Crust In China Research Articles

Composition and Evolution of Continental Crust at Orogenic Belts: Constraints From a 3‐D Crustal Model of Southeast China

AbstractThe chemical composition of continental crust is fundamental and crucial to understanding its origin and evolution. Orogenic belts make up about 30% of the global continental crust, yet their chemical compositions are still poorly constrained. In this study, we report an overall estimate for the continental crust composition of a collisional orogenic belt terrane: the Southeast China. The estimate is obtained through a 3‐D crustal model combining a lithology proportion model and probability density function models for element concentrations of various lithologies. Based on our model, the Southeast China crust is amongst one of the most evolved continental crust composition estimates (71.0% SiO2 content for the upper crust and 64.7% SiO2 for the bulk crust). A strong correlation between SiO2 and the chemical index of alteration (CIA) of different composition models for upper continental crust (UCC) indicates that chemical weathering is a controlling factor for the development of a felsic UCC. In contrast, weak correlations of SiO2 with Ninorm (15.9*Ni/Al2O3) and CIA suggest that igneous differentiation and chemical weathering act together to modify the composition of the bulk continental crust. Orogenic belt terranes like the Southeast China can develop a highly evolved crustal composition, especially for the upper crust, due to extensive intracrustal reworking and differentiation, enhanced chemical weathering, and potential subsequent delamination and removal of thickened crust root. Therefore, an overall estimation of the average composition of continental crust should take into account the more evolved composition of orogenic belts.

National-Scale Cobalt Geochemical Mapping of Exposed Crust in China

Geochemical background is the basis of anomaly evaluation in mineral exploration and environmental investigation. However, the background value obtained from secondary media is inevitably affected by weathering, transportation, and sorting, which leads to secondary depletion or enrichment of chemical elements. This problem can be avoided by the geochemical mapping of the exposed crust. In this paper, more than 38,000 samples of rocks data were collected, and an original method was presented for the first time to produce the cobalt geochemical map of exposed crust across the whole of China. Using a geological map grid of 1:200,000 as the basic calculation unit, the area and content of rock types in each unit were calculated, and then the grid was assigned using the area-weighted average method. Based on this, an geochemical contour map was drawn in ArcGIS. The results show that the median of the exposed crust is 9.74 mg/kg, which is lower than that of the whole crust and soils. This value is explained by the vertical heterogeneity of the crust and the secondary enrichment, respectively. Co anomalies and anomalous centers of exposed crust are distributed in southwest, north, and northwest China, where they are primarily related to Permian Emeishan Large Igneous Province (ELIP), Pacific plate, and the big mantle wedge (BMW), Archean metamorphic basic rocks, and ophiolite belts.

Distribution characteristics of typical geological relics in the Western Sichuan Plateau

Abstract The Western Sichuan Plateau (WSP), located in the eastern margin of the Qinghai–Tibet Plateau, is the most strongly deformed region of the continental crust in China. Frequent tectonic movements shape the unique topography and landform of the WSP and have also produced abundant geological heritage resources. Based on the existing geological heritage survey data in Sichuan Province, the nearest index analysis method of employing a regional spatial point model was used to reveal the distribution rules and the genetic mechanism of typical geological relics in the WSP for the first time. Results indicate that the formation and distribution of geological relics in the WSP are generally controlled by tectonic movement and supplemented by the comprehensive action of external forces such as flowing water. Their distribution shows a condensed spatial distribution pattern and extends along the strike of a fault zone and river strike strip. Finally, based on the characteristics of geological relics in the WSP, some suggestions on the protection and development of regional geological relics were put forward.

