Gilles Ménard

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 France, the NGF, measured during the 1886–1907 period, and the IGN69, measured from 1965 to 1979. The accuracy of these levelings (standard deviation of 1.8mm/√km to 3.8mm/√km) allows us to compute the vertical displacements of the benchmarks between two different eras. The results indicate the occurrence of discrete zones of uplift and subsidence: (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 external Jura. Comparing the spatial distribution of zones of uplift and their 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 Mountains, can be explained by crustal shortening along a major basement-involving thrust fault. This fault ramps up under the Bornes Massif from a depth of 12 km to 7 km, turns into a flat under the Molasse Basin, ramps up to the top of the basement at the north-eastern margin of the internal Jura Mountains, and reaches the surface in the external Jura. The Saleve ramp-anticline is carried by a bifurcation of this thrust. 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 basement-involving thrust fault was already active during the Miocene main folding phase of the Jura Mountains or whether it was activated only during Pliocene-Pleistocene times is subject to debate.

High-precision three-dimensional paleothermometry derived from paleomagnetic data in an Alpine metamorphic unit

In the Dauphinoise zone of the French Western Alps, a posttectonic thermoremanent magnetization, carried by pyrrhotite, has been isolated. The paleomagnetic signature is interpreted as the record of successive partial thermoremanent magnetizations of opposite directions acquired during the early Miocene slow cooling of the epimetamorphic marly limestones of the studied area. A local vertical thermal gradient of 22 ± 2 °C/km was derived from the unblocking temperature of a reversal recorded at different altitudes. Using this vertical gradient, we have produced a paleotemperature map at the time of the reversal (about 20 Ma). The map shows a horizontal regional thermal gradient of 7 °C/km; the higher temperatures were toward the crystalline basement. Paleomagnetic directions demonstrate that this horizontal gradient was present during cooling. Its origin can be explained by large-volume fluid circulation along the major fault that forms the boundary between the crystalline basement and its Liassic cover.

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

Abstract 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.

Acquisition d'aimantations thermorémanentes partielles successives par la pyrrhotite monodomaine lors du refroidissement de la Zone dauphinoise interne (Alpes occidentales, France)

The magnetization of the marls and marly limestones from the internal part of the Dauphinoise zone is carried by monoclinic pyrrhotite. Our experiments show the equality of blocking and unblocking temperatures which suggests that magnetic grains are single domain. Thermal demagnetization analysis shows the presence of colinear components of opposite polarity. They are interpreted as partial thermoremanent magnetizations acquired during different polarity periods of the geomagnetic field at the time of the slow cooling of the studied area.