Structural evolution of two‐stage rifting in the northern East China Sea Shelf Basin

This study focuses on the stratigraphic correlation and structural evolution of the Ieodo and Jeju basins in the northern East China Sea Shelf Basin (ECSSB), which are compared to the Changjiang and Xihu depressions, respectively. Based on multi‐channel seismic reflection data, Cenozoic sedimentary successions resting on the acoustic basement in both basins can be divided into multiple syn‐ and post‐rift units. Basement‐involved structures exhibit different orientations, that is, E–W (ENE–WSW) and NE–SW trending structures in the Ieodo and Jeju basins, respectively. They represent the development of two‐stage rifting. The first rift stage began in the Palaeocene and was only restricted to the Ieodo Basin located west of the Hupijiao Rise. The second rift stage subsequently occurred in the Jeju Basin located east of the Hupijiao Rise during the late Eocene to Miocene. The Eocene post‐rift unit of the Ieodo Basin was uplifted while the Jeju Basin was subsided by bounding faults. The sequential development of rift structures in the basins suggests eastward migration of the Cenozoic rifting in the northern ECSSB. The NE‐trending rift structure of the Jeju Basin parallel to the subduction zone of the Pacific Plate suggests that the second rift stage possibly evolved under the influence of the back‐arc extensional regime. The Hupijiao Rise between the Ieodo and Jeju basins was possibly a geologic basement in the Palaeocene and was an uplifted remnant caused by footwall exhumation during the second rift stage of the Jeju Basin.

In this study, we investigate the northern East China Sea Shelf Basin (ECSSB) consisting of the Ieodo and Jeju basins separated by the Hupijiao Rise, which is one of the representative uplifts ( Figure 2). According to previous studies, both basins evolved from late Cretaceous to Miocene rifting and have undergone approximately coeval tectonic evolution (Cukur, Horozal, Kim, & Han, 2011;Koh FIGURE 1 Topographic map of the East China marginal sea and adjacent trench-arc-basin region with the Ryukyu subduction zone (modified from Suo et al., 2015;Zhou et al., 1989). The contour map is extracted from data by Smith and Sandwell (1997) Lee et al., 2006). The Hupijiao Rise was regarded as a tectonic boundary, such as a transverse fault zone, separating the basins (Figure 2; Zhou et al., 1989). However, there have been few attempts to determine the structural linkage and stratigraphic differences between basins, or their relationship with adjacent basins (i.e., Changjiang and Xihu depressions) in the northern ECSSB.
This paper aims to examine the spatial and temporal distribution of the Ieodo and Jeju basins and the Hupijiao Rise in the northern ECSSB and provides refined seismic stratigraphy in comparison with other depressions of the ECSSB based on seismic interpretation.

| GEOLOGICAL SETTING
The NE trending ECSB along the East Asian continental margin is distributed between the East China mainland and the Ryukyu Trench ( Figure 1; Q. L. Yang, 1992;Zhou et al., 1989). The ECSB is generally divided into the ECSSB landward and the Okinawa Trough seaward, which is separated by the Taiwan-Sinzi Belt (Figure 1; Q. L. Yang, 1992;Zhou et al., 1989). The ECSSB is located in the present-day continental shelf ( Figure 1) and consists mainly of western and eastern depressions bounded by several different rises (the Hupijiao, Haijiao, Yushan, and Fuzhou rises; Q. L. Yang, 1992;Zhou et al., 1989).
The northern ECSSB comprises the Ieodo (formerly known as Socotra), Jeju, Domi, and Sora basins (Figure 1; Oh, Park, & Park, 1997). These basins extend from the Changjiang, Xihu, and Fujiang depressions, respectively, which belong to the region of Chinese territorial waters (G. H. Lee et al., 2006). The Changjiang Depression is located between the western Zhemin and eastern central uplifts. It formed as isolated half-graben in the late Palaeocene (Su et al., 2014;Suo et al., 2015). The Xihu Depression is located between the western central uplifts and is commonly divided into the West Gentle Slope, Central Anticline Belt, and East Sharp Slope (Suo et al., 2015). It formed in the late Eocene, with relatively continuous full-graben geometry (Su et al., 2014;Suo et al., 2015).
The Jeju Basin, the largest basin in the northern ECSSB, has been coevally compared with the Ieodo Basin using seismic interpretation (Cukur et al., 2011;Koh et al., 2016;G. H. Lee et al., 2006). The lowermost sequence of the Ieodo Basin was interpreted to be the same stratigraphic level as the Jeju Basin (Cukur et al., 2011;Koh et al., 2016;G. H. Lee et al., 2006). According to the biostratigraphic study, the Jeju Basin was initiated in the late Eocene, and the tectonic evolution of the Jeju Basin was affected by that of the Xihu Depression (Yun et al., 1999).
Basin-fills in the northern ECSSB were initially dominated by the fluvio-lacustrine deposits and subsequently by coastal plain to shallow marine deposits (KIGAM, 1997;G. H. Lee et al., 2006;Yun et al., 1999).

