洋中脊分段性及其拓展和叠接机制

SEGMENTATION, PROPAGATION AND OVERLAPPING AT MID-OCEANIC RIDGE

各种扩张速率下的洋中脊被转换断层和非转换断层分成许多段(长10到100km不等),而且这种岩浆活动和构造的分段特性在大西洋中脊表现特别明显。洋脊分段特征可以划分为4级。其中,转换断层是1级间断,其错断洋脊距离大于30km,其长度可达1000km左右,存在寿命可达10Ma。相邻转换断层之间的距离间隔也与扩张速率有关:扩张速率最慢处的距离间隔小于200km,中速—快速扩张脊为600～1000km,扩张速率大于140mm a的脊段上未发现转换断层,总体上转换断层间距随扩张速率而增加。叠接拓展中心、斜向剪切带、火山间隔和横向断错等分别为2～4级间断,出现在两条转换断层之间,使洋中脊错断距离逐渐减少。2～4级区段的洋中脊长度也越来越小,存在的寿命也越来越短,直至4级区段的洋中脊长度一般小于10km,存在寿命为102～104a。这种分段性无论在超快速、快速、中速、慢速扩张脊,还是超慢速扩张脊都存在,其分段机制都与洋中脊拓展、叠接、跃迁(或废弃)、死亡过程密切相关,而拓展、叠接过程又受多种动力要素控制。正是洋中脊分段的动力机制控制了中央裂谷的存在与否。

The mid-oceanic ridges with different spreading velocities are segmented into 10 to 100 km long by transform faults and non-transform faults. This segmentation of magmatism and structure appears evidently in the Atlantic mid-oceanic ridge. The segmentation can be divided into four scales. The first-scale offset is transform fault which is about 1 000 km long and 10 Ma in age, offsetting the ridge up to 30 km. The spacing between the adjacent transform faults is related to spreading velocity. The spacing in the segment of slow spreading velocity is less than 200 km. That in the segment of medium-fast spreading velocity is from 600 to 1 000 km. However, there is no transform fault in the segment with more than 140 mm/a spreading velocity. The spacing of transform fault increases with the increase of the velocity. The overlapping center, the oblique shear zone, the volcanic offset or the transverse offset are the second-order to fourth-order offset respectively, being constrained between two transform faults and resulting in the progressive decrease of offset-ridge spacing. The lower-order segment becomes shorter. The existing period in the lower-order segment is also shorter. The fourth-order segment is less than 10 km long and 10^(2)～10^(4 )a in age. The segmentation can occur in any ridge segment of super-fast, fast, medium, slow and super-slow velocity. The segmentation mechanism is closely related to ridge propagation, overlapping, flipping or abandonment, death, which also controls if the mid-ridge rift exists. The propagating and overlapping processes depend on many dynamic factors.