Crustal accretion and transform margin evolution at ultraslow spreading rates

世界海洋下面的地壳是海底扩张的结果--这是一个过程，坚硬的海洋板块以快速(&gt；100毫米/年)、中等(100-55毫米/年)或缓慢(55-20毫米/年)的速度分裂。随着板块分离，下面的地幔上升以填补缺口，并在这样做的同时，由于较低的压力而融化。这些熔岩或岩浆凝固形成约6-8公里厚的大洋地壳，由顶部一层喷发并迅速冷却的岩浆(玄武岩)和一层缓慢冷却的岩浆(辉长岩)组成。在过去的十年里，观察表明，在大洋板块以较慢的速度撕裂时产生的地壳，并不是像我们目前的模型预测的那样由对称的岩浆构造过程形成的，而是穿插着明显的岩浆匮乏时期。在这些岩浆阶段，板块分离被大错动的断层所适应，沿着这些断层，来自下地壳和下地幔的岩石被带到地表。这些下地壳和上地幔岩石出土的区域被称为海洋核心杂岩(OCCS)。大约25%的地球大洋中脊以非常缓慢的速度展开，每年不到20毫米。然而，这些超低海脊大多位于偏远地区，恶劣的天气或冰盖阻碍了他们的调查。因此，在这些最慢的扩张中心，地壳是如何形成和年龄的，目前的模型预测，这些中心应该主要是岩浆匮乏和寒冷的，仍然知之甚少。最近对加勒比超低开曼中部扩张中心进行的海底成像和取样研究，观测到了地球上最深和最热的黑烟热液喷口，以及下地壳和上地幔出土区域与“正常”上洋壳火山喷发的岩石并列在一起。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，我们将确定岩浆建造和岩浆伸展之间的时间和空间相互作用，以及对断裂和热液活动的控制，以及它们之间的关系。作为英国、德国和美国伙伴关系的一部分，我们将使用海底下地震成像来研究MCSC上登特山OCC的地壳结构和岩性，并确定这一区域与同样拥有热液喷口的邻近火山区之间的关系。我们还将研究地壳如何在冷却和老化时发生变化，以及如何在远离脊轴的情况下扩散。利用当地地震的模式，我们将绘制海底以下的断层几何图形，以及这些断层是否在深度上相连。由于MCSC的南端也紧靠着横跨天鹅岛转换带的加勒比海板块的大陆地壳，这也提供了一个独特的机会，不仅可以确定地幔是如何在海脊下上升和熔融的，这种熔体是如何沿海脊分布的，而且这一过程是否受到邻近厚而冷的大陆岩石圈冷却作用的阻碍。为了实现我们的目标，我们将在海床上部署海底地震仪(OBSS)，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，以及不同岩石类型之间的界面。我们还将使用OBSS来记录故障移动时自然出现的信号。我们将测量重力场，以确定地壳密度，作为对我们的地震模型的测试，并将图像更深地成像到地幔的深度，超过该深度，我们的地震信号将穿透。最后，我们将测量磁场的反转，以揭示海底扩张速度和地壳年龄，并结合地震数据确定从当前时间到至少2000万年前的岩浆伸展阶段发生的频率。

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.