The geology and petrology of the Itoigawa–Omi area of the Hida Gaien belt have been reexamined with pioneering metamorphic zonal mapping of glaucophane schist facies done by Banno (1958). A geologic study shows that crystalline schists of ca. 262-381 Ma, 450 Ma garnet amphibolite, amphibolite, 520 Ma jadeitite, albitite, and rodingite, reaching 10 km long and 2-5 km wide, occur as tectonic blocks of serpentinite forming the Omi serpentinite melange (OSM). Along the Omi–River of the eastern side of the OSM, chlorite, garnet, and biotite zones of pelitic schists are again recognized across several tectonic blocks of schist, indicating the slightly broken nature of a large metamorphic unit that resulted probably from exhumation of the serpentinite mélange belt. Kfs and chlorite are common throughout pelitic schists along the Omi–River. Thus, biotite- and oligclase-forming reactions may have been K-feldspar + chlorite → muscovite + biotite + quartz + H2O, and muscovite + clinozoisite + quartz → K-feldspar + anorthite + H2O, respectively. Glaucophane described by Banno (1958) in the chlorite zone of the Omi River has been recognized in composite grains consisting of hornblende core, gluacophane mantle, and actinolite rim. In contrast, glaucophane appears in a few samples along with eclogitic assemblages as plurifacial minerals before, during, and after eclogite facies in the pelitic schists of the western side of OSM (EC unit). Thus, the regional metamorphism described along the Omi–river of OSM (EA unit) is not a retrogressive metamorphism of the EC unit, but a regional hydrothermal recrystallization just before or during exhumation in the serpentinite mélange belt.
206Pb/238U dating of detrital zircons by LA-ICP-MS clarified that the majority of high-P/T Nagasaki metamorphic rocks in western Kyushu contain Late Cretaceous igneous zircon of 89 ± 3 Ma (Turonian-Coniacian) and 86 ± 2 Ma (Coniacian-Santonian). These data indicate that Nagasaki metamorphic rocks are composed mainly of Upper Cretaceous Shimanto metamorphic rocks, i.e. high-P/T metamorphosed accretionary complex of the Cretaceous Shimanto belt. This find strongly supports the subhorizontal geotectonic structure of the Japanese Islands previously inferred from surface geological mapping and deep seismic images.
The geotectonic boundary between the North China (Sino-Korean) and South China (Yangtze) cratons in mainland China forms a remarkable collisional suture, featuring the ca. 230 Ma (Middle Triassic) ultrahigh-pressure (UHP) metamorphic belt in the Dabieshan-Sulu area. Its eastern extension, however, becomes unclear, due to a large-scale tectonic offset related to the Miocene opening of the Bohai basin and Japan Sea. This article reviews the geological characteristics of the mid-Triassic medium-pressure (MP) metamorphic belts in the Korean peninsula and the Japanese Islands, in particular, their protolith assemblage, metamorphic age, and structural horizon. In the Korean peninsula, the Triassic MP metamorphic belt appears twice on the surface, i.e., along the Imjingan belt to the north and along the Ogchon zone to the south. In Japan, the mid-Triassic MP metamorphic belt occurs in north-central Kyushu, in the eastern margin of the Hida mountains in central Japan, and in the northern Kanto area. We conclude that both the MP metamorphic belts in the Korean peninsula and the Japanese Islands correspond to the eastern extension of the collisional suture between the North and South China cratons, on the basis of lines of evidence that include (1) coeval Triassic age of UHP and MP metamorphisms, (2) same protolith assemblage of shelf facies deposited along the passive continental margin, and (3) structural horizon between overlying North China rocks with an Archean heritage and underlying South China rocks with a Proterozoic affinity. This geotectonic correlation among mainland China, Korean peninsula, and the Japanese Islands, in particular, the documentation of the suture in Japan, up to the north of Tokyo, suggests that major parts of the Japanese Islands have developed along the South China margin, whereas the Hida and Oki belts form sole remnants derived from the North China craton.
