Showing papers in "Journal of Geophysical Research in 1984"
TL;DR: In this paper, it was shown that the mean optical path length in a particulate surface is in roughly inverse proportion to the square root of the absorption coefficient, and that absorption bands are Gaussians in shape when plotted as true absorptance vs photon energy, although they have a smaller intensity.
Abstract: The empirical methods and scattering theories that are important for solving remote sensing problems are among the methods for remotely sensed reflectance data analysis presently compared. In the case of the photon mean optical path length concept's implications for reflectance spectra modeling, it is shown that the mean optical path length in a particulate surface is in roughly inverse proportion to the square root of the absorption coefficient. Absorption bands, which are Gaussian in shape when plotted as true absorptance vs photon energy, are also Gaussians in apparent absorptance, although they have a smaller intensity. An apparent continuum in a reflectance spectrum is modeled as a mathematical function that is used to isolate a particular absorption feature for analysis, and it is noted that this continuum should be removed by dividing it into the reflectance spectrum.
1,408 citations
TL;DR: In this article, an analysis of scarp-derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes.
Abstract: Paleoseismological data for the Wasatch and San Andreas fault zones have led to the formulation of the characteristic earthquake model, which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum. Analysis of scarp-derived colluvium in trench exposures across the Wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes. At all of the sites studied, the displacement per event has been consistently large; measured values range from 1.6 to 2.6 m, and the average is about 2 m. On the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry, six major segments are recognized along the Wasatch fault. On the basis of the most likely number of surface faulting events (18) that have occurred on segments of the Wasatch fault zone during the past 8000 years, an average recurrence interval of 400–666 years with a preferred average of 444 years is calculated for the entire zone. Geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south-central segment of the San Andreas fault suggest that the M 8 1857 earthquake is a characteristic earthquake for this segment. Comparisons of earthquake recurrence relationships on both the Wasatch and San Andreas faults based on historical seismicity data and geologic data show that a linear (constant b value) extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes. Only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes. The characteristic earthquake appears to be a fundamental aspect of the behavior of the Wasatch and San Andreas faults and may apply to many other faults as well.
1,381 citations
TL;DR: In this paper, the age of four zircons from thin-sections of one highland breccia, 73217, using the recently constructed ion microprobe SHRIMP was determined.
Abstract: U-Pb age determinations on four lunar zircons from existing thin-sections of one highland breccia, 73217, using the recently constructed ion microprobe SHRIMP, are reported. The analytical reproducibility of SHRIMP is demonstrated, and procedures for measuring Pb/U, Th/U, and corecting for initial Pb are explained. Electron microprobe analyses for the zircons are alsoar reported. The results show that the four zircons survived the lunar cataclysm without any identifiable effects on their U-Pb systematics. All four indicate a single age of 4356 +23 or -14 m.y. The zircons have experienced small variable amounts of Pb loss since crystallization, from almost zero up to about 10 percent. If this occurred during one later event, then age of the latter is between 1100 and 2300 m.y.
1,334 citations
TL;DR: A review of the experimental data on subcritical crack growth in geological materials is presented in this article, where the main parameters describing subcritical cracking growth are the critical stress intensity factor Kc, the sub critical crack growth limit Ko, and the relationship between Ko and Kc.
Abstract: 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 Ko and 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.
1,184 citations
TL;DR: In this paper, the authors presented a method for the inversion of waveform data for the three-dimensional distribution of seismic wave velocities, applied to data from the global digital networks (International Deployment of Accelerometers, Global Digital Seismograph Network).
Abstract: 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.
1,143 citations
TL;DR: In this paper, the authors provide a quantitative framework for the description of weathering and alteration in sedimentary petrology, using the available experimental data on dissolution of silicates to verify quantitatively the usual mineral stability series.
Abstract: The recent literature on the kinetics of water-rock interactions is reviewed. The data are then extended to provide a quantitative framework for the description of weathering and alteration. The available experimental data on dissolution of silicates verifies quantitatively the usual mineral stability series in sedimentary petrology. The rate of hydration of carbonic acid is shown to be a possible limiting factor in water-rock interactions. The framework is developed to enable use of laboratory dissolution experimental results and thermodynamics to arrive at a rate law applicable up to equilibrium and therefore applicable to natural systems. The kinetic justification for the significance of a water-rock ratio is discussed. With a proper treatment of fluid flow, the equations are applied to the weathering profile leading to the development of bauxites from nepheline syenites.
