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

Abstract

Listen

Translate article

In the southwestern part of the Belledonne Massif (External Crystalline Massifs, French Alps), superimposition of three distinct crustal units has been interpreted as the consequence of Late Devonian-Early Carboniferous thrusting toward the ENE under typical collisional metamorphic conditions (9–7 kbar, 600–650 °C). Structural relationships between the different units and the kinematic analysis of microstructures suggest that ductile extensional tectonics with a sinistral component towards the southwest is responsible for the late structure of this domain. Extensional tectonics are responsible for the exhumation of the deep level of the nappe pile (Allemont unit) that recorded an earlier HP-LT tectonometamorphic evolution (10 ± 1 kbar, 550 ± 50 °C) and for the syn-kinematic adiabatic decompression path recorded in the two lowest units (Livet and Allemont). Such isothermal decompression may have been related to rapid thinning (~ 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 affecting the previously thickened Variscan crust during the Upper Carboniferous prior to its restoration to normal thickness. © Elsevier, Paris