Richard
Published: February 2008

Single‐crystal laser fusion 40Ar/39 Ar analyses and several conventional bulk fusion 40K‐40 Ar dates have been used to determine the age of volcaniclastic strata within the Olorgesailie Formation and of associated volcanic and sedimentary units of the southern Kenya rift. In the principal exposures along the southern edge of the Legemunge Plain, the formation spans the interval from approximately 500 to 1000 ka. Deposition continued to the east along the Ol Keju Nyiro river where a tuff near the top of the formation has been dated at 215 ka. In these exposures, the formation is unconformably overlain by sediments dated at 49 ka. A possible source for the Olorgesailie tephra, the Ol Doinyo Nyokie volcanic complex, contains an ash flow dated at ∼1 Ma, extending the known age range of this complex to encompass that of virtually the entire Olorgesailie Formation in the Legemunge Plain. These geologic examples illustrate the importance of the single‐crystal 40Ar/39 Ar dating technique whereby contaminant, altered, or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits. We have presented a computer‐modeling procedure based on an inverse‐isochron analysis that promotes a more objective approach to trimming 40Ar/39 Ar isotope data sets of this type.
Transmission of seismic waves across single natural fractures
Authors: Pyrak‐Nolte Laura J. , Myer Larry R. , Cook Neville G. W.
Published: February 2008

Fractures and other nonwelded contacts are important mechanical and hydrological features of rock masses. Their effects on seismic wave propagation can be modeled as a boundary condition in the seismic wave equation. Seismic stress is continuous across such a boundary, but seismic particle displacement and seismic particle velocity are not. The complete solutions for seismic wave reflection, conversion, and transmission across a displacement and velocity discontinuity between two half‐spaces with different densities and elastic properties are derived for all angles of the incident wave. The ratio between the seismic stress across this boundary and the seismic particle displacement and velocity are described by a specific stiffness and a specific viscosity, respectively. A displacement discontinuity results in frequency‐dependent reflection and transmission coefficients and a frequency‐dependent group time delay. The velocity discontinuity results in frequency‐independent coefficients and zero delay. Results of laboratory experiments on compressional and shear wave transmission across three different natural fractures in a quartz monzonite are described. Measurements were made at different effective stresses under dry and saturated conditions at room temperature. It is shown that the effect of these fractures on the spectral amplitudes for compressional and shear pulses transmitted across these fractures are described well by a displacement discontinuity for compressional pulses under dry and saturated conditions and by a combined displacement and velocity discontinuity for shear wave pulses under dry and saturated conditions. Values of specific stiffness and specific viscosity vary between fractures and increase with increasing effective stress, as does the static specific stiffness of these fractures. Changes in the spectral amplitudes of transmitted pulses are also analyzed in terms of attenuation using the seismic quality factor Q, which is found to be a function of frequency.
The solid‐state flow of polymineralic rocks
Authors: Handy Mark R.
Published: February 2008

Polymineralic rocks display three end‐member types of mechanical and microstructural behavior: (1) strong minerals form a load‐bearing framework that contains spaces filled with weaker minerals; (2) two or more minerals with low relative strengths control bulk rheology and form elongate boudins; (3) one very weak mineral governs bulk rheology, while the stronger minerals form clasts. A concept of strain energy partitioning in polyphase aggregates allows the expression of the bulk strength of a rock in terms of its strain rate, temperature, and the volume proportions and rheological properties of its constituent minerals. Bimineralic strength versus composition relations predicted with this model are highly nonlinear and mimic the rheological behavior of experimentally deformed bimineralic aggregates. Quantitative analysis of microstructures in quartzo‐feldspathic tectonites suggests that foliation development leads to a significant reduction in rock strength. This is due to decreased stress concentration within the deforming aggregate as matrix and clast grain size is reduced and the average spacing between strong minerals increases.
Anisotropic permeability and tortuosity in deformed wet sediments
Authors: Arch Jon , Maltman Alex
Published: February 2008

