Genesis of the Hongzhen metamorphic core complex and its tectonic implications

The Hongzhen metamorphic core complex is situated in the Yangtze plate to the east of the Dabie oro- genic belt. Its ductile detachment zone in the foot wall overprints on the metamorphic complex of the Proterozoic Dongling Group. The present profile of the ductile shear zone with consistent SW-dipping mineral elongation lineation shows antiform and reversed S-shape from northeast to southwest respectively. Exposure structures, microstructures and quartz C-axis fabric all indicate top-to-SW movement for the ductile shear zone. Recrystallisation types of quartz and feldspar in the mylonites demonstrate that the shear zone was developed under the amphibolite facies condition and at mid-crust levels. The metamorphic core complex formed in the Early Cretaceous with a muscovite plateau age of 124.8±1.2 Ma. Regional NE-SW extension along a SW-dipping, gentle detachment zone was responsible for formation of the core complex. Intrusion of the Hongzhen granite with a biotite plateau age of 124.8±1.2 Ma rendered the ductile shear zone curved, uplifted and final localization of the core complex. The Hongzhen metamorphic core complex suggests that the Early Cretaceous magma- tism in this region took place under the condition of regional extension and the eastern Yangtze plate also experienced lithospheric thinning.     

Computation and analysis of the geomagnetic field model in China and its adjacent area for 2003

Based on the geomagnetic data at 135 stations and 35 observatories in China in 2003, the Taylor polynomial model and the spherical cap harmonic model in China and its adjacent area for 2003 were established. In the model calculation, the truncation order of the model and the influences of the boundary restriction on the model calculation were carefully analyzed. The results show that the geomagnetic data used are precise and reliable, and the selection of the truncation order is reasonable. The Taylor polynomial model and the spherical cap harmonic model in China and its adjacent area established in this paper are consistent very well.     

Focal mechanism solutions and its tectonic significance in the trench of the eastern South China Sea

Introduction South China Sea (SCS) is located in the convergence zone between Euro-Asian plate, Pacific plate (Philippine plate) and Indian plate. Interactions of three plates made the crust of this region suffer tectonic stress in many directions and made the South China Sea be in the complex environ-ment of the tectonic stress. There are four different marginal types in the surrounding of the South China Sea: The tectonic zone of the rifting margin in the north of SCS, the NS direct…     

The correlation between the characteristics of seismic wave propagation in Western Caucasus and the geological–tectonic features of the region

The relationship between the characteristics of seismic waves in the Western Caucasus and the geological-tectonic structure of the region is studied for identifying the specificity of seismic propagation in the mountainous regions with a complicated geological structure and forecasting the characteristics of the propagation from the geological and tectonic data. The interpretation is presented for the estimates of the Q-factor of the medium (Q(f) ~ 55f^0.9 in the region of Sochi and Q(f) ~ 90f^0.7 in the region of Anapa), seismic wave enhancement in the upper crustal layers (A(f) ~ 1), and peak ground acceleration residuals, which were previously determined from the records of the local earthquakes and show the distributions of local variations in the parameters of seismic wave radiation and propagation. The obtained characteristics are interpreted in the context of the up-to-date information about the tectonic, geological, and deep structure of the epicentral zones in the Western Caucasus and neighboring territory of the Black Sea. The discrepancies revealed in the low-frequency behavior of the Q-factor in the vicinities of Sochi and Anapa is accounted for by the spatial scale and character of tectonic dislocations of the rocks in these regions. The local variations in the parameters of seismic radiation and propagation are probably related to the geological features of the region such as the fault structures, including the thrusts, shatter zones, oblique seismic boundaries, variations in the thickness and consolidation of the sedimentary cover, as well as the peculiarities in the structure and material composition of the basement.     

Southeastern extension of the Red River fault zone (RRFZ) and its tectonic evolution significance in western South China Sea

The tectonic evolution features in the western South China Sea (SCS) are directly related to the Tethys tectonic province. The Red River fault zone (RRFZ) comprises a large part of the Tethys tectonic province and is the boundary between the Europe Block and the India-Asia Block[1]. It serves as the contact between the uplift of the Qinghai-Tibet Plateau and the SCSopening. The RRFZ, which is an important dividing line in the geology of the SE Asia, is about 1000 km long in the co…     

~(40)Ar/~(39)Ar dating of shear deformation of the Xianshuihe fault zone in west Sichuan and its tectonic significance

TheNW-SEstrikingXianshuihefaultzoneslicesthesoutheasternTibetanPlateauandconnectssoutheastwardwiththeAnninghe-Zemuhe-Xiaojiangfaultzone,whichformahuge,activesinistralstrike-slipfaultzone(fig.1).ThisfaultzoneisanimportantseismicfaultineastTibet[1-5].EarthquakegeologystudiesandoffsetpatternsofyounggeologicalfeatureshaveshownthatlateQuaternarysinistralsliprateoftheXianshuihefaultzonereaches13mm/a[1,2].TheXianshuhefaultzoneconsistsoftwomainbranches,theDaofufaultbranchinthewestandtheXianshuih…     

