南沙海槽前陆盆地是我国南海南缘陆架区重要的含油气盆地,海槽之下陆壳减薄的原因、前陆区逆冲推覆构造的变形机制是南海地球动力学研究的重要科学问题.利用地震、重磁、地热观测资料,依据地震沉积地层分析、重磁反演分析、地幔流应力场分析、热-流变学分析方法,文中计算了南沙海区地壳结构特征、南沙海槽逆冲推覆热-流变学结构.结果表明:南沙海区Moho面深度在18～26 km,其中海槽区Moho面最浅,由海槽中心向东南至陆坡,Moho面由20 km快速下降到26 km深度,说明南沙海区陆壳结构曾发生过强烈的构造变动.南沙海区地壳累积流变强度Fc与岩石圈累积流变强度FL之比小于80％,显示为一个整体陆壳地块,岛礁区大部分地段地壳热流QC与海底热流Q0之比大于60％,为“热壳冷幔”型热结构,而海槽区情况正相反,QC/Q0小于40％,为“冷壳热幔”型热结构.南沙海槽Moho面温度在300～700℃,地壳整体温度较低,地温梯度在垂向上高、低相间成层分布,地壳浅层地温梯度在15～30℃/km,深层地温梯度大于45℃/km.南沙海槽南北两侧应力分布特征不同,北侧挤压,南侧伸展.北侧挤压区,地层挤压收缩量由深向浅减小,南侧伸展区,地层伸展量由深向浅增大,类似手风琴风箱结构.北侧黏滞系数高、流变强度大,南侧黏滞系数低、流变强度小.南侧的黏滞系数、流变强度大约比北侧低2～3个数量级,因此南沙海槽南侧比北侧更容易发生构造变形.由计算结果推测,南沙Moho面起伏或陆壳减薄与“地壳重力均衡作用”和“地幔热隆升作用”有关,海槽东南缘逆冲推覆体构造变形机制主要是“地壳缩短”作用,其次是“重力滑脱”作用.文中没有涉及南沙陆块不同地质时期Moho面、“地壳均衡”、“地幔热隆升”之间的演化关系,也没有涉及南沙海槽基底变形中“弹性挠曲”和“逆冲推覆”之间的关系.%The Nansha trough is a critical foreland oil-gas basin located in the deepwater area to the south of the South China Sea.The mechanism of continental crust thinning and deformation of the deepwater fold-thrust tectonics in the Nansha trough has become an important research subject in geothermics and geodynamics studies.Based on seismic,gravity,magnetic and heat flow data,this paper analyzes the sedimentary strata structure,Moho depth,mantle-flow stress field and therrnal-rheologieal structure of the Nansha trough basin.The results show that the Moho depth is between 18 and 26 km in the Nansha sea area,with the smallest Moho depth lying on the groove center that quickly falls from the trough center southeast to the continental slope by 20-26 km.One possible explanation for this observation is that there had been strong crustal tectonic movement in this region in the geological era.The continental crust appears to be one whole piece with a Fc/FL.ratio (Fc-crust rheological strength,FL lithosphere rheological strength) of less than 80％ in the South China Sea area.Since the reef area has a Qc/Q0 ratio (Qc-crust heat flow,Q0 heat flow of ocean floor) of more than 60％ and the trough area of less than 40％,the thermal structures of the reef and trough areas correspond to the ‘hot-crust-cool-mantle'and ‘cool-crust-hot-mantle'types,respectively.The Nansha trough is a low temperature basin with a Moho surface temperature between 300-700 ℃.There is a stratified distribution of high and low temperature gradients along the vertical direction of the stratigraphic section,where the temperature gradients are 15 30 ℃/km and greater than 45 ℃/km in the upper and lower crust,respectively.The characteristic of stress distribution resembles that of a accordion bellow structure that the north side is squeezed while south side stretched,the shrinkage formation reduced from deep to shallow in the north squeezed zone,and the tension formation increased from deep to shallow in the south stretching zone.Viscosity coefficient and rheological strength are high in the north of the trough,about 2-3 orders of magnitude higher than that in the south;therefore,the tectonic deformation is more likely to occur in the south of the trough.Based on the calculation results,we reason that the thinning mechanism of the Nansha trough continental crust is related to the ‘crustal gravity balance effect'and ‘hot mantle uplift effect'.Overall,the deformation mechanism of the Nansha trough deepwater fold-thrust tectonics is largely due to the ‘crustaI shortening'followed by ‘ gravitational decollement'effects.It is noted that relationships involving ‘Moho rolling',‘crustal isostatic'or ‘hot mantle uplift'in the continental crustal evolution process,or ‘elastic-deflection'or ‘fold-thrust'in the Nansha trough basin sedimentary basement deformation,are notdiscussed in this paper.
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