Dewatering in many accretionary prisms is inferred to occur preferentially along discrete tectonic structures. However, these structures commonly consist of intense particle realignment in ductile shear zones rather than dilative fractures. Such realignment prompts a reduction in porosity within the zones, but there is, nevertheless, natural evidence that the shear zones are able somehow to act as dewatering channels. Experimental investigations indicate that shear zones arise at failure of the material, at which point permeability is reduced across the zones but increases by nearly 2 orders of magnitude parallel to them. The primary fabric of the host sediment also induces an anisotropy of permeability, values differ by an order of magnitude according to flow direction. The tortuosity of the fluid flow path is theoretically analyzed for shear zones produced in different combinations of uniaxial shortening and simple shear. Varying tortuosity explains both the permeability anisotropy of the host material and the ease of fluid flow along the shear zones, despite their reduced porosity.
Evidence for a weak and overpressured décollement beneath sediment‐dominated accretionary prisms
Authors: Byrne Tim , Fisher Don
Published: February 2008

We present structural data from several underthrust sequences in southwest Alaska that suggest that the deformation regime beneath accretionary prisms is characterized by a subvertical σ1 and regionally extensive layer‐parallel shearing. These results contrast markedly with the results obtained from the more shallow parts of accretionary prisms and fold and thrust belts where σ1 plunges gently seaward or toward the deformation front. On the basis of Mohr circle diagrams, we propose that these different stress directions are consistent only with a model in which the two regimes are separated by a very weak décollement and calculate coefficient of friction values that range from 0.25 to 0.08. These values are significantly lower than coefficient of friction values calculated for the basal décollement in the Taiwan fold and thrust belt (e.g., ±0.2 (Barr and Dahlen, 1989a)) and they are also lower than laboratory friction measurements for most rocks (e.g., 0.6–0.8 (Byerlee, 1978)). Consequently, these low values suggest the presence of rocks with unusual material properties (e.g., montmorillonite (Logan and Rauenzahn, 1987)) or anomalous fluid pressures along the décollement. On the basis of the occurrence of shale diapirs, the distribution of early veins, and the absence of montmorillonite in the underthrust sequences, we propose that there was an anomalously high fluid pressure along the décollement fault instead of there being such a low coefficient of friction. These results emphasize the importance of dewatering and fluid flow in controlling the position of the décollement, which in turn determines how the accretionary prism grows. In essence, it is the development of a weak, overpressured décollement within a thick sediment pile that allows plate convergence to be partitioned into offscraping and underthrusting regimes.
Thermophysical properties of the Earth's crust: In situ measurements from continental and ocean drilling
Authors: Williams Colin F. , Anderson Roger N.
Published: February 2008

Knowledge of the variation of thermal conductivity and heat production with depth is critical to understanding the thermal state of the crust. Continental and oceanic drill holes provide unique opportunities for the calibration of empirical and theoretical models relating crustal thermal properties to physical and chemical properties. These calibrations are based on more than 30,000 well log measurements of acoustic velocity (both compressional and shear), density, porosity, radioelement concentrations (K, U, Th), and major element chemistry (Al, Si, Ca, Fe, S, Mg + Na), in formations ranging in age from 5 to 620 Ma and in composition from granitic to gabbroic. The mineralogical data available from induced gamma ray spectroscopy logs, when combined with known mineral thermal conductivities in a model for the thermal conductivity of a crystalline aggregate, enable the in situ application of a laboratory estimation technique and yield continuous conductivity profiles. However, mineralogy‐based conductivity models cannot account for the anisotropic conductivity observed in foliated metamorphic rocks. A phonon conduction model, which utilizes acoustic velocity and bulk density measurements, is accurate within ±15% in both isotropic and anisotropic formations of the upper continental crust and in oceanic crustal layers 2C and 3. In oceanic crustal layers 2A and 2B, however, the effects of fracturing on compressional and shear velocities lead to inaccurate results. The heat flow‐heat production relationship, which provides one method of determining subsurface heat production, has well‐documented limitations. Allis (1979) and Rybach and Buntebarth (1982, 1984), on the basis of laboratory measurements, have proposed quantitative relationships between heat production and rock compositional and physical properties. The well log K, U, and Th data indicate that heat production can be predicted from compositional data within a factor of 2 or 3. Predictions of heat production from acoustic velocity and bulk density measurements are only accurate to within an order of magnitude. Three factors account for these results: (1) the natural variability of velocity and density for a given rock composition, (2) the tendency of U and Th to associate with secondary rather than primary minerals, and (3) U and Th redistribution through fluid flow and metamorphism. Thus seismic velocity based heat production profiles can, at best, be taken as first‐order estimates which may misrepresent the complexity of the continental crust. However, the correlation of formations of known heat production with subsurface structures, when combined with a suite of existing techniques for estimating heat production, may lead to more accurate results.
Utilization of observations of well bore failure to constrain the orientation and magnitude of crustal stresses: Application to continental, Deep Sea Drilling Project, and Ocean Drilling Program boreholes
Authors: Moos Daniel , Zoback Mark D.
Published: February 2008

