CENOZOIC TECTONIC SUBSIDENCE OF THE ZHU Ⅲ DEPRESSION IN THE PEARL RIVER MOUTH BASIN, NORTHERN SOUTH CHINA SEA

The history of tectonic subsidence is the record of basement movement in vertical direction. It is not only the reflection of basin evolution, but also the record of deep geological process. More and more geophysical evidence accumulated in recent years suggest that there is a low velocity anomaly under the Leiqiong area and its adjacent. In order to reveal the movement information of this deep low velocity anomaly body, we calculated the real tectonic subsidence curves as well as the theoretical tectonic subsidence curves for each tectonic units of the Zhu Ⅲ depression by backstripping and strain rate inversion, based on seismic and drilling data. Results show that the Cenozoic tectonic subsidence of the Zhu Ⅲ depression can be divided into five stages: slow syn-rift tectonic subsidence (65~49.5 Ma), rapid syn-rift tectonic subsidence (49.5~30 Ma), slow post-rift thermal subsidence Ⅰ(30~23.03 Ma), accelerated post-rift subsidence (23.03~16.5 Ma), slow post-rift thermal subsidence Ⅱ(16.5~0 Ma). We find that the Zhu Ⅲ depression exists 300~800 m syn-rift subsidence deficit, and the rapid post-rift subsidence might be the compensation to the deficit probably caused by the dynamic support of the influx of warmer asthenosphere material. The results also show that the tectonic subsidence center of the Zhu Ⅲ depression migrated to the northeast during 23.03~10.5 Ma, together with the deep low velocity anomaly body.