The geotectonic evolution of the Japanese Islands has traditionally been studied mainly in Southwest Japan, where pre-Cenozoic basement rocks occur extensively. Now, at the beginning of the 21st century, a major revision is being proposed (see Isozaki et al., this issue, for details). (Yukio ISOZAKI)
This article provides an overview of papers collected in the special issue titled “Geotectonic Evolution of the Japanese Islands under New Paradigms of the Next Generation (Part I-III)” for international readers. Research on the geologic evolution of the Japanese Islands started from the geotectonic subdivision with definitions of elements and boundaries in the late 19th century. Traditionally, some remarkable faults, such as Median Tectonic Line (MTL), Fossa magna, and Tanakura Tectonic Line (TTL), were regarded as the most important; however, these apparent features were in fact formed during the Miocene opening of the Japan Sea, whereas the major structures of Japan were made by the Pacific-type orogenies throughout the Phanerozoic after the initiation of subduction at 520 Ma (the Cambrian). Marine geophysicists have realized that the formation of accretionary complex (AC) along modern active subduction zones is rather exceptional; instead, tectonic erosion takes place dominantly. The estimated addition rate of juvenile arc crust (composed of tonalite–tronjemite–granodiorite suite; TTG) is negative at present for the entire globe, i.e., the total volume of the continental crust is currently decreasing. This indicates that tectonic erosion must also have occurred effectively in the past, and that its geological remnants need to be investigated carefully. A possible proxy geologic body for ancient tectonic erosion is a serpentinite mélange, which often includes calc-alkaline volcanics and coeval high-P/T blueschists within serpentinite matrices as shown in the geology of Japan. The large-scale shortening of a fore-arc crust can occur solely due to tectonic erosion, which may involve more than 150 km-wide areas of missing rocks between volcanic arc and trench. The sporadic occurrence of tectonic blocks of older (520-150 Ma)TTG rocks in serpentinite mélange indicates that four arc crusts, out of the five made by Pacific-type orogeny in Japan during the past 500 million years, have already been consumed. The Pacific-type orogeny was revisited after recent critical discoveries of; (1) orogenic core of high-P/T regional metamorphic belt as a thin (< 2 km) solid high-T intrusion, bounded on the top and bottom by a paired fault; (2) rapid increase of 200-300 km-wide TTG belt formed by slab-melting; (3) these culminated with an approaching mid-oceanic ridge, associated with large-scale tectonic erosion; and (4) formation of an AC after ridge subduction and doming up of a sandwiched set of high-P belt and accretionary belt below and above. The Pacific-type orogeny has been ignored since the “terrane plague”; however, Pacific-type orogeny dominated to increase the TTG crust leading to the births of continents and supercontinent by 1.8 Ga. The extensive tectonic erosion and arc subduction through time, suggesting 10 times more TTG materials, enriched with radiogenic elements, floated at the bottom of the mantle transition zone, which generated heat to warm up the mantle up to 200 K within 100 million years in the Archean, and to 100 K in the Phanerozoic. Subducted TTG and presence of second continents in the mid-mantle may explain more general aspects of mantle dynamics, and the concept may become a frontier of solid Earth science in the 21st century. From the viewpoint of the history of science, the previous geological studies in Japan since the Meiji Era are also summarized briefly, with special reference to the orogeny, tectonics, and geotectonic subdivision of the Japanese Islands.
This is a brief review of tectonic erosion originally proposed by von Huene and Scholl (1991) who have spent most of their academic careers studying marine geophysics along the circum-Pacific subduction zone. Accretionary complex is considered to be formed by trench turbidite resting on the subducting oceanic plate that accretes against a hanging wall at a shallow or deep crustal level due to off-scraping or underplating. This concept was introduced to Japan in the early 1970s and was developed in great detail to propose a new paradigm for accretionary geology that involves ocean-plate stratigraphy. Later, identification of accretionary complex on-land in Japan became the mainstream. A new idea refutes the common occurrence of an on-going accretion process forming accretionary complexes along the circum-Pacific subduction zone. Instead, the concept of tectonic erosion has emerged to explain extensive crustal thinning and subsidence as an on-going process destroying the hanging wall of an older subduction complex or even the basement of the overriding plate at more than half of the active trench. During the past three decades, marine geophysicists and geologists have documented tectonic erosion as a more common process than the formation of an accretional complex in subduction zones, and supeculate that a large volume of the continental crust is subducted into the mantle at both accretionary and erosive convergent margins. A simple calculation of the amount of subducting continental material versus arc producted at the subduction zone suggests a balance, resulting in no growth of continental crust at present (e.g. Clift and Vannuchi, 2004; Scholl and von Huene, 2007). However, considering direct subduction of intra-oceanic arcs and foundering of the continental lower crust into the mantle, we must conclude there is negative growth of the continental crust on the Earth at present.
