bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismic wave motion for a new model of hydraulic fracture with an induced low‐velocity zone
Authors: Mahrer Kenneth D. , Mauk Frederick J.
Published: February 2008

Hydraulic fracture treatments induce microseismicity (i.e., discrete events plus noise) which can be recorded only by seismometers located in or near the treatment well bore. Seismograms recorded in the treatment well bore are composed of interacting phases which to date defy discrete identification and preclude standard inversion techniques to determine source characteristics, propagation path characteristics, or both. To understand the recorded data, synthetic seismograms have been generated using a finite difference approximation for a two‐dimensional hydraulic fracture model which includes a treatment‐induced, low‐velocity zone (LVZ). The LVZ is produced by pore dilatancy in response to formation fluid pressurization caused by pressure diffusion from the treatment fluid and fracture opening. Borehole, seismometer coupling, fluid transport, and dynamic fracture effects are not considered. The models examined showed that a fixed‐aperture fracture plus surrounding LVZ acted as a leaky waveguide confining the seismic energy and altering propagation characteristics which resulted in unconventional seismograms. The seismograms were composed of multiple transient phases caused by phase interactions and conversions at the waveguide boundaries and stationary phases corresponding to the eigenfrequency structure of the fracture‐LVZ waveguide.
Geology of Panamint Valley ‐ Saline Valley Pull‐Apart System, California: Palinspastic evidence for low‐angle geometry of a Neogene Range‐Bounding Fault
Authors: Burchfiel B. C. , Hodges K. V. , Royden L. H.
Published: February 2008

Geological studies support the interpretation that northern Panamint Valley and Saline Valley, southeast California, form paired pull‐apart basins on opposite sides of the right‐slip Hunter Mountain Fault Zone. Eight to ten km of late Cenozoic net slip can be established on the Hunter Mountain Fault Zone. Palinspastic reconstruction of northern Panamint Valley indicates that the valley was formed by movement on a shallow crustal, low‐angle normal fault of 0–15 degree west dip during the last 3.0 Ma. This interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low‐angle faulting and that the most recent extension in the Basin and Range Province is accomodated exclusively by high‐angle faulting. Saline Valley, however, is interpreted to have formed by extension on closely spaced, rotated planar normal faults. Thus, within one geometric system of paired pull‐apart basins, extension appears to have been accommodated in the shallow crust in two different ways.
Internal friction in tholeiitic basalts
Authors: Weiner Adam T. , Manghnani Murli H. , Raj Rishi
Published: February 2008

Internal friction in two tholeiitic basalts from Deccan Traps (India) was measured at 1.3 Hz in the temperature range 500°–900°C. The results gave two attenuation peaks, one at approximately 620°C and the other near 830°C. The peaks are attributed to bimodal grain size distribution. (In contrast, poly crystalline alumina with a single grain size distribution gave one attenuation peak at 810°C.) Grain boundary sliding, in the presence of an intergranular fluid phase, is suggested as the possible mechanism of anelasticity in the basalts. With this assumption, we estimate the viscosity of the palagonite glass in the basalts to be 1.7 × 1013 P in the temperature range 600°–640°C and 2.6–5.0 × 109 P in the temperature range 820°–840°C. These estimates are in general agreement with direct measurement of viscosity in tholeiite basalt by Scarfe (1977)
Precision of two‐color geodimeter measurements: Results from 15 months of observations
Authors: Langbein J. O. , Linker M. F. , McGarr A. F. , Slater L. E.
Published: February 2008

Spectral analysis of the biweekly measurements of line lengths from the 12‐baseline network at Pearblossom, California, reveals that the precision of the prototype two‐color Geodimeter is σ = [a2 + D2b2]1/2, where a = 0.3 mm and b = 0.12 ppm and D is the length of the baseline. In contrast, the computed precision is overestimated by 15–20% when a secular trend is fit to 15 months of measurements. By using spectral analysis, the variance σ2 can be separated into two components: the frequency independent instrumental precision and a frequency dependent part. Evaluation of the same data with regard to their dependence upon fluctuations in weather reveal that at most, weather makes a second‐order contribution of 0.01 ppm to the observed strain changes. This contribution is a factor of 5–10 times smaller than the instrumental precision. Thus the observed strain changes at Pearblossom reported by Langbein et al. (1982) during 1981 are not due to systematic errors resulting from an improper model of refraction in the atmosphere.
A micromagnetic study of pseudo single‐domain remanence in magnetite
Authors: Enkin R. J. , Dunlop D. J.
Published: February 2008

