From the highest (Dent-Blanche klippe) to the deepest unit (Money window) : A kinematic model of the Western Alps

Michel Ballevre (1) , Paola Manzotti (2) , Martin Engi (3)

1 Terre, Temps, Traçage
2 Terre, Temps, Traçage
3 Institute of Geological Sciences [Bern]

The nappe stack of the Western Alps offers us a unique opportunity to discuss orogenic processes at crustal scale. Along and north of the ECORS-CROP seismic line, the nappe stack consists, from top to bottom, of (i) units derived from the Sesia-Dent Blanche continental fragment, (ii) ocean-derived units (Zermatt-Grivola, Tsaté), (iii) units derived from the distal part of the Briançonnais palaeomargin (Monte Rosa - Gran Paradiso), and (iv) units derived from more proximal part of this palaeomargin, now outcropping as windows (Money Unit).We have recently performed detailed mapping, structural analysis, metamorphic petrology and geochronology in both the highest (Dent Blanche) and the deepest (Money) units. As a synthesis, we will now discuss the metamorphic structure of the nappe stack, which is characterized by the following points : – Units involved in the subduction zone at an early stage are thrusted over units involved in the same subduction zone at a later stage, explaining why older eclogite-facies units (e.g. Zermatt-Grivola) are thrust on top of younger eclogite-facies (Monte Rosa- Gran Paradiso) units. – After their subduction as more or less coherent pieces, some continental units are sheared during their exhumation, so that higherpressure subunits (Gran Paradiso) are thrusted over lower pressure ones (Money). – The older nappe boundaries that occur at higher structural levels are deformed (folded and sheared) when the younger nappe boundaries are formed at deeper structural levels. – Late brittle faulting (with up to 3km of vertical displacement) is a necessary requisite for understanding the present location of the nappe boundaries. Geometric and petrologic data are used to construct a consistent kinematic model for this transect of the Western Alps, emphasizing the role of the inherited discontinuities from the pre-orogenic history, namely the Variscan orogeny, the Permian extension and the Jurassic rifting.

Domaines: Sciences de la Terre

Michel Ballevre, Paola Manzotti, Martin Engi. From the highest (Dent-Blanche klippe) to the deepest unit (Money window) : A kinematic model of the Western Alps. 24 ème Réunion des sciences de la Terre 2014, Oct 2014, Pau, France. pp.191. ⟨insu-01080776⟩