Table of Content

20 December 2020, Volume 26 Issue 6

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Microstructure and Effective Viscosity of Ultrahigh-Pressure Metamorphic Rocks from the Dabie Mountains

GU Xiaotong, WANG Qin

2020, 26(6):  601-616.  DOI: 10.16108/j.issn1006-7493.2019046

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Ultrahigh-pressure metamorphic rocks provide a window to investigate deformation mechanisms and rheological contrast of rocks in a subduction channel. Using the electron backscatter diffraction (EBSD) technique, we measured the latticepreferred orientations (LPO) of major minerals from eclogites and felsic gneisses in the Dabie ultrahigh-pressure metamorphic belt. Garnet from eclogites shows nearly random orientation, whereas the LPO of omphacite is characterized by the maximum concentration of [001] axis parallel to the lineation and that of the pole of (100) normal to the foliation. Amphibole from
retrograded eclogites also develops (100)[001] fabric, which may be inherited from omphacite. Quartz in retrograded eclogites and felsic gneisses shows the activation of dominant low-temperature (0001)<a> and medium-temperature {1010}<a> slip systems, which should be formed during exhumation in the middle crust. In contrast, (001)<110> and (010)[100] fabrics in plagioclase resulted from ductile deformation during exhumation in the lower crust under amphibolite facies metamorphism. The effective viscosity of major minerals during subduction and exhumation was calculated using flow laws of dry minerals. The decomposition reaction of albite to jadeite and quartz, and phase transition of quartz to coesite will significantly increase the effective viscosity and density of felsic gneisses and hence facilitate deep subduction of the continental crust. However, during exhumation the changes of effective viscosity across the two phase boundaries are much smaller because of high temperature. Deeper than ~80 km, the effective viscosity of minerals has the sequence of garnet > jadeite > omphacite > coesite. Rheological properties of the upper crust are controlled by coesite and jadeite, whereas those of the lower crust by omphacite and garnet. The strength contrast between ultrahigh-pressure metamorphic rocks will result in decoupling between the upper and lower crust, which allows rapid exhumation of low-density and low-viscosity upper crustal materials in a subduction channel.

Effect of Hydroxyl Group on Nucleation of Calcium Carbonate Phases in Alcohol-Water Mixtures

YANG Yang, ZHU Chen, WANG Hongtao, HUA Hao, JIANG Ruijie, MAO Wei, ZHAO Liang

2020, 26(6):  617-627.  DOI: 10.16108/j.issn1006-7493.2019056

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In this paper, the effect of hydroxyl groups on CaCO3 mineral phases and their interfacial energies in biological-like fluids (alcohol-water mixtures) were studied. The results show that polyhydroxy alcohols are beneficial to the formation of metastable vaterite, and the addition of alcohols can increase the interfacial energies between calcite and vaerite. The decrease of total absoption site density due to the hydroxyl-oriented adsorption of alcohols is most likely the mechanism to increase the mineral interface, and the number of hydrogen bonds in solvents determines the degree of freedom of carbonate ions resulting in
the formation of vaterite, whose CO₃^{2 -} plane is nonparallel. In solvents with weak hydrogen net work, the calcite phase is the thermodynamic-favouriable phase due to the low interfacial energy between water and ethanol-water. The results show that the bio-mineralizing fluid can strongly affect the carbontion process through hydroxyl function group. By controlling the content of hydroxyl group in the fluid, the living organisms can select the formation site of calcium carbonate phase, which extends our understanding about the origine of stable biogenic calcium carbonates.

Mineralogy and Genesis of Asphaltene Amygdalae in the Puge Basalt, Southern Sichuan, China

ZHANG Liangju, RUAN Qingfeng, YANG Yufu, LI Dongsheng, ZENG Weilai

2020, 26(6):  628-638.  DOI: 10.16108/j.issn1006-7493.2020004

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A series of amygdalaes with asphalt, chlorite, quartz, native copper, occur in the Puge basalt, Southern Sichuan, China. The petrological and mineralogical features of different amygdalae were systematically studied by modern analytical techniques. The results show that chlorite in the amygdalae belongs to diabantite, formed in a medium-high temperature, organic-rich hydrothermal environment. Asphalt belongs to petroleum asphalt and was derived from bioclastic
limestone of the Middle Permian Yangxin Formation (P2 y). Chlorite, asphalt, quartz and native copper crystallized in the organic hydrothermal fluid, which are derived from late basic volcanic activity under the tectonic stress after the formation of basalt. A commonly recognized sequence of the processes for the formation of these amygd alaes can be summarized as follows: the later magma stage of Puge basalt → Cu-rich volcanic hydrothermal fluid→tectonic stress→organic ore-fluid →chlorite →the first generation of asphalt →white quartz → smoke-gray quartz, the second generation of asphalt, and native copper →chalcocite→prehnite. The asphaltene amygdalae, crystal cavities and tectonic fracture zone in the basalt can be regarded as important prospecting indicators for Cu mine ralization in the Emeishan Large Igneous Province.

