Structure And Evolution Of Yinggehai And Qiongdongnan Basins, South China Sea:Implications For Cenozoic Tectonics In Southeast Asia

The Yinggehai Basin (YGHB) and Qiongdongnan Basin (QDNB) are in the Northwest corner of the South China Sea positioned at a juncture between a strike-slipping zone and a extensional zone. The basins lie at the southern termination of the largest Tibetan strike-slip zones, the Red River Fault, and are the principal repository of materials eroded from the Red River drainage. The basins are flanked to the East by an oceanic ridge and border the Nansha area to the South, which is being subducted underneath the Borneo Block. Hainan Island, located between the YGHB and QGNB, is composed primarily of granites and basalts. In our study, a combination of geological, geophysical and low-temperature thermochronological methods are used to unravel the basins'history in greater detail. We studied the structure and evolution of the basins and explored the relationships between tectonics, erosion and climate of the peripheral area, which will enrich our knowledge about geodynamics and petroleum geology in Southeast Asia. The highlights of this Ph.D. thesis are shown as follows:1. A key tectonic sequence boundary, T70, was identified in the YGHB and QDNB based on the tectonic-stratigraphy interpretation of a number of available high-resolution seismic datasets. A series of small-scale, NE-SW trending, fault-bounded grabens developed extensively below this boundary. On the contrary, above the T70boundary, the depocenters are trending NE-SW, W-E and SWW-NWW from east to west of the basin. Larger-scale, partly fault-controlled depressions are superimposed clearly over underlying, faulted-bounding grabens on seismic profiles. Analysis of genetic type and geometry of graben-boundary faults, which underlie the T70boundary, indicates that these structures were formed by NW-SE extension. However, above the boundary, the partly fault-controlled depressions extended nearly N-S. We also report that the T70sequence boundary exists in both basins. Dating the sequence boundary by correlation of the regional sequences and biostratigraphical data reveals that it was formed at32Ma, which is consistent with the timing of the initiation of ridge spreading in the South China Sea and onset of sinistral movement along the Red River Fault. Therefore rifting on the northern continental margin of the South China Sea is considered to be a result of proto-South China Sea subduction underneath the Borneo Block. After the initiation of Red River Fault, evolution of the YGHB and QDNB show striking differences, as revealed by our study.2. From tectono-stratigraphical analyses of seismic profiles in the YGHB, we observed that each tectonic unit of the basin had a different deformation history. The Yinggehai Depression, located in the southern part of the basin, is characterized by an extensional lithosphere, thermal cooling and copious mud diapirs. However, on the Lingao Uplift, a very important structure dividing the basin into the Hanoi and Yinggehai Depressions, an inversed sequence boundary was identified on our seismic profiles. We dated the sequence boundary from drill core data at15.5Ma. However, in the Hanoi Depression, the inversed structures formed between15.5and5.5Ma. In our study, we noted that the structures of the QDNB are very different from those of the YGHB. High-quality seismic reflection data enabled us to divide the QDNB into three, first-order structural units, namely Northern Uplift, Central Depression and Southern Uplift. In addition, between Linshui Sag and Songnan Sag we recognized a NW-SE trending grabens and horsts belt, which separates the basin into the Eastern Extension Zone and Western Extension Zone. Moreover, at the junction of YGHB and QDNB, the Zhongjian Uplift, No.1and Zhongjiandong Faults are the main tectonic units in our investigation. At the juncture, the offset of No.1Fault decreases from north to south and gradually disappears, and Zhongjianzhong Fault appears and exhibits an opposite dip direction.3. The Red River Fault system in the Northwest corner of the South China Sea is investigated in detail by combining regional geological data and available seismic profiles. The fault system was interpreted to be composed of three segments:the Yingdong, Dongfang and No.1Fault. In the region of the Lingao Uplift, the Yingdong and Dongfang Faults are typical listric faults. Further south, the two faults disappear and a new fault, No.1Fault, with steeper dip is imaged on the seismic profiles. At the conjunction of YGHB and QDNB, the No.1Fault cut off the No.2Fault of the QDNB and extends southwards. In the middle of the junction zone, the newly described Zhongjiandong Fault with an opposite dip direction than that of the No.l Fault was distinguished on seismic profiles. In addition, we studied the strike-slip Red River Fault system in the YGHB, which has been active during the period of32-5.5Ma. After5.5Ma, no prominent activity is observed in the basin. No.2Fault is the largest-scale fault in the QDNB, streching from west to east. A quantitative, geometric analysis of the No.2Fault system was undertaken in our study and we divided the No.2Fault into three segments. The West and Middle Segment of the No.2Fault are characterized as single fault controlling the evolution of the Ledong, Linshui and Songnan Sags. The East Segment of the No.2Fault was features a dextral oblique fault system, which controls the development of Baodao and Changchang Sags. In addition, the sags in the QDNB can be grouped into half-graben, graben and compound graben types, which can be further divided into ten subtypes.4. In order to better understand how and why the diapirs formed, we compiled regional, high-resolution2D and3D seismic reflection data, as well as drilling data related to diapirs in the YGHB. Eighteen diapirs were identified in our study. The structures of gas chimneys, diapiric faults and palaeo-craters are genetically linked with diapirism. In this thesis, we propose, based on geophysical and geological observations, a