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Journal of the Geological Society

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Organic maturation in the French Subalpine Chains: regional differences in burial history and the size of tectonic loads

Department of Geological Sciences, University of Durham, Durham, DH1 3LE, UK

The French Subalpine Chains of southeastern France are the foreland fold/thrust belt of the Western Alps, formed in the mid- to late Tertiary as a result of the continental collision between the European and Italian-North African plates. With the change from passive margin to destructive margin, certain areas of the Subalpine Chains experienced enhanced temperatures through being buried to greater depths. Burial of these areas has been investigated using vitrinite reflectance (R_o).

Levels of organic maturity of Mesozoic and Tertiary horizons have been established using vitrinite reflectance (R_o) and spore fluorescence. In the southwestern part of the chain, values of R_o for Mesozoic units from the Vercors and Chartreuse massifs are generally low, with all Cretaceous units having values <0.5% R_o. Older Jurassic horizons have higher values, up to 1.25% for the mid-Jurassic (Oxfordian). When compared to values from the same horizons, and from younger Tertiary units (published data) from the northeastern part of the chain, it is clear that this part of the belt has encountered higher temperatures, produced by greater depth of burial. In the Haut-Giffre and Aravis massifs Mesozoic units have R_o values as high as 2.7% and, in terms of hydrocarbon generation, can be considered mature or overmature.

Two causes of greater depth of burial of the northeastern part of the belt are indicated by field studies and by burial history modelling of specific localities of the Subalpine Chains: first, the earlier initiation of the Tertiary foredeep in the northeast compared to the southwest; and second, differences in the size of tectonic loads emplaced onto the Subalpine Chains during the final stages of Alpine compression, with the greater structural deformation of the Subalpine Chains occurring in the northeast.

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