Tsuyoshi Toyoshima

Evolution of the Hidaka metamorphic belt, northern Japan

The Hidaka metamorphic belt is a tilted island-arc assembly of crustal layers developed during early to middle Tertiary age. The P-T estimates of metamorphic rocks in the amphibolite and granulite facies reveal a relatively high geothermal gradient (40°C km−1), possibly caused by the large amount of gabbroic intrusions, at an early evolutional stage of the crust. The crust was subsequently displaced subhorizontally from north to south, giving rise to detachment of lower to upper crustal layers from the lowest crust, and to the formation of a duplex of crustal metamorphic layers. This was followed by dextral transpression in which the crustal layers were uplifted and tilted steeply eastward. Geological setting In Hokkaido, northern Japan, there are two Cretaceous arc-trench systems; one occupies the western part of Hokkaido and the other occupies the eastern part (Fig. 1). The western system consists of the Oshima granitic-volcanic terrane and the Sorachi-Yezo subduction-accretionary terrane on the eastern side where the Kamuikotan serpentinite melange with high-P/T schists occurs. Westward subduction occurred during early to late Cretaceous time (Okada 1982, Kiminami & Kontani 1983, Ishizuka et al. 1983). The eastern system is composed of the Tokoro subduction-accretion terrane which is situated on the western side of the Nemuro volcanic terrane. This northeast-ward subduction was active in the late Cretaceous (Sakakibara 1986, Sakakibara et al. 1986). Late Cretaceous accretionary complexes of both arc-trench systems are juxtaposed in the central part of Hokkaido where the Hidaka metamorphic belt occurs (Fig. 1). A collision between the two arc-trench

High fluorine pargasites in ultrahigh temperature granulites from Tonagh Island in the Archean Napier Complex, East Antarctica

Abstract Pargasites (F/(F+Cl+OH) ratio ( X F ) of up to 0.48) from Tonagh Island in Enderby Land, East Antarctica are closely associated with typical high-grade minerals such as orthopyroxene in quartzo-feldspathic, mafic, and ultramafic granulites, and is regarded as a stable mineral at the peak metamorphic conditions (>1100 °C) calculated for the ultrahigh-temperature Archean Napier Complex. Although experimental investigations have suggested that the upper thermal stability limit of F-free pargasite is below 1050 °C, thermodynamic calculations for the present pargasite+quartz assemblage indicate that the thermal stability limit of pargasite with X F =0.5 is about 150 °C higher than that of the hydroxyl end member. Fluorine substitution in the pargasite therefore allowed the mineral to survive the ultrahigh-temperature metamorphism at Tonagh Island. A positive correlation between the F content of pargasite and coexisting biotite indicates that the minerals approach chemical equilibrium in terms of F–OH distribution. Although the fluorine composition of pargasites ( X F =0.12–0.48) and bulk rock (300–2500 ppm) varies widely, the log( f H 2 O / f HF ) values calculated for these rocks are relatively constant (3.2–3.7), which is consistent with infiltration of an F-bearing fluid during prograde metamorphism. The infiltration of such a fluid is also supported by the higher bulk F content of most of the analyzed samples compared to those of continental and oceanic basaltic rocks, that is, F had been added from an external source. A positive correlation between bulk MgO and F content suggests that F may have been selectively trapped in high- X Mg pargasite in MgO-rich rocks.

Prograde and anatectic reactions in the deep arc crust exposed in the Hidaka metamorphic belt, Hokkaido, Japan

Abstract The Hidaka metamorphic belt is a tilted crustal assembly of a magmatic arc, exposing two third of a total thickness of the arc crust. The igneous activities and metamorphism took place during the Eocene in a tectonically stacked and thickened accretionary prism of the latest Cretaceous to earliest Tertiary age. The lower part of the exposed section consists mainly of mafic rocks with thin intercalations of pelitic and psammitic rocks metamorphosed under amphibolite and granulite and granulite facies, while the upper part consists almost of pelitic and psammitic rocks metamorphosed under amphibolite and greenschist facies, which are gradually changed to very weakly metamorphosed sedimentary rocks toward the top of the section. Reexamination of prograde metamorphism of pelitic granulite facies rocks confirmed a path of increasing temperature with a constant pressure or with slight decrease of pressure, associated with moderate shearing. Norite and diorite veining as a part of mafic intrusives were followed to give rise to the peak metamorphism associating incipient anatexis of pelitic and psammitic gneisses in the basal part of the exposed section. Due to the anatexis, small leucocratic patches or viens are formed, with orthopyroxene and cordierite as residual phases, cutting strong foliations of biotite with garnet and cordierite porphyroblasts. The mafic intrusives played an important role in the metamorphic evolution in the deep part of the Hidaka crust.

