bluuebunny/crossref_metadata_2025 huggingface dataset:
The nature of seismic reflections from deep crustal fault zones
Authors: Jones Terry D. , Nur Amos
Published: February 2008

Deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement. Perhaps the best example is the Consortium for Continental Reflection Profiling (COCORP) Wind River line in Wyoming, which shows the Wind River thrust fault to be a strong reflector from the surface to depths of about 30 km. To identify the physical properties responsible for the seismic reflections from fault zones, we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones. These rocks are characterized by extensive ductile deformation of plastic minerals, brittle deformation of the more rigid minerals, grain size reduction, and development of a strong fabric. Average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock. Seismic anisotropy of 7% or greater is present in several mylonites dependent upon their composition and fabric. Using our data on mylonite properties and models for the crustal structure near the Wind River line, we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the COCORP Wind River reflection data. When anisotropy and the finely laminated structure of the fault zone are considered, our modeling indicates fairly strong reflections from mylonite zones in most cases. We also show that elevated pore pressure in shear zones may produce strong reflections. However, a permeability of the order of 10−16darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive.
Surface and interfacial free energies of quartz
Authors: Parks George A.
Published: February 2008

Water and electrolytes cause significant changes in brittle fracture strength and subcritical fracture propagation velocities in quartz and quartz rocks. The changes may be caused, in part, by changes in surface free energy. Experimental fracture surface energies of quartz range from about 400 mJ m−2 to about 11.5 J m−2. Thermodynamic surface free energies are likely to be lower than fracture surface energies owing to dissipative energy losses and failure to achieve equilibrium surface structure. Thermodynamic surface free energies are sensitive to environmental composition. Reaction of water vapor with pristine fracture surfaces reduces surface energy by hydroxylation, but the extent of reduction is not known. Adsorption of water vapor on the hydroxylated surface and immersion in liquid water reduce surface energy by 75–230 and 72 mJ m−2 respectively. In an electrolyte, the surface free energy is maximum at the point of zero charge, where adsorption of ionic solutes is least. Adsorption of hydrogen ion, hydroxide ion, and electrolytes reduce surface energy as concentration increases, by tens of mJ m−2. All of these surface energy changes are qualitatively consistent with changes in fracture behavior caused by the same variables.
Oxygen diffusion in quartz
Authors: Giletti Bruno J. , Yund Richard A.
Published: February 2008

Diffusional oxygen exchange between quartz and water was studied by determining 18O profiles in the quartz using an ion microprobe. Inverse error function plots of the data are linear as required for a diffusion mechanism. One natural and two synthetic samples were used, and although they had different impurity concentrations (OH−, Na, and Al per 106 Si atoms are 7–4000, 35–835, and 67–127, respectively), the D values were the same within experimental uncertainty. Diffusion parallel to c is about 2 orders of magnitude faster than that normal to c at 700°C, and diffusion normal to the rhombohedron is intermediate. The data indicate a change in slope on an Arrhenius plot at the α‐β boundary. At 100‐MPa (1 kbar) water pressure the preexponential factors (Do, in square centimeters per second) and activation energies (Q, in Kilojoules per mole) for α quartz (500°–550°C) are as follows: Do = 190 and Q = 284 parallel to c, and Do = 8 × 10−2 and Q = 238 normal to the rhombohedron. For β quartz (600°–800°C), Do = 4 × 10−7 and Q = 142 parallel to c, Do = 9 × 10−7 and Q = 155 normal to the rhombohedron; and Do = 1 × 10−4 and Q = 234 normal to c. D increases with PH 2 O, and between 25 and 350 MPa log D versus logƒH 2 O has a slope of approximately 1.1. These D values are all several orders of magnitude larger than previously reported values for oxygen diffusion in water‐free systems.
Subcritical crack growth in geological materials
Authors: Atkinson Barry Kean
Published: February 2008

A review is presented of the experimental data on subcritical crack growth in geological materials. The main parameters describing subcritical crack growth are the critical stress intensity factor Kc, the subcritical crack growth limit Ko, and the stress intensity factor‐crack velocity (K‐v) relationship between Koand Kc. The K‐v data are presented in terms of an equation in which the crack velocity depends on stress intensity factor raised to a power n because this is common practice in experimental studies. These data are presented as tables and in synoptic diagrams. For silicates the value of n increases as the environment becomes depleted in hyroxyl species and with increase in the microstructural complexity of the solid. Values of n as low as 9.5 have been found for tensile cracking of quartz in basic environments and as high as 170 for tensile cracking of basalt in moist air. Insufficient experimental data are available to predict subcritical crack growth behavior at depth in the earth's crust without major extrapolations of the data base. Schematic outlines are presented, therefore, of the probable influence on subcritical crack growth of some key parameters in the crustal environment. These include stress intensity factor, temperature, pressure, activity of corrosive environmental agent, microstructure, and residual strains. In addition, a discussion is presented of the likely magnitude of the subcritical crack growth limit. For stress corrosion tensile crack growth of quartz a limit of approximately 0.2 of the critical stress intensity factor is inferred from theoretical calculations. Further problems discussed with regard to the extrapolation of experimental data to crustal conditions include the choice of a suitable equation to describe crack growth and the magnitude of parameters in these equations. A brief discussion of the double torsion testing method is presented in order to aid the interpretation of experimental results because it is almost the sole method used to study subcritical cracking in rocks.
Subcritical crack growth and other time‐ and environment‐dependent behavior in crustal rocks
Authors: Swanson Peter L.
Published: February 2008

