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31.
Lodolo Emanuele Coren Franco Schreider Anatoly A. Ceccone Giulio 《Marine Geophysical Researches》1997,19(5):439-450
The southwestern part of the Scotia Sea, at the corner of the Shackleton Fracture Zone with the South Scotia Ridge has been investigated, combining marine magnetic profiles, multichannel seismic reflection data, and satellite-derived gravity anomaly data. From the integrated analysis of data, we identified the presence of the oldest part of the crust in this sector, which tentative age is older than anomaly C10 (28.7 Ma). The area is surrounded by structural features clearly imaged by seismic data, which correspond to gravity lows in the satellite-derived map, and presents a rhomboid-shaped geometry. Along its southern boundary, structural features related to convergence and possible incipient subduction beneath the continental South Scotia Ridge have been evidenced from the seismic profile. We interpret this area, now located at the edge of the south-western Scotia Sea, as a relict of ocean-like crust formed during an earlier, possibly diffuse and disorganized episode of spreading at the first onset of the Drake Passage opening. The successive episode of organized seafloor spreading responsible for the opening of the Drake Passage that definitively separated southern South America from the Antarctic Peninsula, instigated ridge-push forces that can account for the subduction-related structures found along the western part of the South Scotia Ridge. This seafloor accretion phase occurred from 27 to about 10 Ma, when spreading stopped in the western Scotia Sea Ridge, as resulted from the identification of the marine magnetic anomalies. 相似文献
32.
A seismic refraction study on old (110 Myr) lithosphere in the northwest Pacific Basin has placed constraints on crustal and uppermantle seismic structure of old oceanic lithosphere, and lithospheric aging processes. No significant lateral variation in structure other than azimuthally anisotropic mantle velocities was found, allowing the application of powerful amplitude modeling techniques. The anisotropy observed is in an opposite sense to that expected, suggesting the tectonic setting of the area may be more complex than originally thought. Upper crustal velocities are generally larger than for younger crust, supporting current theories of decreased porosity with crustal aging. However, there is no evidence for significant thickening of the oceanic crust with age, nor is there any evidence of a lower crustal layer of high or low velocity relative to the velocity of the rest of Layer 3. The compressional and shear wave velocities rule out a large component of serpentinization of mantle materials. The only evidence for a basal crustal layer of olivine gabbro cumulates is a 1.5 km thick Moho transition zone. In the slow direction of anisotropy, upper mantle velocities increase from 8.0 km s-1 to 8.35 km s-1 in the upper 15 km below the Moho. This increase is inconsistent with an homogeneous upper mantle and suggests that compositinal or phase changes occur near the Moho. 相似文献
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34.
Riccardo?Geletti Emanuele?LodoloEmail author Anatoly?A.?Schreider Alina?Polonia 《Marine Geophysical Researches》2005,26(1):17-28
The structural framework of the southern part of the Shackleton Fracture Zone has been investigated through the analysis of
a 130-km-long multichannel seismic reflection profile acquired orthogonally to the fracture zone near 60° S. The Shackleton
Fracture Zone is a 800-km-long, mostly rectilinear and pronounced bathymetric lineation joining the westernmost South Scotia
Ridge to southern South America south of Cape Horn, separating the western Scotia Sea plate from the Antarctic plate. Conventional
processing applied to the seismic data outlines the main structures of the Shackleton Fracture Zone, but only the use of enhanced
techniques, such as accurate velocity analyses and pre-stack depth migration, provides a good definition of the acoustic basement
and the architecture of the sedimentary sequences. In particular, a strong and mostly continuous reflector found at about
8.0 s two-way traveltime is very clear across the entire section and is interpreted as the Moho discontinuity. Data show a
complex system of troughs developed along the eastern flank of the crustal ridge, containing tilted and rotated blocks, and
the presence of a prominent listric normal fault developed within the oceanic crust. Positive flower structures developed
within the oceanic basement indicate strike-slip tectonism and partial reactivation of pre-existing faults. Present-day tectonic
activity is found mostly in correspondence to the relief, whereas fault-induced deformation is negligible across the entire
trough system. This indicates that the E–W-directed stress regime present in the Drake Passage region is mainly dissipated
along a narrow zone within the Shackleton Ridge axis. A reappraisal of all available magnetic anomaly identifications in the
western Scotia Sea and in the former Phoenix plate, in conjunction with new magnetic profiles acquired to the east of the
Shackleton Fracture Zone off the Tierra del Fuego continental margin, has allowed us to propose a simple reconstruction of
Shackleton Fracture Zone development in the general context of the Drake Passage opening. 相似文献
35.
采用金属潜在生态危害系数和危害指数的评估方法,评价了大亚湾海域重金属对海洋生态系统的潜在危害。结果表明,大亚湾海域重金属潜在生态危害轻微,而养殖海区和近岸海区的重金属潜在生态危害则相对较重,大亚湾海域重金属的潜在生态危害较珠江口外浅海轻。 相似文献
36.
人工鱼礁工程的风险评估 总被引:1,自引:0,他引:1
对台风暴潮影响下的嵊泗海域人工鱼礁工程做了风险评估,考虑了台风暴潮中出现的大浪和风暴潮减水对鱼礁联合作用的危害。风险评估分为危险灾害识别、失效概率计算、失效后果评估、风险准则评定和风险管理决策几个主要的步骤。在失效概率的计算中采用基于应用设计点的重点抽样法随机模拟的技术,这一随机模拟技术可以广泛的应用与海洋工程结构的风险评估当中。 相似文献
37.
38.
工程风险分析中的风险当量及其评价标准 总被引:7,自引:2,他引:7
目前,量化、半量化风险分析是工程风险分析的主流。在对工程进行量化和半量化风险分析时,无论采用什么方法,风险评价指标和评价标准都是关键的重要问题。只有在建立了统一的评价指标和评价标准体系的基础上,对工程进行的风险分析能够得出正确和一致的结论。文章通过对国内外风险分析理论和实践的研究和总结,提出了在工程风险分析中确定风险评价指标和评价标准的原则和方法。 相似文献
39.
40.
共转换点道集的抽取与转换波时变静校正 总被引:1,自引:0,他引:1
纵横波速度比固定时,同一地震道中各时间采样点对应不同的转换点位置,因此,常规的整体抽道方法只能抽取特定深度或特定层位条件下的共转换点道集,它不能保证所有P-SV转换波的反射点位置都在同一水平位置处。提出了一种精确抽取共转换点道集的新方法,运用这种方法抽取的共转换点道集中所有数据对应的转换点在地面的投影都对应于同一位置,因此,该道集是一种真正意义上的共转换点道集,在此基础上提出了一种解决短波长问题的转换波时变静校正方法,实际资料处理取得了好的效果。 相似文献