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Geology of Amami Oshima, Central Ryukyu Islands, with Special Reference to Effect of Gravity Transportation on Geologic Struture
Abstract
The geologic structure developed under the strong influence of gravitation is a fundamental characteristic of the geology of Amami Oshima. The Yuwan Formation consists of siliceous slate, sandstone, chert and basalt, and contains exotic masses of bedded chert, basalt and limestone of various sizes. Most of the exotic masses are of the Permian and Triassic, some of which contain again exotic limestone of the Permian and Carboniferous, respectively. The Upper Jurassic sediments of the Yuwan Formation deposited in situ enclose exotic Triassic rocks and the Lower Cretaceous sediments of the formation enclose exotic Permian rocks. Gravity transportation of the exotic Triassic and Permian masses from the northwest took place successively in the Late Jurassic to Early Cretaceous. Albitization was recognized in exotic basalt. The Cenomanian to Turonian Naze Formation consists of slate, phyllite, sandstone and basalt. The Turonian Odana and Ogachi Formations consist mostly of sandstone and slate, but the Odana Formation contains exotic masses of Aptian slate and basalt. The Eocene Wano Formation consists of sandstone, shale and conglomerate. Intraformational deformation is common in these formations. In the western part, the older Yuwan Formation as a whole overlies the younger Odana Formation; the contact between them is a listric surface of southeastward gravity sliding. The Odana Formation slumped a short distance southeastwards over the toes of slides which involved both of the Odana and Naze Formations. In the central part, the Naze Formation slid southeastwards over the Ogachi Formation, which is in turn in contact with the Naze Formation through the slip-surface on the east side.
... Amami-Oshima Island is located in the central Ryukyu Arc, which lies between the Tokara Strait to the north and the Kerama Gap to the south (Fig. 1a). Basement rocks of the island consist of the Eocene Wano Formation and Shimanto accretionary complex in the east and the Chichibu accretionary complex in the west, separated by the Butsuzo Tectonic Line (Osozawa 1984;Takeuchi 1993;Osozawa et al. 2009) (Fig. 1b). The Shimanto and Chichibu accretionary complexes in Amami-Oshima Island are the southwestern extension of the southernmost part of the Shimanto Belt and the Chichibu Belt in the Outer Zone of Southwest Japan (Taira et al. 2016;Wallis et al. 2020). ...
... The Chichibu accretionary complex on Amami-Oshima Island is a mélange characterized by lenses of siliceous mudstone, sandstone, chert, limestone, and basalt in a mudstone matrix (Osozawa 1984). Limestone contains fusulinid, coral, and conodont fossils of Carboniferous to Permian ages, while interbedded limestone and chert contain Triassic radiolarians (Osozawa 1984). ...
... The Chichibu accretionary complex on Amami-Oshima Island is a mélange characterized by lenses of siliceous mudstone, sandstone, chert, limestone, and basalt in a mudstone matrix (Osozawa 1984). Limestone contains fusulinid, coral, and conodont fossils of Carboniferous to Permian ages, while interbedded limestone and chert contain Triassic radiolarians (Osozawa 1984). Basaltic rocks include pillow lava, breccia, and hyaloclastite (Takeuchi 1993). ...
Lithologic heterogeneity and the presence of fluids have been linked to seamount subduction and collocated with slow earthquakes. However, the deformation mechanisms and fluid conditions associated with seamount subduction remain poorly understood. The exhumed Chichibu accretionary complex on Amami-Oshima Island preserves mélange shear zones composed of mudstone-dominated mélange and basalt–limestone mélange deformed under sub-greenschist facies metamorphism. The mudstone-dominated mélange contains sandstone, siliceous mudstone, and basalt lenses in an illitic matrix. The basalt–limestone mélange contains micritic limestone and basalt lenses in a chloritic matrix derived from the mixing of limestone and basalt at the foot of a seamount. The basalt–limestone mélange overlies the mudstone-dominated mélange, possibly representing a submarine landslide from the seamount onto trench-fill terrigenous sediments. The asymmetric S – C fabrics in both mélanges show top-to-SE shear consistent with megathrust-related shear. Quartz-filled shear and extension veins in the mudstone-dominated mélange indicate brittle failure at near-lithostatic fluid pressure and low differential stress. Microstructural observations show that deformation in the mudstone-dominated mélange was accommodated by dislocation creep of quartz and combined quartz pressure solution with frictional sliding of illite, whereas the basalt-limestone mélange was accommodated by frictional sliding of chlorite and dislocation creep of coarse-grained calcite, with possible pressure solution creep and diffusion creep of fine-grained calcite. The mélange shear zones formed in association with seamount subduction record temporal changes in deformation mechanisms, fluid pressure, and stress state during megathrust shear with brittle failure under elevated fluid pressure, potentially linking tremor generation near subducting seamounts.
