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Reply to comment by Y. Rolland et al. on ''Alpine thermal and structural evolution of the highest external crystalline massif: The Mont Blanc''

Abstract : 1. Introduction [1] Leloup et al. [2005] discussed the Cenozoic structural evolution of the Mont Blanc and Aiguilles Rouges ranges by combining new structural, 0Ar/39Ar, and fission track data with published P-T estimates and geochronological data. Our main conclusions were (1) Alpine exhumation of the Aiguilles Rouges was limited to the thickness of the overlying nappes (10 km), while rocks now outcropping in the Mont Blanc have been xhumed 15 to 20 km. (2) Uplift of the two massifs started 22 Myr ago; while at 12 Ma, the Mont Blanc shear zone (MBsz), a reverse fault with a slight right-lateral component, initiated bringing the Mont Blanc above the Chamonix synclinorium and the Aiguilles Rouges; total vertical throw on the MBsz is between 4 and 8 km. (3) Fission track data suggest that relative motion between the Aiguilles Rouges and the Mont Blanc stopped 4 Myr ago. Since that time, uplift of the Mont Blanc has mostly taken place along the Mont Blanc back thrust, a steep north dipping fault zone bounding the southern flank of the range. (4) The highest summits are located where the back thrust intersects the MBsz. (5) Exhumation of the Mont Blanc and Aiguilles Rouges occurred toward the end of motion on the Helvetic basal de´collement (HBD) at the base of the Helvetic nappes. Uplift is linked with a deeper, more external thrust that induced the formation of the Jura arc. [2] While acknowledging that our paper is ‘‘a good step forward in the tectonic comprehension of the Mont Blanc area and provides a good synthesis of preexisting data,’’Rolland et al. [2007] claim that the timing we propose for the thrust and back thrust events is not in agreement with new 40Ar/39Ar data that they publish in their comment. In fact, they raise two main arguments with our observations/ interpretations: [3] 1. Alpine deformation is penetrative within the Mont blanc granite and is not accommodated by the two localized shear zones we describe (the SE dipping Mont Blanc shear zone, or MBsz, in the north and the NW dipping back thrust in the south, Figure 1), but by numerous anastomosed shear zones in the way described by Choukroune and Gapais [1983] in the Aar massif and Gourlay [1986] in the Mont Blanc. All deformations within the Mont Blanc are thus coeval and the Mont Blanc is a transpressive pop-up structure at the rim of a large transpressive fault that runs from the Rhone dextral fault system. [4] 2. The timing of deformation cannot be obtained through 40Ar/39Ar thermochronology due to excess argon and intense fluid circulation. They instead provide a minimum age of 16 Ma for the initiation of top to the SE motions on the SE side of the Mont Blanc (back thrust) based on five phengites 40Ar/39Ar ages from three shear zones (their Figure 3). [5] We will take the opportunity of this reply to address these two points and, in a third point, we briefly discuss possible deformation models of the Mont Blanc range.
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P.H. Leloup, N Arnaud, R Lacassin, E.R Sobel. Reply to comment by Y. Rolland et al. on ''Alpine thermal and structural evolution of the highest external crystalline massif: The Mont Blanc''. Tectonics, American Geophysical Union (AGU), 2007, 26 (2), pp.TC2016. ⟨10.1029/2006TC002022⟩. ⟨insu-01285574⟩

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