Deformation and motion in the Western Alpine Arc

Abstract

We attempt to reconstruct the kinematics (deformation and motion) of the Western Alpine Arc between the Mercantour and Aar Massifs. First, we compile a map showing principal extension (X) direction within three major structural units. In the external zone, X directions are broadly radial to the Alpine Arc; in the internal zone, they are uniform or smoothly varying in some domains; whereas in the intermediate zone they often show much scatter. Next, we use geochronological data and P‐T conditions, estimated from metamorphic mineral reactions, to time the deformation. On each of a series of interval maps, we show X‐directions for strain accumulated during a chosen time interval. We argue that these incremental X‐directions are approximately equivalent to segments of particle paths, showing displacements relative to stable Europe. For 120–100 Ma, the displacement direction was about N150 and coeval with eclogite facies metamorphism; it is interpreted as a direction of overthrusting by the African Plate. Around 40 Ma, the displacement in the south part of the arc became westerly. Finally, between 25 and 15 Ma, displacement directions were generally radial to the arc.

Fold interference pattern at the top of basement domes and apparent vertical extrusion of HP rocks (Ambin and South Vanoise massifs, Western Alps)

“…In the more internal units of the western Penninic domain (e.g. Gran Paradiso), the corresponding D 1 kinematics (or equivalent according to authors) become highly hypothetic and previous observations are often totally contradictory (Choukroune et al (1986) or Platt et al (1989) for different interpretations), due to the post-D 1 intense transposition features (Lefèvre and Michard, 1976;Philippot, 1990;Henry et al, 1993;Rolland et al, 2000;Agard et al, 2001;Wheeler et al, 2001;Reddy et al, 2003). Despite the lack of other critical examples to corroborate our assumption, we propose to regard the geometric and kinematic characteristics of the Ambin and South Vanoise HP-nappes as representative of the early alpine steps of exhumation in the western transect of the Alpine chain (s.s.).…”

“…Structural and microstructural characteristics of the exhumation of the Briançonnais, Piemont and Internal Massifs high pressure rocks, during a complex deformation event globally called D 1 , are still disputed (Caron, 1974(Caron, , 1977Platt et al, 1989;Philippot, 1990;Agard et al, 2001). The kinematic of early deformation in these high pressure domains is poorly known and previous observations are often contradictory (Choukroune et al (1986) or Platt et al (1989) for a different interpretation), due to the post-D 1 intense transposition features (Lefèvre and Michard, 1976;Philippot, 1990;Henry et al, 1993;Rolland et al, 2000;Agard et al, 2001;Wheeler et al, 2001;Reddy et al, 2003).…”

Structural and sedimentary records of the Oligocene revolution in the Western Alpine arc

“…This exhumation is recorded within the lower Oligocene molasse basins on both sides of the renewed relief, sourced from high-pressure metamorphic rocks and oceanic mélanges of various metamorphic grades. The curvature of the arc was progressively acquired, producing radial spreading of transport lineations (Choukroune et al, 1986;Platt et al, 1989b;Lickorish et al, 2002) and southward increasing counter-clockwise rotations of internal units (Collombet et al, 2002). The southern part of the arc now cross-cuts perpendicularly the palaeo-accretionary wedge.…”

“…E-W trending. Pervasive D1 transport lineations (Choukroune et al, 1986) have been re-arranged by arcuate bending during later episodes (Rosenbaum & Lister, 2005). Afterwards, the renewed tectonic activity is marked by the activation of major, west directed crustal thrusts in the Western Alps during early Oligocene (Badertscher & Burkhard, 1998;Tricart et al, 2000;Fügenschuh & Schmid, 2003;Simon-Labric et al, 2009), which cross-cut the previous nappes stack.…”

Dedication to Pierre Choukroune

“…His initiatives in the Hercynides, the Alps (Choukroune et al 1986), and several Archaean cratons Choukroune & Ludden 1997) provided training grounds for a number of young structural geologists from Rennes and elsewhere. None of his PhD students and colleagues at Rennes escaped his thoughtful suggestions and his unusual scientific lucidity and inspiration.…”