Post-middle Miocene rotations recorded in the Bourg d'Oisans area (Western Alps, France) by paleomagnetism

In order to test the occurrence of post metamorphic rotations or tiltings, a paleomagnetic study was undertaken in 44 sites from the inverted 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 around a vertical axis, are recorded. Instead, the dispersion of paleomagnetic data is interpreted as due to a variable tilting around a horizontal N20E axis, consistent with structural observations. This gentle folding in the sedimentary cover is probably associated with thrusting of the Belledonne external crystalline massif over the foreland basement. The age of the tiltings reported in this paper is estimated as being younger than 9-13 Ma.