Focused magmatism versus amagmatic spreading along the ultra‐slow spreading Southwest Indian Ridge: Evidence from TOBI side scan sonar imagery
Focused magmatism versus amagmatic spreading along the ultra‐slow spreading Southwest Indian Ridge: Evidence from TOBI side scan sonar imagery
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DOI：
10.1029/2004gc000738
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发表时间：
2004-10
期刊：
影响因子：
3.7
通讯作者：
D. Sauter;V. Mendel;C. Rommevaux‐Jestin;L. Parson;H. Fujimoto;C. Mével;M. Cannat;K. Tamaki
中科院分区：
文献类型：
--
作者：
D. Sauter;V. Mendel;C. Rommevaux‐Jestin;L. Parson;H. Fujimoto;C. Mével;M. Cannat;K. Tamaki
通过对沿着东经63°40′ ~ 65°40′之间的西南印度洋海岭的拖曳式海底仪（TOBI）侧扫声纳图像的分析，揭示了强烈的岩浆活动聚焦和长的岩浆增生脊段。在沿着轴的不同位置观察到新鲜的火山地形，这些火山地形被高度构造化和沉积的地形分开，沿着轴延伸达82 km。最大的构造化段对应于具有中等磁化异常的显著薄地壳区。我们认为，海底扩张主要是amagmatic在这个构造部分的西南印度洋脊上地幔岩石暴露在海底。这种尺寸的岩浆增生脊段与在大西洋中脊观察到的相当不同，但也在加克尔脊被识别出来，因此可能是超慢速扩张脊的特征。新鲜火山地形的分布、轴向火山脊所覆盖的浅区的出现和高磁化值之间的相关性表明，脊的浅分割主要与玄武岩源层的厚度和/或内在磁化的变化有关。相比之下，重力导出的地壳厚度的强烈沿着轴变化相对于脊的浅分段在长度上收缩，并且仅发生在升高段之下。在这些隆起部分附近，具有新火山结构的小水深隆起与较厚的地壳区域不相对应。这表明一个高度集中的熔体供应下的高架段，可能会饲料火山建设从这些段的中心60公里的地壳中的浅横向熔体迁移，可能通过大型堤防。超慢的扩张速度和海岭的垂直度都不能解释岩浆活动沿着海岭的变化。地幔源区的不均匀性以及西南印度洋脊最东端的低地幔温度可能部分地控制了它的分段。
The analysis of the Towed Ocean Bottom Instrument (TOBI) side scan sonar images along the Southwest Indian Ridge between 63°40′E and 65°40′E reveals strong focusing of magmatic activity and long amagmatic accretionary ridge segments. Fresh‐looking volcanic terrains are observed at distinct locations along the axis separated by highly tectonized and sedimented terrains of an along‐axis extent as much as 82 km. The largest tectonized section corresponds to a dramatically thin crust area with moderate magnetization anomalies. We suggest that seafloor spreading is mainly amagmatic in this tectonized section of the Southwest Indian Ridge with upper mantle rocks exposed at the seafloor. Amagmatic accretionary ridge segments of such dimensions are quite distinct from what is observed at the Mid‐Atlantic Ridge but are also recognized at the Gakkel Ridge and may thus be characteristic of ultra‐slow spreading ridges. The correlation between the distribution of fresh‐looking volcanic terrains, the occurrence of shallow areas crowned by axial volcanic ridges, and high magnetization values suggests a shallow segmentation of the ridge mainly related to variation in the thickness and/or the intrinsic magnetization of the basaltic source layer. By contrast, strong along‐axis variations of the gravity‐derived crustal thickness are shrunken in length relative to this shallow segmentation of the ridge and occur only beneath the elevated segments. Adjacent to these elevated segments, small bathymetric swells with fresh‐looking volcanic constructions do not correspond to thicker crust areas. This suggests a highly focused melt supply beneath the elevated segments which may feed volcanic constructions up to 60 km from the center of these segments by shallow lateral melt migration in the crust, probably through large dikes. Neither the ultra‐slow spreading rate nor the ridge obliquity explains the variation of the magmatic vigor along the ridge. Mantle source heterogeneities together with lower mantle temperatures beneath the easternmost Southwest Indian Ridge could partly control its segmentation.