Estimating present-day displacement fields and tectonic deformation in active mountain belts:: an example from the Chartreuse Massif and the southern Jura Mountains, western Alps

Determination of relative movements between the alpine foreland and the External Crystalline Massif is a key-point for the understanding of the present-day tectonics of the western Alps. In this study we try to test the continuity of the present-day tectonics with the Mio-Pliocene deformation. In particular, we will test if the present-day displacements are localized along the thrusts of the Jura Mountains, or along a blind thrust in the Bas Dauphine Molasse Basin. Definition of relative movements is achieved by several methods, including a comparison of two high precision leveling networks to estimate vertical displacements, horizontal deformation measurements performed by triangulation/triangulation and triangulation/GPS comparison, in situ stress measurements performed in the different tectonic units and geomorphologic observations that constrain the location and the magnitude of the Quaternary deformation. Comparison of leveling data demonstrates: (1) an uplift of the southern Bas Dauphine Molasse Basin relative to its northern part (0.8 mm/year), also revealed by geomorphologic analysis, (2) a significant uplift of the most external jurassian anticlines (0.8 to 2 mm/year), also recorded by the deformation of a paleo-river bed, and (3) an important uplift (up to 2 mm/year) of the Subalpine Massifs. The horizontal strain estimated from comparison of horizontal geodetic data (triangulation, GPS) shows (1) a NW-SE directed shortening between the eastern Chartreuse Massif and the Bas Dauphine Molasses Basin (approximately 3 mm/year), (2) an E-W-directed shortening in the Jura Mountains (approximately 4-3 mm/year) and (3) a dextral strike-slip motion consistent with focal mechanisms along a NNE-SSW direction between the eastern Chartreuse Massif and the eastern Belledonne Massif. These data reveal a present-day strain partitioning between the Belledonne External Crystalline Massif and the Bas Dauphine Molasses Basin. The westward motion of the Subalpine Massifs is partitioned along two southern jurassian thrust-folds, and a dextral NNE-SSW strike-slip shear zone between the Chartreuse Massif and the Belledonne Massif. This strain partitioning is also accompanied by a stress partitioning between the alpine foreland and the External Crystalline Massifs. (C) 1998 Elsevier Science B.V. AU rights reserved.