Researches in biological soil crust of China: A review

Biological soil crust (BSC), a complicated assemblage of cyanobacteria, green algae, diatoms, lichens, mosses, soil microbes and other related microorganisms cemented with soil particles with special architecture, covers more than 40% and even as much as 70% living surface in arid and semi-arid regions. Being an important ecosystem engineer and indicator of desert ecosystem health, BSC plays vital roles in linking the surficial biotic and abiotic components in arid and semi-arid regions and the related studies become a hotspot in the biological-geographic crossing studies in the earth surface process. However, most studies related to BSC came from abroad and mainly focused on hot desert, cold desert, polar and tundra as well as European grassland at the beginning of this century, little information available from Chinese temperate and central Asia desert. In this paper, we reviewed the research achievements in BSC by Chinese researchers since 2000 in the following aspects: (1) The composition, distribution and succession of BSC across Chinese temperate deserts, in particular, the responses of crustal species distribution pattern to climatic, soil properties and vascular plant cover at different spatial scales; (2) eco-physiological responses of BSC to environmental stress, climatic change as well as to natural and anthropological disturbances, large changes in rainfall regimes, UV-B radiation, nitrogen addition, salt and other stresses could result in the conversion of BSC types and further contribute to the changes in the structure and functioning of desert ecosystems; (3) the critical roles of BSC in the soil ecological and hydrological processes, including the dramatic influences of BSC on soil stability, C and N cycling, and other physiochemical properties, as well as infiltration, dew entrapment, soil surface evaporation and soil moisture; (4) the relationships between BSC and vascular plants and soil animals, such as the responses and underlying mechanisms of seed retention, germination, establishment and survival of vascular plants to the succession of BSC, and BSC creating a suitable niche and food web for soil animal and microorganisms, maintaining soil biodiversity; and (5) the cultivation and inoculation of components within BSC as new material applied for soil fertilization and desertification reversal, 6 cyanobacteria and 3 moss species inhabiting BSC were selected from different desert areas and successfully cultured in laboratory and then identified to enhance the stability, fertility and other physiochemical properties of the upper sand soil after their inoculation in the field. Finally, based on the analysis of the new trends in research vision, means and ideas of the international BSC study, we pointed out the key points for future research in BSC.

A source-depleted Early Jurassic granitic pluton from South China: Implication to the Mesozoic juvenile accretion of the South China crust

Source-depleted granites were rarely reported in South China. Hereby we identified such a granitic pluton, the Tiandong pluton, at Northeastern Guangdong province in Southeastern (SE) China. Whole-rock Sr-Nd and zircon Hf isotopes of the Tiandong granites both revealed obviously depleted source signatures, with initial isotopic values of initial 87Sr/86Sr=0.7032–0.7040, εNd(t)=1.1–1.5, and εHf(t)=6–13, respectively. Zircon U-Pb dating implied the granite was intruded in Early Jurassic (188Ma). The dominant minerals of the Tiandong granite consist of K-feldspar, plagioclase, quartz and biotite, with accessory mineral assemblage of apatite+zircon+magnetite. Based on the mineralogy and the depleted isotopic signature, the granites chemically show I-type affinity such as low Zr+Nb+Ce+Y (131.6 to 212.2), 104×Ga/Al (2.12–2.27), A/CNK values <1.1 (0.97–1.03), corundum molecule <1 (0–0.55) and extremely low P2O5 contents (0.05wt%). The one-stage and two-stage depleted mantle Nd model ages (TDM=0.89 to 0.84Ga, T2DM=0.88 to 0.85Ga) are consistent. TDM(Hf) values of 0.31–0.63Ga are also indistinguishable from T2DM(Hf) values of 0.35–0.75Ga. The Nd and Hf isotopic compositions confirm that the Tiandong granites are juvenile crustal accretion but decoupled Nd-Hf isotopic systems. The juvenile crust is likely to originate from a mixed source of the primary asthenospheric mantle and the subordinate EMII. Combined with early studies of adjacent rocks, we propose that the early Jurassic (~200–175Ma) magmatism as evidenced by the Tiandong granites might be driven by upwelling of asthenosphere and subsequent underplating of mafic melts in an intra-plate extensional setting as a response to far-field stress during early stage subduction of the paleo-pacific plate.