The western Pacific region, where the Eurasia, Australia, and Pacific plates currently interact, has been recognized as an important site for constraining the origins and emplacement of ophiolites (particularly for island-arc or supra-subduction zone types), because the spatial distribution of oceanic micro-plates and numerous ophiolitic rocks along their convergent margins infers possible genetic linkages among them. Mafic-ultramafic rocks distributed in the Timor-Tanimbar island chain, eastern Indonesia may be a good example of the on-going emplacement of the marginal basin lithosphere on the continental margin in the arc-continent collision zone, and are recognized as a possible modern analogue for Mesozoic Tethyan-type ophiolites (e.g. Troodos and Oman) in the Alpine-Himalayan orogenic system. Geological occurrence suggests that the buoyant subduction of the Australian continent uplifted fragments of newly formed mantle-crust section, which extends to neighboring pre-emplaced forearc marginal basins. However, from petrological and geochemical points of view, young pillowed basalt, dolerite, and gabbroic cumulate commonly possess island-arc signatures, whereas structurally underlying peridotites are mostly fertile (lherzolitic) in composition. This suggests that the crustal section is not linked to the underlying mantle by a genetic melt-and-residua relationship, as inferred from the lack of complete succession and the presence of abundant crosscutting structures. This inconsistency leads to the emergence of two contrasting models accounting for the unusual occurrence of a fertile mantle in the forearc setting of the Timor-Tanimbar region: (1) thrust-stacked fragments of the subcontinental mantle originally exhumed in the rifting stage of Australia; (2) depth-related heterogeneities in the lithospheric part of the mantle wedge. We note that the current debates on the origins of fertile lherzolites found throughout the Tethyan sutures and western Pacific regions can be settled through a better understanding of Timor-Tanimbar peridotite masses by age-dating studies employing several radiogenic isotope systematics.
The ca. 700 million year-long geotectonic history of the Japanese Islands comprises three distinct intervals; i.e., (1) the age of a passive continental margin off the South China continental margin (ca. 700-520 Ma), (2) the age of an active margin characterized by an arc-trench system (ca. 520-20 Ma), and (3) the age of an island arc off East Asia (20 Ma to the present). These three intervals are chronologically separated by two major boundaries with significant tectonic episodes; i.e., the ca. 520 Ma tectonic inversion from a passive to an active margin by the initiation of subduction from the Pacific side, and the ca. 20 Ma tectonic isolation of the modern island arc system from the Asian margin by the back-arc basin (Japan Sea) opening. Here, the evolutionary history of the Japanese Islands is revised significantly on the basis of new lines of information that derived from a new dating technique of detrital zircon in sandstone. Particularly noteworthy is the recognition of the Early Paleozoic to Middle Mesozoic arc batholiths that were exposed extensively in the past but not at all at present because the pre-Cretaceous granites merely occur as kilometer-size blocks in the modern Japanese Islands. As to these older granites, the remarkable disagreement between the current distribution and the predominance of their clastic grains in younger sandstones suggests the effectiveness of past tectonic erosion processes in the fore-arc domains. The newly documented historical change in sandstone provenance suggests that proto-Japan has experienced not only accretionary growth but also large-scale tectonic erosion in multiple stages. During the ca. 500 million-year history of the Japanese Islands, a large amount of juvenile arc (continental) crust was formed several times, however, most has already disappeared from the Earth's surface. In short, the orogenic growth of Japan, even in a long-lasting active continental margin setting, is explained as the intermittent repetition of ocean-ward continental growth and continent-ward contraction of an active arc-trench system. In contrast to these arc batholiths, the terrigenous flux from the neighboring two major continental blocks (South and North China) was less significant than previously imagined, except for the Jurassic to Early Cretaceous time when the collisional suture between North and South China blocks was selectively eroded to produce abundant terrigenous clastics. It is also significant that the eastern extension of this collisional suture was recognized in Japan as a chain of fragmentary remnants of the Triassic medium-pressure metamorphic belt. On the basis of these new lines of information, the South China-related origin of the main part of Japan is confirmed, whereas the Hida and Oki belts along the Japan Sea are identified as detached fragments of North China block. Summarizing all of these results, a series of revised paleogeographic maps of Japan from the Late Neoproterozoic to the Miocene is illustrated.
The history of supercontinents is briefly reviewed in relation to the origin of the Japanese Islands. The Japanese Islands formed part of the S. China Block, which was a part of supercontinent Rodinia at 1.0 Ga. Rodinia was rifted at 600 Ma, separating S. China Block, N. America, Australia and other continents, to generate the Proto-Pacific Ocean in between. On the other hand, the Hida and Oki islands belong to the N. China Block, which has much longer history than the S. China Block, extending back to 1.9-2.0 Ga with minor older rocks dating back to 3.8 Ga. The 1.8-2.0 Ga high-grade gneiss in the Hida and Oki belts may be part of the 1.8-1.9 Ga Nuna/Columbia supercontinent within which N. China-Japan occurred at the NE corner, as judged from key parallel belts of 1.8-1.9 Ga in N. China. The position of Japan at 1.0 Ga within Rodinia was at the center together with S. China and western margin of N. China. The oldest fossiliferous rocks in Japan may extend back to the Early Cambrian to Ediacaran formed during the rifting of Rodinia directly after Neoproterozoic snowball Earth. Initiation of subduction began ca. 520 Ma, and evolved through five Pacific-type orogenies along the southern margin of S. China. On the other hand, the Hida and Oki belts suffered the Triassic collision orogeny at 230-240 Ma, involving platform sediments up to the Carboniferous age. The final tectonic emplacement above the Jurassic accretionary complex may be related to the extensional event during the opening of the Japan Sea in the Miocene.
