Ryoke Belt Research Articles

Fission track ages on some Ryoke granitic rocks along the Median Tectonic Line, Southwest Japan.

Four apatite fission track (AFT) and two zircon fission track ages were determined on the Ryoke granitic rocks, located close to the Median Tectonic Line (MTL), Southwest Japan. The AFT ages range from ∼9.3 to ∼32 Ma, and thus can be correlated to the “young” AFT ages previously found near the MTL; meanwhile, zircon ages are around 65 Ma. Because some of the localities lie in the western portion of the Ryoke Belt that was not studied previously, it is suggested that the region characterized by the “young” AFT ages, probably representing later thermal events along the MTL, may be extensive to the overall Ryoke Belt.

An initial continental margin plutonism — Cretaceous Older Ryoke granitoids, southwest Japan

AbstractThe emplacement of the Older Ryoke granitoids took place in the Mid-Cretaceous Period in the outermost side of the Inner Zone of southwest Japan, which was located at the eastern margin of the Eurasian Continent before the opening of the Japan Sea. The Ryoke Belt constitutes a long segment of the Cretaceous to Palaeogene felsic magmatic belt of Pacific Asia. The Older Ryoke granitoids represent its initial magmatism, related to the subduction of an oceanic plate underneath the Eurasian plate in that time. Their magmas were generated in the lower to middle crust beneath the Ryoke Belt in the subduction regime. They were emplaced during the Ryoke regional metamorphism and converted to orthogneisses.

反射法地震探査と重力測定による別府湾の地下構造

Multichannel seismic reflection profilings and gravity measurements give new information on the subsurface structure and the tectonic movement in the Beppu Bay area located at the northeast end of the Beppu-Shimabara Graben. Beppu Bay is structurally divided into the eastern and the western areas by a distinct tectonic line. In the eastern area, a graben structure whose axis runs parallel to the Median Tectonic Line is revealed by both the reflection profiles and the Bouguer gravity anomalies. The southern and the northern slopes of the graben are composed of the Sanbagawa Belt basement (SB) and the Ryoke Belt basement (RB), respectively. The surface of SB dips 30 degrees to the north at shallow depths and lowers at greater depths, and then SB lies below RB. The analysis of the seismic reflection and the gravity data suggests a possibility that an unknown formation having the similar density as SB may exist between SB and RB in the north part of this area. Many faults and folding structure in the thick sedimentary sequences overlying RB indicate that RB has been moved relatively to SB by dextral strike-slip movement, and has also slipped down along the surface of SB. On the other hand, the western area is subdivided into the main and the north parts. The main part is characterized by a basin structure buried by thick sediments, where an extremely low Bouguer gravity anomaly is observed. Active listric faulting is developed in the north part; the decolloment surface exists at about 300m depth below the sea floor.

Middle Paleocene terrane juxtaposition along the Median Tectonic Line, southwest Japan: Evidence from 40Ar/ 39Ar mineral ages

The Median Tectonic Line (MTL) is a regional transcurrent fault separating anadalusite-sillimanite-type metamorphic rocks and granitic rocks within the Ryoke terrane from glaucophanic-type metamorphic rocks within the Sambagawa terrane in southwest Japan. The late Cretaceous Izumi Group unconformably overlies Ryoke metamorphic rocks and is tectonically separated from the Sambagawa terrane by the MTL. Recent 40Ar/ 39Ar mineral ages reported for the Sambagawa terrane in this area suggest extensive Late Cretaceous uplift; however, no Sambagawa erosional debris has been described from the Izumi Group. 40Ar/ 39Ar ages have been determined for hornblende and biotite separated from samples systematically collected along a traverse from the Ryoke terrane into the MTL (Kashio area, Chubu district). In addition whole-rock samples of protomylonite within the MTL have been analyzed. Similar 68–70 Ma isotope correlation ages are recorded by hornblende within massive (Ikuta) and foliated (Minakata) granite and protomylonite within the MTL. Biotite from these samples records ages ranging between 66 and 68 Ma. Together the mineral ages suggest relatively rapid post-magmatic cooling through appropriate argon closure temperatures. These also indicate that temperatures maintained during protomylonite formation within the MTL were less than those required to effect even partial rejuvenation of intracrystalline argon systems within hornblende. Protomylonite samples yielded well-defined whole-rock plateau ages of 62–63 Ma which are interpreted to closely date the last phase of ductile flow within the MTL. These results suggest that a significant ductile phase of movement occurred within the MTL in the Middle Paleocene. This may have been associated with significant sinistral displacement and resultant tectonic juxtaposition of previously separated portions of the Sambagawa and Ryoke belts (with unconformably overlying Izumi Group).

