Abstract
Abstract. The Pernambuco shear zone (northeastern Brazil) is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarse-grained (> 50 µm) feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (< 15 µm) localized in C' shear bands. Detailed microstructural observations and electron backscatter diffraction (EBSD) analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (∼ 1–2 mm) monomineralic veins transposed along the shear zone foliation or as thin ribbons ( ≤ 25 µm width) dispersed in the feldspathic mixture. The microstructure and c axis crystallographic-preferred orientation are similar in the thick monomineralic veins and in the thin ribbons, and they suggest dominant subgrain rotation recrystallization and activity of prism < a > and rhomb < a > slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the quartz monomineralic veins to the thin ribbons embedded in the feldspathic C' bands (14 µm vs. 5 µm respectively). The fine-grained feldspar mixture has a weak crystallographic-preferred orientation interpreted as the result of shear zone parallel-oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Once C' shear bands were generated and underwent viscous deformation at constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate 1 order of magnitude faster than the monophase quartz monomineralic veins, as evidenced by applying experimentally and theoretically calibrated flow laws for dislocation creep in quartz and diffusion creep in feldspar. Overall, our data set indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase feldspathic material localized much of the strain. Extensive grain-size reduction and weakening of feldspars is attained in the East Pernambuco mylonites mainly via fracturing which would trigger a switch to diffusion creep and strain localization without a prominent role of metamorphic reactions.
Highlights
Strain localization in shear zones is a fundamental process controlling deformation at plate boundaries and strength evolution of the lithosphere
The proportion and distribution of individual phases were studied on grain boundary maps constructed on SEM-Backscattered electron (BSE) digitized images and on maps derived with electron backscatter diffraction (EBSD) using the Image SXM software package
In the mylonitic granitoid of the Eastern Pernambuco Shear Zone, NE Brazil, grain-size reduction of feldspars was attained via fracturing at P, T conditions typical of the middle crust (∼ 500–550 ◦C, 5 kbar)
Summary
Strain localization in shear zones is a fundamental process controlling deformation at plate boundaries and strength evolution of the lithosphere. The development of localized highstrain zones requires some form of strain weakening as the deformation progresses. Many studies have highlighted the important role of progressive grain-size reduction in promoting weakening and strain localization in the lithosphere (Warren and Hirth, 2006; Herwegh et al, 2011; Kilian et al, 2011; Bercovici and Ricard, 2012; Montesi, 2013; Platt, 2015). Investigating grainsize reduction mechanisms and microstructural modifications during development of shear zones is of paramount importance for our understanding of lithosphere deformation
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have