Stable crack growth strongly influences both the fracture strength of brittle rocks and some of the phenomena precursory to catastrophic failure. Quantification of the time and environment dependence of fracture propagation is attempted with the use of a fracture mechanics technique. We examine some of the difficulties encountered when applying techniques originally developed for simple synthetic materials to complex materials like rocks. A picture of subcritical fracture propagation is developed that embraces the essential ingredients of the microstructure, a microcrack process zone, and the different roles that the environment plays. To do this we examine the results of (1) fracture mechanics experiments on five rock types, (2) optical and scanning electron microscopy, (3) studies of microstructural aspects of fracture in ceramics, and (4) exploratory tests examining the time‐dependent response of rock to the application of water.
Rate‐dependent extensional deformation resulting from crack growth in rock
Authors: Segall Paul
Published: February 2008

Quasi‐static propagation and dilation of macroscopic mode I cracks is considered as a source of inelastic strain in crustal rocks undergoing extensional deformations. The approach here is to first characterize the propagation of a single dilating crack and then to consider how a large two‐dimensional array of similar, parallel cracks accomodates an applied deformation. The driving force for propagation of each crack, Gi, is found as a function of the applied strain and the instantaneous crack lengths ci. The rate of crack propagation is taken to be a function of the instantaneous crack extension force ċi = ċ (Gi). A specific propagation rate extension force relation is developed that both fits the available experimental data and has vanishing propagation rate at a finite value of G. From these results a specific internal state variable constitutive law is derived relating uniaxial stress to the instantaneous uniaxial strain and crack density. For strains less than those necessary to cause crack propagation, the rock is predicted to behave like a linear elastic solid. Following the onset of propagation, the average stress may continue to increase if the strain rate accommodated by crack growth is slow in comparison to the applied strain rate. For constant strain rate boundary conditions the solutions exhibit strain softening, leading to peak stress and then a stress decrease. The peak stress increases nearly logarithmically with increasing applied strain rate. The response to unloading depends strongly on whether or not cracks heal. Unloading that follows sealing of the cracks with mineral precipitates causes the rock to undergo a permanent nonrecoverable strain.
Experimental studies of the role of water in the plasticity of quartzites
Authors: Mainprice D. H. , Paterson M. S.
Published: February 2008

Experiments on several quartzites at 300 MPa confining pressure show them to behave as if near the brittle‐ductile transition at 900°C when oven‐dry and in a more ductile fashion at 1000°C when extra water is added. In both cases the stress‐strain curves are substantially higher than would be expected on the basis of the strength of hydrolytically weakened synthetic crystals containing less “water” than the rocks, as measured by the broad infrared absorption at low temperature. Thus it would appear that, relative to the single crystals, the “effective” concentration of the water‐related species that is active in mechanical weakening within the grains is only a small fraction of the “water” present. The presence of glass in the grain boundaries of “wet” deformed specimens suggests that grain boundary weakening may also be significant in these experiments. Questions of equilibrium and of geological extrapolation are discussed.
Permeability of crystalline rocks: New in situ measurements
Authors: Brace W. F.
Published: February 2008

New in situ measurements from 10 sites, including three in basalt, have about the same range of permeability for crystalline rocks as reported previously, namely, from about 10−18 m2 (1 μdarcy) to 10−13 m2 (100 mdarcy). Highly conductive intervals may be as deep as 3 km.
Geophysical and geological evidence for fracturing, water circulation and chemical alteration in granitic rocks adjacent to major strike‐slip faults
Authors: Stierman Donald J.
Published: February 2008

