POST-MIDDLE MIOCENE ROTATIONS RECORDED IN THE BOURG DOISANS AREA (WESTERN ALPS, FRANCE) BY PALEOMAGNETISM

In order to test the occurrence of post metamorphic rotations or tilti ngs, a paleomagnetic study was undertaken in 44 sites from the inverte d half graben of Bourg d'Oisans (''Zone Dauphinoise'', Western Alps). The magnetization carried by pyrrhotite was acquired after metamorphic culmination, around the Oligo-Miocene limit. No obvious rotations aro und a vertical axis, are recorded. Instead, the dispersion of paleomag netic data is interpreted as due to a variable tilting around a horizo ntal N20E axis, consistent with structural observations. This gentle f olding in the sedimentary cover is probably associated with thrusting of the Belledonne external crystalline massif over the foreland baseme nt. The age of the tiltings reported in this paper is estimated as bei ng younger than 9-13 Ma.