The brittle/ductile transition at upper crustal level: geometry, strain partitioning and fluid circulation. The case study of the Calamita Unit (Elba Island, Northern Apennines, Italy)

This study focuses on the detailed investigation of the brittle/plastic transition in quartz-feldspathic rocks at upper-middle crustal conditions and aims to better understand the role of fluids and temperature during deformation. The Calamita Unit (Elba Island, Italy) is a high metamorphic grade unit (T ~ 650 °C) that has been intruded by a monzogranite body at shallow crustal level (P < 0.2 GPa) and coevally deformed during regional shortening for a limited time span (< 1 Ma). These conditions outline the Calamita Unit as an ideal case study to investigate the brittle/plastic transition at approximately constant pressure (i.e. depth) during temperature decrease, reproducing upper-middle crustal conditions. The Calamita Schists are a metapsammitic complex exposed in the lower part of the Calamita Unit. Pseudosection modelling and Ti-in-biotite thermometry constrain the peak metamorphic assemblage, marked by andalusite + cordierite + biotite + K-feldspar, at upper amphibolite facies conditions (T ~ 600 – 700 °C; P ~ 0.2 GPa), with microstructures suggesting partial melting. The retrograde path is constrained by chlorite geothermometry in the 300 – 500 °C temperature range. Detailed field mapping and structural analysis have revealed at map-scale a pattern of heterogeneous deformation characterized by west-dipping high-strain domains localizing eastward-directed deformation interleaved with relatively low-strain domains. In high-strain domains, mylonitic fabrics are, in turn, overprinted in the brittle regime by non-Andersonian subhorizontal faults associated with Riedel shears networks, formed subparallel to C’ shear bands. Microstructural analysis highlights that temperature decrease and fluid influx controlled the mechanical evolution of the investigated rocks, which are marked by the transition from a high-metamorphic grade foliation to shear bands and mylonites with widespread S-C and S-C’ fabrics, characterized by retrograde, synkinematic white mica and chlorite. Quartz microfabric displays an evolution from fast grain boundary migration, developed close to peak metamorphic conditions, to subgrain rotation and bulging recrystallization, tracking decreasing temperature during deformation. During decreasing temperature, deformation localized in mylonitic quartz ribbons at amphibolite facies conditions (450 °C < T < 600 °C), where recrystallization was accommodated by dislocation creep of quartz under dominant prism slip, causing the development of strong Y-maximum crystallographic preferred orientations (CPO). Secondary rhomb and acute rhomb slip assisted the recrystallization of grains unfavorably oriented for prism slip, with the activation of slip systems whose misorientation axis lies close to the vorticity axis. At greenschist facies conditions (300 °C < T < 450 °C), deformation localized in phyllonitic domains, producing phase mixing of phyllosilicates and tiny quartz grains. Relic, large quartz grains hardened and fractured along synthetic and conjugate shear bands. The propagation of shear bands occurred under fluid-rich conditions and was controlled by cyclic fracturing and precipitation of new quartz and phyllosilicate grains, deposited by circulating fluids. Precipitated new quartz grains developed a CPO parallel to shear bands controlled by the opening of dilatant sites. The nucleation of fine-grained quartz and ‘soft’ phyllosilicates enhanced strain softening and assisted strain partitioning into localized C’ shear bands at the brittle/plastic transition. In the brittle regime (T ~ 300 °C), deformation localized on previously formed C’ shear bands, favorably oriented for reactivation, that acted as ductile precursors for misoriented non-Andersonian faults. Brittle deformation in fault zones was controlled by the cyclical interaction between fracturing, taking advantage of weak crystallographic planes in quartz such as the rhombs, and fluid infiltration, assisting the precipitation of new quartz and phyllosilicate grains, lacking a clear preferred orientation. The data presented in this thesis highlight the role played by fluids during deformation of quartz-feldspathic rocks at the brittle/plastic transition, that efficiently control (1) strain softening of ‘stiff’ domains and (2) strain localization into shear bands that have the potential to act as precursors for non-Andersonian fault zones. The proposed model predicts the development of brittle structures discordantly overprinting ductile fabrics developed in the same kinematic regime, which bears implications for the tectonic evolution of rock volumes (i.e. tectonic units and/or metamorphic complexes) exhumed though the brittle/plastic transition.