Crustal structure of North Atlantic Fracture Zones
Crustal structure of North Atlantic Fracture Zones
复制标题
北大西洋断裂带的地壳结构
DOI：
10.1029/93rg01952
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发表时间：
1993
影响因子：
25.2
通讯作者：
G. Purdy
中科院分区：
文献类型：
--
作者：
R. Detrick;R. White;G. Purdy
地震研究已经确定，沿着缓慢扩张的中大西洋海岭的大偏移距转换显示出异常的地壳结构，这些结构远远超出了通常与洋壳有关的范围。在地震方面，北大西洋的断裂带地壳在厚度和内部结构上都极为不均匀。它通常非常薄（厚度<1-2公里），其特征是压缩波速度低，没有正常的地震层3。一个更渐进的地壳变薄可以从这些断裂带延伸到几十公里。在沿中大西洋海脊沿着较小的海脊轴间断处进行的地震和重力研究也推断地壳异常薄。在大西洋断裂带内发现的地震异常地壳的地质性质，以及这种地壳是如何形成的，仍然存在争议。与现有地震观测结果一致的一种解释是，北大西洋断裂带内的地壳由一个薄的、强烈断裂的、热液蚀变的玄武质部分组成，该部分覆盖着在某些地方被广泛蛇纹化的超镁铁岩。地震地壳视厚度沿着断裂带的变化可能反映了上地幔剖面不同程度的蛇纹化或岩浆地壳厚度的变化。断裂带中存在较薄的地壳部分，可以解释为由于分段扩张中心下方的上涌的三维性质，在靠近山脊偏移的广阔区域内岩浆供应减少，以及由于大规模拆离断层优先形成于靠近段端部的冷脆性岩石圈中的地壳构造肢解。
Seismic studies have established that large-offset transforms along the slow spreading Mid-Atlantic Ridge exhibit anomalous crustal structures that fall well outside the range typically associated with oceanic crust. Seismically, fracture zone crust in the North Atlantic is extremely heterogeneous in both thickness and internal structure. It is frequently quite thin (<1–2 km thick) and is characterized by low compressional wave velocities and the absence of a normal seismic layer 3. A more gradual crustal thinning can extend up to several tens of kilometers from these fracture zones. Anomalously thin crust has also been inferred from both seismic and gravity studies at smaller ridge axis discontinuities along the Mid-Atlantic Ridge. The geological nature of the seismically anomalous crust found within Atlantic fracture zones, and how this crust forms, are still controversial. One interpretation consistent with available seismic observations is that the crust within North Atlantic fracture zones consists of a thin, intensely fractured, and hydrothermally altered basaltic section overlying ultramafics that are extensively serpentinized in places. Variations in apparent seismic crustal thickness along fracture zones may reflect different degrees of serpentinization of the upper mantle section or changes in the thickness of the igneous crust. The existence of a thinner crustal section in fracture zones can be explained by a reduced magma supply within a broad region near ridge offsets due to the three-dimensional nature of upwelling beneath a segmented spreading center and by tectonic dismemberment of the crust by large-scale detachment faults that form preferentially in the cold, brittle lithosphere near the ends of segments.