Penultimate Interglacial emerged reef around Kadavu Island, Southwest Pacific: implications for late Quaternary island-arc tectonics and sea-level history
Pd. Nunn et A. Omura, Penultimate Interglacial emerged reef around Kadavu Island, Southwest Pacific: implications for late Quaternary island-arc tectonics and sea-level history, NZ J GEOL, 42(2), 1999, pp. 219-227
The Kadavu island group is a Pliocene-Quaternary volcanic island are 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 ree
f 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 Nagi
gia Island-revealed it to be of wholly Penultimate Interglacial age (207.2-
223.2 ka). This result is explainable-plausibly not exclusively-by the foll
owing scenario. It is assumed that Penultimate Interglacial sea level reach
ed 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 pa
rt of Kadavu was then uplifted 2 m, perhaps contemporaneously with the most
recent phase of are volcanism. An erosional bench was cut during an early
Last Interglacial sea-level maximum c. 136 000 yr ago when sea level reache
d c. 5 m above its present level. The reason there is no reef dating from t
his time is that reef upgrowth could not "keep up" with rapid sea-level ris
e. Subsequent regression was followed by a slower rise of sea level (with w
hich 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 t
oday.
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 p
oorly 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 vol
canism associated with the nearby convergent plate boundary.