The northern Ryukyu Arc has active backarc rift, neutral-stress forearc, and active accretionary prism. The Okinawa Trough has been shaped by the episodic rifting in the backarc. Paleostresses were inferred in this study from mesoscale faults in Neogene forearc sediments called the Miyazaki Group, southeast Kyushu in the northern Ryukyu Arc. The forearc stress changed from compressional to extensional from the latest Miocene through Early Pliocene time. The stress history is concordant with the transition in tectonic regime from folding to rifting in the backarc. The transition in the stress state occurred simultaneously also with trenchward movement of the volcanic front. These phenomena suggest that the subducting slab under southern Kyushu became steeper in the Early Pliocene. Extensional tectonics ceased sometime in the late Pliocene or early to mid-Pleistocene, concordant with the counterclockwise change of subducting direction of the Philippine Sea Plate. 相似文献
Cretaceous granitic rocks are widely distributed in the northern to central parts of Kyushu Island, southwest Japan arc, and are divided into two types, granite and tonalite-granodiorite. These rocks are clearly defined by their mineral assemblage, that is, the granite contains mostly muscovite and/or garnet and does not have hornblende, whereas the tonalite-granodiorite is always observed with hornblende. Many workers suggest that most of the granite has low initial Sr isotopic ratios (SrI90 Ma: 0.7052-0.7059) whose values are similar to those of tonalite-granodiorite (SrI90 Ma: 0.7050-0.7056).
The SiO2 contents of the granite (67.9 to 78.0 wt.%) are higher than that of tonalite-granodiorite (54.2 to 68.2 wt.%). The granite is characterized by peraluminous compositions, while the tonalite-granodiorite shows metaluminous to peraluminous compositions. The petrogenetic relation between granite and tonalite-granodiorite is examined by a modal batch melting model, which referred to results of already published experiments. The result of the model suggests that the petrogenesis of the peraluminous granitic magma can be explained by an anatexis of metaluminous tonalite in the tonalite-granodiorite under lower to middle crustal conditions and variable fH2O.
Rocks of tonalite to granodiorite in a subduction system generally have low initial Sr isotopic ratios (<0.705), which imply that the magma derived from a highly metaluminous origin such as mafic magma, mafic lower crust and/or subducting slab. In this study, it is regarded that the peraluminous granitic magma can be directly produced by an anatexis of metaluminous tonalite. Therefore, peraluminous granite with low initial Sr isotopic ratios may imply to a first step of recycling of the granitic layer in an active plate margin. 相似文献
Volcanic breccia is a representative heterogeneous rock. As a preliminary study for determining its mechanical properties in laboratory and field experiments, its material properties and structural features were described. Three-dimensional clast-size distribution was also estimated for the study site using the theory of stereology. With this distribution determined, a numerical simulation was performed to investigate the relationship between the volumetric proportion of clasts in a sample and the areal proportion and maximum length of clasts on a lateral surface of the sample. The following observations were made: (1) coefficients of variation for volumetric proportion are dependent on the size of the sample, and became large when the size of the sample is small; (2) samples with a small coefficient of variation can be selected at any size if a restriction is placed on the ratio, Dmax/D0, of the maximum length of clasts to the size of the sample; and (3) the volumetric proportion of clasts in each sample can be estimated from the observed areal proportions within an error range that depends on the whole volumetric proportion. 相似文献
Abstract Upper Paleozoic to Mesozoic sedimentary sequences of chert (Liminangcong Formation), clastics (Guinlo Formation) and a number of limestone units (Coron Formation, Minilog Formation and Malajon Limestone) constitute the accretionary complex of the North Palawan block, Philippines. Based on chert-to-clastic transitions from different stratigraphic sequences around the Calamian Islands, three accretionary belts are delineated: the Northern Busuanga Belt (NBB), the Middle Busuanga Belt (MBB) and the Southern Busuanga Belt (SBB). The accretion events of these belts along the East Asian accretionary complex, indicated by their sedimentary transitions, began with the Middle Jurassic NBB accretion, followed by the Late Jurassic MBB accretion and the Early Cretaceous SBB accretion. Several limestone blocks that formed over the seamounts became juxtaposed with chert–clastic sequences during accretion. During the Late Cretaceous, accretion-subduction along the East Asian margin subsided bringing tectonic stability to the region. The seafloor spreading during the mid-Oligocene disconnected the entire North Palawan block from the Asian mainland and then migrated southward. The collision between the North Palawan block and the Philippine Island Arc system in the middle Miocene generated a megafold structure in the Calamian Islands as a result of the clockwise turn of the accretionary belts in the eastern Calamian from originally northeast–southwest to northwest–southeast. 相似文献
