Stefano Mazzoli

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.

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 (8u2009km, 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.

Active tectonics of the Northern Apennines and Adria geodynamics: new data and a discussion

Abstract The active geodynamic setting of the Northern Apennines is characterised by extension in the axial zone of the chain, and by a more complex tectonic behaviour in the frontal part of the belt. In the latter sector, moderate seismicity occurs, displaying compressional, strike-slip and extensional focal plane solutions with variably oriented P and T axes. For this area, a review of available geological and geophysical data has been integrated by the analysis of seismic reflection lines calibrated with deep well logs. This study confirms that, as already suggested by some previous workers, thrusting and related folding in the study area ceased in Early Pleistocene times. This feature is in contrast with the hypothesis of active thrusting related to a subducting lithospheric slab beneath the chain—an issue which is largely debated based on available geophysical information. Our analysis shows that the Northern Apennines are characterised by an active tectonic setting which is similar to that of the central and southern portions of the belt. These areas all display a Late Quaternary inactivity of the thrust front. NE–SW oriented extension (perpendicular to the strike of the orogen) is well established in their axial zones, whereas a less homogeneous stress field characterises their external sectors and the adjacent foreland. Within this framework, the seismotectonic behaviour of the Northern Apennines—and probably of the whole Italian peninsula between the Po Plain and the Southern Apennines (north of the Calabrian Arc)—may be interpreted as essentially controlled by two main processes. The first of them involves tectonic uplift, possibly related with slab detachment and associated unbending of the foreland plate. The second process consists of a present-day northwestward motion of the Adria block with respect to stable Europe.

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.

The October–November 2002 Molise seismic sequence (southern Italy): an expression of Adria intraplate deformation

A new seismotectonic model for the outer Central-Southern Apennines, including the epicentral area of the 2002 Molise seismic sequence, and adjacent foreland is proposed. The seismic sequence is related to foreland deformation affecting the Adriatic lithosphere. Such deformation is characterized by active NW–SE compression, resulting in the reactivation, mainly as dextral strike-slip faults, of pre-existing east–west oriented structures, both offshore and onshore. One of these structures is interpreted to extend westward and to reach the epicentral area of the recent earthquakes. NW–SE compression, ultimately related with Africa–Europe plate convergence, and uplift would essentially control the active tectonics of the analysed region.

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.

The crustal fault structure responsible for the 1703 earthquake sequence of central Italy

Abstract An active crustal shear zone has been recognized in the central Apennines (Italy) by detailed investigations of the surface fault trace pattern of the axial sectors of the mountain belt. Our data suggest that a 16–18 km thick seismogenic layer rests above the ductile lower part of this shear zone and is affected by an interconnecting Late-Quaternary fault system which includes three main transcurrent (left-lateral) fault sets trending roughly north-south. Several lines of evidence indicate that the central fault set is the seismogenic structure responsible for generating the 1703 seismic sequence of central Italy. Fault slip data, morphotectonic and paleoseismological observations, together with fractal statistics, suggest that coseismic deformation within the epicentral area of the 1703 earthquake sequence is partitioned among a few linked faults, and that the ratio of vertical vs. horizontal slip rates within the active central fault set is about 1:3. Slip rate estimates also show that recurrence times for 1703 type earthquakes are in the range of a millennium. The seismogenic fault structure of the 1703 earthquake sequence may be viewed as a multiple rupture zone with an immature fractal geometry evolving within a crustal volume which deforms in response to transcurrent motion. The main results of this work support our view that the seismogenic potential of this sector of the Apennines can be evaluated by integrating, into a fractal model, the effects due to higher and lower-rank structural components of the Late Quaternary fault system of central Italy.

Fault properties and fluid flow patterns from Quaternary faults in the Apennines, Italy

The study of Quaternary fault zones in the High Agri Valley (southern Italy) and in the axial zones of the central Apennines, allowed us to collect information on the permeability structure, fluid characteristics, and scaling properties of the main fault zones exposed in the area. n nDetailed structural mapping allowed us to derive the appropriate values of the fractal dimension characterising different active fault zone patterns and to evaluate the basic parameters needed for assessing the architecture and related permeability structure of the mapped faults. Scan line and scan area analysis helped in constraining the scaling properties of some of the attributes (i.e. fault zone thickness, fracture spacing etc.) of the main fault zones and in defining their validity range. Our results suggest that, in the Apennines, (i) extrapolation of the architectural indices of a fault zone is admissible over three orders of magnitude, and (ii) the composition of the analysed fluid inclusions and related homogenisation temperatures are associated with fault-driven fluid circulation from both superficial and deep (4–6 km) levels.

Tectonic burial and exhumation in a foreland fold and thrust belt: the Monte Alpi case history (Southern Apennines, Italy)

Abstract The Monte Alpi area of the Southern Apennines represents the only sector of the thrust belt where the reservoir rocks (i.e. Apulian Platform carbonates) for major hydrocarbon accumulations in southern Italy are interpreted to crop out. Tectonic evolution and exhumation of this area were analysed by integrating stratigraphic and structural data with different organic and inorganic parameters which record the burial and thermal evolution of the sediments (vitrinite reflectance, fluid inclusions, and I/S mixed layers in clayey sediments). Our analyses suggest that the presently exposed Monte Alpi structure suffered a loading of ca. 4000xa0m, owing to the emplacement of allochthonous units in Early Pliocene times. Available geological data indicate that erosion of the tectonic load occurred since the Late Pliocene, when the area first emerged. This implies an average exhumation rate in excess of 1xa0mm/year. A model can be constructed which matches the maturity indices and also takes into account intermediate stages of the evolution, resulting from combined structural and fluid inclusion data. By this model, a first stage of exhumation would have taken place at an average rate of about 0.36xa0mm/year. This was controlled by uplift and erosion associated with both: (i) thrusting at depth within the Apulian carbonates (Late Pliocene), and (ii) strike-slip faulting (Early Pleistocene). A second exhumation stage would have occurred in the last 700xa0ky at a much faster rate (ca. 4xa0mm/year) as a result of extensional tectonics.