Tectono-stratigraphic evolution of multiphase rifts in a hangingwall dip-slope setting: A case study in Eocene western slope, Xihu Sag of East China Sea Shelf Basin

The hangingwall dip-slope settings are important components of rift basins, being able to influence offshore sedimentation, but a few studies have examined their properties. Their reactivation of growth faults can contribute to re-shape basin geometry influencing the associated stratigraphic successions. This paper investigates the structural evolution of a hangingwall dip-slope setting to explore the development of multiphase rifts that imprint tectonic properties and stress field migration by utilizing seismic, well-log, and core data from the Xihu Sag, East China Sea Shelf Basin. Three composite sequences (CS1-3) are defined within the Eocene synrift deposits, along with three third-order sequences in CS3. We have reconstructed the evolution of two tectonic phases: rift phase 1 and rift phase 2. Furthermore, we identified two stages within each phase: rift initiation and rift development 1 for rift phase 1, and rift development 2 and rift termination for rift phase 2. However, an immediate decrease in fault activity was recorded in the rift phase 2. Alterations in the stress field throughout intermittent rift phases led to differences in tectono-stratigraphic structures. In rift phase 1, expansion occurred from small isolated depocenters to larger systems, while rift phase 2 experienced contraction. Concurrently, basin geometry evolved from wedge-shaped half-grabens to dish-like geometries. The tectonic change caused sediment overfilling to starvation in rift phase 1 and sediment starvation to balance filling in rift phase 2. This contribution provides insights into hangingwall dip-slope dynamics and rift basin depositional systems.