Gravity and seismic measurements collected on granitic plutons cut by the San Andreas, Garlock, and San Jacinto faults of central and southern California show that these faults are bounded by belts of anomalous rock extending 2–5 km on both sides of the surface traces of the faults. Borehole and petrographic studies show that anomalously low densities and seismic velocities in this zone are due to both fracturing and chemical alteration. Fracturing, water circulation, and alteration are inexorably interrelated. High fracture densities are required to provide the permeability that allows water to reach and react with rocks hundreds to thousands of meters below the surface. Alteration products stable to seismogenic depths include montmorillonite (MO). Since the products of this alteration include expansive clays, this process is self‐limiting. Formation of MO is facilitated by sparse water, whereas kaolinite and other clays can form if the amount of water exceeds the quantity needed for MO. Earthquakes periodically reopen the blocked fractures, a mechanism necessary in generating the broad, deep zone of pervasive alteration inferred from geophysical measurements. These geophysical and petrological patterns are clues not only to the evolution of these faults but also to the dynamics of mechanisms currently operating within the seismogenic zone. This deep circulation model predicts that the strength of a fault segment varies in time. Timing of major earthquakes may be due as much to temporal strength variations as to stress events in the crust. Significant preseismic changes in the physical properties of the crust are probably confined to the zone of fractured rock immediately adjacent to the faults.
Asperities, barriers, characteristic earthquakes and strong motion prediction
Authors: Aki Keiiti
Published: February 2008

Recent results from paleoseismology suggest that earthquakes recurrent on a given fault may often have the same characteristic length and amount of slip. Stable asperities and barriers, which survive many earthquakes, can explain these results. The activity of Volcano Usu, Japan, since 1977 which produced a dacite dome with a total upheaval of 180 m, offered a great opportunity to study the recurrence behavior of a fault. Seismologists at Hokkaido University revealed a family of repeated earthquakes with nearly the same amount of slip generated from the same fault plane on the crater wall, along which the dome moved upward. This family can be explained by the existence of a stable asperity on the crater wall and is called the “asperity‐type” family. They found, however, another type of family of earthquakes, which share the same fault plane but show a great range of variation in the amount of slip. The behavior of the latter family can be explained by the presence of weak barriers over a fault plane defined by strong stable barriers, and this family is referred to as the “barrier‐type” family. This type of earthquake model has been used successfully in the interpretation of observed strong motion acceleration spectra from California, and the results reveal the stability of local stress drop among different earthquakes. The agreement found between local stress drops estimated from strong motion data and those inferred from geological observation supports the possibility for predicting earthquake strong motion directly from geological observation of the causative fault.
Mapping the upper mantle: Three‐dimensional modeling of earth structure by inversion of seismic waveforms
Authors: Woodhouse John H. , Dziewonski Adam M.
Published: February 2008

A method is presented for the inversion of waveform data for the three‐dimensional distribution of seismic wave velocities. The method is applied to data from the global digital networks (International Deployment of Accelerometers, Global Digital Seismograph Network); the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths. The moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity. A global model is constructed for shear wave velocity, expanded up to degree and order 8 in spherical harmonics, and described by a cubic polynomial in depth for the upper 670 km of the earth's mantle. Although no a priori information is incorporated, the model predictions reproduce much of what is known about the dispersion of mantle waves, for example, high phase velocities for shields, low velocities at ridges, and a strong degree 2 pattern for Rayleigh waves. Since the method makes use of complete waveforms, overtone data are also included. It is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently. The model shows that shields and ridges are major features in the depth interval 25–250 km. The ridges of the southern Pacific and the larger shields persist to 350 km, but the SouthEast Indian Rise is underlain by a high‐velocity anomaly at this depth, as is much of the Mid‐Atlantic Ridge. At 450–650 km the major features are a broad region of high velocities incorporating South America, much of the South Atlantic and parts of West Africa, a broad region of low velocities in the central and eastern Pacific, high velocities in the western Pacific, and a low‐velocity anomaly beneath the Red Sea and the Gulf of Aden. In the absence of a crustal correction, degrees 2 and 3 show a high positive correlation with the geoid; paradoxically, this is largely destroyed when the distribution in crustal thickness is taken into account. Spherical harmonic degrees 4–7 show a significant negative correlation.
Focal depths and fault plane solutions of earthquakes and active tectonics of the Himalaya
Authors: Baranowski Jean , Armbruster John , Seeber Leonardo , Molnar Peter
Published: February 2008

We have compared synthetic seismograms with long‐period body waves for nine earthquakes with epicenters in the Himalayan arc to determine depths of foci and to improve fault plane solutions. Focal depths are shallow (10–20 km). Inferred slip vectors are locally perpendicular to the mountain range; they plunge very gently (∼10°( in the eastern sections of the range and more steeply (∼25°) in western sections. Assuming India to be a rigid plate, the radially oriented slip vectors imply that southern Tibet extends at about half the rate of underthrusting in the Himalaya and therefore probably at about 5–10 mm/yr. The shallow depths and gentle dips of the fault planes, at least for the events in the eastern half of the range, are consistent with coherent underthrusting of the Indian plate beneath, at least, the Lesser Himalaya. The steeper dips of fault planes in the western part of the arc might reflect deformation of the overriding thrust plate or simply a steepening of the main underthrusting zone beneath the Greater Himalaya.
Volatile oversaturation during the evolution of silicic magma chambers as an eruption trigger
Authors: Blake Stephen
Published: February 2008

