Slow and fast deformation in the Dora Maira Massif, Italian Alps: Pseudotachylytes and inferences on exhumation history

The Dora Maira Massif (DM), Western Alps is a type example of an ultra-high pressure (UHP) metamorphic terrain. The occurrence of pseudotachylytes within the Val Gilba area of DM led to speculation regarding the role of seismic slip in the exhumation of the massif. In this study, new field, microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented. The formation of pseudotachylyte veins occurred late in the exhumation history at 20.1 ± 0.5 Ma and at shallow depths (<3 km), long after the early stages and rapid exhumation that is characteristic of UHP massifs. Preseismic, coseismic and postseismic deformation features suggest that seismic slip occurred at a period where the geologic strain rate was particularly low. This observation is explained by a stick-slip model along the mylonitic foliation plane in which slip was preferentially along phengite-chlorite rich layers. Frictional sliding instability is promoted by a slow slip rate, by a phenomenon similar to “brake shuttering” in automobiles. An alternative explanation could be provided by climatically driven, accelerated exhumation ca. 20 Ma, as suggested by the 20 Ma-old white micas reported in the Tertiary Piedmont Basin.