1,105 citations
TL;DR: In this article, the authors reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio ϕ = (σ2−σ3)/(σ1 −σ3) between principal stress values (σ1≥σ2≤σ3, compression being positive).
Abstract: Using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations, it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio ϕ = (σ2−σ3)/(σ1−σ3) between principal stress values (σ1≥σ2≥σ3, compression being positive). No assumption is made concerning the orientation of fault planes relative to stress axes, so that reactived faults are taken into account as well as newly created ones. Qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years. The practical limitation of the methods and the necessity for critical field observations are emphasized. These methods can be applied to focal mechanisms of earthquakes. A more complex analysis of heterogeneous data sets, involving an iterative separation of different stress systems, is also discussed. This analysis enables one to distinguish successive faulting events. Careful qualitative study in the field is in all cases essential.
1,080 citations
TL;DR: In this article, a grid search of stress models is performed 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.
Abstract: 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.
1,078 citations
TL;DR: In this article, the authors presented a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates.
Abstract: We present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates. We compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal. A moving-window analysis of the entire seismograms, including the coda, gives δ(t), the difference in arrival times versus running time along the seismogram. The time resolution of the method is an order of magnitude better than the digitization interval. The δ(t) technique is illustrated with a pair of microearthquakes, M = 1.7 and 2.0, that occurred before and after the Coyote Lake, California, earthquake (M = 5.9) of August 6, 1979, and on the same segment of the Calaveras fault that ruptured during the earthquake. The coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet, so that its seismogram appears as a stretched version of the earlier event. We interpret this systematic variation in δ(t) along the coda as a change in the average S velocity in the upper crust in the time interval between the two doublets. S wave velocities appear to have decreased by 0.2% in an oblong region 5–10 km in radius at the south end of the aftershock zone.
928 citations
TL;DR: In this article, the authors used the International Seismological Centre bulletins for the years 1964-1979 to derive a three-dimensional model of lateral variations of the P velocity in the lower mantle.
Abstract: The data from the International Seismological Centre bulletins for the years 1964–1979 are used to derive a three-dimensional model of lateral variations of the P velocity in the lower mantle. Unlike previous studies, in which such perturbations were represented by a blocklike parameterization, we seek the solution in the form δυ(r, θ, ϕ) = Σk = 0K Σl = 0L Σm = 0l ƒk(r)(kAlm cos mϕ + kBlm sin mϕ) plm (cos θ). Some 500,000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients A and B, the maximum K and L used are 4 and 6, respectively. In each iteration, in addition to solving for the three-dimensional structure, we also relocate all the earthquakes, perturb the average travel time curve, and determine station corrections for over 1000 stations. Particular attention is given to the problem of weighting the individual observations in order to avoid, as much as possible, the bias due to the uneven distribution of sources and receivers. The resulting model shows a high level of perturbations just below the 670-km discontinuity and just above the core-mantle boundary, where the maximum perturbations reach 1–1.5% of the average velocity even for this highly smoothed model. At a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries. The model predicts well the large-scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones. The most striking large-scale three-dimensional feature of the model is a ring of high velocities circumscribing the Pacific basin within a depth range from 1000 km to the core-mantle boundary. If the proportionality between perturbations in the velocity and temperature is assumed, the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle. Some caution, however, should be exercised in interpreting the results near the 670-km discontinuity, where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure. At the same time, a significant global increase in the level of perturbations in the lowermost mantle is established beyond question. The results of Dziewonski et al. (1977) on the correlation between the “equivalent geoid” and the observed geoid for the angular order numbers 2 and 3 is confirmed, but there is no significant correlation for degrees from 4 to 6. These calculations are made for the rigid earth. Richards and Hager (this issue) show that introduction of finite viscosity can change the sign of a geoid anomaly. In addition, recent studies of lateral variations in the shear velocity reveal the presence of large-scale anomalies in the upper mantle. Thus, while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive, they point the way in which results from seismology can be used to address some of the basic questions of geodynamics.
773 citations
TL;DR: In this article, a new technique is derived to invert slickenside data for the stress field that caused the faulting episode, simplified by the assumption that the magnitude of the tangential traction on the various fault planes, at the time of rupture, is similar.
Abstract: A new technique is derived to invert slickenside data for the stress field that caused the faulting episode. This inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes, at the time of rupture, is similar. Study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption. Hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion. Application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes. Comparison of the inversion results with the geometry of the folds shows this application to be successful, greatly increasing the number of data sets that can be used to find the paleostress field.
