Crustal Structure of the Jurassic Quiet Zone in the West Pacific Ocean:Insights from 2D Multichannel Seismic Reflection Profiles

The Jurassic oceanic crust is the oldest existing oceanic crust on earth,and although distributed sparsely,carries essential information about the earth's evolution.The area around the Pigafetta Basin in the west Pacific Ocean(also known as the Jurassic Quiet Zone,JQZ)is one of a few areas where the Jurassic oceanic crust is present.This study takes full advantage of high-resolution multichannel seismic reflection profiles in combination with bathymetry,magnetic,and gravity data from the JQZ to examine the structure,deformation,and morphology of the Jurassic oceanic crust.Our results show the following insights:1)The Moho lies at 2–3 s in two-way travel time beneath the seafloor with the segmented feature.The gaps between the Moho segments well correspond to the seamounts on the seafloor,suggesting the upward migration of magma from the mantle has interrupted the pre-existing Moho.2)The oceanic crust is predominantly deformed by crustal-scale thrust faults,normal faults cutting through the top of basement,and vertical seismic disturbance zones in association with migration of thermal fluids.The thrust faults are locally found and interpreted as the results of tectonic inversion.3)Seafloor morphology in the JQZ is characterized by fault scarps,fold scarps,seamounts,and small hills,indicating the occurrence of active faults.4)The oceanic crust in the JQZ and East Pacific Rise has many structural and geometrical variations,such as the thickness of sediments,seafloor topography,basement morphology,fault size and type.

Crustal structure of the northeastern Tibetan plateau,the Ordos block and the Sichuan basin from ambient noise tomography

We apply ambient noise tomography to significant seismic data resources in a region including the northeastern Tibetan plateau, the Ordos block and the Sichuan basin. The seismic data come from about 160 stations of the provincial broadband digital seismograph networks of China. Ambient noise cross-correlations are performed on the data recorded between 2007 and 2009 and high quality inter-station Rayleigh phase velocity dispersion curves are obtained between periods of 6 s to 35 s. Resulting Rayleigh wave phase velocity maps possess a lateral resolution between 100 km and 200 kin. The phase velocities at short periods （〈20 s） are lower in the Sichuan basin, the northwest segment of the Ordos block and the Weihe graben, and outline sedimentary deposits. At intermediate and long periods （〉25 s）, strong high velocity anomalies are observed within the Ordos block and the Sichuan basin and low phase velocities are imaged in the northeastern Tibetan plateau, reflecting the variation of crustal thickness from the Tibetan plateau to the neighboring regions in the east. Crustal and uppermost mantle shear wave velocities vary strongly between the Tibetan plateau, the Sichuan basin and the Ordos block. The Ordos block and the Sichuan basin are dominated by high shear wave velocities in the crust and uppermost mantle. There is a triangle-shaped low velocity zone located in the northeastern Tibetan plateau, whose width narrows towards the eastern margin of the plateau. No low velocity zone is apparent beneath the Qinling orogen, suggesting that mass may not be able to flow eastward through the boundary between the Ordos block and the Sichuan basin in the crust and uppermost mantle.

Crustal structure in Xiaojiang fault zone and its vicinity

Based on the integrative interpretation of travel-time data and amplitude information obtained from the deep seismic sounding experiment on the Chuxiong-Luoping profile, eastern Yunnan province, carried out in January of 2005, we present a 2-D P wave velocity structure along the profile. The crustal structure shows remarkable contrasts between the two sides of the Xiaojiang fault zone, although the whole profile is situated within the Yangtze platform. The average P wave velocities of the crust on the west and east sides of the fault zone are 6.21 km/s and 6.32 km/s, respectively, and the crustal thicknesses are 41 km and 45 km, respectively. These results imply that the crust to the east of the Xiaojiang fault zone presents characteristics of crustal structure in a stable platform, while the crust to the west is complicated with a lower velocity zone in middle of the upper crust. The average velocity of 6.21 km/s is lower than the global continental crustal average （6.30 km/s）, indicating that the region is tectonically active. According to the lateral variation of velocity and depth of interfaces （including the Moho）, it is inferred that the Xiaojiang fault zone has cut through the whole crust. It is also deduced that existence of low velocity zone in middle of the upper crust is conducive to the south-southeastern sliding of the Sichuan- Yunnan （Chuan-Dian） rhombus block.

