Luigi Tortorici

Recent and active tectonics in the Calabrian arc (Southern Italy)

Normal faulting which has developed since the Middle Pleistocene, with an overall ESE-WNW extension, is the dominant mode of deformation which has characterized the Calabrian arc up to the present. Quaternary normal faults extend for a total length of about 180 km along the inner side of the arc. The different normal fault segments, which, during the Pleistocene, controlled the evolution of the main marine sedimentary basins, are varying in length from 10 to 20 km and have fault escarpments showing a very young morphology which define the fronts of the major mountain ranges of the arc. The morphological features of the fault escarpments suggest slip rates of 0.8-1.1 mm/yr for the last 700 k.y. and values of 0.6-0.9 mm/yr for the last 120 k.y., indicating a uniform rate of faulting since the Middle Pleistocene. Crustal seismicity and the mesoseismic areas of historical large events (6.5 ⩽ M ⩽ 7.1) which occurred in the Calabrian arc are located within a narrow belt along the hanging walls of the Quaternary normal fault segments. This suggests that the normal faults which dissect the inner side of the Calabrian arc may be seismically active.

Active faulting in the Calabrian arc and eastern Sicily

The Calabrian arc and eastern Sicily are areas of the central Mediterranean where the effects of Quaternary tectonics are well preserved. The most impressive tectonic feature in this region is represented by a major normal fault belt that runs more or less continuously along the inner side of the Calabrian arc, extending through the Strait of Messina along the Ionian coast of Sicily as far as the Hyblean Plateau. The distinct normal fault segments within the belt, which during Pleistocene times have controlled the evolution of major marine sedimentary basins, have lengths ranging from 10 to 45 km. They exhibit huge fault scarps which defines the fronts of the main mountain ranges of the region (Catena Costiera, Aspromonte, Serre, Peloritani and Hyblean). Morphological features of fault scarps, and the age of the faulted rocks, suggest slip rates of 0.5–1.2 mm/year for the last 700 kyear (Middle Pleistocene-Holocene), reaching values of about 2.0 mm/year in the areas of active volcanism. From a seismological point a view, the Calabrian arc and eastern Sicily represent a very active area which is characterized by crustal earthquakes, the largest of which reached in the last nine centuries an intensity of X–XI (6 < M ≤ 7.4). The occurrence of intermediate and deep focus earthquakes located along the inner side of the arc, beneath the southern Tyrrhenian Sea, is associated to the existence of a slab of Ionian lithosphere. The distribution of crustal seismicity shows that most of the events which have occurred in the area, are located in the hangingwalls of the main Quaternary normal faults hence suggesting a strong relationship between seismic activity and the growth of extensional structures. Geological observations, together with seismological data, indicate that normal faulting in the area results from the development of a main rift zone, related to an overall ESE–WNW extension, which also controls the evolutionary history of the magmatism in this sector of southern Italy.

Late Quaternary slip rates on the Acireale-Piedimonte normal faults and tectonic origin of Mt. Etna (Sicily)

Abstract Mt. Etna is located along the east coast of Sicily, near the boundary between the continental crust of the Hyblean Plateau and the Mesozoic oceanic crust of the Ionian basin. The main active faults near Mt. Etna cut the base of its eastern flank, forming a 30 km long system of NNE- and NNW-trending, ene´chelon fault segments (the Acireale-Piedimonte system), showing dip-slip and oblique (right-lateral) motion. Most segments are associated with shallow-depth seismicity and all have Late Pleistocene to Holocene vertical slip rates ranging between 1 and 2 mm/yr, typical of major active normal faults worldwide. Eruptive fissures, arranged in NNE- to NE-trending zones, cut the highest slopes of the volcano, on the footwall of the normal fault system. Structural analysis suggests that current motions along both the active faults and eruptive fissures are kinematically compatible and simply linked with ongoing, WNW-ESE-directed regional extension. Such extension characterizes, at a greater scale, the active tectonics of southern Italy, where all large, shallow, historical earthquakes have remained confined within a narrow normal fault belt or rift zone stretching from the northern Calabrian Arc to southeastern Sicily. The southernmost, west-dipping, faults of that rift zone in Calabria (Aspromonte) cut across the Calabro-Peloritan thrust belt to join the mostly east-dipping, Acireale-Piedimonte faults, along the western boundary of Ionian oceanic lithosphere, which continues southwards as the Malta escarpment. We thus relate magmatism at Etna to dilational strain on the footwall of an east-facing, crustal-scale normal fault at the bend where the Siculo-Calabrian rift zone veers ≈ 60° eastwards as it begins to cross the Calabrian Arc, leaving the east-facing margin of the Ionian Sea, to catch up with the west-facing margin of the Tyrrhenian Sea farther north.

