Long-term deformation and fluid-enhanced mass transport in a Variscan peridotite shear zone in the Ivrea Zone, northern Italy: a microtextural, petrological and geochemical study of a reactivated shear zone

Altenberger, Uwe

doi:10.1007/s005310050027

Cited by 17 publications

(32 citation statements)

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“…A temperature above 600 • C is also in accord with the general assumption that dynamic recrystallization of olivine in (100), (010) and (001) the regime of dislocation creep at reasonable strain rates of 10 −13 to 10 −15 s −1 requires temperatures above about 650 • C as based on observations from natural systems (e.g. Skrotzki et al, 1990;Altenberger, 1995;Jin et al, 1998) and the extrapolation from experimentally derived flow laws for dislocation creep of olivine ( Fig. 9; Chopra and Paterson, 1981;Hirth and Kohlstedt, 2003).…”

Section: Deformation Processes and Conditions Recorded By Olivine Andsupporting

confidence: 75%

“…Hilairet et al, 2007;Chernak and Hirth, 2010) microfabrics can yield information on the deformation mechanisms, as well as the stress and strain-rate conditions during burial and exhumation (e.g. Skrotzki et al, 1990;van der Wal, 1993;Altenberger, 1995;Jin et al 1998;Zhang et al, 2000;Jung and Karato, 2001;Piepenbreier and Stöckhert, 2001;Stöckhert, 2002;Andreani et al, 2005;Auzende et al, 2006;Jung et al, 2006;Wassmann et al, 2011;Matysiak and Trepmann, 2012). The aim of this study is to use the successively overprinted and modified microfabrics of the partly serpentinized peridotite mylonites to differentiate between the potential record of independent grain-scale deformation processes and implications on the stress conditions.…”

Section: Introductionmentioning

confidence: 99%

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The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

Bial

Trepmann

2013

Solid Earth

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Abstract. We present microfabrics in high-pressure, metamorphic, partly serpentinized peridotite mylonites from the Voltri Massif, in which porphyroclasts and matrix record independent deformation events. The microfabrics are analysed using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and olivine porphyroclasts originating from the mantle protolith embedded in a fine-grained matrix consisting mainly of antigorite and minor olivine and pyroxene. The porphyroclasts record brittle and crystal-plastic deformation of the peridotite at upper-mantle conditions and differential stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolutionprecipitation creep resulting in a pronounced foliation of the antigorite matrix, crenulation cleavages and newly precipitated olivine and pyroxene from the pore fluid at sites of dilation, i.e. in strain shadows next to porphyroclasts and folded fine-grained antigorite layers. Antigorite reveals a pronounced associated shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) with the basal (001) cleavage plane oriented in the foliation plane. In monomineralic antigorite aggregates at sites of stress concentration around porphyroclasts, a characteristically reduced grain size and deflecting CPO as well as sutured grain boundaries indicate also some contribution of crystal-plastic deformation and grain-boundary migration of antigorite. In contrast, the absence of any intragranular deformation features in newly precipitated olivine in strain shadows reveals that stresses were not sufficiently high to allow for significant dislocation creep of olivine at conditions at which antigorite is stable. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation of the peridotite with dislocation creep of olivine in the upper mantle probably related to rifting processes, whereas the serpentinite matrix records dominantly dissolution-precipitation creep and low stresses during subduction and exhumation.

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Section: Deformation Processes and Conditions Recorded By Olivine Andsupporting

confidence: 75%

Section: Introductionmentioning

confidence: 99%

The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

Bial

Trepmann

2013

Solid Earth

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“…Shear zones in the Finero Complex are documented by previous structural and microstructural studies [e.g., Lensch , ; Kruhl , ; Altenberger , ; Kenkmann and Dresen , ]. Kruhl and Voll [] distinguished three stages of deformation and metamorphism in the Finero complex.…”

