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Paleomagnetic study on the Early Triassic red beds from Jiaocheng, Shanxi Province ——Local rotation and tectonic significance

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Science China Earth Sciences 2004年第2期47卷 108-114页

作者： SHI Ruiping HUANG Baochun ZHU Rixiang REN Shoumai Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029China Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029China Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029China Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029China

Paleomagnetic and rock magnetic study has been conducted on the Early Triassicred beds of Liujiagou Formation from Jiaocheng, Shanxi Province. Hematite was shown as themain magnetic mineral. After eradicating an initial viscous component at room temperature to～100℃-200℃, thermal demagnetization shows that most samples contain two remanencecomponents, intermediate-temperature remanence component at 250℃-500℃ and high-tem-perature component at 500℃-680℃. The intermediate-temperature component has a negativefold test at the 95% confidence level. And the pole position of the intermediate-temperaturecomponent in geographic coordinates is correlated with the Middle Jurassic reference pole of theNorth China Block (NCB) within the 95% confidence, suggesting that it might be a remagnetiza-tion component acquired during the Yanshanian period. The high-temperature component con-tains both reversal and normal polarities with positive fold test and C-level positive reversal test atthe 95% confidence level, which suggests that this high-temperature component can be regardedas primary magnetization. Comparison of this newly obtained Early Triassic paleopole with thecoeval mean pole of the Ordos Basin suggests that a locally relative rotation may have happenedbetween the Ordos and the Jiaocheng area of Shanxi Province. This rotation may be related withtwo faults: one is Lishi big fault separating Ordos from Shanxi and the other is Jiaocheng big fault,which is situated in the southeast of sampling locality and was still in motion during the Cenozoic.

关键词： Jiaocheng, Shanxi, paleomagnetism, Early Triassic, local rotation.

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Paleomagnetic study on orogenic belt:An example from Early Cretaceous volcanic rocks,Inner Mongolia,China

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Science China Earth Sciences 2004年第12期47卷 1127-1133页

作者： REN Shoumai 1,2 ,ZHU Rixiang 1 ,HUANG Baochun 1 ,ZHANG Fuqin 1 & WANG Hongqiang 1 1.Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China 2.Strategic Research Center for Oil and Natural Gas,The Ministry of Land and Resources of China,Beijing 100034,China 1. Strategic Research Center for Oil and Natural Gas The Ministry of Land and Resources of China 100034 Beijing China 2. Chinese Academy of Sciences Institute of Geology and Geophysics 100029 Beijing China

We report paleomagnetic results for Early Cretaceous lava flows collected from the Suhongtu area of Inner Mongolia, the middle part of the Tianshan-Mongolia Fold Belt (TMFB). Rock-magnetic experiments for different lava flows indicate that the main magnetic mineral is pseudo-single-domain (PSD) magnetite. The characteristic high-temperature remanence com- ponent is isolated by thermal demagnetization temperature steps between 300°C and 585"C, which yields a mean direction of D = 23.6°, I=56.0° with α 95 = 2.3°. We interpret this high-temperature remanence component as primary magnetization based mainly upon the petrographic analysis, which shows that the shape of magnetite crystal is relatively rounded square or polygon without internal reflection and deuterogenous phenomenon. The correspond- ing pole of the high-temperature remanence component is at 71.1°N, 200.5°E with A 95 = 2.7°. This Early Cretaceous pole is in good agreement with those for Siberia, North China, and Inner Mongolia, suggesting that these continental blocks had already sutured together in the Early Cretaceous, which would further provide constraints on better understanding of the formation and evolution of the TMFB.

关键词： orogenic belt, paleomagnetism, Suhongtu of Inner Mongolia, Eurasia.

