40Ar/39Ar dating of pseudotachylytes: a case study on post-metamorphic brittle fault in the NW Alps

Pseudotachylytes are solidified frictional melts produced by seismic fault slip. Being melts that solidified in seconds or minutes after the seismic slip event, they have always been considered a very favourable tool to date brittle deformation. However, since all pseudotachylytes are composed of inherited clasts, melt-derived matrix and (quite often) also alteration products, it is necessary to discriminate the Ar contribution of these three reservoirs to obtain meaningful ages. This can be done by analyzing Ca/K and Cl/K signatures provided by Ar systematics. Furthermore, microstructural analysis and microCT allow quantifying the clast-to-pseudotachylyte matrix ratios for each sample, and XRPD allows detecting potential alteration phases. Here we present the results of step-heating ⁴⁰Ar/³⁹Ar analyses performed on pseudotachylytes of the Trois Villes Fault and the Quart Fault, which crop out in a region of the Western Alps (Aosta Valley) affected by three different post-metamorphic brittle deformation phases: D1 characterized by NW-SW extension, D2 with NE-SW extension, and D3 showing N-S extension. The relative chronology of these deformation phases is based on consistent cross-cutting relationships. D1 ages of 29–32 Ma have been inferred from syn-kinematic magmatic dikes and hydrothermal veins. However, no absolute ages were so far available for D2 and D3, as direct radiometric dating of fault rocks has never been performed before in the area. Our results are consistent with the relative chronology and greatly improve our understanding of the tectonics of this area.