Detecting the stepwise propagation of the Eastern Sicily thrust belt (Italy): insight from thermal and thermochronological constraints

An extensive dataset of vitrinite reflectance, FTIR parameters on organic matter, illite content in mixed layers illite‐smectite, apatite fission tracks and U‐Th/He dating has been used to reconstruct the stepwise propagation of the Eastern Sicily fold‐and‐thrust belt during Late Palaeogene and Neogene times. The results indicate that the fold‐and‐thrust belt is divisible into two levels of thermal maturity. These levels consist of a less evolved level of thermal maturity that records limited sedimentary burial and minor heating, and a more evolved level of thermal maturity that indicates tectonic burial and exhumation at different times. Deformation and exhumation of shallowly buried units are linked to wedge forward propagation by low‐angle thrusts, whereas the evolution of deeply buried units is associated with tectonic imbrications by duplex formation and steep thrusts. The two tectonic styles alternate during evolution of the fold‐and‐thrust belt under low erosion rates.     

Fractal analysis of mingled/mixed magmas: an example from the Upper Pollara eruption (Salina Island, southern Tyrrhenian Sea, Italy)

Upper Pollara eruption products (13 ka, Salina Island, Italy) include both homogeneous and heterogeneous pumices resulting from mixing/mingling processes between an HK andesite and a high-SiO₂ rhyolite. Representative samples of heterogeneous pumices are collected and analyzed in order to check the correspondence between glass composition and morphological features of the mingling/mixing structures. Image analysis techniques are applied and eight grey color ranges (classes) are extracted from high-resolution scans of pumice. Class 1 (lighter colors) and class 8 (darker colors) show end-member glass compositions, i.e. HK andesite and high-SiO₂ rhyolite, respectively. These two classes show spot- to cluster-like morphological structures. Intermediate classes show an HK dacitic to rhyolitic composition and a banding- to fold-like morphology. Fractal analysis by box-counting of the boundary pattern of eight grey classified images is performed over a length scale of 0.028–1.8 cm. Fractal dimension D is between 1.01 and 1.84. Coupled fractal analysis and geochemical data reveal that D increases as the degree of magma interaction (homogenization) increases. This feature well fits the results from numerical models on the convective mixing of fluids driven by thermal convection. We conclude that the increase of D observed in the Upper Pollara samples reflects the transition from fractal mixing to homogenization. End-member magmas (HK andesite and high-SiO₂ rhyolite) represent isolated mixing regions, while homogenized magmas represent active mixing regions. In the analyzed pumices, isolated and active mixing regions coexist at scales between 10⁻⁴ and 10⁻² m. Morphological and compositional features of the Upper Pollara pumices result from turbulence.     

Volcanological evolution of the Rivi–Capo Volcanic Complex at Salina,Aeolian Islands: magma storage processes and ascent dynamics

Critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation. Here, we present data on densities, and vesicle size and number characteristics, for representative pyroclasts from six silicic eruptions of contrasting size and style from Raoul volcano (Kermadec arc). We use these data to evaluate histories of bubble nucleation, coalescence, and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material. Density/vesicularity distributions show a scarcity of pyroclasts with ～65–75 % vesicularity; however, pyroclasts closest to this vesicularity range have the highest bubble number density (BND) values regardless of eruptive intensity or style. Clasts with vesicularities greater than this 65–75 % “pivotal” vesicularity range have decreasing BNDs with increasing vesicularities, interpreted to reflect continuing bubble growth and coalescence. Clasts with vesicularities less than the pivotal range have BNDs that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. Bubble size distributions (BSDs) and BNDs show variations consistent with 65–75 % representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation, consistent with the closest packing of spheres. We consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit. These processes can act in concert with multiple nucleation events, generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed. Fragmentation preserves the range in vesicularity seen as well as any post-fragmentation/pre-quenching expansion which may have occurred. We demonstrate that differing density pyroclasts from a single eruption interval can have widely varying BND values corresponding to the degree of bubble maturation that has occurred. The modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation.     

