珠江口盆地陆架区岩石圈伸展模拟及裂后沉降分析

Lithospheric Stretching Modeling of the Continental Shelf in the Pearl River Mouth Basin and Analysis of Post-Breakup Subsidence

本文根据伸展盆地发育的挠曲悬臂梁模型,以二维正、反演相结合的方法,计算了珠江口盆地陆架区1530测线北段的岩石圈伸展系数,分析了其裂后沉降规律。由正演模拟,发现盆地1530测线北段的裂陷由北向南逐渐发育,其陆架岩石圈的平均伸展系数为1.2和较大凹陷处的岩石圈理论伸展系数变化在1.08～1.24之间。整条剖面裂后沉降的实测值比理论值大2.5km左右,本文分析造成这一差值的最大可能是裂后异常沉降的存在。由前人成果可知,陆坡区也存在其他大的异常,对于陆架和陆坡区的异常,本文认为它们之间以及它们与其他南海陆缘之间都可能有关联,它们的产生可能是某种共同机制的结果。珠江口盆地陆架区的实测裂后沉降速率明显不同于逐渐减小的理论变化规律,而是存在两期(30～18.5Ma和18.5Ma至今)由快到慢的变化。在30～23.8Ma沉降速率集中在140～190m/Ma,之后23.8～18.5Ma减小至35～65m/Ma。18.5～16Ma的沉降速率迅速增大到300m/Ma,随后16Ma至今又减小至75～110m/Ma。其中18.5～16Ma的沉降速率最大,并与当时陆架坡折的形成和海平面的快速上升相对应,也与前人在陆坡区白云凹陷发现的17.5～15.5Ma裂后重大加速沉积的时间一致,因此本文推测珠江口盆地18.5～17.5Ma可能存在一重大构造事件,引起盆地从陆架到陆坡的裂后快速沉降的发生。但对于构造事件的成因、准确时间及其范围都有待进一步的研究。

Post-rift anomalous subsidences have been discovered in Pearl River Mouth Basin （PRMB） using various modeling methods, but most of the researches concentrated in deep water area of the Baiyun sag in the continental slope always, not the shelf itself. Based on the flexural-cantilever model of stretching basin, lithospheric stretching coefficients of the northern section of the No. 1530 line in the slope shelf of the PRMB and rule of post-break subsidence were calculated in combination with the forward and reverse modeling. It is recognized by the forward modeling that the basin rifted from north to south gradually along the north of 1530 seismic line. The average shelf stretching coefficient is 1.2, with the coefficients ranging from 1.08 to 1.24 among the deeper sags. However, the measured value of whole section in the post-break subsidence in situ is about 2.5 km more than theoretical one, revealing that the difference probably resulted from presence of the post-breakup abnormal subsidence. The previous work also suggests the existence of intensive anomaly in the terrestrial slope. For the anomalies in the continental shelf and terrestrial slope, our study indicated that they might be related to continental margin of South China Sea and resulted from the same mechanism. The research also shows that the practical subsidence rate among the deeper sags is obviously different from the changing rule inferred by theory, and there are two episodes of post-breakup subsidences from fast to slow exist. The subsidence rate was concentrated mainly on the range of 140-190 m/Ma between 30-23.8 Ma and slowed down to 36-65 m/Ma during the period of 23.8-18.5 Ma; it increased to 300 m/Ma between 18.5-16 Ma and then decreased to 75-110 m/Ma since 16 Ma. Among them, the biggest subsidence rate in 18.5-16 Ma is in respond to fast uprising of sea level and the formation of continental shelf and slope, and this is consistent with the accelerated deposition at 17.5-15.5 Ma. Therefore, it is presumed in this study that an important tectonic event occurring in 18.5-17.5 Ma in the PRMB maybe trigger the fast subsidence of the basin from continental shelf to continental slope. But the real mechanism, time and scope of the tectonic event needs further studying.