Showing papers in "Journal of Geophysical Research in 1989"

Magmatism at rift zones: The generation of volcanic continental margins and flood basalts

Abstract: When continents rift to form new ocean basins, the rifting is sometimes accompanied by massive igneous activity. We show that the production of magmatically active rifted margins and the effusion of flood basalts onto the adjacent continents can be explained by a simple model of rifting above a thermal anomaly in the underlying mantle. The igneous rocks are generated by decompression melting of hot asthenospheric mantle as it rises passively beneath the stretched and thinned lithosphere. Mantle plumes generate regions beneath the lithosphere typically 2000 km in diameter with temperatures raised 100–200°C above normal. These relatively small mantle temperature increases are sufficient to cause the generation of huge quantities of melt by decompression: an increase of 100°C above normal doubles the amount of melt whilst a 200°C increase can quadruple it. In the first part of this paper we develop our model to predict the effects of melt generation for varying amounts of stretching with a range of mantle temperatures. The melt generated by decompression migrates rapidly upward, until it is either extruded as basalt flows or intruded into or beneath the crust. Addition of large quantities of new igneous rock to the crust considerably modifies the subsidence in rifted regions. Stretching by a factor of 5 above normal temperature mantle produces immediate subsidence of more than 2 km in order to maintain isostatic equilibrium. If the mantle is 150°C or more hotter than normal, the same amount of stretching results in uplift above sea level. Melt generated from abnormally hot mantle is more magnesian rich than that produced from normal temperature mantle. This causes an increase in seismic velocity of the igneous rocks emplaced in the crust, from typically 6.8 km/s for normal mantle temperatures to 7.2 km/s or higher. There is a concomitant density increase. In the second part of the paper we review volcanic continental margins and flood basalt provinces globally and show that they are always related to the thermal anomaly created by a nearby mantle plume. Our model of melt generation in passively upwelling mantle beneath rifting continental lithosphere can explain all the major rift-related igneous provinces. These include the Tertiary igneous provinces of Britain and Greenland and the associated volcanic continental margins caused by opening of the North Atlantic in the presence of the Iceland plume; the Parana and parts of the Karoo flood basalts together with volcanic continental margins generated when the South Atlantic opened; the Deccan flood basalts of India and the Seychelles-Saya da Malha volcanic province created when the Seychelles split off India above the Reunion hot spot; the Ethiopian and Yemen Traps created by rifting of the Red Sea and Gulf of Aden region above the Afar hot spot; and the oldest and probably originally the largest flood basalt province of the Karoo produced when Gondwana split apart. New continental splits do not always occur above thermal anomalies in the mantle caused by plumes, but when they do, huge quantities of igneous material are added to the continental crust. This is an important method of increasing the volume of the continental crust through geologic time.

The role of sea ice and other fresh water in the Arctic circulation

Abstract: Salinity stratification is critical to the vertical circulation of the high-latitude ocean. We here examine the control of the vertical circulation in the northern seas, and the potential for altering it, by considering the budgets and storage of fresh water in the Arctic Ocean and in the convective regions to the south. We find that the present-day Greenland and Iceland seas, and probably also the Labrador Sea, are rather delicately poised with respect to their ability to sustain convection. Small variations in the fresh water supplied to the convective gyres from the Arctic Ocean via the East Greenland Current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past. The North Atlantic salinity anomaly of the 1960s and 1970s is a recent example; it must have had its origin in an increased fresh water discharge from the Arctic Ocean. Similarly, the freshening and cooling of the deep North Atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the Arctic Ocean into the convective gyres. Finally, we note that because of the temperature dependence of compressibility, a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean.

Mapping small elevation changes over large areas: Differential radar interferometry

Abstract: A technique is described, based on synthetic aperture radar (SAR) interferometry, which uses SAR images for measuring very small (1 cm or less) surface motions with good resolution (10 m) over swaths of up to 50 km. The method was applied to a Seasat data set of an imaging site in Imperial Valley, California, where motion effects were observed that were identified with movements due to the expansion of water-absorbing clays. The technique can be used for accurate measurements of many geophysical phenomena, including swelling and buckling in fault zones, residual displacements from seismic events, and prevolcanic swelling.

A photochemical kinetics mechanism for urban and regional scale computer modeling

Abstract: A new chemical kinetics mechanism for simulating urban and regional photochemistry has been developed and evaluated. The mechanism, called the Carbon Bond Mechanism IV (CBM-IV), was derived by condensing a detailed mechanism that included the most recent kinetic, mechanistic, and photolytic information. The CBM-IV contains extensive improvements to earlier carbon bond mechanisms in the chemical representations of aromatics, biogenic hydrocarbons, peroxyacetyl nitrates, and formaldehyde. The performance of the CBM-IV was evaluated against data from 170 experiments conducted in three different smog chambers. These experiments included NOx-air irradiations of individual organic compounds as well as a number of simple and complex organic mixtures. The results of the evaluation indicate substantial improvement in the ability of the CBM-IV to simulate aromatic and isoprene systems with average overcalculation of ozone concentrations of 1% for the aromatic simulations and 6% for the isoprene simulations. The mechanism also performed well in simulating organic mixture experiments. Maximum ozone concentrations calculated for 68 of these experiments were approximately 2% greater than the observed values while formaldehyde values were low by 9%.

