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

地球表面的三分之二是由海底扩张形成的海洋地壳铺成的，这些海洋地壳是在6万公里长的全球洋中脊（MOR）系统中形成的。随着坚硬的海洋板块以每年<10到160毫米不等的速度被拉开，地球的地幔被从下方拉出，在此过程中部分融化。熔体从地幔中分离出来，上升到地表，形成一个连续的“岩浆”地壳层，通常约6公里厚，由表面的玄武岩和下面缓慢冷却的辉长岩组成。然而，在过去的15年里，我们已经意识到，当扩散速度低于40毫米/年时，这个简单的模型就不可能是正确的。相反，大片的地幔岩石可能暴露在海底，没有岩浆地壳存在。在缓慢扩张的MORs上，如大西洋中脊（MAR）上的板块分离，可能部分是由于巨大的错位——被称为“分离”的异常大的地质断层——在这些断层上可能容纳了数十公里的延伸。当这些断层暴露在海底时，通常会形成平坦或平缓的圆顶状表面，在这些表面上暴露出地幔岩石和/或辉长岩。这些结构被称为“海洋核心复合物”（OCCs）。我们认为OCCs是在岩浆供应减少，海水能够穿透断层进入下面的地幔岩石时形成的。这些岩石被称为“橄榄岩”，主要由矿物橄榄石组成，橄榄石很容易与水反应，产生弱矿物蛇纹石和滑石，润滑断层，使其继续滑动，并发展成一个长期存在的分离体。最近，一些工作人员（包括PI Reston）提出，剥离断裂比以前认为的要普遍得多，以至于多达一半的大西洋海底可能是由这种“构造”扩张形成的。他们认为分离体是区域连续的特征，位于脊轴一侧的所有海底之下，但只在少数地方出现在海面上，即OCCs。但滑脱断层真的如此普遍吗？通过对北冰洋13N区域的详细研究，mcleod和Searle得出了一个完全不同的、不那么极端的观点，即支队是不连续的，局限于单个OCCs。它们散布在火山活跃、岩浆丰富的脊段之间，由岩浆供应的局部减弱引发。在这个模型中，分离体偶尔会被新的岩浆活动“杀死”，这些岩浆活动通常是从相邻的部分横向传递过来的。我们如何区分这些关于海底扩张机制的非常不同的假设？所需的关键数据有：(1)脊轴下分离的次地表几何形状和范围；(2)岩浆地壳的数量和详细分布；(3)与OCCs和火山海底相关的扩张速率的不对称性（它们在区域内应该相似，在局部分离模型中应该不同）。我们建议在13N地区的MAR进行综合地震和海底磁调查，该地区今天在脊轴上有活跃的拆离断层。我们将使用大型海底地震仪（OBSs）阵列，使用“地震断层扫描”（类似于医学CT扫描）来成像与不同岩石类型相关的3D速度变化，并进行多通道反射调查，这将成像地下不连续性-就像简单的x射线一样。然后，我们将离开海底地震仪（将在稍后的巡航中恢复），记录该地区自然微地震的位置。这些将直接显示活动断层的三维几何形状和联系。最后，我们将部署自动机器人车辆Autosub 6000，当我们进行地震实验时，它将被编程为制作非常详细的磁场反转地图（产生海底年龄和扩张速度）和海底地形（帮助结构解释）。

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.