The magnetic structure in fine grains of magnetite has been studied theoretically using a micromagnetic model which is unconstrained except for requiring Bloch‐type domain walls. Stable structures were found by second‐order numerical minimization of exchange, crystalline anisotropy, and demagnetizing energies. Single‐ (SD), two‐ (2D) and three‐domain (3D) structures were studied for particles at room temperature and in zero applied field. Two‐domain structures with relatively large domains have very small net magnetizations because of “skirts” which oppose wall moments. In particles almost filled with domain walls the net magnetization can be quite high because wall moments are uncompensated. Three‐domain structures always have a relatively large remanence due to unbalanced domains. The critical SD‐2D equilibrium size d0 was determined to be 0.084 ± 0.012 μm. The size range over which single‐domain structures can exist is smaller than previously predicted for perfect crystals. Experimental saturation remanence data on submicron precipitated magnetite crystals can be reasonably well fit with the present results, assuming that magnetic structures assume the lowest energy state. A technique for studying the transition mechanism between stable states is developed which demonstrates that the SD reversal mechanism is coherent rotation in particles around the critical superparamagnetic size, while domain wall nucleation, propagation, and denucleation become favored at larger sizes.
Simultaneous travel time inversion for earthquake location and subduction zone structure in the Central Aleutian Islands
Authors: Engdahl E. R. , Gubbins D.
Published: February 2008

A combined location and velocity inversion technique is applied to travel time data from 151 well‐recorded central Aleutian earthquakes. The data include P and S wave arrivals at stations of a local network and P, pP, and sP wave arrivals at teleseismic stations. After correction for crustal structure at the reflection points, the depth phases pP and sP provide important constraints on subduction zone structure not ordinarily resolved by other types of data. The structure is assumed to vary only across the arc and in depth; it is parameterized with cubic splines by specifying the velocity at each point of a gridded arc cross section. To avoid ray tracing, it is assumed that the velocity part of the problem is linear. Preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab, in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab. A slab thickness of 80–100 km and a downdip length of about 400 km, well below the deepest seismic activity, are indicated. The slab is characterized by seismic velocities as much as 11% higher than the surrounding mantle in its upper portions and 4–6% higher at depth. A sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab. The slab anomaly appears to spread out and fall off very slowly with depth; these are probably not real effects but consequences of the omission of three‐dimensional ray tracing.
Effect of a shallow low‐viscosity zone on the formation of midplate swells
Authors: Robinson E. M. , Parsons B.
Published: February 2008

This study uses a finite element numerical method to analyze the effect of a low‐viscosity zone on convection driven by heating from below in the upper mantle. A swell results from the surface expression of the convection cell. The viscosity structure of the upper mantle has been approximated by three layers consisting of a conducting lid which overlies a low‐viscosity layer which in turn overlies a constant viscosity layer extending to the base of the upper mantle. The low‐viscosity zone causes the top boundary layer of the convection cell to thin, and at high viscosity contrasts and Rayleigh numbers, it can cause the boundary layer to go unstable. The low‐viscosity zone also alters the response of the surface observables to the temperature anomalies. In particular, it mitigates the transmission of normal stress to the conducting lid so that the topography and geoid anomalies decrease. The geoid anomaly decreases faster than the topography anomaly, however, so that the depth of compensation can appear to be well within the conducting lid. Because the boundary layer is thinned, the elastic plate thickness also decreases, and since the low viscosity allows the fluid to flow faster in the top layer, the uplift time decreases as well. We have compared the results of this modeling to data at the Hawaii, Bermuda, Cape Verde, and Marquesas swells and have found that it can reproduce their observed anomalies. The viscosity contrasts that are required range from 0.2 to 0.01 and decrease as the age of the swell increases. These magnitudes and the trend with age are consistent with theoretical and other estimates of the viscosity variation in the shallow upper mantle. Convective models can therefore explain the uplift and observed anomalies at midplate swells.
State of stress and relationship of mechanical properties to hydrothermal alteration at Valles Caldera core hole 1, New Mexico
Authors: Dey Thomas N. , Kranz Robert L.
Published: February 2008

We measured the densities, total and microcrack porosities, and ultrasonic velocities of a number of core samples from an 856‐m‐deep core hole near the Banco Bonito vent at Valles Caldera, New Mexico. Reductions in porosity with depth define a zone from about 600 m down where hydrothermal mineralization and recrystallization have been most active. This zone is also reflected in a large decrease in the anisotropy of acoustic velocities. Stress orientation estimates based on microcrack orientations at the 812‐m depth as determined by differential strain curve analysis, as well as anelastic strain recovery measurements on a sample from 472‐m depth, show a horizontal E‐W minimum compression direction and a maximum compression inclined about 30° from vertical.
San Diego Canyon ignimbrites: Pre‐Bandelier tuff explosive rhyolitic volcanism in the Jemez Mountains, New Mexico
Authors: Turbeville B. N. , Self S.
Published: February 2008

Ignimbrites up to 80 m thick underlie the lower Bandelier Tuff in the southwestern part of the Jemez Mountains volcanic field, New Mexico. They were formed during silicic explosive eruptions that preceded the climactic events of the Bandelier Tuffs and collapse of the Valles I (Toledo) and Valles II calderas. Source vents for these units, herein called the San Diego Canyon ignimbrites, probably underlie the southwestern portion of the Valles caldera. Small‐scale caldera collapse may have been associated with the eruption of these ignimbrites, but evidence of an early caldera, or any related vent structure, has since been obliterated. This late Pliocene (about 2.5 Ma), high‐silica rhyolite activity was contemporaneous with eruptions from the predominantly dacitic Tschicoma volcanic center in the northeastern part of the Jemez Mountains and suggests a significantly different history for the Plio‐Pleistocene development of the Jemez Mountains volcanic field than has previously been considered.
Thermal contraction and the state of stress in the oceanic lithosphere
Authors: Haxby William F. , Parmentier E. M.
Published: February 2008

