Upper mantle structure beneath the eastern Pacific Ocean ridges
Upper mantle structure beneath the eastern Pacific Ocean ridges
复制标题
东太平洋海脊下方的上地幔结构
DOI：
--
复制
发表时间：
2005
期刊：
影响因子：
--
通讯作者：
J. Gaherty
中科院分区：
文献类型：
--
作者：
Y. Gu;S. Webb;A. Lerner;J. Gaherty
[1] 我们分析了海底地震仪在区域和远震距离记录的 10 Mw > 5 次地震的垂直分量体波和面波。通过波形建模，我们对东太平洋海脊下方的温度沿轴变化和部分融化施加了新的限制。由此产生的最佳拟合模型显示，不同山脊段之间的平均岩石圈剪切速度存在超过 9% 的变化。我们证明盖速度与板龄的平方根相关，与传导冷却过程一致，但我们发现接近上升轴对年龄的依赖性更快。我们使用半空间冷却模型将平均板块年龄映射到每个剖面的平均岩石圈温度，并且对于高于和低于~1000°C的温度，通过最小二乘线性拟合确定的温度导数（dVs/dT）分别为-1.1和-0.26 m s−1 deg−1。前一种估计值比早期报告确定的值（−0.4 至 −0.7 m s−1 deg−1）更为负面，该报告使用来自更广泛的海底年龄范围的全球或区域数据，但在年轻年龄时分辨率较低。高 ∣dVs/dT∣ 值可能表明在浅地幔深度存在有限的部分熔融，即使考虑了温度和晶粒尺寸变化引起的滞弹性和非谐性的强烈影响。我们的数据还显示地幔结构存在强烈的南北差异：穿过东太平洋隆起（EPR）南部的表面波经历的剪切速度低至∼3.75 km s−1，比EPR北部和加拉帕戈斯扩张中心下方相当深度的平均地幔结构慢0.2 km s−1以上。这种差异不能用简单的传导冷却过程或脊段之间的扩散速率变化来解释。低速区（LVZ）的慢地震速度似乎需要部分融化。速度差异可能仅仅是由南部 EPR 下方的较高温度引起的，但也可能表明由于与北部相比，熔体生产或提取的潜在差异，南部山脊轴下方的 LVZ 存在更多熔体。此外，部分熔化区的宽度或对称性差异可能会影响这两个部分下方观察到的路径平均速度。最后，我们通过分析体波和面波之间的振幅比来确定海洋变换地震的更准确的深度。这些转变地震的深度与洋壳内的脆性变形一致。
[1] We analyze vertical component body and surface waves for 10 Mw > 5 earthquakes, recorded by ocean bottom seismometers at regional and teleseismic distances. Through waveform modeling, we place new constraints on along-axis variation in temperature and partial melt beneath the eastern Pacific ridges. The resulting best fit models show over 9% variation in average lithosphere shear velocities between different ridge segments. We demonstrate that lid velocity correlates with the square root of plate age, consistent with a conductive cooling process, but we find a more rapid dependence on age close to the rise axis. We map the average plate age into a mean lithospheric temperature for each of our profiles using a half-space cooling model, and the temperature derivatives (dVs/dT) determined from least squares linear fits are −1.1 and −0.26 m s−1 deg−1, respectively, for temperatures above and below ∼1000°C. The former estimate is more negative than values determined by earlier reports (−0.4 to −0.7 m s−1 deg−1), using global or regional data from a much wider range of seafloor age but with less resolution at young ages. The high ∣dVs/dT∣ value may suggest the presence of limited partial melt at shallow mantle depths, even after accounting for the strong effect of anelasticity and anharmonicity resulting from temperature and grain size variations. Our data also show a strong north-south difference in mantle structure: the surface waves that traverse the southern East Pacific Rise (EPR) experience shear velocities as low as ∼3.75 km s−1, more than 0.2 km s−1 slower than the average mantle structure at comparable depths beneath the northern EPR and the Galapagos spreading center. This difference cannot be explained by the simple conductive cooling process or spreading rate variations between ridge segments. The slow seismic speeds in the low-velocity zone (LVZ) appear to require partial melt. The velocity difference might be solely caused by higher temperatures under the southern EPR, but it may also suggest more melt in the LVZ beneath the southern ridge axis due to potential differences in melt production or extraction compared to the north. In addition, differences in width or symmetry of the partial melt zones can affect the observed path-averaged velocities beneath these two segments. Finally, we determine more accurate depths for ocean transform earthquakes by analyzing the amplitude ratios between body and surface waves. The depths of these transform earthquakes are consistent with brittle deformation within the oceanic crust.