Tsunami hydrodynamic force on a building using a SPH real-scale numerical simulation

Natural Hazards - One of the most important aspects in tsunami studies is the behaviour of the wave when it approaches the coast. Information on physical parameters that characterize waves is often...     

Volcanic soil formation in Calabria (southern Italy): The Cecita Lake geosol in the late Quaternary geomorphological evolution of the Sila uplands

This paper focuses on the main morphological, physical, chemical and mineralogical features of an andic-like soil, widely outcropping in the Sila upland plateau of Calabria (southern Italy), and its potential role in tephrostratigraphy. A multidisciplinary and multiscale approach allowed identification of this soil as a “masked” distal archive of volcanic products, developed on granite rocks and sediments with a coeval pyroclastic input during pedogenesis. The study demonstrates that the contribution of volcanic parent materials can be successfully hypothesized and assessed even in the absence, limited extent or poor preservation of primary eruptive products. The soil has an Andisol-like appearance, despite laboratory data that do not match the entire suite of diagnostic criteria for the Andisol taxonomic order. Geomorphological, stratigraphic and pedologic results, coupled with tephrostratigraphic and radiometric data, concur to suggest a Late Pleistocene(?) to Holocene age of the Andisol-like soil. In particular, the rhyolitic chemical composition of small-sized glass fragments (identified by SEM–EDS analyses) indicates soil genesis contributed by volcanic ash, probably sourced from Aeolian Arc explosive activity spanning the last 30 ka. Accordingly, the evidence of limited relict clay illuviation and the specific type of pedogenesis allowing the development of andic properties (in turn related to the neoformation of clay minerals from the weathering of volcanic glass) are consistent with a climatic shift from a seasonally-contrasted to a constantly humid pedoenvironment. This change can be ascribed to the Lateglacial(?) or Early–Middle Holocene to Late Holocene transition. Calibrated AMS ¹⁴C dates performed on charcoal fragments sampled from three representative soil profiles, provide Late Holocene ages (3136 ± 19, 343 ± 16 and 92 ± 24 yr BP), in accord with archaeological finds. On the basis of the consistent stratigraphic position, lateral continuity and wide extent, the soil can be considered a good pedostratigraphic marker in the Sila highlands and is informally defined as the “Cecita Lake geosol”. It supplies valuable time constraints for the underlying (occasionally overlying) deposits and/or soils. Moreover, it allows regional-scale morphostratigraphic correlations and detailed reconstruction of Late Pleistocene–Holocene geomorphic events in Calabria, a very suitable region for distal tephra deposition in the central Mediterranean peri-volcanic area. The effects of high-energy volcanic eruptions are interfingered with or superimposed by other geomorphic processes and climatic or anthropogenic signals.     

The influence of variable topography on the depositional behaviour of pyroclastic density currents: The examples of the Upper Pollara eruption (Salina Island,southern Italy)

Stratigraphic reconstruction of the Upper Pollara eruption has allowed for the inference of eruptive mechanisms and the distillation of a sedimentological model for pyroclastic density currents (PDCs) moving across variable topography. The pre-eruptive topography in the study area was characterised by a tuff ring-like morphology, with both inward and outward dipping slopes. Highly viscous, moderately porphyritic, dacitic to rhyolitic magmas fed the eruption, which was characterised by a Vulcanian eruptive style. The stratigraphic succession was divided into five eruption units (EUs), which result from different phases of the eruption separated by stases. Sustained columns occurred only during EU1, while PDC generation dominates EU2–5. Lithofacies analysis of the PDC deposits indicates the prevalence of massive coarse-grained deposits on the inner slopes of the Pollara crater, which are interpreted as the deposits of a flow-boundary zone dominated by granular flow or fluid escape regimes. Dune-bedded, massive to stratified lithofacies dominate the outer slopes of the Pollara crater, and are interpreted as the deposits of PDCs with flow-boundary zones in which traction played a major role. Thin, massive PDC deposits are exposed on the sub-horizontal Malfa terrace, and are interpreted as representative of flow-boundary zones dominated by a granular flow regime. The occurrence of stacked deposits indicates that most of the PDCs were characterised by unsteady pulsatory behaviour, with development of trains of pulses during their transport. The downcurrent lithofacies transitions observed for the Upper Pollara deposits have finally been compared with other similar lithofacies associations which have been described for short-lived PDCs at tuff rings, in order to discuss the influence of pre-eruptive topography on lithofacies association.     