... The former fabric was formed by a mass wasting process in an openocean setting, prior to and independent of the main phase of extensive accretion (Osozawa, 1986;Matsuda and Ogawa, 1993). Clastic dikes truncating exotic blocks are not rare in such Japanese mélanges (e.g., Osozawa, 1984;Hibbard et al., 1992), and they indicate that the matrix was unconsolidated and water-saturated, which led some authors to conclude that the mélanges were formed as a result of mud diapirism (e.g., Wakita, 1988). ...
... The accretionary complex has been divided into four parts (Figs. 1 and 2): the early Cretaceous-Barremian Yuwan complex, the middle Cretaceous-Albian Odana complex, the Cenomanian Naze complex, and the Turonian Ogachi complex, which represent both upper and lower structural components in western and central Amami Oshima. These ages, based mostly on radiolarian fossil evidence (Osozawa, 1984) have been refi ned and modifi ed by Osozawa and Yoshida (1997). Only the Yuwan complex correlates with the southwest Japanese Chichibu zone, while the others belong structurally to the Shimanto zone. ...
... The Naze complex also tectonically overrides the Ogachi complex, although farther east, the Ogachi complex also overlies the other Naze complex, distributed in eastern Amami Oshima (Fig. 2). The Amami Oshima accretionary complexes were accreted progressively from west to east in response to westward subduction, suggesting that the eastern part of the complex is slightly younger than the structurally higher western and central parts; the combined succession is referred to as the Naze complex, given the presence of the same lithologic assemblages in each component (Osozawa, 1984). However, no diagnostic radiolarian fossils have been obtained from the eastern complex. ...
While mélanges have been taken to be a significant component of accretionary complexes, the use of this term has been mostly descriptive, given the lack of consensus concerning their genesis. However, many workers have interpreted asymmetric shear fabrics in mélanges as evidence of a tectonic origin (i.e., giving rise to tectonic mélanges), in contrast to olistostromes, which are considered to be purely sedimentary in origin due (at least in part) to the lack of such shear fabrics. It is still uncertain whether accretionary processes include those that involve the intense shearing of oceanic lithologies along with, for example, the incorporation and entrapment of exotic blocks by muddy matrices during sediment deposition and deformation in axial deep-sea trench settings. Here, we argue that the so-called "block-in-matrix" fabrics that characterize accretionary complexes are primarily sedimentary in origin and are distinct from shear fabrics represented by tectonic mélanges. We also propose that, following their accretion, subsequent recycling of such "oceanic" materials is a common feature within evolving accretionary prisms. Lastly, we draw attention to the observation that D1 pressure-solution cleavage planes - notably widespread in the lower Cretaceous Yuwan complex, Amami Oshima Island, in the Ryukyu arc, SW Japan - are absent from the minor asymmetric shear structures that often characterize tectonic mélanges. In contrast, we suggest that the Yuwan complex represents the effects of broadly symmetric flattening. This scenario is supported by our observation that the number of sedimentary and deformation events differs significantly compared to the single phase of shearing expected in tectonic mélanges. D minus 1 (D-1) pressure-solution and axial-planar cleavages are only observed in exotic blocks and exhibit older accretionary deformation events within a preexisting accretionary prism. Of the existing structural associations, Sd1 is the oldest, predating the D-1 sedimentary episode, and it represents an oceanic sedimentary phase associated with the formation of a distinct type of block-in-matrix fabric produced by the accumulation of debris flows at seamount margins. Sd2 represents a succeeding sedimentary episode characterized by trenchaxis debris flow, including oceanic inclusions derived from collapsed material from an older accretionary prism. The resulting Sd2 block-in-matrix fabric is one of the main lithologic components of the Amami Oshima Yuwan complex. Here, the Sd2 episode typically includes an igneous fabric, consisting of nonexotic intrusive and extrusive basalts within a terrigenous matrix, suggesting that the Yuwan prism may have also been associated with a trench-trench-ridge triple junction. D1 coaxial deformation is observed to overprint all of the episodes represented, although it has not destroyed or significantly sheared their respective fabrics, especially in regard to the main Sd2 block-in-matrix structures, which are therefore considered to represent original sedimentary structures.
