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Pdf/87/9/1259/3418412/I0016-7606-87-9-1259.Pdf by Guest on 01 October 2021 2 J Sedimentology, structural geology, and tectonics of the Shikoku subduction zone, southwestern Japan J. CASEY MOORE Earth Sciences, University of California, Santa Cruz, Santa Cruz, California 95064 DANIEL E. KARIG Department of Geological Sciences, Cornell University, Ithaca, New York 14850 ABSTRACT deep-sea sediment. Studies of uplifted de- (Kanamori and Tsumura, 1971). Kanamori posits provide much detail but fail to reveal (1972) used data from high-magnitude The Shikoku subduction zone is de- deformation and diagenetic-metamorphic shallow earthquakes to postulate recent veloped along the Nankai Trough where processes while they are occurring. The subduction rates of about 3 cm/yr and initi- the Philippine plate is underthrust beneath Deep Sea Drilling Project has recovered the ation of subduction 2 m.y. B.P. The pattern the Asian plate. The landward wall of the best samples from modern subduction of deformation associated with the 1946 Nankai Trough consists of horizontal zones, including cores from the Eastern Nakaido earthquake, as well as the uplift of parallel ridges and basins that trend north- Aleutian Trench (von Huene and Kulm, terraces along the Pacific coast off Shikoku, eastward. A Deep Sea Drilling Project site 1973), the Hellenic Trench (Hsu and Ryan, led Fitch and Scholz (1971) to estimate the (Leg 31, site 298) on the landward flank of 1972), and the Nankai Trough off south- subduction rate as 8 ± 4 cm/yr over a the deepest ridge penetrated 525 m of beds western Japan (Karig, Ingle, and others, period of 1 m.y. or less. in normal stratigraphic position and 86 m 1975). The inner wall of the Nankai of overturned beds (all of Quaternary age), Trough, penetrated to a depth of more than indicating an overturned anticline. The 600 m, reveals a major overturned anti- tight, overturned anticline, which trends cline. These cores represent the deepest and parallel to the Nankai Trough, has an inter- most complete sampling of a modern sub- limb angle of 9°, an axial surface inclined 9° duction zone and are a sequence transi- to 14° landward, and a convergently fan- tional between unconsolidated trench sed- ning axial plane fracture cleavage. A iments and highly deformed lithified mate- coarsening-upward turbidite sequence rial. We discuss the sedimentology, struc- defines a trench facies and demonstrates di- tural geology, and physical properties of rect accretion of deposits from this envi- deposits cored from the inner wall of the ronment. Nankai Trough. We interpret the site of dep- The convergence rate in the Shikoku osition, depth of burial, and structure; subduction zone is estimated to be from 1 further, we provide an overview of the tec- to 2 cm/yr, with a strain rate of about tonic evolution of this subduction zone. 10~13/sec. Tectonic consolidation has re- We informally use the term "Shikoku duced the volume of the subducted and ac- subduction zone" for the dipping seismic creted rocks at least one-third. Olistro- zone and deformed, accreted deposits, and stromes form as a direct consequence of we retain the designation "Nankai Trough" fold evolution in the submarine environ- for the adjacent oceanic deep. ment and can be immediately underthrust, thereby developing a structural fabric. GEOLOGICAL AND GEOPHYSICAL SETTING INTRODUCTION Southwestern Japan has undergone Figure 1. Schematic diagram showing plates A major problem during the formative periodic subduction since the Paleozoic of northwestern Pacific. period of plate tectonic theory was reconcil- (Minato and others, 1965). This margin ing the apparent lack of deformation of sed- forms part of the Asian-Philippine plate A small accretionary prism and the high iments in zones of probable underthrust- boundary (Fig. 1), but prior to the opening incidence of large, shallow earthquakes in- ing in some modern oceanic trenches of the Shikoku Basin in early Miocene time, dicate a short subduction history. A transi- (Scholl and others, 1968, 1970; von Huene it appears that the Pacific plate was in con- tion from turbidite deposits to hemipelagic and Shor, 1969). However, subsequent tact with the Asian plate in this region sediments in the Shikoku Basin at Deep Sea seismic-reflection data from other subduc- (Karig, 1975). During, and probably be- Drilling Project hole 297 marks the de- tion zones indicate that deep-sea deposits cause of, this arc migration, there was a lull velopment in middle Pliocene time (3 m.y. are deformed and accreted in this environ- in volcanic and tectonic activity along this B.P.) of the subduction zone into a trough ment (Chase and Bunce, 1969; Hilde and section of margin. and sediment trap. Moreover, the Nankai others, 1969; Silver, 1969; Holmes and Late Tertiary displacement along this Trough is traceable to a Miocene and others, 1972; Carson and others, 1974; subduction zone has been estimated from younger thrust zone along the southern Beck, 1972; Seely and others, 1974; Kulm both geological and geophysical data. The Fossa Magna (Matsuda, 1962; Kimura, and Fowler, 1974). seismic zone beneath the Nankai Trough 1966; Tsuchi and others, undated), suggest- Marine geophysical techniques delineate reaches a depth of about 70 km and defines ing that slow subduction began approxi- the gross structural framework of accreted a lithospheric slab with a very gentle dip mately 10 m.y. B.P. and that an increasing Geological Society of America Bulletin, v. 87, p. 1259-1268, 12 figs., September 1976, Doc. no. 60907. 