Vp/ Vs ratio structure of the Lofoten continental margin, northern Norway, and its geological implications
Vp/ Vs ratio structure of the Lofoten continental margin, northern Norway, and its geological implications
复制标题
DOI：
10.1111/j.1365-246x.1996.tb05634.x
复制
发表时间：
1996-03
影响因子：
2.8
通讯作者：
S. Kodaira;Martin Bellenberg;T. Iwasaki;T. Kanazawa;H. B. Hirschleber;H. Shimamura
中科院分区：
文献类型：
--
作者：
S. Kodaira;Martin Bellenberg;T. Iwasaki;T. Kanazawa;H. B. Hirschleber;H. Shimamura
1988年，利用三分量海底地震仪在挪威北部近海的罗福滕被动大陆边缘进行了广泛的地震折射和广角反射研究。洛福滕边缘是大陆裂谷和海底扩张与广泛的火山作用形成的，负责陆上溢流玄武岩和向海倾斜的反射体(SDR)。罗福滕边缘Vting高原陡坡向海一侧下方的P波结构具有大洋下地壳较厚的特点，在地壳底部存在一个额外的高速层。本文通过对P-S转换横波资料的研究，得到了P波速度(Vp)与S波速度(Vs)之比向下至上地幔的分布。沉积层的波速比为3.0~5.5。这些高比率可归因于松散的沉积物。在上地壳，海洋侧剖面和陆侧剖面的Vp/Vs值有显著差异，即SDRS向海洋1.86，SDRS向陆地1.76。这种差异可能是由于挤压过程的不同而造成的孔隙度的不同：SDRS海洋一侧的上地壳是由海底喷发形成的，而SDRS的陆地一侧是由海底喷发形成的。中地壳和下地壳的波速比估计分别为1.76~1.78和1.80。对比Vp和Vsv，来自下地壳的数据与以前研究的数据清楚地表明了镁铁片麻岩的存在。这一结果为有关被动火山大陆边缘形成的热点假说提供了新的地震证据。即使在无法识别P波到达的地方，也可以在百公里的偏移量处观察到明显的S波上地幔折射。为了模拟这些S波的到达，在莫霍面以下2公里处需要一个VS=4.8公里的S-1层。
SUMMARY In 1988, an extensive seismic refraction and wide-angle reflection study was performed at the Lofoten passive continental margin, off northern Norway, by using three-component ocean bottom seismographs. The Lofoten margin was formed by continental rifting and sea-floor spreading with extensive volcanism, which is responsible for landward flood basalts and seaward-dipping reflectors (SDR). The P-wave structure beneath the seaward side of the Voting Plateau escarpment at the Lofoten margin is characterized by a thick oceanic lower crust and the existence of an additional high-velocity layer at the bottom of the crust. In this study, the distribution of the P-wave velocity (Vp) to S-wave velocity (Vs) ratio down to the uppermost mantle is obtained by studying P-to-S converted shear-wave data. Vp/Vs ratios of 3.0–5.5 are obtained in the sedimentary layers. These high ratios are attributed to unconsolidated sediments. In the upper crust, a significant difference in Vp/Vs ratios is found between oceanic-side profiles and land-side profiles, i.e. 1.86 oceanward of the SDRs and 1.76 landward of the SDRs. This variation may have been caused by a difference in porosity, due to the difference in extrusive processes: the upper crust of the oceanic side of the SDRs was created by submarine eruptions, while the land side of the SDRs was formed by submarian eruptions. The Vp/Vs ratios in the middle crust and lower crust are estimated as 1.76–1.78 and 1.80, respectively. Comparing Vp and VsV, data from the lower crust to those in previous studies clearly indicates the presence of mafic gneisses. This result represents new seismic evidence for the hotspot hypothesis concerning the formation of passive volcanic continental margins. Clear S-wave upper-mantle refractions can be observed at offsets of up to 100 km, even where P-wave arrivals cannot be identified. In order to model these S-wave arrivals, a VS=4.8 km s-1 layer is necessary 2 km below the Moho.