Geochemistry of the ophiolite and oceanic island basalt in the Kangxian-Pipasi-Nanping tectonic melange zone, South Qinling and their tectonic significance

Detailed studies indicate that Kangxian-Pipasi-Nanping tectonic zone is a complicated melange zone which includes many tectonic slabs of different origins. Ophiolite (MORB-type basalt), oceanic island tholeiite and alkaline basalt have been identified. Moreover, this tectonic melange zone is eastward connected with the Mianlue suture zone. The deformation characteristics, consisting components and volcanic rock geochemical features for the Kangxian-Pipasi-Nanping tectonic melange zone are much similar to those of the Mianlue suture zone and Deerni ophiolite. Therefore, the Kangxian-Pipasi-Nanping tectonic melange zone should be the westward extension part of the Mianlue suture zone. It indicates that the Mianlue suture zone had extended to the Nanping area.     

Petrological, geochemical and chronological constraints for the tectonic setting of the Dongco ophiolite in Tibet

The Dongco ophiolite occurred in the middle-western segment of the Bangong-Nujiang suture zone. The thickness of the ophiolite suite is more than 5 km, which is composed, from bottom to top, of the mantle peridotite, mafic-ultramafic cumulates, basic sills (dykes) and basic lava and tectoni- cally emplaced in Jurassic strata (Mugagongru Group). The Dongco cumulates consist of dunite- troctolite-olivine-gabbro, being a part of DTG series of mafic-ultramafic cumulates. The basic lavas are characterized by being rich in alkali (Na2O K2O), TiO2, P2O5 and a LREE-rich type pattern dip- ping right with [La/Yb]=6.94―16.6 as well as a trace elements spider-diagram with normal anomaly of Th, Nb, Ta, Hf. Therefore, the Dongco basic lavas belong to ocean-island basalt (OIB) and dis- tinctly differ from mid-ocean ridge basalt (MORB) and island-arc basalt (IAB) formed in the plate convergence margin. The basic lavas have higher 87Sr/86Sr (0.704363―0.705007), lower 143Nd/144Nd (0.512708―0.512887) and εNd(t ) from 2.7― 5.8, indicating that they derive from a two-components mixing mantle source of depleted mantle (DM) and enriched mantle (EMI). From above it is ready to see that the Dongco ophiolite forms in oceanic island (OIB) where the mantle source is replaced by a large amount of enriched material, therefore it distinctly differs from these ophiolites formed in island-arc and mid-oecan ridge. Newly obtained SHRIMP U-Pb dating for zircon of the cumulate troctolite is 132 ± 3 Ma and whole-rock dating of ~(39)Ar/~(40)Ar for the basalt is 173.4 ± 2.7 Ma and 140.9 ± 2.8 Ma, indicating that the Dongco ophiolite formed at Early Cretaceous and the middle-western segment of the Bangong-Nujiang oceanic basin was still in the developing and evolving period at Early Cretaceous.     

Thermal Structure and Rheology of the Upper Mantle Derived from Mantle Xenoliths from Gansu Province, Western China

Mantle xenoliths brought up by Cenozoic volcanic rocks onto the earth‘s surface may provide direct information about the upper mantle beneath the volcanic region. This paper presents the study on mantle xenoliths collected from Haoti village, Dangchang County, Gansu Province, western China. The main purpose of the study is to gain an insight into the thermal structure and rbeology of the upper mantle beneath the region. The results show that the upper mantle of the region is composed mainly of spinel lherzolite at shallower depth (52-75km), and garnet iberzolite at greater depth (greater than 75km), instead of harzburgite and dunite as proposed by some previous studies. The upper mantle geotherm derived from the equilibrium temperatures and pressures of xenoliths from the region is lower than that of North China, and is somewhat closer to the Oceanic geotherm. The crust-mantle boundary is determined from the geotherm to be at about 52km, and the Moho seems to be the transition zone of lower crust material with spinel iberzolite. If we take 1280℃ as the temperature of the top of asthenosphere, then the fithospbere-asthenosphere boundary should be at about 120km depth. The differential stress of the upper mantle is determined by using recrystallized grain size piezometry, while the strain rate and equivalent viscosity are determined by using the high temperature flow law of peridotite. The differential stress, strain rate and viscosity profiles constructed on the basis of the obtained values indicate that asthenospberic diapir occurred in this region during the Cenozoic time, resulting in the corresponding thinning of the lithosphere. However, the scale and intensity of the diapir was significantly less than that occurring in the North China region. Moreover, numerous small-scale shear zones with localized deformation might occur in the iithospberic mantle, as evidenced by the extensive occurrence of xenoliths with tabular equigranular texture.     

