Alpine thermal and structural evolution of the highest external crystalline massif: The Mont Blanc.
P. H. Leloup (1), N. Arnaud (2), E. R. Sobel (3), and R. Lacassin (4).
(1), Laboratoire des Sciences de la Terre, UMR CNRS 5570, Ecole
Normale Supérieure, Université Claude Bernard, Villeurbanne,
Phone: +33 4 72 44 62 38 Fax: +33 4 72 44 85 93
(2), Laboratoire de Dynamique de la Lithosphère, UMR CNRS 5573, ISTEEM-USTL, Montpellier, France.
(3), Institut für Geowissenschaften, Universität Potsdam, Potsdam, Germany.
(4), Laboratoire de Tectonique, Me´canique de la Lithosphère, UMR CNRS, 7578, Institut de Physique du Globe de Paris, Paris, France.
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 de´collement (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, NWSE 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
Citation: Leloup, P. H., N. Arnaud, E. R. Sobel, and R. Lacassin
(2005), Alpine thermal and structural evolution of the highest
external crystalline massif: The Mont Blanc, Tectonics, 24, TC4002,
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