Deformation Processes, Textural Evolution and Weakening in Retrograde Serpentinites

Abstract

Serpentinites play a key role in controlling fault rheology in a wide range of geodynamic settings, from oceanic and continental rift zones to subduction zones. In this paper, we provide a summary of the most common deformation mechanisms and frictional strengths of serpentine minerals and serpentinites. We focus on deformation mechanisms in retrograde serpentinites, which show a progressive evolution from undeformed mesh and bastite pseudomorphic textures to foliated, ribbon-like textures formed by lizardite with strong crystallographic and shape preferred orientations. We also discuss the possible mechanical significance of anastomosing slickenfibre veins containing ultraweak fibrous serpentines or relatively strong splintery antigorite. Our review and new observations indicate that pressure solution and frictional sliding are the most important deformation mechanisms in retrograde serpentinite, and that they are frictionally weak (µ~0.3). The mineralogical and microstructural evolution of retrograde serpentinites during shearing suggests that a further reduction of the friction coefficient to µ of 0.15 or less may occur during deformation, resulting in a sort of continuous feedback weakening mechanism.

The internal structure and composition of a plate boundary-scale serpentinite shear zone: The Livingstone Fault, New Zealand

“…11a,b). The porphyroclasts consist of interpenetrating blades of antigorite, a texture that has previously been interpreted as characteristic of prograde metamorphic serpentinites (Wicks and Whittaker, 1977;Wicks, 1984;Viti et al, 2018). The porphyroclasts are surrounded by a matrix of chrysotile, fine-grained lizardite and/or polygonal serpentine (these two varieties are indistinguishable with Raman; Fig.…”

“…The mesh textures show signs of developing into 'ribbon textured' serpentinite, where portions of the mesh structure are preferentially dissolved to form elongate lensoid ribbons ( Fig. 9b) (Viti et al, 2018). In such 10 areas, porphyroclastic magnetite ( Fig.…”

“…The porphyroclasts are surrounded by a matrix of chrysotile, fine-grained lizardite and/or polygonal serpentine (these two varieties are indistinguishable with Raman; Fig. 11b; Tarling et al, 2018). The long axes of the porphyroclasts are subparallel to the scaly foliation, and the porphyroclasts are "truncated" along their foliation-parallel 10 margins by surfaces that are enriched in thin layers or aggregates of lizardite/polygonal serpentine (Fig.…”

“…metasomatism) processes that result in localised brittle deformation within creeping shear zone segments.Solid Earth Discuss., https://doi.data from serpentine minerals and serpentinite fault rocks. Much of this work has been summarised in a series of recent review papers (Guillot and Hattori, 2013; Hirth and Guillot, 2013; Reynard, 2013;Guillot et al, 2015;Viti et al, 2018). However, extrapolation and up-scaling of such data to natural serpentinite shear zones remains problematic because: 1) serpentinite shear zones are structurally and lithologically 5 complex (Strating and Vissers, 1994; Hermann et al, 2000;Singleton and Cloos, 2012), and their physical properties are therefore not adequately represented by laboratory measurements that are often made on small monomineralic samples, and;2) serpentine itself has several sub-types that are challenging to identify unambiguously, particularly in a way that preserves the typically intricate textural relationships between the sub-types.…”

Fault Initiation in Serpentinite

“…However, the mechanisms that control the evolution of strength during the development of serpentinite‐hosted faults are poorly understood. Recent studies focus on deformation of serpentine minerals in the laboratory (Amiguet et al, ; Escartin et al, ; Hilairet et al, ; Moore et al, ; Moore & Lockner, ; Tesei et al, ) and deformation fabrics of naturally deformed serpentinite (Hirauchi & Yamaguchi, ; Tarling et al, ; Viti et al, ). However, much of the inherent complexity in serpentinite‐hosted faults cannot be reproduced in a laboratory, and there is substantial variability in naturally occurring deformation textures (e.g., Wicks, ; Quesnel et al, ).…”