452 research outputs found
Untersuchung der Krustenstruktur des Manihiki Plateaus im Rahmen der Expedition SO-224
Das Manihiki Plateau ist ein untermeerisches Lavaplateau, eine sogenannte âLarge Igneous Provinceâ (LIP), im zentralen Westpazifik (Abb. 1). Es ent-stand in der frĂŒhen Kreide (ca. 125 Ma) wahrscheinlich als ein TeilstĂŒck der âSuper-LIPâ Ontong Java Nui (Chandler et al., 2013; 2012; Taylor, 2006). Dieses vulkanische Plateau bestand neben dem Manihiki Plateau aus dem Ontong Java Plateau und dem Hikurangi Plateau (Abb.1), sowie weiteren TeilstĂŒcken, die mittlerweile subduziert wurden (Larson et al., 2002; Viso et al., 2005). Man geht davon aus, dass Ontong Java Nui ungefĂ€hr 1% der Erd-oberflĂ€che bedeckte. Eine vulkanische Provinz entsteht meist durch eine massive erste vulkanische Phase, gefolgt von mehreren kĂŒrzeren vulkani-schen Phasen (Coffin and Eldholm, 1994). Ontong Java Nui brach zwischen diesen zwei plateaubildenden Phasen auseinander (Hoernle et al., 2010; Timm et al., 2011), und die Teilplateaus durchliefen jeweils eine individuelle tektonische und petrologische Entwicklung.
WĂ€hrend der Expedition SO-224 im Jahr 2012 wurden zwei refraktions- und weitwinkelreflexionsseismische Profile aufgenommen (Fig. 1). Hierzu wurden jeweils 33 Ozeanbodenseismometer ausgebracht. Diese Daten erlauben uns einen Einblick in die Struktur der Kruste und oberen Mantels des Manihiki Pla-teaus. Somit können die Hypothesen ĂŒber die gemeinsame Entstehung des Manihiki Plateaus mit dem Ontong Java Plateau und dem Hikurangi Plateau ĂŒberprĂŒft werden. Ebenso ist es möglich, die Struktur der zwei gröĂten Un-terprovinzen des Manihiki Plateaus, das High Plateau und die Western Plateaus, zu vergleichen.
Bei der Modellierung der Krustenstruktur der beiden Unterprovinzen traten einige Gemeinsamkeiten, aber auch erstaunliche Unterschiede zu Tage (Abb. 2). Generell besteht eine LIP aus einer unteren Kruste, die sehr hohe P-Wellengeschwindigkeiten (7.1 bis 7.7 km/s) aufweist. Diese Schicht ist in bei-den Teilprovinzen vorhanden. Die KrustenmĂ€chtigkeit variiert zwischen 9 und 17 km an den Western Plateaus (Abb. 2a) und betrĂ€gt konstant 20 km am High Plateau (Abb. 2b). Die Struktur der oberen Kruste weist groĂe Unter-schiede zwischen den verschiedenen Teilprovinzen auf. Das High Plateau ist durch basaltische Flussstrukturen geprĂ€gt. Zahlreiche intrusive und extrusive vulkanische Strukturen, wie beispielsweise Tiefseeberge sind hier belegt (Abb. 1 und 2b). Dies deutet auf eine massive vulkanische AktivitĂ€t wĂ€hrend spĂ€te-rer vulkanischer Phasen hin. Im Gegensatz dazu zeigen die Western Pla-teaus nur einen sehr lokalen und geringen Vulkanismus. Mehrere Horst- und Grabensysteme sowie Sedimentbecken können dort identifiziert werden (Abb. 2a). Dieses deutet auf eine starke tektonische Deformation der Western Pla-teaus hin. Auch der graduelle Anstieg der Kruste-Mantelgrenze weist auf eine gedehnte Kruste hin (Abb. 2a). Somit zeigen die beiden Unterprovinzen des Manihiki Plateaus eine unterschiedliche Entwicklung nach ihrer gemeinsamen Entstehung als eines Teils von Ontong Java Nui.
Das High Plateau wurde nur an seinen RÀndern tektonisch beansprucht und durchlief weitere Phasen exzessiver vulkanischer AktivitÀt. Die Western Pla-teaus wurden wahrscheinlich starken DehnungskrÀften im Zusammenhang mit dem Abbruch des Ontong Java Plateaus ausgesetzt. Somit liegt hier eine Dehnung der vorher entstandenen LIP-Kruste und geringer Vulkanismus vor.
