Tectonic evolution of Cretaceous extensional basins in Zhejiang Province, eastern South China: structural and geochronological constraints

摘要

Widespread Cretaceous volcanic basins are common in eastern South China and are crucial to understanding how the Circum-Pacific and Tethyan plate boundaries evolved and interacted with one another in controlling the tectonic evolution of South China. Lithostratigraphic units in these basins are grouped, in ascending order, into the Early Cretaceous volcanic suite (K_(1V)), the Yongkang Group (K_(1-2)), and the Jinqu Group (K_(1-2)). SHRIMP U-Pb zircon geochronological results indicate that (1) the Early Cretaceous volcanic suite (K_(1V)) erupted at 136-129 Ma, (2) the Yongkang Group (K_(1-2)) was deposited from 129 Ma to 91 Ma, and (3) the deposition of the Jinqu Group (K_(1-2)) post-dated 91 Ma. Structural analyses of fault-slip data from these rock units delineate a four-stage tectonic evolution of the basins during Cretaceous to Palaeogene time. The first stage (Early to middle Cretaceous time, 136-91 Ma) was dominated by NW-SE extension, as manifested by voluminous volcanism, initial opening of NE-trending basins, and deposition of the Yongkang Group. This extension was followed during Late Cretaceous time by NW-SE compression that inverted previous rift basins. During the third stage in Late Cretaceous time, possibly since 78.5 Ma, the tectonic stress changed to N-S extension, which led to basin opening and deposition of the Jinqu Group along E-trending faults. This extension probably lasted until early Palaeogene time and was terminated by the latest NE-SW compressional deformation that caused basin inversion again. Geodynamically, the NW-SE-oriented stress fields were associated with plate kinematics along the Circum-Pacific plate boundary, and the extension-compression alternation is interpreted as resulting from variations of the subducted slab dynamics. A drastic change in the tectonic stress field from NW-SE to N-S implies that the Pacific subduction-dominated back-arc extension and shortening were completed in the Late Cretaceous, and simultaneously, that Neo-Tethyan subduction became dominant and exerted a new force on South China. The ongoing Neo-Tethyan subduction might provide plausible geodynamic interpretations for the Late Cretaceous N-S extension-dominated basin rifting, and the subsequent Cenozoic India-Asia collision might explain the early Palaeogene NE-SW compression-dominated basin inversion.