Diagenesis tectonic setting and U-Pb dating for zircon from Tanjianshan Group in northern margin of Qaidam Basin

The shape,texture,content and REE characteristics of zircons from the O2tnd-1,O3tnd-2,O3tnd-3pyroclastic rock of Tanjianshan Group on the north margin of Qaidam Basin indicate that the O2-3tndis the product of volcanism during the transitional period from ocean-land interim crust to oceanic crust.The U-Pb surface ages obtained from O3tnd-2and O3tnd-3can be divided into 9 groups,every age group coincides with the period when significant tectonic-heat event took place at Oulongbuluke micro-continental base of northern Qaidam,suggesting that the base rocks have provided materials for the formation of sedimentary and volcanic rock in O2-3tnd.The volcanic rocks of O3tnd-3formed at 440 Ma,with time gap 46 Ma to those of O1tna-1and O3tnd-3may represent the minor period that Xitieshan back-arc extension have lasted,the scale of back-arc basin that formed in Xitieshan extension may approach to 1 400 km.Based on the test of ithochemistry data for major elements and analysis of Sr isotope geochemistry for the clastic sedimentary rock in O1tna-2and O3tnd-2,the authors get the conclusion that the O1tna-2and O3tnd-2of Tanjianshan Group may form in back-arc basin environment,while the lithology difference between these two formations may reflect the changes of geodynamic processes as the diagenesis tectonic environment transformed from continental margin depression to adjacent sea basin.     

Precambrian crustal evolution in Rangrim Massif,Korean Peninsula

Petrological, chronological and geotectonic geological analysis of Precambrian metamorphic rock in Korean Peninsula shows that the remnants （ 〉3.4 Ga） of continental nuclei crust were formed in the Paleoarchean in the Rangrim Massif. In the massif, the main formation ages of continental crust range from 3.2 Ga to 2.5 Ga, its important growing period was 2.8 - 2. 5 Ga. The subsequent expansion period of the Rangrim Massif was 2. 4 - 2. 2 Ga. The division events occurred in 1.85 Ga and in the Late Paleoproterozoic-Early Mesoproterozoic, respectively. Since then the massif was relatively stable. However, the last division of the Rangrim Massif occurred at 793 Ma.     

Lower Permian formations of different geodynamic environments in A'nyemaqen ophiolitic zone (Buqingshan Mts., eastern Kunlun, Qinghai Province, China)

Lower Permian formations within the Buqingshan Mountains (A'nyemaqen ophiolitic zone, eastern sector of the eastern Kunlun) were formed in the following paleogeodynamic environments (from north to south): (1) shelf and slope of a passive continental margin in a marginal sea; (2) partially Permian metamorphic rocks of subduclion-accretion complexes and volcanogenic rocks of an ensimatic island arc, of the age limited from above by the Asselian - Sakmarian; and (3 ) an island arc slope and oceanic trench. Subduction-accretion complexes and the island arc volcanites are overlain with a sharp angular unconformity by a carbonate-conglomerate sequence, which presents as local molasse of the Early Permian age. Based on fusulinids from the basal limestone, the age of the local molasse is first defined as the Yakhtashian-Bolorian, i.e. Artinskian-Kungurian (?). The thorough investigations revealed that the initial closure of the eastern Paleotethys within the eastern Kunlun corresponded to the Sakmarian-Yakhtashian (Artinskian) boundary, whereas in the western Paleotethys sector (Northern Pamirs) the closure occurred considerably earlier, prior to the Late Bashkirian. Thus, the idea that the Paleotethys in the eastern Kunlun reached its maximum width in the Permian, is highly questionable. During the Early Permian the A'nyemaqen branch of the Paleotethys intensely decreased. Beginning from the Bolorian (Kungurian) and up to the end of the Permian this branch represented its relict in the form of a marginal sea depression. It may be suggested that the Paleotethys closure in the A'nyemaqen took place gradually from the west to the east and covered a long period from the Late Carboniferous to the terminal Early Permian.     

Precambrian crustal evolution in Rangrim Massif , Korean Peninsula

Petrological, chronological and geotectonic geological analysis of Precambrian metamorphic rock in Korean Peninsula shows that the remnants ( > 314 Ga) of continental nuclei crust were formed in the Paleo-archean in the Rangrim Massif. In the massif, the main formation ages of continental crust range from 3.2 Ga to 2.5 Ga, its important growing period was 2.8 - 2.5 Ga. The subsequent expansion period of the Rangrim Massif was 2.4 - 2.2 Ga. The division events occurred in 1.85 Ga and in the Late Paleoproterozoic－Early Mesoproterozoic, respectively. Since then the massif was relatively stable. However, the last division of the Rangrim Massif occurred at 793 Ma.     