The conditions necessary for compressive and tensile failure of well bores drilled into crystalline rock can be adequately represented by simple elastic failure criteria, and analysis of well bore failure can provide constraints on the magnitudes of in situ stresses if the strength of the rock is known. When applied to several boreholes drilled into continental crust where there is relatively complete knowledge of stress magnitudes, these criteria enable us to predict the depth at which compressive failure of the well bores is observed. In oceanic crust, breakouts have been observed at depths below 700 m below sea floor in Deep Sea Drilling Project (DSDP) hole 504B, drilled into 5.9 Ma crust south of the Costa Rica Rift, and near the bottom of DSDP hole 395A, drilled into 7.3 Ma crust west of the Mid–Atlantic Ridge. In both cases the azimuth of maximum horizontal compressive stress is roughly perpendicular to the ridge axis. As the unconfined compressive strengths of basalt samples from DSDP hole 504B are generally above 200 MPa (Bauer and Handin, 1985), the existence of breakouts in DSDP holes 395A and 504B requires a highly compressional stress state, whereShmin∼SvandSHmax≥ 100 Mpa at about 500 m subbasement. These results are consistent with the state of stress inferred from compressional (strike‐slip and reverse faulting) earthquake focal mechanisms in young oceanic crust. As ridge push forces are relatively small in young oceanic crust, we concur with previous suggestions that the high horizontal compressive stresses result from the thermoelastic effects of a convectively cooled upper crustal layer overlying a conductively cooling lithosphere.
Vertical seismic profile synthetics by dynamic ray tracing in laterally varying layered anisotropic structures
Authors: Gajewski Dirk , Pšenčik Ivan
Published: February 2008

Dynamic ray tracing (DRT) is important in evaluating high‐frequency seismic wave fields in complicated structures. Two formulations of the DRT equations for inhomogeneous anisotropic media are presented. One of them is represented by the classical DRT system, suggested by Červený 15 years ago. Both systems are specified in Cartesian coordinates. The DRT equations are supplemented with formulae for the transformation of DRT across a smoothly curved interface between two inhomogeneous anisotropic media. Červený's formulation of the DRT is applied to the computation of vertical seismic profile (VSP) synthetics. The results of DRT are used for the evaluation of the geometrical spreading and of the coefficients of the paraxial ray approximation for travel times and ray amplitudes. In addition, the DRT is also used in the interval ray tracing procedure, a procedure searching for rays starling from the source and terminating in a specified interval on a profile. Results of numerical modeling of VSP measurements in a three‐dimensional laterally varying structure consisting of Isotropic and anisotropic layers separated by curved interfaces are presented. Ray diagrams of selected elementary waves, time‐distance curves, and multisource three‐component VSP synthetics generated for two different source locations calculated for the anisotropic model are compared with the results determined for a reference Isotropic model. The latter is obtained by averaging phase velocities of the anisotropic model. This comparison clearly shows the effects of anisotropy and the lateral variation of the model on seismic wave fields. The reliability of the ray synthetics is briefly discussed.
Anisotropy in seismic velocities and amplitudes from multiple parallel fractures
Authors: Pyrak‐Nolte Laura J. , Myer Larry R. , Cook Neville G. W.
Published: February 2008

