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Abstract
Abstract The Bohai Bay Basin in East Asia is a rift basin created by Cenozoic subduction of the oceanic Pacific plate beneath the Asia continent. Many prior studies suggest that the basin was initially formed in the Paleocene with the development of several NNE-trending extensional grabens, but subsequently impacted by right-lateral shear along these existing NNE-trending structures in the middle Eocene, transforming the Bohai Bay Basin into a transtensional basin and producing EW-trending grabens in the Bozhong and the northeastern Huanghua depressions. However, how this transformation occurred remains to be fully understood. Based on seismic and drilling data, we herein investigated the fault structures, basin architecture, and evolutionary stages of the Huanghua Depression in the central-west Bohai Bay Basin to examine the strain partitioning and evolution mechanism during the Paleogene syn-rifting stage. The results reveal that the Huanghua Depression is composed of three structurally distinctive zones, namely, a dextral transtensional, a NW-SE extensional, and a N-S extensional zones from southwest to northeast, which are separated from each other by two transfer zones. The NW-SE extensional zone is interpreted as a horsetail structure on the northern termination of the dextral transtensional zone. This dextral transtensional zone and the Tan-Lu Fault zone to the east served as strike-slip boundaries within which EW-trending depressions such as the northeastern Huanghua and Bozhong depressions formed in the middle Eocene.
Highlights
During subduction of the oceanic Pacific Plate along the continental margin of East Asia, a number of Cenozoic extensional basins developed from the Sea of Okhotsk in the north to Vietnam in the south [1,2,3,4,5]
The Qikou (QK) Sag to the east of the Huanghua Depression is bounded by EW-trending faults and is the largest and deepest sag in the Huanghua Depression, with 5000-6000 m of syn-rifting strata deposited in the Paleogene
Transfer Zone is located between the Banqiao, Qibei, and Qinan sags to the west and the Beitang and Qikou sags to the east, separating the NW-SE extensional zone from the N-S extensional zone (Figure 3)
Summary
During subduction of the oceanic Pacific Plate along the continental margin of East Asia, a number of Cenozoic extensional basins developed from the Sea of Okhotsk in the north to Vietnam in the south [1,2,3,4,5]. A change of subduction direction of the Pacific oceanic plate from NNW to WNW in the middle Eocene significantly impacted the tectonics of the Bohai Bay Basin [1, 26, 27], initiating dextral strike-slip deformation along these preexisting NNE-trending grabens and generating EW-trending pull-apart or transtensional subbasins in the center of the Bohai Bay Basin [1, 12, 26, 28, 29]. By calculating the coherent properties of the 3D seismic data volume, we identified lateral discontinuities in the strata including fractures, faults, and lithologic boundaries These analyses, together with published results of adjacent depressions, provide constraints for the geometric and kinematic features of the Paleogene dextral transtensional system of the central-east Bohai Bay Basin and have implications for tectonic interactions between subducting west Pacific Plate and its back-arc regions
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