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Initiation and growth of strike-slip faults within effectively intact metagranitoid (Neves area, Eastern Alps, Italy)
No full textThe geometry and progressive development of segmented strike-slip faults developed within effectively intact
metagranitoid was studied in the Neves area of the Tauern Window (Eastern Alps, Italy). The currently exhumed
faults formed in the lower brittle crust under hydrous conditions, as shown by the pervasive presence of chlorite,
epidote and quartz along the faults and within associated veins. The numerous pre-Alpine leucocratic and basic
dykes, as well as earlier Alpine high temperature shear zones and veins (Pennacchioni and Mancktelow, 2007),
provide passive markers useful for estimating the strike-slip offset along the different parts of the brittle faults.
The faults initiated as segmented fractures (on scales ranging from millimeters to hundred meters) arranged in an
en-echelon pattern within shear bands, locally trending sub-parallel to the inferred direction of shortening (around
N-S). Initial (cm-dm) slip accumulation on the fault segments was accommodated by more distributed fracturing
at contractional stepovers and especially by slip on a dominant set of antithetic faults in the stepover. This was
probably associated with a component of out-of-plane (vertical) movement, as slip on the antithetic faults does
not fully account for the overall decrease in slip along the bounding faults towards their tips. This mechanism of
slip accommodation was only effective for a relatively small fault slip. Further slip resulted in the development of
synthetic, sigmoidal by-pass faults crosscutting the earlier antithetic fault set within the relay ramp and connecting
the overstepped master faults. The total in-plane (horizontal) slip along the secondary faults within the stepover
accounts for almost all of the net slip decrease towards the tips of the overstepping faults. However, the lack of
distortion of the overstepping fault segments implies that a component of vertical slip also occurred. This second
stage of evolution is documented in a main stepover of the sinistral NNE-SSW-trending Mesule fault (the major
fault in the area), which has a current maximum offset of about 10 m. Hard linkage allowed the propagation of
seismic fractures, as recorded by pseudotachylyte localized at by-pass contractional bends.
References
Pennacchioni, G., Mancktelow, N.S., 2007. Nucleation and initial growth of a shear zone network within compositionally
and structurally heterogeneous granitoids under amphibolite facies conditions. Journal of Structural
Geology 29, 1757-1780. doi: 10.1016/j.jsg.2007.06.00
Late magmatic healed fractures in granitoids and their influence on subsequent solid-state deformation
No full textLate magmatic fractures developed during final stages of magma crystallization are recorded in many granitoid plutons by planar trails of magmatic biotite and/or leucocratic veins but have not been previously described. Such biotite-rich trails, with lengths of a few centimetres to several tens of centimetres, widths of a few millimetres, and a variable orientation, are ubiquitous in meta-granitoids of the
Neves area (Tauern Window, Eastern Alps, Italy). These pre-Alpine granitoids were metamorphosed to upper amphibolite facies and deformed during the Alpine orogeny, allowing the effect of planar healed magmatic fractures on the nucleation of subsequent solid-state ductile shear zones to be directly assessed. Numerical models considering power-law viscous materials predict that planar arrays of a
weaker mineral (e.g. biotite) oriented at around 45 to the shortening direction should nucleate and localize viscous deformation. However, no discernible localization on the natural small-scale biotite trails is observed, even when they delineate planes that were well-oriented for shear reactivation. In contrast, heterogeneous shear zones nucleated on larger scale (>1e10 m long) planar compositional or structural boundaries such as joints, dykes, quartz veins, alteration layers surrounding veins, and zones of magma mingling outlined by densely packed clusters of basic enclaves, irrespective of their orientation relative
to the imposed shortening direction. Clearly some crucial aspect is missing from purely viscous numerical models.We propose that this missing aspect is the observed interplay between fracture and flow, with new fractures developing and localizing shear at bulk strains too low for discernible localization to occur on the pre-existing healed magmatic fractures. Natural granitoids have a truly elasto-plastoviscous
rheology even at low differential stress (as in the Neves example) and for high grade metamorphic conditions considered as typical of wet “ductile” middle crust
Correction to "Why calcite can be stronger than quartz"
No full textIn the paper “Why calcite can be stronger than quartz” by Neil S. Mancktelow and Giorgio Pennacchioni (Journal of Geophysical Research, 115, B01402, doi:10.1029/2009JB006526, 2010), Figure 10 requires clarification of the parameters used in its calculation to avoid misinterpretation. Figure 10 presents the deformed shapes at shear strain γ = 6 of cylindrical inclusions with an initially circular cross section. The results, calculated by the first author using a personally developed FEM code, were used to demonstrate that for power law materials in simple shear flow, nearly rigid behavior of isolated inclusions is possible even when the effective viscosity ratio is low (~2). In particular, the example with a power law stress exponent of n = 6 in the inclusion and n = 3 in the matrix was considered to be directly relevant to natural examples of coarse calcite clasts in quartz mylonites from the Neves area of the eastern Alps. This fundamental conclusion is correct, as are the calculated shapes for the parameters employed. However, with the aim of remaining concise, details of these parameters were not given: it was simply stated in the caption that “the effective viscosity ratio (μi/μm),” as listed on the left, was “for the case of equal strain rate in inclusion and matrix.” This statement requires clarification because (1) for power law viscous material there is no specific material parameter “viscosity” (or “viscosity ratio”) independent of strain rate, as there is for linear viscous behavior, (2) the strain rate in the inclusion and matrix will not be the same, even at the very start of a numerical experiment, and (3) the strain rate in the inclusion will vary with its axial ratio and orientation
Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia
Full text linkResearch funds from grants CGL2011-22728, CGL2011-23628/BTE, CGL2012-38786 and CGL2014-53388-P by the Spanish government are acknowledged. Pedro Castiñeiras stay at Stanford University was funded by CSIC grant PA1002435. This paper benefitted from the general and specific comments of the reviewers Neil S. Mancktelow and Thomas Blenkinsop. We thank the editor Ernie Rutter for extensive feedback and criticism on the earlier versions of the manuscrip
Single-layer folds developed from initial random perturbations: The effects of probability distribution, fractal dimension, phase, and amplitude
No full textGoing 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
Deformation of an elliptical inclusion in two-dimensional incompressible power-law viscous flow
No full textNonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks
No full textA least-squares method for determining the best-fit point maximum, great circle, and small circle to nondirectional orientation data
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