21 1. The Poitou Threshold encompassing most of the Vienne department, the southern part of Deux-Sèvres, and the northern part of Charente. Its average elevation is 146 meters above sea level, with a gradual rise to the east and southeast toward Limousin, reaching 200 meters at Lathus and 225 meters beyond Isle-Jourdain. To the west, the elevation against the Vendée Massif reaches altitudes of 160 to 190 meters” (Welsch, 1892). The adoption of these concepts likely reflects the influence of geographical studies, particularly the maps produced by Vidal-Lablache, which were widely employed in educational settings during the French Third Republic. But the topographic approach suffers from the fact that no elevation was selected to define the threshold’s limits/boundaries. Tectonic structure of the Poitou Threshold: a brief history of knowledge Welsch (1846, 1892) proposed the first tectonic structure of the Poitou Threshold, including two cross-sections: one cross-section that links the old Hercynian Mountain ranges and another that links the sedimentary basins (see Fig. 2 and 3). In the first cross-section (Fig. 2), Welsch noted only one fault: the Montalembert Fault, located south of the Deux-Sèvres department. The bedrock includes two major anticlines that connect ancient massifs: the Champagné-Saint-Hilaire anticline and the Ligugé anticline. The Poitou area was described as a strait (isthme in French on Fig. 2). The second cross-section (Fig. 3) links the Armorican and Limousin massifs, showing sub-horizontal sedimentary layers between Ménigoute (West) and AvaillesLimouzine (East). The bedrock crops out at both ends. In this figure, the Poitou area was identified as a threshold (Seuil in French in Fig. 3). Glangeaud’s observations refined earlier interpretations by identifying faults that isolate the Champagné-Saint-Hilaire anticline from “collapsed layers” (Glangeaud, 1895). Fournier (1903) later produced a geologic map of the Poitou Threshold. After World War I, Mathieu (1937) conducted additional structural studies for his thesis on Paleozoic terrains in the Vendée region. He identified structural links between the Limousin and Gâtine Hills, revisiting the mapped features of Welsch. Mathieu described four major structural axes, listed from north to south (Fig. 4): A4 – the Ligugé Anticline; A3 – an anticline extending from Le Fouilloux, through Lusignan and Champagné-Saint-Hilaire to Availles-Limouzine; A2 – The Rouillé-Couhé-Civray Axis (also called the SaintSauvant Anticline), and A1 – a complex anticline connecting Mervent to the Montalembert Horst. Axes A5 and A6 were not traced further east due to a lack of structural markers. The major axes were later integrated into the regional geological guide (Gabilly et al., 1978) under the names Essarts-Mervent-Melle-Montalembert Anticline (A1)
RkJQdWJsaXNoZXIy NjA3NzQ=