3,280 research outputs found
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
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
Intermittent fracturing in the middle continental crust as evidence for transient switching of principal stress axes associated with the subduction zone earthquake cycle
Full text linkIn the Neves area, eastern Alps, fractures that localized shear zones in middle continental crust above the Alpine megathrust are commonly oriented at a high angle to the inferred longterm shortening direction. Fractures show a segmentation geometry and, locally, a discernible offset, indicating movement opposite to the sense of subsequent ductile shear and implying a switch of principal stress axes σ 1 and σ 3 during fracturing. We propose that this repeated switch, demonstrated by overprinting relationships and different degrees of fracture reactivation, was due to sporadic co-seismic to early post-seismic rebound in the upper plate of the Alpine continental collision system. Fracturing occurred intermittently in the weak midcrustal rocks due to seismic stress release at high transient strain rates and pore-fluid pressures. Widespread transient fracturing in the hanging wall of the Alpine megathrust regionally controls the orientation of ductile shear zones in the middle crust, as well as the emplacement of magmatic dikes
Minnesota Crookston Names Neil Mancktelow as Sixth Women's Soccer Head Coach in Program History
No full textPlante, Hunter. (2022). Minnesota Crookston Names Neil Mancktelow as Sixth Women's Soccer Head Coach in Program History. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/271415
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
From XY tracking to buckling: Axial plane cleavage fanning and folding during progressive deformation
No full textFolding of axial plane cleavage can occur during progressive deformation without a change in the overall background flow. Two field examples of upright (Lachlan Fold Belt, SE Australia) and recumbent (Naukluft Nappe Complex, central Namibia) folds are presented, in which strongly refracted pressure solution cleavage in competent layers on the fold limbs is buckled as a result of ongoing fold amplification. Finite element modelling confirms that cleavage refraction on limbs can be sufficient for cleavage planes to be subsequently shortened and therefore folded. Cleavage refraction is unequally developed on opposite limbs of asymmetric folds formed by oblique shortening of a layer in coaxial flow or by folding in a more general shear environment. The differences in finite strain on opposite limbs can be quite marked even when the fold shapes themselves are not obviously asymmetric. For folding in simple shear flow, as specifically modelled here, refraction is only strong on the fold limb that rotates against the imposed sense of shear. In known shear environments, this provides a potential kinematic indicator in folded units at relatively low strain (e.g. in simple shear, γ of around one), where other higher-strain indicators, typical of mylonites, are not yet sufficiently developed or are equivocal. © 2005 Elsevier Ltd. All rights reserved
Time‐Lapse Record of an Earthquake in the Dry Felsic Lower Continental Crust Preserved in a Pseudotachylyte‐Bearing Fault
Full text linkISSN:2169-9313ISSN:0148-0227ISSN:2169-9356ISSN:0148-0227ISSN:2169-935
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
The structure and metamorphism of the southern Adelaide fold belt / by Neil Sydney Mancktelow
No full textTypescript (photocopy)Electron microprobe analysis on microfiche at end of Appendices, vol 1.2 v. : ill., maps, (some col., fold.) ; 30 cm.Title page, contents and abstract only. The complete thesis in print form is available from the University Library.Thesis (Ph.D.)--University of Adelaide, Dept. of Geology and Mineralogy, 198
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