Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48′N, using seafloor compliance measurements
Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9°48′N, using seafloor compliance measurements
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DOI：
10.1029/1998jb900087
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发表时间：
1999-02-10
影响因子：
3.9
通讯作者：
Hildebrand, JA
中科院分区：
文献类型：
--
作者：
Crawford, WC;Webb, SC;Hildebrand, JA
横跨东太平洋隆起(北纬9°48‘)的海底柔度测量显示，整个地壳和壳幔交界处的剪切速度都很低，最低的剪切速度集中在隆起轴以下。柔度法利用长波海浪作用下的海底形变探测洋壳。作为频率函数的柔度函数的形状主要由地壳内的低剪切速度区域控制。在北纬9°48‘处，位于海底1.4公里处的浅层同轴熔融透镜的剪切速度小于20m/S，表明该位置的熔融透镜是完全熔融的，而不是连通的晶体碎屑。顺应性数据还要求在海底以下5.4+/-1千米处有第二个剪切速度低于50m/S的轴向熔融透镜。这种“深”熔体可能是在壳幔界面的渗透性或密度势垒上熔融形成的。两个熔体透镜之间的下地壳剪切速度平均为1.7公里/S，表明熔融程度为2.5-18%。熔体在下地壳离轴线至少10公里处持续存在，下地壳低速带的顶部大约在海底以下4公里处。在2B层中，横纵速度比由轴上的0.41增加到离轴10公里处的0.58，表明在轴线上有大量的薄层裂隙，这些裂隙靠近隆起轴线。2B层中较高的轴向孔隙度可以使水热循环向下循环到浅层熔融透镜附近。
Seafloor compliance measurements across the East Pacific Rise at 9 degrees 48'N reveal low shear velocities throughout the crust and at the crust-mantle boundary, with the lowest shear velocities centered beneath the rise axis. The compliance method uses the seafloor deformation under the loading of long wavelength ocean waves to probe the oceanic crust. The shape of the compliance function as a function of frequency is primarily controlled by regions of low shear velocity within the crust. At 9 degrees 48'N, the shear velocity is less than 20 m/s in the shallow on-axis melt lens located 1.4 km beneath the seafloor, demonstrating that the melt lens at this site is fully melt rather than a connected crystal mush. The compliance data also require a second on-axis melt lens 5.4 +/- 1 km beneath the seafloor with shear velocities slower than 50 m/s. This "deep" melt lens may be created by melt pooling at a permeability or density barrier at the crust-mantle interface. The shear velocity in the lower crust between the two melt lenses averages 1.7 km/s, indicating 2.5-18% melt. Melt persists in the lower crust to at least 10 km off-axis, where the top of the lower crustal low-velocity zone is approximately 4 km beneath the seafloor. In seismic layer 2B, the ratio of shear to compressional velocity increases from 0.41 on-axis to 0.58 by 10 km off-axis, indicating that there are abundant thin cracks in the sheeted dikes on-axis and that these cracks close away from the rise axis. High on-axis porosity in layer 2B may allow hydrothermal circulation down to near the shallow melt lens.