Eugenio Fazio

Alpine Metamorphism in the Aspromonte Massif: Implications for a New Framework for the Southern Sector of the Calabria-Peloritani Orogen, Italy

Structural, petrologic, and thermobarometric data presented in this paper contribute to our understanding of the tectono-metamorphic evolution of the lowest tectonic slices of the Aspromonte Massif (southern Calabria, Italy), which crop out in three main tectonic windows. Despite previously being considered different units, they exhibit the following similar features: the same tectonic evolution, analogous blasto-deformation relationships, and absence of Hercynian mineralogical assemblage relics. Similar P-T paths indicate early HP-LT Meso-Alpine metamorphism (400-600° C at 0.95-1.35 GPa), evolving in the Oligocene-Miocene toward a subsequent retrograde shearing event ranging from 480° to 610°C and 0.50 to 0.95 GPa. The latest retrograde evolution is characterized by 350-480°C and 0.32-0.62 GPa. In this new tectonic framework, it is proposed to group the metapelite sequences defining the Madonna di Polsi Unit. Data presented herein suggest that the pre-Alpine geodynamic setting of southern Calabria was a thinned continental margin made up of Hercynian basement and Mesozoic terrigenous-carbonate sedimentary cover. This continental margin evolved during the early Meso-Alpine stage into a subduction zone beneath the European plate, followed by Neo-Alpine syn-convergent exhumation along a deep-seated mylonitic shear zone. These processes are responsible for the Alpine metamorphic overprint on the Hercynian terranes, as well as for Alpine metamorphism of their Mesozoic cover.

Fault-related rocks: deciphering the structural–metamorphic evolution of an accretionary wedge in a collisional belt, NE Sicily

The Alpine chain exposed in the Western Mediterranean area represents a front several kilometres in width, dismembered by more recent tectonics and by opening of the Tyrrhenian Basin. In most exposures of this mountain belt, relics of older metamorphic rocks occur. The deformational sequence of events may be revealed by the recognition of metamorphic records associated with different structures. Within a tract of the Alpine front cropping out in the Peloritani Mountains (NE Sicily), we distinguished two metamorphic complexes characterized by different tectonometamorphic histories. Their present tectonic juxtaposition is a cataclastic thrust linked to the recent Africa-verging Sicilian–Maghrebian fold-and-thrust belt. The Lower Complex is characterized by Hercynian metamorphism (P > 0.2 GPa and T ≈ 350°C) exclusively. It essentially consists of very low-grade metapelites and metavolcanic rocks overlain by an unmetamorphosed sedimentary cover. The Upper Complex, comprising different tectonic slices, consists of medium- to high-grade Hercynian metamorphic rocks (P = 0.3–0.8 GPa and T up to 650°C) with Alpine metamorphic overprint (T > 250°C) affecting also the Mesozoic–Cenozoic cover. Lithotypes, structures, and inferred P–T conditions of investigated rocks suggest the existence of an Alpine accretionary wedge during the Cretaceous deformational collision. Within the Upper Complex, a polyphase Palaeogene mylonitic horizon involving rocks belonging to different tectonic slices fully preserves the tectonometamorphic evolution. For this reason, we focused our attention on these sheared rocks in order to reconstruct the entire tectonic history of this geologically complex area. Our new basic model allows the complex structure of the nappe-pile edifice of the Peloritani Mountains to be simplified, casting new light on the tectonic evolution of this key sector of the southern Calabrian-Peloritani Orogen.