S-wave velocities and anisotropy of typical rocks from Yunkai metamorphic complex and constraints on the composition of the crust beneath Southern China

In order to constrain the interpretation of seismic data from receiver functions and deep profiles of the crust beneath southern China (Cathaysia and Yangtze blocks), we have measured S-wave velocities (Vs) and splitting as a function of hydrostatic confining pressure up to 650MPa for 22 representative samples (i.e., granite, diorite, felsic gneiss and mylonite, amphibolite, schist, and marble) from the Yunkai metamorphic complex (China) that represent the crystalline basement beneath the region. The experimental data were combined with electron backscattering diffraction (EBSD) analysis of rock-forming minerals to constrain variations of Vp/Vs ratios and understand the origin of seismic anisotropy. The crusts beneath the Yangtze and Cathaysia blocks have different average thicknesses (H=35.4±6.3km and 29.8±1.8km, respectively) but display almost the same Vp/Vs values (1.73±0.08 and 1.74±0.04, respectively). These ratios correspond to an average of bimodally distributed granitic and gabbroic lithologies which are dominant, respectively, in the upper and lower crusts, instead a homogeneous andesitic composition of the overall crust. Positive and negative correlations between H and Vp/Vs occur in west and east parts of southern China, respectively. The negative correlation indicates basaltic underplating from a partially molten mantle wedge above the subducting Pacific plate into the southern China crust, whereas the positive correlation implies that much larger thinning strain has taken place in the high temperature mafic lower crust (high temperature) than in the low temperature felsic upper crust during Mesozoic-Cenozoic tectonic extension.

Elemental and Sr–Nd isotopic compositional disparity of riverine sediments around the Yellow Sea: Constraints from grain-size and chemical partitioning

To obtain a better understanding of the source compositions of the river sediments around the Yellow Sea and their relationship with source rocks, elements and strontium-neodymium (Sr–Nd) isotopes of different grain-sizes (silt and clay populations) and chemical (labile and residual phases) fractionations in riverine sediments were studied extensively. These results clearly revealed a systematic compositional disparity between Korean river (KR) and Chinese river (CR) sediments, especially in the residual (detrital) fraction. The geochemical dissimilarity between these might reflect inherited signatures of their source rocks but with minor control from chemical weathering. In particular, the remarkable enrichment of some elements (iron (Fe) and magnesium (Mg)) and the behavior of large ion lithophile elements (e.g., barium (Ba), potassium (K) and Sr) during weathering as well as less-radiogenic Sr isotopic compositions implies that CR sediments might be weathering products of relatively more mafic rocks, with abundant ferromagnesian and plagioclase feldspar minerals, compared with KR sediments derived from silicic granites with relatively higher quartz and potassium feldspar contents. This different petrological rationale is clearly evident in an A–CN–K diagram, which estimated the source rock of CR sediments as granodioritic, a composition that reflects accurately the average composition of weathered continental crust in China. The recognition of such geochemical systematics in two river sediments, especially in grain-size and chemically partitioned data, may contribute to the establishment of provenance tracers for the Yellow Sea and East China Sea sediments with multi-sources as well the dust deposition in the western Pacific.

Geochronology and geochemistry of felsic xenoliths in lamprophyre dikes from the southeastern margin of the North China Craton: implications for the interleaving of the Dabie–Sulu orogenic crust