Geochemistry and LA-ICP-MS zircon U-Pb dating of the Dongjiahe ophiolite complex from the western Bikou terrane

The Dongjiahe ophiolite complex occurring in the western Bikou terrane that is composed chiefly of serpentinite, listwanitizational peridotite, gabbro, cumulus gabbro, and sub-alkaline meta-basalt, possesses a rock association of typical ophiolite sequence. The metaperidotite is depleted in light rare earth element (LREE), whereas the gabbro and meta-basalt from the studied ophiolite sequence, generated by the same parental magmas those have close affinity to the MORB (Mid-ocean ridge basalt), their REE and immobile elements patterns imply an ocean in the northern margin of the Yangtze plate during the Neoproterozoic period. The zircon LA-ICP-MS U-Pb dating for the gabbro yields a weighted mean age of 839.2±8.2Ma, suggesting that the basin occurred during the Neoproterozoic period.     

SHRIMP U-Pb dating and Hf isotopic analyses of Middle Ordovician meta-cumulate gabbro in central Qiangtang, northern Tibetan Plateau

Metabasites consisting of metamorphic ultra-mafic rocks, cumulate gabbro, gabbro (diabase), basalt, and plagiogranite are exposed at the Taoxinghu area in central Qiangtang, northern Tibetan Plateau. Zircon SHRIMP U-Pb dating for the cumulate gabbro yields a weighted mean age of 467±4 Ma, which is the oldest and most reliable magmatic age in this area. Zircon 176Hf/177Hf ratios range from 0.282615 to 0.282657, with εHf(t) values of 5.02±0.28, indicating that the cumulate gabbro was mainly derived from the depleted mantle. In addition, geochemical data of metabasites suggest that they have similar characteristics to those in the mid-ocean ridge basalts (MORB). The Taoxinghu metabasites may represent the fragment of Early Paleozoic ophiolite in the "Central Uplift" of the Qiangtang, northern Tibetan Plateau.     

Diversity history of Ordovician graptolites and its relationship with environmental change

Statistical analysis based on global data indicated that the planktonic graptolites presented a distinguished pattern from that of benthic communities during the great Ordovician biodiversification. The graptolites started to diversify from the beginning of Ordovician and reached an acme in the early Middle Ordovician, but subsequently underwent a steady decline to the end of Ordovician. During the Ordovician, many clades of the graptolites made their originations, flourishing and replacements. However, in different regions and distinct environments across the world, graptolites may presumably display distinct patterns of radiation and evolution, a hypothesis to be tested. Herein a study of the graptolite diversity in the Early to Middle Ordovician in the Upper Yangtze Region (Platform) and Jiangnan Region (Slope), South China, is conducted. The results indicate that graptolites underwent a remarkable increase in both regions, but with distinct magnitude, scope, process and patterns. The diversification of graptolites in the Upper Yangtze Region, though less prominent, is divided into four stages and includes two peaks. In the Jiangnan Region, the graptolite diversification is far more prominent and includes three stages and one peak. Based on the distinct diversity histories and composition of graptolites in the two representative regions, a 'deep-water origin and shallow-water dispersal' model is proposed for the Ordovician graptolite faunas. According to the model, the major graptolite faunas of Ordovician originated in the deep-water region on the continental slope, the source of the graptolite novelties, and subsequently spread into shallow-water region on the shelf. Besides, we also conduct a comparison study of the Ordovician graptolite diversity in South China and other major regions. The results display that the graptolites diversified globally in the Early and Middle Ordovician. At the beginning of Ordovician, graptolites underwent a significant ecological innovation of graptolites: planktonic forms were derived from their benthic ancestors. This derivation resulted in the great guild expansion of graptolites. In late Early Ordovician (Floian), the global expansion of graptolites commenced. From the Floian to the end of Middle Ordovician, the graptolites diversity increased remarkably and displayed three peaks for all the regions. However, the peaks are somewhat distinct in magnitude and timing among regions. The Darriwilian peak is prominent in both South China and Baltic region, but inconspicuous in Australasia. No close relationship between the graptolite diversification and the palaeolatitudes are supported herein. Instead, the graptolite diversification seems to coincide with the global sea-level rises, suggesting a possible intrinsic relationship between them.     

