Natural examples indicate that pseudotachylytes cohexist in the same outcrop with mylonites and ultramylonites. Pseudotachylytes intimately associated with mylonites and ultramylonites can develop in high strain zone close to the brittle-ductile transition (e.g. Passchier, 1982) or entirely within the ductile regime as result of plastic instabilities (e.g. Hobbs et al., 1986; Handy & Brun, 2004). This study report microstructural investigations on two pseudotachylyte veins found within the felsic granulites at the base of the ~20-25 km thick Variscan crustal section outcropping in the Serre Massif (southern Calabria). Felsic granulites consist of quartz, plagioclase, K-feldspar, biotite, garnet, sillimanite and accessory minerals. Stretched minerals and S-C composite foliations are detected in zones crystal-plastic deformation. In places, felsic granulites exhibit an alternance of mylonitic and ultramylonitic bands (a few millimeters thick). Pseudotachylyte fault-veins develop along planes, which have a parallel orientation to the mylonitic and/or ultramylonitic foliation. Locally, pseudotachylyte fault-veins occur along the S-C composite foliations of the felsic granulite. On the other hand, the pseudotachylyte injection-veins cross cut the mylonitic and/or ultramylonitic bands. Microstructural observations indicate that the felsic granulite exhibits a strong grain-size reduction along the S-C composite foliations and near the contact with the pseudotachylyte veins. Back scattered electron (BSE) images, obtained by scanning electron microscopy (SEM) and Field-Emission Gun SEM, show that in proximity of the contact with the pseudotachylyte the garnet of the felsic granulite is fractured and shows rims characterized by a new cristallization of very small euhedral garnet (3-4 m). On the other hand, the ultramylonitic bands display a very fine-grained matrix and have a dark appearance. However, the BSE images reveal a strong penetrative foliation, which is defined by the alignment of biotite and by the shape preferred orientation of quartz, plagioclase and garnet. Moreover, the ultramylonitic bands are characterized by a new crystallization of very small crystals (a few microns in length) of sillimanite and K-feldspar, aligned along the foliation planes. Matrix of the pseudotachylytes is microcrystalline and contains abundant clasts (>50%) made up of quartz, plagioclase, K-feldspar, garnet and rare biotite. Clasts in the matrix are aligned with a parallel orientation to oblique foliation of the mylonitic granulite. At the margin of the vein, garnet of the pseudotachylyte may occur in two habits: 1) garnet microlites with very small sizes (3-4 m) and an idiomorph habit, which formed by direct crystallization from the frictional melt, and 2) garnet clasts (a few ten micrometres in size), with rims characterized by a new crystallization of very small garnets (<2 m) and with a similar aspect to the garnet rims of the host rock. In the vein centre, the matrix is mainly composed of skeletal plagioclase and biotite (a few microns in lenght). Plagioclase and biotite microlites often nucleated on rounded clasts of quartz or plagioclase. Garnet microlites are absent in the vein centre. These data, combined with the indications for the formation depth of the pseudotachylytes (21-23 km) obtained by Altenberger et al. (2010), indicate that during propagation of the seismic rupture the shear deformation was highly heterogeneous and took place through the development of alternating pseudotachylyte and ultramylonite, as result of plastic instabilities. References Altenberger, U., Prosser, G. & Grande, A. (2010): Workshop Physico-chemical processes in seismic faults, 11 Handy, M.R. & Brun, J.P. (2004): Earth Planet. Sc. Lett., 223, 427-441 Hobbs, B.E., Ord, A. & Teyssier, C. (1986): Pure Appl. Geophys., 124, 309-336 Passchier, C.W. (1982): J. Struct. Geol., 4, 69-79

A dike network transecting a basement of intrusive and metamorphic rocks related to the Hercynian orogeny is exposed in the Sila Grande (southern Italy). Dike magmatism, similarly to other regions of the western Mediterranean, such as Sardinia, Corsica, and Catalonia, is of calc-alkaline to alkali-calcic affinity. Zircon U-Pb geochronology indicates that dike magmatism took place between 295 +/- 1 to 277 +/- 1 Ma, after the main late Hercynian emplacement of granitoids (306 +/- 1 Ma). Barometry indicates that the basement underwent exhumation of 8 +/- 3 km before dike injection. The dike network has a geometrical arrangement consistent with a transtensional stress regime that resulted in ductile thinning of the lower crust during the late stage of the Hercynian orogeny and concurrent fracturing of the upper crust that made possible magma ascent through dikes. The proposed tectonic evolution is related to dismemberment of the southern Hercynian belt in the central Mediterranean area as a result of dextral transtension of Gondwana in relation to Laurasia during the Pennsylvanian-Early Permian.