TL;DR: In this paper, the authors present a new model that takes into account the existence of an isotopic kinetic effect at snow formation as a result of the fact that vapor deposition occurs in an environment supersaturated over ice.
Abstract: The classical Rayleigh model assuming isotopic equilibrium fails to explain the deuterium and oxygen 18 contents of polar snow. This model leads to too high temperature-isotope gradients (both for δD and δ18O), to too low δD - δ18O slopes, and consequently to an excessively large range of deuterium excess values (d = δD - 8δ18O). We present a new model that takes into account the existence of an isotopic kinetic effect at snow formation as a result of the fact that vapor deposition occurs in an environment supersaturated over ice. This kinetic effect is thoroughly discussed from a microphysical point of view and tested against experimental data and field observations. This new formulation reconciles predicted and observed values both for the temperature-isotope and δD - δD18O relationships for reasonable values of supersaturation over ice.
TL;DR: In this article, a selfconsistent theory for the mechanics of thin-skinned accretionary Coulomb wedges is developed and applied to the active fold-and-thrust belt of western Taiwan.
Abstract: A self-consistent theory for the mechanics of thin-skinned accretionary Coulomb wedges is developed and applied to the active fold-and-thrust belt of western Taiwan. The state of stress everywhere within a critical wedge is determined by solving the static equilibrium equations subject to the appropriate boundary conditions. The influence of wedge cohesion, which gives rise to a concave curvature of the critical topographic surface and affects the orientation of the principal stresses and Coulomb fracture within the wedge, is considered. The shape of the topographic surface and the angles at which thrust faults step up from the basal decollement in the Taiwanese belt is analyzed taking into account the extensive structural and fluid-pressure data available there. It is concluded that the gross geometry and structure of the Taiwan wedge are consistent with normal laboratory frictional and fracture strengths of sedimentary rocks.
TL;DR: In this article, the authors show that the observed long-wavelength geoid is highly correlated with that predicted by a density model for seismically active subducted slabs and explain the amplitude of the correlation if the density contrasts associated with subduction extend into the lower mantle or if subducts exceeding 350 km in thickness are piled up over horizontal distances of thousands of km at the base of the upper mantle.
Abstract: The total geoid anomaly which is the result of a given density contrast in a convecting viscous earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries, as well as by the density contrast itself is discussed. If the internal density contrasts can be estimated, the depth and variation of viscosity with depth of the convecting system can be constrained. The observed long wavelength geoid is highly correlated with that predicted by a density model for seismically active subducted slabs. The amplitude of the correlation is explained if the density contrasts associated with subduction extend into the lower mantle or if subducted slabs exceeding 350 km in thickness are piled up over horizontal distances of thousands of km at the base of the upper mantle. Mantle wide convection in a mantle that has a viscosity increasing with depth provides the explanation of the long-wavelength geoid anomalies over subduction zones.
TL;DR: In this paper, an analysis of a data set from July 1981 shows that there is satisfactory agreement between the equation resulting from radiative transfer theory and the atmospheric correction algorithm as obtained by analysis of an area of incipient cloud street formation; agreement is also satisfactory between this algorithm and the statistically derived NOAA algorithm used to obtain sea surface temperatures from the satellite data.
Abstract: The Advanced Very High Resolution Radiometer (AVHRR) on the NOAA 7 satellite acquires 1-km spatial resolution data in “split window” channels at 10.8 and 11.9 μm. Data from these spectral channels may be used to estimate surface temperature and the atmospheric correction to radiation from the earth's surface. Analysis of a data set from July 1981 shows that (1) there is satisfactory agreement between the equation resulting from radiative transfer theory and the atmospheric correction algorithm as obtained by analysis of an area of incipient cloud street formation; (2) agreement is also satisfactory between this algorithm and the statistically derived NOAA algorithm used to obtain sea surface temperatures from the satellite data (However, the comparison assumes the NOAA algorithm is valid outside its range of derivation.); (3) in areas of cloud street formation, variations of atmospheric moisture produce radiance temperature differences of order 2–3°C, which if neglected would cause errors in the derivation of surface thermal characteristics. This meteorological variation over distances of 5–10 km would not be inferred from conventional radiosonde measurements or from lower-resolution satellite soundings.