Paleoclimate evolution and aridification mechanism of the eastern Tethys during the Callovian–Oxfordian: evidence from geochemical records of the Qiangtang Basin,Tibetan Plateau

Global climate during the Jurassic has been commonly described as a uniform greenhouse climate for a long time.However,the climate scenario of a cool episode during the Callovian-Oxfordian transition following by a warming trend during the Oxfordian(163.53 to157.4 Ma) is documented in many localities of the western Tethys.It is still unclear if a correlatable climate scenario also occurred in the eastern Tethys during the same time interval.In this study,a detailed geochemical analysis on the 1060 m thick successions(the Xiali and Suowa formations) from the Yanshiping section of the Qiangtang Basin,located in the eastern Tethys margin during the Callovian-Oxfordian periods,was performed.To reveal the climate evolution of the basin,carbonate content and soluble salt concentrations(SO_4~(2-),Cl~-) were chosen as climatic indices.The results show that the overall climate patterns during the deposition of the Xiali and Suowa formations can be divided into three stages:relatively humid(～164.0 to 160.9 Ma),dry(～160.9 to159.6 Ma),semi-dry(～159.6 to 156.8 Ma).A similar warming climate scenario also occurred in eastern Tethys during the Callovian-Oxfordian transition(～160.9 to159.6 Ma).Besides,we clarify that the Jurassic True polar wander(TPW),the motion of the lithosphere and mantle with respect to Earth's spin axis,inducing climatic shifts were responsible for the aridification of the Qiangtang Basin during the Callovian-Oxfordian transition with a review of the paleolatitude of the Xiali formation(19.7+2.8/-2.6° N) and the Suowa formation(20.7+4.1/-3.7° N).It is because the TPW rotations shifted the East Asia blocks(the North and South China,Qiangtang,and Qaidam blocks) from the humid zone to the tropical/subtropical arid zone and triggered the remarkable aridification during the Middle-Late Jurassic(ca.165-155 Ma).     

Formation time of the big mantle wedge beneath eastern China and a new lithospheric thinning mechanism of the North China craton—Geodynamic effects of deep recycled carbon

High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the asthenosphere and it could be easily replaced by convective mantle. The newly formed carbonated silicate melts will migrate upward and metasomatize the overlying lithospheric mantle. Similarly, such metasomatism and partial melting processes repeat, and as a result the cratonic lithosphere in North China would be thinning and the carbonated silicate partial melts will be transformed to high-Si O2 alkali basalts with lower εNdvalues(to-2). As the lithospheric thinning goes on,initial melting depth of carbonated peridotite must decrease from 130 km to close 70 km, because the craton geotherm changed to approach oceanic lithosphere geotherm along with lithospheric thinning of the North China craton. Consequently, the interaction between carbonated silicate melt and cratonic lithosphere is a possible mechanism for lithosphere thinning of the North China craton during the late Cretaceous and Cenozoic. Based on the age statistics of low δ26 Mg basalts in eastern China, the lithospheric thinning processes caused by carbonated metasomatism and partial melting in eastern China are limited in a timespan from 106 to25 Ma, but increased quickly after 25 Ma. Therefore, there are two peak times for the lithospheric thinning of the North China craton: the first peak in 135-115 Ma simultaneously with the cratonic destruction, and the second peak caused by interaction between carbonated silicate melt and lithosphere mainly after 25 Ma. The later decreased the lithospheric thickness to about70 km in the eastern part of North China craton.     

