1,721,198 research outputs found
Western Pacific Model Subduction Zones
No full textnetCDF files for cross sections in the western Pacific through spherical models of plate tectonics and mantle convectionRelated Publication:
Slab horizontal subduction and slab tearing beneath East Asia
Ma, Pengfei China University of Geosciences (Beijing)
Liu, Shaofeng China University of Geosciences (Beijing)
Gurnis, Michael Caltech
Zhang, Bo China University of Geosciences (Beijing)
Geophysical Research Letters
https://doi.org/10.1029/2018GL081703
en
Tectonic plates and mantle convection.
No full textTectonic plates have a fundamental influence on mantle dynamics. Convection models with a continent show that long-wavelength thermal structure is significant throughout the mantle and that the large-scale structure produces periodic motion of the continent. These results may provide explanations to seismically observed mantle structure and supercontinent kinematics. A larger plate results in a greater propensity for slab penetration across an endothermic phase change. A larger plate yields a larger-scale thermal structure and greater negative buoyancy priori to subduction. The larger-scale structure on which the phase change has less influence reduces the degree of layering; the greater negative buoyancy enhances slab penetration. Temperature-dependent rheology stiffens the slabs and enhances slab penetration. Temperature-and stress-dependent rheology and realistic plate margins are the keys to simulating plates. Tectonic faults are incorporated into a viscous medium using a finite element method with constrained elements and Lagrange multipliers. Dynamic topography from subduction zone models with a fault representing a converging plate boundary resembles observed subduction zone topography including outer rise, trench, and back arc basin. The models show that fault dip and age of lithosphere have a greater effect on trench depth than slab dip and slab length. These results explain well the statistics of observed trench depths. The models suggest that the resisting stress on faults in subduction zones is about 15-30 MPa. Convection models in cylindrical geometry with faults, temperature- and stress-dependent rheology, and continents reveal many features of plate tectonics including trench migration and formation of a plate with both continental and oceanic components. The models show that as subduction begins, the overriding continental plate moves oceanward. The overriding plate increases in size at the expense of an oceanic plate. As continents move oceanward, the region vacated by continents becomes occupied by new ocean floor. The continent and the new ocean floor form a coherent plate. When continents move oceanward at a large velocity, subducted slabs have a shallow dip and are deflected at the 670-km phase change. After slabs penetrate the phase change, slab dip becomes steep and the continental motion decreases substantially.PhDGeological Sciences and Scientific ComputingUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104234/1/9501080.pdfDescription of 9501080.pdf : Restricted to UM users only
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Geodynamically Consistent Seismic Velocity Predictions at the Base of the Mantle
Full text linkA model of thermoelastic properties for a chemically homogeneous adiabatic lower mantle is calculated. Constraints provided by this model are used in convection models to study dynamics of a chemically distinct layer at the bottom of the mantle. We find that the layer must be at least 2% denser than the overlying mantle to survive for a geologically significant period of time. Realistic decrease with depth of the thermal expansivity increases layer stability but is unable to prevent it from entrainment. Seismic velocities are computed for an assumed composition by applying the thermal and compositional perturbations obtained in convection simulations to the adiabatic values. The predicted velocity jump at the top of the chemical layer is closer to the CMB in the cold regions than in the hot. The elevation of the discontinuity above CMB in the cold regions decreases with increasing thermal expansivity and increases with increasing density contrast, while in the hot regions we find that the opposite is true. If the density contrast is small, the layer may vanish under downwellings. However, whenever the layer is present in the downwelling regions, it also exists under the upwellings. For a 4% density contrast and realistic values of expansivity, we find that the layer must be more than 400 km thick on average to be consistent with the seismically observed depth of the discontinuity. A simple chemical layer cannot be used to interpret the D″ discontinuity: the required change in composition is large and must be complex, since enrichment in any single mineral probably cannot provide the required impedance contrast. A simple chemical layer cannot explain the spatial intermittance of the discontinuity
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
The EDGES of the Mantle
Full text linkThe core-mantle boundary region is often considered to be the source of narrow
upwellings which drive or influence plate motions and continental breakup,
fuel large igneous provinces and generate volcanic chains. The plume hypothesis
has influenced most fields of geochemistry, petrology, geodynamics and
mantle evolution. The key axioms underlying the plume paradigm are identified:
Axioms are self-evident truths and are seldom stated explicitly. When they
are, they are sometimes not so self-evident. The role of the surface boundary
layer is discussed in connection with large igneous provinces and volcanic
chains. Partial melting is the expected natural state of the upper mantle and only
abnormally high seismic velocities imply absence of melting (slabs, cratons).
Plume theoreticians have underestimated the average temperature of the mantle
and have overestimated melting temperatures. Extensive melting in the upper
mantle does not require abnormal temperatures or plumes. The dynamics and
chemistry of midplate volcanism are explainable by near-surface processes.
"Midplate" volcanism starts at plate boundaries or discontinuities, at lithospheric
"edges", in regions of extension generated by plate processes. The chemistry of
"hotspot" basalts implies contamination by processes and materials that occur
near the surface of the Earth. A buoyant metasomatised layer at the top of the
mantle, near the melting point, removes the need to import heat and chemical
inhomogeneity from D" to explain midplate volcanism. D" is an interesting and
important region, but any connection to surface processes or chemistry is
speculative
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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