Studies of the crustal formation and tectonics at ultra-slow spreading mid-oceanic ridges

该项目的目标是了解西南印度海脊的海底扩张、构造、火山活动、地震活动和热液活动，这是一个超低扩散系统。1997年10月7日至11月3日，富士邮轮(法国-英国-日本国际山脊邮轮)在西南山脊东部进行。测量的主要部分是TOBI深拖侧向扫描Sonnar观测(30 kHz，6千米条带)和海底地震仪观测地壳结构和地震活动。选择了两条各长200公里的一级节段进行顺轴拖曳。设置了一个100千米×100千米箱，用于包括离岸部分的深拖箱测量。TOBI侧扫图像很好地显示了火山特征和断层分布的细节。富士邮轮的主要成果如下。(1)岩浆活动集中在地段中央，反映了地壳的强烈非均质性。(2)发现了一条长约70公里、断裂样式复杂的非火山岩段，但对该段扩张机制的认识还有待进一步研究。(3)北翼为火山构造，南翼为火山构造较少的地段，其外轴部海底地貌对比强烈。(4)观察到大断裂，表明这类断裂对本区的伸展构造起主要作用。(5)在该地区最大的地段之一，观测到强烈的震源地震活动。

The objective of this program is to understand the sea floor spreadingtectonics, volcanism, earthquake activities, and hydrothermal activites of the Southwestern Indian Ridge, an ultraslow spreding system. The FUJI cruise (French-UK-Japan InterRidge cruise) was conducted from October 7 to November 3, 1997 at the eastern part of the Southwestern Ridge. The main parts of the measurements are TOBI deep-tow sidescan sonnar observation (30kHz, 6km swath) and the ocean bottom seismometer observation for the crustal structure and earthquake activities. Two first order segments with the length of 200 km each were selected for along axes towing. One 100km*100km box was set for the deep-tow box survey including offaxis part. The TOBI sidescan image well displayed the detail of volcanic features and fault distribution. The principal results of the FUJI cruise are as follows. (1) Intensive heterogeneity of the crusts were suggested by focused magmatism in the center of the segment. (2) A 70 km long non volcanic segment with a complex fault pattern was identified, although understanding of spreading mechanism of this part needs more survey in this area. (3) Strong contrast of sea bottom features between the offaxes parts of a segment is observed as the northern flank with volcanic construction and as the southern flank with pausity of volcanic construction. (4) Large dtachmen faults are observed suggesitng these type of faults have principal role on the spreading tectonics here. (5) Intensive focused seismic activity was observed y OBS at one of the largest segemts in the area.