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In this study, we investigated the extensional tectonics and post-rift magmatism in the southern South China Sea (SCS) using a newly obtained seismic reflection profile, which crosses the Southwest sector of the oceanic basin (SWSOB), West Liyue Trough and Liyue Basin along the southern margin of the SCS. A wide range of magmatic features was identified in the SWSOB, with a late stage marked by a broad Pleistocene unconformity separating horizontal sediment layers from strata deformed by igneous structures. Along the continental slope, most fault activity continued until Horizon T4 (15.5 Ma), implying that crustal extension continued during seafloor spreading in the SW oceanic sector of the SCS. A low-angle listric normal fault with a large offset, marking the southern boundary of the trough, controlled regional crustal extension. Thick syn-rift sedimentary strata and significant crustal thinning in the southern part of the trough reveal relatively strong extension. Post-rift magmatism occurred in this thinned crust and affected sedimentary strata up to the seafloor. Importantly, a series of high-angle normal faults rapidly deepened the top of the basement, forming a sharp contact between the continental slope and oceanic basin. The distal domain is very narrow (~20 km) with no known detachment faults, suggesting rapid continental breakup that may be related to the presence of rigid blocks near the study area. • A low-angle listric normal fault controlled crustal extension of the southern West Liyue Trough. • The continental slope shows a sharp boundary with the ocean basin via a group of high angle normal faults. • The formation of a narrow distal domain is related to the presence of adjacent rigid crustal blocks. [ABSTRACT FROM AUTHOR]

• We determine hypocentral parameters of the 2020 Pearl River Estuary (PRE) earthquake (M3.5) and its aftershocks, as well as their focal mechanisms. • The 2020 PRE earthquake sequence was mainly caused by rupture of the NW-striking Qi'Ao-Guishan fault in the PRE. • Both the NW- and NE-striking faults in the PRE have seismogenic potential in the GHMGB, and even in the whole coastal area of the northern SCS, which should be paid much attention. The complexity of intraplate earthquakes and their challenge to current earthquake models persist as an important scientific and social problem, both about their causes and the resulting hazards. The Guangdong-Hong Kong-Macao Greater Bay area (GHMGB) is located in the coastal seismic zone of South China. Due to the huge residential population and important economic status, seismic hazard in the GHMGB often causes widespread concern. In this work, waveform data of the 2020 Pearl River Estuary (PRE) earthquake (M L 3.5) and its aftershocks are carefully analyzed using about 6-month data recorded at 124 local seismic stations. A total of 8 aftershocks are identified. We determine the hypocentral parameters of the PRE mainshock and its aftershocks, as well as their focal mechanisms. Our results show that the 2020 PRE earthquake sequence is distributed in a NW-SE direction with a lateral extent of about 3.5 km, which is consistent with the location of the NW-striking Qi'Ao-Guishan fault. The 2020 PRE earthquake and its aftershocks exhibit similar focal mechanisms. According to the stereographic projection results of the P and T axes, the orientation of the principal compressive stress is oblique to the NW-striking Qi'Ao-Guishan fault, hence these earthquakes are mainly characterized by left-lateral strike-slip faulting. In combination with many previous geological and geophysical results, we suggest that the NW-striking faults could have similar seismogenic importance as the NE-striking faults in the GHMGB and even the whole coast of the northern South China Sea, especially at the intersection of the NW- and NE-trending faults. [ABSTRACT FROM AUTHOR]

Abstract: The Cenozoic tectonic subsidence of the deepwater area in the Pearl River Mouth Basin, northern South China Sea is studied by subsidence analysis via backstripping calculations based on data of newly interpreted sequence boundaries. In the subsidence analysis local porosity–depth parameters were estimated based on well data, the paleo-water depth parameters and lithology were estimated for all grid nodes based on well data and sedimentary system maps, and ages of sequence boundaries were adjusted according to the International Chronostratigraphic Chart v2013/01. Sensitivity analysis shows that these are essential to ensure the quality of the subsidence analysis in the slope areas. Maps of tectonic subsidence rates of 18 Cenozoic sequences were constructed and spatial–temporal variations of the tectonic subsidence were discussed. The subsidence was restricted in the Baiyun and Liwan Sags before 30Ma and extended to uplifts after 30Ma. This indicates that the 30Ma unconformity is the breakup unconformity that separated the syn-rift and post-rift sequences. Four substages of post-rift subsidence were identified by cyclic changes in the rate and pattern of tectonic subsidence at boundaries of 23.3Ma, 17.2Ma and 11.9Ma, respectively. Anomalous post-rifting tectonic subsidence was significant and totaled to ~1200m at the center of the Baiyun Sag, and punctuated by rapid subsidence and uplift events. The Baiyun Event was manifested in the study area by a short-lived uplift event in 23.9–23.3Ma followed by a short-lived rapid subsidence in 23.3–19.8Ma in the central study area. This localized rapid subsidence caused northward jump and anti-clockwise rotation of the shelf break. Two-order teeterboard (seesaw)-like subsidence was observed in the study area in the post-rifting stage, suggesting elastic deformations of the crust and lithosphere. [Copyright &y& Elsevier]