Granitoids represent a considerable proportion of the Hercynian crustal sections exposed in Calabria. In the Sila and Serre massif the granitoids are stacked to make a pile of tabular intrusions with a cumulative thickness of 9 to 13 km. Composition typically ranges from tonalite to granite, but minor dioritic and gabbroic bodies are also present. Magma emplacement took place between 300 and 290 ± 10 Ma during the extensional tectonic stage, following collapse of the Hercynian belt. The heat advected by granitoids was considered responsible both for contact metamorphism in the upper crust and regional low-P metamorphism in the lower to intermediate crust. This was satisfactorily reproduced by static and dynamical numerical models. However, a limit of the models is the assumption concerning the instantaneous emplacement of the magmas, following the general consensus on the idea that large magma chambers can be filled through dykes in a short time interval (< 100 kyr in PETFORD et alii, 2000). In recent years, radiometric datings obtained from different levels of the magmatic bodies indicate that pluton growth can be completed in a significantly longer time (>1 Myr, e.g. GLAZNER et alii, 2004; MATZEL et alii, 2006) as an effect of the discontinuous melt feeding. Consequently, the way the pluton grows (downward or upward) needs to be examined since this affects the intensity of thermal perturbation above and below the magmatic body (ANNEN, 2011). On the basis of these arguments we have set up a new 2D thermal model, applied to the Calabria crustal sections, that takes into account the incremental growth of the pluton. We analysed the effects of the end-member processes of pluton accretion at different growth rates. In case of under-accretion, contact metamorphism in the upper crust occurs before regional low-P metamorphism in the intermediate to lower crust. Model indicates that pluton growth must be completed in a short time (< 200 kyr) to reproduce peak temperatures (540-590 °C) in the contact aureole observed in the upper crust. A slower growth rate would result in weaker thermal effects. Consequently, deformation structures related to pluton growth and overprinted by peak temperatures porphyroblasts, must be formed in an even shorter time interval. In case of over-accretion, a reverse time sequence of regional low-P and contact metamorphism is produced and observed peak temperatures can be generated even when pluton construction is completed in 5 Myr (Fig. 1) In comparing data of the Calabria crustal sections with results of under- and over-accretion models, the second option is mandatory (Fig. 1). Model results suggest also that the smooth transition from regional low-P to contact metamorphism may reflect upward pluton growth and concurrent exhumation by extensional tectonics.

Outcrops of the pre-Mesozoic basement, representative of the whole Hercynian continental crust are exposed in Calabria. This is the result of Tertiary geological evolution that brought to the surface different crustal levels. This geological field trip aims to provide a general picture of the continental crust that hopefully may represent a reference frame for geochemical, rheological and geophysical models. The itinerary develops in central and southern parts of Calabria, namely in the Serre massif and in the promontories of Capo Vaticano and Monte Sant’Elia. In three days it is possible to examine compositional and structural features across an entire crust section. Thus rocks affected by very low-grade to granulite facies metamorphism and distinctive features of granitoids emplaced at different structural levels will be examined. The effects of the intense thermal perturbation produced by granitoid emplacement are visible both in the upper and in the lower crust, in a sharp metamorphic aureole and in a migmatitic border zone, respectively. Finally, some cases of Paleozoic rocks with strongly partitioned deformation, produced by Tertiary tectonics in the brittle and the ductile domains, can be observed.

