Gaetano Ortolano

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.

X-Ray Map Analyser

A new semi-automated image processing procedure based on multivariate statistical analysis of X-ray maps of petrological and material science interest has been developed to generate high contrast pseudo-coloured images highlighting the element distribution between and within detected mineral phases.This new tool package, developed in Python and integrated with ArcGis?, generates in only a few minutes several graphical outputs useful for classifying chemically homogeneous zones as well as extracting quantitative information through the statistical data handling of X-ray maps. The code, largely based on the use of functions implemented in ArcGis? 9.3 equipped with Spatial Analyst and Data Management licences, has been suitably integrated with original cyclic functions that hugely reduce the time taken to complete lengthy procedures. In particular these tools, after the acquisition of any kind of multispectral images allow fast and powerful data processing for efficient illustration and documentation of key compositional and microtextural relationships in rocks and materials. Display Omitted

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.

Petro-structural geology of the Eastern Aspromonte Massif crystalline basement (southern Italy-Calabria): an example of interoperable geo-data management from thin section – to field scale

The presented maps provide an example of the synoptic display of multi-scale geological features characterizing the tectono-metamorphic evolution of the crystalline basement terranes in a poly-orogenic-multistadial evolutionary scenario. The petro-structural map of the eastern Aspromonte Massif (southern Calabrian Peloritani Orogen) is characterized by a nappe-like edifice composed by the superimposition of three crystalline basement tectonic units which, sharing the same Alpine-Apennine reworking, underwent a different metamorphic evolution, mostly derived from the Hercynian orogenic cycle. This geological framework is completed by the suture deposition of a Oligocene-Miocene syn-orogenic clastic formation, partly roofed by the back-thrusting of a clay-rich mélange. In order to understand the potential relationships within the complex dataset deriving from the geological investigations of this crystalline basement area, a geo-database, capable of handling multi-scale information from field-derived structural data (i.e. foliation, lineation, fold- and fault-related structures) to micro-scale derived ones (i.e. thin-section analysis, electron microscope and microprobe investigations, thermodynamic modeling outputs) has been constructed, according to the international standards using the Geo-Scientific Markup Language developed by the Commission for the Management and Application of Geo-science Information. Proposed outputs will show the relationships between field-related geological features, showing collected samples and the subsequent laboratory investigations. These are fundamental to achieving reliable results in geological contexts, such as those for reconstructing the tectono-metamorphic evolution of crystalline basement terrains.

Quantitative X-ray Map Analyser (Q-XRMA): A new GIS-based statistical approach to Mineral Image Analysis

Abstract We present a new ArcGIS®-based tool developed in the Python programming language for calibrating EDS/WDS X-ray element maps, with the aim of acquiring quantitative information of petrological interest. The calibration procedure is based on a multiple linear regression technique that takes into account interdependence among elements and is constrained by the stoichiometry of minerals. The procedure requires an appropriate number of spot analyses for use as internal standards and provides several test indexes for a rapid check of calibration accuracy. The code is based on an earlier image-processing tool designed primarily for classifying minerals in X-ray element maps; the original Python code has now been enhanced to yield calibrated maps of mineral end-members or the chemical parameters of each classified mineral. The semi-automated procedure can be used to extract a dataset that is automatically stored within queryable tables. As a case study, the software was applied to an amphibolite-facies garnet-bearing micaschist. The calibrated images obtained for both anhydrous (i.e., garnet and plagioclase) and hydrous (i.e., biotite) phases show a good fit with corresponding electron microprobe analyses. This new GIS-based tool package can thus find useful application in petrology and materials science research. Moreover, the huge quantity of data extracted opens new opportunities for the development of a thin-section microchemical database that, using a GIS platform, can be linked with other major global geoscience databases.

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.