The stretching and subsidence characteristics of passive continental margins have been investigated for decades but are still controversial. We focused our study on the deep water sub-basins of the Pearl River Mouth Basin in the northern South China Sea, the only well-preserved passive margin in the South China Sea. Inverse modeling and forward modeling based on a flexural cantilever model were employed. The stretching factors of the upper crust, lower crust, whole crust, and mantle lithosphere were mapped, and the anomalous post-rift subsidence was isolated from the McKenzie model. The Liwan Sub-basin showed highly depth-dependent stretching that significantly increased with depth. The stretching factor of the lower crust in the center of the Baiyun Sub-basin was higher than that of the mantle lithosphere. Two significant anomalous rapid subsidence events were revealed. The 23.03-19.8 Ma event was characterized by anomalous subsidence at the center, located in the southern Baiyun Sub-basin, with a maximum value in excess of 1200 m. Strong negative P-wave abnormal and magma addition may indicated an upwelling mantle under the Baiyun Sub-basin around 23 Ma. We suggest that the 23.6 Ma southward ridge jump in the South China Sea could have weakened or eliminated the mantle upwelling under the Baiyun Sub-basin and caused rapid subsidence at 23.03-19.8 Ma. The 14.3-11.9 Ma event was characterized by a subsidence zone orientated northeast along the Baiyun Sub-basin, with a maximum value in excess of 600 m. We propose that rapid depositional sediment during this period would have driven the lower crustal flow away from the Baiyun Sub-basin center, causing rapid vertical subsidence and leading to the stretching factor of the lower crust being higher than that of the mantle lithosphere. • The Liwan Sub-basin showed depth-dependent stretching that increased with depth. • The lower crust stretched more than the mantle lithosphere in the Baiyun Sub-basin center. • The 23.6 Ma ridge jump could have caused the 23.03–19.8 Ma rapid subsidence. • The 14.3–11.9 Ma event might be explained by the flow of the lower crust. [ABSTRACT FROM AUTHOR]

As a typical rifted margin, the coastal region of the northern South China Sea has experienced many large crustal earthquakes in the past 400 years. For example, two destructive earthquakes with ~M7.0 in 1600 and ~M7.5 in 1918 occurred close to the NanAo Island. However, the mechanism of these earthquakes and seismotectonics of the region are still unclear. In this work we make the first study of the detailed three-dimensional crustal structure in the source zone of the 1918 NanAo earthquake (M7.5) using newly acquired wide-angle seismic reflection data. Our results show that current seismicity mostly occurred in a midcrustal low-velocity zone or its nearby high-to-low velocity transitional areas. We ascertain locations of the ENE-strike Littoral fault zone and the NW-strike Huanggangshui fault based on the seismic reflection profiles. These faults have caused a large-scale vertical dislocation of the sedimentary basement. The structure of the upper crust is very heterogeneous, containing many high or low velocity anomalies. However, the lower crustal structure is less heterogeneous, mainly characterized by low velocity mostly in the northern part of the study area and high velocity mainly in the southern part. The crust in the seismicity zone shows strong heterogeneity, reflecting highly cracked structural features possibly caused by destructive earthquakes. We infer that the midcrustal low-velocity zone and the intersection between the Littoral fault zone and the Huanggangshui fault could reflect weak structures in the crust and form a local stress buildup, which strongly affected or controlled the generation of intraplate seismicity. The 1918 NanAo earthquake very likely occurred within the current seismicity zone with a depth range of 15–25 km estimated from our tomographic images and previous geological and geophysical results. • A 3D crustal model constrained by wide-angle seismic data is first imaged in the source area of the 1918 Nan'ao earthquake. • Highly cracked structural featuresinthe seismicity zone is revealed, which possibly caused by destructive earthquakes. • We think the midcrustal LVZ and fault intersection could form a local stress buildup to affect intraplate seismicity. [ABSTRACT FROM AUTHOR]

• A large-scale fault zone was identified on the northern South China Sea. • The fault zone accommodated intra-plate deformation during Cenozoic extension. • The fault zone was inherited from a lithospheric-scale suture zone. • The fault zone played a significant role in segmenting the northern SCS margin. Multibeam bathymetric, seismic and borehole data are used to investigate a large-scale strike-slip structure, the Baiyun-Liwan Fault Zone, in the northern South China Sea. This fault zone comprises NW- to NE-striking faults and negative flower structures that were generated by oblique extensional displacement. Notably, the interpreted data reveals that the Baiyun-Liwan Fault Zone was active during the Cenozoic, recording intense magmatism, and accommodating significant intraplate deformation during progressive continental rifting and ocean spreading. It bounds two distinct crustal segments and played a significant role in segmenting the northern margin of the South China Sea. The geometry of faults and strata within the Baiyun-Liwan Fault Zone also controlled local sediment routing and depocentre evolution during the Cenozoic. As basement and syn-rift structures change markedly across the Baiyun-Liwan Fault Zone, we propose this structure to be inherited from a lithospheric-scale fault zone separating the Mesozoic arc from forearc-related terrains. We therefore stress the importance of pre-existing structures in the development of rifted margins, with the example provided by the Baiyun-Liwan Fault Zone having profound implications for palaeogeographic reconstructions in the South China Sea. At present, the Baiyun-Liwan Fault Zone is incised by the Pearl River Canyon and eroded by recurrent submarine landslides, forming a major area of sediment bypass towards the abyssal plain. [ABSTRACT FROM AUTHOR]