The links between Neoproterozoic tectonics, paleoenvironment and Cambrian explosion in the Yangtze Block, China

Abstract

The correlation between Neoproterozoic tectonics, paleoenvironment and early life are crucial for understanding the evolution of the Earth's habitability. The Yangtze Block preserves plenty of essential records to investigate these topics. The early Tonian tectonics of the Yangtze Block was characterized by exterior spreading to its west and north, and coeval interior subduction along its eastern margin, resulting in the passive continental marginal basins on the western and northern margins, and arc-related basins along the eastern margin. In the middle Tonian, the long-term exterior subduction-accretion on the west and north resulted in numerous arc-type igneous rocks and associated clastic deposits. In the east, the continuous interior subduction generated an active continental margin, with back-arc and retro-arc basins filled with volcanic and clastic rocks, and finally led to the collision between the Yangtze and Cathaysia blocks at ca. 820 Ma. Some typical Neoproterozoic microfloras were flourishing. During the late Tonian, the exterior subduction generated the arc-related volcanic basins in the western and outer northern margins, while the interior post-collisional rifting resulted in a rift basin in the eastern margin, filled by shallow-marine to bathyal facies sediments. Influenced by the marginal volcanism, the paleoclimate was warm and humid in the early period, and transition to rather cold and dry in the late period. The microfloras remained the dominant paleontological taxonomy, but some macroalgal fossil fragments appeared. In the Cryogenian, along with termination of the exterior subduction, most parts of the Yangtze Block entered an extensional tectonics. Together with global paleoclimate cooling, the various tectonic backgrounds and paleogeographical frameworks of individual margins accumulated different glacial-interglacial sequences on the eastern, northern and western margins. These multi-stage deposits record paleoclimate cycles of cold-dry glacial periods and warm-humid interglacial periods. Subject to more extreme paleoclimate and paleoenvironment, early life was still dominated by microfloras (Monosphaeritae and Prismatomorphitae of Sphaeromorphida) with minor macroalgal fossil fragments. During the Ediacaran, the entire Yangtze Block evolved into an extensional-subsidence tectonics, with carbonate platform (inner shelf and outer shelf) to abyssal-bathyal (slope-basin) facies gradually developing from northwest to southeast. Episodic atmospheric and oceanic oxygenation events resulted in positive shifts in paleoclimate and paleoenvironment. The advantageous conditions induced several global Ediacaran biotas on the Yangtze Block, which are known as the prologue and first phase of the Cambrian explosion, including the Lantian, Weng'an, Miaohe, Shibantan and Gaojiashan biotas. The transition from the Ediacaran to Cambrian constitutes a critical inflection point in geological history, marking a co-evolution of organisms and paleoclimates that flourished by mutations. Appropriate external conditions made the early life further radiate, and the second and third phases of the Cambrian explosion were unveiled. They are represented by the Meishucun small shelly fossils (SSFs) on the western margin, and the Chengjiang biota and the contemporaneous Qingjiang biota on the western and northern margins of the Yangtze Block, respectively. Such manifestations indicate that the Cambrian explosion in the Yangtze Block was most likely related to the Neoproterozoic subduction and continental arcs, which could provide sufficient nutrients for the later early life radiation.