首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
The nature of the crust of the Okinawa Trough is different from that of the continental shelf in the East China Sea. The crust beneath the Trough is in transformation from continental to oceanic and the depth of MOHO in the northern section of the Trough is deeper than in the southern section. Thick sedimentary strata of Neogene and Pleistocene ages are deposited in the Okinawa Trough, and divided into three layers: the upper horizontal layer, the middle layer lightly folded and the lower deformed layer. They were formed in Pleistocene, Pliocene, and Miocene to Paleogene, respectively. The tectonic movement in the southern section is stronger than that in the northern section. Somevolcanic seamounts appear on the bottom of the Trough. On both slopes of the Trough are developed many normal faults and the intrusive igneous rocks. The Okinawa Trough, the back-arc basin,, is an embryonic marginal basin in rifting and spreading. The formation of the Okinawa Trough started in the early Pliocene. The transform  相似文献   

2.
  相似文献   

3.
This paper presents finite element modeling （FEM） to simulate the present-day stress field and crustal deformation using NE-SW structural section in the central Seismic Gap region of the Garhwal Himalaya. Our study deals with the effect of geometrical characteristics and rock layer parameters on the upper crust. Modeling results show that two types of tectonic regimes developed in the central Seismic Gap region： the geotectonics of the northern part has been controlled by regional compression, whereas southern part is characterized by regional extension. Correspondingly, thrust faults are induced in the northern part and normal faults are extensively developed in the southern front. Those evidences noticeably indicate that the compressive tectonic environment of the Himalaya becomes change into the extensional tectonic regime in its front. The computed shear stress accumulation along the northern fiat of Main Himalayan Thrust （MHT） implies that considerable amount of interseismic stress is building up along the MHT system in the Himalaya, which ultimately release through the possible future great Himalayan earthquake （M 〉 8）. The comparison between our modeled stress field, faulting pattern and horizontal shortening rate with the distribution of the microseismic events, focal mechanism solutions, active faulting and GPS data in the central Seismic Gap region shows good agreement.  相似文献   

4.
The Okinawa Trough is a very active tectonic zone at the margin of the Northwest Pacific and is typical of back-arc rifting at the young stage of tectonic evolution. Many scientists from Japan,China, Germany, France, the U. S.A. and Russia have done a lot of geologic and geophysical investigations there. It is well known that the Okinawa Trough is an active back-arc rift with extremely high heat flow, very strong hydrothermal circulation, strong volcanic and magmatic activity, frequent earthquakes,rapid subsidence and rifting, well-developed fault and central graben. But up to now, there are still some important tectonic problems about the Okinawa Trough that require clarification on some aspects such as the type of its crust, its forming time, its tectonic evolution, the distribution of its central grabens, the relationship between its high heat flow and tectonic activity. Based on the data obtained from seismic sur-vey, geomagnetic and gravity measurements, submarine sampling and heat flow measurements in the last 15 years, the author discusses the following tectonic problems about the Okinawa Trough: (1) If the Okinawa Trough develops oceanic crust or not. (2) Is the South Okinawa Trough tectonically more active than the North Okinawa Trough with shallower water and few investigation data on it. (3) The formation time of the Okinawa Trough and its tectonic evolution. The Okinawa Trough has a very thin continental crust. Up to now, there is no evidence of oceanic crust in the Okinawa Trough. The North, Middle and South Okinawa Trough are all very strongly active areas. From 6 Ma B.P. , the Okinawa Trough began to form. Since 2 Ma, the Okinawa Trough has been very active.  相似文献   

