We document different modes of failure and the hierarchy of the fault development, in the slope-basinal sequence of marls and carbonate grainstones in the eastern part of the Majella Mountain, Central Apennines, Italy. In low-porosity breccia/grainstone units, the early formation of pressure solution seams (pss) and their subsequent shearing are the basic structures leading to fault zone development. In the micrite/marly units, sheared pss may form splays in closing mode (pss) or opening mode fractures (joints) and their subsequent higher order shearing results in faults development. In the high porosity grainstones, the earliest structures are deformation bands of both shearing and compaction modes. However, the later deformation involves pressure solution and the subsequent shearing of the pressure solution seams leading to cataclastic shear zones. We conclude, therefore, that fault zone architecture depends not only on the amount of slip, but also on lithology and rheology, which may have important implications for fluid flow properties of the faults in different carbonate reservoirs.

DUMITRU STĂNICĂ, MARIA STĂNICĂ, LUIGI PICCARDI, EMANUELE TONDI, GIUSEPPE CELLO “Sabba S. Ştefănescu” Institute of Geodynamics of the Romanian Academy, 19-21 Jean-Louis Calderon St, Bucharest 37, 020032, Romania C.N.R. – Istituto di Geoscienze e Georisorse, Via la Pira, 4, 50121 Firenze, Italy 3Dipartimento di Scienze della Terra, Università di Camerino, Via Gentile III da Varano, 62032 Camerino (MC), Italy

In this paper we analyse the geometry, kinematics and dimensional characteristics of fault segments in the north ern-central sector of the Sila Massif (Calabria). The fault zone (named here LF) extends beyond the Ampollino Lake and the Arvo River, and is thought to be the seismogenic structure responsible for the 1638 earthquake. Our data show that LF is composed of two main right-stepping segments arranged en-echelon. The stepover zone corresponds to a relay zone between the two active segments, as proved by the occurrence, in the area, of faulted Holocene sediments. The presence of compressive and extensional jogs with associated push ups and sag ponds along the fault trace, as well as the overall architecture of the fault zone, indicate that the LF is a NW-SE striking left-lateral feature. The structural and dimensional properties of the fault zone, also suggest that active deformation processes associated to LF include mechanical interaction between neighbouring fault segments indicating that those may be considered as part of a single seismogenic structure.

We document different modes of failure and the hierarchy of fault development in the slope-basinal sequence of marls and carbonate grainstones in the eastern part of the Majella Mountain, central Apennines, Italy. In low-porosity breccia/grainstone units, the early formation of pressure solution seams (PS) and their subsequent shearing are the basic structures leading to fault zone development. In the micrite/marly units, sheared pressure solution seams may form splays in closing mode or opening mode (joints) and their subsequent higher order shearing results in fault development. In the high-porosity grainstones, the earliest structures are deformation bands of both shearing and compaction modes. The later deformation, however, involves pressure solution and the subsequent shearing of the pressure solution seams leading to cataclastic shear zones. The resulting fault zone architecture depends on lithology, failure modes, amount of slip, and spatial proximity to the frontal thrust. We distinguish small faults (slip of a few cm up to 1m), intermediate faults (slip between 1 and 10m) and large fault zones (slip>10m). Our observations show that each fault within these dimensional classes has characteristic architecture and damage zone structures controlled primarily by the fundamental failure modes. We conclude therefore that fault zone architecture depends not only on the amount of slip, but also on lithology and rheology, which may have important implications for fluid flow properties of the faults and the associated structures in different intervals of carbonate reservoirs.

Active tectonic processes in south-central Italy have been analyzed by means of spatial and kinematic analysis of fault zones In (i) the axial zones of the Apenninic mountain chain, where most of the seismic strain is released within intramontane basin areas (i.e., Norcia, Fucino, Vallo di Diane, high Agri Valley, etc.); and (ii) in the Gargano promontory, where earthquakes with magnitudes up to 6 and 7 have Occurred. Our work focuses on a better understanding of the most recent deformation processes acting in these areas, and the relative motion between the Adriatic and Tyrrhenian crustal units. In order to derive the information above, we integrated structural and seismological data from the Apennines and the Gargano area. The results of our analysis show that the style of active deformation in both of these areas is broadly consistent with the current stress field. Kinematic data from slip indicators on active fault planes are, in fact, consistent with the geometric proprieties and spatial arrangement of the various fault segments and indicate that most of the active structures are characterized by oblique motion. The overall direction of extension appears to be at fairly high angle to the trend of the Apennines, with a component of horizontal left-lateral motion between Tyrrhenia and Adria.

This study summarises the results of a geological survey carried out in the Monte San Vicino-Cingoli area of the Umbria-Marche Apennines. In this area, the axial culmination of the external Apennine ridge and the widespread occurrence of Messinian and Pliocene deposits provide an almost complete record of the deformation history of the external zones of the chain. The prosecution of the Sibillini Thrust North of the Chienti River valley, never previously ascertained, has been identified within the Miocene units exposed in the western limb of the Cingoli anticline, about 4-5 km away from the mountain front. An important role in the tectonic evolution of the study area is also played by inherited (pre-thrusting) structures. These include both those of the rifted continental margin, of Mesozoic age, and those related to Miocene foreland deformation. The former, well known in the geological literature, are organised in two main fault sets (trending NNE-SSW and WNW-ESE). Synsedimentary Miocene faults, mainly trending NNW-SSE, controlled foredeep turbiditic deposition during early (pre-evaporitic) Messinian times. These faults show very limited or no reversal of slip during contractional deformation. In fact, although mild inversion tectonics has produced some uplift and antiformal folding of hanging wall basin fills, only in one instance (i.e. Avenale Fault) has significant fault reactivation occurred. The onset of contractional deformation within the study area coincides with the end of the deposition of the Gessoso-solfifera Fm. Thrusting appears to have migrated in sequence towards the external zones of our study area up to Early Pliocene times (Globorotalia puncticulata biozone).

On September 26, 1997, at 00.33 h(GMT), a Mw 5.7 earthquake occurred in the axial zone of theUmbria-Marche Apennines of central Italy, in the Colfiorito basin area. At09.40 h (GMT), a Mw 6.0 earthquake again struck the area withinthe Colfiorito basin, a major intramontane basin filled with Quaternarycontinental deposits. The two main shocks, and the associated aftershockswere within a roughly NNW-SSE trending zone of largest damage (Imax10), in which ground deformation has been observed. Along this trend,Cello et al. (1997a) had mapped a few capable faults, showingtranstensional to pure extensional kinematics. Field inspection of themapped faults, carried out after the main shocks, revealed that some ofthem were locally reactivated (for lengths of several hundreds metres andsurface slip in the range of 2–8 cm) during the September 26, 1997earthquakes.