Deep dynamical processes in the central-southern Qinghai-Tibet Plateau—Receiver functions and travel-time residuals analysis of north Hi-Climb

Teleseismic receiver functions and travel-time residuals along the north Hi-Climb broadband seismic array in the central-southern Qinghai-Tibet Plateau show that the lithosphere structures in the central and western Qinghai-Tibet Plateau are different. In the central Qinghai-Tibet Plateau, the Indian Plate is northward subducted beneath the Qiangtang block and arrives at the greatest depth beneath the central-southern Qiangtang block. The delaminated Indian lithospheric slab remains beneath the central Lhasa block to a depth possibly greater than that of the upper interface of the mantle transform zone. In the western Qinghai-Tibet Plateau, the Indian lithospheric plate is gently northward subducted and may have arrived to the south of Tarim plate. Due to the resistance from the gently northward subduction of the Indian mantle lithosphere in the western Qinghai-Tibet Plateau, the upwelling mantle material be-neath the Qiangtang block moves mostly toward the east to bring about the lateral eastward flow of the deep mantle hot material in the central Qinghai-Tibet Plateau.     

Shear-wave velocity structure of the crust and uppermost mantle in the Shanxi rift zone

The Shanxi rift zone is one of the largest and active Cenozoic grabens in the world, studying the velocity structure of the crust and upper mantle in this region may help us to understand the mechanisms of rift processes and the seismogenic environment of active seismicity in continental rifts. In this work, using the broadband seismic data of Shanxi, Hebei, Henan, Shaanxi provinces, and the Inner Mongolia Autonomous Region from February 2009 to November 2011, we have picked out 350 high-quality phase velocity dispersion curves of fundamental mode Rayleigh waves at periods from 8 to 75 s, and Rayleigh wave phase velocity maps have been constructed from 8 to75 s period with horizontal resolution ranging from 40 to50 km by two-station surface-wave tomography. Then,using a genetic algorithm, a 3D shear-wave speed model of the crust and uppermost mantle have been derived from these maps with a spatial resolution of 0.4° 9 0.4°. Four characteristics can be outlined from the results:(1) Except in the Datong volcanic zone, in the depth range of11–30 km, the location of a transition zone between the high- and low-velocity regions is in agreement with the seismicity pattern in the study region, and the earthquakes are mostly concentrated near this transition zone;(2) In the depth range of 31–40 km, shear-wave velocities are higher to the south of the Taiyuan Basin and lower to the north,which is similar to the distribution pattern of Moho depth variations in the Shanxi region;(3) The shear-wave velocity pattern of higher velocities to the south of 38°N and lower velocities to the north is found to be consistent with that from the upper crustal levels to depth of 70 km. At the deeper depths, the spatial scale of the low-velocity anomalies zone in the north is gradually shrinking with depth increasing, the low-velocity anomalies are gradually disappearing beneath the Datong volcanic zone at the depth of 151–200 km. We proposed that the root of the Datong volcano may reach to a depth around 150 km;(4) Along the N–S vertical profile at 112.8°E, the 38°N latitude is the boundary between high and low velocities, arguing the tectonic difference between the Shanxi rift zone and its flanks, in the rift zone the seismic velocity is dominated by low-velocity anomalies while in the flanks it is high.     

Crustal structure beneath Qiangtang and Lhasa terrane from receiver function

To determine the crustal structure in central Tibet, we used teleseismic waveform data recorded by 18 stations in the INDEPTH-Ⅲ seismic array across the central Tibet from the central Lhasa terrane to the central Qiangtang terrane. The S-wave velocity structures beneath stations are determined by inverting the stacked radial receiver function using the GA method. The first order features in the receiver function are modeled. Our results show that the Moho in Qiangtang is about 8 km shallower than that in Lhasa terrane along the INDEPTH-Ⅲ profile. It maybe suggests the northward subduction of the Lhasa mantle lid beneath the Qiangtang terrane is affected by the India-Asia collision. We conclude that there exist low velocity zone in the middle crust across the northern Lhasa and Qiangtang terrane, which can be related to the high temperature upper mantle beneath that.     

