Dating tectonic activity in the Lepontine Dome and Rhone-Simplon Fault regions through hydrothermal monazite-(Ce)

Abstract

Zoned hydrothermal monazite-(Ce) from Alpinetype fissures and clefts is used to gain new insights into the tectonic history of the Lepontine Dome in the Central Alps and the timing of deformation along the RhoneSimplon Fault zone on the dome's western end. Hydrothermal monazites-(Ce) (re)crystallization ages directly date deformation that induces changes in physicochemical conditions of the fissure or cleft fluid. A total of 480 secondary ion mass spectrometry (SIMS) spot analyses from 20 individual crystals, including co-type material of the monazite-(Nd) type locality, record ages for the time of ∼ 19 to 2.7 Ma, with individual grains recording age ranges of 2 to 7.5 Myr. The combination of these age data with geometric considerations and spatial distribution across the Lepontine region gives a more precise young exhumation history for the area. At the northeastern and southwestern edges of the Lepontine Dome, units underwent hydrothermal monazite-(Ce) growth at 19-12.5 and 16.5-10.5 Ma, respectively, while crystallization of monazite-(Ce) in the eastern Lepontine Dome started later, at 15-10 Ma. Fissure monazite-(Ce) along the western limit of the dome reports younger ages of 13-7 Ma. A younger age group around 8-5 Ma is limited to fissures and clefts associated with the Simplon normal fault and related strike-slip faults such as the Rhone Fault. The data set shows that the monazite-(Ce) age record directly links the fluid-induced interaction between fissure mineral and host rock to the Lepontine Dome's evolution in space and time. A comparison between hydrothermal monazite-(Ce) and thermochronometric data suggest that hydrothermal monazite-(Ce) dating may allow us to identify areas of slow exhumation or cooling rates during ongoing tectonic activity.