Age and Evolution of the Volcanoes of Tutuila , American Samoa 1

Tutuila is a basaltic volcanic island within the east southeasterly trending Samoa Island chain in the Pacific Ocean. Potassium-argon ages on 38 whole rock samples oflavas and intrusives demonstrate that the main period of subaerial volcanism occurred over a relatively short interval of about 0.6 Ma in the Early Pleistocene. The major shield volcano, Pago, was built between about 1.54 and 1.28 Ma ago; its large caldera formed approximately 1.27 ± 0.02 Ma ago. Partial filling of the caldera by volcanics occurred from shortly after its formation until about 1.14 Ma ago, and activity on Pago Volcano ended with emplacement of trachyte bodies which have ages of 1.03 ± 0.01 Ma. Construction of the smaller satellitic Olomoana and Taputapu volcanoes, on the eastern and western extensions of the main rift zone through Pago Volcano, took place over much the same time interval as the volcanism on Pago. The youthful basaltic volcanism on the Manu'a Islands, east of Tutuila, allows a rate of migration of the center of volcanism of about 10cmjyr to be estimated. These results are broadly consistent with a hot spot origin for the volcanoes . island chains show a southeasterly migration of shield-building volcanism with time (MeDougall 1964; Jackson et al. 1972; Duncan and McDougall 1976; Jarrard and Clague 1977; Dalrymple et al. 1980; McDougall and Duncan 1980). Similarly there is good evidence for youthful volcanism in the southeastern part of the Samoan chain in the Manu'a Islands. However, it has long been noted (Dana 1849; Daly 1924; Stearns 1944; Chubb 1957) that although the shieldbuilding volcanism on Tutuila and Upolu clearly is of some antiquity, the most northwesterly island, Savai'i, is dominated by very young basaltic volcanism, contrasting with other Pacific island chains. Hawkins and Natland (1975) and Natland (1980) explain this in terms of voluminous rejuvenescent volcanism emanating from a rift zone formed in response to deformation of the Pacific plate adjacent to the Tonga Trench (Figure 1), which marks the boundary between the Pacific and Australian plates. Furthermore, these authors suggest that activity of anure Samoan shield volcanoes similarly was controlled by plate deformation, and thus they do not favor the idea of a hot spot origin for the island chain. 311 1 Manuscript accepted March 1985. 2 Research School of Earth Sciences, Australian National University, G.P .O. Box 4, Canberra, A.C.T. 2601, Australia. TUTUILA LIES IN THE PACIFIC OCEAN at 14°20' S, 170°45' W within the Samoa Islands, which extend as a volcanic chain over at least 370 km from Savai'i in the west northwest to the Manu'a Islands in the east southeast (Figure 1).The islands are the culminations of basaltic volcanoes on a submarine ridge which rises from old (Early Cretaceous) ocean floor more than 4000 m deep. Rose Islet , about 150 km east southeast of the Manu'a Islands, is part of American Samoa, but as a bathymetrically isolated atoll it may not be directly related geologically to the Samoan ridge (Figure 1). The overall trend of the Samoa Island chain is subparallel to that of other young volcanic island chains located on the Pacific lithospheric plate. These chains are thought to represent hot spot or plume traces (Wilson 1963; Morgan 1971), recording movement of the Pacific plate over fixed magma sources in the mantle. Consistent with such models, these 312 PACIFIC SCIENCE, Volume 39, October 1985