Major softening at brittle‐ductile transition due to interplay between chemical and deformation processes: An insight from evolution of shear bands in the South Armorican Shear Zone

Abstract

The formation of S‐C/C′ fabrics in the South Armorican Shear Zone has been evaluated by detailed microstructural study where the focus was given to initiation and early evolution of the C/C′ fabric shear bands. Our observations suggest that the S‐C/C′ fabrics formed at distinct temperature conditions indicating >550°C for the S fabric and 300–350°C at 100–400 MPa for the C/C′ fabric shear bands. The evolving microstructure within shear bands documents switches in deformation mechanisms related to positive feedbacks between deformation and chemical processes and imposes mechanical constraints on the evolution of the brittle‐ductile transition in the continental transform fault domains. Three stages of shear band evolution have been identified. Stage I corresponds to initiation of shear bands via formation of microcracks with possible yielding differential stress of up to 250 MPa. Stage II is associated with subgrain rotation recrystallization and dislocation creep of quartz and coeval dissolution‐precipitation creep of microcline. Recrystallized quartz grains show continual increase in size and decrease in stress and strain rates from 94 MPa to 17–26 MPa and 3.8 × 10−12 s−1–8 × 10−14 s−1 associated with deformation partitioning into weaker microcline layer and shear band widening. The quartz mechanical data allowed us to set some constrains for coeval dissolution‐precipitation of microcline which at our estimated pressure‐temperature conditions suggests creep at 17–26 MPa differential stress and 3.8 × 10−13 s−1 strain rate. Stage III is characterized by localized slip along white mica bands accommodated by dislocation creep at strain rate 3.8 × 10−12 s−1 and stress 9.36 MPa. Our mechanical data point to dynamic evolution of the studied brittle‐ductile transition characterized by major weakening to strengths ~10 MPa. Such nonsteady state evolution may be common in crustal shear zones especially when phase transformations are involved.

Fluid‐Assisted Strain Localization in Quartz at the Brittle/Ductile Transition

“…This occurred under greenschist facies conditions, as indicated by the white mica + chlorite + opaque assemblage that is invariably found on shear bands (Figure 5), although early precursory cracking might have opened even earlier. A similar process was documented in quartz-feldspathic rocks of the South Armorican Shear Zone by Bukovská et al (2016), who described shear band propagation by fluid infiltration promoting reaction softening at lower greenschist facies conditions. Transient surges in pore fluid pressure or strain rate may have also promoted fracturing and fluid ingress, followed by recrystallization and precipitation of new grains, as shown, for example, by Kjøll et al (2015).…”

“…As slip accumulated on synthetic C′ shear bands, more fluids were called in, activating a positive feedback mechanism that enhanced lubrication by continuous authigenesis of soft phyllosilicates organized in subparallel bands (Figures 3c and 3d). A similar process was documented in quartz-feldspathic rocks of the South Armorican Shear Zone by Bukovská et al (2016), who described shear band propagation by fluid infiltration promoting reaction softening at lower greenschist facies conditions. Finally, following the embrittlement of the system below the brittle-ductile transition for quartz (i.e., 310 ± 30°C; Stöckhert et al, 1999;280 ± 30°C;Stipp et al, 2002), the presence of bands of phyllosilicates in shear bands oriented obliquely to the mylonitic foliation formed a weak network acting as ductile precursors (Figure 1c) for faults and shear fractures (Figure 1d), as documented by Papeschi et al (2018) and, in the studied sample, by shear fractures locally connecting and reactivating shear bands (Figure 5d).…”

“…Such evolution is typical of low-grade quartz-feldspathic and schistose rocks and has been largely described in the recent geologic literature (Berthé et al, 1979;Bukovská et al, 2016;van Daalen et al, 1999;Gapais & White, 1982;Gapais, 1989;Gillam et al, 2014;Nishikawa & Takeshita, 1999;. We have shown how cooling promotes general strain hardening forcing deformation partitioning between the mylonitic foliation and sets of shear bands.…”

“…We have shown how cooling promotes general strain hardening forcing deformation partitioning between the mylonitic foliation and sets of shear bands. Such evolution is typical of low-grade quartz-feldspathic and schistose rocks and has been largely described in the recent geologic literature (Berthé et al, 1979;Bukovská et al, 2016;van Daalen et al, 1999;Gapais & White, 1982;Gapais, 1989;Gillam et al, 2014;Nishikawa & Takeshita, 1999;. Other authors also documented the important role played by shear bands in crystalline basements and large shear zones that act as ductile precursors for the development of misoriented, non-Andersonian, faults (Bistacchi et al, 2012;Bolognesi & Bistacchi, 2018;Butler et al, 2008;Ikari et al, 2015;Massironi et al, 2011).…”

“…The precipitation of very fine grained quartz and, in particular, weak phyllosilicates may have assisted progressive strain softening of shear bands, enhancing grain size reduction (e.g., Behrmann & Mainprice, 1987;Fliervoet et al, 1997;White et al, 1980) and reaction softening (e.g., Bukovská et al, 2016;Stünitz & Tullis, 2001;White et al, 1980). In particular, inclusions of phyllosilicates organized in bands (Figure 5e) are widespread along synthetic C′ shear bands that occur indeed not restricted to large quartz porphyroclasts ( Figure 3c) and show larger eastward displacements with respect to the conjugate set.…”