Many rock structures include multiple, near‐parallel, planar discontinuities such as bedding planes or joints. The effects of these nonwelded interfaces on seismic wave propagation are often analyzed using effective moduli, in terms of which seismic wave propagation is independent of frequency and without loss, unless the moduli include imaginary terms. An alternative approach is to treat these interfaces as a boundary condition in the seismic wave equation, across which seismic stress is continuous, but seismic particle displacements are discontinuous. The ratio of the stress to displacement is called the specific stiffness of the interface and characterizes the elastic properties of a fracture. For a completely elastic system this results in frequency‐dependent reflection and transmission coefficients for each interface as well as a frequency‐dependent group time delay. Using multiple, parallel displacement discontinuities and ignoring converted and reflected waves, expressions derived for transmitted wave amplitudes and group velocities show that these depend on frequency, angle of incidence, and polarization in the case of shear waves. Measurements on a laminated steel block show that shear pulses propagating parallel to the laminations and polarized parallel and perpendicular to the plane of the laminations both travel at the velocity for solid steel, although the spectra of these pulses differ considerably. However, the energy of the pulse polarized perpendicular to the laminations may propagate as an interface wave between each pair of laminations. Predictions of the displacement discontinuity model have features quite distinct from many crustal observations to date. We suggest that we are able to model dense populations of coplanar cracks that cannot be treated by effective moduli methods which require a dilute concentration of cracks.
The Cobb‐Eickelberg Seamount Chain: Hotspot volcanism with mid‐ocean ridge basalt affinity
Authors: Desonie Dana L. , Duncan Robert A.
Published: February 2008

Cobb hotspot, currently located beneath Axial seamount on the Juan de Fuca ridge, has the temporal but not the isotopic characteristics usually attributed to a mantle plume. The earlier volcanic products of the hotspot, from eight volcanoes in the Cobb‐Eickelberg seamount (CES) chain, show a westward age progression away from the hotspot and a westward increase in the age difference between the seamounts and the crust on which they formed. These results are consistent with movement of the Pacific plate over a fixed Cobb hotspot and eventual encroachment by the westwardly migrating Juan de Fuca ridge. CES lavas are slightly enriched in alkalies and incompatible elements relative to those of the Juan de Fuca ridge but they have Sr, Nd, and Pb isotopic compositions virtually identical to those found along the ridge. Therefore, Cobb hotspot is a stationary, upper mantle melting anomaly whose volcanic products show strong mid‐ocean ridge basalt (MORE) affinity. These observations can be explained by low degrees of partial melting of entrained heterogeneous upper mantle MORB source material within a thermally driven lower mantle diapir or by an intrinsic MORB‐like composition of the deeper mantle source region from which northeast Pacific plumes rise.
On the nonuniqueness of receiver function inversions
Authors: Ammon Charles J. , Randall George E. , Zandt George
Published: February 2008

To study the resolving power of teleseismic P waveforms for receiver structure, we model synthetic waveforms using a time domain waveform inversion scheme beginning with a range of initial models to estimate the range of acceptable velocity structures. To speed up the waveform inversions, we implement Randall's (1989) efficient algorithms for calculating differential seismograms and include a smoothness constraint on all the resulting velocity models utilizing the “jumping” inversion technique of Shaw and Orcutt (1985). We present the results of more than 235 waveform inversions for one‐dimensional velocity structures that indicate that the primary sensitivity of a receiver function is to high wavenumber velocity changes, and a depth‐velocity product, not simply velocity. The range of slownesses in a typical receiver function study does not appear to be broad enough to remove the depth‐velocity ambiguity; the inclusion of a priori information is necessary. We also present inversion results for station RSCP, located in the Cumberland Plateau, Tennessee. Our results are similar to those from a previous study by Owens et al. (1984) and demonstrate the uncertainties in the resulting velocity estimate more clearly.
Earthquakes, faulting, and stress in the Los Angeles Basin
Authors: Hauksson Egill
Published: February 2008

