西南印度洋脊14°E-25°E区域洋壳增生的构造与岩浆特征

Magmatic and tectonic extension at the Southwest Indian Ridge between 14°E and 25°E

文章利用高精度船载多波束测深及重力数据研究了超慢速西南印度洋脊14°E—25°E区域洋壳增生的构造与岩浆特征。首先采用滤波的方法将原始地形数据分为短波长地形(波长小于20km)和长波长地形(波长大于20km)。然后利用长波长地形剖面获得洋中脊裂谷的深度,利用短波长地形剖面和坡度来识别正断层,并计算出岩浆作用在整个扩张过程中所占的比例,即M值。同时从自由空气重力异常中去除海底地形、参考莫霍面以及板块冷却等重力效应,获取能够表征相对洋壳厚度的剩余地幔布格重力异常(Residual Mantle Bouguer Anomaly,RMBA)。最后在垂直于洋脊的剖面上以10km宽的窗口计算出一系列窗口内的M值、平均RMBA值以及断层的垂直断距,并探讨它们之间的相关性。研究发现在超慢速扩张的西南印度洋脊14°E—25°E区域,岩浆率M值随时间和空间变化明显,裂谷深度呈现较强的两翼不对称性,裂谷深度在一定程度上反映了脊轴附近的平均M值。区域性的平均构造拉伸率(即1-M)处于20%~50%之间,南翼整体处于较强的拉伸状态。统计结果表明,在岩浆作用较强的时期,M值偏大,通常产生较厚的洋壳以及断距较小的断层。

In this study we used the high-resolution shipboard multibeam bathymetry and gravity data to investigate the tectonic and magmatic characteristics of the Southwest Indian Ridge(SWIR)between14°and25°E.First,we filtered the original bathymetry to obtain a short-wavelength bathymetry map(wavelength less than20km),which was used together with the topographic slope map to identify surface normal faults.We also obtained a long-wavelength bathymetry map(wavelength more than20km)that was used to calculate across-ridge axis topographic relief.We also calculated the fraction of plate separation accommodated by magmatic accretion,i.e.,the M factor.We then calculated the Residual Mantle Bouguer Anomaly(RMBA)by removing from the free-air gravity anomaly the gravitational effects of water/crust and crust/mantle interfaces as well as lithospheric plate cooling,assuming a reference crustal thickness of6km.Finally,we calculated the M factor,the mean values of RMBA,and fault throws within10-km-wide running windows along profiles across the ridge axis and investigated the correlations among these parameters.We found that the magma supply varied significantly in time and space at the SWIR between14°and25°E and the axial relief showed strong asymmetry between conjugate ridge flanks that seemed to be controlled by the mean M factor near the ridge axis.Regionally-averaged tectonic extensional strains(i.e.,1-M)were about20%~50%and the southern flank underwent greater average tectonic extensional strains.Areas with thicker crust(i.e.,more negative RMBA)are often associated with greater M values and smaller fault throws,indicating episodes of increased local3D magma supply at this ultraslow spreading ridge.