The Crustal Structure and Assembly of Terranes in the Qaidam-Qilian-Beishan Area, Western China

Abstract: Through a study of the geotransect from Golmud to Ejin Qi published recently, the tectonics of the crust beneath the area from the northern Qinghai-Tibet plateau (Qaidam and the Qilian Mountains) to the border between China and Mongolia and its structure, composition and tectonic evolution have been revealed, and abundant information about the deep structures has been provided. Based on the research into the geotransect, it is suggested that the crust in this area was formed by the assembly of the terranes in different geological stages. Following the formation of the Palaeo-Asian continent, the north part of the corridor of the transect became a part of the huge unifying continent by the end of the Early Permian. In the Mesozoic and Cenozoic, as a result of the compression mainly by the push of the Qinghai-Tibet plateau on the south, the unique crustal structure and geomorphologic features on the northern Qinghai-Tibet plateau were formed. This geotransect together with the Yadong-Golmud geotransect constitutes a long geotransect which runs across the western Chinese continent.

Seismological study on the crustal structure of Tengchong volcanic-geothermal area

Based upon the deep seismic sounding profile conducted in the Tengchong volcanic-geothermal area, a two-dimensional crustal P velocity structure is obtained by use of the finite-difference inversion and the forward travel-time fitting method. The crustal model shows that there is a low velocity zone in upper crust in the Tengchong area, which may be related to the volcanic-geothermal activities, and two intracrustal faults (the LonglingRuili fault and Tengchong fault) exist on the profile, where the Tengchong fault may extend to the Moho discontinuity. Meanwhile, based on teleseismic data recorded by a temporary seismic network, we obtained the S-wave velocity structures beneath the RehaiRetian region in the Tengchong area, which show the low S-wave velocity anomaly in upper crust. The authors discuss the causes of Tengchong volcanic eruption based on the deep crustal structure. The crustal structure in the Tengchong volcanic-geothermal area is characterized by low P-wave and S-wave velocity, low resistivity, high heat-flow value and low Q value. The P-wave velocity in the upper mantle is also low. For this information, it can be induced that the magma in the crust is derived from the upper mantle, and the low velocity anomaly in upper crust in the Tengchong area may be related to the differentiation of magma. The Tengchong volcanoes are close to an active plate boundary and belong to plate boundary volcanoes.

Crustal structure beneath Cameroon from EGM2008

We used the Earth Gravitational Model （EGM2008） data sets to analyze the regional gravity anoma- lies and to study the underground structures in Cameroon. We first created a high-resolution Free-Air anomaly database, then corrected the gravity field of the topographic effect by using ETOPO1 DEM with a resolution of 0.01~ to obtain the Bouguer anomaly, then applied a multi-scale wavelet-analysis technique to separate the gravity-field components into different parts of shallow-to-deep origins, and finally used the logarithmic power spectrum technique to obtain detailed images and corresponding source depths as well as certain lateral inho- mogeneity of structure density. The anomalies of shallow origin show successive elongated gravity ＂highs＂ and ＂lows＂ attributable to subsurface Tertiary and lower Cretaceous undulations. Our results are in good agreement with previous investigations.

The Crustal Structure and Seismic Activity in North China

A layered crustal block model of North China has been constructed based on large amount of data from seismic sounding carried out in recent two decades. Some deep fault zones, such as the Zhangjiakou.Penglai and Tancheng-Lujiang fault zones, divide the upper crust of North China into three upper crustal terranes and nine bolcks. There are distinct differences in velocity and depth distributions, which reflects Cenozoic block faulting in North China in the process of formation of the deep structure. The upper crust shows the features of transition in isostatic adjustment. The existence of a low-velocity layer in the middle crust is characteristic of the crustal structure in North China. There seems to be an increase of rheology of the rocks in the lower crust and a persistence of stable regional stress field. The patterns of the Moho on two sides of the Yanshan-Taihang Mountains are different. The relief of the Moho around Beijing, Shijiazhuang and Guangrao where the deep faults join together shows a quadrantal distribution in some degree. The dynamic sources for seismic activity are the NE-SW horizontal compression and the diapirism of the upper mantle. The middle and upper crust, especially the layered block structure has the most significant effects on seismicity, and the occurrence of earthquakes is more closely related to them than to the Moho.