Since 1920 fourteen moderate‐sized (ML = 4.9–6.4) earthquakes have been reported in the Los Angeles basin. These events are associated with both mappable surficial faults and concealed faults beneath the basin sediments. To determine the style of faulting and state of stress in the basin, single‐event focal mechanisms for 244 earthquakes of M ≥ 2.5 that have occurred during 1977–1989 have been calculated. Fifty‐nine percent of the events are strike‐slip and are mostly located near two of the major, northwest striking right‐lateral strike‐slip faults in the basin, the Newport‐Inglewood fault and the Palos Verdes fault. The 1988 Pasadena and the 1988 Upland earthquakes showed left‐lateral strike‐slip on northeast striking faults. Numerous small earthquakes in the eastern part of the basin show left‐lateral strike‐slip faulting and form a northeast trend near Yorba Linda. Thirty‐two percent of the events have reverse mechanisms and are distributed along two broad zones. The first, the Elysian Park fold and thrust belt, coincides with anticlines along the eastern and northern flank of the Los Angeles basin extending into Santa Monica Bay. The second, the Torrance‐Wilmington fold and thrust belt, coincides with anticlines mapped on the southwest flank of the basin and extends from offshore Newport Beach to the northwest into Santa Monica Bay. Oblique faulting that could be inferred by the merging of strike‐slip and compressional tectonics does not occur in the basin. Instead, the coexistence of zones of thrusting and large strike‐slip faults in the basin suggests that the thrust and strike‐slip movements are mostly decoupled. A few normal faulting mechanisms appear to be related to faulting orthogonal to the axes of plunging anticlines. The trend of the maximum horizontal stress varies from N1°W to N31°E across the basin and consistently forms high angles with the fold axes. The stress field that exists along the flanks of the basin has a vertical minimum stress axis. This stress field and ongoing folding and thrusting suggest that tectonic deformation is concentrated along the flanks of the deep central basin. Today the deformation of the basin consists of uplift and crustal thickening and lateral block movement to accommodate the north‐south compression across the basin.
Physical properties and structure of K2Si4O9 melt quenched from pressures up to 2.4 GPa
Authors: Dickinson James E. , Scarfe Christopher M. , McMillan Paul
Published: February 2008

Melt viscosities and densities of isobarically quenched melts (glasses) have been determined for K2Si4O9 up to pressures of 2.4 GPa. At 1200°C, melt viscosity decreases linearly from 1700 P at 1 atm to 310 P at 2.4 GPa. The increase in glass density with increasing pressure is described by a Birch‐Murnaghan equation of state with values of the 1‐atm bulk modulus and its pressure derivative equal to 32.5 GPa and 7.6, respectively. The structure of the quenched melts was investigated using Raman spectroscopy. Scattering in the 900‐ to 1000‐cm−1 region increases in intensity with increasing pressure, and a new band develops at 940 cm−1, suggesting an increase in Q2 species, perhaps accompanied by the presence of Si in high coordination. In the low‐frequency region the 520‐cm−1 band of the 1‐atm glass increases slightly in frequency with pressure, and a distinct shoulder at 470 cm−1 develops on this band. These observations suggest an increase in the number of Q4 species and a narrowing of the SiOSi bond angle within Q3 groups in the glass. The structural changes are consistent with a shift in the silicate species disproportionation equilibrium, 2Q3 = Q4+Q2, to the right with increasing pressure, and the changes provide a mechanism for explaining the observed variations in viscosity and glass density.
Pacific–North American Plate Motion from very long baseline interferometry compared with motion inferred from magnetic anomalies, transform faults, and earthquake slip vectors
Authors: Argus Donald F. , Gordon Richard G.
Published: February 2008

