Seismic horizon T60 and its relationship with breakup unconformity in the northern continental margin of the South China Sea

Seismic reflection horizon T60, formed at the transition between the Paleogene and Neogene (aged ～23 Ma), serves as a significant geological boundary on the northern continental margin of the South China Sea. However, its research remains limited, often confined to local areas of specific basins. This study utilizes regional two-dimensional seismic profiles, combined with well logging and core data, to investigate the characteristics of the horizon T60 in the central-eastern part of the northern South China Sea margin, and to explore its relationship with the breakup unconformity. The horizon T60 is generally characterized by strong amplitude and high continuity reflections across the study area, with reflection polarity being either positive or negative. On the shelf to upper slope, it typically exhibits positive polarity, attributed to the lower acoustic impedance of the overlying strata compared to the underlying strata (e.g., when the acoustic impedance of the overlying sedimentary strata is lower than that of the underlying volcanic basement, or when compaction results in higher acoustic impedance in the underlying layers). On the lower slope to the abyssal plain, it generally shows negative polarity, attributed to the generally higher calcareous content and consequently higher acoustic impedance of the overlying early Miocene strata. The horizon T60 in the Baiyun, Liwan, and Xingning depressions of the continental slope within the Pearl River Mouth Basin coincides with the breakup unconformity. It is characterized by truncation at the top of the underlying strata, with sediment thickness decreasing from the depression center toward the flanks and well-developed faulting; the overlying strata are mostly conformable, draping the horizon with relatively uniform thickness and little evidence of faulting. In contrast, in the Hanjiang and Lufeng depressions on the shelf and the Taixinan Basin, the burial depth of the horizon T60 is shallower than that of the breakup unconformity. The cessation of rifting along the northern continental margin of the South China Sea, marked by the breakup unconformity, generally exhibits a trend of being earlier in the east and later in the west, as well as earlier in the north and later in the south. These patterns are respectively related to the westward propagation of seafloor spreading in the South China Sea and to the northward decrease in lithospheric heat flow away from the spreading center.