TL;DR: In this article, the authors investigated the present tectonics of the Himalayan continental collision zone using fault plane solutions and well-determined focal depths of medium-sized earthquakes, topography and Landsat imagery in conjunction with seismicity maps, cross sections and available geological information.
Abstract: Fault plane solutions and well-determined focal depths of medium-sized earthquakes, topography, and Landsat imagery in conjunction with seismicity maps, cross sections, and available geological information are used to investigate the present tectonics of the Himalayan continental collision zone. Most of the accurately located epicenters of events along the Himalayan arc (78°E–95°E) that occurred between 1961 and 1981 are concentrated in a narrow zone, about 50 km wide, lying between the northerly dipping Main Boundary (MBT) and Main Central (MCT) thrusts. Most of these events are located just south of the MCT. Though the epicenters of the events are, in general, well located, their depths as determined by teleseismic travel time data are very unreliable. Events with accurately determined depths obtained from identification of surface-reflected phases define a simple, planar zone from about 10-km and 20-km depth, with an apparent dip of about 15°. This result is all the more remarkable considering that the events used were located along about an 1800-km length of the Himalyan arc. Except for one, all available focal mechanisms of events within this zone indicate shallow ( ≲30°), north dipping thrusts. This shallow, north dipping zone apparently defines a part of the detachment that separates the underthrusting Indian plate from the Lesser Himalayan crustal block. The spatial extent and the geometry of this interplate thrust zone strongly indicate that the MBT and nearby subsidiary surface and blind thrusts, rather than the MCT, are currently the most active structures of the Himalayan arc. We suggest that the great Himalayan earthquakes (M>8) occur along the same detachment surface as defined by the thrust-type, medium-sized events. Events located to the south of the MBT and beneath the Ganges foredeep show normal faulting with T axes perpendicular to the Himalayan trend. The above results suggest that the Indian continental plate is underthrusting the Himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones. The November 19, 1980, earthquake that occurred near the MCT (near 88.5°E) shows a predominantly strike-slip focal mechanism. One of the nodal planes of this mechanism is transverse to the Himalayan structural grain, and moreover, this plane has a trend similar to that of the recently mapped Yadong-Gulu rift in the Tethyan Himalaya and in southern Tibet just northeast of the earthquake. We interpret this predominantly left-lateral, strike-slip mechanism to indicate a possible genetic relationship between transverse structural features in the Underthrusting Indian plate (the Kishangang basement fault) and the upper Himalayan blocks and Tibet.
TL;DR: In this paper, the potential and surface deformation Love numbers for internal loading were calculated for a variety of radial Newtonian viscosity variations and flow configurations including both whole mantle and layered convection.
Abstract: In order to obtain a dynamically consistent relationship between the geoid and the earth's response to internal buoyancy forces, we have calculated potential and surface deformation Love numbers for internal loading. These quantities depend on the depth and harmonic degree of loading. They can be integrated as Green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts. Spherically symmetric, self-gravitating flow models are constructed for a variety of radial Newtonian viscosity variations and flow configurations including both whole mantle and layered convection. We demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound, much less than the time scale for significant change in the convective flow pattern, by calculating relaxation times for a series of spherically symmetric viscous earth models. For uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads, resulting in net negative geoid anomalies for positive density contrasts. Geoid anomalies from intermediate-wavelength density contrasts are amplified by up to an order of magnitude. Geoid anomalies are primarily the result of density contrasts in the interior of convecting layers; density contrasts near layer boundaries are almost completely compensated. Layered mantle convection results in smaller geoid anomalies than mantle-wide flow for a given density contrast. Viscosity stratification leads to more complicated spectral signatures. Because of the sensitivity of the dynamic response functions to model parameters, forward models for the geoid can be used to combine several sources of geophysical data (e.g., subducted slab locations, seismic velocity anomalies, surface topography) to constrain better the structure and viscosity of the mantle.
TL;DR: In this article, a method of determining sea ice parameters using Nimbus 7 polarized multispectral radiance data obtained with the NN 7 Scanning Multichannel Microwave Radiometer (SMMR) is presented.
Abstract: A method of determining sea ice parameters using Nimbus 7 polarized multispectral radiance data obtained with the Nimbus 7 Scanning Multichannel Microwave Radiometer (SMMR) is presented. Observed radiances from selected areas in the Arctic region for the period February 3-7, 1979 were used in computing algorithm coefficients. Polar maps of sea ice concentration, multiyear fraction, and ice temperature are illustrated for this period. The variation of the mean and standard deviation of ice concentration and multiyear ice fraction for a region of perennial ice cover over the first 11 months of SMMR operation is also presented. Comparisons are made between the calculated sea ice parameters and information obtained from previous studies using aircraft, submarine and surface observations. The absolute accuracy of the SMMR parameters remains uncertain.