Lamprophyre dikes emplaced in a Jurassic granite at the southeastern margin of the North China Craton (NCC) carry different types of xenoliths. Here, we report a combined study of zircon U–Pb ages and whole-rock geochemistry of the xenoliths, as well as an Ar–Ar age of the lamprophyre, providing constraints on the sources of the magmatism and tectonic evolution in the southeastern margin of NCC. Phlogopite from the lamprophyre dike gave a 40Ar/39Ar plateau age of 116.15 ± 0.33 Ma. The felsic xenoliths can be classified into three groups: monzogranite, banded biotite granitic gneiss, and garnet-bearing gneiss. The internal structures of zircons from the banded biotite granitic gneiss xenolith show complex growth patterns. The mantles of these zircons display low luminance with a concordant SHRIMP U–Pb age of 227 ± 10 Ma. The rims with lighter luminance provide a SHRIMP U–Pb age of 213 ± 10 Ma, both ages are identical within error and significantly different from the ages of the basement rocks in the surrounding Bengbu uplift. However, the age is identical to those of the ultrahigh-pressure metamorphic rocks in the adjacent Dabie–Sulu orogen. In addition, zircon mantle domains of the banded biotite granitic gneiss xenolith have low Th/U ratios (>0.01) and flat HREE patterns ((Yb/Dy)n < 10), which suggest growth in an assemblage with garnet during HP metamorphism. The rim domains show very low Th/U ratios (<0.01) and steep HREE patterns ((Yb/Dy)n > 10), implying growth during exhumation in the absence of a garnet. Our studies show that the banded biotite gneiss represents vestiges of the subducted South China crust injected into or thrust below the North China Craton and provides constraints on the process of underthrusting in a continental collision zone, as well as the Mesozoic tectonic history of the southeastern margin of the NCC.

Quantitative analysis of the shallow crustal tectonic stress field in China mainland based on in situ stress data

The latest hydraulic fracturing and stress relief measurement data in the Chinese mainland were collected. The total of 3856 data entries are measured at 1474 locations. The measured area covers 75–130°E and 18–47°N, and the depth range varies from surface to 4000meters depth, which generally includes each active tectonic block of China and each segment of North–South seismic belt. We investigated the tectonic stress field by removing the effect of gravity. For this, we assume lateral constraints and Heim’s rule. The gravity contribution is removed by using the assumption of lateral constraint and Heim’s rule. Our results show: (1) the maximum and the minimum horizontal principal stress σH, σh and the vertical stress σV in the shallow crust of China all increase linearly with depth: σH=0.0229D+4.738, σh=0.0171D+1.829, σV=0.0272D. Maximum and minimum horizontal tectonic stress varies as a function of depth D linearly 4.738<σT<0.0139D+4.738 and 1.829<σt<0.0162D+1.829. The horizontal tectonic differential stress is σT−σt=0.0058D+2.912. (2) The intermediate value of σT1 (regression value of tectonic stress inferred from the assumption of lateral constraint at 2000 m depth) changes in different areas, the maximum value of which is 45.6MPa, while the minimum value of which is 26.8MPa. Horizontal tectonic differential stress σT−σt increases linearly with depth and the maximum and minimum of σT−σt is 25.3MPa and 13.0MPa, respectively. In general, the stress magnitude is much higher in western than in eastern China. This indicates that the strong Indo-Eurasian collision dominates the present tectonic stress field in Chinese mainland. (3) Compared with other study regions, the northward crustal compression to the Qinghai-Tibet block is relatively lower in magnitude in the shallow subsurface and higher at deeper depth. (4) The orientations of σT in China mainland generally form a radial scattering pattern centered in Tibetan Plateau. From western to eastern China, they rotate gradually clockwise from NS to NNE, NE, NEE, and SE, which is consistent with the result of focal mechanism solutions.