... Except for the cover of mostly marine sediments and limestone, the basement rocks are accretionary complexes distributed in six zones (Fig. 2). Names of these accretionary zones and complexes are modified after Konishi (1963), Ujii and Nishimura (1992), and Osozawa (1984Osozawa ( , 1998a, and are briefly described below, from northwest to southeast (Fig. 2). ...
... The basalt contains secondary actinolite and pumpellyite, which indicates intermediate metamorphism between high and medium P/T (Banno, 1998). The marl includes huge exotic crystalline limestone at the Cape Hedo location (Osozawa, 1984). Asymmetric folds and their axial-planar pressure-solution cleavages are observed in the marl. ...
... The Anne and Yanaza complexes are characterized by blocks in the matrix fabric, while asymmetric melange fabric (e.g., Kimura and Mukai, 1991) is rare, and the fabric is attributed to debris flow (Osozawa, 1984). ...
The Kunigami zone in Okinawa is an extension of the Shimanto zone, Japan. The rocks make up the main part of the Nago metamorphic rocks, and such metamorphic rocks are exceptional in the Shimanto zone. The Anne complex, in the older Motobu zone, is also metamorphosed. The reason for why and how this kind of the metamorphism occurred, and especially why and how the metamorphic rocks were exhumed, is yet uncertain and unresolved. To understand the metamorphic and exhumation process in Okinawa, a structural study is undertaken, and its relation to the Eocene ridge subduction is discussed. We believe exhumation was performed by formation of a D2 extrusion wedge, made up of the Nago metamorphic rocks. The base for this wedge is a subduction thrust, and the roof is a detachment fault. Internally, there exists another Kijoka detachment fault, which is a brittle low-angle fault with top to the northwest shear sense, and the D2 major recumbent folds and thrusts show top to the southeast opposite shear sense in the Kunigami zone. This is the first report that finds detachment faults from the typical and ancient accretionary complex. M2 is mostly retrograde related to exhumation, but its medium P/T-type prograde metamorphism, abnormal at subduction zones, represents a high thermal gradient during ridge subduction. As a result, this ridge subduction is responsible for exhumation. At the time of accretion of the Kunigami zone, D1 ductile contraction and constriction exhibited top to the southeast shear sense, but an opposite and extensional shear sense is recognized in the proto-wedge. During D1, the wedge had already been active and begun to exhume. M1 of the Miyagi complex is accretion related and also of medium P/T-type metamorphism, and is a consequence of Cretaceous ridge subduction without any ability to cause much exhumation.
... The Chichibu accretionary complex constitutes mélange, which is characterized by blocks of basalt, Carboniferous to Permian or Triassic limestone, Permian, Triassic, and Late Jurassic to Early Cretaceous radiolarian chert, Jurassic to Early Cretaceous siliceous mudstone, and sandstone in a mudstone matrix (Fujita, 1989;Koike, 1979;Osozawa, 1984Osozawa, , 1986Osozawa et al., 1979Osozawa et al., , 1983Takeuchi, 1993). Carboniferous to Permian limestone yields fusulinid, coral, and conodont fossils, while interbedded Triassic limestone and bedded chert suggest deposition around the carbon compensation depth (CCD) (Osozawa, 1986;Osozawa et al., 1983). ...