Diese Erkenntnisse können uns genaueren Aufschluss darĂŒber geben, welche Prozesse den Aufbruch der âSuper-LIPâ Ontong Java Nui begĂŒnstigt haben und stellen wichtige Rahmenbedingungen fĂŒr eine plattentektonische Rekon-struktion des zentralen Westpazifiks in der Kreide dar. Durch eine Kartierung der RĂ€nder und Beschaffenheit der Kruste der verschiedenen Teilplateaus Ontong Java Nuis können die ursprĂŒngliche Positionierung der verschiedenen Plateaus zueinander rekonstruiert werden. Dies bildet die Grundlage einer er-folgreichen plattenkinematischen Rekonstruktion
THE URGE TO CHECK SOCIAL NETWORKING SITES: ANTECEDENTS AND CONSEQUENCES
Social networking sites (SNSs), combined with the rapid growth trajectory of mobile devices, and widespread deployment of mobile data services, have evolved as a primary platform for daily social interaction. While the majority of users enjoy frequent interactions with their friends and family members, some users suffer from incessant urges to check up on the lives of others on their social networks. In the last decade, the use of SNSs has received much attention in the IS literature. Not until recently, researchers have begun to examine the dark side of using SNSs. In this study, we attempt to advance existing literature by exploring the role of urges in the context of SNSs. Particularly, we propose a research model that examines the antecedents and consequences of the urge to check SNSs. We will test the model with SNS users using structural equation modeling. We believe that current work will enrich the existing literature on the dark side of SNS use, and raise the awareness in the community regarding this emerging phenomenon
Playing jigsaw with large igneous provinces - a plate-tectonic reconstruction of Omtong Java Nui
Ontong Java Nui is a Cretaceous large igneous province (LIP), which was rifted apart into various smaller plateaus shortly after its emplacement around 125 Ma in the central Pacific. It incorporated the Ontong Java Plateau, the Hikurangi Plateau and the Manihiki Plateau as well as multiple smaller fragments, which have been subducted. Its size has been estimated to be approximately 0.8% of the Earthâs surface. A volcanic edifice of this size has potentially had a great impact on the environment such as its CO2 release. The break-up of the âSuperâ-LIP is poorly constrained, because the break-up and subsequent seafloor spreading occurred within the Cretaceous Quiet Period. The Manihiki Plateau is presumably the centerpiece of this âSuperâ-LIP and shows by its margins and internal fragmentation that its tectonic and volcanic activity is related to the break-up of Ontong Java Nui.
By incorporating two new seismic refraction/wide-angle reflection lines across two of the main sub-plateaus of the Manihiki Plateau, we can classify the break-up modes of the individual margins of the Manihiki Plateau. The Western Plateaus experienced crustal stretching due to the westward motion of the Ontong Java Plateau. The High Plateau shows sharp strike-slip movements at its eastern boundary towards an earlier part of Ontong Java Nui, which is has been subducted, and a rifted margin with a strong volcanic overprint at its southern edges towards the Hikurangi Plateau.
These observations allow us a re-examination of the conjugate margins of the Hikurangi Plateau and the Ontong Java Plateau. The repositioning of the different plateaus leads to the conclusion that Ontong Java Nui was larger (~1.2% of the Earthâs surface at emplacement) than previously anticipated. We use these finding to improve the plate tectonic reconstruction of the Cretaceous Pacific and to illuminate the role of the LIPs within the plate tectonic circuit in the western and central Pacific
TectonoâStratigraphic Evolution of the Kerguelen Large Igneous Province: The Conjugate Williamâs RidgeâBroken Ridge Rifted Margins
AbstractExtensive investigation of continental rift systems has been fundamental for advancing the understanding of extensional tectonics and modes of formation of new ocean basins. However, current rift classification schemes do not account for conjugate end members formed by Large Igneous Province crust, referring to thick mafic crust, sometimes including continental fragments. Here, we investigate the rifting of William's Ridge (Kerguelen Plateau) and Broken Ridge, components of the Kerguelen Large Igneous Province now situated in the Southeast Indian Ocean, and incorporate these end members into the deformation migration concept for rifted margins. We use multichannel seismic reflection profiles and data from scientific drill cores acquired on both conjugate margins to propose, for the first time, a combined tectonoâstratigraphic framework. We interpret seismic patterns, tectonic features, and magnetic anomaly picks to determine an acrossâstrike structural domain classification. This interpretation considers the rift system overall to be âmagmaâpoorâ despite being located proximal to the Kerguelen plume but suggests that synârift interaction between the Kerguelen mantle plume and the lithospheric structure of William's Ridge and Broken Ridge has controlled the alongâstrike segmentation of both conjugates. We integrate seismic reflection and bathymetric data to test the hypothesis of predominantly transform motion, between the Australian and Antarctic plates, in Late Cretaceous and Paleogene time.</jats:p
Hydrogen Epoch of Reionization Array (HERA)
The Hydrogen Epoch of Reionization Array (HERA) is a staged experiment to
measure 21 cm emission from the primordial intergalactic medium (IGM)
throughout cosmic reionization (), and to explore earlier epochs of our
Cosmic Dawn (). During these epochs, early stars and black holes
heated and ionized the IGM, introducing fluctuations in 21 cm emission. HERA is
designed to characterize the evolution of the 21 cm power spectrum to constrain
the timing and morphology of reionization, the properties of the first
galaxies, the evolution of large-scale structure, and the early sources of
heating. The full HERA instrument will be a 350-element interferometer in South
Africa consisting of 14-m parabolic dishes observing from 50 to 250 MHz.
Currently, 19 dishes have been deployed on site and the next 18 are under
construction. HERA has been designated as an SKA Precursor instrument.
In this paper, we summarize HERA's scientific context and provide forecasts
for its key science results. After reviewing the current state of the art in
foreground mitigation, we use the delay-spectrum technique to motivate
high-level performance requirements for the HERA instrument. Next, we present
the HERA instrument design, along with the subsystem specifications that ensure
that HERA meets its performance requirements. Finally, we summarize the
schedule and status of the project. We conclude by suggesting that, given the
realities of foreground contamination, current-generation 21 cm instruments are
approaching their sensitivity limits. HERA is designed to bring both the
sensitivity and the precision to deliver its primary science on the basis of
proven foreground filtering techniques, while developing new subtraction
techniques to unlock new capabilities. The result will be a major step toward
realizing the widely recognized scientific potential of 21 cm cosmology.Comment: 26 pages, 24 figures, 2 table
WSClean : an implementation of a fast, generic wide-field imager for radio astronomy
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.Astronomical widefield imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new widefield interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependencies of CASA's w-projection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarisation correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2-12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square-Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released.Peer reviewedFinal Published versio
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