Abstract:
It has been proved that the NE-trending Longmenshan thrust belt in the eastern margin of the Qinghai-Xizang Plateau was a composite of the expansion and growth of the Mesozoic and Cenozoic foreland thrust belts. However, puzzle is remained for the unidirectional and segmented migration of earth surface failures, aftershocks, and landslides of the Wenchuan earthquake on May 12, 2008. This puzzle challenges the understanding for the texture of the Longmenshan composite thrust belt. Based on the previous studies, this paper applied geological investigation and structural analysis focusing on a possible special texture produced by the composite growth of the Longmenshan thrust belt. The results reveal a composite texture produced by the growth of the Cenozoic foreland thrust belt superimposed over the Mesozoic foreland thrust belt with foundation of the composite thrust wedge. The composite thrust wedge could be graded with formational sequence. The Mesozoic foreland thrust wedges are characterized by thrust fault-related anticlines. Thrust complexes of Bikou, Tangwangzhai and Longwangmiao, which are combined by foreland thrust wedges, were initiated before the late Triassic (237 Ma) and terminated at ~160 Ma. The Cenozoic foreland thrust wedge is composed of thrust fault and sheet, and was staged produced by SE-ward progressive propagation at about 35~10 Ma and 10 Ma, respectively. This thrust wedge propagation might result in the Longquanshan uplift in the east of the western Sichuan basin. Therefore, the Longmenshan foreland thrust belt has characteristics of composite texture and composite growth.