Insights into the tectonic history of the Western Alps through dating of fissure monazite in the Mont Blanc and Aiguilles Rouges Massifs

Abstract

Hydrothermal monazite-(Ce) crystals from sub-horizontal and younger sub-vertical Alpine fissures located in the Mont Blanc and Aiguilles Rouges Massifs were dated by in-situ SIMS Th-U-Pb methods to identify major growth phases during deformation. Sub-horizontal clefts formed in a compressive regime that led to steep foliation and down-dip lineation in the Mont Blanc shear zones. Sub-vertical clefts formed during younger, strike-slip movements along boundaries on both sides of the Mont Blanc Massif and along Permocarboniferous deposits in the Aiguilles Rouges. All analyzed monazite-(Ce) crystals show signs of recrystallized, porous and complexly-zoned grains, indicative of dissolution-reprecipitation processes. Consequently, the crystals show three main growth phases at 11.5–10.5 Ma, 9 Ma and 8–7 Ma, interpreted as representing (re)crystallization during the onset of dextral strike-slip faulting and subsequent steps of activity along the dextral strike-slip fault system. Whereas monazite-(Ce) from the older horizontal clefts may record all three crystallization steps, monazite-(Ce) crystallization in vertical clefts from the Aiguilles Rouges Massif starts only at around 9 Ma. Comparison of monazite-(Ce) crystallization ages with thermochronometers indicates that the anastomizing network of vertical shear zones with down-dip lineation in the Mont Blanc Massif became overprinted at 11.5 Ma and only traces of an older monazite-(Ce) generation survived. Heat advection seems to have been locally sufficient to reset zircon-fission track ages. The vertical clefts of ca. 9 Ma age in the Aiguilles Rouges Massif indicate that strike-slip deformation occurred not only on the south-eastern but also the north-western boundary of the Mont Blanc Massif and may be linked to the increase in exhumation rate around that time. The identified deformation phases show that the external Massifs of the Western Alps (Aar, Mont Blanc, Aiguilles Rouges, Pelvoux) not only share a comparable tectonic position within the Alpine arc but also underwent a very similar tectonic evolution.