Earth Science Frontiers

2019, 26(2): 0-0.

Abstract ( 108 )

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2019, 26(2): 0-0.

Abstract ( 121 )

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Superposed buckle folding at the upper structural levels in western Dabashan: example from the Jianchi area in Zhenba County

ZHANG Zhongyi

2019, 26(2): 1-15.
DOI: 10.13745/j.esf.sf.2019.3.10

Abstract ( 276 )

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Following the 1∶10000 geological mapping and structural analysis, we focused chiefly in this paper on the spatial variations and controlling factors for the superposed fold types, and determined the tectonic nature and the age of folds in the Jianchi area of western Dabashan. Our study indicated the occurrence of two sets of tectonic folds involving undetached Triassic to Jurassic rocks. The types of folds are: (1) NE-EW trending folds (F1). They occur as interrupted NE-EW fold domains composed of outcrop or map-scale fold groups, representing the pre-existing hinge zones of parasitic macrofolds formed regionally on the steeply inclined southern limb of the Micangshan main antiform. (2) NW-NNW trending folds (F2). The F2 folds are curved folds that coincide with the Daba arc and are dominated by a unique structural style of wide spaced anticlines to form a syclinorium at the eastern margin of a foreland depression belt. The west-verging NW-NNW trending folds overprint the earlier NNE-EW ones almost orthogonally to produce two types and four modes of outcrop scale superposition, where two sets of wide-angle superposed flexural-slip striations were observed. The NE-EW folds vanish into Upper-Middle Jurassic syntectonic deposits (Ts1Ts4) and are enveloped by late Middle Jurassic sequences (Ts5 and Ts6). They are interpreted as foreland growth folds formed in association with Late Triassic collision of South Qinling orogeny around 213178 Ma, and occurred as a result of southward-directed progressive non-coaxial shearing during the late-stage development of the Micangshan structure belt. However, the NW-NNW trending folds, which affected the complete Mesozoic and pre-mesozoic rock series, developed as a consequence of the Dabashan foreland deformation during a intraplate orogenic episode (160120 Ma). And the tectonic controlling factor for the folds was to maintain a long-term stable strain field of bulk pure shear. Although the hiatus (about 10 Ma) between the two compressive folding events was brief, the superposed fold sets of different ages were markedly different in styles, tectonic nature and formation mechanism, suggesting a significant change in tectonic driver for the two sets of folds.

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Unconformity characteristics, age constraint and tectonic significance of the Weng'an Dome in southwestern Yangtze Block

SONG Zhidong,YAN Danping，QIU Liang，ZHANG Yixi

2019, 26(2): 16-41.
DOI: 10.13745/j.esf.sf.2019.3.1

Abstract ( 161 )

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The Weng'an Dome is located in the transitional region between the Hubei-Chongqing-Hunan-Guizhou dome group in near N-S direction and the Qianzhong Uplift in the E-W direction. Our research suggests that the Wengan Dome is essentially a superimposed tectonic dome. Caledonian, Indosinian and Yanshanian unconformities are developed and well retained in the Wengan Dome and its surrounding area. Unconformity characteristics corresponding to key tectonic events and age constraint are important factors in reconstructing regional geology and tectonics. Our results can be summarized as follows: (1) The Wengan Dome experienced three major deformations during the Paleozoic and Mesozoic. It is a superimposed tectonic dome with cratonic basement. The S2/AnS2, T2/AnT2 and J2/AnJ2 unconformities are distinguishable within the Weng'an Dome and its surrounding area, and they correspond to the Caledonian, Indosinian and Yanshanian tectonic movements in the Yangtze Block, respectively. And (2) The ages of unconformities, constrained by using detrital zircon U-Pb method, demonstrate that the Caledonian, Indosinian and Yanshanian tectonic events occurred during 426415, 246233 and 168150 Ma, respectively. Through structural and chronological analyses of unconformities, we also explored the tectonic evolution of the dome, which shall contribute to the in-depth study of the dome structure.

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Fluid records within the Ailao ShanRed River shear zone and their geological significance

CHEN Yu, CAO Shuyun，DENG Youguo，CHENG Xuemei，Lv Meixia，DONG Yanlong

2019, 26(2): 42-57.
DOI: 10.13745/j.esf.sf.2019.3.7

Abstract ( 171 )

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Folding during the evolution of mid-lower crustal detachment fault zone: a case study of the Liaonan metamorphic core complex.

