Relative motions between the Pacific plate (P), Indo-Australian plate (IA), New Hebrides (NH) arc microplate and the North Fiji Basin (NFB) microplate are estimated using shallow seismicity; 276 focal mechanism solutions, bathymetry, magnetism and the RM-2 plate model of Minster and Jordan (1978). Within the NFB, we define the western NFB (WNFB), eastern NFB (ENFB) and southern NFB (SNFB) microplates. A 8 cm/y spreading in a N72OE direction occurs along the N-S trending spreading ridge of NFB (WNFB-ENFB boundary). Along the ENFB-IA boundary located at 176"E, a 3 cm/y extension in a N108"E direction is proposed. The WNFB-P boundary which includes the Hazel Holme Extensional Zone (HHEZ), is complex and a general N25 E extension (2 cm/y) is inferred. The Fiji fracture zone (FFZ) is composed of two segments: Along the eastern zone (P-IA boundary) trending N80 O E, a left-lateral strike-slip motion occurs (9.6 cm/y) accommodated by minor N109OE extension (pull-apart); while along the western zone (ENFB-P boundary) a N84OE trending left-lateral strike-slip motion (7 cm/y) is inferred. The WNFB-SNFB boundary corresponds to a N70 O E 'broad fracture zone along which an E-W left-lateral strike-slip motion is proposed. The SNFB microplate moves rapidly eastwards (10.5 cm/y at 172"E) and is almost attached to the IA plate. Within the NFB, an improved model is discussed which includes deformations within the IA and P plates and rotations with Euler poles close to the NFB. Along the New Hebrides trench, the consumption rates are respectively 16, 15, 9 and 12 cm/y at ll", 12.5 O, 15.5O and 20 O S. The minimum rate occurs where the d'Entrecasteaux ridge collides with the New Hebrides arc. Rates of extension at the rear of the NH arc are 7, 5.5 and 2 cm/y at 1l0, 12.5O and 20 OS respectively. Back-arc compression occurs between 13O3O'S and 17OS. The western end of the Hazel Holme extension zone coincides with the 13O3O'S boundary separating the NH back-arc compressive belt and the northern NH back-arc troughs. South of 21°-220S the trend of the slip vector changes along the NH trench from N70"E to N20"W. This implies a N-S motion of convergence along the E-W trending southern part of the NH trench (motion SNFB-IA of 1.5 cm/y at 172OE) and the existence of a N70 O E trending left-lateral fracture zone crossing the southern part of the NFB. This fracture should be considered as a major plate boundary.

Une compilation de la sismicité superficielle du système arc des Nouvelles-Hébrides - bassin nord-fidjien, incluant notamment 276 mécanismes au foyer, permet de définir la tectonique actuelle et de calculer les mouvements relatifs entre les plaques Pacifique et indo-australienne et les microplaques du bassin nord-fidjien et de l'arc des Nouvelles-Hébrides. (Résumé d'auteur)