Imaging the structure of the Northern Lesser Antilles (Guadeloupe – Virgin Island) to assess the tectonic and thermo-mechanical behavior of an arcuate subduction zone that undergoes increasing convergence obliquity

Abstract:

Paradoxically, the Northern Lesser Antilles is the less-investigated and the most tectonically and seismically complex segment of the Lesser Antilles subduction zone:

- The convergence obliquity between the North American and Caribbean plates increases northward from Guadeloupe to Virgin Islands raising questions about the fore-arc tectonic partitioning.

- The margin has undergone the subduction of the rough sediment-starved Atlantic Ocean floor spiked with ridges as well as banks docking, but the resulting tectonic deformation remains hypothetical in the absence of a complete bathymetry and of any seismic line.

- Recent geodetic data and low historical seismic activity suggest a low interplate coupling between Saint-Martin and Anegada, but the sparse onshore seismometers located far from source zone cast doubt on this seismic gap.

To shed new light on these questions, the ANTITHESIS project, 5 Marine Geophysical legs totaling 72 days, aims at recording a complete bathymetric map, deep and shallow seismic reflexion lines, wide-angle seismic data, heat-flow measurements and the seismic activity with a web of sea-bottom seismometers.

Our preliminary results suggest that:

- A frontal sliver of accretionary prism is stretched and expulsed northward by 50km along the left-lateral Bunce fault that limits the prism from the margin basement as far southward as 18.5°N. So far, this structure is the only interpreted sign of tectonic partitioning in the fore-arc.

- The Anegada Passage extends eastward to the accretionary prism through strike-slip faults and pull-apart basins that possibly form a lef-lateral poorly-active system inherited from a past tectonic phase, consistently with geodetic and seismologic data.

- The anomalously cold interplate contact, consistent with a low interseismic coupling, is possibly due to fluid circulation within the shallow crustal aquifer or a depressed thermal structure of the oceanic crust related to the slow-spreading at the medio-Atlantic ridge.