Geochronology,geochemistry and Hf isotopic study of Early Carboniferous granodiorites in Taerqi region,central Daxing'anling and its tectonic implication

Zircon U-Pb age,whole rock geochemical and zircon Hf isotopic data are presented for Late Paleozoic granodiorites from the Taerqi region,central Daxing'anling to constrain its petrogenesis and tectonic implication.LA-ICP-MS zircon U-Pb age data indicates that the Late Paleozoic granodiorites were emplaced with age of333.4 ± 2.2 Ma(Early Carboniferous).Geochemically,the granodiorite samples have Si O2= 60.54%-71.40%,Na2 O = 4.04%--4.66%,K2 O = 1.65%--4.27% and Mg O = 0.96%--3.53%,belonging to medium-K to high-K calc-alkaline I-type granites.They are slightly enriched in large ion lithophile elements(e.g.Rb,Th,U and K) and light rare earth elements,and depleted in high field strength elements(e.g.Nb,Ta and Ti),with εHf(t) values of 8.0--11.8 and Hf two-stage model ages of 586-829 Ma.All these geochemical features suggest that the primary magma was derived from partial melting of Neoproterozoic to Phanerozoic newly accreted lower crust.According to the geochemical data and regional geological investigations,the Early Carboniferous granodiorites formed in an island arc setting linked to the subduction of the Paleo-Asian Oceanic Plate beneath the Xing'an Terrane.This also implies that the Xing'an and Songliao terranes have not amalgamated before the Early Carboniferous.     

THE TECTONICS BENEATH THE OKINAWA TROUGH

The nature of the crust of the Okinawa Trough is different from that of the continental shelf in the East China Sea. The crust beneath the Trough is in transformation from continental to oceanic and the depth of MOHO in the northern section of the Trough is deeper than in the southern section. Thick sedimentary strata of Neogene and Pleistocene ages are deposited in the Okinawa Trough, and divided into three layers: the upper horizontal layer, the middle layer lightly folded and the lower deformed layer. They were formed in Pleistocene, Pliocene, and Miocene to Paleogene, respectively. The tectonic movement in the southern section is stronger than that in the northern section. Somevolcanic seamounts appear on the bottom of the Trough. On both slopes of the Trough are developed many normal faults and the intrusive igneous rocks. The Okinawa Trough, the back-arc basin,, is an embryonic marginal basin in rifting and spreading. The formation of the Okinawa Trough started in the early Pliocene. The transform     

Episodic orogeny deduced from coeval sedimentary sequences in the foreland basin and its implication for uplift process of Longmen Mountain, China

Longmen Mountain located at the boundary between the Sichuan Basin and Tibetan Plateau,representing the steepest gradient of any edges of the plateau.Three endmember models of uplift process and mechanism have been proposed,including crustal thickening,crustal flow,and crustal isostatic rebound.Here we use coeval sedimentary sequences in the foreland basin to restraint uplift process and mechanism in the Longmen Mountain.The more than 10,000 m thick Late TriassicQuaternary strata filled in this foreland basin and can be divided into six megasequences that are distinguished as two distinct types.The first type is the wedge-shaped megasequences which are sedimentary response of strong active thrust loading events,characterized by a high rate of subsidence and sediment accumulation,coarsening-upward succession and a dual-sourced sediment supply.This type includes Late Triassic,Late Jurassic to Early Cretaceous and Late Cretaceous to Paleogene megasequences.The second type is the tabular megasequences,characterized by the low rate of subsidence and sediment accumulation,finingupward succession,and a single-sourced sediment supply,which is sedimentary response of isostatic rebound and erosion unloading.This type includes the Early to Middle Jurassic,Middle Cretaceous and Neogene to Quaternary megasequences.Basing on sedimentary,active tectonic,geomorphic evidence,we infer that the direction has been reversed from SSWdirected sinistral strike-slip to NNE-directed dextral strike-slip during 40-3.6 Ma,and since 3.6 Ma,the Longmen Mountain thrust belt belong to times of isostatic rebound and erosional unloading with NNEdirected dextral strike-slip.This suggests that crustal isostatic rebound is a primary driver for uplift and topography of the present Longmen Mountain.The Wenchuan(Ms8.0) earthquake,which ruptured a large thrust fault with NNE-directed dextral strikeslip along the range front,is an active manifestation of this crustal isostatic rebound process with dextral strike-slipping and shortening.This process may be the cause for the Wenchuan Earthquake and the apparent paradox of high relief,little shortening,the relative dearth of historical seismicity in the region.     

