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在6万公里长的全球大洋中脊(MOR)系统扩张形成的洋壳覆盖的。随着坚硬的海洋板块以每年10到160毫米的速度被拉开，地球的地幔被从下面拉起，在这样做的过程中部分融化。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，由表面的玄武岩和辉长岩组成，辉长岩是其缓慢冷却的对应物。但是，在过去的15年里，我们已经意识到，在年扩张速度低于40毫米的情况下，这个简单的模型不可能是正确的。相反，大片地幔岩石可能暴露在海底，没有岩浆地壳存在。在像中大西洋海脊(MAR)这样缓慢扩张的MORS上，板块分离可能会部分地发生在巨大的错位上--被称为“拆离”的异常大的地质断层--其上可能有数十公里的伸展。在暴露在海底的地方，这些断层通常形成平坦或微微隆起的表面，在这些表面上暴露着地幔岩石和/或辉长岩。这些结构被称为“海洋核心复合体”(OCC)。我们认为，当岩浆供应减少，海水能够沿着断层穿透并接触到下面的地幔岩石时，OCCS就形成了。这些被称为橄榄岩的岩石主要由矿物橄榄石组成，橄榄石很容易与水反应，产生蛇纹石和滑石等弱矿物，对断层进行润滑，使其继续滑动并发展成长期的滑脱。最近，几位研究人员(包括皮雷斯顿)提出，滑脱断层作用比以前认为的要常见得多，以至于多达一半的大西洋海底可能是由这种构造扩张产生的。他们认为分离是地区性的连续特征，位于海脊轴线一侧的所有海底之下，但只出现在少数地方的表面，OCCS。但超脱断层真的这么普遍吗？通过对MAR的13N区域的详细研究，Co-is MacLeod和Searle得出了截然不同且不那么极端的观点，即支队是不连续的，仅限于单个OCC。它们散布在火山活动、富含岩浆的山脊段之间，并由局部岩浆供应减弱触发。在这个模型中，分离部分被新的岩浆作用断断续续地“杀死”，通常从毗邻的部分横向输送。我们如何区分这些关于海底扩张机制的截然不同的假说？所需要的关键数据是：(1)海脊轴线下滑脱的地下几何形状和范围，(2)岩浆壳的数量和详细分布，以及(3)与OCCS和火山海底有关的扩张率的不对称性(它们在区域上应该相似，但在局部滑脱模式上不同)。我们建议通过对13N地区MAR进行全面的地震和海底磁力调查来获得这些数据，那里的滑脱断层今天在山脊轴线上活动。我们将使用一大批海底地震仪(OBS)，通过地震层析成像技术(类似于医用CT扫描)来成像与不同岩石类型相关的3D速度变化，并进行多通道反射测量，这将像简单的X射线一样成像地下不连续面。然后，我们将离开OBSS(将在稍后的巡航中恢复)来记录该地区自然微震的位置。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Autosub 6000，它将被编程为在我们进行地震实验时制作非常详细的磁场反转(产生海底年龄和扩张率)和海底地形(帮助构造解释)地图。

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.