Secondary sedimentary structures of the coal beds and coal-bearing formations provide significant information for depositional environments as well as primary ones. However systematic geological investigation for them has not been reported except for washout. Succeeding to the previous reports (Muraoka, 1997a, b), secondary sedimentary structures which were observed by the writer are described and investigated in terms of their origins and environments in order to utilize to basin analysis for coal exploration. Also the origin of coal beds is attributed to the peat deposits of geologic age from the various phenomena observed in these sedimentary structures. Secondary sedimentary structures of the coal beds are subdivided and described as follows : 1) Washout : 1. Washout by the overlying sandstone bed; 2. Deep washout eroding a few coal beds; 3. Coastal washout; 4. Glacial washout. 2) Mudstone dyke and associated displacement. 3) Fissure filling. 4) Concentration of sulphur caused by bioturbation and degradation of peat. Secondary sedimentary structures of the coal-bearing formations are subdivided and described as follows : 1) Slumping : 1. Rotational slide of the cliff; 2. Normal slumping and reverse slumping; 3. Slumping of large scale which can be esteemed by slump deposits. 2) Slump deposits : 1. Sandstone gravel in mudstone; 2. Sedimentary autobreccia.
Cone-in-cone structure is observed in the Kosho Alternation Member of the Itsukaichimachi Group (Miocene) exposed along the Akigawa River in Akiruno City, Tokyo. This sedimentary structure has long been studied in Europe and North America but there remains many problems still in debate. Study of this structure, however, has mostly been ignored in Japan, and the present cone-in-cone structure was misinterpreted as stromatolites by the previous authors. We describe this material and briefly introduce studies in foreign countries in this occasion. One of the well-preserved cone-in-cone structures in the Kosho Alternation Member is covered with thin banded clay and calcareous ironstones of which iron minerals are mostly siderite. The present samples indicate that the structure was formed by crystallization of calcite in free from the stress field and fibrous crystal behaved similar to ice needles in the ground. Microscopic observation of the structure shows that calcite and siderite were almost syngenetically crystallized immediately below the sea-floor of more than -200m depth.
Gravity anomalies provide much information on subsurface structures, i.e., faults, depressions and dome structures. These structures are considered as an accumulation of the past crustal deformations. The subsurface structures, which are estimated by using gravity anomaly and other geophysical and geological information, can thus become a guidepost to discuss the tectonic model around low and/or high gravity anomalies. The eastern part of central Kyushu, Japan, including Beppu Bay is characterized by gravity low and has complicated structures, as an intersection of the Median Tectonic Line (MTL), the Oita-Kumamoto Tectonic Line (OKTL) and the Kurume-Hiji Line (KHL). Based on the subsurface structures estimated by gravity analysis, we tried to discuss the mechanical formation and the Quaternary tectonic history of this region. Results obtained show the possibility that this region experienced two crustal deformations; (1) the half-graben was formed along the OKTL before 1 Ma ago, and (2) tectonic basins in and around Beppu Bay were formed by right-lateral motions of the MTL and KHL, as a kind of pull-apart basins, in the last 1 Ma.
The Chichio fault, a segment of the Median Tectonic Line active fault system in eastern Shikoku, marks a southern range front of the Sanuki Range with well-defined geomorphic features related to its late Quaternary faulting. We have excavated multiple sites along the Chichio fault to date Holocene surface-rupturing earthquakes. A 4-m-deep trench excavated into floodplain sediments of the Higaidani River at Kamigirai, Ichiba Town, Tokushima Prefecture, contains stratigraphic evidence for the most recent and penultimate earthquakes on the Chichio fault. The fault trace which ruptured during the most recent earthquake offsets all the sediments except for an artificially modified zone immediately below the ground surface. A V-shaped silty deposit near the top of the fault trace is interpreted as cultivated soil for rice farming which filled a coseismic ground fissure associated with the most recent earthquake. From this deposit, we obtained a pottery fragment which was identified as the mould of a Buddhist artifact used around the 16th century A.D. This age constrains the time of the most recent faulting on the Chichio fault to be during or after the 16th century A.D. This earthquake may be correlated to the 1596 Keicho-Kinki earthquake as was proposed by Ishibashi (1989). However, we cannot preclude the possibility that the Chichio fault ruptured during an earthquake separated from the 1596 event. Another fault trace farther south which does not propagate upsection as high as the fault trace during the most recent event suggests the penultimate earthquake. A radiocarbon age and pottery fragments of Yayoi age near the top of the fault trace suggest the time of the penultimate surface-rupturing earthquake at around 2, 000 yr B.P. On the eastern projection of the faults exposed in the trench at Kamigirai, a series of rice paddy dikes are offset right-laterally approximately 7 meters. A contact between gravels and sands exposed on a plan view during an archeological excavation prior to our trenching was also offset right-laterally approximately 6 meters across the fault. These observations suggest that the amount of right-slip and vertical displacements associated with the most recent earthquake on the Chichio fault was 6-7 meters and about 1 meters, respectively.