Crustal electric structure of Haiyuan arcuate tectonic region in the northeastern margin of Qinghai-Xizang Plateau, China

Through the analysis and 2-D inversion for the 5 profiles in Haiyuan arcuate tectonic region (105°～107°E,36°～37.5°N) in the northeastern margin of Qinghai-Xizang Plateau, we have obtained the electric structure within a range of 160 km in width (east-west) and 60 km in depth in the studied area. The results show that the crustal electric structure can be divided into 6 sections, corresponding respectively to Xiji basin (Ⅰ), Xihuashan-Nanhuashan uplift (Ⅱ), Xingrenbu-Haiyuan basin (Ⅲ), Zhongwei-Qingshuihe basin (Ⅳ), Zhongning-Hongsibu basin (Ⅴ) and west-margin zone of Ordos (Ⅵ) from the southwest to the northeast. The crustal electric structure is characterized by a broom-shaped pattern, which scatters to the northwest and shrinks to the southeast. The structures in the top part of Haiyuan arcuate tectonic region are complete and large, however, they diminish from the arc top to the northwest and southeast ends. In the depth from 0 km to 10 km, the resistivity is high in the sections Ⅱ and Ⅵ, but relatively low in the other four sections, showing a similar pattern of basin depression. The electrical basement in the section Ⅲ is the deepest, displaying a "dustpan" shape that is deep in the southwest and shallow in the northeast. A series of discontinuous zones with high conductivity exist in the middle-lower crust in Haiyuan arcuate tectonic region, which is possibly related to the moderate and strong earthquakes in the region. The resistivity distribution in the focal area of the 1920 Haiyuan earthquake is significantly heterogeneous with an obviously high conductivity zone near the hypocenter regime.     

Geochemistry and Sr-Nd-Pb isotopic characteristics of the Mugouriwang Cenozoic volcanic rocks from Tibetan Plateau:Constraints on mantle source of the underplated basic magma

The Mugouriwang Cenozoic volcanic rocks exposed in the north Qiangtang Block of Tibetan Plateau are mainly composed of basalt and andesitic-basalt,both characterized by the lower SiO2 (51%―54%),high refractory elements (i.e. Mg,Cr,Ni) as well as the moderate enrichment in light rare earth elements (LREE) relative to a slight depleted in Eu and high strength field elements (HFSE,i.e. Nb,Ta,Ti). Be-sides,the fairly low Sm/Yb value (3.07―4.35) could signify that the rocks should be derived directly from partial melting of the spinel lherzolite at the upper part of the asthenosphere. These rocks have radiogenic Sr and Pb (87Sr/86Sr = 0.705339 to 0.705667; 208Pb/204Pb = 38.8192 to 38.8937; 207Pb/204Pb = 15.6093 to 15.6245; 206Pb/204Pb = 18.6246 to 18.6383),and non-radiogenic Nd (143Nd/144Nd = 0.512604 to 0.512639; εNd = 0.02 to -0.66) in agreement with those values of the BSE mantle reservoir. The DUPAL anomaly of the rocks can be evidently attested by the △8/4Pb = 66.82 to 74.53 ,△7/4Pb = 9.88 to 11.42,△Sr>50,implying that the Mugouriwang volcanic rock is likely to be generated by partial melting of a Gondwana-bearing asthenospheric mantle ever matasomatised by the fluid from subduction zone. Depending on the previous study on the high-K calc-alkaline intermediate-felsic volcanics in the study area,this paper proposed that the fluids derived from the subducted Lhasa Block metasomatised the asthenosphere beneath the Qiangtang Block,and induced its partial melting,and then the melt under-plated the thickened Qiangtang lithosphere and caused the generation of the Cenozoic adakite-like felsic magmas in the Qiangtang region.     