5.
This paper reports on our research on the gravity data around the Ryukyu T-A-BA system, and presents the results obtained and the calculation methods, such as, Moho calculation, spectrum analysis, smooth filtering, analytic extension, horizontal derivation, correlative analysis, etc. used in this study. In contrast to the information obtained from seismic profiles, the analyses show that oceanic and continental crusts demarcated by the Ryukyu T-A-BA system are very different in gravitational and tectonic characteristics. The structures in the T-A-BA region are very complicated and linearity of the gravity field and structure is very obvious. The Okinawa Trough with its two groups of fault structures in their respective NW and NNE strikes is an active neo-spreading back are basin whose development and evolution is greatly influenced by faulting.  相似文献   

6.
The Liquine-Ofqui Fault Zone （LOFZ） of southern Chilean Andes is one of the largest active strike-slip fault zones. There is an ongoing debate regarding the origin of the stress field along the LOFZ due to its complex geometry. This paper represents a study of the origins of the LOFZ regional stress field. Stress fields are calculated by finite element （FE） analysis. The two possible stress origins, i.e., oblique plate convergence and ridge collision/indenter tectonics of Chile ridge against Peru-Chile trench, have been emphasized in the present study. Three types of boundary conditions for the three particular models have been applied to calculate stress fields. Models are assumed to be elastic and plane stress condition. Modeling results are presented in terms of four parameters, i. e., orientation of maximum horizontal stress （σHmax）, displacement vector, strain distribution, and maximum shear stress （ τmax ） contour line within the model. The results of the first model with oblique plate convergence show inconsistency between the geometric shape of the LOFZ and the distribution of the four parameters. Although more realistic results are obtained from the second model with normal ridge collision, there are few coincident in the LOFZ geometry and regional stress field. The third model with normal and oblique ridge collision is reasonable in understanding the origin of stress field and geometrical condition in the lithosphere of the LOFZ.  相似文献   

7.
The northern margin of the South China Sea, as a typical extensional continental margin, has relatively strong intraplate seismicity. Compared with the active zones of Nanao Island, Yangjiang, and Heyuan, seismicity in the Pearl River Estuary is relatively low. However, a ML4.0 earthquake in 2006 occurred near Dangan Island(DI) offshore Hong Kong, and this site was adjacent to the source of the historical M5.8 earthquake in 1874. To reveal the seismogenic mechanism of intraplate earthquakes in DI, we systematically analyzed the structural characteristics in the source area of the 2006 DI earthquake using integrated 24-channel seismic profiles, onshore–offshore wide-angle seismic tomography, and natural earthquake parameters. We ascertained the locations of NW-and NE-trending faults in the DI sea and found that the NE-trending DI fault mainly dipped southeast at a high angle and cut through the crust with an obvious low-velocity anomaly. The NW-trending fault dipped southwest with a similar high angle. The 2006 DI earthquake was adjacent to the intersection of the NE-and NW-trending faults, which suggested that the intersection of the two faults with different strikes could provide a favorable condition for the generation and triggering of intraplate earthquakes. Crustal velocity model showed that the high-velocity anomaly was imaged in the west of DI, but a distinct entity with low-velocity anomaly in the upper crust and high-velocity anomaly in the lower crust was found in the south of DI. Both the 1874 and 2006 DI earthquakes occurred along the edge of the distinct entity. Two vertical cross-sections nearly perpendicular to the strikes of the intersecting faults revealed good spatial correlations between the 2006 DI earthquake and the low to high speed transition in the distinct entity. This result indicated that the transitional zone might be a weakly structural body that can store strain energy and release it as a brittle failure, resulting in an earthquake-prone area.  相似文献   

