S U M M A R Y The Ordos Block and the Qinling Orogen in central China are key regions that have been affected by Indo-Eurasian collision, convergence of the North China, South China and Siberian cratons, and subduction of the Pacific oceanic plate during the Phanerozoic. Investigation of the crustal structure in this area can therefore provide significant insights into the interactions among these tectonic blocks and plates. The Chinese Academy of Geological Sciences deployed ca. 249 portable broad-band seismic stations in this area from 2011 to 2016, and we retrieved P-wave receiver functions (RFs) from these new stations, and also from all other available permanent and portable seismic stations in China. Crustal thicknesses beneath the stations were retrieved using 1-D S-wave velocity models, which were obtained by linearized inversion of the RFs with initial reference models from regionalized S-wave velocities of surface wave inversions. We demonstrate with synthetic tests that this inversion scheme is superior to a jointly linearized inversion of RFs and surface waves. The results show that crustal thicknesses generally correlate with tectonic domains in the study region. Thus, thick crust is present beneath the Daba Shan region and the area close to the northeastern Tibetan Plateau, intermediate-thickness crust is present beneath the Ordos Block and the central North China Craton, and thin crust exists beneath the North China Plain. However, some notable exceptions exist. The crust beneath the central Qinling is thinner than that beneath the neighbouring Daba Shan region and even the Weihe Basin, implying that subduction and collision of the North China and South China cratons in the Palaeozoic did not produce a thick crust beneath the suture. The relatively thin crust beneath the eastern and central Qinling must be denser than that beneath neighbouring areas. All the basins around the Ordos Block have been considered to be graben. However, our results show that the crust beneath the basins along the southern and northern margins of the Ordos Block is thicker or the same thickness as the crust beneath the flanks of the basins, which suggests these basins are (or were) foreland basins.

Tectonic history of the Ordos Block and Qinling Orogen inferred from crustal thickness

Numerical investigation of effects of artificial excavation pit on transformation and runup processes of tsunami-like wave over fringing reef

The Crustal Anisotropy of West Ordos Block and Its Geodynamic Implications

Crustal structure beneath the central and western North China from receiver function analysis

Dredging materials from reef flats have become an important source of sand and aggregates for meeting the infrastructure needs of coral-lined shores in subtropical and tropical regions, especially for low-lying atoll islands. Dredging at the reef flats can generate artificial excavation pits, which not only have profound influences on coral ecological stability but also deeply affect the hydrodynamic characteristics of coral reefs. To deepen the understanding of the influence of excavation on the wave hydrodynamics of fringing reefs, the wave propagation, wave transformation, wave setup, and wave runup processes of regular waves on fringing reefs with artificial pits have been systematically analyzed using a non-hydrostatic numerical wave solver (NHWAVE). The effects of some significant factors have been carefully investigated. According to the study findings, the existence of artificial pits can result in a slight decrease in the wave height around the artificial pit. The time-mean maximum of wave runup height at the backreef slope can be reduced to some extent when the artificial pit is present. When placed close to the reef edge, the artificial pit can have noticeable effects on the hydrodynamic characteristics of fringing reefs, particularly the wave setup along the reef flat. It is hoped that the study findings can provide further reference for evaluation of the influences of artificial pits on the wave hydrodynamics of fringing reefs. 

Influence of Excavation Pits on the Wave Hydrodynamics of Fringing Reefs under Regular Waves

The low-lying reef islands distributed in the tropical and subtropical coastal regions are highly vulnerable to the devastating damages of surges and waves during the severe weather events. Over the past two decades, extreme waves have caused tremendous loss and damages to the tropical and subtropical coastal regions. Previous research has focused on the wave hydrodynamics of tsunami waves, as well as regular and irregular waves on the fringing reefs. The complex wave hydrodynamics of extreme waves on the fringing reefs are rarely studied. By applying the nonhydrostatic numerical flow solver (NHWAVE), transformation and breaking process of the crest- and trough-focused wave groups on the fringing reef are analyzed in this study. Influences of the major factors, i.e., water depth, significant wave height, peak wave period, forereef slope and backreef slope, and ridge width, are discussed in detail. The results show that there are complex interactions between the fringing reef and the focused wave group. Breaking waves of high intensity can form at the reef crest. Meanwhile, due to the wave breakings at the reef crest and bottom friction of the reef flat, the local wave height can be effectively reduced. Within the complex wave hydrodynamics of focused waves on the fringing reef, most of the wave energy can be dissipated. In addition, hydrodynamic difference between the crest- and trough-focused waves on the fringing reef is very limited. The research results of this study will further help researchers to better understand the wave hydrodynamics of extreme waves over the fringing reefs.

/pdf/numerical-investigation-on-hydrodynamic-processes-of-extreme-3bk305m3.pdf

Numerical Investigation on Hydrodynamic Processes of Extreme Wave Groups on Fringing Reef

The deep structure investigation in Ordos Block and surrounding regions is important to reveal the formation,evolution and destruction of North China Craton.Receiver function analysis is used to image the crustal structure along an N-S broadband seismic observation profile consisting of 32 temporary and permanent broadband seismic stations.The profile crosses the following geologic settings from north to south:Yinshan Oregon,Hetao Graben,Ordos Block,Weihe Graben,Qinling Oregon and Dabashan Oregon.Slant stack(H-κ)method is used to get the crust thickness and poison ratio.Also Kirchhoff migration method is used to image the Moho discontinuity in the study area.The results reveal that the crust thickness in Yinshan Oregon is about 42~44km and the poison ratio is about 0.27.The stable crustal structure indicates that the orogeny in Yinshan is driven by the upwelling of the material from the deep.The uplift of Moho in Hetao Graben reflects the upwelling of the lithospheric material.In Ordos Block,the average crust thickness is 41.2km and the poison ratio 0.27.The Moho slightly goes up from 43.5km in north to 39.0km in south.In the north part of Ordos Block there appears a low velocity layer in 20 km depth.Besides,there is an abrupt small-scale subsidence of Moho 36°N~37°N in the block.These two results challenge the common view that the crust structure in Ordos Block is simple and unbroken.In Weihe Graben the Moho bulge up to the depth of 30 km,which is supposed to be the result of the extrusion and upwelling of the Tibetan Plateau.In Qinling Oregon the average crust thickness is 38.5 km and the Moho varies slightly.In the Dabashan Oregon,the north part of Yangtze Plate,crust thickness attains to 54.5km which is distinctly larger than the North China Craton.

Crustal structures of Ordos block and surrounding regions from receiver functions

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Numerical Investigation on Hydrodynamic Processes of Extreme Wave Groups on Fringing Reef

Crustal structures of Ordos block and surrounding regions from receiver functions