Tapping of an Enriched Asthenospheric Layer at the Samoan Islands along Fractures Produced by Deformation of the Pacific Plate near the Tonga Trench

1J.J. Dieu, 2J.W. Hawkins & 3J.H. Natland

1Rayonier, Inc., 3033 Ingram St, Hoquiam, WA 98550 United States

2Scripps Institution of Oceanography, 9500 Gilman Dr., La Jolla, CA 92093-0220

3RSMAS/MGG University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, jnatland@rsmas.miami.edu

Samoan basalts derive from mantle sources having a strong EMII signature (high 87Sr/86Sr, K, Rb, and Ba). Stratigraphy tied with radiometric dates reveal an increase, maximum, and waning of the influence of the EMII mantle component in the combined history of the shield volcanoes of successively younger islands, Upolu, Tutuila, and Ta`u. This fluctuation is superimposed on, and independent of, Hawaiian-like shield-building, waning alkalic, and post-caldera alkalic stages of volcanism. It is inconsistent with any regular arrangement of EMII within the structure of a postulated mantle plume. The islands are surface expressions of longer east-trending submarine volcanic ridges or lineaments. These and several older shield lineaments appear in satellite-derived bathymetry as en echelon, straight or slightly curving volcanic ridges, each 100-500 km long. They formed in eastward succession on the Pacific Plate as it rode to the west past the strongly curving transform corner of the nearby Tonga trench. They trend more to the east than nested sigmoidal ridges of the mid-plate Hawaiian Islands, and are not nested. Another lineament ~500 km long is superimposed obliquely over the older Upolu and Tutuila lineaments, which it capped with locally extensive Quaternary-Historic post-erosional eruptives over a distance of >200 km. It closely parallels the transform portion of the trench, and formed along a narrow fracture that propagated along the crest of the arch in the Pacific plate that was produced by lateral bending of the plate into that part of the trench (1). The EMII influence along this lineament is strongest to the west, and all lavas along it are distinct from those of the older underlying lineaments in the Pb isotopes (2). The simplest interpretation of Samoan volcanism is that enriched components are irregularly concentrated in a layer or layers at the top of the asthenosphere that has been tapped along fractures produced by shear coupling acting on the edge of the Pacific lithospheric plate where it moves close to or bends toward the transform portion of the Tonga Trench.

(1) Hawkins, JW, & Natland, JH, 1975. Earth Planet. Sci. Lett., 24: 427-439

(2) Hart, S.R., Staudigel, H., Koppers, A.P., Blusztajn, J., Baker, E.., Workman, R., Jackson, M., Hauri, E., Kurz, M., Sims, K., Fornari, D., Saal, A., & Lyons, S., 2000. Geochem. Geophys. Geosyst., 1: Paper 2000GC00108.