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,1,11 Department of Earth Sciences, Basel University, Bernoullistrasse 32, 4056 Basel, Switzerland
New zircon and apatite fission-track data, combined with a structural analysis, provide constraints regarding the large-scale tectonic evolution of part of the French/Italian Alps. The observed migration of cooling toward more external units is interpreted to be due to thrust propagation. In the absence of significant amounts of crustal thinning, the exhumation has been dominantly by erosion.
The most internal unit investigated (the Penninic Houillère zone) started to cool at ca. 35 Ma, after an earlier tectonometamorphic event that only affected Penninic units. Fission-track data from the Cheval Noir unit (replacing the Valaisan unit in the south) constrain the age of metamorphism in these frontal Penninic units to be older than 32 Ma. There, cooling started with the onset of a second stage of the tectonic evolution, i.e., with top-to-the-west-northwest movement along the Roselend thrust at ca. 32 Ma. This Oligocene to early Miocene thrusting led to burial and metamorphism in the external massifs and their cover (Dauphinois unit). During a third tectonic stage, thrust-related culminations formed in the external massifs. Between 20 and 15 Ma, this stage led to the onset of exhumation in the Dauphinois and in the northern frontal Penninic units (Valaisan units), caused by erosion that followed tilting in the backlimb of these culminations. Northward-increasing amounts of Miocene– Pliocene shortening in front of and below the external massifs led to significantly higher amounts of exhumation in the north. Finally, and in very recent times (post–5 Ma), the frontal thrust of the Houillère zone was reactivated as a normal fault with moderate offset (<3 km).
Key Words: fission-track dating • Western Alps • low-grade metamorphism • cooling history • exhumation history
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