ALBERT

Description: We develop a three-dimensional model of shear wave velocity and anisotropy for the Mexico subduction zone using Rayleigh wave phase velocity dispersion measurements. This region is characterized by both steep and flat subduction and a volcanic arc that appears to be oblique to the trench. We give a new interpretation of the volcanic arc obliqueness and the location of the Tzitzio gap in volcanism based on the subduction morphology. We employ the two-station method to measure Rayleigh phase velocity dispersion curves between periods of 16 s to 171 s. The results are then inverted to obtain azimuthally anisotropic phase velocity maps and to model 3-D variations in upper mantle velocity and anisotropy. Our maps reveal lateral variations in phase velocity at all periods, consistent with the presence of flat and steep subduction. We also find that the data are consistent with two layers of anisotropy beneath Mexico: a crustal layer, with the fast directions parallel to the North American absolute plate motion, and a deeper layer that includes the mantle lithosphere and the asthenosphere, with the fast direction interpreted in terms of toroidal mantle flow around the slab edges. Our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure. Our anisotropy results favor a tear, which is also consistent with the geometry of the volcanic belt.

Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Davis, Paul M -- Jackson, David D -- New York, N.Y. -- Science. 2011 Mar 18;331(6023):1400. doi: 10.1126/science.1204861.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21415345" target="_blank"〉PubMed〈/a〉

Description: [1]  We evaluate the mechanisms of attenuation for the Middle America (MA) region in the range of 1.0 ≤  f  ≤ 15 Hz . Our analysis focuses on reconciling laboratory experiments, which suggest that in this frequency range intrinsic attenuation has a weak to zero frequency dependence, with results obtained using the Multiple Lapse Time Widow (MLTW) method. The MLTW method is designed to separate the contributions of scattering by heterogeneities and intrinsic attenuation from total attenuation assuming spherical geometrical spreading and isotropic scattering. Application of the MLTW method suggests that total attenuation in the MA region is concentrated towards the upper crust and is relatively constant throughout the region. It also shows a strong frequency dependence of the energy loss component of the attenuation (apparent intrinsic attenuation). We test the effect that forward and back scattering operators, distribution of the sources, and absorption of energy into the mantle has on the separation of the components of attenuation. We conclude that the inclusion of the frequency-dependent leakage of energy towards the mantle provides a mechanism to satisfy both laboratory experiments and estimates based on the MLTW method. For the Middle America region, we find that the intrinsic attenuation .

Description: The phylogenetic branching order of the green algal groups that gave rise to land plants remains uncertain despite its fundamental importance to understanding plant evolution. Previous studies have demonstrated that land plants evolved from streptophyte algae, but different lineages of streptophytes have been suggested to be the sister group of land plants. To better understand the evolutionary history of land plants and to determine the potential effects of "long-branch attraction" in phylogenetic reconstruction, we analyzed a chloroplast genome data set including three new chloroplast genomes from streptophyte algae: Coleochaetae orbicularis (Coleochaetales), Nitella hookeri (Charales), and Spirogyra communis (Zygnematales). We further applied a site pattern sorting method together with site- and time-heterogeneous models to investigate the branching order among streptophytes and land plants. Our chloroplast phylogenomic analyses support previous hypotheses based on nuclear data in placing Zygnematales alone, or a clade consisting of Coleochaetales plus Zygnematales, as the closest living relatives of land plants.

Description: Very low frequency (VLF) electromagnetic waves that penetrate conductive magma-filled dykes generate secondary fields on the surface that can be used to invert for dyke properties. The model used for the interpretation calculates currents induced in a conductive strip by an inducing field that decays exponentially with depth due to the conductivity of the surrounding medium. The differential equations are integrated to give an inhomogeneous Fredholm equation of the second kind with a kernel consisting of a modified Bessel function of the second kind. Numerical methods are typically used to solve for the induced currents in the strip. In this paper, we apply a modified Galerkin–Chebyshev method, which involves separating the kernel into source and field spectra and integrating the source terms to obtain a matrix equation for the unknown coefficients. The incident wave is expressed as a Chebyshev series. The modified Bessel function is separated into a logarithmic singularity and a non-singular remainder, both of which are expanded in complex Chebyshev polynomials. The Chebyshev coefficients for the remainder are evaluated using a fast Fourier transform, while the logarithmic term and incident field have analytic series. The deconvolution then involves a matrix inversion. The results depend on the ratio of strip-size to skin-depth. For infinite skin-depth and a singular conductivity distribution given by $\tau _0 a/\sqrt{a^2 - z^2 }$ (where 0 is the conductance, a is the half-length and z the distance from the centre), Parker gives an analytic solution. We present a similar analytic series solution for the finite skin-depth case, where the size to skin depth ratio is small. Results are presented for different ratios of size to skin depth that can be compared with numerical solutions. We compare full-space and half-space solutions. A fit of the model to VLF data taken above a magma filled dykes in Hawaii and Mt Etna demonstrates that while properties such as depth to top, conductivity ratio, tilt and dip can be determined, the depth to bottom is indeterminate due to the exponential decay of the inducing field.

