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Low‐temperature frictional characteristics of chlorite‐epidote‐amphibole assemblages: implications for strength and seismic style of retrograde fault zones

Fagereng, Åke and Ikari, Matt J. 2020. Low‐temperature frictional characteristics of chlorite‐epidote‐amphibole assemblages: implications for strength and seismic style of retrograde fault zones. Journal of Geophysical Research. Solid Earth 125 (4) , e2020JB019487. 10.1029/2020JB019487

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Abstract

In retrograde faults exhuming mafic rocks, shearing occurs in metamorphic and/or hydrothermally altered mineral assemblages whose frictional properties are not well known. Here, we present the results of laboratory shearing experiments on chlorite schist, epidotite, and hornblende‐dominated amphibolite and mixtures of these rocks and evaluate their frictional properties and microstructures. The experiments were conducted on powdered rock samples with starting grain size of <125 μm, at room temperature, under fluid‐saturated conditions and applied normal stress of 10 MPa. The results show that chlorite schist is relatively weak (friction coefficient of 0.36), whereas epidotite and amphibolite are strong (friction coefficients of 0.63 and 0.67, respectively). The friction of chlorite schist‐epidotite and chlorite schist‐amphibolite mixtures decreases nearly linearly with increasing chlorite content. Chlorite schist exhibits velocity‐strengthening behavior, epidotite is velocity‐weakening, and the amphibolite shows mostly velocity‐weakening friction. Mixtures show intermediate strength and velocity dependence of friction. Well‐developed striations formed on slip surfaces in samples with ≥50% chlorite schist. The epidotite slip surface exhibits a mixture of very fine particles and coarser crystals. Amphibolite slip surfaces have less very fine grains and are composed of subhedral to euheral needles. Few intragranular fractures are preserved, and we infer wear at contact asperities to be the likely cause of velocity‐weakening in our epidote gouges. Addition of chlorite to epidotite and amphibolite produces a striated slip surface and disrupts contacts between harder grains. Therefore, retrograde chlorite growth is expected to facilitate frictional weakening and stable slip in higher‐grade mineral assemblages exhumed to low‐temperature conditions.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Publisher: American Geophysical Union (AGU)
ISSN: 2169-9313
Date of First Compliant Deposit: 11 June 2020
Date of Acceptance: 2 April 2020
Last Modified: 12 Jun 2020 15:30
URI: http://orca.cf.ac.uk/id/eprint/132351

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