8.
The Longmenshan fault is a thrust fault which runs along the base of the Longmen Mountains in Sichuan province,southwestern China.The southern segment of the fault had two distinct responses to the Ms 8 Wenchuan and Ms 7 Lushan earthquakes.This study determines characteristics of the structural geology of the Longmenshan fault to evaluate how it influenced the two aforementioned earthquakes.This research was done within a Geoinformation Technologies(Gi T) environment based on multi-source remote sensing and crustal movement data extracted from the Global Positioning System(GPS).The spatial distribution of the southern segment of the Longmenshan fault zone was comprehensively analyzed to study both earthquakes.The study revealed that the Wenchuanand Lushan earthquakes occurred on two relatively independent faults.In addition,there was a nearly constant-velocity crustal movement zone between the two epicenters that probably had a compressive stress with slow motion.Furthermore,the central fault and a mountain back fault gradually merged from north to south.The Lushan earthquake was not an aftershock of the Wenchuan earthquake.The research showed that fault zones within 30–50 km of State Highway 318 are intensive and complex.In addition,crustal movement velocity decreased rapidly,with a strong multi-directional shear zone.Thus,activity in that zone was likely stronger than in the northern part over the medium to long term.  相似文献   

9.
MAGMATIC EVOLUTION OF OKINAWA TROUGH DURING ITS EARLY SPREADING STAGE   总被引：2，自引：0，他引：2  
The Okinawa Trough is different from other typical backarc spreading basins because the acidic vol-canic rock(pumice)is extensively distributed in it.Systematic mineralogical and petrochemical study on arepresentative core(Z14—5)to characterize the magmatic evolution of the Trough during its early spreading stage showed that the pumice magma was originally from the mantle,but underwent full crystallizationdifferentiation and was possibly contaminated by crust-derived material.With time,the acidic volcanic ac-tivities of the Okinawa Trough have an evolutional tendency of shifting to relatively basic magma activity.With further spreading of the Trough the magmatic activity will be intensified and the crust of the Troughwill develop from the transitional type to the oceanic type.  相似文献   

10.
This article presents the evidence in support of the direct magma degassing as the principal mechanism of volatiles releasing into the hydrothermal fluids in the Okinawa Trough, as contrastcd to the argument for the hydrothermal stripping of volatiles from the volcanic rocks. Laser Raman microprobe and stepped-heating techniques arc employed to determine the compositions and contents of the volatiles in pumices in the middle Okinawa Trough. The results show that the volatiles are similar to the gases in the hydrothermal fluid,s and hydrothermal minerals in composition, the mean percent content of each component and variation trend. This indicates the direct influence of magma degassing on the hydrothermal fluids. In addition, the contents of volatiles in pumices are rather low and do not support the hydrothermal stripping as the main mechanism to enrich the fluids with gases. The results are consistent with the idea that the direct magma degassing is more important than hydrothermal stripping in supplying gases to the hydrothermal fluids in the Okinawa Trough.  相似文献   

11.
《山地科学学报》2020,17(2):397-409
The vertical distribution of vegetation types along an elevational gradient in mountain areas largely depends on the elevational changes in air temperature and humidity. In this study, we presented the seasonal and diurnal variations in the elevational gradients of air temperature and humidity on the southern and northern slopes in the middle Tianshan Mountain Range using data collected throughout the year via HOBO data loggers. The measurements were conducted at 12 different elevations from 1548 to 3277 m from September 2004 to August 2005. The results showed that the annual mean air temperature decreased along the elevational gradients with temperature lapse rates of(0.71±0.20)°C/100 m and(0.59±0.05)°C/100 m on the northern and southern slopes, respectively. The annual mean absolute humidity significantly decreased with increasing elevation on the northern slope but showed no significant trend on the southern slope. The annual mean relative humidity did not show a significant trend on the northern slope but increased with increasing elevation on the southern slope. The mean air temperature lapse rate exhibited significant seasonal variation, which is steeper insummer and shallower in winter, and this value varied between 0.37°C/100 m and 0.75°C/100 m on the southern slope and between 0.30°C/100 m and 1.02°C/100 m on the northern slope. The mean absolute and relative humidity also exhibited significant seasonal variations on both slopes, with the maximum occurring in summer and the minimum occurring in winter or spring. The monthly diurnal range of air temperature on both slopes was higher in spring than in winter. The annual range of air temperature on the southern slope was higher than that on the northern slope. Our results suggest that significant spatiotemporal variations in humidity and temperature lapse rate are useful when analyzing the relationships between species range sizes and climate in mountain areas.  相似文献   

