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Tectonic implications of fault-scarp-derived volcaniclastic deposits on Macquarie Island: sedimentation at a fossil ridge-transform intersection?
No full textUpper Miocene to lower Pliocene sedimentary rocks on Macquarie Island are dominantly volcaniclastic breccia, sandstone, and siltstone produced by the physical disintegration and tectonic abrasion of oceanic crust in fault zones and mass wasting of these tectonic features. They represent small debris fans and small-scale turbidites deposited at the base of active fault scarps, related to Late Miocene to Early Pliocene seafloor spreading. Most of the sediment is derived from basalts, but diabase and gabbro clasts in some sedimentary rocks indicate that middle and lower oceanic crust was exposed to erosion on the sea floor. A lack of exotic clasts and a low degree of clast roundness are consistent with a local source for the sediment and no input from continental rocks. Spatial relationships between sedimentary rocks and major faults associated with seafloor spreading on the island and correlation between sedimentary clast and adjacent up-thrown block compositions allow us to infer paleotectonic relief for Macquarie Island crust during deposition. Our data support a model involving the deposition of these rocks at the inside corner of a ridge-transform intersection. Furthermore, a tectonic reconstruction of the Australian-Pacific plate boundary for the approximate time that Macquarie Island crust formed (10.9 Ma) also shows that Macquarie Island crust most likely formed near a ridge-transform intersection. This paper describes sedimentation associated with active faulting at a ridge-transform intersection that has been uplifted in situ above sea level along with the surrounding oceanic crust, and demonstrates that high-angle faults have the most pronounced influence, compared with low-angle faults, on sedimentation in this tectonic environment.<br/
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
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From seafloor spreading to uplift: the structural and geochemical evolution of Macquarie Island on the Australian-Pacific plate boundary
Get PDFtextMacquarie Island (54º30’S, 158º54’E) is unique, consisting of a section of
uplifted oceanic crust and upper mantle that still lies within the ocean basin where
it formed. Earlier geophysical studies indicate that between ~40 and 6 Ma, this
plate boundary evolved from a spreading ridge to the modern transpressional
boundary. The rocks of Macquarie Island record both regimes. This study
combines structural, geochemical and geophysical data to describe the evolution
of Macquarie Island and the adjacent Australian-Pacific plate boundary from
spreading to transpression.
The Finch-Langdon fault is the most significant spreading-related
structure on the island, juxtaposing upper crust and intrusive/mantle rocks. On
the basis of structural and petrologic data, I propose that this fault zone formed
near the inside corner of a ridge-transform intersection (RTI) and that structures
on the island are conformable with those in the surrounding seafloor.
Geochemical data for Macquarie Island basalts and peridotites suggest a
complex evolution during the last stages of seafloor spreading. The volcanic
section consists of enriched basalts formed by low degrees of partial melting.
Basalt geochemistry combined with stratigraphic relationships reveal early
intervals of variable enrichment followed by periods of more constant, decreasing
enrichment.
Peridotite and basalt geochemistries differ distinctly. Peridotites show
characteristics of a high degree of melting (heavy rare earth element, or REE, and
Al depletion), whereas low degrees of partial melting are inferred for the basalts.
The mantle rocks also have spoon-shaped REE patterns and anomalous Sr
enrichment. The depletion and trace element patterns are more typical of mantle
rocks in ophiolites than of abyssal mantle.
Ridge propagation proximal to an RTI exposing lower crust/uppermost
mantle would satisfy these structural and geochemical parameters.
Subsequently, transpression along the Australian-Pacific plate boundary
has resulted in transform motion along the plate boundary and vertical
deformation along the ~1500 km long Macquarie Ridge Complex. Uplift faults
on the island are dominantly high-angle, en echelon, normal faults. The
geometries and kinematics of the faults do not match predicted fault patterns for
transpression, but indicate domination by extensional relay zones between stepovers
of faults along the plate boundary.Earth and Planetary Science
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Deformation mechanisms in the Lewisian Gneiss and Cambro-Ordovician sediments at Heilam, Scotland
Get PDFHeilam, a 10 km2 area on the east coast of Loch Eriboll near the Moine Thrust in NW Scotland, is composed of three structural areas: 1) the Arnaboll Nappe, 2) the coastal schuppen zone and 3) the intermediate imbricate zone. Samples of each lithology taken from the zones show a progressive sequence of deformation. The least deformed rocks in the Arnaboll Nappe and in the intermediate imbricate zone have strain shadowed quartz grains and feldspars with deformation lamallae. Near the thrusts of the imbricate zone and in the schuppen zone, micas exhibit a strong preferred orientation, quartz grains show discontinuous undulatory extinction and recrystallization about the margins. Pressure shadows in the quartzites and styolites in the dolomites post-date earlier recrystallization in these areas. Mylonites, notably those along the Arnaboll Thrust, contain micas and quartz with a strong preferred orientation. All minerals show reduction in grain size by either recrystallization or brittle fracture. A sequence of deformation mechanisms affecting the lithologies is 1) initial recrystallization, 2) pressure solution, and 3) minor recrystallization. This sequence is best observed in the mylonites and schuppen zone.Earth and Planetary Science
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
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Microstructures and sense of shear in the Brevard Zone, southern Appalachians
