Timing of the Yanshan Movement: evidence from the Jingxi Basin in the Yanshan fold-and-thrust belt, eastern China

Abstract

Evidence for the Jurassic–Cretaceous Yanshan Movement (Orogeny) was first identified in the Jingxi Basin, a part of the Yanshan fold-and-thrust belt (YFTB) along the northern margin of the North China Craton. The disconformity or angular unconformity within the Jurassic volcano-sedimentary successions in the basin was interpreted to have resulted from a period of incipient shortening during the Yanshan Movement, commonly known as event A. However, the absolute ages of both event A and associated Jurassic units remain controversial. Here, we present the results of zircon U–Pb geochronology for volcanic and clastic rocks from the Jurassic units in the Jingxi Basin. The results provide better constraints on the depositional ages of formations in the basin: (1) Early-to-early Middle Jurassic (175–167 Ma) for the Nandaling Formation; (2) middle Middle Jurassic for the Yaopo Formation; (3) late Middle Jurassic for the Longmen Formation; (4) early Late Jurassic for the Jiulongshan Formation; and (5) late Late Jurassic (157–149 Ma) for the Tiaojishan Formation. Stratigraphic and geochronological correlations indicate that the Jiulongshan Formation and the Tiaojishan Formation are of different depositional ages in different regions. The Longmen Formation in the Jingxi Basin is equivalent to the Haifanggou Formation in the eastern YFTB. Two sedimentary cycles separated by a sedimentary hiatus, shown as disconformity and local angular unconformity, have been identified in the Jurassic sequences of the YFTB. Depositional ages of the rocks above and below the disconformity, which lies between the Yaopo and Longmen formations in the Jingxi Basin, constrain the timing of event A to 167 ± 2 Ma (late Middle Jurassic). This event is the first phase of shortening deformation in eastern China induced by the paleo-Pacific plate subduction. We propose that the shortening period was relatively transient, in contrast to previous suggestions of long-duration shortening.