Rate of detachment of continental slices from the downgoing slab in a subduction zone
Abstract
In the Gran Paradiso Massif (Western Alps), the Money
Unit crops out as a tectonic window below the Gran Paradiso
Unit. In both units, four stages of deformation and
metamorphism have been identified [1]. Stage 1 reflects the
phase of continental crust subduction (~18-20 kbar, 480-520
°C and ~13-18 kbar, 500-530 °C for the Gran Paradiso and
the Money units, respectively). This yields a maximum
difference of ~20 km in the depth reached by these two units
during the early Alpine history. Thrusting of the Gran
Paradiso Unit over the Money Unit (stage 2) led to the
development of the main foliation ( ~12.5 kbar-14.5 kbar and
530-560 °C), identical in both units. The thrust contact was
folded 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) have
been studied in key samples from the Money and Gran
Paradiso Units. Their relative timing of growth and
dissolution have been assessed by combining thermodynamic
modelling, inclusion, textural and chemical (major and traceelement)
data from both major and accessory phases. (U-Th-
Pb) ages of allanite, monazite, and xenotime will be
presented and tied with the evolution of the studied samples.
The new data constrain (i) the age of the high pressure in the
Money and the Gran Paradiso Units, (ii) the timing of the
thrusting of the Gran Paradiso Unit over the lower pressure
Money Unit and (iii) the exhumation rates in the Gran
Paradiso Massif. Preliminary data indicate at most 3 to 5 Ma
between 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, 981-1002016-071