<p>This paper retraces the history of tectonic discoveries in the Simplon area, in the Swiss-Italian Alps, from the second half of the nineteenth century to the end of the twentieth century. The Simplon tunnel project fostered numerous geological studies in this area during the second half of the nineteenth century, hence providing an excellent record of the evolution of thoughts and observations on one and the same geological object. The inverted basement/cover contacts, first recognized and described in this area by Studer (1851), became the main evidence for the innovative interpretations of Gerlach (Die Penninischen Alpen. Beiträge zur Geologie der Schweiz, Denkschriften der Schweizerischen Naturforschenden Gesellschaft 23/1, 1869), describing, for the first time, basement nappes, and even quantifying their amount of displacement, albeit not coining a term for them. Golliez (1894), Schardt (Arch Sci Phys Nat Genève 30:570–583, 1894a) and Schardt (Livret-Guide Géologique dans le Jura et les Alpes de la Suisse dédié au Congrès géologique international, 1894b), in spite of numerous errors, provided for the first time completely interpreted cross sections of the Simplon area, i.e. tracing one and the same tectonic marker continuously, from one end to the other of the section. These cross sections illustrated conceptual interpretations together with the observed structures, and these interpretations were repeatedly modified through time. The turning point for the final interpretation came from Lugeon (Bull Soc Neuchatel Sci Nat 65:25–53, 1901), who did not accept the idea of opposed verging folds, convinced that fold vergence represents the sense of displacement. His global concept of the Alpine nappe system directed everywhere northward, allowed him to conclude that some basement synforms were nothing but refolded anticlines, representing nappe fronts. Lugeon (1901) showed for the first time the superposition of different basement nappes, all showing the same vergence. This nappe stack was suggested to provide the image of deeper, Alpine crustal levels (Argand in Bull Soc Vaudoise Sci Nat 45(166):217–276, 1909; Argand in Les nappes de recouvrement des Alpes Pennines et leurs prolongements structuraux. Beiträge zur Geologische Karte der Schweiz. Neue Folge, XXXI Lieferung, 1911; Argand in Eclog Geol Helv 14:145–191, 1916), as deep as ca. 15 km. These cross sections, displaying the entire orogenic crust were the starting point for a step-wise retro-deformation on the orogenic scale (Argand 1916), thus bringing the temporal element in a debate that had previously been limited to space and geometry. After a long period in which tectonic advancement consisted mainly in detailed, regional structural work (e.g., Bearth in Eclog Geol Helv 49:267–278, 1956a; Bearth in Eclog Geol Helv 49:279–290, 1956b; Bearth in Eclog Geol Helv 50:161–170, 1957), in the 1970’ies the Simplon area became the object of renewed interest and a new structural approach. Milnes (Geol Soc Am Bull 84:269–274, 1973) refined the structural maps of the area, precisely mapping the traces of major axial planes, and showing their mutual overprinting relationships, thus defining a sequence of tectonic events (Milnes in Eclog Geol Helv 67:333–348, 1974a; Milnes in Bull Geol Soc Amer 85:1727–1732, 1974b). In spite of this rigorous and detailed structural work, and some brilliant intuitions (Jäger in Tscherm Min Petr Mitt 11:304–316, 1966), it was only at the beginning of the 1980’s that the Simplon Fault was clearly recognized as a major extensional structure (Mancktelow in Eclog Geol Helv 78:73–96, 1985), cross-cutting and separating the Simplon section into two tectonic units with different exhumation paths. The inferred occurrence of coeval horizontal shortening and extension allowed to shift the debate from the field of structures and kinematics to the one of dynamics.</p> Graphical abstract <p></p>

Evolving geological concepts in the Simplon Alps: milestones in the history of tectonics

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2026年1月22日

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Claudio L. Rosenberg

摘要

This paper retraces the history of tectonic discoveries in the Simplon area, in the Swiss-Italian Alps, from the second half of the nineteenth century to the end of the twentieth century. The Simplon tunnel project fostered numerous geological studies in this area during the second half of the nineteenth century, hence providing an excellent record of the evolution of thoughts and observations on one and the same geological object. The inverted basement/cover contacts, first recognized and described in this area by Studer (1851), became the main evidence for the innovative interpretations of Gerlach (Die Penninischen Alpen. Beiträge zur Geologie der Schweiz, Denkschriften der Schweizerischen Naturforschenden Gesellschaft 23/1, 1869), describing, for the first time, basement nappes, and even quantifying their amount of displacement, albeit not coining a term for them. Golliez (1894), Schardt (Arch Sci Phys Nat Genève 30:570–583, 1894a) and Schardt (Livret-Guide Géologique dans le Jura et les Alpes de la Suisse dédié au Congrès géologique international, 1894b), in spite of numerous errors, provided for the first time completely interpreted cross sections of the Simplon area, i.e. tracing one and the same tectonic marker continuously, from one end to the other of the section. These cross sections illustrated conceptual interpretations together with the observed structures, and these interpretations were repeatedly modified through time. The turning point for the final interpretation came from Lugeon (Bull Soc Neuchatel Sci Nat 65:25–53, 1901), who did not accept the idea of opposed verging folds, convinced that fold vergence represents the sense of displacement. His global concept of the Alpine nappe system directed everywhere northward, allowed him to conclude that some basement synforms were nothing but refolded anticlines, representing nappe fronts. Lugeon (1901) showed for the first time the superposition of different basement nappes, all showing the same vergence. This nappe stack was suggested to provide the image of deeper, Alpine crustal levels (Argand in Bull Soc Vaudoise Sci Nat 45(166):217–276, 1909; Argand in Les nappes de recouvrement des Alpes Pennines et leurs prolongements structuraux. Beiträge zur Geologische Karte der Schweiz. Neue Folge, XXXI Lieferung, 1911; Argand in Eclog Geol Helv 14:145–191, 1916), as deep as ca. 15 km. These cross sections, displaying the entire orogenic crust were the starting point for a step-wise retro-deformation on the orogenic scale (Argand 1916), thus bringing the temporal element in a debate that had previously been limited to space and geometry. After a long period in which tectonic advancement consisted mainly in detailed, regional structural work (e.g., Bearth in Eclog Geol Helv 49:267–278, 1956a; Bearth in Eclog Geol Helv 49:279–290, 1956b; Bearth in Eclog Geol Helv 50:161–170, 1957), in the 1970’ies the Simplon area became the object of renewed interest and a new structural approach. Milnes (Geol Soc Am Bull 84:269–274, 1973) refined the structural maps of the area, precisely mapping the traces of major axial planes, and showing their mutual overprinting relationships, thus defining a sequence of tectonic events (Milnes in Eclog Geol Helv 67:333–348, 1974a; Milnes in Bull Geol Soc Amer 85:1727–1732, 1974b). In spite of this rigorous and detailed structural work, and some brilliant intuitions (Jäger in Tscherm Min Petr Mitt 11:304–316, 1966), it was only at the beginning of the 1980’s that the Simplon Fault was clearly recognized as a major extensional structure (Mancktelow in Eclog Geol Helv 78:73–96, 1985), cross-cutting and separating the Simplon section into two tectonic units with different exhumation paths. The inferred occurrence of coeval horizontal shortening and extension allowed to shift the debate from the field of structures and kinematics to the one of dynamics.

Graphical abstract