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Structural Evolution of the Northern South China Sea Continental Margin

Abstract

The northeastern South China Sea (SCS) continental margin evolved from Andean-type margin in Jurassic to Western Pacific-type margin around 90 Ma. However, no mechanism has been suggested for this transformation of tectonic regime. Here, we provide plausible evidences for this transition from the structural inversion in the western Dongsha-Penghu Uplift of northern SCS margin. Industrial seismic profiles in this region and radiometric dating of igneous rocks from well MZ-1-1 offered new insights to the tectonic history in the northeastern continental margin of SCS. A contractional event (structural inversion) was recorded in the western Dongsha-Penghu Uplift along the highly extended northern SCS continental margin. Combining the radiometric dating results (36 to 48 Ma for volcanic rocks and 91 to 102 Ma for granite) with the seismic profiles, three tectonostratigraphic sequences can be divided: tectonostratigraphic unit III (mainly marine facies), tectonostratigraphic unit II (mainly terrestrial facies), and tectonostratigraphic unit I (acoustic basement). These three tectonostratigraphic sequences are separated by two regional unconformity: rift-onset unconformity (Tg; ca. 65.5 Ma) and breakup unconformity of SCS (T7; ca. 30 Ma). High angel reverse faults formed by reactivation of pre-existing normal faults in the acoustic basement around well MZ-1-1 can be interpreted from seismic profiles and draped by the rift-onset unconformity Tg. The structural inversion revealed a unique collisional event in the Late Cretaceous. The extent of structural inversion was restricted in the western Dongsha-Penghu Uplift around 100 km wide that implies the limited areal distribution of the collision and was unlikely to be caused by micro-continents. From the paleogeography reconstruction of SCS and radiometric dating results, the structural inversion was possibly formed by the collision between volcanic seamounts swarm of northern Palawan blocks (mainly the present Calamian Islands) and the northern SCS continental margin around the Late Cretaceous. We also proposed that this collision event only occurred in the western Dongsha-Penghu Uplift and might have switched off the igneous activities in this margin. Later on, it might also cause the transition of the continental margin from Andean-type to Western Pacific-type.