Seismic tomography of the crust and upper mantle in the Bohai Bay Basin and its adjacent regions

In the Bohai Bay Basin and its adjacent regions(112°―124°E,34°―42°N),there exists abundant gas-petroleum while modern inter-plate seismic activity is robust.Although the tectonic structure of this region is very complicated,plenty of geological,geophysical and geochemical data and results are obtained through previous researches.On the basis of absorbing previous results,especially various kinds of geological and geophysical results,we collect and process the arrival time of P-wave phases of local events and tele-seismic events recorded by the station within this region from 1978 to 2004,build the responding initial model,and image the velocity structure of the crust and upper mantle of this region via tomography.The perturbation images of various depths and velocity profiles imply that the velocity structure of the crust and upper mantle in the Bohai Bay Basin and its adjacent regions is mainly influenced by the surface tectonic units,and is characterized by "Stripped along east-west,and zoned along south-north";some large-scaled faults penetrate Moho and lithosphere,and provide the channels for the basic lava or hot mass upwelling from the mantle.     

Research on Micro-relief of Active Faults on the Coast of Southeast Fujian and Its Adjacent Area

On the southeast coast of Fujian and its adjacent area, the NE-trending Changle-Zhao′an fault zone and several NW-trending faults that are genetically related to the former are well developed. With micro-relief analysis, the paper deals with the Quaternary activity of the faults and the tectonic stress field since the late Pleistocene in this region. The results indicate that the micro-relief of the NE-trending Changle-Zhao′an fault zone and the genetically related NW-trending faults is characterized by vertical and horizontal movements since the Quaternary; the faults in the region have undergone two active stages since the Quaternary, i.e. early Quaternary and late Pleistocene; since the late Pleistocene, the movement of the NE-trending faults showed a right-lateral strike-slip, while that of NW-trending faults a left-lateral strike-slip, indicating a NWW-SEE oriented horizontal principal stress of the regional tectonic stress field     

Early Ordovician strongly peraluminous granite in the middle section of the Yarlung Zangbo junction zone and its geological significance

LA-ICP-MS zircon U-Pb isotopic dating and rock geochemical analysis were done of the Xarru granite in the middle section of the Yarlung Zangbo junction zone.Zircon 206Pb/238U weighted mean ages of 474.9±2.3 and 478.3±1.7 Ma have been obtained for two gneiss granite samples respectively,which represent the formation age of the granite.This is the first discovery of the Early Ordovician magmatism in the Yarlung Zangbo junction zone.The rocks are high-K calcic-alkalic granite,contain tourmaline but not hornblende,with aluminum saturation index(ASI) of A/CNK1.1(1.10–1.20),and are enriched in Rb,Th and U and relatively depleted in Ba,Nb,Sr,Zr,Ti and Eu.They are strongly peraluminous S-type granite,resulting from partial melting of argillaceous components in the crust in a syn-collisional setting.According to previous studies as well as the analysis in this paper,the formation of the Xarru granite is probably related to the Andean-type orogeny in the process of subduction of the Proto-Tethys Ocean towards the Gondwanaland,and it is a product of partial melting of the thickened upper crust as a result of collision between blocks or micro-blocks in the northern margin of the Gondwana supercontinent in the process of oceanic subduction.The Xarru granite is identified as the Early Ordovician granite,indicating that the wall rocks had probably formed in the Cambrian or Precambrian.A crustal basement may exist in the Xarru region.     

Strain and shear types of the Louzidian ductile shear zone in southern Chifeng, Inner Mongolia, China

The Louzidian ductile shear zone at the south of Chifeng strikes NE-SW and dips SE at low-medium- angles. This ductile shear zone is mainly composed of granitic mylonite, which grades structurally upward into a chloritized zone, a microbreccia zone, a brittle fault and a gouge zone. All these zones share similar planar attitudes, but contain different linear attitudes and kinematic indicators. Finite strain measurements were performed on feldspar porphyroclasts using the Fry method. These meas- urements yield Fulin indexes of 1.25―3.30, Lode's parameters of -0.535―-0.112 and strain parameters of 0.41―0.75 for the protomylonite, respectively. These data are plotted within the apparent constric- tional field in Fulin and Hossack diagrams. In contrast, for the mylonite, corresponding parameters are 0.99―1.43, -0.176―-0.004 and 0.63―0.82, respectively, and located in the apparent constrictional field close to the plane strain. The mean kinematic vorticity numbers of the protomylonite and mylonite by using three methods of polar Mohr circle, porphyroclast hyperbolic and oblique foliation, are in the range of 0.67―0.95, suggesting that the ductile shearing is accommodated by general shearing that is dominated by simple shear. Combination of the finite strain and kinematic vorticity indicates that shear type was lengthening shear and resulted in L-tectonite at the initial stage of deformation and the shear type gradually changed into lengthening-thinning shear and produced L-S-tectonite with the uplifting of the shear zone and accumulating of strain. These kinds of shear types only produce a/ab strain facies, so the lineation in the ductile shear zone could not deflect 90° in the progressively deformation.     

