What Causes Shear Band Widening? - an EBSD Based Analysis

Shear band widening has been extensively studied in the past, and work hardening at the core is classically assigned as the cause. 'Knife-sharp' ductile shear bands in mid-crustal granitic rocks often originate from brittle precursors and fluids that percolate through the weak planes usually provide the required ductility. In nominally hydrous minerals like quartz and feldspar, presence of water triggers the rheological switch from viscoelastic in the incipient brittle fractures to viscoplastic. Quartz easily accommodates strain under hydrous greenschist to lower amphibolite facies conditions, whereas feldspars resist it. We studied two shear bands of different thicknesses (SB1: 3mm, SB2: 4.5mm) hosted within two granite samples to assess the role of contrasting resistance to yield in promoting shear zone widening. The core of SB1 is dominated by relict quartz ribbons, whereas that of the SB2 mainly consists of fine-grained recrystallized plagioclase. Thin, thread-like networks of fine biotite grains, deflected by the quartz and feldspar grains, are present within both SB1 and SB2. They are parallel to the shear band margins and suggest fluid activity, which is also supported by the coalesced pores on quartz grain walls and pitted feldspar grain surfaces visible on split surface SEM images. CPO studies and deformation mechanism maps (DMMs) collectively suggest that dislocation and diffusion creep mechanisms deformed the quartz and plagioclase grains, respectively, and the latter accommodated strain 100 times faster than the former. Consequently, the weaker plagioclase grains were more prone to hardening than quartz. We propose that finer grainsize of the plagioclase grains in SB2 and the corresponding higher differential stresses pumped the fluid out of the incipient shear band core and into the adjacent 'drier' regions, promoting shear band widening over time.