Sedimentary environment of Middle Permian Zhesi Formation in Solon area of Inner Mongolia

Through a sampling analysis of trace elements and REE in the section of Middle Permian Zhesi For- mation in Solon area of Inner Mongolia, it is found that the overall sandstone and mudstone samples are charac- terized by LREE-enrichment. By analysis of the discriminant index of the sedimentary environment, the tectonic setting of Zhesi Formation is determined to be a continental island arc environment. Meanwhile, according to the sedimentary structure and lithology combination, Zhesi Formation is divided into the neritic-bathyal reten- tion-reduction environment as well as the delta front and prodelta sedimentary systems.     

Lower Permian formations of different geodynamic environments in A＇nyemaqen ophiolitic zone （Buqingshan Mts., eastern Kunlun, Qinghai Province, China）

Lower Permian formations within the Buqingshan Mountains （A＇nyemaqen ophiolitic zone, eastem sector of the eastern Kunlun） were formed in the following paleogeodynamic environments （from north to south） ： （ 1 ） shelf and slope of a passive continental margin in a marginal sea; （2）partially Permian metamorphic rocks of subduction-accretion complexes and volcanogenic rocks of an ensimatic island arc, of the age limited from above by the Asselian - Sakmarian; and （3） an island are slope and oceanic trench. Subduction-accretion complexes and the island are volcanites are overlain with a sharp angular unconformity by a carbonate-conglomerate sequence, which presents as local molasse of the Early Permian age. Based on fusulinids from the basal limestone, the age of the local molasse is first defined as the Yakhtashian-Bolorian, i.e, Artinskian-Kungurian （？）. The thorough investigations revealed that the initial closure of the eastern Paleotethys within the eastern Kunlun corresponded to the Sakmarian-Yakhtashian （Artinskian） boundary, whereas in the western Paleotethys sector （Northern Pamirs） the closure occurred considerably earlier, prior to the Late Bashkirian. Thus, the idea that the Paleotethys in the eastern Kunlun reached its maximum width in the Permian, is highly questionable. During the Early Permian the A＇nyernaqen branch of the Paleotethys intensely decreased. Beginning from the Bolorian （Kungurian） and up to the end of the Permian this branch represented its relict in the form of a marginal sea depression. It may be suggested that the Paleotethys closure in the A＇nyemaqen took place gradually from the west to the east and covered a long period from the Late Carboniferous to the terminal Early Permian.     

Stratigraphic framework of late Paleozoic in northeastern China

Late Paleozoic strata in northeastern China are distributed in a zonal pattern around the old-land on the Jiamusi-Mongolia Block.They are composed of active deposits in the regular distributed tectonic lithofacies zones.This indicates that the late Paleozoic strata belong to continental margin deposits.According to the strong conformability of the sedimentary strata in the same continental margin and distinct differences among the three continental margins,three stratigraphical regions of the Jiamusi-Mongolia Stratigraphical Province are recognized along the northern,southern and eastern margins of the Jiamusi-Mongolia Block,named respectively as Xing’an Stratigraphical Region,Inner Mongolia grass--Songhua River Stratigraphical Region and Baoqing--Hunchun Stratigraphical Region.Due to the characteristics of continental margin deposits and active sediments,the strata can be correlated on the level of formation by the methods of analysing the rock association in the same stratigraphic region.Therefore,some revisions of the lithological formations of the late Paleozoic strata in northeastern China have been made,and a new chart of lithostratigraphic correlation has been proposed.Furthermore,the present stratigraphic framework is setting on the International Stratigraphic Chart on the level of stage,after comprehensive researches to lithostratigraphy,biostratigraphy and chronostratigraphy,especially the conodont biostratigraphy and isotopic ages of volcanic rocks obtained in recent years.     

Reconstruction of protoliths of metamorphic rocks and tectonic setting of Wolegen Group

Parametamorphic rocks from Arong County in southeastern Inner Mongolia- Daxinganling district are regarded as Proterozoic in age, belonging to the Wolegen Group and composed of volcanoclastic and sand- stone in origin, and have been disputed in tectonic setting. Because of the stability in metamorphism, the rare earth elements indicate the features of their protoliths. The authors integrated the petrologic methods with the geochemical parameters which include ЕREE, ЕLREE/NHREE, δCe, δEu, La/Yb, Sm/Nd, Th/Sc and the standard values of chondrite. The results show that the protoliths of Wolengen Group may be a group of volcanoclastic and continental margin clastic rocks, and their tectonic setting is the continent island arc.     

