Kinematic models of deformation at the front of the Alps: new data from map-view restoration

Abstract

International audienceMap-view restoration of the frontal part of the French Alps was done in order to test two hypotheses about the propagation of deformation. We discussed two models of the ‘rooting' of the Jura-Molasse basal detachment either along basement thrusts beneath the External Basement Massifs (EBM-rooting model) or along basal thrusts of the Subalpine Chains above the External Basement Massifs (SAC-rooting model). The different sequences of deformation are described for the two models with their kinematics and geochronological constraints. The geochronological data available on the uplift timing of the External Basement Massif are then discussed. Due to their range of uncertainty, geochronological data on the exhumation of the External Basement Massifs are not decisive to discriminate between the two models. Using either the maximum or the minimum age in the range of their uncertainty allows fitting the timing constraints of both models. Interpretation of map-view restoration reveals a weakness of the EBM-rooting model linked to the fact that a sharp virgation is predicted between the two main cross sections. This does not discard this model as such virgation may be linked to paleostratigraphic or tectonic effects but this must be taken into account when using this model. Structural data are also discussed as field geology on the mechanism of uplifting of the external basement massif or geophysical data as deep seismic lines shot in the frame of the ECORS and NRP20 research programs. Such data favor one or the other model without discarding one of them. We conclude that, with the available data, both models seem possible and that the less popular one (SAC-rooting model) shows the more coherent restoration field, the reason why we favor it at the present time

Collision kinematics in the western external Alps

“…Few units in the subalpine chains cannot be explained by the activation of a décollement due to basement thrusts below the frontal ECM. Indeed, several thrusts branch on a décollement above the ECM (Figure ) as shown in Deville et al [] and redrawn in Figures and d, in Deville and Chauvière [] and redrawn in Figures and c, and in Affolter et al [] and redrawn in Figures and b. These (small) units suggest that the ECM cover was slightly detached from the basement, which justifies considering it as a unit distinct from the subalpine one in a strict sense.…”

“…Balanced cross section through the Jura‐Bornes‐southwestern Mont Blanc massifs: the Bornes‐Mont Blanc cross section. The subalpine (Bornes) and Jura parts are redrawn from Affolter et al [] and partly constrained by seismic data. For the Bresse graben and Jura relationships, see also Bergerat et al [] and Philippe et al [].…”

“…Alternatively, the subalpine chains could have been emplaced before the ECM uplift, and if so, their shortening would have to be rooted above them. This sequence of tectonic events was proposed by Leloup et al [] or Affolter et al [], for example, and requires the subalpine chain shortening to be older than the ECM uplift.…”

“…Thus, we will only sum up the main implications. The total amount of shortening is about 65.9 km (Table ), among which 29.3 km is linked to the three frontal basement thrusts of the lower part of the Aiguilles Rouges massif, attesting for the shortening in Jura fold‐and‐thrust belt that shortened above a décollement in the Triassic layers [ Burkhard and Sommaruga , ; Affolter et al , ]. The structures in the Aiguilles Rouges massif have been drawn as basement thrusts, although they might have been more distributed shear zones (e.g., Steck et al [], see discussion part).…”

Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology

“…A décollement is activated in the lower Liassic or Triassic rocks (see for example, Deville et al, 1994;Philippe et al, 1998, Affolter et al 2008 and references therein). There, the crust has not been tectonically buried and was rather thick and strong (proximal-most part of the margin).…”