Nephelinites and basanites of the Samoan linear volcanic chain: Their possible tectonic significance

Abstract

The Samoan Islands, along with numerous seamounts and banks, form a 1200 km long linear volcanic chain at the north end of the Tonga Trench. The linear volcanic chain is nearly parallel to the westerly trending part of the Tonga Trench and to the calculated vector of relative plate motion of the Pacific Plate with respect to the India Plate. Bathymetric profiles normal to the trend of the chain suggest that the Pacific Plate is sharply flexed downward into the westerly trending part of the trench and probably is disrupted by faulting due to the sharp curvature of the trench axis near 15°S, 172°W. The linear volcanic chain and young fissures cutting the islands appear to be aligned along the hinge line of the plate flexure. Volcanic rocks from the chain are predominantly alkalic olivine basalt but also include differentiates such as phonolite and quartz trachyte. Basanite and olivine nephelinite cap both isolated and aligned seamounts; on the Samoan Islands they have erupted after erosion of collapsed calderas. The chemistry and mineralogy of the basanite-olivine nephelinite suite suggest that they were derived from near the base of the lithosphere. The linear chain appears to have been volcanically active over most of its length more or less simultaneously. The Samoan Chain is similar in petrology and morphology to late volcanic stages of some other linear volcanic chains. However, it appears that a combination of lithosphere flexure and viscous shear melting at the base of the lithosphere provides a better explanation for its origin than does upwelling from a fixed “hotspot” in the mantle.