The Edifice of Hiuchi Volcano was founded on the Mokake ignimbrite plateau which is directly underlain by the Nanairi pumice. They erupted from near the present summit successively about 350ka. On the plateau, the Ozorizawa cone was first established about 100 ka. Next, the Shibayasugura cone having twin peaks, Shibayasugura (2, 356 m) and Manaitagura (2, 346 m), was built shifting slightly southeast. At 19ka, the Jubeike lava flow and Kumazawa Tashiro lava dome were issued from the northeastern flanks. The latter was accompanied by a devastating blast. Southward-opening horseshoe depression is source of the Nushiri debris avalanche, which dammed up the Nushiri River and formed the Oze marsh about 8ka. The Akanagure lava dome and flows immediately occupied the depression. The youngest volcanic feature at Hiuchi is the Miike lava dome, 300m south of the Manaitagura peak. It emplaced during the 16th century, just before the Shirohikemizu flood descended the Hinoemata River on July 28, 1544. Hiuchi Volcano has erupted 17 × 1012 kg of magma since its birth, 350 thousand years ago. Average discharge rate is 4.9 × 1010 kg per thousand years, an order of magnitude smaller than that of the most active Japanese volcanoes.
Science does not advance at the same speed on its all facets, and is so in metamorphic petrology of Japan. Rock-forming mineralogy and phase petrology became Japanese tradition. Recognition of widespread partial melting in low-pressure metamorphic complex is helping to understand the evolution of crust. The analysis of chemical texture of rock-forming minerals helped to obtain reasonable estimation of P-T path of metamorphic complexes. Dating by K-Ar and Ar-Ar methods performed on as many samples as done for microprobe, greatly modified Japanese metamorphic geology. However experimental petrology, geochemistry of fluid, stable isotope studies and dating by ion-probe mass-spectroscopy are the fields where far more attention should be focussed.
Processes of the granulite facies overprinting on high-pressure (HP) and ultrahigh-pressure (UHP) rocks were considered by comparing the model and actual P-T paths. The model P-T paths based on the thermal structures below the continent-continent collision zone predict that the granulite facies overprinting on the HP and UHP rocks can occur in following three cases. 1) Exhumation at a moderate rate resulting in moderate heating during decompression, which forms a clockwise P-T path. 2) Infinitely fast exhumation of relatively high-T (>700°C) HP/UHP rocks. 3) Isobaric heating at lower-crust depths. Several HP rocks in the western gneiss region and the Bohemian massif recorded paths 1) and 2) and UHP rocks in these areas recorded the path 2). There is only one report of the path 3) in the Bohemian massif. P-T paths of the granulitized coesite-eclogite in eastern China is still under discussion. In the regional gneiss region, granulite facies overprinting on the HP/UHP rocks, i.e., a transition from HP/UHP rocks to the continental crustal material, seems to be not a rare phenomenon.
