Role and extent of detachment faulting at slow-spreading mid-ocean ridges

缓慢扩张的洋中脊滑脱断层的作用和范围

• 批准号: NE/J022551/1
• 负责人: Timothy Reston
• 金额: $ 53.19万
• 依托单位: University of Birmingham
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2013
• 资助国家: 英国
• 起止时间: 2013 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ022551%2F1
• 关键词: Role; extent; detachment; faulting; slow

Two-thirds of the Earth's surface is paved by oceanic crust formed by seafloor spreading at the 60,000 km-long global mid-ocean ridge (MOR) system. As the rigid ocean plates are pulled apart, at rates varying from <10 to 160 mm/year, the Earth's mantle is drawn up from beneath, partly melting as it does so. The melt separates from the mantle and rises to the surface to form a continuous layer of 'magmatic' crust, typically about 6 km thick, made of basalt at the surface and gabbro, its slowly cooled equivalent, beneath.However, over the past 15 years we have come to realise that, at spreading rates below about 40 mm/yr, this simple model cannot be correct. Instead, large tracts of mantle rocks may be exposed on the seafloor, with no magmatic crust being present. Plate separation on slow-spreading MORs such as the Mid-Atlantic Ridge (MAR) may instead be taken up in part on great dislocations - unusually large geological faults known as 'detachments' - on which tens of km of extension may be accommodated. Where exposed on the seafloor these faults typically form flat or gently domed surfaces on which mantle rocks and/or gabbro are exposed. These structures are known as 'oceanic core complexes' (OCCs). We think OCCs form when the magma supply dwindles and seawater is able to penetrate down a fault and access mantle rocks beneath. These rocks, called 'peridotites', are made mostly of the mineral olivine, which reacts easily with water to produce the weak minerals serpentine and talc, lubricating the fault and allowing it to continue slipping and develop into a long-lived detachment.Very recently, several workers (including PI Reston) have proposed that detachment faulting is far more common than previously supposed, to the extent that up to half of all Atlantic seafloor may be generated by such 'tectonic' spreading. They view detachments as regionally continuous features that underlie all the seafloor on one side of the ridge axis, but only emerge at the surface in a few places, the OCCs. But is detachment faulting really so widespread? From a detailed study of the 13N region of the MAR, Co-Is MacLeod and Searle came to the quite different, and much less extreme, view that detachments are discontinuous and restricted to individual OCCs. They are interspersed between volcanically active, magma-rich ridge segments, and triggered by localised waning of magma supply. In this model detachments are episodically 'killed' by renewed magmatism, often delivered laterally from adjoining segments.How can we distinguish these very different hypotheses about the mechanism of seafloor spreading? The key data needed are: (1) the sub-surface geometry and extent of the detachments beneath the ridge axis, (2) the amount and detailed distribution of magmatic crust, and (3) the asymmetry of spreading rates associated with OCCs and volcanic seafloor (they should be similar in the regional and differ in the local detachment models).We propose to obtain these data in a comprehensive seismic and seabed magnetic survey of the MAR in the 13N region, where detachment faults are active at the ridge axis today. We will use a large array of ocean-bottom seismographs (OBSs) to image 3D velocity variations related to different rock types using 'seismic tomography' - akin to medical CT scanning - and conduct a multi-channel reflection survey, which will image sub-surface discontinuities - like a simple X-ray. We will then leave the OBSs (to be recovered on a later cruise) to record the locations of natural micro-earthquakes in the region. These will show directly the 3D geometry and linkage of active faults. Finally, we will deploy the autonomous robot vehicle Autosub 6000, which will be programmed to make very detailed maps of magnetic field reversals (yielding seafloor age and spreading rate) and seafloor topography (helping structural interpretations) while we perform the seismic experiments.

