Sila Massif Research Articles

Weathering of gneiss in Calabria, Southern Italy

The effects of weathering in a Mediterranean climate on the mineralogy and microfabric of Paleozoic gneiss of the Sila Massif, Calabria, southern Italy, have been studied. Field observations show highly weathered rock forms a residual soil. Micromorphological and mineralogical properties of bedrock and saprolite show that the weathering process is characterized by at least two major stages, having two distinct rock microfabrics. In the first stage, the morphological features of the original rock are preserved and weathering is manifested mainly by microfracturing, and large portions of the rock remain unaltered. The second stage of weathering involves further development of microcracks and progressive chemical attack on the minerals. This latter stage occurs along both compositional and microstructural discontinuities, with etch pitting of feldspar, and neoformation of clay minerals and ferruginous products replacing feldspar, biotite, and iron-bearing garnet. The determination of quantitative petrographic indices provides a measure of the various stages of weathering.

Weathering and morphogenesis in a mediterranean climate, Calabria, Italy

Deeply weathered plutonic rocks occur widely in the Sila Massif, Calabria, southern Italy. Three representative weathering profiles developed on the hilltops indicate that sand represents more than 50% by weight throughout most of the weathering profile, and silt- and clay-size fractions are also well represented, filling the interstices among corestones. Both sand, and finer fractions of grus, and soil horizons, are thought to be derived from a combination of granular disintegration and chemical decomposition, developed on relatively flat terrains of the Sila massif. These slopes are now experiencing transport-limited morphodynamics, under a montane-modified Mediterranean climate. The depth of the weathered layers, that have suffered little erosional truncation, typically exceed 15 m, and may reach 50–60 m or more. Major isolated or grouped exfoliation boulders are the most common minor landform feature developed on the Sila massif granite. Boulders have developed as a result of spheroidal weathering and by removal of the sandy-textured granite. The Schmidt hammer (SH) test on boulders and corestones, suggests three distinctive degree of weathering (from moderately to completely weathered rock), and that biotite content is the major controlling factor of the granite mechanical behaviour. Understanding of the dynamics of this weathering system is crucial to the interpretation of the complex suite of variables that control landscape evolution of granitoid terrains.

Sourceland controls on the composition of beach and fluvial sand of the northern Tyrrhenian coast of Calabria, Italy: implications for actualistic petrofacies

The northern Tyrrhenian margin of Calabria in southern Italy provides a natural laboratory for sampling sand at different scales: small drainages (first order), rivers draining mountain ranges (second order), and marine environments (beach to deep-marine; third order). Calabrian mountain ranges represent an uplifted and variable dissected thrust belt constituted by Palaeozoic through Pleistocene plutonic, metamorphic, ophiolitic, carbonate and siliciclastic rocks. The modern setting includes a mountainous coast, having high rates of fluvial discharge, and the deep-marine Paola Basin. The composition of modern fluvial and beach sands is useful for the interpretation of sediment transported into deeper-water environments. Modern beach and fluvial sands of the northern Tyrrhenian margin of Calabria define three distinct petrologic provinces, from north to the south: (1) the Lao Littoral Province has calclithite sand derived from erosion of dominantly carbonates of the southern Apennines; (2) the Coastal Range Littoral Province has quartzolithic sand derived from dominantly metamorphic (schist and phyllite) Coastal Range; and (3) the Santa Eufemia Littoral Province has quarzofeldspathic sand derived from dominantly metamorphic Coastal Range and Sila Massif and plutono-metamorphic Mount Poro provenances. Deep-marine turbidites of the Paola Basin have basinwide quartzolithic turbidite sands having close compositional relations with Coastal Range littoral petrofacies. Only at the northern boundary of the Paola Basin, calclithite turbidite sand record deep-water dispersal of the Lao littoral sands. Comparison of detrital modes from mainland to deep-marine environments contribute to the models of dispersal pathways and geographical extension of actualistic sand petrofacies.

Late Neogene kinematics of intra‐arc oblique shear zones: The Petilia‐Rizzuto Fault Zone (Calabrian Arc, Central Mediterranean)

The kinematics of intra‐arc shear zones play a key role in the secondary shaping of orogenic arcs such as the Calabrian Arc (central Mediterranean). Comparison of the Neogene structural development of the Petilia‐Rizzuto Fault Zone and the basement structure of the bordering Sila massif reveals that the fault zone is the surface expression of a deep NW–SE trending sinistral crustal oblique shear zone. This shear zone continues over a length of more than 130 km across the northern segment of the Calabrian Arc and shows a post‐Eocene sinistral displacement of about 50 km. The late Neogene forearc basin development and syndepositional tectonics along the fault zone are reconstructed in great detail by analyzing the middle Miocene‐Recent tectonic sequence stratigraphy. A strike‐slip cycle can be recognized whereby the subsequent activity of Riedel shears, tensional faults, and P shears, positive flower structures and principle displacement wrench faults, can accurately be traced in time. Observed phenomena are discussed in terms of the activity of a conjugate system of oblique thrust zones within the growing accretionary complex. The evolution of special types of thrust belt basins is illustrated. These include oblique thin‐skinned pull‐apart basins, oblique rhomboidal “harmonica” basins, and “detached slab” basins (new terms introduced here), evolving one into the other. A new feature illustrated is the recurrent basin inversion which generated passive roof duplexes through back‐shear motion and out‐of‐sequence thrusting along the wedge. The fault patterns and the style of inversion tectonics imply an E–W directed axis of effective compressive stress in this part of the arc. This resulted from an interaction of (1) local E–W directed compression related to a differential displacement of two parallel segments of the arc (generated by the migration to the southeast of the Calabrian Arc and opening of the Tyrrhenian backarc basin); (2) alternating NW–SE directed compression and extension (related to pulsating thrust wedge dynamics with phases of accretion and underthrusting respectively) and (3) regional, compressive interplate stress (middle Messinian‐middle Pliocene). All structures are overprinted by post middle Pleistocene extensional faulting (related to rapid uplift of intra‐arc massifs) and reversal along thrust planes and transcurrent faults. This extensional collapse reflects isostatic adjustments in response to plate rupture which was provoked by regional compressive stress.

