Influences of a ridge subduction on seismicity and geodynamics in the central Vanuatu arc.

Abstract:

The central part of the Vanuatu arc is characterized by the subduction of the d'Entrecasteaux ridge under the North Fiji Basin. This ridge influences directly the seismicity and the geodynamics in the proximal region. By analyzing the hypocenters from a local microseismic catalog (2008-2009) and global catalogs we show that the subduction interface, in the first 50 km depth, presents a small dipping angle where the ridge is subducting. This bump highlights the buoyancy of the ridge associated to the excess of fluids present in the seamount. This underplating could explain 20% to 60% of the vertical displacement estimated on the forearc islands from corals datations and that can reach a maximum of 6 mm/yr. The high concentration of hydrous minerals in the subducting ridge might also explain the important activity of intermediate depth earthquakes (half of the total activity in the studied region), we observed a very good correlation between the supposed extension of the ridge in depth and the location of these earthquakes. We propose that they are associated to crust minerals dehydration that causes hydrous fracturation trough preexistent faults. This dehydration process is maintained to a maximum depth of 190 km due to the high thermal parameter of the australian plate.
Using the geometry of the Wadati-Benioff plane derived from earthquakes localisations, we established a 2D mechanical model to explain the horizontal interseismic displacement observed by GPS on islands of the upper plate. We show that the subduction interface alone cannot explain the GPS velocities observed, the system of thrust faults located below the back arc islands of Maewo and Pentecost, plays a major role in the region geodynamics and accommodate as much convergence as the subduction interface (between ~16 and 34 mm/yr). Using the model we were also able to explain the closing of the Aoba basin during interseismic phase (~25 mm/an). Finally, the mechanical model suggests the existence of a 23 km wide locked patch that reaches the trench.