... The Shimanto accretionary complex of Amami-Oshima Island is mainly composed of sandstone and mudstone with basalt and silicic tuff (Osozawa et al., 2009;Takeuchi, 1993). Albian to Turonian radiolarian fossils and ammonite from mudstone indicate that the accretionary age is Early to Middle Cretaceous (Fujita, 1989;Ishikawa & Yamaguchi, 1965;Matsumoto et al., 1966;Osozawa, 1984;Osozawa et al., 1983). 10.1029/2022GC010801 3 of 21 some blocks are elongated parallel to foliation or are wrapped by the foliation. ...
Accreted basaltic rocks are expected to provide information on intraplate volcanism in the oceans. Basaltic rocks, originating from mid‐ocean ridges, plateaus, and seamounts, have been reported from exhumed accretionary complexes. However, basaltic rocks related to petit‐spot volcanism remain poorly documented. We examined basaltic intrusive rocks in an exhumed accretionary complex on Amami‐Oshima Island, Ryukyu Arc. Basaltic sills intruded mélange composed of seamount‐derived micritic limestone and basaltic rocks, which deformed together with the mudstone‐dominated mélange in the subduction zone. A separate basaltic sill intruded pelagic chert before being incorporated as a mélange block in the mudstone matrix during deformation in the subduction zone. Geochemical discrimination diagrams and trace element patterns indicate that the accreted dolerites and nonintrusive basalts likely originate from mid‐ocean ridges and hotspots, respectively. On the other hand, the geochemical characteristics of basaltic sills are compatible with alkaline basalt with enriched in incompatible trace elements relative to other basaltic rocks of hotspot origin and may represent a low degree of partial melting from a deeper mantle source (>90 km). A higher ratio of gadolinium to ytterbium in the basaltic sills relative to hotspot or mid‐ocean ridge‐related basalt indicates both a deeper melting depth, and a magma source which upwelled through thick oceanic lithosphere far from the mid‐ocean ridge. Based on the timing of basaltic intrusion and geochemical features, we suggest that the basaltic intrusive rocks could record past petit‐spot volcanism in the oceanic plate.
... The Ryukyu limestone covers most of Okinoerabu-jima, but metamorphosed mafic tuff, correlated to similar rocks on Okinawa-jima (Osozawa 1984), is the exposed summit of Mt Oyama. Basement is also exposed along an ENE-WSW-trending range making up the spine of the island. ...
... The Yoron-jima basement consists of a metamorphosed accretionary complex of middle Cretaceous age and appears to be a northern continuation of the northern Okinawa-jima complex (Osozawa 1984;Osozawa and Watanabe 2011). The Quaternary geology resembles Okinawa-jima (O1), more than Okinoerabu-jima (A4) or Tokuno-shima (A3). ...
In the Quaternary, the Ryukyu Islands evolved from a continental margin arc to an island arc by backarc spreading of the Okinawa Trough, accompanied by subsidence and isolation of the islands, a process that has continued to the present. Trough-parallel half grabens were filled with marine siltstone. Similar sediments filling orthogonal fault-controlled and west-draining non-tectonic valleys record island separation. New Quaternary nannofossil biostratigraphic data date the deposition of the marine siltstone at 1.552 ± 0.154 Ma. At that time, the entire 1000 km-long island chain comprising the Ryukyu Islands separated from the Asian continent by rifting extending from the Okinawa Trough to the Tsushima Strait. The Tokara, Kerama, and Yonaguni gaps, branched or transverse rifts of the Okinawa Trough, separate the island chain into subgroups of the Osumi, Amami, Okinawa, and Yaeyama islands, and Taiwan. The shallow Taiwan Strait separated Taiwan from the Chinese mainland. The Kuroshio warm current that previously ran offshore of the continental margin arc began to enter the opening backarc basin through the Yonaguni gap and to exit through the Tokara gap, flowing along the axis of the Okinawa Trough. Under influence of the warm current and because of entrapment of continentally sourced detrital sediments by the Okinawa Trough, coral reefs formed around each island. These reefs make up a unit called the Ryukyu limestone. Subsidence continued through the deposition of this limestone, resulting in further isolation of each island. Some islands did not separate from the mainland but emerged above sea level later as a result of volcanic edifice construction or forearc uplift. Following initial isolation, the Japanese islands and Taiwan may have been connected to the mainland by land bridges during some sea level low stands related to glacial periods, whereas the other islands remained isolated. Based on ages of isolation of each island, a Quaternary palaeogeographic map and ‘phylogenetic tree’ of the islands can be drawn showing the separation time of each island from the mainland and from each other. This information should be useful for phylogenetic molecular biologists studying evolution of Ryukyu endemic species and vicariant speciation and could facilitate analysis of the DNA substitution rate.
