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27. Sedimentary Facies Evolution of the Nankai Forearc and Its Implications for the Growth of the Shimanto Accretionary Prism1

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27. Sedimentary Facies Evolution of the Nankai Forearc and Its Implications for the Growth of the Shimanto Accretionary Prism1 Hill, I.A., Taira, A., Firth, J.V., et al., 1993 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 131 27. SEDIMENTARY FACIES EVOLUTION OF THE NANKAI FOREARC AND ITS IMPLICATIONS FOR THE GROWTH OF THE SHIMANTO ACCRETIONARY PRISM1 Asahiko Taira2 and Juichiro Ashi2 ABSTRACT A combination of Deep Sea Drilling Project-Ocean Drilling Program drilling results and site survey data in the Shikoku Basin, Nankai Trough, and Nankai landward slope region provides a unique opportunity to investigate the sedimentary facies evolution in the clastic-dominated accretionary forearc. Here, we consider the facies evolution model based on the drilling results, IZANAGI sidescan images, and seismic reflection profiles. The sedimentary facies model of the Nankai forearc proposed in this paper is composed of two parts: the ocean floor-trench- lower slope sedimentary facies evolution and the upper slope to forearc basin sedimentary facies evolution. The former begins with basal pelagic and hemipelagic mudstones overlain by a coarsening upward sequence of trench turbidites which are, in turn, covered by slope apron slumps and lower slope hemipelagic mudstone. This assemblage is progressively faulted and folded into a consolidated accretionary prism that is then fractured and faulted in the upper slope region. Massive failure of the seafloor in the upper slope region produces olistostrome deposits that contain lithified blocks derived from older accretionary prism dispersed in a mud matrix. The contact between the older accretionary prism and the upper slope olistostrome is a submarine unconformity that is the first stratigraphic evidence for the exhumation of an older prism to the seafloor. The olistostrome beds are then overlain by forearc basin-plain mudstone and turbidites which are progressively covered by coarsening-upward delta-shelf sequences. An application of the forearc facies model to the Shimanto Belt was attempted in order to ensure feasibility of this model. Several characteristic lithologies observed in the Shimanto Belt can be interpreted with this model. The unconformities found in the Shimanto Belt were evaluated to provide an estimate for the exhumation rate of the prism. Comparison with independent measurement of exhumation rate provides a 0.5 to 1 km/m.y. vertical growth of the prism. INTRODUCTION GEOLOGIC SETTINGS Sedimentation in trench-to-forearc regions is controlled by a Nankai Trough complex interaction between tectonics, volcanism, sea-level change, The Nankai Trough extends 700 km from the Suruga Trough, and other factors such as climate and vegetation (Pickering et al., which marks a collision zone between the Izu-Bonin island arc and 1989). Understanding of sedimentary facies and stratigraphic evolu- the Honshu arc, to the northern tip of the Kyushu-Palau Ridge tion in modern trench-forearc regions will aid greatly the interpreta- (Fig. 1). The Nankai accretionary prism defined as the landward tion of ancient accretionary complexes and should help us to gain wedge of the Nankai Trough mainly consists of offscraped and insight into an important objective of convergent margin geology: the underplated materials from the trough fill turbidites and the Shikoku growth and exhumation rate of an accretionary prism. Basin hemipelagic sediments. The Shikoku Basin, a backarc basin Ocean Drilling Program (ODP) Leg 131 drilled a complete strati- of the Izu-Bonin island arc, is situated in the northern corner of the graphic sequence from the slope apron sedimentary cover down to Philippine Sea Plate, which is converging to the northwest or west- oceanic basement at the toe region of the Nankai accretionary prism northwest, based on fault plane mechanisms (Kanamori, 1971) and (Taira, Hill, Firth, et al., 1991). The result, together with previous Deep plate kinematics (Seno, 1977). Estimated values of the convergent Sea Drilling Project (DSDP) data in this region (Karig, Ingle, et al., rate vary from 1 to 2 cm/yr (Ranken et al., 1984) to 3 to 4 cm/yr 1975; Klein, Kobayashi, et al, 1980; Kagami, Karig, Coulbourn, et al., (Seno, 1977). 1986) provides the best example for clastic sedimentation in modern Analysis of magnetic anomalies suggests multiple episodes of trench environments. Shikoku Basin spreading during the late Oligocene to middle Mio- The results of recent swath mapping surveys including SeaBeam cene (Kobayashi and Nakada, 1978; Shih, 1980). The history of the (Le Pichon et al., 1987) and IZANAGI sidescan sonar (Ashi and Taira, opening is mainly divided into two episodes: east-west spreading in press), together with continuously accumulated seismic profile data during 27-19 Ma and north-south spreading during 14-12 Ma (e.g., (Nasu et al., 1982; Aoki et al, 1983; Kaiko I Research Group, 1986; Chamot-Rooke et al., 1987). The Kinan Seamount Chain, which is Taira et al., 1988; Moore et al., 1990), also provide fundamental being subducted at this