12.
The major aim of this study is to put forwad a valid model to explain the-gravity anomalies observed over the Okinawa Trough which the crustal structure and a reasonable model of the downgoing slab themselves cannot explain. The result of inversion indicates that the lithosphere above the center of the trough thins out to 45% of the normal thickness (63 km) of the lithosphere of the East China Sea.Further, the originations of lithospheric force are discussed. The tensional stress of about 160 bar arising from density contrast in the Okinawa Trough is recognized to be the most important causative factor in the stretching of the trough.  相似文献   

13.
Biodiversity patterns of free-living marine nematodes were studied using specific,taxonomic and phylogenetic diversity measures in the southern Yellow Sea,China.The results showed that the average of Shannon-Wiener diversity index(H′) in the study area was 3.17.The higher values were distributed in the east part of Shandong coastal waters and north part of Jiangsu coastal waters,while the lower values were distributed in the southern Yellow Sea Cold Water Mass(YSCWM).The average of taxonomic diversity(Δ) was 62.09 in the study region.The higher values were distributed in the transitional areas between the coastal areas and the southern YSCWM,while the lower values were distributed near the north part of Jiangsu coastal waters and the YSCWM.Results of correlation analysis of species diversity and taxonomic diversity showed that some of the two kinds of diversity index were independent,which suggested that combining the two kinds of diversity indices can reflect the ecological characteristics better.A test for 95% probability funnels of average taxonomic distinctness and variation in taxonomic distinctness suggested that Station 8794(in the YSCWM) was outside of the 95% probability funnels,which may be due to the environmental stress.Results of correlation analysis between marine nematodes biodiversity and environmental variables showed that the sediment characteristics(Md? and Silt-clay fraction) and phaeophorbide a(Pha-a) were the most important factors to determine the biodiversity patterns of marine nematodes.  相似文献   

14.
The climatology subduction rate for the entire Pacific is known, but the mechanism of interannual to decadal variation remains unclear. In this study, we calculated the annual subduction rates of three types of North Pacific subtropical mode waters using a general circulation model (LICOM1.0) for the period of 1958-2001. The model experiments focused on interannual variations of ocean dynamical processes under daily wind forcings and seasonal heat fluxes. The mode water formation region was defined by a potential vorticity minimum at outcrop locations. The model results show that two subduction rate maxima (>100 m/a) were located in the Subtropical Mode Water (STMW) and the Central Mode Water (CMW) formation regions. These regions are consistent with a climatologically calculated value. The subduction rate in the Eastern Subtropical Mode Water (ESTMW) formation region was smaller at about 75 m/a. The subduction rate shows clear interannual and decadal variations associated with oceanic dynamic variabilities. The average subduction rate of the STMW was much smaller during the period of 1981-1990 compared with other periods, while that of the CMW had a negative anomaly before 1975 and a positive anomaly after 1978. The variability agreed with Ekman and geostrophic advections and mixed layer depths. The interannual variability of the subduction rate for the ESTMW was smallest during 1970-1990, as a result of a weak wind stress curl. This paper explores how interannual signals from the atmosphere are stored in different parts of the ocean, and thus may contribute to a better understanding of feedback mechanisms for the Pacific Decadal Oscillation (PDO) event.  相似文献   