ZHENG Yuanyuan，ZHANG Ruoyu，GAN Haonan，LI Hongke，JIANG Hao， ZHANG Juyi，LIU Junlai

2019, 26(2): 58-71.
DOI: 10.13745/j.esf.sf.2019.3.2

Abstract ( 196 )

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Many fold structures are well-preserved in mid-lower crustal detachment fault zones owing to progressive shearing along the fault zone during exhumation of metamorphic core complexes. These fold structures, like host rocks, bear important information on the evolution of the detachment fault zone. In this paper, we studied the various fold structures from the Jinzhou detached fault zone of the Liaonan metamorphic core complex. Based on the relationship between mylonitic foliation and fold, the folds are subdivided into pre-, syn-, and post-shearing folds. The syn-shearing folds can be further grouped into early (a1), mid (a2) and late (a3) syn-shearing folds. The transition from early to late syn-shearing is ascribed to progressive shearing during the detachment faulting. By means of morphological and crystallographic fabric analysis and quartz paleothermometry, we performed preliminary examination on the formation mechanism of folds along the detachment fault to provide constraints to the detachment faulting process in southern Liaoning. Our study shows that differences in macroscopic characteristics or relationship between foliation and folds of different stages, as well as morphological and crystallographic fabric characteristics of quartz grains in the folded rocks, can provide corroborative evidence for determining and restoring formation mechanism of folds. We suggest that the folds from the detachment fault zone result from progressive shearing under the combined effects of buckling and rheological properties of folded layers. During progressive shearing, a1 and a2 stage folds are dominated by buckling, while a3 stage fold is attributed primarily to flexural slip. Folding mainly occurred at a temperature range of 500 to 380 ℃. The detachment fault zone evolution maintained a relatively steady strain state overall from the early to late stage, while a weak contraction occurred in the NE-SW direction during the regional NW-SE extension in the Jinzhou detachment fault zone.

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Discussion on the dynamic mechanism of the Late Cretaceous magmatism-metallogenesis in Xiuwacu, northwestern Yunnan Province.

JIANG Xiaojun, CHEN Zhengyu, LI Wenchang, YU Haijun, LI Chao, ZHANG Xiangfei, WANG Zhongqiang, YAN Qinggao

2019, 26(2): 137-156.
DOI: 10.13745/j.esf.sf.2019.2.7

Abstract ( 177 )

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The Xiuwacu W-Mo deposit is a middle-large porphyry magmatic hydrothermal deposit located at the central section of the Yidun-Geza orogenic belt in northwestern Yunnan Province. Adverse natural environment in this region has delayed studies on Late Cretaceous tectono-magmatic activities in Late Yanshan, with focuses mainly on geochronology, petrogenesis, geodynamic background, fluid characteristics and source of ore-forming materials related to the Late Cretaceous alkali-rich granite, etc. However, discussion on structural stress field and dynamic mechanism is still rare. In this paper, we investigated in detail the structures and alteration-lithofacies of key sections and gallery in the east and west districts, Xiuwacu W-Mo deposits. Taking into account the zircon U-Pb, Lu-Hf and O isotopic characteristics, we believe that the Xiuwacu granitic pluton is a two phase complex pluton divided into east and west ore districts with a near S-N F4 fault as the boundary. The east district is dominated by Late Triassic (212201 Ma) porphyritic biotite granite, while mainly Late Cretaceou porphyritic quartz monzogranite and monzogranitic porphyry are in the west district. The situ-zircon U-Pb, Lu-Hf and O isotopic analysis on the porphyritic quartz monzogranite suggests that the monzogranite formation age is 80.18±0.80 Ma (LA-ICP-MC U-Pb) (negative zircon εHf(t)=-4.49-8.07, variable δ18O ranged 5.98‰7.45‰). Hf isotopic TDM2 model age is 14321239 Ma. Together with known geochemical characteristics, they show that the magmatic-mineralization material source of Late Cretaceous porphyritic quartz monzogranite was derived from the partial melting of thickened lower crust. Furthermore, Late Cretaceous regional tectonic stress field is characterized by NE-SW extension in the Yidun-Geza Area by regional structural analysis on 135 typical Late Cretaceous quartz monzogranite dykes and scheelite-molybdenite quartz veins. In summary, we believe that Late Cretaceous magmatic metallogenesis in this region is formed by the NW-SE extension of Neo-Tethys oceanic subduction resulting in partial melting of pre-existing thickened crust decompression.

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Geochronology, geochemistry and geological significance of volcanic rocks in the Jiega Formation, western Lhasa block, Sailipu, Tibet

WEI Tianwei，KANG Zhiqiang，YANG Feng，RAN Menglan，LI Qiang， WEI Naishao，LIU Di，CAO Yan，CHEN Huan，LI Daixian

2019, 26(2): 157-168.
DOI: 10.13745/j.esf.sf.2019.3.9

Abstract ( 165 )

