印度洋无震海岭及海底高原的初步研究

A PRELIMINARY STUDY ON ASEISMIC RIDGES AND SEA-FLOOR PLATEAU IN THE INDIAN OCEAN

根据1986年10月—1987年5月第三次南极考察和首次环球科学考察所获的印度洋实测重力资料，对印度洋一些典型构造进行了分析研究。初步认为：无震海岭、海底高原和大洋中脊都有着复杂的壳-幔结构，其上都伴有一个布格异常的低值带，但引起这种布格异常低值带的原因却不尽相同。虽然上述3者都是大洋中的隆起地带，但前两者的地壳增厚，莫氏面下拱，软流圈变深，影响布格异常的主要因素是其下存在着一个较大的负山根。相反，在大洋中脊处，地壳变薄，莫氏面上拱，软流圈抬升，影响布格异常的主要因素是其下存在着一个高热、低速、低密度的异常地幔。构造运动剧烈的大洋中脊系，其强大的外力来源为地幔的热动力，它的地壳年龄虽新，但活动历史却很老，并已达到热动力平衡状态，而不是处于均衡状态。

From October, 1986 to May, 1987 Chinese scientists carried out their third Antarctic scientific expedition and first round the world scientific investigation, duringwhich a amount of marine gravity data were obtained by the geophysists from theFirst Institute of Oceanography (FIO), State Oceanic Administration (SOA). Thesedata filled in their data gap in the Atlantic and Indian Ocean. Based on the data aPreliminary study has been made in this paper on the gravity anomalies of the aseismic ridges, sea-floor plateaus and the Mid-Ridge of the Indian Ocean. The resultssuggest that the aseismic ridges, sea -floor plateau and Mid-Ocean Ridge exist witha low value zone of Bouguer anomalies. However the reasons causing the low Bougueranomalies are different. The former two have a crust thickening, a Moho discontinuity arching down and asthenosphere deepening. So the low value zone may be mainly caused by an anti-root lying beneath them. On the contrary in the later case ofthe Mid-Ocean Ridge the crust is becoming thinner, Moho arching up and asthenosphere lifting. So an anomalous mantle with high temperature, low velocity and lowdensity may be main reason of the low Bouguer anomalies. The strong teconic activities of the Mid-Ocean Ridge system seem to be driven by a thermodynamics of themantle. Although the crust of the Mid-Ocean Ridge is new, its activity has a longhistory and has reached to a balance state in thermodynamics. There is almost nodoubt that the free-air anomalies of the low amplitude on the Mid-Ocean Ridge meana lifting of the mantle material with a low velocity and low density, but can notbe seen approximately as a first-order of isostatic anomaly. The Mid-Ocean Ridge cannot be concluded to be in an equilibrium state, either. The studies mentioned abovehave provided with an important basis for the further researches on deep tectogenesis,equilibrium state and dynamic mechanism of the tectonic belts in the Indian Oceanand will be of far reaching importance to the further approach to the formation andevolution of the Indian ocean.