Erwan Gueguen

Repeated reactivation in the Apennine-Maghrebide system, Italy: a possible example of fault-zone weakening?

Abstract Italy owes its complex geological structure to a double switch in tectonic regime, which involved the opening of the Tethys Ocean during Early Mesozoic time, its closure leading to development of the Apennine-Maghrebide fold-and-thrust belt during the Eocene-Recent interval, and the post-orogenic opening of the Tyrrhenian Sea since Miocene time. This history of tectonic inversion is partly preserved within two major fault zones, the Valnerina Line, in the central Apennines, and the Gratteri-Mount Mufara Line, in central-northern Sicily, which were repeatedly reactivated with different kinematic characters. The relatively long life of these structures indicates that strain was localized along anisotropies inherited from early deformation episodes. However, the progressive widening of both fault zones through time may result from strain-hardening fault-rock behaviour during subsequent deformations, thus suggesting that fault reactivation does not imply fault-zone weakening as is often assumed.

Late-Hercynian shearing during crystallization of granitoid magmas (Sila massif, southern Italy): regional implications

Shearing of regional extent, involving granitoids and underlying mid-crustal rocks of the Sila massif (Calabria, Italy), is analysed in this paper. The deformed granitoids are affected by a wide NNW-SSE oriented deformation zone, stretching for about 60 km, from the neighbourhood of Cecita Lake to Cropani village. Meso- and micro-structures in granitoids, close to the boundary with underlying migmatitic paragneiss, indicate that deformation developed from melt-present to solid-state conditions. Simultaneous tectonics and magmatism activated a plutonic accretionary process at mid-crustal levels. This took place at about 300 Ma and involved hybrid magmas with a dominat contribution from a mantle source. The deformation regime remained steady for a long time during magma crystallization and cooling in subsolidus conditions. The regional top-to-the-W sense of shear in the present geographic coordinates, recorded in the deformed granitoids, seems geometrically consistent with the coeval direction of maximum extension found in another sector of the southern Hercynian belt, suggesting the original position of the Sila basement in this context. Magmatic ativity ended with the intrusion of mafic and felsic magams affected by a very weak deformation, ongoing during the final strain increments of the late-Hercynian stage.

The southern Tyrrhenian Sea margin:an example of lithospheric scale strike-slip duplex

The southern Tyrrhenian Sea margin is dominated by deformations whose kinematics are relatively poorly constrained, and different models have been proposed to account for its recent evolution. Analysis of new structural and space geodesy data, combined with available geophysical information, reveal a deformation field characterised by subhorizontal NW-SE directed shortening and SW-NE directed extension. The main recognised regional-scale structure comprises E-W trending fault zones, namely the Ustica-Eolie Line and the Mt. Kumeta-Alcantara Line, connected by the NW-SE trending Marettimo, Trapani, San Vito, Palermo, Gratteri-Mt. Mufara and Eolie faults. This fault network displays a remarkable dextral strike-slip character, and has the features of a strike-slip duplex, that we indicate as the Southern Tyrrhenian Strike-Slip Duplex (STSSD). Analysis of available geophysical data and regional considerations based on platetectonic reconstructions suggest that the STSSD is a primary feature of lithospheric-scale importance. Our strike-slip duplex model differs significantly from previously proposed passive margin interpretations for the Tyrrhenian Sea margin offshore northern Sicily. The results of this investigation show the effectiveness of an integrated, multidisciplinary approach in the study of active deformation margins

Frontal collapse during thrust propagation in mountain belts: a case study in the Lucania Apennines, Southern Italy

In thrust belts, low-angle tectonic contacts are common, and are associated with the stacking of tectonic units and the resulting regional shortening. The Southern Apennines of Italy, where basin and platform sediments are stacked along low-angle regional thrusts caused by the shortening of the Adria passive margin, are no exception. We studied a portion of a north–south-trending, low-angle regional thrust that separates Apennine platform sediments from Lagonegro basin rocks. To the east of the thrust, klippen composed of platform sediments overlying Lagonegro rocks along a low-angle tectonic contact are present. The klippen were first interpreted as remnants of the regional thrust. We performed a detailed structural analysis of the regional thrust, and we examined the structural setting of the klippen. Field evidence and analysis of map patterns revealed that emplacement of low-angle, foreland-propagating thrusts was followed by extensional deformation accommodated by regional east-dipping, low-angle normal faults. At the base of the klippen, we identified low-angle tectonic contacts with an extensional kinematics. We conclude that the klippen were the result of movements of platform sediments and Lagonegro rocks along low-angle normal faults, and not thrusts as previously interpreted. These faults are cut by more recent, high-angle normal faults associated with the opening of the Agri basin. Collectively, we show that evidence of changes in the tectonostratigraphic architecture, fault geometry and kinematics, and fabrics in the thrusts can reveal the presence of low-angle normal faults, the result of an extensional regime. We expect that our findings will contribute to the understanding of the deformation history of part of the Southern Apennines, and of other mountain belts. The results are also important to understand the transition from compression to extension in the Southern Apennines and in similar orogenic belts.