Inversion kinematics at deep-seated gravity slope deformations revealed by trenching techniques

Abstract. We compare data from three deep-seated gravitational slope deformations (DSGSDs) where palaeoseismological techniques were applied in artificial trenches. At all trenches, located in metamorphic rocks of the Italian Alps, there is evidence of extensional deformation given by normal movements along slip planes dipping downhill or uphill, and/or fissures, as expected in gravitational failure. However, we document and illustrate – with the aid of trenching – evidence of reverse movements. The reverse slips occurred mostly along the same planes along which normal slip occurred, and they produced drag folds in unconsolidated Holocene sediments as well as the superimposition of substrate rocks on Holocene sediments. The studied trenches indicate that reverse slip might occur not only at the toe portions of DSGSDs but also in their central-upper portions. When the age relationships between the two deformation kinematics can be determined, they clearly indicate that reverse slips postdate normal ones. Our data suggest that, during the development of long-lived DSGSDs, inversion kinematics may occur in different sectors of the unstable rock mass. The inversion is interpreted as due either to locking of the frontal blocks of a DSGSD or to the relative decrease in the rate of downward movement in the frontal blocks with respect to the rear blocks

Reconstruction of the eruptive activity on the NE sector of Stromboli volcano: timing of flank eruptions since 15 ka

A multidisciplinary geological and compositional investigation allowed us to reconstruct the occurrence of flank eruptions on the lower NE flank of Stromboli volcano since 15 ka. The oldest flank eruption recognised is Roisa, which occurred at ~15 ka during the Vancori period, and has transitional compositional characteristics between the Vancori and Neostromboli phases. Roisa was followed by the San Vincenzo eruption that took place at ~12 ka during the early stage of Neostromboli period. The eruptive fissure of San Vincenzo gave rise to a large scoria cone located below the village of Stromboli, and generated a lava flow, most of which lies below sea level. Most of the flank eruptions outside the barren Sciara del Fuoco occurred in a short time, between ~9 and 7 ka during the Neostromboli period, when six eruptive events produced scoria cones, spatter ramparts and lava flows. The Neostromboli products belong to a potassic series (KS), and cluster in two differently evolved groups. After an eruptive pause of ~5,000 years, the most recent flank eruption involving the NE sector of the island occurred during the Recent Stromboli period with the formation of the large, highly K calc-alkaline lava flow field, named San Bartolo. The trend of eruptive fissures since 15 ka ranges from N30°E to N55°E, and corresponds to the magma intrusions radiating from the main feeding system of the volcano

Dyke-induced graben formation in a heterogeneous succession on Mt. Etna: Insights from field observations and FEM numerical models

The most common way of magma transfer towards the surface is through dyking. Dykes can generate stresses at their tips and the surrounding host rock, initiating surficial deformation, seismic activity, and graben formation. Although scientists can study active deformation and seismicity via volcano monitoring, the conditions under which dykes induce grabens during their emplacement in the shallow crust are still enigmatic. Here, we explore through FEM numerical modelling the conditions that could have been associated with dyke-induced graben formation during the 1928 fissure eruption on Mt. Etna (Italy). We use stratigraphic data of the shallow host rock successions along the western and eastern sections of the fissure that became the basis for several suites of numerical models and sensitivity tests. The layers had dissimilar mechanical properties, which allowed us to investigate the studied processes more realistically. We investigated the boundary conditions using a dyke overpressure range of 1–10 MPa and a local extensional stress field of 0.5–2 MPa. We studied the effect of field-related geometrical parameters by employing a layer thickness range of 0.1–55 m and a variable layer sequence at the existing stratigraphy. We also tested how more compliant pyroclastics, such as scoria, (if present) could have affected the accumulation of stresses around the dyke. Also, we explored how inclined sheets and vertical dykes can generate grabens at the surface. We propose that the mechanical heterogeneity of the flank succession and the local extensional stress field can largely control both the dyke path and dyke-induced graben formation regardless of the tested dyke overpressure values. Similarly, soft materials in the stratigraphy can greatly suppress the shear stresses in the vicinity of a propagating dyke, encouraging narrow grabens at the surface if only the fracturing condition is satisfied, while inclined sheets tend to form semigrabens, respectively. Finally, we provide some insights related to the structural evolution of the 1928 lateral dyking event. All the latter can be theoretically applied in similar case studies worldwide

Geometry, oblique kinematics and extensional strain variation along a diverging plate boundary : the example of the northern Theistareykir Fissure Swarm, NE Iceland

The boundary between the American and European plates emerges in Iceland, an outstanding natural laboratory where it is possible to analyse ongoing rifting processes. In the North Volcanic Zone, we studied with unprecedented detail an active rift, known as the Theistareykir Fissure Swarm (ThFS). We surveyed an area of 85 km2 with 694 measurement sites along 1537 post-Late Glacial Maximum extension fractures. In the southern sector of the study area, fractures strike N30–40° with opening directions about N120°. Fractures in the central sector strike about N00° and opening directions are N90–100°. In the northern sector, fractures strike about N30° with opening directions about N125°. Through a comparison with older faults cropping out in the substrate at the shoulder of the ThFS, we are able to suggest that variations in fracture strike are the effect of substrate structural inheritance as well as the possible interaction with the Tjörnes Fracture Zone. With regard to kinematics, we highlight that most fractures show a small, but systematic, strike-slip component (a more frequent right-lateral component and a less common, left-lateral one). This cannot be explained as the result of fracture strike rotation relative to the regional, tectonic least principal stress. We conclude that the net opening directions can result from the combination of tectonic offsets and events caused by shallow magma chamber inflation and/or dyke intrusions. The latter can produce transcurrent components of displacement along new or already existing fractures