... Taira et al. (1982), for example, recognized a block-in-matrix fabric and distinct differences of radiorarian age between oceanic blocks and a matrix derived from terrigeneous sediment, to conclude a predominantly olistostrome origin for the Shimanto accretionary prism mélange. The lead author studied several fi eld examples and reached a similar conclusion of the predominance of the olistostromal process in these examples (Osozawa, 1984(Osozawa, , 1992. Later studies recognized, however, that many minor structures showed clear evidence for asymmetric shear, and as a result Taira et al. (1992) reinterpreted the Shimanto mélange as dominantly a tectonic mélange. ...
Mélanges represent a significant part of the Miocene Nabae accretionary complex. Such mélanges show sheath folds with D1 axial plane pressure-solution cleavage, whereas the coherent unit shows asymmetric folding with D1 slaty cleavage. In addition, the mélanges are characterized by D1 asymmetric shearing, which includes both thrust and right-lateral-sense components, in contrast to D1 pure shear that characterizes the coherent unit. Thus, this tectonic style acted on the climax of prism development can be referred to as a tectonic mélange. However, because the D1 shear displacement is almost negligible, D0 normal faults and basaltic dikes, operated when matrix sediments were not consolidated, are not disrupted. Oceanic materials such as basalt and chert cannot be incorporated into terrigenous matrix, given the small displacement associated with the D1 shearing. Exotic blocks of chert and sandstone show D minus 1 (D-1) cleavage, which is apparent in the older, probably Eocene, accretionary prism. When this prism was exhumed, it supplied debris to the Miocene trench, and then underwent additional D1 deformation, which included the above asymmetric shearing. This sedimentary and two-way-street tectonic process was recycled within the prism as the latter developed. Thus, as the block-in-matrix fabric was originally sedimentary and labeled D0, the tectonic mélange process that forms block-in-matrix fabric is only conjectural for the Nabae complex. Also it is suggested that these deformations are not progressive nor distinctive for each other.
... Taira et al. (1982), for example, recognized a block-in-matrix fabric and distinct differences of radiorarian age between oceanic blocks and a matrix derived from terrigeneous sediment, to conclude a predominantly olistostrome origin for the Shimanto accretionary prism mélange. The lead author studied several fi eld examples and reached a similar conclusion of the predominance of the olistostromal process in these examples (Osozawa, 1984(Osozawa, , 1992. Later studies recognized, however, that many minor structures showed clear evidence for asymmetric shear, and as a result Taira et al. (1992) reinterpreted the Shimanto mélange as dominantly a tectonic mélange. ...
Abstract Normal faults parallel to the trend of an active ridge are formed in the accretionary prism at trench-trench-ridge triple junction, due to continuous spreading of the subducted ridge. Normal faults are observed in the Nabae and Mugi sub-belts, accretionary zones formed by ridge subduction in the Shimanto Belt. Igneous and sedimentary dykes intrude through the previous normal faults. Using these fault and dyke data, intermediate principal axis of stress relating to the normal faulting is determined, and is fitted to the trend of the subducted ridge. Normal faults formed by ridge subduction are useful for plate reconstruction.