Lithological model of the South China crust based on integrated geophysical data

The structure and petrology of the earth’s crust is critical to understand the growth and evolution of the continents. In this paper, we present the crustal lithological model along the 400-km-long seismic profile between Lianxian, near Hunan Province, and Gangkou Island, near Guangzhou City, South China. This model is based on an integrated geophysical data set including P-wave velocity (VP), S-wave velocity (VS), VP/VS ratio, mass density (ρ) and Lam´ e impedances (ρλ, ρμ), which are compared to those determined by laboratory measurements on a variety of crustal rock samples. The Bouguer gravity anomaly together with the seismic velocity enables us to constrain density. The heat flow and thermal field allow us to carry out corrections for temperature. Pressure correction is based on depth. We directly compare the property parameters determined from the South China seismic data with laboratory measurements made at the same conditions of pressure and temperature. Inversion of the available data for rock lithology indicates that there are substantial differences in the crustal lithology of the Yangtze and Cathaysia blocks. While the average lithology of the upper crust in both blocks is mainly characterized by granite–granodiorite and biotite gneiss, the granite–granodiorite layer is much thicker beneath the Cathaysia block. The middle crust in these two domains is not entirely similar, being granite–granodiorite and granite gneiss as the best fit for the Yangtze block, and granite gneiss and biotite gneiss for the Cathaysia block. The lower crust is composed of biotite gneiss, paragranulite and amphibolite for the Yangtze block, whereas biotite gneiss, paragranulite, diorite, mica quartz schist, amphibolite, green schist facies basalt and hornblende provide the best fit for the Cathaysia block. The results demonstrate that to the east of the Chenzhou–Linwu fault (CLF) that is the southern segment of the Jiangshan–Shaoxing fault, the lithology displays relatively abrupt lateral variations, in contrast to the west of the fault. This suggests that the deformation is well developed in the whole crust beneath the Cathaysia block, in agreement with seismic evidence on the eastward migration of the orogeny and the development of a vast magmatic province. The CLF clearly marks the change in the property parameters of both tectonic blocks, so it appears a natural boundary between the Yangtze and Cathaysia blocks, and it is a crust-scale fault.

Cooling paths of the NE China crust during the Mesozoic extensional tectonics: Example from the south-Liaodong peninsula metamorphic core complex

The south-Liaodong peninsula massif is the easternmost Mesozoic metamorphic core complex, recognized in Eastern China. It provides a good example of the combination of ductile shearing, syn-kinematic plutonism and polyphase exhumation. The Jurassic granodioritic plutons, located at the footwall of the detachment normal fault, and dated here at ca 160 Ma, recorded two different phases of cooling. A slow cooling regime of about 3–10 °C/my prevailing before 122 Ma, was followed by a significant increase in cooling rate of about 40–55 °C/my after that time. By contrast, a single fast cooling path was recorded by the Cretaceous monzogranite situated in the footwall of the detachment normal fault. This result indicates that the Jurassic and Cretaceous plutons recorded different exhumation processes: a Jurassic slow or negligible exhumation and a Cretaceous fast one assisted by normal faulting. These two cooling stages correspond to distinct geodynamic processes during the Jurassic and Cretaceous. Extensional tectonics seems not significant before Early Cretaceous. The second stage, dominated by an extensional regime which develops after ca 120 Ma, is tentatively correlated to the lithosphere removal of the North China Craton.

The principal characteristics of the lithosphere of China

The lithospheric structure of China and its adjacent area is very complex and is marked by several prominent characteristics. Firstly, China’s continental crust is thick in the west but thins to the east, and thick in the south but thins to the north. Secondly, the continental crust of the Qinghai–Tibet Plateau has an average thickness of 60–65 km with a maximum thickness of 80 km, whereas in eastern China the average thickness is 30–35 km, with a minimum thickness of only 5 km in the center of the South China Sea. The average thickness of continental crust in China is 47.6 km, which greatly exceeds the global average thickness of 39.2 km. Thirdly, as with the crust, the lithosphere of China and its adjacent areas shows a general pattern of thicker in the west and south, and thinner in the east and north. The lithosphere of the Qinghai–Tibet Plateau and northwestern China has an average thickness of 165 km, with a maximum thickness of 180–200 km in the central and eastern parts of the Tarim Basin, Pamir, and Changdu areas. In contrast, the vast areas to the east of the Da Hinggan Ling–Taihang–Wuling Mountains, including the marginal seas, are characterized by lithospheric thicknesses of only 50–85 km. Fourthly, in western China the lithosphere and asthenosphere behave as a “layered structure”, reflecting their dynamic background of plate collision and convergence. The lithosphere and asthenosphere in eastern China display a “block mosaic structure”, where the lithosphere is thin and the asthenosphere is very thick, a pattern reflecting the consequences of crustal extension and an upsurge of asthenospheric materials. The latter is responsible for a huge low velocity anomaly at a depth of 85–250 km beneath East Asia and the western Pacific Ocean. Finally, in China there is an age structure of “older in the upper layers and younger in the lower layers” between both the upper and lower crusts and between the crust and the lithospheric mantle.