Nonuniform contraction of the oceanic lithosphere as it cools and thickens following its formation at the axis of a spreading center results in a complex three‐dimensional state of deviatoric stress which can be separated into two parts: a thermal bending stress due to changes in the vertical temperature distribution and a thermal contraction stress due to lateral variations in the vertically averaged temperature. We examine thermal contraction stresses due to temperature changes in a thin, semi‐infinite rectangular plate bounded by the spreading center axis. Two stress‐free boundaries, representing transforms or fracture zones, define the plate width or ridge segment length L. With the bottom of the elastic plate defined by a prescribed temperature, the plate thickens as the square root of age, as expected for a thermal boundary layer due to vertical conductive cooling. Above this prescribed temperature, elastic stresses are assumed to relax quickly. Stresses are obtained by properly accounting for the rate of accumulation of the vertically averaged stress as initially stress‐free material is added to bottom of the cooling, thickening plate. The state of thermal contraction stress calculated from this model is characterized by large tensile stresses at the boundaries of the plate and relatively low stresses in the plate interior. At the ridge axis, ridge‐parallel tensile stresses are about 300 MPa, the same as if the plate were not allowed to contract in this direction. Along the transform boundary, the maximum transform‐parallel tensile stress occurs at a distance L/2 from the ridge‐transform intersection, where its magnitude is comparable to the stress at the ridge axis. The tangential stresses at the plate boundary decrease rapidly with distance from the boundary; at a distance of L/4 from the ridge axis the ridge axis parallel stresses are one tenth of their ridge axis magnitude. The stress magnitudes are independent of both spreading rate and ridge segment length. A large transform‐parallel tensile stress may control the length of transform offsets. Thermal bending moments are influenced by the large thermal contraction stresses near the ridge axis. However, a short distance from the ridge these moments attain their free horizontal contraction values which previous studies have shown to cause observable bending of the plate and a geoid anomaly at fracture zones. Flexure due to thermal bending moments will concentrate bending stresses at a distance from the fracture zone determined by the flexural length of the plate, thus providing a natural length scale controlling the spacing of transforms.
Effects of contact line movement on the dissipation of waves in partially saturated rocks
Authors: Miksis Michael J.
Published: February 2008

A model is presented to describe the attenuation of seismic waves in a partially saturated rock. In particular, we include the dissipation due to contact line movement. In some cases this can dominate the dissipation mechanisms. The model includes both dissipation due to viscosity and contact line movement. This additional dissipation can increase the specific dissipation function by an order of magnitude. This could account for some of the observed difference between theory and experiment. An exact calculation of the dissipation due to contact line movement in an in viscid fluid is also presented.
North America‐Pacific Plate Boundary, An elastic‐plastic megashear: evidence from Very Long Baseline Interferometry
Authors: Ward Steven N.
Published: February 2008

I investigate a number of lithospheric deformation models based upon the concept that the motions of points near the North America‐Pacific plate boundary are a linear combination of North America and Pacific velocities. This concept entails a shear strain transition zone in the vicinity of the boundary. The best of these models fit 95% of the variance in 139 Very Long Baseline Interferometry (VLBI) length and transverse velocity observations taken over 5.5 years and require only 90% of the relative plate motion predicted by RM2. Residual misfits in the megashear models are largely explained by a 6.1‐mm/yr component of Basin and Range spreading perpendicular to the plate boundary. Instantaneous shear deformation associated with plate tectonics is apparently developing in a zone 450 km wide paralleling the San Andreas Fault. Some of this deformation will be recovered through elastic rebound; the rest will be permanently set through plastic processes. Thus depending on the elastic‐plastic composition of the borderlands, the width and pattern of strain accumulation in the megashear over geological time may or may not be significantly different than what is currently observed. Because VLBI data have been assembled for only a small fraction of the earthquake cycle, plastic and elastic strains are indistinguishable. Other information will have to be brought to bear to discriminate elastic and plastic behaviors and reconcile instantaneous and geologic deformation near the North America‐Pacific margin.
What's New at Long Valley
Authors: Goldstein N. E. , Stein R. S.
Published: February 2008

At few places in the conterminous United States does magma, the raw material of the Earth's crust and mantle, appear as tantalizingly close to the surface as at Long Valley caldera in eastern California. Here the search for a site to drill into a magma chamber for the purpose of geothermal energy development coincides with a separate scientific imperative, the need to monitor the caldera for potential hazards due to earthquakes and volcanic eruptions. The region has a proven potential for these hazards; five M ≥ 6 earthquakes have struck since 1978, and phreatic, ash and steam, eruptions have occurred as recently as 550–600 years ago [Miller, 1985]. As we assess the results of additional intermediate depth holes and the results from a widening spectrum of divining tools, the magma seems farther from the surface and less abundant than it did just a few years ago.
Southeast Indian Ridge Between the Rodriguez Triple Junction and the Amsterdam and Saint‐Paul Islands: Detailed Kinematics for the Past 20 m.y.
Authors: Royer Jean‐Yves , Schlich Roland
Published: February 2008