地球表面的三分之二是由海底散布在60,000公里长的全球中山山脊（MOR）系统的海底铺成的。随着刚性海上板的拉开，以<10至160毫米/年的速度不等，地球的披风从下面起草，部分地融化了。熔体与地幔分开并升至表面，形成了连续的“岩浆”地壳，通常约6公里厚，厚度约6公里，由玄武岩和Gabbro制成，其在过去的15年中，它在下面逐渐冷却，我们已经意识到，我们已经意识到的是，这是一个简单的模型，这使得传播速度低于40 mm/yr。取而代之的是，大块地幔岩石可能暴露在海底，没有岩浆地壳。板块分离在慢速的MOR上，例如中大西洋山脊（MAR），可以部分地占用很大的错位 - 异常大的地质断层，称为“分离”），可以容纳数十公里的延伸。在暴露在海底的地方，这些断层通常形成平坦或轻轻的圆顶表面，地幔岩石和/或gabbro暴露在其上。这些结构称为“海洋核心复合物”（OCCS）。我们认为，当岩浆供应减少和海水能够渗透过故障并进入下面的地幔岩石时，我们认为OCC会形成OCC。这些称为“橄榄岩”的岩石主要是由矿物橄榄石制成的，它与水很容易产生薄弱的矿物质蛇形和滑石粉，润滑过错，并允许其继续滑倒并发展成长期寿命。由这种“构造”传播产生。他们将脱离视为区域连续特征，这些特征是山脊轴一侧的所有海底的基础，但仅在几个地方出现在表面上。但是分离的过错真的如此普遍吗？从对3月13N区域的详细研究，Co-is Macleod和Searle到底是完全不同的，非常极端的，认为支队是不连续的，并且仅限于单个OCC。它们散布在火山活跃，岩浆富含岩石的山脊段之间，并由岩浆供应的局部减弱触发。在此模型中，由于新的岩浆作用经常从毗邻的细分市场侧面传递，因此在情节上“杀死”。我们如何区分这些关于海底扩散机制的非常不同的假设？所需的关键数据是：（1）山脊轴下的脱离的地下几何形状和程度，（2）岩浆壳的数量和详细分布，以及（3）（3）与OCC和火山层相关的传播速率的不对称性不对称性（在区域中相似，并在当地分离模型中均具有相似之处。 13N区域，当今山脊轴的脱离断层处于活动状态。我们将使用大量的海底地震仪（OBS）使用“地震层析成像”与不同的岩石类型相关的图像3D速度变化 - 类似于医疗CT扫描 - 并进行了多个通道反射调查，该调查将图像图形子表面的不连续性图像，例如简单的X射线。然后，我们将离开OBS（将在后来的巡游中回收），以记录该地区自然微观的位置。这些将直接显示活跃故障的3D几何和链接。最后，我们将部署自动驾驶机器人汽车AutoSub 6000，该AutoSub 6000将进行编程，以进行非常详细的磁场逆转（产生海底年龄和扩散率）和海底地形（帮助结构解释），同时我们进行地震实验。

项目成果

Detachment Fault Behavior Revealed by Micro-Seismicity at 13°N, Mid-Atlantic Ridge

大西洋中脊北纬 13°微震活动揭示的滑脱断层行为

• 发表时间: 2016
• 作者: Parnell-Turner R

Constraints on crustal structure of adjacent OCCs and segment boundaries at 13°N on the Mid-Atlantic Ridge

大西洋中脊邻近OCC和北纬13°段边界的地壳结构约束

• DOI: 10.1093/gji/ggz074
• 发表时间: 2019
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: Peirce C

Slow slip along the Hikurangi margin linked to fluid-rich sediments trailing subducting seamounts

• DOI: 10.1038/s41561-023-01186-3
• 发表时间: 2023-06-05
• 期刊: NATURE GEOSCIENCE
• 影响因子: 18.3
• 作者: Bangs, Nathan L.;Morgan, Julia K.;Fry, Bill
• 通讯作者: Fry, Bill

Oceanic crust-seismic structure, lithology and the cause of the 2A Event at borehole 504B

洋壳地震结构、岩性及504B钻孔2A事件的成因

• DOI: 10.1093/gji/ggae029
• 发表时间: 2024
• 期刊: Geophysical Journal International
• 影响因子: 2.8
• 作者: Peirce C

Seismicity trends and detachment fault structure at 13°N, Mid-Atlantic Ridge

大西洋中脊北纬13°地震活动趋势和滑脱断层结构

• DOI: 10.1130/g48420.1
• 发表时间: 2020
• 期刊: Geology
• 影响因子: 5.8
• 作者: Parnell-Turner, R.;Sohn, R.A.;Peirce, C.;Reston, T.J.;MacLeod, C.J.;Searle, R.C.;Simão, N.M.
• 通讯作者: Simão, N.M.