Geologic fault model based on the high-resolution seismic reflection profile and aftershock distribution associated with the 2004 Mid-Niigata Prefecture earthquake (M6.8), central Japan

The Mid-Niigata Prefecture earthquake in 2004 (MJMA 6.8) generated surface ruptures along the eastern rim of the Uonuma Hills. To elucidate the structural linkage between the surface ruptures and the source fault at depth, the high-resolution seismic reflection profile across the surface ruptures and nearby active faults, and the data of aftershock distribution are examined. The 5.2-km-long, high-resolution, depth-converted seismic section reveals an emergent thrust beneath the surface ruptures. A two-dimensional model of the fault geometry has been constructed based on the aftershock distribution and the shallow reflection profile. The development of the main geologic structure are well explained by forward modeling using a balanced cross-section method. In detail, the fault system generated the main shock dips at a steep angle (60°) below 5 km depth and more shallowly (30°) near the surface.

Carbonic fluids in ultrahigh-temperature metamorphism: Evidence from Raman spectroscopic study of fluid inclusions in granulites from the Napier complex, East Antarctica

Abstract We report the first quantitative compositional data on fluid inclusions in ultrahigh-temperature (UHT) granulites from the Napier Complex of Enderby Land, East Antarctica. Fluid inclusions in various high-grade minerals such as garnet, orthopyroxene and sapphirine from three UHT localities in the Amundsen Bay area were studied in terms of petrography and microthermometry as well as laser Raman spectroscopy. Measured melting temperatures of inclusions from all the three localities indicate that the trapped fluid phase is dominantly carbonic. Raman analyses confirmed a near pure CO2 composition with only minor dilutants such as N2 (<6.0 mol%), CH4 (<0.3 mol%), and H2O (<0.1 mol%). CH4-bearing fluid associated with sapphirine granulites suggests low oxygen fugacity ( fO2) conditions for the rocks, whereas CH4 was not detected from fluid inclusions in magnetite-bearing high-f O2 garnet granulite. The range of CO2 isochores computed from density measurements in fluid inclusions from the granulites pass through the peak P–T conditions of the Napier metamorphism (T= 1050–1150 °C, P=9–11 kbar) indicating synmetamorphic nature of the fluids. Inclusions in garnet from Bunt Island coexist with carbonate minerals (magnesite) and graphite along with dense CO2-rich fluid, indicating probable derivation from deep-seated primary magmatic sources. The ubiquitous association of carbonic fluids in the UHT mineral assemblages suggests CO2 influx during extreme crustal metamorphism of the Napier Complex. The carbonic fluid probably played an important role in transporting heat from mantle or mantle-derived magmas and in stabilizing the dry mineral assemblages.

Groundwater changes associated with the 2004 Niigata-Chuetsu and 2007 Chuetsu-oki earthquakes

The Geological Survey of Japan, AIST, has been monitoring groundwater in and around the Kinki and Tokai districts for earthquake prediction research. The Niigata Prefectural Office has also been observing groundwater for monitoring land subsidence in Niigata Prefecture. The 2004 Niigata-Chuetsu (MJMA 6.8) and 2007 Chuetsuoki (MJMA 6.8) earthquakes occurred in Niigata Prefecture, Japan, on October 23, 2004 and July 16, 2007, respectively. The two earthquakes have a similar magnitude, epicenter, and mechanism. At many of the observation wells, we detected changes in groundwater level or pressure related to the two earthquakes, but no clear precursory changes. At all of our observation wells in Niigata Prefecture, trend changes were observed after coseismic step-like changes for both of the earthquakes. At some of the stations in and around the Kinki and Tokai districts, coseismic trend changes and/or step-like changes were observed. The pattern of the changes were almost similar for the two earthquakes. Those changes were considered to be caused not by the static crustal deformation but by the ground shaking.

Early Proterozoic Tectonothermal Events in the Napier Complex, East Antarctica: Implications for the Formation of East Gondwana

Abstract Sm-Nd internal isochron ages involving retrograde garnet determined from three ultrahigh-temperature (UHT) gneisses taken from Tonagh Island, the western part of the Napier Complex, East Antarctica gave 1870±82 Ma, 1897±50 Ma and 1557±35 Ma. These ages are younger than the late Archaean timing of UHT metamorphism in the Napier Complex. The ca. 1900 Ma age is considered to reflect an important tectonothermal event in the Napier Complex including a tholeiite dyke intrusion. On the other hand, the ca. 1600 Ma age represents a thermal modification lacking signs of deformational events, and separates from the ca. 1900 Ma event. The related East Gondwana fragments such as the Rayner Complex in Antarctica and the Eastern Ghats Belt in India record extensive tectonothermal event of ca. 1400-1600 Ma, and rare indications of ca. 1900-2000 Ma. It is stressed that the assembly of East Gondwana including the Napier Complex, the Rayner Complex, and the Eastern Ghats Belt, if it existed, should be before ca. 1600 Ma, and may be traced back to ca. 2000 Ma of the supercontinent “Columbia” era.