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Mesozoic magmatic rocks are widely distributed in the Lhasa terrane, and the understanding of their petrogenesis and geodynamic setting may yield important information on the pre-Cenozoic geohistory of the Tibetan Plateau. In this paper, we carried out studies for the first time on the system zircon geochronology and major and trace elemental geochemistry of the Jiega Formation in the Sailipu area in western Lhasa block. The results show that the Jiega Formation has mainly andesite, basalt-andesite and basalt belonging to medium potassium calc alkaline series and is aged at 108.5±1.0 Ma by accurate LA-ICP-MS U-Pb dating. It is enriched in LILE Rb, Th, U and depleted in HFSE Nb, Ta, Ti, typical of island arc volcano rock. The REE patterns revealed enrichment of LREE with some flatness and weak negative Eu anomaly, indicating some low level magmatic differentiation. We conclude that the Jiega Formation volcano rocks were probably formed by partial melting of the metasomatic lithospheric mantle induced by upwelling of asthenosphere from the rotation of subducted slabs under the tectonic setting of the Bangong LakeNu River southward subduction of the Neo-Tethys.

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Tectonic background of the Qamdo Basin and its structural control on coal forming in the Late Triassic

CAO Daiyong, SONG Shiyu, MA Zhikai, PENG Yangwen, QIAO Junwei

2019, 26(2): 169-178.
DOI: 10.13745/j.esf.sf.2018.12.18

Abstract ( 190 )

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The Qamdo Basin in eastern Tibet is a tectonic basin formed from multi-stage coal-forming basins that underwent the superposition transformation of Carboniferous, Permian and Triassic. The coal-bearing strata in the Late Triassic Bagun Formation are widely distributed in the basin. During the Triassic period, the Jinsha and Lancang River ocean basins on either side of the Qamdo Terrane closed successively, resulting in the complex regional tectonic background that largely determined the paleogeographical conditions for coal forming. Different tectonic background yielded specific magma rock types and tectonic-magmatic combinations. The geochemical characteristics of Late Triassic volcanic rocks on either side of Qamdo showed that the bimodal volcanic rock development at the eastern margin of the Qamdo Basin is dominated by stretching stress, forming rift and resulting in large terrain fluctuation and subsidence and fast deposition rate which were disruptive to stable coal-forming environment. The development of intrusive rocks associated with the subduction of Lancang River ocean crust at the western margin of the Qamdo Basin led to a depression under the compressional background; and the relatively simple basement structure provided a good paleogeographical environment for the continuous and stable coal accumulation process. Under the control of later stage tectonic activity, the continuous coal seams were segmented into monoclines or folds with different angles and scales, producing industrial grade coal mining fields in the Bagun, Take Gellar and surrounding areas.

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Provenance analysis of the Yuantongshan Formation in the Hefei Basin and its geological signature: evidences from detrital zircon dating

WANG Yongsheng, TIAN Ziqiang, HU Zhaoqi, BAI Qiao

2019, 26(2): 179-193.
DOI: 10.13745/j.esf.sf.2019.3.3

Abstract ( 142 )

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The Hefei Basin in southeastern North China block was formed during the collision between the South and North China blocks. It transformed from a foreland basin, developed by northward thrusting of the Dabie Orogen, into a fault subsidence basin in an extensional tectonic setting in the Early Cretaceous. The Middle Jurassic Yuantongshan Formation in the Hefei foreland strata features lacustrine facies, which is very different from fluvial facies within the Lower Jurassic Fanghushan Formation. In order to obtain detailed provenance information, we performed detrital zircon LA-ICP MS U-Pb dating of purple siltstone from the Yuantongshan Formation, Feixi district. Zircon dating results of two collected samples revealed two major age peaks at ~2.0 Ga and ~770 Ma and one minor peak at ~2.4 Ga. Histograms of detrital zircon ages for the Yuantongshan Formation and Yangtze block are almost identical, suggesting that the later is provenance of the former. Considering that provenance of strata in a basin should not originate from other side of a watershed, provenance of the Yuantongshan Formation, therefore, is likely rocks cropping out in the Zhangbaling Uplift in the Middle Jurassic. In the early period of the Yanshanian movement, a fold-and-thrust belt occurred in the Lower Yangtze region. The Zhangbaling Uplift raised significantly at that time, causing surface rocks to erode while only the Zhangbaling Group and Feidong Complex preserved. Materials eroded from the Zhangbaling Uplift were transported to and deposited in the Hefei Basin to form the Yuantongshan Formation.

