Extract

Tonga, Kermadec and Lau are groups of islands flanked by oceanic trenches centrally situated in the South Pacific. Rock exposures are relatively shallow and knowledge of the historical sequence of geological events consequently limited. Description therefore relates in large part to the major topographical features and to their geophysical interpretation; the division into structural zones is based essentially on this type of evidence.

This chapter should be read in conjunction with the account of Fiji which immediately adjoins it on the west.

Segment: the area described in detail in this article is shown in Fig. 1.

Zones: four structural zones are suggested for this ‘island arc’. From west to east they are the Lau Ridge zone, Lau Basin zone, Tonga Ridge zone and Tonga Trench zone. All continue southwards, forming the Colville Ridge, Havre Trough, Kermadec Ridge and Kermadec Trench respectively (Fig. 2). The last named lie along the east margin of an active seismic layer (?a thrust sole) which dips westwards beneath the other three zones. The eastern margin of the ‘island arc’ may be taken at the outer edge of the Tonga-Kermadec Trench, whilst the western margin is indefinite. The arc varies from 200 to 1000 km in width and averages 600 km within the 450 km long segment. The general trend of the Lau-Colville Ridge is arcuate; over a 2400 km long stretch it has a radius of curvature of 3500 km. In its southern half, however, it parallels the linear Tonga-Kermadec Ridge.

History: the oldest