南海西北部陆缘张破裂过程与特征

Mechanisms and Characteristics of Rifting and Break-up in the Northwestern Margin of the South China Sea

南海西北部陆缘是被动大陆边缘中度岩浆类型的代表,针对其伸展变形和薄化破裂的研究存在诸多争议,限制了对南海陆缘的全面认知。本研究通过高精度地震剖面分析沉积-断层-岩浆-地壳结构,结合重力反演模型,探讨西北部陆缘变形与破裂过程。结果显示:①西北部陆缘早期(Tg~T80)断层活动速率可达150~500 m/Ma,同张裂期沉积呈楔状,陆缘上地壳以脆性变形为主;晚期(T80~T70)断层活动速率下降,同张裂期沉积更加均厚,陆缘地壳变形的韧性成分占比增加;②从陆架到上陆坡,脆韧转换带与上下地壳分界面深度相近,但从下陆坡到洋陆过渡带(COT),脆韧转换带快速抬升至地壳浅层(5~10 km),上、下地壳拉伸系数均从2~3增加至4~5,推测岩浆侵位(~38 Ma)增加COT地温梯度,导致大部分地壳以韧性方式变形;③西北部陆缘北侧和南侧地壳厚度在~70 km和~30 km范围从~20 km减薄至6~7 km,结合COT的沉积充填、断裂结构和上下地壳厚度特征,提出陆缘地壳早期伸展减薄受控于南倾的单剪式断层,晚期以韧性纯剪方式伸展破裂。该研究为探讨南海北部陆缘横向差异性以及建立中度岩浆型陆缘模型提供新依据。

The structure of the passive continental margin records the deformation process of the lithosphere.The deformation characteristics of the intermediate-magma-type margin differ from those of the archetypical types.To investigate the variation in the structure and deformation of the intermediate magma margin,we explored the northwestern margin of the South China Sea,which preceded the opening of the northwest subbasin of the South China Sea.Based on the high-resolution seismic profiles,we concluded that:(1)In the earlier period(Tg-T80),the fault activity rate reached 150-500 m/Ma,and the wedge-shaped deposition was clearly controlled by the fault.In the late period(T80-T70),the fault activity rate decreased by one-third to two-thirds.The syn-rift sediment was weakly affected by the fault,showing a more uniform thickness and wider distribution.(2)The northwestern continental margin of the SCS was characterized by a large listric fault from shelf to slope.The depth of the brittle-ductile transition zone is similar to that of the Conrad discontinuity.The number and displacement of faults from the lower continental slope to the continent-ocean transition zone(COT)decreased rapidly,and the brittle-ductile transition interface rapidly rose to the shallow crust(5-10 km).According to the‘pull up’reflection of the inner crust in the conjugate continental margin and the tilted syn-rift sediment,we speculate that the continental margin experienced a certain amount of magma emplacement,and the magma emplaced at about T80,which increased the geothermal gradient of the continental margin.Taking the T80 interface as the time limit,the crust in the early stage was extended into a combination of brittle and ductile,and the proportion of ductile crust in the late stage was increased.In the late period,due to the high geothermal gradient,most of the upper crust in the Heshan Sag-Northern Zhongsha Block margin deformed in a ductile manner,resulting in the reduction of the displacement and number of faults,but the upper crust stretching factor increased from 2-3 to 4-5.(3)The crustal thickness of the northern and southern margins of the NWSB rapidly decreased from~20 km thickness to 6-7 km thickness in the range of~70 km and~30 km,respectively.The extension centers of the upper and lower crust stretching factors did not coincide.Combined with the basement reflection and the sediment seaward dipping reflector characteristics of the COT,we propose that the northern margin of the Heshan Depression-Zhongsha Block experienced two stages of earlier simple shear thinning and late pure shear deformation in a strong ductile manner.Finally,magma penetrated the strong thin ductile crust and formed the seafloor spreading center.