Crustal structure beneath China from receiver function analysis

We collected and processed a large amount of high‐quality broadband teleseismic waveform data recorded by the 48 Chinese National Digital Seismic Network stations to estimate large‐scale lateral variations of crustal thickness and Vp/Vs ratio (hence Poisson's ratio) beneath China. A statistical method was used to select mutually coherent receiver functions at each station, which yielded over 200 traces for most of the stations. With the conventional H−κ (the crustal thickness and Vp/Vs ratio) approach, there is a large trade‐off between H and κ. Consequently, multiple maxima are frequently observed in the H−κ domain. We introduced a weight function that measures the coherence between the P‐to‐S conversion and the reverberation phases at each H−κ grid to reduce the trade‐off. A 4th‐root stacking method was further applied to reduce uncorrelated noise relative to the linear stack. These modifications turned out to be very effective in reducing the H−κ trade‐off and yielded reliable estimates of crustal thickness and Vp/Vs ratio. The crust beneath eastern China is as thin as 31–33 km and the underlying Moho is relatively flat and sharp. In the western part of China, the crust is considerably thicker and shows large variations. The Moho is observed at about 51 km depth along the Tian Shan fold system and about 84 km deep beneath the central part of the Tibetan Plateau. The transition occurs at the so‐called N‐S belt between about 100° and 110°E, which is featured by unusually high seismicity and large gravity anomalies. The average Vp/Vs ratio over the mainland China crust is about 1.730 (σ = 0.249), significantly lower than the global average 1.78 (σ = 0.27) of the continental crust. This lower Vp/Vs ratio may suggest a general absence of mafic lowermost crustal layer beneath China.

Crustal Composition of China Continent Constrained from Heat Flow Data and Helium Isotope Ratio of Underground Fluid

Abstract:Based on conservation of energy principle and heat flow data in China continent, the upper limit of 1.3 μW/m3 heat production is obtained for continental crust in China. Furthermore, using the data of heat flow and helium isotope ratio of underground fluid, the heat productions of different tectonic units in China continent are estimated in range of 0.58–1.12 μW/m3 with a median of 0.85 μW/m3. Accordingly, the contents of U, Th and K2O in China crust are in ranges of 0.83–1.76 μg/g, 3.16–6.69 μg/g, and 1.0%–2.12%, respectively. These results indicate that the abundance of radioactive elements in the crust of China continent is much higher than that of Archean crust; and this fact implies China's continental crust is much evolved in chemical composition. Meanwhile, significant lateral variation of crustal composition is also exhibited among different tectonic units in China continent. The crust of eastern China is much enriched in incompatible elements such as U, Th and K than that of western China; and the crust of orogenic belts is more enriched than that of platform regions. It can also be inferred that the crusts of eastern China and orogenic belts are much felsic than those of western China and platform regions, respectively, derived from the positive correlation between the heat production and SiO2 content of bulk crust. This deduction is consistent with the results derived from the crustal seismic velocity data in China. According to the facts of the lower seismic velocity of China than the average value of global crust, and the higher heat production of China continent compared with global crust composition models published by previous studies, it is deduced that the average composition models of global continent crust by Rudnick and Fountain (1995), Rudnick and Gao (2003), Weaver and Tarney (1984), Shaw et al. (1986), and Wedepohl (1995) overestimate the abundance of incompatible elements such as U, Th and K of continental crust.

