The Jura Massif is a thrust belt that formed from the Miocene as part of Alpine orogeny following the thrusting of the external crystalline massifs onto the Jura basement. The massif was built through the detachment and then folding of the sedimentary covers of the Jura paleogeographic domain. These Mesozoic covers correspond mainly to limestone deposits from a shallow epicontinental sea separating the European foreland from the northern passive margin of the Alpine Tethys. They are overlain, particularly in the south, by the Cenozoic molasse of the North Alpine foreland basin, also known as the Swiss Molasse Basin.

Studied since the 18th century, the Jura Massif quickly became a model for the study of limestone massifs. However, it was thanks to numerous seismic surveys conducted during the 1970s and 1980s by oil companies that the internal structure of the Jura Massif was fully understood. The description of the folding of the limestone series and their significant erosion led to the development of the Jura relief model. Its extensive Jurassic sedimentary series is the reason for the name of this geological period, and several Lower Cretaceous stages were also defined in the Jura. Based on actualistic principles, the Jura Massif is considered a geological equivalent of the carbonate platforms of the Bahamas or Barbados.

Geographic and geological framework

The Jura Massif is an arched peripheral mountain chain of the Alps, located between the Western and Central Alps. It extends mainly along the Franco-Swiss border, forming an arc that shifts from a west-east axis in the north to a north-south axis in the south. Strictly speaking, the massif extends from the summit of Lägern (Zurich canton) in Switzerland to the / anticline plateau in the Isère department, France. The folds fade further south, and the massif disappears at the , north of the Vercors Massif, giving way to the Subalpine massifs. The massif spans over 340 km in its inner arc and over 400 km in its outer arc, with a maximum width of 65 km between Besançon and Neuchâtel. Although geographically not part of the Jura Massif, the Salève, Mandallaz Mountain, and ridges all belong to the same geological unit as the Jura.

The Jura Massif defines the outer boundary of the Alpine system.The Alpine system includes all geological units involved in the Alpine orogeny. These units are characterized by the detachment of sedimentary covers and, for the internal units, by metamorphism dating from the Cretaceous to the Paleogene. Beyond this, to the northwest, the sedimentary covers are no longer detached from their basement, and the geological units are less affected by Alpine orogeny. First, there are faulted plateaus (Haute-Saône Plateau and Swabian Jura), which locally extend the external Jura units but do not participate in the curvature of the chain. The massif is also bounded by the Cenozoic West European Rift, which includes the Rhine Graben to the north and the Bresse Graben to the west. The shoulders of the Rhine Graben (the Vosges Massif to the west and the Black Forest to the east), as well as the , act as barriers against which the Jura Massif abuts. Finally, its inner boundary is marked by the Swiss Molasse Basin, or more broadly, the North Alpine foreland basin, which largely corresponds to the Swiss Plateau. This basin separates the Jura from the Prealps ( and ) and the Subalpine massifs (Bornes, Bauges, and Vercors massifs).In geology, the Prealps include only reliefs composed of sedimentary covers belonging to the Penninic structural units (corresponding to the Valaisan, Briançonnais, and Piedmont paleogeographic domains), while reliefs formed by Delphino-Helvetic sedimentary covers are grouped into the Subalpine massifs.

The relief of the Jura Massif resulted from the detachment of its sedimentary cover in the Miocene, forming detachment nappes that subsequently imbricated to create a thrust belt. The Jura paleogeographic domain corresponds to the proximal part of the European passive margin, characterized by shallow marine sedimentation with periodic emergence. Southeastward, it transitions into the Delphino-Helvetic domain, which represents an open marine environment with a higher proportion of pelagic intervals. During the Late Jurassic and Cretaceous, the Jura domain was regularly emergent, allowing the development of coastal continental environments, including swamps and beaches. This emergent threshold served as a corridor between the then-exposed Hercynian reliefs and reinforced the separation from the European foreland, which was dominated by shallow epicontinental sea deposits. The sedimentary covers of the Jura domain extend beyond the Jura Massif itself. Their presence in the Salève, Mandallaz Mountain, and Âge Mountain demonstrates that they continue beneath the North Alpine foreland basinThe North Alpine foreland basin is actually a piggy-back basin, meaning it was passively transported on the Jurassic sedimentary covers. and extend up to the thrust plane of the external crystalline massifs, meaning they also underlie the Subalpine massifs. Thus, the evolution of the Jura Massif and the North Alpine foreland basin are closely linked.

History of geological studies on the Jura Massif

Early studies The first geological studies of the Jura Massif began in the 18th and 19th centuries. However, it was in the early 19th century that the geology of the Jura Mountains truly captured the interest of structural geologists and paleontologists. As Jules Thurmann explains in the introduction to his Essay on the Jurassic Uplifts of Porrentruy (1832), early research introduced only vague notions that did not yet fully explain the massif’s geology. During the period covering the 18th century and the first quarter of the 19th century, the Jurassic terrains of the Jura Massif were poorly known and scarcely studied, in contrast to the Jurassic terrains of England, which served as a reference for studying this geological period.

