南大西洋洋中脊板块构造运动过程:古地磁的约束

Paleomagnetic constraints on plate tectonic process at south mid-Atlantic ridge

海底磁异常的形态与洋中脊两侧板块的微运动或变形密切相关.因此,这方面的研究可为确定板块运动的演化历史、小尺度的动力学过程以及洋中脊分段的机制等提供重要约束.本文对南大西洋一段洋中脊(31°S—34.5°S)两侧的磁异常的偏度进行了系统研究.结果表明研究区域内扩张方向并不总是垂直于洋中脊走向,并且研究区域不同剖面的扩张方向也不一致,具体表现为从北向南,平均扩张方向逐渐增加,依次为33.6°±5.3°、62.8°±13.0°以及94.3°±8.0°.这表明洋中脊的倾斜扩张机制具有复杂性,初步解释应该与转换断层的剪切应力增加有关.深部辉长岩层倾斜和扩张速率不对称性对海底磁异常偏度的影响值得深入研究.另外,由北向南确定的欧拉极向东移动,表明洋中脊两侧的板块在6.5 Ma期间存在剧烈形变.

Shape of marine magnetic anomalies provides great information on micro movement or deformation of plates on both sides of the mid-ridge.Therefore,studies on morphology of marine magnetic anomalies provides crucial constraints for evolution history and small-scale dynamic process of the plate movement,as well as mechanisms for segmentation of spreading axes.In this paper,a systematic study on skewness of marine magnetic anomalies on both sides of the midoceanic ridge（31°S—34.5°S）in South Atlantic was conducted. Results showed that the spreading direction of plates isn′t perpendicular to the trend of the mid-oceanic ridge,and spreading directions for different profiles are inconsistent in three studied areas,and graduallyincrease（from 33.6°±5.3°to 62.8°±13°,and to 94.3°±8°）from north to south.This indicates that mechanism of the oblique spreading is complicated.Our preliminary interpretation is that this complexity is related to increasing in shear stress across transforms.Nevertheless,effects of tilting of gabbro and asymmetric spreading rates on the morphology of marine magnetic anomalies need more attentions.In addition,the eastward displacement of Euler poles in three studied areas indicates that significant deformation between plates on both sides of the mid-ridge occurred at about 6.5 Ma.