Giuseppe Cello

Active tectonics in the central Apennines and possible implications for seismic hazard analysis in peninsular Italy

Abstract The central Apennines fault system (CAFS) of peninsular Italy, overprints earlier structures of a Neogene fold and thrust belt and includes segments characterized by diffuse seismicity distributed within a NNW-SSE-trending zone, 50–60 km wide. The system has been analysed by means of morphotectonic and structural analysis of exposed active fault segments. The resulting fault structure consists of an interconnecting network of roughly N-S-trending, left-lateral, strike-slip segments and mostly NW-SE-oriented, transtensional to normal faults. Evidence for recent activity of CAFS structures is provided by faulted Middle Pleistocene-Holocene deposits (including 30–40-ka-old pyroclastites and 40-ka-old palustrine sediments), fresh scarps in both bedrock and Late Quaternary continental deposits, and decametric lateral offsets locally affecting the post-Wurmian drainage pattern of the area. The regional stress field responsible for the development and evolution of the CAFS, as inferred from fault slip data, is characterized by a NW-SE compression and by a NE-SW extension. The CAFS pattern and its present-day kinematics have been related to left-lateral strike-slip motion on north-south-trending crustal faults. The existence of deep-seated strike-slip faults in the central Apennines has implications for seismic hazard analysis. Motion along these structures suggests, in fact, that coseismic surface faulting is distributed, and that cumulative displacements include normal, transtensional, and strike-slip components. The seismogenic potential of CAFS structures can therefore be best described by multiple-rupture models and be better analysed in terms of partial contributions of lower-rank features constituting congruent structural associations within the system.

Apennine tectonics in southern Italy: a review

Abstract The southern Apennines thrust system developed in Late Cretaceous to Quaternary times in response to deformation processes induced by the convergence between the African and European plates. The structural associations recognized in the area permit us to attribute these processes to three main evolutionary stages: an oceanic subduction, an obduction and a collisional to post-collisional stage. Each stage covers a time period of about 10 7 years and comprises several deformation sequences of similar time scales (about 7–9×10 6 years). In the southern sectors of the chain, in the Calabria–Lucania borderland area, the earliest phases of deformation of the oceanic accretionary wedge are recorded by the metamorphic and structural signature of oceanically-derived rock units. The progressive growth of the southern Apennines thrust system is attested by the overprinting relationships existing among the different structural assemblages characterizing the obduction, collisional and post-collisional stages. The latter includes the Early Pleistocene development of strike-slip faults, which are responsible for strong lateral variations in the crustal structure of the chain, and for the compartimentalization of the system into various sectors displaying different subsurface features.

Structural styles, chronology rates of deformation, and time-space relationships in the Umbria-Marche thrust system (central Apennines, Italy)

Structural interpretation of geological and geophysical data available for the central Apennines (Italy) allowed us to draw, balance, and restore three geological profiles across the external zones of the Umbria-Marche thrust system and to evaluate the timing and rates of deformation in a roughly 4 Ma time window (from late Messinian to late Pleistocene time). From this data set we established a general correlation between spacing L and thickness D of the thrust sheets (which was used as a depth correction factor for deriving local versus regional depths to the sole thrust) and a time-space relationship between the closing time of activity of each thrust and the distance X from a reference point within the system.

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.

Reconstruction of continental margin architecture deformed by the contraction of the Lagonegro Basin, southern Apennines, Italy

Thrust propagation through previously rifted continental margins may result in fold and thrust belts whose structure is strongly controlled by the inherited basin architecture, as it occurs in southern Italy. The Lagonegro units of the southern Apennines comprise a deformed pelagic basin succession showing variable stratigraphic characteristics, mainly lateral variations in both facies and thickness, interpreted to be due to a complex basin topography related to a Triassic rifting event. In contrast to previous studies, cross-section balancing and restoration indicate that the Lagonegro units exposed in the high Agri Valley area suffered relatively limited internal shortening (8 km, i.e. 35%). Early deformation of these rocks, later incorporated into a large-displacement thrust sheet, was dominated by folding around (present-day) roughly north–south-trending axes. The attainment of a regional décollement level was favoured by an early mild inversion of the basin, producing a roughly similar structural elevation of both hanging-wall and footwall successions to Mesozoic faults. Most of the contractional deformation was accommodated by buckling of the Mesozoic syn-rift strata between synsedimentary faults, which represented major mechanical interfaces. Early strain localization in the Lagonegro Basin ahead of the active thrust front was most probably mechanically controlled by a faulted crustal segment which originally lay, within the continental margin, between two massive carbonate platforms.

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.

