Discovery of Late Cretaceous-Paleocene faulted basins developed on the Yandang Low Uplift, East China Sea Shelf Basin

Chuan-sheng Yang , Chang-qing Yang , Lu-ning Shang , Zhong-hui Yan , Yan-qiu Yang

a Qingdao Institute of Marine Geology, China Geological Survey, Ministry of Nutural Resources, Qingdao 266071, China

b Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong University of Science and Technology, Qingdao 266590, China

c Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China

1. Objective

The East China Sea Shelf Basin (ECSSB) is a typical Meso-Cenozoic superimposed basin which consists several tectonic units including the Changjiang Sag, Oujiang Sag,Hupijiao Uplift, Haijiao Uplift, Yandang Low Uplift (YDLU),Minjiang Sag, Taipei Low Uplift, Xihu Sag, Jilong Sag and so on. It has been successively involved into the tectonic evolution of the Tethyan and Pacific domains since Early Mesozoic and recorded the detailed evidences on the tectonic evolution and transition among different types of continental margins in East Asia (Zhang GW et al., 2013). The YDLU is an important secondary structural unit of the ECSSB. It lies between the Oujiang Sag to the west and the Minjiang Sag to the east and extends in NE-SW direction for about 170 km with a 15-50 km width, exhibiting as several discontinuous en-echelon linear basement highs. According to previous studies, the YDLU experienced denudation during the Late Cretaceous and Paleocene periods. Based on the newly acquired and reprocessed seismic profiles perpendicular to the YDLU, this study aims to further reveal the structural characteristics of the YDLU, especially to confirm whether there is small faulted basin developed on the uplift. This work can provide new evidences for the Mesozoic tectonic unit divisions in the ECSSB and is helpful to better understand the subduction of the Pacific Plate and the interaction between oceanic and continental plates (Yang CQ et al., 2016).

2. Methods

The 2D seismic data used in this study was acquired during 2012-2014 and was reprocessed in 2018. The data collection process was significantly influenced by the shallow water depth in this area, and the quality of the raw data was poor due to the surge noise, ship and side interference, and shallow water multiples. In order to improve the signal-noise ratio (SNR) and the resolution of the target strata, and obtain higher imaging quality, we applied several techniques including high SNR processing, low-frequency imaging improving, and high-precision velocity analysis in data processing. Prestack multi-domain denoising was used to suppress the conventional noise. Combined multiple method was used to suppress various types of multiple, highlight effective signal and improve the SNR of seismic data. Bubble suppression, virtual refection suppression and deconvolution were applied to improve the wavelet characteristics, broaden seismic signal frequency, highlight target strata reflection information, and improve wave group characteristics. Based on precise velocity analysis, more velocity pickup points,clearer accurate velocity trend, and better images of complex strata in the study area are obtained. Careful seismic interpretation was conducted following the above data processing. With higher quality, this data set can provide more detailed information for the tectonic analysis than the previous data.

3. Results

After the completely suppression of multiple reflections and the enhancement of effective reflections, the geological structures in the study area are clearly imaged on the reprocessed seismic profiles (Fig. 1b). Meanwhile, the sections offer clearer basement and faults imaging than the previous ones, showing obvious seismic waves and strata. In general, they can be used to describe the structural characteristics of YDLU in detail.

Fig. 1. a-Structural map in the Oujiang Sag showing the plan-view distribution of faults; b-e- interpreted seismic profiles across the Yandang Low Uplift and Oujiang Sag.

As shown in the seismic profiles, the middle part of the YDLU is about 1600 m deep and 6.5 km wide. It dips eastward to the Minjiang Sag where the Cretaceous and Jurassic strata are well preserved. Notably, the Mesozoic strata pinched out westward from about 500 m thick on the eastern edge of the uplift to 0 m (Fig. 1c). Several Meso-Cenozoic basins are identified on the middle YDLU for the first time (Fig. 1b-e).Accordingly, the former conclusion which suggested that the uplift experienced erosion from the Late Cretaceous to the end of the Paleocene should be reconsidered because extension and subsidence may have occurred during this period.

The Meso-Cenozoic basins developed on the YDLU are mainly grabens and half-grabens formed by extension. Their basement structures and sedimentary strata are similar to those of the Oujiang Sag to the west (Fig. 1d). Paleocene strata are preserved in the deeper part of these basins.Furthermore, it is reasonable to infer that thin layered late Cretaceous strata may also be preserved in these basins according to the hazy reflectors beneath the Paleocene sequence. Faults in and around these small basins extend predominantly in NE-NNE directions, with a few small faults run in E-W directions. In summary, we propose that the rifting of these basins was commenced in Late Cretaceous and lasted to the end of the Paleocene controlled by the westwarddipping boundary faults.

4. Conclusion

Based on the careful examination and interpretation of the newly acquired and reprocessed seismic data, this paper have improved the knowledge on the structural characteristics of the YDLU in western East China Sea Shelf Basin. For the first time, this paper confirmed the existence of the small fault basins on the YDLU, which were presumably formed in the Late Cretaceous and Paleocene period. Similar to the Oujiang Sag, the faulted basins exhibit half-graben and graben structures and are filled by Late Cretaceous and Paleocene strata. Meanwhile, it can be inferred that the YDLU acted as the western boundary of the Jurassic proto ECSSB, and also the eastern boundary of the Mesozoic Western Fault Depression. This study further confirmed that the ECSSB has underwent rifting from the Late Cretaceous to the end of the Paleocene caused by lithospheric thinning related to the subduction and retreat of the Izanagi plate.

Acknowledgement

This study has been supported by the National Natural Science Foundation of China (41606050, 41506080 and 41476053) and the China Geological Survey Project(DD20190211 and DD20160153).