The Samoan chain: a shallow lithospheric fracture system
Natland JH. 2006. The Samoan chain: a shallow lithospheric fracture system
Samoan volcanism has occurred mainly along a series of lithospheric fractures that are produced by deformation of the underlying Pacific plate near the Tonga Trench. Most are delineated by aligned volcanic centers 10-50 km apart. The centers coalesce into volcanic ridges that form serially at the eastern end of the chain as the approaching Pacific lithosphere first begins to experience shear stresses near the westward curving corner of the trench. One fracture, the Samoan post-erosional volcanic rift system, is superimposed on at least three of these, has small vents spaced only 1-6 km (average 2 km) apart, has erupted only nephelinite-series mafic lavas, and has been active along a length of > 200 km since the Quaternary. It results from lateral plate bending toward the transform portion of the trench. Although lava compositions systematically are greatly enriched in the radiogenic isotopes of Sr, Nd, and Pb, each lineament is geochemically distinctive. The lavas thus appear to be tapped from a laterally widespread layer of heterogeneously enriched mantle at the base of the lithosphere rather than a geochemically coherent mantle plume.