An early Wisconsin reef on the Daito Ridge, North Philippine Sea.

Abstract

Coralline radiometry by uranium-series has successfully located ancient strand lines during the Last Interglacial State and the following intra-Wisconsin Interstadials, the datum levels to estimate the amount and rate of vertical displacement resulting from local as well as regional neotectonism (e.g. Fig. 8 of Konishi et al., 1974) Together with the celebrated Kita-daito (North Borodino Island) (Sugiyama, 1934, 1936 ; Hanzawa, 1940) , Minami-daito (South Borodino Island) of the Daito Islands represents an emergent dolomitized atoll capping above the otherwise submarine Daito Ridge which is one of the oldest remnant arcs in the Philippine Sea (Konishi et al., 1973). Because of its unique geomorphic configuration and geotectonic location in front of the Ryukyu Trench, this paper reports the occurrence of an emergent fringing reef of the earliest intra-Wisconsin Interstadial (ca. 100,000 years B.P.) from the elevated atoll, Minamidaito, and discusses Late Quaternary tectonic history of the remnant arc. The dating was done by the newly proposed 226Ra-238U method (Komura et al., 1978), the result of which was supported with 14C dates for the same specimens. It is further complemented with oxygen isotopic analysis of the associated gastropods. In spite of the contemporaneous aseismic activity, the observed elevation of the earliest intra-Wisconsin reef at the island suggests a steady slow rate (ca. 0.25 mm per year) of uplift of that part of the Daito remnant arc in the Philippine Marginal Sea for the last 100,000 years, or possibly as long as its two-times of the duration. Acknowledgments. K. Konishi wishes to dedicate this paper to Dr. Teiichi Kobayashi, M. J. A., on the occasion of his Seventyseventh Birthday, who stirred up a fascinating interest of geotectonics to a then freshman student of geology thirty years ago. Expenses for field and laboratory works were funded by the Geodynamics Project through the Ministry of Education, Science and Culture and also from a contract between the Prime Minister's Office and Kanazawa University. We thank to these authorities for their