Ocean Drilling Program Leg 135 Preliminary Report

Leg 135 of the Ocean Drilling Program drilled and cored eight sites across the Lau Basin and adjacent Tonga Ridge to study the geological evolution of a backarc basin and adjacent oceanic arc and forearc. Six sites (Sites 834, 835, 836, 837, 838, and 839) were drilled within the Lau Basin, each within smaller and narrower (<10 km) north-south fault basins and collectively representing a broad east-west transect. Two other sites were drilled on the Tonga Ridge, in the forearc of the Tofua Arc. Site 840 lies on the crest of the Tonga Ridge, a shallow, uplifted carbonate platform overlying older volcanic basement, while Site 841 lies on the arc-trench slope in 4800 m of water. Results from this cruise will provide crucial data toward determining the geological history of the Lau Basin-Tofua/ Tonga Arc oceanic-arc system as well as provide a heretofore uncollected observational base for the understanding of oceanic-arc and backarc spreading systems in general. The principal results of Leg 135 were quite different than anticipated, with two results being of particular significance. First, the Lau Basin is much older than expected (>5.6 Ma instead of 2.5-3 Ma), with seafloor-type spreading having occurred in the backarc only in the last 1 to 2 Ma or so. Prior to this, extension occurred by a combination of repeated extensional rifting ("basin and range") with associated local volcanism, with new crust created, if at all, only within the north-trending fault basins. No consistent variations in the chemistry of the Lau Basin volcanic products were noted either within time or space; the igneous basement rocks collected represent basalts, basaltic andesites, and andesites with affinities to both oceanic-arc and mid-ocean-ridge lavas, and indicate complex heterogeneity of the mantle source. Volcaniclastic sediments collected in the cores also show that volcanism of arc affinity may have occurred throughout much of the Lau Basin during extension, rather than being confined to a narrowly defined arc. The second principal result was the coring of dacitic tuffs, welded tuffs, and lavas of upper Eocene or older age at the base of Hole 84IB. These are in fault contact with overlying upper Eocene to lower Oligocene carbonates and a thick sequence of Miocene proximal and distal volcaniclastic sediments. These subaerially erupted dacitic igneous rocks have subsided more than 6 km since their formation, representing profound tectonic foundering. Similar rhyolites of Late Cretaceous age were cored at DSDP Site 207 on the 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.