TL;DR: In this paper, an exact relation between surface slope α and basal dip β of a non-cohesive critical wedge with uniform properties is derived, and the state of stress within such a wedge has the same orientation everywhere, and α is constant if β is, and vice versa.
Abstract: Active fold-and-thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of Coulomb failure everywhere, overlying a basal decollement where frictional sliding is occurring. Ignoring cohesion, the four strength parameters needed to describe a critical Coulomb wedge are its internal and basal coefficients of friction μ and μb and its internal and basal Hubbert-Rubey fluid pressure ratios λ and λb. An exact relation between surface slope α and basal dip β of a noncohesive critical wedge with uniform properties is derived. The state of stress within such a wedge has the same orientation everywhere, and α is constant if β is, and vice versa. A coefficient of internal friction μ = 1.1 is consistent with the known surface slope, basal dip, and pore fluid pressures in the active fold-and-thrust belt of western Taiwan, assuming that Byerlee's law, μb = 0.85, is valid on the base. The wide variety of tectonic styles observed to occur along convergent margins, including subduction erosion, active accretion, subduction without accretion, and even extension and normal faulting, may be controlled by relatively small spatial or temporal variations in either μb or λb.
TL;DR: A recent review as discussed by the authors summarizes features of calderas and ash flow volcanic fields in western North America, exposed at advantageous levels where both remnants of a Volcanic sequence and upper parts of the cogenetic intrusion are preserved, in comparison with similar rocks elsewhere in the worjd.
Abstract: Large-volume ash flow eruptions and associated caldera collapses provide a direct link with subvolcanic granitic plutons of batholithic dimensions. The eruptive history, structural features, and petrologic evolution of ash flow calderas provide data on early stages of the evolution of an associated subvolcanic magmatic system. Broadly cogenetic, erosionally unroofed granitic plutons provide a record mainly of the late stages of emplacement and crystallization of silicic magmas. This review summarizes features of well-studied calderas and ash flow volcanic fields in western North America, exposed at advantageous levels where both remnants of a Volcanic sequence and upper parts of the cogenetic intrusion are preserved, in comparison with similar rocks elsewhere in the worjd. Primary examples include San Juan, Mogollon-Datil, Marysvale, Latir-Questa, Chiricahua-Turkey Creek, Challis, and Boulder Batholith-Elkhorn Mountains. Most ash flows have erupted from sites of preceding volcanism that records shallow accumulation of caldera-related magma. Structural boundaries of calderas are single ring faults or composite ring fault zones that dip vertically to steeply inward; outward dipping boundary faults favored by some models have not been identified in North American calderas. The area and volume of caldera collapse are roughly proportional to the amount of erupted material. Pyroclastic eruptions of relatively small volume (less than 50–100 km3) may cause incomplete hinged caldera subsidences or structural sags; larger systems are bounded by complete ring faults. Few ash flow vent structures have been related to major calderas; vent geometry, as determined by size analyses of pyroclastic materials, may shift complexly during caldera collapse. Scalloped topographic walls beyond the structural boundaries of most calderas are due to secondary gravitational slumping during subsidence. Most exposed floors are a structurally coherent plate or cylinder bounded by a ring fault or dike, indicating pistonlike caldera collapse; chaotically brecciated floors predicted by models of piecemeal collapse have not been identified. Deviations from circular shape commonly reflect influence of regional structures; some calderas in extensional terranes are elongate in the direction of extension. Large calderas (greater than 100 km3 of erupted material) collapse concurrently with eruption, as indicated by thick intracaldera ash flow fill and interleaved collapse slide breccias. Volumes of intracaldera and outflow tuff tend to be subequal; correlation between them is commonly complicated by contrasts in abundance and size of phenocrysts and lithic fragments, degree of welding, devitrification, alteration, and even chemical composition of magmatie material. Postcollapse volcanism may occur from varied vent geometries within ash flow calderas; ring vent eruptions are most common in resurgent calderas, reflecting renewed magmatic pressure. Large intrusions related to resurgence are exposed centrally within some calderas; ring dikes and other intrusions along bounding ring fractures are especially common in alkalic igneous systems in extensional environments. Subvolcanic magma chambers of calc-alkaline affinities associated with plate-convergent tectonic settings may rise to such high levels that deep cauldron subsidence structures are obliterated. Resurgence within calderas may result in a symmetrical dome or more geometrically complex forms; resurgence is most common in large calderas (greater than 10-km diameter) in cratonic crust and is associated with large silicic intrusions. In addition to resurgence within single calderas, broader magmatic uplift occurs widely within silicic volcanic fields, reflecting isostatic adjustment to emplacement of associated subvolcanic batholiths. Much additional space for shallow batholith emplacement is probably accommodated by gravitationally driven down warping of wall rocks at lower structural levels. Hydrothermal activity and mineralization accompany all stages of ash flow magmatism, becoming dominant late during caldera evolution. Much rich mineralization is millions of years later than caldera collapse, where the caldera served primarily as a structural control for genetically unrelated intrusions and associated hydrothermal systems.