Earthquakes as a self‐organized critical phenomenon

Abstract: The Gutenberg-Richter power law distribution for energy released at earthquakes can be understood as a consequence of the earth crust being in a self-organized critical state. A simple cellular automaton stick-slip type model yields D(E) ≈ E−τ with τ≈1.0 and τ≈1.35 in two and three dimensions, respectively. The size of earthquakes is unpredictable since the evolution of an earthquake depends crucially on minor details of the crust.

Postspinel transformations in the system Mg2SiO4‐Fe2SiO4 and some geophysical implications

Abstract: The high-pressure transformation in MgSiO3 and those in the spinel phases of compositions from Mg2SiO4 to (Mg0.5Fe0.5)2SiO4 in the Mg2SiO4-Fe2SiO4 system were investigated using a uniaxial split-sphere apparatus. The phase boundaries between ilmenite-perovskite in MgSiO3 and between Mg2SiO4 spinel and the assemblage of MgSiO3 perovskite and MgO periclase were determined to be P(GPa) = 26.8–0.0025T(°C) and P(GPa) = 27.6–0.0028T(°C), respectively, in the temperature range 1000–1600°C. The pseudobinary diagrams for the system Mg2SiO4-Fe2SiO4 were determined at temperatures of 1100°C and 1600°C. It was demonstrated that the magnesian spinel (with Fe/Mg + Fe < 0.22 at 1100°C and <0.26 at 1600°C) dissociates into perovskite and magnesiowustite within an extremely narrow pressure interval (<0.15 GPa at 1600°C). The dissociation pressure was found to be almost independent of iron content and to coincide to that at 670 km depth within experimental uncertainties. These experimental results indicate that the sharpness of the 670-km discontinuity may indeed be due to this dissociation in a peridotitic or pyrolitic mantle. The current status of our understanding of deep mantle mineralogy and chemistry is discussed based on recent high-pressure and high-temperature experiments.

Extension during continental convergence, with application to the Tibetan Plateau

Abstract: An explanation in terms of the thermal evolution of thickened continental lithosphere is offered to explain the transition, in the late Tertiary to Quaternary, from north-south compression to east-west extension in the strain rate field of the Tibetan Plateau. The lower part of the lithosphere consists of a thermal boundary layer which, when thickened by horizontal shortening, is colder and denser than its surroundings. Convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere. The convective instability would happen in a time brief compared with the collision time scale but would only occur after there had already been substantial thickening of the lithosphere. The increase in surface height and of potential energy are sufficient for east-west extension to replace north-south compression as the dominant feature of the stress field.

Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres

Abstract: The solution of the generalized two-stream approximation for radiative transfer in homogeneous multiple scattering atmospheres is extended to vertically inhomogeneous atmospheres in a manner which is numerically stable and computationally efficient. It is shown that solar energy deposition rates, photolysis rates, and infrared cooling rates all may be calculated with the simple modifications of a single algorithm. The accuracy of the algorithm is generally better than 10 percent, so that other uncertainties, such as in absorption coefficients, may often dominate the error in calculation of the quantities of interest to atmospheric studies.

Decomposition of magnetotelluric impedance tensors in the presence of local three-dimensional galvanic distortion

Abstract: There are many occasions on which the magnetotelluric impedance tensor is affected by local galvanic distortion (channelling) of electric currents arising from induction in a conductive structure which is approximately two-dimensional (2-D) on a regional scale. Even though the inductive behavior is 2-D, the resulting impedance tensor can be shown to have three-dimensional (3-D) behavior. Conventional procedures for rotating the impedance tensor such as minimizing the mean square modulus of the diagonal elements do not in general recover the principal axes of induction and thus do not recover the correct principal impedances but rather linear combinations of them. This paper presents a decomposition of the impedance tensor which separates the effects of 3-D channeling from those of 2-D induction. Where the impedance tensor is actually the result of regional 1-D or 2-D induction coupled with local frequency independent telluric distortion, the method correctly recovers the principal axes of induction and, except for a static shift (multiplication by a frequency independent real constant), the two principal impedances. Also obtained are two parameters (twist and shear), which partially describe the effects of telluric distortion. It is shown that the tensor operator which describes the telluric distortions can always be factored into the product of three tensor sub-operators (twist, shear, local anisotropy) and a scalar. This product factorization allows assimilation of local anisotropy, if present, into the regional anisotropy. The method of decomposition is given in the paper along with a discussion of the improvements obtained over the conventional method and an example with real data.