Pacific-type orogeny (PTO) has long been recognized as a contrasting accretionary alternative to continent-continent collisional orogeny. However, since the original concept was proposed, there have many new developments, which make it timely to produce a new re-evaluated model, in which we emphasize the following new aspects. First, substantial growth of Tonarite–Trondhjemite–Granite (TTG) crust, and second the reductive effect of tectonic erosion. The modern analog of a Pacific-type orogen developed through six stages of growth exemplified by specific regions; initial stage 1: the southern end of the Andes; stage 2: exhumation to the mid-crustal level at Indonesia outer arc; stage 3: the Barrovian hydration stage at Kii Peninsula, SW Japan; stage 4: the initial stage of surface exposure of the high-P/T regional metamorphic belt at Olympic Peninsula, south of Seattle, USA; stage 5: exposure of the orogenic core at the surface at the Shimanto metamorphic belt, SW Japan; and stage 6: post-orogenic processes including tectonic erosion at the Mariana and Japan trench and the Nankai trough. The fundamental framework of a Pacific-type orogen is an accretionary complex, which includes limited ocean floor material, much terrigenous trench sediment, plus island arc, oceanic plateau, and intra-oceanic basaltic material from the ocean. The classic concept of a PTO stresses the importance of the addition within accreted rocks of new subduction-generated arcs and TTGs, which were added along the continental margins particularly during the Cretaceous. Besides the above additional or positive aspects of a PTO, here we emphasize the negative effects of previously little-considered tectonic erosion caused by subduction over time. The evaluation of such extensive tectonic erosion leads a prospect of the presence of huge quantities of TTG material in the lower transition zone, where many subducted slabs have ponded, as illustrated by mantle tomography. This is confirmed by density profiles of the mantle, which show that TTGs are abundant only along the bottom of the upper mantle accompanied by slab peridotite, lherzolite, and MORB. The major velocity anomaly in the lower transition zone is best explained by the predominance of SiO2 phases, hence TTG, and not by MORB or ultramafic rocks. Reasonable calculations indicate that at a depth range of 520-660 km TTG material amounts to 6-7 times more than the total mass of the surface continental crust. The traditional view is that the Japanese islands evolved since 520 Ma through five Pacific-type orogenies, which grew oceanward, thus creating a continuous accretionary complex ca. 400-500 km wide, with TTG growth at the continental side of each orogen. However, the subducting oceanic lithosphere has produced five times more TTG crust compared with the present TTG crust in the Japan islands. This is explained by the fact that over time tectonic erosion has dominated the increasing arc-TTG crust. Accordingly, Japan has lost four arc-TTG crusts to tectonic erosion. TTG material, such as trench sediment, arc crust, and continental margin crust, was fragmented by tectonic erosion and transported into the bottom of the upper mantle at depths of 520-660 km. Worldwide data suggest that tectonic erosion destroyed and fragmented most of the Pacific-type orogens. (View PDF for the rest of the abstract.)
The Nankai Trough is a convergent plate boundary where the Shikoku Basin subducts beneath southwest Japan. Turbidite and hemipelagic sediments on the trough floor form an accretionary prism landward of the trough axis through a process of sediment compaction. Near the toe of the accretionary prism, a series of frontal thrusts have formed above the decollement, which is a major slipping plane on the plate boundary. In 2002, we discovered an extremely high heat flow at the second frontal thrust near the toe of the Nankai Trough off Muroto using ROV KAIKO during the KR02-10 research cruise of the Japan Agency for Marine-Earth and Technology (JAMSTEC). The background heat flow value was 160 mW/m2 and high values of 250-280 mW/m2 were obtained within 50 m above the second frontal thrust. The heat flow gradually decreases landward from this location. A simple numerical calculation was carried out to test the hypothesis that the heat flow anomaly is caused by an interstitial fluid flow through the decollement/frontal thrust system as a permeable channel. Two parameters—permeability within the fault zone (Kch) and excess pore pressure (ΔP)—at the landward boundary of the decollement were adjusted to produce the best-fit combination to explain the observed heat flow anomaly. The simulation result of 500 combination patterns indicates that Kch and ΔP cannot be determined independently. Instead, only their product (Kch × ΔP) can be estimated. The best-fit result, simulating the highest heat flow of 280 mW/m2 was achieved where the product Kch × ΔP is 8 × 1015, with a surrounding permeability of 10-17 m2. Based on the estimation of excess pore pressure in the decollement of less than 4 MPa, the most appropriate value of channel permeability is estimated to be higher than 2 × 10-15 m2. Our numerical result is generally consistent with the previously estimated fracture permeability, i.e. three orders of magnitude higher than that of ambient sediment. Thus, we attribute the surface heat flow anomaly to the influence of the fluid flow through the permeable channel along the frontal thrust.