Late Quaternary Sedimentation and Faulting in the Sumisu Rift

Late Quaternary sedimentation and faulting in the Sumisu Rift, an active rift in the IzuOgasawara Arc, were studied by relating existing seismic profiles to recently acquired ODP cores. High resolution, single-channel seismic profiles collected by the Geological Survey of Japan show that the surface sediments (about 0.4 or 0.6s) are composed of an alternation of transparent layers, 0.05 to 0.2 s thick, and packages of continuous reflections. The transparent layers correlate with five thick pumiceous beds recovered by ODP drilling (Leg 126) at two sites in the northern part of the south Sumisu Basin. These transparent layers extend to the southeast onto the northwestern flank of Torishima Island, indicating that the volcanic debris in the transparent layers was supplied from volcanoes in and around this island . Near Torishima Island, the transparent layers show both irregular tops and bases and hummocky internal reflections, suggesting a slump or slide origin; the same transparent layers below the basin floor are characterized by flat boundaries except where faulted or tectonically tilted. On the basin floor , the transparent layers were probably transported by high-concentration sediment gravity flows that would tend to cover and smooth bottom irregularities formed by faulting and tilting . Thus, the offset of the top of the thick transparent layer along the faults represents the amount of displacement on the faults after the deposition of the layers. The two transparent layers that correlate with pumiceous beds II and IV of ODP leg 126 are thick and are believed to have had enough volume to blanket the entire basin. Rough estimates of their volume exceed 60 km3. Based on an age of 31,000 and 131,000 B. P. for beds II and V, respectively, the displacement rates on the faults are estimated at about 0.15 to 0.3 cm/y. If the displacement rate has been constant since the beginning of the formation of the Sumisu Rift , about 2,000 m vertical offset of the rift basement along the major fault zone of the eastern boundary of the rift is estimated to have formed in a period of 0.5 to 1 million years. *地 質調査所海洋地質部 **メ モ リアル大学地球科学部 * Marine Geology Department , Geological Survey of Japan ** Earth Science Department , Memorial University, Newfoundland, Canada