The Calabrian Arc and the Ionian Sea in the dynamic evolution of the Central Mediterranean

In the Central Mediterranean area it is possible to recognize three main realms: the Tyrrhenian Sea, the Pelagian block and the Ionian block. These domains are limited by important fracture zones that are easily recognizable on land and offshore. These fracture zones can be grouped into four main trends: (1) the NW-SE trend which is characterized by dextral horizontal movements; (2) the NE-SW trend which is generally characterized by sinistral lateral horizontal movements; (3) the E-W trend which also shows dextral shear movements; and (4) the N-S trend which is characterized by normal faults only. The Tyrrhenian Sea realm is characterized by a thinned crust of oceanic type and is connected to the Apenninic chain by means of listric faults. From a geophysical point of view, the area shows a high heat flow which follows the main structural trends, and positive gravimetric anomalies which are compatible with seismic refraction data and can be explained by the proposed model. The Pelagian block shows a continental crust, about 20 km thick, which progressively increases beneath the Sicilian chain. Here an abrupt interruption with the Tyrrhenian block takes place. In the Pelagian realm, rifting processes developed according to the NW-SE and N-S trend. The N-S trend characterizes the westernmost zone of the realm where volcanic processes and a relatively high heat flow can also be observed. The Ionian block is characterized by the following: a thinned crust (17–20 km) which increases (40 km) towards the Southern Calabrian chain; a thick sedimentary sequence; high positive Bouguer anomalies (+300 mGal); and very low heat flow (50 mW m−2). Geophysical and structural analyses allowed us to evaluate the post-Tortonian evolution of the whole area under consideration. This evolution resulted in continental crust formation according to a rigid—plastic deformation model. According to our model, the Ionian block can be considered as a domain, characterized by a thinned continental crust and affected by intrusive material derived from the upper mantle. Furthermore, this domain, of probably Jurassic age, can be considered as a pelagic basin bordered by the Pelagian block to the west and by the Apulian block to the east-northeast. This basin, according to its position with respect to the Apenninic and Sicilian chains, has not been affected by tectogenesis and still represents a site of continuous sedimentation. From a kinematic point of view, the Ionian realm can be linked with the Pelagian block shifting towards the east-northeast, whereas it is limited towards the north by an E-W fracture zone which regulates the opening of the Tyrrhenian Sea and the counterclockwise rotation of the Apenninic chain. According to the proposed model, the opening of the Tyrrhenian Sea can be considered as a megaextension-fracture which developed parallel to the maximum principal stress direction and evolved as a triangular-shaped structural feature. In preparing the model the authors have taken into account a large set of data collected both on land and offshore.

Quaternary oblique extensional tectonics in the Ethiopian Rift (Horn of Africa)

Abstract The Ethiopian Rift extends in a northeasterly direction, from Southern Ethiopia to the Afar region. It shows a complex fault pattern, characterised by the interplay of a N30°E—N40°E-trending border fault system with the Quaternary Wonji Fault Belt, which is constituted by right-stepping en-echelon NS to N20°E trending faults. The Wonji Fault Belt affects mainly the rift floor, but it also overlaps with some segments of the margins. Its en-echelon arrangement indicates a left-lateral component of displacement along the rift trend. The general fault pattern of the Ethiopian Rift, as well as mesoscopic fault analyses and structural features of some key areas, indicate the occurrence of a roughly E—W extension, which is compatible with the sinistral shear component of motion along the rift. This paper proposes that oblique rifting related to the E—W direction of extension has been active since the beginning of the Quaternary and that it came after an earlier phase of roughly pure extension orthogonal to the rift trend.

From collisional to rifted basins: an example from the southern Calabrian arc (Italy)