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New Early Cretaceous paleomagnetic results from Qilian orogenic belt and its tectonic implications

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Science China Earth Sciences 2002年第6期45卷 565-576页

作者：杨天水林爱明杨振宇孙知明 1. Institute of Geomechanics Chinese Academy of Geological Sciences 100081 Beijing China 2. Department of Earth and Planetary Sciences Shizuoka University Shizuoka Japan

Lower Cretaceous red sedimentary rocks from the depositional basin of East Qilian fold belt have been collected for a paleomagnetic study. Stepwise thermal demagnetization reveals two or three components of magnetization from dark red sandstones. Low-temperature magnetic component is consistent with the present Earth Field direction in geographic coordinates. High-temperature magnetic components are mainly carried by hematite. The mean pole of 19 sites for high-temperature magnetic components after tilt-correction is λ=62.2°N, φ=193.4°E, A95=3.2°, and it passes fold tests at 99% confidence level and reversal tests at 95% confidence level. The paleopole is insignificantly different from that of Halim et al. (1998) from the same sampling area at the 95% confidence level. Compared with paleomagnetic results for North China, South China, and Eurasia, our results suggest that no significant relative latitudinal displacement has taken place between Lanzhou region and these blocks since Cretaceous time. Remarkably, the pole of Lanzhou shows a 20° clockwise rotation with respect to those of North China, South China, and Eurasia. Geological information indicates that the crustal shortening in the western part of Qilian is greater than that in eastern part. In this case, the clockwise rotation of sampling area was related to India/Eurasia collision, and this collision resulted in a left-lateral strike-slip motion of the Altun fault in north Tibetan Plateau after the Cretaceous.

关键词： Qilian Mountain, paleomagnetism, Early Cretaceous, Hekou group.

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Paleomagnetic age and palaeobiological significance of hominoid fossil strata of Yuanmou Basin in Yunnan

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Science China Earth Sciences 2004年第5期47卷 405-411页

作者： Friedrich A. Heller Institute of Geophysics ETH Zurich Switzerland CH-8093

The two sections containing hominoid fossils in the Yuanmou Basin, Yunnan Prov-ince have been studied by means of magnetostratigraphy. The results are as follows. The Xiaohe section records over 8 polarity zones corresponding to Cande-Kent95 time scale, including C3Br.1n, C3Br.1r, C3Br.2n, C3Br.2r, C4n.1n, C4n.1r, C4n.2n and C4n.2r, attaching to Late Mio-cene. This section comprises two layers of hominoid fossils: the lower layer (the 1st layer in fig. 4) which records the lower portion of C4n.2r with paleomagnetic age of 8.20—8.10 Ma and the upper layer (the 16th layer in fig. 4) which records C3Br.1r with paleomagnetic age of 7.20—7.15 Ma. So the age of hominoid fossils in the Xiaohe section is about 8.20—7.15 Ma. The Leilao section records over 11 polarity zones corresponding to Cande-Kent95 time scale, including C3Br, C3Br.1n, C3Br.1r, C3Br.2n, C3Br.2r, C4n.1n, C4n.1r, C4n.2n, C4n.2r, C4n.3n and C4n.3r, attaching to Late Miocene. This section also consists of two layers of hominoid fossils: the lower layer (the 3rd layer in fig. 3) which records C4n.2r with the age of 8.20—8.10 Ma and the upper layer(the 19th layer in fig. 3) which records the middle portion of C3Br with the age of 7.15—7.1 Ma. So the age of hominoid fossils in the Leilao section is about 8.20—7.10 Ma. Sedimentary environments reflected in both sections are extremely similar, and the ages of hominoid fossils are of correspondency. The age of the lower and upper fossil layers is 8.20—8.10 Ma and 7.20—7.10 Ma respectively, which probably imply that hominoids living in Xiaohe and Leilao basins have uniform activity areas. There were hominoids activities all along during 8.00—7.00 MaBP in this field.

关键词： Yuanmou Basin, hominoid, paleomagnetism, chronology.

