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The Mertz Shear Zone (George V Land): implications for australia/Antarctica correlations and East Antarctic Craton/Ross orogen relationships
No full textChapter 7: The Antarctic Continent in Gondwanaland: A Tectonic Review and Potential Research Targets for Future Investigations
No full textThe geological record of the Antarctic continent has, for a long time, had a key role in supercontinent reconstructions. More recently, because of the well-documented relevance of the polar regions' processes in influencing the global changes of both ocean circulation and climate patterns, Antarctica has increasingly been of similar importance in the context of palaeoenvironmental and palaeoclimatic investigations, particularly those focused on the Cenozoic glacial evolution. After a description of the present-day geotectonic setting and of the main geological units before Gondwana amalgamation, the chapter focuses on the tectonic evolution of the Antarctic continent from its inclusion as part of the Gondwana supercontinent to the break-up of this landmass and the repositioning of Antarctica at southern polar latitudes since the Early Cretaceous. The geological evolution of the Antarctic continent is reviewed considering two main time periods: (i) c. 600-450 Ma, covering the processes which were active immediately before and during the amalgamation of Gondwana; and (ii) c. 450-180 Ma, including all the major events that occurred after the final stage of Gondwana amalgamation to the time immediately before the break-up phase. A subsequent section addresses the last 180 Ma during which present-day Antarctica and the other southern continents and surrounding oceanic basins formed as a consequence of the fragmentation of Gondwana. After a general overview of the most significant plate tectonic stages and coeval magmatic products, the attention is then devoted to one of the most investigated regions in Antarctica; the Transantarctic Mountains and the Ross Sea sector of the Western Antarctic Rift System. Some of the main persisting problems and potential research themes for on- and off-shore activities are discussed in the conclusions, which also include some of the most interesting palaeoclimatic issues that are essential to improving our understanding of the polar climate, ice ages and their influences on Earth's climate system in the Cenozoic to present time
The Pressure Character of the Low-Grade Metapelites from Robertson Bay Terrane, northern Victoria Land (Antarctica).
No full textThe geobarometric method based on the b spacing of the white K-micas in metapelites was utilized to determine the pressure character of the metamorphism in Robertson Bay Terrane (RBT) and Bowers Terrane (BT).
Analytical results indicate that no significant differente exists between RBT and BT, and that all b data (240) are close to an average value of 9.027 (3) A.
The conclusion is that both RBT and BT metamorphisms fall in the range of intermediate-pressure (i.e. Barrovian-type) metamorphisms, and can be related to a thermal metamorphic gradient not higher than approximately 15°C/km.
Consequently, plate-tectonic models for northern Victoria Land should take into consideration, as further constraints, the identical metamorphic features of the RRT and BT, and the prevalence of relatively low thermal gradients during the metamorphic climax. This constraint suggests at, in the early Paleozoic, the RBT and the BT both belonged to the same, relatively deep, crustal level, and were both metamorphosed at a considerable depth underneath a thick tectonic cover
An 40Ar-39Ar investigation of the Mertz Glacier area (George V Land, Antarctica): implications for the Ross Orogen - East Antarctic Craton relationship and Gondwana reconstructions
No full textGeorge V Land (Antarctica) includes the boundary between Late Archean–Paleoproterozoic metamorphic terrains of the East
Antarctic craton and the intrusive and metasedimentary rocks of the Early Paleozoic Ross–Delamerian Orogen. This therefore
represents a key region for understanding the tectono-metamorphic evolution of the East Antarctic Craton and the Ross Orogen
and for defining their structural relationship in East Antarctica, with potential implications for Gondwana reconstructions. In the
East Antarctic Craton the outcrops closest to the Ross orogenic belt form the Mertz Shear Zone, a prominent ductile shear zone
up to 5 km wide. Its deformation fabric includes a series of progressive, overprinting shear structures developed under different
metamorphic conditions: from an early medium-P granulite-facies metamorphism, through amphibolite-facies to late greenschistfacies
conditions. 40Ar–39Ar laserprobe data on biotite in mylonitic rocks from the Mertz Shear Zone indicate that the minimum age
for ductile deformation under greenschist-facies conditions is 1502±9 Ma and reveal no evidence of reactivation processes linked
to the Ross Orogeny. 40Ar–39Ar laserprobe data on amphibole, although plagued by excess argon, suggest the presence of a ∼1.7 Ga
old phase of regional-scale retrogression under amphibolite-facies conditions. Results support the correlation between the East
Antarctic Craton in the Mertz Glacier area and the Sleaford Complex of the Gawler Craton in southern Australia, and suggest that
the Mertz Shear Zone may be considered a correlative of the Kalinjala Shear Zone. An erratic immature metasandstone collected east
of Ninnis Glacier (∼180 km east of the Mertz Glacier) and petrographically similar to metasedimentary rocks enclosed as xenoliths
in Cambro–Ordovician granites cropping out along the western side of Ninnis Glacier, yielded detrital white-mica 40Ar–39Ar ages
from ∼530 to 640 Ma and a minimum age of 518±5 Ma. This pattern compares remarkably well with those previously obtained
for the Kanmantoo Group from the Adelaide Rift Complex of southern Australia, thereby suggesting that the segment of the Ross
Orogen exposed east of the Mertz Glacier may represent a continuation of the eastern part of the Delamerian Orogen.
© 2006 Elsevier B.V. All rights reserved
An Ophiolitic complex in the northern Shackleton Range, Antarctica
No full textThree lithotectonic and metamorphic units are exposed in the Bernhardi Heights-Hogbom Outcrops area of the eastern Herberts Mountains (northern Shackleton Range). From N to S, and structurally from top to bottom, they include: i) a high-grade gneiss-migmatite complex containing garnet-bearing meta-intrusives of dioritic to tonalitic composition ('Upper High Grade Gneiss Complex'); ii) a metamorphic 'Ophiolitic Complex' consisting of medium-grade mafic-ultramafic rocks and metasedimentary cover rocks, and iii) a high-grade gneiss complex made up of migmatitic gneisses and rare amphibolites ('Lower High Grade Gneiss Complex'). Prominent rock types in the Ophiolitic Complex are amphibolites of N-type MORB to OIB geochemical and Nd isotope affinity, metagabbros interpreted as metacumulate rocks having been formed in a MORB-type setting and metaperidotites. Single-stage Nd-model ages for the OIB-type amphibolites of around 1.0 Ga constrain a maximum formation age for the igneous protoliths of the ophiolitic unit. The polyphase metamorphic evolution of this complex included an early Barrovian-type metamorphic stage (550°C, 7-8 kbar), followed by decompression with concomitant T increase (up to 650°C, 5-6 kbar) and a late retrogression under greenschist facies conditions. K-Ar amphibole dates for amphibolites which are in the range of 500-490 Ma provide evidence for a late-Pan-African (Ross-age) age of metamorphism. Relict felsic granulites from the Upper High Grade Gneiss Complex record an early high-P granulite facies event (675-750°C, 8.4-11.3 kbar), followed by migmatization under low-P upper amphibolite facies conditions (730-850°C, 5-7 kbar). This early metamorphic event possibly reflects a Proterozoic tectonometamorphic cycle. The subsequent metamorphic stages span from intermediate-P amphibolite facies to greenschist-grade conditions. They document the tectonometamorphic reactivation of the High Grade Gneiss Complex as a consequence of the Ross-age thickening and exhumation history of the northern Shackleton Range crystalline rocks
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
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
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