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Tectonic characteristics, formation and evolution of the Yidong region in the Yihezhuang Uplift, Bohai Bay Basin

WU Kongyou, LI Siyao, TAN Mingyou, LIU Huaqing

2019, 26(2): 194-202.
DOI: 10.13745/j.esf.sf.2018.12.19

Abstract ( 140 )

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Multi-stage tectonic movements increase the complexity of structural geometry within a fault trough, therefore, it is critical to understand the deformation mechanism of fault trough. We show here that the interpretation of the Eastern Yihezhuang Uplift (Jiyang Depression, East China) helped to understand the deformation mechanism of the Yidong fault trough. We investigated the structural characteristics and evolution of the Yidong region using high-resolution 3D seismic and drilling data. The results suggested that the NW- and NE-striking faults cut through the region to form a “two hillone troughtwo step” structural pattern, with the NW-striking Yidong fault trough sandwiched between the Yigu 991 fault and the Yigu 56 fault. In the section perpendicular to the fault trough, the residual thickness of the Paleozoic sediments in the fault trough remains constant, while the Mesozoic is thinning from north to south, and so is the Kongdian Formation from east to west. In the section parallel to the fault trough, multiple imbricated reverse faults formed on the front of the Yigu 991 fault with series of associated SN-trending stepwise normal faults forming at the back margin. This “compress at the front and pull at the back” deformation is the characteristics of gravity sliding tectonics. The analysis of balanced cross sections showed that the region has experienced five stages of evolution. In the rifting stage, the Yigu 991, Yigu 56 and Yidong faults dissected the basement rock into an isolated block, allowing the stratum to glide westward under the action of the tilted block to form the Yidong fault trough. Thus, we propose that the gravity sliding determined the formation of the Yidong fault trough, by which, the rifting at the early Himalayan laid a foundation for the gravity sliding, the wide and gentle Yigu 991 fault plane provided essential condition, and formation tilting derived from the uplift of footwall of the Yidong fault triggered the gravity sliding.

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The structural model and dynamics concerning middle section, Pearl River Mouth Basin in north margin of South China Sea

QI Jiafu, WU Jingfu, MA Bingshan, QUAN Zhizhen, NENG Yuan

2019, 26(2): 203-221.
DOI: 10.13745/j.esf.sf.2019.1.16

Abstract ( 161 )

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The structural style of the Pearl River Mouth Basin (PRMB) at the northern continental margin of the South China Sea (SCS) is quite complex, showing significant spatio-temporal differences influenced by the basement-involved faults striking in NNE, NE, NW and near E-W directions. Based on the structural style changes according to seismic data interpretation, we conclude that the middle section of the PRMB develops a ramp-flat detachment fault that slowly dips to the south; and the Paleogene basin structure is taken as an extensional system of the supra-detachment. The Xijiang Sag in the northern PRMB lies in the head region of the extensional system, from where the boundary normal fault extends to the deep detachment fault in listric geometry, restraining the Paleogene to form a half graben faulted structure. The Panyu Low Uplift in the middle is situated in the low-angle fault zone of the detachment fault, where extensional displacement of the supra-detachment leads to its overlying with the Enping Formation on both sides. The Baiyun Sag in the south overlays on the ramp of the detachment fault; and the Paleogene filling in the sag is characterized by a synform-type “faulted-sag” structure. The Southern Uplift is located in the deeper flat of the detachment fault controlling the Liwan Sag by its branch faults, with a complex “faulted-sag” structure of the Paleogene. Geometry and kinematics of the detachment fault and branch faults on the extensional system control the basin structural style and evolution in different tectonic units. The model demonstrates that both supra- and under-detachment are extensional deformations that could occur in different crustal layers. However, spatio-temporal differences should be expected for different deformation styles and stretch strain distributions. In general, the supra-detachment deformation features brittle extensional structure and the uneven stretch strain distribution is mainly concentrated in the listric normal faults linked to the detachment fault, but the footwall of the detachment is predominantly of ductile stretching deformation with relatively uniform distribution of stretch strain. In the head region of the detachment system (Xijiang Sag), the supra-detachment extension is greater than the under-detachment counterpart; however, the opposite is true in the root region (Baiyun and Liwan sags). The master boundary faults on the northern edge of the Xijiang Sag are listric normal faults striking NE in the Wenchang period as the head of the detachment fault, but cut off by high angle normal faults striking near E-W in the Enping period so that the head of the detachment fault is abandoned or migrated to the Panyu Low Uplift from the northern edge of the Xijiang Sag. The spatio-temporal differences in basin structure style and deformation are influenced by many factors such as pre-existing basement structure, lithosphere rheology, and scale of extension and so on, but the main factor is the response to lithospheric stretching deformation and lithosphere thermal change due to asthenosphere flows. A dynamic model, i.e. the speed up flow of asthenosphere from northwest to southeast drags the overlying lithosphere to divergent motion and causes the continent rifting, can reasonably explain the structural formation and evolution in the middle section of the PRMB. That is, the asthenosphere one-way flow drags the overlying lithosphere to its simple shearing extensional deformation and forms the detachment fault, while the relatively uniform stretch strain of the under-detachment, with diminishing toughness from south to north, is passed onto the supra-detachment with non-uniform brittle extensional deformation enhanced gradually from south to north.

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Morphological characteristics and control factors of oceanic basement in the Southwest Sub-Basin, South China Sea.