Introduction In the Serre Massif of Calabria an about 8 km thick sequence of granulite facies rocks constitutes the lower crust of the former Variscan orogen. The base of this lower crustal section is made up of layered garnet-bearing meta-gabbros which are interspersed with lenses of metaperidotites and layers of metapyroxenites and meta-hornblendites (Moresi et al., 1978; Schenk, 1984). Single zircon conventional and spot U-Pb isotopic analyses place the age of the gabbroic protoliths into the Precambrian (Schenk, 1984, 1990; Micheletti et al. 2008). The main metamorphic overprint under granulite facies conditions of the gabbros has occurred during the Variscan orogeny. Zircon dating constrained the metamorphic peak close to 300 Ma in coincidence with the emplacement of huge masses of granitoid magmas in the middle crust (Schenk, 1984; Caggianelli et al., 2000; Micheletti et al., 2008). Reaction microstructures have induced Schenk (1984) to infer that the lower crust of the Variscan orogen in Calabria was subjected to isothermal decompression immediately after the thermal maximum. He has deduced that the lower crust became decompressed from about 750 to 550 MPa at 800° C and then affected by slow isobaric cooling down to 200° C over the next 250 million years. Acquafredda et al. (2008) have shown that decompression occurred during two different stages and involved a much thicker continental crust than previously assumed. Internally consistent thermodynamic modelling of the pre-decompression mineral assemblage results in 900° C and 1.1 GPa for the metamorphic peak conditions. New data We can now show that metagabbros became deformed under melt present conditions. This is evidenced by the occurrence of cm-thin shear zones wherein the partial melt was drained. Shear-zone development occurred in a general deformation regime, with shortening partitioned next to the shear zone resulting in an intense crenulation of the earlier Variscan foliation. Ti-rich pargasitic amphibole layers were involved in this crenulation deformation and recrystallized. Recrystallized amphiboles have the same chemical composition as those tracing the older layering. Application of the semiquantitative "Ti-in-amphibole" geothermometer of Ernst and Liu (1998) to the recrystallised pargasites results in a deformation temperature of 870° C, whereas application of amphibole-plagioclase thermometry (Holland & Blundy, 1994) results in temperatures of 818 to 857°( in the pressure range 0.5 to 1 GPa). Minimum pressure conditions of 500 MPa can be assessed using the semiquantitative Al-in-amphibole geobarometer of Ernst and Liu (1998). However, application of the crystal structure modelling geobarometer of Nimis & Ulmer (1998) to clinopyroxene in equilibrium with the plagioclase-rich melt of the shear zones shows that pressures during deformation have been rather close to 1 GPa. Melt present deformation of the mafic complex was followed by cooling starting from temperature conditions closely identical to that registered by the recrystallized amphiboles. Reaction seams of quartz+phlogopite around Opx suggest that a dehydration reaction of the type phlogopite (Phl) + quartz (Qtz) = orthopyroxene (Opx) + melt has been crossed backwards during cooling. Peterson & Newton (1989) place this reaction above 800° C. Conclusion The recognition that plagioclase/Cpx melt-bearing shear zones developed at about 1GPa pressure and 870°C temperature suggests that incipient melting of the mafic complex of the lower crust may have triggered the extensional tectonics in Calabria.

The reconstruction of the tectono-metamorphic evolution of the Variscan intermediate and upper crustal section exposed in the southern Serre Massif (Calabria, southern Italy) is crucial for a broader understanding of the crustal dynamics during the Variscan orogeny. In the southern Serre Massif, Variscan tectonics juxtaposed the Mammola Paragneissic unit (hereafter MPu), representative of the intermediate crust, and the Stilo-Pazzano Phyllite unit (SPu), representative of the upper crust. Subsequently, both units were affected by a contact metamorphic event related to the emplacement of the Upper-Carboniferous Serre batholith. Within the MPu, in the Levadio Stream area, garnet-hornblende bearing metandesitic lenses are locally interlayered with the paragneisses. The juxtaposition of the MPu and SPu is marked by a mylonitic shear zone affecting also the metandesite. This study examines via phase equilibria modelling the metamorphic evolution of a sheared garnet-hornblende bearing metandesite. To this purpose, the rock was modelled in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system using the software THERMOCALC v.3.45, with the thermodynamic dataset ds63, including a – x models for both metapelitic and metabasic minerals. The pre-peak metamorphic mineral assemblage (ep-q-pl-mu-chl-bi-rieb) is hosted as primary inclusions in garnets having the following composition: almandine (50-53%) - grossular (29-31%) - spessartine (17-20%). The constructed P-T pseudosection shows that the peak mineral assemblage (g-ep-mu-bi-chl-rieb-ab-sph-q-H2O) was stable in a wide, low-variance P-T field of 6 – 13 kbar and 350 – 525 °C. Garnet isopleth modelling suggests P-T conditions for the regional metamorphic peak of 7.9 – 9.2 kbar and 495 – 510 °C. The subsequent, near isothermal exhumation from about 32 to 10 km depth at 450 – 500 °C brought the metandesite close to the emplacement level of the Serre batolith (at 2.7 kbar). The constructed T-MH2O pseudosection points to T values of 570 – 593 °C for the peak pl-bi-q-g-hb-sph mineral assemblage of the contact metamorphism, under H2O-saturated conditions. The derived P-T path for the garnet-hornblende bearing metandesite is consistent with the one derived by Angì et al. (2010) for the MPu paragneiss, and highlights that comparable peak pressures were reached at the regional metamorphic acme in the southern Serre Massif within the MPu, as well as within the granulite facies rocks cropping out in the north. This suggests that the thermal gradient significantly changed across the Variscan chain exposed in Calabria in response to distinct tectono-magmatic contexts.