Geochemistry and Provenance of the Middle Triassic Clastic Rocks in the Youjiang Basin, Ceheng, Guizhou

CHEN Xing，PENG Chenglong，CHEN Jianshu，WU Kaibin，ZHANG Deming，WANG Wenming，

2020, 26(6):  639-655.  DOI: 10.16108/j.issn1006-7493.2019060

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Owing to the unique sedimentary characteristics, the Youjiang Basin has attracted considerable attention from numerous geologists. The Middle Triassic of the Youjiang Basin was filled with a set of thick flysch sediments, which are predominantly fine-grained sandstones, siltstones, mudstones and transition rocks. However, there are still controversies about the provenance of the clastic rocks. In this paper, detailed petrological and geochemical analyses of the Middle Triassic clastic rocks in Ceheng have been carried out. Results show that the clastic rocks are featured by low compositional maturity and textural maturity, which reflects a proximal sedimentation. The content of SiO2 (average=58.49%) is low with a wide range of variation, which indicates that the quartz contents are relatively low. The high content of Al2O3 (average=12.46%) indicates the high contents of clay minerals and feldspars. The average ratio of Al2O3/SiO2 is 0.22, which is consistent with the graywacke (0.20~0.23), indicating that the clastic rocks are low in maturity and also have proximal sedimentary characteristics. The high content of CaO (average 7.83%) indicates more carbonates. The trace elements are characterized by the enrichment of Rb, Th, La, Nd, Hf and Pb,
but relatively depleted in P, Ti, Sr, Nb and Ta. Rare earth elements are featured by large variation range of ΣREE, enrichment of light rare earth elements and Eu depletion. Analysis of major, trace and rare earth elements reveals that the source rocks of clastic rocks are mainly felsic rocks, with a little mixture of intermediate rocks and quartzose sedimentary rocks. The tectonic setting of the source area is dominated by continental island arc, and partially passive continental margins and active continental margins. The chemical index of alteration (CIA) is 53.93~74.80 (average=65.12). The CIA and Th/U ratios indicate that the source area of clastic rocks mainly experienced moderate chemical weathering.

Study on Plans of Monitoring Shallow Geothermal Field and Characteristics of Geothermal Energy Distribution in Nanjing

ZHANG Tiansheng, LIU Chun, LI Jichen, SHI Bin

2020, 26(6):  656-664.  DOI: 10.16108/j.issn1006-7493.2019072

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As a type of generable energy, shallow geothermal energy has been widely noticed. It’s very important to establish a long-term effective shallow geothermal field monitoring system in the process of developing and utilizing shallow geothermal energy. In order to better monitor shallow geothermal field and the temporal and spatial distribution of energy, four kinds of temperature sensors used in the project, DTS, FBG, Pt100 and iButton, were analyzed and compared from measurement accuracy, applicable range and characteristics through field and indoor tests. Combined with the results of tests and current application,
the advantages and disadvantages of DTS, FBG, Pt100 and iButton on shallow geothermal field monitoring are summarized and a more completed monitoring plan is developed, which provides a reference for monitoring shallow geothermal field and its temporal and spatial distribution of energy. The follows are shown. Distributed temperature sensing fiber is buried in all boreholes, and two typical ones will be selected according to the soil layer distribution of the geothermal borehole and the obtained geothermal distribution data. Along the borehole, FBG temperature sensing string is arranged in the borehole. iButton is arranged at 5cm from the surface of all drilling points; Use Pt100 to monitor the precise temperature along the borehole within 25 m below. According to the obtained data, the general distribution of the shallow geothermal energy in the vertical direction in Nanjing can be summarized, and the distribution is found to be spatially different. The shallow geothermal energy near surface is related to the surface cover, the atmosphere and solar radiation, while the deeper energy is affected by factors such as geological structures and hydrogeological conditions.

