PRESENT-DAY VERTICAL DISPLACEMENTS IN THE NORTH-WESTERN ALPS AND SOUTHERN JURA MOUNTAINS - DATA FROM LEVELING COMPARISONS

Two high-precision leveling networks were successively surveyed in Fra nce, the NGF, measured during the 1886-1907 period, and the IGN69, mea sured from 1965 to 1979. The accuracy of these levelings (standard dev iation of 1.8mm/root km to 3.8mm/root km) allows us to compute the ver tical displacements of the benchmarks between two different eras. The results indicate the occurrence of discrete zones of uplift and subsid ence: (1) a regional uplift (up to 1.4 mm/yr) of the Subalpine Massifs ; (2) an important uplift of the internal Jura (up to 2 mm/yr); (3) a relative subsidence of the southern part of the Jura (0.8 mm/yr); and (4) a relative subsidence of the Bresse Basin with respect to the exte rnal Jura. Comparing the spatial distribution of zones of uplift and t heir respective vertical displacement rates with a regional structural cross section leads to the conclusion that present-day uplift of the Belledonne and Bornes Massifs and of the internal parts of the Jura Mo untains, can be explained by crustal shortening along a major basement -involving thrust fault. This fault ramps up under the Bornes Massif f rom a depth of 12 km to 7 km, turns into a flat under the Molasse Basi n, ramps up to the top of the basement at the northeastern margin of t he internal Jura Mountains, and reaches the surface in the external Ju ra. The Saleve ramp-anticline is carried by a bifurcation of this thru st. Horizontal displacement rates of 6 mm/yr at the Bornes ramp, 2 mm/ yr at the Saleve ramp, and 4 mm/yr at the internal Jura ramp have been determined by inversion of profiles of uplift rates. Whether this bas ement-involving thrust fault was already active during the Miocene mai n folding phase of the Jura Mountains or whether it was activated only during Pliocene-Pleistocene times is subject to debate.