Discriminating the long distance dispersal of fine ash from sustained columns or near ground ash clouds: The example of the Pomici di Avellino eruption (Somma-Vesuvius,Italy)

Ash samples from tephra layers correlated with the Pomici di Avellino (Avellino Pumice) eruption of Somma-Vesuvius were collected in distal archives and their composition and particle morphology investigated in order to infer their behaviour of transportation and deposition. Differences in composition and particle morphologies were recognised for ash particles belonging to the magmatic Plinian and final phreatomagmatic phases of the eruption. The ash particles were dispersed in opposite directions during the two different phases of the eruption, and these directions are also different from that of coarse-grained fallout deposits. In particular, ash generated during magmatic phase and injected in the atmosphere to form a sustained column shows a prevailing SE dispersion, while ash particles generated during the final phreatomagmatic phase and carried by pyroclastic density currents show a general NW dispersion. These opposite dispersions indicate an ash dispersal influenced by both high and low atmosphere dynamics. In particular, the magmatic ash dispersal was first driven by stratospheric wind towards NE and then the falling particles encountered a variable wind field during their settling, which produced the observed preferential SE dispersal. The wind field encountered by the rising ash clouds that accompanied the pyroclastic density currents of the final phreatomagmatic phase was different with respect to that encountered by the magmatic ash, and produced a NW dispersal. These data demonstrate how ash transportation and deposition are greatly influenced by both high and low atmosphere dynamics. In particular, fine-grained particles transported in ash clouds of small-scale pyroclastic density currents may be dispersed over distances and cover areas comparable with those injected into the stratosphere by Plinian, sustained columns. This is a point not completely addressed by present day mitigation plans in case of renewal of activity at Somma-Vesuvius, and can yield important information also for other volcanoes potentially characterised by explosive activity.     

New unspiked K–Ar ages of volcanic rocks of the central and western sector of the Aeolian Islands: reconstruction of the volcanic stages

A geochronological study of the Filicudi, Salina, Lipari and Vulcano Islands (Aeolian Archipelago) using the unspiked potassium–argon technique provides new age data which, combined with stratigraphic correlation, better constrain the temporal evolution of volcanism. The unspiked K–Ar age of the oldest exposed lavas on Filicudi, 219±5 ka, is significantly younger than the previous estimation of 1.02 Ma. In the general context of Aeolian volcanism, this new date suggests that the volcanism of the western sector of the Aeolian Archipelago is younger than previously thought. Geochronological data point out on the rapid transition from calc–alkaline to potassic volcanism. The distribution of the K–Ar ages within the Salina–Lipari–Vulcano group shows that the volcanism started on Lipari and propagated over time northward on Salina and southward on Vulcano. Geochronological and geophysical data suggest that the onset of volcanism in the central sector of the Aeolian Arc may be due to a mantle upwelling structure located below Lipari. A change in the style of the eruptions occurred in the Salina–Lipari–Vulcano system at about 100 ka from the present. Low-energy magmatic eruptions occurred between 188 and about 100 ka. From about 100 ka to the present, higher-energy eruptions and low-energy events due to magma–water interaction also occurred. This change in the style of activity, together with the appearance of evolved products (i.e. rhyolites) during the last 50 ka, is consistent with the formation of magmatic reservoirs located at shallower depth with respect to those of the 188–100-ka period. The new geochronological data and available petrological models reveal that a change in the deep source of the primary magmas occurred in a relatively short time interval.     

