LEG 135

Lau Basin

The Lau Basin comprises the region of lithospheric extension between the Lau Ridge and the Tonga Trench and duirng Leg 135, eight sites (Site 834 to Site 841) were drill to study the evolution of these backarc basins and their relationships to plate subduction processes in oceanic trenches.

The Lau Basin predates 5.6 Ma but seafloor spreading has occurred in the backarc only in the last 1-2 Ma. Prior to this, extension occurred by repeated extensional rifting ("basin and range") and associated local volcanism with new crust created occurred only within the north-trending fault basins. Basalts, basaltic andesites, and andesites display affinities to both oceanic arc and mid- ocean ridge lavas, without spatial or temporal consistency, and reflect a complex heterogeneity of the mantle source. The volcaniclastic sediments also suggest volcanism of arc affinity throughout much of the Lau Basin during extension. A transition from high-energy to pelagic-dominated sedimentation conditions during the early Pleistocene may coincide with the establishment of true seafloor spreading at the eastern Lau spreading center.

Material recovered from the south-central Tonga Platform was predominantly Miocene turbidites and volcaniclastic sediments with textures, and the abundance of turbidites, suggesting that the site was located on the distal end of sediment deposits derived from some former volcanic edifice on the Lau Ridge. Downward coarsening of the sediment implies either diminishing volcanism or a westward retreat of the locus. At ~1 Ma, there is a biostratigraphic hiatus and decrease in sedimentation rate. The Pliocene and younger sedimentation rate is markedly decreased with respect to the Miocene, corresponding to the first stage of breakup of the crust to form the Lau Basin around 5.6 Ma. Paleomagnetic data indicates a 21 deg 11 deg clockwise rotation of the Tonga Arc with respect to the western basin.

Dacitic tuffs, welded tuffs, and lavas of upper Eocene, or older, from the Tonga forearc region are in fault contact with overlying upper Eocene to lower Oligocene carbonates. These subaerially- erupted dacitic rocks have subsided more than 5.4 km since their formation, representing profound tectonic foundering. Similar rhyolites of Late Cretaceous age were recovered at DSDP Site 207 on Lord Howe Rise to the west of the Lau Basin; reconstruction of pre-Eocene plate geometry allows speculation for a common source of the silicic volcanic rocks at both sites.