Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions

Pennacchioni, Giorgio; Mancktelow, Neil S.

doi:10.1016/j.jsg.2007.06.002

Cited by 161 publications

(154 citation statements)

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“…2e), as described for ductile deformation developed during post-magmatic cooling of many other plutons (e.g., Pennacchioni 2005, Pennacchioni & Zucchi 2013. This is consistent with the general observation that planar compositional or structural heterogeneities localize shear deformation in an isotropic rock (Pennacchioni & Mancktelow 2007, Mancktelow & Pennacchioni 2013. Many pegmatites (but also the leucocratic microgranite dikes) distinguished in the stereoplot of Figure 4c show the effect of ductile deformation.…”

Section: Codera Areasupporting

confidence: 86%

Tertiary Pegmatite Dikes of the Central Alps

Guastoni¹,

Pennacchioni²,

Pozzi³

et al. 2014

Can Mineral

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Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. AbSTRACTThe largest field of Alpine Oligocene pegmatite dikes is in the Central Alps within the Southern Steep Belt (SSB) of the Alpine nappes; it extends for about 100 km in an E-W direction and 15 km in a N-S direction north of the Periadriatic Fault, from the Bergell pluton (to the east) to the Ossola valley (to the west). The pegmatite field geographically overlaps (1) the highest temperature domain of the Lepontine Barrovian metamorphic dome and (2) the zone of Alpine migmatization. We have studied pegmatites in two areas: (1) the Codera area on the western border of the Bergell pluton and (2) the Bodengo area between the Mera and the Mesolcina valleys. Most pegmatites show a simple mineral assemblage consisting of K-feldspar, quartz, and muscovite ± biotite, and only a minor percentage of the dikes (< 5%) contains Sn-Nb-Ta-Y-REE-U oxide, Y-REE phosphate, Mn-Fe-phosphate, Ti-Zr-silicate, Be-Y-REE-U-silicate and oxide minerals (beryl, chrysoberyl, bertrandite, bavenite, and milarite), garnet (almandine-spessartine), tourmaline (schorl to rare elbaite), bismuthinite, magnetite, and rarely dumortierite and helvite. The mineral assemblages, geological context, and chemical compositions allow the distinction between LCT (lithium, cesium, tantalum) and mixed LCT-NYF (niobium, yttrium, fluorine) pegmatites (with only one exception of an NYF dike in the Bodengo area). The LCT pegmatites of the Central Alps did not reach a high degree of geochemical evolution. The most fractionated pegmatites are found in the Codera area and contain Mn-rich elbaite, triplite, pink-beryl, and Cs-Rb-rich feldspar. In the Bodengo area pegmatites locally contain miarolitic cavities and the most evolved pegmatites correspond to the berylcolumbite-phosphate type. From a structural point of view two main types of pegmatites can be distinguished: (1) pegmatites that were involved in ductile deformation and (2) pegmatites that postdated the main ductile deformation of the SSB. Many pegmatites of the Codera valley belong to the first structural type: they were emplaced at relatively high ambient temperature (ca. 500 °C) and locally show a pervasive recrystallization of quartz and a mylonitic structure. The Codera dikes trend about 70° and are steeply dipping. In the Bodengo area the main set of pegmatites (trending approximately N-S to NNE-SSW) crosscuts the ductile deformation structures of the SSB, but the area also includes an earlier generation of boudinaged and folded pegmatite dikes. The undeformed ...

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Section: Codera Areasupporting

confidence: 86%

Tertiary Pegmatite Dikes of the Central Alps

Guastoni¹,

Pennacchioni²,

Pozzi³

et al. 2014

Can Mineral

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“…Handy and Stünitz, 2002;Pennacchioni and Mancktelow, 2007;Fusseis and Handy, 2008). Shear zones and style of strain accommodation and localization processes in granitoids have been described by different authors (Ramsay and Graham, 1970;Burg andLaurent, 1985, 1986;Gapais et al, 1987;Goncalves et al, 2016, and references therein).…”

Section: Introductionmentioning

confidence: 97%

“…In this context, the role of brittle precursors in nucleating shear zones has received increasing consideration as having in some cases a fundamental role in the localization process (e.g. Segall and Simpson, 1986;Mancktelow and Pennacchioni, 2005;Pennacchioni and Mancktelow, 2007;Menegon and Pennacchioni, 2010;Pennacchioni and Zucchi, 2013).…”

Section: Introductionmentioning

confidence: 99%

“…Guermani and Pennacchioni, 1998;Pennacchioni and Mancktelow 2007); most of these studies consider deformation at shallow to intermediate crustal depths (Fusseis and Handy, 2008;Mazzoli et al, 2009;, although case studies under conditions of upper amphibolite to granulite (White, 1996;Pennacchioni and Cesare, 1997;Pittarello et al, 2013;Altenberger et al, 2013) and eclogite facies (Austrheim and Boundy, 1994;Austrheim, 2013, and references therein) were also investigated.…”