Numerical modeling of neotectonic movements and state of stresses in the central Seismic Gap region,Garhwal Himalaya

This paper presents finite element modeling （FEM） to simulate the present-day stress field and crustal deformation using NE-SW structural section in the central Seismic Gap region of the Garhwal Himalaya. Our study deals with the effect of geometrical characteristics and rock layer parameters on the upper crust. Modeling results show that two types of tectonic regimes developed in the central Seismic Gap region： the geotectonics of the northern part has been controlled by regional compression, whereas southern part is characterized by regional extension. Correspondingly, thrust faults are induced in the northern part and normal faults are extensively developed in the southern front. Those evidences noticeably indicate that the compressive tectonic environment of the Himalaya becomes change into the extensional tectonic regime in its front. The computed shear stress accumulation along the northern fiat of Main Himalayan Thrust （MHT） implies that considerable amount of interseismic stress is building up along the MHT system in the Himalaya, which ultimately release through the possible future great Himalayan earthquake （M 〉 8）. The comparison between our modeled stress field, faulting pattern and horizontal shortening rate with the distribution of the microseismic events, focal mechanism solutions, active faulting and GPS data in the central Seismic Gap region shows good agreement.     

Seismic sequence and depositional evolution of slope basins in mid-northern margin of the South China Sea

As one of the biggest marginal seas in the western Pacific margin, the South China Sea (SCS) experienced continental rifting and seafloor spreading during the Cenozoic. The northern continental margin of the SCS is classified as a passive continental margin. However, its depositional and structural evolution remains controversial, especially in the deep slope area. The lack of data hindered the correlation between continental shelf and oceanic basin, and prevented the establishment of sequence stratigraphic frame of the whole margin. The slope basins in the mid-northern margin of SCS developed in the Cenozoic; the sediments and basin infill recorded the geological history of the continental margin and the SCS spreading. Using multi-channel seismic dataset acquired in three survey cruises during 1987 to 2004, combined with the data of ODP Leg 184 core and industrial wells, we carried out the sequence stratigraphic division and correlation of the Cenozoic in the middle-northern margin of SCS with seismic profiles and sedimentary facies. We interpreted the seismic reflection properties including continuity, amplitude, frequency, reflection terminals, and 15 sequence boundaries of the Cenozoic in the study area, and correlated the well data in geological age. The depositional environment changed from river and lake, shallow bay to open-deep sea, in correspondence to tectonic events of syn-rifting, early drifting, and late drifting stages of basin evolution.     

Tectonic Characteristics and Evolution of Bohai Bay Basin,China

Synthetical analyzing the deep geophysical data within Bohai bay basin the authors detect the deep crustal struc-ture presenting high geothermal flux, thinned crust and arched Moho discontinuity, and the basin basement belongs to rigid continental crust. The development of the basin was controlled by two - dimensional faults in NNE and NWW directions. The tectonic units of the basin can be subdivided into three structural divisions: the east, middle and west division. The basin is considered as a continental rift. The tectonic background and regional right條ateral stress field during the late Cretaceous and Paleogene were a compound result of the Kula Plate W-directional subducting under Eurasia Continental Plate in 80-74Ma and the Philippine sea Plate W-directional subducting under the Eurasia Continental Plate since 60Ma, the long -rang effect of the India Continental Plate wedging into the Eurasia Continental Plate and of the Siberia Plate SE梔irectional relatively moving.     

A PRELIMINARY STUDY ON THE TECTONICS AND EVOLUTION OF THE FUJIAN COAST CHANGLE-NANAO METAMORPHISM ZONE

The Fujian coast Changle-Nanao metamorphism zone rocks are composed of gneiss, schist andhornblendite which had gone through metamorphism of amphilbolite facies, and followed the large-scaleintrusive mass of gneissic granite. The zone was orginally composed of Early Palaeozoiccontinental margin and island arc volcanic and sedimentary rocks. In 180-150 Ma, the zone undermentductile shear deformation and amphibolite facies metamorphism; and in 130-80 Ma, rose rap1dly andco1lided with the Southeastem China continen1al rnargln volonic rocks a10ng the NE trending ductileshear belt. The above geologic setting laid the present Fujian coast tectonic foundation whose formationand evolution are known to be interrlatal with the sueduction and collision of the Taiwan CentralRange.     