Description: The Diligencia basin in the Orocopia Mountains of southeastern California has been one of the primary areas used to test the hypothesis of more than 300 km of dextral slip along the combined San Andreas/San Gabriel fault system. The Orocopia Mountains have also been the focus of research on deposition, deformation, metamorphism, uplift and exposure of the Orocopia Schist, which resulted from flat-slab subduction during the latest Cretaceous/Paleogene Laramide orogeny. The uppermost Oligocene/Lower Miocene Diligencia Formation consists of more than 1500 m of nonmarine strata, including basalt flows and intrusions dated at 24–21 Ma. The base of the Diligencia Formation sits nonconformably on Proterozoic augen gneiss and related units along the southern basin boundary, where low-gradient alluvial fans extended into playa-lacustrine environments to the northeast. The northern basal conglomerate of the Diligencia Formation, which was derived from granitic rocks in the Hayfield Mountains to the north, sits unconformably on the Eocene Maniobra Formation. The northern basal conglomerate is overlain by more than 300 m of mostly red sandstone, conglomerate, mudrock and tuff. The basal conglomerate thins and fines westward; paleocurrent measurements suggest deposition on alluvial fans derived from the northeast, an interpretation consistent with a NW-SE–trending normal fault (present orientation) as the controlling structure of the half graben formed during early Diligencia deposition. This fault is hereby named the Diligencia fault, and is interpreted as a SW-dipping normal fault, antithetic to the Orocopia Mountains detachment and related faults. Deposition of the upper Diligencia Formation was influenced by a NE-dipping normal fault, synthetic with, and closer to, the exposed detachment faults. The Diligencia Formation is nonconformable on Mesozoic granitoids in the northwest part of the basin. Palinspastic restoration of the Orocopia Mountain area includes the following phases, each of which corresponds with microplate-capture events along the southern California continental margin: (1) Reversal of 240 km of dextral slip on the San Andreas fault (including the Punchbowl and other fault strands) in order to align the San Francisquito–Fenner–Orocopia Mountains detachment-fault system at 6 Ma. (2) Reversal of N-S shortening and 90° of clockwise rotation of the Diligencia basin and Orocopia Mountains, and 40 km of dextral slip on the San Gabriel fault between 12 and 6 Ma. (3) Reversal of 40° of clockwise rotation of the San Gabriel block (including Soledad basin and Sierra Pelona) and 30 km of dextral slip on the Canton fault between 18 and 12 Ma. These palinspastic restorations result in a coherent set of SW-NE–trending normal faults, basins (including Diligenica basin) and antiformal structures consistent with NW-SE–directed crustal extension from 24 to 18 Ma, likely resulting from the unstable configuration of the Mendocino triple junction.

Description: Ensembl Genomes ( http://www.ensemblgenomes.org ) is an integrating resource for genome-scale data from non-vertebrate species. The project exploits and extends technologies for genome annotation, analysis and dissemination, developed in the context of the vertebrate-focused Ensembl project, and provides a complementary set of resources for non-vertebrate species through a consistent set of programmatic and interactive interfaces. These provide access to data including reference sequence, gene models, transcriptional data, polymorphisms and comparative analysis. This article provides an update to the previous publications about the resource, with a focus on recent developments. These include the addition of important new genomes (and related data sets) including crop plants, vectors of human disease and eukaryotic pathogens. In addition, the resource has scaled up its representation of bacterial genomes, and now includes the genomes of over 9000 bacteria. Specific extensions to the web and programmatic interfaces have been developed to support users in navigating these large data sets. Looking forward, analytic tools to allow targeted selection of data for visualization and download are likely to become increasingly important in future as the number of available genomes increases within all domains of life, and some of the challenges faced in representing bacterial data are likely to become commonplace for eukaryotes in future.