Can crustal extension be distinguished from thrusting in the internal parts of mountain belts? A case history of the Entrelor shear zone, Western Alps

Criteria for the distinguishing between structures that accommodate crustal-scale extension or crustal-scale shortening may be applied to late orogenic shear zones developed in the internal parts of mountain belts. This is done for a large shear zone that has been mapped in the NW Italian Alps, around the internal basement massif of Gran Paradiso. The structure involved is termed the Entrelor shear zone. It carries greenschist facies European continental rocks (the Briançonnais—Grand St. Bernard unit) onto an old subduction complex of oceanic (Piemonte) and continental (Gran Paradiso) material that preserve eclogitic assemblages. Evidence for the map-scale continuity of the Entrelor shear zone is presented together with kinematic data to show how folds, stretching lineations and shear criteria relate to a regional episode of ESE-directed shearing. However, folds initiated systematically oblique to the shear direction, on a NE-SW axis. With continued shearing, the fold population remains skewed E-W of the population of stretching lineations. The shear zone may be traced for 70 km around the dome of the Gran Paradiso massif and displays a minimum displacement of 20 km. This Entrelor structure shows variable offset of metamorphic grade. However, for much of its length, greenschist facies rocks are carried in its hanging-wall onto eclogites in the footwall. Tracing these units regionally shows that this apparent extensional offset of metamorphic facies is the result of restacking of an originally ‘inverted’ metamorphic sequence (i.e. HP Piemonte on LP Briançonnais). This contractional behavior is consistent with the profile of the shear zone relative to the modern surface. The Entrelor shear climbs up from generally buried in the west to generally eroded in the east. The conclusion that the Entrelor shear zone is probably a thrust rather than an extensional structure relies on linking outcrop data into a semi-regional context. False interpretations may result from examining only small parts of the structure. However, although these map and structural criteria are consistent with contractional kinematics associated with crustal shortening, confirmation is required through linked petrological and isotopic studies. The Entrelor shear zone is thus a good illustration of the need for caution in inferring the contractional or extensional nature of structures developed late in the history of the internal parts of mountain belts.