Geomorphology and Geodynamics of the Cook-Austral Island-Seamount Chain in the South Pacific Ocean: Implications for Hotspots and Plumes

Abstract

Among Pacific hotspot tracks, the Cook-Austral island-seamount chain is distinctly anomalous in geodynamic behavior, exhibiting repetitive episodes of volcanism at multiple sites, uplift of selected islands long after initial immersion by subsidence, and multiple alignments of volcanic edifices. Cook-Austral islands include a variety of disparate geomorphic types: volcanic islands without reefs, with fringing reefs, and with barrier reefs enclosing shallow lagoons, low-lying atolls; and makatea islands composed of volcanic cores surrounded by annular limestone tablelands.

Neogene hotspot volcanism along the chain, built across Cretaceous-Paleogene seafloor, has occurred above three different mantle source regions, now located near its southeastern end, center, and northwestern end. Each hotspot has given rise to a separate age trend of volcanism along the same broad hotspot track, as the Pacific plate drifted over them, and each was initiated by eruption through the same region of lithosphere now located near the northwestern end of the compound chain. Age-distance trends of hotspot volcanism along the nearby Society and Marquesas chains depart less from expectation, but both the orientation and age trend of the Marquesas hotspot track also are anomalous. Lithospheric properties have seemingly influenced loci of South Pacific hotspot magmatism more than plume theory implies.

Cook-Austral-Society-Marquesas volcanic suites are distinctly more alkalic than typical Hawaiian tholeiites, and display marked isotopic heterogeneities that reflect derivation from multiple mantle sources, even for individual volcanoes. As the volcanic assemblages all were erupted from the SOPITA-Superswell region of anomalously hot mantle including diverse recycled components, hotspots that built the several island chains may reflect bleeding of magma through lithospheric flaws from a buoyant subjacent pillow of atypical mantle, poised to melt readily from varied stimuli.

Monotonic island subsidence is expected following cessation of hotspot volcanism, but episodes of post-immersion uplift have produced six makatea islands along the Cook-Austral chain. Makateas are tablelands of Neogene limestone, representing uplifted fringing reefs, and form annular girdles surrounding volcanic island cores. Despite limited erosional degradation, makatea surfaces provide a measure of net post-immersion island uplift. Paleoshoreline indicators of changing relative sea levels on Cook-Austral islands reflect the combined influence of seafloor thermotectonic subsidence, thermal rejuvenation associated with renewed hotspot activity, flexural bulges surrounding the isostatic loads of subsided volcanic edifices built out of sequence, the last-interglacial eustatic highstand in global sea level, and a mid-Holocene hydro-isostatic highstand in regional sea level.

Key paleoshoreline indicators include emergent reef flats and microatolls recording former low-tide levels, paleobeachrock recording paleotidal range, and shoreline notches formed by solution and bioerosion at former high-tide levels. The elevations of paleonotches with respect to modern shoreline notches incised into the same seacliffs provide the most precise measures of shoreline emergence. Steady but slow thermotectonic subsidence is undetectable for mid-Holocene paleoshoreline features, but has measurably altered the elevations of last-interglacial features.

In the Cook Islands, paleoshoreline features on Rarotonga and Aitutaki are compatible with theoretical expectations for thermotectonic subsidence since the last interglacial, and with a post-mid-Holocene drawdown in hydro-isostatic sea level intermediate between amounts calculated and observed regionally. Four makatea islands, apparently uplifted along the flexural bulge surrounding Rarotonga, display slightly greater post-mid-Holocene emergence, unexpected amounts of post-last-interglacial emergence, and net makatea uplifts similar but not identical to calculated inferences. Excess elevations of last-interglacial terraces on the makatea islands may conceivably reflect continued magmatic inflation of the Rarotonga volcanic edifice long after cessation of surface volcanism.

In the Austral Islands, makatea uplift reflects either thermal rejuvenation of lithosphere by hotspot volcanism or concurrent passage of the islands over an asthenospheric bump, or both. As for the Cook makateas, post-last-interglacial emergence of Rurutu implies persistence of uplift beyond the expected time interval, whereas post-mid-Holocene emergence of Tubuai can be ascribed entirely, within inherent uncertainty limits, to hydro-isostasy alone.

The distribution of subsided and uplifted islands along the Cook-Austral chain reflects multiple hotspot activity, probably related to multiple mantle diapirs of local character rather than to deepseated plumes. Rapid Pacific plate motion can generate elongate hotspot tracks from transient hotspot activity unrelated to columnar advective plumes.