Pulsed emplacement of the Mount Kinabalu granite, northern Borneo

Abstract: High-precision U–Pb ion microprobe analyses provide new constraints on the emplacement and origin of the Kinabalu granite in Sabah, northern Borneo. The granite is a sheeted laccolith-like body comprising dyke-fed granitic units that young downwards, each emplaced beneath the previous sheet. Analyses of concentric growth zones in zircons indicate crystallization between 7.85 ± 0.08 and 7.22 ± 0.07 Ma, and show that the entire pluton was emplaced and crystallized within less than 800 ka. Several pulses of magmatism are recognized, each lasting for a maximum of 250 ka, and possibly as briefly as 30 ka. The oldest ages coincide with the highest elevations whereas the youngest ages are found at lower elevations around the edge of the body. Based on these new age data and field observations we identify the biotite granodiorite, hornblende granite and porphyritic facies as the Upper, Middle and Lower Units respectively. Inherited zircon ages indicate different protoliths for the Upper and Middle Units. The Upper Unit is derived from attenuated continental crust of the South China margin subducted beneath Sabah. The Middle Unit is sourced from melting of the crystalline basement in Sabah with little or no contribution from South China crust. Supplementary material: Full U–Pb ion microprobe analytical data, and modal and major element composition data are available at http://www.geolsoc.org.uk/SUP18385 .

New Algorithm and Results of WAAS Ionospheric Delay Correction

电离层介质的色散性是影响电磁波信号进行卫星导航定位精度的重要因素之一.配合北斗二代分系绩研制任务,提出了一种新的电离层折射校正算法,并利用2000年7月1日到3日的双频GPS观测数据对6个用户站进行试算,进一步将试算所得均方根误差和电离层网格算法得到的误差进行比较.结果表明,对于中纬区域的用户站,估算的TEC误差约为0.5 m左右;而低纬用户误差相对增大,为1 m左右.文中给出的算法与电离层网格模型所提供的精度相差不多,在未来中国自主的卫星增强系统中采用新方法进行电离层进行修正是可行的及有效的.

Frequency-dependent Shear-wave Q Models for the Crust of China and Surrounding Regions

— Starting with fundamental-mode Rayleigh-wave attenuation coefficient values (γ R ) predicted by previously determined frequency-independent models of shear-wave Q (Q μ ), we have obtained frequency-dependent Q μ models that explain measured values of γ R as well as of Lg coda Q and its frequency dependence at 1 Hz (Q o and η, respectively) for China and some adjacent regions. The process combines trial-and-error selection of a model for the depth distribution of the frequency dependence parameter (ζ) for Q μ with a formal inversion for the depth distribution of Q μ at 1 Hz. Fifteen of the derived models have depth distributions of ζ that are constant, or nearly constant, between the surface and a depth of 30 km. ζ distributions that vary with depth in the upper 30 km are necessary to explain the remaining seven models. ζ values for the depth-independent models vary between 0.4 and 0.7 everywhere except in the western portion of the Tibetan Plateau where they range between about 0.1 and 0.3 for three paths. These low ζ values lie in a region where QLg and crustal Q μ are very low and suggest that they should also be low for high-frequency propagation. The models in which ζ varies with depth all show a decrease in that value ranging between 0.55 and 0.8 in the upper 15 km of the crust and (with two exceptions where ζ=0.0) between 0.3 and 0.55 in the depth range 15–30 km. The distribution of ζ values between 0.6 and 0.8 (the higher part of the range) in the upper crust indicates that high-frequency waves will propagate most efficiently, relative to low-frequency waves, in a band that includes, and strikes north-northeastward from the path between event 212/97 and KMI to the path between event 180/95 and station HIA in the north.

Shear waveQstructure and its lateral variation in the crust of China and surrounding regions

