Basin response to the Jurassic geodynamic turnover from flat subduction to normal subduction in South China

The Mesozoic South China experienced the transformation from the Triassic large-scale intracontinental compression deformation to the Cretaceous massive magmatic activity and regional large-scale tectonic extension. This understanding has become a consensus in the academic community, but the dynamic mechanism of this transformation is still hotly debated on whether it was controlled by the collision between land blocks or the subduction of the paleo-Pacific plate. The Jurassic is a critical period for the transformation of these two major tectonic processes. A deep understanding of their tectonic background and evolution is crucial for resolving the above-mentioned problem.

Although scholars generally believed that the subduction of the paleo-Pacific plate has had an important impact on the geological evolution of South China, there are currently two completely different understandings of the Jurassic tectonic background and dynamic mechanism in South China. Some believed that the Jurassic South China was mainly controlled by intracontinental extension due to the delamination of flat-subducted paleo-Pacific slabs (Li and Li, 2007). Their evidence included the occurrence of a large number of A-type granites, bimodal magmatic rocks, and various basalt and other rift basins (Li and Li, 2007; Meng et al., 2012, 2015; Huang et al., 2015). Others argued that it was dominated by contractional tectonics due to the Andean-type subduction of the paleo-Pacific plate. Their evidence included the calc-alkaline arc magmatic belt along the eastern continental margin, synchronous extensive thrust-fold deformation and so on (Xiao et al., 2005; Li et al., 2012; Xu et al., 2017; Yuan et al., 2017; Li et al., 2018; Zhang Wei et al., 2018).

Northwestern Zhejiang, located near the ancient merged belt of the Yangtze and Cathaysia blocks in eastern South China (Xiao and He, 2005), has a relatively continuous Early-Middle Jurassic strata (Wang Longwu et al., 2004). It is one of the ideal windows for unraveling the structure-sedimentary evolution and the temporal and spatial changes of provenance in South China during the late Triassic to the Middle Jurassic.

Here, we present new detrital zircon U-Pb ages and sedimentary data from the Jurassic basin in NW Zhejiang to constrain the Jurassic tectonic evolution. The continuous Jurassic succession archives an abrupt sedimentary change from high-energy coastal environment to proximal and fast-filling terrestrial environment. This lithostratigraphic change was also accompanied by the shift of detrital provenance. Age spectra of the detrital zircons from the bottom of the Jurassic show strong 1.0–0.7 Ga and 500–400 Ma populations, which are inferred to mainly derive from the Yangtze block. In contrast, samples from the overlying Lower-Middle Jurassic were dominated by age groups of 2.0–1.7 Ga and 300–170 Ma, which were possibly sourced from the Cathaysia block. The switch of the sedimentary and provenance characteristics reveals that an Earliest Jurassic broad sag basin in the inland shifted into an Early-Middle Jurassic retro-arc foreland basin along the coastal region. Abundant Jurassic-aged zircons are compatible with the re-initiation of "normal subduction” in the Early Jurassic. The re-initiation of “normal subduction” resulted in the generation of accretionary orogeny, continental arcs and retroarc foreland basin in the eastern South China margin, which contrasts to the extensional regime in the inland. The basin response and distinct tectonic regimes between the hinterland and continental margin in the Early-Middle Jurassic support a geodynamic turnover from the flat to normal subduction.

Figure 1 Schematic diagram of the structural evolution and deep dynamic interpretation of the early Mesozoic basins in South China (Zhang et al., 2021)

The research is one of the main achievements of Zhang Wei's master's thesis of Zhejiang University. He is currently a Ph.D candidate at Okayama Material Center in Japan; the corresponding authors are Professor Zhang Feng-Qi and Associate Professor Zhu Kong-Yang of Zhejiang University. This research was jointly funded by the National Natural Science Foundation of China (41772205, 41330207, 41720104003).

Article information: Zhang Wei, Zhang Feng-Qi, Dilek Yildirim, Zhu Kong-Yang, Wu Hong-Xiang, Chen Dong-Xu, Chen Han-Lin. 2021. Basin response to the Jurassic geodynamic turnover from flat subduction to normal subduction in South China. GSA Bulletin, Online. https://doi.org/10.1130/B36059.1

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