Section: Geological Settingmentioning

confidence: 84%

“…Grieco et al . [] found a first metasomatic event to occur at about 208 Ma, coinciding in time with extensional tectonics recorded by high‐temperature shear zones in the Ivrea‐Verbano Zone [e.g., Brodie , , ; Brodie and Rutter , ; Altenberger , , , ; Zingg et al ., ; Handy and Zingg , ; Handy and Stünitz , ]. Other authors suggest an ancient subduction zone setting, in which fluids infiltrated the mantle wedge [e.g., Lu et al ., ; Zanetti et al ., , ].…”

Section: Geological Settingmentioning

confidence: 98%

The deformation record of olivine in mylonitic peridotites from the Finero Complex, Ivrea Zone: Separate deformation cycles during exhumation

Matysiak

Trepmann

2015

Tectonics

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Mylonitic peridotites from the Finero complex are investigated to detect characteristic olivine microfabrics that can resolve separate deformation cycles at different metamorphic conditions. The heterogeneous olivine microstructures are characterized by deformed porphyroclasts surrounded by varying amounts of recrystallized grains. A well-developed but only locally preserved foam structure is present in recrystallized grain aggregates. This indicates an early stage of dynamic recrystallization and subsequent recovery and recrystallization at quasi-static stress conditions, where the strain energy was reduced such that a reduction in surface energy controlled grain boundary migration. Ultramylonites record a renewed stage of localized deformation and recrystallization by a second generation of recrystallized grains that do not show a foam structure. This second generation of recrystallized grains as well as sutured grain and kink band boundaries of porphyroclasts indicate that these microstructures developed during a stage of localized deformation after development of the foam structure. The heterogeneity of the microfabrics is interpreted to represent several (at least two) cycles of localized deformation separated by a marked hiatus with quasi-static recrystallization and recovery and eventually grain growth. The second deformation cycle did not only result in reactivation of preexisting shear zones but instead also locally affected the host rock that was not deformed in the first stage. Such stress cycles can result from sudden increases in differential stress imposed by seismic events, i.e., high stress-loading rates, during exhumation of the Finero complex.

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“…The metamorphism in the Ivrea zone reaches granulite facies in the lower part and amphibolite facies in the upper part. Pre-Alpine extensional crustal shearing after peak metamorphic conditions in the lower to middle crust produced high-temperature ductile shear zones (Brodie and Rutter 1987, Altenberger 1991, 1995a. Extension probably occurred between the Permian and the Liassic (e.g.…”

Section: Ivrea Zonementioning

confidence: 99%

Strain localization mechanisms in deep-seated layered rocks

Altenberger¹

1997

Geologische Rundschau

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More than 160 shear zones of the Norwegian Caledonides and Italian Southern Alps, formed under high-temperature or high-pressure conditions, are analysed with respect to mechanisms of strain localization. In metabasic rocks with a pre-existing (magmatic) layering strain localizes preferentially in mafic layers, although experiments in monomineralic rocks would predict a location in the weaker feldspar-rich layers. In addition, in some of these zones amphibole recrystallizes dynamically, whereas feldspars only show undulatory extinction. These are evidences for strength inversion between feldspars and mafic minerals. This inverse strength behaviour is caused by a reduced grain size of feldspars and sometimes amphiboles in the pre-existing mafic layers, i.e. prior to shearing. The localization of the shear zones is therefore a grain-size-sensitive process (grain-size-dependent softening), nearly independent of the composition of the deformed material. The numerous amphibole-amphibole contacts favour dynamical recrystallization by grain-boundary migration and the rare feldspar-feldspar contacts prevent or suppress the same process in feldspars. Therefore, the feigned inverse strength is mainly caused by the interaction of reduced grain size and grain-boundary effects in the original rock. This behaviour can lead to the preferential localization of shear zones in mafic layers, although they are not favourably oriented with respect to the stress system.

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Long-term deformation and fluid-enhanced mass transport in a Variscan peridotite shear zone in the Ivrea Zone, northern Italy: a microtextural, petrological and geochemical study of a reactivated shear zone

Cited by 17 publications

References 0 publications

The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

The deformation record of olivine in mylonitic peridotites from the Finero Complex, Ivrea Zone: Separate deformation cycles during exhumation

Strain localization mechanisms in deep-seated layered rocks

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