Weakening of gneiss surfaces colonized by endolithic lichens in the temperate climate area of northwest Italy

A role of lithobionts in geomorphological processes is increasingly argued, but the spatio‐temporal scale of their impact is largely unexplored in many ecosystems. This study first characterizes in the temperate zone (northwest Italy) the relationships between lithobiontic communities including endolithic lichens and the hardness of their siliceous rock substrate (Villarfocchiardo Gneiss). The communities are characterized, on humid and xeric quarry surfaces exposed for decades and natural outcrops exposed for centuries, in terms of lichen and microbial constituents, using a combined morphological and molecular approach, and with regard to their development on and within the gneiss. A lichen species belonging to Acarosporaceae (Polysporina‐Sarcogyne‐Acarospora group, needing taxonomic revision) chasmoendolithically colonizes both the humid and xeric quarry surfaces, on which epilithic cyanobacterial biofilms and epilithic pioneer lichens respectively occur. Light and electron microscopic observations show the development of the endolithic thalli within rock microcracks and the hyphal penetration along crystal boundaries down to depths of 1 to 3 mm, more pronounced within the humid surfaces. Such colonization patterns are likely related to biogeophysical deterioration, while no chemical alteration characterizes minerals contacted by the endolithic lichen. By contrast, on natural outcrops, where the endolithic colonization is negligible, a reddish rind below epilithic lichens indicates chemical weathering processes. Schmidt Hammer measurements highlight that the endolithic lichens deeply affect the hardness of the gneiss (down to ?60% with respect to fresh controls and surfaces only colonized by cyanobacteria), exerting a significantly higher weakening effect with respect to the associated epilithic lithobionts. The phenomenon is more remarkable on humid than on xeric quarry surfaces and natural outcrops, where epilithic lichens are likely involved in long‐term hardening processes supporting surface stabilization. Endolithic lichens are thus active biogeomorphological agents at the upper millimetric layer of siliceous rocks in temperate areas, exerting their weakening action during the early decade‐scaled stages of surface exposure. Copyright © 2015 John Wiley & Sons, Ltd.     

The Pomici di Avellino eruption of Somma–Vesuvius (3.9 ka BP). Part II: sedimentology and physical volcanology of pyroclastic density current deposits

Pyroclastic density currents (PDCs) generated during the Plinian eruption of the Pomici di Avellino (PdA) of Somma–Vesuvius were investigated through field and laboratory studies, which allowed the detailed reconstruction of their eruptive and transportation dynamics and the calculation of key physical parameters of the currents. PDCs were generated during all the three phases that characterised the eruption, with eruptive dynamics driven by both magmatic and phreatomagmatic fragmentation. Flows generated during phases 1 and 2 (EU1 and EU3pf, magmatic fragmentation) have small dispersal areas and affected only part of the volcano slopes. Lithofacies analysis demonstrates that the flow-boundary zones were dominated by granular-flow regimes, which sometimes show transitions to traction regimes. PDCs generated during eruptive phase 3 (EU5, phreatomagmatic fragmentation) were the most voluminous and widespread in the whole of Somma–Vesuvius’ eruptive history, and affected a wide area around the volcano with deposit thicknesses of a few centimetres up to more than 25 km from source. Lithofacies analysis shows that the flow-boundary zones of EU5 PDCs were dominated by granular flows and traction regimes. Deposits of EU5 PDC show strong lithofacies variation northwards, from proximally thick, massive to stratified beds towards dominantly alternating beds of coarse and fine ash in distal reaches. The EU5 lithofacies also show strong lateral variability in proximal areas, passing from the western and northern to the eastern and southern volcano slopes, where the deposits are stacked beds of massive, accretionary lapilli-bearing fine ash. The sedimentological model developed for the PDCs of the PdA eruption explains these strong lithofacies variations in the light of the volcano’s morphology at the time of the eruption. In particular, the EU5 PDCs survived to pass over the break in slope between the volcano sides and the surrounding volcaniclastic apron–alluvial plain, with development of new flows from the previously suspended load. Pulses were developed within individual currents, leading to stepwise deposition on both the volcano slopes and the surrounding volcaniclastic apron and alluvial plain. Physical parameters including velocity, density and concentration profile with height were calculated for a flow of the phreatomagmatic phase of the eruption by applying a sedimentological method, and the values of the dynamic pressure were derived. Some hazard considerations are summarised on the assumption that, although not very probable, similar PDCs could develop during future eruptions of Somma–Vesuvius.