Alessandro Tibaldi

Volcano-tectonic activity along structures of the unstable NE flank of Mt. Etna (Italy) and their possible origin

Abstract We describe the prehistoric–historic activity of the main and minor volcano-tectonic structures of the northeastern flank of Mt. Etna, giving new data which provide a clearer view of how destabilized volcanoes react to magma pressure and gravity forces. Data are mostly based on new lithostratigraphic surveys and on offset measurements of late Pleistocene–Holocene deposits and historic man-made features. The North-East Rift (NE Rift) is a volcanic rift that has guided mostly explosive, but also secondary effusive, activity since the end of the Pleistocene. Monogenetic cones and explosive craters are strongly elongated and aligned along 66 main surface fractures and several others, hidden, but inferred, from sinkhole alignments. The opening direction is locally oblique, with a left-lateral component in the north-eastern segment of the rift. At the NE tip of the rift, the movements are relieved by the normal left-lateral Pernicana Fault (PF), an 11-km-long structure comprising E–W fault segments connected by right-stepping en echelon faults. Our study demonstrates that the very high slip rates at a centennial scale of these structures are well constrained and similar (NE Rift=0.6–1.8 cm/a, PF=0.5–2.2 cm/a) and have been also recognized in the Latest Pleistocene–Holocene (2.7 cm/a). The E–W Fiumefreddo Fault (FF) has normal left-lateral kinematics and a historic slip rate of 0.1 cm/a. Ripa della Naca Faults strike NE and are covered by unfaulted

Influence of cone morphology on dykes, Stromboli, Italy

Abstract The island of Stromboli represents the emerged part of a 2.6-km-high composite volcano elongated in a NE direction. Over the period 100–13 ka BP the morphological evolution of the volcano was characterised by a series of summit caldera collapses between major building phases. From 13 ka BP onward, a series of large lateral collapses occurred northwestwards in one sector of the cone. The results of the field study of the total (94) dyke population is discussed in this paper. They show that during the last 100 ka the majority of dykes were injected with a NNEGSSW to ENE–WSW strike along a NE–SW weakness zone crossing the volcano summit. This is interpreted as a volcanic rift the geometry of which is controlled by regional tectonic stresses. Dykes injected prior to ∼13 ka BP at lower altitudes are generally steeper than those injected at higher elevations. After 13 ka BP, dykes were injected along the NE–SW weakness zone and parallel and close to the margins of the earliest sector collapse, at dips of 20–80° inclined towards the collapse depression. I suggest that this is linked to unbuttressing of the collapse depression or to seaward gravity instability of the depression infill deposits. Extensive emplacement of long dykes along the NE–SW weakness zone occurred only when the volcano regained a conical shape. Some of the dykes emplaced close to the original surface of the cone bent to assume a dip direction parallel to the local slope. I propose that this local change of dyke geometry can be explained by the direct influence of the free surface of the cone leading to a rotation of the least principal stress which assumes an orientation perpendicular to the slope.

Stress pattern evolution in the central sector of the Mexican Volcanic Belt

Abstract Stratigraphic studies and detailed field analyses of brittle and plastic deformations allow the reconstruction of the stress pattern and the tectonic evolution of the central sector of the Mexican Volcanic Belt. Several compressional phases affected the metamorphic basement, while the last one also involved the volcanic cover of Early Miocene age. In Middle Miocene-Early Pliocene times and ENE-WSW extensional phase developed, representing the southern protraction of the Basin and Range Province of the western United States. In Pliocene-Quaternary times there then followed the development of a complicated deformational cycle showing the superficial evidence of a sinistral lateral shear zone. At the beginning the area was affected by a NE-SW compression including reverse, dextral and sinistral strike-slip faulting. This was followed by Early Pleistocene sinistral transtension characterized by a NW-SE direction of least principal stress and sinistral normal-slip faulting. During Late Pleistocene-Holocene times the transtension was concentrated along E-W faults. Tectonic control in the distribution of volcanism has also been recognized. The Basin and Range phase controlled the deposition of the Miocene volcanic and clastic units, particularly the effusion of basaltic lavas connected with the latest extensional movements of the same phase. The Mexican Volcanic Belt sequence is strictly linked with the development of a large E-W sinistral lateral shear zone, active from the Pliocene to the Present, which shows the extensional component progressively prevailing over the shear one.