Estimating values of permeability (k), ef cient porosity (P) and hydraulic conductivity (K) by analysing eld outcrops as analogue of geothermal reservoirs, is a timely theme useful for predictions during geothermal ex- ploration programs. In this paper we present a methodology providing k, P and K values, based on geomet- ric analysis of quartz-tourmaline faults-vein arrays hosted in micaschist exposed in south-eastern Elba Island (Tuscan Archipelago, Italy), considered as the analogue of rock hosting the so-called “deep reservoir” in the Larderello geothermal eld. The methodology is based on the integration among structural geology, uid inclu- sions results and numerical analyses. Through a detailed structural mapping, scan-lines and scan-boxes analy- ses, we have reconstructed three superposed faulting events, developed in an extensional setting and framed in the Neogene evolution of inner Northern Apennines. Geometrical data of the fault-veins array were processed by reviewing the basic parallel-plate-model-equation for k evaluation. Fluid inclusion analyses provided those salinity and pressure-temperature values necessary for de ning density and viscosity of the parent geothermal uids. Then, permeability, density and viscosity were joined to get hydraulic conductivity (K). Permeability is estimated between 5 × 10− 13 and 5 × 10− 17 m2 with variations among the different generation of faults, while the hydraulic conductivity is encompassed between 1.31 × 10− 8 and 2.4 × 10− 13 m/s. The obtained permeabil- ity and hydraulic conductivity values are comparable with those from several geothermal areas, and in particular from the Larderello geothermal eld. The main conclusion is that the proposed integrated approach provides a reliable methodology to obtain crucial values, normally obtained after drilling, for developing numerical ow models of geothermal uid path in active geothermal systems by eld and laboratory analyses of analogue, ex- humed, geothermal systems.

A thermo-rheological model of the Monte Capanne pluton, Elba Island, Italy is proposed as having general relevance for the thermal and tectonic evolution of upper crustal granites and their surrounding rocks in extensional regions. The thermal evolution of the pluton and country rocks is followed for 1 myr after emplacement, which occurred at c. 6.9 Ma. The pluton completely crystallized in c. 210 kyr (±20%). The adjacent rocks reached a thermal peak of 550 °C (±10%), maintaining a temperature higher than 500 °C for c. 100 kyr. The temperature distribution is used to construct a model for the time-dependent rheology of the pluton and surrounding rocks. A series of 2D cross-sections shows an upward migration of the regional brittle−ductile transition, and the formation of a ductile horizon above the pluton. The former is a combined effect of unroofing and middle crust heating; the latter is the result of temperature increase in rheologically weak country rocks. This ductile horizon has a potential role in the tectonic evolution of the region, since it could favour the formation of upper crustal shear zones and listric faults rooting in the transient brittle−ductile transition and playing a major role in further post-emplacement extension.