Microstructure and elastic anisotropy of naturally deformed leucogneiss from a shear zone in Montalto (southern Calabria, Italy)

Abstract A strain gradient was mesoscopically recognized in sheared leucogneisses cropping out near Mount Montalto (Calabria, southern Italy) in the Aspromonte–Peloritani Unit on the basis of field observations. In order to investigate the relationship between textural and physical anisotropy, a microstructural and petrophysical study was carried out on selected mylonites exhibiting different stages of deformation. The main mineral assemblage is Qtz+Pl+Kfs+Wm, displaying S–C and shear-band textures; mica-fish and ribbon-like quartz are widespread. As strain increases K-feldspar, biotite and premylonitic low phengite white mica transformed to synmylonitic high phengite white mica and quartz, accompanied by an increasing albitization. Different quartz c-axis patterns are ascribable to non-coaxial progressive deformation; we suggest that deformation proceeded under greenschist- up to amphibolite-facies conditions owing to a local increase in shearing temperature. Laboratory seismic measurements were carried out on sample cubes (43 mm edged) cut according to the structural frame (foliation, lineation) of the rock. At 400 MPa and room temperature the averages of compressional (Vp) and shear-wave velocities (Vs) are very similar: 5.70–5.91 and 3.36–3.55 km s−1, respectively. Seismic anisotropy and shear-wave splitting are related to the modal amounts of constituent minerals (in particular mica) and their crystallographic preferred orientation. Importantly, anisotropy is lowest in the most strained rock.

Microstructural, compositional and petrophysical properties of mylonitic granodiorites from an extensional shear zone (Rhodope Core complex, Greece)

At the southern boundary of the Rhodope Massif, NE Greece, the Kavala Shear Zone (KSZ) represents an example of the Eastern Mediterranean deep-seated extensional tectonic setting. During Miocene time, extensional deformation favoured syntectonic emplacement and subsequent exhumation of plutonic bodies. This paper deals with the strain-related changes in macroscopic, geochemical and microstructural properties of the lithotypes collected along the KSZ, comprising granitoids from the pluton, aplitic dykes and host rock gneisses. Moreover, we investigated the evolution of seismic anisotropy on a suite of granitoid mylonites as a result of progressive strain. Isotropic compressional and shear wave velocities ( V p , V s ) and densities calculated from modal proportions and single-crystal elastic properties at given pressure–temperature ( P – T ) conditions are compared to respective experimental data including the directional dependence (anisotropy) of wave velocities. Compared to the calculated isotropic velocities, which are similar for all of the investigated mylonites (average values: V p ~ 5.87 km s −1 , V s ~ 3.4 km s −1 , V p / V s = 1.73 and density = 2.65 g cm −3 ), the seismic measurements give evidence for marked P-wave velocity anisotropy up to 6.92% (at 400 MPa) in the most deformed rock due to marked microstructural changes with progressive strain, as highlighted by the alignment of mica, chlorite minerals and quartz ribbons. The highest P- and S-wave velocities are parallel to the foliation plane and lowest normal to the foliation plane. Importantly, V p remains constant within the foliation with progressive strain, but decreases normal to foliation. The potential of the observed seismic anisotropy of the KSZ mylonites with respect to detectable seismic reflections is briefly discussed.

The Calabria-Peloritani Orogen, a composite terrane in Central Mediterranean; its overall architecture and geodynamic significance for a pre-Alpine scenario around the Tethyan basin

The Calabria-Peloritani Orogen is an arcuate segment of the peri-Mediterranean orogenic Alpine nappe system that comprises the whole Calabria and the north-eastern sector of Sicily. It comprises the Sila and Catena Costiera Massifs in northern Calabria, the Serre and Aspromonte Massifs in central and southern Calabria, and the Peloritani Mountains in Sicily. In Sila and Catena Costiera Massifs, three tectonic complexes are recognisable: a) the basal Apennine Complex, which consists of carbonate platform sequences of passive continental margin; b) the intermediate Liguride Complex, made of oceanic-derived units, affected by HP/LT metamorphism; and c) the upper Calabride Complex, which represents a nearly entire section of continental crust. The Catanzaro Line separates the northern sector from the Serre Massif that also represents a nearly entire segment of Variscan continental crust unaffected by Alpine metamorphism. Further to the south, the Palmi Line separates the Serre from the Aspromonte Massif and the Peloritani Mountains. These two latter nappe edifices consist of either Variscan metamorphic units, Variscan units with Alpine overprint and units of continental derivation that are exclusively affected by Alpine metamorphism. The comparison between the geological evolutions of the various chain sectors, as well as their structural setting and their direction of tectonic transport, indicates that the Calabria-Peloritani Orogen is a composite terrane derived from the amalgamation of crustal blocks of different continental provenance. Northern Calabria represents a fragment of the Adria palaeomargin, whereas southern Calabria and northeastern Sicily are relics of an accretionary wedge resulting from the deformation of the European continental margin. As a consequence, nowadays a segment of the Europe-Adria collisional suture crops out in central Calabria.

