1,720,986 research outputs found
Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model
No full textDirect seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges. On the East Pacific Rise, at 17°S, there were two long-term broadband ocean bottom seismometer deployments, the MELT and GLIMPSE experiments. These arrays spanned young Pacific plate seafloor ranging in age from 0 to 8 Ma. Combining these two data sets, we describe the increase in Rayleigh wave phase velocities for 16–100 s period as function of distance from the ridge with two parameterizations: an arbitrary function of seafloor age and a simple polynomial function of age on the Pacific and Nazca plates. Although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the Pacific plate can be resolved at periods of ? 50 s, the three parameter polynomial model fits the data almost as well, indicating a simple pattern of the evolution of structure with age.To compare our observations to the predictions for the conductive cooling of the lithosphere, we convert a thermal half-space cooling model to shear velocity using anharmonic and anelastic contributions. The patterns in the velocities are not consistent with simple conductive cooling. The conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range. We observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend. The asthenospheric low velocity zone is asymmetric, dipping to the west with the lowest velocities beneath the Pacific plate west of the spreading center. The conductive cooling model also over predicts the shear velocities in the lithosphere by ~ 0.2 km/s. These anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required.<br/
P and S Delays Beneath Intraplate Volcanic Ridges and Gravity Lineations Near the East Pacific Rise
No full textThe Gravity Lineations Intraplate Melting Petrology and Seismic Expedition (GLIMPSE) explored the origin of two intraplate, non–hot spot ridge systems associated with gravity lineations in the south central Pacific. Using the 11 month ocean bottom seismometer (OBS) deployment of the GLIMPSE experiment, we determined the average P and S station delays for the region. Across the array, station delays varied by 1.22 s for P waves and 2.13 s for S waves. The presence of ultraslow shear velocity (<400 m/s) sediment layers of variable thickness throughout the region introduced S delays, determined from P-to-S-converted phases generated at the crust-sediment boundary, of 0.08 to 0.43 s. Consequently, we corrected the S delays for these sediments to examine seismic anomalies due to crustal and mantle structure. We used the sediment delay times and the relative amplitudes of the converted phases to estimate the average regional shear velocity and thickness of the sediment layer beneath the stations. The range of sediment thickness in the region is 10–70 m, and the average shear velocity in the sediments is 140 m/s. Both the average P and sediment-corrected S delays show a positive correlation with seafloor age, with faster arrivals on older seafloor where the seismically fast lithosphere has thickened away from the spreading axis. The delay times also correlate with the residual mantle Bouguer gravity anomaly (rMBA), with slow arrivals coinciding with rMBA lows and the associated ridge systems. The delays are much too large to be caused by variations in crustal thickness and must represent anomalies within the mantle. These observations qualitatively support the either the channelized asthenospheric return flow model or the small-scale convection hypothesis for the origin of the gravity lineations. These models predict both positively correlated density and velocity anomalies caused by thermal anomalies and the pressure-release melting that migrates to the surface to form the volcanic ridges above the upwelling limbs of convection cells or channels. <br/
Analysis of gravity and topography in the GLIMPSE study region: Isostatic compensation and uplift of the Sojourn and Hotu Matua Ridge systems
No full textThe Gravity Lineations Intraplate Melting Petrologic and Seismic Expedition (GLIMPSE) Experiment investigated the formation of a series of non–hot spot, intraplate volcanic ridges in the South Pacific and their relationship to cross-grain gravity lineaments detected by satellite altimetry. Using shipboard gravity measurements and a simple model of surface loading of a thin elastic plate, we estimate effective elastic thicknesses ranging from ?2 km beneath the Sojourn Ridge to a maximum of 10 km beneath the Southern Cross Seamount. These elastic thicknesses are lower than predicted for the 3–9 Ma seafloor on which the volcanoes lie, perhaps due to reheating and thinning of the plate during emplacement. Anomalously low apparent densities estimated for the Matua and Southern Cross seamounts of 2050 and 2250 kg m?3, respectively, probably are artifacts caused by the assumption of only surface loading, ignoring the presence of subsurface loading in the form of underplated crust and/or low-density mantle. Using satellite free-air gravity and shipboard bathymetry, we calculate the age-detrended, residual mantle Bouguer anomaly (rMBA). The rMBA corrects the free-air anomaly for the direct effects of topography, including the thickening of the crust beneath the seamounts and volcanic ridges due to surface loading of the volcanic edifices. There are broad, negative rMBA anomalies along the Sojourn and Brown ridges and the Hotu Matua seamount chain that extend nearly to the East Pacific Rise. These negative rMBA anomalies connect to negative free-air anomalies in the western part of the study area that have been recognized previously as the beginnings of the cross-grain gravity lineaments. Subtracting the topographic effects of surface loading by the ridges and seamounts from the observed topography reveals that the ridges are built on broad bands of anomalously elevated seafloor. This swell topography and the negative rMBA anomalies contradict the predictions of lithospheric cracking models for the origin of gravity lineaments and associated volcanic ridges, favoring models with a dynamic mantle component such as small-scale convection or channelized asthenospheric return flow. <br/