Scaling turbulent dissipation in the thermocline

Abstract: By comparing observations from six diverse sites in the mid-latitude thermocline, we find that, to within a factor of 2, 〈eIW〉=7×10‐10〈N2/N02〉〈S104/SGM4〉 W kg‐1, where 〈eIW〉 is the average dissipation rate attributable to internal waves; N0 = 0.0052 s−1 is a reference buoyancy frequency; S10 is the observed shear having vertical wavelengths greater than 10 m; and SGM is the corresponding shear in the Garrett and Munk spectrum of internal waves. The functional form agrees with estimates by McComas and Muller and by Henyey, Wright, and Flatte of the rate of energy transfer within the internal wave spectrum, provided the energy density of the internal waves is treated as a variable instead of one of the constant parameters. Following Garrett and Munk, we assume that 〈S104/SGM4〉=〈EIW2/EGM2〉, where EIW is the observed energy density and EGM is the energy density used by Garrett and Munk. The magnitude of eIW is twice that of Henyey et al. and one third that of McComas and Muller. Thus the observations agree with predictions sufficiently well to suggest that (1) a first-order understanding of the link between internal waves and turbulence has been achieved, although Henyey et al. made some ad hoc assumptions and Garrett and Munk's model does not match important features in the internal wave spectrum reported by Pinkel, and (2) the simplest way to obtain average dissipation rates over large space and time scales is to measure 〈N2/N02〉〈S104/SGM4〉. Even though the observations were taken at latitudes of 42°−11.5°, the variability in the Coriolis parameter ƒ was too limited for a conclusive test of the ƒ dependence also predicted for 〈eIW〉 by the wave-wave interaction models. An exception to the scaling occurs east of Barbados in the thermohaline staircase that is apparently formed and maintained by salt fingers. Although e in the staircase is very low compared with rates at mid-latitude sites, it is 8 times larger than predicted for e due only to internal waves.

Prodigious submarine landslides on the Hawaiian Ridge

Abstract: The extensive area covered by major submarine mass wasting deposits on or near the Hawaiian Ridge has been delimited by systematic mapping of the Hawaiian exclusive economic zone using the side-looking sonar system GLORIA. These surveys show that slumps and debris avalanche deposits are exposed over about 100,000 km2 of the ridge and adjacent seafloor from Kauai to Hawaii, covering an area more than 5 times the land area of the islands. Some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume, ranking them among the largest on Earth. The slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts, culminate near the end of subaerial shield building, and apparently continue long after dormancy. Consequently, landslide debris is an important element in the internal structure of the volcanoes. The dynamic behavior of the volcanoes can be modulated by slope failure, and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems. The landslides are of two general types, slumps and debris avalanches. The slumps are slow moving, wide (up to 110 km), and thick (about 10 km) with transverse blocky ridges and steep toes. The debris avalanches are fast moving, long (up to 230 km) compared to width, and thinner (0.05–2 km); they commonly have a well-defined amphitheater at their head and hummocky terrain in the lower part. Oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high-level wave deposits (such as the 365-m elevation Hulopoe Gravel on Lanai) that are found on several islands. Most present-day submarine canyons were originally carved subaerially in the upper parts of debris avalanches. Subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters.

Atmospheric carbon dioxide at Mauna Loa Observatory: 2. Analysis of the NOAA GMCC data, 1974–1985

Abstract: The first 12 years (1974--1985) of continuous atmospheric CO/sub 2/ measurements from the NOAA GMCC programs at the Mauna Loa Observatory in Hawaii are analyzed. Hourly and daily variations in the concentration of CO/sub 2/ due to local sources and sinks are described, with subsequent selection of data representing background concentrations. A digital filtering technique using the fast Fourier transform and low-pass filters was used to smooth the selected data and to separate the seasonal cycle from the long-term increase in CO/sub 2/. The amplitude of the seasonal cycle was found to be increasing at a rate of 0.05/plus minus/0.02 ppm yr/sup /minus/1/. The average growth rate of CO/sub 2/ was 1.42/plus minus/0.02 ppm yr/sup /minus/1/, and the fraction of CO/sub 2/ remaining in the atmosphere from fossil fuel combustion was 59%. A comparison between the Mauna Loa continuous CO/sub 2/ data and the CO/sub 2/ flask sample data from the sea level site at Cape Kumukahi, Hawaii, showed that the amplitude of the seasonal cycle at Cape Kumakahi was 23% larger than at Mauna Loa, with the phase of the cycle at Mauna Loa lagging the cycle at Cape Kumukahi by about 1-2 weeks. /copyright/ American Geophysical Union 1989

Regular and chaotic charged particle motion in magnetotaillike field reversals: 1. Basic theory of trapped motion