Quartz preferred orientation in naturally deformed mylonitic rocks (Montalto shear zone–Italy): a comparison of results by different techniques, their advantages and limitations

Abstract In the geologic record, the quartz c-axis patterns are widely adopted in the investigation of crystallographic preferred orientations (CPO) of naturally deformed rocks. To this aim, in the present work, four different methods for measuring quartz c-axis orientations in naturally sheared rocks were applied and compared: the classical universal stage technique, the computer-integrated polarization microscopy method (CIP), the time-of-flight (TOF) neutron diffraction analysis , and the electron backscatter diffraction (EBSD). Microstructural analysis and CPO patterns of quartz, together with the ones obtained for feldspars and micas in mylonitic granitoid rocks, have been then considered to solve structural and geological questions related to the Montalto crustal scale shear zone (Calabria, southern Italy). Results obtained by applying the different techniques are discussed, and the advantages as well as limitations of each method are highlighted. Importantly, our findings suggest that patterns obtained by means of different techniques are quite similar. In particular, for such mylonites, a subsimple shear (40% simple shear vs 60% pure shear) by shape analysis of porphyroclasts was inferred. A general tendency of an asymmetric c-maximum near to the Z direction (normal to foliation) suggesting dominant basal slip, consistent with fabric patterns related to dynamically recrystallization under greenschist facies, is recognized. Rhombohedral slip was likely active as documented by pole figures of positive and negative rhombs (TOF), which reveal also potential mechanical Dauphiné twinning. Results showed that the most complete CPO characterization on deformed rocks is given by the TOF (from which also other quartz crystallographic axes can be obtained as well as various mineral phases may be investigated). However, this use is restricted by the fact that (a) there are very few TOF facilities around the world and (b) there is loss of any domainal reference, since TOF is a bulk type analysis. EBSD is a widely used technique, which allows an excellent microstructural control of the user covering a good amount of investigated grains. CIP and US are not expensive techniques with respect the other kind of investigations and even if they might be considered obsolete and/or time-consuming, they have the advantage to provide a fine and grain by grain “first round” inspection on the investigated rock fabric.

Strain rates of the syn-tectonic Symvolon pluton (Southern Rhodope Core Complex, Greece): an integrated approach combining quartz paleopiezometry, flow laws and PT pseudosections

We studied the mylonitic granodiorites belonging to the Symvolon pluton from the Kavala Shear Zone (Rhodope Massif, NE Greece), to determine strain rates and refining the tectonic exhumation model of this area. With this purpose, we combined a quantitative microstructural analysis with an image assisted thermodynamic modelling. In particular, three samples exhibiting increasing stages of deformation were selected to estimate PT conditions of the mylonitic event by means of a quantitative mineral-chemical study realized via statistical data handling of X-ray maps. Obtained PT constraints were then used in conjunction with quartz paleopiezometry combined with flow laws, to estimate the amount of mean strain rate achieved during the shearing evolutionary stage of these rocks. Such multifaceted approach allows us to infer strain partitioning and pluton cooling rateduring the mylonitic evolution, obtaining an average strain rate of 7.5 * 10-12 (s-1).

