Detailed Density Structure Research Articles

Multilayer Densities of the Crust and Upper Mantle in the South China Sea Using Gravity Multiscale Analysis

The South China Sea (SCS) is the result of several major tectonic plates’ interaction, and a more detailed density structure is essential to study the tectonic evolution of the marginal seas. In the past decades, density studies in the SCS have focused more on profiles, and the lateral density structure has been less mentioned by scholars. Previously, gravity data have been used many times to study the SCS; this can satisfy the demands of large-scale lateral studies, but a point that cannot be ignored is that the gravity method has certain deficiencies in vertical resolution. Therefore, this paper applies a gravity multiscale analysis method to isolate the Bouguer gravity anomaly signal in the SCS, and then uses the power spectrum method to estimate the mean field source depth, which corresponds to the decomposed gravity anomaly signal. On this basis, we modeled stratified crustal and upper mantle structures in the SCS to determine the lateral density distribution at different depths. The results show the following: (1) There are two large high-density materials in the SCS. The first one is located near the Yinggehai Basin, where the depth ranges from 42.4 km to 71.2 km, with a diameter of nearly 220 km; the second one is located near the northwest sub-basin, where the depth ranges from 106.8 km to 128.8 km, which is probably part of the Paleo-Pacific remnant in the SCS. (2) The proto South China Sea subduction zone is located in the northwestern part of the Sulu Sea and has a northeast–southwest trend.

Crustal structure and fault geometries of the Garhwal Himalaya, India: Insight from new high-resolution gravity data modeling and PSO inversion

A high-resolution gravity survey from the Indo-Gangetic plain to the MCT zone is carried out across the Garhwal Himalaya, India, which lies in the central seismic gap. The gravity measurements along a ∼205 km transect with dense station interval (<650 m) are modeled using the power spectrum, gravity isolation technique, wavelet analysis, PSO inversion, 2D-forward modeling, and constraints from previous results. The spectral analysis suggests two dominant layer interfaces in the lithosphere at 44 km and 12 km, inferred as the Moho and the Himalayan decollement thrust, respectively. The gravity isolation technique contributes to assessing the previously known geophysical and geological information and separating individual gravity responses despite the complexity of the tectonics of the Himalaya and further deriving the gravity effect of the upper crustal region, including main fault/thrust zones. The computed residual gravity anomaly shows prominent signatures of the main tectonics structures. Wavelet analysis and the Particle Swarm Optimization (PSO) inversion constrain the geometries of the intracrustal fault/thrust zones. Our model shows an increase in the Moho depth towards the north (from ∼40.4 km to 43.5 km). The derived MHT geometry clearly shows the ramp structure and associated Lesser Himalayan Duplex structures towards the north. The proposed detailed density structure discerns the geometries of different crustal structures across the Garhwal Himalaya and gives insight to the upper crustal fault zones.

Arc crustal differentiation mechanisms

The detailed vertical compositional and thermal structure of the entire Kohistan arc section is constructed using ∼63P/T constraints and 209 plutonic whole rock analyses. The aim is to better understand the nature of the chemical differentiation in the plutonic arc crust at different crustal levels.Results indicate that a distinct difference in character exists between the upper and lower plutonic arc crust. Plutonic rocks in the lower crust (⩾25–30km) have whole-rock chemical characteristics that indicate mineral accumulation and residual melt loss. In these cumulate rocks there is a systematic chemical stratification of decreasing incompatible trace elements with depth (e.g. Rb, K2O). In contrast, the plutonic rocks of the upper crust represent mainly frozen liquids and show no systematic relationship between chemical composition and intrusion depth. These results clearly indicate that magmatic differentiation in the Kohistan arc dominantly occurred in the deeper arc crust (⩾30km) consistent with the lower crustal ‘hot zone model’. To understand the role of density barriers for melt stagnation in the crust, and thereby for the observed chemical stratification, the detailed density structure of the plutonic arc crust is compared to melt densities calculated for plutonic rocks with near liquid compositions. Results indicate that melts are consistently less dense than rocks, and there is no evidence for a “neutral buoyancy line” controlling melt emplacement. It is speculated that melt stagnation in the lower arc crust and subsequent chemical differentiation of a felsic upper and mafic lower continental crust is dominantly controlled by temperature and to a lesser extent to density or rheological barriers.