We use geodetic measurements from very long baseline interferometry (VLBI) to test whether the Pacific‐North American plate velocity averaged over several years (1984–1987) of direct observation equals that averaged over millions of years as inferred from other data. We furthermore test whether the Pacific‐North America velocity estimated from VLBI parallels the San Andreas fault, transform faults and earthquake slip vectors in the Gulf of California, and earthquake slip vectors along the Queen Charlotte fault, along the Alaskan peninsula, and along the Kamchatkan peninsula. We assume that VLBI sites at Fairbanks (Alaska), Fort Davis (Texas), Richmond (Florida), Westford (Massachusetts), and Platteville (Colorado) lie on the North American plate and that VLBI sites on the Hawaiian island of Kauai, on the Marshall island of Kwajalein, and at Vandenberg Air Force Base (California) lie on the Pacific plate. The Euler vector (0.806°/m.y. about 47.5°N, 79.9°W) that we estimate from VLBI is well constrained; its three‐dimensional 95% confidence ellipsoid axes are only 5–15% as long as the Euler vector itself. The VLBI Euler vector is nearly identical to the Pacific‐North America Euler vector of plate motion model NUVEL‐1, which is determined from transform fault azimuths, earthquake slip vectors, and spreading rates from marine magnetic anomalies that integrate motions since 3 Ma. The VLBI Euler vector has a rotation rate only 0.023°/m.y. faster than that of NUVEL‐1, and the VLBI Euler pole lies only 2° from that of NUVEL‐1. The VLBI Euler vector predicts that Pacific‐North America motion is on average 10°±4° clockwise of earthquake slip vectors along the Queen Charlotte fault, suggesting that these slip vectors are biased measures of the direction of relative plate motion. The VLBI Euler vector predicts that Pacific‐North America motion is 6°±4° clockwise of the San Andreas fault in central California, reinforcing the conclusion from plate motion models that the San Andreas fault does not parallel Pacific‐North American plate motion. The VLBI Euler vector predicts that Pacific‐North America motion is parallel to most transform faults and earthquake slip vectors in the Gulf of California and to most earthquake slip vectors along the Alaskan and Kamchatkan peninsulas. The Pacific‐North America velocity averaged over several years equals its velocity averaged over millions of years with the difference along the mutual boundary nowhere exceeding 4±7 mm/yr. Therefore the motions between the Pacific and North American plates appear to be steady. Moreover, our results imply that one can combine rates averaged over intervals as long as millions of years with rates averaged over intervals as short as a few years when constructing models of lithosphere kinematics.
Terrestrial heat flow in east and southern Africa
Authors: Nyblade Andrew A. , Pollack Henry N. , Jones D. L. , Podmore Francis , Mushayandebvu Martin
Published: February 2008

We report 26 new heat flow and 13 radiogenic heat production measurements from Zimbabwe, Zambia and Tanzania, together with details and some revisions of 18 previous heat flow measurements by other investigators from Kenya and Tanzania. These measurements come from Archean cratons, Proterozoic mobile belts, and Mesozoic and Cenozoic rifts. Heat flow data from eight new sites in the Archean Zimbabwe Craton are consistent with previous measurements in the Archean Kaapvaal‐Zimbabwe Craton and Limpopo Belt (Kalahari Craton) and do not change the mean heat flow of 47±2 mW m−2 (standard error of the mean) in the Kalahari Craton based on 53 previous measurements. Eight new sites in the Archean Tanzania Craton give a mean heat flow of 34±4 mW m−2. The mean heat flow from nine sites in the Proterozoic Mozambique Belt to the east of the Tanzania Craton in Kenya and Tanzania is 47±4 mW m−2. Twelve measurements in the Mesozoic rifted continental margin in east Africa give a mean heat flow of 68±4 mW m−2; four measurements in the Mesozoic Luangwa and Zambezi Rifts range from 44 to 110 mW m−2 with a mean of 76±14 mW m−2. In comparing heat flow in east and southern Africa, we observe a common heat flow pattern of increasing heat flow away from the centers of the Archean cratons. This pattern suggests a fundamental difference in lithospheric thermal structure between the Archean cratons and the Proterozoic and early Paleozoic mobile belts which surround them. Superimposed on this common pattern are two regional variations in heat flow. Heat flow in the Tanzania Craton is lower by about 13 mW m−2 than in the Kalahari Craton, and in the Mozambique Belt in east Africa heat flow is somewhat lower than in the southern African mobile belts at similar distances from the Archean cratonic margin. The two regional variations can be explained in several ways, none of which can as yet be elevated to a preferred status: (1) by variations in crustal heat production, (2) by thin‐skinned thrusting of the Mozambique Belt over the Tanzania Cratonic margin, (3) by lateral heat transfer from beneath the rift flanks into the rifts, or (4) by lower mantle heat flow beneath all of eastern Africa prior to the Cenozoic development of the East African rift system.
Paleointensity high at 9000 years ago from volcanic rocks in Japan
Authors: Tanaka Hidefumi
Published: February 2008

