Lau Basin

The Lau Basin is a back-arc basin (also addressed as “interarc basin”) at the Australian-Pacific plate boundary. It is formed by the Pacific plate subducting under the Australian plate. The Tonga-Kermadec Ridge, a frontal arc, and the Lau-Colville Ridge, a remnant arc, sit on the western and eastern sides of the basin respectively.

Lau Basin is a young basin (<= 5 m.y. old) that separates a previously continuous island arc by extensional rifting. During the Pliocene, the Pacific plate was subducting beneath the Australian plate. The slab of the Pacific plate melted as it was thrust down, and then rose to form the original Tonga-Kermadec Ridge. Around 25 Ma B.P, the Pacific plate started to drift away from the Australian plate, thus splitting the volcanic ridge. The rifting was initially caused by extension until 6 Ma B.P, by which time seafloor spreading started in this region and eventually formed the Lau Basin between the separated ridges.

The V-shaped Lau Basin was opened by two southward propagating spreading centers: the Central Lau Spreading Center (CLSC) and the East Lau Spreading Center (ELSC). The initial ELSC was oriented north-south and has a spreading rate of ~100mm/yr. The northeastern tip of ELSC propagated southward faster than the other part and produced a oriented 170 degree. The ELSC rotated 15–25 degree clockwise and continued to propagate towards the south. Then the CLSC, as well as an extensional transform zone (ETZ) linking the two spreading centers were formed. The CLSC propagated southwards and replaced the northern segment ELSC. The region of overlap of CLSC and ELSC is characterized by strike-slip earthquakes. Recent measurements have shown that the opening rates are increasing at ELSC and CLSC. At present, the spreading rate of Lau Basin is about 150mm/year. It is an example of a fast-spreading back-arc basin.

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