LATE QUATERNARY SEA-LEVEL HISTORY ON SAIPAN, COMMONWEALTH OF THE NORTHERN MARIANA ISLANDS, USA: A TEST OF TECTONIC UPLIFT MODELS AND GLACIAL ISOSTATIC ADJUSTMENT PROCESSES

S. Uyeda and H. Kanamori (1979, JGR 84, 1049-1061) proposed a tectonic model with two end members of a subduction-boundary continuum: the “Chilean” type (shallow dip of the subducting plate, great thrust earthquake events, as well as compression and uplift of the overriding plate) and a “Mariana” type (steep dip of the subducting plate, no great thrust earthquake events, back-arc extension, and no uplift). This model has been used to explain variable rates of Quaternary uplift around the Pacific Rim, yet no uplift rates have been determined for the Mariana Islands. We studied the late Quaternary Tanapag Limestone, which rims much of the eastern and southern coasts of Saipan, Northern Mariana Islands, with elevations of 20-25 m. Ten well-preserved corals (Acropora, Goniastrea) yield U-series ages ranging from ~134 ka to ~127 ka, correlating this emergent reef terrace with the last interglacial period, when sea level was ~6 m above present. Data yield relatively low uplift rates of 0.11-0.15 m/ka, consistent with the Uyeda-Kanamori model. Despite this low uplift rate, we found a low-elevation (+4.2 m) reef with corals in growth position below a notch cut into older limestone. Calibrated radiocarbon ages of corals from this reef range from 3.1-3.9 ka. The occurrence of this young reef is not likely due to co-seismic uplift, but is consistent with similar observations on tectonically stable or slowly uplifting islands elsewhere in the equatorial Pacific Ocean. This Holocene high stand of sea has been interpreted to be the result of glacial isostatic adjustment processes.