1259 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/9/1259/3418412/i0016-7606-87-9-1259.pdf by guest on 01 October 2021 2 J. C. MOORE 130 135 140 SEDIMENTOLOGY 40° 40° The sedimentology of the sequence cored at site 298 allows interpretation of the site of deposition and the position of the rocks before tectonic transport. Seventeen cores sampling 154 m (25 percent) of the 611 m drilled are described and interpreted. The recovery in all cores averaged 43 percent; recovery was below average in the upper portion of the hole. 35° 35° Lithology The cored sequence was divided into two units (Fig. 4). Unit I comprises mud (clayey silt) and silty sand of late Pleistocene—Holocene age. Both sediment types include irregular clasts of mud, mud- stone, and sandstone as much as 10 cm across. The deepest occurrence of the 30° 30° lithified clasts (mudstone and sandstone) defines the base of Unit I, at 194.20 m. Sand beds as much as 1 m thick are both massive and graded, and they generally show basal layers of the Bouma A interval. Unit II extends from 194.2 m to at least Figure 2. Location map of southwestern Japan, showing position of sites 297, 298, and 299. Line the base of hole 298 at 611 m (minimum through site 298 indicates location of Figure 11. thickness 416.8 m). It is composed of mud-mudstone, silt-siltstone, and sand of subduction rate allowed a trough to form at As neither the basement nor the underlying early and late Pleistocene age. Most of the 3 m.y. B.P. Shikoku Basin section seem to be deformed mud and mudstone includes 50 to 60 per- The Nankai Trough is a shallow (Fig. 2) or uplifted on the profile of Hilde and cent silt-sized particles and 40 to 50 percent trench with a small but well-developed ac- others, there must be thickening and de- clay-sized particles. Sand and silt-siltstone cretionary prism (Ludwig and others, 1973; formation, primarily in the basal part of beds represent only 4 percent of Unit II, Karig and Sharman, 1975). The lower the turbidite unit. The contact between the whereas sand beds comprise 48 percent of trench slope is acoustically opaque, and on step area and the lower slope, on the profile Unit I (Fig. 5). In Unit II, these sand and reflection profiles (Hilde and others, 1969; of Hilde and others, appears to be a silt-siltstone beds compose the lower layer Ludwig and others, 1973), it appears as a shallow-dipping thrust, but acoustic- of graded beds, and locally show cross- series of hyperbolic reflectors. A detailed diffraction effects from point reflectors off lamination and basal scour marks. Bouma survey over the lowermost slope section by the ship track might also be responsible. intervals of the C-D-E and D-E type are Karig before the drilling of DSDP hole 298 most common. (Leg 31) demonstrates that these hyper- Muddy deposits show fissility at about boles defined nearly horizontal ridges, 275 m and form mudstone below 300 m. oriented subparallel to the trench for dis- Silt is generally consolidated to siltstone in tances of at least 10 km (Fig. 3). Sediment Unit II, whereas sand is usually unconsoli- ponded between the ridges increases in dated. thickness upslope. The sediment in the inter-ridge ponds is deposited as flat lying Depositional Processes and or slightly tilted seaward and shows in- Site of Accumulation creasing tilt landward with depth. In the inter-ridge area of hole 298, there is no evi- The sand beds of Unit I are broadly simi- dence of significant ponding of sediment. lar to density current deposits reported Toward the upper part of the slope, opaque from submarine fan channels (Nelson and basement is covered with a continuous Kulm, 1973). Therefore, it is likely that layer of sediment. these massive sands are the product of a Near hole 298, flat-lying sediment of the channelized or somewhat laterally re- trench floor is separated from the steep stricted flow path. lower slope by a step, about 3 km wide and The mudstone clasts which define Unit I 50 m high. The Glomar Challenger are enigmatic in that they are hard lithified seismic-reflection profile (Karig, Ingle, and rock and occur in mud layers as well as sand layers.
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