Nappe stacking and first evidence of Late Variscan extension in the Belledonne Massif (External Crystalline Massifs, French Alps)

In the southwestern part of the Belledonne Massif (External Crystalline Mas sifs, French Alps), superimposition of three distinct crustal units has bee n interpreted as the consequence of Late Devonian-Early Carboniferous thrus ting toward the ENE under typical collisional metamorphic conditions (9-7 k bar, 600-650 degrees C). Structural relationships between the different uni ts and the kinematic analysis of microstructures suggest that ductile exten sional tectonics with a sinistral component towards the southwest is respon sible for the late structure of this domain. Extensional tectonics are resp onsible for the exhumation of the deep level of the nappe pile (Allemont un it) that recorded an earlier HP-LT tectonometamorphic evolution (10 +/- 1 k bar, 550 +/- 50 degrees C) and for the syn-kinematic adiabatic decompressio n path recorded in the two lowest units (Livet and Allemont). Such isotherm al decompression may have been related to rapid thinning (similar to 3mm y( -1)) and led to local decompressional melting at the base of the nappe pile . The thinning is best explained by extensional tectonics processes affecti ng the previously thickened Variscan crust during the Upper Carboniferous p rior to its restoration to normal thickness. (C) Elsevier, Paris.