Anomalous seismic crustal structure of oceanic fracture zones
Anomalous seismic crustal structure of oceanic fracture zones
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DOI：
10.1111/j.1365-246x.1984.tb02868.x
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发表时间：
1984-12
影响因子：
2.8
通讯作者：
R. White;R. Detrick;M. Sinha;M. Cormier
中科院分区：
文献类型：
--
作者：
R. White;R. Detrick;M. Sinha;M. Cormier
总结。在断裂带内发现的地壳地震结构不在北大西洋正常洋壳观察到的速度结构范围之内。断裂带的地壳通常很薄，其特点是地壳速度低，明显缺乏具有大洋第三层典型速度的折射波。在大偏移距和小偏移距的断裂带中都存在异常地壳。由于它们是洋盆中最常见的构造特征之一，而且在缓慢扩展的海脊上分布得特别紧密，因此断裂带是地震地壳不均质性的主要来源。我们将异常地壳解释为覆盖在超基性岩上的薄的、强烈破裂、断层和热液蚀变的玄武岩和辉长岩部分，在某些地方，广泛的蛇纹岩作用。在断裂带内发现的异常薄的地壳和断裂带两侧数十公里范围内地壳的逐渐减薄可以用两个主要过程来解释：首先，在山脊变形交界处与扩张中心相对的冷岩石圈边缘改变了扩张中心的正常侵入和挤压过程，导致异常而薄的火成岩段的吸积；其次，每个扩张脊段来自单独的地壳下岩浆供应点，因此当岩浆沿扩张中心横向向下流动时，它会产生一个厚度递减的地壳段，最终形成山脊段两端断裂带的非常薄的地壳。
Summary. The seismic structure of crust found within fracture zones falls outside the range of velocity structures observed for normal oceanic crust in the North Atlantic. The crust in fracture zones is frequently very thin and is characterized by low crustal velocities and by the conspicuous absence of a refractor with a velocity typical of oceanic layer 3. Anomalous crust is present in both large- and small-offset fracture zones. Since they are among the most common tectonic features in the ocean basins, and are particularly closely spaced on slow-spreading ridges, fracture zones represent a major source of seismic crustal heterogeneity. We interpret the anomalous crust as a thin, intensely fractured, faulted and hydrothermally altered basaltic and gabbroic section overlying ultramafics that, in places, are extensively serpentinized. The unusually thin crust found within fracture zones and the gradual crustal thinning over a distance of several tens of kilometres on either side of the fracture zones can be explained by two main processes; firstly the cold lithosphere edge opposite the spreading centre at the ridgetransform intersection modifies the normal intrusive and extrusive processes of the spreading centre leading to the accretion of an anomalous and thin igneous section; and secondly each spreading ridge segment is fed from a separate subcrustal magma supply point, so as the magma flows laterally down the spreading centre it generates a crustal section of decreasing thickness, culminating in the very thin crust of the fracture zones at either end of the ridge segment.