GEOMETRY AND STRUCTURE OF THE VITIAZ TRENCH LINEAMENT (SW PACIFIC)

Swath bathymetric, sonar imagery and seismic reflection data collected during the SOPACMAPS cruise Leg 3 over segments of the Vitiaz Trench Lineament and adjacent areas provide new insights on the geometry and the stuctural evolution of this seismically inactive lineament. The Vi tiaz Trench Lineament, although largely unknown, is one of the most im portant tectonic Feature in the SW Pacific because it separates the Cr etaceous crust of the Pacific Plate to the north from the Cenozoic lit hosphere of the North Fiji and Lau Basins to the south. The lineament is considered to be the convergent plate boundary between the Pacific and Australian Plates during midde to late Tertiary time when the Viti az Are was a continuous east-facing are from the Tonga to the Solomon Islands before the development of the North Fiji and Lau Basins. Progr essive reversal and cessation of subduction from west to east in the L ate Miocene-Lower Plioene have been also proposed. However, precise st ructures and age of initiation an. cessation of deformation along the Vitiaz Trench Lineament are unknown. The lineament consists of the Vit iaz Trench and three discontinuous and elongated troughs (Alexa, Rotum a and Home Troughs) which connect the Vitiaz Trench to the northern en d of the Tonga Trench. Our survey of the Alexa and Rotuma Troughs reve als that the lineament is composed of a series of WNW-ESE and ENE-WSW trending segments in front of large volcanic massifs belonging to the Melanesian Border Plateau, a WNW trending volcanic belt of seamounts a nd ridges on Pacific crust. The Plateau and Pacific plate lying immedi ately north of the lineament have been affected by intense normal faul ting, collapse, and volcanism as evidenced by a series of tilted block s, grabens, horsts and ridges trending N 120 degrees to N 100 degrees and N 60 degrees-70 degrees. This tectonism includes several normal fa ulting episodes, the latest being very recent and possibly still activ e. The trend of the fault scarps and volcanic ridges parallels the dif ferent segments of the Vitiaz Trench Lineament, suggesting that tecton ics and volcanism are related to crustal motion along the lineament. A lthough the superficial observed features are mainly extensional, they are interpreted as the result of shortening along the Vitiaz Trench L ineament. The fabric north of the lineament would result from subducti on-induced normal faulting on the outer wall of the trench and the zig -zag geometry of the Vitiaz Trench Lineament might be due to collision of large volcanic edifices of the Melanesian Border Plateau with the trench, provoking trench segmentation along left-lateral ENE-WSW trend ing transform zones. The newly acquired bathymetric and seismic data s uggest that crustal motion (tectonism associated with volcanism) conti nued up to recent times along the Vitiaz Trench Lineament and was acti ve during the development of the North Fiji Basin.