Abstract: We give a systematic theoretical analysis of trapped nonadiabatic charged particle motion in two-dimensional taillike magnetic field reversals. Particle dynamics is shown to be controlled by the curvature parameter κ, i.e., the ratio κ² = Rmin/ρmax between the minimum radius of curvature of the magnetic field and the maximum Larmor radius in it for a particle of given energy. κ≫1 corresponds to the usual adiabatic case with the magnetic moment μ as a first-order invariant of motion. As κ decreases toward unity, the particle motion becomes stochastic due to deterministic chaos, caused by the overlapping of nonlinear resonances between the bounce- and the gyro-motion. We determine the threshold of deterministic chaos and derive the related pitch angle diffusion coefficient which describes statistically the particle behavior in the limit κ → 1. Such behavior, which for κ ≅ 1 becomes strongly chaotic, applies, e.g., to thermal electrons in Earth's magnetotail and makes its collisionless tearing mode instability possible. We also show that in sharply curved field reversals, i.e., for κ 1. Both types of trapped particle motion in sharply curved magnetic field reversals κ<1 are closely connected with fast oscillations perpendicular to the reversal plane. However, the trajectories are adiabatic only in the case that they permanently remain crossing the reversal plane. The adiabatic are of a ring type, i.e., they resemble rings in phase space and also in real physical space. For ring-type orbits the action integral over the fast oscillations is an adiabatic invariant in the usual sense. On the other hand, the most common particle trajectories in a sharply curved field reversal with κ<1 are essentially of a cucumberlike quasi-adiabatic type. For quasi-adiabatic cucumberlike orbits the action integral over the fast oscillations is an adiabatic invariant only in a piecemeal way between successive traversals in the phase space of the fast motion of a separatrix between orbits which do and those, which do not cross the reversal plane. Due to the effect of separatrix traversals the slow motion shifts between different cucumber orbits with a conservation of the action integral on average but with its chaotic phase space diffusion even for very small perturbation parameters κ. The case κ<1 is applicable, e.g., to thermal ions and high-energy electrons in Earth's magnetotail. Our findings lead to a systematic interpretation of particle observations in Earth's magnetotail and of numerous numerical calculations, carried out in the past. They also explain rather well, e.g., the pitch angle diffusion of plasma sheet particles, the isotropization of the plasma sheet electron distribution immediately before a substorm and provide with the transition to chaos a mechanism for the onset of a large-scale tail instability and the explosion of isolated substorms. Further implications for magnetotail physics, such as acceleration processes and the influence of the particle escape from the field reversal will be discussed in a second related paper.

Migration of fluid phases and genesis of basalt magmas in subduction zones

Abstract: Dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of H2O in the production of magmas at convergent plate boundaries. The subduction of oceanic lithosphere, occurring with increasing pressures and rising temperatures, causes liberation of H2O from the slab. Amphibole, which can be stable to the highest PT conditions among hydrous phases in the slab, decomposes at around 90 km depth. It follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab-derived H2O is not a direct trigger for the production of arc magmas. Instead, the H2O released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals (serpentine, talc, amphibole, chlorite, and phlogopite). This hydrated peridotite is dragged downward on the slab toward higher PT regions and releases H2O to shallower potential magma source regions in the mantle wedge. Combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge, it is concluded that those minerals decompose beneath the forearc region. On the other hand, high PT experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front. Phlogopite in the peridotite decomposes to release H2O at a deeper level (about 200 km). H2O liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites. Along with the migration of H2O through the above processes, subduction components, especially large ion lithophile elements, can be overprinted on the magma source region, which governs the geochemical characteristics of arc magmas.

Forcing of late Cenozoic northern hemisphere climate by plateau uplift in southern Asia and the American west

Abstract: Geologic evidence indicates that net vertical uplift occurred on a large (kilometer) scale and at accelerating rates during the middle and late Cenozoic in plateaus of southern Asia and the American west. Based on this evidence, General Circulation Model sensitivity tests were run to isolate the unique effects of plateau uplift on climate. The experiments simulated significant climatic changes in many places, some far from the uplifted regions. The basic direction of most of these simulated responses to progressive uplift is borne out by changes found in the geologic record: winter cooling of North America, northern Europe, northern Asia, and the Arctic Ocean; summer drying of the North American west coast, the Eurasian interior, and the Mediterranean; winter drying of the North American northern plains and the interior of Asia; and changes over the North Atlantic Ocean conducive to increased formation of deep water. The modeled changes result from increased orographic diversion of westerly winds, from cyclonic and anticyclonic surface flow induced by summer heating and winter cooling of the uplifted plateaus, and from the intensification of vertical circulation cells in the atmosphere caused by exchanges of mass between the summer-heated (and winter-cooled) plateaus and the mid-latitude oceans. Disagreements between the geologic record and the model simulations in Alaska and the Southern Rockies and plains may be related mainly to the lack of narrow mountain barriers in the model orography. Taken together, the observed regional trends comprise much of the pattern of “late Cenozoic climatic deterioration” in the northern hemisphere that culminated in the Plio-Pleistocene ice ages. The success of the uplift sensitivity experiment in simulating the correct pattern and sign of most of the observed regional climatic trends points to uplift as an important forcing function of late Cenozoic climatic change in the northern hemisphere at time scales longer than orbital variations; however, the modest amplitude of the uplift-induced cooling simulated at high latitudes indicates a probable need for additional climatic forcing.