Virtual geosite communication through a webgis platform: A case study from santorini island (Greece)

We document and show a state-of-the-art methodology that could allow geoheritage sites (geosites) to become accessible to scientific and non-scientific audiences through immersive and non-immersive virtual reality applications. This is achieved through a dedicated WebGIS platform, particularly handy in communicating geoscience during the COVID-19 era. For this application, we selected nine volcanic outcrops in Santorini, Greece. The latter are mainly associated with several geological processes (e.g., dyking, explosive, and effusive eruptions). In particular, they have been associated with the famous Late Bronze Age (LBA) eruption, which made them ideal for geoher-itage popularization objectives since they combine scientific and educational purposes with ge-otourism applications. Initially, we transformed these stunning volcanological outcrops into geo-spatial models—the so called virtual outcrops (VOs) here defined as virtual geosites (VGs)—through UAV-based photogrammetry and 3D modeling. In the next step, we uploaded them on an online platform that is fully accessible for Earth science teaching and communication. The nine VGs are currently accessible on a PC, a smartphone, or a tablet. Each one includes a detailed description and plenty of annotations available for the viewers during 3D exploration. We hope this work will be regarded as a forward model application for Earth sciences' popularization and make geoheritage open to the scientific community and the lay public. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Surface deformation during the 1928 fissure eruption of Mt. Etna (Italy): Insights from field data and FEM numerical modelling

The 1928 CE volcanic activity on eastern Etna, Italy, produced wide surface deformation and high effusion rates along fissures, with excess volumes of about 50 million m(3) of lavas. This, in conjunction with the low elevation of the main eruptive vents (1150 m a.s.l.), caused the destruction of the Mascali town. Our research focuses on a multidisciplinary study from field observations and Finite Element Method modelling through COMSOL Multi-physics (R), with the aim of reconstructing the geometry, kinematics and origin of the system of faults and fissures formed during the 1928 event. We collected quantitative measurements from 438 sites of azimuth values, opening direction and aperture amount of dry fissures, and attitude and vertical offsets of faults. From west to east, four volcanotectonic settings have been identified, related to dike propagation in the same direction: 1) a sequence of 8 eruptive vents, surrounded by a 385-m wide graben, 2) a 2.5-km long single eruptive fissure, 3) a half-graben as wide as 74 m and a symmetric, 39-m-wide graben without evidence of eruption, 4) alignment of lower vents along the pre-existing Ripe della Naca faults. Field data, along with historical aerial photos, became inputs to FEM numerical models. The latter allowed us to investigate the connection between diking and surface deformation during the 1928 event, subject to a range of overpressure values (1-20 MPa), host rock properties (1-30 GPa) and geometrical complexity (stratigraphic sequence, layer thickness). In addition, we studied the distribution of tensile and shear stresses above the dike tip and gained insights into dike-induced graben scenarios. Our multidisciplinary study reports that soft (e.g. tuff) layers can act as temporary stress barriers and control the surface deformation scenarios (dike-induced graben, single fracture or eruptive fissures) above a propagating dike by suppressing the distribution of shear stresses towards the surface

Tectonic control over the abuli samsari volcanic ridge, lesser caucasus, Georgia

The purpose of the present work is to integrate previous research focused on the Abuli Samsari Volcanic Ridge, situated in the Javakheti Highland, Georgia. Through a synergic approach, consisting in the collection and analysis of field and satellite data, combined with the results of previously published research, we have been able to define the overall structure of the volcanic ridge, which, on its northern sector, is cut across by two parallel pipelines, carrying oil and gas from the Caspian Sea to the western countries. Despite the likelihood of seismic or volcanic events in the area, geohazard assessment had never been adequately performed for this section of the pipelines’ route across Southern Georgia. The most relevant outcomes of our effort, aimed at filling this critical gap, consist in: the identification and mapping of eruptive centers and tectonic lineaments; the reconstruction of magma pathways; the definition of the expected moment magnitude for possible earthquakes; the assessment of orientation of the maximum horizontal stress from the Late Miocene to the present day. We have used these results to evaluate the current seismic and volcanic hazards affecting the Abuli Samsari Volcanic Ridge, which may have major impacts on the security of the pipelines. The calculated, about N-S directed maximum horizontal stress may play a key role in volcanic reactivation, which might occur in the form of fissure eruptions and the formation of new vents and monogenetic as well as composite volcanoes. As the track of the pipelines lies just north of the younger volcanic edifice in the ridge we suggest that these lifelines could be severely affected by possible future volcanic eruptions, which might bring about a major interruption in oil delivery from the Caspian Sea towards the west