The Southeast Indian Ridge has the fastest spreading rates of the three mid‐oceanic ridge systems of the Indian Ocean and has recorded the movements of Antarctica relative to Australia and India since the Late Cretaceous. New bathymetric and magnetic data have been collected by the R/V Marion Dufresne (1983) and the R/V Jean Charcot (1984), on the western part of this ridge, between the Rodriguez Triple Junction (25.5°S, 70.0°E) and the Amsterdam and Saint‐Paul islands (38°S, 78°E). These data bring additional information on the seafloor magnetic pattern produced by the Southeast Indian Ridge during the past 20 m.y. A new tectonic chart is proposed for the area around the Amsterdam and Saint‐Paul islands. We have mapped 17 isochrons ranging from anomalies 6 to 1 (20.5–0.7 Ma) based on the compilation of all the data available in this area (25 cruises). Their distribution clearly shows asymmetric features. Reconstructions at short time intervals show that stage poles of rotation describe oscillatory movements along a direction parallel to the Southeast Indian Ridge axis. Observed changes in spreading rates and the stability of the spreading directions since the Miocene support this result.
Sea Beam survey of an active strike‐slip fault: The San Clemente fault in the California Continental Borderland
Authors: Legg Mark R. , Luyendyk Bruce P. , Mammerickx Jacqueline , de Moustier Christian , Tyce Robert C.
Published: February 2008

The San Clemente fault, located in the California Continental Borderland, is an active, northwest trending, right‐lateral, wrench fault. Sea Beam data are used to map the major tectonic landforms associated with active submarine faulting in detail unavailable using conventional echo‐sounding or seismic reflection data. In the area between North San Clemente Basin and Fortymile Bank, the major late Cenozoic faults are delineated by alignments of numerous tectonic landforms, including scarps, linear trenches, benches, and sags. Character and spatial patterns of these landforms are consistent with dextral wrench faulting, although vertical offsets may be substantial locally. The main trace of the San Clemente fault cuts a straight path directly across the rugged topography of the region, evidence of a steeply dipping fault surface. Basins or sags located at each right step in the en echelon pattern of faults are manifestations of pull‐apart basin development in a right‐slip fault zone. Seismic reflection profiles show offset reflectors and a graben in late Quaternary turbidites of the Navy Fan, where the fault zone follows a more northerly trend. Modern tectonic activity along the San Clemente fault zone is demonstrated by numerous earthquakes with epicenters located along the fault's trend. The average strike of the San Clemente fault is parallel to the predicted Pacific‐North American relative plate motion vector at this location. Therefore we conclude that the San Clemente fault zone is a part of the broad Pacific‐North American transform plate boundary and that the southern California region may be considered as a broad shear zone.
Wave field processing of data from a large‐aperture seismic experiment in southwestern Oklahoma
Authors: Chang Wen‐Fong , McMechan George A. , Keller G. Randy
Published: February 2008

Data from a wide‐aperture survey performed in January 1985 in southwestern Oklahoma have been processed using prestack imaging, and the images are interpreted to reveal a new picture of the crustal structure in the Wichita Uplift and the Anadarko Basin. As the data were in recording apertures of approximately 90 km, it was necessary to develop new processing concepts and software. A complete new imaging system, based on common source gathers, was conceived, implemented, tested, and applied to these data. The heart of the system consists of velocity estimation by slant stacking, and acoustic prestack reverse time, finite difference migration. The migration produces reflectivity images in regions of 50 km × 100 km, has no dip restrictions, and operates in an arbitrarily complicated two‐dimensional velocity distribution. For comparison the data were also processed using standard common midpoint processing, but this is a less valid approach for wide‐aperture data. The main ambiguities and limitations in processing and interpretation are a consequence of the small number of shots. Processing and interpretation of the data provide a new picture of the subsurface structure of southwestern Oklahoma. The environment is clearly compressive with thrust faulting as the main accommodating mechanism in the upper and central crust. Beneath a possible detachment zone at ≈30 km depth the lower crust has responded to the compression by thickening by ≈50%. The Moho is interpreted to be ≈45 km in depth beneath the thrust zone, shallowing to ≈40 km beneath the Wichita Uplift and the Anadarko Basin.
Bulk moduli of magnesiowüstites from static compression measurements
Authors: Richet Pascal , Mao Ho‐Kwang , Bell Peter M.
Published: February 2008