Large, shallow silicic magma chambers can apparently generate their own potential to initiate caldera‐producing eruptions. Differentiation of an initially water‐poor siliceous magma yields a stably graded zone of extremely fractionated low‐density liquids of low temperature, low crystallinity, and high water content at the chamber top, After protracted evolution of such a system, the uppermost magma in the chamber will become oversaturated in water, generating an excess pressure. By equating the magma and chamber volumes at some pressure above the stable lithostatic value, this excess magmatic pressure can be evaluated as a function of the magma's compressibility, water content and solubility, lithostatic pressure (chamber depth), and some assumed measure of the chamber's ability to expand. Critical overpressures between 5 and 25 MPa are believed to be sufficient to rupture most chambers, implying that for rhyolite magma at 850°C, the critical water contents are in the range 3–6 wt % for chambers buried at depths of 1.5–7 km. The vesicularities of the magma in the oversaturated layer are calculated to be ≲1% but may reach ∼10% in cases where the oversaturated layer is a small (less than a few percent) part of the chamber and when the chamber can dilate during the increase in pressure. It follows from this study that fractionated magmas evolving at shallow depth cannot attain water contents much above 6 or 7 wt %, since in so doing they become oversaturated and are ultimately erupted.
On the mechanics of caldera resurgence
Authors: Marsh Bruce D.
Published: February 2008

The mechanisms suggested to cause caldera resurgence can be categorized as (1) viscous rebound of magma, (2) regional detumescence, and (3) pressure against the base of the caldera block due to magmatic vesiculation and convection or addition of new magma. The critical observation that resurgence takes of the order of 103–105 years insists that the causative mechanism be regulated by a highly viscous (e.g., ∼1022 Pa s) structural member of the crust. The relatively low viscosity magma and a purely elastic crust cannot be important in achieving this time scale. The most likely mechanism is the coupled effect of regional detumescence and magmatic vesiculation. Detumescence compresses the magma chamber, squeezing magma upward against and through the caldera block. Vesiculation also compresses the chamber and tends to dome the caldera block. The times of both detumescence and doming of the caldera block by magmatic pressure are probably inversely proportional to the size of the magma chamber, but the rate of cooling of the chamber is proportional to its size. Because solidification spreads viscous relaxation over a wide region of the crust and curtails resurgence, there is likely to be a minimum volume of magma that can undergo resurgent doming. It is difficult to be exact about this critical size until more precise models have been developed.
Catastrophic isotopic modification of rhyolitic magma at times of caldera subsidence, Yellowstone Plateau Volcanic Field
Authors: Hildreth Wes , Christiansen Robert L. , O'Neil James R.
Published: February 2008

The Yellowstone Plateau volcanic field has undergone repeated eruption of rhyolitic magma strongly depleted in 18O. Large calderas subsided 2.0, 1.3, and 0.6 Ma ago, on eruption of ash flow sheets that represent at least 2500, 280, and 1000 km3 of zoned magma. More than 60 other rhyolite lavas and tuffs permit reconstruction of the long‐term chemical and isotopic evolution of the silicic system. Narrow δ18O ranges in the ash flow sheets contrast with wide δ18O variations in postcaldera lavas of the first and third caldera cycles. Earliest postcollapse lavas are 3 to 6‰ lighter than the preceding ash flow sheets. The O18 depletions were short‐lived events that immediately followed caldera subsidence; hundreds of cubic kilometers of magma were drastically 18O depleted and thousands were depleted by 1–2‰. Sequences of postcaldera lavas record partial recovery toward precaldera δ18O values; secular trends between collapse events thus reflect gradual reenrichment of the roofmost magma in δ18O. Much of the subcaldera reservoir was affected, because lavas that erupted as far apart as 115 km reflect the same pattern of depletion and partial recovery. Contemporaneous extracaldera rhyolites have the highest δ18O values in the volcanic field and show no effects of the repeated depletions. Sr and Pb isotope ratios of intracaldera rhyolites jump to more radiogenic values at times of caldera formation and show a longterm zigzag pattern like that of δ18O. Although some contamination by foundering roof rocks seenis to be required, water was probably the predominant contaminant. Even if roof rocks had been strongly depleted in O18 before engulfment, their assimilation would have been far from sufficient to account for the large O18 shift. The low‐ O18 lavas contain no xenocrysts and show no trace element or phenocryst evidence of massive contamination. Their Fe‐Ti‐oxide temperatures indicate no cooling relative to the caldera‐forming ash flow magma, and their whole‐rock, glass, and phenoeryst chemistry suggests compositional continuity with the ash flow sequence. Oxygen exchange between the magma and a mass of low‐O18 water greatly exceeding solubility limits may require (1) recurrent explosive activity to sustain access and mixing of water with the magma and (2) convection of the magma reservoir to prevent local saturation.
Large‐volume rhyolite ash flow eruptions and submarine caldera collapse in the Lower Mesozoic Sierra Nevada, California
Authors: Busby‐Spera Cathy J.
Published: February 2008