The system Mg2SiO4-Fe2SiO4 at high pressures and temperatures: Precise determination of stabilities of olivine, modified spinel, and spinel

Abstract: High-pressure and high-temperature phase relations in the system Mg2SiO4-Fe2SiO4 were thoroughly reinvestigated at pressures up to 21 GPa and at temperatures of 1200°C and 1600°C using a uniaxial split-sphere apparatus. Both normal and reverse experiments were performed on the univariant reactions. Both exsolving and dissolving experiments were performed to determine the divariant loops at 1600°C. The run products were examined using microfocused X ray diffractometry, scanning electron microscopy, and electron probe microanalysis. The phase diagrams thus constructed were compared with those previously proposed using an experimental or thermochemical method. Phase changes in mantle olivine with Mg number of 89 occur in the following order: (α) − (α+β) − (β) − (β+γ) − (γ). The α+β loop narrows with increasing temperature. The applicability of an isochemical peridotitic mantle model is discussed in the light of the location and thickness of the 400-km discontinuity.

A new magnetic coordinate system for conjugate studies at high latitudes

Abstract: Studying magnetically conjugate phenomena at very high latitudes requires a magnetic coordinate system that is smooth and well defined at the geographic poles In addition, it should provide for accurate comparisons at different altitudes In this report the authors present a variation on the corrected geomagnetic coordinate system that is well defined and smooth over the entire globe It provides an analytic expression relating geographic coordinates, including altitude, to the magnetic coordinates The coordinate system is produced by tracing magnetic field lines using the IGRF85 reference magnetic field model with time derivatives updating the model to 1988 An expansion of the relationship in terms of spherical harmonics has been determined, which then provides the required well-defined and smooth relationship over the entire globe Independent expansions for different altitudes show a smooth functional relationship of the coefficients of the expansion with altitude, and therefore simple interpolation schemes can be used to provide an appropriate expansion at any altitude between 0 km and approximately 600 km By reversing the process, the inverse expansions relating the magnetic coordinates to geographic coordinates have also been determined The effects of the seasonal variation in the Sun's declination along with the variation in the Sun's declinationmore » along with the variation in the Sun's apparent position due to the eccentricity of the Earth's orbit result in a variation of nearly 1 hour of magnetic local time for a fixed UT over the course of a year In many applications this variation may be important and should be included when presenting data in terms of magnetic latitude and MLT« less

Average plasma properties in the central plasma sheet

Abstract: Using four months of tail data obtained by the three-dimensional plasma instrument on board the AMPTE/IRM satellite in 1986, we have done a statistical survey on the behavior of ion and electron moments in the central plasma sheet. Almost 80,000 spin averages of plasma density, ion bulk velocity, ion and electron temperature, and plasma β were analyzed with respect to differences between their values in the inner and outer central plasma sheet as well as their dependence on magnetic activity. The ion temperature increases with increasing magnetic activity while the ion density decreases during disturbed intervals, except in the neutral sheet neighborhood at smaller radial distances. The ion and electron temperatures in the central plasma sheet are highly correlated, with Ti/Te being constant over a wide range of temperatures and about twice as large as in the distant tail. The average ion flow speeds in the central plasma sheet are below 100 km/s and nearly identical to those found in the plasma sheet boundary layer, although the distribution functions usually are quite different. High-speed flows do occur, but in bursts of most often less than 1 min duration with intermittent intervals of nearly stagnant plasma. The distribution of flow directions strongly favors sunward flow for velocities above 300 km/s, indicating that a near-earth neutral line is rarely, if ever, located inside of XGSM = −19 RE.

Large‐scale ice flow over a viscous basal sediment: Theory and application to ice stream B, Antarctica

Abstract: Recent seismic studies of ice stream B, Antarctica and field analysis of mid-latitude glacial deposits suggest that deformable basal sediments (e.g., water-saturated till) are important in determining ice sheet flow. If the ratio of till viscosity to effective ice viscosity is small, vertical shear associated with horizontal flow is confined to the deforming bed alone. Ice flow over a deformable bed is thus akin to that of floating ice shelves, because ice shelves flow over inviscid seawater. For some Antarctic ice streams, and possibly for portions of the late Wisconsin ice sheets in North America and Eurasia, basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity. Instead, basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients. Comparison of the observed flow of ice stream B to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile. These simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance. While bed deformation is critical to ice stream existence, sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate.

A 3-year continuous record on the influence of daytime, season, and fertilizer treatment on methane emission rates from an Italian rice paddy