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New paleomagnetic and magnetic fabric results for Early Cretaceous rocks from the Turpan intramontane basin,east Tianshan,northwest China

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Science China Earth Sciences 2004年第6期47卷 540-550页

作者： HUANG Baochun WANG Yongcheng ZHU Rixiang Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029 China Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029 China Institute of Geology and Geophysics Chinese Academy of Sciences Beijing 100029 China

In order to further constrain the tectonic configuration of the Tianshan fold belt and the blocks of its northern and southern sides prior to the Eocene India/Eurasia collision and to reveal their general kinematic characteristics after the collision,we carried out pa- leomagnetic study for Early Cretaceous rocks collected from the Turpan intramontane basin of east Tianshan,northwest China.A high-temperature component (HTC)with dual polarities can be isolated from most samples after progressive thermal *** demagnetization behavior and some rock magnetic features of the samples indicate that this HTC resides mainly in *** positive fold and reversal tests suggest that the HTC may be a primary mag- netization acquired at or close to the time of formation of the *** to previous results obtained from Cretaceous rocks in western China,paleomagnetic results from the Turpan in- tramontane basin retain also the inclination shallowing relative to stable ***, the anisotropy of magnetic susceptibility (AMS)measurements showed weak anisotropy degrees for Early Cretaceous Shengjinkou sandstones from the Turpan *** weak anisotropy degrees are independent of inclinations of the HTC,confirming that the effect of sedimentary compaction in most of these rocks can generally be neglected. Comparison of the Early Cretaceous results from Turpan with the coeval ones from Tarim, Junggar and stable Eurasia suggests that the Turpan intramontane basin displaced with no or slightly relative tectonic movement compared with Tarim but with a distinguishable latitudinal conver-gence relative to both Junggar and stable Eurasia since the India/Eurasia collision.

关键词： Turpan Basin, Early Cretaceous, paleomagnetism, kinematics.

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Paleomagnetic results of the Cretaceous marine sediments in Tongyouluke, southwest Tarim

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Science China Earth Sciences 2005年第3期48卷 406-416页

作者： SHEN Zhongyue1, CHEN Hanlin1, FANG Dajun1, DING Jinghai1, ZHANG Shiben2, HUANG Zhibin2 & LI Meng2 1. Department of Earth Sciences, Zhejiang University, Hangzhou 310027 China 2. Tarim Oil Field Company, PetroChina Company Limited, Korla 830011, China 1. Department of Earth Sciences Zhejiang University 310027 Hangzhou China 2. Tarim Oil Field Company PetroChina Company Limited 830011 Korla China

Paleomagnetic and rock magnetic studies on samples of 18 sites from the Creta- ceous marine sedimentary rocks in the Tongyouluke section, Akto County, southwest Tarim, China show that the magnetic carriers of the Lower Cretaceous are dominated by hematite with some magnetite, while the magnetic carriers of the Upper Cretaceous are characterized by a combination of magnetite and titanomagnetite as well as hematite and goethite. Stepwise thermal demagnetization is performed and vector analysis is used to isolate magnetic components, which illustrates a single magnetic component or double magnetic components. The high temperature stable components are dual polarities and pass polarity test, reversal test and consistency test. The overall mean direction of the Lower Cretaceous is D = 27.0°, I = 42.0°, α95=6.5° with pole position at φ = 190.3°, λ=63.1°, dp=4.9°, dm=8.0°. The overall mean direction of the Upper Cretaceous is D = 29.1°, I = 39.4°, α95=11.2° with pole position at φ =190.9°, λ=60.3°, dp= 8.0°, dm=13.4°. Compared with the inclination of the Early Cretaceous from red beds of north Tarim, the contemporary inclination of southwest Tarim is 10.0°±7.8° sharper, but it is still 8.1° ±8.9°, shallower than that of Early Cretaceous basalts in southwest Tarim. Although these pa- leomagnetic data show slightly larger confidence limit, the paleolatitude of the marine Cretaceous tends to lie between that of terrestrial red beds and volcanic rocks.

关键词： Tarim paleomagnetism, Cretaceous, magnetic inclination, red beds, marine sedimentary rock.

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