Penultimate Interglacial emerged reef around Kadavu Island, Southwest Pacific: Implications for late Quaternary island-arc tectonics and sea-level history

Abstract

The Kadavu island group is a Pliocene-Quaternary volcanic island arc in the Southwest Pacific, along which the last eruptions occurred during the late Quaternary. Recent investigations have focused on late Quaternary tectonic history with a view to illuminating regional plate interactions. The main island (Kadavu) is divisible into three structural blocks, the westernmost of which is the youngest and the least submergent. This block is surrounded by fragments of emerged reef limestone reaching 7.1 m above the modern reef surface (minimum emergence magnitude), cut by a prominent erosional (wave-cut) bench at 2.6–3.4 m above the modern shore platform. Th/U dating of this limestone at the largest, highest outcrop—offshore Nagigia Island—revealed it to be of wholly Penultimate Interglacial age (207.2–223.2 ka). This result is explainable—plausibly not exclusively—by the following scenario. It is assumed that Penultimate Interglacial sea level reached a maximum of c. 10 m (relative to the present) around Kadavu, as it did around the Ryukyu islands of southern Japan, before falling. The western part of Kadavu was then uplifted 2 m, perhaps contemporaneously with the most recent phase of arc volcanism. An erosional bench was cut during an early Last Interglacial sea-level maximum c. 136 000 yr ago when sea level reached c. 5 m above its present level. The reason there is no reef dating from this time is that reef upgrowth could not “keep up” with rapid sea-level rise. Subsequent regression was followed by a slower rise of sea level (with which reef upgrowth did keep pace) to a second Last Interglacial maximum c. 120 000 yr ago, which reached some 2 m above present sea level. Subsequent subsidence of c. 2 m has carried this reef below present sea level, leaving the Penultimate Interglacial reef the only one emerged in western Kadavu today. The inferred 2 m uplift of western Kadavu between c. 200 and 130 000 yr ago may have been linked to the last phase of magmatic activity here, a time poorly constrained by available K-Ar dates. Post-120 ka subsidence may have been linked to a southward or southwestward shift in the axis of active volcanism associated with the nearby convergent plate boundary. © 1999 Taylor & Francis Group, LLC.