Crustal magnetization and accretion at the Southwest Indian Ridge near the Atlantis II fracture zone, 0–25 Ma
Crustal magnetization and accretion at the Southwest Indian Ridge near the Atlantis II fracture zone, 0–25 Ma
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亚特兰蒂斯 II 断裂带附近西南印度洋脊的地壳磁化和吸积，0-25 Ma
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发表时间：
2003
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通讯作者：
H. Kinoshita
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作者：
A. Hosford;M. Tivey;Takeshi Matsumoto;H. Dick;H. Schouten;H. Kinoshita
[1]我们分析了西南印度洋脊（SWIR）两侧0 - 25myr -old海底的地球物理数据。在研究区内，即使在缺乏玄武岩上地壳的海底，线状的海洋磁异常也是一致的和可识别的。至少自20 Ma以来，14 km/Myr的全部扩张速率几乎保持不变，但地壳增生一直高度不对称，南极和非洲两侧的扩张速率分别为8.5 km/Myr和5.5 km/Myr。这种不对称性可能是SWIR大偏置裂缝带之间约400公里宽的走廊所特有的。与大西洋中脊相反，地壳磁化振幅与现今裂谷沿线的海底地形直接相关。这种模式似乎主要是地壳厚度沿轴变化的函数，而不是磁矿物学。离轴处，古板块末端的磁化振幅大于古板块中点的磁化振幅，表明下地壳或蛇纹石化的上地幔中存在磁化诱导成分。在吸积的20myr范围内，古段中点磁源层的改变使磁化幅度降低了70-80%。磁性和海洋钻探计划（ODP） 735B孔的数据表明，下地壳冷却得足够快，可以锁定与上覆地壳同步的初级热磁化。因此，侵入性下地壳内的磁极边界可能比以前海洋地壳磁化模型所设想的更陡峭。随着地壳的老化，下地壳在保存海洋磁条方面变得越来越重要。
[1] We analyze geophysical data that extend from 0 to 25-Myr-old seafloor on both flanks of the Southwest Indian Ridge (SWIR). Lineated marine magnetic anomalies are consistent and identifiable within the study area, even over seafloor lacking a basaltic upper crust. The full spreading rate of 14 km/Myr has remained nearly constant since at least 20 Ma, but crustal accretion has been highly asymmetric, with half rates of 8.5 and 5.5 km/Myr on the Antarctic and African flanks, respectively. This asymmetry may be unique to a ∼400 km wide corridor between large-offset fracture zones of the SWIR. In contrast to the Mid-Atlantic Ridge, crustal magnetization amplitudes correlate directly with seafloor topography along the present-day rift valleys. This pattern appears to be primarily a function of along-axis variations in crustal thickness, rather than magnetic mineralogy. Off-axis, magnetization amplitudes at paleo-segment ends are more positive than at paleo-segment midpoints, suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle. Alteration of the magnetic source layer at paleo-segment midpoints reduces magnetization amplitudes by 70–80% within 20 Myr of accretion. Magnetic and Ocean Drilling Program (ODP) Hole 735B data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust. Thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization. As the crust ages, the lower crust becomes increasingly important in preserving marine magnetic stripes.