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Evolution of shear zones through the brittle-ductile transition: The Calamita Schists (Elba Island, Italy)
Language
English
Obiettivo Specifico
1T. Struttura della Terra
Status
Published
JCR Journal
JCR Journal
Peer review journal
Yes
Title of the book
Issue/vol(year)
/113 (2018)
Publisher
Elsevier
Pages (printed)
100-114
Issued date
August 2018
Subjects
Structural geology, brittle ductile transition
Abstract
A network of shear zones that evolved through the brittle-ductile transition is exposed in the Calamita Schists,
Elba Island, Italy. The shear zones formed during Late Miocene contractional deformation coeval with high grade
contact metamorphism (∼650 °C) related to the emplacement of plutonic rocks at shallow crustal levels
(∼7–10 Km). An early stage high metamorphic grade foliation was overprinted by mylonitic deformation that
progressively localized on low-metamorphic grade shear bands producing S-C mylonites during cooling of
contact aureole.
Localization of deformation on shear bands was driven by temperature decrease that triggered strain partitioning
between ‘hard’ high grade relics and ‘soft’ shear bands. Softening of shear bands occurred likely due to
fluid influx and retrograde growth of fine-grained phyllosilicates. The interconnection of anastomosing shear
bands and passive rotation of the relic high grade foliation caused widening of the shear bands producing
mylonites with a composite mylonitic foliation and C′ shear bands. An estimate of the vorticity number Wk of the
flow of ∼0.3–0.5 was obtained from the orientation of C′ shear bands measured at the meso- and thin sectionscale.
Close to the brittle-ductile transition, the growth of soft phyllosilicates allowed C′ shear bands to act as
precursory structures to brittle deformation localized into an array of low-angle faults and shear fractures.
Elba Island, Italy. The shear zones formed during Late Miocene contractional deformation coeval with high grade
contact metamorphism (∼650 °C) related to the emplacement of plutonic rocks at shallow crustal levels
(∼7–10 Km). An early stage high metamorphic grade foliation was overprinted by mylonitic deformation that
progressively localized on low-metamorphic grade shear bands producing S-C mylonites during cooling of
contact aureole.
Localization of deformation on shear bands was driven by temperature decrease that triggered strain partitioning
between ‘hard’ high grade relics and ‘soft’ shear bands. Softening of shear bands occurred likely due to
fluid influx and retrograde growth of fine-grained phyllosilicates. The interconnection of anastomosing shear
bands and passive rotation of the relic high grade foliation caused widening of the shear bands producing
mylonites with a composite mylonitic foliation and C′ shear bands. An estimate of the vorticity number Wk of the
flow of ∼0.3–0.5 was obtained from the orientation of C′ shear bands measured at the meso- and thin sectionscale.
Close to the brittle-ductile transition, the growth of soft phyllosilicates allowed C′ shear bands to act as
precursory structures to brittle deformation localized into an array of low-angle faults and shear fractures.
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article
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