Fission track thermochronologic analysis of the Ryoke belt and the median tectonic line, southwest Japan

Fission track (FT) ages were determined on 27 apatite and 21 zircon samples collected from three traverses across the eastern part of the Ryoke belt. FT zircon ages are reasonably consistent, averaging 58 Ma in the three traverses, suggesting uniform uplift and cooling of the region through the zircon isotherm (∼240°C). FT apatite ages range from 60 to 10 Ma and are divided into two groups: “old” apatites, located far from the Median Tectonic Line (MTL), cluster around 54 Ma in the three traverses, also indicating uniform uplift and cooling through the apatite isotherm (∼105°C); and “young” apatites, located near the MTL, show age differences between the three traverses. FT length data of nine apatites reveal later thermal overprinting events near the MTL, probably due to shear heating induced by fault movements. The observed uplift and cooling rates of the Ryoke belt and the timing of motion along the MTL are discussed in the tectonic framework.

510 フィッション・トラック熱年代学 : 嶺家帯と中央構造線への応用

510 フィッション・トラック熱年代学 : 嶺家帯と中央構造線への応用

Carbon contents of the magnetite-series and ilmenite-series granitoids in Japan.

Representative magnetite-series (n=75) and ilmenite-series (n=143) granitoids, and metamorphic rocks (n=11) from the Ryoke Belt, Sanyo Belt, Sanin Belt, Hidaka Belt, Outer Zone of Southwest Japan and Green Tuff Belt, were analyzed for total and noncarbonate carbon contents by combustion and infrared absorption photometry. Noncarbonate carbon contents of the ilmenite-series granitoids (58 and 61 ppm in the average for granodiorite and monzogranite, respectively) are slightly higher than those of the magnetite-series granitoids (39 and 35 ppm), especially of the sodic type (13 ppm for tonalite). Some ilmenite-series granitoids have noncarbonate carbon contents as high as 200-300 ppm at few places. In the carbonate vs noncarbonate carbon contents diagram, common ilmenite-series granitoids have values between those of the sodic granitoids and metamorphic rocks indicating that the ilmenite-series granitoids were formed by mixing of both the igneous and sedimentary source materials. A rather ambiguous separation of the magnetite series from the ilmenite series in these contents may also be due to remobilization of carbon after the granitoids solidified.

Strontium isotopic and chemical variations of the granitic rocks in the Tsukuba district, Japan

Initial 87Sr/86Sr ratios, major and trace element compositions have been determined for the Paleogene granitic rocks in the Tsukuba district, Japan. Isotopic ages strongly suggest that the granitic rocks (seven units) were continuously emplaced and solidified during a short time interval. Initial 87Sr/86Sr ratios for seven granitic units vary from 0.7082 to 0.7132, while sedimentary and metasedimentary country rocks have ratios at the time of granitic magma emplacement ranging from 0.7149 to 0.7298. Continuous linear arrays for the granitic rocks in the diagrams of initial 87Sr/86Sr ratios versus some chemical parameters can be explained by either of following two processes. One is the assimilation — fractional crystallization (AFC) process between the parental magma (SiO2 of 68% and initial ratio of 0.7078) and sedimentary country rocks, and the other is magma mixing process between above parental magma and sediment derived acidic magma (melt) (SiO2 of 75%). The high initial ratios (0.7078–0.7098) for basic rocks such as gabbro or diorite in the Tsukuba district and the similar characteristics observed in the rocks of Ryoke belt (SW Japan) suggest that the parental magma had the same source region as the basic rocks, probably the lower crustal source.

Petrogenesis of basic rocks in the Ryoke Belt of the Kinki and Setouchi districts, Southwest Japan

Petrogenesis of basic rocks in the Ryoke Belt of the Kinki and Setouchi districts, Southwest Japan

FISSION TRACK THERMOCHRONOLOGIC ANALYSIS OF THE RYOKE BELT AND THE MEDIAN TECTONIC LINE, SOUTHWEST JAPAN

Fission track (FT) ages were determined on 27 apatite and 21 zircon samples collected from three traverses across the eastern part of the Ryoke belt. FT zircon ages are reasonably consistent, averaging 58 Ma in the three traverses, suggesting uniform uplift and cooling of the region through the zircon isotherm (∼240°C). FT apatite ages range from 60 to 10 Ma and are divided into two groups: “old” apatites, located far from the Median Tectonic Line (MTL), cluster around 54 Ma in the three traverses, also indicating uniform uplift and cooling through the apatite isotherm (∼105°C); and “young” apatites, located near the MTL, show age differences between the three traverses. FT length data of nine apatites reveal later thermal overprinting events near the MTL, probably due to shear heating induced by fault movements. The observed uplift and cooling rates of the Ryoke belt and the timing of motion along the MTL are discussed in the tectonic framework.

