A geomorphological description of the tectonic troughs behind the volcanic arc is provided from data gathered in the New Hebrides island arc. From bathymetry and seismic reflection profiles, a representative plan of these troughs is made. From front to rear, the arc sequence is generally a small depression, followed by a wide, deep depression with a narrow ridge along either side. Stratigraphy and tectonics clearly show that the troughs were induced by crustal stretching; sedimentary formations observed are thought to be Plio-Quaternary. Magnetic anomalies on several profiles, and some gravimetric data, show information on the deep structure and genesis of the troughs. We conclude from the data that these recently faulted basins are symmetrical about the longitudinal axis where rising magma is causing uplift.Three possible mechanisms are suggested for the formation of the troughs. The most likely is that transcurrent faults occurred in the curved parts of the arcs, after which convection cells appeared in the asthenosphere above the Benioff zone. The rising columns of the converting cells caused tensional tectonic movement beneath the troughs along pre-existing fault lines. Ascending material then formed intrusions, creating magnetic anomalies.