Strain localization and sheath fold development during progressive deformation in a ductile shear zone: a case study of macro-to micro-scale structures from the Aspromonte Massif, Calabria

In the southern sector of the Calabrian Peloritani Orogen (CPO; southern Italy), a crustal-scale shear zone (Montalto Shear Zone - MSZ) has developed under greenschist facies conditions (0.3-0.6 GPa for 350-550 °C) during Alpine orogenesis linked to the collision between the Adria and European plates. Deformation produced a mylonitic horizon up to 800 meters thick and a dominant top-to-NE sense of shear has been recognized. This high-strain zone is localized at the contact between two tectono-metamorphic units, the Aspromonte Peloritani Unit (APU), and the Madonna di Polsi Unit (MPU) that are characterized by a strong lithological contrast, with leucocratic ortho- and para-gneiss of the APU being thrust onto garnet-bearing phyllites (MPU). Mesoscopic structures typical of ductile shear zonesare observed and include mylonitic foliation, stretching lineation, and sheath folds showing the typical eye-type cross sections, which occur at a range of outcrop to thin section scales. Several deformational phases have previously been interpreted from mesoscopic structures in this area. After careful field investigation, we propose an alternative model that interprets structural features in terms of progressivedeformation developed during the same tectonic event. In particular, a folding phase, with sub-vertical axes and ca. 2 meters wavelength folds, which in past reconstructions were interpreted to be generated during a post-shear compressive episode, have now been considered the result of incremental strain coeval with the main shear zone activity (i.e. syn-shear). This new interpretation simplifies the tectonic history of the MSZ, and allows the removal of a previously identified tectonic phase.

Eye-type folds at the Palmi shear zone (Calabria, Italy)

very complex structures often occurring in ductile high strain shear zones dominated by simple shear (Alsop and Carreras 2007) displaying more than 90° hinge-line curvature and usually a typical elliptical Y-Z cross section of the finite strain ellipsoid (Alsop and Holdsworth 2004, 2006). Their overall vergence and minor related en-echelon tongues help to define the shear sense or flow transport direction (Alsop and Carreras 2007). The photograph (width of view ~2 m) shows a sheath fold truncated upwards by a sub-horizontal dipping minor shear plane occurring in the calc-silicates bearing skarns. The morphology shown here resembles fairly a box type fold due to the closure of lower limbs, nevertheless this geometric shape of sheath fold cross section is commonly observed (Reber et al. 2012, 2013). Our research group is presently working on the detail of such eyetype folds by doing a careful collection of structural elements such as axial surfaces, interlimb/apical angles, hinge orientations, by also considering transport direction deduced form minor associated Sand Z-type folds. The Palmi shear zone (Calabria, southern Italy) trends—WNW and is of crustal-scale. It developed during the Eocene strike-slip tectonics (Ortolano et al. 2013; Festa et al. 2016) affecting the CalabriaPeloritani terrane (Cirrincione et al. 2015) in the central Mediterranean area during the Alpine Orogeny (51–56 Ma, biotite Rb–Sr age; Prosser et al. 2003). The mylonitic rocks consist of remnants of the southern European Hercynian metamorphic basement: migmatitic paragneisses, skarns and tonalites. Marked differences of lithologies sheared and attained greenschist facies (0.6 GPa, 410 °C: Prosser et al. 2003). This produced a strong rheological contrast and strain localized within carbonates. At this outcrop (SP102 road; geographic coordinates: N 38°22′52.06′′N; 15°51′44.15′′E) beautiful examples of eye-type folds are visible at various scales. Eye-shaped folds are usually absent in moderately strained shear zones (Mukherjee 2013, 2014a, 2015). However, flanking structures (review in Mukherjee 2014b) and intrafolial folds (review in Mukherjee et al. 2015) on high strain can resemble/produce sheath folds. Eye-type folds are

Garnet growth in natural high pressure regime: a key tool in unravelling the pressure-temperature path of rocks involved in mountain belt orogeny

A study of a natural system concerning the growth of minerals (garnet) into metapelite host rocks from Calabria (southern Italy), which underwent pressure up to 1.2 GPa, is presented. Through the application of a thermodynamic model it was possible to determine P and T conditions achieved during metamorphic evolution of these rocks useful to reconstruct the related tectonic settings of poly-orogenic metamorphic complexes.