DING Hanghang, DING Weiwei, FANG Yinxia

2019, 26(2): 222-232.
DOI: 10.13745/j.esf.sf.2019.1.15

Abstract ( 210 )

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We interpreted two multi-channel seismic profiles across the Southwest Sub-Basin (SWSB) of the South China Sea to clarify the oceanic basement types based on the regional morphological and structural features. We calculated half-spreading rates in different areas and analyzed their relationship with the basement types. Our geological interpretations recognized two typical types of the oceanic basement: type Ⅰ represents flat basement influenced by localized volcanic eruptions; and type Ⅱ features rotated blocks controlled by normal faulting. The calculated spreading rates showed that the half-spreading rate in the northeastern SWSB, represented by seismic profile N3, was between 13 and 36 mm/a; whereas in the middle SWSB, represented by the seismic profile NH973-1, the rate was largely stable at about 19 mm/a. The correlation between basement type and half-spreading rate is clear: typeⅠbasement corresponds to higher spreading rate and type II to lower spreading rate. The spreading rate in the northeastern SWSB varies periodically, following the inter-phase distribution pattern of the two basement types. The middle SWSB is dominated by type Ⅱ basement with a stable spreading rate. We suggest that the longer spreading history and higher magma budget in the northeastern SWSB may be the controlling factors for the complex basement morphology in this area.

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Discovery and the geological significance of retrograded eclogites from the northern margin of the central Qilian block

SONG Zhijie, ZHANG Hongyuan, HOU Di, LIU Changfeng, LIU Wencan, WU Chen

2019, 26(2): 233-248.
DOI: 10.13745/j.esf.sf.2018.7.3

Abstract ( 244 )

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In the 1∶50000 regional geological survey, we discovered retrograded eclogites, occurring as tectonic lenses in the metamorphic basement of the Dayanglong area in the western part of the northern margin of the central Qilian block. Petrographic and mineralogic examinations revealed that garnets in eclogites preserved prograde metamorphic zonation, exhibiting annulus characteristics in both their chemical composition and mineral inclusions, and that Dayanglong retrograded eclogites belong to C-type eclogites. The prograde metamorphic assemblages (M1) are mineral inclusions (such as biotite, amphibole and plagioclase) preserved in the garnet core. The p-T metamorphic conditions were estimated to be 568580 ℃ and 0.800.82 GPa, with peak metamorphism (M2) taking place at 669±5 ℃ and 2.1±0.2 GPa, consistent with high-pressure (HP) metamorphism. The M3 stage is characterized by the amphibole+plagioclase+biotite corona around garnet porphyroblasts, indicating an isothermal decompression process with estimated p-T metamorphic conditions of 681705 ℃ and 0.680.71 GPa. The p-T conditions for the subsequent retrogression under amphibolite-facies environment were estimated by coarse-grained matrix amphiboles to be 500545 ℃ and 0.380.43 GPa in the M4 stage. The estimated metamorphic conditions revealed a clockwise p-T path, suggesting that the subducted slab has undergone a process of rapid subduction and exhumation. Most of the cathodoluminescence (CL) images of zircons from retrograded eclogites showed low luminous intensity with no zoning, weak zoning or spongy zoning with 5 μm wide strong CL belt at edges, indicating that most zircons are metamorphic or inherited zircons. Results from LA-ICP-MS zircon U-Pb dating showed that the 485±22 Ma zircon age may represent the top of the HP metamorphic time. According to geochemical analysis and tectonic discrimination, the protolith of retrograded eclogites is MORB, which likely came from the North Qilian oceanic crust. The above results, combined with arc magmatic rocks in the northern margin of the central Qilian block, confirm that the North Qilian oceanic plate had experienced south-north bi-dive subduction during the Late CambrianMiddle Ordovician.

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Uplifting evolution of the Central Uplift Belt, Qiangtang: constraints from tectono-thermochronology

ZHAO Zhen, LU Lu, WU Zhenhan

2019, 26(2): 249-263.
DOI: 10.13745/j.esf.sf.2018.9.7

Abstract ( 205 )

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Based on our measurements of fission track ages of apatites (11343 Ma) and zircons (169103 Ma), U-Pb ages of zircons (215206 Ma) and K-Ar ages of biobites (186178 Ma) of granite samples, we performed apatite fission track thermal history modeling and track age spectra calculation (TASC) to obtain up-to-date histories of uplifting and cooling of the Qiangtang Central Uplift Belt (QCUB) since the Late Triassic. The QCUB experienced four major uplifting-cooling episodes, namely the Early Jurassic, Late JurassicEarly Cretaceous, Late Cretaceousearly Miocene and later Miocene. The four episodes correspond to the dynamics of the post-collision stretching of the South and North Qiangtang blocks, Lhasa-South Qiangtang collision, Neo-Tethys ocean subduction, and Indo-Asian collision and N-S strike-slip movement, respectively, and experienced upliftings of 11.4, 2.85, 4.35 and 0.85 km, respectively. The QCUB probably was at sea level in the Early Cretaceous, and rapidly uplifted to 22.5 km in the Late Cretaceous; during 9020 Ma, the long term thrusting systems caused further uplifting to ~5 km, followed by differential surface uplifting due to the denudation effect of the Great Lakes and the south-north strike-slip action in the Miocene.