Geology of Los Azufres Caldera, Mexico, and its relationships with regional tectonics

Ferrari, L., Gardufio, V.H., Pasquar6, G. and Tibaldi, A., 1991. Geology of Los Azufres caldera, Mexico, and its relationships with regional tectonics. In: S.P Verma (Editor), Calderas: Genesis, Structure and Unrest. J. Volcanol. Geotherm. Res., 47: 129-148. The Los Azufres geothermal field is one of several silicic centres of the Mexican Volcanic Belt (MVB) for which a caldera structure was suggested. Geological and structural surveys in a wide area surrounding Los Azufres reveal that this complex is situated in an area of unusual concentration of acid volcanism, consisting of four pyroclastic units and several dome complexes. Although a complete caldera structure is not morphologically evident, several facts suggest a long collapse history at Los A zufres: - the geothermal field lies at the centre of a subcircular depressed area (27 × 26 km in size) filled by a distinctive fluviolaeustrine sequence: - Middle to Late Miocene rocks bound to the south and to the north of this depression and are encountered only at depth inside it; - four large ignimbritic suites of latest Miocene and Pliocenc age outcrop outside the depression; - volume estimations of these pyroclastic products are comparable with the missing volume of the depressed area: - no alternative caldera structures exist in a radius of 200 km from Los Azufres; - dacitic to rhyolitic lavas, principally extruded as dome complexes, were emplaced inside this depression during the Pleistocene. Los Azufres is therefore interpreted as a nested caldera of latest Miocene and Pliocene age, The recent regional tectonic evolution of the central sector of MVB comprises a Late Miocene-Early Pliocene left-lateral transcurrent phase, followed by a Late Pliocene-Quatern ary transtensional one. The silicic volcanism occurring between 6.1 and 2.8 Ma can be linked to the first phase, while in the adjacent areas of MVB a volcanic hiatus can be recognized. The following transtensional phase reached the Los Azufres area only during the Pleistocene and disrupted the caldera structure. Normal faults, developed during this period, controlled the uprising of basic magma which partly interacted with the remaining differentiated one and produced part of the recent intracaldera cycle.

Control of rock rheology on deformation style and slip-rate along the active Pernicana Fault, Mt. Etna, Italy

Abstract The already known part of the active left-lateral strike-slip Pernicana Fault, on the eastern flank of Mt. Etna Volcano, has a Holocene high slip-rate of 1.5–2.7 cm/yr, but its surface trace abruptly disappears eastward approaching the lower slope of the volcano. Using geological, geomorphological and structural surveys we found that the Pernicana deformation zone extends 8 km further to the east, beyond this point, through a different mechanism. As the thickness of the volcanic pile decreases, the single E–W- to ESE-striking main fault plane gives way to a set of faults, arranged in a splay structure, which partitions the fault motion. Here, slip-rates are 0.4 to 1.4 cm/yr on each fault. Further east, where the lava succession is very thin and unfaulted, we suggest that lavas are decoupled from plastic-deforming sub-Etnean clay deposits which absorb the strain, or that lavas have a too low mass to slide. On the very lowest slope of the volcano (i.e. further east), lavas are once again thicker and the Pernicana deformation zone behaves at the surface once again in a brittle manner, locally known as the Fiumefreddo Fault, with a low slip-rate of

Landslides triggered by earthquakes and their relations with faults and mountain slope geometry: an example from Ecuador

Abstract A large number of landslides occurred during two seismic events (respectively, 6.9 and 6.1 Msw) on 5 March 1987 in the Ecuadorian Andes. These landslides have been mapped, digitized, and coregistered with topography at 1:50,000 scale. Geometry of coseismic and Holocene faulting has been assessed integrating field and geophysical data. Landslide distribution and Holocene tectonic features have been compared with earthquake foci, geological deposits, slope area, inclination and orientation, and vegetation cover. The macro-seismic field deduced by landslide distribution is ellipse-shaped with the major axis striking NNE and coinciding with the trend of the coseismic and Holocene faults. Landslide distribution also shows a correlation with respect to the dip-direction of these faults and the orientation of mountain slopes. Slopes parallel to the coseismic fault planes, sloping towards WNW, are almost unaffected by landsliding, while this increases on the slopes at high angle to the fault plane and lying along its strike. If these results could be confirmed in other areas, a method of reconstruction of geometry of seismogenetic faults from topographic effects appears feasible.