| STRUCTURE OF THE ACOUSTIC BASEMENT
In order to interpret the seismic stratigraphy and reveal the stratigraphic

The acoustic basement is composed of Precambrian gneiss and
Mesozoic granitoid (Su et al., 2014;Zhou et al., 1989). The Precambrian gneiss generally consists of Neoproterozoic rocks whose Cathaysia Block extended to the northern ECSSB of the basement (Su et al., 2014;Zhou et al., 1989). Depositional ages of the Ieodo Basin are not clearly defined due to the absence of wells in the basin.
We infer that the pre-rift unit in the Ieodo Basin was formed in the The fanning geometry and configuration of related reflections indicate a syn-rift stacking pattern (Nottvedt, Gabrielsen, & Steel, 1995;Schlische, 1991). The syn-rift phase in the Ieodo Basin occurred prior to the early Eocene, as indicated by the overlying early Eocene unconformity. The syn-rift unit in the Changjiang Depression, stratigraphically equivalent to the Ieodo Basin, was deposited in the Palaeocene (Su et al., 2014;. Hence, we suggest that the syn-rift unit 1 in the Ieodo Basin was most likely deposited in the Palaeocene.

| Post-rift unit 1 (early Eocene)
The post-rift unit 1 overlies the early Eocene unconformity and is identified in the Ieodo Basin (Figure 4). Internal reflections close to

| Post-rift unit 2 (Oligocene to Miocene)
The overlying post-rift unit 2 is regionally observed in the Ieodo and Jeju basins, whereas the post-rift unit 1 is confined to the Ieodo Basin ( Figures 5 and 6a). The post-rift unit 2 consists of the lower and upper

| Post-rift unit 3 (Pliocene to the present)
The  The lateral linkage between the depressions of the east rift region in the ECSSB is much better than that of the west rift region (Suo et al., 2015). At that time (Eocene), the rate of the Pacific-Eurasia convergence reached a minimum, indicating a widespread extension (Northrup, Royden, & Burchfiel, 1995). The NE-striking extensional structure parallel to the subduction zone further suggests that the second rift in the Jeju Basin was most likely related to back-arc extension due to roll-back of the subduction of the Pacific Plate (Li et al., 2009;Sibuet & Hsu, 1997).
The syn-rift unit in the Jeju Basin was affected by structural deformation, leading to the formation of the early Oligocene unconformity. The early Oligocene deformation is consistent with the Yuquan Movement, which has been well identified in the east rift region of the ECSSB, for example, the Xihu Depression (Q. L. Yang, 1992;Zhou et al., 1989).
Following the Yuquan Movement, the Ieodo and Jeju basins underwent post-rift subsidence during the Oligocene to Miocene, forming the post-rift unit 2. This overall subsidence was temporarily interrupted by the early Miocene regional uplifting and folding, resulting in the formation of the early Miocene unconformity. The unconformity corresponds to the tectonic event, named the Huagang Movement, that caused the regional deformation of the Xihu Depression (Q. L. Yang, 1992;Zhou et al., 1989) and is known to be the main cause of the uplift of the Taiwan-Sinzi Belt (Q. L. Yang, 1992;Zhou et al., 1989). In the Jeju Basin, however, it is not certain whether the movement affected the regional uplifting of the Taiwan-Sinzi Belt due to the structural overprint of the late Miocene deformation.
At the end of the Miocene post-rift subsidence, the entire region of the northern ECSSB was strongly deformed by regional contraction, as indicated by the late Miocene unconformity terminating faultreactivated folds and tilted or upturned strata (Figure 9c). Such strong deformation was recorded not only in the Jeju Basin but also in the Xihu Depression, named the Longjing Movement (Q. L. Yang, 1992; G. Zhou et al., 1989). The Longjing Movement resulted from the back-arc opening of the Okinawa Trough by the subduction of the Philippine Sea Plate, forming the trench-arc-basin system in the middle Miocene (Gungor et al., 2012;Ren et al., 2002; or it resulted from the collision of the Luzon arc in the Philippines (Gungor et al., 2012;Ren et al., 2002;.
The tectonic evolution of the Ieodo and Jeju basins in this study reveals the development of a two-stage rift in the northern ECSSB; the Palaeocene to early Eocene first rift in the Ieodo Basin, and the late Eocene to Miocene second rift in the Jeju Basin. We suggest the syn-rift depocentre in the northern ECSSB migrated from the Ieodo Basin to the Jeju Basin. This is supported by recent studies on the rift migration of the ECSSB from the western depressions (e.g., Changjiang Depression) to the eastern depressions (e.g., Xihu Depression; Su et al., 2014;Y. H. Suo et al., 2015;. The eastward rift migration of the ECSSB has been reported to result from the back-arc extension due to the subduction of the Pacific Plate beneath the Eurasian Plate (Hall, Ali, & Anderson, 1995;Northrup et al., 1995;Sibuet et al., 2004;Sibuet & Hsu, 1997;Su et al., 2014) or from extrusion tectonics due to the collision between the Eurasian and Indian plates (Liu, Cui, & Liu, 2004;Suo et al., 2014;Suo et al., 2015). Alternatively, the interaction between the back-arc extension and the far-field effect of the Indo-Asian collision has been proposed as the source of the eastward rift migration (Ren et al., 2002;Suo et al., 2014;.
In this study, although there is insufficient data to determine which force was the dominant factor controlling the overall evolution of the basin, the structural orientation and distribution of the second rifting, parallel to the subduction zone of the Pacific Plate, suggests that the second rifting was possibly developed under the influence of the back-arc extensional regime.

