Dual-vergence structure from multiple migration of widely spaced OBSs
Dual-vergence structure from multiple migration of widely spaced OBSs
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DOI：
10.1016/j.tecto.2017.04.005
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发表时间：
2017-10
期刊：
影响因子：
2.9
通讯作者：
S. Yelisetti;G. Spence;M. Scherwath;M. Riedel;D. Klaeschen
中科院分区：
文献类型：
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作者：
S. Yelisetti;G. Spence;M. Scherwath;M. Riedel;D. Klaeschen
利用几台分布广泛的海底地震仪(OBSS)的数据获得了卡斯卡迪亚盆地北部和额脊地区的详细结构。使用镜像成像，其中下行多次波(镜像信号)被迁移，因为它们提供的信息比仅使用主信号成像的区域大得多。具体地说，基尔霍夫时间偏移被应用于水听器和垂直检波器数据。我们的结果表明，在卡斯卡迪亚北部边缘，以前的单通道或多通道地震(MCS)数据中没有观察到的显著构造。结果表明，在这些水深(2.0-2.5公里)内，OBS在其在海底的位置两侧可以成像长达5公里，因此5公里的OBS间距足以提供两次偏移叠加。结果还显示，仅使用一个小型气枪源(120英寸)，就可以在海底下5-6公里处找到火成岩洋壳的顶部。具体地说，OBS偏移结果清楚地显示了反射体的连续性，这使得能够识别前缘逆冲和主逆冲断层。这些断层首次表明，在这一边缘存在着双汇聚结构。到目前为止，这种构造已在约0.5%的现代收敛边缘中观察到，可能与俯冲引起的水平挤压和超压导致的低基面剪应力有关。对该区域以前的MCS数据的重新分析扩大了OBS的迁移结果，并进一步表明，在温哥华岛中部周围，辐合度从向海洋转向向陆地。此外，断裂几何学分析表明，由于断裂和褶皱作用而导致的缩短总量约为3公里，这表明逆冲至少在肯塔基州∼65年前就开始了。
The detailed structure of the northern Cascadia basin and frontal ridge region was obtained using data from several widely spaced ocean bottom seismometers (OBSs). Mirror imaging was used in which the downgoing multiples (mirror signal) are migrated as they provide information about a much larger area than imaging with primary signal alone. Specifically, Kirchhoff time migration was applied to hydrophone and vertical geophone data. Our results indicate remarkable structures that were not observed on the northern Cascadia margin in previous single-channel or multi-channel seismic (MCS) data. Results show that, in these water depths (2.0–2.5 km), an OBS can image up to 5 km on either side of its position on the seafloor and hence an OBS spacing of 5 km is sufficient to provide a two-fold migration stack. Results also show the top of the igneous oceanic crust at 5–6 km beneath the seafloor using only a small airgun source (120 in.3). Specifically, OBS migration results clearly show the continuity of reflectors which enabled the identification of frontal thrusts and a main thrust fault. These faults indicate, for the first time on this margin, the presence of a dual-vergence structure. These kinds of structures have so far been observed in < 0.5% of modern convergent margins and could be related to horizontal compression associated with subduction and low basal shear stress resulting from over-pressure. Reanalysis of previous MCS data from this region augmented the OBS migration results and further suggests that the vergence switches from seaward to landward around central Vancouver Island. Furthermore, fault geometry analyses indicate that the total amount of shortening accommodated due to faulting and folding is about 3 km, which suggest that thrusting would have started at least ∼ 65 ky ago.