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

Abstract

A mylonitic quartzite with conjugate and synthetic shear bands was investigated by Electron Backscatter Diffraction and optical microscopy to obtain insights on recrystallization mechanisms and strain localization in quartz at plastic to semibrittle conditions close to the brittle‐ductile transition. The mylonitic quartzite deformed during late Miocene thrusting coeval with contact metamorphism in the high‐strain domains of the Calamita Schists (Elba Island, Italy). Mylonitic deformation occurred from amphibolite to lower greenschist facies conditions during cooling of the aureole. Dynamic recrystallization, dominated by the activity of dislocation creep by prism <a> slip, produced recrystallized quartz layers mantling relic large quartz porphyroclasts. Under decreasing temperature and fluid‐rich conditions, quartz porphyroclasts acted as relatively rigid bodies and fractured along synthetic and conjugate C′ shear bands. Shear bands developed along kinematically favored orientations, just locally assisted by weak crystallographic planes in quartz. Fracturing along shear bands was assisted by cataclasis and fluid infiltration enhancing fracture propagation and healing by recrystallization and authigenesis of new quartz and phyllosilicate grains. The process enhanced the propagation of and strain localization in shear bands, with the development of bands of weak phyllosilicates. Furthermore, we observed the development of a c axis preferred orientation (CPO) related to dissolution and precipitation of new grains with their c axis oriented parallel to shear bands. This study highlights the importance of the interplay between brittle and crystal‐plastic processes and fluid ingress in the semibrittle regime to understand deformation partitioning and strain localization.

Deformation and Material Transfer in a Fossil Subduction Channel: Evidence From the Island of Elba (Italy)

“…This early Miocene collision event is responsible for the current architecture of the nappe stack, characterized by overall N‐S striking and W‐dipping thrusts and foliations (Figures 3a and 3b) and general ENE‐vergence (Figures 2 and 3; Barberi et al., 1969; Keller & Coward, 1996; Massa et al., 2017; Perrin, 1975 and references therein). The continental units, stacked during the Oligocene–early Miocene, comprise from top to bottom (Figure 3a): (a) the anchizone‐facies Tuscan Nappe (illite crystallinity/Δ2 θ = 0.25 – 0.39; Pandeli et al., 2001), (b) the greenschist‐facies Rio Marina Unit (Deschamps et al., 1983), and the amphibolite‐facies (c) Ortano and (d) Calamita Units (Musumeci et al., 2011; Musumeci & Vaselli, 2012; Papeschi & Musumeci, 2019; Papeschi et al., 2017, 2018). Amphibolite‐facies metamorphism—at P < 0.2 GPa and T ≤ 650°C–700°C—developed during the late Miocene emplacement of the igneous rocks in the upper crust which overprinted the structural and metamorphic fabric of the Ortano and Calamita Units (Duranti et al., 1992; Papeschi et al., 2019).…”

Deformation-induced Japan twinning in quartz during incipient mylonitization

“…In many cases, new grains in MSZs show an abrupt change in crystallographic orientation, with a large c-axis misorientation angle, with respect to the host grain, inconsistent with recrystallization by subgrain rotation (SGR). Despite the different interpretations for explaining the crystallographic preferred orientation (CPO) along MSZs (Hippertt and Egydio-Silva, 1996;van Daalen et al, 1999;Vernooij et al, 2006;Trepmann et al, 2007Trepmann et al, , 2017Menegon et al, 2011;Papeschi and Musumeci, 2019), the process remains elusive for many microstructures. The initial stages of deformation of coarse quartz play an important role in shear localization and strain softening and therefore have an influence on the rheology of continental crust rocks (Poirier, 1980;White et al, 1980;Muto et al, 2011).…”

The origin and localization of “vermiculite” along the intra‐terrane shear zones in the Bundelkhand Craton, India: Mechanism and implication

“…Thus, it can be construed that the presence of the vermiculite ± epidote veins played a critical role in the transformation of the brittle fractures into the ductile shear zones in the Bundelkhand granitoid. It is also suggested by earlier workers that without the presence of a fluid phase, which contributes in enhancing fluid-rock interaction, the rheological transformation of precursor fractures to the ductile shear zones is not possible (e.g.,Fossen & Cavalcante, 2017;Mancktelow & Pennacchioni, 2005;Papeschi & Musumeci, 2019;Pennacchioni & Mancktelow, 2007). Although the timing of the brittle fractures vis-à-vis the ductile shear zones remains unclear, it can be ascertained that the percolation of the vermiculite veins through the pre-existing fractures and associated channels predates the nucleation of the ductile shear zones (e.g.,Mancktelow & Pennacchioni, 2005).Therefore, the localization of vermiculite veins is controlled by the fracture system developed within the Bundelkhand granitoid, which paved the way for the percolation of the parent material of the vermiculites (Figure8a).…”