Abstract
The alpine structural evolution of the Mont Blanc, highest point of the Alps (4810 m), and of the surrounding area has been reexamined. The Mont Blanc and the Aiguilles Rouges external crystalline massifs are windows of Variscan basement within the Penninic and Helvetic nappes. New structural, 40Ar/39Ar, and fission track data combined with a compilation of earlier P-T estimates and geochronological data give constraints on the amount and timing of the Mont Blanc and Aiguilles Rouges massifs exhumation. 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 exhumed from 15 to 20 km depth. Uplift of the two massifs started ∼22 Myr ago, probably above an incipient thrust: the Alpine sole thrust. At ∼12 Ma, the NE-SW trending Mont Blanc shear zone (MBsz) initiated. It is a major steep reverse fault with a dextral component, whose existence has been overlooked by most authors, that brings the Mont Blanc above the Aiguilles Rouges. Total vertical throw on the MBsz is estimated to be between 4 and 8 km. 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 bounding the southern flank of the range. The "European roof" is located where the back thrust intersects the MBsz. Uplift of the Mont Blanc and Aiguilles Rouges occurred toward the end of motion on the Helvetic basal décollement (HBD) at the base of the Helvetic nappes but is coeval with the Jura thin-skinned belt. Northwestward thrusting and uplift of the external crystalline massifs above the Alpine sole thrust deformed the overlying Helvetic nappes and formed a backstop, inducing the formation of the Jura arc. In that part of the external Alps, ∼NW-SE shortening with minor dextral NE-SW motions appears to have been continuous from ∼22 Ma until at least ∼4 Ma but may be still active today. A sequential history of the alpine structural evolution of the units now outcropping NW of the Pennine thrust is proposed.