Mesozoic evolution of the Hefei basin in eastern China: Sedimentary response to deformations in the adjacent Dabieshan and along the Tanlu fault

摘要

This paper presents a study of Jurassic–Early Cretaceous sedimentary evolution of the Hefei basin in eastern China and explores the relationship between clastic sedimentation and coeval deformation of the Dabieshan to the south and the Tanlu fault to the east. The Hefei basin experienced a two-stage evolution. The basin was initiated in the Early Jurassic and expanded in the Middle and Late Jurassic. The synsedimentary Jinzhai normal fault is considered to be a border fault of the basin because it controlled Middle to Upper Jurassic proximal alluvial deposits. Persistent N- to NE-directed paleo-flows in the southern Hefei basin indicate that sediments came from the Dabieshan, and the presence of Triassic coesite-bearing detrital zircon in Lower Jurassic sediments documents exhumation of ultrahigh-pressure rocks of the Dabieshan to the surface as early as the Early Jurassic. Occurrences of eclogite clasts in an Upper Jurassic unit indicate continued denudation of the Dabieshan at that time. Thickening of Jurassic clastic units to the southern and southeast parts of the basin suggests that basin subsidence and depositional loci were under the coupled control of the Jinzhai normal fault on the south and the NE-striking left-lateral transtensional Tanlu fault on the east. Jurassic extensional subsidence of the Hefei basin is in marked contrast to the coeval development of a contractional foreland basin south of the Dabieshan, which combines to indicate contemporaneous extension and contraction on the north and south sides of the Dabieshan, respectively. Vigorous volcanism and uplift of the southern Hefei basin characterized the second stage of development of the Hefei basin in the Early Cretaceous, and this led to a synchronous shift of its main depocenter to the north. This younger depocenter is characterized by lacustrine and fluvial-deltaic sedimentation, where alluvial and fan-deltaic coarse-grained deposition mainly occurred along the eastern edge of the basin. Early Cretaceous subsidence is attributed to E-W extension across the middle segment of the Tanlu fault, and the Zhangbaling massif on the east acted as a footwall and provided a source for sediment to the northern basin. A model is accordingly advanced to account for how the Hefei basin developed in response to the tectonic exhumation of the Dabieshan and the deformation of the Tanlu fault in the Mesozoic. It illustrates that the Hefei basin initiated and evolved during Jurassic time in an extensional setting that was triggered by southerly upward extrusion of ultra-high-pressure rocks of the Dabieshan. Early Cretaceous development of the basin was controlled by magmatism-related uplift of the Dabieshan on the south and orthogonal normal faulting of the middle segment of the Tanlu fault on the east. This study provides an independent constraint upon the exhumation processes of ultrahigh-pressure rocks of the Dabieshan.