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

地球表面的三分之二是由60 000公里长的全球洋中脊（莫尔）系统的海底扩张形成的洋壳铺成的。当坚硬的海洋板块以每年小于10毫米到160毫米的速度被拉开时，地球的地幔从下面被拉上来，部分地融化。熔融物从地幔中分离出来，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，我们逐渐认识到，在扩张速度低于40毫米/年的情况下，这个简单的模型是不正确的。相反，大片的地幔岩可能暴露在海底，没有岩浆地壳存在。板块在缓慢扩张的MORs上的分离，如大西洋中脊（MAR），可能部分地被大位错--被称为“地错”的异常大的地质断层--所吸收，在这些断层上可以容纳数十公里的延伸。在海底暴露的地方，这些断层通常形成平坦或轻微的圆顶表面，地幔岩和/或辉长岩暴露在其上。这些结构被称为“海洋核心复合物”（ocean core complex，OCC）。我们认为，当岩浆供应减少，海水能够渗透到断层并进入下面的地幔岩石时，OCC就形成了。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，并发展成为一个长期的剥离。（包括PI雷斯顿）提出，剥离断层比以前想象的要普遍得多，在某种程度上，多达一半的大西洋海底可能是由这种“构造”扩张产生的。他们认为深冲是区域性的连续特征，位于脊轴一侧的所有海底之下，但只在少数地方出现在表面，即OCC。但断层剥离真的如此普遍吗？通过对MAR的13 N区域的详细研究，Co-Is麦克劳德和Searle得出了完全不同的观点，但远没有那么极端，他们认为Determination是不连续的，并局限于个别OCC。它们散布在火山活跃、富含岩浆的山脊段之间，并由岩浆供应的局部减弱引发。在这个模型中，碎屑岩被更新的岩浆作用所“杀死”，这些岩浆作用通常是从相邻的板块横向传递过来的。我们如何区分这些关于海底扩张机制的非常不同的假设？所需的关键数据是：（1）洋脊轴下的次表层几何形状和伸展范围;（2）岩浆壳的数量和详细分布;与OCC和火山海底有关的扩张速率的不对称性（它们应在区域上相似，但在局部分离模型上不同）我们建议在13 N区域的MAR进行全面的地震和海底磁力调查时获得这些数据，该区域的滑脱断层在今天的山脊轴线上活动。我们将使用一个大型海底地震仪阵列（OBS），使用“地震层析成像”（类似于医学CT扫描）对与不同岩石类型相关的3D速度变化进行成像，并进行多通道反射调查，这将像简单的X射线一样对地下不连续性进行成像。然后，我们将留下OBS（将在稍后的巡航中恢复），以记录该地区天然微地震的位置。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，它将被编程为在我们进行地震实验的同时绘制非常详细的磁场反转（产生海底年龄和扩展速率）和海底地形（帮助结构解释）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.