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1.
The Yarlung–Tsangpo Suture Zone (YTSZ), as the southernmost and youngest among the sutures that subdivides the Tibetan Plateau into several east–west trending blocks, marks where the Neo‐Tethys was consumed as the Indian continent moved northward and collided against the Eurasian continent. Mélanges in the YTSZ represent the remnants of the oceanic plate through subduction and collision. Mélanges are characterized by a highly sheared volcanoclastic or siliceous mudstone matrix including blocks of chert, claystone, and basalt. Detailed radiolarian analyses are conducted on the mélange near Zhongba County. Macroscopic, mesoscopic, and microscopic observations are combined in order to elucidate the relationships among age, lithology, and structure of blocks in the mélange. Reconstructed ocean plate stratigraphy includes Lower Jurassic limestone within the chert sequence accumulated at a depth near the CCD (Unit 2), Upper Jurassic thin‐bedded chert interbedded with claystone deposited in the wide ocean basin (Unit 3), and Lower Cretaceous chert with siliceous mudstone (Units 4 and 5), representing the middle parts of ocean plate stratigraphy. The results highlight the fabric of brecciated chert on mesoscopic scale, which is thought to be due to localized overpressure. The formation of mesoscopic and microscopic block‐in‐matrix fabrics in the mélange is proposed for the chert and siliceous mudstone bearing different extents of consolidation and competence during the progressive deformation of accreted sediments at shallow‐level subduction.  相似文献   
2.
We performed shock recovery experiments on an olivine‐phyric basalt at shock pressures of 22.2–48.5 GPa to compare with shock features in Martian meteorites (RBT 04261 and NWA 1950). Highly shocked olivine in the recovered basalt at 39.5 and 48.5 GPa shows shock‐induced planar deformation features (PDFs) composed of abundant streaks of defects. Similar PDFs were observed in olivine in RBT 04261 and NWA 1950 while those in NWA 1950 were composed of amorphous lamellae. Based on the present results and previous studies, the width and the abundance of lamellar fine‐structures increased with raising shock pressure. Therefore, these features could be used as shock pressure indicators while the estimated pressures may be lower limits due to no information of temperature dependence. For Martian meteorites that experienced heavy shocks, the minimum peak shock pressures of RBT 04261 and NWA 1950 are estimated to be 39.5–48.5 GPa and 48.5–56 GPa, respectively, which are found consistent with those estimated by postshock temperatures expected by the presence of brown olivine. We also investigated shock‐recovered basalts preheated at 750 and 800 °C in order to check the temperature effects on shock features. The results indicate a reduction in vitrifying pressure of plagioclase and a pressure increase for PDFs formation in olivine. Further temperature‐controlled shock recovery experiments will provide us better constraints to understand and to characterize various features found in natural shock events.  相似文献   
3.
The 137°E repeat hydrographic section for 50 winters during 1967–2016 has been analyzed to examine interannual to interdecadal variations and long-term changes of salinity and temperature in the surface and intermediate layers of the western North Pacific, with a particular focus on freshening in the subtropical gyre. Rapid freshening on both isobars and isopycnals began in the mid-1990s and persisted for the last 20 years in the upper main thermocline/halocline in the western subtropical gyre. In addition, significant decadal variability of salinity existed in the subtropical mode water (STMW), as previously reported for the shallower layers. An analysis of the 144°E repeat hydrographic section during 1984–2013 supplemented by Argo profiling float data in 2014 and 2015 revealed that the freshening trend and decadal variability observed at 137°E originated in the winter mixed layer in the Kuroshio Extension (KE) region and was transmitted southwestward to 137°E 1–2 years later in association with the subduction and advection of STMW. The mechanism of these changes and variations in the source region was further investigated. In addition to the surface freshwater flux in the KE region pointed out by previous studies, the decadal KE variability in association with the Pacific Decadal Oscillation likely contributes to the decadal salinity variability through water exchange between the subtropics and the subarctic across the KE. Interdecadal change in both the surface freshwater flux and the KE state, however, failed to explain the rapid freshening for the last 20 years.  相似文献   
4.