Inhomogeneous progressive deformation during low P/ T type Ryoke regional metamorphism in central Japan

The progressive synmetamorphic deformation path of the Ryoke low P/T type regional metamorphic belt changes from flattening to uniaxial extension. The flattening-type deformation dominates in the northern region of the Ryoke belt, whereas in the southern region the uniaxial extension deformation is predominant. The strain magnitudes of both regions increase with the increasing metamorphic temperature. The axial ratios of a 1 a 2 ( a 1 is the long axis and a 2 is the intermediate axis of strain ellipsoid) reached 3bout 3 and 4. The gradual change in deformation history suggests that the Ryoke low P/T type metamorphic belt underwent a progressive tangential shear with successive change in magnitude of longitudinal shortening during progressive deformation/metamorphism.

Deformation of quartz in an inner shear zone of the Ryoke belt:an example in the Kishiwada area, Osaka Prefecture

Deformation of quartz in an inner shear zone of the Ryoke belt:an example in the Kishiwada area, Osaka Prefecture

The Ryoke Belt in the Hiki Hills, northeastern marginal area of the Kanto Mountains

The Ryoke Belt in the Hiki Hills, northeastern marginal area of the Kanto Mountains

関東平野の基盤構造

The basement rocks distributed in Central Honshu are divided from the Sea of Japan to the Pacific side into the Hida, Hida Marginal-Joetsu, Mino-Ashio, RyOke, Sambagawa, Chichibu, and Shimanto Belts. The Median Tectonic Line forms the boundary between the Ryoke and the Sambagawa Belts. The Hida and the Mino-Ashio Belts are characterized by some Helium-rich natural gases. Some geologists has postulated the existence of the Kashiwazaki-Choshi Tectonic Line trending NW-SE from Kashiwazaki in the Sea of Japan side to the southern part of the Kashima-nada in the Pacific side. The line is confirmed on the subsurface geology known from many deep wells and some geophysical data. Although there are three tectonic belts, that is the Sambagawa, Chichibu, and Shimanto Belts on the southern side of the Kashiwazaki-Choshi Tectonic Line, the Mino-Ashio and Ryoke Belts are recognized on the northern side of the line.At present Northeast Japan belongs to the north American plate. The front of the plate was situated at the Itoigawa-Shizuoka Tectonic Line by the middle Miocene age. Afterward the front has migrated from the line to the Kashiwazaki-Chashi Tectonic Line, on which the nest of earthquakes on the southwestern part of Ibaraki Prefecture rests.

領家帯南縁部に分布する泉南流紋岩類および巣山大砕岩のフィッション・トラック年代

領家帯南縁部に分布する泉南流紋岩類および巣山大砕岩のフィッション・トラック年代

Structure of granitic rocks of the southern Ryoke belt, Kinki district, Japan

Structure of granitic rocks (Cretaceous in age) distributed in the southern part of the Ryoke belt in the Kinki district has been studied. The granitic rocks intruded through three stages. The first stage granites (gneissose tonalite to granodiorite) distributed in the northern part of the studied area have undergone both D1 and D2 deformation. The D1 deformation is characterized by overspread foliation (S1) trending at low to moderate angles to the general trend of the Ryoke belt. The D2 deformation is of the left-lateral sense and resulted in the formation of strongly mylonitized belt of several kilometers wide along the Median Tectonic Line. Upright folds of left-hand echelon are also associated with the D2 deformation. The second stage granites (massive or schistose granite to granodiorite) distributed in the southern part of the studied area have undergone minor and local de formation (D3) characterized by branching shear belts caused by shear stress of left-lateral sense. The third stage granites (massive to weakly schistose tonalite to granodiorite), occurring as a pluton between the first stage and second stage granites, have undergone minor and local deformation (D4) caused by shear stress of left-lateral sense.