Abstract: CH4 emission rates have been measured in an Italian rice paddy between 1984 and 1986, covering three vegetation periods. For these measurements a fully automated, computerized sampling and analyzing system was developed which allowed the simultaneous determination of CH4 emission rates at 16 different field plots. CH4 emission rates showed strong diurnal and seasonal variations. Diurnal changes correlated with changes in soil temperature. During the season, CH4 emission rates showed a first maximum in May–June before tillering and a second maximum in July during the reproductive stage of the rice plants. In 1985 and 1986 two maxima were observed during summer in addition to the first maximum in the rate of CH4 emission during spring. Application of mineral and/or organic fertilizer strongly influenced the CH4 emission rates, depending on the type, rate, and mode of fertilizer application. Thus the rates decreased by at most 40% and 60% after fertilization by deep incorporation with 200 kg N/ha urea and 200 kg N/ha ammonium sulfate, respectively. Application of 200 kg N/ha calcium cyanamide led to a reduction of the first maximum of CH4 emission but caused the second maximum to increase, the overall result being that the seasonally averaged CH4 emission rate was comparable to that observed in unfertilized fields. Application of rice straw at a rate of 12 t/ha enhanced the rate of CH4 emission by a factor of 2 compared with the control. Higher application rates of rice straw did not cause a further increase in CH4 emission. The complete records of CH4 emissions over three vegetation periods indicate an average seasonal CH4 emission rate from unfertilized fields of 0.28 g CH4/m2 d, with a range of 0.16–0.38 g CH4/m2 d. Based on this value and applying the observed temperature dependence of the CH4 emission rates, the global annual CH4 emission from rice paddies is estimated to be in the range of 50–150 Tg, with a likely average of 100 Tg. This figure represents between 19% and 25% of the global CH4 emission, indicating that rice paddies are one of the most important individual sources of atmospheric CH4.

Carrier Phase Ambiguity Resolution for the Global Positioning System Applied to Geodetic Baselines up to 2000 km

Abstract: A technique for resolving the ambiguities in the GPS carrier phase data (which are biased by an integer number of cycles) is described which can be applied to geodetic baselines up to 2000 km in length and can be used with dual-frequency P code receivers. The results of such application demonstrated that a factor of 3 improvement in baseline accuracy could be obtained, giving centimeter-level agreement with coordinates inferred by very-long-baseline interferometry in the western United States. It was found that a method using pseudorange data is more reliable than one using ionospheric constraints for baselines longer than 200 km. It is recommended that future GPS networks have a wide spectrum of baseline lengths (ranging from baselines shorter than 100 km to those longer than 1000 km) and that GPS receivers be used which can acquire dual-frequency P code data.

Seismic velocities in mantle minerals and the mineralogy of the upper mantle

Abstract: Comparison of seismic velocities in mantle minerals, under mantle conditions, with seismic data is a first step toward constraining mantle chemistry. The calculation, however, is uncertain due to lack of data on certain physical properties. “Global” systematics have not proved very useful in estimating these properties, particularly for the shear parameters. A new approach to elasticity estimation is used in this study to produce estimates of unknown quantities, primarily pressure and temperature derivatives of elastic moduli, from the structural and chemical trends evident in the large amount of elasticity data now available. These trends suggest that the derivatives of unmeasured high-pressure phases can be estimated from “analogous” low-pressure phases. Using these predictions and the best available measurements, seismic velocities are computed along high-temperature adiabats for a set of mantle minerals using third-order finite strain theory. The calculation of density and moduli at high temperature, to initiate the adiabat, must be done with care since parameters such as thermal expansion are not independent of temperature. Both compressional and shear seismic profiles are well-matched by a mineralogy dominated by clinopyroxene and garnet and with an olivine content of approximately 40% by volume. Between 670 and 1000 km, perovskite alone provides a good fit to the seismic velocities. Combining seismic velocities with recent phase equilibria data for a hypothetical pure olivine mantle suggests that a mineralogy with a maximum of 35% olivine (shear profile) or 40–53% olivine (compressional profile) by volume can satisfy the constraint imposed by the 400-km discontinuity. Other features of the upper mantle can then be matched by appropriate combinations of pyroxenes, garnets, and their high-pressure equivalents. While mantle models with a substantially larger fraction of olivine cannot be ruled out, they are acceptable only if the derivatives of the spinel phases are substantially different from olivine and deviate from trends in the larger data set.

Olivine-modified spinel-spinel transitions in the system Mg2SiO4-Fe2SiO4: Calorimetric measurements, thermochemical calculation, and geophysical application

Abstract: The olivine(α)-modified spinel(β)-spinel (γ) transitions in the system Mg2SiO4-Fe2SiO4 were studied by high-temperature solution calorimetry. Enthalpies of the β-γ and a α-γ transitions in Mg2SiO4 at 975 K and of the α-γ transition in Fe2SiO4 at 298 K were measured. The γ solid solution showed a positive enthalpy of mixing. Phase relations at high pressures and high temperatures were calculated from these thermochemical data including correction for the effect of nonideality of α, β, and γ solid solutions. The calculated phase diagrams agree well with those determined experimentally by Katsura and Ito very recently. The α − (Mg0.89, Fe0.11)2SiO4 transforms to β through a region of α+β without passing through the α+γ phase field at around 400 km depth in the mantle with an interval of about 18(±5) km. Temperatures at 390 and 650 km depths are estimated to be about 1673 and 1873 K, respectively, assuming an adiabatic geotherm.

