冲绳海槽南段与中、北段构造活动性对比的热模拟研究

Thermal modeling of tectonic activity contrast between the south part and the middle and north parts of the Okinawa trough

受新生代太平洋板块弧后扩张剧烈活动的地缘特性影响,冲绳海槽构造特征复杂,南、中、北段在热液活动、断裂性质、火成岩特性、扩张时代等方面存在显著不同,因此认识该区各段构造活动性对查清其复杂地质特征具有重要意义.本文依据前人通过磁异常反演得到的居里面深度资料,利用热模拟的方法,对冲绳海槽各段深、浅构造活动性进行了探讨.模拟结果表明,南段软流层构造活动强度约为中、北段的6倍,而岩石圈浅层构造活动却相对较弱.该结论与前人所得到的地质地球物理资料相符,主要表现为:相对于中、北段,南段在海槽总体演化历程上裂陷较深;海底火成岩岩浆源区较深,结晶分异程度较弱,同化混染程度较强;切穿沉积基底的大型断裂较为发育,而沉积层内部的小型断裂分布相对稀疏;沉积层岩浆侵入活动较弱,海底所呈现出的热液活动区数量较少;现代地震活动较多,震源深度较大.根据模拟结果与实际资料的对比分析我们可以推测:(1)冲绳海槽北段可能还有一些热液区没有被探测到,也可能在历史演化进程中失去活力,或者被第四系沉积物覆盖;(2)南段存在孕育更多热液活动区的潜力.

Influenced by the strong back-arc extensional effect along the margin of the Pacific plate in the Cenozoic,the tectonics of the Okinawa trough is very complicated and segmented.Each segment differs from each other in hydrothermal activity,fault characteristics,volcanic rocks and expanding periods.Therefore,recognizing the tectonic activity of each segment of the Okinawa Trough is important to clarify its complex geological features.On the basis of Curie depth derived from magnetic anomaly inversion in previous work,this paper used thermal modeling to explore the deep and shallow tectonic activity of the Okinawa trough.The modeling results indicate that the deep tectonic activity intensity in the asthenosphere of the south segment is about six times that of the middle and north segments,while the shallow tectonic activityintensity is relatively weak.This conclusion is in accordance with lots of existing geological and geophysical data.Compared with the middle and northern Okinawa trough,the southern Okinawa trough（1）underwent a deeper rifting;（2）has a deeper magma source for forming seafloor volcanic rocks,a weaker extent of crystallization differentiation,and a stronger extent of assimilation and contamination with continental crust;（3）developed more large faults cutting through sedimentary basements and fewer small fractures in the interior of sediments;（4）produced weaker magmatic intrusions through sediments;（5）developed fewer hydrothermal activity areas in the seafloor;and（6）possesses stronger current seismicity and deeper hypocenters.Based on the comparative analysis of modeling results and real data,we inferred that（1）in the northern Okinawa trough,some hydrothermal activity areas have not been found so far,or have lost energy during the evolutionary process,or have been covered by the Quaternary strata,and（2）the southern Okinawa trough has a potential to develop more hydrothermal activity areas.