Microstructures and flow mechanisms in regional metamorphic rocks of Japan

A number of microstructural features indicate a difference in the dominant deformation mechanism between the higher temperature Ryoke and the lower temperature Sambagawa and Shimanto metamorphic belts of Japan. The microstructures of metacherts containing deformed radiolaria are divided into two types: in both the Sambagawa and Shimanto belts the quartz grains are tabular while in the Ryoke belt they are equiaxed. TEM studies of these metacherts revealed that the tabular grains contain abundant subboundaries consisting of large numbers of network dislocations and bowe-out dislocations, while the equiaxed grains contain no subboundaries and have low densities of dislocations which are not bowed-out. There is a corresponding difference in the textures (lattice preferred orientation of quartz): the Ryoke metacherts display randomly distributed c-axes of quartz, while the Sambagawa and Shimanto metacherts show conspicuous crossed girdle patterns with some asymmetry. There is a third difference between these regions: in the metacherts of the Ryoke metamorphic belt, the strain magnitudes determined from deformed radiolaria increase with increasing volume fraction of mica in the same metamorphic P and T conditions, while in the Sambagawa and the Shimanto metamorphic cherts the strain magnitudes decrease with increasing the mica fraction.

Hydrogen isotope study of the granitic rocks of the Ryoke belt, Central Japan

Among many Cretaceous granitic masses in the Ryoke belt of central Japan, two discordant plutons, Kisokoma and Shinshiro, and the largest batholith, Inagawa, were investigated for the D/H ratio in coexisting biotite and hornblende. The relationships between δD- and X Fe- [= Fe/(Fe + Mg)] values for coexisting biotite and hornblende in the three granitic masses are markedly different. In the Kisokoma pluton, tielines between coexisting biotite and hornblende on the δ D— X Fe diagram are distributed in a narrow area and slopes of the tielines are essentially parallel to that derived from T. Suzuoki and S. Epstein's experimental work. In the Shinshiro pluton the tielines are distributed in two areas though the slopes are also almost parallel to that of Suzuoki and Epstein. In the Inagawa batholith, most of the tielines are nearly vertical with highly scattered δ D-values. The X Fe-values of the minerals are ∼ 0.5 in the Kisokoma and Shinshiro plutons, and ∼ 0.7 in the Inagawa batholith. Such a difference in the δ D— X Fe relationship is explained by the different geological conditions of intrusion of the three granitic masses. Namely, the Kisokoma and Shinshiro plutons are discordant, and the erosion level of the former is shallow and that of the latter is deep. The Inagawa batholith intruded during the geologic movement and interacted with the country rocks. On the basis of Suzuoki and Epstein's experimental work, the δ D-value of the water in a single granitic magma can be estimated to be as low as −60 to −70‰ in the initial stage, and as high as ∼ −30‰ in the later stage, because of the early crystallization of biotite and hornblende, whose δ D-values are usually lower than the value of water in the magma. When the experimental results for hornblende by C.M. Graham et al. and those for biotite by Suzuoki and Epstein are combined, the temperature of the hydrogen exchange equilibration between biotite and magmatic water is calculated to be from 400° to 630°C for the Kisokoma pluton, and from 650° to 800°C for the Shinshiro pluton. It is impossible to estimate the equilibration temperature for the Inagawa batholith, because there are no experimental data on hydrogen isotope fractionation between hydrous silicates, such as biotite and hornblende, with higher X Fe-values than 0.7 and coexisting water.

Mesozoic strata and their soft-sediment deformation in the Ryoke belt, northern Kiso Mountains, central Japan

Mesozoic strata and their soft-sediment deformation in the Ryoke belt, northern Kiso Mountains, central Japan

Implications of mylonitic microstructures for the geotectonic evolution of the Median Tectonic Line, central Japan

The Median Tectonic Line (MTL), the most prominent onshore fault in Japan, demarcates the Cretaceous Hiji quartz dioritic gneiss of the Ryoke belt on the west from the high P/T type Sambagawa metamorphic rocks on the east in the Takato area. Toward the MTL, the Hiji gneiss grades into strongly mylonitized rocks characterized by grain-size reduction of quartz. In the mylonitic rocks, the development of fluxion banding (Sm) is remarkably influenced by the existence of K-feldspar, forming a myrmekitic intergrowth. Brittle microstructures indicative of truly cataclastic deformation are observed only in mylonitic rocks close to the MTL. Early deep-level ductile deformation apparently gave way to shallower, brittle deformation at a later stage. The attitude of stretching lineations ( Lm) and asymmetric microstructures observed in the mylonites suggest that sinistral strike-slip shearing with a subordinate component of vertical-slip took place during mylonitization in mid-Cretaceous time. The mylonitic rocks and their protolith, supposed to have constituted the eastern limb of the shear zone, were probably eroded out and lost by upheaval of the Sambagawa belt relative to the Ryoke belt.