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

• DOI: 10.1007/s11434-007-0437-0
• Publication Date: 2007-10-06T08:14:26Z
• Authors: Hui Zhang; XiaoDong Tan; YuLin Han
• Keywords: 01 natural sciences; 0105 earth and related environmental sciences

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 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 degrees C, an intermediate temperature component (ITC) at 200-360 degrees C and a high temperature component (HTC) at 400-460 degrees C. 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 degrees, Inc = 45.2 degrees (alpha(95) = 4.5 degrees, N = 34), corresponding to a paleopole at 79.3 degrees N, 219.5 degrees E (d(p) = 3.6 degrees, d(m) = 5.7 degrees), 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 degrees, Inc =49.0 degrees (alpha(95) = 3.6 degrees, N = 23), corresponding to a paleopole at 69.2 degrees N, 195.5 degrees E (d(p) =3.1 degrees, d(m) = 4.8 degrees), suggests a clockwise rotation of 12.8 degrees +/- 3.5 degrees. 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 degrees +/- 5.8 degrees clockwise rotation, confirming that at least 50% of the observed clockwise rotations in the eastern middle Yangtze fold belt can be attributed to oroclinal bending. The remagnetization data and geological evidence observed in the middle Yangtze fold belt suggest that collision between SCB and North China Block (NCB) probably lasted till the early period of Early Cretaceous.