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Strain localization on different scales and the importance of brittle precursors during deformation in the lower crust (Davenport Shear Zone, Central Australia)
No full textHigh strain rocks in the Musgrave Ranges (Central Australia) provide a rather unique insight into the development
of lower crustal shear zones during the 550 Ma Petermann Orogeny, allowing common models for lower crustal
deformation to be critically evaluated. The observed structures in the study area are, from south to north: (1)
The Mann Fault, which is poorly exposed but evident on airborne geomagnetic maps. This regional scale fault
with a component of dextral shear shows a step-over resulting in the formation of a pull-apart basin. (2) The
Davenport Shear Zone, accommodating the horizontal extension in a 7 km wide WNW-ESE-trending mylonitic
zone developed under subeclogitic, lower crustal conditions. This high strain zone is bounded to the north by a
more than 50 km long, continuous, sheared dolerite dyke. North of this dyke, the ∼ 1200 Ma Musgravian fabric is
still preserved, only slightly rotated and typically N-S trending. (3) The Woodroffe Thrust, marking the northern
boundary of the Musgrave Ranges, brings these lower crustal rocks on top of amphibolite facies units, with a
top-to-north sense of movement.
Strain in the Davenport Shear Zone is very heterogeneously distributed, with localization and partitioning
from the kilometre down to the millimetre scale. Pseudotachylyte is commonly associated with dykes, especially
on the boundaries, and is often sheared. The orientation of sheared dykes and localized shear zones is typically at a
high angle to either side of the shortening direction, resulting in a variable sense of shear and a major component of
flattening, with a nearly horizontal extension direction. Detailed outcrop-scale mapping shows that compositional
inhomogeneities, such as quartz veins, are generally not exploited, even when favourably oriented for shear
reactivation. Ultramylonitic shear zones are sometimes only a few millimetres wide but extend for several metres
and are generally oblique to the background foliation. Pseudotachylyte often predates or is coeval with localized
shearing and fracturing clearly played a major role in the nucleation of mesoscale discrete shear zones. In order
to constrain the conditions of pseudotachylyte formation, and to establish whether they developed under lower
crustal subeclogitic conditions, garnet-bearing sheared pseudotachylytes were sampled for geothermobarometric
analysis
Pseudotachylyte formation vs. mylonitization – repeated cycles of seismic fracture and aseismic creep in the middle crust (Woodroffe Thrust, Central Australia)
No full textThe Musgrave Ranges in Central Australia provide excellent exposure of the shallowly south-dipping Woodroffe
Thrust, which placed
∼
1200 Ma granulites onto amphibolite facies gneisses. This
∼
400 km long E-W structure
developed under mid-crustal conditions during the intracratonic Petermann Orogeny around 550 Ma. From
field observations and measurements, the shortening direction is constrained to be N-S and the movement
sense top-to-north. Ductile deformation during this process almost entirely localized in the footwall rocks,
developing a zone of mylonites, ultramylonites and sheared pseudotachylytes, several hundred metres wide, with
pseudotachylyte abundance rapidly decreasing further into the footwall. In contrast, the hanging wall behaved in
a predominantly brittle manner, producing significant volumes of pseudotachylyte breccia and isolated veins, but
was otherwise mostly unaffected and only weakly foliated. The difference in rheological behaviour is reflected
in the pseudotachylyte fabric, which is dominantly sheared in the footwall and largely unsheared in the hanging
wall. Low-strain domains in the footwall show that localized shearing initiated along pseudotachylyte veins and
that shear zones and mylonitic foliations were in turn exploited by subsequent pseudotachylyte veins. Neither
phyllonitization nor synkinematic growth of new muscovite is observed. In contrast to models with a simple
brittle-to-viscous transition, these observations show that a continuous cycle of brittle fracturing and shearing is
active in dry mid-crustal environments. The products of multiple earthquakes and ductile overprint, repeatedly
exploiting the same structural discontinuity, are composite layers of sheared pseudotachylyte. In the Woodroffe
Thrust, these layers are numerous and frequently observed parallel to the foliation in the footwall mylonites. The
thickest of these sheared pseudotachylyte horizons (
∼
15 m thick) mark the immediate contact to the hanging
wall and almost entirely consist of pseudotachylyte matrix. Particularly in the footwall, but locally also in the
hanging wall, shear strain can additionally be concentrated along the margins of dolerite dykes, whose mineral
assemblages will be studied to determine the metamorphic conditions that were active during development of the
Woodroffe Thrust
Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)
Full text linkGarnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lowercrustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼ 1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼ 550 Ma overprint under sub-eclogitic conditions (600–700 °C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lowercrustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte
Interplay between seismic fracture and aseismic creep in the Woodroffe Thrust, central Australia – Inferences for the rheology of relatively dry continental mid-crustal levels
No full textThe over 600 km long Woodroffe Thrust developed at lower to mid-crustal levels during the intracontinental Petermann Orogeny at ca. 560–520 Ma. Ductile deformation with a top-to-north shear sense was accommodated along a shallowly (≤30°) south-dipping surface. Metamorphic conditions during deformation are established along a 60 km N-S transect, providing an ideal framework for studying variation in microstructure and crystallographic preferred orientations with changing temperature (ca. 520–620 °C) and pressure/depth in dominantly dry felsic crust. In the Woodroffe Thrust mylonites, dynamic recrystallization of quartz was dominated by subgrain rotation, whereas feldspar underwent grain size reduction by neocrystallization. Differential stress, estimated from quartz grain size piezometry, decreases with increasing metamorphic grade (i.e., deeper structural levels), and indicates a long-term average strain rate of around 10−11–10−12 s−1. We propose a qualitative rheological model to explain the observed cyclic interplay between ductile shearing (mylonitization) and brittle fracturing (pseudotachylyte formation) in the relatively dry middle crust. The model involves the downward migration of earthquake ruptures from the overlying seismogenic zone, which transiently triggers seismic slip at mid-crustal levels
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe 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
Full text linkWe 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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