Structural interpretation of available geological and geophysical data carried out along a regional transect extending across the southern Calabrian arc from the Tyrrhenian margins to the Ionian off-shore, demonstrates that a complex interplay between compressional and extensional processes has controlled the evolution of Upper Miocene-Pleistocene sedimentary basins developing in this region. Our data indicate that an Upper Miocene-Lower Pliocene succession outcropping in the southern Calabrian arc represents the infilling of perched basins developed between crystalline basement thrust sheets. A similar tectonic pattern characterises the Plio-Pleistocene basins occurring on the frontal part of the arc in the Ionian off-shore. Upper Pliocene-Pleistocene sediments outcropping along the Tyrrhenian side of the arc reveal that they in fact represent the infilling of extension-related basins. Time-space migration of compressional- and extensional-related sedimentary basins can be explained as the result of different tectonic processes developing on the upper plate as a result of the underplating of the Ionian domain beneath the Calabrian block. In this framework Upper Miocene-Lower Pliocene and Upper Pliocene-Pleistocene perched basins represent, therefore, fore-arc basins developed on the frontal accretionary wedge and/or on the hangingwall buttress of the arc during its southeastwards migration. Late Pliocene-Early Pleistocene extensional tectonics occurring along the Tyrrhenian side of the arc, may indeed be related to accommodation processes which characterised the rear of the wedge to maintain a stable geometry as a result of the underplating of the Ionian crust.

Geometry of the neotectonic stress field in southern Italy: Geological and seismological evidence

The neotectonic regime in southern Italy has been evaluated by making a comparison between all the available structural and seismological data. The area investigated can be subdivided into four distinct zones which are characterized by different stress regimes. In the Southern Apennines the tensile axis of the stress field is oriented approximately NE-SW while the maximum principal stress (σ1) is subvertical. In Northern Calabria, the tensile axis is ESE-WNW and the σ1 axis is almost vertical. In the Catanzaro trough both the tensile axis and the σ1 axis are subhorizontal and act E-W and N-S, respectively. Finally, the Strait of Messina zone is characterized by a tensile axis oriented E-W and by σ1 being subvertical.

Structural evolution of the Lucanian Apennines, southern Italy

Abstract In this paper we present a study of an entire segment of the Southern Apennines of Italy, extending from the Pollino mountain range to the south, to the Agri Valley to the north, in which all the units of the chain are represented. Combining regional information with detailed structural data obtained from the different tectonic units forming the orogenic belt, the structure as well as the tectonic evolution of this portion of the orogenic belt are proposed. The overall architecture of this segment of the Southern Apennines mountain chain represents the result of a complete orogenic cycle in which oceanic subduction, syn-collisional and post-collisional events are well recorded. Structurally, this segment of the orogenic belt exhibits two distinct structural levels, separated by a major detachment surface (the sole thrust of the Accretionary wedge), that occur at the surface in the allochthonous nappes and at depth in the Adria continental margin. The upper level is characterized by an imbricated fan system affecting the allochthonous terranes emplaced during oceanic subduction and the first stages of continent–continent collision. The lower level is represented by a duplex geometry that has developed in the post-collisional stage, during evolution of the foreland migrating thrust system, involving the carbonate platform–basin system of the Adria block. During the last stages of the mountain building process, when the foreland thrust migration was locked by the thickening of the colliding continental crusts, the Southern Apennines were thus affected by a severe strike-slip tectonics that dissects the entire mountain belt deeply modifying the previous thrust geometry.

Pleistocene strike‐slip tectonics in the Lucanian Apennine (southern Italy)

Structural studies carried out in the Lucanian Apennines (Southern Italy) show that strike-slip faulting was the principal mode of deformation of this area during middle-upper Pleistocene time. W-NW to E-SE trending left strike-slip fault systems dissect the entire Apennine mountain belt and affect the preexisting thrust geometry. Strike-slip faults, activated by a roughly E-W shortening, are characterized by different geometries representing the surface response to lateral motion occurring along deep-seated structures. The occurrence of different structural patterns which characterize different segments of strike-slip system is related to (1) the depth of a major decoupling surface which separates the upper tectonic multilayered horizon (Apennines thrust belt system) from the lower rigid horizon (Apulian belt) in which strike-slip structures have originated and (2) the geometric relationships between the strike-slip faults and the thrust belt pattern which characterize the upper horizon. The different segments of the strike-slip system are interpreted as internal deformation developed within a crustal shear zone. This zone, which corresponds to the boundary between the Apulian block and the Apennine chain, is characterized by sinistral movement as a response to the northwesterly convergent motion of the African plate with respect to Europe.

Transtensional tectonics in the Sicily Channel

Abstract Structural, geophysical and volcanological data available for the Sicily Channel demonstrate that the whole area is characterized by the occurrence of transtensional structures activated in a dextral shear zone trending roughly WNW-ESE. These data allowed us to put forward a kinematic model of the area which accounts for both the existence of discrete tectonic depressions and for the localized volcanic activity of this sector of the Pelagian Sea. The proposed model is somewhat different from other rifting mechanisms available for the Sicily Channel in that it may explain the occurrence of extensional features and the associated volcanism in a zone of continental collision through the development of large-scale pull-apart basins involving deep crustal levels.