Crustal Structure of the Chuan-Dian Block Revealed by Deep Seismic Sounding and its Implications for the Outward Expansion of the East Tibetan Plateau

The Chuan-Dian Block(CDB)is located in the southeastern margin of the Tibetan Plateau,with a complex geological structure and active regional faults.The present tectonic condition with strong crustal deformation is closely related to the ongoing collision of the India and Eurasia plates since 65 Ma.The study of the crustal structure of this area is key to revealing the evolution and deep geodynamics of the lateral collision zone of the Tibetan Plateau.Deep seismic sounding is the most efficient method with which to unravel the velocity structure of the whole crust.Since the 1980s,19 deep seismic sounding profiles have been captured within the CDB area.In this study,we systematically integrate the research results of the 19 profiles in this area,then image the 3D crustal velocity,by sampling with a 5 km spacing and 2D/3D Kriging interpolation.The results show the following.(1)The Moho depth in the study area deepens from 30 km in the south to 66 km in the north,whereas there is no apparent variation from west to east.The Pn wave velocity is higher in stable tectonic units,such as 7.95 km/s in the Lanping-Simao block and 7.94 km/s in the western margin of the Yangtze block,than in active or mobile tectonic units,such as 7.81 km/s in the Baoshan block,7.72 km/s in the Tengchong block and 7.82 km/s in the Zhongdian block.(2)The crustal nature of the Tengchong block,the northern Lanping-Simao block and the Zhongdian block reflects a type of orogenic belt,having relatively strong tectonic activities,whereas the crustal nature of the central Lanping-Simao block and the western margin of the Yangtze block represents a type of platform.The different features of the upper-middle crust velocity,Moho depth and Pn wave velocity to both sides of the Red River fault zone and the Xianshuihe fault zone,reflect that they are clearly ultra-crustal.(3)Based on the distribution of the low velocity zones in the crust,the crustal material of the Tibetan Plateau is flowing in a NW–SE direction to the north of 26°N and to the west of 101°E,then diverting to flowing eastwards to the east of 101°E.

Basic Features of the Crustal Structure in the Lower Yangtze and Its Neighboring Area in the Chinese Mainland： Review of Deep Seismic Sounding Research

The Deep Seismic Sounding( DSS) projects carried out from the 1970 s in the lower Yangtze region and its neighboring area were reviewed in this paper,then the basic wave group features of those wide angle reflection / refraction record sections,and of the crustal structure are summarized. It shows that there were in total five clear wave groups on the record sections,which include the first arrival Pg,the reflection P1 from the bottom interface of the upper crust,the reflection P3 from the bottom interface of the middle crust,the strong reflection Pm from the Moho boundary,and the refraction Pn from uppermost mantle. In general,these phases are easily consistently traced and compared,despite some first arrivals being delayed or arriving earlier than normal due to the shallow sedimentary cover or bedrocks. In particular,in the Dabie Mountain region the seismic events of a few gathered shots always have weak reflection energy,are twisted,or exhibit disorganized waveforms, which could be attributed to the disruption variations of reflection depth,the broken Moho,and the discontinuity of the reflection boundary within crust. The regional crustal structures are composed of the upper,middle and lower crust,of which the middle and lower layers can be divided into two weak reflection ones. The crustal thickness of the North China and Yangtze platform are 30km- 36 km,and the Moho exhibits a flat geometry despite some local uplifts. The average pressure velocity in lower crust beneath this two tectonic area is 6. 7 ± 0. 3km / s. Nevertheless,beneath the Dabieshan area the crustal thickness is 32km- 41 km,the Moho bends down sharply andtakes an abrupt 4km- 7km dislocation in the vertical direction. The average pressure velocity in the lower crust beneath the Dabieshan area is 6. 8 ± 0. 2km / s.