Petrology of basic and intermediate orogenic granitoids from the Sila Massif (Calabria, southern Italy)

AbstractHercynian gabbroic, dioritic and tonalitic rocks crop out in the neighbourhood of Rovale (Sila Grande, Calabria). They make up a crude rectangular outcrop with the western part consisting of gabbroic rocks and the eastern of dioritic and tonalitic rocks. They come into contact with medium to high grade metapelites on the western side and with heterogeneous granodiorites on the other sides. In the gabbroic body both opx ± ol bearing cumulates and amphibole differentiates occur and are characterized by the widespread presence of brown pargasite. Sporadic magmatic to subsolidus corona textures between olivine and plagioclase or orthopyroxene and plagioclase can be observed and their preservation clearly suggests a post‐tectonic emplacement for the gabbroic magma. Diorites and tonalites display hypidiomorphic textures free of olivine and orthopyroxene and bearing green Mg‐hornblende. The granitoids, on the basis of chemical data, display orogenic features of the continent‐continent collision type. The gabbroic rocks have high Al tholeiitic composition and fractionation of orthopyroxene and plagioclase played an important part in their evolution.The Rb/Sr isochron method did not give a precise emplacement age for the granitoids as a whole. Initial 87Sr/86Sr ratios (at 290 Ma) are higher in the gabbroic body (0.7091–0.7095) than in diorites and tonalites (0.7083–0.7092). Thus gabbroic rocks appear more displaced than diorites and tonalites towards crustal isotopic composition. The eNd data seem to confirm this feature, thus suggesting that the gabbroic rocks and diorites derived from distinct mantle magma batches. Interestingly, small isotropic gabbroic masses occur within the diorites and show general features that allow them to be considered as possibly parental with respect to the host diorites. The evolution to the dioritic composition might have occurred through fractionation and minor mixing with a more acidic component such as the northern granodiorites.Geochemical, Sr and Nd isotopic data indicate a scenario of a composite plutonic body formed by distinct magma batches of mixed crust and mantle origin.

Late magmatic to subsolidus coronas in gabbroic rocks from the sila massif (Calabria, Italy)

Corona textures between olivine and plagioclase or orthopyroxene and plagioclase are present in Hercynian gabbroic rocks from the Calabrian Sila Massif. They have been studied through optical and SEM investigations together with EDS and WDS analyses. Textural features indicate the existence of two extreme corona types formed during late magmatic stages or during subsolidus cooling. Magmatic coronas are characterized by an inner orthopyroxene layer and an outer orange-brown amphibole layer that might be in optical continuity with orthopyroxene and amphibole poikilites respectively. Subsolidus coronas consist of an inner layer of colourless amphibole and an outer layer of amphibole ± spinel. They sometimes form a collar also around plagioclase enclosed in olivine. A large spectrum in the composition of corona amphiboles from Ti-bearing pargasite to Mg-hornblende was observed. The variation in Ti content of amphibole was interpreted as a consequence of the different conditions of crystallization from late magmatic to subsolidus with temperatures ranging from 880°C to 580°C. The significant gahnite component in spinel possibly indicates that subsolidus reactions occurred in an open system. The pressure of formation constrained by the mineral assemblage of metamorphic basement rocks and by the neighbouring diorites has been estimated at 4 kbar.

Paleomagnetic evidence for non-apenninic origin of the sila nappes (Calabria)

The magnetization of Lower Permian rocks from Sila has a mean direction D = 56.5°, I= +20.4° with α 95 = 9.1° after correction for Upper Neogene tilting. A further correction for the attitude of the nappes after their Middle Miocene emplacement establishes paleolatitudes consistent with those from the Lower Permian Tethys. The remarkable internal consistency of the data has not supported the distinction of units with opposite vergences within the Sila crystalline nappes. The declination indicates that the Sila massif has rotated counter-clockwise by about 90° relative to the Apennines, Sardinia and the Southern Alps and therefore the well-known Apenninic rotation alone does not account for the total change of direction in tectonic transport. Accordingly, the structural trends of tectonic phases older than the emplacement time of the Calabrian nappes should no longer be referred to present-day geographic coordinates. The post-Late Cretaceous motion relative to the north Calabrian Apennines enhances the geotectonic role of the northern boundary of the Calabrian—Peloritan arc, since its sinistral-shear character permits both tectonic transport from the west and counter-clockwise motion during tectonic transport.