The Mineral King roof pendant in the southern Sierra Nevada, California, provides a view of a Triassic and early Jurassic submarine caldera complex which was tilted to vertical and intruded by middle Cretaceous granitoids. An estimated 90 km3 of rhyolitic and 10 km3 of andesitic volcanic rocks are interstratified with marine sandstones, siltstones, limestones, and conglomerates. Periods of volcanotectonic activity alternated with periods of marine sedimentation, Volcanic eruptions at Mineral King resulted in (1) deposition of widespread, very thick sheets of rhyolite ash flow tuff and (2) construction of small (1–8 km3) andesitic stratocones. Growth faults, fault talus aprons, slide blocks, slumps, and debris flows found in association with these volcanic deposits indicate that the eruptive phases represent periods of great seismic activity. Interuptive phases were periods of volcanic and tectonic quiescence during which sequences of shallow to deep marine sediments were deposited. Four different times at Mineral King, eruption of large volumes of rhyolite ash flow tuff resulted in catastrophic caldera collapse and ponding of single ash flow tuffs to thicknesses greater than 0.5 km. Only minimum volumes can be estimated for three of these intracaldera tuffs because they are truncated by younger granites. Volumes are estimated to be at least 25 km3 each, placing these tuffs in the lower size range of epicontinental ring structures. The fourth caldera, the Vandever Mountain caldera, is entirely preserved in the roof pendant. The vertically dipping strata of the Vandever Mountain caldera provide an unusual opportunity to examine a submarine caldera in cross section. Evidence for caldera subsidence during the eruption of the Vandever Mountain rhyolite ash flow tuff includes the following: (1) the great thickness of the ash flow tuff and the rapid accumulation of the tuff during progressive draw‐down of a compositionally zoned magma chamber, (2) high‐angle faults which pond the ash flow tuff and offset underlying but not overlying stratigraphy, and (3) megabreccia and slide blocks up to 0.5 km long that were derived from the caldera wall and floor and incorporated into the ash flow tuff as it filled the caldera. Disruption of the caldera floor was intense near the caldera margin but negligible along most of its length, owing to pistonlike subsidence of an intact cylinder of crust. Small‐volume ash flow eruptions occurred along an incipient, “leaky” ring fracture zone prior to the caldera‐forming eruption of the Vandever Mountain ash flow tuff. The Vandever Mountain caldera was not affected by resurgent doming or late stage rhyolite magmatism in the cross‐sectional view provided by the Mineral King roof pendant.
A geophysical study of the Manila Trench, Luzon, Philippines: 1. Crustal structure, gravity, and regional tectonic evolution
Authors: Hayes Dennis E. , Lewis Stephen D.
Published: February 2008

The Manila Trench subduction zone is an active convergent plate margin between the South China Sea and the northern Philippines. The trench trends northerly and is associated with a volcanic arc, an east dipping Benioff zone beneath Luzon, and a well‐developed fore arc basin system. The Luzon Trough fore arc basins lie landward of the Manila Trench and contain up to 4.5 km of Cenozoic sediments. Offscraping and accretion of turbidite sediments deposited in the Manila Trench have resulted in the upbuilding and outbuilding of an accretionary prism which forms the seaward margin of the fore arc basin system. The uplifted Zambales ophiolite and its offshore extension form the landward side, of the fore arc basin. Multichannel seismic reflection profiles reveal that folding and thrust faulting of trench strata occur at the base of the trench slope. The major structural décollement at the Manila Trench usually forms near the unconformity separating the hemipelagic sediments from the turbidite sediments. Subduction and perhaps underplating of the basal hemipelagic trench sediments accompany the deformation and accretion of the overlying clastic deposits. Faulting and uplift occur within a narrow zone of the fore arc where seamounts associated with the relict spreading center of the South China Sea basin have presumably been subducted. North of Lingayan Gulf the fore arc is disrupted by active fault systems that trend north and northwest offshore from the Luzon Central Valley and from the southern Cordillera Central. Based on regional geological and tectonic observations, we infer that subduction probably began at the Manila Trench in late Oligocene to middle Miocene time. The long‐term average convergence rate at the Manila Trench is estimated to be between 10 and 20 mm/yr and may be slowing in the north owing to the collision of Taiwan with Eurasia.
An improved method for determining the regional stress tensor using earthquake focal mechanism data: Application to the San Fernando Earthquake Sequence
Authors: Gephart John W. , Forsyth Donald W.
Published: February 2008

The orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and R = (σ2–σ1)/(σ3–σ1), a measure of relative stress magnitudes, under the assumption of uniform stress in the source region. This analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation. In the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions; the method allows for errors in orientations of both the fault planes and slip directions. We have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model; thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind. By using a grid search of stress models rather than a linearization scheme, we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses. The method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population. The technique has been applied to 76 events from the San Fernando earthquake sequence for which we have found best fit stresses (plunge and azimuth): σ1, = 7,187; σ2 = 27,281; σ3 = 62,84; and R = 0.65. The average misfit between this stress model and all the data is about 8°, and all but eight of the aftershocks have misfits of less than 20°. These values are considerably less than the uncertainty of the focal mechanism determinations; therefore significant stress inhomogeneities are not required by the data. Our analysis does not support the suggestion of a change in stresses during the aftershock sequence, as proposed by others on the basis of an apparent change in focal mechanisms.
The E ring of Saturn and satellite Enceladus
Authors: Pang Kevin D. , Voge Charles C. , Rhoads Jack W. , Ajello Joseph M.
Published: February 2008

Saturn's outermost (E) ring appears to be closely related to its satellite Enceladus. The radial optical profile of the E ring and systematic trends in the albedos of the seven Saturnian satellites residing within the E ring suggest that Enceladus bears a generative relationship to the E ring. An examination of the micro physical properties of the ring particles elucidates this relationship. Matching the phase function of the E ring requires dielectric spheres, and its blue color, a narrow size distribution. Limits are also placed on their complex refractive index. All observed optical and infrared properties of Saturn's E ring can be explained interms of Mie scattering by a narrow size distribution of ice spheres with an effective diameter of 2–2.5 μm and an effective variance of 0.1–0.15. The spherical shape of the ring particles and their narrow size distribution imply that they have a molten origin, followed by quick freezing. Continued replenishment of the E ring by volcanic eruptions on Enceladus seems quite plausible, considering all available evidence. The recent discovery of tectonic activity on Enceladus supports our conclusions. Although both exogenic and endogenic hypotheses on the origin of the E ring are consistent with the gross macrostructure of the E ring and systematic trends in the albedos of the satellites, only episodic volcanic injections of particles into the E‐ring can explain the coexistence of the diverse ring structures. We compared the relationship between Enceladus and the E ring with that between Io and Io's torus and discovered many parallels, which are useful in understanding outer solar system volcanism and ring formation.
Age progressive volcanism in the New England Seamounts and the opening of the central Atlantic Ocean
Authors: Duncan R. A.
Published: February 2008

Radiometric ages (K‐Ar and 40Ar‐39Ar methods) have been determined on dredged volcanic rocks from seven of the New England Seamounts, a prominent northwest‐southeast trending volcanic lineament in the northwestern Atlantic Ocean. The 40Ar‐39Ar total fusion and incremental heating ages show an increase in seamount construction age from southeast to northwest that is consistent with northwestward motion of the North American plate over a New England hot spot between 103 and 82 Ma. A linear volcano migration rate of 4.7 cm/yr fits the seamount age distribution. These ages fall within a longer age progression from the Corner Seamounts (70 to 75 Ma), at the eastern end of the New England Seamounts, to the youngest phase of volcanism in the White Mountain Igneous Province, New England (100 to 124 Ma). The New England hot spot, estimated from the new radiometric ages and motion of North America in the hot spot reference frame to be near 28°N, 33°W, probably generated a short line of mid to late‐Tertiary age seamounts on the African plate but appears to be presently inactive. The hot spot reference frame is used to calculate the motion of the North American plate away from the African plate from early Cretaceous time to the present. Prominent magnetic anomalies recorded in the central Atlantic seafloor give the positions of the spreading ridge for a range of known ages. Hot spots that now underlie the African plate in the eastern central Atlantic (New England, Canary, Madeira, Cape Verde, Azores) could have produced Cretaceous seamount and island chains on the North American plate during the early opening of the central Atlantic. Each of these hot spots has been overridden by spreading ridges at predictable times. Some of these hot spot crossings are expressed as geochemical anomalies in the oceanic crust now far removed from spreading ridges.
Tapping of magmas from ubiquitous mantle heterogeneities: An alternative to mantle plumes?
Authors: Sleep Norman H.
Published: February 2008

