Paleomagnetic Study Of The Cretaceous Redbeds In The Yuanma Basin,West Hunan And Its Implications For The Regional Structural Deformation

As a geological unit formed by the collision between the Yangtze plate and Cathaysia plate in the middle and late Proterozoic,the South China plate has been influenced by the compression of the Pacific plate and the Indian plate since the Late Cretaceous.The western part of the South China plate is the rigid Sichuan basin,the eastern part is the Cathaysia plate,and the central part is a series of NE-SW continental deformation belts.The Yuan-ma basin is located in the middle of the South China plate,with the Xuefengshan tectonic belt in the east and the eastern Sichuan fold belt in the west.The studies about the Cretaceous red sandstone are very helpful to understand the tectonic evolution of the South China plate since the late Mesozoic.The eastern Sichuan fold belt,as a"Jurassic mountain type"fold in the middle of the South China plate,has been a hotspot issue in the study of intracontinental deformation.Paleomagnetic study of the Yuanma basin can reveal the deformation mechanism of the eastern Sichuan fold belt since the Cenozoic.The paleomagnetic samples from the Mayang section of the Yuanma basin show that the tilt-corrected site-mean direction is Ds=6.8°,Is=55.7°,k=56.8,?₉₅=3.3°.The paleomagnetic sample of the Jishou section showed that the tilt-corrected site-mean direction is Ds=15.6°,Is=42.9°,k=118.6,?₉₅=2.6°.The paleomagnetic results from the Yuanling section showed that the tilt-corrected site-mean direction is Ds=5.7°,Is=43.6°,k=156.4,?₉₅=2.9°.These results all passed the fold test,and indicated that since the Late Cretaceous,the area in the south of the Yuanma basin had undergone a 3.9±4.5°insignificant counterclockwise tectonic rotation.Since the late Early Cretaceous,a clockwise tectonic rotation of 4.1±3.0°has occurred in the central of the Yuanma basin.Since the late Early Cretaceous,an insignificant counterclockwise rotation of 0.6±2.9°has occurred in the north of the Yuanma basin.Combined with previous paleomagnetic results from the Yichang area,the correlation analysis of Qiyueshan fault in the eastern Sichuan fold belt,and the important structure in the west foothills of the Xuefeng orogenic belt,including the Yuanma basin,was carried out.The results show that the linear correlation between the structural trace changes of the Qiyueshan fault,and the rotation of the red bed basins in the west foot of the Xuefeng Mountain since the Cretaceous is R=-0.98343.The east Sichuan fold belt on the southeast side of the fault zone is the result of right-handed shear deformation.The reason for the dextral shear may be the result of the compression between Indian plate and the Eurasian continent.The dextral shear deformation is most likely occurred in the Zhangjiajie-Huayuan fault,and its displacement is estimated to be 50?93km.After tectonic reconstruction,the linear correlation between the structural track change of the Qiyueshan fault and the tectonic rotation of the redbed basins in the west foot of Xuefeng mountain is R=0.99908.Meanwhile magnetic fabric results also support this finding.It shows that since the Cenozoic,the central Yuanma Basin has been subjected to NE-SW-direction compression,while the southern part has been affected by local tectonic effect of NEE-SWW-direction compression.The compression in the south of the Yuanma Basin was greater than that in the north,indicating that the central part of the South China Plate was affected by NE-SW tectonic stress due to the Indo-Asia collision.Under this compressive stress field,a series of NE-SW-trending faults in the northwest of the Yuanma Basin were likely to undergo structural re-activation and thus tectonic movement.