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1

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Exhumation in a convergent orogen: the western Tauern window (1997)

Fiigenschuh, Bernhard ; Seward, Diane ; Mancktelow, Neil

Oxford, UK : Blackwell Publishing Ltd

Terra nova 9 (1997), S. 0

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ISSN: 1365-3121

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Unroofing of the western Tauern window involved both low-angle detachment faulting (Brenner Fault) and enhanced footwall erosion, contemporaneous with upright antiformal folding. This combination reflects orogen-parallel (˜E–W) extension during continued ˜N–S Alpine convergence. New fission-track ages establish the relative chronology of folding and faulting and demonstrate that displacement was not always accommodated on the same surface. During exhumation, some units migrated from the footwall to the hanging wall of the main detachment fault, due to the interplay between folding and faulting. The region can effectively be divided into 3 distinct domains. (1) The Penninic units of the western Tauern Window were always in the footwall to the fault, with maximum exhumation in the core of the dome, due to folding and erosion. (2) The Lower Austroalpine unit north of the Tauern Window was first part of the hanging wall to the Brenner Fault. At a later stage this unit was exhumed by a further 4–5 km as part of the footwall to a more discrete, through-going fault (the Silltal Fault). (3) The Middle and Upper Austroalpine units west of the Tauern Window were always within the hanging wall.Exhumation of the footwall from an initial depth of ∼ 25 km led to a transition in mechanical behaviour. The curviplanar (folded) ductile shear zone marking the boundary to the Tauern window was eventually transected by a more planar discrete brittle fault (Silltal Fault, with unit 2 now in the footwall), along which the pre-existing mylonites were passively exhumed to the surface.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1365-3121.1997.tb00015.x

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An SEM study of porosity and grain boundary microstructure in quartz mylonites, Simplon Fault Zone, Central Alps (1998)

Mancktelow, Neil S. ; Grujic, Djordje ; Johnson, Eric L.

Springer

Contributions to mineralogy and petrology 131 (1998), S. 71-85

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract A backscattered and secondary electron SEM study of the grain boundary microstructure in quartz mylonites sampled along the length of the retrograde Simplon Fault Zone established three characteristic components. (1) Fine isolated pores (≤ 1 μm diameter) are scattered across two-grain interfaces, preferentially concentrated on surfaces in extension. Pores are uncommon on three-grain junctions and there is no evidence for fluid interconnectivity along three- and four-grain junctions. The fine porosity may develop by accumulation of original, mainly intragranular fluid inclusions to the grain boundary during deformation and recrystallization and by cavitation of grain boundaries during grain boundary sliding. Dynamic cavitation implies that the “ductile” mylonitic deformation is at least locally dilatant and therefore pressure sensitive. (2) Large “vug”-like pores (up to mm-scale) extend along multi-grain boundaries. Observed in all samples, they are most common in the higher initial temperature, coarse-grained samples with a microstructure dominated by grain boundary migration recrystallization. Grains bordering this connected porosity develop perfect crystal faces, undecorated by fine pores or pits. The irregular “lobate” optical microstructure of many migrating grain boundaries actually consists of a series of straight crystal faces. The coarse porosity is probably due to accumulation during dynamic recrystallization of (CO2-rich ?) fluid with a high wetting angle against quartz. (3) In one sample, interconnected sinuous ridges, ≤ 0.2 μm high, are observed to follow three- and four-grain junctions and disjoint into more isolated worms and spheroidal globules. On two-grain interfaces, these are transitional to more branching vein-like or convoluted brain-like forms. The brain-like and globular forms have been observed, with varying frequency, through the range of samples, with the globules attaining sizes of up to 60 μm. Vein structures have also been observed on intragranular fractures. These topologies do not match across adjoining surfaces and must have developed into free space. The ridge-vein-brain-spheroid structure is distinctly different to that previously observed on experimentally healed microcracks and its origin is not unequivocally established. They could represent unstable meniscus necking of a thin grain-boundary phase of low viscosity, developed due to quasi-adiabatic shear and/or local stress-induced dilatancy during microcracking.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s004100050379

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3

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A least-squares method for determining the best-fit point maximum, great circle, and small circle to nondirectional orientation data (1981)

Mancktelow, Neil S.