Relationship between the regional tectonic activity and crustal structure in the eastern Tibetan plateau discovered by gravity anomaly

The eastern Tibetan plateau has been getting more and more attention because it combines active faults,uplifting, and large earthquakes together in a high-population region. Based on the previous researches, the most of Cenozoic tectonic activities were related to the regional structure of the local blocks within the crustal scale. Thus,a better understanding of the crustal structure of the regional tectonic blocks is an important topic for further study. In this paper, we combined the simple Bouguer gravity anomaly with the Moho depths from previous studies to investigate the crustal structure in this area. To highlight the crustal structures, the gravity anomaly caused by the Moho relief has been reduced by forward modeling calculations. A total horizontal derivative（THD） had been applied on the gravity residuals. The results indicated that the crustal gravity residual is compatible with the topography and the geological settings of the regional blocks,including the Sichuan basin, the Chuxiong basin, the Xiaojiang fault, and the Jinhe fault, as well as the Longmenshan fault zone. The THD emphasized the west margin of Yangtze block, i.e., the Longriba fault zone and the Xiaojiang fault cut through the Yangtze block. The checkboard pattern of the gravity residual in the SongpanGarze fold belt and Chuandian fragment shows that the crust is undergoing a southward and SE-directed extrusion,which is coincident with the flowing direction indicatedfrom the GPS measurements. By integrating the interpretations, the stepwise extensional mechanism of the eastern Tibetan plateau is supported by the southeastward crustal deformation, and the extrusion of Chuandian fragment is achieved by Xianshuihe fault.

Oceanic crustal structure and tectonic origin of the southern Kyushu-Palau Ridge in the Philippine Sea

A new high-resolution velocity model of the southern Kyushu-Palau Ridge(KPR) was derived from an activesource wide-angle seismic reflection/refraction profile. The result shows that the KPR crust can be divided into the upper crust with the P-wave velocity less than 6.1 m/s, and lower crust with P-wave velocity between 6.1 km/s and 7.2 km/s. The crustal thickness of the KPR reaches 12.0 km in the center, which gradually decreases to 5.0–6.0 km at sides. The velocity structure of the KPR is similar to the structures of the adjacent West Philippine Basin and Parece Vela Basin(PVB), indicating a typical oceanic crust. Isostatic analysis shows that some regional compensation occurs during the loading of the KPR, which implies that the KPR was built mainly by magmatism during the splitting of the Izu-Bonin-Mariana arc and the following back-arc seafloor spreading of the PVB during30–28 Ma BP. The absence of the thick middle crust(6.0–6.5 km/s) and high velocity lower-crustal layers(7.2–7.6 km/s) suggest that arc magmatism plays a less important role in the KPR formation.

Crustal structure beneath Liaoning province and the Bohai Sea and its adjacent region in China based on ambient noise tomography

The velocity structure of the crust beneath Liaoning province and the Bohai sea in China was imaged using ambient seismic noise recorded by 73 regional broadband stations. All available three-component time series from the 12-month span between January and December 2013 were cross-correlated to yield empirical Green＇s functions for Rayleigh and Love waves. Phase- velocity dispersion curves for the Rayleigh waves and the Love waves were measured by applying the frequency- time analysis method. Dispersion measurements of the Rayleigh wave and the Love wave were then utilized to construct 2D phase-velocity maps for the Rayleigh wave at 8-35 s periods and the Love wave at 9-32 s periods, respectively. Both Rayleigh and Love phase-velocity maps show significant lateral variations that are correlated well with known geological features and tectonics units in the study region. Next, phase dispersion curves of the Rayleigh wave and the Love wave extracted from each cell of the 2D Rayleigh wave and Love wave phase-velocity maps, respectively, were inverted simultaneously to determine the 3D shear wave velocity structures. The horizontal shear wave velocity images clearly and intuitively exhibit that the earthquake swarms in the Haicheng region and the Tangshan region are mainly clustered in the transition zone between the low- and high-velocity zones in the upper crust, coinciding with fault zones, and their distribution is very closely associated with these faults. The vertical shear wave velocity image reveals that the lower crust downward to the uppermost mantle is featured by distinctly high velocities, with even a high-velocity thinner layer existing at the bottom of the lower crust near Moho in central and northern the Bohai sea along the Tanlu fault, and these phenomena could be caused by the intrusion of mantle material, indicating the Tanlu fault could be just as the uprising channel of deep materials.

