Geomorphology and geodynamics of the Cook Austral island seamount chain inthe South Pacific Ocean: Implications for hotspots and plumes

Among Pacific hotspot tracks, the Cook-Austral island-seamount chain is dis tinctly 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. Coo k-Austral islands include a variety of disparate geomorphic types: volcanic islands without reefs, with fringing reefs, and with barrier reefs enclosi ng shallow lagoons, low-lying atolls and makatea islands composed of volcan ic cores surrounded by annular limestone tablelands. Neogene hotspot volcanism along the chain, built across Cretaceous-Paleogen e seafloor, has occurred above three different mantle source regions, now l ocated near its southeastern end, center, and northwestern end. Each hotspo t 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 initiat ed by eruption through the same region of Lithosphere now located near the northwestern end of the compound chain. Age-distance trends of hotspot volc anism along the nearby Society and Marquesas chains depart less from expect ation, but both the orientation and age trend of the Marquesas hotspot trac k also are anomalous. Lithospheric properties have seemingly influenced loc i 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 heterogeneiti es that reflect derivation from multiple mantle sources, even for individua l volcanoes. As the volcanic assemblages all were erupted from the SOPITA-S uperswell region of anomalously hot mantle including diverse recycled compo nents, hotspots that built the several island chains may reflect bleeding o f magma through lithospheric flaws from a buoyant subjacent pillow of atypi cal mantle, poised to melt readily from varied stimuli. Monotonic island subsidence is expected following cessation of hotspot volc anism, but episodes of post-immersion uplift have produced six makatea isla nds along the Cook-Austral chain. Makateas are tablelands of Neogene limest one, representing uplifted fringing reefs, and form annular girdles surroun ding volcanic island cores. Despite limited erosional degradation, makatea surfaces provide a measure of net post-immersion island uplift. Paleoshorel ine indicators of changing relative sea levels on Cook-Austral islands refl ect the combined influence of seafloor thermotectonic subsidence, thermal r ejuvenation associated with renewed hotspot activity, flexural bulges surro unding the isostatic loads of subsided volcanic edifices built out of seque nce, the last-interglacial eustatic highstand in global sea level, and a mi d-Holocene hydro-isostatic highstand in regional sea level. Key paleoshoreline indicators include emergent reef flats and microatolls r ecording former low-tide levels, paleobeachrock recording paleotidal range, and shoreline notches formed by solution and bioerosion at former high-tid e levels. The elevations of paleonotches with respect to modern shoreline n otches incised into the same seacliffs provide the most precise measures of shoreline emergence. Steady but slow thermotectonic subsidence is undetect able for mid-Holocene paleoshoreline features, but has measurably altered t he elevations of last-interglacial features. In the Cook Islands, paleoshoreline features on Rarotonga and Aitutaki are compatible with theoretical expectations for thermotectonic subsidence sinc e the last interglacial, and with a post-mid-Holocene drawdown in hydro-iso static sea level intermediate between amounts calculated and observed regio nally. Four makatea islands, apparently uplifted along the flexural bulge s urrounding Rarotonga, display slightly greater post-mid-Holocene emergence, unexpected amounts of post-last-interglacial emergence, and net makatea up lifts similar but not identical to calculated inferences. Excess elevations of last-interglacial terraces on the makatea islands may conceivably refle ct continued magmatic inflation of the Rarotonga volcanic edifice long afte r 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 o ver an asthenospheric bump, or both. As for the Cook makateas, post-last-in terglacial emergence of Rurutu implies persistence of uplift beyond the exp ected time interval, whereas post-mid-Holocene emergence of Tubuai can be a scribed entirely, within inherent uncertainty limits, to hydro-isostasy alo ne. The distribution of subsided and uplifted islands along the Cook-Austral ch ain reflects multiple hotspot activity, probably related to multiple mantle diapirs of local character rather than to deep-seated plumes. Rapid Pacifi c plate motion can generate elongate hotspot tracks from transient hotspot activity unrelated to columnar advective plumes.