The geotectonic subdivision and relevant definitions of geotectonic units in the Japanese Islands are revised on the basis of new data, particularly with detrital zircon dating of U-Pb ages and seismic profiling of the deep arc crust across the islands. In addition to the final confirmation of the subhorizontal structures of the Paleozoic to Cenozoic accretionary complexes and their high-P/T metamorphosed equivalents, several new aspects were recognized; i.e., detection of the eastern extension of the collisional suture between the Sino-Korean and Yangtze cratons in the Higo belt with medium-pressure-type metamorphism in SW Japan, and separation of the traditional Sanbagawa belt into two distinct metamorphic belts characterized by mutually different ages of protolith AC-formation and peak metamorphism. The occurrence and consumption of 4 Paleozoic to Mesozoic granite batholiths, as major provenances for the ancient Japanese Islands, are documented by detrital zircon dating of Paleozoic–Mesozoic sandstones. With respect to these new findings, the definitions of unit boundaries were thoroughly revised in terms of chronological spectrum in “ocean plate stratigraphy–metamorphism”. The geological significance of 5 major tectonic lines (faults) of the Pacific-type (or Miyashiro-type) orogen in Japan, i.e., the Nagato–Hida marginal TL, Osayama–Omi TL, Ishigaki–Kuga TL, Paleo–Median TL, and Butsuzo TL, is discussed. The current revision of the geotectonic subdivision and definitions of component units and their mutual boundaries leads to the following conclusions, which challenge the conventional understanding of the orogenic history of the Japanese Islands. (1) Proto-Japan in the Early Paleozoic was located closer to the South China (Yangtze) craton rather than the North China (Sino–Korean) craton. (2) Ever since 520 Ma, subduction of past Pacific ocean floors formed mature arc-trench systems with a full set of granite batholith, fore-arc basin, accretionary complex, and high-P/T metamorphosed equivalents at least 5 times; however, the former 4 sets were almost completely destroyed, with the exception of smaller tectonic blocks that currently occur within serpentinite mélange. (3) Tectonic erosion played a significant role in consuming ancient fore-arc crusts including 4 granite batholiths of the Paleozoic to mid-Mesozoic. (4) Serpentine mélange represents the former Wadati–Benioff plane along which tectonic erosion took place. (5) The Japanese Islands, which basically developed along the Yangtze continental margin, have experienced multiple episodes of oceanward growth and continentward retreat due to alternating subduction-accretion and tectonic erosion. (6) Net production of juvenile crust occurred on a large scale along the Japan margin during the 500 million year-long oceanic subduction regime since the Cambrian; however, intensive tectonic erosion effectively erased the older crusts from the surface and enriched the underlying sub-arc mantle with heat-generating continental material.
Primary arc magmas are thought to originate through interactions among subducting slab, slab-derived fluid (slab-fluid) or slab-derived melt, and mantle wedge. In most subduction zones, slab-fluid is thought to play a major role in lowering the solidus temperature of the mantle wedge, inducing partial melting to produce arc magmas. The slab-fluid is composed of an aqueous fluid derived from materials associated with the subducting slab, such as sediment and altered oceanic crust. The chemical composition of slab-fluid depends on the mobility of elements during dehydration, and affects the chemical and isotopic compositions of the fluid-added mantle wedge and arc volcanic rocks. Accordingly, geochemical analyses of arc volcanic rocks provide valuable information on slab-fluid, fluid-added mantle, and melting conditions. Examples from the Japan arcs support the validity of this approach in deciphering the composition and amount of slab-fluid, including both across-arc and along-arc variations: the amount of slab-fluid is largest in the Central Japan arc due to overlapping subduction of the two plates, while it is significantly less in the other arcs. In the Northeast Japan arc and the Izu-Bonin arcs the amount of slab-fluid decreases from the front to the rear arc. Accurate identification of slab-fluid also resolves a large-scale along-arc variation in the isotopic composition of mantle wedge, involving the Indian-type and Pacific-type MORB mantles, which contributes to an understanding of the large-scale mantles flow.
The Japanese Islands have experienced subduction of the old Pacific Plate and the young Philippine Sea Plate, as well as back-arc basin opening, such as at the Kurile basin, Japan Sea, Shikoku basin, and Okinawa trough. This paper summarizes progress in understanding mantle dynamics beneath Japan in the past ∼10 years in terms of igneous petrology and geochemistry, seismology, and numerical simulation. The discussion is separated into four regions: Hokkaido, northeast and central Japan, southwest Japan (Chugoku and Shikoku districts), and Kyushu, from northeast to the southwest. Now that a massive amount of volcanic eruption age data and high-resolution seismic analyses have been accumulated for every region, we can discuss mantle dynamics within mantle wedge and back arc mantle in more detail than in the 20th century. Constraints related to water content, temperature, and melting process in the mantle from a natural sample are, however, still missing despite the importance of these factors when modeling mantle dynamics.