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Superposed folding since the Permian on both sides of the Xar Moron Suture, southeastern Inner Mongolia: implications for syn- and post-collision geodynamic process

ZHANG Yuqing, ZHANG Changhou, HOU Liyu, ZHANG Yipeng, HUANG Yingzhu, CHEN Hanlin, CHANG Lizhong

2019, 26(2): 264-280.
DOI: 10.13745/j.esf.sf.2018.9.6

Abstract ( 170 )

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Multi-scale fold interference patterns developed in the Permian strata on both sides of the Xar Moron fault, southeastern Inner Mongolia. These superimposed folds and the deformation within the Jurassic strata recorded multiple tectonic events in the research area since the Permian. Research on these deformations is crucial for a better understanding of the tectonic evolution from the Paleo-Asian system to the Paleo-Pacific system along the northern margin of the North China Block. In this study, we employed detailed field mapping to decipher the geometry of these superimposed structures and their evolution. Folds distributed in the Permian strata are characterized by sinuous axial trace with southeast verging. Folds in the Mesozoic strata, however, are trending southeast with northwest verging. Three tectonic deformation phases were identified in this research: (D1) East-northeast trending folds in the Permian strata implying a north-northwest-south-southeast compression from the end of the Permian to the Middle Triassic; (D2) Regional shear events during the Late Triassic resulting in the formation of the Type2a and Type1d fold interference patterns proposed by simon, 2004; and (D3) Northwest-verging folds within the Middle Jurassic strata thought to be developed under a northwest-southeast compression during the Late Jurassic. Furthermore, D3 deformation led to fold tightening within the Permian strata. We interpret these three deformation phases (D1, D2, D3) as the consequences of the final closure of the Paleo-Asian Ocean and collisional orogeny, the east-northeast extrusion of blocks between the dextral strike-slip of the Xar Moron fault in northern China and the sinistral strike-slip East Gobi Fault in southeastern Mongolia, and the westward subduction of the Paleo-Pacific oceanic plate under the Eurasian plate, respectively.

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Astrochronology of the Upper Ordovician Pagoda Formation, South China and its geological implications

MA Xueying, DENG Shenghui, LU Yuanzheng, WU Huaichun, LUO Zhong, FAN Ru, LI Xin, FANG Qiang

2019, 26(2): 281-291.
DOI: 10.13745/j.esf.sf.2019.3.8

Abstract ( 204 )

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We conducted high resolution cyclostratigraphic studies on the magnetic susceptibility (MS) series of the Upper Ordovician Pagoda Formation in three locations including the Qiaoting section in Nanjiang, Sichuan, the Sanquan section in Nanchuan, Chongqing, and the Liangcun section in Xishui, Guizhou. We identified the hierarchy of Milankovitch 405 ka long eccentricity, 100 ka short eccentricity, 30 ka obliquity and 20 ka precession cycles with strong eccentricity and precessional signals. We also established floating astronomical time scales by tuning the 405 ka eccentricity cycles and determined the precise durations of the Pagoda Formation to be 4.17, 5.64 and 5.05 Ma for the Qiaoting, Sanquan and Liangcun sections, respectively. The average sedimentary rates of the Pagoda Formation were calculated to be 7.67, 4.68 and 6.37 m·Ma-1 for the three sections, respectively, decreasing from north to south in the Sichuan Basin, indicating that the studied area uplifted to become continent before the sedimentary time of the Pagoda Formation and transgressed from south to north. The conodont Hamarodus brevirameus and Protopanderodus insculptus zones in the Pagoda Formation lasted 3.62 and 1.43 Ma for the Liangcun section, respectively, while the trilobites Niuchangella angustilimbata, Dislobosaspis xixiangensis and unnamed zones lasted 2.83, 0.41 and 0.81 Ma for the Qiaoting section, respectively.

