In order to study the tidal current ridges, a three-dimensional numerical model is, for the first time, applied to studying the hydrodynamic circumstance with Houbolt’s spiral assumption in the radial sandbank area. It successfully reveals that the radial sandbanks are molded by to-and-fro tidal current as well as the subtransverse circulation current, and that both contribute to maintaining the interior dynamic balance of the sandbanks. It is also found that the subtransverse circulation current would not always appear in pair within the ridges. These discoveries enrich the theoretical results in hydrodynamics of tidal current ridges. 相似文献
Reverse-time migration has attracted more and more attention owing to the advantages of high imaging accuracy, no dip restriction, and adaptation to complex velocity models. Cross-correlation imaging method is typically used in conventional reverse-time migration that produces images with strong low-frequency noise. Wavefield decomposition imaging can suppress such noise; however, some residual noise persists in the imaging results. We propose a 2D multidirectional wavefield decomposition method based on the traditional wavefield decomposition method. First, source wavefields and receiver wavefields are separated into eight subwavefields, respectively. Second, cross-correlation imaging is applied to selected subwavefields to produce subimages. Finally, the subimages are stacked to generate the final image. Numerical examples suggest that the proposed method can eliminate the low-frequency noise effectively and produce high-quality imaging profiles. 相似文献
The Yanshan movement/orogeny has been proposed for 90 years, which is of special significance in the history of geological research in China. This study conducted a review by synthesizing major achievements regarding episodic deformation features, sedimentary and magmatic records of the Yanshan orogeny in China, and clarified the episodic tectono-magmatism and its geodynamic origins. The tectonic implications of the Yanshan orogeny are discussed in the context of global plate tectonics and supercontinent reconstruction. Lines of evidence from structural, sedimentary and magmatic data suggest that the Yanshan orogeny represents a regional-scale tectonic event that affected the entire China continent in late Mesozoic period. Numerous age and structural constraints consistently indicate that the Yanshan orogeny was initiated in the Jurassic(at ～170±5 Ma). and was characterized by alternating stages of crustal shortening at ～170–136 Ma, crustal extension at ～135–90 Ma, and weak shortening at ～80 Ma. The 170–136 Ma crustal shortening was reflected in the generation of two regional stratigraphic unconformities(the Tiaojishan and Zhangjiakou unconformities), which were initially named the A and B episodes of "the Yanshan Orogeny" by Mr.Wong Wenhao in 1928. Geodynamically, the Yanshan orogeny in East Asia was associated with nearly coeval oceanic subduction and continental convergence in the Paleo-Pacific, Neo-Tethys, and Mongol-Okhotsk tectonic domains. As a consequence, three giant accretionary-collisional tectonic systems were formed along the continental margins of East Asia, i.e., the Mongol-Okhotsk, Bangonghu-Nujiang, and SE China subduction-and collision-related accretionary systems. The Yanshan orogeny induced widespread crustal-scale folding and thrusting, tectonic reactivation of long-lived zones of crustal weakness,and extensive magmatism and mineralization in intraplate regions. Based on the time principle of supercontinent assembly and break-up, we propose that the mid-Late Jurassic multi-plate convergence in East Asia might represent the initiation of the assembly of the Amasia supercontinent, and the Yanshan orogeny might be the first "stirrings" that is a prerequisite for the birth of the Amasia supercontinent. 相似文献
Saindak is one of the typical porphyry Cu deposits (PCDs) in the Chagai magmatic arc in Pakistan. Ore-forming porphyries at Saindak PCD are mainly composed of tonalite. Here, we use geochemistry of apatite enclosed in plagioclase phenocrysts from the ore-forming tonalite to constrain the releasing and recharging processes of S and Cl in the underlying parental magma chamber during PCD mineralization. Although apatite inclusions have homogeneous intra-grain S and Cl compositions, there is significant inter-grain S and Cl variations in apatite inclusions located from core to rim in the hosting plagioclase. Such inter-grain S and Cl variation in apatites are coupled with the core-to-rim trends of An, FeO and Mg contents of the hosting plagioclase phenocryst. It indicates that the Saindak PCD likely formed by episodic injection of primitive magmas during the growth of an underlying magma chamber, rather than by one major injection or by addition of mafic melt derived from different source region. Each primitive melt injection introduced essential ore-forming materials such as S and Cl, which were rapidly and effectively released to the coexisting fluids, causing mineralization. Once primitive melt injection stops, signaling the end of growth of underlying magma chamber, mineralization will cease quickly although the hydrothermal system can still survive for a long time. However, the later released fluids are relatively depleted in ore-forming materials, and thus have lower capability to generate mineralization. Accordingly, predominant porphyry-type mineralizations occurred during the growth rather than waning stage of a magmatic system.
