Ductile shearing on top of the Eocene extruding wedge: the Combin Shear Zone

Abstract

International audienceA popular model for the exhumation of HP-UHP rocks is the ‘extruding wedge’ model, where a crustal sliceis bounded at its base by a ‘thrust shear-sense’ fault and to the top by a ‘normal shear-sense’ fault. In theWestern Alps, three main ‘normal shear-sense’ shear zones may have bounded extruding wedges at differenttimes during the Alpine orogeny, namely the Late Cretaceous-Palaeocene Fobello-Rimella Shear Zoneduring extrusion of the Cretaceous Sesia-Dent Blanche stack, the late Eocene Combin Shear Zone duringextrusion of the Briançonnais-Piemonte-Liguria (‘Penninic’) stack, and, finally, the Frontal Penninic Faultduring Oligo-Miocene extrusion of the Helvetic stack.The available knowledge on the Combin Shear Zone (CSZ), the best exposed of these three shear zones, willbe reviewed based on a synthetic structural map of the NW Alps. A general agreement has been achieved onthe following points.1. Geological mapping has established the geometry and continuity of the CSZ from the frontal part of theDent 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 estimateddownthrow of the northern block of c. 2.5 km with respect to the southern block. As a consequence, thesections observed north (Monte Rosa) and South (Gran Paradiso) of the Aosta Fault display differentstructural levels in the Alpine nappe stack.3. The CSZ has been folded (Vanzone phase) during the final part of its history (i.e. when displacementalong the CSZ was no more taking place), due to the indentation of the Adriatic mantle. This offers us theunique opportunity to study the change in deformation mechanisms along the shear zone (for a distanceparallel 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 lithologiespervasively reworked by the ductile shear changes along strike (dominantly calcschists from the topmostoceanic 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 ofsouth-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 asin its footwall (Zermatt). Two examples of kilometre-scale overprinting structures (Ollomont and Cignana)will be discussed in detail