Differences of Polygonal Faults with Irregularly Polygonal Geometries: A Case Study from the Changchang Sag of Qiongdongnan Basin, Northern South China Sea

Polygonal faults (PFs) generally have a classic polygonal geometry in map view. However, under the influence of tectonic faults, diapirs, channels, and slopes, the classic polygonal geometry of PFs is not preserved, demonstrating differences (different characters) in map-view 3D seismic data covering an area of 334 km2 of the Changchang (CC) sag, are used to document the map-view and cross-sectional characteristics of PFs. These data also help investigate the irregularly polygonal geometries of PFs due to the presence of influence factors, such as transtensional faults, submarine fans, channels, diapirs/gas chimneys, and the basal slope within the lower-middle Miocene strata. Results show that various irregularly polygonal geometries of PFs can be classified into en-echelon and arcuate PFs, channel-segmenting and -bounding PFs, radial PFs, and rectangular PFs in map-view. En-echelon and arcuate PFs are induced by transtensional faults and exhibit a unique ‘flower’ structure in NE- and SE-trending cross-sections in the NW area of the study area. This finding is documented for the first time. Channel-segmenting PFs occur in the (northwest) low-amplitude muddy channel and are inhibited in the (southeast) high-amplitude sandy channel in the SW area. Radial PFs are radially aligned around a gas chimney/diapir containing some high-amplitude anomalies (HAAs) in the middle area. The presence of intrusive sandstones with HAAs along the periphery of the diapirs restricts the occurrence of PFs. Two high-amplitude submarine fans act as a mechanical barrier to the propagation of PFs. Meanwhile, the (moderate) slope in the NE area induces rectangular PFs. Additionally, the geneses of the PFs in the current study are comprehensively discussed. This study adds to our understanding of the differences between PFs with irregularly polygonal geometries.