Crustal structure along the Zhenkang-Luxi deep seismic sounding profile in Yunnan derived from receiver functions

The crustal thicknesses and the Poisson’s ratios under the seismic stations can be calculated by receiver function method with H-κ stacking effectively. But the stacking results are affected to some extent by the average crustal P-wave velocity. To eliminate this effect and get more accurate crustal structure along the Zhenkang-Luxi deep seismic sounding profile which lies in Yunnan Province, we calculate the receiver functions from the teleseismic events recorded by 11 temporary stations as well as 5 permanent ones along the profile and carry out the stacking with Vp obtained from the profile in this study. Our study shows that the crustal thicknesses along the Zhenkang-Luxi profile range from 34.8 km to 41.8 km with an average of 39 km. The crust is thicker in the middle part of the profile and thinner in both sides in general. Dramatic changes of crustal thickness about 3 km are detected across both the Lancangjiang fault and the Xiaojiang fault, which implies that these faults cut through the Moho. The lowest Poisson’s ratio under the stations is 0.22 and the highest is 0.27 with the mean of 0.25, which is lower than the global average value 0.27 in the continental crust. It suggests that most of the crust along the profile lacks mafic component, but contains more felsic substance. The low Poisson’s ratio also indicates that there is no satisfying condition for partial melting. We deduce that the material flow in the middle-lower crust in the southeastern margin of the Tibetan plateau may occur only in the north region of 24°N.

Crustal structure of northeastern Tibetan plateau and Ordos block:Waveform interpretation of the Maqen-Jingbian seismic refraction profile

The Maqen-Jingbian wide-angle seismic reflection and refraction experiment was carried out in 1998, which aims at determining detailed structure in the crust and top of the upper mantle and understanding structural relation between the northeastern Tibetan plateau and the Ordos block. The 1-D crustal models inferred by waveform inversion show strong variations in crustal structure, which can be classified into four different types： ① an Ordos platform with the Proterozoic crust and two high-velocity layers in the northeast section, ② a transitional crust between the northeastern Tibetan plateau and the Ordos block across the Haiyuan earthquake zone, ③ the Qilian orogenic zone in the central part, and ④ the Qinling orogenic zone in the southwestern section. The Moho depth increases from -42 km to -62 km from the NE part to the SW part of the profile. The crystalline crust consists of the upper crust and lower crust in northeastern Tibetan plateau. There is an obviously low P-wave velocity layer dipping northeastward, which is 12-13 km thick, at the bottom of the upper crust in Qinling orogenic zone and Haiyuan earthquake zone. The lower crust is characterized by alternating high and low P-wave velocity layers. Beneath Ordos block, i.e., the NE part of the profile, the crust shows quite a smooth increase in P-wave ve- locity down to the Moho at a depth of about 42 km.

Crustal structure of the Qiangtang and Songpan-Ganzi terranes(eastern Tibet) from the 2-D normalized full gradient of gravity anomaly

Numerous geophysical studies have revealed the lithospheric structure of the Qiangtang and the Songpan-Ganzi terranes in the eastern Tibetan Plateau.However,crust-mantle evolution and crustal response to the Indian lithospheric subduction are still controversial.Answering these questions requires additional information regarding crustal structure.In this study,the 2-D normalized full gradient(NFG)of the Bouguer gravity anomaly was used to investigate anomalous sources and interpret the crustal structure underneath the Qiangtang and Songpan-Ganzi terranes.The NFG-derived structures with loworder harmonic numbers(N=33 and N=43)showed that an anomalous source beneath the southern Qiangtang terrane had a characteristic northeastward-dipping shape,suggesting the northeastward motion of the crustal material induced by underthrusting Indian lithospheric mantle.The NFG images with harmonic number N=53 showed a large-scale anomalous source in the lower crust of the transformational zone from the Qiangtang terrane to the Songpan-Ganzi terrane,consistent with thickening crust and resistance of lower crustal flow.The anomalous source demonstrated by the NFG results with harmonic number N=71,located in the upper crust underneath the Ganzi-Yushu fault,suggested a seismogenic body of the 2010 M6.9 Yushu event.

