The magnetic structure of Axial Seamount, Juan de Fuca Ridge
The magnetic structure of Axial Seamount, Juan de Fuca Ridge
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胡安德富卡海脊轴状海山的磁结构
DOI：
10.1029/jb095ib08p12735
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发表时间：
1990
影响因子：
--
通讯作者：
H. Johnson
中科院分区：
文献类型：
--
作者：
M. Tivey;H. Johnson
Axial Seamount是位于东太平洋Juan de Fuca（JDF）中脊上的一座大型火山活动海山，位于46°N，130′W。海表磁异常资料表明，Axial完全位于Brunhes正极性期形成的地壳内。然而，edibrium并没有产生一个简单的正磁异常，与平均海底上方的过量质量有关。几个明显的负磁异常与海山有关，包括位于山顶以西的圆形低气压和火山东南侧的细长低气压（走向西北）。这两个异常构成了一个NW-SE磁槽的一部分，该磁槽斜向于N20°E的JDF脊的总体走向延伸穿过海山沉积物。在与North Helium盆地有关的海山北方侧翼观察到第三个负异常，North Helium盆地是一个深深的狭长盆地，使海山东北侧翼凹陷。反演的海洋表面磁性数据和地形的正演模拟，由疏浚样品的磁化值的约束下，表明测深的影响可以解释氦盆地异常，但地形本身不能产生低磁化值，延伸到整个海山峰会。地壳岩石的磁化强度在海山的空间上没有变化，因此山顶的负异常可以用磁源层有效厚度的变化来解释。均匀磁化的简单正演模型表明，异常源层必须薄50%（厚度小于700 m），以产生观测到的磁响应。变薄的源层与在中轴线顶峰下方的深度处存在大量岩浆房或大量地壳蚀变区相一致。异常的形状意味着这两个源的趋势必须倾斜于一般的山脊趋势，但平行于绝对板块运动的方向。
Axial Seamount is a large, volcanically active seamount located in the eastern Pacific on the central Juan de Fuca (JDF) ridge at 46°N, 130′W. Sea surface magnetic anomaly data show that Axial lies completely within crust formed during the Brunhes normal polarity epoch. The edifice, however, does not produce a simple positive magnetic anomaly related to the excess mass above mean seafloor. Several pronounced negative magnetic anomalies are associated with the seamount, including both a circular-shaped low located west of the summit and an elongate low (trending NW) on the southeast flank of the volcano. These two anomalies form part of a NW-SE magnetic trough which extends across the seamount edifice oblique to the general JDF ridge trend of N20°E. A third negative anomaly is observed on the northern flank of the seamount associated with North Helium Basin, a deep elongate basin which indents the northeast flank of the seamount. Both inversion of sea surface magnetics data and forward modelling of topography, constrained by magnetization values of dredged samples, show that bathymetric effects can explain the Helium Basin anomaly, but topography alone cannot produce the low magnetization values which extend across the seamount summit. Magnetization of crustal rocks does not vary spatially over the seamount, so that negative anomalies at the summit can be explained by variations in the effective thickness of the magnetic source layer. A simple forward model of uniform magnetization shows that the anomaly source layer would have to thin by 50% (to less than 700 m thick) in order to generate the observed magnetic response. A thinned source layer is consistent with the presence of either a substantial magma chamber at depth beneath the summit of Axial or a zone of substantial crustal alteration. The shape of the anomaly implies the trend of either of these sources must be oriented obliquely to the general ridge trend but parallel to the direction of the absolute plate motion.