Magnetization and tectonic evolution of ultraslow-spreading rate lower oceanic crust, Atlantis Bank, SW Indian Ridge (IODP Expedition 360)

南印度洋中脊亚特兰蒂斯浅滩超慢速扩张速度下洋地壳的磁化作用和构造演化（IODP Expedition 360）

• 批准号: NE/N019210/1
• 负责人: Antony Morris
• 金额: $ 14.73万
• 依托单位: Plymouth University
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2015
• 资助国家: 英国
• 起止时间: 2015 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FN019210%2F1
• 关键词: Magnetization; tectonic; evolution; ultraslow; spreading

Generation of ocean crust by seafloor spreading at mid-ocean ridges is responsible for forming two-thirds of the Earth's surface. The lower crust is the key to understanding this process. It consists of the rock gabbro, a slowly-cooled, coarse-grained igneous rock formed by crystallization of magma derived from melting of the mantle at mid-ocean ridges. The lower crust therefore acts as a critical layer, receiving magma from below, crystallizing it to form gabbros and passing remaining melt upwards to form the upper crust. However, we have surprisingly few samples of lower crustal gabbros on which to test different hypotheses for seafloor spreading. These have come from a limited number of deep drill holes in the world's oceans, and the volume of all samples combined would fit inside a 2.5 m box! In addition to sampling, an important tool in determining the age of oceanic crust is the interpretation of subtle variations in the strength of the Earth's magnetic field produced by the magnetization of oceanic crustal rocks on the seafloor. These "marine magnetic anomalies" provide a global tape recording of seafloor spreading in the form of positive and negative variations in field strength that are distributed symmetrically on either side of mid-ocean ridges as a result of past reversals of the direction of the field during spreading. However, to date we have never sampled across one of these important reversal boundaries in the lower crust, and retrieving such samples would provide a major step forward in understanding of the magnetic signal in the oceans.Significant new samples of the lower crust can only be obtained by scientific oceanic drilling. International Ocean Discovery Program (IODP) Expedition 360 "SW Indian Ridge Lower Crust and Moho" will use the drillship JOIDES Resolution to recover core samples from a new 1.3 km deep hole into lower oceanic crust at Atlantis Bank, where these rocks have been brought near to the seafloor by movement on a major fault system that cuts the crust. By describing and analyzing core samples of gabbros from this special tectonic setting, the expedition will address important questions such as: how the igneous lower crust forms (or accretes); how magma migrates, crystallizes and evolves (changes composition) during accretion; and how tectonic deformation (faulting) of the crust contributes to seafloor spreading by taking up stretching of the oceanic plates. Importantly, Atlantis Bank also provides a unique opportunity to understand the source of marine magnetic anomalies as a reversal boundary is believed to lie a few hundred metres beneath the seafloor at this location.This project will analyse the magnetic properties of the lower crustal rocks recovered at Atlantis Bank. The gabbros acquired a magnetization in the direction of the Earth's field shortly after they crystallized. The direction of the magnetization trapped in the rocks can be used as a marker to determine whether the rocks have been tilted during faulting, by comparing with the known present day direction of the Earth's field at the site. However, a major problem with this approach is that IODP samples are free to spin in the core barrel during drilling, and are not oriented relative to present day north. To overcome this, we will examine oriented geophysical "pictures" of the inside of the borehole wall and match up geological structures of known orientation in these images with corresponding structures on the core samples to reorient core pieces. This will allow us to restore the magnetic data obtained from the cores to a true geographic reference framework and hence to use their magnetization directions to determine the extent to which tectonic tilting has contributed to the structural development of the massif. We will also look at how the magnetization of the gabbros changes as the reversal boundary in the hole is approached and crossed, shedding new light on the nature of source of marine magnetic anomalies.

海底散布在海脊中的海壳产生，负责形成地球表面的三分之二。下层地壳是理解这一过程的关键。它由岩石加布罗（Rock Gabbro）组成，岩石是一种缓慢冷的，粗粒的火成岩，由岩浆的结晶形成，这些岩浆是由中山脊在地幔熔化中得出的。因此，下层地壳充当临界层，从下面接收岩浆，将其结晶以形成gabbros并将其剩余的向上融化以形成上皮。但是，令人惊讶的是，下层地壳gabbros的样品很少，可以在上测试不同假设的海底扩散。这些来自世界海洋中有限数量的深钻孔，所有样品组合的体积都适合2.5 m的盒子内！除了采样外，确定海洋壳时代的重要工具是解释了海底磁性磁化强度产生的地球磁场强度的微妙变化。这些“海洋磁异常”提供了海底传播的全局胶带记录，以田间强度的正变化和负变化的形式，由于过去在中脊的两侧分布在田间脊的两侧，这是由于过去在场期间的方向逆转而导致的。蔓延。但是，迄今通过科学海洋钻探获得。国际海洋发现计划（IODP）Expedition 360“ SW Indian Ridge Lower Crust and Moho”将使用Drillship Cain Resolution从一个新的1.3公里深的孔中恢复核心样品，进入Atlantis Bank的下海洋壳，这些岩石已将这些岩石带到附近通过在切割外壳的主要断层系统上移动到海底。通过描述和分析此特殊构造环境中GABBRO的核心样本，该探险将解决重要的问题，例如：火成岩下层地壳如何形式（或增生）；岩浆在积聚过程中如何迁移，结晶和发展（变化组成）；地壳的构造变形（断层）如何通过占据海洋板的伸展而导致海底扩散。重要的是，亚特兰蒂斯银行还提供了一个独特的机会来了解海洋磁异常的来源，因为据信逆转边界位于该位置的海底几百米。亚特兰蒂斯银行。 Gabbros结晶后不久就在地球场的方向上获得了磁化。被困在岩石中的磁化强度的方向可以用作标记，以确定在断层期间岩石是否已与现场现场的已知当天方向进行比较。但是，这种方法的一个主要问题是IODP样品在钻孔过程中可以自由旋转，并且相对于当今的北方而言，且不定向。为了克服这一点，我们将检查钻孔壁内部的定向地球物理“图片”，并在这些图像中与已知取向的地质结构相匹配，并在核心样品上具有相应的结构，以重新定向核心块。这将使我们能够恢复从核心获得的磁数据到真正的地理参考框架，从而使用其磁化方向来确定构造倾斜有助于构造地块的结构发展的程度。我们还将研究Gabbros的磁化如何随着孔中的逆转边界而变化，并散发出有关海洋磁异常源本质的新灯。

项目成果

Dynamic Accretion Beneath a Slow‐Spreading Ridge Segment: IODP Hole 1473A and the Atlantis Bank Oceanic Core Complex

• DOI: 10.1029/2018jb016858
• 发表时间: 2019-12
• 期刊: Journal of Geophysical Research: Solid Earth
• 作者: H. J. Dick;C. J. MacLeod;P. Blum;N. Abe;D. K. Blackman;J. Bowles;M. Cheadle;K. Cho

Magnetic Mineral Populations in Lower Oceanic Crustal Gabbros (Atlantis Bank, SW Indian Ridge): Implications for Marine Magnetic Anomalies

• DOI: 10.1029/2019gc008847
• 发表时间: 2020-03
• 期刊: Geochemistry
• 影响因子: 3.7
• 作者: J. Bowles;A. Morris;M. Tivey;I. Lascu