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非转换断层不连续(NTD)在超慢速扩张洋中脊高频度出现,其地壳速度结构与相邻的岩浆扩张中心(NVR)差别明显.结合表层断层多和减薄的地壳,说明NTD在超慢速洋中脊扩张理论、热液活动甚至矿产资源方面有十分重要的意义.本文使用2条海底广角地震测线的数据,使用射线追踪正反演的方法,获得了西南印度洋中脊28~29扩张段之间NTD下方的地壳及上地幔速度结构.结果表明:(1)NTD下方地壳较薄(3.2~4.5 km),洋壳层2厚约2.0 km,洋壳层3较薄(1.0 km)甚至缺失;(2)洋壳层2表层速度横向差异大,表明存在较多断裂;(3)洋壳层3出现低速区,成因可能是受浅部大断裂或地幔蛇纹石化作用影响;(4)上地幔顶部速度存在低速异常(7.2 km/s),结合NTD地壳较薄(<5 km)和表层断裂多的特征,认为是上地幔发生大规模蛇纹石化作用导致低速,为进一步理解和区分莫霍面与蛇纹石化前缘(Serpentinization Front)提供了一个很好的例证.
Non-converted fault discontinuities (NTDs) occur at high frequency over the ultra-slow spreading mid-ocean ridge, and the crustal velocity structures are significantly different from the adjacent magma expansion centers (NVRs). Combined with the extensive and thinning crust of surface faults, NTD It is of great significance in the theory of ultra-slow mid-ocean ridge extension, hydrothermal activity and even mineral resources.In this paper, we use the data of two seafloor wide-angle seismic surveys and use the method of ray tracing forward and inversions to obtain the mid-south Indian Ocean ridge The results show that: (1) the crust beneath NTD is thin (3.2 ~ 4.5 km), the thickness of ocean shell 2 is about 2.0 km, and the thickness of ocean shell 3 is thinner (1.0 km) or even absent; (2) Lateral velocity difference of surface layer 2 in the oceanic crust is large, indicating that there are more faults; (3) Low velocity zones occur in the crust 3, which may be caused by shallow major faults or mantle serpentinization (4) There is a low velocity anomaly (7.2 km / s) at the top of the upper mantle. Considering the features of the thin crust (<5 km) and surface faults in the NTD, it is considered that large-scale serpentinization of the upper mantle leads to low velocity , For further understanding and differentiation between the Moho and the Serpentinization Front A good example.