Plio-pleistocene volcanism in the Andean Cordillera of Central Chile (33∘ – 40∘ S. L.)

Recent field studies in the Andean Cordillera of Central Chile (33–40 S. L.) have shown a widespread distribution around the Chile-Argentina border of pliopleistocene volcanic and pyroclastic flow (5-1 m. y. K/Ar). The older ages are found in Rio Blanco (Santiago) area and the younger in the Cola de Zorro area. They cover horizontal to sub-horizontally, with strong unconformity their meso-cenozoic basement.In the Rio Blanco area the most silicic rocks are dactic and rhyolitic ignimbrites and lava flows. They are similar to the Rhyolitic Formation (Zeil andPichler, 1967) of northern Chile.South of 36 S. L. most of the rocks are calcalkaline basalts and andesites except for the samples from the Pino Hachado area, which plot nearRittman's alkaline field. Chemical inhomogenety of the plio-pleistocene volcanic rocks is thus present in the Andes of Central Chile.

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Zusammenfassung Neue Arbeiten über die Geologie der Cordillera de los Andes im zentralen Abschnitt Chiles (33–40 S) haben eine enorme Verbreitung von plio/pleistozänen Vulkaniten (5-1 m. a.) vor allem in der Grenzregion mit Argentinien ergeben. Die höheren Alter fand man im Gebiet von Rio Blanco (Santiago) und die jüngsten im Gebiet von Cola de Zorro.Die Vulkanite bedecken mit einer horizontalen bis subhorizontalen Verbreitung und mit einer scharfen Diskordanz alle Gesteinsserien des Mesozoikums und Känozoikums.Im Gebiet von Rio Blanco handelt es sich um Ignimbrite rhyolitischer und dacitischer Zusammensetzung. Die kieselsäurereichsten Proben sind ähnlich den Gesteinen der Rhyolit-Formation vonZeil &Pichler (1967) in Nordchile.Südlich von 36 S hat die Mehrheit der Proben eine andesitische und basaltische Zusammensetzung. Es sind kalkalkaline Gesteine mit Ausnahme der Proben von Pino Hachado, die einen starken alkalinen Trend nach dem Diagramm vonRittmann haben. Auf diese Weise ergibt sich eine chemische Inhomogenität bei den plio/pleistozänen Vulkaniten von Zentralchile.

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Resumen Los más recientes trabajos de geología de campo en la Cordillera de los Andes de Chile Central (33–40 L. S.) han evidenciado enormes extensiones de rocas volcánicas pliopleistocénicas (5-1 m. a.) sobre todo en el área limítrofe con Argentina. Las mayores edades fueron encontradas en el area de Rio Blanco (Santiago) y las más jóvenes en el area de Cola de Zorro.Las rocas volcánicas plio-pleistocénicas cubren con disposición horizontal a subhorizontal y con fuerte discordancia a todas las rocas subyacentes del Meso y Cenozoico.En el area de Río Blanco las rocas son ignimbritas de composición riolítica a dacítica y corresponden a las muestras más silícicas similar a rocas de la Formación Riolítica del norte de Chile deZeil yPichler (1967).Al sur del paralelo 36 S la mayoría de las muestras son de composición andesítica y basáltica de naturaleza calcoalcalina excepto las muestras de Pino Hachado que presentan fuerte afînidad alcalina según el diagrama deRittmann. De esta manera se muestra que existe inhomogeneidad química en las rocas volcánicas plios-pleistocénicas de Chile Central.

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Sulla turbolenza delle correnti marine nello stretto di messina

Riassunto Gli Autori espongono i risultati di un tentativo di valutazione del grado di turbolenza verticale delle correnti marine davanti alle imboccature Nord e Sud dello Stretto di Messina. I valori massimi ottenuti per il coefficiente di diffusione verticale della salinitàK _z sono dell'ordine di 10³–10⁴ cm²/sec.

