Strain Partitioning in Crustal Shear Zones: the Effect of Interconnected Micaceous Layers on Quartz Deformation

Many studies model the strength of deforming crust on the assumption that abundant and interconnected quartz defines the weakest phase and controls the bulk rheology. This simplification discards rheological variation resulting from strain partitioning into well-developed layers of other weak phases, such as micas. This is important, as recent analytical models have suggested the dominant role of schists in strain localisation. We assessed changes in quartz deformation behaviour based on the geometric and spatial arrangement of interconnected mica, in two sets of naturally deformed rocks: Ms-quartzite mylonites (Main Central Thrust, Himachal Pradesh, NW Indian Himalayas) and polymineralic Bt-granite mylonites (El Pichao Shear Zone, Sierras Pampeanas, NW Argentina). Both Ms-quartzite and Bt-granite samples contain fabric domains where micas are well-connected. Several fabric domains were chosen in each rock type, based on the modal proportions, spacing, shape and connectivity of micas. In domains where mica connectivity is poor, quartz exhibits well-defined <a>-rhomb maxima, indicative of deformation via dislocation creep, whereas in domains with high mica connectivity quartz exhibits weak CPO maxima with evidence of several active slip systems. We suggest that with increased mica connectivity: (i) strain compatibility requirements force the activation of secondary slip systems in quartz; and (ii) the efficacy of strain accommodation in quartz is lessened, resulting in a weaker CPO. Decreased spacing between micas may also play a minor role in inhibiting effective recovery in quartz. Changes in quartz deformation emerge as functions of both mica pinning and increased localisation of strain between sheared micas. We further suggest that the dominant phase controlling crustal rheology can switch from quartz to mica where there is development of inter-connected micaceous layers during deformation.