CRETACEOUS EXTENSION IN EASTERN ASIA AND THE CLOCKWISE ROTATION OF KOREA AND CONTIGUOUS AREAS

Clockwise crustal rotation (ca. 22° to 34°) of a large area including eastern Liaoning province and the Korean Peninsula with respect to a western, non-rotated North China block has been documented by paleomagnetic studies over the past decade. However, the timing of this rotation and the reasons for it are controversial. Lin et al. (2003) believed that rotation was largely post-Early Cretaceous in age, and was the consequence of extension within a crustal domain that tapered southwards towards the Bohai Sea and its Gulf of Liaoning. This domain largely coincides with the Songliao basin of eastern China, the extensional history of which is obscured by thick, younger sedimentation. Our field studies near the Gulf of Liaoning support the Lin et al. kinematic hypothesis, but not their timing of rotation. Earlier recognition of the Liaonan metamorphic core compex (LMCC) east of the Gulf has been complimented by the recent discovery of the Waziyu complex northwest of the Gulf (WMCC; Darby et al., 2004). Both complexes have master detachment faults that root westward and were active in the Early Cretaceous between ca. 130-115 Ma. North China paleomagnetic studies indicate that supradetachment strata (Yixian Fm., ca. 134-120 Ma) in the upper plate of the Waziyu detachment fault have not been rotated (Zhu et al., 1999). Such non-rotation supports our premise that the lower plates of the WMCC and the LMCC have been displaced ESE-ward in an absolute sense away from the stable North China block, thus contributing to the rotation of Korea and contiguous areas. Rotation is inferred to have affected only the upper crust above mid-crustal levels into which we believe the two detachment systems flattened. If so, the regional Tan Lu fault that lies between the two core complexes was truncated at mid-crustal depth, since in areas to the south it forms a Mesozoic plate boundary between North and South China lithospheric blocks. Lin et al. (2003) were unaware of the LMCC and WMCC and argued that most of the regional block rotation was post Early Cretaceous and, in part, early Cenozoic. However, the Early Cretaceous ages of the two Liaoning mcc's and a Songliao basin mcc (Xujiaweizi) discovered by recent drilling through its younger stratigraphic cover, support our and some Korean workers' conclusions that most of the clockwise rotation was Early Cretaceous.