Paleomagnetism of late Mesozoic rocks from northeastern China: the role of the Tan-Lu fault in the North China Block

Abstract

Paleomagnetic studies were performed on Jurassic, Cretaceous and Tertiary rocks sampled from the Qitaihe area in Heilongjiang and Benxi area in Liaoning Provinces, northeast China. Both locations are near the Tancheng-Lujiang (Tan-Lu) fault system; Benxi is close to but on the eastern side of the fault while Qitahe lies between two major branches of the northwestern extension of this fault. In Mesozoic rocks, secondary magnetization in the present field direction was observed, but it was possible to retrieve the primary components by taking the high-temperature portion of the demagnetizing spectra. The Mesozoic poles thus obtained, especially those for the Cretaceous, deviate from the paleomagnetic poles of similar ages from the central part of the North China Block (NCB), Siberian Block or South China Block (SCB). Although the distances to the poles (flattening) are quite similar, the Benxi pole suggests a small clockwise rotation with respect to the central NCB poles, while the the Qitaihe poles indicate a much larger rotation in the opposite direction. It is shown that the deviation of the Benxi pole is similar to that observed for the Korean Peninsula and Shangdong Province, which all lie to the east of the Tan-Lu fault in the NCB. The Qitaihe pole position is quite different from the poles either west or east of the Tan-Lu fault. From these observations, it is concluded that a left-lateral strike-slip movement at the Tan-Lu fault system since the Cretaceous is the cause of systematic deviation in the position of the poles obtained from east of the fault including the Benxi area, while anomalous direction of Qitaihe rocks may represent a small scale rotation within the Tan-Lu fault system. The estimation of the movement along the Tan-Lu fault depends on which branch of the fault system is considered most active. If the main branch is assumed to be the place of slip, the movement can be represented by an Euler pole which lies to the south of Honshu Island (20°N, 150°E), with an estimated total displacement of 800 km since the Cretaceous.