The static compression of two magnesiowüstites with 60 and 80 mol % FeO has been measured up to 500 kbar under quasi‐hydrostatic conditions. Within the framework of a Birch‐Murnaghan equation of state, these and other data for magnesiowüstites are consistent with linear variations of the 1‐bar isothermal bulk modulus and of its 1‐bar pressure derivative, from pure wüstite (Ko = 1.545 ± 0.01 Mbar, Ko′ = 2.75 ± 0.3) to periclase (Ko = 1.600 ± 0.005 Mbar, Ko′ = 4.85 ± 0.2). The composition with 80% FeO, but not that with 60% FeO, exhibits a second‐order like phase change analogous to the magnetic transition already described for wüstite at around 150 kbar. The lack of a significant dependence of the bulk moduli on nonstoichiometry suggests that the effect of pressure on the defect structure is not as simple as assumed previously.
Seasat altimetry and the South Atlantic Geoid: 2. Short‐wavelength undulations
Authors: Gibert Dominique , Courtillot Vincent , Olivet Jean‐Louis
Published: February 2008

This paper is a discussion of the short‐wavelength (25–100 km) undulations of the geoid recovered from Seasat altimeter data in the South Atlantic. These undulations are known to be highly correlated to seafloor topography, and they considerably improve our knowledge about global bathymetry of the South Atlantic, which remains poorly surveyed. The orientation of fracture zones (FZ) with respect to the Seasat profiles ensures an efficient recovery of the FZ pattern in the younger (<65 Ma) parts of the South Atlantic between 20°N and 50°S. We used 22 well‐marked FZs regularly distributed between 12°N and 47°S to compute Eulerian poles describing the finite relative movement between Africa and South America. From the present back to 35 Ma, FZ trends south of the equator are well accounted for with a single rotation, and standard statistical tests do not indicate a need for distinct and successive rotations. However, these tests indicate an improvement of the fit when distinct poles are adjusted to separate subsets of our 22 FZs (either FZs north and south of the equator, or FZs north and south of Tristan da Cunha). This points out the possibility of the existence of either “corridors” in which slight relative movements occur, or diffuse elastic deformation over the entire South Atlantic, although this is really a second‐order effect. We also present and discuss a map of the roughness of the geoid (for the 25‐ to 110‐km wave band). Strong disagreements between the roughness map and the GEBCO maps exist in regions of sparse ship track coverage; for example, the Rio Grande Rise has an associated roughness feature which appears elongated and more symmetric with respect to the Walvis Ridge than what is displayed on the GEBCO maps. The roughness map also clearly documents a long‐wavelength (∼400 km) segmentation of the Mid‐Atlantic Ridge between the equator and 35°S.
Shock temperatures in silica glass: Implications for modes of shock‐induced deformation, phase transformation, and melting with pressure
Authors: Schmitt Douglas R. , Ahrens Thomas J.
Published: February 2008

Gray body temperatures and emittances of silica glass under shock compression between 10 and 30 GPa are determined. Observed radiative temperatures are higher than computed continuum temperatures for shock‐compressed silica glass; however, below ∼26 GPa observed emittances are <0.02. This suggests that fused quartz deforms heterogeneously in this shock pressure range as has been observed in other minerals. Between 10 and 16 GPa, radiative temperatures decrease from 4400 K to 3200 K, whereas above 16–30 GPa, gray body temperatures of ∼3000 K with low emittances are observed. The emittances increase with pressure from 0.02 to 0.9. The pressure range from 10 to 16 GPa coincides with the permanent densification region, while the 16–30 GPa range coincides with the inferred mixed phase region along the silica glass Hugoniot. The differing radiative behaviors may relate to these modes of deformation. Based upon earlier shock recovery experiments and a proposed model of heterogeneous deformation under shock compression, the temperatures associated with low emittances in the mixed phase region probably represent the melting temperature of the high‐pressure phase, stishovite, which can be expected to crystallize from a melt in hot zones. Above 20 GPa the melting temperature of stishovite would therefore be 3000 K±200 K and almost independent of pressure to 30 GPa. The effects of pressure on melting relations for the system SiO2–Mg2SiO4 are considered together with the proposed stishovite melting curve and suggested maximum solidus temperatures within the mantle of ∼2370 K at 12.5 GPa and ∼2530 K at 20.0 GPa. Using the proposed stishovite melting temperatures Tm and estimates of upper mantle temperatures T, the effective viscosity, which can be considered a function of the homologous temperature T/Tm, appears to remain nearly constant from 200 to 600 km depth in the Earth.
Pb, Sr, and Nd isotopic characteristics of Tertiary Red Sea rift volcanics from the central Saudi Arabian Coastal Plain
Authors: Hegner E. , Pallister J. S.
Published: February 2008

Alkalic to subalkalic bimodal Tertiary igneous rocks from the Red Sea coastal plain at Al Lith, Saudi Arabia, represent early rift volcanism in the central part of the Red Sea rift. Initial 143Nd/144Nd and 87Sr/86Sr plot within or near the enriched mid‐ocean ridge basalt (MORB) field and show a variation in ε‐Nd values from +4.1 to +7.7. The data are intermediate between more depleted basalts from the Red Sea spreading axis and less depleted alkalic basalts from the southeastern Arabian shield and carbonatites from the East African rift. Pb isotopic compositions in alkalic and tholeiitic basalts show a variation in 206Pb/204Pb from 18.0 to 18.9 and plot in and slightly above the MORB envelope. Relatively low 206Pb/204Pb and variable 207Pb/204Pb and 208Pb/204Pb in tholeiitic volcanics are compatible with a component from either old lithospheric mantle or lower crust containing ancient sediment. The isotopic compositions suggest mixing of asthenospheric and lithospheric sources but show no evidence for changing proportions of these components during early rift volcanism at Al Lith. Volcanism in the central part of the Red Sea is tholeiitic, consistent with a larger degree of melting or melting at increasingly shallower depths as rifting progresses.
Compressible convection with constant and variable viscosity: The effect on slab formation, geoid, and Topography
Authors: Schmeling Harro
Published: February 2008