15.
An annual investigation on phytoplankton communities was conducted to reveal the effects of nutrients on phytoplankton assemblages in Lake Taihu,East China. A total of 78 phytoplankton taxa were identified. Phytoplankton biomass was higher in the northern part of the lake than in the southern part. Cyanobacteria and Bacillariophyta alternated dominance in the northern area,where algal blooms often appear,and co-dominated in the southern area. In the northern part,the proportions of cyanobacteria and Bacillariophyta varied significantly in total biovolume,both along the phosphorus(P) gradient,and between total nitrogen levels(≤3 mg/L and 3 mg/L TN). The proportions of cyanobacteria and Bacillariophyta had no signif icant variations in total biovolume along P and N(nitrogen) gradients in the southern part. Correlation analysis and CCA results revealed that P was the key factor regulating phytoplankton community structure. Nitrogen was also important for the phytoplankton distribution pattern. It was concluded that nutrient structure was heterogeneous in space and shaped the distribution pattern of phytoplankton in the lake. Both exogenous P and internally sourced Prelease needs to be considered. N reduction should be considered simultaneously with P control to efficiently reduce eutrophication and algal blooms.  相似文献   

16.
The Kaikoura earthquake on November 14,2016 is one of the largest and most complex earthquakes in New Zealand since 1947.Despite the fact that it has ruptured about 12 separate faults,triggered 2132 aftershocks within one week of the mainshock and induced considerable stress changes,few studies have been conducted to comprensively investigate the characteristics.The current study examines the horizontal and vertical displacements as well as the stress and gravity changes,aftershock distributions and also find out whether these changes affect the surrounding regions along the complex fault systems.The study covers the entire area affected by the Kaikoura event,which includes the northern part of the South Island and the southern part of the North Island.The dislocation theory was employed to evaluate the coseismic slip model on the multiple faults.The displacement results revealed that the maximum horizontal displacement is about 6 m and the vertical about 2 m,which are reasonably consistent with earlier study findings.Besides,the stress and gravity changes are quite complicated and inhomogeneous as evidenced by our coseismic model,demonstrating the complexity of the Kaikoura earthquake as well.Almost all the aftershocks are distributed in places where the stress and gravity change are found to be significant.In order to investigate the stability of our stress change models,we applied different friction coefficients and receiver fault parameters.The results justify the friction coefficient(μ=0.4)and the receiver fault parameters(230°,70°,150°)are suitable to define good stress change estimates.According to the stress change results at 15 km depth,the northern parts of the mainshock region,Hundalee fault,Humps fault and Jordan thrust areas together with the Wellington area are closer to failure and situated in a seismic risk zone.The multidimensional analysis adopted in this paper is helpful for making decisions and applications of stress and gravity change models in assessing seismic hazards.  相似文献   

17.
The Okinawa Trough is a natural laboratory for the study of air-sea interaction and paleoenvironmental change. It has been demonstrated that present offshore export of particles in the bottom nepheloid layer occur primarily with downwelling from the northeast winter monsoon, which is inhibited by a transverse circulation pattern in summer. This current system was very different during the Last Glacial Maximum owing to low sea level(-120 m) and exposure of a large shelf area. We collected sediment core Oki01 from the middle Okinawa Trough during 2012 using R/V K exue No. 1 to elucidate the timing and cause of the current system transition in the East China Sea. Clay mineral, dry density, and elemental(Ti, Ca) composition of core Oki01 was analyzed. The results indicate that clay minerals derived mainly from the Huanghe(Yellow) and the Changjiang(Yangtze) Rivers during 16.0–11.6 ka, and the modern current system in the East China Sea formed beginning in the early Holocene. Therefore, mixing of East China Sea continental shelf, Changjiang River and partially Taiwan Island sediment are the major contributors. The decrease of log(Ti/Ca) and alternating provenance since the early Holocene indicate less sediment from the East China in summer because of resistance of the modern current system, i.e., a "water barrier" and upwelling. Conversely, sediment delivery persists in winter and log(Ti/Ca) indicates the winter monsoon signal since the early Holocene. Our evidence also suggests that sediment from Taiwan Island could be transported by the Kuroshio Current to the middle Okinawa Trough, where it mingles with winter monsooninduced export of sediment from the Changjiang River and East China Sea continental shelf. Although the present research advances understanding of the evolutionary history of paleoenvironmental change in the Okinawa Trough, more sediment cores should be retrieved over wide areas to construct a larger scenario.  相似文献   

