Crustal accretion and transform margin evolution at ultraslow spreading rates

超慢扩张速率下的地壳吸积和变换边缘演化

• 批准号: NE/K011162/1
• 负责人: Christine Peirce
• 金额: $ 23.52万
• 依托单位: Durham 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%2FK011162%2F1
• 关键词: Crustal; accretion; transform; margin; evolution

The crust that underlies the world's oceans forms as a result of seafloor spreading - a process that sees the rigid oceanic plates pulled apart at fast (>100 mm/yr), intermediate (100-55 mm/yr) or slow (55-20 mm/yr) rates. As plates separate the mantle beneath rises to fill the gap and as it does so it melts due to the lower pressure. This molten rock, or magma, solidifies to form the ~6-8 km thick oceanic crust, comprising a layer of erupted and rapidly cooled magma (basalt) at the top and a layer of slowly cooled magma (gabbro) beneath.Over the last decade, observations have shown that the crust created where oceanic plates are pulled apart at slower rates, does not form by such a simple process of symmetrical, magmatic construction as our current models predict, but instead the magmatic construction is interspersed with periods of apparent magma-starvation. During these amagmatic phases plate separation is accommodated by large-offset faults along which rocks from the lower crust and the upper mantle beneath are brought to the surface. These regions of exhumed lower crust and upper mantle rocks are called oceanic core complexes (OCCs).About 25% of the Earth's mid-ocean ridges spread at very slow rates of less than 20 mm/yr. However, most of these ultraslow ridges are located in remote areas that have poor weather or ice cover that impedes their investigation. Consequently, how the crust forms and ages at these slowest spreading centres, which current models predict should be predominantly magma-starved and cold, remains poorly understood. Recent seabed imaging and sampling studies of the ultraslow Mid-Cayman Spreading Centre (MCSC) in the Caribbean, have observed the deepest and hottest black smoker hydrothermal vents on Earth, and regions of exhumed lower crust and upper mantle juxtaposed against volcanically erupted rocks of the "normal" upper oceanic crust. Here we will establish the crustal context of these contrasting observations that challenge the predictions of traditional models, and we will determine the time and space interplay between magmatic construction and amagmatic extension and the controls on, and relationship between, faulting and hydrothermal activity.As part of a British, German and American partnership, we will use sub-seabed seismic imaging to study the structure and lithology of the crust at the Mt Dent OCC on the MCSC and determine the relationship between this and the adjacent volcanic domain that also hosts hydrothermal vents. We will also investigate how the crust changes as it cools and ages as it spreads away from the ridge axis. Using the pattern of local earthquakes we will map sub-seabed fault geometries and whether or not these faults are connected at depth. As the southern tip of the MCSC also abuts against the continental crust of the Caribbean plate across the Swan Island Transform Zone, this also provides a unique opportunity to determine not only how the mantle rises up and melts beneath the ridge and how this melt is distributed along-ridge, but also if this process is impeded by the cooling affect of adjacent thick, cold continental lithosphere. To achieve our goals we will deploy ocean-bottom seismographs (OBSs) onto the seabed to determine the variation in velocity associated with, and the interfaces between the different rock types deep into the crust and upper mantle using man-made seismic signals. We will also use the OBSs to record the signals that occur naturally when faults move. We will measure the gravity field to determine crustal density as a test of our seismic models, and to image deeper into the mantle to depths beyond which our seismic signals will penetrate. Finally, we will measure reversals in the magnetic field to reveal seafloor spreading rate and crustal age and, jointly with the seismic data, determine how frequently phases of amagmatic extension have occurred from the current time to at least 20 million years ago.

