摘要
利用中国大陆东南缘四条北西向人工地震测深剖面的初至Pg波走时,使用有限差分层析成像方法反演剖面基底速度结构,并通过初始模型选取、射线数分布、走时拟合等手段分析结果的可靠度.研究结果表明,剖面表层速度比华北克拉通的地表速度高,闽东火山断拗带地表速度普遍高于西侧的闽西南坳陷带.相邻构造带的分界线两侧均有明显的速度横向变化,剖面东段的闽东火山断拗带内的东南沿海一带,速度变化较大,地层的褶皱变形要强于西段.基底埋深自西向东呈逐渐变浅的趋势,剖面东段的闽东火山断拗带基底起伏剧烈,基底深度与闽西南坳陷带相比较浅,与闽西北隆起带相比较深.西段的闽西北隆起带基底变化较为平缓,基底深度相对较浅;闽西南坳陷带基底深度明显加深,在龙岩新泉盆地内最深接近4km.基底形态的起伏变化揭示出东南缘基底是坳陷与隆起并存的构造特征,与研究区近代的地质构造特征相吻合.邵武—河源、政和—大埔和长乐—诏安断裂带都切割基底,其中,政和—大埔断裂4km深度下方有比较明显的低速异常带,结合本文以及现有的研究结果,进一步确认政和—大埔断裂是不同断块构造单元的分界.
Using first-arrival Pg travel time from four NW-trending DSS profiles,finite-difference tomography inversion is conducted to image basement structure beneath the southeastern margin of China mainland.The reliability of the inversion results is analyzed in terms of initial velocity model selection,ray number distribution and travel-time fitting.The results show that the surface velocity of the profiles is higher than that of the North China Craton,and that of volcanic depressions in Eastern Fujian is generally higher than the west side of the depression belt in southwestern Fujian.Both sides of the boundary between adjacent tectonic belts exhibit obvious lateral changes of velocity.The variation of the velocity is larger in the eastern profile section of the volcanic depression in Eastern Fujian along the southeast coast,where folding deformation is stronger than the western section.The thickness of crystalline substrate has a thinning trend from west to east.The relief of the basement of the the eastern profile of the volcanic depression in Eastern Fujian is dramatic,while the basement depth is relatively shallow in the depression belt in Southwestern Fujian and is deeper than the uplift belt in northwestern Fujian.The basement changes in the northwestern uplift belt of the western segment are relatively gentle and the basement depth is relatively shallow,and the basement depth in the depression belt in Southwestern Fujian is significantly deeper,and it is the deepest in the Xinquan of Longyan Basin,which is nearly 4 km.It is The relief of the basement reveals a structural feature of the coexistence of depression and uplift,consistent with the characteristics of modern geological structure in the study area.The Shaowu-Heyuan,Zhenghe-Dapu and Changle-Zhaoan faults all cut through basement.Among them,there is a relatively low-velocity anomaly zone at 4 km depth below the Zhenghe-Dapu fault.Combined with other evidence from previous geophysical investigations,it is further confirmed that the Zhenghe-Dapu fault is the boundary between different fault-block structure units.