Exploratory seismic site response surveys in a complex geologic area: a case study from Mt. Etna volcano (southern Italy)

A preliminary study targeting to evaluate the local seismic response was performed in the eastern flank of Mt. Etna (southern Italy) using ambient noise measurements. The obtained spectral ratios were subdivided through cluster analysis into different classes of fundamental frequency permitting to draw an iso-frequency contour map. The analysis set into evidence the extreme heterogeneity of lava sequences, which makes difficult to identify a single seismic bedrock formation. Another important outcome, concerning the local seismic effects in terms of frequency and azimuth, is the important role played by the fracture fields associated with the main structural systems of the area. The existence of two zones with strong directional effects striking WNW–ESE and NW–SE, nearly orthogonal to the orientation of the main fracture fields, corroborate such hypothesis.     

Studying travertines for neotectonics investigations: Middle–Late Pleistocene syn-tectonic travertine deposition at Serre di Rapolano (Northern Apennines, Italy)

Middle–Late Pleistocene tectonic activity has been inferred through studies on travertine deposits exposed in a tract of the hinterland Northern Apennines. A detailed study on the relationships between tectonics and travertine deposition coupled with ²³⁰Th/²³⁴U age determination of travertines at Cava Oliviera quarry, located close to Serre di Rapolano village (southern Tuscany, Northern Apennines), allowed us to recognise Pleistocene faults, whose activity has been referred to 157–24 ka, at least. Travertine deposition was tectonically controlled by WSW-ENE striking, oblique and normal faults, associated to a main fault (named as the Violante Fault). This structure dissected a regional normal fault (known as the Rapolano Fault) Early–Middle Pliocene in age, which bounded the eastern side of the Pliocene Siena Basin, and gave rise to space accommodation for clayey and sandy marine sediments. Hydrothermal circulation (and related travertine deposition) was favoured by the damaging enhancement due to the fault–fault intersection. Tectonic activity has been also documented by deformation recorded by travertines, which suggest a main tectonic event between 64 ± 5 and 40 ± 5 ka. The tectonic activity described for the study area agrees with the Quaternary tectonic evolution documented in the surrounding areas (e.g. Mt. Amiata and Mt. Vulsini), as well as the Tyrrhenian margin of the Central Apennines, indicating that a widespread tectonic activity affected the inner part of the Apennines until the latest Quaternary.     

Types of eruptions of Etna volcano AD 1670–2003: implications for short-term eruptive behaviour

Analysis of the historical records of Etnas eruptive activity for the past three centuries shows that, after the large 1669 eruption, a period of about 60 years of low-level activity followed. Starting from 1727, explosive activity (strombolian, lava fountaining and subplinian) at the summit crater increased exponentially to the present day. Since 1763, the frequency of flank eruptions also increased and this value remained high until 1960; afterward it further increased sharply. In fact, the number of summit and flank eruptions between 1961 and 2003 was four times greater than that of the pre-1960 period. This long-term trend of escalating activity rules out a pattern of cyclic behaviour of the volcano. We propose instead that the 1670–2003 period most likely characterises a single eruptive cycle which began after the large 1669 eruption and which is still continuing.On the basis of the eruptive style, two distinct types of flank eruptions are recognised: Class A and Class B. Class A eruptions are mostly effusive with associated weak strombolian activity; Class B eruptions are characterised by effusive activity accompanied by intense, long-lasting, strombolian and lava fountaining activity that produces copious tephra fallouts, as during the 2001 and 2002–2003 eruptions. Over the past three centuries, seven Class B eruptions have taken place with vents located mainly on the south-eastern flank, indicating that this sector of the volcano is a preferential zone for the intrusion of volatile-rich magma rising from the deeper region of the Etna plumbing system.Electronic Supplementary Material Supplementary material is available for this article at Editorial responsibility: M. Carroll     

Diatom assemblage changes in lacustrine sediments from Isla de los Estados, southernmost South America, in response to shifts in the southwesterly wind belt during the last deglaciation

