Ductile shearing on top of the Eocene extruding wedge: the Combin Shear Zone
Résumé
A popular model for the exhumation of HP-UHP rocks is the ‘extruding wedge’ model, where a crustal slice
is bounded at its base by a ‘thrust shear-sense’ fault and to the top by a ‘normal shear-sense’ fault. In the
Western Alps, three main ‘normal shear-sense’ shear zones may have bounded extruding wedges at different
times during the Alpine orogeny, namely the Late Cretaceous-Palaeocene Fobello-Rimella Shear Zone
during extrusion of the Cretaceous Sesia-Dent Blanche stack, the late Eocene Combin Shear Zone during
extrusion of the Briançonnais-Piemonte-Liguria (‘Penninic’) stack, and, finally, the Frontal Penninic Fault
during Oligo-Miocene extrusion of the Helvetic stack.
The available knowledge on the Combin Shear Zone (CSZ), the best exposed of these three shear zones, will
be reviewed based on a synthetic structural map of the NW Alps. A general agreement has been achieved on
the following points.
1. Geological mapping has established the geometry and continuity of the CSZ from the frontal part of the
Dent Blanche Tectonic System to the western boundary of the Sesia Zone.
2. The CSZ has been cut during the Miocene by the brittle Aosta-Ranzola Fault, with an estimated
downthrow of the northern block of c. 2.5 km with respect to the southern block. As a consequence, the
sections observed north (Monte Rosa) and South (Gran Paradiso) of the Aosta Fault display different
structural levels in the Alpine nappe stack.
3. The CSZ has been folded (Vanzone phase) during the final part of its history (i.e. when displacement
along the CSZ was no more taking place), due to the indentation of the Adriatic mantle. This offers us the
unique opportunity to study the change in deformation mechanisms along the shear zone (for a distance
parallel to its displacement of about 50 km).
4. The main ductile deformation along the CSZ was taking place at greenschist-facies conditions,
overprinting eclogite-facies to greenschist-facies deformations of Cretaceous to Middle Eocene age.
More controversial are the following issues.
5. The thickness of the ductile shear zone increases from NW (frontal part of the Dent Blanche) to SE
(frontal part of the Sesia Zone), from a few hundred metres to several kilometres. The type of lithologies
pervasively reworked by the ductile shear changes along strike (dominantly calcschists from the topmost
oceanic units in the Combin Zone, possibly up to the whole of the ‘gneiss minuti’ in the frontal Sesia Zone).
6. A major consequence of the ductile displacement along the CSZ is the development in its footwall of
south-east-verging, kilometre-scale, folds (Mischabel phase). The sedimentary sequences of the Pancherot-
Cime Bianche-Bettaforca Unit may be used to estimate the minimum amount of ‘normal shear sense’
displacement of the order of 15-20 km.
7 The CSZ is cutting and reworking eclogite-facies structures developed in its hangingwall (Sesia) as well as
in its footwall (Zermatt). Two examples of kilometre-scale overprinting structures (Ollomont and Cignana)
will be discussed in detail.