This study concerns the geology of the sedimentary units situated between the crystalline basements of the Mont-Blanc, Aiguilles-Rouges, and Belledonne Massifs (French Alps, Savoie).
Formation names are proposed for Triassic and Liassic rocks of the Dauphiné zone in this area. Descriptions of their lithostratigraphic content, an inventory of their faunas, and synthetic schemes of longitudinal and transversal stratigraphic variations are given. The Liassic formations of the Mont Joly area mark the transition from the North-Helvetic domaine (Morcles-Doldenhorn) nappe in Switzerland to the classical Dauphinois Lias of the French Alps
The following tectonic units have been defined: 1) the Aiguilles-Rouges and the external part of the Belledonne Massifs together with their autochthonous sedimentary cover; 2) the parautochtonous Unit of Vervex; 3) the Morcles Nappe, which can be divided into two units: the Sangle Unit and the Mont Joly Unit which form large recumbent anticlines, linked by the Mont d'Arbois Syncline; and 4) the internal part of the Mont-Blanc Massif together with its sedimentary cover.
The Sangle Unit and the Mont Joly Unit are extensions of the Morcles Nappe in France. The external part of the Mont-Blanc Massif and the internai part of the Belledonne Massif form the crystalline cores of these anticlines and therefore represent the homeland of these units.
The Mont Joly Unit corresponds to the Aravis Massif (middle Jurassic to Tertiary Subalpine Chain), the Sangle Unit to the Bornes Massif (frontal part of the Subalpine Chain). The Mont d'Arbois Syncline can be related to the Thônes Syncline. This means that the Aravis and Bornes Massifs belong to one single tectonic element which corresponds to the Morcles Nappe. The Diablerets and Wildhorn Nappes are not present in the Subalpine Chain to the contrary of what has been claimed in some recent papers.
The hypothetical "Mont-Blanc Thrust" which should separate this crystalline massif from the Mesozoic rocks of the Chamonix Zone, does not exist. The vertical faults of N-S direction mapped by sorne authors at the SW end of the Aiguilles-Rouges and Mont-Blanc Massifs show no alpine movement.
The Roselette Nappe, which lies between the Mont-Blanc and Belledonne Massifs, does not belong to the Ultrahelvetic but could be a continuation of the helvetic Diablerets and Wildhorn Nappes.