The Geology of the Lau Basin

Abstract

The Lau Basin comprises oceanic crust that separates the remnant Lau Ridge volcanic arc from the active Tofua arc. The basin is situated above mantle exhibiting strong seismic wave attenuation; it overlies the west dipping seismic zone of the Tonga Trench subduction system and is presently opening at rates as high as 1.6 cm/yr. Seaward rollback of the trench axis, coupled with upwelling high temperature mantle diapirs, is proposed as the main driving force to cause crustal extension in this region of oceanic plate convergence. The trapezoidal shape of the basin suggests that it has opened progressively from north to south; the trace of the Louisville seamount chain may help constrain the southern apex of the basin. The basin has opened in two tectonic styles. Initially, beginning at about 6 Ma, attenuation and rifting of the forearc or outer part of the Lau Ridge formed a series of half-grabens that were partly sedimented and received basaltic flows. This extension and magmatism overlapped with Lau Ridge arc volcanism. The second stage of opening (about 5.5-5 Ma) was promoted by a southward-propagating rift system that formed new crust by seafloor spreading. A second propagator (about 1.5 Ma) has overtaken the first and forms an overlapping ridge system. A small three-limbed spreading system in the northeastern basin forms a triple junction. A fourth spreading system is recognized in the northwestern part of the basin. In spite of the suprasubduction zone (SSZ) setting and the proximity to volcanic arcs, the basin is dominated by mid-ocean ridge basalt (MORB)-like crust. Isotope data indicate that both Pacific and Indian MORB-source mantle have been involved; Pacific source formed some of the oldest crust, the Indian source feeds the modem spreading ridges. Helium-isotope data suggest a Samoan "plume" component is important in generating seamounts of the northern part of the basin. Lau Basin crust exhibits a general depletion in high-field-strength elements, and some samples show varied enrichment in large-ionic-radius lithophile elements, but the source must be like the MORB-source in most respects. However, mixing of MORB-like and arc1ike magmas has been important in the basin's history. Locally, crust is transitional to arc compositions but this occurs largely in areas of older crust or where rifting first encountered older crust/mantle. Heterogeneity of the SSZ