| Role of the Hupijiao Rise between the Ieodo and Jeju basins
The Hupijiao Rise is a basement high between the Ieodo and Jeju basins (Han et al., 2015). It is one of the intra-basinal basement highs in the northern ECSSB, separating western depressions from eastern depressions (Figure 1). The basement highs have been interpreted as regional uplifts, tectonic boundaries, or relic arcs related to the development of NW trending transverse faults (Sibuet et al., 2004;Zhou et al., 1989). The uplift timing of the Hupijiao Rise was not defined but was probably constrained by the late Miocene unconformity (Han et al., 2015). In this study, we observe that the uplift caused upturned reflections in the pre-rift unit and post-rift unit 1 of the Ieodo Basin along the western margin of the Hupijiao Rise (Figure 4).
In the pre-rift unit, the upturned reflections can be interpreted to be a consequence of uplifting due to the early Eocene tectonic inversion of the Oujiang Movement (Q. L. Yang, 1992;Zhou et al., 1989). In addition, the early Eocene FIGURE 10 Free-air gravity anomalies in the northern ECSSB from data by Sandwell and Smith (2009). The negative anomalies coincide with basins. However, the Hupijiao Rise does not show the strong positive anomaly such as those of the Taiwan-Sinzi Belt. The contour in the map denotes the top of the acoustic basement [Colour figure can be viewed at wileyonlinelibrary.com] deformation in the Ieodo Basin was possibly associated with intrabasinal deformation related to igneous intrusion (Han et al., 2015).
In the post-rift unit 1 of the Ieodo Basin, the upturned strata are identified in the footwalls of the basin-bounding normal faults between the Ieodo and Jeju basins, indicating the large amount of footwall uplift and erosion (Figure 4a). Footwall uplift with normal faulting commonly occurs under extension and records a significant amount of erosion due to exhumation (C. Lee et al., 2016;Wernicke & Axen, 1988). In the Changjiang Depression close to the north-western Xihu Depression, the folding of the Eocene strata was also affected by the extension (F. Wang, Zhu, Hu, Xu, & Zhao, 2005; G. . We infer that the Hupijiao Rise was partly uplifted and eroded, owing to the isostatic adjustment in the late Eocene syn-rift stage. In addition, the positive anomaly of the footwall block is observed in a free-air gravity anomaly ( Figure 10). In the gravity map (Sandwell & Smith, 2009), the positive anomaly locally coincides with the footwalls of basin-bounding faults. However, the anomaly in the Hupijiao Rise is not much stronger than that of the Taiwan-Sinzi Belt which exhibits a constant and strong positive anomaly, indicating regional uplift.
In the flattened surface of the early Miocene unconformity We suggest that the Hupijiao Rise is not a regional uplift, but most likely is a geologic basement and was locally faulted and uplifted due to the isostatic rebound of the footwall during the late Eocene to earliest Oligocene subsidence of the Jeju Basin. The Ieodo and Jeju basins are largely separated by basement highs, the Hupijiao Rise. The Hupijiao Rise was locally faulted and uplifted due to the isostatic rebound of the footwall during the late Eocene to earliest Oligocene subsidence of the Jeju Basin, and then it remained stable while the Taiwan-Sinzi Belt was uplifted during the late Miocene. We suggest that the Hupijiao Rise is not a regional uplift zone inherited from the tectonic event but a basement high formed during the second syn-rift stage.