Late stage thermal history of the Songliao Basin and its tectonic implications: Evidence from apatite fission track (AFT) analyses

Apatite Fission Track (AFT) data from the Songliao Basin indicates that the late stage tectonic movements in the Songliao Basin have zoning in space and episodes in time. The late stage tectonic movements started from the east part of the basin and migrated westward. AFT ages in the east part of the basin are older than those in the west part of the basin, suggesting that the uplift occurred earlier in the east than in the west. The denudation thickness in the east part of the basin is significantly greater than that in the centre and west. The thermal history evolved two episodes of rapid cooling and subsequent slow cooling processes. Age-depth relationship derived from the AFT data indicates a four-episode denudation history. Further Monte Carlo random simulation of the AFT data reveals the four changing points of the thermal evolution at 65 Ma, 43.5 Ma, 28 Ma and 15 Ma, respectively. The uplifting and denudation rates from different episodes of evolution are proportional to the plate convergence rate. Based on the above analyses and the regional geologic background, it is concluded that the late stage thermal events in the Songliao Basin are the far field response to the subduction of the Pacific Plate under the Eurasian Plate. The first episode of the rapid cooling probably started at the end of the Nenjiang Formation, climaxed at the end of the Cretaceous and ceased at the Late Eocene. The subsequent slow cooling lasts another 15 Ma. The first episode of the evolution is the far field response to the major episode of the Yanshan Movement and subsequent series of the tectonic reorganization, especially the directional change of the Pacific Movement and also the subduction of the Indian Plate underneath the Eurasian Plate. While the second episode of the evolution is the far field response to the extension and closure of the Sea of Japan. Extension led to the migration and converging of the mantle heat flow to the Sea of Japan and resulted in the rapid cooling of the Songliao Basin.     

Parameter estimation of the stochastic AMR model and its application to the study of several strong earthquakes

Introduction According to many published papers, seismicity in time-space domain shows some characteristics, such as doughnut epicenter distribution (Mogi, 1969) or quiescence of seismic activity before large earthquakes (WANG, et al, 2002), and aftershock decay (Ogata, 1998). In recent years, more and more seismologists (Lynnr, Steven, 1990) have found that many strong earthquakes are preceded by enhancing regional seismicity and accelerating strain energy release (ZHANG, et al, 2001). T…     

Seismogenic model of earthquakes in groups in tectonic block and analysis for some features of earthquake precursory field

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Petrogenesis and tectonic implications of Late-Triassic high εNd(t)-εHf(t) granites in the Ailaoshan tectonic zone(SW China)

High εNd(t)-εHf(t) granites are robust evidence for crustal growth. In this paper we report results of petrologic, geochronological and geochemical investigations on the Huashiban granites from the Ailaoshan tectonic zone in western Yunnan(SW China). Zircon grains separated from the two samples(10HH-119 A and 10HH-120A) yield the weighted mean 206Pb/238 U ages of 229.9 ± 2.0 Ma and 229.3 ± 2.3 Ma, respectively, interpreted as the crystallization ages of the granites. Based on our results, in combination with the existing U-Pb geochronological data for the Ailaoshan metamorphic rocks, we propose that the Ailaoshan Group might be a rock complex composed of the Mesoproterozoic, Neoproterozoic, Hercynian, Indosinian and Himalayan components, rather than a part of the crystalline basement of the Yangtze block. The zircon grains show highly depleted Lu-Hf isotope compositions, with positive εHf(t) values ranging from 8.4 to 13.1. The Huashiban granites have high SiO2(72.66 wt%–73.70 wt%), low Mg#(0.28–0.34) with A/CNK=1.01–1.05, and can be classified as peralumious high-K calc-alkaline I-type granites. A synthesis of these data indicates that the Ailaoshan tectonic zone had evolved into a post-collisional setting by the Late-Triassic(229 Ma). Genesis of the Huashiban high εNd(t)-εHf(t) granites involved into two processes:(1) underplating of the sub-arc mantle into the lower crust, and(2) remelting of the juvenile crustal materials in response to the upwelling of the asthenospheric mantle in the post-collisional setting.     

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.     