Section: Introductionmentioning

confidence: 99%

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Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

Molli

Menegon

Malasoma³

2017

Solid Earth

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Abstract. The switching in deformation mode (from distributed to localized) and mechanisms (viscous versus frictional) represent a relevant issue in the frame of crustal deformation, being also connected with the concept of the brittle-"ductile" transition and seismogenesis. In a subduction environment, switching in deformation mode and mechanisms and scale of localization may be inferred along the subduction interface, in a transition zone between the highly coupled (seismogenic zone) and decoupled deeper aseismic domain (stable slip). However, the role of brittle precursors in nucleating crystal-plastic shear zones has received more and more consideration being now recognized as fundamental in some cases for the localization of deformation and shear zone development, thus representing a case in which switching deformation mechanisms and scale and style of localization (deformation mode) interact and relate to each other. This contribution analyses an example of a millimetrescale shear zone localized by brittle precursor formed within a host granitic protomylonite. The studied structures, developed in ambient pressure-temperature (P -T ) conditions of low-grade blueschist facies (temperature T of ca. 300 • C and pressure P ≥ 0.70 GPa) during involvement of Corsican continental crust in the Alpine subduction. We used a multidisciplinary approach by combining detailed microstructural and petrographic analyses, crystallographic preferred orientation by electron backscatter diffraction (EBSD), and palaeopiezometric studies on a selected sample to support an evolutionary model and deformation path for subducted continental crust. We infer that the studied structures, possibly formed by transient instability associated with fluctuations of pore fluid pressure and episodic strain rate variations, may be considered as a small-scale example of fault behaviour associated with a cycle of interseismic creep and coseismic rupture or a new analogue for episodic tremors and slow-slip structures. Our case study represents, therefore, a fossil example of association of fault structures related to stick-slip strain accommodation during subduction of continental crust.

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“…In the brittle-ductile transition regime and at lower temperatures, shear zone formation often occurs at brittle precursors (Brander et al, 2012;Fusseis and Handy, 2008;Pennacchioni and Mancktelow, 2007;Pittarello et al, 2012), or by ductile fracturing (Dimanov et al, 2007;Menegon et al, 2013;Rybacki et al, 2008;Rybacki et al, 2010;Shigematsu et al, 2009;Spiess et al, 2012;White, 2012). Ultimately, strain weakening associated with localization may result from (e.g., Burlini and Bruhn, 2005): (1) grain size reduction by dynamic recrystallization or metamorphic reaction that produce new stress-free grains and/or a switch to grain size-sensitive deformation mechanisms, (2) structural softening in polyphase materials induced by the formation of interconnected weak layers at high strain, (3) geometric softening resulting from shape and crystallographic preferred orientation, (4) chemical weakening resulting in a change of point defects concentration, (5) fluid-induced dissolution-precipitation creep or reduction of the effective pressure causing embrittlement, (6) partial melting, leading to mechanical weakening and fast diffusion along wet grain boundaries, (7) transformation plasticity caused by mineral phase change, and (8) shear heating.…”

Section: Introductionmentioning

confidence: 99%

Strain localization during high temperature creep of marble: The effect of inclusions

et al. 2014

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The deformation of rocks in the Earth's middle and lower crust is often localized in ductile shear zones. To better understand the initiation and propagation of high-temperature shear zones induced by the presence of structural and material heterogeneities, we performed deformation experiments in the dislocation creep regime on Carrara marble samples containing weak (limestone) or strong (novaculite) second phase inclusions. The samples were mostly deformed in torsion at a bulk shear strain rate of ≈ 1.9x10 -4 s -1 to bulk shear strains γ between 0.02 and 2.9 using a Paterson-type gas deformation apparatus at 900°C temperature and 400 MPa confining pressure. At low strain, twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at γ > 0.1 -0.2.Peak shear stress at the imposed conditions is about 20 MPa, which is ≈8% lower than the strength of intact samples. Strain progressively localized within the matrix with increasing bulk strain, but decayed rapidly with increasing distance from the inclusion tip.Microstructural analysis shows twinning and recrystallization within this process zone, with a strong crystallographic preferred orientation, dominated by {r} and (c) slip in .Recrystallization-induced weakening starts at local shear strain of about 1 in the process zone, corresponding to a bulk shear strain of about 0.1. In contrast, torsion of a sample containing strong inclusions deformed at similar stress as inclusion-free samples but do not show localization. The experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips. Recrystallizationinduced grain size reduction may only locally promote grain boundary diffusion creep. Accordingly, the bulk strength of the twisted aggregate is close to or slightly below the lower (isostress) strength bound, determined from the flow strength and volume fraction of matrix and inclusions.

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Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions

Cited by 161 publications

References 65 publications

Tertiary Pegmatite Dikes of the Central Alps

Tertiary Pegmatite Dikes of the Central Alps

Switching deformation mode and mechanisms during subduction of continental crust: a case study from Alpine Corsica

Strain localization during high temperature creep of marble: The effect of inclusions

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