The deformation record of olivine in mylonitic peridotites from the Finero Complex, Ivrea Zone: Separate deformation cycles during exhumation

Abstract

Mylonitic peridotites from the Finero complex are investigated to detect characteristic olivine microfabrics that can resolve separate deformation cycles at different metamorphic conditions. The heterogeneous olivine microstructures are characterized by deformed porphyroclasts surrounded by varying amounts of recrystallized grains. A well-developed but only locally preserved foam structure is present in recrystallized grain aggregates. This indicates an early stage of dynamic recrystallization and subsequent recovery and recrystallization at quasi-static stress conditions, where the strain energy was reduced such that a reduction in surface energy controlled grain boundary migration. Ultramylonites record a renewed stage of localized deformation and recrystallization by a second generation of recrystallized grains that do not show a foam structure. This second generation of recrystallized grains as well as sutured grain and kink band boundaries of porphyroclasts indicate that these microstructures developed during a stage of localized deformation after development of the foam structure. The heterogeneity of the microfabrics is interpreted to represent several (at least two) cycles of localized deformation separated by a marked hiatus with quasi-static recrystallization and recovery and eventually grain growth. The second deformation cycle did not only result in reactivation of preexisting shear zones but instead also locally affected the host rock that was not deformed in the first stage. Such stress cycles can result from sudden increases in differential stress imposed by seismic events, i.e., high stress-loading rates, during exhumation of the Finero complex.

Porphyroclasts: Source and Sink of Major and Trace Elements During Deformation-Induced Metasomatism (Finero, Ivrea-Verbano Zone, Italy)

“…Altenberger [15] suggested that the shear zones were related to the Variscan cycle. Matysiak and Trepmann [33] argued that the development of mylonites was connected to extensional tectonics in Jurassic times by analogy with what was reported for other shear zones in the Ivrea-Verbano Zone. The rare and unexpected occurrence of a single zircon grain within the fine-grained matrix of an ultramafic ultramylonitic sample (30F; Figure 10b,c) allowed us to discuss the age of activity of the shear zone.…”

“…Chromite-rich layers/pockets mainly occur in dunite, where they are broadly parallel to the high-temperature (HT) foliation [19]. In recent decades, the deformation processes within the mantle peridotites (Ph-Pd) and mafic/ultramafic rocks of the Finero Mafic Complex were the subject of multiple microstructural [1,2,[14][15][16][28][29][30][31][32][33] and geochemical/geochronological studies [17][18][19][20][21][22][23][24][25]27,33,34]. According to Tommasi et al [16] the mantle peridotite experienced a pervasive HT deformation in the presence of hydrous melts with crystallization of pargasite at T of about 980-1080 • C and P <2 GPa.…”

“…The Finero Complex was affected by different pre-Alpine tectonic events under granuliteto greenschist-facies conditions producing pervasive or localized foliations and isoclinal to open folds [1,15,16,28,29,32,41,47]. Discrete shear zones and pseudotachylites have received considerable attention over the past 40 years [1,15,16,29,33], but the age of the different deformation events is still debated. Altenberger [15] suggested that the shear zones were related to the Variscan cycle.…”

Strain Localization in the Root of Detachment Faults at a Melt‐Starved Mid‐Ocean Ridge: A Microstructural Study of Abyssal Peridotites From the Southwest Indian Ridge

“…Stresses estimated based on recrystallized grain sizes (80–270 MPa) are higher than those inferred in the MDZs from the Bogota Transform. Preservation of small grain size (1.5–84 µm) may result from rapid exhumation after deformation and recrystallization (Matysiak & Trepmann, 2015), which is more likely in an extensional than in a transform setting. Postseismic relaxation characteristic times are small relative to those associated with the activity of the detachment faults and we do not observe evidence for a peak in stress followed by a relaxation period.…”

Formation of the Iberian‐European Convergent Plate Boundary Fault and Its Effect on Intraplate Deformation in Central Europe

“…During exhumation and cooling of rocks the deformation conditions change and inherited ductile shear fabrics may not remain weak in the changing environment. Moreover, weak shear fabrics may be annealed during quasi‐static recrystallization processes, which would also help to recover the strength of the lithosphere (e.g., Matysiak & Trepmann, ; Trepmann et al, ). We therefore propose that the lithospheric‐scale reactivation of a rift‐inherited extensional detachment fault is unlikely and was probably restricted to the upper ~5–10 km.…”