Median Tectonic Line (MTL) and Fossa Magna (Itoigawa-Shizuoka Tectonic Line) had long been considered to be the most critical fault boundaries controlling development of the Japanese Islands since Naumann (1885) and Kobayashi (1941). After the appearance of plate tectonics, several new interpretations emerged, e.g., sub-surface Benioff plane for the MTL. In this paper, we propose that those tectonic lines, major faults, and Tanakura Tectonic Line (TTL) were formed through a process at micro-plate boundaries during the opening of the Japan Sea in the Miocene. MTL could have been formed along the consuming boundary between the PHS plate and Japan Sea microplate, which has shifted southward to the Nankai trough, accompanying large-scale tectonic erosion. Fossa Magna was formed as a gigantic transform fault with a transtension component in the Medial-Japan Sea when opening was initiated. The eastern and western boundaries of the Japan Sea must be a strike-slip fault, corresponding to TTL to the east, and a newly proposed strike-slip fault called the West Kyushu Tectonic Line, respectively. Fossa Magna, a medial region defined by two NS-trending Miocene parallel faults in central Honshu, defined by Nauman (1885) could be interpreted to be the largest transform fault in the Medial-Japan Sea to offset the spreading axis when the Japan Sea opened. It should be emphasized that large-scale tectonic erosion occurred in front of consuming plate boundaries facing the PHS and PAC plates oceanward during the opening of the Japan Sea. The volume of tectonic erosion is calculated to be 17,581,500 km3, which is equivalent to 2/3 of the present-day Japan arc crust, which is sufficient to reach the depth of the megalith between the upper and lower mantle boundary, even with 10 km thickness of materials eroded and transported along the Benioff zone. Although MTL, Fossa Magna, and TTL are remarkable in the geology of Japan, these young faults never affected the orogeneses of Japan back to 520 Ma, which grew the continental crust of Japan. We propose that microplate boundary processes decreased the volume of the Japan crust.
This paper reviews the geology, petrology, and tectonics of the Proto-Izu-Mariana arc against the Honshu arc, Japan, since 15 Ma, to formulate current topics related to the Pacific-type orogeny. Since the first pioneering work by Sugimura (1972), which placed the plate boundary on-land Japan north of the Izu peninsula collision-accretion tectonics of the Proto-Izu-Mariana arc against the Honshu arc started to be extensively investigated through multi-disciplinary methods. Aoike (1999) proposed a comprehensive scenario of five successive accretions of Proto-Izu to the present, with all bounded by top- and bottom-faults since 15 Ma. Oroclinal bending of the Honshu arc began simultaneously with the collision and indentation of the Proto-Izu arc, as is well documented by the Neogene paleomagnetic declination of sedimentary rocks in the Kanto district (Niitsuma et al., see summary, 1989). The entire Izu-Mariana arc is subducting without any accretion now under Honshu, as is well documented by seismic tomography imaging by Hasegawa et al. (see Hasegawa et al., 2010). Tanzawa Tonalite–Trondhjemite–Granodiolite (TTG) pluton intruded into the already-accreted Tanzawa Group at 4 Ma (Tani et al., 2010), resulting in a contact metamorphic aureole ranging from amphibolite through greenschist to zeolite facies at a depth of ca. 10 km, and tectonically denudated along the Kan-nawa fault to be exposed on the surface by 1.0 Ma. The Proto-Izu-Mariana arc was built on the Pre-Paleogene, presumably Cretaceous MORB crust on which boninitic and fractionated silicic lava flows accumulated since 48 Ma, which is older than the bending of the Hawaii–Emperor seamount chain at 43 Ma. Hence, we need another plate off Proto-Ogasawara arc, called North New Guinea plate (Seno, 1985), and ridge subduction underneath Ogasawara arc to initiate subduction magmatism. Both calk-alkali rock series (CA) and coeval blueschist–eclogite rocks are exposed along the inner wall of the Izu–Mariana trench. This strongly suggests that extensive tectonic erosion occurred, resulting in the coexistence of two rock units that were formed 100-150 km from each other horizontally, i.e., rocks on the volcanic front and rocks on the descending oceanic slab. Petrogenesis of juvenile crust and TTG rocks are being debate with two models: basaltic arc magma fractionation (Kawate and Arima, 1998) and delaminated mafic lower crust (Nakajima and Arima, 1998). In the latter model, a difficulty is derived from density at the Moho depth and small size of delaminated residue. Instead, extensive tectonic erosion that transports not only the upper crust but also the lower mafic crust together with parts of the descending slab could be more plausible than these two models in order to present a thinner mafic lower crust.
