1,720,962 research outputs found
Neogene paleomagnetism and geodynamics of the Hikurangi margin, East Coast, New Zealand
Get PDFVertical-axis rotations are an important component of Neogene deformation in the New Zealand plate boundary region, and potentially offer fundamental insights into the rheology of continental crust. Extensive paleomagnetic sampling along the Hikurangi margin, on the East Coast of the North Island, has provided new insights into the patterns, rates and timings of tectonic rotation, and also an improved understanding of the magnetic signature of New Zealand Cenozoic mudstones. Rigorous field tests reveal numerous late remagnetizations, which haveoften formed several million years after deposition and can be irregularly distributed within an outcrop. Scanning electron microscopy and rock magnetic analyses indicate that the remanence carrier is predominantly the ferrimagnetic iron sulphide, greigite, which is present as a mixed population of single domain and superparamagnetic grains that are characteristic of arrested authigenic growth. Strong viscous overprints are the result of later, usually recent, oxidation of these sulphides. The recognition of late-forming magnetizations leads to a completely new view of the Neogene tectonic evolution of the Hikurangi margin, with no tectonic rotations being evident prior to 8–10 Ma; coherent rotation of most of the Hikurangi margin since that time refutes the existence of the independently rotating ‘domains’ that were inferred from earlier paleomagnetic data. This pattern is more consistent with the short-term velocity field, and allows all Neogene rotation to be more simply explained as a large-scale response to realignment of the subducting Pacific plate. Tectonic rotations have been accommodated by a variety of structures since 10 Ma; in the Late Miocene and Pliocene, rates of tectonic rotation were 3–4 times faster than presently observed and possibly involved a much larger region, before initiation of the North Island Dextral Fault Belt and the Taupo Volcanic Zone at 1-2 Ma instigated the current tectonic regime. Collision of the Hikurangi Plateau in the Late Miocene is interpreted to have caused both the initiation of tectonic rotation, and the widespread remagnetization of sediments, making it a key event in the Neogene evolution of the plate boundary region
Widespread remagnetizations and a new view of Neogene tectonic rotations within the Australia-Pacific plate boundary zone, New Zealand
No full textLarge, clockwise, vertical axis tectonic rotations of the Hikurangi margin, East Coast, New Zealand, have been inferred over both geological and contemporary timescales, from paleomagnetic and geodetic data, respectively. Previous interpretations of paleomagnetic data have laterally divided the margin into independently rotating domains; this is not a feature of the short-term velocity field, and it is also difficult to reconcile with the large-scale boundary forces driving the rotation. New paleomagnetic results, rigorously constrained by field tests, demonstrate that late diagenetic growth of the iron sulfide greigite has remagnetized up to 65% of sampled localities on the Hikurangi margin. When these remagnetizations are accounted for, similar rates, magnitudes, and timings of tectonic rotation can be inferred for the entire Hikurangi margin south of the Raukumara Peninsula in the last 7–10 Ma. Numerous large (50–80°) declination anomalies from magnetizations acquired in the late Miocene require much greater rates of rotation (8–14° Ma?1) than the presently observed rate of 3–4° Ma?1, which is only likely to be characteristic of the tectonic regime established since 1–2 Ma. These new results are consistent with both long- and short-term deformation on the Hikurangi margin being driven by realignment of the subducting Pacific plate, with collision of the Hikurangi Plateau in the late Miocene potentially being key to both the initiation of tectonic rotations and the widespread remagnetization of Neogene sediments. However, accommodating faster, more coherent rotation of the Hikurangi margin in Neogene reconstructions of the New Zealand plate boundary region, particularly in the late Miocene, remains a challenge
Magnetite dissolution, diachronous greigite formation, and secondary magnetizations from pyrite oxidation: Unravelling complex magnetizations in Neogene marine sediments from New Zealand
No full textDetailed rock magnetic and electron microscope analyses indicate that the magnetic signature of Neogene marine sediments from the east coast of New Zealand is dominated by the authigenic iron sulphide greigite. The greigite is present as a mixed population of stable single domain and superparamagnetic grains, which is consistent with authigenic growth from solution. This growth can result from pyritization reactions soon after deposition, which also leads to dissolution of most detrital magnetite; however, where constrained by field tests, our data suggest that remanence acquisition can occur > 1 Myr after deposition, and can vary in timing at the outcrop scale. Strong viscous overprints result from oxidation of the iron sulphides, probably during percolation of oxic ground water. This process can sometimes destroy any ancient remanent magnetization. This complex magnetic behaviour, particularly the presence of late-forming magnetizations carried by greigite, means that the remanence in New Zealand Cenozoic sediments, and in similar sediments elsewhere, cannot be assumed to be primary without confirmation by field tests. The reversals test should be employed with caution in such sediments, as patchy remagnetizations can lead to false polarity stratigraphies
Tectonic and geochronological implications of variably timed magnetizations carried by authigenic greigite in marine sediments from New Zealand
No full textDetailed scanning electron microscope observations, coupled with elemental microanalysis, confirm the presence of a chemical remanent magnetization carried by authigenic greigite (Fe3S4) in uplifted Neogene marine sediments from the Hikurangi Margin of New Zealand. Normal polarity samples from the studied section have declinations that are deflected 60° clockwise of reversed polarity samples, indicating the presence of two distinct magnetizations separated by several million years of tectonic rotation about a vertical axis. However, although multiple generations of iron sulfide growth are observed petrographically, we see no clear differences in the relative timing of greigite formation between samples carrying these two magnetizations. Not only can the diagenetic growth of greigite in fine-grained marine sediments occur long after deposition, obscuring tectonic and magnetostratigraphic information, but such remagnetizations are also difficult to distinguish from a more primary signal in the absence of constraints from field tests. Our observations emphasize that considerable care is necessary when interpreting paleomagnetic data from greigite-bearing sediments
Relocation of the tectonic boundary between the Raukumara and Wairoa Domains (East Coast, North Island, New Zealand): implications for the rotation history of the Hikurangi margin
No full textPaleomagnetic studies of Neogene marine sediments have documented large clockwise rotations of the Hikurangi margin (East Coast, North Island) during the Neogene, with the exception of the Raukumara Peninsula, which is unrotated with respect to the Australian plate. Immediately south of the Raukumara Peninsula, the Wairoa region has been rotated clockwise by 50–60°; the boundary between these domains is associated with a change in regional structural trends. However, a declination of 70 ± 14° reported from Otaian (19–22 Ma) sediments in the Rakauroa area is located to the north of this change. Characterisation of how differential rotations have been accommodated along the Hikurangi margin has been frustrated by this apparent mismatch between paleomagnetic and structural data. Paleomagnetic analysis of two new Rakauroa localities has yielded declinations of 16 ± 7° and 19 ± 9°, consistent with expected values for the Australian plate. This region is therefore not part of the Wairoa Domain. A strong viscous magnetic overprint was observed in many samples, the incomplete removal of which resulted in the misidentification of a large declination anomaly in the previous study. The paleomagnetically defined boundary between the Raukumara and Wairoa Domains now coincides with the area where regional structural trends alter. Reassignment of the Rakauroa area to the Raukumara Domain also results in a revised rotation history for the Wairoa Domain, suggesting rotation rates of 4–5°/m.y. since the late Miocene (5–10 Ma), and potentially no earlier rotation. No reliable record of early and middle Miocene vertical axis rotation on the Hikurangi margin now exists north of Marlborough; further studies are required to properly constrain the rotation history for this time interval
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
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