Long‐lived heterogeneities in the mantle are indicated by Nd‐Sr‐Pb isotopic systematics. Explanations for these variations have invoked either distinct layers in the earth that are preferentially tapped by some source regions (such as plumes rising from the base of the mantle) or small‐scale heterogeneities which are ubiquitously distributed through the mantle and tapped differentially in various source regions. Consideration of the thermal and mechanical aspects of the latter hypothesis provides an explanation for the differences between mid‐oceanic ridge basalts and off‐axis volcanism. The melting behavior in a region with heterogeneities is modified by lateral conduction of heat, so that the first melted regions are tapped preferentially at off‐axis volcanoes where small degrees of melting occur. A small (less than a few kilometers), easily melted heterogeneity draws heat from its surroundings as it melts during ascent. An increase of melting by a factor of 2 over adiabatic ascent is probable in the most easily melted regions if the melted and unmelted regions have comparable volume. The increase is larger in the earliest stages if melting is confined to small isolated heterogeneities. The depleted iso‐topic ratios of mid‐oceanic ridge basalts can be explained as follows. Heterogeneities with enriched components melt before those with depleted components as material ascends beneath the ridge. Then the enriched melts are removed from the source region before mid‐oceanic ridge basalts are tapped. The first enriched components to melt enter fractures that carry them upward and outward away from the source region of basalts at the ridge axis. This occurs because the direction of shear strain in the upwelling is such that the axis of tension plunges at 45° away from the ridge axis and dikes of melt propagate up perpendicular to the axis of tension. In contrast with the ridge, volcanoes away from the ridge axis preferentially tap the earlier melted components. At the greater source depths of these volcanoes only non‐mid‐oceanic ridge basalt (MORB) components are significantly molten. Another enriched off‐axis source region is the layer formed by the first enriched magma which moved laterally and avoided the source region of basalts at the ridge axis. For small‐scale ubiquitous heterogeneities to explain the differences between mid‐oceanic ridge basalts and off‐axis volcanism, the MORB component must produce a greater volume of melt than the other components. If isotopic mass balances preclude the MORB component from being this abundant in the whole mantle, then the lower mantle is probably a separate reservoir from the upper mantle. In that case, it is conceivable that passive ubiquitously distributed blobs derived from the lower mantle at earlier times act as small‐scale heterogeneities. Both this blob model and the small‐scale heterogeneity model are possible alternatives to active mantle plumes.
Preferential rifting of continents: A source of displaced terranes
Authors: Vink Gregory E. , Morgan W. Jason , Zhao Wu‐Ling
Published: February 2008

Lithospheric rifting, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential rifting of continents, we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth. Comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3. Also, a thickened crust can halve the total strength of normal continental lithosphere. Because the weakest area acts as a stress guide, any rifting close to an ocean‐continent boundary would prefer a continental pathway. This results in the formation of small continental fragments or microplates that, once accreted back to a continent during subduction, are seen as displaced terranes. In addition, the large crustal thicknesses associated with suture zones would, ironically, make such areas likely locations for future rifting episodes. This results in the tendency, described as the Wilson Cycle, of new oceans to open along the suture where a former ocean had closed.
Mechanics of fold‐and‐thrust belts and accretionary wedges: Cohesive Coulomb Theory
Authors: Dahlen F. A. , Suppe John , Davis Dan
Published: February 2008

A critically tapered fold‐and‐thrust belt or submarine accretionary wedge is one that is on the verge of Coulomb failure everywhere, including its base where frictional sliding along a decollement is assumed to be occurring. Cohesion within a wedge can add significantly to the overall strength near the toe; the effect of this is to decrease the near‐toe taper, leading to a critical topographic profile that is concave upward if the decollement is planar. We obtain an approximate self‐consistent solution for the state of stress within a thin‐skinned cohesive critical Coulomb wedge, and determine the relationship between the wedge taper and its strength and basal friction. The theory is then applied to the presently deforming fold‐and‐thrust belt of western Taiwan. Fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step‐up angles of thrust faults from the basal decollement are used to constrain the Taiwan wedge strength parameters. An attractive assertion fully consistent with all the observations is that the mechanics of fold‐and‐thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory. In particular, if Byerlee's law µb = 0.85 is adopted as the coefficient of sliding friction on the base, we find a coefficient of internal friction µ = 0.9–1.0 in the wedge and a wedge cohesion So = 5–20 MPa. Other solutions having strengths and ambient stresses up to 4 times lower than this can also, however, satisfy the data.
Relative motions between oceanic plates of the Pacific Basin
Authors: Engebretson David C. , Cox Allan , Gordon Richard G.
Published: February 2008

Appendix tables are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C., 20009. Document B84‐012; $2.50. Payment must accompany order.