Upper crustal structure beneath Southwest Iberia north of the convergent boundary between the Eurasian and African plates

The 3-D P- and S-wave velocity models of the upper crust beneath Southwest Iberia are determined by inverting arrival time data from local earthquakes using a seismic tomo^raphy method. We used a total of 3085 P- and 2780 S-wave high quality arrival times from 886 local earthquakes recorded by a per- manent seismic network, which is operated by the Institute of Meteorology （IM）, Lisbon, Portugal. The computed P- and S-wave velocities are used to determine the 3-D distributions of Vp/Vs ratio. The 3-D velocity and Vp/Vs ratio images display clear lateral heterogeneities in the study area. Significant veloc- ity variations up to ~6% are revealed in the upper crust beneath Southwest lberia, At 4 km depth, both P- and S-wave velocity take average to high values relative to the initial velocity model, while at 12 km, low P-wave velocities are clearly visible along the coast and in the southern parts. High S-wave velocities at 12 km depth are imaged in the central parts, and average values along the coast; although some scattered patches of low and high S-wave velocities are also revealed. The Vp/Vs rztio is generally high at depths of 4 and 12 km along the coastal parts with some regions of high Vp/Vs ratio in the north at 4 km depth, and low Vp/Vs ratio in the central southern parts at a depth of 12 km, The imaged low velocity and high Vp/Vs ratios are related to the thick saturated and unconsolidated sediments covering the region; whereas the high velocity regions are generally associated with the Mesozoic basement rocks.

Crustal structure of the central Tibetan plateau and geological interpretation

Based on teleseismic data obtained from 225 stations from two networks in the central Tibetan plateau, we have generated detailed crustal structure images using P-wave receiver function techniques with more accurate piercing-depth-correction and time-depth-correction than what have previously been available. Our images indicate an undulatory Moho beneath the Tibetan plateau with a steep jump beneath the northern Himalaya, and obviously different structures in proximity to the Bangong-Nujiang suture. In several sections of the Tibetan plateau, the lower crust is characterized by pervasive high-velocity regions, which are consistent with the preservation of eclogite bodies beneath the plateau, whose presence affects the dynamics of the Tibetan plateau.

The geological structure background and the crustal structure in the northeastern margin of the Qinghai-Tibetan plateau

The geological structure background, the crustal structure and the shape of Moho in the northeastern margin of the Qinghai-Tibetan plateau are studied. Based on artificial seismic sounding profile as well as geological data. The main results are summarized as follows: (1) The geotectonic subdivisions and the characteristics of main deep and large faults in the northeastern margin of the Qinghai-Tibetan plateau are presented; (2) The general features of the Moho are obtained mainly based on artificial seismic sounding data; (3) There exists well corresponding relation between surface faults and some features of the Moho, which suggests that such complex crustal structure might be the preparation environment of strong earthquakes.

Constraints on the crustal structure beneath the Sinai subplate,SE Mediterranean,from analysis of local and regional travel times

The Sinai Peninsula has been recognized as a subplate of the African Plate located at the triple junction of the Gulf of Suez rift,the Dead Sea Transform fault,and the Red Sea rift.The upper and lower crustal structures of this tectonically active,rapidly developing region are yet poorly understood because of many limitations.For this reason,a set of P- and S-wave travel times recorded at 14 seismic stations belonging to the Egyptian National Seismographic Network（ENSN） from 111 local and regional events are analyzed to investigate the crustal structures and the locations of the seismogenic zones beneath central and southern Sinai.Because the velocity model used for routine earthquake location by ENSN is one-dimensional,the travel-time residuals will show lateral heterogeneity of the velocity structures and unmodeled vertical structures.Seismic activity is strong along the eastern and southern borders of the study area but low to moderate along the northern boundary and the Gulf of Suez to the west.The crustal V_P/V_S ratio is 1.74 from shallow（depth≤10 km） earthquakes and 1.76 from deeper（depth 〉 10 km） crustal events.The majority of the regional and local travel-time residuals are positive relative to the Preliminary Reference Earth Model（PREM）,implying that the seismic stations are located above widely distributed,tectonically-induced low-velocity zones.These low-velocity zones are mostly related to the local crustal faults affecting the sedimentary section and the basement complex as well as the rifting processes prevailing in the northern Red Sea region and the ascending of hot mantle materials along crustal fractures.The delineation of these low-velocity zones and the locations of big crustal earthquakes enable the identification of areas prone to intense seismotectonic activities,which should be excluded from major future development projects and large constructions in central and southern Sinai.

A note on crustal structure between Antarctic Peninsula and East Antarctic craton across the shelf of southern Weddell Sea

Analysis of gravity data based on the Airy isostasy, magnetic depth estimates and few seismic refraction data taken together indicates a thinning of the crust between the Antarctic Peninsula and the East Antarctic craton below the Filchner and Ronne ice shelves.