【摘要】 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.更多还原

【Abstract】 Non-transform discontinuities(NTDs) are frequently found on ultra-slow spreading ridges. They have crustal structures that differ significantly from those of neighboring neo-volcanic ridges(NVRs). For an ultra-slow spreading ridge with weakened lithosphere and thinned crust, NTDs play an important role in understanding the mechanisms of ridge spreading, hydrothermal venting and mineral resource deposition. In this paper, we present an extensive ocean bottom seismometer(OBS) experiment that was carried out in 2010 at the Southwest Indian Ridge(SWIR) during R/V Dayang Yihao cruise DY115-21. The seismic source was a four-air-gun array with a total volume of or 98.32 L(6000 cubic inches) and a seismic trace spacing of ~250 m. A total of 10832 shots were fired and 52 survey lines(about 2650 km in length) were acquired. Thirty-eight OBSs, each containing a three-component geophone and a hydrophone, successfully recovered information from the crust and upper mantle in NVR and NTD settings. In this paper, we use two wide-angle seismic profiles and a ray-tracing method to constrain the velocity structure of the crust and upper mantle of the NTD between segments 28 and 29 of the SWIR. The processing methods include relocations of shot and OBS positions, time drift corrections and filtering. The widely used ray-theoretical travel-time inversion codes of Zelt and Smith(1992) were used for the modeling. The method includes a damped least square inversion of picked travel times, and we computed the statistical misfit, resolution and uncertainty of model parameters to ensure that the final velocity model had the best fit to our data. The main conclusions are as follows:(1) NTD has a thinner crust(3.2–4.5 km), with an oceanic layer 2 that is 2 km thick and an oceanic layer 3 that is very thin(<1 km) or absent.(2) Oceanic layer 2 has significant lateral velocity variation, indicating the presence of various faults.(3) The uppermost mantle has low velocities(7.2 km/s), indicative of large and deep faults or the presence of serpentinization. We suggest that the low velocity zone in the upmost mantle, together with the thinner crust(<5 km) of the NTD and the presence of faults, result from large-scale serpentinization of the uppermost mantle, providing a good example for investigations of the difference between the Moho discontinuity and a serpentinization front.更多还原