LATE CENOZOIC NEOTECTONICS IN THE DONGSHA ISLANDS REGION AND ITS RESPONDS TO COLLISION BETWEEN CHINESE CONTINENTAL MARGIN AND LUZON

Dongsha Islands and adjacent sea area located at northern continental margin of the South China Sea (SCS), which is connected to the Manila Trench to the east. In order to discuss the deformation characteristics of Dongsha Islands and adjacent sea area since Late Cenozoic, and to reveal the tectonics shaped by the collision between Luzon arc and southeast Chinese continental margin, especially the responds caused by the collision, according to the analyses of seismic stratigraphy in combination with gravity, magnetic and drilling wells data, three post-rift sequences (Ⅴ, Ⅵ, Ⅶ) have been discerned. Sequence Ⅰ—Ⅲ deposited before the SCS rifting overlying the acoustic basement with subparallel, discontinuous and medium to high amplitude reflectors and typical faulted basin sedimentation. Terrestrial sandstones, siltstones are the major lithofacies. Sequence Ⅰ, Ⅱ and Ⅲ correspond to Shenhu, Wanshan and Enping formations, respectively. Sequence Ⅳ appeared in the shelf margin of the Dongsha rise, and is comprised of carbonate facies and carbonate reef facies. Well-logging showed that the bottom of this sequence is composed of chaotic, hyperbolic and high amplitude reflectors. The seismic facies may vary from fluvial to estuarine facies corresponding to Zhujiang formation. The upper part is characterized by continuous and high amplitude parallel reflectors. Sequences Ⅳ and Ⅴ are divided by regional unconformity. Sequence Ⅴ and Ⅵ are marked by very continuous, high amplitude and parallel reflectors representing Miocene and Pliocene deltaic mudstones and siltstones making Yuehai formation and Hanjinag formation. The seismic exterior of the sequence changes from parallel to slanting across structural cover. The unconformity on the top (T 1) is the boundary between Miocene and Pliocene. Sequence Ⅶ downlaping and terminating on Sequence Ⅵ in the east of the study area, formed after the spreading of the SCS, and consists of parallel, continuous and medium to high amplitude reflectors. This sequence is absent in the upper continental slope. The Pleistocene strata cover directly the Sequence Ⅳ distributing westward steadily. The sequence thickness of the Wanshan antilcline is only 95 m. The inner reflector is characterized by cross slanting and bending. Welllogging suggested that the lithofacies are Pliocene marine mudstones aggregated on the continental margin. The inner reflector interface (T 0) is the unconformity surface within Pleistocene. Extensive tectonic uplift, magma activity, erosion, and earthquake took place in the maritime area, where most faults are still active during the Pliocene and Quaternary in the northwestern South China Sea. Two groups of faults trending NEE and NW developed during the late Cenozoic. Faulted blocks uplifted intensively and magma intruded along the faults. Widely distributed NEE faults controlled the formation of the Dongsha rise and the boundary between the uplift zone and the subsidence zone. These faults resulted from the reactivation of old fractures and are still very active. The NEE normal faults and the NW faults run across each other and the vertical offset usually reaches 1000—1500m, maximum 3000m. NW-SE extending faults widely distributed in the Dongsha area and controlled the pattern of the basement. They have the same formation and development characteristics as those on the China continental margin before Miocene; and they were mostly formed in Yanshan and Himalaya: representing left strike-slip fault and controlling diverse uplift of the basement. Newer faulting showed distinct differences. To the west of the Dongsha area it appeared as a right lateral shear fault and had the same characteristics as those of the Honghe fault, while to the east it appeared as a left lateral shear fault. The lateral activity of the NW fault could be related to the left lateral stress when NW oriented subduction of the Luzon arc occurred. Steep incline strata and short-axis developed faulting, syn-depositional anticline and diapir can be detected on multi-channel seismi