Cooling history of the Dauphinoise Zone (Western Alps, France) deduced from the thermopaleomagnetic record: geodynamic implications
Résumé
A post-tectonic thermoremanent magnetization, carried by pyrrhotite, has been previously demonstrated in the Dauphinoise Zone of the French Western Alps. The paleomagnetic signal is interpreted to record successive partial thermoremanent magnetizations acquired in normal and reverse geomagnetic fields during the slow cooling of the epimetamorphic marly limestones. A comparison between the derived magnetic polarity sequence versus temperature (thermopaleomagnetic record) and the magnetostratigraphic time scale, taking into account radiochronological constraints, allows us to derive 21 age/temperature couples between 24.1 Ma/302 °C and 20.2 Ma/182 °C, for the Bourg d'Oisans area. Combining these thermopaleomagnetic data with other P, T, t and structural data, a geodynamical model is proposed for the tectono-metamorphic evolution of the Dauphinoise Zone. The thickening contemporaneous with the main syncleavage folding occurs around 26.5 Ma. Depending on the hypothesis considered to explain the excess of heat, the Bourg d'Oisans area was buried to 8.6 km (fluids hypothesis) or 12.5 km (tectonic hypothesis). In both cases, these depths imply the existence of overlying units, which might come from the internal zones, actually cropping out eastward of the Penninic front. A tectonic denudation is proposed just before 24.1 Ma in order to explain our observations. After this denudation, a faster cooling occurs. It is probably related to the dissipation of the thermal anomaly, induced by the tectonic denudation, and with the increase of the erosion due to the underthrust of a more external crustal slice. This interpretation is supported by the similar fluctuations of cooling rate derived here and subsidence of the Molasse basin. Afterwards, the cooling slowed down. During this time, the studied area seems to be passively moved westward in the hanging wall of the External Crystalline Massifs thrust, without significant deformations.