This paper considers the importance of “the process model” in petrology, taking a transition model of coesite inclusion as an example. The process model, as a new trend in petrology, deals a dynamics of physico-chemical processes in rocks and also in the crust, in contrast to the static analysis of the equilibrium state in the traditional petrology. It will provide a unified view and a perspective in interpretations of natural data, when combined with the results of the traditional petrology such as phase equilibrium analyses and pressure-temperature-time analyses. The process model stimulates the traditional petrology, leading to the establishments of more and more quantitative petrology.
Chronological information is indispensable for the detailed analyses of a temperature-pressure-time path of metamorphic rocks and for modeling of whole metamorphic processes from subduction to exhumation. The succession of prograde and retrograde metamorphic reactions should have been recorded by chemical zoning and reaction texture of metamorphic minerals including epidote group minerals and titanite. These minerals, containing substantial amounts of ThO2 (up to 2.5 wt%) and UO2 (up to 0.6 wt%), are useful for providing reliable chronological information on individual metamorphic reactions. CHIME (chemical Th-U-total Pb isochron method) is a nondestructive age determination method having high spatial resolution. Although the CHIME coupled with EPMA (electron probe micro-analyzer) is a powerful tool for subgrain dating of Th-and/or U-rich minerals, the EPMA is hard to analyze Pb in young epidote group minerals as well as titanite. The detection sensitivity of the micro-PIXE (Proton Induced X-ray Emission) analysis, a proton analogue of EPMA, is an order of magnitude higher than that of EPMA. Thus, the CHIME coupled with micro-PIXE will make it possible to obtain subgrain age data of epidote group minerals and titanite as young as 10Ma. Our analytical capabilities for the age determination are much greater than seemed possible ten years ago, they become certainly possible powerful tools for Earth Sciences as EPMA has been for the last twenty-five years.
There are two types of orogenic belts : the continent-continent collision type such as Alps and the subduction related (Cordilleran) type of southwestern Japan. Their metamorphic rocks have contrasting geochronological characteristics. Alpine metamorphic rocks have recorded three major metamorphic event : 1) the Cretaceous ultra-high and related high P/ T metamorphism in continental material such as the Sesia-Lanzo and the Pennitic Internal Crystalline Massifs, 2) the Eocene high P/T metamorphism in the ophiolites and calc-schists of the Mesozoic Tethys and 3) medium P/T metamorphism, in which both types of high P/ T metamorphic rocks were variably reset by Oligocene thermal events. However, almost all geochronological data reported from the Alpine metamorphic belt show mixed ages which are due to the mixture of minerals formed in the three metamorphic events, requiring a petrological and chronological re-scrutiny by both expert petrologist and geochronologist with a conscientious strategy. On the contrary, the southwest Japan has several metamorphic belts in which the rocks record generally mono-metamorphism, suggesting a simple history in comparison with Alpine rocks. However, recent study has revealed a complex history of the low P/ T metamorphic belt : the low P/ T metamorphism took place in the pre-dating accretionary complexes which have suffered the subduction related metamorphism or in the geologic unit consisting of the predating high grade metamorphic rocks. This gave us the discordant age relation similar to that in Alps. Thus, orogenic belts are so heterogeneous in geochronology, indicating that the belts have recorded the complicated history of earth crust. Conversely, the advanced geochronology and petrology will make it possible to peruse the complicated history of earth crust.
Recent discussions on partial melting in high-grade metamorphic rocks are briefly reviewed. Garnets in high-grade pelitic rocks commonly contain euhedral calcic plagioclase inclusions. In addition, they display compositional growth zoning for phosphorus as well as major elements (Fe, Mn, Mg and Ca) and other minor elements (e.g., Y), though modified to various degrees by volume diffusion. The phosphorus zonation is defined by a low-P core with a distinct high-P rim, and correlates with the presence or absence of euhedral calcic plagioclase inclusions. Apatite, xenotime and other phosphate minerals as well as K-feldspar are present in the rock matrix and as inclusions in the garnet rim, whereas these minerals are usually absent in the garnet core. This suggests phosphate undersaturation during the growth of the core. All these features are satisfactorily explained by the following partial melting reaction. Biotite + sillimanite + sodic plagioclase + K-feldspar + quartz + phosphates (+H2O) =granitic melt +garnet core+calcic plagioclase+rutile (or ilmenite). The garnet rim and the rock matrix may have formed from the granitic melt upon cooling. These data would be a new indicator for previous partial melting of pelitic gneisses and granulites.