SUMMARY We have obtained three-layer crustal models of shear wave Q (Qμ) for several surface wave paths in China and peripheral regions using a single-station, multimode method in which amplitude spectra of fundamental-mode and higher-mode Rayleigh waves, computed for known source depths and mechanisms, are compared to measured spectra. The three layers, totaling 60 km in thickness, roughly comprise the entire crust in the Tibetan plateau and include both the crust and part of the uppermost mantle in other regions where the crust is thinner. Qμ in the shallowest layer, 10 km thick, is lowest (about 40) in the western portion of the Tibetan plateau and highest (about 250) in southeastern China. In the middle layer, at 10–30 km depth, Qμ is lowest (60–80) beneath Tibet and the Pamir thrust system and highest (120–140) in central China and parts of the Sino-Korean platform. Uncertainties of Qμ in the deepest layer, at 30–60 km depth, are much greater than in the upper two layers but available results suggest that Qμ is lowest (about 80) under the Pamir thrust system and highest (about 180) under southern Mongolia. The densities of event and station coverage, although wanting in some regions, allow us to develop the first regionalized maps of crustal Qμ variation for continents. The maps, for depth ranges 0–10 km and 10–30 km, generally show good agreement with Q results obtained earlier with other phases and good correlation with the tectonics of this active region.

The 3‐D Structure of Shear Waves in the Crust and Uppermantle Beneath East Chinese Continen‐tal Inverted by Rayleigh Wave Data

AbstractThe phase velocity dispersion of seismic surface waves is computed from the clear and reliable Rayleigh wave records of 21 seismic stations and 43 paths in east Chinese continent. We obtain the distribution of phase velocity dispersion by Talantola probability inversion method. The image of 3‐D shear wave velocity in the crust and upper mantle is inverted by the phase velocity dispersion curves in the investigated region. Then we obtain the S wave velocity structure in the crust and upper mantle of east Chinese continent. The results show that the over all tendency of the thickness of east China crust is thin in the east and thick in the west. To the west of 105° E, the thickness of crust gradually increases to 55km. There is an ‘h’ form slope belt of the crust thickness. In the region of west Henan and south Shanxi provinces the crust is relatively thin. The thickness of crust in Dabie mountain and Taishan mountain are thicker than surrounding region. But in the Qinling mountain region the crust is not thicker. In North China region the top interface of the low velocity layer (LVL) of the upper mantle is rather shallow. It is about 80–90km. In the region of Eerduosi depression, Sichuan platform trough and Guizhou‐Hunan region it is about 120–130km. In the east Yangzi block as well as east and center South China fold system velocity in the top upper mantle is relatively low and the velocity contrast of LVL is not evident. In the west of Yunnan‐Guizhou fold system there is no LVL within the top 200km of the upper mantle.

Petrophysical studies on rocks from the Dabie ultrahigh-pressure (UHP) metamorphic belt, Central China: implications for the composition and delamination of the lower crust

The Dabie–Sulu ultrahigh-pressure (UHP) metamorphic belt of Central China represents a zone in which upper and lower continental crust has been subducted and then rapidly exhumed from the mantle back into the crust. About 30 UHP rock samples representing major lithologies were collected from surface exposures and their physical properties were measured in the laboratory at pressures up to 600 MPa (room temperature) and temperatures up to 600°C (600 MPa confining pressure). The experimentally determined data include compressional ( V p) and shear wave velocities ( V s), velocity anisotropy (shear wave splitting), density, and intrinsic pressure and temperature derivatives of V p and V s. Using a regional geotherm, velocity–depth profiles were calculated for the different lithologies spanning compositions from felsic through intermediate to mafic, and metamorphic grades up to granulite and eclogite facies. The velocity data, in particular the Poisson ratio ( V p/ V s ratio), were used to interpret lithologically the seismic models evaluated from seismic refraction data. The seismic velocity profiles reveal a four-layer structure (upper, middle, upper-lower and lowermost crust) with an average thickness of 34 km. From our results, we infer that a mixture of about 90% felsic gneiss with variable amounts of high V p amphibolite/gabbro constitute the middle crust. Intermediate granulite and mafic granulite fit both the V p and Poisson ratio of the upper-lower and lowermost crust, respectively. From the combined V p and Poisson ratio ( V p/ V s) data, we conclude that eclogite is not a major constituent of the present Dabie deep crust. Combining the laboratory-derived in-situ seismic data ( V p and Poisson ratio) with refraction seismic data, along with evidence from geological and geochemical investigations, suggests that lower crustal delamination played an important role in the modification of the East China crust.