A study on characteristics of tectonic block motion and tectonic setting of strong earthquakes in northern part of the Shanxi fault depression zone

Introduction Shanxi fault depression zone (SFDZ) is one of important Cenozoic fault basin zones and strong earthquake belts in Chinese mainland. Its northern part has aroused wide research interests due to the complicated tectonics and high activity of strong earthquakes there. Early researches on this depression zone were carried out since 60s of last century (DENG, et al, 1973; DENG, YOU, 1985; LU, DING, 1985; XU, 1990; XU, et al, 1996, 2002). In 90s of last century, the geologica…     

3-D rheologic model of earthquake preparation (II): Strain field and its applications

Introduction Based on the elastic theory of the hard inclusion (Dobrovolskii, 1991), we developed an inclusion theory of rheologic medium, and applied the results of bulk-strain field of a rheologic inclusion model to explain the spatial-temporal evolution process of earthquake precursors (SONG, et al, 2000). In the former paper (SONG, et al, 2003), we derived the viscoelastic displacement field of the rheologic inclusion model on the basis of the analytic expression of displacement field o…     

New paleontological evidence on time determination of the east part of the Eastern Kunlun Mélange and its tectonic significance

The area studied is located in the middle ofQinghai Province, at the south end of Qaidam Basinand the eastern part of the Eastern Kunlun Mountainsin Dulan County, Haixi Prefecture and Madoi County,Golog Tibetan Autonomous Prefecture, Qinghai Prov-ince (fig. 1). The area is situated tectonically in theeast of the Eastern Kunlun Mountains at the junctionof the Eastern Kunlun Orogenic Belt with the WesternQingling which is also the junction of the QaidamPlate with the South China Plat…     

Mesozoic radiolarian chert from the middle sector of the Yarlung Zangbo suture zone,Tibet and its tectonic implications

The middle sector of the Yarlung Zangbo suture zone stretches over 200 km long from Ngamring through Geding to Rinbung, roughly along Yarlung Zangbo River valley (Fig. 1). This belt resulted from the closure of the Tethyan ocean and the collision be- tween Indian plate and Lhasa block[1―8]. Lots of works demonstrated that rifting of the Tethyan basin in southern Tibet started from Triassic time. Initial oce- anic crust appeared in the Late Jurassic, and then ex- perienced a rapid sprea…     

A nested model of the Cariaco Basin (Venezuela): description of the basin’s interior hydrography and interactions with the open ocean

A high-resolution (1/60°), three-dimensional numerical circulation model of the Cariaco Basin (Venezuela) is constructed by nesting the Regional Ocean Modeling System (ROMS) in the 1/12° global Hybrid Coordinate Ocean Model (HYCOM). A new bathymetry, computed by merging DBDB2 data and in situ depth measurements using optimal interpolation, is described. This new bathymetry corrects the depth of the channels that connect the Cariaco Basin with the open ocean and which play a very important role in the basin circulation. Results from a 2004 ROMS hindcast are presented. Observations (temperature, salinity, and currents) are used to validate the model results before using the model to describe the annual cycle of the Cariaco Basin and the interactions between the basin and the open ocean. Two modes of interaction are described, the first being the meanders and eddies that travel westward with the Caribbean Current, and the second being a subsurface eastward current that flows along the north coast of South America. The circulation path within the basin is directly related to the intensity of this current. Both mechanisms described play a role in the ventilation of the basin. The present study is also an example of the feasibility of one of the objectives of GODAE (Global Ocean Data Assimilation Experiment): downscaling from a large-scale model to a regional model. In particular, the nesting ratio of 5 used in this work demonstrates that a high-resolution model can be successfully nested in HYCOM.     

Inversion of gravity and topography data for the crust thickness of China and its adjacent region

Introduction Since the middle of the century, gravitational isostasy has been a fundamental hypothesis for inverting the gravity data to find the crust thickness. Geophysicists have done a lot of researches on using gravity data to investigate the depth of Moho discontinuity. Since 1980, the International Lithosphere Program emphasized the importance of investigating the Moho depth variation. Thereafter a lot of results have been published in the world (Braitenberg et al, 2000; Kaban et al,…     

A new species of Macrocnemus (Reptilia: Protorosauria) from the Middle Triassic of southwestern China and its palaeogeographical implication

A new species of the terrestrial protorosaur Macrocnemus is briefly described and named Macrocne-mus fuyuanensis sp. nov. The specimen was found from the marine Triassic of Yunnan Province, southwestern China. It differs from the type species M. bassanii in having the following characters: (1) 17 or 18 dorsal vertebrae; (2) relatively long humerus; and (3) femur longer than tibia. M. fuyuanensis is the only record of the genus outside Monte San Giorgio area and the first definite terrestrial reptile found from the marine Triassic of China. The age of the fossil-bearing Zhuganpo Member of the Falang Formation is believed to be the Ladinian based on associated vertebrate fauna. The existence of Mac-rocnemus in the limestone suggests the presence of a terrestrial ecosystem which probably originated from the Anisian and well developed in the Ladinian in this region. The island system along the north-coast of Tethyan during the Triassic was probably an important route for the exchanges of reptiles, especially the terrestrial reptiles between the west and east.     

Distribution of ~(226)Ra in the Arctic Ocean and the Bering Sea and its hydrologic implications

The Arctic Ocean, the northernmost parts of the earth, covers the total surface area of 14.79 million square kilometers and amounts to only about 4% of global ocean surface area. Although its surface area is the smallest in the four major oceans, the Arct…