TL;DR: In this article, a specific deformation model, based on advective transport rate control, is developed, which can produce strain at competitive rates but with stress and temperature dependences of unusual form.
Abstract: Evidence is presented to support the conclusion that pore fluid pressures tresses PF during regional metamorphism are generally greater than or equal to the minimum principal compressive stress S3. The resultant very low effective stresses σ lead to significantly increased porosity and permeability, even at moderate to high metamorphic temperatures. Permeabilities between 10−18 and 10−15 m2 and are considered to be common, resulting in rapid fluid migration and the dominance of advective (infiltrative) over diffusive mass transport, even over relatively small distances. In view of the importance of intergranular mass transport to rock deformation during metamorphism, a mobile, high-pressure fluid will have substantial rheological effects, especially in polyphase rocks. The fluid is capable of influencing the rate of dislocation creep in a number of ways. More importantly, advective mass transport along fluid pressure gradients can give rise to a solution transfer deformation mechanism that competes with conventional pressure solution. The rate of deformation by advective mass transport could be controlled by a number of processes, including dissolution kinetics, advective transport rates, and the rate of crack growth. A specific deformation model, based on advective transport rate control,is developed, which can produce strain at competitive rates but with stress and temperature dependences of unusual form.
TL;DR: Partition coefficients of Hf, Zr, and REE between olivine, orthopyroxene, clinopyroxenes, plagioclase, garnet, amphibole, ilmenite, phlogopite, and liquid are presented in this article.
Abstract: Partition coefficients of Hf, Zr, and REE between olivine, orthopyroxene, clinopyroxene, plagioclase, garnet, amphibole, ilmenite, phlogopite, and liquid are presented. Samples consist of megacrysts in kimberlite, phenocrysts in alkaline basalts, tholeiitic basalts and andesitic to dacitic rocks, and synthetic garnet and clinopyroxene in Hawaiian tholeiites. The Hf-Lu and Zr-Lu elemental fractionations are as large as the Lu-Sm or Lu-Nd fractionation. The Hf and Zr partition coefficients between mafic phenocrysts and liquids are smaller than the Lu partition coefficients, but are similar to the Nd or Sm partition coefficients. The Hf and Zr partition coefficients between ilmenite, phlogopite, and liquid are larger than the Lu partition coefficients for these minerals and their corresponding liquids. The Hf-Zr elemental fractionation does not occur except for extreme fractionation involving Zr-minerals and extremely low fO2. These data have an important bearing on chronological and petrogenetic tracer studies involving the Lu-Hf isotopic system.
TL;DR: In this article, the authors show that for a given rock, flow stress can be correlated with the intensity of a residual water-related broad absorption, and that the amount of water that has been found to be of importance for the strength of these dunites are of the order of 0.01 wt %, amounts much less than most estimates of the water content of the earth's undepleted upper mantle.
Abstract: Specimens of two natural dunites (Anita Bay, of 100 μm grain size, and Aheirn, of 900 μm grain size) have been deformed in experiments at temperatures between 1200° and 1400°C and constant strain rates between 10−4 and 10−6 s−1 in a gas-medium deformation apparatus at 300 MPa confining pressure. Prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200°C for >60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water. The predried specimens are much stronger than specimens tested under wet conditions in an earlier study. The predried specimens also do not show the apparent grain size dependence seen under wet conditions. Infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock, flow stress can be correlated with the intensity of a residual “water related” broad absorption. The addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component. The weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water-related species in the grain boundaries, but intragranular weakening may also be contributing. It is suggested that the concentration of the water-related species in the grain boundaries is governed by its concentration in the small melt fraction present. The amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0.01 wt %, amounts much less than most estimates of the water content of the earth's undepleted upper mantle. Such water may have an important role in determining upper mantle flow behavior.