The Japanese Islands consist fundamentally of late Paleozoic to Cenozoic accretionary complexes that formed in situ in a subduction zone along the East Asian continental margin, i.e. 2.0 Ga Yangtze craton (South China) and 450 Ma fore-arc ophiolite. Recent research utilizing microfossil and radiometric dating has distinguished several major accretionary complexes, including high-P/T metamorphosed parts, and subordinate ophiolites. In particular, recognition of oceanic plate stratigraphy and age of subduction-related metamorphism for individual accretionary complex allows the geotectonic subdivision of the Japanese Islands be emended with a new definition of geotectonic units and their mutual boundaries. Removing the effect of arcrelated magmatism and secondary tectonic modification by microplate activities such as backarc basin opening, fore-arc sliver movement, and arc-collision, a remarkable oceanward younging polarity is recognized among the accretionary complexes. This polarity in growth is well observed in Southwest Japan where seven distinct units occur, i.e. from the Japan Sea side to the Pacific side: 400-300 Ma high-P/T schists, Permian (250 Ma) accretionary complex, 230-180 Ma high-P/T schists, Jurassic (180-140 Ma) accretionary complex, 100 Ma high-P/T schists, Late Cretaceous (80 Ma) accretionary complex, and Tertiary (50-20 Ma) accretionary complex. The sinuous surface trajectories of these geotectonic boundaries and occurrence of several tectonic outliers and windows indicate that all these complexes, including high-P/T schists, occur as subhorizontal (or gently northward dipping) thin tectonic unit, i. e. nappe. Thus the Japanese Islands form a huge pile of nappes that become younger structurally downward to the modern Nankai accretionary complex. What is remarkable in this subhorizontal orogen is that high-P/T units are tectonically intercalated between low-P units, e. g. the thin nappe of 100 Ma Sanbagawa blueschists between Jurassic and Late Cretaceous accretionary complexes of the prehnite-pumpellyite facies. Uplift of the Sanbagawa high-P/T unit appears to correlate with the arrival of the Kula/Pacific spreading ridge at the trench, suggesting that this high-P/T accretionary complex may have been extruded and uplifted into low-P domain in fore-arc by buoyant subduction of the spreading ridge at the trench. Evidence of ridge subduction at that time is supported by reconstructed paleoplate motion and the coeval climax of arc-related Ry-oke magmatism associated with low-P/T regional metamorphism. Formation of older high-P/T blueschist nappes sandwiched between low-P units can be explained likewise. Subduction of major spreading ridges seems most critical for the episodic oceanward development not only of subhorizontal high-P/T nappes but also of continent side granitic belts.
The pre-Eocene metamorphic rocks found in the Yaeyama Islands (the southern part of the Ryukyu Arc) have disputed relationships with the metamorphic formations of Japan and Taiwan. They are divided into the Tomuru and Fusaki formations. 1.(1) The Tomuru Formation is composed of high pressure (HP) metamorphic rocks derived from oceanic sediments and the uppermost oceanic crust. The first deformation phase is characterized by a synmetamorphic NW-SE trending stretching lineation (L1). Kinematic analysis shows that the deformation corresponds to a subhorizontal ductile shear directed along L1 from the northwest to the southeast.2.(2) The Fusaki Formation or chaotic formation is an olistostrome of Triassic or Lower Jurassic radiolarite olistoliths. The age of the matrix is assumed to be late Jurassic. It is slightly schistose and only a S-vergent deformation phase is observed. It is assumed that the HP schists overthrust the Mesozoic olistostrome. 39Ar-40Ar step-heating experiments have been carried out on phengite and Na amphibole from the Tomuru Formation. Phengite and zoned barroisite-crossite give well defined plateaus at 225 ± 4.8 Ma and 237 ± 6.3 Ma respectively. They are interpreted as the age of the ductile deformation. Crossite yields an integrated data of 104.8 ± 8.3 Ma, interpreted as the age of the thrusting of the HP schists above the chaotic formation. The lithostratigraphy, petrology, microtectonics and 39Ar-40Ar ages of the HP schists and the chaotic formation are similar to the Permian Sangun HP schists and the Jurassic Tanba olistostrome of the Inner Belt of southwestern Japan respectively. There, the Sangun schists also overthrust the Tanba olistostrome during the Mesozoic orogeny. The same Tanba-like Mesozoic olistostrome is also found in Taiwan, but was intensely deformed and metamorphosed in Miocene times. The geodynamic collision model proposed to account for the late Permian to early Triassic orogen of southwestern Japan is extended to the South Ryukyu and Taiwan. Conversely, the Mesozoic Sanbagawa HP schists and the upper Jurassic to lower Miocene turbidites and olistostrome (Sanbosan-Shimanto zones) of southwestern Japan are neither exposed in the south Ryukyu Arc, nor in Taiwan. Hypotheses are discussed that call upon tectonic erosion or non-deposition to account for this absence.
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