Seismic imaging of a possible paleoarc in the Izu‐Bonin intraoceanic arc and its implications for arc evolution processes
Seismic imaging of a possible paleoarc in the Izu‐Bonin intraoceanic arc and its implications for arc evolution processes
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DOI：
10.1029/2008gc002073
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发表时间：
2008-10
期刊：
影响因子：
3.7
通讯作者：
S. Kodaira;Takeshi Sato;N. Takahashi;M. Yamashita;T. No;Y. Kaneda
中科院分区：
文献类型：
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作者：
S. Kodaira;Takeshi Sato;N. Takahashi;M. Yamashita;T. No;Y. Kaneda
洋内弧中的地壳演化过程，包括地壳增生和裂谷作用，已经讨论了很长时间。为了研究伊豆-小笠原洋内岛弧的地壳演化，我们在后弧（即，西七户山脊），位于现今火山前缘以西约150公里处。在这项研究中，地震速度和反射率图像是使用广角地震数据。对于地震速度成像，我们应用了折射层析成像，其中使用了93，535个拾取。根据折射层析成像的初始模型计算出的总均方根（rms）失配为483.1 ms，根据最终模型计算出的均方根失配减少到66.7 ms。所得到的地震图像显示出地壳厚度沿着地震剖面的显著变化：北方为薄壳（厚10-15 km），中部为三个离散的厚壳段（厚20-25 km），南部为中厚壳（厚10 - 15 km）。这些变化主要是由于中地壳厚度的变化，地震速度为6.0- 6.8km/s。地壳厚度的这种变化与海底地形无关，海底地形的特点是中新世后的跨弧海山链。它确实与沿着当今伊豆-博宁弧火山前沿观察到的地壳变化有很好的相关性。这些发现表明，形成跨弧海山链的岩浆活动对弧后地壳的影响很小，弧后地壳的主要部分是在弧后与火山前缘分离之前形成的。通过关联沿着后弧的结构变化（即，平均地震速度的变化以及中地壳的厚度）和那些沿着现今火山前缘的变化，我们发现将后弧（古弧）与现今火山前缘分开的裂谷方向是北-东北。
Crustal evolution processes in intraoceanic arcs, including crustal accretion and rifting, have been long discussed. To examine crustal evolution in the Izu‐Bonin intraoceanic island arc, we conducted an active‐source wide‐angle seismic study along a north‐south profile (500 km long) within a possible paleoarc in the rear arc (i.e., the Nishi‐shichito ridge) about 150 km west of the present‐day volcanic front. In this study, the seismic velocity and reflectivity images are obtained using the wide‐angle seismic data. For the seismic velocity imaging, we applied refraction tomography in which 93,535 picks were used. The overall root‐mean‐square (rms) misfit calculated from the initial model of the refraction tomography was 483.1 ms, and those calculated from the final model were reduced to 66.7 ms. The resultant seismic image shows marked variations of crustal thickness along the seismic profile: thin crust (10–15 km thick) in the northern part, three discrete thick crustal segments (20–25 km thick) in the central part, and a moderately thick crust (∼15 km thick) in the southern part. These variations are mainly attributed to thickness variations of the middle crust having seismic velocity of 6.0–6.8 km/s. This variation of crustal thickness does not correlate with seafloor topography, which is characterized by post‐Miocene across‐arc seamount chains. It does correlate well with crustal variations observed along the present‐day volcanic front of the Izu‐Bonin arc. These findings suggest that the magmatic activity that created the across‐arc seamount chains had little effect on the rear‐arc crust and that the main part of the rear‐arc crust was created before the rear arc separated from the volcanic front. By correlating the structural variations along the rear arc (i.e., the variation of the average seismic velocity as well as the thickness of the middle crust) and those along the present‐day volcanic front, we found that the direction of rifting to separate the rear arc (paleoarc) from the present‐day volcanic front was north‐northeast.