Get PDFThe Brevard Zone, which separates the Blue Ridge and Inner Piedmont geologic provinces in the southern Appalachians, is a major structural feature with a multiple deformation history. Microstructures in oriented thin sections from rocks in the Brevard Zone in Tugaloo, Whetstone and Tamassee quadrangles, South Carolina, Rosman quadrangle, North Carolina, and in the sheared Ben Hill Granite in Atlanta, Georgia, indicate that there were at least two early ductile deformations and a later, locally developed, brittle deformation. The oldest recognizable microstructures arc a prominent foliation (S₁), quartz ribbons and garnets. The age of these features and the sense of shear during their formation is unknown. The remainder of the observed microstructures are categorized into groups A, B, C and D on the basis of orientation, overprinting relationships and direction of motion as indicated by sense of shear criteria present. Group A is the oldest of these microstructures and group D is the youngest. Group A features consists of northwest-verging, tight to isoclinal F₂ folds, a weakly developed, axial planar foliation (S₂), and scattered F₃ folds, coaxial with F₂. The F₁ folds of Roper and Dunn (1973) are not observed due to later deformation. Group A microstructures are ductile features which formed during a west- to northwest-directed thrusting motion. Group B features include type II s-c mylonites, c-surfaces, scattered folds and garnet pressure shadows. The orientation of these features indicates that they formed during a period of dextral strike-slip shearing with a possible thrust component. Group C contains an extensional crenulation cleavage (ECC) which is relatively younger than features in groups A and B. The orientation of ECC is incompatible with dextral motion, thus they suggest a change in the direction of bulk motion in the Brevard Zone, the direction of which is unknown. Along strike a notable change in deformation conditions occurred during the ductile deformation(s) which formed features in groups A, B and C. This change is reflected in highly recrystallized quartz textures in Tugaloo, relative to partially recrystallized textures in Rosman quadrangle. Retrograde metamorphism postdates the formation of features in groups A, B and C. Group D contains the youngest microstructures which formed during a localized brittle deformation. Brecciation is visible in thin section and outcrop, however no sense of shear direction can be determined. Drag folds and faults are present in several outcrops but their geometry is highly variable. The bulk motion during brittle deformation is unknown. Sense of shear criteria in group A are compatible with tectonic models for both the Taconic and Alleghanian orogenies in the southern Appalachians. However, group A probably formed in the Taconic because the most intense ductile deformation has been reported for this time period. Microstructures in group B and C are found in the sheared, Permian Ben Hill Granite in Atlanta and thus are Alleghanian in age. Rocks containing group B and C in the northeastern study areas cannot be radiometrically dated with confidence, however, their orientations, deformation conditions and sense of shear is similar to group B and C in the Ben Hill Granite indicating that they are also Alleghanian in age. The dextral strike-slip motion indicated by groups B is compatible with the results of previous workers (Reed and Bryant, 1964; Bobyarchick, 1983) elsewhere along the Brevard Zone who have also demonstrated Alleghanian dextral motion. Thus the results of this study confirm an episode of ductile, dextral strike-slip motion in the Brevard Zone during the Alleghanian. Group C and D may also be Alleghanian or they may be the result of a separate and more recent deformation, possibly related to the Triassic opening of the present-day Atlantic Ocean.Earth and Planetary Science
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Kinematic and geometric evolution of the Buckskin-Rawhide metamorphic core complex, west-central Arizona
Get PDFtextReconstructing the structural evolution of metamorphic core complexes is critical to understanding how large-magnitude extension is accommodated in the middle to upper crust. This dissertation focuses on the Miocene geometric and kinematic evolution of the Buckskin-Rawhide metamorphic core complex in west-central Arizona, addressing controversial topics including the geometric development of mid-crustal shear zones, the formation of detachment fault corrugations, and the transition from detachment faulting to more distributed deformation. Detailed microstructural data from mylonites in the lower plate of the Buckskin-Rawhide detachment fault indicate that early Miocene mylonitization was characterized by consistent top-NE-directed shear and ~450-500°C deformation temperatures that varied by [less-than or equal to]50°C across a distance of ~35 km in the extension direction. The relatively uniform deformation conditions and strain recorded in mylonitized ~22-21 Ma granitoids are incompatible with models in which the lower plate shear zone represents the down-dip continuation of a detachment fault. Instead, lower plate mylonites initiated as a subhorizontal shear zone that was captured and rapidly exhumed by a moderately to gently dipping detachment fault system. Structural data and geologic mapping demonstrate that the prominent NE-trending Buckskin-Rawhide detachment fault corrugations are folds produced by extension-perpendicular (NW-SE) shortening during core complex extension. Dominant NE-directed slip on the detachment fault was progressively overprinted by NW- and SE-directed slip associated with corrugation folding. Orientation patterns of upper plate bedding across the corrugations are compatible with folding about a NE-trending axis. Extension-perpendicular shortening in the lower plate is recorded by synmylonitic constriction and folding. Upright m-scale and km-scale lower plate folds parallel the detachment fault corrugations and developed primarily by postmylonitic flexural slip that was coeval with detachment faulting. The total amount of NW-SE shortening across the lower plate is ~10%, but the amount of NW-SE shortening recorded by the younger detachment fault is only ~1%. The relatively late-stage development of corrugations in the Buckskin-Rawhide metamorphic core complex suggests that extension-perpendicular shortening was primarily driven by a reduction of vertical stresses through crustal thinning and tectonic denudation. Brittle fault data document the transition from large-magnitude, NE-directed extension to distributed E-W extension and right-lateral faulting. Following exhumation to brittle conditions, lower plate mylonites were extended up to ~20-30% by NE-dipping, syndetachment normal faults. Towards the end of detachment faulting, the extension direction rotated clockwise, and some portions of the Buckskin detachment fault record a transition from dominant top-NE slip to ENE- and E-directed slip. After detachment faulting ceased, E-W extension was accommodated primarily by steeply NE-dipping, right-lateral and oblique right-lateral-normal faults. The cumulative amount of right-lateral shear across the core complex is probably 7-9 km, which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction. Postdetachment right-lateral/transtensional faulting across the Buckskin-Rawhide metamorphic core complex reflects the increasing influence of the Pacific-North American transform plate boundary towards the end of the middle Miocene.Earth and Planetary Science
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