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Characteristics of uranium content and its geological and mineralization significance for the provenance areas, northern Northwest China

WANG Wenqing, LIU Chiyang, WANG Jianqiang, MA Huanhuan, GUAN Yuzhao

2019, 26(2): 292-303.
DOI: 10.13745/j.esf.sf.2018.9.1

Abstract ( 259 )

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The provenance areas of major basins in northern Northwest China, including the peripheral regions of the Yine, Beishan group, Santanghu, and Juggar basins, are situated primarily on the south margin of the mid-western Central Asian Tectonic Zone and neighboring areas. Dividing 1416 collected samples, including intermediate, acidic and alkaline rocks in the study area into subgroups, we analyzed rock age and uranium contents of all samples and calculated their paleo-uranium contents and the amount of uranium migration during geological history. The result showed that major and lesser magmatic activities occurred in the Early CarboniferousEarly Triassic (349248 Ma) and Middle OrdovicianEarly Devonian (470396 Ma), respectively, which correspond to the tectonic evolution stages and major events of the beginning subduction (O2S1) and closure (CT1) of the Paleo-Asian Ocean and formation of the Central Asian-Mongolian collision fold belt (CT1), respectively. The uranium-rich rocks were mainly formed in the Early CarboniferousEarly Triassic (334250 Ma), consistent with the Late Paleozoic magmatic activity. And regionally, the younging of magmatic and uranium-rich rock age to the east matches the evolution history of the Paleo-Asian Ocean closure and formation of the Central Asian Tectonic Zone from west to east. Variable uranium contents were observed for several provenance areas. Comprehensive comparison and evaluation showed that the areas including the Liuyuan in Beishan, and AltayYundu and southeastern Qinghe along the northeast margin of the Junggar Basin, had no less uranium than the Zongnaishan area; the Kalamaili area on the northeast margin of the Junggar Basin was relatively rich in uranium. In addition, the periphery of the Yine Basin including Yabulaishan, Bayannuorigong, Qinggele and Shalazhashan, Yantan and Mazongshan areas in Beishan, the Santanghu Basin periphery, and part of the Junggar Basin including northwestern Altay on the northeast and northern west margins, had moderate uranium content. The rest of areas had poor uranium content. Our study indicates the uranium-rich provenance areas in the study area began to rise in the Late Triassic and developed later on to provide abundance of sediments and uranium to the adjacent MesozoicCenozoic basins and became the dominant uranium source for the sandstone host, coal-type and mudstone-type uranium deposits in the basins. The deep source rocks in the basin are rich in uranium, which can reduce the threshold temperature of hydrocarbon generation and increase hydrocarbon production, and both aspects have significant geological and mineralization implications.

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The pH constraint on transportation and precipitation of iron materials for iron quartzite formation: an example of the Ryongyon iron deposit in the Korean Peninsula

KIM Yunsong, LI Guowu, YIN Jingwu, KIM Byongsong, KIM Cholsu

2019, 26(2): 304-311.
DOI: 10.13745/j.esf.2018.12.4

Abstract ( 116 )

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Many accomplishments are made in the mechanistic study of iron source material migration for the formation of large-scale iron ore deposits during the Precambrian period. However, a few factors have yet to be fully resolved. In particular, still in debate are multiple theories on the genesis of cast iron deposits (non-Algoma type) of continental source. We have investigated the continental migration of iron source materials for the Ryongyon iron ore deposits in the Korean Peninsula as an example. From our geochemical study and the work of previous researchers, we showed that the iron ore was not Algoma type but formed from migration and sedimentation of weathered continental source materials under strong acidic conditions. As we know, during the Precambrian time, oxygen was depleted in the atmosphere and water with no ozone layers formed therefore no plants grew on land. Then, how did such a strong acidic environment occur? To clarify, we performed thermodynamic calculations on prevailing materials (at the time) that could convert water into acid in the environment. The results revealed that, when sulphide (such as pyrite) weathering occurred, it developed an iron source capable of medium degree weathering and transport. This result is consistent with the fact that the pH value was less than 3.5 for waters from the sulphide-rich stratum where iron content was much higher than in non-sulphide containing strata. Thus we conclude that sulphide weathering played a non-negligible role in the formation of continental source iron deposits.

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Quick assessment of earthquake-triggered landslide hazards: a case study of the 2017 M_S 7.0 Jiuzhaigou earthquake

CHEN Xiaoli, SHAN Xinjian, ZHANG Ling, LIU Chunguo, HAN Nana4, LAN Jian

2019, 26(2): 312-320.
DOI: 10.13745/j.esf.sf.2018.9.11

Abstract ( 208 )

