We present an example of interaction between magmatism and tectonics at shallow crustal levels. In the Late Miocene the metamorphic units of the eastern Elba Island (northern Apennines) were intruded at very shallow crustal levels by a large pluton (> 60 km2 ) with the development of an hectometre-sized contact aureole defined by growth of low-pressure/high-temperature mineral assemblages (P max < 0.2 GPa, T max ~650 °C). Structural data show that the contact aureole is associated with a km-sized antiform of the foliation and by several metre-to decametre-thick high-strain domains consisting of strongly foliated rocks containing synkinematic HT/LP mineral assemblages and ductile shear zones of variable thickness. These shear zones are characterized by a mylonitic foliation variably overprinted by cataclasis. Quartz microfabrics indicate that the dynamic crystallization processes progressively changed from grain boundary migration, associated with the thermal peak of contact metamorphism, to subgrain rotation and bulging recrystallization, the latter mostly associated with the cataclastic overprint. These transitions of recrystallization mechanisms in quartz are related to a progressive decrease of temperature during deformation. Deformation accompanied the development and cooling of the contact aureole, which recorded the switch from high temperature ductile to low temperature brittle conditions. The geometry of the studied deformation structures is consistent with the constraints of the regional tectonic evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement of igneous rocks.
Exhumed high-pressure/low-temperature (HP-LT) rocks and terranes occur worldwide in both active and fossil collision zones reflecting complex subduction zone processes (
A network of shear zones that evolved through the brittle-ductile transition is exposed in the Calamita Schists, Elba Island, Italy. The shear zones formed during Late Miocene contractional deformation coeval with high grade contact metamorphism (~650 °C) related to the emplacement of plutonic rocks at shallow crustal levels (~7-10 Km). An early stage high metamorphic grade foliation was overprinted by mylonitic deformation that progressively localized on lowmetamorphic grade shear bands producing S-C mylonites during cooling of contact aureole.Localization of deformation on shear bands was driven by temperature decrease that triggered strain partitioning between 'hard' high grade relics and 'soft' shear bands. Softening of shear bands occurred likely due to fluid influx and retrograde growth of fine-grained phyllosilicates. The interconnection of anastomosing shear bands and passive rotation of the relic high grade foliation caused widening of the shear bands producing mylonites with a composite mylonitic foliation and C' shear bands. An estimate of the vorticity number Wk of the flow of ~0.3 -0.5 was obtained from the orientation of C' shear bands measured at the meso-and thin section-scale. Close to the brittleductile transition, the growth of soft phyllosilicates allowed C' shear bands to act as precursory structures to brittle deformation localized into an array of low-angle faults and shear fractures.
IntroductionShear zones are tabular high-strain zones localized within domains of relatively lower strain that accommodate deformation in the lithosphere (Ramsay and Graham, 1970;Sibson, 1977;Ramsay, 1980;Lister and Snoke, 1984). They are usually distinguished in plastic (ductile or viscous) or brittle shear zones, according to their dominant deformation mechanism (Fossen and Cavalcante, 2017). However, brittle and ductile deformation mechanisms interplay during deformation, accompanying nucleation, growth and evolution of shear zones (e.g. Fusseis et al., 2006;
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