Failure Model Analysis for Piedmont Gentle Slopes Due to Temporary Confined Water

WANG Bo, XU Baotian, LIU Haibo

2020, 26(6):  665-673.  DOI: 10.16108/j.issn1006-7493.2019073

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A large number of gently inclined landslides occurred after heavy rainfall in Nanjing and Zhenjiang area. The temporary confined water was formed by the rise of water level in the slopes after rainfall. Landslide deformations showed uplift in the leading edge and tension crack in the trailing edge of the slope. According to the in-situ geological survey of Shilong Road landslide in Jiangning, Nanjing, the stratigraphic structure of the landslide is characterized by the dualistic geological structure that the upper part is fine and the lower part is coarse. This study chose Shilong Road landslide as an example to analyze its failure characteristics. Based on the basic principles of simple confined water distribution model and simply supported beam model, this paper attempted to analyze the mechanical mechanism of uplift and failure in aquiclude of front section of the slope under the action of temporary confined water. Aiming at the uplift characteristics of leading edge of the landslide, a one-dimensional steady flow model was constructed to analyse the distribution characteristics of confined water in the permeable layer of front slope. The simply supported beam model was introduced to analyse the influence of confined water uplift pressure on the deformation and stability, and the law of cracks development in aquiclude. The criterion of failure after uplift of aquiclude was determined, and the influence of uplifted pressure head at the foot of slope and thickness of aquiclude on the failure characteristics of aquiclude was analysed in detail. The results show that confined water head in the permeable layer is triangular distribution. The deepest crack in aquiclude develops at the maximum bending moment of uplift pressure on simply supported beam model. The maximum damage area of aquiclude is positively correlated with the uplift pressure head at the foot of slope and negatively correlated with the thickness of aquiclude. The critical uplift pressure head and the critical thickness of aquiclude are approximately linear when
the aquiclude occurs uplift-tension failure.

Experimental Study on Mechanical Properties and Deformation Characteristics of Cemented Marine Muck in Qingdao Coastal Area

LI Peng，NIE Ning，LIU Qiang，HAO Shengli，WU Shaofang，YANG Junjie

2020, 26(6):  674-679.  DOI: 10.16108/j.issn1006-7493.2019076

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In order to study the mechanical and deformation characteristics of cement-soil formed by marine muck in coastal area and Jiaozhou bay of Qingdao, a series of mixed proportion of tests were carried out. The test results show that the size of the samples has little effect on the compressive strength of cement-soil. The unconfined compressive strength of cement-soil approximately increases hyperbolically with age. The long-term strength of cement-soil can be predicted by the total cementwater ratio in cement-soil and the unconfined compressive strength of cement-soil at any time. The unconfined compressive strength of cement-soil increases linearly with the increase in cement mixing ratio that can be predicted with empirical formula. When high sulphate resistant cement is used, it can play an obvious anti-erosion role when its content exceeds 15%. In addition, the cement-water ratio should be increased properly to obtain higher cement-soil strength.

Research Progress and Prospect of the Tectonic Evolution of Okinawa Trough and Its Relationship with Magmatism, Hydrothermal Activities and Sedimentation

LIU Wei，SUN Yujie，ZHENG Hanyue，XIN Mengran，HE Wei，WEI Tong，GUO Shiyue，CHEN Sifang,LIU Wenwen，FENG Yuge

2020, 26(6):  680-690.  DOI: 10.16108/j.issn1006-7493.2019084

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The Okinawa Trough is a typical back-arc basin in the western Pacific Trench Arc Back-Arc system. Its unique tectonic location, magmatism, hydrothermal activities and sedimentary records have been a focus of academic research. Based on previous research, we review the controls of magmatic, hydrothermal and sedimentary activities by tectonic evolution of the Okinawa Trough. The subduction of the Philippine Sea plate led to the formation of the Okinawa Trough and the occurrence of arc volcanism, back-arc volcanism and cross-back arc volcanism. These three types of volcanism resulted in three different zones of hydrothermal activity. The magmatism and fault system caused by subduction of Philippine Sea plate provide heat sources and channels for the development of hydrothermal fluid in the Okinawa Trough. The subsidence of Zhe -Min Uplift at ~416 ka brought about the sediments in the northern Okinawa Trough to change from coarse-to fine-grained. Finally, we also provide the outlook of further research based on the current research status of the Okinawa Trough.