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Landslides triggered by strong earthquakes in mountainous areas can cause severe casualties and also pose threats to socio-economic development in the affected region. Therefore, an effective evaluation model, not only for a quick assessment of landslide distribution, but also for identification of most hazardous areas, is needed to provide critical assistance in the deployment and execution of rescue efforts. On August 8, 2017, a MS 7.0 earthquake struck Jiuzhaigou County, Aba Prefecture, Sichuan Province. The earthquake triggered massive landslides causing casualties and property loss. After this event, we quickly analyzed the pre- and post-earthquake remote sensing images (produced by the Beijing No.2 satellite) in the affected region and interpreted the landslide distribution. Although, in some areas, the post-seismic images were unable to provide detailed interpretation of landslide due to cloud coverage, they met the primary requirements for landslide interpretation at locations of high seismic intensity where landslides mainly occurred. In total, we interpreted 194 landslides with individual area >700 m2. These landslides were mainly distributed along the long axis of seismic intensity, with an accumulative disaster area of 5.6 km2 and impact range exceeding 600 km2. Based on topography and lithology analyses of affected region, as well as peak ground acceleration data, we used the Newmark rigid block model to predict the landslide hazardous area after the earthquake. The prediction results were divided into five levels according to degree of potential danger, i.e. high, moderately high, moderate, low and very low. Comparison study shows that the predicted landslide hazard zones agreed with the actual distribution of earthquake-triggered landslides that interpreted landslides mainly distributed in areas evaluated as high danger zone, validating the effectiveness of using the Newmark rigid block model to quickly evaluate earthquake-triggered landslides. In addition, we also discussed limitations and made suggestions for future improvement in the model.

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The U.S. Frontier Observatory For Research in Geothermal Energy project and comparison of typical EGS site exploration status in China and U.S

ZHANG Senqi, WEN Dongguang, XU Tianfu, FU Lei, JIA Xiaofeng, SUN Sheng, WENG Wei, ZHANG Yang, YANG Tao, Joseph MOORE, JIANG Shu, Rick ALLIS, John MCLENNAN

2019, 26(2): 321-334.
DOI: 10.13745/j.esf.sf.2019.2.9

Abstract ( 384 )

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The Frontier Observatory for Research in Geothermal Energy (FORGE) initiative by the United States Department of Energy is to form a new generation of enhanced geothermal system(EGS) test platform based on the construction practice of EGS demonstration project. It is guided by the constraints of classic hot dry rock exploration and development and the conception of “Reproducible Results=Tremendous Potential”. It aims to achieve a new breakthrough in U.S. hot dry rock exploration and development technology in order to meet the green electricity consumption of 100 million households in the United States. A comparison of typical EGS site exploration status in China and U.S. demonstrated that the Milford EGS site in Salt Lake City, Utah and Gonghe EGS sites in Qinghai Province are competitive in technological development. However, the Milford EGS site is more advanced in natural fracture system investigation, in situ stress field test, parameter acquisition and fracturing scheme establishment. Thus, we have suggested relevant institutions to speed up the Gonghe EGS site exploration and development to quickly improve EGS technology level and achieve EGS engineering in China.

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N-nitrosamine containing underground waters and regional cancer incidence in the Pearl River Delta region

ZHAO Lunshan, CEN Kuang, LIU Xiuli, WU Xuefang, ZHU Xuetao, WEI Junxiao, CHEN Yuan, LUN Zhiying

2019, 26(2): 335-349.
DOI: 10.13745/j.esf.sf.2019.1.19

Abstract ( 227 )

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Localized/regional nasopharyngeal and hepatocellular carcinoma in South China are the typical cancer types by regional malignant tumor, ravaging public health in the area for hundreds of years. In this paper, taking the centuries old regional carcinoma epidemic in the Pearl River Delta as an example, applying epidemiology and ecogeochemical methodology, we determined that both environmental and drinking waters contained N-nitrosamine, a known carcinogen. We investigated the formation mechanism of N-nitrosamines compounds in natural waters under the constraints of regional geological, climate, hydrologic and ecological conditions. We developed a solid-phase extraction method (SPE) adaptable to gas chromatography-mass spectrometric (GC-MS) analysis to detect nine trace N-nitrosamine compounds in environmental and drinking waters. The detection limit ranged from 0.34 to 2.19 ng/L. In two representative cancer high-risk areas, Shunde and Sihui, we performed for the first time quantitive analysis of nine trace N-nitrosamines in different water occurrences. The results showed that the N-nitrosamine concentrations in local waters ranged from 0.34 to 307 ng/L. And the detection rate of 40% was much higher than the 15% rate set by healthy drinking water standard for N-nitrosamines at 10 ng/L. The testing results indicated NDMA and NDBA were the main pollutants, while other nitrosamines were detected at lower concentrations. In the less industrialized Sihui area, nitrosamine concentrations share the same statistical characteristics for different water occurrences such as well, tap, underground and spring waters: the average and peak values of NDMA and NDBA barely exceeded the standards at 21.77 and 15.54 ng/L, respectively, higher than other nitrosamines. This was a typical nitrosamine composition profile for the natural regional underground water. The water samples were taken from wells at 020 m depths. The underground water is ammonium-rich and contains nitrosamines formed under unique regional ecogeological and climate conditions, and it may be the original culprit of the regional carcinoma. Among the five endemic regions studied, the industrialized Shunde area has highest nitrosamine content, with nitrosamine concentration correlating positively with hepatocellular carcinoma morbidity. Our study confirmed that cumulative effect of industrial pollution can increase cancer morbidity.

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