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Summary The Authors present the results of some calculations carried out to obtain values of the vertical turbulence (by means of the coefficient of eddy-diffusion of salinityK _z ) of the currents in the Strait of Messina. The maximum values ofK _z are of the order of 10³–10⁴ cm²/sec.

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Comunicazione presentata il 24 Aprile 1957 alla Quinta Assemblea Generale della «Società Italiana di Geofisica e Meteorologia» (Genova: 23–25 Aprile 1957).     

Relationship between some trace elements and magmatic processes

Relationships between magmatic differentiation, volcanological evolution and occurrence of some trace elements have been pointed out by a petrographic and volcanological study of the Quaternary magmatic products of the Northern Latium alkali-potassic volcanoes (Sabatini, Vico, Vulsini).Several hundred analyses of U, Th, Be, Zr, Rb, Sr have been carried out. The above elements are contained in exceptional quantity in the examined alkaline rocks. The distribution pattern of these elements helps for a better understanding of some types of differentiation and, above all, allows to identify the processes related to the uplift and outflow of magmas which are undetectable with the usual petrochemical methods.In particular, the trace elements considered allow us to distinguish a deepseated magma evolution from superficial differentiations related to the volcanic conditions. Moreover, a loss of alkalies from the upper levels of the magmatic chambers has been ascertained which determines peculiar deformations in the normal petrogenetic trends.-- (: Sabatini, Vico, Vulsini). , . .

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Zusammenfassung Das petrographische und vulkanologische Studium der quartären Vulkanite der Mediterranen Sippe des nördlichen Latiums (die Vulkane Sabatini, Vico und Vulsini) haben uns die Gelegenheit gegeben, die Verhältnisse zwischen Magmenentwicklung, vulkanologischer Evolution und Verteilung einiger Spurenelemente zu bestätigen.Zu diesem Zweck sind Hunderte von Analysen von U, Th, Be, Zr, Sr, Rb — Elemente welche in ungemein großen Mengen in den untersuchten alkalischen Gesteinen enthalten sind — vorgenommen worden. Das Studium der Verteilung dieser Elemente hat ein gründliches Verstehen einiger Differentiationstypen erlaubt, und vor allem den Ablauf des Prozesses des Magmen-Aufstiegs und Abflusses gezeigt, welcher nicht mittels der normalen petrochemischen Untersuchungen ersichtlich ist.Besonders die in Betracht gezogenen Spurenelemente erlauben die Unterscheidung der magmatischen Evolution in den tiefen Lagen von den oberflächlicheren Differentiationen, welche an das vulkanische Geschehen gebunden sind.In dieser Hinsicht ist ein wichtiges Schlußergebnis die Bestätigung des Alkaliverlustes seitens der oberflächlichen Sektoren der Magmaherde, ein Verlust welcher eigenartige Deformationen von den normalen petrogenetischen Linien gezeigt hat.

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Résumé L'étude pétrographique et volcanologique des manifestations magmatiques quaternaires de la province alcalin-potassique du Latium septentrional (Volcans Sabatini, Vico et Vulsini) nous a permis de vérifier les rapports entre la différenciation magmatique, l'évolution volcanologique et la distribution de quelques éléments en traces.Dans ce but on a exécuté plusieurs centaines d'analyses de U, Th, Be, Zn, Rb, Sr, éléments qui sont présents en quantité exceptionellement élevé dans les roches alcalines examinées.L'étude de la distribution de ces éléments a permis une compréhension plus approfondie de certains types de differenciation et surtout, elle a indiqué l'existence de processus liés à la remontée et effusion des magmas, qui ne sont pas relevables par les examens pétrochimiques normaux.En particulier les éléments en traces considérés permettent de distinguer l'évolution magmatique de milieu profond des différenciations superficielles liées aux situations volcaniques.A ce propos une conclusion importante a été la constatation de la perte d'alcali par les portions plus superficielles des réservoirs magmatiques; une perte qui a déterminé des déformations particulières qui les éloignent des lignes pétrogénétiques normales.

