Rate of detachment of continentalslices from the downgoing slab in asubduction zone

International audienceIn the Gran Paradiso Massif (Western Alps), the MoneyUnit crops out as a tectonic window below the Gran ParadisoUnit. In both units, four stages of deformation andmetamorphism have been identified [1]. Stage 1 reflects thephase of continental crust subduction (~18-20 kbar, 480-520°C and ~13-18 kbar, 500-530 °C for the Gran Paradiso andthe Money units, respectively). This yields a maximumdifference of ~20 km in the depth reached by these two unitsduring the early Alpine history. Thrusting of the GranParadiso Unit over the Money Unit (stage 2) led to thedevelopment of the main foliation ( ~12.5 kbar-14.5 kbar and530-560 °C), identical in both units. The thrust contact wasfolded during stage 3 together with the entire Money Unit,and then both units were exhumed together (stage 4).Accessory phases (i.e. monazite, allanite, and xenotime) havebeen studied in key samples from the Money and GranParadiso Units. Their relative timing of growth anddissolution have been assessed by combining thermodynamicmodelling, inclusion, textural and chemical (major and traceelement)data from both major and accessory phases. (U-Th-Pb) ages of allanite, monazite, and xenotime will bepresented and tied with the evolution of the studied samples.The new data constrain (i) the age of the high pressure in theMoney and the Gran Paradiso Units, (ii) the timing of thethrusting of the Gran Paradiso Unit over the lower pressureMoney Unit and (iii) the exhumation rates in the GranParadiso Massif. Preliminary data indicate at most 3 to 5 Mabetween the peak pressure (stage 1) and the thrusting event(stage 2), i.e. a displacement of the order of 2 to 4 mm/year.[1] Manzotti et al. (2015) J. Metamorph. Geol. 33, 981-1002016-07