南海西南海盆壳幔结构重力反演与热模拟分析

Gravity inversion and thermal modeling about the crust-mantle structure of Southwest basin in the South China Sea

壳幔结构及扩张期后的岩浆活动是研究南海西南海盆形成演化的关键.本文针对NH973-1剖面开展壳幔密度结构重力反演,并依据重力反演的壳幔模型,定量模拟海底扩张期后的壳幔热结构与热演化过程.重力反演表明:西南海盆中央残余扩张脊之下存在一个较深的凹陷带,其下Moho面比两侧略深,呈现扩张期后的热沉降特点.热模拟发现:海盆扩张终止后,壳幔并非均衡一致降温,而是以"地壳增温-地幔降温"方式进入热沉降阶段,海底扩张终结后,地壳继续增温约7 Ma之后才与地幔一同缓慢降温,同步进入后期热沉降.沿NH973-1剖面南、中、北段的地壳增温特点不同,在海盆扩张中心北侧约70 km之下7～9 km深度处,在9.6～4.6 Ma期间温度增幅一直保持在200℃以上,将该处地壳温度抬升至1100℃以上,具备了产生扩张期后岩浆熔融的温度条件.P-T图解分析也支持此期间如果地壳具备局部构造断裂形成的含水条件和减压条件,就可能因部分熔融产生岩浆活动.

The crust-mantle structure and magmatic activity is one of the key problems to study the formation and evolution of Southwest Sub-basin of South China Sea. According to gravity data supplied by the NH973-1 profile, we get the density model of crust-mantle construction with gravity inversion method. And then, we learn the temperature structure and thermal evolution consequence after sea floor spreading. Gravity inversion indicated that there is a deeper sunken sedimentation area and a depressed Moho surface region under the SW sub-basin center. Thermal modeling shows that temperature variation is not a consistent degradation between crust and mantle, but in the way of crust warming with mantle cooling. After the termination of seafloor spreading, the crust warming lasted for about 7 Ma, followed by slow cooling, together with the mantle. Then, crust thermal evolution mode was changed to a new way that its temperature declines as same as mantle. In this way, crust and mantle entered the stage of thermal subsidence synchronously. Along profile NH973-1, the characteristics of crustal warming are differentamong the south, middle and north part. At the depth of 7~9 km under the place about 70 km to the north of oceanic basin spreading center, the rate of temperature increase sustained above 200~C between 9.6 Ma and 4.6 Ma. This appending temperature would drive up the local temperature value to 1100~C which is of advantage to crust magmatic activity in the end time of seabed spreading. Based on the P-T diagram, we can know that magmatic partial melting would be likely to occur in this time if there was wet system and decompressing condition in crust.