Determination of Mesozoic Tectostratigraphic Units and Its Significance in Jilantai Sag, Hetao Basin

YANG Dexiang，QU Zhenghui，CHEN Shuguang，LI Zhuangfu，WU Jianping，WANG Jianguang，WU Han

2020, 26(6):  691-703.  DOI: 10.16108/j.issn1006-7493.2020028

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Significant discoveries of petroleum exploration have been achieved in 2017 in the Linhe Depression, Hetao Basin. Based on a literature review of the tectonic evolution of the study area since the Mesozoic, combined methods of well-seismic comparison and basin analogy, the Mesozoic tectostratigraphic units of Jilantai Sag have been determined, and their occurrence and significance have also been studied. The results show that, except for the Triassic, the Mesozoic strata were well developed in the study area. Particularly, the Cretaceous strata can be further divided into three tectostratigraphic units, i.e., K1^I、K1^II and K1
^III; The K1 ^I is characterized by abroad sedimentation unit covering the whole study area, while the other tectostratigraphic units are wedge-shaped with major axes aligning along the NNE main faults; The Mesozoic basin type of the Jilantai Sag have experienced rift stage during the Early-Middle Jurassic, rift stage during the early Early Cretaceous, transitional stage from rift to depression during the middle Early Cretaceous, depression stage during the late Early Cretaceous; The strata of the K1 ^II and K1 ^III tectostratigraphic units located between the faults F₃ and F ₇, and the Jtectostratigraphic unit located in the east of fault F ₇ show great potential of petroleum exploration.

Paleoenvironment Changes Inferred from the Analysis of Organic Elements and Carbon Isotope Characteristics of Jurassic Argillaceous Rocks in the Dameigou, North Qaidam Basin

LI Wenshuai，ZHANG Xiaoli，FENG Qiao，JIAO Ke，CHEN Yan，ZOU Kaizhen，CUI Han

2020, 26(6):  704-711.  DOI: 10.16108/j.issn1006-7493.2019085

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The Jurassic strata in the Dameigou area of the North Qaidam Basin recorded the evolution of paleoenvironment in the Northwest. In this paper, the organic geochemical analysis of the Dameigou section is performed to reconstruct the environmental evolution of the Jurassic in the Dameigou area. The analysis reveals that the H/C ratio shows a good negative correlation with the C/N ratio and δ ¹³C. The C/N ratios and δ ¹³C values decrease gradually and the H/C ratio gradually increases during the Early Jurassic. In the Middle Jurassic, the C/N ratio and δ ¹³C remained stable in the early stage and the latter was sharply negative, while the H/C ratio was reversed. During the Late Jurassic, the C/N ratio and δ ¹³C remained largely unchanged and the H/C ratio was stable in the high value range. The C/N ratio of organic elements in the early-middle Jurassic is greater than 15, indicating that organic matter is mainly from terrestrial higher plants. The paleoclimate of the Dameigou section is characterized by two cycles: in the Early Jurassic Xiaomeigou Formation, the paleoclimate was  haracterized by warm-humidity-dry heat and the paleoclimate evolved from warm wet to dry hot in the late Middle Jurassic.

Rayleigh Wave Tomography in the Northeastern Tibetan Plateau: A Comparison between Ray Theory and Eikonal Equation

HAO Shijie，HUANG Zhouchuan，WANG Liangshu，XU Mingjie，MI Ning，YU Dayong

2020, 26(6):  712-720.  DOI: 10.16108/j.issn1006-7493.2019094

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The Northeastern margin of the Tibetan Plateau is under ongoing uplifting, whose tectonic structures have always been a research hotspot. Previous studies of surface wave tomography in this region show significant differences, which may be related to different tomography methods. We performed Rayleigh wave phase velocity tomography using both ray-theory and Eikonalequation methods in this study. We compared the tomography results of 20 s, 30 s and 60 s obtained from the two methods which showed relatively consistent velocity features. The overall velocity characteristics are consistent with the distribution of main structural units in the study area. The Northeastern of the Tibetan Plateau shows a low velocity anomaly, while the Ordos Block shows a high velocity anomaly. In the Yinchuan-Hetao Graben, Eikonal-equation based tomography shows more obvious low velocity anomalies in 20 s and 30 s, and the distribution of low velocity anomalies is better consistent with the contour of the graben, indicating that the Eikonal-equation based tomography can make use of the data more sufficiently.