Ozone destruction through heterogeneous chemistry following the eruption of El Chichón

Abstract: It is now well established that heterogeneous reactions provide an important mechanism for Antarctic ozone depletion. Recent laboratory studies suggest that the same reactions that occur on HNO3/H2O ice clouds in the cold Antarctic stratosphere can also take place on sulfuric acid particles (e.g., volcanic and background aerosols) typical of lower latitudes, albeit at slower rates. The reduction in stratospheric ozone observed at northern mid-latitudes in late 1982 through 1983 following the volcanic eruption of El Chichon is investigated in terms of ozone loss through heterogeneous chemistry on the aerosol which formed in the stratosphere. The rates of the relevant heterogeneous reactions are believed to be critically dependent on (1) the aerosol surface area density and (2) the percent by weight sulfuric acid in the liquid particles. Direct measurements of both of these important quantities for El Chichon aerosol are described and used as a basis for model calculations of their possible effects on ozone and other trace species. The observed volcanic particle surface area reached a maximum at mid-latitudes of about 50 μm2 cm−3 (above a typical background value of about 0.75) at an altitude of 18–20 km in early 1983. This enhancement of surface area is about the same as that encountered in stratospheric clouds in the Antarctic, suggesting a possible basis for ozone depletion through heterogeneous chemistry. Observations of NO2 and HNO3 also suggest that heterogeneous reactions on both background and volcanic aerosol play a significant role in partitioning reactive nitrogen species in middle and high latitudes in winter. It is shown that heterogeneous reactions similar to those occurring in Antarctica may have been responsible for at least a portion of the anomalous ozone reduction observed at mid-latitudes in early 1983.

Cosmic ray production rates of 10Be and 26Al in quartz from glacially polished rocks

Abstract: The concentrations of Be-10 and Al-26 in quartz crystals extracted from glacially polished granitic surfaces from the Sierra Nevada range are studied. These surfaces are identified with the glacial advance during the Tioga period about 11,000 yr ago. The measurements yield the most accurate estimates to date for the absolute production rates of three nuclides in SiO2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43.8-44.6 N and altitudes 2.1-3.6 km.

Late Cenozoic right‐lateral strike‐slip faulting in southern Tibet

Abstract: Field observations collected during the Sino-French Himalaya-Tibet cooperation project (1980–1982) show that right-lateral faulting between southern and northern Tibet, roughly along the “chord” joining the eastern and western syntaxes of the Himalayan arc, is a major component of the active tectonics north of the Himalayas Just north of the well developed rifts of southern Tibet, we studied three right-lateral, 100–200 km long, strike-slip faults forming an en echelon array along that chord Evidence for earthquake surface breaks was found on each of these faults For the M = 8, Beng Co (or Damxung) strike-slip earthquake of November 18, 1951, rupture parameters could be constrained to be ū = 8 ± 2 m and L = 90 km Offset glacial and postglacial morphological markers (105 years and 104 years BP) imply displacement rates of the order of 1–2 cm/yr on the three faults The en echelon fault zone appears to extend westward ≈2000 km across the plateau to the Karakorum fault and is thus referred to as the Karakorum-Jiali fault zone (KJFZ) Fast rates of slip on it and on faults along the northern rim of Tibet appear to contrast with the slower rates of deformations in the interior of the plateau Our estimates of slip rates on the strike-slip and normal faults of southeastern Tibet suggest that eastward extrusion of North Tibet between the right-lateral KJFZ and the left-lateral Altyn Tagh and Kun Lun faults absorbs at least 30% of the present convergence between India and Asia We propose an instantaneous velocity model of the active tectonics of Asia compatible with such faulting rates and geometries Evidence for Tertiary right-lateral strike-slip movements in southern Tibet and around the eastern syntaxis of the Himalayas is also found in the structural record Offsets of various large-scale geological features imply that these movements have been large

Flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension

Abstract: We suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound. This mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes, and magmatic thickening of the crust. Our hypothesis is based on two critical concepts. First, the lithosphere retains finite mechanical strength or flexural rigidity during extension. Second, isostatic rebound (uplift) of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation. We develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks. We first investigate the isostatic consequences of finite simple slip on an initially planar, dipping normal fault cutting the entire lithosphere. When the lithosphere retains flexural rigidity during extension, the topography resulting from this model resembles a half graben, and the footwall rift flank is flexurally uplifted. This simple normal faulting model explains free-air gravity anomalies and topography observed at rift flanks in oceanic lithosphere (such as Broken Ridge in the eastern Indian Ocean, the Caroline ridges-Sorol Trough in the western equatorial Pacific, and the Coriolis Trough behind the New Hebrides island arc). We then investigate a general kinematic model for lithospheric extension where simple slip on a surface of arbitrary shape is accompanied by pure shear extension in the upper and lower plates. When the simple slip component is not zero or the distribution of pure shear in the upper and lower plates is not identical, the surface of slip can be regarded as a detachment. By simplification, our general model accounts for pure shear extension of the lithosphere that is uniform with depth. In this case, detachments have no meaning in the geologic sense. However, the kinematics of depth-independent pure shear may nevertheless be described in terms of a surface, which we term a kinematic reference surface, at some depth in the lithosphere. We speculate that the depth of this surface may be rheologically controlled. The magnitude of rift flank uplift by flexure depends critically on the depth of this reference surface. In contrast, if local isostasy is assumed when the lithosphere undergoes a given amount of depth-independent pure shear, the resulting topography will be the same regardless of how the kinematics of that extension are formulated. The basin and rift flank topography and free-air gravity anomaly over young continental rifts, such as the Rhine graben, can be satisfied using our general extensional model with a small amount (<5 km) of extension along a listric-shaped detachment soling into the crust-mantle boundary. Because the flexural rebound mechanism explains the observed topography and gravity anomaly over both oceanic and continental extensional domains, we suggest that rheological differences between the two lithospheric types may not be important in their overall response to extension.

