Strain localization within polymineralic mid- to lower-crustal rocks: from melt injection to fluid-present solid-state flow

Mid- to lower-crustal shear zones accommodate large strains by high-temperature viscous flow. Yet, strain localization is strongly modulated by transient thermal and chemical perturbations such as the presence of syn-kinematic melts or fluids.

We aim at unravelling the general strain localization behavior, the role of fluid/melt presence on the rheology of polymineralic shear zones, with focus on potential changes from mid- to lower-crustal levels.

For this purpose, we use the Cossato-Mergozzo-Brissago (CMB) and the Pogallo shear zone systems in the Southern Alps, Northern Italy (Handy, 1987) as a natural laboratory. Field observations and targeted sampling were combined with quantitative microstructural analysis of polymineralic mylonites and ultramylonites. Quartz paleopiezometry (monomineralic quartz bands) provides differential stress and Ti-in-biotite (Henry et al., 2005) provides temperature for felsic lithologies. Such data from natural mylonites are used as input for granitoid shear-zone flow laws (Nevskaya et al., 2025b) to derive strain rates and compare rheology across crustal depths.

Field and microstructural observations indicate two endmember microfabric types. Type I (CMB) fabrics occur within a broad (~2-3 km) belt of felsic mylonites (grain size ~50-100 µm). Inside the mylonites, many dykes developed with episodic pulses of melt injection and syn-kinematic back veining (Handy and Streit, 1999). These mylonites commonly contain quartz-feldspars-mica domains with steady-state grain sizes stabilized by pinning and dissolution-precipitation processes. Ti-in-biotite thermometry indicates lower-crustal temperatures of ~680-730 °C.

Type II (Pogallo) fabrics also represent microstructural steady states characterized by fine- to ultrafine-grained ultramylonites (grain size <5 µm). These fabrics developed in narrower (~500 m) shear zones, where dykes are clearly pre-kinematic boudinaged/lenticular, indicating pure solid-state deformation. Recrystallization of hydrous phases (sheet silicates) and the occurrence of syn-kinematic quartz veins indicate presence of aqueous fluids during shearing. Type II fabrics are consistent with deformation at lower temperatures and/or higher strain rates relative to Type I fabrics. New geo-thermobarometry data yields pressure-temperature estimates indicating deformation at ~5-6 kbar and ~550-600 °C.

In sum, both Type I and Type II polymineralic fabrics record microstructural steady states in which grain size is stabilized by cycles of nucleation and growth, grain-boundary pinning by neighbouring phases, and dissolution-precipitation processes. Systematic decreases in steady-state grain size correlate with changing deformation conditions (temperature and differential stress). These observations are consistent with recent deformation experiments on granitoid ultramylonites, in which pinning-controlled dissolution-precipitation creep (pc-DPC) was identified as a dominant deformation mechanism (Nevskaya et al., 2025a, b). By combining our constraints on deformation conditions and microstructural parameters with the granitoid flow law of Nevskaya et al. (2025b), we assess how strain localization and effective rheology evolve from mid- to lower-crustal levels.