A CLOSE-UP ON THE THERMAL STRUCTURE OF THE W. ALPS PALEO-SUBDUCTION ZONE USING GARNET PETROCHRONOLOGY

The Western Alps comprise among the largest and best-preserved coherent sequences of lithospheric fragments from the Tethyan seafloor which have been subducted to eclogite facies and later exhumed shortly before the entrance of the European continental margin into the subduction zone. Sub-seafloor hydrothermally-altered basalts from the ~60-km thick Zermatt-Saas (ZSO) ophiolite record high-pressure metamorphism at P-T conditions ~1.9-2.3 GPa and ~500-575°C.

We present the results from a study combining zoned Sm-Nd garnet geochronology, mineral trace element geochemistry, phase equilibiria and diffusion modeling of a pyrite-rich chlorite-talcschist from the Servette locality of the ZSO (St. Marcel Valley, Italy), in order to further constrain parameters for the thermal structure of the Western Alps.

Major and trace element zoning in garnet elucidates two distinct growth generations. Garnet cores display evidence for reaction overstepping and rapid nucleation and growth. Zoning in garnet mantles and rims (Ca and Mn-rich annuli) suggest that garnet growth slowed, undergoing several distinct resorption/growth episodes of minor amplitude. Through modeling of intra-crystal diffusive relaxation of these growth annuli, constraints are made on the rate of subsequent (isothermal) exhumation.

Two cm-sized garnets were microdrilled to separate core and rim growth generations and were analyzed via ID-TIMS in order to determine the garnet growth duration. Garnet cores were dated to 46.9 ± 1.6 Ma, signifying an approximate age for the initiation of garnet growth. Garnet rims were dated to 43.5 ± 1.3 Ma. This growth duration (3.4 ± 2.1 Ma), when coupled to P-T constraints, results in burial and heating rate estimates of ~4-5 km/Ma and ~15-20°C/Ma.

Geometric calculations varying slab velocity and dip are in agreement with the P-T-t trajectory deduced here, showing that subduction occurred at a velocity of ~2-3 cm/yr, along a ~10-15˚ slab dip. These results highlight that a significant decrease of the thermal regime from ~10°C /km down to ~3°C/km existed at c. 45 Ma. This deflection departs from most existing thermal models, thus raising the need to improve our assessment of the thermal structure, with implications on the seismicity and stress distribution of deeper segments of the subduction interface.