Stress-strain curves for all experiments are available with entire manuscript on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009- Document B84-008, $2.50. Payment must accompany order.
TL;DR: In this article, a time domain inversion routine was used to determine the vertical velocity structure of the mid-period passband of Regional Seismic Test Network station RSCP.
Abstract: Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of Regional Seismic Test Network station RSCP are inverted for vertical velocity structure beneath the Cumberland Plateau, Tennessee. The detailed broadband receiver functions are obtained by stacking source-equalizd horizontal components of teleseismic P waveforms. The resulting receiver functions are most sensitive to the shear velocity structure near the station. A time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin, flat-lying, homogeneous layers. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. The results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the East Continent Gravity High located northeast of RSCP. The results from events arriving from the northeast show a high-velocity midcrustal layer not present in results from the southeast azimuth. This velocity structure can be shown to support the idea that this feature is part of a Keweenawan rift system. Another interesting feature of the derived velocity models is the indication that the crust-mantle boundary beneath the Cumberland Plateau is a thick, probably laminated transition zonemore » between the depths of 40 and 55 km, a result consistent with interpretations of early refraction work in the area.« less
TL;DR: In this paper, the authors constructed a faulting model for the 1979 Imperial Valley earthquake by comparing synthetic particle velocities with the near-source strong motion data, by trial and error.
Abstract: By comparing synthetic particle velocities with the near-source strong motion data we have constructed, by trial and error, a faulting model for the 1979 Imperial Valley earthquake The calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the Imperial Valley and the spatial variation of the slip rate parameters on the fault plane The independent slip rate parameters are (1) the strike-slip rate amplitude, (2) the dip-slip rate amplitude, (3) the duration that slip rate is nonzero (the rise time of the slip function) and (4) the rupture time, which determines when the slip rate is initiated Our faulting model has the following principal features: (1) Faulting occurred on the Imperial fault and on the Brawley fault, rupture on the Brawley fault being triggered by rupture on the Imperial fault (2) The Imperial fault is a plane 35 km long and 13 km wide with a strike of 323°, measured clockwise from north, and a dip of 80° NE The Brawley fault is a 10 km long and 8 km wide plane with a strike of 360° and a dip of 90° (3) Faulting on the Imperial fault is primarily right-lateral strike slip with a small component of normal dip slip in the sediments at its northern end The larger strike-slip rates are generally confined between depths of 5 and 13 km with maximum values of about 10 m/s The duration varies on the fault with a maximum of 19 s, which is considerably shorter than the total time for the rupture to take place (4) The rupture velocity on the Imperial fault is highly variable Locally, it exceeds the shear wave velocity and, in one instance, the compressional wave velocity The average rupture velocity, though, is less than the shear wave velocity (5) Although the slip on the Brawley fault contributes only about 4% of the total moment, it greatly affects the ground motion at nearby stations (6) The total seismic moment is 67×1018 N m where the Imperial fault contributes 64×1018 N m and the Brawley fault contributes 27×1017 N m In the process of trying almost 300 faulting models, we found that given the elastic parameters of the medium, the synthetic seismograms were most sensitive to the specification of the rupture time Although the slip rate amplitudes are linearly related to the data, the rupture time and the duration are not The parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault
TL;DR: In this paper, the authors reanalyze the flow model proposed by Wyllie and Rose (1950) in which the complicated flow network through the pore phase of rock is replaced by a single representative conduit.
Abstract: We reanalyze the flow model proposed by Wyllie and Rose (1950) in which the complicated flow network through the pore phase of rock is replaced by a single representative conduit. Although the model is a very simple representation of the complicated pore phase in rock, we find that it provides an adequate simulation of how the transport properties vary with external pressure. Expressions derived for fluid permeability k and formation factor F are combined to give an expression for the mean hydraulic radius of the pore phase. Using this expression, we show that the exponent r in the empirical relationship k ∝ F−r must fall in the range 1≤r≤3. Also, we use the expression for hydraulic radius to estimate the crack area per unit volume and the standard deviation of the height of the asperities on the rhicrocrack surfaces for two granites. The values are in reasonable agreement with other estimates.