世界海洋下方的地壳是由于海底扩张而形成的，在这一过程中，坚硬的海洋板块以快（>100毫米/年）、中（100-55毫米/年）或慢（55-20毫米/年）的速度被拉开。当板块分离时，下面的地幔会上升以填充间隙，并且当它这样做时，它会由于较低的压力而熔化。这种熔融岩石或岩浆凝固形成约 6-8 公里厚的海洋地壳，包括顶部的一层喷发和快速冷却的岩浆（玄武岩）和下面一层缓慢冷却的岩浆（辉长岩）。在过去的十年中，观察表明，海洋板块以较慢的速度拉开时形成的地壳，并不是通过像这样简单的对称岩浆构造过程形成的。 我们目前的模型预测，但岩浆构造却散布着明显的岩浆匮乏时期。在这些非岩浆阶段期间，大偏移断层适应了板块分离，沿着这些断层，来自下地壳和上地幔的岩石被带到地表。这些被挖出的下地壳和上地幔岩石区域被称为大洋核心复合体 (OCC)。大约 25% 的地球洋中脊以每年 20 毫米以下的非常缓慢的速度扩张。然而，这些超慢山脊大多数位于偏远地区，天气恶劣或冰盖阻碍了他们的调查。因此，人们对地壳如何在这些扩张最慢的中心形成和老化仍然知之甚少，目前的模型预测这些中心应该主要缺乏岩浆且寒冷。最近对加勒比海超慢速中开曼扩散中心（MCSC）进行的海底成像和采样研究，观察到了地球上最深、最热的黑烟热液喷口，以及与“正常”上洋地壳的火山喷发岩石并列的下地壳和上地幔区域。在这里，我们将建立这些对比观测的地壳背景，挑战传统模型的预测，我们将确定岩浆构造和非岩浆延伸之间的时间和空间相互作用，以及断层和热液活动的控制和关系。作为英国、德国和美国合作伙伴关系的一部分，我们将使用海底地震成像来研究 MCSC 上 Mt Dent OCC 的地壳结构和岩性，并确定 该区域与附近也有热液喷口的火山区域之间的关系。我们还将研究地壳在远离山脊轴时冷却和老化时如何变化。利用当地地震的模式，我们将绘制海底断层几何形状以及这些断层是否在深度上相连。由于 MCSC 的南端也跨越天鹅岛转换带紧邻加勒比板块的大陆地壳，这也提供了一个独特的机会，不仅可以确定地幔如何在山脊下上升和熔化以及这种熔化物如何沿山脊分布，还可以确定这一过程是否受到邻近厚而冷的大陆岩石圈的冷却影响的阻碍。为了实现我们的目标，我们将在海底部署海底地震仪（OBS），利用人造地震信号确定与地壳和上地幔深处不同岩石类型相关的速度变化以及它们之间的界面。我们还将使用OBS来记录断层移动时自然发生的信号。我们将测量重力场以确定地壳密度，作为对我们地震模型的测试，并对地幔深处进行成像，直至我们的地震信号将穿透的深度。最后，我们将测量磁场的反转，以揭示海底扩张速率和地壳年龄，并结合地震数据，确定从当前到至少 2000 万年前无岩浆伸展阶段发生的频率。

项目成果

Meteor-Berichte CAYSEIS - magma-starved oceanic crustal accretion and transform margin formation in the Cayman Trough revealed by seismic and seismological data

Meteor-Berichte CAYSEIS - 地震和地震数据揭示了开曼海槽中缺乏岩浆的海洋地壳增生和转换边缘形成

• 发表时间: 2016
• 作者: Grevemeyer I

Seismic investigation of an active ocean-continent transform margin: the interaction between the Swan Islands Fault Zone and the ultraslow-spreading Mid-Cayman Spreading Centre

活跃洋陆转换边缘的地震调查：天鹅群岛断层带与超慢速扩张的开曼中部扩张中心之间的相互作用

• DOI: 10.1093/gji/ggz283
• 发表时间: 2019
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: Castiello G

Characterising the evolution of transform continental margins - a geophysical study of the Swan Island transform margin-Cayman Trough intersection, Caribbean Sea

表征变换大陆边缘的演化——加勒比海天鹅岛变换边缘-开曼海槽交汇处的地球物理研究

• 发表时间: 2016
• 作者: Castiello G

Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity

• DOI: 10.1130/g46577.1
• 发表时间: 2019-11-01
• 期刊: GEOLOGY
• 影响因子: 5.8
• 作者: Grevemeyer, Ingo;Hayman, Nicholas W.;Dannowski, Anke
• 通讯作者: Dannowski, Anke

Episodic magmatism and serpentinized mantle exhumation at an ultraslow-spreading centre

• DOI: 10.1038/s41561-018-0124-6
• 发表时间: 2018-06-01
• 期刊: NATURE GEOSCIENCE
• 影响因子: 18.3
• 作者: Grevemeyer, Ingo;Hayman, Nicholas W.;Papenberg, Cord
• 通讯作者: Papenberg, Cord