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Authors are indepded to Dr. S.Smcana and to Dr. G.Falchi for the analyses performance.     

Plio-Quaternary volcanotectonic activity in the northern sector of the Main Ethiopian Rift: relationships with oblique rifting

Deformation and magmatism within the 90 km wide northern Ethiopian Rift system is concentrated along a narrow zone - the Wonji Fault Belt. Two key areas (the Nazret-Dera and Asela-Ziway areas), located along the eastern margin of the north-northeast to northeast trending Main Ethiopian Rift, have been investigated in order to reconstruct the recent tectonomagmatic evolution of the northern branch of the Main Ethiopian Rift. In these areas, Early Pleistocene volcanic products (Wonji Group) overlie Pliocene volcanic rocks (Eastern Margin Unit). Detailed stratigraphical reconstructions have revealed the presence of several tectonomagmatic units which can be correlated between the two study areas. The stratigraphical and petrological study of these units outlined (1) the bimodal composition (basalts-pantellerites) of the oldest and youngest units and the unimodal character (pantellerites) of the products erupted during the intervening period; (2) the mainly fissural origin of the ignimbrites and oldest basalts; and (3) a mafic/felsic volumetric ratio of 1:5.The geological data suggest that, around the Pliocene-Quaternary boundary, a change in the stress field occurred in this Main Ethiopian Rift sector, passing from a direction of extension roughly orthogonal to the rift shoulders, to oblique rifting related to an east-west trending extension. In this framework the change in the style of volcanism observed in the Nazret-Dera and Asela-Ziway areas can be related to the change of the stress field. A new geodynamic model is presented for the Late Pliocene to Recent evolution of this sector of the Main Ethiopian Rift. According to this model, a large volume of rhyolitic products was erupted during an oblique rifting phase, following a previous period of pure extension. The change in the tectonic regime favoured partial melting of the underplated basalts as a decrease in the pressure and an elevation of isotherms occurred.     

Distribuzione geografica dell'attivita' magnetica terrestre

Riassunto Richiamate le caratteristiche secondo cui le perturbazioni regolari ed irregolari del campo terrestre concorrono a definire l'attività geomagnetica, se ne analizza la distribuzione geografica mediante le grandezze:HR _H ,ZR _Z , e (10^–4.²). Di queste sono illustrati e discussi gli andamenti in funzione della latitudine geomagnetica, dedotti dai valori medi calcolati per quasi tutte le stazioni funzionanti durante l'Anno Polare 1932–33 ed il triennio successivo 1933–36. Si rileva particolarmente il massimo assoluto di attività della zone anulare delle aurore, e quello relativo delle regioni equatoriali. Segue un'indagine, a carattere piuttosto indicativo, sulla ripartizione geografica del grado di variazione dell'attività nel tempo, introducendo a tale scopo opportuni indici ed esponendone il comportamento. Conclude la Nota un cenno sugli andamenti stagionali dell'attività e della variabilità geomagnetica.     

Quantifying Organization of Atmospheric Turbulent Eddy Motion Using Nonlinear Time Series Analysis

Using three methods from nonlinear dynamics, we contrast the level of organization inthe vertical wind velocity (w) time series collected in the atmospheric surface layer(ASL) and the canopy sublayer (CSL) for a wide range of atmospheric stability ()conditions. The nonlinear methods applied include a modified Shannon entropy, waveletthresholding, and mutual information content. Time series measurements collected overa pine forest, a hardwood forest, a grass-covered forest clearing, and a bare soil, desertsurface were used for this purpose. The results from applying all three nonlinear timeseries measures suggest that w in the CSL is more organized than that in the ASL, and that as the flows in both layers evolve from near-neutral to near-convective conditions, the level of organization increases. Furthermore, we found that the degree of organization in w associated with changes in is more significant than the transition from CSL to ASL.     