Springer

Mathematical geology 13 (1981), S. 507-521

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ISSN: 1573-8868

Keywords: Structural analysis ; structural geology ; fabric diagrams ; orientation data ; least squares

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Mathematics

Notes: Abstract Given a set of nondirectional orientation data (fold axes, lineations, dip and dip direction of bedding, universal stage readings of crystallographic axes, etc.),the “best-fit” line (point maximum),plane (great circle),or cone (small circle)can be determined by minimizing the sum of the squares of the angular residuals using a simplex convergence technique. Stereoplots of the angular deviation over the complete lower hemisphere for these distributions may also be generated when consideration of the constraint on the best-fit position is important; for example in comparing different data sets of the same structural element. The routines are available as a FORTRAN coded computer program.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF01034502

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Multiple faults in ductile simple shear: analogue models of flanking structure systems (2007)

Exner, Ulrike ; Grasemann, Bernhard ; Mancktelow, Neil S.

In: Geological Society Special Publication 253: 381-395.

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Publication Date: 2007-10-08

Description: Rotational behaviour and deformation around multiple faults was investigated in analogue experiments using a linear viscous matrix material under simple shear boundary conditions. Previous analogue and numerical studies have shown that, for single faults, characteristic deformation geometries are produced in initially straight marker lines parallel to the shear zone boundary (flanking structures). Observations from several natural shear zones suggest that not only single faults, but often several parallel or conjugate fault planes are subjected to progressive shear resulting in distinctive deflection geometries. If the distance between faults is on the order of their length, or less, then the perturbation flow fields interfere and coalescence, and finite deflection structures develop that are distinctly different from those around single fractures. In particular, coeval contractional and extensional geometries may develop across conjugate faults, although for bulk simple shear the total length of marker lines parallel to the shear zone boundary cannot change. This advises caution in inferring shear-zone parallel contraction or extension from secondary slip surfaces. In contrast to single flanking structures, conjugate flanking structure systems occurring in natural shear zones are reliable shear sense indicators due to their triclinic symmetry.

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Intermittent fracturing in the middle continental crust as evidence for transient switching of principal stress axes associated with the subduction zone earthquake cycle (2020)

Mancktelow, Neil ; Pennacchioni, Giorgio

Geological Society of America

In: Geology. 2020; 48(11): 1072-1076. Published 2020 Jul 21. doi: 10.1130/g47625.1.

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Publication Date: 2020-07-21

Description: In 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 long-term 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 σ1 and σ3 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.

Print ISSN: 0091-7613

Electronic ISSN: 1943-2682

Topics: Geosciences

Published by Geological Society of America

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The not-so-simple effects of boundary conditions on models of simple shear (2011)

Frehner, Marcel ; Exner, Ulrike ; Mancktelow, Neil S. ; Grujic, Djordje

Geological Society of America (GSA)

In: Geology 39: 719-722.

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Publication Date: 2011-08-01

Description: Analog modeling of geological processes, such as folding instabilities or the behavior of inclusions in a matrix, often employs a linear simple-shear rig. In theory, a homogeneous plane-strain flow is prescribed at the boundaries of such deformation rigs but, in practice, the resulting internal deformation of the analog material (commonly paraffin wax or silicone putties) often deviates strongly from the intended homogeneous strain field. This can easily lead to misinterpretation of such analog experiments. We present a numerical finite-element approach to quantify the influence of imperfect simple-shear boundary conditions on the internal deformation of a homogeneous viscous analog material. The results demonstrate that imperfect boundary conditions in the vorticity-normal plane can cause the heterogeneous strain observed in some analog experiments. However, in other experiments, the analog material lies on top of a weak lubricating material or is sandwiched between two such materials. These layers lead to a viscous drag force acting on the analog material, resulting in imperfect simple-shear boundary conditions in the third dimension. For this experimental configuration, the numerical results show that the lubricating layers are responsible for the heterogeneous strain observed in analog models. The resulting errors in internal strain can be as high as 100%, and these difficult-to-avoid boundary effects must be considered when interpreting analog simple-shear experiments.

Print ISSN: 0091-7613

Electronic ISSN: 1943-2682

Topics: Geosciences