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• The slab-unfolding links plates and slabs without assumed vertical slab sinking. • The Lesser Antilles slab was laterally transported up to ∼900 km after subduction. • We reconstruct up to 18 mm/yr lateral slab transport since 50 Ma. • A new pre-subduction ridge-transform system of the Proto-Caribbean is presented. The link between surface tectonic plates and mantle slabs is fundamental for paleo-tectonic reconstructions and for our understanding of mantle dynamics. Many seismic tomography-based studies have assumed vertical slab sinking and projected mantle features to the surface to reconstruct paleo-trench locations or explain tectonic features. Here, we used a slab-unfolding approach that does not require assumptions about sinking paths or rates to re-interpret the seismic structure of the Lesser Antilles slab underneath the Caribbean. A recent study invoked mainly vertical slab sinking and a highly folded and deformed slab to explain seismic Caribbean mantle structures. However, our results show that the upper-mantle Lesser Antilles slab structure can be better explained by limited intra-slab deformation and up to ∼900 km lateral slab transport towards the northwest after subduction. Our results indicate that such lateral slab transport can occur even with probable weaknesses in the slab that originate from a subducted fossil ridge-transform system. We ascribe the lateral slab transport in the mantle to a kinematic connection with the North American plate, which has migrated northwestward since the Eocene. [ABSTRACT FROM AUTHOR]

Fluids are important for mass transfer at the slab–mantle interface in subduction zones. However, it is usually difficult to trace fluids from specific sources in a subducting slab, especially those derived from dehydration of serpentinite. Coesite-bearing whiteschist at Dora-Maira in the Western Alps is characterized by strong Mg enrichment relative to the country rocks, which requires infiltration of Mg-rich fluids into the supracrustal rock. In order to constrain the origin of such Mg-rich fluids, we have performed an integrated study of whole-rock Mg and O isotopes, zircon U–Pb ages and O isotopes for the whiteschist and related rocks. Zircons in the whiteschist show two groups of U–Pb ages at ∼262 Ma and ∼34 Ma, respectively, for relict and newly grown domains. The Permian U–Pb ages of relict magmatic domains are consistent with the protolith age of host metagranite, suggesting that their common protolith is the Permian granite. The Tertiary U–Pb ages occur in coesite-bearing metamorphic domains, consistent with the known age for ultrahigh-pressure metamorphism. The metamorphic domains have δ 18 O values of 5.8 – 6.8 ‰ , whereas the relict magmatic domains have high δ 18 O values of ∼ 10 ‰ . Such high δ 18 O values are also characteristic of the metagranite, indicating that the whiteschist protolith underwent metasomatism by metamorphic fluids with low δ 18 O value of ∼ 2 – 4 ‰ . The whiteschist mostly has whole-rock δ 26 Mg values of −0.07 to 0.72‰, considerably higher than country-rock δ 26 Mg values of −0.54 to − 0.11 ‰ . Thus, the metamorphic fluids are not only rich in Mg but also heavy in Mg isotopes. They were probably derived from the breakdown of Mg-rich hydrous minerals such as talc and antigorite in serpentinite at the slab–mantle interface in the subduction channel. Therefore, the dehydration of mantle wedge serpentinite during the subduction and exhumation of continental crust can provide the Mg-rich fluids responsible for the metasomatism of crustal rocks at subarc depths. [ABSTRACT FROM AUTHOR]