CAN CRUSTAL EXTENSION BE DISTINGUISHED FROM THRUSTING IN THE INTERNALPARTS OF MOUNTAIN BELTS - A CASE-HISTORY OF THE ENTRELOR SHEAR ZONE, WESTERN ALPS

Criteria for the distinguishing between structures that accommodate cr ustal-scale extension or crustal-scale shortening may be applied to la te orogenic shear zones developed in the internal parts of mountain be lts. This is done for a large shear zone that has been mapped in the N W Italian Alps, around the internal basement massif of Gran Paradiso. The structure involved is termed the Entrelor shear zone. It carries g reenschist facies European continental rocks (the Brianconnais-Grand S t. Bernard unit) onto an old subduction complex of oceanic (Piemonte) and continental (Gran Paradiso) material that preserve eclogitic assem blages. Evidence for the map-scale continuity of the Entrelor shear zo ne is presented together with kinematic data to show how folds, stretc hing 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 fo ld population remains skewed E-W of the population of stretching linea tions. The shear zone may be traced for 70 km around the dome of the G ran Paradise massif and displays a minimum displacement of 20 km. This Entrelor structure shows variable offset of metamorphic grade. Howeve r, for much of its length, greenschist facies rocks are carried in its hanging-wall onto eclogites in the footwall. Tracing these units regi onally shows that this apparent extensional offset of metamorphic faci es is the result of restacking of an originally 'inverted' metamorphic sequence (i.e. HP Piemonte on LP Brianconnais). This contractional be havior is consistent with the profile of the shear zone relative to th e modern surface. The Entrelor shear climbs up from generally buried i n the west to generally eroded in the east. The conclusion that the En trelor shear zone is probably a thrust rather than an extensional stru cture relies on linking outcrop data into a semi-regional context. Fal se interpretations may result from examining only small parts of the s tructure. However, although these map and structural criteria are cons istent with contractional kinematics associated with crustal shortenin g, confirmation is required through linked petrological and isotopic s tudies. The Entrelor shear zone is thus a good illustration of the nee d for caution in inferring the contractional or extensional nature of structures developed late in the history of the internal parts of moun tain belts. Copyright (C) 1996 Elsevier Science Ltd.