The dem of mt. etna: geomorphological and structural implications

AbstractA 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.

Deformation at Stromboli volcano (Italy) revealed by rock mechanics and structural geology

Abstract We approach the reconstruction of the recent structural evolution of Stromboli volcano (Italy) and the analysis of the interplay between tectonics, gravity and volcanic deformation. By tying together structural, lithostratigraphic and rock mechanics data, we establish that since 100 ka BP, the edifice has faulted and jointed mainly along NE-striking planes. Faults mostly dip to the NW with normal displacement. Taking also into account the presence of a NW-trending regional least principal stress and of tectonic earthquake hypocenters inside the cone, we suggest that this fracturing can be related to the transmission of tectonic forces from the basement to the cone. Dyking concentrated along a main NE-trending weakness zone (NEZ) across the volcano summit, resembling a volcanic rift, whose geometry is governed by the tectonic field. In the past 13 ka, Stromboli experienced a reorganisation of the strain field, which was linked with the development of four sector collapses affecting the NW flank, alternating with growth phases. The tectonic strain field interplayed with dyking and fracturing related to unbuttressing along the collapse shoulders. We propose that tectonics control the geometry of dykes inside the cone and that these, in turn, contribute to destabilise the cone flanks.

A giant deep-seated slope deformation in the Italian Alps studied by paleoseismological and morphometric techniques

This work proves the existence of a large deep-seated gravitational slope deformation (DGSD) in a hilly region of the southwestern Alps, whereas DGSD are usually linked with high relief energy in mountain environments. Moreover, we describe the usefulness of applying paleoseismological techniques by means of trench excavation to date and understand the deformation history and genesis of recent morphostructures, and we found evidence of causative relationships between DGSD and surface landslides. The studied DGSD of Mt. Croce della Tola–Mt. Scincina, Italy, is the largest of the western Alps: it is 7-km long and involves a minimum area of 16 km2 with a volume ≥3.5 km3 probably extending further NE beneath Lake Maggiore surface. Several parallel scarps, representing the surface expression of slip-planes, affect the upper part of the slope, whereas the lower part presents a convex profile. DGSD at Mt. Croce della Tola started in the last interglacial period (120–40 ka BP) and the following glacial phase was not able to significantly modify the general slope geometry. Post-glacial deformation also occurred, especially at Mt. Scincina, after 25 ka BP. Post-glacial and active surface landslides developed on the convex lower part of the slope, suggesting that they resulted from instability due to the new profile assumed by the slopes during the deep-seated deformation. The occurrence of Mt. Scincina DGSD with a very low topographic gradient is interpreted as an effect induced by on other large DGSD.

Latest Pleistocene-Holocene tectonics of the Ecuadorian Andes

Abstract A structural study was carried out in the most tectonically active areas of the Ecuadorian Andes, including part of the Interandean Valley, the Cordillera Real and the Subandean Zone. Since the Pleistocene, the region has undergone complex wrench tectonics which are still active. Apparently, two different stress states have coexisted within different parts of the Andean range during the latest Pleistocene and Holocene: 1. (1) The external part of the chain, including the Cordillera Real and the westernmost part of the Subandean Zone, has been subject to a east-northeast-west-southwest direction of greatest principal stress. Associated motions are partitioned between right-lateral oblique thrusting and pure right-lateral strike-slip faulting along north-south to north-northeast trending structures. As a whole, these define a right-lateral transpressive shear zone. 2. (2) In the Interandean Valley, which lies above a major Tertiary suture zone, deformations are consistent with a north-south direction of greatest horizontal principal stress associated with recent development of a left-lateral transtensional shear zone. In the first zone, fault displacements are at least an order of magnitude larger than in the second zone. The present tectonics of the area is explained by a kinematic model, in which the wedge-shaped Cordillera Real crustal block, bounded to the east by a right-lateral shear zone and to the west by a left-lateral shear zone, is being uplifted and extruded northward with respect to the Cordillera Occidental and to the stable South American foreland. This motion is the result of oblique convergence between the Nazca and South American plates.