18.
Based on a ship survey during January 1998, the characteristics of the flow, the thermohaline properties and the volume transport of the Arabian Sea are discussed. A strong westward flow exists between 10.5?N and 11?N, part of which turns to the south as the Somali current near the coast at about 10?N and the rest turns north. At the passage between the African continent and the So- cotra Island, the northern branch separates into two flows: the left one enters the passage and the right one flows eastward along the southern slope of the island. Off the island the flow separates once more, most of it meandering northeast and a small fraction flow- ing southeast. Volume transport calculation suggests that the tidal transport is one or two orders of magnitude smaller than the total transport in this region and it becomes more important near the coast. The average velocity of the flow in the upper layer (0-150 m) is about 20 cm s-1, with a maximum of 53 cm s-1 appearing east of the Socotra Island, and the subsurface layer (200-800 m) has an aver- age velocity of 8.6 cm s-1; the velocity becomes smaller at greater depths. The depth of the seasonal thermocline is about 100 m, above which there is a layer with well mixed temperature and dissolved oxygen. High-salinity and oxygen-rich water appears near the surface of the northern Arabian Sea; a salinity maximum and oxygen minimum at 100 m depth along 8?N testifies the subduction of surface water from the northern Arabian Sea. Waters from the Red Sea and the Persian Gulf also influence the salinity of the area.  相似文献   

19.
Based on the EOF analyses of Absolute Dynamic Topography satellite data,it is found that,in summer,the northern South China Sea(SCS) is dominated by an anticyclonic gyre whilst by a cyclonic one in winter.A connected single-layer and two-layer model is employed here to investigate the dynamic mechanism of the circulation in the northern SCS.Numerical experiments show that the nonlinear term,the pressure torque and the planetary vorticity advection play important roles in the circulation of the northern SCS,whilst the contribution by seasonal wind stress curl is local and limited.Only a small part of the Kuroshio water intrudes into the SCS,it then induces a positive vorticity band extending southwestward from the west of the Luzon Strait(LS) and a negative vorticity band along the 200 m isobath of the northern basin.The positive vorticity field induced by the local summer wind stress curl is weaker than that induced in winter in the northern SCS.Besides the Kuroshio intrusion and monsoon,the water transports via the Sunda Shelf and the Sibutu Passage are also important to the circulation in the northern SCS,and the induced vorticity field in summer is almost contrary to that in winter.The strength variations of these three key factors(Kuroshio,monsoon and the water transports via the Sunda Shelf and the Sibutu Passage) determine the seasonal variations of the vorticity and eddy fields in the northern SCS.As for the water exchange via the LS,the Kuroshio intrusion brings about a net inflow into the SCS,and the monsoon has a less effect,whilst the water transports via the Sunda Shelf and the Sibutu Passage are the most important influencing factors,thus,the water exchange of the SCS with the Pacific via the LS changes dramatically from an outflow of the SCS in summer to an inflow into the SCS in winter.  相似文献   

20.
Chemical and isotopic data of the lava samples dredged in the southern Bach Ridge and the northern Italian Ridge of the Musicians Seamounts province, northeast of Hawaii. Although most of the samples analyzed are generally altered, a few are fresh. The latter exhibits similar geochemical and isotopic characteristics to normal MORB （Mid-Ocean Ridge Basalts）. There are systematic geochemical trends from hotspot to mid-ocean ridge in the province. Incompatible element and isotopic variations suggest that the flow field had at least two distinct parental magmas, one with higher and one with lower MgO concentrations. The two parental magmas could be related by a magma mixing model. The major and trace element modeling shows that the two parental magmas could not have been produced by different degrees of melting of a homogeneous mantle source, but they are consistent with melting of a generally depleted mantle containing variable volumes of embedded enriched heterogeneity enriched interbeds.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号