Mass loss from inhomogeneous hot star winds

<i>Context. <i/>The mass-loss rate is a key parameter of hot, massive stars. Small-scale inhomogeneities (clumping) in the winds of these stars are conventionally included in spectral analyses by assuming optically thin clumps, a void inter-clump medium, and a smooth velocity field. To reconcile investigations of different diagnostics (in particular, unsaturated UV resonance lines vs. /radio emission) within such models, a highly clumped wind with very low mass-loss rates needs to be invoked, where the resonance lines seem to indicate rates an order of magnitude (or even more) lower than previously accepted values. If found to be realistic, this would challenge the radiative line-driven wind theory and have dramatic consequences for the evolution of massive stars. <i>Aims. <i/>We investigate basic properties of the formation of resonance lines in small-scale inhomogeneous hot star winds with non-monotonic velocity fields.<i>Methods. <i/>We study inhomogeneous wind structures by means of 2D stochastic and pseudo-2D radiation-hydrodynamic wind models, constructed by assembling 1D snapshots in radially independent slices. A Monte-Carlo radiative transfer code, which treats the resonance line formation in an axially symmetric spherical wind (without resorting to the Sobolev approximation), is presented and used to produce synthetic line spectra.<i>Results. <i/>The optically thin clumping limit is only valid for very weak lines. The detailed density structure, the inter-clump medium, and the non-monotonic velocity field are all important for the line formation. We confirm previous findings that radiation-hydrodynamic wind models reproduce observed characteristics of strong lines (e.g., the black troughs) without applying the highly supersonic “microturbulence” needed in smooth models. For intermediate strong lines, the velocity spans of the clumps are of central importance. Current radiation-hydrodynamic models predict spans that are too large to reproduce observed profiles unless a very low mass-loss rate is invoked. By simulating lower spans in 2D stochastic models, the profile strengths become drastically reduced, and are consistent with higher mass-loss rates. To simultaneously meet the constraints from strong lines, the inter-clump medium must be non-void. A first comparison to the observed Phosphorus V doublet in the O6 supergiant <i>λ<i/> Cep confirms that line profiles calculated from a stochastic 2D model reproduce observations with a mass-loss rate approximately ten times higher than that derived from the same lines but assuming optically thin clumping. Tentatively this may resolve discrepancies between theoretical predictions, evolutionary constraints, and recent derived mass-loss rates, and suggests a re-investigation of the clump structure predicted by current radiation-hydrodynamic models.

THREE-DIMENSIONAL SIMULATIONS OF BI-DIRECTED MAGNETOHYDRODYNAMIC JETS INTERACTING WITH CLUSTER ENVIRONMENTS

We report on a series of three-dimensional magnetohydrodynamic simulations of active galactic nucleus (AGN) jet propagation in realistic models of magnetized galaxy clusters. We are primarily interested in the details of energy transfer between jets and the intracluster medium (ICM) to help clarify what role such flows could have in the reheating of cluster cores. Our simulated jets feature a range of intermittency behaviors, including intermittent jets that periodically switch on and off and one model jet that shuts down completely, naturally creating a relic plume. The ICM into which these jets propagate incorporates tangled magnetic field geometries and density substructure designed to mimic some likely features of real galaxy clusters. We find that our jets are characteristically at least 60% efficient at transferring thermal energy to the ICM. Irreversible heat energy is not uniformly distributed, however, instead residing preferentially in regions very near the jet/cocoon boundaries. While intermittency affects the details of how, when, and where this energy is deposited, all of our models generically fail to heat the cluster cores uniformly. Both the detailed density structure and nominally weak magnetic fields in the ICM play interesting roles in perturbing the flows, particularly when the jets are non-steady. Still, this perturbation is never sufficient to isotropize the jet energy deposition, suggesting that some other ingredient is required for AGN jets to successfully reheat cluster cores.

