Abstract

Zircon and apatite fission-track analysis has established the continuation of a zone of rapid change in mineral ages extending from the Simplon fault zone in the Simplon Alps, through the Rhone Valley, and along the southeastern side of the Mont Blanc and Belledonne massifs. Apatite fission-track ages to the northwest of this zone generally range from 1.4 to 6 Ma, whereas those to the southeast range from 6 to 13 Ma: corresponding values for zircon are 11-13 and 16-26 Ma, respectively. The zone corresponds to the position of the Frontal Pennine thrust in the Mont Blanc-Belledonne region. This structure, a prominent feature on recent deep seismic profiles, may have been reactivated in the Neogene with an important normal-fault component. This is suggested by apatite and zircon fission-track ages in the footwall that are younger than those in the hanging wall and by the observed elimination of the lower Pennine (or Simplon) nappes along this fault zone in the Rhone Valley-Mont Blanc-Belledonne region, resulting in the juxtaposition of North Penninic and Helvetic-Ultra-helvetic nappes at the current erosion level. The distribution of fission-track ages reflects this movement and the related Neogene exhumation of the Mont Blanc and Belledonne external massifs, which has continued until less than 1.4 m.y. ago (i.e., it is almost certainly still active)and has tilted the overlying nappe sequence.

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