TL;DR: A simple model for ocean chemistry is presented and shown to account for the present level of atmospheric CO2 as mentioned in this paper, which is suggested that these changes may explain the apparent control on climate exercised by secular variations in the orbital parameters of the earth.
Abstract: Approximately half of the nitrogen and phosphorus entering deep waters of the contemporary ocean are transported from the surface in inorganic form as preformed nutrients. A simple model for ocean chemistry is presented and shown to account for the present level of atmospheric CO2. Fluctuations in preformed nutrients, modulated by changes in insolation and circulation at high latitudes, can result in significant variations in CO2. It is suggested that these changes may account for the apparent control on climate exercised by secular variations in the orbital parameters of the earth.
TL;DR: In this paper, the average rotational strain tensors for different parts of Asia were estimated for the last 80 years based on estimates of source dimensions, average displacements, seismic moments, and orientations of faults of all large earthquakes.
Abstract: We determine approximate average rates of deformation in portions of eastern and central Asia for the last 80 years. These rates are based on estimates of source dimensions, average displacements, seismic moments, and orientations of faults of all large earthquakes (M>7.5) and as many other major earthquakes (M≥7.0) as possible in Asia since 1900. A major part of this study is the compilation of these data. For separate parts of Asia we combine seismic moment tensors to estimate the average rotational strain (or deformation) tensors for these regions for the last 80 years. We estimate that shortening across the Tien Shan has occurred at 11 mm/yr (between 5 and 19 mm/yr). Left-lateral shear on easterly trending planes in Mongolia, in northwestern Tibet, Gansu, and Ningxia, and on the Xianshuihe fault in western Sichuan has been very rapid, more than 10 mm/yr in each region, and apparently as rapid as 40 mm/yr in northwestern Tibet, Gansu, and Ningxia. Slower rates of a few to 10 mm/yr appear to characterize east-west extension and conjugate north-south shortening of Tibet. The earthquake history of the Himalaya is consistent with rates of underthrusting of more than 10 mm/yr but is probably less than the convergence rate of India with Eurasia of about 50 mm/yr. The overall strain field is consistent with a large part of India's penetration into Eurasia being absorbed by the extrusion of material out of India's way. As a result, southeast China moved at about 21 mm/yr east-southeast with respect to Eurasia in the last 80 years with an uncertainty of about a factor of 2 in this estimated rate.
TL;DR: In this article, coupled gas and aqueous-phase photochemistry of a stratiform cloud in a remote region of the marine atmosphere is investigated with a time-dependent box model.
Abstract: The coupled gas- and aqueous-phase photochemistry of a stratiform cloud in a remote region of the marine atmosphere is investigated with a time-dependent box model. Both scavenging of ambient acidic aerosols and gases as well as aqueous-phase chemical reactions within droplets are found to be important sources of acidity to cloud water and can lead to pH levels in cloud water in the remote marine atmosphere well below 5.6. The major sources of acidity via aqueous-phase chemical reactions are the generation of sulfuric acid from dissolved SO2 and the generation of formic acid from dissolved formaldehyde. In both cases, aqueous-phase free radicals can play a significant role either directly by oxidizing dissolved SO2 and HCHO or indirectly by producing the aqueous-phase oxidant H2O2. The rate of SO2 conversion to sulfuric acid is sensitive to a variety of parameters including the accommodation or sticking coefficient for SO2, H2O2, HO2, and OH, the liquid water content, and the ambient levels of SO2, HNO3, and other acidic or basic gases. Because high levels of SO2 tend to deplete cloud water of H2O3, the possibility exists that the pH of precipitation in polluted regions will respond nonlinearly to reduced SO2 emissions.
TL;DR: In this article, the large extensional structures in the crust of the eastern Basin-range and its transition into the Middle Rocky Mountains and Colorado Plateau have been studied using seismic reflection data, surface geology, velocity models from refraction data, and earthquake data.
Abstract: Using 1500 km of industry-released seismic reflection data, surface geology, velocity models from refraction data, and earthquake data, the large extensional structures in the crust of the eastern Basin-Range and its transition into the Middle Rocky Mountains and Colorado Plateau have been studied. It is suggested that the close spatial correlation between normal faults and thrust fault segmentation along the Wasatch Front reflects major east-trending structural and lithological boundaries inherited from tectonic processes associated with the evolution of the cordilleran miogeocline, which began in the Precambrian.