Stress field changes in the Rhone Valley from the Miocene to the present

Abstract

Study of the deformation of Neogene and Quaternary deposits in the Rhône Valley shows that the end-Miocene E-W greatest principal compression ( σ 1) that deformed the Western Alps did not continue into the Quaternary, at least between Vienne and Montélimar. The Middle to Upper Miocene molasse on the east side of the Rhône Valley was affected by E-W compression at the end of the Miocene, causing reverse faulting and N-S folding (St. Lattier and Varacieux anticlines). In contrast, the Pliocene sands and clays of the Rhône Valley are unaffected by folding and show only small normal faults with a maximum throw of l m trending between N-S and NW-SE. These were caused by E-W to NE-SW extension, which is incompatible with the E-W compression that characterises the latest-Miocene Alpine phase. These structures are the surface expression of extension that was probably induced locally by the overall compressive regime that exists to the present day in this part of Western Europe [the “transcurrent extensional regime” of Philip (1983), USTL thesis, Montpellier]. This stress field results from the collision of the African and European plates that began in the Tertiary. West of the frontal Pennine thrust, the present-day direction of compression, which is deduced from calculated focal mechanisms and from in-situ stress measurements, varies between N-S and NW-SE. In the Grenoble region and the northern Massif Central, the focal mechanisms indicate transcurrent movement, with σ 1 horizontal and oriented between N-S and NW-SE, and with σ 2 vertical. In the southwestern Massif Central and the Vendée, however, they indicate an extensional tectonic regime, with σ 1 vertical and σ 2 horizontal and oriented NW-SE, σ 3 trends NE-SW. This orientation is similar to that which would have caused the small Pliocene normal faulting observed in the Rhône Valley between Vienne and Montélimar. In this part of the Rhône Valley, west of the frontal Pennine thrust, the E-W stress resulting from the collision of Apulia (Italy) with the West European plate has not been in evidence since the Pliocene. The E-W compression has been replaced by N-S to NW-SE compression resulting directly from the collision between the African and European plates transmitted through the Straits of Gibraltar and the Iberian microplate in the west and the Corsica-Sardina block in the east. The piercing effect of the Apulian block is no longer dominant in the Rhône Valley.