Lesser Antillean Arc Initiation and Migration as a Proxy of Slab Dynamics: Geothermochronology, Thermobarometry and Structure of Saint Martin Granodiorites

In subduction zones, volcanic arc initiation, cessation, migration and associated upper plate deformation -i.e faulting and vertical motions- reflect large-scale slab dynamics. At the northeastern edge of the Caribbean plate, the Greater Caribbean subduction zone waned out during the Mid Eocene, following the subduction of the Bahamas bank. This arc cessation was contemporaneous with (i) a plate boundary re-organization (evolving from subduction to transform), (ii) upper plate deformation and (iii) arc initiation in the Lesser Antilles. As part of the GAARANTI project that aims at unraveling the relationships between the evolution of terrestrial Caribbean biodiversity and vertical motions resulting from the Lesser Antilles subduction zone dynamic, we study the Saint Martin granodiorites, one of the two Oligocene plutons outcropping in the Lesser Antillean forearc. We investigate the birth and evolution of the Lesser Antillean arc and its thermo-mechanical impact on the Caribbean upper plate. In order to characterize the P,T,t path of the pluton we performed several thermochronological analyses covering a wide range of temperature (U-Pb on zircon -Tc 850°C, Ar/Ar on amphibole -Tc 550°C- and biotite -Tc 325°C-, zircon and apatite fission-tracks -Tc 250 and 110°C, respectively as well as U-Th/He on apatite -Tc 60°C) coupled with in-situ thermobarometry analyses (Al in hornblendes) and structural data. Geochronology and thermobarometry reveal that the granodiorites emplaced at ca. 28 Ma, at a depth of 5 km. Based on the age difference between amphibole and biotite Ar/Ar ages, we show that the northern pluton cooled faster than the southern one. Preliminary thermochronological results show a fast cooling between 29 and 25 Ma and then a continuous and slow cooling since 25 Ma and inverse modeling points to a 10 Ma cooling event. Our investigations give insights on the thermo-mechanical evolution of the arc-forearc region of the Lesser Antilles subduction zone. Considering a mean high of 1200m for the volcanic edifice, the pluton emplaced at shallow depth (ca. 4 km) within the Caribbean plate. The pluton is bounded by N-S faults that could possibly be responsible for the 10 Ma exhumation event. This thermal event may be contemporaneous with the westward arc migration during Miocene times and may reflect slab flattening.