Fabric controls on strain accommodation in naturally deformed mylonites: the influence of interconnected micaceous layers

Nicholas J R Hunter, Pavlina Hasalova, Roberto F Weinberg, Christopher J L Wilson

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21 Citations (Scopus)

Abstract

We present microstructural analyses demonstrating how the geometrical distribution and interconnectivity of mica influences quartz crystallographic preferred orientation (CPO) development in naturally deformed rocks. We use a polymineralic (Qtz + Pl + Kfs + Bt + Ms ± Grt ± Tur) mylonite from the Zanskar Shear Zone, a section of the South Tibetan Detachment (NW Himalaya), to demonstrate how quartz CPO intensity decreases from quartz-dominated domains to micaceous domains, independently of whether or not quartz grains are pinned by mica grains. We then use a bimineralic (Qtz + Ms) mylonite from the Main Central Thrust (NW Himalaya) to show how increasing mica grain connectivity is concomitant with a systematic weakening of quartz CPO. Our results draw distinctions between CPO weakening due to: (i) second phase drag, leading to ineffective recovery in quartz; and (ii) increased transmission and localisation of strain between interconnected mica grains. In the latter case, well-connected micaceous layers take up most of the strain, weakening the rock and preventing straining of the stronger quartz matrix. Our findings suggest that rock weakening in quartz-rich crustal rocks is influenced not only by the presence of mica-rich layers but also the degree of mica grain connectivity, which allows for more effective strain localization through the entire rock mass.
Original languageEnglish
Pages (from-to)180-193
Number of pages14
JournalJournal of Structural Geology
Volume83
DOIs
Publication statusPublished - 2016

Keywords

  • Crystallographic preferred orientation
  • Main Central Thrust
  • Mylonite
  • Second phase
  • Strain localisation
  • Strain partitioning
  • Zanskar shear zone

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