New geochemical and Sr–Nd isotopic data for the Iratsu eclogite and surrounding metamorphic rocks of the Sanbagawa belt, Japan, show that, while the protoliths of the metamorphic rocks formed in a variety of tectonic settings, the Iratsu body represents a deeply subducted and accreted island arc. The igneous protoliths of eclogites and garnet amphibolites were probably generated from a mantle source that had components of both a depleted mantle modified by slab-released fluid (as seen in a negative Nb anomaly) and an enriched mantle, similar to that of ocean island basalts (OIB). Fractional crystallization modeling indicates that the protoliths of some garnet clinopyroxenites from the Iratsu body are cumulates from a basaltic magma that crystallized under high O2 and H2O fugacities in the middle to lower crust. The source characteristics and crystallization conditions suggest that the protoliths of the Iratsu rocks formed in an oceanic island arc. Quartz eclogites from the marginal zone of the Iratsu body have geochemical signatures similar to turbidites from the Izu–Bonin island arc (as seen in a negative Nb anomaly and a concave REE pattern). The protoliths might be volcaniclastic turbidites that formed in a setting proximal to the oceanic island arc. Geochemical and isotopic signatures of the surrounding mafic schists are similar to normal (N-) and enriched (E-) mid-ocean-ridge basalt (MORB), and distinct from the rocks from the Iratsu body. The protoliths of the mafic schists likely formed in a plume-influenced mid-ocean ridge or back-arc basin. Pelitic schists from the surrounding rocks and pelitic gneisses from the marginal zone of the Iratsu body have evolved, continental geochemical signatures (as seen in a negative εNd(t) value (~?5)), consistent with their origin as continent-derived trench-fill turbidites.  相似文献   
5.
Understanding diverse characteristics of human mobility provides profound knowledge of urban dynamics and complexity. Human movements are recorded in a variety of data sources and each describes unique mobility characteristics. Revealing similarity and difference in mobility data sources facilitates grasping comprehensive human mobility patterns. This study introduces a new method to measure similarities on two origin–destination (OD) matrices by spatially extending an image‐assessment tool, the structural similarity index (SSIM). The new measurement, spatially weighted SSIM (SpSSIM), utilizes weight matrices to overcome the SSIM sensitivity issue due to the ordering of OD pairs by explicitly defining spatial adjacency. To evaluate SpSSIM, we compared performances between SSIM and SpSSIM with resampling the orders of OD pairs and conducted bootstrapping to test the statistical significance of SpSSIM. As a case study, we compared OD matrices generated from three data sources in San Diego County, CA: U.S. Census‐based Longitudinal Employer–Household Dynamics Origin–Destination employment statistics, Twitter, and Instagram. The case study demonstrated that SpSSIM was able to capture similarities of mobility patterns between datasets that varied by distance. Some regions showed local dissimilarity while the global index indicated they were similar. The results enhance the understanding of complex mobility patterns from various datasets, including social media.  相似文献   
6.
Gully erosion is a major cause of soil loss and severe land degradation in sub-humid Ethiopia. The objective of this study was to investigate the role and the effect of subsurface water level change on gully headcut retreat, gully formation and expansion in high rainfall tropical regions in the Ethiopian highlands. During the rainy seasons of 2017–2019, the expansion rate of 16 fixed gullies was measured and subsurface water levels were measured by piezometers installed near gully heads. During the study period, headcut retreats ranged from 0.70 to 2.35 m, with a mean value of 1.49 ± 0.56 m year−1, and average depth of the surface water level varied between 1.12 and 2.82 m, with a mean value of 2.62 m. Gully cross-section areas ranged from 2.90 to 20.90 m2, with an average of 9.31 ± 4.80 m2. Volumetric retreat of gully headcuts ranged from 4.49 to 40.55 m3 and averaged 13.34 ± 9.10 m3. Soil loss from individual gullies ranged from 5.79 to 52.31 t year−1 and averaged 17.21 ± 11.74 t year−1. The headcut retreat rate and sediment yield were closely related over the three study seasons. Elevated subsurface water levels facilitated the slumping of gully banks and heads, causing high sediment yield. When the soil was saturated, bank collapse and headcut retreat were favoured by the combination of elevated subsurface water and high rainfall. This study indicates that area exclosures are effective in controlling subsurface water level, thus reducing gully headcut retreat and associated soil loss.  相似文献   
7.
This work studies periodic solutions applicable, as an extended phase, to the JAXA asteroid rendezvous mission Hayabusa 2 when it is close to target asteroid 1999 JU3. The motion of a spacecraft close to a small asteroid can be approximated with the equations of Hill’s problem modified to account for the strong solar radiation pressure. The identification of families of periodic solutions in such systems is just starting and the field is largely unexplored. We find several periodic orbits using a grid search, then apply numerical continuation and bifurcation theory to a subset of these to explore the changes in the orbit families when the orbital energy is varied. This analysis gives information on their stability and bifurcations. We then compare the various families on the basis of the restrictions and requirements of the specific mission considered, such as the pointing of the solar panels and instruments. We also use information about their resilience against parameter errors and their ground tracks to identify one particularly promising type of solution.  相似文献   
8.
