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. 2017 Jun 8;8:15736. doi: 10.1038/ncomms15736

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Copyright © 2017, The Author(s)

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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(a) Water distribution in grain boundaries during strain localization, grain size reduction and related water pumping (top part). While water equally distributes on olivine grain boundaries during incipient strain localization, giving rise to an overall E-type fabric, the fluid subsequently accumulates in ultramylonites because of ‘dynamic' pumping. The blue arrows refer to water migration through grain boundaries. Accordingly, where water is no longer available at grain boundaries, olivine grains are progressively dried out through water exsolution, leading to strain partitioning (middle and right panels) and E-type fabric that is then overprinted by D-type fabric (change in dislocation slip-system). The D-type fabric also arises in ultramylonites as fluid distributes in new grain boundaries. We here attribute water pumping to creep cavitation and related phase nucleation in layers dominated by grain-size-sensitive creep (ultramylonites). (b) Large-scale fluid pumping and strain partitioning in mylonitic complexes. From left to right, water converges towards the centre of the mylonitic complexes, which progressively widen through lateral production of further ultramylonite layers. As a consequence, the olivine fabric switches from an E-type to a D-type LPO in the centre of the protolith and water accumulates in the mylonites, where the density of ultramylonites increases. This gives rise to cracking (black stars) due to high fluid pressure in these zones.