西北印度洋脊的厘定及其地质构造特征

西北印度洋的洋脊系统目前以"中印度洋脊"和"卡尔斯伯格脊"分别指示南北两段,两者的分界点被认为是澳大利亚板块与印度板块的板块边界与洋脊的交点,但具体分布位置不明确.基于已有的地质、地球物理和地球化学等多方面特征,认为卡尔斯伯格脊和中印度洋脊可以统一称为"西北印度洋脊",从罗德里格斯三联点一直延伸到欧文断裂带.新的洋脊厘定将有助于更全面地了解整个西北印度洋的洋脊演化和地球动力学过程.西北印度洋脊地形上南北两端断裂较少,中间断层密集,形似吸管的弯折部位,调节洋脊的转向.重力异常显示沿脊轴方向两端高中间低的特征,表明两端岩浆供给相对充足,而中间断层密集区岩浆量少.磁异常特征显示清晰的分带性,指示多阶段的洋脊扩张历史.岩石地球化学特征显示南北两个同位素相对富集洋脊段,可能与热点作用相关,或与残留岩石圈或地壳物质对亏损软流圈地幔的富集改造有关.

Definition of Northwest Indian Ridge and Its Geologic and Tectonic Signatures

The existing nomenclatures for the ridge system of Northwest Indian Ocean include Carlsberg Ridge and Central Indian Ridge from north to south. The demarcation point between them is thought to be the intersection of Australia-Indian plate boundary and the ridge axis, which is yet poorly constrained. Based on the current knowledge of geological, geophysical and geochemical investigation and research results of the whole ridge segment, a new terminology for the ridge system of Northwest Indian Ocean is proposed, i.e. the Northwest Indian Ridge (NWIR). The whole ridge extends from Rodrigues Triple Junction to Owen transform fault, which includes Carlsberg Ridge and Central Indian Ridge, no matter where the boundary is. The new term may promote the thorough recognition of evolution history of Northwest Indian Ridge, as well as its geodynamic process. The Northwest Indian Ridge is especially full of faults in the middle segment but less on both ends, just like the shape of a straw and its bend is an analog for the turning point of the ridge. The gravity anomaly shows that the magma supplies at both ends are relatively abundant, while the magma in the middle part is rather limited. The magnetic anomaly in the Northwest Indian Ocean is zonated and indicates a multi-stage ridge spreading processes. Besides, the most prominent feature in petrogeochemistry is the presence of relatively isotopically enriched ridge basalts from two ridge segments, which may be related to the hotspot intervention and/or contamination of depleted asthenosphere by residual lithosphere or crustal materials.