The spatial representativeness of heat fluxes on the basis of single-tower measurements, and the mechanism of the so-called energy imbalance problem, are investigated through numerical experiments using large-eddy simulation (LES). LES experiments are done for the daytime atmospheric boundary layer heated over a flat surface, as a best-case scenario completely free of sensor errors and the uncertainties of field conditions. Imbalance is defined as the deviation of the `turbulent' heat flux at a grid point from the horizontally averaged `total' heat flux. Both the theoretical and numerical results of the present study suggest the limitation of single-tower measurements and the necessity of horizontally-distributed observation networks.The temporally averaged `turbulent' flux based on a point measurement systematically underestimates the `total' flux (negative imbalance). This is attributed to local advection effects caused by the existence of turbulent organized structures (TOS), whose time scale is much longer than that of thermal plumes. The temporal and spatial change of TOS patterns causes low-frequency trends in the velocity and temperature data resulting in large scatter of the flux estimates. The influences of geostrophic wind speed, averaging time, observation height, computational domain size and resolution on tower-measured fluxes are also discussed. Finally, it is suggested that a weak inhomogenity in surface heating may reduce the negative bias of flux estimates. 相似文献
Stable carbon and oxygen isotope measurements of biogenic carbonate provide information for reconstructing past oceanic environments. In particular, 18O/16O ratios correlate with the temperature and salinity of seawater and 13C/12C is a proxy for dissolved inorganic carbon in seawater and symbiont photosynthesis. Here, we report 13C/12C and 18O/16O values for skeletons of corals (genus Porites) with various growth rates. In faster-growing corals, 13C/12C and 18O/16O showed out-of-phase annual fluctuations. In slower-growing corals, the isotopes fluctuated in phase. We developed a simple vector notation to show two patterns of 13C/12C annual fluctuation, each with a different offset in relation to 18O/16O annual fluctuation. The phase offset between 13C/12C and 18O/16O annual fluctuations depends on the relative intensities of kinetic isotope effects on calcification and metabolic isotope effects such as photosynthesis. This model might improve our ability to infer past climate and oceanographic conditions from coral skeletons. 相似文献
Analytical data for minor and trace elements published or communicated to us, on seventeen GSJ (Geological Survey of Japan) reference samples, "Igneous rock series" received up to April 1994 are compiled. The data were evaluated statistically in consideration of analytical methods. Based on the selected available data, 1994 recommended and proposed values for 65 minor and trace elements are presented. 相似文献
Airborne aerosol collections were performed over Wakasa bay (36°00′N, 135°30′E) in March and Kumano open sea (34°00′N, 136°50′E) and Seto (35°10′N, 137°10′E) in July 2001 at altitudes between 1.0 and 5.8 km. The particles were individually analyzed using transmission electron microscopy (TEM). Relatively large mineral-dust (mostly clay) particles were abundant in the March samples. They also dominated in July in the mid-troposphere higher than 4 km altitude, whereas sea salt and ammonium sulfate were more abundant at lower altitudes. Ca-coated grid samples show many traces of aqueous sulfate droplets. The proportions of former sulfate droplets to the total collected particles apparently increased with increasing relative humidity at the time of sampling. TEM analysis revealed that a significant fraction of these former droplets enclose mineral-dust particles as well as sea salt, soot, and fly ash. Some enclose mixtures of mineral-dust, sea-salt, soot, and fly ash particles. The results provide evidence that mineral dust from the Asian continent could acquire coatings of sulfate while being transported in the free troposphere. The mineral-dust particles probably acquired the sulfate coatings either through heterogeneous uptake of gaseous SO2 and subsequent oxidation or through coagulation with cloud or fog droplets. The presence of the mixed particles in sulfate droplets also indicates that aggregation of particles of different origins occurred through cloud processing. Such sulfate-coated dust particles would affect cloud formation, precipitation, and chemistry of the free troposphere. 相似文献