The study by Jules Thurmann The first major geological study of the Jura Massif was conducted by Swiss geologist Jules Thurmann, considered the father of Jura geology, during the second quarter of the 19th century. His four major publications on the subject are: Essai sur les soulèvements jurassiques, published in multiple volumes between 1832 and 1836; Esquisses orographiques de la chaîne du Jura, published in 1852; Résumé des lois orographiques générales du système des Monts-Jura, published in 1853; and Nouveaux principes d'orographie jurassique, published posthumously in 1857.Jules Thurmann died on July 25, 1855. Unlike other geologists of his time, who attempted to apply the characteristics of English Jurassic formations to the continent, Thurmann relied on paleontology to identify the different rock layers of the massif.

Early tectonic explanations Following Thurmann, many geologists attempted to explain the Jura’s tectonics. The massif was the subject of some of the earliest studies by Marcel Alexandre Bertrand in the 1880s, during which he created geological maps of the regions of Besançon, Lons-le-Saunier, and Pontarlier. The French geologist concluded that the Jura was the reference region for straight and regular folds. Following Bertrand’s work, the dominant tectonic model for the Jura between the late 19th century and mid-20th century described it as a simple, autochthonous fold chain formed in one or two phases, depending on the author. According to this model, the Jura’s formation resulted from tangential thrusting created by the "pushing mass" of the Alpine front from the southeast.

At the beginning of the 20th century, Louis Rollier, a professor at the University of Zurich, linked the formation of the Jura Massif to the presence of geosynclines formed at the start of the Cenozoic. He attributed the regularity and rounded shape of the foldsAccording to the author, the Jurassic and Cretaceous sedimentary overload in the Jura should have formed powerful folds with ridge-like shapes morphologically comparable to those in the Alps. to the presence of marginal Tertiary basins, which forced the massif to fold according to the "general contraction movement of the Earth's crust," preserving the contours of these basins. He also rejected the hypothesis that surrounding crystalline massifs (Vosges, Black Forest, and the ) influenced the Jura’s folding. Instead, he argued that the collapse and sedimentary overload of the Tertiary basins, composed of heavy sediments, squeezed the Jura, forcing it to fold—effectively "sandwiched" between the Swiss basin to the east and the Bresse and Rhine basins to the west and north, respectively. According to Rollier, the deepening of certain folds resulted from a more pronounced folding of the crystalline basement compared to the sedimentary cover. In conclusion, Rollier refuted the hypothesis that the Jura’s formation was caused by Alpine thrusting, attributing it instead to the influence of the surrounding Tertiary basins.

The mid-20th century shift The model of a simple, straight-folded chain was challenged by studies conducted in the second quarter of the 20th century, which showed that most Jura folds exhibit a complex structure that the prevailing formation model could not explain. Additionally, it became clear that the straight, regular folds—previously considered characteristic of Jura topography—were in the minority and primarily concentrated in the northern and western external Jura (regions of Delémont, Clerval, and the Revermont). In contrast, the folds of the High Jura Chain were of the "box fold" type, a point that Swiss geologists had emphasized for years.

In 1941, Maurice Lugeon proposed a new hypothesis regarding the formation of the Jura Massif, following several studies in the Alps in which he introduced the role of gravity to explain the emplacement and deformation of nappes. According to him, the sedimentary cover of the Jura detached from the Hercynian basement at the level of the Middle Triassic, as August Buxtorf had already pointed out. This shear plane was, moreover, shared with the molasse and the Helvetic nappes, implying that they underwent the same deformation history. However, Maurice Lugeon rejected the idea that the Jura had deformed as if compressed in a vise and instead invoked gravity as the sole explanation. Gravitational displacement implied the presence of a slope along which the Jura cover would have moved. These slopes corresponded to depressions located along the Alps (i.e., the present Swiss Molasse Basin) and between the Massif Central and the Vosges (i.e., the Bresse and Rhine grabens), which he defined as "subsurface folds." According to his hypothesis, the Jura was thus positioned on a counter-slope of the Hercynian peneplain, sliding along the plastic Triassic layers. However, since the consequences of such a movement appeared paradoxical, Maurice Lugeon abandoned this first hypothesis and formulated a new one based on the transmission of compressional forces. According to this second hypothesis, the Mesozoic layers were folded to the northwest of the Swiss Molasse Basin, where the molasse thinned, thereby weakening the system. The force driving this process was attributed to the weight of the Prealpine nappes, located at the southeastern edge of the Molasse Basin, which would have pushed the molasse and the Mesozoic layers toward the northwest, causing a basal detachment at the level of the Triassic layers.