Cenozoic tectonic events and their implications for constraining the structure and stratigraphic styles from rifting to collision at the southeastern margin of the South China Sea

The southeastern margin of the South China Sea (SCS) connects the Southwest sub-basin of the SCS to the north and the collision zone of Nansha-Liyue blocks and Palawan Island to the south, and it records a complex process of the superimposed regional tectonics. Based on recently collected 2D seismic profiles, constraint by wells and outcrop data, our study establishes a stratigraphic framework across different structural units related to various tectonic settings. We identified six major unconformities from the Cenozoic sediment succession and analyzed their geological significance through a comprehensive interpretation of the regional seismic profiles and related well data. Results show the unconformity T7 represents the cessation of the initial rifting along the whole continental margin, which corresponds to the N-S spreading in the central part of the SCS oceanic basin. Reflector T4 is the breakup unconformity that represents the structural evolution transitioning from rifting to post-rifting in the study area. It also synchronizes with the initial deposition of the Nido carbonate platform. Reflector T3 is the most significant regional structural unconformity that corresponds to the cessation of seafloor spreading in the Southwest sub-basin and collision between the Nansha-Liyue blocks and the Cagayan Arc, choking the proto-SCS subduction zone. These tectonic events, also referred to as Sabah Orogeny, indicate the inception of peripheral foreland basin developed from northwest Palawan to the Reed Bank. Major tectonic events recorded in the study area include southward subduction and closure of the proto-SCS as well as opening and cessation of ridge spreading of the SCS, all of which lead to the Nansha-Liyue-North Palawan blocks successively rifting from the South China continent, subsequently drifting southward and ultimately colliding with the Cagayan volcanic arc. Therefore, we propose that the southeastern margin of the SCS has undergone a multi-stage tectonic evolution from the rifting basin through the post-rifting depression to the collisional foreland basin in the Cenozoic.