Cretaceous synfolding remagnetization components revealing tectonic rotation of the middle Yangtze fold belt  

作　　者：ZHANG Hui TAN XiaoDong HAN YuLin 

机构地区：[1]CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China. [2]College of Earth Sciences, Graduate University of Chinese Academy of Sciences, Beijing 100049, China.

出　　处：《Chinese Science Bulletin》2007年第20期2837-2846,共10页

基　　金：Supported by the National Natural Science Foundation of China (Grant No. 40374023);the "Hundred Talents Project" of CAS

摘　　要：To reveal the deformation process of the middle Yangtze fold belt, we conducted a paleomagnetic study on Middle Triassic limestones and Middle to Late Jurassic sandstones from Wanzhou, Chongqing. Stepwise thermal and alternating field demagnetization were used to isolate the multi-component re- manent magnetizations. The Jurassic samples were overprinted by recent geomagnetic field, while three magnetization components were isolated from the Middle Triassic samples. A low temperature component (LTC) was isolated at temperatures below 200℃, an intermediate temperature component (ITC) at 200―360℃ and a high temperature component (HTC) at 400―460℃. The LTC is distributed around the present-day Earth magnetic field, probably a viscous component. Stepwise unfolding indi- cates that the maximum precision parameters of ITC and HTC components are achieved at 33±8% and 50±27% (with 95% confidence) unfolding, respectively. The best-clustered ITC mean direction, Dec = 11.2°, Inc = 45.2° (α95 = 4.5°, N = 34), corresponding to a paleopole at 79.3°N, 219.5°E (dp = 3.6°, dm = 5.7°), is consistent with the Cretaceous reference direction of the South China Block (SCB). The best-clustered HTC mean direction (taking 70% unfolding), Dec = 24.2°, Inc =49.0° (α95 = 3.6°, N = 23), corresponding to a paleopole at 69.2°N, 195.5°E (dp =3.1°, dm = 4.8°), suggests a clockwise rotation of 12.8°±3.5°. These synfolding remagnetization components clearly reveal that a clockwise rotation happened at the middle stage of folding, thus supporting that at least part of the variation in fold axis strikes is due to orocline rotation. Combined with published data, our analysis indicates that the Wan- zhou-Xiangxi segment of the middle Yangtze fold belt experienced oroclinal bending. Furthermore, a published post-folding component isolated from the Middle Triassic Puqi Formation suggests a 27.5°±5.8° clockwise rotation, confirming that at least 50% of the observed clockwise rotations in the eastern middle Yangtze fold bTo reveal the deformation process of the middle Yangtze fold belt, we conducted a paleomagnetic study on Middle Triassic limestones and Middle to Late Jurassic sandstones from Wanzhou, Chongqing. Stepwise thermal and alternating field demagnetization were used to isolate the multi-component remanent magnetizations. The Jurassic samples were overprinted by recent geomagnetic field, while three magnetization components were isolated from the Middle Triassic samples. A low temperature component （LTC） was isolated at temperatures below 200℃, an intermediate temperature component （ITC） at 200-360℃ and a high temperature component （HTC） at 400-460℃. The LTC is distributed around the present-day Earth magnetic field, probably a viscous component. Stepwise unfolding indicates that the maximum precision parameters of ITC and HTC components are achieved at 33±8% and 50±27% （with 95% confidence） unfolding, respectively. The best-clustered ITC mean direction, Dec = 11.2°, Inc = 45.2° （α95 = 4.5°, N = 34）, corresponding to a paleopole at 79.3°N, 219.5°E （dp= 3.6°, dm= 5.7°）, is consistent with the Cretaceous reference direction of the South China Block （SCB）. The best-clustered HTC mean direction （taking 70% unfolding）, Dec = 24.2°, Inc =49.0° （α95 = 3.6°, N = 23）, corresponding to a paleopole at 69.2°N, 195.5°E （dp =3.1°, dm =4.8°）, suggests a clockwise rotation of 12.8°±3.5°. These synfolding remagnetization components clearly reveal that a clockwise rotation happened at the middle stage of folding, thus supporting that at least part of the variation in fold axis strikes is due to orocline rotation. Combined with published data, our analysis indicates that the Wanzhou-Xiangxi segment of the middle Yangtze fold belt experienced oroclinal bending. Furthermore, a published post-folding component isolated from the Middle Triassic Puqi Formation suggests a 27.5°±5.8° clockwise rotation, confirming that at least 50% of the observed clockwise rotations in the ea

关 键 词：古地磁 再磁化 中生代 马蹄形山系 褶皱带 

分 类 号：P5[天文地球—地质学]