Microstructural evolution and rheology of quartz in a mid-crustal shear zone Jeffrey M. Rahl a, ⁎, Philip Skemer b a Department of Geology, Washington and Lee University, Lexington, VA 24450, USA b Department of Earth and Planetary Sciences, Washington University in St. Louis, Saint Louis, MO 63130, USA abstract article info Article history: Received 28 June 2015 Received in revised form 27 April 2016 Accepted 10 May 2016 Available online 13 May 2016 We present microstructural and crystallographic preferred orientation (CPO) data on quartz deformed in the middle crust to explore the interaction and feedback between dynamic recrystallization, deformation processes, and CPO evolution. The sample investigated here is a moderately deformed quartz-rich mylonite from the Blue Ridge in Virginia. We have created high-resolution crystallographic orientation maps using electron backscatter diffraction (EBSD) of 51 isolated quartz porphyroclasts with recrystallized grain fractions ranging from 10 to 100%. Recrystallized grains are internally undeformed and display crystallographic orientations dispersed around the orientation of the associated parent porphyroclast. We document a systematic decrease in fabric intensity with recrystallization, suggesting that progressive deformation of the recrystallized domains involves processes that can weaken a pre-existing CPO. Relationships between recrystallization fraction and shear strain suggest that complete microstructural re-equilibration requires strains in excess of γ = 5. Variation in the degree of re- crystallization implies that strain was accumulated heterogeneously, and that a steady-state microstructure and rheology were not achieved. © 2016 Elsevier B.V. All rights reserved. Keywords: Quartz CPO Mylonite Deformation mechanisms Recrystallization 1. Introduction Quartz exerts an important control on the rheology of the continen- tal crust, and under certain conditions it may represent the strongest mechanical element of the lithosphere (e.g., Kohlstedt et al., 1995; Bürgmann and Dresen, 2008). At conditions typical of the middle to lower crust, quartz deformation generally proceeds by dislocation creep processes (Hirth and Tullis, 1992). As dislocations accumulate in a crystal, the associated strain energy promotes dynamic recrystalliza- tion, in which the migration of grain boundaries and/or the migration of dislocations to sub-grain boundaries creates crystals with a lower free dislocation density (e.g., Urai et al., 1986). Likewise, during disloca- tion creep, a crystallographic preferred orientation (CPO) is developed. The preferential alignment of grains with particular crystallographic ori- entation may introduce anisotropy in both seismological and rheologi- cal properties (e.g., Kocks et al., 2000). Feedbacks between deformation processes, dynamic recrystallization, and CPO develop- ment are known to have a profound effect on the rheology and anisot- ropy of many important minerals in Earth, including olivine (Warren and Hirth, 2006; Précigout et al., 2007; Knoll et al., 2009; Kaczmarek and Tommasi, 2011; Skemer et al., 2013), pyroxene (Raimbourg et al., 2008; Skemer and Karato, 2008), and feldspar (Jiang et al., 2000; Mehl and Hirth, 2008). Although the effects of dynamic recrystallization on the rheology of quartz-rich shear zones are likely to be profound, significant questions persist. Grain-size reduction due to dynamic recrystallization may in- duce strain-weakening through transitions to grain-size sensitive defor- mation mechanisms (Rutter and Brodie, 1988; Kilian et al., 2011a; Linckens et al., 2011), but due to uncertainties with empirical flow laws the deformation conditions where this occurs are poorly known (Behr and Platt, 2011). CPO is commonly used to draw inferences about deformation, including deformation temperature (Schmid and Casey, 1986; Stipp et al., 2002b; Toy et al., 2008; Law, 2014), shear sense (Kilian et al., 2011b; Toy et al., 2012), or strain (Carreras and García Celma, 1982). However, experimental data relating dynamic re- crystallization to strain and CPO evolution are limited (Heilbronner and Tullis, 2002; Heilbronner and Tullis, 2006; Muto et al., 2011), and it is unclear how quartz CPO evolves over a wide range of deformation conditions and deformation mechanisms. In this contribution, we use a quartz-rich mid-crustal mylonite as a natural laboratory to investigate the coupled processes of dynamic re- crystallization, grain-size reduction, and the evolution of CPO. The sam- ple targeted in this study is from the Rockfish Valley Deformation Zone in the Blue Ridge province of Virginia, a 1–3 km thick zone of mylonitic deformation representative of mid-crustal shear zones (Bailey and Simpson, 1993). Isolated quartz porphyroclasts within the sample Tectonophysics 680 (2016) 129–139 ⁎ Corresponding author. E-mail address: [email protected] (J.M. Rahl). http://dx.doi.org/10.1016/j.tecto.2016.05.022 0040-1951/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto
Microstructural evolution and rheology of quartz in a mid-crustal shear zone
Jun 23, 2023
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