The Izumi Group in southwestern Japan is considered to represent deposits in a forearc basin along an active volcanic arc during the late Late Cretaceous. The group consists mainly of felsic volcanic and plutonic detritus, and overlies a Lower to Upper Cretaceous plutono‐metamorphic complex (the Ryoke complex). In order to reconstruct the depositional environments and constrain the age of deposition, sedimentary facies and U–Pb dating of zircon grains in tuff were studied for a drilled core obtained from the basal part of the Izumi Group. On the basis of the lithofacies associations, the core was subdivided into six units from base to top, as follows: mudstone‐dominated unit nonconformably deposited on the Ryoke granodiorite; tuffaceous mudstone‐dominated unit; tuff unit; tuffaceous sandstone–mudstone unit; sandstone–mudstone unit; and sandstone‐dominated unit. This succession suggests that the depositional system changed from non‐volcanic muddy slope or basin floor, to volcaniclastic sandy submarine fan. Based on a review of published radiometric age data of the surrounding region of the Ryoke complex and the Sanyo Belt which was an active volcanic front during deposition of the Izumi Group, the U–Pb age (82.7 ±0.5 Ma) of zircon grains in the tuff unit corresponds to those of felsic volcanic and pyroclastic rocks in the Sanyo Belt.  相似文献   
9.
Micro-scale distributions of trace and minor elements in, for example, coral skeletons are crucial as geochemical tracers of past environmental conditions, because they have the inherent advantage of accounting for confounding diagenetic and physiological effects. To extract reproducible paleoceanographic records from coral skeletons, a selective measurement of specific ultrastructures at high spatial resolution is required. Compared to warm-water reef-building corals, such data are limited in cold-water corals and, to the best of the authors’ knowledge, the latter have to date not been examined by means of micro-X-ray fluorescence. This technique was used for micrometer-scale imaging of P, Mg, Sr, O, and Fe intensities (counts per unit time) in a fossil specimen (as yet unknown age) of the cold-water coral Desmophyllum sp. from surface sediments of the NW Pacific. Cross plots confirmed that the micro-XRF signals were associated with corresponding trends in elemental concentration (ppm). Two major structural components of the septum—centers of calcification (COCs) and the surrounding fibrous aragonite portion—differed in composition. The COCs were characterized by higher intensities of P and Mg (650 and 220 counts per 5 s, respectively), and lower intensities of Sr (2,800) and O (580; corresponding values for the fibrous aragonite are 370, 180, 3,300 and 620 counts per 5 s, respectively). Oxygen intensity values were mostly homogeneous, but slightly lower in COCs and substantially higher in a well-defined patch in the fibrous aragonite. The mostly homogeneous P signals in the fibrous aragonite confirm the utility of this structural component and of coral septa in general for tracer studies of oceanic P. Nevertheless, spot occurrences of elevated P (>950 counts per 5 s) spanning tens of micrometers in specific parts of the fibrous region of the septum would cause overestimates of oceanic P, and should evidently not be overlooked in future research. The distribution of Fe showed no correlation with P, indicating no significant contamination in the form of P-bearing diagenetic ferromanganese precipitates. Such hotspots plausibly reflect the presence of other mineral phases, such as crystalline hydroxylapatite inclusions or contamination with organic material. The P signal intensity was positively correlated with Mg (r=0.553, p<0.001), and negatively with Sr (r=–0.489, p<0.001) and O (r=–0.311, p<0.001). There was no discernible evidence of control by water temperature in the Sr distribution pattern. These findings establish micro-X-ray fluorescence as a highly suitable pre-screening tool in cold-water coral sclerochronology, which can serve to refine sampling strategies without sample damage, and complement other micrometer-scale spatial distribution analyses of elements (notably, Ca) based on well-known approaches involving micro-milling, electron microprobes, secondary ion mass spectrometry, and laser ablation.  相似文献   
10.
Devolatilization reactions during prograde metamorphism are a key control on the fluid distribution within subduction zones. Garnets in Mn-rich quartz schist within the Sanbagawa metamorphic belt of Japan are characterized by skeletal structures containing abundant quartz inclusions. Each quartz inclusion was angular-shaped, and showed random crystallographic orientations, suggesting that these quartz inclusions were trapped via grain boundary cracking during garnet growth. Such skeletal garnet within the quartz schist formed related to decarbonation reactions with a positive total volume change (?V t > 0), whereas the euhedral garnet within the pelitic schists formed as a result of dehydration reaction with negative ?V t values. Coupled hydrological–chemical–mechanical processes during metamorphic devolatilization reactions were investigated by a distinct element method (DEM) numerical simulation on a foliated rock that contained reactive minerals and non-reactive matrix minerals. Negative ?V t reactions cause a decrease in fluid pressure and do not produce fractures within the matrix. In contrast, a fluid pressure increase by positive ?V t reactions results in hydrofracturing of the matrix. This fracturing preferentially occurs along grain boundaries and causes episodic fluid pulses associated with the development of the fracture network. The precipitation of garnet within grain boundary fractures could explain the formation of the skeletal garnet. Our DEM model also suggests a strong influence of reaction-induced fracturing on anisotropic fluid flow, meaning that dominant fluid flow directions could easily change in response to changes in stress configuration and the magnitude of differential stress during prograde metamorphism within a subduction zone.  相似文献   
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