study area (Fig. 1), is interpreted to be formed information on sedimentary evolution of the Nankai forearc. by an episode of off-ridge volcanism and/or an expression of an The purpose of this chapter is thus to delineate the sedimentary aborted stage of seafloor spreading in the Shikoku Basin (Klein and facies evolution model of the Nankai trench and forearc based on a Kobayashi, 1980; Chamot-Rooke et al., 1987). synthesis of drilling results and site surveys. Then we attempt to apply The Nankai Trough off Shikoku is characterized by a broad, flat this model to the Shimanto Belt as a case study for calibration of the seafloor in water depths between 4500 and 4900 m (Fig. 2), and the exhumation rate of the prism. trough floor is filled by a turbidite sequence overlying the Shikoku Basin hemipelagic mudstone. The cross section along the trough axis, however, shows considerable irregularities in the oceanic basement (Le Pichon et al., 1987). This suggests that the trench-filling process ' Hill, I.A., Taira, A., Firth, J.V., et al., 1993. Proc. ODP, Sci. Results, 131: College is closely controlled by basement relief, with a large variation in Station, TX (Ocean Drilling Program). 2 Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo trench-fill thickness observed (Le Pichon et al., 1987). The turbidite 164,Japan. sediments transported laterally along the trough axis from the Suruga 331 A. TAIRA, J. ASHI Overlying the unconformity are several exposures of cover se- quences from Cretaceous to Tertiary age. They represent various depositional environments from slope basin to shelf/coastal zone. Among them, well-developed ones belong to middle Miocene forearc basin sequences that we consider as a main case study. DATA SOURCE One of the important data sources for this paper is the IZANAGI acoustic image. The IZANAGI seafloor imaging system includes 11- to 12-kHz acoustic backscattering measurement and a bathymetric mapping system in a single body towed at depths of 120 m or less at speeds up to 8 kt. The IZANAGI survey was carried out using the Shinsan Maru, operated by the Ocean Research Institute, University of Tokyo, in conjunction with Seafloor Survey International, K-Ma- rine, and Sanyo Techno Marine, between 8 June and 23 June 1989. The 3.5-kHz sub-bottom profiles were obtained simultaneously. The survey area covered both the Nankai Trough and the accretionary prism off Shikoku. Closely spaced seismic reflection profiles have been obtained in the studied area by various organizations. All seismic reflection lines, including unpublished data, have been obtained during the past 10 yr (e.g., Nasu et al., 1982; Leggett et al, 1985; Kaiko I Research Group, 1986; Taira et al., 1988; Moore et al., 1990). The bathymetry off Shikoku was provided by the SeaBeam map constructed during the O DSDP& ODP Site Kaiko project in 1984 (see Le Pichon et al, 1987) and the IZANAGI SUT: Suruga Trough data based on a phase-difference technique. SAT: Sagami Trough ICZ : Izu Collision Zone SEDIMENTARY FACIES IN THE NANKAI FOREARC Figure 1. Location map showing the Nankai Trough, Shikoku Basin, and Shikoku Basin Facies related physiographic features, and the DSDP-ODP drilling sites. The distri- On DSDP legs, four sites, 297, 442,443, and 444, were occupied bution of a Cretaceous-Tertiary accretionary prism, the Shimanto Belt, is also by the Glomar Challenger in the Shikoku Basin (Karig, Ingle, et al., shown. The rectangle is the area shown in Figure 2. 1975; Klein, Kobayashi, et al., 1980). They provide a basic frame- work for stratigraphic evolution (Figs. 1 and 3). The Shikoku Basin Trough are interpreted to be the direct consequence of the Izu collision can be roughly divided by the north-northwest-south-southeast trend- in Pliocene-Pleistocene times (Taira and Niitsuma, 1986). ing Kinan Seamount Chain into two basins: the eastern Shikoku Basin Basically, four modes of sediment transportation processes and the western Shikoku Basin. The lithology of the Shikoku Basin dominate the trench to forearc region of the Nankai Trough: hemi- sediments is dominated by hemipelagic mudstone intercalated by pelagic background sedimentation for the entire region except the volcanic ash layers. It seems that the depth of the Shikoku Basin has shelf area, axial trench turbidites, coarse shelf elastics from the been close to the carbonate compensation depth (CCD) throughout hinterlands of southwest Honshu and Shikoku, and volcanic ash its evolution to the present (White et al., 1980). The hemipelagite thus mainly from Kyushu and the Izu-Bonin arc. The shelf sands are shows a considerable variation in preservation of calcareous plank- transported to several canyons that cross-cut the accretionary tonic organisms. The basin-wide correlation of the history of CCD prism. Compared to axial transport of coarse elastics, the amount fluctuation has not yet been established. of canyon-fed sediments to the trough floor is subordinate (Taira The thickness variation and lithologic characteristics of the and Niitsuma, 1986). Shikoku Basin sediments indicate two dominant patterns: a general southward decrease of total sediment thickness and westward de- Shimanto Belt crease of volcanic materials.
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