Isla de los Estados (54° 45′S, 63° 10′–64° 46′W) lies east of the main island of Tierra del Fuego and is the southeastern-most point in Argentina. Because of its geographic position near the latitudes of the Southern Hemisphere Westerlies and the strong influence of the Antarctic Circumpolar Current (ACC), the area is suitable for paleoecological and paleoclimate research. The island is not far north of the Subantarctic Front, which limits the northern boundary of the ACC. Paleoenvironmental study in this geographic location can shed light on past changes in atmospheric and marine circulation patterns. Diatom analysis of the lower part of a sediment sequence from Laguna Cascada (54° 45′ 51.3′′S, 64° 20′ 20.07′′W) enabled inference of changing lake conditions between 16 and 11.1 cal ka BP. Between 16 and 14.4 cal ka BP fragilarioid diatom species, often a pioneer group, dominated the record. Their presence shows seasonally open-water conditions from the onset of sedimentation. In zone II (14.4–12.8 cal ka BP), the dominance of planktonic/tychoplanktonic Aulacoseira spp. might represent longer ice-free periods and windier conditions, which would have kept this heavy species suspended in the water column. This period corresponds to the Antarctic Cold Reversal, when the Southern Hemisphere Westerlies were possibly centered on the latitudes of Tierra del Fuego, resulting in windy and wet conditions. Zone III (12.8–11.1 cal ka BP) is dominated by benthic diatom taxa that are mainly associated with peat and wetland vegetation. This suggests that climate conditions had become milder and less windy, favoring aquatic productivity and terrestrial vegetation development. This change in environmental conditions may have been a consequence of the southward movement of the Southern Hemisphere Westerlies at the start of the Antarctic Holocene thermal optimum.     

The occurrence of Mt Barca flank eruption in the evolution of the NW periphery of Etna volcano (Italy)

Geological surveys, tephrostratigraphic study, and ⁴⁰Ar/³⁹Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.     

Long-term uplift rate of the Etna volcano basement (southern Italy) based on biochronological data from Pleistocene sediments

The eastern flank of Mt. Etna volcano rests on Pleistocene marine sediments, which unconformably cover the Apenninic–Maghrebian Chain units. A quantitative biostratigraphic analysis was carried out based on the calcareous nannofossil content of the Pleistocene deposits outcropping along the S and NE periphery of the volcano. Sediments were constrained to the MNN19e and MNN19f biozones, deposited from 1.2 to 0.589 Ma. According to the depth of deposition and the present altitude of the Pleistocene succession, uplift rates are estimated between 1.1 and 1.7 mm yr⁻¹ for the northeastern sector of the Etna edifice, and between 0.36 and 0.61 mm yr⁻¹ for the southern one. This inhomogeneous long-term uplift rate affecting the Etna region, probably results from a buried thrust below the northern flank of Etna, which is related to the post-Tortonian geodynamic evolution of NE Sicily.     

The DEM or Mt. Etna: geomorphological and structural implications

Abstract

A Digital Elevation Model (DEM) of Mt. Etna is presented; it has altimetric and planimetric resolution of 1 m and 5 m, respectively, and covers an area of about 120 km . This 3-D view of Mt. Etna allowed both recognition and location of the main morphostructural and volcano-tectonic features of the volcano. A slope map has been generated from the DEM; on the basis of slope distributions and surface textures, five acclivity domains have been recognized. The largest domain, south of the summit craters, reflects the occurrence of old plateau lavas, distinct from central volcanoes which built the present Etnean volcanic system. Interaction between the central volcanoes, with their summit calderas and failed slopes, produced the other recognised domains. Furthermore, newly identified relevant morphostructural lines are discussed. © Elsevier, Paris     

The 18.7 ka phreatomagmatic flank eruption on Etna (Italy): relationship between eruptive activity and sedimentary basement setting

This paper documents a phreatomagmatic flank eruption that occurred 18 700 ± 100 a BP , on the lower north-eastern slope of Etna during the Ellittico volcano activity, which produced fall and surge deposits. This type of eruption is connected to a sedimentary basement ridge at Etna. The interaction between the rising magma and the shallow groundwater hosted in the volcanic pile overlying the impermeable sediments resulted in phreatomagmatic instead of strombolian activity. Three eruptive phases are distinguished based on field and analytical data: (i) an explosive phreatomagmatic opening, (ii) a main phase producing coarse lithic-rich fallout and a strombolian deposit, and (iii) the final pulsating surge-forming phase. The discovery of this phreatomagmatic flank eruption, which occurred at lower altitude, raises important issues for previous hazard assessments at Etna.     

An unsuccessful attempt at U/Th dating of soil calcretes from the Doukkâli area (western Morocco) and environmental implications

A large number of calcrete samples from topsoils of the Doukkâli area, western Morocco, were studied by U-disequilibrium series methods. The ²³⁰Th/²³⁴U ages are rather uniformly distributed between>350 and 2 ka BP. Homologous samples for occurrence and structure display quite different ages, the ²³⁰Th/²³⁴U ages are considered apparent. This is explained by repeated deposition of secondary calcite in the calcrete pores, which caused lowering of the original ages. However, the apparent ages may be considered minimum ages of the calcrete formation, indicating that calcium carbonate mobilization and deposition has been taking place repeatedly since >350 ka.

The age range obtained is quite comparable with that of calcretes from southern Spain, suggesting similar conditions for the calcrete formation in the two areas.