Investigation of a Vine‐Matthews Magnetic Lineation from a submersible: The source and character of marine magnetic anomalies
Investigation of a Vine‐Matthews Magnetic Lineation from a submersible: The source and character of marine magnetic anomalies
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DOI：
10.1029/jb088ib04p03403
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发表时间：
1983-04
影响因子：
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通讯作者：
K. Macdonald;S. P. Miller;B. Luyendyk;T. Atwater;L. Shure
中科院分区：
文献类型：
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作者：
K. Macdonald;S. P. Miller;B. Luyendyk;T. Atwater;L. Shure
我们在这里报告了直接绘制海底藤马修斯磁条边界的首次尝试。我们的目标是研究反转过渡带记录的大洋地壳增生过程，并研究瓦恩-马修斯磁异常磁源的形成。我们的深拖曳和基于 ALVIN 的磁力研究重点关注北纬 21° 附近东太平洋隆起侧翼的松山/布伦赫斯反转转变。虽然东太平洋海隆的海平面磁异常清晰度低于平均水平，但深拖数据的“三维”反演揭示了宽度不足 1.8 公里的尖锐的走向线性极性转变（Macdonald 等人，1980a）。通过在 ALVIN 上安装垂直磁梯度仪并沿极性过渡区和跨极性过渡区进行 280 次可靠的极性测定，增强了这些测量结果。即使在跨越边界两侧的长距离穿越中，我们也发现几乎每个磁性目标都具有正确的极性，即与区域磁线的极性相同。磁线极性的这种均匀性令人惊讶。露头火山部分的磁极性转变沿走向是急剧且线性的，在某些情况下由不同年龄的相对流锋的清晰地质接触来描绘。绘制在过渡带内的几个弱磁化露头可能是在地磁场反转期间喷发的。从 ALVIN 绘制的海底反转边界相对于深拖曳和海平面磁数据反演得出的平均边界位置，向西北移动 250 m 至 500 m。这种偏移量提供了一种估计地壳形成期间熔岩流远离扩散轴的溢出的方法。深拖和 ALVIN 测量的结合表明，大约 0.7 m.y.以前地壳增生带（磁化火山岩、侵入岩和深成岩）宽2000-2800米，而近代火山活动带仅宽1000-2000米。最近反转的确定与当前扩散中心的潜水观测结果非常吻合，该中心的近期火山活动带（新火山带）宽度在 600 至 2000 米之间变化。这张关于磁线形成和地壳增生过程的非常有序的图景似乎与复杂的深海钻探项目（DSDP）来自大西洋的磁力结果相冲突。我们认为地壳生成过程和由此产生的磁结构随扩散速率的变化而显着变化。在缓慢扩张的大西洋中脊，主要的火山活动可能很少发生（约 104 年），岩浆房可能处于非稳态，新火山带的移动或宽度变化很大。这种零星的、开始和停止的扩散过程将导致高度异质且复杂的地壳和磁结构，如 DSDP 孔中所见。此外，显着的断层和倾斜可能会破坏缓慢扩张的地壳。对于中等至快速扩张的中心，更频繁的火山活动（~50-600年）、近乎稳定状态的岩浆室和狭窄、稳定的新火山带将创造出不太复杂的磁性和地壳结构，无论是从阿尔文观测到的还是从太平洋清晰的海平面磁异常推断的。
We report here the first attempt to map directly the boundary of a Vine-Matthews magnetic stripe on the seafloor. Our objectives are to study the processes of oceanic crustal accretion as recorded in the reversal transition zone and to investigate the formation of the magnetic source of Vine-Matthews magnetic anomalies. Our deep-tow and ALVIN-based magnetic studies focus on the Matuyama/Brunhes reversal transition on the flanks of the East Pacific Rise near 21°N. While the sea level magnetic anomalies are less than average in clarity for the East Pacific Rise, a ‘three-dimensional’ inversion of deep-tow data reveals a sharp, strike-linear polarity transition less than 1.8 km wide (Macdonald et al., 1980a). These measurements have been augmented by mounting a vertical magnetic gradiometer on ALVIN and making 280 reliable polarity determinations along and across the polarity transition zone. Even on long traverses across both sides of the boundary, we find that nearly every magnetic target has the correct polarity, i.e., the same polarity as the regional magnetic lineation. This homogeneity in polarity of the magnetic lineations is surprising. The magnetic polarity transition in the outcropping volcanic section is sharp and linear along strike, delineated in some cases by a clear geologic contact of opposing flow fronts of different ages. Several weakly magnetized outcrops mapped within the transition zone may have erupted during the time in which the geomagnetic field was reversing. The reversal boundary mapped on the seafloor from ALVIN is displaced 250 m to 500 m NW away from the spreading axis relative to the position of the average boundary as derived from inversion of the deep-tow and sea level magnetic data. This offset provides a means for estimating the spillover of lava flows away from the spreading axis during the time the crust was formed. The combination of deep-tow and ALVIN measurements suggests that circa 0.7 m.y. ago the crustal accretion zone (magnetized volcanic, intrusive, and plutonic rocks) was 2000–2800 m wide, while the zone of recent volcanism alone was only 1000–2000 m wide. The determination for the most recent reversal agrees well with submersible observations at the present spreading center where the zone of recent volcanism (neovolcanic zone) varies between 600 and 2000 m in width. This very orderly picture for the formation of magnetic lineations and crustal accretion processes appears to conflict with complex Deep Sea Drilling Project (DSDP) magnetic results from the Atlantic. We suggest that the crustal generating processes and resulting magnetic structure vary significantly with spreading rate. On the slow-spreading Mid-Atlantic Ridge, major episodes of volcanism are likely to be infrequent (∼104 years), the magma chamber may be non-steady state, and the neovolcanic zone shifts or varies in width considerably. This sporadic, start and stop spreading process will contribute to a highly heterogeneous and complex crustal and magnetic structure as seen in DSDP holes. In addition, significant faulting and tilting may disrupt slow-spreading crust. For intermediate- to fast-spreading centers, more frequent volcanism (∼50–600 years), a nearly steady state magma chamber, and a narrow, stable neovolcanic zone will create a less complex magnetic and crustal structure both as seen from ALVIN and as inferred from clear sea level magnetic anomalies in the Pacific.