A synthetic stratigraphic model of foreland basin development

Abstract: The processes of erosion and deposition must be included in foreland basin models to predict correctly basin geometry and stratigraphy. We present a synthetic stratigraphic model of the development of nonmarine foreland basins that predicts progressive geometry, topography, and facies patterns. In the model, steady crustal shortening occurs according to a wedgethickening model, erosion and deposition follow a diffusive process, and the lithosphere is compensated elastically. Erosion and deposition are controlled by the transport coefficients r of the diffusion equation. For a range of thrust velocities and lithospheric rigidities, transport coefficients are of order 102-103 m/yr in the mountain belt; these values are much higher than those derived from the study of scarp degradation. In the sedimentary basin, transport coefficients of order 104 m2/yr are appropriate and are compatible with previous studies of fluvial and deltaic deposition. Rapid thrusting results in a narrow underfilled basin, while slow thrusting results in a wide overfilled basin. In addition, by varying the erosional and depositional transport coefficients while holding other parameters .constant, we generate both overfilled and underfilled basins. These results suggest that changes in the rate of thrust loading, the climate, or the source rock lithology can create stratigraphic signatures that have been interpreted to record viscoelastic relaxation of the lithosphere. Clearly, to understand either the long-term behavior of the lithosphere or to interpret orogenic history from preserved foreland strata, the manner in which a basin was filled must be considered. We apply the model to the evolution of the modern sub-Andean foreland and find that an erosional transport coefficient of 3000 m2/yr and a depositional transport coefficient of 20,000 m2/yr successfully predict the observed basin geometry.

The 40Ar/39Ar thermochronometry for slowly cooled samples having a distribution of diffusion domain sizes

Abstract: Many 40Ar/39Ar age spectra for alkali feldspars are significantly different from the model age spectra calculated for slowly cooled samples composed of diffusion domains of a single size, and the Arrhenius plots for these samples show departures from linearity that are inconsistent with diffusion from domains of equal size. The most plausible explanation for these discrepancies is the existence of a distribution of diffusion domain sizes. We have extended the single-diffusion-domain closure model of Dodson so that it applies to minerals with a distribution of domain sizes and have used it to explain many commonly observed features of 40Ar/39Ar age spectra and Arrhenius plots for 39Ar loss during step heating. For samples with a distribution of diffusion domain sizes, the form of the 39Ar Arrhenius curve is a function of the heating schedule (i.e., the temperature and duration of the steps used), and thus different heating schedules will result in different curves for the same sample. This effect can be used to confirm the existence of a distribution of diffusion domain sizes and to optimize the information contained in the Arrhenius plot. The multiple diffusion domain size model is used to reinterpret the age spectra, Arrhenius plots, and cooling history of three feldspars from the Chain of Ponds pluton, northwestern Maine, earlier interpreted assuming a single domain size. Interpreting the 40Ar/39Ar and 39Ar released during step heating in terms of a single domain size gives rise to a large discrepancy between the cooling rate determined from the age and closure temperature of the three samples compared to the cooling rate required to explain the shape of the individual age spectra. The single domain size model fails also to account for the observed departures from linearity of the Arrhenius plots. We show that a particular domain size distribution in each sample can explain in detail both the shape of the age spectra and the Arrhenius plots, and results in the three samples defining a common cooling history. There is thus good evidence for the three alkali feldspar samples studied here that the thermally activated diffusion measured by 39Ar release during step heating in the laboratory is also the mechanism responsible for argon loss or retention in the natural setting.

Sensitivity of climate to late Cenozoic uplift in southern Asia and the American west: Numerical experiments

Abstract: Experiments with general circulation models with no mountains, half mountains and full (modern) mountains show the sensitivity of atmospheric circulation patterns to progressive amounts of uplift. The experiments were motivated by geologic evidence for large, kilometer-scale uplift in the late Cenozoic in southern Asia and the American west, particularly within the last 10 m.y. In January the amplitude of the mid-latitude, upper tropospheric planetary waves increased with uplift, and the low-level winds were progressively blocked or diverted around the topographic features. In July the progressive uplift caused monsoonlike circulations to develop in the vicinity of the Colorado and Tibetan plateaus. Atmospheric heating rates, midtroposphere vertical motion, and upper-tropospheric planetary wave amplitudes varied approximately linearly with progressive uplift. These results suggest that geologically recent kilometer-scale uplift may have had climatic consequences comparable in magnitude and pattern to those of earlier stages of uplift. Comparison of the simulated climatic effects of uplift with the geologic evidence for late Cenozoic climatic change, especially for the last 10 m.y., is given in accompanying papers.