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地质学报  2010 

珠江口盆地陆架区岩石圈伸展模拟及裂后沉降分析

, PP. 1135-1145

Keywords: 伸展系数,裂后异常沉降,正反演模拟,陆架,珠江口盆地

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Abstract:

目前已通过不同的模拟方法发现珠江口盆地存在裂后沉降异常,但研究重点多放在了陆坡白云凹陷深水区,对陆架区的研究和分析较少。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它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。

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