When a Plateau Suppresses a Plume: Disappearance of the Samoan Plume Under the Ontong Java Plateau

Abstract

The Samoan hotspot generated an age-progressive volcanic track that can be traced back to 24 Ma at Alexa Bank, but the trace of the older portion (>24 Ma) of the hotspot track is unclear. We show that six seamounts located in and around the Magellan Seamount chain—north of the Ontong-Java Plateau (OJP)—have ages (87–106 Ma), geochemistry, and locations consistent with absolute plate motion model reconstructions of the Samoan hotspot track in the late Cretaceous, and three additional seamounts have geochemistry and locations consistent with a Samoan origin. However, a large segment of the Samoan hotspot (24–87 Ma) remains unidentified. Absolute plate motion models show that, from ∼60 to 30 Ma, the OJP passed over the Samoan plume. The exceptional thickness of the OJP lithosphere may have largely suppressed Samoan plume melting because the inferred volcanic trace of the Samoan hotspot wanes, and then disappears, on the OJP. Fortunately, 44 Ma volcanism at Malaita Island, located on the southern margin of the OJP, has a location, age, and geochemistry consistent with a Samoan plume origin, and provides a “missing link” bridging the younger and older segments of the Samoan hotspot. Our synthesis of geochemical, geochronological, and plate motion model evidence reveals that Samoa exhibits a clear hotspot age progression for over 100 Myr. Passage of ancient plateaus over young plumes—here called “plume-plateau” interaction—may be relatively common: the OJP also passed over the putative Rarotonga hotspot, and the Society and Pitcairn hotspots were overtopped by the Manihiki Plateau.