Dynamics of intrathermocline vortices in a gyre flow over a seamount chain

The interaction of meddies with a complex distribution of seamounts is studied in a three-layer quasi-geostrophic model on the f-plane. This study aims at understanding if and how this seamount chain can represent a barrier to the propagation of these eddies and how it can be involved in their decay. The eddies are idealized as vortex patches in the middle layer, interacting with a regional cyclonic current and with ten idealized seamounts. The numerical code is based on the contour surgery technique. The initial position, radius, shape, number and polarity of the eddies are varied. The main results are the following: (1) Though they do not describe the unsteady flow, the streamlines of the regional and topographic flow provide a useful estimate of the vortex trajectories, in particular towards the major seamounts, where stronger velocity shears are expected. (2) The tallest and widest seamounts which have the largest vorticity reservoir are able to considerably erode the vortices, but also to draw anticyclones towards the seamount top. The ability of narrower seamounts to erode vortices is related to their multiplicity. (3) Only 1/3 of the anticyclones with about 30-km radius reach the southern boundary of the seamount chain, and their erosion is larger than 50 %. The other anticyclones are either completely eroded or trapped over a wide seamount top. Cyclones are less affected by seamounts because they oppose the topographic draft towards the seamount top and they drift along the side of the seamount. (4) Large vortices resist topographic erosion more efficiently. The rate of erosion grows from a few percent to about 35–50 % as the vortex radius decreases from about 60 to 30 km. Small cyclones are not eroded, contrary to small anticyclones (which completely decay), in relation with the different trajectories of these eddies in the vicinity of the seamounts. (5) The detailed vortex shape does not appear critical for their evolution, if they are close enough to the seamount chain initially. The interaction between a group of vortices initially north of the seamount chain can modify their trajectory to such an extent that they finally avoid collision with seamounts. (6) Finally, meddy trajectories across the Horseshoe Seamounts (data from the AMUSE experiment) show qualitative similarity with the vortex paths in the model. Several events of vortex decay also occur at comparable locations (in particular over the wide and tall seamounts) in the model and observations.     

Early Neoproterozoic Granite-Gneisses of the Junggar Alataw (Southeastern Kazakhstan): Age, Petrogenesis and Tectonic Implications

The combined petrographic, petrological, geochemical and geochronological study of the Neoproterozoic gneisses of the Sarychabyn and Baskan complexes of the Junggar Alataw of South Kazakhstan elucidate the Precambrian tectonic evolution of the Aktau–Yili terrane. It is one of the largest Precambrian crustal blocks in the western Central Asian orogenic belt. The U-Pb single-grain zircon ages indicate that granite-gneisses formed from the same source and crystallised in the early Neoproterozoic ca. 930–920 Ma. The chemical composition of gneisses corresponds to A2-type granites. The whole-rock Nd isotopic characteristics (εNd(t) = ?4.9 to ?1.0 and TNd(DM-2st) = 1.9 to 1.7 Ga) indicate the involvement of Paleoproterozoic crustal rocks in magma generation. Early Neoproterozoic ca. 930–920 Ma A-type granitoids in the Aktau–Yili terrane of South and Central Kazakhstan might reflect within-plate magmatism adjacent to the collisional belt or a local extension setting in back-arc areas of the continental arc.     

Sur le comportement geochimique des elements de la lithosphere en fonction du temps

Résumé Les roches magmatiques sont considérées comme un mélange d'atomes dans une phase précristalline. Si l'on applique à la composition chimique de celles-ci la statistique mathématique, on voit apparaître des régularités qui forment un cadre souple dans lequel se produisent différentes variations. La diminution exponentielle des concentrations de magnesium en fonction du temps à travers tout le précambrien et le remplacement progressif de cet élément par l'aluminium donne le schéma de l'évolution.La classification dimensionelle des atomes permet de lier l'histoire des éléments majeurs de la lithosphère à celle des éléments mineurs et d'aborder ainsi de façon nouvelle les rapports qui existent entre les roches et les gîtes métallifères.

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Magmatic rocks are considered as an atomical mixture in a precristalline phase. If you apply to their chemical composition the mathematical statistic, regularities appear that shape a supple frame in which various compositions come forward.Exponential decrease of magnesium concentrations in relation with time throughout the Precambrian, and the progressive substitution of this element by aluminium give the scheme of evolution.Dimensional classification of atoms allows to link the history of lithosphere's major elements with that of minor elements and to approach in a new way the connexion between rocks and metalliferous deposits.

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Zusammenfassung Die magmatischen Gesteine werden in einer vorkristallinen Phase als Atommischungen betrachtet.Die auf ihren Chemismus angewandte mathematische Statistik enthüllt das Vorhandensein von Regelmäßigkeiten, welche die Veränderungen ihrer Zusammensetzungen abgrenzen.Die exponentielle Abnahme der Magnesiumkonzentrationen und das Ersetzen dieses Elementes durch Al, während des ganzen Vorkambriums, gibt ein Schema des Evolutionsprozesses.Eine Atomdurchmesserklassifikation gestattet, die Geschichte der Hauptelemente der Lithosphäre mit derjenigen der Spurenelemente zu verbinden und die Frage der Beziehungen zwischen den magmatischen Gesteinen und den Metallagerstätten auf neuere Weise zu behandeln.

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Propagating PZ 90 to WGS 84 Transformation Parameters

The 29^th Research Institute recently published its latest transformation parameters for WGS and PZ 90. Because these tranformation parameter estimates were derived from stations located within Russia, variance-covariance propagation was carried out to study the benefits of a better global distribution of stations. An example that includes two stations in North America is presented. The transformation improves significantly. ? 1999 John Wiley & Sons, Inc.     

On the role of diffractions in velocity model building: a full-waveform inversion example

Studia Geophysica et Geodaetica - Imaging of small-scale heterogeneities is important for the geological exploration in complex environments. It requires a processing sequence tuned to...     

Rapid morphological changes at the summit of an active volcano: reappraisal of the poorly documented 1964 eruption of Mount Etna (Italy)

While the eruptive record of Mount Etna is reasonably complete for the past 400 years, the activity of the early and late 1960s, which took place at the summit, is poorly documented in the scientific literature. From 1955 to 1971, the Central and Northeast Craters were the sites of long-lived mild Strombolian and effusive activity, and numerous brief episodes of vigorous eruptive activity, which led to repeated overflows of lava onto the external flanks of the volcano. A reconstruction of the sequence of the more important of these events based on research in largely obscure and nearly inaccessible sources permits a better understanding of the eruption dynamics and rough estimates of erupted volumes and of the changes to the morphology of the summit area. During the first half of 1964, the activity culminated in a series of highly dynamic events at the Central Crater including the opening of a fissure on the E flank of the central summit cone, lava fountains, voluminous tephra emission, prolonged strong activity with continuous lava overflows, and growth of large pyroclastic intracrater cones. Among the most notable processes during this eruption was the breaching of a section of the crater wall, which was caused by lateral pressure of lava ponding within the crater. Comparison with the apparently similar summit activity of 1999 allows us to state that (a) lava overflows from large pit craters at the summit are often accompanied by breaching of the crater walls, which represents a significant hazard to nearby observers, and that (b) eruptive activity in 1999 was much more complex and voluminous than in 1964. For 1960s standards however, the 1964 activity was the most important summit eruption in terms of intensity and output rates for about 100 years, causing profound changes to the summit morphology and obliterating definitively the former Central Crater.