Quantifying the Rheology of Quartz-bearing Polyphase aggregates deformed under mid-crustal conditions: An EBSD-based Application

Microstructural observations such as grain and subgrain sizes, pole figures (CPO), grain boundary irregularities, and thermometry (opening angles, Ti-in-Quartz) from the mineral quartz have become one of the most reliable proxies for deciphering the mechanical properties, including stress, strain rate, and deformation conditions (temperature) of the continental crust. These relationships were constrained from experimental studies on monomineralic quartz aggregates. Consequently, many field-based studies from continental shear zones focus on analyzing sporadically occurring quartzites and quartz veins. However, crustal rocks are predominantly polymineralic, yet, for simplification, most rheological models rely on homogeneous single-phase approximations. Interactions among multiple mineral phases can disrupt steady-state grain sizes, leading to violations of the piezometric relationships commonly applied to quartz mylonites. Experimental studies further show that polymineralic aggregates deform at significantly lower stresses than their monomineralic counterparts, implying that previous studies have likely overestimated the strength of the crust. In addition, experiments demonstrate that the presence of a secondary phase results in markedly different quartz CPO from that expected in single-phase quartzite. These observations raise an important question: to what extent can quantitative microstructural data from polymineralic rocks be used to infer realistic mechanical properties of the continental crust? Addressing this gap is crucial for developing rheological models that accurately reflect the deformation processes occurring in nature.

In this study, we focus on performing high-resolution EBSD analysis of quartz-bearing mylonites formed from metapelites and granites during thrust-sense shearing along the Main Central Thrust (MCT) shear zone (Western Himalaya, India), which runs along the entire Himalayan Mountain belt. From south to north, these samples record an increasing peak-metamorphic temperature and pressure condition; from 535 °C and 5.8 kbar to 683 °C and 11 kbar. Although strain is inhomogeneously distributed within the ~4 km thick shear zone, an overall increase in deformation intensity is recorded towards the north. The Crystallographic Vorticity Axis (CVA) analysis of quartz reveals monoclinic simple-shear flow kinematics consistent with earlier studies; however, the secondary phases (plagioclase) exhibit pure shear-dominated deformation. Depending on the proportion of the secondary phase (30 to 70%), quartz grains form either continuous layers (monophase domain) or isolated quartz aggregates (polyphase domain). Overall, the CPO pattern in the monophase domain exhibits a transition from a type-II crossed-girdle to an asymmetric type-I pattern, towards the north. The mixed polyphase domain exhibits a random CPO. Within the monophase domains, the fabric strength (M-index, B-index) is higher for the thicker domains (> 163μm). Thereafter, we segregate our analysis into two types of quartz grains: (i) quartz surrounded by quartz grains (Q-Q), and (ii) quartz surrounded by other phases (Q-S). Within a thin section, the Q-Q grains exhibit higher fabric strength, larger recrystallized grain sizes, but lower aspect ratios compared to the Q-S grains. The low-angle boundary density increases towards the north (0.0042 to 0.0095µm^-1), but the density is always higher for Q-Q grains than Q-S grains. Our study suggests Q-Q grains can be used for piezometry. We will discuss these results in terms of deformation mechanisms and strain partitioning between mono and polyphase domains.