Toward Mapping the Detailed Density Structure of Classical Be Circumstellar Disks

The first results from a near-contemporaneous optical and infrared spectroscopic observing program designed to probe the detailed density structure of classical Be circumstellar disks are presented. We report the discovery of asymmetrical infrared emission lines of He I, O I, Fe II, and the Brackett, Paschen, and Pfund series lines of H I which exhibit an opposite V/R orientation (V $>$ R) to that observed for the optical Balmer H$\alpha$ line (V $<$ R) in the classical Be star $\zeta$ Tau. We interpret these data as evidence that the density wave which characterizes $\zeta$ Tau's disk has a significantly different average azimuthal morphology in the inner disk region as compared to the outer disk region. A follow-up multi-wavelength observational campaign to trace the temporal evolution of these line profile morphologies, along with detailed theoretical modeling, is suggested to test this hypothesis.

Three-Dimensional Imaging of Nerve Tissue by X-Ray Phase-Contrast Microtomography

We show that promising information about the three-dimensional (3D) structure of a peripheral nerve can be obtained by x-ray phase-contrast microtomography (p- μCT; Beckmann, F., U. Bonse, F. Busch, and O. Günnewig, 1997. J. Comp. Assist. Tomogr. 21:539–553). P- μCT measures electronic charge density, which for most substances is proportional to mass density in fairly good approximation. The true point-by-point variation of density is thus determined in 3D at presently 1 mg/cm 3 standard error (SE). The intracranial part of the rat trigeminal nerve analyzed for the presence of early schwannoma “microtumors” displayed a detailed density structure on p- μCT density maps. The average density of brain and nerve tissue was measured to range from 0.990 to 0.994 g/cm 3 and from 1.020 to 1.035 g/cm 3, respectively. The brain-nerve interface was well delineated. Within the nerve tissue, a pattern of nerve fibers could be seen that followed the nerve axis and contrasted against the bulk by 7 to 10 mg/cm 3 density modulation. Based on the fact that regions of tumor growth have an increased number density of cell nuclei, and hence of the higher z element phosphorus, it may become possible to detect very early neural “microtumors” through increases of average density on the order of 10 to 15 mg/cm 3 by using this method.

Numerical study of the behavior of heated water discharged into the ocean

Matsuno and Nagata (1987) showed numerically that the spreading characteristics of the discharged heated water from power plants of existing scale is significantly influenced by the earth's rotation effect. Although the effect of the enhancement of the Coriolis parameter on the spreading characteristics of the formed warm water mass was discussed in order to demonstrate the rotation effect, other parameters such as the density difference between the discharged heated water and ambient water and the vertical eddy viscosity and diffusivity were fixed. In this paper, the dependence of the spreading characteristics on these parameters is examined. Then, it is shown that the overall shape of the formed warm water mass and density and velocity structure strongly depend on these parameters. Also, it is indicated that the behavior of the warm water mass under the rotation effects is too complicated to be described with a few parameters. For example, the internal radius of deformation seems to be one of the determinative parameters, and the increase of the density difference between discharged water and ambient water has a similar effect on the shape of the formed warm water mass as a decrease of the Coriolis parameter. However, a change of the two values has different effects on the detailed density structure and current structure of the warm water mass. The Prandtl number seems to determine some aspects of the veloczty field such as velocity magnitude and width of the southward flowing current zone. However, other features such as the thickness of the warm water mass are not determined by the Prandtl number.

On a self-consistent representation of earth models, with an application to the computing of internal flattening

Most realistic Earth models published as yet have been given in tabulated form, with the noticeable exception of three simple parametric Earth models derived by Dziewonski et al. (1975). Simple interpolation in these tables may lead to inconsistencies, when we consider certain effects which depend crucially on detailed density structure. We establish algorithmic formulae, which may be used to compute all the mechanical properties of a model in an entirely consistent way, once the density as well as P- and S- wave velocities are known. We then use this formulation to integrate Clairaut’s equation in a very efficient way, and thus obtain the hydrostatic flattening to the first order in smallness at any point inside the model. For most geodynamic purposes, we may suffice with this approximation. Finally, we show the results of some calculations of hydrostatic flattening to the first and second order, using an iterative technique of solving the integral figure equations, for an Earth model consistent with all geophysical data available at present. We find that the hydrostatic flattening at the surface should be about 1/298.8, instead of 1/296.961 as quoted by Nakiboglu (1979) for essentially the same model. Moreover, from our results, we estimate the actual flattening of the coremantle boundary to be about 1/390.3.