Multi-year drought frequency analysis at multiple sites by operational hydrology - A comparison of methods

This paper compares two generators of yearly water availabilities from sources located at multiple sites with regard to their ability to reproduce the characteristics of historical critical periods and to provide reliable results in terms of the return period of critical sequences of different length. The two models are a novel multi-site Markov mixture model explicitly accounting for drought occurrences and a multivariate ARMA. In the case of the multisite Markov mixture model parameter estimation is limited to a search in the parameter space guided by the value of parameter λ to show the sensitivity of the model to this parameter. Application to two of the longest time series of streamflows available in Sicily (Italy) shows that the models can provide quite different results in terms of estimated return periods of historic droughts, although they seem to perform more uniformly when it comes to simulate drought-related statistics such as drought length, severity and intensity. The role of parameter selection for the multisite Markov mixture model and of the marginal probability of generated flows in providing results consistent with the characteristics of the observed series is discussed. Both models are applied to the system of sources supplying the city of Palermo (Sicily) and its environs showing the applicability of the newly developed multisite Markov mixture model to medium-to-large scale water resources systems.     

Eruptive history and petrologic evolution of the Albano multiple maar (Alban Hills, Central Italy)

A comprehensive volcanological study of the Albano multiple maar (Alban Hills, Italy) using (i) ⁴⁰Ar/³⁹Ar geochronology of the most complete stratigraphic section and other proximal and distal outcrops and (ii) petrographic observations, phase analyses of major and trace elements, and Sr and O isotopic analyses of the pyroclastic deposits shows that volcanic activity at Albano was strongly discontinuous, with a first eruptive cycle at 69±1 ka producing at least two eruptions, and a second cycle with two peaks at 39±1 and 36±1 ka producing at least four eruptions. Contrary to previous studies, we did not find evidence of magmatic or hydromagmatic eruptions younger than 36±1 ka. The activity of Albano was fed by a new batch of primary magma compositionally different from that of the older activity of the Alban Hills; moreover, the REE and ⁸⁷Sr/⁸⁶Sr data indicate that the Albano magma originated from an enriched metasomatized mantle. According to the modeled liquid line of descent, this magma differentiated under the influence of magma/limestone wall rock interaction. Our detailed eruptive and petrologic reconstruction of the Albano Maar evolution substantiates the dormant state of the Alban Hills Volcanic District. Electronic Supplementary Material Supplementary material is available for this article at Editorial responsibility: J. Donnelly-Nolan An erratum to this article can be found at     

Crustal attenuation in the Southern Andean retroarc (38°–39°30′ S) determined from tectonic and gravimetric studies: The Lonco-Luán asthenospheric anomaly

The western retroarc of the Southern Andes between 38° and 40° S is formed by a NNW-elongated ridge not associated with stacked thrust sheets. On the contrary, during the last 4–3 Ma this ridge was affected by extensional deformation, regional uplift and related folding on a very broad scale. Receiver function analysis shows that the drainage divide area and adjacent retroarc lie over an attenuated crust. Expected crustal thickness at these latitudes is around 38 km, whereas in this part of the retroarc the thickness is less than 32 km. The causes for such attenuation have been linked to a moderate steepening of the subducted Nazca plate beneath the South American plate, which is suggested by a westward shift and narrowing of the magmatic arc during the last 4 to 5 Ma. Gravimetric studies show that the upper plate did not react homogeneously to slab steepening, but ancient sutures and lithospheric discontinuities deeply buried under Mesozoic to Cenozoic sequences in the retroarc were locally reactivated. These processes resulted in an asthenospheric anomaly that correlates at the surface with the area of Pliocene to Quaternary doming, widespread extension and three radial troughs. Two of the troughs have accommodated substantial amounts of extension, but the third was probably aborted at an early stage. Moreover, the presence of an anomalous concentration of calderas and large volcanic centers over the proposed asthenospheric anomaly, and their age distribution, may indicate minor migration of the asthenospheric anomaly between 4 and 2 Ma through the western South American plate.