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Fluid-controlled deformation in blueschist facies conditions; plastic vs brittle behaviour in a brecciated mylonite (Voltri Massif, Western Alps, Italy)

Cristina Malatesta, L. Federico, L. Crispini and G. Capponi
Fluid-controlled deformation in blueschist facies conditions; plastic vs brittle behaviour in a brecciated mylonite (Voltri Massif, Western Alps, Italy) (in Birth and death of oceanic basins; geodynamic processes from rifting to continental collision in Mediterranean and circum-Mediterranean orogens, G. Capponi (editor), Andrea Festa (editor) and Gisella Rebay (editor))
Geological Magazine (February 2018) 155 (2): 335-355

Abstract

A blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals. The mylonitic foliation formed at metamorphic conditions of T = 220-310 degrees C and P = 6.5-10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolate-tablet foliation boudinage; (iii) veins; (iv) breccia. The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle-ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overall P-T metamorphic conditions (blueschist-facies). A strong fluid-rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input. Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface. We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or 'slow earthquakes' triggered by high pore-fluid pressure.


ISSN: 0016-7568
EISSN: 1469-5081
Coden: GEMGA4
Serial Title: Geological Magazine
Serial Volume: 155
Serial Issue: 2
Title: Fluid-controlled deformation in blueschist facies conditions; plastic vs brittle behaviour in a brecciated mylonite (Voltri Massif, Western Alps, Italy)
Title: Birth and death of oceanic basins; geodynamic processes from rifting to continental collision in Mediterranean and circum-Mediterranean orogens
Author(s): Malatesta, CristinaFederico, L.Crispini, L.Capponi, G.
Author(s): Capponi, G.editor
Author(s): Festa, Andreaeditor
Author(s): Rebay, Gisellaeditor
Affiliation: University of Genoa, Department of Earth, Environmental and Life Sciences, Genoa, Italy
Affiliation: University of Genova, Department of Earth, Environment and Life Sciences, Genova, Italy
Pages: 335-355
Published: 201802
Text Language: English
Publisher: Cambridge University Press, London, United Kingdom
References: 113
Accession Number: 2018-053018
Categories: Igneous and metamorphic petrologyStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, geol. sketch map
N44°10'00" - N44°40'00", E07°45'00" - E08°15'00"
Country of Publication: United Kingdom
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Abstract, Copyright, Cambridge University Press. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 2018
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