Two‐dimensional, Cartesian finite difference models of compressible convection with constant and variable viscosity and fixed bottom temperature are presented. Density variations according to the Adams Williamson equation of state are included. In the case of constant viscosity convection, viscous and adiabatic heating damp the flow. Compressible density variations hardly influence geoid undulations; however, the lower thermal boundary layer becomes thinner, and the mean cell temperature increases. In the case of variable viscosity, the nonlinear coupling between compression, adiabatic and viscous heating, and a temperature‐, pressure‐, and stress‐dependent rheology leads to important consequences: The upwelling flow broadens and plumes are retarded. The flow strongly concentrates toward the downwelling region and becomes mechanically decoupled from the interior of the cell by a low‐viscosity region. This mechanism seems to be important for the formation of subducting slabs. Extrapolated to mantle conditions, two low‐viscosity regions are predicted flanking the slab on either side and inhibiting an early dispersal and mixing of slab material into the mantle. This process might be aided by an increase of negative buoyancy forces with depth as observed in the models. Further results are as follows: Increasing the dissipation number in variable viscosity convection may either damp or speed up convection, depending on the rheology. Models with internal heating and a fixed bottom temperature show that the threshold to time‐dependent variable viscosity convection is drastically reduced if the anelastic liquid approximation is applied instead of the extended Boussinesq approximation.
The impacted Martian crust: Structure, hydrology, and some geologic implications
Authors: MacKinnon David J. , Tanaka Kenneth L.
Published: February 2008

We propose that the heavily bombarded Martian crust generally consists of a 10‐km‐thick zone of highly permeable, fractured basement rock overlain by a 1‐ to 2‐km‐thick, relatively impermeable ejecta zone. Seismic, drill core, and observational measurements of terrestrial impact and explosion craters demonstrate that the subjacent rocks are intensely fractured into meter‐sized (or larger) blocks. Gravity measurements over terrestrial and lunar craters indicate that bulking of a few percent occurred in the upper few kilometers of the underlying bedrock. These measurements suggest that fractured Martian basement rocks may have an open fracture porosity of about 1% and permeabilities of at least 103 darcies; these values would decrease with depth due to decreasing fracture intensity and increasing lithostatic pressure. Theoretical impact fragmentation and mixing models and laboratory and field studies indicate that impact ejecta are composed of well‐mixed, poorly sorted clasts that follow power law distributions. On the basis of these distributions, packing theory, experimental relations, and measurements on analogous materials, we estimate that the average porosity of unaltered ejecta within the ejecta zone is 10–20% and their maximum permeability is about 10−2 darcy. Subsequent alteration of ejecta by cementation and compression may reduce porosity and permeability, whereas local fracturing may increase them. Comminution may produce an abundance of clay‐sized material that may effectively retain water or ice and increase ground ice retention in equatorial areas. Our model is consistent with observed 1‐ to 3‐km‐deep mechanical discontinuities in the Martian crust and the susceptibility to erosion of unconsolidated ejecta by sapping and other processes. We further propose that debris flows made up of ejecta and followed by catastrophic floods may explain the development of Chryse region outflow channels with much less water than was required by flooding alone.
Seismic reflection imaging problems resulting from a rough surface at the top of the accretionary prism at convergent margins
Authors: Coltrin George , Backus Milo , Shipley Thomas H. , Cloos Mark
Published: February 2008

Modern seismic reflection profiling has contributed substantially to our understanding of deformation processes at actively convergent plate margins. Nonetheless, the complexity of most seismic profiles leads to great uncertainty about internal prism structures because interpretations of particular profiles can vary from one showing dominantly bedded but faulted sediments to another showing chaotic mélange. The steep dips of bedding and faults form a complex three‐dimensional geometry that causes widely recognized but poorly understood imaging problems. In this report, the specific effects on seismic wave propagation of roughness of a few hundred meters amplitude at the interface between the tectonized sediments of a prism and the overlying slope sediments are systematically analyzed through normal incidence and offset synthetic modeling. For a surface roughness and velocity contrast similar to that between the prism and slope cover off Costa Rica, the modeling shows that there are important focusing and defocusing effects because of wave front distortion at the interface. The synthetic seismic profiles of simple continuous interfaces within a model prism produce discontinuous reflections beneath the interface with amplitudes reduced by a factor of ∼2, and phase changes occurring for over 30% of the rays traced through the models. The significance of these effects increases with the depth and the dip of the reflecting interface. The distortions are greater for dip lines than for strike lines as the structure changes more rapidly in the dip direction. The study demonstrates that wherever there is surface roughness comparable to that found off Costa Rica, the detection of even simple structures within the prism will be quite difficult with only two‐dimensional reflection methods. Three‐dimensional imaging techniques will be necessary at many convergent plate margins to differentiate between models of the prism accretion and deformation.
Ejecta scaling laws for craters in dry alluvial sites
Authors: Lee C. K. B. , Mazzola T. A.
Published: February 2008

