SPOT data and the Montpellier igneous rocks as keys to a new large-scale tectonic interpretation of the Bas Languedoc (southern France)

Abstract

The alkaline intrusions of Montpellier, in the south of France, are of absarokitic composition, consisting of very dark basanites containing orthoclase, biotite, analcite and in places Na-ferrisalite. The dykes cut country rocks up to Early Oligocene in age and have yielded K-Ar ages of 31 to 23 Ma (Stampian-early Aquitanian). Interpretation and analysis of Spot-1 XS and panchromatic remote sensing data indicate an original network of conjugate fracture systems, the fractures trending 020–040°, and 170° that acted as channelways for the ascending magma. This alkaline magnetic district defines a NE- to NNE-trending tectono-magmatic corridor formed in a thinned part of the European plate during Oligocene anorogenic extension. The regional fracture system formed in two stages: (a) reactivation of pre-existing large-scale Riedel shears (which formed initially during the end-Eocene to Early Oligocene Pyrenean stage, extending eastwards from the wall of the Cévennes Fault); (b) synchronous re-opening of the 130° and 170° fractures, whose roots are to be found in the Variscan basement. Satellite data, in particular those of Spot, with its high spatial resolution and stereoscopic capability, in combination with a detailed study of the Montpellier Tertiary igneous rocks have advanced the geological knowledge of the Montpellier area. The new scheme has direct consequences for mineral exploration, especially exploration for oil.