The tectono-metamorphic evolution of the Hercynian intermediate-upper crust outcropping in eastern Sila (Calabria, Italy) has been reconstructed, integrating microstructural analysis, P-T pseudosections, mineral isopleths and geochronological data. The studied rocks belong to a nearly complete crustal section that comprises granulite facies metamorphic rocks at the base and granitoids in the intermediate levels. Clockwise P-T paths have been constrained for metapelites of the basal level of the intermediate-upper crust (Umbriatico area). These rocks show noticeable porphyroblastic textures documenting the progressive change from medium-P metamorphic assemblages (garnet-and staurolite-bearing assemblages) towards low-P/high-T metamorphic assemblages (fibrolite- and cordierite-bearing assemblages). Peak-metamorphic conditions of similar to 590 degrees C and 0.35 GPa are estimated by integrating microstructural observations with P-T pseudosections calculated for bulk-rock and reaction-domain compositions. The top level of the intermediate-upper crust (Campana area) recorded only the major heating phase at low-P (similar to 550 degrees C and 0.25 GPa), as documented by the static growth of biotite spots and of cordierite and andalusite porphyroblasts in metapelites. In situ U-Th-Pb dating of monazite from schists containing low-P/high-T metamorphic assemblages gave a weighted mean U-Pb concordia age of 299 +/- 3 Ma, which has been interpreted as the timing of peak metamorphism. In the framework of the whole Hercynian crustal section the peak of low-P/high-T metamorphism in the intermediate-to-upper crust took place concurrently with granulite facies metamorphism in the lower crust and with emplacement of the granitoids in the intermediate levels. In addition, decompression is a distinctive trait of the P-T evolution both in the lower and upper crust. It is proposed that post-collisional extension, together with exhumation, is the most suitable tectonic setting in which magmatic and metamorphic processes can be active simultaneously in different levels of the continental crust.

Fractal-geometry-based analysis techniques offer simple and efficient ways for analyzing magmatic fabrics that are otherwise difficult to describe quantitatively. This study shows an application of two different methods on flow patterns observed in a syntectonic magmatic body injected into the lower crust. XZ and YZ rock cuts are scanned, and the scans are automatically transferred to binary patterns of mafic and felsic minerals. These are analyzed by box-counting as well as the modified Cantor-dust method. Box-counting leads to characterization of the entire patterns, proves their fractality in two different scale ranges, and yields information about magma mingling and grain-aggregate forming processes. The modified Cantor-dust method quantifies the anisotropy of pattern complexity and represents a potentially powerful method for determination of shear sense during magmatic flow. Both methods represent useful tools specifically for analyzing diffuse magmatic fabrics and for connecting field-related studies with analyses on the microscale.

Granitoids represent a large proportion of the Hercynian crustal sections exposed in Calabria. In the Sila and Serre massif the granitoid stack has a cumulative thickness of 9 to 13 km and includes tabular intrusions of tonalitic to granitic composition with minor dioritic and gabbroic bodies. Emplacement of the granitoids occurred sometime between 300 and 290 ± 10 Ma during the extensional tectonic stage, following collapse of the Hercynian chain. The heat advected by granitoids was considered responsible both for contact metamorphism in the upper crust and regional low-P metamorphism in the lower to intermediate crust. This was shown by static and dynamical numerical models. A main limitation of the models was the instantaneous emplacement assumed for the magmas. In the light of the general consensus existing on the idea that large magma chambers can be filled through dykes in a short time (< 100 kyr in Petford, et al., 2000), the instantaneous emplacement was considered a permissible simplification. In recent years, the rapid growth of plutons has been questioned, on the basis of radiometric dating at different levels of the magmatic body (e.g., Glazner et al., 2004). Even though the melt flow entering in the magma chamber can be high, feeding may be discontinuous. Consequently, pluton accretion by repeated melt injections can be completed in some million years (Matzel et al., 2006). Another interesting point raised is the way the pluton grows (downward or upward) because this affects the intensity of thermal perturbation above and below the magmatic body (Annen, 2011). On the basis of these arguments we have formulated a new 2D thermal model, to be applied on the Calabria crustal sections, that takes into account the incremental growth of the pluton. We examined the consequences of the end-member processes of pluton accretion and of different growth rates. In case of under-accretion, contact metamorphism in the upper crust occurs before regional low-P metamorphism in the intermediate to lower crust. Model indicates that pluton growth must be completed in a short time (< 200 kyr) to reproduce observed peak temperatures (540-590 °C) in the contact aureole. Deformation structures related to pluton growth, overprinted by peak T minerals, must be formed in an even shorter time interval. In case of over-accretion, an opposite sequence of low-P and contact metamorphism is produced and peak temperatures can be generated even when pluton construction lasts for 5 Myr. In comparing data of the Calabria crustal sections with results of underand over-accretion models, the second option is preferred. Model results suggest also that the smooth transition from regional low-P to contact metamorphism may reflect upward pluton growth and concurrent exhumation by extensional tectonics.