Five problems of anatexites and granites have been discussed to study the generation of granitic body. The first is that partial melting takes place in non-equilibrium conditions originated from element-transfer in crystal and from melt-migration in migmatite. Minerals in non-equilibrium relation generally coexist in a migmatite. The second, the partial melting of migmatites is strongly affected by water-mobility in rocks and by increasing rate of temperature. The third, melt-convergence leads to occurrence of vein and to melt-depletion around veins. The fourth, the mechanism of melt-transfer is different between voluminous melt and little melt. Voluminous melt (>50 % melt) can move together with containing crystals. The fifth, there are some limiting relations between crustal thickness to melt and the size and volume of granitic body.
The author picks up the study topics of metamorphic rocks in consideration of the paradigm of plume tectonics. First of all, He refers to minerals which are considered to have derived from the transitional zone and the lower mantle, i.e., inclusions in diamond in kimberlite. A problem of transitional zone origin of the Alpe Arami peridotite is introduced. Subsequently, ultra high pressure (UHP) metamorphic rocks exhumed by continent-continent collision are also briefly introduced. In addition, he reviews a discussion on diamond of possible UHP origin in the New England Fold Belt (NEFB), Australia. The author emphasizes that tectonics of regional metamorphic belts in global scale relates closely to the formation and subsequent fragmentation of a Late Proterozoic supercontinent, Rodinia, and amalgamation forming new continents. Chronological study, especially by SHRIMP, is effective to estimate linkage among fragmented continental blocks, and an example in northern Japan is referred. Introduced is the structural studies on the metamorphic belts along the plate collision zones which reveal non-rigidness of the lithosphere. Active deformation in the mountains is explained by not only extrinsic force along the plate boundary but also intrinsic force (gravitational potential energy) acting on the continental lithosphere. Study on deformation of metamorphic rocks composing the lower crust is important to test the effect of the intrinsic force and to reveal the influences of regional surface subsidence behind the subduction zone and mountain building along the collision zones. Modelling by experimental work and computer simulation increase the weights in studies of tectonics of metamorphic belts.
The strategic justification for supporting scientific activities has largely depending on the understanding that research provided both national security in a time of global hostility involving the improvement of the economy, greater social equity, better health, better race relations, and enhancement of environmental quality, and widespread health benefits. In terms of curriculum in the Earth Sciences, the important point here is that study courses must evolve to accommodate the changing, enlarging body of knowledge as well as the ultimate career needs of the future degree holders. Growing subdisciplines needed in America include hydrology, hydrogeology, neotectonics, continental margin oceanography, engineering geology, geologic hazards, environmental science, aqueous geochemistry, and material sciences. Metamorphic petrologists are especially well-equipped with the fundamental knowledge from physics and chemistry as well as geology to treat the problems society wishes to address and solve. The situation provides us with a natural advantage, but to realize it, we must move our emphasis away from the traditional topics of metamorphic petrology.
The model concept and the role of modeling were reviewed in the context of geological sciences. Modeling is a powerful tool for analyzing and understanding natural phenomena. Any model should be presented in a quantitative way, so that it may be tested and evaluated with reference to observational data sets. The role of quantitative models may be summarized as follows : (1) A model provides tools for measuring the nature in terms of well defined physical parameters, (2) It provides means for evaluating relative importance among conceivable elementary processes in a complex phenomenon. (3) It provides a theoretical basis for experimental planning and natural observations.
Metamorphic petrology in the modern sense started at 1950s with Eskora's concept and open system thermodynamics of Thompson and Korzhinskii. The time scale of 50 years after foundation of the discipline seems to be slightly long compared with disciplines of biological sciences. The aim of the discipline, then may be sharpened to be evaluated the possibility of future development of this field. This should be performed from the viewpoints of science management and science marketing and contributions. Here I will try to evaluate these in the metamorphic petrology with structural geology disciplines.