Estimating emplacement rates for seaward-dipping reflectors associated with the U.S. East Coast Magnetic Anomaly
Estimating emplacement rates for seaward-dipping reflectors associated with the U.S. East Coast Magnetic Anomaly
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估计与美国东海岸磁异常相关的向海倾斜反射体的就位率
DOI：
10.1093/gji/ggy360
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发表时间：
2018
影响因子：
2.8
通讯作者：
Buck, W R
中科院分区：
文献类型：
--
作者：
Davis, J K;Bécel, A;Buck, W R
沿着北美东部边缘，在与被动边缘大部分平行的宽、正、高幅度磁异常--东海岸磁异常(ECMA)附近发现了火山向海倾斜反射器(SDR)包。特别提款权的年龄和就位率在很大程度上是未知的，但这些包裹以前已包括在迅速就位的中大西洋岩浆省(CAMP)的体积估计中。在这里，我们编译了SDR玄武岩的磁性，并正演模拟了SDR包以不同的伸展速率放置时所产生的总场磁异常。我们发现，为了匹配ECMA的宽、正和高幅度特征，SDR必须要么只保留单个磁性计时器并以不合理的高全程延伸率(≥200 mm yr-1)安放，要么以足够低的延伸率(≤20 mm yr-1)安放，以允许感应性磁异常主导整个磁异常信号。陆上钻探现场发现玄武岩流动具有正常和反向剩余磁极。这表明，ECMA特别提款权的部署速度约为2000万毫米/年或更短。因此，我们支持ECMA最好被解释为大陆地壳和岩浆SDR物质之间的磁化率反差引起的诱发异常的说法。根据SDR结构域的宽度和中大西洋早期的延伸率估计，我们估计SDR包的就位至少需要6 Myr，可能需要长达31 Myr。这一结果表明，要么应该增加目前估计的600基尔营地持续时间，要么应该将海上特别提款权材料排除在火成岩省数量估计之外。最后，如果其他火山边缘的SDR包以与ECMA附近观测到的相似的伸展速率侵位，那么我们预计来自包的磁异常可能类似地由感应磁异常信号主导。由于感应磁异常几乎完全是地球物质的正磁异常，这可能解释了为什么在观测到SDR的火山被动边缘没有宽、高幅度的负磁异常。
Along the eastern margin of North America, packages of volcanic seaward dipping reflectors (SDRs) are found proximal to a wide, positive, high amplitude magnetic anomaly, the East Coast Magnetic Anomaly (ECMA) that parallels much of the passive margin. The age and emplacement rates of the SDRs are largely unknown, but the packages have previously been included in volumetric estimates of the rapidly emplaced Central Atlantic Magmatic Province (CAMP). Here we compile the magnetic properties of SDR basalts and forward model the total-field magnetic anomaly that would be produced by SDR packages emplaced at various extension rates. We find that in order to match the wide, positive, and high amplitude character of the ECMA, the SDRs must either: preserve only a single magnetic chron and have been emplaced at unreasonably high full rates of extension (≥200 mm yr–1), or have been emplaced at extension rates low enough (≤20 mm yr–1) to allow for an induced magnetic anomaly to dominate the overall magnetic anomaly signal. An onshore drill site reveals basalt flows with both normal and reversed remanent magnetic polarity. This suggests that the ECMA SDRs were emplaced at rates of extension of around 20 mm yr–1or less. Thus, we support suggestions that the ECMA is best interpreted as an induced anomaly arising from a susceptibility contrast between continental crust and magmatic SDR material. Based on the width of the SDR domain and extension rate estimates for the early Central Atlantic, we estimate the emplacement of the SDR packages took at least 6 Myr and may have taken as long as 31 Myr. This result suggests that either the current 600 kyr estimated duration of CAMP should be increased, or the offshore SDR material should be excluded from estimates of the igneous province volume. Finally, if packages of SDRs at other volcanic margins were emplaced at similar extension rates to those observed near the ECMA, then we expect that the magnetic anomaly from the packages may be similarly dominated by an induced magnetic anomaly signal. Because induced magnetic anomalies are almost exclusively positive for earth materials, this may explain the lack of wide, high amplitude, negative magnetic anomalies at volcanic passive margins where SDRs are observed.
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