Six paleointensities were determined from young lava flows of Fuji and Oshima Volcanoes whose ages are around 8000 to 15000 years B.P. Two lava flows which have well‐defined ages at 8970±390 years B.P. and 9260±430 years B.P. give large paleointensities of 67±12 and 65±6 μT, respectively. Another lava flow of less accurate age also gives large paleointensity of 63±6 μT. The new paleointensity data are consistent with the trend of averaged paleointensity variation in Japan for the last 9000 years. They form a peak of paleointensity high at around 9000 years B.P. comparable to the well‐documented one at 2000 years B.P. This suggests that there exists a dominant component with a characteristic period of about 7000 years in paleointensity variation. Large paleointensity at about 8000–10000 years ago is also reported in France and Iceland. Paleointensity variation averaged every 1000 years for the last 14000 years is quite similar in Europe and Japan. The sine‐curve‐like variation of paleointensity observed in both Europe and Japan probably reflects the nature of the dipole moment if we consider much difference of longitude between the two regions.
Enclaves from the A‐type granite of the Mégantic Complex, White Mountain Magma Series: Clues to granite magmagenesis
Authors: Bédard Jean
Published: February 2008

The subsolvus biotite‐hornblende‐granite at Mégantic contains mafic to leucocratic enclaves. The groundmass of leucocratic enclaves is microgranitic; that of mafic enclaves is meladioritic. Apatite is commonly acicular in the enclaves, suggesting quenching of mafic magmas commingled with the granitic host. Mafic enclaves contain mafic clots (hornblende cores, biotite rims), some with relic cores of viAl‐Cr‐rich diopsidic augite similar to phenocrysts in associated basaltic dykes. The clots are interpreted to be pseudomorphs after mafic phenocrysts. The enclaves also contain granitic microxenoliths, xenocrysts of quartz with hornblende reaction rims, partially resorbed oligoclase and perthite megacrysts and biotite xenocrysts. The xenocrysts record hybridization between a mafic enclave magma and a granitic magma and/or rock. Enclaves define mixing lines on many variation diagrams. However, Rb, Sr, Ba, Zr, Nb, Y, REE, Zn, Pb, and F do not fit this pattern and appear to have been redistributed by residual granitic melts or exsolved volatiles. The data suggest that the granite is anatectic, heat being provided by crystallization of associated basalts. Mixing calculations suggest that as little as 5% hybridization with basalt or hawaiite can account for the Sr isotopic heterogeneity of the granites.
Melting relations in the iron‐sulfur system at ultra‐high pressures: Implications for the thermal state of the Earth
Authors: Williams Quentin , Jeanloz Raymond
Published: February 2008

The melting temperatures of FeS‐troilite and of a 10 wt % sulfur iron alloy have been measured to pressures of 120 and 90 GPa, respectively. Our results document that FeS melts at a temperature of 4100 (± 300) K at the pressure of the core‐mantle boundary. Eutectic‐like behavior persists in the iron‐sulfur system to the highest pressures of our measurements, in marked contrast to the solid‐solution‐like behavior observed at high pressures in the iron‐iron oxide system. Iron with 10 wt % sulfur melts at a similar temperature as FeS at core‐mantle boundary conditions. If the sole alloying elements of iron within the core are sulfur and oxygen and the outer core is entirely liquid, the minimum temperature at the top of the outer core is 4900 (± 400) K. Calculations of mantle geotheims dictate that there must be a temperature increase of between 1000 and 2000 K across thermal boundary layers within the mantle. If D″ is compositionally stratified, it could accommodate the bulk of this temperature jump.