Distribution of recent volcanism and the morphology of seamounts and ridges in the GLIMPSE study area: Implications for the lithospheric cracking hypothesis for the origin of intraplate, non–hot spot volcanic chains
No full textLithospheric cracking by remotely applied stresses or thermoelastic stresses has been suggested to be the mechanism responsible for the formation of intraplate volcanic ridges in the Pacific that clearly do not form above fixed hot spots. As part of the Gravity Lineations Intraplate Melting Petrology and Seismic Expedition (GLIMPSE) project designed to investigate the origin of these features, we have mapped two volcanic chains that are actively forming to the west of the East Pacific Rise using multibeam echo sounding and side-scan sonar. Side-scan sonar reveals the distribution of rough seafloor corresponding to recent, unsedimented lava flows. In the Hotu Matua volcanic complex, recent flows and volcanic edifices are distributed over a region 450 km long and up to 65 km wide, with an apparent, irregular age progression from older flows in the west to younger in the east. The 550-km-long Southern Cross Seamount/Sojourn Ridge/Brown Ridge chain appears to have been recently active only at its eastern end near the East Pacific Rise. A third region of recent flows is found 120 km north of Southern Cross Seamount in seafloor approximately 9 Myr old. No indication of lithospheric extension in the form of faulting or graben formation paralleling the trend of the volcanic chains is found in the vicinity of recent flows or anywhere else in the study area. Thermoelastic cracking could be a factor in the formation of a few small, very narrow volcanic ridges, but most of the volcanic activity is broadly distributed in wide swaths with no indication of formation along narrow cracks. The Sojourn and Brown chains appear to begin as distributed zones of small seamounts that later develop into segmented ridges, perhaps under the influence of membrane stresses from self-loading. We suggest that the linear volcanic chains are created by moving melting anomalies in the asthenosphere and that lithospheric cracking plays at most a secondary role. <br/
Variations in shear-wave splitting in young Pacific seafloor
No full textAnalysis of SKS phases recorded by ocean-bottom seismometers in the GLIMPSE Experiment has expanded the region of in situ measurements of shear-wave splitting in young seafloor near the southern East Pacific Rise. Average splitting times at individual GLIMPSE sites range from 1.1 to 2.2 s with fast direction azimuths trending approximately in the direction of absolute plate motion and fossil spreading for the Pacific plate. The GLIMPSE study area at 12 to 14°S is adjacent to the MELT Experiment. The new observations demonstrate that splitting delays are not uniform on the Pacific plate and that the contrast between the Nazca and Pacific plates is not a fundamental consequence of the asymmetric plate motion. The variations in degree of splitting could be caused by small-scale convection, variations in water content, or rate of deformation in the asthenospheric return flow to the East Pacific Rise
Mantle heterogeneity and off axis volcanism on young Pacific lithosphere
No full textPlate tectonics and mantle plumes explain most volcanism on earth, but there are numerous actively forming linear volcanic chains in the middle of tectonic plates that are not explained by these theories. Using the multidisciplinary geophysical dataset of the MELT and GLIMPSE experiments, we show that associated with 3 volcanic chains west of the East Pacific Rise there are low seismic velocities and densities in the asthenosphere that extend to the East Pacific Rise spreading center. Analogous to the Hawaiian swell, the low-density anomalies produce swells beneath the volcanoes on young seafloor. The associated gravity anomalies are part of a set of gravity lineaments that have been previously interpreted as being due to thermo-elastic cracking of the lithosphere or small-scale convection. The correlation between the surface volcanism and subsurface density and velocity anomalies and their extension to the spreading center suggest that pre-existing, buoyant or fertile asthenospheric mantle heterogeneities are stretched in the direction of plate motion by shear between the plate and the underlying mantle. These heterogeneities seed small-scale convection, producing upwelling and pressure release melting, forming volcanic chains that extend nearly to the ridge axis
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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
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