Paléotectonique linéamentaire et tectonique active des Alpes maritimes franco-italiennes : une synthèse

Abstract

This study intends to establish the relationship between the seismicity of the French-Italian Maritime Alpes and its regionai structural history. The seismotectonic aim of this work involves a deeper examination of strain within the surrounding area of regional lineaments so as to highlight their eventual role during the different stages of the Alpine Orogeny. In order to achieve this a more detailed examination was made as to the proximity of the main lineaments, fault corridors or thrust faults, all being areas where the stress field is strongly disrupted and is a discontinuous spatial function, but for which it is not always possible to define the notion of trajectory stress. It seemed inadequate to establish continuous trajectory stress maps for an area well known for its geological discontinuities, so research was concentrated on the regional as well as the general kinematics of opposing mosaic crustal blocks which allows only a probability approach to seismic risk zones. An analyse was carried out of striated fractures and pebbles to determine the finite strain of the crystalline rocks which form the Argentera Massif and the overlying Permian to Quaternary sediments in the French-ltalian Maritime Alps. An interpretation of the data obtained was carried out using the method of "diedres droits" (Pegoraro, 1972; Angelier and Mechler, 1977) carefully automated by us. It is important to remember that this method gives useful information concerning the shortening and lengthening positions for tectonic trends defined for decametre sized areas. The results obtained using this method were compared to focal earth quake mechanisms which have occurred during the last three decades in the French-Italian Maritime Alps in order to attempt to specify the tectonic factors of regional seismic risk. Shortening trend variability close to the main regional faults. A spatial study of regional ductile finite strain (Guardia et al., 1985; Ivaldi et al., 1986, 1991) and brittle strain in the French-Italian Maritime Alps has revealed not only extensive areas where there is continuous or progressively evolving strain trends, but also restricted zones where the strain trend is discontinuous or rapidly changing. The strain trend discontinuities are aligned along roughly rectilinear corridors and have the appearance of a regional palaeotectonic framework comprising of four major lineament direction: N40°E, N90°E, N120-140°E and N160-170°E. These tectonic corridors have continually influenced both the regional alpine sedimentation and strain throughout all periods. We suggest that these tectonic corridors correspond to a lineament framework which cuts the regional alpine crust into decakilometre crustal blocks. Recurrent faulting in the French-Italion Maritime Alps, throughout the whole of the Alpine Orogeny, along these lineament directions accounts for the marked variability of the shortening trends observed within the sectors lying between ENE and WNW (fig. 1, 2, 3). The shortening trends clearly show that the local strain which is more intense in the hectometric to kilometric width lineament corridors is closely dependant on recurrent faulting of major discontinuities of the continental crust. The two lineament groups which have the most influence on the local strain trend are N40°E and N120°-140° and induce respectively the shortening NW-SE and NE-SW. Taking into account the convergent motion of the alpine and mediterranean tectonic plates these two lineament groups have, since the Paleogene and Neogene, functioned and still function most often as senestral and dextral strike-slip faults during any period of crustal strain in the past (Guardia and Ivaldi, 1985-87). These reactivated faults are characterised by overlaps towards the south and west and have provoked respectively progressive anticlockwise and clockwise rotations of alpine shortening trends. Their passage disrupts the strain field homogenity caused by the movement of the E-W and N-S faults. Lastly, the close spatial association of N40°E senestral strike-slip faults and N120°-140 or N160-170°E dextral strike-slip faults determines reverse backthrustings towards the north (fig. 4). These active lineament groups concern not only the studied alpine socle range but also the pliocene quaternary sedimentary formation covering. The seismicity of the French-Italian Maritime Alps is a current reflection of its regional tectonic history During the last 600 years four historical earthquakes of MSK intensity equal to or higher than VIII have occurred in the French-Italian Maritime Alps. Also within the region there is a flot inconsiderable amount of microseismic activity. Earthquakes maps have been constructed (fig. 5) and their potential magnitude deduced from the earthquake energy (Gutenberg and Richter, 1956) simultaneously over a surface area of 6 km2 (fig. 6). These maps have been made from 1366 earthquakes which occurred between 1977 to 1992 in the French-Italian Maritime Alps and which were recorded by the IPG of Strasbourg. Earthquake magnitudes range from 0.2 to 5.1 with focal depths of between 0.1 and 28.1 km. The location of individual epicentres shows on average an error of about ± 3 km, even so this value is strongly reduced through using of a large number of earthquakes. This large number, and the high quality of the instrumental data recorded during recent years, has allowed greater precision on the restriction of focal solutions (fig. 7). By a comparison of field structural data with the instrumental data il can be shown that the seismicity of the French-Italian Maritime Alps reflects ils tectonic history. Earthquakes generated not only on land but also at sea in the last few decades reflect, in terms of the spatial distribution of their focal mechanisms and their potential magnitude areas, the regional fault pattern and the motion of opposing crustal fault blocks. Seismicity is particularly pronounced along the main N40°E, N120-140°E and N160-170°E lineaments and is even greater where these lineaments intersect each other. The lineament corridors and their junctions correspond to a field area in which elastic strain, stored by the motion and confrontation of opposing crustal fault blocks, advantageously drifts away. This observation is important in explaining the causes of earthquakes as well as anticipating the regional pattern of earthquake hazards. The breaking up of the alpine crust through combined lineament corridors N40° and N120-140°E generates a regional block pattern and whose present movement il is possible to determine through our knowledge of focal mechanisms. For every known mechanism the nodal plane which is retained as the active plane is in accord with one of the focal lineament directions. According to this choice il is possible to outline a regional geodynamical model which takes into account the large crustal blocks materialised by the Mohorovicic discontinuity revealed by the results of the "European Geotraverse" programme (Blundell et al., 1992). In this model (fig. 8, 9) the penninic thrust between Triora and Demonte (fig. 1) correspond at depth to a major Mohorovicic discontinuity and whose surface is strongly uplifted towards the east. To the north of Demonte the frontal penninic thrust is no longer perpendicular to the Mohorovicic discontinuily, but justs out itself towards the West. The Mohorovicic surfaces join up towards the south at the basement of the thinned and oceanized mediterranean crust of the Ligurian-Provençal Basin.