The existing factor‐of‐20 scatter in the ejecta thickness data, for surface bursts (chemical and nuclear) in dry alluvium, is reduced to a scatter a factor of ∼4 by a simple yield correction. This correction accounts for the mismatch in the scaling laws that govern the formation of the ejecta blanket and of the apparent crater. Craters from surface bursts obey strength scaling, in which linear crater dimensions scale as cube root of the yield. Their ejecta blankets, on the other hand, obey gravity scaling, in which linear ejecta blanket dimensions scale as yield to a power of less than a third. For this reason, geometric scaling (scaling the range and the ejecta thickness by the crater radius) is appropriate for craters in the gravity regime. However, it is not the appropriate scaling rule for ejecta from surface bursts and is the reason for the observed factor‐of‐20 scatter. A simple correlation is proposed that agrees with the data to better than a factor of 2 in the continuous ejecta region, and slightly more than a factor of 2 in the discrete ejecta region. For buried bursts, the craters obey 1/3.4 scaling, a transition between the strength and gravity regimes, while the ejecta blankets scale with gravity. Despite this slight mismatch in the scaling, field data for buried bursts agree with a proposed geometric‐scaled correlation to approximately a factor of 2 in the continuous ejecta region and approximately a factor of 5 in the discrete ejecta region.
Elastic properties of contacting surfaces under normal and shear loads: 1. Theory
Authors: Yoshioka Naoto , Scholz Christopher H.
Published: February 2008

A theory of contact between two elastic rough surfaces is presented that predicts, from knowledge of the surface topography, both the normal and shear deformation. The theory was modified from one developed by others in order to properly account for initial overlap of the two surfaces and to allow for calculation of shear stiffnesses under no‐slip conditions. Since the theory is critically dependent on the surface topography, a new method is introduced for the derivation of the real topographic parameters. The result of theoretical calculation shows that the normal closure nonlinearly increases with normal stress and is larger for rougher surfaces and that the shear stiffness also nonlinearly increases with normal stress and is greater for smoother surfaces than rougher ones. The verification of the theory will be made by comparing it with experimental data in a companion paper.
Upper mantle P velocity structure of the Canadian Shield
Authors: LeFevre L. Victoria , Helmberger Donald V.
Published: February 2008

Waveform modeling of the phases Pnl, P, and PP is used to determine the P wave upper mantle structure beneath the Canadian shield. Long‐period recordings at World‐Wide Standard Seismograph Network, long‐range seismic measurement, and Canadian network stations in North America are used, with earthquakes within and just outside the shield as sources. The P data set is incomplete due to the scarcity of large events within the shield, and the inclusion of PP provides much more data. Using the WKBJ approach to calculate the mantle response, the PP waveforms can be modeled almost as well as the P. The phase Pnl persists to large distances in shield regions, and the interference of Pnl with triplication range P helps constrain the structure in the upper 400 km. Important features of our model, which is designated S25, are (1) a thick, high‐Q, high‐velocity lid to 165 km, (2) a deep low‐velocity zone, and (3) discontinuities of 5% at 405 km and 4% at 660 km. The lid and low‐velocity zone create an unusual B branch arrival, not seen in other regions, which persists well beyond 30°. Only slight lateral variation is noted within the shield region. The major features of the model are similar to those of Grand and Helmberger's S wave model for the Canadian shield.
Wave field processing of data from the 1986 Program for Array Seismic Studies of the Continental Lithosphere Ouachita Experiment
Authors: Chang Wen‐Fong , McMechan George A.
Published: February 2008

In May 1986, a wide‐aperture seismic field experiment was conducted in southwestern Arkansas and northwestern Louisiana. This experiment was sponsored by the Incorporated Research Institutions for Seismology as part of the Program for Array Seismic Studies of the Continental Lithosphere. The survey line runs from the south edge of the Ouachita Mountains, out into the Gulf Coastal Plain. The experiment involved 21 shot points and two major deployments, each of 400 recorders, with an effective aperture of 200 km. The goal of the experiment was to attempt to provide constraints on the diverse development sequences that have been proposed to explain the structure of the area. The data were processed using a previously developed, prestack common source acoustic finite difference migration to produce images of reflectivity in a slice 200 km long and 40 km deep. This migration algorithm has no dip restrictions and uses a two‐dimensional, variable velocity distribution for wave extrapolation. Interpretation of the results of the prestack processing, together with the results of complementary two‐dimensional ray trace modeling (not shown here) of the data, provides an enhanced picture of the structure of the Paleozoic continental margin that is consistent with the observations in most previous geophysical and geological studies. The main physical and temporal features of the model are crustal thinning during Precambrian rifting, a continent‐ocean boundary sedimentary wedge and carbonate bank, thrusting of deepwater sediments during the Ouachita orogeny, and Triassic rifting.
Empirical Green's Functions from small earthquakes: A waveform study of locally recorded aftershocks of the 1971 San Fernando Earthquake
Authors: Hutchings Lawrence , Wu Francis
Published: February 2008

