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Inconsistent correlation of seismic layer 2a and lava layer thickness in oceanic crust

机译：大洋地壳中2a地震层与熔岩层厚度的相关性不一致

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At mid-ocean ridges with fast to intermediate spreading rates, the upper section of oceanic crust is composed of lavas overlying a sheeted dyke complex. These units are formed by dykes intruding into rocks overlying a magma chamber, with lavas erupting at the ocean floor. Seismic reflection data acquired over young oceanic crust commonly image a reflector known as 'layer 2A', which is typically interpreted as defining the geologic boundary between lavas and dykes. An alternative hypothesis is that the reflector is associated with an alteration boundary within the lava unit. Many studies have used mapped variability in layer 2A thickness to make inferences regarding the geology of the oceanic crust, including volcanic construction, dyke intrusion and faulting. However, there has been no link between the geologic and seis-mological structure of oceanic crust except at a few deep drill holes. Here we show that, although the layer 2A reflector is imaged near the top of the sheeted dyke complex at fast-spreading crust located adjacent to the Hess Deep rift, it is imaged significantly above the sheeted dykes section at intermediate-spreading crust located near the Blanco transform fault. Although the lavas and underlying transition zone thicknesses differ by about a factor of two, the shallow seismic structure is remarkably similar at the two locations. This implies that seismic layer 2A cannot be used reliably to map the boundary between lavas and dykes in young oceanic crust. Instead we argue that the seismic layer 2A reflector corresponds to an alteration boundary that can be located either within the lava section or near the top of the sheeted dyke complex of oceanic crust.
机译：在洋中脊，扩散速度快到中等，洋壳上部由熔岩覆盖，覆盖在成片的堤防复合体上。这些单元是由堤坝侵入岩浆室上方的岩石中形成的，熔岩在海底喷发。在年轻的洋壳上采集的地震反射数据通常为反射层成像，称为“ 2A层”，通常被解释为定义熔岩和堤坝之间的地质边界。另一种假设是，反射器与熔岩单元内的蚀变边界相关。许多研究已经使用2A层厚度的映射变异性来推断大洋地壳的地质情况，包括火山构造，堤防侵入和断层。但是，除了几个深孔，大洋地壳的地质结构和地震学结构之间没有联系。在这里，我们显示，尽管2A层反射器成像在靠近Hess Deep裂谷的快速扩散地壳的片状堤复合体顶部附近，但它显着成像在靠近Hess深裂谷的片状堤部分上方。布兰科变换故障。尽管熔岩和下层过渡带厚度相差约两倍，但浅地震结构在这两个位置非常相似。这意味着地震层2A无法可靠地用于绘制年轻洋壳中熔岩和堤坝之间的边界。取而代之的是，我们认为地震层2A的反射层对应于蚀变边界，该边界可以位于熔岩剖面内，也可以位于大洋地壳薄片状堤复杂的顶部附近。

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