Provenance analysis using zircon U-Pb dating for sandstone enables us to reconstruct the paleogeographic history of the Japanese Islands. We determined the U-Pb ages of detrital zircons (50-200 grains) in 14 Phanerozoic sandstones (Silurian, Permian, Triassic, Jurassic, Cretaceous, and Neogene) and two modern river-sands collected from 13 areas in Japan. The measurements clarified two groups of Precambrian clusters (2500-1500 Ma and 900-800 Ma) and five distinct peaks of the Phanerozoic (520-400 Ma, 280-210 Ma, 190-160 Ma, 110-90 Ma, and 80-60 Ma). The Precambrian clusters from Paleozoic-Mesozoic sandstones suggest that proto-Japan was located close to the Yangtze (South China) craton, and the interaction with the Sino-Korean (North China) craton started sometime in the middle Triassic. The dominance of 520-400 Ma detrital zircons and scarcity of Precambrian ones in Paleozoic sandstone suggest that proto-Japan is likely to have formed an intra-oceanic island arc system isolated from large continental blocks. The strong peaks of 520-400 Ma, 280-210 Ma, and 190-160 Ma show a clear contrast with current small exposures of corresponding granites in Japan, indicating that all these granitic bodies may have been eroded away probably by tectonic erosion. The detrital zircon age spectra of the Miocene sandstone clarified that the final separation of the Japanese Island arc system from the Asian continental margin occurred around 16 Ma.
The Japanese Islands have long been considered to be the most evolved of all the island arcs in the oceans. A simple scenario has been implicitly accepted for the growth of the Japanese Islands: since subduction started sometime around 520 Ma, the TTG crust has increased over time in association with the steady-state growth of the accretionary prism in front. Here, we show very different dynamic growths of TTG crusts over time than previously thought, i.e., four times more TTG crusts than at present must have gone into the deep mantle due to tectonic erosion, which occurred six times since subduction was initiated at 520 Ma. Tectonic erosion is a major process that has controlled the development history of the Japanese islands. It can be traced as a serpentinite mélange belt, which indicates the upper boundary of past extensive tectonic erosion.
Recent progress in our understanding of the consuming plate boundary indicates the ubiquitous occurrence of tectonic erosion of the hanging wall of the continental margin, sediment-trapped subduction, and direct subduction of immature oceanic arcs into deep mantle. Geological studies have estimated the volume of subducted tonalite-trondhjemite-granodiorite (TTG) materials to about seven times the surface total volume of continental crust. To reveal the fate of subducted crusts and how they recycle within the Earth, we studied high-pressure densities and elastic properties of TTG by means of the first principles computation method and compared them to those of peridotite. We found that TTG is gravitationally stable and its seismic velocities are remarkably faster than peridotite in the depth range from 300 to 800 km, especially from 300 to 670 km. We, therefore, propose SiO2-rich second continents in the mantle transition zone, which used to form the TTG crust on the Earth's surface. Our proposed model may provide reasonable explanations of seismological observations such as the splitting of the 670 km discontinuity and seismic scatterers in the uppermost part of the lower mantle. The difference in seismic velocities between PREM model and experimental results in the lower part of the transition zone can be explained by 25 volumetric% of TTG, which would correspond to about six times the present volume of the continental crust. Formation and dynamics of those second continents would have controlled the Earth's thermal history over geologic time.
It has long been believed that granite remains on the surface of solid Earth indefinitely due to its low density, hence continents increased in volumes to cover about 30% of the Earth's surface over time. However, recent studies on the accretion history of continents reveals that at least 80% of the granite that ever formed had been subducted into the deep mantle to form “second continents” at the mantle transition zone (410-660 km). These second continents would affect mantle dynamics in two ways. First, as the second continents are gravitationally stable at the depth of the mantle transition zone, they act as a barrier to descending cold slabs. The stagnation of cold slab would be partly due to the pre-existing second continents. Another effect of the second continents on mantle dynamics relates to their chemical component. Because granite is enriched with incompatible elements, including long-lived radiogenic elements K, Th, and U, the second continents act as heat sources in the mantle. In particular, the heat generation of the second continents is a key to understanding the formation-breakup cycle of supercontinents. Granite that piled up beneath a supercontinent during continent accretion would cause thermal instability to form a superplume beneath the supercontinent. A numerical study on the thermal evolution of subducted granite gives the characteristic time scale of thermal instability which is consistent with the lives of supercontinents suggested by geological studies.