Upper crust structure of eastern A’nyemaqên suture zone: Results of Barkam-Luqu-Gulang deep seismic sounding profile

Barkam-Luqu-Gulang deep seismic sounding profile runs from north of Sichuan Province to south of Gansu Prov- ince. It is located at the northeastern edge of Tibetan Plateau and crosses eastern A’nyemaqên suture zone. The upper crust structures around eastern A’nyemaqên suture zone and its adjacent area are reconstructed based on the arrival times of refracted Pg and Sg waves by using finite difference method, ray tracing inversion, time-term method and travel-time curve analysis. The results show that the depth variation of basement along profile is very strong as indicated by Pg and Sg waves. The basement rose in Zoigê basin and depressed in eastern A’nyemaqên suture zone, and it gradually rose again northward and then depressed. The results also indicate that eastern A’nyemaqên suture zone behaves as inhomogeneous low velocity structures in the upper crust and is inclined to- ward the south. Hoh Sai Hu-Maqên fault, Wudu-Diebu fault and Zhouqu-Liangdang fault are characterized by low velocity distributions with various scales. The distinct variation in basement depth occurred near Hoh Sai Hu-Maqên fault and Zhouqu-Liangdang fault, which are main tectonic boundaries of A’nyemaqên suture zone. Wudu-Diebu fault, located at the depth variation zone of the basement, possibly has the same deep tectonic back- ground with Zhouqu-Liangdang fault. The strongly depressed basement characterized by low velocity distribution and lateral inhomogeneity in A’nyemaqên suture zone implies crushed zone features under pinching action.     

Structure and physical properties of the garth＇s interior

The studies on the structure and physical properties of the Earth‘s interior done by Chinese geophysicists from 1999 to 2002 were reviewed in this paper. It includes several research areas: the structure of the Earth‘s interiors using seismic tomography, anisotropy of the upper mantle in China and its adjacent areas, quality factor Qβ for S waves, subduction zone, mantle discontinuities, physical properties of Earth‘s materials and others. The review concerns mainly the contents, the methods and the results of the studies. It can be seen that new progress in the study on the structure and physical properties of the Earth‘s interior has been made in the last 4 years in China. It is shown on three aspects: advancement made on some preexistent areas; pioneering on some new fields and new methods adopted.     

Using large dynamite shots to image the structure of the Moho from deep seismic reflection experiment between the Sichuan basin and Qinling orogen

The Qinling orogen was formed as a result of the collision between the North and South China blocks. The Qinling orogen represents the location at which the southern and northern parts of the Chinese mainland collided, and it's also the intersection of the Central China orogen and the north-south tectonic belt. There is evidence of strong deformation in this orogen, and it has had a long and complex geological history. We investigated the structure of the Moho in the southern Qinling orogen using large dynamite shot imaging techniques. By integrating the analysis of the single-shot and the move-out corrections profile, we determined the structure of the Moho beneath the northern Dabashan thrust belt and the southern Qinling orogen, including the mantle suture beneath Fenghuang mountain. The Moho is divided into two parts by the mantle suture zone beneath Fenghuang mountain:(1) from Ziyang to Hanyin, the north-dipping Moho is at about45–55 km depth and the depth increases rapidly; and(2)from Hanyin to Ningshan, the south-dipping Moho is at about 40–45 km depth and shallows slowly. The mantle suture is located beneath Fenghuang mountain, and the Moho overlaps at this location: the shallower Moho is connected to the northern part of China, and the deeper Moho is connected to the southern part. This may indicate that the lithosphere in the Sichuan basin subducts to the Qinling block and that the subduction frontier reaches at least as far as Fenghuang mountain.