Contrasting metamorphic records and their implications for tectonic process in the central Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

Abstract Metamorphic rocks in the central part of Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, are classified into three types based on petrological characteristics. (i) The Austkampane area preserves c. 800 °C and 0.5–0.6 GPa peak metamorphic conditions followed by decompression and subsequent isobaric cooling and later hydration (A-type). (ii) The Brattnipene and eastern Menipa area preserve peak P–T conditions of c. 800 °C and 0.7–0.8 GPa with subsequent isobaric cooling and later hydration (B-type). (iii) The area including Lunckeryggen, southern Walnumfjella and western Menipa preserves an amphibolite–facies peak metamorphic condition with signatures of prograde metamorphism (L-type), which are typically unaffected by the retrograde hydration event. Peak granulite–facies metamorphism of A- and B-type rocks are contemporaneous at c. 640–600 Ma, but a difference in the P–T paths between these rocks can be explained by thrusting of the A-type rock unit onto the B-type rock unit. By contrast, the timing of the metamorphism of the L-type rocks is significantly younger at c. 550 Ma, possibly related to the intrusion of pegmatites and granitoids. These metamorphic records in the central part of the Sør Rondane Mountains can be a test ground for the regional tectonic processes proposed for the orogeny related to Gondwana formation. Supplementary material: Representative mineral compositions are listed at www.geolsoc.org.uk/SUP18623

Pseudotachylytes, related fault rocks, asperities, and crustal structures in the Hidaka metamorphic belt, Hokkaido, northern Japan

Many pseudotachylytes and their related fault rocks are found in the Hidaka metamorphic belt representing an ancient crustal section. On the basis of field observations of the pseudotachylytes and related fault rocks, nature of seismogenic faulting in the Hidaka crust is examined. The field observations suggest the following conclusions. (1) Two structural types of pseudotachylytes are distinguished: layer-parallel and layer-oblique. The latter are scattered in the metamorphic belt, but the former occur only in the southern part of the metamorphic belt. (2) An abundance of the layer-parallel pseudotachylytes suggests that earthquakes occurred repeatedly and frequently in the southern part, where complicated and duplicated crustal structures occur with many low-temperature thin mylonite zones. The southern part with such crustal structures was an ancient seismogenic area containing asperities and having a radius of a few tens of kilometers in the Hidaka crust. In the seismogenic area, the layer-parallel pseudotachylytes resulted from seismic slip on the mylonitic foliation within the low-temperature mylonite zones with strong preferred orientation of micas. (3) The layer-parallel pseudotachylytes and the subsequent layer-oblique pseudotachylytes post-date the latest and very-low-temperature mylonitization in the metamorphic belt. The former pseudotachylytes formed just above the upper side of the brittle-plastic transition zone.

Heterogeneous material distribution, an important reason for generation of strain-localized mylonite and frictional slip zones in the Hidaka metamorphic belt, Hokkaido, Japan

Lithological heterogeneity of low P/T metamorphic rocks in southern area of Hidaka metamorphic belt (HMB) was formed through historical development of HMB while these rocks had been laid in ductile lower crust. Many strain-localized mylonite zones (<100 m in thickness) are preferentially developed within S-type tonalite and pelitic gneiss, which are characterized by a large modal amount of phyllosilicates (biotite+muscovite+chlorite) and quartz, compared to other lithofacies in HMB. Mylonitic foliations are more conspicuous with close to the center of the shear zone associated with increase in amounts of phyllosilicate minerals, indicating fluidenhanced weakening mechanisms were operated in plastic shear zones. Pseudotachylyte veins are observed exclusively in these mylonite zones, which were generated during exhumation stage of HMB. We conclude the seismic slip zones in southern HMB had been initiated in the ductile lower crust by concentration of localized plastic shear zones within the phyllosilicate- and quartz-rich lithofacies, which were heterogeneously formed by old metamorphic and magmatic events. Then these zones were further weakened by fluid-enhanced plastic deformation, and finally seismic slips occurred at the bottom of seismogenic upper crust, during exhumation of HMB.