24 A new concept of karst development based on hydrogeology and geophysics and Champagné-Saint-Hilaire Anticline (A3). This structural framework, based on a model of parallel fault alignments extending from the Limousin to the Vendée along a south-Armorican trend N120°E/N140°E (Fig. 5), prevailed until the early 1990s. In the 1980s, this structural model was reinforced by the identification of the Limousin Tonalitic Line (LTL) linking the Parthenay Massif to Limousin granites and the Availles-Limouzine Fault (Dhoste, 1983; Peiffer, 1987; Cuney et al., 2001). North of the threshold, the Marche Fault line is well documented only in the southern Berry area. This fault system between the Massif Central and the Paris basin was long unknown westward beneath the Poitou Threshold. Early indications came from gravimetric anomalies (Goguel, 1954) and a magnetic anomaly map of the Paris Basin (Debeglia, 1980). Figure 6 shows the correspondence between mapped faults and the gravity map of Martelet et al. (2009). The light gravimetric anomaly indicates the Eo-Variscan suture between Gondwana (or Central Armorica) and Laurussia and appears to cross the threshold without faulting in the sedimentary coverage. This contact is also highlighted by a negative Bouguer anomaly (Baptiste, 2017). The Nort-surErdre Fault marks the Ligérienne Province boundary and coincides with the light Bouguer anomaly. This fault is interpreted as an Eo-Variscan suture (Dercourt, 1998) and extends towards the Loudun fault. Goguel’s (1954) heavy gravimetric anomaly north of Poitiers is aligned with the eastern extension of the Thouars fault (Weber, 1973). This anomaly corresponds to a contact of Jurassic and Upper Cretaceous sediments, suggesting that the Thouars fault is a heritage that gave rise to the Thouars granitic intrusion. The Mirebeau fault is aligned with the Thouars fault. However, no faults are mapped east of Mirebeau on the Vouneuil-sur-Vienne geological map (Bourgueil et al., 1976). Seismic activity in this area (e.g., a magnitude 3.2 earthquake in Saint-Léger-La-Pallu on June 14, 2019) reinforces the hypothesis of a fault that extends from the Armorican Massif and is masked by Eocene formations. Further studies, including a study on the Parthenay Fault (Poncet, 1993) and ANDRA’s recognition of the Charroux Granite in the late 1990s, revised the structural map of the bedrock of the Poitou Threshold. These studies revealed three main structures originating from the Armorican Massif (Fig. 7): (i) the Vasles-Availles Fault Zone, passing through Champagné-Saint-Hilaire (formerly the Pouzauges-Oradour axis of Cariou et al., 1989); (ii) the Parthenay Fault, trending N160°E, which connects to the Saint-Maixent and Lezay grabens rather than the Vasles Fault and Champagné-Saint-Hilaire Horst (Poncet, 1993; Cuney et al., 2001; Rolin and Colchen, 2001); (iii) the Basse Marche Fault, south of Poitiers, is linked to the Ligugé Granite and extends to the Thouars Fault (Rolin and Colchen, 2001) via the Mirebeau fault. Rolin et al. (2009) consider these fault systems to be shear zones. Figure 7 is a schematic map of Paleozoic faults and granites from Rolin and Colchen (2001) and
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