西南印度洋中脊49.5°E离轴地壳结构

Off-axis crustal structure at the Southwest Indian Ridge (49.5°E)

超慢速扩张西南印度洋中脊岩浆的集中供给在空间维度上表现为岩浆扩张段(NVR)与相邻的非转换断层不连续带(NTD)地壳结构的差异,而在时间维度上表现为离轴与沿轴地壳结构的差异.为了进一步揭示岩浆集中供给的时空分布特征,本文选取西南印度洋中脊热液区2010年海底地震仪深部探测中平行于洋中脊距轴部偏北约10 km的离轴测线d0d10,使用射线追踪正演和反演的方法,得到了NVR和NTD北侧离轴区域的地壳及上地幔P波速度结构,并与轴部速度结构进行了对比分析.研究结果表明:(1)NTD北侧离轴区域的地壳厚度约5.2 km,其厚度明显大于轴部NTD下方地壳厚度(～3.2 km),由此推测洋脊轴部NTD区域形成的地壳在不断减薄;(2)NVR北侧离轴区域的地壳厚度约7.0 km,其厚度亦大于轴部NVR地壳厚度(～5.8 km),表明在洋中脊演化过程中洋脊轴区域的岩浆供给在不断减少,其活动性在不断减弱.

In order to reveal the spatial and temporal distribution characteristics of magma supply anomalies,this study selects one off-axis profile(line d0d10)of wide angle seismic exploration in the hydrothermal vent zone of the Southwest Indian Ridge(SWIR)(49.5°E)for further analysis,which was collected in 2010,located about 10 km north of the mid-ocean ridge(MOR).The P-wave crust and upper mantle velocity structures beneath off-axis neo-volcanic ridge(NVR)and non-transform discontinuity(NTD)are inverted by the ray tracing forward modeling and inversion methods.Comparing with the on-axis velocity structure indicates that(1)the crustal thickness of off-axis NTD is about 5.2 km,larger than that beneath the on-axis NTD(~3.2 km).These features imply that the crust formed in the NTD at the MOR is thinning.(2)The crustal thickness of off-axis NVR is about 7.0 km,also larger than that of the on-axis NVR(~5.8 km).These results indicate that the magma supply of on-axis region is decreasing and the activity is weakening during the MOR evolution process.