Gold mineralization in Jiaodong Peninsula and destruction of North China Craton: Insights from Mesozoic granite

Large-scale tectonic magmatism and extensive gold mineralization have occurred in the eastern North China Craton(NCC)(Jiaodong and Liaodong peninsulas) since Mesozoic, which indicated that the region experienced decratonization process. The genesis type of granites evolved from S-type to I-type and A-type during Late Jurassic to Early Cretaceous, and thus reflects the evolution of geodynamics in the Late Mesozoic, indicating the varied subduction process of the Paleo-Pacific Plate(PPP) and the c...     

Geochemistry and zircon U-Pb chronology of quartz porphyry in Yuejinshan Fe-polymetallic deposit of Heilongjiang

Quartz porphyry in Yuejinshan Fe-polymetallic deposit is one of the rock masses, which formed the granite belt relating to the late Yanshanian skarn-type Fe-polymetallic deposits in the eastern Heilongjiang, and is also closely related to the metallogenic space of the Fe-polymetallic deposits. Quartz porphyry has the zircon U-Pb concordia age of 125.0±1.1 Ma, overall showing high Si(SiO_2=74.48%--75.00%), rich alkaline(Na_2O+K_2O=7.93%--8.17%, K_2O/Na_2O=1.39--1.46), and poor Mg(MgO=0.27%--0.31%), with the A/CNK value being 0.95--0.99, having the characteristics of obvious enrichment of LREE and medium Eu negative anomaly(0.69--0.74), indicating that the rock belongs to quasi-aluminous high potassium calc-alkaline series and has the same characteristics as those of the I-type granite. The rock is also characterized by the enrichment of LILE and active incompatible elements, and depletion of HFSE such as Nb, Ta, P and Ti, with the Mg~# value being 32--37, indicating that the rock originated from partial melting of crustal materials. It can be concluded from the above characteristics or data and from the distribution of contemporary magmatic rocks in the area, that quartz porphyry in Yuejinshan Fe-polymetallic deposit was formed in a tectonic background of the subduction of the paleo-Pacific plate in late Yanshanian.     

Mapping of geomorphic dynamic parameters for analysis of landslide hazards: A case of Yangbi river basin on the upper Lancang-Mekong of China

Landslides are common hazards in orogenic belt areas. However, it is difficult to quantitatively express the driving effects of tectonic uplift and stream erosion on the occurrence of landslides on large spatial scales by conducting field investigations. In this study, we analyzed a relatively large region that extends over the Yangbi River basin on the upper Lancang-Mekong in China. A series of quantitative indices, including kernel density of the landslide(KDL), hypsometric integral(HI), steepness index(ksn), stream power(?), and stream power gradient(ω) were used to explore the promoting effects of tectonic uplift and stream action intensity on landslides by mapping geomorphic dynamic parameters combined with actual landslide data. The analysis showed that the HI value in the highest landslide risk area was approximately 0.47, and that the KDL in the region can be expressed as a function of steepness or stream power gradient of the channel network, namely, KDL = 0.0127 Ln ksn-0.0167(R~2 = 0.72, P 0.001) and KDL = 0.0219 Ln ω-0.0558(R~2 = 0.21, P 0.02). Therefore, the lower reach of the Yangbi River basin, with higher steepness and stream power gradient, usually has a high uplifting rate and stream incision that drives landslides and causes the entire river network system to be in a stage of longterm active erosion. Furthermore, the results suggest that sediments were being rapidly discharged from the steep tributary channels to the mainstream. This practical situation highlights that the downstream area of the river basin is a high-risk area for landslide hazards, especially in association with heavy rainfall and earthquakes.     

THE GRAVITY FIELD AND DEEP TECTONICS OF THE RYUKYU T-A-BA SYSTEM

This paper reports on our research on the gravity data around the Ryukyu T-A-BA system, and presents the results obtained and the calculation methods, such as, Moho calculation, spectrum analysis, smooth filtering, analytic extension, horizontal derivation, correlative analysis, etc. used in this study. In contrast to the information obtained from seismic profiles, the analyses show that oceanic and continental crusts demarcated by the Ryukyu T-A-BA system are very different in gravitational and tectonic characteristics. The structures in the T-A-BA region are very complicated and linearity of the gravity field and structure is very obvious. The Okinawa Trough with its two groups of fault structures in their respective NW and NNE strikes is an active neo-spreading back are basin whose development and evolution is greatly influenced by faulting.