Late Cenozoic tectonic history of Lau Ridge, Southwest Pacific, and associated shoreline displacements: Review and analysis

Abstract

Abstract The islands of the Lau Ridge in eastern Fiji have long been a focus for the attention of geologists owing to their spectacular sequences of preserved shorelines. A review of past work is given, and 107 shoreline levels are recognised across 32 Lau islands. The data are tabulated and plotted against their longitudes and latitudes. No shoreline groups are recognised from a plot of height against longitude, which appears to repudiate claims that the Lau Ridge was tilted eastwards during the opening of the back-arc Lau Basin, east of the ridge. In the height/latitude plot, three groups of shorelines (24) are clearly identified between 21 m and 80 m above present mean sea level. Above the level of shoreline 4, another shoreline group (5a) is recognised. Below 27 m, three shoreline groups (lb-Id) are identified with slightly less certainty. The average correlation coefficient for the seven shoreline groups (lb-5a) is −0.809. The gradients of regression lines show a consistent decrease with decreasing height through shoreline group 4 (1 m/12.58 km) to shoreline group lb (1 m/30.72 km). A belief that uplift alone has been responsible for relative shoreline emergence is weakened by the absence of any clear geotectonic cause. It is possible that contributory causes included (1) residual movements of the Lau Ridge resulting from Pliocene collision with the Viti Levu-Vanua Levu arc segment, (2) interaction between the spreading Lau Basin and the remnant arc (Lau Ridge) after this time, and (3) uplift associated with late Cenozoic thermal events in Lau. The reasonably consistent decrease in gradient exhibited by shoreline groups 1b-5a is partially explained by sea-level changes resulting principally from geoid deformation. A decrease in oceanic geoid gradient in Lau could have been caused by the contraction or movement northwestwards of the geoid high presently centred over New Guinea. It is concluded that both uplift and geoidal-eustatic change may have contributed to the displacement of shorelines in Lau since the early Pliocene. The cause of neither movement is clear.