New laser ablation inductively coupled plasma mass spectrometry U-Pb dating on zircon and monazite was performed to estimate the time required for the building of the Serre batholith in Calabria. Age spectra from the bottom and top of the pluton are characterized by two main peaks at 306 and 295 Ma, resulting from a mutual interference between serial intrusive events. On this basis, the emplacement of the top granodiorite layer postdates by about 10 m.yr. emplacement of the lower tonalite layer. These results have been incorporated into a two-dimensional numerical thermal model, assuming overaccretion of a batholith in an extensional tectonic regime. With this approach it was possible to reproduce pressure-temperature paths for various levels of the continental crust and define timing for low-pressure regional and contact metamorphism. In a unique tectonomagmatic scenario the model reproduces re- gional low-pressure metamorphic effects in the lower to intermediate continental crust and, with a time lag of about 6 m.yr., contact metamorphism in the upper crust. Finally, we propose a conceptual model for the emplacement of the Serre batholith in an extensional tectonic setting. Space for magma can be created by lower crust thinning and rock uplift at the bottom and top of the batholith, respectively.

Abstract – Tectonic and thermal perturbations, related to emplacement of granodiorite in the upper continental crust, have been investigated in the late-Hercynian basement exposed in southern Calabria (Italy). Here, the structural aureole is marked by the presence of a major rim fold adjacent to the intrusive contact for a length of at least 20 km. Geometrical analysis of the structural aureole and related foliations, lineations and crenulations reveals that the perturbed zone is at least 3000 m wide and characterized by an open synform trending nearly parallel to the intrusive contact. This pattern is compatible with a laccolith-like mode of magma emplacement, related to the accretion of the pluton that shouldered weak phyllitic and slaty wall rocks. The metamorphic aureole, about 1800 m wide, is characterized by biotite, cordierite and andalusite that appear sequentially in spotted schists and hornfelses approaching the intrusive contact. The peak assemblage equilibrated between 535 and 590 ◦C at pressures between 175 and 200 MPa, confirmed by Al-in-hornblende barometry on granodiorite. Microstructural analysis allowed the inference of a time lag between the thermal and tectonic perturbations. With the aid of thermal modelling it was possible to quantify the time required to reach the peak temperature at a distance from the intrusive contact where cordierite spots and andalusite porphyroblasts clearly overprint crenulations. This estimate represents the time limit to accomplish deformation in the inner portion of the aureole and thus indicates a minimum strain rate of 4 × 10−14 s−1 within the country rocks during granodiorite intrusion.

New microstructural data on the mylonites from the well-exposed Palmi shear zone (southern Calabria) are presented with the aim to shed light on both the kinematics and the geometry of the southwestern branch of the Alpine belt during Eocene. In the study area, located between the Sardinia–Corsica block and the Calabria–Peloritani terrane, previous large-scale geodynamic reconstructions suggest the presence of strike–slip or transform fault zones dissecting the southwestern branch of the Alpine belt. However, there are no field data supporting the oc- currence of these structures. This paper uses vorticity analysis technique based on the aspect ratio and the long axis orientation of rigid porphyroclasts in mylonitic marbles and mylonitic granitoids, to estimate the contribution of pure and simple shear of deformation during the movement of the Palmi shear zone. Porphyroclasts aspect ratio and orientation were measured on thin sections using image analysis. Es- timates of the vorticity number, Wm, indicate that the Palmi shear zone recorded general shear with a contribution of pure shear of c. 65%. Then, the Palmi shear zone can be interpreted as a segment of a left-lateral transpressive bend along the southern termination of the Eocene Alpine front.