Résumé
Active deformation at the boundary between the Eurasia and Africa plates varies in style. The belt between the Alpine mountain range and the Mediterranean Sea, for example, differs markedly in its western and eastern parts. In the western part, around southeast France, the mountains are higher, but the seismicity lower, than in the eastern part, around northern Italy and Greece. Yet the inter-plate convergence rate of 6 mm/yr varies by less than 15% between these two areas. To better understand the behaviour of this complex plate boundary, we use geodesy to map the spatial distribution of the deformation. In this paper, we focus on southeast France, a tectonic crossroads between three different domains (Alps, Ligurian Sea, and Massif Central) which exhibits a moderate level of seismicity. Here, the geodetic measurements imply low rates of horizontal deformation. By combining historical triangulation measurements mostly from 1947 to 1983 with Global Positioning System (GPS) surveys in 1993 and 1994, we estimate the rate of angular shear in triangular subnetworks covering the study area. The estimated strain rates in thirteen of nineteen triangles are smaller than their (1 standard deviation) uncertainties of about 0.1 microradian/yr. This value bounds the rate of deformation for a 100-km wide zone in Provence, between Marseilles to the south and the Ventoux massif to the north. The geodetic estimates place an upper bound of 1 to 2 mm/yr on the slip rates of two seismically active structures, the Durance fault and the Nı̂mes fault, assuming a fault zone ∼20 km wide in each case. We also find strain rates as high as 0.20±0.07 microradian/yr in three subnetworks near the epicentre of the magnitude 5.3 Haute-Ubaye earthquake in 1959, in a region which includes the higher summits. This may be interpreted either as pure shear with compression oriented NE–SW across this region or right-lateral simple shear along NNW–SSE-trending faults. Given that this earthquake is the only one of its magnitude recorded in the study area during the time interval spanned by the geodetic measurements, we infer that most of the geodetically observed deformation occurs aseismically. On the whole, the geodetic results suggest that the rate of north–south shortening across the 100-km wide study area is not more than 1 or 2 mm/yr, in agreement with kinematic models based on other types of geophysical data. Since this deformation represents only a small part of the convergence rate of 6 mm/yr predicted by the NUVEL-1 model for the entire boundary between the African and Eurasian plates, the remaining deformation must be accommodated elsewhere. This study illustrates how a careful analysis of historical geodetic data can measure the rate of tectonic deformation, even where it is low, and thus meaningfully bound geophysical models.