Pre-orogenic tectonics in the Umbria-Marche sector of the Afro-Adriatic continental margin

Abstract The pre-orogenic deformation of the Afro-Adriatic paleomargin during Jurassic–Paleogene times is recorded in the Umbria–Marche regions of Central Italy by facies differences, lateral thickness variations and, most importantly, by faults. Structural data suggest the persistence of normal faulting not only during rifting (Late Trias–Bathonian) but also during drifting and more precisely in Late Cretaceous times. Syn- and post-rift extension caused the development of mainly NNW–SSE- and ENE–WSW-trending structures. Post break-up normal faults are found in several localities of the Apennines and generally coincide with older, syn-rift sites of stretching. One-dimensional numerical modelling of subsidence suggests that thinning during rifting was about 15%, compatible with the stretching factor reconstructed from geological sections. Late Cretaceous thinning was obviously less but still significant and estimated at ca 5%. The acquisition and analysis of the stratigraphic and structural data presented in this study allowed us to correlate the subsidence history of the Umbria–Marche basin with the tectonic events that occurred in pre-orogenic times, in this sector of the Apennines, thus deriving useful indications about the pre-Oligocene evolution of this segment of the perimediterranean mountain belt.

Structure and deformation processes in the Strait of Sicily “rift zone”

Abstract The Strait of Sicily “rift zone” (SSRZ) is a young tectonic feature (Early PlioceneRecent) cutting across the Pelagian Sea. It is characterized by a thin continental-type crust which progressively thickens towards the African continental margin and beneath the Sicilian chain. Geophysical data available for the whole area show that extensional faulting is active in many places along structures trending roughly NNW-SSE, but that the dominant mode of deformation, at a regional scale, can be best described in terms of strike-slip tectonics. Strike-slip faults trending NW-SE are seismically documented west of the island of Pantelleria, in the eastern sector of the Malta graben and along the southern border of the Medina graben. Similar features have also been mapped on land in Malta and Pantelleria. Kinematic analysis of these structures suggest that the SSRZ cannot be considered as a classical intracontinental rift, because the deformation does not appear to be dominated by a generalized thermal instability but rather by compressive stresses in a transcurrent tectonic regime. Therefore an alternative model which accounts for the activation of large-scale right-lateral strike-slip faults seems to be more suitable. Within this main dextral shear zone the progressive development of pull-apart basins could be responsible for the incipient separation of the Sicilian microplate from the African continent and for the thinning of the crust and the associated volcanic activity of the northwestern sector of the SSRZ.

Structural signature of tectonic processes in the Calabrian Arc, southern Italy: Evidence from the oceanic‐derived Diamante‐Terranova unit

The Diamante-Terranova unit is an oceanic-derived element within the thrust and nappe structure of the northern sector of the Calabrian Arc. Its metamorphic and structural signature reveals a long history of SE dipping subduction and collision-related deformation of an oceanic lithospheric section underlying the narrow Diamante-Terranova basin. The deformation history of the Diamante-Terranova unit may be related to three main tectonic phases. The first one is characterized by a structural association which includes features that developed during a progressive deformation related to the evolution of a deep-seated shear zone. Kinematic indicators characterizing subduction-related structures clearly show a top-to-the-northwest sense of overthrust shear. The second tectonic phase that can be recognized in the area is characterized by structures that may be related to the onset of shear deformation induced by a postcollisional northeastward motion of the overriding plate. The third tectonic phase is recorded by brittle features related to the latest tectonic events affecting the Calabrian Arc during the opening of the Tyrrhenian Sea. Shear sense indicators and available radiometric and stratigraphic information, together with plate kinematic data, allowed us to infer the time evolution of this sector of the Calabrian Arc and to assess that the Diamante-Terranova unit cannot be considered as an “Eoalpine” element since its deformation, related to subduction-collision processes, occurred prior to the end of the Cretaceous.

Fault zone characteristics and scaling properties of the Val d’Agri Fault System (Southern Apennines, Italy)

Abstract Systematic in situ analysis of active fault zones in Val d’Agri (southern Italy) suggests that the acquisition of quantitative data on fault-related structural discontinuities is fundamental for (1) discriminating between the latter and older regional features, (2) defining the architecture and related permeability structure of faults, and (3) performing simple statistics in order to evaluate the validity range within which fault characteristics may be considered to be fractals. This type of information can be integrated with regional seismotectonic analysis in order to asses the present-day conditions of deformation characterising the area, and to constrain the possible kinematics of the seismogenic structures controlling earthquake activity in this sector of the southern Apennines. The results of our study are also of interest for modelling earthquake sources, since a knowledge of the permeability structure and scaling properties of a faulted rock volume is potentially relevant for simulating the time and space dependent behaviour of fault zones during a seismic cycle.