three-stage model for diapirism:initiation, emplacement and collapse. During each stage, different diapiric types are described, including buried diapirs, piercing diapirs and collapsed diapirs. In addition, associated with large sediment supply from the Red River and large subsidence rate of YGHB, we suggest that a high paleogeothermal gradient to be the result of crustal thinning. Additionally, large volumes of sediment from the Tibetan plateau delivered by the Red River to the basin, majorly affect diapirism.5. The denudation history of Hainan Island aids in estimating the sediment flux from the island into the surrounding basins. However, the lack of low-temperature thermochronological data from Hainan Island obstructs us from studying the exhumation history in this region. Thus we collected rocks from places with scarce low-temperature thermochronological data. Ages from (U-Th-Sm)/He dating on Apatite range between35-16Ma. More specifically, most ages fall between30-25Ma. We used Gaussian the linear inversion method to study the topographic evolution and erosional history of Hainan Island since35Ma. The results from our numerical modeling show low erosion rates of0.05-0.2km/Ma on Hainan Island. Thus the sediments supplied from Hainan to the YGHB were calculated to be0.3-0.4km3/Ma, which is considered to be a small contribution compared to the total amount of sediments in the YGHB.6. Sedimentation rates within the YGHB are likely to be the most reliable record of changing erosion rates onshore, which in turn is linked to Tibetan Plateau uplift and monsoonal strength in SE Asia. We reconstructed sediment accumulation rates using a seismic framework and a better age control. This permits improved spatiotemporal constraints and enhanced stratigraphic resolution, with13separable stratigraphic units younger than23Ma. Considering the amount of sediments in the YGHB supplied by the Hainan Island, we estimated the sediments supplied by the Red River at different time intervals. Correlation with the event of Tibetan Plateau uplift and monsoon climate, we favor major headwater capture away from the Red River during the period between Oligocene and Middle Miocene, most likely the Oligocene. High rates of sediment accumulation since5.5Ma, even as tectonics appear to have slowed, suggest frequent climatic changes.7. We present one multichannel deep-penetration seismic reflection line collected during the late2000s. A numerical backstripping calculation using2D models of flexural cantilever and sediment decompaction were employed. The results demonstrate different stretching factors for the upper crust, the whole crust and the whole lithosphere, which indicate depth-dependent stretching occurred on the ocean-continent transition zone of the northern margin of the South China Sea. Compared with the faulting on the upper crust, the larger stretching factor of the whole lithosphere has an important effect on the fast subsidence recognized in recent years. In our study, three stages of basin evolution of the ocean-continent transition zone of the South China Sea were established. They are rifted continent stage, post-rifted thermal subsidence stage and post-rifted fast subsidence stage. This bears great significance for the geodynamics of the continental margins and deepwater petroleum exploration in the South China Sea.8. Based on regional tectonic and geophysical data, we support the tectonic model that SE Asia can be divided into a collision-extrusion tectonic province and a proto-South China Sea slab-pull tectonic province. These two tectonic provinces are separated by a transform boundary beginning with the Red River Fault extending southward along the Vietnamese margin and merging with the trench along the Lupar Line. We have further clarified the distinct structures, evolution and dynamic settings of the two tectonic provinces. The collision-extrusion tectonic province is considered to result from crustal shortening associated with the ongoing India-Eurasian collision, whereas the proto-South China Sea slab pull tectonic province was mainly driven by proto-South China Sea subduction toward the Borneo Block. Our focus areas, YGHB and QDNB, are situated along the junction of these two tectonic provinces.9. A new compilation of high-resolution2D seismic profiles tied to wells with biostratigraphic age control was used to better constrain the evolution of the YGHB and QDNB. We divided the Cenozoic evolution of the basins into four episodes, namely fault-controlled grabens episode (55-32Ma), fault-sag grabens episode (32-23Ma), post-rifted thermal subsidence episode (23-5.5Ma) and post-rifted fast subsidence episode (5.5-OMa). We recognized the offshore Red River Fault became active after32Ma. Before the onset of strike-slip, widespread crustal extension occurred across the northern continental margin of the South China Sea. Fault-bounded grabens developed at the bottom of the YGHB and QDNB. We noted the two basins have a very different evolution after32Ma. In the YGHB, fault-bounded grabens are typically overlain by successions deposited in a pull-apart regime dominated by the Red River Fault. After15.5Ma the basin experienced distinct structural inversion. In the Lingao Uplift, a clear unconformity was dated at15.5Ma, while after that time no erosion was documented in the geological and geophysical data. In the Hanoi Depression, a strong erosional unconformity was terminated at5.5Ma. In contrast, in the Yinggehai Depression, there is no evidence for inversion or erosion. A transition from extension to compression migrating from south to north between15.5and5.5Ma, known as zipper tectonism, indicates a reversal of slip sense along the Red River Fault. However, QDNB was interpreted as a typical extensional basin. The evolution of the QDNB before32Ma was similar to that of the YGHB. During the period of32-23Ma, the QDNB was controlled by lithospheric thermal cooling and faulting. Between23-5.5Ma, the basin was completely controlled by the lithospheric thermal cooling. The last tectonic episode was a fast subsidence event starting after5.5Ma.