Seismograms from 52 aftershocks of the 1971 San Fernando earthquake recorded at 25 stations distributed across the San Fernando Valley are examined to identify empirical Green's functions, and characterize the dependence of their waveforms on moment, focal mechanism, source and recording site spatial variations, recording site geology, and recorded frequency band. Recording distances ranged from 3.0 to 33.0 km, hypocentral separations ranged from 0.22 to 28.4 km, and recording site separations ranged from 0.185 to 24.2 km. The recording site geologies are diorite gneiss, marine and nonmarine sediments, and alluvium of varying thicknesses. Waveforms of events with moment below about 1.5×1021 dyn cm are independent of the source‐time function and are termed empirical Green's functions. Waveforms recorded at a particular station from events located within 1.0 to 3.0 km of each other, depending upon site geology, with very similar focal mechanism solutions are nearly identical for frequencies up to 10 Hz. There is no correlation to waveforms between recording sites at least 1.2 km apart, and waveforms are clearly distinctive for two sites 0.185 km apart. The geologic conditions of the recording site dominate the character of empirical Green's functions. Even for source spatial separations of up to 20.0 km, the empirical Green's functions at a particular site are consistent in frequency content, amplification, and energy distribution. Therefore, it is shown that empirical Green's functions can be used to obtain site response functions. The observations of empirical Green's functions are used as a basis for developing the theory for using empirical Green's functions in deconvolution for source pulses and synthesis of seismograms of larger earthquakes.
Possible effects of breaking gravity waves on the circulation of the middle atmosphere of Mars
Authors: Barnes Jeffrey R.
Published: February 2008

A simplified (quasi‐geostrophic, beta‐plane) dynamical model of the zonal‐mean flow is used to investigate the possible effects of breaking internal gravity waves on the circulation in the middle atmospheric region (∼10–100 km altitude) of Mars. The gravity wave parameterization employed is basically the saturation parameterization of Lindzen (1981) which has been utilized in several terrestrial modeling studies. Numerical simulations are performed for a range of plausible values of the gravity wave parameters and for different representations of the mean flow thermal forcing, with a focus on the effects of topographically forced stationary gravity waves on the winter hemisphere circulation. The results of the model simulations show that breaking gravity waves (of intermediate horizontal scales, ∼100–1000 km wavelengths) could act to “close off” the winter westerly circumpolar jet in the ∼40–80 km altitude region and induce strong high‐latitude warming at these levels. The latter could explain recent thermal measurements indicating that Martian winter polar latitudes may be very warm, far above radiative equilibrium, in this middle atmospheric region [Deming et al., 1986]. For the most plausible range of parameter values breaking gravity waves do not induce much warming below ∼40 km, consistent with available data showing that winter high latitudes are normally very cold at lower levels. Several “dust storm” cases indicate that breaking gravity waves possibly could act to induce a very large polar warming at relatively low altitudes, such as was observed during the 1977 winter solstice dust storm. However, quite large gravity wave amplitudes and/or relatively long horizontal wavelengths are required for such strong low‐level effects; thermal forcing associated with dust heating might be a source for such anomalous gravity wave activity during a global dust storm. Summer simulations indicate that breaking gravity waves might alter the mean flow at somewhat higher levels than in the winter hemisphere, as a consequence of the presence of westerlies in the lower atmosphere. This could explain the presence of a warm summer pole in the recent 50–80 km thermal measurements of Deming et al. Several aspects of the results relating to the middle atmospheric circulation and the polar warming phenomenon are discussed. The possible importance of breaking gravity waves for constituent transports in the middle atmosphere of Mars is also briefly considered.
H2O grain size and the amount of dust in Mars' Residual north polar cap
Authors: Kieffer Hugh H.
Published: February 2008

In Mars' north polar cap the probable composition of material residual from the annual condensation cycle is a mixture of fine dust and H2O grains of comparable size and abundance. However, metamorphism of such material will gradually lower its albedo by increasing the size of the H2O grains only. If the cap is undergoing net annual sublimation (as inferred from water vapor observations), late summer observations should be of old ice with H2O grain sizes of 100 μm or more. Ice of this granularity containing 30% fine dust has a reflectivity similar to that of dust alone; the observed albedo and computed ice grain size imply dust concentrations of 1 part per 1000 or less. The brightness of the icy areas conflicts with what would be expected for a residual cap deposited by an annual cycle similar to that observed by Viking and aged for thousands of years. The residual cap surface cannot be “old dirty” ice. It could be old, coarse, and clean; or it could be young, fine, and dirty. This brings into question both the source of the late summer water vapor and the formation rate of laminated terrain.