Relative motion poles describing the displacement histories between the Pacific plate and once adjacent oceanic plates (Farallon, Kula, Izanagi I, Izanagi II, and Phoenix) were derived for the late Mesozoic and Cenozoic eras. Because fracture zone and magnetic anomaly data are generally available from the Pacific plate but not from adjacent plates, a new method of analysis for onesided data was required. This analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions. The following are the main conclusions drawn from our analysis: (1) For time intervals of the order of 107 years, termed stages, relative motion poles for plate pairs remained nearly fixed. Between stages, shifts in poles were commonly both large and abrupt. Within stages, rates of plate motion were commonly observed to change markedly, indicating that plates changed speed more frequently than they changed direction. (2) The relative motions of all of the plates analyzed changed at about chron M11 (135 Ma), chron 34 (85 Ma), and chron 25 (56 Ma). (3) During the Early Cretaceous there were five oceanic plates in the Pacific basin rather than the four recognized by previous workers. (4) To determine the number of Farallon plates that existed to the east of the Pacific plate during the time interval from chron 34 (85 Ma) to chron 25 (56 Ma), fracture zones and magnetic anomalies that record Pacific‐Farallon spreading from the northern, central, and southern Pacific plate were analyzed separately and collectively. The analysis shows that a single Pacific‐Farallon relative motion pole and a single rate are consistent with all of the data. (5) Spreading rates along the Pacific‐Kula ridge decreased markedly between chrons 32b and 25 (72–56 Ma), probably in response to the arrival of buoyant Kula lithosphere at a subduction zone northwest of the Bering Sea. (6) Soon after chron 25 (56 Ma), a major reorganization is recorded along the Pacific‐Farallon boundary. The Kula‐Farallon boundary is also thought to have changed during this reorganization. (7) Kula‐Pacific anomalies north of anomaly 25 are modeled by assuming continued Kula‐Pacific spreading after chron 25 (56 Ma). (8) Probably by chron 18 (43 Ma), Pacific‐Kula spreading had ceased and the Kula‐Farallon ridge system had evolved into alignment with the Pacific‐Farallon spreading direction. However, the timing of these events is uncertain because Pacific‐Kula data younger than chron 25 (56 Ma) are sparse.
Coupled thermal‐hydraulic‐mechanical phenomena in saturated fractured porous rocks: Numerical approach
Authors: Noorishad J. , Tsang C. F. , Witherspoon P. A.
Published: February 2008

The fundamentals of the theory of consolidation and thermoelasticity are recast into the formulation of a phenomenon called thermohydroelasticity. Subsequently, a variational principle and Galerkin formulation are combined with the finite element method to develop a new technique to investigate coupled thermal‐hydraulic‐mechanical behavior of liquid‐saturated, fractured porous rocks. A code‐to‐code verification of the method is performed. Finally, the environment of a heater emplaced in hard rock is simulated. The effects of the coupled thermal stresses in the fractured rock are evident from the dramatic reduction of permeability due to the deformation of the fractures. These results can improve the under‐standing of observations and displacement measurements made in the in situ experiments at the Stripa mine in Sweden.
An experimental approach to thermal convection in a two‐layered mantle
Authors: Olson Peter
Published: February 2008

If the 650 km discontinuity marks a compositional boundary, as has been suggested, then the upper and lower mantle may be convecting separately. A series of laboratory experiments on two‐layered convection were made in order to determine how thermal convection interacts with a stable density discontinuity. The working fluid consisted of two superposed layers of GLOBE 1132 syrup, a glucose solution with a Newtonian viscosity which depends strongly on temperature. The initial density contrast between layers ranged from 0.5% to 8%. A uniform heat flux was supplied to the base of the lower layer. By varying the heat flux, Rayleigh numbers between 4× 04 and 1×107 were obtained. In every case, two‐layered convection was observed, but in no case did a steady state result. Instead, a slow mixing between the layers occurred, driven by viscous stresses acting on the density interface. The mixing mechanism was provided by convective eddies which entrained fluid across the discontinuity in the form of thin schlieren. Mixing continued until the density contrast across the discontinuity became small enough to permit overturning. The mixing rate was determined by monitoring changes in dye concentration in each layer. It is found that the mixing rate is governed by the bulk Richardson number Ri, a measure of the ratio of between interfacial buoyancy and viscous forces. Mixing rate data from experiments covering the range 80<Ri<3300 are consistent with a power law of the form
Ri−1, where Δρ is the density jump across the discontinuity and
is the scale for convective strain rate. Applying this mixing law to the mantle indicates that mass exchange between the upper and lower mantle could occur by this mechanism at a rate of 1018–1019 kg per million years. Convectively driven entrainment across the 650 km density discontinuity can provide a mechanism for interaction between the upper and lower mantle and may be an important source of mantle heterogeneity.
Structural trends in the northern Red Sea
Authors: Mart Yossi , Hall John K.
Published: February 2008

The northern Red Sea is underlain by a series of Plio‐Pleistocene sedimentary sequences that unconformably overlie evaporitic series of late Miocene age. A series of bathyal rifts and cogenetic diapirs, trending approximately N‐S, were recently discovered in that region between latitude 26°30′N and 28°N. The rifts are up to 60 km long and 7 km wide, and the diapirs commonly ascended along the rifts' boundary faults. The diapirs stem from the late Miocene evaporites; thus the discerned structural trends were developed during the Plio‐Pleistocene. The encountered structural patterns deviate considerably from the NW‐SE structural trends that prevail in the central Red Sea as well as in the faults that border the northern Red Sea and the Gulf of Suez. The tectonic activity that formed the Suez rift in the middle to late Miocene was not rejuvenated there during the Plio‐Pleistocene, and the new data indicate that the axis of the tectonic extension at the northern Red Sea jumped to show a northward orientation. It is suggested that incipient spreading center extends from the Red Sea northward to the Gulf of Elat and the Dead Sea rift.