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

本项目的目的是了解西南印度脊（一个超低速扩张系统）的海底扩张构造、火山活动、地震活动和热液活动。1997年10月7日至11月3日在西南山脊东部进行了富士巡航（法国-英国-日本山脊间巡航）。测量的主要部分是TOBI深拖曳边扫描声纳观测（30kHz， 6km波段）和海底地震仪观测地壳结构和地震活动。选取两个一阶段，每段长度为200 km，进行沿轴牵引。深拖箱测量设一个100km*100km箱，包括离轴部分。TOBI侧面扫描图像较好地显示了火山特征和断层分布的细节。富士邮轮的主要结果如下。(1)地壳非均质性较强，岩浆活动集中在中段。(2)发现了一条长70 km、构造复杂的非火山段，但对该段扩张机制的认识仍需进一步深入研究。(3)某一段的北翼为火山构造，南翼为火山构造较弱，两段的海底特征对比强烈。(4)大型分离断裂在该区扩张构造中起主要作用。(5) OBS在该地区最大的一段观测到强烈的集中地震活动。

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.