Heterogeneous brittle-ductile deformation at shallow crustal levels under high thermal conditions: The case of a synkinematic contact aureole in the inner northern Apennines, southeastern Elba Island, Italy

Abstract

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.

New Constraints on the Evolution of the Inner Northern Apennines by K‐Ar Dating of Late Miocene‐Early Pliocene Compression on the Island of Elba, Italy

“…For the purpose of this study, we adopt a simplified classification of the nappe stack, which can be subdivided into an Upper and a Lower Complex separated by the Capo Norsi‐Monte Arco Thrust (CN‐MAT), a N‐S trending and west dipping, top‐to‐the‐east thrust fault. A Mid‐Ordovician basement with Jurassic‐Oligocene cover rocks and a Carboniferous basement with Triassic‐Jurassic metasediments (referred to as Ortano and Calamita Unit, respectively; e.g., Musumeci et al, ; Papeschi et al, ) form the footwall to the CN‐MAT. Their regional metamorphic grade reaches greenschist to upper amphibolite facies.…”

Fluid‐Assisted Strain Localization in Quartz at the Brittle/Ductile Transition

“…We have shown how cooling promotes general strain hardening forcing deformation partitioning between the mylonitic foliation and sets of shear bands. We document an example of ductile-brittle shear zones, where the progressive evolution from (1) west dipping mylonites to (2) S-C mylonites with subhorizontal shear bands reactivated as faults occurred in a very short time span (<1 Ma; Musumeci et al, 2015;Papeschi et al, 2017). Other authors also documented the important role played by shear bands in crystalline basements and large shear zones that act as ductile precursors for the development of misoriented, non-Andersonian, faults (Bistacchi et al, 2012;Bolognesi & Bistacchi, 2018;Butler et al, 2008;Ikari et al, 2015;Massironi et al, 2011).…”

“…Close to peak metamorphic conditions, deformation was likely accommodated by grain boundary migration recrystallization, as indicated by relic amoeboid grains and dissection microstructures (see in detail Papeschi et al, 2017). Close to peak metamorphic conditions, deformation was likely accommodated by grain boundary migration recrystallization, as indicated by relic amoeboid grains and dissection microstructures (see in detail Papeschi et al, 2017).…”

“…Deformation, related to the activity of thrust sheets coeval with pluton emplacement, affected the Calamita Schists at upper crustal conditions during postmagmatic cooling. These latter crosscut the mylonitic foliation exploiting precursory ductile shear bands (Papeschi et al, 2017(Papeschi et al, , 2018. Deformation was heterogeneously distributed in top to the east ductile shear zones that recorded the transition from an upper amphibolite facies foliation to a greenschist facies mylonitic foliation, overprinted at the brittle/ductile transition by brittle fault zones and shear fractures.…”

“…Such evolution is typical of low-grade quartz-feldspathic and schistose rocks and has been largely described in the recent geologic literature (Berthé et al, 1979;Bukovská et al, 2016;van Daalen et al, 1999;Gapais & White, 1982;Gapais, 1989;Gillam et al, 2014;Nishikawa & Takeshita, 1999;. As the system undergoes progressive embrittlement, shear bands propagate as shear fractures and finally join up to develop discrete, non-Andersonian, fault zones (e.g., Papeschi et al, 2017Papeschi et al, , 2018. However, brittle deformation is generally described as a separate event that overprints an earlier, ductile stage.…”