3. Sites selection for paleoseismological studies
3.3. Southern limit of the Roussillon neogene basin

3.3.1. Le Boulou area

The southern margin of Roussillon neogene basin is characterised by a sharp topographic contrast between the Palaeozoic axial zone of the eastern Pyrenees (Alberes massif) and the neogene fill up of the basin (Figure 11). This limit corresponds to the Tech fault system composed of two 20km long segments dipping north and oriented ENE–WSW, offset by smaller NE–SW segments. This major fault often cuts the hercynian structures of the Alberes gneissic and shaly block. Since the average topographic relief is about 750–800m, the vertical throw along the fault (with a width of more than 75m of gouge) since the lower Miocene is at least 3600m and about 1600m since the upper Miocene. The northern active front of the Alberes massif presents reverse faults affecting to Miocene and Pliocene series (Philip et al., 1992) and strike-slip component along the valleys of the Tec and Tet faults (Geo-Ter, 1998).

The main ENE-WSW Le Boulou normal segmented fault is clearly identified from satellite imagery and the Alberes block being uplifted along this fault and tilted toward the south. From the satellite and aerial photographs, the Montesquieu lineament, trending E-W, sub parallel to the Boulou fault, is also identified a few hundred of meters to the north along the contact between the Palaeozoic shales and the Mio-Pliocene conglomerate deposits. The surficial trace of the fault is coherent with a south dipping accident extending from the Tech valley to the village of Montesquieu (Calvet, 1994). Along this major structure, there is no field evidence of extensive brittle deformation affecting the plio-quaternary deposits (alluvial fans or terraces) which locally seal the fault eventhough the morphology of the scarp could be interpreted as typical of recently active normal faults. Up to now the only field evidences of post Miocene movements were found along the strike-slip NE-SW structure (Maureillas fault) which offsets the main Boulou fault and along the Montesquieu reverse fault (Figure 22).

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.. Figure 22 : Post-Miocene NE-SW strike-slip fault along the Maureillas lineament
and Post-Miocene W-E reverse fault along the Montesquieu lineament.

This western prolongation already detected through the DEM analysis seems to be confirmed by the presence of an E-W topographic linear scarp which affect the würmian terrace. Due to the presence of underground lifelines a first campaign of reconnaissance trenches has been performed during the summer 1998 in order to determine the best feasible site for a specific paleoseismicity study.

Three preliminary excavations were chosen to quantify the thickness of the different quaternary layers, to identify the paleotopography of the surface of the upper Miocene deposits and to optimise the final location of the paleoseismological trench (Figure 23). The first results obtained through the analysis of these preliminary trenches lead to focus the upcoming studies on a very precise 600 meters long E-W segment located between the Balmourene torrent and the most eastern preliminary trench (T2).

3.3.1.1. Upcoming studies

Until now no geophysical investigations have been performed on this area in the frame of Paleosis project. However due to the fact that this specific site will exactly correspond to the entrance of the future TGV railway tunnel to be realised in the next 5 years across the Alberes massif between France and Spain, a huge amount of geotechnical and geophysical data are now available from the French railways company SNCF. These analysis are under process and the results will be integrated in the near future in this European Paleosis programme. These data campaigns of georadar prospections are projected some selected sites along the Montesquieu fault (Figure 23).

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Figure 23 : Location of the exploration trenches and prospective georadar
profiles in the western prolongation of the Montesquieu fault.